arXiv subjects

Majid Hashemi

Publications and source records attributed to Majid Hashemi.

At least 19 recordsLinked to original sources

arXiv2025

Observability of 2HDM charged Higgs boson in a misaligned scenario

The large hadron collider precision measurement of $h_{125}$ properties suggests that a misaligned two Higgs doublet model type 1 is preferred among other types of the model. In this analysis, a lepton collider potential for charged Higgs boson observation is evaluated and compared with HL-LHC expectations. We focus on $H^{\pm} \to W^{\pm}h_{125}$ decay which is exclusively activated in misalignment scenario. An event analysis including fast detector simulation is performed and it is shown that some parts of the parameter space which are out of HL-LHC reach can be probed by a high energy lepton collider with signal significance exceeding $2σ$ and $5σ$ for exclusion or discovery. The region with highest sensitivity is in the middle range charged Higgs boson mass, i.e., $350 < m_{H^{\pm}} <450$ GeV and $\tan β\gtrsim4$ at non-zero $\cos(β-α)$ values.

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arXiv2024

Charged Higgs decay to $W^{\pm}H$ at a high energy lepton collider

In this work, we present a search strategy for heavy charged Higgs boson at Compact Linear Collider (CLIC) as a future $e^+e^-$ collider. The signal is charged Higgs boson pair production in two Higgs doublet model (2HDM) followed by $H^{\pm}\to W^{\pm}H$ and $H\to b\bar{b}$. Here, $H$ denotes the heavy CP-even neutral Higgs boson of the model. The collider center of mass energy is chosen to be $\sqrt{s}=1400$ GeV as the second stage of CLIC operation. In this case, $m_{H^+}<\sqrt{s}/2$ can be explored due to the pair production. It is shown that the signal of charged Higgs in the mass range 250 GeV $<m_{H^+}<$ 650 GeV in fully hadronic final state, containing four $b$-jets from neutral Higgs and four jets from $W$ bosons, can well be observed on top of the standard model background. Finally 5$σ$ contours are presented in ($m_{H^{\pm}}$, $m_H$) space for different $\tanβ$ values.

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arXiv2023

Heavy neutral 2HDM Higgs Boson Pair Production at CLIC Energies

In this work, the neutral Higgs boson pair production is analyzed at Compact Linear Collider (CLIC) to be operating at center of mass energies $\sqrt{s}$=1400 GeV (stage 2) and $\sqrt{s}$=3000 GeV (stage 3). The Higgs bosons to be searched for are neutral CP-even (H) and CP-odd (A) within the framework of two Higgs doublet model (2HDM) in the mass range 300 < mH/A < 1000 GeV. All types of the CP-conserving model are studied and the signal observability is evaluated taking into account the main SM background processes like ZZ, tt and the SM-like Higgs boson associated production (hZ). Results are presented for a set of model parameters and Higgs boson masses in terms of signal distributions over the background as well as the integrated luminosity needed for 5σ discovery. It is shown that the heavy mass region is well observable at CLIC in types 3 (flipped) and 4 (lepton-specific) in the regions not excluded by LHC so far, while in type 1 the signal observation is challenging due to the large jet multiplicity in the tt final state.

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arXiv2022

Parameter dependence and analysis of the 2HDM neutral Higgs boson pair production and decay at future lepton colliders

In this work, we present a study of the neutral Higgs bosons in the two Higgs doublet model (2HDM) in terms of their production processes and decay channels as a function of the model parameters. The analysis is performed for all four types of the 2HDM and the most promising processes and decay channels are identified for each type of the model. Several Higgs boson mass scenarios below and above the threshold of decay to gauge boson pair are introduced and the corresponding categories of final states are analyzed. The event analysis including collider beam spectrum and detector simulation shows that future lepton colliders have the potential to explore regions in the 2HDM parameter space, which have not yet been excluded by LHC, in a few weeks of operation. Final results are presented in terms of the signal distributions on top of the background and 95% CL exclusion and 5$σ$ contours based on center of mass energy of $\sqrt{s}$ = 500 GeV at the Compact Linear Collider (CLIC) or International Linear Collider (ILC).

