Measurement of the CP Violation
Parameter sin21 in B0d 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)
← bg = 0.425
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
周長3km
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
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
Spec of the Belle
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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 sr ~ 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
 m± 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/y(l+ l-) KS(p+ p-)
J/y (l+ l-) KS(p0 p0)
y(2S)(l+ l-) KS(p+ p-)
y(2S)(J/y p+ p-) KS(p+ p-)
C1(J/y g) KS(p+ p-)
C(K+ K- p0) KS(p+ p-)
C(KS K- p+) KS(p+ p-)
 f = +1
– J/y(l+ l-) p0
– J/y(l+ l-) KL
• For the measurement of A(t), CP eigenstate
mode is used.
Selection criteria
• J/y, y(2S) →l+ l– opposite charged tracks are positively identified as
lepton.
– For J/y(l+ l- KS(p+ p-) 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/y, My(2S) [-12.5s, 3s] (s ~ 12 MeV)
– m+ m- (radiative tail smaller than e+e-)
• Accept MJ/y, My(2S) [-5s, 3s] (s ~ 12 MeV)
• KS → p+ p-
– The candidate is opposite charged track pairs that
have an invariant mass within MKS [±4s] (s ~ 4
MeV)
• KS → p0 p0
– reconstructed from 4g within MKS [±3s] (s ~ 9.3
MeV)
 p0 of the J/y p0 mode
– reconstructed from 2g lager than 100MeV within Mp0
[±3s] (s ~ 4.9 MeV)
Reconstruct of B (other than J/y 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.
Reconstruction of J/y 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
• 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
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*- → D0 p•Relative probability of b-flavor is determined by using MC, for each track
in one of these categories.
q = 1 : ftag is likely B0d
•Combining the result ↑ to determine a b-flavor ‘q’.
q = -1 : ftag is likely B0d
• Evaluating each event flavor-tagging dilution factor ‘r’ to correct for
wrong-flavor assignment.
r = 0 : no flavor discrimination
r = 1 : perfect 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.
(NOF – NSF)
(NOF + NSF)
= (1 – 2wl)cos(DmdDt)
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 Dt.
•
Total effective tagging efficiency
 Slfl(1 – 2wl)2 = 0.270
+0.021
-0.022
⇔
good agreement with MC
0.274
Determination of the Dt
• The fCP vertex is determined by using lepton tracks (J/y
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
r 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
• The reliability of the Dt determination and
R(Dt) parametrization is confirmed, and in
good agreement with world average value.
• Algorithm OK
Determination of sin21
• sin21 is obtained by an unbinned maximumlikelihood 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
• pdf for background is
– ft : the fraction of the background
 tbkg : effective lifetime
 d(Dt) : Dirac delta function
– fCP modes,+0.11
except J/y KL
• ft = 0.10 -0.05
– J/y KL mode
tbkg = 1.75
+1.15
-0.82
ps
• J/y K*(KL p0) background pdf is fitted Psig with f = -0.46
• Non-CP background are fitted Pbkg with ft = -1, tbkg = tB
• To obtain the likelihood value of each
event as a function of sin21, the pdfs are
convolved.
• fsig : probability that the event is signal
• The most probable sin21 is defined as the
value that maximizes the likelihood
function L = PiLi.
+0.32
+0.09
• We obtain sin21 = 0.58 -0.34 (stat) -0.10(syst)
• Fig.3(b) shows the asymmetry obtained by
performing the fit to events in Dt bins
separately, together with acurve that
represents sin21sin(DmdDt) for sin21.
• 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.
Summary
• Measurement of the standard model CP
violation parameter sin21 based on 10.5 fb-1
data sample collected by Belle:
+0.32
+0.09
sin21 = 0.58 -0.34 (stat) -0.10(syst)