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Charming Charm, Beautiful Bottom, and Quark-Gluon Plasma in the Large Hadron Collider Era

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Charming Charm, Beautiful Bottom, and Quark-Gluon Plasma in the Large Hadron Collider Era Charming Charm, Beautiful Bottom, and Quark-Gluon Plasma in the Large Hadron Collider Era Santosh K. Das1 and Raghunath Sahoo2 The primordial matter of quarks and gluons, which filled our universe just after few micro-seconds of its creation through Big Bang, is expected to be created in the laboratory by colliding nuclei at relativistic energies. The ongoing nuclear collision programs at the Relativistic Heavy Ion Collider (RHIC) and the Large Hadron Collider (LHC) are two experimental facilities, where matter in the state of Quark-Gluon Plasma (QGP) can be created and characterized. Heavy quarks, mainly charm and bottom quarks, are considered as novel probes to characterize QGP, and hence the QCD matter. Heavy quark diffusion coefficients play a significant role to understand the properties of QCD matter. Experimental measurements of nuclear suppression factor and elliptic flow are able to constrain the heavy quark diffusion coefficients, which is a key ingredient for the phenomenological study and disentangle different energy loss models. We give a general perspective of heavy quark diffusion coefficient in QGP and discuss its potential as a probe to disentangle different hadronization mechanisms, as well as to probe the initial electromagnetic field produced in non-central collisions. Experimental perspective on future measurements are discussed with special emphasis on heavy-flavors as next generation probes. Key Words: Big Bang, quark gluon plasma, heavy-ion collisions, heavy flavors The second half of twentieth century was a energy interactions in cosmic rays and tremendous success in particle physics accelerators. Although the free existence of through the discovery of Quark Model of these fundamental particles like quarks and hadrons by Murray Gell-Mann and George the force carrier of strong interaction- the Zweig, the Standard Model of Particle gluons is not observed because of Physics by Glashow, Salam and Weinberg confinement property of strong interaction, (and many others) through unification of the quest to create a matter with partonic fundamental forces. The endeavours in degrees of freedom has been a frontier of basic science, while looking for the research since decades. For an illustration, fundamental constituents of matter, have Fig. 1 shows the confinement of quarks contributed immensely to the developments inside hadrons and quark matter consisting in particle detection and accelerator of deconfined colored quarks and gluons. technologies, which have given direct and indirect social benefits of colossal nature. Theoretical calculations based on lattice As the present day understanding of QCD (lQCD) predict that hadronic matter constituents of matter, we have 6-quarks, 6- at high temperatures and densities dissolves leptons, their antiparticles and the force into a deconfined state of quarks and gluons carriers. However, among these, we only - called Quark Gluon Plasma (QGP)1. In encounter the light quarks – up and down, this state, hadrons like protons and and electrons in normal nuclear matter. neutrons, which form the normal matter Other heavy particles are created in high- around us, lose their identity and their 1 Santosh K. Das is at Indian Institute of Technology Goa, India, Email: [email protected] 2 Raghunath Sahoo, FInstP is at Indian Institute of Technology Indore, India, Email: [email protected] (Corresponding Author) 1 constituents -- quarks and gluons, can move up, down and strange quarks, which are freely in this plasma. The major goals of the taken as light quarks (flavors). Although ongoing nucleus-nucleus collision charm, bottom and top are considered as programmes at Relativistic Heavy Ion heavy-quarks, because of huge mass of top Collider (RHIC) at the Brookhaven National Laboratory, USA and the Large quark and its low production cross section, Hadron Collider (LHC) at CERN, it is not considered for the present Switzerland at Giga and Terra electron volt discussions. energies are to create and characterize QGP in the laboratory. This state of matter is found to behave like a nearly perfect fluid having a remarkably small value of shear viscosity to entropy density ratio. QGP is believed to be the primordial matter formed at the infancy of our universe just after few micro-seconds of the creation of the universe through Big Bang. This primordial matter of quarks and gluons, because of large concentration of energy and high temperature, expands with time (like an Fig.1: Pictorial representation of expanding fireball) through various confinement of quarks inside hadrons complex processes. These processes (proton, neutron etc.) and the deconfined include, formation of composite hadrons quark matter called quark-gluon plasma. 