Cascade Decay Research Articles

Leveraging Intramolecular π-Stacking to Access an Exceptionally Long-Lived 3MC Excited State in an Fe(II) Carbene Complex.

The ability to manipulate excited-state decay cascades using molecular structure is essential to the application of abundant-metal photosensitizers and chromophores. Ligand design has yielded some spectacular results elongating charge-transfer excited state lifetimes of Fe(II) coordination complexes, but triplet metal-centered (3MC) excited states─recently demonstrated to be critical to the photoactivity of isoelectronic Co(III) polypyridyls─have to date remained elusive, with temporally isolable examples limited to the picosecond regime. With this report, we show how strong-field donors and intramolecular π-stacking can conspire to stabilize a long-lived 3MC excited state for a remarkable 4.1 ± 0.3 ns in fluid solution at ambient temperature. Analysis of variable-temperature time-resolved absorption data using theoretical models ranging from Arrhenius to semiclassical Marcus theory, combined with computational modeling and X-ray crystallography, reveal a Jahn-Teller stabilized excited state with a high activation barrier for ground-state recovery. The net result is a chromophore with a 3MC excited-state lifetime that is orders of magnitude longer than anything yet observed for an Fe(II) complex.

Study of lepton flavor universality and angular distributions in D→KJ∗(→Kπ)ℓνℓ

The decay of the D meson into multibody final states is a complex process that provides valuable insights into the fundamental interactions within the Standard Model of particle physics. This study focuses on the decay cascade D+→KJ∗ℓ+ν→K±π∓ℓ+ν where the KJ∗ resonance encompasses the K∗(892),K∗(1410),K0∗(1430) states. We employ the helicity amplitude technique to derive the angular distributions for the decay chain, enabling the extraction of one-dimensional and two-dimensional distributions. Utilizing form factors for the D→K∗ transition derived from the quark model, we calculate the differential and integrated partial decay widths, explicitly considering the electron and muon masses. Decay branching fractions are calculated, the ratios of the branching fractions are found to be Br(D+→K∗(892)(→K-π+)μ+νμ)Br(D+→K∗(892)(→K-π+)e+νe)=0.975, Br(D+→K∗(1410)(→K-π+)μ+νμ)Br(D+→K∗(1410)(→K-π+)e+νe)=0.714 and Br(D+→K0∗(1430)(→K+π-)μ+νμ)Br(D+→K0∗(1430)(→K+π-)e+νe)=0.774. Results in this work will serve a calibration for the study of c→s in D meson decays in future and provide useful information towards the understanding of the properties of the K∗ meson, as well as Kπ system.

Quantum Molecular Charge-Transfer Model for Multistep Auger-Meitner Decay Cascade Dynamics.

The fragmentation of molecular cations following inner-shell decay processes in molecules containing heavy elements underpins the X-ray damage effects observed in X-ray scattering measurements of biological and chemical materials, as well as in medical applications involving Auger electron-emitting radionuclides. Traditionally, these processes are modeled using simulations that describe the electronic structure at an atomic level, thereby omitting molecular bonding effects. This work addresses the gap by introducing a novel approach that couples Auger-Meitner decay to nuclear dynamics across multiple decay steps, by developing a decay spawning dynamics algorithm and applying it to potential energy surfaces characterized with ab initio molecular dynamics simulations. We showcase the approach on a model decay cascade following K-shell ionization of IBr and subsequent Kβ fluorescence decay. We examine two competing channels that undergo two decay steps, resulting in ion pairs with a total 3+ charge state. This approach provides a continuous description of the electron transfer dynamics occurring during the multistep decay cascade and molecular fragmentation, revealing the combined inner-shell decay and charge transfer time scale to be approximately 75 fs. Our computed kinetic energies of ion fragments show good agreement with experimental data.

Enantioselective One-Photon Excitation of Formic Acid.

We use one-photon excitation to promote K-shell electrons of formic acid (which has a planar equilibrium structure) to an antibonding π^{*} orbital. The excited molecule is known to have a (chiral) pyramidal equilibrium structure. In our experiment, we determine the handedness of the excited molecule by imaging the momenta of charged fragments, which occur after its Coulomb explosion triggered by Auger-Meitner decay cascades succeeding the excitation. We find that the handedness of the excited molecule depends on its spatial orientation with respect to the propagation (or polarization) direction of the exciting photon. The effect is largely independent of the exact polarization properties of the light driving the 1s→π^{*} excitation.

