Introduction
τ mass
BSM taudecaysZ0

Latest results on dark sector and tau physics at Belle II∼ Lomonosov Conference ∼Dark photons
Axion-likeparticle
Outro
BackupEwan Hill (ehill@mail.ubc.ca)
on behalf of theBelle II Collaboration
University of British Columbia

August 24 2021

Ewan Hill
1 / 23

The SuperKEKB accelerator is pushing the collider luminosity
frontier for Belle II to study lots of different kinds of physics
Introduction
Frontiers
Accelerator
Luminosity
Belle II Detector
B Factory
Physics goals++

τ mass
BSM taudecaysZ0
Dark photons
Axion-likeparticle
Outro
BackupEwan Hill
2 / 23Belle
B-physics, matter-antimatter asymmetry,
precision measurements, direct searches for
new physics, ...

Cosmic Particles

Fermilab image

The SuperKEKB particle collider accelerates beams of electronsand positrons, stores them in a ring, and collides them.
Introduction

I e− beam: 7 GeV

Frontiers
Accelerator

I e+ beam: 4 GeV

Luminosity
Belle II Detector

I Centre of mass energy:√s = 10.58 GeV

B Factory
Physics goals++

τ massI Continuous collision ofbeams in storage rings

BSM taudecaysI World record instantaneous
luminosities
June 21 2021:

Z0
Dark photons
Axion-likeparticle34
Lpeak
cm−2 s−1
inst. = 3.12 × 10

Outro

= 31.2 nb−1 s−1BackupEwan Hill
3 / 23arXiv:1809.01958

Recorded 213 fb−1 of data but the target = 50 ab−1 . Early daysfor experiment but enough data for initial or new studies.
Luminosity projection:

Introduction
Frontiers
Accelerator
Luminosity
Belle II Detector
B Factory
Physics goals++

τ mass
BSM taudecaysZ0
Dark photons
Axion-likeparticle
Outro
BackupEwan Hill
4 / 23https://confluence.desy.de/display/BI/Belle+II+Luminosity

Various sub-detectors measure the trajectories of charged particles,
the energies of particles, and perform particle identification
Introduction
Frontiers
Accelerator
Luminosity
Belle II Detector
B Factory
Physics goals++

τ mass
BSM taudecaysZ0
Dark photons
Axion-likeparticle
Outro
Backup

Belle II detector:
I Asymmetric particle
KL and muon detector
beam energies +
detector
EM Calorimeter
I Cylindrical layout oflayers of detectors
Belle II Detector [735 collaborators, 101 institutes,
electrons (7 GeV)
23 nations] Particle Identification
I Solenoid (1.5 T) bends
trajectories of charged
Vertex Detector
2 layers Si Pixels (DEPFET) +4 layers Si double sided strip DSSD
particles in φI
Particle identification
positrons (4 GeV)
Central Drift Chamber
detectors to distinguish
K ± from π ± etc.
Resistive Plate Counter (barrel outer layers)
Scintillator + WLSF + MPPC
(end-caps , inner 2 barrel layers)

CsI(Tl), waveform sampling electronics

Time-of-Propagation counter (barrel)
Prox. focusing Aerogel RICH (forward)

Smaller cell size, long lever arm

Belle II TDR, arXiv:1011.0352

Ewan Hill
5 / 23

SuperKEKB will produce a large number of B 0 and B ± mesons tostudy B-physics
Introduction
Frontiers
Accelerator
Luminosity
Belle II Detector
B Factory

I The collider centre of mass energy = 10.58 GeV ∼ m [Υ (4S)]→ large cross-section for producing Υ (4S)
I Υ (4S) → B B̄ : > 96% of decays

Physics goals++

τ massI SuperKEKB designed to be a “B factory” .... b-quarks !!!

2 Theory

BSM taudecaysΥ (4S) = bb̄ meson
B ≡ B ± , B 0 , B¯0
B + = ub̄ meson
B 0 = db̄ meson

Z0
Dark photons
Axion-likeparticle
Outro
BackupFigure 2.8: Feynman diagram of the B meson pair production via the ⌥(4S) resonance as done
at Belle II [9]Ewan Hill
6 / 23Main goal to study B-physics but ...

as continuum background. The cross section ratio of the continuum events and the
production of a ⌥(4S) is about 3 : 1. From the ⌥(4S), a B B̄ pair is produced in morethan 96% of the cases [19]. The production of B + B and B 0 B̄ 0 pairs is expected to beOther major goals include performing precision measurements, andsearches for new physics
Introduction
Frontiers
Accelerator
Luminosity
Belle II Detector
B Factory

Other major Belle II physics goals:

Physics goals++

τ mass
BSM taudecaysZ0
Dark photons
Axion-likeparticle
Outro
BackupEwan Hill
7 / 23

I Search for new physics through precision measurements that are sensitive tothe presence of heavy virtual particles (e.g. through studies of the τ -lepton)
I σ [e+ e− → Υ (4S)] = 1.05 nb
I σ [e+ e− → τ + τ − ] = 0.92 nb

I SuperKEKB makes lots of τ ’s too !

