Jet quenching in relativistic heavy ion collisions at the LHC

Transcription

1 Jet quenching in relativistic heavy ion collisions at the LHC Aaron Angerami Columbia University Columbia Particle Physics Seminar Wednesday, 04/0/203

2 Modern Nuclear Physics High energy physics has established QCD as the theory of the strong interaction Categorical imperative of modern nuclear physics: Given QCD, what types of matter and physical phenomena does the theory admit, and how can they be understood in terms of the theory s two main features: asymptotic freedom and color confinement Natural place to start is to study the phase structure of QCD

3 QCD at High emperatures Nuclear matter must become weakly coupled (asymptotic freedom) Degrees of freedom are deconfined color charges his is the Quark Gluon Plasma (QGP) Quark Gluon Plasma As matter cools, degrees of freedom return to color neutral hadrons System evolves through confinement transition Cold Nuclei

4 QCD at High emperatures Nuclear matter must become weakly coupled (asymptotic freedom) Degrees of freedom are deconfined color charges his is the Quark Gluon Plasma (QGP) Quark Gluon Plasma As matter cools, degrees of freedom return to color neutral hadrons System evolves through confinement transition In a heavy ion collision it may be possible to jump from ordinary nuclear matter to deconfined medium Cold Nuclei

5 Insights from the Lattice Cold Nuclei Quark Gluon Plasma System does not reach SB limit of ideal gas Interactions still important at high Is system strongly coupled? Rapid change in degrees of freedom near C~200 MeV Smooth cross-over, not st or 2 nd order phase transition

6 Heavy Ion Collisions : Bulk Observables Measurements of global observables (dnch/dη, de/dη) and identified particle spectra have led to the conclusions*: - At snn ~ 00 GeV, initial temperature of matter is above C~200 MeV - Observables consistent with matter that rapidly equilibrates and expands hydrodynamically with low viscosity Elliptic flow measurements constrain kinematic viscosity to entropy density ratio (η/s) to be small - Near quantum limit conjectured through AdS/CF correspondence (η/s > /4π) Low viscosity and resulting collective behavior consistent with a strongly coupled system ( sqgp )!! * See RHIC white papers: nucl-ex/040003, nucl-ex/050009, nucl-ex/040020, nucl-ex/040022

7 Heavy Ion Collisions : Hard Probes Processes with large momentum transfer ( q 2 ) - Are amenable to perturbative QCD calculations - Produce particles are not in equilibrium with the bulk Jets, photons, W, Z act as external probes of medium Jets carry color charge and experience jet quenching Energy loss of a parton or modification of its parton shower through interactions with medium RHIC program has yielded indirect evidence for the phenomenon of jet quenching - In terms of single particles from jet whose kinematics are not the same as the parton suffering energy loss - Indicate that the collinear factorization used to calculate hard processes in QCD is explicitly broken in HIC Scales introduced by medium break required separation of scales between hard and soft components of process

8 Jet Quenching omographic view - raditional approach taken in HI field - Jets as external probe - Provide access to medium transport coefficients Use jets to learn about QGP Jet phenomenology perspective - More HEP-like approach - New regime of QCD radiation Use QGP to learn about jets Unique opportunity to study universal phenomena of radiation and diffusion in a fundamental theory!

9 Nuclear Geometry Radial density profile for spherically symmetric nucleus Most heavy ions spherically symmetric Radial profile well described by Woods- Saxon distribution (r) = 0 +e (r R)/a In transverse plane projectile sees density of nucleons described by nuclear thickness A (b) = Z (b,z)dz

10 Nuclear Geometry y b = impact parameter Ψ 2 z In CM frame, nuclei appear as Lorentz contracted pancakes In transverse plane, overlap region is anisotropic - Higher density gradients in direction of reaction plane angle (Ψ 2 ) defined by impact parameter vector Per collision flux of nucleons increases with collision centrality

11 Aside: Elliptic Flow Sharper density gradients in-plane result in higher pressure gradients (through hydrodynamics) In-plane gets more push from collective expansion than out-of-plane dn / +2v 2 cos 2( 2) d Measure v2 Out-of-plane In-plane v SAR ALICE v 2 {4} MC-KLN Reaction Plane RHIC: η/s=0.6 LHC: η/s=0.6 LHC: η/s=0.20 LHC: η/s= centrality Use robust model (hydro + lattice EOS) to extract medium transport coefficients (η/s) Blueprint for quantitative extraction of medium parameters

