Hard Probes: High-p T and jets II Marco van Leeuwen, Utrecht University Topical lectures NIKHEF June 2009

2 Part I: Intermediate p T

3 Baryon excess STAR Preliminary B. Mohanty (STAR), QM08 High p T : Au+Au similar to p+p  Fragmentation dominates Baryon/meson = Intermediate p T, 2 – 6 GeV Large baryon/meson ration in Au+Au

4 Hadronisation through coalescence fragmenting parton: p h = z p, z<1 recombining partons: p 1 +p 2 =p h Fries, Muller et al Hwa, Yang et al Meson p T =2p T,parton Recombination of thermal (‘bulk’) partons produces baryons at larger p T Recombination enhances baryon/meson ratios Hot matter Baryon p T =3p T,parton

5 Near-side ‘Ridge’ d+Au, 200 GeV 3 < p t,trigger < 4 GeV p t,assoc. > 2 GeV Au+Au 0-10% STAR preliminary  trigger d+Au: ‘jet’-peak, symmetric in ,  Au+Au: extra correlation strength at large   ‘Ridge’ Unexpected – what can it be?

6 Mechanisms for ridge formation Long. flow Jet broadening Three categories Medium responseTrigger effect Long. flow Gluons from fragmentation/energy loss couple to longitudinal flow Extra yield due to medium heating/drag or propagating parton Trigger selects existing structure in the medium (underlying event, color flux tubes) Different scenarios suggest different behaviour, e.g. multiplicity, p T -dependence,  extent, baryon content Experimental tests ongoing

7 Near-side Ridge 3 < p t,trig < 4 GeV/c Au+Au 0-10% STAR preliminary associated  trigger `Ridge’: associated yield at large , small  Ridge softer than jet – medium response? J. Putschke et al, QM06 Weak dependence of ridge yield on p T,trig  Relative contribution reduces with p T,trig 4 < p t,trig < 6 GeV/c Au+Au 0-10% STAR preliminary Jet-like peak p t,assoc. > 2 GeV/c

8 Associated yields from coalescence Baryon p T =3p T,parton Meson p T =2p T,parton Expect large baryon/meson ratio associated with high-p T trigger Expect reduced associated yield with baryon triggers 3 < p T < 4 GeV (Hwa, Yang) Hot matter Baryon p T =3p T,parton Meson p T =2p T,parton Hard parton Hot matter Recombination of thermal (‘bulk’) partons ‘Shower-thermal’ recombination No jet structure/associated yield

9 Associated baryon/meson ratios Baryon/meson ratio in ridge close to Au+Au inclusive, in jet close to p+p Different production mechanisms for ridge and jet? p+p, d+Au: B/M  0.3 Au+Au: Baryon enhancement Inclusive spectra p T trig > 4.0 GeV/c 2.0 < p T Assoc < p T trig C. Suarez et al, QM08 p+p /  + +  - Ridge (large  ): Baryon enhancement Jet (small  ) B/M  0.3 Associated yields

10 More medium effects: away-side 3.0 < p T trig < 4.0 GeV/c 1.3 < p T assoc < 1.8 GeV/c Au+Au 0-10% d+Au Away-side: Strong broadening in central Au+Au ‘Dip’ at  =  M. Horner, M. van Leeuwen, et al

% 4.0 < p T trig < 6.0 GeV/c 6.0 < p T trig < 10.0 GeV/c 3.0 < p T trig < 4.0 GeV/c Preliminary Au+Au 0-12% 1.3 < p T assoc < 1.8 GeV/c Low p T trig : broad shape, two peaksHigh p T trig : broad shape, single peak Away-side shapes Fragmentation becomes ‘cleaner’ as p T trig goes up Suggests kinematic effect? M. Horner, M. van Leeuwen, et al

12 Shockwave/Mach Cone Gyulassy et al arXiv: T. Renk, J. Ruppert B. Betz, QM09, PRC79, Mach-cone/shockwave in the QGP? Exciting possibility! Are more mundane possibilities ruled out? – Not clear yet Proves that QGP is really ‘bulk matter’ Measure speed of sound?

13 Summary High p T : –Relatively clean jet-fragmentation –Hadron production suppressed by factor 4-5 Intermediate p T : –Large baryon/meson ratio – Coalescence? –Near side: ridge – Interplay of jets and bulk? –Away side: double hump – Mach cones?

14 Parton energy from  -jet and jet reconstruction Qualitatively: `known’ from e + e - known pQCDxPDF extract Full deconvolution large uncertainties (+ not transparent) Fix/measure E jet to take one factor out Two approaches:  -jet -Jet reconstruction  Second-generation measurements at RHIC – first generation at LHC?

15 Perturbative QCD processes Hadron production Heavy flavours Jet production –e + e - → jets –p(bar)+p → jets Direct photon production Measurement difficulty Theory difficulty

16 Jet reconstruction algorithms Two categories of jet algorithms: Sequential recombination k T, anti-k T, Durham –Define distance measure, e.g. d ij = min(p Ti,p Tj )*R ij –Cluster closest Cone –Draw Cone radius R around starting point –Iterate until stable ,  jet = particles For a complete discussion, see: Sum particles inside jet Different prescriptions exist, most natural: E-scheme, sum 4-vectors Jet is an object defined by jet algorithm If parameters are right, may approximate parton

17 Collinear and infrared safety Illustration by G. Salam Jets should not be sensitive to soft effects (hadronisation and E-loss) -Collinear safe -Infrared safe

