Quark Structure and

RHIC Highlights

Abby Bickley

University of Colorado

July 8, 2005

Standard Model: Particles

• Provides a description of the fundamental particles and forces that govern matter

• Quarks and leptons as identified as the elementary particles

• Each quark and lepton has an antimatter partner which is referred to as an antiquark or antilepton

Elementary Particles: Quarks• Spin 1/2 fermions

• Exist in the bound state as hadrons

– Baryon: 3 bound quarks

– Meson: 2 bound quarks

• Never observed in isolation

• Naturally occur in three familiesName Symbol Charge Rest Mass

(MeV/c2)

Up u +2/3 1.5 - 4

Down d -1/3 4-8

Strange s -1/3 80-130

Charm c +2/3 1150-1350

Bottom b -1/3 4100-4400

Top t +2/3 1743005100

Elementary Particles: Quarks• We know that the nucleus of an atom is composed

of nucleons (protons & neutrons)• But these nucleons also have a quark substructure

– Proton = uud

– Neutron = udd

• The antimatter equivalent to the proton is the antiproton (uud)

• The most common mesons are pions and kaons+: ud, -: ud, K+: us, K-: us

Elementary Particles: Leptons• Spin 1/2 fermions

• Point-like => no substructure

• Never bound

Standard Model - Forces• Standard model includes the forces that govern the

interactions between matter• Each force is conveyed by a mediating (or exchange)

particle• Weak force governs radioactive decay• Strong force binds quarks in hadrons and nucleons in the

nucleus• Gravitational force has not yet been incorporated into the

standard model

Quantum Chromodynamics• Theory that describes the properties of the

strong force• Color = property associated with interaction

(analogous to electric charge)• Every quark carries a color charge of red or

green or blue• Every gluon (exchange particle) also carries

a color charge– Results in important consequences for RHIC

Quantum Chromodynamics• Coupling between color carriers

INCREASES with distance– (opposite behavior to the more familiar

electromagnetic force)

• Confinement:– At large distances the QCD potential is large

and confines quarks inside bound state it is not possible to separate bound quarks

• Asymptotic Freedom:– At very small distances the QCD potential is

weak and quarks behave as if they are unbound

Confinement• The energy required to pull apart a quark antiquark pair is greater

than the rest mass of the pair• As energy is introduced to the system a new quark antiquark pair

will be produced from the vacuum instead of separating the original pair

Asymptotic Freedom• Quarks behave as if they are unbound or free when

separated by only very small distances

• Theory tells us that it might be possible to achieve this state in systems of extreme temperature and/or density

• It is this deconfined state that is known as the Quark Gluon Plasma (QGP)

• Conceptually the QGP can be visualized as a soup of freely moving quarks and gluons

Phase Diagrams of MatterH2O

Temperature (C)

Solid

Liquid

Gas

3740.01

Pre

ssur

e (a

tm)

0.006

225

Triple Point

Critical Point

Nuclear Matter

Quark Gluon Plasma

Hadron Gas

Nuclei

Baryochemical Potential (GeV)

Tem

pera

t ur e

(M

eV)

10

170Phase

Transition

Neutron Stars

170 MeV ~ 2 x 1012 C

Cosmology

Collision Evolution

Geometry Production Formation Freezeout Freezeout

0 fm/c ~2 fm/c ~7 fm/c >7fm/c

Time

1 fm/c ~ 3x10-24 seconds

Characterized by alignment of colliding nuclei.

Collision Evolution

Geometry Production Formation Freezeout Freezeout

0 fm/c ~2 fm/c ~7 fm/c >7fm/c

Time

1 fm/c ~ 3x10-24 seconds

Quarks and gluons generated;Rescattering may lead to thermal equilibrium.

Collision Evolution

Geometry Production Formation Freezeout Freezeout

0 fm/c ~2 fm/c ~7 fm/c >7fm/c

Time

1 fm/c ~ 3x10-24 seconds

Quarks and gluons combine to form particles,but inelastic collisions continue.

Collision Evolution

Geometry Production Formation Freezeout Freezeout

0 fm/c ~2 fm/c ~7 fm/c >7fm/c

Time

1 fm/c ~ 3x10-24 seconds

Inelastic collisions cease; Final particle yields fixed.

Collision Evolution

Geometry Production Formation Freezeout Freezeout

0 fm/c ~2 fm/c ~7 fm/c >7fm/c

Time

1 fm/c ~ 3x10-24 seconds

Elastic collisions cease;Particles travel to detector.

