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Lectures on B Physics

Lectures on B Physics. Bob Kowalewski University of Victoria Currently at La Sapienza and the Laboratorio Nazionale di Frascati. Overview of the lectures . Lecture 1: History, facilities, B production and decay, CKM matrix Lecture 2: Semileptonic and radiative B decays

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Lectures on B Physics

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  1. Lectures on B Physics Bob Kowalewski University of Victoria Currently at La Sapienza and theLaboratorio Nazionale di Frascati Kowalewski --- Perugia lectures

  2. Overview of the lectures • Lecture 1: History, facilities, B production and decay, CKM matrix • Lecture 2: Semileptonic and radiative B decays • Lecture 3: Oscillations and CP violation • Lecture 4: CP violation Kowalewski --- Perugia lectures

  3. Lecture 1 • History of B physics: 1977 – 2004 • Significant facilities, past and present • B meson production and decay • CKM matrix Kowalewski --- Perugia lectures

  4. Historical context • 1974 was an exciting year for particle physics, with the discovery of the (2nd generation) charm quark (J/ψ) and the (3rd generation) τ lepton • The search for a 3rd generation of quarks was motivated by symmetry with the lepton sector as well as by the insight of Kobayashi and Maskawa (in 1973) that a 3x3 quark mixing matrix has an irreducible imaginary parameter that can lead to CP violation Kowalewski --- Perugia lectures

  5. Upsilon experiment at FNAL • 400 GeV proton beam incident on target • Look for muon pairs; measure invariant mass Kowalewski --- Perugia lectures

  6. Initial results Kowalewski --- Perugia lectures

  7. Discovery of the b quark • 1977: Lederman et al. discover Υ resonances in μ+μ- mass spectrum  Υ(1S), Υ(2S), Υ(3S) • Interpreted as bound states of a new quark, b, the first quark of the 3rd generation: • Electromagnetic decay seen (μ+μ-) • Decay width is narrow • Lederman receives Nobel Prize in 1988 for this work. Kowalewski --- Perugia lectures

  8. Later data • States seen are the first 3 radial excitations of the vector bb stateΥ(1S),Υ(2S),Υ(3S) • Observed width is experimental resoln • Quantum numbers JPC=1-- • b mass ~ 4.6 GeV Kowalewski --- Perugia lectures

  9. Limitations of technique • Only muon pairs are recorded! • Limited mass resolution • Not well suited for fine-grained study • No clear signature for separating b-flavored particles (i.e. bq - B mesons) from background • Need e+e- experiment to examine in detail Kowalewski --- Perugia lectures

  10. First e+e- facilities • At the time of Υ discovery, Cornell was building CESR, a 16 GeV center-of-mass e+e- collider • CESR was subsequently redesigned to run in the Υ energy range: 10-11 GeV • The CLEO and CUSB detectors started collecting data in 1979 Kowalewski --- Perugia lectures

  11. e+e- takes over 3S 10.28 10.44 • 3 narrow Υ States seen immediately;observed width = beam energy spread • Broader Υ(4S) resonance seen at 10.58 GeV; above BB threshold 1S 2S 9.40 9.50 9.96 10.02 • B0 and B+ discovered by CLEO (1982) • B* mesons at CUSB (1985) • ARGUS detector (DORIS-II) starts at DESY (1982) 2mB Kowalewski --- Perugia lectures

  12. CESR and CLEO Kowalewski --- Perugia lectures

  13. DORIS-II and ARGUS Kowalewski --- Perugia lectures

  14. Initial findings • B mesons have significant semileptonic branching fractions: BF(BXℓν) ~ 10% • B mesons are spin 0 • B+ and B0 have mB = 5.279 GeV (Δm<1 MeV) • B decay dominated by bc transition (|Vcb| >> |Vub|) • B mesons have long (~1.5 ps) lifetimes (|Vcb|<<1) • FCNC decays not observed (constrain topless models) Kowalewski --- Perugia lectures

