“Study of jet substructure in heavy ion collisions: effect of combinatorial background on the subjet distance ΔRg\Delta R_g”

In this project we chose and studied one observable in the context of jet physics, the subjet distance $\Delta R_g$. A toy model was used together with a Monte Carlo event generator (Pythia) to simulate the effects of the combinatorial background that is present in heavy ion collisions. The combinatorial background is subtracted with the constituent subtraction method. The effect of detector efficiency was studied as well. It was possible to recreate the shape of $\Delta R_g$ observed experimentally using the toy model. In addition, the jets $p_T$ and area distribution are presented in this report, as well as the dependence of $\Delta R_g$ on $p_T$. An interesting difference was found when comparing the event-by-event and the jet-by-jet subtraction methods of the constituent subtraction algorithm. This difference is not yet fully understood; there seems to be no simple explanation of the effect, so we suspect that it is a rather subtle implication of the algorithm that deserves further study

Systematic study of flow vector decorrelation in sNN=5.02\mathbf{\sqrt{\textit{s}_{_{\bf NN}}}=5.02} TeV Pb-Pb collisions

International audienceMeasurements of the $p_{\rm T}$-dependent flow vector fluctuations in Pb-Pb collisions at $\sqrt{s_{_{\rm NN}}} = 5.02~\mathrm{TeV}$ using azimuthal correlations with the ALICE experiment at the LHC are presented. A four-particle correlation approach [1] is used to quantify the effects of flow angle and magnitude fluctuations separately. This paper extends previous studies to additional centrality intervals and provides measurements of the $p_{\rm T}$-dependent flow vector fluctuations at $\sqrt{s_{_{\rm NN}}} = 5.02~\mathrm{TeV}$ with two-particle correlations. Significant $p_{\rm T}$-dependent fluctuations of the $\vec{V}_{2}$ flow vector in Pb-Pb collisions are found across different centrality ranges, with the largest fluctuations of up to $\sim$15% being present in the 5% most central collisions. In parallel, no evidence of significant $p_{\rm T}$-dependent fluctuations of $\vec{V}_{3}$ or $\vec{V}_{4}$ is found. Additionally, evidence of flow angle and magnitude fluctuations is observed with more than $5\sigma$ significance in central collisions. These observations in Pb-Pb collisions indicate where the classical picture of hydrodynamic modeling with a common symmetry plane breaks down. This has implications for hard probes at high $p_{\rm T}$, which might be biased by $p_{\rm T}$-dependent flow angle fluctuations of at least 23% in central collisions. Given the presented results, existing theoretical models should be re-examined to improve our understanding of initial conditions, quark--gluon plasma (QGP) properties, and the dynamic evolution of the created system

Measurement of Ωc0\Omega^0_{\rm c} baryon production and branching-fraction ratio BR(Ωc0→Ω−e+νe)/BR(Ωc0→Ω−π+){\rm BR(\Omega^0_c \rightarrow \Omega^- e^+\nu_e)} / {\rm BR(\Omega^0_c \rightarrow \Omega^- \pi^+)} in pp collisions at s\sqrt{s} = 13 TeV

International audienceThe inclusive production of the charm-strange baryon $\Omega^{0}_{\rm c}$ is measured for the first time via its semileptonic decay into $\Omega^{-}\rm e^{+}\nu_{e}$ at midrapidity ($|y|<0.8$) in proton$-$proton (pp) collisions at the centre-of-mass energy $\sqrt{s}=13$ TeV with the ALICE detector at the LHC. The transverse momentum ($p_{\rm T}$) differential cross section multiplied by the branching ratio is presented in the interval $2<p_{\rm T}<12~{\rm GeV}/c$. The branching-fraction ratio ${\rm BR}(\Omega^0_{\rm c} \rightarrow \Omega^{-}{\rm e}^{+}\nu_{\rm e})/ {\rm BR}(\Omega^0_{\rm c} \rightarrow \Omega^{-}{\pi}^{+})$ is measured to be 1.12 $\pm$ 0.22 (stat.) $\pm$ 0.27 (syst.). Comparisons with other experimental measurements, as well as with theoretical calculations, are presented

Exclusive four pion photoproduction in ultraperipheral Pb-Pb collisions at sNN=5.02\sqrt{s_{\rm NN}} = 5.02 TeV