CP violation induced by neutral meson mixing interference

We investigate a long-overlooked CP violation effect—the double-mixing CP asymmetry—in a type of cascade decay that involves at least two mixing neutral mesons in the decay chain. It is induced by the interference between different oscillation paths of the neutral mesons in the decay process. The double-mixing CP asymmetry is of critical importance for phenomenology, providing opportunities for clean determination of Cabibbo-Kobayashi-Maskawa phase angles free of uncertainties induced by the strong dynamics. To illustrate this point, we perform a phenomenological analysis on two examples: Bs0→ρ0K→ρ0(π−ℓ+νℓ) and B0→D0K→D0(π+ℓ−ν¯ℓ). Our results demonstrate that the double-mixing CP asymmetry can be numerically significant in the absence of strong phases, as shown by the former example. Additionally, the latter example showcases a direct extraction of weak and strong phases from data, without the need for theoretical inputs. Published by the American Physical Society 2024

Decay-angular-distribution correlated CP violation in heavy hadron cascade decays

CP violation in baryon decay processes is still undiscovered to date. We present a general analysis of the decay-angular distributions and the corresponding CP asymmetries in cascade decays of the type H→R(→ab)c, where H is a heavy hadron that decays through weak interactions H→Rc and the resonance R decays strongly via R→ab. Based on the analysis, we propose to search for CP violation in the decay-angular distributions in the cascade decay processes B→BM, with B or M subsequently decaying through strong interactions, where B is the mother baryon, B and M are the daughter baryon and meson, respectively, and M has to be spin nonzero. We also present some typical decay channels in which the search for such kinds of CP asymmetries can be performed. Published by the American Physical Society 2024

Cascade energy reemission by the silver atom ionized by 0.01–100 keV photons. Possible application of silver-based radiosensitizing agents in photon beam radiation therapy

A method of straightforward construction and analysis of the cascade decay trees is applied to study the cascade energy reemission by silver atoms upon photoionization in the incident photon energy range of 0.01–100 keV. Cascade electrons and photons emitted during the cascade relaxation of an inner-shell vacancy carry away a significant part of the energy initially acquired by the silver atom in a photoionization act. The energies redistributed through following channels were calculated as functions of incident photon energy: energy absorbed by the silver atom itself (a), and energies carried away by the cascade electrons (b) and photons (c). Channel (a) is dominant only at small incident photon energies up to the M5 ionization threshold. Channel (b) dominates between M and K thresholds making 57–85 % of the acquired energy. Above the K threshold, the weight of channel (b) is still significant, 24 %, however, dominant is channel (c).

Collider signatures of near-continuum dark matter

In this paper we study a near-continuum dark matter model, in which dark sector consists of a tower of closely spaced states with weak-scale masses. We construct a five-dimensional model which naturally realizes this spectrum. The dark matter is described by a bulk field, which interacts with the brane-localized Standard Model sector via a Z portal. We then study collider signatures of this model. Near-continuum dark matter states produced in a collider undergo cascade decays, resulting in events with high multiplicity of jets and leptons, large missing energy, and displaced vertices. A custom-built Monte Carlo tool described in this paper allows for detailed simulation of the signal events. We present results of such simulations for the case of electron-positron collisions.

Cascade energy reemission by the iodine atom irradiated by 0.01–100 keV photons. Role of photo- and cascade-produced electrons in radiosensitization using iodine-containing agents

Cascade decays of single K to O vacancies in an isolated iodine atom are simulated by straightforward construction of the cascade decay trees. Cascade-emitted electron and photon spectra are calculated and analyzed to determine the portions of energies (a) absorbed by the iodine atom after ionization and complete cascade relaxation, (b) emitted by cascade electrons, (c) emitted by cascade photons. These quantities are calculated as functions of incident photon energy following photoionization of the iodine atom in the 0.01–100 keV energy range. Cascade electrons and photoelectrons are shown to be principal energy transmitters from an ionized iodine atom to the environment. Presented data are relevant for developing the strategies of photon activation radiation therapy when iodine is used as a radiosensitizing agent.

Cross sections of cascade production of photoions after photoionization of the silver atom in the 0.07–100 keV incident photon energy range

The formation of multiply charged Agq+ ions resulting from photoionization and subsequent cascade decay of inner-shell vacancy states is studied using the method of construction and analysis of the cascade decay trees. Cross sections for the cascade production of Agq+ ions, yields of Agq+ ions, and mean photoion charge are calculated as functions of incident photon energy in the range of 70 eV to 100 keV. The results of this study are relevant for developing the strategies of radiotherapy using Ag-based radiosensitizers.