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arXiv2021

Kinematic corrections and reconstruction methods for neutral Higgs boson decay to $b\bar{b}$ in 2HDM type-I at future lepton colliders

In this paper, an approach for neutral Higgs bosons search is described based on 2HDM type-I at electron-positron linear colliders operating at $ \sqrt{s}=1$ TeV. The beam is assumed to be unpolarized and fast detector simulation is included. The signal process produces a fully hadronic final state through $ e^{+}e^{-}\rightarrow AH\rightarrow b\bar{b}b\bar{b} $ where both CP-even and CP-odd Higgs bosons ($H$ and $A$) are assumed to decay to a pair of $b$-jets. Several benchmark scenarios are introduced as the baseline for the analysis taking $ m_{H/A}$ in the range 150--300 GeV. In order to avoid Higgs boson conversion $A \to ZH$, Higgs boson masses are chosen with $m_A-m_H < m_Z$. It is shown that with a proper kinematic correction applied on final state $b$-jet four momenta, true combinations of $b$-jets can be found for simultaneous reconstruction of both Higgs bosons through $b\bar{b}$ invariant mass calculation. Results show that observable signals can be achieved with statistical significance exceeding $5σ$ well before the target integrated luminosity of 8 $ab^{-1}$.

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arXiv2018

Possibility of observing Higgs bosons at the ILC in the lepton-specific 2HDM

The Higgs boson pair production at a linear $e^+e^-$ collider is analyzed in the $4τ$ final state in the context of lepton-specific or type IV 2HDMs. Both beams are assumed to be unpolarized. The Higgs boson pairs (HA) are produced through off-shell $Z^*$ production and decay to $τ$-jets which is the main decay channel for neutral Higgs bosons in type IV. Using a simplified detector simulation based on SiD detector at ILC, the 4$τ$ signal is studied through the $τ$-jet pair invariant mass reconstruction. Several benchmark scenarios are considered for center of mass energies of 500 and 1000 GeV at integrated luminosity of 500$fb^{-1}$. Among Standard Model (SM) background processes, the main background is $e^+e^- \to ZZ$ followed by $Z \to ττ$. This background is however, well under control. With the luminosity assumed in the analysis, striking signals are obtained beyond the reach of LHC. Such signals would allow for precise determination of masses and cross sections and alredy much lower luminosities are sufficient for discovery.

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arXiv2018

Search for neutral Higgs bosons decaying to $b\bar{b}$ in the flipped 2HDM at future $e^-e^+$ linear colliders

In this study, assuming the Type-Y (flipped) 2HDM at the SM-like scenario as the theoretical framework, observability of the additional heavy neutral Higgs bosons $H$ and $A$ is investigated through the signal process chain $e^- e^+ \rightarrow AH \rightarrow b\bar{b}b\bar{b}$ at a linear collider operating at the center-of-mass energy of 1.5 TeV. The assumed signal process is highly motivated by the enhancements in the $A/H\rightarrow b\bar{b}$ decays at relatively high $\tb$ values. Such enhancements result in the dominance of the mentioned decay modes even for Higgs masses above the threshold of the on-shell top quark pair production. Taking advantage of such a unique feature, several benchmark scenarios are studied. Simulating the detector response based on the SiD detector at the ILC, simulated events are analyzed to reconstruct the $H$ and $A$ Higgs bosons. The top quark pair production and $Z/γ$ production are the main SM background processes and are well under control. Results indicate that, the $H$ and $A$ Higgs bosons are observable with signals exceeding $5σ$ with possibility of mass measurement in all the tested scenarios. Specifically, the parameter space region enclosed with the mass ranges $m_H$=150-500 GeV and $m_A$=230-580 GeV with the $A/H$ mass splitting of 80 GeV is observable at the integrated luminosity of 500 $fb^{-1}$.

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arXiv2018

Search for neutral Higgs bosons within Type-I 2HDM at future linear colliders

In this study, we investigate observability of the neutral scalar ($H$) and pseudoscalar ($A$) Higgs bosons in the framework of the Type-I 2HDM at SM-like scenario at a linear collider operating at $\sqrt s=$ 500 and 1000 GeV. The signal process chain $e^- e^+ \rightarrow A H \rightarrow ZHH\rightarrow jj b\bar{b}b\bar{b}$ where $jj$ is a di-jet resulting from the $Z$ boson decay and $b\bar{b}$ is a $b$ quark pair, is assumed and several benchmark scenarios with different mass hypotheses are studied. The assumed signal process is mainly motivated by the possible enhancements the decay modes $A\rightarrow ZH$ and $H\rightarrow b\bar{b}$ may receive in the Type-I. Event generation is performed for the assumed scenarios separately and the beamstrahlung effects are taken into account. The detector response is simulated based on the SiD detector at the ILC and the simulated events are analyzed to obtain candidate mass distributions of the Higgs bosons. According to the results, the top quark pair production process has the most contribution to the total background and is, however, well-controlled. Results indicate that, in all of the considered scenarios, both of the Higgs bosons $H$ and $A$ are observable with signals exceeding $5σ$ with possibility of mass measurement. To be specific, at $\sqrt s=500$ GeV, the region of parameter space with $m_H=150$ GeV and $200\leq m_A \leq 250$ GeV is observable at the integrated luminosity of 500 $fb^{-1}$. Also, at $\sqrt s=1000$ GeV, the region with $150\leq m_H \leq 250$ GeV and $200\leq m_A \leq 330$ GeV with a mass splitting of 50-100 GeV between the $H$ and $A$ Higgs bosons is observable at the same integrated luminosity.

