pakhlov pavel (itep, moscow) why b physics is still interesting belle detector measurement of sin2 ...
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Pakhlov Pavel (ITEP, Moscow)
Why B physics is still Why B physics is still
interestinginteresting
Belle detectorBelle detector
Measurement of sin2Measurement of sin2
Rare B decays Rare B decays
Future plansFuture plans
University of Lausanne11/12/2001
Pakhlov Pavel (ITEP, Moscow) 2
B-physicsB-physics • Test of the Standard Model (SM) mechanisms
• hadronic models, CKM, etc…
• Measurement of the fundamental parameters of SM• exact values can be a hint for new phenomenology
• Search for physics beyond SM• large b-quark mass
• SM decays suppression (Vcb<<1)
• large contribution from loop and box diagrams
Pakhlov Pavel (ITEP, Moscow) 3
Why CP is importantWhy CP is important
• Test CKM mechanism as a single source of CP violation:• During 35 years CP violation measured only in K system. CKM
theory single (free) parameter; Experiment single measurement .
• Measurement of CKM matrix elements:• sin2: constrain on CKM matrix elements without QCD
uncertainties.• sin2 from BDK, xd/xs, B : free of model uncertainties, but
requires much more statistics.
• New physics:• Need overfull constrain on CKM parameters (better if without
QCD uncertainties) to test the SM• … or to observe very large discrepancy
Pakhlov Pavel (ITEP, Moscow) 4
CPV in B mesons at eCPV in B mesons at e++ee-- colliders colliders
• B0 B0 are produced and remain in coherent p-state until one of them decays.
• If the first B decays to flavor specific mode, the state of the other B is orthogonal at this time.
• Then the second B propagates (oscillating).• If it decays to a CP-final state: two contributions from
B and B interfere.• If the first B decays to the CP final state and the
second to a specific flavor state – the interference results into opposite sign of CP asymmetry. The time integrated asymmetry vanishes!
Pakhlov Pavel (ITEP, Moscow) 5
Mixing-induced CP asymmetryMixing-induced CP asymmetry
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The Belle collaborationThe Belle collaboration
~300 authors from 50 institutions (13 countries)
Group from the University of Lausanne joined Belle recently
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KEKB asymmetric eKEKB asymmetric e++ee-- collider collider
• Two separate rings• e+ : 3.5GeV• e- : 8.0GeV
• Ecm : 10.58GeV c 200m
• Luminosity:• design: 1034 cm-2s-1
• achieved: 5.46x1033 cm-2s-1
• Crossing angle 22mrad• Beam size:
x=100m y=3m
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KEKB/Belle luminosity summaryKEKB/Belle luminosity summary
May 1999- December 2001: accumulated: 44.3 /fb
analyzed: 32.8 /fb
KEKB records on the luminosity peak: 5.466 × 1033 /cm2/sec (World record) per day: 282.5 /pb (> whole ARGUS statistics) per month: 6120. /pb
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The Belle detectorThe Belle detector
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B-meson reconstructionB-meson reconstruction
• Reconstruct all low background (cc)K0 CP-final states:
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BB00 J/ J/ K KSS
KKSS ++-- 4 MeV/c2
J/J/ ee++ee--
11 MeV/c2
J/J/ ++--
10 MeV/c2
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BB00 J/ J/ K KSS
Energy difference: E=EJ/KS
-Ecm/2
Beam-constrained mass:
Mbc= (Ecm/2)2- pJ/KS
2457
events~3% bkgd
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All full reconstructed modesAll full reconstructed modes
Decay mode: signal
bkgd
J/KS, KS+- 457 11.9
J/KS, KS00 76 9.4
(2S)KS, (2S)+- 39 1.2
(2S)KS, (2S)J/+- 46 2.1
c1 KS, c1 J/ 24 2.4
c KS, c KSK 23 11.3
c KS, c K+K-0 41 13.6
All CP=-1 modes 747 58
J/K*0, K*0 KS0 41 6.7
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BB00 J/ J/ K KLL
• KL momentum is not measured, only direction is known
• Reconstruct J/+-
• Assume M(J/KL)=MB to calculate P(KL)
• Remove full reconstructed J/K, J/K*.
• Cut on B-candidate likelihood based on kinematics and event shape.
• Look at P(J/KL)
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BB00 J/ J/ K KLL
• Main background from B J/ X (quasy two-body) – well understood: use MC.
• Some backgrounds are CP-finale states: special care when CP-fit.