12 Leading Order Picture of Jet Production Initial partons No net transverse momentum (collinear) p φ At leading order, outgoing partons are: back-to-back ( Δφ=π ) with equal transverse momenta ( p ) Large q 2 (momentum transfer in hard scattering) required to produce high p partons

13 he Parton Shower Large q 2 processes may receive higher order corrections (large logs) from processes with additional radiation Resummation yields DGLAP evolution equation Radiation pattern from virtuality evolution know as a parton shower (PS) DGLAP splitting functions give probability for splitting but process happens randomly on per jet basis Internal jet structure varies greatly from jet to jet MC implementations of PS (e.g. PYHIA) give good descriptions of jet structure PS shower involves particular subset of higher order corrections to LO - Does not get full next-to-leading order (NLO) behavior

14 Leading Order + Parton Shower Incoming parton kinematics not fixed May have evolved from different kinematics φ Outgoing parton may emerge with large virtuality Must radiate to get back on shell φ2 Need a jet reconstruction algorithm to cluster final state particles into jets Expect some spread in p balance (asymmetry) and Δφ (acoplanarity) due to initial and final state parton showers as well as bonafide NLO processes

15 Building Intuition for Jets Expect highly collimated jets of particles Steeply falling (power-law) production spectrum Momentum balance mostly respected by dijet system but expect some acoplanarity Number of Events ALAS s=7 ev anti-k t jets R=0.6 p >00 GeV y <2.8 Leading two jets: y <0.8 p max >0 GeV - Data L dt=36 pb 2 jets 3 jets 4 jets 5 jets PYHIA 0 /2 2 /3 5 /6 [radians]

16 Expectations for Jet Quenching Intuition from single particle energy loss in electromagnetic media Low energies dominated by energy loss due to elastic scattering with medium Collisional Higher energies dominated by medium induced bremsstrahlung Radiative - Virtuality set by scale of medium (radiation length) - Coherence effects due to interference between multiple scattering centers (LPM effect)

17 Expectations for Jet Quenching Can construct QCD analogs for single particle energy loss but this is only part of the story - Jet is not on shell vacuum radiation - Ensemble of partons at different virtualities How are various components of PS affected by quenching? - Jet may not lose energy but simply have it redistributed - Alternatively, radiation at larger angles takes energy out of nominal jet cone In the past theoretical approaches focused on leading parton - BDMPS-Z : QCD analog of LPM + interference between vacuum and medium-induced radiation from leading parton No canonical paradigm (in analogy with hydrodynamics) - State of the art : technical details of interplay between vacuum and medium-induced radiation as applied to full PS

18 Hard Scattering Rates in Nuclear Collisions Medium-induced energy loss will systematically reduce outgoing jet energy Expect the number of jets observed at a given p to be reduced in collisions with larger medium effects Jet suppression due to quenching - Compare hard scattering rates in different centrality classes Rate increases with centrality due to geometric enhancement, which is unrelated to quenching - Per collision flux of nucleons increases with centrality - Normalize by per collision luminosity AA or by the effective number of binary nucleon collisions Ncoll =AA σnn inel R AA = h AA i N evt dn jet dp d NN jet dp cent R CP = hn coll i hn coll i N evt dn jet dp N evt dn jet dp cent periph

19 9

20 he ALAS Detector ±40 m

21 ALAS Calorimeter EM barrel and end cap use accordion design for uniform radiation length Liquid argon end-caps.7 η < 3.2 Steel-scintillator hadronic calorimeters ile barrel η <.0 Extended tile barrel 0.8 < η <.7 r φ Hadronic end-cap (HEC) EM end-cap (EMEC) Cells in Layer 3 ϕ η = X 0 rigger ower η = 0. η = 0 4.3X 0 ϕ=0.0245x4 36.8mmx4 =47.3mm 500 mm 470 mm ϕ.7x 0 6X mm/8 = 4.69 mm η = η ϕ = η = Strip cells in Layer Square cells in Layer 2 rigger ower ϕ = Liquid argon EM barrel η <.5 High granularity precision calorimeter Fine η segmentation in first layer for γ π 0 separation Presampler layer to measure early showers Liquid argon Forward calorimeter (FCal) combined EM and hadronic 3.2 < η < 4.9

22 Heavy Ions in ALAS - otal Integrated Luminosity [ub ] 2 s NN = 2.76 ev LHC Delivered (Pb+Pb) ALAS Online Luminosity ALAS Recorded - otal Delivered: 9.69 ub - otal Recorded: 9.7 ub 0 02/ 09/ 6/ 23/ 30/ 07/2 Day in 200 Z 200 Z 20 L dt =7µ b L dt = 40 µ b

