arXiv subjects

Qaisar Shafi

Publications and source records attributed to Qaisar Shafi.

At least 127 records · Page 7Linked to original sources

arXiv2013

Particle Spectroscopy of Supersymmetric SU(5) in Light of 125 GeV Higgs and Muon g-2 Data

The discovery of the Higgs boson at the Large Hadron Collider (LHC) has a great impact on the minimal supersymmetric extension of the Standard Model (MSSM). In the context of the constrained MSSM (CMSSM) and its extension with non-universal masses for the MSSM Higgs doublets (NUHM2), sparticles with masses $ > 1$ TeV are necessary to reproduce the observed Higgs boson mass of 125-126 GeV. On the other hand, there appears to be a significant amount of discrepancy between the measured muon $g-2$ and the Standard Model prediction. A successful explanation of this discrepancy in the MSSM requires new contributions involving relatively light sparticles with masses $ < 1$ TeV. In this paper, we attempt to accommodate the two conflicting requirements in an SU(5) inspired extension of the CMSSM. We assign non-universal but flavor blind soft supersymmetry breaking masses to the scalar components in $\bar 5$ and 10 matter supermultiplets. The two MSSM Higgs doublets in the 5, $\bar5$ representations of SU(5) are also assigned unequal soft mass$^2$ at $M_{GUT}$. We identify parameter regions which can simultaneously accommodate the observed Higgs boson mass and the muon $g-2$ data, and which are compatible with other phenomenological constraints such as neutralino dark matter relic abundance and rare B-meson decays. Some regions of the allowed parameter space will be explored at the upgraded LHC and by dark matter direct detection experiments.

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arXiv2013

SO(10) as a Framework for Natural Supersymmetry

We consider an SO(10) grand unified theory in which the ratio of the SU(2)_W and SU(3)_c gaugino masses satisfy M_2/M_3 \approx 3, which results in the realization of natural supersymmetry. In the MSSM parameter space this relation looks artificial, but in the SO(10) case it results from a field with a designated vacuum expectation value. We consider two models, namely M_1:M_2:M_3=-1/5:3:1 (Case I), and M_1:M_2:M_3=-5:3:1 (Case II). Focusing on ameliorating the little hierarchy problem, we explore the parameter space of these models which yield small fine-tuning measuring parameters (natural supersymmetry) at the electroweak scale (Δ_{EW}) as well as at the high scale (Δ_{HS}). Although both models allow for the solution of the little hierarchy problem, the predicted sparticle spectra can differ markedly in the two cases. Depending on the ratio of the bino mass to the other gaugino masses, Case I leads to stau lepton masses of around a 100 GeV, while in Case II, the stau slepton masses are in the several TeV range. In Case I, the bino-like neutralino can be as light as 90 GeV, while the gluino is heavier than 2 TeV or so. In Case II, due to gluino-bino near degeneracy, the bino cannot be lighter than a TeV or so. Having a light neutralino with sizable bino-higgsino mixture in Case I allows the direct dark matter search experiments to test this class of models.

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arXiv2013

A Predictive Yukawa Unified SO(10) Model: Higgs and Sparticle Masses

We revisit a class of supersymmetric SO(10) models with t-b-tau Yukawa coupling unification condition, with emphasis on the prediction of the Higgs mass. We discuss qualitative features in this model that lead to a Higgs mass prediction close to 125 GeV. We show this with two distinct computing packages, Isajet and SuSpect, and also show that they yield similar global features in the parameter space of this model. We find that t-b-tau Yukawa coupling unification prefers values of the CP-odd Higgs mass m_{A} to be around 600 GeV, with all colored sparticle masses above 3 TeV. We also briefly discuss prospects for testing this scenario with the ongoing and planned direct dark matter detection experiments. In this class of models with t-b-tau Yukawa unification, the neutralino dark matter particle is heavy (m_{\tildeχ_1^{0}} \gtrsim 400 \rm \ GeV), which coannihilates with a stau to yield the correct relic abundance.

