CP Violation Measuring matterantimatter asymmetry with BaBar

1
CP ViolationMeasuring matter/anti-matter
asymmetry with BaBar

• Wouter Verkerke
• University of California, Santa Barbara

2
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 b
• The angle a
• The angle g

3
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

4
The Cabibbo-Kobayashi-Maskawa matrix

• 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

Mixes the left-handed charge 1/3 quark mass
eigenstates d,s,b to give the weak
eigenstates d,s,b.
l3
l
l
l2
l3
l2
lcos(qc)0.22
CP
The phase changes sign under CP.
Transition amplitude violates CP if Vub ? Vub,
i.e. if Vub has a non-zero phase
5
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 Vi3Vi10
• Testing the Standard Model
• Measure angles, sides in as many ways possible
• SM predicts all angles are large

CKM phases (in Wolfenstein convention)
6
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 B?f via two amplitudes a1 a2
  A. weak phase diff. g ? 0, no
  CP-invariant phase diff.

B?f
Aa1a2
Aa1a2
a2
A
g
a1
a1
-g
A
a2
AA ? No observable CP asymmetry
7
Observing CP violation

• Example process B?f via two amplitudes a1 a2
  A. weak phase diff. g ? 0,
  CP-invariant phase diff. d ? 0

B?f
Aa1a2
Aa1a2
g
d
d
A
-g
a2
A
a2
a1
a1
A?A ? Need also CP-invariant phase
for observable CP violation
8
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 Dmd?0.5 ps-1, t1.5 ps
• Interfere B ? B ? f with B ? f
• Mixing mechanism introduces weak phase of 2b and
  a CP-invariant phase of p/2, so no large strong
  phases in decay required

N(B0)-N(B0) N(B0)N(B0)
2p ? Dmd ? 2tB
9
ACP(t) from interference between mixingdecay and
decay

• Time dependent CP asymmetry takes
  S?sin(Dmdt)C?cos(Dmdt) form
• C0 means no CP violation in decay process
• If C0, coefficient S measures sine of mixing
  phase

mixing
decay
If only single real decay amplitude contributes
10
CKM Angle measurements from Bd decays

• Sources of phases in Bd amplitudes
• The standard techniques for the angles

b?u
In Wolfenstein phase convention.
t?d
B0 mixing single b?u decay
The distinction between a and g measurements is
in the technique.
B0 mixing single b?c decay
Interfere b?c and b?u in B decay.
11
The PEP-II B factory specifications

• Produces B0B0 and BB- 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

?(4S)
BB threshold
12
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.

13
The BaBar experiment

• Outstanding K? ID
• Precision tracking (Dt measurement)
• High resolution calorimeter
• Data collection efficiency gt95

Electromagnetic Calorimeter (EMC)
1.5 T Solenoid
Detector for Internally reflected Cherenkov
radiation (DIRC)
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, p0 and g
reco. IFR Layers of RPCs within iron. Muon
and neutral hadron (KL)
Drift chamber (DCH)
Instrumented Flux Return (IFR)
Silicon Vertex Detector (SVT)
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Silicon Vertex Detector
Readout chips
Beam bending magnets
Beam pipe
Layer 1,2
Layer 3
Layer 4
Layer 5
15
Cerenkov Particle Identification system

• Cerenkov light in quartz
• Transmitted by internal reflection
• Rings projected in standoff box
• Thin (in X0) in detection volume, yet precise

16
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 s(DE) ? 15 MeV
2
17
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 t0

Vertexing
Tag-side vertexing 95 efficient
B-Flavor Tagging
sz ? 170 mm
sz ? 70 mm
Dt1.6 ps ? Dz ?250 mm
Exclusive B Meson Reconstruction
? Dz/gbc
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Flavor tagging
Determine flavor of Btag ? BCP(Dt0)from partial
decay products
Leptons Cleanest tag. Correct gt95
Full tagging algorithm combines all in neural
network Four categories based on particle
content and NN output. Tagging performance
e-
e
W-
W
n
n
b
b
c
c
Kaons Second best. Correct 80-90
efficiency
mistake rate
W-
W
c
c
K-
s
s
b
K
b
u
W-
u
28
W
d
d
19
Putting it all together sin(2b) from B0 ? J/y KS

