arXiv subjects

Mojtaba Mohammadi Najafabadi

Publications and source records attributed to Mojtaba Mohammadi Najafabadi.

At least 37 records · Page 2Linked to original sources

arXiv2019

Loop induced singlet scalar production through the vector like top quark at future lepton colliders

In this paper, we explore the signature of a simplified model which includes a new singlet scalar state and a vector like quark at future lepton colliders. In particular, we study the production of the new singlet scalar in association with a photon, which proceeds through loop level diagrams involving vector like top quark partner, at future $e^-e^+$ colliders with different center-of-mass energies from 500 GeV to 3 TeV. To show the sensitivity of the process, the exclusion limits on the parameter space of the model are presented considering the decay of the singlet scalar into a pair of Higgs boson, followed by the decay of Higgs bosons into $b\bar{b}$ pairs. The results are compared to those obtained from the LHC, electroweak precision data and other channels at lepton colliders and it is shown that a notable sensitivity to the parameter space of the simplified model could be achieved.

hep-ph↗

arXiv2019

New Probes for Axion-like Particles at Hadron Colliders

Axion-like particles (ALPs) appear from spontaneous global symmetry breaking in many extensions of the Standard Model (SM). In this paper, we find bounds on ALP ($a$) model parameters at the LHC from the ALP production associated with a photon and a jet ($j+γ+a$) as well as single top and top quark pairs ($t+j+a$, $t\bar{t}+a$) in a model independent approach. In particular, it is shown that the ALP production associated with a photon plus a jet at the LHC is a promising channel with significant sensitivity to probe the ALP couplings to gluons and electroweak gauge bosons. The prospects are presented at the High Luminosity LHC including a realistic detector simulation and pile up effects. Furthermore, the ALP model is examined through its contributions to the top quark (chromo)magnetic dipole moments. It is shown that the top quark magnetic and chromomagnetic dipole moments enable us to probe the ALP couplings to top quark and gauge bosons at a time. The constraints are complementary to those obtained from direct searches, as they are sensitive to light ALPs.

hep-ph↗

arXiv2018

Constraining top quark flavor violation and dipole moments through three and four top quark productions at the LHC

In this paper, we examine the sensitivity of the three-top quark production at the LHC to the top quark flavor-changing neutral currents (FCNC) as well as the sensitivity of the four-top production to the strong and weak dipole moments of the top quark. Upper limits at $95\%$ CL on the branching fractions of $\mathcal{B}(t\rightarrow qX)$, where $X = g, Z, γ, H$ and $q=u,c$, are set by performing an analysis on three-top events in the same-sign dilepton channel. We consider the main sources of the background processes and a realistic detector simulation is performed at the center-of-mass energy of 14 TeV. In the second part of this work, based on the recent upper limits which have been set on the four-top quark cross section by the ATLAS and CMS collaborations from 13 TeV data, we constrain the top quark strong and weak dipole moments. The bounds on the top quark dipole moments are presented using the future LHC prospects for four-top quark cross section measurement.

hep-ph↗

arXiv2018

Same-sign top pair plus $W$ production in flavor changing vector and scalar models

We investigate the prospect of the LHC for discovering new physics effects via new strategies in the same-sign top pair and same-sign top pair associated with a $W$ boson signatures. Significant enhancement in production of same-sign top quarks (plus a $W$ boson) is a joint property of several models beyond the standard model. We concentrate on the leptonic (electron and muon) decay of the top quarks and study the exclusion reach of the LHC data for a simplified model approach where top quark flavor changing could occur through a $Z'$ or a neutral scalar $ϕ$ exchange. Less background contributions and clean signature are the advantages of the leptonic decay mode of the top quarks in the same-sign production processes. A combination is performed on both same-sign top pair and same-sign top pair plus a $W$ boson production modes which enables us to reach a large fraction of the model parameter space. Assuming the couplings of new physics of the order of $10^{-2}$, the mass of a flavor changing $Z'$ or a neutral scalar above 1 TeV could be excluded. We propose a momentum dependent charge asymmetry and angular observables in the same-sign top process which provide the possibility of separation of new physics signal from the SM backgrounds as well as discrimination of the flavor changing $tuX$ from $tcX$, where $X = Z',ϕ$.

