Annual Progress Report for JFY 2010
March 24, 2011

CDF Experiment

Spokespersons
Japan F. Ukegawa (University of Tsukuba)
U.S.A. R. Roser (Fermilab) and G. Punzi (University of Pisa)

The CDF experiment at the Fermilab Tevatron proton-antiproton collider continued toaccumulate high-statistics data and produce rich physics results in 2010. The accelerator
performance has been superb. The instantaneous luminosity at the beginning of storesconsistently exceeds L = 3 × 1032 cm−2 s−1 , with a record luminosity of 4.02 × 1032 cm−2 s−1
in April 2010 (Figure 1 (top)). A weekly integrated luminosity in excess of 60 pb−1 has also
been achieved regularly. The integrated luminosity in the calendar year 2010 amounted toabout 2.5 fb−1 and the Run II total to date is now about 10.5 fb−1 (Figure 1 (bottom)).
The upgraded CDF detector has been operational very smoothly and eﬃciently. The
integrated luminosity recorded by CDF amounts to 8.8 fb−1 , roughly 80 times that of Run I(Figure 2). CDF expects to collect a total of about 10 fb−1 at the end of US ﬁscal year 2011.
CDF has analyzed up to 7.1 fb−1 of data thus far, depending on data sets and physics topics, and many important and interesting results have emerged. In this report we summarize
these physics results, with an emphasis on contributions from the Japan group.

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Physics Results

1.1

Top quark physics

The top quark and its properties have been studied extensively since its discovery by CDF
in 1995. With increased statistics of Run-II data, these studies are now performed with highprecision.
The top quark analyses with Run-II data include measurements of the mass and production cross section using various decay channels.
The top quark mass is extracted using various techniques and channels. By combining
all measurements, CDF ﬁnds the top quark mass to be mt = 173.1 ± 0.7 ± 0.9 GeV/c2 . The
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precision has long surpassed the original Run-II (2 fb−1 ) goal of 3 GeV/c2 and is now better
than 1%. Using also the measurements by the D0 experiment, we derive the combined top
quark mass of mt = 173.3 ± 0.6 ± 0.9 GeV/c2 (July 2010).
A measurement of the W boson mass has been performed by CDF. Combining it with top
quark mass measurements provides indirect information on the Higgs boson mass throughelectroweak radiative corrections. Figure 3 shows the results as of July 2010. A constraint
31
2
on the standard model Higgs boson mass is obtained to be 96 +− 24 GeV/c at 68% CL, andis less than 171 GeV/c2 at 95% CL (Figure 4).
Other production and decay properties of the top quark are also studied, including tt̄ spincorrelations and forward-backward asymmetry at production, and the W ± boson helicity inthe decay.
The tt̄ forward-backward asymmetry at production in proton-antiproton collisions is ofinterest. Since the initial state is CP symmetric, and the strong interaction conserves parity,
one naively expects there should exist no forward-backward asymmetry. However, it ispredicted that the interferences between the lowest order and higher order processes produce
a small asymmetry, of order 10%. And if a new strong interaction beyond the standard model
exists and it violates parity (such as axi-gluons), an asymmetry will be naturally introduced.
At CDF, a measurement using the lepton plus jet ﬁnal state produced an intriguing result,
AFB = +0.475 ± 0.114, to be compared with a QCD prediction of +0.088 ± 0.013. A similar
analysis is performed using the di-lepton ﬁnal state, giving AFB = +0.42 ± 0.15 ± 0.05 [1],to be compared with a QCD prediction of +0.06 ± 0.01. The observed raw asymmetry is
shown in Figure 5. Both of the channels give asymmetries larger than the predictions, andthey are larger at higher tt̄ mass regions. Therefore it could be a hint of new physics at highmass. The di-lepton analysis is performed by Y. Takeuchi (Tsukuba).

