Properties of charged-particle production at mid-rapidity for Au+Au collisions at RHIC Aneta Iordanova University of Illinois at Chicago

Outline Overview Analysis Technique Centrality determination Centrality determination Tracklet reconstruction method Tracklet reconstruction methodResultsConclusions

Overview This thesis presents results on the number of charged particles (multiplicity) produced in Au+Au collisions √s NN = 200, 19.6 and 62.4 GeV √s NN = 200, 19.6 and 62.4 GeV Data taken at RHIC with PHOBOS “Vertex Tracklets” multiplicity analysis

Relativistic Heavy Ion Collider Four dedicated experiments BRAHMS BRAHMS PHOBOS PHOBOS PHENIX PHENIX STAR STAR Au+Au collisions at four √ s NN 19.6, 62.4, 130, 200 GeV 19.6, 62.4, 130, 200 GeV

High density (  ), equilibrated state (   ~1 fm/c) e: average energy per particle dN/dy: number of all produced particles close to the interaction point Dense Matter at RHIC

Study the dense matter at RHIC N ch (multiplicity) can be used to estimate  Account for the neutral particles Account for the neutral particles Starting point in search for QGP Vary the √ s NN  Bj >  QGP for all measurements in Au+Au  Bj >  QGP for all measurements in Au+Au Learn more how the particles are produced in Au+Au collisions Compare with simpler collision systems (p+p) Compare with simpler collision systems (p+p) Concentrate on the particle production evolution with centrality Concentrate on the particle production evolution with centrality  QGP  QGP ~1GeV/fm 3,  nucl ~0.15GeV/fm 3

4  Multiplicity Array Octagon, Vertex and Ring Counters Mid-rapidity Spectrometer TOF wall for high momentum PID Triggering Scintillator Paddle Counters Zero Degree Calorimeter (ZDC) Octagon Detector Vertex Detector

ZDC N ZDC P Au x z PP PN Positive Paddle Counter 16 scintillator slats Collision Selection Main Trigger Coincidence of the Paddle Counters (  t = 10ns) Coincidence of the Paddle Counters (  t = 10ns) Timing signals of Paddles and ZDC reject background Timing signals of Paddles and ZDC reject background Trigger sensitive to ~97% of the inelastic cross-section for 200 GeV Trigger sensitive to ~97% of the inelastic cross-section for 200 GeV Negative Zero Degree Calorimeter  t[ns]  z[m]

Pseudorapidity Z,  Y X  Charged particle pseudorapidity density Mid-rapidity  ~3  ~1

Centrality Determination

peripheral Gold (Au), A=197 Participants (N part ) Spectators (A-N part ) b b = impact parameter N part /2 → “Participant pairs” Collision Classification central b b R~7fm

Centrality Determination at 200 GeV Summed Paddle Signals ‘Paddle Mean’ ‘Paddle Mean’ Zero Degree Calorimeter Energy (at ±18 m) Observe monotonic anti- correlation of Paddle Mean with ZDC Energy (number of spectator neutrons) for central data Paddle Counters at 3.2 < |η| < 4.5 Central 50% ZDC Sum (a.u.) Paddle Mean (a.u.) 200GeV Au+Au

Centrality Determination at 200 GeV Paddle Mean also monotonic with N part from MC simulations (HIJING+GEANT) Glauber model used to calculate N Glauber model used to calculate N part With this information Estimate the trigger efficiency (from Data and MC) Estimate the trigger efficiency (from Data and MC) Divide data into bins of inelastic cross-section Divide data into bins of inelastic cross-section N part Paddle Mean (a.u.) Entries

Centrality Determination at 200 and 19.6 GeV Measured pseudorapidity distributions Paddles cover relatively ‘different’ region in  for 19.6 than 200 GeV Results for top 25% of the inelastic cross-section

Centrality Determination at 19.6 GeV Summed Paddle Signal No longer monotonic No longer monotonic Need a new signal Summed charge deposited in Octagon Summed charge deposited in OctagonMonotonic with spectators (Data) with spectators (Data) with N part (MC) with N part (MC) ZDC Sum (a.u.) Paddle Mean (a.u.) EOct (Summed Charge in Octagon) EOct (a.u.)

