arXiv subjects

Amitava Datta

Publications and source records attributed to Amitava Datta.

At least 55 records · Page 3Linked to original sources

arXiv2012

New limits on top squark NLSP from ATLAS 4.7 $fb^{-1}$ data

Using the ATLAS 4.7 $fb^{-1}$ data on new physics search in the jets + $\met$ channel, we obtain new limits on the lighter top squark ($\tilde t_1$) considering all its decay modes assuming that it is the next to lightest supersymmetric particle (NLSP). If the decay $\lstop$ $\ra c \lspone$ dominates and the production of dark matter relic density is due to NLSP - LSP co-annihilation then the lower limit on $\mlstop $ is 240 GeV. The limit changes to 200 GeV if the decay $\lstop$ $\ra b W \lspone$ dominates. Combining these results it follows that $\lstop $ NLSP induced baryogenesis is now constrained more tightly.

hep-ph↗

arXiv2012

Many faces of low mass neutralino dark matter in the unconstrained MSSM, LHC data and new signals

If all strongly interacting sparticles (the squarks and the gluinos) in an unconstrained minimal supersymmetric standard model (MSSM) are heavier than the corresponding mass lower limits in the minimal supergravity (mSUGRA) model, obtained by the current LHC experiments, then the existing data allow a variety of electroweak (EW) sectors with light sparticles yielding dark matter (DM) relic density allowed by the WMAP data. Some of the sparticles may lie just above the existing lower bounds from LEP and lead to many novel DM producing mechanisms not common in mSUGRA. This is illustrated by revisiting the above squark-gluino mass limits obtained by the ATLAS Collaboration, with an unconstrained EW sector with masses not correlated with the strong sector. Using their selection criteria and the corresponding cross section limits, we find at the generator level using Pythia, that the changes in the mass limits, if any, are by at most 10-12% in most scenarios. In some cases, however, the relaxation of the gluino mass limits are larger ($\approx 20%$). If a subset of the strongly interacting sparticles in an unconstrained MSSM are within the reach of the LHC, then signals sensitive to the EW sector may be obtained. This is illustrated by simulating the $blj$$\etslash$, $l= e and μ$, and $bτj$$\etslash$ signals in i) the light stop scenario and ii) the light stop-gluino scenario with various light EW sectors allowed by the WMAP data. Some of the more general models may be realized with non-universal scalar and gaugino masses.

hep-ph↗

arXiv2011

Enriching the exploration of the mUED model with event shape variables at the CERN LHC

We propose a new search strategy based on the event shape variables for new physics models where the separations among the masses of the particles in the spectrum are small. Collider signature of these models, characterised by low $p_T$ leptons/jets and low missing $p_T$, are known to be difficult to look for. The conventional search strategies involving hard cuts may not work in such situations. As a case study, we have investigated the hitherto neglected jets + missing $E_T$ signature -known to be a challenging one - arising from the pair productions and decay of $n =1$ KK-excitations of gluons and quarks in the minimal Universal Extra Dimension (mUED) model. Judicious use of the event shape variables, enables us to reduce the Standard Model backgrounds to a negligible level . We have shown that in mUED, $R^{-1}$ upto $850 ~\rm GeV$, can be explored or ruled out with 12 fb$^{-1}$ of integrated luminosity at the 7 TeV run of the LHC. We also discuss the prospects of employing these variables for searching other beyond standard model physics with compressed or partially compressed spectra.

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arXiv2011

Low mass neutralino dark matter in mSUGRA and more general models in the light of LHC data

The $bτj \etslash$ signal at the ongoing LHC experiments is simulated with Pythia in the mSUGRA and other models of SUSY breaking. Special attention is given on the compatibility of this signature with the low mass neutralino dark matter (LMNDM) scenario consistent with WMAP data. In the mSUGRA model the above signal as well as the LMNDM scenario are strongly disfavoured due to the constraints from the on going SUSY searches at the LHC. This tension, however, originates from the model dependent correlations among the parameters in the strong and electroweak sectors of mSUGRA. That there is no serious conflict between the LMNDM scenario and the LHC data is demonstrated by constructing generic phenomenological models such that the strong sector is unconstrained or mildly constrained by the existing LHC data and parameters in the electroweak sector, unrelated to the strong sector,yield DM relic density consistent with the WMAP data. The proposed models, fairly insensitive to the conventional SUSY searches in the jets + $\etslash$ and other channels, yield observable signal in the suggested channel for $\lum \gsim 1 \ifb$ of data. They are also consistent with the LMNDM scenario and can be tested by the direct dark matter search experiments in the near future. Some of these models can be realized by non-universal scalar and gaugino masses at the GUT scale.

