Search for Dark Matter Archives

DarkSide-20k as a Neutrino Observatory

Author: Lucy Kotsiopoulou

Beyond the Dark matter search

The goal of DarkSide-20k is ultimately to try and detect particle-like dark matter. But what if it could also detect neutrinos? Neutrinos are the lightest particles with the smallest mass of all the elementary particles within the Standard Model, so light that they were believed to be massless until about twenty years ago. They come in three flavours, the electron neutrino νe, muon neutrino νμ and tau neutrino ντ, and only interact via the Weak force and Gravity. In fact, they interact so weakly with matter that every second 100 trillion neutrinos pass through your body. Nearly twenty years ago, it was experimentally observed that they oscillate between their flavours and are in actuality massive. They are very tiny indeed… but not as tiny as originally thought! As it happens, there is a high probability that they all will be released from a very special galactic source: a supernova explosion. Looking further back, nearly thirty years ago the most recent near to Earth supernova, SN 1987A in the Large Magellanic Cloud, exploded, providing the only occasion on which supernova neutrinos have been detected on Earth. After travelling for 168,000 years, and sweeping through Earth in around 13 seconds, a total of 25 supernova neutrino events were detected by three neutrino detectors at the time: IMB, Baksan and Kamiokande II. Now, in preparation for the next supernova explosion, specifically a Type II core-collapse one, as SN 1987A, the only thing one can do is wait. Or maybe not?!

Exclusion limits at 90% C.L. set by DarkSide-50 on axio-electric coupling constant, compared to results from other experiments (SuperCDMS, XENON1T, XENONnT, PandaX-II). Figure from Phys. Rev. Lett. 130, 101002

Fig. 1: Sensitivity to neutrinos from a core-collapse supernova according to its distance from the Earth, for a detection in DarkSide-20k and the future ARGO uniquely performed through Coherent Elastic Neutrino Nucleus Scattering (Ref: JCAP 03 (2021) 043)

The visible light when there is a supernova explosion in the sky can be as bright as the moon and visible even in daylight. Nonetheless, this light represents less than 0.01% of the total energy released from the star’s final moments. 1% becomes the kinetic energy of the debris of the star being plunged outwards, while 99% is neutrinos of all flavours launching into outer space. Astronomers gather plentiful information about the explosion by detecting the light through telescopes, and have been doing so for centuries. However, this light can last from only a few seconds up to a few days, and often it is not in the visible spectrum, leaving astronomers unprepared as to where to turn their telescopes. Neutrinos however are different; as they only interact very weakly with the matter within the star just before it explodes, they can escape before the photons of light and reach Earth earlier, alerting us to incoming photons. Therefore neutrino and dark matter detectors can form a crucial role in predicting the next supernova, mainly within the joint effort of the Supernova Neutrino Early Warning System (SNEWS). Many neutrino experiments of all kinds and dark matter experiments are part of this programme and are poised for action. DarkSide-20k is now working on becoming its newest member!

The impact of DarkSide-20k within SNEWS

Supernova neutrinos have an energy of about 15 MeV, which for a neutrino is considered low. As DarkSide-20k is a dual-phase TPC liquid argon detector, the detection method will be based on interactions between the incoming neutrino and the argon nuclei floating within the confines of the detector. There are two ways in which a supernova neutrino of the above energies could interact: either by scattering off the entire nucleus or by actually breaking it apart. The first process is called Coherent Elastic neutrino (ν) – Nucleus Scattering (CEνNS) and the signal that will be detected by DarkSide-20k’s TPC is the small recoil energy from the scattered argon nucleus, thanks to the amplified signal in the gas pocket at the top of the TPC, capable of detecting recoils as low as 0.5 keV. For a more technical description, our published paper can be found at the end of this article as well as on our Publications page. This paper focuses only on the CEνNS interactions, where our experiment can detect all three types of neutrinos. Thanks to CEνNS only,DarkSide-20k will be able to discover a supernova explosion as far out as the edge of the Milky Way galaxy for a star eleven times the size of our Sun or bigger. The other process however, in which the nucleus is broken apart by the neutrino, at this energies can only occur with electron neutrinos. This reaction, called the Charged Current Interaction (CCI) or electron neutrino absorption in argon, releases an electron and an excited potassium nucleus. By combining the information from both interactions, one can compare the total flux of neutrinos reaching Earth with only the electron neutrinos, yielding possible answers to how the neutrinos oscillated within the supernova before reaching Earth.

