Medium modifications to the initial-final coherence pattern

We have recently released a new article where we investigated medium modifications to the color coherence pattern between the initial and final state radiation. All the details of our new work can be found in the link listed below

http://arxiv.org/abs/arXiv:1308.2186

The experimental results of high pT hadrons produced in high energy nuclear collisions at RHIC and LHC requires a correct understanding of the medium modifications to the color coherence among different emitters in a parton branching. These problems have been addressed in a series of papers by studying the antenna radiation. It is natural to study another different configuration relevant from an antenna.

In our new work, we determine analytically the different contributions to the soft gluon spectrum due to a highly energetic parton created in the remote past. This parton suffers a hard scattering and afterwards it crosses a QCD medium of finite size. Within controlled approximations, we show that the presence of a QCD medium indeed modifies the inter- ference pattern among both emitters compared with the vacuum case. We observe that depending on the interplay of the relevant kinematic variables of our problem there is a partial suppression of the interferences either in the coherent or incoherent limit. As a con- sequence, we would observe that both emitters can radiate gluons at large angles and there is a maximal angle completely determined by the medium properties. Our setup might have phenomenological consequences in the dense-dilute regime, e.g., pA collisions. 

Bayesian reweighting of nuclear parton densities

We have released a work to the arxiv where we study the impact of data coming from the proton-lead run at the LHC. The full article is available at

http://arxiv.org/abs/1309.5371

Understanding the behaviour of partons inside a nucleus is crucial for the correct interpretation of experimental results in high energy collisions when at least one of the initial particles colliding is a nucleus. The LHC proton-lead run posses a high sensitivity to formerly unexplored kinematical regions and therefore will play a fundamental role in the determination of the nuclear parton densities, in particular for the gluon.

Obtaining the information from the data should be done in principle through the global analysis of all (new and old) published data. Unfortunately such a procedure is quite time consuming and expediency-lacking.

In our work we bypass this obstacle by means of a reweighting technique (Bayesian reweighting) and analyze the potential constraining power of the LHC proton-lead run data. The most relevant result is a public code that allows any user to appraise the impact on the EPS09 nuclear parton densities of any new data set. The code can be downloaded from:

http://igfae.usc.es/hotlhc/index.php/software

By using this code we estimate a reduction of the nuclear gluon uncertainty up to a factor of two using some proton-nucleus observables as examples of the potentialities of these data

We have released a paper to the arxiv in which a new picture of jet quenching dictated by color coherent parton branching is presented. The paper can be find at

http://arxiv.org/abs/arXiv:1210.7765

The description of the parton shower of jets in the presence of a medium is one of the main goals of the HotLHC project. Following the philosophy of the vacuum studies, we are performing analytical calculations in limiting cases where controlled calculations are possible to acquire a better and better understanding of the whole problem. One of the essential ingredients is color coherence. The effect of color coherence among different emitters in the medium have been studied in a series of papers in the setup known as the antenna radiation. Based on this knowledge, we propose a new picture in which the jet is composed by a reduced number of effective emitters. Each of this effective emitter is an extended object, with typical size determined by the medium color correlation length, inside which regular vacuum branching can appear. 

Two main consequences of the appearance of color coherence are a reduced energy loss, due to a reduced number of effective emitters, and an unmodified fragmentation functions for each emitter. 

This new picture provides a consistent understanding of the present data on reconstructed jet observables and constitute the basis for future developments. 

In order to check the relevance of this effect to the actual LHC kinematics we have made a proof-of-principle study reclustering the reconstructed jets from a Monte Carlo event generator with the typical medium characteristic angular scale. We find that at least one-half of the jets measured at the LHC may be unresolved by the medium, hence composed of only one effective emitter.