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CP Violation: Recent Measurements and Perspectives for Dedicated Experiments

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Instituto de Física

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

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

- 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

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

Q = -1/3

Q = -1

Q = 0

Current understanding of Matter: The Standard ModelThree 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!

SM at work

- neutral currents, charm, W and Z bosons;

g

W-

W-

b

e-

c

ne

Weak Interactionscan 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: 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

Exists

P

C

nR

CP

nL

nR

Doesn’t

Exist

Doesn’t

Exist

C

P

Exists

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

Weak decay

phase

mixing phase

mixing phase

Standard Model: CKM matrixThe quark electroweak eigenstates are connected to the mass eigenstates by the CKM matrix :

=

phenomenological applications: Wolfenstein parameterization

VtdVtb+VcdVcb+Vud Vub= 0

(,)

In SM:

Vtd

Vub

Vcb

(1,0)

(0,0)

VtdVud+VtsVus+Vtb Vub= 0

In SM:

Vtd

Vub

Vts

d

Decay Diagram

B0

B0

W-

W-

d

b

u,c,t

d

p-

Mixing Diagram

u

b

W-

u

b

B0

p+

d

d

CP Violation in B DecaysIn 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

fCP

B0

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

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

Experimental bounds on the Unitarity Triangle

Bd mixing: md

Bs mixing: ms / md

bul, Bl :Vub

Kaon mixing & BK decays: K

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

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

Generic experimental issues

f

B

p

B

1 cm

triggering

decay time resolution

particle ID

neutrals detection

flavour tagging

systematic effects

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

Flavour tagging

- w: wrong tag fraction
- : tagging efficiency
- N: total untagged

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)

- secondary vertex
- software triggers

- 17 siliconvertex detectors
- 11 tracking stations
- two RICH for hadron identification
- a normal conductor magnet (4 Tm)
- hadronic and eletromagneticcalorimeters
- muondetectors

Trigger strategy

(four levels)

Important final states with and

Calorimetry- 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

Essential for hadronic PID

Aerogel

flavour tag with

kaons

(b c K)

background suppression

two body B

decay products

- 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

ATLAS

165k

18

0.017

CMS

433k

16

0.015

Standard Model:

strong indication of New Physics!

Observation of direct asymmetries (10% level):

Systematic errors in CP measurements

- ratios
- robust

asymmetries

est

sys

high statistical precision

- tagging efficiencies

- production asymmetries

- final state acceptance

- mistag rate

CP eigenstates

Control channels

ATLAS:

Monte Carlo

Detector cross-checks

- experimental:
- background with
- similar topologies

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

C

events/107s

BTeV

23.7 k

29

0.024

--

--

--

LHCb

12.3 k

17

0.09

0.07

-0.49

--

approximately

1 year

5 year

(degrees)

|P/T|=0.1

0.05

4-fold discrete ambiguity in

0.02

(degrees)

- Penguins

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

3.3k

LHCb

doubly Cabibbo suppressed

comparable decay

amplitudes

color suppressed

Cabibbo allowed

unknows:

=65o (1.13 rad)

b=2.2x10-6

()=10o

Vtb

Vtd

*

*

Vud

Vtd

*

Vcb

Vcd

*

Vub

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

inclusive reconstruction

exclusive reconstruction

~ 260k / year S/B ~ 3

~ 83k / year S/B ~ 12

Mixing

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

SM:

flavour specific state

fit proper time distributions for

untagged:

tagged:

tagged

43fs

72k

BTeV

43fs

34.5k

LHCb

Vtb

*

*

Vcs

*

Hadron identification: background

Interference of direct and mixing induced decays

Theoretically clean (no pinguins)

Vts

Vub

Vts

- amplitudes about same magnitude
- four rates

Vus

Vcb

*

- two asymmetries

- 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

(xS=40)

- CP admixture
- clean experimental signature
- full angular analysis

events

370k (5y)

LHCb

0.03

0.03

600k (3y)

CMS

(xS=40)

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

- makes use of penguins (sensitive to new physics...)
- four observables:
- seven unknowns:

contour plots in the

and

planes

- U-spin symmetry:
- input and

d()

(5y)

events/1y

BTeV

--

--

32.9k

0.034

LHCb

9.5k

- observation of the decay
- measurement of its BR

- SM : BR ~
- high sensitivity search

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

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)

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) Bdr 10 3- 6

2+ Bd D* -- > 5

-2 Bs DsK 6-15 3-14

Bd DK* -- 10

B- DK- 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:

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 SMparameters and search of New Physics;

Exciting times: understanding the origin of CP violation in the SM and beyond.

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