Transcript Measurement of CP Violating Asymmetry with the BaBar detector

Observation Of CP Violation in B
Decays with the BaBar Detector
Vivek Sharma
University of California, San Diego
(On Behalf of the BaBar Collaboration)
FNAL Joint Theory & Experiment Seminar
FNAL : b quark Physics started here !
(1S)
(1S)
(2S)
(3S)
7/17/2015
Vivek Sharma
2
Outline Of This Talk



CP Violation, CKM Matrix and the Unitarity Triangle
CP Violation in B Decays : The Three possibilities
Observation of CP Violation in the interference of Decay and
Mixing  Sin2b


The PEP-II B Factory & The BaBar Detector
The three linked steps towards the sin2b measurement




B Lifetime
B Mixing
CP Asymmetry
The Way forward : Summary and Outlook
7/17/2015
Vivek Sharma
3
CP Violation : Inner Space, Outer Space



Search for CP Violation mechanism has been a concern of
particle physics since its discovery in the KL system
3 Generation CKM mixing matrix (via the phase) provides an
elegant explanation for this effect which needs to be probed
critically and cleanly
The B meson system is an excellent new laboratory for
understanding & testing the mechanism behind CP Violation
SM via CKM phase does incorporate
enough CPV to explain cosmic
matter-antimatter asymmetry
So, if there are beyond the SM
phenomena, their effect may be
measurable in B system
7/17/2015
Vivek Sharma
4
CP Violation in the Standard Model
Mass Eigenstate s  Weak Eigenstate s  Quark Mixing
VCKM
Vud
  Vcd

 Vtd
Vus
Vcs
Vts
Vub 
Vcb 

Vtb 
CKM Matrix
Complex matrix described by
4 independent parameters
Wolfenstein parametrization:
VCKM
 1  2 / 2

A3   i


2
2



1  / 2
A
 3

2


A

1



i


A

1


CP Violation:


*
J  Im Vik Vjk
Vj Vi*  0
J  A 26
7/17/2015
phase
  0  no CPV from SM
Vivek Sharma
5
Unitarity Triangle
Choice of parameters:


  1  2 / 2  
  1  2 / 2 
, A,  and 
Unitarity : 1  R t  R u  0
At the 1% level : Vus
  Vus  sin c
  0.2205  0.0018
At the 5% level : Vcb
A  Vcb /2
A  0.83  0.06
| Vub | and | Vtd |
  -  plane

 ,  
Ru g
Rt
b
0, 0

Vud Vub
Ru 


Vcd Vcb
Rt 
Vtd Vtb

Vcd Vcb
1, 0 
 2  2 e i γ
  (1   ) 2   2 e i b
*
g  arg Vub
,     g  b
7/17/2015
Vivek Sharma
6
Three Forms of CP Violation in B Decays
Direct
CP Violation
–Total amplitude for a decay and its CP conjugate have different
magnitudes
–Difficult to relate measurements to CKM matrix elements due to
hadronic uncertainties
–Relatively small asymmetries expected in B decays

CP Violation in Mixing
–Would give rise to a charge asymmetry in semi-leptonic decays
–Expected to be small in Standard Model (DG<<DM)
CP
Violation in the interference of mixed and unmixed decays
–Typically use a final state that is a CP eigenstate (fCP)
–Large time dependent asymmetries expected in Standard Model

–Asymmetries can be directly related to CKM parameters in many cases,
7/17/2015without hadronic uncertainties
Vivek Sharma
7
CP Violation in interference between Mixing and Decay
Flavor
eigenstate
Flavor
eigenstate
B0(t)
Initial
state
fCP
B0
B0(t)
Initial
state
fCP
B0
Time evolution of initial pure B0/B0 states: Mixing
Mass eigenstates: BH , L  p B0  q B 0
p
1
q
fCP is a CP eigenstate CP f CP  CP f CP
7/17/2015
Vivek Sharma
8
CP from Interference of Mixing and Decay
B0
mixing
~ e  2 ib
B0
λ fCP  η fCP
Af CP
f CP
q A fCP

p A fCP
amplitude
ratio
 e  2ib
CP eigenvalue
Af CP
0
 f CP  1  Pr ob( Bphys
( t )  f CP )  Pr ob(B0phys ( t )  f CP )
Define Time-dependent CP Observable:
G( B0phys (t )  f CP ) G( B0phys (t )  f CP )
Af (t ) 
CP
G( B0phys (t )  f CP )  G( B0phys (t )  f CP )
 C f cos (Dmd t )  S f sin (Dmd t )
CP
7/17/2015
CP
Vivek Sharma
C f CP 
Sf CP 
1  |  f CP | 2
1  |  f CP | 2
 2 Im  f CP
1  |  f CP | 2
9
The “Golden” Decay Mode: B0 J/yK0S
c
J
/
y
c
0
b
0
B d
b
u,c,t
W
u,c,t
K mixing
W
B
s
0
K0
d
d
λ J/y K0  CP e
0
0
BCP  J /y K S
0
0
BCP  J /y K L
i 2 b
L,S



Theoretically clean mode to measure sin2b
Clean experimental signature
“Large” branching fraction compared to other CP eigenstates
“Golden Modes”
Time-dependent CP asymmetry
ACP (t )  CP sin 2b sin(Dm t )
7/17/2015
Vivek Sharma
 CP = -1
 B0  J/y K0S
 B0  y(2s) K0S
CP = +1
 B0  J/y K0L
10
Decay Time Distribution in B fCP
f  : Bphys  f CP
0
f  : Bphys  f CP
0
f (B  fCP , t )  G4 eG |Dt| [1  CfCP cos(Dmd t )
7/17/2015
Vivek Sharma
SfCP sin(Dmd t )]
11
Decay Time Evolution & ACP for B0 J/yK0S


t spectrum and the observed
asymmetry for a perfect
detector
(assuming sin2b = 0.6)
Visible difference between
B0 and B0 decay rates
sin 2b
ACP (t )   f sin 2b sin( Δmd t )


In this ideal case, the amplitude
of the oscillation is the CP Asymmetry
time-integrated asymmetry is 0
t
7/17/2015
Vivek Sharma
12
Exptal Requirements For CPV Measurement



