Decay Threshold Research Articles

Convenient Location of a Near-Threshold Proton-Emitting Resonance in ^{11}B.

The presence of clusterlike narrow resonances in the vicinity of reaction or decay thresholds is a ubiquitous phenomenon with profound consequences. We argue that the continuum coupling, present in the open quantum system description of the atomic nucleus, can profoundly impact the nature of near-threshold states. In this Letter, we discuss the structure of the recently observed near-threshold resonance in ^{11}B, whose very existence explains the puzzling beta-delayed proton emission of the neutron-rich ^{11}Be.

Acoustic reflex decay and acoustic reflex latency threshold test findings in patients with cerebellopontine angle tumors: Correlation with tumor type, size, and extent

Context: This was a comparative prospective study. Aims: (1) To characterize acoustic reflex decay (ARD) and acoustic reflex latency threshold (ARLT) profiles in cerebellopontine angle (CPA) tumors. (2) To correlate ARD and ARLT test results with the type, size, and extent of tumors. Study Design: Comparative prospective study. Setting: Primary versus Tertiary referral center. Subjects: The clinical group comprised 98 patients with CPA tumor and the control group comprised 100 normal hearing subjects. Subjects and Methods: ARD and ARLT were estimated at 500 Hz and 1000 Hz both ipsilaterally and contralaterally for both the groups. Statistical Analysis Used: Statistical analysis for clinical group versus control group was carried out using analysis of covariance for age and gender covariates. For discrete clinical groups, analysis of variance followed by post hoc (Turkey) test was used. An independent sample t-test was used for the statistical analysis of tumor ear versus nontumor ear. Results: Test results indicated that the proportion of ears with abnormal reflex decay (0%–25%) was less compared to those with normal findings (75%–100%) in the patient group reflecting the poor efficacy of ARD to identify the presence of CPA tumors. Statistical analysis for ARLT was possible mainly for nontumor ear stimulation and it did not give any clear indications regarding the presence of brain stem compression. Conclusions: In the present study, an estimation of ARD and ARLT did not appear to be useful indices for CPA tumor identification.

Performance of wood and wood-based materials regarding fungal decay

Due to the increasingly stringent energy efficiency requirements, timber frame houses are becoming more and more popular across Europe. Depending on the circumstances, timber frame walls might be exposed to high relative humidity or condensation conditions, leading to a risk of fungal degradation. In order to assess the durability of a timber frame construction in a reliable way, a clear view on the potential risk of fungal growth is essential. Various experiments are reported in literature to define the minimal moisture threshold for fungal decay, yet all very different in set-up and hence leading to contradictory conclusions. Therefore, the present paper aims at a contribution to a better understanding of the influence of moisture conditions on the onset and progress of wood decay in timber frame walls. Based on the information that is still lacking in international literature, a new test methodology has been elaborated within this work and is reported in this paper.

A low climate threshold for south Greenland Ice Sheet demise during the Late Pleistocene

The Greenland Ice Sheet (GIS) has been losing mass at an accelerating rate over the recent decades. Models suggest a possible temperature threshold between 0.8 and 3.2 °C, beyond which GIS decline becomes irreversible. The duration of warmth above a given threshold is also a critical determinant for GIS survival, underlining the role of ocean warming, as its inertia prolongs warmth and triggers longer-term feedbacks. The exact point at which these feedbacks are triggered remains equivocal. Late Pleistocene interglacials provide potential case examples for constraining the past response of the GIS to a range of climate states, including conditions warmer than present. However, little is known about the magnitude and duration of warming near Greenland during these periods. Using high-resolution multiproxy surface ocean climate records off southern Greenland, we show that the previous 4 interglacials over the last ∼450 ka all reached warmer than present climate conditions and exceeded the modeled temperature threshold for GIS collapse but by different magnitudes and durations. Complete deglaciation of the southern GIS in Marine Isotope Stage 11c (MIS 11c; 394.7 to 424.2 ka) occurred under climates only slightly warmer than present (∼0.5 ± 1.6 °C), placing the temperature threshold for major GIS retreat in the lower end of model estimates and within projections for this century.