1.2

Bottom quark physics

Since the observation of the particle-antiparticle oscillations of the Bs0 meson and a measurement of their frequency ∆ms in 2006, CDF has turned its attention to more detailed studies
of B-hadron decays and searches for possible new eﬀects originationg from physics beyond
the standard model and the CKM theory.
For example, we have looked for CP violation in Bs0 -B̄s0 mixing, which modulates withthe frequency ∆ms . The decay mode Bs0 → J/ψ ϕ is the most useful for this purpose. A
measurement based on a 2.8 fb−1 dataset is published, and an update using a 5.2 fb−1 dataset
is now available. A constraint on the phase βs is obtained and a wide range of the parameter
space has been already eliminated.
Another interesting ﬁeld of study is the B-hadron decays through the FCNC processesb → s. H. Miyake (Tsukuba) has observed signals for B + → K + µ+ µ− , Bd0 → K ∗0 µ+ µ− ,and Bs0 → ϕ µ+ µ− . The last mode is the ﬁrst observation of the mode, and represents
the rarest decay mode ever for the Bs0 meson. The muon pair invariant mass distributions2

are examined, and the corresponding diﬀerential branching fractions are determined. Theinteresting observables in these decays are the polarization and the the lepton angle forwardbackward asymmetry. The standard model processes have both vector and axial-vector
couplings in these decays, and therefore an asymmetry is expected to be non-zero and has
been observed in previous measurements. However, the existence of new physics could change
drastically those distributions, and a hint of possible eﬀects has been observed by the Belland BaBar experiments. The CDF asymmetry distribution [2] follows the trend of theBelle/BaBar measurements, and remains to be an important subject to be studied withhigher statistics. An analysis using 6.7 fb−1 of data and additional trigger paths and decay
modes is under way, and we expect an increase of a factor of two in signal statistics. The
new result will be available very soon.

1.3

New particle searches

Searches for the Higgs boson, supersymmetric particles and other new particles have been
performed. Cross sections for the standard model Higgs boson production at the Tevatron isshown in Figure 6 (left) for various subprocesses. The largest contribution comes from single
production through gluon fusion, with the top quark in the loop. Associated production
with a weak boson, W ± H and Z 0 H, is smaller. The decay branching fractions of the Higgs
boson are shown in Figure 6 (right). At a low mass, below 135 GeV/c2 , the decay to bb̄ pairs
is dominant, while at a high mass the decays to weak boson pairs become dominant.
CDF has searched for the Higgs boson production using many diﬀerent ﬁnal states. The
ﬁrst, aimed at a low mass Higgs, looks for the associated production p̄p → V HX, where
V is a weak gauge boson W ± or Z 0 , with the W ± (Z 0 ) boson decaying into ℓ± ν (ℓ+ ℓ− ,ν ν̄) pairs, and the Higgs boson to bb̄ pairs. The b quark jets are identiﬁed with displaced
vertices. The main background comes from heavy ﬂavor (bb̄, cc̄) production in association
with a vector boson V , and top quark pair production. Various kinematic distributions,
including the di-jet mass distribution (Figure 7 (left)), are examined for possible productionof the Higgs boson. Also an artiﬁcial neural network discriminant is employed (Figure 7(right)). No excess of events over known backgrounds is found in 5.7 fb−1 of data, andupper limits on the Higgs boson production cross section are set, in the range of 2.0 to
44 times the standard model value, depending on the assumed Higgs boson mass (100 to
150 GeV/c2 ) [3]. Y. Nagai (Tsukuba) has performed the analysis. M. Kurata (Tsukuba) is
applying the Dynamical Likelihood Method to the Higgs search in this channel.
The second analysis looks for single production of the Higgs boson followed by its decays
to W boson pairs. The channel is sensitive to a high mass Higgs. The W + W − ﬁnal stateis identiﬁed with their decays to leptons, thus the signature is two energetic leptons and alarge missing transverse energy and nothing else. The dominant background comes fromelectroweak W + W − production, which leads to the same signature. However, there exists a
diﬀerence in event topology, for example the lepton azimuthal angle correlations. Again, no3

