We study the discovery prospects of the charged Higgs boson in the context of multi Higgs models ... more We study the discovery prospects of the charged Higgs boson in the context of multi Higgs models in certain BSM scenarios. We classify models into three categories based on the charged Higgs coupling properties: gaugophobic, fermiophobic, and chromophobic. In each case, we identify viable modes of discovery, and present LHC analysis for discovery. We find that extensions of the Standard Model in which the charged Higgs does not couple to colored particles offer the best possible avenues for discovery.
We study an extension of the minimal supersymmetric standard model (MSSM) with additional right-h... more We study an extension of the minimal supersymmetric standard model (MSSM) with additional right-handed singlet neutrino superfields. While such an extension incorporates a mechanism for the neutrino mass, it also opens up the possibility of having the right-sneutrinos (ν) as the lightest supersymmetric particle (LSP). In this work, we focus on the viability of rather small (500 GeV) higgsino mass parameter (µ), an important ingredient for "naturalness", in the presence of such a LSP. For simplicity, we assume that the bino and wino mass parameters are much heavier; thus we only consider (almost) pure and compressed higgsino-like states, with small O(10 −2) gaugino admixture which nevertheless still affect the decay of the low-lying higgsino-like states, thus significantly affecting the proposed signatures at colliders. Considering only prompt decays of the higgino-like states, especially the lightest chargino, we discuss the importance of leptonic channels consisting of up to two leptons with large missing transverse energy to probe this scenario at the Large Hadron Collider (LHC). In addition we also comment on the dark matter predictions for the studied scenario.
We propose a left-right symmetric framework with a universal seesaw mechanism for the generation ... more We propose a left-right symmetric framework with a universal seesaw mechanism for the generation of masses of the Standard Model quarks and leptons. Heavy vectorlike singlet quarks and leptons are required for generation of Standard-Model-like quark and lepton masses through a seesaw mechanism. A softly broken Z 2 symmetry distinguishes the lepton sector and the quark sector of the model. This leads to the presence of some lepton-specific interactions that can produce unique collider signatures which can be explored at the current Large Hadron Collider run and also future colliders.
Several supersymmetric models with extended gauge structures predict light doubly-charged Higgsin... more Several supersymmetric models with extended gauge structures predict light doubly-charged Higgsinos. Their distinctive signature at the large hadron collider is highlighted by studying their production and decay characteristics.
We propose a non-supersymmetric SU(5) model in which only the third family of fermions are unifie... more We propose a non-supersymmetric SU(5) model in which only the third family of fermions are unified. The model remedies the non-unification of the three Standard Model couplings in non-supersymmetric SU(5). It also provides a mechanism for baryon number violation which is needed for the baryon asymmetry of the Universe and is not present in the Standard Model. Current experimental constraints on the leptoquark gauge bosons, mediating such baryon and lepton violating interactions in our model, allow their masses to be at the TeV scale. These can be searched for as a (b\tau) or (tt) resonance at the Large Hadron Collider as predicted in our model.
Physical Review D Particles and Fields, Feb 1, 2008
Recently proposed idea of unparticles arising due to a scale invariant sector in the theory can g... more Recently proposed idea of unparticles arising due to a scale invariant sector in the theory can give rise to effective operators with different Lorentz structures. We show that by using the different polarization options at the future linear e + e − colliders, the nature of these effective operators can be easily understood. The unique feature of a complex phase in the propagator of the unparticle can also be understood uniquely for the different spins by exploiting the initial beam polarizations at the International Linear Collider (ILC).
With the upcoming run of the Large Hadron Collider (LHC) at much higher center of mass energies, ... more With the upcoming run of the Large Hadron Collider (LHC) at much higher center of mass energies, the search for Beyond Standard Model (BSM) physics will again take center stage. New colored particles predicted in many BSM scenarios are expected to be produced with large cross sections thus making them interesting prospects as a doorway to hints of new physics. We consider the resonant production of such a colored particle, the diquark, a particle having the quantum number of two quarks. The diquark can be either a scalar or vector. We focus on the vector diquark which has much larger production cross section compared to the scalar ones. In this work we calculate the next-to-leading order (NLO) QCD corrections to the on-shell vector diquark production at the LHC produced through the fusion of two quarks as well as the NLO corrections to its decay width. We present full analytic results for the one-loop NLO calculation and do a numerical study to show that the NLO corrections can reduce the scale uncertainties in the cross sections which can be appreciable and therefore modify the expected search limits for such particles. We also use the dijet result from LHC to obtain current limits on the mass and coupling strengths of the vector diquarks.
