# Presentation: 524006

## Recent* Results from A+A Collisions at RHICPaul Stankus, ORNL

- First colliding nuclear beams
- First accelerator designed for high-energy heavy ions
- * Recent = July ‘02, “Quark Matter” in Nantes, France

## Goal

- To persuade (some of) you that pQCD in A+A collisions
- at RHIC could be a very rich and interesting field:
- 	Exotic initial state effects
- 	Exotic collision effects
- 	Exotic final state effects
- 	Many channels, processes and probes
- 	Dramatic data available now! with much
- 	   more to follow

## Exotic (p & non-p)QCD effects

***Initial State:***
- High density
- of partons at low
- and modest *x*;
- Unusual shadowing?“Colored Glass
-   Condensate”?

***Collision Overlap:***
- Multiple scattering
- of partons; large
*k*T smearing?
*Very* high density
- of gluons; Classical
- color fields?

***Final State:***
- Hard-scattered
- products exit
- through excited
- medium;
- Medium Effects?
- Is fragmentation
- modified?

## Our Original Motivation

- To use hard scattering products as probes to measure the properties of dense, highly excited QCD matter (what you would call final-state-effects)
- We originally conceived of hard-scattering products as “calibrated sources” created within HI collisions (we have since learned better!)

- We can measure a full
- palette of hard-scattering
- products:

- q: fast color triplet

- g: fast color octet

- Q: slow color triplet

- QQbar: slow color
- singlet/octet

- Virtual photon: colorless

- Real photon: colorless

**Unknown Medium**

- Induced
- gluon
- radiation?

- Energy
- Loss?

- Dissociation?

- Controls

## Jet Prelude: Why is this hard?

- Cannot look at “true”, traditional calorimetric jets; soft particle energy density dET/dd~ 100 GeV/unit-radian
- Next best thing: leading particles = high-PT hadrons, and also high-PT pairs, either same side (leading and next-leading) or opposite side (leading and opposite leading)
- Ambiguity between hadrons from jet fragmentation source and hadrons from multi-collisional/”thermal” source, even out to several GeV/c (and beyond?)
- Model “thermal” source: hadron gas with  temperature, chemical (ie flavor) equilibration, baryon density and overall flow velocity in radial direction

## Thermal Model can fit hadron spectra out to at least 4 GeV/c -- do you believe it?

- description by hydrodynamical source
- perfect description possible
- Tch  = 172 ± 2 MeV
-   B = 37 ± 4 MeV
- Tkin = 123 ± 6 MeV
-  < T > = 0.45 ± 0.02

- spectra of pions and (anti)protons

- T.P., nucl-th/ 0207012

- T. Peitzman

## History of High-Energy A+A Beams

**BNL-AGS**: mid 80’s, early 90’s
-         O+A, Si+A   15 AGeV/c     s1/2NN ~ **6 GeV**
-         Au+A            11 AGeV/c    s1/2NN ~ **5 GeV**
**CERN-SPS**: mid 80’s, early 90’s
-         O+A, S+A   200 AGeV/c    s1/2NN ~ **20 GeV**
-         Pb+A           160 AGeV/c    s1/2NN ~ **17 GeV**
**BNL-RHIC**: early 00’s
-         Au+Au                                 s1/2NN ~ **130 GeV**
-         Au+Au, p+p                         s1/2NN ~ **200 GeV**

**Finally: enough energy for copious hard scattering processes!**

## Nomenclature: Centrality

- Characterize A+A collision
- intuitively in Glauber model:
- Here NParticipant = 4
-          NCollision = 3
- <NColl> = <TAB>N+N inel

- 40 mbarn

- Describe classes of events by
- percentile of impact parameter
- distribution:

- Peripheral; 60%-80%
- <NCollisions> = 20 +- 5

- Central; 0%-10%
- <NCollisions> = 850 +- 20

## Quantifying Nuclear Effects

- R = eA(x,Q2)/A ep(x,Q2)   General DIS

- RA = pA(PT)/A pp(PT)  Hadron PT spectra

- pA(xF) = A pp(xF)    eg  DY, J/

- R

- RA

- 

- Shadowing, EMC, etc.

- Cronin effect

**?**

- x

- PT

- xF

- Absorption, initial state energy loss

- (This space available!)

