B Physics and CP Violation

1
B Physics and CP Violation

• Jeffrey D. Richman
• UC Santa Barbara
• CTEQ Summer School
• Madison, June 7-8, 2002

2
Outline (Lecture 1)

• Overview of B decays
• Why B physics is interesting overview of decay
  diagrams introductory discussion of CP
  violation.
• Accelerators and b-quark production
• The BaBar Detector
• Identifying B decays
• B-meson lifetimes and mixing
• CP Violation (CPv) and the CKM matrix
• the CKM hierarchy and the prediction of large CP
  asymmetries in B decays

3
Outline (Lecture 2)

• CP Asymmetries
• sin(2b) the golden measurement
• the struggle for the other angles
• Rare decays
• Penguins are everywhere!
• Semileptonic decays, decay dynamics, and the
  magnitudes of CKM elements.
• Heavy-quark symmetry and Vcb
• Prospects and future directions
• A reference J. Richman, Les Houches lectures,
  1997.
• http//hep.ucsb.edu/papers/driver_houches12.ps
• (or send e-mail asking for a copy
  richman_at_charm.physics.ucsb.edu)

4
Remarks/disclaimers

• I will be unashamedly pedagogical, and I will not
  aim for the level of impartiality that is
  customary in a review talk or article.
• I will be unashamedly selective many important
  topics have been left out.
• There will be a strong bias towards recent
  results from ee- colliders at the Y(4S). This is
  probably not too misleading for now, since BaBar,
  Belle, and CLEO have to some extent defined the
  state of the art, especially in CPv and rare
  decays. However, soon-to-come measurements from
  the Fermilab Tevatron (CDF, D0) will be of major
  importance.
• My own background in b physics BaBar, CLEO
• I strongly encourage you to ask questions!

5
Goals of B (and Bs) Physics

1. Can CP violation be understood quantitatively
   within the Standard Model, or is new physics
   needed? Perform a comprehensive set of
   measurements to check for the presence non-SM
   CP-violating phases.
2. Make precise measurements of the Standard Model
   CKM parameters Vcb , Vub , Vtd , Vts ,
   a, b, g,...
3. Map out and understand rare B decays, especially
   processes with loops that can be very sensitive
   to particles outside the Standard Model.
4. Understand the dynamics of B decays underlying
   weak interaction process with overlay of complex
   strong interaction effects. Progress HQET,
   lattice QCD, many measurements to test
   predictions.

6
Overview of B Decays

• b is the heaviest quark that forms bound states
  with other quarks (t-quark decays too rapidly).
• m(b)ltm(t) gt the b-quark is forced to decay
  outside of its own generation
• Dominant decays are CKM suppressed
• Relatively long B lifetime
• Silicon tracking systems have been essential
  tools.
• Largest single branching fraction
• Many interesting rare decay processes are
  experimentally accessible (b-gtuW, gluonic
  penguins, electroweak penguins).

7
Leptonic and Semileptonic Decays

• Leptonic B decay not yet observed!
• Largest expected mode is
• Ignoring photon radiation

• Used to measure magnitudes of CKM elements Vcb
  and Vub
• Amplitude can be rigorously parametrized in terms
  of form factors.

8
Hadronic Decays Tree Diagrams

• Theoretical predictions very difficult.
• Naïve factorization model works reasonably well
  in predicting pattern of decays.

• Color suppressed
• Naïve factorization model probably breaks down.
  (New data on B-gtD0p0 and B-gtD0p0.)

• The color allowed and color suppressed amplitudes
  interfere constructively in charged B decays.
  (Opp. effect for D.)

9
Processes with loops sensitivity to new particles

• Both gluonic and electoweak penguins have been
  observed!
• The SM mixing rate is dominated by tt (off-shell)
  intermediate states.

10
Processes used for sin2b measurement
A color suppressed decay! However, in this case,
the rate is enhanced by the relatively large
decay constant of the J/y
11
Decay modes for sin2b measurement
12
The C, P, and T Transformations

• C, P, and T are discrete transformations there
  is no continuously varying parameter, and these
  operations cannot be constructed from successive
  infinitesimal transformations.
• In all well-behaved quantum field theories, CPT
  is conserved. A particle and its antiparticle
  must have equal mass and mean lifetime.

