arXiv subjects

Pran Nath

Publications and source records attributed to Pran Nath.

At least 73 records · Page 4Linked to original sources

arXiv2016

Leptonic $g-2$ moments, CP phases and the Higgs boson mass constraint

Higgs boson mass measurement at $\sim 125$ GeV points to a high scale for SUSY specifically the scalar masses. If all the scalars are heavy, supersymmetric contribution to the leptonic $g-2$ moments will be significantly reduced. On the other hand the Brookhaven experiment indicates a 3sigma deviation from the standard model prediction. Here we analyze the leptonic $g-2$ moments in an extended MSSM model with inclusion of a vector like leptonic generation which brings in new sources of CP violation. In this work we consider the contributions to the leptonic $g-2$ moments arising from the exchange of charginos and neutralinos, sleptons and mirror sleptons, and from the exchange of $W$ and $Z$ bosons and of leptons and mirror leptons. We focus specifically on the $g-2$ moments for the muon and the electron where sensitive measurements exist. Here it is shown that one can get consistency with the current data on $g-2$ under the Higgs boson mass constraint. Dependence of the moments on CP phases from the extended sector are analyzed and it is shown that they are sensitively dependent on the phases from the new sector. It is shown that the corrections to the leptonic moments arising from the extended MSSM sector will be non-vanishing even if the SUSY scale extends into the PeV region.

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arXiv2016

ATLAS Diboson Excess from Stueckelberg Mechanism

We discuss the diboson excess seen by the ATLAS detector around 2 TeV in the LHC run I at $\sqrt{s}=8$ TeV. We explore the possibility that such an excess can arise from a $Z'$ boson which acquires mass through a $U(1)_X$ Stueckelberg extension. The corresponding $Z'$ gauge boson is leptophobic with a mass of around 2 TeV and has interactions with $SU(2)_L$ Yang-Mills fields and gauge fields of the hypercharge. The analysis predicts $Z'$ decays into $WW$ and $ZZ$ as well as into $Zγ$. Further three-body as well as four-body decays of the $Z'$ such as $WWZ, WWγ, WWZZ$ etc are predicted. In the analysis we use the helicity formalism which allows us to exhibit the helicity structure of the $Z'$ decay processes in an transparent manner. In particular, we are able to show the set of vanishing helicity amplitudes in the decay of the massive $Z'$ into two vector bosons due to angular momentum conservation with a special choice of the reference momenta. The residual set of non-vanishing helicity amplitudes are identified. The parameter space of the model compatible with the diboson excess seen by the ATLAS experiment at $\sqrt s=8$ TeV is exhibited. Estimate of the diboson excess expected at $\sqrt s= 13$ TeV with 20 fb$^{-1}$ of integrated luminosity at LHC run II is also given. It is shown that the $WW$, $ZZ$ and $Zγ$ modes are predicted to be in the approximate ratio $1:\cos^2θ_W (1+ α\tan^2θ_W)^2/2: (1-α)^2\sin^2θ_W/2$ where $α$ is the relative strength of the couplings of hypercharge gauge fields to the couplings of the Yang-Mills gauge fields. Thus observation of the $Zγ$ mode as well as three-body and four-body decay modes of the $Z'$ will provide a definite test of the model and of a possible new source of interaction beyond the standard model.

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arXiv2016

Light Stops and Observation of Supersymmetry at LHC RUN-II

Light stops consistent with the Higgs boson mass of $\sim126\,{\rm GeV}$ are investigated within the framework of minimal supergravity. It is shown that models with light stops which are also consistent with the thermal relic density constraints require stop coannihilation with the neutralino LSP. The analysis shows that the residual set of parameter points with light stops satisfying both the Higgs mass and the relic density constraints lie within a series of thin strips in the $m_0-m_{1/2}$ plane for different values of $A_0/m_0$. Consequently, this region of minimal supergravity parameter space makes a number of very precise predictions. It is found that light stops of mass down to 400~GeV or lower can exist consistent with all constraints. A signal analysis for this class of models at LHC RUN-II is carried out and the dominant signals for their detection identified. Also computed is the minimum integrated luminosity for $5σ$ discovery of the models analyzed. If supersymmetry is realized in this manner, the stop masses can be as low as 400~GeV or lower, and the mass gap between the lightest neutralino and lightest stop will be approximately 30-40~GeV. We have optimized the ATLAS signal regions specifically for stop searches in the parameter space and find that a stop with mass $\sim 375\,{\rm GeV}$ can be discovered with as little as $\sim$ 60~fb$^{-1}$ of integrated luminosity at RUN-II of the LHC; the integrated luminosity needed for discovery could be further reduced with more efficient signature analyses. The direct detection of dark matter in this class of models is also discussed. It is found that dark matter cross sections lie close to, but above, coherent neutrino scattering and would require multi-ton detectors such as LZ to see a signal of dark matter for this class of models.

