Search

We show how in the standard electroweak model three SU(2)L Nambu monopoles, each carrying electromagnetic (EM) and Z magnetic fluxes, can merge (through Z strings) with a single U(1)Y Dirac monopole to yield a composite monopole that only carries EM magnetic flux. Compatibility with the Dirac quantization condition requires this composite monopole to carry six quanta (12π/e) of magnetic charge, independent of the electroweak mixing angle θw. The Dirac monopole is not regular at the origin and the energy of the composite monopole is therefore divergent. We discuss how this problem is cured by embedding U(1)Y in a grand unified group such as SU(5). A second composite configuration with only one Nambu monopole and a colored U(1)Y Dirac monopole that has minimal EM charge of 4π/e is also described. Finally, there exists a configuration with an EM charge of 8π/e as well as screened color magnetic charge. [ABSTRACT FROM AUTHOR]

We propose a pseudo-Goldstone boson dark matter (pGDM) particle in SO(10) grand unified theory (GUT). Due to its Goldstone nature, this pGDM evades the direct DM detection experiments which, otherwise, severely constrain the parameter space of DM models. In SO(10), the pGDM is embedded as a linear combination of the Standard Model (SM) singlet scalars in 16H and ̅126H representations. We consider two scenarios for the intermediate route of SO(10) symmetry breaking (SB) to the SM: SU(5)×U(1)X and Pati-Salam the SU(4)c×SU(2)L×SU(2)R(4-2-2) gauge groups. The vacuum expectation value of ̅126H, which triggers the breaking of U(1)X and 4-2-2 symmetry in the two scenarios, respectively, determines the pGDM lifetime whose astrophysical lower bound provides one of the most stringent constraints. For the 4-2-2 route to SO(10), the successful SM gauge coupling unification requires the 4-2-2 breaking scale to be O(1011) GeV, and most of the parameter space is excluded. For the SU(5)×U(1)X route, on the other hand, the U(1)X breaking scale can be significantly higher, and a wide range of the parameter space is allowed. Furthermore, the proton lifetime in the SU(5) case is predicted to be 4.53×1034 years, which lies well within the sensitivity reach of the Hyper-Kamiokande experiment. We also examine the constraints on the model parameter space from the Large Hadron Collider and the indirect DM search by Fermi-LAT and MAGIC experiments. [ABSTRACT FROM AUTHOR]

We investigate the impact of Coleman-Weinberg inflation on the stochastic gravity wave background spectrum emitted by intermediate-scale cosmic strings. The string network is partially inflated and reenters the horizon at later times after the end of inflation, such that the short string loops are not produced. This leads to a significant modification of the gravity wave spectrum that we explore in detail. We find that Coleman-Weinberg inflation can help to satisfy the Parkes Pulsar Timing Array (PPTA) bound for dimensionless string tension values in the range Gμ1.1×10-10. We also identify the modified gravity wave spectra which, in the case of inflation, are compatible with the North American Nanohertz Observatory for Gravitational Waves (NANOGrav) data. We then discuss the formation of monopoles and strings at the same breaking scale and the compatibility of the Monopole, Astrophysics and Cosmic Ray Observatory (MACRO) bound with the PPTA bound, and also with the NANOGrav data. Finally, an example of a realistic nonsupersymmetric E6 model incorporating successful Coleman-Weinberg inflation is presented in which monopoles and strings both survive inflation and are present at an observable level. [ABSTRACT FROM AUTHOR]

We propose a SU(5)×U(1)X×U(1)PQ model, where U(1)X is the generalization of the B-L (baryon minus lepton number) gauge symmetry and U(1)PQ is the global Peccei-Quinn (PQ) symmetry. There are four fermions families in ¯5+10 representations of SU(5), a mirror family in 5+10 representations, and three SU(5) singlet Majorana fermions. The U(1)X related anomalies all cancel in the presence of the Majorana neutrinos. The SU(5) symmetry is broken at MGUT≃(6-9)×1015 GeV and the proton lifetime τp is estimated to be well within the expected sensitivity of the future hyper-Kamiokande experiment, τp≲1.3×1035 years. The SU(5) breaking also triggers the breaking of the PQ symmetry, resulting in axion dark matter (DM), with the axion decay constant fa of order MGUT or somewhat larger. The CASPEr experiment can search for such an axion DM candidate. The Hubble parameter during inflation must be low, Hinf≲109 GeV, in order to successfully resolve the axion domain wall, axion DM isocurvature and SU(5) monopole problems. With the identification of the U(1)X breaking Higgs field with the inflaton field, we implement inflection-point inflation, which is capable of realizing the desired value for Hinf. The vectorlike fermions in the model are essential for achieving successful unification of the SM gauge couplings as well as the phenomenological viability of both axion DM and inflation scenario. [ABSTRACT FROM AUTHOR]