# Presentation: 354143

## Can we discover the critical point at RHIC?Experimental Overview

- Gunther Roland

*BNL*
* **March 9 2006*

- Thanks to
- Burak Alver, Ed Wenger, Siarhei Vaurynovich, Wei LI

## Exploring the QCD Phasediagram

## Exploring the QCD Phasediagram

## Exploring the QCD Phasediagram

**Temperature (MeV)**

**Quark-Gluon Plasma**

**Hadron Gas**

**Phase Boundary**

**0**

**200**

**0**

**Atomic Nuclei**

**1**

- Plot from M. Stephanov,
- Correlations ‘05

- Challenge: Guidance on exact location and
-  strength of correlation signals is limited

## Experimental Search for the Critical Point

- Theoretical guidance is limited
- Experimental strategy:
- Search in different observables, correlate observables
- Maximize experimental sensitivity
- Fine grained exploration of μB, T space
- Side effect:
- Sensitivity to critical phenomena at 1st order transition, Onset of Deconfinement, EOS softest point,  changes in bulk dynamics etc

- ➡NA49

## Experimental Search for the Critical Point

- This idea/strategy is not new
- This talk (this workshop)
- What is known from excitation functions?
- What could be done better at RHIC (and how good)?

- RHIC

- SPS

- AGS

- First of many
- experimental talks

## Fixed target vs collider experiments

- If rates O(1Hz) or greater:
- √s scan of _hadronic_ observables feasible

## Suitable hadronic observables

- Here: Simple observables
- Multiplicities and multiplicity correlations
- Spectra and momentum correlations
- Net charge correlations
- Elliptic flow
- Particle ratios and ratio fluctuations

## Excitation Function: Mid-rapidity Multiplicities 

## Excitation Function: Total Multiplicities 

- RHIC

- SPS

- AGS

- Compilation by PHOBOS
- nucl-ex/0301017
- submitted to PRC

- e+e-

- log2(s)
- Wit Busza,
-  Zakopane ‘04

## Fraction of produced hadrons in Nch

- Change from baryon- to meson dominated system
- over the energy range of interest!

- RHIC

- SPS

- AGS

- Excitation Function: Total Multiplicities

- c.f.  Gazdzicki, Steinberg

## Excitation Function: Momentum Spectra

- Compilation by NA49
- Plot from Claudia Hoehne, QM’05

- filled symbol:  particle
- open symbol: antiparticle

- - Structure in energy dependence of <mT>
- - Reminiscent of  Van Hove’s T vs ε prediction (1982)
- - Surprisingly difficult measurement
- 		● Decay corrections, PID acceptance

- “The Kink”

## Excitation Function: Momentum Fluctuations

- RHIC
- STAR

- SPS
- CERES

- AGS?

- Compilation by STAR
- STAR PRC 72 044902 (2005)

- Central A+A

- “The Kink”

- Monotonic energy dependence over measured range
- No results near “kink” region

## Excitation Function: Momentum Fluctuations

- STAR

- Interpretation connected to choice of variables

- Connection to
- 2-particle correlations

- Scaling: Connection
- between <pT>, N, fluctuations?

- ΣpT≈

## Excitation Function: Charge Fluctuations

- QGP

- Little (no) √s  dependence of charge fluctuations

- Plot from Claude Pruneau
- RHIC Users meeting workshop ‘04

- NA49, PRC 70 064903 (2004)

- PHENIX: PRL 89 082301 (2002)

## N.B. Multiplicity Fluctuations

- effective cluster size ≈ 2-2.5
- for 200 GeV Au+Au

- Analysis of forward/backward multiplicity correlations
- Particles are produced in clusters of 2-3

- PHOBOS, Peter Steinberg, QM’05

## N.B. Multiplicity Fluctuations

- effective cluster size ≈ 2-2.5
- for 200 GeV Au+Au

- “Cluster” in Δη,Δφ space via
- 2-particle correlations
- (pythia p+p @200 GeV, η<3)

- Lesson:
- Hadronization cannot be ignored in
- discussion of early stage fluctuations

- PHOBOS, Peter Steinberg, QM’05

## Excitation Function: Elliptic flow

- Low Density Limit:
- STAR, PRC 66 034904 (2002)
- Voloshin, Poskanzer, PLB 474 27 (2000)
- Heiselberg, Levy, PRC 59 2716, (1999)

- Cu+Cu: PHOBOS QM 2005

- Cu+Cu

- Au+Au

- “Participant Eccentricity”
- provides universal scaling
- Approach to equilibrium or
- sampling different regions of EOS?

