cp violation measuring matter/anti-matter asymmetry with babar
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CP Violation Measuring matter/anti-matter asymmetry with BaBar. Wouter Verkerke University of California, Santa Barbara. Outline of this talk. Introduction to CP violation A quick review of the fundamentals. CP-violating observables Experiment and analysis techniques - PowerPoint PPT PresentationTRANSCRIPT
Wouter Verkerke, UCSB
CP ViolationMeasuring matter/anti-matter asymmetry with BaBar
Wouter Verkerke
University of California, Santa Barbara
Wouter Verkerke, UCSB
Outline of this talk
• Introduction to CP violation– A quick review of the fundamentals.
– CP-violating observables
• Experiment and analysis techniques– Accelerator and detector (PEP-II and BaBar)
– Event selection, measuring time dependent CP asymmetries
• Selection of (recent) BaBar CP violation results– The angle
– The angle
– The angle
Wouter Verkerke, UCSB
Why is CP violation interesting?
• It is of fundamental importance– Needed for matter/anti-matter
asymmetry in the universe
– Standard Model CP-violation in quark sector is far too small to explain matter asymmetry in the universe
• History tells us that studying symmetry violation can be very fruitful
• CP violating processes sensitive to phases from New Physics
• Can CP-violation measurements at the B factories break the Standard Model in this decade?
– Measure phases of CKM elements in as many ways as possible
Wouter Verkerke, UCSB
• In the Standard Model, the CKM matrix elements Vij describe the electroweak coupling strength of the W to quarks
– CKM mechanism introduces quark flavor mixing
– Complex phases in Vij are the origin of SM CP violation
The Cabibbo-Kobayashi-Maskawa matrix
u
d
t
c
bs
CP The phase changes sign under CP.
Transition amplitude violates CP if Vub ≠ Vub*, i.e. if Vub has a non-zero phase
Mixes the left-handed charge –1/3 quark mass eigenstates d,s,b to give the weak
eigenstates d’,s,b’.
3 2
2
3
=cos(c)=0.22
Wouter Verkerke, UCSB
The Unitarity Triangle – Visualizing CKM information from Bd decays
• The CKM matrix Vij is unitary with 4 independent fundamental parameters
• Unitarity constraint from 1st and 3rd
columns: i V*i3Vi1=0
• Testing the Standard Model– Measure angles, sides in as many ways possible– SM predicts all angles are large
β
-i
-i
γ1 1
1 1 1
1 1
e
e
CKM phases (in Wolfenstein convention)
u
d
t
c
bs
Wouter Verkerke, UCSB
Observing CP violation
• So far talking about amplitudes, but Amplitudes ≠ Observables.
• CP-violating asymmetries can be observed from interference of two amplitudes with relative CP-violating phase
– But additional requirements exist to observe a CP asymmetry!
• Example: process Bf via two amplitudes a1 + a2 = A.
weak phase diff. 0, no CP-invariant phase diff.
Bf
A
A
Bf
a1 a1
a2
a2
A=a1+a2A=a1+a2
+
-
|A|=|A| No observable CP asymmetry
Wouter Verkerke, UCSB
Observing CP violation
• Example: process Bf via two amplitudes a1 + a2 = A. weak phase diff. 0, CP-invariant phase diff. 0
BfBfA=a1+a2
A=a1+a2
+-
|A||A| Need also CP-invariant phase for observable CP violation
a1 a1
a2
a2
AA
Wouter Verkerke, UCSB
CP violation: decay amplitudes vs. mixing amplitudes
• Interference between two decay amplitudes gives two decay time independent observables
– CP violated if BF(B f) ≠ BF(B f)
– CP-invariant phases provided by strong interaction part.
