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B Physics and CP Violation

B Physics and CP Violation. Jeffrey D. Richman UC Santa Barbara CTEQ Summer School Madison, June 7-8, 2002. Outline (Lecture 1). Overview of B decays Why B physics is interesting; overview of decay diagrams; introductory discussion of CP violation.

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B Physics and CP Violation

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  1. B Physics and CP Violation Jeffrey D. Richman UC Santa Barbara CTEQ Summer School Madison, June 7-8, 2002

  2. Outline (Lecture 1) • Overview of B decays • Why B physics is interesting; overview of decay diagrams; introductory discussion of CP violation. • Accelerators and b-quark production • The BaBar Detector • Identifying B decays • B-meson lifetimes and mixing • CP Violation (CPv) and the CKM matrix • the CKM hierarchy and the prediction of large CP asymmetries in B decays

  3. Outline (Lecture 2) • CP Asymmetries: • sin(2b): the golden measurement • the struggle for the other angles • Rare decays • Penguins are everywhere! • Semileptonic decays, decay dynamics, and the magnitudes of CKM elements. • Heavy-quark symmetry and Vcb • Prospects and future directions A reference: J. Richman, Les Houches lectures, 1997. http://hep.ucsb.edu/papers/driver_houches12.ps (or send e-mail asking for a copy: richman@charm.physics.ucsb.edu)

  4. I will be unashamedly pedagogical, and I will not aim for the level of impartiality that is customary in a review talk or article. I will be unashamedly selective: many important topics have been left out. There will be a strong bias towards recent results from e+e- colliders at the Y(4S). This is probably not too misleading for now, since BaBar, Belle, and CLEO have to some extent defined the state of the art, especially in CPv and rare decays. However, soon-to-come measurements from the Fermilab Tevatron (CDF, D0) will be of major importance. My own background in b physics: BaBar, CLEO I strongly encourage you to ask questions! Remarks/disclaimers

  5. Goals of B (and Bs) Physics • Can CP violation be understood quantitatively within the Standard Model, or is new physics needed? Perform a comprehensive set of measurements to check for the presence non-SM CP-violating phases. • Make precise measurements of the Standard Model CKM parameters: |Vcb |,|Vub |,|Vtd |,|Vts |,a, b, g,... • Map out and understand rare B decays, especially processes with loops that can be very sensitive to particles outside the Standard Model. • Understand the dynamics of B decays: underlying weak interaction process with overlay of complex strong interaction effects. Progress: HQET, lattice QCD, many measurements to test predictions.

  6. Overview of B Decays • b is the heaviest quark that forms bound states with other quarks (t-quark decays too rapidly). • m(b)<m(t) => the b-quark is forced to decay outside of its own generation • Dominant decays are CKM suppressed: • Relatively long B lifetime: Silicon tracking systems have been essential tools. • Largest single branching fraction: • Many interesting rare decay processes are experimentally accessible (b->uW, gluonic penguins, electroweak penguins).

  7. Leptonic B+ decay not yet observed! Largest expected mode is: Ignoring photon radiation: Used to measure magnitudes of CKM elements: Vcb andVub Amplitude can be rigorously parametrized in terms of form factors. Leptonic and Semileptonic Decays

  8. Theoretical predictions very difficult. Naïve factorization model works reasonably well in predicting pattern of decays. Hadronic Decays: Tree Diagrams • “Color suppressed” • Naïve factorization model probably breaks down. (New data on B->D0p0 and B->D*0p0.) • The color allowed and color suppressed amplitudes interfere constructively in charged B decays. (Opp. effect for D+.)

