Chapter 1
Physics Motivation
In this chapter, we give an overview of the physics motivation for the SuperKEKB
asymmetric B factory. The overview covers the e+ e− environment, achievements at
Belle, and the range of physics achievable at SuperKEKB with the Belle II experiment.
The SuperKEKB physics program is diverse, and the range of physics topics that canbe studied is very broad. This chapter provides justifications for the design integrated
luminosity, and plans for running at diﬀerent centre-of-mass energies.

1.1

Overview

The SuperKEKB facility designed to collide electrons and positrons at centre-of-massenergies in the regions of the Υ resonances. Most of the data will be collected at theΥ(4S) resonance, which is just above threshold for B-meson pair production where
no fragmentation particles are produced. The accelerator is designed with asymmetricbeam energies to provide a boost to the centre-of-mass system and thereby allow fortime-dependent charge-parity (CP ) symmetry violation measurements. The boost isslightly less than that at KEKB, which is advantageous for analyses with neutrinos in
the final state that require good detector hermeticity.
SuperKEKB has a design luminosity of 8 × 1035 cm−2 s−1 , about 40 times larger thatof KEKB. This luminosity will produce 5 × 1010 b, c and τ pairs, at a rate of about 10
ab−1 per year (see Table 1.1).

1.1.1

The Intensity Frontier

The Standard Model (SM) is, at the current level of experimental precision and at theenergies reached so far, is the best tested theory. Despite its tremendous success in2

Table 1.1: Beauty, Υ, charm and τ yields. Per year integrals are at design luminosityand are for guidance only.
ChannelBelle
BaBar
Belle II (per year)
B B̄
7.7 × 108 4.8 × 108
1.1 × 1010
Bs(∗) B̄s(∗) 7.0 × 106−
6.0 × 108
Υ(1S)
1.0 × 108
1.8 × 1011
Υ(2S)
1.7 × 108 0.9 × 107
7.0 × 1010
Υ(3S)
1.0 × 107 1.0 × 108
3.7 × 1010
3.6 × 107−
3.0 × 109
Υ(5S)ττ
1.0 × 109 0.6 × 109
1.0 × 1010

describing the fundamental particles and their interactions, excluding gravity, it does
not provide answers to many fundamental questions.
The SM does not explain why there should be only three generations of elementary
fermions and why there is an observed hierarchy in the fermion masses. The masses
and mixing parameters of the SM bosons and fermions are not predicted and must
therefore be determined experimentally. The origin of mass of fundamental particles
is explained within the SM by spontaneous electroweak symmetry breaking, resulting
in a scalar particle, the Higgs boson. However, the Higgs boson does not account forneutrino masses. It is also not yet clear whether there is a only single SM Higgs bosonor whether there may be a more elaborate Higgs sector with other Higgs-like particle
as in supersymmetry or other NP models.
Studies of symmetries have often illuminated our understanding of nature. At thecosmological scale, there is the unresolved problem with the matter-antimatter asymmetry in the universe. While the violation of CP symmetry (CP V) is a necessary
condition for the evolution of a matter-dominated universe, the observed CP V within
the quark sector that originates from the complex phase of the Cabibbo-KobayashiMaskawa (CKM) matrix is many orders of magnitude too small to explain the dominance of matter in the universe. Hence, there must exist undiscovered sources of the CP
asymmetry. Furthermore, the elements of the CKM matrix exhibit a roughly diagonalhierarchy, even though the SM does not require this. This may indicate the presence ofa new mechanism, such as a flavour symmetry, that exists unbroken at a higher energy
scale. Considering the open questions that in the SM remain unanswered, it is fairto conclude that the present theory is an extremely successful but phenomenologicaldescription of subatomic processes at the energy scales up to O(1 TeV). Many New
Physics (NP) scenarios have been proposed to explain these shortcoming of the SM, in3many of them new particles and new processes arise.
Experiments in high energy physics are designed to address the above questions
through searches of NP using complementary approaches. One approach is at theenergy frontier with the main experiments being ATLAS and CMS at the Large Hadron
Collider (LHC) at CERN. The second is the intensity frontier, exemplified by the LHCb
experiment at the LHC, the BESIII experiment at the BEPCII charm factory, and theBelle II experiment at SuperKEKB.
At the energy frontier, the LHC experiments will be able to discover new particles
produced in proton-proton collisions at a centre-of-mass energy of up to 14 TeV. Sensitivity to the direct production of a specific new particle depends on the cross section
and on the size of the data sample. At the intensity frontier, signatures of new particles or processes can be observed through measurements of suppressed flavour physics
reactions or from deviations from SM predictions. An observed discrepancy can beinterpreted in terms of NP models. This is the approach of Belle II.
The sensitivity of Belle II to NP depends on the strength of the flavour violating
couplings of the NP. The mass reach for new particle/process eﬀects can be as high as
O(100 TeV/c2 ) if the couplings are enhanced compared to the SM [1]. Sensitivity to
the contribution of a new particle or process to a particular flavour physics reaction
depends on the NP model and on the size of the data sample. In the past, measurements
of processes quantum corrections have given access to high mass scale physics before
accelerators were available to directly probe these scales.
The history of particle physics is instructive on this point. The suppression of
KL → µ+ µ− decays allowed theorists to deduce the existence of the charm quark,through the GIM mechanism, and calculate its mass from the observed rate of K − K̄
oscillations. Later, the unexpected discovery of large B − B̄ oscillations indicatedthat the top quark was heavier than the mass scale at which the direct searches werebeing carried out, contrary to the theoretical prejudices of the time. The unexpected
observation of CP V in K meson decays fifty years ago was used to predict the existenceof a third generation of quarks, and revolutionised how we think about extensions of theSM. About two orders of magnitude more data are now needed to determine whether
the observed CP V eﬀects in B meson decays are consistent with the SM.
The high-energy and intensity frontiers are associated with the complementary direct versus indirect nature of the contribution of new particles or processes to theensemble of available measurements and the distinct predictions from NP models inthese two regimes. Belle II and SuperKEKB will exploit our strengths at the intensity
frontier by moving beyond a simple observation of a NP eﬀect to its detailed characterisation through over-constraining measurements in several related flavour physics

4reactions.

1.1.2

Motivations for Belle II

Further study of the quark sector, beyond the first generation B factories, is necessary
to reveal NP at high mass scales that may manifest in flavour observables. For example, some rare flavour changing neutral current (FCNC) processes are sensitive tomass scales as high as 100 TeV, well beyond the reach of direct searches at the LHC.
Conversely, if new particles are found at the LHC, measurements in the flavour sector
may allow diagnosis of their nature. There are several important questions that can
only be addressed by further study of loop processes and rare decays in flavour physics,
described in turn below.
• Are there new CP violating phases? The answer to this question will require
new measurements of time-dependent CP V in b → s decays such as B → ϕK 0 ,η ′ K 0 with a data sample of order 50 ab−1 . If there is NP in b → d transitions,precise measurements of CKM parameters in mixing and in tree processes will
be required.
• Are there right-handed currents from NP? In this case, the experimental approach
involves measurement of time-dependent CP V in B → K ∗0 (→ KS0 π 0 )γ, or relatedmodes.
Another interesting approach involves triple-product CP V asymmetries in B →V V decays. Data samples with luminosity of order 50 ab−1 are required for
either approach. Right handed currents will also be probed in charged weakinteractions, for example B → V ℓν, V = D∗ , π.
• Are there quark FCNCs beyond the SM ? It is of great interest to measure b → sν ν̄transitions such as B → K (∗) ν ν̄, part of a class of decays with large missing
energy. It is desirable to measure FCNCs with comparable accuracy in b → d,b → s and c → u transitions.
• Are there sources of LFV beyond the SM? Neutrino experiments have found large
mixing between the νµ and ντ , raising the question: are there flavour changing
processes such as τ → µγ visible at the 10−8 level? LFV in charged lepton decay
is a key prediction in many neutrino mass generation mechanisms.
• Are there new operators with quarks enhanced by NP? Experimentally, it is crucial
to measure forward-backward asymmetries as a function of the q 2 of the dilepton,
AFB (q 2 ), in inclusive b → sℓ+ ℓ− decays and in charged weak interactions. Another5example of this approach to NP is measuring the rates and asymmetries in all
B → Kπ modes to a precision that we can determine whether or not there areenhanced electroweak penguins.
• Does nature have multiple Higgs bosons? Many extensions to the SM, such as
two-Higgs-doublet models (2HDM), predict charged Higgs bosons in addition
to a neutral SM-like Higgs. The charged Higgs will be searched for in flavour
transitions to τ leptons, including B → τ ν and B → D(∗) τ ν.
• Does NP enhance CP V via D0 − D̄0 mixing to an observable level? The SM
predicts negligible CP V in this case. Hence CP V in the D system would be a“smoking gun” for NP.
It is worth noting that not only will Belle II measure the current array of CKM
observables with unprecedented precision, it will also allow measurements of a large
number of new observables and new modes relevant to NP in the quark sector.