23 Centrality Partition FCal E distribution into ranges corresponding to fixed percentiles of the total Use Glauber Model to determine Ncoll and Npart in collisions in the same percentile FCal E has strong linear correlation with total event activity at mid-rapidity

24 Jet Reconstruction in Heavy Ion Collisions In addition to normal issues in a HEP jet measurement, HI collisions have the added complication of an underlying event (UE) which has - Large dynamic range Peripheral collisions pp-like Central collisions Npart > 300, ~00 GeV underneath jet - Global event correlations, e.g. flow - Large uncorrelated UE fluctuations Perform event-by-event background determination and subtraction - Subtract average E density, ρ, modulated by measured elliptic flow!! E subtr E A jet [ + 2v 2 cos 2( jet 2)] - Use iterative procedure to ensure background is not biased by the jets themselves

25 First look at jets in Pb+Pb at the LHC Highly asymmetric dijet events appear to be a manifest feature of heavy ion collisions at the LHC ALAS result is first direct observation of jet quenching hep-ex/0.682

26 Dijet asymmetry E 2 A J = E E + E2 Back-to-back structure preserved Significant fraction of events with enhanced dijet asymmetry attributed to different path lengths of jets in the QGP hep-ex/0.682

27 he nuclear modifications R V, R S, R G for Carbon (upper group of panels) and Beyond Asymmetry Dijet asymmetry is striking signature of jet quenching but it is also difficult to understand quantitatively Dependent on energy loss of two jets with path-length correlation Supplement the picture by studying inclusive quenching Hard scattering rates in HI R AA, R CP 2 =.69 GeV 2 ) (, Pb 2 =00 GeV 2 ) (, Pb Q 2 =.69 GeV Q 2 =00 GeV Pb Pb Pb EPS09NLO EPS09NLO Must separate quenching from cold nuclear effects Nucleus has different parton distribution function Check rates of color neutral processes, W ±! `± `, Z 0! `+`

28 Data/JEPHOX Photon and Z Production Rates ALAS-CONF PRL 0, (203) P H Y S I C A L R E V ] -2 [GeV Z ll y <2.5 dn 7 0 dp p N events Pb+Pb 0-0%(+6) 0-20%(+4) 20-40%(+2) ALAS = 2.76 ev Data 20 - L int = 0.5 nb s NN Data / Model 2 0%-5% 2 5%-0% 2 0%-20% hep-ex/ %-40% 2 ALAS Preliminary Pb+Pb s NN = 2.76 ev - = 33 µb η <.3 photon p 0%-5% ( 00) 40-80% 5%-0% ( 20) 0%-20% ( 5) 20%-40% 40%-80% L int [GeV] 2 40%-80% Z p [GeV] Z

29 Jet Suppression Medium effects may cause jet energy to be transported outside the nominal jet cone Can lost energy be recovered by expanding size of jet definition (radius)? Measure single jet suppression with multiple jet sizes = jet 2 Jets produced with different angles wrt to event plane (Δφ) will see different path lengths and density profiles in the medium Measure single jet suppression as a function of Δφ : v2 jet

30 Results: RCP vs p in Centrality Bins hep-ex/ R CP 2.5 ALAS anti-k t R = 0.2 Pb+Pb L dt s NN - = 7 µb = 2.76 ev R CP 2.5 ALAS anti-k t R = 0.4 Pb+Pb L dt s NN - = 7 µb = 2.76 ev % % % % % % % [GeV] R = 0.2 p R = % Result fully unfolded (SVD method) for finite jet energy resolution Use % as peripheral p [GeV]

31 Results: RCP vs Npart in p bins hep-ex/ Centrality dependence as represented by Npart Suppression turns on differently for high and low p jets

32 Results: RCP vs R hep-ex/ p R CP % centrality ALAS 0-0 % Centrality 58 < p 89 < 50 < 38 < p p p < 82 GeV < 03 GeV < 58 GeV < 44 GeV Pb+Pb L dt s NN - = 7 µb = 2.76 ev R CP ALAS 89 < % % 0-20 % 0-0 % 89 < p <03 GeV p < 03 GeV Pb+Pb L dt s NN - = 7 µb = 2.76 ev 0 R = 0.2 R = 0.3 R = 0.4 R = R = 0.2 R = 0.3 R = 0.4 R = 0.5 R R

