AION/Magis

Abstract:

The MALTA2 sensor is the second prototype in the MALTA family of Depleted Monolithic Active Pixel Sensors, developed using a modified 180nm CMOS imaging process and optimized for operation in the high-radiation and high hit-rate conditions of future collider experiments. With a matrix of 224 × 512 pixels and a 36.4µm pitch, MALTA2 is designed for fast charge collection, low noise, and low power consumption. MALTA2 incorporates a low doped n − layer extending across the pixel matrix, which enhances the lateral depletion and charge collection by drift. This work evaluates a MALTA2 variant with very high n − layer doping for improved radiation hardness. The sensor has been neutron irradiated up to a fluence of 5 × 1 0 15 1 MeV n eq cm −2 and characterized in test beam campaigns at the CERN SPS using a 120 GeV/c mixed hadron beam. Hit detection efficiencies above 95% were achieved up to a fluence of 3 × 1 0 15 1 MeV n eq cm −2 . At 5 × 1 0 15 1 MeV n eq cm −2 , the hit detection efficiency measured was 64.9 ± 0.2 % at a substrate bias voltage of -60 V. To investigate this degradation, the effective active depth has been extracted as 20.3 ± 0.2 μ m from cluster size measurements as a function of the beam incident angle. The results from the very high doping will guide the development of prototypes featuring ultra-high n − layer doping, aiming to achieve greater radiation hardness in next-generation silicon tracking detectors.

Abstract:

The first observation of a new meson state decaying into $B_c^{+}$ and a photon is presented using a dataset of $pp$ collisions at a center-of-mass energy $\sqrt{s}=13\mathrm{TeV}$ collected by the ATLAS detector at the Large Hadron Collider, corresponding to an integrated luminosity of $140{\mathrm{fb}}^{-1}$ . The $B_c^{+}$ mesons are reconstructed from a three-muon final state from $B_c^{+}\to J/\psi ({\mu }^{+}{\mu }^{-}){\mu }^{+}{\nu }_{\mu }X$ decays, along with photons reconstructed via conversions to electron-positron pairs in the detector material. The new state is observed with a significance that exceeds 8 standard deviations. The mass difference between the new meson state and the ground-state $B_c^{+}$ meson is measured to be $64.5\pm 1.4(\mathrm{stat}){}_{-1.4}^{+1.0}(\mathrm{syst})\mathrm{MeV}$ . This corresponds to a mass for the observed state of $6339.0\pm 1.4(\mathrm{stat}){}_{-1.4}^{+1.0}(\mathrm{syst})\pm 0.3(m_{B_c^{+}})\mathrm{MeV}$ , where the last uncertainty is due to the precision of the best knowledge of the $B_c^{+}$ meson mass. The low mass difference value matches the theory expectations for the lowest vector state of the $\overline{b}c$ system, the $B_c^{*+}$ meson.