charge dependent correlations and chiral magnetic effect · charge separation along the orbital...

S.A. VoloshinStrong and Electroweak Matter 2010, McGill University, Montreal, July 29-July 2, 2010page 1

Sergei A. Voloshin

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Charge dependent correlations and chiral magnetic effect

Local strong parity violation and new perspectives in experimental study of non-perturbative QCD

Sergei A. Voloshin

L or BOutline:

QCD vacuum. Chiral Magnetic Effect Anisotropic flow Observable Experimental results Future directions

Summary

Local strong parity violation and new perspectives in experimental study of non-perturbative QCD

Sergei A. Voloshin

L or BOutline:

QCD vacuum. Chiral Magnetic Effect Anisotropic flow Observable Experimental results Future directions

Summary

STAR results:

QCD vacuum topology

NCS = -2 -1 0 1 2

Energy of gluonic field is periodic in NCS direction (~ a generalized coordinate)

Instantons and sphalerons are localized (in space and time) solutions describing transitions between different vacua via tunneling or go-over-barrier

Defines the confinement radius, hadron size.

Defines the size of constituent quark, size of the “small” instantons

Quark propagating in the background of the quark condensate obtains dynamical “constituent quark” mass

Quark interactions with instantons - change chirality (P and T odd),- lead to quark condensate

EDM of QCD matter

Charge separation along the orbital momentum:EDM of the QCD matter (~ the neutron EDM)(Local Parity Violation)

Chiral magnetic effect: NL≠NR ⊕ magnetic field or Induction of the electric field parallel to the (static) magnetic field

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The asymmetry is too small to observein a single event, but is measurable by correlation techniques

S.A. VoloshinStrong and Electroweak Matter 2010, McGill University, Montreal, July 29-July 2, 2010page

Anisotropic flow X

XZ – the reaction plane

Picture: © UrQMD

Anisotropic flow X

Picture: © UrQMD

E877, PRL 73, 2532 (1994)

Anisotropic flow X

Picture: © UrQMD

E877, PRC 55 (1997) 1420E877, PRL 73, 2532 (1994)

Anisotropic flow, RHIC

STAR Collaboration , PRL, 86 (2000) 402

v2 = 〈cos(2(ϕ − ΨRP )〉

STAR Collaboration , PRL, 101 (2008) 252101 Au+Au, 200 GeV

Picture: © UrQMD

Anisotropic flow, RHIC

STAR Collaboration , PRL, 86 (2000) 402

v2 = 〈cos(2(ϕ − ΨRP )〉

STAR Collaboration , PRL, 101 (2008) 252101 Au+Au, 200 GeV

Picture: © UrQMD

Observable

The effect is too small to see in a single event The sign of topological charge varies and〈a〉= 0 ➔ one has to measure correlations, 〈aα aβ〉, P -even quantity (!)

Consider only first harmonic 〈aα aβ〉 is expected ~ 10-4

〈aα aβ〉can not be measured as 〈sin φα sin φβ〉due to large contribution from effects notrelated to the orientation of the reaction plane➔ the difference in corr’s in- and out-of-plane

Single particle effective distribution

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Bin ≈ Bout, v1= 0

Observable

The effect is too small to see in a single event The sign of topological charge varies and〈a〉= 0 ➔ one has to measure correlations, 〈aα aβ〉, P -even quantity (!)

Consider only first harmonic 〈aα aβ〉 is expected ~ 10-4

〈aα aβ〉can not be measured as 〈sin φα sin φβ〉due to large contribution from effects notrelated to the orientation of the reaction plane➔ the difference in corr’s in- and out-of-plane

A practical approach: three particle correlations

Single particle effective distribution

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Bin ≈ Bout, v1= 0

Charge separation: expectations/predictions

Same charge particles are preferentially emitted in the same direction, along or opposite to the system orbital momentum and magnetic field.

Unlike-sign particles are emitted in the opposite directions.

“Quenching” in a dense medium can lead to suppressionof unlike-sign (“back-to-back”) correlations.

The effect has a “typical” Δη width of order ~ 1.

The magnitude of asymmetry ~ 10─2 for midcentral collisions 10─4 for correlations.