33 Quantitative statement of R dependence hep-ex/ Ratios of RCP to RCP with R=0.2! Measure relative suppression with respect to most suppressed R value (R=0.2)! Variation with R is significant Note switch log scale to focus on low p behavior Many systematics cancel, correlated between different R Statistical correlation between different R values included and propagated through unfolding

34 Dependence on Δφ ALAS-CONF Red curve: fit to +2v jet 2 meas cos 2

35 Results: v2 vs p ALAS-CONF Corrected experimental effects Jet energy resolution (unfolded) EP resolution 5% modulation in jet yield at low p, decreases with p to ~.5%

36 Results: v2 vs Npart ALAS-CONF Modulation is smallest in most central collisions where initial collision geometry is most symmetric, i.e. vs

37 Conclusions: Hard Scattering Rates Production of color-neutral probes consistent with binary Z! e + e,z! µ + µ and scaling, three independent checks: Jets are suppressed by factor of two in central collisions and show no p dependence for 38 < p < 20 GeV - Roughly same as single particle RAA for p > 30 GeV Jets with larger R show less suppression more so at low p Centrality dependence of suppression turns on differently for high and low p jets Significant modulation of jet yield with respect to event plane - 5% at low p decreasing to ~.5% at 200 GeV - Modulation is smallest in most central collisions where initial collision geometry is most symmetric

38 Asymmetry: Differential Energy Loss γ/z jet correlations provide clean probe since γ and Z ( or leptonic decay products) do not suffer energy loss Do NO expect jets recoiling against γ/z to have same p as γ/z - Effects like initial state parton shower cause broadening of distribution - Focus on xj = p jet / p γ/z Unmodified xj and AJ distributions in are different γ and Z jet events - Large virtuality required to produce Z - Potentially provide different handles on energy loss since intrinsic are different!!

39 γ-jet: xj Distributions ALAS-CONF R = 0.2 R = 0.3

40 Z jet Correlations ALAS-CONF % centrality 20 80% centrality Mostly proof of principle due to low statistics but hints at potential of the measurement when more data comes

41 D(z) Jet Structure: Fragmentation Function Distribution of charged hadrons within a jet ALAS Preliminary Pb+Pb s NN =2.76 ev - L int =0.4 nb ) D(p ALAS Preliminary Pb+Pb s NN =2.76 ev - L int =0.4 nb ALAS-CONF p jet > 00 GeV p hadron > 2 GeV D(z) 6 0-0% % % % % % % - anti-k R=0.4 jet p > 00 GeV 0 z D(p) 6 0-0% % % % % % % 0 z = p cos(δr)/p jet anti-k R=0.4 jet p > 00 GeV p 2 0 [GeV]

42 Jet Structure: Centrality Dependence ALAS-CONF Ratio = D 0 0 % / D % Similar trends in D(z) and D(p) distributions Enhancement at low z/p Suppression at moderate z/p High p behavior may exhibit additional enhancement

43 he Average Jet empting to interpret moderate z fragments losing energy and contributing to excess at low z - Would conclude all jets are quenched in the same way Not every jet has distribution of fragments like this In fact none do! Jets with fragments near z~ are kinematically restricted from having additional fragments except at lowest z No guarantee that jets contributing to depletion are same jets contributing to excess Are jets with different parton showers/z distributions quenched differently/more likely to suffer less energy loss?

44 Average ypical But must be careful with interpretation of average especially in the case of jet properties Ensemble averaged distribution may not be characteristic of individual event-by-event jet properties Measurements of jet properties carry detailed information on quenching but characteristics of quenching may vary greatly from case to case Utilize differential measurements to make stronger conclusions these are experimentally accessible Cautious interpretation of average properties until we have an answer to: Key question: Is quenching driven by average energy loss effects or by significant event-by-event variation not well represented by the average? Can be addressed by additional measurements

45 Additional Slides

46 Single hadron RCP ALAS-CONF

47 Single hadron RCP : η dependence ALAS-CONF

48 W Yields ALAS-CONF

49 Jets In Heavy Ion Collisions Apply IRC safe jet definition to measured E distribution in calorimeter In addition to jet signal, also have contribution from underlying event (UE) Define jet measurement as energy correlated with single QCD hard scattering, need to separate from uncorrelated UE contribution de total d d = deue d d + dejet d d Construct estimate of UE background, subtract and run jet finding Average depends strongly on centrality, must determine event-by-event Must be modulated to include flow effects +2v 2 cos [2 ( 2)] Jets must be excluded from the estimate of the background

