We show that experiments at the Forward Physics Facility, planned to operate near the ATLAS interaction point during the LHC high-luminosity era, will be able to probe predictions of Little String Theory by searching for anomalous U (1) gauge bosons living in the bulk. The interaction of the abelian broken gauge symmetry with the Standard Model is generated at the one-loop level through kinetic mixing with the photon. Gauge invariant generation of mass for the U(1) gauge boson proceeds via the Higgs mechanism in spontaneous symmetry breaking, or else through anomaly-cancellation appealing to Stuckelberg-mass terms. We demonstrate that FASER2 will be able to probe string scales over roughly two orders of magnitude: 10(5) less than or similar to M-s/TeV less than or similar to 10(7).

It was recently shown that the N = 2 string topological amplitudes in the heterotic weak coupling limit generate a six-dimensional Melvin space, providing a description of the Omega-background in string theory, where string propagation can be exactly studied. In this work, we generalise the analysis to the refined case of the Omega-background with two independent deformation parameters. The Melvin space is now ten-dimensional and is extended by an action on the internal K3 compactification manifold of the heterotic superstring, corresponding to an SU(2)(R) rotation in the field theory description. We identify the class of heterotic topological amplitudes realising this background as the scattering of two anti-self-dual gravitons and arbitrary numbers of anti-self-dual graviphotons, self-dual vector fields of the dilaton multiplet, together with self-dual magnetic fluxes along the K3. In the field theory limit, our result correctly reproduces the perturbative part of the Nekrasov free energy in the case where both equivariant parameters are turned on.

This article is an addendum to [ 1]. We extend our computation of the onshell scattering amplitudes in an arbitrary inflaton background (phi) over bar (1). Although the effective Einstein frame cutoff for (phi) over bar (1) >> MP /root xi turns out to be <<(phi)over bar> or xi(phi) over bar (1) for the U(1) model, this is not the case for the realistic doublet Higgs model where the effective Einstein frame cutoff turns out to be the standard MP /root xi for both the Palatini and metric formulations. However, as it has been pointed out in [1] the background (phi) over bar (1) is the effective Jordan frame cutoff in all cases for both the Palatini and metric formalisms.