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arXiv2013

$R$-Symmetry Breaking in Supersymmetric Hybrid Inflation

We consider a supersymmetric hybrid inflation scenario in which the U(1) $R$-symmetry is explicitly broken by Planck scale suppressed operators in the superpotential. We provide an example with minimal Kähler potential, with the $R$-symmetry breaking term relevant during inflation being $αS^4$, where $S$ denotes the well-known gauge singlet inflaton superfield. The inflationary potential takes into account the radiative and supergravity corrections, as well as the soft supersymmetry breaking terms. For successful inflation, with the scalar spectral index in the currently preferred range, $n_s \approx 0.97 \pm 0.010$, $| α|$ $\lesssim 10^{-7}$. The tensor to scalar ratio $r \lesssim 10^{-4}$, while $| \text{d} n_s / \text{d} \ln k | \sim \mathcal{O}(10^{-3})-\mathcal{O}(10^{-4})$

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arXiv2012

Non-Universal Gaugino Masses and Natural Supersymmetry

We demonstrate that natural supersymmetry is readily realized in the framework of SU(4)_c \times SU(2)_L \times SU(2)_R with non-universal gaugino masses. Focusing on ameliorating the little hierarchy problem, we explore the parameter space of this model which yields small fine-tuning measuring parameters (natural supersymmetry) at the electroweak scale (Δ_{EW}) as well as at high scale (Δ_{HS}). It is possible to have both Δ_{EW} and Δ_{HS} less than 100 in these models, (2 % or better fine-tuning), while keeping the light CP-even (Standard Model-like) Higgs mass in the 123 GeV-127 GeV range. The light stop quark mass lies in the range 700 GeV <m_{\tilde{t}_{1}}< 1500 GeV, and the range for the light stau lepton mass is 900 GeV <m_{\tildeτ_{1}}< 1300 GeV. The first two family squarks are in the mass range 3000 GeV <m_{\tilde{t}_{1}}< 4500 GeV, and for the gluino we find 2500 GeV <m_{\tilde{g}_{1}}< 3500 GeV. We do not find any solution with natural supersymmetry which yields significant enhancement for Higgs production and decay in the diphoton channel.

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arXiv2012

Inverse Seesaw in NMSSM and 126 GeV Higgs Boson

We consider extensions of the next-to-minimal supersymmetric model (NMSSM) in which the observed neutrino masses are generated through a TeV scale inverse seesaw mechanism. The new particles associated with this mechanism can have sizable couplings to the Higgs field which can yield a large contribution to the mass of the lightest CP-even Higgs boson. With this new contribution, a 126 GeV Higgs is possible along with order of 200 GeV masses for the stop quarks for a broad range of \tanβ. The Higgs production and decay in the diphoton channel can be enhanced due to this new contribution. It is also possible to solve the little hierarchy problem in this model without invoking a maximal value for the NMSSM trilinear coupling and without severe restrictions on the value of \tanβ.

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arXiv2012

Revisiting mGMSB in light of a 125 GeV Higgs

We explore the implications of a 124-126 GeV CP-even Higgs boson on the fundamental parameter space and sparticle spectroscopy of the minimal gauge mediated supersymmetry breaking (mGMSB) scenario. The above mass for the Higgs boson yields stringent lower bounds on the sparticle masses in this class of models. The lightest neutralino and stau masses lie close to 1.5 TeV and 800 GeV respectively, while the majority of the sparticle masses are in the several to multi-TeV range. We show that with a single pair of 5+\bar{5} SU(5) messenger multiplets, the lower limit on the gravitino mass is \sim 360 eV. This is reduced to about 60 eV if five pairs of 5+\bar{5} messenger fields are introduced. Non-standard cosmology and non-standard gravitino production mechanisms are required in order to satisfy cosmological observations.

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arXiv2012

Smooth Hybrid Inflation and Non-Thermal Type II Leptogenesis

We consider a smooth hybrid inflation scenario based on a supersymmetric SU(2)_L x SU(2)_R x U(1)_B-L model. The Higgs triplets involved in the model play a key role in inflation as well as in explaining the observed baryon asymmetry of the universe. We show that the baryon asymmetry can originate via non-thermal triplet leptogenesis from the decay of SU(2)_L triplets, whose tiny vacuum expectation values also provide masses for the light neutrinos.

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arXiv2012

Higgs Boson Production and Decay: Effects from Light Third Generation and Vectorlike Matter

We study the implications of light third generation sparticles on the production cross section and decay widths of a light CP-even Higgs boson. For simplicity, we consider scenarios in which only one of the sfermions from the third generation is light. For each case, we attempt to explain the apparently large enhancement in the Higgs production and decay in the diphoton channel with small deviations in the ZZ channel. In the MSSM framework we find that only a light stau can explain these observations while keeping the lightest CP-even Higgs boson mass in the interval 123 GeV \lesssim m_h \lesssim 127 GeV. For the light stop scenario, the observations related to the diphoton and ZZ channel can be accommodated but, in order to satisfy the Higgs mass bound, one needs to go beyond the MSSM. In particular, we invoke vector like particles with masses around a TeV. These new particles preserve gauge coupling unification and provide additional contributions to the Higgs mass. With these new contributions a 126 GeV Higgs mass is easily achieved. We also find that with only a light sbottom quark, the above mentioned excess is hard to accommodate.