We study the viability of having two relatively light top squarks ('stops') in the framework of t... more We study the viability of having two relatively light top squarks ('stops') in the framework of the Next-to-Minimal Supersymmetric Standard Model (NMSSM). Such light stops render the NMSSM rather 'natural'. These are shown to be allowed by the relevant direct searches at the Large Hadron Collider (LHC) and to be compatible with the latest LHC results on the Higgs sector, other low energy electroweak constraints and recent constraints from the dark matter (DM) sector. We propose dedicated searches for such light stops at the LHC within a 'simplified' scenario that may have a bino-like or a singlino-like neutralino LSP as the DM candidate and point out various final states carrying the imprint of their collective presence. Under certain circumstances, in such a scenario, presence of two light stops may give rise to final states which are not so typical in their search. Thorough studies at the detector level reveal the status of such a scenario after the 8 TeV run of the LHC and shed light on the prospects of its 13 and 14 TeV runs. In favorable regions of the NMSSM parameter space, with low-lying spectra, signals with significance 5σ is possible with a few tens to a few hundreds of fb −1 of integrated luminosity in diverse final states.
We consider an extension of the Standard Model involving a singlet Higgs and down type vector-lik... more We consider an extension of the Standard Model involving a singlet Higgs and down type vector-like quarks in the light of the current LHC Higgs data. For a good range of the parameters of the Higgs potential, and a mass range for the heavy vector-like quark, we find that the singlet heavy Higgs arising from the production and decay of the vector-like quarks give rise to (2b~4t) signal. The subsequent decay of the top quarks to $b W^{+}$ give rise to a final state with six b quarks, two same-sign charged leptons and missing transverse momenta with observable cross-sections at the 14 TeV run of the Large Hadron Collider. The Standard Model background for such a final state is practically negligible.
ABSTRACT We consider an extension of the Standard Model involving a singlet Higgs and down type v... more ABSTRACT We consider an extension of the Standard Model involving a singlet Higgs and down type vector-like quarks in the light of the current LHC Higgs data. For a good range of the parameters of the Higgs potential, and a mass range for the heavy vector-like quark, we find that the singlet heavy Higgs arising from the production and decay of the vector-like quarks give rise to (2b~4t) signal. The subsequent decay of the top quarks to $b W^{+}$ give rise to a final state with six b quarks, two same-sign charged leptons and missing transverse momenta with observable cross-sections at the 14 TeV run of the Large Hadron Collider. The Standard Model background for such a final state is practically negligible.
New physics at the TeV scale is highly anticipated at the LHC. New particles with color, if withi... more New physics at the TeV scale is highly anticipated at the LHC. New particles with color, if within the LHC energy reach, will be copiously produced. One such particle is a diquark, having the quantum numbers of two quarks, and can be a scalar or a vector. It will decay to two light quarks, or two top quarks, or a top and a light quark, (up or down type depending on the quantum number of the produced diquark). If singly produced, it can be looked for as a dijet resonance, or as giving extra contribution to the single top production or tt production. In this work, we consider a color sextet vector diquark having the quantum number of (ud) type, its resonance production , and the subsequent decay to tb, giving rise to excess contribution to the single top production. Even though the diquark mass is large, its strong resonance production dominate the weak production of tb for a wide range of the diquark mass. Also its subsequent decay to tb produce a very hard b-jet compared to the usual electroweak production. In addition, the missing energy in the final state event is much larger from the massive diquark decays. Thus, with suitable cuts, the final state with b,b and a charged lepton together with large missing energy stands out compared to the Standard Model background. We make a detailed study of both the signal and the background. We find that such a diquark is accessible at the 7 TeV LHC upto a mass of about 3.3 TeV with the luminosity 1 fb −1 , while the reach goes up to about 4.3 TeV with a luminosity of 10 fb −1 .