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arXiv2016

Gluino Coannihilation and Observability of Gluinos at LHC RUN II

The observability of a gluino at LHC RUN II is analyzed for the case where the gluino lies in the gluino-neutralino coannihilation region and the mass gap between the gluino and the neutralino is small. The analysis is carried out under the Higgs boson mass constraint and the constraint of dark matter relic density consistent with the WMAP and Planck experiment. It is shown that in this case a gluino with mass much smaller than the current lower limit of $\sim 1500$ GeV as given by LHC RUN II at 3.2 fb$^{-1}$ of integrated luminosity would have escaped detection. The analysis is done using the signal regions used by the ATLAS Collaboration where an optimization of signal regions was carried out to determine the best regions for gluino discovery in the gluino-neutralino coannihilation region. It is shown that under the Higgs boson mass constraint and the relic density constraint, a gluino mass of $\sim 700$ GeV would require 14 fb$^{-1}$ of integrated luminosity for discovery and a gluino of mass $\sim 1250$ GeV would require 3400 fb$^{-1}$ of integrated luminosity for discovery at LHC RUN II. An analysis of dark matter for this case is also given. It is found that for the range of gluino masses considered, the neutralino mass lies within less than 100 GeV of the gluino mass. Thus a measurement of the gluino mass in the gluino-neutralino coannihilation region will provide a determination of the neutralino mass. In this region the neutralino is dominantly a gaugino and the spin-independent proton-neutralino cross section is small but much larger than the neutrino floor lying in the range $\sim (1-10)\times 10^{-47}$ cm$^{2}$. Thus a significant part of the parameter space of the model will lie within the reach of the next generation LUX-ZEPLIN dark matter experiment.

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arXiv2016

An Analysis of B-L=-2 Operators from Matter-Higgs Interactions in a Class of Supersymmetric SO(10) Models

Recently interest in GUT baryogenesis has been resurrected due to the observation that B-violating dimension seven operators that arise in grand unified theories that also violate B-L produce baryon asymmetry that cannot be wiped out by sphaleron processes. While a general analysis of such higher dimensional operators from a bottom up approach exists in the literature, a full analysis of them derived from grand unification does not exist. In this work we present a complete analysis of B-L=-2 operators within a realistic SO(10) grand unification where the doublet-triplet splitting is automatic via a missing partner mechanism. Specifically we compute all allowed dimension five, dimension seven and dimension nine operators arising from matter-Higgs interactions. The relative strength of all the allowed B-L=-2 operators is given. Such interactions are useful in the study of neutrino masses, baryogenesis, proton decay and $n-\bar n$ oscillations within a common realistic grand unification framework.

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arXiv2015

Chromoelectric Dipole Moments of Quarks in MSSM Extensions

An analysis is given of the chromoelectric dipole moment of quarks and of the neutron in an MSSM extension where the matter sector contains an extra vectorlike generation of quarks and mirror quarks. The analysis includes contributions to the CEDM from the exchange of the $W$ and the $Z$ bosons, from the exchange of charginos and neutralinos and the gluino. Their contribution to the EDM of quarks is investigated. The interference between the MSSM sector and the new sector with vectorlike quarks is investigated. It is shown that inclusion of the vectorlike quarks can modify the quark EDMs in a significant way. Further, this interference also provides a probe of the vectorlike quark sector. These results are of interest as in the future measurements on the neutron EDM could see an improvement up to two orders of magnitude over the current experimental limits and provide a window to new physics beyond the standard model.

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arXiv2015

$μ\to e γ$ Decay in an MSSM Extension

An analysis is given of the decay $μ\to e+ γ$ in an MSSM extension with a vectorlike generation. Here mixing with the mirrors allows the possibility of this decay. The analysis is done at the one loop level with the exchange of charginos and neutralinos and of sleptons and mirror sleptons in the loops. A one loop analysis with W and Z boson exchange and mirror leptons and neutrinos is also considered. The effects of CP violating phases from the new sector on the decay $μ\to e γ$ are analyzed in detail. The constraints arising from the current upper limit on the branching ratio ${\cal{B}}(μ\to {e} γ)$ from the MEG experiment of $2.4\times 10^{-12}$ (at 90\% CL) on the parameter space of SUSY models and on vectorlike models are explored. Further, the MEG experiment is likely to improve the upper limit by an order of magnitude in the coming years. The improved limits will allow one to probe a much larger domain of the parameter space of the extended models.