Centrality Measures Centrality defined at 200 and 19.6 GeV Not exactly the same (no choice) Not exactly the same (no choice) 200 GeV → away from mid-rapidity 200 GeV → away from mid-rapidity 19.6 GeV → at mid-rapidity 19.6 GeV → at mid-rapidity Does this matter? Had to check Had to check From our d+Au experience this could be critical From our d+Au experience this could be critical Results for top 25% of the inelastic cross-section

Matched Centrality Measures Select the “same” regions at 200 and 19.6 GeV Now have two centrality methods at each energy One at mid-rapidity One at mid-rapidity One away from mid-rapidity One away from mid-rapidity Mechanism for comparing ‘like’ regions to see systematic effects Regions are ‘matched’ according to the ratio of beam rapidities (a) with (c) (b) with (d)

Centrality determination at 62.4 GeV Used the developed techniques Two methods Two methods One at mid-rapidity One at mid-rapidity One away from mid- rapidity One away from mid- rapidity To compare with 200 and 19.6 GeV To compare with 200 and 19.6 GeV 62.4 GeV 62.4 GeV not a measurement!

Multiplicity measurement at mid-rapidity (|  |<1)

Vertex Detector Top Bottom 62.1mm 50.4mm Z,  Beam pipe  1 channel Y X 8192 silicon pads Outer Layer: 2 × 2048 pads, 0.47mm × 24.1mm Inner Layer: 2 × 2048 pads, 0.47mm × 12.0mm

First Pass Second Pass  Seed Layer Search Layer Reconstructed Vertex hit  Search,  Search Determine  Seed,  Seed |  | = |  Search –  Seed | < 0.3 |  | =|  Search –  Seed | < 0.1 smallest  combination. Tracklets with a common hit in the “Search Layer” smallest  combination. Top Vertex Tracklet Reconstruction

Combinatorial Background Reconstructed tracklets have a difference in angle  close to zero Others are random combinations (combinatorial background )  = N bg_tracklets /N reconstructed  =0.76  =0.76 Data Tracklets/Background for 80 to 100 Hits in Outer Vertex Layer, 19.6 GeV Signal:Background = 3:1

Combinatorial Background Combinatorial background: formed by rotating Inner Vertex Detector layers about the beam pipe formed by rotating Inner Vertex Detector layers about the beam pipe Z,  Beam pipe 

Combinatorial Background Reconstructed tracklets have a difference in angle  close to zero Others are random combinations (combinatorial background )  = N bg_tracklets /N reconstructed  =0.76  =0.76 Data Tracklets/Background for 80 to 100 Hits in Outer Vertex Layer, 19.6 GeV Signal:Background = 3:1

Acceptance + Efficiency Correction Factor Multiplicity Determination

Acceptance and Efficiency Correction Factor  ’ depends on: Z-vertex position Z-vertex position multiplicity in detector (hits) multiplicity in detector (hits) Hits in Outer Vertex Layer / GeV 62.4 GeV 200 GeV  ’ corrects for: azimuthal acceptance of the detector azimuthal acceptance of the detector tracklet reconstruction efficiency tracklet reconstruction efficiency secondary decays secondary decays Includes combinatorial background Includes combinatorial background

Results

Charged particle multiplicity as a function of centrality Different results from the centrality determination Centrality determined at mid-rapidity Centrality determined at mid-rapidity Larger for central events Larger dN ch /d  for central events Similar dependence with centrality Important when compare to other experiments Same effect observed for 62.4 and 200 GeV results Same effect observed for 62.4 and 200 GeV results 19.6 GeV Mid-rapidity method Away from mid-rapidity

Dividing by /2 Minimizing the centrality determination effects Minimizing the centrality determination effects Direct comparison to multiplicity obtained at the same center-of- mass energy from other collision systems Two centrality methods 19.6 GeV

Measured pseudorapidity density per participant pair as a function of Measured pseudorapidity density per participant pair as a function of Multiplicity in Au+Au collisions (dN ch /d  ) per participant pair( /2) higher than the corresponding values for inelastic Percentile cross-section 0-50% for 200, 62.4 GeV 0-50% for 200, 62.4 GeV 0-40% for 19.6 GeV 0-40% for 19.6 GeV 200 GeV (UA5) 90 % C.L and 62.4 GeV (ISR) p(p)+p

Divide by the corresponding p+p multiplicity Compare particle production in Au+Au to collisions collisions Multiplicity/ /2 is ~40% higher Multiplicity/ /2 is ~40% higher Remarkable similarities between the data sets Similar centrality dependence Similar centrality dependence Observed level above value of 1 (participant scaling) Observed level above value of 1 (participant scaling) p(p)+p

N part /2 ~ A “Participants” Centrality Scaling Participant scaling Multiplicity in Au+Au proportional to number of participating pairs (N part /2) Multiplicity in Au+Au proportional to number of participating pairs (N part /2) Every pair is equivalent to 1 p+p collision, produces the same number of particles as p+p at this energy Every pair is equivalent to 1 p+p collision, produces the same number of particles as p+p at this energy dN ch(Au+Au) /d  = dN ch(p+p) /d  x N part /2

