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Reflections on Beauty: CP Asymmetries in B Meson Decay

Reflections on Beauty: CP Asymmetries in B Meson Decay. • Weak interactions & the b-quark: CKM matrix • B (eauty) mesons & CP • B meson production: e + e – ->  (4S) • Belle/Babar experiment. K. Kinoshita University of Cincinnati. Symmetry of Physical Laws.

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Reflections on Beauty: CP Asymmetries in B Meson Decay

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  1. Reflections on Beauty:CP Asymmetries in B Meson Decay • Weak interactions & the b-quark: CKM matrix • B(eauty) mesons & CP • B meson production: e+e– -> (4S) • Belle/Babar experiment K. Kinoshita University of Cincinnati

  2. Symmetry of Physical Laws In interaction-free universe (4-d, relativistic QM) • massless particles • symmetric in transformations P(r<—>-r), C(particle<—> antiparticle), T(t<—>-t) Add interactions: emission/absorption of field quantum • mass via self-interaction • interaction strength/probability a “charge” g2a“coupling constant” • symmetry info in vertex Forces: Strong, Electromagnetic, Weak, Gravitational coupling ~ 10-5, quanta W±, Z0

  3. Weak Interaction The only force known to • allow particle to change identity • violate P symmetry (maximally) right-handed particles, left-handed antiparticles. no coupling to LH particles, RH antiparticles. • violate CP symmetry (a little) Why is CP violation of interest? • matter-antimatter asymmetry in universe requires CP violating interactions (Sakharov 1967) What is source of observed CP asymmetry?

  4. We have an interesting possibility ... Standard Model =12 fermion flavors (+antifermion) + strong, EM, weak forces, unification of EM+weak distinguished ONLY by mass (?) • fermions: 3 generations x 2 types x 2 ea (doublets) • all stable, if not for weak interaction

  5. Weak couplings • Z0 • "neutral current” • DQ = 0 • l±<–>l± • q+2/3 <–> q'+2/3 • NO generation x-ing • –> no flavor-changing • single coupling strength • W± • "charged current” • DQ = ±1 • l–<–> n • q+2/3 <–> q'–1/3 • (small) generation x-ing, quark only • all different strengths Large # of fundamental "charges" – can this be simplified?

  6. GIM mechanism Explains • suppression of flavor-changing neutral currents • multiplicity of charged current couplings • for >2 generations, CP violation Picture • strong doublets, “degenerate”generations, perturbed by weak force: new doublets no generation x-ing, universal W-coupling (=gF, seen in leptons) d', s', b' are linear combinations of d, s, b: u c t d' s' b' Cabibbo-Kobayashi-Maskawa (CKM) matrix } complex preserves metric “ orthogonality = unitary For 3 x 3,unitarity constrains {9 real+9 imaginary} dof to 4 free parameters, incl. 1 irreducible imaginary part

  7. Unitarity of CKM (Wolfenstein parametrization): dsb uVud Vus Vub cVcd Vcs Vcb tVtd Vts Vtb 1/gF x W-couplings: 1-l2/2 ll3A(r-ih) -l 1-l2/2l2A l3A(1-r-ih)-l2A 1 @ • from decay rates, • l = 0.220 ± 0.002 • A =0.81 ± 0.08 • |r-ih| =0.36 ± 0.09 • |1-r-ih| = 0.79 ± 0.19 Unitarity condition: Vji*Vjk=dik {i=1,k=3}: Vub*Vud+Vcb*Vcd+Vtb*Vtd=0 Vub*Vud Vcb*Vcd Vtb*Vtd Vcb*Vcd => + 1 + = 0 -(rih) -(1-rih) a (r, h): "unitarity triangle"   Self-consistent if CKM is correct

  8. Complex couplings revealed via CP asymmetry – t-integrated rates Gµ |<f|Hint|i>|2 => not sensitive to phase: CP{ } = – need interference between processes: • e.g., decays to CP eigenstate - paths w/wo mixing interfere • -> CP-dependent oscillation in decay time distributions {cc}+{Ks,KL,π0}

