# Presentation: 593103

## The little bang at RHIC
John P. Sullivan
Los Alamos National Laboratory

**John P. Sullivan**
**Los Alamos National Laboratory**

## Outline

- What are we doing and why?
- What is RHIC?
- Where is RHIC?
- Brief description of PHENIX detector
- What do the collisions look like? Geometry?
- Measuring temperature
- Measuring density
- How close have we come to the early universe?
- Are there any signals?
- The future

## Phase Transitions

- We hope to recreate a small piece of matter above 1012 °K,  consisting of a plasma of quarks and gluons. We reach this state by colliding Au nuclei.

## Boiling Nuclei

- Fundamental Method:
- Collide heavy nuclei at the highest possible energies:

- Fundamental Goals:
- Create (new) dense forms of matter
- Re-create the _*quark-gluon phase transition*_

## The Relativistic Heavy Ion Collider at BNL

- 106 ns crossing time
- 6 “intersection regions” for experiments
- Maximum Energy for Au+Au:
- 200 GeV per nucleon-nucleon collision
- v/c ~ 0.99999
- Design Luminosity (measure of intensity)
- Au-Au 2x1026 cm-2s-1
- Translation: Multiply by the Au+Au interaction cross section (~6.8x10-24 cm-2) to get event rate
- Event rate ~1360/sec
- Capable of colliding any nuclear species on any other nuclear species

**The Relativistic Heavy Ion Collider at BNL**

## You can see RHIC from space

- Picture taken in
- 1982 when the ring was under construction

## The accelerator complex

**RHIC**

## A virtual tour of RHIC

## PHENIX detector

- Tale of the Tape:
- Began Operation June 2000
- 12 Detector subsystems
- 4 Spectrometer arms
- Total weight = 3000T
- 315,000 readout channels
-  >125 Varieties of custom
-       printed circuit boards
- 13 ASICs designed
-       specifically for PHENIX** **

- The PHENIX Experiment is designed to probe fundamental features of
- the strong nuclear force including:
- The detection and characterization of the Quark-Gluon Plasma
- The spin structure of the nucleons** **

## The PHENIX Collaboration

- A strongly international venture:
- 11 nations
- Brazil, China, France, Germany, India, Israel, Japan, South Korea, Russia, Sweden, United States
- 51 institutions

## A real Au+Au event

- Looking down the beam line, a reconstructed event from the STAR collaboration:

## Phase Diagrams

**Nuclear Matter**

## Model calculation

- Animation my Jeff Mitchell (BNL)
- VNI collision model by K. Kinder-Geiger, R. Longacre

- Starts just
- prior to the
- collision
- Beam directions
- Beam nuclei
- Lorentz
- contracted
- in lab frame

## Measuring the collision geometry

- Zero Degree Calorimeters (ZDC’s) are about 18 m from the interaction region, one on each side. They measure “spectator” neutrons.

- Events are characterized via a 2D plot of these two detectors.

- The beam beam counters (BBC) are at +-3.5 m. They measure charged “participants”.

## Determining N(participants)

- 5-10%

- 10-15%

- 15-20%

- BBC/BBCmax
- 2R    impact parameter     0

- ZDC/ZDC max

- BBC vs ZDC centrality is correlated with the number of charged particles in the central detector arms:

## Geometry

- b = impact parameter

- Two nuclei, one going into the page, the other out:

- b

- “central”
- collision
- (b       0)

- “peripheral”
- collision
- (b      2R)

- There is an almond-
- shaped overlap region,
- nucleons in this volume
- are called “participants”,
- the others continue on at
- ~ their original momentum and are called “spectators”.