13
P and C violation in Weak Interactionsis Maximal
(V-A)
Allowed
Not Allowed
Allowed
P
C
14
A First Look at CP violation

• The discovery of CP violation in 1964 was based
  on the demonstration that the mass eigenstate KL
  is not an eigenstate of CP, so
  .
• The lifetime separation between BH and BL is
  tiny, so we must use a different method, in which
  we compare the rates for CP-conjugate processes.

Remove Ks from beam using lifetime difference.
CPv small in kaon system!
15
The Legacy of Kaon Physics
...the effect is telling us that at some
tiny level there is a fundamental asymmetry
between matter and antimatter, and it is telling
us that at some tiny level interactions will show
an asymmetry under the reversal of time. We know
that improvements in detector technology and
quality of accelerators will permit even more
sensitive experiments in coming decades. We are
hopeful then, that at some epoch, perhaps
distant, this cryptic message from nature will be
deciphered. ...J.W. Cronin, Nobel Prize
lecture. J.W. Cronin and V.L. Fitch, Nobel
Prize 1980. J.W. Cronin, Rev. Mod. Phys. 53, 373
(1981). J.H. Christenson, J.W. Cronin, V.L.
Fitch, and R. Turlay, Phys. Rev. Lett. 13,
138 (1964).
16
CP violation and alien civilizations

• We can use our knowledge of CP violation to
  determine whether alien civilizations are made of
  matter or antimatter, without having to touch
  them.

We have these inside of us
Long-lived neutral kaon
17
CP Violation and Cosmology

• A. Sakharov noted (1967) that CP violation has an
  important connection to cosmology.
• 3 conditions for an asymmetry between N(baryons)
  and N(anti-baryons) in the universe (assuming
  equal numbers initially due to thermal
  equilibrium).
• baryon-number-violating process
• both C and CP violation (helicities not relevant
  to particle populations)
• departure from thermal equilibrium

18
How can CP asymmetries arise? (I)

• When we talk about CP violation, we need to talk
  about the phases of QM amplitudes.
• This is usually very confusing.
• some phases are physical others are not.
• many treatments invoke specific phase
  conventions, which acquire a magical aura.
• Need to consider two types of phases
• CP-conserving phases dont change sign under CP.
  (Sometimes called strong phases since they can
  arise from strong, final-state interactions.)
• CP-violating phases these do change sign under
  CP.

19
How can CP asymmetries arise? (II)

• Suppose a decay can occur through two different
  processes, with amplitudes A1 and A2.
• First, consider the case in which there is a
  (relative) CP-violating phase between A1 and A2
  only.

No CP asymmetry! (Decay rate is different from
what is would be without the phase.)
20
How can CP asymmetries arise? (III)

• Next, introduce a CP-conserving phase in addition
  to the CP-violating phase.
• Now have a CP asymmetry

21
Measuring a CP-violating phase

• To extract the CP-violating phase from an
  observed CP asymmetry, we need to know the value
  of the CP-conserving phase.
• In direct CP-violating processes we usually do
  not know the relative CP-conserving phase because
  it is produced by strong-interaction dynamics
  that we do not understand.

22
B production at the Y(4S)
No accompanying pions! The B-meson energy is
known from the beam energy.
Rate of events vs. total energy in ee- CM frame
TM
(CLEO, CLNS 02/1775)
23
The New ee- B factories

• The machines have unequal (asymmetric) energy
  e and e- beams, so two separate storage rings
  are required.
• PEP-II E(e-)8.992 GeV E(e)3.120 GeV
  bg0.55
• The machines must bring the beams from the
  separate rings into collision.
• KEK-B -11 mrad crossing angle
• PEP-II magnetic separation
• With two separate rings, the machines can store
  huge numbers of beam bunches without parasitic
  collisions.
• KEK-B 1224 bunches/beam I(e)716 mA I(e-)895
  mA
• PEP-II 831 bunches/beam I(e)418 mA I(e-)688
  mA
• CESR (single ring) 36 bunches/beam
  I(e)I(e-)365 mA

24
PEP-II ee- Ring and BaBar Detector
LER (e, 3.1 GeV)
Linac
HER (e-, 9.0 GeV)
BaBar
BaBar
PEP-II ring C2.2 km
May 26, 1999 1st events recorded by BaBar
25
The Y(4S) Boost

• The purpose of asymmetric beam energies is to
  boost the B0B0 system relative to the lab frame.
• By measuring Dz, we can follow time-dependent
  effects in B decays.
• The distance scale is much smaller than in the
  kaon decay experiments that first discovered CP
  violation!

