Measurement of the inclusive top quark pair in association with a radiated photon production cross section in the dilepton channel for pp collisions at 8 TeV

Abstract

Here is presented the top-quark pair plus photon production cross section measured in pp collisions at a centre-of-mass energy of √ s = 8 TeV with the CMS detector at the Large Hadron Collider, using data recorded in 2012 corresponding to an integrated luminosity of Lint = 19:6 fb−1. The measurement is performed in the dilepton decay channel. The fiducial region is defined by the final state of the process pp → W⁺W⁻b bl γ, with a minimum photon transverse energy of ET( γ) > 25 GeV and lies within a pseudorapidity range of η ( γ)| < 1:4442, with a separation of ∆R(γ; all) > 0:3 between the photon and quarks or leptons in η — space. Signal events are simulated using the MadGraph event generator. Data-driven templates are constructed in order to measure the photon identification efficiency using the supercluster footprint removal technique, and a fit to the charged hadron isolation is performed. The ratio of tt + γ cross section relative to the inclusive t t cross section, R = t t+ t t, is exploited in order to cancel various sources of systematic uncertainties. The combination of dilepton final state decay channels ratio in the fiducial area is measured to be R = 0:00115 ± 0:00023(stat:). Using a recent CMS t t cross section measurement at 8 TeV in the dilepton channel of t t = 245:6± 1:3(stat:)± +6:6/5:5 (syst:) ± 6:5(lumi:) pb, the top pair plus photon production cross section is calculated to be CMS / t t+γ = 944 154(stat:+syst:) fb. Being in agreement with the t t+γ SM expectation of SM / t t+γ = 861 ± 71(scale) ± 30(PDF) fb, this is the most accurate measurement of the t t +γ process to date, and the first at a centre-of-mass energy of 8 TeV in the dilepton channel.