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arXiv2018

Analysis of b quark pair production signal from neutral 2HDM Higgs bosons at future Linear Colliders

In this paper, the b quark pair production events are analyzed as a source of neutral Higgs bosons of the two Higgs doublet model type I at linear colliders. The production mechanism is e+e- -> Z/gamma -> HA -> bbbb assuming a fully hadronic final state. The analysis aim is to identify both scalar and pseudo-scalar Higgs bosons in different benchmark points accommodating moderate boson masses. Due to pair production of Higgs bosons, the analysis is most suitable for a linear collider operating at sqrt(s) = 1 TeV. Results show that in selected benchmark points, signal peaks are observable in the b-jet pair invariant mass distributions at integrated luminosity of 500 fb-1.

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arXiv2018

Observability of 2HDM neutral Higgs bosons with different masses at future $e^+e^-$ linear colliders

Assuming two Higgs doublet model (2HDM) at SM-like scenario as the theoretical framework, this study addresses the question of observability of heavy neutral CP-even and CP-odd Higgs bosons $H$ and $A$ at a linear collider operating at $\sqrt{s}=1$ TeV. The signal production channel is $e^- e^+ \rightarrow A H \rightarrow ZHH$ with subsequent leptonic decay of the $Z$ boson and Higgs bosons decays into $b$ quark pairs. Therefore, to be specific, type-I 2HDM is used to allow dominant Higgs boson decay to b-quark pairs below the top quark pair production threshold. Two benchmark points with mass ranges $150\leq m_H\leq 200$ and $250\leq m_A\leq 300$ are simulated. The relevant energy and momentum smearing is applied and appropriate selection cuts are imposed to enrich signal events. Results indicate that both Higgs bosons are observable with signals exceeding $5σ$ significance with possibility of mass measurement at the integrated luminosity of 500 $fb^{-1}$.

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arXiv2018

Capability of future linear colliders to discover heavy neutral CP-even and CP-odd Higgs bosons within Type-I 2HDM

In this study, assuming the type-I 2HDM at SM-like scenario, observability of heavy neutral CP-even and CP-odd Higgs bosons $H$ and $A$ at a linear collider operating at $\sqrt{s}=1$ TeV is investigated through the signal process chain $e^- e^+ \rightarrow A H \rightarrow ZHH$ where the $Z$ boson experiences a leptonic ($e^-e^+$ or $μ^-μ^+$) decay and the $H$ Higgs boson is assumed to decay into a di-photon. This signal process is motivated especially by the clean signature which leptonic and photonic events can provide at colliders, and also by the enhancement due to the charged Higgs-mediated contribution to $H$ di-photon decay at large $\tanβ$ values. Simulation is based on four benchmark points according to which the Higgs mass $m_H$ ($m_A$) varies within the range $150$-$300$ ($200$-$400$) GeV. Energy smearing of jets and photons are performed, and momentum smearing is also applied to leptons. Results indicate that, for all of the assumed benchmark points, the Higgs bosons $H$ and $A$ are observable with measurable masses, and with signals exceeding $5σ$ at integrated luminosities $111$ and $201\,fb^{-1}$ respectively. Such luminosities are easily accessible to future linear colliders.

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arXiv2017

Analysis of top quark pair production signal from neutral 2HDM Higgs bosons at LHC

In this paper, the top quark pair production events are analyzed as a source of neutral Higgs bosons of two Higgs doublet model type I at LHC. The production mechanism is $pp \to H/A \to t\bar{t}$ assuming a fully hadronic final state through $t \to Wb \to jjb$. In order to distinguish the signal from the main background which is the standard model $t\bar{t}$, we benefit from the fact that the top quarks in signal events acquire a large Lorentz boost due to the heavy neutral Higgs boson. This feature leads to three collinear jets (a fat jet) which is a discriminating tool for identification of the top quarks from the Higgs boson resonances. Events with two identified top jets are selected and the invariant mass of the top pair is calculated for both signal and background. It is shown that the low $\tanβ$ region has still some parts which can be covered by this analysis and has not been excluded yet by flavor physics data.

