Searcharxiv⌕ Search

arXiv subjects

Antonio Ortiz

Publications and source records attributed to Antonio Ortiz.

54 records · Page 3Linked to original sources
arXiv2016

Heavy-ion physics at the LHC: Review of Run I results

In this work we review what we consider are, some of the most relevant results of heavy-ion physics at the LHC. This paper is not intended to cover all the many important results of the experiments, instead we present a brief overview of the current status on the characterization of the hot and dense QCD medium produced in the heavy-ion collisions. Recent exciting results which are still under debate are discussed too, leading to intriguing questions like whether we have a real or fake QGP formation in small systems.

hep-ex↗
arXiv2016

Jet effects in high-multiplicity pp events

The study of the high-multiplicity pp events has become important because we need to understand the origin of the fluid-like features which have been found in such small systems. In this work we concentrate on the radial flow signatures. To this end, the role of jets in high-multiplicity pp collisions is investigated using PYTHIA 8.

hep-ph↗
arXiv2015

Light flavor results in p-Pb collisions with ALICE

Particle ratios provide insight into the hadrochemistry of the event and the mechanisms for particle production. In Pb-Pb collisions the relative multi-strange baryon yields exhibit an enhancement with respect to pp collisions, whereas the short-lived K$^{*0}$ resonance is suppressed in the most central events due to re-scattering of its decay daughter particles. Measurements in p-Pb allow us to investigate the development of these effects as a function of the system size. We report comprehensive results on light-flavor hadron production measured with the ALICE detector in p-Pb collisions at $\sqrt{s_{\rm NN}}=5.02$ TeV, covering a wide range of particle species which includes long-lived hadrons, resonances and multi-strange baryons. The measurements include the transverse momentum spectra and the ratios of spectra among different species, and extend over a very large transverse momentum region, from $\approx$100 MeV/$c$ to $\approx$20 GeV/$c$, depending on the particle species.

hep-ex↗
arXiv2015

Mean pt scaling with m/nq at the LHC: Absence of (hydro) flow in small systems?

In this work, a study of the average transverse momentum (pt) as a function of the mid-rapidity charged hadron multiplicity (Nch) and hadron mass (m) in p-Pb and Pb-Pb collisions at LHC energies is presented. For the events producing low Nch, the average pt is found to scale with the reduced hadron mass, i.e., mass divided by the number of quark constituents (m/nq), this scaling also holds for inelastic pp collisions at RHIC and LHC energies. The scaling is broken in high multiplicity p-Pb and Pb-Pb collisions, where, for < 60 the average pt is higher for baryons than that for mesons, though they increase linearly with m/nq. This behavior is qualitatively well reproduced by Pythia 8, but not by hydro calculations, where an universal scaling with the hadron mass (and not with m/nq) is predicted for all the multiplicity event classes. Only the central (0-60%) Pb-Pb collisions behave as expected from hydro.

hep-ph↗
arXiv2015

Mid-rapidity charged hadron transverse spherocity in pp collisions simulated with Pythia

The pp collisions have been studied for a long time, however, there are still some effects which are not completely understood, such as the long range angular correlations and the flow patterns in high multiplicity events, which were recently discovered at the LHC. In a recent work it was demonstrated that in Pythia 8, multi-parton interactions and color reconnection can give some of the observed effects similar to the collective flow well known from heavy-ion collisions. Now using the same model, a study based on mid-rapidity charged hadron transverse spherocity is presented. The main purpose of this work is to show that a differential study combining multiplicity and event shapes opens the possibility to understand better the features of data, specially at high multiplicity.

hep-ph↗
arXiv2015

Overview of ALICE results

The ALICE detector was designed to study the physics of matter under extreme conditions of high energy density. Different results were reported by the experiment using data from the successful run I of the LHC. The goal of the present work is to present an overview of recent ALICE results. This comprises selected results from several analyses of pp, p-pb and Pb-Pb data at the LHC energies.

hep-ex↗
arXiv2014

Study of the PYTHIA $ΔηΔϕ$ correlation as a function of multiplicity and transverse sphericity in proton-proton collisions at 7 TeV