We propose a non-supersymmetric SU(5) model in which only the third family of fermions are unifie... more We propose a non-supersymmetric SU(5) model in which only the third family of fermions are unified. The model remedies the non-unification of the three Standard Model couplings in non-supersymmetric SU(5). It also provides a mechanism for baryon number violation which is needed for the baryon asymmetry of the Universe and is not present in the Standard Model. Current experimental constraints on the leptoquark gauge bosons, mediating such baryon and lepton violating interactions in our model, allow their masses to be at the TeV scale. These can be searched for as a (bτ) or (tt) resonance at the Large Hadron Collider as predicted in our model.
A radion in a scenario with a warped extra dimension can be lighter than the Higgs boson, even if... more A radion in a scenario with a warped extra dimension can be lighter than the Higgs boson, even if the Kaluza-Klein excitation modes of the graviton turn out to be in the multi-TeV region. The discovery of such a light radion would be gateway to new physics. We show how the twophoton mode of decay can enable us to probe a radion in the mass range 60-110 GeV. We take into account the diphoton background, including fragmentation effects, and include cuts designed to suppress the background to the maximum possible extent. Our conclusion is that, with an integrated luminosity of 3000 f b −1 or less, the next run of the Large Hadron Collider should be able to detect a radion in this mass range, with a significance of 5 standard deviations or more.
The idea of unifying quarks and leptons in a gauge symmetry is very appealing. However, such an u... more The idea of unifying quarks and leptons in a gauge symmetry is very appealing. However, such an unification gives rise to leptoquark type gauge bosons for which current collider limits push their masses well beyond the TeV scale. We present a model in the framework of extra dimensions which breaks such quark-lepton unification symmetry via compactification at the TeV scale. These color triplet leptoquark gauge bosons, as well as the new quarks present in the model, can be produced at the LHC with distinctive final state signatures. These final state signals include high p T multi-jets and multi-leptons with missing energy, monojets with missing energy, as well as the heavy charged particles passing through the detectors, which we also discuss briefly. The model also has a neutral Standard Model singlet heavy lepton which is stable, and can be a possible candidate for the dark matter.
We study the 'single photon' process e + e − → γνν with contributions due to exchange of massive ... more We study the 'single photon' process e + e − → γνν with contributions due to exchange of massive gravitons in the Randall-Sundrum model of low-scale quantum gravity. It is shown that for significant regions in the parameter space, this process unambiguously highlights the resonance structure of the graviton sector. Even in the non-resonant part of the parameter space, we show that comparison with the benchmark process e + e − → µ + µ − can clearly distinguish signals for warped gravity from similar signals for large extra dimensions.
We study single top production in association with a charged Higgs in the type II two Higgs doubl... more We study single top production in association with a charged Higgs in the type II two Higgs doublet model at the Large Hadron Collider. The polarization of the top, reflected in the angular distributions of its decay products, can be a sensitive probe of new physics in its production. We present theoretically expected polarizations of the top for top charged-Higgs production, which is significantly different from that in the closely related process of tW production in the Standard Model. We then show that an azimuthal asymmetry, constructed from the decay lepton angular distribution in the laboratory frame, is a sensitive probe of top polarization and can be used to constrain parameters involved in top charged-Higgs production.
The Littest Higgs model with T-parity (LHT), apart from offering a viable solution to the natural... more The Littest Higgs model with T-parity (LHT), apart from offering a viable solution to the naturalness problem of the Standard Model, also predicts a set of new fermions as well as a candidate for dark matter. We explore the possibility of discovering the heavy T-odd quark Q H at the LHC in a final state comprising two hard jets with a large missing transverse momentum. Also discussed is the role of heavy flavor tagging.
We study the production of a single charged slepton in association with a top quark in a R-parity... more We study the production of a single charged slepton in association with a top quark in a R-parity violating supersymmetric model with lepton number violating interactions at the Large Hadron Collider. We find that the longitudinal polarization asymmetry of the top quark in such a production mode is significantly different from that in the production of a single top or a top pair in the Standard Model for a wide range of slepton masses. Our signal analysis shows that the top-slepton associate production leads to final states with distinct kinematic signatures, which differ from the Standard Model background.
