Top Secrets: a new way to peek at the dark side of the Universe

12 January 2024

Photo: J. Niedziela & CMS/CERN

It is proposed to use the CMS detector to search for long-lived ALPs (in green) up to farther lifetimes. ALPs are produced together with pairs of top quarks (yellow) and can reveal the link between well-known and dark sector particles.

A team of scientists from the Cluster of Excellence Quantum Universe have proposed an innovative approach to search for particles connecting our visible world with the elusive world of Dark Matter. The study proposes new strategies to search for particles at the Large Hadron Collider (LHC), using collisions that simultaneously include both known, well-understood particles and undiscovered particles produced away from the LHC collision point. The results were recently published in the Journal of High Energy Physics.

Everything in our surroundings is made of particles - the tiniest building blocks of matter. Particle physics attempts to identify these building blocks and explain the interactions between them. One phenomenon that is still not explained by our current particle physics description is Dark Matter, which does not emit or absorb light, making it extremely difficult to observe. However, through its gravitational interactions, we know that it exists and constitutes 85% of the mass of the Universe. While a wide range of experiments, such as astronomical searches for Dark Matter in our galaxy, have made great efforts to unveil the mystery of Dark Matter, its true nature remains unexplained. Therefore, the next step in the journey of understanding this type of matter is to study its properties in a laboratory.

To study Dark Matter in a laboratory, particle physicists could try to produce it at the LHC and analyse how it affects the behaviour of other known particles. Despite many searches in the last decade, we have not yet seen any evidence of Dark Matter production at colliders. Therefore, applying new search strategies is essential to finding Dark Matter.

A new idea to link dark and visible matter is to search for particles called Axion-Like Particles (ALPs). The axion was first introduced to explain one of the most well-known questions in particle physics, called the strong CP problem (the puzzle that matter and antimatter behave the same for the strong nuclear force but not in the weak nuclear force). Since then, axions have evolved to the more general class of ALPs, which have become widely included in theories of new physics. For instance, ALPs could act as mediators in the interactions between known and dark particles. The Quantum Universe cluster of excellence has a well-established program focusing on ALPs from both the theoretical and experimental perspective, but searching ALPs at the LHC is relatively unexplored.

One of the properties of ALPs is that they interact more with heavier particles, making it interesting to explore how ALPs interact with the heaviest known particle - the top quark. This preferred interaction of ALPs suppresses their decay to other, lighter particles. By restricting ALPs to interact solely with top quarks, we further constrain their possible decay channels and, as a result, allow them to have long lifetimes and to travel macroscopic distances before converting into other particles.

Such long-lived particles could be created at the LHC, and the products of their decays could be registered by detectors such as ATLAS or CMS. Explicitly searching for particles decaying away from the collision point allows us to reject many typical and uninteresting processes. Due to quantum mechanical effects, long-lived particles with a given average lifetime can exhibit different decay lengths within the detector, or even outside the detector limits.

The presented study proposes to search for long-lived ALPs produced together with pairs of top quarks. This provides an opportunity to select cases that likely contain ALPs, because of their preferential interactions with heavy particles. As a result, the study estimates that long-lived ALPs with mean lifetimes up to 20 meters could be detected at the LHC, despite the limiting size of the detector. Furthermore, the upcoming upgrade of the LHC, High-Luminosity LHC, will further extend the discovery potential of ALP with lifetimes up to 300 meters, thanks to the increase in the amount of collected data.

Employing this new strategy, the secrets of top quarks are examined by Cluster of Excellence Quantum Universe researchers Juliette Alimena (DESY), Freya Blekman (DESY, UHH), Jeremi Niedziela (DESY), and Lovisa Rygaard (DESY, UHH). The authors conclude that experiments at the LHC should be able to discover low-mass ALPs, if they exist and have roughly the same mass as the proton. It would also be possible to observe long-lived ALPs, with mean lifetimes far beyond the limits of the detector size. Moreover, the High-Luminosity LHC shows great promise to further extend the discovery potential.

The next step is to examine the LHC data to see if these particles truly exist!