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We report measurement of initial G2-chromatid breaks in normal human fibroblasts exposed to various types of high-LET particles. Exponentially growing AG 1522 cells were exposed to gamma rays or heavy ions. Chromosomes were prematurely condensed by calyculin A. Chromatid-type breaks and isochromatid-type breaks were scored separately. The dose response curves for the induction of total chromatid breaks (chromatid-type + isochromatid-type) and chromatid-type breaks were linear for each type of radiation. However, dose response curves for the induction of isochromatid-type breaks were linear for high-LET radiations and linear-quadratic for gamma rays. Relative biological effectiveness (RBE), calculated from total breaks, showed a LET dependent tendency with a peak at 55 keV/micrometer silicon (2.7) or 80 keV/micrometer carbon (2.7) and then decreased with LET (1.5 at 440 keV/micrometer). RBE for chromatid-type break peaked at 55 keV/micrometer (2.4) then decreased rapidly with LET. The RBE of 440 keV/micrometer iron particles was 0.7. The RBE calculated from induction of isochromatid-type breaks was much higher for high-LET radiations. It is concluded that the increased production of isochromatid-type breaks, induced by the densely ionizing track structure, is a signature of high-LET radiation exposure.

Abstract A search for a chargino-neutralino pair decaying via the 125 GeV Higgs boson into photons is presented. The study is based on the data collected between 2015 and 2018 with the ATLAS detector at the LHC, corresponding to an integrated luminosity of 139 fb−1 of pp collisions at a centre-of-mass energy of 13 TeV. No significant excess over the expected background is observed. Upper limits at 95% confidence level for a massless $$ {\tilde{\chi}}_1^0 $$ χ ˜ 1 0 are set on several electroweakino production cross-sections and the visible cross-section for beyond the Standard Model processes. In the context of simplified supersymmetric models, 95% confidence-level limits of up to 310 GeV in $$ m\left({\tilde{\chi}}_1^{\pm }/{\tilde{\chi}}_2^0\right) $$ m χ ˜ 1 ± / χ ˜ 2 0 , where $$ m\left({\tilde{\chi}}_1^0\right) $$ m χ ˜ 1 0 = 0.5 GeV, are set. Limits at 95% confidence level are also set on the $$ {\tilde{\chi}}_1^{\pm }{\tilde{\chi}}_2^0 $$ χ ˜ 1 ± χ ˜ 2 0 cross-section in the mass plane of $$ m\left({\tilde{\chi}}_1^{\pm }/{\tilde{\chi}}_2^0\right) $$ m χ ˜ 1 ± / χ ˜ 2 0 and $$ m\left({\tilde{\chi}}_1^0\right) $$ m χ ˜ 1 0 , and on scenarios with gravitino as the lightest supersymmetric particle. Upper limits at the 95% confidence-level are set on the higgsino production cross-section. Higgsino masses below 380 GeV are excluded for the case of the higgsino fully decaying into a Higgs boson and a gravitino.