Jets as Probes of Strongly Coupled Quark-Gluon Plasma

Abstract

In this thesis we have studied how high energetic excitations propagate through a non- abelian strongly coupled plasma. This new state of matter is produced at heavy ion collisions in our accelerators and allows us to study a stage of the evolution of our Universe that occurred during the first microseconds after the Big Bang. In this extreme conditions of temperature and density the ordinary matter that we are made of behaves as a an almost perfect fluid, the most perfect known by mankind up to now in fact. The theory of strong interactions is tested at an energy scale that even though it is high enough to melt hadrons, it does not get to the point where the coupling constant is low enough to allow a perturbative description. In the plasma, the partonic field content, the quarks and gluons, cease to be the relevant degrees of freedom and a microscopic description in terms of quasi-particles is not possible. A very useful tool to put to test the actual behaviour of this strongly coupled fluid is the analysis of jet modifications as a result of their interactions with the plasma.

In a first introductory part we have given the concepts needed to picture how heavy ion collisions develop as we are able to understand it today. At weak coupling, the main mechanism responsible for energy loss is induced gluon emission and interesting interference phenomena occur that lead to a dependence on path length of as the squared distance. These are known as coherence effects and their study becomes richer by considering multi-gluon emission, as it is done done in Part III. The strongly coupled picture uses holography to map a dressed excitation moving through a strongly coupled plasma into a string propagating in a higher dimensional space containing a black hole. Since the non-abelian theory in which the calculation is done is not QCD, but N = 4 SYM, we take these results as an insight to describe energetic parton propagation in a model of jet quenching in heavy ion collisions.

Even though we assume that the exchanges with the medium are soft enough to include non-perturbative effects, as described by gauge/gravity duality, the energetic partons that are produced in the collision generally have a high virtuality which they relax by successive splittings. The latter occur at length scales that are not resolvable by the medium, and they should proceed as in vacuum. This observation motivates us to adopt a hybrid description for the interplay between the multi scale jet and the QGP, using each description at the scale it is supposed to be valid. This phenomenological description has proven to be very successful in describing dijet and photon-jet data at different centralities, and predictions have been made for a wide range of observables for the coming data from run 2 of LHC, including a new observable, the ratio of the fragmentation functions of the leading and subleading jet in a dijet pair, which is highly sensitive to the specific energy loss mechanism.

In the next part of the work we extend our hybrid model by the inclusion of two effects, broadening and medium response, which should help us better describe intra-jet observables. The

first effect, broadening, is due to the Brownian motion that probes experience in a thermal bath, and it will tend to broaden the distribution of particles within the jet. As it turns out, the observable quantifying such modifications, the jet shapes, are rather insensitive to the inclusion of this effect. However, by restricting the range of the tracks entering this analysis,we have been able to produce a new observable which shows a remarkable dependence on the precise strength of the broadening mechanism.

The second effect involves overall energy-momentum conservation. The rapidly thermalized energy deposited by the energetic partons modifies the plasma, inducing temperature and velocity fluctuations in the surrounding fluid cells. This perturbation propagates long distances in the form of a wake and eventually decays into soft hadrons, whose orientations keep a correlation with the jet direction and therefore produce a net effect even after background subtraction. The observable consequences are best noticed in intra-jet measurements such as jet shapes and fragmentation functions, where it is clearly seen that the inclusion of such physics is in good agreement with the observed experimental trend, and it becomes simply unavoidable when comparisons against global measurements are performed.

Finally, we compute the inclusive two gluon stimulated emission within the context of perturbative QCD. By studying the full answer in different kinematical limits we arrive to the conclusion that jet propagation is perceived from the point of view of the plasma as a set of effective emitters depending on the resolution power, which for a thin plasma it is of the order of the Debye screening mass. This physics is a missing piece of the Monte Carlo jet quenching model presented in this thesis and its inclusion is expected to have important consequences for the more differential observables, a task that will be undertaken in future work.

These are very exciting times for the physics of strong nuclear interactions. We have seen how the very fundamental questions about the nature of the high temperature, strongly coupled phase of ordinary matter can be addressed by the study of jet quenching and its observable consequences. This thesis represents an effort in the confrontation of the seductive ideas of holography with experiments. Having the means to quantitatively confront new ideas, as we have done throughout the presented work, new observables, and new data is critical if we are eventually to understand the properties of the strongly coupled liquid quark-gluon plasma that Nature has served us.

En aquesta tesi estudiem com excitacions d’alta energia es propaguen a través d’un plasma no abelià fortament acoblat. Aquest nou estat de la matèria es produeix a les col·lisions de ions pesats als nostres acceleradors i permet l’estudi d’una etapa de l’evolució del nostre Univers que va tenir lloc durant els primers microsegons després del Big Bang. En aquestes condicions extremes de temperatura i densitat, la matèria ordinària de la qual estem fets es comporta com un fluid perfecte, de fet el més perfecte conegut per la humanitat a dia d’avui. La teoria de les interaccions fortes és posada a prova a una escala d’energia que tot i ser prou alta per fondre els hadrons, no arriba al punt en què la constant d’acoblament és prou feble com per permetre una descripció pertorbativa. Al plasma, el contingut en camps partonics, els quarks i els gluons, deixen de ser els graus de llibertat rellevants i una descripciò microscòpica en termes de quasi-partícules no és possible. Una eina molt útil per posar a prova l’autèntic comportament d’aquest fluid fortament acoblat és l’anàlisi de les modificacions dels jets com a resultat de la seva interacció amb el plasma.

Tot i que assumim que els intercanvis amb el medi són prou suaus com per incloure efectes no pertorbatius, tal com descriu la dualitat gauge/gravetat, els partons energètics que es produeixen a la col·lisió generalment tenen virtualitats altes, les quals relaxen a través de particions successives. Aquestes últimes ocorren a escales de longitut que no són resolubles pel medi, i per tant haurien de procedir com en el buit. Aquesta observació ens motiva a adoptar una descripció híbrida per a la interacció entre el jet i el QGP, fent servir cada descripció a l’escala on es suposa que es vàlida. Aquesta descripció fenomenològica ha demostrat ser molt exitosa en la descripció de les dades de dijets i fotó-jet per diferents centralitats, i hem produït prediccions per a un ample ventall d'observables a l'energia del run 2 del LHC, incloent un nou observable molt sensible al mecanisme específic de pèrdua d'energia. També incorporem els efectes d'eixamplament i resposta del medi per descriure els observables intra-jet tot obtenint importants millores qualitatives en la descripció de les correlacions a llarg abast entre el jets i el plasma.

Finalment, calculem l’emissió estimulada de dos gluons en el context de QCD pertorbativa. Estudiant la resposta completa en diferents límits arribem a la conclusió que des del punt de vista del plasma la propagació del jet és percebuda com una col·lecció d’emissors efectius depenenent del poder de resolució, el qual per un plasma prim és de l’ordre de l’invers de la massa d’apantallament de Debye. La seva inclusió en el model híbrid s’espera que tingui conseqüències importants pels observables més diferencials, una tasca que s’emprendrà en un treball futur.