– Strong phases usually unknown this can complicate things…
• Interference between mixing and decay amplitudes introduces decay-time dependent CP violating observables
– Bd mixing experimentally very accessible: Mixing freq md0.5 ps-1, =1.5 ps
– Interfere ‘B B f’ with ‘B f’
– Mixing mechanism introduces weak phase of 2 and a CP-invariant phase of /2, so no large strong phases in decay required
2 md 2B
N(B
0)-
N(B
0)
N(B
0)+
N(B
0)
Wouter Verkerke, UCSB
ACP(t) from interference between mixing+decay and decay
• Time dependent CP asymmetry takes Ssin(mdt)+Ccos(mdt) form
• C=0 means no CP violation in decay process
• If C=0, coefficient S measures sine of mixing phase
mixing decay
If only single real decay amplitude contributes
20
02
0
0
)(
)(
)(
)( iif e
fBA
fBAe
fBA
fBA
p
q
00 BpBqB
Wouter Verkerke, UCSB
CKM Angle measurements from Bd decays
• Sources of phases in Bd amplitudes*
• The standard techniques for the angles:
*In Wolfenstein phase convention.
Amplitude Rel. Magnitude Weak phase
‘bc’ Dominant 0
‘bu’ Suppressed
B=2 (mixing) Time dependent
B0 mixing + single bc decay
B0 mixing + single bu decay
Interfere bc and bu in B± decay.
The distinction between and measurements is in the technique.
β
-i
-i
γ1 1
1 1 1
1 1
e
e
bu
td
Wouter Verkerke, UCSB
The PEP-II B factory – specifications
• Produces B0B0 and B+B- pairs via Y(4s) resonance (10.58 GeV)
• Asymmetric beam energies– Low energy beam 3.1 GeV
– High energy beam 9.0 GeV
• Boost separates B and B and allows measurement of B0 life times
• Clean environment– ~28% of all hadronic interactions is BB
BB threshold
(4S)
Wouter Verkerke, UCSB
The PEP-II B factory – performance
• Operates with 1600 bunches– Beam currents of 1-2 amps!
• Continuous ‘trickle’ injection– Reduces data taking interruption
for ‘top offs’
• High luminosity– 6.6x1033 cm-2s-1
– ~7 BB pairs per second
– ~135 M BB pairs since day 1.
• Daily delivered luminosity still increasing
• Projected luminosity milestone– 500M BB pairs by fall 2006.
Wouter Verkerke, UCSBSilicon VertexDetector (SVT)
Drift chamber (DCH)
ElectromagneticCalorimeter (EMC)
1.5 T Solenoid
InstrumentedFlux Return (IFR)
SVT: 5 layers double-sided Si. DCH: 40 layers in 10 super- layers, axial and stereo.
DIRC: Array of precisely machined quartz bars. .
EMC: Crystal calorimeter (CsI(Tl)) Very good energy resolution. Electron ID, 0 and reco.
IFR: Layers of RPCs within iron. Muon and neutral hadron (KL)
The BaBar experiment
• Outstanding K ID• Precision tracking (t measurement)• High resolution calorimeter• Data collection efficiency >95%
Detector forInternally reflectedCherenkov radiation(DIRC)
Wouter Verkerke, UCSB
Silicon Vertex Detector
Beam pipe
Layer 1,2Layer 3
Layer 4Layer 5
Beam bending magnets
Readoutchips
Wouter Verkerke, UCSB
Čerenkov Particle Identification system
• Čerenkov light in quartz– Transmitted by internal reflection– Rings projected in standoff box
– Thin (in X0) in detection volume, yet precise…
Wouter Verkerke, UCSB
Selecting B decays for CP analysis
• Exploit kinematic constraints from beam energies– Beam energy substituted mass has better resolution than invariant mass
– Sufficient for relatively abundant & clean modes
(mES) 3 MeV
(E) 15 MeV
mes>5.27 GeV
N = 1506Purity = 92%
mes (GeV)mes
E
2
SKccB )(0
Wouter Verkerke, UCSB
Measuring (time dependent) CP asymmetries
• B0B0 system from Y(4s) evolves as coherent system– All time dependent asymmetries integrate to zero!
