Transcript Status of Belle - University of Cincinnati

Reflections on Beauty:
CP Asymmetries in B Meson Decay
• Weak interactions & the b-quark: CKM matrix
• B(eauty) mesons & CP
• B meson production: e+e– -> (4S)
• Belle/Babar experiment
K. Kinoshita
University of Cincinnati
Symmetry of Physical Laws
In interaction-free universe (4-d, relativistic QM)
• massless particles
• symmetric in transformations
P(r<—>-r), C(particle<—> antiparticle), T(t<—>-t)
Add interactions: emission/absorption of field quantum
• mass via self-interaction
• interaction strength/probability
field
2
a “charge” g a “coupling constant”
particle
particle'
• symmetry info in vertex
vertex
Forces: Strong, Electromagnetic, Weak, Gravitational
coupling ~ 10-5, quanta W±, Z0
2
Weak Interaction
The only force known to
• allow particle to change identity
• violate P symmetry (maximally)
right-handed particles, left-handed antiparticles.
no coupling to LH particles, RH antiparticles.
• violate CP symmetry (a little)
Why is CP violation of interest?
• matter-antimatter asymmetry in universe
requires CP violating interactions (Sakharov 1967)
What is source of observed CP asymmetry?
3
We have an interesting possibility ...
Standard Model = 12 fermion flavors (+antifermion)
+ strong, EM, weak forces, unification of EM+weak
distinguished ONLY by mass (?)
fermions: 3 generations x 2 types x 2 ea (doublets)
all stable, if not for weak interaction
Gener ation
type
Q/|e|
lepton
–1
0
+2/3
–1/3
(no strong)
quark
(strong)
1
2
3
up
down
charm
strange
truth
beauty
 tau
e electron
µ muon
e neutrino µ neutrino  neutrino
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Weak couplings
Z0
±
W, Z
"neutral current”
l
DQ = 0
±
q+2/3
0
–
<–> l ±
<–> q'+2/3
–
µ
–
"charged current”
DQ = ±1
f'
f
l
q+2/3
NO generation x-ing
–> no flavor-changing
single coupling strength
e
W±
u
– <–>

<–> q'–1/3
generation x-ing,
quark only
all different strengths
(small)
c
t
seen
suppressed
e
µ


d
s
b
not seen
Large # of fundamental "charges" – can this be simplified?
5
GIM mechanism
Explains
• suppression of flavor-changing neutral currents
• multiplicity of charged current couplings
• for >2 generations, CP violation
Picture
• strong doublets, “degenerate”generations, perturbed by weak force:
new doublets
u
c
t
d' universal
s'
b'
no generation x-ing,
W-coupling
(=gF, seen in leptons)
d', s', b' are linear combinations of d, s, b:
d'
s' =
b'
M
d
s
b
Cabibbo-Kobayashi-Maskawa (CKM) matrix
complex
preserves metric
“ orthogonality
}
= unitary
For 3 x 3, unitarity constrains {9 real+9 imaginary} dof to
4 free parameters, incl. 1 irreducible imaginary part
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Unitarity of CKM
1/gF
u
x W-couplings:
c
t
d
Vud
Vcd
Vtd
s
Vus
Vcs
Vts
b
(Wolfenstein parametrization):
1-l2/2
l
l3A(r-ih)
Vub
-l
1-l2/2 l2A
Vcb @
l3A(1-r-ih) -l2A
1
Vtb
from decay rates,
Unitarity condition:
Vji*Vjk=dik {i=1,k=3}: Vub*Vud+Vcb*Vcd+Vtb*Vtd=0
=> Vub*Vud + 1 + Vtb*Vtd = 0
Vcb*Vcd
Vcb*Vcd
-(rih)
-(1-r-ih)
(r, h): "unitarity triangle"
Self-consistent if CKM is correct
(r,h)
1.0

