The Physics of Ultraperipheral Collisions at the LHC (0706.3356v2)

Abstract: We discuss the physics of large impact parameter interactions at the LHC: ultraperipheral collisions (UPCs). The dominant processes in UPCs are photon-nucleon (nucleus) interactions. The current LHC detector configurations can explore small $x$ hard phenomena with nuclei and nucleons at photon-nucleon center-of-mass energies above 1 TeV, extending the $x$ range of HERA by a factor of ten. In particular, it will be possible to probe diffractive and inclusive parton densities in nuclei using several processes. The interaction of small dipoles with protons and nuclei can be investigated in elastic and quasi-elastic $J/\psi$ and $\Upsilon$ production as well as in high $t$ $\rho^0$ production accompanied by a rapidity gap. Several of these phenomena provide clean signatures of the onset of the new high gluon density QCD regime. The LHC is in the kinematic range where nonlinear effects are several times larger than at HERA. Two-photon processes in UPCs are also studied. In addition, while UPCs play a role in limiting the maximum beam luminosity, they can also be used a luminosity monitor by measuring mutual electromagnetic dissociation of the beam nuclei. We also review similar studies at HERA and RHIC as well as describe the potential use of the LHC detectors for UPC measurements.

• The paper demonstrates that ultraperipheral collisions can effectively probe high gluon densities and refine nuclear parton distribution functions via photon-induced processes.
• It employs photoproduction models, including the Vector Meson Dominance Model and leading-twist nuclear shadowing, to accurately predict vector meson cross-sections.
• The study leverages LHC capabilities to explore new QCD regimes, revealing transitions from color transparency to opacity through diffractive and high-t processes.

Insights into Ultraperipheral Collisions at the LHC

This paper presents an extensive paper on the physics of ultraperipheral collisions (UPCs) at the Large Hadron Collider (LHC) with a focus on photon-nucleon interactions. It outlines several processes accessible in UPCs, such as diffractive and inclusive parton density studies, which can probe quantum chromodynamics (QCD) at high gluon densities, and proposes UPCs as a tool to measure nuclear parton distribution functions (nPDFs).

Experimental and Theoretical Framework

UPCs occur when ions interact through their electromagnetic fields rather than through direct collisions, allowing investigations of photon-induced processes at unprecedented energies. The paper elaborates on the capabilities of current LHC detectors to explore small Bjorken-$x$ phenomena at photon-nucleon center-of-mass energies surpassing 1 TeV. This extension beyond the capabilities of HERA facilitates probing new QCD regimes, attributed to the high density of gluons.

Key Processes in UPCs

Ultraperipheral processes provide an opportunity to paper photon-photon and photon-nucleus interactions. The photon emissions are coherent, producing virtual photons of energy greater than their virtuality. In coherent photoproduction, the photon interacts with the entire nucleus, maintaining its integrity, leading to processes like vector meson photoproduction. The production cross-sections are calculated by integrating the photon flux over its energy distribution and the photon-nucleon cross-sections, delivering results that are sensitive to photon flux and nuclear absorption effects.

Photoproduction of Vector Mesons

The paper outlines calculations for the photoproduction of vector mesons such as $\rho^0$, $J/\psi$, and $\Upsilon$. These processes are detailed within the framework of the Vector Meson Dominance Model (VDM), where the photoproduction is compared to elastic scattering. For light mesons, the paper suggests using photoproduction as a measure of the interaction profile at high $W_{\gamma p}$ energies. The paper reports successful predictions of photoproduction cross-sections by incorporating effects such as leading-twist nuclear shadowing and compares them against existing HERA data to showcase predictive accuracies beyond the HERA $x$ range.

Diffraction and High $t$ Processes

Exploring large $t$ processes with rapidity gaps provides insight into the dynamics of color transparency and opacity. The paper of high-mass diffractive processes investigates transitions from color transparency, where cross-section scales with nuclear size, to color opacity, exhibiting saturation effects. This transition is explored in contexts like diffractive dijet production and heavy meson production.

Implications and Future Directions

The LHC opens possibilities to probe extremely low $x$ regimes, which are pivotal in understanding unitarization effects in QCD. The results underscore the importance of leading-twist shadowing in determining the nuclear gluon distributions, with resonance suppression observed at large energies in heavier nuclei. The paper advocates exploring UPCs as they promise cleaner backgrounds for small $x$ QCD studies compared to direct hadronic collisions due to the limited number of secondary interactions.

Conclusion

The comprehensive investigation into UPCs at the LHC displays its potential to substantially advance our understanding of high-density QCD environments. These studies are indispensable for bridging the empirical data from HERA to LHC scales and refining models of nuclear shadowing, diffraction, and partonic interactions at high energies. As the LHC continues to deliver rich data, UPCs offer a robust platform for probing fundamental physics, offering new domains of exploration in QCD and potential confirmations of models predicting saturation effects in high-density environments.