DSpace Repository :: Browsing by Author "Das, Arindam"

Browsing by Author "Das, Arindam"

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Bounds on heavy Majorana neutrinos in type-I seesaw and implications for collider searches
(Elsevier, 2017-11-10) Das, Arindam; Okada, Nobuchika; Korea Institute for Advanced Study (KIAS); Seoul National University (SNU); University of Alabama Tuscaloosa
The neutrino masses and flavor mixings, which are missing in the Standard Model (SM), can be naturally incorporated in the type-I seesaw extension of the SM with heavy Majorana neutrinos being singlet under the SM gauge group. If the heavy Majorana neutrinos are around the electroweak scale and their mixings with the SM neutrinos are sizable, they can be produced at high energy colliders, leaving characteristic signatures with lepton-number violations. Employing the general parametrization for the neutrino Dirac mass matrix in the minimal seesaw scenario, we perform a parameter scan and identify allowed regions to satisfy a variety of experimental constraints from the neutrino oscillation data, the electroweak precision measurements and the lepton-flavor violating processes. We find that the resultant mixing parameters between the heavy neutrinos and the SM neutrinos are more severely constrained than those obtained from the current search for heavy Majorana neutrinos at the LHC. Such parameter regions can be explored at the High-Luminosity LHC and a 100 TeV pp-collider in the future. (c) 2017 The Author(s). Published by Elsevier B.V.
Classically conformal U(1)' extended standard model, electroweak vacuum stability, and LHC Run-2 bounds
(American Physical Society, 2016-06-29) Das, Arindam; Oda, Satsuki; Okada, Nobuchika; Takahashi, Dai-Suke; University of Alabama Tuscaloosa; Okinawa Institute of Science & Technology Graduate University
We consider the minimal U(1)' extension of the standard model (SM) with the classically conformal invariance, where an anomaly-free U(1)' gauge symmetry is introduced along with three generations of right-handed neutrinos and a U(1)' Higgs field. Since the classically conformal symmetry forbids all dimensional parameters in the model, the U(1)' gauge symmetry is broken by the Coleman-Weinberg mechanism, generating the mass terms of the U(1)' gauge boson (Z' boson) and the right-handed neutrinos. Through a mixing quartic coupling between the U(1)' Higgs field and the SM Higgs doublet field, the radiative U(1)' gauge symmetry breaking also triggers the breaking of the electroweak symmetry. In this model context, we first investigate the electroweak vacuum instability problem in the SM. Employing the renormalization group equations at the two-loop level and the central values for the world average masses of the top quark (m(t) = 173.34 GeV) and the Higgs boson (m(h) = 125.09 GeV), we perform parameter scans to identify the parameter region for resolving the electroweak vacuum instability problem. Next we interpret the recent ATLAS and CMS search limits at the LHC Run-2 for the sequential Z' boson to constrain the parameter region in our model. Combining the constraints from the electroweak vacuum stability and the LHC Run-2 results, we find a bound on the Z' boson mass as m(Z') greater than or similar to 3.5 TeV. We also calculate self-energy corrections to the SM Higgs doublet field through the heavy states, the right-handed neutrinos and the Z' boson, and find the naturalness bound as m(Z') less than or similar to 7 TeV, in order to reproduce the right electroweak scale for the fine-tuning level better than 10%. The resultant mass range of 3.5 TeV less than or similar to m(Z') less than or similar to 7 TeV will be explored at the LHC Run-2 in the near future.