  15. Early discoveries – B0 mixing • B0 and B0 mix to produce mass eigenstates; Δm~0.5 ps-1. First seen by ARGUS (1987) • At Υ(4S), ~1 B0 in 6 decays as B0 • Confirmed by CLEO in 1988 • Initial B flavor cannot be determined; need1 B to decay first Kowalewski --- Perugia lectures

  16. Fast-forward 14 years… • The flavor oscillation is now mapped out over ~1.5 full periods • Δm= (0.502±0.006) ps-1 5.0 10.0 15.0 1 2 4 dileptons 20.7 fb-1 1 2 4 Belledileptons29.4 fb-1 unmixed mixed Kowalewski --- Perugia lectures

  17. Early discoveries – buℓν • bu transitions observed by CLEO (1989). • Signature is an excess of leptons with momenta above the kinematically allowed range for bc decays. • bu rate ~ 1/50 bc rate bc qq Kowalewski --- Perugia lectures

  18. 15 years later… Data (continuum sub) MC for BB background S/B ~ 1/25 at 2.0 GeV! Kowalewski --- Perugia lectures

  19. Radiative Penguin decays • 1993 – exclusive decay BK*γ seen in CLEO • 1995 – inclusive bsγ process measured (much harder!) • Rate probes new physics BaBar B0K*0γ Kowalewski --- Perugia lectures

  20. Contributions from higher energy e+e- machines • Full range of b-flavored hadron states produced • The PEP (SLAC) and PETRA (DESY) experiments (√s~30 GeV) made early measurements of the average B lifetime • LEP experiments and SLD made numerous contributions in Z decays: • Precise B lifetimes; lifetime differences • Discovery of Bs and Λb • Discovery of P-wave B mesons (B**) Kowalewski --- Perugia lectures

  21. P-wave B** Discovery • Resonant structure appears in the unlike-sign B+π±distribution • Mass resolution insufficient to separate states Excess in B+π- combinations B+ π+combinations agree with MC B+π± invariant mass Kowalewski --- Perugia lectures

  22. Hadron colliders for b physics • Fermilab Tevatron experiments CDF and D0 have made important contributions to • Bs decays • b-hadron lifetimes • Future hadron facilities (LHC-b, B-TeV and, possibly, ATLAS and CMS at LHC) may make a number of important measurements • Bs oscillations and CP violation • Leptonic and some radiative B decays Kowalewski --- Perugia lectures

  23. The B factory era • CESR had an impressive history…but new challenges require new facilities B factories>100 fb-1 / year Kowalewski --- Perugia lectures

  24. B factory design goals • Major physics motivation: CP violation in B decays • Requires asymmetric beam energies (Odone) • Requires high luminosity: • KEK-B proposed at KEK; luminosity target 1 ×1034 cm-2 s-1 • PEP-2 proposed at SLAC; luminosity target 0.3×1034 cm-2 s-1 • Peak luminosity of 1034 cm-2 s-1 gives integrated luminosity per year of ~ 150 fb-1 or ~2×108 Υ(4S)decays Kowalewski --- Perugia lectures

  25. PEP-II and KEK-B Jonathan Dorfan Pier Oddone Kowalewski --- Perugia lectures

  26. B factories: PEP-II and KEK-B BaBarBelleLmax (1033/cm2/s)9.2 13.9 best day (pb-1)681 944 total (fb-1)244 338 • Both B factories are running well: Belle Kowalewski --- Perugia lectures

  27. B factory detectors • Belle and BaBar are similar in performance; some different choices made for Cherenkov, silicon detectors • Slightly different boost, interaction region geometry (crossing angle) CsI (Tl) BaBar DIRC e+ (3.1 GeV) Belle e- (9 GeV) IFR SVT DCH Kowalewski --- Perugia lectures