Cross sections of cascade production of photoions after photoionization of the iodine atom in the 0.01–100 keV incident photon energy range

Ion yields upon the cascade decays of single vacancies in K to O shells of an isolated iodine atom are calculated by the method of construction and analysis of the cascade decay trees. Branching ratios needed for the simulations are calculated based on Pauli–Fock partial transition widths. Partial photoionization cross sections are calculated with accounting for the core relaxation effect and used to calculate the cross sections of I q+ photoions production in the 0.01–100 keV incident photon energy range. Accounting for the spin–orbit splitting and the splitting due to electron–electron interaction is shown to be important when calculating mean energies and probabilities of the transitions between energetically closely lying ionic configurations. On the other hand, inclusion of the additional shake-off electron ejections, in spite of complicating severely the decay trees, is found to affect little calculated cascade ions production probabilities.

Two-dimensional total absorption spectroscopy with conditional generative adversarial networks

We explore the use of machine learning techniques to remove the response of large volume γ-ray detectors from experimental spectra. Segmented γ-ray total absorption spectrometers (TAS) allow for the simultaneous measurement of individual γ-ray energy (Eγ) and total excitation energy (Ex). Analysis of TAS detector data is complicated by the fact that the Ex and Eγ quantities are correlated, and therefore, techniques that simply unfold using Ex and Eγ response functions independently are not as accurate. In this work, we investigate the use of conditional generative adversarial networks (cGANs) to simultaneously unfold Ex and Eγ data in TAS detectors. Specifically, we employ a Pix2Pix cGAN, a generative modeling technique based on recent advances in deep learning, to treat (Ex,Eγ) matrix unfolding as an image-to-image translation problem. We present results for simulated and experimental matrices of single-γ and double-γ decay cascades. Our model demonstrates characterization capabilities within detector resolution limits for upwards of 93% of simulated test cases.

Monte Carlo Simulation of Energy Dissipation during the Cascade Decay of Inner-Shell Vacancies in an Iron Atom Placed in Water

We have performed the Monte Carlo simulation of the processes of secondary ionization of water induced by cascade decays of inner-shell vacancies in an iron atom placed in water. We have obtained the spectra of electrons and photons emitted during the decay of vacancies in the K and L shells of the iron atom. The dependences of the number of secondary ionization events and the energy absorbed as a result of these processes on the radius of the sphere in which such processes occur have been calculated. The decay of a single 1s vacancy in an iron atom generates on the average 232 events of secondary ionization induced by an electron impact, in which the energy of 3274 eV is absorbed, as well as 18 secondary photoionization events, in which the energy of 256 eV is absorbed. The dependences of the dose absorbed in water on the distance from the iron atom have been calculated.

Monte Carlo Simulation of Energy Dissipation during the Cascade Decay of Inner-Shell Vacancies in an Iron Atom Placed in Water

Monte Carlo Simulation of Energy Dissipation during the Cascade Decay of Inner-Shell Vacancies in an Iron Atom Placed in Water

New physics in multi-electron muon decays

We study the exotic muon decays with five charged tracks in the final state. First, we investigate the Standard Model rate for μ+ → 3e+ 2e− 2ν (B\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$ \\mathcal{B} $$\\end{document} = 4.0 × 10−10) and find that the Mu3e experiment should have tens to hundreds of signal events per 1015μ+ decays, depending on the signal selection strategy. We then turn to a neutrinoless μ+ → 3e+ 2e− decay that may arise in new-physics models with lepton-flavor-violating effective operators involving a dark Higgs hd. Following its production in μ+ → e+hd decays, the dark Higgs can undergo a decay cascade to two e+e− pairs through two dark photons, hd→ γdγd→ 2(e+e−). We show that a μ+→ 3e+ 2e− search at the Mu3e experiment, with potential sensitivity to the branching ratio at the O\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$ \\mathcal{O} $$\\end{document}(10−12) level or below, can explore new regions of parameter space and new physics scales as high as Λ ∼ 1015 GeV.

Toward extracting gamma from Brightarrow DK without binning

B^pm rightarrow DK^pm transitions are known to provide theoretically clean information about the CKM angle gamma , with the most precise available methods exploiting the cascade decay of the neutral D into CP self-conjugate states. Such analyses currently require binning in the D decay Dalitz plot, while a recently proposed method replaces this binning with the truncation of a Fourier series expansion. In this paper, we present a proof of principle of a novel alternative to these two methods, in which no approximations at the level of the data representation are required. In particular, our new strategy makes no assumptions about the amplitude and strong phase variation over the Dalitz plot. This comes at the cost of a degree of ambiguity in the choice of test statistic quantifying the compatibility of the data with a given value of gamma , with improved choices of test statistic yielding higher sensitivity. While our current proof-of-principle implementation does not demonstrate optimal sensitivity to gamma , its conceptually novel approach opens the door to new strategies for gamma extraction. More studies are required to see if these can be competitive with the existing methods.

