Search for massive neutral gauge bosons decaying into muon pairs in $pp$ collisions at $\sqrt{s}=7$ TeV with the ATLAS detector at the LHC

A search is performed for deviations from the standard model (SM) prediction in the invariant mass distribution of muon pairs produced in pp collisions at a center of mass energy of $\sqrt{s}=7$ TeV at the LHC. This search is based on 5 fb$^{-1}$ of data collected by the ATLAS experiment in 2011. The main contribution to muon pair production in the SM is the Drell-Yan process, with additional small contributions from diboson and $t\bar t$ production and decay. No significant deviation from the SM expectation has been observed and therefore, direct limits at the 95% confidence level are set on two benchmark hypotheses. The first hypothesis assumes the simplest scenario where the SM is extended to five dimensions and a single spatial extra dimension (ED) is compactified on a $S^1/Z_2$ orbifold. As a result, a Kaluza-Klein (KK) tower of heavy resonant copies of the SM $\gamma/Z$ particles is expected. However, this model also predicts each new massive resonance to be preceded, at much lower masses, by a large deficit with respect to the SM prediction. This is due to a strong destructive interference with the $\gamma/Z$ particles, and this feature is found to have a very large exclusion power. Another hypothesis which is considered here, consists of a class of purely resonant $Z'$ models, inspired by higher-symmetry breaking, such as the $E_6$ group. The $Z'$ bosons also interfere with the $\gamma/Z$ particles although this effect is much smaller than for the KK. The direct limits are set on the KK and $Z'$ masses, where the KK mass is equivalent to the inverse of the ED compactification scale. Two methods are used to set limits on the masses $M_{\rm KK}$ and $M_{Z'}$. In the first method, which is valid only for the $Z'$ case, the limit calculation uses the cross section times branching fraction ($\sigma B$) as a free parameter, while neglecting the interference with the $\gamma/Z$. In the second method, the interference is taken into account by introducing a coupling-strength parameter, $g$. For a single ED, the lower limit on $M_{\rm KK}$ is 3.93 TeV from dimuons, and it extends to 4.71 TeV when dielectrons are included. This is the first direct limit on this model and it is almost 1 TeV higher than previous indirect limits. The limit on $M_{Z'}$ is ranging between 1.79 TeV and 2.25 TeV depending on the $Z'$ model assumed.