measurements of cp asymmetries and branching fractions in b 0 p + p - , k + p - , k + k -
DESCRIPTION
Measurements of CP asymmetries and branching fractions in B 0 p + p - , K + p - , K + K -. Paul Dauncey Imperial College, University of London For the BaBar Collaboration. CP violation in the Standard Model. CP violation arises due to complex CKM matrix; e.g. Wolfenstein parametrisation:. - PowerPoint PPT PresentationTRANSCRIPT
16 May 2002 Paul Dauncey - BaBar 1
Measurements of CP asymmetries and branching fractions in B0 +,K+,K+K
Paul Dauncey
Imperial College, University of London
For the BaBar Collaboration
16 May 2002 Paul Dauncey - BaBar 2
CP violation arises due to complex CKM matrix; e.g. Wolfenstein parametrisation:
)(O
1A)i1(A
A21
1
)i(A21
1
VVV
VVV
VVV
V 4
23
22
32
tbtstd
cbcscd
ubusud
is third internal angle
CP violation in the Standard Model
CKM unitarity; represent as triangle in , plane
Vtd e-i
Vub e-i
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Direct CP violationCP violation requires interference between at least two amplitudes
with different phases, e.g. B0 K+
B0 K+
T
P
CP violation gives a non-zero asymmetry:
AK(B0 K(B0 K+(B0 K(B0 K+)
In principle leads tomeasurement
Relative
weak phase ei
– –
16 May 2002 Paul Dauncey - BaBar 4
Indirect CP violationMixing gives two paths if final state accessible from B0 and B0, e.g.
B0 J/ K0S
B0 J/ K0S
CP violation gives a time-dependent asymmetry:
Amplitude = Im(ei) = sin2Leads tomeasurement: see talk by S. Rahatlou
B0
T
TM ––
–
Relative
weak phase ei2
16 May 2002 Paul Dauncey - BaBar 5
CP violation in B0 +
B0 +has both effects
B0 +
If both tree and penguin significant: parametrise as sin2eff
Interpretation also depends on strong amplitudes
B0
T
T
PM
P
–
–
–Relative
weak phase ei
No relative weak phase
Relative
weak phase ei2
16 May 2002 Paul Dauncey - BaBar 6
Experimental realityAnalysis is different from B0 J/ K0
S… No clean or K sample; need to determine particle type
Particle identification needed Analyse both modes simultaneously; also include KK
No clean signal sample; high backgrounds from udsc Branching fractions are ~ 10–6 – 10–5
Need to use kinematics and event shapes to distinguish modes Use unbinned maximum likelihood fit to extract signals
…but some elements are identical: Need to “tag” the other B to find if B0 or B0
Use lepton charge, K charge or neural networks Inefficiency and impurity (“dilution”)
Cannot measure time directly Time difference t between two B decays
–
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The BaBar detector
DIRC PID)144 quartz bars
11000 PMs
1.5T solenoid EMC6580 CsI(Tl) crystals
Drift Chamber40 stereo layers
Instrumented Flux Returniron / RPCs (muon / neutral hadrons)
Silicon Vertex Tracker5 layers, double sided strips
e+ (3.1GeV)
e (9 GeV)
Pep-II delivers boosted e+e Y(4s) BB, on and off the Y(4s)
Integrated luminosity: 55.6 fb-1 on peak, 60.2 0.7 million BB events
–
–
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Particle identification with the DIRCDetector of Internally Reflected Cherenkov light (DIRC): essential
for this analysis to distinguish K from Reconstruct Cherenkov
angle cosC = 1/n
from rings seen in PMTs
Cherenkov light transmitted down quartz bars by internal reflection
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Analysis is done in two stages: Direct CP; extract branching fractions for K and KK and
also the K decay CP asymmetry AK
Maximum likelihood fit to kinematic/event shape quantities Requires no tag or vertex measurement Separate fit reduces systematic error
Indirect CP; extract CP asymmetry sin2eff
Fix branching fractions and AK to above results Requires tag to determine if B0 or B0
Requires vertex information to find time dependence
All results are preliminary
Analysis overview
–
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B candidate mass using beam “energy substitution”
mES = (Ebeam2 – pB
2) in CM
Select 5.2 < mES < 5.3 GeV/c2
Kinematic variables - mES
Signal Monte Carlo; Gaussian
Background; ARGUS threshold function
Depends on reconstructed momentum of B candidate, pB ~ 325 MeV
Same for all signal modes Resolution dominated by
beam energy uncertainty mES) ~ 2.6 MeV/c2
Signal shape from MC, background from fit
16 May 2002 Paul Dauncey - BaBar 11
Kinematic variables - E
Background; quadratic function
KKK
B candidate energy difference from beam energy
E = EB – Ebeam in CM
Select | E | < 0.15 GeV
Depends on masses of B decay products; mass assumed
K and KK shifted to non-zero average; ~ 45 MeV per K
Resolution dominated by tracking
E) ~ 26 MeV Signal shape parametrised from
MC; background from fit
Signals: Gaussian
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DIRC c mean and resolution parametrised from data using D*+ D0+ (K–+)+ decays
Particle identification - c
