Precision electroweak
physics
Roberto TenchiniINFN-Pisa
5th Rencontres du Vietnam Particle Physics and Astrophysics Hanoi August 5 to August 11, 2004
21 Years of W and Z Physics
W Mass History
77
78
79
80
81
82
83
84
1 2 3 4 5 6 7 8 9
Years
GeV
/c2
1986UA1UA2
1990UA2CDF
2000TeVLEP
=1.5 GeV
=400 MeV =100 MeV
=39 MeV
Only Published Results
(*) Preliminary average 2004 : =34 MeV, weight of LEP 2/3, Tevatron 1/3
(*)
21 Years of W and Z PhysicsZ Mass History
88
89
90
91
92
93
94
95
1 2 3 4 5 6 7 8 9
Years
GeV
/c2
1986UA1UA2
1990LEP
=1.7 GeV
=31 MeV
Only Published Results
91,08
91,1
91,12
91,14
91,16
91,18
91,2
=2.1 MeV
(2001)Pietrzyk Burkhardt,
046.0936.128
1)M( Z s
21 Years of W and Z Physics
Standard deviations
-2
0
2
4
6
8
10
12
14
1 2 3 4 5 6 7 8 9
Years
Nu
mb
er o
f si
gm
as
W Mass vs Tree Level
77
78
79
80
81
82
83
84
1 2 3 4 5 6 7 8 9
Years
GeV
/c2
E.W. Tree level SM relation
F
Z
Z
W
W
G
M
M
MM
1
2
)(1 2
22
Strong Evidence of pure E.W.Higher Order Corrections
1986 20021986 2002
(with running QED)
Tree level not enough : more parameters
• Radiative corrections in function of three more parameters Mtop, Mhiggs, s
• An Observable is written as
sHiggstopZFZii MmMGMfO ,,,,,
Example :
rGM
MM
FZ
WW
1
1
21
2
22
one loop radiative
corrections
Outlook of the rest of the talk
• Z Couplings: asymmetries at the Z and measurement of sin2()W – (a crisis ?)
• W Couplings: W Branching Ratios – (three LEP exp are final)
• Measurement of the W mass – (some hot issues)
• Constraints on the SM Higgs
NEW
MEANS FIRST TIMEAT THIS CONFERENCE
eA
A
tot
rBrFlBlF
etot
rl
4
3,,,,polFB
LR
A
A
Asymmetries at the Z pole• Z decay into a ff pair. With unpolarized beam
from fermion direction and helicity 3
asymmetries
etot
LBLFRBRF
ftot
LBLFRBRF
fetot
A
A
AABF
4
3,,,,FBpol
,,,,pol
4
3 FB
A
A
A
Can measure for e,,,c,b
Can measure with ’s
• Z with beam polarization (SLC) :
Al
Vlleff
AfVf
AfVff
g
g
gg
ggA
14
1sin
)()(
2
2
22
Electroweak Couplings :from deep inelastic scattering to LEP-SLC
Huge increase in precision
Asymmetries at the Z and
• Consistency is at 6.2%
• Long standing difference between Alr and AFB(b)
leff
2sin
NEW FINAL
b anomalous couplings ?No independent evidence
Model) (Standard 0.2158 R
0.000650.21646 R
Model) (Standard 935.0
(SLD) 020.0923.0)(A
(b)A
b
b
FB
4
3 FB
pol
b
b
betot
A
bA
AABF
measuresAf
Vf
g
g
measures 22VfAf gg
Plots like this one includeb asymmetry !
NEW FINAL
Again on the b asymmetry
•Two techniques•use semileptonic b decays•use weighted charge of particles in the hemisphere
•Very different systematic effects•LEP average still statistically dominated
AFB(b)
STATISTICS 0.00156
UNCORRELATED SYSTEMATIC 0.00061
QCD CORRECTION 0.00030LIGHT QUARK FRAGMENTATION 0.00013SEMILEPTONIC DECAYS MODELLING 0.00013CHARM FRAGMENTATION 0.00006BOTTOM FRAGMENTATION 0.00003
TOTAL SYSTEMATIC ERROR 0.00073 Example: Stability of resultas a function of the b purity
NEW FINAL
The b asymmetry: results
0.00250.0996 (b)A0FB
0.00190.1000 (b)A0FB
0.00160.0998 (b)A0FB
LEP average
Leptons only
Inclusive
NEW FINAL
My conclusion on “sin2() crisis”
• There is no evidence of problems related to the b asymmetry measurement
• Will solve the ALR-b asymmmetry discrepancy with future linear colliders if– Polarization of both beams available– Very high statistics run at the Z will take place
WW Production at LEP2
Three diagrams contribute at Born level (CC03 diagrams) :
Actually look for subsequent W decay into lepton-(anti) neutrino and quark1- (anti) quark2.
