CP Violation:

1
CP Violation Recent Measurements and
Perspectives for Dedicated Experiments

• Outline
• Introduction
• CP violation in the B sector
• BaBar and Belle
• Future experiments BTeV and LHCb
• Strategies to measure the CP viol. parameters
• Conclusions

LAFEX/CBPF March, 2001
2
Motivations
CP violation is one of the fundamental phenomena
in particle physics
CP is one of the less experimentally constrained
parts of SM
SM with 3 generations and the CKM ansatz can
accomodate CP
CP asymmetries in the B system are expected to
be large.
Observations of CP in the B system
can test the consistency of SM lead to the
discovery of new physics
Cosmology needs additional sources of CP
violation other than what is provided by the SM
3
Symmetry in Physics

• The symmetry, or invariance, of the physical laws
  describing a system undergoing some operation is
  one of the most important concepts in physics.
• Symmetries are closely linked to the dynamics of
  the system
• Different classes of symmetries

Lagrangian invariant under an operation limits
the possible functional form it can take.
continuous X discrete, global X local, etc.
Examples of Symmetry Operations
Translation in Space Translation in Time Rotation
in Space Lorentz Transformation Reflection of
Space (P) Charge Conjugation (C) Reversal of Time
(T) Interchange of Identical Particles Gauge
Transformations
4
Three Discrete Symmetries

• Parity, P
• x ? -x L ? L
• Charge Conjugation, C
• e ? e- K- ? K g ? g
• Time Reversal, T
• t ? -t
• CPT Theorem
• One of the most important and generally valid
  theorems in quantum field theory.
• All interactions are invariant under combined C,
  P and T
• Only assumptions are local interactions which are
  Lorentz invariant, and Pauli spin-statistics
  theorem
• Implies particle and anti-particle have equal
  masses and lifetimes

5
Current understanding of Matter The Standard
Model
Three generations of fermions
Quarks
Leptons
especified by gauge symmetries SU(3)C ? SU(2)L ?
U(1)Y
Interactions (bosons)
(QED)
Eletroweak
H
Higgs
Z
Weak
W
g
Strong
(QCD)
Very successful when compared to experimental
data!
6
SM at work

• neutral currents, charm, W and Z bosons

7
Weak Interactions
can change the flavour of leptons and quarks
g universal weak coupling
matrix rotates the quark states from a basis in
which they are mass eigenstates to one in which
they are weak eigenstates

• VCKM 3?3 complex unitary matrix
• four independent parameters (3 numbers, 1
  complex phase)
• effects due to complex phase CP violating
  observables
• result of interference between different
  amplitude
• all CP violating observables are dependent
  upon one
• parameter

8
Symmetry and Interactions
CP Symmetry and the Weak Interaction

• Despite the maximal violation of C and P
  symmetry, the combined operation, CP, is almost
  exactly conserved

9
Standard Model CKM matrix
The quark electroweak eigenstates are connected
to the mass eigenstates by the CKM matrix

phenomenological applications Wolfenstein
parameterization
10
Unitarity triangles
Vtd Vtb?Vcd Vcb?Vud Vub? 0
(?,?)
In SM
?
??Vtd
??Vub?
?
?
??Vcb
(1,0)
(0,0)
Vtd Vud?Vts Vus?Vtb Vub? 0
In SM
??Vtd
??Vub?
??Vts
???
??
11
CP Violation in B Decays
In order to generate a CP violating observable,
we must have interference between at least two
different amplitudes

B decays two different types of amplitudes
decay
mixing
Three possible manifestations of CP
violation Direct CP violation (interference
between two decay amplitudes) Indirect CP
violation (interference between two mixing
amplitudes) CP violation in the
interferencebetween mixed and unmixed decays
12
CP Violation in B Decays

• Direct CP Violation
• Can occur in both neutral and charged B decays
• Total amplitude for a decay and its CP conjugate
  have different magnitudes
• Difficult to relate measurements to CKM matrix
  elements due to hadronic uncertainties
• Relatively small asymmetries expected in B decays
• Indirect CP Violation
• Only in neutral B decays
• Would give rise to a charge asymmetry in
  semi-leptonic decays (like d in K decays)
• Expected to be small in Standard Model
• CP Violation in the interference of mixed and
  unmixed decays
• Typically use a final state that is a CP
  eigenstate (fCP)
• Large time dependent asymmetries expected in
  Standard Model
• Asymmetries can be directly related to CKM
  parameters in many cases, without hadronic
  uncertainties

