QCD at LHC in pp. Department of Theoretical Physics, Lund University

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1 3rd Nordic LHC and Beyond Workshop 2-3 February 2009 Lund, Sweden QCD at LHC in pp Torbjörn Sjöstrand Department of Theoretical Physics, Lund University Introduction: the structure of an event Multiple Interactions Hadronization Observables and Summary

2 The structure of an event Warning: schematic only, everything simplified, nothing to scale,... p p/p Incoming beams: parton densities

3 W + p u d g p/p Hard subprocess: described by matrix elements

4 c s W + p u d g p/p Resonance decays: correlated with hard subprocess

5 c s W + p u d g p/p Initial-state radiation: spacelike parton showers

6 c s W + p u d g p/p Final-state radiation: timelike parton showers

7 c s W + p u d g p/p Multiple parton parton interactions...

8 c s W + p u d g p/p... with its initial- and final-state radiation

9 Beam remnants and other outgoing partons

10 Everything is connected by colour confinement strings Recall! Not to scale: strings are of hadronic widths

11 The strings fragment to produce primary hadrons

12 Many hadrons are unstable and decay further

13 ALICE simulated event

14 What is multiple interactions? Cross section for 2 2 interactions is dominated by t-channel gluon exchange, so diverges like dˆσ/dp 2 1/p4 for p Integrated cross section above ptmin for pp at 14 TeV jet cross section total cross section 1000 integrate QCD 2 2 qq qq qq q q qq gg qg qg gg gg gg qq with CTEQ 5L PDF s sigma (mb) ptmin (GeV)

15 σ int (p min ) = dx 1 dx 2 dp 2 f 1(x 1, p 2 ) f 2(x 2, p 2 ) dˆσ p min dp 2 Half a solution to σ int (p min ) > σ tot : many interactions per event P n σ tot = σ int = σ n n=0 n=0 n σ n σ int > σ tot n > 1 n = n If interactions occur independently then Poissonian statistics P n = n n e n n! but energy momentum conservation large n suppressed

16 Other half of solution: perturbative QCD not valid at small p since q,g not asymptotic states (confinement!). Naively breakdown at p min h r p 0.2 GeV fm 0.7 fm 0.3 GeV Λ QCD... but better replace r p by (unknown) colour screening length d in hadron r r r r d resolved λ 1/p d screened

17 so modify dˆσ/dp 2 dˆσ dp 2 α2 s (p2 ) p 4 α2 s (p2 ) p 4 θ(p p min ) (simpler) or α2 s(p p2 ) (p p2 )2 (more physical) where p min or p 0 are free parameters, empirically of order 2 GeV Typically 2 3 interactions/event at the Tevatron, 4 5 at the LHC, but may be more in interesting high-p ones. 0 p 2

18 PYTHIA implementation (1) Simple scenario (1985): first model for event properties based on perturbative multiple interactions no longer used (no impact-parameter dependence) (2) Impact-parameter-dependence (1987): still in frequent use (Tune A, Tune DWT, ATLAS tune,... ) double Gaussian matter distribution, interactions ordered in decreasing p, PDF s rescaled for momentum conservation, but no showers for subsequent interactions and simplified flavours (3) Improved handling of PDFs and beam remnants (2004) Trace flavour content of remnant, including baryon number (junction) u Study colour (re)arrangement u among outgoing partons (ongoing!) Allow radiation for all interactions d

19 (4) Evolution interleaved with ISR (2004) Transverse-momentum-ordered showers dp dp = ( dpmi dp + dp ISR dp with ISR sum over all previous MI p ) exp ( p i 1 p ( dpmi dp + ) ) dp ISR dp dp (5) Rescattering (in progress) p max p 1 hard int. p 1 ISR p 2 mult. int. ISR is 3 3 instead of 4 4: p 3 mult. int. ISR p min interaction number

20 CDF 3-jet + prompt photon analysis Yellow region = double parton scattering (DPS) The rest = PYTHIA showers σ DPS = σ Aσ B σ eff for A B = σ eff = 14.5 ± mb Strong enhancement relative to naive expectations!