QGP Signals: J/ Suppression• J/ suppression

– J/ particle is a meson consisting of a cc pair– Commonly referred to as hidden charm– Color screening = dynamic screening of long

range confining potential in the medium– End result of color screening is the reduction of

the number of J/ particles produced in the collision

– Consequently an enhancement in the number of open charm particles might be expected

QGP Signals: J/ Suppression• J/ particle can not be measured directly

since it’s half life is too small• Must instead measure its decay products

J/ e++ e- BR = 6%J/ ++ - BR = 6%

• These can be measured in the PHENIX central arm detectors (RICH & EMC) and muon arm detectors, respectively

• Compare results from very heavy dense systems with lighter less dense collisions

• Measure as a function of collision centrality

QGP Signals: J/ Suppression • First Au+Au 200

GeV collision data taken in the fall of 2001

• PHENIX collected ~50 Million minimum bias events

• Suggestive of suppression but limited by insufficient statistics

QGP Signals: Jet Suppression• Hard scattering processes lead to the

emission of high energy sprays of back to back particles

• One of these jets must pass through the bulk of the medium in order to be detected

• In the presence of a QGP expect “away side” jet to be suppressed relative to the “near side” jet

Sometimes a high energy photon is created in the collision. We expect it to pass through the plasma without pause.

Probes of the Medium

Sometimes we produce a high energy quark or gluon.

If the plasma is dense enough we expect the quark or gluon to be swallowed up.

Probes of the Medium

Jet Quenching !

Jet correlations in proton-proton reactions.

Strong back-to-back peaks.

Azimuthal Angular Correlations

Jet Quenching !

Jet correlations in proton-proton reactions.

Strong back-to-back peaks.

Jet correlations in central Gold-Gold.

Away side jet disappears for particles pT > 2 GeV

Azimuthal Angular Correlations

Jet Quenching !

Jet correlations in proton-proton reactions.

Strong back-to-back peaks.

Jet correlations in central Gold-Gold.

Away side jet disappears for particles pT > 2 GeV

Jet correlations in central Gold-Gold.

Away side jet reappears for particles pT>200 MeV

Azimuthal Angular Correlations

Superfluidity & RHIC

aka “The Perfect Liquid”

Properties of Superfluidity?

• Ideal superfluid: – Viscosity of the fluid is zero– Experiences no resistance to flow– Only hypothetically possible at absolute zero where

no excitations exist

• Real superfluid:– Characterized as a two-fluid system

• Superfluid component - fraction of liquid in ground state

• Normal component - fraction of liquid in excited state; experiences finite viscosity

Example 1: 4He• Observation:

– Early 1930’s => liquid state does not solidify as absolute zero is approached

– Late 1930’s => below the temperature of 2.17 K resistance to flow decreases by a factor of >1500

• Explanation:– Obeys Bose statistics

– Wavefunction of system is symmetric to the exchange of any two atoms

– A finite fraction of the atoms occupy a single one particle state

– Superfluid component is a Bose-Einstein Condensate

– Normal component carries the entropy of the system

Example 2: 3He• Observation:

– Pre-1970 => liquid state does not solidify as absolute zero is approached

– Early 1970’s => below the temperature of 3 mK three distinct phases exist that exhibit properties of superfluidity

• Explanation:– Obeys Fermi statistics– Fermions pair up into Cooper pairs

• “a sort of giant diatomic quasi-molecule whose characteristic ‘radius’ is very much larger than the typical interatomic distance”

– Cooper pairs obey Bose statistics and undergo Bose-Einstein Condensation

Viscosity Bound• Recent work by Son et al. has shown that a lower

viscosity bound may exist even in superfluids

• Finite viscosity results from normal component

hep-th/0405231

Relevance to RHIC

• Hydrodynamics can be used to describe the collision medium formed in a heavy ion collision

• A hydro representation is favored over perturbative calculations by the strong collective effects observed

• The viscosity of the collision system results in a deviation in the observed particle distributions relative to that predicted by hydro

• Figure shows deviation of measured elliptic flow from hydro predictions

Relevance to RHIC

• Model estimates of the viscosity of QGP very small– (/s) ~1/10– (/s) >1 for water

• Heavy ion collision system formed at RHIC could be used to test the viscosity bound

• How could QGP be considered a superfluid????– In a strongly coupled system bound pairs of quarks and gluons

could be formed and experience analogous properties to Cooper pairs

References

• Leggett, A.J., Reviews of Modern Physics, Vol. 71, No. 2, Centenary 1999.

• Wilks, J., “The Theory of Liquid 4He”, www.iop.org/EJ/article/0034-4885/20/1/302/rpv20i1p38.pdf

• Shuryak, E., hep-ph/0312227