Single Decay Channel Research Articles

Measurement of the Higgs boson mass with H → γγ decays in 140 fb−1 of [formula omitted] TeV pp collisions with the ATLAS detector

The mass of the Higgs boson is measured in the H→γγ decay channel, exploiting the high resolution of the invariant mass of photon pairs reconstructed from the decays of Higgs bosons produced in proton–proton collisions at a centre-of-mass energy s=13 TeV. The dataset was collected between 2015 and 2018 by the ATLAS detector at the Large Hadron Collider, and corresponds to an integrated luminosity of 140 fb−1. The measured value of the Higgs boson mass is 125.17±0.11(stat.)±0.09(syst.) GeV and is based on an improved energy scale calibration for photons, whose impact on the measurement is about four times smaller than in the previous publication. A combination with the corresponding measurement using 7 and 8 TeV pp collision ATLAS data results in a Higgs boson mass measurement of 125.22±0.11(stat.)±0.09(syst.) GeV. With an uncertainty of 1.1 per mille, this is currently the most precise measurement of the mass of the Higgs boson from a single decay channel.

Exciton Dynamics in Colloidal CdS Quantum Dots with Intense and Stokes Shifted Photoluminescence in a Single Decay Channel.

CdS quantum dots (QDs), synthesized by a sol-gel method, exhibit significantly Stokes shifted bright photoluminescence (PL), predominantly from the trap states. Surprisingly, the PL decay at the emission maximum is single-exponential. This is an unusual observation for as-prepared QDs and indicates a narrow distribution in the nature of trap states. A closer look reveals an additional fast component for the decays at shorter emission wavelengths, presumably due to the band edge emission, which remains elusive in the steady-state spectra. Indeed, a significantly narrower and blue-shifted emission band is observed in the decay-associated spectra. The contribution of this component to the steady-state PL intensity is shown to be overwhelmed by that of the significantly stronger trap emission. Exciton dynamics in the quantum dots is elucidated using transient absorption spectra, in which the stimulated emission is observed even at low pump power.

Multichannel decay law

It is well known, both theoretically and experimentally, that the survival probability for an unstable quantum state, formed at t=0, is not a simple exponential function, even if the latter is a good approximation for intermediate times. Typically, unstable quantum states/particles can decay in more than a single decay channel. In this work, the general expression for the probability that an unstable state decays into a certain i-th channel between the initial time t=0 and an arbitrary t>0 is provided, both for nonrelativistic quantum states and for relativistic particles. These partial decay probabilities are also not exponential and their ratio turns out to be not a simple constant, as it would be in the exponential limit. Quite remarkably, these deviations may last relatively long, thus making them potentially interesting in applications. Thus, multichannel decays represent a promising and yet unexplored framework to search for deviations from the exponential decay law in quantum mechanical systems, such as quantum tunneling, and in the context of particle decays.

Capturing nonexponential dynamics in the presence of two decay channels

The most unstable quantum states and elementary particles possess more than a single decay channel. At the same time, it is well known that typically the decay law is not simply exponential. Therefore, it is natural to ask how to spot the non-exponential decay when (at least) two decay channels are opened. In this work, we study the tunneling phenomenon of an initially localized particle in two spatially opposite directions through two different barriers, mimicking two decay channels. In this framework, through a specific quantum mechanical examples which can be accurately solved, we study general properties of a two-channel decay that apply for various unstable quantum states (among which also for unstable particles). Apart from small deviations at early times, the survival probability and the partial tunneling probability along the chosen direction are very well described by the exponential-decay model. In contrast, the ratios of the decay probabilities and probability currents are evidently not a simple constant (as they would be in the exponential limit) but display time-persisting oscillations. Hence, these ratios are optimal witnesses of deviations from the exponential decay law.

Towards model-independent exclusion of light Stops

Understanding the extent to which experimental searches are sensitive to Light Stops (LST) scenarios is essential to resolve questions about naturalness, electroweak baryogenesis and Dark Matter. In this paper we characterize the reach on LST scenarios in two ways. We extend experimental searches to cover specific gaps in the LST parameter space, showing for the first time that assuming a single decay channel one can exclude the region of $m_{\tilde{t}}<m_{top}$, which in its turn excludes electroweak baryogenesis in MSSM. Also, we explore the extent to which searches are weakened in a more generic scenario when more than one decay channel takes place, even after their combination. This study highlights the need for a more comprehensive exploration of the LST parameter space.

Atomically resolved real-space imaging of hot electron dynamics.

The dynamics of hot electrons are central to understanding the properties of many electronic devices. But their ultra-short lifetime, typically 100 fs or less, and correspondingly short transport length-scale in the nanometre range constrain real-space investigations. Here we report variable temperature and voltage measurements of the nonlocal manipulation of adsorbed molecules on the Si(111)-7 × 7 surface in the scanning tunnelling microscope. The range of the nonlocal effect increases with temperature and, at constant temperature, is invariant over a wide range of electron energies. The measurements probe, in real space, the underlying hot electron dynamics on the 10 nm scale and are well described by a two-dimensional diffusive model with a single decay channel, consistent with 2-photon photo-emission (2PPE) measurements of the real time dynamics.

