Hard QCD Results with Jets at the LHC

0.44 MB·English

by  Sven Menke

Hard QCD Results with Jets at the LHC
SvenMenke1,aonbehalfoftheATLASandCMScollaborations
Max-Planck-Institutfu¨rPhysik,Fo¨hringerRing6,80805Mu¨nchen,Germany
√
Abstract. HardQCDresultsinproton-protoncollisionsat s=7TeVwithjetsfromdatarecordeduptothe
endof2010bytheCMSandATLASexperimentsattheLHCarereported.Inclusivejetanddi-jetcrosssection
measurementsaswellasobservablessensitivetomulti-jetactivityareshownandcomparedtosimulationsbased
onleadinglogpartonshowersaswellasNLOQCDpredictions.Novelapproachestoidentifyhighlyboosted
2
1 massivefinalstatesbyexploitingthejetsubstructurearetestedonthedominantQCDbackground.
0
2
1 Introduction uncertainty are validated with in-situ methods using p
n ⊥
a balanceindi-jetandγ-jeteventsandthemomentumpro-
J TheATLAS[1]andCMS[2]experimentsbothhaverich jectionfractionmethodinγ-jetevents.TheMonnteCarlo

5 QCDprogramsinvolvinghigh p jetsaimingtoprobethe based correction factors are validated with single particle
⊥
2 structure of the colliding protons, to measure the strong test-beamdataandE/pmeasurementsofisolatedhadrons
coupling constant and to test the standard model (SM) at in collision data which are then extrapolated using frag-
] the shortest distance scales accessible today in the high- mentationpredictionstothejet-level.ThesystematicJES
x
e center-of-mass proton-proton collisions of the LHC. Fur- uncertaintyistypically3−6%forbothATLASandCMS
- thermoredeviationsfromtheSMwouldindicatethepres- over a large range of pseudo-rapidities and p⊥, with the
p enceofnewphysicsbeyondtheSM.Thedatasetsrecorded largervaluesatlarge|η|,verylowandveryhigh p⊥.
e
h uptotheendof2010andcorrespondingtointegratedlu-
[ minositiesofL (cid:39) 35pb−1 perexperimenthavebeenused
byATLASandCMStoupdatetheirmeasurementsofin- 3 Inclusive jet cross section
2
clusive jet and di-jet cross sections, as well as to mea-
v measurements
sure multi-jet and angular di-jet distributions. New meth-
8
ods based on the sub-structure of jets to detect heavily
7
9 boostedmassiveobjectsendingupinsinglejetsattheLHC TheinclusivejetcrosssectionismeasuredbyATLAS[10]
4 have also been tested on the dominant QCD background and CMS [11] as a function of transverse jet momentum
. andcomparedtoexpectations. p⊥ andjetrapidityy.Thedataiscorrectedbin-by-binfor
1 migrationeffectsin p duetothesteeplyfallingspectrum
0 ⊥
in p and the finite p resolution. In CMS the corrected
2 ⊥ ⊥
1 2 Jet reconstruction and calibration
:
v
arXi Tgr0e.oh7creo.itnIihnnsmtpfrruua[tcr3ett]iodo-insthamuensdojeeddcteoablwllyginiotbehroaittrdhh-ismsaetafxaenprceeAern4impt-iav-ereknac⊥tmtsojeeritsntercststlheue0ms.t4iemnrc≤iinnluggRsiaev≤lie-- 2σ [pb/TeV]|yd|m/dd12max1111111000000011111220246801 ×NS yLNsOote npm QpaeCtriDct. uc(CnocrTre.ErtQa in6t.i6e)s 2100 .... 1283 <<<< |||||yyyyy|||||mmmmmaaaaaxxxxx <<<<< 22100.....81283 (((((××××× 111110000086420)))))
ther from stable particles in generator-level simulations, 108
106
partonsinNLOcalculations,topologicalcalorimeterclus-
104
ters [4,5] in ATLAS or particle flow (PF) objects [6,7] in 102 asn t=i k7t TjeetVs,, ∫ RL =d t0 =.4 37 pb 1
CMSinfullsimulationsanddata.Topologicalclusterscan 1 ATLASPreliminary
10 2 10 1 2×10 1 1 2 3 45
becalibratedpriortothejetmaking[5]inATLASorleft m12 [TeV]
attheelectromagnetic(EM)scale.ThePFobjectsusein-
Fig.1.InclusivejetcrosssectionfromCMS(left)forAnti-k jets
formationfromallCMSsubsystemsandarecalibratedto ⊥
withR=0.5asfunctionof p for6rapidityintervalsscaledfor
correspondtostableparticleslikeγ’s,leptons,chargedand ⊥
easierviewing.Thedatapoints(symbols)arecomparedtoNLO
neutral hadrons. In all cases residual jet-level corrections
predictions (solid lines) corrected for non-perturbative effects.
