Penetrating Probes of the Quark-Gluon Plasma in Nuclear Collisions at RHIC and the LHC Prof. Brian A. Cole. Columbia University PHENIX and ATLAS

Fundamental Interactions Consider: Only “matter” that we can study in the lab has properties determined almost entirely by EM “force”. Caveat: nuclei – but the force is not truly fundamental (e.g. no single gauge boson)

RHIC Physics - Background Size scales Atom – m Nucleus – m Nucleon – m (1 fm) Quarks, electrons –Have no “size” per se Mass Scales (MeV/c 2 ) Electron – Quark (up) ~ 5 Nucleon – 939 Au Nucleus – 1.8 x 10 5 Physical Constants ħ c  0.2 GeV fm p CuAu

Strong Interactions at a Glance Hadrons are composed of quarks Quarks carry “color” charge. –Gluons mediate interaction. gluons also carry “color”.  Gluons couple to large r gluon field between quarks collapses into “flux tube”  “Confining” potential What about other combinations? –e.g. penta-quark –Or, how about chilioi-quark ….

QCD Thermodynamics (on Lattice) Rapid cross-over from “hadronic matter” to “Quark-Gluon Plasma” at T  170 MeV (  ~ 1 GeV/fm 3 ). Energy density / T 4 saturates rapidly –Except in real-world case of 2+1 flavor ?? Pressure / T 4 saturates less rapidly.  Strongly coupled RHIC ? Energy Density / T 4 Pressure / T 4

“Primordial” Quark Gluon Plasma The Early Universe, Kolb and Turner Thermodynamic degeneracy factor QCD Transition

Relativistic Heavy Ion Collider  Run 1 (2000): Au-Au  S NN = 130 GeV  Run 2 (2001): Au-Au, p-p  S NN = 200 GeV  Run 3 (2003): d-Au, p-p  S NN = 200 GeV  Run 4 (2004): Au-Au  S NN = 200, 64 GeV, p-p  S NN = 200 GeV  Run 5 (2005): Cu-Cu  S NN = 200, 64 GeV, p-p  S NN = 200 GeV STAR

RHIC Initial Conditions 200 GeV per nucleon,  = E/m  100. –Au diameter, d  14 fm, contracted d/   0.2 fm  Crossing time < 0.2 fm/c. Add QM:  E  ħ c /  t –Fluctuations with  E > 1 GeV are “on shell”  These are primarily gluons (~ 200)  RHIC is a gluon collider! (10 GeV/fm 3 )

RHIC: Au+Au Collision Simulation

Use self-generated quarks/gluons/photons as probes of the medium (classic physics technique!) Penetrating Probes of Created Matter z t Collisions between partons

Perturbative quantum chromo-dynamics Factorization: separation of  into –Short-distance physics: – calculable using perturbation expansion ** –Long-distance physics:  ’s – universal, can be measured separately. large momentum transfer – high p T outgoing particles p-p di-jet Event STAR From Collins, Soper, Sterman Phys. Lett. B438: , 1998

pQCD – Single Hadron Production Add fragmentation to hadrons D(z) – fractional momentum dist. of particles created by outgoing quark or gluon (i.e. in a jet) KKP Kretzer data vs pQCD Phys. Rev. Lett. 91, (2003)

How to directly probe medium ? Use quarks & gluons from high-Q 2 scattering –“Created” at very early times (~ 0.1 fm). –Propagate through earliest, highest  matter. (QCD) Energy loss of (color) charged particle –~ Entirely due to radiation –Virtual gluon(s) of quark multiply scatter. e.g. GLV (Gyulassy, Levai, Vitev) formalism Experimentally measure using:  (Leading) high-p  hadrons  Di-jet correlations

Central Arms PHENIX RHIC Two spectrometers measuring at 90° Two (forward/backward muon) spectrometers Optimized for rare/penetrating probes. Drift Chamber Pad Chambers RICH EM Calorimeter Time of flight

Central Arms In Action  0  The pion is the lightest and most copiously produced meson.  0 :

Au-Au  0 Spectra From PHENIX Calculations with no energy loss Calculations with energy loss Observe only 20% of expected high p T  Energy density ~15 Gev/fm 3  100 x normal nuclear energy density!!  Reminder: critical energy density ~ 1 GeV/fm 3 Transverse Momentum spectrum Expected R AA  Observed/Expected Using p-p data as baseline

PHENIX: Au-Au High-p T  0 Suppression  constancy for pT > 4 GeV/c for all centralities? We are now measuring out to truly high p T

 0 Suppression: dE/dx Comparisons Quark & gluon dE/dx analysis: Turbide et al (McGill) –Essentially an ab initio calculation –Compared to precision (relatively) data

Crucial Control Measurement: Deuteron-Au

Prompt Photon Production Prompt photons provide an independent control measurement for jet quenching. –Produced in hard scattering processes –But, no final-state effects (???)

