arXiv subjects

Keith A. Olive

Publications and source records attributed to Keith A. Olive.

At least 127 records · Page 7Linked to original sources

arXiv2015

Gluino Coannihilation Revisited

Some variants of the MSSM feature a strip in parameter space where the lightest neutralino is identified as the lightest supersymmetric particle (LSP), the gluino is the next-to-lightest supersymmetric particle (NLSP) and is nearly degenerate with the LSP, and the relic cold dark matter density is brought into the range allowed by astrophysics and cosmology by coannihilation with the gluino NLSP. We calculate the relic density along this gluino coannihilation strip in the MSSM, including the effects of gluino-gluino bound states and initial-state Sommerfeld enhancement, and taking into account the decoupling of the gluino and LSP densities that occurs for large values of the squark mass. We find that bound-state effects can increase the maximum LSP mass for which the relic cold dark matter density lies within the range favoured by astrophysics and cosmology by as much as ~ 50% if the squark to gluino mass ratio is 1.1, and that the LSP may weigh up to ~ 8 TeV for a wide range of the squark to gluino mass ratio \lesssim 100.

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arXiv2015

No-Scale Inflation

Supersymmetry is the most natural framework for physics above the TeV scale, and the corresponding framework for early-Universe cosmology, including inflation, is supergravity. No-scale supergravity emerges from generic string compactifications and yields a non-negative potential, and is therefore a plausible framework for constructing models of inflation. No-scale inflation yields naturally predictions similar to those of the Starobinsky model based on $R + R^2$ gravity, with a tilted spectrum of scalar perturbations: $n_s \sim 0.96$, and small values of the tensor-to-scalar perturbation ratio $r < 0.1$, as favoured by Planck and other data on the cosmic microwave background (CMB). Detailed measurements of the CMB may provide insights into the embedding of inflation within string theory as well as its links to collider physics.

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arXiv2015

The origin of dispersion in DLA metallicities

Recent chemical abundance measurements of damped Ly-alpha absorbers (DLAs) revealed an intrinsic scatter in their metallicity of ~0.5 dex out to z~5. In order to explore the origin of this scatter, we build a semi-analytic model which traces the chemical evolution of the interstellar matter in small regions of the Universe with different mean density, from over- to underdense regions. We show that the different histories of structure formation in these regions, namely halo abundance, mass and stellar content, is reflected in the chemical properties of the protogalaxies, and in particular of DLAs. We calculate mean metallicity-redshift relations and show that the metallicity dispersion arising from this environmental effect amounts to ~0.25 dex and is an important contributor to the observed overall intrinsic scatter.

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arXiv2015

Calculations of Inflaton Decays and Reheating: with Applications to No-Scale Inflation Models

We discuss inflaton decays and reheating in no-scale Starobinsky-like models of inflation, calculating the effective equation-of-state parameter, $w$, during the epoch of inflaton decay, the reheating temperature, $T_{\rm reh}$, and the number of inflationary e-folds, $N_*$, comparing analytical approximations with numerical calculations. We then illustrate these results with applications to models based on no-scale supergravity and motivated by generic string compactifications, including scenarios where the inflaton is identified as an untwisted-sector matter field with direct Yukawa couplings to MSSM fields, and where the inflaton decays via gravitational-strength interactions. Finally, we use our results to discuss the constraints on these models imposed by present measurements of the scalar spectral index $n_s$ and the tensor-to-scalar perturbation ratio $r$, converting them into constraints on $N_*$, the inflaton decay rate and other parameters of specific no-scale inflationary models.

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arXiv2015

Dark Matter and Gauge Coupling Unification in Non-supersymmetric SO(10) Grand Unified Models

Unlike minimal SU(5), SO(10) provides a straightforward path towards gauge coupling unification by modifying the renormalization group evolution of the gauge couplings above some intermediate scale which may also be related to the seesaw mechanism for neutrino masses. Unification can be achieved for several different choices of the intermediate gauge group below the SO(10) breaking scale. In this work, we consider in detail the possibility that SO(10) unification may also provide a natural dark matter candidate, stability being guaranteed by a leftover $Z_2$ symmetry. We systematically examine the possible intermediate gauge groups which allow a non-degenerate, fermionic, Standard Model singlet dark matter candidate while at the same time respecting gauge coupling unification. Our analysis is done at the two-loop level. Surprisingly, despite the richness of SO(10), we find that only two models survive the analysis of phenomenological constraints, which include suitable neutrino masses, proton decay, and reheating.