• Special case: J/K*0, K*0 KL0 - mixture =-1 and =+1: from angular analysis B J/ K*
569 events=
346 signal+
223 bkgd
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Flavor taggingFlavor tagging
• Use tracks from the accompanying B specific flavor• Inclusive leptons
• High momentum leptons b c - • Intermediate momentum leptons s +
• Inclusive hadrons
• High momentum pions B0 D*- + (X), D-+
(X)• Intermediate momentum kaons K+
X• Low momentum pions D0 -
• Take into account correlations
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Multi-dimensional tagging likelihoodMulti-dimensional tagging likelihood
• Each class of tagging / each specific tagging particle have different tagging purity. If sum all events with different purity loose accuracy.
• Divide all events into several categories depending on tagging purities, then average over independent measurements: • Calculate probability for each track to belong to one of the
flavor tagging class.• Combine information about all tracks into likelihood for event
to be B0 or B0. • Divide the whole likelihood range into 6 bins of different
purities.• Extract dilution factor for each bin from the data. Use specific
flavor channels: B0 D(*)-+ ,B0 D(*)-+
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Determination of wrong tag fractionDetermination of wrong tag fraction
• A(t)measured~(1-2)sin(md t); =Nwrong/N.
• For sin(2) we need to know !
• Get from B0B0 mixing amplitude ~(1-2)cos(md t);
• Fit to the data with free (r). Efficiency >99%
effective=27.01.2%
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• CP-side vertexing: • use prompt tracks from J/,
(2S), c
• Reject poorly fit vertices z 75m
• Tagging B vertexing:• Use well fit tracks• Iterative procedure: fit discard worst track z 140m
• Efficiency: 87%, t 1.5ps
• not-Gaussian tails 3%,
Vertex reconstructionVertex reconstruction
1137 events areused for CP fit
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Check vertex resolution function Check vertex resolution function
BBDD((**)-)-++BB00 =1.55=1.550.02 0.02
pspsBB+ + =1.64=1.640.03 0.03
psps
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CP-fitCP-fit
• Each event has individual sensitivity to sin2 (depending on signal purity, tagging dilution, z and vertex fit accuracy, etc…)
• For each event calculate its likelihood as a function of sin2:
signal fraction wrong tag fraction PDG free
parameter
• Calculate sin2 which maximize the total likelihood Li
Resolution function
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sin2sin2 results results
sin2=0.990.140.06
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Cross-checksCross-checks
Mode dependencies:Check asymmetries in Non-CP sample.“sin2”=0.050.
04
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Compare with other experimentsCompare with other experiments
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Rare B-decaysRare B-decays
• New level of sensitivity to the rare B-decays:• Cabibbo-suppressed decays: B D(*)K+,D*+D(*)-
• Color-suppressed decays: B0 D(*)0X0
• Hadronic penguin decays: B+ K+K+K-,K++-
• Electro-weak penguin decays: B K+-
• To be used in future for CP violation study
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Cabibbo-suppressed decaysCabibbo-suppressed decays
• B D(*)K+ (10.4 /fb): • Large background
from Cabibbo-allowed is suppressed by PID
• Remaining background disentangled by E
• Branching ratios: Br(DK-)/BR(D-)~0.068-0.079
• B- D(*)0K- (D0 CP) to be used in future for measuring angle
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Color-suppressed DecaysColor-suppressed Decays
B D(*)0 X0 have been observed for the first time:Mode Br (X10-4)
D00 3.1 0.4 0.5
D*00 2.7 0.8 0.6
D00 1.4 0.5 0.3
D*00 2.0 0.9 0.4
D00 1.8 0.5 0.4
D*00 3.1 1.3 0.8
D00 D*00
D*00
D*00
D00
D00
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Electroweak penguin decaysElectroweak penguin decays
• FCNC are forbidden at tree level, but loop or box induced.
• Sensitive to the New Physics.
• B K K++- - observed observed for the first timefor the first time
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Future plansFuture plans
• Nearest Belle upgrade – summer 2002: reduce beampipe 2cm 1.5cm; new 4 layers silicon detector:• Better vertex resolution• Higher efficiency for slow track reconstruction
• e+e- machines competitive with LHCB and BTeV if
L~1035. SuperKEKB is under investigation
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SummarySummary
• Belle observed large CP-violation in B decays. sin2=0.990.140.06
• Many new rare B decays observed for the first time.
• KEKB/Belle continue successful operation. • Many new interesting results are expected in
future.• KEKB/Belle plan to compete with LHC in B-
physics after LHC starts.