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LHC Days in Split: Heavy Ion Theory

This talk will discuss the emergence of collective phenomena and properties of matter from the fundamental Lagrangian in heavy ion collisions at the LHC. It will cover topics such as the flow in PbPb and pPb reaction planes, hydrodynamic behavior, initial conditions, pre-equilibrium evolution, and alternative explanations for collectivity. Additionally, it will explore the implications of fluid dynamics for soft particle production and jet quenching.

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LHC Days in Split: Heavy Ion Theory

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  1. LHC Days in Split: Heavy Ion Theory pp@LHC: PbPb@LHC: pPb@LHC: + LQCD QCD precision EWK precision Higgs physics BSM physics Urs Achim Wiedemann CERN PH-TH Thu 22/09, afternoon How do collective phenomena and properties of matter emerge from this fundamental Lagrangian? Thu 22/09, morning Mo 19/09, afternoon + LBSM Tue 22/09, afternoon Urs Wiedemann CERN TH Department

  2. Flow in PbPb and pPb Reaction plane • Azimuthal asymmetries vm persist unattenuated in many-particle correlations collectivity CMS, PRL 115 (2015) 012301 • pT-dependence of vm • PID-dependence of vm • rapidity (in)dependence of vm • centrality dependence • correlations of reaction • plane orientations support fluid dynamic picture of heavy ion collisions

  3. Fluid dynamic modeling of heavy ion collsions • Few model parameters characterize • Initial conditions • Hydro evolution • Particlization • Bayesian analysis • Leads to very good posteriors • Best parameters indicate a minimally dissipative fluid prior J.E. Bernhard et al. , arXiv:1605.03954v2 posterior

  4. How can hydrodynamic behavior arise? Initial conditions Pre-equilibrium stage Hydrodynamic expansion • non-equilibrated • over-saturated • anisotropic • determined by effective kinetic theory • (if dynamics is perturbative) • qualitatively understood in models of gauge-gravity duality • (if dynamics is non-perturbative) leads to rapid hydrodynamization • controlled matching on pre-equilibrium stage • determined by very few thermal equilibrium properties • (calculable from 1st principles in QCD) in more detail …

  5. 1. Initial conditions • Gluon distributions grow rapidly at small x. • Small-x growth in incoming nuclei can reach maximal parton densities Venugopalan McLerran; Jalilian-Marian,Kovner,Leonidov,Weigert; Balitsky; Kovchegov;… • Phase space density of produced partons is over-occupied (thermal distribution fgluon ~ 1) • Initial momentum distribution of • is anisotropic

  6. 2a. Pre-equilibrium evolution (perturbative) Under longitudinal expansion, initially overoccupied systems become underoccupied before reaching local thermal equilibrium. R.Baier, A.H. Mueller, D. Schiff, D.T. Son, 2001 • QCD effective kinetic theory: • to order and for • modes with satisfy Boltzmann equation Berges, Eppelbaum, Kurkela, Moore, Schlichting, Venugopalan, … 2->2 collision kernel LPM splitting term Kurkela, arXiv:1601.03283

  7. 2a. Hydrodynamization of kineticevolution • Dissipative hydrodynamics describes long-time behavior of QCD effective kinetic theory Kurkela&Zhu, PRL115 (2015)182301 Sub-fermi hydrodynamization time scale

  8. 2b. Pre-equilibrium evolution (non-perturbative) Gauge-gravity duality gives access to pre-equilibrium dynamics of a class of non-abelian plasmas in the strong coupling limit • Strongly coupled non-abelian plasmas equilibrate fast Chesler, Yaffe, PRL 102 (2009) 211601 Heller, Janik Witaszczyk, PRL 108 (2012) 201602 Free energy Pressure anisotropy Sub-fermi hydrodynamization time scale

  9. Take-away message on fluid dynamics • Theory has reached recently an improved understanding of • how QCD fluid dynamics can emerge in pre-equilibrium dynamics. • Rapid hydrodynamization (on sub-fm time scale) • Hydrodynamization prior to thermalization • Anisotropic hydrodynamics • Anomalous hydrodynamics • Thermal fluctuations in relativistic hydrodynamics • This extends standard textbook treatments of relativistic fluid dynamics in various ways. • Data on soft particle production are well-described by fluid dynamic simulations.

  10. CMS Coll. arXiv:1606.06198 • Flow is also seen in pp@LHC. Are there alternatives to fluid dynamics that account for collectivity in small systems?

  11. Alternatives to fluid dynamics? • Azimuthal anisotropy from interference? • Color field domains in incoming hadrons? Kovner, Lublinsky, Dumitru, Gelis, Jalilian-Marian, Lappi, Venugopalan, Dusling, … • Still open questions • Scaling of higher order cumulants • System size dependence • Odd harmonics • Studies typically done in CGC saturation model, but physics conclusions may hold more generally.

  12. Hard Probes: Jet fragmentation vs. Jet thermalization

  13. Medium-induced gluon radiation: naïve estimate Medium characterized by BDMPS transport coefficient: Brownian motion Characteristic gluon energy Phase accumulated in medium: , where Number of coherent scatterings: Non-abelian LPM-effect Gluon energy distribution: Average energy loss Soft gluons emitted first. Fromation time

  14. Phenomenology of parton energy loss • Supplementing parton showers with LPM-type medium-induced splittings accounts for “jet quenching observables”: • RAA of single inclusive hadrons • RAA of heavy flavor • RAA of jets • Dijet asymmetries • Medium-modified jet fragmentation functions TALK BY Y. PACHMAYER G. Milhano, K. Zapp, arXiv:1512.08107

  15. Parametric understanding of jet quenching • Clarifies sensitivities to: A. Kurkela & UAW, PLB740 (2015) 172 • color field strength of medium, • i.e. jet quenching parameter • constituents of medium • accessible via large-angle • scattering Longitudinal phase space => • thermalization: • the LPM splitting function is • exactly the C1->2 kernel of the • Boltzmann equation underlying • hydrodynamization Angular phase space =>

  16. Jet grooming: “Soft drop” procedure Measuring medium-modified SPLITting functions Larkoski, Marzani, Soyez Thaler (2014,2015) • remove soft junk • to probe first perturbative splitting • Could medium-modified splitting functions be testable by jet grooming? Studies of medium-mod color antenna: Mehtar-Tani,Salgado, Tywoniuk, …. • First data on groomed jet splitting functions in heavy ion collisions CMS-PAS-HIN-16-006 p.t.o.

  17. Slide from Marta Verweij CMS-PAS-HIN-16-006 PbPb vs pppT,jet: 140-160 GeV Strong modification of splitting observed in central PbPb collisions Branching more imbalanced in central PbPb Marta Verweij

  18. This was not an overview. I did not try to tell everything, I tried to say something: I gave an example of how the LHC heavy ion program addresses the fundamental question: How do collective phenomena and properties of matter emerge from the fundamental Lagrangian?

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