measurements of sin2 from b-factories masahiro morii harvard university the babar collaboration...
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Measurements of sin2from B-Factories
Masahiro MoriiHarvard University
The BABAR Collaboration
BEACH 2002, Vancouver, June 25-29, 2002
BEACH 2002, May 25-29, 2002 M. Morii, Harvard University 2
Introduction CP violation in B0 decays gives access to the angles
of the Unitarity Triangle
sin2 measured to ±0.08 dominated by B0 J/KS
Where does this leave us?
*
2 *
*
1 *
*
3 *
arg
arg
arg
td tb
ud ub
cd cb
td tb
ud ub
cd cb
V V
V V
V V
V V
V V
V V
1
See D. Marlow’s talk
BEACH 2002, May 25-29, 2002 M. Morii, Harvard University 3
Unitarity Triangle and sin2 Measured sin2 agrees with indirect constraints
Shrinking (sin2) alonemay not reveal new physics
Must measure the sidesand the other angles
*
*ud ub
cd cb
V V
V V
*
*td tb
cd cb
V V
V V
Next possibility at the B Factories?
BEACH 2002, May 25-29, 2002 M. Morii, Harvard University 4
Measuring sin2 Time-dependent CP asymmetry in B0 fCP is
0 0
0 0
( ( ) ) ( ( ) )sin( ) cos( )
( ( ) ) ( ( ) ) CP CP
phys CP phys CPf d f d
phys CP phys CP
B t f B t fS m t C m t
B t f B t f
2
2 2
12Im
1 1CP
CP CP
CP
ff f
f
AqS C
p A
CKM phase appears here
b cc
s
W
cbVb u
u
d
W
ubV
0SB J K 0B
sin 2 sin 2
Easy!
BEACH 2002, May 25-29, 2002 M. Morii, Harvard University 5
Penguin Pollution Unlike J/ KS, mode suffers from significant
pollution from the penguin diagrams with a different weak phase
To estimate eff – , we need: P/T ratio – about 1/3 from BR(B K)/BR(B ) = strong phase difference between P and T
b uu
d
W
ubV
effsin 2b u
u
d
W
T = Tree P = Penguin
gtbV *
tdV
BEACH 2002, May 25-29, 2002 M. Morii, Harvard University 6
Mode BABAR BR106
Belle BR106
0
0 1.11.0
0 0 0
2 0 2 5.4 0.7 0.5 5.1 1.1 0.4
1 1 0 4.1 0.8 7.0 2.2 0.8
0 1 1 3.3 5.6
T C P
B
B
B
Taming Penguins Take advantage of the isospin symmetry
T C PT C P A
b uu
d
ddb u
u
d
dd
b u
u
d
dd
All preliminary
BEACH 2002, May 25-29, 2002 M. Morii, Harvard University 7
B0 00 Branching Ratio BABAR: Preliminary 54 fb-1
BR(00) < 3.3×10–6 (90% CL) Belle: Preliminary 31.7 M BB
2.2“bump” in the signal Fitted BR= (2.9 ± 1.5 ± 0.6)×10–6 BR(00) < 5.6×10–6 (90% CL)
CLEO: 9.13 fb-1
BR(00) < 5.7×10–6 (90% CL)
BELLE
Expect first observation in the near future
BEACH 2002, May 25-29, 2002 M. Morii, Harvard University 8
0 0
0 0
( ) ( )sin( ) cos( )
( ) ( )tag tag
d dtag tag
N B N BS m t C m t
N B N B
z c t
CP Asymmetry in B0
Same method as sin2 measurements Difference: the direct CP term cannot be neglected
9 GeV3.1 GeV4S
Btag
BCP
Tagusing l±, K±
Moving with = 0.55
e e
0 0orB B
CP final
state
# of events with 0 0 tagB B
BEACH 2002, May 25-29, 2002 M. Morii, Harvard University 9
Challenges Specific to B0 +
Topology B0 hh simple to reconstruct Particle ID must separate ± from K±
DIRC (BABAR) and Aerogel (Belle) Significant background from continuum
Event-shape variables Fisher discriminant Common with other CP measurements
Flavor tagging Vertex reconstruction
And, of course, as much as possibledtL
( ) ( )BR BR K
BEACH 2002, May 25-29, 2002 M. Morii, Harvard University 10
B0 Reconstruction
mbc (or mES) and E peak cleanly for the two-body signal K and KK peaks shifted in E Additional discrimination
2 2ES bm E p
CME E E
bE
MC
off-resonance data
MC
MC
BELLE BELLE
2 2( 2) ( )bc CMm E p p 2CME E E E
BEACH 2002, May 25-29, 2002 M. Morii, Harvard University 11
Whole event is jettyThe other B decays spherically
Continuum Background Most of the background come from continuum
Use event shape variables that represent “jettiness” to suppress them
Signal udsc background
Examples
BEACH 2002, May 25-29, 2002 M. Morii, Harvard University 12
Sphericity Angle Angle S between the sphericity axes of the B candidate and
the rest of the event
Cut at 0.8 removes 83% ofthe continuum background
SBABAR
MC
cos S
background
0.8
reject
BEACH 2002, May 25-29, 2002 M. Morii, Harvard University 13
Fisher Discriminant BABAR uses the “CLEO” Fisher
Momentum flow in 9 cones around the candidate axis
Output of Fisher goes into the likelihood fit
MC
D0data
Bkg MC
mES sideband data
BEACH 2002, May 25-29, 2002 M. Morii, Harvard University 14
Bkg MC
off-res. data
Fisher Discriminant Belle’s Fisher discriminant uses:
Modified Fox-Wolfram moments B flight direction
Output is turned intoa likelihood ratio R Cut at 0.825 removes
95% of continuumbackground
MC
D0data
reject
BEACH 2002, May 25-29, 2002 M. Morii, Harvard University 15
Event Sample – BABAR BABAR 55.6 fb-1 preliminary
enhanced for these plots with a cut on Fisher
Kcontinuum
16 715 9( ) 124N
BEACH 2002, May 25-29, 2002 M. Morii, Harvard University 16
Event Sample – Belle Belle 41.8 fb-1
K
Continuum
( ) 78.5 13.8(stat)N
BEACH 2002, May 25-29, 2002 M. Morii, Harvard University 17
Maximum Likelihood Fit Start from the physics function:
Fold in t resolution and mis-tag probabilities Multiply by PDFs for mES, E BABAR uses particle ID and Fisher in the fit
Belle uses these variables in event selection Add PDFs for background (K, KK, continuum)
Feed the candidates and turn the crank…
( ) 1 sin( ) cos( )4
t
d d
ef t m C m tS t
0
0
tag
tag
B
B
BEACH 2002, May 25-29, 2002 M. Morii, Harvard University 18
BABAR
BELLE
CP Fit Results
BABAR and Belle disagree by >2 Belle 1.2 outside the physical
boundary Is there any problem?