Estimating relative cellulolytic and ligninolytic enzyme activities as functions of lignin and cellulose content in decomposing plant litter

Extracellular enzymes catalyze plant litter decomposition, including enzymes that degrade holocellulose (E2) and lignin (E3). To estimate relative enzyme activities associated with observed patterns of hollocellulose (C2) and lignin (C3) decay, we set observed decay rates equal to reverse Michaelis-Menten equations. Results were consistent with empirical studies, showing a negative relationship of E2/(E2+E3) to litter lignin content, C3/(C2+C3), above a minimum threshold at which lignin begins to decay. This threshold was previously reported to be 40% lignin content, but our results demonstrated substantial variability with litter type and environment. To our knowledge, this is the first mechanistic explanation of microbial allocation of cellulolytic and ligninolytic enzymes as a function of the lignin concentration of the lignocellulose complex but raises further questions about factors controlling the threshold for lignin decay, such as nitrogen availability.

Heavy Bernoulli-percolation clusters are indistinguishable

We prove that the heavy clusters are indistinguishable for Bernoulli percolation on quasi-transitive nonunimodular graphs. As an application, we show that the uniqueness threshold of any quasi-transitive graph is also the threshold for connectivity decay. This resolves a question of Lyons and Schramm (Ann. Probab.27 (1999) 1809–1836) in the Bernoulli percolation case and confirms a conjecture of Schonmann (Comm. Math. Phys. 219 (2001) 271–322). We also prove that every infinite cluster of Bernoulli percolation on a nonamenable quasi-transitive graph is transient almost surely.

Spectroscopy, lifetime and decay modes of the [formula omitted] tetraquark

We present the first full-fledged study of the flavor-exotic isoscalar Tbb−≡bbu¯d¯ tetraquark with spin and parity JP=1+. We report accurate solutions of the four-body problem in a quark model, characterizing the structure of the state as a function of the ratio MQ/mq of the heavy to light quark masses. For such a standard constituent model, Tbb− lies approximately 150 MeV below the strong decay threshold B−B⁎¯0 and 105 MeV below the electromagnetic decay threshold B−B¯0γ. We evaluate the lifetime of Tbb−, identifying the promising decay modes where the tetraquark might be looked for in future experiments. Its total decay width is Γ≈87×10−15 GeV and therefore its lifetime τ≈ 7.6 ps. The promising final states are B⁎−D⁎+ℓ−ν¯ℓ and B⁎¯0D⁎0ℓ−ν¯ℓ among the semileptonic decays, and B⁎−D⁎+Ds⁎−, B⁎¯0D⁎0Ds⁎−, and B⁎−D⁎+ρ− among the nonleptonic ones. The semileptonic decay to the isoscalar JP=0+ tetraquark Tbc0 is also relevant but it is not found to be dominant. There is a broad consensus about the existence of this tetraquark, and its detection will validate our understanding of the low-energy realizations of Quantum Chromodynamics (QCD) in the multiquark sector.

High spin structure of Ar39 and the FSU cross-shell interaction

High-spin states in $^{39}\mathrm{Ar}$ were populated using the $^{27}\mathrm{Al}(^{14}\mathrm{C},\phantom{\rule{4pt}{0ex}}pn)$ reaction at 25.6 MeV. The de-exciting $\ensuremath{\gamma}$ rays were measured with the FSU $\ensuremath{\gamma}$ detector array along with evaporation protons in a Si $E\text{\ensuremath{-}}\mathrm{\ensuremath{\Delta}}E$ telescope. The known high-spin level scheme was extended up to over 11 MeV with a dozen new levels above the neutron decay threshold. The decay pattern appears somewhat atypical for heavy-ion fusion-evaporation reactions. The structure of $^{39}\mathrm{Ar}$ is discussed in terms of the new FSU cross-shell $spsdpf$ interaction fitted to a wide range of nuclei. This interaction has proved quite successful in accounting for the level scheme of $^{39}\mathrm{Ar}$, including the previously suggested fully aligned $\ensuremath{\pi}{f}_{7/2}\ensuremath{\bigotimes}\ensuremath{\nu}{f}_{7/2}17/{2}^{+}$ state and previously discovered analogs of the lowest states in $^{39}\mathrm{Cl}$.

