arXiv subjects

Majid Hashemi

Publications and source records attributed to Majid Hashemi.

33 records · Page 2Linked to original sources

arXiv2016

Observability of s-channel Heavy Charged Higgs at LHC Using Top Tagging Technique

In this paper, the question of observability of a heavy charged Higgs in the mass range 400 GeV < mH < 1000 GeV, is addressed. The production process is set to pp -> H+ -> tb at 14 TeV LHC. The analysis benefits from top tagging technique which is based on finding a fat jet as a result of the boosted top quark decay in signal events. A detailed hadron level analysis is performed and selection efficiencies are presented with different charged Higgs mass hypotheses. Finally running toy experiments and using pseudo-data, a fit over signal plus background distributions is performed to assess possibility of reconstructing the charged Higgs peak and its invariant mass measurement. It is shown that the charged Higgs mass can well be reconstructed in the mass range 500 GeV to 1 TeV, with a signal significance which depends on tanbeta. Eventually 5sigma discovery and 95% C.L. exclusion contours are also provided.

hep-ph↗

arXiv2015

Charged Higgs Study through Triple or Double Higgs Production in a General Two Higgs Doublet Model at Future Lepton Collide

In this study the charged Higgs signal through triple or double Higgs production in a general two Higgs doublet model (2HDM) is investigated. The main production process is $e^+e^- \rightarrow H^+H^-H^0$ followed by the charged Higgs decay to a pair of $τν$ and the neutral Higgs decay to $b\bar{b}$. The alternative process $H^+W^-H^0$ is also included as a source of charged Higgs signal in the analysis having comparable cross-section. The focus is on a future $e^+e^-$ linear collider operating at $\sqrt{s}=1.5$ TeV. The final state under consideration ($τ^+ τ^- b \bar{b} E^{\textnormal{miss}}_{T}$) is suitable for electroweak background rejection using the $b-$tagging tools. It is shown that although the signal cross section is small, with a reasonable background suppression, high signal significance values are achievable at an integrated luminosity $500 fb^{-1}$ depending on the charged Higgs mass, $\tanβ~$ and the CP-odd neutral Higgs mass. Finally results are quoted in terms of the signal significance for charged Higgs in the mass range $170<m_{H^{\pm}}<400$ GeV.

hep-ph↗

arXiv2015

The t-channel Charged Higgs Production in Single Top Events at LHC

In this paper, the t-channel charged Higgs production at LHC is studied. Production process is a t-channel single top event with charged Higgs exchange. Therefore the signal is similar with Standard Model single top production in terms of the final state. In the first step, the signal cross section is calculated and compared to the other main production processes which are used for a heavy charged Higgs search at LHC, i.e., pp -> tbH- and pp -> H+ -> tb. In the next step, an event generation and analysis is applied on signal and background events, in order to estimate the signal significance. The signal cross section is typically smaller than the associated production (tbH-) and resonance production (tb) by a factor of 10-3 and ranges from 10 fb to 1 fb for charged Higgs mass from 200 to 500 GeV at tanbeta = 50. Due to the small cross section of signal events and large SM background, the signal significance is small even after a dedicated kinematic analysis and selection of events, however, tanbeta values above 120 can be excluded at an integrated luminosity of 3000 fb-1.

hep-ph↗

arXiv2015

Charged Higgs Detection in the $τν$ Decay Mode at Future Linear Colliders

Charged Higgs phenomenology and detection chance at future linear colliders through the production process $e^{-}e^{+} \ra H^{+}H^{-} \ra τ^{+}ντ\barν$ is studied. Both charged Higgs bosons are considered to decay to $τν$ and the hadronic decay of $τ$ leptons is analyzed taking into account spin and kinematics effects using a proper simulation of the $τ$ lepton decay. It is shown that within the MSSM framework, with \tanb = 10, a wide range of charged Higgs masses would have detectable signal beyond the $5σ$ statistical significance at linear colliders with $\sqrt{s} = 500$ and 1000 GeV.

hep-ph↗

arXiv2015

Enhancement of Charged Higgs Production in Association With W at Muon Colliders in General Two Higgs Doublet Models

The charged Higgs associated production with a W boson has a smooth cross section as a function of the charged Higgs mass at muon colliders. The cross section in Minimal Supersymmetric Standard Model is about 25fb in the range 200 GeV < mH < 400 GeV with tanbeta = 50. This is much larger than the corresponding cross section at an e+e- collider which reaches a fraction of femtobarn. The observability of this channel at a muon collider has been recently studied in an earlier work leading to the result that with 1 ab-1, a 5sigma signal can be observed throughout the aforementioned mass range. In this paper, results of a study based on a general two Higgs doublet model (type II and III) are presented and the cross section of this process in the most sensitive parameter space is evaluated. It is concluded that the cross section of this process increases with increasing neutral Higgs boson masses involved in the s-channel diagram and can be as large as several picobarn with tanbeta = 50. The region of "physical Higgs boson mass" parameter space which could lead to a 5 sigma signal at 50fb-1 is specified.

