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Apolinario L.,LIP
Few-Body Systems | Year: 2017

The description of the in-medium modifications of partonic showers has been at the forefront of current theoretical and experimental efforts in heavy-ion collisions. It provides a unique laboratory to extend our knowledge frontier of the theory of the strong interactions, and to assess the properties of the hot and dense medium (QGP) that is produced in ultra-relativistic heavy-ion collisions at RHIC and the LHC. The theory of jet quenching, a commonly used alias for the modifications of the parton branching resulting from the interactions with the QGP, has been significantly developed over the last years. Within a weak coupling approach, several elementary processes that build up the parton shower evolution, such as single gluon emissions, interference effects between successive emissions and corrections to radiative energy loss of massive quarks, have been addressed both at eikonal accuracy and beyond by taking into account the Brownian motion that high-energy particles experience when traversing a hot and dense medium. In this work, by using the setup of single gluon emission from a color correlated quark–antiquark pair in a singlet state (qq¯ antenna), we calculate the in-medium gluon radiation spectrum beyond the eikonal approximation. The results show that we are able to factorize broadening effects from the modifications of the radiation process itself. This constitutes the final proof that a probabilistic picture of the parton shower evolution holds even in the presence of a QGP. © 2017, Springer-Verlag Wien.

The characteristics of extensive air showers are sensitive to the details of hadronic interactions at energies and in kinematic regions beyond those tested by human-made accelerators. Uncertainties on extrapolations of the hadronic interaction models in these regions hamper the interpretation of the ultra high energy cosmic ray data in terms of primary mass composition. We report on how the Pierre Auger Observatory is able to constrain the hadronic interaction models by measuring the muon content and muon production depth of air showers and also by measuring the proton-air cross section for particle production at a center-of-mass energy per nucleon of 57 TeV. © 2015 Published by Elsevier B.V.

Faccioli P.,LIP | Lourenco C.,CERN | Seixas J.,LIP | Seixas J.,University of Lisbon
Physical Review Letters | Year: 2010

The covariance properties of angular momentum eigenstates imply the existence of a rotation-invariant relation among the parameters of the difermion decay distribution of inclusively observed vector mesons. This relation is a generalization of the Lam-Tung identity, a result specific to Drell-Yan production in perturbative QCD, here shown to be equivalent to the dynamical condition that the dilepton is always produced transversely polarized with respect to quantization axes belonging to the production plane. © 2010 The American Physical Society.

The accumulation of 19 fb-1 of data at 8 TeV has been a unique window for Supersymmetry searches at the LHC, allowing the CMS collaboration to search for specific supersymmetric particles. This article covers the search for supersymmetric particles in a variety of production-and decay-modes as well as channels. It focuses on the search for the lightest scalar top (stop) in the single lepton channel, stop which might be the only observable in the case where Supersymmetry is realized in nature, and which can play a unique role in "naturalness" © Owned by the authors, published by EDP Sciences, 2014.

Santos H.,LIP
Nuclear and Particle Physics Proceedings | Year: 2016

The phenomenon of events containing highly asymmetric dijet pairs is one of the most striking results in heavy ion physics. It has provided the first direct observation of in-medium jet energy loss at the LHC. New results showing the variation of the dijet asymmetry with the angle between the leading jet and the second order Event Plane are presented. This observable effectively probes the path-length dependence of the dijet asymmetry at fixed centrality and can provide a better understanding of the correlation of the parton energy-loss with the underlying geometry. Correlated production of nearby jets is also shown. Two neighbouring jets originating from the same hard scattering should have more similar path lengths in the medium compared to the two jets in the dijet event topology, therefore measuring neighbouring jets may probe differences in quenching that do not result from different path length. The production of nearby jets is quantified using the rate, RδR, of "neighbouring" jets that accompany "test" jets. The centrality dependence of RδR as well as the ratios of the RδR values in collisions of different centrality and the values in the 40-80% centrality range are presented. © 2016 Elsevier B.V.

