The other window on New Physics:
CP violation at the B factories
Gabriella Sciolla
M.I.T.
Outline:
„
The physics of CP violation
„
„
CPV in the B system
„
„
“How would you like to live in Looking-glass House?”
L. Carroll
CPV in the Standard Model and the Unitarity Triangle
Constraining the Unitarity Triangle at the B factories
„
„
What is CP and why is it interesting?
Measurements of angles and sides
Conclusion
„
Summary & Prospects
The matter dominated Universe
The Big Bang model predicts:
„
„
matter and anti-matter produced in equal amounts
matter and anti-matter annihilated into pure energy
But this goes against experimental evidence:
„
„
The Universe exists
It is made of (almost) only matter
How is this possible?
A. Sakharov’s 3 conditions (1967):
- Baryon number non conservation
- Thermal non equilibrium
- C and CP violation
March 21, 2006
What can we learn from CPV?
G. Sciolla – M.I.T.
The CP symmetry
CP = C × P
C: Charge Conjugation
Particle Æ Anti-particle
P: Parity
Inverts space coordinates
Is Nature CP symmetric?
Wu et al., 1957
March 21, 2006
but CP was expected to be conserved
CP|ν>L = |ν>R
What can we learn from CPV?
G. Sciolla – M.I.T.
CP violation in K decays
In 1964 Fitch and Cronin discovered CP violation in the
decays of KL mesons: KLÆπ+π-
KL
March 21, 2006
π+π−π0
CP=-1
}
0
0
0
πππ
π+π−
CP=+1
}
0
0
ππ
What can we learn from CPV?
G. Sciolla – M.I.T.
CP violation in the Standard Model
In 1973 the Kobayashi-Maskawa mechanism explained
CPV and predicted the existence of third quark family.
CP violation originates from a complex phase in the quark
mixing matrix (CKM matrix).
⎛Vud Vus
⎜
V = ⎜Vcd Vcs
⎜V V
ts
⎝ td
March 21, 2006
1 2
⎛
λ
⎜ 1− λ
2
Vub ⎞ ⎜
⎟ ⎜
1 2
1− λ
Vcb ⎟ =
−λ
⎜
2
⎟
Vtb ⎠ ⎜ Aλ3 (1 − ρ − iη ) − Aλ2
⎜
⎝
⎞
Aλ3 ( ρ − iη ) ⎟
⎟
⎟ + O(λ6 )
Aλ2
⎟
⎟
1
⎟
⎠
What can we learn from CPV?
G. Sciolla – M.I.T.
Pros and Cons of CKM
Pros:
9
9
9
Elegant and simple explanation of CPV in SM
It is very predictive: only one CPV phase
It accommodates all experimental results
„
„
„
Indirect CP violation in KÆππ and KLÆπlν
Direct CP violation in KÆππ
CP violation in the B system
CKM
Cons:
/ nB/nγ predicted by CKM « observed value
„
…by orders of magnitude!
Æ New sources of CPV must exist besides CKM!
CPV as a probe for New Physics:
.Any extension of SM provides new sources of CP violation
March 21, 2006
What can we learn from CPV?
G. Sciolla – M.I.T.
Standard Model or New Physics?
Measure CP violation in channels theoretically
very well understood and look for deviations
w.r.t. Standard Model prediction
What can we learn from kaons?
„ Experimental results very hard to interpret theoretically:
„
„
„
Loose constraints from εK measurement
No constraints from ε’/ε yet…
…or clear theory but very hard to reach experimentally:
„
BF(KLÆπ0νν)~10-11!
Can B mesons do better?
March 21, 2006
What can we learn from CPV?
G. Sciolla – M.I.T.
The Unitarity Triangle
Unitarity of CKM implies: V†V = 1 Æ 6 unitarity conditions
*
*
*
Of particular interest: Vud Vub + Vcd Vcb + Vtd Vtb = 0
(ρ,η)
α
Vub* Vud
Vcd Vcb*
γ
V tb*V td
V cd V cb*
β
(0,0)
(1,0)
All sides are ~ O(1) Æ possible to measure both sides and angles!
„
„
CP asymmetries in B meson decays measure α, β and γ
Sides from B mixing, rare B decays, Semileptonic B decays
March 21, 2006
What can we learn from CPV?
