PhD Defence Lars Beemster

same sign w pair production in double parton interactions

The LHC physics program is now in full swing and has already turned heads in Stockholm. More discoveries are literally in the making, as the size of the recorded data set and the available energy range is ever increasing. So what is the LHC, and in specific ATLAS, looking for? Basically the entire accelerator and detector complex is one gigantic microscope. It searches for the smallest building blocks of nature, or elementary particles. Much like a Lego kit is build out of small elementary building blocks. Quite a few of those elementary particles are already discovered, like the familiar electrons and photons (light particles). Some examples of less familiar particles but similar in the fact that they are elementary are quarks, W and Z bosons, and the recently discovered Higgs boson among others. All these elementary particles interact with each other, bouncing off of each other and even destroying one another. A lot of these processes are already studied and documented in the Standard Model of particle physics, but not all.

One previously unseen process that is about to be uncovered is W boson pair production in Double Parton Interactions. The process consists of two semi-separate productions of a W boson in a single proton-proton collision. Since this type of process requires the interaction of four initial particles, the dependence of these four parents on one another can be investigated in this case.

The results of the analysis show that the W boson pair production in DPI events is still an elusive process. The main contamination of the signal comes from the difficulty to properly reconstruct events with a different signature. The two main background processes are identified, and should be the main focus of future research into this process. When more data is acquired the cuts can be tightened, improving the signal to noise ratio. When the noise level is low enough real physics can be done to unravel more of the proton structure mysteries. The amount of data used in this analysis is recorded over the course of about a year. The ATLAS detector is planned to run for at least a couple of decades at increasing event rates, so the problem of too little signal events should be solved in the coming years. This gives positive prospects for the physics potential of the W boson pair production in DPI events.