Measurement of D0 production in Proton-Proton Collisions at s = 7 TeV with the ALICE Detector

Driven by the curiosity of basic structure of the material world, from the beginning of Daltons atomic theory in 1808, scientists have been established to explore the microscopic world. Later, scientists found that atom consists of atomic nuclei and electrons. Until the inelastic electron experiments, scientists realized that the nucleus have basic structure, that is, quarks and gluons. So far there have not found much smaller structure in electron. The Standard Model of particle physics is the best tool, currently, to describe the fundamental structure of matter and the fundamental interaction forces among them. The basic components of this theory are three types of particles: leptons, quarks and gauge bosons. There are six different leptons and the corresponding antileptons, six different quarks and their corresponding antiquarks and four types of gauge bosons as the force-carrying particles, which mediate the fundamental interaction force and can be grouped to the electromagnetic, weak, and strong interactions. The electroweak interactions is described by the Yang-Mills gauge theory. While the quantum chromodynamics (QCD) is a theory of the strong interaction (color force), a fundamental force describing the interactions of the quarks and gluons. According to the quantum chromodynamics (QCD), a deconfined quark-gluon plasma (QGP) will be formed at extremely high temperature and/or density. This deconfined phase (QGP) consists of free quarks and gluons that compose hadronic matter. Quarks are confined in hadronic matter, but quarks are deconfined in the QGP. In nature, the QGP probably have existed in the first few microseconds after the Big-Bang and still exists in the cores of heavy neutron stars. Fortunately, high energy heavy ion collisions provide a unique opportunity to study the properties of such deconfined QGP and the transition is expected to occur at a temperature of about 175 MeV and an energy density of 0.7 GeV/fm3. Super Proton Synchrotron (SPS) experiments at CERN first tried to create the QGP using Pb-Pb collisions (√ sNN = 17.6 GeV). Relativistic Heavy Ion Collider (RHIC) keeps this effort by Au-Au collisions at Brookhaven National Laboratory at √ sNN = 200 GeV. Several indirect evidences for a ‘new state of matter’ (QGP) were announced, for example: collective flow, jet quenching and J/ψ depression, etc. A Large Ion Collider Experiment (ALICE) is one of the four experiments at Large Hadron Collider (LHC) the biggest accelerator in the world at the moment. ALICE has been carrying on the experimental heavy-ion program by SPS and RHIC from 2010. The main target of ALICE is the study the heavy-ion collisions at the center-of-mass energy of 5.5 TeV per nucleon with lead and study the properties of the hadronic matter at the extremely high energy densities. In this thesis one focus on charm physics measured with ALICE experiment. This is because heavy-quarks (charm and bottom) provide a reliable tool to probe the dynamic properties of the collision system evolution. Heavy quarks are characterized by early production which takes place on the timescale of the order of 1/mQ according to the pQCD. Thus, their production kinematics is not influenced by medium effects and due to the long decay length they undergo the thermalization phase of the quark-gluon plasma. They interact strongly with the hot and dense matter produced in heavy-ion collisions and lose energy when they transverse the medium. It provides the system evolution dynamic information to measure some typical observables. The physics framework is discussed in chapter 1 and chapter 2, where one summarize the status of the experimental studies of deconfinement in heavy-ion collisions and present how charm particles can serve as probes of deconfined matter (QGP). The ALICE experimental framework is described in Chapter 3, along with layout, main sub-systems and their expected performance. The main studies of this thesis are summarized in the following two parts. • The track impact parameter, defined as the distance of closest approach of the particle trajectory to the primary vertex (see Fig.4.1).The track impact parameter is a critical variable for the separation of physics signals from backgrounds, especially for the selection of physics signals which are characterized by the secondary vertex with a small displacement from the primary vertex. This is, in particular, the case for the detection of particles with open charm and open beauty, namely D0 (cτ ∼= 123μm), D+ (τ ∼= 315μm) and B mesons (cτ ∼ 500μm), and so on. The main requirement applied for the selection of such particles is the presence of one or more daughter tracks (decay products) which