We study the modifications of jets created in heavy-ion collisions at LHC energies. The inherent hierarchy of scales governing the jet evolution allows to distinguish a leading jet structure, which interacts coherently with the medium as a single color charge, from softer sub-structures that will be sensitive to effects of color decoherence. We argue how this separation comes about and show that this picture is consistent with experimental data on reconstructed jets at the LHC, providing a quantitative description simultaneously of the jet nuclear modification factor, the missing energy in di-jet events and the modification of the fragmentation functions. In particular, we demonstrate that effects due to color decoherence are manifest in the excess of soft particles measured in fragmentation functions in Pb-Pb compared to proton-proton collisions.
Using a pQCD picture for jet evolution in a dense QCD medium, in which medium-induced parton branchings are factorized from vacuum-like emissions, we study two jet substructure observables: the $z_g$ and the Soft Drop multiplicity distributions. We compute the respective nuclear modification factors using a Monte-Carlo implementation of the parton showers. Our results are in qualitative agreement with LHC data for Pb+Pb collisions. We identify the physical mechanisms explaining our results: incoherent jet energy loss, semi-hard medium-induced emissions and the bias introduced by the steeply fa…
Electroweak boson-tagged jet measurements provide a promising experimental channel to accurately study the physics of jet production and propagation in dense QCD medium. In this talk, we present theoretical predictions for the nuclear-induced attenuation of the differential cross section for isolated photon-tagged and $\rm Z^0$-tagged jet production in heavy ion collisions, and provide theoretical interpretations to the recent LHC data. We demonstrate quantitatively the significance of collisional and radiative energy losses, as revealed in the strong momentum asymmetry $dσ/dx_{VJ}$ and nuclea…
We analyze the recent LHCb measurement of the distribution of the fraction of the transverse momentum, $z(J/ψ)$, carried by the $J/ψ$ within a jet. LHCb data is compared to analytic calculations using the fragmenting jet function (FJF) formalism for studying $J/ψ$ in jets. Logarithms in the FJFs are resummed using DGLAP evolution. We also convolve hard QCD partonic cross sections, showered with PYTHIA, with leading order Non Relativistic Quantum Chromodynamics (NRQCD) fragmentation functions and obtain consistent results. Both approaches use Madgraph to calculate the hard process that creates …
We investigate the forward-jet energy spectrum within the Color Glass Condensate framework at 5 TeV center-of-mass energy. In particular, we focus on the kinematic range covered by the CMS-CASTOR calorimeter. We show that our saturation-model calculations are compatible with the CASTOR measurements and that to optimally reproduce the data, effects of multi-parton interactions need to be included. We predict a significant nuclear suppression - reaching down to 50% at the lowest considered jet energies $E_{\rm jet} \sim 500 \, {\rm GeV}$.
We compare the jet-path length and beam-energy dependence of the pion nuclear modification factor and a parton-jet nuclear modification factor at RHIC and LHC. We contrast predictions based on a linear pQCD and a highly non-linear hybrid-AdS holographic model of jet-energy loss. We find that both models require a reduction of the jet-medium coupling from RHIC to LHC to account for the measured pion nuclear modification factor. In case of the parton-jet nuclear modification factor, however, which serves as a lower bound for the LO jet nuclear modification factor of reconstructed jets, the extra…
We present POWHEG predictions for photon-jet correlations at RHIC and the LHC. We show that the total transverse-momentum spectrum of photons and jets is modified not only by medium, but also by higher-order QCD effects, as is the distribution in their relative azimuthal angle. At the LHC, photon-jet measurements in the forward region allow to probe parton momentum fractions of lead ions down to 10$^{-4}$.
We investigate the effect of PT-broadening on jet substructure observables in heavy-ion collisions at the LHC. As an example, we focus on the opening angle of the two branches that satisfy the soft drop grooming condition in a highly energetic jet. The medium modification of the angular distribution can provide important information on the jet transport properties of hot QCD matter. In addition, we take into account a change of the overall fraction of quark and gluon jets in heavy-ion collisions. We comment on the comparison to a recent measurement from the ALICE Collaboration.
We present a method for improving the phenomenological description of Mueller-Navelet jets at LHC, which is based on matching the BFKL resummation with fixed order calculations. We point out the need of a consistent identification of jets between experimental measurements and theoretical descriptions. We hope as well to motivate an extensive analysis of MN jets at LHC in run 2.
