Decay Length Research Articles

Reinterpretation of searches for long-lived particles from meson decays

Many models beyond the Standard Model predict light and feebly interacting particles that are often long-lived. These long-lived particles (LLPs) in many cases can be produced from meson decays. In this work, we propose a simple and quick reinterpretation method for models predicting LLPs produced from meson decays. With the method, we are not required to run Monte-Carlo simulation, implement detector geometries and efficiencies, or apply experimental cuts in an event analysis, as typically done in recasting and reinterpretation works. The main ingredients our method requires are only the theoretical input, allowing for computation of the production and decay rates of the LLPs. There are two conditions for the method to work: firstly, the LLPs in the models considered should be produced from a set of mesons with similar mass and lifetime (or the same meson) and second, the LLPs should, in general, have a lab-frame decay length much larger than the distance between the interaction point and the detector. As an example, we use this method to reinterpret exclusion bounds on heavy neutral leptons (HNLs) in the minimal “3+1” scenario, into those for HNLs in the general effective-field-theory framework as well as for axion-like particles. We are able to reproduce existing results, and obtain new bounds via reinterpretation of past experimental results, in particular, from CHARM and Belle.

Theory of glide symmetry protected helical edge states in a WTe2 monolayer

Helical edge states in quantum spin Hall (QSH) materials are central building blocks of topological matter design and engineering. Despite their principal topological protection against elastic backscattering, the level of operational stability depends on manifold parameters such as the band gap of the given semiconductor system in the 'inverted' regime, temperature, disorder, and crystal orientation. We theoretically investigate electronic and transport properties of QSH edge states in large gap 1-T' WTe$_{2}$ monolayers. We explore the impact of edge termination, disorder, temperature, and interactions on experimentally addressable edge state observables, such as local density of states and conductance. We show that conductance quantization can remain surprisingly robust even for heavily disordered samples because of an anomalously small edge state decay length and additional protection related to the large direct gap allowed by glide symmetry. From the simulation of temperature-dependent resistance, we find that moderate disorder enhances the stability of conductance by localizing bulk states. We evaluate the edge state velocity and Luttinger liquid parameter as functions of the chemical potential, finding prospects for physics beyond linear helical Luttinger liquids in samples with ultra-clean and well-defined edges.

Mesoscopic theory for ionic liquids and concentrated electrolytes in a slit with charged walls

Concentrated electrolytes and ionic liquids confined between parallel charged plates are studied within a theory combining density functional and field-theoretic methods, extended in this work to the slit geometry. We obtain charge- and density profiles, as well as the excess grand potential as a function of the slit width L in the self-consistent Gaussian approximation. For large density of ions, the obtained disjoining pressure exhibits oscillatory asymptotic decay for increasing L, with the decay length that agrees with the results of recent experiments. On a general level, we show that if the charge density decays with a distance from a charged surface in the oscillatory way, then the electrostatic energy of the slit with charged walls also decays with L in an oscillatory way with the same characteristic lengths. The oscillatory decay disagrees with the asymptotic repulsion obtained in the surface force balance experiments. We argue that in the experiment, the lack of the attraction between the confining surfaces for large L may follow from slow ordering of the isotropic fluid absorbed in the expanding slit into layers with alternating charge.

Development of a high-speed small-angle infrared thermography system in EAST.

A high-speed infrared small-angle infrared thermography system (SATS) has been developed and installed on the Experimental Advanced Superconducting Tokamak (EAST) for measuring the surface temperature of the divertor target to calculate the high heat flux induced by Edge Localized Modes (ELMs) and providing observation means for the further physical study of some key parameters, such as power decay length λq and the characteristic time of different types of ELMs. An endoscopic optical system is applied to realize the SATS for clear imaging of the divertor plate area and protection from the harm of impurity deposition and latent tungsten ablation during discharge. The field of view (FOV) of the endoscopic optical system is designed to be 13° in the horizontal direction and 9° in the vertical direction. As a consequence, ∼35° of coverage of the lower-outer divertor and a small part of the lower-inner divertor in toroidal are covered by the FOV with a spatial resolution of around 2 mm/pixel. This paper presents a detailed description of the new SATS and the preliminary experimental diagnostic results. The radial distribution of heat flux induced by an ELM crash was demonstrated.

