# Presentation: 463848

## Relativistic Heavy-Ion Physics: Experimental Overview

**ICPAQGP**
**Jaipur, India, Nov. 26-30, 2001**
**Itzhak Tserruya**
**Weizmann Institute**

- Itzhak Tserruya - Jaipur 2001

- 26 Nov. 01

## Outline

**•  **_**Introduction**_
**     ***** From AGS ********  SPS ********  RHIC**

**•  **_**Global Observables**_
**    ***** **** ****dN****ch****/d********,    ****dE****T****/d********,   <p****T****>**

- •  _**Summary**_

**•  **_**Special probes**_
**   ***** Low-Mass Dileptons and Photons  **
**   ***** J/******** Suppression  **
**   ***** ****High p****t**** Suppression**

**• **_**The Hadronic System**_
**    ***** **** ****particle spectra, ratios, baryon density,**
**        ****flow, HBT and strangeness**

- Itzhak Tserruya - Jaipur 2001

## 15 Years of  SPS Programme

**  ****1986 - 1987 : Oxygen @ 60 & 200 GeV/nucleon**
**  ****1987 - 1992 : Sulphur @ 200 GeV/nucleon**
**  ****1994 - 2000 : Lead @ 40, 80 & 158 GeV/nucleon**
**  ****2002 - 2003 : Indium and Lead @ 158 GeV/nucleon**

**And proton beams for**
** ****pp and pA reference**
** ****studies**

- Itzhak Tserruya - Jaipur 2001

- 26 Nov. 01

## RHIC: 2 Weeks and 4 Experiments

- Itzhak Tserruya - Jaipur 2001

- 26 Nov. 01

## Physics Menu

|  |  | SPS                                             | RHIC |  |  |  |
| --- | --- | --- | --- | --- | --- | --- |
|  |  |  | BRAHMS      | PHENIX | PHOBOS | STAR |
| Global
 and 
Hadrons | Multiplicity
ET
Spectra and ratios
Flow & Correlations
Multi – strange | NA49, WA80
NA49, WA80
All
NA44+ ...
WA 97 | X

X | X
X
X
X | X

X
 | X
X
X
X
X |
| Rare /
Penetrating
Probes | Photons
Low-mass e+e-
Vector mesons  
  l+l-
       K+K-
J/ e+e-
               + -
   + -
Open charm
High pT | WA98, CERES
CERES
NA50
NA50
NA49

NA50

NA60
(CERES, WA98) |  | X
 X*
X
X
X
X
X
 X*
 X*
X |  | X

X
X

 X*
X |

_**Physics Menu**_

- * Needs upgrade

## What is New / Different at RHIC?

**•  ****Collider **
-      *** No target, no delta electrons ! ******** less background**

**•  ****Dedicated and flexible machine**
*** Will run 20-30 weeks per year   ******** higher statistics**
*** Energy scan, beam species and  (polarized) pp ******** more systematics**

**•  ****High Energy  ****(********s****NN**** = 200 GeV  at RHIC  vs ********s****NN**** = 17 GeV at SPS)**
*** Higher initial energy density ******** longer lifetime**
*** Access to perturbative phenomena  **
**       ****(hard processes and Jets)**

**•   ****Comprehensive detectors **
*** Measure same observables in several detectors with**
**   ****significant overlap for comparisons.**

**  ****new probes?  **

## Global Observables

**•  ****WHAT ?**
**   ********* dN****ch****/d********,    ****dE****T****/d********,   <p****T****>**
**   ********* Reflect conditions well after freeze-out **
**      ****and resonance decays**

**•  ****WHY ?**
**   ********* “Easy” measurements**
**   ***** Characterize collision geometry **
**   ***** Constrain models  **
**   ********* Initial conditions     **

- Itzhak Tserruya - Jaipur 2001

- 26 Nov. 01

## PHOBOS:    ||<1 , 1%?
    2 layers of Si detectors close to vertex (B=0)
    dNch/d = 555 ± 12 ± 35 (6% most central) PRL
    dNch/d = 579 ± 1 ± 22 (6% most central) 

PHENIX:    ||<0.35 ,    = 90o
    2 layers of PC at 2.5 and 5 m from vertex (B=0)
   dNch/d = 622  ± 1 ± 41 (5% most central)

