# Presentation: 125583

## Recent Discoveries at RHIC Do they indicate a new state of matter?

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

- W.A. Zajc

- 30-Jul-03

**Notes:**
RHIC runs to date, L’s?

Several success stories: v2, High pT, control, PID, 

PID matters?

S, T, mu, rho, deconfine as barn, mb, etc.

Quo vadis

DeBye

Upsilon

Decade

## FERMI-Lab

- W.A. Zajc

- 30-Jul-03

## Fermi’s Vision

- RHIC

**From Fermi notes on Thermodynamics**

**(Almost) included RHIC physics**
**See also remarks in his “statistical model” paper**

- W.A. Zajc

- 30-Jul-03

## Relevant Thermal Physics

**Need better control of dimensional analysis:**

**Relevant Thermal Physics**

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

- W.A. Zajc

- 30-Jul-03

## Slightly More Refined Estimate

**Slightly More Refined Estimate**

***Compare***

****** ****Phase transition at T ~ 140 MeV****      with latent heat ~0.8 GeV / fm****3**** **

***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 / fm****3**** **

- W.A. Zajc

- 30-Jul-03

## (1970) TH   (2000) TC

**That is: The ‘Hagedorn temperature’ ****T****H**** is now understood as a precursor of ****T****C**
**T****C**** =  *****Phase transition temperature*****  of QCD**

- F. Karsch, hep-ph/0103314

**Current estimates from lattice calculations:****T****C**** ~ 150-170 MeV****  ****L**** ~ 0.7********0.3 GeV / fm****3****(latent heat)**** **

**Study ****confining potential****in Lattice QCD ****at various temperatures**

- W.A. Zajc

- 30-Jul-03

## Making Something from Nothing

**Experimental method:**
**Energetic collisions of heavy nuclei**
**Experimental measurements:****Use probes that are**
**Auto-generated**
**Sensitive to all time/length scales**

**Explore non-perturbative “vacuum”  ****that confines color flux by **_***melting it***_

**Particle production**
**Our ‘perturbative’ region ****is filled with**** **
**gluons**
**quark-antiquark pairs**
**    ****which screen the “bare” interaction**
**A Quark-Gluon Plasma (QGP)**

- W.A. Zajc

- 30-Jul-03

## Previous Attempts

**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

- 30-Jul-03

## The Landscape of QCD

**The Landscape of QCD**

- W.A. Zajc

- 30-Jul-03

## 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***_**) **

- W.A. Zajc

- 30-Jul-03

## RHIC Runs To Date

**Run-1 (2000):**
**Au-Au at 130 GeV     ~ 1 ********b****-1 **** ****(p-p equivalent:   ~ 0.04 pb****-1****)**
**Run-2 (2001-2):**
** ****Au-Au at 200  GeV   ~ 24 ********b****-1 ****(p-p equivalent:          ~ 1 pb****-1****)**
** ****p-p      at 200  GeV   0.15 pb****-1**
**Run-3 (2002-3): **
**d-Au     at 200 GeV          2.7 nb****-1 ****(p-p equivalent:               ~ 1 pb****-1****)**
** ****p-p       at 200 GeV       0.35 pb****-1**

**RHIC Successes (to date) based on ability to deliver physics at ~all scales:**
- barn      : Multiplicity (Entropy)
- millibarn: Flavor yields (temperature)
- microbarn: Charm (transport)
- nanobarn: Jets (density)
- picobarn: J/Psi (deconfinement)

- W.A. Zajc

- 30-Jul-03

## How is RHIC Different?

**Different from p-p, e-p colliders**
**Atomic weight A introduces new scale Q****2**** ~ A****1/3**** Q****0****2**
**Different from previous (fixed target) heavy ion facilities**
**E****CM****  increased by  order-of-magnitude**
**Accessible x (parton momentum fraction)****decreases by ~ same factor **
**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**

**Jargon Alert:**
******s = Center-of-mass energy (per nucleon collision)**
**p****T**** = transverse momentum = |p| sin ******** **
**Q****2**** = (momentum transfer)****2**

- W.A. Zajc

- 30-Jul-03

## 1 RHIC Event

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

**Q. How to take the measure ****     of  such complexity??****      (Is it possible?)**
**A. (*****Yes*****.) Begin with ****     single-particle momentum****     spectra**

- W.A. Zajc

- 30-Jul-03

## RHIC’s Experiments

**RHIC’s Experiments**

- W.A. Zajc

- 30-Jul-03

## Kinematics 101

**Fundamental single-particle observable:**
**Momentum Spectrum**

- W.A. Zajc

- 30-Jul-03

## (PID) Acceptances

**(PID) Acceptances**

**STAR Acceptance**

**PHOBOS Acceptance**

- W.A. Zajc

- 30-Jul-03

## Longitudinal Dynamics

**Longitudinal Dynamics**

**From the RHIC design manual:**
**Emphasis on higher beam energy needed  to develop “baryon-free” central region**
**This theoretical argument is nicely confirmed by measurements from BRAHMS**
**Aids in (future) comparisons to**
**lattice gauge theory **
**conditions in the early universe**

