# Presentation: 456978

## Introduction toRHIC Science

**W.A. Zajc****Columbia University**

- W.A. Zajc

- 19-Apr-01

## Conclusions

**RHIC is a hadron collider of unprecedented versatility.**
**The four RHIC experiments have a broad coverage ideally suited to exploit RHIC’s potential.**
**RHIC and its experiments provide a superb environment for the study of QCD as a fundamental theory:**
**Phase transition(s) associated with**
**Confinement**
**Chiral symmetry breaking**
**Origin of proton spin**

- W.A. Zajc

- 19-Apr-01

## Why is RHIC?

**Why is RHIC?**

**We have a theory of the strong interaction:**

**Where does the proton get its spin?**

**How does nuclear matter “melt”?**

**To understand fundamental features of the strong interaction:**

**(This works well except when the interaction is strong...)**

- W.A. Zajc

- 19-Apr-01

## Phase Diagrams

**Water**

**Nuclear Matter**

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- 19-Apr-01

## Previous Attempts

** ****First attempt at QGP formation **_***was***_** successful**
**                                            ****(~10****10**** years ago)**

**  ****( Effective number of degrees-of-freedom per relativistic particle )****                                                     ****                                                   **

- W.A. Zajc

- 19-Apr-01

## Making Something from Nothing

**Explore non-perturbative “vacuum” ****by **_***melting it***_
**Temperature scale **
**Particle production**
**Our ‘perturbative’ region ****is filled with**** **
**gluons**
**quark-antiquark pairs**
**A Quark-Gluon Plasma (QGP)**
**Experimental method:**
**Energetic collisions of heavy nuclei**
**Experimental measurements:****Use probes that are**
**Auto-generated**
**Sensitive to all time/length scales**

- W.A. Zajc

- 19-Apr-01

## RHIC Specifications

**3.83 km circumference**
**Two independent rings**
**120 bunches/ring**
**106 ns crossing time**
**Capable of colliding ****~any nuclear species ****on ****~any other species**
**Energy:**
**500 GeV for p-p**
**200 GeV for Au-Au****(per N-N collision)**
**Luminosity**
**Au-Au: 2 x 10****26**** cm****-2**** s****-1**
**   ****p-p  : 2 x 10****32**** cm****-2**** s****-1**** ****(**_***polarized***_**) **

**1**

**3**

**4**

**1’**

**2**

**6**

**5**

- W.A. Zajc

- 19-Apr-01

## RHIC Luminosity

**It’s high!**
**It’s an equal opportunity parton collider:**
**Can accelerate essentially ****all species**
**Designed for p-p to Au-Au**
**Asymmetric collisions ****(esp. p-A) allowed**
**Good news / bad news:**
**Permits many handles ****on systematics**
**Permits *****in situ***** ****measurements of ****“background” p-p and p-A physics**
***Detectors must handle unparalleled dynamic range in rates and ******track densities***

- W.A. Zajc

- 19-Apr-01

## How is RHIC Different?

**It’s a **_**collider**_
**Detector systematics independent of E****CM**
**(No thick targets!)**
**It’s **_**dedicated**_
**Heavy ions will run 20-30 weeks/year**
**It’s **_**high energy**_
**Access to perturbative phenomena**
**Jets**
**Non-linear dE/dx**
**Its detectors are **_**comprehensive**_
**~All final state species measured with a suite of detectors that nonetheless have significant overlap for comparisons**

- W.A. Zajc

- 19-Apr-01

## What’s Different from “Ordinary” Colliders?

**Obviously:**
**Multiplicities**
**(Cross sections)**
**But also:**
**Hermeticity requirements**
**Rates**
**Low p****T**** physics**
**High p****T**** physics**
**Signals**

