QCD Results from the Fermilab Tevatron proton-antiproton Collider

3.6 MB·English

by  Andreas Warburton

QCD Results from the Fermilab Tevatron pp¯ Collider
Andreas Warburton∗†
(onbehalfoftheCDFandDØCollaborations)
RutherfordPhysicsBuilding,McGillUniversity
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Montréal,Québec H3A2T8
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0 Canada
2 E-mail: [email protected]

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Selected recent quantum chromodynamics (QCD) measurements are reviewed for Fermilab
J
7 RunIITevatronproton-antiprotoncollisionsstudiedbytheColliderDetectoratFermilab(CDF)
√
1 andDØCollaborationsatacentre-of-massenergyof s=1.96TeV.TantamounttoRutherford

scatteringstudiesattheTeVscale,inclusivejetanddijetproductioncross-sectionmeasurements
]
x
are used to seek and constrain new particle physics phenomena, test perturbative QCD calcula-
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- tions, inform parton distribution function (PDF) determinations, and extract a precise value of
p
e thestrongcouplingconstant,αs(mZ)=0.1161+−00..00004418. Inclusivephotonproductioncross-section
h
measurementsrevealaninabilityofnext-to-leading-order(NLO)perturbativeQCD(pQCD)cal-
[

culationstodescribelow-energyphotonsarisingdirectlyinthehardscatter. Eventswithγ+3-jet
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v configurationsareusedtomeasuretheincreasinglyimportantdoublepartonscattering(DPS)phe-
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nomenon,withanobtainedeffectiveinteractioncrosssectionofσ =16.4±2.3mb. Observa-
0 eff
8 tionsofcentralexclusiveparticleproductiondemonstratetheviabilityofobservingtheStandard
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Model Higgs boson using similar techniques at the Large Hadron Collider (LHC). Three areas
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ofinquiryintolowerenergyQCD,crucialtounderstandinghigh-energycolliderphenomena,are
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0 discussed: the examination of intra-jet track kinematics to infer that jet formation is dominated
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bypQCD,andnothadronization,effects;detailedstudiesoftheunderlyingeventanditsuniver-
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v sality; andinclusiveminimum-biascharged-particlemomentumandmultiplicitymeasurements,
i
X whichareshowntochallengetheMonteCarlogenerators.
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XXthHadronColliderPhysicsSymposium
November16–20,2009
Evian,France
∗Speaker.
†SupportedbytheNaturalSciencesandEngineeringResearchCouncilofCanada.
(cid:13)c Copyrightownedbytheauthor(s)underthetermsoftheCreativeCommonsAttribution-NonCommercial-ShareAlikeLicence. http://pos.sissa.it/
QCDTevatronResults AndreasWarburton
1. Introduction
Quantum chromodynamics (QCD), the theory of the strong interaction between quarks and
gluons, isintrinsictoexperimentalstudiesofhadroncollisions. Thispaperreviewsseveralrecent
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QCDresultsfromtheCDFandDØexperimentsinanalysesof s=1.96TeV pp¯collisionscover-
ingupto2.7fb−1 oftime-integratedluminosity. Thegeneralapproachhasbeentosearchfornew
physics phenomena, test QCD theory, enable electroweak and exotic measurements by informing
MonteCarlo(MC)backgroundmodels,andlaythegroundworkfortheLHCeraof ppcollisions.
2. InclusiveJetandDijetProduction
Heedful testing of perturbative QCD (pQCD) at the shortest distance scales ever studied in
colliderexperimentsisprovidedthroughmeasurementsofdifferentialinclusivejetanddijetcross
sections. RecentCDF[1]andDØ[2]results,whichagreemutuallyandwithnext-to-leading-order
(NLO)pQCDcalculationsovereightordersofmagnitude,areshowninFig.1. Articulationofthe
Tevatron measurements into several rapidity regions has revealed that the experimental precision
now exceeds that of the parton distribution function (PDF) uncertainties and that softer gluons at
highFeynmanx(≥0.25)needtobeincludedinthePDFfits.
