arXiv subjects

Q. Shafi

Publications and source records attributed to Q. Shafi.

At least 19 recordsLinked to original sources

arXiv2020

Confronting Grand Unification with Lepton Flavour Violation, Dark Matter and LHC Data

We explore possible signatures for charged lepton flavour violation (LFV), sparticle discovery at the LHC and dark matter (DM) searches in grand unified theories (GUTs) based on SU(5), flipped SU(5) (FSU(5)) and SU(4)$_c \times $SU(2)$_L \times $SU(2)$_R$ (4-2-2). We assume that soft supersymmetry-breaking terms preserve the group symmetry at some high input scale, and focus on the non-universal effects on different matter representations generated by gauge interactions at lower scales, as well as the charged LFV induced in Type-1 see-saw models of neutrino masses. We identify the different mechanisms that control the relic DM density in the various GUT models, and contrast their LFV and LHC signatures. The SU(5) and 4-2-2 models offer good detection prospects both at the LHC and in LFV searches, though with different LSP compositions, and the SU(5) and FSU(5) models offer LFV within the current reach. The 4-2-2 model allows chargino and gluino coannihilations with neutralinos, and the former offer good detection prospects for both the LHC and LFV, while gluino coannihilations lead to lower LFV rates. Our results indicate that LFV is a powerful tool that complements LHC and DM searches, providing significant insights into the sparticle spectra and neutrino mass parameters in different models.

hep-ph↗

arXiv2019

Monopoles, Strings, and Necklaces in $SO(10)$ and $E_6$

We employ a variety of symmetry breaking patterns in $SO(10)$ and $E_6$ Grand Unified Theories to demonstrate the appearance of topological defects including magnetic monopoles, strings, and necklaces. We show that independent of the symmetry breaking pattern, a topologically stable superheavy monopole carrying a single unit of Dirac charge as well as color magnetic charge is always present. Lighter intermediate mass topologically stable monopoles carrying two or three quanta of Dirac charge can appear in $SO(10)$ and $E_6$ models respectively. These lighter monopoles as well as topologically stable intermediate scale strings can survive an inflationary epoch. We also show the appearance of a novel necklace configuration in $SO(10)$ broken to the Standard Model via $SU(4)_c\times SU(2)_L\times SU(2)_R$. It consists of $SU(4)_c$ and $SU(2)_R$ monopoles connected by flux tubes. Necklaces consisting of monopoles and antimonopoles joined together by flux tubes are also identified. Even in the absence of topologically stable strings, a monopole-string system can temporarily appear. This system decays by emitting gravity waves and we provide an example in which the spectrum of these waves is strongly peaked around $10^{-4}~{\rm Hz}$ with $\Omega_{\rm gw}h^2\simeq 10^{-12}$. This spectrum should be within the detection capability of LISA.

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arXiv2018

Confronting SUSY GUT with Dark Matter, Sparticle Spectroscopy and Muon $(g-2)$

We explore the implications of LHC and cold dark matter searches for supersymmetric particle mass spectra in two different grand unified models with left-right symmetry, $SO(10)$ and $ SU(4)_c \times SU(2)_L \times SU(2)_R$ (4-2-2). We identify characteristic differences between the two scenarios, which imply distinct correlations between experimental measurements and the particular structure of the GUT group. The gauge structure of 4-2-2 enhances significantly the allowed parameter space as compared to $SO(10)$, giving rise to a variety of coannihilation scenarios compatible with the LHC data, LSP dark matter and the ongoing muon g-2 experiment.

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arXiv2018

Dark Matter, Sparticle Spectroscopy and Muon $(g-2)$ in $SU(4)_c \times SU(2)_L \times SU(2)_R$

We explore the sparticle mass spectra including LSP dark matter within the framework of supersymmetric $SU(4)_c \times SU(2)_L \times SU(2)_R$ (422) models, taking into account the constraints from extensive LHC and cold dark matter searches. The soft supersymmetry-breaking parameters at $M_{GUT}$ can be non-universal, but consistent with the 422 symmetry. We identify a variety of coannihilation scenarios compatible with LSP dark matter, and study the implications for future supersymmetry searches and the ongoing muon g-2 experiment.

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arXiv2017

Light sterile neutrinos, dark matter, and new resonances in a $U(1)$ extension of the MSSM

