The search for DM is one of the main targets of the LHC experiments. Investigations into the validity of EFTs commonly used in interpreting such searches have been presented. A measure of the validity of an EFT, RtotΛ , was introduced. It indicates the fraction of events for which the defined condition of validity for the EFT is fulfilled and depends on the DM mass and the assumed cut-off scale. The analysis for the full list of EFT operators used by the ATLAS and CMS collaborations, connecting fermion DM particles and quarks or gluons and originating from the exchange of heavy mediators in the s-channel, has been
106 Chapter 5. Validity of Effective Field Theory Dark Matter Models at the LHC performed analytically, assuming collision energies of 8 and 14 TeV. The analytical results were completed by performing numerical event simulations which reproduce the experimental situation as closely as possible. The results indicate that the range of validity of the EFT is significantly limited in the parameter space of (mDM,Λ) that is probed by LHC searches.
While these findings are valid for s-channel processes, a similar analysis exists also for t-channel scenarios [123], where comparable results are obtained.
The advantage of avoiding too much model dependence still holds for the EFT approach;
however, the presented results clearly demand an alternative to the EFT interpretation, such as through identifying a set ofSimplified Models, which are able to reproduce the EFT operators in the heavy mediator limit. This allows for a consistent analysis of the current and future LHC data by consistently taking into account the possibility of an on-shell production of the mediator. Furthermore, comparisons to direct and indirect searches can be presented in a more comprehensive way by using such Simplified Models.
In the following chapter, the ATLASMonojet Analysis of 20.3fb−1 of 8 TeVppcollision data is presented, which adapted the rescaling procedure introduced here and presented the EFT limits alongside with these modified bounds. Furthermore, a first step towards the consistent use of Simplified Models is made there by considering a Z0-like model for interpretation.
Subsequently, a study is presented that conducted a detailed re-interpretation of this and two other DM searches in terms of a set of Simplified Models and also shows comparisons to direct detection results.
Chapter 6
Search for Dark Matter in Events with an Energetic Jet and Large Missing
Transverse Energy
The simplest possible scenario for the production of Dark Matter (DM) at the LHC is given by assuming a process in which two incoming partons would lead to a final state with two DM particles1. However, such a final state could not be detected in the experiments: since the DM particles are only interacting weakly with the detector material, they would escape without leaving a signal.
On the other hand, if the radiation of an object like a jet, a photon or even a vector boson from the initial partons is assumed, the final state presents a very unique “mono-X” signature:
one energetic object is the only activity in the event. It is recoiling against the invisible, undetected particles which leads to a significant momentum imbalance in the transverse plane. This scenario is sketched in Fig. 6.1.
Even without having a signal scenario in mind, event topologies featuring an energetic object and large missing transverse energy (ETmiss) are distinct signatures to look for physics beyond the Standard Model at the LHC. Various possible final states have been studied: mono-jet, mono-photon, mono-W/Z and mono-Higgs [51, 124–133]. Due to the large probability of radiating a gluon or a quark off the incoming partons, the mono-jet channel generally has the highest cross section and is most sensitive to possible signals.
Such searches were commonly interpreted in terms of an effective field theory (EFT) of DM production, as introduced in Sec. 2.6. Such models are useful to reduce the model dependence
1A final state featuring only one DM particle might be considered even simpler. However, almost all of the proposed DM models require that these new particles have some kind of conserved charge (e.g. R-parity in SUSY) such that they are stable. This would require them to be produced in pairs.
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108 Chapter 6. Search for Dark Matter in Monojet-like Events
q
¯ q
χ
¯ χ g
Figure 6.1: Sketch of the pair production of DM particles, χχ, associated with a jet from¯ initial-state radiation of a gluon, g.
of the interpretation of the results and allow for a straight-forward comparison between collider searches and direct or indirect detection experiments. However, the assumptions entering in the EFT formulation are often not justified at LHC energies, as outlined in Chapter 5. There, a method is introduced with which the obtained EFT limits can be modified, allowing to judge the impact of the limited EFT validity on the resulting limits.
In the following, the search for new physics in events with an energetic jet and largeETmiss, performed on the full 8 TeV dataset of 20.3fb−1, is presented. There are two major improve-ments with respect to its precursor. First, a veto on isolated tracks is introduced, allowing for a powerful rejection of electroweak backgrounds. Second, a dedicated optimisation for DM signals is performed, leading to the replacement of the veto on additional jets2 by a topological cut on the balance between the leading jetpT and theETmiss. The analysis results were published in 2015 [134].
After an introduction to the analysis strategy in Sec. 6.1, and details on the data and simulation samples used (Sec. 6.2), the event selection is introduced in Sec. 6.3. In particular, the veto on isolated tracks, which I developed, is discussed in detail in Sec. 6.3.3, as well as the optimisation for DM signals, to which I contributed (Sec. 6.3.4). The background estimation is explained in Sec. 6.4 and the sources of systematic uncertainties are discussed in Section 6.5 before the results are presented in Sec. 6.6. Finally, the resulting model-independent limits on events from new physics and the DM interpretation in terms of EFT and Simplified Models are presented in Sec. 6.7. I played a significant role also in defining the interpretation strategy, the presentation of results, the comparisons to direct and indirect DM searches and I calculated the relic density constraints. Conclusions with an outlook on new results from this channel is given in Sec. 6.8.
2Events containing more than two jets were vetoed before.
Chapter 6. Search for Dark Matter in Monojet-like Events 109
6.1 Analysis Strategy
The analysis looks for evidence of new physics in events with one energetic jet and largeETmiss. While the occurrence of leptons (electrons and muons) is vetoed, additional jets are allowed under certain conditions. Such events are referred to as “monojet-like” in the following. Nine signal regions (SRs) are defined by an increasing, inclusive lower ETmiss cut.
The analysis faces a large, irreducible background fromZ+jets events in which theZ decays to – invisible – neutrinos. To a smaller extend,Z/γ∗(→`+`−)andW(→`ν)events, in which the lepton(s) are not identified or out of acceptance, contribute to the background of the signal regions. The background contributions from these electroweak processes are estimated from simulation. Their normalisation is extracted from data in background-enriched control regions (CRs). In this approach, so-called transfer factors are used to extrapolate each background from CRs to SRs. This method significantly reduces the impact of systematic uncertainties on the final result.