Crosscheck systematics
BABAR (preliminary) Belle (hep-ex/0204002)
S –0.01 ± 0.37 ± 0.07
C –0.02 ± 0.29 ± 0.07
0.38+0.161.210.27 0.13
0.310.94 0.090.25
Belle usesA C
2 2 1S C
BEACH 2002, May 25-29, 2002 M. Morii, Harvard University 19
CP Asymmetries – BABAR
enhanced for these plots with a cut on Fisher No significant asymmetry
BEACH 2002, May 25-29, 2002 M. Morii, Harvard University 20
CP Asymmetries – Belle
Rate difference (= C)
t-dependent asymmetry (= S and C)
Subtract bkg0 tagB
0 tagB
BEACH 2002, May 25-29, 2002 M. Morii, Harvard University 21
Crosschecks Both experiment made extensive crosschecks, e.g.
Asymmetry in background? Look for asymmetries
in K or mass sideband Vertex resolution of the
2-body decays? Measure B lifetime with , K Measure mixing with K
Likelihood values and errors? Toy Monte Carlo studies
BELLE
BEACH 2002, May 25-29, 2002 M. Morii, Harvard University 22
Monte Carlo Fit Test Generate ~1000 “toy” experiments
Belle used (0.7, 0.7)for the central values
Fit and compare: Likelihood values Pull distributions Errors
Lowest probability: 5.4%
BABAR
S C
(S) (C)
MC
Measured
S C
BABAR
BELLEBELLE
Measured
Everything looks reasonable
BEACH 2002, May 25-29, 2002 M. Morii, Harvard University 23
BABAR
BELLE
Interpretation
How well do we know ?(*Gronau and Rosner, PRD65, 093012) Average BABAR and Belle Assume = 26°, P/T = 0.28
BABAR (preliminary) Belle (hep-ex/0204002) Average*
S –0.01 ± 0.37 ± 0.07 –0.66 ± 0.26
C –0.02 ± 0.29 ± 0.07 –0.49 ± 0.21
0.38+0.161.210.27 0.13
0.310.94 0.090.25
NB: Large uncertainty
BEACH 2002, May 25-29, 2002 M. Morii, Harvard University 24
Measured S ±1 corresponds to
Interpretation
Gronau and RosnerPRD65, 093012
[89 ,138 ]
Indirect:
BABAR + Belle
3021(97 )
Accuracy comparableto the indirect constraints
We are starting to measure
BEACH 2002, May 25-29, 2002 M. Morii, Harvard University 25
Summary BABAR and Belle measured sin2eff using B0 +
Direct constraint on is reaching useful accuracy Things to watch out for:
sin2eff with higher statistics Resolve “discrepancy”
BR(B0 0 ) Better bound on eff –
BABAR (preliminary) Belle (hep-ex/0204002)
S –0.01 ± 0.37 ± 0.07
C –0.02 ± 0.29 ± 0.07
0.38+0.161.210.27 0.13
0.310.94 0.090.25
BEACH 2002, May 25-29, 2002 M. Morii, Harvard University 26
Bound on eff – Full isospin analysis (Gronau & London, 1990) requires
and separately Too hard for BABAR/Belle Upper limits on average BR
Use BR(00) to put upper bound on eff – Grossman and Quinn, 1998; Charles, 1998
0 0 0( )BR B 0 0 0( )BR B
Gronau, London, Sinha, SinhaPLB 514:315-320, 2001
0 0( ) ( )BR BR
eff2( ) Allowed
was assumed
0( )1.3
( )
BR
BR
BEACH 2002, May 25-29, 2002 M. Morii, Harvard University 27
GLSS Bound on eff –
If I use and
GLSS bound weakerfor smaller BR()
Better measurements ofBR() and BR() will give us a betterhandle on the penguinsin the near future
0( )0.91 0.25
( )
BR
BR
6( ) 5.3 10BR
0 0 6( ) 10BR
eff2( )
BABAR90% CL
Small
Large
BEACH 2002, May 25-29, 2002 M. Morii, Harvard University 28
B Flight Direction Angle B of the B candidate momentum relative to the beam
axis
Signal Background ~flat
eeB
Bp BELLE
cos B
21 cos B
BEACH 2002, May 25-29, 2002 M. Morii, Harvard University 29
Systematic Errors BABAR:
Dominated by the shape of the particle ID variable Belle:
Uncertainties of the background fractions Fit bias near the physical boundary for S Wrong tag fraction for C
All measurements are statistically limited