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