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arXiv2017

Search for heavy neutral CP-even Higgs within Type-IV 2HDM at a future linear collider

In this paper, the production process $e^- e^+ \rightarrow A H$ is analyzed in the context of the type IV 2HDM and the question of observability of a neutral CP-even Higgs boson $H$ at a linear collider operating at $\sqrt{s}=1$ TeV is addressed. The CP-odd Higgs is assumed to experience a gauge-Higgs decay as $A\rightarrow ZH$ with hadronic decay of $Z$ boson as the signature of signal events. The production chain is thus $e^+e^- \rightarrow AH \rightarrow ZHH \rightarrow jj\ell\ell\ell\ell$ where $\ell$ is a $τ$ or $μ$. Four benchmark points with different mass hypotheses are assumed for the analysis. The Higgs mass $m_H$ is assumed to vary within the range 150-300 GeV in increments of 50 GeV. The anti-$k_t$ algorithm is used to perform the jet reconstruction. Results indicate that the neutral CP-even Higgs $H$ is observable through this production mechanism using the di-muon invariant mass distribution with possibility of mass measurement. The corresponding signal significances exceed $5σ$ at integrated luminosity of 3000 $fb^{-1}$.

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arXiv2017

Leptophilic neutral Higgs bosons in two Higgs doublet model at a linear collider

This paper addresses the question of observability of neutral Higgs bosons through the leptonic decay in a two Higgs doublet model (2HDM). Both scalar and pseudoscalar Higgs bosons ($H,~A$) are considered. The model is set to type IV to enhance the leptonic decay. In such a scenario, a signal production process like $e^+e^- \to A^0H^0 \to ττμμ$ or $μμττ$ would provide a clear signal on top of the background in a di-muon invariant mass distribution far from the $Z$ boson pole mass. The analysis is based on a $τ$-id algorithm which preselects events if they have two $τ$ jets by requiring a hadronic $τ$ decay. Several benchmark points are defined for the search, requiring a linear collider operating at $\sqrt{s} =$ 0.5 and 1 TeV. It is shown that the signal can be observed on top of the background in all benchmark points at an integrated luminosity of 1000 $fb^{-1}$.

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arXiv2016

Observability of Inert Scalars at the LHC

In this work we investigate the observability of Inert Doublet Model scalars at the LHC operating at the center of mass energy of 14 TeV. The signal production process is $pp \to AH^\pm \to ZHW^{\pm}H$ leading to two different final states of $\ell^+\ell^-Hjj H$ and $\ell^+\ell^-H\ell^\pmνH$ based on the hadronic and leptonic decay channels of the W boson. All the relevant background processes are considered and an event selection is designed to distinguish the signal from the large Standard Model background. We found that signals of the selected search channels are well observable at the LHC with integrated luminosity of 300 fb$^{-1}$.

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arXiv2016

The $s$-channel Charged Higgs in the Fully Hadronic Final State at LHC

With the current measurements performed by CMS and ATLAS experiments, the light charged Higgs scenario ($m_{H^{\pm}}$ $<$ 160 GeV), is excluded for most of the parameter space in the context of MSSM. However, there is still possibility to look for heavy charged Higgs boson particularly in the $s$-channel single top production process where the charged Higgs may appear as a heavy resonance state and decay to $t\bar{b}$. The production process under consideration in this paper is $pp \ra H^{\pm} \ra t\bar{b}~+~h.c.$, where the top quark decays to $W^{+}b$ and $W^{+}$ boson subsequently decays to two light jets. It is shown that despite the presence of large QCD and electroweak background events, the charged Higgs signal can be extracted and observed at a large area of MSSM parameter space ($m_{H^{\pm}}$,tan$β$) at LHC. The observability of charged Higgs is potentially demonstrated with 5$σ$ contours and $95\%$ confidence level exclusion curves at different integrated LHC luminosities assuming a nominal center of mass energy of $\sqrt{s}$ = 14 TeV.

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arXiv2016

Production of Inert Scalars at the high energy $e^+ e^-$ colliders

We investigate the phenomenology of the light charged and neutral scalars in Inert Doublet Model at future $e^+ e^-$ colliders with center of mass energies of 0.5 and 1 TeV, and integrated luminosity of 500~fb$^{-1}$. The analysis covers two production processes, $e^{+}e^{-} \rightarrow H^{+}H^{-}$ and $e^{+}e^{-}\rightarrow AH$, and consists of signal selections, cross section determinations as well as dark matter mass measurements. Several benchmark points are studied with focus on $H^{\pm} \rightarrow W^{\pm}H$ and $A \rightarrow ZH$ decays. It is concluded that the signal will be well observable in different final states allowing for mass determination of all new scalars with statistical precision of the order of few hundred MeV.

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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.

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