  9. Both gluonic and electoweak penguins have been observed! The SM mixing rate is dominated by tt (off-shell) intermediate states. Processes with loops: sensitivity to new particles

  10. c b W+ c s d d Processes used for sin2b measurement A color suppressed decay! However, in this case, the rate is enhanced by the relatively large decay constant of the J/y:

  11. Decay modes for sin2b measurement

  12. The C, P, and T Transformations • C, P, and T are discrete transformations: there is no continuously varying parameter, and these operations cannot be constructed from successive infinitesimal transformations. • In all well-behaved quantum field theories, CPT is conserved. A particle and its antiparticle must have equal mass and mean lifetime.

  13. P and C violation in Weak Interactionsis Maximal (V-A) Allowed Not Allowed Allowed P C

  14. A First Look at CP violation • The discovery of CP violation in 1964 was based on the demonstration that the mass eigenstate KL is not an eigenstate of CP, so . • The lifetime separation between BH and BL is tiny, so we must use a different method, in which we compare the rates for CP-conjugate processes. Remove Ks from beam using lifetime difference. CPv small in kaon system!

  15. The Legacy of Kaon Physics “...the effect is telling us that at some tiny level there is a fundamental asymmetry between matter and antimatter, and it is telling us that at some tiny level interactions will show an asymmetry under the reversal of time. We know that improvements in detector technology and quality of accelerators will permit even more sensitive experiments in coming decades. We are hopeful then, that at some epoch, perhaps distant, this cryptic message from nature will be deciphered.” ...J.W. Cronin, Nobel Prize lecture*. J.W. Cronin and V.L. Fitch, Nobel Prize 1980. *J.W. Cronin, Rev. Mod. Phys. 53, 373 (1981). J.H. Christenson, J.W. Cronin, V.L. Fitch, and R. Turlay, Phys. Rev. Lett. 13, 138 (1964).

  16. CP violation and alien civilizations • We can use our knowledge of CP violation to determine whether alien civilizations are made of matter or antimatter, without having to touch them. We have these inside of us Long-lived neutral kaon

  17. CP Violation and Cosmology • A. Sakharov noted (1967) that CP violation has an important connection to cosmology. • 3 conditions for an asymmetry between N(baryons) and N(anti-baryons) in the universe (assuming equal numbers initially due to thermal equilibrium). • baryon-number-violating process • both C and CP violation (helicities not relevant to particle populations) • departure from thermal equilibrium

  18. How can CP asymmetries arise? (I) • When we talk about CP violation, we need to talk about the phases of QM amplitudes. • This is usually very confusing. • some phases are physical; others are not. • many treatments invoke specific phase conventions, which acquire a magical aura. • Need to consider two types of phases • CP-conserving phases:don’t change sign under CP. (Sometimes called strong phases since they can arise from strong, final-state interactions.) • CP-violating phases:these do change sign under CP.

  19. How can CP asymmetries arise? (II) • Suppose a decay can occur through two different processes, with amplitudes A1 and A2. • First, consider the case in which there is a (relative) CP-violating phase between A1 and A2 only. No CP asymmetry! (Decay rate is different from what is would be without the phase.)

  20. How can CP asymmetries arise? (III) • Next, introduce a CP-conserving phase in addition to the CP-violating phase. • Now have a CP asymmetry

  21. Measuring a CP-violating phase • To extract the CP-violating phase from an observed CP asymmetry, we need to know the value of the CP-conserving phase. • In direct CP-violating processes we usually do not know the relative CP-conserving phase because it is produced by strong-interaction dynamics that we do not understand.

  22. B production at the Y(4S) No accompanying pions! The B-meson energy is known from the beam energy. Rate of events vs. total energy in e+e- CM frame: TM (CLEO, CLNS 02/1775)

  23. The machines have unequal (“asymmetric”) energy e+ and e- beams, so two separate storage rings are required. PEP-II: E(e-)=8.992 GeV E(e+)=3.120 GeV bg=0.55 The machines must bring the beams from the separate rings into collision. KEK-B: +-11 mrad crossing angle PEP-II: magnetic separation With two separate rings, the machines can store huge numbers of beam bunches without parasitic collisions. KEK-B: 1224 bunches/beam; I(e+)=716 mA; I(e-)=895 mA PEP-II: 831 bunches/beam; I(e+)=418 mA; I(e-)=688 mA CESR (single ring): 36 bunches/beam; I(e+)=I(e-)=365 mA The New e+e- B factories