The study and understanding of the details of proton-proton collisions is important since they are the benchmark for the comparison with heavy ion results. In the present work we have studied the $ΔηΔϕ$ correlations as a function of the event multiplicity and transverse sphericity in pp collisions at $\sqrt{s}$ = 7 TeV using PYTHIA 8.180. The results show large variation in the shape of the correlation as a function of the sphericity and multiplicity indicating the importance of slicing the data in bins of both observables to better understand the complexity of the interactions. Motivated by the radial flow patterns originated by color reconnection (CR), results with and without CR together with dihadron correlations are discussed.

hep-ph↗
arXiv2014

Production of $π$/K/p from intermediate to high $p_{\rm T}$ in pp, p-Pb and Pb-Pb collisions measured by ALICE

In this work the results on the transverse momentum distributions ($\sim0.3<p_{\rm T}<15$ GeV/$c$) of charged pions, kaons and (anti)protons, measured in $\rm pp$ and $\rm Pb-Pb$ collisions at $\sqrt{s_{\rm NN}}=2.76$ TeV and in $\rm p-Pb$ collisions at $\sqrt{s_{\rm NN}}=5.02$ TeV, are presented. The evolution of the spectral shapes and particle ratios with multiplicity is shown and the similarities among the different systems are discussed.

hep-ex↗
arXiv2014

Disentangling the soft and hard components of the pp collisions using the sphero(i)city approach

A new method to extract information from the pp data is proposed. The approach is based on the use of the event structure variables: sphericity and spherocity, to split the data into enhanced soft and hard processes samples corresponding to events with large and low numbers of multi-parton interactions, respectively. The present study was developed in the framework of Pythia 8.180 for inelastic pp collisions at sqrt(s) =7 TeV. As an application of the method, a study of the identified particle transverse momentum spectra and their ratios; is presented for soft (isotropic) and hard (jetty-like) events. The flow-like effect on these observables due to multi-parton interactions and color reconnection is relevant for soft events suggesting that partons inside the jet do not feel color reconnection and its flow-like consequences.

hep-ph↗
arXiv2014

Results on identified particle production in pp, p-Pb and Pb-Pb collisions measured with ALICE at the LHC

Using the unique capabilities of the ALICE detectors for particle identification, different measurements have been performed to study the properties of the hot and dense matter created in the Pb-Pb collisions at sqrt(sNN)= 2.76 TeV. The analysis of the p-Pb data at sqrt(sNN) = 5.02 TeV reveals that the suppression of high pt hadrons observed in heavy nuclei collisions can not be explained as due to initial state effects. The systems created in the p-Pb collisions do not show evidence of jet quenching but, surprisingly, exhibit characteristics of flow. In this paper a review of the main results on identified particle production measured in different colliding systems is presented, data are also compared to models.

hep-ex↗
arXiv2013

Color reconnection and flow-like patterns in pp collisions

Increasingly, with the data collected at the LHC we are confronted with the possible existence of flow in pp collisions. In this work we show that PYTHIA 8 produces flow-like effects in events with multiple hard subcollisions due to color string formations between final partons from independent hard scatterings, the so called color reconnection. We present studies of different identified hadron observables in pp collisions at 7 TeV with the tune 4C. Studies have been done both for minimum bias and multiplicity intervals in events with and without color reconnection to isolate the flow-like effect.

hep-ph↗
arXiv2011

Transverse sphericity of charged particles in minimum bias p-p collision in ALICE at the LHC

The study of the sphericity of primary charged particles in minimum bias proton-proton collisions at 0.9, 2.76 and 7 TeV with the ALICE detector at the LHC is presented. The mean sphericity as a function of multiplicity is reported for events with different hardness. The ALICE results show a growing transverse sphericity with multiplicity for all collision energies, with a steeper rise at low multiplicity. The tendency of the MC generators is opposite at high multiplicity.

hep-ex↗
arXiv2011

Three-hadron angular correlations from pQCD at RHIC and LHC

We study three-hadron azimuthal angular correlations in high energy proton-proton and central nucleus-nucleus collisions at RHIC and LHC at mid-rapidity. We use the LO parton matrix elements for 2 \rightarrow 3 processes and include the effect of parton energy loss in the Quark-Gluon Plasma using the modified fragmentation function approach. For the case when the produced hadrons have either same or not too different momenta, we observe two away side peaks at 2pi/3 and 4pi/3. We consider the dependence of the angular correlations on energy loss parameters that have been used in studies of single inclusive hadron production at RHIC. Our results on the angular dependence of the cross section agree well with preliminary data by the PHENIX collaboration. We comment on the possible contribution of 2 \rightarrow 3 processes to di-hadron angular correlations and how a comparison of the two processes may help characterize the plasma further.