18 Collinear safety Note also: detector effects, such as splitting clusters in calorimeter (  0 decay) Illustration by G. Salam

19 Infrared safety Infrared safety also implies robustness against soft background in heavy ion collisions Illustration by G. Salam

20 Current best jet algorithms Only three good choices: –k T algorithm (sequential recombination, non- circular jets) –Anti-k T algoritm (sequential recombination, circular jets) –SISCone algorithm (Infrared Safe Cone) + some minor variations: Durham algo, different combination schemes These are all available in the FastJet package: Really no excuse to use anything else (and potentially run into trouble)

21 Relating jets and single hadrons High-p T hadrons from jet fragmentation Qualitatively: Single hadrons are suppressed: -Suppression of jet yield (out-of-cone radiation) R AA jets < 1 -Modification of fragment distribution (in-cone radiation) softening of fragmentation function and/or broadening of jet structure

22 Jet finding in heavy ion events  η p t per grid cell [GeV] STAR preliminary ~ 21 GeV FastJet:Cacciari, Salam and Soyez; arXiv: Jets clearly visible in heavy ion events at RHIC Use different algorithms to estimate systematic uncertainties: Cone-type algorithms simple cone, iterative cone, infrared safe SISCone Sequential recombination algorithms k T, Cambridge, inverse k T Combinatorial background Needs to be subtracted

23 p+pAu+Au central STAR Preliminary Jet spectra STAR Preliminary Note kinematic reach out to 50 GeV Jet energy depends on R, affects spectra k T, anti-k T give similar results Take ratios to compare p+p, Au+Au

24 Jet R AA at RHIC Jet R AA >> 0.2 (hadron R AA ) Jet finding recovers most of the energy loss  measure of initial parton energy M. Ploskon, STAR, QM09 Some dependence on jet-algorithm? Under study…

25 Radius dependence RAA depends on jet radius: Small R jet is single hadron M. Ploskon, STAR, QM09 Jet broadening due to E-loss?

26 Fragmentation functions STAR Preliminary p t,rec (AuAu)>25 GeV 20<p t,rec (AuAu)<25 GeV Use recoil jet to avoid biases Suppression of fragmentation also small (>> 0.2) E. Bruna, STAR, QM09

27 Di-jet spectra 27 Elena Bruna for the STAR Collaboration - QM09 STAR Preliminary E. Bruna, STAR, QM09 Jet I AA Away-side jet yield suppressed  partons absorbed... due to large path lentgh (trigger bias)

28 Emerging picture from jet results Jet R AA ~ 1 for sufficiently large R – unbiased parton selection Away side jet fragmentation ummodified – away-side jet emerges without E-loss Jet I AA ~ 0.2 – Many jets are absorded (large E-loss) Study vs R, E to quantify P(  E) and broadening

29 From RHIC to LHC -Larger p T -reach: typical parton energy > typical  E -Energy dependenc of E-loss with high- energy jets Larger initial density  = GeV/fm 3 at RHIC  ~ 100 GeV/fm 3 at LHC 10k/year Large cross sections for hard processes Including heavy flavours Validate understanding of RHIC data Direct access to energy loss dynamics, P(  E) And others, e.g. gluon saturation

30 Energy loss distribution Brick L = 2 fm,  E/E = 0.2 E = 10 GeV Typical examples with fixed L  E/E> = 0.2 R 8 ~ R AA = 0.2 Significant probability to lose no energy (P(0)) Broad distribution, large E-loss (several GeV, up to  E/E = 1) Broad distribution; typical energy loss ~5 GeV

31 R AA at LHC S. Wicks, W. Horowitz, QM2006 T. Renk, QM2006 Expected rise of R AA with p T depends on energy loss formalism Nuclear modification factor R AA at LHC sensitive to radiation spectrum P(  E) LHC: typical parton energy > typical  E GLVBDMPS RHIC

32 Summary Intermediate p T : interplay between soft and hard physics –Parton coalescence –The ridge –Shock waves? Jets –First measurements from RHIC – much larger reach at LHC –Powerful tool to diagnose energy loss Absorption vs continuous energy loss

33 Extra slides

34 Summary of jet results – Evidence that di-jet rates are suppressed A.Recover a fraction of the jet energy  shift towards smaller energies B.Do not reconstruct jet  Biased jet population selected p Trec (recoil)>25 GeV –No strong modification of FF (two approaches lead to a similar conclusion) High-energy recoil jets are biased (non interacting) 20<p Trec (recoil)<25 GeV –di-jet rates less suppressed A.“Feed-down” from high-energy jets B.More complete jet energy recovered –Indication of modification of FF Elena Bruna for the STAR Collaboration - QM09 34 STAR Preliminary p t,rec (AuAu)>25 GeV STAR Preliminary 20<p t,rec (AuAu)<25 GeV

35 pQCD illustrated CDF, PRD75, jet spectrum ~ parton spectrum fragmentation

36 Path Length III: v 2 at high-p T PQM: Dainese, Loizides, Paic, Eur Phys J C38, 461 Models tend to predict low v 2 v 2 for  0 Agreement improves for high-p T Promising measurements, not much new in recent years Main issue: how do jets influence the event-plane (non-flow)

37 Fragmentation bias in di-hadrons LEP: Quarks: D(z) ~ exp(-8.2 z) Gluons: D(z) ~ exp(-11.4 z) Test case: Use two different frag functions Calculate away-side spectra p Tt p Ta Away-side spectra not sensitive to slope of fragmentation function More detailed analysis shows: mainly depends on power n of partons spectrum

38 Di-jet spectra unfolding