W Mass results from Tevatron and LHC

0.18 MB·English

by  Alex Melnitchouk

W Mass results from Tevatron and LHC
AlexMelnitchoukforATLAS,CDF,CMS,D0andLHCbCollaborations,a
UniversityofMississippi,University,Mississippi,38677,USA
Abstract. MostrecentresultsofWbosonmassmeasurementsfromTevatronexperiments(CDFandD0)inpp¯
collisionsat √s=1.96GeVarereported,using0.2 fb 1and1.0 fb 1datacollectedatCDFandD0,respectively.
2 − −
ThemeasurementsofW bosonpropertiesatLHCexperiments(ATLAS,CMS,andLHCb)in ppcollisionsat
1
√s = 7TeV,usingdatacollectedbeforeSummer2011,arepresented.Thesemeasurementsareessentialatthe
0
2 preparationstageoftheWbosonmassmeasurementsatLHC.ChallengesforWmassmeasurementattheLHC
incomparisonwiththeTevatronareoutlined.ProspectsforWmassprecisionwithupcomingmeasurementsand
n
itsimplicationsarediscussed.
a
J

4
1 Introduction calorimeter and plug calorimeter (η < 2.8) while elec-
2 | |
trons in D0 [5] are reconstructed in the central and end-

] MeasurementoftheW bosonmass(MW)providesuswith cap calorimeters (η < 1.05 and 1.5 < η < 3.2). Here
x a uniquely powerful key to uncovering the origin of the η= lntan(θ/2,an|d|θisthepolaranglew|it|hrespecttothe
e −
electroweak symmetry breaking and learning about new protondirection.BothCDFandD0requiretightelectrons
-
p physics.Atthelooplevel,Wbosonisconnectedwiththe in the centralcalorimeter(η < 1.05)forW eνcandi-
| | →
e topquarkandtheHiggsbosonviatheradiativecorrections dates.Electronenergiesaremeasuredwiththecalorimeter,
h totheWmass.Henceprecisemeasurementsofthemasses while electron direction is measured with tracking detec-
[
of the top quark and W boson allow us to constrain the tors,usingtracksthatarematchedtoelectronclusterinthe
1 mostprobablemassrangeofthe Higgsbosonmass. Cur- calorimeter.
v rentworldaverageforWmassis80.399 0.023GeV[1]. Muons are identified by a track in the muon system
±
4 Current world average for top quark mass is 173.2 0.9 matched to a track in the central tracking system. Mea-
3 GeV [2]. These measurements combined with other±pre- surements include the muons reconstructed in the central
1 cisionmeasurementstellusthatthemassoftheStandard- muon extension sub-detector which extends the coverage
5
ModelHiggsbosonis lowerthan161GeV at95%confi- from η <0.6to η <1.
. | | | |
1 dence level[3]. With improvedprecisionof the W boson
0 mass measurementtighter constraintscould be placed on
2 the Higgsbosonmass. Compatibilityof such tightercon- 3 Overviewof W mass measurement
1
straintsfromprecisiondatawiththeresultsfromongoing
:
v direct Higgs boson searches or lack thereof would be a W boson mass is measured using three transverse kine-
i criticalpieceofinformationforunderstandingelectroweak maticvariables:thetransversemass
X
symmetrybreakingmechanism. m = 2pe,µpν(1 cos∆φ), the the transverse momen-
r T q T T −
a tumofthelepton1( pe,µ)andneutrino(pν)transversemo-
T T
mentum,where∆φistheopeninganglebetweentheelec-
2 Identification of Electrons and Muons at
tron(muon)andneutrinomomentaintheplanetransverse
CDF and D0 to the beam. Neutrino transverse momentum (pν) is in-
T
ferred from the imbalance of transverse energy. We also
Electronsare identified as an electromagnetic(EM) clus- refertothisobservableasmissingE (MET).
T
terreconstructedwithasimpleconealgorithm.Toreduce A sophisticated parametrizedMonte Carlo simulation