26
From CESR (1 ring, E symmetric) toPEP-II (2
rings, E asymmetric)
Top view of PEP-II interaction region showing
beam trajectories.
Pretzel orbits in CESR (36 bunches, 20 mm
excursions)
(10X expansion of vertical scale)
27
The race between BaBar/PEP-II and Belle/KEK-B
Belle
Exceeds design luminosity!
28
ee- vs. pp and pp

• Production cross sections
• Y(4S)
• pp at Tevatron
• pp at LHC
• b fraction (ratio of b cross section to total
  hadronic cross section)
• Y(4S) 0.25
• pp at Tevatron 0.002
• pp at LHC 0.0063
• Comments
• Triggering so far, most B branching fractions
  have been measured at ee- machines, because
  CDF, D0 triggers were very selective in Run 1.
  Also, PID g detection are better at Y(4S)
  experiments so far.)
• Hadron colliders produce Bs and b-baryons. (LEP
  also.)
• New displaced-vertex triggers at hadron-collider
  experiments should make a dramatic improvement.

29
The BABAR Detector
1.5 T solenoid
DIRC (particle ID)
CsI (Tl) Electromagnetic Calorimeter
e (3.1GeV)
Drift Chamber
Instrumented Flux Return
e- (9 GeV)
Silicon Vertex Tracker

• SVT 97 efficiency, 15mm z resol. (inner layers,
  perpendicular tracks)
• Tracking s(pT)/pT 0.13 PT ? 0.45
• DIRC K-p separation gt3.4s for Plt3.5GeV/c
• EMC ?E/E 1.33 E-1/4 ? 2.1

30
BaBar Detector
center line
DIRC quartz bars standoff box PM tubes
Superconducting magnet (1.5 T)
Drift chamber
e-
e
CsI crystals
Muon detector B-flux return
Silicon Vertex Tracker
31
BaBar Event Display(view normal to beams)
EM Calorimeter 6580 CsI(Tl) crystals (5 g
energy res.)
Cerenkov ring imaging detectors 144 quartz bars
(measure velocity)
Tracking volume B1.5 T
Rdrift chamber80.9 cm
Silicon Vertex Tracker 5 layers 15-30 mm res.
(40 measurement points, each with 100-200 mm res.
on charged tracks)
32
Innermost Detector Subsystem Silicon Vertex
Tracker
Installed SVT Modules
Be beam pipe R2.79 cm
(B mesons move 0.25 mm along beam direction.)
33
BaBar Silicon Vertex Tracker

• 5 layers of double-sided silicon-strip detectors
  (340)

80 e-/hole pairs/mm
34
Particle Identification (DIRC)(Detector of
Internally Reflected Cherenkov Light)

• Measure angle of Cherenkov cone
• Transmitted by internal reflection
• Detected by PMTs

No. light bounces (typical)300
35
Particle Identification with the DIRC.

• DIRC ?c resolution and K-? separation measured in
  data ? D? D0? ? (K-?)? decays

gt9s
s(qc) ? 2.2 mrad
K/p Separation
2.5s
36
Particle Identification
E/p from E.M.Calorimeter
Shower Shape
0.8 lt p lt 1.2 GeV/c E/p gt 0.5
1 lt p lt 2 GeV/c

• Electrons p gt 0.5 GeV
• shower shapes in EMC
• E/p match
• Muons p gt 1 GeV
• Penetration in iron of IFR
• Kaons
• dE/dx in SVT, DCH
• ??C in DRC

e
e
p
p
qc from Cerenkov Detector
dE/dx from Dch
0.8 lt p lt 1.2 GeV/c
0.5 lt p lt 0.55 GeV/c
e
e
p
e
p
p
37
Identifying B Decays in BaBar

• Select candidate daughter particles using
  particle ID, etc.
• Compute the total 4-momentum (E, p)(E1E2E3,
  p1 p2 p3)
• Compute invariant mass m2E2-p2

?mes ? 3 MeV s DE ? 15 MeV
Gives 10x improvement in mass resolution.
All Ks CP modes Nsig ? 750 Purity 95
DE
mes
38
sin2b Signal and Control Samples
J/Y Ks (Ks ? pp-)
Bflav mixing sample
J/Y Ks (Ks ? pp-)
CP-1
Y(2s) Ks
J/Y Ks (Ks ? p0p0)
J/Y KL
J/Y Ks (Ks ? p0p0)
CP1
J/Y K0 (K0 ? Ksp0)
?c1 Ks
J/Y K0 (K0 ? Ksp0)
39
The Lorentz Boost