International audienceThe intense photon fluxes from relativistic nuclei provide an opportunity to study photonuclear interactions in ultraperipheral collisions. The measurement of coherently photoproduced $\pi^+\pi^-\pi^+\pi^-$ final states in ultraperipheral Pb-Pb collisions at $\sqrt{s_{\mathrm{NN}}}=5.02$ TeV is presented for the first time. The cross section, d$\sigma$/d$y$, times the branching ratio ($\rho\rightarrow \pi^+ \pi^+ \pi^- \pi^-$) is found to be $47.8\pm2.3~\rm{(stat.)}\pm7.7~\rm{(syst.)}$ mb in the rapidity interval $|y| < 0.5$. The invariant mass distribution is not well described with a single Breit-Wigner resonance. The production of two interfering resonances, $\rho(1450)$ and $\rho(1700)$, provides a good description of the data. The values of the masses ($m$) and widths ($\Gamma$) of the resonances extracted from the fit are $m_{1}=1385\pm14~\rm{(stat.)}\pm3~\rm{(syst.)}$ MeV/$c^2$, $\Gamma_{1}=431\pm36~\rm{(stat.)}\pm82~\rm{(syst.)}$ MeV/$c^2$, $m_{2}=1663\pm13~\rm{(stat.)}\pm22~\rm{(syst.)}$ MeV/$c^2$ and $\Gamma_{2}=357 \pm31~\rm{(stat.)}\pm49~\rm{(syst.)}$ MeV/$c^2$, respectively. The measured cross sections times the branching ratios are compared to recent theoretical predictions

Exclusive four pion photoproduction in ultraperipheral Pb-Pb collisions at sNN=5.02\sqrt{s_{\rm NN}} = 5.02 TeV

International audienceThe intense photon fluxes from relativistic nuclei provide an opportunity to study photonuclear interactions in ultraperipheral collisions. The measurement of coherently photoproduced $\pi^+\pi^-\pi^+\pi^-$ final states in ultraperipheral Pb-Pb collisions at $\sqrt{s_{\mathrm{NN}}}=5.02$ TeV is presented for the first time. The cross section, d$\sigma$/d$y$, times the branching ratio ($\rho\rightarrow \pi^+ \pi^+ \pi^- \pi^-$) is found to be $47.8\pm2.3~\rm{(stat.)}\pm7.7~\rm{(syst.)}$ mb in the rapidity interval $|y| < 0.5$. The invariant mass distribution is not well described with a single Breit-Wigner resonance. The production of two interfering resonances, $\rho(1450)$ and $\rho(1700)$, provides a good description of the data. The values of the masses ($m$) and widths ($\Gamma$) of the resonances extracted from the fit are $m_{1}=1385\pm14~\rm{(stat.)}\pm3~\rm{(syst.)}$ MeV/$c^2$, $\Gamma_{1}=431\pm36~\rm{(stat.)}\pm82~\rm{(syst.)}$ MeV/$c^2$, $m_{2}=1663\pm13~\rm{(stat.)}\pm22~\rm{(syst.)}$ MeV/$c^2$ and $\Gamma_{2}=357 \pm31~\rm{(stat.)}\pm49~\rm{(syst.)}$ MeV/$c^2$, respectively. The measured cross sections times the branching ratios are compared to recent theoretical predictions

Measurement of Ωc0\Omega^0_{\rm c} baryon production and branching-fraction ratio BR(Ωc0→Ω−e+νe)/BR(Ωc0→Ω−π+){\rm BR(\Omega^0_c \rightarrow \Omega^- e^+\nu_e)} / {\rm BR(\Omega^0_c \rightarrow \Omega^- \pi^+)} in pp collisions at s\sqrt{s} = 13 TeV

International audienceThe inclusive production of the charm-strange baryon $\Omega^{0}_{\rm c}$ is measured for the first time via its semileptonic decay into $\Omega^{-}\rm e^{+}\nu_{e}$ at midrapidity ($|y|<0.8$) in proton$-$proton (pp) collisions at the centre-of-mass energy $\sqrt{s}=13$ TeV with the ALICE detector at the LHC. The transverse momentum ($p_{\rm T}$) differential cross section multiplied by the branching ratio is presented in the interval $2<p_{\rm T}<12~{\rm GeV}/c$. The branching-fraction ratio ${\rm BR}(\Omega^0_{\rm c} \rightarrow \Omega^{-}{\rm e}^{+}\nu_{\rm e})/ {\rm BR}(\Omega^0_{\rm c} \rightarrow \Omega^{-}{\pi}^{+})$ is measured to be 1.12 $\pm$ 0.22 (stat.) $\pm$ 0.27 (syst.). Comparisons with other experimental measurements, as well as with theoretical calculations, are presented

Systematic study of flow vector decorrelation in sNN=5.02\mathbf{\sqrt{\textit{s}_{_{\bf NN}}}=5.02} TeV Pb-Pb collisions