excess of events is observed and upper limits on Higgs production have been placed. Thisis presently the most sensitive search channel at CDF, and the upper limit for the mass of
160 GeV/c2 is 0.83 times the standard model value using 7.1 fb−1 of data (March 2011). An
upper limit of less than unity means that the Higgs boson at the assumed mass is excluded.
An attempt is being made to improve the sensitivity further by using the same production
mode and a diﬀerent decay channel, where one of the W bosons decays leptonically and theother hadronically to two quark jets. Y. Sudo and K. Sato (Tsukuba) are working on theanalysis.
The third method looks for associated production of the Higgs boson with a weak boson
W ± or Z 0 , where the Higgs boson decays to W + W − pair and one or both of the W bosons
can be oﬀ mass shell. There exist three W bosons in the ﬁnal state, where two of them havethe same charge. They are required to decay leptonically, leading to a very distinctive ﬁnal
state of isolated, same-charge lepton pairs, such as e+ µ+ .
The results from diﬀerent analyses as of summer 2010 (luminosity up to 5.9 fb−1 ) are
combined and summarized in Figure 8, where the ratio of the obtained upper limits on
production cross sections to the standard model predictions is presented. The limits areat about unity for a Higgs mass near 160 GeV/c2 , and are less than two at low mass [4].
Figure 9 shows the limits where the results from both CDF and D0 experiments are combined.
Again data up to 5.9 fb−1 have been analyzed. Mass ranges 100 < mH < 109 GeV/c2 and158 < mH < 175 GeV/c2 have been excluded at 95% CL [5]. It is expected that the exclusion
region becomes wider as more data are analyzed and improvements to analysis techniques
are implemented.
If the results from both indirect and direct searches for the Higgs boson are combined,
we obtain a likelihood distribution shown in Figure 10. The constraint on the Higgs mass2
2
is mH = 120.6 +17.9− 5.2 GeV/c , and an upper limit at 95% CL is 155.3 GeV/c . Obviously it
favors a light Higgs boson, which may require a lot of data to observe even at the LHC.
Tevatron running will be terminated at the end of the US ﬁscal year 2011. As mentioned
earlier, CDF will accumulate roughly 10 fb−1 of data that are good for analysis. After
that, CDF will continue data analyses and continue to produce important physics results,
at least for a few more years. Some of them will be competitive with and others will becomplementary to LHC results.

References[1] “Measurement of the forward backward asymmetry in the dilepton decay channel using
5.1 fb−1 ”,
CDF Collaboration, CDF public note 10436 (2011).

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[2] “Measurement of the forward-backward asymmetry in the B → K (∗) µ+ µ− decay andﬁrst observation of the Bs0 → ϕµ+ µ− decay”,
CDF Collaboration, submitted to Phys. Rev. Lett., arXiv:1101.1028v2 [hep-ex] (2011).
[3] “Search for standard model Higgs boson production in association with a W ± boson at
CDF with 5.7 fb−1 ”,
CDF Collaboration, CDF public note 10239 (2010).
[4] “Combined upper limit on standard model Higgs boson production in up to 5.9 fb−1 ofdata,
CDF Collaboration, CDF public note 10223 (2010).
[5] “Combined CDF and D0 upper limits on standard model Higgs-boson production withup to 6.7 fb−1 of data”,
CDF and D0 Collaborations, arXiv:01007.4587 [hep-ex] (2010).

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Figure 1: Tevatron performance in Run II. Top: instantaneous luminosity at the beginningof each store. Bottom: integrated luminosity delivered by week.

-1

Luminosity (pb )
01/03

01/04

01/05

01/06

01/07

01/08

01/09

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7000

01/10

8000
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Delivered
Acquired

2000

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store number

Figure 2: Integrated luminosity (top) delivered to and (bottom) recorded by CDF in Tevatron
Run II.

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July 2010

80.5

LEP2 and Tevatron (prel.)
LEP1 and SLD

mW [GeV]

68% CL

80.4

80.3

∆α

mH [GeV]
114
300
150

1000
175

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mt [GeV]987Tevatron 95% CL

LEP 95% CL

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G fitter SM

Nov 10∆χ 2

Figure 3: Measurements of the top quark and W boson masses as of July 2010, and indirect
information on the Higgs boson mass.