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arXiv2011

Revealing the footprints of squark gluino production through Higgs search experiments at the Large Hadron Collider at 7 TeV and 14 TeV

The invariant mass distribution of the di-photons from the decay of the lighter scalar Higgs boson(h) to be carefully measured by dedicated h search experiments at the LHC may be distorted by the di-photons associated with the squark-gluino events with much larger cross sections in Gauge Mediated Supersymmetry Breaking (GMSB) models. This distortion if observed by the experiments at the Large Hadron Collider at 7 TeV or 14 TeV, would disfavour not only the standard model but various two Higgs doublet models with comparable h - masses and couplings but without a sector consisting of new heavy particles decaying into photons. The minimal GMSB (mGMSB) model constrained by the mass bound on h from LEP and that on the lightest neutralino from the Tevatron, produce negligible effects. But in the currently popular general GMSB(GGMSB) models the tail of the above distribution may show statistically significant excess of events even in the early stages of the LHC experiments with integrated luminosity insufficient for the discovery of h. We illustrate the above points by introducing several benchmark points in various GMSB models - minimal as well as non-minimal. The same conclusion follows from a detailed parameter scan in a simplified GGMSB model recently employed by the CMS collaboration to interpret their searches in the di-photon + $\etslash$ channel. Other observables like the effective mass distribution of the di-photon + X events may also reveal the presence of new heavy particles beyond the Higgs sector. The contamination of the h mass peak and simple remedies are also discussed.

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arXiv2011

SUSY signals with small and large trilinear couplings at the LHC 7 TeV runs and neutralino dark matter

We propose the signal $1b+1l+N_j+\etslash$ along with appropriate selection criteria for the LHC 7 TeV run, where the number of jets $(N_j)$ is $\ge$ 2 or 4. These signals can complement the canonical $Jets + \etslash$ signature since they are sensitive to the trilinear soft breaking parameter $(A_0)$ and low values of the parameter tan$β$ in the minimal supergravity (mSUGRA) model. A large region of this mSUGRA parameter space within the reach of the ongoing experiments at the LHC is disfavoured by the bound on the lightest Higgs boson mass ($m_h \ge 114.4 $ GeV) unless $A_0$ has moderate to large negative values. Interestingly this parameter space with $A_0 \ne 0$ is also consistent with the observed dark matter relic density. A natural consequence of large $A_0$ is the existence of a light top squark ($\lstop$). The proposed signals primarily stem from direct $\lstop \lstop^*$ production and/or $\tilde g \ra \tilde t_{1} t $, if all squark-gluino events are considered. A thorough analysis of the signals and the corresponding backgrounds are presented using the event generator Pythia. We finally compare the signal size for $A_0 = 0$ and $A_0 \ne 0$.

hep-ph↗

arXiv2010

SUSY darkmatter at the LHC - 7 TeV

We have analysed the early LHC signatures of the minimal supergravity (mSUGRA) model. Our emphasis is on the 7 - $TeV$ run corresponding to an integrated luminosity of $\sim 1.0 ~fb^{-1}$ although we have also discussed briefly the prospects at LHC-10 $TeV$. We focus on the parameter space yielding relatively light squark and gluinos consistent with the darkmatter relic density data and the LEP bounds on the lightest Higgs scalar mass. This parameter space is only allowed for non-vanishing trilinear soft breaking term $A_0$. A significant region of the parameter space with large to moderate negative values of $A_0$ consistent with the stability of the scalar potential and relic density production via neutralino annihilation and/or neutralino - stau coannihilation yields observable signal via the jets + missing transverse energy channel. The one lepton + jets + missing energy signal is also viable over a smaller but non-trivial parameter space. The ratio of the size of the two signals - free from theoretical uncertainties - may distinguish between different relic density generating mechanisms. With efficient $τ$-tagging facilities at 7 $TeV$ the discriminating power may increase significantly. We also comment on other dark matter relic density allowed mSUGRA scenarios and variants there of in the context of LHC-7 $TeV$.

