arXiv subjects

Karim Ghorbani

Publications and source records attributed to Karim Ghorbani.

33 records · Page 2Linked to original sources

arXiv2017

The LHC upper bounds for $pp \to \text{diboson}$, $t\bar t$ cross section on fermionic dark matter

The ATLAS report in August 2016 provided an upper limit for the $pp\to$ diboson and $t\bar t$ cross sections. We consider a pseudoscalar-mediated fermionic dark matter together with gluon and photon effective operators interacting with the pseudoscalar. Choosing the resonance mass being $m_ρ=200, 750$ GeV and $2$ TeV, beside the relic density and the invisible Higgs decay constraints we constrain more the space of parameters with the diboson and $t\bar t$ cross section upper bounds. We finally provide some benchmarks consistent with all the constraints. Having exploited a pseudoscalar mediator, the DM-nucleon cross section is velocity suppressed so that the model evades easily the bounds put by the future direct detection experiments such as XENON1T.

hep-ph↗

arXiv2016

Scalar Dark Matter in Scale Invariant Standard Model

We investigate single and two-component scalar dark matter scenarios in classically scale invariant standard model which is free of the hierarchy problem in the Higgs sector. We show that despite the very restricted space of parameters imposed by the scale invariance symmetry, both single and two-component scalar dark matter models overcome the direct and indirect constraints provided by the Planck/WMAP observational data and the LUX/Xenon100 experiment. We comment also on the radiative mass corrections of the classically massless scalon that plays a crucial role in our study.

hep-ph↗

arXiv2016

Scalar Split WIMPs in the Future Direct Detection Experiments

We consider a simple renormalizable dark matter model consisting of two real scalars with a mass splitting $δ$, interacting with the SM particles through the Higgs portal. We find a viable parameter space respecting all the bounds imposed by invisible Higgs decay experiments at the LHC, the direct detection experiments by XENON100 and LUX and the dark matter relic abundance provided by WMAP and Planck. Despite the singlet scalar dark matter model that is fragile against the future direct detection experiments, the scalar split model introduced here survives such forthcoming bounds. We emphasize on the role of the co-annihilation processes and the mixing effects in this feature. For $m_{\text{DM}} \sim 63$ GeV in this model we can explain as well the observed gamma-ray excess in the analyses of the Fermi-LAT data at Galactic latitudes $2^{\circ} \leq |b| \leq 20^{\circ}$ and Galactic longitudes $|l| < 20^{\circ}$.

hep-ph↗

arXiv2015

Two-portal Dark Matter

We propose a renormalizable dark matter model in which a fermionic dark matter (DM) candidate communicates with the standard model particles through two distinct portals: Higgs and vector portals. The dark sector is charged under a $U(1)'$ gauge symmetry while the standard model has a leptophobic interaction with the dark vector boson. The leading contribution of DM-nucleon elastic scattering cross section begins at one-loop level. The model meets all the constraints imposed by direct detection experiments provided by LUX and XENON100, observed relic abundance according to WMAP and Planck, and the invisible Higgs decay width measured at the LHC. It turns out that the dark matter mass in the viable parameter space can take values from a few GeV up to 1 TeV. This is a new feature which is absent in the models with only one portal. In addition, we can find in the constrained regions of the parameter space a DM mass of $\sim 34$ GeV annihilating into $b$ quark pair, which explains the Fermi-LAT gamma-ray excess.

hep-ph↗

arXiv2014

Fermionic dark matter with pseudo-scalar Yukawa interaction

We consider a renormalizable extension of the standard model whose fermionic dark matter (DM) candidate interacts with a real singlet pseudo-scalar via a pseudo-scalar Yukawa term while we assume that the full Lagrangian is CP-conserved in the classical level. When the pseudo-scalar boson develops a non-zero vacuum expectation value, spontaneous CP-violation occurs and this provides a CP-violated interaction of the dark sector with the SM particles through mixing between the Higgs-like boson and the SM-like Higgs boson. This scenario suggests a minimal number of free parameters. Focusing mainly on the indirect detection observables, we calculate the dark matter annihilation cross section and then compute the DM relic density in the range up to $m_{\text{DM}} = 300$ GeV. We then find viable regions in the parameter space constrained by the observed DM relic abundance as well as invisible Higgs decay width in the light of 125 GeV Higgs discovery at the LHC. We find that within the constrained region of the parameter space, there exists a model with dark matter mass $m_{\text{DM}} \sim 38$ GeV annihilating predominantly into $b$ quarks, which can explain the Fermi-LAT galactic gamma-ray excess.

hep-ph↗

arXiv2014

Leading Chiral Logarithms of $K_{S} \to γγ$ and $K_{S} \to γ~l^+ l^-$ at two Loops

We obtain the leading divergences at two-loop order for the decays $K_{S} \to γγ$ and $K_{S} \to γ~ł^+ ł^-$ using only one-loop diagrams. We then find the double chiral logarithmic corrections to the decay branching ratio of $K_{S} \to γγ$ and to the decay rate for $K_{S} \to γ~ł^+ ł^-$. It turns out that these effects are numerically small and therefore make a very small enhancement on the branching ratio and decay rate. We also derive an expression for the corrections of type $\log μ~\times$ LEC. Numerical analysis done for the process $K_{S} \to γγ$ shows that these single logarithmic effects can be sizable but come with opposite signs with respect to the double chiral logarithms.

