arXiv subjects

Seyed Yaser Ayazi

Publications and source records attributed to Seyed Yaser Ayazi.

26 records · Page 2Linked to original sources

arXiv2012

Top Quark Forward-Backward Asymmetry and $W'$-Boson with General Couplings

The measured forward-backward asymmetry in top pair events at the Fermilab Tevatron collider deviates significantly from the standard model expectation. Several models have been proposed to describe the observed asymmetry which grows with the rapidity difference of the top pairs and also with the $t\bar{t}$ invariant mass. The presence of a heavy charged gauge boson $W'$ with the coupling $W'-t-d$ (left-handed, right-handed, and a mixture of left and right-handed couplings) could generate the desired top forward-backward asymmetry keeping the top pair cross section consistent with the standard model prediction. Such $W'$-boson makes contribution to the electric dipole moment of the neutron through contribution to the $d$-quark electric dipole moment, recently measured charge asymmetry ($A_{C}$) by the LHC experiments, and the total cross section of top pair at the LHC and Tevatron. We show that the upper bounds on neutron and top electric dipole moments disfavour any $W'$ with a mass below 240 GeV which could explain the Tevatron forward-backward asymmetry. It is shown that the charge asymmetry provides an allowed region in the parameters space with no overlap with the allowed region where the asymmetry could be described.

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arXiv2012

Single top quark production in $t$-channel at the LHC in Noncommutative Space-Time

We study the production cross section of the $t$-channel single top quark at the LHC in the noncommutative space-time. It is shown that the deviation of the $t$-channel single top cross section from the Standard Model value because of noncommutativity is significant when $|\vecθ| \gtrsim 10^{-4}$ GeV$^{-2}$. Using the present experimental precision in measurement of the $t$-channel cross section, we apply upper limit on the noncommutative parameter. When a single top quark decays, there is a significant amount of angular correlation, in the top quark rest frame between the top spin direction and the direction of the charged lepton momentum from its decay. We study the effect of noncommutativity on the spin correlation and we find that depending on the noncommutative scale, the angular correlation can enhance considerably. Then, we provide limits on the noncommutative scale for various possible relative uncertainties on the spin correlation measurement.

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arXiv2011

Constraints on Randall-Sundrum model from top-antitop production at the LHC

We study the top pair production cross section at the LHC in the context of Randall-Sundrum model including the Kaluza-Klein (KK) excited gravitons. It is shown that the recent measurement of the cross section of this process at the LHC restricts the parameter space in Randall-Sundrum (RS) model considerably. We show that the coupling parameter ($\frac{k}{\bar{M}_{pl}}$) is excluded by this measurement from 0.03 to 0.22 depending on the mass of first KK excited graviton ($m_1$). We also study the effect of KK excitations on the spin correlation of the top pairs. It is shown that the spin asymmetry in $t\bar{t}$ events is sensitive to the RS model parameters with a reasonable choice of model parameters.

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arXiv2010

The Impact of Kaluza-Klein Excited W Boson on the Single Top at LHC and Comparison with other Models

We study the s-channel single top quark production at the LHC in the context of extra dimension theories, including the Kaluza-Klein (KK) decomposition. It is shown that the presence of the first KK excitation of $W$ gauge boson can reduce the total cross section of s-channel single top production considerably if $M_{W_{KK}}\sim2.2 \rm TeV$ ($3.5 \rm TeV$) for $7\rm TeV$ ($14\rm TeV$) in proton-proton collisions. Then the results will be compared with the impacts of other beyond Standard Model (SM) theories on the cross section of single top s-channel. The possibility of distinguishing different models via their effects on the production cross section of the s-channel is discussed.

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arXiv2009

A Window on the CP-violating Phases of MSSM from Lepton Flavor Violating Processes

It has recently been shown that by measuring the transverse polarization of the final particles in the LFV processes $μ\to eγ$, $μ\to eee$ and $μN\to e N$, one can derive information on the CP-violating phases of the underlying theory. We derive formulas for the transverse polarization of the final particles in terms of the couplings of the effective potential leading to these processes. We then study the dependence of the polarizations of $e$ and $γ$ in the $μ\to e γ$ and $μN \to e N$ on the parameters of the Minimal Supersymmetric Standard Model (MSSM). We show that combining the information on various observables in the $μ\to eγ$ and $μN\to e N$ search experiments with the information on the electric dipole moment of the electron can help us to solve the degeneracies in parameter space and to determine the values of certain phases.

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arXiv2007

Electron Electric Dipole Moment from Lepton Flavor Violation

The general Minimal Supersymmetric Standard Model introduces new sources for Lepton Flavor Violation (LFV) as well as CP-violation. In this paper, we show that when both sources are present, the electric dipole moment of the electron, $d_e$, receives a contribution from the phase of the trilinear $A$-term of staus, $ϕ_{A_τ}$. For $ϕ_{A_τ}=π/2$, the value of $d_e$, depending on the ratios of the LFV mass elements, can range between zero and three orders of magnitude above the present bound. We show that the present bound on $d_e$ rules out a large portion of the CP-violating and the LFV parameter space which is consistent with the bounds on the LFV rare decays. We show that studying the correlation between $d_e$ and the P-odd asymmetry in $τ\to eγ$ helps us to derive a more conclusive bound on $ϕ_{A_τ}$ We also discuss the possibility of cancelation among the contributions of different CP-violating phases to $d_e$.

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arXiv2006

Revisiting electroweak baryogenesis in context of cancelation scenario in the MSSM

We study electric dipole moment bounds on CP-violating phases in the MSSM with special emphasis on the cancelation scenario. We find that in the range favored by electroweak baryogenesis ({\it i.e.,} $|μ|\simeq M_1$ or $|μ|\simeq M_2$), $\sin [θ_μ+θ_{M_1}]$ can be as large as ${\cal O}(1)$, even for slepton masses below 500 GeV. Such large values of the phases promise a successful electroweak baryogenesis. We also discuss the possibility of large CP-odd effects at a linear collider.

hep-ph↗

arXiv2006

Reconciling large CP-violating phases with bounds on the electric dipole moments in the MSSM

The possibility of cancelation between different contributions to d_e, d_n and d_{Hg} has been reconsidered with special emphasis on the region that is phenomenologically interesting (intermediate values of \tan βand sub-TeV sfermion masses). It is found that in the range favored by electroweak baryogenesis (\it i.e., |μ|\simeq M_1 or |μ|\simeq M_2), \sin [θ_μ+θ_{M_1}]\sim 1 can be compatible with the EDM bounds even for slepton masses below 500 GeV. Such large values of the phases promise a successful electroweak baryogenesis. The possibility of large CP-odd effects at linear collider has also been discussed.

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