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arXiv2015

Big Bang Nucleosynthesis: 2015

Big-bang nucleosynthesis (BBN) describes the production of the lightest nuclides via a dynamic interplay among the four fundamental forces during the first seconds of cosmic time. We briefly overview the essentials of this physics, and present new calculations of light element abundances through li6 and li7, with updated nuclear reactions and uncertainties including those in the neutron lifetime. We provide fits to these results as a function of baryon density and of the number of neutrino flavors, N_nu. We review recent developments in BBN, particularly new, precision Planck cosmic microwave background (CMB) measurements that now probe the baryon density, helium content, and the effective number of degrees of freedom, n_eff. These measurements allow for a tight test of BBN and of cosmology using CMB data alone. Our likelihood analysis convolves the 2015 Planck data chains with our BBN output and observational data. Adding astronomical measurements of light elements strengthens the power of BBN. We include a new determination of the primordial helium abundance in our likelihood analysis. New D/H observations are now more precise than the corresponding theoretical predictions, and are consistent with the Standard Model and the Planck baryon density. Moreover, D/H now provides a tight measurement of N_nu when combined with the CMB baryon density, and provides a 2sigma upper limit N_nu < 3.2. The new precision of the CMB and of D/H observations together leave D/H predictions as the largest source of uncertainties. Future improvement in BBN calculations will therefore rely on improved nuclear cross section data. In contrast with D/H and he4, li7 predictions continue to disagree with observations, perhaps pointing to new physics.

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arXiv2015

Phenomenological Aspects of No-Scale Inflation Models

We discuss phenomenological aspects of no-scale supergravity inflationary models motivated by compactified string models, in which the inflaton may be identified either as a Kähler modulus or an untwisted matter field, focusing on models that make predictions for the scalar spectral index $n_s$ and the tensor-to-scalar ratio $r$ that are similar to the Starobinsky model. We discuss possible patterns of soft supersymmetry breaking, exhibiting examples of the pure no-scale type $m_0 = B_0 = A_0 = 0$, of the CMSSM type with universal $A_0$ and $m_0 \ne 0$ at a high scale, and of the mSUGRA type with $A_0 = B_0 + m_0$ boundary conditions at the high input scale. These may be combined with a non-trivial gauge kinetic function that generates gaugino masses $m_{1/2} \ne 0$, or one may have a pure gravity mediation scenario where trilinear terms and gaugino masses are generated through anomalies. We also discuss inflaton decays and reheating, showing possible decay channels for the inflaton when it is either an untwisted matter field or a Kähler modulus. Reheating is very efficient if a matter field inflaton is directly coupled to MSSM fields, and both candidates lead to sufficient reheating in the presence of a non-trivial gauge kinetic function.

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arXiv2015

SU(5) Grand Unification in Pure Gravity Mediation

We discuss the proton lifetime in pure gravity mediation models with non-universal Higgs soft masses. Pure gravity mediation offers a simple framework for studying SU(5) grand unified theories with a split supersymmetry like spectra. We find that for much of the parameter space gauge coupling unification is quite good leading to rather long lifetimes for the proton. However, for $m_{3/2}\sim 60$ TeV and $\tanβ\sim 4$, for which gauge coupling unification is also good, the proton lifetime is short enough that it could be in reach of future experiments.

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arXiv2015

The effects of He I 10830 on helium abundance determinations

Observations of helium and hydrogen emission lines from metal-poor extragalactic H II regions provide an independent method for determining the primordial helium abundance, Y_p. Traditionally, the emission lines employed are in the visible wavelength range, and the number of suitable lines is limited. Furthermore, when using these lines, large systematic uncertainties in helium abundance determinations arise due to the degeneracy of physical parameters, such as temperature and density. Recently, Izotov, Thuan, & Guseva (2014) have pioneered adding the He 10830 infrared emission line in helium abundance determinations. The strong electron density dependence of He 10830 makes it ideal for better constraining density, potentially breaking the degeneracy with temperature. We revisit our analysis of the dataset published by Izotov, Thuan, & Stasinska (2007) and incorporate the newly available observations of He 10830 by scaling them using the observed-to-theoretical Paschen-gamma ratio. The solutions are better constrained, in particular for electron density, temperature, and the neutral hydrogen fraction, improving the model fit to data, with the result that more spectra now pass screening for quality and reliability, in addition to a standard 95% confidence level cut. Furthermore, the addition of He 10830 decreases the uncertainty on the helium abundance for all galaxies, with reductions in the uncertainty ranging from 10-80%. Overall, we find a reduction in the uncertainty on Y_p by over 50%. From a regression to zero metallicity, we determine Y_p = 0.2449 +/- 0.0040, consistent with the BBN result, Y_p = 0.2470 +/- 0.0002, based on the Planck determination of the baryon density. The dramatic improvement in the uncertainty from incorporating He 10830 strongly supports the case for simultaneous (thus not requiring scaling) observations of visible and infrared helium emission line spectra.