Displaced or invisible? ALPs from B decays at Belle II

At colliders, neutral long-lived particles can be detected through displaced decay products or as missing energy. Which search strategy is better depends on the particle’s decay length just as on the detector properties. We investigate the complementarity of displaced and invisible signatures for the Belle II experiment. Focusing on axion-like particles a produced from meson decays, we present a new search strategy for two-body decays with missing energy . With 50 ab−1 of data, Belle II can probe light invisible resonances with branching ratio mathcal{B} (B+→ K+a) ≳ 10−7 and decay length cτa ≳ 1 m. For axion-like particles, we expect the sensitivity of to small couplings to improve by up to two orders of magnitude compared to previous searches at collider and fixed-target experiments. For sub-GeV particles, at Belle II and searches at beam-dump experiments are most sensitive; for heavier particles, searches for displaced vertices at Belle II, long-lived particle experiments at the LHC, and future fixed-target experiments can probe the smallest couplings.

Universal Evolution of Fickian Non-Gaussian Diffusion in Two- and Three-Dimensional Glass-Forming Liquids.

Recent works show that glass-forming liquids display Fickian non-Gaussian Diffusion, with non-Gaussian displacement distributions persisting even at very long times, when linearity in the mean square displacement (Fickianity) has already been attained. Such non-Gaussian deviations temporarily exhibit distinctive exponential tails, with a decay length λ growing in time as a power-law. We herein carefully examine data from four different glass-forming systems with isotropic interactions, both in two and three dimensions, namely, three numerical models of molecular liquids and one experimentally investigated colloidal suspension. Drawing on the identification of a proper time range for reliable exponential fits, we find that a scaling law λ(t)∝tα, with α≃1/3, holds for all considered systems, independently from dimensionality. We further show that, for each system, data at different temperatures/concentration can be collapsed onto a master-curve, identifying a characteristic time for the disappearance of exponential tails and the recovery of Gaussianity. We find that such characteristic time is always related through a power-law to the onset time of Fickianity. The present findings suggest that FnGD in glass-formers may be characterized by a "universal" evolution of the distribution tails, independent from system dimensionality, at least for liquids with isotropic potential.

Near or far detectors? A case study for long-lived particle searches at electron-positron colliders

We explore the discovery potential for long-lived particles at the 250-GeV ILC. The goal is to investigate possible gains of a dedicated far detector over the main detector ILD. For concreteness, we perform our study for sub-GeV axion-like particles $a$ produced via $e^+e^- \to a \gamma$ or $e^+e^- \to Z \gamma \to (a\gamma)\gamma$ and decaying into pairs of charged leptons. In the ideal case of zero background and perfect detection efficiency, we find that far detectors placed in the planned underground cavities or a large cuboid on the ground can enhance the sensitivity to long-lived pseudo-scalars at best moderately. On the other hand, the ILD itself is a perfect environment to search for long-lived particles, due to its excellent angular coverage and radial thickness. For long-lived particles produced with cross sections of a few picobarns, the ILD could probe lifetimes up to 300$\,$ns, or proper decay lengths up to 100$\,$m, in 250$\,$fb$^{-1}$ of data. For axion-like particles produced through weak interactions, the ILC can reach an even higher sensitivity than searches for displaced vertices in meson decays at Belle II. Our findings apply similarly to other proposed electron-positron experiments with a high angular coverage, such as the FCC-ee and CEPC.

Spatial structures of rf ring-shaped magnetized sputtering plasmas with two facing cylindrical ZnO/Al2O3 targets

Spatial structures of the ion flux to the substrate are measured in an rf ring-shaped magnetized sputtering plasma with two facing cylindrical ZnO/Al2O3 targets at various argon gas pressures of 0.13, 0.67, and 0.93 Pa. Spatial distributions of the Hall parameter and Larmor radius of electrons and ions are also discussed by using simulated values of the magnetic flux density. The magnitude of the ion flux for 0.13 and 0.67 Pa is of the order of 1020 m−2 s−1, while for 0.93 Pa it is of the order of 1021 m−2 s−1 at a fixed rf power of 20 W. The radial profile of the ion flux has a peak at the position of the ring-shaped groove near an rf electrode and then becomes uniform further away from the electrode at all gas pressures. It is found that the axial profile of the deviation from a uniform profile estimated from the radial profile of the ion flux has two decay characteristics (1st decay length of 13.9–17.5 mm and 2nd decay length of 52.6–66.7 mm) and their decay lengths decrease with increasing the gas pressure.

Pure Spin Current Injection into a Helimagnet

The injection of a pure spin current into a conducting helimagnet is investigated. The characteristic decay lengths for the spin current injected into the helimagnet are determined, and their physical meaning is described. It is shown that instead of the spin diffusion length, helimagnets are characterized by the decay length that is always smaller than the spin diffusion length, the difference in these lengths being determined by the ratio of the helimagnet spiral period to the spin diffusion length. We predict the existence of the “effect of the chiral polarization of a pure spin current,” i.e., the emergence of the spin current with longitudinal (transverse) polarization, which depends on the spiral chirality, upon the injection of a pure spin current with the transverse (longitudinal) polarization relative to the spiral axis.

