QCD Chirality 2017, UCLA, March 27-30, CME Theory: what next? D. Kharzeev

Transcription

1 QCD Chirality 2017, UCLA, March 27-30, 2017 CME Theory: what next? D. Kharzeev 1

2 Many new theoretical and experimental developments since QCD Chirality 2016 Excellent talks at this Workshop demonstrate that the interest in the anomaly-induced macroscopic quantum phenomena continues to grow Reasons: 1. Rich, fundamental physics; link between relativistic field theory, fluid mechanics, and many-body theory 2. Diverse applications (heavy ion collisions, condensed matter, quantum optics, cosmology,.) 3. Fun problem to work on! 2

3 Theory news It is no longer possible to review all developments in the theory of CME and related phenomena in a short talk so I will not attempt it. Resources and reviews: talks at this Workshop, QCD Chirality 2017 talks at Chiral Matter 2016 (December 5-8, 2016, RIKEN): DK, J.Liao, S.Voloshin, G. Wang, Prog.Part.Nucl.Phys. 88 (2016)1; DK, Prog.Part.Nucl.Phys. 75 (2014) 133; Strongly interacting matter in magnetic fields, Springer 2013 Eds. DK, K.Landsteiner, A. Schmitt, H.-U.Yee, 3 Lect.Notes Phys. 871 (2013) pp

4 Instead, I would like to provoke a discussion on how the theorists can help in the experimental study of CME in heavy ion collisions. Let me mention some of the recent experimental results first: UU collisions identified hadrons Talk by P. Tribedy Talk by G. Wang Event shape engineering Talk by J. Onderwaater Λ polarization vorticity Talks by M.Konyushikhin, N.Niida pa collisions challenge to theory! Talks by W.Li, 4 P.Tribedy

5 Surprising scaling of pa and AA results at different energies: But: different dependence on rapidity difference between α and β Talk by W.Li 5

6 Some comments: 1. The scaling is a challenge to both CME and background interpretations, since background scales as v 2 /N, and v 2 is different (~ 30%?) in pa and AA at the same multiplicity. Even more challenging for RHIC vs LHC comparison. 2. In pa, one expects much weakened, but non-zero correlations between magnetic field B and reaction plane due to the gradient of nuclear density. For a black disk: This configuration yields B orthogonal to the reaction plane; Its contribution is suppressed by (R N /R A ) 2 6

7 This configuration yields B orthogonal to the reaction plane (RP); Its contribution is suppressed by (R N /R A ) 2 The proton is always much smaller than the nucleus or is it? The proton size grows with energy: Gribov diffusion; Shrinkage of diffraction peak. Even at LHC, still a relatively modest size growth. But: the second term is due to the number of parton splittings in high multiplicity N events, can expect larger than average size of the proton, Can this effect give a sizeable correlation between B and RP? 7

8 Average Multiplicity: High Multiplicity: Can this effect give a sizeable correlation between B and RP? 8

9 3. Even in pa collisions, vorticity has to be correlated with the reconstructed reaction plane: Perhaps, the Chiral Vortical Effect (CVE)? Can this be studied in high multiplicity pp collisions? (small B, high vorticity, can check scaling expected for background vs CVE) 9

10 What can theorists do to help finding the truth? Build a fully quantitative description of anomalyinduced phenomena in heavy ion collisions, suggest new observables What is the appropriate framework? Comparison to the RHIC and LHC data established relativistic hydrodynamics as a truly effective theory of heavy ion collisions extend this theory to include the effects of anomalies and dynamical electromagnetic fields (Chiral Magnetohydrodynamics, CMHD). 10

11 Hydrodynamics and symmetries Hydrodynamics: an effective low-energy TOE. States that the response of the fluid to slowly varying perturbations is completely determined by conservation laws (energy, momentum, charge,...) Conservation laws are a consequence of symmetries of the underlying theory What happens to hydrodynamics when these symmetries are broken by quantum effects (anomalies of QCD and QED)? 11 Son, Surowka; Landsteiner, Megias, Pena-Benitez; Sadofyev, Isachenkov; Kalaydzhyan, Kirsch; DK, Yee; Zakharov; Jensen, Loganayagam, Yarom; Neiman, Oz;.