We study the discovery prospects of the charged Higgs boson in the context of multi Higgs models ... more We study the discovery prospects of the charged Higgs boson in the context of multi Higgs models in certain BSM scenarios. We classify models into three categories based on the charged Higgs coupling properties: gaugophobic, fermiophobic, and chromophobic. In each case, we identify viable modes of discovery, and present LHC analysis for discovery. We find that extensions of the Standard Model in which the charged Higgs does not couple to colored particles offer the best possible avenues for discovery.
We study an extension of the minimal supersymmetric standard model (MSSM) with additional right-h... more We study an extension of the minimal supersymmetric standard model (MSSM) with additional right-handed singlet neutrino superfields. While such an extension incorporates a mechanism for the neutrino mass, it also opens up the possibility of having the right-sneutrinos (ν) as the lightest supersymmetric particle (LSP). In this work, we focus on the viability of rather small (500 GeV) higgsino mass parameter (µ), an important ingredient for "naturalness", in the presence of such a LSP. For simplicity, we assume that the bino and wino mass parameters are much heavier; thus we only consider (almost) pure and compressed higgsino-like states, with small O(10 −2) gaugino admixture which nevertheless still affect the decay of the low-lying higgsino-like states, thus significantly affecting the proposed signatures at colliders. Considering only prompt decays of the higgino-like states, especially the lightest chargino, we discuss the importance of leptonic channels consisting of up to two leptons with large missing transverse energy to probe this scenario at the Large Hadron Collider (LHC). In addition we also comment on the dark matter predictions for the studied scenario.
We propose a left-right symmetric framework with a universal seesaw mechanism for the generation ... more We propose a left-right symmetric framework with a universal seesaw mechanism for the generation of masses of the Standard Model quarks and leptons. Heavy vectorlike singlet quarks and leptons are required for generation of Standard-Model-like quark and lepton masses through a seesaw mechanism. A softly broken Z 2 symmetry distinguishes the lepton sector and the quark sector of the model. This leads to the presence of some lepton-specific interactions that can produce unique collider signatures which can be explored at the current Large Hadron Collider run and also future colliders.
Several supersymmetric models with extended gauge structures predict light doubly-charged Higgsin... more Several supersymmetric models with extended gauge structures predict light doubly-charged Higgsinos. Their distinctive signature at the large hadron collider is highlighted by studying their production and decay characteristics.
We propose a non-supersymmetric SU(5) model in which only the third family of fermions are unifie... more We propose a non-supersymmetric SU(5) model in which only the third family of fermions are unified. The model remedies the non-unification of the three Standard Model couplings in non-supersymmetric SU(5). It also provides a mechanism for baryon number violation which is needed for the baryon asymmetry of the Universe and is not present in the Standard Model. Current experimental constraints on the leptoquark gauge bosons, mediating such baryon and lepton violating interactions in our model, allow their masses to be at the TeV scale. These can be searched for as a (b\tau) or (tt) resonance at the Large Hadron Collider as predicted in our model.
Physical Review D Particles and Fields, Feb 1, 2008
Recently proposed idea of unparticles arising due to a scale invariant sector in the theory can g... more Recently proposed idea of unparticles arising due to a scale invariant sector in the theory can give rise to effective operators with different Lorentz structures. We show that by using the different polarization options at the future linear e + e − colliders, the nature of these effective operators can be easily understood. The unique feature of a complex phase in the propagator of the unparticle can also be understood uniquely for the different spins by exploiting the initial beam polarizations at the International Linear Collider (ILC).
With the upcoming run of the Large Hadron Collider (LHC) at much higher center of mass energies, ... more With the upcoming run of the Large Hadron Collider (LHC) at much higher center of mass energies, the search for Beyond Standard Model (BSM) physics will again take center stage. New colored particles predicted in many BSM scenarios are expected to be produced with large cross sections thus making them interesting prospects as a doorway to hints of new physics. We consider the resonant production of such a colored particle, the diquark, a particle having the quantum number of two quarks. The diquark can be either a scalar or vector. We focus on the vector diquark which has much larger production cross section compared to the scalar ones. In this work we calculate the next-to-leading order (NLO) QCD corrections to the on-shell vector diquark production at the LHC produced through the fusion of two quarks as well as the NLO corrections to its decay width. We present full analytic results for the one-loop NLO calculation and do a numerical study to show that the NLO corrections can reduce the scale uncertainties in the cross sections which can be appreciable and therefore modify the expected search limits for such particles. We also use the dijet result from LHC to obtain current limits on the mass and coupling strengths of the vector diquarks.
