arXiv subjects

Hooman Davoudiasl

Publications and source records attributed to Hooman Davoudiasl.

135 records · Page 8Linked to original sources

arXiv2000

Signatures of Extra Dimensions at e-gamma and gamma-gamma Colliders

We study the processes γ\e -> γ\e and γγ-> γγ, in the context of the proposal for Weak Scale Quantum Gravity (WSQG) with large extra dimensions. With an ultraviolet cutoff $M_S \sim 1$ TeV for the effective gravity theory, the cross sections obtained for these processes at the Next Linear Collider (NLC), with the \e γan γγoptions, deviate from the predictions of the Standard Model significantly. Our results suggest that, for typical proposed NLC energies and luminosities, the predictions of WSQG can be tested in the range 1 TeV \lsim M_S \lsim 10 TeV, making \e γan γγcolliders important tools for probing WSQG.

hep-ph↗

arXiv1999

Compton Scattering at the NLC and Large Extra Dimensions

We study Compton scattering, gamma e -> gamma e, in the context of the recent proposal for Weak Scale Quantum Gravity (WSQG) with large extra dimensions. It is shown that, with an ultraviolet cutoff $M_S \sim 1$ TeV for the effective gravity theory, the cross section for this process at the Next Linear Collider (NLC) deviates from the prediction of the Standard Model significantly. Our results suggest that, for typical proposed NLC energies and luminosities, WSQG can be tested in the range 4 TeV$\lsim M_S \lsim$ 16 TeV, making gamma e -> gamma e an important test channel.

hep-ph↗

arXiv1999

gamma gamma -> gamma gamma as a Test of Weak Scale Quantum Gravity at the NLC

Recently, it has been proposed that the fundamental scale of quantum gravity can be close to the weak scale if there are large extra dimensions. This proposal has important phenomenological implications for processes at the TeV scale. We study the process gamma gamma -> gamma gamma, assuming an ultraviolet cutoff M_S \sim 1 TeV for the effective gravity theory. We find that, at center of mass energies \sqrt(s) \sim 1 TeV, the contribution of gravitationally mediated scattering to the cross section is comparable to that coming from the one-loop Feynman diagrams of the Standard Model. We thus conclude that the effects of weak scale quantum gravity can be studied at the Next Linear Collider (NLC), in the photon collider mode. Our results suggest that, for typical proposed NLC energies and luminosities, the range 1 TeV \lsim M_S \lsim 10 TeV can be probed.

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arXiv1998

Diffusion and Decoherence of Squarks and Quarks During the Electroweak Phase Transition

To estimate the diffusion constant $D$ of particles in a plasma, we develop a method that is based on the mean free path $λ$ for scatterings with momentum transfer $q >~ T$. Using this method, we estimate $λ$ and $D$ for squarks and quarks during the electroweak phase transition. Assuming that Debye and magnetic screening lengths provide suitable infrared cutoffs, our calculations yield $λ<~ 18/T$ and $D <~ 5/T$ for both squarks and quarks. Our estimate of $λ$ suggests that suppressions of charge transport due to decoherence of these strongly interacting particles during the electroweak phase transition are not severe and that these particles may contribute significantly to electroweak baryogenesis.

hep-ph↗

arXiv1997

Electroweak Baryogenesis Using Baryon Number Carrying Scalars

We describe a new mechanism for the generation of the baryon asymmetry of the universe during a first order electroweak phase transition. The mechanism requires the existence of two (or more) baryon number carrying scalar fields with masses and CP violating mixing which vary with the Higgs field expectation value. This mechanism can be implemented using squarks in supersymmetric theories or using leptoquarks. Our central observation is that reflection of these scalars from a bubble wall can yield a significant net baryon number flux into the symmetric phase, balanced by a flux of opposite sign into the broken phase. For generic parameter choices, scalars with incident energies in a specific, but not narrow, range yield order one reflection asymmetries (between the probability of reflection of the scalars and of their antiparticles). The interesting energies are those for which there are two propagating scalars in the symmetric phase but only one in the broken phase. Electroweak sphaleron processes drive the baryon number in the symmetric phase toward zero, but do not act in the broken phase. Our estimate of the resulting baryon asymmetry is consistent with cosmological observations for a range of mass parameters and CP violating phases in a supersymmetric implementation, as long as the bubble walls are not too fast and not too thick.