hep-ph↗

arXiv2018

Study of Higgs Effective Couplings at Electron-Proton Colliders

We perform a search for beyond the standard model dimension-six operators relevant to the Higgs boson at the Large Hadron Electron Collider (LHeC) and the Future Circular Hadron Electron Collider (FCC-he). With a large amount of data (few ab$^{-1}$) and collisions at TeV scale, both LHeC and FCC-he provide excellent opportunities to search for the BSM effects. The study is done through the process $e^-p \to h j ν_e$ where the Higgs boson decays into a pair of $b \bar{b}$ and we consider the main sources of background processes including a realistic simulation of detector effects. For the FCC-he case, in some signal scenarios to obtain an efficient event reconstruction and to have a good background rejection, jet substructure techniques are employed to reconstruct the boosted Higgs boson in the final state. In order to assess the sensitivity to the dimension-six operators, a shape analysis on the differential cross sections is performed. Stringent bounds are found on the Wilson coefficients of dimension-six operators with the integrated luminosities of 1 ab$^{-1}$ and 10 ab$^{-1}$ which in some cases show improvements with respect to the high-luminosity LHC results.

hep-ph↗

arXiv2017

Study of top quark dipole interactions in $t\bar{t}$ production associated with two heavy gauge bosons at the LHC

In this paper, we investigate the prospects of measuring the strong and weak dipole moments of the top quark at the Large Hadron Collider (LHC). Measurements of these couplings provide an excellent opportunity to probe new physics interactions as they have quite small magnitudes in the Standard Model. Our analyses are through studying the production cross sections of $t\bar{t}WW$ and $t\bar{t}ZZ$ processes in the same sign dilepton and four-lepton final states, respectively. The sensitivities to strong and weak top quark dipole interactions at the $95\%$ confidence level for various integrated luminosity scenarios are derived and compared with other studies. In addition to using the total cross sections, a novel handle based on an angular observable is introduced which is found to be sensitive to variations of the top quark strong dipole moments. We also investigate the sensitivity of the invariant mass of the system to the strong and weak dipole moments of the top quark.

hep-ph↗

arXiv2017

Probing Higgs boson couplings in H+$γ$ production at the LHC

In this paper, we examine the potential of Higgs boson production associated with a photon at the LHC to probe the new physics effects in the framework of the standard model effective field theory. It is shown that the differential kinematic distributions such as photon transverse momentum and invariant mass of Higgs+$γ$ in Higgs associated production are powerful variables to explore the coefficients of dimension six operators. The analysis is performed in the decay channel of Higgs boson into a $b\bar{b}$ pair including the main sources of background processes and a realistic simulation of the detector effects. We provide constraints at $95\%$ confidence level on the Wilson coefficients of dimension-six operators affecting Higgs boson plus a photon production. We show to what extent these limits could be improved at the high luminosity LHC. The effect of these constraints on a well-motivated beyond standard model scenario is presented.

hep-ph↗

arXiv2017

Single top quark production as a probe of anomalous $tqγ$ and $tqZ$ couplings at the FCC-ee

In this paper, a detailed study to probe the top quark Flavour-Changing Neutral Currents (FCNC) $tqγ$ and $tqZ$ at the future $e^{-}e^{+}$ collider FCC-ee in three different center-of-mass energies of 240, 350 and 500 GeV is presented. A set of useful variables are proposed and used in a multivariate technique to separate signal $e^- e^+ \rightarrow Z/γ\rightarrow t \bar{q} ~ ( \bar{t} q )$ from standard model background processes. The study includes a fast detector simulation based on the {\sc delphes} package to consider the detector effects. The $3 σ$ discovery regions and the upper limits on the FCNC branching ratios at 95\% confidence level (CL) in terms of the integrated luminosity are presented. It is shown that with 300 fb$^{-1}$ of integrated luminosity of data, FCC-ee would be able to exclude the effective coupling strengths above ${\cal O} (10^{-4}-10^{-5})$ which is corresponding to branching fraction of ${\cal O}(0.01-0.001)$\%. We show that moving to a high-luminosity regime leads to a significant improvement on the upper bounds on the top quark FCNC couplings to a photon or a $Z$ boson.

hep-ph↗

arXiv2017

Constraining Higgs boson effective couplings at electron-positron colliders

We probe the dimension-six operators contributing to Higgs production in association with a $Z$ boson at the future high-luminosity electron-positron colliders. Potential constraints on dimension-six operators in the Higgs sector are determined by performing a shape analysis on the differential angular distribution of the Higgs and $Z$ boson decay products. The analysis is performed at the center-of-mass energies of 350 and 500 GeV including a realistic detector simulation and the main sources of background processes. The 68\% and 95\% confidence level upper limits are obtained on the contributing anomalous couplings considering only the decay of the Higgs boson into a pair of $b$-quarks and leptonic $Z$ boson decay. Our results show that angular observables provide a great sensitivity to the anomalous couplings, in particular, at the high-luminosity regime.