the background of jets faking electrons, electron candi- isusedformodelingthesevariablesasa functionof M .
W
dates are requiredto have a large fractionof their energy M isextractedfromabinnedmaximum-likelihoodfitbe-
W
deposited in the EM section of the calorimeter and pass tween the data and simulation. Fast simulation includes
energyisolationandshowershaperequirements.Electron modelsofelectron(muon),recoilsystem,andbackgrounds.
candidatesareclassifiedastightifatrackismatchedspa- Electronefficiencies,resolutionandenergyscale parame-
tially to EM cluster and if the track transverse momen- terizationsaretunedtoZ eedata.
tum is close to the transverse energy of the EM cluster. →
InCDF[4]electronsarereconstructedbothinthecentral 1 electronormuoninthecontextofthisusage,D0usesonly
electron channel, whereas CDF uses both electron and muon
a e-mail:[email protected] channelsforM measurement
W
EPJWebofConferences
Recoilsystemrepresentsenergydepositedinthecalorime- CDF II ∫ L dt ≈ 200 pb-1
ter from all sources exceptthe electron(s).Recoil system p 0
p/
consistsofthreemajorcomponents:hardrecoil(particles ∆ Scale correction = (-1.64±0.01 ±0.06 )x10-3
stat slope
thatcollectivelybalancethe p oftheW ofZ boson),un-
T
derlying event, and additional interactions. Contribution
-0.001
from the third component depends on the instantaneous
luminosity. Hard recoil is modeled using the full detec-
tor simulation, while the other two components are de-
scribed by real data events. Full recoil model is tuned to -0.002
Z ee data, using imbalance between the Z boson mo-
me→ntum measured with electrons(muons)and with recoil J/Ψ→µµ data
system. Sourcesof backgroundsto W e,µν eventsin-
cludeW →τν→e,µνν,QCD,andZ →→ee,µµprocesses. -0.0030 0.2 0.4 <1/pµT> (GeV-01).6
∫
CDF II L dt ≈ 200 pb-1
4 Lepton Energy Scale Calibration nts / 0.01 4000 SE = 1 ± 0.00025stat
Dominant uncertainties in M measurements come from eve
W χ2/dof = 17 / 16
leptonenergyscalemeasurements.Tofirstorderfractional
errorontheleptonenergyscaletranslatestofractionaler-
2000
rorontheWmass[6].
D0determineselectronenergyscaleusinghighp elec-
T
tronsfromZ eedecays.Precisionofsuchcalibrationis
→
limitedmostlybythesizeoftheZ eesample.
→
CDF relies on tracking detector for both electron and
0
1 1.5
muon energy scale calibration. First tracking detector is E/p (W→eν)
calibrated using J/ψ µµ events. J/ψ invariant mass is
→ Fig.1.Top:fractionalmuonmomentumcorrectionasafunction
measuredasafunctionofmuonmomentum.Fig.1shows
ofinversemomentum.Bottom:ratioofelectronenergymeasured
the correction needed to make measured J/ψ mass to be
inthecalorimetertoelectronmomentummeasuredbythetrack-
atitsPDGvalue(overalloffset)andindependentofmuon
ingsysteminW eνevents.
momentum (slope). This correction was implemented in →
the simulation by adjusting the energy-loss model. Then
tracker calibration is transported to the calorimeter using
W eν electrons near the peak of the E/p distribution,
→
shownalsoinFig.1.
CDF Run 0/I 80.436 ± 0.081
5 Results and Prospects D0 Run I 80.478 ± 0.083
CDF Run II 80.413 ± 0.048
M resultsfromD0[7]andCDF[8]alongwithotherM
W W
measurementsandcombinationsareshowninfFig.2.D0 Tevatron 2007 80.432 ± 0.039
result 80.401 0.021(stat) 0.038(syst) GeV = 80.401 D0 Run II 80.402 ± 0.043
± ±
0.043GeV agreeswith theworld averageandthe indi-
±vidual measurements and is more precise than any other Tevatron 2009 80.420 ± 0.031
