highlights from star at rhic

Highlights from STAR at RHIC

Evan FinchYale University

Outline of Your Next Hour…

Heavy-ion collisions-why? The STAR experiment at RHIC (the Relativistic Heavy-Ion

Collider) 2½ crucial results from RHIC (as of ~4 years ago) LOCAL QCD PARITY VIOLATION

Theory What we’re looking for Experimental Observables, Results and Backgrounds Future

Whirlwind tour of what else STAR is doing and will do soon…

Why Heavy Ion Collisions? To (re)-create the Quark Gluon Plasma

To study the QCD vacuum state

The QCD Vacuum Quark Confinement

T.D. Lee (1994)

The QCD Vacuum Chiral Symmetry breaking

If the light quark masses are zero, the QCD Lagrangian

Has no coupling between right and left handed quarks, it is unchanged not just by rotations of u-d-s, but also uR-dR-sR, uL-dL-sL independently.

This is inconsistent with the observe hadronic spectrum, leading to the understanding that there is a condensate in the vacuum coupling left and right handed pairs.

The QCD Vacuum The UA(1) problem the Strong CP Problem

UA(1) problem: expect 9 Goldstone bosons from spontaneous breaking of chiral symmetry by the vauum. Why is the η’ so heavy?

Answer from modern theory: UA(1) is not really a symmetry of the quantum theory because of1) Chiral anomaly 2) Topological properties of the QCD vacuum It is generally understood that effectively, these add another term to the QCD Lagragian which can be written as θEcBc (P, CP odd) why is there no (global) parity, CP violation in QCD? (Strong CP Problem)

The QCD Vacuum Local CP Violation in QCD?

It has been proposed some time ago that by exciting the vacuum, we may change its symmetry properties…

In particular, it has been proposed that there may be regions of excited vacuum within heavy ion collisions in which the P, CP symmetries are violated by QCD even if these symmetries are conserved overall.

Lee and Wick, PRD9, 2291 (1974)Morley and Schmidt, Z.Phys.C26,627 (1985)Kharzeev, Pisarski, and Tytgat, PRL81, 512(1998)

(how to study?) The QCD Vacuum

RHIC Year System Energy

2000 Au+Au 1302001 Au+Au 200

2002 p+p 2002003 d+Au 2002004 Au+Au 62.4

2002005 Cu+Cu 62.4

2002006 p+p 62.4

2002007 Au+Au 2002008 p+p 20020082009

2010

d+Aup+p

Au+Au

20020050020062277.511

STAR at RHIC

STAR at RHIC

ZDC

Barrel EM Calorimeter

Magnet

Coils

ZDC

FTPC west

Main TPC

FTPC east

STAR TPC Performance

STAR TPC PerformanceParticle identification through

energy loss with dE/dx resolution ~7%

Momentum resolution (for track traversing entire TPC) as good as 2%

Single track efficiency ~80% in central collisions

(how to study?) QGP, The QCD Vacuum

Strategy: dump a lot of energy into an “extended area” create a strongly interaction system in a deconfined, high temperature state.

Crucial first question: is it reasonable to consider the result of a heavy ion collision to be a bulk system with thermodynamic properties?

A non-central collision results in an almond-shaped region of hot-matter (in the transverse plane)

(cross section view) : Higher pressure in the x-direction than the y-direction.

Leading to this picture in momentum space: particles being pushed out in the x and –x directions, giving an anisotropy in the momentum space azimuthal distribution

Elliptic flow…

€

dNdϕ

∝ 1+ 2v1 cos(ϕ − ΨRP ) + 2v2 cos[2(ϕ − ΨRP )]+ ...{ }

v2 vs hydrodynamic model calculationsIdeal (i.e. NO viscosity) hydrodynamics fits the RHIC v2 data very well- (not the case in HI collisions at lower energies.)And to get the mass dependence roughly right requires an equation of state which includes a phase transitionFrom theoretical fits to v2 results, it is argued that the systems is a collective system at very early times and that the viscosity is extremely low (more on these later)Other results (most strongly, HBT correlations) support this picture of collectivity, but do not necessearily give good agreemtent with hydro )

Elliptic flow-experimental issues Main issue: in each collision, you have to find the

reaction plane.