1.1.3

Advantages of SuperKEKB

There are many experimental reasons to choose SuperKEKB to address puzzles inflavour physics: they are discussed in turn.
• Running on the Υ(4S) resonance produces a very clean sample of B 0 B̄ 0 pairs
in a quantum correlated 1−− state. The low background environment allows forreconstruction of final states containing photons from B and π 0 , ρ± , η, η ′ etc
decays. Neutral KL0 mesons are also eﬃciently reconstructed.
• Detection of the decay products of one B allows the flavour of the other B to betagged.
• Due to low track multiplicities and detector occupancy, the B, D and τ reconstruction eﬃciency is high and the trigger bias is low. This substantiallyreduces correction and systematic uncertainties in many types of measurements,
e.g. Dalitz plot analyses.
• By utilising asymmetric beam energies, the Lorentz boost of the e+ e− system canbe made large enough so that B or D mesons travel an appreciable distance before
decaying, allowing precision measurements of lifetimes, mixing parameters, and
CP V. Note that measurement of the D lifetime provides a measurement of themixing parameter yCP , while measurement of the B lifetime (which is alreadywell measured), can be used to determine the decay time resolution function fromdata.
6• Since the absolute delivered luminosity is measured with Bhabha scattering, ane+ e− experiment measures absolute branching fractions.
• Since the initial state is completely known, “missing mass” analyses can be performed to infer the existence of new particles via energy/momentum conservation
rather than reconstructing their final states. By fully reconstructing a B decay in
a hadronic or semileptonic final state, rare decays with neutrinos can be observedor measured with minimal model dependence. Similar approaches can be applied
to charm physics.
• In addition to producing large samples of B and D decays, an e+ e− machine
produces large samples of τ leptons allowing for measurements of rare τ decays
and searches for lepton flavour and lepton numbers violation τ decays in a very
low background environment, often with zero expected background.

1.1.4

Belle achievements

The legacy of the B-factories laid the groundwork for many areas that will be further exploited at SuperKEKB. To illustrate the expected sensitivity of Belle II and
SuperKEKB to NP phenomena, we examine some of the major achievements at the
Belle experiment.
The Belle experiment accomplished its main mission, which was the verification of
Kobayashi and Maskawa’s proposal that a single irreducible complex phase can explain
CP violating phenomena. Belle’s (and BaBar’s) observation of large time-dependent
CP asymmetries in modes such as B → J/ψKS0 demonstrated that the KM proposal
was correct and laid the foundation for their 2008 Nobel Prize in Physics. The results
provided a theoretically clean measurement of one of the angles of the unitarity triangle(UT), β. After the accumulation of the ∼1 ab−1 data set, the measurements of βbecame precision results and important calibrations for NP. To check the consistencyof the SM, Belle measured the other two angles of the UT, α and γ. The results forthe sides and angles of the UT are consistent. However, NP contributions of order 10%
the size of the SM amplitude, are still allowed. In parallel with the work on fixing the
weak interaction parameters of the UT, Belle also completed a decade of studies andpublications on rare decays and QCD.
In rare decays Belle established the existence of a number of highly suppressed
processes including b → dγ and b → sℓ+ ℓ− . In addition, as the data sample has
increased, there have been a number of intriguing hints of NP in various channels, e.g.
exclusive hadronic b → s CP violating modes, and B → D(∗) τ ν, with no compelling7

evidence at the current level of sensitivity. Exploration of NP will require the luminosityof SuperKEKB.
In the τ sector searches for lepton-flavour-violating (LFV) decays and CP V havereached an interesting sensitivity at Belle but so far no NP signals have been found. The
foundation for Belle II explorations of the charm sector have also been established. The
highlights include two classes of unexpected and unanticipated results: the discovery of
D0 − D̄0 mixing and the existence of a large number of new charmonium-like resonances.
The latter was completely unexpected by the theoretical community and was guided
by Belle data. Belle is also the world’s leading two-photon facility.
KEKB’s capabilities to operate in a range of centre of mass energies allowed Belle torecord a number of unique large data sets at the Υ(1S), Υ(2S), and Υ(5S) resonances.
The Υ(5S) data were used to study some properties and decays of Bs mesons. It also
led to the discovery a series of peculiar bottomonium-like resonances and to finding
missing bottomonia states.
A timeline of specific Belle highlights are listed below.
• 2002: Belle observed mixing-induced time-dependent CP V (TCP V) in neutral B
mesons and provided a measurement of sin 2β through analysis of B 0 → J/ψK 0
and related decays.
• 2003: Belle presented evidence of TCP V in B 0 → π + π − decays and a measurement of the UT angle α. Now α (combining with BaBar) is also measured
precisely to a level of approximately 4%.
• In the same year, Belle measured TCP V in the penguin-dominated modes B 0 →ϕKS0 , K + K − KS0 and η ′ KS0 and observed that the parameter sin 2β deviated somewhat from the value measured in tree-dominated B 0 → J/ψKS0 decays. The
discrepancy has become less significant with more data, but is still statisticallylimited.
• In 2003 Belle discovered a new neutral resonance named X(3872), produced in B
decays, exhibiting properties that were inconsistent with conventional mesons. It
has been joined by discovery of a large number of other interesting new particles,
including the electrically charged Z(4430)+ in 2007.
• 2004: Data revealed for the first time direct CP V (DCP V) in B decays to π + π −and K + π − .
• In the same year, Belle released the first measurement of γ, using a new method: atime-independent measurement of the Dalitz distribution in B → D(∗) K decays.
8This approach has proven to be the most sensitive method and will be exploited
by Belle II.
• 2005: The first measurements of TCP V in B 0 → K ∗0 γ were reported. The CP
asymmetry in this decay is strongly suppressed by fermion helicity conservationin the SM, but can be enhanced in some NP models.
• 2006: The purely leptonic decay B → τ ν, was observed for the first time. The
branching fraction is sensitive to the magnitude of the least well known CKM matrix element, Vub and the B decay constant. In NP scenarios, it can be enhancedby the contribution of a charged Higgs boson.
• 2007: Belle and BaBar found the first evidence for the phenomenon of mixing inthe system of neutral charmed mesons D0 . The large rate can be accommodated
by NP but may also be explained by diﬃcult to calculate long-distance strong
interaction eﬀects in the SM.
• 2008: A measurement of the branching fraction and photon energy spectrum forfully inclusive B → Xs γ (radiative penguin diagram) decays was performed withan energy threshold of 1.7 GeV. This result is in agreement with the SM andprovides strong constraints on charged Higgs bosons from NP.
• In the same year, a measurement of DCP V in B + → K + π 0 , proved it to be
diﬀerent than the same quantity in B 0 → K + π − decays, contrary to the naive
expectation from the presence of electroweak penguin diagrams. In combination
with other B → Kπ measurements and with the larger Belle II data set, strong
interaction eﬀects can be controlled, and therefore the validity of the SM can betested in a model-independent way.
• 2011: Belle made first observations of bottomonium and bottomonium-like resonances, hb and Zb respectively. The latter is a charged bottomonium-like particle
and therefore the first beauty hadron found to contain four quarks.
Belle successfully established the KM model, measuring weak interaction parameters,
and observed suppressed SM processes. At the next stage in Belle II at SuperKEKB,
the focus will shift to NP exploration.