hep-ph↗

arXiv2016

Constraining the monochromatic gamma-rays from dark matter annihilation by the LHC

The installation of forward detectors in CMS and ATLAS turn the LHC to an effective photon-photon collider. The elastic scattering of the beam-protons via the emission of photons, which can be identified by tagging the intact protons in the forward detectors, provides a powerful diagnostic of the central production of new particles through photon-photon annihilation. In this letter we study the central production of dark matter particles and the potential of LHC to constrain the cross section of this process. By virtue of the crossing symmetry, this limit can immediately be used to constrain the production of monochromatic gamma-rays in dark matter annihilation, a smoking gun signal under investigation in indirect dark matter searches. We show that with the integrated luminosity $\mathcal{L}=30~{\rm fb}^{-1}$ in LHC at center-of-mass energy $\sqrt{s}=$ 13 TeV, for dark matter masses $\sim (50-600)$ GeV, a model-independent constraint on the cross section of dark matter annihilation to monochromatic gamma-rays at the same order of magnitude as the current Fermi-LAT and the future limits from CTA, can be obtained.

hep-ph↗

arXiv2016

Measuring anomalous WW$γ$ and t$\bar{\text{t}}γ$ couplings using top+$γ$ production at the LHC

We consider the electroweak production of a top quark in association with a photon at the LHC to probe the electroweak top quark couplings (t$\bar{\text{t}}γ$) as well as the triple gauge boson couplings (WW$γ$). The study is based on the modifications of the t$\bar{\text{t}}γ$ and WW$γ$ interactions via heavy degrees of freedom in the form of dimension-six operators which we add to the standard model Lagrangian. A binned angular asymmetry in single top quark plus photon events and cross section ratio are proposed to probe the anomalous t$\bar{\text{t}}γ$ and WW$γ$ couplings. It is shown that the proposed angular asymmetry can distinguish anomalous t$\bar{\text{t}}γ$, WW$γ$ couplings from the standard model prediction and yield a great sensitivity.

hep-ph↗

arXiv2016

Constraints on top quark flavor changing neutral currents using diphoton events at the LHC

In this paper we show that the diphoton mass spectrum in proton-proton collisions at the LHC is sensitive to the top quark flavor changing neutral current in the vertices of $tuγ$ and $tcγ$. The diphoton mass spectrum measured by the CMS experiment at the LHC at a center-of-mass energy of 8 TeV and an integrated luminosity of 19.5 fb$^{-1}$ is used as an example to set limits on these FCNC couplings. It is also shown that the angular distribution of the diphotons is sensitive to anomalous $tuγ$ and $tcγ$ couplings and it is a powerful tool to probe any value of the branching fraction of top quark rare decay to an up-type quark plus a photon down to the order of $10^{-4}$. We also show that the $tuγ$ FCNC coupling has a significant contribution to the neutron electric dipole moment (EDM) and the upper bound on neutron EDM can be used to constrain the $tuγ$ FCNC coupling.

hep-ph↗

arXiv2016

Direct and Indirect Searches for Top-Higgs FCNC Couplings

Large top quark flavor changing through neutral currents is expected by many extensions of the standard model. Direct and indirect searches for flavor changing neutral currents (FCNC) in the top quark decays to an up type quark (up,charm) and a Higgs boson are presented. We probe the observability of the top-Higgs FCNC couplings through the process e$^-$e$^+ \rightarrow t (\rightarrow \ell ν_{\ell} b) \,\, \bar{t} (\rightarrow q H)$, where $\ell$ = e, $μ$ and $q$ reflects up and charm quarks. It is shown that the branching ratio $Br(t\rightarrow qH)$ can be probed down to $1.12\times 10^{-3}$ at $95\%$ C.L. at the center-of-mass energy of 500 GeV with the integrated luminosity of 3000 fb$^{-1}$. We also update the constraint on the top-Higgs FCNC coupling using the electroweak precision observables related to $Z\rightarrow c\bar{c}$ decay.

hep-ph↗

arXiv2015

Double Higgs Production with a Jet Substructure Analysis to Probe Extra Dimensions

In this paper, we perform a comprehensive study to probe the effects of large extra dimensions through double Higgs production in proton-proton collisions at the center-of-mass energies of 14, 33, 100 TeV. We concentrate on the channel in which both Higgs bosons decay into $b\bar{b}$ pair and take into account the main background contributions through realistic Monte-Carlo simulations. In order to achieve an efficient event reconstruction and a good background rejection, jet substructure techniques are used to efficiently capture the boosted Higgs bosons in the final state. The expected limits on the model parameters are obtained based on the invariant mass and the angular properties of the final state objects. Depending on the number of extra dimensions, bounds up to 6.1, 12.5, 28.1 TeV are set on the model parameter at proton-proton collisions with the center-of-mass energies of 14, 33, and 100 TeV, respectively.

