w physics at lep e.barberio southern methodist university pic2003 zeuthen 28 th june 2003
TRANSCRIPT
June 2003 E.Barberio
the LEP program
LEP1: 18 Million Z boson decays (89-95)LEP2: 36 Thousand W pairs (96-00)
• W pair production• triple and quartic gauge couplings• W mass and width measurements • final state interactions
this talk:
June 2003 E.Barberio
WW events
semileptonic channel
43.8%missing energylow background
hadronic channel
45.6%large backgroundambiguity in assigning jets to W
leptonic channel 10.6%large missing energy
WWll
WWqql
WWqqqq
June 2003 E.Barberio
W pair cross section
1% measurement
clear evidence of WW and WWZ vertices: probe of the non-Abelian structure of the Standard Model
+ +
theo
WW
σ
σ=0.9980.006(stat)0.007(syst)
preliminary LEP
June 2003 E.Barberio
W branching fractions
SM: 10.83%
= 1.000 0.021
= 1.052 0.029
= 1.052 0.028
test of lepton universality at 3% (less precise than LEP1)
SM: 67.51%
hadronic branching fraction:Br(Wqq’) = 67.92 0.27%
June 2003 E.Barberio
triple gauge couplings WW WWZ
W
W
W
W
Zgeneral WW and WWZinteraction: 14 parameters
electric quadrupole moment
magnetic dipole moment
1W
m2e
W
2W
m
eW
q
applying C and P invariance& use low-energy constraints we are left with 3 parameters
relation with the static W properties:
SM values
0
1z1
g
1
June 2003 E.Barberio
measuring the coupling at LEP2
sensitive observables
WW production:most constraining
We-
e+
W+
W-
W
f
f
W decay angles (helicity)
W+W- production angle cosW
W rest frame and of W decay products
June 2003 E.Barberio
Single W
single W production +
8% precision
but it is very constraining for k
smaller cross section than WW:
OPAL preliminary
- single W- WW angles- WW
- combined
k
June 2003 E.Barberio
TGC 1-parameter fit results
dominant systematics O(em)
g1Z,: 0.015 : 0.039
(partial statistics)
- ALEPH- DELPHI- L3- OPAL - LEP
g1Z, k 2-5% measurement
June 2003 E.Barberio
TGC 3-D parameter fit results
joint minimization of statistical error
2D contour: 3rd parameter at the minimum
June 2003 E.Barberio
W polarizationin the SM W boson longitudinally polarized
spin density matrix
evidence for WL at 5 level !
OPAL
cosW
LL=00d/dcosWdcosW
T=(+++--)d/dcosWdcosW
L/ =0.2430.0270.012
SM: 0.240 at s=197 GeV
cosh*
L/ =0.2100.0330.016
unfold decay angle distribution
June 2003 E.Barberio
Quartic Gauge Couplingin SM these couplings exist but too small to be seen at LEP
look for anomalous contributionsparameterised by additional termsin the Lagrangian
couplings a0, ac, an; physics scale
-0.020 < a0/2 < 0.020 GeV-
2
-0.053 < ac/2 < 0.037 GeV-
2
-0.16 < an/2 < 0.15 GeV-2
new OPAL analysis of WW
June 2003 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
mass of the W boson
direct reconstruction :
mW from the invariant mass calculated using the W decay
productsWW qqqq and WW qql (ALEPH and OPAL also WW ll)
raw mass
measure mW and mtop prediction of mH
June 2003 E.Barberio
mW spectrum
had
ron
isat
ionW
p
rod
ucti
on
an
d d
ecay
Pert
.QC
Dd
eca
y
W
ob
serv
ati
on
(D
ETEC
TO
R)
mW extraction calibrated
with Monte Carlo simulation
reconstructed mass distorted! - initial state radiation E0<Ebeam
- mW(jet/recon. lepton) mW(quark/lepton)
June 2003 E.Barberio
LEP: latest results
direct measurements
mH<210 GeV @ 95% C.L. SM fitmH > 114 GeV direct limit
mW(GeV)
mWworld=80.4260.034 GeVW constrained to SM
relationship with mW:
June 2003 E.Barberio
Systematic errors
WWqqqq weight channel in the combination: 9%
experimentschannelsyears
qqlv qqqq comb. corr.e c y
CR - 90 9 e - y
BE - 35 3 e - y
other 4 5 4 - - -
rad. corrections 8 8 8
fragmentation 19 18 18 e c y
detector 14 10 14 - c y
LEP energy 17 17 17 e c y
systematics 31 101 31statistical 32 35 29