Response of the FAst TIMing Array (FATIMA) for DESPEC at FAIR Phase-0

The Monte-Carlo simulated response for γ-ray detection of the FAst TIMing Array (FATIMA) for exploitation within the DEcay SPECtroscopy (DESPEC) experimental system at the FAIR Phase-0 facility at Darmstadt, Germany is presented. In this configuration, FATIMA consisted of 36 LaBr3(Ce) detectors surrounding the AIDA, position-sensitive charged-particle active stopper. The decay of the Iπ=8+ isomer-fed decay cascade in 96Pd, measured in the first DESPEC experiment at the FAIR-0 facility was used to validate the simulations. The experimental data yielded in-situ full-energy peak efficiency values for FATIMA of 11.2(11)%, 6.8(7)%, 3.8(4)% and 2.1(4)% at 106, 325, 684 and 1415 keV respectively, consistent with the values derived from the simulated response.

The parameter-free Monte Carlo approach for diffractive dissociation of relativistic oxygen nuclei into four alpha particles in interactions with protons at 3.25GeV/c per nucleon

In this paper, we have investigated the processes of formation of four alpha particles in the final state in the reactions [Formula: see text] of diffractive dissociation of oxygen-16 nuclei in interactions with protons at an incident momentum of 3.25[Formula: see text]GeV/[Formula: see text] per nucleon. A Monte Carlo model is proposed to describe the kinematic characteristics of the produced alpha particles. The proposed Monte Carlo model does not have any free parameters. The algorithm of the model takes into account the energetically allowed channels of direct (or cascade) decays of the excited oxygen-16 nuclei into the final four alpha particles. To form the masses of the excited oxygen-16, carbon-12, and beryllium-8 nuclei, the experimental tables of their excitation levels have been used. All decays have been generated isotropically in the rest frame of each unstable nucleus — the excited oxygen-16, carbon-12, and beryllium-8 nuclei. The results of the Monte Carlo model calculations have proved to be in good agreement with distributions of various kinematic characteristics of the final state alpha particles taken from the experiment. The good agreement is reached if, after simulation of the mass of the excited oxygen-16, the energetically allowed decay channel is selected for which the sum of the total energy, released in this channel, and the masses of the four secondary alpha particles turn out to be less, but closest to the mass of the excited oxygen-16 nucleus. At a high level of excitation, there are several channels with the same energy release, which have been generated with equal probabilities.

Search for a Dark-Matter-Induced Cosmic Axion Background with ADMX.

We report the first result of a direct search for a cosmic axion background (CaB)-a relativistic background of axions that is not dark matter-performed with the axion haloscope, the Axion Dark Matter eXperiment (ADMX). Conventional haloscope analyses search for a signal with a narrow bandwidth, as predicted for dark matter, whereas the CaB will be broad. We introduce a novel analysis strategy, which searches for a CaB induced daily modulation in the power measured by the haloscope. Using this, we repurpose data collected to search for dark matter to set a limit on the axion photon coupling of a CaB originating from dark matter cascade decay via a mediator in the 800-995MHz frequency range. We find that the present sensitivity is limited by fluctuations in the cavity readout as the instrument scans across dark matter masses. Nevertheless, we suggest that these challenges can be surmounted using superconducting qubits as single photon counters, and allow ADMX to operate as a telescope searching for axions emerging from the decay of dark matter. The daily modulation analysis technique we introduce can be deployed for various broadband rf signals, such as other forms of a CaB or even high-frequency gravitational waves.

Probing dark QCD sector through the Higgs portal with machine learning at the LHC

The QCD-like dark sector with GeV-scale dark hadrons has the potential to generate new signatures at the Large Hadron Collider (LHC). In this paper, we consider a singlet scalar mediator in the tens of GeV-scale that connects the dark sector and the Standard Model (SM) sector via the Higgs portal. We focus on the Higgs-strahlung process, qoverline{q} ′ → W* → WH, to produce a highly boosted Higgs boson. Our scenario predicts two different processes that can generate dark mesons: (1) the cascade decay from the Higgs boson to two light scalar mediators and then to four dark mesons; (2) the Higgs boson decaying to two dark quarks, which then undergo a QCD-like shower and hadronization to produce dark mesons. We apply machine learning techniques, such as Convolutional Neural Network (CNN) and Energy Flow Network (EFN), to the fat-jet structure to distinguish these signal processes from large SM backgrounds. We find that the branching ratio of the Higgs boson to two light scalar mediators can be constrained to be less than about 10% at 14 TeV LHC with mathcal{L} = 3000 fb−1.