arXiv subjects

Stuart Raby

Publications and source records attributed to Stuart Raby.

At least 37 records · Page 2Linked to original sources

arXiv2017

Vector-like Leptons: Muon g-2 Anomaly, Lepton Flavor Violation, Higgs Decays, and Lepton Non-Universality

In this paper, we consider the Standard Model (SM) with one family of vector-like (VL) leptons, which couple to all three families of the SM leptons. We study the constraints on this model coming from the heavy charged lepton mass bound, electroweak precision data, the muon anomalous magnetic moment, lepton flavor violation, Higgs decay constraints and a recently measured lepton non-universality observable, $R_{K^{*0}}$. We find that the strongest constraints are coming from the muon $g-2, \ R_{μμ}=Γ(h\toμμ)/Γ(h\toμμ)_\text{SM}, \ R_{γγ}$ and $\text{BR}(μ\to eγ)$. Although VL leptons couple to all three families of the SM leptons, the ratio of electron-VL to muon-VL coupling is constrained to be $\langleλ_e/λ_μ\rangle\lesssim10^{-4}$. We also find that our model cannot fit the observed value of $R_{K^{*0}}$.

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arXiv2017

A Pati-Salam SUSY GUT with Yukawa Unification

Previous studies of a three family Yukawa unified supersymmetric grand unified theory (SUSY GUT) with SO(10) or Pati-Salam (PS) gauge symmetry proposed by Raby and students show that this model is able to fit low energy and inflation observables. However, the fit to low energy observables is not great especially for $\sin2β$, and up and down quark masses. In this paper, we show that by choosing PS as the gauge group and modifying the Yukawa sector, the low energy fit improves significantly while other qualities of the model are maintained. In particular, the lightest SUSY particle is the neutralino with mass of order 300 - 500 GeV, the lightest stop and sbottom have mass of order 3 - 5 TeV and the CP odd Higgs mass is of order 5 - 6 TeV, so we are in the decoupling limit for the light Higgs. In addition, we reinterpret gluino simplified model analyses by the ATLAS and CMS collaborations and find that the most stringent gluino mass bound for our model is $M_{\tilde{g}}\sim1.9\,\text{TeV}$. The current best fit point, consistent with this bound, with gluino mass $M_{\tilde{g}}=1.9\,\text{TeV}$ has $χ^2/\text{dof}\approx1.12$, compared to the best fit point of the previous model with $χ^2/\text{dof}=1.90$. We find that predictions for the electric dipole moment of the electron, the branching ratio $BR(μ\rightarrow e γ)$ and the CP violating angle in the lepton sector, $\sinδ$, are affected significantly as compared to previous results. In summary, we are unable to rule out this model or place an upper bound on gluino mass accessible by this run of the LHC because the $χ^2/\text{dof}$ of this model is well below $2σ$ even for a gluino mass as high as 2.7 TeV. On the bright side, this means that this model is still viable and we might find low energy SUSY particles in the near future.

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arXiv2017

Leptogenesis after Inflation in a Pati-Salam Model

In this talk I discuss a supersymmetric Pati-Salam model of fermion masses and mixing angles which fits low energy data. The model is then extended to include an inflationary sector which is shown to be consistent with Bicep2-Keck-Planck data. The energy scale during inflation is associated with the PS symmetry breaking scale. Finally, the model is shown to be consistent with the observed baryon-to-entropy ratio necessary for Big Bang Nucleosynthesis. It turns out that only the heaviest right-handed neutrino decays produce the correct sign of the baryon-to-entropy ratio. Nevertheless, we obtain the observed value due to the process of instant preheating.

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arXiv2016

Reheating and Leptogenesis after Pati-Salam F-term Subcritical Hybrid Inflation

In this paper, we extend the analysis of a Pati-Salam subcritical hybrid $F$-term inflation model, proposed by two of us, by studying the reheating and the baryogenesis (via leptogenesis) of the model. This SUSY GUT model is able to fit low energy electroweak precision data, LHC data, $b$-physics data, in addition to inflation observables such as the tensor-to-scalar ratio and the scalar spectral index. The reheating mechanism of this model is instant preheating due to the bosonic and fermionic broad parametric resonance, while the baryon-to-entropy ratio is obtained from the CP asymmetric right-handed (s)neutrinos decay. The phases in the neutrino Yukawa matrices are fixed by fitting to the low energy observables. With these phases, the heaviest right-handed (s)neutrinos decay to produce a lepton asymmetry with the correct sign, while the two lighter right-handed (s)neutrinos decay to produce the wrong sign. Consequently, the baryogenesis analysis is necessarily performed by including all three families of the right-handed (s)neutrinos.