areneededtoaccountforparticlelossesnotdetectableon
Experimental uncertainties are indicated by the yellow bands;
clusterorPFobjectlevel[8,9]withlargercorrections(up DoubledifferentialcrosssectionfromATLAS(right)forAnti-k
⊥
toafactorof2)forEM-scaleinputsandsmallcorrections di-jeteventswithR=0.4asfunctionofdi-jetmassm for5in-
(onthelevelof5−10%)foralreadycalibratedinputs.The 12
tervalsofmaximumrapidity|y |withsystematicexperimental
max
jet-levelcalibrationsareMonteCarlo(MC)basedcorrec-
uncertainties(greyband).NLOpredictionswithNPcorrections
tion functions in |η| and p⊥. Jet energy scale (JES) and anduncertaintiesareshownaswell(yellowband).
a e-mail:[email protected]
EPJWebofConferences
JJ
σσ22/d|y|(CTEQ 6.6)/dp/d|y|/d/dpd0TT0001111.......468246821 AanTti kLtDCC AaTT jetE1at0SQs. 6R.P 6=0r.4e (l0i.m0<|iyn|<a0NNM.3NNrS)yPP T∆DDWPFFD2F 022(C0..018L((911000)00)) σσ22/d|y|(CTEQ 6.6)/dp/d|y|/d/dpd0TT0001111.......468246821 AanTti kLtDCH AaTE jetERatSQAs. P 6RD.P 6F=0 1r.4.e0 (l0i.m0<|iyn|<a0HG.3ErJ)RyR ∆ A2PPD0F0D(8CFL 19.05) ⊗Ratio to NLO (CT10) NP 1.125 CCMNHAMTNEBS1RPKST0WDA MD ⊗FP02a2 D90tN.a 0F1⊗P81 ⊗ , .N ⊗0∆ PN P⊗NPD PNFP CL68 ⊗Ratio to NLO (CT10) NP 1.125 CCMNHAMTNEBS1RPKTS0WDAM D⊗FP02a2 D90tN.a 0F1⊗P81 ⊗ , .N ⊗0∆ PN P⊗NPD PNFP CL68
102 103 pT[GeV] 102 103 pT[GeV] 0.5 0.5
σσ22/d|y|(CTEQ 6.6)/dp/d|y|/d/dpd0TT012...55521 AanTti kLCCtD ATTa jteE1at0SQs. 6R.P6 =0r.4e (l2i.m1<|iyn|<a2NNM.8NNrS)yPP T∆DDWPFFD2F 022(C0..018L((911000)00)) σσ22/d|y|(CTEQ 6.6)/dp/d|y|/d/dpd0TT012...55521 AanTti kLCHtD ATEa jteERatSQAs. P 6RD.P6 F=0 1r.4.e0 (l2i.m1<|iyn|<a2HG.8ErJ)RyR ∆ A2PPD0F0D(8CFL 19.05) CjFeiMtg.mS3a0.fsoRsrCD0a.Mi0(j0e tAS≤.tM2 iM| y oLa|nm s=jas jx3t o )<6 iA 0p-nf.b5tko-i1- k0t ⊥ Tv.√ h5Rse ==e0 j7r.7 eTdetVNso1uLwbOMliJetJh (pTderRVie3ff)dei=cretino0tni.a7s0lfadoC0D2si..Mi07jr-e S≤tj aM| y eLa||m ys=tas x3f 1 | <6 ucA 2pn.br5nt<-i1-o k c T√ sRst =s=i00 o7.7 Ts.ne5Ve2co(tlifeoMftnJthJ )(eTferaVo4d)nmid-
102 pT[GeV] 102 pT[GeV] 2.0<|y|<2.5(right).ThereferenceNLOpredictionusesCT10
whichiscomparedtotheratiosusingMSTW 2008,NNPDF 2.1,
Fig.2.RatiooftheinclusivejetcrosssectionfromATLASfor
HERAPDF 1.0andABKM09instead.Errorbarsindicatestatistical
Anti-k jets with R = 0.4 as function of p over NLO predic-
⊥ ⊥ errors.Thegreybandshowstheexperimentalsystematicuncer-
tionsfor|y| < 0.3(top)and2.1 < |y| < 2.8(bottom).Therefer-
taintiesandtheyellowbandthetypicalPDFuncertainty(CT10).