hep-ph↗
arXiv2011

The broad away side of azimuthal correlations: 3 vs 2 final state particles in high energy nuclear collisions

In high energy heavy ion collisions at RHIC there are important aspects of the medium induced dynamics, that are still not well understood. In particular, there is a broadening and even a double hump structure of the away-side peak appearing in azimuthal correlation studies in Au+Au collisions which is absent in p+p collisions at the same energies. These features are already present but suppressed in p+p collisions: 2 to 3 parton processes produce such structures but are suppressed with respect to 2 to 2 processes. We argue that in A+A collisions the different geometry for the trajectories of 3 as opposed to 2 particles in the final state, together with the medium induced energy loss effects on the different cross sections, create a scenario that enhances processes with 3 particles in the final state, which gives on average this double hump structure.

nucl-th↗
arXiv2009

Event Shape Analysis in ALICE

The jets are the final state manifestation of the hard parton scattering. Since at LHC energies the production of hard processes in proton-proton collisions will be copious and varied, it is important to develop methods to identify them through the study of their final states. In the present work we describe a method based on the use of some shape variables to discriminate events according their topologies. A very attractive feature of this analysis is the possibility of using the tracking information of the TPC+ITS in order to identify specific events like jets. Through the correlation between the quantities: thrust and recoil, calculated in minimum bias simulations of proton-proton collisions at 10 TeV, we show the sensitivity of the method to select specific topologies and high multiplicity. The presented results were obtained both at level generator and after reconstruction. It remains that with any kind of jet reconstruction algorithm one will confronted in general with overlapping jets. The present method determines areas where one does encounter special topologies of jets in an event. The aim is not to supplant the usual jet reconstruction algorithms, but rather to allow an easy selection of events allowing then the application of algorithms.

hep-ex↗
arXiv2009

Three and two-hadron correlations in \sqrt{s_{NN}}=200 GeV proton-proton and nucleus-nucleus collisions

We compare the azimuthal correlations arising from three and two hadron production in high energy proton-proton and nucleus-nucleus collisions at \sqrt{s_{NN}}=200 GeV, using the leading order matrix elements for two-to-three and two-to-two parton-processes in perturbative QCD. We first compute the two and three hadron production cross sections in mid-rapidity proton-proton collisions. Then we consider Au + Au collisions including parton energy loss using the modified fragmentation function approach. By examining the geometrical paths the hard partons follow through the medium, we show that the two away-side partons produced in two-to-three processes have in average a smaller and a greater path length than the average path length of the away-side parton in two-to-two processes. Therefore there is a large probability that in the former processes one of the particles escapes while the other gets absorbed. This effect leads to an enhancement in the azimuthal correlations of the two-to-three with respect to the two-to-two parton-processes when comparing to the same processes in proton-proton collisions since in average the particle with the shortest path length looses less energy with respect to the away side particle in two-to-two processes. We argue that this phenomenon may be responsible for the shape of the away-side in azimuthal correlations observed in mid-rapidity Au + Au collisions at RHIC.

nucl-th↗
arXiv2009

Fine structure in the azimuthal transverse momentum correlations at $\sqrt{s_{NN}}=200$ GeV using the event shape analysis

The experimental results on transverse momentum azimuthal hadron correlations at RHIC have opened a rich field for parton energy loss analysis in heavy-ion collisions. Recently, a considerable amount of work has beendevoted to study the shapes of the ``away-side'' jet which exhibit an interesting and unexpected ``double hump'' structure not observed in the analogous treatment of $pp$ data. Driven by the possibility that the latter result might just mean that such structure exists already in the case of $pp$ collisions, but that its relative intensity could be small, here we use the Event Shape Analysis to show that it is possible to identify and select well defined event topologies in $pp$ collisions, among which, a double hump structure for the away-side jet emerges. Using two shape parameters, the sphericity in the transverse plane and the recoil to analyze a sample of PYTHIA generated $pp$ collisions at $\sqrt{s_{NN}}=200$ GeV, we show that this structure corresponds to two jets emitted in the backward hemisphere. Finally, we show that Q-PYTHIA qualitatively reproduces the decrease in the yield of dijet events and the increase of the double hump structure in the away side observed in heavy ion collisions. The implications for the treatment of parton energy loss in heavy-ion collisions are discussed

hep-ph↗