List of monojet-like models and questions

Introduction

This page contains is a minimal collection of simplified models that stems from the existing literature and experimental experience, accounting for the discussion on the dmf-lhc-contributors mailing list, in particular from this thread.

We would like to make a list that is practical and economical for LHC experiments, as each of the models will have to be scanned in its free parameters. However, the list should be exhaustive, and whenever a choice or an assumption is made (e.g. restrict our choice to Dirac fermion DM, or assume MFV), that choice should be well motivated in our documentation and we should know what to expect when that particular assumption is relaxed so that future work can continue to investigate further models.

Each of the models is described as follows:

• Dark Matter type: the type of DM particle considered
• Mediator type(s): the type of interaction mediated by the mediator particle
• Mediator mass(es): whether there are constraints on the masses of the mediators or these are free parameters
• Channel: exchange of the mediator: s or t channel
• SM Couplings : whether there are constraints on the couplings between mediator and SM or these are free parameters
• DM Couplings: whether there are constraints on the couplings between mediator and SM or these are free parameters
• Includes lepton couplings: whether the model is leptophobic
• Main signatures: main experimental signatures produced by the model
• ME implementation ready: whether there is a generator ready
• References: (not yet exhaustive) list of model description - feel free to add links from your literature review
• Main questions: questions that need answered in the choice for this model

We still have not included in this list the choice of grid points for the scan as we first want to settle on the basic parameters, but that will be the next step.

Note to contributors: Please modify/correct/add to this list and give your answer to the questions, sending an e-mail to the dmf-lhc-contributors when you do so. We will summarize the results at the next Forum meeting.

List of models

t-channel (squark-exchange) model (#1, from arXiv:1402.2285)

• Dark Matter type: Dirac fermion
• Mediator type(s): Three colored scalars (note: either all mediators or only up-type can contribute, but only x-sec will change)
• Mediator mass(es): Same mass for all three mediators: M_med - Maximal Flavor Symmetry assumption
• Channel: t-channel
• SM Couplings : Same DM and SM coupling for all three mediators: g
• DM Couplings: Same DM and SM coupling for all three mediators: g
• Includes lepton couplings: N/A
• Width: Calculable, or left as free parameters
• Main signatures: Monojet, dijet+MET
• ME implementation ready: Yes, Madgraph
• References: arXiv:1402.2285
• Main questions:
  • Similarities of this model with the other t-channel model below - Lagrangians are similar, is there only a difference in the choice of couplings?
  • Differences with similar pMSSM models
  • Why there are no vector mediator t-channel models? Possibility (Mrenna): harder to write down without invoking new quantum numbers.

t-channel (squark-exchange) model (#2, from DM@LHCv2 proceedings)

• Dark Matter type: Dirac fermion
• Mediator type(s): Three colored scalars
• Mediator mass(es): Either degenerate masses (M_med) or split between first/second and third generation (M_1,2, M_3)
• Channel: t-channel
• SM Couplings : Either degenerate coupling for SM and DM (g) or split between first/second and third generation (g_1,2, g_3)
• DM Couplings: same as SM couplings
• Includes lepton couplings: N/A
• Width: Calculable, or left as free parameters
• Main signatures: Monojet, dijet+MET
• ME implementation ready: Yes, Madgraph
• References: DM@LHC v2 proceedings, talk "Aspects of DM searches" at U Washington by Tim Tait, 1308.2679
• Main questions:
  • Similarities of this model with the other t-channel model above - Lagrangians are similar, is there only a difference in the choice of couplings?
  • Differences with similar pMSSM models
  • Why there are no vector mediator t-channel models?

Scalar mediator, s-channel model from M. Buckley's talk and 1410.6497 and talk of P. Harris and 1411.0535

