Transcript No Slide Title

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
João R. T. de Mello Neto
Instituto de Física
LAFEX/CBPF
March, 2001
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
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
Lagrangian invariant under an operation limits the
possible functional form it can take.
• Different classes of symmetries:
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
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
me+ - memaver
mK 0 - mK 0
maver
 8 10-9
 8 10-18
Current understanding of Matter:
The Standard Model
Three generations of fermions
Quarks
u  c  t 
     
d   s b
e

e
-
Leptons



Interactions (bosons)

 - 


 
 



-
Q = +2/3
Q = -1/3



Q = -1
Q=0
especified by gauge symmetries
SU(3)C  SU(2)L  U(1)Y
g
(QED)
Z
W
Weak
g
Strong (QCD)
Eletroweak
H
Higgs
Very successful when compared to experimental data!
SM at work
• neutral currents, charm, W and Z bosons;
Weak Interactions
can change the flavour of leptons and quarks
W-
eg
W-
b
gVcb
e
c
g: universal weak coupling
VCKM
Vud

  Vcd
V
 td
Vus Vub 

Vcs Vcb 
Vts Vtb 
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
Symmetry and Interactions
Interaction
Conserved Quantity
Strong
Electromagnetic
Weak
Energy/Momentum
Yes
Yes
Yes
Electric Charge
Yes
Yes
Yes
Baryon no., Lepton no.
Yes
Yes
Yes
Flavor Quantum #
Yes
Yes
No
Isospin
Yes
No
No
Parity P, charge conjugation C
Yes
Yes
No
CP
CPT
Yes
Yes
Almost
Yes
Yes
Yes
CP Symmetry and the Weak
Interaction
• Despite the maximal violation of C and P symmetry, the
combined operation, CP, is almost exactly conserved
Exists
L
P
Doesn’t
Exist
R
C
C
CP L
R
P
Exists
Doesn’t
Exist
Standard Model: CKM matrix
The quark electroweak eigenstates are connected to the
mass eigenstates by the CKM matrix :
VCKM
 v ud

=  v cd
v
 td
v us
v cs
v ts
v ub 

v cb 

v tb 
phenomenological applications: Wolfenstein parameterization
=
 1 - 2 / 2

 -
 - V e - i
 td
Bd - Bd
mixing phase

- Vub e -ig 

1 - 2 / 2
A2 
Vts e
ig
Bs - Bs
mixing phase
1


Weak decay
phase
Unitarity triangles



Vtd Vtb +Vcd Vcb +Vud Vub =

(,)
In SM:
   -  -g
 Vtd
 Vub
 Vcb
g
0

(1,0)
(0,0)


Vtd Vud +Vts Vus +Vtb
 Vub
 Vtd
 Vts
g
g

Vub = 0
In SM:
g  2  0.03
CP Violation in B Decays
In order to generate a CP violating observable, we must
have interference between at least two different
amplitudes
d
B decays: two different types
of amplitudes
WB0
decay
b
u
u
d
d
+
Decay Diagram
mixing
u,c,t
b
B0
d
d
W-
Wu,c,t
Mixing Diagram
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 interference
between mixed and unmixed decays
B0
b
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  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
B0
fCP
B0
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:
( B  f ) - ( B  f )
A(t ) 
( B  f ) + ( B  f )
For decays to CP eigenstates where one decay diagram
dominates, this asymmetry simplifies to:
AfCP (t )  - Im( fCP ) sin(mt)
Requires a time-dependent measurement
Peak asymmetry is at t = 2.3
M  0.7 for B0
1
0,7
0,9
0,6
0,5
Decay Rate
0,7
0,6
0,4
0,5
0,3
0,4
0,3
0,2
0,2
0,1
0,1
0
0
0
1
2
Decay time in lifetimes
3
4
Rate Asymmetry
0,8
Experimental bounds on the
Unitarity Triangle
Bd mixing: md
Bs mixing: ms / md
bul, Bl :Vub
Kaon mixing & BK decays: K
B factories
e+e- - (4s)
g = 0.56
B0
zCP
B0
ztag
Measurements of sin(2)
Measurements before 2005
BaBar, Belle
CDF, D0
HERA-B
Will establish significant evidence
for CP violation in the B sector
theory
low statistics
theory
bu
Bd  
Bd   + - 0

