# Presentation: 261749

## Experiments at RHIC:Exciting Discoveries & Future Plans

*Barbara Jacak*
*Stony Brook *
*University*
*June 20, 2006*

## outline

**introduction**
**plasmas and strong coupling**
**collective effects**
**hydrodynamics and viscosity**
**transmission of probes by the quark-gluon plasma**
**heavy quark probes**
**diffusion**
**	**** ****color screening **
**Color Glass Condensate (discussed by Dima)**
**Plasma Physics of the Quark Gluon Plasma at RHIC II**

## required conditions (per lattice)

## plasma

**ionized gas which is macroscopically neutral**
**exhibits collective effects**
**interactions among charges of multiple particles**
**spreads charge out into characteristic (Debye) length, ********D**
**multiple particles inside this length**
**	****they screen each other**
**plasma size > ********D**
**	**
**“****normal” plasmas are electromagnetic (e + ions)**
**quark-gluon plasma interacts via strong interaction**
**color forces rather than EM**
**exchanged particles: g instead of ******

##  Screening: Debye Length

**distance over which the influence of an individual charged particle is felt by the other particles in the plasma**
**charged particles arrange themselves so as to effectively shield any electrostatic fields within a distance ********D**
******D**** =   ********0****kT**
**           ****  ****-------**
**	****	****   ****n****e****e****2**
**Debye sphere = sphere with radius ********D**
**number electrons inside Debye sphere is typically large**
**N****D****= N/V****D****= ********V****D       ****V****D****= 4/3 ******** ********D****3****  **

**1/2**

***in strongly coupled plasmas it’s ************ 1***

## Debye screening in QCD: a tricky concept

**in leading order QCD  (O. Philipsen, hep-ph/0010327)**
** ****vv**

## don’t give up!  ask lattice QCD

**running coupling**

**coupling drops off for r > 0.3 fm**

***Karsch, et al.***

## Implications of D ~ 0.3 fm

**can use to estimate Coupling parameter, ******
****** ****= <PE>/<KE>  but also ******** = 1/N****D**
**for ********D**** = 0.3fm and ******** = 15 GeV/fm****3**
**	****V****D**** = 4/3 ******** ********D****3**** = 0.113 fm****3**
**	****E****D ****= 1.7 GeV**
**to convert to number of particles, use gT or g****2****T**
**	****for T ~ 2T****c**** and g****2**** = 4**
**	****get N****D ****= 1.2 – 2.5    **
**	******** ****~ 1**
**NB: for ******** ~ 1**
**plasma is NOT fully screened – it’s strongly coupled!**
**other strongly coupled plasmas behave as liquids, even crystals for ******** ****≥**** 150**
**	****dusty plasmas, cold atoms+ions , warm dense matter**

## to study experimentally:look at radiated  & “probe” particles

**to study experimentally:****look at radiated  & “probe” particles**

**as a function of transverse momentum**
**p****T**** = p sin ********with respect to beam direction)**
**	****90****° is where the action is (max T, ********)**
**	****	****	****midway between the two beams!**
**p****T**** < 1.5 GeV/c**
**“****thermal” particles **
**radiated from bulk of the medium**
**internal plasma probes**
**p****T**** > 3 GeV/c**
**jets (hard scattered q or g)**
**heavy quarks, direct photons **
**produced early→“external” probe**

## RHIC at Brookhaven National Laboratory

**Collide Au + Au ions for maximum volume**
******s = 200 GeV/nucleon pair, p+p and d+A to compare**

## 4 complementary experiments

## collective effects

***a basic feature distinguishing plasmas from ordinary matter***
**simultaneous interaction of each charged particle with a considerable number of others**
**due to long range of (electromagnetic) forces**
**magnetic fields generated by moving charges give rise to magnetic interactions**

## search for collectivity in QGPuse “internal” probes – emitted particles

- dN/d ~ 1 + 2 v2(pT) cos (2) + ...
-     “elliptic flow”

- momentum space

## v2 is large & reproduced by hydrodynamics

** ****large pressure ****	****buildup **
** ****anisotropy ******** ****	****happens fast **
** ****fast equilibration!**

## Elliptic flow scales with number of quarks

***implication: quarks are the relevant degrees of freedom when the pressure is built up.***
***Hadronization: quark coalescence***