Central lead-lead collisions at the LHC energies may pose a particular challenge for jet identification as multiple jets are produced per each collision event. We simulate the jet evolution in central Pb-Pb events at $\sqrt{s_{\rm NN}} = 2.76$ GeV collision energy with EPOS3 initial state, which typically contains multiple hard scatterings in each event. Therefore the partons from different jets have a significant chance to overlap in momentum space. We find that 30% of the jets with $p_\perp > 50$ GeV, identified by the standard anti-$k_\perp$ jet finding algorithm with jet cone size R=0.3, c…
We review the status of NNLO calculations for jet cross sections at the LHC. In particular, we describe how perturbative stability and convergence can be used as criteria to select the most appropriate scales in the theoretical description of di-jet and single jet inclusive production.
Diffractive processes possible to be measured at the LHC are listed and briefly discussed. This includes soft (elastic scattering, exclusive meson pair production, diffractive bremsstrahlung) and hard (single and double Pomeron exchange jets, $γ$+jet, W/Z, jet-gap-jet, exclusive jets) processes as well as Beyond Stanrad Model phenomena (anomalous gauge couplings, magnetic monopoles).
Jet-medium interaction involves two important effects: jet energy loss and medium response. The search for jet-induced medium excitations is one of the hot topics in jet quenching study in relativistic nuclear collisions. In this work, we perform a systematic study on how the lost energy from hard jets evolves with the bulk medium and redistributes in the final state of heavy-ion collisions via a multi-phase transport model. In particular, the ($Δη, Δϕ$) distribution of charged particles with respect to the jet axis and jet shape function are studied for various Pb+Pb collision centralities an…
Jets with a large radius $R\gtrsim 1$ and grooming algorithms are widely used to fully capture the decay products of boosted heavy particles at the Large Hadron Collider (LHC). Unlike most discriminating variables used in such studies, the jet radius is usually not optimized for specific physics scenarios. This is because every jet configuration must be calibrated, insitu, to account for detector response and other experimental effects. One solution to enhance the availability of large-$R$ jet configurations used by the LHC experiments is {\it jet re-clustering}. Jet re-clustering introduces a…
We review the history of jets in high energy physics, and describe in more detail the developments of the past ten years, discussing new algorithms for jet finding and their main characteristics, and summarising the status of perturbative calculations for jet cross sections in hadroproduction. We also describe the emergence of jet grooming and tagging techniques and their application to boosted jets analyses.
We propose the study of new observables in LHC inclusive events with three tagged jets, one in the forward direction, one in the backward direction and both well-separated in rapidity from the each other (Mueller-Navelet jets), together with a third jet tagged in central regions of rapidity. Since non-tagged associated mini-jet multiplicity is allowed, we argue that projecting the cross sections on azimuthal-angle components can provide several distinct tests of the BFKL dynamics. Realistic LHC kinematical cuts are introduced.
Several important processes and analyses at the LHC are sensitive to higher-order perturbative corrections beyond what can currently be calculated at fixed order. One important class of logarithmic corrections are those which appear when the centre-of-mass energy of a QCD collision is much larger than the transverse momenta of the observed jets. We describe the High Energy Jets (HEJ) framework, which includes the dominant high-energy logarithms to provide all-order predictions for several LHC processes including Higgs, $W$, or $Z$ boson production in association with jets. We will summarise so…
We propose a simple way to test the Abelian decomposition of QCD, the existence of two types of gluons, experimentally at LHC. The Abelian decomposition decomposes the gluons to the color neutral neurons and colored chromons gauge independently. This refines the Feynman diagram in a way that the color conservation is explicit, and generalizes the quark model to the quark and chromon model. We predict that the neuron jet has the color factor 3/4 and has a sharpest jet radius and smallest particle multiplicity, while the chromon jet with the color factor 9/4 remains the broadest jet. Moreover, t…
We compute the next-to-leading order (NLO) QCD corrections to the gluon-fusion subprocess of diphoton-plus-jet production at the LHC. We compute fully differential distributions by combining two-loop virtual corrections with one-loop real radiation using antenna subtraction to cancel infrared divergences. We observe significant corrections at NLO which demonstrate the importance of combining these corrections with the quark-induced diphoton-plus-jet channel at next-to-next-to-leading order (NNLO).
