XENON1T: leading experiments in the direct search for dark matter

XENON1T, one of the flagship experiments in the direct search for dark matter, operating from 2016 to 2018 at the Gran Sasso National Laboratories (LNGS) of the INFN National Institute of Nuclear Physics, presented theanalysis of its latest data at an online seminar from the LNGS on June 17, showing an unexpected excess of events.

The observed signal has high statistical significance; it could be due to the presence a tiny amount of tritium, an isotope of hydrogen. But it could also be a sign of something much more exciting, such as theexistence of new particles, for example solar axions. Or, another interesting hypothesis, it could imply new properties of neutrinos.

Coordinated by Elena Aprile, of Columbia University in New York, the XENON project hunts for dark matter particles, and among them their most credited version, the so-called WIMPs (Weakly Interacting Massive Particles). For these the XENON1T detector has obtained the most stringent limits on their probability of interaction with ordinary matter, over a wide range of possible masses.

In general, the liquid Xenon experimental technique is also sensitive to other types of particles and interactions, which are very rare and can also explain other open problems in physics and astrophysics. In 2019, for example, again using XENON1T data, scientists published on the cover of Nature a measurement of the rarest nuclear decay that has ever been directly observed.

The construction of the XENON1T detector used 3.2 tons of ultra-pure liquid xenon, 2 t of which constituted the sensitive region of the detector. When a particle passes through the xenon, it generates a weak light signal and releases a handful of electrons, thus revealing its presence. The ratio of the light produced to the number of electrons released makes it possible to identify the type of interaction, i.e., the nature of the particle. Most of these interactions are due to particles whose existence is known and the number of interactions produced by them in the detector can be calculated with great accuracy.

Events produced by known particles constitute a noise floor for dark matter detection and their number determines the sensitivity of the detector. XENON1T was the most sensitive detector, with the lowest background ever obtained in the search for dark matter.

However, when the XENON1T data were compared with the expected background, an excess of 53 events was observed compared with the 232 expected to be observed. The excess is mainly present at low energy, below 7 keV, and is due to events distributed uniformly throughout the sensitive volume of the detector and throughout the data acquisition period.

Three at present appear to be explanations for the observed excess.

Itis possible that an infinitesimal amount of Tritium was present in the detector; this would constitute an additional source not considered in the calculation of background events. A few tritium atoms out of ¹⁰²⁵ (10,000,000,000,000,000,000,000!) xenon atoms would be enough to explain the observed excess. Tritium is a hydrogen isotope that spontaneously decays by emitting an electron with energy contained in the energy range of the observed signal; it may be naturally present in trace amounts in the materials used to construct the detector. At present, there are no independent measurements to confirm or refute the presence of tritium in the detector, so a definitive answer to this explanation is not yet possible.

A second, and much more exciting, possibility is that the excess is due to the existence of a new particle, namely solar axions. Indeed, the observed excess has an energy spectrum very similar to that predicted for axions produced in the Sun and interacting in the XENON1T detector. Axions are a hypothetical particle proposed to explain a particular symmetry in strong nuclear interactions (i.e., the forces that hold the nuclei of atoms together), and the Sun could be a powerful source of these particles. Solar axions are not candidates for dark matter, but their discovery would mark the first observation of a theoretically well-motivated and as yet unobserved class of particles, with a major impact in the understanding of particle physics and astrophysical phenomena. In addition to solar axions, which could explain the excess of events observed by XENON1T, the existence of other axions that would have been produced in the primordial universe has also been hypothesized and could, the latter yes, constitute a source of dark matter.

Alternatively, the excess could be due to neutrinos, billions of which pass through our bodies undisturbed every second. This interpretation would imply that the magnetic moment-a property of all elementary particles related to their probability of interaction-of the neutrino is much larger than predicted by the Standard Model.

Of the three possible explanations considered by the XENON collaboration, the observed excess seems to favor that of the signal from solar axions. In statistical terms, the solar axion hypothesis has a significance of 3.5 sigma, equal to a probability of 2 in 10,000 that the excess is due to a random fluctuation in the background, rather than a new signal. Although this significance is quite high, however, it is still not sufficient to declare the discovery--for which at least 5 sigma is required--of solar axions, The significance of the tritium and neutrino magnetic moment hypotheses correspondsto 3.2 sigma, so they are also well compatible with the experimental data.

Right now the XENON collaboration is proceeding with the construction of XENONnT, a new and more sensitive detector, successor to XENON1T. XENONnT will have 3 times more xenon mass and a further reduced expected background compared with XENON1T. With future data from XENONnT, the XENON collaboration expects to find out whether the excess measured by XENON1T is due to a simple statistical fluctuation, a new component of the background, or something more interesting: the signal of a new particle or an interaction beyond the Standard Model.

A group of Neapolitan physicists from the Department of Physics "Ettore Pancini" under the responsibility of Michele Iacovacci is actively participating in the XENON project. Their contribution is part of the program to reduce background events by building a system capable of identifying neutrons that in the 8 tons of xenon liquid of XENONnT could simulate events from dark matter interactions. We are talking about very few events, about 5 in 4 years of operation, which the intervention of the neutron veto will reduce to about 1 event. Reduced funding and increased detector mass will allow XENONnT to improve sensitivity to rare events by a factor of 10 and hopefully measure the first dark matter interactions.

 

LINK
The XENON1T experiment
The construction of XENON1T at LNGS (movie)

International head of XENON collaboration.
Elena Aprile | xe-pr@lngs.infn.it|+39 3494703313 | +1 212 854 3258

Head of XENON group in Naples
Michele Iacovacci, Department of Physics "E. Pancini," michele.iacovacci@unina.it,+39 081676128 | +39 3388853113