hep-ph↗

arXiv2014

Off-diagonal Yukawa Couplings in the s-channel Charged Higgs Production at LHC

The search for the heavy charged Higgs (mH+ > mtop) has been mainly based on the off-shell top pair production process. However, resonance production in s-channel single top events is an important channel to search for this particle. In a previous work, it was shown that this process, i.e., qq' -> H+ -> tb + h.c., can lead to comparable results to what is already obtained from LHC searches through gb -> tH- process. What was obtained was, however, based on diagonal Yukawa couplings between incoming quarks assuming cs as the main incoming pair due to the CKM matrix element being close to unity. The aim of this paper is to show that off-diagonal couplings, like cb, may lead to substantial contributions to the cross section, even if the corresponding CKM matrix element is two orders of magnitude smaller. For this reason, the cross section is calculated for each initial state including all diagonal and off-diagonal terms, and all is finally added together to get the total cross section which is observed to be ~ 2.7 times larger than what is obtained from cs initial state. Results are eventually reflected into 95% C.L. exclusion and $5σ$ discovery contours at different integrated luminosities of LHC. A reasonable coverage of the parameter space is obtained by the 95% C.L. exclusion contour.

hep-ph↗

arXiv2014

Observability of Charged Higgs through Triple or Double Higgs Production in a General Two Higgs Doublet Model at an e+e- Linear Collider

In this paper the triple Higgs production in a general two Higgs doublet model (2HDM) is studied. The production process is e+e- -> H+H-H0 followed by the charged Higgs decay to a pair of tau nu and the neutral Higgs decay to bb. The alternative process H+W-H0 is also included as a source of charged Higgs signal in the analysis. The focus is on a future e+e- linear collider operating at sqrt(s) = 1.5 TeV. The final state under consideration (tau tau bb ETmiss) is suitable for electroweak background rejection using the b-tagging tools. It is shown that although the signal cross section is small, with a reasonable background suppression, high signal significance values are achievable at an integrated luminosity 500 fb-1 depending on the charged Higgs mass, tanbeta and the CP-odd neutral Higgs mass. Finally results are quoted in terms of the signal significance for charged Higgs in the mass range 170 < mH+ < 400 GeV.

hep-ph↗

arXiv2013

Observability of Heavy Charged Higgs through s-channel Single Top Events at LHC

The charged Higgs bosons can be produced as a resonance in s-channel single top events. The light charged Higgs in such events preferably decays to a pair of $τν$ thus making it difficult to distinguish from the large single W events producing the same final state. However, the heavy charged Higgs decay to a pair of $t \bar{b}$ can be extracted from the SM background events. The final state under consideration in this paper contains the top quark decay to $W^+ b$ followed by $W^+$ decay to electrons or muons. It is shown that this signal can be observed at LHC at a large area of MSSM phase space ($m(H^+), tanβ$). Finally $5σ$ and 95% CL exclusion contours are presented at different integrated luminosities of LHC assuming a nominal center of mass energy of $\sqrt{s}$ = 14 TeV.

hep-ph↗

arXiv2013

Charged Higgs Pair Production in a General Two Higgs Doublet Model at $e^+e^-$ and $μ^+μ^-$ Linear Colliders

In this paper, charged Higgs pair production through $\ell^+ \ell^- \rightarrow H^+ H^-$ where $\ell = e$ or $μ$, is studied within the framework of a general Two Higgs Doublet Model (2HDM). The analysis is relevant to a future $e^+e^-$ or $μ^+μ^-$ collider operating at center of mass energy of $\sqrt{s}=500$ GeV. Two different scenarios of small and large $α$ values is studied. Here $α$ is the parameter which diagonalizes the neutral CP-even Higgs boson mass matrix. Within the Minimal Supersymmetric Standard Model (MSSM), cross section of this process is almost the same at $e^+e^-$ and $μ^+μ^-$ colliders. It is shown that at $e^+e^-$ colliders within a general 2HDM, cross section is not sensitive to the mass of neutral Higgs bosons, however, it can acquire large values up to several picobarn at $μ^+μ^-$ colliders with the presence of heavy neutral Higgs bosons. A scan over Higgs boson mass parameter space is performed to analyze the effect of large masses of neutral Higgs bosons involved in the s-channel propagator and thus in the total cross section of this process.

hep-ph↗

arXiv2013

Single Top Events as a Source of Light Charged Higgs in the Fully Hadronic Final State at LHC