  28. The collaborations • By any pre-LHC standard, this is big science; BaBar has ~ 600 members, Belle ~ 300 (not all pictured in either case!) KEKB / Belle Pep2 / BaBar Kowalewski --- Perugia lectures

  29. B meson production • Production in e+e- at Υ(4S) {Z} • cross-section ~1.1nb, purity (bb / Σiqiqi) ~ 0.3 {7nb, 0.22} • simple initial state: BB in p-wave, decay products overlap{b quark hadronizes to B+: B0: Bs: b-baryon ~ 0.4, 0.4, 0.1, 0.1; b and b jets separated} • “easy” to trigger, apply kinematic constraints • Production at hadron machines (gluon fusion) • cross-sections much higher (×104) • All b hadrons are produced • triggering harder, purity (b / Σiqi) ~ (few/103) Kowalewski --- Perugia lectures

  30. Y(4S) experiments • e+e- → Y(4S) → B+B- or B0B0; roughly 50% each • B nearly at rest (βγ ~ 0.06) in 4S frame; no flight info • B energy = ½ c.m. energy; valuable constraint, since σE~50 MeV for reconstruction, ~5 MeV for e+e- beams on peak BB off peak (q=u,d,s,c) qq 2mB Kowalewski --- Perugia lectures

  31. Asymmetric B factories • Boost CM along beam (z) axis • Separation of B and B decay ~ βγcτB ~ 250 μm • Boost imposes asymmetry in detector design • Required luminosity is large since CP eigenstates have small product BF to states with clean signatures; e.g. BF(B0J/ψ(ℓ+ℓ-) KS) < 10-4 • Angular coverage is a compromise between luminosity (quadrupole magnets close to IR) and detector acceptance Kowalewski --- Perugia lectures

  32. B decay basics • B mesons are the lightest b-flavored particles; they must decay weakly (Δb=1) • The 0th order picture is of a free b quark weak decay • Putting back the light quark we get the spectator (or external W emission) decays • Other decay diagrams are suppressed either by color matching or some power of 1/mB. Kowalewski --- Perugia lectures

  33. b quark decay c e νeb • Charged-current Lagrangian in SM: • Since mb<< MW, the effective 4-fermion interaction is • CKM suppressed (|Vcb|<<1) → long lifetime ~ 1.5ps ×3 for color Kowalewski --- Perugia lectures

  34. b quarks and B mesons… • The b quark decay is simple • B meson decay is not… Vcb Kowalewski --- Perugia lectures

  35. Spectator decays Semileptonic ~ 26% Hadronic ~ 73% Theoretical predictions tend to have large uncertainties. Factorization (W decay products do not mix with other quarks) partly works single hadronic current; ~reliable theory Heavy Quark Expansion BF form factors Kowalewski --- Perugia lectures

  36. b W+ l+,n b B0 l–,l’–,n d W– u Leptonic < 10-4, 7,11τ, μ, e Leptonic decays B+ • Suppressed by helicity (like πeν) • measures fB×|Vub| Helicity suppressed; FCNC In SM: B(B0m+m–) ~ 8×10-11B(B0nn) ~ zero Kowalewski --- Perugia lectures

  37. n,ℓ ℓ,ν n,ℓ n,ℓ n,ℓ s,d b q q q Non-spectator decays Colour-suppressed; Includes all bcc q’q EW penguins; 2nd order weak Large mt enhances these loop diagrams gluonic penguins; 2nd order weak W exchange Kowalewski --- Perugia lectures

  38. Box diagrams • 2nd order Δb=2 transition takes B0→B0 making decay eigenstates distinct from flavour eigenstates • Large mt makes up for Weak suppression B0 → B0: (B0→B0)/ B0 = 0.18 Kowalewski --- Perugia lectures

