Measurement of dielectrons in pp, p(-)Pb and Pb-Pb collisions with ALICE at the LHC

Dielectrons are penetrating probes which carry unaffected information on the hot and dense medium created in ultra-relativistic heavy-ion collisions. The low-mass region of the dielectron spectrum (m(pi)(0) < m(ee) < m(p)) is sensitive to virtual photon production and in-medium modifications of low-mass vector meson spectral functions. These modifications are connected to the restoration of chiral symmetry, which is expected at high temperature. Dielectrons are also sensitive to heavy-flavor production. In these proceedings, we present the preliminary results of dielectron measurements in pp collisions at root s = 7 TeV, in p-Pb collisions at root s(NN) = 5.02 TeV and in central (0-10%) Pb-Pb collisions at root s(NN) = 2.76 TeV with the ALICE experiment at the LHC. The expected scenario and perspectives for the dielectron measurement after the ALICE upgrade are also discussed

Low-mass dielectron measurement in Pb--Pb collisions at sNN=2.76 TeV\sqrt{s_{\mathrm{NN}}}=2.76\ \mathrm{TeV} with ALICE at the LHC

In ordinary matter, quarks and gluons are confined inside hadrons by the strong interaction. At extreme conditions of temperature and energy density, a new state of matter is formed, called quark-gluon plasma (QGP). This is made of deconfined quasi-free quarks and gluons. Based on the current cosmological picture, the quark-gluon plasma was the state of our universe few $\mu$s after the Big Bang. Moreover, there is evidence that a degenerate state of matter with similar properties to the QGP exists in the inner core of neutron stars and other compact astrophysical objects.Microscopic and extremely short-lived quantities of such a nuclear plasma can be created in ultra-relativistic heavy ion collisions. Its properties can be studied through several experimental probes using dedicated detectors installed around the collision region. This interesting branch of research is part of the experimental program of the Large Hadron Collider (LHC) at CERN, where lead ion beams are accelerated to unprecedented energies.The QGP properties, in principle, can be described by Quantum-Chromo Dynamics (QCD), the theory of the strong interaction. However, a description of the system based on QCD first principles is extremely complicated due to the relatively low energy scale involved (compared to $\Lambda_{QCD}$), which does not allow a perturbative approach. Further complications arise from many-body properties of QCD which are anyhow extremely interesting to explore.The deconfined medium created in heavy-ion collisions rapidly evolves, passing through several thermodynamic stages.Photons and dileptons are unique tools to study the properties of heavy-ion collisions. These particles are continuously emitted and they cross the medium with negligible interaction, thus carrying undisturbed information on their production source.Electromagnetic probes provide complementary information to hadronic probes, allowing to constrain the theoretical models used for the description of the system in the early stages. Thermal photons and dileptons carry information on the system temperature. Moreover, in-medium modifications of low-mass vector mesons spectral functions can be studied through their dilepton decay channels. These effects have since long been proposed as signatures of chiral symmetry restoration.Dileptons are also sensitive to heavy-flavor production, which gives a significant contribution to the intermediate mass region of the dilepton spectrum ($m_{\phi} < m_{l^{+}l^{-}} < m_{J /\psi}$).In this thesis, the dielectron production in Pb--Pb collisions at $\sqrt{s_{\mathrm{NN}}}=2.76\ \mathrm{TeV}$ with the ALICE experiment at the LHC has been studied. ALICE is the detector at the LHC dedicated to the study of heavy-ion collisions. The ALICE excellent tracking and particle identification capabilities make this experiment well suited for dielectron measurements. The main focus of this work has been the study of the low-mass region of the dielectron invariant mass spectrum, where contributions from thermal dileptons and from modified low-mass vector mesons are expected. The fraction of virtual direct photons has been measured, and the dielectron spectrum has been compared to the expected contributions from hadron decays, thermal dileptons and in-medium $\rho^{0}$ and $\omega$, resulting in good agreement within the experimental uncertainties.The future perspectives for the dielectron measurement and the predicted scenario after the ALICE upgrade are also presented

Region of interest-specific loss functions improve T2 quantification with ultrafast T2 mapping MRI sequences in knee, hip and lumbar spine

