ATLAS Measurements of Jets in Heavy-Ion Collisions

Transcription

1 ALAS Measurements of Jets in Heavy-Ion Collisions Aaron Angerami, Columbia University for the ALAS Collaboration PANIC Cambridge, Massachusetts 7/8/

2 Jets in Heavy Ion Collisions Jets provide a powerful tool for determining the degrees of freedom and their effective scales in heavy ion collisions Results from the RHIC program show that high p particle production is suppressed and that usual factorization of hard processes is broken in nuclear collisions q not dominant scale new relevant scales: temperature, system size? Single particle suppression doesn t tell us: Does energy remain in jet but redistributed among fragments? Or is the jet energy being transferred to the medium? Need to go beyond single particles, look at full jets

3 Radiative Energy Loss Medium interactions induce additional radiation modifying usual vacuum fragmentation high z region of fragmentation function sensitive to medium induced radiation ) M/V s (z,q Ratio of modified fragmentation functions to vacuum. - - z

4 Radiative Energy Loss Broadening of radiation may cause energy deposition outside jet cone Predictions of radiative energy loss suggest energy can be recovered by expanding jet cone

5 Di-jet Asymmetry: Original Result From Nov. PRL ~ μb - E A J = E E + E First indication of suppression of full jets Momentum balance from hard process not contained in jets E > GeV E > 5 GeV 5

6 Understanding Asymmetry Asymmetry results are startling, does this constrain energy loss picture? Difficult with a two-jet observable by itself. Supplement with single jet observables: Jet spectra/central to peripheral ratio (RCP) Longitudinal and transverse fragment distributions (z and j) Known to be sensitive to quantitative details of energy loss Less sensitive to global event features, hopefully can disentangle potentially complicated medium effects 6

7 Data Pb+Pb snn=.76 ev Lint = mb- Model -% -% -% -% - -5% 7-8% 6-7% - 5-6% E in barrel strongly correlated with FCal (/Nevt)dNevt/dΣE [ev-] ALAS Detector: Calorimeter - - FCal E used to determine centrality. < η <.9 FCal ΣE [ev] Analysis uses jets in barrel: η <.8 Pb Ion run: 7μb-, 5 M events 7

8 Jet Reconstruction Jets reconstructed using anti-kt algorithm with two choices of R parameter (R=. and R=.) Inputs are.x. (ΔηxΔϕ) calorimeter towers Average background computed per calorimeter sampling layer per. η strip for each event Potential jets excluded from averaging to prevent bias R=.{ jets utilize an iterative procedure to determine background background modulated by elliptic flow before subtraction 8

9 Jet Spectra R=. ] - [GeV - ALAS Preliminary Centrality ] - [GeV ALAS Preliminary dn jet de N evt % - % - % - % -5 % 5-6 % 6-8 % dn jet de N evt N coll -8 Centrality - % - % 5-6 % 6-8 % Pb+Pb - = 7 µ b R =. =.76 ev 5 5 E [GeV] Centrality dependence largely removed by geometry Ncoll -9 - Pb+Pb - = 7 µ b R =. =.76 ev 5 5 E [GeV] Raw spectra with bin-by-bin unfolding, additional % systematic uncertainty from jet energy scale 9

10 RCP R=. Jets R CP Centrality - % Centrality - % Pb+Pb - = 7 µ b =.76 ev R CP < 5 GeV < E < 5 GeV 5 < E Pb+Pb - = 7 µ b =.76 ev Gradual turn on of medium effects with increasing centrality R =. R = Centrality 5-6 %.5 5 < E < GeV E [GeV] 5 6 Centrality [%] Clear factor of ~ suppression indicated in most central collisions Entire jet, not just leading fragment

11 RCP R=. Jets R CP.5 Centrality - % R CP.5 < 75 GeV 5 < E Centrality - % Pb+Pb - = 7 µ b =.76 ev.5 < GeV 75 < E Pb+Pb - = 7 µ b =.76 ev R =. R = Centrality 5-6 %.5 < E < 5 GeV E [GeV] 5 6 Centrality [%] Slightly more suppression observed Not much energy recovered by larger cone size:..

12 Jet Fragments: ransverse Structure ] - [(GeV/c) dn/dj /j /N jets - - R=. E,jet > GeV R =., -%, p > GeV R =., -8%, p > GeV Pb+Pb =.76 ev - =7 µb [GeV/c] All charged particles with p > GeV and within jet radius j ] - [(GeV/c) dn/dj /j /N jets R=. E,jet =75- GeV R =., -%, p > GeV R =., -8%, p > GeV Pb+Pb =.76 ev - =7 µb [GeV/c] j = p sin R R = ( jet ) +( jet ) j Lack of broadening consistent with RCP measurement Some change in shape with centrality, not enough to indicate a strong effect

13 Jet Fragments: Longitudinal Structure dn/dz /N jets E,jet > GeV - - R=. Pb+Pb =.76 ev - =7 µb R =., -%, p > GeV R =., -8%, p > GeV z = p E jet cos R z dn/dz /N jets E,jet =75- GeV R =., -%, p > GeV R =., -8%, p > GeV - - R=. Pb+Pb =.76 ev - =7 µb z Large modification at high z not indicated Slight redistribution of jet s energy among fragments seen at mid-low z (.)

