First Measurement Of Cross Section Research Articles

First measurement of the cross section for top quark pair production with additional charm jets using dileptonic final states in pp collisions at [formula omitted

The first measurement of the inclusive cross section for top quark pairs (tt‾) produced in association with two additional charm jets is presented. The analysis uses the dileptonic final states of tt‾ events produced in proton-proton collisions at a centre-of-mass energy of 13 TeV. The data correspond to an integrated luminosity of 41.5fb−1, recorded by the CMS experiment at the LHC. A new charm jet identification algorithm provides input to a neural network that is trained to distinguish among tt‾ events with two additional charm (tt‾cc‾), bottom (tt‾bb‾), and light-flavour or gluon (tt‾LL) jets. By means of a template fitting procedure, the inclusive tt‾cc‾, tt‾bb‾, and tt‾LL cross sections are simultaneously measured, together with their ratios to the inclusive tt‾ + two jets cross section. This provides measurements of the tt‾cc‾ and tt‾bb‾ cross sections of 10.1±1.2(stat)±1.4(syst)pb and 4.54±0.35(stat)±0.56(syst)pb, respectively, in the full phase space. The results are compared and found to be consistent with predictions from two different matrix element generators with next-to-leading order accuracy in quantum chromodynamics, interfaced with a parton shower simulation.

Observation of isolated high-ET photons in deep inelastic scattering

First measurements of cross sections for isolated prompt photon production in deep inelastic ep scattering have been made using the ZEUS detector at the HERA electron–proton collider using an integrated luminosity of 121 pb−1. A signal for isolated photons in the transverse energy and rapidity ranges 5<ETγ<10GeV and −0.7<ηγ<0.9 was observed for virtualities of the exchanged photon of Q2>35GeV2. Cross sections are presented for inclusive prompt photons and for those accompanied by a single jet in the range ETjet⩾6GeV and −1.5⩽ηjet<1.8. Calculations at order α3αs describe the data reasonably well.

Formation of negative ions by electron impact on SiF4 and CF4

First measurements of cross sections for the formation of negative ions by electron attachment to SiF4 and CF4 are reported for an electron impact energy range of 0 to 50 eV. The energies at which the various ions appear and the positions at which the ionization efficiency curves peak have been obtained and compared with previous measurements. Thermochemical data have been used to predict and identify the various channels of dissociation. Cross sections for the production of ion pairs through the process of polar dissociation have also been measured.