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Bose-Einstein correlations of charged hadrons in proton-proton collisions at 13 Tev
No full textBose-Einstein correlations of charged hadrons are measured over a broad multiplicity range, from a few particles up to about 250 reconstructed charged hadrons in proton-proton collisions at = 13 TeV. The results are based on data collected using the CMS detector at the LHC during runs with a special low-pileup configuration. Three analysis techniques with different degrees of dependence on simulations are used to remove the non-Bose-Einstein background from the correlation functions. All three methods give consistent results. The measured lengths of homogeneity are studied as functions of particle multiplicity as well as average pair transverse momentum and mass. The results are compared with data from both CMS and ATLAS at = 7 TeV, as well as with theoretical predictions.[graphic not available: see fulltext
Search for supersymmetry in pp collisions at 13 TeV with 137 fb in final states with a single lepton using the sum of masses of large-radius jets
No full textResults are reported from a search for new physics beyond the standard model in proton-proton collisions in final states with a single lepton; multiple jets, including at least one jet tagged as originating from the hadronization of a bottom quark; and large missing transverse momentum. The search uses a sample of proton-proton collision data at s=13 TeV, corresponding to 137 fb-1, recorded by the CMS experiment at the LHC. The signal region is divided into categories characterized by the total number of jets, the number of bottom quark jets, the missing transverse momentum, and the sum of masses of large-radius jets. The observed event yields in the signal regions are consistent with estimates of standard model backgrounds based on event yields in the control regions. The results are interpreted in the context of simplified models of supersymmetry involving gluino pair production in which each gluino decays into a top quark-antiquark pair and a stable, unobserved neutralino, which generates missing transverse momentum in the event. Scenarios with gluino masses up to about 2150 GeV are excluded at 95% confidence level (or more) for neutralino masses up to 700 GeV. The highest excluded neutralino mass is about 1250 GeV, which holds for gluino masses around 1850 GeV
Search for physics beyond the standard model in multilepton final states in proton-proton collisions at 13 TeV
No full textA search for physics beyond the standard model in events with at least three charged leptons (electrons or muons) is presented. The data sample corresponds to an integrated luminosity of 137 fb of proton-proton collisions at = 13 TeV, collected with the CMS detector at the LHC in 2016–2018. The two targeted signal processes are pair production of type-III seesaw heavy fermions and production of a light scalar or pseudoscalar boson in association with a pair of top quarks. The heavy fermions may be manifested as an excess of events with large values of leptonic transverse momenta or missing transverse momentum. The light scalars or pseudoscalars may create a localized excess in the dilepton mass spectra. The results exclude heavy fermions of the type-III seesaw model for masses below 880 GeV at 95% confidence level in the scenario of equal branching fractions to each lepton flavor. This is the most restrictive limit on the flavor-democratic scenario of the type-III seesaw model to date. Assuming a Yukawa coupling of unit strength to top quarks, branching fractions of new scalar (pseudoscalar) bosons to dielectrons or dimuons above 0.004 (0.03) and 0.04 (0.03) are excluded at 95% confidence level for masses in the range 15–75 and 108–340 GeV, respectively. These are the first limits in these channels on an extension of the standard model with scalar or pseudoscalar particles.[graphic not available: see fulltext
Dark Matter bound-state formation at higher order: a non-equilibrium quantum field theory approach
No full textThe formation of meta-stable dark matter bound states in coannihilating scenarios could efficiently occur through the scattering with a variety of Standard Model bath particles, where light bosons during the electroweak cross over or even massless photons and gluons are exchanged in the t-channel. The amplitudes for those higher-order processes, however, are divergent in the collinear direction of the in- and out-going bath particles if the mediator is massless. To address the issue of collinear divergences, we derive the bound-state formation collision term in the framework of non-equilibrium quantum field theory. The main result is an expression for a more general cross section, which allows to compute higher-order bound-state formation processes inside the primordial plasma background in a comprehensive manner. Based on this result, we show that next-to-leading order contributions, including the bath-particle scattering, are i) collinear finite and ii) generically dominate over the on-shell emission for temperatures larger than the absolute value of the binding energy. Based on a simplified model, we demonstrate that the impact of these new effects on the thermal relic abundance is significant enough to make it worthwhile to study more realistic coannihilation scenarios
H.E.S.S. and Fermi-LAT observations of PSR B1259-63/LS 2883 during its 2014 and 2017 periastron passages
