LHC Days in Split: Heavy Ion Theory

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## LHC Days in Split: Heavy Ion Theory

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**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**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**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**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 …**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**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**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**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**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.**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?**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.**Hard Probes: Jet fragmentation vs. Jet**thermalization**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**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**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 =>**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.**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**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?