Searching for axion-like particles with proton tagging at the LHC

TL;DR

Baldenegro et al. investigate axion-like particles (ALPs) with photon couplings by exploiting central exclusive diphoton production in proton-proton collisions at the LHC, with forward proton tagging to fully constrain the event. They implement ALP-induced light-by-light scattering within an EFT framework, compute helicity amplitudes for CP-even and CP-odd couplings, and evaluate production rates via the equivalent photon approximation. A realistic analysis includes 13 TeV collisions, 300 fb^-1, stringent photon and proton-selection cuts, and a background model dominated by non-exclusive photon pairs overlapped with soft diffraction, which largely suppresses background to near zero. The resulting projected bounds on the ALP-photon coupling f^{-1} reach down to a few 10^-2–10^-1 TeV^-1 for masses 0.6–2 TeV, demonstrating that proton-tagged exclusive diphoton production provides a competitive and complementary probe, especially for broad resonances.

Abstract

The existence of an axion-like particle (ALP) would induce anomalous scattering of light by light. This process can be probed at the Large Hadron Collider in central exclusive production of photon pairs in proton-proton collisions by tagging the surviving protons using forward proton detectors. Using a detailed simulation, we estimate the expected bounds on the ALP--photon coupling for a wide range of masses. We show that the proposed search is competitive and complementary to other collider bounds for masses above 600 GeV, especially for resonant ALP production between 600 GeV and 2 TeV. Our results are also valid for a CP-even scalar, and the efficiency of the search is independent of the width of the ALP.

Figures (7)

• Figure 1: Schematic diagram of an axion-like particle production in two-photon coherent emission in proton-proton collisions. The scattered intact protons are tagged with the forward proton detectors and the photon pair is detected in the central detector.
• Figure 2: Schematic diagram of the proton tagging method at the LHC in central exclusive processes. The central detector (circle) collects the photon pair. The LHC magnets (blue) act as a precise momentum spectrometer on the outgoing intact protons. The protons pass through the forward detectors (black boxes) and their kinematic information is reconstructed offline. The dashed line represents the beamline.
• Figure 3: The dominant background for central exclusive diphoton production comes from non-exclusive photon pair production (left) overlapped with uncorrelated protons coming from soft diffractive processes in the additional interactions per bunch crossing (right).
• Figure 4: Differential yield as a function of the photon pair invariant mass for exclusive diphoton candidates with two tagged protons within the acceptance $0.015<\xi_{1,2}<0.15$. No elastic or exclusive offline selection is applied for the diphoton candidates in this plot. We assume there are in average 50 secondary interactions per bunch crossing. For illustrative purposes, we show an instance of a resonant ALP production with $m_{a} = 1200$ GeV and a coupling value $f^{-1} = 0.1$ TeV$^{-1}$.
• Figure 5: Distributions of the ratio of the diphoton mass reconstructed with the forward detectors $m_{pp} = \sqrt{\xi_1\xi_2 s}$ to the reconstructed diphoton mass $m_{\gamma\gamma}$ (left) and the difference of the diphoton rapidity $y_{\gamma\gamma}$ and the rapidity reconstructed with the forward detectors $y_{pp}=\frac{1}{2}\log(\frac{\xi_1}{\xi_2})$ distribution (right). Diphoton candidates in these plots have passed the elastic selection and the mass lowerbound of 600 GeV. A strong correlation between the forward-backward and central information can be seen for the signal (light blue), while for the background (red line) we see these variables are uncorrelated. We select diphoton candidates lying inside the dashed vertical lines. The width of the signal in these plots is caused mainly by the $\xi_{1,2}$ resolution. The integrated luminosity is 300 fb$^{-1}$ and the average number of pileup interactions is $\mu = 50$. The intact protons lie within the acceptance $0.015<\xi_{1,2}<0.15$.