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arXiv2016

Axion star collisions with Neutron stars and Fast Radio Bursts

Axions may make a significant contribution to the dark matter of the universe. It has been suggested that these dark matter axions may condense into localized clumps, called "axion stars." In this paper we argue that collisions of dilute axion stars with neutron stars, of the type known as "magnetars," may be the origin of most of the observed fast radio bursts. This idea is a variation of an idea originally proposed by Iwazaki. However, instead of the surface effect of Iwazaki, we propose a perhaps stronger volume effect caused by the induced time dependent electric dipole moment of neutrons.

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arXiv2016

Yukawa Unification in an SO(10) SUSY GUT: SUSY on the Edge

In this paper we analyze Yukawa unification in a three family SO(10) SUSY GUT. We perform a global $χ^2$ analysis and show that SUSY effects do not decouple even though the universal scalar mass parameter at the GUT scale, $m_{16}$, is found to lie between 15 and 30 TeV with the best fit given for $m_{16} \approx 25$ TeV. Note, SUSY effects don't decouple since stops and bottoms have mass of order 5 TeV, due to RG running from $M_{GUT}$. The model has many testable predictions. Gauginos are the lightest sparticles and the light Higgs boson is very much Standard Model-like. The model is consistent with flavor and CP observables with the $BR(μ\to eγ)$ close to the experimental upper bound. With such a large value of $m_{16}$ we clearly cannot be considered "natural" SUSY nor are we "Split" SUSY. We are thus in the region in between or "SUSY on the Edge."

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arXiv2016

A Pati-Salam Version of Subcritical Hybrid Inflation

In this paper we present a model of subcritical hybrid inflation with a Pati-Salam [PS] symmetry group. Both the inflaton and waterfall fields contribute to the necessary e-foldings of inflation, while only the waterfall field spontaneously breaks PS hence monopoles produced during inflation are diluted during the inflationary epoch. The model is able to produce a tensor-to-scalar ratio, $r < 0.09$ consistent with the latest BICEP2/Keck and Planck data, as well as scalar density perturbations and spectral index, $n_s$, consistent with Planck data. For particular values of the parameters, we find $r = 0.084$ and $n_s = 0.0963$. The energy density during inflation is directly related to the PS breaking scale, $v_{PS}$. The model also incorporates a $\mathbb{Z}_4^R$ symmetry which can resolve the $μ$ problem and suppress dimension 5 operators for proton decay, leaving over an exact $R$-parity. Finally the model allows for a complete three family extension with a $D_4$ family symmetry which reproduces low energy precision electroweak and LHC data.

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arXiv2015

Basis invariant description of chemical equilibrium with implications for a recent axionic leptogenesis model

We provide a systematic treatment of chemical equilibrium in the presence of a specific type of time dependent background. The type of time dependent background we consider appears, for example, in recently proposed axion/Majoron leptogenesis models [1,2]. In describing the chemical equilibrium we use quantities which are invariant under redefinition of fermion phases (we refer to this redefinition as a change of basis for short), and therefore it is a basis invariant treatment. The change of the anomaly terms due to the change of the path integral measure [3,4] under a basis change is taken into account. We find it is useful to go back and forth between different bases, and there are insights which can be more easily obtained in one basis rather than another. A toy model is provided to illustrate the ideas. For the axion leptogenesis model [1], our result suggests that at $T > 10^{13}$ GeV , when sphaleron processes decouple, and $Γ_{B+L} << H < Γ_L$ (where $H$ is the Hubble parameter at temperature $T$ and $Γ_L$ is the $ΔL = 2$ lepton number violating interaction rate), the amount of $B-L$ created is controlled by the smallness of the sphaleron interaction rate, $Γ_{B+L}$. Therefore it is not as efficient as described. In addition, we notice an interesting modification of gauge boson dispersion relations at subleading order.

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arXiv2015

Gluino LOSP with Axino LSP

In this letter we have presented a novel version of "long-lived" gluinos in supersymmetric models with the gluino the lightest ordinary supersymmetric particle [LOSP] and axino LSP. Within certain ranges of the axion decay constant $f_a < 1 \times 10^{10}$ GeV, the gluino mass bounds are reduced to less than 1000 GeV. The best limits can be obtained by looking for decaying R-hadrons in the detector where the gluino decays to a gluon and axino in the calorimeters. SUSY models with a gluino LOSP can occur over a significant region of parameter space in either {\em mirage-mediation} or general gauge-mediated SUSY breaking models. The gluino LOSP is not constrained by cosmology, but in this scenario the axion/axino may be good dark matter candidates.