## at high pT v2 reflects opacity of medium

**at high p****T**** v****2**** reflects opacity of medium**

- approximately expected level from jet quenching

## transmission of probes which interact with plasma

**for QGP: fast g and quarks**
**  ****probes must carry color charge**

**transmission of probes which interact with plasma**

**EM plasma: **
**x-ray transmission**

## nuclear modification factor

**photons escape plasma**
**pions and other hadrons: strong interaction, absorbed**

## flat RAA via radiative energy loss only

## RAA wrt reaction plane – more discriminating

**Energy loss depends **
**on the path-length, **
**expansion, collisions(?)**

## dihadrons: away side suppressed (low pT)

## some away side particles “reappear” at higher pT

- STAR nucl-ex/0604018

**p****T**** trigger > 8 GeV/c**

## away side yield: some jets escape, some eaten

- STAR nucl-ex/0604018

**Note similarity of**
**away side jet**
**fragmentation.**
**Only yield changes**

## how does plasma respond to deposited energy?

*E. Shuryak: *g radiates energy
- kick particles in the plasma, accelerate
- them in jet direction: a sound wave

## not an experi-mental artefact!

***J. Jia***

## generally

***generally***
***a phenomenon***
***in crystals but***
***not liquids***

## 3 particle correlations support cone-like structure

***J. Ulery, HP06***

## to further test interaction: heavy quarks 

**~ same E loss as u,d quarks**
****** ****energy loss not all radiative**
**need collisions!  **

## RAA of e± from heavy flavors was a shock

**Wicks****, Horowitz, Djordjevic, & Gyulassy, nucl-th/0512076**

## diffusion = transport of particles by collisions

**Moore & Teaney**
**PRC71, 064904, ‘05**

**D ~ 3/(2********T) is small!**
**   → ****strong interaction of c quarks **
**larger D →less charm e loss**
** ****fewer collisions, smaller v****2**

**D = 1/3 <v> ********mfp****  = <v>/ 3******
**D ******** collision time**
**                →***** *****relaxation time **

## aim to measure the screening length

**J/********bound state of c and cbar quarks)**
***Tests screening & confinement: ***
***	******do bound c + ******c****** survive the medium?***
***    ******or does QGP screening kill them?***
**Look at R****AA**** for J/******
**different bound states probe different lengths**

## At RHIC:

**CuCu**
****
**200 GeV/c**

**AuAu**
****
**200 GeV/c**

**dAu**
****
**200 GeV/c**

***J/************     *********
***muon arm***
***1.2 < |y| < 2.2***

**measured/expected**

## At RHIC:

**CuCu**
****
**200 GeV/c**

**AuAu**
****
**200 GeV/c**

**dAu**
****
**200 GeV/c**

**AuAu**
**ee**
**200 GeV/c**

**CuCu**
**ee**
**200 GeV/c**

***J/************     *********
***muon arm***
***1.2 < |y| < 2.2***

***J/************     ******ee***
***Central arm***
***-0.35 < y < 0.35***

## At RHIC:

**CuCu**
****
**200 GeV/c**

**AuAu**
****
**200 GeV/c**

**dAu**
****
**200 GeV/c**

**AuAu**
**ee**
**200 GeV/c**

**CuCu**
****
**62 GeV/c**

***J/************     *********
***muon arm***
***1.2 < |y| < 2.2***

***J/************     ******ee***
***Central arm***
***-0.35 < y < 0.35***

***Factor ~3***
***suppression***
***in central events***

- !

**CuCu**
**ee**
**200 GeV/c**

## RAA vs Npart: PHENIX and NA50

**NA50 data normalized at NA50 p+p point.**
**Suppression similar in the two experiments, although the collision energy is 10 times higher (200GeV in PHENIX & 17GeV in NA50) **** **

## What suppression should we expect?

**Models that were successful in describing SPS data**
**fail to describe data at RHIC **
**- but lattice QCD says bound states until ~2T****c**** -**

## Regeneration  Narrowing of pT and y?

- Recombined only

**Regeneration ******** Narrowing of p****T**** and y?**

**p****T**** broadening in between Thews direct & in-medium formation: some regeneration**
**Recombination ****→**** narrower rapidity distribution with increasing N****part**
**BUT:**** From p+p to central Au+Au : ****no significant change**** in y distribution.**