I Direct searches for physics beyond the standard model (e.g. Axion-likeparticles, Z 0 , dark photon)

Introduction
τ massmτ set up
Pseudomass method
Tau mass

BSM taudecaysZ0
Dark photons
Axion-likeparticle
Outro
BackupEwan Hill
8 / 23TAU MASS MEASUREMENTThe tau mass is a SM quantity that needs measuring and will help
test the SM.
Introduction
τ massmτ set up
Pseudomass method
Tau mass

BSM taudecaysZ0
Dark photons
Axion-likeparticle
Outro
Backup

I mτ is a SM quantity that needs
measuring.
I Deviations of relations involving thelepton masses in the SM could signal
new physics: e.g. test lepton
universality

I Measure mτ in:e+ e− → τ + τ − events in 4-track
final states
I Only one π 0 → γγ allowed in final
state (on the 1-prong side)π+

I B (τ → e or µ) ∝ m5τ ττSM branching ratio ofτ → e or µ is highly sensitive tothe tau mass.

e
τ+
γ/Z
e

Ewan Hill
9 / 23

τ−

π−
π+
ντ

ντ
ν`
`−

0

Data / MC

Measure mτ in just the 3-prong decays by determining theendpoint of the distribution of Mmin ≤ mτ
1.5

Mmin [GeV/c2]

1

0.5

0.4

0.6

0.8

1

1.2

1.4

1.6

1.8

2

2.2

Mmin [GeV/c2]mτ set up
Pseudomass method
Tau mass

BSM taudecaysZ0
Dark photons
Axion-likeparticle
Outro

600

I Pseudomass, Mmin , method developedby the ARGUS Collaboration

Data

Belle II (Preliminary)∫

MC total0

Ldt = 8.8 fb-1

τ(→ πππν ) τ(→ e, µ, π, ππ )ττ BGeell (l=e,µ) & eehhll(γ ) (l=e, µ)qq (q=u,d,s,c)

500

I Fit Mmin distribution to determine
end-point

400
300
200
100
0

I Correct for endpoint bias,
0.72 ± 0.12 MeV/c2 (from MC), to getmτ

BackupMmin ≡

1.5

Mmin [GeV/c2]

1
0.5
1.7

1.72

1.74

1.76

1.78

1.8

1.82

1.84

Mmin [GeV/c2]q Figure 4. Distribution of the pseudomass for e+e ! ⌧ +⌧ process with subsequen
COM − E+ !
m3π 2 + 2 Ebeam
−⇡+|p
(⌧3π
⇡ 3π
⇡ +|)
⌫¯⌧ )≤
andm
1-prong(⌧ ! ` ⌫¯` ⌫⌧ , ⌧ ! h ⌫⌧ or ⌧ ! ⇡ ⇡ 0 ⌫⌧ ) deca
3π (EτEwan Hill
10 / 23700

I Measure mτ (3-prong decay)
Eventsτ massData / MCIntroduction

ARGUS Collaboration, Phys. Lett. B 292, 221 (1992)

entire range (up), and in the range 1.70 to 1.85 GeV/c2 (bottom). The description of t
the same as that of Fig. 2.

• Momentum shift due to the B-field map: The leading source of un
for thisarXiv:2008.04665
measurement comes from a momentum scale factor of 0.056+0.0
0.0Mmin [GeV/c ]First Belle II tau mass measurement:mτ = 1777.28 ± 0.75 (stat.) ± 0.33 (syst.) MeV/c2

Figure 5. The pseudomass (Mmin ) distribution in the data sample (black points) and the resultsof the fit (blue line).

Introduction
τ massPDG average

1776.86 ± 0.12 MeV/c2

BES III (2014)

1776.91 ± 0.12 ± 0.13 MeV/c2

ARGUS (1992)

1776.3 ± 2.4 ± 1.4 MeV/c2

I Largest systematic uncertainty:
momentum shift due to B field map= 0.29 MeV/c2mτ set up
Pseudomass method
Tau mass

BSM taudecaysBelle (2007)

1776.61 ± 0.13 ± 0.35 MeV/c2

Dark photons

BaBar (2009)

1776.68 ± 0.12 ± 0.41 MeV/c2

Axion-likeparticle

Belle II (2020)

1777.28 ± 0.75 ± 0.33 MeV/c2

Z0

Outro

1773

Backup

1774

1775

1776

1777

1778

mτ [MeV/c2]

1779

1780

I Second largest systematic: estimator
bias for conversion from end-point tomass = 0.12 MeV/c2
I Each remaining systematic< 0.1 MeV/c2
1781

I Comparatively small overall σsyst. ; BES
III better having done an energy scan.

