Holography and the problem of parton energy loss in a quark-gluon plasma Daniel Pablos SEWM ‘18 Barcelona 25th May 2018
Absence of quasiparticles? Most satisfactory description of QGP involves an almost ideal liquid phase pp pPb PbPb Weller & Romatschke - PLB ‘17 studies of QGP formation in small systems suggest common hydrodynamic origin for flow effects! 2 Daniel Pablos McGill / JETSCAPE
Absence of quasiparticles? pp pPb PbPb η s ∼ 0 . 08 Small value of shear viscosity over entropy density ratio challenges quasiparticle description T ∼ 1 1 Bernhard et al. - PRC ‘16 τ qp ∼ 5 η York & Moore - PRD ‘08 s T Predicted by Policastro, Son and Starinets (2001) s = 1 η for a large class of non-abelian gauge theories 4 π at strong coupling which have a gravity dual 3 Daniel Pablos McGill / JETSCAPE
Absence of quasiparticles? pp pPb PbPb Hydrodynamics at work with large gradients at very early times Natural situation at strong coupling Chesler - PRL ‘15, JHEP ‘16 R ∼ 1 Even for system sizes of order hydrodynamic expansion is well behaved T Appealing picture of hydrodynamization for all system sizes within strong coupling 4 Daniel Pablos McGill / JETSCAPE
Absence of quasiparticles? would like to have description of jet/QGP interaction in harmony with sQGP hypothesis traditional pQCD computations need to assume separation of scales E λ D ⌧ λ m.f.p. g ⌧ 1 for ex: q neglect correlations among scatterers T But the typical virtuality exchanged between the jet and the medium √ q ∼ ET g ∼ 2 β 1 loop ( q ) There is motivation to explore an alternative, non-perturbative, description of jet quenching that does not assume the presence of quasiparticles 5 Daniel Pablos McGill / JETSCAPE
Holography: a non-perturbative tool quarks are dual to open strings attached to probe flavour branes having a plasma in the gauge theory represents a black hole in the bulk bulk metric perturbations encode boundary stress energy variations J Friess, et al., PRD75 (2007) ! N = 4 SYM and QCD have very different vacuums but ? N = 4 T > T c T 6 = 0 and QCD share similarities 6 Daniel Pablos McGill / JETSCAPE
Proxies for jets as falling strings Chesler et al. - PRD ‘09 boundary dressed quarks are open strings attached to a D7 flavour brane charged under U(1) gauge field sourcing baryon current at boundary depth of string endpoint determines horizon localisation of excitation at boundary presence of string perturbs metric satisfies linearised Einstein’s equations string sourced near boundary expression of energy-momentum tensor Chesler & Rajagopal, JHEP ‘16 hydro (long wavelength) non-hydro (jet modes) 7 Daniel Pablos McGill / JETSCAPE
Null falling string approximation Chesler & Rajagopal - PRD ‘15, JHEP ‘16 σ ∗ endpoint angle string null geodesics profile Nambu-Goto action Schwarzschild-AdS Solve E.O.M. by finding null geodesic profile: x geo ( t ) , u geo ( t ) 1. 2. 0 ( σ ) (peaks at the endpoint) Π τ Find energy carried by each geodesic: 3. Construct the string energy-momentum tensor: 8 Daniel Pablos McGill / JETSCAPE
Null falling strings Unambiguous determination of boundary jet properties: Pieces of string falling below horizon energy-momentum into hydro modes with rate: Chesler & Rajagopal - PRD ‘15, JHEP ‘16 E 1 / 3 x therm = 1 in as the jet loses energy … it also gets wider T 4 / 3 2 κ Also, quite interestingly: Fractional energy loss only depends on initial jet opening angle most energy at endpoint: Bragg-like peak 9 Daniel Pablos McGill / JETSCAPE
Hybrid strong/weak coupling approach Pablos et al. - JHEP ‘14, ‘16, ‘17 High energy jet starts with a high virtuality, much greater than medium scale Parton shower well approximated by vacuum-like splittings (late stages?) Plasma-jet interaction dominated by temperature scale Use non-perturbative holographic prescription for partonic energy loss Energy flowing into hydro modes: O (1) free parameter Estimate the hadronic spectra coming from medium response (assume small perturbation, instantaneous hydrodynamization) Lost jet energy converted into soft particles at large angles (corr. bkgd.) 10 Daniel Pablos McGill / JETSCAPE
Jet induced medium excitations string acts as a perturbation in the large Nc limit agreement between hydrodynamics & wake of a quark in gauge/gravity duality energy-momentum conservation in the jet+plasma interplay Chesler & Yaffe - PRD ‘08 wake hadron distribution estimate small perturbation on top of hydro only valid for soft hadrons (within hybrid model) no extra free parameter } 11 Daniel Pablos McGill / JETSCAPE
