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CP Violation: Recent Measurements and Perspectives for Dedicated Experiments

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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CP Violation: Recent Measurements and Perspectives for Dedicated Experiments

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  1. 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

  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. 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!

  6. SM at work • neutral currents, charm, W and Z bosons;

  7. 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

  8. 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

  9. = 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

  10. 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  

  11. 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

  12. 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

  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. B0zCP e+e- (4s) = 0.56 B0ztag 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. p (p) Generic experimental issues f B p B 1 cm triggering decay time resolution particle ID neutrals detection flavour tagging systematic effects

  21. 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

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

  25. 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

  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 ATLAS 165k 18 0.017 CMS 433k 16 0.015 Standard Model: strong indication of New Physics! Observation of direct asymmetries (10% level):

  29. 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

  30. (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 --

  31. CP conserving strong phase approximately 1 year 5 year (degrees) |P/T|=0.1 0.05 4-fold discrete ambiguity in  0.02  (degrees)

  32. Tree terms • 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

  33. color allowed doubly Cabibbo suppressed comparable decay amplitudes color suppressed Cabibbo allowed unknows: =65o (1.13 rad) b=2.2x10-6 ()=10o

  34. 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

  35. addition of channel: 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 fit proper time distributions for untagged: tagged: tagged 43fs 72k BTeV 43fs 34.5k LHCb

  37. Mixing Amplitude fit method: A, A determined for each by a ML fit

  38. 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

  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 (xS=40) • CP admixture • clean experimental signature • full angular analysis events 370k (5y) LHCb 0.03 0.03 600k (3y) CMS (xS=40)

  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. related by U-spin symmetry • 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

  43. 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

  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* -- > 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 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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