1.2

NP-sensitivity at Belle II

To illustrate the unprecedented sensitivity to the presence of NP eﬀects at the intensity
frontier that the improved performance of the Belle II detector and the much larger
9data set will oﬀer, we expand on some of the previous listed measurements. The
measurements presented are just few examples of the broad physics program of Belle II.
Belle II is able to perform measurements in B 0(±) and Bs(∗) meson decays, charm physics,
τ lepton physics, spectroscopy, and electroweak (weak mixing angle) measurements.
A large number of planned measurements will over-constrain the SM as well as its
extensions and will shed light on the nature of NP.

1.2.1

CKM matrix and the Unitarity Triangle

SuperKEKB can improve on all the measurements of the Unitarity Triangle (UT)
angles, α, β, γ to a precision of about 1◦ , 0.3◦ and 1.5◦ , respectively. This is suﬃcientto perform a percent level determination of the apex of the UT using only angles. The
determinations of β and γ will still be theoretically clean at the end of data taking.
The measurement of α will be theory limited and achieving such high precision requires
a suite of measurements with varying strong phases [2–4]. The measurement of γ will
be done via a range of methods using B → D(∗) K (∗) decays.
At the end of the B factory era the tension between inclusive and exclusive determinations of |Vub | [5–7] and |Vcb | [8–11] still remain, and are still the least well measured
side length parameters. The challenge at SuperKEKB is to determine new techniques
to understand this tension, based on the larger data sets at hand.
The angle γ and |Vub | are determined from tree-level processes and thus considered
to have minimal sensitivity to contributions from NP. They provide the SM “reference”
determination of the CKM UT (its apex, the values of ρ̄ and η̄. The CKM matrix element, |Vub | is measured from inclusive and exclusive b → uℓν processes. There is an
on-going eﬀort to improve the theory predictions using both the continuum methods
and lattice QCD, and a factor of a few improvement on the errors seems feasible. For
example, the present theory error on |Vub | from exclusive B → πℓν can be reduced
from present 8.7% to 2% by 2018 [12]. The persistent inclusive-exclusive puzzle must
be understood before using this precision as a test of the SM. The theoretical uncertainties in the measurement of γ from B → DK decays are even smaller. All therequired hadronic matrix elements can be measured in the cascade B → DK, D → f
decay, if enough final states f are taken into account. The irreducible theoretical errors thus enter only at the level of one-loop electroweak corrections and are below
O(10−6 ). The present experimental errors are ±12◦ in the average of the Belle [13–16]
and BaBar measurements (LHCb also provides a competitive result). The tree-level
determinations of ρ̄ and η̄ can then be compared with the measurements from loopinduced FCNCs, for example with the time-dependent CP asymmetry in B → J/ψKS0
and related modes determining the angle β.
101.2.2CP violation

One of the main strengths of Belle II is the ability to make precision measurements of
CP V, and this capability will be exploited to search for NP. The diﬀerence between
B 0 and B̄ 0 decay rates to a common self-conjugate state is sensitive to both direct
CP V (i.e. occurring in the B 0 and B̄ 0 decay amplitudes), and indirect CP V frominterference between the B → f decay and B → B̄ 0 → f mixing amplitudes. The
indirect CP V was originally measured at Belle and BaBar for all charged final statessuch as J/ψK0 and π + π − ; at Belle II this measurement will be extended to precision
studies of more challenging final states such as B → KS0 KS0 , B → K 0 π 0 and B → Xs,d γ.
b → ss̄s Penguin decays
An important search for NP is to compare the time-dependent CP asymmetries ofpenguin dominated b → ss̄s processes with the tree dominated b → cc̄s decays.
Observables that probe this are the diﬀerences of CP asymmetries, which include
SJ/ψKS0 − SϕKS0 and SJ/ψKS0 − Sη′ KS0 . Statistically limited studies of this class of decays
were performed in Belle [17–19]. CP asymmetry measurements have small theoreticaluncertainties as they involve ratios of rates, from which the leading amplitudes cancel.
The value of sin β as measured in B 0 → ϕKS0 and similar b → s transitions iscurrently in agreement with the value measured in B → J/ψKS0 decays, the current
world average diﬀerence being ∆S = sin 2βϕKS0 − sin 2βJ/ψKS0 = 0.07 ± 0.13. While the
CKM matrix elements included in the amplitudes of these decays are approximately
real, the possibility of B 0 − B̄ 0 mixing before the decay introduces an additional factor(Vtb∗ Vtd )2 ∝ e−2iβ . Hence, the decay time distribution of both decays is sensitive tosin 2β, and the diﬀerence in the value measured in the two decays is expected to vanish
within small corrections, ∆S = 0.03 ± 0.01. However, NP can contribute through loops
to B → ϕKS0 decays, and change the expectation for ∆S.
In B → K + K − KS0 decays, ϕKS0 is just one of several intermediate resonant contributions to the final state. The value of sin 2β(ϕKS0 ) is determined with a decay timedependent Dalitz plot analysis, where the accuracy of K + K − KS0 vertex determinationand particle identification are crucial for background suppression. With L = 10 ab−1of data, the experimental and theoretical uncertainties will be comparable.
Radiative B decays
Radiative decays were the cornerstone of NP searches in the B factory era. While thebranching fraction expectation appears to be reaching the theory barrier, the study of
CP V both oﬀers a theoretically clean way of probing NP that is less sensitive to decay
11dynamics. CP V in inclusive B → Xs+d γ is expected to reach 0.5% precision. Radiative
decays b → sγ are also sensitive probes of new right-handed weak currents, absent in
the SM. The helicity structure of the eﬀective Hamiltonian that describes this loop
process allows only for bR → sL γL and bL → sR γR decays, where the subscript denotes
the handedness of the particle. The amplitude of the former (latter) process depends
on the helicity flip and is proportional to mb (ms ). In mesons the bL → sR γR transition,for example, can proceed directly or via B̄ 0 → B 0 mixing; the interference leads to asmall time dependent CP asymmetry that is proportional to ms /mb . In various NP
models (e.g. left-right symmetric models) the right-handed currents are not suppressedand can lead to a sizeable CP asymmetry. A prominent example of such transitions
is B 0 → KS0 π 0 γ [20]. Within the SM, the decay time dependent CP asymmetry inthis decay is estimated to be S ≡ −2(ms /mb ) sin 2β ≡ −0.04; some SM predictions
allow for a value of |S| up to 0.1. On the other hand, in L − R symmetric models,the asymmetry can be as large as S ≡ 0.67 cos 2β ≡ 0.5. The decay-time dependence
in B 0 → KS0 π 0 γ is measured through reconstruction of the B meson decay vertex
using only pions from KS0 → π + π − decay that are constrained to the e+ e− interaction
region. The Belle II vertex detector will improve the vertex position resolution and,more importantly, increase the reconstruction eﬃciency of KS0 decays with charged pion
hits in the silicon detectors. With a data set corresponding to 50 ab−1 , the sensitivityof the measurement will reach 0.035 and thus test a range of NP predictions.
Charmless radiative decays will come to the fore in Belle II, such as S(B → ργ),
which should be measurable with a precision of better than 0.1. This will be complemented by DCP V studies of b → d transitions [21, 22].
Charmless B decays
Charmless 2-body B meson decays are another example of rare SM processes in which
the possible contribution of NP could be observed in the future. The decays B → Kπ
proceed through a tree diagram but are suppressed by the small CKM matrix element|Vub |. Thus, the contribution of the loop penguin diagram is of similar magnitude.
The interference of the two leads to a direct CP asymmetry. Neglecting additional
K + π0diagrams contributing to B + decays only, the asymmetries ACP
in B ± → K ± π 0+−
πin B 0 (B̄ 0 ) → K ± π ∓ decays are expected to be the same. However,decays and AKCP
a precise CP measurement by Belle showed a significant diﬀerence between the two,
+ π−
+ π0
− AK
= 0.164 ± 0.035 ± 0.013 [23]. The diﬀerence could be due to the∆A = AKCP
CP
neglected diagrams contributing to charged B meson decays, for which the theoreticaluncertainty is large, or to some unknown NP eﬀect that violates isospin. To resolve thisissue, a sum rule has been proposed demanding precision measurements of all isospin
12

states.
The most demanding of these measurements is the all-neutral final state K 0 π 0 . It
requires vertex reconstruction of the charged pions from the neutral kaon decays anddepends crucially on a vertex detector with a large radial acceptance. Reconstructionof the neutral pion requires very good electromagnetic calorimetry. For the final stateswith charged kaons and pions, an excellent separation between the two particle species
must be provided by the particle identification system. The main systematic uncertainty contributions (tag side interference) in the Belle measurement of B 0 → K 0 π 0are expected to be reduced with a larger data sample. At Belle II the uncertainty on
A(K 0 π 0 ) from time dependent analyses is expected to be reach approximately ∼ 3%,
suﬃcient for NP studies.