hep-ph↗

arXiv2010

The interplay between the charged Higgs and squark-gluino events at the LHC

In some extensions of the standard model with extended Higgs sectors, events from new particle production may pass the selection criteria for Higgs search in different channels at the LHC - 14 TeV and mimic Higgs signals. This intriguing possibility is illustrated by PYTHIA based simulations using several representative points in the parameter space of the minimal supersymmetric standard model (MSSM) including a point in the minimal supergravity model consistent with the Dark matter (DM) relic density data. Our simulations explore the interplay between the charged Higgs signal and typical squark-gluino events. We argue that the standard selections like the one based on the polarization properties of the $τ$'s from charged Higgs decay, though adequate for handling the SM background, may not be very efficient in the presence of SUSY backgrounds. We then propose an alternative search strategy based on pure kinematics which sufficiently controls both the SM and the MSSM backgrounds. For charged Higgs masses ($H^{\pm}$) in the deep decoupling regime (600 GeV $\lsim m_{H^{\pm}} \lsim$ 800 GeV) this method works well and extends the LHC reach close to 800 GeV for an integrated luminosity of 30 $fb^{-1}$ with or without the SUSY background. For a lighter charged Higgs a judicious combination of the old selections and some of the cuts proposed by us may disentangle the Higgs signal from the squark-gluino backgrounds quite effectively.

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arXiv2010

Application of Graphics Processing Units to Search Pipeline for Gravitational Waves from Coalescing Binaries of Compact Objects

We report a novel application of graphics processing units (GPUs) for the purpose of accelerating the search pipelines for gravitational waves from coalescing binaries of compact objects. A speed-up of 16 fold has been achieved compared with a single central processing unit (CPU). We show that substantial improvements are possible and discuss the reduction in CPU count required for the detection of inspiral sources afforded by the use of GPUs.

gr-qc↗

arXiv2009

Tracking down the elusive charginos / neutralinos through $τ$ leptons at the Large Hadron Collider

An unconstrained minimal supersymmetric standard model with the superpartners of the strongly interacting particles very heavy (close to the kinematic reach of the LHC or even beyond it) and a relatively light electroweak sector is considered. Using the event generator Pythia it is shown that the 1$τ$-jet (tagged) + 2$l$ and 2$τ$-jets (tagged) + 1$l$ signals with a reasonably hard $\etslash$ spectrum either by themselves or in combination with the conventional $3l$ signal, which is known to be of rather modest size with a soft $\etslash$ spectrum, may appreciably extend the reach of chargino-neutralino search at the LHC with 10 fb$^{-1}$ of integrated luminosity. This is especially so if the lighter chargino and the second lightest neutralino decays via two body leptonic modes with large BRs. The theoretical motivation of this scenario, yielding large values of the fine-tuning parameters but consistent with various indirect constraints including the dark matter relic density, is briefly discussed. It is shown that in the minimal supergravity (mSUGRA) model with an universal scalar mass at the GUT scale, the signals involving $τ$-jets are not viable. Theoretically well-motivated variations of these boundary conditions are, however, adequate for reviving these signals.

hep-ph↗

arXiv2009

Probing R-parity violating models of neutrino mass at the LHC via top squark decays

It is shown that the R-parity violating decays of the lighter top squarks (${\widetilde t}_1$) triggered by the lepton number violating couplings $λ^{\prime}_{i33}$, where the lepton family index i = 1-3, can be observed at the LHC via the dilepton di-jet channel even if the coupling is as small as 10$^{-4}$ or 10$^{-5}$, which is the case in several models of neutrino mass, provided it is the next lightest supersymmetric particle(NLSP) the lightest neutralino being the lightest supersymmetric particle(LSP). We have first obtained a fairly model independent estimate of the minimum observable value of the parameter ($P_{ij} \equiv BR(\widetilde t \to l_i^+ b) \times BR(\widetilde t^* \to l_j^- \bar b$)) at the LHC for an integrated luminosity of 10fb$^{-1}$ as a function of $\mlstop$ by a standard Pythia based analysis. We have then computed the parameter $P_{ij}$ in several representative models of neutrino mass constrained by the neutrino oscillation data and have found that the theoretical predictions are above the estimated minimum observable levels for a wide region of the parameter space.

