Jet Physics at Hadron-Hadron CollidersJoey HustonMichigan State University(CDF, CTEQ and ATLAS)CTEQ SS04 Madison, WisconsinA good reference for QCD Available at fine bookstores, CambridgeUniversity Press, orat Fermilab Keith Ellis hasoffered to personallysign any copy thatyou bring to himQCD at hadron colliders First of all, my apologies for any repetition ofmaterial that may have been presented byothers at this SS◆ especially since Nikos appears to have had 3lectures worth of material in his 1 lecture Some motherhood statements… Basically all interactions at hadron-hadron colliders aregoverned by or modified by QCD◆ both standard model physics◆ …as well as possible new physics▲ new physics has backgrounds from standard modelphysics▲ so an understanding of QCD has to precede any searchesfor new physicsConsider jet production at hadron colliders Jet production at the Tevatroncurrently probes the most“violent” collisions currentlyachievable◆ smallest distance scales(10-17 cm)◆ some of greatest sensitivity tonew physics->quarkcompositeness New version of Rutherfordscattering◆ production of jets at high pTindicates that there must bepoint-like constituents withinproton, i.e. quarks◆ If we observe a deviation fromthe expected jet cross sections atthe highest jet pT’s (smallestdistance scales), this may also bean indication of something insidethe quarksQCD at hadron colliders Collisions that produce jets at high transversemomentum take place at small impact parameter (theproton and the anti-proton are hitting head-on) and aremore accurately hard collisions between partons(quarks and gluon) inside the two protons(uud)pfxaσ^a bfpxb(uud)DDcdjetjetSo what’s with the blobs, etc? The proton has 2 valence up quarks and 1 valence down quark (but alsolots of sea quarks and gluons) All share the momentum of the proton; the collisions is between 1 partonfrom the proton carrying a momentum fraction xa and one parton from theanti-proton carrying a momentum fraction xb The distribution of momentum carried by a given parton is given by partondistribution function (pdf’s), fparton(x,Q2)(uud)pfxaσ^a bfpxb(uud)DDcdjeta measure of the hardness of the collision (also sometimes called µF)Blobs, continued So one parton carrying a momentum fraction xa collides with anotherparton carrying a momentum fraction xb The collision is given by the red blob; that’s where most of the hard QCDcomes in◆ to leading order, σ α αs2◆ to next-to-leading order, σ α αs3◆ to next-to-next-to-leading order,σ α αs4…you get the idea(uud)pfxaσ^a bfpxb(uud)DDcdjetway easy to calculatebeen there, done that, for over 10 years nowbest estimate is 2 years awaynowBlobs, continued c and d are the outgoing partons, but problem: they carry color; wedon’t observe the quarks or gluons themselves (don’t see nakedcolor) but instead the partons fragment into jets of hadrons (pions,kaons, and protons) it’s the jets of hadrons that we can observe experimentally; D(z,Q2)describes how the partons fragment into hadrons(uud)pfxaσ^a bfpxb(uud)DDcdjetfraction of momentum carriedby hadronNote thehardness of the collisiongets involvedagainJetsThe exact definition of that localizedcluster is important for precisionmeasurements.Original description of fragmentation done byFeynman and FieldWho’s now working with me on CDFand who has a famous sisterRick’s famous sisterRick’s wife Rick’s sister RickTrivia question: who is Max Born’s famous grand-daughter?Olivia Newton-John John Travola is notrelated to any famousphysicist◆ as far as I knowJet Fragmentation Functions The parton that gives rise tothe jet fragments into a fairlylarge number of particles(increasing as the jet ETincreases)◆ say 20 particles for a jet withET of 100 GeV Most of the jet particles have asmall fraction (z) of the totaljet momentum◆ and the gluon fragmentationis softer than the quarkfragmentation We can’t calculate thefragmentation functionsperturbatively; have tomeasure them◆ mosly at LEP◆ we can calculate how theychange with Q, thoughBlobs, continued The cross section for jet production to occur depends on aconvolution of fparton(xa,Q2), fparton(xb,Q2) and the hard scatteringcross section σ The parton distributionfunctions can bedetermined from one processand then applied universallyfactorization(uud)pfxaσ^a bfpxb(uud)DDcdjet^Parton distribution function fits Since all hadron-hadron crosssections depend on pdf’s, thedetermination of them is animportant aspect of QCDphenomenology◆ non-perturbative physics,although evolution of pdf’swith Q2 can be calculatedperturbatively Can determine pdf’s usingdata from DIS (deep-inelasticscattering), DY (Drell-Yan pairproduction), and theproduction of jets There are two major groupsthat provide pdf’s to the world◆ MRST◆ CTEQxf(x,Qo) = Ao xA1 (1-x)A2 P(x)small x behaviorlarge x behaviorin betweenJet production at LO There are many subprocessesthat contribute to jet production athadron colliders◆ gg scattering◆ gq scattering◆ qq (or qq scattering)◆ qq scatteringmostly gg scattering at low ET, which corresponds to low values of parton x (where the gluon dominates); we’ll see later that jet production is important in global pdf fits, especially at high xJet production beyond LO Calculating jet crosssections at LO is agraduate studentexercise Several programs (EKS,JETRAD) for NLOavailable◆ this is the first order atwhich you can believe thenormalization of the crosssection Calculating the crosssection to NLO is a 5year (or more) exerciseinvolving ten’s of peopleElements of NNLO calculationSomething to look forward to MC @ NLO combinesthe best features ofparton shower MonteCarlos and NLOcalculations The hard cross section iscalculated to NLO andthen passed on toHerwig for additionalgluon radiation andhadronization Inclusive jet productionwill be included by thisfallat NLOJet algorithms beyond LOA complication Remember that it’s not onlythe two main partons that areinteracting with each other;the rest of the proton and anti-proton are interacting as well(but softly) The result is a kind of lowenergy underlying event fillingthe detector◆ this underlying event must besubtracted from the jet energy asbest as possible◆ the assumption that we make isthat the underlying event in a jetevent is similar to the ambientlevel