Angular analysis and CP violation studies in B decays at CMS

Angular distributions of the decay B0 → K*0μ+μ− are a clean probe of physics beyond the standard model. In particular the forward-backward asymmetry and other variables which can be determined as a function of q2 provide fruitful information from a single decay channel. An angular and proper decay time analysis is applied to the Bs → J/ψϕ decays. The Bs signal candidates are reconstructed and used to extract the mixing phase ϕs. The theoretical prediction of the ϕs angle is particularly robust, thus any deviation from the prediction can be a smoking gun signal of new physics.

Mode-coupling theory for multiple decay channels

We investigate the properties of a class of mode-coupling equations for the glass transition where the density mode decays into multiple relaxation channels. We prove the existence and uniqueness of the solutions for Newtonian as well as Brownian dynamics and demonstrate that they fulfil the requirements of correlation functions; in the latter case the solutions are purely relaxational. Furthermore, we construct an effective mode-coupling functional, which allows us to map the theory to the case of a single decay channel, such that the covariance principle found for the mode-coupling theory for simple liquids is properly generalized. This in turn allows establishment of the maximum theorem stating that long-time limits of mode-coupling solutions can be calculated as maximal solutions of a fixed-point equation without relying on the dynamic solutions.

Efficient two-dimensional atom localization via spontaneous emission in a single decay channel

We investigate the two-dimensional atom localization behaviors in a four-level atomic system via controlled spontaneous emission in a single decay channel. It is found that the detecting probability and precision of atom localization behaviors can be significantly improved via adjusting the system parameters. More importantly, the two-dimensional atom localization patterns reveal that the maximal probability of finding an atom within the sub-half-wavelength domain of the standing waves can reach unity when the corresponding conditions are satisfied. As a result, our scheme may be helpful in laser cooling or the atom nano-lithography via atom localization.

Decays of near BPS heterotic strings

The decay of highly excited massive string states in compactified heterotic string theories is discussed. We calculate the decay rate and spectrum of states carrying momentum and winding in the compactified direction. The longest lived states in the spectrum are near Bogomol'nyi-Prasad-Sommerfield (BPS) states whose decay is dominated by a single decay channel of massless radiation which brings the state closer to being BPS.

Near-threshold site-selected dissociative ionization of core-excited carbon dioxide

The ionization and fragmentation of C and O 1s core excited CO2 has been studied using soft X-rays, threshold electron analysis to define the initial core hole and a triple coincidence technique to examine the final dissociated ion state. The excitation of the O 1s electron results in a single decay channel, CO++O+, corresponding to the breaking of a single C-O bond. At the C 1s edge, the same channel is seen together with a weaker channel, corresponding to the triply ionized three-body fragmentation O++C++O+. Moreover, the angular distribution of ions from the CO++O+ channel is seen to depend on the excited-core state.

The study of ionic fragmentation by photoelectron‐photoion coincidence spectroscopy

AbstractSince its invention, a decade ago, photoelectron‐photoion coincidence spectroscopy has continuously provided extremely important information on ionic fragmentation processes. The outstanding reputation of the technique is essentially founded on the fact that internal energy selected cations can be studied. In striking contrast to the importance of the method and to the reliability of the data obtained, only about 150 coincidence studies have been published up to now. This is due to the rather intricate experimental technique involved and, probably even more, to the main drawback of this method, i.e. the enormous length of time involved in the measurements. The systems investigated so far are comprehensively compiled in the present article. The two experimental variants currently in use, termed fixed and variable wavelength technique, are compared. Evidence for the complementary character of the two experimental versions is presented. It is recalled that the primary source of information is in any case a time‐of‐flight distribution. The different ways of analysing these distributions are summarized and the accuracy of the derived results is discussed. The current state of the art is exemplified by reviewing the outcomes for a number of selected examples. The presented results on di‐, tri‐ and tetra‐atomic cations reveal clearly the more recently achieved experimental progress. Data on such smaller ionic species are particularly useful for studying predissociation mechanisms, a domain which becomes more and more accessible to modern ab initio calculations of fragmentation pathways. The reported results for O2+ and CO2+ reveal that branching ratios for the formation of electronically and vibrationally excited dissociation products can now be quantified for sufficiently small systems. The possibility of distinct behaviour of isoenergetic molecular ions depending on the method of preparation is outlined in the case of formaldehyde cation. The same cation is used to exemplify a puzzling isotope effect originating in a rate‐determining radiationless transition that precedes a significantly faster dissociative step. A variety of data on larger systems grouped according to topic are critically reviewed. The determination and accuracy of rate‐energy functions is discussed, at first for the case of a single decay channel; then the analogous analysis for a series of competing fragmentations is outlined. The importance of competitive radiative and non‐radiative/dissociative depletion of an excited electronic state is pointed out.