Front face shaping of the inner wall tiles in the COMPASS Upgrade tokamak

The central column of the COMPASS Upgrade tokamak, where the plasma discharges will be started and shut down, is covered by a continuous first wall composed of 8 bulk tungsten guard limiters with recessed Inconel tiles in between. In diverted configurations, the plasma will be limited for a short duration (∼0.2 s–0.4 s) during the plasma current ramp-up and ramp-down phases but high heat fluxes due to short power decay lengths are foreseen. An adapted shaping is therefore needed to spread the heat loads over a maximum area. A novel method taking into account the different magnetic equilibria and the incident angle of the field lines was used to design a roof shape able to ensure a constant heat flux on the toroidal direction, accounting for the λqnear−SOL and λqfar−SOL. Due to the diversity of plasma scenarios considered on COMPASS-U, the optimized shape of the inner wall tiles strongly differs from one scenario to another, thus a compromise has to be found among all foreseen scenarios. The tile shaping also needs to take into account possible short wave misalignments. Calculations with the 3D magnetic field line tracing code PFCFlux validates the design, confirming the constant toroidal profile of deposited heat flux for the COMPASS-U workhorse scenarios, or slightly decreasing or increasing heat flux profiles (from the center to the sides of the tile) for scenarios with smaller or higher λqnear−SOL, respectively, within the expected large range (1.7 mm<λqnear−SOL<5 mm), but still providing acceptable loads. Estimated heat fluxes are in the range of 5 MW/m2 to 35 MW/m2 on the 8 guard limiters, receiving 70%–80% of the power deposited to the inner wall. In case of shortwave misalignments up to 0.5 mm in the radial direction, calculations show that leading edges are still protected, confirming the robustness of the design.

Colloidal interactions for a polystyrene particle and a smooth silicon surface: Atomic force microscopy, XDLVO theory, and Surface Element Integration

In this study Liftshitz van der Waals, electrostatic and acid base interaction forces of a 4 µm polystyrene particle with flat silicon surfaces were experimentally determined using interfacial interaction energy measurements and atomic force microscopy (AFM). Interfacial forces computed using analytical expressions based on the Surface Element Integration (SEI) technique were compared with the standard Derjaguin Approximation (DA) and Continuum Mechanics Adhesion approaches, namely: Derjaguin, Muller and Toporov (DMT), Johnson, Kendall and Roberts (JKR) and Maugis and Pollock (MP) theories in order to assess the effect of curvature on a sphere-plate geometry. For acid base interactions, the SEI was found to deviate from the DA at small particles radii (less than 100 nm) and over a wide ratio of decay length, λ, to particle size, R. Accounting for curvature for acid base interaction requires geometric factors that are functions of λ/R. For van der Waals force of interaction, the SEI and DA showed large variation at large ratios of interparticle separation, D, to particle radius, R. For Double layer interaction force large variation between SEI and DA was observed for small κa values, and good agreement observed at large κa values. The AFM forces of adhesion of polystyrene in 0.01 mM and 100 mM KCl electrolyte solutions were found to be 2.38 ± 2.15 nN and 9.29 ± 6.89 nN, respectively. These results were found to be consistent with the SEI technique that gave adhesion forces of 4.8 and 10.2 nN at the low and high electrolyte concentrations, respectively. These data are also aligned with Maugis and Pollock (MP) theory that accounts for elasto-plastic deformation, Pe.

Thermal transport imaging in the quantum Hall edge channel

Research focused on heat transport in the quantum Hall (QH) edge channel has successfully addressed fundamental theoretical questions surrounding the QH physics. However, the picture of the edge channel is complicated by the phenomenon of energy dissipation out of the edge, and theories treating this dissipation are lacking. More experimental data is also needed to determine the coupling mechanism by which energy leaves the edge channel. We developed a method to map the heat transport in the QH edge to study the dissipation of heat. We locally heated the QH edge and locally detected the temperature increase while continuously varying the distance between heater and thermometer. We thereby obtained the thermal decay length of the edge state.

