PI: Christine Davies
Axion-like particles (ALPs) frequently appear as light spinless particles in extensions of the Standard Model of particle physics. How best to find them, if they exist, depends on their mass and the strength of their coupling to known particles.
Figure 1. Diagrams showing how a vector meson, V (here the J/𝛙), decays to an axion-like particle, α, and a photon. HPQCD have calculated the diagram on the left using lattice QCD for the first time.
If the ALP mass is in the range of a few MeV/c2 to a few GeV/c2, a good place to search is in decays of a heavy vector (spin 1) meson, such as the J/ψ containing a charm-anticharm quark pair. Figure 1 shows two decay routes with the ALP, a, coupling to a quark (left) or photon (right). The right-hand diagram needs the J/ψ annihilation amplitude, already known to 0.4% from HPQCD (arxiv:2005.01845). HPQCD has now determined the left-hand diagram for the J/ψ, using DiRAC@Cambridge (arxiv:2502.06721).
Figure 2. Branching ratio for 𝐽/ψ → 𝛄𝒂 for an ALP coupling only to c quarks, divided by that coupling. The lattice QCD results (in green) are much more accurate than earlier estimates from perturbation theory. The x-axis is the ALP mass in GeV, covering the range from zero up to the 𝐽/ψ mass.
Figure 3. Example of coupling values that can be excluded (coloured shading) for an ALP mass of 0.25 GeV. These use HPQCD’s results and experimental searches from BESIII for an ALP decaying to 𝜇⁺𝜇⁻(teal) and 𝛄𝛄 (green). We assume here that quark couplings are flavour universal i.e 𝒄𝑢𝑢 = 𝒄𝒄𝒄 and photon and quark couplings have the same sign.
Figure 2 shows the normalised probability for J/ψ decay to γa from lattice QCD as a function of the ALP mass, with much improved accuracy (better than 2%) over earlier estimates from perturbation theory.
The limits that can be placed on ALP couplings to photons and quarks using the new HPQCD results are demonstrated in Figure 3. The coloured areas are excluded by combining HPQCD’s calculations with BESIII experimental search results for J/ψ→γa. The lattice QCD results give much more reliable limits than previously possible.