arXiv subjects

Stefan Wallner

Publications and source records attributed to Stefan Wallner.

8 recordsLinked to original sources

arXiv2024

Role of $ρ-ω$ interference in semileptonic $B \to π^+ π^- \ell \bar ν_\ell$ decays

It is long known that interference effects play an important role in understanding the shape of the $π^+π^-$ spectrum of resonances near the threshold. In this manuscript we investigate the role of the $ρ-ω$ interference in the study of semileptonic $B \to π^+ π^- \ell \bar ν_\ell$ decays. We determine for the first time the strong phase between $B \to ρ^0 \ell \bar ν_\ell$ and $B \to ω\ell \bar ν_\ell$ from a recent Belle measurement of the $m_{ππ}$ spectrum of $B \to π^+ π^- \ell \bar ν_\ell$. We find $ ϕ_{ρ-ω} = \left( -46_{-67}^{+155} \right)\unicode{xb0}$ and extract the branching fraction of $\mathcal{B}(B \to ρ^0 \ell \bar ν_\ell) = \left(1.41_{-0.38}^{+0.49} \right) \times 10^{-4} $. In addition, we set a limit on the $S$-wave component within a mass window ranging from $2 m_π$ to $1.02 \, \mathrm{GeV}$ of $ 0.51 \times 10^{-4} \,\, \mathrm{at} \, \, 90\% \, \mathrm{CL} $. We also determine the absolute value of the Cabibbo-Kobayashi-Maskawa matrix element of $|V_{ub}|_{ρ-ω} = \left( 3.03^{+0.49}_{-0.44} \right) \times 10^{-3}$, which takes into account the $ρ-ω$ interference.

hep-ph↗

arXiv2022

Long-term trends of light pollution assessed from SQM measurements and an empirical atmospheric model

We present long-term (4-10 years) trends of light pollution observed at 26 locations, covering rural, intermediate and urban sites, including the three major European metropolitan areas of Stockholm, Berlin and Vienna. Our analysis is based on i) night sky brightness (NSB) measurements obtained with Sky Quality Meters (SQMs) and ii) a rich set of atmospheric data products provided by the European Centre for Medium-Range Weather Forecasts. We describe the SQM data reduction routine in which we filter for moon- and clear-sky data and correct for the SQM "aging" effect using an updated version of the twilight method of Puschnig et al. (2021). Our clear-sky, aging-corrected data reveals short- and long-term (seasonal) variations due to atmospheric changes. To assess long-term anthropogenic NSB trends, we establish an empirical atmospheric model via multi-variate penalized linear regression. Our modeling approach allows to quantitatively investigate the importance of different atmospheric parameters, revealing that surface albedo and vegetation have by far the largest impact on zenithal NSB. Additionally, the NSB is sensitive to black carbon and organic matter aerosols at urban and rural sites respectively. Snow depth was found to be important for some sites, while the total column of ozone leaves impact on some rural places. The average increase in light pollution at our 11 rural sites is 1.7 percent per year. At our nine urban sites we measure an increase of 1.8 percent per year and for the remaining six intermediate sites we find an average increase of 3.7 percent per year. These numbers correspond to doubling times of 41, 39 and 19 years. We estimate that our method is capable of detecting trend slopes shallower/steeper than 1.5 percent per year.

astro-ph.IM↗

arXiv2020

Correcting Sky Quality Meter measurements for aging effects using twilight as calibrator