• Need to explicitly measure time dependence
– B0 mesons guaranteed to have opposite flavor at time of 1st decay
• Can use ‘other B0’ to tag flavor of B0CP at t=0
B-Flavor Tagging
Vertexing
t=1.6 ps z 250 mz
170 m
z 70 m
z/c
Tag-side vertexing~95%efficient
Exclusive B Meson
Reconstruction
Wouter Verkerke, UCSB
Flavor tagging
Leptons : Cleanest tag. Correct >95%
Kaons : Second best. Correct 80-90%
b c
e
W
b c
e+
W+
b
Wc s
u
d
K
W+b
W+c s
u
d
K+
W
Full tagging algorithm combines all in neural network
Four categories based on particle content and NN output.
Tagging performance
= 28%
Determine flavor of Btag BCP(t=0)from partial decay products
efficiency mistake rate
Wouter Verkerke, UCSB
B0(t) B0(t) ACP(t) = Ssin(mdt)+Ccos(mdt)
sin2
Dsin2
Putting it all together: sin(2) from B0 J/ KS
• Effect of detector imperfections
– Dilution of ACP amplitude due imperfect tagging
– Blurring of ACP sine wave due to finite t resolution
• Measured & Accounted for in simultaneously unbinned maximum likelihood fit to control samples
– measures t resolution and mistag rates.
– Propagates errors
Actual sin2 result on 88 fb-1
Imperfect flavor tagging
Finite t resolution
t t
Wouter Verkerke, UCSB
• Interference between mixing and single real decay– Interfering amplitudes of comparable magnitude
the observable asymmetry is large (ACP of order 1)
• Extraordinarily clean theory prediction (~1% level)– Single real decay amplitude all hadronic uncertainty cancel
– ACP(t) = sin(2) sin(md t)
• Experimentally easy– ‘Large’ branching fraction O(10-4)
– Clear signature (J/ l+l- and KS +-)
B-factory ‘flagship’ measurement: sin2 from J/ KS
B0 Mixing……followed by………Decay Decay
B0
b
d ds
ccW+
Vcb
Vcs
J/
Ks
*
cc
d
s
W+
Vcb
Vcs
J/
Ks
*
Wouter Verkerke, UCSB
‘Golden’ measurement of sin2sin2 = 0.76 0.074
Combined result (88 fb-1, 2001)sin2 = 0.741 0.067 0.034 || = 0.948 0.051 0.030
(stat) (syst)
B0 (cc) KS (CP=-1)
B0 (cc) KL (CP=+1)
sin2 = 0.72 0.16
No evidence for cos(mt) term
Wouter Verkerke, UCSB
Constraints on the apex of the Unitarity Triangle.
Standard Model interpretation
Method as in Höcker et al, Eur.Phys.J.C21:225-259,2001
= (1-2/2) = (1-2/2)
Wouter Verkerke, UCSB
Standard Model interpretation
Method as in Höcker et al, Eur.Phys.J.C21:225-259,2001
Latest results including the Belle experiment.
One solution for is very consistent with the other constraints.
4-fold ambiguity because we measure sin(2), not
1
2
4
3
The CKM model for CP violation has passed its first precision test!
There is still room for improvement: measurement is statistics dominatedSummer ’04 data 2-3 x 88fb-1
Wouter Verkerke, UCSB
B-factory measurements of sin2
• Going beyond the ‘golden’ modes– Consistency requires S=sin2, C=0
for all B0 decay modes for which the weak phase is zero.
– Decay modes dominated by the bs penguin may meet these criteria
– Measure ACP(t) from interference between mixing + bs decayand bs decay
• Loop diagrams are sensitive to contributions from new physics– Look for deviations of S=sin2
Standard model expectation for sin(2) from bs penguins
B0K0 B0’K0 B00K0
Experimentally best modes:
SM contributions that spoil S = sin2• u-quark penguin (weak phase = !)
