Jet Production In Heavy Ion Collisions Research Articles

Deep learning assisted jet tomography for the study of Mach cones in QGP

Mach cones are expected to form in the expanding quark-gluon plasma (QGP) when energetic quarks and gluons traverse the hot medium at a velocity faster than the speed of sound in high-energy heavy-ion collisions. The shape of the Mach cone and the associated diffusion wake are sensitive to the initial jet production location and the propagation direction of the parton shower relative to the radial flow because of the distortion caused by the collective expansion of the QGP and the large density gradient. The shape of jet-induced Mach cones and their distortions in heavy-ion collisions provide a unique and direct probe of the dynamical evolution and the equation of state of QGP. However, it is difficult to identify the Mach cone and the diffusion wake in current experimental measurements of final hadron distributions because they are averaged over all possible initial jet production locations and parton-shower propagation directions. To overcome this difficulty, we develop a deep learning assisted jet tomography which uses the full information of the final hadrons from jets to localize the initial jet production positions. This method can help to constrain the initial regions of jet production in heavy-ion collisions and enable a differential study of Mach-cones with different path lengths and orientations relative to the radial flow of the QGP in heavy-ion collisions.

Emerging Quantum Order in an Expanding Gas

In the everyday world, being “far from equilibrium” is a common state of affairs. In biology it is crucial for life. In economics it is the driving force for market activity. In many-body physics it generates a near-endless zoo of complex effects, rich both conceptually and phenomenologically. Indeed, nonequilibrium physics encompasses many fields including the cosmology of the early Universe [1], quark-gluon jet production in heavy-ion collisions [2], glass formation in dense fluids [3], and laserdriven symmetry breaking in solids [4, 5]. Yet questions about nonequilibrium phenomena can often appear untameable, because the nonequilibrium effects lack a universal organizing principle. For this reason, experiments on the dynamics of quantum systems that can be controlled at a microscopic level and are well isolated from their environment have become increasingly important for obtaining concrete answers [6–8]. In a new study, Ulrich Schneider, from the Ludwig Maximilian University of Munich in Germany, and colleagues have used a cold atomic gas to carefully realize, for the first time, a nonequilibrium phenomenon known as dynamical quasicondensation [9]. This is where delicate quantum long-range order, normally the hallmark of low-temperature equilibrium systems, is seen to spontaneously emerge in an expanding gas. Their experiment convincingly verifies this phenomenon, opening the door to further studies in more complex systems. It also creates enticing links to other phenomena such as thermalization and equilibration. Long-range order takes place when remote portions of a physical system are correlated in some way. For instance, a crystalline solid has long-range order due to the regular arrangement of its constituent atoms. But order can come in many forms and, moreover, can be quantum mechanical in origin. Bose-Einstein condensation is one such example. In these systems, below a certain temperature, the statistics of bosons conspire to make them all bunch up in the same quantum ground state. The resulting equilibrium state then possesses long-range phase ordering. This means that if matter waves from two distant parts of the system overlap, interference fringes will appear. True condensation requires such ordering to be independent of distance. However, if bosons are constrained to move around in a two-dimensional (2D) plane or one-dimensional (1D) tube, they are said to form quasicondensates instead, because their long-range phase ordering drops off with distance as a power law. In 2004, Marcos Rigol and Alejandro Muramatsu [10] predicted that in a 1D setting a quasicondensate of bosons could emerge dynamically. They considered a 1D lattice populated with bosons that can hop between neighboring sites. Importantly, these bosons were required to strongly repel each other, preventing any two from being on the same site—a feature that gains them the name of “hard-core bosons.” Next, they took the initial state of the system to have a central region with one boson located on each site, surrounded by an otherwise empty and infinitely large lattice. This state is highly excited and evolves in time, with the bosons suddenly expanding into the empty regions. They found that this expansion quickly results in a 50:50 split of the bosons occupying the momentum states k = ±π/2a, where a is the lattice spacing. Consequently, the bosons bunch up to form quasicondensates traveling to the left and right at the only finite momenta consistent with energy conservation [10]. To cleanly realize and measure the delicate effects of dynamical quasicondensation, Schneider and colleagues exploited and refined several techniques in the field of cold atoms. Specifically, they prepared a Bose-Einstein condensate of around 100,000 bosonic potassium atoms and then slowly loaded them into a three-dimensional (3D) periodic arrangement of laser light known as an optical lattice. For low laser intensities, the potential

High-$\pt$ processes measured with ALICE at the LHC

From studies of single-particle spectra, particle correlations, and jet production in heavy-ion collisions we can obtain information about the density and the dynamic properties of the Quark-Gluon Plasma (QGP). The observed suppression of high-pT particle production (RAA) and away-side jets (IAA) is generally attributed to energy loss of partons as they propagate through the plasma. We present the results obtained from the analysis of Pb-Pb collisions at sqrt (sNN) = 2.76 TeV recorded by ALICE in November 2010. The nuclear modification factors RAA and IAA, and the status of full jet reconstruction in Pb-Pb is presented. Comparison with the RHIC measurements at lower collision energy and with theory models is shown.

Scaling behavior of the azimuthal and centrality dependencies of jet production in heavy-ion collisions

For heavy-ion collisions at RHIC a scaling behavior is found in the dependencies on azimuthal angle $\phi$ and impact parameter $b$ for pion production at high $p_T$ essentially independent of the hadronization process. The scaling variable is in terms of a dynamical path length $\xi$ that takes into account detailed properties of geometry, medium density and probability of hard scattering. It is shown in the recombination model how the nuclear modification factor depends on the average $\bar\xi(\phi,b)$. The data for $\pi^0$ production at $p_T=$ 4-5 and 7-8 GeV/c at RHIC are shown to exhibit the same scaling behavior as found in the model calculation. Extension to back-to-back dijet production has been carried out, showing the existence of $\bar\xi$ scaling also in the away-side yield per trigger. At LHC the hard-parton density can be high enough to realize the likelihood of recombination of shower partons arising from neighboring jets. It is shown that such 2-jet recombination can cause strong violation of $\bar\xi$ scaling. Furthermore, the large value of $R_{AA}$ that exceeds 1 can become a striking signature of such a hadronization process at high energy.