Review of Jet Measurements in Heavy Ion Collisions

Abstract

A hot, dense medium called a Quark Gluon Plasma (QGP) is created in ultrarelativistic heavy ion collisions. Early in the collision, hard parton scatterings generate high momentum partons that traverse the medium, which then fragment into sprays of particle called jets. Understanding how these partons interact with the QGP and fragment into final state particles provides critical insight into quantum chromodynamics. Experimental measurements from high momentum hadrons, two particle correlations, and full jet reconstruction at the Relativistic Heavy Ion Collider (RHIC) and the Large Hadron Collider (LHC) continue to improve our understanding of energy loss in the QGP. Run 2 at the LHC recently began and there is a jet detector at RHIC under development. Now is the perfect time to reflect on what the experimental measurements have taught us so far, the limitations of the techniques used for studying jets, how the techniques can be improved, and how to move forward with the wealth of experimental data such that a complete description of energy loss in the QGP can be achieved. Measurements of jets to date clearly indicate that hard partons lose energy. Detailed comparisons of the nuclear modification factor between data and model calculations led to quantitative constraints on the opacity of the medium to hard probes. However, while there is substantial evidence for softening and broadening jets through medium interactions, the difficulties comparing measurements to theoretical calculations limit further quantitative constraints on energy loss mechanisms. Since jets are algorithmic descriptions of the initial parton, the same jet definitions must be used, including the treatment of the underlying heavy ion background, when making data and theory comparisons. We call for an agreement between theorists and experimentalists on the appropriate treatment of the background, Monte Carlo generators that enable experimental algorithms to be applied to theoretical calculations, and a clear understanding of which observables are most sensitive to the properties of the medium, even in the presence of background. This will enable us to determine the best strategy for the field to improve quantitative constraints on properties of the medium in the face of these challenges.

Diffusive dynamics of critical fluctuations near the QCD critical point

“…In the heavy-ion collisions strongly interacting matter at extreme temperatures T and densities is created [12][13][14]. The successful description of collective effects by conventional fluid dynamical simulations [15][16][17][18][19][20][21] and the modification of high-energetic probes measured in heavy-ion collisions compared to proton-proton collisions [22] are convincing indications for the formation of a new state of matter, the quark-gluon plasma (QGP). At the highest beam energies √ s NN at the LHC the QGP is almost baryon free, i.e.…”

How does the Quark-Gluon Plasma know the collision energy?

“…One way to approach such questions is through studying jet quenching (see for example [11,12] for reviews), the effect of the plasma on the propagation of highly energetic partons produced by the collision. This is conventionally described by the parameter q, the mean squared transverse momentum acquired by a hard parton per unit distance travelled.…”

“…For the central RHIC collisions we have B = α = 0; it is now easy to solve ( 10) and ( 11) for r h and M * , and then s C RHIC /ǫ can be found from equation (12). The energy density for central LHC collisions is estimated [51] to be around 2.3 times as large as in central RHIC collisions; using this, and following the discussion of the "thickness function" for nuclei given in [4], we find that the LHC plasmas in which we are interested arise when the impact parameter of the collision is b ≈ 12 fm (with an assumed nuclear radius of around 7 fm).…”

“…We compute α ≈ 0.36, α ≈ 0.13, and α = 0 respectively with these data. Solving ( 9), (10), and (11) in these cases and using (12), we compute values for s P LHC /ǫ which are still smaller than the value for 2.76 TeV collisions, and the final results (using (14) again) are as follows:…”

Hard Substructure of Quenched Jets: a Monte Carlo Study

“…The suppression of the jet yield in heavy-ion collisions with respect to the reference measured in proton-proton collisions due to the partonic energy loss in the quarkgluon plasma (QGP) is a well-established phenomenon both at RHIC and LHC; see [1] for a recent review. While the measurement of jet quenching in terms of the nuclear modification factor is essential for inferring the QGP transport coefficients, it contains a limited amount of information.…”