• The asymmetric beam energies of PEP-II allow us
  to measure quantities that depend on decay time.

e-
e
9.0 GeV
3.1 GeV
40
Measurement of Decay Time Distributions
B0 decay time distribution
(linear scale)
background
41

• B0 and anti-B0 mesons spontaneously oscillate
  into one another! (Mixing also occurs with
  neutral kaons.)
• Neutral B mesons can be regarded as a coupled,
  two-state system.
• To find the mass eigenstates we must find the
  linear combinations of these states that
  diagonalize the effective Hamiltonian.

42
Interpretation of the Effective Hamiltonian

• The effective Hamiltonian for the two-state
  system is not Hermitian since the mesons decay.

Quark masses, strong, and EM interactions
Decays
43
CP Violation in Mixing

• Compare mixing for particle and antiparticle

off-shell
off-shell
on-shell
on-shell
CP-conserving phase
44
CP violation in mixing, continued

• To produce a CP asymmetry in mixing, M12 and G12
  must not be collinear and both must be nonzero

No CP violation in mixing
CP violation in mixing
45
Time evolution of states that are initially
flavor eigenstates
General case allows CP violation.
46
CP Violation in B Mixing is Small

• When CP violation in mixing is absent (or very
  small), we have
• In the neutral B-meson system, the states that
  both B0 and B0 can decay into have small
  branching fractions, since
• normally lead to different final states. Can
  have (Cabibbo suppressed) and
  (b-gtu is CKM suppressed). So the SM
  predicts

not yet observed
47
Time evolution of states that are initially
flavor eigenstates
In these formulas, we have assumed that
DG/Gltlt1 and have set
48
The Oscillation Frequency (Dm)

• In the neutral B-meson system, the mixing
  amplitude is completely dominated by off-shell
  intermediate states (Dm) contrast with the
  neutral kaon system.
• Calculation of the mixing frequency
• Time-dependent mixing probabilities and asymmetry

49
Tagging
CP asymmetry is between B0 ? fcp and B0 ? fcp
Must tag flavor at Dt0 (when flavor of two Bs
is opposite). Use decay products of other (tag) B.
Leptons Cleanest tag. Correct 91
Kaons Second best. Correct 82
W
W-
c
c
s
K-
K
s
b
b
W-
u
u
W
d
d
50
Effect of Mistagging and Dt Resolution
wProb. for wrong tag
No mistagging and perfect Dt
D1-2w0.5
Nomix
Mix
Dt
Dt
D1-2w0.5
Dt res 99 at 1 ps 1 at 8 ps
Dt
Dt
51

• Measure mixing on control sample
• constrain model of Dt resolution
• measure dilution D (1-2w)

T2p/Dm
NoMix(t)-Mix(t) NoMix(t)Mix(t)
D
Dm (0.516 ? 0.016 ? 0.010) ps-1
52
CP violation in the Standard Model

• In the SM, the couplings of quarks to the W are
  universal up to factors that are elements of a
  unitary, 3x3 rotation matrix Vij of the quark
  fields. This matrix originates in the Higgs
  sector (mass generation of quarks).

W-
W-
ne
e-
u
b
W
53
The Standard Model Unitarity Triangle
Cabibbo-Kobayashi-Maskawa (CKM) matrix
Col 1Col 30
1 of 6 equal-area triangles orientation is just
an unphysical phase
Weak interaction eigenstates
Quark mass eigenstates
V has only 4 real parameters, including 1
CP-violating phase.
CPv
If just 2 quark generations no CP phase
allowed!
54
The Structure of the CKM Matrix
The CKM matrix exhibits a simple, hierarchical
structure (which we do not understand) with 4
real parameters.
0.04
(All unitarity triangles have same area,
corresponding to the sizes of interference terms
between 1st order weak amps. But we care about
CP asymmetries, so the angles of the triangles
also matter.)
55
End of Lecture 1
56
Outline (Lecture 2)

• CP Asymmetries
• sin(2b) the golden measurement
• the struggle for the other angles
• Rare decays
• Penguins are everywhere!
• Semileptonic decays, decay dynamics, and the
  magnitudes of CKM elements.
• Heavy-quark symmetry and Vcb
• Prospects and future directions
• A reference J. Richman, Les Houches lectures,
  1997.
• http//hep.ucsb.edu/papers/driver_houches12.ps
• (or send e-mail asking for a copy
  richman_at_charm.physics.ucsb.edu)