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arXiv2015

Threshold Corrections to the Bottom Quark Mass Revisited

Threshold corrections to the bottom quark mass are often estimated under the approximation that tan$β$ enhanced contributions are the most dominant. In this work we revisit this common approximation made to the estimation of the supersymmetric threshold corrections to the bottom quark mass. We calculate the full one-loop supersymmetric corrections to the bottom quark mass and survey a large part of the phenomenological MSSM parameter space to study the validity of considering only the tan$β$ enhanced corrections. Our analysis demonstrates that this approximation underestimates the size of the threshold corrections by $\sim$12.5% for most of the considered parameter space. We discuss the consequences for fitting the bottom quark mass and for the effective couplings to Higgses. We find that it is important to consider the additional contributions when fitting the bottom quark mass but the modifications to the effective Higgs couplings are typically $\mathcal{O}$(few)% for the majority of the parameter space considered.

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arXiv2014

Chaotic Hybrid Inflation with a Gauged B - L

In this paper we present a novel formulation of chaotic hybrid inflation in supergravity. The model includes a waterfall field which spontaneously breaks a gauged $U_1(B-L)$ at a GUT scale. This allows for the possibility of future model building which includes the standard formulation of baryogenesis via leptogenesis with the waterfall field decaying into right-handed neutrinos. We have not considered the following issues in this short paper, i.e. supersymmetry breaking, dark matter or the gravitino or moduli problems. Our focus is on showing the compatibility of the present model with Planck, WMAP and Bicep2 data.

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arXiv2014

Degenerate gaugino mass region and mono-boson collider signatures

In this paper we discuss search strategies at the LHC for light electroweak gauginos which are mostly Wino-like, Higgsino-like or an admixture. These states are typically degenerate with decay products that are less energetic and hence difficult to detect. In addition, their production cross-sections at a hadron collider are suppressed compared to colored states such as the gluinos. In order to detect these states one needs to trigger on initial or final state radiation. Many previous analyses have focussed on mono-jet and mono-photon triggers. In the paper we argue and show that these triggers are unlikely to succeed, due to the large background from QCD backgrounds for the mono-jet searches and the fact that the $p_T$ distribution of the mono-photons are rapidly decreasing functions of $p_T$. We show this with both an analytic calculation of photons in the initial state radiation and also a detailed numerical analysis. We then argue that mono-Z triggers, from Z decaying into charged leptons may well provide the best search strategy, in particular for Higgsino-like and mixed cases.

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arXiv2014

LHC Phenomenology of SO(10) Models with Yukawa Unification II

In this paper we study Yukawa-unified SO(10) SUSY GUTs with two types of SO(10) boundary conditions: (i) universal gaugino masses and (ii) non-universal gaugino masses with effective "mirage" mediation. With these boundary conditions, we perform a global chi^2 analysis to obtain the parameters consistent with 11 low energy observables, including the top, bottom, and tau masses. Both boundary conditions have universal scalar masses and "just so" splitting for the up- and down-type Higgs masses. In these models, the third family scalars are lighter than the first two families and the gauginos are lighter than all the scalars. We therefore focus on the gluino phenomenology in these models. In particular, we estimate the lowest allowed gluino mass in our models coming from the most recent LHC data and compare these to limits obtained using simplified models. We find that the lower bound on M_gluino in Yukawa-unified SO(10) SUSY GUTs is generically ~1.2 TEV at the 1 sigma level unless there is considerable degeneracy between the gluino and the LSP, in which case the bounds are much weaker. Hence many of our benchmark points are not ruled out by the present LHC data and are still viable models which can be tested at LHC 14.

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arXiv2013

SO(10) Yukawa Unification after the First Run of the LHC

In this talk we discuss SO(10) Yukawa unification and its ramifications for phenomenology. The initial constraints come from fitting the top, bottom and tau masses, requiring large $\tanβ\sim 50$ and particular values for soft SUSY breaking parameters. We perform a global $χ^2$ analysis, fitting the recently observed `Higgs' with mass of order 125 GeV in addition to fermion masses and mixing angles and several flavor violating observables. We discuss two distinct GUT scale boundary conditions for soft SUSY breaking masses. In both cases we have a universal cubic scalar parameter, $A_0$. In the first case we consider universal gaugino masses, and universal scalar masses, $m_{16}$, for squarks and sleptons; while in the latter case we have non-universal gaugino masses and either universal scalar masses, $m_{16}$, for squarks and sleptons or D-term splitting of scalar masses. We discuss the spectrum of SUSY particle masses and consequences for the LHC.

