Detecting Ghostly Neutrinos that Skim Earth’s Crust

Paper Title: Trinity: An Imaging Air Cherenkov Telescope to Search for Ultra-High-Energy Neutrinos

Authors: Anthony M. Brown, Mahdi Bagheri, Michele Doro, Eliza Gazda, Dave Kieda, Chaoxian Lin, Yasar Onel, Nepomuk Otte, Ignacio Taboada, Andrew Wang

First Author’s Institution: Department of Physics, Durham University, Durham, United Kingdom

Status: published in Proceedings of the 37th International Cosmic Ray Conference (ICRC 2021) [open access]

Neutrinos, the Introverts of the Standard Model

Neutrinos are some of the most abundant particles in the universe, second only to photons, yet they remain one of the particles we know the least about. Unlike photons, neutrinos are incredibly antisocial. As neutral particles with very little mass, it is extremely unlikely for a neutrino to interact with any matter that it passes through. This makes them excellent candidates as messengers for astrophysical events because they are easily able to pass through any matter in their way, like the dust and gas residing within and between galaxies. However, this also means that it’s difficult to detect them because they can just as easily pass through Earth when they arrive. Most detectors get around this by having large detector volumes to increase the likelihood of a neutrino interacting within the detector, such as IceCube and Super-Kamiokande.

Some of the highest-energy neutrinos (fittingly called “ultra-high-energy”, or UHE) can provide excellent information about high-energy astrophysical processes like supernovae and active galactic nuclei. Using UHE neutrinos, we can probe the nuclear reactions that create high-energy particles in these “cosmic accelerators,” and we can search for likely sources of cosmic rays. The only problem is that higher energy neutrinos are much rarer than lower energy neutrinos, meaning that we need detectors that can observe enormous volumes to detect a significant number of UHE neutrinos. To accomplish this, scientists have proposed Trinity, a new detector comprised of an array of three telescopes that will search the atmosphere for signals of neutrinos passing through Earth.

Scratching the Surface

All neutrino detectors operate under the same basic principle. When a neutrino interacts with matter, it can produce a particle moving very close to the speed of light. When this happens in a transparent medium (ice, water, air, etc.), the particle can be moving faster than the speed of light in that medium. This creates an effect analogous to a sonic boom (check out this cool animation), but with light waves instead of sound waves. The light waves produced in this fashion are referred to as Cherenkov radiation, which have wavelengths in the blue, violet, and ultraviolet areas of the electromagnetic spectrum. Cherenkov radiation is what gives nuclear reactors their characteristic blue glow!

A diagram of the Earth-skimming detection technique used by Trinity. A high-energy tau neutrino enters Earth's crust at a shallow angle from the left. The neutrino interacts and produces a tau lepton, which travels out of the crust and into the atmosphere. The tau decays in the atmosphere, releasing Cherenkov radiation that travels through the atmosphere to a detector sitting on a mountain.
Figure 1: Diagram of the Earth-skimming technique used by Trinity. An Earth-skimming tau (τ) neutrino can produce a τ lepton, which releases Cherenkov radiation when it decays in the atmosphere. This radiation gets detected by the cameras in each Trinity telescope. (Figure 1 from today’s paper.)

How the neutrinos interact is where Trinity differs from other detectors. Trinity uses a novel detection method called the Earth skimming technique, shown in Figure 1. When some UHE neutrinos pass into Earth’s crust, they will interact and produce a fast-moving particle called a tau (τ), which is like a heavy but unstable cousin of the electron. This τ can survive long enough to exit Earth’s crust and decay in the atmosphere, which is where the Cherenkov radiation is produced. The Cherenkov photons typically have a wavelength of 300-400 nanometers, putting them in the blue to UV range of the electromagnetic spectrum, through Trinity expects to detect redder photons (wavelengths between 600-700 nanometers) from distant events because blue light scatters more easily in the atmosphere, making it less likely to travel the long distance to the detector. (This wavelength dependence is also why the sky is blue, and why sunsets and lunar eclipses are red!) By placing Trinity high on a mountain, scientists can observe the Cherenkov radiation from τ’s produced from high-energy neutrinos skimming through Earth’s surface.