1.2.3

Rare B decays

SuperKEKB and Belle II are very well equipped to study a broad variety of rare B
decays. Excellent reconstruction of charged and neutral particles, and particle identification, with a near 100% eﬃcient trigger allow the study of decay chains involving
neutral and very weakly interacting particles. The latter can be studied using hadronicand semileptonic tagging techniques, developed for the B factories. Numerous rare B
decays that were observed with low statistics at Belle and BaBar, or not at all, will
become accessible at Belle II.
Leptonic decays
The highest profile example of a rare leptonic B decays is B → τ ν, which in the
SM results from a W -exchange diagram and has an expected branching fraction of−4(0.74+0.09[24]. This mode is sensitive to models and that predict the ex−0.07 ) × 10
istence of a charged Higgs. The eﬀect of a possible charged Higgs boson on thepartial leptonic decay width of B mesons is given by Γ(B + → τ + ν) = ΓSM (B + →τ + ν)[1 − (m2B /m2H ) tan2 β]2 , where ΓSM (B + → τ + ν) denotes the SM partial decay
width, and tan β denotes the ratio of the vacuum expectation values of the two Higgs
fields and is a free parameter of the models.
The final state contains multiple neutrinos and thus is measurable only in a e+ e−
experiment. Experimentally the leptonic branching fraction measurement consists of(partial) reconstruction of the accompanying B meson in the event, called the tagging B
meson (Btag ). Btag can be fully reconstructed in a number of hadronic decays (hadronictagging) or partially reconstructed in semileptonic decays (semileptonic tagging), where
the hadronic system (and the charged lepton) of the final state is detected while the13neutrino escapes detection. The hadronic tagging method has better purity in the
Btag sample, but suﬀers from a lower eﬃciency compared to semileptonic tagging. The
remaining particles in the event are assigned to the signal B meson (Bsig ); if they
are consistent with a possible τ decay, the undetected part of the event consists of
one or more neutrinos from (semi-)leptonic decays. The signature of such event isthus a little or no residual energy detected in the electromagnetic calorimeter, after
removing the contributions from the particles used in the reconstruction of Btag andthe τ from Bsig → τ ν. The current average branching fraction from Belle [25, 26] andBaBar [27, 28] is (1.14 ± 0.22) × 10−4 , slightly higher than the SM expectation. BelleII should reduce this uncertainty to around 3%. The related channel B → µν will
be measured to about 6% precision, and will increase the overall sensitivity to NP in
leptonic B decays.
Semileptonic decays with tau leptons
In the B factory era, semileptonic B decays were used to precisely determine the CKM
parameters. It turns out that semileptonic decays with a τ lepton are also very sensitive
to NP, with the added advantage over B → τ ν of being sensitive to the handednessof the NP current through polarisation studies. Compared to ordinary semileptonic
decays B 0 → D∗+ ℓ− ν with ℓ = e or µ, B → D(∗) τ ν decays, occurring through aquark-level b → cτ ν process, are kinematically suppressed because of the large τ mass.
The predicted branching fractions, based on the SM, are approximately 1.4% and 0.7%
for B → D∗ τ ν and B → Dτ ν decays, respectively. On the other hand, the large τ
lepton mass makes them sensitive to interactions with a charged Higgs. Therefore,
these B → D(∗) τ ν modes can be a very eﬀective probe to search for indirect evidenceof charged Higgs or other NP hypotheses beyond the SM. Furthermore, in practicethe ratio of branching fractions, B(B → D(∗) τ ν)/B(B → Dℓν) is measured, which
minimises SM form factor and some experimental normalisation uncertainties. Thus
this has the potential to be a very precise test of the SM.
An interesting anomaly in the B-factory data is the deviation from the SM of the
B → D(∗) τ ν decay rates observed by by both BaBar and Belle. In the BaBar analysis
alone, the diﬀerence from the SM prediction in the rates was approximately 3.4σ [29].
No such deviation is seen in B → τ ν. Although the B → D(∗) τ ν anomaly could
be accommodated with a NP charged Higgs exchange, it would require non-minimal
flavour structure. To settle this case the SuperKEKB dataset will be required.

14

Electroweak penguins
SuperKEKB will provide access to studies of electroweak penguin processes in all lepton
species final states, i.e. charged e, µ and τ pairs, and neutral ν pairs. The latter,
involving ν ν̄ are theoretically very clean, and Belle II should reach the SM level in
B → K (∗) ν ν̄; while the current constraints are an order of magnitude weaker [30].
Hadronic and semileptonic tagging will be used to isolate the non-signal B mesonin the event. The signature is a single kaon candidate accompanied by momentum
imbalance.
There is also a long list of interesting measurements in b → sγ and b → sℓ+ ℓ− inboth inclusive and exclusive analysis, of rates, CP asymmetries, isospin asymmetries,
angular distributions, triple product correlations, exploiting the possibility of hadronicand semileptonic tagging [31–35]. Belle II can uniquely measure inclusive processes suchas B → Xs+d ℓ+ ℓ− for which theoretical predictions have less uncertainty than thosefor exclusive processes, measured by Belle and BaBar [36, 37]. In exclusive analyses,
Belle II will precisely measure electron final states, and search for as-yet-unseen tau
final states, thereby complementing studies of muon modes at LHCb [38].
Furthermore, the branching fraction of b → sγ [39–41] can also be experimentally
improved upon by a factor of two or more.

1.2.4

B physics at Υ(5S)

It is expected that SuperKEKB will produce of order 5 ab−1 of data near the Υ(5S).
The flavour physics program for this data set is to perform searches for NP, and precision tests of the SM under SU(3) transformation. Key NP modes are those with neutral
final states or missing energy, such as Bs → γγ [42, 43] and Bs → τ τ . The latter will
exploit hadronic tagging, as done in analogous searches at Υ(4S) [44]. SuperKEKB
will also contribute to the general understanding of Bs decays, with inclusive and comprehensive studies of decay final states only possible at SuperKEKB [45]. Precision
measurements of absolute branching fractions will have an important impact on NP
reach, e.g. in Bs → µµ.