Also shown in Fig. 1 are recently extracted correlated values of the strong coupling constant
α by DØ [3]. Inclusive cross sections for jets with transverse momentum 50< p <145 GeV,
s T
correspondingtox≤0.25,wereusedtoensurenegligiblecorrelationsbetweenPDFsandextracted
α valuesandtoderiveameanofα (m )=0.1161+0.0041[3]. ThepreciseDØmeasurementslieat
s s Z −0.0048
energies intermediate to results from the HERA experiments (Fig. 1) and to those from an earlier
CDFRunIresult[4],whichspanned40< p <250GeV.
T
Inclusive dijet observables are favoured harbingers of new physics phenomena. Fig. 2 shows
recentresultsfromCDF[5]andDØ[6]ofdijetinvariantmass(M )distributions,forwhichStan-
jj
dardModel(SM)extensionspredictresonantenhancements. Bothexperimentsdemonstrateamass
reach out beyond 1.2 TeV/c2 and test pQCD predictions, with no indications of resonances. CDF
setsthetightest95%confidencelevel(CL)massexclusionlimitstodateonseveralexoticparticle
speciesthatdecaytotwojets: excitedquarks,colour-octetρ mesons,axigluons,flavour-universal
T
colorons,andW(cid:48)andZ(cid:48)bosons[5]. DØdemonstratesPDFsensitivityatforwardmaximumrapidi-
tiesintherange2.0<|y |<2.4,showingsofterhigh-xgluonstobefavoured[6].
max
Figure 1: [left] CDF [1] and [centre] DØ [2] inclusive jet differential cross sections, for multiple rapidity
regions,comparedtoNLOQCD;[right]derivedα (p )andα (m )valuesasafunctionofjet p [3].
s T s Z T
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QCDTevatronResults AndreasWarburton
Figure2: [left]CDF[5]and[centre]DØ[6]dijet-massdifferentialcrosssectionscomparedtoNLOQCD
predictions;[right]DØdijetangulardistributions[7].
Figure3: [left]CDF[8]and[right]DØ[9]inclusivephotoncrosssectionscomparedtoNLOQCD.
Fig. 2 also shows consistency between pQCD and a DØ measurement of the dijet angular
distribution, an observable that is complementary to M in its capacity to reveal new phenom-
jj
ena[7]. RutherfordandQCDscatteringarelargelyinsensitiveto χ , whichisexpectedtopeak
dijet
at low values (central rapidities) for new physics such as composite quarks, large extra spatial
dimensions (LEDs), and TeV−1 scale extra dimensions. DØ has set the most stringent 95% CL
limitsonthequarkcompositenessscaleΛ>2.9TeV,theADDLED(GRW)effectivePlanckscale
M >1.6TeV,andtheTeV−1 EDcompactificationscaleM >1.6TeV[7].
S C
3. InclusivePhotonProductionandDoublePartonScattering
Photonsemergingdirectlyfrom pp¯collisionsareunaffectedbyfragmentationandhadroniza-
tion, and therefore serve as potent probes of the hard parton scattering dynamics, with potential
sensitivity to gluon PDFs. Fig. 3 depicts recent CDF [8] and DØ [9] inclusive photon production
cross-section measurements, which agree within experimental uncertainties and show that NLO
pQCD has difficulty describing the behaviour in data at low photon p . Further theoretical effort
T
willbenecessarytounderstandthisdiscrepantregion.
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QCDTevatronResults AndreasWarburton
Vector meson photoproduction
Figure 4: [left] CDF central exclusive µ+µ− invariant mass distribution for pp¯ → p[µ+µ−(γ)]p¯ pro-
cesses[13];[right]CDFk distributionofparticlesinaconeof0.5radaroundthejetaxisinadijetinvariant
T
massbinwith(cid:104)Q(cid:105)=19GeV,correspondingtodijetmassesintherange66<M <95GeV/c2[14].
jj
DØhasstudiedγ+3-jeteventstomeasuredoublepartonscattering(DPS),wherebytwopairs
of partons undergo hard interactions in a single pp¯ collision. DPS is not only a background to
many rare processes, especially at higher luminosities, but also provides insight into the spatial
distribution of partons in the colliding hadrons. The DPS cross section is expressed as σγ+3jet ≡
DPS
σ σ /σ , where σ is the effective interaction region that decreases for less uniform spatial
γj jj eff eff
partondistributions. DØmeasuresameanofσ =16.4±2.3mb[10],whichisconsistentwithan
eff
earlierCDFresult[11],andfindsσ tobeindependentofjet p inthesecondinteraction. Ifmore
eff T
precisestudiescanrevealaσ sensitivitytojet p ,thiscouldindicateadynamicaldeparturefrom