Collider phenomenology of heavy neutrinos
(University of Alabama Libraries, 2016) Das, Arindam; Okada, Nobuchika; University of Alabama Tuscaloosa
The existence of the neutrino mass has been established by the neutrino oscillation experiments. The so-called seesaw extension of the Standard Model is probably the simplest idea to naturally explain the existence of tiny neutrino mass through the lepton number violating Majorana mass term. There is another alternative way, commonly known as the inverse seesaw mechanism, where the small neutrino mass is obtained by the tiny lepton number violating parameters. In this work we investigate the signatures of such heavy neutrinos, having mass in the Electroweak scale at the high energy colliders. Based on a simple realization of inverse seesaw model we fix the model parameters to reproduce the neutrino oscillation data and to satisfy the other experimental constraints. We assume two flavor structures of the model and the different types of hierarchical light neutrino mass spectra. For completeness we consider the general parameterization for the model parameters by introducing an arbitrary orthogonal matrix and the nonzero Dirac and Majorana phases. Due to the smallness of the lepton number violating parameter this model can manifest the trilepton plus missing energy at the Large Hadron Collider(LHC). Using the recent LHC results for anomalous production of the multilepton events at $8$ TeV with a luminosity of $19.5$ fb$^{-1}$, we derive the direct upper bounds on the light-heavy neutrino mixing parameter as a function of the heavy neutrino mass. Using a variety of initial states such as quark-quark, quark-gluon and gluon-gluon as well as photon mediated processes for the Majorana heavy neutrinos we obtain direct upper bounds on the light-heavy neutrino mixing angles from the current LHC data at $8$ TeV. For the pseudo-Dirac heavy neutrinos produced from the various initial states using the recent anomalous multilepton search by the LHC at $8$ TeV with $19.5$ fb$^{-1}$ luminosity, we obtain upper bounds on the mixing angles.
Direct bounds on electroweak scale pseudo-Dirac neutrinos from root s=8 TeV LHC data
(Elsevier, 2014) Das, Arindam; Bhupal Dev, P.S.; Okada, Nobuchika; University of Alabama Tuscaloosa; University of Manchester
Seesaw models with a small lepton number breaking can naturally accommodate electroweak-scale pseudo-Dirac neutrinos with a sizable mixing with the active neutrinos, while satisfying the light neutrino oscillation data. Due to the smallness of the lepton number breaking parameter, the 'smoking gun' collider signature of same-sign dileptons is suppressed, and the heavy neutrinos in these models would manifest at the LHC dominantly through lepton number conserving trilepton final states. Using the recent CMS results for anomalous production of multilepton events at the $\sqrt{s}$ = 8 TeV LHC with an integrated luminosity of 19.5 fb⁻¹, we derive direct upper bounds on the light-heavy neutrino mixing parameter as a function of the heavy Dirac neutrino mass. These limits extend the collider sensitivity to higher heavy neutrino masses up to about 500 GeV. In the lower mass range, our limits are comparable to the existing indirect constraints derived from Higgs and electroweak precision data.
Electroweak vacuum stability in classically conformal B - L extension of the standard model
(Springer, 2017) Das, Arindam; Okada, Nobuchika; Papapietro, Nathan; University of Alabama Tuscaloosa
We consider the minimal $U(1)_{B-L}$ extension of the standard model (SM) with the classically conformal invariance, where an anomaly-free $U(1)_{B-L}$ gauge symmetry is introduced along with three generations of right-handed neutrinos and a $U(1)_{B-L}$ Higgs field. Because of the classically conformal symmetry, all dimensional parameters are forbidden. The$B-L$ gauge symmetry is radiatively broken through the Coleman–Weinberg mechanism, generating the mass for the $U(1)_{B-L}$ gauge boson ($Z′$ boson) and the right-handed neutrinos. Through a small negative coupling between the SM Higgs doublet and the $B-L$ Higgs field, the negative mass term for the SM Higgs doublet is generated and the electroweak symmetry is broken. In this model context, we investigate the electroweak vacuum instability problem in the SM. It is well known that in the classically conformal $U(1)_{B-L}$ extension of the SM, the electroweak vacuum remains unstable in the renormalization group analysis at the one-loop level. In this paper, we extend the analysis to the two-loop level, and perform parameter scans. We identify a parameter region which not only solve the vacuum instability problem, but also satisfy the recent ATLAS and CMS bounds from search for $Z′$ boson resonance at the LHC Run-2. Considering self-energy corrections to the SM Higgs doublet through the right-handed neutrinos and the $Z′$ boson, we derive the naturalness bound on the model parameters to realize the electroweak scale without fine-tunings.
