Breit-Wigner Peak Research Articles

Enhancement of p-wave dark matter annihilation by quasi-bound states

We scrutinize the Sommerfeld enhancement in dark matter pair annihilation for p-wave and higher-ℓ partial waves. For the Yukawa potential these feature a super-resonant Breit-Wigner peak in their velocity-dependence close to Sommerfeld resonances as well as a universal scaling with velocity for all ℓ ≥ 1 that differs from the s-wave case. We provide a quantum mechanical explanation for these phenomena in terms of quasi-bound states sustained by the centrifugal barrier of the partial-wave potential, and give approximate WKB expressions capturing the main effects. The impact of quasi-bound states is exemplified for wino dark matter and models with light mediators, with a focus on indirect detection signals. We note that quasi-bound states can also explain similar peaks in the bound-state formation and self-scattering cross sections.

Coupled-channel meson-meson scattering in the diabatic framework

We apply the diabatic framework, a QCD-based formalism for the unified study of quarkoniumlike systems in terms of heavy quark-antiquark and open-flavor meson-meson components, to the description of coupled-channel meson-meson scattering. For this purpose, we first introduce a numerical scheme to find the solutions of the diabatic Schr\"odinger equation for energies in the continuum, then we derive a general formula for calculating the meson-meson scattering amplitudes from these solutions. We thus obtain a completely nonperturbative procedure for the calculation of open-flavor meson-meson scattering cross sections from the diabatic potential, which is directly connected to lattice QCD calculations. A comprehensive analysis of various elastic cross sections for open-charm and open-bottom meson-meson pairs is performed in a wide range of the center-of-mass energies. The relevant structures are identified, showing a spectrum of quasiconventional and unconventional quarkoniumlike states. In addition to the customary Breit-Wigner peaks, we obtain nontrivial structures such as threshold cusps and minimums. Finally, our results are compared with existing data and with results from our previous bound-state--based analysis, finding full compatibility with both.

Cross section of the process e+e−→pp¯ in the vicinity of charmonium ψ(3770) including three-gluon and D -meson loop contributions

The total cross section of the process ${e}^{+}{e}^{\ensuremath{-}}\ensuremath{\rightarrow}p\overline{p}$ is calculated within the energy range close to the mass of $\ensuremath{\psi}(3770)$ charmonium state. It was shown that the main contribution to this cross section comes from three gluon mechanism which is also responsible of the large phase with respect to the Born amplitude. This phase provides the characteristic dip behavior of the cross section in contrast to the usual Breit-Wigner peak shape. Okubo--Zweig--Iizuka-allowed mechanism with $D$-mesons in the intermediate state was also estimated and gives relatively small contribution to the cross section and to the phase.

Heavy quarkonia spectroscopy at zero and finite temperature in bottom-up AdS/QCD

S-wave states of charmonium and bottomonium are described using bottom-up AdS/QCD. We propose a holographic model that unifies the description of masses and decay constants, leading to a precise match with experimental data on heavy quarkonia. Finite temperature effects are considered by calculating the current-current spectral functions of heavy vector mesons. The identification of quasi-particle states as Breit-Wigner resonances in the holographic spectral function was made. We develop a prescription to subtract background contributions from the spectral function to isolate the Breit-Wigner peak. The quasi-particle holographic thermal evolution is described, allowing us to estimate the melting temperature for vector charmonia and bottomonia. Our holographic model predicts that $J/\Psi$ melts at $415$ MeV $(\sim 2.92 ~T_c)$ and $\Upsilon$ melts at $465$ MeV $(\sim 3.27~ T_c)$)

On a possible large width 750 GeV diphoton resonance at ATLAS and CMS

The ATLAS and CMS experiments at the LHC have reported an excess of diphoton events with invariant mass around 750 GeV, with local significance of about $3.6 \sigma$ and $2.6 \sigma$, respectively. We entertain the possibility that this excess is due to new physics, in which case the data suggest a new particle with 13 TeV LHC production cross section times diphoton branching ratio of about 5~fb. Interestingly, ATLAS reports a mild preference for a sizeable width for the signal of about 45 GeV; this result appears consistent with CMS, and is further supported by improving the compatibility of the 8 TeV and 13 TeV analyses. We focus on the possibility that the new state is a scalar. First, we show that, in addition to the new state that is needed directly to produce the diphoton bump, yet more new particles beyond the Standard Model are needed to induce diphoton decay rate of the right size. Second, we note that if the excess is attributed to the Breit-Wigner peak of a single new state, then the signal strength and width -- taken together -- suggest a total LHC production cross section of order $10^5$ fb. Restricting to perturbative models without ad-hoc introduction of many new states or exotic charges, we reach the following conclusions: (i) Gluon-fusion cannot explain the required large production cross section. (ii) Tree level production from initial state quarks cannot explain the required branching ratio to two photons. (iii) Tree level production is constrained by flavor data as well as LHC Run-I and Tevatron dijet analyses. Insisting on a large width we are led to suggest that more than one scalar states, nearly degenerate in mass, could conspire to produce an observed wide bump.

