W physics at LEP. E.Barberio Southern Methodist University PIC2003 Zeuthen 28 th June 2003. the LEP program. LEP1: 18 Million Z boson decays (8995 ) LEP2: 36 Thousand W pairs (9600). W pair production triple and quartic gauge couplings W mass and width measurements
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LEP1: 18 Million Z boson decays (8995)
LEP2: 36 Thousand W pairs (9600)
this talk:
E.Barberio
WWlnln
leptonic channel
10.6%
large missing energy
semileptonic channel
43.8%
missing energy
low background
WWqqln
hadronic channel
45.6%
large background
ambiguity in assigning jets to W
WWqqqq
E.Barberio
=0.9980.006(stat)0.007(syst)
W pair cross section+
+
preliminary LEP
clear evidence of WWg and
WWZ vertices: probe of the
nonAbelian structure of the
Standard Model
1%measurement
E.Barberio
= 1.052 0.029
= 1.052 0.028
SM: 10.83%
hadronic branching fraction:
Br(Wqq’) = 67.92 0.27%
SM: 67.51%
W branching fractionstest of lepton universality at 3%
(less precise than LEP1)
E.Barberio
W
g
Z
W
W
SM values
triple gauge couplings WWg WWZgeneral WWg and WWZ
interaction: 14 parameters
applying C and P invariance
& use lowenergy constraints
we are left with 3 parameters
relation with the static W properties:
magnetic dipole moment
electric quadrupole moment
E.Barberio
W
qW
e
e+
f
W+
q
W
f
measuring the coupling at LEP2WW production:
most constraining
sensitive observables
W+W production angle cosW
W decay angles (helicity)
W rest frame
q and of W
decay products
E.Barberio
E.Barberio
OPAL preliminary
8% precision
kg
Single Wsingle W production
smaller cross section
than WW:
but it is very constraining for kg:
E.Barberio
 ALEPH
 DELPHI
 L3
 OPAL
 LEP
(partial statistics)
g1Z, kg 25% measurement
dominant systematics O(em)
g1Z,lg: 0.015 kg: 0.039
E.Barberio
2D contour: 3rd parameter
at the minimum
joint minimization of
statistical error
E.Barberio
sL/s =0.2430.0270.012
SM: 0.240 at s=197 GeV
cosqh*
OPAL
L
sL=r00ds/dcosqWdcosqW
sT=(r+++r)ds/dcosqWdcosqW
cosqW
W polarizationin the SM W boson longitudinally polarized
unfold decay angle distribution
spin density matrix
sL/s =0.2100.0330.016
evidence for WL at 5s level !
E.Barberio
in SM these couplings exist but too
small to be seen at LEP
look for anomalous contributions
parameterised by additional terms
in the Lagrangian
new OPAL analysis of WWg
E.Barberio
excellent mass resolution comes from
kinematic fit:
constrain total (E,p) to (s,0)
need for precise knowledge of the
beam energy from LEP
raw mass
mass of the W bosonmeasure mW and mtop prediction of mH
direct reconstruction:
mW from the invariant mass calculated using the W decay products
WW qqqq and WW qqln
(ALEPH and OPAL also WW lnln)
E.Barberio
mW spectrum
W
observation (DETECTOR)
W production and decay
Pert.QCD
hadronisation
decay
reconstructed mass distorted!
 initial state radiation E0<Ebeam
 mW(jet/recon. lepton) mW(quark/lepton)
mW extraction calibrated
with Monte Carlo simulation
E.Barberio
mWworld=80.4260.034 GeV
GW constrained to SM relationship with mW:
mH<210 GeV @ 95% C.L. SM fit
mH > 114 GeV direct limit
mW(GeV)
E.Barberio
qqqq
comb.
corr.
e c y
rad. corrections
8
8
8
e c y
fragmentation
19
18
18

c y
detector
1
4
10
1
4
e c y
LEP energy
17
17
17
e

y
CR

90
9
BE

35
3
e

y



other
4
5
4
systematics
31
101
31
statistical
3
2
3
5
29
total
44
107
4
3
Systematic errorsexperiments
channels
years
WWqqqq weight channel in
the combination: 9%
crossLEP effort in progress to address these errors
derive them from data whenever is possible
E.Barberio
mW calibrated on Monte Carlo with O() photon radiation but not all diagrams are completely included:
a new OPAL analysis tries to estimate on data the contribution of real production using WWg events
estimated mass shift due
to real photon production
from data ~ 68 MeV
E.Barberio
possible interaction between the two W decays products not in the simulation apparent shift in mw
Colour Reconnection (CR):
and in heavy meson decays
Bose Einstein Correlation (BEC):
only phenomenological models
fm
E.Barberio
W
L3
30%
CR: particle flow in 4jet events at LEP2CR: modifies particle flow between
Ws:
RN=(A+C)/(B+D) is used to compare with models:
various models and parameters!
one experiment can exclude only extreme cases LEP combination
E.Barberio
between various models SK1 gives the largest mW bias:
vary reconnection fraction
preferred value in data Precmin~49%
mass bias calculated from Precmin+1s used in the mW combination: mass shift increases (90 MeV) but data driven
r=RNdata/RNnoCR
r=0 no CR, r0 CR
E.Barberio
strategies to reduce CR bias:
 hybrid cone jet cone algorithm
 remove low energy particle pcut
all CR model used behave as SK1!
it also reduces BEC systematics!
systematics are under study
SK1 parameter
most probably LEP will use these strategies for the final mW trade statistics for systematics:
~ factor 23 in CR shift, 2 in BEC shift
~ 20% loss in statistics
E.Barberio
 higher sensitivity than colour flow
 mass difference still use the
qqqq channel to measure mW!
mW(noCR)–mWCR to study CR
combination with colour flow
(almost uncorrelated)
use this combination to get the CR systematics for the W mass:
the exact procedure is under discussion
all experiments are working on similar
analyses
it will be difficult to achieve a
5s discovery for CR in WW events
E.Barberio
hadronic parts of qqln
rotate/boost
Bose Einstein Correlationsmeasure BEC between W comparing r(Q) (2particle density) in 4q
and ‘mixed’ WW events:
R2(Q)=ρ(4q) /ρ(mix WW)noBE
mix ‘WW’
event
ALEPH, L3: no sign of BEC between Ws
DELPHI: small BEC between Ws
propagate results on BEC between Ws
into mW systematics: work in progress
however mass shift due to BEC is
expected to be smaller than CR
E.Barberio
fit simultaneously for mW and GW
direct measurement of GW
Gwworld=2.1390.069 GeV
SM 2.095 GeV
E.Barberio
E.Barberio
∝Vqq2
W
q’
CKM unitarity and VcsCKM unitarity for elements not involving the top quark
flavour changing transitions
W onshell
dominated by the error on the Br
measurement of Vcs the least know CKM element before LEP2 (11%):
Vcs = 0.966 ± 0.013
dominated by the error on the Br
E.Barberio