Vertexing detectors and vertexing performance in Run 2 in ALICE

The ALICE apparatus includes an Inner Tracking System for high-precision vertexing and tracking at midrapidity. Thanks to this detector, remarkable results were obtained with Run 2 data in all collision systems studied at the LHC. In this contribution, the role of the ITS in the event, track and secondary vertex reconstruction is highlighted, and the comparison of the resolution on the track impact parameter to the collision point in data and Monte Carlo simulations is reported. Finally, some of the main physics results obtained in pp and Pb--Pb collisions relying on the excellent ALICE vertexing and tracking capabilities are presented

Misura del rapporto Λ/K⁰ in collisioni piombo-piombo a LHC con ALICE

Il lavoro proposto consiste nell'analizzare un campione di dati presi recentemente all'acceleratore LHC dall'esperimento ALICE per estrarre una grandezza caratteristica della formazione del plasma di quark e gluoni (QGP) nelle collisioni tra ioni pesanti a energie ultrarelativistiche. Obiettivo finale: misura del rapporto di produzione tra barioni strani (Λ) e mesoni strani (K⁰S) in differenti classi di centralità della collisione

Misura di elettroni da decadimenti di adroni pesanti e produzione di barioni pesanti con l'esperimento ALICE a LHC

The ALICE experiment at the LHC at CERN is devoted to the study of collisions of heavy nuclei accelerated at ultra-relativistic energies. These collisions are employed to reproduce in laboratory the quark-gluon plasma (QGP), a deconfined state of the strongly-interacting matter. %characterised by the quark and gluon degrees of freedom. The existence of this state is predicted by the QCD theory under extreme conditions of energy-density and temperature. The QGP is supposed to constitute the early universe in the first $\sim 10$ \mjs after the Big Bang. Charm and beauty quarks are unique probes for investigating the QGP. Given a mass of the GeV order, they are produced in the hard-scattering processes in the early stages of the nucleus-nucleus collision, experiencing the full evolution of the system. Charm and beauty quarks lose energy by interacting with the plasma constituents. These phenomena can be studied by measuring the heavy-flavour hadron production and exploited to infer properties of the system. In this thesis, the production of electrons from charm and beauty hadron decays in central (0--10\%), semicentral (30--50\%) and peripheral (60--80\%) Pb--Pb collisions at \sNNsqrt=5.02 TeV measured with the ALICE experiment is presented. These results are compared with those in pp collisions by means of the nuclear modification factor ($R_{\rm AA}$) and a significant suppression with respect to what expected in absence of a deconfined medium is observed in central and semicentral events. This behaviour indicates that charm and beauty quarks are subject to in-medium energy loss. The observed effect decreases in more peripheral events. The measurement is pushed down to $p_{\rm T}^{\rm e^-}=500$ MeV/$c$, where the heavy quark production is sensitive to the shadowing effects. The $R_{\rm AA}$ does not overcome unity, signalling that the production of heavy-flavour hadrons is suppressed. The QGP formation in heavy-ion collisions is expected to induce a modification of the heavy quark hadronisation mechanisms. In order to disentangle the effects of the medium produced in Pb--Pb collisions, a deep comprehension of the mechanisms that govern the hadronisation in pp collisions is required. Recent results on baryon-to-meson production ratios in pp collisions at the LHC showed an enhancement with respect to $\rm e^+e^-$ and $\rm e^-p$ collisions. %, indicating that the hadronisation via fragmentation is not enough. In this thesis, the measurement of the $\Lambda_{\rm c}^+$ and $\Sigma_{\rm c}^{0,++}$ baryon production cross section in pp collisions at $\sqrt{s}=13$ TeV with the ALICE experiment is described. The ratio to the $\rm D^0$ meson production of both baryons is significantly higher than what measured in $\rm e^+e^-$ and $\rm e^-p$ collisions. The measurements are described by several model calculations assuming different mechanisms for the charm quark hadronisation. Moreover, the first measurement of the prompt $\Lambda_{\rm c}^+$ feed-down from $\Sigma_{\rm c}^{0,+,++}$ decays in pp collisions is presented and observed to be $\sim 2$ times larger than $\rm e^+e^-$ collisions in the interval $2<p_{\rm T}<12$ GeV/$c$. The investigation of the charm quark hadronisation mechanisms in hadronic collisions can significantly benefit from production measurements of more charm baryons. In the last Chapter, a few studies on the capabilities to perform such measurements in the future with the ALICE experiment are discussed. New frontiers in the heavy-flavour hadron production measurements will be allowed by the significantly higher statistics that the experiment will collect in the upcoming years, as well as the improved pointing resolution provided by the upgraded ITS detector.The ALICE experiment at the LHC at CERN is devoted to the study of