G. Sciolla – M.I.T.
Redundancy, redundancy, redundancy!
3 ways to look for New Physics:
a) Sides vs angles
b) Measurement of same angle using channels with different
sensitivity to NP
c) Measurement of same sides using channels with different
sensitivity to NP
α
γ
March 21, 2006
β
What can we learn from CPV?
G. Sciolla – M.I.T.
Redundancy, redundancy, redundancy!
3 ways to look for New Physics:
a) Sides vs angles
b) Measurement of same angle using channels with different
sensitivity to NP
c) Measurement of same sides using channels with different
sensitivity to NP
α
γ
March 21, 2006
β
What can we learn from CPV?
G. Sciolla – M.I.T.
Redundancy, redundancy, redundancy!
3 ways to look for New Physics:
a) Sides vs angles
b) Measurement of same angle using channels with different
sensitivity to NP
c) Measurement of same sides using channels with different
sensitivity to NP
α
γ
March 21, 2006
β
What can we learn from CPV?
G. Sciolla – M.I.T.
How to measure the angles?
Time dependent CP asymmetry in B0 decays.
N ( B 0 (t) → fCP ) − N ( B0 (t) → fCP )
ACP(t) =
N ( B 0 (t) → fCP ) + N ( B0 (t) → fCP )
Questions:
„ How is ACP(t) related to the UT angles?
„ How can we measure ACP(t)?
March 21, 2006
What can we learn from CPV?
G. Sciolla – M.I.T.
The B0 system
The B0 system:
B mesons
Flavor eigenstates
B0 and B0
Mass eigenstates
BH and BL
| BL ⟩ = p | B 0 ⟩ + q | B 0 ⟩
| BH ⟩ = p | B 0 ⟩ − q | B 0 ⟩
( p 2 + q 2 = 1)
The K0 system:
K mesons
Flavor eigenstates
K0 and K0
Mass eigenstates
KS and KL
March 21, 2006
| K S ⟩ = p' | K 0 ⟩ + q' | K 0 ⟩
| K L ⟩ = p' | K 0 ⟩ − q' | K 0 ⟩
What can we learn from CPV?
G. Sciolla – M.I.T.
Time evolution of B0
„
Starting from pure B0(B0) state, and after time t
⎧
⎫
q
∆mt
∆mt
s in
B 0 ( t ) = e − im B t e − Γ t ⎨ c o s
B0 − i
B0 ⎬
2
2
p
⎩
⎭
⎧
⎫
p
∆mt
∆mt
s in
B 0 ( t ) = e − im B t e − Γ t ⎨ − i
B 0 + cos
B0 ⎬
2
2
q
⎩
⎭
„
Interested in Prob(B0(t)Æf) and Prob(B0(t)Æf) :
„
Calculate amplitudes
f H B 0 (t )
„
Use
Af = f H B0
and
f H B 0 (t )
Af = f H B 0
B0
~ e − 2 iβ
Take square to get decay rates…
March 21, 2006
What can we learn from CPV?
A f CP
mixing
„
t
t =0
B0
ay
dec A
f CP
f CP
G. Sciolla – M.I.T.
CP violation in interference between
mixing and decays in B0
Time dependent CP asymmetry for B0 Æ fCP:
N ( B 0 (t ) → f CP ) − N ( B 0 (t ) → f CP )
ACP (t ) =
= S f sin(∆mt ) − C f cos(∆mt )
N ( B 0 (t ) → f CP ) + N ( B 0 (t ) → f CP )
where
Sf =
For B0
2 Im λ f
1+ λf
2
Cf =
1− λf
2
1+ λf
2
q Af
λf = ⋅
p Af
q
~1, so when only 1 diagram contributes to the final state:|λ|=1
p
ACP(t) = -ηf Imλ sin(∆mt)
(CP violation in interference between mixing and decays in B0 )
March 21, 2006
What can we learn from CPV?
G. Sciolla – M.I.T.
CP violation in B0 decays: sin2β
For some lucky modes, Imλ is directly and simply related
to the angles of the Unitarity Triangle.