We study the viability of having two relatively light top squarks ('stops') in the framework of t... more We study the viability of having two relatively light top squarks ('stops') in the framework of the Next-to-Minimal Supersymmetric Standard Model (NMSSM). Such light stops render the NMSSM rather 'natural'. These are shown to be allowed by the relevant direct searches at the Large Hadron Collider (LHC) and to be compatible with the latest LHC results on the Higgs sector, other low energy electroweak constraints and recent constraints from the dark matter (DM) sector. We propose dedicated searches for such light stops at the LHC within a 'simplified' scenario that may have a bino-like or a singlino-like neutralino LSP as the DM candidate and point out various final states carrying the imprint of their collective presence. Under certain circumstances, in such a scenario, presence of two light stops may give rise to final states which are not so typical in their search. Thorough studies at the detector level reveal the status of such a scenario after the 8 TeV run of the LHC and shed light on the prospects of its 13 and 14 TeV runs. In favorable regions of the NMSSM parameter space, with low-lying spectra, signals with significance 5σ is possible with a few tens to a few hundreds of fb −1 of integrated luminosity in diverse final states.
We consider an extension of the Standard Model involving a singlet Higgs and down type vector-lik... more We consider an extension of the Standard Model involving a singlet Higgs and down type vector-like quarks in the light of the current LHC Higgs data. For a good range of the parameters of the Higgs potential, and a mass range for the heavy vector-like quark, we find that the singlet heavy Higgs arising from the production and decay of the vector-like quarks give rise to (2b~4t) signal. The subsequent decay of the top quarks to $b W^{+}$ give rise to a final state with six b quarks, two same-sign charged leptons and missing transverse momenta with observable cross-sections at the 14 TeV run of the Large Hadron Collider. The Standard Model background for such a final state is practically negligible.
ABSTRACT We consider an extension of the Standard Model involving a singlet Higgs and down type v... more ABSTRACT We consider an extension of the Standard Model involving a singlet Higgs and down type vector-like quarks in the light of the current LHC Higgs data. For a good range of the parameters of the Higgs potential, and a mass range for the heavy vector-like quark, we find that the singlet heavy Higgs arising from the production and decay of the vector-like quarks give rise to (2b~4t) signal. The subsequent decay of the top quarks to $b W^{+}$ give rise to a final state with six b quarks, two same-sign charged leptons and missing transverse momenta with observable cross-sections at the 14 TeV run of the Large Hadron Collider. The Standard Model background for such a final state is practically negligible.
New physics at the TeV scale is highly anticipated at the LHC. New particles with color, if withi... more New physics at the TeV scale is highly anticipated at the LHC. New particles with color, if within the LHC energy reach, will be copiously produced. One such particle is a diquark, having the quantum numbers of two quarks, and can be a scalar or a vector. It will decay to two light quarks, or two top quarks, or a top and a light quark, (up or down type depending on the quantum number of the produced diquark). If singly produced, it can be looked for as a dijet resonance, or as giving extra contribution to the single top production or tt production. In this work, we consider a color sextet vector diquark having the quantum number of (ud) type, its resonance production , and the subsequent decay to tb, giving rise to excess contribution to the single top production. Even though the diquark mass is large, its strong resonance production dominate the weak production of tb for a wide range of the diquark mass. Also its subsequent decay to tb produce a very hard b-jet compared to the usual electroweak production. In addition, the missing energy in the final state event is much larger from the massive diquark decays. Thus, with suitable cuts, the final state with b,b and a charged lepton together with large missing energy stands out compared to the Standard Model background. We make a detailed study of both the signal and the background. We find that such a diquark is accessible at the 7 TeV LHC upto a mass of about 3.3 TeV with the luminosity 1 fb −1 , while the reach goes up to about 4.3 TeV with a luminosity of 10 fb −1 .