Colourlegend:Figure 6. The
comparisonof the ⌧ mass measurements obtained in this analysis (in blue text)
Systematic
uncertainty from
andvarious
Totalexperiments.
uncertainty
with the PDG
average and measurements
The green and blue bands
indicate the systematic and total uncertainties, respectively.

mτ = 1777.28 ± 0.75 (stat.) ± 0.33 (syst.) MeV/c2

The leading source of systematic uncertainty is the momentum scale factor, which

Improving systematics (B-field re-mapped): will be systematics dominated after ∼ 300 fb−1

expected to be reduced in the near future. With the present level of the systematic
EwanisHill
arXiv:2008.04665
11 / 23

Introduction
τ mass
BSM taudecays
LFV τ decays

τ → eαZ0
Dark photons
Axion-likeparticle
Outro
BackupEwan Hill
12 / 23

LEPTON-FLAVOUR VIOLATING TAU DECAYSTheir branching fractions are enhanced as highas current
sensitivity
Belle II will push the exclusion limits
of experimental
many lepton
flavour⇒Observation of LFV is a clear signature of New Physics
violating τ -decays.

n in tau decay
Introduction
τ mass

BSM taued.
decays
LFV τ decays

8

Add neutrino oscillations to SM:
Branching ratio ∼ O 10−54

τ → eαTau lepton : the heaviest charged lepton- Opens
many possible LFV decay modes which depeNewphysics:Branching ratio ∼ O 10−10 − O 10−7
K0
K0

Z0

s.Dark photons
Axion-like
hparticle
Outro
BackupSUSY

Higgs-mediation LFV

I Search for lepton flavour violating τ decay

ew Physics (NP) I Test lepton flavour conservation in SM
I Dozens of possible lepton flavour violating τ decay channels to be studied...
I Projection: extend the exclusion limits by 1-2 orders of magnitude with

50 NP
ab−1models: see backup slides
which depend on
Ewan Hill
13 / 23

The Belle II physics bookℓBelle II starting search for τ → eα, where α is invisible
Introduction
τ mass
BSM taudecaysExample lepton flavour violating τ decay thatconnects to dark matter studies:
I Search for τ → eα, where α is invisible

LFV τ decays

τ → eαI General search but α possibly a DM candidate
in some models [1, 2, 3]→3π𝞶(tag) I Previous searches:
Z

ℓh

h
h

signal side
"3-prong"

ν6&0Dark photons
Axion-likeparticlest vector
Outro

Backupreducible)

C.&Cecchi&
I Mark III (1985, 9.4
pb−1 )I ARGUS (1995, 476 pb−1 )p ⋅ T̂
T = max(∑ |p | )I Since two-body decay, search for resonance ine± momentum measuredi in approximation ofτ rest frame
I Require other τ to have 3-prong
decay foriibetter approximation of τ rest frame
I Current data set should give order ofmagnitude improvement in exclusionmise for the SM.
Ewan Hill

BELLE2-NOTE-PL-2020-018
14 / 23 µµ(γ),ee(γ),
ee𝓵𝓵, beam

C.&Cecchi&

e+

⌧
⌧ℓ

signale sideehhtag side⌫¯⌧"1-prong"

α

(NP)

6&

Introduction
τ mass
BSM taudecaysZ0
Dark sector

Dark photons
Axion-likeparticle
Outro
BackupEwan Hill
15 / 23

SEARCH FOR THE Z 0

constant gused to compute 90% credibility level (C.L.) upper limits
g0 ≤ 1 areon the standard Z0 cross section. We assume flat priors for
with
BFðZ
all positive+ −values of the cross section, while Poissonian
likelihoods are assumed for the number of observed andthe light b
simulated events. Gaussian smearing is used to model the
The finsystematic uncertainties. Results are cross-checked withshown inlog-flat priors and with a frequentist procedure based on
Again, noSearchfor resonanceinandmassof tosystem
the Feldman-Cousins
approach [30]are foundbecance [28]the LFV
recoiling again muon pair:using the1checked w(Fig. 5).
found in t
−1

Search for invisibly decaying Z 0 in µ µ events.

Dark sector

I potentially addresses(g − 2)µ anomaly

Dark photons
Axion-likeparticleµ

Outro
Backup

e

ν, χ
ν, χ

γ/Z
µ
Ewan Hill
16 / 23

10−4

Phys. Rev. D, 89, 113004. June 201480(g-2)µ±2σ

10−2

Lµ -Lτ (obs.) 90% CL UL

Belle II 2018

10−3

Z0

e

10

Lµ -Lτ , BF(Z'∫ Ldt = 276 pb
123inv)=1 (obs.) 90% CL UL

Lµ -Lτ expected UL-10e±µ± invisible) [fb]Z0

Lµ -Lτ , BF(Z'4inv)=1 expected UL

56

MZ [GeV/c2]

7

8
+ -

BSM taudecaysLµ − Lτ model:
I Z 0 does not interact with 1st
generation leptons
I includes dark matter candidate

90% C.L. upper limits on coupling constant g0 . Dark blue

FIG. 3.
filled areas show the exclusion regions for g0 at 90% C.L.,
assuming the Lμ − Lτ predicted BF for Z0 → invisible; light−1
blue
areas are for BFðZ0 →µinvisibleÞ ¼ 1. The solid and dashed lines
are the expected sensitivities in the two hypotheses. The red bandshowsII,thePhys.
regionRev.
that could
the muonApril
anomalousBelle
Lett. explain
124, 141801.
2020, magnetic
moment ðg−2Þμ #2σ [1,5]. The step at M Z0 ¼ 2mμ for the Lμ − Lτ
BELLE2-NOTE-PL-2020-012

