CERN Accelerating science

Abstract Our diverse universe consists of a handful of ’elementary’ or ’fundamental’ particles. These elementary particles can be classified as (i) matter particles, the fermions and (ii) mediator particles, the bosons. There are only four distinct ways for fundamental particles to interact, (i) strong interaction (ii) electromagnetic interaction (iii) weak interaction and (iv) gravitational interaction. At present, Quantum Chromodynamics (QCD) is the best theory that we have to understand the strong force. It describes interaction between quarks and gluons (together called partons) inside a nucleon. QCD involves two very important phenomena related to strong interaction: asymptotic freedom and confinement. At high energy density or high temperature, the coupling strength of the QCD decreases and almost vanishes. Thus at high temperature, the quarks and gluons essentially behave as interaction free. This property is referred to as asymptotic freedom. On the other hand Quarks and Gluons are never observed freely or isolated but are always found in confined colourless composite particles, the confinement is an additional property required for QCD. QCD also suggests that u and d quarks must have the same mass. This symmetry is called chiral symmetry and in observed phenomenon the chiral symmetry must be broken spontaneously. To answer such questions the ideal approach would be to directly solve QCD equations, which result from the QCD Lagrangian but it is a very challenging task. But an alternative approach is to create some physical conditions in which the QCD equations are most simple. One such system is achieved when QCD matter is subjected to high temperature via heavy-ion collision. At this high temperature, Quarks and Gluons move freely in the matter and this state is referred as Quark Gluon Plasma (QGP) state. The Relativistic Heavy Ion Collider (RHIC) at Brookhaven National Laboratory (BNL) and the Large Hadron Collider (LHC) at CERN are both designed for heavy ion collision experiments. A Large Ion Collider Experiment (ALICE) at CERN is a special set of detectors designed to study QGP. Direct observation of QGP is very difficult because of its very short lifetime ($\sim10 fm/c$). It then quickly thermalises and expands to form hadron gas. Therefore an indirect method is required to look for the formation of the QGP. QCD theory provides a large number of such observables which can be used as an evidence not only to qualify but also to quantify the QGP produced and hence also a test to QCD theory. One such probe is the study of heavy flavour hadrons which are made up of at least a charm or beauty quark. The large masses of heavy flavors have several implications: (i) Heavy flavours are produced in initial hard scattering processes. Due to their small formation time ($\Delta t\sim0.1 fm/c$) as compared to the formation time of QGP ( $\Delta t\sim0.3 fm/c$) at the LHC, they go through all the stages that occur as the hot and dense medium created in heavy-ion collisions evolves over time. As a result, heavy quarks can investigate the qualities of dense matter formed early in a collision. (ii) Heavy quark thermalization is “delayed” in comparison to light quark thermalization. Heavy quarks might “thermalize” to some extent, but not totally, on a time scale comparable to the QGP lifetime. Therefore, their spectra should be significantly modified, but still retain memory about their interaction history, and, hence, represent an ”optimal” probe. (iii)RHIC and, in particular, LHC experiments enable extremely low parton momentum fractions x, when gluon saturation effects become significant. Since they have enormous masses, heavy quarks are useful tools for studying gluon saturation because charm productions can be calculated using perturbative QCD and their yield is sensitive to the initial gluon density. As a result, measuring open charm and beauty creation allows researchers to investigate QGP features and the colour charge and mass dependency of parton in-medium energy loss. The study of open charm mesons (mesons made up of one light quark and one charm quark) is a useful tool for probing pQCD theoretical models to reproduce data. In the present work, the study of open heavy flavour (D$^+$-meson) yield in pp collisions at 13 TeV as a function of charged-particle multiplicity and transverse spherocity gives insight into the mechanism influencing their production in hadronic collisions at these energies and is a tool to test the influence of Multiple Parton Interactions (MPI). Moreover, analyzing the charm production processes using pp data could help in learning the basic differences between hard and soft processes of particle production. Along with that, they also serve as a reference for the similar measurements in p-Pb and Pb-Pb collisions. This thesis is organised as follows: Chapter 1 gives a brief introduction about the field of high energy physics. The goal of high-energy