Thanks to such a high-statistics, multi-channel, multi-messenger observation from the next event we will be able to constrain the details of the last moments of a star’s life, while also providing an independent inference of the nature of neutrinos and their mass, yielding answers to the neutrino mass hierarchy problem: examining order of their masses, and which are the lightest.

Sources:

The DarkSide-20k Collaboration, “Sensitivity of future liquid argon dark matter search experiments to core-collapse supernova neutrinos” (2020), https://arxiv.org/pdf/2011.07819
Horiuchi S, Kneller JP, “What can be learned from a future supernova neutrino detection?” (2018), https://arxiv.org/pdf/1709.01515.
Phillips, AC, “The Physics of Stars”, 2nd Edition (1999).

Search for sub-GeV Dark Matter

Search for Dark Matter

Author: Stefano Piacentini

Light Dark Matter Candidates: a General Overview

Traditionally, experiments located in underground laboratories have focused on searching for dark matter (DM) particles with masses ranging from a GeV/c² to a few TeV/c² by looking for their interactions with ordinary matter via elastic scattering off atomic nuclei. These interactions are usually called nuclear recoils (NR). In principle those “heavy” DM particles can also undergo scattering off atomic electrons, the so-called electron recoils (ER), but the cross section of such interactions is kinematically disfavoured given the large difference in mass between the two particles – DM and the electron. The possible existence of DM particles in this mass region is strongly supported by theoretical motivations – see the WIMP section. However, the lack of uncontroversial evidence of direct detection of heavy DM particles motivates the search for particle candidates outside the traditional mass range. In this regard, the sub-GeV/c² mass region is well theoretically motivated and there are a plethora of theories providing viable DM candidates. Being predicted in a mass range which is much smaller than the mass of the nucleons, those light particles are usually searched for by looking at their scattering off, or absorption by, atomic electrons. In the following sections we will shortly describe light DM candidates that have been tested so far with DarkSide detectors

The Axion-like particles

Axions have been theorised by R. D. Peccei and H. R. Quinn as an ingenious solution to the so-called strong CP problem of the QCD (Quantum ChromoDynamics), namely the lack of predicted CP violation in strong interactions. The solution proposed by Peccei and Quinn is to add an additional spontaneously broken chiral symmetry to the Standard Model of particle physics. With this assumption, the presence of a new neutral massive bosonic pseudo-scalar particle, called “QCD axion”, is predicted. More in general, we usually refer to any pseudo-scalar bosonic particle that arises from the spontaneous breaking of a global symmetry as “axion-like particles” (ALPs), even if not necessarily addressing the strong CP problem. ALPs and axions are promising DM candidates and, with masses spanning several orders of magnitude in the sub-GeV/c² mass region, they can satisfy the stability requirements needed to explain the abundance of DM that we evaluate from observations today in the universe. Based on the most relevant theories, they are coupled to electrons and photons and can be produced by astrophysical sources, like our Sun, being therefore in principle detectable on Earth. In our detectors, the ALPs can interact with electrons via two mechanisms: the axio-electric effect, analogous to the photoelectric effect but with axions replacing photons, and the “inverse Primakoff” effect, where the ALPs are converted into regular photons during the interaction with the electrons. Figure 1 shows the constraints set by DarkSide-50 on the axio-electric coupling constant gAe.

