Karim Ghorbani - Academia.edu (original) (raw)

Papers by Karim Ghorbani

In this research we consider a U (1) X gauge boson acting as a dark matter candidate. The vector ... more In this research we consider a U (1) X gauge boson acting as a dark matter candidate. The vector dark matter gets mass when a complex singlet scalar breaks the gauge symmetry spontaneously. The dark matter candidates communicate with the SM particles via a scalar-Higgs portal. The focus in this work is on the dark matter mass smaller than 10 GeV. This parameter space is not studied thoroughly before. Dark matter annihilation via forbidden channel and near pole are studied in order to place constraints from observed relic density and CMB. Other bounds from colliders, beam-dump experiments, and astrophysical observations are imposed. Taking into account all the bounds including the direct detection upper limits, the viable space is achieved.

$Research paper thumbnail of Leading Chiral Logarithms of <math xmlns="http://www.w3.org/1998/Math/MathML"><semantics><mrow><msub><mi>K</mi><mi>S</mi></msub><mo>→</mo><mi>γ</mi><mi>γ</mi></mrow><annotation encoding="application/x-tex">K_{S} \to \gamma \gamma</annotation></semantics></math>KS→γγ at two Loops$

We obtain the leading divergences at two-loop order for the decays K S → γγ and K S → γ l + l − u... more We obtain the leading divergences at two-loop order for the decays K S → γγ and K S → γ l + l − using only one-loop diagrams. We then find the double chiral logarithmic corrections to the decay branching ratio of K S → γγ and to the decay rate for K S → γ l + l −. 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 × LEC. Numerical analysis done for the process K S → γγ shows that these single logarithmic effects can be sizable but come with opposite signs with respect to the double chiral logarithms.

Journal of Physics G: Nuclear and Particle Physics, 2010

The idea of the meson cloud approach in the chiral quark model has been extended to include gluon... more The idea of the meson cloud approach in the chiral quark model has been extended to include gluon cloud in order to achieve the parton densities in the nucleon, based on the constitute quark framework. The splitting function of the quark to the quark-meson and quark-gluon at low Q 2 value are used to obtain parton densities in the constituent quark. The phenomenological constituent model is employed to extract the parton distributions in the proton at low Q 2 value. Since we have access to the parton densities at low Q 2 , we are able to obtain F 2 (x, Q 2) structure function at low Q 2 value. The result is in good agreement with available experimental data and some theoretical models. To confirm the validity of our calculations, the fraction of total momentum of proton which is carried by gluon at high Q 2 and also the Gottfried sum rule are computed. The results are in good agreement with what are expected.

Nuclear Physics B

The recently reported measurement of the W-boson mass by CDF-II collaboration is significantly he... more The recently reported measurement of the W-boson mass by CDF-II collaboration is significantly heavier than that of the Standard Model prediction. We study this anomaly in the scale invariant Two-Higgs-Doublet-Model (SI-2HDM) with a Z2 symmetry to avoid the flavor-changing-neutralcurrent (FCNC). In this scenario the Higgs particle is the classically massless scalon in the SI-2HDM gaining its mass by radiative corrections, hence being naturally light. Moreover, because of the scale symmetry the model is more predictive respect to the generic 2HDM. We show that the oblique parameters depending on the masses of the charged and CP-even (CP-odd) neutral scalar components of the SI-2HDM denoted respectively by M H ± and M h (MA), are large enough to accommodate the W-boson mass anomaly in the model. There are as well viable regions in the mass spectrum of the SI-2HDM that evade the experimental bounds from colliders on the charged Higgs and neutral scalars.

We obtain the leading divergences at two-loop order for the decays K_S→γγ and K_S→γ ł^+ ł^- using... more We obtain the leading divergences at two-loop order for the decays K_S→γγ and K_S→γ ł^+ ł^- using only one-loop diagrams. We then find the double chiral logarithmic corrections to the decay branching ratio of K_S→γγ and to the decay rate for K_S→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 μ × LEC. Numerical analysis done for the process K_S→γγ shows that these single logarithmic effects can be sizable but come with opposite signs with respect to the double chiral logarithms.

