Studies of the decay B c K at the BaBar experiment
Nick Barlow
University of Manchester
December 2003
2
The BaBar Collaboration
• The BaBar collaboration consists of– Nearly 600 physicists– From 10 countries– 75 Institutions
• The c analyses presented here were mainly performed by:– Me– Witold Kozanecki (Saclay)– Stefania Ricciardi (RHUL)– Frank Jackson (Manchester)– Gautier Hamel de Monchenault (Saclay)
3
Outline
• The PEP-II B-factory, and BaBar detector• CP violation in the quark sector of the
S.M.– CKM matrix and Unitarity Triangle– 3 Types of CP violation in B decays
• Measuring the CKM parameter sin2 using neutral B decays to charmonium + K.(Specifically Bc KS)– Reconstructing B candidates– B flavour tagging– Time-independent and time-dependent fits
4
Motivation for the BaBar experiment
• CP violation is interesting because– It is one of the least tested areas of the
Standard Model– Has some relevance to the matter/antimatter
asymmetry in the Universe
• Before BaBar and Belle, CP violation had only been observed in the kaon system– Hadronic uncertainties make comparison with
Standard Model parameters difficult
• High luminosity of PEP-II facilitates numerous other precision measurements in B physics
• (and tau physics…)
5
The PEP-II B-factory
e+e-(4S) BBbar
9 GeV electron beam
3.1 GeV positron beam
Boost (4S) in lab frame: 0.55
6
PEP-II performance
• Design peak luminosity = 3.0*1033cm-2s-1
• Record peak luminosity = 6.8* 1033cm-2s-1
• 24 hr record integrated = 484.3 pb-1
• 7 day record integrated = 2.49 fb-1
• Records improving all the time, largely thanks to trickle injection…
• Results in this talk use 81fb-1 (Runs 1 and 2).
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Trickle injection
• Inject charge continuously into LER, while BaBar is taking data
• Will increase integrated luminosity by at least 15%– If backgrounds
are under control…
8
The BaBar detector
e+ (3.1GeV)
e- (9GeV)
Silicon Vertex Tracker
Drift CHamber
Electro-Magnetic Calorimeter
Detector of Internally Reflected Cherenkov light
Instrumented Flux Return
1.5T solenoid
9
Tracking and Vertex finding
• SVT consists of 5 layers of double-sided silicon strip detectors– Single hit resolution ~80 microns.
• DCH consists of 40 layers (10 superlayers) of wires, arranged in axial and stereo layers in helium/isobutane gas mixture– Measures momentum
and position of charged particles
(pT)/PT=0.13%PT0.45%
10
Particle Identification
• Charged hadrons (, K) identified using dE/dx in the DCH and SVT, and Cherenkov angle in the DIRC– K- separation>3.4 for
P<3.5GeV/c
• Leptons identified using the IFR () and EMC (electrons)
11
Neutral particles
• EMC consists of 6480 CsI(Tl) crystals, arranged in a barrel+endcap configuration.
• Measures energy of electrons and photons over a wide energy range.
12
CKM matrix and Unitarity Triangle
• Quark mixing is described by the CKM matrix
)(O
1A)i1(A
A21
1
)i(A21
1
VVV
VVV
VVV
V 4
23
22
32
tbtstd
cbcscd
ubusud
Wolfenstein parametrization
Complex phase CP violation0*** tbtdcbcdubud VVVVVV
Unitarity condition:
Can be represented as a triangle on complex plane:
Im
Re1
(,)
*
*
cbcd
tbtd
VV
VV
*
*
cbcd
ubud
VV
VV
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Unitarity triangle cont.
• Angles are given by:
Im
Re1
(,)
*
*
cbcd
tbtd
VV
VV
*
*
cbcd
ubud
VV
VV
*
*
argubud
tbtd
VV
VV
*
*
argcbcd
ubud
VV
VV
*
*
argtbtd
cbcd
VV
VV
14
Existing constraints on Unitarity Triangle
Errors are dominated by
theoretical uncertainties!
Constraints come from- CP violation in K0 mesons - Vub and Vcb measurement - Bd and Bs mixing http://ckmfitter.in2p3.fr
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3 Types of CP violation in B decays
1. CP violation in decay (direct CP violation)
– Amplitude for a decay is not same as amplitude for CP conjugate decay
2. CP violation in mixing (indirect CP violation)
– Mass eigenstates cannot be chosen to be CP eigenstates
3. CP violation in interference between decays with and without mixing
– Time-dependent asymmetry in decays of and to common CP eigenstate.