?

Most straightforward way to do this (and requiring least statistics; used in most early v2 measurements) is to basically add up the particles’ momentum vectors and take the sum to define the reaction plane azimuthal angle.When such a reaction plane is used, v2 can suffer a large contribution from other two particle correlations.

More advanced methods are now used to try to overcome this. multi-particle correlationsforward detectors for reaction plane2-D fits of 2 particle correlations. (using longitudinal

information)

multiparticle method is used

Peripheral

Central

“Jet” quenchingUsing high pT particles to probe the density of the system

In a p-p collision, when we detect one high pT particle, we tend to find others near it in azimuth and 180° away.

Tran

sver

se p

lane

Hard scattering

Au-Au medium

In a Au-Au collision, the particles at 180° disappear; “quenched by the medium”?And they return somewhat when the almond is sideways

“Jet” quenching Another view of this effect: there are fewer hadrons at pT~few GeV in

central Au+Au collisions than we expect from p+p …AND it’s not an initial state effect, because there is no such quenching in d+Au data.

Results imply a very high gluon density in the medium, consistent with expectations of Quark-Gluon Plasma and are roughly consistent with other estimations of gluon/energy density of the medium.


Heavy-ion collisions-why? The STAR experiment at RHIC 2½ crucial results from RHIC (as of ~4 years ago) LOCAL PARITY VIOLATION

Theory->What we’re looking for Results and Backgrounds Future


Elliptic flow, jet quenching in Au+Au (and not in d+Au)

Back to Local Parity Violation…

Global P/CP violation is “expected” in QCD but not observed.

Model calculations have indicated that there may be local regions in heavy ion collisions in which these symmetries are violated.

Subsequent work has suggested a specific mechanism by which this may take place (the Chiral Magnetic Effect), and an experimental signal to search for.

Chiral Magnetic Effect…Two ingredients:① Each parity violating region is characterized by a topological charge

(integer number) related to the net chirality of quarks (NL-NR) emitted from the region.

② There is also a huge (electromagnetic) magnetic field formed in a heavy ion collision.

The combined effect of the two is to separate charge along the magnetic field

Chiral Magnetic Effect Prediction that we want to search for experimentally

is charge separation along the direction of of the collision angular momentum vector (i.e. perpendicular to the reaction plane).

€

€

This separation is expected to change sign event-by-event (LOCAL parity violation)

How to look for this?If has the same sign in each event…:

€

dN±

dϕ∝

1+ 2v1 cos(ϕ − ΨRP ) + 2v2 cos[2(ϕ − ΨRP )]+ ...+2a± sin(ϕ − ΨRP ) + ... ⎧ ⎨ ⎩

⎫ ⎬ ⎭

€

r E •

r B

And parity violation is signaled by nonzero a+=−a−≠0

How to look for this?If has the same sign in each event…:

€

dN±

dϕ∝

1+ 2v1 cos(ϕ − ΨRP ) + 2v2 cos[2(ϕ − ΨRP )]+ ...+2a± sin(ϕ ± − ΨRP ) + ... ⎧ ⎨ ⎩

⎫ ⎬ ⎭

€

r E •

r B

It’s not, so we expect over many events

€

sin(ϕα − ΨRP ) = aα = 0Instead, we look at

€

sin(ϕα − ΨRP )sin(ϕ β − ΨRP ) = aα aβ + Bout

Sensitive to (signal)2, but will accumulate event to event Also sensitive to background correlations

to the extent that they have non-zero projection along the direction of the angular momentum vector

α,β=+,−

How to look for this?Observable we use (proposed in S. Voloshin):

P-violation term

Non-flow 2-particle correlations projected “out-of-reaction-plane”

= ( v1,αv1,β + Bin ) − ( <aαaβ> + Bout )

Directed flow (small)

Non-flow 2-particle correlations projected onto the reaction-plane

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€

cos(ϕα − ΨRP )cos(ϕ β − ΨRP ) − sin(ϕα − ΨRP )sin(ϕ β − ΨRP )

Main point: This observable is sensitive to the parity violating charge separation. It is parity even and as such is sensitive to physics backgrounds. Naïve expectation is:

€

a+a+ = a−a− > 0

a+a− = − a+a+

First Look: A Suggestive Signal

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<cos

(φα+

φ β−2

Ψ RP)>

“<−a+a+>”,”<−a−a−>”

“<−a+a−>”

Peripheral

Central

Theoretical “Expectations”

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<cos

(φα+

φ β−2

Ψ RP)>

Calculation-with significant uncertainty in magnitude of what same sign signal should look likeMeasured values are roughly in line with initial estimates of signal size due to the Chiral Magnetic Effect (Local Parity Violation).