- W.A. Zajc

- 30-Jul-03

## Transverse Dynamics

**The ability to access “jet” physics also clearly anticipated in RHIC design manual**
**(vintage: ISAJET)**
**a new perturbative probe of the colliding matter**
**Most studies to date have focused on single-particle****“high pT” spectra**
**Please keep in mind**
**“****High pT” is lower than you think**

- W.A. Zajc

- 30-Jul-03

## Focus on some slice of collision:
Assume 3 nucleons struck in A, and 5 in B
Do we weight this contribution as
Npart ( = 3 + 5) ? 
Ncoll ( = 3 x 5 ) ?
Answer is a function of pT :
Low pT  large cross sections   yield  ~Npart
Soft, non-perturbative, “wounded nucleons”, ...
High pT  small cross sections  yield ~Ncoll
Hard, perturbative, “binary scaling”, point-like, A*B, ...

**Focus on some slice of collision:**
**Assume 3 nucleons struck in A, and 5 in B**
**Do we weight this contribution as**
**Npart ( = 3 + 5) ? **
**Ncoll ( = 3 x 5 ) ?**
**Answer is a function of p****T ****:**
**Low p****T ******** large cross sections **** ******** yield  ~N****part**
**Soft, non-perturbative, “wounded nucleons”, ...**
**High p****T ******** ****small cross sections**** ******** ****yield ~N****coll**
**Hard, perturbative, ****“binary scaling”, ****point-like, A*B, ...**

**Predicting p****T**** Distributions at RHIC**

- W.A. Zajc

- 30-Jul-03

## Luminosity

**Consider collision of ‘****A****’ ions per bunch****with                                 ‘****B****’ ions per bunch:**
**Luminosity**

-                               A

-                               B

- Cross-sectional area ‘S’

- W.A. Zajc

- 30-Jul-03

## Change scale by ~ 109

**Consider collision of ‘****A****’ nucleons per nucleus****with                                 ‘****B****’ nucleons per nucleus:**
**‘****Luminosity’**

-                             A

-                               B

- Cross-sectional area ‘S’

*Provided:*
- No shadowing
- Small     cross-sections

- W.A. Zajc

- 30-Jul-03

## Q. Why did we build RHIC?

**Q. Why did we build RHIC?**

**A: To gain access to ‘small’ cross-sections* ****that are**
**A) Fundamental**
**B) Calculable**
**C) Interesting**
**which then allow us to use**
**N****coll   ****( aka A*B or “binary” or “point-like”)**
**scaling of yields as our**
** *****baseline hypothesis***
**for probing a new state of matter**
**(This of course one of  many possible answers...)**

- }

**p+p → ********0****+X (200 GeV)**

- W.A. Zajc

- 30-Jul-03

## Systematizing our Knowledge

**Systematizing our Knowledge**

**All four RHIC experiments have carefully developed techniques for determining **
**the number of participating nucleons ****N****PART**** in each collision****(and thus the impact parameter)**
**The number of binary nucleon-nucleon collisions ****N****COLL**** as a function of impact parameter**
**This effort has been essential in making the QCD connection**
**Soft physics   ~ ****N****PART**
**Hard physics  ~ ****N****COLL**
**Often express impact parameter b in terms of “centrality”, e.g., ****10-20%**** most central collisions**

- W.A. Zajc

- 30-Jul-03

## Example of Ncoll Scaling

**Q: Are there rare probes at RHIC that scale as the number of binary collisions?**
**A: Yes, charm production (for N****coll**** from 71 to 975)**

- PHENIX Run-2 Preliminary Data presented at Quark Matter 2002

- W.A. Zajc

- 30-Jul-03

## ‘Jets’  at  RHIC

**Production rate  calculable in pQCD**
**But strong reduction predicted due to dE/dx ~ path-length ****(due to non-Abelian nature of medium)**
***However:***
**“****Traditional” jet methodology very difficult at RHIC**
**Dominated by the soft background**
**Investigate by (systematics of) *****high-p******T****** single particles***

**‘****Jets’  at  RHIC**

- W.A. Zajc

- 30-Jul-03

## Another Example of Ncoll Scaling

**PHENIX (Run-2) data on ********0**** production ****in peripheral collisions:**
**Excellent agreement ****between ****PHENIX measured ********0****’s ****in p-p****and****PHENIX measured ********0****’s ****in Au-Au  *****peripheral*****collisions scaled by ****the number of collisions****over ~ 5 decades**

**PHENIX Preliminary**

- W.A. Zajc

- 30-Jul-03

## Central Collisions Are Profoundly Different

**Central Collisions Are *****Profoundly Different***

**Q: ****Do all processes that *****should *****scale with N****coll**** do just that?**
**A: No! **
******Central collisions ****are *****different *****.****(Huge deficit at high p****T****)**
**This is a *****clear***** discovery****of *****new***** behavior at RHIC**
**Suppression of ****low-x gluons in ****the initial *****state*****?**
**Energy loss in ****a new state of *****matter*****? **