STAR:    ||<1.8 ,     = 2
   Tracking in TPC,  pt>100 MeV  (B#0)
   dNch/d = 567 ± 1 ± 38 (5% most central)
BRAHMS  ||<4.7
Si strips, scintillators and Cherenkov counters  
 dNch/d = 553 ± 1 ± 36 (5% most central)

**    ****2 layers of Si detectors close to vertex (B=0)**
**    ****dN****ch****/d******** = 555** ± 12 ± 35 (6% most central) PRL
-     **dN****ch****/d******** = 579 **± 1 ± 22 (6% most central)
_**PHENIX**__**:**_**    |********|<0.35 ,   ******** = 90****o**
**    ****2 layers of PC at 2.5 and 5 m from vertex (B=0)**
**   ****dN****ch****/d******** = 622**  ± 1 ± 41 (5% most central)
_**STAR**__**:**_**    |********|<1.8 ,    ******** = 2******
**   ****Tracking in TPC,  p****t****>100 MeV**** **** (B****#****0)**
**   ****dN****ch****/d******** = 567 **± 1 ± 38 (5% most central)
_**BRAHMS  **_**|********|<4.7**
**Si strips, scintillators and Cherenkov counters  **
** ****dN****ch****/d******** = 553 **± 1 ± 36 (5% most central)

_**RHIC : dN**__**ch**__**/d**__****__** at **__****__**s**__**NN**__** = 130 GeV **_

## sNN Dependence

**      ****From SPS to RHIC :**
**   ****dN****ch****/dy and dE****T****/d******** increase by **
**     ****~70% at  *********s******NN *****= 130 GeV**** **
**             ****and **
**an additional  ~15% at  *********s******NN *****= 200 GeV**

-    ln(*s**NN** *) **dependence from **
**   ****AGS to RHIC**

## dN/d: Predictions

** *****  PHOBOS: 15 % increase in dN****ch****/d******** **from *s**NN*** = 130 to 200 GeV.**
-  *  Dramatic increase predicted by** HIJING with quenching not observed**

- Itzhak Tserruya - Jaipur 2001

- 26 Nov. 01

## Rapidity Distribution

_**Rapidity Distribution**_

- Enhancement of particle production for central  collisions at mid-rapidity.
-  Particle production scales with Npart at high rapidities ( >3).

**BRAHMS  Au-Au ********s****NN****=130 GeV preliminary**

** ****Same scaling observed at SPS**

## Centrality determination

**Use combination of **
**Zero Degree Calorimeters**
**Beam-Beam Counters**
- (sensitive to 92% of geom)
** ****to define centrality classes**
**Glauber modeling**
** ****to extract N-participants**

**BBC**

**ZDC**

**ZDC **

- Au

- Au

**BBC**

**Central Multiplicity Detectors**

- Itzhak Tserruya - Jaipur 2001

- 26 Nov. 01

## Centrality Dependence : Comparison to      CERN Results                   and  Models

**Saturation models** **reproduce the scaling with centrality and energy dependence!**

**Hard processes contribution**
**increases with centrality:**
** ****~50%**** ****for most central collisions**

**  ****Evidence of hard processes?**

_**New regime **_
_**at RHIC?**_

## Transverse energy per charged particle

**dE****T**** ****/dN****ch**** independent of *********s******NN***

**dE****T**** ****/dN****ch**** independent of  ****N****part**

**• ****Consistent with very moderate increase of <p****T****> from AGS to RHIC:**
**         ****NA49    <p****T****>****h****- = 385 MeV          RHIC  <p****T****>  ********450  MeV **

** • ****UA1:  ****<p****T****>  =392  MeV          dE****T**** / dN****ch**** = 0.9**

## Global Observables: Summary 

**       ********* Stronger increase than at the CERN SPS **
**       ***** ****Evidence for role of hard processes ?**
**       ***** Saturation models?**

_**Global Observables: Summary **_

**  ****•  ****E****T****  per N****ch****  independent of centrality and of energy**

**•   ****dN****ch****/d*****y***** , dE****T****/d*****y*****  and ********Bj****:  **ln(*s**NN** *)** dependence from AGS to RHIC**
**    ******** ****~**** 9****0%**** ****increase from **** ****SPS at *********s******NN*****= 17.2 GeV****  to ****RHIC**** **** **
**    ****at  *********s******NN*****= ****200 GeV**** **** **** **