Geoneutrinos in large direct detection experiments

Geoneutrinos can provide a unique insight into Earth's interior, its central engine and its formation history. We study the detection of geoneutrinos in large direct detection experiments, which has been considered non-feasible. We compute the geoneutrino-induced electron and nuclear recoil spectra in different materials, under several optimistic assumptions. We identify germanium as the most promising target element due to the low nuclear recoil energy threshold that could be achieved. The minimum exposure required for detection would be $\mathcal{O}(10)$ tonne-years. The realistic low thresholds achievable in germanium and silicon permit the detection of $^{40}$K geoneutrinos. These are particularly important to determine Earth's formation history but they are below the kinematic threshold of inverse beta decay, the detection process used in scintillator-based experiments.

Evidence for charm-bottom tetraquarks and the mass dependence of heavy-light tetraquark states from lattice QCD

We continue our study of heavy-light four-quark states and find evidence from lattice QCD for the existence of a strong-interaction-stable $I(J^P)=0(1^+)$ $ud\bar{c}\bar{b}$ tetraquark with mass in the range of 15 to 61 MeV below $\bar{D}B^*$ threshold. Since this range includes the electromagnetic $\bar{D}B\gamma$ decay threshold, current uncertainties do not allow us to determine whether such a state would decay electromagnetically, or only weakly. We also perform a study at fixed pion mass, with NRQCD for the heavy quarks, simulating $qq^\prime \bar{b}^\prime \bar{b}$ and $q q^\prime \bar{b}^\prime\bar{b}^\prime$ tetraquarks with $q,\, q^\prime =ud$ or $\ell s$ and variable, unphysical $m_{b^\prime}$ in order to investigate the heavy mass-dependence of such tetraquark states. We find that the dependence of the binding energy follows a phenomenologically-expected form and that, though NRQCD breaks down before $m_{b^\prime}=m_c$ is reached, the results at higher $m_{b^\prime}$ clearly identify the $ud\bar{b}^\prime \bar{b}$ channel as the most likely to support a strong-interaction-stable tetraquark state at $m_{b^\prime}=m_c$. This observation serves to motivate the direct $ud\bar{c}\bar{b}$ simulation. Throughout we use dynamical $n_f=2+1$ ensembles with pion masses $m_\pi=$415, 299, and 164 MeV reaching down almost to the physical point, a relativistic heavy quark prescription for the charm quark, and NRQCD for the bottom quark(s).

First Observation of ^{20}B and ^{21}B.

The most neutron-rich boron isotopes ^{20}B and ^{21}B have been observed for the first time following proton removal from ^{22}N and ^{22}C at energies around 230 MeV/nucleon. Both nuclei were found to exist as resonances which were detected through their decay into ^{19}B and one or two neutrons. Two-proton removal from ^{22}N populated a prominent resonancelike structure in ^{20}B at around 2.5MeV above the one-neutron decay threshold, which is interpreted as arising from the closely spaced 1^{-},2^{-} ground-state doublet predicted by the shell model. In the case of proton removal from ^{22}C, the ^{19}B plus one- and two-neutron channels were consistent with the population of a resonance in ^{21}B 2.47±0.19 MeV above the two-neutron decay threshold, which is found to exhibit direct two-neutron decay. The ground-state mass excesses determined for ^{20,21}B are found to be in agreement with mass surface extrapolations derived within the latest atomic-mass evaluations.