**PHENIX Preliminary**

- W.A. Zajc

- 30-Jul-03

## Energy Loss of Fast Partons

**Many approaches**
**1983: Bjorken  **
**1991: Thoma and Gyulassy (1991)**
**1993: Brodsky and Hoyer (1993)**
**1997: BDMPS- depends on path length(!)**
**1998: BDMS**
**Numerical values range from **
**        ****~ 0.1 GeV / fm (Bj, elastic scattering of partons)**
**~several GeV / fm (BDMPS, non-linear interactions of  gluons)**

- W.A. Zajc

- 30-Jul-03

- W.A. Zajc

- 30-Jul-03

## Systematizing Our Expectations

**Describe in terms of *****scaled ratio*****   R****AA****= 1 for “baseline expectations”**
**Will present most of remaining plots on suppression in terms of this ratio**

- W.A. Zajc

- 30-Jul-03

## Is the Suppression New?

**Yes- in the sense that an *****enhancement***** is observed in proton-nucleus collisions:**
**Known since 1975 that ****yields increase as   A********,   ******** > 1**** **

**J.W. Cronin et al.,****Phys. Rev. D11, 3105 (1975)**
**D. Antreasyan et al.,****Phys. Rev. D19, 764 (1979)**

- W.A. Zajc

- 30-Jul-03

## Is The Suppression Unique to RHIC?

**Is The Suppression Unique to RHIC?**

**Yes- all previous *****nucleus-nucleus***** measurements see *****enhancement*****, ****not *****suppression*****.  **
** **
**Effect at RHIC is *****qualitatively***** new physics made accessible by RHIC’s ability to produce **
**(copious) perturbative probes**
**(New states of matter?)**** **
**Run-2 results show that this effect persists (increases) to ****the highest available ****transverse momenta**
**Describe in terms of *****scaled ratio*****   R****AA****= 1 for “baseline expectations”**

**ISR 31 GeV**

**RHIC 200 GeV**

**SPS 17 GeV**

- W.A. Zajc

- 30-Jul-03

## Is The Suppression Always Seen at RHIC?

**NO!**
**Run-3: ***a crucial control measurement***  via d-Au collisions**

- W.A. Zajc

- 30-Jul-03

## First Conclusion

**The combined data from Runs 1-3 at RHIC on  ****p-p, Au-Au and d-Au collisions establish that ***a new effect*** (a new state of matter?) ****is produced in central Au-Au collisions**

**Au + Au Experiment**

**d + Au Control Experiment**

- Preliminary Data

- Final Data

- W.A. Zajc

- 30-Jul-03

## Theoretical Understanding?

**Theoretical Understanding?**

**Both **
**Au-Au suppression **(I. Vitev and M. Gyulassy, hep-ph/0208108)
**d-Au enhancement** (I. Vitev, nucl-th/0302002** **)
**understood in an approach that combines multiple scattering with absorption in *****a dense partonic medium***
****** ****Our ****high p****T**** probes****have been ****calibrated****and are now****being used to****explore the ****precise properties****of the medium**

**Au-Au**

**d-Au**

- W.A. Zajc

- 30-Jul-03

## Further Evidence

**Further Evidence**

**STAR azimuthal correlation function shows ****~ complete absence of “away-side” jet**

**Surface emission only (?)**
**That is, “partner” in hard scatter is absorbed in the dense medium**

- W.A. Zajc

- 30-Jul-03

## Other New Effects	

**Other New Effects****	**

**Comparison of Au-Au, d-Au and p-p data indicate**
**Dense matter uniquely formed in Au-Au collisions**
**How dense? Sufficient to “extinguish” jets **
**Q. Are there other anomalies observed in these collisions?**
**A. Yes- the fragmentation function is drastically modified:**
**Q. How to ****understand this?**
**A. Competition****between**
***Fragmentation***
**and**
***Recombination***
_***at the quark level***_
**(next slide)**

**Peripheral**

**Central**

- W.A. Zajc

- 30-Jul-03

## Recombination

**The *****in vacuo***** ****fragmentation**** of a high momentum quark to produce hadrons *****competes***** with the *****in medium***** ****recombination ****of lower momentum quarks to produce hadrons**
**Example:**
**Fragmentation: D****q→h****(z) **
**produces a 6 GeV/c ******** ****from a 10 GeV/c quark**
**Recombination:**
**produces a 6 GeV/c ******** ****from **_***two***_** 3 GeV/c quarks**
**produces a 6 GeV/c proton**** ****from **_***three***_** 2 GeV/c quarks **

- Fries, et al, nucl-th/0301087

- Greco, Ko, Levai, nucl-th/0301093

- W.A. Zajc

- 30-Jul-03

## Recombination Meets Data

**Provides a “natural” explanation of**
**Spectrum of charged hadrons**
**Enhancements seen in p/******
**Momentum scale for same**

- Fries, et al, nucl-th/0301087

**...requires the assumption of a thermalized parton phase... (which) may be appropriately called a quark-gluon plasma**
**Fries *****et al.*****, nucl-th/0301087**