**•  ****<p****T****> **** very moderate increase O(20%)  ****between  SPS and RHIC **

**•  ****Increase of particle production predicted by HIJING with **
**    ****quenching not observed.**** **

**  ****•  ****Rapidity distribution:**** same properties as observed in SPS **

## The Hadronic System 

**•    ****Particle ratios and chemical equilibrium     **

**•    ****Particle spectra ******** radial flow and thermal freeze out**

**•    ****Baryon stopping and baryon density. **

**•    ****Elliptic flow**

**•    ****HBT **

**•   ****Stangeness and multi-strangeness production**

- Itzhak Tserruya - Jaipur 2001

- 26 Nov. 01

## Particle spectra : thermal freeze-out

_**Particle spectra : thermal freeze-out**_

**T****fo**** (RHIC) = 0.13 GeV as at SPS**
**ß****r**** (RHIC) = 0.52c       ß****r**** (SPS) = 0.4c **
**Stronger radial flow at RHIC ******** higher pressure             **

- Simple hydrodynamics:
- Teff = Tfo +  mass * 2r

**Notes:**

 Thermal equilibrium and radial flow. Quote and compare to SPS – Also figure from SPS?
Look at Nu Xu QM01 slides and slide 21 in particular.

## Particle ratios: chemical freeze-out

_**Particle ratios: chemical freeze-out**_

- Simple statistical thermal model works well
-   evidence for chemical equilibrium

**Chemical freezeout**

** ****RHIC Au-Au ********s****NN****=130 GeV **

**SPS  Pb-Pb ********s****NN****=17 GeV **

|  | RHIC | SPS |
| --- | --- | --- |
| Tch(MeV)
 B (MeV) | 1757
516 | 1685
270 |

**Strong decrease of **
**baryo-chemical potential**
****** ****low net-baryon density**

- Itzhak Tserruya - Jaipur 2001

- 26 Nov. 01

## Baryon Density at Mid-Rapidity

_**Baryon Density at Mid-Rapidity**_

**  ****Nice agreement between the four**
**experiments.**
**   ****Average p/p = 0.61********0.03**

**   ****Ratio ******** 1**

##  Elliptic Flow (I)

**V****2**

**  ****in reasonable agreement  with hydrodynamics**
**  ****implies early pressure buildup **
****** ****compatible with early equilibration **

**Very strong elliptic flow  (********50% stronger than at CERN)**

_** **__**Elliptic Flow (I)**_

_**Centrality dependence**_**            **_**Energy dependence**_

**E****beam ****/A (GeV)**

- central

- peripheral

## Elliptic Flow (II) : pT Dependence

_**Elliptic Flow (II) : p**__**T **__**Dependence**_

-  **Hydrodynamic calculations:**
-    in reasonable agreement at low pt .
-   fail at high pt .

- v2 vs pT and particle mass

**OR**
**Evidence of QGP and mixed phase? **(Sorge PRL 82, 2048 (1999)

**Jet quenching propagating through anisotropic matter? require large gluon densities **(Gyulassy, Vitev and Wang (nucl-th/00012092)

## HBT

- STAR, PRL 87, 082301

**The big puzzle:**
**  ****Almost all parameters have the **
**    ****same values at AGS, SPS and RHIC**
**   ****If QGP is formed R****o **** >> R****s**
**    ****Observation: R****o**** ******** R****s**
**  ****Revise picture of space-time evolution?**

- Itzhak Tserruya - Jaipur 2001

- 26 Nov. 01

## Strangeness Production 

_**Strangeness Production **_

**factor 17**

-   Kaons carry about 75 % of the global     strangeness content of the final state
-  At SPS  factor  2  global strangeness enhancement  wrt elementary collisions

-   Hierarchy of enhancement   >  > 
-   Factor  17  enhancement for triple strange 

**  ****Interpretation still unclear :**
**     ****-- statistical thermal model ?**
**     ****-- QGP signature?**
**  ****RHIC data and 40 GeV SPS data will be very interesting.**

