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Information. Please switch off mobile phones. Corrections : Page 23 : Light systems are Ni+Ni (FOPI), Si+Al(E802), S+S(NA35) Page 97 : Acceptance plot (2.8 < y < 2.95) is wrong. Stopping in central GeV Au+Au collisions at RHIC. Peter Harald Lindenov Christiansen

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  1. Information Please switch off mobile phones. Corrections : Page 23 : Light systems are Ni+Ni (FOPI), Si+Al(E802), S+S(NA35) Page 97 : Acceptance plot (2.8 < y < 2.95) is wrong

  2. Stopping in central GeV Au+Au collisionsat RHIC Peter Harald Lindenov Christiansen Niels Bohr Institute Faculty of Science, University of Copenhagen

  3. Outline of talk • Introduction to heavy ion physics • Stopping • The BRAHMS experiment • Analysis • Results • Conclusions

  4. Quantum Chromo Dynamics (QCD) 3 color charges (red, green, blue) Hadrons have to be colorless Baryons have all 3 colors Mesons has a color and an anti-color A single quark cannot be observed because it has color! The quarks are confined inside the hadrons! This talk will be about proton and anti-protons Hadrons Baryons Mesons

  5. QCD potential Gluons carries color Gluons can interact with gluons

  6. Quark Gluon Plasma Deconfinemt Confinemt ? Lattice QCD calculations

  7. Experimental heavy ion physics

  8. Simulations 1 2 3 4

  9. Relativistic Heavy Ion Collider PHOBOS First heavy ion collider in the world. L=2*1026cm-2s-1 R=1200Hz PHENIX STAR Data presented here is from the first Au+Au run at GeV

  10. A Real Collision

  11. What is stopping ? • Energy conservation. Kinetic energy of initial baryons is used to create a hot and dense zone. • Baryon (qqq) number conservation. • Before: 2*197 baryons  After:2*197 net-baryons (baryons-anti-baryons) • Stopping is the study of the energy loss suffered by the baryons in the collision. The energy loss happens in 3 ways : • Initial interactions • Rescattering of partons and hadrons • Decays

  12. How to measure stopping Use rapidity variable  Distributions are boost invariant Physical space BEFORE COLLISION “Velocity” space AFTER COLLISION 2 extreme final states Full stopping Full transparency

  13. Two physics pictures Stopping – excited nucleons Transparency – excited color field

  14. p+p collisions Because of the target and projectile symmetry the rapidity loss is symmetric around mid-rapidity. target projectile p+p collisions exhibits a large degree of transparency.

  15. A+A Collisions Geometry participants spectators ? b is the impact parameter. Geometric Glauber model calculations can be used to calculate the collision geometry.

  16. Au+Au collisions at AGS E917 • A+A collisions is more than a sum of p+p collisions • p+p picture is recovered in peripheral collisions • In central collisions the rapidity distibution peaks at mid-rapidity • Can be described by two Gaussians.

  17. Energy dependence Au+Au Pb+Pb E866/E877 NA49 ?   Energy

  18. How to quantify stopping Use rapidity loss : For symmetric collisions the last term is calculated as : MAX Relative rapidity loss independent of beam energy! What happens at RHIC ? MIN

  19. What happens at RHIC ? Will there be stopping ? Or transparency ? BRAHMS can tell!

  20. The BRAHMS detector

  21. A BRAHMS event D2 T2 FFS 6 deg D1 T1 BEAM MRS 90 deg D5 TPM1 TPM2

  22. Event reconstruction - Global • Interaction Point • Centrality

  23. Event reconstruction - Tracks • Local tracking • Matching (momentum) • Particle identification

  24. Proton PID using TOF 2cuts p K  m2 momentum dependence parameterized by :

  25. Proton PID in the FS  Ring Imaging CHerenkov K p The ring radius in the RICH depends on the velocity. The RICH is used to identify protons directly and as a VETO counter for pions and kaons. Important to correct for contamination.

  26. Proton and anti-proton acceptance A single spectrometer setting covers a small fraction of phase space, but by combining different settings pT-spectra can be obtained at many different rapidities. MRS(0<y<1), FFS(1<y<2), FS(2.0<y<3.5?)