0B 0B0B
0B
fCP
mixing
decay
tfA
fA
0t
t
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CP violation in neutral B decays
• Consider neutral B decays into a CP eigenstate fCP that is accessible to both B0 and B0:
• Can define measurable quantity :
CP
CP
CP
f
ff A
A
p
q
violationCP 2 Type1p
q
violationCP 1 Type1CP
CP
f
f
A
A
violationCP 3 Type0Im
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CP violation in neutral B decays
• Time dependent asymmetry given by:
• If || = 1 (no direct CP violation), this reduces to:
2
2
00
00
1
)sin(Im2)cos(1
))(())((
))(())((
CP
CPCP
CP
f
BfBf
CPCP
CPCPf
tmtm
ftBftB
ftBftBa
)sin(Im tma Bff CPCP
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BCharmonium K0
• For BCharmonium K0, both ‘tree’ and dominant ‘penguin’ diagrams have same weak phase, so no direct CP violation, || = 1
d
c
dB
J/c
K
b
s
c
d
c
d
b
c
s
= chK sin2
Kf
f
BchK p
q
A
A
p
q
CP
CP
0
*
*
*
*
*
*
00 ImImcdcs
cdcs
cscb
cscb
tdtb
tdtbchKchK VV
VV
VV
VV
VV
VV
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BCharmonium K0
• BJ/KS is the ‘Golden channel’ at BaBar• The J/ decays to leptons – clean reconstruction with
good efficiency
• The c has a much larger intrinsic width than the J/, and only decays hadronically• Need to work harder to reduce background
• Less than half the total width is accounted for by known decay channels
• Although J/ and c have opposite intrinsic CP, the J/ is a vector, so (-1)L term in CP of 2-body state means that BJ/KS and BcKS decays have the same CP eigenvalue (-1)
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Experimental method
• BBbar pair evolve as coherent P-wave state.– Always exactly 1 B0 and 1 B0
• Boosted wrt lab frame: can measure distance z between B decay vertices.
e+
(4S)
Fully reconstructe
d B decay
(CP eigenstate)
e.g. BJ/Ks, B cKs
Tag flavour of B-meson using e.g. charge of a primary lepton
t z/( c)
Btag
BCP
e-
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Time-dependent asymmetry
• In principle, just measure time difference t for events where Btag is tagged as a B0 or a B0.
• In practice, tagging and t determination are not perfect – need to modify Probability Density Functions (PDFs) with mistag fractions (w) and resolution functions. B0
tagsB0 tags
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Time dependent PDF
P(t) exp(–|t|/B) ( 1 ± (1-2w) CP sin2sin(mt) ) R(t)
dilution Resolution function
• Can extract sin2b from time-dependent fit to t distributions of tagged events, BUT• Need to know mistag fraction and resolution
function• Need to add other PDFs for background events,
which in general will have different time dependence
CP eigenvalue
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Neutral B-meson mixing
• Neutral B mesons can mix– Btag and Bflav can
be opposite flavour (no mixing)or same flavour (mixing)
e+
(4S)
Fully reconstructed B
decay
(flavour eigenstate)e.g. BD- +
Tag flavour of B-meson using e.g. charge of a primary lepton
t z/( c)
Btag
Bflav
e-
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B-mixing (2)
• PDF for B-mixing is given by:
tRtmDeBBBBP t
cos14
)( 0000
Resolution function
Dilution = 1-2w, where w is fraction of wrongly tagged events
Real probability of B-mixing
unmixed
mixed
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B flavour tagging
• Fully reconstructing Btag not practical – would be too inefficient.
• Instead, use Neural Net tagging algorithms based on different physics processes– Primary lepton from W in bc transition– Charged kaons from bcs transition– Charge of slow pions from D* decays– Charge of highest pT track
• Divide into 4 hierarchical, mutually exclusive ‘Tagging Categories’, – Lepton, Kaon1, Kaon2, Other
• Each tagging category has its own mistag fraction, t resolution function, measured on real data using B mixing sample.
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Tagging performance
• Important parameter is effective tagging efficiency Q=eff(1-2w)2
– This is what goes into the error on sin2b
Category Efficiency
(%)
Mistag
(%)
QualityQ(%)
Lepton 10.2 0.2
2.7 0.3 9.2 0.2
Kaon1 17.8 0.2
8.8 0.4 12.1 0.3
Kaon2 16.3 0.2
20.4 0.6
5.7 0.2
Other 22.7 0.2
31.3 0.6
3.2 0.2
Total 67.1 0.5
30.2 0.5
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Measuring t
• Resolution on t is dominated by determination of z decay position of Btag
– Use constraints from knowledge of the beamspot position, and the momentum of Brec
t= z /( c) + small correction due to small momentum of Bs in Y(4S) frame
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t resolution
• Different PDF used for each tagging category– All use triple Gaussian (core, tail, outliers),
and event-by-event errors. t resolution usually better for lepton
category8 free parameters:•Relative fractions of tail and outliers
•Scale factor for width of core Gaussian
•Bias for tail Gaussian
•Bias factors for core Gaussians in each tagging category
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Data samples
• BCP sample – reconstructed B0 c KS events– Used for measuring sin2b
• Bflav sample – events where 1 B is reconstructed into a flavour eigenstate e.g. D*0-
– Used for measuring mistag fractions and t resolution
• Bsig+ sample - reconstructed B+ c K+ events– Used for cross-checks, higher statistics than neutral
mode.