25 out of quarks and gluons, called Figure courtesy . hadronization (corresponding to a system temperature, which marks the quark-hadron phase transition: deconfinement transition); chemical freezeout, where the particle abundancies are frozen (inelastic collision stops) and kinetic freeze-out (ceasing of elastic collisions) followed by free streaming of the secondary particles to reach the detectors sitting all around the interaction point of the nuclear collisions in the experimental area. The study of the properties of QGP is a field of high contemporary interest and remarkable progress has been made in the last decades Fig.2: Pictorial representation of three towards the understanding of the properties generations of quarks and their masses. of strongly interacting Quark Gluon Plasma. In this article, we shall focus on Heavy quarks2,3,4, mainly charm and heavy flavour particles as a powerful probe bottom quarks, thanks to their large masses, of this primordial matter and their enriched are considered as novel probes to measurements at the LHC energies in view characterize the properties of QGP phase. of the upgrade programmes. First of all they are created in hard processes which are accessible to A pictorial representation of three perturbative QCD (pQCD) calculations and generations of quarks and their masses are therefore, their initial distribution is theoretically known and could be verified given in Fig. 2 for illustration. The masses by experiments. They are produced in the of charm and bottom quarks are higher than 2 early stages of the collisions and witness the element. The heavy quarks drag, and entire space-time evolution of QGP phase momentum diffusion coefficients are and can act as an effective probe of the related through the fluctuation-dissipation created matter. The spectra of the heavy theorem (FDT), Dp=M γ T. Here T is the quarks are too hard to come to equilibrium temperature of the thermal bath and M is with the QGP phase during the expansion. the HQ mass. The spatial diffusion Therefore, the heavy quark momentum coefficient, Ds, can be calculated from the spectra carry the information of their diffusion coefficients in momentum space 2 interaction with the plasma constituents in the static limit (p → 0): Ds=T /Dp. The during the expansion and hence, the plasma major aim of all the theoretical studies are properties. Heavy quarks (HQ), as colored to extract the heavy quark spatial diffusion objects, interact strongly with the plasma coefficients (Ds), which quantify the constituents and their momentum spectra interaction of heavy quarks with the are strongly modified. This is expressed by medium, that is directly related to the 5,6 the nuclear suppression factor , RAA(pT), thermalization time and can be evaluated the ratio of the measured heavy quarks using lattice QCD. The Ds, Ds=T/M* γ (in momentum spectra (yields) in nucleus- the limit of momentum, p → 0), is directly nucleus to proton-proton collisions, related to the heavy quark drag coefficient rescaled by the number of binary collisions. γ. Drag coefficient appearing in Ds is If there were no interactions with the QGP expected to scale with the mass, hence, (as well as no cold nuclear matter effects provides a measure of the QCD interaction like shadowing) one would indeed find RAA ideally independent of the mass of the = 1. Another observable is the elliptic quark. One can estimate the heavy quark 7,8 -1 flow , v2(pT ) = < cos(2Φ) >. It is a thermalization time, τth= γ (p→0) by: measure of the anisotropy in the angular 2 distribution of the produced heavy mesons τth=M/2πT (2 π TDs) as a response to the initial anisotropy in coordinate space in non-central nucleus- A simultaneous description of both RAA(pT) nucleus collisions, transferred by the bulk and v2(pT) is a top challenge to all the to the heavy quarks. theoretical models and is known as “heavy quark puzzle”. Several theoretical efforts have been made to study RAA and v2 of the heavy quarks Before the first experimental results, it was measured in experiments within the Fokker expected that HQ interaction with the QGP Planck (FP) approach and the relativistic medium could be characterized by means of Boltzmann approach. Heavy quarks drag perturbative QCD, which led to the (γ) and momentum diffusion coefficients expectations of a smaller modification of (Dp) are the key inputs, to solve the FP their spectra and a small v2. However, the equation to study the momentum evolution first observations of non-photonic electrons of heavy quarks in QGP medium to coming from heavy meson decays compute its RAA and v2, which contain the measured at the highest RHIC energy have microscopic details mentioning how the shown a surprisingly large modification of heavy quarks interact with the hot QGP their spectra and a quite large v2, indicating medium. Heavy quark drag coefficient is quite strong interactions between HQ and responsible for the heavy quark energy loss the QGP medium. This is substantially and diffusion coefficient is responsible for beyond the expectations from perturbative its momentum broadening. The drag and QCD interactions. These observations diffusion coefficients of the heavy quarks triggered several studies in which non- can be computed starting from the heavy perturbative effects have been quark - light quark scattering matrix incorporated.
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