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arXiv2013

LHC Phenomenology of SO(10) Models with Yukawa Unification

In this paper we study an SO(10) SUSY GUT with Yukawa unification for the third generation. We perform a global chi^2 analysis given to obtain the GUT boundary conditions consistent with 11 low energy observables, including the top, bottom and tau masses. We assume a universal mass, m_{16}, for squarks and sleptons and a universal gaugino mass, M_{1/2}. We then analyze the phenomenological consequences for the LHC for 15 benchmark models with fixed m_{16} = 20 TeV and with varying values of the gluino mass. The goal of the present work is to (i) evaluate the lower bound on the gluino mass in our model coming from the most recent published data of CMS and (ii) to compare this bound with similar bounds obtained by CMS using simplified models. The bottom line is that the bounds coming from the same sign di-lepton analysis are comparable for our model and the simplified model studied assuming BR(gluino -> t tbar neutralino) = 100%. However the bounds coming from the purely hadronic analyses for our model are 10 - 20% lower than obtained for the simplified models. This is due to the fact that for our models the branching ratio for the decay gluino -> g neutralino is significant. Thus there are significantly fewer b-jets. We find a lower bound on the gluino mass in our models with M_{gluino} > 1000 GeV. Finally, there is a theoretical upper bound on the gluino mass which increases with the value of m_{16}. For m_{16} <= 30 TeV, the gluino mass satisfies M_{gluino} <= 2.8 TeV at 90% CL. Thus, unless we further increase the amount of fine-tuning, we expect gluinos to be discovered at LHC 14.

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arXiv2013

Yukawa Unification Predictions with effective "Mirage" Mediation

In this letter we analyze the consequences, for the LHC, of gauge and third family Yukawa coupling unification with a particular set of boundary conditions defined at the GUT scale, which we characterize as effective "mirage" mediation. We perform a global chi-squared analysis including the observables M_W, M_Z, G_F, alpha_em, alpha_s(M_Z), M_top, m_b(m_b), M_tau, BR(B -> X_s gamma), BR(B_s -> mu^+ mu^-) and M_{h}. The fit is performed in the MSSM in terms of 10 GUT scale parameters, while tanb and mu are fixed at the weak scale. We find good fits to the low energy data and a SUSY spectrum which is dramatically different than previously studied in the context of Yukawa unification.

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arXiv2013

Gluino bounds: Simplified Models vs a Particular SO(10) Model (A Snowmass white paper)

We consider the results from the first run of LHC studied in the context of simplified models and re-interpret them for a particular SO(10) model with a non-simplified topology. Hadronic searches have been designed to obtain the best sensitivity for the simplified models. They require multiple b-jets in the final state. But we show that the bounds obtained from these searches are weaker in the case of the particular model studied here, since there are fewer b-jets.

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arXiv2012

Yukawa Unification Predictions for the LHC

This paper is divided into two parts. In the first part we analyze the consequences, for the LHC, of gauge and third family Yukawa coupling unification with a particular set of boundary conditions defined at the GUT scale. We perform a global chi^2 analysis including the observables M_W, M_Z, G_F, 1/alpha_em, alpha_s(M_Z), M_t,m_b(m_b), M_tau, BR(B -> X_s gamma), BR(B_s -> mu^+ mu^-) and M_h. The fit is performed in the MSSM in terms of 9 GUT scale parameters, while tan beta and mu are fixed at the weak scale. Good fits suggest an upper bound on the gluino mass, M_gluino \lesssim 2 TeV. This constraint comes predominantly from fitting the bottom quark and Higgs masses (assuming a 125 GeV Higgs). Gluinos should be visible at the LHC in the 14 TeV run but they cannot be described by the typical simplified models. This is because the branching ratios for gluino -> t tbar neutralino, b bbar neutralino, t bbar chargino^-, b tbar chargino^+, g neutralino are comparable. Stops and sbottoms may also be visible. Charginos and neutralinos can be light with the LSP predominantly bino-like. In the second part of the paper we analyze a complete three family model and discuss the quality of the global chi^2 fits and the differences between the third family analysis and the full three family analysis for overlapping observables. We note that the light Higgs in our model couples to matter like the Standard Model Higgs. Any deviation from this would rule out this model.

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