physics is to study matter that interacts strongly at extremely high energy densities. Moreover, a discussion of the Standard Model of particle physics is given, which is currently the best description about the elementary particles and interactions we have. In addition to that a general introduction to the Quantum Chromodynamics (QCD) is given. QCD is a quantum field theory of the strong force. Similar to QED, quantum chromodynamics predicts the existence of force-carrying particles called gluons, which transmit strong forces between matter particles that carry "color", a strong form of "charge". A brief presentation about Quark Gluon Plasma (QGP) is also made and its phase diagram is discussed. QGP is composed of asymptotic free quarks and gluons and proof of its discovery has been published in heavy ion collision analysis experiments. A brief introduction about the Ultrarelativistic heavy-ion collisions and stages involved in its evolution will also be discussed. Relativistic collisions of heavy ions provide a unique opportunity to study the most interesting aspects of strong interactions in the laboratory. In these collisions, the energy density is so high that a transition from hadronic state to QGP state occurs. The analysis of collision involves the study of different quantities and parameters which are related to the properties of the different particles produced in the collision. These parameters and quantities are also described in this chapter. Besides that, this chapter also discusses about the experimental signatures related to the formation of QGP medium in relativistic collisions. In this chapter 2, the ALICE Experimental setup and its main characteristics is discussed. ALICE is one of the four main detector setup at the LHC at CERN. It is designed to handle very large particle production rate occuring in most central Pb-Pb collisions. Due to its very good tracking, vertexing and PID capabilities ALICE is very well suited for the open-charm studies. The main detectors used for the present analysis are the Inner Tracking System (ITS), the Time Projection Chamber (TPC) and the Time Of Flight (TOF). All these detectors and their performances will be described in detail in this chapter. Chapter 3 introduces ALICE data taking and computing framework. In ALICE the collection and processing of data is controlled by five central online systems which are (i) Data Acquisition system (DAQ) (ii) Central Trigger Processor (CTP) (iii) the High-Level Trigger (HLT) system (iv) the Detector Control System (DCS) and (v) the Experiment Control System (ECS). During the data taking period, the configuration of different detectors is controlled by the DAQ which work in collaboration with the CTP and the HLT system to select interesting physics events, provide an efficient access to these events and finally to store this data for later analysis. The DCS allows for the monitoring of the detector hardware from a central interface whereas ECS controls all these operations. The software package required for the data analysis in this work will also be discussed. The software used for this is called "AliRoot" which is an extension of "Root" and is based on the C++ language. Further, a detailed description of the simulations and reconstruction techniques of data, primary and secondary vertex reconstruction will be discussed. Chapter 4 emphasizes the importance of heavy flavour study in understanding the QCD as well as QGP. The heavy flavour have large masses as compared to the transition temperature which is around T$\sim$ 160 MeV. Due to this, heavy flavour is produced in the hard scattering processes which occur at the very early stages of the collision. The heavy flavour, therefore, observes the production of the QGP medium and evolution of QGP medium in spacetime in a heavy-ion collision. This chapter is, therefore, dedicated to the discussion of different kinds of processes involved in the production of heavy flavour in pp, pA ans AA collisions. The different observables like nuclear modification factor ($R_{AA}$) and elliptic flow variable ($v_2$) which play a significant role for understanding the production and behaviour of heavy flavour are also discussed. It also describes different effects like Cold Nuclear Matter (CNM) effects and Hot Nuclear Matter (HNM) effects that occur due the differences in the collision systems such pp, pA and AA. A summary of the results from the study of open heavy flavours in pp, pA,and AA collisions performed in different experiments like RHIC and LHC is also presented. Chapter 5 presents the analysis strategy for the D$^+$ meson self-normalized yield as a function of charged-particle multiplicity. The data used in this work has been collected by the ALICE Experiment during the Run-II of the LHC at CERN. This data is collected with pp collisions at at $\sqrt{s}$ = 13 TeV in the year 2016, 2017 and 2018. For the analysis, two different event triggers namely Minimum Bias and