BR (B fCP) ~ 10-4  Need to record and reconstruct a
large # of B Mesons
Determine the flavor of the initial B meson to separate B0
from B0 ( B Flavor Tagging)
Define and measure a ‘time’ in order to study the timedependent asymmetry


B Mesons must travel a measurable distance before decaying
Vertex Reconstruction: A high precision tracking system to measure
the distance between the B decay points
BaBar Detector @ PEP-II B Factory as example
7/17/2015
Vivek Sharma
13
The Asymmetric Energy Collider @ U4S) : PEP-II
ee
 U(4S )  BB at
s  10.58 GeV
BB threshold
Cleanest source of B0 mesons:
 U ( 4S)  1.05 nb
( 24 % of  had )
The U (4S)  B0 B0 system
evolves coherently
until one of the
B0 mesons decays,
so:
ACP (t)  ACP (Δt)  sin (DmBd Δt)
Dt : proper time difference
PEP-II
BABAR
between the two B decays
ACP(Dt) integrates to zero over all Dt
Study of CPV
Off
On
measure of Dt
ECM  MU (4S)
7/17/2015
Vivek Sharma
(MeV)
14
PEP-II Asymmetric Energy B-Factory at SLAC
Collides 9 GeV e- on 3.1 GeV e+
U(4S) boost in lab frame : bg = 0.56
7/17/2015
Vivek Sharma
15
PEP-II Performance Has Been Spectacular !
Records from this week!
PEP-II top luminosity
4.21 x 1033cm-2s-1
(design: 3.0 x 1033)
30/fb analyzed
for CP
Top recorded Lumi/week: 1.4 fb-1
Top recorded Lumi/24h: 282 pb-1
Top recorded Lumi/8h: 96 pb-1
BABAR logging efficiency: > 96%
October 3, 2001
October 99
PEP-II delivered:
50.6 fb-1
BABAR recorded:
48.0 fb (includes 5.15 fb-1 off peak)
90 million B’s recorded, being analysed !!
7/17/2015
Vivek Sharma
16
The BaBar Detector
Electromagnetic Calorimeter
6580 CsI(Tl) crystals
1.5 T solenoid
e+ (3.1 GeV)
Cerenkov Detector
(DIRC)
144 quartz bars
11000 PMTs
e- (9 GeV)
Drift Chamber
40 stereo layers
Instrumented Flux Return
iron / RPCs (muon / neutral hadrons)
Silicon Vertex Tracker
5 layers, double sided strips
SVT:
97% efficiency, 15 mm z hit resolution (inner layers, perp. tracks)
SVT+DCH:(pT)/pT = 0.13 %  pT + 0.45 %
DIRC:
K- separation 4.2  @ 3.0 GeV/c  2.5  @ 4.0 GeV/c
EMC:
E/E = 2.3 %E-1/4  1.9 %
7/17/2015
Vivek Sharma
17
B Event Topology at the Boosted (4S)
z
Flavor
Tagging
(bg)U(4S) = 0.56
e
Btag
U  4s 
-
Tag vertex
reconstruction
e+
μ-
Brec
K s0
Coherent BB pair
Δz
Δt 
< βγ  c
7/17/2015
μ+
Dz
π-
π
+
Exclusive B Meson and
Vertex Reconstruction
Start the Clock
Vivek Sharma
18
Sin2b Analysis Strategy
Factorize the time-dependent analysis in 3 building blocks
Obtain All analysis ingredients from DATA (not MC)
Measurements
 B±/B0 Lifetimes
 B0 B0-Mixing
 CP-Asymmetry
7/17/2015
Analysis Ingredient
(a) Reconstruction of B
mesons in flavor eigenstates
(b) B vertex reconstruction
(c) B Flavor Tagging + a + b
Reconstruction of neutral B
mesons in CP eigenstates
+a+b+c
Vivek Sharma
19
0
+
Measurement of the B and B Lifetime
Tag B
z ~ 110 mm
K+
K0
g
Reco B
z ~ 65 mm
U(4s)
bg = 0.56
3. Reconstruct Inclusively
the vertex of the “other”
B meson (BTAG)
Dz
-
-
D-
+
Dt  Dz/gbc
1. Fully reconstruct one B meson
in self tagging (BREC)
2. Reconstruct the decay vertex
4. compute the proper time difference Dt
5. Fit the Dt spectra
7/17/2015
Vivek Sharma
20
Fully-Reconstructed Hadronic B Decay sample
Flavor Eigenstates Bflav : for lifetime and mixing measurements
Self-tagging hadronic decays
bc u d
30 fb-1
“Open Charm” decays
N
(  ) 

 9400
purity 83%

1
B  D π /ρ /a
B  D(  )0π 
0
B0 / B 0
Neutral
B Mesons
Hadronic decays into final states
with Charmonium
b  (c c ) s
Charged
B Mesons
N
B / B
 8500
purity 85 %
B0  J /y K *0 ( K   )
B  J /y K  , y (2S ) K 
7/17/2015
cm
2
mES = (Ebeam
)2 - (pcm
B )
Vivek Sharma
[GeV]
21
Recoil (Tag) side Vertex and Dz Reconstruction
BREC direction

Reconstruct Brec vertex from


charged Brec daughters
Determine BTag vertex from



charged tracks not
belonging to Brec
Brec vertex and
momentum
beam spot and U(4S)
momentum
BREC Vertex
BREC daughters
Interaction Point
Beam spot
TAG Vertex
z
BTAG direction
TAG tracks, V0s

High efficiency (97%)

Average Dz resolution is 180 mm (<|Dz|> ~ bgct = 260 mm)

Dt resolution function characterized from data
7/17/2015
Vivek Sharma
22
tB Measurement at Boosted (4S): Unique
true Dt
Dt resolution
measured Dt
LEP/CDF
e-t/t

B production
point known
eg. from
beam spot
BaBar
Either Brec or
Btag can decay
first (this
analysis)
e-|Dt|/t
=

Resolution
function
=
lifetime
Resolution
Function +
Lifetime
Need to disentangle resolution function from physics
7/17/2015
Vivek Sharma
23
Dt Resolution Function