- No Recombination

- Thews *et al.*

***slide from T. Ullrich, HP06***

## Probe experimentally: onium spectroscopy

**Probe experimentally: onium spectroscopy**

**40% of J/******** from ******** and ********’ decays**
** ****they are screened but direct J/******** not?**

## moments of the distribution function of particles f(x,v)
0th moment → particle density (n)
higher moments are <velocity> & temperature, 
	pressure tensor, heat flux tensor
opacity/transmission is a probe of choice
Transport properties (e.g. diffusion, viscosity)
Screening
Collective Effects
hydrodynamic expansion, shock propagation, plasma waves
→ density correlations inside plasma
Radiation
bremsstrahlung, blackbody, collisional and recombination
Plasma oscillations, instabilities 

**0****th**** moment ****→ particle density (n)**
**higher moments are**** <velocity> & temperature, **
**	****pressure tensor, heat flux tensor**
**opacity/transmission is a probe of choice**
**Transport properties (e.g. diffusion, viscosity)**
**Screening**
**Collective Effects**
**hydrodynamic expansion, shock propagation, plasma waves**
**→ ****density correlations inside plasma**
**Radiation**
**bremsstrahlung, blackbody, collisional and recombination**
**Plasma oscillations, instabilities **

**Plasma Physics of the QGP with RHIC II**

***plasma diagnostics:***

## moments of the distribution function of particles f(x,v)
0th moment → particle density (n)
higher moments are <velocity> & temperature, 
	pressure tensor, heat flux tensor
opacity/transmission is the first probe
Transport properties (e.g. diffusion, viscosity)
Screening
Collective Effects
hydrodynamic expansion velocity, shock propagation
→ density correlations inside plasma
Radiation
bremsstrahlung, blackbody, collisional and recombination
Plasma oscillations, instabilities 

**0****th**** moment ****→ particle density (n)**
**higher moments are**** <velocity> & temperature, **
**	****pressure tensor, heat flux tensor**
**opacity/transmission**** ****is the first probe**
**Transport properties (e.g. diffusion, viscosity)**
**Screening**
**Collective Effects**
**hydrodynamic expansion velocity, ****shock propagation**
**→ ****density correlations inside plasma**
**Radiation**
**bremsstrahlung, ****blackbody****, collisional and recombination**
**Plasma oscillations, instabilities **

**Plasma properties & “diagnostics”**

## next step in transmission study: jet tomography

**jet quenching vs. system size, energy**
**→ **_***parton & energy density for EOS***_
**→ ****vary p****T**** to probe medium coupling,**
**	**** ****early development of system**
**golden channel: ********-jet correlations**
****** ****fixes jet energy **
**flavor-tagged jets to sort out g vs. q energy loss**
**need detector upgrades (calorimeter coverage, DAQ)**
***must have RHIC II’s increased luminosity***** ****x10 *****for*****:**
**statistics for  clean ********-jet & multi-hadron correlations**
**system scan in a finite time**
**tool of choice to study medium response/conductivity**

***small ************, low rate***

##  measure D & B decays; onium spectroscopy

***inner trackers for PHENIX and STAR***

**STAR**

**PHENIX**

***+ RHIC II luminosity!***

## need high luminosity to scan energy & system

**pin down viscosity (and the collision dynamics)**
**sort out via 3D hydro +**
**measure v2 vs. v3, v4**
**c, ******** flows to separate late stage dissipation from early viscous effects**
**thermalization**
**plasma temperature via radiated ***********
**c, ******** flows**** **
**can we identify signals of early plasma instability?**
**probe 2q correlations via baryon production?**
**direct search for density correlations poses a**
**	****challenge to experiment & theory both!**

## conclusion - discoveries

**The matter created shows collective flows**
**developed early, with quarks/gluons the likely d.o.f.**
**magnitude implies very low viscosity**
**QGP behaves as a liquid**
**	****similar to other strongly coupled plasmas!**
**Very opaque to color charged probes**
**even charm quarks lose energy and flow!**
**another result of strong coupling**
**J/******** suppressed, but only partially**
**perhaps screening + recombination from thermal bath?**
**or sequential melting of ******** and ********`**
**Evidence of CGC initial state**
***The matter behaves as expected for a plasma!***