Vub
g
Vtd
Vcb
Bd , Bs

well measured
no precise/direct
measurement
no access to g
mixing
Bd  J K S
 (sin(2 ))  0.05
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
Bd , Bs
Hadronic b production
B hadrons at Tevatron
  - ln(tan( / 2))
g
for larger  the B
boost  gincreses rapidly

b pair production 
at LHC
• b quark pair produced
preferentially at low 
• highly correlated
tagging
low pt cuts
LHC and Tevatron experiments
Tevatron
LHC
2.0 TeV pp
14 TeV pp
bb cross section
 100 b
 500 b
Inelastic cross section
 50 mb
 80 mb
Ratio bb/inelastic
0.2%
0.6%
Bunch spacing
132 ns
25 ns
BTeV
LHCb
Energy/collision mode
Detector configuration Two-arm forward Single-arm forward
Running luminosity
bb events per 107
Interactions/crossing
Average B momentum
Mean flight path
2x1032 cm-2s-1
2x1032 cm-2s-1
2x1011
1x1012
~ 2.0
0.5 (~ 30% single)
40 GeV/c
80 GeV/c
3.6 mm
7 mm
Generic experimental issues
f
B f
B
p
p (p)
B
1 cm
( B  f ) - ( B  f )
A(t ) 
( B  f ) + ( B  f )
triggering
decay time resolution
particle ID
neutrals detection
flavour tagging
systematic effects
Flavour tagging
For a given decay channel
B f
signal B
f
B
other B
B
SS: look directly at particles accompanying the signal B
b Bo
s s
s
+
u K
u
OS: deduce the initial flavour of the signal
meson by identifying the other b hadron
semileptonic decay
bl
kaon tag
bcs
jet charge
Qjet  c
Flavour tagging
• w: wrong tag fraction
  ( NR + NW ) N
•  : tagging efficiency
w  Nw ( Nw + NR )
• N: total untagged
D  1- 2 w
A ~
1
D 2N
BteV
LHCb
(%) D 
(%) D


4.5
0.66
e
--
--
K
18
0.52
Vertex charge
32
0.36 0.60 0.16

88
0.16
--
--
K
40
0.26
--
--
BS
--
--
40
0.40
0.11 0.34
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/0 detection and e identification in lead-tungsten crystal
calorimeter
–  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 )
Trigger strategy
(three levels)
• pioneering pixel vertex trigger
• software triggers
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
• “high” pt  , e, g , h
Trigger strategy
• secondary vertex
(four levels)
• software triggers
Calorimetry
Important final states with
0
 and g
Use 2x11,850 lead-tungsten crystals (PbWO4)
• technology developed for LHC by CMS
• radiation hard
• fast scintillation (99% of light in <100 ns)
Excellent energy, angular resolution and photon efficiency
Pions with 10 GeV
 (M )  2.6 MeV/c2
Particle Id
Essential for hadronic PID
Aerogel
flavour tag with
kaons
(b  c K)
background
suppression
two body B
decay products
Strategies for measurements of
CKM angles and rare decays

 ,g
Bd0  J KS
B    (B  K K )

0
Bd  
0
d
B  
+
0
d
+
-
+
0
s
g
-

B  DK
B  K
0
d
0
d
2 + g
Bd0  D*- +
Bd0( s )  J KS0 , D(+s ) D(-s )
g - 2g

s
B D K
0
s

Rare
g
B  J 
Bs0  J 
0
s
-
Bs0(d )   +  Bd0  K 0  +  -
(/)
xs
B D 
0
s
s
+

ACP (t ) 
B  J KS
0
d
 ( B 0  J / K S0 ) - ( B 0  J / K S0 )
 ( B 0  J / K S0 ) + ( B 0  J / K S0 )
dir
mix
 A
cosmd t + A
sin md t
Penguins:
• expected to be small
• same weak phase as tree
amplitude
ACP (t )  D sin(2 ) sin mt
dilution factor:
• tagging
• background
events /1y
(M) / MeV/c2
 (sin(2 ))
BTeV
88k
7
0.025
LHCb
80.5k
9.3
0.021
ATLAS
CMS
165k
433k
18
16
0.017
0.015
dir
ACP
( Bd0  J KS0 )  0
Standard Model:
Observation of direct
asymmetries (10% level):
ACP (B+  J K + )  0
strong indication of
New Physics!
Systematic errors in CP
measurements
asymmetries
• ratios
• robust
high statistical precision
• tagging efficiencies
• production asymmetries
 
