# Presentation: 935116

## J/ Suppression andCollision Dynamicsin Heavy Ion Collisions

- Mike Bennett
- Los Alamos National Laboratory

## Nuclear Matter Phase Diagram

- At high temperature and density, nuclear matter is expected to undergo a phase transition to a Quark-Gluon Plasma
- Recreates the state of matter in the universe a few microseconds after the Big Bang

## The Phase Transition(s)

- The phase transition is actually two transitions:
- 	Deconfinement Transition:
- 	Quarks and Gluons are no longer confined to hadrons
- 	Chiral Symmetry Restoration:
- 	Quark Condensate goes to ~0
- Recent Lattice Calculations suggest a transition temperature of ~150-200 MeV--should be accessible experimentally

- Deconfinement Transition

- Chiral Restoration

- Laermann
- QM 96

- Schafer
- QM 96

## Signatures of the QGP

- Deconfinement Probes:
- 	J/,  ‘  Suppression
- 	Increased dE/dx of partons (Jet Quenching)
- 	Strangeness, antibaryon enhancement
- 	Direct photons 2-5 GeV from gluon-	quark Compton scattering
- 	Enhanced dilepton pairs 1-3 GeV from
- 		quark-antiquark annihilation
- Chiral Symmetry Probes:
- 	Change in mass, width and BR
- 	Disoriented Chiral Condensates

- S. Nagamiya, PHENIX

## Recent CERN Announcement

- “Circumstantial Evidence” for QGP includes:
- J/ Suppression
- Enhanced Production of Strange Particles
- Temperature ~180 MeV from particle abundance ratios
- Energy density ~2-4 GeV from extrapolating back final state energy

- 

## Debye Screening

- In a deconfined medium, attraction between c and cbar is screened (Matsui and Satz)
- As Debye length decreases with increasing temperature, different states are screened

**c**

- c

- C-Cbar screened in a QGP

## “Normal” J/ Suppression

- Initial expectation was J/ would not interact in normal nuclear matter
- Yield in pA data far exceeded expectations
- A dependence of pA data indicated absorption well  beyond expectation
- These puzzles can be resolved by “color octet model”--explains “normal J/ Suppression”

- 

## “Anomalous” J/ Suppression

- Yields from p-A and A-A (through S) described by absorption cross section of 6-8 mb--consistent with predictions for c-cbar-g color octet state
- Yields from Pb-Pb collisions display absorption beyond this level, so-called “anomalous suppression”
- Plotted against “L”, the mean length through nuclear material.  This is not an ideal parameter--not a measured quantity, saturation for large systems
- Need to look at J/, DY individually, as a function of centrality

- L. Ramello, Quark Matter ‘97

- NA38, NA50 J/ to DY ratio

## Comparison to Simple Glauber

- NA50 Drell-Yan

-  = 0
-  = 6.2 mb
-  = 9.0 mb

- Simple Glauber model, with production from all N-N collisions equally likely	  MJB, J.L. Nagle, Physics Letters B465, 21 (1999)
- Collision dynamics based on observed A-A systematics: 				ET = constant  *  Wounded nucleons,  smeared by 94% /E resolution
- Drell-Yan yields are fit very well
- J/yields are not fit well with absorption cross sections from 6-9 mb

- NA50 J/

## “Explaining” Anomalous Suppression

- Absorption by Hadronic Co-Movers
- 	Inelastic scattering by hadrons at similar momentum
- Gluon Shadowing
- 	Increased EMC effect in larger systems
- Initial State Energy Loss
- 	Reduced Production in Later Collisions
- Quark-Gluon Plasma

## Geometry of Energy Loss

- Nucleons lose energy as they traverse the colliding nucleus
- Production of  J/ and Drell-Yan have steep  energy dependence
- Affects J/ and DY differently
- Reduces total yield
- Reduces Cronin effect, changes pt spectrum
- Mimics QGP signal

- Absorption only

- Absorption + Energy Loss

## Energy Loss in Min Bias Collisions

- J/ yield per N-N Collision, plotted against Mean Number of N-N Collisions
- Absorption only gives simple exponential
- Energy loss suppresses from simple exponential
- Want to look at detailed centrality dependence, for both J/ and Drell-Yan

- Frankel & Frati, hep-ph/9710532

## The Model and Parameters

- Glauber Formalism, using 30mb N-N cross section
- Disregarding energy loss, all N-N Collisions contribute equally
- J/ produced “at rest”, absorption cross section 7.1 mb		(MJB, J.L.Nagle, PRC59,2713)
- Production of J/ and DY depends on energy of N-N Collision
- Stopping in p-A collisions suggest nucleons lose ~40% of their momentum per collision  at t=

- Comparison to NA49 Central Pb+Pb
- using 33% momentum loss per collision

## The “L Parameter” and Absorption Fits

- At  fixed impact parameter, J/ path lengths vary widely; each centrality bin represents a variety of impact parameters
- A simple average over path lengths underestimates absorption cross section; using an iterative process, a refit  gives 7.1 ± 0.6 mb
- Consistent with an fit with different methodology (7.3 ± 0.6 mb, Kharzeev et al, ZPC74, 307 (1997)

- MJB & JLN
- PRC, May 99

## Time Scales and Collision Dynamics

- At CERN energies, nuclei cross in ~0.1 fm/c
- Most energy loss is via soft interactions, with a time scale of a few fm/c
- Some fraction of this energy loss is at short time scale, treat as a variable parameter

## J/ Yields with Energy Loss

- Several values of Energy Loss 0%, 5%, 10% and 15% momentum per collision (0%, 15%, 30%, 50% of total t= loss)
- Normalization chosen to give best fit in lowest two ET bins
- Highest Energy Loss matches spectral shape well

## Drell-Yan Yields with Energy Loss

- Several values of Energy Loss 0%, 5%, 10% and 15% momentum per collision
- Normalization chosen to give best fit in lowest ET bins
- Hard to reconcile any energy loss with data
- Is it reasonable to assume same energy loss is applicable for both J/ and DY?

