CP Violation: Recent Measurements

and Perspectives for Dedicated Experiments

LAFEX/CBPFMarch, 2001

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

SM with 3 generations and the CKM ansatz can accomodate CP

CP is one of the less experimentally constrained parts of SM

Observations of CP in the B system can:test the consistency of SMlead to the discovery of new physics

Cosmology needs additional sources of CP violation other than what is provided by the SM

Motivations

CP violation is one of the fundamental phenomena in particle physics

CP asymmetries in the B system are expected to be large.

• 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:

Symmetry in Physics

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

Examples of Symmetry OperationsExamples of Symmetry Operations

Lagrangian invariant under an operation limits the possible functional form it can take.

continuous X discrete, global X local, etc.

Three Discrete Symmetries

• Parity, P

• x x L L

• Charge Conjugation, C

• e e K K

• 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

9108

aver

ee

m

mm

108 1800

aver

KK

m

mm

Current understanding of Matter: The Standard Model

Quarks

Leptons

Three generations of fermions

d

u

s

c

b

t Q = +2/3

Q = -1/3

e

e

Q = -1

Q = 0

Interactions (bosons)

Z

W

g

(QED)

Weak

Strong

Eletroweak

(QCD)

H Higgs

Very successful when compared to experimental data!

especified by gauge symmetries SU(3)C SU(2)L U(1)Y

SM at work

• neutral currents, charm, W and Z bosons;

Weak Interactions

can change the flavour of leptons and quarks

b W

cgVcb

e W

eg

g: universal weak coupling

tbtstd

cbcscd

ubusud

CKM

VVV

VVV

VVV

V

matrix rotates the quark states from a basis in which they are mass eigenstates to one in which they are weak eigenstates

• VCKM: 33 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

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

Symmetry and InteractionsInteraction

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 Yes Yes Almost

CPT Yes Yes Yes

CP Symmetry and the Weak Interaction

L

R L

R

C

C

P

P

CP

Exists

Exists

Doesn’tExist

Doesn’tExist

Standard Model: CKM matrix

CKMV =

tbtstd

cbcscd

ubusud

vvv

vvv

vvv

The quark electroweak eigenstates are connected to the mass eigenstates by the CKM matrix :

=

mixing phase

Weak decay phase

dd BB mixing phase

ss BB

1

2/1

2/122

2

its

itd

iub

eVeV

A

eV

phenomenological applications: Wolfenstein parameterization

In SM:

03.02

In SM:

(0,0)

Vub

Vcb

Vtd

(,)

(1,0)

Vtd Vtb+Vcd Vcb

+Vud Vub= 0

Unitarity triangles

Vtd Vud+Vts Vus

+Vtb Vub= 0

Vub

Vtd

Vts

CP Violation in B Decays

d

bW

d

uu

d

B0

Decay Diagram

B0 B0

b

b d

du,c,t

u,c,t

W W

Mixing Diagram

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

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

B0

fCP

CP Assymmetry in B decays

)()(

)()()(

fBfB

fBfBtA

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:

)sin()Im()( mttACPCP ff

Requires a time-dependent measurementPeak asymmetry is at t = 2.3

0

0,1

0,2

0,3

0,4

0,5

0,6

0,7

0,8

0,9

1

0 1 2 3 4

Decay time in lifetimes

Dec

ay R

ate

0

0,1

0,2

0,3

0,4

0,5

0,6

0,7

Rat

e A

sym

met

ry

M0.7 for B0

Experimental bounds on the Unitarity Triangle

Bd mixing: md

Bs mixing: ms / md

bul, Bl :Vub

Kaon mixing & BK decays: K

B factories

e+e- (4s) = 0.56

B0

zCP

B0

ztag

Measurements of sin(2)

theory

well measured

05.0))2(sin(

ub

Measurements before 2005

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

sd BB ,

theory low statistics

mixing

Sd KJB no precise/direct measurement

sd BB ,

dB0 dB

BaBar, BelleCDF, D0HERA-B

Will establish significant evidence for CP violation in the B sector

Vtd

no access to

Vub

Vcb

well measured

Hadronic b production

• b quark pair produced preferentially at low • highly correlated

tagging low pt cuts

))2/ln(tan(

B hadrons at Tevatron

for larger the B boost increses rapidly

b pair production at LHC

LHC and Tevatron experiments

Tevatron LHC

Energy/collision mode 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

Detector configuration Two-arm forward Single-arm forward

Running luminosity 2x1032 cm-2s-1 2x1032 cm-2s-1

bb events per 107 2x1011 1x1012

Interactions/crossing ~ 2.0 0.5 (~ 30% single)

