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QCD at Colliders

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QCD at Colliders Particle Physics Dr Victoria Martin, Spring Semester 2012 Lecture 10: QCD at Colliders !Renormalisation in QCD !Asymptotic Freedom and Confinement in QCD !Lepton and Hadron Colliders !R = (e+e!!hadrons)/(e+e!"µ+µ!) !Measuring Jets !Fragmentation 1 From Last Lecture: QCD Summary • QCD: Quantum Chromodymanics is the quantum description of the strong force. • Gluons are the propagators of the QCD and carry colour and anti-colour, described by 8 Gell-Mann matrices, !. • For M calculate the appropriate colour factor from the ! matrices. 2 2 • The coupling constant #S is large at small q (confinement) and large at high q (asymptotic freedom). • Mesons and baryons are held together by QCD. • In high energy collisions, jets are the signatures of quark and gluon production. 2 Gluon self-Interactions and Confinement , Gluon self-interactions are believed to give e+ q rise to colour confinement , Qualitative picture: •Compare QED with QCD •In QCD “gluon self-interactions squeeze lines of force into Gluona flux tube self-Interactions” ande- Confinementq , + , What happens whenGluon try self-interactions to separate two are believedcoloured to giveobjects e.g. qqe q rise to colour confinement , Qualitativeq picture: q •Compare QED with QCD •In QCD “gluon self-interactions squeeze lines of force into a flux tube” e- q •Form a flux tube, What of happensinteracting when gluons try to separate of approximately two coloured constant objects e.g. qq energy density q q •Require infinite energy to separate coloured objects to infinity •Form a flux tube of interacting gluons of approximately constant •Coloured quarks and gluons are always confined within colourless states energy density •In this way QCD provides a plausible explanation of confinement – but not yet proven (although there has been recent progress with Lattice QCD) Prof. M.A. Thomson •Require infinite Michaelmasenergy to 2011separate coloured objects to infinity 257 •Coloured quarks and gluons are always confined within colourless states •In this way QCD provides a plausible explanation of confinement – but not yet proven (although there has been recent progress with Lattice QCD) Prof. M.A. Thomson Michaelmas 2011 257 From HadronisationLast Lecture: and Jets Jets , •Consider a Considerquark and a quark anti-quark and anti-quark produced produced in electron in electron positron positron annihilation annihilation i) Initially Quarks separate at Hadronisationq q and Jets high velocity (i) Initially Quarks separate,Consider at a quark and anti-quarkq producedq in electron positron annihilation ii) Colour flux tube formsi) Initially Quarks separate at q q high velocitybetween quarks high velocity q q q q q q (ii) Colouriii) fluxEnergy tube stored forms inii) theColour flux tube forms flux tube sufficient betweento quarks q q betweenproduce quarksqq pairsiii) Energy stored in the q q flux tube sufficient to (iii) Energyiv) storedProcess continues in the produceflux tubeqq pairs until quarks pair sufficient to produceiv) Process qq ! pairs continues, up into jets of until quarks pair Might worry that coupling becomes colourless hadronsup into jets of infinite at (iv) Process continues untilcolourless quarks hadrons , This process is called hadronisation. It is not (yet) calculable. pair up into jets of, colourlessThis process is called hadronisation. It is not (yet) calculable. , The main consequence, is that at collider experiments quarks and gluons hadrons The main consequencei.e. isat that at collider experiments quarks and gluons observed as jets observedof particles as jets of particles e+ e+ q • Butq quantum gravity effects would come ! ! in way below this energy and it is “ ” OPAL Collaboration, Eur. Phys. J. C33 (2004) highly unlikely that QED as is would – be valid in this regime – e q q e Prof. M.A. Thomson Michaelmas 2011 258 Prof. M.A. Thomson , In MichaelmasQED, running 2011 coupling increases 258 This process is called hadronisationvery slowly. It is not (yet) calculable. • •Atomic physics: •The main consequence is that at collider experiments quarks and gluons observed as jets of particles•High energy physics: 3 Prof. M.A. Thomson Michaelmas 2011 277 Running Coupling Running of ! • Renormalisation effects QCD. The observeds (renormalised) value of the QCD Similar to QED but also have gluon loops coupling constant #S depends on diagrams such as: + + + +… Fermion Loop Boson Loops , Remembering adding amplitudesSimilar, so can to get QED negative interferenceNo equivalent and the insum QED, as no can be smaller than the original diagram alone photon-photon interactions • Bosonic, loopsBosonic interfere loops “interfere negatively negatively ”with the fermion loops. • #S can be written terms of the value at a reference scale µ: 2 2 αS(µ ) α (q )= = no. of colours S with 2 2 1+ αS (µ ) (11n 2n ) ln =q no. of quark flavours 12π C − f µ2 nC=3 is the number of colours • ! 