1.2.5

Charm Physics

Indirect searches for NP with charm quarks provide complementary, and analogous
constraints to searches in B decays. At tree level, NP contributions can be discerned.
Some highly calculable processes, e.g. leptonic decays of Ds mesons (Ds → µν, Ds →τ ν), can be used to determine decay constant fDs and CKM parameters |Vcs | [46].
Studies of processes forbidden in the SM at tree level in the charm sector are promising15

avenues for NP searches. Examples include D0 − D̄0 mixing, or inclusive and exclusivetransitions mediated by c → uγ [47] or c → uℓℓ.
FCNC decays in the charm system, such as D → µµ have received renewed interest, after the measurement of Bs → µµ. While heavily GIM-suppressed, long distancecontributions from D0 → γγ [48, 49], for example, also contribute and must be constrained from experiment. Direct constraint on the decay D0 → γγ would limit thesecontributions to the dimuon mode to below 10−10 .
Searches for CP violation in charm decays [50, 51] and oscillations [52] is anotherimportant example involving a FCNC. Unlike B decays, where golden channels canbe readily identified in time-dependent measurements, in D0 mixing, hadronic eﬀects
are larger and therefore a more challenging to tackle in any given mode [53]. Thisissue can be resolved by measuring a multitude of final states, only possible with
SuperKEKB. For example, to resolve ACP in D → KK or Kπ, penguin contributions
must be isolated through an isospin analysis with neutral modes. Belle II is also able
to determine absolute ACP for each given mode, rather than ∆ACP at hadron colliders,thus providing further experimental information. SuperKEKB will be sensitive to bothof the mixing parameters xD and yD in a number of golden CP self-conjugate decay
modes, hence allowing mixing induced CP violation to be observed.
The observation of processes forbidden or extremely suppressed in the SM, provide
high-impact discovery potential. Examples include lepton and baryon number violatingtransitions, and D decays to ν ν̄ [54]. The latter are precisely calculable in the SM due
to the absence of long distance contributions, and thereby provide a clean NP probe.
Such challenging modes can be studied using charm tagging based on the hadronic fullreconstruction technique developed at Belle.
There are also examples of where Belle II will perform important measurements incharm that impact on results from other results, e.g. the strong phase, ϕ, as a functionof position in the Dalitz plot for D0 → KS0 π + π − decays that feed into D0 mixing and γ
measurements. In B decays, Decays such as B → ργ have yielded measurements of the
CKM matrix element |Vtd |, and probes of NP in short-range electroweak processes,
whereas long-range contributions are suppressed. The situation is reversed in thecharm sector, where radiative decays are expected to be dominated largely by nonperturbative processes by up to 3 orders of magnitude. Given the expected dominanceof long-range processes, radiative charm decays, B(D0 → V γ), V = K ∗ , ϕ, provide an
excellent laboratory in which to test QCD-based calculations that currently limit thesensitivity of searches of NP in the charm sector.

161.2.6

Tau physics

Belle II will collect vast quantities of τ lepton decays that can be used in a number of
NP searches and SM precision tests. These include searches for LFV, CP violation,
precision measurements of the electric dipole moment and g − 2 of the τ , and precision
SM quantities such as |Vus |. The expected sensitivities to rare and forbidden τ decays
will be unrivalled. A high luminosity τ factory at the Υ(4S) peak is an ideal facility formost τ physics measurements as the data analysis can exploit correlated production
without collision background. Lepton flavour changing processes are highly suppressedin the SM, with very small but finite branching fractions due to the light mass of theneutrinos. However, in many scenarios of physics beyond the SM, large enhancementsof charged LFV are predicted, often dependent on the size of the neutrino mixing
parameters. To determine the underlying nature of any NP aﬀecting the lepton sector
we must combine results from both τ and µ decay studies with results from the neutrino
sector. One must constrain all three types of possible transitions between charged
lepton generations, two of which are studied in Belle II. The Belle II design integrated
luminosity of 50 ab−1 provides a LFV sensitivity seven times better than Belle forbackground limited modes such as τ → µγ and up to 50 times better for the cleanest
searches such as τ → eee to limits of 5 × 1010 .
It is also expected that Belle II will measure CP asymmetries in τ decays at a levelthat bounds many models of NP in a complementary way to the LFV searches. For
example, CP V in τ → KS0 πν, which is very precisely predicted in the SM, is expected
to be measured with 10−4 precision, an order of magnitude better than Belle. In somemodels this is equivalent to a B(τ → ℓℓℓ) limit of 10−10 [55].

1.2.7

Quarkonium(like) Spectroscopy

Over the past decade, the B-factories and hadron machines have discovered a large
number of states that were not predicted by conventional meson interpretation, andare instead only described by a larger number of constituents. The actual identificationof such states represents a revolution in our understanding of particle physics, and inparticular our understanding of QCD in the low energy regime. The existence of a
new fundamental type of state implies the existence of a number of new states. Under
the assumption of the molecular model there are more than 15 new states predicted
for each J P C . To build a complete picture of new states requires a comprehensive
overview aﬀorded by SuperKEKB. One of the cleanest way of studying new particles
is to produce them near resonance, achievable by adjusting the machine energy. The
clean environment of the machine implies good detection capabilities for all neutral
17

and charged particles, crucial for fully evaluating charged and neutral exotic hadrons.
Charmonium(like)√In e+ e− collisions at CM energies near s = 10.58 GeV, there are a number of ways to
produce final states that contain a cc̄ quark pair. These include: i) B-meson decays,in which b → cc̄s is a favoured transition; ii) γγ fusion, which is proportional to thesquare of the quark charge and, thus, favours production of cc̄ and uū pairs over ss̄ anddd¯ pairs; iii) near-threshold s-channel cc̄ production via initial-state radiation; and iv)
cc̄ associated production with J/ψ mesons in e+ e− annihilation, which Belle found to√be the dominant mechanism for J/ψ productions in e+ e− annihilation near s = 10
GeV. Belle exploited all four of these processes to make a series of discoveries in the
spectroscopy and interactions of cc̄ charmonium mesons.
In the last decade, the observations of the spin-singlet charmonium states hc (1P )and ηc (2S) have completed the charmonium multiplets below the open-charm threshold. Experiments have also revealed a large number of unexpected cc̄ mesons above
the open charm threshold, known as the XY Z states. Both neutral and charged states
have been found, including the Z(4430)0 [56]. These states may be manifestations ofeither tetra-quarks, cc̄g hybrids, meson molecules or other exotica. Many of the XY Z
states are narrow and some are manifestly exotic, indicating a gap in our understandingof the QCD spectrum.
Bottomonium (like)
Recent observations of the spin-singlet states hb (1P ), hb (2P ) [57], and ηb (2S) [58] havecontributed greatly to our understanding of bottomonium multiplets below the openbottom threshold. The unexpected discovery of narrow, manifestly exotic bb̄ mesons
above the open-bottom threshold has revealed a gap in our understanding of QCD.
These include the Zb (10610) and Zb (10650), where the former is only 2.6 ± 2.2 MeV
above the mB + mB ∗ mass threshold and the latter is only 2.0 ± 1.6 MeV above 2mB 2 .
Combined with a J P = 1+ assignment, it is suggestive of B B̄ ∗ and B ∗ B̄ ∗ S-wave
molecule-like states.
Future data will probably reveal additional bb̄ threshold states. Comparisons ofthese bottomonium states with the XY Z states in the charmonium system will provide
additional insight. The next phase of measurement with Belle II will systematicallyinvestigates the spectroscopy of these new states in both charmonium and bottomonium.

181.2.8

Direct searches for NP

Most of the NP searches at Belle II are indirect. However there are models that predict
NP particles at the MeV−GeV scale. Bottomonium decays can are very sensitive to
new particles at this mass scale that may have escaped detection up to now due to small
couplings with ordinary matter. This includes Weakly and non-Weakly Interacting
Massive Particles that couple to the SM via new U (1) gauge symmetries [59–61].
These models often predict a rich sector of hidden particles, that include dark mattercandidates, dark photons (A′ ) and dark Higgs (h′ ) at the GeV scale. These may decay
to SM photons via a process known as kinetic mixing, with coupling strength ϵ. There
are two associated scenarios that Belle II can probe: searches for dark matter, andsearches for new gauge bosons.
The former, low mass dark matter(χ) scenario, predicts invisible radiative decays ofΥ(1S) mesons through kinetic mixing with the hidden sector. Such signatures can bemeasured in the Υ(3S) → π + π − invisible decay [62]. The sensitivity of this search islimited by similarly behaved background that needs to be subtracted and by Υ(1S) →ℓℓ where the leptons are undetected. In the SM, invisible Υ(1S) decays proceed throughthe ν ν̄ final state with a branching fraction B(Υ(1S) → ν ν̄) ∼ (1×10−5 ), well below thecurrent experimental sensitivity. Calculations based on the thermal DM relic density
predict a rate Υ(1S) → χχ larger by one or two orders of magnitude than that of
Υ(1S) → ν ν̄.
Dark photons can for example be searched for at Belle II in various Υ and B
reactions through their coupling to lepton pairs. Dark Higgs candidates will be searched
for in “Higgstrahlung” processes, e+ e− → A′ h′, h′ → A′ A′ . Belle II will have the best
sensitivity in the mass range 100 MeV−2 GeV and is thus complementary to thededicated JLAB dark photon search and other planned experiments in Europe [63].
It is anticipated that in 2017 as many as 500 million Υ(3S) mesons will be produced
on resonance, for searches of radiative Υ transitions to DM. The data set will be anorder of magnitude greater than those currently available. Ultimately Belle II may
reach limits 2 orders of magnitude lower than previous direct NP searches at the B
factories.

