Vector mediator, Dirac fermion DM

Jon Butterworth

This model is one of those considered in the ATLAS search summary [21]. We have so far compared two scenarios, first one with the coupling of the mediator to quarks set to 0.25, and zero coupling to leptons (also discussed in [92]):

ATLAS Searches




(Contur 1.0.x, Rivet 3.1.0, Herwig 7.2, 03/02/2020)

You can see the switch-on of \(t\bar{t}\) production at \(M_{Z^{\prime}} = 350\) GeV in the heatmaps for the ATLAS semileptonic top measurements below. Also shown is the heatmap for the boosted hadronic top measurement [19], which has a higher threshold due to the high transverse momentum cut on the tops.

../../_images/ATLAS_13_LMETJETMesh.png ../../_images/ATLAS_13_TTHADMesh.png

The low \(M_{\rm Z^\prime}\) region is interesting. The lowest mass bin in the above plots is 10 GeV, so the impression the sensitivity goes down to zero is misleading. However, extending the lower bin to 5 GeV and expanding the low mass region by using a log scale shows it does indeed go pretty low. The sensitivity here comes from \(\gamma +\) jets and \(W,Z +\) jets measurements, where the \(Z^\prime\) decays hadronically. It arises from marginal contributions from many analyses, so given the rather crude treatment of correlations and background in Contur at present, such a situation should be treated with caution. Nevertheless it illustrates the fact that these measurements to probe the model in that region, and is worth following up.


(Contur 1.0.x, Rivet 3.1.0, Herwig 7.2, 4/2/2020, using correlation info)

Second with the mediator coupling to quarks set to 0.1, and a coupling on 0.01 to leptons.

ATLAS Searches




(Contur 1.1.x, Rivet 3.1.x, Herwig 7.2, 4/3/2020, using correlation info)

Again the sensitivity apparently extends to lower masses than seen in the ATLAS combined search results, with the caveat as above - the lowest mass point generated is \(M_{\rm Z^\prime}= 10\) GeV Below, the \(M_{\rm Z^\prime}\) is generated on a log scale for \(M_{\rm Z^\prime} \ge 5\) GeV, so the low mass regime can be seen in more detail.


(Contur 1.0.x, Rivet 3.1.x, Herwig 7.2, 05/03/2020, using correlation info)


In the first case the measurments in Contur have similar sensitivity to the searches at \(M_{\rm Z^\prime}\), despite having less data available. The top measurements are especially powerful, particularly the ATLAS fully-hadronic boosted top measurement [19].

In the second case, the limits from Contur aren’t as strong, presumably because the dijet measurements currently available don’t have as much integrated luminosity as was used in the search.

In both cases, \(\gamma +\) jets and \(W,Z +\) jets measurements extend the reach to lower \(M_{\rm Z^\prime}\).

The model files are available in the DMsimp_s_spin1 directory here