hep-ph↗

arXiv2008

CP-violating Higgs at Tevatron

We analyze the prospect for observing the intermediate neutral Higgs ($h_2$) boson in its decay to two lighter Higgs bosons ($h_1$) at the Tevatron in the framework of the CP violating MSSM using the PYTHIA event generator. We consider the lepton+ 4-jets+ $\met$ channel from $p \bar p \ra W h_2 \ra W h_1 h_1 \ra l ν_l b \bar b b\bar b$, with two or three tagged $b$ jets. We found that it is very hard to observe this signature in the LEP-allowed region of parameter space, due to the small signal efficiency.

hep-ph↗

arXiv2008

Search for Top Squarks at Tevatron Inspired by Dark Matter and Electroweak Baryogenesis

The search for the top squark ($\widetilde{t}_{1}$) within the kinematic reach of Tevatron Run II is of great contemporary interest. Such a $\widetilde{t}_{1}$ can explain the baryon asymmetry of the universe provided $120 ~GeV/c^{2} \leq m_{\widetilde{t}_{1}} \leq m_t$. Moreover if $Δm \equiv m_{\widetilde{t}_{1}}$ - $m_{\widetildeχ_1^0}$ is small, where $\widetildeχ_1^0$ is the LSP, the dark matter relic density as obtained from the WMAP data may be explained via $\widetilde{t}_{1}$- LSP coannihilation. In this scenario the decay $\widetilde{t}_{1} \to c \widetildeχ_1^0$ is likely to occur with 100 % branching ratio but for small $Δm$ the conventional di-jet + $\not E_T$ signal becomes unobservable. We propose a new search strategy based on the $2j + \not E_T$ signature accompanied by an isolated cluster of energy which arises from a decaying heavy particle with characteristic decay length. Our preliminary simulations with PYTHIA indicate that for $100 ~GeV/c^{2} \le m_{\widetilde{t}_{1}} \le 130 ~GeV/c^{2}$ this signal may be observable while somewhat lager $m_{\widetilde{t}_{1}}$ may still provide hints of new physics.

hep-ph↗

arXiv2008

Lepton Flavours at the Early LHC Experiments as the Footprints of the Dark Matter Producing Mechanisms

The mSUGRA parameter space corresponding to light sleptons well within the reach of LHC and relatively light squarks and gluinos (mass $\le$ 1 TeV) has three regions consistent with the WMAP data on dark matter relic density and direct mass bounds from LEP 2. Each region can lead to distinct leptonic signatures from squark-gluino events during the early LHC experiments (integrated luminosity $\sim 10 ~fb^{-1}$ or even smaller). In the much studied stau-LSP coannihilation region with a vanishing common trilinear coupling ($A_0$) at the GUT scale a large fraction of the final states contain electrons and / or muons and $e$ - $μ$ - $τ$ universality holds to a good approximation. In the not so well studied scenarios with non-vanishing $A_0$ both LSP pair annihilation and stau-LSP coannihilation could contribute significantly to the dark matter relic density for even smaller squark-gluino masses. Our simulations indicate that the corresponding signatures are final states rich in $τ$-leptons while final states with electrons and muons are suppressed leading to a violation of lepton universality. These features may be observed to a lesser extent even in the modified parameter space (with non-zero $A_0$) where the coannihilation process dominates. We also show that the generic $m$-leptons + $n$-jets+ $\not! E_T$ signatures without flavour tagging can also discriminate among the three scenarios. However, the signals become more informative if the $τ$ and $b$-jet tagging facilities at the LHC experiments are utilized.

hep-ph↗

arXiv2007

Associated Higgs Production in CP-violating supersymmetry: probing the `open hole' at the Large Hadron Collider

A benchmark CP-violating supersymmetric scenario (known in the literature as `CPX-scenario') is studied in the context of the Large Hadron Collider (LHC). It is shown that the LHC, with low to moderate accumulated luminosity, will be able to probe the existing `hole' in the $m_{h_1}$-$\tanβ$ plane, which cannot be ruled out by the Large Electron Positron Collider data. This can be done through associated production of Higgs bosons with top quark and top squark pairs leading to the signal \emph{dilepton + $\leq{5}$ jets (including 3 b-jets) + missing ${p_T}$}. Efficient discrimination of such a CP-violating supersymmetric scenario from other contending ones is also possible at the LHC with a moderate volume of data.