astro-ph.CO↗

arXiv2014

Light Higgsinos in Pure Gravity Mediation

Pure gravity mediation (PGM), with two free parameters, is a minimalistic approach to supergravity models, yet is capable of incorporating radiative electroweak symmetry breaking, a Higgs mass in agreement with the experimental measurement, without violating any phenomenological constraints. The model may also contain a viable dark matter candidate in the form of a wino. Here, we extend the minimal model by allowing the $μ$-term to be a free parameter (equivalent to allowing the two Higgs soft masses, $m_1$ and $m_2$, to differ from other scalar masses) which are set by the gravitino mass. In particular, we examine the region of parameter space where $μ\ll m_{3/2}$ in which case, the Higgsino becomes the lightest supersymmetric particle (LSP) and a dark matter candidate. We also consider a generalization of PGM which incorporates a Peccei-Quinn symmetry which determines the $μ$-term dynamically. In this case, we show that the dark matter may either be in the form of an axion and/or a neutralino, and that the LSP may be either a wino, bino, or Higgsino.

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arXiv2014

Two-Field Analysis of No-Scale Supergravity Inflation

Since the building-blocks of supersymmetric models include chiral superfields containing pairs of effective scalar fields, a two-field approach is particularly appropriate for models of inflation based on supergravity. In this paper, we generalize the two-field analysis of the inflationary power spectrum to supergravity models with arbitrary Kähler potential. We show how two-field effects in the context of no-scale supergravity can alter the model predictions for the scalar spectral index $n_s$ and the tensor-to-scalar ratio $r$, yielding results that interpolate between the Planck-friendly Starobinsky model and BICEP2-friendly predictions. In particular, we show that two-field effects in a chaotic no-scale inflation model with a quadratic potential are capable of reducing $r$ to very small values $\ll 0.1$. We also calculate the non-Gaussianity measure $f_{\rm NL}$, finding that is well below the current experimental sensitivity.

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arXiv2014

A No-Scale Inflationary Model to Fit Them All

The magnitude of B-mode polarization in the cosmic microwave background as measured by BICEP2 favours models of chaotic inflation with a quadratic $m^2 ϕ^2/2$ potential, whereas data from the Planck satellite favour a small value of the tensor-to-scalar perturbation ratio $r$ that is highly consistent with the Starobinsky $R + R^2$ model. Reality may lie somewhere between these two scenarios. In this paper we propose a minimal two-field no-scale supergravity model that interpolates between quadratic and Starobinsky-like inflation as limiting cases, while retaining the successful prediction $n_s \simeq 0.96$.

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arXiv2014

Universality in Pure Gravity Mediation with Vector Multiplets

We consider models of Pure Gravity Mediation in which scalar mass universality is imposed at the grand unified scale and gaugino masses are generated through loops. The minimal model requires a very restricted range for $\tan β\approx 2-3$ and scalar masses (set by the gravitino mass) of order 300 TeV - 1.5 PeV in order to obtain a Higgs mass near 126 GeV. Here we augment the minimal model with one or more sets of vector multiplets (either a ${10}$ and $\bar {10}$ pair or one or more $5$ and $\bar 5$ pairs). If coupled to the MSSM Higgs, these allow for radiative electroweak symmetry breaking over a significantly larger range of $\tan β\approx 2-40$ and can fit the Higgs mass with much smaller values of the gravitino mass. In these models, the lightest supersymmetric particle (LSP) is often the bino, and in order to satisfy the relic density constraint, the bino must be nearly degenerate with either the wino or gluino. In the models considered here, bino gluino coannihilations determine the relic density and since the two are nearly degenerate, LHC limits on the gluino mass are greatly relaxed allowing light relatively gravitinos and gluino masses well within the reach of the LHC.