57
Decay rates for B0(t) and B0 (t) to fCP
58
Calculating the CP Asymmetry
If there is just one direct decay amplitude, we
will see that
If CP violation is due to interference between
mixing and one direct decay amp pure sin(Dm t)
time dependence.
59
Calculating l

• Piece from mixing (a)

• Piece from decay

if just one direct decay amplitude to fCP
Hadronic physics divides out!
60
Calculating l for specific final states
61
Why it is magic
CP violating phase
CP conserving phase!
62
asdf
Graphical Analysis
63
Analogy Double-Slit Experiments with Matter
and Antimatter
source
In the double-slit experiment, there are two
paths to the same point on the screen. In
the B experiment, we must choose final states
that both a B0 and a B0 can decay into. We
perform the B experiment twice (starting from B0
and from B0). We then compare the results.
64
CP violation due to interference between mixing
and decay non-exponential decay law
65
Ingredients of the CP Asymmetry Measurement
Determine initial state tag using other B.
Measure Dt dependence
Reconstruct the final state system.
66
The Lorentz Boost

• The asymmetric beam energies of PEP-II allow us
  to measure quantities that depend on decay time.

e-
e
9.0 GeV
3.1 GeV
67
Tagging
We must classify each neutral B according to
whether it started as a B0 or a B0. The start
time is defined as the decay time of the
accompanying B meson (tag B). We use
flavor-specific final states of the tag B.
Leptons Cleanest tag. Correct 91,
Efficiency 11
Kaons Second cleanest. Correct 82,
Efficiency 35
W
W-
c
c
s
K-
s
K
b
b
W-
u
u
W
d
d
68
The Correlated State

• At the Y(4S), the two neutral B mesons evolve as
  a correlated quantum state until one of them
  decays.
• As a consequence, the asymmetry of
  time-integrated rates is identically zero!
• At the Y(4S), we must measure the CP asymmetry as
  a function of time. The experiment would not work
  with the silicon vertex detector.

69
Experimental aspects of the sin2b measurement
Acp(Dt)
F(Dt)
F(Dt)
sin2b
Everything perfect ?
D sin2b
Add tag mistakes ? Dilution D1-2w
Add imperfect Dt resolution
?
Dt(ps)
Dt(ps)
Must understand tagging/mistagging and Dt
resolution !!
70
Blind Analysis

• The whole analysis is performed blind.
• All studies are performed in such a way as
• to hide information on the value of the
• final answer.
• Avoids any subconscious experimenter bias
• e.g. agreement with the Standard Model!

When we are ready, we have an unblinding party..
71
Fit results
sin2b (cc) Ks CP -1 0.76 ? 0.10 ? 0.04 J/Y
KL CP 1 0.73 ? 0.19 ? 0.07 All modes 0.75 ?
0.09 ? 0.04
(stat)
(syst)
56 fb-1 62 M BB pairs.
72
CP asymmetry in CP -1 and 1 modes
J/Y KL CP 1
Note likelihood curves are normalized to the
total number of tagged events, not B0 and anti-B0
separately.
73
Crosscheck fit Bflav events as a CP sample
Expect no CP asymmetry
ACP -0.004 ? 0.027
74
sin2b fit results
Systematic errors CP -1 background 0.019 Dt
resolution and detector effects 0.015 Dmd and tB
(PDG 2000) 0.014 Monte Carlo statistics 0.014 J/Y
KL background 0.013 Signal mistag
fractions 0.007 Total systematic error 0.04
sin2b (cc) Ks CP -1 0.76 ? 0.10 ? 0.04 J/Y
KL CP 1 0.73 ? 0.19 ? 0.07 All modes 0.75 ?
0.09 ? 0.04
Fit without l1 constraint (CP-1 only) l
0.92 ? 0.06 (stat) ? 0.03 (syst) Iml/l 0.76 ?
0.10
(stat)
(syst)
75
Cross checks
sin2b by decay mode
sin2b in sub-samples
Individual modes and sub-samples are all
consistent.
76
CKM interpretation
h
Our sin2b measurement is consistent with current
Standard Model constraints from measurements of
other parameters.
Method as in Höcker et al, Eur.Phys.J.C21225-25
9,2001 (also other recent global CKM matrix
analyses)
r
77
Michael Peskins viewpoint
78
Conclusions so far...