enceNLOpredictionisCTEQ 6.6whichiscomparedtothera-
Non-perturbative uncertainties are dominant at low masses and
tiosusingCT10,MSTW 2008,NNPDF 2.0andNNPDF 2.1(left);
notshowninthefigures.
HERAPDF 1.0,HERAPDF 1.5andGJR08(right).Error barsin-
dicatestatisticalerrors.Thelightshadedbandshowstheexper-
imentalsystematicuncertaintiesexcludingacommon3.4%un-
certaintyfromtheluminositymeasurement.Theotherbandsin-
dicatetherespectivetheoreticaluncertainties. R = 0.4. Similar results are obtained by CMS [14] for
R = 0.7. Both ATLAS and CMS use full simulations to
obtain the bin-by-bin migration corrections for the distri-
butions. Dominant experimental systematic uncertainties
stemfromtheJESuncertaintyandareintherangeof15−
spectraareobtainedbyfittingamodifiedpower-lawfunc-
30%forATLASandaround15%atlowmassesand60%
tion with Gaussian smearing in p to the observed spec-
⊥ at high masses for CMS. As is the case for the inclusive
tra.InATLASthecorrectionfactorsareobtainedfromfull
detectorsimulationsincludingdetectorinefficiencies.Typ- jet cross section measurement a comprehensive compar-
ison to NLO pQCD predictions has been made by both
icalcorrectionsareinthe10−15%rangebutcanextend
ATLAS [10] and CMS [13]. Figure 3 shows the ratio of
to30−50%attheedgesofthephasespace.TheNLOper-
themeasureddoubledifferentialdi-jetcrosssectiontothat
turbativeQCD(pQCD)predictionsonparton-levelonthe
otherhandarecorrectedfornon-perturbative(NP)effects predicted in CT10-based MC simulation for two rapidity
bins. The agreement with HERAPDF is best, but all tested
due to hadronisation and the underlying event activities.
PDFsetsagreewithinuncertainties.
These corrections are obtained by comparing simulations
with leading log generators (PYTHIA/HERWIG) which are
runwithandwithouttheseeffectsenabled.Thecorrections
dependstronglyonjetsize.ForR=0.5,0.6theunderlying 4 Angular and multi-jet variables
eventeffectsdominateandcorrectionsarearound1.2−1.4
at small p⊥. For R = 0.4 hadronisation effects are domi- Duetotheirsensitivitytonewphysicsandtheirabilityto
nant and corrections of about 0.8 are obtained at low p⊥. probe mass scales without explicitly relying on JES cal-
Thecorrectionsapproachunityatlarger p⊥ forallusedR ibrations the angular distributions of multi-jet events are
values. of particular interest. The azimuthal de-correlation ∆φ of
Figure1(left)showstheinclusivejetcrosssectionmea- the two most energetic jets as measured by ATLAS [15]
surement for jets with size R = 0.5 as a function of jet is shown in the left plot of figure 4. Values close to π are
transversemomentummeasuredbyCMS.Theexperimen- expectedfordi-jeteventswhilesmallervaluesindicatethe
tal uncertainties are in the range 10−20% and are dom- presenceofadditionaljets.NLOpQCDcalculationsusing
inatedbytheuncertaintiesonJESandresolution.Similar NLOJet++andMSTW 2008agreewiththedatafor∆φ<π.