  24. PEP-II e+e- Ring and BaBar Detector LER (e+, 3.1 GeV) Linac HER (e-, 9.0 GeV) BaBar BaBar PEP-II ring: C=2.2 km May 26, 1999: 1st events recorded by BaBar

  25. The Y(4S) Boost • The purpose of asymmetric beam energies is to boost the B0B0 system relative to the lab frame. • By measuring Dz, we can follow time-dependent effects in B decays. • The distance scale is much smaller than in the kaon decay experiments that first discovered CP violation!

  26. From CESR (1 ring, E symmetric) toPEP-II (2 rings, E asymmetric) Top view of PEP-II interaction region showing beam trajectories. Pretzel orbits in CESR (36 bunches, 20 mm excursions) (10X expansion of vertical scale)

  27. The race between BaBar/PEP-II and Belle/KEK-B Belle Exceeds design luminosity!

  28. Production cross sections Y(4S): pp at Tevatron: pp at LHC: b fraction (ratio of b cross section to total hadronic cross section) Y(4S): 0.25 pp at Tevatron: 0.002 pp at LHC: 0.0063 Comments Triggering: so far, most B branching fractions have been measured at e+e- machines, because CDF, D0 triggers were very selective in Run 1. Also, PID & g detection arebetter at Y(4S) experiments so far.) Hadron colliders produce Bs and b-baryons. (LEP also.) New displaced-vertex triggers at hadron-collider experiments should make a dramatic improvement. e+e- vs. pp and pp

  29. The BABAR Detector 1.5 T solenoid DIRC (particle ID) CsI (Tl) Electromagnetic Calorimeter e+ (3.1GeV) Drift Chamber Instrumented Flux Return e- (9 GeV) SiliconVertex Tracker • SVT: 97% efficiency, 15mm z resol. (inner layers, perpendicular tracks) • Tracking : s(pT)/pT = 0.13% PT 0.45% • DIRC : K-p separation >3.4s for P<3.5GeV/c • EMC: E/E = 1.33% E-1/4  2.1%

  30. BaBar Detector center line DIRC: quartz bars standoff box PM tubes Superconducting magnet (1.5 T) Drift chamber e- e+ CsI crystals Muon detector & B-flux return Silicon Vertex Tracker

  31. BaBar Event Display(view normal to beams) EM Calorimeter: 6580 CsI(Tl) crystals (5% g energy res.) Cerenkov ring imaging detectors: 144 quartz bars (measure velocity) Tracking volume: B=1.5 T Rdrift chamber=80.9 cm Silicon Vertex Tracker 5 layers: 15-30 mm res. (40 measurement points, each with 100-200 mm res. on charged tracks)

  32. Innermost Detector Subsystem: Silicon Vertex Tracker Installed SVT Modules Be beam pipe: R=2.79 cm (B mesons move 0.25 mm along beam direction.)

  33. 50mm 300mm BaBar Silicon Vertex Tracker • 5 layers of double-sided silicon-strip detectors (340) 80 e-/hole pairs/mm

  34. Quartz bar Active Detector Surface Particle Cherenkov light Particle Identification (DIRC)(Detector of Internally Reflected Cherenkov Light) • Measure angle of Cherenkov cone • Transmitted by internal reflection • Detected by PMTs No. light bounces (typical)=300

  35. Particle Identification with the DIRC. • DIRC c resolution and K- separation measured in data  D*+ D0+ (K-+)+ decays >9s s(qc)  2.2 mrad K/p Separation 2.5s