## Elliptic flow

- The matter formed in the collision is initially very hot
- (kT ~ 200 MeV or ~2.4 x 1012 °K)
- There is a lot of pressure pushing the material out from
- the center. The matter tends to “flow” outward. The details can be used to estimate the initial pressure.
- A remnant of the initial almond-shaped overlap region
- (the “participants”) can be seen in the elliptic flow

- Determine via a correlation function  method
- C() = R()/B()
- R() = number of pairs in real events
- B() = number of pairs in “mixed” events

## Correlation Functions

**Correlation Functions**

- V2 shows clear centrality and p**T** dependence

## How do we determine temperature?

- Boltzmann distribution: dN/dp3 ~ exp(-E/T)
- E = energy(mass + kinetic) = mc2, = (1-b2)-1⁄2
- T = temperature (really kT)
- Relativistically, dN/dp3 depends on the frame of reference, therefore, the Lorentz-invariant form is usually used: dN/(dp3/) ~ E dN/dp3
- dN/dp3 can be expressed as p2dpd(cosθ)dφ
- At θ = 90o (transverse to the beam), then p = pT, pick d(cosθ) and dφ independent of pT, then:
- dN/dp3 ~ (1/p2) dN/dpT
- We measure the number of particles in a bin of pT divided by pT2 and miscellaneous factors which are not important for this discussion.

## Particle identification via TOF

- 1.5< p < 1.6 GeV/c

- Mass squared [GeV2]

- +

- K+

- p

- /K separation	     < 1.6 GeV/c
- Proton separation   < 3.5 GeV/c

## MT Spectra

- MT Spectra

- mT**2**** = p****T****2**** + m****2****  (=E at 90°)**
**Slopes of  transverse mass spectra increase with particle mass**
**Not all particles have the same slope, so it is hard  to call this slope a temperature.**

- E dN/dp3  =

## Transverse Momentum Spectra

- Excellent agreement between charged and neutral pions

## 5% Central Single Particle Spectra

- T = 1224 MeV
- t = 0.72 0.01
- 2/dof = 30.0/40.0

## Fitting the Single Particle Spectra

- Exclude  resonances by fitting pt > 0.5 GeV/c
- The resonance region decreases T by ~20 MeV.  This is no surprise!  Sollfrank and Heinz also observed this in their study of S+S collisions at CERN energies.
- NA44 also had a lower pt cut-off for pions in Pb+Pb collisions.*   *

- Simultaneous fit (mt -m0 ) <  1 GeV (see arrows)

- PHENIX Preliminary

- PHENIX Preliminary

- PHENIX Preliminary

## Cooling then particle formation

- The “temperature” we see is close to our expectations for the phase transition. It is hard to see a higher temperature from pions, kaon, protons, etc because they are produced after the system cools to the phase transition. We can only say clearly that the temperature was at least this high.

## How do we measure density?

- Initially, the transverse size is given by the collision geometry
- can ~ “measure” the radii via two-particle interferometry
- measure the total number of particles (~5000 max)
- Gives ~ density

## Two particle wavefunction

## Average over positions in source

- This plot tries to indicate how the average of many two particle wavefunctions (which each have a 1 + cos(q*dR) form) results in a 1 + exp(-q2R2) form for the result. It is not quantitative. It is clear that C2 = 2 at q = 0 and that C2 = 1 at large q.

## Correlation function

- Typically fit data with:
- C2 = 1 +  exp ( -q2R2)

- Experimental definition:
- C2(q) ~ A(q)/B(q)
- A(q) = actual distribution
- B(q) = background distribution -- mix real events

## 3-D correlation result

- Num. of pairs: 3.1 millions

- Num. of pairs: 3.3 millions

- + +

- - -

- (Errors are statistical only)

-  = 0.395  0.026
- Rside = 4.42  0.22
- Rout = 4.45  0.22    [fm]
- Rlong = 5.28  0.32

- = 0.399  0.026
- Rside = 4.41  0.22
- Rout = 4.30  0.24   [fm]
- Rlong =5.13  0.26

## KT dependence of radius parameters

- We can see
- “x-p correlation”
- if there is collective expansion of the source

- 0.1    0.2     0.3    0.4    0.5    0.6   0.7

- KT [GeV/c]

- STAR’s result (     +      - )
-                 PRL 87 982301(2001)

- KT dependence of radius parameters

- PHENIX’s result ( + ,  - )

## Determining Density

**PHENIX**

- EMCAL

- R2

- Determining *Density*

- Bjorken ~ 6.8 particles/fm3 = 30 - 40 times normal nuclear density

- “Bjorken” formula for density:

- time to thermalize the system (t0 ~  1 fm/c)

- ~6.5 fm

- What density is achieved?