• Effect of detector imperfections
• Dilution of ACP amplitude due imperfect tagging
• Blurring of ACP sine wave due to finite Dt
  resolution
• Measured Accounted for in simultaneously
  unbinned maximum likelihood fit to control
  samples
• measures Dt resolution and mistag rates.
• Propagates errors

Imperfect flavor tagging
Finite Dt resolution
? Actual sin2b result on 88 fb-1
Dt
Dt
20
B-factory flagship measurement sin2b from J/y
KS

• 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(2b) sin(Dmd t)
• Experimentally easy
• Large branching fraction O(10-4)
• Clear signature (J/y ? ll- and KS ? pp-)

Decay
B0 Mixingfollowed byDecay
d
Ks
s
Vcs

c
W
J/Y
c
f 0
Vcb
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Golden measurement of sin2b
sin2b 0.76 ? 0.074
B0 ? (cc) KS (CP-1)
No evidence for cos(DmDt) term
sin2b 0.72 ? 0.16
B0 ? (cc) KL (CP1)
22
Standard Model interpretation
Constraints on the apex of the Unitarity Triangle.
h
r
Method as in Höcker et al, Eur.Phys.J.C21225-25
9,2001
23
Standard Model interpretation
4-fold ambiguity because we measure sin(2b), not b
One solution for b is very consistent with the
other constraints.
2
1
Latest results including the Belle experiment.
h
The CKM model for CP violation has passed its
first precision test!
3
There is still room for improvement measurement
is statistics dominated Summer 04 data ? 2-3 x
88fb-1
4
r
Method as in Höcker et al, Eur.Phys.J.C21225-25
9,2001
24
B-factory measurements of sin2b

• Going beyond the golden modes
• Consistency requires Ssin2b, C0 for all B0
  decay modes for which the weak phase is zero.
• Decay modes dominated by the b?s penguin may
  meet these criteria
• Measure ACP(t) from interference between mixing
  b?s decayand b?s decay
• Loop diagrams are sensitive to contributions
  from new physics
• Look for deviations of Ssin2b

f0
f0
f???
f???
25
Standard model expectation for sin(2b) from b?s
penguins
I
Experimentally best modes
B0?fK0
B0?hK0
B0?p0K0
(I)
(I, II III)
(II III)
f g / 0

• SM contributions that spoil S sin2b
• u-quark penguin (weak phase g!) but
  relative CKM factor of 0.02
• u-quark tree (different phase)

II
f g / 0
these limits will improve with additional data
f g
III
Grossman, Ligeti, Nir, Quinn. PRD 68, 015004
(2003) and Gronau, Grossman, Rosner
hep-ph/0310020
26
b?s penguin measurements

• Experimentally more difficult
• Branching fractions smaller, more irreducible
  background

B0 ? hKS
B0 ? fKS
B0 ? KSp0
27
sin2b from b?s penguin measurements
hKs BaBar 0.02 ? 0.34 ? 0.03
fKs BaBar 0.45 ? 0.43 ? 0.07
p0Ks BaBar 0.48 (0.38) ? 0.11
0.47
b?s penguin average Babar 0.27 ? 0.22
sin2b from B0 ? (cc) KS
28
sin2b from b?s penguin measurements
(My naïve averages)
hKs BaBar 0.02 ? 0.34 ? 0.03 Belle 0.43 ? 0.27
? 0.05 Ave 0.27 ? 0.21
fKs BaBar 0.45 ? 0.43 ? 0.07 Belle 0.96 ?
0.50 (0.09) Ave 0.14 ? 0.33
0.11
p0Ks Babar 0.48 (0.38) ? 0.11
0.47
KK-Ks non-resonant Belle 0.51 ? 0.26 ? 0.05
(0.18)
0.00
b?s penguin average Babar and Belle 0.27 ? 0.15
sin2b from B0 ? (cc) KS
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sin2b b?s penguin modes

• Current naïve world averages
• S 0.27 0.15 (3s below J/yKs 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.