PHENIX p-p Prompt  Production Absolute comparison, no fudge factors. pQCD very well reproduces prompt  cross- section. Points: PHENIX Curve: PQCD

Au-Au Prompt Photon Production Large background to prompt  measurement –Primarily hadron decays (e.g.  )  total /  background Pions are quenched Photons aren’t Calculations of hard scattering rates in A+A collisions OK. High-p T hadron suppression must be due to jet quenching.

STAR Experiment: “Jet” Observations Number of pairs Angle between high energy particles 0º0º180º proton-proton jet event  In Au-Au collisions we see only one “jet” at a time !  How can this happen ?  Jet quenching! q q Analyze by measuring (azimuthal) angle between pairs of particles

But, Have We Created “Matter” ? “Pressure” converts spatial anisotropy to momentum anisotropy. Requires early thermalization. Unique to heavy ion collisions Answer: yes  dN/d  x y z

“Elliptic Flow” Parameterize azimuthal anisotropy by “v 2 ” parameter – Compare to “eccentricity”: Data consistent w/ hydrodynamic calculations

PHENIX QGP “White Paper” Already a year old …

“Perfect Fluid?” My view: Perfect fluid is reasonable interpretation of available data but there is room for skepticism.

Perfect Fluid? (2) What we do know The matter created in Au+Au collisions must thermalize and generate pressure in time < 1 fm/c. “perturbative” quark/gluon scattering cannot achieve such early thermalization. Thus, “collective” effects are needed. Possibilities: –Residual long-range interactions –Colored bound states (Shuryak) –“Color instabilities” – e.g. runaway growth of local chromo-magnetic fields –Thermalization ab initio ? (Hawking-like radiation from deceleration of initial gluons/fields)

(di)Jet Angular Correlations (PHENIX) PHENIX (nucl-ex/ ): moderate p T

Di-jet Distortion vs “Impact Parameter”

Origin of di-jet Distortion? Mach cone? Jets may travel faster than the speed of sound in the medium. While depositing energy via gluon radiation. QCD “sonic boom”  Intensity

Mach Cone (2) Talk by J. Ruppert at Quark Matter Suggestive, but there are other possibilities (Cherenkov, coherent gluon radiation, di-jet quenching, …

Photon Production off the Plasma (?) The plasma mediates a jet-photon conversion

Jet-Conversion Photons: Data There may already be room for jet-conversion contribution in the PHENIX prompt photon data. More detailed studies underway – stay tuned.

What I Didn’t Show You Charm quarks also are quenched –And show rapid thermalization! Large charm quark elliptic flow signal –Can only be established at the quark level. Large baryon excess for 2 < p T < 5 GeV/c – Hadron formation by quark recombination We see final state particle flavor distributions consistent with “freeze-out” from chemically equilibrated system. We are rapidly approaching stage where QGP is ONLY viable interpretation of data.  End of the RHIC

RHIC: Penetrating Probes We are directly observing results of colored probes propagating in colored matter. – Calibrated by photon measurements, d-Au. – ~ 100x energy density of normal nuclei – >> energy density for “phase transition” Matter exhibits strong collective motion – Due to chromo-magnetic instabilities?? – Due to colored bound states?? We are studying first “fundamental” matter that interacts non electromagnetically! We may be seeing collective response of medium to energetic particles (Mach cone?)

New Opportunity: LHC w/ ATLAS

Why Heavy LHC? Low x – Gluon production from saturated initial state Energy density – ~ 50 GeV/fm 3 (?) Rate – “copious” jet production above 100 GeV Jets – Full jet reconstruction Detector – necessary detector “for free”!

Why ATLAS? Calorimetery!

Simulated Pb+Pb Event in ATLAS (No Jet)

Pb+Pb Jet Simulations in ATLAS

Heavy Ion Initial Conditions: Modern At very high energies, strong gluon production from gluon fields with large occupation #’s (~ classical) R. Venugopalan, LHC Heavy Ion PANIC

Jets as Color Antennas Studies of modified jets in heavy ion collisions may shed light on a “fundamental” problem in (particle) physcs A high-energy quark/gluon acts like a “color antenna” In vacuum, radiation strongly affected by quantum interference. But, in medium thermal gluons “regulate” radiation.

LHC Physics Opportunity Create & study quark-gluon plasma at T = 0.8~GeV Study particle production from strong gluon fields. New program with w/ new discoveries ~ guaranteed –If RHIC is any guide … p T reach, rates, detector capabilities at LHC allow for qualitatively different (better!) measurements. Overlap w/ many other sub-fields of physics –Particle physics –Plasma physics –Fluid/hydro dynamics –Thermal field theory, lattice & non-lattice –String theory (!?) – AdS-CFT correspondence –General relativity ???