Example:
B0ÆJ/ΨKS: the “golden mode”
c
B0
J /ψ
c
s
d
b
d
K0
⎛ Vtb*Vtd ⎞ ⎛ Vcs*Vcb ⎞ ⎛ Vcd* Vcs ⎞
λ =⎜
* ⎟⎜
* ⎟⎜
* ⎟
⎝ VtbVtd ⎠ ⎝ VcsVcb ⎠ ⎝ Vcd Vcs ⎠
η
ACP(t) = sin2β sin∆mt
March 21, 2006
What can we learn from CPV?
α
γ
β
ρ
G. Sciolla – M.I.T.
How to measure the angles?
Time dependent CP asymmetry in B0 decays.
N ( B 0 (t) → fCP ) − N ( B0 (t) → fCP )
ACP(t) =
N ( B 0 (t) → fCP ) + N ( B0 (t) → fCP )
Questions:
„ How is ACP(t) related to the UT angles?
„ How can we measure ACP(t)?
March 21, 2006
What can we learn from CPV?
9
G. Sciolla – M.I.T.
How to measure the CP asymmetry
N ( B 0 (t ) → fCP ) − N ( B0 (t ) → fCP )
ACP (t ) =
N ( B 0 (t ) → fCP ) + N ( B0 (t ) → fCP )
.
e-
Flavor tagging
(coherent state)
B0
Υ(4S)
eExclusive B
reconstruction
e+
Excellent PID
µ+
B0
µ- π
Br~ 10-5 -10-6
∆z~ βγc ∆t
March 21, 2006
What can we learn from CPV?
π+
Precise ∆t determination
Sciolla – M.I.T.
B has G.boost
The B factories: PEP-II and KEK-B
„
Asymmetric beams
„
„
9.0 GeV e- beam
3.1 GeV e+ beam
„
Currents: 1-2 A
„
Peak Luminosity:
„
„
„
Design:
3 x 1033 cm-2s-1
Achieved: 12 x 1033 cm-2s-1
Integrated Luminosity
>400 fb-1
Belle: ~600 fb-1
>1 ab-1 in total
„
March 21, 2006
What can we learn from CPV?
!
G. Sciolla – M.I.T.
The BABAR Detector
1.5 T solenoid
CsI(Tl) EMC
DIRC (PID)
e+ (3.1GeV)
Silicon Vertex Tracker
Drift Chamber
Instrumented Flux Return
e- (9GeV)
„
„
„
„
SVT:
Tracking:
DIRC:
EMC:
March 21, 2006
97% efficiency, 15µm z resolution
σ(pT)/pT = 0.13% pT ⊕ 0.45%
K-π separation: > 4.2σ @ p=3 GeV/c
σE/E = 2.3% E-1/4 ⊕ 1.9%
What can we learn from CPV?
G. Sciolla – M.I.T.
The measurements
V V
|V V |
*
ub ud
*
cd cb
α
Vtb*Vtd
| VcdVcb* |
The angle β
γ
March 21, 2006
ββ
What can we learn from CPV?
G. Sciolla – M.I.T.
BaBar 2006 (347M BB)
The golden mode for sin2β:
B0Æ charmonium
„
„
„
Theoretically clean
Experimentally clean
Relatively large BF (~10-4)
c
b
c
s
d
CP sample:
K0
d
J/ψ K s , Ψ(2s)K s , χcK s , ηcK s
J/ ψ K L
NCP =−1 = 6028
Purity = 92%
N CP = + 1 = 4324
Purity = 55%
mES [GeV]
March 21, 2006
What can we learn from CPV?
∆E [MeV]
G. Sciolla – M.I.T.
Gold plated event
at BaBar
B0 Æ J/Ψ KS
B0 Æ K-X
Zoom on
interaction region
March 21, 2006
What can we learn from CPV?
G. Sciolla – M.I.T.
B flavor tagging
The principle:
D*-
B0
ν
l+
π−
D0
π−(soft)
K+
D*-
B0
W+
π+(hard)
Information combined in a NN-based algorithm
Q=εtag(1-2w)2
.
March 21, 2006
What can we learn from CPV?
G. Sciolla – M.I.T.
The importance of tagging
Flavor tagging is a crucial ingredient in CP measurements
since
σ (sin 2 β ) ∝ 1 / Q
obs
ACP
( t ) = (1 − 2ω ) ACP ( t )
ÆExtract tagging purity and efficiency directly from data
in time dependent B0 mixing measurement
eApply tagging algorithm
B0
e-
Υ4S
e+
B0
March 21, 2006
π+
Dπ+
πK-
What can we learn from CPV?