Real process is defined by exp. cuts4321-- ffffWWee
WW production at LEP2: the backgrounds
•Selections needed to •Extract events from background •Classify WW decays to different channels (fully hadronic, fully leptonic, semileptonics) •(Semi)leptonic decays can further be separated in e,, channels
BR
Signatures - 2 energetic, aco-
planar leptons - large missing E, p
- 2 hadronic jets - missing E & p - isolated lepton
- 4 hadronic jets - low missing E, p
Backgrounds ZZ, Zee, Z We, qq() qq()
Efficiency ~35-65% ~65-90% ~85%
Purity ~80-95% ~94-99% ~85%
BR
Signatures - 2 energetic, aco-
planar leptons - large missing E, p
- 2 hadronic jets - missing E & p - isolated lepton
- 4 hadronic jets - low missing E, p
Backgrounds ZZ, Zee, Z We, qq() qq()
Efficiency ~35-65% ~65-90% ~85%
Purity ~80-95% ~94-99% ~85%
Event selection
q
q
q
q
q q
Semileptonic (qqSemileptonic (qqll)) Hadronic (4q) Hadronic (4q) LeptonicLeptonic
11% 44% 45%
Lepton=e,,
Cross Sections in all channels
Nj selected events in each selection (j)L total luminosity j expected events function of i
WW for each channel (i) ij efficiency matrix j
bkg background cross section j
4f four fermion interference correction
j
j
eN
Pj j
Nj
ij
j
!
)(
)( 4 fj
bkgj
WWiijj L
Likelihood
EXAMPLE OF PROCEDURE
Four Fermion Interference
ee- νeνeeefor diagrams f-4 two:Example
Small Correction taken from Monte Carlo (~ 1% relative)
SIGNAL (following the CC03 definition)
Single W
Most important(*) 4-f (non WW) processes can be directly measured
(*) for the interference
Total WW Cross Section
• Precision reached by LEP experiments a challenge to theoretical predictions– Predictions with enhanced
O() radiative corrections are needed !
87.042.97KORALW):(Theory
)LEP(R
89.032.99YFSWW):(Theory
)LEP(R
)O(without
)O(with
Strong evidence of Triple Gauge Couplings
NEW 3 exp FINAL
W decays : Branching Ratios
Standard Model : 10.8%Standard Model : 67.5%Test of lepton universality:
result higher than e+
NEW ADL FINAL
W Leptonic Couplings
NEW ADL FINAL
014.0037.1)g(
)g(
015.0034.1)g(
)g(
010.0997.0)g(
)g(
e
e
• If electron and muon couplings are assumed to be the same and combined the result is 3 higher
•was 2.3 in summer 2003•is 2.6if final results only are used
W Mass at LEP from direct reconstruction
• Above threshold the W mass is measured from direct reconstruction of the jet-jet invariant mass in the fully hadronic and semileptonic channels• Event reconstructed as 2 (semileptonic candidate) or 4 (hadronic candidate) jets with iterative procedure• In the hadronic channel 3 jet-jet combinations from 4 jet • WW boson pairs at LEP
– 161 – 209 GeV centre of mass energy– ~700 pb-1 by each experiments– ~4500 qqqq , ~4000 lqq events for
each experiments.
LEP2 EnergySystematics:Systematics:
• At LEP2: Ebeam~20MeV (E/E~10-4)
mW~17MeVmW~17MeV
– Error coming mainly from extrapolation.
Resonant depolarizationResonant depolarization:– only works up to 60GeV extrapolation
• Kinematic fit imposing energy-momentum conservation
RECENT FINAL
Reconstructed MW
L3 L3 qqqq
DELPHIDELPHIeeqqqq
OPALOPAL qqqq
LEP: Systematic Uncertainties for MW
• QED effects (ISR, FSR, etc.)• Fragmentation• Detector effects • Uncertainty on the LEP beam energy• Colour Reconnection• Bose-Einstein correlations
(Weight of qqqq in LEP combination: 0.09)
Effort to reduce this error by designing
4q analyses less affected by CR effects
Final State Interactions
Separation of W decays vertex at LEP2 ~0.1 fm small with respect to the hadronization scale ~1fm
Interconnection phenomena : Final State is no longer factorized into two separated W’s
can bias the W mass in the fully hadronic channel
)10ΛΓ ( QCDW
NEED DATA to control these non-perturbative effects
Present LEP result: Colour Reconnection constrained by the particle flow analysis
• Most CR models predict a modified particle flow in W+W- events:
CR:
No CR:
W-
W+
W-
W+
• The ratio of particle flow between the inter and intra-W regions is built:
(A + B) / (C + D)
A
B
C
D
•Data
-SK1(extreme parameter)-Jetset
• Measurement sensitive only to extreme scenarios,
Colour Reconnection Systematic error ~ 90 MeV
Final LEP result: Colour Rec. constrained by Mass variation when soft particles
are excluded
Mass Shift predicted by model
Example: difference between Mass measured in standard analysis
andMass measured using only particles in the core of the jet
Models
• This correlation is a general feature of All Models : CR is expected to affect mainly– low momentum particles– particles away from the jet
core
• By studying Mass Stability (or Measuring the W mass only with particles in the jet core ) expect to reduce CR Systematic Error to ~ 50 MeV
mass difference (no FSI syst)
MW(qqqq-lqq)= +2243 MeV
MW at LEP : 4q and lqq 80.411±0.032(stat) ±0.030(syst)GeV/c2
80.420±0.035(stat) ±0.101(syst)GeV/c2
• Before LEP: measurements at hadron colliders (SppS, Tevatron Run I)• After LEP : measurements at hadron colliders (Tevatron Run II, LHC)• Drell-Yan single W production (quark-antiquark annihilation)• W decay to leptons (e or ) + neutrino• Fit to MW
T , the transverse mass distribution
)cos1(2MTW T
leptoneT pp
W mass from hadron colliders: the past and the near future !