30
The angle a from B ? pp

• Determination of a Observe ACP(t) of B0 ? CP
  eigenstate decay dominated by b?u
• Interference between mixingb?u decay and b?u
  decay
• Textbook example is B0 ? pp-.
• If the above b?u tree diagram dominates the decay
  
• ACP(t)sin(2a)sin(DmdDt).

B0 Mixing
b?u decay
Vub
f g
sin2a
31
The angle a - the penguin problem

• Turns out the dominant tree assumption for pp-
  is bad.
• There exists a penguin diagram for the decay as
  well
• Magnitude of penguin can be estimated from B ?
  Kp- (dominated by SU(3) variation of this
  penguin)
• Penguin amplitude is large, contribution to B ?
  pp- could be 30!
• Including the penguin component (P) in l
• Coefficients from time-dependent analysis

tree decay
penguin decay
s
Vtd/Vts
/ K
Vub
f 0
f 0
f g
Ratio of amplitudes P/T and strong phase
difference d can not be reliably calculated
Unknown phase shift
32
Disentangling the penguin determining 2k

• Gronau London Use isospin relations
• Measure all isospin variations of B ? pp
• B0 ? pp- , B0 ? pp-,
  B0 ? p0p0 , B0 ? p0p0 B- ?
  p-p0 B ? pp0
• Weak phase offset 2k can bederived from isospin
  triangles
• Complicated

2k
-
33
Disentangling the penguin the Grossman-Quinn
bound

• Easy alternative to isospin Grossman-Quinn bound
  
• Look at isospin triangles and construct upper
  limit on k
• Minimum required input BF(B? ? p?p0) and limit
  on BF(B0 ? p0p0)
• Works best if B0 ? p0p0 is small
• Experimental advantage no flavor tagging in
  B?p0p0
• Measure B0 ? p0p0!

10-5
10-6
34
the Grossman-Quinn bound on k for B0 ? pp

• B0 ? p0p0 is observed! (4.2s)
• GQ Bound using world averages
• p0p0 (1.90.5)x10-6
• pp0 (5.30.8)x10-6
• p0p0 large, thus GQ bound not very constraining
• Isospin analysis required for p0p0!

Plots are after cut on signal probability ratio
not including variable shown, optimized with
S/sqrt(SB) .
BELLE (1.70.60.2)x10-6, 3.4s
35
Alternatives to B ? pp for determination of a

• There are other final states of b?u tree diagram,
  e.g.
• B ? r p (Dalitz analysis required)
• B ? r r (Vector-vector ? multiple amplitudes)
• B ? rr- analysis
• 3 helicity amplitudes Longitudinal (CP-even), 2
  transverse (mixed CP)
• Looks intractable, but entirely longitudinally
  polarized!
• rr- is basically a CP-even state with same
  formalism as pp-.

As predicted by G.Kramer, W.F.Palmer, PRD 45,
193 (1992). R.Aleksan et al., PLB 356, 95
(1995).
36
the Grossman-Quinn bound for B0 ? rr

• The Grossman-Quinn bound for B0 ? rr

(BaBar) (Belle)
(assuming full longitudinal polarization)
37
Alpha summary

• The pp system large penguin pollution
• We have seen B0?p0p0!
• Current GQ bound
• Full isospin analysis required!
• The rr system small penguin pollution
• Polarization is fully longitudinal (as
  predicted).
• Current GQ bound
• Bound may improve as additional data becomes
  available
• Time-dependent rr- results (measures sin(2a2k))
  coming soon.
• There are more techniques than pp and rr
• e.g. Dalitz analysis of rp

38
The angle g

• Measuring g Measuring the
  phase of the Vub
• Main problem Vub is very small O(l3)
• Either decay rate or observable asymmetry is
  always very small.
• Conventional wisdom measuring g 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 g (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 )

39
The angle g B ? DK

• Strategy I interfere b?u and b?c 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 g

fg
color suppression
f0
Ru is the left side of the Unitarity Triangle
(0.4).
FCS is (color) suppression factor(0.2-0.5,
naively?1/3)
40
g 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(1rb), small ACP O(rb)
• ADS small BF O(rb2), large ACP O(1)

Branching fractionssmall (0.1-1)
CKM favored
Doubly Cabibbo suppressed (by factor O(100))
41
B ? DK Observables Gronau-London-Wyler

