String Cosmology - Large-Field Inflation in String Theory

This is a short review of string cosmology. We wish to connect string-scale physics as closely as possible to observables accessible to current or near-future experiments. Our possible best hope to do so is a description of inflation in string theory. The energy scale of inflation can be as high as that of Grand Unification (GUT). If this is the case, this is the closest we can possibly get in energy scales to string-scale physics. Hence, GUT-scale inflation may be our best candidate phenomenon to preserve traces of string-scale dynamics. Our chance to look for such traces is the primordial gravitational wave, or tensor mode signal produced during inflation. For GUT-scale inflation this is strong enough to be potentially visible as a B-mode polarization of the cosmic microwave background (CMB). Moreover, a GUT-scale inflation model has a trans-Planckian excursion of the inflaton scalar field during the observable amount of inflation. Such large-field models of inflation have a clear need for symmetry protection against quantum corrections. This makes them ideal candidates for a description in a candidate fundamental theory like string theory. At the same time the need of large-field inflation models for UV completion makes them particularly susceptible to preserve imprints of their string-scale dynamics in the inflationary observables, the spectral index $n_s$ and the fractional tensor mode power $r$. Hence, we will focus this review on axion monodromy inflation as a mechanism of large-field inflation in string theory.Comment: 34 pages, 1 figure, LaTeX, v2: typos fixed, 3 references added, agrees with published version in "Perspectives in String Phenomenology" and "International Journal of Modern Physics A

Supersymmetry and Unification: Heavy Top Was the Key

I review the unification of gauge couplings of strong, weak and electro-magnetic interactions. I start by recalling the history of the most important prediction of low-energy supersymmetry: the correct value of the weak mixing angle tied to a large top quark mass. I then turn to the discussion of the present day situation of the minimal supersymmetric Grand Unified Theories based on SU(5) and SO(10) groups, and I show why the minimal SU(5) is in accord with experiment. For the sake of completeness I also summarize the problems and possible solutions of the minimal ordinary SU(5). One version, based on the minimal Georgi-Glashow model, augmented by the adjoint fermion, predicts a light fermion triplet to lie below TeV or so. Its (lepton number violating) decays offer a hope of probing neutrino (Majorana) masses and mixings at the LHC.Comment: To be published in the proceedings of the Scientific and Human Legacy of Julius Wess, Memorial Workshop held in Donji Milanovac, Serbia, August 201

Elements of F-ast Proton Decay

Gauge coupling unification in the Minimal Supersymmetric Standard Model (MSSM) strongly suggests the existence of a Grand Unified Theory (GUT), which could be probed by the observation of proton decay. Proton lifetime in the p \to (e+|mu+) pi0 dimension six mode is proportional in the fourth power to the GUT mass scale, and inversely proportional in the fourth power to the GUT coupling. We provide an updated dictionary of solutions for the relevant unification parameters with generic beta-function coefficients, significantly upgrading the level of detail with which second order effects are treated, and correcting subtle published errors. F-lipped SU(5) with strict MSSM field content is known to survive existing null detection limits for proton decay approaching 10^34 years, and indeed, the lifetime predicted by prior studies can be so long that successful detection is not currently plausible. Recently studied classes of F-theory derived GUT models postulate additional vector-like multiplets at the TeV scale which modify the renormalization group to yield a substantial increase in the SU(3)_C X SU(2)_L unified coupling. We find the conjunction of these models with the F-resh analysis employed to be comparatively F-ast proton decay, only narrowly evading existing detection limits, and likely falling within the observable range of proposed next generation detectors such as DUSEL and Hyper-Kamiokande. The TeV-scale vector multiplets are themselves suitable for cross correlation by the Large Hadron Collider. Their presence moreover magnifies the gap between the dual mass scales of Flipped SU(5), allowing for an elongated second stage renormalization, pushing grand unification to the doorstep of the reduced Planck mass.Comment: V2, As published in Nuclear Physics B; 57 pages, 7 figures, 12 table

Charged-fermion masses in SO(10): analysis with scalars in 10+120

We consider the scenario in which the mass matrices of the charged fermions in the SO(10) Grand Unified Theory are generated exclusively by renormalizable Yukawa couplings to one $\mathbf{10} \oplus \mathbf{120}$ representation of scalars. We analyze, partly analytically and partly numerically, this scenario in the three-generations case. We demonstrate that it leads to unification of the $b$ and $\tau$ masses at the GUT scale. Testing this scenario against the mass values at the GUT scale, obtained from the renormalization-group evolution in the minimal SUSY extension of the Standard Model, we find that it is not viable: either the down-quark mass or the top-quark mass must be unrealistically low. If we include the CKM mixing angles in the test, then, in order that the mixing angles are well reproduced, either the top-quark mass or the strange-quark mass together with the down-quark mass must be very low. We conclude that, assuming a SUSY SO(10) scenario, charged-fermion mass generation based exclusively on one $\mathbf{10} \oplus \mathbf{120}$ representation of scalars is in contradiction with experiment.Comment: 18 pages, 3 eps figures; references added and corrected. We have also corrected an error in the code for the CKM matrix; the ensuing results are sharper at eliminating our scenario. In version 3 we have rectified a statement concerning Ref.[18] and added one reference and some phrases; final version for Nucl. Phys.

F-enomenology

The advantages of Flipped SU(5) over conventional Supersymmetric GUTs, like SU(5), are discussed. Recent values of the strong coupling at M_Z, sin-squared theta-Weinberg, g-2 of the muon, and the lower limit on the proton lifetime for the (K+, anti-neutrino) mode point directly to Flipped SU(5) as the simplest way to avoid potential pitfalls. It is shown that "F(lipped)-enomenology" accomodates easily all presently available low-energy data, favoring a rather "light" supersymmetric spectrum while yielding the right amount of Cold Dark Matter and a proton lifetime in the ((e+/muon+), pi-zero) mode which is beyond the present experimental limit yet still possibly accessible to a further round of experiments.Comment: 22 pages; 3 figures and 2 diagrams prepared with feynmf.mf & feynmf.sty; Invited talk given at: 1st Intl. Conf. on String Phenomenology, Oxford, England, July 6-11, 2002, -and- NeSS 2002, Washington D.C., USA, September 19-21, 200