Particle production in nuclear collisions over a broad centrality range Aneta Iordanova University of Illinois at Chicago for the PHOBOS collaboration

2c2cr, Prague, 8 Sept 2005Aneta Iordanova Phobos Collaboration Burak Alver, Birger Back, Mark Baker, Maarten Ballintijn, Donald Barton, Russell Betts, Richard Bindel, Wit Busza (Spokesperson), Zhengwei Chai, Vasundhara Chetluru, Edmundo García, Tomasz Gburek, Kristjan Gulbrandsen, Clive Halliwell, Joshua Hamblen, Ian Harnarine, Conor Henderson, David Hofman, Richard Hollis, Roman Hołyński, Burt Holzman, Aneta Iordanova, Jay Kane, Piotr Kulinich, Chia Ming Kuo, Wei Li, Willis Lin, Constantin Loizides, Steven Manly, Alice Mignerey, Gerrit van Nieuwenhuizen, Rachid Nouicer, Andrzej Olszewski, Robert Pak, Corey Reed, Eric Richardson, Christof Roland, Gunther Roland, Joe Sagerer, Iouri Sedykh, Chadd Smith, Maciej Stankiewicz, Peter Steinberg, George Stephans, Andrei Sukhanov, Artur Szostak, Marguerite Belt Tonjes, Adam Trzupek, Sergei Vaurynovich, Robin Verdier, Gábor Veres, Peter Walters, Edward Wenger, Donald Willhelm, Frank Wolfs, Barbara Wosiek, Krzysztof Woźniak, Shaun Wyngaardt, 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

3c2cr, Prague, 8 Sept 2005Aneta Iordanova

4c2cr, Prague, 8 Sept 2005Aneta Iordanova PHOBOS detector

5c2cr, Prague, 8 Sept 2005Aneta Iordanova Au+Au √s NN = 19.6, 56, 62.4, 130 and 200 GeV √s NN = 19.6, 56, 62.4, 130 and 200 GeVCu+Cu √s NN = 22.4, 62.4 and 200 GeV √s NN = 22.4, 62.4 and 200 GeVd+Au √s NN = 200 GeV √s NN = 200 GeV Au+Au d+Au Au+Au Cu+Cu All-inclusive count, Including tests Relativistic heavy ion collisions at PHOBOS

6c2cr, Prague, 8 Sept 2005Aneta Iordanova We have created a state of matter at RHIC with high energy-density, that is nearly net-baryon free and interacting very strongly. Transition to this high-energy state of matter does not create abrupt changes in observables at RHIC energies. The data exhibit many “simple” scaling behaviors. The data exhibit a remarkable factorization of collision energy and geometry. “White Paper”: nucl-ex/ , Nucl. Phys. A 757, 28 PHOBOS observations

7c2cr, Prague, 8 Sept 2005Aneta Iordanova Outline Description of a heavy ion collision Control parameters Control parametersEnergy Colliding system size (centrality N part ) Species Global features of charged particle production (observables) dN ch /d  shapes (particle distributions in  ) dN ch /d  shapes (particle distributions in  ) Particle distributions in p T Particle distributions in p T Observed scaling and factorization Limiting fragmentation (Extended longitudinal scaling) Mid-rapidity factorization

8c2cr, Prague, 8 Sept 2005Aneta Iordanova A description of a collision Two high energy nuclei collide Produce particles with different properties emission angle (  distribution) emission angle (  distribution) momentum (p T distribution) momentum (p T distribution) species (PID spectra and particle ratios) species (PID spectra and particle ratios) We can define some terminology for the collision for the collision for the particles for the particles

9c2cr, Prague, 8 Sept 2005Aneta Iordanova Centrality of the collision Centrality Describes the interacting volume of the collision Describes the interacting volume of the collision Simplest case: p+A collisions Consider the “A” as a continuous sphere of matter Consider the “A” as a continuous sphere of matter Proton passes through a medium density region Proton passes through a high density region Proton passes through a very low density region definition for collisions

10c2cr, Prague, 8 Sept 2005Aneta Iordanova Centrality of the collision The nucleus is not continuous matter Discrete nucleons Have different density of nucleons dependent on distance from center Use this as a basis to classify collisions in A+A collisions definition for collisions

11c2cr, Prague, 8 Sept 2005Aneta Iordanova Impact parameter, b distance between colliding nuclei, perpendicular to the beam-axis distance between colliding nuclei, perpendicular to the beam-axis large b are classified as PERIPHERAL large b are classified as PERIPHERAL small b are classified as CENTRAL small b are classified as CENTRAL b Classifying centrality: Impact parameter CANNOT measure impact parameter: MUST derive it from models and connect with a measured quantity from data definition for collisions