  9. CP Asymmetry of B -> J/y Ks tree (real Vij) mixing+tree arg(Vtd2) = 2f1 -> * No theoretical uncertainty

  10. Measure time dependence - what’s needed? • •B pair production  e+e– -> (4S) -> BB • •Measure decay-time difference Asymmetric energy e+e–(@KEKB:g b ct200mm) good vertexing  silicon strip vertex detector • •Find CP eigenstate decays high quality ~ detector  Belle/Babar • •Tag other B’s flavor good hadron id  dE/dx, Aerogel, TOF, DIRC • good lepton id  CsI, multilayer µ • •Lots of B mesons ~108 (Br(BfCP) ~ 103) very high Luminosity KEKB/PEP2

  11. BB pair production: Upsilon e+e– -> (4S) -> BB + 20 MeV B+:B0~1:1 CLEO KEKB: 8.0 GeV e– + 3.5 GeV e+ IP size = 77µmx2.0µmx4.0mm Event rate Cross section ~ 1 nb = 10-33 cm2 dN dt = s x L ~ 10 s–1 ~108yr–1 Luminosity (collision rate) 1034 cm–2s–1(design; currently 5.5x1033 @KEKB) Currently@Belle: 3x107 BB events (published), 4.8x107 on tape

  12. Time measurement at (4S) (4S): CP=-1, conserved until first B decay (t=0) identify b/b {flavor tag} Reconstruct CP=±1 mode @ t=Dt +e- -> (4S) t=0 Dz≈Dtbgc J/y ~200 µm  B2 Ks e- e+ B1 flavor tag: e, µ, K±, ...

  13. Detector: e.g. Belle Designed to measure CP asymmetry Charged tracking/vertexing - SVD: 3-layer DSSD Si µstrip (~55 µm) – CDC: 50 layers (He-ethane) Hadron identification – CDC: dE/dx (~7%) – TOF: time-of-flight (~95 ps) – ACC: Threshold Cerenkov (aerogel) Electron/photon – ECL: CsI calorimeter (1.5%@1 GeV) Muon/KL – KLM: Resistive plate counter/iron

  14. Belle Collaboration 274 authors, 45 institutions many nations

  15. CP mode reconstruction J/l + l B0 J/ Ks(+ )“golden mode” Ks+ 1lepton+1”not-hadron” ~4MeV/c2 Ks mass4

  16. B0 J/ Ks(continued) 457 events ~3% background ~10MeV DE • Kinematics for final selection: • DE E*cand–E*beam0 (E*beams 1/2/2) • 10-50 MeV res, depends on mode • Mbc (Beam-constrained mass) • Mbc(E*beam2-p*cand2 )1/2 ~3MeV/c2 Signal region Mbc

  17. Other charmonium+K B0 J/ Ks(00 ) J/ KS(+& 00) (2S)(l+l & J/+)KS c1(J/) KS c(KSK+-, K+K-0) KS J/ KL J/ K*0 (KLp0) (mostly)  x f =-1 76 events ~ 9 bkg  x f=+1

  18. Other charmonium c1 1st observation of inclusive Bc2X c2 M(ll)  M(ll)

  19. J/y KL • • J/y: {tight mass cut} • 1.42<py*<2.00 GeV/c • • KL: {KLM/ECL cluster w/o track, • >1 KLM superlayers (resolution~ 3° (1.5° if ECL)} within 45˚ of expected lab direction • • Require cand to have B mass, calculate momentum in CMS (pB*) (~0.3 GeV for signal) • • backgrounds: random (from data), “feeddown,” known • modes - estimate via MC