Kharzeev, McLerran, Warringa, Nucl.Phys.A803,227(2008)

“<−a+a−>”

“<−a+a+>”

<−a+a+>


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<cos

(φα+

φ β−2

Ψ RP)>

Calculation of reduction of signal expected in opposite-sign correlations.To explain this reduction of signal, the assumption is that when particles are emitted in opposite directions, the correlation has a better chance of being destroyed by interactions in the medium

“<−a+a−>”

“<−a+a+>”

PHYSICS BACKGROUNDS 2 types I’ll discuss:

Type 1: 3-particle clusters Causes us get the reaction plane angle

(ΨRP)wrong. Method for how to beat this down is very

straightforward : find plane in a way uncorrelated with ‘signal’ particles.

Type 2: 2-particle clusters with reaction plane dependence. Cannot disentangle just by better

measurement of reaction plane.

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Physics backgrounds- type 2

Same-side, in-plane pairs

Opposite-side, in-plane pairs .

Same-side, out-of-plane pairs

Opposite-side, out-of-plane pairs .

<cos(φα+φβ−2ΨRP)> measures, roughly speaking…

Potential problems include clusters (jets/ minijets / resonances) whose production or properties depends on orientation with respect to the reaction plane. For example, a resonance which decays generally with a small opening angle and has positive v2 gives a positive contribution.

probability that is from a cluster

probability that is from the same clusterclustA

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Physics backgrounds-type 2

Lines represent STAR measurements for {–aαaβ+[non P-odd effects]}

Various symbols represent event generator calculations of [non P-odd effects]

These predicted backgrounds are not zero, but generally same charge ~ opp. Charge

But, these models also do not do a good job predicting other, more mundane, correlations

<cos

(φα+

φ β−2

Ψ RP)>

[non -odd cos( 2 effects]RP a a P

Physics backgrounds-type 1

Lines represent STAR measurement . Symbols represent model calculations of this background, which may be large for opposite charged correlations.

This background can be constrained experimentally (see next slide)…

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3-particle clusters distort the measurement of the reaction plane…

UrQMD

Beating down “type 1” background

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ZDC

Barrel EM Calorimeter

Magnet

Coils

ZDC

FTPC west

Main TPC

FTPC east

Measure signal particles in main TPC (|rapidity|<1) and reaction plane in FTPCs (2.7<|rapidity|<3.7. Then only clusters that are ~2 units wide in rapidity can cause a problem.

Beating down “type 1” background

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Measure signal particles in main TPC (|rapidity|<1) and reaction plane in FTPCs (2.7<|rapidity|<3.7. Then only clusters that are ~2 units wide in rapidity can cause a problem.

We find that using the FTPC reaction plane gives the same answer either the clusters are wide in rapidity, or this background is small. Next step: use ZDC at beam rapidity to measure the reaction plane.

Results- 200 GeV AuAu and CuCu

unlike sign in CuCu compared to AuAu consistent with the idea of less quenching in smaller system (N.B. there is a large potential 3-particle background on all unlike-sign points)

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pt dependence of signal

pt difference: signal is roughly constant for a pt difference from 0 to 2 GeV/c. Would seem to rule out causes like HBT, Coloumb

Average pt : signal grows with pt up to 2 GeV/c where the measurement runs out of steam. Not as initially expected.

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Summary of STAR Local Parity Violation measurements

STAR results agree with the magnitude and gross features of the theoretical predictions for local P-violation in heavy-ion collisions.

The particular observable used in this analysis is P-even andis sensitive to non-parity-violating effects.With the systematics checks discussed in this paper, we have not identified effects that would explain the observed same-charge correlations.