Figure 1. Exclusion limits at 90% C.L. set by DarkSide-50 on axio-electric coupling constant, compared to results from other experiments (SuperCDMS, XENON1T, XENONnT, PandaX-II). Figure from Phys. Rev. Lett. 130, 101002

Sterile neutrinos

According to the Standard Model of particle physics, there are three species of neutrinos – 𝜈e, 𝜈𝞵, and 𝜈𝞽 – which can interact with electrons, muons, and tauons via weak interactions. Since they have a very small mass, not greater than O(0.2 eV/c2), they could only contribute to a small fraction of the total amount of DM in our universe. However, there are theories that enlarge the set of neutrinos by adding a new neutrino particle that has no weak, strong or electromagnetic interactions with the other particles of the Standard Model: the so-called “sterile” neutrino 𝜈s. The only interaction between the sterile neutrinos and the other particles of the standard model can happen only via oscillations to standard model neutrinos. In the case in which the mass of this particle is of the order 1 keV/c2 or above, the sterile neutrino is a viable DM candidate, as it’s neutral, heavy enough, interacting very weakly, and stable over a cosmological timescale. Some of their decays can produce monochromatic O(keV) X-ray emissions, and today there are hints of observations of such peaks in the X-ray spectrum of galaxies or galaxy clusters with an expected high DM density.

In DarkSide detectors, the sterile neutrinos could be detected via the process 𝜈s + e → 𝜈e + e (and its antineutrinos counterpart), parameterised by the mixing angle |Ue4|2 between this fourth-family lepton and the electronic one. This absorption process by argon shell electrons would result in a monoenergetic signal at the particle’s rest mass. Figure 2 reports the results of the search for such DM candidates using the DarkSide-50 data.

Figure 2: Exclusion limits at 90% C.L. set by DarkSide-50 on mixing angle |Ue4|2 for sterile neutrinos. Figure from Phys. Rev. Lett. 130, 101002

Dark photons

The dark photon is a hypothetical vector-boson particle which has the role of force carrier for the so-called “hidden sector”, a new set of particles that are not directly interacting with the Standard Model particles. The dark photon, which acts as a regular Standard Model photon in the hidden sector, can interact with the Standard Model one via a mechanism known as kinetic mixing. The coupling between the regular photon and the dark photon is therefore the only mechanism through which the hidden sector is connected to the Standard Model.

The dark photons and the particles in the hidden sector are naturally viable dark matter candidates: massive, neutral, very weakly coupled to the Standard Model particles, and stable over a cosmological timescale. Like ALPs, in the DarkSide detectors they could be detected via their absorption by argon electrons. Also in this case, the absorption would result in a monoenergetic signal at the particle’s rest mass. Figure 3 shows the constraints on the dark photon kinetic mixing parameter 𝜅 obtained analysing the DarkSide-50 dataset.

References

[1] Phys. Rev. Lett. 130 (2023)

[2] Phys. Rev. D 107 (2023)

Wimps!

Search for Dark Matter

There is a wide range of astronomical evidence that the visible stars and gas in all galaxies — including our own — are immersed in a much larger cloud of non luminous matter, typically containing much greater (by orders of magnitude) amounts of mass. The existence of this “dark matter” is consistent with evidence from large-scale galaxy surveys and cosmic microwave background measurements, which indicate that the majority of matter in the universe is non-baryonic. The nature of this non-baryonic component is still totally unknown, and the resolution of the “dark matter puzzle” is of fundamental importance to cosmology, astrophysics, and elementary particle physics. One leading explanation, motivated by supersymmetry theory, is that dark matter is comprised of as-yet-undiscovered Weakly Interacting Massive Particles (WIMPs) formed in the early universe and subsequently gravitationally clustered in association with baryonic matter.

Schematic representation of rotating disc galaxies in the distant Universe and the present day. Observations with ESO's Very Large Telescope suggest that such massive star-forming disc galaxies in the early Universe were less influenced by dark matter. As a result the outer parts of distant galaxies rotate more slowly than comparable regions of galaxies in the local Universe. Their rotations curves, rather than being flat, drop with increasing radius.