I Simulations: can probe(g − 2) band with ∼ 50 fb
0

ε σ(e eτ massg

Introduction

70
60
50
40
30
20

0

0

FIG. 5. 9
ϵ × σ½eþ e−Introduction
τ mass
BSM taudecaysZ0
Dark photons
Axion-likeparticle
Outro
BackupEwan Hill
17 / 23

SEARCH FOR DARK PHOTONSDark photon model could produce events with missing energy ordisplaced vertices
Introduction

Inelastic DM modelτ massIf χ2 decays outside of detector:

BSM taudecays

e−

Z0

χ1
γ

e− , µ− , hadronA"e+ , µ+ , hadron

χ2

Dark photons

A"∗

e+

Axion-likeparticle

γ
χ1

Outro

I Only directly detect initial stateradiation: γI Single photon trigger with
0.5 GeV threshold
I Large background from e+ e− → γγ (γ)

Figure 2. The Feynman diagram depicting the photon and displaced fermion signature in the
1 scenario.
context of the inelastic DM

I χ = DM candidate

with

JHEP02(20BackupI Single photon search

I Some cosmic muons background

I Will have competitive results with the!√" √√(8x2 − 4xdata+ 2) logset.1−4x+1+ 1 − 4x(2x + log x)
existing
2 x

If χ2 decay in the detector:√.
(2.13)
4x2 1 − 4x
I search for displaced vertex
At the hadronic scale the operator Oq matches onto the DM-nucleon operator
F3 (x) =Ewan Hill
18 / 23

ON = NN χ1 χnumber:
1
Journal of High Energy Physics,
Article39 (2020)(2.14)

Introduction
τ mass
BSM taudecaysZ0
Dark photons
Axion-likeparticle
Motivation
Results

Outro
BackupEwan Hill
19 / 23

SEARCH FOR AXION-LIKE PARTICLESBelle II sensitive to axion-like particles as portals to dark matter.
Introduction
τ massAn Axion-like particle, a

BSM taudecays

I couples to bosons. Here focus on a → γγ

Z0I could be a “portal” or “mediator” to connect SM to Dark Matter candidates ifma ∼ O 1 GeV/c2

Dark photons
Axion-likeparticle
Motivationγ

Results

DM

Outro

e+

Backup

a

a

γ∗

γ

e−

DM
Ewan Hill
20 / 23γ

After selecting clean events with self-consistent photons, no excess
observed and exclusions set

PHYSICAL REVIEW LETTE
Introduction
τ mass
BSM taudecaysZ

0

Dark photons
Axion-likeparticle
Motivation
Results

Outro
Backup

I 445 ± 3 pb−1 of data taken in 2018
I Search for bump on largee+ e− → γγγ background
I Require that the photon t/∆t are allconsistent with each other
I No tracks from the interaction point√√
I 0.88 s ≤ mγγγ ≤ 1.03 s
I No significant excesses observed
I Even with a small data set, results
exclude previously unexplored parts ofphase space.

Ewan Hill
21 / 23

Belle II, Physics Review Letters 125, 161806 (2020)

FIG. 5. Upper limit (95% C.L.) on the ALP-photon couplingfrom this analysis and previous constraints from electron beamdump experiments and eþ e− → γ þ invisible [6,9], proton beamdump experiments [8], eþ e− → γγ [11], a photon-beam experiment [12], and heavy-ion collisions [13].

where αQED is the electromagnetic coupling [6]. ThisIntroduction
τ mass
BSM taudecaysZ0
Dark photons
Axion-likeparticle
Outro
End/Future

BackupEwan Hill
22 / 23

OUTRO

Early Belle II results show signs of promise for the future
Introduction
τ mass
BSM taudecaysZ

0

Dark photons
Axion-likeparticle
Outro
End/Future

BackupSummary:

To get to the future:

I Some early results already probing theunexplored

I Remove “draft” pixel detector and insert
full one

I Other early measurements show promise forthe future

I Upgrades to accelerator (shorter term)

I The collider has set a new world record for
instantaneous luminosity

I Polarized beams?

I There is still a lot of work to be done toreach target of 50 ab−1

I Upgrades to detector (longer term)
I Me: job applications

For all the latest Belle II results see:https://confluence.desy.de/display/BI/Journal+Publications
https://docs.belle2.org/
https://arxiv.org/archive/hep-ex
Ewan Hill
23 / 23:D

AVENGERS: BELLE II - POST-CREDITS SCENE.... i.e.
BACKUP SLIDESIntroduction
τ mass
BSM taudecaysZ0
Dark photons
Axion-likeparticle
Outro
BackupSuperKEKB
Tau mass

τ LFV exclusions
Axion-like particle

Ewan Hill
24 / 23

The higher luminosities are largely achieved by squeezing thebeams to be even smaller at the collision point
Introduction
τ mass
BSM taudecaysZ0
Dark photons
Axion-likeparticle
Outro
BackupSuperKEKB
Tau mass

