Pakhlov Pavel (ITEP, Moscow) Why B physics is still interesting Belle detector Measurement of sin2 Rare B decays Future plans University of Lausanne

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


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


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!


Mixing-induced CP asymmetryMixing-induced CP asymmetry


The Belle collaborationThe Belle collaboration

~300 authors from 50 institutions (13 countries)

Group from the University of Lausanne joined Belle recently


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


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


The Belle detectorThe Belle detector


B-meson reconstructionB-meson reconstruction

• Reconstruct all low background (cc)K0 CP-final states:


BB00 J/ J/ K KSS

KKSS ++-- 4 MeV/c2

J/J/ ee++ee--

11 MeV/c2

J/J/ ++--

10 MeV/c2


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


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


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)


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


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


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(*)-+


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%


• 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


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


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


sin2sin2 results results

sin2=0.990.140.06


Cross-checksCross-checks

Mode dependencies:Check asymmetries in Non-CP sample.“sin2”=0.050.

04


Compare with other experimentsCompare with other experiments


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


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


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


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


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


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.

Documents

Pakhlov Pavel (ITEP, Moscow) Why B physics is still interesting Belle detector Measurement of sin2 Rare B decays Future plans University of Lausanne