Post on 21-Dec-2015
1
Daniela Bortoletto
Purdue University Introduction
SM expectations Previous measurements
The measurement of sin 2 at CDF Signal reconstruction Flavor tagging methods Fit results and cross checks
Future prospects
The measurement of sin(2)
University of Southampton 25-29 July 199
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SM with 3 generations and the CKM ansatz can accomodate CP
if the complex phase is 0CP. Only =0.21960.023, A=0.8190.035 are measured precisely.
CP is one of the less well-tested parts of SM ( , / in the Kaon system)
CP asymmetries in the B system are expected to be large. Independent observations of CP in the B system can: test the SM
Introduction
)(O
1A)i1(A
A2/1
)i(A2/1
VVV
VVV
VVV
V 4
23
22
32
tbtstd
cbcscd
ubusud
BB00
H0
2
B
BLHB
00L
0
MBqBpBdbB
)10(OMMMBqBpBdbB
lead to the discovery of new physics
3
The goal of B-physics is to over-constrain the unitarity triangle to test the CKM ansatz or to expose new physics
B Physics and CKM matrix
Unitarity triangle
B
BJ/K0sBK
(,)
(1,0)
(1--i)(+i)
*cbcd
*tbtd
VV
VV*
cbcd
*ubud
VV
VV
(0,0)
B/ B0-B0 mixing
Vud Vub*+Vcd Vcb
*+Vtd+Vtb*=0
4
Possible manifestations of CP violation can be classified as: CP violation in the decay: It occurs
in B0/B+decays if |A(f)/|A(f)|1 CP violation in mixing: It occurs
when the neutral mass eigenstates are not CP eigenstates (|q/p|1)
CP violation in the interference between decays with and without mixing
Mixing: Vtd introduces a complex phase in the box diagram
Interfering amplitudes: direct decay B0 f B0 B0 mixing followed by B0
f
CP violation in B decays
B0
B0
f B0
B0
f
Mie
Mie
Vtd
V*td
b
b
d
dt
tB0 B0W W
Box Diagram
2i2*
td2tdd
2i2td
2tdd
e|A|Vm)BB(A
e|A|Vm)BB(A
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Determination of sin(2)
Color suppressed modes bccs. Dominant penguin contribution has the same weak phase Negligible theoretical uncertainty
Cabibbo suppressed modes bccd such as B0/B0 DD,D*D*. Large theoretical uncertainties due to the penguin contribution
Penguin only or penguin dominated modes bsss or dds. Tree contributions absent or Cabbibbo and color suppressed penguin diagrams dominate even larger theoretical uncertainties
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B-factories at the (4S) : B0 and B0 mesons are produced in a coherent C=-1 state time integrated CP
asymmetry = 0. Determination of CP needs A(t ) where t =t(CP)-t(tag) or z =c t Need good z resolution
pp and pp colliders: time integrated asymmetry does not vanish
Since xd=0.732 0.0032 (PDG98)
Experimental considerations
2sin5.02sinx1
xA
2d
dCP
ACP is Maximum at t=2.2 lifetimes
ACP(t)
t
Measurement of the asymmetry as a function of proper time ACP(t) is more powerful Combinatoric background
dominates small ct region
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B0/B0J/K0s
For B0/B0 J/K0S we have CP(K0
s)=1 and CP(J/K0S )= -1. To reach a
common final state the K0 must mix additional phase Asymmetry is directly related to sin2.