are displaced from the primary vertex (e.g. for D0 → K−π+ two displaced tracks are required, for B → e± +X one electron-tagged displaced track is required. How to select the fit function and define the fit range is the subject of section 4.1. In this section the particle track impact parameter distribution and fitting procedure are introduced. The final measured particles mainly come from two different parts. Particles coming from the primary vertex have an impact parameter distribution with gaussian shape. Particles coming from weak decay have an exponential distribution of impact parameter, as is the case for particles scattered from the detector materials. So, the fit function, combined by gaussian with exponential tail, was used as the analysis tool and extract the impact parameter resolution. Section 4.2 focuses on the cause which affect the impact parameter resolution. The main effects on impact parameter resolution are discussed, including primary vertex selection and diamond constraint, small-angle multiple scattering and particle species (particle identification). The resolution of track impact parameter is the convolution of the resolution of primary vertex with that of tracks. The primary vertex and the variables associated to the tracks will affect the impact parameter resolution. For the primary vertex, one mainly discuss two aspects: the ‘diamond constraint’ on primary vertex distribution and the effect of current track on the primary vertex. The emitted particles with small transverse momenta will be deflected by many small-angle scatterings (Coulomb scattering) when the particles traverse the beam pipe, detectors and equipments. The track impact parameter resolution contributed by the uncertainty of the track fit can be regarded as a sum of spatial precision of tracking detectors and multiple scattering. The formula on impact parameter resolution distribution with polar angle was given out, see text for detail. Within the error range, the result of ESDPID is agreement with that of PDGPID. The resolution distribution for different kinds of particle have the same trend which is larger at low pt than at high pt and have clear mass order at low pt. The value of resolution for protons is the biggest one among three kinds of particle, kaon comes second and it is the smallest for pion at the same pt. Because the proton has larger mass, so it will undergo more multiple scattering when it traverse the beam pipe, detector and support equipment. Finally, one consider the different selection conditions affecting the impact parameter resolution, as well as magnet and charge effects on the resolution and mean of impact parameter. The barrel detectors in ALICE are embedded in a large solenoidal magnet providing a magnetic field < 0.5 T in positive and negative value, and they allow to reconstruct track in the pseudorapidity range |η| < 0.9. So, the magnetic field and the particle charge will affect the impact parameter resolution and mean. • ALICE is a general-purpose heavy-ion experiment designed to study the physics of strongly interacting matter and the quark gluon-plasma in nucleus-nucleus collisions at the LHC. The measurement of open charm and open beauty production allows one to investigate the mechanisms of heavy-quark production, propagation and, at low momenta, hadronisation in the hot and dense medium formed in high-energy nucleus-nucleus collisions. It is an important task in ALICE to measure charm production via the exclusive reconstruction of selected D meson decay channels at central rapidity. The measurement of the cross-section for charm production in p-p collisions is not only a fundamental reference to investigate medium properties in heavy-ion collisions, but an key test of pQCD predictions in a new energy domain as well. In chapter 5, the analysis procedure and the final D0 cross-section for the D0 → K−π+ channel are presented. First, the analysis strategy is recalled, as well as the detailed steps of analysis are given according to the analysis strategy, see in section 5.1. In p-p collisions, if all the possible pairs are considered as ‘candidate’ D0, the signal over combinatorial background ratio is ∼ 10−4. It is then mandatory to preselect the reconstructed tracks and candidates on the basis of the typical kinematical and geometrical properties characterizing the signal tracks and reconstructed vertices. Beside two kinds of variables : single track variables and pair variables, particle identification, in particular for the charged kaon, is applied for background rejection and improving the ration of signal-to-background, see detail in section 5.2. Then, the pt-differential cross sections for prompt D0 at LHC √ s = 7 GeV, obtained from the yields extracted by fitting the invariant