Periodic Reporting for period 4 - DARKJETS (Discovery strategies for Dark Matter and new phenomena in hadronic signatures with the ATLAS detector at the Large Hadron Collider)
Periodo di rendicontazione: 2020-08-01 al 2021-07-31
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 “collision events”), 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 humankind 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. This project concluded in 2021 having achieved all its objectives.
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 “collision events”), 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 humankind 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. This project concluded in 2021 having achieved all its objectives.
With the DARKJETS team, we have introduced and successfully published a search for particles that propagate the interactions between standard matter and dark matter, using a new real-time analysis technique for the ATLAS detector, called "Trigger Level Analysis". With this technique, we have reached a sensitivity to these particles with masses between 400 GeV and 1 TeV that would not be achievable using traditional analysis techniques. The results were published in Physics Review Letters. We then pushed the reach of the ATLAS detector for these particles further, using traditional data taking but a process that had never before used at the LHC in the search for these particles. This extended the sensitivity to these dark matter mediators to masses above 250 GeV, in a publication on Physics Letters B.
We have searched for new particles in the context of Supersymmetry using a technique that is complementary to both the approaches above and can reach masses of 100 GeV, published in EPJC. As a follow-up of that search, we also have started a future-looking search program for new particles from dark matter models whose complexity resembles that of the Standard Model, and the first paper in this line of research is being finalised.
As a necessary step for the success of all these results, the team has been very active in the operations and performance of the experiment, with tasks that benefitted the entire collaboration such as shifts to monitor the quality of the data and contributions to the operations, calibration and performance of the experiments, leading to 3 peer-reviewed publications. The team members were appointed to positions of responsibility in ATLAS data taking and computing for the community.
Finally, the results have been interpreted in terms of dark matter models and compared to different experiments. The broad adoption of those models by the global community and their use as standard benchmarks for comparisons of different searches and experiments was enabled by the PI and team, via the leadership of international prioritisation efforts and the creation of different initiatives that connect different communities of researchers investigating the nature of dark matter (e.g. Update of the European Strategy of Particle Physics, Initiative for Dark Matter in Europe and Beyond, Snowmass for the input to the prioritisation of US particle physics). These efforts led to 7 peer-reviewed papers, a whitepaper for the European Strategy of Particle Physics, and five letters of intent that will lead to Snowmass whitepapers.
We have searched for new particles in the context of Supersymmetry using a technique that is complementary to both the approaches above and can reach masses of 100 GeV, published in EPJC. As a follow-up of that search, we also have started a future-looking search program for new particles from dark matter models whose complexity resembles that of the Standard Model, and the first paper in this line of research is being finalised.
As a necessary step for the success of all these results, the team has been very active in the operations and performance of the experiment, with tasks that benefitted the entire collaboration such as shifts to monitor the quality of the data and contributions to the operations, calibration and performance of the experiments, leading to 3 peer-reviewed publications. The team members were appointed to positions of responsibility in ATLAS data taking and computing for the community.
Finally, the results have been interpreted in terms of dark matter models and compared to different experiments. The broad adoption of those models by the global community and their use as standard benchmarks for comparisons of different searches and experiments was enabled by the PI and team, via the leadership of international prioritisation efforts and the creation of different initiatives that connect different communities of researchers investigating the nature of dark matter (e.g. Update of the European Strategy of Particle Physics, Initiative for Dark Matter in Europe and Beyond, Snowmass for the input to the prioritisation of US particle physics). These efforts led to 7 peer-reviewed papers, a whitepaper for the European Strategy of Particle Physics, and five letters of intent that will lead to Snowmass whitepapers.
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 some of 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 in ATLAS.
Another new search that had never been done at the LHC has also been published: the search for dark matter mediators that are produced in association with other particles (photons or jets of particles), published for the first time at the LHC with major contributions from the team (and the PI as editor).
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. The team has used that as a stepping stone to prototype a new search for a different kind of dark matter processes that will also 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. This search is part of a pioneering new line of LHC searches for new dark matter models.
In the general context of dark matter at colliders, this project was instrumental to a paradigm shift for collider searches where we refocused our search targets from effective field theories to models with a mediator/portal particle, so to exploit the capability of colliders and accelerators to also detect the visible decay products of the mediator/portal particles. This change of perspective helps the contextualization of collider searches when comparing with other experiments, and can highlight strengths, shortcomings and complementarity of different searches. There is much more work to do to make those models theoretically robust and fully highlight their shortcomings as well as their strengths, and this continues beyond this project.
Another new search that had never been done at the LHC has also been published: the search for dark matter mediators that are produced in association with other particles (photons or jets of particles), published for the first time at the LHC with major contributions from the team (and the PI as editor).
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. The team has used that as a stepping stone to prototype a new search for a different kind of dark matter processes that will also 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. This search is part of a pioneering new line of LHC searches for new dark matter models.
In the general context of dark matter at colliders, this project was instrumental to a paradigm shift for collider searches where we refocused our search targets from effective field theories to models with a mediator/portal particle, so to exploit the capability of colliders and accelerators to also detect the visible decay products of the mediator/portal particles. This change of perspective helps the contextualization of collider searches when comparing with other experiments, and can highlight strengths, shortcomings and complementarity of different searches. There is much more work to do to make those models theoretically robust and fully highlight their shortcomings as well as their strengths, and this continues beyond this project.