ACP(t)=sin[2(M- D)]sin mdt =sin2 sin mdt and
sin2 =
0
S00
S0
0S
00S
0
CPK/JBK/JB
K/JBK/JB)t(A
*cscd
cs*cd
*cbcs
cb*cs
*tdtb
td*tb
VV
VV
VV
VV
VV
VVIm
B0 B0 Mixing Ratio of K0-K0 mixing
)f(A
)f(A
22 )1(
)1(2
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Vub/Vcb=0.093 from semileptonic decays
K=2.2810-3
B0-B0 mixing md=0.472 ps-1
Limit on Bs-Bs mixing ms >12.4 ps-1
Indirect determination of sin2 In SM the asymmetries in the B system are expected to be large
S. Mele CERN-EP-98-133, 1998 findssin2=0.75 0.09
Parodi et al. sin2=0.725 0.06
Ali et al. 0.52<sin2<0.94
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Measurement of ACP(t) requires:
Reconstruct the signal B0/B0J/K0S
Measure proper decay time (not critical in pp colliders but useful) Flavor tagging to determine if we have a B0(bd) or B0(bd) at
production Tagging algorithms are characterized by an efficiency and a dilution
D. The measured asymmetry is AobsCP=D ACP
Ntot = total number of events
NW= number of wrong tags
NR=number of right tags
D=2P-1 (P=prob. of correct tag) and D=1 if NW=0 D=0 if NW=NR
Best tagging methods has highest D2
Measurement accuracy
WR
WR
NN
NND
tot
tag
N
N
Crucial factor
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Assume you have 200 events N=200 100 are tagged Ntag=100
tagging efficiency =Ntag/Ntot=50%
Of those 100 events 60 are right sign NR=60
40 are wrong sign NW=40
Dilution D=(NR-NW)/(NR+NW)=(60-40)/100=20%
Effective tagging efficiency D2=( 0.5)(0.2)2=2%
Statistical power of this sample ND2=200*0.02=4 events
Tagging
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Previous Measurements
sin2=3.2 0.5
Opal Zbb D. Ackerstaff et al. Euro. Phys. Jour. C5, 379 (1998) (Jan-1998)
Flavor tagging techniques:
Jet charge on opposite side jet
Jet charge on same side B
Vertex charge of a significantly separated vertex in the opposite hemisphere
24 J/K0S candidates
Purity 60 %
1.82.0
00
00
BB
BBA
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Previous Measurements
sin2=1.8 1.1 0.3
CDF ppbb Abe et al. PRL. 81, 5513 (1998) (June 1998)
198 17 B0/B0 J/K0S candidates with both
muons in the SVX ( S/B 1.2). Measure asymmetry with Same side tagging
Dsin2=0.31 1.1 0.3.
Using D=0.166 0.018 (data) 0.013 (MC) from mixing measurement + MC
00
00
BB
BBA
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Run I CDF detector
Crucial components for B physics: Silicon vertex detector
proper time measurements impact parameter
resolution:
d=(13+40/pT) m
typical 2D vertex error (r-) 60 m
Central tracking chamber mass resolution. B=1.4T, R=1.4m (pT/pT)2=(0.0066)2(0.0009pT)2
typical J/K0S mass resolution
10 MeV/c2
Lepton detection (triggering and tagging)
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CDF updated measurement
Add candidate events not fully reconstructed in the SVX Double the signal to 400 events but
additional signal has larger (ct)
Use more flavor tag methods to establish b flavor at production
Check D2 with mixing analysis
Use a maximum likelihood method to combine the tags. Weight the events: in mass (B peak versus sidebands) in lifetime (more analyzing power at
longer lifetimes) in tagging probability Account for detector biases
B
background
c
(B0)=1.5610-12 s
)mtcos(D)t(N)t(N
)t(N)t(N)t(A
mixedunmixed
mixedunmixed
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Signal J/ -+ require two central tracks with
matching hits in the muon chambers K0
S -+ use long lifetime c(K0S)=2.7 cm
to reject background by requiring Lxy/(Lxy)>5
Perform 4-track fit assuming B J/ K0S
Constrain -+ and -+ to m(K0S) and
m(J/) world average respectively K0
S points to B vertex and B points to primary vertex
Background cc production prompt J/ ( not from b
decays) + random K0S or fake
bb production J/+X, random K0S or
fake
J/K0S Event selection
B decay
+
-
+
-
primary
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J/K0S Signal sample
CDF run1, L=110 pb-1 202 events with both muons in SVX
(ct) 60 m. 193 with one or both muons NOT in
SVX (ct) 300-900 m
Plot normalized mass
M-MB/ error on M
Both in SVX
One or Both not in SVX
395 31 events
S/B=0.7
S/B=0.9
S/B=0.5
202 18 events
19326
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We must determine if we had a B0 or a B0 at the time of production. Opposite-side flavor tagging (OST) bb produced by QCD Identify the
flavor of the other b in the event to infer the flavor of the B0 /B0 J/K0S.
