Evidence for the WWW Production in pp Collisions with the ATLAS Detector

Abstract

This thesis presents the first evidence for the production of three $W$ bosons ($W^{pm}W^{pm}W^{mp}$) using $79.8;text{fb}^{-1}$ of $pp$ collision data at a center-of-mass energy of $13;$TeV recorded by the ac{ATLAS} detector at the ac{LHC} between 2015 and 2017. The search is conducted using the semi-leptonic ($W^{pm}W^{pm}W^{mp} rightarrow ell^{pm}nu ell^{pm} nu jj$) and fully-leptonic ($W^{pm}W^{pm}W^{mp} rightarrow ell^{pm}nu ell^{pm} nu ell^{mp} nu$) channels where $ell=e, mu$. The semi-leptonic channel is examined in a region where the two same-sign $W$ bosons decay leptonically while the third $W$ boson decays hadronically, and the fully-leptonic channel is examined in a region where all three $W$ bosons decay leptonically and without same-flavor opposite-sign lepton pairs.

In the semi-leptonic channel, a total of 798 candidate events are observed in data, to be compared with the prediction of $sim 68$ signal events and $sim 700$ background events. In the fully-leptonic channel, a total of 27 candidate events are observed in data, to be compared with the prediction of $sim 11$ signal events and $sim 23$ background events. The first evidence for $W^{pm}W^{pm}W^{mp}$ production is observed by rejecting the background-only hypothesis with an observed (expected) significance of 3.3 standard deviations (2.4 standard deviations). The fiducial $W^{pm}W^{pm}W^{mp}$ cross sections are measured to be $ 1.82^{+0.33}_{-0.32} text{(stat.)} ^{+0.40}_{-0.36} text{(syst.)}~si{fb}$ for the $ell^{pm}nu ell^{pm} nu jj$ channel and $0.30^{+0.054}_{-0.052} text{(stat.)} ^{+0.065}_{-0.058} text{(syst.)}~si{fb}$ in the $ell^{pm}nu ell^{pm} nu ell^{mp} nuchannel$. The inclusive $W^{pm}W^{pm}W^{mp}$ production cross section is found to be $690^{+125}_{-120} text{(stat.)} ^{+150}_{-135} text{(syst.)}~si{ fb}$.

The $W^{pm}W^{pm}W^{mp}$ search results are combined with the $WWZ$ and $WZZ$ search results using a binned maximum-likelihood fit. The first evidence for the production of three massive gauge bosons is observed with an observed (expected) significance of 4.0 standard deviations (3.1 standard deviations).