Dijet Imbalance Research Articles

Probing Parton Saturation and the Gluon Dipole via Diffractive Jet Production at the Electron-Ion Collider.

We demonstrate that hard dijet production via coherent inelastic diffraction is a promising channel for probing gluon saturation at the Electron-Ion Collider. By inelastic diffraction, we mean a process in which the two hard jets-a quark-antiquark pair generated by the decay of the virtual photon-are accompanied by a softer gluon jet, emitted by the quark or the antiquark. This process can be described as the elastic scattering of an effective gluon-gluon dipole. The cross section takes a factorized form, between a hard factor and a unintegrated ("Pomeron") gluon distribution describing the transverse momentum imbalance between the hard dijets. The dominant contribution comes from the black disk limit and leads to a dijet imbalance of the order of the target saturation momentum Q_{s} evaluated at the rapidity gap. Integrating out the dijet imbalance, we obtain a collinear factorization where the initial condition for the Dokshitzer-Gribov-Lipatov-Altarelli-Parisi evolution is set by gluon saturation.

New results on fully corrected dijet asymmetry in Pb + Pb collisions with ATLAS

The phenomenon of events containing highly asymmetric dijet pairs is one of the most striking results in heavy ion physics, providing the first direct observation of in-medium jet energy loss at the Large Hadron Collider. Detailed measurements of a centrality-dependent dijet imbalance in 2.76 TeV Pb + Pb collisions using data collected by the ATLAS detector in the 2011 LHC heavy ion run are presented. The new analysis provides a measurement, fully corrected for detector effects to the particle level, of the centrality- and leading jet transverse momentum-(pT-) dependence of the dijet pT balance distribution, compared to an analogous measurement in pp collisions at the same center-of-mass energy.

Average Transverse Momentum Quantities Approaching the Lightfront

In this contribution to Light Cone 2014, three average transverse momentum quantities are discussed: the Sivers shift, the dijet imbalance, and the $p_T$ broadening. The definitions of these quantities involve integrals over all transverse momenta that are overly sensitive to the region of large transverse momenta, which conveys little information about the transverse momentum distributions of quarks and gluons inside hadrons. TMD factorization naturally suggests alternative definitions of such integrated quantities, using Bessel-weighting and rapidity cut-offs, with the conventional definitions as limiting cases. The regularized quantities are given in terms of integrals over the TMDs of interest that are well-defined and moreover have the advantage of being amenable to lattice evaluations.

Towards a hybrid strong/weak coupling approach to jet quenching

Abstract We explore a novel hybrid model containing both strong and weak coupling physics for high energy jets traversing a deconfined medium. This model is based on supplementing a perturbative DGLAP shower with strongly coupled energy loss rate. We embed this system into a realistic hydrodynamic evolution of hot QCD plasma. We confront our results with LHC data, obtaining good agreement for jet R A A , dijet imbalance A J and fragmentation functions.

Jets and high pT hadrons in CMS

Abstract We present the recent experimental results on jets and high p T hadrons measured by the CMS Collaboration using various observables: jet and high p T charged hadron yields, dijet imbalance, jet shapes and fragmentation functions. The measurements of dijet p T and pseudorapidity distributions in pPb collisions using the data delivered by LHC at early 2013 are also presented.

Energy dependence of the dijet imbalance in Pb-Pb collisions at 2.76ATeV

The appearance of monojets is among the most striking signature of jet quenching in the context of ultrarelativistic heavy-ion collisions at the Large Hadron Collider. Experimentally, the asymmetry of back-to-back jets is quantified in terms of the dijet imbalance distribution by the ATLAS and CMS collaborations. Recently, the CMS collaboration has also studied the trigger jet momentum (P_T) dependence of the imbalance in the range between 120 and 500 GeV which is found to decrease with jet P_T. In this work, results from the in-medium shower code YaJEM are compared with this data set. These results suggest that the main effects observed in the data are the kinematical collimation of jets and the increase in the probability to produce more collimated quark jets with jet P_T, whereas there is no indication for any non-trivial energy dependence of the shower-medium interaction mechanism itself. The data furthermore can rule out models in which the jet shape is collimated due to the medium modification.

Dijet imbalance in 2.76 TeV PbPb collisions in CMS

We present the measurement of dijet production in PbPb collisions at a nucleon-nucleon center-of-mass energy of 2.76 TeV that is studied with the CMS detector at the LHC. We use data corresponding to an integrated luminosity of 150 μb−1. Jets are reconstructed using the anti-kT algorithm on particle flow objects. The dijet momentum balance and angular correlations are studied as a function of collision centrality and leading jet momentum. For the most peripheral PbPb collisions, good agreement of the dijet momentum balance distributions with pp data and reference calculations at the same collision energy is found. However, more central collisions show a strong imbalance of leading and subleading jet transverse momenta, confirming the previous observations made using a smaller dataset. The extended jet transverse momentum range significantly increases the discriminatory power of the measurement for different models of parton energy loss in hot QCD matter.

Study of jet quenching using dijets in PbPb collisions with CMS

Jets are an important tool to probe the hot, dense medium that is produced in ultra-relativistic heavy ion collisions. The large collision energy at the LHC provides copious production of dijets with energies that can be cleanly identified above the heavy ion background. The multipurpose CMS detector is well designed to measure these hard scattering processes with its high resolution calorimeters and high precision silicon tracker. Jet quenching is observed by a significant imbalance of dijet transverse momentum in PbPb collisions at TeV. The fraction of unbalanced dijets is found to increase strongly with increasing collision centrality. The dijet imbalance persists to the highest jet momenta studied. The redistribution of the quenched jet energy is studied using the transverse momentum balance of charged tracks projected onto the direction of the leading jet. In contrast to pp collisions, a large fraction of the momentum balance for asymmetric jets is found to be carried by low momentum particles at large angular distance from the jet axis.

Jet fragmentation functions measured in Pb–Pb collisions with CMS

The energy loss of partons in high-energy nuclear collisions is one of the most interesting observables to probe the nature of the produced medium. Analyzing data from Pb–Pb collisions at a center-of-mass energy of 2.76 TeV, recorded with the compact muon solenoid (CMS) detector, parton energy loss is observed as a significant imbalance of dijet transverse momentum. To gain further understanding of the parton energy loss mechanism and how it manifests itself in the distribution of final state particles, fragmentation functions are studied for the leading and subleading jet in bins of dijet imbalance.

Dijet imbalance in hadronic collisions

The imbalance of dijets produced in hadronic collisions has been used to extract the average transverse momentum of partons inside the hadrons. In this paper we discuss new contributions to the dijet imbalance that could complicate or even hamper this extraction. They are due to polarization of initial state partons inside unpolarized hadrons that can arise in the presence of nonzero parton transverse momentum. Transversely polarized quarks and linearly polarized gluons produce specific azimuthal dependences of the two jets that in principle are not suppressed. Their effects cannot be isolated just by looking at the angular deviation from the back-to-back situation; rather they enter jet broadening observables. In this way they directly affect the extraction of the average transverse momentum of unpolarized partons that is thought to be extracted. We discuss appropriately weighted cross sections to isolate the additional contributions.

Hadronic dijet imbalance and transverse-momentum-dependent parton distributions

We compare several recent theoretical studies of the single transverse-spin asymmetry in dijet correlations at hadron colliders. We show that the results of these studies are all consistent. To establish this, we investigate, in particular, the two-gluon exchange contributions to the relevant initial and final-state interactions in the context of a simplifying model. Overall, the results confirm that the dijet imbalance obeys, at best, a nonstandard or ``generalized'' transverse-momentum-dependent factorization.