τ LFV exclusions
Axion-like particle

Ewan Hill
25 / 23

Instantaneous luminosity ofSuperKEKB ×30 that of KEKB
(old collider):I ×1.5: more particles per
beam (increased current,
number of bunches, etc.)
I ×20: squeezing the beams(“nano-beam” collision
scheme)SuperKEKB set a world record for instantaneous luminosity inJune 2020 while on our way to target nominal specifications
Introduction
τ mass
BSM taudecaysZ0
Dark photons
Axion-likeparticle
Outro
BackupSuperKEKB
Tau mass

τ LFV exclusions
Axion-like particle

https://www.kek.jp/en/newsroom/2020/06/26/1400/
Ewan Hill
26 / 23https://www.bnl.gov/newsroom/news.php?a=117285

Look at di-tau events with one 1-prong tau decay and one tau
decay to 3 charged pions
Introduction
τ mass
BSM taudecaysZ0

Measure tau mass in di-tau events: e+ e− → τ + τ −Require four track final state: Require one 1-prong decay and one 3-prong decay of
the two taus:

Dark photons
Axion-likeparticle
Outro
BackupSuperKEKB
Tau mass

τ LFV exclusions
Axion-like particle

I Selected 1-prong tau decays:I τ − → (1 or 2 ν) ≤ 1 π 0 (1 charged particle)
I τ − → ν τ h− ,h− ≡ π − or K −−−0
I τ → ντ π π
I τ − → ντ `− ν̄` ,`− ≡ e− or µ−

I Selected 3-prong tau decays:
I τ + → ν̄τ π + π − π +I Results in at most one π 0 in the final state.
Ewan Hill
27 / 23

Assume charge conjugates throughout

A simple selection to pick out clean events and good charged pions
is usedIntroduction

8.8 fb−1 of data taken in 2019τ mass
BSM taudecaysZ0
Dark photons
Axion-likeparticle
Outro
BackupSuperKEKB
Tau mass

τ LFV exclusions
Axion-like particle

Some of the event selections:I EECL /plab < 0.8 for charged pions
I Enhances the selection of τ + → ν̄τ π + π − π +I For π 0 → γγ:
I Require EECL (γ) > 100 MeV
I Require 0.115 < mγγ < 0.152 GeV/c2
I Reject events with a photon of E > 200 MeV that is not the daughter of a π 0
I Reduces background contamination from e+ e− → q q̄ processes.
After selections:
I Efficiency of reconstructing signal events = 16.6%

Ewan Hill
28 / 23

I Purity of sample = 84.5% (over non-zoomed Mmin window).

Measure mτ in just the 3-prong decays by determining theendpoint of the distribution of Mmin ≤ mτ
Introduction
τ massMeasure mτ (3-prong decay)

BSM taudecaysPseudomass, Mmin , method developed by the ARGUS Collaboration.

Z0

Take pτ = p3π + pντDark photons
Axion-likeparticle
Outro
BackupSuperKEKB
Tau mass

τ LFV exclusions
Axion-like particle... Assume cos α (p3π , pντ ) = 1, mν = 0 ...
qCOM − E
Mmin ≡ m3π 2 + 2 Ebeam
3π (E3π − |p3π |) ≤ mτI Fit the Mmin distribution for the end-point → mτ .

I Apply corrections to compensate for the neutrino assumptions etc.

Apply somewhat simple event selection to 8.8 fb−1 of data taken in 2019....
Ewan Hill
29 / 23

ARGUS Collaboration, Phys. Lett. B 292, 221 (1992)

Data / MC

1.5

Mmin [GeV/c2]

Correct end-point position by 0.72 MeV/c2 to get tau massmeasurement.
M [GeV/c ]
1

0.5

0.4

0.6

0.8

1

1.2

1.4

1.6

1.8

2

2.2

2

min

Introduction

700

τ mass600

BSM taudecays∫Dark photons

MC total0

Ldt = 8.8 fb-1

τ(→ πππν ) τ(→ e, µ, π, ππ )ττ BGeell (l=e,µ) & eehhll(γ ) (l=e, µ)qq (q=u,d,s,c)

500

EventsZ0

Data

Belle II (Preliminary)

400
300
200

Axion-likeparticle

100

Outro

SuperKEKB
Tau mass

τ LFV exclusions

0

Data / MC

Backup1.5

Mmin [GeV/c2]

1
0.5
1.7

1.72

1.74

1.76

1.78

1.8

1.82

1.84

Mmin [GeV/c2]

Axion-like particle

MC

I Fit of+ e ! ⌧ + ⌧end-point.
Mthe
distributionfor
to edetermine
Figure 4. Distribution
process with subsequent 3-prong
min pseudomass
0 ⌫ truth(⌧ + ! ⇡ + ⇡ ⇡ + ⌫I¯⌧ ) Difference
and 1-prongbetween(⌧ ! `measured⌫¯` ⌫⌧ , ⌧ MC
! hend-point⌫⌧ or ⌧ and! ⇡m⇡MC) decaysin theis 0.72± 0.12 MeV/c2 .
τ ⌧entire range (up),Iand in the range 1.70 to 1.85 GeV/c2 (bottom). The description of the plot is
Use this measured bias in MC to convert measured end-point in data to mτ measurement.
the same as that of Fig. 2.