## Hadron Gas: Summary

**Particle spectra and particle ratios follow the AGS and SPS**
**    ****systematics.  **
**Almost the same decoupling temperatures at SPS and RHIC but **
**    ****larger radial flow at RHIC.**
**Net baryon density at RHIC is much smaller  than at AGS and SPS,****but total baryon density is slightly higher than a the SPS.**
**Anomalous high p****T ****elliptic flow.**
**What do we learn from HBT?**
**Waiting for results on strangeness.**

- Itzhak Tserruya - Jaipur 2001

- 26 Nov. 01

##  Electromagnetic Probes 

**•  ****e.m. probes are emitted through the entire history of the collision **_** **_
**   **Large mfp no final state interaction
-                          carry information from place of creation to detectors.
-    **C****hallenge: disentangle the different components:**
-        ** **_**Thermal radiation**_** from **
**           ****QGP:     *****q q   ************   *************   ************   l ******+****** ******l ******-***
***           *****HG:      ***********+************ ******- ****** ************   *************  ************   l ******+****** ******l****** -***
-         _** **__**Chiral symmetry restoration**_
-          *** Best candidates: ********-meson decay**
**                                        ****combined study of ******** ******** l****+ ****l****- **** and ******** ******** K****+ ****K****-***** ***       ** **
**•****  **_**Strangeness Enhancement**_
-          ***   ***********  *****meson yield measured via its leptonic decay channels***** ***
**•  **_**Charm Enhancement**_
**        *****  semileptonic decays of charmed mesons**

## Low-Mass e+e- pairs

_**Low-Mass **__***e***__***+***__***e***__**-**__** pairs**_

**Main CERES Result:**
**Strong enhancement of low-mass pairs in A-A collisions**
**(wrt to expected yield from known sources)**

**Enhancement factor ****(.25 <m<.7GeV/c****2)****:**
**           ****2.6 ****± 0.5 ****(stat) ****± 0.6 ****(syst)**

**An (even stronger ?) enhancement observed  at 40 A GeV**

- Itzhak Tserruya - Jaipur 2001

- 26 Nov. 01

## Onset of Chiral Symmetry Restoration?

**Dropping ********-meson mass**

-  (Rapp, Wambach et al)

**In-medium ********-meson broadening**

- (G.E. Brown et al)

**What happens as chiral symmetry is restored?  Dropping masses or line broadening?**

- Itzhak Tserruya - Jaipur 2001

- 26 Nov. 01

## Quark – Hadron Duality

**In-medium ********+**** ********-**** ann. rates  ********  perturbative qbarq ann. rates**
**      ****quark – hadron duality down to m ****~**** 0.5 GeV/c****2**

- R. Rapp

**Thermal radiation from the plasma?**

- Itzhak Tserruya - Jaipur 2001

- 26 Nov. 01

## Direct Photons 

**WA98 **

## Low-mass e+e- Pairs: Prospects at RHIC

**  ******** ****At SPS energies, both scenarios (********-mass dropping and ********-width broadening) rely on a high baryon density at mid rapidity. **

****** ****What can  we expect at RHIC?**

- Itzhak Tserruya - Jaipur 2001

- 26 Nov. 01

## Lepton Probes at RHIC : the beginningSingle Electron Spectra

- Expected contributions from:
-     0,  ’ Dalitz decays
-  conversions
-    resonance decays
-    charm and bottom
- Remaining signal is then from
-    thermal radiation
-    new physics

**D****0****      K****-****  ********+**

**Charm    **

**Beauty**

**Drell-Yan   **

******0****,******** Dalitz**

****** ****conversions**

**Simulation**

**Now Real Data !**

- Itzhak Tserruya - Jaipur 2001

- 26 Nov. 01

## Hard  Probes (I): J/  suppression

**•  **_**An “old” signature of QGP formation:**_
**    ****(**Matsui and Satz PL** B178,** (1986) 416).
***  At high enough color density, the screening radius < binding radius ******** J/******** will melt **

**•  **_**One of the first observations:**_
*** J/******** suppression in 200 A GeV S-Au collisions **
*** Alternative explanation: absorption in nuclear medium:**
**  ****J/******** + *****N ***** ******** *****DD*****           *********abs ****** ~ *****6mb**