- W.A. Zajc

- 19-Apr-01

## Other Differences

**Event characterization**
**Impact parameter b ****is well-defined in heavy ion collisions**
**Event multiplicity predominantly determined by collision geometry**
**Characterize this by global measures of multiplicity and/or transverse energy**
**Models**
**HEP has SM**
**Reliable predictions of baseline phenomena**
**HI has only Sub-SM’s...**
**Even the baseline physics at RHIC and beyond is intrinsically unknown**

- b

- W.A. Zajc

- 19-Apr-01

## Uniqueness of RHIC

**Substantial increase in E****CM**** ******
**Access to high Q****2**** probes**
**Dominance of mini-jets**
**Dominance of gluon collisions**
**    **
**    **

**To what extent do gluons **
**Affect  particle production (saturation)?**
**Determine initial entropy production?**
**Control particle ratios?**
**    **
**    **

- W.A. Zajc

- 19-Apr-01

**Notes:**
This slide to argue that it’s not enough to just do the first 1000 collisions and declare victory– 
If you want to get to qualitatively new regime of RHIC, you need some moderately hard processes recorded.

## Gluon saturation at RHIC

- So for     **x < ******** /2Rm**
- Transverse gluon area    **x A G(x,p****T****2****) • ******** ******** ****2**** /p****T****2**** > ********R****2**with a saturation scale in **p****T****2****  ~  A****1/3**

**Gluon saturation at RHIC**

**When do the gluons overlap significantly?**

- W.A. Zajc

- 19-Apr-01

## RHIC’s Experiments

- W.A. Zajc

- 19-Apr-01

## (PID) Acceptances

**(PID) Acceptances**

**STAR Acceptance**

**PHOBOS Acceptance**

- W.A. Zajc

- 19-Apr-01

## Run-1 Results

**All four experiments worked**
**All four experiments produced ****quality data within a few months ****of initial RHIC operation**
**Particle yields**
**Rapidity and p****T**** spectra**
**Flow**
**Source sizes**
**(Etc.)**
**This from a data set equivalent to ****1-3 days running of RHIC at design luminosity**

**RHIC worked ****(i.e, achieved its Year-1 goals):**
**Stable operation at 130 GeV**
**Delivery of 10% of design luminosity**

- W.A. Zajc

- 19-Apr-01

## PHOBOS

**An experiment with a philosophy:**
**Global phenomena **
**large spatial sizes **
**small momenta**
**Minimize the number of technologies:**
**All Si-strip tracking**
**Si multiplicity detection**
**PMT-based TOF**
**Unbiased global look at very large number of collisions (~10****9****)**

- W.A. Zajc

- 19-Apr-01

## PHOBOS Result on dNch/d

**Constrains (determines!) ****maximum multiplicities ****at RHIC energies**
**Does not constrain ****centrality dependence ****of same **
**Does not (quite) ****distinguish between **
**“****Saturation” models, ****dominated by gg ********g**
**“****Cascade” models, ****dominated by ****gg ********gg, gg ********ggq**
**( X.N. Wang and M. Gyulassy, nucl-th/0008014 )**

- W.A. Zajc

- 19-Apr-01

## BRAHMS

**An experiment with an emphasis:**
**Quality PID spectra over a broad range of rapidity and p****T **
**Special emphasis:**
**Where do the baryons go?**
**How is directed energy transferred to the reaction products?**
**Two magnetic dipole spectrometers in “classic” fixed-target configuration**

- W.A. Zajc

- 19-Apr-01

## BRAHMS Results

**First paper to be submitted this week to PRL**
**Anti-proton to proton yields as a function of rapidity :**

**                  ****(Two points are reflected about y=0)**
**Clear evidence for development of (nearly) baryon-free central region**

- W.A. Zajc

- 19-Apr-01

## Approaching the Early Universe

**Early Universe:**
**Anti-proton/proton = 0.999999999**

- E866
- Au+Au

- NA44
- Pb+Pb

-  pbar/p

- We’ve created “pure” matterapproaching this value

- For the first timein heavy ion collisions, more baryons are pair-produced than brought in from initial state