1.3

Centre-of-mass energies

There are a multitude of physics topics unique to the physics program of Belle II,with rare decays and CP asymmetries in B decays at the forefront. The programprovides simultaneous studies of a wide range of areas in b-quark, c-quark, tau lepton,
two-photon, quarkonium and exotic physics. The latter two topics have come to fore19

in recent time, concerning puzzles in our understanding of QCD in describing 4-quark
states, and the search for a dark sector and light Higgs. Open questions will be addressed with extended run periods at Υ(1S), Υ(2S), Υ(3S), Υ(5S), near the Υ(6S),
and fine energy scans in intermediate regions. Measurements at Υ(5S) also oﬀer uniqueinsight into Bs decays. The physics runs will commence on the Υ(4S) to calibrate thedetector. It will be the primary energy point for the duration of the experiment, as
was the case for KEKB.

1.4

Summary

The physics goals of Belle II, as a next generation flavour factory, are to search for
NP in the flavour sector at the precision frontier, and to further reveal the natureof QCD in describing matter. A summary of the expected sensitivities for individual
observables at selected integrated luminosities is given in Tables 1.2 and 1.3.
The physics motivation for the e+ e− SuperKEKB is independent of results fromthe LHC: if LHC finds NP, precision flavour input is essential to further understandthose discoveries. On the other hand, if the LHC finds no evidence for NP, the highstatistics b, charm and τ samples provide a unique way to probe for NP beyond theTeV scale. On the interplay between e+ e− machines and LHCb: the two experiments
are highly complementary. LHCb will have high statistics samples of both Bs andB mesons and will produce measurements that dominate the all-charged final states.
However, SuperKEKB will dominate B measurements of final states with neutrinos,
or multiple photons. The e+ e− program also includes extensive precision studies of thetau and a number of other non-flavour physics topics.

20

Table 1.2: Expected errors on several selected flavour observables with an integrated
luminosity of 5 ab−1 and 50 ab−1 of Belle II data. The current results from Belle, orfrom BaBar where relevant (denoted with a †) are also given. Items marked with a‡ are estimates based on similar measurements. Errors given in % represent relative
errors.
Observables
UT angles

sin 2βα [◦ ]γ [◦ ]

Belle(2014)
0.667 ± 0.023 ± 0.012 [64]85 ± 4 (Belle+BaBar) [24]68 ± 14 [13]Gluonic penguins

S(B → ϕK 0 )
S(B → η ′ K 0 )
S(B → KS0 KS0 KS0 )A(B → K 0 π 0 )0.90+0.09−0.19 [19]0.68 ± 0.07 ± 0.03 [65]0.30 ± 0.32 ± 0.08 [17]−0.05 ± 0.14 ± 0.05 [66]

0.053
0.028
0.100
0.07

0.018
0.011
0.033
0.04

UT sides|Vcb | incl.
|Vcb | excl.
|Vub | incl.
|Vub | excl. (had. tag.)

41.6 · 10−3 (1 ± 1.8%) [8]1.2%−3
37.5 · 10 (1 ± 3.0%ex. ± 2.7%th. ) [10] 1.8%
4.47 · 10−3 (1 ± 6.0%ex. ± 2.5%th. ) [5] 3.4%
3.52 · 10−3 (1 ± 8.2%) [7]
4.7%

1.4%
3.0%
2.4%

B(B → τ ν) [10−6 ]
B(B → µν) [10−6 ]R(B → Dτ ν)
R(B → D∗ τ ν)†
B(B → K ∗+ νν) [10−6 ]
B(B → K + νν) [10−6 ]96(1 ± 27%) [26]
< 1.7 [67]0.440(1 ± 16.5%) [29]†0.332(1 ± 9.0%) [29]†< 40 [30]< 55 [30]

3%
7%
2.5%
1.6%
30%
30%

Missing E decaysRad. & EW penguins B(B → Xs γ)
3.45 · 10−4 (1 ± 4.3% ± 11.6%)−2
ACP (B → Xs,d γ) [10 ] 2.2 ± 4.0 ± 0.8 [68]−0.10 ± 0.31 ± 0.07 [20]S(B → KS0 π 0 γ)
S(B → ργ)
−0.83 ± 0.65 ± 0.18 [21]C7 /C9 (B → Xs ℓℓ)∼20% [36]−6
B(Bs → γγ) [10 ]< 8.7 [42]B(Bs → τ τ ) [10−3 ]−21

Belle II
5 ab−150 ab−1
0.012
0.008
2
1
6
1.5

10%
20%
5.2%
2.9%
< 15
< 21

7%
6%
1
0.5
0.11
0.035
0.23
0.07
10%
5%
0.3
−
< 2 [44]‡ −Table 1.3: Continued from previous page
Observables

Belle
Belle II−1
(2014)
5 ab
50 ab−1
5.31 · 10−3 (1 ± 5.3% ± 3.8%) [46] 2.9%
0.9%
−3
3.6%
5.70 · 10 (1 ± 3.7% ± 5.4%) [46] 3.5%
< 1.5 [49]30%
25%Charm Rare

B(Ds → µν)
B(Ds → τ ν)
B(D0 → γγ) [10−6 ]Charm CPACP (D0 → K + K − ) [10−2 ] −0.32 ± 0.21 ± 0.09 [69]ACP (D0 → π 0 π 0 ) [10−2 ]−0.03 ± 0.64 ± 0.10 [70]00 0
−2ACP (D → KS π ) [10 ]−0.21 ± 0.16 ± 0.09 [70]

0.11
0.29
0.08

0.06
0.09
0.03

Charm Mixing

x(D0 → KS0 π + π − ) [10−2 ]y(D0 → KS0 π + π − ) [10−2 ]|q/p|(D0 → KS0 π + π − )ϕ(D0 → KS0 π + π − ) [◦ ]0.56 ± 0.19 ± 0.07
[52]
0.13
0.05
0.30 ± 0.15 ± 0.08 [52]0.90 ± 0.16
± 0.08
[52]
0.15
0.06
4
−6 ± 11 ± 5 [52]

0.14
0.08
0.10
6

0.11
0.05
0.07
4

Tau

τ → µγ [10−9 ]τ → eγ [10−9 ]τ → µµµ [10−9 ]< 45 [71]< 120 [71]< 21.0 [72]

< 4.6
< 12
< 4.5

< 0.5
< 1.2
< 0.5

22

Bibliography[1] J. Charles, S. Descotes-Genon, Z. Ligeti, S. Monteil, M. Papucci, et al., Futuresensitivity to new physics in Bd , Bs and K mixings,
Phys.Rev. D89 (2014) 033016, arXiv:1309.2293 [hep-ph].
[2] A. Somov et al., Belle Collaboration, Improved measurement of CP -violating
parameters in B → ρ+ ρ− decays, Phys.Rev. D76 (2007) 011104,
arXiv:hep-ex/0702009 [hep-ex].
[3] A. Kusaka et al., Belle Collaboration, Measurement of CP Asymmetry in aTime-Dependent Dalitz Analysis of B 0 → (ρπ)0 and a Constraint on the CKM
Angle ϕ2 , Phys.Rev.Lett. 98 (2007) 221602, arXiv:hep-ex/0701015 [hep-ex].
[4] I. Adachi et al., Belle Collaboration, Measurement of the CP Violation
Parameters in B 0 → π + π − Decays, arXiv:1302.0551 [hep-ex].
[5] P. Urquijo et al., Belle Collaboration, Measurement Of |Vub | From Inclusive
Charmless Semileptonic B Decays, Phys.Rev.Lett. 104 (2010) 021801,
arXiv:0907.0379 [hep-ex].
[6] H. Ha et al., Belle Collaboration, Measurement of the decay B 0 → π − ℓ+ ν anddetermination of |Vub |, Phys.Rev. D83 (2011) 071101,
arXiv:1012.0090 [hep-ex].
[7] A. Sibidanov et al., Belle Collaboration, Study of Exclusive B → Xu ℓν Decays
and Extraction of |Vub | using Full Reconstruction Tagging at the Belle
Experiment, Phys.Rev. D88 (2013) 032005, arXiv:1306.2781 [hep-ex].
[8] C. Schwanda et al., Belle Collaboration, Measurement of the Moments of the
Photon Energy Spectrum in B → Xs γ Decays and Determination of |Vcb | and mb
at Belle, Phys.Rev. D78 (2008) 032016, arXiv:0803.2158 [hep-ex].