  39. CKM matrix • Kobayashi and Maskawa noted that a 3rd generation results in an irreducible phase in mixing matrix: • Observed smallness of off-diagonal terms suggests a parameterization in powers of sinθC 3 x 3 unitary matrix. Only phase differences are physical, → 3 real angles and 1 imaginary phase Kowalewski --- Perugia lectures

  40. d s b u c t Wolfenstein++ parameterization Buras, Lautenbacher, Ostermaier, PRD 50 (1994) 3433. • shown here to O(λ5) where λ=sinθ12=0.22 • Vus, Vcb and Vub have simple forms by definition • Free parameters A, ρ and η are order unity • Unitarity triangle of interest is VudV*ub+VcdV*cb+VtdV*tb=0 • Note that |Vts /Vcb| = 1 + O(λ2) all terms O(λ3) Kowalewski --- Perugia lectures

  41. A Unitarity Triangle  Rt Ru g b Kowalewski --- Perugia lectures

  42. B decays – a window on the quark sector • The only 3rd generation quark we can study in detail • Investigate flavour-changing processes, oscillationsCKM matrix Cabibbo angle B lifetime, decay CP Asymmetries (phase) BdBd and BsBs oscillations =1 Kowalewski --- Perugia lectures

  43. Surveying the unitarity triangle • The sides of the triangle are measured in b→uℓν and b→cℓν transitions (Ru) and in Bd0-Bd0 and Bs0-Bs0 oscillations (Rt) • CP asymmetries measure the angles • Vub, Vcb and Vtd measure the sides Rt  Ru g b GET A BETTER PICTURE Kowalewski --- Perugia lectures

  44. End of Lecture 1 Kowalewski --- Perugia lectures

  45. Lecture 2 – Semileptonic and Radiative B Decays • B meson decays – role of QCD • Heavy Quark symmetry • Exclusive semileptonic decays • Inclusive semileptonic decays • Radiative decays • p.s. – se parlo troppo velocenon esitate a dirmelo Kowalewski --- Perugia lectures

  46. Surveying the unitarity triangle • The sides of the triangle are measured in b→uℓν and b→cℓν transitions (Ru) and in Bd0-Bd0 and Bs0-Bs0 oscillations (Rt) • CP asymmetries measure the angles • Today we’ll talk about the rings Rt  Ru g b GET A BETTER PICTURE Kowalewski --- Perugia lectures

  47. Recall: • The b quark decay is simple • B meson decay is less so… Vcb Kowalewski --- Perugia lectures

  48. B hadron decay – parton model • Bound b quark is virtual and has some “Fermi momentum” • b quark then has pb = pF and Eb = MB - pF, somb =√( MB2 - 2MBpF ) • Parton model usually assigns pF from a Gaussian with r.m.s. of ~ 0.5 GeV • pF ~ 0.5 GeV, corresponds to mb ~ 4.8 GeV gives a reasonable description of some inclusive spectra (e.g. pe) • Ad-hoc model; hard to assign uncertainties to predictions Kowalewski --- Perugia lectures

  49. Xhνe e Beyond parton model… B • Parton model had some successes, but did not provide quantitative estimates of theoretical uncertainties. • How does QCD modify the weak decay of the b quark? • QCD becomes non-perturbative at ΛQCD ~ 0.5 GeV but is perturbative for mb: αs(mb)~0.22 • Modern approaches, based on heavy quark symmetry: • use the operator product expansion (OPE) to separate short- and long-distance physics • Leads to effective field theories, e.g. HQE, SCET… • Used to calculate form factors in lattice QCD Kowalewski --- Perugia lectures

  50. Heavy Quarks in QCD • Heavy Quarks have mQ >> ΛQCD (or Compton wavelength λQ << 1/ΛQCD ) • Soft gluons (p ~ ΛQCD) cannot probe the quantum numbers of a heavy quark → Heavy Quark Symmetry • γbinding e- and N in atoms can’t probe nuclear mass, spin… isotopes have similar chemistry! b Kowalewski --- Perugia lectures

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