**“****Extra” protons sampled ****from ~p****T****/3**

- W.A. Zajc

- 30-Jul-03

## Hydrodynamics of Elliptic Flow

**  ****( as per a hydrodynamic description)**

**Hydrodynamics of Elliptic Flow**

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

- W.A. Zajc

- 30-Jul-03

## Recombination Tested

**The *****complicated***** observed flow pattern in ****v****2****(p****T****)**** d****2****n/dp****T****d******** ~ 1 + 2 ****v****2****(p****T****) ****cos (2 ********)**
**is predicted to be *****simple***** at the quark level under**** ****p****T**** → p****T ****/ n , v****2**** → v****2**** / n ,   n = 2,3 for meson,baryon**
_***if***_** the flow pattern is established at the quark level**

- W.A. Zajc

- 30-Jul-03

## Second Conclusions

**Suppression at high p****T**** is characteristic of dense matter formation in Au-Au collisions**
**(lack of suppression for heavy quarks, as observed in N****coll**** scaling of charm yields, also predicted)**
**Recombination models operating at the parton level describe**
**“****Anomalous” baryon/meson yields****(i.e., jet fragmentation is augmented by “other” partons)**
**Elliptic flow patterns for different mesons and baryons****(results from one primordial flow pattern established at the parton level)**
**Is there evidence that these (deconfined?) partons are also thermalized?**

- W.A. Zajc

- 30-Jul-03

## Results on Particle Composition

**Results on Particle Composition**

- BRAHMS: 10% central
- PHOBOS: 10%
- PHENIX: 5%
- STAR: 5%

**200 GeV/A Au+Au**

- W.A. Zajc

- 30-Jul-03

## Is there a ‘Temperature’?

**Is there a ‘Temperature’?**

**Apparently:**
**Assume distributions described by one temperature ****T**** and ****one ( baryon) chemical potential ************: **
**One ratio (e.g.,  ********p / p**** ) determines ******** / T**** :**
**A second ratio (e.g., ****K / ******** ) provides ****T**** ******** ******
**Then predict all other hadronic yields and ratios:**

- W.A. Zajc

- 30-Jul-03

## Previous figure  RHIC has net baryon density ~ 0:
TCH = 179 ± 4 MeV, B = 51 ± 4 MeV (M. Kaneta and N. Xu, nucl-ex/0104021)

**T****CH**** = 179 ****± 4 ****MeV, ********B**** = 51 ****± 4 ****MeV ****(M. Kaneta and N. Xu, nucl-ex/0104021)**

**Locating RHIC on Phase Diagram**

- RHIC is as close as we’ll get to the early universe for some time

- Previous Heavy Ion Experiments (CERN SPS)

- W.A. Zajc

- 30-Jul-03

## Questions

**Do those many particles in the *****final state***** have anything to do with a *****state of matter*****?**
**For example: Is there a well-defined**
**Energy density            ******
**Temperature                T     **
**Chemical potential      ******
**Size                               R**
**Transport coefficient   ******
**Answer: Yes (apparently)**
**The first round of RHIC experiments have determined ~all of these parameters (and more)**

- W.A. Zajc

- 30-Jul-03

## Open Questions

**Is the quark-gluon plasma being formed in RHIC collisions? ****To be determined:**
**Does charmonium show the expected suppression from (color) Debye screening?**
**Is there direct (photon) radiation from the plasma?**
**Do the suppression effects extend to the highest p****T****’s?**
**What is the suppression pattern in cold nuclear matter? ****(proton-nucleus collisions)**
**What are the gluon and sea-quark contributions to the proton spin? ****(polarized proton running)**

- First results now available!

- W.A. Zajc

- 30-Jul-03

## Screening by the QGP

**                           ****In pictures:**

- W.A. Zajc

- 30-Jul-03

## 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

- 30-Jul-03

## J/ Measurements To Date

**p-p results:**
**~comparable ****to other hadron facilities ****(especially at low p****T****)**
**Au-Au results:**
**A limit only**
**To be addressed in Run-4**

- W.A. Zajc

- 30-Jul-03

## Looking Ahead

**Looking Ahead**

**Runs 1-5: ***EXPLORATION*
**Well underway!**
**“****Complete” data sets for full energy**
**Au-Au**
**d-Au**
**200 GeV p-p**
**“****Complete” data set for A-A comparison**
**Strong start on ********G physics**
**Runs 5-10: ***CHARACTERIZATION*
**Ion program**
**Species scans**
**Energy “scans”**
**d-A, p-A**
**Spin program**
**“****Complete” program of ********G(x) at 200 GeV**
**500 GeV running, sea quark contributions**
**Study of G(x) via direct photons, heavy flavor (energy scan?)**
**Upgrades (as available) to extend reach of both programs**
**Runs 11-15: ***EXPLOITATION*
**Full upgrades available**
**Repeat “complete” measurements with x10-100 sensitivity**

- W.A. Zajc

- 30-Jul-03

## On The Nature of Discovery

**Discovery of Top**
**“****Discovery” of QCD**
**“****Discovery(?)” that gluon Is massless**
**It is clear that RHIC physics is on the cusp**
**“****Evidence for” QGP is abundant**
**“****Discovery of” same is imminent**

- W.A. Zajc

- 30-Jul-03

## Closed Questions

**Has the accelerator worked?**** **
**Have the experiments worked?**
**Are the data analyzable?**
**Are they being analyzed?**
**Do the data validate the premise of RHIC?**
**Collective, ~thermal behavior**
**Contact with basic QCD phenomenology**
**Are there new phenomena?**
**Are there prospects for a long and fruitful experimental program?**

