arXiv subjects

M. Kraus

Publications and source records attributed to M. Kraus.

At least 55 records · Page 3Linked to original sources

arXiv2018

Top quark mass studies with $t\bar{t}j$ at the LHC

A precise measurement of the top quark mass, a fundamental parameter of the Standard Model, is among the most important goals of top quark studies at the Large Hadron Collider. Apart from the standard methods, numerous new observables and reconstruction techniques are employed to improve the overall precision and to provide different sensitivities to various systematic uncertainties. Recently, the normalised inverse invariant mass distribution of the $t\bar{t}$ system and the leading extra jet not coming from the top quark decays has been proposed for the $pp \to t\bar{t}j$ production process, denoted as ${\cal R}(m_t^{pole},ρ_s)$. In this paper, a thorough study of different theoretical predictions for this observable, however, with top quark decays included, is carried out. We focus on fixed order NLO QCD calculations for the di-lepton top quark decay channel at the LHC with $\sqrt{s}=13$ TeV. First, the impact on the extraction of $m_t$ is investigated and afterwards the associated uncertainties are quantified. In one approach we include all interferences, off-shell effects and non-resonant backgrounds. This is contrasted with a different approach with top quark decays in the narrow width approximation. In the latter case, two cases are employed: NLO QCD corrections to the $pp\to t\bar{t}j$ production process with leading order decays and the more sophisticated case with QCD corrections and jet radiation present also in top quark decays. The top quark mass sensitivity of ${\cal R}(m_t^{pole},ρ_s)$ is investigated and compared to other observables: the invariant mass of the top anti-top pair, the minimal invariant mass of the $b$-jet and a charged lepton as well as the total transverse momentum of the $t\bar{t}j$ system.

hep-ph↗

arXiv2017

Detection of new eruptions in the Magellanic Clouds LBVs R 40 and R 110

We performed a spectroscopic and photometric analysis to study new eruptions in two luminous blue variables (LBVs) in the Magellanic Clouds. We detected a strong new eruption in the LBV R40 that reached $V \sim 9.2$ in 2016, which is around $1.3$ mag brighter than the minimum registered in 1985. During this new eruption, the star changed from an A-type to a late F-type spectrum. Based on photometric and spectroscopic empirical calibrations and synthetic spectral modeling, we determine that R\,40 reached $T_{\mathrm{eff}} = 5800-6300$~K during this new eruption. This object is thereby probably one of the coolest identified LBVs. We could also identify an enrichment of nitrogen and r- and s-process elements. We detected a weak eruption in the LBV R 110 with a maximum of $V \sim 9.9$ mag in 2011, that is, around $1.0$ mag brighter than in the quiescent phase. On the other hand, this new eruption is about $0.2$ mag fainter than the first eruption detected in 1990, but the temperature did not decrease below 8500 K. Spitzer spectra show indications of cool dust in the circumstellar environment of both stars, but no hot or warm dust was present, except by the probable presence of PAHs in R\,110. We also discuss a possible post-red supergiant nature for both stars.

astro-ph.SR↗

arXiv2017

Off-shell $t\bar{t}j$ production and top quark mass studies at the LHC

Precision studies of the properties of the top quark represent a cornerstone of the LHC physics program. In this contribution we focus on the production of $t\bar{t}$ pairs in association with one hard jet and in particular on its connection with precision measurements of the top quark mass at the LHC. We report a summary of a full calculation of the process $pp \to e^+ν_eμ^-\barν_μb \bar{b}j$ at NLO QCD accuracy, which describes $t\bar{t}j$ production with leptonic decays beyond the Narrow Width Approximation (NWA), and discuss the impact of the off-shell effects through comparisons with NWA. Finally we explore the sensitivity of $t\bar{t}j$ in the context of top-quark mass extraction with the template method, considering two benchmark observables as case studies.

hep-ph↗

arXiv2017

A pulsed, mono-energetic and angular-selective UV photo-electron source for the commissioning of the KATRIN experiment