21 without multiple interactions

22 with multiple interactions

23 Jet pedestal effect Events with hard scale (jet, W/Z,... ) have more underlying activity! Events with n interactions have n chances that one of them is hard, so trigger bias : hard scale central collision more interactions larger underlying activity. Centrality effect saturates at p hard 10 GeV. Studied in detail by Rick Field, comparing with CDF data: MAX/MIN Transverse Densities Jet #1 Direction Jet #1 Direction Toward-Side Jet TransMIN very sensitive to the beam-beam remnants! Toward Toward TransMAX TransMIN TransMAX TransMIN Away Jet #3 Away Away-Side Jet Define the MAX and MIN transverse regions on an event-by-event basis with MAX (MIN) having the largest (smallest) density.

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26 Colour correlations p (n ch ) is very sensitive to colour flow p p long strings to remnants much n ch /interaction p (n ch ) flat p p short strings (more central) less n ch /interaction p (n ch ) rising

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28 <p T > [GeV/c] CDF Run II Pythia Pythia Pythia Deferred FSR Pythia No reconnection Pythia Deferred FSR, No reconnection <p T > [GeV/c] CDF Run II Pythia No reconnection Pythia No reconnection + Rescattering Pythia 8.114, RR * p 0 = 4.34 Pythia Rescattering, RR * p 0 = <p T > [GeV/c] Charged particle multiplicity CDF Run II Pythia No reconnection Pythia No reconnection + ES1 Pythia No reconnection + ES2 Pythia 8.114, RR * p 0 = 5.56 Pythia ES1, RR * p 0 = 4.35 Pythia ES2, RR * p 0 = Charged particle multiplicity Charged particle multiplicity dummy can describe data but need reconnection rescattering not important here enhanced screening gives significant effect but still need reconnection

29 Onium production Standard perturbative QCD fails to describe charmonium and bottomonium production, though improved in NNLO Non-relativistic QCD, a.k.a. colour octet model does better job, but ill-behaved at p 0 Apply same regularization as for multiple interactions: W = ( p 2 ) 2 p p2 ( αs (p p2 ) α s (p 2 ) ) 3 (Bargiotti; Kraan)

30 LHC predictions: pp collisions at s = 14 TeV dn chg /d at = PYTHIA tuned PYTHIA default PHOJET1.12 pp - interactions Transverse < N chg > PYTHIA tuned PHOJET1.12 CDF data Central Region (min-bias dn chg /d ~ 7) dn chg /d ~ 30 LHC UA5 and CDF data 4 LHC 6 4 dn chg /d ~ 15 x x s (GeV) PYTHIA models favour ln 2 (s); PHOJET suggests a ln(s) dependence. Tevatron P t (leading jet in GeV) A. M. Moraes Minimum-bias and the Underlying Event at the LHC 5 th November 2004

31 Multiple Interactions Outlook Issues requiring further thought and study: Multi-parton PDF s f a1 a 2 a 3 (x 1, Q 2 1, x 2, Q 2 2, x 3, Q 2 3,...) Close-packing in initial state, especially small x Impact-parameter picture and (x, b) correlations e.g. large-x partons more central!, valence quarks more central? Details of colour-screening mechanism Rescattering: one parton scattering several times Intertwining: one parton splits in two that scatter separately Colour sharing: two FS IS dipoles become one FS FS one Colour reconnection: required for p (n charged ) Collective effects (e.g. QGP, cf. Hadronization above) Relation to diffraction: eikonalization, multi-gap topologies,... Action items: Vigorous experimental program at LHC Study energy dependence: RHIC (pp) Tevatron LHC Develop new frameworks and refine existing ones Much work ahead!