- 

- 

- 

- 

- 

- 

- 

- 

- 

- W.A. Zajc

- 30-Jul-03

## Run-2 Results on Proton Production

**Suppression for ********0****’s**
**Strict(?) N****coll**** scaling for protons**

- W.A. Zajc

- 30-Jul-03

## Run-2 results on J/ Production

**First results on J/******** yields**
**Measured in pp via **
**e+e- (central arms)**
******+********- (South Muon arm)**

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

- W.A. Zajc

- 30-Jul-03

## Phase Diagrams

**Phase Diagrams**

**Water**

**Nuclear Matter**

- W.A. Zajc

- 30-Jul-03

## Dynamics 101

**Dynamics 101**

**Q. How to estimate initial energy density?**
**A. From *****rapidity density***** of transverse energy ****E****T**
**“*****Highly relativistic nucleus-nucleus collisions: ******The central rapidity region*****”, J.D. Bjorken,**** Phys. Rev. **_**D27**_**, 140 (1983).**
**Assumes**
**~ 1-d hydrodynamic expansion**
**Invariance in y along “central rapidity plateau”****(I.e., flat rapidity distribution)**
**Then**
**    **
**since boost-invariance of matter ********                     where ******** ~ 1 fm/c**

- W.A. Zajc

- 30-Jul-03

## 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 ?**

- W.A. Zajc

- 30-Jul-03

## Hydrodynamic Behavior

**Superimposed on the thermal (~Boltzmann) distributions:**
**Collective velocity fields from **
**Momentum spectra ~ **
**‘****Test’ by investigating description for different mass particles:**
**Excellent description of particle production ****(P. Kolb and U. Heinz, hep-ph/0204061)**

- W.A. Zajc

- 30-Jul-03

## Measuring Space-Time Dimensions

**Can we “image” the particle source?**
**No – But ****Hanbury-Brown—Twiss (HBT)**** measurements provide ****3-D measure of spatial dimensions**

**F.B. Yano and S.E. Koonin, ****Phys.Lett.B78:556-559,1978**

- W.A. Zajc

- 30-Jul-03

## Sizes

**Measurements of R****SIDE**
**‘****Large’ source**
**Transverse momentum dependence ******** strongly expanding source**
**In rough agreement with hydrodynamic description**
**But**
**Complete disagreement with predictions for ****R****OUT ****/ R****SIDE**
**‘****Freeze-out’ time development not understood?**
**Problems in formalism?**
**An outstanding open question**

- W.A. Zajc

- 30-Jul-03

## Making the QCD Connection

**Making the QCD Connection**

**A surprising connection has emerged between  *****soft***** ****phenomena (charged multiplicity) and QCD **
**The measured multiplicities at RHIC are  *****low*****  compared to ****(pre-data) calculations**
**This is (now) understood* as a manifestation of saturation in the *****initial ****** *****state gluon distributions**
**N****ch****   ~ Nuclear Gluon Density****          ~ A xG (x, Q****2****)*****not*****         ~ A xG (x, Q****2****) ******** xG (x, Q****2****)****(little or no “cascading” ?)**
**‘****Understood’ in the sense that this is an area of intense theoretical activity and discussion**

- Eskola, QM2001

- W.A. Zajc

- 30-Jul-03

## Production via gluon fields

**Apply the “method of virtual quanta” to each nucleus**
**Particle “production” is the realization of ****“****pre-existing****” (dense) gluon fields**
**But with *****essential*****modifications from QCD:**
**Non-linear******** *****saturation***** in gluon densities**
***Saturation scale***** Q****S****2**** ****in momentum space *****increases***** with nuclear****size and/or overlap**
**Strong fields******** ~classical dynamics**

- W.A. Zajc

- 30-Jul-03

## Gluon Saturation in Nuclei

- 1 J.P Blaizot, A.H. Mueller, Nucl. Phys. B289, 847 (1987)

**Saturation condition  ******** ~ 1****   ********  Q****S****2**** ~ ********S**** A xG(x,Q****2****) ****/**** ********R****2**** ~ A****1/3**

**Gluon Saturation in Nuclei**

-  D. Kharzeev, nucl-th/0107033

**So for     ******** /mx  ~ 2R , ****~***** all***** constituents contribute****Parton density  ******** ~ A xG(x,Q****2****) ****/**** ********R****2**

**Parton cross section ****   ******** ~  ********S**** ******** ****2**** ****/**** Q****2**** **

**At rest**

**Moving with p****z**

- W.A. Zajc

- 30-Jul-03

## Large nucleus (A) at low momentum fraction  x  gluon distribution saturates ~ 1/s(QS2)  with  QS2 ~ A1/3
A collision* puts these gluons ‘on-shell’ ~ A xg(x,Q2) / R2
Parton-hadron duality maps gluons directly to charged hadrons

 Each collision varies the  effective  A , i.e, the number of participants NPART

Shattering the ‘Color Glass Condensate’)