• Dark Matter type: Dirac fermion, scalar
• Mediator type: Scalar, pseudoscalar (complex case)
• Mediator mass: M_phi, M_a
• Channel: s-channel
• SM Couplings : Yukawa-like, with prefactor g_q_i. Simplest MFV renormalizable case: g_u = g_d = g_l = 1. Phenomenology will differ with different choices.
• DM Couplings: no Yukawa structure, g_DM, can be = g_SM
• Includes lepton couplings: yes
• Width: Minimal width calculable, other choices are model-dependent. Suggestion 1411.0535 to use the mediator width as free parameter ranging from the minimal width to 2 or 3 times the minimal width
• Main signatures: >=2jet+Met (also with HF)
• ME implementation ready: yes, for Dirac fermions (MCFM/Sherpa(soon)/Powheg(soon))
• References: arXiv: 1410.6497, arXiv: 1411.0535
• Main questions:
  • are the Yukawa coupling pre-factors independent, so different assumptions can be generated independently?
  • does the kinematic change in case of other DM types
  • we cannot necessarily neglect DM or mediator couplings to the Higgs
    • Do we want to consider Higgs-portal models separately, where the Higgs is an additional mediator in addition, or do we treat those as specific cases of the more general one (see http://arxiv.org/abs/1411.0535 and http://arxiv.org/abs/1410.6497)? We could mention further less-simplified models in the write-up (Khoze and collaborators / Lin and collaborators are working on similar model - see attachment)
    • How will the width/kinematics change due to those couplings?
    • Do we use invisible decays of the Higgs / Higgs couplings to constrain the parameter space we choose for scans of those models?

Scalar mediator, s-channel top loop model from U. Haisch and E. Re's talks and 1410.6497

• Dark Matter type: Dirac fermion, scalar
• Mediator type: Scalar, pseudoscalar (complex case)
• Mediator masses: M_phi, M_a
• Channel: s-channel, but also EFT
• SM Couplings : only coupling to top
• DM Couplings: no Yukawa structure, g_DM
• Includes lepton couplings: no
• Width: Calculable
• Main signatures: 2jet+Met, can exploit angular correlations
• ME implementation ready: in progress, Powheg
• References: arXiv:1311.7131
• Main questions:
  • validity of EFT (do we want to include it?)

Vector mediator, includes models commonly used so far by ATLAS and CMS

• Dark Matter type: Dirac fermion
• Mediator type: Vector
• Mediator masses: M_V
• Channel: s-channel
• SM Couplings : g_q, universal coupling for quarks only (this would be the most model-independent assumption, theorists will worry about anomalies and correlations between quark and lepton couplings)
• DM Couplings: g_DM
• Coupling possibilities:
  • Vector coupling to DM, axial vector coupling to SM (DM@LHC v2 proceedings)
  • Vector coupling to DM, vector coupling to SM (1308.6799 and DM@LHC v2 proceedings)
  • Axial vector coupling to DM, axial vector coupling to SM (1308.6799)
  • Axial vector couplings to DM, vector couplings to DM (Haisch/Cacciapaglia's suggestion) - is it distinguishable from others?
• Includes lepton couplings: no
• Width: Calculable
• Main signatures: monojet, >=2jet+Met (NLO)
• ME implementation ready: yes, Powheg/MCFM (in progress)
• References: arXiv:1311.7131, DM@LHC v2 proceedings
• Main questions:
  • width assumptions might change the nature of the mediator and its constraints from direct mediator searches. Consider this as minimal width?
  • DM@LHCv 2 proceedings reckon it's difficult to get purely axial vector couplings to all quarks consistent with SM Yukawa and MFV. Agreement?
  • if g_DM = g_q = 1, is this model still observable at the LHC?
  • should we have a choice of couplings g_DM = g_q that allows us to compare monojet and dijet searches?

General choices and considerations for all models:

• m_DM, m_Med are free parameters to scan
• Consider all 6 quarks for width calculation
• Consider loop generation of mediator as well when necessary, specialized event generators available
• All width calculations should be considered as minimal and scanned with one or two wider assumptions
• It will be difficult to do generator-level scans for experiments, full simulation is needed in many cases and desired by the collaborations. We can however provide truth-level acceptances and reconstruction correction factors so that it is the theorists doing the scan and reinterpreting the experimental results.

General questions:

• Why do we restrict ourselves to Dirac DM? What would change with scalar (real/complex) or vector, or Majorana fermion DM?
• How seriously do we consider other constraints on simplified models? E.g. arXiv: 1501.03490
• How simplified should our models be? Should we consider "less-simplified" models (eg, scalar mediator mixing with the Higgs, does not introduce too many extra parameters) within the original starting list?

-- CaterinaDoglioni - 2015-01-24