but relative CKM factor of ~0.02• u-quark tree (different phase)
u /
u /
*Grossman, Ligeti, Nir, Quinn. PRD 68, 015004 (2003) and Gronau, Grossman, Rosner hep-ph/0310020
I
II
III
(I) (I, II & III) (II & III)
SM sin2 from SU(3)
B0K0 <0.25
B0’K0 <0.35
B00K0 <0.20
these limits will improvewith additionaldata
Wouter Verkerke, UCSB
bs penguin measurements
Mode BF(Bf)
x10-6
iBFi
x10-6
Reco. Efficiency
Purity Tagged signalEvents
81fb-1 115fb-1
J/Ks 440 36.0 44% 97% 940
’Ks 33 10.6 23% ~60% 110
Ks 4 1.4 42% ~80% ~34 ~48
0Ks 6 4.1 17% ~50% ~83
Experimentally more difficult• Branching fractions smaller, more irreducible background
B0 ’KS B0 KS B0 KS0
Wouter Verkerke, UCSB
sin2 from bs penguin measurements
’ ’Ks
BaBar 0.02 0.34 0.03
Ks
BaBar 0.45 0.43 0.07
bs penguin average
Babar 0.27 0.22
0Ks
BaBar 0.48 (+0.38) 0.11–0.47
sin2 from B0 (cc) KS
Wouter Verkerke, UCSB
sin2 from bs penguin measurements
’ ’Ks
BaBar 0.02 0.34 0.03Belle 0.43 0.27 0.05Ave 0.27 0.21
Ks
BaBar 0.45 0.43 0.07Belle –0.96 0.50 (+0.09)Ave –0.14 0.33
K+K-Ks non-resonantBelle 0.51 0.26 0.05 (+0.18)
bs penguin average
Babar and Belle 0.27 0.15
–0.00
–0.11
0Ks
Babar 0.48 (+0.38) 0.11–0.47
sin2 from B0 (cc) KS
(My naïve averages)
Wouter Verkerke, UCSB
sin2 : bs penguin modes
• Current naïve world averages
S = 0.27 ± 0.15 (~3 below J/Ks S = 0.74 ± 0.05).
C = 0.10 ± 0.09
• Still very early in the game– Measurements are statistics limited.
Errors smaller by factor 2 in 2-3 years.
– Standard Model pollution limits from SU(3) analysis will also improve with more data.
Wouter Verkerke, UCSB
The angle from B
• Determination of : Observe ACP(t) of B0 CP eigenstate decay dominated by bu
– Interference between mixing+bu decay and bu decay
– Textbook example is B0 +.
• If the above bu tree diagram dominates the decay
ACP(t)=sin(2)sin(mdt).
bu decay
Vub
B0 Mixing
sin2
Wouter Verkerke, UCSB
The angle - the penguin problem
• Turns out the dominant tree assumption for is bad.– There exists a penguin diagram for the decay as well
– Magnitude of penguin can be estimated from B K+- (dominated by SU(3) variation of this penguin)
– Penguin amplitude is large, contribution to B could be ~30%!
• Including the penguin component (P) in
• Coefficients from time-dependent analysis
penguin decay
tree decay Vub
Vtd/Vts
s
Ratio of amplitudes |P/T| and strong phase difference can not be reliably calculated
)22sin(1,sin 2
CSC
Unknown phase shift
Wouter Verkerke, UCSB
Disentangling the penguin: determining 2
• Gronau & London: Use isospin relations
– Measure all isospin variations of B
B0 +- , B0 +-, B0 00 , B0 00
B- -0 = B+ +0
– Weak phase offset 2 can bederived from isospin triangles
• Complicated…
-
Wouter Verkerke, UCSB
Disentangling the penguin: the Grossman-Quinn bound
• Easy alternative to isospin: Grossman-Quinn bound – Look at isospin triangles and construct upper limit on
– Minimum required input: BF(B 0) and limit on BF(B0 00)
– Works best if B0 00 is small
– Experimental advantage: no flavor tagging in B00
• Measure B0 00!
‘~10-5’ ‘~10-6’
)()(
)()()2/(sin
00
0000002
BBBB
BBBB
Wouter Verkerke, UCSB
the Grossman-Quinn bound on for B0
• B0 00 is observed! (4.2)
• GQ Bound using world averages
– 00: (1.9±0.5)x10-6
– ±0: (5.3±0.8)x10-6
• 00 large, thus GQ bound not very constraining– Isospin analysis required for 00!