- W.A. Zajc

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## PID Overlaps

- W.A. Zajc

- 19-Apr-01

## Agreement between experiments

- W.A. Zajc

- 19-Apr-01

## STAR 

- W.A. Zajc

- 19-Apr-01

## STAR Event

**Data Taken June 25, 2000.**
**Pictures from Level 3 online display**.

- W.A. Zajc

- 19-Apr-01

## Centrality Dependence of Elliptic Flow

**    ****Evidence that initial spatial asymmetry is efficiently translated to momentum space**
**  ****( as per a hydrodynamic description)**

**    ****Parameterize azimuthal asymmetry of charged particles**** ****as**** **
**                                                                                                                          ****1 + 2 ****v****2**** ****cos (2 ********)**

- W.A. Zajc

- 19-Apr-01

## PHENIX 

- W.A. Zajc

- 19-Apr-01

## Excellent consistency between two analyses
Yields grow significantly faster than Nparticipants
Evidence for term ~ Ncollisions
Qualitatively consistent with HIJING
Inconsistent with some saturation models

**Excellent consistency between two analyses**
**Yields grow significantly faster than Nparticipants**
**Evidence for term ~ Ncollisions**
**Qualitatively consistent with HIJING**
**Inconsistent with some saturation models**

**PHENIX Results**

**      ****Evidence for highest energy densities yet achieved ****(~ 5 GeV/fm****3****)**

- W.A. Zajc

- 19-Apr-01

## Physics, Consistency between Experiments

**Physics, Consistency between Experiments**

**Trend **
**incompatible with final-state gluon ****saturation model**
**Good agreement with model ****based on *****initial-state***** saturation ****(Kharzeev and Nardi, nucl-th/0012025)**

**Excellent agreement between (non-trivial) PHENIX and PHOBOS analyses of this systematic variation with nuclear overlap.**

- W.A. Zajc

- 19-Apr-01

## Identified Particle Spectra

**To what extent do gluons (aka pions) dominate particle production at y=0?**
**Completely, at least for p****T**** < 2 GeV/c**

- W.A. Zajc

- 19-Apr-01

## What’s left?

***The all-important spin program***

- W.A. Zajc

- 19-Apr-01

## A polarized hadron collider is uniquely suited to some spin measurements:
G via
Direct photons
Hign pT pions
J/ production
                via
W+/W- production
Polarized Drell-Yan
RHIC has been equipped 
To provide polarized beams of protons
To make spin measurementsof same in (at least)PHENIX and STAR

**A polarized hadron collider is uniquely suited to some spin measurements:**
******G via**
**Direct photons**
**Hign p****T**** pions**
**J/******** production**
**                ****via**
**W****+****/W****-**** production**
**Polarized Drell-Yan**
**RHIC has been equipped **
**To provide polarized ****beams of protons**
**To make spin measurements****of same in (at least)****PHENIX and STAR**

**RHIC Spin Physics**

- W.A. Zajc

- 19-Apr-01

## RHIC Spin Potential

**Wide range of measurements in ****many channels to address**
******G**
**Sea quark polarization**

- W.A. Zajc

- 19-Apr-01

## Connections

**QCD is a fundamental theory valid in both the weak and the strong coupling limit**
**Both aspects are important at RHIC:**
**Initial state in ion-ion collisions determined by low-x gluons**
**Thermalization determined by interplay between**
**(Relatively) few hard gluons carrying most of the energy**
**“****Bath” of  numerous but very soft gluons ****(Baier, Mueller, Schiff and Son)**
**Final state multiplicities very sensitive to saturation in gluon distributions**
**Also are subtle connections between**
**Chiral symmetry of QCD**
**Spin structure of the nucleon**
**Chiral symmetry restoration in heavy ion collisions**
**“****To know the inside of the proton, you must know the    ****outside of the proton” (R. Mawhinney)**
**“****Deconfinement is chirality by other means” ****(with apologies to Clauswitz)**

