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Register a new userComparing energy loss phenomenology
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M. van Leeuwen
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High-pT particle production is suppressed in heavy ion collisions due to parton energy loss in dense QCD matter. Here we present a systematic comparison of two different theoretical approximations to parton energy loss calculations: the opacity expansion and the multiple-soft scattering approximation for the simple case of a quark traversing a homogeneous piece of matter with fixed length (the TECHQM brick problem), with focus on the range of parameters that is relevant for interpreting RHIC measurements of high-pT hadron suppression.
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The similarities and differences between three commonly used formalisms for radiative parton energy loss in hot strongly interacting matter are discussed. The single gluon emission spectra are evaluated for a model system consisting of a homogeneous medium with a fixed length, the `TECHQM brick. Sizable quantitative differences are found and the origins of these differences are discussed.
In this work we complete the investigation of the recently introduced energy-energy correlation (EEC) function in hadronic Higgs decays at next-to-leading order (NLO) in fixed-order perturbation theory in the limit of vanishing light quark masses. The full analytic NLO result for the previously unknown EEC in the $H to q bar{q} + X$ channel is given in terms of classical polylogarithms and cross-checked against a numerical calculation. In addition to that, we discuss further corrections to predictions of the Higgs EEC event shape variable, including quark mass corrections, effects of parton shower and hadronization. We also estimate the statistical error on the measurements of the Higgs EEC at future Higgs factories and compare with the current perturbative uncertainty.
QCD jets, produced copiously in heavy-ion collisions at LHC and also at RHIC, serve as probes of the dynamics of the quark-gluon plasma (QGP). Jet fragmentation in the medium is interesting in its own right and, in order to extract pertinent information about the QGP, it has to be well understood. We present a brief overview of the physics involved and argue that jet substructure observables provide new opportunities for understanding the nature of the modifications.
We compute the in-medium energy loss probability distribution of two neighboring subjets at leading order, in the large-$N_c$ approximation. Our result exhibits a gradual onset of color decoherence of the system and accounts for two expected limiting cases. When the angular separation is smaller than the characteristic angle for medium-induced radiation, the two-pronged substructure lose energy coherently as a single color charge, namely that of the parent parton. At large angular separation the two subjets lose energy independently. Our result is a first step towards quantifying effects of energy loss as a result of the fluctuation of the multi-parton jet substructure and therefore goes beyond the standard approach to jet quenching based on single parton energy loss. We briefly discuss applications to jet observables in heavy-ion collisions.
We consider propagation of high energy earth-skimming taus produced in interactions of astrophysical tau neutrinos. For astrophysical tau neutrinos we take generic power-law flux, $E^{-2}$ and the cosmogenic flux initiated by the protons. We calculate tau energy loss in several approaches, such as dipole models and the phenomenological approach in which parameterization of the $F_2$ is used. We evaluate the tau neutrino charged-current cross section using the same approaches for consistency. We find that uncertainty in the neutrino cross section and in the tau energy loss partially compensate giving very small theoretical uncertainty in the emerging tau flux for distances ranging from $2$ km to $100$ km and for the energy range between $10^6$ GeV and $10^{11}$ GeV, focusing on energies above $10^8$ GeV. When we consider uncertainties in the neutrino cross section, inelasticity in neutrino interactions and the tau energy loss, which are not correlated, i.e. they are not all calculated in the same approach, theoretical uncertainty ranges from about $30%$ and $60 %$ at $10^8$ GeV to about factors of 3.3 and 3.8 at $10^{11}$ GeV for the $E^{-2}$ flux and the cosmogenic flux, respectively, for the distance of 10 km rock. The spread in predictions significantly increases for much larger distances, e.g., $sim 1,000$ km. Most of the uncertainty comes from the treatment of photonuclear interactions of the tau in transit through large distances. We also consider Monte Carlo calculation of the tau propagation and we find that the result for the emerging tau flux is in agreement with the result obtained using analytic approach. Our results are relevant to several experiments that are looking for skimming astrophysical taus, such as the Pierre Auger Observatory, HAWC and Ashra. We evaluate the aperture for the Auger and discuss briefly application to the the other two experiments.
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