Light scalars in neutron star mergers

Abstract

Due to their unique set of multimessenger signals, neutron star mergers have emerged as a novel environment for studies of new physics beyond the Standard Model (SM). As a case study, we consider the simplest extension of the SM scalar sector involving a light CP-even scalar singlet S mixing with the SM Higgs boson. These S particles can be produced abundantly in neutron star mergers via the nucleon bremsstrahlung process. We show that the S particles may either be trapped in or stream freely out of the merger remnant, depending on the S mass, its mixing with the SM Higgs boson, and the temperature and baryon density in the merger. In the free-streaming region, the scalar S will provide an extra channel to cool down the merger remnant, with cooling timescales as small as O(ms). On the other hand, in the trapped region, the Bose gas of S particles could contribute a larger thermal conductivity than the trapped neutrinos in some parts of the parameter space, thus leading to faster thermal equilibration than expected. Therefore, future observations of the early postmerger phase of a neutron star merger could effectively probe a unique range of the S parameter space, largely complementary to the existing and future laboratory and supernova limits. In view of these results, we hope the merger simulation community will be motivated to implement the effects of light CP-even scalars into their simulations in both the free-streaming and trapped regimes.

Light scalar explanation for 18 TeV GRB 221009A

“…In this work, we will use the 18 TeV GRB γ-ray flux observation and the number of the detected events in the 2000 second observation window to set new astrophysical bounds. These limits complement the existing bounds derived from stellar cooling and luminosity measurements of the Sun, white dwarfs, red giants and supernovae [57][58][59].…”

“…However, we know that the dominant mode for light scalar production is nucleon-nucleon bremsstrahlung via pion exchange. [54,59,63,[65][66][67][68][69]. Possible sources for very high energy scalars in high nucleon density environments could be via the interaction of AGN jets with…”

Improved stellar limits on a light CP-even scalar

“…(2.1), it is found that the contributions from the S-nucleon diagrams are partially canceled out. As a result, the matrix element for the process (2.3) is dominated by the SN N and Sππ diagrams for the mass ranges of m S 10 MeV and m S 10 MeV [54,55], respectively.…”

“…stream from the merger [55]. For the free streaming regions, the scalar S might take away sizable energy from the stars, thus contributing to energy loss.…”

“…The corresponding SN1987A limits on the light CP-even scalar S have been studied in refs. [38,[49][50][51][52][53][54][55]. Assuming constant baryon density n B = 1.2 × 10 38 cm −3 and temperature T = 30 MeV within the supernova core with radius R c = 10 km, and applying the observed neutrino luminosity of L ν = 3 × 10 53 erg/sec, the mixing angle sin θ is excluded in the range of 5.9 × 10 −7 to 7.0 × 10 −6 with scalar mass up to 148 MeV [54].…”