The two types of Cherenkov gluons at LHC energies

Beside comparatively low energy Cherenkov gluons observed at RHIC, there could be high energy gluons at LHC, related to the high energy region of positive real part of the forward scattering amplitude. In both cases they give rise to particles emitted along some cone. The characteristics of the cones produced by these two types of gluons are different. Therefore different experiments are needed to detect them. The cosmic ray event which initiated this idea is described in detail. RHIC experiments [1, 2] have shown the two-bump structure of the azimuthal angle distribution near the away-side jets. This can be interpreted as the emission of Cherenkov gluons [3, 4, 5, 6]. The notion of Cherenkov gluons was first introduced in [7, 8] to explain some cosmic ray data [9]. Analogous to Cherenkov photons, the Cherenkov gluons can be emitted in hadronic collisions provided the nuclear index of refraction n exceeds 1. The partons moving in such nuclear medium would emit them. These gluons should be emitted at the cone surface with the cone angle θ in the rest system of the infinite medium defined by the relation cos θ = 1 βn , (1) where β is the ratio of the velocities of the parton-emitter and light which can be replaced by 1 for relativistic partons. The gluon propagation in a nuclear medium can be described by the nuclear index of refraction. It is well known [10] that the excess ∆n R (E) of n over 1 is proportional to the real part of the forward scattering amplitude ReF (E) and to the number of scatterers ν. It must be positive for Cherenkov gluons to be emitted: ∆n R (E) = n − 1 ∝ ν(E)ReF (E) > 0. (2) Here we must use the forward scattering amplitude of a gluon propagating in strong fields of the quark-gluon medium formed in heavy-ion collision. QCD estimates of F (E) are rather indefinite. Therefore, we have to rely on general properties of hadronic reactions known from experiment. According to our knowledge about hadronic reactions, ReF (E) is positive for any colliding hadrons either within the lower wings of the Breit-Wigner resonances 1 or at very high energies [12, 13]. Jet energies available in RHIC experiments [1, 2] are sufficient only for emission of gluons with rather low energy. The resonance production is the common property of hadronic interactions at low energies. Thus one …