w physics at lep e.barberio southern methodist university dallas (usa) september 2003
Post on 05-Jan-2016
215 Views
Preview:
TRANSCRIPT
W physics at LEP
E.Barberio
Southern Methodist UniversityDallas (USA)
September 2003
Nikhef, September 12th, 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:
Nikhef, September 12th, 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
Nikhef, September 12th, 2003 E.Barberio
W branching fractions
= 0.997 0.021
= 1.058 0.028
= 1.061 0.028
test of lepton universality at 3% (less precise than LEP1)
SM: 67.51%
SM: Wl and Wqq couplings are equal, but QCD correction enhance hadronic branching fraction:Br(Wqq’) = 67.8 0.28%
SM: 10.83%
Nikhef, September 12th, 2003 E.Barberio
CKM unitarity and Vcs
ji,
2
ijs
ji V)πα
(1)qq'BR(W
|Vcs| = 0.989 ± 0.014dominated 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’
Nikhef, September 12th, 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=0.9780.006(stat)0.007(syst)
preliminary LEP
Nikhef, September 12th, 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
Nikhef, September 12th, 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
Nikhef, September 12th, 2003 E.Barberio
WW production/decay angular distributions
W1
Nikhef, September 12th, 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
Nikhef, September 12th, 2003 E.Barberio
TGC 1-parameter fit results
(almost final)
- ALEPH- DELPHI- L3- OPAL - LEP
g1Z, k 2-5% measurement
dominant systematics O(em)
g1Z,: 0.015 :0.039
Nikhef, September 12th, 2003 E.Barberio
TGC 3-D parameter fit results
2D contour: 3rd parameterat the minimum
Nikhef, September 12th, 2003 E.Barberio
W polarisationin the SM W boson longitudinally polarised
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
Nikhef, September 12th, 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
e.g. OPAL
s GeV
Nikhef, September 12th, 2003 E.Barberio
Standard Model parameterse.w. process at tree level are computed from three parameters , GF , mZ and the CKM matrix elements Vij
contrary to ‘exact gauge symmetry’ theories (QED or QCD) the effect of heavy particles do not decouple: mtop was predicted by LEP1/SLD
sensitivity to mHiggs or to any kind of “heavy new physics” at energies not accessible
vacuum fluctuations modify the value of the observables -> when higher orders are included, observables are predicted as:
O(em,s,mW, mZ, mHiggs, mtop ,Vij) on-shell renormalization scheme
em = 0.004 ppmG= 9 ppmmZ = 23 ppm
very well measured!
Nikhef, September 12th, 2003 E.Barberio
measurement of the W mass
measure mW and mtop prediction of mH or new heavy objects which couple with the W as the Higgs does
r radiative corrections
r = - + rew 3%
Δr)1() θsin1(G 2
)(mα π m
W2
F
ZEM2W
from data + theoryfrom decay from LEP
tree level mW= 80.937 GeV wrong by 10
Nikhef, September 12th, 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
products
WW qqqq and WW qql (ALEPH and OPAL also WW ll)
raw mass
Nikhef, September 12th, 2003 E.Barberio
reconstructed mass distributions
DELPHIeqq
ALEPH 4q
L3 qq
OPAL qq
Nikhef, September 12th, 2003 E.Barberio
mW spectrum
mW extraction calibrated
with Monte Carlo simulation
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)
reconstructed mass distorted! - initial state radiation E0<Ebeam
- mW(jet/recon. lepton) mW(quark/lepton)
Nikhef, September 12th, 2003 E.Barberio
LEP: latest results
mW(GeV)
mWworld=80.4260.034 GeVW constrained to SM
relationship with mW: direct measurements
mH<210 GeV @ 95% C.L. SM fitmH > 114 GeV direct limit
Nikhef, September 12th, 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
Nikhef, September 12th, 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
Nikhef, September 12th, 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 also at the hadronization phase
• 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
Nikhef, September 12th, 2003 E.Barberio
expected effects of color reconnection
effects:
- change in particle particle multiplicity
- depletion of soft momenta particles
- anomalies in the particle flow /string effect modified
- rapidity gaps
- change in the reconstructed value of mW : the most sensitive observable unfortunately
It affects:
- interaction between decay products at the parton level
- final hadronic color singlets do not correspond to the initial W bosons
the effect should be present in the data, but how strong it is ?
