Observing light-by-light scattering at the Large Hadron Collider (1305.7142v3)

Abstract: Elastic light-by-light scattering, $\gamma\gamma\to\gamma\gamma$, is open to study at the Large Hadron Collider thanks to the large quasi-real photon fluxes available in electromagnetic interactions of protons (p) and lead (Pb) ions. The $\gamma\gamma\to\gamma\gamma$ cross sections for diphoton masses $m_{\gamma\gamma} > 5$ GeV amount to 105 fb, 260 pb, and 370 nb in p-p, p-Pb, and Pb-Pb collisions at nucleon-nucleon center-of-mass energies $\sqrt{s_{NN}}$ = 14 TeV, 8.8 TeV, and 5.5 TeV respectively. Such a measurement has no substantial backgrounds in Pb-Pb collisions where one expects about 70 signal events per run, after typical detector acceptance and reconstruction efficiency selections.

• The paper reports that light-by-light scattering can be observed at the LHC, with PbPb collisions yielding cross sections as high as 370 nb.
• It deploys Standard Model analyses with Monte Carlo simulations and the equivalent photon approximation to simulate realistic photon emissions.
• The results imply significant prospects for testing quantum electrodynamics and probing potential new physics beyond the Standard Model.

Observing Light-by-Light Scattering at the Large Hadron Collider

The paper presented in the paper, "Observing light-by-light scattering at the Large Hadron Collider" by David d'Enterria and Gustavo G. Silveira, explores the feasibility of observing elastic light-by-light (LbyL) scattering ($\gamma\gamma \to \gamma\gamma$) using the Large Hadron Collider (LHC). This elusive, yet fundamental quantum-mechanical process remains unobserved directly in the laboratory setting despite indirect confirmations through measurements of the anomalous magnetic moments of the electron and muon.

Theoretical Framework and Methodology

Photon-photon scattering is modeled within the Standard Model (SM) framework and processed through virtual one-loop diagrams involving charged fermions and bosons. The LHC offers substantial photon fluxes in ultraperipheral collisions (UPCs) involving protons (p) and lead (Pb) ions, which can be exploited to investigate this process. The analyses utilize realistic simulation techniques for photon emissions employing the equivalent photon approximation (EPA) and Monte Carlo setups through tools like \textsc{MadGraph}.

Notably, $\gamma\gamma$ cross sections with diphoton masses greater than 5 GeV reach 105 fb in \pp\ collisions, 260 pb in \pPb\ collisions, and 370 nb in \PbPb\ collisions. These values were determined for distinct center-of-mass energy conditions at the LHC and leverage the Z$^4$ scaling factor, enhancing photon fluxes significantly in ion-ion collisions.

Results and Implications

The derived cross sections suggest that \PbPb\ collisions provide a substantial signal (about 70 expected events per one-year LHC run) after accounting for typical detector acceptances and efficiencies. In contrast, \pp\ and \pPb\ collisions predict lower countable events under realistic operational conditions, posing additional challenges due to higher backgrounds, notably from central exclusive production (CEP) in hadronic collisions.

Practical and Theoretical Consequences

Conducting these measurements could remarkably advance the validation of quantum field theories and set the stage for exploring beyond-standard-model physics, such as probing new charged particle contributions, supersymmetric partners, or quantum gravity effects. The calculational setup could also aid in understanding potential anomalous gauge couplings.

Future Directions

Progress in this domain will hinge on advancements in detector efficiencies and resolution at the forward LHC locations, alongside enhancing run-specific luminosities for ions and protons. While the current model estimates focus predominantly on Standard Model physics, incorporation of alternative theoretical frameworks or dynamical extensions could reveal deviations or alignments with experimental findings, underscoring the apparatus's capacity as not just a testing ground for known physics, but as a discovery machine for new phenomena.

Thus, while direct light-by-light scattering remains experimentally challenging, and while signal-background separation and luminosity considerations are pivotal, the paper underscores the promising prospects at the LHC for realizing this quantum phenomenon, potentially marking a notable scientific milestone.