Proton-Nucleus Collisions at the LHC: Scientific Opportunities and Requirements (1105.3919v1)

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.

Scientific Opportunities in Proton-Nucleus Collisions at the LHC

The paper "Proton-Nucleus Collisions at the LHC: Scientific Opportunities and Requirements" outlines the potential scientific benefits and experimental requirements for conducting proton-nucleus (p+A) collisions at the Large Hadron Collider (LHC). This document is particularly central to expanding our understanding of quantum chromodynamics (QCD) and the conditions required for nuclear matter effects, critical for interpreting heavy-ion collision data at the LHC.

Proton-nucleus collisions are a cornerstone for benchmarking the results obtained in nucleus-nucleus (A+A) collisions, providing reference measurements necessary to disentangle initial state effects from final state phenomena inherent to the dense medium produced in heavy-ion collisions. With the LHC's capability to collide asymmetric nuclear beams, unique investigative domains in QCD become accessible, particularly probing small-$x$ physics and examining nuclear parton distribution functions (nPDFs) under high-energy conditions.

Key Findings and Observations

1. Feasibility and Planning:
   • The paper establishes the feasibility of conducting p+A runs at the LHC without significant modifications to the collider setup. A nominal beam energy of $\sqrt{s}=8.8$ TeV is proposed with an estimated luminosity of $L=10^{29}$ cm$^{-2}$s$^{-1}$ for p+Pb collisions. This allows operations that reduce systematic uncertainties inherent in nuclear collision systems and support the paper of rapidity asymmetric detectors.
2. Nuclear Parton Distribution Functions:
   • p+A collisions offer unmatched possibilities to constrain nuclear PDFs, critical for interpreting A+A data at LHC energies. Currently, nPDFs face significant uncertainties, particularly for gluons, emphasizing a need for proton-nucleus benchmarking to validate theoretical models and reduce systematics.
3. Small-$x$ and Saturation Phenomena:
   • The kinematic reach in p+A collisions extends the ability to probe small-$x$ physics several magnitudes greater than current capabilities, providing insight into the saturation phenomena of partonic densities. This advances the paper of high-density QCD regimes potentially observable at the LHC.
4. Ultra-peripheral Collisions and Astrophysics:
   • Explores opportunities for photon-induced reactions and electroweak studies extending beyond standard QCD analyses, including high-energy photon-proton and photon-nucleus interactions. Furthermore, p+A data serves astroparticle physics by improving models for interpreting cosmic-ray air shower events.

Experimental Considerations

The asymmetric nature of p+A collisions implies rapidity shifts and energy interpolation for benchmarking purposes, necessitating specific experimental setups. Ensuring consistent baseline measurements across varied energies demands adequate scheduling and technical readiness of the LHC experiments involved. The paper anticipates experimental challenges related to luminosity requirements and synchronization of beam operations for ideal data retrieval.

Implications and Future Directions

Proton-nucleus collisions at the LHC stand to provide profound insights into QCD and the nature of nuclear matter under extreme conditions, staying central to the heavy-ion research program. The outcomes have broad theoretical implications, particularly refining nPDFs and testing saturation models. Practically, p+A runs will inform methodologies in high-energy nuclear physics, potentially influencing future designs of dedicated electron-ion colliders. The breadth of scientific inquiry achievable through these collisions highlights necessary preparations and forethought for subsequent LHC operational phases.

In conclusion, the outlined paper is a comprehensive evaluation of the scientific and technical prerequisites for exploiting the p+A collision mode at the LHC, underscoring its role in advancing the field’s understanding of high-energy physics phenomena.