**A collision* puts these gluons ‘on-shell’ ********~ A xg(x,Q****2****) / ********R****2**
**Parton-hadron duality maps gluons directly to charged hadrons**
** **
** ****Each collision varies ****the  *****effective*****  ****A**** , ****i.e, the number ****of participants ****N****PART**
**Shattering the ‘Color Glass Condensate’)**
**      **

**Saturation in Multiplicity**

- W.A. Zajc

- 30-Jul-03

## Initial State Partons?

**Procedure:**
**Determine scale Q****S****2**** as function of nuclear overlap**
**Q****S****2**** ~ A****1/3**** ~ N****part****1/3**
**Assume final-state multiplicities proportional to number of *****initial-state***** gluons in this saturated regime:**
**dN/d******** = c N****part**** xG(x, Q****S****2****)**
**Note that DGLAP requires evolution of xG(x, Q****S****2****) :**
**  ****xG(x, Q****S****2****) ~ ln (Q****S****2**** / ********QCD****2****)**
**Results:**
**“****Running” of the multiplicity yield****directly from  from xG(x, Q****2****)  +DGLAP**

- W.A. Zajc

- 30-Jul-03

## Extensions of Saturation Approach*

**Extensions of Saturation Approach*******

**Use HERA data, counting rules**
** ****x G(x,Q****2****) ~ x****-********(1-x)****4**
***Describe*****  rapidity dependence:**
**y ~ ln(1/x) **** ******** Q****S****2****(s,y) = Q****S****2****(s,y=0)e********y**

***Predict*****  energy dependence: **
**x = Q****S**** / ********s **** ******** Q****S****2****(s,y) = Q****S****2****(s****0****,y) (s/s****0****) ********/2 **
**Predict****10-14%****  increase between**** ********s = 130 and 200 GeV**
**Versus ****14********6%****  reported by ****PHOBOS**

** ********* D. Kharzeev and E. Levin, nucl-th/0108006**

- W.A. Zajc

- 30-Jul-03

## RHIC and HERA (?)

**The gluon densities at low *****x***** and their Q****2**** evolution are the *****same*****  as those used in saturation models applied  at HERA in e-p collisions:**
**A.M. Stasto, K. Golec-Biernat, J. Kwiecinski, Phys. Rev. Lett. 86, 596 (2001)**
**J. Bartels, K. Golec-Biernat, H. Kowalski hep-ph/0203258**
**More detailed understanding of**
**the precise connection**
**implications for other RHIC observables (e.g., d-Au collisions)**
**   ****an area of  *****intense*****  investigation**

- W.A. Zajc

- 30-Jul-03

## New Data, New Tests

**(Just completed) RHIC Run-3a was with d-Au collisions**
**A “decisive” test of **
**initial state (gluon saturation)**
**versus **
**final-state effects (parton energy loss)**

**PHENIX Preliminary,**
******s = 200 GeV, ********0****’s**
**dAu min bias**

- W.A. Zajc

- 30-Jul-03

## Enhancement, not Suppression

**The (preliminary) results on “Cronin” effects in d-Au collisions at RHIC show**
**An *****enhancement*****, not the *****suppression***** seen in Au-Au events**
**Trends that will be hard to accommodate within the “pure” gluon saturation approaches**
**Good agreement with *****predictions***** based on QCD-based calculations of multiple scattering and energy loss****in dense partonic matter****(I. Vitev, nucl-th 0302002)**
**Science in progress...**

**PHENIX Preliminary,**
******s = 200 GeV, ********0****’s dAu min bias**

**Au-Au 0-10%**

- W.A. Zajc

- 30-Jul-03

## Making the Connection(?)

**“****Final-state descriptions” provide excellent description of the data in terms of **
**Input:**
**Temperature T**
**Pressure  T******** **
**Transport properties of medium R/******
**Output**
**Species abundances**
**Spectra**
**Collective flow**
**“****jet quenching”**
**“****Initial-state descriptions” provide excellent description of the data in terms of**
**Input:**
**Saturation scale Q****S****2**
**Measured gluon structure functions xG(x,Q****2****)**
**Output**
**Multiplicities**
**Spectra**
**Collective flow**
**“****jet quenching”**
**Can these be reconciled via “parton-hadron duality”?**
**Possible explanation of (mysterious) “pre-existing equilibrium” required in old Hagedorn approach to statistical production of particles**

- W.A. Zajc

- 30-Jul-03

## Relevant Nuclear Physics

**Normal nuclei have   ~ constant density:**
******0**** ~ 0.16 GeV / fm****3**
**M****PROTON**** ~ M****NEUTRON**** ~ 1 GeV**
**R(A) = 0.92 A****1/3 ****(rms)**** , ****where A = Atomic**** ****Number**

**<r****2****PROTON****>****1/2**** ~ <r****2****NEUTRON****>****1/2**** ~ 0.86 fm**
**Nuclei ~ close-packed “spheres” of protons and neutrons**
**Nuclear potential**
**Short range ( ~ 1 fm)**
**Modest strength ( ~ 50 MeV depth)**
**Nuclei are loosely bound**
**Treat as ~free Fermi gas of protons and neutrons**
**Nuclear physics is the “Large A, small Q****2”**** limit of QCD**