• There are more observables sensitive to g than
  ACP
• Absolute decay rate also sensitive to g, 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, db, g)

42
B ? DK GLW results
GLW method large BF, small ACP

• Result for B- ? D0 K- in 115 fb-1
• Results for CP-odd modesin progress (R-, A-)

D0p- background
43
B ? DK The Atwood-Dunietz-Soni method

• Two observables, similar to GLW technique
• Ratio of branching fractions and ACP
• D0 ? Kp- 2 observables (A, R), 3 unknowns (rb,
  dbdd, g)
• Insufficient information to solve for g
• Can add other D0 decay modes, e.g. D0 ? Kp-p0 ?
  4 observables (2xA, 2xR), 4 unknowns (rb,
  dbdDKp, dbdDKpp0, g)
• Expected BF is 5?10-7 very hard!
• Expect observable O(10) events in 100M BB events
• Unknown values of g, rb, db add O(10) uncertainty
  of BF estimate
• Measurement not attempted until now

44
g from Atwood-Dunietz-Soni method B- ? K p-D0
K- results
MC yield prediction with BF7x10-5 12 evts
ADS method small BF, large ACP

• Newly developed background suppression
  techniques give us sensitivity in BF O(10-7)
  range BF 5x10-7 ? 10 events
• But we dont see a signal!
• Destructive interference, rb is small, or just
  unlucky?
• Cannot constrain g with this measurement
• But BF proportional to rb2 ? results sets upper
  limit on rb

Yield in 115 fb-1 of data1.1 ? 3.0 evts
No assumptions rb lt 0.22 (90 C.L.)

• from CKM fit rb lt 0.19 (90 C.L.)
• (95 C.I. region)

45
B ? DK prospects for B-factories at 500 fb-1

• Combine information ong from various sources
• Example study
• Assume g75o, db30o,
  dd15o
• Consider various scenarios
• GLW alone

g75o, db30o, dd15o
rb0.3
Dc2
3s
2s
GLW
1s
g
46
B ? DK prospects for B-factories at 500 fb-1

• Combine information ong from various sources
• Scenarios
• GLW alone
• GLWADS(Kp)
• GLWADS(Kp)dd from CLEO-c
• ADS/GLW combination powerful
• There are additional information not usedin
  this study, e.g.
• GLW D0K,D0K,D0K
• ADS Kpp0,K3p
• sin(2bg) from Dp, D0K0, DKp,

g75o, db30o, dd15o
Dc2
rb0.3
3s
GLWADSCLEO-c
GLWADS
2s
11o
GLW
1s
g
47
B ? DK prospects for B-factories at 500 fb-1

• Combine information ong from various sources
• rb is critical parameter

g75o, db30o, dd15o
3s
rb0.1
2s
1s
67o
3s
rb0.2
2s
D?2
1s
23o
3s
rb0.3
2s
1s
11o
g
48
Gamma summary

• The B ? DK program is underway
• Measurements for GLW methods in progress (B ?
  D()0 K()-)
• First measurement of ADS method (B ? Kp-K-)
• ADS and GLW techniques powerful when combined
• Final results depends strongly on rb
• Other g methods in progress as well
• Dalitz analyses of B- ? D0(KSpp-)K-, B ? DKp
• Time dependent analysis of B ? Dp- (mixing Vub
  decay)
• sin(2bg)gt0.57 (95 C.L.))
• Analysis of B0 ? D()0 K()0
• There is no golden mode to measure g
• All techniques are difficult and to 1st order
  equally sensitive.
• Combine all the measurements and hope for the
  best

49
Concluding remarks

• The CKM model for CP violation passed its first
  test (sin2b).
• Future measurements of sin2b from B0 ? (cc)KS
  will continue improve constraints on apex of
  unitarity triangle
• The b?s penguin measurement of sin2b offers a
  window to new physics.
• Another 2-3 years worth of data will clarify
  current 3s discrepancy
• We are cautiously optimistic that we can measure
  a now that B ? rr decay turns out have little
  penguin pollution
• Measurement of g just starting. Success depends
  on many unknowns
• BaBar is projected to double its current dataset
  by 2006