Dz
event-by-event (Dt) from vertex errors
Charm Lifetime induced bias leads to
 Small correlation between the lifetime
and the Resolution Function
parameters
R  (1  ftail  f outlier )G ( S Dt , mcore  0)
 ftail G ( S Dt , m  0)  exp(Dt /  Dtt bias )
~0.6 ps
Signal
MC (B0)
 f outlier G ( outlier , moutlier )
tracks from long-lived D’s
in tag vertex
asymmetric Resolution
Function
7/17/2015
Vivek Sharma
Dt (meas-true)/Dt
24
B Lifetime Likelihood Fit


Simultaneous unbinned maximum
likelihood fit to B0/B+ samples
Use data to extract the properties of
background events



Mass distribution provides the
signal probability
Use the events in the sideband
(mES < 5.27) to determine the
Dt structure of the background
events under the signal peak
B0 mES
B0 Bkg Dt
19 free parameters



7/17/2015
t(B+) and t(B0)
Dt signal resolution
empirical background
description
2
5
12
Vivek Sharma
25
B Lifetime Results:Calibrating The BaBar Clock
t0
20 fb-1
B0/ B0
= 1.546  0.032  0.022 ps
PDG: 1.548
t
 0.032 ps
= 1.673  0.032  0.022 ps
PDG: 1.653
 0.028 ps
t/t0 = 1.082  0.026  0.011
PDG: 1.062

B
signal
+ bkg

7/17/2015
Dt (ps)
2 % statistical error
1.5% systematic error
Main source of systematic
error

background
To Appear in PRL
Precision measurement !


 0.029

Parameterization of the Dt
resolution function
Description of events with large
measured Dt (outliers)
Vivek Sharma
26
Sin2b Analysis Strategy (Part II)
Measurements
Analysis Ingredient
 B±/B0 Lifetimes
(a) Reconstruction of B mesons
in flavor eigenstates
(b) B vertex reconstruction
 B0 B0-Mixing
(c) B Flavor Tagging (+ a + b)
 CP-Asymmetries
 Reconstruction of neutral
B mesons in CP eigenstates
(+ a + b + c)
7/17/2015
Vivek Sharma

27
B0B0 Mixing with Fully Reconstructed B Mesons
Tag B
z ~ 110 mm
K+
K0
g
Reco B
z ~ 65 mm
U(4s)
bg = 0.56
3. Reconstruct Inclusively
the vertex of the “other”
B meson (BTAG)
4. Determine the flavor of
BTAG to separate Mixed and
Unmixed events
Dz
-
-
D-
+
Dt  Dz/gbc

1. Fully reconstruct one B meson
in flavor eigenstate (BREC)

2. Reconstruct the decay vertex 
5. compute the proper time difference Dt

6. Fit the Dt spectra of mixed and unmixed events
7/17/2015
Vivek Sharma
28
Dt Spectrum of Mixed and Unmixed B Events
realistic
mis-tagging & finite time resolution
perfect
flavor tagging & time resolution
60
60
UnMixed
Mixed
50
40
50
40
30
30
20
20
10
10
0
-8
-6
-4
-2
0
2468
Decay Time Difference (reco-tag) (ps)
 | Δ t |/τ Bd
e
f Unmix (Δ t)  
Mix
 4τ Bd

-6
-4
-2
0
2468
Decay Time Difference (reco-tag) (ps)

 1+
_ 1  2w cos( Δmd Δ t )
w: the fraction of wrongly tagged
events
Dmd: oscillation frequency
7/17/2015
0
-8
UnMixed
Mixed



  ResolutionFunction


0
0
Unmixed: B0flav Btag
or B0flav Btag
Mixed:
Vivek Sharma
0
0
B0flav Btag
or B0flav Btag
29
Extraction of Dmd and mistag fraction
Fraction of Mixed Events
Sensitive to mistag fraction
measurement because the mixing has
not started yet
B0
At t=0 the observed ‘mixed’ events are
only due to wrongly tagged events
B+
Sensitive to Dmd measurement
when the amplitude of the
oscillation is at its maximum
7/17/2015
Vivek Sharma
30
B Flavor Tagging Methods
Hierarchical Tagging Categories
For electrons, muons and Kaons use the charge correlation
l
n
b
W-
D, D*
d

l B
d
0
kaons
0
c
B0
 Q<0 B
l   B0
-
b
c
 Q 0 B
s
K
B0
d
Lepton Tag
Multivariate analysis exploiting the other
kinematic information of the event, e.g.,
 Momentum spectrum of the charged particles
 Information from non-identified leptons and kaons
 Soft  from D* decay
Neural Network
*0
0
kaons
d
Kaon Tag
NN output
Each category is characterized by the probability of
giving the wrong answer (mistag fraction w)
7/17/2015
Vivek Sharma
31
B Flavor Tagging Performance Using B Mixing
The large sample of fully reconstructed hadronic B
decays provides the precise determination of the
tagging parameters required in the CP fit
Fraction of
tagged events e
(%)
Wrong tag
fraction w (%)
Q = e(1-2w)2
(%)
Lepton
10.9 0.3
8.9  1.3
7.4  0.5
Kaon
35.8 0.5
17.6  1.0
15.0  0.9
NT1
7.8 0.3
22.0  2.1
2.5  0.4
NT2
13.8 0.3
35.1  1.9
1.2  0.3
ALL
68.4 0.7
Tagging
category
Highest “efficiency”
7/17/2015
26.1  1.2
The error on sin2b the quality factor Q
1
  sin 2b  
Q
Vivek Sharma
Smallest mistag fraction
32
Dt Resolution Function
R  Dt   1  ftail  f outl   Gcore  Dt , Score , core,i 
Core
 ftail  Gtail  Dt , Stail , tail 
Outlier
Tail
 foutl  Goutl  Dt , outl  8 ps, outl  0 
core  Score  evt
Dt
tail  Stail  evt
Dt
Use the event-by-event
uncertainty on Dt
R(Dt)
B0 flavour
sample
CP sample
Dt Residual (ps)
Different bias
For each tagging
category
7/17/2015
Dt (ps)
Vivek Sharma
33
Mixing Likelihood Fit
Unbinned maximum likelihood fit to flavor-tagged neutral B sample
 | Δ t |/τ Bd
e
f Unmix (Δ t)  
Mix
 4τ Bd