MRI T2 mapping sequences quantitatively assess tissue health and depict early degenerative changes in musculoskeletal (MSK) tissues like cartilage and intervertebral discs (IVDs) but require long acquisition times. In MSK imaging, small features in cartilage and IVDs are crucial for diagnoses and must be preserved when reconstructing accelerated data. To these ends, we propose region of interest-specific postprocessing of accelerated acquisitions: a recurrent UNet deep learning architecture that provides T2 maps in knee cartilage, hip cartilage, and lumbar spine IVDs from accelerated T2-prepared snapshot gradient-echo acquisitions, optimizing for cartilage and IVD performance with a multi-component loss function that most heavily penalizes errors in those regions. Quantification errors in knee and hip cartilage were under 10% and 9% from acceleration factors R = 2 through 10, respectively, with bias for both under 3&nbsp;ms for most of R = 2 through 12. In IVDs, mean quantification errors were under 12% from R = 2 through 6. A Gray Level Co-Occurrence Matrix-based scheme showed knee and hip pipelines outperformed state-of-the-art models, retaining smooth textures for most R and sharper ones through moderate R. Our methodology yields robust T2 maps while offering new approaches for optimizing and evaluating reconstruction algorithms to facilitate better preservation of small, clinically relevant features

Production of omega mesons in pp collisions at s=7 TeV

The invariant differential cross section of inclusive omega (782) meson production at midrapidity (|y| pi+pi-pi 0 decay channel. The measured omega production cross section is compared to various calculations: PYTHIA 8.2 Monash 2013 describes the data, while PYTHIA 8.2 Tune 4C overestimates the data by about 50%. A recent NLO calculation, which includes a model describing the fragmentation of the whole vector-meson nonet, describes the data within uncertainties below 6 GeV/c, while it overestimates the data by up to 50% for higher pT. The omega/pi 0 ratio is in agreement with previous measurements at lower collision energies and the PYTHIA calculations. In addition, the measurement is compatible with transverse mass scaling within the measured pT range and the ratio is constant with C omega/pi 0=0.67 +/- 0.03(stat) +/- 0.04(sys) above a transverse momentum of 2.5 GeV/c

Measurements of mixed harmonic cumulants in Pb-Pb collisions at root s(NN)=5.02 TeV

Correlations between moments of different flow coefficients are measured in Pb-Pb collisions at root s(NN) = 5.02 TeV recorded with the ALICE detector. These new measurements are based on multiparticle mixed harmonic cumulants calculated using charged particles in the pseudorapidity region vertical bar eta vertical bar < 0.8 with the transverse momentum range 0.2 < p(T) < 5.0 GeV/c. The centrality dependence of correlations between two flow coefficients as well as the correlations between three flow coefficients, both in terms of their second moments, are shown. In addition, a collection of mixed harmonic cumulants involving higher moments of v(2) and v(3) is measured for the first time, where the characteristic signature of negative, positive and negative signs of four-, six- and eight-particle cumulants are observed, respectively. The measurements are compared to the hydrodynamic calculations using iEBE-VISHNU with AMPT and TRENTo initial conditions. It is shown that the measurements carried out using the LHC Run 2 data in 2015 have the precision to explore the details of initial-state fluctuations and probe the nonlinear hydrodynamic response of v(2) and v(3) to their corresponding initial anisotropy coefficients epsilon(2) and epsilon(3). These new studies on correlations between three flow coefficients as well as correlations between higher moments of two different flow coefficients will pave the way to tighten constraints on initial-state models and help to extract precise information on the dynamic evolution of the hot and dense matter created in heavy-ion collisions at the LHC. (C) 2021 The Author. Published by Elsevier B.V

Transverse-momentum and event-shape dependence of D-meson flow harmonics in Pb-Pb collisions at root(S)(NN)=5 . 02 TeV