collisions of heavy nuclei accelerated at ultra-relativistic energies. These collisions are employed to reproduce in laboratory the quark-gluon plasma (QGP), a deconfined state of the strongly-interacting matter. %characterised by the quark and gluon degrees of freedom. The existence of this state is predicted by the QCD theory under extreme conditions of energy-density and temperature. The QGP is supposed to constitute the early universe in the first $\sim 10$ \mjs after the Big Bang. Charm and beauty quarks are unique probes for investigating the QGP. Given a mass of the GeV order, they are produced in the hard-scattering processes in the early stages of the nucleus-nucleus collision, experiencing the full evolution of the system. Charm and beauty quarks lose energy by interacting with the plasma constituents. These phenomena can be studied by measuring the heavy-flavour hadron production and exploited to infer properties of the system. In this thesis, the production of electrons from charm and beauty hadron decays in central (0--10\%), semicentral (30--50\%) and peripheral (60--80\%) Pb--Pb collisions at \sNNsqrt=5.02 TeV measured with the ALICE experiment is presented. These results are compared with those in pp collisions by means of the nuclear modification factor ($R_{\rm AA}$) and a significant suppression with respect to what expected in absence of a deconfined medium is observed in central and semicentral events. This behaviour indicates that charm and beauty quarks are subject to in-medium energy loss. The observed effect decreases in more peripheral events. The measurement is pushed down to $p_{\rm T}^{\rm e^-}=500$ MeV/$c$, where the heavy quark production is sensitive to the shadowing effects. The $R_{\rm AA}$ does not overcome unity, signalling that the production of heavy-flavour hadrons is suppressed. The QGP formation in heavy-ion collisions is expected to induce a modification of the heavy quark hadronisation mechanisms. In order to disentangle the effects of the medium produced in Pb--Pb collisions, a deep comprehension of the mechanisms that govern the hadronisation in pp collisions is required. Recent results on baryon-to-meson production ratios in pp collisions at the LHC showed an enhancement with respect to $\rm e^+e^-$ and $\rm e^-p$ collisions. %, indicating that the hadronisation via fragmentation is not enough. In this thesis, the measurement of the $\Lambda_{\rm c}^+$ and $\Sigma_{\rm c}^{0,++}$ baryon production cross section in pp collisions at $\sqrt{s}=13$ TeV with the ALICE experiment is described. The ratio to the $\rm D^0$ meson production of both baryons is significantly higher than what measured in $\rm e^+e^-$ and $\rm e^-p$ collisions. The measurements are described by several model calculations assuming different mechanisms for the charm quark hadronisation. Moreover, the first measurement of the prompt $\Lambda_{\rm c}^+$ feed-down from $\Sigma_{\rm c}^{0,+,++}$ decays in pp collisions is presented and observed to be $\sim 2$ times larger than $\rm e^+e^-$ collisions in the interval $2<p_{\rm T}<12$ GeV/$c$. The investigation of the charm quark hadronisation mechanisms in hadronic collisions can significantly benefit from production measurements of more charm baryons. In the last Chapter, a few studies on the capabilities to perform such measurements in the future with the ALICE experiment are discussed. New frontiers in the heavy-flavour hadron production measurements will be allowed by the significantly higher statistics that the experiment will collect in the upcoming years, as well as the improved pointing resolution provided by the upgraded ITS detector

Constraining Hadronization Processes with Charm Baryons in pppp and pp–Pb Collisions with ALICE

In this contribution, we present the latest measurements of D0, D+, and D+ s mesons together with the final measurements of Λ+c , Ξ0,+ c , Σ0,++ c , and the first measurement of Ω0 c baryons performed with the ALICE detector at midrapidity in pp collisions at √ s = 5.02 and √ s = 13 TeV. Recent measurements of charm-baryon production at midrapidity in small systems show a baryon-to-meson ratio significantly higher than that in e+e− and e−p collisions, suggesting that the fragmentation of charm is not universal across different collision systems. Thus, measurements of charm-baryon production are crucial to study the charm-quark hadronization in a partonrich environment like the one produced in pp collisions at the LHC energies. Furthermore, the recent Λ+c /D0 yield ratio, measured down to pT = 0 in p–Pb collisions will be discussed. The measurement of charm baryons in p-nucleus collisions provides important information about a possible additional modification of hadronization mechanisms, on cold nuclear matter effects, and on the possible presence of collective effects that could modify the production of heavy-flavour hadrons. Finally, the first measurements of charm fragmentation fractions and charm production cross section at midrapidity per unit of rapidity will be shown for pp and p–Pb collisions using all measured single-charm ground-state hadrons

