Search for effects related to Chiral Magnetic Wave at STAR

1. Search for effects related to Chiral Magnetic Wave at STAR Gang Wang (UCLA) for STAR Collaboration

2. Motivation • CSE + CME  Chiral Magnetic Wave: • gapless collective excitation • signature of Chiral Symmetry Restoration Peak magnetic field ~ 1015 Tesla ! (Kharzeev et al. NPA 803 (2008) 227)

3. Observables Y. Burnier, D. E. Kharzeev, J. Liao and H-U Yee, Phys. Rev. Lett. 107, 052303 (2011) Formation of electric quadrupole: , where charge asymmetry is defined as . Then π-v2 should have a positive slope, and π+ v2 should have a negative slope with the same magnitude. The integrated v2 of π-is not necessarily bigger than π+: (other physics) only the A± dependency matters for CMW testing.

4. Observables CMW + Parity-odd domain, => charge separation across RP This charge separation effect needs to be beyond statistical fluctuation and conventional physics background. Kharzeev, PLB633:260 (2006) Kharzeev, McLerran, Warringa, NPA803:227 (2008)

5. STAR

6. STAR: excellent PID and tracking The correlation measurements at STAR are accurate to relative 0.1%.

7. Observed charge asymmetry • N+ (N-) is the number of positive (negative) particles within |η|<1. • The distribution was divided into 5 bins, with roughly equal counts. • Tracking efficiency was corrected later.

8. Charge asymmetry dependency • v2 was measured with the Q-cumulant method. • Clear A± dependency • v2(A±) slopes for π±: • opposite sign • similar magnitude • v2 difference vs A± may have a non-zero intercept: other physics?

9. Correction for tracking efficiency • Fit with a straight line to extract the slope . • Do the same for all centralities

10. Slope vs centrality • Statistical errors only • Smooth trend • Positive and significant for most centrality bins • Systematic uncertainty: • weak decays (Λ et al.) • tracking efficiency • <A±> bin center effect • different v2 methods

11. Theoretical calculations Y. Burnier, D. E. Kharzeev, J. Liao and H-U Yee, private communication. • Very similar trend between data and theoretical calculations • To compare the magnitude, the acceptance effects need investigation.

12. UrQMD Hongwei Ke The slopes from UrQMD are consistent with 0 using the same approach.

13. Outlook • Further studies of the weak decay contributions • DCA (Distance of Closest Approach) cut on particles • simulation such as UrQMD and AMPT • Beam energy scan (39 GeV, 27 GeV and 19.6 GeV) • different charge asymmetry distribution • different magnetic field? • Kaon v2 • opposite v2 ordering than pion (v2(K+) > v2(K-)) • what about A± dependency? • Handle on the magnetic field • trigger on spectators with Zero Degree Calorimeters • is the signal still there when we turn off B?

14. CMW + Local Parity Violation P-even quantity: still sensitive to charge separation A direct measurement ofthe P-odd quantity “a” should yield zero. S. Voloshin, PRC 70 (2004) 057901 Non-flow/non-parity effects: largely cancel out Directed flow: vanishes if measured in a symmetric rapidity range

15. STAR Preliminary Results with different EPs Lost in the medium? The correlators using TPC/ZDC event planesare consistent with each other.

16. Non-zero Radial flow? Dilution effect Weaker B field STAR Preliminary Momentum conservation? The factor Npart is used to compensate for dilution effect.

17. (OS - SS)*Npart IfNspec is a measure for B, the signal (OS-SS)*Npart is roughly proportional to the magnetic field in central/mid-central collisions. Experimentally, we may trigger on Nspec to control B.

18. Physics background for LPV: Possible physics background charge conservation/cluster + v2 + - ΨRP + - Phys. Rev. C72 (2005) 014904 Qualitatively similar trend! Need to trigger on very small Nspec.

19. Beam energy scan As we lower the beam energy, changes start to show from the peripheral collisions. 27 GeV and 19.6 GeV coming soon...

20. If we consider OS-SS to be signal... The signal seems to be disappearing at 7.7 GeV, but the statistical errors are large.

21. Outlook • Further studies of identified particles • Kπ correlation • pπ, Λπ... • ΛΛ correlation: vorticity • Beam energy scan (27 GeV and 19.6 GeV) • Is there a smooth transition from 200 GeV to 7.7 GeV? • different magnetic field? • Handle on the magnetic field • trigger on spectators with ZDCs • is there still signal when we turn off B? • U+U collisions • different v2 trend from Au+Au? See also Kent Riley's talk! Phys.Rev.Lett. 105 (2010) 172301

22. Backup slides

23. Multi-component Coalescence (MCC) + Quark Transport John Campbell & Mike Lisa, preliminary study, publication in preparation.

24. In the quark-gluon medium, there could be multiple P-odd domains. The net effect is like a random walk,but one-dimensional. Dilution effect What do we know about the position Rn after n steps? Rnfollows a Gaussian distribution:mean = 0, and rms= Our measurement of PV is like Rn2, expected to be n. Compared with going in one fixed direction, where Rn2 = n2, the "random-walk"measurement is diluted by a factor ~ n ~ Nch.

25. charge conservation/cluster + v2 Possible physics background Phys.Rev.C83:014913,2011 Phys. Rev. C72 (2005) 014904 Qualitatively similar trend! Need to trigger on very small Nspec.

26. Balance function Phys.Rev.C83:014913,2011