Threshold Anomalies Research Articles

Abnormal threshold behaviors of photo-pion production off the proton in the GZK region

The cosmic-ray spectrum structures help to study the acceleration and propagation mechanism of ultra-high energy cosmic rays, and these structures were predicted to culminate in a cut-off, named the Greisen–Zatsepin–Kuzmin (GZK) cut-off, near 5×1019eV\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$5\ imes 10^{19}~\ extrm{eV}$$\\end{document} as a result of the inelastic interaction of protons with the 2.73K\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$2.73~\ extrm{K}$$\\end{document} black body radiation. The confirmation of the existence of GZK cut-off was tortuous, leading to activities to explore new physics, such as the cosmic-ray new components, unidentified cosmic-ray origins, unknown propagation mechanism and the modification of fundamental physics concepts like the tiny Lorentz invariance violation (LV). The confirmation of the GZK cut-off provides an opportunity to constrain the LV effect. We use a phenomenological framework to restudy the GZK mechanism under the Planck scale deformation of the proton and pion dispersion relations. Restudying the photon induced pion production of the proton p+γ→p+π0\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$\ extrm{p}+\\gamma \\rightarrow \ extrm{p}+\\pi ^0$$\\end{document}, we predict abnormal threshold behaviors of this reaction under different LV modifications. Therefore we can study the LV effects not only from the conventional GZK cut-off, but also from potentially threshold anomalies of the pion production process. We divide the LV parameter space into three regions, and analyze the constraints from current observations in each region. The current observations have set strict restrictions on a certain LV region. However, for others LV regions, further experimental observations and theoretical researches are still needed, and we also find survival space for some theoretical explorations that permit specific LV effects.

Threshold anomalies in low-energy reactions with loosely-bound projectiles

Recent results concerning data for fusion and elastic scattering of low-energy neutron-halo 6He and weakly-bound 6Li projectiles, colliding with medium mass targets, are briefly reviewed. The extended optical model (EOM) was used, which includes two complex polarization potentials, one associated to fusion couplings and the other one associated to direct reaction couplings. Four systems having 6Li projectiles and one with 6He were analyzed. The extracted threshold anomalies are discussed.

Searching Lorentz invariance violation from cosmic photon attenuation

Lorentz invariance violation (LIV) can change the threshold behavior predicted by special relativity and cause threshold anomalies which affect the propagation of cosmic photons. In this work, we focus on the threshold anomaly effect on cosmic photon attenuations by extragalactic background light (EBL) and discuss how to identify LIV from observations of very high energy (VHE) photons propagated from long distance in the universe. We point out that the Large High Altitude Air Shower Observatory (LHAASO), one of the most sensitive gamma-ray detector arrays currently operating at TeV and PeV energies, is an ideal facility for performing such LIV searching. We apply the proposed strategy to discuss the newly observed gamma-ray burst GRB 221009A to demonstrate the predictive ability of our suggestions.

Seasonally Modulated El Niño Precipitation Response in the Eastern Pacific and Its Dependence on El Niño Flavors

Abstract Many previous studies have shown that El Niño exhibits a strong seasonality in its teleconnections and regional climate impacts. Aside from the seasonal synchronization of El Niño anomalous sea surface temperature (SST) itself, seasonal differences in its associated climate impacts could also stem from the local background seasonal cycle. During the El Niño developing boreal autumn (August–October) and decaying boreal spring (February–April), the El Niño–associated precipitation anomalies display remarkably different patterns over the eastern tropical Pacific despite similar SST anomaly amplitudes. This strong seasonality can be largely attributed to the seasonal cycle of the eastern tropical Pacific SST background state with the cold tongue being strongest in autumn and weakest in spring. Therefore, the El Niño–associated SST in spring is likely to exceed the convection threshold on both sides of the equator, leading to an approximately symmetric precipitation response about the equator. In contrast, this symmetric precipitation response is absent in autumn since the SST near and south of the equator remains below the threshold and pronounced eastern tropical precipitation anomalies can only be observed in the warm Northern Hemisphere. This seasonality is mainly embodied in eastern Pacific (EP) El Niño events rather than central Pacific (CP) El Niño events since the westward-shifted warm SST anomalies for the latter cannot establish an effective cooperation with the cold tongue SST annual cycle. This study has important implications for regional climate prediction by involving the different local precipitation responses over the tropical eastern Pacific.