The elliptic and triangular flow coefficients v(2) and v(3) of prompt D-0 , D+ , and D*(+) mesons were measured at midrapidity (|y| < 0.8) in Pb-Pb collisions at the centre-of-mass energy per nucleon pair of root(S)(NN) = 5.02 TeV with the ALICE detector at the LHC. The D mesons were reconstructed via their hadronic decays in the transverse momentum interval 1 < p(T) < 36 GeV/c in central (0-10%) and semi-central (30-50%) collisions. Compared to pions, protons, and J/psi mesons, the average D-meson v(n) harmonics are compatible within uncertainties with a mass hierarchy for p(T) less than or similar to 3 GeV/c , and are similar to those of charged pions for higher p(T). The coupling of the charm quark to the light quarks in the underlying medium is further investigated with the application of the event-shape engineering (ESE) technique to the D-meson v(2) and p(T)-differential yields. The D-meson v(2) is correlated with average bulk elliptic flow in both central and semi-central collisions. Within the current precision, the ratios of per-event D meson yields in the ESE-selected and unbiased samples are found to be compatible with unity. All the measurements are found to be reasonably well described by theoretical calculations including the effects of charm-quark transport and the recombination of charm quarks with light quarks in a hydrodynamically expanding medium. (C) 2020 The Author(s). Published by Elsevier B.V

Measurement of isolated photon-hadron correlations in √ s NN = 5.02 TeV p p and p -Pb collisions

This paper presents isolated photon-hadron correlations using pp and p-Pb data collected by the ALICE detector at the LHC. For photons with vertical bar eta vertical bar < 0.67 and 12 < p(T) < 40 GeV/c, the associated yield of charged particles in the range vertical bar eta vertical bar < 0.80 and 0.5 < p(T) < 10 GeV/c is presented. These momenta are much lower than previous measurements at the LHC. No significant difference between pp and p-Pb is observed, with PYTHIA 8.2 describing both data sets within uncertainties. This measurement constrains nuclear effects on the parton fragmentation in p-Pb collisions, and provides a benchmark for future studies of Pb-Pb collisions

Jet-associated deuteron production in pp collisions at s=13 TeV

Deuteron production in high-energy collisions is sensitive to the space–time evolution of the collision system, and is typically described by a coalescence mechanism. For the first time, we present results on jet-associated deuteron production in pp collisions at s=13 TeV, providing an opportunity to test the established picture for deuteron production in events with a hard scattering. Using a trigger particle with high transverse-momentum (pT>5 GeV/c) as a proxy for the presence of a jet at midrapidity, we observe a measurable population of deuterons being produced around the jet proxy. The associated deuteron yield measured in a narrow angular range around the trigger particle differs by 2.4–4.8 standard deviations from the uncorrelated background. The data are described by PYTHIA model calculations featuring baryon coalescence

Kaon femtoscopy in Pb-Pb collisions at root s(NN)=2.76 TeV

We present the results of three-dimensional femtoscopic analyses for charged and neutral kaons recorded by ALICE in Pb-Pb collisions at root s(NN) = 2.76 TeV. Femtoscopy is used to measure the space-time characteristics of particle production from the effects of quantum statistics and final-state interactions in two-particle correlations. Kaon femtoscopy is an important supplement to that of pions because it allows one to distinguish between different model scenarios working equally well for pions. In particular, we compare the measured three-dimensional kaon radii with a purely hydrodynamical calculation and a model where the hydrodynamic phase is followed by a hadronic rescattering stage. The former predicts an approximate transverse mass (m(T)) scaling of source radii obtained from pion and kaon correlations. This m(T) scaling appears to be broken in our data, which indicates the importance of the hadronic rescattering phase at LHC energies. A k(T) scaling of pion and kaon source radii is observed instead. The time of maximal emission of the system is estimated by using the three-dimensional femtoscopic analysis for kaons. The measured emission time is larger than that of pions. Our observation is well supported by the hydrokinetic model predictions

Jet-associated deuteron production in pp collisions at s=13 TeV

Deuteron production in high-energy collisions is sensitive to the space–time evolution of the collision system, and is typically described by a coalescence mechanism. For the first time, we present results on jet-associated deuteron production in pp collisions at s=13 TeV, providing an opportunity to test the established picture for deuteron production in events with a hard scattering. Using a trigger particle with high transverse-momentum (pT>5 GeV/c) as a proxy for the presence of a jet at midrapidity, we observe a measurable population of deuterons being produced around the jet proxy. The associated deuteron yield measured in a narrow angular range around the trigger particle differs by 2.4–4.8 standard deviations from the uncorrelated background. The data are described by PYTHIA model calculations featuring baryon coalescence