Measurement of the ZZ production cross section and limits to the anomalous triple gauge couplings with forward electrons with the ATLAS detector

Measurements of the self coupling between bosons are important to test the electroweak sector of the Standard Model (SM). The production of pairs of Z bosons through the s-channel is forbidden in the SM. The presence of physics, beyond the SM, could lead to a deviation of the expected production cross section of pairs of Z bosons due to the so called anomalous Triple Gauge Couplings (aTGC). Proton-proton data collisions at the Large Hadron Collider (LHC) recorded by the ATLAS detector at a center of mass energy of 8 TeV were analyzed corresponding to an integrated luminosity of 20.3 fb-1. Pairs of Z bosons decaying into two electron-positron pairs are searched for in the data sample. The effect of the inclusion of detector regions corresponding to high values of the pseudorapidity was studied to enlarge the phase space available for the measurement of the ZZ production. The number of ZZ candidates was determined and the ZZ production cross section was measured to be: rn7.3±1.0(Stat.)±0.4(Sys.)±0.2(lumi.)pb, which is consistent with the SM expectation value of 7.2±0.3pb. Limits on the aTGCs were derived using the observed yield, which are twice as stringent as previous limits obtained by ATLAS at a center of mass energy of 7 TeV.

Measurement of the e + e - pi + pi - cross section using initial state radiation at BESIII

The future goal of modern physics is the discovery of physics beyond the Standard Model. One of the most significant hints for New Physics can be seen in the anomalous magnetic moment of the muon - one of the most precise measured variables in modern physics and the main motivation of this work. This variable is associated with the coupling of the muon, an elementary particle, to an external electromagnetic field and is defined as a = (g - 2)/2, whereas g is the gyromagnetic factor of the muon. The muon anomaly has been measured with a relative accuracy of 0.5·10-6. However, a difference between the direct measurement and the Standard Model prediction of 3.6 standard deviations can be observed. This could be a hint for the existence of New Physics. Unfortunately, it is, yet, not significant enough to claim an observation and, thus, more precise measurements and calculations have to be performed.rnThe muon anomaly has three contributions, whereas the ones from quantum electrodynamics and weak interaction can be determined from perturbative calculations. This cannot be done in case of the hadronic contributions at low energies. The leading order contribution - the hadronic vacuum polarization - can be computed via a dispersion integral, which needs as input hadronic cross section measurements from electron-positron annihilations. Hence, it is essential for a precise prediction of the muon anomaly to measure these hadronic cross sections, σ(e+e-→hadrons), with high accuracy. With a contribution of more than 70%, the final state containing two charged pions is the most important one in this context.rnIn this thesis, a new measurement of the σ(e+e-→π+π-) cross section and the pion form factor is performed with an accuracy of 0.9% in the dominant ρ(770) resonance region between 600 and rn900 MeV at the BESIII experiment. The two-pion contribution to the leading-order (LO) hadronic vacuum polarization contribution to (g - 2) from the BESIII result, obtained in this work, is computed to be a(ππ,LO,600-900 MeV) = (368.2±2.5stat±3.3sys)·10-10. With the result presented in this thesis, we make an important contribution on the way to solve the (g - 2) puzzle.