Spin-flip effects in a parallel-coupled double quantum dot molecule

We investigate theoretically the electronic transport through a parallel-coupled double quantum dot (DQD) molecule attached to metallic electrodes, in which the spin-flip scattering on each quantum dot is considered. Special attention is paid to the effects of the intradot spin-flip processes on the linear conductance by using the equation of motion approach for Green’s functions. When a weak spin-flip scattering on each quantum dot is present, the single Fano peak splits into two Fano peaks, and the Breit–Wigner resonance may be suppressed slightly. When the spin-flip scattering strength on each quantum dot becomes strong, the linear conductance spectrum consists of two Breit–Wigner peaks and two Fano peaks due to the quantum interference effects. The positions and shapes of these resonant peaks can be controlled by using the magnetic flux through the quantum device.

Fano resonance and persistent current in mesoscopic open rings: Influence of coupling and Aharonov-Bohm flux

Electronic transport and persistent current in a mesoscopic ring connected to current leads is investigated using an $S$-matrix approach. In the presence of an impurity in one arm, the transmission probability of the ring may exhibit resonances of the Fano type in addition to Breit-Wigner peaks. It is shown that the transition from the weak to the strong coupling regime leads to Fano line shapes with gradually smaller asymmetry parameters, but the positions of the transmission zeros and ones remain unaffected. This results in a modification of the resonance width. The Breit-Wigner line shape is sensitive to the coupling and disappears in the strong coupling limit. The presence of an Aharonov-Bohm flux alters the amplitudes of the Fano resonances, turning them progressively into broad oscillations. The effect of coupling on the persistent current is also investigated for various impurity strengths and positions. It is shown that increasing the coupling causes gradual suppression of the persistent current, while increasing the impurity strength may lead to negligible persistent current. Furthermore, it is shown that placing the impurity commensurately in the arm causes systematic collapse of certain Fano resonances. For those impurity positions, a divergent persistent current is obtained. The temperature smearing of the Fano resonances is also analyzed.

Ac gate-induced Fano resonance and peak splitting of conductance in a quantum dot

We have investigated the conductance of a quantum dot system suffering an anti-symmetric ac gate voltage which induces the transition between dot levels in the linear regime at zero temperature in the rotating wave approximation. Interesting Fano resonances appear on one side of the displaced resonant tunnelling peaks for the nonresonant case or the peak splitting for the resonant case. The line shape of conductance (vs Fermi energy) near each level of the quantum dot can be decomposed into two profiles: a Breit-Wigner peak and a Fano profile, or a Breit-Wigner peak and a dip in both cases.

Continuum level density in a microscopic cluster model: Parameters of resonances

The positions and widths of nuclear resonance states of the nuclei ${}^{8}\mathrm{Be},$ ${}^{5}\mathrm{He},$ and ${}^{5}\mathrm{Li}$ have been calculated in the microscopic cluster model using a real square integrable basis. The imposition of Gamow or scattering asymptotic boundary conditions onto the wave function is avoided. The approach is based on the notion of the continuum level density. This density is smoothed by the Strutinsky averaging procedure and it is calculated by making use of the eigenvalues of the full and the free Hamiltonian matrices. The continuum level density is connected to the S matrix and has a Breit-Wigner peak around the resonance energy. This approach is compared with the complex scaling method and with the exact calculation of the scattering phase shift.

Possibility of experimental separation of resonances and cusps from background in electron scattering

It is shown, with the help of model calculations, that a laser field can be used to suppress the background in electron scattering cross sections and to pick out only those parts which are rapidly varying as functions of the incoming electron energy. Therefore it becomes possible to measure the pure Breit-Wigner peaks and threshold effects, independent of the fact that interference between the resonances and the background in radiationless electron scattering is strong. The results are interpreted within the low-frequency limit for free-free transitions. The ac Stark shift of the resonance is also observed.

Dispersion relations and resonance reactions

For one-channel scattering, a resonance formalism is developed which avoids continuing the scattering function into a region where it has resonance poles. In a non-relativistic terminology, it is assumed that for any particular partial wave the scattering function S is analytic in the energy plane cut from −∞ to −1 and from 0 to ∞. It is further assumed that S satisfies a dispersion relation, and that it takes complex conjugate values at complex conjugate points. Under these assumptions, S can be written as a Blaschke product times an exponential function. The Blaschke product contains the zeros of S. At energies below the threshold for inelastic processes, it describes the resonances. If in this energy region S has isolated zeros close to the real axis, these give rise to Breit-Wigner peaks in the cross section. The exponential function corresponds to the potential scattering. If refers to a scattering process in which the real part of the phase shift is connected with the absorption at higher energies through a dispersion relation. The formalism can be extended to energies at which more than one channel is involved. This generalization is, however, not discussed.