The NA64 experiment at CERN is at the forefront of the search for dark matter, particularly within thermal dark matter models involving sub-GeV particles. This experiment has uncovered a significant breakthrough: the identification of an interaction mediated by the elusive dark photon A0.
Image Credit: Dong liu/Shutterstock.com
This research was published in the journal APS Physical Review Letters on the 16th October 2023.
The Dark Matter Enigma
Dark matter, an invisible entity constituting a significant fraction of the universe's mass, remains one of science's great enigmas. Thermal dark matter models, particularly those featuring sub-GeV particles, emerge as compelling candidates for solving the riddle.
According to these models, dark and ordinary matter were once in thermal equilibrium, mutually neutralizing at equivalent rates.
As the universe expanded and cooled, the equilibrium between dark matter and standard model particles was disrupted, resulting in an enigmatic remnant of dark matter density. To address this situation, a new form of interaction that connects dark matter and standard model particles is required.
The Dark Photon's Role
Central to this interaction is the dark photon A0, serving as the mediator between dark matter χ and ordinary matter. The crux of this interaction lies in the kinetic mixing term (ϵ/2)F0μνFμν, where Fμν and F0μν represent the stress tensors of the photon and dark photon fields, and ϵ refers to the mixing strength.
The dark photon A0, linked to the spontaneously broken UD(1) gauge group, possesses a dark coupling strength eD. This coupling finds expression as:
Lint = -eDA0μJμD, with JD representing the dark matter current.
The mixing term, in turn, engenders an interaction Lint = ϵeA0μJμem between A0 and the electromagnetic current Jμem, where e stands for the electromagnetic coupling.
Understanding these intricate connections is pivotal as they lay the foundation for unraveling the mysteries of dark matter and its interaction with the standard model of particle physics.
Navigating the Parameter Space
The exploration of thermal scalar and fermionic dark matter models hinges on a meticulous study of the parameter space. This revolves around the predicted existence of sub-GeV χ particles, manifesting in diverse forms: scalar, Majorana, or pseudo-Dirac particles, all coupled to the dark photon A0.
Crucial to this exploration is the determination of parameter values defining the annihilation cross-section, the relic dark matter density, and the dark coupling strength αD. These parameters unveil the intricate interplay between the dark photon's mixing ϵ and the coupling αD, offering valuable insights into the nature of dark matter.
The NA64 Experiment
In the quest for sub-GeV dark matter, the NA64 experiment at CERN takes center stage. The experiment's ingenious design orchestrates collisions between 100 GeV electrons and an active target to unearth the secrets of sub-GeV dark matter. The experimental setup comprises scintillator counters, a magnetic spectrometer, calorimeters, and a suite of detectors, all meticulously crafted for identifying and characterizing particles.
The magnetic spectrometer, a key component, ensures the precise reconstruction of the incoming electron's momentum, achieving an impressive 1% accuracy. This, combined with an array of detectors such as Micromegas and straw-tube chambers, enables a comprehensive understanding of particle interactions within the experiment.
Overcoming Background Challenges
The pursuit of sub-GeV dark matter is not without its challenges, notably background signals that can obscure sought-after discoveries. These background sources span dimuon losses, mistagged μ, π, K decays, escaping neutrals, and punchthrough of neutral hadrons.
Researchers employed innovative techniques to mitigate these background signals. Well-considered selection criteria, rigorously supported by simulations and data analysis, help identify and reject unwanted signals, ensuring clarity for accurate observations.
Statistical Analysis and Revealing Results
Statistical analysis plays a pivotal role in the quest for understanding dark matter. By employing advanced techniques, researchers have determined upper limits on the mixing strength ϵ, unearthing valuable insights.
The results of this experiment are nothing short of groundbreaking. The team has established the 90% confidence level exclusion limits for ϵ, unveiling the intricate relationship between ϵ and the A0 mass. These results are poised to reshape our understanding of dark matter, opening new avenues for exploration and research.
Implications for Dark Matter Models
The significance of the NA64 experiment's findings extends beyond the laboratory, offering constraints on thermal dark matter models. The (y; mχ) and (αD; mχ) parameter planes for low-mass dark matter particles now charted provide insights into the challenges facing certain dark matter scenarios and offer the motivation for further exploration.
With each breakthrough, we inch closer to unraveling the enigma of dark matter.
Andreev, Yu.M. et al. (2023) ‘Search for Light Dark matter with NA64 at CERN’, Physical Review Letters, 131(16). doi:10.1103/physrevlett.131.161801. https://journals.aps.org/prl/abstract/10.1103/PhysRevLett.131.161801