Enhanced pair production of heavy Majorana neutrinos at the LHC
(American Physical Society, 2018) Das, Arindam; Okada, Nobuchika; Raut, Digesh; Korea Institute for Advanced Study (KIAS); University of Alabama Tuscaloosa
Towards experimental confirmations of the type-I seesaw mechanism, we explore a prospect of discovering the heavy Majorana right-handed neutrinos (RHNs) from a resonant production of a new massive gauge boson (${Z}^{\prime }$) and its subsequent decay into a pair of RHNs (${Z}^{\prime }\to NN$) at the future high luminosity runs at the Large Hadron Collider (LHC). Recent simulation studies have shown that the discovery of the RHNs through this process is promising in the future. However, the current LHC data very severely constrains the production cross section of the ${Z}^{\prime }$ boson into a dilepton final states, $pp\to {Z}^{\prime }\to {\ell }^{+}{\ell }^{-}$ ($\ell =e$ or $\mu $). Extrapolating the current bound to the future, we find that a significant enhancement of the branching ratio $\mathrm{BR}\left({Z}^{\prime }\to NN\right)$ over $\mathrm{BR}\left({Z}^{\prime }\to {\ell }^{+}{\ell }^{-}\right)$ is necessary for the future discovery of RHNs. As a well-motivated simple extension of the standard model (SM) to incorporate the ${Z}^{\prime }$ boson and the type-I seesaw mechanism, we consider the minimal ${U\left(1\right)}_{X}$ model, which is a generalization of the well-known minimal $B-L$ model without extending the particle content. We point out that this model can yield a significant enhancement up to $\mathrm{BR}\left({Z}^{\prime }\to NN\right)/\mathrm{BR}\left({Z}^{\prime }\to {\ell }^{+}{\ell }^{-}\right)\simeq 5$ (per generation). This is in sharp contrast with the minimal $B-L$ model, a benchmark scenario commonly used in simulation studies, which predicts $\mathrm{BR}\left({Z}^{\prime }\to NN\right)/\mathrm{BR}\left({Z}^{\prime }\to {\ell }^{+}{\ell }^{-}\right)\simeq 0.5$ (per generation). With such an enhancement and a realistic model-parameter choice to reproduce the neutrino oscillation data, we conclude that the possibility of discovering RHNs with, for example, a $300\text{}\text{}{\mathrm{fb}}^{-1}$ luminosity implies that the ${Z}^{\prime }$ boson will be discovered with a luminosity of $170.5\text{}\text{}{\mathrm{fb}}^{-1}$ ($125\text{}\text{}{\mathrm{fb}}^{-1}$) for the normal (inverted) hierarchy of the light neutrino mass pattern.
Heavy Majorana neutrino pair productions at the LHC in minimal U(1) extended Standard Model
(Springer, 2018) Das, Arindam; Okada, Nobuchika; Raut, Digesh; Korea Institute for Advanced Study (KIAS); University of Alabama Tuscaloosa
In our recent paper (Das et al. in Phys Rev D 97:115023, 2018) we explored a prospect of discovering the heavy Majorana right-handed neutrinos (RHNs) at the future LHC in the context of the minimal non-exotic U(1) extended Standard Model (SM), where a pair of RHNs are created via decay of resonantly produced massive U(1) gauge boson ( ${Z}^{\prime }$ ). We have pointed out that this model can yield a significant enhancement of the branching ratio of the ${Z}^{\prime }$ boson to a pair of RHNs, which is crucial for discovering the RHNs under the very severe LHC Run-2 constraint from the search for the ${Z}^{\prime }$ boson with dilepton final states. In this paper, we perform a general parameter scan to evaluate the maximum production rate of the same-sign dilepton final states (smoking gun signature of Majorana RHNs production) at the LHC, while reproducing the neutrino oscillation data. We also consider the minimal non-exotic U(1) model with an alternative charge assignment. In this case, we find a further enhancement of the branching ratio of the ${Z}^{\prime }$ boson to a pair of RHNs compared to the conventional case, which opens up a possibility of discovering the RHNs even before the ${Z}^{\prime }$ boson at the future LHC experiment.