A general formula is derived for the finite volume dependence of vacuum expectation values analog... more A general formula is derived for the finite volume dependence of vacuum expectation values analogous to Lüscher’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 〈¯qq 〉 is evaluated and compared with the result from the Lüscher formula. Due to the size of the p 6 result Quantum Chromo Dynamics (QCD) at low energy remains a difficult problem. One of the ways to deal with this problem is to numerically evaluate the functional integral of QCD. This approach, known as lattice QCD, has now reached the stage where realistic calculations with fairly light quark masses are now possible. One side effect of this is

Abstract: We calculate the decay η → 3π at next-to-next-to-leading order or order p 6 in Chiral P... more Abstract: We calculate the decay η → 3π 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.

We calculate the two form-factors for the four Kaon to pion transitions via a vector current to o... more 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 formfactors 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.

In all calculations done in this thesis, the evaluation of higher order quantum corrections has b... more In all calculations done in this thesis, the evaluation of higher order quantum corrections has been central. In the first paper included in this thesis we have investigated the effects of finite volume meson propagators for quark-antiquark vacuum expectation value at two-loop level. In the second paper we have improved the earlier calculations for the eta decay to three pions by implementing two-loop quantum corrections. The low energy constants of order p6 are estimated by means of a resonance chiral Lagrangian. In addition, the experimental charged decay rate leads to the determination for the isospin breaking quantities. Finally, we have studied the isospin breaking effects on the vector and scalar form-factor of semi-leptonic decays. Semi-leptonic kaon decays, both rare and weak semi-leptonic decays. Semi-leptonic kaon decays provide a rich laboratory to find the CKM matrix element Vus and the possible new physics beyond the Standard Model in flavor sector. (Less)

We calculate the scalar semileptonic kaon decay in finite volume at momentum transfer tm = (mK −m... more We calculate the scalar semileptonic kaon decay in finite volume at momentum transfer tm = (mK −m�) 2 , applying Chiral Perturbation Theory. First we obtain the hadronic matrix element to be calculated in finite volume. It turns out possible to evaluate the scalar form factor at the endpoint in a finite box. Finite size effects estimated in this paper are believed to be useful for Lattice data for extrapolation to large lattice size.

$Research paper thumbnail of Isospin breaking in <math xmlns="http://www.w3.org/1998/Math/MathML"><semantics><mrow><mi>K</mi><mi>π</mi></mrow><annotation encoding="application/x-tex">K\pi</annotation></semantics></math>Kπ vector form-factors for the weak and rare decays <math xmlns="http://www.w3.org/1998/Math/MathML"><semantics><mrow><msub><mi>K</mi><mrow><mi mathvariant="normal">ℓ</mi><mn>3</mn></mrow></msub></mrow><annotation encoding="application/x-tex">K_{\ell3}</annotation></semantics></math>Kℓ3, <math xmlns="http://www.w3.org/1998/Math/MathML"><semantics><mrow><mi>K</mi><mo>→</mo><mi>π</mi><mi>ν</mi><mover accent="true"><mi>ν</mi><mo>ˉ</mo></mover></mrow><annotation encoding="application/x-tex">K\to\pi\nu\bar\nu</annotation></semantics></math>K→πννˉ and <math xmlns="http://www.w3.org/1998/Math/MathML"><semantics><mrow><mi>K</mi><mo>→</mo><mi>π</mi><msup><mi mathvariant="normal">ℓ</mi><mo>+</mo></msup><msup><mi mathvariant="normal">ℓ</mi><mo>−</mo></msup></mrow><annotation encoding="application/x-tex">K\to\pi\ell^+\ell^-</annotation></semantics></math>K→πℓ+ℓ−$

We calculate the two form-factors for the four Kaon to pion transitions via a vector current to o... more We calculate the two form-factors for the four Kaon to pion transitions via a vector current to order p6p^6p6 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=0t=0t=0 and for varying ttt and for the scalar form-factors at the Callan-Treiman point.

We study a model with a vector dark matter candidate interacting with the SM charged leptons thro... more We study a model with a vector dark matter candidate interacting with the SM charged leptons through a scalar portal. The dark matter candidate acquires mass when the scalar breaks spontaneously an abelian gauge symmetry. The scalar mediator being leptophilic only induces elastic scattering of dark matter with electrons at the leading order (tree level). Given the recent results from Xenon1T upper bounds where the strongest sensitivity lies in the range ∼ O(1) GeV, we find the viable space in the parameter space respecting constraints from the observed relic density, muon g-2 anomaly, e+e− colliders and astrophysical observables. It is shown that the current upper bounds of Xenon1T is partially sensitive to the viable parameter space, such that it excludes the scalar mediator with masses ms . 3 MeV for DM masses mDM ∼ 0.1− 10 GeV. karim1.ghorbani@gmail.com