 1  1  2w cos( Δmd Δt )
Fit Parameters
Dmd
Mistag fractions for B0 and B0 tags
Signal resolution function(scale factor,bias,fractions)
Empirical description of background Dt
B lifetime fixed to the PDG value
34 total free parameters
7/17/2015



 R


1
8
9
16
tB = 1.548 ps
All Dt parameters
extracted from data
Vivek Sharma
34
B0B0 Mixing Fit Result
N( unmixed )  N( mixed )
 1  2w  cos( Δmd Δt )
N( unmixed )  N( mixed )
Asymmetry
Asymmetry(Dt ) 
1
BABAR
0.8
20 fb-1
0.6
0.4
0.2
0
C.L. 28 %
-0.2
-0.4
-0.6
-0.8
-1
0
2468
10
12
|Dt| (ps)
Dmd = 0.519 ± 0.020 (stat) ± 0.016 (syst) h ps-1
7/17/2015
Vivek Sharma
Preliminary
35
Dmd Measurement in Comparison
preliminary
Precision Dmd measurement
 4% statistical error
 3% systematic error
dominated by MC
correction
7/17/2015
Vivek Sharma
36
Sin2b Analysis Using (I) and (II)
Measurements
Analysis Ingredient
 B±/B0 Lifetimes
(a)Reconstruction of B mesons
in flavor eigenstates
(b)B vertex reconstruction
 B0 B0-Mixing
(c)Flavor Tagging + a + b
 CP-Asymmetries
 Reconstruction of neutral
B mesons in CP eigenstates
+a+b+c
7/17/2015
Vivek Sharma


37
Measurement of CP Asymmetry : Sin2b
Tag B
z ~ 110 mm
-
K0
g
CP B
z ~ 65 mm
+
Ks0
m-
U(4s)
bg = 0.56
3. Reconstruct Inclusively
the vertex of the “other”
B meson (BTAG)
4. Determine the flavor of
BTAG to separate Mixed and
Unmixed events
Dz
Dt  Dz/gbc

m+
1. Fully reconstruct one B meson
in CP eigenstate (BCP)
2. Reconstruct the decay vertex 

5. compute the proper time difference Dt

6. Fit the Dt spectra of B0 and B0 tagged events
7/17/2015
Vivek Sharma
38
The fully Reconstructed CP Sample
J/y KS
Before tagging requirement
KS+ -
1999-2001 data
32 x 106 BB pairs
29 fb-1 on peak
y(2S) KS
J/y K*
cc1 KS
tagged
events
Purity
CP
[J/y, y(2S),
cc1] KS
480
96%
-1
J/y KL
273
51%
+1
J/y K*0(KS0)
50
74%
mixed
Full CP sample
803
80%
Sample
J/y KS
K S  0 0
J/y KL
After flavor tagging
7/17/2015
2
cm 2
mES = (Ecm
beam ) -(p B )
Vivek Sharma
cm
DE  EJ /y  EKL - Ebeam
39
Dt Spectrum of CP Events
perfect
flavor tagging & time resolution
B0tag  B 0
realistic
mis-tagging & finite time resolution
B0tag  B0
B0tag  B 0
B0tag  B0
CP PDF

 |Δt|/τB
Mistag fractions w

d


e
f
(Δt)



1
And
CP,

4 τBd 



Resolution function R
determined from the
flavor sample
7/17/2015










η f sin 2 β(1 2w)sin( Δmd Δt )   R


Mixing PDF







|Δt|/τBd
f mixing,(Δt)  e
4τ
Vivek Sharma
Bd















 1 (1 2w)cos( Δmd Δt)  R
40
Sin2b Likelihood Fit
Combined unbinned maximum likelihood fit to Dt spectra
of flavor and CP sample
Fit Parameters
tagged CP samples
Sin2b
1
Mistag fractions for B0 and B0 tags in each Cat. 8
tagged flavor sample
Signal resolution function
16
Empirical description of background Dt
20
B lifetime fixed to the PDG value
tB = 1.548 ps
Mixing Frequency fixed to the PDG value
Dmd = 0.472 ps-1
Global correlation coefficient for sin2b: 13%
Different Dt resolution function parameters for Run1 and Run2
 All Dt parameters extracted
45 total free parameters
7/17/2015
from data
 Correct estimate of the error and
correlations
Vivek Sharma
41
Blind analysis !
•The sin2b analysis was done blind to
eliminate possible experimenters’ bias
–The amplitude in the asymmetry ACP(Dt) was hidden by
arbitrarily flipping its sign and by adding an arbitrary
offset
–The CP asymmetry in the Dt distribution was hidden by
multiplying Dt by the sign of the tag and by adding an
arbitrary offset
–The blinded aproach allows systematic studies of
tagging, vertex resolution and their correlations to be
done while keeping the value of sin2b hidden
–The result was unblinded 1 week before public
announcement this summer!
7/17/2015
Vivek Sharma
42
Improvements Between Run1 and Run2

First publication in March 2001
sin(2b) = 0.34 ± 0.20 (stat) ± 0.05 (syst)
PRL 86 (2001) 2515

Changes since then:







More data (run 2): 23 32 BB pairs
Significantly (30%) improved reconstruction efficiency in Run 2
Optimized selection criteria takes into account CP asymmetry of background
in J/yKL
Additional decay modes cC1KS and J/yK*0
Better alignment of Tracking system ( Kalman )
Improved vertex resolution for reconstructed and tag B
Statistical Power of the analysis almost doubled w.r.t March
Publication
7/17/2015
Vivek Sharma
43
Raw CP Asymmetry in Clean Charmonium Modes
All tags
sin2b=0.56 ± 0.15
Kaon tags
In
f = -1
events
Raw ACP
7/17/2015
sin2b=0.59 ± 0.20
Raw ACP
Vivek Sharma
44
Raw CP Asymmetry for J/y KL
Background
contribution
sin2b=0.70±0.34
7/17/2015
Vivek Sharma
45
Sin2b Results : July 5th, 2001
Phys. Rev. Lett. 87 091801 (2001)
Calibration:Null result in
flavor samples
Combined fit to all modes
Sin2b = 0.59 ± 0.14
Consistency of CP
channels P(c2) = 8%
Goodness of fit (CP Sample):
P(Lmax>Lobs) > 27%
7/17/2015
Vivek Sharma
46
Run1 – Run2 Comparison
Run1
7/17/2015
Run1-Run2 change for PRL modes: 1.8
Run2
Vivek Sharma
47
sin 2b
Consistency Checks
1.5
all CP modes
B A B AR
1
0.5
0
-5
Combined CP=-1
05
Dt (ps)
sin2b measured in
several Dt bins
sin2b vs. J/y decay mode and
tagging category and flavor for
 = -1 events
7/17/2015
Vivek Sharma
48
CP Asymmetry Corrected For B Oscillation
Sin 2bvalue, fitted in bins of Dt
f   1 modes
sin 2b, fitted in bins of Dt
and multiplied by sine(Dm Dt)
ACP (t )  sin 2b sin( Δmd t )
7/17/2015
0.56  sin Δm Bd Δ t
Vivek Sharma
49
Major Sources of Systematic Error in Sin2b
Measurement is Statistics Dominated
Error/Sample
KS
KL
K*0
Total
Statistical
0.15
0.34
1.01
0.14
Systematic
0.05
0.10
0.16
0.05
Signal resolution and vertex reconstruction
0.03
 Resolution model, outliers, residual misalignment of the
Silicon Vertex Detector
 Flavor Tagging
0.03
 possible differences between BCP and Bflavor samples

Background Characterization:
0.02 (overall)
+
 Signal probability, fraction of B background in the signal
region, CP content of background
*0
 Total 0.09 for J/Y KL channel; 0.11 for J/Y K

Total Systematic Uncertainty:
0.05 for total sample

7/17/2015
Vivek Sharma
50
Search for Direct CP Violation
Without SM Prejudice :
Af (t )  Cf cos(Dmd t ) - Sf sin(Dmd t )
CP
CP
If more than one
amplitude present then ||
might be different from 1
CP
CfCP 
SfCP 
1  | λ fCP | 2
1  | λ fCP | 2
2 Im λ fCP
1  | λ fCP | 2
To probe new physics
(only use CP=-1 sample that contains no CP background)
|| = 0.93 ± 0.09 (stat) ± 0.03 (syst)
No evidence of direct CP violation due to decay amplitude
interference (SCP unchanged in Value)
7/17/2015
Vivek Sharma
51
Observation of CP Violation In B Meson System
sin 2β  0.59  0.14 (stat)  0.05 (syst)
Probability of obtaining observed result if
CP is an exact symmetry ( No CPV)
5
Prob ( sin 2b  0 ) < 3  10
Prob ( sin 2b  0 ) < 2  10 4
Full Sample
(f  1)
No evidence for direct CPV (“Sine” term unchanged in the fit)
|  | 0.93  0.09  0.03
7/17/2015
Vivek Sharma
52
The Unitarity Triangle and This Measurement
BaBar sin2b
One solution for b is
consistent with
measurements of
sides of Unitarity
Triangle
(with 30/fb)
Error on sin2b is dominated
by statistics
 will decrease as
1
Luminosity
7/17/2015
Example: Höcker et al, hep-ph/0104062
(also Vivek
other
recent global CKM matrix analyses)
Sharma
53
Summary Of Time-Dependent Measurements

BaBar has observed CP violation in the B0 system at 4.1 level
sin2b = 0.59 ± 0.14 ± 0.05



Probability to observe an equal or larger value if
no CP violation exists is < 3 x 10-5
Corresponding probability for the CP = -1 modes only is < 2 x 10-4
New precision measurements of B0/B+ lifetimes and
B0B0 Oscillation frequency Dmd
t0 = 1.546  0.032  0.022 ps
t = 1.673  0.032  0.022 ps
t0 /t = 1.082  0.026  0.011
Dmd = 0.519 ± 0.020 ± 0.016 h ps-1
7/17/2015
Vivek Sharma
54
Luminosity Projection to Summer 2002
Project 100 fb-1 by
Jun 2002
We are Here
7/17/2015
Vivek Sharma
Hope to analyze
Data very Quickly
As demonstrated
Already
55
Luminosity Plans for BABAR & PEP II
Expect 550 fb-1 By 2006
600
Integrated Lumi [fb-1]
14
400
12
10
300
8
200
6
4
100
Peak Lumi [10**33]
16
500
0
7/17/2015
18
Yearly Lumi
Cumulative Lumi
Peak Lumi
2
0
1999 2000 2001 2002 2003 2004 2005 2006 Year
Yearly Lumi
2
23
40
45
Cumulative Lumi
2
25
65
110 172 272 372 542
Peak Lumi
1
2
4
5
62
6
100 100 170
8.5
Vivek Sharma
11
16
56
Prognostications on Future Sin2b Precision

In the Charmonium Modes

Add more sub-modes “drops in the bucket” :




Select Y hadrons, not just Y e+ e- or m m ,
smarter event selection (bremstrahlung recovery)
Expect for charmonium modes:
sin 2b  0.08 for 100 fb 1
Add new CP modes :


b sss  B  fKS
 Compare with sin2bfrom b c c s
 sin2 b  0.23
Cabibbo Suppressed B  Y 0
 Look for difference in sin2bmeasured from b ccs


for 500 fb1
bound u-quark penguin pollution
Cabibbo suppressed b ccd  B  D(*+) D(*-)
 May contain (small but unknown) penguin pollution