Long-lived TeV-scale right-handed neutrino production at the LHC in gauged U(1)(X) model
(Elsevier, 2019) Das, Arindam; Dev, P.S. Bhupal; Okada, Nobuchika; Osaka University; Washington University (WUSTL); United States Department of Energy (DOE); University of Chicago; Fermi National Accelerator Laboratory; University of Alabama Tuscaloosa
A gauged $U{\left(1\right)}_{X}$ extension of the Standard Model is a simple and consistent framework to naturally incorporate three right-handed neutrinos (RHNs) for generating the observed light neutrino masses and mixing by the type-I seesaw mechanism. We examine the collider testability of the $U{\left(1\right)}_{X}$ model, both in its minimal form with the conventional charges, as well as with an alternative charge assignment, via the resonant production of the $U{\left(1\right)}_{X}$ gauge boson (${Z}^{\prime }$) and its subsequent decay into a pair of RHNs. We first derive an updated upper limit on the new gauge coupling ${g}_{X}$ as a function of the ${Z}^{\prime }$-boson mass from the latest LHC dilepton searches. Then we identify the maximum possible cross section for the RHN pair-production under these constraints. Finally, we investigate the possibility of having one of the RHNs long-lived, even for a TeV-scale mass. Employing the general parametrization for the light neutrino mass matrix to reproduce the observed neutrino oscillation data, we perform a parameter scan and find a simple formula for the maximum RHN lifetime as a function of the lightest neutrino mass eigenvalue (${m}_{\mathrm{lightest}}$). We find that for ${m}_{\mathrm{lightest}}\lesssim {10}^{-5}$ eV, one of the RHNs in the minimal $U{\left(1\right)}_{X}$ scenario can be long-lived with a displaced-vertex signature which can be searched for at the LHC and/or with a dedicated long-lived particle detector, such as MATHUSLA. In other words, once a long-lived RHN is observed, we can set an upper bound on the lightest neutrino mass in this model.
Probing the seesaw mechanism at the 250 GeV ILC
(Elsevier, 2019) Das, Arindam; Okada, Nobuchika; Okada, Satomi; Raut, Digesh; Osaka University; University of Alabama Tuscaloosa; University of Delaware
We consider a gauged U(1)${}_{B-L}$ (Baryon-minus-Lepton number) extension of the Standard Model (SM), which is anomaly-free in the presence of three Right-Handed Neutrinos (RHNs). Associated with the U(1)${}_{B-L}$ symmetry breaking the RHNs acquire their Majorana masses and then play the crucial role to generate the neutrino mass matrix by the seesaw mechanism. Towards the experimental confirmation of the seesaw mechanism, we investigate a RHN pair production through the U(1)${}_{B-L}$ gauge boson (${Z}^{\prime }$) at the 250 GeV International Linear Collider (ILC). The ${Z}^{\prime }$ gauge boson has been searched at the Large Hadron Collider (LHC) Run-2 and its production cross section is already severely constrained. The constraint will become more stringent by the future experiments with the High-Luminosity upgrade of the LHC (HL-LHC). We find a possibility that even after a null ${Z}^{\prime }$ boson search result at the HL-LHC, the 250 GeV ILC can search for the RHN pair production through the final state with same-sign dileptons plus jets, which is a “smoking-gun” signature from the Majorana nature of RHNs. In addition, some of RHNs are long-lived and leave a clean signature with a displaced vertex. Therefore, the 250 GeV ILC can operate as not only a Higgs Factory but also a RHN discovery machine to explore the origin of the Majorana neutrino mass generation, namely the seesaw mechanism.
Production of heavy neutrino in next-to-leading order QCD at the LHC and beyond
(2016) Das, Arindam; Konar, Partha; Majhi, Swapan; University of Alabama Tuscaloosa
Majorana and pseudo-Dirac heavy neutrinos are introduced into the type-I and inverse seesaw models, respectively, in explaining the naturally small neutrino mass. TeV scale heavy neutrinos can also be accommodated to have a sizable mixing with the Standard Model light neutrinos, through which they can be produced and detected at the high energy colliders. In this paper we consider the Next-to-Leading Order QCD corrections to the heavy neutrino production, and study the scale variation in cross-sections as well as the kinematic distributions with different final states at 14 TeV LHC and also in the context of 100 TeV hadron collider. The repertoire of the Majorana neutrino is realized through the characteristic signature of the same-sign dilepton pair, whereas, due to a small lepton number violation, the pseudo-Dirac heavy neutrino can manifest the trileptons associated with missing energy in the final state. Using the $\sqrt{s}$=8TeV, 20.3 fb⁻¹ and 19.7 fb⁻¹ data at the ATLAS and CMS respectively, we obtain prospective scale dependent upper bounds of the light-heavy neutrino mixing angles for the Majorana heavy neutrinos at the 14 TeV LHC and 100 TeV collider. Further exploiting a recent study on the anomalous multilepton search by CMS at $\sqrt{s}$=8TeV with 19.5 fb⁻¹ data, we also obtain the prospective scale dependent upper bounds on the mixing angles for the pseudo-Dirac neutrinos. We thus project a scale dependent prospective reach using the NLO processes at the 14 TeV LHC.

Browsing by Author "Das, Arindam"

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