Figure 1: Schematic representation of rotating disc galaxies in the distant Universe and the present day. Observations with ESO's Very Large Telescope suggest that such massive star-forming disc galaxies in the early Universe were less influenced by dark matter. As a result the outer parts of distant galaxies rotate more slowly than comparable regions of galaxies in the local Universe. Their rotations curves, rather than being flat, drop with increasing radius.

Direct detection

According to the WIMP standard scenario, galaxies like ours are submersed in a “halo” filled with dark matter, in thermal equilibrium with the standard luminous matter that we observe everyday with telescopes and antennas. As we rotate together with the Sun, around the center of our Galaxy, we are hit by an apparent “wind” of WIMPs, too feebly interacting with standard matter to be sensed or seen. In principle, WIMPs could be detected in terrestrial experiments through their collisions with ordinary nuclei, giving observable low-energy (below 100 keV) nuclear recoils. The predicted collision rates are extremely small and require ultra-low background detectors with large (1–100 tonnes) target masses, located in deep underground sites to eliminate neutron background coming from cosmic ray muons.

Noble elements like argon are ideal targets to investigate WIMPs: stable, chemically inert, still gets into an excited state whenever some particle scatters on either is nucleus (Nuclear Recoil) or kicks an electron (Electron Recoil).

De-Excitation light is released, together with ionization electrons and heat. While we do not look at the latter, DarkSide detectors are designed to measure both the “scintillation” and the ionization signal, as widely explained with DarkSide-50 experiment.

Why argon?

We need a target that is stable, transparent to its own scintillation light, and relatively easy to extract and purify from radioactive contaminants. These impurities—emitting electrons, alpha particles, gamma rays, or neutrinos—could otherwise obscure the rare and still-undiscovered WIMP signal we’re searching for.

Argon stands out as the only noble element that enables clear discrimination between nuclear recoils—from neutrons or potential WIMP interactions—and electron recoils, which arise from trace-level radioactivity in the detector materials. By analyzing the shape of the recorded waveforms, or more practically, by measuring the fraction of prompt scintillation light relative to the total pulse, we can distinguish electron recoil backgrounds from nuclear recoils with extraordinary precision: a misclassification occurs only once in ten million events.

This powerful pulse shape discrimination is key to suppressing backgrounds and paves the way for an instrumentally background-free WIMP search in next-generation experiments like DarkSide-20k.

$Figure 2: How does the Pulse shape discrimination look like in DarkSide-50 data? This is one of the crucial graphics in our data analysis, showing the amount of recorded scintillation light, S1, evaluated as total photoelectrons recorded by our photosensors, compared with the Pulse-shape discrimination parameter, f90 in DarkSide-50, corresponding to the fraction of the prompt scintillation light. Most of the backgrounds, electron recoils mainly from the beta-decay of the 39-Argon, sits at f90 about 0.3, as only 30 percent of the scintillation light in these events is released in the first 90 ns. On the other hands, WIMPs, giving Nuclear recoils, will have f90 centered at about 0.7, as shown in the violet area. Picture from Phys. Rev. D 98, 102006 (2018).$

Figure 2: How does the Pulse shape discrimination look like in DarkSide-50 data? This is one of the crucial graphics in our data analysis, showing the amount of recorded scintillation light, S1, evaluated as total photoelectrons recorded by our photosensors, compared with the Pulse-shape discrimination parameter, f90 in DarkSide-50, corresponding to the fraction of the prompt scintillation light. Most of the backgrounds, electron recoils mainly from the beta-decay of the 39-Argon, sits at f90 about 0.3, as only 30 percent of the scintillation light in these events is released in the first 90 ns. On the other hands, WIMPs, giving Nuclear recoils, will have f90 centered at about 0.7, as shown in the violet area. Picture from Phys. Rev. D 98, 102006 (2018).