Ewan Hill
30 / 23

arXiv:2008.04665

Backgrounds small and flat in the end-point region
Introduction

12000

τ mass

10000

SuperKEKB
Tau massττ BG

eell (l=e,µ) & eehhll(γ ) (l=e, µ)qq (q=u,d,s,c)

9000

Belle II (Preliminary)8000∫Ldt = 8.8 fb

7000

Events6000

Data

MC total0-1

τ(→ πππν ) τ(→ e, µ, π, ππ )ττ BGeell (l=e,µ) & eehhll(γ ) (l=e, µ)qq (q=u,d,s,c)

6000
5000
4000
3000
2000

2000

Axion-likeparticle

1000
0

0

Data / MC

Backupτ(→ πππν ) τ(→ e, µ, π, ππ )4000

Dark photons

Outro

-1

1.5

Data / MCZ0

MC total0∫Ldt = 8.8 fb

8000

Events

BSM taudecaysData

Belle II (Preliminary)M3π [GeV/c2]

1
0.5
0.4

0.6

0.8

1

1.2

1.4

1.6

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Mmin [GeV/c2]

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0.4

2

0.6

0.8

1

1.2

1.4

1.6

1.8

2

2.2

Mmin [GeV/c2]2

M3π [GeV/c ]τ LFV exclusions
700
Figure 3. The invariant mass M3⇡ of the three tracks on the 3-prong side after the selection
Belle II (Preliminary)
criteria. The description of the plot is the same as that of Fig. 2.
600
Ldt = 8.8 fb-1

Axion-like particle

I Dominant background in these plots is from other 3-prong tau decays but does∫
not contaminate the end-point region where fit is performed.
I Small
flatmass
backgroundin thefit backgroundregionextracting the ⌧ mass.
Unlike theandinvariantdistribution where
the main
Events

500
400
300

contamination is due to misidentified ⌧ + ! ⇡ + ⇡ ⇡ + ⌫¯⌧ decays, in the region of interest
Ewan
Hill pseudomass distribution all background contributions are negligible except200forof thearXiv:2008.04665
31
e+/e23 ! q q̄. While⌧ + ! ⇡ + ⇡ ⇡ + ⌫¯⌧ decays show the endpoint behaviour, the background
100

Data

MC total0τ(→ πππν ) τ(→ e, µ, π, ππ )ττ BGeell (l=e,µ) & eehhll(γ ) (l=e, µ)qq (q=u,d,s,c)

Tau mass measurement systematics
Introduction
τ mass
BSM taudecaysZ0
Dark photons
Axion-likeparticle
Outro
BackupSuperKEKB
Tau mass

τ LFV exclusions
Axion-like particle

Ewan Hill
32 / 23

Systematic uncertainty

MeV/c2

Momentum shift due to the B-field map 0.29
Estimator bias
0.12
Choice of p.d.f.
0.08
Fit window
0.04
Beam energy shifts
0.03
Mass dependence of bias
0.02
Trigger eﬃciency
 0.01
Initial parameters
 0.01
Background processes
 0.01
Tracking eﬃciency
 0.01
Table I. Summary of systematic uncertainties.
arXiv:2008.04665

and about 300 fb 1 of data would be needed to become systematically dominated, as
illustrated in Fig. 7. The systematic uncertainties can be reduced further by increasing
the Monte Carlo statistics in the estimation of the bias, which is currently the secondlargest systematic uncertainty. Thus, a better systematic precision is expected in the
future.

Future of the Belle II tau mass measurement

Introduction

BSM taudecaysZ0
Dark photons
Axion-likeparticle
Outro
BackupSuperKEKB

1

Stat. Precision [MeV/c2]

τ mass0.8

I Magnetic field has been remapped

Belle II
luminosity projection

0.75 MeV/c2 @ 8.76 [fb-1]

0.6
0.4
0.31 MeV/c2 @ 50 [fb-1]
0.22 MeV/c2 @ 100 [fb-1]

0.2

I After improvements in the momentum
scale factor systematic uncertainty,
expect a future total systematic
uncertainty of ∼ 0.15 MeV/c2 .

I After that, need ∼ 300 fb−1 of data forthe measurement to become
systematically dominated.

0.13 MeV/c2 @ 300 [fb-1]

Tau mass

τ LFV exclusions
Axion-like particle

0
10

102

-1

103

Luminosity [fb ]Figure 7. Projection of the statistical uncertainty as a function of luminosity for the ⌧ mass2
τmeasurement. The black
dot represents the statistical uncertainty in this measurement and the
Ewan
red Hill
triangles mark the projected statistical uncertainties for 50, 100 and 300 fb 1 .

m = 1777.28 ± 0.75 (stat.) ± 0.33 (syst.) MeV/c

33 / 23

arXiv:2008.04665

Test of lepton universality
Introduction
τ mass
BSM taudecaysZ0

m5

I Uncertainties on mµ are much smaller thanuncertainties on mτI ττ is the lifetime of the τ