• We have observed CP violation in the neutral-B
  meson system.
• The asymmetry is large, unlike the O(10-3)
  effects observed in the neutral-K system.
• The asymmetry displays consistent behavior across
  all observed channels, including CP odd and CP
  even final states.
• The time dependence of the asymmetry agrees with
  the expectation based on interfering amplitudes
  involving mixing and direct decay.

79
Conclusions so far...

• With the present data sample, the region allowed
  by the measurement is consistent with the
  Standard Model CKM framework constrained by
• CP-violation measurements in K decay
• non-CP-violating observables in B decay

80
Hadronic Rare B Decays Towards sin(2a)

• B-gtpp would measure sin(2a)
• except there is a second direct decay amplitude!

81
Hadronic Rare B Decays B-gtpp-, B-gtKp-
mES
DE
B-gtpp-
mES
DE
B-gtKp-
82
Mixing and CP Asymmetry Measurement in B-gtpp
Mixing
83
Belle Mixing and Asymmetry Measurement in B-gtpp
84
B ? K()ll- in the SM and Beyond

• Flavor changing neutral current (b to s)
  proceeds via penguin or box diagrams in the
  SM.
• New physics at the EW scale (SUSY, technicolor,
  4th generation quarks, etc.) can compete with
  small SM rate.
• Complementary to studying b to s g due to
  presence of W and Z diagrams.

85
Branching Fraction Predictions in the Standard
Model
New Ali et al. predictions lower by 30-40
long-distance contribution from y resonances
excluded
86
Decay rate vs. q2 in the SM and SUSY
J/yK
Pole from Kg, even in mm-
y(2S)K
SUSY models
SM nonres
SM nonres
q2
q2
constructive interf.
destructive
87
Generator-level q2 Distributions from Form-Factor
Models
Ali et al. 2000 (solid line)
Colangelo 1999 (dashed line)
Melikhov 1997 (dotted line)
Shapes are very similar!
88
J/y and Large Sideband Control Sample Study B
Likelihood Variable
J/y Sample signal-like
Large SB Sample background-like
log LB
-10
4
log LB
off resonance
89
Kll- Fit Regions, Unblinded Run 12 data (56.4
fb-1)
DE
mES
90
Fit Results (preliminary)
B(B?Kee)/B(B?Kmm)1.21 from Ali, et al, is used
in combined Kll fit.
91
Belle results (29.1 fb-1)
Bkgd shape fixed from MC
92
Results

• We obtain the following preliminary results

• The statistical significance for B ? K ll- is
  computed to be gt 4s including systematic
  uncertainties.

BaBar and Belle results are both higher than
typical theoretical predictions, but the
uncertainties are still very large.
93
Measuring Magnitudes of CKM Elements with
Semileptonic B Decays
Expt.
Need input from theory!
Expt.
94
Kinematic Configurations in Semileptonic Decay

• b-gtcln processes are dominant and are much easier
  to understand than b-gtuln decays.
• reliable theoretical predictions for b-gtcln at
  zero recoil (Heavy Quark Symmetry/HQET).
• zero recoil b-gtc without disturbing the light
  degrees of freedom
• expansion in LQCD/mQ

zero recoil
95
Semileptonic decays Dalitz plot

• Effect of V-A coupling on lepton angular
  distribution and energy spectrum.

zero recoil
96
Contributions of different helicities to the rate
Max recoil
Zero recoil
97
New CLEO measurement of Vcb
98
CLEO Measurement of Vcb w distribution and
extrapolation to zero recoil
99
Systematic Errors on CLEO Vcb Measurement
100
Recent Vcb measurements

• Uncorrected for common inputs

• Corrected for common inputs

(Compilation by Artuso and Barberio,
hep-ph/0205163, May 2002.)
101
Recent Vcb measurements
102
Form Factor at Zero Recoil and Vcb

• The experimental extrapolation to zero recoil
  velocity of the daughter hadron provides the
  quantity
• Zero recoil form factor (consensus value)
• World average Vcb

103
Bumps in the road Crystal Ball observation of
the z(8.3) (1984)
Photon energy spectrum.
104
First observation of exclusive B decay

• CLEO I data (1983)

105
Some free advice

• Almost every measurement is very hard, even if it
  is of a quantity that no one cares about. So, try
  to find an important measurement that will have
  real scientific impact.
• Never determine your event-selection criteria
  using the same event sample that you will use to
  measure your signal.
• Dont use more cuts than you need. A simple
  analysis is easier to understand, check,
  duplicate, and present.
• Look at all the distributions you can think of
  for your signal and compare them with what you
  expect.
• Look at the distributions of events that you
  exclude. Do you understand the properties of your
  background?

