Measurement of transverse momentum relative to dijet systems in PbPb and pp collisions at s N N = 2.76 $$ \sqrt{s_{\mathrm{NN}}}=2.76 $$ TeV

Abstract

An analysis of dijet events in PbPb and pp collisions is performed to explore the properties of energy loss by partons traveling in a quark-gluon plasma. Data are collected at a nucleon-nucleon center-of-mass energy of 2.76 TeV at the LHC. The distribution of transverse momentum (p T ) surrounding dijet systems is measured by selecting charged particles in different ranges of p T and at different angular cones of pseudorapidity and azimuth. The measurement is performed as a function of centrality of the PbPb collisions, the p T asymmetry of the jets in the dijet pair, and the distance parameter R used in the anti-k T jet clustering algorithm. In events with unbalanced dijets, PbPb collisions show an enhanced multiplicity in the hemisphere of the subleading jet, with the p T imbalance compensated by an excess of low-p T particles at large angles from the jet axes.

Event-by-event fluctuations in the medium-induced jet evolution

“…Quite remarkably, the above numbers agree quite well with the phenomenology of dijet asymmetry at the LHC [1][2][3][4][5][6][7][8][9]. Such an agreement should be taken with a grain of salt, since our present description is rather idealized: we have ignored several important dynamical ingredients, like the elastic collisions off the medium constituents, the longitudinal expansion of the medium, the virtualities of the partons in the jet, the gluon fragmentation into hadrons, and the additional sources of fluctuations, besides the randomness of the branching process.…”

“…the latter. Accordingly, the 'missing' energy is only slowly recovered when increasing the angular opening ∆R of the subleading jet; for instance, for the relatively large value ∆R = 0.8, there is still a substantial energy imbalance, ∆E 20 ÷ 30 GeV [2,8].…”

“…A remarkable phenomenon discovered and, by now, abundantly studied in Pb+Pb collisions at the LHC is the di-jet asymmetry -a strong energy imbalance between two jets which propagate nearly back-to-back in the plane transverse to the collision axis [1][2][3][4][5][6][7][8][9]. Imbalanced di-jets are also observed in proton-proton collisions, where they are mostly associated with 3-jet events, but the respective events in (central) Pb+Pb collisions look quite different.…”

Angular distributions of the quenched energy flow from dijets with different radius parameters in CMS

“…1 is for an inclusive A J selection, with balanced and imbalanced A J available in Ref. [8]. In the first bin ∆, there is a large imbalance towards leading jet side made up of high-p T tracks, most pronounced in the central PbPb binning.…”

“…In this analysis, the missing p T technique is used to characterize the angular distribution of the energy lost by the parton. By choosing a new azimuthal axis that is symmetric with respect to the dijet system, the contribution from underlying event cancels completely even when mapping in η-φ space, and the momentum flow of the dijet system can be studied in detail [8].…”