hep-ph↗

arXiv2007

Non-zero trilinear parameter in the mSUGRA model - dark matter and collider signals at Tevatron and LHC

Phenomenologically viable and interesting regions of parameter space in the minimal super-gravity (mSUGRA) model with small $m_0$ and small $m_{1/2}$ consistent with the WMAP data on dark matter relic density and the bound on the mass of the lightest Higgs scalar $ m_h>$ 114 GeV from LEP2 open up if the rather adhoc assumption $A_0$=0, where $A_0$ is the common trilinear soft breaking parameter, employed in most of the existing analyses is relaxed. Since this region corresponds to relatively light squarks and gluinos which are likely to be probed extensively in the very early stages of the LHC experiments, the consequences of moderate or large negative values of $A_0$ are examined in detail. We find that in this region several processes including lightest supersymmetric particle (LSP) pair annihilation, LSP - lighter tau slepton (${\tilde τ}_1$) coannihilation and LSP - lighter top squark (${\tilde t}_1$) coannihilation contribute to the observed dark matter relic density. %\sout{The possibility that a relic density producing ${\tilde t}_1$ can be %observed at the current experiments at the Tevatron is wide open.} The possibility that a ${\tilde t}_1$ that can participate in coannihilation with the lightest neutralino to satisfy the WMAP bound on relic density and at the same time be observed at the current experiments at the Tevatron is wide open. At the LHC a large number of squark - gluino events lead to a very distinctive semi-inclusive signature $τ^\pm$+X$_τ$ (anything without a tau lepton) with a characteristic size much larger than $e^\pm$+X$_e$ or $μ^\pm$+X$_μ$ events.

hep-ph↗

arXiv2007

Focus Point SUSY at the LHC Revisited

The estimation of the backgrounds for gluino signals in focus point supersymmetry is extended by including the backgrounds from the production of four third generation quarks in the analysis. We find that these backgrounds are negligible if one uses the strong selection criteria proposed in the literature (including this analysis) for heavy gluino searches. Softer selection criteria often recommended for lighter gluino searches yield backgrounds which are small but numerically significant. We have also repeated the more conventional background calculations and compared our results with the other groups. We find that the size of the total residual background estimated by different groups using different event generators and hard kinematical cuts agree approximately. In view of the theoretical uncertainties in the leading order signal and background cross sections mainly due to the choice of the QCD scale, the gluino mass reach at the LHC cannot be pinpointed. However, requiring a signal with $\rm\geq 3$ tagged b-jets (instead of the standard choice of $\rm\geq 2$) it is shown that gluino masses close to 2 TeV can be probed at the LHC for a range of reasonable choices of the QCD scale for an integrated luminosity of 300 fb$^{-1}$.

hep-ph↗

arXiv2007

New signals of a R-parity violating model of neutrino mass at the Tevatron

In a variety models of neutrino masses and mixings the lighter top squark decays into competing R - parity violating and R - parity conserving channels. Using Pythia we have estimated in a model independent way the minimum value of P $\equiv$ BR($\widetilde t_1 \to c \widetilde χ_1^0 $) $\times$ BR($\widetilde t_1 \to l^+_i b$), where $l_i = e^+$ and $μ^+$, corresponding to an observable signal involving the final state 1$l$ + jets +$\met$ (carried by the neutrinos from the $\tilde χ_1^0$ decay) at Tevatron Run II. For the kinematical cuts designed in this paper P depends on $m_{\widetilde t_1}$ only. We then compute P for representative choices of the model parameters constrained by the oscillation data and find that over a significant region of the allowed parameter space P is indeed larger than $P_{min}$. This signal is complementary to the dilepton + dijet signal studied in several earlier experimental and phenomenological analyses and may be observed even if BR($\widetilde t_1 \to l^+_i b$) is an order of magnitude smaller than BR($\widetilde t_1 \to c \widetilde χ_1^0$). The invariant mass distribution of the hardest lepton and the hardest jet may determine $m_{\widetilde t_1}$ and reveal the lepton number violating nature of the underlying interaction. The invariant mass distribution of the two lowest energy jets may determine $m_{\widetilde χ_1^0}$.

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