Transcript Document

Time-Dependent Particle-Antiparticle Asymmetries
in the Neutral B-Meson System
Michael D. Sokoloff
University of Cincinnati
The story of CP Violation has changed
qualitatively in the past two years.
Babar and BELLE have observed timedependent CP violation in neutral B-mesons,
in accord with the Standard Model.
B
B
0
The ensemble of these and other results
appear to validate the Kobayashi-Maskawa
mechanism as the source of CP violation in the
electroweak sector.
Michael D. Sokoloff
fCP
B
0
B
New Physics may yet be manifest in CP violation
measurements to come.
Seminar at NKU, 26 October 2004
0
1
0
fCP
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.
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Special Relativity
•
Energy and Momentum
– Energy and momentum form a four-vector (t,x,y,z). 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
QuickT i me™ and a
T IFF (Uncompressed) decompressor
are needed to see this pi cture.
•
•
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.
From the reconstructed
energy and momentum of the
candidate parent, we can
calculate 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 = hn ; p = hn/c
These quanta are called photons: g
m
In relativistic quantum field theory: A
g
To calculate cross-sections and decay rates we use perturbation theory
based on Feynman Diagrams:
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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
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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.
•
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Weak Charged Current Interactions
neutrino scattering
charm decay
l
~
l
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.
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Weak Phases in the Standard Model
b = f1; a = f2; g = f3
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Particle-Antiparticle Mixing
•
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.
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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.
•
B
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Elements of Macroscopic CP Violation
B
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Some Relevant Feynman Diagrams
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The PEP-II Storage Ring at SLAC
Total: 244 fb-1 (Jul 31st 04)
•
PEP-II is SLAC’s e+eB factory running at the (4S)
c.m. energy
•
The (4S) resonance decays to
charged and neutral B-anti-B
pairs
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BaBar Detector
All subsystems
crucial for CP
analysis
97% efficiency, 15 mm 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 %
SVT:
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Belle Detector
Aerogel Cherenkov cnt.
n=1.015~1.030
SC solenoid
1.5T
3.5GeV e+
CsI(Tl) 16X0
TOF counter
8GeV eTracking + dE/dx
small cell + He/C2H5
Si vtx. det.
3 lyr. DSSD
m / KL detection
14/15 lyr. RPC+Fe
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Experimental Technique at the (4S) Resonance
e+e-  (4S)  B B
m-
Boost: bg = 0.55
e-
-
K
Btag
(4S)
Flavor tag and
vertex
reconstruction
e+
B0
B
KS
0
Coherent L=1 state
t 
z
bg c
m-
Brec
z
Start the Clock
-
+
m
+

Exclusive B meson and vertex
reconstruction
Stop the Clock
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Identifying Fully Reconstructed B’s
(
)
For fits, both Belle and Babar characterize signals
and backgrounds with PDF’s which utilize Mbc, E,
tagging category, etc.
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Tagging Errors and Finite t Resolution
perfect
tagging & time resolution
Btag= B
Btag= B 0
0
(f+)


B0  D(*)-+/ r+/ a +
Ntagged=23618
Purity=84%
typical
mistagging & finite time
resolution
(f-)
1
fUnmixed
Mixed

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 - t / 

B
e

(t) = 
1 (1- 2w) cos(md t)  R
 4 B



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Effective Tagging Efficiency Q
Q=e(1-2w)2
Babar Tagging Performance
Category
r = estimated tagging dilution
Q (%)
e (%)
w (%)
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
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hep-ex/020825 v1
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sin2b Golden Sample: (cc)KS
and (cc)KL
85 x 106 BB evts
2938 events used to
measure sin2f1
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sin(2b) Fit Results
CP odd: sin 2f1 = 0.716  0.083
CP even: sin 2f1 = 0.78  0.17
|lf| = 0.948  0.051 (stat)  0.017 (sys)
Scss = sin(2f1 ) = 0.759  0.074 (stat)  0.032 (sys)
sin(2f1 ) = 0.719  0.074 (stat)  0.035 (sys)
asumming |lf| = 1 (hep-ex/020825, v1)
Summer 2002
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sin(2b) Fit Results
hf =+1
hf =-1
sin2b = 0.755  0.074
sin2b = 0.723  0.158
sin2b = 0.741  0.067 (stat)  0.034 (sys) with |lf| = 1
|lf| = 0.948  0.051 (stat)  0.017 (syst)
hf =-1
Sf = 0.759  0.074 (stat)  0.032 (syst)
}
Summer 2002
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Golden modes with a lepton tag
The best of the best!
Ntagged = 220
Purity = 98%
Mistag fraction 3.3%
t 20% better than
background
other tag
categories
Consistent results
across mode,
data sample,
tagging category
sin2b = 0.79  0.11
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Standard Model Comparison
One solution for b is in
excellent agreement
with measurements of
unitarity triangle apex
r = r (1-l2/2)
h = h (1-l2/2)
Method as in Höcker et al,
Eur. Phys.J.C21:225-259,2001
sin2bb == 0.741
0.722  0.067
0.040 (stat)
(stat)  0.034
0.023 (sys)
(sys)
sin2
sin2
sin2ff11== 0.719
0.728  0.074
0.056 (stat)
(stat)  0.035
0.023 (sys)
(sys)
Nir@ICHEP2002: Im(lyK) = 0.725
0.734  0.037
0.054
HFAG@ICHEP2004:
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sin2b from the Penguin Decay b  sss
2.4 from s-penguin
to sin2b (cc)
2.7 from s-penguin
to sin2b (cc)
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B   to Measure sin2aeff
With Penguins (P):
No Penguins (Tree only):
l
Vtb*Vtd Vud* Vub
=
VtbVtd* V V *
ud ub
mixing
decay
l = e
Seminar at NKU, 26 October 2004
1+ P /T ei e - ig
C   sin( )
2 ia
l = e
C  = 0
S  = sin(2a )
i ig
2 ia 1+ P /T e e
2
S  = 1 - C 
sin(2aeff )

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B   CP Asymmetry Results
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B   CP Asymmetry Results
PRL 93, 021601 (2004)
152M BB pairs
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Time-Dependent CP Violation in B-Decays
A Summary
Babar and BELLE have observed timedependent CP violation in neutral B-mesons,
in accord with the Standard Model.
HFAG@ICHEP2004: Im(lyK) = 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.
Seminar at NKU, 26 October 2004
Michael D. Sokoloff
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