An estimate of the antineutrino energy loss by beta decay on strongly screening iron group nuclei in presupernova

The research on supernovae explosions is one of the most exciting fields in astrophysics. Recent studies show that the electron capture and beta decay play key roles in supernovae explosions. In this paper, the half-lives of some iron group nuclei by β−-decay are investigated due to shell effect, pair effect, and the changes of the decay energy. We also discuss the influence of strong electron screening (SES) on the electron energy, beta decay threshold energy, and the antineutrino energy loss rates (AELR) in β− decay. The AELR increases, and the screening enhancement factor can get to 4.684 in SES due to Q-value correction, shell and pairing effect. The AELR can be suppressed at lower density and higher temperature environment (e.g., 0.1 < ρ7 < 800, T9 > 30). The screening enhancement factor decreases about 14.10% (e.g., for 56Ni).

Searching for states analogous to the C12 Hoyle state in heavier nuclei using the thick target inverse kinematics technique

Identification of alpha cluster states analogous to the 12C Hoyle state in heavier alpha- conjugate nuclei can provide tests of the existence of alpha condensates in nuclei. Such states are predicted for 16O, 20Ne, 24Mg, 28Si etc. at excitation energies slightly above the multi-alpha particle decay threshold, but have not yet been experimentally identified. The Thick Target Inverse Kinematics (TTIK) technique can be used to study the breakup of excited self-conjugate nuclei into many alpha particles. The reaction 20Ne+{\alpha} was studied using a 20Ne beam at 12 MeV/nucleon from the K150 cyclotron at Texas A&M University. The TTIK method was used to study both single {\alpha}-particle emission and multiple {\alpha}-particle decays. Events with alpha multiplicity up to four were analyzed. The analysis of the three {\alpha} - particle emission data allowed the identification of the Hoyle state and other 12C excited states decaying into three alpha particles. The results are shown and compared with other data available in the literature. Although the statistics for events with alpha multiplicity four is low, the data show a structure at about 15.2 MeV that could indicate the existence in 16O of a state analogous to the 12C Hoyle state. This structure is confirmed by the re-analysis of alpha multiplicity four events from a previous experiment performed at 9.7 MeV/nucleon with a similar setup but lower granularity. Moreover, the reconstructed excitation energy of 24Mg for these events peaks at around 34 MeV, very close to the predicted excitation energy for an excited state analogous to the 12C Hoyle state in 24Mg.

Microscopic dynamics of superfluid He4 : A comprehensive study by inelastic neutron scattering

The dynamic structure factor of superfluid $^4$He has been investigated at very low temperatures by inelastic neutron scattering. The measurements combine different incoming energies resulting in an unprecedentedly large dynamic range with excellent energy resolution, covering wave vectors $Q$ up to 5 \AA$^{-1}$ and energies $\omega$ up to 15 meV. A detailed description of the dynamics of superfluid $^4$He is obtained from saturated vapor pressure up to solidification. The single-excitation spectrum is substantially modified at high pressures, as the maxon energy exceeds the roton-roton decay threshold. A highly structured multi-excitation spectrum is observed at low energies, where clear thresholds and branches have been identified. Strong phonon emission branches are observed when the phonon or roton group velocities exceed the sound velocity. The spectrum is found to display strong multi-excitations whenever the single-excitations face disintegration following Pitaevskii's type a or b criteria. At intermediate energies, an interesting pattern in the dynamic structure factor is observed in the vicinity of the recoil energy. All these features, which evolve significantly with pressure, are in very good agreement with the Dynamic Many-body calculations, even at the highest densities, where the correlations are strongest.