Centrality Scaling Collision scaling Multiplicity proportional to N coll Multiplicity proportional to N coll Each collision contributes with a multiplicity of 1 p+p collision Each collision contributes with a multiplicity of 1 p+p collision L~A 1/3 N coll = # of NN collisions: ~A 4/3 “Collisions” dN ch(Au+Au) /d  = dN ch(p+p) /d  x N coll

Divide by the corresponding p+p Remarkable similarities between the two data sets Similar N part dependence Similar N part dependence Observed level above participant scaling depends on the p+p reference Observed level above participant scaling depends on the p+p reference

Ratio of two data sets – systematic errors Most of the systematic errors on the individual measurements at two energies will cancel in the ratio Analyses performed with the same method Analyses performed with the same method Detector Detector Centrality determination Centrality determination Percentile cross-section used in ratio top 40% (R 200/19.6 ) top 40% (R 200/19.6 ) top 50% (R 200/62.4 ) top 50% (R 200/62.4 ) Errors are estimated as 1- .

Ratio of two data sets – systematic errors R  Most of geometry/efficiency effects cancel in the ratio Most of geometry/efficiency effects cancel in the ratio Contribution from secondary decays Contribution from secondary decays R   is found to be the same for Data/MC for the two data sets  is found to be the same for Data/MC for the two data sets Uncertainty from measured y- beam position Uncertainty from measured y- beam position R Npart Nucleon-nucleon inelastic cross-section Nucleon-nucleon inelastic cross-section MC simulations of the detector response MC simulations of the detector response Glauber model calculations Glauber model calculations RRRR R  19.6 R  200 R Npart 2%0.4%0.4% 200 and 19.6 GeV

Ratio of two data sets – systematic and statistical errors R Nrec Counting statistics Counting statistics Uncertainty in trigger efficiency (centrality bin position) Uncertainty in trigger efficiency (centrality bin position) central events 0% mid-central events 6% Final 1-  systematic and statistical error Centrality dependent Centrality dependent central events 3% mid-central events 7% RRRR R  19.6 R  200 R Npart R Nrec R central 2%0.4%0.4%2.2%3.0% 200 and 19.6 GeV

Final 1-  systematic and statistical error on the ratio R Nrec 200 and 62.4 GeV Counting statistics Counting statistics Uncertainty in trigger efficiency (centrality bin position) Uncertainty in trigger efficiency (centrality bin position) central events 0% mid-central events 3% Final 1-  systematic and statistical error Centrality dependent Centrality dependent central events 3% mid-central events 4% 200 and 19.6 GeV 200 and 62.4 GeV Major difference due to the way the efficiency is estimated

Ratio for the data sets Data ratio 200/19.6 for centrality derived at mid- rapidity No centrality (geometry) dependence No centrality (geometry) dependence R = 2.03 ± 0.02 ± 0.05 (simple scale-factor between 19.6 and 200 GeV) R = 2.03 ± 0.02 ± 0.05 (simple scale-factor between 19.6 and 200 GeV) Data ratio 200/62.4 for centrality derived at mid- rapidity No centrality dependence No centrality dependence R = 1.39 ± 0.01 ± 0.02 R = 1.39 ± 0.01 ±  errors R 200/19.6 R 200/62.4

Models for particle production Multiplicity in Au+Au can be assumed to arise from two types of processes “hard+soft” HIJING HIJING Cut off scale p T ~2 GeV Above this scale processes calculated with QCD  hard grows with energy, will result in enhanced multiplicity KLN saturation model KLN saturation model Initial state of fast moving nuclei Number of gluons is large (QCD self interaction) At some momentum scale this number is expected to saturate - limits multiplicity Charged particle multiplicity grows slowly with centrality

Ratio for the data sets Models Hijing Hijing Increasing trend at mid- rapidity with centrality Saturation Model (KLN) Saturation Model (KLN) flat centrality dependence as in data 1-  errors

Other ‘Geometry Scaling’ observations in PHOBOS Multiplicity 200/130 GeV mid- rapidity ratio 200/130 GeV mid- rapidity ratio Phys.Rev.C (R) (2002) 19.6 to 200 GeV N ch / 19.6 to 200 GeV N ch / Plot from QM 2002 talks

Conclusions Measured charged-particle pseudorapidity density at mid-rapidity for Au+Au collisions at 200, 19.6 and 62.4 GeV Centrality, derived from different  -regions for each of the Centrality, derived from different  -regions for each of the Au+Au collision energies, yield consistent results Au+Au collision energies, yield consistent results An increase in particle production per participant pair for Au+Au compared to the corresponding values for p+p collisions An increase in particle production per participant pair for Au+Au compared to the corresponding values for p+p collisions The ratio of the measured yields for the top 40% in the cross section gives a simple scaling factor between two energies. The ratio of the measured yields for the top 40% in the cross section gives a simple scaling factor between two energies.