7/17/2015
 sin 2 b  0.25 for 500 fb1
D*D* mode requires angular analysis (in progress)
Vivek Sharma
57
New Modes for “Sin2b”: 20 fb-1
b  sss
B0  f K S0
b  ccd
Go Back
B0  D D
~ 32 events
~ 11 events
B  Y 0
7/17/2015
Vivek Sharma
58
CP violation in B0   decays
Decay distributions f+(f-) when tag = B0(B0)
e(  Dt /t )
f ( Dt ) 
[1  S f sin( Dm d Dt )  C f cos( Dm d Dt )]
4t
tree diagram
u
b
1||2
Cf 
1||2
penguin diagram
u
d
b
d
2 Im(  )
Sf 
1||2
u
u
For single weak phase
For additional weak phase
q Af

  f e  2 i ( b  g )   f e 2 i
p Af
|  |  1  must fit for direct CP
Im ()  sin2  need to relate
asymmetry to 
C 0, S= sin2eff
C0, S= sin2
7/17/2015
Vivek Sharma
59
L= 30.4 fb-1
15
0
K 
KK
217  18
4.3  64..33
2
B A B AR
+ -
Kp
139 K
3 
20
0
pp
23 
2 K
5
-0.1
00
DE
60
B A B AR
+ -
40
Kp
.1
(GeV)
126 K
3 
20
0
5.25
2
.3
(GeV/c )
mES
7/17/2015
10
.3
mES
60
40
B A B AR
+ -
(GeV/c )
Events/20 MeV

2

65 
12
11
15
0
5.25
Events/2 MeV/c
 

pp
20 
1 K
5
Total Yields (fit):

B A B AR
+ -
10
Events/20 MeV
Events/2 MeV/c
For Illustration
purposes:
Events after likelihood
ratio cuts
2
CP Sample B0  
Vivek Sharma
-0.1
00
DE
.1
(GeV)
Lepton Photon 2001
60
CP Asymmetry Fit and Results
S (   )  0.030.53
0.56 ( stat )  0.11( syst )
C (   )  0.250.45
0.47 ( stat )  0.14( syst )
ACP ( K  )  0.07  0.08( stat )  0.02( syst )
Preliminary Results (PRD Bound)
Observation of CP Asymmetry
(time Dependent or in Decay)
Will be a Major Achievement !
7/17/2015
Vivek Sharma
61
BaBar Aim : Multiple Measurements and Tests to
Overconstrain the Unitarity Triangle
Sin2b is just one focus of BaBar: Work in progress on Many Fronts
An Exciting era of B physics in Progress !
7/17/2015
Vivek Sharma
62
Summary Of Time-Dependent Measurements

BaBar has observed CP violation in the B0 system at 4.1 level
sin2b = 0.59 ± 0.14 ± 0.05




Probability to observe an equal or larger value if
no CP violation exists is < 3 x 10-5
Corresponding probability for the CP = -1 modes only is < 2 x 10-4
With anticipated 100 fb-1 by next summer, we expect the precision
in sin2bto be ~ 0.08
Searches for CP violation in B0   decays started
S (  )  0.03


0.53
0.56
( stat )  0.11( syst )
C (   )  0.250.45
0.47 ( stat )  0.14( syst )
ACP ( K  )  0.07  0.08( stat )  0.02( syst )
7/17/2015
Vivek Sharma
63
Backup Slides
7/17/2015
Vivek Sharma
64
Consistency Check: Run1 vs. Run2
Run 1
Difference for modes used in the old PRL: 1.8 
Run 2
PRL modes
0.32±0.18
PRL modes
0.83±0.23
J/YKL
0.71±0.42
J/YKL
0.68±0.58
J/YK*0
1.26±1.22
J/YK*0
0.15±1.62
ccKs
1.14±1.25
+0.4
ccKs
3.20 -0.7
Y(2S)Ks
0.40±0.49
Y(2S)Ks
1.85±1.21
J/YKs2p0
0.13±0.65
J/YKs2p0
1.62±0.74
JYKs
0.23±0.24
JYKs
0.72±0.27
Average
0.45±0.18
Average
0.82±0.22
-1 -0.5 00 .5 11 .522
7/17/2015
sin2b
.533
.5
-1 -0.5 00 .5 11 .522
Vivek Sharma
sin2b
.533
.5
65
c c K+
Y2SK+
J/YK+
J/YK*0(K+-)
J/YKs(00)
ccKs(+-)
Y2SKs(+-)
J/YKs(+-)
Improved Particle Reconstruction
KS Golden modes ~30% larger
than run 1: efficiency improved
7/17/2015
Vivek Sharma
66
Additional Channels: J/YK*0(KS0)

Improved understanding of the background and its effective
CP
(Angular analysis
paper about to be
submitted)
55 signal events
before tagging; 37
after
7/17/2015
Vivek Sharma
67
Improved KL Selection

Original analysis was
optimized for S2/(S+B)


Fine for BF measurements, but
not for CP
Need to optimize accounting
for the background asymmetry
(S+AB/ASB)2/(S+B)


7/17/2015
Re-optimized with Monte Carlo
Expect 10% improvement on the
error
Vivek Sharma
68
Resulting KL Yields: Run1

For data the improvement is betterOld
than expected New
In the IFR selection the signal
yield has not changed while the
background is halved
Run1
7/17/2015
Vivek Sharma
69
CP Sample: Non-KL Modes
Present Sample: 725
PRL Sample: 425
Before tagging and
vertexing requirements
NNOW=672
7/17/2015
Vivek Sharma
70
CP Sample: J/Y KL
Run1+
Run2
7/17/2015
N/%
EMC
IFR
Run1
77/52
96/68
Run2
49/59
32/55
Run1+Run2
128/56
129/65
Vivek Sharma
71
Improved Vertex Performance

We expect some vertex improvements in Run2 from:


Better use of layer 1 SVT hits in the Kalman fit
Better SVT internal alignment
Improvement in resolution leads to
… 3-4% on sin2b error
run2
run1
NEW
OLD
7/17/2015
Vivek Sharma
(sin2b)
72
Tagging Performance from Data

Obtained from “mixing fit” to data samples
NT2
L
K
K
L
NT1
NT1 NT2
Q = eD2=
26.1 1.2 %
7/17/2015
Vivek Sharma
73
Likelihood Fit Method

Global unbinned maximum likelihood fit to data:
•
•

Mistag rates, Dt resolutions = tagged flavour sample
sin2b = tagged CP samples
45 parameters for mistag rates, Dt resolution & backgrounds: floated to
obtain an empirical description from data
parameter
#params
sin2b
1
Determining
subsample
CP
w & Dw
42=8
flavour
Dt resolution
8  2 = 16
flavour and CP
Background t
4+2+3=9
sidebands
Background w 4  2 = 8
sidebands
Background Dt 3
sidebands
Separate Dt resolutions
for run1 and run2
Largest correlation
with sin2 b : 13%
tB = 1.548 ps and Dmd = 0.472 ps-1 fixed
7/17/2015
Vivek Sharma
74
The New World Average
New sin2b world
average is 8
significant
Measurements assumed
to be uncorrelated
7/17/2015
Vivek Sharma
75
Run1 – Run2 Comparison