Nikhef, September 12th, 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
Nikhef, September 12th, 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
Nikhef, September 12th, 2003 E.Barberio
using mw for CR?
mW is the most sensitive observable and we can use it
to measure/limit CR
CR affects more particles in the interjet region
variable used mass difference: e.g. mW(k<0)-mW(K>0)this allows to use the qqqq channel to measure mW
exclude/change the weight of soft inter-W particles from jets!
strategies to reduce CR bias: - hybrid cone jet cone
algorithm- remove low energy particle pcut
- jet direction from pk : K>0 decreases sensitivity; K<0 enhance it
Nikhef, September 12th, 2003 E.Barberio
mW and CR
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
Delphi (this summer): cone and pcut
Nikhef, September 12th, 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
Nikhef, September 12th, 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
Nikhef, September 12th, 2003 E.Barberio
measuring the W widthfit simultaneously for mW and W direct measurement of W
SM 2.095 GeV
wworld=2.139 0.069 GeV
Nikhef, September 12th, 2003 E.Barberio
conclusions and outlookmeasurements at the Z peak demonstrate that the SM is a quantum field theorymeasurements above the WW threshold demonstrate that the SM is a non-abelian gaunge theory
and as for the Z, measurements of the W properties at LEP has brought the quantitative test of the SM to a high level of accuracy:no deviation are observed within that accuracy
LEP2 achievements were better than foreseen:• triple gauge coupling are now well determined: 5% measurement!
• 5 evidence of the longitudinal polarisation of the W
• measurement the W mass 42 MeV and 91 MeV for the width, with good prospects to improve mW to meet the 35 MeV error
…BUT LEP did not see the Higgs….
Nikhef, September 12th, 2003 E.Barberio
global fit of the SM to data
limit from direct searchesmH> 114.4 GeV
deduce mH which gives best 2
radiative corrections~ log mH mH
ew < 219 GeV 95% C.L.
GeV96m 6038H
mH
largest discrepancy: 3 P(2) ~ 4.4% allP(2) ~ 27.3% without NuTeV
Nikhef, September 12th, 2003 E.Barberio
LHC and the electroweak interaction
5
full mass range accessible in1 year ( 5) final word
~1 year ~3 years
~ 4 years
LHC pp, s=14 TeV, start 2007?
LEPlimit~50% ~ 35% ~25% ~10%
2002 LEP2+Run15.1 GeV33 MeV
2006 LEP2+Run22.5 GeV25 MeV
2009 ? LHC1.5 GeV15 MeV
??? LC ? 0.2 GeV 7 MeV
mtop
mW
H
H
mm
if Higgs discovered comparison of measured mH with indirect measurement
Nikhef, September 12th, 2003 E.Barberio
mW at hadron colliders:Tevatron
pTv is inferred from the recoil
system balancing the W
the non-zero pT is due to gluon
radiation from quarks
single W production through qq annihilation:
)cos1(pp2m TlT
TW
mW measurement is performed in the leptonic channels using the transverse mass:
p = Ebeam=s/2sx1 p x2 p
p p
W
l
s sxx s 21
Nikhef, September 12th, 2003 E.Barberio
Systematics: key issues
pTW distribution Z bosons (fully reconstruct) plus models/theory
for difference between Z and W (different initial state quarks)
recoil pT distribution Z bosons with study of underlying event ET distributions from proton remnants and multiple interactions
HENCE major limitation on systematics from Z statistics…
calibration, energy scales and resolutions:challenge for detector alignment and calibration,use Z, , J/ mass peaks
Nikhef, September 12th, 2003 E.Barberio
Tevatron results
error source CDF CDFe D0lepton E scale 85 75 56lepton E resl 20 25 19
PTW distrib. 20 15 15
recoil model 35 37 35selection bias 18 - 12backgrounds 25 5 9PDFs / lumi 15 15 8radiative corrn 11 11 12statisitcs 100 65 60total 144 113 84
RunI (~100 pb-1, 15-30k events per channel): CDF W and e, D0 We
Tevatron (+UA2): mW= 80.454 0.059 GeV main systematics ‘almost’ uncorrelated
Nikhef, September 12th, 2003 E.Barberio
mW at hadron machines: LHC
mtop~2 GeV requires mW ~ 15 MeV systematics mW(MeV
)statistics 2E-p scale 15?energy resolution
5?
recoil model 5?lepton id 5pT
W 5
parton distr.func.
10?
W width 7radiative decays 10background 5total 25
statistical error for 10 fb-1
mW<2 MeVW l: 3 x 108 eventsZ ll: 3 x 107 events
plus unknown effects ..…
one LHC experiment
Nikhef, September 12th, 2003 E.Barberio
conclusions and outlook• LEP gave a very solid ground to the Standard Model
of electroweak interactions
• however: the Higgs is still missing……
• Tevatron is exploring a higher energy region and will
reduce the uncertainties on mtop and mW
(measurement uncorrelated with LEP) but has little chances to see the Higgs
• LHC will explore a higher energy region: it will cover the full allowed range for the Higgs
• if we find the Higgs at LHC we will need another e+e- machine for precision measurements
Nikhef, September 12th, 2003 E.Barberio
event rate and particle multiplicity
• L = luminosity = 1034 cm-2 s-1
• bunch spacing = 25 ns• 22 events / bunch
LHC events previous machines in 1 year total statistics
Z 108 LEP: 107 in ~10 yrsW 109 FNAL: 107 in ~7 yrstop 108 FNAL: 105 in ~7 yrs
top related