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Measurement of the CP Violation Parameter sin2  1 in B 0 d Meson Decays

Measurement of the CP Violation Parameter sin2  1 in B 0 d Meson Decays. 6/15 Kentaro Negishi. Belle 実験. ← bg = 0.425. KEKB 加速器：電子 (e - )8.0GeV 、陽電子 (e + )3.5GeV 　重心エネルギー 10.6GeV の非対称衝突型加速器 (10.6GeV = B 中間子一対がしきい値で生成 ). e - e + 衝突器として世界一のルミノシティ ピークルミノシティ :1.7×10 34 /cm 2 /s

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Measurement of the CP Violation Parameter sin2  1 in B 0 d Meson Decays

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  1. Measurement of the CP Violation Parameter sin21 in B0d Meson Decays 6/15 Kentaro Negishi

  2. Belle実験 ← bg = 0.425 KEKB加速器：電子(e-)8.0GeV、陽電子(e+)3.5GeV 　重心エネルギー10.6GeVの非対称衝突型加速器 (10.6GeV = B中間子一対がしきい値で生成) e-e+衝突器として世界一のルミノシティ ピークルミノシティ:1.7×1034/cm2/s これまでに約8億個のB中間子を生成 本論文でのデータは10.5 fb-1 周長3km

  3. B中間子の崩壊はBelle検出器でとらえる Belle検出器はいくつかのサブ検出器からなる

  4. Spec of the Belle • 3-layer SVD • 50-layer CDC • 1188 ACC • 128 TOF • 8736 CsI(Tl) crystals ECL • 1.5 T • 14-layer of 4.7-cm-thick iron KLM • Resolution • Momentum for charged trk (spt/pt)2 = (0.0019pt)2 + (0.0034)2 pt [GeV] • Impact parameter srf ~ sz = 55 mm • Specific ionization sdE/dx = 6.9 % (for minimum ionizing pions) • TOF flight-time sTOF = 95 ps • K± identification efficiency ~ 85 %, p± fake rate ~ 10 %, p < 3.5 GeV • Energy for g (sE/E)2 = (0.013)2 + (0.0007/E)2 + (0.008/E1/4)2 E [GeV] Eg > 20 MeV • e± identification efficiency >90 %, hadron fake rate ~ 0.3 %, p > 1GeV • ± identification efficiency >90 %, hadron fake rate < 2 %, p > 1GeV • KL angle 1.5° ~ 3°

  5. Motivation • The variable time-dependent asymmetry shows that the measurement of decays B0 and B0 to CP eigenstates is sensitive to f1.

  6. Decay and subdecay mode • f = -1 • J/y(l+ l-) KS(+-) • J/y(l+ l-) KS(00) • (2S)(l+ l-) KS(+-) • (2S)(J/y+-) KS(+-) • C1(J/y) KS(+-) • C(K+ K-0) KS(+-) • C(KS K- +) KS(+-) • f = +1 • J/(l+ l-) 0 • J/(l+ l-) KL • For the measurement of A(t), CP eigenstate mode is used.

  7. Selection criteria • J/y, (2S) →l+ l- • opposite charged tracks are positively identified as lepton. • For J/y(l+ l- KS(+-) mode, the requirement for one of the tracks is relax. • e+ e- • Including every g detected within 0.05 rad of e direction in invariant mass calculation. (radiative tail) • Accept MJ/, M(2S) [-12.5s, 3s] (s ~ 12 MeV) • m+m- (radiative tail smaller than e+e-) • Accept MJ/, M(2S) [-5s, 3s] (s ~ 12 MeV)

  8. KS → +- • The candidate is opposite charged track pairs that have an invariant mass within MKS [±4s] (s ~ 4 MeV) • KS → 00 • reconstructed from 4g within MKS [±3s] (s ~ 9.3 MeV) • 0 of the J/y 0 mode • reconstructed from 2g lager than 100MeV within M0 [±3s] (s ~ 4.9 MeV)

  9. Reconstruct of B (other than J/ KL) • Mbc fit, after DE cut. • DE selection depends on the each mode. (corresponding to ~ ±3s) • For Mbc fit, the B signal region is defined as 5.270 < Mbc < 5.290 GeV.

