The Quark Gluon Plasma what is it and why should it exist ?

The first second of the universe from D.J. Schwarz, astro-ph/ AP NP HEP time

History of the early universe The tools need to study the early history of the universe: - Accelerator based high energy and nuclear experiments (LHC, RHIC) - Observational cosmology (LSST, HAWC, etc.)

Going back in time… Age Energy Matter in universe Age Energy Matter in universe GeV grand unified theory of all forces GeV grand unified theory of all forces s10 14 GeV 1 st phase transition (strong: q,g + electroweak: g, l,n) s10 2 GeV 2 nd phase transition (strong: q,g + electro: g + weak: l,n) s0.2 GeV 3 rd phase transition (strong:hadrons + electro:g + weak: l,n) 3 min.0.1 MeV nuclei 6*10 5 years0.3 eV atoms Now 3*10 -4 eV = 3 K (13.7 billion years) RHIC, LHC & FAIR FRIB & FAIR

Strong color field Force grows with separation !!! Analogies and differences between QED and QCD to study structure of an atom… “white” proton …separate constituents Imagine our understanding of atoms or QED if we could not isolate charged objects!! nucleus electron quark quark-antiquark pair created from vacuum “white” proton (confined quarks) “white”  0 (confined quarks) Confinement: fundamental & crucial (but not understood!) feature of strong force - colored objects (quarks) have  energy in normal vacuum neutral atom To understand the strong force and the phenomenon of confinement: Create and study a system of deconfined colored quarks (and gluons) F ~ 1/r 2 F ~ r

The main features of Quantum Chromodynamics (QCD) (Nobel Prize 2004) Confinement Confinement At large distances the effective coupling between quarks is large, resulting in confinement.At large distances the effective coupling between quarks is large, resulting in confinement. Free quarks are not observed in nature.Free quarks are not observed in nature. Asymptotic freedom Asymptotic freedom At short distances the effective coupling between quarks decreases logarithmically.At short distances the effective coupling between quarks decreases logarithmically. Under such conditions quarks and gluons appear to be quasi- free.Under such conditions quarks and gluons appear to be quasi- free. (Hidden) chiral symmetry (Hidden) chiral symmetry Connected with the quark massesConnected with the quark masses When confined quarks have a large dynamical mass - constituent massWhen confined quarks have a large dynamical mass - constituent mass In the small coupling limit (some) quarks have small mass - current massIn the small coupling limit (some) quarks have small mass - current mass

Theoretical and computational QCD in vacuum and in medium In vacuum: - asymptotically free quarks have current mass - confined quarks have constituent mass - baryonic mass is sum of valence quark constituent masses Masses can be computed as a function of the evolving coupling strength or the ‘level of asymptotic freedom’, i.e. dynamic masses. But the universe was not a vacuum at the time of hadronization, it was likely a plasma of quarks and gluons. Is the mass generation mechanism the same ? B.Mueller (2004): New Discoveries at RHIC

The evolution of luminous matter Electro-weak symmetry breaking via Higgs field (  m of W, Z,  Mechanism to generate current quark masses (but does not explain their magnitude) Standard model is symmetric. All degrees of freedom are massless. QCD phase transition (I): chiral symmetry breaking via dynamical quarks. Mechanism to generate constituent quark masses (but does not explain hadronization) QCD phase transition (II): Confinement to hadrons. Mechanism to generate hadron properties (but does not explain hadron masses)

What can we do in the laboratory ? a.) Re-create the conditions as close as possible to the Big Bang, i.e. a condition of maximum density and minimum volume in an expanding macroscopic system. Is statistical thermodynamics applicable ? b.) Measure a phase transition, characterize the new phase, measure the de-excitation of the new phase into ‘ordinary’ matter – ‘do we come out the way went in ?’ (degrees of freedom, stable or metastable matter, homogeneity) c.) Learn about hadronization (how do particles acquire mass) – complementary to the Higgs search but with the same goal. The relevant theory is Quantum Chromo Dynamics

Generating a deconfined state Nuclear Matter (confined) Hadronic Matter (confined) Quark Gluon Plasma deconfined ! Present understanding of Quantum Chromodynamics (QCD) heating compression  deconfined color matter !