No full textPSR B1259-63/LS 2883 is a gamma-ray binary system consisting of a pulsar in an eccentric orbit around a bright Oe stellar-type companion star that features a dense circumstellar disc. The high- and very-high-energy (HE, VHE) gamma-ray emission from PSR B1259-63/LS 2883 around the times of its periastron passage are characterised, in particular, at the time of the HE gamma-ray flares reported to have occurred in 2011, 2014, and 2017. Spectra and light curves were derived from observations conducted with the H.E.S.S.-II array in 2014 and 2017. A local double-peak profile with asymmetric peaks in the VHE light curve is measured, with a flux minimum at the time of periastron and two peaks coinciding with the times at which the neutron star crosses the companion's circumstellar disc ( d). A high VHE gamma-ray flux is also observed at the times of the HE gamma-ray flares ( d) and at phases before the first disc crossing ( d). PSR B1259-63/LS 2883 displays periodic flux variability at VHE gamma-rays without clear signatures of super-orbital modulation in the time span covered by H.E.S.S. observations. In contrast, the photon index of the measured power-law spectra remains unchanged within uncertainties for about 200 d around periastron. Lower limits on exponential cut-off energies up to TeV are placed. At HE gamma-rays, PSR B1259-63/LS 2883 has now been detected also before and after periastron, close to the disc crossing times. Repetitive flares with distinct variability patterns are detected in this energy range. Such outbursts are not observed at VHEs, although a relatively high emission level is measured. The spectra obtained in both energy regimes displays a similar slope, although a common physical origin either in terms of a related particle population, emission mechanism, or emitter location is ruled out
Towards Real-time Ion-specific StructuralSensitivity in Nanoporous Carbon Electrodesusing In Situ Anomalous Small-angle X-rayScattering
No full textCurrent in situ techniques to study ion charge storage and electrical double-layer formation in nanoporous electrodes are either chemically sensitive to element-specific concentration changes or structurally sensitive to rearrangements of ions and solvent molecules; but rarely can they cover both. Here we introduce in situ anomalous small-angle X-ray scattering (ASAXS) as a unique method to extract both real-time structural and ion-specific chemical information from one single experiment. Using a 1 M RbBr aqueous electrolyte and a hierarchical micro- and mesoporous carbon electrode, we identify different charging mechanisms for positive and negative applied potentials. We are able not only to track the global concentration change of each ion species individually, but also to observe their individual local rearrangement within the pore space
A mechanochemical route for the synthesis of and its structural re-investigation using structure solution from powder diffraction data
No full textA new and solvent-free synthesis route has been adopted and optimized to prepare crystalline VNbO5 from the mechanochemical reaction between Nb2O5 and V2O5 as starting reagents. The substantially amorphous mixture of equimolar pentoxide V and Nb metals observed after extended mechanical treatment transforms into a crystalline powder following calcination under mild conditions at 710 K. The structure solution of the X-ray diffraction pattern using a global optimization approach, combined with Rietveld refinement, points to a space group P212121 (no. 19) different from Pnma (no. 62) previously proposed in the literature assuming it to be isostructural to VTaO5. The new space group helps to describe weak peaks that remained previously unaccounted for and allows more reliable determination of atomic fractional coordinates and interatomic distance distribution. The as-prepared VNbO5 has been tested as a dopant (5 wt%) for the purpose of solid state hydrogen storage, decreasing significantly the release of hydrogen of MgH2/Mg (620 K) and further enhancing the hydrogen sorption kinetic properties
Light-induced spin transition in the spin-crossover complex FePt2 detected by optical pump -coherent resonant nuclear elastic scattering
No full textWe report the results of optical pump-nuclear resonance probe experiments on the SCO complex [FeII(L-PtII(t-but-tpy))2](BF4)2 with L being 2,6-di(pyrazol-1-yl)-4-(trimethylsilylethynyl)pyridine) and t-but-tpy being 4,4′,4″-Tri-tert-Butyl-2,2′:6′,2″-terpyridine using a novel experimental set-up at the beamline P01, Petra III, DESY Hamburg. We investigate the changes in the spin state of the complex when it is excited by laser pulses of 766 nm wavelength and pulse width < 100 ps. Our simulations of the nuclear forward scattering data indicate a dominant low spin state along with some high spin fraction in the absence of laser pulses. We observe clear changes in the time-spectrum following the instant at which the laser pulse hits the sample. Furthermore, these alterations are recorded as the relative timing of the laser pulses with respect to the synchrotron pulses is varied
VERITAS Detection of LS 5039 and HESS J1825-137
No full textWith 8 h of observations, VERITAS confirms the detection of two very high energy gamma-ray sources. The gamma-ray binary LS 5039 is detected with a statistical significance of 8.8 σ . The measured flux above 1 TeV is (2.5±0.4)×10−12cm−2s−1 near inferior conjunction and (7.8±2.8)×10−13cm−2s−1 near superior conjunction. The pulsar wind nebula HESS J1825-137 is detected with a statistical significance of 6.7 σ and a measured flux above 1 TeV of (3.9±0.8)×10−12cm−2s−1
Low-Temperature Carbon Dioxide Gas Sensor Based on Yolk−Shell Ceria Nanospheres
No full textMonitoring carbon dioxide (CO2) levels is extremely important in a wide range of applications. Although metal oxide-based chemoresistive sensors have emerged as a promising approach for CO2 detection, the development of efficient CO2 sensors at low temperature remains a challenge. Herein, we report a low-temperature hollow nanostructured CeO2-based sensor for CO2 detection. We monitor the changes in the electrical resistance after CO2 pulses in a relative humidity of 70% and show the high performance of the sensor at 100 °C. The yolk–shell nanospheres have not only 2 times higher sensitivity but also significantly increased stability and reversibility, faster response times, and greater CO2 adsorption capacity than commercial ceria nanoparticles. The improvements in the CO2 sensing performance are attributed to hollow and porous structure of the yolk–shell nanoparticles, allowing for enhanced gas diffusion and high specific surface area. We present an easy strategy to enhance the electrical and sensing properties of metal oxides at a low operating temperature that is desirable for practical applications of CO2 sensors