Experimental results from forward rapidities at RHIC Bedanga

Experimental results from forward rapidities at RHIC Bedanga Mohanty Variable Energy Cyclotron Centre, Kolkata Outline : Ø Introduction Ø Particle production Ø Longitudinal Scaling Ø Nuclear stopping Ø Color Glass Condensate ØSummary Bedanga Mohanty QGP MEET 2006, February 5 th – 7 th, Kolkata 1

Introduction What is rapidity ? What is pseudorapidity ? Rapidity as a “relativistic velocity” forward rapidity Midrapidity y>0 y=infinity : y=0 t forward rapidity: Q small Y=0 Y = 1. 5 z Y=2 Y=4 Bedanga Mohanty QGP MEET 2006, February 5 th – 7 th, Kolkata 2

Experimental challenges Bedanga Mohanty QGP MEET 2006, February 5 th – 7 th, Kolkata 3

Why look at forward rapidity ? Observable look different at forward rapidity Rapidity distribution Transverse momentum distribution Bedanga Mohanty Azimuthal distribution Particle ratio QGP MEET 2006, February 5 th – 7 th, Kolkata 4

Physics issues at forward rapidity Ø Particle production -- Collision centrality -- Pseudorapidity distribution -- Transverse momentum spectra Ø Longitudinal Scaling -- System size dependence -- Energy dependence -- Centrality dependence -- Identified particle Ø Baryon production -- Stopping or Transparency -- Energy used for particle production Ø Color Glass Condensate (initial state) -- Nuclear modification factor -- Correlations Bedanga Mohanty Forward Detectors at RHIC STAR photon detector STAR Forward TPC STAR Forward 0 detector STAR Forward meson spectrometer PHENIX stations BRAHMS spectrometers PHOBOS multiplicity array QGP MEET 2006, February 5 th – 7 th, Kolkata 5

Centrality, trigger and forward rapidity All four RHIC experiments have 2 ZDCs The purpose of ZDCs are Ø To detect beam neutrons Ø Measure their total energy (hence multiplicity). Ø The ZDC coincidence is a minimal bias selection Ø The neutron multiplicity is also known to be correlated with event geometry or a of measure collision centrality s. NN/2 b Collisions s. NN/2 Participant Bedanga Mohanty spectator QGP MEET 2006, February 5 th – 7 th, Kolkata 6

Evolution of particle production Midrapidity Forward rapidity Full rapidity Wounded Nucleon “Scaling” But d. N/dh = A Npart + B Ncoll Scaling at forward rapidity different from midrapidity Bedanga Mohanty QGP MEET 2006, February 5 th – 7 th, Kolkata 7

Momentum spectra at forward rapidity Gluon to fragmentation more in KKP than Kretzer At higher rapidity and lower p. T data prefers Kretzer FF than KKP FF Higher rapidity sensitive to g fragmentation PDF : CTEQ 6 Bedanga Mohanty QGP MEET 2006, February 5 th – 7 th, Kolkata 8

Ratio of p. T spectra at forward rapidity Midrapidity Forward rapidity Rd. Au = d 2 N/dp. Td (d+Au) NColl d 2 N/dp Td (p+p) RAA = d 2 N/dp. Td (A+A) NColld 2 N/dp. Td (p+p) At midrapidity, RAA is suppressed and Rd. Au is enhanced for high p. T Bedanga Mohanty QGP MEET 2006, February 5 th – 7 th, Kolkata 9

Ratio of p. T spectra at forward rapidity Midrapidity Forward rapidity d+ Au collisions at 200 Ge. V Cronin-like enhancement at =0 Clear suppression as changes from 0 to 4. 0 Bedanga Mohanty QGP MEET 2006, February 5 th – 7 th, Kolkata 10

Scanning the phase diagram Continuous Critical endpoint T Quark-Gluon Plasma Meson dominated Chiral symmetry restored Hadronic matter Baryon dominated Chiral symmetry broken Nuclei Bedanga Mohanty 1 st order Colour superconductor Neutron stars B QGP MEET 2006, February 5 th – 7 th, Kolkata 11

Scanning the phase diagram Ejiri et al. Fodor-Katz For a given center of mass energy the phase diagram can be explored by varying centrality (temperature) and rapidity (baryon chemical potential) Tch peripheral This temperature is Tch As a representation we show the variation of Tfo with centrality Bedanga Mohanty central Need of higher rapidity measurements QGP MEET 2006, February 5 th – 7 th, Kolkata 12

Scanning the phase diagram Baryon chemical potential increases with rapidity Bedanga Mohanty QGP MEET 2006, February 5 th – 7 th, Kolkata 13