total 44 107 43
cross-LEP effort in progress to address these errorsderive them from data whenever is possible
June 2003 E.Barberio
radiative corrections
a new OPAL analysis tries to estimate on data the contribution of real production using WW events
mW calibrated on Monte Carlo with O() photon radiation but not all diagrams are completely included:
estimated mass shift dueto real photon production from data ~ 6-8 MeV
June 2003 E.Barberio
final state interactions (only 4q)
possible interaction between the two W decays products not in the simulation apparent shift in mw
only phenomenological modelsfm
Colour Reconnection (CR):
• W decay~0.1fm<< hadronization scale~1fm colour flow between Ws
• seen at ep,pp colliders (rapidity gaps) and in heavy meson decays
Bose Einstein Correlation (BEC):
• favours production of pairs/multiplets of identical particles close together
• well established in single Z and W
June 2003 E.Barberio
CR: particle flow in 4-jet events at LEP2
L330%
RN=(A+C)/(B+D) is
used to compare with models:
various models and parameters! one experiment can exclude only extreme cases LEP combination
CR: modifies particle flow between Ws:
W W
June 2003 E.Barberio
particle flow: LEP combination
r=RNdata/RN
no-CR
r=0 no CR, r0 CR
preferred value in data Precmin
~49%r
between various models SK1 gives the largest mW
bias: vary reconnection fraction
mass bias calculated from Prec
min+1 used in the mW
combination: mass shift increases (90 MeV) but data driven
June 2003 E.Barberio
mW and CR
strategies to reduce CR bias: - hybrid cone jet cone
algorithm- remove low energy particle pcut
SK1 parameter
most probably LEP will use these strategies for the final mW
trade statistics for systematics:
all CR model used behave as SK1!
it also reduces BEC systematics!systematics are under study
~ factor 2-3 in CR shift, 2 in BEC shift ~ 20% loss in statistics
June 2003 E.Barberio
CR with mW
combination with colour flow (almost uncorrelated)
mW(no-CR)–mW
CR to study CR
- higher sensitivity than colour flow- mass difference still use the qqqq channel to measure mW!
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 5discovery for CR in WW events
June 2003 E.Barberio
Bose Einstein Correlations
hadronic parts of qqln
rotate/boost
mix ‘WW’ event
measure BEC between W comparing (Q) (2-particle density) in 4q and ‘mixed’ WW events:
R2(Q)=ρ(4q) /ρ(mix WW)noBE
Δρ = ρ(4q)- ρ(mix WW)
ALEPH, L3: no sign of BEC between WsDELPHI: small BEC between Ws
propagate results on BEC between Ws into mW systematics: work in progresshowever mass shift due to BEC is expected to be smaller than CR
June 2003 E.Barberio
measuring the W widthfit simultaneously for mW and W direct measurement of W
SM 2.095 GeV
wworld=2.1390.069 GeV
June 2003 E.Barberio
conclusions and outlook• LEP met the expectations and exceeded them
• many properties of the W boson are measured • triple gauge coupling are well determined
•5 evidence of the longitudinal polarisation of the W
• for the measurements of the W mass and width
there are good prospects to improve the results and for mW to meet the 35 MeV error goal
• so far good agreement with the Standard Model predictions• final analyses still going on …
0.0240.020λ
0.0550.943
0.998g
γ
λ
0.0230.025
Z1
κ
091.0150.2Γ
0.046GeV80.426m
W
W
June 2003 E.Barberio
CKM unitarity and Vcs
ji,
2
ijs
ji V)πα
(1)qq'BR(W
|Vcs| = 0.966 ± 0.013dominated by the error on the Br
measurement of Vcs the least know CKM element before LEP2 (11%):
flavour changing transitionsW on-shell
b)s,(d,j
c)(u,i 0.0252.039V2
ij
CKM unitarity for elements not involving the top quark
2V2
ij
dominated by the error on the Br
∝|Vqq|2
qW
q’