Average B momentum 40 GeV/c 80 GeV/c

Mean flight path 3.6 mm 7 mm

Generic experimental issues

p (p) p

B

B

triggering

flavour tagging

particle ID

1 cm

f

)()(

)()()(

fBfB

fBfBtA

decay time resolution

fB

neutrals detection

systematic effects

u

Flavour tagging

For a given decay channel fB

fB

B

signal B

other B

SS: look directly at particles accompanying the signal B

OS: deduce the initial flavour of the signal meson by identifying the other b hadron

bssu

osB

K

lb

scb

cQ jet

semileptonic decay

kaon tag

jet charge

Flavour tagging

NDA

2

1~

• w: wrong tag fraction

• : tagging efficiency

• N: total untagged

NNN WR )(

wD 21

)( Rww NNNw

BteV(%) D

LHCb(%) D

4.5 0.66

e -- --

K 18 0.52

40 0.40

Vertex charge 32 0.36 0.60 0.16

88 0.16 -- --

K 40 0.26 -- --

BS -- -- 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 – 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

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

The LHCb Detector

Trigger strategy(four levels)

• “high” pt , e, , h • secondary vertex• software triggers

Calorimetry

Important final states with and 0

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

2MeV/c 6.2)( M

Particle Id

Essential for hadronic PID

Aerogel

flavour tag with kaons(b c K)

background suppressiontwo body Bdecay products

Strategies for measurements of CKM angles and rare decays

Sd KJB 0

0

dB

2 *0 DBd

sx ss DB0

2KDB ss

0

0dB )( 0 KKBs

DKBd

0

KBd 0

JBs 0

(/)0 JBs

)()(00

)( , ssSsd DDKJB

Rare

0)(dsB

00 KBd

,

0dB

Sd KJB 0

)/()/(

)/()/()(

0000

0000

SS

SSCP

KJBKJB

KJBKJBtA

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

mtDtACP sin)2sin()(

dilution factor: • tagging• background

0)( 00 SddirCP KJBA

0)( KJBACP Standard Model:

Observation of direct asymmetries (10% level):

strong indication of New Physics!

tmAtmA dmix

ddir sincos

80.5k 9.3

18 0.017

BTeV

LHCb

events /1y

88k

(M) / MeV/c2 ))2(sin(

7 0.025

0.021

ATLAS 165k

CMS 433k 16 0.015

Systematic errors in CP measurements

high statistical precisionasymmetries • ratios• robust

• production asymmetries • tagging efficiencies

• mistag rate • final state acceptance

Control channels

Monte Carlo Detector cross-checks

ffffff ss 00

CP eigenstates Sd KJB /0

KJB /

00 / KJBd

)( taa(t)

ff

00 ff

00 ff

ATLAS: 005.0010.0)2sin( sysest

ss DB 0ss ff

)2sin( 0dB

)()(

)()()(

dd

dd

BB

BBtA

• experimental: background with similar topologies

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

tmAtmA dmix

ddir sincos

BTeV

LHCb

events/107s

23.7 k

12.3 k

(MeV)

29

17

)(tA0.024

--

dirA mixA-- --

0.09 0.07

C

--

-0.49

)2sin( 0dB

PeTeBA iid

)( 0

sinsin2)( 0

T

PBA d

dir

CP conserving strong phase

)sin()2cos(cos2)2sin()( 0 T

PBA d

mix

approximately

4-fold discrete ambiguity in

(de

gree

s)

030

(degrees)

|P/T|=0.1

0.05

0.02

1 year5 year

0dB

Time dependent Dalitz plot analysis• • Tree terms• Penguins

Helicity effects: corners Cuts: lower corner eliminated

Unbinned loglikelihood analysis: 9 parameters

Under investigation:

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

)(

BTeV

LHCb

events/1y

cos(2) and sin(2 ) no ambiguity

10.8k

3.3k

(MeV)

28

50 3o-6o

~10

KDB 0

color allowed doubly Cabibbo suppressed

color suppressed Cabibbo allowed

comparable decayamplitudes

K K B) (

K K B) (

K K K B) (

K K K B) (

unknows:

,,,b

=65o (1.13 rad)b=2.2x10-6

()=10o

2 *0 DBd

four time dependent decay rates:no penguin diagrams:clean det. of

*0 DBd

* D

*0 DBd

* D

two asymmetries• weak phase• strong phase difference between tree diagrams

exclusive reconstruction *D~ 83k / year S/B ~ 12

inclusive reconstruction

*D~ 260k / year S/B ~ 3

b

d dc

du

Vud

Vcb0dB

*D

*

cdu

Vub0

dB

*D

bd d

Vcd*

2

b dd b

0dB 0

dB

Vtb*

Vtb*Vtd

Vtd

small asymmetry: suppressedVub

2 *0 DBd

uncertainty due to:

_*0_*0 (( DBADBA dd

~ 360k / year

requires fullangular analysis

addition of channel: 1* aD

Mixing00ss BB

ss DB0

• very important for flavour dynamics• future hadron experiments: fully explore the Bs mixingSM: 1ps )263.14( sM

%2010/ ss

flavour specific state

untagged: fit proper time distributions for sss / ,

tagged: sM

BTeV

LHCb

tagged

34.5k

72k

e)proper tim(

43fs

43fs sD

KKDs* ,

00ss BB Mixing

Amplitude fit method: )cos( tMA s

A, A determined for each by a ML fit sM

2 KDB ss0

Theoretically clean (no pinguins)

Hadron identification: backgroundsD

Interference of direct and mixing induced decays

b c

Vus

Vcb*

b ss b

0sB 0

sB

Vtb*

Vtb*Vts

Vts

csu

Vub0

sB K

sD

bs s

Vcs*

sD

K

0sB s s

us

• amplitudes about same magnitude• four rates

)( fBs )( fBs )( fBs )( fBs

• two asymmetries

BTeV

LHCb

)2( oo 156

Sensitivity to: /s sx)(

)(

KDBA

KDBA

ss

ss

oo 143

events/1y

13.1k

6k

2 KDB ss0

(/)0 JBs JBs 0

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

(/)0 JBs

• CP eigenstate• direct extraction of )2sin(

BTeV

events/1y

9.2k

))2(sin( 0.033(xS=40)

JBs 0

• CP admixture• clean experimental signature• full angular analysis

LHCb

CMS

events

370k (5y)

600k (3y)

))2(sin(

(xS=40)

0.03

0.03

2MeV/c )11(19

Sensitivity to New Physics

Transversity analysisA. Dighe hep-ph/0102159 (CERN-TH/2001-034)

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

0dB KKBs

0 ,

• related by U-spin symmetry• makes use of penguins (sensitive to new physics...)• four observables:

• seven unknowns:

mixKK

dirKK

mixdir AAAA ,,,

,,,,,, dd

)( su

utpen

ucc

ctpen

b

i

AA

A

Rde

1 tpen

upen

utpen AAA

• U-spin symmetry:• input and

dd

contour plots in the and planes

d d

3.0ddo53

o76

BTeV

LHCb

events/1y

32.9k

)(

9.5k o9.1

--

dirKK

mixKK AA ,

0.034--

d()

degrees)(

(5y)

Rare B decays

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

In the SM:

Excellent probe of indirect effects of new physics!

SBSM : BR ~ • observation of the decay• measurement of its BR

910

510

LHCb

ATLAS

CMS

width MeV/c2 signal backg

26

26

62

33

2721

10

933

dBSM : BR ~ • high sensitivity search

1010

KBd

0dB

sB

• measure branching ratios• study decay kinematics

events/1yBTeV

LHCb

2.2k

S/B

11

4.5k 16

(3y)

Rare B decays KBd

Forward-backward asymmetry )(sAFB

)( _ pps

can be calculated in SM and other models 0)( 0 sAFB

A. Ali et al., Phys. Rev. D61074024 (2000)

LHCbFBA

%8.5%4.2 (1y)

Physics summary (partial)

Parameter Channels BTeV LHCb

sin(2) BdJ/Ks 0.025 0.021

Bd A(t) 0.024 --

Amix -- 0.07

Adir -- 0.09

sin(2) Bd 10 3- 6

2+ Bd D -- > 5

-2 Bs DsK 6-15 3-14

Bd DK -- 10 B- DK- 10 --

sin(2) Bs J/ -- 0.03 (5y)

Bs J/ 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)

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

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.

CP violation is one of the most active and interesting topicsin today’s particle physics;

The precision beauty CP measurements era already started - Belle and BaBar;