EMC  

Standard Model


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The Standard Model (SM) is a theory describing fundamental particles and the interaction between them. These fundamental particles are all fermions, having spin 1/2, and are grouped into three so called generations. There are nine quarks, which have a charge of either -1/3 or 2/3 and nine leptons, which consist of three massive leptons, the electron, the muon and the tau and their respective neutrinos. The force carriers called the gauge bosons, all have spin 1. The massless photon is the mediator of the electromagnetic force, the massless gluon is the mediator of the strong force and the massive W+- and Z bosons are the mediators of the weak force.  Latest results from ATLAS and CMS prove the existence of a Standard Model Higgs boson, a scalar boson with spin 0, which give mass to the fermions and the gauge bosons.

What we are working on

W+jets measurements

A precision measurement of the production of a W boson in assiciation with jets (W+jets) is not only a powerful test of perturbative quantumchromodynamic (QCD) but also an important measurement for new physics searches where W+jets events are a major background. Understanding W+jets production in multiple regions of phase space, such as events with high jet momenta or large scalar sum pT has become an important test of next-to-leading order calculations and tree-level Matrix Element generators. Measuring the cross section in multiple differential distributions like jet momenta, the angular properties of the jets, the scalar sum pT and the jet multiplicity is an extensive test of the Monte Carlo generator which are used heavily for background estimates in new physics searches. Due to lack of statistics in the data, these generators so far have only been tested at moderate jet energies and jet multiplicities, using proton-antiproton collisions at the Tevatron and in the first data sets at the LHC. 
 
We are focusing on several measurements of W+jets at 7, 8 and 14 TeV center of mass energies. The goal of these publications is to probe extreme phase spaces (i.e. events with large jet momenta or large scale sum pT). These are areas where the Standard Model itself is least understood and with the large data sets from the LHC can be measured for the time. Additionally the measurement of the ratio of W+jets at two different center of mass energies is a precise test of perturbative QCD, since many of the dominant experimental and theoretical uncertainties cancel in this ratio. The measurement of this ratio is also a sensitive search for new physics through deviations from a known Standard Model cross section.
 

Analysis of Wyy final states

While diboson final states have already been measured at the LHC, the dataset has so far been too small to study final states containing three vector bosons. The signature comprising one W boson and two photons will be among the first triboson final states to be detected by ATLAS. The extraction of the cross section will be unprecedented as it is so small, that it couldn't be measured by previous experiments. A very attractive feature of studying the Wyy final state is, that it is sensitive to the so called quartic gauge couplings, describing a four vector boson interaction. This is due to the fact that the process pp -> Wyy comprises four boson vertices. The gauge couplings are precicely described by the Standard Model of particle physics and a deviation from these predictions would hint to new phenomena aside from the to date established theory.

 
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