## Cronin Effect

- Prior N-N Collisions broaden transverse momentum (“Cronin effect”)
-  J/: <pt2>pp = 1.23 ± 0.05 GeV2  (NA3);			 pt2=0.125 GeV2  (fit to pA + AA, Kharzeev et al, PLB 405, 14 (1997))
- DY: <pt2>pp = 1.38 ± 0.07 GeV2  (NA3);			 pt2=0.056 GeV2  (fit to pA + AA, Gavin and Gyulassy, PLB 214, 241 (1988))

- <pt2>N = <pt2>pp + N  pt2

##  Drell-Yan <pt2> with Energy Loss

- Several values of Energy Loss 0%, 5%, 10% and 15% momentum per collision
- Spectra not very sensitive to energy loss

## J/ <pt2> with Energy Loss

- Several values of Energy Loss 0%, 5%, 10% and 15% momentum per collision
- Large values of Energy Loss do not fit data
- Not consistent with Energy Loss required to fit J/ yields

## Is QGP necessary to fit J/ <pt2>?

- Must take error in pp data into account
- pp data taken at 200 GeV; scaling to 158 GeV (linear in s) reduces pp “intercept” to 1.13 GeV2--changes normalization, not shape
- J.L.Nagle, MJB, Phys. Lett. B465, 21 (1999)
- D.Kharzeev, M.Nardi, H.Satz, Phys. Lett. B405, 14 (1997). Concluded QGP necessary to fit data, but shown here  rescaled for pp energy.

## Conclusions (Part 1)

- Within normalization uncertainty,  J/ <pt2> spectrum is consistent with a normal hadronic scenario
- J/  Yields are not consistent with a simple Glauber calculation.  Adding Energy Loss can fit the J/ yield shape ...BUT
- Energy Loss cannot consistently fit both J/ and Drell-Yan yields
- Energy Loss cannot consistently fit both J/ yields and J/        <pt2> spectra
- Energy Loss does not appear to explain “anomalous” J/ suppression

## Requirements for Analysis

- J/ Measurement
- 	Yields and Transverse Momenta Spectra
- 	Both over a large range of system size, from pp, pA, several AA
- Benchmark measurement
- 	Drell-Yan over same range of geometries
- Collision Dynamics
- 	Energy loss systematics from pA, AA
- 	Geometric dependence of Et, secondary multiplicity

## PHENIX Experiment at RHIC

## Dileptons in PHENIX

- Dimuon spectrum

- Dielectron Spectrum

## RHIC --  A Versatile Accelerator

- Heavy Ion Program    Vary Species and Beam Energy
- p-A Program, Spin Program    polarized protons
- Four Experiments  Complementary Physics,  Common Centrality Measurement

## Extrapolating to RHIC

- Large uncertainty in extrapolating collision dynamics from AGS/SPS to RHIC
- Proton dN/dy measured over large phase space by BRAHMS
- Charged Particle Multiplicity measured over large phase space by PHENIX MVD, PHOBOS, STAR
- These distributions are essential to understanding the environment in which the J/ is produced

## Hadrons in PHENIX

- Measure transverse momentum in central arms to ~1%
- Identified hadrons in central arms
- 	/K to 2.5 GeV using TOF
- 	K/p to 3.5 GeV using TOF
- Simulations ongoing to assess full PID capability of PHENIX

## Hard Processes and Multiplicity

- Hard Processes generate ~50% of particle multiplicity at RHIC (Gyulassy)
- Simple extrapolation from AGS/SPS not valid
- Interesting physics in measuring multiplicity
- Measurement of charged particle transvere momenta spectra constrain hadronic comover models

- HIJING with
- various parton
- processes

- VNI Parton
- Cascade with
- hadronic
- cascade

## Baryon Structure at RHIC?

- Recent revival of old idea--baryon junction
- “ball” of soft gluons is basis of baryons, with 3 valence quarks held by color “strings”
- Not ruled out by existing data
- Observables at RHIC--
- 	Antibaryon/baryon ratio at
- 		mid-rapidity (PHENIX)
- 	Baryon stopping (BRAHMS)
- 	Hard forward mesons (BRAHMS)
- Impact on J/ yield is under investigation

## Multiplicity in PHENIX

- Measure Charged Particle Multiplicity accurately over large pseudorapidity range
- Measure dN/d, dN/dd
- Sensitive to localized fluctuations on an event-by-event basis

## PHENIX MVD

- Inner and Outer Hexagonal Barrels of 200 micron pitch Si Microstrips
- Si Pad Endcaps 2mm2 to 4.5mm2
- Multichip Module Electronics, 256 Channels in ~4.5cm2
- ~35,000 Total Channels

## MVD Construction

## MVD Construction (II)

## Conclusions (Part 2)

- A full understanding of J/ suppression will require systematic measurement of yields over numerous geometries AND an understanding of the collision dynamics
- PHENIX is well situated to measure J/and higher mass states in both muon and electron channels
- PHENIX is well situated to investigate collision dynamics via global variables and hadron spectra
- Expect collision dynamics to be the most interesting physics early in the RHIC program
- These measurements will set the context for later physics analyses