In the last decade numerous Sky Quality Meters (SQMs) were installed throughout the globe, aiming to assess the temporal change of the night sky brightness (NSB), and thus the change in light pollution. However, it has become clear that SQM readings may be affected by aging effects such as degradation of the sensor sensitivity and/or loss of transmissivity of optical components (filter, housing window). To date, the magnitude of the darkening has not been assessed in a systematic way. We report for the first time on the quantification of the SQM aging effect and describe the applied method. We combine long-term SQM measurements obtained between 2011 and 2019 in Potsdam-Babelsberg (23 km to the southwest of the center of Berlin), Vienna and Stockholm with a readily available empirical twilight model, which serves as calibrator. Twilight SQM observations, calibrated for changing sun altitudes, reveal a linear degradation of the measurement systems (SQM + housing window) with the following slopes: 34$\pm$4, 46$\pm$2 and 53$\pm$2 milli-mag$_{\rm SQM}$ arcsec$^{-2}$ yr$^{-1}$ for Stockholm, Potsdam-Babelsberg and Vienna. With the highest slope found in Vienna (latitude $\sim$48$^\circ$) and the lowest one found in Stockholm (latitude $\sim$59$^\circ$), we find an indication for the dependence of the trend on solar irradiance (which is a function of geographic latitude).

astro-ph.IM↗

arXiv2019

Wave-Selection Techniques for Partial-Wave Analysis in Light-Meson Spectroscopy

The light-meson spectrum can be studied by analyzing data from diffractive dissociation of pion or kaon beams. The contributions of the various states that are produced in these reactions are disentangled by the means of partial-wave analysis. A challenge in these analyses is that the partial-wave expansion has to be truncated, i.e. that only a finite subset of the infinitely many partial-wave amplitudes can be inferred from the data. In recent years, different groups have applied regularization techniques in order to determine the contributing waves from the data. However, to obtain meaningful results the choice of the regularization term is crucial. We present our recent developments of wave-selection methods for partial-wave analyses based on simulated data for diffractively produced three-pion events.

hep-ex↗

arXiv2019

Circalunar variations of the night sky brightness -- an FFT perspective on the impact of light pollution

Circa-monthly activity conducted by moonlight is observed in many species on Earth. Given the vast amount of artificial light at night (ALAN) that pollutes large areas around the globe, the synchronization to the circalunar cycle is often strongly perturbed. Using two-year data from a network of 23 photometers (Sky Quality Meters; SQM) in Austria (latitude ~48°), we quantify how light pollution impacts the recognition of the circalunar periodicity. We do so via frequency analysis of nightly mean sky brightnesses using Fast Fourier Transforms. A very tight linear relation between the mean zenithal night sky brightness (NSB) given in mag$_{SQM}$ and the amplitude of the circalunar signal is found, indicating that for sites with a mean zenithal NSB brighter than 16.5 mag$_{SQM}$ the lunar rhythm practically vanishes. This finding implies that the circalunar rhythm is still detectable (within the broad bandpass of the SQM) at most places around the globe, but its amplitude against the light polluted sky is strongly reduced. We find that the circalunar contrast in zenith is reduced compared to ALAN-free sites by factors of 1/9 in the state capital of Linz (~200,000 inhabitants) and 1/3 in small towns, e.g. Freistadt and Mattighofen, with less than 10,000 inhabitants. Only two of our sites, both situated in national parks (Bodinggraben and Zöblboden), show natural circalunar amplitudes. At our urban sites we further detect a strong seasonal signal that is linked to the amplification of anthropogenic skyglow during the winter months due to climatological conditions.

astro-ph.IM↗

arXiv2019

A revisit to the enigmatic variable star 21 Comae

The magnetic chemically peculiar (Ap/CP2) star 21 Com has been extensively studied in the past, albeit with widely differing and sometimes contradictory results, in particular concerning the occurrence of short term variability between about 5 to 90 minutes. We have performed a new investigation of 21 Com using MOST satellite and high-cadence ground-based photometry, time series spectroscopy, and evolutionary and pulsational modeling. Our analysis confirms that 21 Com is a classical CP2 star showing increased abundances of, in particular, Cr and Sr. From spectroscopic analysis, we have derived Teff = 8900(200) K, log g = 3.9(2), and vsini = 63(2) km/s. Our modeling efforts suggest that 21 Com is a main sequence (MS) star seen equator-on with a mass of 2.29(10) M(Sun) and a radius of R = 2.6(2) R(Sun). Our extensive photometric data confirm the existence of rotational light variability with a period of 2.05219(2) d. However, no significant frequencies with a semi-amplitude exceeding 0.2 mmag were found in the frequency range from 5 to 399 c/d. Our RV data also do not indicate short-term variability. We calculated pulsational models assuming different metallicities and ages, which do not predict the occurrence of unstable modes. The star 18 Com, often employed as comparison star for 21 Com in the past, has been identified as a periodic variable (P = 1.41645 d). While it is impossible to assess whether 21 Com has exhibited short-term variability in the past, the new observational data and several issues/inconsistencies identified in previous studies strongly suggest that 21 Com is neither a delta Scuti nor a roAp pulsator but a "well-behaved" CP2 star exhibiting its trademark rotational variability.

