1Michael D. Sokoloff
Time-Dependent Particle-Antiparticle Asymmetries in the Neutral B-Meson System
Michael D. SokoloffUniversity of Cincinnati
The story of CP Violation has changed qualitatively in the past two years.
Babar and BELLE have observed time-dependent CP violation in neutral B-mesons, in accord with the Standard Model.
The ensemble of these and other results appear to validate the Kobayashi-Maskawa mechanism as the source of CP violation in the electroweak sector.
New Physics may yet be manifest in CP violation measurements to come.
B0B 0fCP
B0
B 0 fCP
2Michael D. Sokoloff
The Nature of Particle Physics
• Particle physicists study the fundamental constituents of matter and their interactions.
• Our understanding of these issues is built upon certain fundmental principles– The laws of physics are the same everywhere– The laws of physics are the same at all times– The laws of physics are the same in all inertial reference
systems (the special theory of relativity)– The laws of physics should describe how the wave function
of a system evolves in time (quantum mechanics)
• These principles do not tell us what types of fundamental particles exist, or how they interact, but they restrict the types of theories that are allowed by Nature.
• In the past 30 years we have developed a Standard Model of particle phyiscs to describe the electromagnetic, weak nuclear, and strong nuclear interactions of constituents in terms of quantum field theories.
3Michael D. Sokoloff
Special Relativity• Energy and Momentum
– Energy and momentum form a four-vector (E,px,py,pz). The Lorentz invariant quantity defined by energy and momentum is mass:
– For the special case when an object is at rest so that its momentum is zero
• When a particle decays in the laboratory, we can measure the energy and momenta of it decay products (its daughter particles), albeit imperfectly.
• The energy of the parent is exactly the sum of the energies of its daughters. Similarly, each component of the parent’s momentum is the sum of the corresponding components of the daughters’ momenta.
QuickTime™ and aTIFF (Uncompressed) decompressor
are needed to see this picture.
From the reconstructedenergy and momentum of thecandidate parent, we cancalculate its invariant mass.
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Classical Field Theory (E&M)
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Fields and Quanta
• Electromagnetic fields transfer energy and momentum from one charged particle to another.
• Electromagnetic energy/momentum is quantized:– E = hp = hc
• These quanta are called photons: • In relativistic quantum field theory: A • To calculate cross-sections and decay rates we use
perturbation theory based on Feynman Diagrams:
6Michael D. Sokoloff
Strong Nuclear Interactions of Quarks and Gluons
Each quark carries one of three strong charges, and each antiquark carries an anticharge. For convenience, we call these colors:
Just as photons are the quanta of EM fields, gluons are the quanta of strong nuclear fields; however, while photons are electrically neutral, gluons carry color-anticolor quantum numbers.
The Nobel Prize in Physics 2004
Gross Politzer Wilczek
7Michael D. Sokoloff
Baryons and Mesons
• Quarks are never observed as free particles.– Baryons consist of three quarks, each with a
different color (strong nuclear) charge proton = neutron =– Mesons consist of quark-antiquark pairs with
canceling color-anticolor charges
• Baryons and meson (collectively known as hadrons) have net color charge zero.
• A Van der Waals-types of strong interaction creates an attractive force which extends a short distance (~ 1 fm) to bind nucleii together.
8Michael D. Sokoloff
Weak Charged Current Interactions
neutrino scattering charm decay
As a first approximation, the weak charged current interaction couples fermions of the same generation. The Standard Model explain couplings between quark generations in terms of the Cabibbo-Kobayashi-Maskawa (CKM) matirx.
f
f
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Weak Phases in the Standard Model
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• A second order weak charged current process, a box diagram amplitude, provides a mechanism by which particles oscillate into antiparticles.
• Particles decay exponentially with characteristic times
• Neutral B-mesons mix sinusoidally with characteristic times
• Experimentally
which makes mixing observation relatively easy.
Particle-Antiparticle Mixing
11Michael D. Sokoloff
Time-Dependent CP Violation
• Both particles and antiparticles can decay to common final states which are CP eigenstates. As an example,
• The final state is invariant under charge and parity conjugation; that is, it remains .
• The Standard Model predicts that the CKM phase will produce a time dependent asymmetry in the decays of and to this final state, and that the asymmetry will vary sinusoidally.