The KATRIN experiment aims to determine the neutrino mass scale with a sensitivity of 200 meV/c^2 (90% C.L.) by a precision measurement of the shape of the tritium $β$-spectrum in the endpoint region. The energy analysis of the decay electrons is achieved by a MAC-E filter spectrometer. To determine the transmission properties of the KATRIN main spectrometer, a mono-energetic and angular-selective electron source has been developed. In preparation for the second commissioning phase of the main spectrometer, a measurement phase was carried out at the KATRIN monitor spectrometer where the device was operated in a MAC-E filter setup for testing. The results of these measurements are compared with simulations using the particle-tracking software "Kassiopeia", which was developed in the KATRIN collaboration over recent years.

physics.ins-det↗

arXiv2016

Off-shell Top Quarks with One Jet at the LHC: A comprehensive analysis at NLO QCD

We present a comprehensive study of the production of top quark pairs in association with one hard jet in the di-lepton decay channel at the LHC. Our predictions, accurate at NLO in QCD, focus on the LHC Run II with a center-of-mass energy of 13 TeV. All resonant and non-resonant contributions at the perturbative order ${\cal O}(α_s^4 α^4)$ are taken into account, including irreducible backgrounds to $t\bar{t}j$ production, interferences and off-shell effects of the top quark and the $W$ gauge boson. We extensively investigate the dependence of our results upon variation of renormalisation and factorisation scales and parton distribution functions in the quest for an accurate estimate of the theoretical uncertainties. Additionally, we explore a few possibilities for a dynamical scale choice with the goal of stabilizing the perturbative convergence of the differential cross sections far away from the $t\bar{t}$ threshold. Results presented here are particularly relevant for searches of new physics as well as for precise measurements of the top-quark fiducial cross sections and top-quark properties at the LHC.

hep-ph↗

arXiv2016

Clumpy molecular structures revolving the B[e] supergiant MWC 137

The peculiar emission-line star MWC 137 with its extended optical nebula was recently classified as B[e] supergiant. To study the spatial distribution of its circumstellar molecular gas on small and large scales, we obtained near-infrared and radio observations using SINFONI and APEX, respectively. We find that the hot CO gas is arranged in moving clumpy ring and shell structures close to the star, while a cold CO envelope is encircling the borders of the optical nebula from the south to the west.

astro-ph.SR↗

arXiv2016

The Gamma-ray Cherenkov Telescope for the Cherenkov Telescope Array

The Cherenkov Telescope Array (CTA) is a forthcoming ground-based observatory for very-high-energy gamma rays. CTA will consist of two arrays of imaging atmospheric Cherenkov telescopes in the Northern and Southern hemispheres, and will combine telescopes of different types to achieve unprecedented performance and energy coverage. The Gamma-ray Cherenkov Telescope (GCT) is one of the small-sized telescopes proposed for CTA to explore the energy range from a few TeV to hundreds of TeV with a field of view $\gtrsim 8^\circ$ and angular resolution of a few arcminutes. The GCT design features dual-mirror Schwarzschild-Couder optics and a compact camera based on densely-pixelated photodetectors as well as custom electronics. In this contribution we provide an overview of the GCT project with focus on prototype development and testing that is currently ongoing. We present results obtained during the first on-telescope campaign in late 2015 at the Observatoire de Paris-Meudon, during which we recorded the first Cherenkov images from atmospheric showers with the GCT multi-anode photomultiplier camera prototype. We also discuss the development of a second GCT camera prototype with silicon photomultipliers as photosensors, and plans toward a contribution to the realisation of CTA.

astro-ph.IM↗

arXiv2016

Inauguration and First Light of the GCT-M Prototype for the Cherenkov Telescope Array

