b physics at lep andrea sciabà infn-cnaf xxxiii international symposium on multiparticle dynamics...
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b physics at LEP
Andrea Sciabà
INFN-CNAF
XXXIII International Symposium on Multiparticle Dynamics
September 5-11, 2003Krakow
ISMD2003Andrea SciabàINFN-CNAF
b physics
Goals: CKM matrix
B oscillations (|Vtd|, |Vts|) B semileptonic BRs (|Vcb|, |Vub|) b lifetimes CP violation
masses Electroweak
sin2 θW (asymmetries) Rb (sensitive to new physics, mtop)
QCD processes fragmentation two-photon events, gluon splitting, etc.
New physics (CP violation, rare decays, etc.) …
B 0 oscillations
V udV ub
*
A B
C
Vtd V
tb *
VcdVcb*
b cℓν
b
uℓν
ISMD2003Andrea SciabàINFN-CNAF
b physics at LEP LEP data sample features and
advantages ~ 3.6×106 Z0 bb events very clean environment high heavy quark tagging power due to
silicon microvertex detectors production of many species of b hadrons high boost of b hadrons
… but many results being superseded by pp colliders and b factories
ISMD2003Andrea SciabàINFN-CNAF
b physics at LEP
LEP data still contributes significantly to b semileptonic BRs Bs oscillations Asymmetries, Rb (no more measurements until Z
factories)
Bs, Λb, excited b hadron masses Fragmentation heavy quark production in 2-photon collisions gluon splitting into heavy quarks
Only a few topics will be covered here
ISMD2003Andrea SciabàINFN-CNAF
Bs oscillationsBs oscillations: Basic concepts
Main motivation:
md |Vtd| gives a large error
md/ ms |Vtd| with a 3 times better theoretical error!
BBss,d,d00BBss,d,d
00
bb
bb
s,ds,d
WW-- WW++
VVtbtb
VVttqq
VVttqq**
VVttbb**
tt
tt
s,ds,d
15% theor. error
22*
qqq BBBtbtqq fBMVVm
2
2
std
ts
B
B
d
s
V
V
m
m
m
m
d
s
|Vcb|2
SU(3) ~ 5% error
)cos( )( 0s
0s BB
tmetP s
t
s
s
1
2
1
Oscillation probability:
fit A for any assumed value ω of Δms
ω <<Δms A 0
ω ≃Δms A 1
ω excluded at 95%CL if A+1.65σA<1
A
2
2
121
1)(exp)(~ tB
As
fNS
ISMD2003Andrea SciabàINFN-CNAF
Strategy Flavour tagging at production
jet charges, vertex charges, kaons, leptons, etc.
typical mistag 25% Flavour tagging at decay
lepton or D charge identify final state (b, bc,
etc.) Distinguish Bs from other hadron
species Measure Bs decay length,
momentum t = (m/p) L (p/p) t
Maximum likelihood fit to amplitude A at given ms
Production flavour tagging
Decay flavour tagging
ISMD2003Andrea SciabàINFN-CNAF
Oscillation analyses Exclusive analyses
Fully reconstructed Bs (A, D)
Semi-exclusive analyses Ds
+- pairs (A, D, O) Ds
+h- pairs (D) Φ- pairs (D)
Inclusive analyses semileptonic decays (A, D, O) inclusive vertices (D)
Very low statistics
High purity
Very high p.t. resolution!
Low statistics
Fair purity
Good p.t. resolution
High statistics
Low purity
Low p.t. resolution
ISMD2003Andrea SciabàINFN-CNAF
Fully reconstructed Bs (ALEPH)
s
1s
s
s
D
aD
D
B 0
KK
KKD
S
0s
0
000 a1
ss DD
,Ds0
,aDs0
1
total
and 0 reconstruction efficiency and purity increase
180 m
p/p 0.5 – 3 %
Well reconstructedMissing ’s or ’s
t0.08 ps
Excellent proper time resolution!
Add a photon
ISMD2003Andrea SciabàINFN-CNAF
Ds+- pairs (DELPHI)
(updated)
KKD
D
D
KKD
KKD
D
*s
s
s
Ss
*s
s
0
0
0
0
KKD
KKD
s
s
;
;eX ss DB0
Semileptonic modes
Hadronic modes Event-by event:
proper time resolution Bs purity
Sensitivity: ms = 8.6 ps-1
ms > 4.9 ps-1 at 95% CL
ISMD2003Andrea SciabàINFN-CNAF
Inclusive analysis (ALEPH) Topological reconstruction
of the D vertex Extensive event-by-event
treatment of: proper time resolutions and
corrections sample composition flavour tags huge sensitivity
enhancement!