Change in central value ~1.8 in uncorrelated error
30% efficiency improvement for all KS modes
15% improvement due to vertexing/alignment
7/17/2015
Vivek Sharma
76
Silicon Vertex Detector (SVT)
Dipole
Readout
permanent magnet chips
(21 cm from I.P.)
Beam pipe (Beryllium)
Layer 1,2 1% R.L.
Layer 3
Layer 4
Layer 5
7/17/2015
Vivek Sharma
77
200 mm
SVT: precise B vertex, Dz measurement
• 5 Layer AC-coupled
double sided silicon detector
• SVT Located in high radiation area
• Radiation hard readout electronics (2Mrad)
• Up to 98% hit reconstruction efficiency
• Hit resolution ~15 μm at 00

7/17/2015
Vivek Sharma
78
Drift Chamber (DCH)



• Track reconstruction efficiency 98% for p>200 MeV/c,
>500 mrad, and nominal DCH voltage.
• PID up to p=0.7 GeV/c: (dE/dx) =7.5 % (Bhabha)
• SVT + DCH: impact parameter resolution
65 µm in z
55 µm in transverse plane
at p=1.0 GeV/c
7/17/2015
Vivek Sharma
40 axial and stereo layers inside
1.5 Tesla magnetic field
80:20 helium:isobutane
Measurement of charged particle
momentum and ionization
loss dE/dx for PID
Reconstruction of the decay J/Ym+m- in
selected BB events.
Mass resolution: 11.4 MeV/c2.
79
PID performance, 0 reconstruction
EMC: e±,g,0 ID
Muons
Electrons
Egg>300MeV
Mgg(GeV/c2)
 (E)
E
7/17/2015
Vivek Sharma
2.3%
 4
 1.9%
E
80
Ring imaging Cherenkov detector (DIRC)
DETECTION OF INTERNALLY
REFLECTED CHERENKOV LIGHT
– 144 synthetic fused silica radiator bars
– Photons transmitted by internal reflection
– Rings expand in standoff region
(1.2m distance, 6000 l purified water)
– Detected by ~11000 conventional PMTs
– Essential for PID 0.7-4.3 GeV/c
e+
Typical performance:
• number of detected photons:
Barbox
20-50
Compensating coil
• average track c resolution:
2.4mrad
(in e+e– m+m– events, 3–9 GeV/c)
7/17/2015
Support tube
(Al)
e-
Assembly flange
Standoff
box
Vivek Sharma
81
Tight Kaon ID
7/17/2015
Vivek Sharma
82
B0   Asymmetry Result
Decay distributions f+(f-) when tag = B0(B0)
Dt /t
e
f  (Dt) 
[1 S sin(Dmd Dt) C cos(Dmd Dt)]
4t


0.53
0.56
( stat )  0.11 ( syst )


0.45
0.47
( stat )  0.14 ( syst )
S (  )  0.03
C (  )  0.25
Preliminary

ACP ( K  )  0.07  0.08 ( stat )  0.02 ( syst )
A = N(K-+)-N(K+-)/N(K-+)+N(K+-)



Measurement compatible with no CP in B0  
Statistically limited due to small branching fraction
Need ~500/fb for (S) ~ 0.10-0.15
7/17/2015
Vivek Sharma
83
B Lifetime : Systematic uncertainties
t0 (ps)
t (ps)
(t/t0)
MC statistics
0.009
0.006
0.006
stat. limitations of MC validation studies
Resolution parameterization
0.011
0.009
0.003
some included in stat. error (free parameters), study different
parameterizations,
SVT alignment algorithm
Common resolution parameters
0.004
0.005
0.006
different resol. parameters for charged and neutral B
(different D0/D+ mix)
Beam spot
0.002
0.002
cancels
Dt outliers
0.011
0.011
0.005
Absolute z scale
0.008
0.008
cancels
absolute z scale estimated to better than 0.5% using
secondary interactions in beam pipe
Boost
0.006
0.006
cancels
propagate errors on PEP-II boost
Signal probability
0.003
0.003
0.003
propagate errors from mES lineshape fit
Background modeling
0.005
0.009
0.003
compare Dt distribution of background events in signal
region and in sideband;
wrong-charge contamination of signal
Total in quadrature
0.022
0.022
0.011
Systematic effect
7/17/2015
Vivek Sharma
Comment
propagate errors on BS position and size
vary mean and width of outlier PDF
84
Systematic Error : Absolute Z Length Scale
Estimation of the absolute z scale from measurement
of length of Be beam pipe ( + Tantalum foil wrapped
around it ) using off-beam electroproduction reactions
therein.
<r> (mm)
Use the beam pipe as a “ruler”. The beam pipe radius
increases at two points in z close to its extremities.
The distance between these
two points is known from
an independent measurement.
z (mm)
7/17/2015
Vivek Sharma
85
B Lifetime: Dt Resolution Parameters
Parameterization of resolution function:
f * G(s) + (1- f) * G(s)  E(k)
(3 parameters)
Monte Carlo
7/17/2015
Data
Parameter
B0/B0
B
B0/B0 and B
f
0.709  0.008
0.685  0.007
0.69  0.07
k
1.027  0.021
0.881  0.015
1.04  0.24
s
1.063  0.005
1.057  0.003
1.21  0.07
Vivek Sharma
86
PEP-II asymmetric collider
• Asymmetric collider
operating at the (4S)
resonance
Goal
Achieved
nb-1/s
=1033 cm-2 s-1
3.0
3.28
pb1/day
135
184
fb1/week
0.8
1.03
fb1/month
3.3
3.8
7/17/2015
E(e-) = 9.0 GeV, E(e+) = 3.1 GeV
bg0.56
Vivek Sharma
87
Is it possible to measure a large asymmetry ?