50 Jet Reconstruction Define average background excluding cells ΔR < 0.4 from jets Calculate event plane angle from FCal w k E k sin (2 k ) 2 = 2 tan k w k E k cos (2 k ) Calculate v2 per sampling layer:!! v 2i = j i E j cos [2 ( j 2)] j Average over η excluding bins within 0.4 of seeds k i E j Also reconstruct track jets, run anti-kt R=0.4 on particles p > 4 GeV

51 Performance truth σ [ E ]/E or truth /E E Efficiency 0.4 ALAS simulation R = anti-k t truth σ [ E ]/E truth σ [ E ]/E E E /E /E + fit, 0-0% + fit, 60-80% truth, 0-0% truth ε, 0-0% ε, 60-80% ε', 0-0% ε', 60-80%, 60-80% Reconstruction capabilities evaluated using MC Use PYHIA dijets embedded into HIJING events Validated using data, extract systematics truth E [GeV]

52 JES Validation: rack Jet Matching Matching between track jets and calo jets to study calorimetric response in MC and data Limits effects of possible medium-modified fragmentation on JES All values not shown 0.5% R 0-0 % 0-20 % % % % % % 0.5 % 0.5 % 0.5 % 0.5 % 0.5 % % 0.5 % 0.5 % 0.5 % 0.5 % 0.5 % %.0 % 0.5 % 0.5 % 0.5 % 0.5 % %.5 %.0 % 0.5 % 0.5 % 0.5 % JES uncertainty constant above 70 GeV (table) Grows linearly, doubling from its nominal value at 30 GeV

53 Performance: Jet Energy Resolution Extract σ through statistical RMS or Gaussian fit Low E: dominated by UE fluctuations High E: limited by intrinsic detector resolution Described by functional form: ( E ) E = E a E b ce a: sampling fluctuations c: proportional to energy e.g. holes b: UE fluctuations } truth σ [ E ]/E or truth /E E Efficiency centrality independent centrality dependent 0.4 ALAS simulation R = anti-k t truth σ [ E ]/E truth σ [ E ]/E E E /E /E + fit, 0-0% + fit, 60-80% truth, 0-0% truth ε, 0-0% ε, 60-80% ε', 0-0% ε', 60-80%, 60-80% truth E [GeV]

54 Fluctuations Analysis Uncorrelated UE fluctuations underneath jet not subtracted Effect on jet spectrum corrected by unfolding MC must provide accurate description of UE fluctuations Study distributions of E sum in groups of rectangular groups of towers approximately same size as jets (e.g. 7x7 R=0.4)

55 Performance: Jet Energy Resolution Fixed by fluctuation analysis ( E ) E = E a E b ce Free parameters in fit Fit results give a and c values in agreement for all centralities Establishes quantitative relationship between UE fluctuations and ΔE fluctuations (JER) truth σ [ E ]/E or truth /E E Efficiency 0.4 ALAS simulation R = anti-k t truth σ [ E ]/E truth σ [ E ]/E E E /E /E + fit, 0-0% + fit, 60-80% truth, 0-0% truth ε, 0-0% ε, 60-80% ε', 0-0% ε', 60-80%, 60-80% truth E [GeV]

56 Unfolding R CP corrected measured ALAS jets Pb+Pb L dt s NN - = 7 µb = 2.76 ev anti-k t 0-0 % R = 0.2, measured R = 0.2, corrected R = 0.4, measured R = 0.4, corrected p [GeV] UE and detector effects result in finite JER Jet spectrum is steeply falling Result is significant bin migration Use MC to generate response matrix Contains information about bin migration SVD unfolding Invert response using curvature constraint on result to regularize unfolding Unfolding checks Hocker and Kartvelishvili:! hep-ph/ Apply to MC, look for bias Refold data, check refolded looks like input

57 Overview of Systematic Uncertainties JES: Relative energy scale differences central and peripheral JER: Possible disagreement between data and MC in UE fluctuations Efficiency: cover possible MC/data differences, 5% for p < 00 GeV X ini : Sensitivity to power in power law: +0.5, -0.5 Rcoll: sensitive to centrality determination, σnn Regularization: Sensitivity to choice of k:+/-

58 Measured Yield Before Corrections Red curve: fit to +2v jet 2 meas cos 2

59 Azimuthal Dependence of Jet Performance

60 wo-jet Observables: Dijet Asymmetry E 2 A J = E E + E2 E > 00 GeV E 2 > 25 GeV Updated from published result Contributions to second peak mostly from events where second jet consistent with background level

61 D(z) Jet Structure: R Dependence R = 0.2 R = 0.3 ALAS-CONF Behavior persists for smaller radii Robust against UE effects which increase with R D(p)