astro-ph.SR↗

arXiv2018

Light-Quark Resonances at COMPASS

The main goal of the spectroscopy program at COMPASS is to explore the light-meson spectrum in the mass range below about $2\,\text{GeV}/c^2$ using diffractive dissociation reactions. Our flagship channel is the production of three charged pions in the reaction: $π^- + p \to π^-π^-π^+ + p_\text{recoil}$, for which COMPASS has acquired the so far world's largest dataset of roughly $50\,\text{M}$ exclusive events using an $190\,\text{GeV}/c$ $π^-$ beam. In order to extract the parameters of the $π_J$ and $a_J$ resonances that appear in the $π^-π^-π^+$ system, we performed the so far most comprehensive resonance-model fit, using Breit-Wigner parametrizations. This method in combination with the high statistical precision of our data allows us to study ground and excited states. We study the $a_4(2040)$ resonance in the $ρ(770)πG$ and $f_2(1270)πF$ decays. In addition to the ground state resonance $a_1(1260)$, we have found evidence for the $a_1(1640)$. We also study the spectrum of $π_2$ states by simultaneously describing four $J^{PC}=2^{-+}$ waves using three $π_2$ resonances, the $π_2(1670)$, the $π_2(1880)$, and the $π_2(2005)$. Using a novel analysis approach, where the resonance-model fit is performed simultaneously in narrow bins of the squared four-momentum transfer $t'$ between the beam pion and the target proton, allows us to study the $t'$ dependence of resonant and non-resonant components included in our model. We observe that for most of the partial waves, the non-resonant components show a steeper $t'$ spectrum compared to the resonances and that the $t'$ spectrum of most of the resonances becomes shallower with increasing resonance mass. We also study the $t'$ dependence of the relative phases between resonance components. The pattern we observe is consistent with a common production mechanism of these states.

hep-ex↗

arXiv2017

Recent Results on Light-Meson Spectroscopy from COMPASS

The main goal of the spectroscopy program at COMPASS is to explore the light-meson spectrum below about $2\,\text{GeV}/c^2$ in diffractive production. Our flagship channel is the decay into three charged pions: $p + π^-\to π^-π^-π^+ + p_\text{recoil}$, for which COMPASS has acquired the so far world's largest dataset of roughly $50\,\text{M}$ exclusive events using an $190\,\text{GeV}/c$ $π^-$ beam. Based on this dataset, we performed an extensive partial-wave analysis. In order to extract the resonance parameters of the $π_J$ and $a_J$ states that appear in the $π^-π^-π^+$ system, we performed the so far largest resonance-model fit, using Breit-Wigner resonances and non-resonant contributions. This method in combination with the high statistical precision of our measurement allows us to study ground and excited states. We have found an evidence of the $a_1(1640)$ and $a_2(1700)$ in our data, which are the first excitations of the $a_1(1260)$ and $a_2(1320)$, respectively. The relative strength of the excited states with respect to the corresponding ground state is larger in the $f_2(1270)\,π$ decay mode compared to the $ρ(770)\,π$ decay mode. We also study the spectrum of $π_2$ states in our data. Therefore, we simultaneously describe four $J^{PC}=2^{-+}$ waves in the resonance-model fit by using three $π_2$ resonances, the $π_2(1670)$, the $π_2(1880)$, and the $π_2(2005)$. Within the limits of our model, we can conclude that the $π_2(2005)$ is required to describe all four $2^{-+}$ waves properly.

hep-ex↗