The Gamma-ray Cherenkov Telescope (GCT) is a candidate for the Small Size Telescopes (SSTs) of the Cherenkov Telescope Array (CTA). Its purpose is to extend the sensitivity of CTA to gamma-ray energies reaching 300 TeV. Its dual-mirror optical design and curved focal plane enables the use of a compact camera of 0.4 m diameter, while achieving a field of view of above 8 degrees. Through the use of the digitising TARGET ASICs, the Cherenkov flash is sampled once per nanosecond continuously and then digitised when triggering conditions are met within the analogue outputs of the photosensors. Entire waveforms (typically covering 96 ns) for all 2048 pixels are then stored for analysis, allowing for a broad spectrum of investigations to be performed on the data. Two prototypes of the GCT camera are under development, with differing photosensors: Multi-Anode Photomultipliers (MAPMs) and Silicon Photomultipliers (SiPMs). During November 2015, the GCT MAPM (GCT-M) prototype camera was integrated onto the GCT structure at the Observatoire de Paris-Meudon, where it observed the first Cherenkov light detected by a prototype instrument for CTA.

astro-ph.IM↗

arXiv2016

TARGET: A Digitizing And Trigger ASIC For The Cherenkov Telescope Array

The future ground-based gamma-ray observatory Cherenkov Telescope Array (CTA) will feature multiple types of imaging atmospheric Cherenkov telescopes, each with thousands of pixels. To be affordable, camera concepts for these telescopes have to feature low cost per channel and at the same time meet the requirements for CTA in order to achieve the desired scientific goals. We present the concept of the TeV Array Readout Electronics with GSa/s sampling and Event Trigger (TARGET) Application Specific Circuit (ASIC), envisaged to be used in the cameras of various CTA telescopes, e.g. the Gamma-ray Cherenkov Telescope (GCT), a proposed 2-Mirror Small-Sized Telescope, and the Schwarzschild-Couder Telescope (SCT), a proposed Medium-Sized Telescope. In the latest version of this readout concept the sampling and trigger parts are split into dedicated ASICs, TARGET C and T5TEA, both providing 16 parallel input channels. TARGET C features a tunable sampling rate (usually 1 GSa/s), a 16k sample deep buffer for each channel and on-demand digitization and transmission of waveforms with typical spans of ~100 ns. The trigger ASIC, T5TEA, provides 4 low voltage differential signal (LVDS) trigger outputs and can generate a pedestal voltage independently for each channel. Trigger signals are generated by T5TEA based on the analog sum of the input in four independent groups of four adjacent channels and compared to a threshold set by the user. Thus, T5TEA generates four LVDS trigger outputs, as well as 16 pedestal voltages fed to TARGET C independently for each channel. We show preliminary results of the characterization and testing of TARGET C and T5TEA.

astro-ph.IM↗

arXiv2016

B[e] Supergiants' circumstellar environment: disks or rings?

B[e] Supergiants are a phase in the evolution of some massive stars for which we have observational evidence but no predictions by any stellar evolution model. The mass-loss during this phase creates a complex circumstellar environment with atomic, molecular, and dust regions usually found in rings or disk-like structures. However, the detailed structure and the formation of the circumstellar environment are not well-understood, requiring further investigation. To address that we initiated an observing campaign to obtain a homogeneous set of high-resolution spectra in both the optical and NIR (using MPG-ESO/FEROS, GEMINI/Phoenix and VLT/CRIRES, respectively). We monitor a number of Galactic B[e] Supergiants, for which we examined the [OI] and [CaII] emission lines and the bandheads of the CO and SiO molecules to probe the structure and the kinematics of their formation regions. We find that the emission from each tracer forms either in a single or in multiple equatorial rings.

astro-ph.SR↗

arXiv2016

The GCT camera for the Cherenkov Telescope Array

The Gamma-ray Cherenkov Telescope (GCT) is proposed for the Small-Sized Telescope component of the Cherenkov Telescope Array (CTA). GCT's dual-mirror Schwarzschild-Couder (SC) optical system allows the use of a compact camera with small form-factor photosensors. The GCT camera is ~0.4 m in diameter and has 2048 pixels; each pixel has a ~0.2 degree angular size, resulting in a wide field-of-view. The design of the GCT camera is high performance at low cost, with the camera housing 32 front-end electronics modules providing full waveform information for all of the camera's 2048 pixels. The first GCT camera prototype, CHEC-M, was commissioned during 2015, culminating in the first Cherenkov images recorded by a SC telescope and the first light of a CTA prototype. In this contribution we give a detailed description of the GCT camera and present preliminary results from CHEC-M's commissioning.