- W.A. Zajc

- 30-Jul-03

## 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

- 30-Jul-03

## Simple Chemistry

**Assume **
**chemical description appropriate ******** (********,T)**
**Boltzmann approximation valid      ******** n(********,T) ~ e ********/ T**** **
**Feed down negligible                        ******** use “raw” ratios**
**Then **

- W.A. Zajc

- 30-Jul-03

## 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

- 30-Jul-03

## Spin in PHENIX

**An integral part of our collaboration**
**~11 of 57 institutions**
**An integral part of our detector**
**South Muon Arm, high p****T**** triggers, ...**
**An integral part of our run plan**
**Commitment to significant allocation of running ~each RHIC year**
**Run-3 Run Coordinator (M. Grosse Perdekamp)**
**An integral part of our physics**
**Confinement ******** mixing of helicity components**

**Bag****Wall**

- W.A. Zajc

- 30-Jul-03

## Goals of the Spin Program

**Determine the complete spin structure of the nucleon**
**In particular, contributions from **
**Gluon polarization  (********G)**
**Sea-quark polarization (**u , d** )**
**Why RHIC?**
**High energy ******** factorization**
**Polarized hadrons ******** gq, gg collisions**
**High energy ******** new probes (W’s)**

- W.A. Zajc

- 30-Jul-03

## RHIC Reduces Scale Dependence

**Already encountered this as part of our p-p comparison data**
**Now note the direct relevance of this to understanding spin measurements at RHIC**

- PHENIXPreliminary

- W.A. Zajc

- 30-Jul-03

## First Spin Physics (Run-2)

**New components**
**Level-1 triggers**
**EmCal **
**Muon**
**Spin-sorted scalers (4 x 120)**
**Luminosity**
**    ****with**
**polarization ******** 25%**
****** ****only very modest *****effective***** luminosity**

- W.A. Zajc

- 30-Jul-03

## Projected Statistical Error from Run-2 AN Measurement

**Projected Statistical Error from Run-2 A****N**** Measurement**

**Even with this very modest start**
**~ x 10 previous best ****(E704) measurement**
**High p****T**** at x=0 permits ****pQCD treatment**
**Should lead to bounds ****on the product **
**(transversity quark distributions) **
**x **
**(Collins-Heppelman **
** ****fragmentation function)**

- W.A. Zajc

- 30-Jul-03

## PHENIX Beam Requirements* for Spin Measurements

**Gluon polarization**
**   ****-Inclusive hadron production                  200 GeV                               40%                             3  pb****-1**
**   ****-high x prompt photon production         200 GeV                               50%                         320 pb****-1****                        **
**   ****-low x prompt photon production          500 GeV                               70%                         800 pb****-1**** **
**   **
**Quark polarization**
** ****(W-production)                                                                               same   run                                        **
**Transverse spin physics**
**   ****-Collins Heppelman   FF                         200 GeV                               50%                                30 pb****-1**
**    ****-Interference               FF                          200 GeV                               70%                             300 pb****-1**
**   **
**Scheduling priorities:     a) Follow RHIC luminosity and polarization profile**
**                                               ****b) Competing programs at CERN  and DESY. ****                                           (COMPASS has started production running in July 2002!)**
**Note the requirement for ~10****3 ****(~10****4****) improvement in P****2****L  (P****4****L)**

**PHENIX Beam Requirements* ****for Spin Measurements**

- W.A. Zajc

- 30-Jul-03

## Impact of Projected Run-3 PHENIX ALL Measurement

**Impact of Projected Run-3 ****PHENIX A****LL**** Measurement**

**Comparison of  projected PHENIX A****LL**** measurements (200 GeV, 3pb****-1****) with asymmetries predicted for two different assumed gluon polarizations:**** **

- W. Vogelsang

- Theoretical interpretation possible in pQCD as demonstrated by comparison between measured PHENIX π0   cross section and NLO pQCD.
- No detector changes required.
- Luminosity requirements are consistent with CAD projections for luminosity.
- Competes with ongoing efforts at CERN.

- W.A. Zajc

- 30-Jul-03

## Impact of full PHENIX G Measurement in Direct Photon Production

**Impact of full PHENIX *********G***** Measurement in Direct Photon Production**

**If the projected PHENIX Prompt Photon Data (200GeV, 320pb****-1****) are added to a global QCD analysis of existing DIS data:**

**AAC Preliminary**

- M. Hirai, H.Kobayashi, M. Miyama *et al. (The Asymmetry Analysis Collaboration)*

- Present DIS

- W.A. Zajc

- 30-Jul-03

## Projected Impact of PHENIX 500 GeV Spin Measurement for Quark Polarization 

- Projected results on quark and anti-quark distributions for 800 pb-1 . Unique capability at RHIC in W-production.
- Flavor decomposition will reduce the uncertainty in current pol-PDF models.
- Requires increased selectivity of the muon trigger (rejection = 50 x present value)

**Projected Impact of PHENIX 500 GeV ****Spin Measurement for Quark Polarization**** **

- J. Soffer and N. Saito

- W.A. Zajc

- 30-Jul-03

## RHIC Spin Potential

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

- W.A. Zajc

- 30-Jul-03

## A*B scaling at RHIC?

**NO!**
**Both STAR and PHENIX data see deficit of particles at high transverse momenta,*****relative to expected A*B scaling:***