Effect is likely to be most pronounced at pt ≤ ~ 1 GeV/c, though radial flow can move it to higher pt

Asymmetry is proportional to the strength of magnetic field

“Signature” of deconfinement and chiral symmetry restoration

Kharzeev, PLB 633 260 (2006) [hep-ph/0406125]Kharzeev, Zhitnitsky, NPA 797 67 (2007)Kharzeev, McLerran, Warringa, NPA 803 227 (2008)Fukushima, Kharzeev, Waringa, PRD 78, 074033

STAR Experiment and Data Set

This analysis used the data taken during RHIC Run IVand based on (after all quality cuts)Au+Au 200 GeV ~ 10.6 M Minimum Bias eventsAu+Au 62 GeV ~ 7 M Minimum Bias eventsCu+Cu 200 GeV ~ 30 M Minimum Bias eventsCu+Cu 62 GeV ~ 19 M Minimum Bias events

Tracking done by two Forward TPCs (East and West) and STAR Main TPC.

Tracks used: | η | < 1.0 (Main TPC) -3.9 < η < -2.9 (FTPC East) 2.9 < η < 3.9 (FTPC West)

0.15 < pT < 2.0 GeV/c(unless specified otherwise)

ZDC-SMD (spectator neutrons) is used for the first order reaction plane determination

Results presented/discussedin this talk are for charged particles in the main TPC region (|η| < 1.0)

Central Trigger Barrel

Silicon Vertex Tracker

Barrel EM Calorimeter

Magnet

FTPC west

Main TPC

FTPC east

Backgrounds

Event generators: the signal is not zero, but different from expectations (e.g. same charge ~ opp. charge)

(+,+) and (-,-) results are combined as “same charge”HIJING+v2 = added “afterburner” to generate flowMEVSIM: flow as in experiment, number of resonances maximum what is consistent with experiment

“Flowing clusters”/RP dependent fragmentation

Global polarization, v1 fluctuations, …

I. Physics (RP dependent). Can not be suppressed

II. RP independent. (depends on method and in general can be greatly reduced)

Backgrounds

LPV theory

Data vs models

Large difference in like-sign vs unlike-sign correlations in the data compared to models.

Bigger amplitude in like-sign correlations compared to unlike-sign.

Like-sign and unlike-sign correlations are consistent with theoretical expectations

… but the unlike-sign correlations can be dominated by effects not related to the RP orientation.

The “base line” can be shifted from zero.

〈 +,+ 〉 and 〈–,–〉 results agree within errors and are combined in this plot and all plots below.

Data vs models

Large difference in like-sign vs unlike-sign correlations in the data compared to models.

Bigger amplitude in like-sign correlations compared to unlike-sign.

Like-sign and unlike-sign correlations are consistent with theoretical expectations

… but the unlike-sign correlations can be dominated by effects not related to the RP orientation.

The “base line” can be shifted from zero.

First – let us understand uncertainties and then move to more “differential” results

〈 +,+ 〉 and 〈–,–〉 results agree within errors and are combined in this plot and all plots below.

Testing the technique. (RP from TPC and FTPC)Testing:

Using ZDC-SMD for the (first harmonic) event plane yields similar agreement, though with larger uncertainties (Run VII – next slide).

Center points: obtained using v2{FTPC}bands: lower limits - using v2{2}, upper – v2{4}; where v2{4} is not available it is assumed that using v2{FTPC} suppresses non-flow by 50%.

| η | < 1.0 (Main TPC) 2.9 < |η| < 3.9 (FTPC)

Testing the technique. (RP from ZDC-SMD)Testing: | η | < 1.0 (Main TPC)

2.9 < |η| < 3.9 (FTPC)

G. Wang (STAR),talk at 2010 April APS Meeting

Uncertainties at small multiplicities

Thick lines: uncertainties due to 3-particle correlations not related to the reaction plane orientation for the case of particle c taken from TPC region (from HIJING; UrQMD givesabout factor of 2 smaller values).

Note: such uncertainty in principle can be decreased, e.g. if use particle c separated from (α,β)by a rapidity gap.

-Uncertainty is smaller for the same charge signal than for opposite charge.

- Even smaller (consistent with zero) for triplets with all particles of the same charge (not shown)

(RP independent background)

Charge combinations

The results are independent of the charge of “c” particle. (Note results for 3 particles all of the same charge)

ReversedFull Field

Full Field

PHENIX (from talk of R. Lacey)

PHENIX: results

PHENIX/STAR comparison

Au+Au and Cu+Cu @ 200 GeV

+/– signal in Cu+Cu is stronger,qualitatively in agreement with “theory”,but keep in mind large uncertainties dueto correlations not related to RP

The signal is multiplied by Npart to remove “trivial” dilution due to multiplicity increasein more central collisions.

Opposite charge correlations scale with Npart,(suppression of the back-to-back correlations ?)