astro-ph.IM↗

arXiv2016

Inhomogeneous molecular ring around the B[e] supergiant LHA 120-S 73

We aim to improve our knowledge on the structure and dynamics of the circumstellar disk of the LMC B[e] supergiant LHA 120-S 73. High-resolution optical and near-IR spectroscopic data were obtained over a period of 16 and 7 years, respectively. The spectra cover the diagnostic emission lines from [CaII] and [OI], as well as the CO bands. These features trace the disk at different distances from the star. We analyzed the kinematics of the individual emission regions by modeling their emission profiles. A low-resolution mid-infrared spectrum was obtained as well, which provides information on the composition of the dusty disk. All diagnostic emission features display double-peaked line profiles, which we interpret as due to Keplerian rotation. We find that LHA 120-S 73 is surrounded by at least four individual rings of material with alternating densities (or by a disk with strongly non-monotonic radial density distribution). Moreover, we find that the molecular ring must have gaps or at least strong density inhomogeneities, or in other words, a clumpy structure. The mid-infrared spectrum displays features of oxygen- and carbon-rich grain species, which indicates a long-lived, stable dusty disk. We cannot confirm the previously reported high value for the stellar rotation velocity. The line profile of HeI 5876 A is strongly variable in both width and shape and resembles of those seen in non-radially pulsating stars. A proper determination of the real underlying stellar rotation velocity is hence not possible. The existence of multiple stable and clumpy rings of alternating density recalls ring structures around planets. Although there is currently insufficient observational evidence, it is tempting to propose a scenario with one (or more) minor bodies or planets revolving around LHA 120-S 73 and stabilizing the ring system, in analogy to the shepherd moons in planetary systems.

astro-ph.SR↗

arXiv2016

Commissioning of the vacuum system of the KATRIN Main Spectrometer

The KATRIN experiment will probe the neutrino mass by measuring the beta-electron energy spectrum near the endpoint of tritium beta-decay. An integral energy analysis will be performed by an electro-static spectrometer (Main Spectrometer), an ultra-high vacuum vessel with a length of 23.2 m, a volume of 1240 m^3, and a complex inner electrode system with about 120000 individual parts. The strong magnetic field that guides the beta-electrons is provided by super-conducting solenoids at both ends of the spectrometer. Its influence on turbo-molecular pumps and vacuum gauges had to be considered. A system consisting of 6 turbo-molecular pumps and 3 km of non-evaporable getter strips has been deployed and was tested during the commissioning of the spectrometer. In this paper the configuration, the commissioning with bake-out at 300°C, and the performance of this system are presented in detail. The vacuum system has to maintain a pressure in the 10^{-11} mbar range. It is demonstrated that the performance of the system is already close to these stringent functional requirements for the KATRIN experiment, which will start at the end of 2016.

physics.ins-det↗

arXiv2016

Top Quark Pair Production in Association with a Jet with NLO QCD Off-Shell Effects at the Large Hadron Collider

We present a complete description of top quark pair production in association with a jet in the dilepton channel. Our calculation is accurate to next-to-leading order in QCD (NLO) and includes all non-resonant diagrams, interferences and off-shell effects of the top quark. Moreover, non-resonant and off-shell effects due to the finite W gauge boson width are taken into account. This calculation constitutes the first fully realistic NLO computation for top quark pair production with a final state jet in hadronic collisions. Numerical results for differential distributions as well as total cross sections are presented for the Large Hadron Collider (LHC) at 8 TeV. With our inclusive cuts, NLO predictions reduce the unphysical scale dependence by more than a factor of 3 and lower the total rate by about 13% compared to leading order QCD (LO) predictions. In addition, the size of the top quark off-shell effects is estimated to be below 2%.

hep-ph↗

arXiv2015

TARGET: toward a solution for the readout electronics of the Cherenkov Telescope Array