## RHIC Year-1 High-PT Hadrons

- Charged and neutral hadron spectra out to pT~4-5 GeV/c
- Nominally expect production through hard scattering, scale spectra from N+N by number of binary collisions
- Peripheral reasonably well reproduced; but central significantly below binary scaling

## Last Year’s Big News

**Observe:**
- RHIC spectra fall below binary scaling at all pT for central events
- Previous highest energy A+A collisions exceed binary scaling (Cronin expectation)
**Suspect:** scattered parton interaction in dense medium; **but** must keep an open mind

## “The cover of the Rolling Stone”

- (Almost) No one reads PRL on paper these days.
- Cover artists thought the graph looked better without numbers on the axes.
- (We were pleased nonetheless.)

## Charged Hadron Spectra

- C. Jorgensen, BRAHMS
- Parallel Saturday

- C. Roland,
- PHOBOS
- Parallel Saturday

- J. Jia, PHENIX
- Parallel Saturday

- J. Klay, STAR
- Parallel Saturday

- 200 GeV results from all experiments

- T.Peitzman

## RAA Comparison to pT = 6 GeV/c

- RAA Comparison to pT = 6 GeV/c

**Similar Suppression in all centralities at 200 GeV**

- J.Klay

## Central/Peripheral Comparison

**At 130 GeV, the ****suppression increases up to p****T**** = 6 GeV/c.  **

**With higher p****T**** data from 200 GeV, we see that**** ****the suppression has saturated at p****T**** ~6 GeV/c**

- 0.5

- 0.5

- J.Klay

## High pt suppression at 130 GeV

**130GeV**

**PHENIX (nucl-ex/0207010)**

**Consistent with STAR(nucl-ex/0206011)**

- Detailed pT and centrality dependence
- Peripheral RAA  1
- Central RAA saturates ~ 0.6 at   pt >2GeV/C

- J.Jia

## Ratio central/peripheral

- Lower ratio for     200 GeV
- more suppression or change in proton yield?
- Similar shape for  130 and 200 GeV
- increase to 2 GeV/c
- decrease to 4 GeV/c

- colored bracket represent the systematic error.
- thick black line is uncertainty of the scaling factor from N. collisions

- J.Jia

## Charged particle pT spectra from 200 GeV

- Charged particle pT spectra from 200 GeV

**p****T**** <2 GeV/c,  increase of inverse slope ******** flow**

**p****T**** >2 GeV/c,  decrease of inverse slope   ******** suppression **

- h+ + h-

- J.Jia

## Comparison with NN references I

- RAA for peripheral collisions
- ~ 0.75 ± 0.3  for pT>2GeV/c
- consistent with 1
- similar to 130 GeV

- Shaded band is syst. error from NN

- Common syst. Scaling error

- RAA for central collisions
- significantly below 1
- 200 GeV below 130 GeV data
- ~ 0.2 ± 0.08 from 4 to 8 GeV/c

- Calculate RAA: divide data by NN references

- J.Jia

##   PHENIX Overview

-   PHENIX Overview

- Most of central arms used to measure the pion spectrum
- Powerful cross-checks of results

- S.Mioduszewski

## Comparison with UA1 Fitting

- UA1 data are only up to 6GeV/c and extrapolated to higher pT
- The extrapolation is below our data at high pT
- Now have pp data to use as important reference for Au+Au collision and jet quenching measurement.

- PHENIX Preliminary

- UA1 data

*extrapolation*

*p**T** dependent systematic error*

*Normalization systematic error 30% is not included here.*

- H. Torii

## NLO pQCD calculation
CTEQ5M pdf
Potter-Kniehl-Kramer fragmentation function
 = pT/2, pT, 2pT
Consistent with data within the scale dependence.

- NLO pQCD calculation
- CTEQ5M pdf
- Potter-Kniehl-Kramer fragmentation function
-  = pT/2, pT, 2pT
- Consistent with data within the scale dependence.

- Comparison with QCD Calculation

- PHENIX Preliminary

*p**T** dependent systematic error*

*Normalization systematic error 30% is not included here.*

- H. Torii

## Nuclear Modification Factor

- SPS – “Cronin” effect
- RHIC - suppression
- Our own measure of the p+p spectrum  reduces the uncertainty!