Simultaneous analysis of elastic scattering and fusion in He6+Zn64 : A transition in direct reaction mechanisms, striking threshold anomalies, and halo effects

Using the extended optical model, a simultaneous analysis is carried out of elastic scattering and fusion reactions induced by the neutron halo $^{6}\mathrm{He}$ projectile on a $^{64}\mathrm{Zn}$ target. Dynamic polarization potentials ${U}_{F}$, ${U}_{D}$ are introduced which can be associated to fusion and direct couplings, respectively. Appropriate fitting parameters leading to dispersive potentials are found which provide a good simultaneous description of the data. An unexpected local maximum is uncovered in the energy dependence of the total reaction cross sections, indicating a transition in the involved direct reaction processes, and a possible underlying mechanism is discussed. ${U}_{F}$ is found to follow the normal threshold anomaly but with an unusually large threshold energy. ${U}_{D}$, on the other hand, exhibits a breakup threshold anomaly, but with a strikingly high value of the peak energy, which can be associated to the above-mentioned transition. The results also are discussed within the context of possible halo effects.

Ultrahigh-energy photons from LHAASO as probes of Lorentz symmetry violations

The Large High Altitude Air Shower Observatory (LHAASO) is one of the most sensitive gamma-ray detector arrays currently operating at TeV and PeV energies. Recently the LHAASO experiment detected ultra-high-energy (UHE; ${E}_{\ensuremath{\gamma}}\ensuremath{\gtrsim}100\text{ }\text{ }\mathrm{TeV}$) photon emissions up to 1.4 PeV from twelve astrophysical gamma-ray sources. We point out that the detection of cosmic photons at such energies can constrain the photon self-decay motivated by superluminal Lorentz symmetry violation (LV) to a higher level, thus can put strong constraints to certain LV frameworks. Meanwhile, we suggest that the current observation of the PeV-scale photon with LHAASO may provide hints to permit a subluminal type of Lorentz violation in the proximity of the Planckian regime, and may be compatible with the light speed variation at the scale of $3.6\ifmmode\times\else\texttimes\fi{}{10}^{17}\text{ }\text{ }\mathrm{GeV}$ recently suggested from gamma-ray burst (GRB) time delays. We further propose detecting PeV photons coming from extragalactic sources with future experiments, based on LV-induced threshold anomalies of ${e}^{+}{e}^{\ensuremath{-}}$ pair-production, as a crucial test of subluminal Lorentz violation. We comment that these observations are consistent with a D-brane/string-inspired quantum-gravity framework, the space-time foam model.

Threshold anomalies of ultra-high energy cosmic photons due to Lorentz invariance violation

From special relativity, photon annihilation process γγ→e+e− prevents cosmic photons with energies above a threshold to propagate a long distance in cosmic space due to their annihilation with low energy cosmic background photons. However, modifications of the photon dispersion relation from Lorentz invariance violation (LIV) can cause novel phenomena beyond special relativity to happen. In this paper, we point out that these phenomena include optical transparency, threshold reduction and reappearance of ultra-high energy photons in cosmic space. The recent observation of near and above PeV photon events by the LHAASO Collaboration reveals the necessity to consider the threshold anomalies. Future observations of above threshold photons from extragalactic sources can testify LIV properties of photons.

Systematic study of elastic scattering and fusion induced by weakly bound 6Li on medium mass targets. Threshold anomalies

Using the extended optical model, a comparative and systematic analysis of different 6Li-induced reactions at sub- and above-barrier energies is carried out. Collisions with 58Ni, 59Co, 64Ni and 64Zn are analyzed. For each selected system, the São Paulo potential is used as a bare potential, a surface polarization potential is added to simulate the effects of direct reactions and a volume potential is considered to account for couplings to fusion. Simultaneous fits to the corresponding elastic scattering and fusion data of each system are optimized at the different energies measured, and the validity of the respective dispersion relations is verified. Based on the potentials obtained for each selected system, the elastic scattering data are properly described and the fusion cross sections are well reproduced at the energies around and above the barrier. For the 6Li + (58Ni, 64Zn) systems, some abnormal strength values are found for the fusion polarization potentials at sub-barrier energies, which would invalidate the dispersion relation. A possible explanation is put forward and, as a result, it is concluded that the constriction of dispersive potentials might help identify unphysical data. For all systems, the final energy dependence of the surface polarization potential is found to be consistent with the Breakup Threshold Anomaly, while the fusion polarization potential follows the Threshold Anomaly.

Does an Early Spring Indicate an Early Summer? Relationships Between Intraseasonal Growing Degree Day Thresholds

AbstractSpring heat accumulation plays a major role in the timing of events such as leaf‐out, leaf expansion, flowering, and insect hatch in temperate systems. Accordingly, heat accumulation can serve as a proxy for the timing of plant and insect phenological activity and can be used in a predictive way when the timing of heat accumulation thresholds being reached can be anticipated. This has strong value for a host of planning and management applications. If relationships exist between earlier‐ and later‐season thresholds at a location, then the timing of later‐season phenological events that are forced by the accumulation of warmth could be anticipated based on when earlier‐season thresholds are met. Using high‐resolution daily temperature data, we calculated the coherence in pairs of spring‐season heat accumulation (growing degree day) threshold anomalies over 1948–2016. Overall, relationships between thresholds spanning the entire spring season were relatively low, while later season thresholds exhibited much higher correlations. This pattern is generally the result of decreasing variability in heat accumulation with season progression. However, correlation strengths did not follow latitudinal or gradients, revealing that within‐season heat accumulation and interannual variability in threshold timing are unique to the specified base temperature and thresholds being compared. We show that the relationships between earlier‐ and later‐season heat accumulation thresholds were sufficient to accurately predict the timing of phenological events in plants in two case examples.