12c2cr, Prague, 8 Sept 2005Aneta Iordanova Participants (N part ) → nucleons inside overlap volume Spectators → nucleons outside overlap volume characteristic nuclear size The geometrical overlap number of interacting nucleons (participants) number of interacting nucleons (participants) (3d) nucleus distribution based on measurement N part /2 ~ A Classifying centrality: geometry of the collision/system size Number of participants = Number of wounded nucleons definition for collisions

13c2cr, Prague, 8 Sept 2005Aneta Iordanova Measuring centrality: A+A collisions Data Divide data based on the number of particles produced in a region of phase space Divide data based on the number of particles produced in a region of phase space This signal is from the “paddle” counters 3.2<|η|< <|η|<4.5 definition for collisions

14c2cr, Prague, 8 Sept 2005Aneta Iordanova Measuring centrality: A+A collisions Monte-Carlo simulations simulate the same signal from the paddles simulate the same signal from the paddles correlate this to the (known) number of participants correlate this to the (known) number of participants find the average N part (wounded nucleons) associated with a given fraction of cross section find the average N part (wounded nucleons) associated with a given fraction of cross section MC definition for collisions

15c2cr, Prague, 8 Sept 2005Aneta Iordanova Centrality datasets d+AuCu+CuAu+Au Lowest N part Highest N part definition for collisions

16c2cr, Prague, 8 Sept 2005Aneta Iordanova Angular distribution of produced charged particles Particles are produced predominantly at small angles they all go down they all go down the interaction axis Particles can be assigned an “angle” − pseudorapidity (η) similar to the characteristic velocity − rapidity (y) similar to the characteristic velocity − rapidity (y) PHOBOS Data η = ±1 η = ±3 m«p, then E≈p definition for particles

17c2cr, Prague, 8 Sept 2005Aneta Iordanova θ and η distributions η is logarithmic! ~45 0 → η=1 ~45 0 → η=1 This is a little confusing at first sight it seems that ALL the particles are at 90 0 it seems that ALL the particles are at 90 0 they are not! they are not! PHOBOS cannot measure y over whole region – requires particle identification η = ±1 η = ±3 definition for particles

Data comparison of Au+Au and Cu+Cu

19c2cr, Prague, 8 Sept 2005Aneta Iordanova Charged hadron dN ch /d  distribution centrality Au+Au : PRL 91, (2003) 19.6 GeV62.4 GeV130 GeV 200 GeV preliminary dN/d  distribution With energy: Height increases Height increases Width increases (in  space) Width increases (in  space) With centrality(0-50% central): Height increases Height increases Au+Au data

20c2cr, Prague, 8 Sept 2005Aneta Iordanova Mid-rapidity dN ch /d  versus energy Central Au+Au collisions produce higher multiplicity than p+p collisions per colliding pair produce higher multiplicity than p+p collisions per colliding pair slower particle production versus energy than expected slower particle production versus energy than expected 6% central collisions, N part ~ 340 dN/d  distribution

21c2cr, Prague, 8 Sept 2005Aneta Iordanova d+Au Charged hadron dN ch /d  distribution centrality Au+Au : PRL 91, (2003) d+Au : arxiv nucl-ex/ GeV62.4 GeV130 GeV 200 GeV preliminary dN/d  distribution With species: Same systematic trends Same systematic trends preliminary Au+Au data Cu+Cu data

22c2cr, Prague, 8 Sept 2005Aneta Iordanova Comparison of Au+Au and Cu+Cu For the same collision system size (N part ) dN ch /d  shape is very similar for central Cu+Cu with mid-central Au+Au collisions central Cu+Cu with mid-central Au+Au collisions mid-central Cu+Cu with peripheral Au+Au collisions mid-central Cu+Cu with peripheral Au+Au collisions 200 GeV Cu+Cu Preliminary 3-6%, N part = 100 central Au+Au 35-40%, N part = 99 dN/d  distribution Cu+Cu Preliminary 15-25%, N part = 61 Au+Au 45-55%, N part = 56 midcentral

23c2cr, Prague, 8 Sept 2005Aneta Iordanova Comparison of Au+Au, d+Au and p+p collisions at 200 GeV Scale by N part /2 allows for a direct comparison to p+p allows for a direct comparison to p+p i.e. normalized to 1 participating pair i.e. normalized to 1 participating pair Multiplicity obtained at the same center-of-mass energy is similar in p+p and d+Au Multiplicity is larger for Au+Au collisions at the same energy All at √s = 200 GeV dN/d  distribution d+Au : PRL 93, (2004)