B0B 0
fCPB0
B 0 fCP
12Michael D. Sokoloff
Elements of Macroscopic CP Violation
B0B 0
fCP
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Some Relevant Feynman Diagrams
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Elements of Macroscopic CP Violation
B0B 0
fCP
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The PEP-II Storage Ring at SLAC
• PEP-II is SLAC’s ee
B factory running at the (4S) c.m. energy
• The (4S) resonance decays to charged and neutral B-anti-B pairs
Total: 244 fb-1 (Jul 31st 04)
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BaBar Detector
SVT: 97% efficiency, 15 m z hit resolution (inner layers, transverse 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 %
All subsystems crucial for CP analysis
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/ KL detection 14/15 lyr. RPC+Fe
Tracking + dE/dx small cell + He/C2H5
CsI(Tl) 16X0
Aerogel Cherenkov cnt. n=1.015~1.030
Si vtx. det. 3 lyr. DSSD
TOF counter
SC solenoid1.5T
Belle DetectorBelle Detector
8GeV e
3.5GeV e
18Michael D. Sokoloff
+e-e
Boost: = 0.55
Start the Clock
Coherent L=1 state
B0
B0
Experimental Technique at the (4S) Resonance
4S
e+e- (4S) B B
Exclusive B meson and vertex reconstruction
Exclusive B meson and vertex reconstruction
zΔ
€
Δt ≈Δzβγ c
Btag
Brec
K-
Flavor tag and vertex
reconstruction
Flavor tag and vertex
reconstruction
Stop the Clock
KS
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Identifying Fully Reconstructed B’s
For fits, both Belle and Babar characterize signals and backgrounds with PDF’s which utilize M
bc, ΔE, tagging category, etc.
( )
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Tagging Errors and Finite Δt Resolution
€
Btag= B0
perfect tagging & time
resolution
€
Btag= B 0
(f-)(f+)
B0 D(*)-+/ +/ a1+
Ntagged=23618Purity=84%
€
fUnmixedMixed
(Δt) =e−Δt /τ
B
4τ B
1± (1− 2w) cos(ΔmdΔt)[ ]
⎧
⎨ ⎪
⎩ ⎪
⎫
⎬ ⎪
⎭ ⎪⊗R
typical mistagging & finite
time resolution
21Michael D. Sokoloff
Effective Tagging Efficiency QQ=(1-2w)2
Babar Tagging Performance
Category
w Q (%)
Lepton 9.0.
3.3 0.6
7.9 0.3
Kaon I 16.7 0.2 9.9 0.7
10.7 0.4
Kaon II 19.8 0.3 20.9 0.8
6.7 0.4
Inclusive
20.0 0.3 31.6 0.9
2.7 0.3
Total 65.6 0.5 28.1 0.7
r = estimated tagging dilution
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hep-ex/020825 v1Summer 2002
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sin2 Golden Sample: (cc)KS and (cc)KL
85 x 106 BB evts
2938 events used to
measure sin21Summer
2002
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|f| = 0.948 0.051 (stat) 0.017 (sys) Scss = sin(2
1 ) = 0.759 0.074 (stat) 0.032
(sys)
sin(21 ) = 0.719 0.074 (stat) 0.035 (sys)
asumming |f| = 1 (hep-ex/020825, v1)
CP odd: sin 21 = 0.716 0.083
CP even: sin 21 = 0.78 0.17
sin(2) Fit Results
Summer 2002
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sin2 = 0.723 0.158 sin2 = 0.755 0.074
sin(2) Fit Results
f =-1 f =+1
sin2 = 0.741 0.067 (stat) 0.034 (sys) with
Summer 2002
|f| = 1
|f| = 0.948 0.051 (stat) 0.017 (syst) Sf = 0.759 0.074 (stat) 0.032 (syst)
}f =-1
25Michael D. Sokoloff
The best of the best!
Ntagged = 220Purity = 98%Mistag fraction
3.3%
Δt 20% better than other tag categories
sin2 = 0.79 0.11
Golden modes with a lepton tag
Consistent results across mode, data sample, tagging category
background
Summer 2002
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Standard Model Comparison
One solution for is in excellent
agreement with measurements of unitarity triangle
apex
Method as in Höcker et al, Eur. Phys.J.C21:225-259,2001
= (1-2/2)
= (1-2/2)
Nir@ICHEP2002: Im(K) = 0.734 0.054
sin2 = 0.741 0.067 (stat) 0.034 (sys)sin21= 0.719 0.074 (stat) 0.035 (sys)
sin2 = 0.722 0.040 (stat) 0.023 (sys)
sin21= 0.728 0.056 (stat) 0.023 (sys)
HFAG@ICHEP2004: Im(K) = 0.725 0.037
27Michael D. Sokoloff
2.7 from s-penguin to sin2 (cc)
2.4 from s-penguin to sin2 (cc)
sin2 from the Penguin Decay b sss
28Michael D. Sokoloff
B to Measure sin2eff
mixing
*
*
ubud
ubud
VV
VV= *
*
tdtb
tdtb
VV
VV
decay
No Penguins (Tree only):
€
=e i
C = 0S = (sin )
With Penguins (P):
€
=e i + P /T eiδ ei
+ P /T eiδ e−i
C ∝ (sin δ )
S = −C (sin eff )
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B CP Asymmetry Results
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B CP Asymmetry Results
PRL 93, 021601 (2004)
152M BB pairs
31Michael D. Sokoloff
Time-Dependent CP Violation in B-DecaysA January 2005 Summary
Babar and BELLE have observed time-dependent CP violation in neutral B-mesons, in accord with the Standard Model.
HFAG@ICHEP2004: Im(K) = 0.725
0.037
The ensemble of these and other results appear to validate the Kobayashi-Maskawa mechanism as the source of CP violation in the electroweak sector.
New Physics may yet be manifest in CP violation measurements to come. Lots of experimental work is being done. Several “> 2.5””” effects are stimulating theoretical work.