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arXiv2012

Impact of LHC Searches on NLSP Top Squark and Gluino Mass

We explore the implications of 7 TeV LHC searches for a scenario in which one of the stops is the next-to lightest supersymmetric particle (NLSP). The NLSP stop (\tilde{t}_1) is assumed to decay exclusively into neutralino and charm quark. We consider processes where the stops are pair produced together with a hard QCD jet. We also consider stop quarks from gluino decays, \tilde{g}\to t\tilde{t}_1^\ast+\bar{t}\tilde{t}_1. We show that the monojet ATLAS and CMS searches corresponding to 1 fb^{-1} of integrated luminosity are sensitive to stop masses of up to 160 GeV, with the 20% neutralino-stop coannihilation region essentially ruled out for M_{\tilde{t}_1}\lesssim 140 GeV. The region M_{\tilde{t}_1}\lesssim 130 GeV is excluded with even relatively larger mass difference, M_{\tilde{t}_1}-M_{\tildeχ_1^0}\sim 40 GeV, by the multi-jets search. The b-jet and same-sign dilepton searches are sensitive to a heavier gluino because they only pick up gluino pair production events followed by top quarks decaying into b-jets and same-sign dileptons, respectively. We find that the LHC data places a lower limit on the gluino mass in this scenario of about 600 GeV (700 GeV) from b-jets (same-sign dileptons) searches.

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arXiv2012

Higgs Boson Mass from t-b-tau Yukawa Unification

We employ the Yukawa coupling unification condition, y_t= y_b= y_tau at M_GUT, inspired by supersymmetric SO(10) models, to estimate the lightest Higgs boson mass as well as masses of the associated squarks and gluino. We employ non-universal soft masses, dictated by SO(10) symmetry, for the gauginos. Furthermore, the soft masses for the two scalar Higgs doublets are set equal at M_GUT, and in some examples these are equal to the soft masses for scalars in the matter multiplets. For mu > 0, M_2 > 0, where M_2 denotes the SU(2) gaugino mass, essentially perfect t-b-tau Yukawa unification is possible, and it predicts a Higgs mass of 122 - 124 GeV with a theoretical uncertainty of about 3 GeV. The corresponding gluino and the first two family squarks have masses 3 TeV. We present some LHC testable benchmark points which also show the presence of neutralino-stau coannihilation in this scenario. The well-known MSSM parameter tan beta~47.

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arXiv2012

125 GeV Higgs, Type III Seesaw and Gauge-Higgs Unification

Recently, both the ATLAS and CMS experiments have observed an excess of events that could be the first evidence for a 125 GeV Higgs boson. This is a few GeV below the (absolute) vacuum stability bound on the Higgs mass in the Standard Model (SM), assuming a Planck mass ultraviolet (UV) cutoff. In this paper, we study some implications of a 125 GeV Higgs boson for new physics in terms of the vacuum stability bound. We first consider the seesaw extension of the SM and find that in type III seesaw, the vacuum stability bound on the Higgs mass can be as low as 125 GeV for the seesaw scale around a TeV. Next we dicuss some alternative new physics models which provide an effective ultraviolet cutoff lower than the Planck mass. An effective cutoff Λ\simeq 10^{11} GeV leads to a vacuum stability bound on the Higgs mass of 125 GeV. In a gauge-Higgs unification scenario with five-dimensional flat spacetime, the so-called gauge-Higgs condition allows us to predict a Higgs mass of 125 GeV, with the compactification scale of the extra-dimension being identified as the cutoff scale Λ\simeq 10^{11} GeV. Identifying the compactification scale with the unification scale of the SM SU(2) gauge coupling and the top quark Yukawa coupling yields a Higgs mass of 121\pm 2 GeV.

