Abstract
A search is performed for narrow resonances decaying into $WW$, $WZ$, or $ZZ$ boson pairs using 20.3 fb$^{-1}$ of proton-proton collision data at a centre-of-mass energy of $\sqrt{s}$ = 8 TeV recorded with the ATLAS detector at the Large Hadron Collider. Diboson resonances with masses in the range from 1.3 to 3.0 TeV are sought after using the invariant mass distribution of dijets where both jets are tagged as a boson jet, compatible with a highly boosted $W$ or $Z$ boson decaying to quarks, using jet mass and substructure properties. The largest deviation from a smoothly falling background in the observed dijet invariant mass distribution occurs around 2 TeV in the $WZ$ channel, with a global significance of 2.5 standard deviations. Exclusion limits at the 95% confidence level are set on the production cross section times branching ratio for the $WZ$ final state of a new heavy gauge boson, $W'$, and for the $WW$ and $ZZ$ final states of Kaluza--Klein excitations of the graviton in a bulk Randall--Sundrum model, as a function of the resonance mass. $W'$ bosons with couplings predicted by the extended gauge model in the mass range from 1.3 to 1.5 TeV are excluded at 95% confidence level.
Highlights
Background modelThe search for high-mass diboson resonances is carried out by looking for resonance structures on a smoothly falling dijet invariant mass spectrum, empirically characterised by the function dn dx p1(1 − x)p2+ξp3 xp3, (6.1) √where x = mjj/ s, and mjj is the dijet invariant mass, p1 is a normalisation factor, p2 and p3 are dimensionless shape parameters, and ξ is a dimensionless constant chosen after fitting to minimise the correlations between p2 and p3
W bosons with couplings predicted by the extended gauge model in the mass range from 1.3 to 1.5 TeV are excluded at 95% confidence level
The dijet mass distributions after all three tagging selections are well-described by the background model over the entire mass range explored, with the exception of a few bins near mjj = 2 TeV which contain more events than predicted by the background model
Summary
The ATLAS detector [23] surrounds nearly the entire solid angle around the ATLAS collision point It has an approximately cylindrical geometry and consists of an inner tracking detector surrounded by electromagnetic and hadronic calorimeters and a muon spectrometer. The final filter in the high-level trigger requires a jet to satisfy a higher transverse momentum (pT) threshold, reconstructed with the anti-kt algorithm [24] and a large radius parameter (R = 1.0). This search is performed using the dataset collected in 2012 from 8 TeV LHC pp collisions using a single-jet trigger with a nominal pT threshold of 360 GeV. The uncertainty is derived following the methodology detailed in ref. [25]
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