hep-ph↗

arXiv2013

Kaon semi-leptonic form factor at zero momentum transfer in finite volume

Using Chiral Perturbation Theory, we obtain the kaon semi-leptonic vector form factor in finite volume at a generic momentum transfer, $q^2$, up to one loop order. At first we confirm the lattice observation that the contribution of the heavy Pseudo-Goldstone boson in the finite volume corrections at zero momentum transfer is unimportant. We then evaluate the form factor at $q^2=0$ numerically and compare our results with the present lattice data. It turns out that our ChPT results are comparable with the lattice data to some extend. The formula for the finite volume corrections obtained for the form factor at momentum transfer $q^2$, provides a tool for lattice data in order to extrapolate at large lattice size.

hep-ph↗

arXiv2012

Chiral and volume extrapolation of pion and kaon electromagnetic form factor within SU(3) ChPT

We calculate the pion and kaon electromagnetic form factors in finite volume for a generic momentum transfer in space-like region, using SU(3) chiral perturbation theory. To this end, we first find the hadronic matrix element in a new form which is suitable for our calculation in finite volume. We present our numerical findings for the chiral behavior and finite volume corrections and compare these for pion and kaon form factors. As a result, for the pion electromagnetic form factor we find the finite volume correction $ΔF^π_{em} = 0.0041$ at momentum transfer $q^2 = -0.1358$ and for the kaon form factor $ΔF^{K}_{em} = 0.0029$ at $q^2 = -0.1357$ with $m_π = 0.3128$ and $L = 2.6 fm$ as the linear size of the lattice.

hep-ph↗

arXiv2011

Pion mass dependence of the $K_{l3}$ semileptonic scalar form factor within finite volume

We calculate the scalar semileptonic kaon decay in finite volume at the momentum transfer $t_{m} = (m_{K} - m_π)^2$, using chiral perturbation theory. At first we obtain the hadronic matrix element to be calculated in finite volume. We then evaluate the finite size effects for two volumes with $L = 1.83 fm$ and $L= 2.73 fm$ and find that the difference between the finite volume corrections of the two volumes are larger than the difference as quoted in \cite{Boyle2007a}. It appears then that the pion masses used for the scalar form factor in ChPT are large which result in large finite volume corrections. If appropriate values for pion mass are used, we believe that the finite size effects estimated in this paper can be useful for Lattice data to extrapolate at large lattice size.

hep-ph↗

arXiv2009

$K_{S}$ $\to$ $γγ$ at one-loop revisited

We have calculated the decay amplitude for the process $K_{S}$ $\to$ $γγ$ at one loop order in chiral perturbation theory. As a new improvement we have included the weak mass term which is only relevant for processes with external fields in the final state. This term was ignored in earlier publications for this decay. We find that the inclusion of $G_{8}^{\prime}$ brings the theoretical decay rate into a good agreement with experiment.

hep-ph↗

arXiv2007

Isospin breaking in $Kπ$ vector form-factors for the weak and rare decays $K_{\ell3}$, $K\toπν\barν$ and $K\toπ\ell^+\ell^-$

We calculate the two form-factors for the four Kaon to pion transitions via a vector current to order $p^6$ in Chiral Perturbation Theory to first order in isospin breaking via the quark masses. In addition we derive relations between these form-factors valid to first order in the up-down quark-mass difference but to all orders in Chiral Perturbation Theory. We present numerical results for all eight form-factors at $t=0$ and for varying $t$ and for the scalar form-factors at the Callan-Treiman point.

hep-ph↗

arXiv2007

$η\to 3 π$ at Two Loops In Chiral Perturbation Theory

We calculate the decay $η\to3π$ at next-to-next-to-leading order or order $p^6$ in Chiral Perturbation Theory. The corrections are somewhat larger than was indicated by dispersive estimates. We present numerical results for the Dalitz plot parameters, the ratio $r$ of the neutral to charged decay and the total decay rate. In addition we derive an inequality between the slope parameters of the charged and neutral decay. The experimental charged decay rate leads to central values for the isospin breaking quantities $R=42.2$ and $Q=23.2$.

hep-ph↗

arXiv2006

Quark anti-quark expectation value in finite volume

We have computed the quark anti-quark expectation value in finite volume at two loop in chiral perturbation theory and compare it with a formula obtained in analogy to the Luscher formula for pion mass in finite volume. We observe that due to the small finite size correction at two loop it is not possible to obtain conclusions on the accuracy of the extended Luscher formula.

hep-lat↗

arXiv2006

Finite Volume Dependence of the Quark-Antiquark Vacuum Expectation Value

A general formula is derived for the finite volume dependence of vacuum expectation values analogous to Luscher's formula for the masses. The result involves no integrals over kinematic quantities and depends only on the matrix element between pions at zero momentum transfer thus presenting a new way to calculate the latter, i.e. pion sigma terms. The full order $p^6$ correction to the vacuum condensate $<\bar q q>$ is evaluated and compared with the result from the Luscher formula. Due to the size of the $p^6$ result no conclusion about the accuracy of the Luscher formula can be drawn.

hep-lat↗

arXiv2005

Two Loop Partially Quenched and Finite Volume Chiral Perturbation Theory Results

This talk presents some results relevant for lattice QCD at higher order in ChPT. First we discuss the finite volume corrections at two loops for the quark condensate as well as a Lüscherlike finite volume formula for it. The latter allows for an alternative determination of meson sigma terms. The second set of results presented here are the calculations at two loops in partially quenched chiral perturbation theory of masses and decay constants of the charged mesons. We present results for all relevant quark mass combinations for the cases with two and three sea quarks.

hep-lat↗