Johanna Gramling

Simplified models for dark matter searches at the LHC

This document outlines a set of simplified models for dark matter and its interactions with Standard Model particles. It is intended to summarize the main characteristics that these simplified models have when applied to dark matter searches at the LHC, and to provide a number of useful expressions for reference. The list of models includes both s-channel and t-channel scenarios. For s-channel, spin-0 and spin-1 mediation is discussed, and also realizations where the Higgs particle provides a portal between the dark and visible sectors. The guiding principles underpinning the proposed simplified models are spelled out, and some suggestions for implementation are presented.

On the Validity of the Effective Field Theory for Dark Matter Searches at the LHC, Part II: Complete Analysis for the

We generalize in several directions our recent analysis of the limitations to the use of the effective field theory approach to study dark matter at the LHC. Firstly, we study the full list of operators connecting fermion DM to quarks and gluons, corresponding to integrating out a heavy mediator in the s-channel; secondly, we provide analytical results for the validity of the EFT description for both √s = 8 TeV and 14 TeV; thirdly, we make use of a MonteCarlo event generator approach to assess the validity of our analytical conclusions. We apply our results to revisit the current collider bounds on the ultraviolet cut-off scale of the effective field theory and show that these bounds are weakened once the validity conditions of the effective field theory are imposed.

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The study of collision events with missing energy as searches for the dark matter (DM) component of the Universe are an essential part of the extensive program looking for new physics at the LHC. Given the unknown nature of DM, the interpretation of such searches should be made broad and inclusive. This report reviews the usage of simplified models in the interpretation of missing energy searches. We begin with a brief discussion of the utility and limitation of the effective field theory approach to this problem. The bulk of the report is then devoted to several different simplified models and their signatures, including s-channel and t-channel processes. A common feature of simplified models for DM is the presence of additional particles that mediate the interactions between the Standard Model and the particle that makes up DM. We consider these in detail and emphasize the importance of their inclusion as final states in any coherent interpretation. We also review some of the experimental progress in the field, new signatures, and other aspects of the searches themselves. We conclude with comments and recommendations regarding the use of simplified models in Run-II of the LHC.

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A bstractThe use of simplified models as a tool for interpreting dark matter collider searches has become increasingly prevalent, and while early Run II results are beginning to appear, we look to see what further information can be extracted from the Run I dataset. We consider three ‘standard’ simplified models that couple quarks to fermionic singlet dark matter: an s-channel vector mediator with vector or axial-vector couplings, and a t-channel scalar mediator. Upper limits on the couplings are calculated and compared across three alternate channels, namely mono-jet, mono-Z (leptonic) and mono-W/Z (hadronic). The strongest limits are observed in the mono-jet channel, however the computational simplicity and absence of significant t-channel model width effects in the mono-boson channels make these a straightforward and competitive alternative. We also include a comparison with relic density and direct detection constraints.

Simplified Models for Dark Matter and Missing Energy Searches at the LHC

There are different approaches to search for Dark Matter (DM) at the LHC. One strategy is to assume theoretically well-motivated complete models that provide a DM candidate, such as SUSY (introduced in Chap. 4) or extra dimensions. The searches are optimised to target specific decay scenarios and final states that are expected for these models. The characteristics of these final states might depend on specific choices made for the model parameters. Most importantly, the interpretation of the experimental results and the conclusions drawn from them strongly depend on the specific model and its details. This model dependence might be reduced by the use of Simplified Models that capture only parts of the model characteristics, as in the case for the Stop Analsysis that is presented in Chap. 9. The price to pay for more generality is always a loss of completeness.

Collide and conquer: constraints on simplified dark matter models using mono- X collider searches

The supersymmetric partner of the top quark, the stop, might be significantly lighter than the other squarks, as was motivated in Chap. 4. Hence, the search for stop production at the LHC presents a well-motivated approach to look for SUSY at the LHC. The stop can decay in different ways, depending on the SUSY particle mass spectrum, in particular on the masses of the stops \(\tilde{t}_1\) and \(\tilde{t}_2\), the charginos \(\displaystyle \tilde{\chi }^\pm \) and the lightest neutralino \(\displaystyle \tilde{\chi }^0_1\), and other model parameters. The analysis presented in the following considers two possible stop decay scenarios, illustrated in Fig. 9.1. Both scenarios assume R-parity conservation, hence the stops are produced in pairs. In the first scenario, the stop decays into a top quark and the lightest neutralino: \({\tilde{t}^{}_{1}}\rightarrow t+\displaystyle \tilde{\chi }^0_1 \) (tN), the second scenario assumes the decay into a b-quark and the lightest chargino, where the latter decays further into a W boson and the lightest neutralino: \({\tilde{t}^{}_{1}}\rightarrow b+\displaystyle \tilde{\chi }^\pm _1 \) (bC). Furthermore, a mixed decay scenario where both decay channels are allowed with various branching ratio (\(\mathcal {BR}\)) assumptions is considered in the interpretation.

Validity of Effective Field Theory Dark Matter Models at the LHC

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 particles. 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.

Search for New Physics in Events with Missing Energy and Top Quarks

In order to learn more about fundamental interactions, particle physics offers three directions: increasing the energy, increasing the intensity or increasing the precision. The Large Hadron Collider (LHC) clearly pushes the energy frontier by achieving unprecedented collision energies. But it also provides very large datasets to test very weak interactions and the excellent performance of its experiments allows to improve the precision of measurements on some parameters and properties significantly. Being a hadron collider, the momentum transfer in the collisions is not fixed to one exact energy (as opposed to electron-positron colliders) and it is an excellent machine for discovering new particles.

Search for Dark Matter in Monojet-like Events

Formulated in the 1960s, the Standard Model of particle physics describes subatomic particle interactions with remarkable success. Its prediction of the top quark and the Higgs boson were the most recent triumphs. Over several orders of magnitude in production cross section, the measurements performed at the Large Hadron Collider (LHC) at CERN precisely confirm the predictions of the Standard Model.

The ATLAS Detector at the LHC

Collider searches for Dark Matter (DM) in events with large missing transverse energy (\(E_{\mathrm {T}}^{\mathrm {miss}}\)) have been commonly interpreted within effective field theory (EFT) models of DM pair production. The advantage of this approach is that limits derived in terms of an EFT are applicable to a broad range of complete theories and depend only on the specification of a few parameters, namely the cutoff scale, \(\Lambda \), and the DM mass, \(m_{\chi } \).