Compton-like dark photon production in electron-nucleus collisions

The Compton-like production of massive dark photons is investigated in ultrarelativistic electron-ion collisions considering the kinetic mixing between the dark photon and the Standard Model photon. The quasi-real photons in the heavy ion are described by the EPA approximation and the model is employed to calculate the integrated cross section and event rates as a function of the dark photon mass, mγ′, and mixing parameter, ε. Predictions are shown for electron-ion colliders (EICs) in the mass range 100 ≤ mγ′ ≤ 500 MeV. Numerical results are provided within the kinematic coverage of the planned machines Electron-ion collider in China (EicC), A Polarized Electron-Ion Collider at Jefferson Lab (JLEIC), Electron Ion Collider/USA (EIC), Large Hadron Electron Collider (LHeC) and Future Circular Collider (FCC-eA). It complements existing search strategies for dark photons in the considered mass interval.

Point break: using machine learning to uncover a critical mass in women's representation

Decades of research has debated whether women first need to reach a “critical mass” in the legislature before they can effectively influence legislative outcomes. This study contributes to the debate using supervised tree-based machine learning to study the relationship between increasing variation in women's legislative representation and the allocation of government expenditures in three policy areas: education, healthcare, and defense. We find that women's representation predicts spending in all three areas. We also find evidence of critical mass effects as the relationships between women's representation and government spending are nonlinear. However, beyond critical mass, our research points to a potential critical mass interval or critical limit point in women's representation. We offer guidance on how these results can inform future research using standard parametric models.

Measurement of light-by-light scattering and search for axion-like particles with 2.2 nb−1 of Pb+Pb data with the ATLAS detector

This paper describes a measurement of light-by-light scattering based on Pb+Pb collision data recorded by the ATLAS experiment during Run 2 of the LHC. The study uses 2.2 nb−1 of integrated luminosity collected in 2015 and 2018 at $$ \sqrt{s_{\mathrm{NN}}} $$ s NN = 5.02 TeV. Light-by-light scattering candidates are selected in events with two photons produced exclusively, each with transverse energy $$ {E}_{\mathrm{T}}^{\gamma } $$ E T γ > 2.5 GeV, pseudorapidity |ηγ| < 2.37, diphoton invariant mass mγγ> 5 GeV, and with small diphoton transverse momentum and diphoton acoplanarity. The integrated and differential fiducial cross sections are measured and compared with theoretical predictions. The diphoton invariant mass distribution is used to set limits on the production of axion-like particles. This result provides the most stringent limits to date on axion-like particle production for masses in the range 6–100 GeV. Cross sections above 2 to 70 nb are excluded at the 95% CL in that mass interval.

Kinematic age determinations of planetary nebula central stars

Central stars of planetary nebulae (CSPN) have a relatively large mass interval, so that it is expected that these stars also have different ages, typically above 1 Gyr. Apart from the properties of the CSPN themselves, the problem of age determination is also important in the context of the chemical evolution of the Galaxy, for instance in the understanding of the time variation of chemical abundance gradients. In this work, we estimated the ages of a sample of CSPN on the basis of some correlations between their kinematic properties and the expected ages. According to these correlations, the observed dispersions in the U, V, W velocities are uniquely defined by the stellar ages. The adopted correlations were derived from the recent Geneva-Copenhagen survey of galactic stars. Preliminary results suggest the most CSPN in the galactic disk have ages under 3 Gyr. These results are also compared with some recent age distributions based on independent correlations involving the nebular chemical abundances.

The Substellar Population in σ Orionis

The σ Orionis cluster (~3 Myr, 350 pc) is an ideal site to investigate the early evolution of substellar (brown dwarf and planetary mass) objects. To date, the cluster photometric and spectroscopic sequence of free-floaters is known for a wide mass range from 1 M⊙ down to roughly 3 MJup. The substellar domain covers spectral types that go from mid-M classes to the recently defined “methane” T-types, i.e., surface temperatures between ~3000K and 800 K. We derive a rising initial substellar mass function in the mass interval of 150–5 MJup (dN/dM ~ M-α, with α = 0.9 ± 0.4). We also find evidence for a extension of this mass function toward lower masses down to 2–3 MJup. This indicates that the population of isolated planetary mass objects with masses below the deuterium burning threshold is rather abundant in the cluster.

Effects of data incompleteness and metallicity on the mass functions of young LMC star clusters

The young globular clusters in the Magellanic Clouds offer a good number statistic and a reasonably wide mass interval which are required for the derivation of any statistically reliable slope of the Initial Mass Function (IMF). Elson et al. (1989) and Mateo (1988) are amongst those few who utilized this potential first. These authors, however, arrive at different conclusions. Elson et al. find quite flat mass function slopes in comparison with the values given by Mateo. Here we present IMF slopes based on B, V CCD photometry for four young LMC clusters, NGC 1711, 2004, 2164 and 2214 and discuss the effects on them of cluster metallicity and of uncertainties in the incompleteness of the data.

Remarks on the Size Distribution of Colliding and Fragmenting Particles

This paper is concerned with some aspects of determining the evolution of the size distribution of a finite number of mutually colliding and fragmenting particles such as the asteroids or interplanetary dust. If n(m, t) is the number of particles per unit volume per mass interval at time t, then n = dn/dt is the rate at which that number changes with time. This rate can be calculated if the laws are known according to which the colliding bodies erode one another and fragment and if the influence of collisions on the motion of the particles is known. To reduce the complexity of the problem, one assumes that the speed of approach between the bodies is always the same vcoll and that they, as well as the debris, occupy a fixed volume (“particles in a box”). Only collisions between two bodies are considered, and the way in which erosion and fragmentation occurs at a given value of vcoll depends only on their masses. The particles are assumed to be spherical.