RHIC @ y =3 and SPS radial flow drops by 30% 62. 4 ”SPS”-like hadron chemistry 62. 4 dn/dy drops by a factor of 3 Recreating the matter at SPS in forward rapidity at RHIC! Bedanga Mohanty QGP MEET 2006, February 5 th – 7 th, Kolkata 14

Energy for particle production Energy p p K+ K + - : : : 3108 428 1628 1093 5888 6117 • Fit , K and p distributions (in Ge. V) 0 n n K 0 : : : : total energy of , K and p 6004 3729 513 1628 1093 1879 342 (d. N/dy and m. T vs y) • Assume reasonable distribution for particles we don’t detect ( 0, n, …) • Calculate the total energy… sum: 33. 4 Te. V produced: 24. 8 Te. V NB: the method is very sensitive to the tails of the d. N/dy dist. ( 10 -15%) Bedanga Mohanty 35 Te. V (Ebeam Npart) of which 25 Te. V are carried by produced particles. ~ 71 % of the beam energy QGP MEET 2006, February 5 th – 7 th, Kolkata 15

Summary on particle production Ø Ø Ø Forward rapidity region provides information on collision centrality, it is important to have centrality information from a rapidity region different from where you are studying your physics Detectors at forward rapidity provides trigger information in experiments at RHIC Scaling of particle production different at midrapidity and forward rapidity The ratio of p. T spectra in d. Au collisions w. r. t pp collisions at high p. T decreases from above unity to much below with increase as we move from midrapidity to forward rapidity Since baryon chemical potential increases as we go from midrapidity to forward rapidity – we can scan the QCD phase diagram Forward rapidity helps in determining the amount of energy of the beam spend in particle production ~ 72% in central Au. Au collisions at RHIC Bedanga Mohanty QGP MEET 2006, February 5 th – 7 th, Kolkata 16

Longitudinal scaling e-+e+ collisions DELPHI, PLB 459 (1999) yjet ln( s/Mj) Mj ~ 1 Ge. V Longitudinal scaling observed in elementary collisions Bedanga Mohanty QGP MEET 2006, February 5 th – 7 th, Kolkata 17

Longitudinal scaling - I p+p collisions Over a factor of ~50 variation in √s and Data show limiting fragmentation at high CDF (900) Phys. Rev D 41 (1990) 2330 UA 5 (200, 546) Z. Phys. C 43 1 (1989) ISR (23. 6, 45. 2) Nucl. Phys B 129 365 (1977) Bedanga Mohanty QGP MEET 2006, February 5 th – 7 th, Kolkata 18

Longitudinal scaling - II p(d)+A collisions Data show limiting fragmentation at high for beam and target rapidities PHOBOS, Phys. Rev. C (2005) Bedanga Mohanty QGP MEET 2006, February 5 th – 7 th, Kolkata 19

Longitudinal scaling - III A+A collisions PHOBOS, PRL STAR PMD, PRL 200 Ge. V 130 Ge. V 19. 6 Ge. V h = h - ybeam Bedanga Mohanty Photons and charged particles follows limiting fragmentation at high in nucleus-nucleus collisions QGP MEET 2006, February 5 th – 7 th, Kolkata 20

Longitudinal scaling - IV Centrality dependence PHOBOS, PRL STAR PMD, PRL 200 Ge. V 130 Ge. V 19. 6 Ge. V Scaling of charged particles centrality dependent that for photons centrality independent Bedanga Mohanty QGP MEET 2006, February 5 th – 7 th, Kolkata 21

Longitudinal scaling - IV Understanding Centrality dependence STAR PMD+FTPC, PRC The difference between inclusive charged particles and inclusive photons may be due to : (a) Beam remnants (b) Baryons transport Study negatively charged and positively charged hadrons separately : Beam protons are +ve charged Bedanga Mohanty Study identified particle longitudinal scaling QGP MEET 2006, February 5 th – 7 th, Kolkata 22

Longitudinal scaling – V Understanding Centrality dependence d. Np/dh/0. 5 Npart STAR PMD+FTPC, PRC h - ybeam Beam fragments have some contribution production follows energy independent limiting fragmentation Phys. Rev. Lett. 95 (2005) 062301 Bedanga Mohanty Net protons production do not follow energy independent limiting fragmentation STAR nuclex/0511026 QGP MEET 2006, February 5 th – 7 th, Kolkata 23

Longitudinal scaling – VI Extended scaling peripheral central But the centrality and energy dependence factorizes PHOBOS : Nucl. Phys. A 757 28 (2005) Bedanga Mohanty QGP MEET 2006, February 5 th – 7 th, Kolkata 24