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arXiv2015

The Neutron Electric Dipole Moment and Probe of PeV Scale Physics

The experimental limit on the neutron electric dipole moment is used as a possible probe of new physics beyond the standard model. Within MSSM we use the current experimental limit on the neutron EDM and possible future improvement as a probe of high scale SUSY. Quantitative analyses show that scalar masses as large as a PeV and larger could be probed in improved experiment far above the scales accessible at future colliders. We also discuss the neutron EDM as a probe of new physics models beyond MSSM. Specifically we consider an MSSM extension with a particle content including a vectorlike multiplet. Such an extension brings in new sources of CP violation beyond those in MSSM. These CP phases contribute to the EDM of the quarks and to the neutron EDM. These contributions are analyzed in this work where we include the supersymmetric loop diagrams involving the neutralinos, charginos, the gluino, squark and mirror squark exchange diagrams at the one loop level. We also take into account the contributions from the $W$, $Z$, quark and mirror quark exchanges arising from the mixings of the vectorlike generation with the three generations. It is shown that the experimental limit on the neutron EDM can be used to probe such new physics models. In the future one expects the neutron EDM to improve an order of magnitude or more allowing one to extend the probe of high scale SUSY and of new physics models. For the MSSM the probe of high scales could go up to and beyond PeV scale masses.

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arXiv2015

Reminiscences of my work with Richard Lewis Arnowitt

This article contains reminiscences of the collaborative work that Richard Arnowitt and I did together which stretched over many years and encompasses several areas of particle theory. The article is an extended version of my talk at the Memorial Symposium in honor of Richard Arnowitt at Texas A\&M, College Station, Texas, September 19-20, 2014.

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arXiv2015

Supersymmetry after the Higgs

A brief review is given of the implications of a 126 GeV Higgs boson for the discovery of supersymmetry. Thus a 126 GeV Higgs boson is problematic within the Standard Model because of vacuum instability pointing to new physics beyond the Standard Model. The problem of vacuum stability is overcome in the SUGRA GUT model but the 126 GeV Higgs mass implies that the average SUSY scale lies in the several TeV region. The largeness of the SUSY scale relieves the tension on SUGRA models since it helps suppress flavor changing neutral currents and CP violating effects and also helps in extending the proton life time arising from baryon and lepton number violating dimension five operators. The geometry of radiative breaking of the electroweak symmetry and fine tuning in view of the large SUSY scale are analyzed.Consistency with the Brookhaven $g_μ-2$ result is discussed. It is also shown that a large SUSY scale implied by the 126 GeV Higgs boson mass allows for light gauginos (gluino, charginos, neutralinos) and sleptons. These along with the lighter third generation squarks are the prime candidates for discovery at RUN II of the LHC. Implication of the 126 GeV Higgs boson for the direct search for dark matter is discussed. Also discussed are the sparticles mass hierarchies and their relationship with the simplified models under the Higgs boson mass constraint.

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arXiv2014

Sparticle Mass Hierarchies, Simplified Models from SUGRA Unification, and Benchmarks for LHC Run-II SUSY Searches

Sparticle mass hierarchies contain significant information regarding the origin and nature of supersymmetry breaking. The hierarchical patterns are severely constrained by electroweak symmetry breaking as well as by the astrophysical and particle physics data. They are further constrained by the Higgs boson mass measurement. The sparticle mass hierarchies can be used to generate simplified models consistent with the high scale models. In this work we consider supergravity models with universal boundary conditions for soft parameters at the unification scale as well as supergravity models with nonuniversalities and delineate the list of sparticle mass hierarchies for the five lightest sparticles. Simplified models can be obtained by a truncation of these, retaining a smaller set of lightest particles. The mass hierarchies and their truncated versions enlarge significantly the list of simplified models currently being used in the literature. Benchmarks for a variety of supergravity unified models appropriate for SUSY searches at future colliders are also presented. The signature analysis of two benchmark models has been carried out and a discussion of the searches needed for their discovery at LHC RUN-II is given. An analysis of the spin independent neutralino-proton cross section exhibiting the Higgs boson mass dependence and the hierarchical patterns is also carried out. It is seen that a knowledge of the spin independent neutralino-proton cross section and the neutralino mass will narrow down the list of the allowed sparticle mass hierarchies. Thus dark matter experiments along with analyses for the LHC Run-II will provide strong clues to the nature of symmetry breaking at the unification scale.