We present $\psi'$MSSM, a model based on a $U(1)_{\psi'}$ extension of the minimal supersymmetric standard model. The gauge symmetry $U(1)_{\psi'}$, also known as $U(1)_N$, is a linear combination of the $U(1)_\chi$ and $U(1)_\psi$ subgroups of $E_6$. The model predicts the existence of three sterile neutrinos with masses $\lesssim 0.1~{\rm eV}$, if the $U(1)_{\psi'}$ breaking scale is of order 10 TeV. Their contribution to the effective number of neutrinos at nucleosynthesis is $\Delta N_{\nu}\simeq 0.29$. The model can provide a variety of possible cold dark matter candidates including the lightest sterile sneutrino. If the $U(1)_{\psi'}$ breaking scale is increased to $10^3~{\rm TeV}$, the sterile neutrinos, which are stable on account of a $Z_2$ symmetry, become viable warm dark matter candidates. The observed value of the standard model Higgs boson mass can be obtained with relatively light stop quarks thanks to the D-term contribution from $U(1)_{\psi'}$. The model predicts diquark and diphoton resonances which may be found at an updated LHC. The well-known $\mu$ problem is resolved and the observed baryon asymmetry of the universe can be generated via leptogenesis. The breaking of $U(1)_{\psi'}$ produces superconducting strings that may be present in our galaxy. A $U(1)$ R symmetry plays a key role in keeping the proton stable and providing the light sterile neutrinos.

hep-ph↗

arXiv2016

Axion, mu Term, and Supersymmetric Hybrid Inflation

We show how successful supersymmetric hybrid inflation is realized in realistic models where the resolution of the minimal supersymmetric standard model mu problem is intimately linked with axion physics. The scalar fields that accompany the axion, such as the saxion, are closely monitored during and after inflation to ensure that the axion isocurvature perturbations lie below the observational limits. The scalar spectral index n_s is about 0.96 - 0.97, while the tensor-to-scalar ratio r, a canonical measure of gravity waves, lies well below the observable range in our example. The axion domain walls are inflated away, and depending on the axion decay constant f_a and the magnitude of the mu parameter, the axions and/or the lightest supersymmetric particle compose the dark matter in the universe. Non-thermal leptogenesis is naturally implemented in this class of models.

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arXiv2016

Diphoton resonances in a U(1)_{B-L} extension of the minimal supesymmetric standard model

Inspired by the 750 GeV diphoton state recently reported by ATLAS and CMS, we propose a U(1)_{B-L} extension of the MSSM which predicts the existence of four spin zero resonance states that are degenerate in mass in the supersymmetric limit. Vector-like fields, a gauge singlet field, as well as the MSSM Higgsinos are prevented from acquiring arbitrary large masses by a U(1) R-symmetry. Indeed, these masses can be considerably lighter than the Z' gauge boson mass. Depending on kinematics the resonance states could decay into right handed neutrinos and sneutrinos, and/or MSSM Higgs fields and Higgsinos with total decay widths in the multi-GeV range.

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arXiv2014

Effects of Neutrino Inverse Seesaw Mechanism on the Sparticle Spectrum in CMSSM and NUHM2

We study the implications of the inverse seesaw mechanism (ISS) on the sparticle spectrum in the Constrained Minimal Supersymmetric Standard Model (CMSSM) and Non-Universal Higgs Model (NUHM2). Employing the maximal value of the Dirac Yukawa coupling involving the up type Higgs doublet provides a 2-3 GeV enhancement of the lightest CP-even Higgs boson mass. This effect permits one to have lighter colored sparticles in the CMSSM and NUHM2 scenarios with LSP neutralino, which can be tested at LHC14. We present a variety of LHC testable benchmark points with the desired LSP neutralino dark matter relic abundance.

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arXiv2013

Baryon Number Violation

This report, prepared for the Community Planning Study - Snowmass 2013 - summarizes the theoretical motivations and the experimental efforts to search for baryon number violation, focussing on nucleon decay and neutron-antineutron oscillations. Present and future nucleon decay search experiments using large underground detectors, as well as planned neutron-antineutron oscillation search experiments with free neutron beams are highlighted.

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arXiv2012

Non-Minimal Chaotic Inflation, Peccei-Quinn Phase Transition and non-Thermal Leptogenesis

We consider a phenomenological extension of the minimal supersymmetric standard model (MSSM) which incorporates non-minimal chaotic inflation, driven by a quadratic potential in conjunction with a linear term in the frame function. Inflation is followed by a Peccei-Quinn phase transition, based on renormalizable superpotential terms, which resolves the strong CP and mu problems of MSSM and provide masses lower than about 10^12 GeV for the right-handed (RH) (s)neutrinos. Baryogenesis occurs via non-thermal leptogenesis, realized by the out-of-equilibrium decay of the RH sneutrinos which are produced by the inflaton's decay. Confronting our scenario with the current observational data on the inflationary observables, the light neutrino masses, the baryon asymmetry of the universe and the gravitino limit on the reheat temperature, we constrain the strength of the gravitational coupling to rather large values (~45-2950) and the Dirac neutrino masses to values between about 1 and 10 GeV.

hep-ph↗

arXiv2011

Inflation in Supersymmetric SU(5)

We analyze the adjoint field inflation in supersymmetric (SUSY) SU(5) model. In minimal SUSY SU(5) hybrid inflation monopoles are produced at the end of inflation. We therefore explore the non-minimal model of inflation based on SUSY SU(5), like shifted hybrid, which provides a natural solution for the monopole problem. We find that the supergravity corrections with non-minimal Kahler potential are crucial to realize the central value of the scalar spectral index n_s ~ 0.96 consistent with the seven year WMAP data. The tensor to scalar ratio r is quite small, taking on values r < 10^{-5}. Due to R-symmetry massless SU(3) octet and SU(2) triplet Higgs bosons are present and could spoil for gauge coupling unification. To keep gauge coupling unification intact, light vector-like particles are added which are expected to be observed at LHC.

