Measurements of sin2α from B-Factories Masahiro Morii Harvard University The BABAR Collaboration BEACH 2002, Vancouver, June 25-29, 2002
Measurements of sin2αfrom 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
! sin2β measured to ±0.08 dominated by B0 → J/ψ 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 VV V
V VV V
V VV V
α φ
β φ
γ φ
= ≡ −
= ≡ −
= ≡ − ρ 1
η
α
βγ
See D. Marlow’s talk
BEACH 2002, May 25-29, 2002 M. Morii, Harvard University 3
Unitarity Triangle and sin2β
! Measured sin2β agrees with indirect constraints! Shrinking σ(sin2β) alone
may not reveal new physics
! Must measure the sidesand the other angles
*
*ud ub
cd cb
V VV V α
βγ
*
*td tb
cd cb
V VV V
Next possibility at the B Factories?
BEACH 2002, May 25-29, 2002 M. Morii, Harvard University 4
Measuring sin2α! 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 fΓ → − Γ →
= ∆ + ∆Γ → + Γ →
2
2 2
12 Im1 1
CP
CP CP
CP
ff f
f
AqS Cp A
λλ λλ λ
−= − = =
+ +
CKM phase appears here
b cc
s
W
cbVb u
u
dW
ubV
0SB J Kψ→ 0B π π+ −→
sin 2β sin 2α
Easy!
BEACH 2002, May 25-29, 2002 M. Morii, Harvard University 5
Penguin Pollution
! Unlike J/ψ KS, π +π − mode suffers from significant pollution from the penguin diagrams with a different weak phase
! To estimate αeff – α, we need:! P/T ratio – about 1/3 from BR(B → Kπ)/BR(B → ππ)! δ = strong phase difference between P and T
b uu
dW
ubVeffsin 2α
b uu
d
W
T = Tree P = Penguin
gtbV *
tdV
BEACH 2002, May 25-29, 2002 M. Morii, Harvard University 6
Belle BR×106BABAR BR×106Mode0
0 1.11.0
0 0 0
2 0 2 5.4 0.7 0.5 5.1 1.1 0.41 1 0 4.1 0.8 7.0 2.2 0.80 1 1 3.3 5.6
T C P
BBB
α α α
π ππ ππ π
+ −
+ + +−
→ ± ± ± ±→ ± ± ±→ − < <
Taming Penguins! Take advantage of the isospin symmetry
T C PT C Pα α α= ⋅ + ⋅ + ⋅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 " π0π0 Branching Ratio
! BABAR: Preliminary 54 fb-1
! BR(π 0π 0) < 3.3×10–6 (90% CL)
! Belle: Preliminary 31.7 M BB! 2.2σ “bump” in the signal! Fitted BR= (2.9 ± 1.5 ± 0.6)×10–6
! BR(π 0π 0) < 5.6×10–6 (90% CL)
! CLEO: 9.13 fb-1
! BR(π 0π 0) < 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 ππ ππ
−= ∆ ∆ − ∆ ∆
+
z c tγβ∆ = ∆
CP Asymmetry in B0 " π+π−
! Same method as sin2β measurements! Difference: the direct CP term cannot be neglected
9 GeV3.1 GeVϒ4S
Btag
BCP
Tagusing l±, K±
Moving withβγ = 0.55
e− e+
π −
0 0orB B
π +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 → π +π −
! Topology B0 → h+h− simple to reconstruct! Particle ID must separate π ± 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 possibledt∫L
( ) ( )BR BR Kπ π π+ − + −<
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! Kπ and KK peaks shifted in ∆E " Additional discrimination
2 2ES bm E pππ= −
!