Nunesa A.S.,LIP
Nuclear and Particle Physics Proceedings | Year: 2015

During its 2011 data taking campaign, the COMPASS experiment at CERN has collected data of 200 GeV polarised muons scattering off a target with polarised protons. These data allow the extension of the phase-space coverage of the COMPASS data obtained in 2007 with a beam of 160 GeV polarised muons, namely to access lower values of x for any given value of Q2. The data in the DIS region were used to extract the longitudinal double spin asymmetry Ap 1 and the spin dependent structure function gp 1. Thereafter, our world data NLO QCD fits of polarised parton distributions were updated. It was also possible to improve our test on the Bjorken sum rule. The asymmetries Ap 1 and the structure function gp 1 were likewise extracted from the two beam energy data sets for Q2 < 1 (GeV/c)2, thus complementing our extraction of similar quantities in the non-perturbative region for the deuteron. © 2015 Elsevier B.V.

The Fluorescence Detector of the Pierre Auger Observatory measures the atmospheric depth, X max, where the longitudinal profile of the high energy air showers reaches its maximum. This is sensitive to the nuclear mass composition of the cosmic rays. Due to its hybrid design, the Pierre Auger Observatory also provides independent experimental observables obtained from the Surface Detector for the study of the nuclear mass composition. We present X max-distributions and an update of the average and RMS values in different energy bins and compare them to the predictions for different nuclear masses of the primary particles and hadronic interaction models. We also present the results of the composition-sensitive parameters derived from the ground level component.

Wohri H.K.,LIP
Nuclear Physics A | Year: 2011

We present a selection of the first physics results from the CMS experiment in proton-proton collisions, based on data collected at various beam energies during December 2009 and 2010. The measurements include particle production in the soft-QCD regime as well as high-pT spectra and correlations. Inclusive jet production and b-jet production have been studied down to pT below 20 GeV/c, exploiting a novel technique based on the full reconstruction of the event ("particle flow"). Studies of heavy-flavour (open charm and beauty) production have profited from the excellent vertexing performance of the detector, which enables a precise reconstruction of the associated secondary vertices. This capability has also been used to measure the production of J/Ψ mesons promptly produced, independently from those coming from the decay of b hadrons. The production cross sections of the J/Ψ and Υ states, in pp collisions at 7 TeV, have been measured in several rapidity intervals and down to very low pT. © 2011 Elsevier B.V.

Franco C.,LIP
Nuclear and Particle Physics Proceedings | Year: 2016

The COMPASS experiment [COMPASS, P. Abbon et al., The COMPASS experiment at CERN, Nucl. Inst. Meth. A577, 455 (2007)] at CERN is one of the leading experiments studying the nucleon spin. These studies are being carried on since 2002, by measuring hadrons produced in deep inelastic scattering (DIS) of 160 GeV/c and 200 GeV/c polarised muons off different polarised targets (NH3 for polarised protons and 6LiD for polarised deuterons). One of the main goals is to determine how the total longitudinal spin projection of the nucleon, 1/2, is distributed among its constituents, quarks and gluons. We review here the recent results on the quark and gluon helicities obtained by COMPASS. The other major goal, whose fulfilment is needed for a complete understanding of the nucleon spin, is the determination of the transverse momentum dependent parton distributions (TMDs). Regarding this topic, the latest results on the Collins and Sivers asymmetries will be shown. The former is sensitive to the transverse spin structure of the nucleon, while the latter reflects the correlations between the quarks transverse momentum and the nucleon spin. This overview will conclude with a summary of the approved plans of COMPASS for the near future: the study of TMDs with a pioneering polarised Drell-Yan experiment and the measurement of generalised parton distributions (GPDs). © 2016.

Maneira J.,LIP
Journal of Physics: Conference Series | Year: 2013

SNO+ is a multi-purpose Neutrino Physics experiment, succeeding to the Sudbury Neutrino Observatory by replacing heavy water with liquid scintillator. Its scientific goals are the search for neutrinoless double-beta decay, the study of solar neutrinos and antineutrinos from reactors and the Earth's natural radioactivity, as well as supernovae neutrinos. The experimental advantages of SNO+ are the possibility of loading large quantities of double-beta decaying isotope in the liquid scintillator volume, and the very low backgrounds allowed by the deep underground location and radiopurity of the employed materials. The installation of the detector at SNOLAB is being completed, and commissioning has already started, with a dry run. Filling with water and later, with scintillator, will start next year. This talk will summarize the Physics goals of SNO+, as well as the main detector developments. © Published under licence by IOP Publishing Ltd.

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