The answer is… yes!
Suppose at a given time t’ you have
Asymmetry (t ) 

N B0  N B 0
N B0  N B 0
5  (2)

 3.5
5  (2)
Nevents < 0 is possible in a likelihood fit



The signal PDF can be negative in some regions
Requires having NO OBSERVED event in those regions
The only constraint on the PDF is the normalization
PDF

1

7/17/2015
Vivek Sharma
88
Large sin2bin B0  cC1KS

Lepton tags

B0 tags
fit for B0/B0 Dt PDFs, not for ACP
Large sin2b possible , because

No events where PDF<0
(eg. lepton tags)


B0 tags

B0 tags
Sum of signal + background PDFs
positive (eg. Kaon tags)
Note: a single lepton B0-tag at
Dt = -/2Dm would bring sin2b
from 2.6 to ~1/(1-2wlep)  1.1
B0 tags
Measure sin2bunbiased for low stat.
samples and probability to obtain
sin2b2.6 when true value 0.7
is 1~2%
Dt [ps]
7/17/2015
Kaon tags
Dt [ps]
Vivek Sharma
89
15
lepton
5
5.225
5.25
5.275
40
5.3
5.2
20
10
5.25
5.275
60
5.3
GeV/c2
mES (Kaon)
BABAR
40
20
NT2
NT1
5.225
5.225
GeV/c
BABAR
5.2
kaon
2
30
0
5.25
5.275
mES (NT1)
7/17/2015
60
0
5.2
BABAR
80
20
mES (Lepton)
Candidates per 2.0 MeV/c2
mES distributions
for the different
tagging categories
BABAR
10
0
Sum of +-/K+-:
No particle ID used
until the fit is performed
Candidates per 2.0 MeV/c2
20
Candidates per 2.0 MeV/c2
9741 two-prong
candidates in 30.4 fb-1
(97% background,
almost entirely from
continuum)
Candidates per 2.0 MeV/c2
CP Data Sample: Likelihood Fit
Vivek Sharma
5.3
GeV/c2
0
5.2
5.225
5.25
mES (NT2)
5.275
5.3
GeV/c2
90
CP Asymmetry Fit and Results

Extended ML fit to the BRs and CP done simultaneously:






5 tagging categories (leptons, K, NT1, NT2, untagged)
8 event species (Sig and Bkg: +- , K+ , K-+ , K+K-)
Discriminating variables (mES, DE , F, c1 , c2 , Dt)
Dilutions, R(Dt) for the signal taken from sin2b analysis
Dmd, B0 lifetime fixed as in sin2b analysis
R(Dt) for the background taken from sidebands in mES
distribution
Preliminary Results
53
S(    )  0.03 00..56
(stat)  0.11(syst )
45
C(    )  0.25 00..47
(stat)  0.14(syst )
A CP (K    )  0.07  0.08(stat)  0.02(syst )
7/17/2015
Vivek Sharma
91
CP Violation in the Standard Model


The weak interaction between quarks regulated by the
Cabibbo-Kobayashi-Maskawa matrix
With 3 generations of quarks, the SM can accommodate CP violation through
complex coupling constants: 3 angles and a complex phase
Unitarity of the
CKM Matrix
Unitarity Triangle
V V V V V V  0
*
ud ub
7/17/2015
*
cd cb
*
td tb
Vud V*ub

g
Vivek Sharma
Vtd V*tb
b
Vcd V*cb
92
Direct and Indirect CP Violation Mechanisms

Direct CP Violation: Interference of
two decay amplitudes

Can occur in both neutral and
charged B decays

Total amplitude for a decay and
its CP conjugate have different
magnitudes

GB  f   G B  f


7/17/2015


Indirect CP Violation

Only in neutral B decays

Charge asymmetry in
semileptonic B decays

Expected to be small in Standard
Model
0
Acp 
Large hadronic uncertainties
=> difficult measure CKM matrix
elements
Relatively small asymmetries
expected in B decays s
K
u
W
u
b
0
B

d
d
Decay Diagram
Vivek Sharma
G( B 0  Xl n )  G( B  Xl n )
0
G( B  Xl n )  G( B  Xl n )

0
u,c,t
b
B0
d

W
~ 0.01
d
W
u,c,t
Mixing Diagram
B0
b
93
Time evolution of B0 mesons into a final CP eigenstate
The decay distribution for events with a B0 (f+) and B0bar tags (f-)
f (B  fCP , t )  G4 eG |Dt| [1  CfCP cos(Dmd t )
0
f  : Bphys
 f CP
f  : Bphys  f CP
0
λ fCP
CfCP 
q A fCP
 
p A fCP
1  | λ fCP | 2
1  | λ fCP |
2
SfCP 
SfCP sin(Dmd t )]
2 Im λ fCP
1  | λ fCP | 2
Amplitude ratio
Weak Phase
In order to have CP Violation
0
 f CP  1  Pr ob( Bphys
( t )  f CP )  Pr ob(B0phys ( t )  f CP )
7/17/2015
Vivek Sharma
94
Time-dependent CP Asymmetry
From the time evolution of the B0 and B0 states we can define the
time-dependent asymmetry :
G( B0phys (t )  f CP ) G( B0phys (t )  f CP )
Af (t ) 
CP
G( B0phys (t )  f CP )  G( B0phys (t )  f CP )
 Cf cos(Dmd t ) - Sf sin(Dmd t )
CP
CfCP 
CP
1  | λ fCP | 2
1  | λ fCP |
SfCP 
2
1  | λ fCP | 2
Sensitive to the phase
of  even without direct
CP Violation
Probe of direct CP violation
since it requires
λ fCP  1
7/17/2015
2 Im λ fCP
Vivek Sharma
95
CP Violation in B Decays
To generate a CP violating observable, we must have:
–Interference between at least two different amplitudes
–All 3 quark generations involved
d 


u
W
u
b

d
d
Decay Diagram
In B decays, can consider two different types of amplitudes:
–Those responsible for decay
B0
–Those responsible for mixing
This gives rise to three possible manifestations of CP violation:
–Direct CP violation
u,c,t
b
•(interference between two decay amplitudes)
–Indirect CP violation
•(interference between two mixing amplitudes)
B0
d
W
d
W
u,c,t
Mixing Diagram
B0
b
– CP violation in the interference between mixed and unmixed decays
7/17/2015
Vivek Sharma
96