## conclusion – future plans

**figure out the plasma physics of this new kind of matter:**
**temperature**
**transport properties**
**collective excitations**
**	****expansion dynamics**
**	****density waves**
**	****instabilities?**
**screening length**
**need**
**detector upgrades (planning, construction underway)**
**high luminosity of RHIC II (x10 via electron cooling)**
**	****low ******** probes, scan properties with system & energy**

## Energy density of matter

***high***** energy density: **
****** ****> 10****11 ****J/m****3**
**P > 1 Mbar**
**I > 3 X 10****15****W/cm****2**** **
**Fields > 500 Tesla  **

**backup slides**

## conclusions

**the matter formed at RHIC is a “perfect” fluid**
**shows collective flows with small viscosity**
**huge interaction cross sections, very opaque **
**multiple collisions affect even heavy charm quarks**
**color is partially, but not completely, screened**
**this is like other strongly coupled plasmas**
**as it *****should***** be  ****→ it *****is***** a plasma!**
**neutrality scale > interparticle distance **
**How does this super high energy density plasma work?**
**□ ****map properties of the new stuff at RHIC**
**	**** ****how does the plasma transport the “lost” energy?**
**	**** ****radiation rate?**
**	**** ****initial temperature achieved? (theory says ~380 MeV) **
**□ ****collide Pb+Pb at the**** LHC for higher T****initial ****	**** **
**	**** ****reach ~ 800 MeV: is coupling strong or weak?**

## collective effects

***a basic feature distinguishing plasmas from ordinary matter***
**simultaneous interaction of each charged particle with a considerable number of others**
**due to long range of the forces**
**EM plasma: charge-charge & charge-neutral interactions**
**charge-neutral dominates in weakly ionized plasmas**
**neutrals interact via distortion of e cloud by charges**
**very sensitive to coupling, viscosity...**
**magnetic fields generated by moving charges give rise to magnetic interactions**

## not an experi-mental artefact,part I

***J. Jia***

## the ridge

***J. Putschke***

## hadrochemistry of jet-associated particles

**hadrochemistry of jet-associated particles**

***jet & ridge similar but not***
*** ******identical for Npart<50***
***K trigger ******typical meson******??***

***J. Bielcikova***

***jet core yields unchanged***
***chemistry constant***

## plasma properties known, so far

**	****Extract from models, constrained by data**

| Energy loss <dE/dz> (GeV/fm) | 7-10 |  | 0.5 in cold matter |
| --- | --- | --- | --- |
| Energy density (GeV/fm3)
 | 14-20 |  | >5.5 from ET data
above hadronic E density! |
| dN(gluon)/dy | ~1000 |  | From energy loss, hydro huge! |
| T (MeV) | 380-400 |  | Experimentally unknown as yet |
| Equilibration time0 (fm/c) | 0.6 |  | From hydro initial condition; cascade agrees very fast!
NB: plasma folks have same problem & use same  technique |
| Opacity (L/mean free path) | 3.5 |  | Based on energy loss theory |

## can get better agreement with data

**if add formation of “extra” J/******** by coalescence of **
**c and anti-c from the plasma**
**caveat: not necessarily unique or correct explanation!**

##  baryon puzzle...

**baryons enhanced for pT <  5 GeV/c**

## At CERN (√s = 17 GeV):

**  ****NA50 and NA60 show suppression in Pb+Pb & In+In**
** ****suppression follows system size**
**  ****Normal nuclear absorption from p+A data: ******** = 4.18±0.35 mb**

**At**** ****CERN (****√s = 17 GeV)****:**

## Is the energy density high enough?

********* *********** ****5.5 *****GeV/fm******3***** (200 GeV Au+Au)**
** ****	*****well above predicted transition!***

- PRL87, 052301 (2001)

- Colliding system expands:

**value is lower limit: **
**longitudinal expansion rate, formation time overestimated**

## Saturation of gluons in initial state(colored glass condensate) 

- Wavefunction of low x (very soft) gluons overlap and the self-coupling gluons fuse.
- Saturation at higher x at RHIC vs. HERA due to nuclear size
-  suppressed jet cross section; no back-back pairs

- Mueller, McLerran, Kharzeev, ...

