Absolute Energy Calibration of X-ray TESs with 0.04 eV Uncertainty at 6.4 keV in a Hadron-Beam Environment

A performance evaluation of superconducting transition-edge sensors (TESs) in the environment of a pion beam line at a particle accelerator is presented. Averaged across the 209 functioning sensors in the array, the achieved energy resolution is 5.2 eV FWHM at Co $K_{\alpha}$ (6.9 keV) when the pion beam is off and 7.3 eV at a beam rate of 1.45 MHz. Absolute energy uncertainty of $\pm$0.04 eV is demonstrated for Fe $K_{\alpha}$ (6.4 keV) with in-situ energy calibration obtained from other nearby known x-ray lines. To achieve this small uncertainty, it is essential to consider the non-Gaussian energy response of the TESs and thermal cross-talk pile-up effects due to charged-particle hits in the silicon substrate of the TES array.Comment: Accepted for publication in J. Low Temperature Physics, special issue for the proceedings of the Low Temperature Detectors 16 conferenc

Search for Bs0→γγB_{s}^{0}\rightarrow\gamma\gamma and a measurement of the branching fraction for Bs0→ϕγB_{s}^{0}\rightarrow\phi\gamma

We search for the decay $B_{s}^{0}\rightarrow\gamma\gamma$ and measure the branching fraction for $B_{s}^{0}\rightarrow\phi\gamma$ using 121.4~$\textrm{fb}^{-1}$ of data collected at the $\Upsilon(\mathrm{5}S)$ resonance with the Belle detector at the KEKB asymmetric-energy $e^{+}e^{-}$ collider. The $B_{s}^{0}\rightarrow\phi\gamma$ branching fraction is measured to be $(3.6 \pm 0.5 (\mathrm{stat.}) \pm 0.3 (\mathrm{syst.}) \pm 0.6 (f_{s})) \times 10^{-5}$, where $f_{s}$ is the fraction of $B_{s}^{(*)}\bar{B}_{s}^{(*)}$ in $b\bar{b}$ events. Our result is in good agreement with the theoretical predictions as well as with a recent measurement from LHCb. We observe no statistically significant signal for the decay $B_{s}^{0}\rightarrow\gamma\gamma$ and set a $90\%$ confidence-level upper limit on its branching fraction at $3.1 \times 10^{-6}$. This constitutes a significant improvement over the previous result.Comment: 6 pages, 3 figure