12 CMHD is a natural extension of the existing hydrodynamical approach to QCD plasma: A striking finding of RHIC and LHC heavy ion experiments is the quantum fluid behavior of QCD plasma. Two ways in which quantum physics affects hydrodynamics: 1. Quantum effects on conventional transport coefficients (KSS bound on shear viscosity-to-entropy ratio, bulk viscosity from conformal anomaly, ) 2. New transport coefficients of entirely quantum nature (Chiral magnetic conductivity, chiral vortical conductivity,..) 12

13 To apply CMHD, we also need: 1. Initial conditions that a) allow to describe topological transitions in magnetic background at early stage of the collision and b) can be connected to parton-based weak coupling description of hard processes in high energy collisions => classical Yang-Mills looks appropriate 2. Chiral kinetic theory to understand the matching between YM and hydrodynamical descriptions 3. Careful evaluation of the initial magnetic field 13

14 Very significant progress since QCD Chirality 2016, helped by the establishment of BEST Collaboration a) theoretical understanding of the CMHD s behavior (self-similar inverse cascade of magnetic helicity, coupling to Alfven waves, CME from magnetic reconnections, ) Talks by Y.Hirono, J.Liao, A.Sadofyev, S.Sen, K.Tuchin, X.Xia, Y.Yin b) real-time first principle lattice calculations of the initial state in heavy ion collisions, CME and CMW seen Talk by M.Mace c) first relativistic MHD calculations in heavy ion physics ECHO-QGP (F. Becattini, L. Del Zanna, G. Inghirami, ); Talk by A.Srivastava d) further development of the Chiral kinetic theory 14 Talks by S.Pu, Q.Wang, D.Yang,

15 Talk by M. Mace: Chiral Magnetic Wave in real time! Anomalous transport in real time :axial charge B :vector charge Static U(1) magnetic field in z-dir 15

16 CMHD (background magnetic field) Electric charge Chiral charge Talk by Y.Hirono Y.Hirono, T.Hirano, DK, arxiv: (3+1) ideal CMHD (Chiral MagnetoHydroDynamics)

17 Magnetic field evolution in ideal MHD [G. Inghirami et al. EPJC 16] Talk by Y.Hirono MHD Vacuum 17

18 Talk by Y.Hirono; CMHD with dynamical MHD magnetic field from ECHO-QGP Initial condition: B field evolution in transverse plane [Tuchin PRC 13] [Li-Sheng-Wang PRD 16] 18

19 Anomalous viscous hydrodynamics Anomalous currents as perturbations on top of conventional (2+1)D VISHNU viscous hydrodynamics; background magnetic field. Y.Jiang, S.Shi, Y.Yin, J.Liao, Arxiv: Talk by J.Liao 19

20 What next? Immediate goals for quantitative CME theory: 1. Combine an anomalous hydrodynamical code with a MHD code full CMHD 2. Produce coarse-grained initial conditions for the chiral and electric charge densities and currents (in addition to the energy density) from real-time lattice QCDxQED simulations. 3. Back-reaction on magnetic field at early stages initial condition for magnetic field from QCDxQED (big effect) 4. Phenomenology of backgrounds! 20

21 arxiv: See also: DK, J.Liao, S.Voloshin, G.Wang, Prog.Part.Nucl.Phys.88(2016)1 Approved dedicated 2018 CME run at RHIC with Zr (Z=40), Ru (Z=44) isobars not much time left!

22 Summary The physics of anomaly-induced macroscopic phenomena is very rich, diverse and fundamental A fully quantitative description of anomalous transport in strongly coupled systems (such as the quark-gluon plasma) Chiral magnetohydrodynamics is a very rich theory with the behavior that still has to be understood a lot of work to be done, both in theory and experiment! 22