Mandate of the ATLAS-CMS Dark Matter Forum

Run 1 searches for 'mono-X' signatures at ATLAS and CMS employed an effective field theory [1] to quantify the reach of the analyses and to allow comparison with direct dark matter detection experiments. After Run 1 it is now clear, however, that in certain cases [2] the LHC can directly probe additional mediating particles that the EFT neglects. While the EFT integrates out the degrees of freedom of the (heavy) intermediate particle, concrete "simplified" model with low-­mass mediators can provide a complementary interpretation. Those models can be used both to interpret mono-­X searches and to guide the design of complementary searches for additional signatures.

To understand what signal models should be used for Run 2, groups of experimenters from both collaborations have held separate discussions with small groups of theorists [3,4]. Another joint discussion was held at the DM@LHC workshop in Oxford [5], in September 2014.

All three discussions concluded that simplified models with an explicit mediator between SM and DM particles constitute a desirable alternative to the EFT, and each proposed minimal sets of simplified models that should be used. Yet while these proposals largely agree on the overall approach, they differ in the priority and details of the different simplified models.

To harmonize the proposals, ATLAS and CMS organize a forum, the ATLAS-­CMS Dark Matter Forum, whereby both collaborations will agree on a minimal common set of Run-2 dark matter interpretations with the participation of interested theorists and experimenters.

As Run 2 data-­taking rapidly approaches, it is important to narrow the scope and timescale of this forum to quickly achieve the following goals:

1. A prioritized, small set of benchmark simplified models should be agreed upon by both collaborations for Run-2 searches.
2. The matrix element implementation of the simplified models should be standardized, and other common technical details (order of the calculation, showering) harmonized as much as practical. It would be desirable to have a common choice of LO/NLO, ME-­parton shower matching and merging, factorization and renormalization scales for each of the simplified models. This will also lead to a single set of theory uncertainties, which will be easier to deal with when comparing results from the two collaborations.
3. On the same timescale, the forum could also discuss the conditions under which the EFT interpretation may still be desirable.
4. An arxiv document should be prepared summarising these items, suitable both as a reference for the internal ATLAS and CMS audiences and as an explanation for theory and non-collider readers.

Everyone is welcome to contribute to the forum. The list of authors for the summary document will be limited to those who contribute.

ATLAS has appointed Caterina Doglioni and Antonio Boveia to organize the discussions and edit the document. CMS has similarly appointed Steven Lowette, Steve Mrenna, and Sarah Malik. Theorists might be added once the experimental discussion has started. Recommendations according to the above items 1 and 2 should be finalized by February 2015 at the latest for the purpose of timely MC production. The other items should be finalised within the following two months. The document may also address how to compare the collider results to non-­collider experiments.

It is not anticipated that the experiments will formally approve the document but it will be circulated to their relevant internal communities.

Once the document is finalized, the ATLAS and CMS management should also consider if a continuation of the activity in a more structured environment is envisaged and under which form.

[1] http://arxiv.org/abs/1008.1783
[2] http://arxiv.org/abs/1307.2253
[3] http://arxiv.org/abs/1409.2893
[4] http://arxiv.org/abs/1409.4075
[5] http://indico.cern.ch/event/312657/

-- CaterinaDoglioni - 2014-12-05