Search results for dark photon leptonic decays manage to exclude new regions

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Dark photons are hypothetical particles that resemble light particles (i.e., photons), but interact weakly with normal matter, which would make them impossible or very difficult to detect using conventional experimental methods. These particles are dark matter candidates, meaning that they could contribute to the invisible and elusive form of matter accounting for approximately 85% of the universe's mass.

The NA62 Collaboration, a large research collaboration involving scientists at various institutes worldwide, has published the results of a new search for dark photons, specifically for their leptonic decays. Their findings, published in Physical Review Letters, were derived by analyzing data collected by the NA62 detector at CERN configured in beam-dump mode.

"Dark matter searches are currently one of the hot topics in the high energy physics community. We look for weakly interacting particles in a number of different facilities ranging from accelerator experiments to tabletop laboratory setups," Alina Kleimenova and Stefan Ghinescu, part of the NA62 Collaboration, told Phys.org.

"While LHC experiments rely on the high collision energy, smashing protons at about 14 trillion electron volts, NA62, being a fixed-target experiment, focuses on the high intensity approach with a quintillion (10¹⁸) of protons on target per year. This intensity creates a unique opportunity to probe various rare processes and beyond Standard Model scenarios."

Dark photons, also referred to as A', are among the hypothetical particles beyond the Standard Model whose existence could be probed by the NA62 detector. These particles could act as mediators between known visible matter and dark matter.

Specifically, dark photons might couple to ordinary matter, as they could mix with photons described by the Standard Model. The coupling, however, would be extremely weak, which would explain why they have not been detected so far.

"This feeble interaction translates into a long lifetime, meaning in NA62 settings, A' would travel from tenths of centimeters to hundreds of meters before decaying," said Kleimenova and Ghinescu.

"Theoretically, if the dark photon is the lightest dark matter particle and has a mass below approximately 700 MeV, it would primarily decay into lepton pairs, such as electrons or muons. NA62 has all the necessary ingredients to be able to possibly see these decay signatures, featuring a very long beam line (over 80 m from the target to the decay volume), precise tracking, timing and particle identification systems and the possibility to collect these data in an almost background-free mode."

The primary objective of the recent study by the NA62 Collaboration was to investigate the sensitivity of the NA62 detector at CERN to dark photon decays. By analyzing the data collected by the detector while it was configured in so-called dump mode, the team hoped to identify signals that could be associated with dark photons.

"NA62 is a kaon experiment dedicated to precision measurements and studies of rare kaon decays," explained the authors. "The experiment can also be operated in 'dump mode.' In this mode, we can remove the target used to produce kaons and dump the 400 GeV proton beam onto an absorber at twice the usual intensity."

Theoretical predictions suggest that interactions between protons and dump material in the NA62 detector could produce various particles in hidden sectors of the light spectrum with masses around 1 GeV, including dark photons. These particles could then travel and decay in the instrumented region of the NA62 experiment.

"What we search for in our analysis is an event with only two opposite charged lepton tracks, which form a vertex inside the NA62 instrumented region," said the authors. "Since this event should originate from the proton-dump collision, we trace the two-lepton vertex 80 meters back to the front plane of the absorber and check if this traced position is compatible with the location of the primary proton interaction point."

As part of their recent study, the researchers analyzed a data sample of 1.4×10¹⁷ protons on dump collected by the NA62 detector in 2021. In the meantime, however, the detector has collected additional data and is expected to reach approximately 10¹⁸ protons on dump by the end of the NA62 experiment.

"Unfortunately, we did not find any evidence of dark photons, but we managed to exclude new regions in the dark photon mass and interaction strength parameter space," said the authors. "In addition, our results can be reinterpreted within other models, for example those involving axion-like particles."

While the team did not detect dark photon decays yet, their recent findings could inform future searches for these elusive particles. Kleimenova, Ghinescu and their colleagues are now working on combining their results with the findings of the collaboration's hadronic final states analysis.

"This current effort would conclude a comprehensive search for dark matter mediators using data collected by NA62 in 2021," add the authors.

"Our ultimate objective is to extend this analysis to the entire NA62 dump dataset. Furthermore, there are a few more Hidden sector scenarios which could be investigated by NA62, for example, Heavy Neutral Leptons (HNLs). HNLs are particularly interesting because they can address several key problems in particle physics and cosmology, such as the origin of neutrino masses, the matter-antimatter asymmetry in the universe, and the nature of dark matter."

More information: E. Cortina Gil et al, Search for Leptonic Decays of Dark Photons at NA62, Physical Review Letters (2024). DOI: 10.1103/PhysRevLett.133.111802.

Journal information: Physical Review Letters

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