Diffractive photoproduction of vector mesons at the LHC

We confront saturation-based results for diffractive $\psi(2s)$ and $\rho$ production at HERA and $J/\psi$ photoproduction with all available data including recent ones from HERA, ALICE and LHCb, finding a good agreement. We show that the $t$-distribution of differential cross-section of photoproduction of vector mesons offers a unique opportunity to discriminate among saturation and non-saturation models. This is due to emergence of a pronounced dip (or multiple dips) in the $t$-distribution of diffractive photoproduction of vector mesons at relatively large, but potentially accessible $|t|$ that can be traced back to the unitarity features of colour dipole amplitude in the saturation regime. We provide various predictions for exclusive (photo)-production of different vector mesons including the ratio of $\psi(2s)/J/\psi$ at HERA, the LHC and at future colliders.Comment: 7 pages, 9 figures, to appear in proceedings of DIS 2014, XXII. International Workshop on Deep-Inelastic Scattering and Related Subjects, 28 April - 2 May 2014, Warsaw, Polan

An analysis of the influence of background subtraction and quenching on jet observables in heavy-ion collisions

Subtraction of the large background in reconstruction is a key ingredient in jet studies in high-energy heavy-ion collisions at RHIC and the LHC. Here we address the question to which extent the most commonly used subtraction techniques are able to eliminate the effects of the background on the most commonly discussed observables at present: single inclusive jet distributions, dijet asymmetry and azimuthal distributions. We consider two different background subtraction methods, an area-based one implemented through the FastJet pack- age and a pedestal subtraction method, that resemble the ones used by the experimental collaborations at the LHC. We also analyze different ways of defining the optimal parame- ters in the second method. We use a toy model that easily allows variations of the background characteristics: average background level and fluctuations and azimuthal structure, but cross- checks are also done with a Monte Carlo simulator. Furthermore, we consider the influence of quenching using Q-PYTHIA on the dijet observables with the different background subtrac- tion methods and, additionally, we examine the missing momentum of particles. The average background level and fluctuations affect both single inclusive spectra and dijet asymmetries, although differently for different subtraction setups. A large azimuthal modulation of the background has a visible effect on the azimuthal dijet distributions. Quenching, as imple- mented in Q-PYTHIA, substantially affects the dijet asymmetry but little the azimuthal dijet distributions. Besides, the missing momentum characteristics observed in the experiment are qualitatively reproduced by Q-PYTHIA.Comment: 29 pages, 43 figures Accepted by JHE

Electron-Ion Physics with the LHeC

The Large Hadron Electron Collider (LHeC) project is the proposal to use the existing LHC proton/ion beams and construct a new electron beam line to perform high-energy electron-proton/ion collisions. In this talk, we consider some of the physics topics that could be studied in the electron-ion mode. In particular, we estimate how much the current nuclear parton distribution fits could be improved with the deeply inelastic scattering measurements at the LHeC by including pseudodata into a global analysis. In addition, we discuss briefly other topics that would help to better understand some aspects of heavy-ion collisions, namely small-$x$ physics and hadron production with a nuclear target.Comment: Talk presented at DIS2015 conference, April 27 - May 1, 2015, Southern Methodist University, Dallas, Texas. v2: Updated reference lis

Background subtraction and jet quenching on jet reconstruction

In order to assess the ability of jet observables to constrain the characteristics of the medium produced in heavy-ion collisions at the LHC, we investigate the influence of background subtraction and jet quenching on jet reconstruction, with focus on the dijet asymmetry as currently studied by ATLAS and CMS. Using a toy model, we examine the influence of different background subtraction methods on dijet momentum imbalance and azimuthal distributions. We compare the usual jet-area based background subtraction technique and a variant of the noise-pedestal subtraction method used by CMS. The purpose of this work is to understand what are the differences between the two techniques, given the same event configuration. We analyze the influence of the quenching effect using the Q-PYTHIA Monte Carlo on the previous observables and to what extent Q-PYTHIA is able to reproduce the CMS data for the average missing transverse momentum that seems to indicate the presence of large angle emission of soft particles.Comment: 4 pages, 3 figures, Proceedings for Hard Probes 201