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arXiv2009

Observable n - anti-n Oscillations with New Physics at LHC

We explore extensions of the MSSM in which TeV scale vector-like multiplets can mediate observable n-anti-n oscillations, without causing conflict with the proton decay experiments, with a U(1) symmetry playing an important role. The colored vector-like particles, in particular, may be found at the LHC through some decay modes arising from their direct couplings to quarks.

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arXiv2007

From New to Large Field Inflation: Comparison with WMAP3

We briefly summarize and update a class of inflationary models from the early eighties based on a quartic (Coleman-Weinberg) potential for a gauge singlet scalar (inflaton) field. For vacuum energy scales comparable to the grand unification scale, the scalar spectral index n_s=0.94-0.97, in very good agreement with the WMAP three year results. The tensor to scalar ratio r<~0.14 while alpha=dn_s/d ln k=~-10^-3. For vacuum energy scales >~10^16 GeV, the inflationary scenario switches from `new' to `large-field' (V\proptoϕ^2) inflation. An SO(10) version naturally explains the observed baryon asymmetry via non-thermal leptogenesis.

astro-ph↗

arXiv2006

Coleman-Weinberg Potential In Good Agreement With WMAP

We briefly summarize and update a class of inflationary models from the early eighties based on a quartic (Coleman-Weinberg) potential for a gauge singlet scalar (inflaton) field. For vacuum energy scales comparable to the grand unification scale, the scalar spectral index n_s=0.94-0.97, in very good agreement with the WMAP three year results. The tensor to scalar ratio r<~0.14, while alpha=dn/dlnk is =~-10^-3. An SO(10) version naturally explains the observed baryon asymmetry via non-thermal leptogenesis.

astro-ph↗

arXiv2006

Supersymmetric Hybrid Inflation with Non-Minimal Kahler potential

Minimal supersymmetric hybrid inflation based on a minimal Kahler potential predicts a spectral index n_s\gsim 0.98. On the other hand, WMAP three year data prefers a central value n_s \approx 0.95. We propose a class of supersymmetric hybrid inflation models based on the same minimal superpotential but with a non-minimal Kahler potential. Including radiative corrections using the one-loop effective potential, we show that the prediction for the spectral index is sensitive to the small non-minimal corrections, and can lead to a significantly red-tilted spectrum, in agreement with WMAP.

hep-ph↗

arXiv2005

U(1)_B-L: Neutrino Physics and Inflation

A gauged U(1)_B-L symmetry predicts three right handed handed neutrinos and its spontaneous breaking automatically yields the seesaw mechanism. In a supersymmetric setting this breaking can be nicely linked with inflation to yield deltaT/ T proportional to (M_B-L / M_P)^2, where M_B-L (M_P) denote the B-L breaking (Planck) scale. Thus M_B-L is estimated to be of order 10^16 GeV, and the heaviest right handed neutrino mass is less than or of order 10^14 GeV. A second right handed neutrino turns out to have a mass of order 10-10^2 T_r, where T_r (<~10^9 GeV) denotes the reheat temperature. A U(1) R symmetry plays an essential role in implementing inflation and leptogenesis, resolving the MSSM mu problem and eliminating dimension five nucleon decay. An unbroken Z_2 subgroup plays the role of matter parity. The scalar spectral index n_s=0.99+-0.01 for the simplest models, while in smooth hybrid inflation n_s>=0.97. The tensor to scalar ratio r is negligible, and dn_s/dlnk<~10^-3.

hep-ph↗

arXiv2005

Reheat temperature in supersymmetric hybrid inflation models

The allowed range of parameters for supersymmetric hybrid inflation and its extensions are investigated. The lower bound on the reheat temperature T_r in these models with hierarchical right handed neutrinos is found to be 3*10^7 GeV. (T_r as low as 100 GeV is possible for quasi degenerate right handed neutrinos.) We also present revised estimates for the scalar spectral index and the symmetry breaking scale associated with inflation.

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arXiv2004

New inflation, preinflation, and leptogenesis

We present a new inflation model in which the inflationary scenario including subsequent reheating is determined in part by a U(1)_R\times Z_n symmetry. A preinflation epoch can be introduced to yield, among other things, a running spectral index indicated by the WMAP analysis. The inflaton decay into right handed neutrinos, whose masses can be hierarchical because of the Z_n symmetry, leads to a reheat temperature >~10^8 GeV (~10^4-10^6 GeV in some cases), followed by non-thermal leptogenesis.

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