Quality Assurance of Diamond Radiation Detectors

Semiconductor radiation detectors are devices used to detect electromagnetic and particle radiation. The signal formation is based on the transportation of charges between the valence band and conduction band. The interaction between the detector material and the radiation generates free electrons and holes that move in opposite directions in the electric field applied between the electrodes. The movement of charges induces a current in the external electrical circuit, which can be used for particle identification, measurement of energy or momentum, timing, or tracking. There are several different detector materials and designs and, new options are continuously developed. Diamond is a detector material that has received a great amount of interest in many fields. This is due to its many unique properties. Many of them arise from the diamond crystal structure and the strength of the bond between the carbon atoms. The tight and rigid structure makes diamond a strong and durable material, which allows operation of diamond detectors in harsh radiation environments. This, combined with the fast signal formation and short response time makes diamond detector an excellent choice for high energy physics applications. The diamond structure leads also to a wide band gap. Thanks to the wide band bap, diamond detectors have low leakage current and they can be operated even in high temperatures without protection from surrounding light. Especially electrical properties of semiconductors strongly depend on the concentration of impurities and crystal defects. Determination of electrical properties can therefore be used to study the crystal quality of the material. The electrical properties of the material determine the safe operational region of the device and knowledge of the leakage current and the charge carrier transportation mechanism are required for optimized operation of detectors. Characterization of electrical properties is therefore an important part of semiconductor device fabrication. Electrical characterization should be done at different stages of the fabrication in order to detect problems at an early stage and to get an idea of what could have caused them. This work describes the quality assurance process of single crystal CVD (chemical vapour deposition) diamond detectors for the PPS-detectors for the CMS-experiment. The quality assurance process includes visual inspection of the diamond surfaces and dimensions by optical and cross polarized light microscopy, and electrical characterization by measurement of leakage current and CCE (charge collection efficiency). The CCE measurement setup was improved with a stage controller, which allows automatic measurement of CCE in several positions on the diamond detector. The operation of the new setup and the reproducibility of the results were studied by repeated measurements of a reference diamond. The setup could successfully be used to measure CCE over the whole diamond surface. However, the measurement uncertainty is quite large. Further work is needed to reduce the measurement uncertainty and to determine the correlation between observed defects and the measured electrical properties

Search for invisible decays of the Higgs boson produced via vector boson fusion in proton-proton collisions at root s=13 TeV

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Observation of B-0 ->psi(2S)K-S(0)pi(+)pi(-) and B-s(0)->psi(2S)K-S(0) decays

Using a data sample of root s = 13 TeV protonproton collisions collected by the CMS experiment at the LHC in 2017 and 2018 with an integrated luminosity of 103 fb(-1), the B-s(0) -> psi(2S)K-S(0) and B-0 -> psi(2S)K-S(0) pi(+) pi(-) decays are observed with significances exceeding 5 standard deviations. The resulting branching fraction ratios, measured for the first time, correspond to B(B-s(0) -> psi(2S)K-S(0))/B(B-0 Zeta -> psi(2S)K-S(0)) = (3.33 +/- 0.69(stat) +/- 0.11 (syst) +/- 0.34 (f(s)/f(d))) x 10(-2) and B(B-0 -> psi(2S)K-S(0) pi(+) pi(-))/B(B-0 -> psi(2S)K-S(0)) = 0.480 +/- 0.013 (stat) +/- 0.032 (syst), where the last uncertainty in the first ratio is related to the uncertainty in the ratio of production cross sections of B-s(0) and B-0 mesons, f(s)/f(d).Peer reviewe

Search for resonant production of strongly coupled dark matter in proton-proton collisions at 13 TeV

The first collider search for dark matter arising from a strongly coupled hidden sector is presented and uses a data sample corresponding to 138 fb(-1), collected with the CMS detector at the CERN LHC, at root s = 13 TeV. The hidden sector is hypothesized to couple to the standard model (SM) via a heavy leptophobic Z' mediator produced as a resonance in proton-proton collisions. The mediator decay results in two "semivisible" jets, containing both visible matter and invisible dark matter. The final state therefore includes moderate missing energy aligned with one of the jets, a signature ignored by most dark matter searches. No structure in the dijet transverse mass spectra compatible with the signal is observed. Assuming the Z' boson has a universal coupling of 0.25 to the SM quarks, an inclusive search, relevant to any model that exhibits this kinematic behavior, excludes mediator masses of 1.5-4.0 TeV at 95% confidence level, depending on the other signal model parameters. To enhance the sensitivity of the search for this particular class of hidden sector models, a boosted decision tree (BDT) is trained using jet substructure variables to distinguish between semivisible jets and SM jets from background processes. When the BDT is employed to identify each jet in the dijet system as semivisible, the mediator mass exclusion increases to 5.1 TeV, for wider ranges of the other signal model parameters. These limits exclude a wide range of strongly coupled hidden sector models for the first time.Peer reviewe

Measurement of the Bs0 = μ+μ- decay properties and search for the B0 → μ+μ- decay in proton-proton collisions at √s=13 TeV

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Search for nonresonant Higgs boson pair production in the four leptons plus twob jets final state in proton-proton collisions at √s=13 TeV

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Search for a heavy composite Majorana neutrino in events with dilepton signatures from proton-proton collisions at √s=13 Tev

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Search for a charged Higgs boson decaying into a heavy neutral Higgs boson and a W boson in proton-proton collisions at √s=13 TeV

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Azimuthal anisotropy of dijet events in PbPb collisions at √sNN=5.02 TeV

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