106
More free advice

• When possible, use data rather than Monte Carlo
  events to measure efficiencies and background
  levels.
• Do not use Monte Carlo samples blindly. Find out
  where the information came from that went into
  the MC. The MC may do well in someone elses
  analysis, but in may never have been checked for
  the modes or region of phase space relevant to
  your analysis.
• Be careful not to underestimate the systematic
  errors associated with ignorance of
• signal efficiency
• background shapes, composition, and normalization

107
Yet more advice

• Dont be afraid to
• ask any question
• pursue a crazy idea
• jump into something you dont already understand
• question what people say is established fact
• look into the details and assumptions

108
Conclusions

• We have two remarkable new facilities for B
  physics
• KEK-B/Belle
• PEP-II/BaBar
• The performance of these accelerators is a major
  achievement for the laboratories.
• The clear observation of CP asymmetries in the B
  meson system is a milestone for particle physics.
• The measurement of sin(2b) is very well
  accomodated by the SM. It suggests that the
  dominant source of CP violation in B decays is
  due to the CKM phase. In spite of this, we have a
  long way to go before we have fully tested the
  SM/CKM framework.

109
Conclusions (continued)

• Hadron-collider experiments will soon start to
  play a major role the observation and precise
  measurement of Bs mixing is one of the next major
  goals.
• We are just beginning to scratch the surface of
  rare B decays. They have interesting sensitivity
  to new physics.
• The next few years will be very exciting.

110
Backup slides
111
PEP-II

• Very high current, multibunch operation
• 2 rings helps avoid beam instabilities and
  parasitic beam crossings (crossings not at the
  IP)
• I(e)1.3 A (LER), I(e-)0.7 A (HER)
• Bunch spacing 6.3-10.5 ns
• Beam spot
• sx120 mm sy5.6 mm sz9 mm
• Number bunches/beam 553-829 (to 1658)
• High-quality vacuum to keep beam-related
  backgrounds tolerable for experiments

112
PEP-II/BaBar Construction

• 1993 Start of PEP-II construction
• 1994 Start of BaBar construction
• Summer 1998 1st ee- collisions in PEP-II
• Spring 1999 BaBar moves on beamline
• May 26, 1999 1st events recorded by BaBar
• Oct 29, 2000 PEP-II achieves design luminosity
• Intense competition with KEK-B/Belle in Japan

113
PEP-II/BaBar

• The Standard Model predicts O(1) CP asymmetries
  in B decays! However, these asymmetries occur in
  processes that are relatively rare, so a large
  data sample is required.
• To perform these measurements, a two-ring ee-
  storage ring with unequal beam energies was built
  by SLAC/LBNL/LLNL with unprecedented luminosity.
  We now have gt60 MU (4S) events.

114
The BaBar Collaboration(9 countries)
115
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116
BaBar DIRC quartz bar
Overall length (4 bars) 4.9 m
No. light bounces (typical)300
3.5 cm
Surface roughness (r.m.s.) 0.5 nm
l (typical) 400 nm
117
BaBar DIRC Principle
s(qC) 3 mrad
Number of Cherenkov photons20-60
118
Experimental aspects of CP measurement
Acp(Dt)
F(Dt)
F(Dt)
True Dt, Perfect tagging
sin2b
True Dt, Imperfect tagging
D sin2b
D (1-2w) where w is mistag fraction.
Must measure flavor tag Dilution.
Measured Dt, Imperfect tagging
Must measure Dt resolution properties.
Dt(ps)
Dt(ps)
119
B0 mixing measurement D and R(Dt,Dt)
Amix(Dt)
Fmix(Dt)
Fnomix(Dt)
True Dt, Perfect tagging
True Dt, Imperfect tagging
D
Amplitude of mixing asymmetry is the dilution
factor D.
Measured Dt, Imperfect tagging
Mixing sample has 10x statistics of CP sample.
Shape of Dt determines resolution function
R(Dt,Dt)
Dt(ps)
Dt(ps)
120
B-gtKg