严力青年研究员，复旦大学，于2023年3月23日16:00在物质科研楼C座11层1124做题为”The direct photon puzzle and the weak magnetic photon emission ”的报告, 欢迎感兴趣者参加。.pdf

The direct photon puzzle and the weak magnetic photon emission Li Yan Institute of Modern Physics at Fudan University Mar. 23, 2023 @ USTC with Jing-An Sun, arXiv: 2302.07696 Outline • Direct photons and direct photon puzzle in heavy-ion experiments. • Magnetic field in heavy-ion collisions. • Weak magnetic photon emission (WMPE). 2 Standard picture of Heavy-ion collisions • QCD phase diagram 3 Standard picture of Heavy-ion collisions • Different stages in one heavy-ion collision event initial stage (pre-equilibrium) + QGP + Hadron Gas + freeze out 4 Hadron probes • Hadron spectra: pions, kions, protons, etc., where vn (n-th order flow) contains information of QGP collective flow. vn of charged hadrons has been measured precisely for n<6. Flow provides probes of transport properties of QGP, e.g., η/s, ζ/s. 5 Hadron probes from hydro (standard model) • Hydro: initial condition + hydro EoM + QCD EoS + hadronization • Hadron spectra and flow paradigm: medium response to geometry [C. Gale et al, 2016.00216] [PHOBOS MC-Glauber, C. Loizides et al, 1408.2549] 6 Flow paradigm • Anisotropic flow developped as system expands, • A finite time scale is required. time 7 Direct photons in hadron colliders: e.g., • Photons from partonic scatterings (subtract secondary decays). • Can be calculated via pQCD, + • cross-section ~ • Isotropic emmission. + ... , access to gluon PDF in hadron 8 Direct photon cross section in pp 9 Direct photons in heavy-ion collisions • Produced during the whole system evolution (exclude hadron decay) probe access to information of different evolution stages! “historian” 10 Direct photon production in heavy-ion collisions [G. David, 1907.08893] Hard production: pp ⊗ Ncoll 11 Direct photon production in heavy-ion collisions [G. David, 1907.08893] Thermal radiation 12 Direct photon production in heavy-ion collisions [G. David, 1907.08893] Jet conversion, etc. 13 Direct photon yields from heavy-ion collisions • Hard process dominates at large pT • Excess of direct γ above pQCD scaled expectation at small pT: thermal radiations. • Non-prompt (direct - pQCD scaled photons) spectrum used to estimate QGP temperature. • We focus on the QGP thermal radiation [PHENIX collaboration, 2203.12354] 14 Direct photon yields from heavy-ion collisions • Hard process dominates at large pT • Excess of direct γ above pQCD scaled expectation at small pT: thermal radiations. • Non-prompt (direct - pQCD scaled photons) spectrum used to estimate QGP temperature. • We focus on the QGP thermal radiation [Turbide et al, PRC 69 014902 (2004)] 15 Thermal radiation from equilibriated QGP • Leading order radiation: and • Production rate: • Photon spectrum requires space-time integral: require medium info. 16 Direct photon elliptic flow (before 2011) • Naive expectation: Temperature Momentum anisotropy prompt + preequilibrium γ dominant thermal γ ~ T4 hadrons Time of QGP evolution 17 Direct photon elliptic flow (after 2011) • Experimental observation: [PHENIX collarboration, PRL 109, 122302 (2012)] 18 Direct photon elliptic flow (after 2011) • Confirmation at the LHC: [ALICE collarboration, 1805.04403] 19 Experiments vs theories [PHENIX, 1509.07752, R. Ryu et al, PRL 115 132301(2015), J. Paquet et al, PRC 93 044906 (2016), O. Linnyk et al, PRC 92, 054914 (2015)] 20 Direct photon puzzle • PHENIX collarboration, PRL 109, 122302 (2012): • Direct photon puzzle: “Not too much of a puzzle left for yields.” [K. Reygers, Quark Matter 2022 plenary talk] 21 Up-to-date (conventional) hydro prediction [C. Gale et al, 2016.00216] • Chemical equilibration in QGP • EbE 2+1D hydro for medium expansion • NNLO pQCD for prompt photons • LO thermal rate (AMY) with also dissipative corrections: f + δf • Hadron gas γ production • But, no magnetic field! 22 Electromagnetic field in heavy-ion collisions • From the relativistic motion of nucleus • B field is dominated out of plane, • B field is extremely strong initially, which has lead to many novel effects. [Skokov and Bzdak 2012, Deng and Huang 2012, Kharzeev 2008, Tuchin 2010, Skokov and Mclerann 2013, ...] • Life time of B is crucial, but unknown. 