distributionsforR = 0.4andR = 0.6areobtainedbyAT- Leadinglogsimulations(PYTHIA,HERWIG,SHERPAagree
LAS,withuncertaintiesintherangeof10−30%.Different withthedataandgiveagooddescriptionoftheperturba-
NLO predictions are tested [12,13] by comparing the ra- tively diverging point ∆φ = π. The right side plot of fig-
tiosofdatatoNLOMCpredictionsforvariousPDFsets. ure4showsthedistributionofχ =exp|y −y |,theex-
dijet 1 2
Figure 2 shows an example from ATLAS for the rapidity ponentialoftherapiditydifferencebetweenthetwolead-
region|y|<0.3forR=0.4andR=0.6.CMSobtainssim- ing jets in p , as measured by CMS [16] for different di-
⊥
ilarcomparisonsforR = 0.5.TheNLOpredictionsarein jetmassintervals.Thedistributioninχ isexpectedtobe
dijet
general systematically above the data but still compatible almostflatforQCDwhilenewphysics(suchasquarkcom-
with the measurement within the assigned uncertainties. positeness) would cause excess events at small χ . The
dijet
Thedeviationsbecomelargeratlarge|y|and p⊥. comparison to NLO pQCD calculations with NLOJet++
The double differential cross section in the maximum and the CTEQ 6.6 PDF-set shows good agreement with
jet rapidity |y | and di-jet mass m for di-jet events as thedata,andalowerlimitonthecontactinteractionscale
max 12
measuredbyATLAS[10]isshowninFigure1(right)for forleft-handedquarksofΛ+ =5.6TeV(Λ− =6.7TeV)for
HadronColliderPhysicsSymposium2011
1] [radians/d d1/φ∆σσ111111100000003456789 aA1 1D654322 n16 000161aT t00 000000it a< L<<<<<<<k t∫ppp ppppppAjLemTmTmTmTmTmTmTmTmTtdaSaaaaaaaasxxxxxxxxxt ≤≤≤≤≤≤≤≤>=R 3128654328=66100001600 s000000000.p6 = bGGGGGGGGG 71 epeeeeeeeeVVVVVVVV VTT> (((((((((1e×××××××××0111111111V0000000000 017654328G)))))))))eV |y|<0.8 /d d1//d d1//d d1//d d1//d d1//d d1//d d1//d d1//d d1//d d1/χσσχσσχσσχσσχσσχσσχσσχσσχσσχσσdijetdijetdijetdijetdijetdijetdijetdijetdijetdijetdijetdijetdijetdijetdijetdijetdijetdijetdijetdijetdijetdijetdijetdijetdijetdijetdijetdijetdijetdijet0000000000000000000000000000000000000000........................................4567456745674567456745674567456745674567 DΛΛQ+-aC t==Da 55 p TTreeedVVicti11Mo..n84jj <<> MM2.2jjjj T<< e21LV..s28 = C=TT 3ee M7VV6 S TpebV-(((1+++000...435))) Gap fraction345 2211197418520000000 ≤≤ ≤≤≤≤≤ pp pppppTT TTTTT<< <<<<<19 20222110 74185G 00000Ge GGGGGeVVeeeee VVVVV ( +( +0(((((0)+++++.322115).).))55)) LQAe0aT =d Li2n0gADHPP GOOaEpStTeJWWa Vd (HH2pij0eaEE1trGG t0so en++l e lPHecYvEtieoTRlnH)WIAIG RR32320000....686811 sC= M7 TSeV aLnintt=i-3K6T pRb=-10.5
102 1.1 < Mjj < 1.4 TeV (+0.25)
1110010 10 321 π/2 2π/3PNscDLaOFle &p u5Q nαπCcs ./Du6n cO.(α4s) π 000000000000000000000000000000..............................232323232323232323231111111111 00000.....58632 5555< <<<< M MMMMjj jjjjjjjj< <<<< 0 1000.6....18535 5 5 TTT TTeeeVeVeVVV((((++++0000....1210))55)) 21 0000....242400 00..55 11DPPPMAHSYYYLyaEAsPtTTTRDatGHHHeWGmIIIEAAARIGaN668At + i+Pcttt11+uuu HUP nnnt.. ueeeYn+55n cT ZD2PeeHC26r Y2tITaAT.3i6Hn tItyAun6e t uD22n6eT D6T 22..55
