Exclusive, elastic and total cross-section measurements in ATLAS

<!--HTML--><div style="line-height:1.38;margin-bottom:0pt;margin-top:0pt;"><span style="font-variant-east-asian:normal;font-variant-ligatures:normal;font-variant-position:normal;vertical-align:baseline;white-space:pre-wrap;">Elastic scattering of protons, where the incoming particles barely bounce off each other, has intrigued particle physicists for decades. Its non-perturbative nature prohibits calculations from first principles. However, by measuring this process, fundamental quantities of strong interactions can be determined among which, most importantly, the total cross section. The seminar presents the most precise determination to date of the total cross section and the so-called rho-parameter, the ratio of the real to imaginary parts of the forward elastic scattering amplitude.&nbsp;</span></div><div style="line-height:1.38;margin-bottom:0pt;margin-top:0pt;"><span style="font-variant-east-asian:normal;font-variant-ligatures:normal;font-variant-position:normal;vertical-align:baseline;white-space:pre-wrap;">The measurement is performed using data collected at a center-of-mass energy of 13 TeV with a dedicated special LHC optics with beta* equal to 2.5 km. The scattering angles of the protons are measured with the ALFA detectors, housed in roman pots placed 240 m from the ATLAS interaction point. Such setup allows access to the Coulomb-nuclear interference region at small momentum transfer. The new high precision measurement provides important information on the energy evolution of the total cross section and rho parameter, which is necessary to improve the modeling of hadronic scattering at high energy. The measurement of exclusive production of charged pion pairs at the LHC using 7 TeV data is also discussed. This represents the first use of proton tagging to measure an exclusive hadronic final state at the LHC.</span></div>