Transcript Document

Recent results from Belle:
Reflections on Beauty
Kay Kinoshita
University of Cincinnati
Belle Collaboration
APR01 APS
Outline
•CP asymmetry in Standard Model
•B decays and CP
•Asymmetric e+e- collisions at (4S)
•KEKB and Belle:
time-dependent measurement
•Measurement of sin2f1
•Summary
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Belle Collaboration
274 authors, 45 institutions
many nations
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CP Asymmetry in Standard Model
Weak interaction of quarks (u,c,t), (d,s,b)
{mass <- CKM matrix -> weak} eigenstates
CKM - 3x3, unitary by definition -> 4 free parameters
d
s
b
Vud Vus Vub
Vcd Vcs Vcb
Vtd Vts Vtb
u
c
1-l2/2
l
l3A(r-ih)
-l
1-l2/2
l2 A
=
l3A(1-r-ih) -l2A
1
t
irreducibly complex
-> CP violation
VudVub*
VcdVcb*
VtdVtb*+VcdVcb*+VudVub*=0
“unitarity triangle”
rh
1.0
f g

f
a
Vtd Vtb*
VcdVcb*
b f
0.0
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CP phenomenology
•before 1998: seen only in K system
•predicted for b-hadrons in Standard Model, e.g.:
rh
1.0
B0->ππ
B0->rπ
f
VudVub*
VcdVcb*
B0-> D*π
B0->D*r
Vtd Vtb*
VcdVcb*
first result
B0->J/Ks
B0->D(*) D(*)
f

f
0.0
1.0
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CP Asymmetry of B -> J/ Ks
B
Decays to CP eigenstate:
paths w/wo mixing interfereB
f CP
{cc}+{Ks,KL,π0}
CP
eigenstate
CP = ±1
CP-dependent oscillation in decay time distributions
No theoretical uncertainty
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Time measurement at (4S)
(4S): CP=-1, conserved
until first B decay (t=0)
identify b/b {flavor tag}
B
B
+e-
D
t=0

e-
B1
e+ B2
Dz≈Dtbgc
~200 µm
Reconstruct
CP=±1 mode @ t=Dt
-> (4S)
J/
“CP side”
Ks
flavor tag: e, µ, K±, ...
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Experimental considerations
• B0 lifetime = 1.548±0.032 ps, ct=464±10 µm multiply by bg
mixing Dm = 0.47±0.02 ps–1; cT~4.0 mm } for lab length
(decay in flight)
0
0
need to distinguish B vs `B (flavor tag), high efficiency
Kaons
W
W
–
K
leptons
b
W–
c
e–