## Why no energy loss for charm quarks?

**“****dead cone” predicted by Kharzeev and Dokshitzer, Phys. Lett. B519, 199 (1991)**
**Gluon bremsstrahlung:**
**k****T****2**** = ********2**** t****form****/********transverse momentum of radiated gluon**
**	****	****	****           ********p****T**** in single scatt. ********mean free path**
****** ****~ k****T ****/ ********gluon energy**
**But radiation is suppressed below angles ********0****= M****q****/E****q**
**	****soft gluon distribution is**
**	****dP = ********s****C****F****/******** d********/******** k****T****2 ****dk****T****2****/(k****T****2****+ ********2**** ********0****2****)****	****2**

**not small for**
**heavy quarks!**
**causes a dead cone**

## FONLL Predictions

**Mateo Cacciari provided a prediction using the Fixed Order Next Leading Logarithm pQCD approach**
**His calculation agrees perfectly with our “poor man’s” HVQLIB+PYTHIA predictions**
**Data exceed the central theory curve by a factor of 2-3**
**Possible explanations:**
**NNLO contribution **
**Fragmentation mechanisms need to be studied in more details**

## use this technique to measure viscosity

**melt crystal with laser light**
**induce a shear flow (laminar)**
**image the dust to get velocity**
**study:**
**  ****spatial profiles v****x****(y)**
**  ****moments, fluctuations ****→**** T(x,y)**
**  ****curvature of velocity profile**
**    ****→**** ****drag forces**
**  ****viscous transport of drag in**
**    ******** ****direction from laser**
**compare to viscous hydro.**
**  ****extract ******
**    ****shear viscosity/mass density**
**PE vs. KE competition governs**
**  ****coupling & phase of matter**
*Csernai,Kapusta,McLerran nucl-th/0604032*

## can get better agreement with data

**if add formation of “extra” J/******** by coalescence of **
**c and anti-c from the plasma**
**caveat: not necessarily unique or correct explanation!**

## screening and thermal masses

***Screening mass, m******D******, defines inverse length scale***
***Inside this distance, an equilibrated plasma is sensitive to insertion of a static source***
***Outside it’s not.***

**T dependence of electric &**
**magnetic screening masses**
**Quenched lattice study**
**of gluon propagator**
**  ****figure shows:**
** ****m****D,m****= 3Tc,   m****D,e****= 6Tc at 2Tc**
**  ********D**** ~ 0.4 & 0.2 fm**

***magnetic screening mass significant***
***not very gauge-dependent, but DOES***
***grow w/ lattice size (long range is important)***

- Nakamura, Saito & Sakai, hep-lat/0311024

## use to get  & compare to molecular dynamics

*B. Liu and J. Goree, cond-mat/0502009*

**minimum arises because kinetic part of ******** decreases with ******** & potential part increases**

***MD: solve the***
***equations of motion***
***for massive particles***
***subject to (screened)***
***interaction potential***
***follow evolution of***
***particle distribution***
***function (&correlations)***
***solve coupled diff.eq’s***
***over nearby space***
***density-density***
***correlations ******→****** *********

## RHIC’s “perfect fluid”

**one that exhibits ideal, non-dissipative hydrodynamics**
**“****not-quite-ideal” =  can support a shear stress, ******** ****≠**** 0**
**Viscosity ******** is**
****** ****~ <v>/******** ~ ****√****kT/******
**so viscosity *****increases***** with T**
**	****	****	****  *****decreases***** with ******

## mesons show common pT dependence

**mesons show common p****T ****dependence**

## Fast equilibration, high opacity (even for charm): how?

**multiple collisions using free q,g scattering cross sections    ****	**_***doesn’t work!***_
**     ****need ******** x50  in the medium**

- Molnar

## Color glass condensate?

- Hadron Punch Through

**Better agreement with BRAHMS data**
**“****normal” shadowing cannot explain **(**R. Vogt hep-ph/0405060**)
**      ...****sign of CGC?**

**Kharzeev, hep-ph/0405045**

**Centrality, p****T**
** ****dependence **
**~ correct**

## Compare with BRAHMS

- Overall consistent.

**nucl-ex/0411054**

## deposited energy doesn’t thermalize so fast

**deposited energy doesn’t thermalize so fast**

***T. Renk***

****** ****distribution +**
**longitudinal expansion **
**depopulate ******** region ****&**** shift Mach peak**

****

****