- W.A. Zajc

- 19-Apr-01

**Notes:**
"War is politics by other means" -- Clauswitz
Inside of proton

QCD: 1. New state, 2. New field strengths

Relation between low x and equilibration, saturation and multiplicity 

## An Interesting Technical Connection

**RHIC experiments are coping with data volumes at the leading edge of current HEP experience:**
**Run-1**
**~1 Tb per day (recording rate per experiment)**
**Data volumes ~ 10 Tb (per experiment, from last year)**
**Run-2 and beyond**
**Roughly same recording rate (up to x2 higher?)**
**Greatly increased volumes (multi-100 Tb)**
**RHIC Computing Facility is a real-world existence proof for data analysis on this scale:**
**HPSS capacity of ~1.5 petabytes**
**HPSS optimized file mover**
**Port of Objectivity to Linux**
**“****Grand Challenge” query processor**
**ROOT**
**Key members of development team ****(Brun, Rademakers, Buncic, Fine) ****have been supported by NP funds**
**NP supported contributions to ROOT**
**Multithreading**
**Port to SOLARIS**
**Objectivity encapsulation**
**StEvent**

- W.A. Zajc

- 19-Apr-01

## What’s Left?

**Most of the program:**
**Energy scans**
**Species scans**
**All the systematic studies required before laying claim to new physics**
**Vital spin program**
***Example***** (A-A) program to do this:**
**Run-2: **
**Au+Au, crude p-p comparison run**
**First look at J/******** production, high p****T**
**Run-3: **
**High luminosity Au+Au       (60%) of HI time**
**High luminosity light ions  (40%) of HI time**
**Detailed examination of A*B scaling of J/******** yield**
**Run-4: **
**p-d/p-p comparisons**
**Baseline data for rare processes**
**Run-5:**
**“****Complete” p-A program with p-Au**
**Energy scans**
**Systematic mapping of parameter space**

- W.A. Zajc

- 19-Apr-01

## Run-2 Goals

**Au-Au running**
**Achieve design values for**
**Energy (200 GeV)**
**Luminosity (2 x 10****26**** cm****-2**** s****-1****)**
**Interaction region (20 cm)**
**~ 10 week physics run **
**~ 100 x existing data sets from Run-1**
**p-p running**
**Commission **
**proton collisions at 200 GeV****(5 x 10****30**** cm****-2**** s****-1****)**
**Polarization for same ( ******** 50%)**
**~ 5 weeks physics run**
**(Additional heavy ion running to be determined) **

- W.A. Zajc

- 19-Apr-01

## Run-2 (A-A) Results

**Significantly enhanced detectors**
**                                 ******
**Much greater integrated luminosity**
**Greatly extended reach in observables**
**p****T**** to 20 GeV/c                 (currently 5 GeV/c)**
***Spectra***** of ********’s  and ********’s  (currently mass peaks only)**

**J/********                                       (no current data)**
**Extended understanding of RHIC physics**
**Access truly perturbative regime**
**Understand detailed hadro-chemistry**
**Understand (Debye?) screening in hot system**

- W.A. Zajc

- 19-Apr-01

## Physics Upgrades

**Significant portions of RHIC physics are luminosity limited**
**New physics opportunities have appeared since initial design of RHIC experiments**
**Initial survey of RHIC terrain now provides crucial information for pursuit of ****background-limited signals**
**New technologies have emerged since initial design of RHIC experiments**

- W.A. Zajc

- 19-Apr-01

## Control (1)

**Q. How to establish the observed suppression at high p****T**** as a ****plasma effect?**
**Answers:**
**Study it out to highest possible transverse momenta**
**Study it as a function of flavor and/or color charge of probe**
***Control  *****initial state geometry**
***Control ***** initial state parton kinematics  ********  photon-tagged jets**