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arXiv2014

Electron EDM as a Sensitive Probe of PeV Scale Physics

We give a quantitative analysis of the electric dipole moments as a probe of high scale physics. We focus on the electric dipole moment of the electron since the limit on it is the most stringent. Further, theoretical computations of it are free of QCD uncertainties. The analysis presented here first explores the probe of high scales via electron EDM within MSSM where the contributions to the electric dipole moment (EDM) arise from the chargino and the neutralino exchanges in loops. Here it is shown that the electron EDM can probe mass scales from tens of TeV into the PeV range.The analysis is then extended to include a vectorlike generation which can mix with the three ordinary generations. Here new CP phases arise and it is shown that the electron EDM now has not only a supersymmetric contribution from the exchange of charginos and neutralinos but also a non-supersymmetric contribution from the exchange of W and Z bosons. It is further shown that the interference of the supersymmetric and the non-supersymmetric contribution leads to the remarkable phenomenon where the electron EDM as a function of the slepton mass first falls and become vanishingly small and then rises again as the slepton mass increases This phenomenon arises as a consequence of cancellation between the SUSY and the non-SUSY contribution at low scales while at high scales the SUSY contribution dies out and the EDM is controlled by the non-SUSY contribution alone. The high mass scales that can be probed by the EDM are far in excess of what accelerators will be able to probe. The sensitivity of the EDM to CP phases both in the SUSY and the non-SUSY sectors are also discussed.

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arXiv2014

3.5 keV Galactic Emission Line as a Signal from the Hidden Sector

An emission line with energy of $E\sim 3.5$ keV has been observed in galaxy clusters by two experiments. The emission line is consistent with the decay of a dark matter particle with a mass of $\sim 7$ keV. In this work we discuss the possibility that the dark particle responsible for the emission is a real scalar ($ρ$) which arises naturally in a $U(1)_X$ Stueckelberg of MSSM. In the MSSM Stueckelberg extension $ρ$ couples only to other scalars carrying a $U(1)_X$ quantum number. Under the assumption that there exists a vectorlike leptonic generation carrying both $SU(2)_L\times U(1)_Y$ and $U(1)_X$ quantum numbers, we compute the decay of the $ρ$ into two photons via a triangle loop involving scalars. The relic density of the $ρ$ arises via the decay $H^0\to h^0+ ρ$ at the loop level involving scalars, and via the annihilation processes of the vectorlike scalars into $ρ+ h^0$. It is shown that the galactic data can be explained within a multicomponent dark matter model where the 7 keV dark matter is a subdominant component constituting only $(1-10)$\% of the matter relic density with the rest being supersymmetric dark matter such as the neutralino. Thus the direct detection experiments remain viable searches for WIMPs. The fact that the dark scalar $ρ$ with no interactions with the standard model particles arises from a Stueckelberg extension of a hidden $U(1)_X$ implies that the 3.5 KeV galactic line emission is a signal from the hidden sector.

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arXiv2014

Probe of New Physics using Precision Measurement of the Electron Magnetic Moment

The anomalous magnetic moment of the electron is determined experimentally with an accuracy of $2.8\times 10^{-13}$ and the uncertainty may decrease by an order of magnitude in the future. While the current data is in excellent agreement with the standard model, the possible future improvement in the error in $Δa_e= a_e^{\text{exp}}- a_e^{\text{theory}}$ has recently drawn interest in the electron anomalous magnetic moment as a possible probe of new physics beyond the standard model. In this work we give an analysis of such physics in an extension of the minimal supersymmetric standard model with a vector multiplet. In the extended model the electroweak contribution to the anomalous magnetic moment of the electron include loop diagrams involving in addition to the exchange of W and Z, the exchange of charginos, sneutrinos and mirror sneutrinos, and the exchange of neutralinos, sleptons and mirror sleptons. The analysis shows that a contribution to the electron magnetic moment much larger than expected by $m_e^2/m_μ^2$ scaling of the deviation of the muon anomalous magnetic moment over the standard model prediction, i.e., $Δa_μ= 3 \times 10^{-9}$ as given by the Brookhaven experiment, can be gotten within the MSSM extension. Effects of CP violating phases in the extended MSSM model on the corrections to the supersymmetric electroweak contributions to $a_e$ are also investigated. The analysis points to the possibility of detection of new physics effects with modest improvement on the error in $Δa_e= a_e^{\text{exp}} - a_e^{\text{theory}}$.