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arXiv2014

Universality in Pure Gravity Mediation

If low energy supersymmetry is realized in nature, the apparent discovery of a Higgs boson with mass around 125 GeV points to a supersymmetric mass spectrum in the TeV or multi-TeV range. Multi-TeV scalar masses are a necessary component of supersymmetric models with pure gravity mediation or in any model with strong moduli stabilization. Here, we show that full scalar mass universality remains viable as long as the ratio of Higgs vevs, tan beta is relatively small (\lesssim 2.5). We discuss in detail the low energy (observable) consequences of these models.

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arXiv2014

The Extent of the Stop Coannihilation Strip

Many supersymmetric models such as the CMSSM feature a strip in parameter space where the lightest neutralino χis identified as the lightest supersymmetric particle (LSP), the lighter stop squark \tilde t_1 is the next-to-lightest supersymmetric particle (NLSP), and the relic χcold dark matter density is brought into the range allowed by astrophysics and cosmology by coannihilation with the lighter stop squark \tilde t_1 NLSP. We calculate the stop coannihilation strip in the CMSSM, incorporating Sommerfeld enhancement effects, and explore the relevant phenomenological constraints and phenomenological signatures. In particular, we show that the \tilde t_1 may weigh several TeV, and its lifetime may be in the nanosecond range, features that are more general than the specific CMSSM scenarios that we study in this paper.

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arXiv2014

Resurrecting Quadratic Inflation in No-Scale Supergravity in Light of BICEP2

The magnitude of primordial tensor perturbations reported by the BICEP2 experiment is consistent with simple models of chaotic inflation driven by a single scalar field with a power-law potential \propto ϕ^n: n \simeq 2, in contrast to the WMAP and Planck results, which favored models resembling the Starobinsky R + R^2 model if running of the scalar spectral index could be neglected. While models of inflation with a quadratic potential may be constructed in simple N=1 supergravity, these constructions are more challenging in no-scale supergravity. We discuss here how quadratic inflation can be accommodated within supergravity, focussing primarily on the no-scale case. We also argue that the quadratic inflaton may be identified with the supersymmetric partner of a singlet (right-handed) neutrino, whose subsequent decay could have generated the baryon asymmetry via leptogenesis.

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arXiv2014

Peccei-Quinn Symmetric Pure Gravity Mediation

Successful models of Pure Gravity Mediation (PGM) with radiative electroweak symmetry breaking can be expressed with as few as two free parameters which can be taken as the gravitino mass and $\tan β$. These models easily support a 125-126 GeV Higgs mass at the expense of a scalar spectrum in the multi-TeV range and a much lighter wino as the lightest supersymmetric particle. In these models, it is also quite generic that the Higgs mixing mass parameter, $μ$, which is determined by the minimization of the Higgs potential is also in the multi-TeV range. For $μ> 0$, the thermal relic density of winos is too small to account for the dark matter. The same is true for $μ< 0$ unless the gravitino mass is of order 500 TeV. Here, we consider the origin of a multi-TeV $μ$ parameter arising from the breakdown of a Peccei-Quinn (PQ) symmetry. A coupling of the PQ-symmetry breaking field, $P$, to the MSSM Higgs doublets, naturally leads to a value of $μ\sim \langle P \rangle^2 /M_P \sim \mathcal{O}(100)$ TeV and of order that is required in PGM models. In this case, axions make up the dark matter or some fraction of the dark matter with the remainder made up from thermal or non-thermal winos. We also provide solutions to the problem of isocurvature fluctuations with axion dark matter in this context.

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arXiv2014

A No-Scale Framework for Sub-Planckian Physics

We propose a minimal model framework for physics below the Planck scale with the following features: (i) it is based on no-scale supergravity, as favoured in many string compactifications, (ii) it incorporates Starobinsky-like inflation, and hence is compatible with constraints from the Planck satellite, (iii) the inflaton may be identified with a singlet field in a see-saw model for neutrino masses, providing an efficient scenario for reheating and leptogenesis, (iv) supersymmetry breaking occurs with an arbitrary scale and a cosmological constant that vanishes before radiative corrections, (v) regions of the model parameter space are compatible with all LHC, Higgs and dark matter constraints.

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