hep-ph↗

arXiv2014

Probing the Top Quark Flavour-Changing Neutral Current at a Future Electron-Positron Collider

We present a study to examine the sensitivity of a future $e^{-} e^{+}$ collider to the anomalous top flavour-changing neutral current (FCNC) to the gluon. To separate signal from background a multivariate analysis is performed on top-quark pair and background events, where one top quark is considered to follow the dominant Standard Model (SM) decay, $t \rightarrow Wb$, and the other top decays through FCNC, $t\rightarrow qg$, where $q$ is a $u-$ or a $c-$quark. The analysis of fully-hadronic FCNC decay of the $t \bar{t}$ pair is also presented. The $95\%$ confidence level limits on the top-quark anomalous couplings are obtained for different values of the center-of-mass energies and integrated luminosities.

hep-ex↗

arXiv2014

Exploring the Anomalous Higgs-top Couplings

Top quark with its large Yukawa coupling is crucially important to explore TeV scale physics. Therefore, the study of Higgs-top sector is highly motivated to look for any deviations from the standard model predictions. The most general lowest order Lagrangian for the Higgs-top Yukawa coupling has scalar ($κ$) and pseudoscalar ($\tildeκ$) components. Currently, these couplings are constrained indirectly using the present experimental limits on the Higgs-$γ$-$γ$ and Higgs-gluon-gluon couplings. Furthermore, stronger bounds on $κ$ and $\tildeκ$ are obtained using the limits on the electric dipole moments (EDM). In this work, we propose an asymmetry-like observable $O_ϕ$ in $t\bar{t}H$ production at the LHC to probe the Higgs-top coupling and to distinguish between the scalar and pseudoscalar components. We also show that the presence of the pseudoscalar component in the Higgs-top Yukawa coupling leads to a sizeable value for the top quark EDM. It is shown that a limit of $10^{-19}$ e.cm, which is achievable by the future $e^{-}e^{+}$ collider, allows us to exclude a significant region in the $(κ,\tildeκ)$ plane.

hep-ph↗

arXiv2014

Direct photon production as a probe of quarks chromoelectric and chromomagnetic dipole moments at the LHC

In this paper we show that the $γ+$jet invariant mass distribution in proton-proton collisions at the LHC is significantly sensitive to the quarks chromoelectric ($κ$) and chromomagnetic ($\tildeκ$) dipole moments. It is shown that the presence of $κ$ or $\tildeκ$ leads to an increment of the cross section of $γ+$jet process in particular in the tail of $γ+$jet invariant mass distribution. Using the measured $γ+$jet invariant mass distribution by the CMS experiment at the center-of-mass energy of 8 TeV, we derive bounds on the quarks chromoelectric and chromomagnetic dipole moments. In extraction of the limits, we consider both theoretical and systematic uncertainties. The uncertainties originating from variation of the renomalization/factorization scales and the choice of proton parton distribution functions are taken into account as a function of the $γ+$jet invariant mass. We exclude $κ$ or $\tildeκ$ above $10^{-5}$ at 95$\%$ confidence level. This is the most stringent direct upper limit on $κ$ or $\tildeκ$.

hep-ph↗

arXiv2014

Probing the Anomalous FCNC Interactions in Top-Higgs Final State and Charge Ratio Approach

We study the anomalous production of a single top quark in association with a Higgs boson at the LHC originating from flavor changing neutral current (FCNC) interactions in $tqg$ and $tqH$ vertices. We derive the discovery potentials and $68\%$ C.L. upper limits considering leptonic decay of the top quark and the Higgs boson decay into a $b\bar{b}$ pair with 10 fb$^{-1}$ integrated luminosity of data in proton-proton collisions at the center-of-mass energy of 14 TeV. We propose a charge ratio for the lepton in top quark decay in terms of lepton $p_{T}$ and $η$ as a strong tool to observe the signal. In particular, we show that the charge ratio increases significantly at large $p_{T}$ of the charged lepton. While the main background from $t\bar{t}$ is nearly charge symmetric and $W+jets$ background has much smaller charge ratio with respect to the signal. We show that this feature can also be used in the probe of anomalous single top production with a $Z-$boson or a photon which are under the attention of the experimental collaborations.

hep-ph↗