24c2cr, Prague, 8 Sept 2005Aneta Iordanova Comparison of d+Au and p+p More detailed comparison of d+Au and p+p shows more peripheral collisions tend toward p+p shape more peripheral collisions tend toward p+p shape limiting slopes in d+Au and p+p are the same, but offset limiting slopes in d+Au and p+p are the same, but offset dN/d  distribution 2 dN ch N part dη η PHOBOS d+Au and p+p Data at √s = 200 GeV d+Au : arxiv nucl-ex/ p+p : 2004 J.Phys.G 30 S

scaling limiting fragmentation

26c2cr, Prague, 8 Sept 2005Aneta Iordanova Limiting Fragmentation Term for particles produced at high η. particles produced close to the beam rapidity of one of the colliding nuclei particles produced close to the beam rapidity of one of the colliding nuclei same “Limiting” distribution of charged- particles in this region independent of energy same “Limiting” distribution of charged- particles in this region independent of energy Center-of-mass System limiting fragmentation

27c2cr, Prague, 8 Sept 2005Aneta Iordanova Au+Au dN ch /dη vs η limiting fragmentation Scaling by N part /2 Distributions are relatively the same Distributions are relatively the same ~ same for each energy for 0-6% central ~ same for each energy for 0-6% central y beam grows with energy Shift each η by y beam (Prelim) y beam ~3.05 y beam ~4.20 y beam ~4.90 y beam ~5.37

28c2cr, Prague, 8 Sept 2005Aneta Iordanova Au+Au dN ch /dη vs η–y beam Region of ‘overlap’ for each energy for each energy close to rapidity of one projectile close to rapidity of one projectileExpected narrow fragmentation region narrow fragmentation regionObserved extended longitudinal scaling extended longitudinal scaling Fragmentation Region grows with energy grows with energy limiting fragmentation

29c2cr, Prague, 8 Sept 2005Aneta Iordanova Extended longitudinal scaling is also observed to hold for Cu+Cu data Cu+Cu preliminary Cu+Cu dN ch /dη vs η–y beam limiting fragmentation

30c2cr, Prague, 8 Sept 2005Aneta Iordanova Centrality Dependence Au+Au Centrality data divided into distinct multiplicity bins Central 0-6% N part ~ 340 Peripheral 35-40% N part ~ 100 limiting fragmentation

31c2cr, Prague, 8 Sept 2005Aneta Iordanova Centrality + Energy Dependence Au+Au Observations reduction at η~0 increase at η>y beam important observation for the total yield limiting fragmentation

32c2cr, Prague, 8 Sept 2005Aneta Iordanova Smaller systems This observation is not peculiar to Au+Au first observed in p+p first observed in p+p also in d+Au and new Cu+Cu also in d+Au and new Cu+Cu All exhibit similar features limiting fragmentation

33c2cr, Prague, 8 Sept 2005Aneta Iordanova p+p Collection of many data over a factor of ~50 in √s reasonable Limiting Fragmentation agreement! η’ = η-y beam CDF (900) → Phys.Rev D41 7(1990) 2332 UA5 (200,546) → Z.Phys.C 43 (1989) 1 ISR (23.6,45.2) → Nucl.Phys B (1977) limiting fragmentation

34c2cr, Prague, 8 Sept 2005Aneta Iordanova d+Au 50-70% Centrality, PHOBOS data d+Au data from nucl-ex/ p+Em referenced therein limiting fragmentation

35c2cr, Prague, 8 Sept 2005Aneta Iordanova d+Au Limiting fragmentation in both projectile rest frame projectile rest frame target rest frame target rest frame Centrality/system size dependence systematic comparison with lower energy data systematic comparison with lower energy data Limiting fragmentation in each centrality bin Limiting fragmentation in each centrality bin limiting fragmentation

scaling universality of total charged

37c2cr, Prague, 8 Sept 2005Aneta Iordanova “Universality” of total charge in e + +e -, p+p and A+A collisions Total charged particles produced scale differently in A+A and p+p collisions arxiv nucl-ex/ , submitted to PRL

38c2cr, Prague, 8 Sept 2005Aneta Iordanova e + +e - scale as p+p after consideration of the “leading hadron” reduce √s by ½ reduce √s by ½ “Universality” of total charge in e + +e -, p+p and A+A collisions

39c2cr, Prague, 8 Sept 2005Aneta Iordanova In this regime all data are consistent Can examine this more closely by dividing by e + +e - e + +e - fit e + +e - fit “Universality” of total charge in e + +e -, p+p and A+A collisions

