Highlights from STAR at RHIC

1. Highlights from STAR at RHIC Evan Finch Yale University

2. 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…

3. Why Heavy Ion Collisions? • To (re)-create the Quark Gluon Plasma • To study the QCD vacuum state

4. The QCD Vacuum • Quark Confinement T.D. Lee (1994)

5. 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-sLindependently. 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.

6. 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 of 1) 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 θEcBc (P, CP odd)  why is there no (global) parity, CP violation in QCD? (Strong CP Problem)

7. 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)

8. (how to study?) The QCD Vacuum

9. RHIC

10. STAR at RHIC

11. STAR at RHIC

12. Magnet Main TPC Coils FTPC east FTPC west ZDC ZDC Barrel EM Calorimeter STAR at RHIC

13. STAR TPC Performance

14. STAR TPC Performance Particle 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

15. (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?

16. Elliptic flow… A non-central collision results in an almond-shaped region of hot-matter (in the transverse plane) 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 (cross section view) : Higher pressure in the x-direction than the y-direction.

17. v2vs hydrodynamic model calculations • Ideal (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 transition • From 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 )

18. 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 correlations • forward detectors for reaction plane • 2-D fits of 2 particle correlations. (using longitudinal information) multiparticle method is used ? Central Peripheral

19. “Jet” quenching Using high pT particles to probe the density of the system Hard scattering Transverse plane Au-Au medium In a p-p collision, when we detect one high pT particle, we tend to find others near it in azimuth and 180° away. In a Au-Au collision, the particles at 180° disappear; “quenched by the medium”? And they return somewhat when the almond is sideways

20. “Jet” quenching • Another view of this effect: there are fewer hadrons at pT~fewGeV 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.

21. Outline of Your Next Hour… • 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 • Whirlwind tour of what else STAR is doing and will do soon… Elliptic flow, jet quenching in Au+Au (and not in d+Au)

22. 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.

23. Chiral Magnetic Effect… Two ingredients: 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. 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

24. 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)

25. How to look for this? Ifhas the same sign in each event…: And parity violation is signaled by nonzero a+=−a−≠0

26. How to look for this? Ifhas the same sign in each event…: It’s not, so we expect over many events α,β=+,− Instead, we look at 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

27. How to look for this? Observable we use (proposed in S. Voloshin): = ( v1,αv1,β+ Bin ) − ( <aαaβ> + Bout ) Non-flow 2-particle correlations projected “out-of-reaction-plane” Non-flow 2-particle correlations projected onto the reaction-plane P-violation term Directed flow (small) 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:

28. First Look: A Suggestive Signal “<−a+a−>” <cos(φα+φβ−2ΨRP)> “<−a+a+>”,”<−a−a−>” Peripheral Central

29. Theoretical “Expectations” Kharzeev, McLerran, Warringa, Nucl.Phys.A803,227(2008) “<−a+a−>” <cos(φα+φβ−2ΨRP)> <−a+a+> “<−a+a+>” • Calculation-with significant uncertainty in magnitude of what same sign signal should look like • Measured values are roughly in line with initial estimates of signal size due to the Chiral Magnetic Effect (Local Parity Violation).

30. Theoretical “Expectations” “<−a+a−>” <cos(φα+φβ−2ΨRP)> “<−a+a+>” • 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

31. 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.

32. Physics backgrounds- type 2 <cos(φα+φβ−2ΨRP)> measures, roughly speaking… Same-side, in-plane pairs Opposite-side, in-plane pairs . Opposite-side, out-of-plane pairs . Same-side, out-of-plane pairs 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.

33. 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+φβ−2ΨRP)>

34. Physics backgrounds-type 1 3-particle clusters distort the measurement of the reaction plane… 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)… UrQMD

35. Magnet Main TPC Coils FTPC east FTPC west ZDC ZDC Barrel EM Calorimeter Beating down “type 1” background 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.

36. Beating down “type 1” background 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. 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.

37. Results- 200 GeVAuAu 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)

38. 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.

39. 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 and • is 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 models • that we have studied (HIJING, HIJING+v2, UrQMD, • MEVSIM), which span a broad range of hadronic physics.

40. LPV Future: Experiments Dedicated 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 neutral • particles • 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. Turn off Chiral Magnetic Effect, see what backgrounds remain Look for critical behavior, as LPVpredicted to dependstronglyon deconfinement and chiral symmetry restoration Colliding isobaric nuclei (the same mass number anddifferent charge) and by that controlling the magnetic field Quarks emerging from P-odd region expected to be equally distributed among light flavors.

41. Future theoretical directions • 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 (equal number of q-qbarpairs of different flavors?). page

42. Outline of Your Next Hour… • 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 • Whirlwind tour of what else STAR is doing and will do soon… Elliptic flow, jet quenching in Au+Au (and not in d+Au)

43. 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.

44. 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.

45. charm: thermalized? • Non-photonic electron signal suggests that they are… • Much better measurements coming with STAR heavy flavor tracker.

46. 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.

47. Critical Point Search • Main Idea: if collision system passes near a critical point, correlation length growslook for enhanced fluctuations in particle type production (using new STAR TOF system), pT correlations, etc. as a function of incident beam energy.

48. Summary Several 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!

49. PHENIX “direct photons” results • NCQ scaling • NPE – energy loss of heavy quarks? • Estruct stuff (2-D correlations) • Charm flow? • Full jet reconstruction

50. Theoretical “Expectations” 1.0 0.5 0.0 λ/R = 0.1 λ/R = 0.2 λ/R = 0.3 <cos(φα+φβ−2ΨRP)> 2 1.5 1.0 0.5 0 b/R • “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