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Report

Teaser, summary, work performed and final results

Periodic Reporting for period 2 - DARKJETS (Discovery strategies for Dark Matter and new phenomena in hadronic signatures with the ATLAS detector at the Large Hadron Collider)

Teaser

Summary

The Standard Model of particle physics is incomplete. One notably missing piece is dark matter, an invisible substance composing 85% of the matter in the universe. Dark matter does not emit nor reflect light, nor have we yet observed any known particle interacting with it. Dark matter is massive, since it is subject to gravity. It is through its gravitational effects on other matter in space that astronomers inferred its existence.

What is dark datter made of?

This is one of the unanswered questions that is missing from the understanding of the universe in which we live. A compelling hypothesis is that dark matter is comprised of particles that only interact weakly with conventional particles and have a much larger mass with respect to the constituents of ordinary matter. In this case, dark matter can be produced in the collision of other particles (called â€œcollision eventsâ€), such as those produced at the Large Hadron Collider (LHC), at the CERN laboratory in Geneva.

It is with this machine that my team and I are searching for particles that are produced alongside Dark Matter and for subtle effects due to the presence of dark matter particles, using one of the largest particle detector mankind has ever built, the ATLAS experiment.

One of the challenges faced by searches for these elusive dark matter particles and associated processes is the overwhelming amount of data necessary to be sensitive to processes this rare. Traditional data taking techniques cannot cope with this amount of data. Consequently, up to the advent of this project, the ATLAS experiment discarded the vast majority of collision events that could have contained new particles mediating a new force between known particles and dark matter with a mass below the tera-electronvolt (TeV). The main objectives of this project are to gain sensitivity to such resonances in searches at the ATLAS detector and lay a solid theoretical foundation for the interpretation of these results in a global context including dark matter experiments worldwide.

Work performed

We have reached one of the main objectives of this project by deploying a new data technique that allows to collect more data than ever before and used it to set the strongest constraints to date on models of dark matter mediated by these new particles, using LHC data recorded by the ATLAS detector in 2015 and 2016. The post-doctoral researcher in this project is now responsible for the overall data taking of the kind of physics object used in this search. This effort has led to two public notes and two peer-reviewed papers that have been presented and discussed at international conferences. These results have been disseminated to the general public and CERN community through the official ATLAS experiment website and the CERN courier, and we are preparing a press release for Swedish newspaper in collaboration with Lund University.

The second objective of this project, a new search that looks for a dark matter mediator particle produced in association with another known particle, has been done and made public as a conference note. One of the PhD student in this project is contributing significantly to an upcoming peer-reviewed publication, extending the original idea in the project to different associated particles and using identification techniques that helps distinguishing signal from background.

The third objective of the project, consisting of searches where two new particles are produced simultaneously and decay into four jets of particles in the detectors, has also yielded a first peer-reviewed publication containing new constraints on the theory of supersymmetry. The technical work needed to perform this search, which has been performed by a PhD student in this project, has also been submitted to a journal and is undergoing peer-review. The detector board that will improve this search is on track for installation in time for the next round of LHC data taking, and the PhD student working on the project is making progress towards its commissioning.

The fourth objective of this project is the interpretation of the results from the various searches into a common theoretical framework that allows the contextualization of LHC results within the global landscape of dark matter searches. While being PI of this project, I have been selected to be one of the two LHC-wide representatives of the Dark Matter Forum and Working Group, groups that are of interest for hundreds of physicists actively working under my leadership and subscribed to the mailing lists. I led these groups that are of interest for hundreds of physicists to develop a set of community, consensus-driven recommendations on the theoretical framework to be used for the interpretation of dark matter searches at the LHC. The dissemination of the work took place through international conferences and whitepapers published on the arXiv high energy physics paper repository.

Image sources:
* https://twiki.cern.ch/twiki/pub/AtlasPublic/TriggerOperationPublicResults
* https://atlas.web.cern.ch/Atlas/GROUPS/PHYSICS/CombinedSummaryPlots/EXOTICS/ATLAS_DarkMatterCoupling_Summary/ATLAS_DarkMatterCoupling_Summary.png
* https://arxiv.org/abs/1804.03496

Final results

The main contribution of the DARKJETS team to the advancement of the experimental state of the art in high energy physics is the introduction of a data taking technique that is new to the ATLAS detector. The amount of experimental data that can be recorded is generally limited by constraints in recording the selected collision events to permanent storage. The more data, the more chances there are to detect rare processes: traditional techniques that are only able to record a limited number of events decrease the sensitivity of the analysis to the presence of new particles. We have overcome this limitation by deploying a technique called Trigger-object Level Analysis for recording only the subset of information relevant to the searches. As this limited amount of information requires a much smaller event size to be recorded to disk, we can record order of magnitude more data (see figure) and obtain the most stringent constraints on the characteristics of this kind of dark matter mediators. In order to perform this search, we had to develop a new calibration technique that advances the state of the art. So far, we have used about one third of the dataset that will be collected by the end of the full LHC data taking period up to 2018 (called Run-2) and we expect to repeat this search with the full dataset.

A new search that had never been done at the LHC has also been made public: the search for dark matter mediators that are produced in association with other particles (photons or jets of particles). By the end of the project, we expect two additional peer-reviewed publication: one that is underway as mentioned above, and one containing an expanded suite of signatures of a new particle associated to known particles with the full LHC dataset.

The search for new particles produced in pairs and decaying into four jets of particles has been published for the first time at the ATLAS experiment with contributions by the PhD student. We are now working on an improved version of that search with the full dataset, as well as prototyping a new search for a different kind of dark matter processes that will benefit from the work on the detector hardware that the PhD student is working on and that will be installed in the detector by the end of the project for the next round of data taking.

Finally, by the end of the project we will contribute to a â€œlegacyâ€ paper on the ATLAS results of dark matter searches.