eff T
γ+3jet
thenaïveassumptionthatσ dependsonanuncorrelatedproductofPDFsinσ andσ [12].
DPS γj jj
4. CentralExclusiveParticleProduction
Central exclusive processes are those where the colliding hadrons emerge intact but impart
γ/g combinations that interact at higher order to produce fully measurable states at low rapidi-
ties, with surrounding rapidity regions devoid of particles. CDF recently reported observations of
pp¯→p[dijet]p¯,indijeteventswithE (jet)>10GeV;and pp¯→p(cid:2)µ+µ−(γ),J/ψ,ψ(2S),χ0(cid:3)p¯,
T c
with two oppositely charged central muons and either no other particles or one additional photon
detected(seeFig.4)[13]. ConsiderationoftheSMHiggsH →bb¯ signatureandquantum-number
analogiesbetweenthescalarχ0mesonandtheHbosonpermitsthesemeasurementstodemonstrate
c
theviabilityofexclusiveSMHiggsproductionthrough pHpprocessesattheLHC[13].
5. JetFragmentation,UnderlyingEvent,andMinimumBiasStudies
SoftQCDinteractionsinhadroncollisions,studiedthroughcharged-particleobservables,are
experimentally and theoretically challenging yet crucial to understanding high p phenomena. A
T
CDF study of the transverse momenta (k ) of intra-jet particles with respect to their jet axis has
T
provided insight into which stage of jet formation principally determines jet characteristics [14].
Fig.4showsak distributionresemblingresummedpQCDpredictions,indicatingsupportforlocal
T
parton-hadrondualityandade-emphasisofnon-perturbativehadronizationeffectsinjetformation.
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QCDTevatronResults AndreasWarburton
Figure5: CDF[16]track p differentialcrosssection[left],averagetrack p vs. multiplicity[centre],and
T T
charged-particlemultiplicity[right]distributionscomparedwithvarioustheoreticalpredictions.
The underlying event (UE) consists of the beam-beam remnants minus the hard-scattering
productsandisbecomingincreasinglyimportanttothediscoveryandprecisionpotentialathadron
colliders. CDF has conducted an extensive program of UE studies that exploit jet and Drell-Yan
event activity topologies to maximize the sensitivity of UE observables [15]. Refs. [15] contain
several distributions of UE-sensitive observables, corrected to the particle level, that suggest the
UEmaybeuniversal(independentofthehardprocess)andinformMCtuninganddevelopment.
OtherimportantinputsintoMCtuningariseininclusiveinelastic pp¯collisions,studiedusinga
minimum-biastriggerunderlowluminosityconditions. CDFhasmeasuredaninclusivedifferential
charged-particle p crosssectionover11ordersofmagnitude(seeFig.5),findingpooragreement
T
with PYTHIA at higher momenta [16]. Fig. 5 also shows the measured correlation between the
mean p and multiplicity of charged particles in minimum-bias events, observables to which the
T
MCtuningparametersfavouringmultiplepartoninteractions(MPI)areparticularlysensitive. The
indicatedcharged-particlemultiplicitydistributionisthemostpreciseandextensiveevermeasured
inthe|η|≤1pseudorapidityrange;existingMCcalculationsdescribethehigh-multiplicityregion
poorly.
6. ConcludingRemarks
√
The Tevatron experiments now provide precision QCD physics at s=1.96 TeV, and mea-
surements common to both CDF and DØ are mutually consistent. The Rutherford scattering ap-
proachtostudyingthefundamentalconstituentsofmatterhasnowsoundlyenteredtheTeVregime,
withstringentconstraintsonnewphysics,testsofpQCD,precisemeasurementsofα ,insightinto
s
jet fragmentation processes, and information on high-x PDF gluon content. Theoretical improve-
ments are called for in several places, in particular to describe the observed photon cross section
and inclusive particle production event characteristics. These results are based on less than one
thirdoftheanticipatedcompleteRunIIsample, withmoremeasurementsexpectedinthecoming
yearstoilluminatetheLargeHadronCollidereraofQCDrediscoveryanddiscovery,nowbegun!
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QCDTevatronResults AndreasWarburton
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