• Energy Research

Abstract The underlying event activity in proton-proton collisions at forward pseudo-rapidity (−6.6 < η < −5.2) is studied with the CMS detector at the LHC, using a novel observable: the ratio of the forward energy density, dE/dη, for events with a charged-particle jet produced at central pseudorapidity (|η jet| < 2) to the forward energy density for inclusive events. This forward energy density ratio is measured as a function of the central jet transverse momentum, p T, at three different pp centre-of-mass energies ( $ \sqrt{s}=0.9 $ , 2.76, and 7 TeV). In addition, the $ \sqrt{s} $ evolution of the forward energy density is studied in inclusive events and in events with a central jet. The results are compared to those of Monte Carlo event generators for pp collisions and are discussed in terms of the underlying event. Whereas the dependence of the forward energy density ratio on jet p T at each $ \sqrt{s} $ separately can be well reproduced by some models, all models fail to simultaneously describe the increase of the forward energy density with $ \sqrt{s} $ in both inclusive events and in events with a central jet.

Abstract The production of Z bosons in association with a high-energy photon (Zγ production) is studied in the neutrino decay channel of the Z boson using pp collisions at $$ \sqrt{s}=13 $$ s = 13 TeV. The analysis uses a data sample with an integrated luminosity of 36.1fb−1 collected by the ATLAS detector at the LHC in 2015 and 2016. Candidate Zγ events with invisible decays of the Z boson are selected by requiring significant transverse momentum (p T) of the dineutrino system in conjunction with a single isolated photon with large transverse energy (E T). The rate of Zγ production is measured as a function of photon E T, dineutrino system p T and jet multiplicity. Evidence of anomalous triple gauge-boson couplings is sought in Zγ production with photon E T greater than 600 GeV. No excess is observed relative to the Standard Model expectation, and upper limits are set on the strength of ZZγ and Zγγ couplings.