STAR QM2009 Experimental study of spontaneous strong parity violation… S.A. Voloshinpage 1 Experimental study of spontaneous strong parity violation in

STAR

QM2009 Experimental study of spontaneous strong parity violation… S.A. Voloshin

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Experimental study of spontaneous strong parity violation

in heavy ion collisions at RHIC

Sergei A. Voloshin

The Collaboration

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STAR

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“Message” of this talk

The physics of the spontaneous 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. Talk by D. Kharzeev : TIP – Topology Induced Parity violation.

TIP Magnetic field Charge separation along the orbital momentum. Asymmetries ~10-2, within reach of the experiment.

STAR detects a signal that is generally in agreement with mostly qualitative theoretical predictions.

The observable is P -even and might have contributions from physical effects not related to the strong parity violation.

Detector/acceptance effects / event generators: Posters by E. Finch (STAR) and I. Selyuzhenkov (STAR).

A dedicated program: theory and experiment.

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QCD vacuum topology

topological charge

winding number

Chern-Simons number

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

The volume of the box is 2.4 by 2.4 by 3.6 fm.The topological charge densityAnimation by Derek Leinweber

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

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Chiral symmetry breaking

Quark propagating in the background of the quark condensate obtains dynamical

“constituent quark” mass

u d s c b t

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

3 42 2

30

230 MeV , 850 MeV

500 MeV/fm

qq g G

in

n

e n If θ ≠ 0 QCD vacuum breaks CP.Experiment (neutron EDM): θ < 10─10

strong CP problem

T.D. Lee, PRD 8 1226 (1973)Morley, Schmidt, Z.Phys. C26, 627 (1985)Kharzeev, Pisarski, Tytgat, PRL81:512(1998)Kharzeev, Pisarski, PRD61:111901(2000)Voloshin, PRC62:044901(2000)Kharzeev, Krasnitz, Venugopalan, PLB545:298(2002)Finch, Chikanian, Longacre, Sandweiss, Thomas, PRC65 (2002)

In HIC there can be created metastable P-odd domains leading to observable effects

NCS = -2 -1 0 1

2

Strong CP problem

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EDM of QCD matter

Charge separation along the orbital momentum:EDM of the QCD matter ~ the neutron EDM

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

Theory: charge separation in HIC requires Deconfinement

(needed for quarks to diffuse after initial “impulse”

from interaction with gluonic configurations) Chiral symmetry restoration (propagation in a chirally broken phase kills the correlations)

Topologically non-trivial gluonic fields in HIC: - sphalerons, - glasma (McLerran, Venugopalan, Kharzeev)- “turning points” (Shuryak)

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The asymmetry is too small to observe in a single event but should be measurable by correlation techniques

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

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Charge separation: expectations/predictions

Same charge particles are preferentially emitted in the same direction, along or opposite to the system orbital momentum (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 localized at pt 1 GeV/c, though radial flow can move it to higher pt

Asymmetry is directly proportional to the strength of magnetic field

Signature of deconfinement and chiral symmetry restoration

Detailed predictions for collision energy and atomic number dependencies are possible, but require more calculations.

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

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Observable

The effect is too small to see in a single eventThe sign of Q varies and a = 0 one has to measure correlations, aα aβ , P -even quantity (!) aα aβ is expected to be ~ 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

Effective particle distribution

Reaction Plane

1s

2s

1c2c1

2

1 2 1 2 1 2cos 2 RP c c s s

1 2 1 2 1 2cos c c s s

X

Y TRANSVERSE PLANE

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Bin ≈ Bout, v1= 0

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“Flowing clusters” decays / RP dependent fragmentation

Global polarization, v1 fluctuations, detector

effects,… Posters: E. Finch, I. Selyuzhenkov

Physics backgrounds

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

probability that is from a cluster

probability that is from the same cluster

clustA

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

No difference between (+,+) and (-,-) results has been observed and those 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

Cluster production (e.g. as reported by PHOBOS) is not described well by the event generators !

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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 (spectatorneutrons) 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

ZDC

Barrel EM Calorimeter

Magnet

Coils

ZDC

FTPC west

Main TPC

FTPC east

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Dividing out RP resolution. (Reaction plane from TPC and FTPC)

Testing:

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

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)

STAR Preliminary STAR Preliminary

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Uncertainties at small multiplicities

Bands: 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).

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)

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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.

We proceed with more “differential” look and compare with theoretical predictions

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

STAR Preliminary

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Transverse momentum and Δη dependences (AuAu200).

Typical “hadronic” width,consistent with “theory”

Does the signal extend to too high transverse momenta?

STAR PreliminarySTAR Preliminary

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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 (not shown)

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Au+Au and Cu+Cu @ 200 GeV

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

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Au+Au and Cu+Cu: 200 vs 62 GeV

+/– signal at lower energy is stronger,qualitatively in agreement with “theory”

Uncertainties due to non-RP effects arenot shown!

200 GeV 62 GeV

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Summary of the STAR results

The results agree with theoretical expectations, though the signal persists to higher transverse momenta than expected

The observable is P-even and can have contributions from other physics effects

Studies are still on-going, but to date no other effects have been identified that would explain the observed same-sign correlations

The observed signal may provide evidence for the spontaneous parity violation

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Building a program?

Theory: Theoretical guidance and detailed calculations are needed ▪ Dependence on collision energy, centrality, system size, PID, etc. ▪ Understanding physics background !

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

Experiment: Beam energy scan / Critical point search

High statistics PID studies / properties of the clusters

Isobaric beams

Look for critical behavior, as TIP predicted to depend stronglyon deconfinement and chiral symmetry restoration

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

e.g. GSI: FOPI Collab. Note that such studies will be also very valuable for understanding the initial conditions, origin of the directed flow, etc.Ru+ Zr96 96

44 40

e.g. sphaleron “bubble” decays isotropically (?), into q-qbar pairs of different flavors. We can check this!

Do these contribute to clusters seen at ISR, RHIC?

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Summary

The effect of the spontaneous parity violation (TIP – Topology Induced Parity violation) is an integral part of QCD.

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

The effect can be studied via correlation techniques (P-even)

The data is generally in agreement with theoretical expectations

The development in this field will depend strongly on the theory. Detailed calculations for the signal and backgrounds are needed

Theory: the effect can serve as a signature of QGP formation. RHIC critical point search/beam energy scan can answer this question

Rich future program: identified particle correlations, isobaric beams…

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The STAR Collaboration

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Charge combinations

FullField ReversedFullField

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STAR QM2009 Experimental study of spontaneous strong parity violation… S.A. Voloshinpage 1 Experimental study of spontaneous strong parity violation in