40c2cr, Prague, 8 Sept 2005Aneta Iordanova All data are consistent with e + +e - Scaling breaks down for low energy A+A “Universality” of total charge in e + +e -, p+p and A+A collisions

factorization at midrapidity in energy and centrality

42c2cr, Prague, 8 Sept 2005Aneta Iordanova Mid-rapidity dN ch /d  Information about the created matter →energy density Particle production in A+A: interplay between collision centrality (geometry) and collision energy? balance of binary/soft processes balance of binary/soft processes Mid-rapidity factorization

43c2cr, Prague, 8 Sept 2005Aneta Iordanova Au+Au PHOBOS Multiplicity in Au+Au collisions (dN ch /d  ) per participant pair (N part /2) higher than the corresponding values for inelastic p+p Percentile cross-section 0-50% for 200, 130 and 62.4 GeV 0-50% for 200, 130 and 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) Mid-rapidity factorization Measured pseudorapidity density per participant pair as a function of N part

44c2cr, Prague, 8 Sept 2005Aneta Iordanova Divide by the corresponding p+p multiplicity Compare particle production in Au+Au to p+p collisions multiplicity per pair is ~40% higher multiplicity per pair 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) Mid-rapidity factorization

45c2cr, Prague, 8 Sept 2005Aneta Iordanova Participant 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 multiplicity from coherent “soft” collisions multiplicity from coherent “soft” collisions dN ch(Au+Au) /d  = dN ch(p+p) /d  x N part /2 Participants − in overlap region “Wounded Nucleon Model”

46c2cr, Prague, 8 Sept 2005Aneta Iordanova Collision 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 multiplicity from binary collisions multiplicity from binary collisions dN ch(Au+Au) /d  = dN ch(p+p) /d  x N coll projectile “nucleon”

47c2cr, Prague, 8 Sept 2005Aneta Iordanova Divide by the corresponding p+p Remarkable similarities between all 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 Data is dominated by N part scaling closer to N part than N coll closer to N part than N coll Mid-rapidity factorization

48c2cr, Prague, 8 Sept 2005Aneta Iordanova Midrapidity Ratios Define a ratio 200/19.6 for midrapidity data similar for other energies similar for other energies Mid-rapidity factorization

49c2cr, Prague, 8 Sept 2005Aneta Iordanova Midrapidity Ratios 200/ / /130 Cu+Cu preliminary Au+Au PHOBOS HIJING Saturation Data ratio 200/X no centrality (geometry) dependence no centrality (geometry) dependenceModels HIJING HIJING disagrees with data Saturation Model (KLN) Saturation Model (KLN) flat centrality dependence as in data Mid-rapidity factorization KLN 200/19.6 Phys.Rev.C (R) /62.4 Preliminary 200/130 Phys.Rev.C (R) 2002 HIJING:Comput.Phys.Commun. 83 (1994) 307,v 1.35 KLN: Phys.Lett.B (2001) & arXiv:hep-ph/

factorization at midrapidity versus p T

51c2cr, Prague, 8 Sept 2005Aneta Iordanova Transverse Momentum Factorization So far discussed factorization of bulk matter ~p T <1GeV/c ~p T <1GeV/c Can look at differential factorization for p T bins p T factorization

52c2cr, Prague, 8 Sept 2005Aneta Iordanova Ratio of yield for 200 and 62.4 GeV is centrality independent for all measured p T bins Transverse Momentum Factorization p T factorization Data from PRL 94, (2005)

factorization versus η

54c2cr, Prague, 8 Sept 2005Aneta Iordanova Factorization of Extended Longitudinal Scaling Peripheral Central dN/d  factorization

55c2cr, Prague, 8 Sept 2005Aneta Iordanova Factorization of Extended Longitudinal Scaling Peripheral Central dN/d  factorization Ratio Peripheral / Central

56c2cr, Prague, 8 Sept 2005Aneta Iordanova Summary PHOBOS has measured multiplicity for large systematic dataset large systematic dataset -5.4<η< <η<5.4 2 to 360 participants 2 to 360 participants √s = 19.6 to 200 GeV √s = 19.6 to 200 GeV Au+Au and Cu+Cu data are similar Scaling and factorization properties are found in the Au+Au data extended longitudinal scaling extended longitudinal scaling mid-rapidity factorization in bulk yield and versus p T mid-rapidity factorization in bulk yield and versus p T seem to persist into the Cu+Cu collision system seem to persist into the Cu+Cu collision system Observed factorization in energy and centrality is seen in multiplicity and versus p T implies that this is set at an early stage of the collision and is connected with the geometry of the collision implies that this is set at an early stage of the collision and is connected with the geometry of the collision

The End