  10. Reconstruction of J/ KL mode • Requiring the observed KL direction to be within 45°from the direction expected for a two-body decay. • Using likelihood fit for suppression of background. The likelihood depend on ↓ • J/y momentum at CM, • angle between KL and its nearest charged track, • multiplicity of the charged tracks, • The kinematics obtained by B+ → J/y K*+ hypothesis

  11. Removing event that are reconstructed as • B0 → J/y KS • B0 → J/y K*0 • B+ → J/y K+ • B+ → J/y K*+ • In this mode, result is obtained as the pBcms distribution fit. • pBcms calculated for B → J/y KL two-body decay hypothesis. • The B signal region is defined as 0.2 ≦ pBcms ≦ 0.45 GeV

  12. q = 1 : ftag is likely B0d q = -1 : ftag is likely B0d Identification of the B flavor • Here, it is need to identify the B flavor. • Tracks are selected in several categories that distinguish the b-flavor. • l (pl high) from b → c l-n • l (pl low) from c → s l+n • K± from b → c → s ; B0 → D(‘) → K(‘) • p (pp high) from B → D(*)- (p+, r+, a1+, etc) • p (pp low) from D*- → D0p- • Relative probability of b-flavor is determined by using MC, for each track in one of these categories. • Combining the result ↑ to determine a b-flavor ‘q’.

  13. (NOF – NSF) (NOF + NSF) = (1 – 2wl)cos(DmdDt) NOF : number of opposite to tagged sample flavor events NSF : number of same flavor events • Evaluating each event flavor-tagging dilution factor ‘r’ to correct for wrong-flavor assignment. • The probabilities for an incorrect flavor assignment ‘wl’ are measured by self-tagging mode reconstruction. • wl are determined from the amplitudes of the time-dependent B0d-B0d mixing oscillations. r = 0 : no flavor discrimination r = 1 : perfect flavor assignment

  14. These tagging algorithm are verified to be a possible bias in the flavor tagging by measuring the effective tagging efficiency for B self-tagging samples, and different Dt. • Total effective tagging efficiency ⇔ good agreement with MC • Slfl(1 – 2wl)2 = 0.270 0.274 +0.021 -0.022

  15. Determination of the Dt • The fCP vertex is determined by using lepton tracks (J/y(2S)) or prompt tracks (C). • The ftag vertex is determined by tracks not assigned to fCP, and requirements (dr < 0.5 mm, dz < 1.8 mm, sdz < 0.5 mm) • dr, dz are the distances of the closest approach to the fCP vertex in the rf plane, and z direction. sdz is error of dz. • The resolution function R(Dt) is parameterized as a sum of two Gaussian. • SVD vertex resolution • charmed meson lifetimes • effect of B motion at CM • incompleteness of reconstructed tracks

  16. The reliability of the Dt determination and R(Dt) parametrization is confirmed, and in good agreement with world average value. • Algorithm OK

  17. Determination of sin2f1 • sin2f1 is obtained by an unbinned maximum-likelihood fitting to the observed Dt distributions. • Pdf for signal is • tB0d : B0d lifetime ~ (1.530 ± 0.009)10-12 s • Dmd : B0d mass difference ~ (0.507 ± 0.005)10-12 ps-1

  18. pdf for background is • ft : the fraction of the background • tbkg : effective lifetime • d(Dt) : Dirac delta function • fCP modes, except J/y KL • ft = 0.10 tbkg = 1.75 ps • J/y KL mode • J/y K*(KLp0) background pdf is fitted Psig with xf = -0.46 • Non-CP background are fitted Pbkg with ft = -1, tbkg = tB +0.11 -0.05 +1.15 -0.82

  19. To obtain the likelihood value of each event as a function of sin2f1, the pdfs are convolved. • fsig : probability that the event is signal

  20. The most probable sin2f1 is defined as the value that maximizes the likelihood function L = PiLi.

  21. +0.32 -0.34 +0.09 -0.10 • We obtain sin2f1 = 0.58 (stat) (syst) • Fig.3(b) shows the asymmetry obtained by performing the fit to events in Dt bins separately, together with acurve that represents sin2f1sin(DmdDt) for sin2f1.

  22. Check for a possible fit bias by applying the same fit to non-CP eigenstates. • B0d → D(*)-p+ • B0d → D*-r+ • B0d → J/y K*0(K+p-) • B0d → D*- l+n • B+ → J/y K+ • It can not be possible to find asymmetry.

  23. Summary • Measurement of the standard model CP violation parameter sin2f1 based on 10.5 fb-1 data sample collected by Belle: sin2f1 = 0.58 (stat) (syst) +0.32 -0.34 +0.09 -0.10

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