- W.A. Zajc

- 30-Jul-03

## Is There An Ultimate Temperature?

**Is There An Ultimate Temperature?**

- From the National Research Council Committee on *The Physics of the Universe* report

**No.**** ****7****Are there new states of matter at ultrahigh temperatures and densities?**

- W.A. Zajc

- 30-Jul-03

## Discovery of Jet Quenching?

**Some would (have) claimed just that**

**To be done ****before this can ****be experimentally ****established:**
**Improve p****T**** ****range (Run-2)**
**Measure p-p ****spectrum (Run-2)**
***Study in “cold nuclear matter” with p-A collisions***** (Run-2?)**
**Vary system size (Run-2??)**

- W.A. Zajc

- 30-Jul-03

## ~1970:  An Ultimate Temperature?

**The very rapid increase of ****hadron levels with mass**

**~ equivalent to an exponential level density**

**and would thus imply a “limiting temperature”****T****H**** ~ 170 MeV**

**Hagedorn,**
- _[S. ](http://www.slac.stanford.edu/spires/find/hep/www?rawcmd=FIND+AUTHOR+FRAUTSCHI+AND+title+statistical)__[Fraustchi](http://www.slac.stanford.edu/spires/find/hep/www?rawcmd=FIND+AUTHOR+FRAUTSCHI+AND+title+statistical)_**, Phys.Rev.D3:2821-2834,1971**

- W.A. Zajc

- 30-Jul-03

## QCD is not QED

**QED (Abelian):**
**Photons have do not carry charge**
**Flux is not confined ******** 1/r potential   ******** 1/r****2**** force**
**QCD (Non-Abelian):**
**Gluons carry charge (red, green, blue)********(anti-red, anti-green, anti-blue)**
**Flux tubes form         ******** potential ~ r   ******** constant force**

**+**

**+...**

- W.A. Zajc

- 30-Jul-03

## Rare Processes

**Particle production via *****rare processes*****  should scale with ****N****coll ****, the number of underlying binary nucleon-nucleon collisions**
**Roughly: Small ********  ******** no shadowing ******** *****per nucleon*****  luminosity is relevant quantity**
**Take scaling with ****N****coll**** as our *****null hypothesis*****for hard processes**

- W.A. Zajc

- 30-Jul-03

## Open Charm as a Rare Process

**Via analysis of single-electron spectrum:**
**Measure electron p****T**** spectrum**
**Quantify (or bound) all other sources of e’s**
**Remaining excess: from semi-leptonic decays of D’s**

- W.A. Zajc

- 30-Jul-03

**Open Charm Yields**

**Charm cross-sections in Au+Au at RHIC, *****extracted  assuming*****  ****N****coll****  *****scaling,*****in good agreement with world’s data, NLO pQCD PYTHIA**

- W.A. Zajc

- 30-Jul-03

## Where do the Baryons Go?

**There is a ****(not-quite-perfect) correspondence between longitudinal momentum (rapidity) and the angle of emission in the ****center-of-mass**

- W.A. Zajc

- 30-Jul-03

## BRAHMS Results

**Anti-proton/proton ratio as a function of rapidity:**

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

- W.A. Zajc

- 30-Jul-03

## Centrality dependence of change

**suppression stronger with centrality & increased p****T**

- W.A. Zajc

- 30-Jul-03

**http://www.sandia.gov/media/NRgallery00-03.htm#testmov**

- W.A. Zajc

- 30-Jul-03

## A*B scaling at RHIC?

**NO!**
**Compare *****per collision*****  yield R****AA**** of ********0****’s for**
**Pb+Pb CERN SPS (17 GeV)**
**exceed unity at p****T**** ~ 2 GeV/c**
**> A*B scaling**
**R****AA**** > 1 ‘Cronin effect’**
**Au+Au RHIC (130 GeV)**
**Never reach unity**
**< A*B scaling*****(as distinct from charm yields)***
**Clear deficit even at the highest p****T**

**     ****Deficit ****opposite sign**** of ****enhancements**** previously seen in nuclear collisions ****(Cronin effect)**

- W.A. Zajc

- 30-Jul-03

## More Jargon

**Cronin *****et al*****. showed yield in p-A goes as**
**Use this to define “nuclear modification factor” R****pA**** :**
**Extension to ****     A-B collisions: **

- W.A. Zajc

- 30-Jul-03

## Rare processes at RHIC

**Both PHENIX and STAR have measured charged particle spectra out to “small” cross sections**

- W.A. Zajc

- 30-Jul-03

## Experimental Gauge Theory

**QCD is the only fundamental gauge *****theory *****amenable to  *****experimental*****  study in both**
**Weak and strong coupling limits**
**Particle and bulk limits**
**RHIC**
**(Strong, bulk     ) limit : heavy ion collisions**
**(Strong, particle) limit : spin physics**
**(Weak  , particle)  limit : W’s as helicity probes**
**(Weak  , bulk     ) limit  : high p****T**** probes of plasma state **

- W.A. Zajc

- 30-Jul-03