VudVub*
VcdVcb*
a


0.0
l = 0.220 ± 0.002
A = 0.81 ± 0.08
|r-ih| = 0.36 ± 0.09
|1-r-ih| = 0.79 ± 0.19
Vtd Vtb*
VcdVcb*

1
1.0
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Complex couplings revealed via CP asymmetry
– t-integrated rates G  |<f|Hint|i>|2 => not sensitive to phase:
CP{
}=
V
V*
xy
y
x
xy
x
y
– need interference between processes:
e.g., decays to CP eigenstate - paths w/wo mixing interfere
B
->
f CP
{cc}+{Ks,KL,π0}
CP
B eigenstate
CP-dependent oscillation in decay time distributions
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CP Asymmetry of B -> J/y Ks
tree (real Vij)
mixing+tree
d
c
c
s
t
b
b
W
d
W
b
}y
t
d
b
W
s
d

arg(Vtd2) = 21
->
c
s
d
c} y
c
c
W
} Ks
W
t
b
*2
V
td
} Ks
b
W
t

d
2
V
td
c
c
s
}y
} Ks
* No theoretical uncertainty
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Measure time dependence - what’s needed?
•B pair production  e+e– -> (4S) -> BB
•Measure decay-time difference
Asymmetric energy e+e– (@KEKB:   c200mm)
good vertexing  silicon strip vertex detector
•Find CP eigenstate decays
high quality ~ detector  Belle/Babar
•Tag other B’s flavor
good hadron id  dE/dx, Aerogel, TOF, DIRC
good lepton id  CsI, multilayer µ
•Lots of B mesons ~108
(Br (BfCP) ~ 10-3)
very high Luminosity