Energy loss and (de)coherence effects beyond eikonal approximation

The parton branching process is known to be modified in the presence of a medium. Colour decoherence processes are known to determine the process of energy loss when the density of the medium is large enough to break the correlations between partons emitted from the same parent. In order to improve existing calculations that consider eikonal trajectories for both the emitter and the hardest emitted parton, we provide in this work, the calculation of all finite energy corrections for the gluon radiation off a quark in a QCD medium that exist in the small angle approximation and for static scattering centres. Using the path integral formalism, all particles are allowed to undergo Brownian motion in the transverse plane and the offspring allowed to carry an arbitrary fraction of the initial energy. The result is a general expression that contains both coherence and decoherence regimes that are controlled by the density of the medium and by the amount of broadening that each parton acquires independently.Comment: 4 pages, to appear in the proceedings of the Quark Matter 2014 conferenc

Diffractive Dijet Production in Deep Inelastic Scattering and Photon-Hadron Collisions in the Color Glass Condensate

We study exclusive dijet production in coherent diffractive processes in deep inelastic scattering and real (and virtual) photon-hadron ($\gamma^{(*)}$-h) collisions in the Color Glass Condensate formalism at leading order. We show that the diffractive dijet cross section is sensitive to the color-dipole orientation in the transverse plane, and is a good probe of possible correlations between the $q\bar{q}$-dipole transverse separation vector \r and the dipole impact parameter \b. We also investigate the diffractive dijet azimuthal angle correlations and $t$-distributions in $\gamma^{(*)}$-h collisions and show that they are sensitive to gluon saturation effects in the small-$x$ region. In particular, we show that the $t$-distribution of diffractive dijet photo-production off a proton target exhibits a dip-type structure in the saturation region. This effect is similar to diffractive vector meson production. Besides, at variance with the inclusive case, the effect of saturation leads to stronger azimuthal correlations between the jets.Comment: 15 pages, 10 figures; v2: a clarifying Appendix added, 3 new plots added, references added. The version to appear in PL

Monte Carlo for Jet Showers in the Medium

The most commonly employed formalisms of radiative energy loss have been derived in the high- energy approximation. In its present form, it is reliable only for the medium modifications of inclusive particle spectra. Modifications to this formalism are expected to be important for less inclusive measurements. This is especially relevant for reconstructed jets in heavy-ion collisions, which are becoming available only recently. We present some ideas to overcome this limitation. Specifically, we show an implementation of radiative energy loss within a jet parton shower. This implementation has been done within the PYTHIA Monte Carlo event generator. We present the publicly available routine Q-PYTHIA and discuss some of the obtained physics results.Comment: 4 pages, 1 figures - To appear in the conference proceedings for Quark Matter 2009, March 30 - April 4, Knoxville, Tennessee. Final version with minor typos correcte

In-medium jet evolution: interplay between broadening and decoherence effects

The description of the modifications of the coherence pattern in a parton shower, in the presence of a QGP, has been actively addressed in recent studies. Among the several achievements, finite energy corrections, transverse momentum broadening due to medium interactions and interference effects between successive emissions have been extensively improved as they seem to be essential features for a correct description of the results obtained in heavy-ion collisions. In this work, based on the insights of our previous work [1], we explore the physical interplay between broadening and decoherence, by generalising previous studies of medium-modifications of the antenna spectrum [2, 3, 4] - so far restricted to the case where transverse motion is neglected. The result allow us to identify two quantities controlling the decoherence of a medium modified shower that can be used as building blocks for a successful future generation of jet quenching Monte Carlo simulators: a generalisation of the $\Delta_{med}$ parameter of the works of [2, 4] - that controls the interplay between the transverse scale of the hard probe and the transverse resolution of the medium - and of the $\Delta_{coh}$ in [1] - that dictates the interferences between two emitters as a function of the transverse momentum broadening acquired by multiple scatterings with the medium.Comment: Proceedings for Quark Matter 2015 (corrected version