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Colliding Shock Waves in AdS 5. Yuri Kovchegov The Ohio State University Based on the work done with Javier Albacete and Anastasios Taliotis, arXiv:0805.2927 [hep-th], arXiv:0902.3046 [hep-th], arXiv:0705.1234 [hep-ph]. Outline.
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Colliding Shock Waves in AdS5 Yuri Kovchegov The Ohio State University Based on the work done with Javier Albacete and Anastasios Taliotis, arXiv:0805.2927 [hep-th], arXiv:0902.3046 [hep-th], arXiv:0705.1234 [hep-ph]
Outline • Problem of isotropization/thermalization in heavy ion collisions • AdS/CFT techniques • Bjorken hydrodynamics in AdS • Colliding shock waves in AdS: • Collisions at large coupling: complete nuclear stopping • Mimicking small-coupling effects: unphysical shock waves • Proton-nucleus collisions
Notations proper time rapidity QGP CGC valid up to times t ~ 1/QS The matter distribution due to classical gluon fields is rapidity-independent.
Most General Rapidity-Independent Energy-Momentum Tensor The most general rapidity-independent energy-momentum tensor for a high energy collision of two very large nuclei is (at x3 =0) which, due to gives
Color Glass at Very Early Times (Lappi ’06 Fukushima ‘07) In CGC at very early times we get, at the leading log level, such that, since Energy-momentum tensor is
Color Glass at Later Times: “Free Streaming” At late times classical CGC gives free streaming, which is characterized by the following energy-momentum tensor: such that and • The total energy E~ e t is conserved, as expected for • non-interacting particles.
Classical Fields from numerical simulations by Krasnitz, Nara, Venugopalan ‘01 • CGC classical gluon field leads to energy density scaling as
Much later Times: Bjorken Hydrodynamics In the case of ideal hydrodynamics, the energy-momentum tensor is symmetric in all three spatial directions (isotropization): such that Using the ideal gas equation of state, , yields Bjorken, ‘83 • The total energy E~ e t is not conserved
If then, as , one gets . Rapidity-Independent Energy-Momentum Tensor Deviations from the scaling of energy density, like are due to longitudinal pressure , which does work in the longitudinal direction modifying the energy density scaling with tau. • Positive longitudinal pressure and isotropization ↔ deviations from
The Problem • Can one show in an analytic calculation that the energy-momentum tensor of the medium produced in heavy ion collisions is isotropic over a parametrically long time? • That is, can one start from a collision of two nuclei and obtain hydrodynamics? • Let us proceed assuming that strong-coupling dynamics from AdS/CFT would help accomplish this goal.
AdS/CFT Approach z=0 Our 4d world 5d (super) gravity lives here in the AdS space 5th dimension AdS5 space – a 5-dim space with a cosmological constant L= -6/L2. (L is the radius of the AdS space.) z
AdS/CFT Correspondence (Gauge-Gravity Duality) Large-Nc, large l=g2 Nc N=4 SYM theory in our 4 space-time dimensions Weakly coupled supergravity in 5d anti-de Sitter space! • Can solve Einstein equations of supergravity in 5d to learn about energy-momentum tensor in our 4d world in the limit of strong coupling! • Can calculate Wilson loops by extremizing string configurations. • Can calculate e.v.’s of operators, correlators, etc.
Holographic renormalization de Haro, Skenderis, Solodukhin ‘00 • Energy-momentum tensor is dual to the metric in AdS. Using Fefferman-Graham coordinates one can write the metric as with z the 5th dimension variable and the 4d metric. • Expand near the boundary of the AdS space: • For Minkowski world and with
Single Nucleus in AdS/CFT An ultrarelativistic nucleus is a shock wave in 4d with the energy-momentum tensor
Shock wave in AdS Need the metric dual to a shock wave and solving Einstein equations The metric of a shock wave in AdS corresponding to the ultrarelativistic nucleus in 4d is (note that T_ _ can be any function of x^-): Janik, Peschanksi ‘05
Diagrammatic interpretation The metric of a shock wave in AdS corresponding to the ultrarelativistic nucleus in 4d can be represented as a graviton exchange between the boundary of the AdS space and the bulk: cf. classical Yang-Mills field of a single ultrarelativistic nucleus in CGC in covariant gauge (McLerran-Venugopalan model): the gluon field is given by 1-gluon exchange (Jalilian-Marian, Kovner, McLerran, Weigert ’96, Yu.K. ’96)
Asymptotic geometry • Janik and Peschanski ’05 showed that in the rapidity-independent case the geometry of AdS space at late proper times t is given by the following metricwith e0 a constant. • In 4d gauge theory this gives Bjorken hydrodynamics: with
Bjorken hydrodynamics in AdS • Looks like a proof of thermalization at large coupling. • It almost is: however, one needs to first understand what initial conditions lead to this Bjorken hydrodynamics. • Is it a weakly- or strongly-coupled heavy ion collision which leads to such asymptotics? If yes, is the initial energy-momentum tensor similar to that in CGC? Or does one need some pre-cooked isotropic initial conditions to obtain Janik and Peschanski’s late-time asymptotics?