We propose a non-supersymmetric SU(5) model in which only the third family of fermions are unifie... more We propose a non-supersymmetric SU(5) model in which only the third family of fermions are unified. The model remedies the non-unification of the three Standard Model couplings in non-supersymmetric SU(5). It also provides a mechanism for baryon number violation which is needed for the baryon asymmetry of the Universe and is not present in the Standard Model. Current experimental constraints on the leptoquark gauge bosons, mediating such baryon and lepton violating interactions in our model, allow their masses to be at the TeV scale. These can be searched for as a (bτ) or (tt) resonance at the Large Hadron Collider as predicted in our model.
A radion in a scenario with a warped extra dimension can be lighter than the Higgs boson, even if... more A radion in a scenario with a warped extra dimension can be lighter than the Higgs boson, even if the Kaluza-Klein excitation modes of the graviton turn out to be in the multi-TeV region. The discovery of such a light radion would be gateway to new physics. We show how the twophoton mode of decay can enable us to probe a radion in the mass range 60-110 GeV. We take into account the diphoton background, including fragmentation effects, and include cuts designed to suppress the background to the maximum possible extent. Our conclusion is that, with an integrated luminosity of 3000 f b −1 or less, the next run of the Large Hadron Collider should be able to detect a radion in this mass range, with a significance of 5 standard deviations or more.
The idea of unifying quarks and leptons in a gauge symmetry is very appealing. However, such an u... more The idea of unifying quarks and leptons in a gauge symmetry is very appealing. However, such an unification gives rise to leptoquark type gauge bosons for which current collider limits push their masses well beyond the TeV scale. We present a model in the framework of extra dimensions which breaks such quark-lepton unification symmetry via compactification at the TeV scale. These color triplet leptoquark gauge bosons, as well as the new quarks present in the model, can be produced at the LHC with distinctive final state signatures. These final state signals include high p T multi-jets and multi-leptons with missing energy, monojets with missing energy, as well as the heavy charged particles passing through the detectors, which we also discuss briefly. The model also has a neutral Standard Model singlet heavy lepton which is stable, and can be a possible candidate for the dark matter.
We study the 'single photon' process e + e − → γνν with contributions due to exchange of massive ... more We study the 'single photon' process e + e − → γνν with contributions due to exchange of massive gravitons in the Randall-Sundrum model of low-scale quantum gravity. It is shown that for significant regions in the parameter space, this process unambiguously highlights the resonance structure of the graviton sector. Even in the non-resonant part of the parameter space, we show that comparison with the benchmark process e + e − → µ + µ − can clearly distinguish signals for warped gravity from similar signals for large extra dimensions.
We study single top production in association with a charged Higgs in the type II two Higgs doubl... more We study single top production in association with a charged Higgs in the type II two Higgs doublet model at the Large Hadron Collider. The polarization of the top, reflected in the angular distributions of its decay products, can be a sensitive probe of new physics in its production. We present theoretically expected polarizations of the top for top charged-Higgs production, which is significantly different from that in the closely related process of tW production in the Standard Model. We then show that an azimuthal asymmetry, constructed from the decay lepton angular distribution in the laboratory frame, is a sensitive probe of top polarization and can be used to constrain parameters involved in top charged-Higgs production.
The Littest Higgs model with T-parity (LHT), apart from offering a viable solution to the natural... more The Littest Higgs model with T-parity (LHT), apart from offering a viable solution to the naturalness problem of the Standard Model, also predicts a set of new fermions as well as a candidate for dark matter. We explore the possibility of discovering the heavy T-odd quark Q H at the LHC in a final state comprising two hard jets with a large missing transverse momentum. Also discussed is the role of heavy flavor tagging.
We study the production of a single charged slepton in association with a top quark in a R-parity... more We study the production of a single charged slepton in association with a top quark in a R-parity violating supersymmetric model with lepton number violating interactions at the Large Hadron Collider. We find that the longitudinal polarization asymmetry of the top quark in such a production mode is significantly different from that in the production of a single top or a top pair in the Standard Model for a wide range of slepton masses. Our signal analysis shows that the top-slepton associate production leads to final states with distinct kinematic signatures, which differ from the Standard Model background.
Uploads
Papers by Santosh Rai