Plots are after cut on signal probability ratio not including variable shown, optimized with S/sqrt(S+B) .
[BELLE: (1.7±0.6±0.2)x10-6, 3.4]
.).%90(47o LC
)22sin(1 2
CS
Wouter Verkerke, UCSB
Alternatives to B for determination of
• There are other final states of bu tree diagram, e.g.– B (Dalitz analysis required)
– B (Vector-vector multiple amplitudes)
• B +- analysis– 3 helicity amplitudes: Longitudinal (CP-even), 2 transverse (mixed CP)
– Looks intractable, but entirely longitudinally polarized*!
– + is basically a CP-even state with same formalism as +.
*As predicted by G.Kramer, W.F.Palmer, PRD 45, 193 (1992). R.Aleksan et al., PLB 356, 95 (1995).
Wouter Verkerke, UCSB
the Grossman-Quinn bound for B0
• The Grossman-Quinn bound for B0
(assuming full longitudinal polarization)
(BaBar)
(Belle)
Wouter Verkerke, UCSB
Alpha summary
• The system: large penguin pollution– We have seen B000!
– Current GQ bound:
– Full isospin analysis required!
• The system: small penguin pollution– Polarization is fully longitudinal (as predicted).
– Current GQ bound:
– Bound may improve as additional data becomes available
– Time-dependent + results (measures sin(2+2)) coming soon.
• There are more techniques than and – e.g. Dalitz analysis of
Wouter Verkerke, UCSB
The angle
• Measuring = Measuring the phase of the Vub
– Main problem: Vub is very small: O(3)
– Either decay rate or observable asymmetry is always very small.
• Conventional wisdom: measuring at B factories is difficult/impossible.– Gamma is the least constrained angle of the Unitarity Triangle
• Current attitude: we should try.– There are new ideas to measure Dalitz decays, 3-body decays,…)
– New experimental data suggest color suppression is less severe, which eases small rate/asymmetry problem somewhat
– B-Factories produce more luminosity than expected(BaBar & Belle approaching O(200) fb-1 by Summer ’04 time )
Wouter Verkerke, UCSB
The angle : B DK
• Strategy I: interfere bu and bc decay amplitudes– D0/D0 must decay to common final state to interfere
• Ratio of decay B amplitudes rb is small: O(10-1)
• rb is not well measured, but important
– rb large more interference more sensitivity to
colorsuppression
Ru is the left side of the Unitarity Triangle (~0.4).
]2.008.0[)(
)(
)(
)(0
0
CSub FRcbA
ubAr
KDBA
KDBA
FCS is (color) suppression factor([0.2-0.5], naively1/3)
Wouter Verkerke, UCSB
from B DK – Two approaches
• Approach I: D0/D0 decay to common CP eigenstate
– ‘Gronau, London & Wyler’
– D0/D0 decay rate same
• Approach II: D0/D0 decay to common flavor eigenstate
– ‘Atwood, Dunietz & Soni’
– Use D0/D0 decay rate asymmetry to compensate B decay asymmetry`
• Complementary in sensitivity
– GLW: large BF: O(1±rb), small ACP: O(rb)
– ADS: small BF: O(rb2), large ACP: O(1)
00000
000
,intoe.g.decays,2/)(
,intoe.g.decays,2/)(
SSCP
CP
KKDDD
KKDDD
Branching fractions
small (0.1%-1%)
KD
KD0
0CKM favored
Doubly Cabibbo suppressed (by factor O(100))
Wouter Verkerke, UCSB
B DK Observables – Gronau-London-Wyler
• There are more observables sensitive to than ACP
– Absolute decay rate also sensitive to , but hard to calculatedue to hadronic uncertainties
– GLW: measure ratio of branching fractions: hadronic uncertainties cancel!