Detection Capabilities

Trinity detection capabilities. Trinity is expected to detect neutrinos between 1 PeV and 1 EeV, with increasing sensitivity with more livetime and larger telescopes. The solid orange/red lines represent the sensitivities for the Trinity Demonstrator, a single Trinity telescope, and the full Trinity array. The dashed curves at higher energies represent upper limits on the flux of UHE neutrinos from other experiments. The gray area at the bottom represents allowed fluxes based on predictions from data using UHE cosmic rays. The blue datapoints at lower energies are IceCube measurements of astrophysical neutrinos, and the blue shaded region is a fir to this data.
Figure 2: Trinity‘s integral sensitivity, or overall ability to detect neutrinos. The solid orange/red lines represent the sensitivity for the Trinity Demonstrator, a single Trinity telescope, and the full Trinity array. The dashed curves represent upper limits on the flux of UHE neutrinos from other experiments. The gray area represents allowed fluxes based on predictions from data using UHE cosmic rays. The blue datapoints are IceCube measurements of astrophysical neutrinos, and the blue shaded region is a fit to this data. (Figure 5 from today’s paper.)

Trinity will be capable of measuring UHE neutrinos at the PeV scale (1015 eV, or 106 GeV) ‒ all the way down to 1 PeV ‒ allowing them to fill in the gap between TeV-scale measurements from detectors like IceCube and EeV-scale measurements from detectors like Auger. This region of Trinity‘s sensitivity also overlaps with astrophysical neutrino measurements from IceCube (see Figure 2). This has two main consequences:

  1. Trinity is guaranteed to measure astrophysical neutrinos in this energy range (we know they exist because they’ve been detected by IceCube);
  2. Trinity and IceCube’s combined performance can lead to improved measurements.

Additionally, Trinity’s sensitivity at lower energies should result in a larger number of neutrino detections than what other observatories see in this energy range. This means Trinity can provide important measurements for the UHE neutrino spectrum, which is not well-constrained with measurements from other detectors.

The other primary benefit of Trinity is scalability. Since each telescope can operate independently and does not require a significant amount of funding or materials to
construct, it is very feasible to expand the planned Trinity project to more than three telescopes.

Demonstrating Feasibility

Image of the Trinity Demonstrator. The telescope consists of a hexagonal arrangement of mirrors that direct light to the detectors. It is installed in a shed with a garage-style door that opens and closes depending on whether the Demonstrator is in use.
Figure 3: Image of the Trinity Demonstrator installed at Frisco Peak, Utah. (Image from the Trinity Collaboration website.)

As a precursor to the full Trinity array of telescopes, the Trinity Demonstrator is a smaller-scale telescope that serves as a proof-of-concept for the full detector. An image of the Demonstrator is shown in Figure 3. Positioned atop Frisco Peak, Utah at an elevation of 9,500 ft., the Demonstrator recently had first light on October 3rd, 2023. The Demonstrator allows the Trinity Collaboration to start collecting data and to set up pipelines for data processing and data analysis. Having these pipelines in place means that the Trinity Collaboration can immediately begin analyzing some of the highest-energy particles in the universe at full capacity when the full Trinity telescope array comes online. The Trinity Collaboration plans to conduct their initial assessment of the Demonstrator over its first year of operations, which they will use to inform development and construction of the full-sized Trinity telescopes. Their current timeline estimates deployment of the Trinity array in 2026 or 2027.

Astrobite edited by Emma Clarke and Nathalie Korhonen Cuestas

Featured image credit: Figure 1 from today’s paper.

About Brandon Pries

I am a graduate student in physics at Georgia Institute of Technology (Georgia Tech). I do research in computational astrophysics with John Wise, using machine learning to study the formation and evolution of supermassive black holes in the early universe. I've also done extensive research with the IceCube Collaboration as an undergraduate at Michigan State University, studying applications of neural networks to event reconstructions and searching for signals of neutrinos from dark matter annihilation.

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