Ridge effect, azimuthal correlations, and other novel features of gluonic string collisions in high energy photon-mediated reactions

Abstract

One of the remarkable features of high-multiplicity hadronic events in proton-proton collisions at the LHC is the fact that the produced particles appear as two "ridges", opposite in azimuthal angle ϕ, with approximately flat rapidity distributions. This phenomenon can be identified with the inelastic collision of gluonic flux tubes associated with the QCD interactions responsible for quark confinement in hadrons.In this paper, we analyze the ridge phenomena when the collision involves a flux tube connecting the quark and antiquark of a high energy real or virtual photon. We discuss gluonic tube string collisions in the context of two examples: electron-proton scattering at a future electron-ion collider or the peripheral scattering of protons accessible at the LHC. A striking prediction of our analysis is that the azimuthal angle of the produced ridges will be correlated with the scattering plane of the electron or proton producing the virtual photon. In the case of ep → eX, the final state X is expected to exhibit maximal multiplicity when the elliptic flow in X is aligned with the electron scattering plane. In the pp → ppX example, the multiplicity and elliptic flow in X are estimated to exhibit correlated oscillations as functions of the azimuthal angle Φ between the proton scattering planes. In the minimum-bias event samples, the amplitude of oscillations is expected to be on the order of 2% to 4% of the mean values. In the events with highest multiplicity, the oscillations can be three times larger than in the minimum-bias event samples.

The Large Hadron–Electron Collider at the HL-LHC

“…In this respect, measurements in ep and eA collisions at the LHeC at considerable centreof-mass energies will offer crucial additional information. For example, the collision of the virtual photon with the proton at the LHeC can be considered to be a high-energy collision of two jets or 'flux tubes', as discussed in references [564,565] and illustrated in figure 93. This can lead to the production of 'ridges' and other novel configurations of gluons and quarks and will be uniquely measured at the LHeC.…”

Particle correlations from the initial state

“…A string-based model is also proposed in [103,104]. There, valence diquark-quark flux tubes or strings in the incoming protons overlap and produce more particles in the transverse than in the longitudinal direction of the flux tube.…”

Exploring the energy frontier with deep inelastic scattering at the LHC

“…TheLHeCasanelectronioncollider, with its huge kinematic range and high precision, will all have profound implications to our understandingofallstagesofheavyioncollisions at high energies; the wave function of the colliding nuclei; the particle production mechanism; the initial spatial and momentum distributions of produced partons prior to the emergence of a collective behaviour. This includes correlations such as those revealed by the ridgephenomenon, which may be explained both in perturbative frameworks, like the colour glass condensate, or in non-perturbative ones, like the inelastic collision of gluonic flux tubes associated with the QCDinteractionsresponsibleforquarkconfinementin hadrons and be ideally studied in electron-hadron scattering [49]. Furthermore, eA collisions will establish a baselinerepresentingthenormal(cold)nuclearmedium, relative to which the effects of the hot dense medium can be contrasted for hard probes such as jets and quarkonia.…”