- Add neutral
- particles

## Central Events – What’s Going On?

- “Standard” predictions _overestimate_ the cross-section for 0 by at least *********5***

- dE/dx=0.25 GeV/fm

- dE/dx = 0 (pQCD)

*Predictions  *including  (plasma-like!) energy loss ***consistent ***with 0

## Approaching the early universe?

**E866**
**Au+Au**

**AGS**

**NA44**
**Pb+Pb**

**RHIC**
**Au+Au**

**In the Early Universe, the**
**	****Anti-proton/proton = 0.999999999**

- Anti-ptoron/proton

**s****1/2**** [GeV]**

## What is ahead?

- The first high statistics Au+Au run ended ~Thanksgiving 2001.
- Analysis is in progress, and the first round of results is
- expected this summer.
- The plasma phase is hard to measure and only a careful
- study of all the signals will be conclusive.
- Many of the most important signals require the higher statistics data from the new run.
- For example, direct photon radiation from the plasma and “melting” of the J/Ψ (c-cbar resonance)

## Summary

- We understand the collision geometry (b)
- We understand the Temperature (T)
- We understand the density ()
- Anti-particle/particle ratios approaching 1
- Elliptic flow results: initial spatial asymmetry translates to similar asymmetry in momentum
- The temperature and density measurements suggest that we are at or above the QGP phase transition. We need to look for signals.

## Relevant Thermal Physics

- Q. How to liberate quarks and gluons from ~1 fm confinement scale?
- A. Create an energy density

## Rough Estimate

*Compare*

****** **Phase transition at T ~ 140 MeV      with latent heat ~0.8 GeV / fm3** **

**Select  system with higher pressure:**

- Compare to best estimates (Karsch, QM01) from lattice calculations:T ~ 150-170 MeV  latent heat ~ 0.70.3 GeV / fm3** **

## Physics Implications (??)

- Slide from
- _[seminar ](http://nt3.phys.columbia.edu/people/gyulassy/Talks/KFKI_020901/MGsem2901.html)_
- given a few

## Determining Energy Density

- For the most central events:

**PHENIX**

- EMCAL

- R2

- Determining Energy *Density*

- Bjorken~ 4.6 GeV/fm3

-      ~30 times normal nuclear density     ~_1.5 to 2 times higher_ than         any previous experiments

- Bjorken formula for thermalized energy density

- time to thermalize the system (0 ~  1 fm/c)

- ~6.5 fm

- What is the energy density achieved?
- How does it compare to the expected phase transition value ?

## Shape of Things Now

## Collision energy dependence

- -/+, K-/K+ and pbar/p vs. collision energy.
- anti-particle/particle ratiosare dramatically increasing from SPS and AGS energies and approaching unity.
- (p-pbar)/(Npart pair) is dramatically decreasing from AGS and SPS energy RHIC : factor 7 smaller than AGS energy.

- Net proton
- ( p-p) @ y=0

- PHENIX Preliminary

- PHENIX Preliminary

## Particle correlations

## Multiplicity Vertex Detector

- Two concentric barrels of 300m Si strips
- Two endplates of Si pads
- Total  coverage of  -2.5 <** **< +2.5
- 28,672 Si strips, 6048 Si pads
- Determines event vertex and measures particle multiplicity/event
- Electronics is bare die on ceramic Multi- Chip Module

**Vertes resolution       5 mm RMS**