– Experimental bonus: many systematic uncertainties cancel as well
– Bottom line: 2 observables each for CP+ and CP- decays
• 3 independent observables (R+, R-, A+=-A-), 3 unknowns (rb, b, )
Wouter Verkerke, UCSB
2
2
0
00
10)20.035.031.8(
10)5.06.18.8(
)(
)()(
KDBB
KDBBKDBBR CPCP
B DK : GLW results
• Result for B- D0 K- in 115 fb-1
• Results for CP-odd modesin progress (R-, A-)
06.017.007.0 A
D0
background
06.017.007.0 A
GLW method: large BF, small ACP
Wouter Verkerke, UCSB
B DK : The Atwood-Dunietz-Soni method
• Two observables, similar to GLW technique
– Ratio of branching fractions and ACP
• D0 K+-: 2 observables (A, R), 3 unknowns (rb, b+d, )
– Insufficient information to solve for
– Can add other D0 decay modes, e.g. D0 K+-0 4 observables (2xA, 2xR), 4 unknowns (rb, b+DKp, b+DKpp0, )
• Expected BF is ~510-7 – very hard!– Expect observable O(10) events in 100M BB events
– Unknown values of , rb, b add O(10) uncertainty of BF estimate
– Measurement not attempted until now
Wouter Verkerke, UCSB
from Atwood-Dunietz-Soni method: B- [K+ -]D0 K- : results
• Newly developed background suppression techniques give us sensitivity in BF = O(10-7) range
BF 5x10-7 ~10 events
• But we don’t see a signal!– Destructive interference,
rb is small, or just unlucky?
• Cannot constrain with this measurement…
– But BF proportional to rb2 results sets upper limit on rb
MC yield prediction with BF=7x10-5: 12 evts
Yield in 115 fb-1 of data:1.1 3.0 evts
No assumptions: rb < 0.22 (90% C.L.) from CKM fit : rb < 0.19 (90% C.L.)(95% C.I. region)
ADS method: small BF, large ACP
Wouter Verkerke, UCSB
B DK : prospects for B-factories at 500 fb-1
• Combine information on from various sources
• Example study– Assume =75o,
b=30o, d=15o
– Consider various scenarios
• GLW alone
rb=0.3
=75o, b=30o, d=15o
3
2
1GLW
2
Wouter Verkerke, UCSB
2
B DK : prospects for B-factories at 500 fb-1
• Combine information on from various sources
• Scenarios
– GLW alone
– GLW+ADS(K)
– GLW+ADS(K)+d from CLEO-c
• ADS/GLW combination powerful
• There are additional information not usedin this study, e.g.
– GLW: D*0K,D0K*,D*0K*
– ADS: K0,K3
– sin() from D*, D0K0, DK,…
rb=0.3
=75o, b=30o, d=15o
3
2
111o
GLW
GLW+ADS
GLW+ADS+CLEO-c
Wouter Verkerke, UCSB
B DK : prospects for B-factories at 500 fb-1
• Combine information on from various sources
• rb is critical parameter
2
rb=0.1
rb=0.2
rb=0.3
=75o, b=30o, d=15o
12
3
3
3
2
2
1
111o
23o
67o
Wouter Verkerke, UCSB
Gamma summary
• The B DK program is underway– Measurements for GLW methods in progress (B D(*)0 K(*)-)
– First measurement of ADS method (B [K+-]K-)
– ADS and GLW techniques powerful when combined
– Final results depends strongly on rb
• Other methods in progress as well– Dalitz analyses of B- D0(KS)K-, B DK
– Time dependent analysis of B D*- (mixing + Vub decay)• |sin(2+)|>0.57 (95% C.L.))
– Analysis of B0 D(*)0 K(*)0
• There is no ‘golden’ mode to measure – All techniques are difficult and to 1st order equally sensitive.
– Combine all the measurements and hope for the best
Wouter Verkerke, UCSB
Concluding remarks
• The CKM model for CP violation passed it’s first test (sin2).
– Future measurements of sin2 from B0 (cc)KS will continue improve constraints on apex of unitarity triangle
• The bs penguin measurement of sin2 offers a window to new physics.
– Another 2-3 years worth of data will clarify current 3 discrepancy
• We are cautiously optimistic that we can measure now that B decay turns out have little penguin pollution
• Measurement of just starting. Success depends on many unknowns…
• BaBar is projected to double its current dataset by 2006