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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]

AbstractThis paper presents a measurement of b-jet production in Pb+Pb and pp collisions at $$\sqrt{s_{_\text {NN}}}=5.02$$ s NN = 5.02  TeV with the ATLAS detector at the LHC. The measurement uses 260 $$\text {pb}^{-1}$$ pb - 1 of pp collisions collected in 2017 and 1.4 $$\text {nb}^{-1}$$ nb - 1 of Pb+Pb collisions collected in 2018. In both collision systems, jets are reconstructed via the anti-$$k_{t}$$ k t algorithm. The b-jets are identified from a sample of jets containing muons from the semileptonic decay of b-quarks using template fits of the muon momentum relative to the jet axis. In pp collisions, b-jets are reconstructed for radius parameters $$R= 0.2$$ R = 0.2 and $$R= 0.4$$ R = 0.4 , and only $$R= 0.2$$ R = 0.2 jets are used in Pb+Pb collisions. For comparison, inclusive $$R= 0.2$$ R = 0.2 jets are also measured using 1.7 $$\text {nb}^{-1}$$ nb - 1 of Pb+Pb collisions collected in 2018 and the same pp collision data as the b-jet measurement. The nuclear modification factor, $$R_\text {AA}$$ R AA , is calculated for both b-jets and inclusive jets with $$R= 0.2$$ R = 0.2 over the transverse momentum range of 80–290 GeV. The nuclear modification factor for b-jets decreases from peripheral to central collisions. The ratio of the b-jet $$R_\text {AA}$$ R AA to inclusive jet $$R_\text {AA}$$ R AA is also presented and suggests that the $$R_\text {AA}$$ R AA for b-jets is larger than that for inclusive jets in central Pb+Pb collisions. The measurements are compared with theoretical calculations and suggest a role for mass and colour-charge effects in partonic energy loss in heavy-ion collisions.

Abstract A search for new phenomena has been performed in final states with at least one isolated high-momentum photon, jets and missing transverse momentum in proton–proton collisions at a centre-of-mass energy of $$ \sqrt{s} $$ s = 13 TeV. The data, collected by the ATLAS experiment at the CERN LHC, correspond to an integrated luminosity of 139 fb−1. The experimental results are interpreted in a supersymmetric model in which pair-produced gluinos decay into neutralinos, which in turn decay into a gravitino, at least one photon, and jets. No significant deviations from the predictions of the Standard Model are observed. Upper limits are set on the visible cross section due to physics beyond the Standard Model, and lower limits are set on the masses of the gluinos and neutralinos, all at 95% confidence level. Visible cross sections greater than 0.022 fb are excluded and pair-produced gluinos with masses up to 2200 GeV are excluded for most of the NLSP masses investigated.

Les résultats d'une recherche de nouveaux phénomènes dans des événements avec une grande quantité de mouvement transverse manquante et un boson de Higgs se désintégrant en deux photons sont rapportés. Les données des collisions proton-proton à une énergie de centre de masse de 8 TeV et correspondant à une luminosité intégrée de $ 20.3\text{ }\ text {} {\mathrm{fb}}^{\ ensuremath {-}1}$ ont été collectées avec le détecteur ATLAS au LHC. Les données observées sont bien décrites par les arrière-plans attendus du modèle standard. Des limites supérieures sur la section transversale des événements avec un grand élan transversal manquant et un candidat boson de Higgs sont également placées. Les limites d'exclusion sont présentées pour les modèles de physique au-delà du modèle standard présentant des candidats de matière noire. Se informan los resultados de una búsqueda de nuevos fenómenos en eventos con gran momento transversal faltante y un bosón de Higgs en descomposición a dos fotones. Los datos de las colisiones protón-protón a una energía del centro de masa de 8 TeV y correspondientes a una luminosidad integrada de $ 20.3\text{ }\ text {} {\mathrm{fb}}^{\ ensuremath {-}1}$ se han recopilado con el detector ATLAS en el LHC. Los datos observados están bien descritos por los antecedentes del modelo estándar esperado. También se colocan límites superiores en la sección transversal de eventos con gran falta de impulso transversal y un candidato al bosón de Higgs. Los límites de exclusión se presentan para modelos de física más allá del modelo estándar con candidatos de materia oscura. Results of a search for new phenomena in events with large missing transverse momentum and a Higgs boson decaying to two photons are reported. Data from proton-proton collisions at a center-of-mass energy of 8 TeV and corresponding to an integrated luminosity of $20.3\text{ }\text{ }{\mathrm{fb}}^{\ensuremath{-}1}$ have been collected with the ATLAS detector at the LHC. The observed data are well described by the expected standard model backgrounds. Upper limits on the cross section of events with large missing transverse momentum and a Higgs boson candidate are also placed. Exclusion limits are presented for models of physics beyond the standard model featuring dark-matter candidates. تم الإبلاغ عن نتائج البحث عن ظواهر جديدة في أحداث ذات زخم عرضي كبير مفقود وبوزون هيغز يتحلل إلى فوتونين. تم جمع البيانات من اصطدامات البروتون- البروتون عند طاقة مركز الكتلة البالغة 8 تيرا إلكترون فولت والمطابقة لمعان متكامل قدره 20.3 $\ text{}\ text {} {\ mathrm {fb }}^{\ ensuremath{-} 1 }$ باستخدام كاشف أطلس في مصادم الهدرونات الكبير. يتم وصف البيانات المرصودة بشكل جيد من خلال خلفيات النموذج القياسي المتوقعة. يتم أيضًا وضع حدود عليا على المقطع العرضي للأحداث ذات الزخم العرضي الكبير المفقود ومرشح بوزون هيغز. يتم تقديم حدود الاستبعاد لنماذج الفيزياء التي تتجاوز النموذج القياسي الذي يضم مرشحين للمادة المظلمة.

AbstractCross-sections for the production of a Z boson in association with two photons are measured in proton–proton collisions at a centre-of-mass energy of 13 TeV. The data used correspond to an integrated luminosity of 139 fb$$^{-1}$$ - 1 recorded by the ATLAS experiment during Run 2 of the LHC. The measurements use the electron and muon decay channels of the Z boson, and a fiducial phase-space region where the photons are not radiated from the leptons. The integrated $$Z(\rightarrow \ell \ell )\gamma \gamma $$ Z ( → ℓ ℓ ) γ γ cross-section is measured with a precision of 12% and differential cross-sections are measured as a function of six kinematic variables of the $$Z\gamma \gamma $$ Z γ γ system. The data are compared with predictions from MC event generators which are accurate to up to next-to-leading order in QCD. The cross-section measurements are used to set limits on the coupling strengths of dimension-8 operators in the framework of an effective field theory.