f0 f0 f s f s f + f • mistag rate
• final state acceptance
 
a(t) a (t )
CP eigenstates Bd  J /K S
Control channels
B  J /K

B  J /K
0
d
Bs0  Ds
ATLAS:

0
0

   
f+ f-

f0 f0
f 0 f 0 
 
fs fs
sin(2 )  0.010 0.005
est
Monte Carlo
sys
Detector cross-checks
sin(2 )
B  
+
0
d
-
• experimental:
background with
similar topologies
• theoretical: penguin diagrams make it harder to interpret
observables in term of 
( Bd   + - ) - ( Bd   + - )
A(t ) 
( Bd   + - ) + ( Bd   + - )
 Adir cosmd t + Amix sin md t
BTeV
LHCb
events/107s
23.7 k
12.3 k
 (MeV) A(t ) Adir Amix
29
17
0.024
--
--
0.09
C
-0.07 -0.49
--
sin(2 )
B  
+
0
d
-
A(Bd0   + - )  eig T + e-i P
approximately
P
A ( B    )  2 sin  sin 
T
dir
0
d
+
CP conserving
strong phase
-
Amix ( Bd0   + - )  - sin(2 ) - 2
P
cos cos(2 ) sin( )
T
(degrees
)
  300
1 year
5 year
|P/T|=0.1
0.05
4-fold discrete
ambiguity in

0.02
 (degrees)

B  
0
d
•
Time dependent Dalitz plot analysis • Tree terms
• Penguins
Helicity effects: corners
Cuts: lower corner eliminated
Unbinned loglikelihood analysis: 9 parameters
cos(2) and sin(2 )
no ambiguity
• background
Under investigation: • Dalitz plot acceptance
• other resonances
• EW penguins
events/1y
(MeV)
 ( )
BTeV
10.8k
28
~10
LHCb
3.3k
50
3o-6o
g
-
B D K
color allowed
0
-
doubly Cabibbo suppressed
comparable decay
amplitudes
B -  ( K + - ) K -
B +  ( K - + ) K +
color suppressed
unknows:
b,  ,  , g
=65o (1.13 rad)
b=2.2x10-6
()=10o
Cabibbo allowed
B-  (K + K - )K B+  (K + K - )K +
2 + g B  D 
*-
0
d
Vud
Bd0
b
d
V*
cb
d
u
c

0
d
B
b
d
+
Vtb*
Vtd
Vtd
Vtb*
*D
d
*
Vcd
Bd0
b
d
Vub
d 0
b Bd
2
c
*D
d
u

d

four time dependent decay rates:
B D 
*-
0
d
D 
*+
two asymmetries
exclusive
D
*
+
Bd0  D*- +
 D*+ -
-
no penguin diagrams:
clean det. of
g
small asymmetry:
Vub suppressed
• weak phase
• strong phase difference between tree diagrams
reconstruction
~ 83k / year S/B ~ 12
inclusive
D*
reconstruction
~ 260k / year S/B ~ 3
2 + g
B D 
*-
0
d
+
uncertainty due to:
  A( Bd0  D *+ _
addition of
A( Bd0  D *+ _
D*- a1 channel:
~ 360k / year
requires full
angular analysis
Bs0 - Bs0 Mixing
• very important for flavour dynamics
• future hadron experiments: fully explore the Bs
mixing
SM: M  (14.3 - 26) ps-1
s
s / s  10 - 20%
B D 
0
s
untagged:
tagged:
s
+
flavour specific state
fit proper time distributions for s , s / s
M s
tagged
BTeV Ds   - , K * K - 72k
34.5k
Ds   LHCb
 (proper time)
43fs
43fs
B - B Mixing
0
s
0
s
Amplitude fit method:
A cos(M st )
A, A determined for each M s by a ML fit
g - 2g B  D K

s
0
s

Interference of direct and mixing induced decays
Theoretically clean (no pinguins)
Bs0
b
s
Vtb*
Vts
Vts
Vtb*
Vus
0
s
b
V*cb
B s
s
b
Bs0
u +
s K
c s Ds
Bs0
Vcs*
b
s
Vub