3σ

6
5
2σ

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Theory uncertainty
Fit including theory errors
Fit excluding theory errors

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1σ

1
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MH [GeV]Figure 4: Likelihood distribution for the Higgs boson mass derived from global ﬁts to electroweak measurements.
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Events

yt difference

CDF II Preliminary∫ L dt = 5.1 fb

150

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Data

Aobs= 0.138 ± 0.054

100

Data-Bkg

Asub= 0.205 ± 0.073 ± 0.021
Bkg(± 1σ)

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0

-2

0

2

∆ yt

1.0

Branching Ratio

Cross Section (pb)Figure 5: Study of tt̄ forward-backward asymmetry in production using the di-lepton ﬁnal
state. The distribution of the diﬀerence in the t and t̄ rapidities after background subtraction
is shown.

s = 1.96 TeV
gg→H

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_
bb
WW

0.1 ττ

gg

ZZ

WH
0.1

_
cc

ZH

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Higgs Mass (GeV/c )

10-3
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γγ

Zγ

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Higgs Mass (GeV/c2)

Figure 6: Left: Cross section for the production of the standard model Higgs boson at
Tevatron. right: Branching fractions of the standard model Higgs boson to various ﬁnal
states. Both plots show dependences on the Higgs boson mass.

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Central Lepton
ST+ST

All Lepton Types
ST+ST

CDF Run II Preliminary (5.7 fb-1)

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Data
NonW
Z+jetsdiboson(WW,WZ,ZZ)
Single top (t-ch)Single top (s-ch)tt
W+c c/c
W+bb
W+LF
Higgs WH (115GeV) × 5
Higgs ZH (115GeV) × 50

60
50

CDF Run II Preliminary (5.7 fb-1)

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10

Data
NonW
Z+jetsdiboson(WW,WZ,ZZ)
Single top (t-ch)Single top (s-ch)102tt
W+c c/c
W+bb
W+LF
Higgs WH/ZH (115GeV) × 5

3

40
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10-1

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Dijet mass (NN b-jet Energy corrected) (GeV/c2)0

0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1
BNN output (M = 115)
H

Figure 7: Higgs boson search in the pp̄ → W ± HX → (ℓ± ν) (bb̄) X channel. Left: Invariant
mass distribution of the two b-quark candidate jets produced in association with a W boson.
Right: Output of the Bayesian neural network discriminant.

95% CL Limit/SM

CDF Run II Preliminary, <L> = 5.6-5.9 fb
-1

10

3

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2

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WH+ZH+VBF→jjbb 4.0 fb Obs-1
WH+ZH+VBF→jjbb 4.0 fb Exp-1
H→ττ 2.3 fb Obs-1
H→ττ 2.3 fb Exp-1
ZH→llbb 5.7 fb Obs-1
ZH→llbb 5.7 fb Exp

LEP
Excl.

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WH+ZH→METbb 5.7 fb Obs-1
WH+ZH→METbb 5.7 fb Exp-1
WH→lνbb2j 5.7 fb Obs-1
WH→lνbb2j 5.7 fb Exp-1
H→γγ 5.4 fb Obs-1
H→γγ 5.4 fb Exp-1
WH→lνbb3j 5.6 fb Obs-1
WH→lνbb3j 5.6 fb Exp-1
H→WW 5.9 fb Obs-1
H→WW 5.9 fb Exp
Combined Obs
Combined Exp

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SM=1

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July 19, 2010

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2mH (GeV/c )Figure 8: Summary of CDF results on the searches for the standard model Higgs boson.
The experimental upper limits on production cross sections, normalized to the theoreticalpredictions, are shown for all search channels.

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Tevatron Run II Preliminary, <L> = 5.9 fb

95% CL Limit/SM-1

LEP Exclusion

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TevatronExclusion

Expected
Observed±1σ Expected±2σ Expected

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SM=1
Tevatron Exclusion

July 19, 2010

100 110 120 130 140 150 160 170 180 190 200
2mH(GeV/c )

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G fitter SM
Tevatron exclusion at 95% CL

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Nov 10LEP exclusion at 95% CL∆χ 2Figure 9: Summary of combined CDF and D0 results on the searches for the standard model
Higgs boson. The experimental upper limits on production cross sections, normalized tothe theoretical predictions, are shown after combining all search channels. The mass ranges
100 < mH < 109 GeV/c2 and 158 < mH < 175 GeV/c2 have been excluded at 95% CL.

3σ

2σ
Theory uncertainty
Fit including theory errors
Fit excluding theory errors

1σ
200

250

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MH [GeV]Figure 10: Likelihood distribution for the Higgs boson mass including both indirect measurements and direct searches.

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