electroweak b physics at lep v. ciulli infn firenze
TRANSCRIPT
Electroweak b physics at LEP
V. CiulliINFN Firenze
Introduction
Two electroweak observable in Zbb decays:
• Partial decay width: Rb = bb /had
measures
• Forward-bakward b asymmetry: AFB(b)\ measures
Often results given as
AbVb gg
22 )()( AbVb gg
22 )()(
2
AfVf
AfVff gg
ggA
The Electroweak FitGlobal fit probability ~
4.5%(28% without NuTev)
Measurements using Z QQ decays
• Rb anomaly solved long ago…
• Forward-backward b asymmetry pull > 2
Rb vs Rc (LEP+SLD)
Summer 1996 Summer 2003
Mt = 174.3 5.1 GeV
AFB(b) • Very stable over the years• Errors significantly reduced
– New analysis techniques– Full data reprocessing
Asymmetries at the Z pole• From fermion direction and helicity:
• With polarized beams
e- e+
f
f
e
f
ff
fe
A
A
AA
4
3)B(A)F(AA
A
4
3A
polpolpolFB
tot
RLpol
tot
BFFB
e-
e+
f
f
f
e
A
A
4
3)B(A)F(AA
A
LRLRLRFB
tot
RLLR
f = e,b,c
f = only
any f
f = e,b,c
AFB(b) measured by means of two different techniques• b-tagging from semi-leptonic b decays
b quark/anti-quark separation from lepton charge
• Inclusive b-tagging (impact parameter, secondary decay vertex, etc…) b quark/anti-quark separation from jet-charge, e.g.
The two measurements are systematically uncorrelated
Statistical correlation in the same experiment: 25% - 40%
AFB (b) with leptons
• Lepton momentum and transverse momentum separate different sources
• b-quark direction measured from event thrust, signed by the lepton charge
• Asymmetry measured from fit to the angular distribution
• Separation is improved in recent measurements by the use of more variables and neural networks
thrustlepQx cos
xAx
dx
dN
N FB3
81
8
31 2
OPAL final (2003)
AFB (b) with leptons - 2
• AFB diluted by mixing and bcl
• Both mixing and semi-leptonic BR’s measured on same data– Modeling effects, important for those measurements,
cancel out in AFB
• AFB(c) also measured (correlation < 20%)
• The measurement is largerly dominated by statistical error
Source A(b)(%) A(c)(%) BR’s 0.034 0.189 Detector 0.015 0.088 Lepton modeling 0.090 0.210 Bkg asymmetries 0.002 0.072 B and D physics 0.032 0.166 Mixing 0.132 0.113 Total 0.169 0.369
ALEPH final (2002)
0.00170.00410.0952 b)(AFB
AFB(b) with jet-charge• AFB measured from
forward-backward charge asymmetry
measured on data !
• Vertex and identified particles also used to tag the charge
2 2
AFB(b) with jet-charge - 2• Main systematics
– b purity (efficiencies from double tag methods)– hemisphere correlations (from MC, but much
less important than for Rb)
– charge tagging calibration
charge separation vs cosData vs MC
QCD corrections• Measured asymmetry must be corrected for
gluon radiation effects
Thrust axis quark direction
correction ~3.5% > AFB/AFB 1.7%
QCD corrections
• Bias due to event selection – Leptons: 50-70% of total correction– Jet-charge: ~25% only (corrections almost
completely absorbed into charge correlations between the hemispheres)
Lepton momentum
The six electroweak observables
measured at LEP and SLC are fitted together with
Taking into account statistical and systematical correlationsBR’s and mixing mostly correlated to asymmetriesCharm hadron production correlated to Rc
Rb, Rc, AFB(b), AFB(c), Ab and Ac
LEP + SLD Combination
BR(b l), BR(b c l), BR(c l), , f(D+), f(Ds), f(cbarioni), P(cD*+)xBR(D*+ D0+)
Results
Source AFB(b)(%) Statistics 0.15 Internal systematics 0.06 Common systematics 0.04 Total systematics 0.07 Total 0.16
parameter fit:2/d.o.f.=51/91(91/91 if only stat errors are used)
Inclusive: A0FB(b)=0.10000.0019
Leptons: A0FB(b)=0.10000.0025
A0FB(b)=0.09970.0016
AFB(b) vs centre-of-mass energy
Peak data only (+0.6):A0
FB(b)=0.10060.0017
Ab: are LEP and SLD compatible?
700186300
Hm
1.53.174 tm
• LEP only (indirect): Ab=0.8980.021
• SLD direct: Ab=0.9250.020
• Agree within 0.8 • LEP+SLD: Ab=0.9030.013 (0.935 SM)
• From all asymmetries (including quark ones):
• Final precision < 10-3
but average prob = 7%
• Most sensitive measurements ALR
AFB(b)disagree by 2.9 !( polarisation in the middle of the two)
sin2elfefp t
b-quark couplings anomaly?
Large deviations in the right coupling gRb only!
Difficult to explain…
Conclusions
• LEP b-asymmetry measurements almost final – 2nd iteration using reprocessed data and
improved techniques– only DELPHI inclusive still preliminary
• LEP results are consistent with SLD Ab
measurement• O(3) discrepancy between “hadronic”
and “leptonic” mixing angle measurements confirmed
• Interpretation of this electroweak result difficult
Backup slides