KEKB/PEP2
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BB pair production: Upsilon
e+
e–
b
(4S)
b
B+:B0~1:1
}B
u d
or
u d
}B
e+e– -> (4S) -> BB + 20 MeV
CLEO
KEKB:
8.0 GeV e– + 3.5 GeV e+
IP size = 77µmx2.0µmx4.0mm
Event rate
Cross section ~ 1 nb = 10-33 cm2
dN
=sxL
dt
~ 10 s–1
~108yr–1
Luminosity (collision rate)
1034 cm–2s–1 (design; currently 5.5x1033 @KEKB)
Currently@Belle: 3x107 BB events (published), 4.8x107 on tape
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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-
B2
e+ B1
Dz≈Dtc
~200 µm
Reconstruct
CP=±1 mode @ t=Dt
-> (4S)
J/y
Ks
flavor tag: e, µ, K±, ...
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Detector: e.g. Belle
Designed to measure CP asymmetry
Charged tracking/vertexing
- SVD: 3-layer DSSD Si µstrip (~55 µm)
– CDC: 50 layers (He-ethane)
Hadron identification
– CDC: dE/dx (~7%)
– TOF: time-of-flight (~95 ps)
– ACC: Threshold Cerenkov (aerogel)
Electron/photon
– ECL: CsI calorimeter (1.5%@1 GeV)
Muon/KL
– KLM: Resistive plate counter/iron
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Belle Collaboration
274 authors, 45 institutions
many nations
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CP mode reconstruction
B0  J/y Ks(+- )
J/yl + l -
“golden mode”
Ks+-
1lepton+1”not-hadron”
s~4MeV/c2
Ks mass4s
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B0  J/y Ks
(continued)
457 events
~3% background
Kinematics for final selection:
DE E*cand–E*beam 0 (E*beam s 1/2/2)
10-50 MeV res, depends on mode
Mbc (Beam-constrained mass)
Mbc (E*beam2-p*cand2 )1/2
s~10MeV
DE
Signal region
s~3MeV/c2
Mbc
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Other charmonium+K
B0  J/y Ks(00 )
J/y KS(+- & 00)
y(2S)(l+l- & J/y+-) KS
c1(J/y) KS
hc(KSK-, K+K-0) KS
J/y KL
J/y K*0 (KL0) (mostly)
x f =-1
76 events
~ 9 bkg
x f=+1
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Other charmonium
c1
c2
1st observation
of inclusive
B c2 X
M(ll-) - M(ll-)
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J/y KL
• J/y: {tight mass cut}
1.42<py*<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)
Creator: HIGZ Version 1 .23/07
Preview: This EPS picture was not saved with a preview (TIFF or PICT) included
Co mment: This EPS picture will print to a postscript p rinter but not to other types
printers
“feeddown,” known
modes - estimate via MC
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CP candidates
Fully reconstructed modes
750 candidates
~58 bkg
J/y KL
Nsig = 346 events
Nbkg = 223 evts
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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+}
* multidimensional likelihood, e>99%
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-
• BCP: sz~75 µm (rms)
use only tracks from J/y
• Btag: sz~140 µm (rms)
h
remaining tracks, excluding Ks;
iterate, excluding tracks w. poor 2/n
resolution includes physics (e.g.
charm)
• Overall eff. = 87%
m
m-
K-
Dz
K-
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Raw Dt distributions
distribution in Dt~Dz/c, unbinned max. likelihood fit
•CP is violated!!!
•seen in raw data
•large effect
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Prepare to fit for sin21
• B0 lifetime = 1.548±0.032 ps, c=464±10 µmmultiply by =0
} for lab length
mixing Dm = 0.47±0.02 ps–1; cT~4.0 mm
(decay in flight)
only ~ 1 cycle of oscillation measurable
• True CP asymmetry is diluted:
background to CP reconstruction
incorrect flavor tag rate
vertex resolution - not exactly as modeled
all need checks in data
-> Use same methods to make other (better known)
physics measurements: B0 mixing, B lifetime,
D lifetime, null CP
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Wrong tag fraction via mixing
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:
• "effective tagging efficiency"
eeff=S(1-2wl)2etag, l =(27.0±2.2)%
99.4% of candidates tagged
(goodl agreement w MC)
Flavor tags classified by
(MC) Purity - 6 bins
25
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/c
• form, parameters from
- Monte Carlo
tail fraction: 1.8%
- fits for D0K-π+, BD*l lifetimes
• validate: B lifetime, same fitting
0=1.55±0.02 ps (PDG2000: 1.548±0.032 ps)
+=1.64±0.03 ps (PDG2000: 1.653±0.028 ps)
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Fitting Dt distribution
• distribution in Dt~Dz/c
• unbinned max. likelihood fit, includes
- signal root distribution (analytic)
- wrong tag fraction (const)
- background: right & wrong tag (MC, parametrized)
- detector & tagging Dz resolution
(parametrized,evt-by-evt)
27
Results
All modes combined: sin21=0.99±0.14(stat)+0.06(sys)
NB-NB
NB+NB
binned in Dt
curve from
unbinned fit
28
Control sample: B0 non-CP states
use: B0D(*)-π+, D*-r+, D*-l+, J/yK*(K+π-)
“sin21”
0.050.04
(statistical
error only)
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fit CP –1 and CP+1 separately:
CP=-1
sin21
NB-NB
NB+NB
0.840.17
CP=+1
1.310.23
(statistical
errors only)
30
Systematic errors
Vertex algorithm
Flavor tagging
0.04
0.03
Resolution function
0.02
KL background fraction
0.02
Background shapes
0.01
Dmd and B0 errors
0.01
Total
0.06
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Compare with other experiments
32
Result in context
(locating tip of unitarity triangle)
Belle’s 1s band
(two sol’ns)
BaBar
Vub
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Summary/Prospects
Successful run of Belle in 2000-1
• sin 21: 30.5 fb–1 on (4S), 1137 tagged events
• 19 papers published or submitted
Next
• higher precision on sin21
data as of 1/23/02 - 48 fb–1; anticipate 100 fb–1 by summer
• Lum: peak 5.5x1033cm–2s–1; 24 hrs 311 pb–1;
month 6120 pb–1
• other angles - need >300 fb–1 - within sight!
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