• amplitudes about same
magnitude ( Bs  f )
• four rates (Bs  f )
(Bs  f )
(Bs  f )
• two asymmetries
Hadron identification:
Ds 
c Ds s
u +
sK
background
g - 2g
Sensitivity to:
s / 
events/1y

s
B D K
0
s
xs

A( Bs  Ds- K + )

A( Bs  Ds+ K - )
 (g - 2g )
BTeV
13.1k
6o - 15o
LHCb
6k
3o - 14o

g
B  J 
0
s
(/)
B  J 
0
s
• dominated by one phase only
• very small CP violating effects (SM)
• sensitive probe for CP violating effects beyond the SM
Bs0  J   (/)
  19(11) MeV/c2
• CP eigenstate
• direct extraction of sin(2g )
events/1y
BTeV
 (sin(2g ))
9.2k
0.033
(xS=40)
B  J 
0
s
• CP admixture
• clean experimental signature
• full angular analysis
events
LHCb
370k (5y)
CMS
600k (3y)
 (sin(2g ))
0.03
0.03
(xS=40)
Sensitivity to New Physics
Transversity analysis
A. Dighe
hep-ph/0102159 (CERN-TH/2001-034)
• simpler angular analysis with the transversity angle
• accuracy similar for same number of events
• if g is large the advantage of J / is lost
 ,g
B  
+
0
d
-
Bs0  K + K -
• related by U-spin symmetry (u  s)
• makes use of penguins (sensitive to new physics...)
• four observables: Adir , Amix , Adir , Amix

• seven unknowns:

KK
g ,  , g , d , , d , 
ct


A
1 
pen
i

de 
u
ut
Rb  Acc + Apen 
• U-spin symmetry: d   d
• input
KK
Autpen  Aupen - Atpen

 and g
contour plots in the
g - d and g - d 
planes
d()
g  76
o
d  d   0.3
o
  53
g (degrees)
BTeV
LHCb
mix
dir
, AKK
(5y)  (g )
events/1y AKK
--32.9k
9.5k
0.034
1.9o
Rare B decays
In the SM:
• flavour changing neutral currents
only at loop level
• very small BR ~ 10-5 or smaller
Excellent probe of indirect effects of new physics!
-9
BS   +  -
SM : BR ~ 10
• observation of the decay
• measurement of its BR
width
MeV/c2
LHCb
26
ATLAS
CMS
Bd   +  Bd   0  +  Bs   +  -
Bd  K  +  -
signal
33
27
21
62
26
backg
10
(3y)
93
3
SM : BR ~ 10-10
• high sensitivity search
• measure branching ratios
• study decay kinematics
events/1y
BTeV
LHCb
2.2k
4.5k
S/B
11
16
Rare B decays
Bd  K  +  Forward-backward asymmetry
AFB (s)
s  ( p  + + p _ )
can be calculated in SM and other models
LHCb
AFB (s0 )  0
AFB (1y)
2.4% - 5.8%
A. Ali et al., Phys. Rev. D61
074024 (2000)
Physics summary (partial)
Parameter
sin(2)

sin(2)
2+g
g-2g
g
sin(2g)
Channels
BdJ/Ks
Bd A(t)
Amix
Adir
Bd
Bd  D
Bs DsK
Bd  DK
B- DKBs  J/
Bs  J/
BTeV
0.025
0.024
--10
-6-15
-10
-0.033
Bs oscil.
xs
Bs  Ds (up to) 75
Rare Decays
Bs  
-Bd  K  2.2k (0.2k)
LHCb
0.021
-0.07
0.09
3- 6
> 5
3-14
10
-0.03 (5y)
-(up to) 75
11(3.3)
22.4k(1.4k)
Other physics topics: Bc mesons, baryons, charm,
tau, b production, etc
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;
Conclusions
CP violation is one of the most active and interesting topics
in today’s 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.