1. STAR QM2009 Experimental study of spontaneous strong parity violation… S.A. Voloshinpage 1 Experimental study of spontaneous strong parity violation in heavy ion collisions at RHIC Sergei A. Voloshin The Collaboration L or B STAR

2. QM2009 Experimental study of spontaneous strong parity violation… S.A. Voloshinpage 2

3. STAR QM2009 Experimental study of spontaneous strong parity violation… S.A. Voloshinpage 3 L or B “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.

4. STAR QM2009 Experimental study of spontaneous strong parity violation… S.A. Voloshinpage 4 QCD vacuum topology topological charge winding number Chern-Simons number N CS = -2 -1 0 1 2 Energy of gluonic field is periodic in N CS 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 density Animation 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.

5. STAR QM2009 Experimental study of spontaneous strong parity violation… S.A. Voloshinpage 5 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 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 N CS = -2 -1 0 1 2 Strong CP problem

6. STAR QM2009 Experimental study of spontaneous strong parity violation… S.A. Voloshinpage 6 EDM of QCD matter Charge separation along the orbital momentum: EDM of the QCD matter ~ the neutron EDM Chiral magnetic effect: N L ≠N R  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) L or B 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

7. STAR QM2009 Experimental study of spontaneous strong parity violation… S.A. Voloshinpage 7 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 suppression of 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 p t  1 GeV/c, though radial flow can move it to higher p t  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

8. STAR QM2009 Experimental study of spontaneous strong parity violation… S.A. Voloshinpage 8 Observable  The effect is too small to see in a single event  The sign of Q varies and  a  = 0  one has to measure correlations,  a α a β , P -even quantity (!)   a α a β  is expected to be ~ 10 -4   a α a β  can not be measured as  sin φ α sin φ β  due to large contribution from effects not related 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 X Y TRANSVERSE PLANE L or B B in ≈ B out, v 1 = 0

9. STAR QM2009 Experimental study of spontaneous strong parity violation… S.A. Voloshinpage 9  “Flowing clusters” decays / RP dependent fragmentation  Global polarization, v 1 fluctuations, detector effects,… Posters: E. Finch, I. Selyuzhenkov Physics backgrounds 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 flow MEVSIM: 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 !

10. STAR QM2009 Experimental study of spontaneous strong parity violation… S.A. Voloshinpage 10 STAR Experiment and Data Set This analysis used the data taken during RHIC Run IV and based on (after all quality cuts) Au+Au 200 GeV ~ 10.6 M Minimum Bias events Au+Au 62 GeV ~ 7 M Minimum Bias events Cu+Cu 200 GeV ~ 30 M Minimum Bias events Cu+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/discussed in 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

11. STAR QM2009 Experimental study of spontaneous strong parity violation… S.A. Voloshinpage 11 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 v 2 {FTPC} bands: lower limits - using v 2 {2}, upper – v 2 {4}; where v 2 {4} is not available it is assumed that using v 2 {FTPC} suppresses non-flow by 50%. | η | < 1.0 (Main TPC) 2.9 < |η| < 3.9 (FTPC) STAR Preliminary

12. STAR QM2009 Experimental study of spontaneous strong parity violation… S.A. Voloshinpage 12 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)

13. STAR QM2009 Experimental study of spontaneous strong parity violation… S.A. Voloshinpage 13 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

14. STAR QM2009 Experimental study of spontaneous strong parity violation… S.A. Voloshinpage 14 Transverse momentum and Δη dependences (AuAu200). Typical “hadronic” width, consistent with “theory” Does the signal extend to too high transverse momenta? STAR Preliminary

15. STAR QM2009 Experimental study of spontaneous strong parity violation… S.A. Voloshinpage 15 Au+Au and Cu+Cu @ 200 GeV +/– signal in Cu+Cu is stronger, qualitatively in agreement with “theory”, but keep in mind large uncertainties due to correlations not related to RP (not shown)

16. STAR QM2009 Experimental study of spontaneous strong parity violation… S.A. Voloshinpage 16 Au+Au and Cu+Cu @ 200 GeV Opposite charge correlations scale with N part, (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 N part The signal is multiplied by N part to remove “trivial” dilution due to multiplicity increase in more central collisions

17. STAR QM2009 Experimental study of spontaneous strong parity violation… S.A. Voloshinpage 17 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 are not shown! 200 GeV62 GeV

18. STAR QM2009 Experimental study of spontaneous strong parity violation… S.A. Voloshinpage 18 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

19. STAR QM2009 Experimental study of spontaneous strong parity violation… S.A. Voloshinpage 19 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 strongly on deconfinement and chiral symmetry restoration Colliding isobaric nuclei (the same mass number and different 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. 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?

20. STAR QM2009 Experimental study of spontaneous strong parity violation… S.A. Voloshinpage 20 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…

21. STAR QM2009 Experimental study of spontaneous strong parity violation… S.A. Voloshinpage 21 The STAR Collaboration

22. STAR QM2009 Experimental study of spontaneous strong parity violation… S.A. Voloshinpage 22 Charge combinations FullField ReversedFullField

23. STAR QM2009 Experimental study of spontaneous strong parity violation… S.A. Voloshinpage 23