ALPtraum: ALP production in proton beam dump experiments

TL;DR

The paper addresses the challenge of reliably predicting ALP production in proton fixed-target experiments, where the composite nature of the proton and target complicates calculations. It demonstrates that Primakoff production of ALPs with photon coupling $g_{a\gamma}$ can be computed accurately in a momentum regime where simple electromagnetic form factors suffice, enabling precise rates and angular distributions. By reanalyzing CHARM and NuCal, the work derives new constraints on the ALP parameter space and, using realistic detector geometries, shows that NA62 in dump mode and the proposed SHiP facility can probe previously unexplored regions of $m_a$ and $g_{a\gamma}$. The results provide a practical, geometry-aware framework that leverages existing and near-future experiments to search for ALPs in the MeV–GeV range, with broad applicability to scalar ALPs and other weak couplings.

Abstract

With their high beam energy and intensity, existing and near-future proton beam dumps provide an excellent opportunity to search for new very weakly coupled particles in the MeV to GeV mass range. One particularly interesting example is a so-called axion-like particle (ALP), i.e. a pseudoscalar coupled to two photons. The challenge in proton beam dumps is to reliably calculate the production of the new particles from the interactions of two composite objects, the proton and the target atoms. In this work we argue that Primakoff production of ALPs proceeds in a momentum range where production rates and angular distributions can be determined to sufficient precision using simple electromagnetic form factors. Reanalysing past proton beam dump experiments for this production channel, we derive novel constraints on the parameter space for ALPs. We show that the NA62 experiment at CERN could probe unexplored parameter space by running in 'dump mode' for a few days and discuss opportunities for future experiments such as SHiP.

Figures (9)

• Figure 1: Summary of constraints on the ALP parameter space (compilation from Jaeckel:2015jla and references therein; in particular SLAC electron fixed target limits are from Riordan:1987awBjorken:1988asHewett:2012ns). The new limits from the proton beam dump experiments CHARM and NuCal, derived in the present paper, are shown in turquoise and orange.
• Figure 2: Primakoff production of ALPs in proton-nucleus collisions.
• Figure 3: Predictions for the differential ALP-production cross section from 400 GeV protons on a copper target in the laboratory frame at $g_{a \gamma}= 10^{-4}\ {\rm GeV}^{-1}$ for $m_a = 50\:\text{MeV}$ (left) and $m_a = 500\:\text{MeV}$ (right) .
• Figure 4: Predictions for the differential ALP-production cross section in the laboratory frame as a function of $E_a$ (left) and $\theta$ (right) obtained in two different ways. Blue (dashed): Using equivalent photon spectra for both the proton beam and the target nuclei in the centre-of-mass frame and calculating the probability for photon fusion. Orange (dotted): Using equivalent photon spectra only for the proton beam in the laboratory frame and calculating the probability for ALP-emission and photon absorption. Note that, in contrast to figure \ref{['fig:alp-distribution']}, we show $\mathrm{d}^2 \sigma_{pN} / \mathrm{d} E_a \, \mathrm{d} \cos \theta = (\sin \theta)^{-1} \, \mathrm{d}^2 \sigma_{pN} / \mathrm{d} E_a \, \mathrm{d} \theta$, which remains finite for $\theta \rightarrow 0$.
• Figure 5: Layout of NA62, sketch taken from Valente:1293104. The SPS proton beam (from left), hits the target and the kaons of the secondary hadron beam are identified and measured before entering the vacuum decay region downstream. For a potential ALP search, most preferentially the beam should directly impinge on the TAX (see text), the liquid krypton calorimeter for photon detection is placed approximately at a distance of $241\:\text{m}$ behind the target. Overall exact geometric information is available at BEATCH. The bar below the diagram indicates the effective length of the absorber $D$ and the decay volume $L$ we have used in our calculations.