The observed signal cannot be described by the background modelsthat we have studied (HIJING, HIJING+v2, UrQMD,MEVSIM), which span a broad range of hadronic physics.

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LPV Future: ExperimentsDedicated experimental and theoretical program focused on local parity violation, and more generally on non-perturbative QCD: structure of the vacuum, hadronization, etc.

Experiment:

U+U central body-body collisions

Correlations among neutralparticles

Beam energy scan / Critical point search

Isobaric beams

High statistic PID studies/ Properties of clusters

Parity-forbidden decays (η,η’)

Such collisions (“easy” to trigger on) will have low magnetic field and large elliptic flow – test of the LPV effect.

Colliding isobaric nuclei (the same mass number anddifferent charge) and by that controlling the magnetic field

Look for critical behavior, as LPV predicted to depend strongly on deconfinement and chiral symmetry restoration

Quarks emerging from P-odd region expected to be equally distributed among light flavors.

Turn off Chiral Magnetic Effect, see what backgrounds remain

Future theoretical directionsTheory:

Confirmation and detail study of the effect in Lattice QCD

Theoretical guidance and detailed calculations are needed: ▪ Dependence on collision energy, centrality, system size, magnetic field, PID, etc.

▪ Understanding physics background !

▪ Size/effective mass of the clusters, quark composition (equal number of q-qbarpairs of different flavors?).


Heavy-ion collisions-why? The STAR experiment at RHIC 2½ crucial results from RHIC (as of ~4 years ago) LOCAL PARITY VIOLATION

Theory->What we’re looking for Results and Backgrounds Future


Elliptic flow, jet quenching in Au+Au (and not in d+Au)

PHENIX “direct photons” results

Photons are a wonderful probe because they emerge from the early medium unscathed, but background is very challenging.•RAA measurement of photons shows that reference line from binary scaling is correct!!•Provides access to temperature of the early system!!-fits to spectrum give T~230MeV, extrapolation via hydro gives higher temp.

v2 and constituent quark coalescence

In “intermediate” pT range, v2 values for mesons baryons behave as if flow is established at a partonic level, and then mesons and baryons are simply formed by momentum space coalescence.

charm: thermalized? Non-photonic

electron signal suggests that they are…

Much better measurements coming with STAR heavy flavor tracker.

Seeing the full jet… Fairly large disagreement about the medium

properties deduced from different models of jet quenching, all of which are consistent with the RAA data.

A much stricter test for theory would be possible if we could determine experimentally where the jet energy goes. This needs a very careful study of the background subtraction in a heavy-ion event.

Critical Point Search Main Idea: if collision system passes near a critical

point, correlation length growslook for enhanced fluctuations in particle type production (using new STAR TOF system), pT correlations, etc. as a function of incident beam energy.

SummarySeveral STAR/RHIC results with related theory work

point to a deconfined state of matter existing in RHIC collisions in which the degrees of freedom are partonic.

STAR results concerning local parity violation: Are we seeing effect of vacuum being excited to a state with different symmetry properties? Results are very intriguing, but need better modeling of backgrounds.

Lots of STAR work I didn’t even remotely cover!

PHENIX “direct photons” results

NCQ scaling NPE – energy loss of heavy quarks? Estruct stuff (2-D correlations) Charm flow? Full jet reconstruction


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<cos

(φα+

φ β−2

Ψ RP)>

“Qualitative” calculation of reduction of signal expected in opposite-sign correlations.To explain this reduction of signal, the assumption is that when particles are emitted in opposite directions, the correlation has a better chance of being destroyed by interactions in the medium

λ/R = 0.1λ/R = 0.2λ/R = 0.3

2 1.5 1.0 0.5 0 b/R

1.0

0.5

0.0 €

a+a−

a+a+

Using ZDC-SMD for reaction plane

Acceptance/Efficiency CorrectionsDone by “recentering” (e.g. replacing cosnφi by cosnφi-

<cosnφ>) and double-checked by explicit cumulant calculation.