(C) = 2.2 mrad 9
2.5
K/ Separation
Momentum range of decays
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Background suppression - Fisher
Signal Monte Carlo
Background
Cut on angle between B candidate and sphericity of the other tracks in the event: |cos s| < 0.8
F - optimized linear combination of energy flow into nine cones around candidate (CLEO Fisher discriminant). Signal shape from MC, background from fit
Cut
Signal Monte Carlo
Background
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Five input variables per event; assume independent (uncorrelated) PDFs for each:
mES
F Discriminate signal from background E C
+ Discriminate different signal modes
C–
Eight fit parameters: four for signal, four for background N(+ N(K+ N(+K N(K+ K
Branching fraction maximum likelihood fit
Fit directly for N(K) and AK
N(K+N(K) (1–AK
N(+KN(K) (1+AK
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Branching fraction results
Mode
Yield
(events)
Branching Fraction
(10-6)
KAsymmetry, AK
πKB0
ππB0
KKB0
01.006.005.0
4.07.04.5
8.01.18.17
C.L.) (90% 1.1
716915124
1524403 C.L.) (90% 6.15
No significant direct CP violation seen in B0 K+
90% C.L. –0.14 AK 0.05
Main systematics:
Branching fractions – uncertainty in shape of C PDF
Asymmetry - possible charge bias in track and C reconstruction
Fit results are:
16 May 2002 Paul Dauncey - BaBar 16
Branching fraction projections
Fit sample: 17585 candidates, ~ 38%
B0K
B0
Likelihood projections: remove one variable from fit and cut on fit probabilities to give enhanced signal samples:
Background and crossfeed
16 May 2002 Paul Dauncey - BaBar 17
Measuring indirect CP violationExtend the branching fraction analysis to extract indirect CP
information: Needs extra information on:
Time of decay (vertexing); reconstruct “other” B decay point and find time difference of B decays
Flavour of the decaying B0 (tagging); use tracks whose charge carries flavour information
Use identical techniques to sin2 analysis
Fit for CP violation asymmetries while holding branching fractions and K asymmetry to previously determined values Vary these parameters within determined errors for systematics; small
effect for asymmetries
16 May 2002 Paul Dauncey - BaBar 18
Asymmetric beam energies mean Y(4s) is boosted:Vertexing and t
(4s)
= 0.55 +z
t z/c
-
Fully reconstructed BBest vertex for “other” B
t is time difference between the two decays: Resolution is dominated by the “other” B, (t) ~ 0.8 ps Independent of signal type Can use same resolution model as sin2 analysis Exploit mixing to measure signal performance (dilutions) and
efficiencies
Signal resolution determined from large sample of fully reconstructed B’s, background shape determined from fit
16 May 2002 Paul Dauncey - BaBar 19
The signal modes depend differently on t: general form allows for both tree and penguin: rates
f(t) ~ 1 Ssin(mdt) Ccos(mdt) for B0(B0) tag
SImand C For pure tree, ei2so Ssinand C With some penguin contribution,CandSC
sineff
Ktime dependence due to B0 mixing: rates f(t) ~ 1 cos(mdt) for unmixed (mixed) B0
KK general form similar to in principle Model with simple exponential
Actual PDFs used in fit: Diluted due to mistags Convolved with t resolution functions
Signal yield time dependences
–
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Flavour tagging
Gev/c2mES
Fit sample: 9220 tagged candidates
Separate into tag categories
Lepton; eff ~ 8%
Neural Net 1; eff ~ 2%
Kaon;
eff ~ 14%
Neural Net 2; eff ~ 1%
Use same flavour tagging as sin2 measurement:
Untagged events (~ 33%) retained in the fit to determine background shapes
16 May 2002 Paul Dauncey - BaBar 21
Fit results in:
S = –0.01 ± 0.37 ± 0.07
C= –0.02 ± 0.29 ± 0.07
No significant indirect CP violation seen in B0 +
90% C.L. –0.66 S 0.62
90% C.L. –0.54 C 0.48
Main systematic: Uncertainty in shape of C PDF
Time-dependent fit results
Enhanced B sample: t distributions and asymmetry between mixed and unmixed events.
Fit result
Fitted background
16 May 2002 Paul Dauncey - BaBar 22
Fit checked using “toy” studies of simulated experiments All parameters unbiased All errors consistent with expectations Likelihood value (goodness of fit) consistent with expectations
Fit holds B lifetime and mixing constant: cross check by fitting B = 1.66 ± 0.09 ps (c.f. PDG value 1.54 ± 0.02 ps)
md = 0.517 ± 0.062 ps–1 (c.f. PDG value 0.479 ± 0.012 ps–1)
Time-dependent fit cross checks
Enhanced B Ksample: asymmetry between unmixed and mixed events.
16 May 2002 Paul Dauncey - BaBar 23
Summary of BaBar preliminary results
Branching fractions:
B(B0 ) = (5.4 ± 0.7 ± 0.4) 10–6
B(B0 K) = (17.8 ± 1.1 ± 0.8) 10–6
B(B0 KK)= 1.1 10–6 (90% C.L.)
CP asymmetries; no evidence for CP violation:
S= –0.01 ± 0.37 ± 0.07 or 90% C.L. –0.66 S 0.62
C= –0.02 ± 0.29 ± 0.07 or 90% C.L. –0.54 C 0.48
AK= –0.05 ± 0.06 ± 0.01 or 90% C.L. –0.14 AK 0.05