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arXiv2012

125 GeV Higgs Boson from t-b-tau Yukawa Unification

We identify a class of supersymmetric SU(4)_c x SU(2)_L x SU(2)_R models in which imposing essentially perfect t-b-tau Yukawa coupling unification at M_GUT yields a mass close to 122-126 GeV for the lightest CP-even (SM-like) Higgs boson. The squark and gluino masses in these models exceed 3 TeV, but the stau and charginos in some cases can be considerably lighter. We display some benchmark points corresponding to neutralino-stau and bino-wino coannihilations as well as A-resonance. The well-known MSSM parameter tan beta is around 46-52.

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arXiv2012

Higgs and Sparticle Spectroscopy with Gauge-Yukawa Unification

We explore the Higgs and sparticle spectroscopy of supersymmetric SU(4)_c x SU(2)_L x SU(2)_R models in which the three MSSM gauge couplings and third family (t-b-tau) Yukawa couplings are all unified at M_GUT. This class of models can be obtained via compactification of a higher dimensional theory. Allowing for opposite sign gaugino masses and varying m_t within 1 sigma of its current central value yields a variety of gauge-Yukawa unification as well as WMAP compatible neutralino dark matter solutions. They include mixed bino-Higgsino dark matter, stau and gluino coannihilation scenarios, and the A-resonance solution.

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arXiv2012

Simplified Smooth Inflation with Observable Gravity Waves

We consider a simplified version of supersymmetric smooth hybrid inflation which contains a single ultraviolet cutoff m_P = 2.4 x 10^{18} GeV, instead of the two cutoffs m_P and M_* ~ few x 10^{17} GeV that are normally employed. With global supersymmetry the scalar spectral index n_s ~ 0.97, which is in very good agreement with the WMAP observations. With a non-minimal Kahler potential, the supergravity version of the model is compatible with the current central values of n_s and also yields potentially observable gravity waves (tensor to scalar ratio r <~ 0.02).

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arXiv2012

A heavier gluino from t-b-tau Yukawa-unified SUSY

Supersymmetric models with t-b-tau Yukawa coupling unification and unified gaugino masses at the GUT scale with mu >0 show a mild preference for light gluino masses \lesssim 500 GeV. This range of gluino mass is now essentially ruled out by LHC searches. We show that a heavier gluino mass ~ 0.5-3 TeV can also be compatible with excellent t-b-tau Yukawa coupling unification in supersymmetric models with non-universal Higgs masses (NUHM2). The gluino in such models is the lightest colored sparticle, while the squark sector displays an inverted mass hierarchy with m_{\tilde q} ~ 5-20 TeV. We present some LHC testable benchmark points for which the lightest Higgs boson mass ~ 125 GeV. We also discuss LHC signatures of Yukawa-unified models with heavier gluinos. We expect gluino pair production followed by decay to final states containing four b-jets plus four W-bosons plus missing E_T to occur at possibly observable rates at LHC.

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arXiv2012

Sparticle mass spectra from SU(5) SUSY GUT models with $b-τ$ Yukawa coupling unification

Supersymmetric grand unified models based on the gauge group SU(5) often require in addition to gauge coupling unification, the unification of b-quark and $τ$-lepton Yukawa couplings. We examine SU(5) SUSY GUT parameter space under the condition of $b-τ$ Yukawa coupling unification using 2-loop MSSM RGEs including full 1-loop threshold effects. The Yukawa-unified solutions break down into two classes. Solutions with low tanβ~3-11 are characterized by gluino mass ~1-4 TeV and squark mass ~1-5 TeV. Many of these solutions would be beyond LHC reach, although they contain a light Higgs scalar with mass <123 GeV and so may be excluded should the LHC Higgs hint persist. The second class of solutions occurs at large tanβ~35-60, and are a subset of $t-b-τ$ unified solutions. Constraining only $b-τ$ unification to ~5% favors a rather light gluino with mass ~0.5-2 TeV, which should ultimately be accessible to LHC searches. While our $b-τ$ unified solutions can be consistent with a picture of neutralino-only cold dark matter, invoking additional moduli or Peccei-Quinn superfields can allow for all of our Yukawa-unified solutions to be consistent with the measured dark matter abundance.

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arXiv2011

Neutralino-Sbottom Coannihilation in SU(5)

We identify within the SU(5) framework the minimum number of soft supersymmetry breaking parameters which can yield a bottom squrak (sbottom) as the next to lightest supersymmetric particle. We focus in particular on the neutralino-sbottom coannihilation scenario which gives rise to the desired neutralino dark matter relic density. We find solutions in which the sbottom mass is greater than or of order 210 GeV, while the gluino and the first two family squarks are heavier than 1 TeV. Some benchmark points which can be tested at the LHC are presented.

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