Longitudinal scaling – VII Predictive power Assuming: d. N/d grows log(s) and linear scaling at high holds The most central pseudorapidity density at midrapidity in LHC ~ 6 x 400/2 ~ 1200 Acta Phys. Polon. B 35 2873 (2004 ) Bedanga Mohanty QGP MEET 2006, February 5 th – 7 th, Kolkata 25

Longitudinal scaling – VIII Azimuthal Anisotropy Reaction plane (YR) yy PRL 94, 122303 zz Drawing by M. Kaneta x x (defines YR) Anisotropy is sensitive to EOS …… We observe a simple scaling at high ? ! Bedanga Mohanty QGP MEET 2006, February 5 th – 7 th, Kolkata 26

Summary of scaling studies Ø Ø Ø Forward rapidity region shows certain universality of particle production. Energy independent behavior observed for e+e-, pp, p. A, AA collisions when rapidity shifted by beam rapidity referred to as Limiting Fragmentation The longitudinal scaling breaks at forward rapidity for inclusive charged particles when we study it’s centrality dependence in Au+Au collisions at RHIC. It is maintained for inclusive photons Detailed study reveals that apart from beam remnants, it is the baryons and its transport in rapidity that breaks the scaling Surprisingly the scaling is restored in energy when we look at the ratio of peripheral to central yields in rapidity shifted by beam rapidity. This extended scaling reflects some kind of factorization in the dependences. Quantities which crucially depends on the EOS and systems evolution (v 2, v 1) surprisingly shows longitudinal scaling at forward rapidity Bedanga Mohanty QGP MEET 2006, February 5 th – 7 th, Kolkata 27

Baryon stopping and nuclear transparency Anti-baryons - all from pair production Baryons - pair production + transported B/B ratio = 1 - Transparent collision B/B ratio ~ 0 - Full stopping, little pair production Measure p/p, / , K-/K+ (uud/uud) (uds/uds) (us/us) Landau or Bjorken hydrodynamics Bedanga Mohanty QGP MEET 2006, February 5 th – 7 th, Kolkata 28

Baryon stopping and nuclear transparency Net protons Indication of increasing transparency with energy AGS energies : Landau hydrodynamics applicable RHIC energies : Bjorken hydrodynamics applicable Bedanga Mohanty QGP MEET 2006, February 5 th – 7 th, Kolkata 29

Baryon stopping and nuclear transparency Which matters : Target or projectile ? Eur. Phys. J. C 2: 643 (1998) 2 d 197 Au 16 O 197 Au 32 S 197 Au Projectile size is irrelevant. Bedanga Mohanty QGP MEET 2006, February 5 th – 7 th, Kolkata 30

Baryon stopping and nuclear transparency Baryon production in d. Au collisions at RHIC 0 -20% 20 -40% 40 -100% (stat. Errors only) Difference (~baryon transport) is centrality dependent but…but their ratio (~baryon transport + pair production) is not The increase in baryon stopping with collision centrality is not reflected in a decrease of the anti-baryon to baryon ratio. Bedanga Mohanty QGP MEET 2006, February 5 – 7 , Kolkata 31 th th

Baryon stopping and nuclear transparency Gaussians in pz: Net-baryon after feed-down & neutron corrections 2. 03 0. 16 2. 00 0. 10 6 order polynomial Lack of measurement Bedanga Mohanty QGP MEET 2006, February 5 th – 7 th, Kolkata 32

Baryon stopping and nuclear transparency • Upper limit to rapidity loss? • Energy loss: E = 25. 7 2. 1 Te. V E/nucleon = 72 6 Ge. V Bedanga Mohanty QGP MEET 2006, February 5 th – 7 th, Kolkata 33

Summary on baryon production Ø Net-baryon poor midrapidity region Ø d. N(net-protons)/dy = 7 Ø Total-baryon rich midrapidity region Ø d. N(all baryons)/dy 65 Ø Largest observed rapidity loss Ø < y> = 2 Ø as large as in p. A Ø Stopping power Ø central Pb+Pb at RHIC: 72% Ø central S+S at SPS: 58% Ø p+p collisions: 50% Bedanga Mohanty QGP MEET 2006, February 5 th – 7 th, Kolkata 34

From CGC to Quark Gluon Plasma Manifestation of CGC at RHIC Øhadron multiplicities Øhigh p. T suppression at forward rapidity ØDisappearance of back-to-back correlations in azimuthal angles between jets in forward and midrapidity L. Mc. Lerran, T. Ludlam, Physics Today, October 2003 Bedanga Mohanty QGP MEET 2006, February 5 th – 7 th, Kolkata 35

CGC : Hadron multiplicity Multiplicity distribution Consistent with data for pp, d. Au and Au. Au collisions Bedanga Mohanty QGP MEET 2006, February 5 th – 7 th, Kolkata 36