TARGET is an application specific integrated circuit (ASIC) designed to read out signals recorded by the photosensors in cameras of very-high-energy gamma-ray telescopes exploiting the imaging of Cherenkov radiation from atmospheric showers. TARGET capabilities include sampling at a high rate (typically 1 GSample/s), digitization, and triggering on the sum of four adjacent pixels. The small size, large number of channels read out per ASIC (16), low cost per channel, and deep buffer for trigger latency (~16 $μ$s at 1 GSample/s) make TARGET ideally suited for the readout in systems with a large number of telescopes instrumented with compact photosensors like multi-anode or silicon photomultipliers combined with dual-mirror optics. The possible advantages of such systems are better sensitivity, a larger field of view, and improved angular resolution. The two latest generations of TARGET ASICs, TARGET 5 and TARGET 7, are soon to be used for the first time in two prototypes of small-sized and medium-sized dual-mirror telescopes proposed in the framework of the Cherenkov Telescope Array (CTA) project. In this contribution we report on the performance of the TARGET ASICs and discuss future developments.

astro-ph.IM↗

arXiv2015

The first GCT camera for the Cherenkov Telescope Array

The Gamma Cherenkov Telescope (GCT) is proposed to be part of the Small Size Telescope (SST) array of the Cherenkov Telescope Array (CTA). The GCT dual-mirror optical design allows the use of a compact camera of diameter roughly 0.4 m. The curved focal plane is equipped with 2048 pixels of ~0.2° angular size, resulting in a field of view of ~9°. The GCT camera is designed to record the flashes of Cherenkov light from electromagnetic cascades, which last only a few tens of nanoseconds. Modules based on custom ASICs provide the required fast electronics, facilitating sampling and digitisation as well as first level of triggering. The first GCT camera prototype is currently being commissioned in the UK. On-telescope tests are planned later this year. Here we give a detailed description of the camera prototype and present recent progress with testing and commissioning.

astro-ph.IM↗

arXiv2015

Disk tracing for B[e] supergiants in the Magellanic Clouds

B[e] supergiants are evolved massive stars with a complex circumstellar environment. A number of important emission features probe the structure and the kinematics of the circumstellar material. In our survey of Magellanic Cloud B[e] supergiants we focus on the [OI] and [CaII] emission lines, which we identified in four more objects.

astro-ph.SR↗

arXiv2015

Interplay between pulsations and mass loss in the blue supergiant 55 Cygnus = HD 198478

Blue supergiant stars are known to display photometric and spectroscopic variability that is suggested to be linked to stellar pulsations. Pulsational activity in massive stars strongly depends on the star's evolutionary stage and is assumed to be connected with mass-loss episodes, the appearance of macroturbulent line broadening, and the formation of clumps in the wind. To investigate a possible interplay between pulsations and mass-loss, we carried out an observational campaign of the supergiant 55 Cyg over a period of five years to search for photospheric activity and cyclic mass-loss variability in the stellar wind. We modeled the H, He I, Si II and Si III lines using the nonlocal thermal equilibrium atmosphere code FASTWIND and derived the photospheric and wind parameters. In addition, we searched for variability in the intensity and radial velocity of photospheric lines and performed a moment analysis of the line profiles to derive frequencies and amplitudes of the variations. The Halpha line varies with time in both intensity and shape, displaying various types of profiles: P Cygni, pure emission, almost complete absence, and double or multiple peaked. The star undergoes episodes of variable mass-loss rates that change by a factor of 1.7-2 on different timescales. We also observe changes in the ionization rate of Si II and determine a multiperiodic oscillation in the He I absorption lines, with periods ranging from a few hours to 22.5 days. We interpret the photospheric line variations in terms of oscillations in p-, g-, and strange modes. We suggest that these pulsations can lead to phases of enhanced mass loss. Furthermore, they can mislead the determination of the stellar rotation. We classify the star as a post-red supergiant, belonging to the group of alpha Cyg variables.

astro-ph.SR↗