∆φ [radians] 2222222222 4444444444 6666666666 8888888888 11111111110000000000 11111111112222222222 11111111114444444444χχχχχχχχχχ 11111111116666666666 00 1 2 3 4 5 ∆y6 HHTT ((TTeeVV))
ddddddddddiiiiiiiiiijjjjjjjjjjeeeeeeeeeetttttttttt
Fig.4.Azimuthalde-correlation∆φofthetwomostenergeticjets Fig. 5. Gap-fraction (left) for the two leading jets in p as a
⊥
asmeasuredbyATLAS(left)forAnti-k⊥ jetswithR = 0.6for function of ∆y for various intervals of p¯⊥ as measured by AT-
differentpmaxintervals(blackmarkers)comparedtoNLOpQCD LAS(markersandyellowerrorband)comparedtoHEJcalcula-
⊥
calculations(redlines)withassociatederrors(hatchpattern);The tions(blueband)andNLOsimulationswithPOWHEGinterfaced
distributionofχ (blackpoints)fordifferentM rangesasmea- toPYTHIA(reddashes)andHERWIG(bluedot-dashes);Ratioof
dijet jj
suredbyCMS(right)forAnti-k⊥jetswithR=0.5comparedto inclusive3-jetoverthe2-jetcrosssectionsR32asfunctionofto-
NLO pQCD calculations (shaded band) and predictions includ- taltransversemomentumH (right)asmeasuredbyCMS(black
⊥
ingcontactinteractions(coloredlines)forcompositenessscales dots and yellow uncertainty band) compared to various simu-
ofΛ+/−=5TeV. lations using different PYTHIA tunes, MADGRAPH, ALPGEN and
HERWIG++.
destructive(constructive)interferencehasbeenobtainedat
5 Jet mass and sub-structure
95% CL. A complementary study of the rapidity gap be-
tweenthetwojetswitheitherleading p orthelargestra-
⊥
In the high energy regime of LHC, heavy objects with
piditygap∆yhasbeendonebyATLAS[17].Theso-called
massesO(100GeV),canreceivelargeLorentzboostssuch
gap-fractionisdefinedasthefractionofeventswithoutad-
thattheirdecayproductsaremeasuredinasinglejet.Sev-
ditionaljetactivityintherapidityintervalbetweenthetwo
eralapproachesareconsideredtoexplorethesub-structure
jets.Anyadditionaljetwithinthegaphastohaveatrans-
of these jets with the aim to identify such heavy objects.
verse momentum above a veto scale p > Q , with the
⊥ 0
default choice Q = 20GeV to stay far away from Λ . Amongthemare:
0 QCD
The gap-fraction is shown in the left plot of figure 5 for C/Afiltering: Theclusteringoflarge(R(cid:39)1.2)Cambridge-
the choice of leading jets in p⊥ as a function of ∆y for Aachen (C/A) [19] type jets is reversed until a large
various intervals of the average transverse momentum of dropinjet-massisobserved.Theremainingconstituents
the two leading jets p¯⊥. The comparison with HEJ calcu- arere-clusteredwithasmallerRparameter.
lationsshowssomedeviationsinthelarge p¯⊥ regionsbut Jetpruning: C/Aork⊥[20,21]jet-clusteringisperformed
theagreementimprovesasp¯⊥approachesQ0,whichisex- ontheconstituentsofalargejetandineachclustering
pectedsinceHEJisdesignedtogiveagooddescriptionof step the softer of the two clusters being combined is
QCDinthelimitwherealljetshavesimilar p⊥.Thebest discarded if it’s transverse momentum is below a cer-
descriptionisachievedwithPOWHEGinterfacedtoPYTHIA tainfractionoftheoriginaljet p andtheangulardis-
⊥
although deviations are observed at large ∆y. POWHEG in- tancebetweenthetwoclustersislarge.