International audienceMeasurements of the $p_{\rm T}$-dependent flow vector fluctuations in Pb-Pb collisions at $\sqrt{s_{_{\rm NN}}} = 5.02~\mathrm{TeV}$ using azimuthal correlations with the ALICE experiment at the LHC are presented. A four-particle correlation approach [1] is used to quantify the effects of flow angle and magnitude fluctuations separately. This paper extends previous studies to additional centrality intervals and provides measurements of the $p_{\rm T}$-dependent flow vector fluctuations at $\sqrt{s_{_{\rm NN}}} = 5.02~\mathrm{TeV}$ with two-particle correlations. Significant $p_{\rm T}$-dependent fluctuations of the $\vec{V}_{2}$ flow vector in Pb-Pb collisions are found across different centrality ranges, with the largest fluctuations of up to $\sim$15% being present in the 5% most central collisions. In parallel, no evidence of significant $p_{\rm T}$-dependent fluctuations of $\vec{V}_{3}$ or $\vec{V}_{4}$ is found. Additionally, evidence of flow angle and magnitude fluctuations is observed with more than $5\sigma$ significance in central collisions. These observations in Pb-Pb collisions indicate where the classical picture of hydrodynamic modeling with a common symmetry plane breaks down. This has implications for hard probes at high $p_{\rm T}$, which might be biased by $p_{\rm T}$-dependent flow angle fluctuations of at least 23% in central collisions. Given the presented results, existing theoretical models should be re-examined to improve our understanding of initial conditions, quark--gluon plasma (QGP) properties, and the dynamic evolution of the created system

Systematic study of flow vector decorrelation in sNN=5.02\mathbf{\sqrt{\textit{s}_{_{\bf NN}}}=5.02} TeV Pb-Pb collisions

International audienceMeasurements of the $p_{\rm T}$-dependent flow vector fluctuations in Pb-Pb collisions at $\sqrt{s_{_{\rm NN}}} = 5.02~\mathrm{TeV}$ using azimuthal correlations with the ALICE experiment at the LHC are presented. A four-particle correlation approach [1] is used to quantify the effects of flow angle and magnitude fluctuations separately. This paper extends previous studies to additional centrality intervals and provides measurements of the $p_{\rm T}$-dependent flow vector fluctuations at $\sqrt{s_{_{\rm NN}}} = 5.02~\mathrm{TeV}$ with two-particle correlations. Significant $p_{\rm T}$-dependent fluctuations of the $\vec{V}_{2}$ flow vector in Pb-Pb collisions are found across different centrality ranges, with the largest fluctuations of up to $\sim$15% being present in the 5% most central collisions. In parallel, no evidence of significant $p_{\rm T}$-dependent fluctuations of $\vec{V}_{3}$ or $\vec{V}_{4}$ is found. Additionally, evidence of flow angle and magnitude fluctuations is observed with more than $5\sigma$ significance in central collisions. These observations in Pb-Pb collisions indicate where the classical picture of hydrodynamic modeling with a common symmetry plane breaks down. This has implications for hard probes at high $p_{\rm T}$, which might be biased by $p_{\rm T}$-dependent flow angle fluctuations of at least 23% in central collisions. Given the presented results, existing theoretical models should be re-examined to improve our understanding of initial conditions, quark--gluon plasma (QGP) properties, and the dynamic evolution of the created system

Exclusive four pion photoproduction in ultraperipheral Pb-Pb collisions at sNN=5.02\sqrt{s_{\rm NN}} = 5.02 TeV

International audienceThe intense photon fluxes from relativistic nuclei provide an opportunity to study photonuclear interactions in ultraperipheral collisions. The measurement of coherently photoproduced $\pi^+\pi^-\pi^+\pi^-$ final states in ultraperipheral Pb-Pb collisions at $\sqrt{s_{\mathrm{NN}}}=5.02$ TeV is presented for the first time. The cross section, d$\sigma$/d$y$, times the branching ratio ($\rho\rightarrow \pi^+ \pi^+ \pi^- \pi^-$) is found to be $47.8\pm2.3~\rm{(stat.)}\pm7.7~\rm{(syst.)}$ mb in the rapidity interval $|y| < 0.5$. The invariant mass distribution is not well described with a single Breit-Wigner resonance. The production of two interfering resonances, $\rho(1450)$ and $\rho(1700)$, provides a good description of the data. The values of the masses ($m$) and widths ($\Gamma$) of the resonances extracted from the fit are $m_{1}=1385\pm14~\rm{(stat.)}\pm3~\rm{(syst.)}$ MeV/$c^2$, $\Gamma_{1}=431\pm36~\rm{(stat.)}\pm82~\rm{(syst.)}$ MeV/$c^2$, $m_{2}=1663\pm13~\rm{(stat.)}\pm22~\rm{(syst.)}$ MeV/$c^2$ and $\Gamma_{2}=357 \pm31~\rm{(stat.)}\pm49~\rm{(syst.)}$ MeV/$c^2$, respectively. The measured cross sections times the branching ratios are compared to recent theoretical predictions