Diagnostics of nonergodic extended states and many body localization proximity effect through real-space and Fock-space excitations

We provide real-space and Fock-space (FS) characterizations of ergodic, nonergodic extended (NEE) and many-body localized (MBL) phases in an interacting quasiperiodic system, namely, the generalized Aubry-Andr\'e-Harper model, which possesses a mobility edge in the noninteracting limit. We show that a mobility edge in the single-particle (SP) excitations survives even in the presence of interaction in the NEE phase. In contrast, all single-particle excitations get localized in the MBL phase due to the MBL proximity effect. We give complementary insights into the distinction of the NEE states from the ergodic and MBL states by computing local FS self-energies and decay length associated, respectively, with the local and the nonlocal FS propagators. Based on a finite-size scaling analysis of the typical local self-energy across the NEE to ergodic transition, we show that MBL and NEE states exhibit qualitatively similar multifractal character. However, we find that the NEE and MBL states can be distinguished in terms of the distribution of local self-energy and the decay of the nonlocal propagator in the FS, whereas the typical local FS self-energy cannot tell them apart.

Spatial nonlocality effect on the surface plasmon propagation in plasmonic nanospheres waveguide

Spatial nonlocality affects the plasmonic characteristics of nanostructures. We used the quasi-static hydrodynamic Drude model to obtain the surface plasmon excitation energies in various metallic nanosphere structures. The surface scattering and radiation damping rates were phenomenologically incorporated into this model. We demonstrate that spatial nonlocality increases the surface plasmon frequencies and total plasmon damping rates in a single nanosphere. This effect was amplified for small nanospheres and higher multipole excitation. In addition, we find that spatial nonlocality reduces the interaction energy between two nanospheres. We extended this model to a linear periodic chain of nanospheres. Then we obtain the dispersion relation of surface plasmon excitation energies using Bloch’s theorem. We also show that spatial nonlocality decreases the group velocities and energy decay lengths of the propagating surface plasmon excitations. Finally, we demonstrated that the effect of spatial nonlocality is significant for very small nanospheres separated by short distances.

Multi-code estimation of DTT edge transport parameters

The main goal of the Divertor Tokamak Test facility (DTT) is to operate with a high value of power-exhaust-relevant parameter P S O L /R in plasma scenarios similar to those foreseen for the Demonstration Fusion Power Plant (DEMO) in terms of low collisionality and neutral opacity. For these unique characteristics, accurate modelling of the principal scenario is necessary for machine designing. In edge numerical codes, cross-field transport profiles have a high impact on modelling results. This work aims at providing a coherent set of transport parameters for DTT full-power (FP) single-null (SN) scenario edge modelling. To evaluate such parameters for DTT, a transport analysis on the current machine has been performed using SOLEDGE2D-EIRENE and SOLPS-ITER. The transport parameters to be used in the simulations of the DTT single-null scenario were selected using two complementary methods. The first is the modelling of JET and Alcator C-Mod (C-Mod) with SOLEDGE2D-EIRENE and SOLPS-ITER, validating transport parameters by comparing modelling results to experimental data from pulses which are considered DTT-relevant . JET pulses were selected with the highest auxiliary input power (from 29 to 33 MW), plasma current and toroidal field to better match DTT parameters; nitrogen and neon seeded pulses were selected to check possible seeding material dependencies. The considered C-Mod pulse better matches DTT plasma density and neutral opacity. Transport parameters are then scaled to DTT according to scaling laws. The second method derives the transport parameters by tuning their values inside the DTT separatrix to reproduce the pedestal profiles predicted by the EPED model via the Europed code and applied in DTT. The derived set of DTT transport parameters is consistent with the results obtained by modelling present machines, reproduces the expected heat flux decay length in detached conditions and, inside the separatrix, reproduces the predicted pedestal using transport parameters which are coherent with what is predicted by the quasi-linear turbulent model QuaLiKiz. • Edge and divertor transport parameters of the DTT single-null scenario were studied. • The edge transport parameters of JET and C-Mod pulses were derived by SOLEDGE2D-EIRENE and SOLPS modelling. • Transport parameters were scaled from present machines to DTT. • First principles core modelling with JETTO confirms the validity of the previous estimations.

Analysis of edge transport in L-mode negative triangularity TCV discharges

One of the major problems for future tokamak devices are ELMs (Edge Localized Modes) as they can lead to large, uncontrolled heat fluxes at the machine targets. For this reason, different techniques and alternative magnetic configurations are under study to mitigate or avoid these phenomena. One of the most promising among these studies is the Negative Triangularity (NT) configuration, which exhibits a global confinement comparable with H-Mode operation and, staying in L-mode, could enable an easier power exhaust dissipation due to a possible bigger heat flux decay length with respect to the conventional Positive triangularity (PT) H-mode. In this work, the fluid code SOLEDGE2D-EIRENE is used to study edge transport. Studies are made on discharges performed in the TCV device (Tokamak à configuration variable), which can create a variety of different plasma geometries thanks to 16 independently powered poloidal field coils and its open vacuum vessel. In order to understand if and how power and particle exhaust in NT differs with respect to those of the PT shape, four discharges in single null magnetic divertor configuration with fixed lower triangularity (δbot=+0.5) but with different upper triangularity (from δup=-0.28 to δup=+0.45) have been modelled. All of them are ohmically heated, L-mode deuterium plasmas, and in the high recycling regime. Moreover, these discharges were previously used in [4] to measure the heat flux decay length by IRT (Infrared Thermography), allowing us to make comparisons with modelling results.