Threshold Effects and Anomalies in the Physical Characteristics of LiNbO3:ZnO Crystals

We have studied the structural and optical characteristics and radiation hardness of LiNbO3:ZnO crystals containing ~0.15–5.9 mol % ZnO. The results demonstrate that anomalies in composition dependences of their structural and optical characteristics and radiation hardness are closely interrelated with threshold effects in the LiNbO3:ZnO crystals. We have confirmed the presence and determined the exact position of two concentration thresholds: at low dopant concentrations (~1.17 mol % ZnO in the crystals) and intermediate ones (~4.8 mol % ZnO in the crystals). Composition dependences of structural parameters for the crystals near the main concentration threshold (~6.8 mol % ZnO in the melt) have been determined with appreciably higher accuracy than before.

Channel-closing effects in strong-field ionization by a bicircular field

Channel-closing effects, such as threshold anomalies and resonantlike intensity-dependent enhancements in strong-field ionization by a bicircular laser field are analyzed. A bicircular field consists of two coplanar corotating or counter-rotating circularly polarized fields having different frequencies. For the total detachment rate of a negative ion by a bicircular field we observe threshold anomalies and explain them using the Wigner threshold law and energy and angular momentum conservation. For the corotating bicircular case, these effects are negligible, while for the counter-rotating case they are pronounced and their position depends on the magnetic quantum number of the initial state. For high-order above-threshold ionization of rare-gas atoms by a counter-rotating bicircular laser field we observe very pronounced intensity-dependent enhancements. We find all four types of threshold anomalies known from collision theory. Contrary to the case of linear polarization, channel-closing effects for a bicircular field are visible also in the cutoff region of the electron energy spectrum, which is explained using quantum-orbit theory.

Spin-orbit-induced resonances and threshold anomalies in a reduced-dimension Fermi gas

We calculate the reflection and transmission probabilities in a one-dimensional Fermi gas with an equal mixing of the Rashba and Dresselhaus spin-orbit coupling (RD-SOC) produced by an external Raman laser field. These probabilities are computed over multiple relevant energy ranges within the pseudo-potential approximation. Strong scattering resonances are found whenever the incident energy approaches either a scattering threshold or a quasi-bound state attached to one of the energetically closed higher dispersion branches. A striking difference is demonstrated between two very different regimes set by the Raman laser intensity, namely between scattering for the single- minimum dispersion versus the double-minimum dispersion at the lowest threshold. The presence of RD-SOC together with the Raman field fundamentally changes the scattering behavior and enables the realization of very different one-dimensional theoretical models in a single experimental setup when combined with a confinement-induced resonance.

Threshold phenomena in few-nucleon systems

Experimental data on threshold anomalies in few-nucleon systems are reviewed. The wealth of spectroscopic information obtained in studies of threshold anomalies is illustrated by the 7Li + t system near the 7Li(t,n)9Be*(Ex = 1.4. MeV, T = 3/2) threshold. Analysis of the excitation functions of the 7Li + t reaction channels allows energies of the lowest isospin T = 2 levels in the 10Be nucleus to be obtained; their spins and parities to be established; and the quantum characteristics, mass, and energy of the first excited state of the nucleon-unstable 10Li nucleus to be determined.

Optical fano resonances in photonic crystal slabs near diffraction threshold anomalies

Optical Fano resonances due to resonant eigenmodes in a layered periodically-modulated structure (photonic crystal slab) are investigated theoretically. The special attention is focused on the behavior of the resonances near a diffraction threshold. A new formulation of the resonant mode approximation for the optical scattering matrix near the diffraction threshold anomalies is proposed.

Threshold anomalies in Horava–Lifshitz-type theories

Recently the study of threshold kinematic requirements for particle-production processes has played a very significant role in the phenomenology of theories with departures from Poincaré symmetry. We here specialize these threshold studies to the case of a class of violations of Poincaré symmetry which has been much discussed in the literature on Horava–Lifshitz scenarios. These involve modifications of the energy–momentum (“dispersion”) relation that may be different for different types of particles, but always involve even powers of energy–momentum in the correction terms. We establish the requirements for compatibility with the observed cosmic-ray spectrum, which is sensitive to the photopion-production threshold. We find that the implications for the electron–positron pair-production threshold are rather intriguing, in light of some recent studies of TeV emissions by Blazars. Our findings should also provide additional motivation for examining the fate of the law of energy–momentum conservation in Horava–Lifshitz-type theories.

