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João R. T. de Mello Neto. 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

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Slide1 l.jpg

João R. T. de Mello Neto

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


Slide2 l.jpg

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


Slide3 l.jpg

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


Three discrete symmetries l.jpg

+

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


  • Current understanding of matter the standard model l.jpg

    Q = +2/3

    Q = -1/3

    Q = -1

    Q = 0

    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!


    Sm at work l.jpg
    SM at work

    • neutral currents, charm, W and Z bosons;


    Weak interactions l.jpg

    gVcb

    g

    W-

    W-

    b

    e-

    c

    ne

    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


    Slide8 l.jpg

    nL

    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


    Standard model ckm matrix l.jpg

    =

    Weak decay

    phase

    mixing phase

    mixing phase

    Standard Model: CKM matrix

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

    =

    phenomenological applications: Wolfenstein parameterization


    Slide10 l.jpg

    Unitarity triangles

    VtdVtb+VcdVcb+Vud Vub= 0

    (,)

    In SM:

    Vtd

    Vub

    Vcb

    (1,0)

    (0,0)

    VtdVud+VtsVus+Vtb Vub= 0

    In SM:

    Vtd

    Vub

    Vts

    

    


    Cp violation in b decays l.jpg

    u,c,t

    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 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


    Cp violation in b decays12 l.jpg

    B0

    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 l.jpg
    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 l.jpg
    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 l.jpg

    B0zCP

    e+e- (4s) = 0.56

    B0ztag

    B factories



    Slide17 l.jpg

    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


    Hadronic b production l.jpg
    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 l.jpg

    p (p)

    Generic experimental issues

    f

    B

    p

    B

    1 cm

    triggering

    decay time resolution

    particle ID

    neutrals detection

    flavour tagging

    systematic effects


    Flavour tagging l.jpg

    b

    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 tagging22 l.jpg
    Flavour tagging

    • w: wrong tag fraction

    •  : tagging efficiency

    • N: total untagged


    The btev detector l.jpg
    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)


    The lhcb detector l.jpg

    The LHCb Detector

    • 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)


    Calorimetry l.jpg

    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


    Slide26 l.jpg

    Particle Id

    Essential for hadronic PID

    Aerogel

    flavour tag with

    kaons

    (b  c K)

    background suppression

    two body B

    decay products



    Slide28 l.jpg

    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

    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 l.jpg
    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


    Slide30 l.jpg

    (MeV)

    • 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

    --


    Slide31 l.jpg

    CP conserving strong phase

    approximately

    1 year

    5 year

    (degrees)

    |P/T|=0.1

    0.05

    4-fold discrete ambiguity in 

    0.02

     (degrees)


    Slide32 l.jpg

    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


    Slide33 l.jpg

    color allowed

    doubly Cabibbo suppressed

    comparable decay

    amplitudes

    color suppressed

    Cabibbo allowed

    unknows:

    =65o (1.13 rad)

    b=2.2x10-6

    ()=10o


    Slide34 l.jpg

    Vtb

    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


    Slide35 l.jpg

    addition of channel:

    uncertainty due to:

    ~ 360k / year

    requires full

    angular analysis


    Mixing l.jpg
    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


    Slide37 l.jpg

    Mixing

    Amplitude fit method:

    A, A determined for each by a ML fit


    Slide38 l.jpg

    Vtb

    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


    Slide39 l.jpg

    Sensitivity to:

    events/1y

    13.1k

    BTeV

    6k

    LHCb


    Slide40 l.jpg

    • 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 l.jpg
    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


    Slide42 l.jpg

    contour plots in the

    and

    planes

    • U-spin symmetry:

    • input and

    d()

    (5y)

    events/1y

    BTeV

    --

    --

    32.9k

    0.034

    LHCb

    9.5k


    Rare b decays l.jpg

    • SM : BR ~

    • observation of the decay

    • measurement of its BR

    • SM : BR ~

    • high sensitivity search

    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


    Rare b decays44 l.jpg
    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 l.jpg
    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* -- > 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:


    References l.jpg
    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 l.jpg
    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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