23[9] C. Schwanda et al., Belle Collaboration, Moments of the Hadronic Invariant
Mass Spectrum in B → Xc ℓν Decays at Belle, Phys.Rev. D75 (2007) 032005,
arXiv:hep-ex/0611044 [hep-ex].
[10] W. Dungel et al., Belle Collaboration, Measurement of the form factors of the
decay B 0 → D∗− ℓ+ ν and determination of the CKM matrix element |Vcb |,Phys.Rev. D82 (2010) 112007, arXiv:1010.5620 [hep-ex].
[11] P. Urquijo et al., Belle Collaboration, Moments of the electron energy spectrum
and partial branching fraction of B → Xc eν decays at Belle,
Phys.Rev. D75 (2007) 032001, arXiv:hep-ex/0610012 [hep-ex].
[12] USQCD Collaboration, T. Blum et al., Lattice QCD at the intensity frontier ,2013. http://www.usqcd.org/documents/13flavor.pdf.
[13] K. Trabelsi, Belle collaboration, Study of direct CP in charmed B decays andmeasurement of the CKM angle gamma at Belle, (2013) ,arXiv:1301.2033 [hep-ex].
[14] Y. Horii et al., Belle Collaboration, Evidence for the Suppressed Decay
B → DK − , D → K + π − , Phys.Rev.Lett. 106 (2011) 231803,
arXiv:1103.5951 [hep-ex].
[15] A. Poluektov et al., Belle Collaboration, Evidence for direct CP violation in the
decay B → D(∗) K, D → Ks π + π− and measurement of the CKM phase phi3 ,Phys.Rev. D81 (2010) 112002, arXiv:1003.3360 [hep-ex].
[16] H. Aihara et al., Belle collaboration, First Measurement of ϕ3 with aModel-independent Dalitz Plot Analysis of B → DK, D → Ks ππ Decay,
Phys.Rev. D85 (2012) 112014, arXiv:1204.6561 [hep-ex].
[17] K.-F. Chen et al., Belle Collaboration, Observation of time-dependent CP
violation in B 0 → η ′ K 0 decays and improved measurements of CP asymmetries
in B0 → ϕK 0 , KS0 KS0 KS0 and B 0 → J/ψK 0 decays,
Phys.Rev.Lett. 98 (2007) 031802, arXiv:hep-ex/0608039 [hep-ex].
[18] H. Sahoo et al., Belle Collaboration, First Observation of Radiative B 0 → ϕK 0 γ
Decays and Measurements of Their Time-Dependent CP Violation,
Phys.Rev. D84 (2011) 071101, arXiv:1104.5590 [hep-ex].
[19] Y. Nakahama et al., Belle Collaboration, Measurement of CP violating
asymmetries in B 0 → K + K − KS0 decays with a time-dependent Dalitz approach,Phys.Rev. D82 (2010) 073011, arXiv:1007.3848 [hep-ex].
24[20] Y. Ushiroda et al., Belle Collaboration, Time-Dependent CP Asymmetries in
B 0 → KS0 π 0 γ transitions, Phys.Rev. D74 (2006) 111104,
arXiv:hep-ex/0608017 [hep-ex].
[21] N. Taniguchi et al., Belle Collaboration, Measurement of branching fractions,
isospin and CP-violating asymmetries for exclusive b → dγ modes,
Phys.Rev.Lett. 101 (2008) 111801, arXiv:0804.4770 [hep-ex].
[22] P. del Amo Sanchez et al., BaBar Collaboration, Study of B → Xγ decays anddetermination of |Vtd /Vts |, Phys.Rev. D82 (2010) 051101,
arXiv:1005.4087 [hep-ex].
[23] Y.-T. Duh, T.-Y. Wu, P. Chang, G. Mohanty, Y. Unno, et al., Belle
Collaboration, Measurements of Branching Fractions and Direct CP
Asymmetries for B → Kπ, B → ππ and B → KK Decays,
Phys.Rev. D87 (2013) 031103, arXiv:1210.1348 [hep-ex].
[24] J. Charles et al., CKMFitter Group, , Eur. Phys. J. C 41 (2005) 1–131,
arXiv:0406184 [hep-ph].
[25] K. Hara et al., Belle collaboration, Evidence for B − → τ − ν̄ with a Semileptonic
Tagging Method , Phys.Rev. D82 (2010) 071101, arXiv:1006.4201 [hep-ex].
[26] I. Adachi et al., Belle Collaboration, Measurement of B − → τ − ν̄τ with a
Hadronic Tagging Method Using the Full Data Sample of Belle,
Phys.Rev.Lett. 110 (2013) 131801, arXiv:1208.4678 [hep-ex].
[27] J. Lees et al., BaBar Collaboration, Evidence of B → τ ν decays with hadronic B
tags, Phys.Rev. D88 (2013) 031102, arXiv:1207.0698 [hep-ex].
[28] B. Aubert et al., BaBar Collaboration, A Search for B + → ℓ+ νℓ Recoiling
Against B − → D0 ℓ− ν̄X, Phys.Rev. D81 (2010) 051101,
arXiv:0912.2453 [hep-ex].
[29] J. Lees et al., BaBar Collaboration, Evidence for an excess of B̄ → D(∗) τ − ν̄τdecays, Phys.Rev.Lett. 109 (2012) 101802, arXiv:1205.5442 [hep-ex].
[30] O. Lutz et al., Belle Collaboration, Search for B → h(∗) ν ν̄ with the full BelleΥ(4S) data sample, Phys.Rev. D87 (2013) 111103,
arXiv:1303.3719 [hep-ex].

25[31] J. Lees et al., BaBar Collaboration, Precision Measurement of the B → Xs γ
Photon Energy Spectrum, Branching Fraction, and Direct CP Asymmetry
ACP (B → Xs+d γ), Phys.Rev.Lett. 109 (2012) 191801,
arXiv:1207.2690 [hep-ex].
[32] S. Nishida et al., Belle Collaboration, Measurement of the CP asymmetry in
B → Xs γ, Phys.Rev.Lett. 93 (2004) 031803, arXiv:hep-ex/0308038 [hep-ex].
[33] B. Aubert et al., BaBar Collaboration, A Measurement of CP Asymmetry inb → sγ using a Sum of Exclusive Final States,
Phys.Rev.Lett. 101 (2008) 171804, arXiv:0805.4796 [hep-ex].
[34] T. Hurth, E. Lunghi, and W. Porod, Untagged B̄ → X(s+d) γ CP asymmetry as aprobe for new physics, Nucl.Phys. B704 (2005) 56–74,
arXiv:hep-ph/0312260 [hep-ph].
[35] M. Benzke, S. J. Lee, M. Neubert, and G. Paz, Long-Distance Dominance of the
CP Asymmetry in B → Xs,d + γ Decays, Phys.Rev.Lett. 106 (2011) 141801,
arXiv:1012.3167 [hep-ph].
[36] Y. Sato et al., Belle Collaboration, Measurement of the Lepton
Forward-Backward Asymmetry in Inclusive B → Xs ℓ+ ℓ− Decays,
arXiv:1402.7134 [hep-ex].
[37] J. Lees et al., BaBar Collaboration, Measurement of the B → Xs ℓ+ ℓ− branching
fraction from a sum of exclusive final states, arXiv:1312.5364 [hep-ex].
[38] R. Aaij et al., LHCb Collaboration, Diﬀerential branching fraction and angularanalysis of the decay B 0 → K ∗0 µ+ µ− , JHEP 1308 (2013) 131,
arXiv:1304.6325.
[39] S. J. Lee, M. Neubert, and G. Paz, Enhanced Non-local Power Corrections to theB̄ → Xs γ Decay Rate, Phys.Rev. D75 (2007) 114005,
arXiv:hep-ph/0609224 [hep-ph].
[40] M. Misiak, H. Asatrian, K. Bieri, M. Czakon, A. Czarnecki, et al., Estimate ofB(B̄ → X(s) γ) at O(α(s)2 ), Phys.Rev.Lett. 98 (2007) 022002,
arXiv:hep-ph/0609232 [hep-ph].
[41] T. Hermann, M. Misiak, and M. Steinhauser, B̄ → Xs γ in the Two Higgs
Doublet Model up to Next-to-Next-to-Leading Order in QCD,JHEP 1211 (2012) 036, arXiv:1208.2788 [hep-ph].
26