End

Other ‘Geometry Scaling’ observations in PHOBOS Charged hadron p T spectra Ratio of yield for 200 and 62.4 GeV is centrality independent for all measured p T bins Ratio of yield for 200 and 62.4 GeV is centrality independent for all measured p T bins

Other ‘Geometry Scaling’ observations in Charged hadron p T spectra Ratio of yield for 200 and 62.4 GeV is centrality independent for all measured p T bins Ratio of yield for 200 and 62.4 GeV is centrality independent for all measured p T bins

Other ‘Geometry Scaling’ observations in Charged hadron p T spectra Ratio of yield for 200 and 62.4 GeV is centrality independent for all measured p T bins Ratio of yield for 200 and 62.4 GeV is centrality independent for all measured p T bins

Collaboration Collaboration Birger Back, Mark Baker, Maarten Ballintijn, Donald Barton, Russell Betts, Abigail Bickley, Richard Bindel, Wit Busza (Spokesperson), Alan Carroll, Zhengwei Chai, Patrick Decowski, Edmundo García, Tomasz Gburek, Nigel George, Kristjan Gulbrandsen, Clive Halliwell, Joshua Hamblen, Adam Harrington, Michael Hauer, Conor Henderson, David Hofman, Richard Hollis, Roman Hołyński, Burt Holzman, Aneta Iordanova, Jay Kane, Nazim Khan, Piotr Kulinich, Chia Ming Kuo, Willis Lin, Steven Manly, Alice Mignerey, Gerrit van Nieuwenhuizen, Rachid Nouicer, Andrzej Olszewski, Robert Pak, Inkyu Park, Heinz Pernegger, Corey Reed, Michael Reuter, Christof Roland, Gunther Roland, Joe Sagerer, Helen Seals, Iouri Sedykh, Wojtek Skulski, Chadd Smith, Maciej Stankiewicz, Peter Steinberg, George Stephans, Andrei Sukhanov, Marguerite Belt Tonjes, Adam Trzupek, Carla Vale, Sergei Vaurynovich, Robin Verdier, Gábor Veres, Peter Walters, Edward Wenger, Frank Wolfs, Barbara Wosiek, Krzysztof Woźniak, Alan Wuosmaa, Bolek Wysłouch ARGONNE NATIONAL LABORATORYBROOKHAVEN NATIONAL LABORATORY INSTITUTE OF NUCLEAR PHYSICS PAN, KRAKOWMASSACHUSETTS INSTITUTE OF TECHNOLOGY NATIONAL CENTRAL UNIVERSITY, TAIWANUNIVERSITY OF ILLINOIS AT CHICAGO UNIVERSITY OF MARYLANDUNIVERSITY OF ROCHESTER

Comparison of Centrality Methods mid-rapidity / away from mid-rapidity

Comparison of Centrality Methods fixed cross-section / fixed N part

Published(200) and Preliminary results(19.6) /paper

Backup Centrality distributions

Other experiments

Other ‘Geometry Scaling’ observations in Charged hadron p T spectra R cp 200 and 62.4 GeV R cp 200 and 62.4 GeV Normalized by N coll Normalized by N part /2 62.4GeV 200GeV Au+Au spectra p+p spectra

Estimate the energy density Absolute maximum: Total available energy:E tot = A x energy ~350 x 100 = GeV Volume of instant overlap: Equilibrated state after  ~1 fm/s Not all collisions will reach same  ! R2R2 cc  max = GeV E tot = N particles x energy

Glauber Monte Carlo Au+Au 200 GeV 62.4 GeV 19.6 GeV Centrality Scaling Collision scaling Multiplicity proportional to N coll Multiplicity proportional to N coll Each collision contributes with a multiplicity of 1 p+p collision Each collision contributes with a multiplicity of 1 p+p collision

Ratio for the data sets Data ratio Au+Au1 (fixed fraction of cross-section) Au+Au1 (fixed fraction of cross-section) No centrality dependence R = 2.03 ± 0.02 ± 0.05 Au+Au2 (fixed ) Au+Au2 (fixed ) No centrality dependence 1-  errors

inelastic data points inelastic data points 200 GeV Measured by UA5 Measured by UA5 Z.Phys C33 1 (1986) dN/d  = 2.29±0.08 dN/d  = 2.29± GeV Measured by ISR Measured by ISR Nucl.Phys. B (1977) dN/d  = 1.90± GeV Extrapolated from ISR data Extrapolated from ISR data Nucl.Phys. B (1977) dN/d  = 1.27±0.13 dN/d  = 1.27±0.13