14 Jet Fragments: Longitudinal Structure Modest modification at mid-low z High z behavior unlike prediction Ratio of D(z) Ratio of D(z) (-%)/(-8%).8 (-%)/(-8%) R =. R =. Pb+Pb =.76 ev - =7 µb z z Pb+Pb =.76 ev - =7 µb z (-%)/(-8%).8 (-%)/(-8%) R =. R =. Pb+Pb s =.76 ev NN - =7 µb R= Pb+Pb s =.76 ev NN - =7 µb z

15 wo-jet Observables: Di-jet Asymmetry J ) dn/da evt (/N Centrality -% Pb+Pb =.76 ev = 7 µ b Centrality -% Centrality -% E > GeV E > 5 GeV R =. Centrality -6% Centrality -% HIJING+PYHIA Pb+Pb Data p+p Data Centrality 6-8% E A J = E E + E E > GeV E > 5 GeV Updated from published result Contributions to second peak mostly from events where second jet consistent with background level A J 5

16 ) dn/da evt J Di-jet Asymmetry: R=. (/N Centrality -% Pb+Pb =.76 ev = 7 µ b Centrality -% Centrality -% E > GeV E > 5 GeV R =. Centrality -6% Centrality -% HIJING+PYHIA Pb+Pb Data Centrality 6-8% E A J = E E + E E > GeV E > 5 GeV Distribution flatter, peak smeared out Less sensitive to background fluctuations A J 6

17 ) dn/da (/N J Asymmetry: Energy Dependence, R=. 75 < E < GeV <E <5 GeV 5<E <5 GeV -% evt 5 75 < E < GeV Centrality -% R =. E > 5 GeV < E < 5 GeV Centrality -% Pb+Pb =.76 ev - = 7 µ b 5 5 < E < 5 GeV Centrality -% HIJING+PYHIA Pb+Pb Data Increasing jet energy stretches peak out 5 75 < E < GeV Centrality -% < E < 5 GeV Centrality -% < E < 5 GeV Centrality -% -% 5 75 < E < GeV Centrality 6-8% 5 < E < 5 GeV Centrality 6-8% < E < 5 GeV Centrality 6-8% Peak at low values of AJ restored in peripheral collisions 6-8% A J 7

18 Summary Di-jet asymmetry indicates significant distortion in momentum balance contained in jets that increases with centrality, nearly flat away from peak RCP measurements support the interpretation that asymmetry is caused by significant suppression of high energy jets Small variation between results of different cone sizes shows little room for suppression coming from out-of-cone energy loss j and z distributions provide complementary result indicating that the suppression mechanism does not modify longitudinal or transverse distributions of charged particle jet fragments as would be expected with radiation-dominated energy loss scenario Results not obviously consistent with radiation-dominated energy loss except possibly at large angles 8

19 Supporting Slides

20 Jet Spectra R=. dn jet ] - [GeV de evt N ALAS Preliminary Centrality - % - % - % - % -5 % 5-6 % 6-8 % dn jet ] - [GeV de evt N coll N ALAS Preliminary Centrality - % - % 5-6 % 6-8 % Pb+Pb - = 7 µ b R =. =.76 ev E [GeV] - - Pb+Pb - = 7 µ b R =. =.76 ev E [GeV]

21 Jet Fragments: Longitudinal Structure R=. Ratio of D(z) Ratio of D(z) (-%)/(-8%).8 (-%)/(-8%) R =. R =. Pb+Pb =.76 ev - =7 µb z z Pb+Pb =.76 ev - =7 µb z (-%)/(-8%).8 (-%)/(-8%) R =. R =. Pb+Pb s =.76 ev NN - =7 µb Pb+Pb s =.76 ev NN - =7 µb z

22 -7 Yields vs Centrality ] X - [GeV 8 6 R =. -5 ] X - [GeV.5 R =. dn de evt N dn de evt N coll N < E < 5 GeV coll N.5 5 < E < 75 GeV -7 ] X - [GeV dn de evt N coll N Pb+Pb - = 7 µ b 5 < E < 5 GeV =.76 ev -6 ] X - [GeV dn de evt N -7 ] X.5 - coll N Pb+Pb - = 7 µ b 75 < E < GeV =.76 ev -7 ] X Centrality [%] 6 [GeV - [GeV dn de evt N coll N.5 5 < E < GeV Centrality [%] dn de evt N coll N < E < 5 GeV Centrality [%]

23 Jet Performance: Resolution from MC σ(δ E /E ) σ(δ E /E ) σ(δ E /E ) Centrality -% R =. R = E [GeV] E [GeV] Centrality -% Centrality 6-8% E [GeV] σ(δ E /E ) σ(δ E /E ) Centrality -% ALAS Simulation E [GeV]... Centrality -5% E [GeV] E E = σ(δ E /E ) σ(δ E /E ) Centrality -% E [GeV]... E truth Centrality 5-6% E [GeV] E reco E truth E truth E reco E truth

24 Jet Resolution: Background Fluctuations Standard deviation of 7x7 towers E [MeV] 8 6 η <.8 7x7 towers Pb+Pb data s MC NN =.76 ev Rescaled MC FCal ΣE [ev]

25 ALAS Detector: Inner Detector Jets in heavy ion collisions R SC Pixels 5