13
CP Assymmetry in B decays
To observe C P violation in the interference
between mixed and unmixed decays, one can measure
the time dependent asymmetry
For decays to CP eigenstates where one decay
diagram dominates, this asymmetry simplifies to

Requires a time-dependent measurement Peak
asymmetry is at t 2.3t
DMt 0.7 for B0
14
Experimental bounds on the Unitarity Triangle
Bd mixing ?md
Bs mixing ?ms / ?md
b?ul?, B??l? Vub
Kaon mixing BK decays ?K
15
B factories
16
Measurements of sin(2?)
17
Measurements before 2005
BaBar, Belle
Will establish significant evidence for CP
violation in the B sector
CDF, D0
HERA-B
theory low statistics
theory
Vtd
Vub?
mixing
Vcb
well measured
no precise/direct measurement
no access to
??
well measured
Constraints from the unitarity triangle

• consistency with the SM (within errors)
• inconsistency with the SM ( not well
  understood)

Next generation of experiments

• precise measurements in several channels
• constrain the CKM matrix in several ways
• look for New Physics

18
Hadronic b production
B hadrons at Tevatron
for larger the B boost increses
rapidly
b pair production ? at LHC

• b quark pair produced preferentially at low ?
• highly correlated

tagging low pt cuts
19
LHC and Tevatron experiments
20
Generic experimental issues
f
B
p
B
1 cm
triggering
decay time resolution
particle ID
neutrals detection
flavour tagging
systematic effects
21
Flavour tagging
For a given decay channel
signal B
other B
SS look directly at particles accompanying the
signal B
s
s
u
u
OS deduce the initial flavour of the signal
meson by identifying the other b hadron
semileptonic decay
kaon tag
jet charge
22
Flavour tagging

• w wrong tag fraction
• ? tagging efficiency
• N total untagged

23
The BTeV detector

• Central pixel vertex detector in dipole magnetic
  field (1.6 T)
• Each of two arms
• tracking stations (silicon strips straws)
• hadron identification by RICH
• g/p0 detection and e identification in
  lead-tungsten crystal calorimeter
• m triggering and identification in muon system
  with toroidal magnetic field
• Designed for luminosity 2 x 1032 cm-2s-1
  ( 2 x 1011 bb events per 107 s )

• pioneering pixel vertex trigger
• software triggers

Trigger strategy (three levels)
24
The LHCb Detector

• 17 silicon vertex detectors
• 11 tracking stations
• two RICH for hadron identification
• a normal conductor magnet (4 Tm)
• hadronic and eletromagnetic calorimeters
• muon detectors

Trigger strategy (four levels)
25
Calorimetry

• Use 2x11,850 lead-tungsten crystals (PbWO4)
• technology developed for LHC by CMS
• radiation hard
• fast scintillation (99 of light in lt100 ns)

Excellent energy, angular resolution and photon
efficiency
Pions with 10 GeV
26
Particle Id
Essential for hadronic PID
Aerogel
flavour tag with kaons (b ? c ?K)
background suppression two body B decay products
27
Strategies for measurements of CKM angles and
rare decays
Rare
28
Penguins

• expected to be small
• same weak phase as tree
• amplitude

• tagging
• background

dilution factor
?(M) / MeV/c2
events /1y
BTeV
0.025
7
88k
LHCb
9.3
0.021
80.5k
18
0.017
ATLAS
165k
CMS
433k
16
0.015
Standard Model
strong indication of New Physics!
Observation of direct asymmetries (10 level)
29
Systematic errors in CP measurements
high statistical precision

• tagging efficiencies

• production asymmetries

• final state acceptance

• mistag rate

CP eigenstates
Control channels
ATLAS
Monte Carlo
Detector cross-checks
30

• experimental
• background with
• similar topologies

• theoretical penguin diagrams make it harder to
  interpret
• observables in term of

events/107s
C
BTeV
23.7 k
29
0.024
--
--
--
LHCb
12.3 k
17
0.09
0.07
-0.49
--
31
approximately
1 year
5 year
??(degrees)
P/T0.1
0.05
4-fold discrete ambiguity in ?
0.02
? (degrees)
32
Time dependent Dalitz plot analysis
Helicity effects corners
Cuts lower corner eliminated
Unbinned loglikelihood analysis 9 parameters
cos(2?) and sin(2 ?) no ambiguity