c is diluted:
b
s
• True CP asymmetry
background to CP reconstruction
incorrect flavor tag rate
vertex resolution
• Bottom line: need >few x 10 fb–1 @ (4S) {>107 B events},
vertexing@<40µm, hadron ID, lepton ID
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Beams: KEKB
bg = 0.425
sE*beam) = 2.6 MeV
IP size = 77µm(x) x 2.0µm(y) x 4.0mm(z)
Lmax = 3.4 X 1033 cm–2s–1 (design: 1x1034)
Data (6/1999–12/2000)
Ldt = 10.5 fb–1@(4S), 0.6 fb–1off
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Belle detector
Charged tracking/vertexing
- SVD: 3-layer DSSD Si µstrip
– CDC: 50 layers (He-ethane)
Hadron identification
– CDC: dE/dx
– TOF: time-of-flight
– ACC: Threshold Cerenkov (aerogel)
Electron/photon
– ECL: CsI calorimeter
Muon/KL
– KLM: Resistive plate counter/iron
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CP mode reconstruction
J/l + l - )
B0  J/ Ks(+- )
as an example
Ks+-
1lepton+1”not-hadron”
s~4MeV/c2
Ks mass4s
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CP mode (continued)
Kinematics for final selection:
DE E*cand–E*beam0 (E*beams0.5/2)
10-50 MeV res, depends on mode
Mbc (Beam-constrained mass)
Mbc(E*beam2-p*cand2 )0.5
Signal region
s~3MeV/c2
s~10MeV
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J/ KL
• J/: {tight mass cut}
1.42<p*<2.00 GeV/c
• KL: {KLM/ECL cluster w/o track,
>1 KLM superlayers (resolution~ 3°
(1.5° if ECL)} within 45˚ of
expected lab direction
• Require cand to have B mass,
calculate momentum in CMS
(pB*) (~0.3 GeV for signal)
• backgrounds: random (from data),
Title: r404_f1_e61383_b det_xy_ncol_nopt_stecl_nomo m.ps (Portrait A 4)
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“feeddown,” known
modes - estimate via MC
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CP candidates
Fully reconstructed modes
J/ KL
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CP candidates - numbers
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Flavor tagging
l-
l+
b
c
s
–
K
*+
0
D
D
+
π
– high-p lepton (p*>1.1 GeV): b->l– net K charge b->K–
– medium-p lepton, b->c-> l+
– soft π b->c{D*+->D0π+}
* all into multidimensional likelihood
Significance of CP asymmetry depends on
– tagging efficiency e
– wrong-tag fraction w (measured w data)
- effective efficiency = e(1-2w)
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Dz: vertex reconstruction
Constrained to measured IP in r-f
• BCP: sz~88 µm (rms)
use only tracks from J/
• Btag: sz~164 µm (rms)
h
remaining tracks, excluding Ks;
iterate, excluding tracks w. poor c2/n
resolution includes physics (e.g.
charm)
• Overall eff. = 87%
m+
mK-
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Prepare to fit: Wrong tag fraction
Same fit method, but
CP->flavor-specific
• BD*-l+, D(*)-π+, D*-r++flavor tag
• separate same-, opp-flavor events
• fit to Dz: mixing asymmetry, w:
Bins of dilution parameter (MC)
• "effective tagging efficiency"
eeff=S(1-2wl)2etag, l =(27.0±2.2)%
99.4% of candidates tagged
(good agreement w MC)
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Dt resolution function
• Double Gaussian, parameters calculated eventby-event, includes effects of
- detector resolution
- poorly measured tracks
- bias from e.g. charm
- approximation of Dt=Dz/bgc
• form, params determined by
tail fraction: 1.8%
- Monte Carlo
- fits for D0K-π+, BD*l lifetimes
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Verifying Dt resolution
Dt used in other measurements, serve as checks
• B0 mixing w. dileptons
Dmd=0.463±0.008±0.016 ps-1 (5.9 fb–1) PRL86,3228
(PDG2000: 0.472±0.017 ps-1)
• B lifetimes
Reconstructed B + flavor tag vertex
BDX semileptonic+hadronic modes.
t0=1.56±0.04 ps
(PDG2000: 1.548±0.032 ps)
t+=1.66±0.04 ps
(PDG2000: 1.653±0.028 ps)
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Dt resolution
B0 mixing w. dileptons
Same sign
b
l-
- 2 primaries, mixed event
- Primary+2ndary, unmixed & B+B- Backgrounds
c
l+
s
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Opposite sign
- 2 primaries, unmixed & B+B- Primary+2ndary, mixed&unmixed
- Backgrounds
Asymmetry in signal (2 primaries)
Nopp-Nsame
Nopp+Nsame
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Fitting Dt distribution
• distribution in Dt~Dz/bgc
• unbinned max. likelihood fit, includes
- signal root distribution (analytic)
- wrong tag fraction (const)
- background: right & wrong tag (MC,
parametrized)
- detector & tagging resolution
(parametrized,evt-by-evt)
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Results
All modes combined :
+0.32
sin2f1=0.58-0.34
binned in Dt
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Results
likelihood
Uncombined results are consistent
Mode
Fit
(stat. err.)
Non-CP
0.065  0.075
B K S
1.21+-..40
47
B K L - 0.04  0.60
CP = -1
0.82+-..36
41
CP = +1
0.10+-..57
60
All CP
0.58+-..32
34
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Systematic errors
Flavor tagging
–0.07
+0.05
Physics parameters
Background Fraction
except KL
Background Fraction for
–0.04
+0.03
–0.02
+0.03
KL
–0.05
+0.05
Background Shape
–0.01
+0.01
Resolution Function
–0.01
+0.01
IP measurement
–0.01
+0.02
Total
–0.10
+0.09
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Result in context
+0.32+0.09
sin2f1=0.58-0.34-0.10
{PRL 86, 2509 (2001)}
• Probability of observing
sin2f1>0.58 if CP is
conserved: 4.9%
• Feldman-Cousins
confidence
interval
CKM,
from
rates
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Summary/Prospects
Successful run of Belle in 2000
Results on
• sin 2f1: 10.5 fb–1 on (4S), 282 tagged events
• + 17 papers at Osaka ICHEP 2000
• 2 publications, 4 submitted, more soon
Next
• some improvements to analysis
• data as of 4/01 - 20.5 fb–1; anticipate 30 fb–1 by summer
• Luminosity: peak 3.41x1033cm–2s–1; 24 hrs 198 pb–1;
month 4047 pb–1
• KEKB continuing to improve performance
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