Grand Unification and Subcritical Hybrid Inflation

We consider hybrid inflation for small couplings of the inflaton to matter such that the critical value of the inflaton field exceeds the Planck mass. It has recently been shown that inflation then continues at subcritical inflaton field values where quantum fluctuations generate an effective inflaton mass. The effective inflaton potential interpolates between a quadratic potential at small field values and a plateau at large field values. An analysis of the allowed parameter space leads to predictions for the scalar spectral index n_s and the tensor-to-scalar ratio r similar to those of natural inflation. Using the range for n_s and r favoured by the Planck data, we find that the energy scale of the plateau is constrained to the interval (1.6 - 2.4) * 10^16 GeV which includes the energy scale of gauge coupling unification in the supersymmetric standard model. The tensor-to-scalar ratio is predicted to have the lower bound r > 0.049 for 60 e-folds before the end of inflation.Comment: 5 pages, 3 figures, published version, a few typos are correcte

Flavour mixings in flux compactifications

A multiplicity of quark-lepton families can naturally arise as zero-modes in flux compactifications. The flavour structure of quark and lepton mass matrices is then determined by the wave function profiles of the zero-modes. We consider a supersymmetric $SO(10)\times U(1)$ model in six dimensions compactified on the orbifold $T^2/{\mathbb Z_2}$ with Abelian magnetic flux. A bulk $\mathbf{16}$-plet charged under the $U(1)$ provides the quark-lepton generations whereas two uncharged $\mathbf{10}$-plets yield two Higgs doublets. Bulk anomaly cancellation requires the presence of additional $\mathbf{16}$- and $\mathbf{10}$-plets. The corresponding zero-modes form vectorlike split multiplets that are needed to obtain a successful flavour phenomenology. We analyze the pattern of flavour mixings for the two heaviest families of the Standard Model and discuss possible generalizations to three and more generations.Comment: 28 pages, 3 figures, 3 table

Proton decay in flux compactifications

We study proton decay in a six-dimensional orbifold GUT model with gauge group $SO(10)\times U(1)_A$. Magnetic $U(1)_A$ flux in the compact dimensions determines the multiplicity of quark-lepton generations, and it also breaks supersymmetry by giving universal GUT scale masses to scalar quarks and leptons. The model can successfully account for quark and lepton masses and mixings. Our analysis of proton decay leads to the conclusion that the proton lifetime must be close to the current experimental lower bound. Moreover, we find that the branching ratios for the decay channels $p \rightarrow e^+\pi^0$ and $p\rightarrow \mu^+\pi^0$ are of similar size, in fact the latter one can even be dominant. This is due to flavour non-diagonal couplings of heavy vector bosons together with large off-diagonal Higgs couplings, which appears to be a generic feature of flux compactifications.Comment: 26 pages, 3 figures, 2 table

Flavour physics without flavour symmetries

We quantitatively analyze a quark-lepton flavour model derived from a six-dimensional supersymmetric theory with $SO(10)\times U(1)$ gauge symmetry, compactified on an orbifold with magnetic flux. Two bulk $\mathbf{16}$-plets charged under the $U(1)$ provide the three quark-lepton generations whereas two uncharged $\mathbf{10}$-plets yield two Higgs doublets. At the orbifold fixed points mass matrices are generated with rank one or two. Moreover, the zero modes mix with heavy vectorlike split multiplets. The model possesses no flavour symmetries. Nevertheless, there exist a number of relations between Yukawa couplings, remnants of the underlying GUT symmetry and the wave function profiles of the zero modes, which lead to a prediction of the light neutrino mass scale, $m_{\nu_1} \sim 10^{-3}$ eV and heavy Majorana neutrino masses in the range from $10^{12}$ GeV to $10^{14}$ GeV. The model successfully includes thermal leptogenesis.Comment: Minor additions; Published versio

Metastable strings and grand unification

The structure of the Standard Model (SM) of particle physics points toward grand unified theories (GUTs) where strong and electroweak interactions are unified in a non-Abelian GUT group. The spontaneous breaking of the GUT symmetry to the SM symmetry, together with cosmic inflation, generically leads to metastable topological defects, the most prominent example being cosmic strings. The gravitational-wave background (GWB) produced by a cosmic string network is one of the candidates for an explanation of the GWB recently observed by pulsar timing array (PTA) experiments. We review some properties of the predicted GWB with emphasis on potential implications for GUT model building. The most striking prediction is a GWB in the LIGO-Virgo-KAGRA band that could be discovered in the near future.Comment: 15 pages, 6 figures; Rubakov conference, Yerevan. arXiv admin note: text overlap with arXiv:2307.0469

Probing Lumps of Wee Partons in Deep Inelastic Scattering

Recently, the ZEUS collaboration has reported on several remarkable properties of events with a large rapidity gap in deep inelastic scattering. We suggest that the mechanism underlying these events is the scattering of electrons off lumps of wee partons inside the proton. Based on an effective lagrangian approach the $Q^2$-, $x$- and $W$-distributions are evaluated. For sufficiently small invariant mass of the detected hadronic system, the mechanism implies leading twist behaviour. The $x$- and $W$-distributions are determined by the Lipatov exponent which governs the behaviour of parton densities at small $x$.Comment: 8 pages latex, DESY 94-10

Supersymmetric Moduli Stabilization and High-Scale Inflation

We study the back-reaction of moduli fields on the inflaton potential in generic models of F-term inflation. We derive the moduli corrections as a power series in the ratio of Hubble scale and modulus mass. The general result is illustrated with two examples, hybrid inflation and chaotic inflation. We find that in both cases the decoupling of moduli dynamics and inflation requires moduli masses close to the scale of grand unification. For smaller moduli masses the CMB observables are strongly affected.Comment: 5 page