Lepton with high p
Topologically rec. “D” track
“B” track
Photon
Primary vertex
Bs
“D”
Sensitivity: ms = 13.6 ps-1
ms > 11.9 ps-1 at 95% CL
ISMD2003Andrea SciabàINFN-CNAF
Oscillation results Almost final LEP results
on Bs oscillations Lower limit on ms: 14.4
ps-1
Now, we must wait for Tevatron to measure ms
ISMD2003Andrea SciabàINFN-CNAF
b quark fragmentation The fragmentation function
models the non-perturbative confinement of the quark into colourless hadrons
different choices of the variable:
not accessible experimentally
ISR, FSR, hard gluon emission effects not unfolded
Model-dependent analyses (fit to a given fragmentation model)
Model-independent analyses (xB spectrum reconstruction)
quark||
hadron||
)(
)(
pE
pEz
beam
hadron
E
ExB
ISMD2003Andrea SciabàINFN-CNAF
Inclusive analyses Event selection based on:
b-tagging secondary vertex
reconstruction Direct xB reconstruction
(OPAL) neural networks to distinguish
particles from b decay and from hadronization
σE≃5 GeV
xB, z reconstructed using neural networks (DELPHI), input variables correlated to: kinematics (track rapidity,
multiplicity, etc.) jet/total energy b-tagging
prelim
inar
y
ISMD2003Andrea SciabàINFN-CNAF
Exclusive analyses (ALEPH) B D(*)ℓν decays
Much lower statistics ~ 3400 events
Eν reconstructed from missing energy
σE ~ 8%
xB spectra not efficiency-corrected
ISMD2003Andrea SciabàINFN-CNAF
Model dependent results Favoured models:
Lund / Lund-Bowler Kartvelishvili
Disfavoured models: Peterson et al. Collins
ISMD2003Andrea SciabàINFN-CNAF
Model independent results
-0.0033
-0.0052
Mean values for xB:
OPAL: 0.7193±0.0016(stat.)+0.0038(syst.)
DELPHI: 0.7153±0.0007(stat.)+0.0049(syst.)
ALEPH: 0.716±0.006(stat.)±0.006(syst.)
All measurements consistent bias w.r.t. old measurements due to
theassumption of the Peterson model
ISMD2003Andrea SciabàINFN-CNAF
AbFB measurements (I)
Forward-backward asymmetry at the Z pole:
Related to sin2θW
AbFB most sensitive to sin2θW
Need to measure: cosθ (from thrust axis) quark charge quark flavour
QCD corrections to be calculated gluon radiation thrust axis approximation most important systematic error
BF
BFqFBA
coscos
cosqFBAd
d
3
81 2
22 1
2
1
2
4
3
)()(qe
ee
ee
AV
AV
AV
AVqFB gg
gg
gg
ggA
AA
f
f
A
Vf
g
g1
4
12effsin
effsin 23
21
q
q
A
V
I
Q
g
g
q
q
e- e+f
f
ISMD2003Andrea SciabàINFN-CNAF
AbFB measurements (II)
Use a lepton tag lepton charge
correlated to the quark charge
need to distinguish
correct for the dilution by B mixing
background
cbb
cbb
c
bobs )( FBFB AA 21
bb FBBF AQQ
Use the jet/vertex/kaon charge b-tagging to enhance
purity purity from data quark charge estimator
Q
charge separation δ from data effect of B mixing
included
ISMD2003Andrea SciabàINFN-CNAF
AbFB with lepton tag
(DELPHI) b-tagging (ηEVT)
Track I.P., sec.vertex mass and energy, track rapidities
Flavour composition vs. ηEVT from data with standard double counting method
Lepton p and p
Charge tagging Lepton charge, opposite jet charge as
cross check Uses Q× Qopp (-/+: right/wrong correlation)
Probability for a lepton of coming from b, bc , c , bckgnd function of (p, p), (ηEVT, Q× Qopp)
2-dim fit of the obs asymmetry to AbFB,
AcFB over bins of (cosT, Pb-Pbc , Pc)
(syst) 0063.0(stat) 0086.00728.0
(syst) 0024.0(stat) 0052.01021.0,0
FB
,0FB
c
b
A
A
New!
ISMD2003Andrea SciabàINFN-CNAF
AbFB with lepton tag
(OPAL) Neural networks to
distinguish b (NETb) and c (NETc) Lepton p and p, lepton jet
energy, (∑p)jet
Jet decay length significances, lepton i.p. significance
Likelihood fit to the number of single-lepton and double-lepton events Bins of cos, NETb and NETc
Background fraction fitted from data
average mixing parameter fitted from data using same-sign dilepton events
AbFB and Ac
FB fitted(syst) .(stat) ..(syst) .(stat) ..