2 Nobel! Prize" for Physics, 2004 S decreases with Q (Gross, Politzer, Wilczek) • nf=6Prof. is M.A. the Thomson number of quark flavoursMichaelmas 2011 278 • Conventional to choose a reference of !, defined by: ! ~ 220 MeV 12π ln Λ2 = ln µ2 − (11n 2n )α (µ2) C − f S 4 Running of #S 12π α (q2)= S 2 (11n 2n ) ln q C − f Λ2 This calculation won the Nobel! " Prize for Physics 2004 for Gross, Politzer and Wilczek •#S is found to decrease with increasing q2 "The more energetic the interaction 2 (high q ), the weaker #S. #S (q=mZ) ~ 0.12 Predicted shape of the running versus measurements "The less energetic the interaction 2 (low q ), the stronger #S. 2 2 #S (q =1 GeV ) ~ 1 5 Asymptotic Freedom and Confinement 2 2 • At high energy, q >> ! , #S is small, e.g. #S (q=mZ) ~ 0.12. "Quarks and gluons behave like free objects at high energy or short distances. "This is known as asymptotic freedom. "e.g. in electron-proton scattering with high q2 we found that we could consider the scattering from the individual quarks. "Use pertubation theory to calculate processes. However due to moderately large #S need to calculate the more than just the simplest diagrams. 2 4 "Leading order (#S ), Next-to-leading order (#S ), Next-to-next-to- 6 leading order (#S ) 2 2 • At low energy, q ~ ! , #S is large, e.g. #S (q=1 GeV) ~ 1. "Quarks and gluons are locked (confined) inside mesons and baryons. "Cannot use perturbation theory to obtain sensible results. "Many approaches to calculating QCD non-perturbatively, e.g. lattice QCD, MC techniques. 6 Gluon self-Interactions and Confinement , Gluon self-interactions are believed to give e+ q rise to colour confinement , Qualitative picture: •Compare QED with QCD •In QCD “gluon self-interactions squeeze lines of force into a flux tube”Colliders e- q , What happens when try to separate two coloured objects e.g. qq •Collider experimentsq collide beams of particlesq e.g. e+e!, pp,! e!p, pp Key •parametersForm a flux tube (see of interacting also lecture gluons of 4)approximately constant • energy density 2 •centre•Require of infinite mass energy energy: to separate √s = coloured(p aobjects+ pb to) infinity •Coloured quarks and gluons are always confined within colourless states •In this way QCD provides a plausible explanation! of confinement – but •Integratednot yet proven luminosity (although there"! dt has = !been # timerecent to progress run experiment with Lattice QCD) Prof. M.A. Thomson Michaelmas 2011 257 Hadronisation and Jets ,Consider a quark and anti-quark produced in electron positron annihilation i) Initially Quarks separate at q q high velocity q q ii) Colour flux tube forms between quarks iii) Energy stored in the q q q q flux tube sufficient to produce qq pairs 7 iv) Process continues until quarks pair 22 up into jets of Evidence for Gluons colourless hadrons , This process is called hadronisation+ ! . It isIn notQED (yet), electr calculable.ons can radiate photons. In QCD , The main consequence ise thate at collider!quarks hadronsexperimentscan radiate quarksgluons.and gluons observed as jets of particles + e q e+ q ! ! √α γ S g √α Qq√α e– q 1 - 2 Prof. M.A. Thomson Michaelmas 2011e q q 258 • Electromagnetic production of qq ! pair, strong1 interactions cause q and q ! to fragment into two jets giving an extra factor of in the matrix element, i.e. an extra factor of in cross • In CMP frameartic jetsle arePh producedysics back-to-back.section. Angular distribution (1+cos2"), sameIn asQED e+wee!"canµdetect+µ! the photons. In QCD we • never see free gluons due to confinement. Dr M.A. Thomson Experimentally detect gluons as an additional jet: 3-Jet Events. e+ q g √α γ S g √α q Qq√α 1 q e- q2 q ! Angular distribution of gluon jet depends on • Emission of a hard gluon in final stategluon givesspin three jets (rate measures #s) • Observation of three jet events is DrdirectM.A. Thomson evidence for gluons Lent 2004 8 Part II, Lent Term 2004 HANDOUT III Dr M.A. Thomson Lent 2004 Lepton Colliders PETRA: Positron-Elektron- LEP: Large Electron Positron Collider Tandem-Ring-Anlage • At CERN • The world’s highest energy e+e! collider, 27 km circumference. • LHC was built in LEP tunnel • Ran from 1989 to 2000 • Centre of mass energy, #s=89 to 206 GeV • Four experimental collision points: Aleph, Delphi, L3, Opal • Highlight: beautiful confirmation of the electroweak model • At DESY, Hamburg • ran 1978 to 1986 • e+e! collider, 2.3 km • #s = 14 to 46 GeV. • Two experimental collision points: TASSO and JADE. • Highlight: discovery of the gluon! 9 Jet Events at Lepton Colliders Two jet event from LEP Three jet event from LEP Three jet event from Petra 10 Gluon self-Interactions and Confinement , Gluon self-interactions are believed to give e+ q rise to colour confinement , Qualitative picture: •Compare QED with QCD •In QCD “gluon self-interactions squeeze lines of force into a flux tube” e- q , What happens when try to separate two coloured objects e.g.
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