The single top production proceeds through different channels at LHC. The t-channel production which has a cross section at the level of one third of the ttbar, can be regarded as a source of light charged Higgs boson through top quark decay to charged Higgs. Due to the low jet multiplicity and characteristic kinematic features of this process, a signal selection can be designed to extract charged Higgs signal from the background processes. In this paper, taking MSSM as the theoretical framework, The light charged Higgs signal from single top events is analyzed. The charged Higgs is assumed to decay to tau nu and the hadronic decay of tau leptons is analyzed. Different charged Higgs mass hypotheses are analyzed and a scan over the parameter space (mH+,tanbeta) is performed. It is shown that this process has the potential to exclude large areas of parameter space, while with enough data, a 5 sigma discovery would also be possible. Finally 95% C.L. exclusion and 5 sigma contours are provided for different integrated luminosities of LHC.

hep-ph↗

arXiv2012

Charged Higgs Observability Through Associated Production With W at a Muon Collider

The observability of a charged Higgs boson produced in association with a W boson at future muon colliders is studied. The analysis is performed within the MSSM framework. The charged Higgs is assumed to decay to tb and a fully hadronic final state is analyzed, i.e., mu+mu- \rightarrow H\pmW\mp \rightarrow tbW \rightarrow WbbW \rightarrow jjjjbb. The main background is tt production in fully hadronic final state which is an irreducible background with very similar kinematic features. It is shown that although the discovery potential is almost the same for a charged Higgs mass in the range 200 GeV < mH\pm < 400 GeV, the signal significance is about 1sigma for tanbeta = 50 at integrated luminosity of 50 fb-1. The signal rate is well above that at e+e- linear colliders with the same center of mass energy and enough data (O(1 ab-1)) will provide the same discovery potential for all heavy charged Higgs masses up to mH\pm \sim 400 GeV, however, the muon collider cannot add anything to the LHC findings.

hep-ph↗

arXiv2012

Observability of Light Charged Higgs Decay to Muon in Top Quark Pair Events at LHC

In this paper the charged Higgs signal through the decay to a pair of muon and neutrino (H -> mu nu) is analyzed. The analysis attempts to estimate the amount of muonic signal of the charged Higgs at LHC at a center of mass energy of 14 TeV. The signal process is the top quark pair production with one of the top quarks decaying to a charged Higgs (non SM anomalous top quark decay) and the other decaying to a W boson which is assumed to decay hadronically to two light jets. Due to the small branching ratio of charged Higgs decay to muon, results are quoted for data corresponding to an integrated luminosity of 300 fb^-1 which is expected to be collected at the LHC high luminosity regime. It is shown that a signal significance close to 5 down to below 1 is achievable for a charged Higgs mass in the range 80 GeV < m(H \pm) < 150 GeV taking the top quark pair production with both top quarks decaying to W bosons as the main irreducible background.

hep-ph↗

arXiv2011

Possibility of observing MSSM charged Higgs in association with a W boson at LHC

Possibility of observing associated production of charged Higgs and W boson in the framework of MSSM at LHC is studied. Both leptonic and hadronic decays of W boson are studied while the charged Higgs boson is considered to decay to a $τ$ lepton and a neutrino. Therefore two search categories are defined based on the leptonic and hadronic final states, i.e. $\ell τ+E^{miss}_{T}$ and $jj τ+E^{miss}_{T}$ where $\ell=e$ or $μ$ and $j$ is a light jet from $W$ decay. The discovery chance of the two categories is evaluated at an integrated luminosity of 300 \invfb at LHC. It is shown that both leptonic and hadronic final states have the chance of discovery at high \tanb. Finally $5σ$ and $3σ$ contours are provided for both search categories.

hep-ph↗

arXiv2010

SLIM at LHC: LHC search power for a model linking dark matter and neutrino mass

Recently a model has been proposed that links dark matter and neutrino masses. The dark matter candidate which is dubbed as SLIM has a mass of MeV scale and can show up at low energy experiments. The model also has a high energy sector which consists of a scalar doublet, $(ϕ^-, ϕ^0)$. We discuss the potential of the LHC for discovering the new scalars. We focus on the $ϕ^+ϕ^-$ and $ϕ^{\pm} ϕ^0$ production and the subsequent decay of the charged scalar to a charged lepton and the SLIM which appears as missing energy. Identifying the background, we estimate the signal significance and find that it can exceed $5 σ$ at 30 ${\rm fb}^{-1}$ for the 14 TeV run at the LHC. We comment on the possibility of extracting the flavor structure of the Yukawa couplings which also determine the neutrino mass matrix. Finally, we discuss the prospects of this search at the current 7 TeV run of the LHC.

hep-ph↗

arXiv2006

Possibility of tanbeta measurement in CMS at LHC

The achievable tanbeta determination accuracy for CMS at LHC is presented. Using MSSM H/A->tau tau decay in the associated production process gg->bbH/A the event rates are measured at large tanbeta and the systematic and statistical errors are estimated. Due to sensitivity of the above event rates to tanbeta it is shown that it is possible to determine constraints on tanbeta for a given set of SUSY parameters and uncertainties.

hep-ph↗