• Numerous Monte Carlo samples– Used for cross-checks, and tuning the fit – can vary
generated value of sin2b.
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Reconstruction of B c K candidates
• Use decay channels c KSK+- and c K+K-0
• Most BaBar (and Belle, CLEO) B analyses use the variables mES and E, which are (nearly) uncorrelated. E is the measured – expected energy of the
reconstructed B candidate
– mES is the beam-energy substituted B-mass
(assumes E=0)**beamB EEE
2*2*BbeamES pEm
mES (GeV5.35.2
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Fighting background
• First run a ‘skim’ – loose cuts on topological variables, charged track multiplicity etc. to reduce dataset to a manageable size
• Apply cuts on reconstructed masses of KS, 0 etc.– All cuts optimized to maximise S/sqrt(S+B)
• Estimated from signal MC and E sidebands of on-resonance data
• Use a ‘Fisher Discriminant’ to fight background from u,d,s,c quark events– Linear combination of ‘event shape’ variables– Light quark events tend to have jet-like structure in CM
frame, while BBbar events tend to be more spherical, as B mesons are produced almost at rest in CM frame
32
Types of background
• Combinatorial background, from light quark events or BBbar events where Brec is reconstructed using tracks from both Bs– Can be studied using off-resonance data, MC,
and E sidebands of on-resonance data– Modeled using a threshold function (ARGUS
function) in mES
• Peaking background from BBbar events– In our case, studies on high-statistics BBbar MC indicate that
this is dominated by decays to the same final state as our signal, but without an intermediate c resonance:
• Neither of these backgroundsis expected to have any structure in mX – the mass of the
reconstructed c candidate.
B+
K+
K-
0
K+
C
B+
K*+
33
Time independent fit
• Extract signal and background yields using 2D unbinned maximum likelihood fit to mES and mX
c signal: Breit-Wigner Gaussian2 in mX Gaussian1 in mES
J/ signal: Gaussian2 in mX Gaussian1 in mES
Peaking background: Gaussian1 in mES linear in mX
Combinatorial background: Argus shaped in mES, linear in mX
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Results of time independent fit
• KsKPi neutral channel: Tagging
categoryFraction of CP signal
Fraction of peaking bg
Lepton 0.38 ±0.09
0.18±0.06
Kaon 1 0.11±0.03
0.04±0.01
Kaon 2 0.16±0.04
0.05±0.02
Other 0.12±0.03
0.03±0.01
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The sin2 fit
• The value of sin2 is obtained from a simultaneous fit to the Bflav and BCP samples – Takes into account potential small correlations
between sin2b and resolution functions etc., and uses statistical power of BCP sample.
• Altogether there are 34 free parameters– Mainly due to resolution functions for each
tagging category for signal and background, and the composition and time-dependence of the backgrounds in both the BCP and Bflav samples.
36
Results on MC
B0 c KS signal, generated with sin2b=-0.7
sin2b=-0.744±0.074
B+ c K+ signal, no CP violation expected
sin2b=-0.062±0.059
37
Results on Bsig+ sample
• No CP violation expected:
sin2b=0.11±0.16
38
Results on BCP sample
• i.e. the actual result!
sin2b=0.51±0.33
39
More cross-checks
• Can use ‘Toy MC’ experiments, where events are generated to according to a PDF, to check validity of result:– Compare maximised likelihood found in fit to data
with distribution of values found in 500 Toy MC experiments
– Compare statistical error in fit to data with those found in 500 Toy MC experiments
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Systematic errors
Source Error on sin2
Fraction of peaking bg 0.02
CP of peaking bg 0.04
Dilution & t resolution 0.03
Combinatorial bg description 0.03
(mES) 0.0015
(mX) 0.02
mB B (PDG 2000) 0.01
Total systematic error 0.08
41
Comparison with other decay modes
CP =-1 CP =+1
sin2 = 0.755 0.074
sin2 = 0.723 0.158
42
Constraints on the Unitarity triangle
http://ckmfitter.in2p3.fr
BcKS only All BaBar results
43
Conclusions
• BaBar is a good place to do B physics– Very high luminosity of PEP-II– Clean environment of e+e- collisions
• BaBar and Belle have made precision measurements of CP violation in B mesons– Values of sin2b agree well with S.M. expectations
• It is (just about) possible to measure sin2b using the decay channel BcKS – Helped reduce the statistical error by 0.01 for the
last publication!!– Studies on MC indicate it will be advantageous to
include this channel in the main sin2b measurement for the forseeable future
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45
Backup slides
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What is an c ?
c is the ground state of the charmonium system– Mass = (2979.7 1.5) MeV/c2 (PDG 2002)– Width is very poorly known:– BaBar has just made a new measurement
using c from 2 photon production:
KK
eeee
SC
C
0
Use J/psi from ISR to measure detector resolution
2MeV/c8.05.23.33 TOTC
This is the value we use in our analyses
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