Bflavor exclusive
reconstruction
G. Sciolla – M.I.T.
Time dependent mixing fit
mixed
unmixed
(
)
−
(t )
N
t
N
mixing
∝ (1 − 2ω ) cos(∆mt)
(t ) = mixed
A
unmixed
(t ) + N
(t )
N
1-2ω
π/∆md
30 fb-1
.
∆md = 0.516 ± 0.016(stat) ± 0.010(syst) h ps-1
March 21, 2006
What can we learn from CPV?
G. Sciolla – M.I.T.
hep-ex/0607107 (BaBar 2006)
The golden mode for sin2β:
CP fit in BÆ charmonium K0
Unbinned maximum likelihood fit to ∆t distribution
CP=-1
March 21, 2006
CP=+1
sin(2β) = 0.710
± 0.034 CPV? ± 0.019syst
G. Sciolla – M.I.T.
What can we learn from stat
Belle 2006
The golden mode for sin2β:
Belle’s BÆ charmonium K0
0
B
_ tag
B0 tag
_
532 M BB pairs
sin(2β)
= 0.642 ± 0.031
± 0.017 CPV?
March 21, 2006
Whatstat
can we learn from syst
G. Sciolla – M.I.T.
How well do we know sin(2β)?
.
BABAR
PRL 94, 161803 (2005)
0.722 ± 0.040 ± 0.023
Belle
0.652 ± 0.039 ± 0.020
BELLE-CONF-0569
HFAG Average
0.685 ± 0.032
ICHEP 2006
0.5
0.6
0.7
0.8
sin2β
March 21, 2006
What can we learn from CPV?
G. Sciolla – M.I.T.
How to look for New Physics
„
Beautiful measurement:
„
„
„
Error on sin2β is 0.026!
Error on β < 1 degree!
But by itself is useless! To test the SM we need
to compare it with other measurements:
„
„
Opposite side in the UT
Independent measurement of sin2β using channels
mediated by other Feynman diagrams
March 21, 2006
What can we learn from CPV?
G. Sciolla – M.I.T.
The left side: Rb
α
VV
V
Rb ∝V V
V
*
ub
ub ud
*
cd cb
Vtb*Vtd
VcdVcb*
cb
γ
β
NB: β is the best measured quantity in the Unitarity Triangle
β = 21.2 ± 1.0 degrees
Æ precise measurement of Rb is needed for accurate tests of SM
March 21, 2006
What can we learn from CPV?
G. Sciolla – M.I.T.
Semileptonic B Decays
ν
Hadron level
Parton level
W−
b
l
Vub , Vcb
b
B
u, c
Γ(b → uAν ) =
„
νν
Vub
u
G
2
5
V
m
ub
b
192π 2
2
F
XXuu , X c
Sensitive to hadronic effects
„
„
Al−
Theory error not negligible
Prob(bÆc)/Prob(bÆu)~50
„ Vcb precisely measured (±2%)
„
Vub is the challenge
March 21, 2006
What can we learn from CPV?
G. Sciolla – M.I.T.
Two approaches to Vub
Inclusive B → Xu l ν
B
Exclusive B → π l ν
ν
Al-−
ν
W−
B
Vub
Xu
Vub
Al-−l
π
Exclusive B → π lν
Inclusive B → Xulν
„
Hadronic final state is not specified
„
„
bÆc l ν background is suppressed
using kinematical variables
Better S/B but lower branching
fraction (10-4)
„
Needs form factor calculation from
Lattice QCD
„
Partial rate is measured
Æ theoretical uncertainties ~5%
March 21, 2006
Æ uncertainty of ~ 12%
What can we learn from CPV?
G. Sciolla – M.I.T.
|Vub| from Inclusive B → Xu l ν
Close collaboration
between theorists and
experimentalists led to
Precision on Vub: ±7.3%
c.f.r.: precision on β: ±4.7%
World Average 4.49 ± 0.33
χ2/dof = 6.1/6
March 21, 2006
Goal in 2008: error on Vub: ±5%
What can we learn from CPV?
G. Sciolla – M.I.T.
Unitarity Triangle constraints
η
95% CL
from sides
ρ
sin2β vs indirect UT constraints: very good agreement!
