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Elementary Particle Physics

Two models, one analysis

Munich, April 7, 2021

Just in time for this year's “Rencontres de Moriond” conference, The ATLAS collaboration released two new results with searches for the supersymmetric partners of the top and the bottom quarks. These searches have been carried out in the group of Dr. Alexander Mann at the chair of elementary particle physics at the Ludwig-Maximilians-Universität.

Picture: Photos of the massive mural of the ATLAS detector at CERN Point 1 painted by artist Josef Kristofoletti (Copyright CERN 2010)

Pictures: 1 2 3

Conferences are important occasions for scientific exchange. Researchers from all over the world meet in one place to present and discuss their latest findings. Due to the coronavirus situation, this year’s edition of the “Rencontres de Moriond”, one of most important international conference series in high-energy physics, took place in a virtual format. Just in time for this conference, the ATLAS collaboration released two new results with searches for the supersymmetric partners of the top and the bottom quarks, called top squarks and bottom squarks for short. These searches have been carried out in the group of Dr. Alexander Mann at the chair of elementary particle physics at the Ludwig-Maximilians-Universität.

In these two searches, the researchers look for new phenomena which are not described by the current Standard Model of particle physics. Specifically, they target collision events produced by the Large Hadron Collider at CERN which sport one or two jets that arise from the hadronization of b-quarks in addition to one or two tau leptons that decay hadronically. These are highly complex types of signatures, both of which can only be reliably identified through the use of advanced machine-learning techniques. Events like these would arise from the production of pairs of supersymmetric top or bottom squarks, which then promptly decay into Standard Model particles that are registered in the ATLAS detector.

The search for top squarks is described in this preliminary conference note for Moriond. One of its special features is that it cannot only be used to search for the traces of supersymmetric particles, but also to search for another promising extension of the Standard Model: the existence of leptoquarks. Leptoquarks are hypothetical particles that have received a lot of attention by theorists recently, because they can provide a potential explanation of the intriguing anomalies that are observed in the decays of different types of B mesons. They can decay into a lepton and a quark, hence the name, and a feat which would set them apart from all particles that have ever been observed.

The search for bottom squarks is a joint project with researchers from the university of Bergen. It is documented in a paper submitted to Physics Review D and already available as a preprint from the arXiv webpage. A nice summary of the results can also be found in this ATLAS briefing.

As the result of both analyses the detector data is found to be in very good agreement with the background prediction obtained from the Standard Model. Alas, this good agreement rules out the existence of both the supersymmetric particles or leptoquarks up to a certain mass, at the conventional confidence level of 95%: if the supersymmetric particles or leptoquarks predicted in these new models existed with a mass below these bounds, the chances to miss them are low, as they would have lead to a discrepancy between the observed yields and the Standard Model prediction. But they could still be heavier, because then the rate at which they are produced in the proton-proton collisions at the Large Hadron Collider are too low to find them in the current dataset. The search will thus continue when the Large Hadron Collider will be fired up for the next time, which is planned for 2022.

Further reading:

  • ATLAS-CONF-2021-008: ATLAS Collaboration, Search for new phenomena in pp collisions in final states with tau leptons, b-jets, and missing transverse momentum with the ATLAS detector
  • arXiv:2103.08189: ATLAS Collaboration, Search for bottom-squark pair production in pp collision events at √s=13 TeV with hadronically decaying τ-leptons, b-jets and missing transverse momentum using the ATLAS detector
  • Overview of further ATLAS results for the Moriond conferences: here