  27. Constructing pT-spectra Invariant yield : • DATA : Measured protons and anti-protons • ACC : Geometrical acceptance • CORRections • Tracking efficiency • PID efficiency (slat efficiency) • Multiple scattering and nuclear absorption correction

  28. Data selection • Global cuts : • Interaction point (BB & ZDC agrees, and close to nominal IP) • Centrality : 0-5 % shown here • Track cuts : • Pointing (Track points back to the IP) • Magnet fiducial cut (No intersection) • PID cuts : • TOF (TOFW, H1, H2) and RICH

  29. Acceptance correction ~0.5% Simulation with pions. Pions are stopped when they hit the magnet and all physical processes except energy loss have been turned off.

  30. Acceptance correction The acceptance correction should correct for the limited geometrical coverage of the spectrometers. The correction is calculated by simulation.

  31. A pT-spectrum

  32. Extracting dN/dy Fit pT spectra and use the fit to extrapolate into regions where we don’t measure to get dN/dy. The difference between the fits depends on the fit-range. In the following mT-exponentials are used at all rapidities.

  33. Rapidity Coverage

  34. Examples of pT-spectra 0-5% central collisions

  35. Rapidity densities dN/dy Reflected Full stopping Full transparency Plots has statistical errors only. Typical systematic errors are : ~1.0 (0<y<1) ~2.6 (y~2) ~1.6 (y~3) Net-proton distrubution is far from full stopping.

  36. Net-proton energy dependence The shape of the net-proton distribution measured at RHIC is different from what is observed at lower energies. At RHIC the mid-rapidity region is almost net-proton free. Pair production dominates at RHIC.

  37. Comparison to Models I Net-protons measured includes protons from hyperon decays e.g. Λp+-. To compare with models the protons from hyperon decays have to be removed. BRAHMS does not measure Λ, instead we use models and simulations to correct : HIJING : s = 0.9/0.4 C~0.75 at all rapidities

  38. Comparison to Models II Hijing (Strings, no rescattering) UrQMD (Transport calculation, resonance excitations, rescattering) Hijing describes the data best, BUT Hijing does not reproduce Λ/p (y=0) or p-bar/p (0<y<3)

  39. Rapidity Loss Estimates Beam rapidity All net-protons at y = 3.5 Maximal rel. rap. loss = 0.24 All net-protons at y = 5.0 Minimal rel. rap. loss = 0.16 Example of processes : p+pn+p+π+(pn) n+nn+p+π -(n p) p+N  +K++N (p )  p+ π -( p) 29 net-protons measured (0 < y <3) Estimate total : 350 participants 140 initial protons Assume 140 total  70 (y>0)  41 outside acceptance (y>3)

  40. Rapidity Loss (MCM fit) Fit the data with the MCM inspired function :

  41. Rapidity Loss Results BLUE is DATA RED is MODELS Constant relative rapidity loss is broken at RHIC.

  42. Net-proton energy dependence

  43. Conclusions • The observed net-proton yield increases from 7.3±0.5(stat.) ±1.0(syst.) at y = 0 to at 12.9±0.4(stat.) ±1.6(syst.) at y=3. • The collisions exhibits a large degree of transparency. This has not been observed in collisions at lower energies. • Hijing reproduce the observed net-proton yields while UrQMD over predicts the stopping power. This suggests that the same string physics as p+p can describe the results. • Scaling of rapidity loss is broken at RHIC. The relative rapidity loss is lower than what was observed in collisions at SIS, AGS, and SPS energies.

  44. Phase diagram of hadronic matter

  45. Model predictions • Geometric Glauber model calculations can be used to calculate the collision geometry. • Most interactions are soft so pQCD can not be used. • The physics learned from p+p collisions can be used as a starting point, but there are important differences : • Formation times, Off-shell cross sections, Rescattering • The models chosen are : • MCM (Simple) • Hijing (Strings) • UrQMD (Transport)

  46. Multi Chain Model B is the projectile(y=Y), A is the target(y=0) r is the ratio of protons to nucleons W is the number of participants P(n) is the fraction of nucleons that has n binary collisions Q are the fragmentation functions that contains the physics SIS AGS SPS RHIC

  47. Hijing Energy lost in hard scatterings is resolved first. All the soft scatterings results in string excitations. The strings decays after all collisions have been resolved according to Lund string model (JETSET). The strings can be (de)excited by more scatterings after they are created with a modified probability. Figure is taken from Phys. Lett. B 443, p 45

  48. UrQMD Transport theory. Only 1234 scatterings. All particle production from decays. Propagate as free particle between scatterings. Reduced cross section of strings and decay time of strings is important. Strings decay time . σ=1GeV/fm σ=3GeV/fm

  49. Tracking 1

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