Measurement of the CP Violation Parameter sin21 in B0

d Meson Decays

6/15 Kentaro Negishi

Belle実験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

← = 0.425

Belle検出器はいくつかのサブ検出器からなる

B中間子の崩壊は Belle検出器でとらえる

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 (pt/pt)2 = (0.0019pt)2 + (0.0034)2 pt [GeV]– Impact parameter r ~ z = 55 m– Specific ionization dE/dx = 6.9 % (for minimum ionizing pions)– TOF flight-time TOF = 95 ps– K± identification efficiency ~ 85 %, ± fake rate ~ 10 %, p < 3.5 GeV– Energy for (E/E)2 = (0.013)2 + (0.0007/E)2 + (0.008/E1/4)2 E [GeV] E > 20 M

eV– 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°

Motivation

• The variable

time-dependent asymmetry shows that the measurement of decays B0 and B0 to CP eigenstates is sensitive to 1.

Decay and subdecay mode f = -1

– J/(l+ l-) KS(+ -)– J/(l+ l-) KS(0 0) (2S)(l+ l-) KS(+ -) (2S)(J/ + -) KS(+ -) C1(J/ ) 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.

Selection criteria

• J/, (2S) →l+ l- – opposite charged tracks are positively identified as lept

on.– For J/(l+ l- KS(+ -) mode, the requirement for one o

f 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) [-12.5, 3] ( ~ 12 MeV)

– + - (radiative tail smaller than e+e-)• Accept MJ/, M(2S) [-5, 3] ( ~ 12 MeV)

• KS → + -

– The candidate is opposite charged track pairs that have an invariant mass within MKS [±4] ( ~ 4 MeV)

• KS → 0 0

– reconstructed from 4 within MKS [±3] ( ~ 9.3 MeV)

0 of the J/0 mode– reconstructed from 2 lager than 100MeV within M0

[±3] ( ~ 4.9 MeV)

Reconstruct of B (other than J/ KL)

• Mbc fit, after E cut.E selection depends on the each mode.

(corresponding to ~ ±3)• For Mbc fit, the B signal region is defined a

s 5.270 < Mbc < 5.290 GeV.

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/ momentum at CM,– angle between KL and its nearest charged track,– multiplicity of the charged tracks,– The kinematics obtained by B+ → J/ K*+ hypothesis

• Removing event that are reconstructed as– B0 → J/ KS

– B0 → J/ K*0

– B+ → J/ K+

– B+ → J/ K*+

• In this mode, result is obtained as the pBcms distribution fit.

• pBcms calculated for B → J/ KL two-body decay hypothesi

s.

• The B signal region is defined as 0.2 p≦ Bcms 0.45 GeV≦

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-

•l (pl low) from c → s l+

•K± from b → c → s ; B0 → D(‘) → K(‘)

• (p high) from B → D(*)- (+, +, a1+, etc)

• (p low) from D*- → D0 -

•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’.

q = 1 : ftag is likely B0d

q = -1 : ftag is likely B0d

• 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-B0

d mixing oscillations.

r = 0 : no flavor discrimination

r = 1 : perfect flavor assignment

(NOF – NSF)

(NOF + NSF)= (1 – 2wl)cos(mdt)

NOF : number of opposite to tagged sample flavor events

NSF : number of same flavor events

• 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 t.

• Total effective tagging efficiency ⇔ good agreement with MC lfl(1 – 2wl)2 = 0.270 0.274

+0.021

-0.022

Determination of the t• The fCP vertex is determined by using lepton tracks (J/ (2

S)) or prompt tracks (C).• The ftag vertex is determined by tracks not assigned to fCP, an

d requirements (r < 0.5 mm, z < 1.8 mm, z < 0.5 mm) r, z are the distances of the closest approach to the fCP vertex in the

r plane, and z direction. z is error of z.

• The resolution function R(t) is parameterized as a sum of two Gaussian.– SVD vertex resolution – charmed meson lifetimes– effect of B motion at CM– incompleteness of reconstructed tracks

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

• Algorithm OK

Determination of sin21

• sin21 is obtained by an unbinned maximum-likelihood fitting to the observed t distributions.

• Pdf for signal is

B0d : B0d lifetime ~ (1.530 ± 0.009)10-12 s

md : B0d mass difference ~ (0.507 ± 0.005)10-12 ps-1

• pdf for background is

– f : the fraction of the background bkg : effective lifetime (t) : Dirac delta function

– fCP modes, except J/ KL

• f = 0.10 bkg = 1.75 ps

– J/ KL mode• J/ K*(KL 0) background pdf is fitted Psig with f = -0.46• Non-CP background are fitted Pbkg with f = -1, bkg = B

+0.11

-0.05

+1.15

-0.82

• To obtain the likelihood value of each event as a function of sin21, the pdfs are convolved.

• fsig : probability that the event is signal

• The most probable sin21 is defined as the value that maximizes the likelihood function L = iLi.

• We obtain sin21 = 0.58 (stat) (syst)• Fig.3(b) shows the asymmetry obtained by

performing the fit to events in t bins separately, together with acurve that represents sin21sin(mdt) for sin21.

+0.32

-0.34

+0.09

-0.10

• Check for a possible fit bias by applying the same fit to non-CP eigenstates.– B0

d → D(*)- +

– B0d → D*- +

– B0d → J/ K*0(K+ -)

– B0d → D*- l+

– B+ → J/ K+

• It can not be possible to find asymmetry.

Summary

• Measurement of the standard model CP violation parameter sin21 based on 10.5 fb-1 data sample collected by Belle:

sin21 = 0.58 (stat) (syst)+0.32

-0.34

+0.09

-0.10