Colliding shock waves in AdS Considered by Nastase; Shuryak, Sin, Zahed; Kajantie, Louko, Tahkokkalio; Grumiller, Romatschke; Gubser, Pufu, Yarom. I will follow J. Albacete, A. Taliotis, Yu.K. arXiv:0805.2927 [hep-th], arXiv:0902.3046 [hep-th]
McLerran-Venugopalan model in AdS • Imagine a collision of two shock waves in AdS: • We know the metric of bothshock waves, and know thatnothing happens before the collision. • Need to find a metric in theforward light cone! (cf. classical fields in CGC) ? empty AdS5 1-graviton part higher order graviton exchanges
Heavy ion collisions in AdS empty AdS5 1-graviton part higher order graviton exchanges
Expansion Parameter • Depends on the exact form of the energy-momentum tensor of the colliding shock waves. • For the parameter in 4d is m t3 :the expansion is good for early times t only. • For that we will also considerthe expansion parameter in 4d is L2t2. Also valid for early times only. • In the bulk the expansion is valid at small-z by the same token.
What to expect • There is one important constraint of non-negativity of energy density. It can be derived by requiring thatfor any time-like tm. • This gives (in rapidity-independent case)along with Janik, Peschanksi ‘05
Physical shock waves Simple dimensional analysis: The same result comes out of detailed calculations. Grumiller, Romatschke ‘08 Albacete, Taliotis, Yu.K. ‘08 Each graviton gives , hence get no rapidity dependence:
Physical shock waves: problem 1 • Energy density at mid-rapidity grows with time!? This violates condition. This means in some frames energy density at some rapidity is negative! • I do not know of a good explanation: it may be due to some Casimir-like forces between the receding nuclei. (see e.g. work by Kajantie, Tahkokkalio, Louko ‘08)
Physical shock waves: problem 2 • Delta-functions are unwieldy. We will smear the shock wave:with and . (L is the typical transverse momentum scale in the shock.) • Look at the energy-momentum tensor of a nucleus after collision: • Looks like by the light-cone timethe nucleus will run out of momentum and stop!
Physical shock waves • We conclude that describing the whole collision in the strong coupling framework leads to nuclei stopping shortly after the collision. • This would not lead to Bjorken hydrodynamics. It is very likely to lead to Landau-like hydrodynamics. • While Landau hydrodynamics is possible, it is Bjorken hydrodynamics which describes RHIC data rather well. Also baryon stopping data contradicts the conclusion of nuclear stopping at RHIC. • What do we do? We know that the initial stages of the collisions are weakly coupled (CGC)!
Unphysical shock waves • One can show that the conclusion about nuclear stopping holds for any energy-momentum tensor of the nuclei such that • To mimic weak coupling effects in the gravity dual we propose using unphysical shock waves with not positive-definite energy-momentum tensor:
Unphysical shock waves • Namely we take • This gives: • Almost like CGC at early times: • Energy density is now non-negative everywhere in the forward light cone! • The system may lead to Bjorken hydro. cf. Taliotis, Yu.K. ‘07
Will this lead to Bjorken hydro? • Not clear at this point. But if yes, the transition may look like this: (our work) Janik, Peschanski ‘05
Isotropization time • One can estimate this isotropization time from AdS/CFT (Yu.K, Taliotis ‘07) obtainingwhere e0 is the coefficient in Bjorken energy-scaling: • For central Au+Au collisions at RHIC at hydrodynamics requires e=15 GeV/fm3 at t=0.6 fm/c (Heinz, Kolb ‘03), giving e0=38 fm-8/3. This leads toin good agreement with hydrodynamics!
Landau vs Bjorken Bjorken hydro: describes RHIC data well. The picture of nuclei going through each other almost without stopping agrees with our perturbative/CGC understanding of collisions. Can we show that it happens in AA collisions using field theory? Landau hydro: results from strong coupling dynamics at all times in the collision. While possible, contradicts baryon stopping data at RHIC.
pA Setup • Consider pA collisions:
pA Setup • In terms of graviton exchanges need to resum diagrams like this: cf. gluon production in pA collisions in CGC!
Eikonal Approximation • Note that the nucleus is Lorentz-contracted. Hence all and are small.
Physical Shocks • Summing all these graphs forthe delta-function shock wavesyields the transverse pressure: • Note the applicability region:
Physical Shocks • The full energy-momentum tensor can be easily constructed too. In the forward light cone we get:
Physical Shocks: the Medium • Is this Bjorken hydro? Or a free-streaming medium? • Appears to be neither. At late timesNot a free streaming medium. • For ideal hydrodynamics expectsuch that: • However, we getNot hydrodynamics either.
Physical Shocks: the Medium • Most likely this is an artifact of the approximation, this is a “virtual” medium on its way to thermalization.
Proton Stopping • What about the proton? Dueto our earlier result about shock wave stopping we should be able to see how it stops. • And we do:T++ goes to zero as x+ grows large!
Proton Stopping • We get complete proton stopping (arbitrary units): T++ of the proton X+
More On Stopping: AA Case • Contour plot of transverse pressure for AA collisions. (Albacete,Yu.K., Taliotis, in preparation)
Conclusions • We have constructed graviton expansion for the collision of two shock waves in AdS, with the goal of obtaining energy-momentum tensor of the produced strongly-coupled matter in the gauge theory. • We have solved the pA scattering problem in AdS. • Real shock waves stop: Landau hydrodynamics. • Delta-prime shock waves don’t stop, but it is not clear what they lead to. Hopefully some form of ideal hydrodynamics. • Wherefore art thou Bjorken hydro?
Delta-prime shocks • For delta-prime shock waves the result is surprising. The all-order eikonal answer for pA is given by LO+NLO terms: • That is, graviton exchange series terminates at NLO. +
Delta-prime shocks • The answer for transverse pressure iswith the shock waves • As p goes negative at late times, this is clearly not hydrodynamics and not free streaming.