At CDF 60% loss in efficiency due the acceptance of the other B0. Lepton tagging :
b +X b b -X b
Jet charge tag :
Q(b-jet) > 0.2 b Q(b-jet) <- 0.2 b
Flavor tagging methods
B0(bd) J/K0S
+-
+
-
Opposite side b
+
Q(b-jet)>0.2
K0S
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Identify the flavor of the B0/B0J/K0S through
the charge of the opposite b-jet Jet definition allows for wide low PT jets:
Cluster tracks by invariant mass (Invariant mass cutoff 5 GeV/c2 )
remove track close to primary B Weight tracks by momentum and impact
parameter
pT= track momentum TP = probability track comes from primary
vertex (low Tp more likely track comes from B )
Jet Charge Flavor tagging
i iPTi
i iPTii
jet ))T(2(p
))T(2(pqQ
Qjet>0.2 b
Qjet<-0.2 b
|Qjet|<0.2 no tag
=(40.2 3.9)%
Qjet in BJ/K
-QK*QJet
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Soft Lepton Flavor tagging
Identify the flavor of the B0/B0J/K0S
through the semileptonic decay of the opposite B. b - X b + X
Electron: central track (PT>1 GeV/c) matched to EM cluster
Muon: central track (PT>2 GeV/c) matched to muon stub
Efficiency 6% Source of mistags:
Sequential decay b c X Mixing Fake leptons
Opposite side tagging was used at CDF to study B0 B0 mixing Ph. D. Thesis O. Long and M. Peters
md=0.50 0.05(stat)+0.05(sys) ps-1
md=0.464 0.018 ps-1 (PDG)
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Same side tagging
du
b
B0-
su
b
BS K-
ds
b
B0K0
us
b
B-K+
ud
b
B-+
No K/ separation higher correlation for charged B
Problems with opposite side tagging Opposite b-hadron is central only 40 % of the time If opposite b-hadron is B0
d or B0s mixing degrades tagging
Same side flavor tagging (SST). Exploits the correlation between the charge of nearby and the b quark charge due to fragmentation or B** production (Gronau,Nippe,Rosner)
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Correlation due to excited B** production
B**+ (I=1/2) resonance B**- B0 - Implementation of SST: Search for
track with minimum Ptrel in b-jet cone
SST has higher efficiency ( 70 %) than OST
Same side tagging
Candidate track Pt>400 MeV/c d/<3 wrt primary vertex
PB
B0 J/K0S
+-
+
-
Same side pion negative charge
d
b b
d
u
B0
-
B**-
Ptrrel
PB+ Ptr
Ptr
Cone R=0.7
B direction
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Tagger calibration
Use BJ/ K sample to determine the efficiency and the dilution D of the sample:
Charge of the K b or b
Decay mode and trigger
analogous to B J/ K0S
B+/B- does not mix
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Calibration Jet Charge Tagging
Sample of 988 J/K events 273 right-sign events 175 wrong-sign events
Tagging efficiency: =Ntag/Ntot=(44.9 2.2)%
Tagging dilution:
D=NR-NW/NR+NW= (21.5 6.6)%
Mistag fraction: w=(39.23.3)%
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Calibration of Soft Lepton Tagging
Sample of 988 J/K events 54 right-sign events 12 wrong-sign events
Tagging efficiency: =Ntag/Ntot=(6.5 1.0)%
Tagging dilution:
D=NR-NW/NR+NW=(62.5 14.6)%
Mistag fraction: w=(18.87.3)%
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Same Side Tagging Calibration
D+=0.270.03(stat)+0.02(syst)
D0=0.180.03(stat)+0.02(syst)
D=0.1660.022 both muons in SVX
D=0.1740.036 one/both muons NOT in SVX
Use inclusive + D* sample. This sample was used for the determination of B0/B0 mixing in F. Abe at al Phys. Rev. Lett. 80, 2057(1998) and Phys. Rev. D 59 (1999)
Use MC to scale for different PT spectrum in J/ K0
S wrt + D/D* sample