ττ
τ
BτSM
` ∝ Bµe τµ m5
µ
...
5
BτSM
e ∝ m τ ττ

I Bτ ` is the branching ratio of τ decaying to `νν

0.1790Dark photons
Axion-likeparticle

I We can measure Bτ e , mτ , and ττ

0.1785●

Backup

B ʹτe

Outro
0.1780SuperKEKB
Tau mass

τ LFV exclusions

0.1775

Axion-like particle

HFLAV
Spring 20170.1770
289

290

291

ττ(fs)
arXiv:1804.08436
Ewan Hill
34 / 23

Using semi-hadronic processes

292

SM SM
FigureTest ofprediction of the
I 1.TheBτthee equation is what the Standard Model
relation between⌧ leptonic
says onthehowBτ e branchingvaries with ττ after inputting
fractions and the ⌧ lifetime and mass. B0⌧emτ .
denotes the statistical average of
point
Be = IB(⌧ The! e¯⌫ered⌫⌧ ) andthe Bise SMdata measurement , B data measurement =prediction τfromτ the Bµ measurement
τe
Be (Bµ ) = ((290.3
Bµ · ( f⌧e / f±
The yellow
band ± 0.04) %).
⌧µ ).0.5)
fs, (17.85
represents the uncertainty from theSM⌧
I The yellow line is Bτ e , based on the measured
lifetime.

value of mτ with a width corresponding to theτ lifetime uncertainty, which is dominated bythe τ mass uncertainty.

τ mass
BSM taudecaysZ0
Dark photons
Axion-likeparticle
Outro
BackupSuperKEKB
Tau mass

τ LFV exclusions
Axion-like particle

lγ

lP0

0

lS

lV0

lll

lhh

Λh

10-5
10-6
10-7

CLEO
BaBar
Belle
LHCb
Belle II

10-8
10-9
10-10

e- γ
µ- γ
e-- π00
µ -π
e η
µ- η
e- η'
µ - η'
e- KS0µ - -KS0
e f0
µ- f
e- ρ0
µ - ρ00e- K*µ - K*
e K*
µ- K*
e- φ
µ- φ
e-- ω
µ ω
e- e+ eµ - e+ ee- µ + µ µ -- µ + µ -e µ+ e
µ - e+ µ e- π+ πµ - π+ π-e-- π+ K µ π+ K
e- K ++ πµ - K π-e-- K ++ K µ K K
e- KS0 KS0µ - KS0 KS0π- e+ ππ- µ + π-π- e+ K π-- µ + K K- e+ K K µ+ K
π- Λ
π-- Λ
K Λ
K Λ

Introduction

90% C.L. upper limits for LFV τ decays(projected) exclusion limits for lepton flavour violating τ decays

Fig. 177: Current 90% C.L. upper limits for the branching fraction of ⌧ LFV decays obtained in the CLEO,
Ewan Hill

BaBar, and Belle experiments. Purple boxes, blue inverted triangles, green triangles and yellow boxes show
CLEO, BaBar, Belle and LHCb results, respectively, while red circles express the Belle II future prospects,
where they are extrapolated from Belle results assuming the integrated luminosity of 50 ab 1 .

35 / 23

The Belle II physics booka

Introduction
τ mass
BSM taudecaysZ0
Dark photons
Axion-likeparticle
Outro
BackupSuperKEKB
Tau mass