,FB
,FB
003800052006290001500040009810
0
0
c
b
AA
ISMD2003Andrea SciabàINFN-CNAF
AbFB with incl. tag
(DELPHI) high purity b-tagging (b-tag)
lifetime and sec vertex info, rapidities b-tag selection efficiency from data
counting events with 0,1,2 tagged hemi sample composition (b, c, uds)
charge tagging (flavhem) jet charge, sec. vertex charge flavour tag combining P(same Q|B) for
all tracks prob. of charge mistag from data
(unlike/like-sign double tagged events) for charm events, mistag from data using
charge of reconstructed D* (new!) fit to no. of single and double tagged
events in bins of (cosθ, b-tag)
Main systematics from jet charge hem. correlations
(syst) .(stat) ..,FB 0014000300097800 bA
New!
ISMD2003Andrea SciabàINFN-CNAF
AbFB with incl. tag
(OPAL) b events tagged with
NN (secondary vertex info) leptons (p, p, isolation) flavour composition from data (singly
and doubly tagged hemispheres) Pf
charge tagging estimator Q from jet charge (always) sec. vertex charge (if any) kaon charge (from bc s) (if any)
all charge estimators available
f
FBfffBF ACPsQQ flavours
ff BF QQ
4
2 f
ffP flavours
(syst) .(stat) ..,FB 0018000340100200 bA
ISMD2003Andrea SciabàINFN-CNAF
AbFB summary
AbFB measurements all LEP
almost finalized well compatible (also with SLD) sin2lept
eff: 2.9σ discrepancy with ALR (SLD) need to wait for linear colliders
SLD Ab
LEP A bFB
LE
P+
SL
D
Abe
bFB AAA
4
3
ISMD2003Andrea SciabàINFN-CNAF
Open b production in events Test of perturbative QCD
at LO, only direct and single resolved terms (of the same order)
more robust calculations because of the large quark mass
2-3 orders of magnitude suppression w.r.t open charm because of larger mass, smaller charge
photons mostly quasi-real anti-tagged events
Analysis strategy select anti-tagged photon-photon events reconstruct lepton candidates reconstruct jets measure lepton p w.r.t. jet axis
e-
e+
e+
e-
b
b
e-
e+
e+
e-
b
b
direct term
single resolved term
ISMD2003Andrea SciabàINFN-CNAF
bbX in L3 627 pb-1, 189 GeV < s < 209 GeV muons AND electrons large
statistics Fit to p distribution:
p spectra from MC (PYTHIA for )
Nb, Nc, Nuds left free Consistent e and results
Also measurement of
consistent with previous L3 results
(syst)pb.(stat)..)Xbbeeee((syst)pb.(stat)..)Xbbeeee(
electrons
muons
324261232352013
(syst)pb.(stat)..)Xbbeeee( 3271812
muons
electrons
(stat)pb)Xcceeee( 117998
435 events
3850 events
ISMD2003Andrea SciabàINFN-CNAF
bbX in DELPHI (new)
pb (syst) .(stat) ..)Xbbeeee( 4333914
pb (syst) (stat) )Xcceeee( 206191937
463 pb-1, 189 GeV < s < 209 GeV only muons Fit to p distribution:
p spectra from MC (PYTHIA for )
Nc fixed to LEP average for (e+e-cc)
Nuds measured selecting hadrons instead of muons
Nb left free
Study of K-lepton correlations (first time!) bK--X cK-+X increased purity of c (K±) and b (K±±)
events measured cross sections:
651 events
ISMD2003Andrea SciabàINFN-CNAF
bbX in OPAL 371 pb-1, 189 GeV < s < 202
GeV only muons Fit to p distribution:
p spectra from MC (PYTHIA for )
Nc fixed to OPAL measurement for (e+e-cc) with D* mesons
Nuds measured selecting hadrons instead of muons
Nb left free
xT=2pT/Wvis distribution shows the consistency of the 1:1 ratio of direct to single-resolved contributions
pb (syst) (stat) ..)Xbbeeee( 843552214 .-.
444 events
ISMD2003Andrea SciabàINFN-CNAF
bb(cc) summary Consistent results among LEP
experiments Good agreement with Drees
et al. model for charm data
EXCESS for beauty data:4 discrepancy
No satisfactory explanation at present
ISMD2003Andrea SciabàINFN-CNAF
Conclusions
LEP HF physics program almost finalized Bs oscillations (ms> 14.4 ps-1 at 95% CL) heavy quark asymmetries (difficult
interpretation) Heavy quark fragmentation function well
measured and consistent (both in average and shape)
Large discrepancy in bbX rate (but not in ccX rate)
Impossible to show many other results…
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