  20. CP candidates J/y KL Fully reconstructed modes 750 candidates ~58 bkg Nsig = 346 events Nbkg = 223 evts

  21. Flavor tagging l- l+ b c s K– D*+ D0 π+ • – high-p lepton (p*>1.1 GeV): b->l- • – net K charge b->K– • – medium-p lepton, b->c-> l+ • – soft π b->c{D*+->D0π+} • * multidimensional likelihood, e>99% • Significance of CP asymmetry depends on • – tagging efficiency e • – wrong-tag fraction w (measured w data) • - effective efficiency = e(1-2w)

  22. Dz: vertex reconstruction • Constrained to measured IP in r-f • • BCP: sz~75 µm (rms) • use only tracks from J/y • • Btag: sz~140 µm (rms) • remaining tracks, excluding Ks; • iterate, excluding tracks w. poor c2/n • resolution includes physics (e.g. charm) • • Overall eff. = 87% m+ m- K- Dz K-

  23. Raw Dt distributions distribution in Dt~Dz/bgc, unbinned max. likelihood fit • CP is violated!!! • seen in raw data • large effect

  24. Prepare to fit for sin2f1 • B0 lifetime = 1.548±0.032 ps, ct=464±10 µm mixing Dm = 0.47±0.02 ps–1; cT~4.0 mm only ~ 1 cycle of oscillation measurable • True CP asymmetry is diluted: background to CP reconstruction incorrect flavor tag rate vertex resolution - not exactly as modeled all need checks in data -> Use same methods to make other (better known) physics measurements: B0 mixing, B lifetime, D lifetime, null CP multiply by bg for lab length (decay in flight) }

  25. Wrong tag fraction via mixing • Same fit method, but • CP->flavor-specific • • BD*-l+n, D(*)-π+, D*-r++flavor tag • • separate same-, opp-flavor events • • fit to Dz: mixing asymmetry, w: • • "effective tagging efficiency" • eeff=S(1-2wl)2etag, l =(27.0±2.2)% • 99.4% of candidates tagged • (good agreement w MC) l Flavor tags classified by (MC) Purity - 6 bins

  26. Dt resolution function • • Double Gaussian, parameters calculated event-by-event, includes effects of • - detector resolution • - poorly measured tracks • - bias from e.g. charm • - approximation of Dt=Dz/bgc • • form, parameters from • - Monte Carlo • - fits for D0K-π+, BD*ln lifetimes • • validate: B lifetime, same fitting • t0=1.55±0.02 ps(PDG2000: 1.548±0.032 ps) • t+=1.64±0.03 ps (PDG2000: 1.653±0.028 ps) tail fraction: 1.8%

  27. Fitting Dt distribution • • distribution in Dt~Dz/bgc • • unbinned max. likelihood fit, includes • - signal root distribution (analytic) • - wrong tag fraction (const) • - background: right & wrong tag (MC, parametrized) • - detector & tagging Dz resolution • (parametrized,evt-by-evt)

  28. Results All modes combined: sin2f1=0.99±0.14(stat)+0.06(sys) binned in Dt NB-NB NB+NB xf weighted by CP sensitivity curve from unbinned fit

  29. Control sample: B0 non-CP states use: B0D(*)-π+, D*-r+, D*-l+n, J/yK*(K+π-) “sin2f1” 0.050.04 (statistical error only)

  30. fit CP –1 and CP+1 separately: CP=-1 sin2f1 opposite!! 0.840.17 NB-NB NB+NB CP=+1 1.310.23 weighted by CP sensitivity (statistical errors only)

  31. Systematic errors

  32. Compare with other experiments

  33. Result in context Belle’s 1s band (two sol’ns) (locating tip of unitarity triangle) BaBar Vub

  34. Summary/Prospects Successful run of Belle in 2000-1 • sin 2f1: 30.5 fb–1 on (4S), 1137 tagged events • 19 papers published or submitted Next • higher precision on sin2f1 data as of 1/23/02 - 48 fb–1; anticipate 100 fb–1 by summer • Lum: peak 5.5x1033cm–2s–1; 24 hrs 311 pb–1; month 6120 pb–1 • other angles - need >300 fb–1 - within sight!

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