CKM mechanism is the dominant source of CPV at low energies
„
New Physics does not show up in the golden mode ÆSM reference
„
Compare with sin2β in independent modes with different sensitivity to NP
March 21, 2006
What can we learn from CPV?
G. Sciolla – M.I.T.
How to look for New Physics (2)
„
Beautiful measurement:
„
„
„
Error on sin2β is 0.026!
Error on β < 1 degree!
But by itself is useless! To test the SM we need
to compare it with other measurements:
„
„
Opposite side in the UT
Independent measurement of sin2β using channels
mediated by other Feynman diagrams
March 21, 2006
What can we learn from CPV?
G. Sciolla – M.I.T.
Another way to look for New Physics:
sin2β in Penguin Modes
Decays dominated by gluonic penguin diagrams
0
„ The typical example: B ÆφKS
NP
SM
d
„
„
b
s
d
)23
~ +(δRR
bR
~
g
sR
s
s
d
d
No tree level contributions
Impact of New Physics could be significant
„
„
d
~
g
New particles could participate in the loop Æ new CPV phases
NP affects each channel differently; low branching fractions
„
Measure ACP in as many bÆsqq penguins as possible!
„
φ K0, K+ K− KS, η′ KS, KS π0, KS KS KS, ω KS, f0(980) KS
March 21, 2006
What can we learn from CPV?
G. Sciolla – M.I.T.
BaBar ICHEP 2006
Belle ICHEP 2006
The golden penguin: B0 Æ φK0
N φK S = 307 ± 21
N φK L = 114 ± 14
S φK = +0.50 ± 0.21 ± 0.05
0
March 21, 2006
C φK = −0.07 ± 0.15 ± 0.06
0
What can we learn from CPV?
G. Sciolla – M.I.T.
BaBar ICHEP 2006
Belle ICHEP 2006
The silver penguin: B0 Æ η’KS
„
„
Relative large BF ~ 6 x 10-5
Theoretically less clean than φKS
„
Tree diagram possible
… but Cabibbo and color suppressed…
„
|∆S| =|SΨK-SPenguin|~ 0.01QCDFact Æ 0.1SU(3)
Nη ' K S = 1421 ± 46
Sη′Ks = 0.64 ± 0.10 ± 0.04
Cη′Ks = −0.01 ± 0.07 ± 0.05
March 21, 2006
What can we learn from CPV?
G. Sciolla – M.I.T.
BaBar ICHEP 2006
Belle ICHEP 2006
A new golden penguin: B0ÆKSKSKS
„
Theoretically clean
„
„
Penguin dominated CP=+1 eigenstate
Experimentally challenging
„
B decay vertex uses KS pseudo-particles
and beam spot constraint
N K S K S K S = 176 ± 17
Add babar plot
−S K S K S K S = +0.66 ± 0.26 ± 0.08
C K S K S K S = −0.14 ± 0.22 ± 0.05
March 21, 2006
What can we learn from CPV?
G. Sciolla – M.I.T.
“sin2β” in penguins
ƒ No significant shift observed
in each mode
ƒ although a trend is visible…
ƒ Naïve average: 0.52±0.05
ƒ ~2.6σ from J/ΨKs
ƒ Statistical errors still large…
March 21, 2006
What can we learn from CPV?
G. Sciolla – M.I.T.
The measurements
V V
|V V |
*
ub ud
*
cd cb
α
γ
March 21, 2006
Vtb*Vtd
| VcdVcb* | The side Rt
β
What can we learn from CPV?
G. Sciolla – M.I.T.
The measurement of Rt
Rt = λ −1
α
γ
„
Vtd
Vts
b
Bd0(s)
d (s )
β
t
W
Vtd (s)
Vtd(s)
W
t
d(s)
Bd0( s )
b
Bs/Bd oscillations
∆md
V
∝ td
∆ms
Vts
„
„
„
2
Theory error <5%
∆md is precisely measured
But Bs mixing is very hard…
March 21, 2006
What can we learn from CPV?
G. Sciolla – M.I.T.
BS mixing at the Tevatron
„
„
.
„
.
„
.
Flavor tagging
QSST~4%
QOST~1%
.
Time reconstruction
σ(ct)~26-70 µm
Reconstruction of BS decay
in hadronic or SL modes
March 21, 2006
What can we learn from CPV?