**RHIC:  one 15 GeV photon / hour**
**(Central Au-Au into ********y = 1)**

- W.A. Zajc

- 19-Apr-01

## Screening by the QGP

**                           ****In pictures:**

- W.A. Zajc

- 19-Apr-01

## Screening by the QGP

**Screening by the QGP**

**In first-order finger physics:**
**Follow usual derivation of Debye screening**
**Now put in QGP scales and assumptions:**
**Hadrons with radii greater than ****~ ********D**** will be dissolved**
**Study “onium” bound states**

- W.A. Zajc

- 19-Apr-01

## Control (2)

**Control (2)**

**A plasma should exhibit a thermal (Debye) screening length ******** ~ 1 /gT**
**Q. How to establish that the (to be observed) charmonium suppression pattern results from this mechanism?**
**Answers: **
**Study vs. p****T**
**Study vs. centrality**
**Study in lighter systems**
**Study vs. a *****control *****(a vector meson that should not be suppressed, the Upsilon)**

- W.A. Zajc

- 19-Apr-01

## Vector Meson Rates

**10 weeks of Au-Au running at design luminosity:**
**30K J/******** ‘s **
**Enough for rough **
**centrality dependence**
**p****T**** spectra**
**But very modest ****with respect to 500K**** J/******** ‘s  in CERN Pb-Pb****data set**
**x 4 luminosity growth ****produces ‘CERN-like’ ****production rate**

**Upsilon rate ~ 10****-3**** J/******

**Major luminosity upgrade required to access this ****important physics**

- W.A. Zajc

- 19-Apr-01

## “New” Physics (1)

**Increased understanding of open charm significance**
**Saturation of  u,d,s  abundances important in establishing thermal properties of system**
**Chemical equilibrium  ******** no further information on *****dynamics***
**Charm (probably) does not chemically equilibrate**
**Important probe of early dynamics**
**Important complement to charmonium measurements**

**Major interest in pursuit of “open charm” as a plasma diagnostic**
**Currently only very modest capabilities via measurement of ****high p****T**** leptons**

**Important to extend with *****direct *****detection via displaced vertices**

- W.A. Zajc

- 19-Apr-01

## “New” Physics (2)

**Increased appreciation for role of proton-nucleus studies in calibrating plasma signals**
***Example:***** Strangeness enhancement in p-A, studied versus “centrality”**

****** ****FNAL E866**

******  ****MMS+MMN**

****** ****MMS**

****** ****Central**

**PHENIX**

**Increased appreciation for versatility of RHIC as hadron dynamics laboratory**
***Example:***** Study of Drell-Yan production in p-d vs. p-p ****(15 weeks each)**

- W.A. Zajc

- 19-Apr-01

## “Old” Physics

**Example: Detection of low mass di-lepton pairs as probes of**
**Thermal radiation: ********* ******** e****+****e****-**
**Chiral restoration: ************ e****+****e****-**
**Deferred in initial design of RHIC experiments**
**Unknown backgrounds from mundane sources**
**(Conversion and Dalitz pairs)**
**Uncertain technical capabilities for ****“hadron blind” detectors**
**Now feasible ****(and remains compelling)**

- W.A. Zajc

- 19-Apr-01

## Strengthening the Program

**Upgrades**
**Extend physics reach of RHIC**
**Apply lessons of (recent) past**
**Access new observables**
***Increase ******(already beneficial) overlap and complementarity of the RHIC experiments***

- W.A. Zajc

- 19-Apr-01

## Publication Summary (to date)

**BRAHMS**
**Anti-proton/proton ratio versus rapidity**
**PHENIX**
**Charged multiplicity versus participants**
**Transverse energy production (also versus participants)**
**PHOBOS**
**Charged multiplicity in central collisions**
**Anti-proton/proton ration at  y=0**
**STAR**
**Elliptic flow**
**Anti-proton/proton ration at y=0**
**All submissions to PRL**
**4 accepted**
**3 submitted this week **

- W.A. Zajc

- 19-Apr-01

## RHIC/BNL/RBRC Physics

**QCD**

**Spin ****Dynamics**

**Lattice****Studies**

**Heavy Ion****Collisions**

- W.A. Zajc

- 19-Apr-01