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Acceptance/Efficiency Corrections

“recentering” correction done

Full Field

“Reversed” Full Field

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Acceptance Correction Checks With simulations, we check various patterns of

inefficiencies to ensure that acceptance corrections perform adequately.

pT>150MeV/c

pT>1GeV/c

Trac

k fin

ding

effi

cien

cy (%

)

100-

100-

0-

0-

Phi (degrees)

Uncorrected Corrected

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Scaling by Nch2, AuAu and CuCu from

HIJINGSTAR Preliminary

α,β unlike sign:200 GeV AuAu: Data, HIJING opposite sign correlations both scale as Nch

-2, as expected for 3 (or more) particle clusters.200 Gev Cu-Cu: also scale as roughly N-2. Some overall scaling difference due to matching of HIJING, data multiplicity distributionsα,β like sign:

HIJING scales as N-2

Data does not

Full Cumulant – Analysis and Results

Real Terms)2cos(va 2

21 kji

jkikji cos2cos2coscos

kjikji 2coscoscos22coscos

Imaginary Terms enter via cross-terms to create additional real terms

kjikji

jkikji

kjikji

iiiii

iiii

ii

eeeee

eeee

ee

22

22

22

jkikji sin2sin2sinsin

kjikji 2cossinsin22sinsin

kjikji 2sinsincos22sincossin2

Rxn plane resolutions

Motivation One way this may be realized within a heavy ion

collision (Chiral Magnetic Effect):

CP violation creates a net chirality of quarks/anti-quarks within a domain.

The strong (electro-)magnetic field of the collision acts on this to create a separation of charge along the angular momentum vector of the collision.

Experimentally, this is what we will look for!

A correlation of a vector(E) and pseudovector(B) P violation 58


Expectations for correlations from Chiral Magnetic Effect.

Magnitude: <a+a+> ~10-4 for mid-centrality Au-Au, with a suppression for <a+a-> by a factor of a few (both are very rough calculations (not predictions)) .

System dependence unknown, but would expect less “quenching” in smaller or less dense systems.

For given system, falling signal with Nch. “bulk” phenomenon -> “low” pt.

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<a+a+>,<a-a->

aaaa

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Expectations for correlations from Chiral Magnetic Effect.

Magnitude: <a+a+> ~10-4 for mid-centrality Au-Au, with a suppression for <a+a-> by a factor of a few (both are very rough calculations (not predictions)) .

A dependence unknown but would expect less quenching in smaller or less dense systems. For given A, expect |a| to scale with Z.

For given system, falling signal in <cos(Δφ++Δφ-)> with Nch.

“bulk” phenomenon -> “low” pt.

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Reaction-plane independent background

HIJING (quenching off) predicts that this background is about as large as measured signal for unlike-sign in several peripheral bins in all systems measured, but not significant background for like-sign correlations over most of centrality range. UrQMD predicts a considerably smaller 3-particle cluster background.

STAR Preliminary

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Note: HIJING correlations and data unlike-sign correlations scale very closely as 1/N2, consistent with a large contribution from 3(or more)-particle clusters. Like sign data correlations have very different scaling.

But some signs of age…

Get A. Poskanzer slides of TPC, FTPC acceptance (from recentering presentation)

How do these models do with other (not sensitive to LPV) correlations?

Reaction plane independent two-particle correlations are NOT predicted well by these models How far should we trust these models to calculate background to our LPV measurement?

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62 GeV ResultsNothing strikingly different from the 200 GeV results. Signal is somewhat larger (less combinatoric dilution) and again shows consistency with “less quenching in less dense systems”STAR Preliminary

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pt dependence

page 65

The transverse momentum dependence of the signal shown in the previousslides is fully consistent with a picture in which particles from a LPV cluster decay has pt distribution only slightly “harder” than the bulk.

η dependence of signal

STAR Preliminary

66

pt dependence of signal

pt difference: signal is roughly constant for a pt difference from 0 to 2 GeV/c. Would seem to rule out causes like HBT, Coloumb

Average pt : signal grows with pt up to 2 GeV/c where the measurement runs out of steam. Not as initially expected.

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highlights from star at rhic

Documents

qcd vacuumstrategy

qcd lagrangian

qcd lagragian

qcd vacuum state

rhic star

qcd vacuumlocal cp violation

qcd vacuumquark confinement

qcd vacuumthe ua1 problem