terfaced to HERWIG tends to predict smaller gap fractions
Forthejetsubstructurealgorithmstobeusefultheyhave
over the full phase space and the deviations increase for
larger∆yasforthePOWHEG+PYTHIAcase. to be tested on QCD jets as this will be the main back-
ground.C/AFilteringisusefulforthedecaysofheavypar-
The right hand plot in figure 5 shows the cross sec-
ticles to two low mass objects and the QCD behavior has
tion ratio of three-jet over two-jet events R as a func-
tionofthetotaltransversemomentumsum H32 = (cid:80) p beenstudiedinATLASin[22].Themassdropm1/mjet of
⊥ jets ⊥ theleadingsubjetisrequiredtobesmallerthan0.67(light
asmeasuredbyCMS[18].Manysystematicuncertainties (cid:16) (cid:17)2
suchasthoseduetotheJESandthejetselectionefficiency subjet)andthe p⊥asymmetry min(p1⊥,p2⊥)×∆R1,2/mjet
largely cancel in this ratio, while the uncertainty due to largerthan0.09(fairlysymmetric).Onceareversedclus-
the integrated luminosity vanishes entirely. Therefore R teringstepwiththesepropertiesisfoundthecurrentjetis
32
provides a stringent test of QCD predictions. Events with re-clusteredwithC/AandR=min(0.3,∆R /2)findingn
1,2
two or more Anti-k jets with R = 0.5 with |y| < 2.5 new subjets of which the leading min(3,n) are combined
⊥
and p > 50GeV and H > 0.2TeV are selected and togivethefinalC/Afilteredjet.Figure6showsthespec-
⊥ ⊥
compared to various PYTHIA6, PYTHIA8 and HERWIG++ trum of jet masses for C/A jets with R = 1.2 before and
based tunes and to simulations using the multi-parton fi- afterthefilteringprocedureineventswithexactlyonepri-
nalstategeneratorsMADPGRAPHandALPGENinterfacedto mary vertex (to remove pile-up) and at least one jet with
PYTHIA6.Allpredictionsdescribetheobservedratiowell p > 300GeV and |y| < 2. The agreement with all three
⊥
in the region H > 0.5TeV but, with the exception of predictionsisgoodalthoughHERWIG++producesjetswith
⊥
MADPGRAPH,overshootbetween10−30%atlowerH . larger mass (before filtering) compared to data. Jet prun-
⊥
EPJWebofConferences
σd 1 σdm [GeV]000...0000.00106821 ATLAS Preliminary CNPaVm =b r1id, gpAPHHeTTyeet Lrr h>wwAAiaii Sgga3J+ c20i+m0h01me 0Gyn D eaRVta=,, 1L| y.=2| 3 <j5ep 2tbs 1 σd 1 σdm [GeV]00..000.000681 ATLAS Preliminary CSNPpaVlmi t=/bF r1iildt,e gpAPHHreTTyeeet Lrr hd>wwAAia ii Sggwa3J+ c20ii+mt0h0h1me 0GRyn D qeaRqVt a=>,, 1L |0 y.=2.| 3 3 <j5ep 2tbs 1 AIo’fdcAlkiTkneLoAtowStlheaandndkgCtmhMeeSJnetftosPrerpfroorvmidainncgemanedwSiMth/QthCeDmgartoeuripasl
0.004
0.004 presented here. In particular I benefited greatly from dis-
0.002 0.002 cussionswithJ.Butterworth,M.Campanelli,A.Davison,
MC / DataMC / Data0111101111..........8246882468011 50 100 150 200 250 300 MC / DataMC / Data0111101111..........8246882468011 50 100 150 200 250 300 A.DiCiaccio,K.Kousouris,andM.Voutilainen.
00..66 00..66
00..44 00..44
00..22 5500 110000 115500 220000 225500 330000 00..22 5500 110000 115500 220000 225500 330000
JJeett MMaassss [[GGeeVV]] JJeett MMaassss [[GGeeVV]] References
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arXiv:1102.2020
ent PYTHIA tunes and HERWIG++. The overall agreement
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Both ATLAS and CMS have made comprehensive stud- hep-ph/9305266
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NLO pQCD calculations has been found and constraints (2011),ATLAS-CONF-2011-073
onnewphysicsweresetbytheobservedagreement.Novel
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techniques to identify massive boosted objects were suc- (2011),CMS-PAS-JME-10-013
cessfully tested on the large QCD background expected.
The challenge will be to continue the studies presented
hereundertheincreasedpile-upconditionsinthedatataken
beyond2010.