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Combining Dilution: Define D=qD where q=-1 (b-quark), q=+1 (b-quark) and q=0 (no
tagging) then Deff=(D1+D2)/(1+D1D2) Tags agree Deff=(D1+D2)/(1+D1D2) Example SST and JCT D=36.8%
Tags disagree Deff=(D1-D2)/(1-D1D2) Example SST and JCT D=5.1%
Each event is weighted by the dilution in the fit
Same side SVX =(35.53.7)% D= (16.6 2.2 )% Same side non-SVX =(38.13.9)% D= (17.4 3.6 )% Soft lepton all = (5.61.8)% D= (62.5 14.6)% D2= (2.2 1.0)% Jet charge all = (40.2 3.9)% D= (23.5 6.9 )% D2= (2.2 1.3)% (if SLT do
not use Jet charge)
D2= (6.3 1.7)%
Flavor Tagging Summary
Combined flavor tagging power including correlations and multiple tags: A sample of 400 events has the statistical power of 25 perfectly tagged events
D2= (2.1 0.5)%
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Results
Muons from J/ decay in Silicon vertex detector High resolution ct Asymmetry vs ct
Data with low resolution ct measurement Time integrated ACP
ACP=0.47 sin2 If md is fixed to the PDG world
average (md=0.4640.018 ps-1), the minimization of the likelihood function yields:
sin2=0.790.39(stat)0.16(syst) Statistical error >systematics.
Float
md
sin2=0.79+0.41
-0.44(stat.+sys.)
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Systematic errors : Dilution 0.16 (limited by the statistics
of the calibration sample) Other sources 0.02
Cross checks: Float md :
Measure time integrated asymmetry: sin2=0.71 0.63
Only SVX events and SST: sin2=1.771.02
Verify errors and pulls with toy MC
Systematic errors and cross checks
1 contours
44.041.088.02sin
Mean:0.44
=1.01
error
Pull
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As a check we can apply the multiple flavor tagging algorithm to the measurement of mixing in B0J/K0* decays.
The data is consistent with the expected oscillations
Measurements:md=(0.400.18) ps-1
DK=0.96 0.38 dilution due to incorrect K- assignments
Expectation:md=(0.4640.018) ps-1
DK=0.8 0.3
Cross checks
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Measurement
Feldman-Cousin frequentist (PRD 57, 3873, 1998)
0<sin2<1 at 93 % CL Bayesian (assuming flat prior
probability in sin2)
0<sin2 <1 at 95 % CL Assume true value sin2=0.
Probability of observing sin 2 >0.79 =3.6 %.
Confidence Limits on sin(2)
Scan of the likelihood function
sin2
sin2=0.79+0.41
-0.44(stat.+sys.)
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Results in and plane
CDF sin2 measurements fourfold ambiguity {, /2- , +, 3/2-} Solid lines are the 1 bounds, dashed lines two solutions for for
<1, >0 (shown) two solutions for >1, <0 (not-shown)
1 bounds
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B-factories at(4S)
pp colliders:
BABAR estimates J/K0
S
(bb ) 50 b but (bb)/(total) 0.001
Tagging factor 0.063 (Run1) 0.097 (Run II-with Kaon tagging)
N(B0 /B0 J/K0S) =400 /100 pb-1
(Run 1) 15000 /2 fb-1 (run II +e triggers)
S/B =0.9 in B0/B0J/K0S
( sin2)=0.4 0.08 in Run II
(bb ) 1.05 nb but (bb)/(total) 0.26
Tagging factor 0.25-0.3(MC) N(B0/ B0 J/K0
S) =660 / 30fb-1
S/B=16 in B0/B0J/K0S
(sin2)=0.12
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CDF reach in run II for sin2
Run I value with Run II projected error
sin2=0.79 0.084
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Summary
CDF measures:
Mixing mediated CP will be measured precisely by CDF/D0 /BaBar/Belle/HeraB by the beginning of the new century
Precise determination of sin2 is a key step towards understanding quark mixing and CP
sin2=0.79+0.41
-0.44