Expectations from Lattice QCD  /T 4 ~ # degrees of freedom confined: few d.o.f. deconfined: many d.o.f. T C ≈ 173 MeV ≈ 2  K ≈ 130,000  T[Sun’s core]  C  0.7 GeV/fm 3

The phase diagram of QCD Temperature baryon density Neutron stars Early universe nuclei nucleon gas hadron gas colour superconductor quark-gluon plasma TcTc 00 critical point ? vacuum CFL

RHIC BRAHMS PHOBOS PHENIX STAR AGS TANDEMS Relativistic Heavy Ion Collider (RHIC) 1 mile v =  c  s NN =200 GeV

Study all phases of a heavy ion collision If the QGP was formed, it will only live for s !!!! BUT does matter come out of this phase the same way it went in ???

Study all phases of a heavy ion collision If the QGP was formed, it will only live for s !!!! BUT does matter come out of this phase the same way it went in ???

Proving the existence of a new phase of matter Can we prove that we have a phase that behaves different than elementary pp collisions ? Three steps: a.) prove that the phase is partonic b.) prove that the phase is collective c.) prove that the phase characteristics (state variables) are different from the QCD vacuum

Proof (a) for partonic medium creation Shooting a high momentum particle through a dense medium p p ? Au+Au idea: p+p same  s NN = 200 GeV as reference ?: what happens in Au+Au to jets which pass through medium? Prediction: scattered quarks radiate energy (~ GeV/fm) in the colored medium:  “quenches” high p T particles  “kills” jet partner on other side

STAR, nucl-ex/ energy loss pQCD + Shadowing + Cronin pQCD + Shadowing + Cronin + Energy Loss R AA and high-pT suppression Deduced initial gluon density at   = 0.2 fm/c dN glue /dy ≈  ≈ 15 GeV/fm 3, eloss = 15*cold nuclear matter (compared to HERMES eA) (e.g. X.N. Wang nucl-th/ ) SYSTEM NEEDS TO BE PARTONIC

Proof (b): is the matter behaving collective ? elliptic (anisotropic) flow Dashed lines: hard sphere radii of nuclei Reaction plane In-plane Out-of-plane Y X Flow Y X Time Directed flowElliptic flow Mid-central collision

Proof (c): new phase leads to new matter production mechanism The medium consists of constituent quarks ? Massive quasiparticles instead of current quarks ? baryons mesons

Elliptic flow exists and its magnitude is described by ideal hydrodynamics ! Hydrodynamics: strong coupling, small mean free path: many interactions NOT plasma-like system behaves liquid-like Strong collective flow: elliptic expansion with mass ordering x y z

A surprise: ideal liquid behavior First time in heavy-ion collisions we created a system which is in quantitative agreement with ideal hydrodynamic model. The new phase behaves like an ideal liquid rather than a plasma. Not anticipated. In stark contrast to pQCD.

How strong is the coupling ? Simple pQCD processes do not generate sufficient interaction strength. Navier-Stokes type calculation of viscosity yield a near perfect liquid Viscous force ~ 0. We have made a sQGP not the anticipated wQGP. ?

Experimental verification at RHIC Lacey et al., PRL 98 (2007) The quantum limit has been reached at RHIC and has been independently verified in several measurements of collective effects RB, J.Phys.G35: (2008)

Hirano, Gyulassy (2006) Lessons from RHIC: The Quark Soup AIP Science Story of 2006

The future is bright A three prong approach: lower energy better facilityhigher energy FAIR: Facility for Antiproton & Ion Reseach LHC: Large Hadron Collider with ALICE, CMS, ATLAS RHIC-II: RHIC upgrade with higher luminosity and upgraded detectors

Measuring pp/AA in all LHC experiments  ALICE  Dedicated & most versatile heavy ion detector  Important for particle identified fragmentation measurements in pp  CMS/ATLAS  Dedicated & most versatile p+p detectors  Important for calorimeter based jet measurements in A+A.