• background
• Dalitz plot acceptance
• other resonances
• EW penguins

Under investigation
events/1y
?(MeV)
10.8k
28
10
BTeV
50
3o-6o
LHCb
3.3k
33
color allowed
doubly Cabibbo suppressed
comparable decay amplitudes
color suppressed
Cabibbo allowed
unknows
?65o (1.13 rad) b2.2x10-6
?(?)10o
34
2?
?
four time dependent decay rates
no penguin diagrams clean det. of
small asymmetry suppressed
Vub

• weak phase
• strong phase difference between tree diagrams

two asymmetries
exclusive reconstruction
inclusive reconstruction
260k / year S/B 3
83k / year S/B 12
35
uncertainty due to
360k / year
requires full angular analysis
36
Mixing

• very important for flavour dynamics
• future hadron experiments fully explore the Bs
  mixing

SM
flavour specific state
untagged
fit proper time distributions for
tagged
tagged
43fs
72k
BTeV
43fs
34.5k
LHCb
37
Mixing
Amplitude fit method
A, ?A determined for each by a ML fit
38
Interference of direct and mixing induced decays
Theoretically clean (no pinguins)
Vts
Vub
Vts

• amplitudes about same magnitude
• four rates

Vus
Vcb

• two asymmetries

39
Sensitivity to
events/1y
13.1k
BTeV
6k
LHCb
40

• dominated by one phase only
• very small CP violating effects (SM)
• sensitive probe for CP violating effects beyond
  the SM

• CP eigenstate
• direct extraction of

events/1y
0.033
9.2k
BTeV
(xS40)

• CP admixture
• clean experimental signature
• full angular analysis

events
370k (5y)
LHCb
0.03
0.03
600k (3y)
CMS
(xS40)
41
Sensitivity to New Physics
Transversity analysis
hep-ph/0102159 (CERN-TH/2001-034)
A. Dighe

• simpler angular analysis with the transversity
  angle
• accuracy similar for same number of events
• if is large the advantage of
  is lost

42
d(?)
(5y)
events/1y
BTeV
--
--
32.9k
0.034
LHCb
9.5k
43
Rare B decays
In the SM

• flavour changing neutral currents
• only at loop level
• very small BR or smaller

Excellent probe of indirect effects of new
physics!
width MeV/c2
signal
backg
LHCb
26
33
10
(3y)
93
27
62
ATLAS
3
21
26
CMS

• measure branching ratios
• study decay kinematics

S/B
events/1y
BTeV
2.2k
11
16
4.5k
LHCb
44
Rare B decays
Forward-backward asymmetry
can be calculated in SM and other models
(1y)
LHCb
A. Ali et al., Phys. Rev. D61 074024 (2000)
45
Physics summary (partial)
Parameter Channels BTeV
LHCb sin(2?) Bd?J/?Ks 0.025 0.021 ? Bd???
A(t) 0.024 -- Amix
-- 0.07 Adir
-- 0.09 sin(2?) Bd?r? 10? 3?- 6? 2?? Bd ?
D? -- gt 5? ?-2?? Bs ?DsK 6?-15? 3?-14?
? Bd ? DK -- 10? B- ?D?K- 10?
-- sin(2??) Bs ? J/yf -- 0.03
(5y) Bs ? J/y? 0.033 -- Bs oscil. xs Bs ?
Ds? (up to) 75 (up to) 75 Rare Decays Bs ?
?? -- 11(3.3) Bd ? K ?? 2.2k (0.2k)
22.4k(1.4k)
Bc mesons, baryons, charm, tau, b production,
etc
Other physics topics
46
References
CERN yellow report, Proc. of the Workshop on
Standard Model Physics (and more) at the LHC, May
2000, CERN 2000-004
BTeV Proposal , May 2000
LHCb Proposal, February 98
47
Conclusions
CP violation is one of the most active and
interesting topics in todays particle physics
The precision beauty CP measurements era already
started - Belle and BaBar
BTeV and LHCb are second generation beauty CP
violation experiments
Both are well prepared to make crucial
measurements in flavour physics with huge amount
of statistics
Impressive number of different strategies for
measurements of SM parameters and search of New
Physics
Exciting times understanding the origin of CP
violation in the SM and beyond.