CME E Eππ∆ = −
bE =
π+π− MC
off-resonance data
π+π− MC
Κ+π− MC
BELLE BELLE
2 2( 2) ( )bc CMm Eπ π+ −= − +p p 2CME E E E
π π+ −∆ = + −
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
π +
π −
π +
π −
Signal udsc background
Examples
BEACH 2002, May 25-29, 2002 M. Morii, Harvard University 12
Sphericity Angle! Angle θS between the sphericity axes of the B candidate and
the rest of the event
! Cut at 0.8 removes 83% ofthe continuum background
π +
π −
SθBABAR
π+π− MC
cos Sθ
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
π +
π −
π+π− MC
D0π+ data
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
π+π− MC
D0π+ data
reject
BEACH 2002, May 25-29, 2002 M. Morii, Harvard University 15
Event Sample – BABAR! BABAR 55.6 fb-1 preliminary
! π+π− enhanced for these plots with a cut on Fisher
Kπ + continuum
16 715 9( ) 124N π π+ − + +
− −=
BEACH 2002, May 25-29, 2002 M. Morii, Harvard University 16
Event Sample – Belle! Belle 41.8 fb-1
Kπ
Continuum
( ) 78.5 13.8(stat)N π π+ − = ±
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 (Kπ, KK, continuum)
! Feed the candidates and turn the crank…
[ ]( ) 1 sin( ) cos( )4
t
d def t m C m tS tππ ππ
τ
τ
∆
±
−
∆ = ∆ ∆ ∆ ∆± ∓0
0
tagtag
BB
+−
BEACH 2002, May 25-29, 2002 M. Morii, Harvard University 18
BABAR
BELLE
CP Fit Results
! BABAR and Belle disagree by >2σ! Belle 1.2σ outside the physical
boundary
! Is there any problem?! Crosscheck systematics
–0.02 ± 0.29 ± 0.07Cππ
–0.01 ± 0.37 ± 0.07Sππ
Belle (hep-ex/0204002)BABAR (preliminary)0.38+0.161.21 0.27 0.13
+− − −0.310.94 0.090.25
+− ±−
Belle usesA Cππ ππ≡ −
2 2 1S Cππ ππ+ =
BEACH 2002, May 25-29, 2002 M. Morii, Harvard University 19
CP Asymmetries – BABAR
! π+π− 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 (= Cππ)! ∆t-dependent asymmetry (= Sππ and Cππ)
Subtract bkg0 tagB0 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 Kπ or mass sideband! Vertex resolution of the
2-body decays?! Measure B lifetime with ππ, Kπ! Measure mixing with Kπ
! 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
Sππ Cππ
σ(Sππ) σ(Cππ)
MC
Measured
Sππ Cππ
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 α?(*Gronau and Rosner, PRD65, 093012)
! Average BABAR and Belle! Assume β = 26°, P/T = 0.28
–0.49 ± 0.21
–0.66 ± 0.26
Average*
–0.02 ± 0.29 ± 0.07Cππ
–0.01 ± 0.37 ± 0.07Sππ
Belle (hep-ex/0204002)BABAR (preliminary)0.38+0.161.21 0.27 0.13
+− − −0.310.94 0.090.25
+− ±−
NB: Large uncertainty
BEACH 2002, May 25-29, 2002 M. Morii, Harvard University 24
! Measured Sππ ±1σcorresponds to
Interpretation
Gronau and RosnerPRD65, 093012
[89 ,138 ]α ∈ ° °
Indirect:
BABAR + Belle
3021(97 )α +
−= °
Accuracy comparableto the indirect constraints
We are starting to measure α
BEACH 2002, May 25-29, 2002 M. Morii, Harvard University 25
Summary
! BABAR and Belle measured sin2αeff using B0 → π +π −
! Direct constraint on α is reaching useful accuracy! Things to watch out for:
! sin2αeff with higher statistics " Resolve “discrepancy”! BR(B0 → π 0π 0) " Better bound on αeff – α
–0.02 ± 0.29 ± 0.07Cππ
–0.01 ± 0.37 ± 0.07Sππ
Belle (hep-ex/0204002)BABAR (preliminary)0.38+0.161.21 0.27 0.13
+− − −0.310.94 0.090.25
+− ±−