32 Initiators and Remnants p u g s initiators: in to hard interaction Need to assign: correlated flavours correlated x i = p zi /p ztot s u d beam remnants correlated primordial k i correlated colours correlated showers PDF after preceding MI/ISR activity: 0) Squeeze range 0 < x < 1 into 0 < x < 1 x i (ISR: i i current ) 1) Valence quarks: scale down by number already kicked out 2) Introduce companion quark q/q to each kicked-out sea quark q/q, with x based on assumed g qq splitting 3) Gluon and other sea: rescale for total momentum conservation

33 Beam drag Colour flow connects hard scattering to beam remnants. Can have consequences, e.g. in π p A(x F ) A(x F ) = #D #D + #D + #D + Pair production All channels WA92, 350 GeV WA82, 340 GeV E791, 500 GeV E769, 250 GeV (also B asymmetries at LHC, but small) x F (a) π p + u u d c c ud If low-mass string e.g.: cd: D, D cud: Λ + c, Σ + c, Σ + c flavour asymmetries c D d Can give D drag to larger x F than c quark for any string mass

34 Fragmentation of junction topology Encountered in R-parity violating SUSY decays χ 0 1 uds, or when 2 valence quarks kicked out of proton beam d (g) junction lab frame d (g) rest frame s (b) J x u (r) z 120 J u (r) d s q 4 q 5 q 4 q 5 q 3 flavour space q 3 q 2 q 2 qq 1 qq 1 u s (b) More complicated (but solved) with gluon emission and massive quarks

35 Rapidity spectrum of original baryon number: Tevatron: y - Junction Baryons LHC: y - Junction Baryons dn/dy 0.6 Old MI - Tune A New MI - Ran New MI - Rap New MI - Lam dn/dy 0.6 Old MI - Tune A New MI - Ran New MI - Rap New MI - Lam y y

36 Strangeness content and p of original baryon number: P( S ) Tevatron: JB strangeness Old MI - Tune A New MI - Ran New MI - Rap New MI - Lam dn/p LHC: p - Junction Baryons Old MI - Tune A New MI - Ran New MI - Rap New MI - Lam S p

37 Observables Everything hangs together, but... Measure Main physics interest Prior knowledge (jet universality!?) LEP hadrons hadronization LEP jets Final-State Radiation Multiple interactions (MI) dn ch /dη η=0 MI rate multiplicity distribution MI fluctuations forward-backward correlations MI fluctuations, string lengths p (n ch ) reconnection rate (or other physics) jet pedestal impact-parameter picture γ + 3 jets hard double parton scattering onium production low-p regularization of MI (1/σ)dσ/dn jet ; E > 3,5,7; lumpiness (particle clustering), R = 0.4, 0.7, 1.0 screening/emergence of jets energy dependence screening relation to small-x gluons

38 Measure Main physics interest Jets (ISR/FSR = Initial/Final-State Radiation) dσ/dp jet PDFs, K factors (1/σ)dσ/dp Z ISR + primordial k ϕ(jet1, jet2) ISR (1/E )de /dr inside jet FSR, hadronization hard multijets ISR + FSR (+MI) activity between jets colour flow, coherence Hadronization flavour composition jet universality charge/flavour correlations (de-)correlations between strings, (disoriented chiral condensate) baryon flow junction framework charm, bottom flow beam drag (needs lower energies) dσ/dp hadron fragmentation function (q vs. g) Other aspects rapidity gaps Pomerons / reconnection dn ch /dη for fix n ch ditto combined picture background for BSM searches

39 Summary and Outlook Sad lack of communication between pp and AA communities (and e + e at LEP/ILC) This meeting brings promise of better dialogue RHIC data from 200 & 500 GeV extremely valuable reference points Much already there make it visible: HepData & Rivet LHC detectors have complementary strengths all can contribute pp simpler than AA, but may contain the seeds of QGP etc. QCD physics in pp remains big challenge!