# Vector or Axial-Vector mediator, Dirac fermion DM¶

Jon Butterworth

This class of models is one of those considered in the ATLAS search summary [22], updated here: https://atlas.web.cern.ch/Atlas/GROUPS/PHYSICS/PUBNOTES/ATL-PHYS-PUB-2020-021/ We compare equivalent scenarios here (see [114] for an early set of results):

## Vector mediator¶

ATLAS Searches

Contur

(Contur 2.0.x, Rivet 3.1.4, Herwig 7.2, 24/05/2021, using correlation info)

The white line delineates the region of 95% exclusion (the dashed line is 68%). The colours indicate which analysis pool contributes most to the exclusion. (See the measurements page in the sidebar for the definition of the analsysis pools.)

You can see the switch-on of $$t\bar{t}$$ production at $$M_{Z^{\prime}} = 350$$ GeV, when the lepton and missing energy analyses become important, with the boosted hadronic top measurement [20], taking over above a higher threshold, due to the high transverse momentum cut on the tops. As with the ATLAS search result, the dijet channel dominates at the highest $$M_{\rm Z^\prime}$$ region, but the exclusion dies away around 2.5 TeV, about a TeV lower than the search result. This is likely because the measurement currently only includes 3.2/fb of data, while the search uses the full run 2 139/fb.

The low $$M_{\rm Z^\prime}$$ region is interesting. The lowest mass bin in the above plot is 10 GeV, so the impression the sensitivity goes down to zero is misleading. However, extending the lower bin to 4 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 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 2.0.x, Rivet 3.1.4, Herwig 7.2, 24/5/2021, using correlation info)

Another scenario has the mediator coupling to quarks set to 0.1, and a coupling of 0.01 to leptons.

ATLAS Searches

Contur

(Contur 2.0.x, Rivet 3.1.4, Herwig 7.2, 24/5/2021, using correlation info)

The sensitivity is limited at higher $$M_{\rm Z^\prime}$$ compared to the searches because the missing energy measurement only uses 36/fb of data whereas the search uses 139/fb. Also, the only 13 TeV high-mass-dilepton measurement, which is from CMS, only uses 3.2/fb. There is a 139/fb 13 TeV resonance search from ATLAS available in Rivet, however, and where the signal is actualy resonant, this can be expected to give reasonable results in Contur. It is included in the scan below:

The dileptons now dominate almost everywhere and the overall exclusion is close to that obtained by the searches.

(Contur 2.0.x, Rivet 3.1.4, Herwig 7.2, 24/5/2021, using correlation info)

As with the first set of model parameters, 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 $$M_{\rm Z^\prime} \ge 4$$ GeV, so the low mass regime can be seen in more detail.

(Contur 2.0.x, Rivet 3.1.4, Herwig 7.2, 24/04/2021, using correlation info)

## Axial-Vector mediator¶

As with the vector case, the coupling to quarks is set to 0.25, no coupling to leptons.

ATLAS Searches

Contur

(Contur 2.0.x, Rivet 3.1.4, Herwig 7.2, 24/04/2021, using correlation info)

As with the vector case, the coupling to quarks is set to 0.1, but now the coupling to leptons is also 0.1.

ATLAS Searches

Contur

(Contur 2.0.x, Rivet 3.1.4, Herwig 7.2, 24/04/2021, using correlation info)

Also, as before, including the 13 TeV dilepton resonance search extends the sensitivity, in this case to cover the whole plane. (The slight extension of sensitivity at low DM mass and high mediator mass seems to be because Herwig predicts a slightly higher cross section in that region than the POWHEG samples used in the ATLAS combination [72].

## Summary¶

When the coupling to the SM quarks is 0.25, the measurments in Contur have less sensitivity than the searches at high $$M_{\rm Z^\prime}$$, because the dijet measurement in the end drives this region, and the available measurement only uses 36/fb, while the searches use 139/fb. The top measurements are quite powerful, particularly the ATLAS fully-hadronic boosted top measurement [20].

When the coupling to the SM quarks is reduced to 0.1, and some coupling to the SM leptons is introduced, the limits from measurements at higher DM masses aren’t as strong as the searches, primarily due to the lack of high-luminosity dilepton measurements. When the ATLAS full run two dilepton resonance search is used, however, the sensitivity is similar.

In both cases, $$\gamma +$$ jets and $$W,Z +$$ jets measurements extend the reach to lower $$M_{\rm Z^\prime}$$, beyond that of the searches.

The model files are available in the DMsimp_s_spin1 directory here