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arXiv2014

Cogenesis in a universe with vanishing $B-L$ within a gauged $U(1)_x$ extension

We consider a gauged $U(1)_x$ extension of the standard model and of the minimal supersymmetric standard model where the dark matter fields are charged under $U(1)_x$ and carry lepton number while the standard model fields and fields of the minimal supersymmetric standard model are neutral under $U(1)_x$. We consider leptogenesis in this class of models with all fundamental interactions having no violation of lepton number, and the total $B-L$ in the universe vanishes. Such leptogenesis leads to equal and opposite lepton numbers in the visible sector and in the dark sector, and thus also produces asymmetric dark matter. Part of the lepton numbers generated in the leptonic sector subsequently transfer to the baryonic sector via sphaleron interactions. The stability of the dark particles is protected by the $U(1)_x$ gauge symmetry. A kinetic mixing between the $U(1)_x$ and the $U(1)_Y$ gauge bosons allows for dissipation of the symmetric component of dark matter. The case when $U(1)_x$ is $U(1)_{B-L}$ is also discussed for the supersymmetric case. This case is particularly interesting in that we have a gauged $U(1)_{B-L}$ which ensures the conservation of $B-L$ with an initial condition of a vanishing $B-L$ in the universe. Phenomenological implications of the proposed extensions are discussed, which include implications for electroweak physics, neutrino masses and mixings, and lepton flavor changing processes such as $\ell_i \to \ell_j γ$. We also briefly discuss the direct detection of the dark matter in the model.

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arXiv2014

Large Neutrino Magnetic Dipole Moments in MSSM Extensions

An analysis of the Dirac neutrino magnetic moment with standard model interactions gives $μ_ν\sim 3 \times 10^{-19} μ_B (m_ν/1 eV)$. The observation of a significantly larger magnetic moment will provide a clear signal of new physics beyond the standard model. The current experimental limits on the neutrino magnetic moments are orders of magnitude larger than the prediction with the standard model interactions and thus its test appears out of reach. Here we give an analysis of the Dirac neutrino magnetic moments within the framework of a minimal supersymmetric standard model extension with a vectorlike lepton generation. Specifically we compute the moments arising from the exchange of W and the charged leptons in the loop, as well as from the exchange of charginos, charged sleptons and charged mirror sleptons. It is shown that the neutrino moment in this case can be several orders of magnitude larger than the one with standard model like interactions, lying close to and below the current experimental upper limits and should be accessible in improved future experiment. A correlated prediction of the heaviest neutrino lifetimes from radiative decays to the lighter neutrinos via exchange of charginos and sleptons in the loops is also made. The predicted lifetimes are several orders of magnitudes smaller than the one with the standard model interactions and also lie close to the current experimental limits from analyses using the cosmic background neutrino data.

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arXiv2013

Baryogenesis from dark matter

We consider the possibility that some primordial fields decay purely into the dark sector creating asymmetric dark matter. This asymmetry is subsequently transmuted into leptons and baryons. Within this paradigm we compute the amount of asymmetric dark matter created from the out of equilibrium decays of the primordial fields with CP violating Yukawa couplings. The dark matter asymmetry is then transferred to the visible sector by the asymmetry transfer equation and generates an excess of $B-L$. Baryogenesis occurs via sphaleron processes which conserve $B-L$ but violate $B+L$. A mechanism for the annihilation of the symmetric component of dark matter is also discussed. The model leads to multi-component dark matter consisting of both bosonic and fermionic components.

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arXiv2013

Radiative Decays of Cosmic Background Neutrinos in Extensions of MSSM with a Vector Like Lepton Generation

An analysis of radiative decays of the neutrinos $ν_j\to ν_l γ$ is discussed in MSSM extensions with a vector like lepton generation. Specifically we compute neutrino decays arising from the exchange of charginos and charged sleptons where the photon is emitted by the charged particle in the loop. It is shown that while the lifetime of the neutrino decay in the Standard Model is $\sim 10^{43}$ yrs for a neutrino mass of 50 meV, the current lower limit from experiment from the analysis of the Cosmic Infrared Background is $\sim 10^{12}$ yrs and thus beyond the reach of experiment in the foreseeable future. However, in the extensions with a vector like lepton generation the lifetime for the decays can be as low as $\sim 10^{12}- 10^{14}$ yrs and thus within reach of future improved experiments. The effect of CP phases on the neutrino lifetime is also analyzed. It is shown that while both the magnetic and the electric transition dipole moments contribute to the neutrino lifetime, often the electric dipole moment dominates even for moderate size CP phases.

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