- “Hydro-
- Limit”

## Now to the main event.....

## Excitation Function: Particle Ratios 

- Excitation Function: Particle Ratios

- Non-monotonic behavior at AGS/SPS boundary

- NA49 Phys.Rev.C66:054902,2002
- Plot from Claudia Hoehne, QM’05

- “The Horn”

## Excitation Function: K/π Fluctuations

- Lesson:  Keeping acceptance/experimental technique constant is essential for precision studies

- “The Horn”

- RHIC
- STAR
- Preliminary

- SPS
- NA49
- Preliminary

- AGS?

- NA49, Christof Roland, QM’04

- STAR, Supriya Das, VI Workshop ‘05

## Excitation Function: K/π Fluctuations

- RHIC

- SPS

- AGS?

- Normalized by dN/dy

- Scaling by dN/dy or (dN/dy)1/2 does not work over full energy range

## Why use a collider?

- Can  RHIC low-√s scan improve on SPS measurements?

## Event-by-event fit of K/π (NA49)

- One Event:

- Event Ensemble:

**<dE/dx>**

**<dE/dx>**

**                 **

**log(p****tot**** [GeV])**

**                 **

**log(p****tot**** [GeV])**

- Slide from Christof Roland,
- Correlations ‘05, MIT

## Extracting dynamical fluctuations (NA49)

**Compare data to **
**mixed events:**
- σ2**data ****-  **σ2**mix****  = **σ2**dynamic**

**Process the relative widths of the distributions:**σ** = RMS/Mean * 100  [%]**

- Slide from Christof Roland,
- Correlations ‘05, MIT

## Challenge: Changing acceptance (NA49)

**20 GeV: **

**40 GeV: **

**160 GeV: **

- Slide from Christof Roland,
- Correlations ‘05, MIT

## Mid-rapidity spatial track density

**“****You prefer your collider”**
**Les Horribles Cernettes**

- Fixed
- target

- Collider

- Collider

- Fixed
- target

- Track density only rise logarithmically at collider

- from this:Occupancy
- Double hits
- Space Charge
- Baseline shifts etc

## What can be achieved at RHIC?

- “Ideal Detector”

- * >80% tracking + PID efficiency
-   from 0.1 < pT < 2.0 GeV/c,
-   over 2+ units of rapidity
- * Large acceptance trigger/centrality
-    counters
- * High resolution vertexing
- Assume 100k central sample at each √s

- * 2π ToF PID coverage near y=0 possible
- * Acceptance in (pT,y) independent of √s
- * Track density grows logarithmically with√s
- * Consistent centrality determination from Nch
- But: give up very low pT , luminosity vs fixed target

- PID + trigger

- Tracking

- Vertexing

- Trigger

- Tracking

- Vertexing

- Trigger

- PID + trigger

## K/π fluctuations at low-√s RHIC

- Estimate that RHIC low-√s scan could
- allow reduction of statistical errors to ~ 1/4 of NA49
- Significant reduction of systematics of √s dependence

## What about low pT?

- Median Kaon pT ~ 0.5 GeV
- (constant over SPS range)

- Critical fluctuations:
- Long Wavelength
- ➡ look at low pT

- A low pT measurement (0.1 - 0.5 GeV/c) would capture ~ 45% (75%) of all kaons (pions)

- Fraction of particles below pT

- based on NA49 <mT>

## Low pT K/π Fluctuations at √s RHIC

- Low pT K/π Fluctuations at √s RHIC

- Simulation
- Statistical errors only

- Assumptions:
- * Ideal RHIC detector (see above)
- * 100k central Au+Au at each √s
- * K/π Fluctuations from NA49 are correct
- * 0.1 < pT < 0.5 GeV/c

- Fluctuations
- only at low pT

- Fluctuations
- at all pT

- Conclusion:
- RHIC low-√s scan should allow precision measurement  of (low pT) K/π fluctuations

## Summary

- Strong theoretical motivation for A+A at √s from 5-50 GeV
- Rich existing data set in this range
- Non-trivial structure/change in yields, ratios,  fluctuations/correlations
- Limited statistical/systematic significance
- Low-√s scan at RHIC could confirm and _improve_ experimental results

## Summary cntd

- To improve on SPS, low-√s run needs to be done right
- Sufficient run time (e.g. 1 RHIC run)
- Large acceptance ToF PID
- High resolution vertexing
- Careful event selection/triggering
- Sufficient number of energies (≥ 5)

## Slide 34

## Correlations and Fluctuations, revisited

- centrality →

- Central A+A

- Text

- p+p

- Net-charge correlations (c.f. Balance Function)

- Net-charge Fluctuations

- Global scaling, even
- though underlying
- correlations change