CGC : Longitudinal scaling Au+Au 0 -6% central Consistent with energy independence observed in data at forward rapidity CGC calculation Q 2 s 0=2 Ge. V 2 Q 2=5. 3 Ge. V 2 Nucl. Phys. A 757 28 (2005) Phys. Rev. C 70 027902 (2004) Bedanga Mohanty QGP MEET 2006, February 5 th – 7 th, Kolkata 37

CGC : p. T spectra 0 ü ü Data : G. Rakness, nucl-ex/0501026 D. A. Morozov, , hep-ex/0505024 B. Mohanty (STAR) QM 2005 Bedanga Mohanty A model that treats Au nucleus as a CGC is consistent with d. Au data The model includes low x. Bj evolution of the Au wave function Model: A. Dumitru, A. Hayashigaki, J. Jalilian-Marian, hep-ph/0506308 QGP MEET 2006, February 5 th – 7 th, Kolkata 38

CGC : Nuclear modification factor Saturation tells us that the Cronin-peak disappears at high : y=0 As y grows Phys. Rev. D 68 , 094013 (2003) Nucl. Phys. A 739, 319 (2004) Bedanga Mohanty Consistent with observations from data QGP MEET 2006, February 5 th – 7 th, Kolkata 39

CGC : Nuclear modifiaction factor D. Kharzeev, Y. V. Kovchegov, K. Tuchin, hep-ph/0405054 (2004) Bedanga Mohanty • CGC model describes Rd. Au and RCP • Suppression comes in at y > 0. 6 QGP MEET 2006, February 5 th – 7 th, Kolkata 40

Other models also explain Rd. Au Vs. with only the recombination of soft and shower partons: Phys. Rev. C 71 024902 (2005)no multiple scattering, and no gluon saturation put in explicitly see R. Vogt, hep-ph/0405060 (2004), Phys. Rev. C 70 (2004) 064902 Bedanga Mohanty See also R. Vogt, 41 QGP MEET 2006, February 5 th – 7 th, Kolkata hep-ph/0405060 (2004)

CGC : Back-to-back Correlations Disappearance of back-to-back correlations in d. Au collisions predicted by KLM seems to be observed in preliminary STAR data. Bedanga Mohanty QGP MEET 2006, February 5 th – 7 th, Kolkata 42

v 2(h) from CGC+Hydro Qualitatively consistent with rapidity dependence of azimuthal anisotropy at RHIC when we use CGC as initial condition for hydrodynamical calculations Bedanga Mohanty QGP MEET 2006, February 5 th – 7 th, Kolkata 43

Some other forward physics topics I could not cover ………. • Forward Spin Physics – Large Rapidity g-jet may provide interesting corners of phase space to probe gluon polarization (ALL). • Other opportunities are present in diffractive physics and Ultra peripheral collisions. • Lambda polarization has a prominent xf dependence • J/y production in PHENIX forward rapidity R. Bellwied, Nucl. Phys. A 698 (2002) 499 Bedanga Mohanty QGP MEET 2006, February 5 th – 7 th, Kolkata 44

Summary Ø Detectors at forward rapidity region provides the experiment : centrality and trigger ØForward rapidity region provides rich information on particle production certain universality (species, energy) observed Ø Forward rapidity region provides information on nuclear stopping, baryon transport and energy for particle production Ø Forward rapidity provides a chance to scan the QCD phase diagram Ø Forward rapidity provides the best place to study the possible initial conditions at RHIC : CGC ØForward rapidity provides testing ground for NLO p. QCD Measurements at forward rapidity (Kinematical limits and detector constraints) is also an experimental challenge Bedanga Mohanty QGP MEET 2006, February 5 th – 7 th, Kolkata 45

Initial and final effects - d. Au • Initial effects Wang, Levai, Kopeliovich, Accardi “Cronin effect” broaden p. T Initial state elastic multiple scattering leading to Cronin enhancement (RAA>1) • Especially at forward rapidities: Eskola, Kolhinen, Vogt, Nucl. Phys. A 696 (2001) 729 -746 HIJING D. Kharzeev et al. , PLB 561 (2003) 93 • Others B. Kopeliovich et al. , hepph/0501260 J. Qiu, I, Vitev, hep-ph/0405068 R. Hwa et al. , nuclth/0410111 D. E. Kahana, S. Kahana, nucl-th/0406074 Bedanga Mohanty Nuclear shadowing depletion of low-x partons Shadowing Anti Shadowing Gluon saturation depletion of low-x gluons due to gluon fusion ”Color Glass Condensate (CGC)” gg g r/ Suppression due to dominance of projectile valence quarks, energy loss, coherent multiple scattering, energy conservation, parton recombination, . . . QGP MEET 2006, February 5 th – 7 th, Kolkata 46