-   Effect of nuclear medium on yields

- PHENIX Preliminary

- D. d’Enterria talk

- binary scaling

- S.Mioduszewski

## Suppression in Inclusive Photons

- Photons (primarily from 0 decays) also show suppression
-  Not an artifact of extraction of  0 peak yield

- Klaus Reygers talk

- S.Mioduszewski

## Hadron Species Ratios in Run-1

**(Anti)Baryon/pion ratios rise well above values in p+p**
**Suspect** radial hydrodynamical flow boosting baryons while mesons are suppressed; **but**
- Similar effect seen in p+A/p+p (Cronin);
- Could potentially be a modification to the fragmentation process

## pbar/ and p/ 

*By Takao Sakaguchi at Quark Matter 2002, July 18-24 at Nantes, France*

- pbar/ , p/ ratios
*p**T*<2GeV, pbar/-, p/+
*p**T*>1GeV, use 0 with -, +
- Point-by-Point Errors include point-by-point statistics+systematic errors
- Bands: *p**T* independent systematic errors
- Decreasing at much more high *p**T*?

**pbar/pi**

**p/pi**

- T.Sakaguchi

## Comparison with Year-1 Data

- Data Compared to Year-1
- Both Year-1 and Year-2 are consistent within systematic errors

*By Takao Sakaguchi at Quark Matter 2002, July 18-24 at Nantes, France*

- Another hint.
- More  rather than protons?

- T.Sakaguchi

## Particle Composition at high pT

- 0/(h++h-)/2  ratio ~ 0.5 up to 9 GeV/c
-   do protons continue to make up a large fraction of charged                                                      hadron yield?

- S.Mioduszewski

## Interlude: “Elliptic Flow”

- b

- 

- +

- -

- dN/d**v****2** cos(2)

- The impact parameter vector defines the “reaction plane direction” in non-central collisions.  Low PT particles “feel” this geometry and show a quadrupole distribution relative to the event plane direction.
- Event plane directions were first measured with recoiling beam fragments, but can also be derived from low-PT  distributions

## How to sense geometry

- Pressure gradients lead
- to collective motion

- High-Pt
- hadrons

- Fragmentation

- Heavy flavor quark endures;
- Is there medium interaction?
- Thermal production?

- Hard-scattered partons travel
- through early medium; modification?

- Difference in pressure gradients
- can lead to anisotropic motion

- Identical pair correlations reveal space-time geometry

- Br?

## Comparison v2 (pT) with models (130 GeV)

-  qualitative agreement with “jet-quenching” scenario

- Adler *et al*., nucl-ex/0206006

- K.Filimonov

## v2(pT) up to 12 GeV/c

-  Statistical errors only

-  Finite v2 up to 12 GeV/c in mid-peripheral bin

- K.Filimonov

## Sources of azimuthal correlations

- Au+Au
- flow
- p+p and Au+Au collisions:
- dijets
- momentum conservation
- jets
- resonances

- Small 

- All 

- STAR Preliminary
- Au+Au @ 200 GeV/c
- 0-5% most central
- 4<pT(trig)<6 GeV/c
- 2<pT(assoc.)<pT(trig)

- D.Hardtke

## Relative Charge Dependence

- Strong dynamical charge correlations in jet fragmentation 
- Compare ++ and -- charged azimuthal correlations to +- azimuthal correlations

- STAR Preliminary @ 200 GeV/c
- 0-10% most central Au+Au
- p+p minimum bias
- 4<pT(trig)<6 GeV/c
- 2<pT(assoc.)<pT(trig)

| System | (+-)/(++ & --) |
| --- | --- |
| p+p | 2.7+-0.6 |
| 0-10% Au+Au | 2.4+-0.6 |
| Jetset | 2.6+-0.7 |

- ||<0.5 - ||>0.5 (scaled)

- 0<||<1.4

- Same particle production mechanism for pT>4 GeV/c in pp
- and central Au+Au

- Au+Au

- p+p

- D.Hardtke

## Excess Above Flat Background, p-p

**Excess Above Flat Background, p-p**

**Data points (black) are background subtracted and acceptance corrected.**
**Blue is the PYTHIA curve * a****pythia**** * <********>**

- M.Chiu

## Excess from Flat Bkg, Au-Au

**black = associated charged particles, ****green = mixed,**** purple = subtracted**

**In AuAu collisions there is a statistically significant excess from a flat distribution at all centralities and all pt bins.**
**So what is that excess?  Try both ****PYTHIA**** only and also ****PYTHIA**** + elliptic flow contribution.**