Gamma-ray flux limits from brown dwarfs: Implications for dark matter annihilating into long-lived mediators

Brown dwarfs (BDs) are celestial objects representing the link between the least massive main-sequence stars and giant gas planets. In the first part of this article, we perform a model-independent search of a gamma-ray signal from the direction of nine nearby BDs in 13 years of \Fermi-LAT data. We find no significant excess of gamma rays, and we, therefore, set 95\% confidence level upper limits on the gamma-ray flux with a binned-likelihood approach. In the second part of the paper, we interpret these bounds within an exotic mechanism proposed for gamma-ray production in BDs: If the dark matter (DM) of the universe is constituted of particles with non-negligible couplings to the standard model, BDs may efficiently accumulate them through scatterings. DM particles eventually thermalize, and can annihilate into light, long-lived, mediators which later decay into photons outside the BD. Within this framework, we set a stacked upper limit on the DM-nucleon elastic scattering cross section at the level $\sim 10^{-38}$ cm$^{2}$ for DM masses below 10 GeV. Our limits are comparable to similar bounds from the capture of DM particles in celestial objects, but have the advantage of covering a larger portion of the parameter space in mediator decay length and DM mass. They also depend only on the local DM abundance, as opposed to the inner Galaxy profile, and are thus more robust.

Boosted displaced decay of right-handed neutrinos at CMS, ATLAS and MATHUSLA

We investigate boosted displaced signatures in the Type-I seesaw mechanism associated with the B − L gauge symmetry. Such events arise from decays of right-handed neutrinos depending on their Yukawa couplings and masses. Considering two scenarios: (a) three degenerate right-handed neutrinos whose Yukawa couplings are reconstructed from the observed neutrino masses and mixing; (b) only one right-handed neutrino which decouples from the observed neutrino mass generation and thus its coupling can be arbitrarily small, a detailed PYTHIA based simulation is performed to determine the parameter regions of the B − L gauge boson mass, the neutrino Yukawa couplings, and the right-handed neutrino mass sensitive to CMS, ATLAS, proposed FCC-hh detector and MATHUSLA at the centre of mass energies of 14, 27 and 100 TeV via displaced signatures. We also show in detail how the boost effect enhances the displaced decay lengths, especially for the longitudinal ones, and hinders the probe of Majorana nature of neutrinos.

On the Formation of a Plasma Cloud at the Ablation of a Pellet in a High-Temperature Magnetized Toroidal Plasma

The investigation of cold secondary plasma clouds near pellets ablating in the hot plasma of magnetic confinement devices (tokamaks and stellarators) provides valuable information on the physical characteristics of a pellet cloud. In this work, the characteristic sizes of emitting clouds around fusible polystyrene pellets and refractory carbon pellets have been analyzed. The calculation of the ionization length of C+ ions in both carbon and hydrocarbon clouds has shown that the contribution of only hot electrons is insufficient to ensure the experimentally observed decay lengths of the CII line intensity. Taking into account the strong shielding of the electron flux of the background plasma in the hydrocarbon pellet cloud, the ionization of C+ ions in this cloud is determined predominantly by electrons of the cold plasma of the cloud. Shielding near a refractory carbon pellet is weak because its ablation rate is lower. The contributions from hot electrons of the surrounding plasma and cold electrons of the pellet cloud to the ionization of C+ ions are comparable in the case of carbon pellets.

Direct Observation of Lateral Field Confinement in Symmetry‐Protected THz Bound States in the Continuum

AbstractElectromagnetic field confinement on sub‐wavelength scales is possible at the expense of increasing optical losses. Examples are surface plasmon polaritons and resonant structures, which suffer from absorption losses and radiative outcoupling, respectively. Here, the first experimental evidence of strong electromagnetic field confinement along the lateral (out‐of‐plane) direction in a near lossless metasurface that supports a symmetry‐protected Bound State in the Continuum is shown, which fully suppresses the radiative losses of resonant scatterers. This experimental demonstration, supported by coupled dipole calculations, is measured by a double near‐field probe technique to locally excite and detect the time evolution and confinement of the electric field onto the surface, achieving an amplitude decay length from the metasurface of λ/38 at 0.39 THz.