• Energy Research

We measure the photon-energy spectrum in radiative bottom-meson ($B$) decays into inclusive final states involving a strange hadron and a photon. We use SuperKEKB electron-positron collisions corresponding to $189~\mathrm{fb}^{-1}$ of integrated luminosity collected at the $Υ(4S)$ resonance by the Belle II experiment. The partner $B$ candidates are fully reconstructed using a large number of hadronic channels. The $B \rightarrow X_s γ$ partial branching fractions are measured as a function of photon energy in the signal $B$ meson rest frame in eight bins above $1.8~\mathrm{GeV}$. The background-subtracted signal yield for this photon energy region is $343 \pm 122$ events. Integrated branching fractions for three photon energy thresholds of $1.8~\mathrm{GeV}$, $2.0~\mathrm{GeV}$, and $2.1~\mathrm{GeV}$ are also reported, and found to be in agreement with world averages. 14 pages, 3 figures

The production of a prompt photon in association with a $Z$ boson is studied in proton-proton collisions at a centre-of-mass energy $\sqrt{s} =$ 13 TeV. The analysis uses a data sample with an integrated luminosity of 139 fb$^{-1}$ collected by the ATLAS detector at the LHC from 2015 to 2018. The production cross-section for the process $pp \rightarrow \ell^+\ell^-\gamma+X$ ($\ell = e, \mu$) is measured within a fiducial phase-space region defined by kinematic requirements on the photon and the leptons, and by isolation requirements on the photon. An experimental precision of 2.9% is achieved for the fiducial cross-section. Differential cross-sections are measured as a function of each of six kinematic variables characterising the $\ell^+\ell^-\gamma$ system. The data are compared with theoretical predictions based on next-to-leading-order and next-to-next-to-leading-order perturbative QCD calculations. The impact of next-to-leading-order electroweak corrections is also considered. Journal of high energy physics 2020(3), 54 (2020). doi:10.1007/JHEP03(2020)054 Published by SISSA, [Trieste]

Measurements of dijet $p_T$ correlations in Pb+Pb and pp collisions at a nucleon–nucleon centre-of-mass energy of $\sqrt{{s}NN}$ = 2.76 TeV are presented. The measurements are performed with the ATLAS detector at the Large Hadron Collider using Pb + Pb and pp data samples corresponding to integrated luminosities of 0.14 nb$^{−1}$ and 4.0 pb$^{−1}$ , respectively. Jets are reconstructed using the anti-$k_t$ algorithm with radius parameter values R = 0.3 and R = 0.4 . A background subtraction procedure is applied to correct the jets for the large underlying event present in Pb+Pb collisions. The leading and sub-leading jet transverse momenta are denoted $p_{{T}_{1}}$ and $p_{{T}_{2}}$ . An unfolding procedure is applied to the two-dimensional ( $p_{{T}_{1}}, p_{{T}_{2}}$ ) distributions to account for experimental effects in the measurement of both jets. Distributions of (1/N)dN/$dx_J$ , where $x_J$ = $p_{{T}_{1}} / p_{{T}_{2}}$ , are presented as a function of $p_{{T}_{1}}$ and collision centrality. The distributions are found to be similar in peripheral Pb+Pb collisions and pp collisions, but highly modified in central Pb + Pb collisions. Similar features are present in both the R = 0.3 and R = 0.4 results, indicating that the effects of the underlying event are properly accounted for in the measurement. The results are qualitatively consistent with expectations from partonic energy loss models. Physics letters / B 774, 379 - 402 (2017). doi:10.1016/j.physletb.2017.09.078 Published by North-Holland Publ., Amsterdam