Phenomenology at LHC 12 Daniel Pablos McGill / JETSCAPE
Constraining the free parameter Use hadron and jet suppression data for central collisions at LHC to fit the free parameter Hadron suppression # inelastic collisions per heavy ion collision estimated through the Glauber model JHEP 1704 (2017) 039 Perform a global fit and extract the Jet suppression best value of κ SC ATLAS-CONF-2017-009 13 Daniel Pablos McGill / JETSCAPE
Fit results 0 . 8 2 σ Consistent, but 1 σ 0 . 7 some tension between * Global Fit hadrons & jets 0 . 6 preferred value κ 0 . 5 0 . 4 0 . 3 CMS Had 5.02 CMS Had 2.76 RHIC Had 0.20 ATLAS Had 5.02 ATLAS Had 2.76 CMS Jets R=0.2 2.76 CMS Jets R=0.3 2.76 CMS Jets R=0.4 2.76 ATLAS Jets R=0.4 5.02 ATLAS Jets R=0.4 2.76 In preparation * with LHC data only Jets 0-10% Hadrons 0-5% 14 Daniel Pablos McGill / JETSCAPE
Fit results 0 . 8 2 σ Consistent, but 1 σ 0 . 7 some tension between * Global Fit hadrons & jets 0 . 6 preferred value κ 0 . 5 Finite resolution effects 0 . 4 (a.k.a. coherence in pQCD) 0 . 3 affect hadron & jet relative suppression CMS Had 5.02 CMS Had 2.76 RHIC Had 0.20 ATLAS Had 5.02 ATLAS Had 2.76 CMS Jets R=0.2 2.76 CMS Jets R=0.3 2.76 CMS Jets R=0.4 2.76 ATLAS Jets R=0.4 5.02 ATLAS Jets R=0.4 2.76 (see back-up) In preparation * with LHC data only Jets 0-10% Hadrons 0-5% 15 Daniel Pablos McGill / JETSCAPE
Hadron and Jet suppression Hadrons 1 . 4 Jets R = 0 . 4 1 . 2 Which is the observable that relates hadrons and jets? 1 0 . 8 R AA 0 . 6 0 . 4 0 . 2 0 10 100 1000 Hadron or Jet p T [GeV] In preparation 16 Daniel Pablos McGill / JETSCAPE
Connection between hadrons and jets Hadrons 1 . 4 ATLAS Jets R = 0 . 4 1 . 2 jet fragmentation 1 High z functions (FFs) enhancement 0 . 8 R AA 0 . 6 ATLAS-CONF-2017-005 0 . 4 Count the average number of hadrons, 0 . 2 per jet, with energy fraction z 0 10 100 1000 Hadron or Jet p T [GeV] In preparation 17 Daniel Pablos McGill / JETSCAPE
Connection between hadrons and jets Hadrons 1 . 8 1 . 4 Actual jet FFs HYBRID Jets R = 0 . 4 1 . 6 MODEL High z Jets ⊗ FF actual 1 . 4 1 . 2 Jet FFs ratio enhancement 1 . 2 1 1 0 . 8 PbPb jet FFs 0 . 6 0 . 8 R AA 0 . 5 1 1 . 5 2 2 . 5 3 3 . 5 4 ln(1/z) 0 . 6 0 . 4 Count the average number of hadrons, 0 . 2 per jet, with energy fraction z 0 10 100 1000 Hadron or Jet p T [GeV] In preparation 18 Daniel Pablos McGill / JETSCAPE
Connection between hadrons and jets Hadrons 1 . 8 1 . 4 Actual jet FFs Vacuum jet FFs Jets R = 0 . 4 1 . 6 Jets ⊗ FF actual 1 . 4 1 . 2 Jet FFs ratio Jets ⊗ FF vacuum 1 . 2 Flat 1 FFs ratio 1 0 . 8 PbPb jet FFs 0 . 6 0 . 8 R AA 0 . 5 1 1 . 5 2 2 . 5 3 3 . 5 4 ln(1/z) Vacuum jet FFs 0 . 6 0 . 4 Jet substructure is important 0 . 2 for jet quenching phenomenology 0 10 100 1000 Hadron or Jet p T [GeV] In preparation 19 Daniel Pablos McGill / JETSCAPE
Jet narrowing Wider, more active jets lose more energy than narrower, hard fragmenting ones bias inclusive jet sample to narrower ones, Steeply falling jet spectrum explains high z enhancement ∆ E narrow < ∆ E wide High p T hadrons belong to such subsample of narrow jets, which get less quenched, R jet R R had R and so AA > AA Effect seen in the literature, for different models, on different observables - see for instance: Brewer et al. - JHEP ‘18 Milhano & Zapp - EPJ ‘16 Pablos et al. - JHEP ‘17 20 Daniel Pablos McGill / JETSCAPE
R AA vs R Pablos et al. - JHEP ‘16 1 . 2 R=0.1 Wider jets lose more energy R=0.2 1 R=0.3 R=0.4 R jet decreases with increasing jet radius R=0.5 AA 0 . 8 R=0.6 WITHOUT MEDIUM RESPONSE R=0.7 R=0.8 R AA 0 . 6 0 . 4 0 . 2 0 10 100 Jet p T [GeV] 21 Daniel Pablos McGill / JETSCAPE
R AA vs R Pablos et al. - JHEP ‘16 1 . 2 R=0.1 Opening the jet radius allows R=0.2 1 R=0.3 the recovery of some of the lost energy R=0.4 R=0.5 Hint of ordering reversal around R ∼ 0 . 7 0 . 8 R=0.6 R=0.7 WITH MEDIUM RESPONSE R=0.8 R AA 0 . 6 0 . 4 Upcoming precise data from CMS 0 . 2 Stay tuned! 0 10 100 Jet p T [GeV] Characteristic behaviour of strong coupling: efficient energy transfer into hydro modes 22 Daniel Pablos McGill / JETSCAPE
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