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arXiv1997

Perturbative Corrections to a Sum Rule for the Heavy Quark Kinetic Energy

We calculate the perturbative corrections to order α_s^2β_0 to the sum rule derived from the second moment of the time-ordered product of b \to c currents near zero recoil. This sum rule yields a bound on λ_1, the expectation value of the b quark kinetic energy operator inside the B meson. The perturbative corrections significantly weaken the bound relative to the tree level result, yielding λ_1 < -0.15 GeV^2.

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arXiv1996

Non-linear Sigma Model Solutions for the Disoriented Chiral Condensate at O(p^4)

Boost-invariant (1+1) dimensional solutions for the Disoriented Chiral Condensate (DCC) are obtained numerically, in the context of the $SU(2)_L \otimes SU(2)_R$ non-linear sigma model at O(p^4) in the momentum expansion. We ignore the mass terms in the Lagrangian, as we are mainly interested in the behavior of the solutions for small values of proper time $τ$. The solutions obtained at O(p^4) are matched to those of O(p^2) at a late proper time $τ\gg 1/m_π$, where $m_π= 140$ MeV is the mass of the pion. We find that at O(p^4) the solutions for the DCC do not have singular behavior at early proper times $τ\ll 1/m_π$. The solutions indicate that for $τ\lsim (0.5-0.8)$ fm the O(p^4) corrections become important. We take the sizes of the field derivatives to be indicators of the validity of the momentum expansion. Thus, we deduce that the O(p^4) solutions can be used to represent the qualitative behavior of the DCC down to proper times of about 0.2 fm. Since below $\sim 0.2$ fm the formalism is not reliable, we conclude that the inclusion of higher order terms beyond O(p^4) is not needed to extend the validity of the solutions to earlier proper times.

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arXiv1996

The Decay $D^0\to \bar K^{*0} π^- e^+ ν_e$ in the Context of Chiral Perturbation Theory

We study the decay $D^0\rightarrow \bar K^{*0} π^- e^+ ν_e$, using $SU(2)_L \otimes SU(2)_R$ chiral perturbation theory for heavy charmed mesons and vector mesons, in the kinematic regime where $p_M \cdot p_π/m_M$ (here $M = D^0$ or $\bar K^{*0}$) is much smaller than the chiral symmetry breaking scale, $Λ_{χSB}$ ( $Λ_{χSB} \sim $ 1 GeV). We present the leading diagrams and amplitude, and calculate the rate, in the region where, to leading order in our calculations, the $\bar K^{*0}$ is at zero recoil in the $D^0$ rest frame. The rate thus calculated is given in terms of a known form factor and depends on the $DD^* π$ coupling constant $g_D$ of the heavy (charmed) meson chiral perturbation theory Lagrangian. A measurement of the above decay, in the aforementioned kinematic regime, can result in the extraction of an experimental value for $g_D$, accurate at the level of our approximations, and give us a measure of the validity of approaches based on chiral perturbation theory in studying similar processes.

hep-ph↗

arXiv1995

Chiral Perturbation Theory for $τ\to ρπν_τ$, $τ\to K^* πν_τ$, and $τ\to ωπν_τ$

We use heavy vector meson $SU(2)_L \times SU(2)_R$ chiral perturbation theory to predict differential decay distributions for $τ\rightarrow ρπν_τ$ and $τ\rightarrow K^* πν_τ$ in the kinematic region where $p_V \cdot p_π/m_V$ (here $V = ρ$ or $K^*$) is much smaller than the chiral symmetry breaking scale. Using the large number of colors limit we also predict the rate for $τ\rightarrow ωπν_τ$ in this region (now $V = ω$). Comparing our prediction with experimental data, we determine one of the coupling constants in the heavy vector meson chiral Lagrangian.

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