Neutron Halos in Excited States of 12B

An experiment was done to search for states with a neutron halo in 12B. The measurements were carried out at the cyclotron of the University of Jyvaskyla (Finland) using Large Scattering Chamber (LSC). The idea of the work was to search for two states with the expected neutron halo, 1 ̄ and 2 ̄. Differential cross sections with excitation of 12B states, including abovementioned states, were observed. The preliminary calculations on halo radii by the method of asymptotic normalization coefficients for the 2 ̄ and 1 ̄ states which are in a discrete spectrum gave following values: 5.6 fm and 7.4 fm, which is much larger than the radius of the valence neutron in the ground state. But strictly the presence of a neutron halo can be confirmed only for 1 ̄ state. The 2 ̄ state can be considered only as candidate for halo. An unexpected result was obtained for the 3 ̄, 3.39 MeV state, which is in continuum 19 keV above the decay threshold 12B → 11B + n, preliminary estimation for its halo radius is ∼ 6.5 fm. This indicates that the halo can be present in this state as well. But strict conditions for neutron halo are not fulfilled in the same way as for 2 ̄ state. Until now, the neutron halo in unbound states has been observed only for the members of the rotational bands.

Collectivization of anti-analog strength above charged particle thresholds

Ten years ago, highly excited states were found in $^{9}\mathrm{Li}$ and $^{10}\mathrm{Be}$ a few hundred kilovolts above the proton decay threshold. These physical states are too low in energy to be the isospin-stretched configuration of the decay channel (the isobaric analog or ${T}_{g}$). However, these states can be understood by a continuum cognizant shell model as strongly mixed states of lower isospin (${T}_{l}$), where the mixing is largely mediated by the open neutron channels but ushered in energy to be just above the proton threshold.

Width of a two-body coupled-channel resonance

We study the width of a two-body resonance in a coupled-channel system. We demonstrate how the width does not come only determined by the available phase space for its decay to the detection channel, but it greatly depends on the relative position of the mass of the resonance with respect to the masses of the coupled-channels generating the state. Our results are consistent with the experimental observation of narrow hadrons lying well above their lowest decay threshold.

Hadronic molecules

Hadrons are composite particles made of quark and gluons. Interestingly, some excited hadronic states resemble the deuteron viewed as a barely bound neutron-proton system. Such hadronic molecules are spatially extended systems lying very close to decay thresholds. This work reviews the current experimental evidence for hadronic molecules and related theoretical descriptions, including effective field theories and lattice quantum chromodynamics.

Stability of Heavy Tetraquarks

We discuss the stability of tetraquark systems with two different masses. After some reminders about the stability of very asymmetric $$QQ{\bar{q}} {\bar{q}}$$ tetraquarks, we demonstrate that in the all-heavy limit $$q \rightarrow Q$$ , the system becomes unstable for standard color-additive models. We also analyze the consequences of symmetry breaking for $$Qq{\bar{Q}}{\bar{q}}$$ configurations: we find a kind of metastability between the lowest threshold $$Q{\bar{Q}} + q{\bar{q}}$$ and the highest one, $$Q{\bar{q}}+{\bar{Q}} q$$ , and we calculate the width of the resonance. Our results are consistent with the experimental observation of narrow hadrons lying well above their lowest decay threshold.

Stability of tetrons

We consider the interactions in a mesonic system, referred here to as ‘tetron’, consisting of two heavy quarks and two lighter antiquarks (which may still be heavy in the scale of QCD), i.e. generally QaQbq¯cq¯d, and study the existence of bound states below the threshold for decay into heavy meson pairs. At a small ratio of the lighter to heavier quark masses an expansion parameter arises for treatment of the binding in such systems. We find that in the limit where all the quarks and antiquarks are so heavy that a Coulomb-like approximation can be applied to the gluon exchange between all of them, such bound states arise when this parameter is below a certain critical value. We find the parametric dependence of the critical mass ratio on the number of colors Nc, and confirm this dependence by numerical calculations. In particular there are no stable tetrons when all constituents have the same mass. We discuss an application of a similar expansion in the large Nc limit to realistic systems where the antiquarks are light and their interactions are nonperturbative. In this case our findings are in agreement with the recent claims from a phenomenological analysis that a stable bbu¯d¯ tetron is likely to exist, unlike those where one or both bottom quarks are replaced by the charmed quark.