10.1007/s11448-008-2006-4

Effective-range theory is developed to describe the processes of multiphoton bremsstrahlung absorption and the emission of laser radiation that accompanies the scattering of electrons from atoms in a laser field. It is found that the cross sections for multiphoton absorption in the plateau region increases resonantly for electron energies corresponding to the thresholds of induced bremsstrahlung (i.e., multiples of the photon energy). It is shown that this effect is caused by the laser-field modified threshold phenomena in the cross sections for multichannel reactions (Baz’ threshold anomalies).

Wavelength dependence of channel-closing enhancements in high-order above-threshold ionization and harmonic generation

We investigate the laser-wavelength dependence of high-order above-threshold ionization and high-order harmonic generation rates. For both processes we have found pronounced enhancements in the spectra for those wavelengths for which the channel-closing condition is satisfied. These enhancements are explained in two complementary ways. One is in terms of the constructive interference of a large number of long quantum orbits, while the second interpretation is based on threshold anomalies. There are two types of these enhancements, which leave very different footprints in the spectra. When the ground-state parity of the atom changes, the two types are interchanged.

Supergap anomalies in cotunneling between normal-superconducting and between two superconducting leads via a small quantum dot

Cotunneling current through a resonant level coupled to either normal and superconducting or to two superconducting leads is studied for the domain of bias voltages $V$, exceeding the superconducting gap $2\ensuremath{\Delta}$. Due to the on-site repulsion in the resonant level, cotunneling of an electron is accompanied by the creation of a quasiparticle in a superconducting lead. Energy conservation imposes a threshold for this inelastic transport channel: ${V}_{c}=3\ensuremath{\Delta}$ for normal-superconductor case and ${\stackrel{\ifmmode \tilde{}\else \~{}\fi{}}{V}}_{c}=4\ensuremath{\Delta}$ for the superconductor-superconductor (S-S) case. We demonstrate that the behavior of the current near the respective thresholds is nonanalytic, namely, $\ensuremath{\delta}{I}^{in}(V)\ensuremath{\propto}{(V\ensuremath{-}{V}_{c})}^{3/2}\ensuremath{\Theta}(V\ensuremath{-}{V}_{c})$ and $\ensuremath{\delta}{I}^{in}(V)\ensuremath{\propto}(V\ensuremath{-}{\stackrel{\ifmmode \tilde{}\else \~{}\fi{}}{V}}_{c})\ensuremath{\Theta}(V\ensuremath{-}{\stackrel{\ifmmode \tilde{}\else \~{}\fi{}}{V}}_{c})$. Stronger anomaly for the S-S leads is the consequence of the enhanced density of states at the edges of the gap. In addition, the enhanced density of states makes the threshold anomalies for two-electron cotunneling processes in the Coulomb-blockaded regions more pronounced than for the normal-normal leads.

Threshold phenomena in electron-atom scattering in a laser field

Effective-range theory is developed to describe the processes of multiphoton bremsstrahlung absorption and the emission of laser radiation that accompanies the scattering of electrons from atoms in a laser field. It is found that the cross sections for multiphoton absorption in the plateau region increases resonantly for electron energies corresponding to the thresholds of induced bremsstrahlung (i.e., multiples of the photon energy). It is shown that this effect is caused by the laser-field modified threshold phenomena in the cross sections for multichannel reactions (Baz’ threshold anomalies).

Intensity-dependent enhancements in high-order above-threshold ionization

The very pronounced intensity-dependent enhancements of groups of peaks of high-order above-threshold-ionization spectra of rare-gas atoms are investigated using an improved version of the strong-field approximation, which realistically models the respective atom. Two types of enhancements are found and explained in terms of constructive interference of the contributions of a large number of long quantum orbits. The first type is observed for intensities slightly below channel closings. Its intensity dependence is comparatively smooth and it is generated by comparatively few (of the order of 20) orbits. The second type occurs precisely at channel closings and exhibits an extremely sharp intensity dependence. It requires constructive interference of a very large number of long orbits (several hundreds) and generates cusps in the electron spectrum at integer multiples of the laser-photon energy. An interpretation of these enhancements as a threshold phenomenon is also given. An interplay of different types of the threshold anomalies is observed. The position of both types of enhancements, in the photoelectron-energy---laser-intensity plane, shifts to the next channel closing intensity with the change of the ground-state parity. The enhancements gradually disappear with decreasing laser pulse duration. This confirms the interpretation of enhancements as a consequence of the interference of long strong-laser-field-induced quantum orbits.