-
TeVatron RUN IIW and Z cross sections
Present MW at LEP and TeVatron
ElectroWeak fit results• Electroweak theory
tested at one loop level• Indications for a light
Higgs
Summer 2003
Winter 20042
Higgs
2top
GeV/c 219M
GeV/c 1.53.174M
2Higgs
2top
GeV/c 251M
GeV/c 3.40.178M
Conclusions
•Asymmetries at the Z are final (including quarks)
•W cross sections at LEP are essentially final
•The LEP W mass result is still PRELIMINARY
–LEP collaborations are still working on this measurement, final results for the end of this year
– Activities to reduce the uncertainty due to CR effects and gain information from the 4q channel
• Reducing the LEP final error (~ 35 MeV ) will be the challenge for Tevatron II and LHC
•In spite of the increased top mass there is still evidence for a light Higgs
Backup Slides
Impact parameter
Secondary vtx
Transverse momentum
Momentum
AFB(b) from semileptonic decays
• Can fit independently b and c asymmetries, mixing and background composition
AFB(b) QCD corrections : cross-check
No evidence of bias due to gluon emission
W Leptonic Branching Ratios
NEW ADL FINAL
028.0076.1)BR(W
)BR(W
029.0070.1)BR(W
)BR(W
020.0994.0)BR(W
)BR(W
e
e
• If the Branching Ratios to electron and muon are assumed to measure the same quantity and combined the result is 3 higher
•was 2.3 in summer 2003•is 2.6if final results only are used
Measuring BE in W+W- events
• Inter W, BEI confirmed
• Between W’s, BEB, disfavoured
MW down from ~35 to ~15 MeV
Final
BEBBEI
eg
Error on W mass (Run I)
Theory improvementsImprove PDF constraints with measurements (W charge asymmetry, Z rapidity distribution)
These errors are determined using CDF/D∅ data,scale with luminosityDetector improvements for Run II will also help
40 MeV per experiment with 2 fb-1 feasible
Running of QED:
')'('
)'(
3)(
)()()(1
)0()(
24
)5(
)5(
dssss
sRss
ssss
m
hadhad
tophadl
046.0936.128
1)M( Z s
)61(0359895.137)0(1
ldat threshoDominat βchannelt
βchannel s 3
GeV 22.040.80M
pb45.069.3
W
WW
dominating is productionW e
: dependence model Small
e
LEP average:
Mw from the threshold method at 161 GeV
Kinematic FitsKinematic fit used to improve reconstructed four momenta
pairingsjet -jet 3
fit-5Cor -4C a use
jets-4 force :qqqq mm4C :fit 5C
s)(0,E),P( :fit 4C
rec2rec1
fit P-5C2C
orfit P-4C1C
: qqor eqq
info. Mor
fit -EP-5C1C
Spectrometer
Based on measurement of lepton bending angle
Spectrometer
Based on measurement of lepton bending angle
LEP Energy (2)• 3 methods used to cross-check extrapolation:
Systematics:Systematics:
Radiative returnRadiative returnEnergy lossBased on the frequency of the field provided to beam to compensate from syncroton radiation.
Energy lossBased on the frequency of the field provided to beam to compensate from syncroton radiation.
E loss by sync.
Jet velocity & massLast jet reconstruction(with ECAL cleaning)
Z peak jets boost
(T>0.8)
jetjet Mlog
)log(
P
MeV 20180M
%2.05.1)(: DATA/MC
jet MeV 205M %2.00.0)(: DATA/MC jet
Jet boost (and mass) now well calibrated !
Removing low energy neutrals(<1.5 GeV , and <2 GeV if mixed)
Radiative Z peak
MeV 54Total
MeV 24anglePolar
MeV 16 tracksForward
MeV 12stat MC
MeV 16background
MeV 7ISR
MeV 20methodFit
MeV 30rsCalorimete
MeV 19ionFragmentat
effect mSource 12
MeV (syst) 58 (stat) 3243E
MeV (syst) 54 (stat) 3040m
b
Z
Systematic uncertainties
Unbinned likelihood fit to a pdf built
with a fit to the MC reference
2GeV/cZm
Forward-backward Asymmetryof
The pull of individualpseudo-observablesin the global fit
bbZ
Deep Inelastic N scattering
Prob(2) 42% 14%
Prob(2 ) 14 % 1.7%
Observables vs top and Higgs mass
2tFmGr )/log( 222
WHWF MMMGr
Electroweak observables in the (mtop , mhiggs ) plane