  36. Particle Identification E/p from E.M.Calorimeter Shower Shape 0.8 < p < 1.2 GeV/c E/p > 0.5 1 < p < 2 GeV/c • Electrons – p* > 0.5 GeV • shower shapes in EMC • E/p match • Muons – p* > 1 GeV • Penetration in iron of IFR • Kaons • dE/dx in SVT, DCH • C in DRC e e p p qc from Cerenkov Detector dE/dx from Dch 0.8 < p < 1.2 GeV/c 0.5 < p < 0.55 GeV/c e e p p e p

  37. Identifying B Decays in BaBar • Select “candidate daughter particles” using particle ID, etc. • Compute the total 4-momentum: (E, p)=(E1+E2+E3, p1+ p2 +p3) • Compute invariant mass: m2=E2-|p|2 mes 3 MeV sDE  15 MeV Gives 10x improvement in mass resolution. All Ks CP modes Nsig 750 Purity 95% DE mes

  38. sin2b Signal and Control Samples J/Y Ks (Ks p+p-) Bflav mixing sample J/Y Ks (Ks p+p-) CP=-1 Y(2s) Ks J/Y Ks (Ksp0p0) J/Y KL J/Y Ks (Ksp0p0) CP=+1 J/Y K*0 (K*0  Ksp0) c1 Ks J/Y K*0 (K*0  Ksp0)

  39. Tag B sz ~ 170 mm CP B sz ~ 70 mm J/Y U(4s) K0 bg = 0.56 Dz Dt @Dz/gbc gbctB@ 250 mm The Lorentz Boost • The asymmetric beam energies of PEP-II allow us to measure quantities that depend on decay time. e- e+ 9.0 GeV 3.1 GeV

  40. Measurement of Decay Time Distributions B0 decay time distribution (linear scale) background

  41. B0 and anti-B0 mesons spontaneously oscillate into one another! (Mixing also occurs with neutral kaons.) • Neutral B mesons can be regarded as a coupled, two-state system. • To find the mass eigenstates we must find the linear combinations of these states that diagonalize the effective Hamiltonian.

  42. Interpretation of the Effective Hamiltonian • The effective Hamiltonian for the two-state system is not Hermitian since the mesons decay. Quark masses, strong, and EM interactions Decays

  43. CP Violation inMixing • Compare mixing for particle and antiparticle off-shell off-shell on-shell on-shell CP-conserving phase

  44. CP violation in mixing, continued • To produce a CP asymmetry in mixing, M12 and G12 must not be collinear and both must be nonzero: No CP violation in mixing CP violation in mixing

  45. Time evolution of states that are initially flavor eigenstates General case; allows CP violation.

  46. CP Violation in B Mixing is Small • When CP violation in mixing is absent (or very small), we have • In the neutral B-meson system, the states that both B0 and B0 can decay into have small branching fractions, since normally lead to different final states. Can have (Cabibbo suppressed) and (b->u is CKM suppressed). So the SM predicts not yet observed

  47. Time evolution of states that are initially flavor eigenstates In these formulas, we have assumed that DG/G<<1and have set

  48. The Oscillation Frequency (Dm) • In the neutral B-meson system, the mixing amplitude is completely dominated by off-shell intermediate states (Dm) [contrast with the neutral kaon system]. • Calculation of the mixing frequency • Time-dependent mixing probabilities and asymmetry

  49. e- e+ W- W+ n n b b c c Tagging CP asymmetry is between B0 fcp and B0  fcp Must tag flavor at Dt=0 (when flavor of two Bs is opposite). Use decay products of other (tag) B. Leptons : Cleanest tag. Correct 91% Kaons : Second best. Correct 82% W+ W- c c s K- K+ s b b W- u u W+ d d

  50. Effect of Mistagging and Dt Resolution w=Prob. for wrong tag No mistagging and perfect Dt D=1-2w=0.5 Nomix Mix Dt Dt D=1-2w=0.5 Dt res: 99% at 1 ps; 1% at 8 ps Dt Dt

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