G. Sciolla – M.I.T.
D0 result
BsƵDs: 5601±102
„
.
„
„
.
.
17 < ∆ms < 21 ps-1 @ 90% CL
. assuming Gaussian errors
Most probable value of ∆ms = 19 ps-1
„
March 21, 2006
What can we learn from CPV?
G. Sciolla – M.I.T.
CDF result
3,700 events
+0.42
−0.21
∆mS = 17.33
„
± 0.07 ps
-1
⇒
Vtd
= 0.208+−0.008
0.007
Vts
Probability of random fluctuation: ~0.5%
March 21, 2006
What can we learn from CPV?
G. Sciolla – M.I.T.
Impact of ∆mS on Unitarity Triangle
„
March 21, 2006
What can we learn from CPV?
G. Sciolla – M.I.T.
But to test the SB we need redundancy…
Rt from BÆργ/ωγ vs. BÆΚ∗γ
„
Radiative penguin decays with bÆdγ and bÆsγ
Vtd,Vts
„
„
Ratio of BF measures |Vtd/Vts|
Ali and Parkomenko
BÆK*γ well established; BÆρ(ω)γ is the challenge!
„
Theory error:
8% for ρ0γ
15% for average
Standard Model expectation:
0
-6
+
-6
„ BÆρ γ/ωγ: ~ 0.5 x 10 ; BÆρ γ ~ 1 x 10
March 21, 2006
What can we learn from CPV?
G. Sciolla – M.I.T.
BÆργ MC
BÆργ/ωγ: analysis
„
π+
π−
In principle a simple analysis
„
„
„
„
e + e − → qq
Two body decay: pγ~mB/2
Exclusive meson reconstruction
0
+ −
„ ρ Æπ π
Pion identification is a must to
+
+ 0
„ ρ Æπ π
reject BÆ K*γ background
+
−
0
„ ωÆπ π π
Exclusively reconstruct B meson
„ mES and ∆E
γ
+ −
e e → bb
mES (GeV)/c2
BÆργ MC
B
0
But huge continuum background!
„
Eg: γ from π0 vs pions in opposite jets
„
NN for continuum suppression is key
„
„
„
B
0
Shape variables (e.g.: R2)
Properties of B decay (e.g.: ∆z)
“Tagging”-like variables (e.g.: pCMS of leptons)
March 21, 2006
What can we learn from CPV?
(GeV)/c
G. Sciolla∆E
– M.I.T.
2
Background Rejection Efficiency
NN Output and performance
Black: background
Red: signal MC
NN Output
March 21, 2006
What can we learn from CPV?
Rejects >98% Bgd
keeping ~50% Sig
Signal Efficiency
G. Sciolla – M.I.T.
SM expectation
1.0 x 10-6
0.5 x 10-6
0.5 x 10-6
Recent Results
„
Belle - Summer 2005: 370 fb-1
Combined BF
„
+0.34 +0.10
1.32-0.31
−0.09
5.1(5.4)
BaBar - Summer 2006: 316 fb
Combined BF
March 21, 2006
–1
6.3σ
What can we learn from CPV?
1.01 ± 0.21 ± 0.08
G. Sciolla – M.I.T.
UT constraints
„
From BF(ργ)/BF(K*γ) one can extract Vtd/Vts:
ICHEP 2006 BaBar only
In agreement with BS mixing results
March 21, 2006
What can we learn from CPV?
G. Sciolla – M.I.T.
Conclusion
„
Precise and redundant measurements of sides and angles
of UT allowed quantitative test of CKM mechanism
„
„
No New Physics?
„
„
CKM works beautifully!
If ΛNP~1 TeV Æ effects in CP ~ mW /ΛNP ~ 10%
„ Precision ~ 3% needed: more data coming…
Not seeing New Physics does mean something
„
„
CPV is just part of the puzzle
„ Many other NP studies at B factories: BÆτν, BÆsγ, BÆsll…
Constraints on New Physics models coming from B physics will help
interpret the discoveries at the LHC
March 21, 2006
What can we learn from CPV?
G. Sciolla – M.I.T.
What have we learned?
95% CL
UT in 2001
March 21, 2006
What can we learn from CPV?
G. Sciolla – M.I.T.