- M.Chiu

## Fitting Pythia + 2vchvtrigcos(2),pt dependence

**v****ch****v****trig**

- 1/Ntrig dN/d****

**0.3-0.6 GeV**

**0.6-1 GeV**

**1- 2 GeV**

**2- 4 GeV**

- PHENIX Preliminary

- PHENIX Preliminary

- PHENIX Preliminary

- PHENIX Preliminary

**For lower pt, ambiguity between the contribution from the elliptic flow component and the jet-like component.**
**At higher pt (2 GeV and above), the jet-like component dominates over any elliptic flow component.**

**Fitting Pythia + 2v****ch****v****trig****cos(2********),****pt dependence**

**a****pythia**

**20-40% Cent**

- M.Chiu

## Peripheral Au+Au data vs. pp+flow

- D.Hardtke

## Central Au+Au data vs. pp+flow

- D.Hardtke

## Ratio vs. # participants

- D.Hardtke

## Npart Scaling at high pT

- Npart scaling describes data at pT 4.25 GeV/c

- Normalized to yield
- at Npart = 65

- Npart Scaling at high pT

- Ncoll-scaling

**PHOBOS Preliminary**

- C.Roland

## Central/Semi-peripheral collision at y2

** ****Indicates suppression of high p****T**** ****pions at y********2.**

** ****Sets in at lower p****T  ****(compared to y=0)?**

- Central/Semi-peripheral collision at y2

- C.Jorgensen

## Separation of non-photonic e±: cocktail method

**PYTHIA**

**direct ******
**(J****.**** Alam et al. PRC 63(2001)021901)**

- R.Averbeck

## Energy dependence of charm production 

- R.Averbeck

## Centrality dependence at 200 GeV 

- Centrality dependence at 200 GeV

- R.Averbeck

## p-p    ee Invariant Mass Spectrum

- Same plots as previously but now on a linear scale.

**p-p    ee Invariant Mass Spectrum**

- This analysis

- All triggers

**N****J/******** = 24 **_**+**_** 6 (stat) **_**+**_** 4 (sys)**
**Bd********/dy = 52 **_**+**_** 13 (stat) **_**+**_** 18 (sys) nb**

- A.Frawley

## pp    Invariant Mass Spectrum

**1.2 < y < 1.7  ****N****J/******** = 26 **_**+**_** 6 **_**+**_** 2.6 (sys)****   B d********/dy = 49 **_**+**_** 22% **_**+**_** 29% (sys) nb**
**1.7 < y < 2.2  ****N****J/******** = 10 **_**+**_** 4 **_**+**_** 1.0**** ****(sys)****   B d********/dy = 23 **_**+**_** 37% **_**+**_** 29% (sys) nb**

- A.Frawley

## * See J.F. Amundson et al., Phys. Lett. B 390 (1997) 323.

********** ****(pp->J/********) = 3.8 **_**+**_** 0.6 (stat) **_**+**_** 1.3 (sys) ********b**

- Gaussian and PYTHIA shape fits give essentially the same integral.
- The quoted result is the average of the two fits.

- A.Frawley

## Au-Au     ee Invariant Mass Spectra

**N****J/******** = 10.8 **_**+**_** 3.2 (stat) + 3.8 - 2.8 (sys)**

**N****J/******** = 5.9 **_**+**_** 2.4 (stat) **_**+**_** 0.7 (sys)**

- A.Frawley

## Are our data consistent with binary scaling?

- Are our data consistent with **binary scaling**?
- A confidence level of 16% says that a truly flat distribution would produce a fit as poor as this **in 16% of cases**.
- So it **probably** trends down with increasing Npart, but don't bet the farm!

- A.Frawley

## Scorecard

- Jets/High-PT hadrons: Lots of action!  Singles yields, spectra, composition do not agree with pQCD expectations, while high-PT pair correlations are a mixed bag.  And this is only the beginning!
- Open charm and J/ seem to agree with Pythia predictions, but statistics are limited.
- Lots more to come!  From existing data: direct photons, more detailed pair correlations; in later years ’, Y, DY, g+jet, double direct photons; **plus** p+A/d+A; etc.etc....
- Plenty for any theorist to work with! And many fundamental QCD issues potentially involved.