Maurizio Giannotti

Revisiting the bound on axion-photon coupling from globular clusters.

We derive a strong bound on the axion-photon coupling g(aγ) from the analysis of a sample of 39 Galactic Globular Clusters. As recognized long ago, the R parameter, i.e., the number ratio of stars in horizontal over red giant branch of old stellar clusters, would be reduced by the axion production from photon conversions occurring in stellar cores. In this regard, we have compared the measured R with state-of-the-art stellar models obtained under different assumptions for g(aγ). We show that the estimated value of g(aγ) substantially depends on the adopted He mass fraction Y, an effect often neglected in previous investigations. Taking as a benchmark for our study the most recent determinations of the He abundance in H ii regions with O/H in the same range of the Galactic Globular Clusters, we obtain an upper bound g(aγ)<0.66×10(-10)  GeV(-1) at 95% confidence level. This result significantly improves the constraints from previous analyses and is currently the strongest limit on the axion-photon coupling in a wide mass range.

Revisiting the SN1987A gamma-ray limit on ultralight axion-like particles

We revise the bound from the supernova SN1987A on the coupling of ultralight axion-like particles (ALPs) to photons. In a core-collapse supernova, ALPs would be emitted via the Primakoff process, and eventually convert into gamma rays in the magnetic field of the Milky Way. The lack of a gamma-ray signal in the GRS instrument of the SMM satellite in coincidence with the observation of the neutrinos emitted from SN1987A therefore provides a strong bound on their coupling to photons. Due to the large uncertainty associated with the current bound, we revise this argument, based on state-of-the-art physical inputs both for the supernova models and for the Milky-Way magnetic field. Furthermore, we provide major amendments, such as the consistent treatment of nucleon-degeneracy effects and of the reduction of the nuclear masses in the hot and dense nuclear medium of the supernova. With these improvements, we obtain a new upper limit on the photon-ALP coupling: gaγ 5.3 × 10-12 GeV-1, for ma 4.4 × 10-10 eV, and we also give its dependence at larger ALP masses ma. Moreover, we discuss how much the Fermi-LAT satellite experiment could improve this bound, should a close-enough supernova explode in the near future.

Constraining the Axion-Photon Coupling with Massive Stars

We point out that stars in the mass window ~8-12M([circumpunct]) can serve as sensitive probes of the axion-photon interaction, g(Aγγ). Specifically, for these stars axion energy losses from the helium-burning core would shorten and eventually eliminate the blue loop phase of the evolution. This would contradict observational data, since the blue loops are required, e.g., to account for the existence of Cepheid stars. Using the MESA stellar evolution code, modified to include the extra cooling, we conservatively find g(Aγγ)</~0.8×10(-10) GeV(-1), which compares favorably with the existing bounds.

The Response of primordial abundances to a general modification of G(N) and/or of the early Universe expansion rate

We discuss the effects of a possible time variation of the Newton constant G{sub N} on light elements production in big bang nucleosynthesis (BBN). We provide analytical estimates for the dependence of primordial abundances on the value of the Newton constant during BBN. The accuracy of these estimates is then tested by numerical methods. Moreover, we determine numerically the response of each element to an arbitrary time-dependent modification of the early universe expansion rate. Finally, we determine the bounds on possible variations of G{sub N} which can be obtained from the comparison of theoretical predictions and observational data.

Cool WISPs for stellar cooling excesses

Several stellar systems (white dwarfs, red giants, horizontal branch stars and possibly the neutron star in the supernova remnant Cassiopeia A) show a mild preference for a non-standard cooling mechanism when compared with theoretical models. This exotic cooling could be provided by Weakly Interacting Slim Particles (WISPs), produced in the hot cores and abandoning the star unimpeded, contributing directly to the energy loss. Taken individually, these excesses do not show a strong statistical weight. However, if one mechanism could consistently explain several of them, the hint could be significant. We analyze the hints in terms of neutrino anomalous magnetic moments, minicharged particles, hidden photons and axion-like particles (ALPs). Among them, the ALP or a massless HP represent the best solution. Interestingly, the hinted ALP parameter space is accessible to the next generation proposed ALP searches, such as ALPS II and IAXO and the massless HP requires a multi TeV energy scale of new physics that might be accessible at the LHC.

Probing axions with the neutrino signal from the next galactic supernova

We study the impact of axion emission in simulations of massive star explosions, as an additional source of energy loss complementary to the standard neutrino emission. The inclusion of this channel shortens the cooling time of the nascent protoneutron star and hence the duration of the neutrino signal. We treat the axion-matter coupling strength as a free parameter to study its impact on the protoneutron star evolution as well as on the neutrino signal. We furthermore analyze the observability of the enhanced cooling in current and next-generation underground neutrino detectors, showing that values of the axion mass

THE IMPACT OF NEUTRINO MAGNETIC MOMENTS ON THE EVOLUTION OF MASSIVE STARS

{m}_{a}\ensuremath{\gtrsim}8\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}3}\text{ }\text{ }\mathrm{eV}

Mirror world, supersymmetric axion and gamma ray bursts

can be probed. Therefore a galactic supernova neutrino observation would provide a valuable possibility to probe axion masses in a range within reach of the planned helioscope experiment, the International Axion Observatory.

Electromagnetic effects in K(l3) decays

We explore the sensitivity of massive stars to neutrino magnetic moments. We find that the additional cooling due to the neutrino magnetic moments brings about qualitative changes to the structure and evolution of stars in the mass window 7 M ? M 18 M ?, rather than simply changing the timescales for the burning. We describe some of the consequences of this modified evolution: the shifts in the threshold masses for creating core-collapse supernovae and oxygen-neon-magnesium white dwarfs and the appearance of a new type of supernova in which a partial carbon-oxygen core explodes within a massive star. The resulting sensitivity to the magnetic moment is at the level of (2-4) ? 10?11 ?B.

Stellar Recipes for Axion Hunters

A modification of the relation between axion mass and the PQ constant permits a relaxation of the astrophysical constraints, considerably enlarging the allowed axion parameter space. We develop this idea in this paper, discussing a model for an ultramassive axion, which essentially represents a supersymmetric Weinberg-Wilczek axion of the mirror world. The experimental and astrophysical limits allow a PQ scale fa ~ 104?106 GeV and a mass ma ~ MeV, which can be accessible for future experiments. On a phenomenological ground, such an ultramassive axion turns out to be quite interesting. It can be produced during the gravitational collapse or during the merging of two compact objects, and its subsequent decay into e+e? provides an efficient mechanism for the transfer of the gravitational energy of the collapsing system to the electron-positron plasma. This could resolve the energy budget problem in the Gamma Ray Bursts and also help in understanding the SN type-II explosion phenomena.