Measurement of heavy-flavour decay electrons and heavy-flavour baryon production with ALICE experiment at LHC

The ALICE experiment at the LHC at CERN is devoted to the study of collisions of heavy nuclei accelerated at ultra-relativistic energies. These collisions are employed to reproduce in laboratory the quark-gluon plasma (QGP), a deconfined state of the strongly-interacting matter. The existence of this state is predicted by the QCD theory under extreme conditions of energy-density and temperature. The QGP is supposed to constitute the early universe in the first $\sim 10$ $\mu$s after the Big Bang. Charm and beauty quarks are unique probes for investigating the QGP. Given a mass of the GeV order, they are produced in the hard-scattering processes in the early stages of the nucleus-nucleus collision, experiencing the full evolution of the system. Charm and beauty quarks lose energy by interacting with the plasma constituents. These phenomena can be studied by measuring the heavy-flavour hadron production and exploited to infer properties of the system. In this thesis, the production of electrons from charm and beauty hadron decays in central (0-10%), semicentral (30-50%) and peripheral (60-80%) Pb-Pb collisions at $\sqrt{s_{\rm NN}}=5.02$ TeV measured with the ALICE experiment is presented. These results are compared with those in pp collisions by means of the nuclear modification factor ($R_{\rm AA}$) and a significant suppression with respect to what expected in absence of a deconfined medium is observed in central and semicentral events. This behaviour indicates that charm and beauty quarks are subject to in-medium energy loss. The observed effect decreases in more peripheral events. The measurement is pushed down to $p_{\rm T}^{\rm e^-}=500$ MeV/$c$, where the heavy quark production is sensitive to the shadowing effects. The $R_{\rm AA}$ does not overcome unity, signalling that the production of heavy-flavour hadrons is suppressed. The QGP formation in heavy-ion collisions is expected to induce a modification of the heavy quark hadronisation mechanisms. In order to disentangle the effects of the medium produced in Pb-Pb collisions, a deep comprehension of the mechanisms that govern the hadronisation in pp collisions is required. Recent results on baryon-to-meson production ratios in pp collisions at the LHC showed an enhancement with respect to $\rm e^+e^-$ and $\rm e^-p$ collisions. In this thesis, the measurement of the $\Lambda_{\rm c}^{+}$ and $\Sigma_{\rm c}^{0.++}$ baryon production cross section in pp collisions at $\sqrt{s}=13$ TeV with the ALICE experiment is described. The ratio to the $\rm D^{0}$ meson production of both baryons is significantly higher than what measured in $\rm e^+e^-$ and $\rm e^-p$ collisions. The measurements are described by several model calculations assuming different mechanisms for the charm quark hadronisation. Moreover, the first measurement of the prompt $\Lambda_{\rm c}^{+}$ feed-down from $\Sigma_{\rm c}^{0,+,++}$ decays in pp collisions is presented and observed to be $\sim 2$ times larger than $\rm e^+e^-$ collisions in the interval $2 < p_{\rm T} < 12$ GeV/$c$. The investigation of the charm quark hadronisation mechanisms in hadronic collisions can significantly benefit from production measurements of more charm baryons. In the last Chapter, a few studies on the capabilities to perform such measurements in the future with the ALICE experiment are discussed. New frontiers in the heavy-flavour hadron production measurements will be allowed by the significantly higher statistics that the experiment will collect in the upcoming years, as well as the improved pointing resolution provided by the upgraded ITS detector

Studies on the hadronization of charm and beauty quarks

In this contribution, the latest results on hadronization studies of charm and beauty quarks obtained with the data collected with the ALICE experiment at the LHC are presented. Measurements of prompt and non-prompt charm-hadron production in pp, p-Pb and Pb-Pb are shown. The results are also compared with theoretical models that consider different implementations of the heavy-quark hadronization across collision systems

Misura di elettroni da decadimenti di adroni con charm e beauty in collisioni Pb-Pb a LHC con ALICE

