Proton–nucleus collisions at the LHC: scientific opportunities and requirements

Abstract

Proton-nucleus (p+A) collisions have long been recognized as a crucial component of the physics programme with nuclear beams at high energies, in particular for their reference role to interpret and understand nucleus-nucleus data as well as for their potential to elucidate the partonic structure of matter at low parton fractional momenta (small-x). Here, we summarize the main motivations that make a proton-nucleus run a decisive ingredient for a successful heavy-ion programme at the Large Hadron Collider (LHC) and we present unique scientific opportunities arising from these collisions. We also review the status of ongoing discussions about operation plans for the p+A mode at the LHC.

Study of Higgs boson production and itsbb¯decay inγ−γprocesses in proton-nucleus collisions at the LHC

“…F(θ c , σ * , σ z ) is a small reduction factor from the half-crossing angle, θ c , and bunch length σ z , which we neglect in this discussion. In Table IV we list the beam parameters relevant for the p A running mode [41,42]. We note that, as mentioned in the discussion of Table I, the values quoted are for now only unofficial (but plausible) estimates.…”

“…Note that these "maximum" photon energies do not have to be interpreted as a hard cut-off but as an indication of the energy (ω > γ β/b) from which the photon flux is exponentially suppressed. Table I summarizes the most relevant parameters for ultraperipheral pp, p A, and AA collisions at the LHC [18,41]. It has to be noted that whereas proton-proton and nucleus-nucleus collisions are obviously part of the approved baseline physics programme of the LHC, proton-nucleus collisions are still considered as an upgrade of the heavyion programme [41,42].…”

“…Table I summarizes the most relevant parameters for ultraperipheral pp, p A, and AA collisions at the LHC [18,41]. It has to be noted that whereas proton-proton and nucleus-nucleus collisions are obviously part of the approved baseline physics programme of the LHC, proton-nucleus collisions are still considered as an upgrade of the heavyion programme [41,42]. Detailed studies [42] have nonetheless shown that is a perfectly feasible mode of operation of the collider, and its physics possibilities have been discussed vastly in the literature as a crucial baseline for the interpretation of the AA data [41,43,44].…”

Resummation in nonlinear equation for high energy factorizable gluon density and its extension to include coherence

“…This will offer an unprecedented possibility to test the partonic structure at high densities at relatively "cleaner" environment as compared to AA collisions [2]. The particularly interesting phenomenon which is believed to be understood with studies of observables produced in p-Pb is a phenomenon of gluon saturation [3].…”

Erratum: Observing Light-by-Light Scattering at the Large Hadron Collider [Phys. Rev. Lett.111, 080405 (2013)]

“…energy $s À1 from its peak at ffiffiffiffiffiffiffi s p % 3m e [30], which favors the comparatively softer Pb photon beams for the observation of LbyL scattering. In Table I, we summarize the most relevant parameters for ultraperipheral p-p, p-Pb, and Pb-Pb collisions at the LHC [22,31].…”