23 Theoretically expected decay of magnetic field A brief summary: • Vacuum evolution of B is well solved, as well as B before tau_0. • Medium contribution is uncertain, due to the electrical conductivity, as well as B after tau_0. vacuum + medium • tau_0 is related to chemical equilibration of QGP, approximately, tau_0 ~ 1 fm/c. • Life time of B is extremely short unless with a unreasonbly large conductivity. [A. Huang et al, 2212.08579, J-J. Zhang et al, 2201.06171, U. Gursoy et al, 1401.3805, L Yan and X. Huang, 2104.00831, K. Hattori and X. Huang, 1609.00747, and many others] vacuum 24 Strong Magnetic field and direct photon v2 • Conformal anomaly [G. Basar et al., 2012] • Quark with Landau level excitations [A. Ayala et al, 1704.02433, X. Wang et al, 2006.16254] • Some other mechanism involving induced inelastic emission (synchrotronlike), etc. [K. Tuchin, PRC 91, 014902., 2015, B. Zakharov, EPJC 76 (2016)] • Strong magnetic field only exists at very early times: tau~0.1fm/c. 25 • Charged carriers in QGP (quarks) needs time to generate: tau>tau_0. Magnetic field: strong vs weak • Strong B leads to MHD: B becomes hydro d.o.f. • Strong B alters dynamics of quark scattering amplitudes, (Landau levels) • In realistic HIC, these conditions reduce to a criterion, weak field condition, , thus 26 Weak magnetic photon emission (WMPE) in QGP • Weak field strength assumption: • Dynamical evolution of QGP follows hydro, instead of MHD. • However, QGP is slightly off-equilibrium due to weak B field. • Photon production via quark scatterings not changed substantially. • Our goal: 1. find a novel mechanism of photon emission in weak B field. 2. photon emission must be significantly anisotropic. 3. do not change direct photon yields substantially. 27 Weak magnetic photon emission in QGP 1. Start with the production rate from kinetic theory*: * small angle approximation is not a necessary step, but good for illustration 28 Weak magnetic photon emission in QGP 2. Shift in fq due to the presence of a weak B field: where in background rate quark distribution also gets corrected by viscosities. 29 Weak magnetic photon emission in QGP 3. Determining fEM according to electrical conductivity of QGP, 30 Weak magnetic photon emission in QGP 3. Determining fEM according to electrical conductivity of QGP, properties of background QGP properties of external E&B field, e.g., 31 Origin of photon elliptic emission anisotropy • External B field naturally contains a dipole moment, • An extra dipole moment comes from background QGP, e.g., a tilted fireball, [P. Bozek et al., 1101.3354] • Coupled effects give rise to [STAR collaboration, PRL 101, 252301 (2008)] 32 Origin of photon elliptic emission anisotropy • External B field naturally contains a dipole moment, • An extra dipole moment comes from background QGP, e.g., a tilted fireball, [P. Bozek et al., 1101.3354] • Coupled effects give rise to 33 WMPE on top of Bjorken flow • Bjorken flow: boost invariant along z, analytical but unphysical. • Correction of quark distribution due to B field, which implies already , irrespective of magnitude of By • After integral over space-time and even rapidity window as experiments, only odd components remain, so here for photons v1=0. 34 WMPE under realistic conditions • Electrical conductivity = LO pQCD evaluation (AMY), to be consistent with background photon results. • One tilted fireball initial condition, which has been used to reproduce charged hadron v1 : single-shot • Solving medium evolution using 3+1 D hydro (MUSIC), with finite shear/bulk viscosities, from tau0=0.4 fm/c. 35 WMPE under realistic conditions • Space-time profile of external B field as in vacuum: “worst-case” scenario [K. Hattori and X. Huang, 1609.00747] here is the initial field strength when QGP evolves hydrodynamically. 36 Ideal hydro and weak field condition • Weak B field indeed gives rise to a large elliptic flow of direct photons! 37 Viscous hydro and WMPE • Detailed shape of elliptic flow is sensitive to viscosity. • Experimental data of elliptic flow can be reproduced! • Direct photon yields receive small enhancement (~10%) . 38 Confronts experiment at RHIC • Good agreement for all centralities. • Initial field strength extracted and grows as centrality increases: correct trend! 39 Confronts experiment at LHC • Good agreement for all centralities. • Initial field strength extracted and grows as centrality increases: correct trend! 40 Event-by-event simulations (preliminary) • Extract initial field strength is reduced significantly: • Surprisingly, vacuum evolution of B(t=0.4 fm/c) gives 41 Summary • Weak magnetic photon emission can be a solution to “direct photon puzzle”! • A weak magnetic field evolves along with QGP in HIC. • Direct photon spectrum can be used to extract B field strength at early time. 42