Same charge signal is suggestive of correlations with the reaction plane

Opposite charge corr’s are somewhat stronger in CuCu compared to AuAu at the same Npart

The signal is multiplied by Npart to remove “trivial” dilution due to multiplicity increasein more central collisions.

Δη dependences (AuAu200).

Typical “hadronic” width,consistent with “theory”.

Not color flux tubes?

Correlations at large Δη (and large Δpt, see next slide) --it is not HBT or Coulomb.

Transverse momentum dependences (AuAu200).

Signal persiststo too high pt ?

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

Two-particle correlations

Little comments…Where are they coming from?How large is the contribution to them,from the same LPV physics?

Requires differential analysis.

Two-particle correlations

Little comments…Where are they coming from?How large is the contribution to them,from the same LPV physics?

Requires differential analysis.

Solid – 2part correlations

Open -- <a+b-2Psi>

Two-particle correlations and background

Open -- <a+b-2Psi>

Two “remarkable” cancellations:-- <cos(φ1+φ2)> , opposite sign, very close to zero-- <cos(φ1+φ2)>, same sign, very close to <cos(φ1-φ2)>

Two-particle correlations and background

Open -- <a+b-2Psi>

Two “remarkable” cancellations:-- <cos(φ1+φ2)> , opposite sign, very close to zero-- <cos(φ1+φ2)>, same sign, very close to <cos(φ1-φ2)>

Consider 2-particle correlations of a type ~[a + 2b cos(φ1-φ2)]

It leads to <cos(φ1- φ2)> ≈ b and <cos(φ1+φ2)> = 2 b v2

Summary of STAR/PHENIX results

Measured correlations agree with the magnitude and gross features of the theoretical predictions for Chiral Magnetic Effect

Observables are P-even and might be sensitive to non-parity-violating effects.

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

Future program

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

Experiment:

U+U central body-body collisions

Beam energy scan / Critical point search

Isobaric beams

High statistics PID studies / properties of the clusters

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

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

Note that such studies will be also very valuable for understanding the initial conditions, baryonstopping, origin of the directed flow, etc.

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

in particular with neutral particles; see also next slides

Future program. Needs for theory.

Theory:

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 (e.g. equal number of q-qbarpairs of different flavors?).

Central U+U collisions

In both cases the magneticfield is small, but elliptic flow is large in body-body.

All (“physics”) background effects scale with elliptic flow.

Correlations due to chiral magneticeffect scale with (square of ) the magnetic field.

Magnetic field:

Elliptic flow:

Sum runs over allspectators,Calculated for t=0.

Nuclear parameters:

Central U+U and Au+Au (Nspect<20)

How to select events with large v2?

Central U+U and Au+Au (Nspect<20)

How to select events with large v2?

Selecting on multiplicity

Note the possibility to perform testof CME “working” on fluctuations in Au+Au collisions

Sphaleron clusters

Clusters in multi-particle production

cluster = sphaleron?

Pomeron out of instantons?

We have to identify other features of the “topological bubbles”, e.g. instanton “bubble” - decays isotropically, - into equal number of q-qbar pairs of all flavors.

t’Hooft’s interaction

pp2pp Phase II

Look for: Mass distribution, multiplicities, quark flavor content of the clusters (PID correlations, KKπ vs πππ, etc.), angular distributions, unusual behavior in HBT parameters (production by coherent field)

Conclusions The physics of the local strong parity violation is not an exotics but an integral part of QCD: quark interactions with topologically non-trivial gluonic configurations - instantons, sphalerons, etc., the same physics as that of the chiral symmetry breaking.

Observable effects are within reach of the experiment !

In non-central nuclear collisions LPV leads to the charge separation along the system’s orbital momentum/magnetic field.

STAR and PHENIX detect a signal that is in agreement with (mostly qualitative) theoretical predictions.

A dedicated program aimed on a direct experimental study of non-perturbative QCD effects is developing: U+U collisions, beam energy scan, identified particle correlations, isobaric beams…

Instantons in QCD

Interacting with instanton quarks change chirality !

Instantons in QCD

Interacting with instanton quarks change chirality !

Topological transitions have never been observed directly (e.g. at the level of quarks in DIS). An observation of the local strong parity violation would be a clear proof for the existence of such physics.

P-odd domains in heavy ion collisions. First look.

where α is the fraction of particles originating from the P-odd domain

Difference in the orientation of the event planedetermined with positive or negative particles!

This analysis has to be repeated using RHIC data

charge dependent correlations and chiral magnetic effect · charge separation along the orbital...

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