[42] J. Wicht et al., Belle Collaboration, Observation of Bs0 → ϕγ and Search for
Bs0 → γγ Decays at Belle, Phys.Rev.Lett. 100 (2008) 121801,
arXiv:0712.2659 [hep-ex].
[43] A. Gemintern, S. Bar-Shalom, and G. Eilam, B → Xs γγ and Bs → γγ insupersymmetry with broken R-parity, Phys.Rev. D70 (2004) 035008,
arXiv:hep-ph/0404152 [hep-ph].
[44] B. Aubert et al., BaBar Collaboration, A search for the rare decay B 0 → τ + τ − at
BABAR, Phys.Rev.Lett. 96 (2006) 241802, arXiv:hep-ex/0511015 [hep-ex].
[45] C. Oswald et al., Belle Collaboration, Measurement of the inclusive semileptonic
branching fraction B(Bs0 → X − ℓ+ νℓ ) at Belle,
Phys.Rev. D87 (2013) no. 7, 072008, arXiv:1212.6400 [hep-ex].
[46] A. Zupanc et al., Belle Collaboration, Measurements of branching fractions of
leptonic and hadronic Ds+ meson decays and extraction of the Ds+ meson decay
constant, arXiv:1307.6240 [hep-ex].
[47] S. Prelovsek and D. Wyler, c → uγ in the minimal supersymmetric standard
model , Phys.Lett. B500 (2001) 304–312, arXiv:hep-ph/0012116 [hep-ph].
[48] S. Fajfer, P. Singer, and J. Zupan, The Rare decay D0 → γγ,
Phys.Rev. D64 (2001) 074008, arXiv:hep-ph/0104236 [hep-ph].
[49] J. Lees et al., BaBar Collaboration, Search for the Decay D0 → γγ andMeasurement of the Branching Fraction for D0 → π 0 π 0 ,Phys.Rev. D85 (2012) 091107, arXiv:1110.6480 [hep-ex].
[50] M. Staric et al., Belle Collaboration, Search for CP Violation in D± Meson
Decays to ϕπ ± , Phys.Rev.Lett. 108 (2012) 071801, arXiv:1110.0694 [hep-ex].
[51] B. Ko et al., Belle Collaboration, Search for CP Violation in the Decay
D+ → KS0 K + , JHEP 1302 (2013) 098, arXiv:1212.6112 [hep-ex].
[52] K. Abe et al., Belle Collaboration, Measurement of D0 − D̄0 Mixing Parametersin D0 → KS0 π + π − decays, Phys.Rev.Lett. 99 (2007) 131803,
arXiv:0704.1000 [hep-ex].
[53] Y. Grossman, Y. Nir, and G. Perez, Testing New Indirect CP Violation,
Phys.Rev.Lett. 103 (2009) 071602, arXiv:arXiv:0904.0305 [hep-ph].

27

[54] G. Burdman, E. Golowich, J. L. Hewett, and S. Pakvasa, Rare charm decays inthe standard model and beyond , Phys.Rev. D66 (2002) 014009,
arXiv:hep-ph/0112235 [hep-ph].
[55] I. Bigi, ’Ceterum censeo Fabricam Super Saporis esse faciendam’ (’Moreover I
advise a Super-Flavour Factory has to be built’), arXiv:0804.4612 [hep-ph].
[56] R. Mizuk et al., Belle Collaboration, Dalitz analysis of B → Kπ + ψ ′ decays andthe Z(4430)+ , Phys.Rev. D80 (2009) 031104, arXiv:0905.2869 [hep-ex].
[57] I. Adachi et al., Belle Collaboration, First observation of the P -wave spin-singlet
bottomonium states hb (1P ) and hb (2P ), Phys.Rev.Lett. 108 (2012) 032001,
arXiv:1103.3419 [hep-ex].
[58] R. Mizuk et al., Belle Collaboration, Evidence for the ηb (2S) and observation ofhb (1P ) → ηb (1S)γ and hb (2P ) → ηb (1S)γ, Phys.Rev.Lett. 109 (2012) 232002,
arXiv:1205.6351 [hep-ex].
[59] M. Reece and L.-T. Wang, Searching for the light dark gauge boson in GeV-scale
experiments, JHEP 0907 (2009) 051, arXiv:0904.1743 [hep-ph].
[60] R. Essig, P. Schuster, and N. Toro, Probing Dark Forces and Light Hidden
Sectors at Low-Energy e+ e− Colliders, Phys.Rev. D80 (2009) 015003,
arXiv:0903.3941 [hep-ph].
[61] B. Batell, M. Pospelov, and A. Ritz, Probing a Secluded U(1) at B-factories,
Phys.Rev. D79 (2009) 115008, arXiv:0903.0363 [hep-ph].
[62] B. Aubert et al., BaBar Collaboration, A Search for Invisible Decays of theΥ(1S), Phys.Rev.Lett. 103 (2009) 251801, arXiv:0908.2840 [hep-ex].
[63] R. Essig, J. A. Jaros, W. Wester, P. H. Adrian, S. Andreas, et al., Dark Sectors
and New, Light, Weakly-Coupled Particles, arXiv:1311.0029 [hep-ph].
[64] I. Adachi et al., Precise measurement of the CP violation parameter sin 2ϕ1 in
B 0 → (cc̄)K 0 decays, Phys.Rev.Lett. 108 (2012) 171802,
arXiv:arXiv:1201.4643 [hep-ex].
[65] L. Santelj et al., Belle Collaboration, Measurement of time-dependent CP
violation in B 0 → η ′ KS0 , DIS 2014 Conference (2013) .

28

[66] Y. Chao et al., Belle Collaboration, Measurements of time-dependent CP
violation in B 0 → ωKS0 , f0 (980)KS0 , KS0 π 0 and K + K − KS0 decays,
Phys.Rev. D76 (2007) 091103, arXiv:hep-ex/0609006 [hep-ex].
[67] N. Satoyama et al., Belle Collaboration, A Search for the rare leptonic decays
B + → µ+ ν and B + → e+ ν, Phys.Lett. B647 (2007) 67–73,
arXiv:hep-ex/0611045 [hep-ex].
[68] L. Pesantez et al., Belle Collaboration, ACP in B → Xs γ decays, DIS 2014
Conference (2014) .
[69] B. R. Ko, Belle Collaboration, CP violation and mixing in the charm sector at
Belle, and current HFAG averages, arXiv:arXiv:1212.5320 [hep-ex].
[70] N. Nisar et al., Belle Collaboration, Search for CP violation in D0 → π 0 π 0decays, arXiv:1404.1266 [hep-ex].
[71] K. Hayasaka et al., Belle Collaboration, New search for τ → µγ and τ → eγ
decays at Belle, Phys.Lett. B666 (2008) 16–22, arXiv:0705.0650 [hep-ex].
[72] K. Hayasaka, K. Inami, Y. Miyazaki, K. Arinstein, V. Aulchenko, et al., Searchfor Lepton Flavor Violating Tau Decays into Three Leptons with 719 Million
Produced τ + τ − Pairs, Phys.Lett. B687 (2010) 139–143,
arXiv:1001.3221 [hep-ex].

29