ALICE (A Large Ion Collider Experiment) è uno dei quattro maggiori esperimenti all'acceleratore LHC (Large Hadron Collider) del CERN. Esso è specializzato nello studio del Quark-Gluon Plasma (QGP), lo stato della materia che ha caratterizzato l'universo nei primi microsecondi dopo il Big Bang, in cui i quark e i gluoni non sono confinati all'interno di adroni, bensì liberi. Considerato che nella materia ordinaria non ci sono le condizioni per formare questo stato esotico, l'esperimento sfrutta le collisioni tra ioni pesanti, in particolare Pb-Pb, tramite le quali si riescono a ricreare in un piccolo volume le densità di energia e le temperature tipiche del QGP. A causa della loro massa elevata (m_c≈1.5 GeV/c^2, m_b≈4.8 GeV/c^2), in questi processi i quark charm e beauty vengono prodotti principalmente nelle collisioni partoniche iniziali in cui avviene un grande trasferimento di impulso, le quali hanno tempi caratteristici inferiori rispetto a quelli di formazione del mezzo deconfinato (∼fs). Di conseguenza, essi preesistono alla formazione del QGP e poi interagiscono con esso. Attraverso lo studio delle proprietà fisiche degli adroni con quark pesanti, è possibile quindi studiare alcune proprietà del sistema interagente, in particolare le caratteristiche dei meccanismi di perdita di energia nel mezzo e l'eventuale termalizzazione dei partoni. In questo lavoro si presenta un'analisi di adroni contenenti charm e beauty attraverso canali di decadimento semi-elettronico in collisioni Pb-Pb registrate nel 2015 all'energia nel centro di massa per coppia di nucleoni di √(s_NN )=5.02 TeV con ALICE. Gli elettroni di tali decadimenti vengono individuati tramite selezioni sulle tracce (distanza della traccia dal vertice primario, numero di cluster nei rivelatori, …) e identificazione delle particelle (PID) sfruttando i segnali dei rivelatori TPC, ITS e TOF con cui, in particolare, si distinguono gli elettroni dalla contaminazione dovuta agli adroni. Il principale contributo di fondo allo spettro in impulso trasverso (p_T) degli elettroni da adroni con heavy-flavour è dovuto agli elettroni derivanti da conversione di fotoni γ→e^+ e^- e da decadimenti Dalitz dei mesoni leggeri π^0,η→e^+ e^- γ, individuati attraverso un'analisi di massa invariante. Infine, una volta applicate le correzioni sperimentali (efficienza di ricostruzione, accettanza angolare, …) si ottiene lo spettro in impulso trasverso degli elettroni derivanti da decadimenti di adroni con heavy-flavour, quindi si stima il fattore di modificazione nucleare R_AA, definito come il rapporto tra dN_AA/dp_T e la sezione d'urto misurata in collisioni protone-protone dσ_pp/dp_T, normalizzato per la funzione di sovrapposizione nucleare T_AA. Nel caso di eventi Pb-Pb il fattore di modificazione nucleare devia dall'unità, dimostrando che i quark pesanti interagiscono con il QGP, perdendo energia per irraggiamento di gluoni e attraverso collisioni elastiche con i partoni liberi costituenti

Heavy-flavour hadron production

The conventional description of heavy-flavour hadron production in pp collisions is based on a factorisation approach, assuming universal fragmentation functions among collision systems. Recent results on heavy-flavour baryon measurements from the LHC experiments show tensions with model calculations based on this approach and employing fragmentation functions constrained from $\rm e^+e^-$ and $\rm e^-p$ collision experiments. In this contribution, the most recent results from ALICE, ATLAS, CMS and LHCb experiments on the heavy-flavour hadron production in pp collisions at the TeV scale are reported. The comparison with the theoretical predictions that address the baryon enhancement in hadronic collisions at the LHC is also discussed.The conventional description of heavy-flavour hadron production in pp collisions is based on a factorisation approach, assuming universal fragmentation functions among collision systems. Recent results on heavy-flavour baryon measurements from the LHC experiments show tensions with model calculations based on this approach and employing fragmentation functions constrained from $\rm e^+e^-$ and $\rm e^-p$ collision experiments. In this contribution, the most recent results from ALICE, ATLAS, CMS and LHCb experiments on the heavy-flavour hadron production in pp collisions at the TeV scale are reported. The comparison with the theoretical predictions that address the baryon enhancement in hadronic collisions at the LHC is also discussed