**•  **_**Anomalous suppression in Pb-Pb collisions**_
** **

- Itzhak Tserruya - Jaipur 2001

- 26 Nov. 01

## Anomalous J/ suppression in Pb-Pb collisions

_**Anomalous J/**__****__** suppression in Pb-Pb collisions**_

**Normal nuclear absorption:**
**        *********abs****** = *****6.4 ± 0.8 mb**

**NA50**

**Anomalous absorption in Pb-Pb for**
**E****T ****> 40 GeV or N****part ****>100 or b < 8****fm**

- Itzhak Tserruya - Jaipur 2001

- 26 Nov. 01

## J/  suppression: Evidence of deconfinement ?

_**J/**__****__**  suppression: Evidence of deconfinement ?**_

**Conventional models ruled out**

**Two-step pattern: **
**successive melting of charmonium states**
** ********c**** **** (binding energy ******** 250 MeV) and  **
** ****J/******** (650 MeV)**

**NA50**

- Also: Capella et al.

- Hadronic models: cold nuclear + “comover” dissociation

- QGP models: energy density thresholds + ET fluctuations

- “thresholds” and high ET behavior favour QGP models

## Jets and minijets expected to contribute to particle production at RHIC energies

_**Hard Probes (II): Jet Quenching at RHIC?**_

**schematic view of  jet production**

- Itzhak Tserruya - Jaipur 2001

- 26 Nov. 01

## Charged pT Spectra

- Itzhak Tserruya - Jaipur 2001

- 26 Nov. 01

## pT Distributions in pp collisions

**Data very well described by power law:**
**         ********pp**** = d****2****N/dp****t****2**** = A (p****0****+p****t****)****-n**

**  ****Scale to central (semi-inclusive)**
**   ****collisions: **
**    ****Y****AA**** (b) = ********pp**** T****AA****(b) = Y****pp ****N****coll**
****** ****Nuclear modification factor:**

** ******** ****Interpolate existing data to **
*******s******NN*****  = 130 GeV/c**

**  ****Scale to minimum bias (inclusive) **
**   ****AA collisions: **
**        ********AA**** = A****2**** ********pp**** **

- Itzhak Tserruya - Jaipur 2001

- 26 Nov. 01

##  pT Spectra: comparison to pp  

**Yield in central collisions is significantly reduced relative to scaled pp yield**

_** **__**p**_**T**_** Spectra: comparison to pp  **_

- Itzhak Tserruya - Jaipur 2001

- 26 Nov. 01

## Central to pp ratio

** ****Ratio at high p****T**** significantly **
**  ****below 1**
******o ****suppression more**
**    ****pronounced ?**

_**Central to pp ratio**_

**                 **_**SPS**_
**Known nuclear effects:**
**  ****Low p****T**** :****  ****soft processes ******** N****part**
**        ****R ******** N****part ****/ N****cooll  ****~ 0.2**
** ****High p****T**** : broadening due to **
**  ****rescattering (Cronin effect)**
**         ******** ****R > 1.**

## Central to Peripheral Ratio

**Same behavior observed in the ratio of central to peripheral collisions****  **

**N****coll**** 10% central = 905 ± 96**
**N****coll****  60-80% peripheral = 20 ± 6**

- Itzhak Tserruya - Jaipur 2001

- 26 Nov. 01

##  Spectra of Identified Hadrons

**proton/antiproton contribution dominates above p****T**** > 2 GeV   **

_** **__**Spectra of Identified Hadrons**_

**This is not the expected jet fragmentation function D(z)!**

- Itzhak Tserruya - Jaipur 2001

- 26 Nov. 01

**Good agreement with**
** ****peripheral collisions**

- Calculations:
- X.N. Wang

_** **_

_**Comparison to Theory **_

** ****p-QCD over estimates the  cross-section  for ********0**** at least a  factor of 5**
** ****Consistent with parton energy loss**
**of dE/dx = 0.25 GeV/fm**

- Red: “Cronin” + shadowing

- Green: dE/dx = 0.25GeV/fm

- Black : No nuclear effect

-       Data:

## Summary

**  **** ********* **_**Outstanding start of RHIC machine and experiments**_
-   ** ******* _**A  very strong  field:**_
**          ****•  ****15 years of AGS and SPS notorious achievements**
**          ****•  ****A blooming present with RHIC and some more SPS**
**          ****•  ****A bright future with RHIC and LHC**

- Itzhak Tserruya - Jaipur 2001

- 26 Nov. 01