mass spectra. The fit function used to reproduce the invariant mass distributions is the sum of a Gaussian for D0 peak and an exponential or second order polynomial for the background. The amount of signal and background is then extracted by subtraction of the background fit from the total or by counting the excess of entries in the histogram with respect to the background function, see in section 5.3. In order to evaluate the total number of D0 mesons effectively produced and decayed in the D0 → K−π+ channel, (ND0→K π+ tot ) the raw signal yield is divided by an efficiency correction factor (ϵ) that accounts for selection cuts, for PID efficiency, for track and primary vertex reconstruction efficiency, and for the detector acceptance. The procedure and the tools used to compute the efficiency corrections is the subject of section 5.4. At LHC energies, a relevant fragmentation fraction of D0 mesons comes from the decay of B mesons. On average, the reconstructed tracks coming from ‘secondary’ D0 are well displaced from the primary vertex, because of the relatively long B lifetime (cτ ≃ 460-490 μm). Thus, the selection further enhances their contribution to the raw signal yield (up to 15%) and it is important to subtract this fraction. To determine its amount different methods are available and will be detailed in the text. The best way is to extract it directly from data exploiting the different shapes of the impact parameter distribution of secondary D0, but this requires large statistics. Alternatively, or as a cross check it is possible to rely on Monte Carlo estimates based on pQCD calculations, but this can add a bias to the measurement, or on the measurement of beauty production at the LHC, see detail in section 5.5. In the section 5.6, the raw yield, corrected for the efficiency, is divided by the decay channel branching ratio (BR(D0 → K−π+) = 3.80 ± 0.09%) to get the total number of produced D0 mesons ND0 tot . The latter number is divided by the integrated luminosity LINT to obtain the cross section for D0 meson production. A factor 1/2 must be considered because both D0 and ¯D0 mesons are reconstructed and a factor 1/(2 ymax) because the measurement is performed in the rapidity range −ymax < y < +ymax. Several sources of systematic uncertainties were considered, namely those affecting the signal extraction from the invariant mass spectra, as well as the statistical uncertainties, the detail see section 5.7. Finally, the measured D0 meson production cross sections are compared to two theoretical predictions, namely FONLL and GM-VFNS. Our measurement of D0 at LHC energies are reproduced by both models within their theoretical uncertainties.

L’argomento di questa tesi è la misura della produzione di charm in collisioni protone-protone a LHC con l’esperimento ALICE. Lo scopo dell’esperimento è lo studio dello stato della materia di QCD che si forma in collisioni nucleo-nucleo ad alta energia. I quark pesanti (charm e beauty) sono ritenuti delle sonde privilegiate per lo studio delle proprietà di questo stato della materia, attraverso il confronto delle loro quantità di produzione di interazioni piombo-piombo e protone-protone. I primi due capitoli della tesi introducono le motivazioni per la misura. In seguito, viene descritto l’esperimento ALICE con i suoi rivelatori e le loro prestazioni. Le attività svolte durante la tesi sono presentate negli ultimi due capitoli: la misura della risoluzione in parametro d’impatto delle tracce ricostruite; la misura della sezione d’urto di produzione del mesone D0 in collisioni pp all’energia di 7 TeV nel centro di massa. Il parametro d’impatto di una traccia è definito come la distanza tra la traccia e il vertice primario dell’interazione pp. Questa quantità è cruciale per la separazione di vertici di decadimento secondari di particelle con charm e beauty, come i mesoni D. La risoluzione, determinata principalmente dal rivelatore a pixel di silicio (il più preciso e più vicino al punto di interazione), è stata misurata attraverso un metodo appositamente sviluppato ed è risultata coerente con le prestazioni attese. La produzione di charm in collisioni pp è stata misurata ricostruendo i decadimenti adronici del mesone D0 (D0-> K-pi+). La selezione del segnale rispetto al fondo combinatorio utilizza l’identificazione del kaone e di tagli geometrici legati alla separazione del vertice di decadimento dal vertice primario di interazione. La sezione d’urto di produzione è stata misurata nell’intervallo da 2 a 12 GeV/c di momento trasverso. I calcoli di QCD perturbativa descrivono bene i valori sperimentali. La misura ha fornito il riferimento per la prima valutazione preliminare degli effetti della materia di QCD sulla produzione di charm in collisioni nucleo-nucleo.