MW measurement from a single experiment. CDF result LEP2 average 80.376 ± 0.033
80.413 0.034(stat) 0.034(syst)GeV=80.413 0.048
GeV. Fi±g. 3 shows a±comparison of observables b±etween World average 80.399 ± 0.023
D0 1fb 1 W eν data and fastsimulation. Fig. 4 shows July 09
corresp−onding→muonchannelplotsforCDF0.2fb 1 mea- 80 80.2 80.4 80.6
− m (GeV)
surement. In both CDF and D0 measurements dominant W
experimentalsystematicerrorisduetoleptonenergyscale,
whereas dominanttheoretical error is due to PDFs. Cur- Fig.2. SummaryofthemeasurementsoftheWbosonmassand
rentlybothCDFandD0experimentsperformedWboson their average. The result from the Tevatron corresponds to the
values which includes corrections to the same W boson width
mass measurements only on small fraction of their data.
and PDFs. The LEP II results are from [9]. An estimate of the
These measurementlead to 31 MeV W mass uncertainty
worldaverageoftheTevatronandLEPresultsismadeassuming
fromTevatronandtoworldaverageuncertaintyof23MeV.
nocorrelationsbetweentheTevatronandLEPuncertainties.
Based on electroweak fits most probable Higgs mass
valueis92GeV,massregionabove161GeVisexcluded
at 95% confidence level. If world average uncertainty is
XXIIndHadronColliderPhysicsSymposium
GeV10000 (a) D0, 1 fb-1 DFAatSaT MC GeV20000 (b) D0, 1 fb-1 DFAatSaT MC GeV20000 (c) D0, 1 fb-1 DFAatSaT MC
0.5 7500 χ2B/daocfk =g 4r8o/u49nd 0.5 15000 χ2B/daocfk =g 3r9o/u31nd 0.5 15000 χ2B/daocfk =g 3r2o/u31nd
s/ s/ s/
nt nt nt
ve 5000 ve10000 ve10000
E E E
2500 5000 5000
χ χ χ
2 2 2
0 0 0
-2 -2 -2
50 60 70 80 90 100 25 30 35 40 45 50 55 60 25 30 35 40 45 50 55 60
mT (GeV) peT (GeV) ET (GeV)
Fig.3. Eelectronm ,p ,andMETdistributionsinW eνD0dataandfastsimulation(fastmc).Addedbackgroundisshownaswell.
SignedχdistributionTsarTeshowninthebottomofparto→feachplot.Signedχisdefinedasχ =[N (fastmc)]/σ foreachpointinthe
i i− i i
distribution,N isthedatayieldinbiniandσ isthestatisticaluncertaintyinbini.
i i
∫ ∫ ∫
CDF II L dt ≈ 200 pb-1 CDF II L dt ≈ 200 pb-1 CDF II L dt ≈ 200 pb-1
events / 0.5 GeV 1000 events / 0.25 GeV 1000 MW = χ(28/0d3o2f 1= ± 7 626 /s t6at2) MeV events / 0.25 GeV 1000 MW = χ(28/0d3o9f 6= ± 4 646 /s 6ta2t) MeV
500 MW = (80349 ± 54stat) MeV 500 500
χ2/dof = 59 / 48
060 70 80 90 mT(µν) (GeV1)00 030 40 50pT(µ) (GeV) 030 40 50pµT(ν) (GeV)
Fig. 4. Distributions of M observables in CDF measurement (muon channel). Blue – data. Red – fast simulation. Fit results and
W
statisticalerrorsareindicated.Left:m .Middle:muon p .Right:neutrino p .
T T T
reduced to 15 MeV then most probable value and exclu- quarks.Qualitativelythesametypeofasymmetryasafunc-
sion limit would become 71 GeV and 117 GeV respec- tionofW bosonrapidityisexpectedasincaseofproton-
tively[10].Theseestimatesaremadewiththeassumptions antiprotoncollisions.Howevertheshapeisexpectedtodif-
ofnochangeinthecentralvaluesofWbosonmassandtop fercomparedtoproton-antiprotoncollisionssincevalence
quarkmassandwithtopquarkmassuncertaintyof1GeV. quarks and sea quarks have different momentum fraction
WiththefullTevatrondatasetprecisionof15MeVmaybe distributions.BesidesthetotalnumberofproducedW+ is
possible[6]. expected to exceed that of W since proton contains two
−
valence u quarks and one valence d quark. The inclusive
ratio of cross sections for W+ and W boson production
−
6 W boson measurementat the LHC was measured by CMS to be 1.43 0.05 [11]. Moreover,
