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Search for light pseudoscalar bosons, pair-produced in Higgs boson decays in the four-electron final state in proton-proton collisions at 13 TeV
No full textA search for pairs of light neutral pseudoscalar bosons (A) resulting from the decay of a Higgs boson is performed. The search is conducted using LHC proton-proton collision data at 13 TeV, collected with the CMS detector in 2016--2018 and corresponding to an integrated luminosity of 138 fb. The A boson decays into a highly collimated electron-positron pair. A novel multivariate algorithm using tracks and calorimeter information is developed to identify these distinctive signatures, and events are selected with two such merged electron-positron pairs. No significant excess above the standard model background predictions is observed. Upper limits on the branching fraction for are set at 95% confidence level, for masses between 10 and 100 MeV and proper decay lengths below 100 m, reaching branching fraction sensitivities as low as 10. This is the first search for Higgs boson decays to four electrons via light pseudoscalars at the LHC. It significantly improves the experimental sensitivity to axion-like particles with masses below 100 MeV.A search for pairs of light neutral pseudoscalar bosons (A) resulting from the decay of a Higgs boson is performed. The search is conducted using LHC proton-proton collision data at = 13 TeV, collected with the CMS detector in 20162018 and corresponding to an integrated luminosity of 138 fb. The A boson decays into a highly collimated electron-positron pair. A novel multivariate algorithm using tracks and calorimeter information is developed to identify these distinctive signatures, and events are selected with two such merged electron-positron pairs. No significant excess above the standard model background predictions is observed. Upper limits on the branching fraction for H AA 4e are set at 95% confidence level, for masses between 10 and 100 MeV and proper decay lengths below 100 m, reaching branching fraction sensitivities as low as 10. This is the first search for Higgs boson decays to four electrons via light pseudoscalars at the LHC. It significantly improves the experimental sensitivity to axion-like particles with masses below 100 MeV
First Measurement of the Quadrupole Moment of the 21+ State in Sn110
No full textThe Sn isotopic chain, exhibiting double shell closures at Sn100 and Sn132, is a key testing ground for theoretical models of the atomic nucleus. It was originally predicted that the transitional matrix elements between the first 2+ state and the 0+ ground state for the even-even isotopes in this chain should show a simple dependence on the neutron number. This prediction was, however, disproven experimentally in some of the first experiments with postaccelerated radioactive beams, a situation that has remained unresolved ever since. Subsequent theoretical work has suggested that the explanation can be found in proton excitations across the Z=50 shell gap, with an accompanying experimental signature that the first excited 2+ state in Sn110 should have a distinct oblate shape. In this Letter, we present the first measurements of the spectroscopic quadrupole moment of the 21+ state, B(E2;41+→21+) and B(E2;42+→21+) values for Sn110, as well as the B(E2;21+→01+) value with significantly improved precision compared to previous results. From the same experiment, half-lives of the 21+ and 41+ states were measured using the Doppler shift attenuation method. Our combined result, Q(21+)=20(8) efm2 for Sn110, is the largest positive value known among the Sn isotopes, indicating an oblate shape of the state by more than 2σ. Comparison of the E2 transition strengths and quadrupole moments with recent shell model calculations are presented
Measurements and identification strategies for non-collision backgrounds in ATLAS experiment
No full textDuring nominal LHC collisions, protons can interact with residual gas in the beam pipe or with upstream collimators, producing showers of background particles known as Beam-Induced Backgrounds (BIB). These particles do not originate from the real proton-proton interaction point. BIB can significantly impact detector performance and mimic signals in the searches for missing energy or for certain types of new physics, such as neutral long-lived particles. The ATLAS Non-Collision Background group plays a key role in developing tools to identify and reject these backgrounds. The characteristic features of BIB, as observed by the ATLAS beam conditions monitor, the inner detector or as fake jets in calorimeters, are studied in detail in order to identify them and estimate their residual contribution. Series of dedicated pressure bump tests were performed during LHC Run 2 by introducing local pressure bumps at different locations with a gas density higher by orders of magnitude than during normal operation, and then the rates of beam-gas events are estimated from the pressure measurements and pressure bump profiles obtained from calculations. In this paper, an overview of the origins of BIB, the ATLAS BIB online monitoring system, the pressure bump tests, and recent results based on measurements from LHC Run 2 data are provided
Validation of the SiPM-on-Tile Readout Chain for the CMS High Granularity Calorimeter
No full textFor the upcoming high-luminosity LHC, the endcap calorimeters of the CMS experiment will be replaced by the high-granularity calorimeter (HGCAL), a sampling calorimeter using silicon sensors in the front and plastic scintillators read out by SiPMs in the back. We have built and tested a complete slice of scintillator tile modules and readout electronics under realistic installation, grounding and powering conditions. Using this system, we validated the powering scheme, assessed the system stability and demonstrated data readout with the Serenity back-end hardware. The successful validation of the SiPM-on-Tile front-end as a complete system is an important milestone towards the construction and operation of HGCAL
Analytical Soft Functions for Heavy-Quark Final States at Hadron Colliders
No full textWe present the first computation of the complete two-loop, fully-differential soft function describing the production of a heavy-quark pair in association with a color-singlet system at hadron colliders. This result constitutes one of the most complex soft functions known to date and it is obtained in closed analytic form for generic multi-dimensional kinematics. This allows us to obtain novel analytic results for the transverse-momentum-dependent and threshold soft functions in this class of processes. We further obtain a decomposition of the soft function into dipole and tripole color correlators, thereby supplying essential building blocks for processes involving a heavy-quark pair produced together with additional light jets at both hadron and lepton colliders. These results represent a key ingredient for advancing precision predictions for heavy-quark physics at the LHC
