Overview of the Search for Chiral Effects in Heavy Ion Collisions

1. Overview ofthe Search for Chiral Effects in Heavy Ion Collisions HUAN ZHONG HUANG (黄焕中） Fudan University & University of California, Los Angeles August 2017 @Fudan

2. OUTLINE • CME and Charge Separation Across the RP • Search for Chiral Vortical Effect • Future Perspective

3. QCD Domain Formation Non-Abelian Gauge Theory Dynamical by nature The volume of the box is 2.4 by 2.4 by 3.6 fm. The topological charge density Animation by Derek Leinweber

4. Vacuum Structure and Excitation Energy of gluonic field is periodic in NCSdirection (~ a generalized coordinate) 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. NCS = -2 -1 0 1 2 Instantons and sphalerons are localized (in space and time) solutions describing transitions between different vacua via tunneling or go-over-barrier QCD vacuum transition  nonzero topological charge  chirality imbalance (local parity violation)

5. γ correlator background effects P-even quantity: sensitive to charge separation fluctuation Directed flow A quantitative measure for extra charge fluctuation. S. Voloshin, PRC 70 (2004) 057901

6. Charge Dependent g Measure OS SS Phys. Rev. Lett. 103(2009)251601 Phys. Rev. C 81(2010)54908 Phys. Rev. C 88 (2013) 64911 • γos > γss, consistent with CME expectation • Consistent between different years (2004 and 2007) • Confirmed with 1st-order EP (from spectator neutron v1) • Not explained by known event generators

7. g more sensitive to in-plane separation + + + + + + + + + + + + + + + + + + + + + + - - - - - - - - - - - - - - - - - - - - - - In-plane back-to-back correlation could be an artifact due to g definition Explore other correlators (Roy Lacey’s Talk)

8. Similar signals in U+U • Use γOS-γSS to quantify the signal • Npart accounts for dilution effects Deformed nuclei: U+U • A dedicated trigger for events with 0-1% spectator neutrons • With magnetic field suppressed, the charge separation signal (mostly background) disappears, while v2 is still ~2.5% 70-80% 0-5% Extrapolate to intermediate centrality? Isobar collisions may work better. 0-1%

10. Phys. Rev. Lett 113 (2014) 052302 Beam Energy Scan At lower beam energies, charge separation starts to diminish.

13. Phys. Rev. Lett 113 (2014) 052302 • Against CME expectation, δOS > δSS • Indicate overwhelming background, larger than any possible CME effect. • Try combining information from γ and δ to retrieve the CME contribution, H H Measure A. Bzdak, V. Koch and J. Liao, Lect. Notes Phys. 871,503 (2013).

14. There is likely a beam energy dependence Compare with RHIC data CMS data

15. Phys. Rev. Lett 113 (2014) 052302 Difficult to Remove Charge Separation A. Bzdak, V. Koch and J. Liao, Lect. Notes Phys. 871,503 (2013). • κ ≈ 2 - v2,F/v2,Ω≈ 1.2: F and Ω denote full phase space and finite detector acceptance, respectively • CME signal (ΔH) decreases to 0 from 19.6 to 7.7 GeV • The decomposition of g into F and H is not unique

16. Agree with CMS Statement ! At RHIC DH ~ 20% Dg correlator To be precise, maybe useful to specify “at the LHC energies” in the CMS statement! Congratulations to CMS for the nice study!

17. Chiral Vortical Effect Chiral Magnetic Effect vs Chiral Vortical Effect Chirality Imbalance (μA) Magnetic Field (ω μe)Fluid Vorticity (ω μB) ↓↓ Electric Charge (je)Baryon Number (jB) correlate Λ–p to search for the Chiral Vortical Effect D. Kharzeev, D. T. Son, PRL106 (2011) 062301

18. same baryon number: • opposite baryon number: • “same B”is systematically lower than “oppo B” in the mid-central and peripheral collisions, consistent with the CVE expectation. Λ-proton correlation

19. Baryon-Baryon Correlation L-p correlation – different from L-h and KS-p correlation ! CVE?

20. STAR Measurement for Lambda Polarization WRT the Reaction Plane • Larger effect at lower beam energy ! • Difference between Lambda and Anti-Lambda?

21. Discovery Yet ? There is a charge separation effect -- separate CME and background ?! There is an extra-v2 due to charge asymmetry -- electric quadrupole due to CMW or ? There is a baryon-baryon separation effect -- CVE or ? More insight and towards a definitive answer: -- establish B field and its consequence -- effect correlating CME/CVE/CMW

22. Outlook: Isobars Isobars are atoms (nuclides) of different chemical elements that have the same number of nucleons. For example, 9644Ruthenium and 9640Zirconium: Up to 10% variation in B field

23. Isobars: B field • Clear difference in the B field for the same centrality • The ratio is close to 1.1 for peripheral events • Reduces to 1.07 for central events Courtesy of Xu-Guang Huang and Wei-Tian Deng

24. Isobars: charge separation • Projection from 1.2B events shows difference in ΔH • The ratio is 5σ above 1 (3σ with 400M events) • If it's v2-driven, the ratio will follow eccentricity (be 1 or below 1)

25. Experimental Window of Opportunity • Isobaric running to see B field effect • @200 GeV in 2018 • 2) Au+Au data from Beam Energy Scan II • to observe B magnitude and life-time • difference 2019 + • 3) Develop an experimental QCD chirality • effect research program in China • (Fudan, SINAP + ?) • Complement to theoretical work by • Qun Wang et al and Defu Hou et al

26. THE END

27. Isobars: Δv2(Ach) slope • The slope parameter is also expected to differ • With 1.2B events, the ratio is 1σ above 1 • Need more statistics

28. Differential C and S Correlations

29. Differential C and S Correlations

30. Charge Separation Out-of vs. In RP There should be more out-of-plane charge fluctuation than in-plane. Indeed, we can visualize this effect, which is on percent level! relative difference in RMS 200 GeV Au+Au Phys. Rev. C 88 (2013) 64911

31. K0S-hadron correlation • Correlations of K0S-h- and K0S-h+ consistent with each other: no charge-dependent separation

32. Λ-hadron correlation • Correlations of Λ-h±also show no charge-dependent separation (protons and antiprotons have been excluded from h±) • Separation observed for h±-h± is due to electric charge • s quarks participate in the chiral dynamics in a similar way as u/d