the shapes of W+ and W p spec±tra are expected to be
− T
different. Hence, they need to be measured and modeled
W mass measurementsat the LHC is expectedto involve
separately.RecentlyATLASmeasuredW boson p spec-
additionalchallengesin comparisonwith the correspond- T
trum [15], shown in Fig. 5, which can be consideredfirst
ingmeasurementsattheTevatron.First,muchhighernum-
steptowardsunderstandingthisobservableattheprecision
berofadditionalinteractions,whichproduceinthedetec-
needed for W mass measurement. Another input needed
tor largeenergydeposits, uncorrelatedwith the W boson.
for W mass measurementis precise knowledge of parton
Second,specificsoftheW bosonproductionmechanisms.
distributionfunctions(PDFs).SinceW chargeasymmetry
Incaseofproton-antiprotoncollisionsW+(W )ispro-
− as a functionof rapidityis drivenby partondistributions,
ducedwith valenceu and d¯(d and u¯) quarks.Total num-
bymeasuringtheasymmetrypartondistributionfunctions
ber of producedW+ and W is the same. As the u quark
− can be constrained. Asymmetry in the W boson rapidity
tendstocarryahigherfractionoftheproton’smomentum
distributionhastraditionallybeenstudiedintermsofcharged
than the d quark, the W+(W ) is boosted, on average, in
− lepton asymmetry, as W boson rapidity cannot be deter-
theproton(anti-proton)direction.Henceasymmetryinthe
minedontheevent-by-eventbasis,sinceneutrinoescapes
production rate between W+ and W as a function of W
− the detection. Charged lepton asymmetry,is the convolu-
rapidityis observed.However,W boson p spectrum,in-
T tionofW productionandV-A(vector-axialvector)decay
tegratedoverall rapiditiesis identicalfor W+ andW . in ±
− asymmetries.Asymmetryisdefinedasaratioofdifference
caseofproton-antiprotoncollisions.
andsumofpositivelychargedandnegativelychargedlep-
Incaseofproton-protoncollisionsW+(W )isproduced
− tons.ATLAS[12],CMS[13]andLHCb[14]alreadyper-
with valence u and sea d¯ quarks (valence d and sea u¯)
EPJWebofConferences
-1]V 10-1 13. CMSCollaboration,JINST3S08004(2008)
Ge ATLAS 14. LHCbCollaboration,JINST3S08005(2008)
W ) [pT10-2 ∫Ldt ≈ 31 pb-1 1156.. ALTHLCAbSpuCbollilcanbootreatLioHnC,abr-XCiOv:N11F0-280.61310-0839
d
σ / dfid 10-3 Data 2010, s = 7 TeV 1178.. ACTMLSApSupbulibclniconteoCteMAST-LPAASS--CEOWNKF--1210-1010-5129
σ ) ( fid 10-4
1/
( 10-5 Data
RESBOS
10-6
S 1.6
O 1.4
B
S 1.2
RE 1
Data / 000...4680 50 100 150 200 250 300
pW [GeV]
T
Fig.5.NormalizeddifferentialcrosssectionasafunctionofW
boson p obtained fromthecombined electronandmuon mea-
T
surements,comparedtotheRESBOSprediction.
formed first measurements of lepton charge asymmetries
[16,17,18]
Since sea quarksare involved in W boson production
at the LHC, both charmand strange quarks,unlike at the
Tevatron,contributesignificantly.PDFsofbothcharmand
strangequarksarecurrentlyverypoorlyconstrained.Sig-
nificantimprovementsinPDFprecisionwouldbeneeded
forpreciseW massmeasurementattheLHC.
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∫
≈ -1
CDF II L dt 200 pb
V
e
G 1500

5
2
0. M = (80473 ± 57 ) MeV

/ W stat
s
t
n
e
v χ2/dof = 63 / 62
e
1000
500
0
30 40 50
pe(ν) (GeV)
T
∫
≈ -1
CDF II L dt 200 pb
V
e
G

5
.
0
1500
/

s
t
n
e
v
e
1000
±
M = (80493 48 ) MeV
W stat
500
χ2/dof = 86 / 48
0
60 70 80 90 100
m (eν) (GeV)
T
∫
≈ -1
CDF II L dt 200 pb
V
e
G
1500

5
2
0. M = (80451 ± 58 ) MeV

/ W stat
s
t
n
e
v χ2/dof = 63 / 62
e
1000
500
0
30 40 50
E (e) (GeV)
T