τ LFV exclusions
Axion-like particle

Ewan Hill
36 / 23

selection asasa asystematic
uncertainty.
addbackgroundthese threePDF (0.5 GeV=c2 ), in
half of the resulting mass-resolution difference
right) changes
in theWedefault
uncertainties
no correlations
systematic uncertainty. The effect of a $2
mm shift ofin quadrature
the photon assumingenergy selection
criteria (4.0 GeV=c2 ), and in theamongstthem.
total relativedetermination
uncertainty method
due to (6.85
the GeV=c2 ).
the interaction point relative to the calorimeter
is foundto Theinvariant-mass
selection
is approximately 5.5% for ALP masses
have a negligible impact on the mass resolution
andefficiency
is notabove 0.5 GeV=c2 , multiplied
and increases
toapproximately
8%
foryield, are shown inincluded as a systematic uncertainty.
by the sign of the signal
the lightest
considered.
Aslocaladditional
system-including systematic
We describe the backgrounds by polynomialsof ALP
the masses
Fig. 3.
The largest
significance,
PHYSICALREVIEW
LETTERS 125, 161806 (2checkswe vary uncertainties,
the photon-timing
selectionandminimum complexity consistent with theaticdatafeatures.
is foundnearbym!1
a ¼ 0.477 GeV=c with a
Polynomials of second to fifth order are used:second for classifier
the shower-shapevalue of selection
S ¼ 2.8σ.by !5% to accountcandidate. We corre2
possible between
datadividing
and simulation
0.2 < ma ≤ 0.5 GeV=c2 , fourth for 0.5<mafor≤6.85GeV=c,
By
the signalsamples,yield bythe
the signal efficiency
that ranges from 0
invariantmass Mγγγ
by !0.002
GeV=ctoand fifth for 6.85 < ma ≤ 9.7 GeV=c2. The
backgroundandselection
the integrated
luminosity,
we2 obtain
the ALP cross
We vary the energ
polynomial parameters are not fixed by simulation(b)
account but
for areuncertainties
energy,and confidence
thesection inσ a(a).theWebeamcomputethe 95%level (C.L.)
separation selection
free parameters of each data fit. Each fit ispolar-angle-acceptance
performed in a
selectionby σpropagating
the effectupper
limits onof ma using a one-sided
a as a functionof !5 mrad, and ta
mass range that corresponds to −20σ CB to þ30σ
M2γγshift, ofof a CB!2formm
the interaction
point relative
to the[31]. For each ma fit
frequentist
profile-likelihood
methodsignal selection ef
calorimeterfor maximal
result,we reportpossible
the leastmisalignment
stringent of all 95% C.L. upperand −25σ CB to þ25σ CB for M2recoil. In addition,
thetofitaccountselection as a syste
2 4 For all of
limitsthe variations
the ECL.
thesedeterminedchecks, wefrom
find thatthey have ofa background model
ranges are constrained between M2γγ > 0ofGeV=c anduncertainties in q
andsignal
fit range.
We efficiency,
convert thesocrosseffectselection
we dosection limit to theM2recoil < 100.5 GeV2 =c4 . The choice ofnegligible
the orderof on theamongst them. The
coupling limit
using with them.
not associate
any systematic
uncertainty
background polynomial and fit range is optimized
based
selection efficiency
Weχ 2 extractof ma byon the following conditions: giving a reduced
close tothe signal yield as a function"#
2
above 0.5 GeV=c2 ,a series of independent binned
maximumgaγγ
αQED
m2a 3
one, providing locally smooth fit results, performing
and being conσ¼;
1−
the lightest ALP malikelihood
fits. We use 100 bins fora each fit
24 range. The
ssistent with minimal variations between adjacent
fit ranges.
2
atic checks we vary
Peaking backgrounds from eþ e− → Pγ fits
are areveryperformed
small in the range 0.2 < ma < 6.85 GeV=c
the shower-shape c
2
2
for the M
spectrum, and in the range 6.85 < ma <compared
expected (b)statisticalonγγ theFIG. 1. M2recoil distribution(a) andtoMthe
together uncertainty
γγ distribution
for possible betwe
Photonenergycuts:
9.7 GeV=c
for the M2recoil spectrum. The resolution of
withI
the stacked
contributions
fromthe different
SM adequately
signalyield
andfound tosimulated
be modeledby2 theinvariant mass M
background samples.
For M2 ≤ 16backgroundGeV2 =c4, theselection is
M2γγ worsens with increasing ma , while that of M 2recoil
PDF.
I mpolynomial
>
4
GeV:
E
>
0.65
GeV
a
γ
2
2
4
(b)
account for uncert
16 systematic
GeV =c , ituncertainties
is Eγ > 0.65
GeV.
Eγ > 1.0 GeV; for M >Thedue
to the signal
efficiency
improves
with increasing ma (see Fig. 2). The transitionI
polar-angle-accepta
m
≤
4
GeV
:
E
>
1
GeV
Simulation is normalized
luminosity.
The inset
showsa totheγ in (b) areandsignal mass
resolution
includedas Gaussian
between
M 2γγ and M2recoil fits is determined as the point of
2 =c4 .
of a !2 mm shift
an enlargement
low-massregionshaping
M 2γγ <
1 GeV
Iof thenuisance
parameters
witha effects
width
equal
the systematic
Helps
avoid
ontoequal
thesensitivity obtained using background simulations.
calorimeter to acco
uncertainty. The systematic uncertainty due The
to thebacksignal
probability density function (PDF) has two
background
mass
distributionof the ECL. For all oground shape, which is the dominant sourceof systematiccomponents:
a peaking contribution from correctly reconnegligible effect on
is estimatedrepeating all
fits withsignal
alter- photons and a combinatorial-background
I Look at uncertainty,
structedmγγ , andsimilar byquantity
calculated
not associate any s
native fit ranges changed by $5σ CB and contribution
with the polyfrom
the
other
two
combinationsofphotons.
from recoil
photonenergyby $1. For each mass value ma ,We extract thenomialorders modified
We model the peaking contribution using a Crystal Ball
performing a serie
we report the smallest of all signal significancevalues[29]. The mass-dependent CB parameters(CB) function
likelihood
fits. We
determined from each background model.used
Theinlocal
sig-to data are fixed to those obtained by fitting
the fits
fits are performednificance
is given
by For the simulated M 2
simulated
events.
ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi systematic uncertainties
pffiffiffiffiffiffiincluding
recoil distribution, the2 2for the M2γγ spect
S ¼ 2LetterslnðL=L125,
L (2020)is the maximum
likelihood
M2
distribution(a) and MM
bkg Þ, where
Belle II, Physics Review
161806
CB meanis found FIG.
to be1.unbiased.
For the simulated
γγ distribution (b) together
FIG. 4.

Expectedand observed upper limits(95% C.L.) on therecoilγγ