High-Multiplicity triggers are used. For the Minimum Bias trigger, we have around 1600 million events and the High-Multiplicity trigger gives us around 332 million events. Before the extraction of the raw yield of D$^+$ meson, the z-vertex correction has been applied. This z-vertex correction is needed because the efficiency of the detector varies along z-axis (beam-axis or longitudinal axis) and also changes over time. The z-vertex correction removes this inefficiency. After this, the D$^+$ meson candidates are reconstructed in the central pseudorapidity ($|\eta| <$1) region from their charged hadronic decay channel $D^+ \rightarrow K^- \pi^+ \pi^+$. Daughter particles are identified using PID information from TPC and TOF to reduce background at low $p_T$. Different selection criteria are applied on the angle between the reconstructed momentum and the line joining the primary and the secondary vertex. D-meson candidates were then filtered by applying kinematical and topological cuts and the D$^+$ candidates decay vertex was calculated. This gives us the invariant mass spectra. This invariant mass spectra contains both the signal as well as background. To get the raw spectra, the invariant mass spectra is fitted with gaussian function for the signal and exponential function for the background. The raw spectra has been corrected for the detector efficiency and acceptance. For this, the Monte Carlo datasets (which is anchored to the pp collision data) are used. The raw spectra is then corrected to get the corrected spectra in different intervals of event multiplicity and $p_T$. To get the self-normalized yield, the ratio of the corrected spectra in different multiplicity and $p_T$ intervals is taken with respect to the corrected spectra in the Minimum Bias sample. After the calculation of self-normalized yields, the systematic errors from different sources are calculated. These include systematics from raw yield extraction, kinematical and topological variations,Monte Carlo $p_T$ shape and feed-down subtraction. The systematics related to the raw yield extraction are calculated by multitrial approach. In this approach, we vary the different parameters like background fitting functions, lower and upper range of invariant mass spectra, fixing and non-fixing of sigma values. The ratio between the production of the mesons in a multiplicity and $p_T$ interval class over the multiplicity integrated class is calculated. The assigned systematic was estimated considering the r.m.s of the distribution of the plotted ratios. For the systematics related topological and kinematical variations, five different sets of loose and five different sets of tight cuts with respect to the standard cuts are used. The ratio of the self-normalized yields for different varied sets are calculated with respect to the self-normalized values for the standard cuts. The r.m.s of these values are assigned as the systematics. For the MC $p_T$ shape systematics, the efficiency is further calculated using $p_T$ weights. The ratio of efficiency with and without $p_T$ weights is the assigned systematic. To calculate the contribution from the beauty production, we used the $N_b$ method in which the b quark mass and the factorization and renormalization scales in the FONLL calculations were varied. The envelope of these variations was taken and assigned directly as systematic. The final results of the D$^+$-meson self-normalized yield including systematics is presented. In addition to that the results for the average D mesons (D$^0$, D$^+$, and D$^{*+}$) self-normalized yields are also calculated and these results are compared to theoretical models and other heavy flavour measurements. In Chapter 6 the analysis strategy for the D$^+$- meson self-normalized yield as a function of transverse spherocity is discussed. The data sample used for this analysis is the same Minimum Bias data that has been used for the analysis in previous chapter. In this case, additional sets of cuts related the spherocity calculations are used. Here the strategy of D$^+$ meson reconstruction in pp collisions and the details of cut selection strategy, signal extraction, particle identification and data quality selections will be discussed. The spectra is plotted in different classes of multiplicity, transverse spherocity and $p_T$. The detailed discussion will be made on the self-normalized yield measurements and the description of systematic uncertainties affecting our results. The sources of systematics and their calculations are same as in the case of analysis of D$^+$ meson as a function of charged-particle multiplicity. The final results for the self-normalized yields including the systematics values are presented as a function of transverse spherocity in different intervals of tracklet multiplicity and $p_T$. In chapter 7, the results presented in chapter 5 and 6 are summarized. The physics conclusion for the whole work done pertaining to the thesis will be given.