Millions of barely noticeable “ghost” particles called neutrinos fly through our bodies every second. With almost zero mass and no charge, these particles do not even source us.
However, they contain secrets that could unlock the origin of matter from the very first moments of the universe.
These particles are incredibly difficult to detect, but that may soon change. For the first time, researchers have shown that particle collisions, such as those at CERN, can be used to detect these ghosts in droves. Especially for the tau neutrino, this could bring their total recorded number from only a handful to thousands.
The results were published Wednesday in the journal Physical Review D.
What is new – Particle collides, such as the Large Hadron Collider (LHC) at CERN, work by slamming beams of hadron particles (such as protons, etc.) into each other at almost the speed of light. As a result of these collisions, subatomic particles such as quarks or bosons are secreted and captured by detectors such as ATLAS. Neutrinos have always been a part of this process, but until this time there have been no experiments designed to detect them.
Part of the problem is that the LHC uses magnets to direct particles around in a circular collision path with detectors scattered around these curves. These magnetic traffic signs do not affect uncharged particles such as neutrinos. As a result, they shoot off the edge of the detector trajectory, like a drift with a blown tire.
In this subatomic slide, Jonathan Feng, a physicist at the University of California Irvine, saw an opportunity to capture new data. Enter: PHASES (ForwONErd Sear ExpeRiment).
Feng, the lead researcher in the FASER collaboration, originally dreamed of FASER as a way to capture a whole new class of supersymmetric particles. And while this performance may well still be in FASER’s future, the first results of experiments released this month have confirmed the detection of six neutrinos at the LHC.
What is a neutrino and why is it important?
As mentioned above, a neutrino is an incredibly light (at least 6 million times lighter than an electron) particle that has no charge and almost no interaction with matter as we know it. Therefore, these particles can sail right through us without us noticing it.
High-energy nuclear physics, which take place in the heart of our sun, rain neutrinos down on Earth, providing most of them we can detect. But these small particles can also be created from atomic decay, cosmic rays, and the collision of terrestrial particle rays. While neutrinos can be abundant, they are difficult to catch.
Some notable experiments studying neutrinos include the Fermilabs MiniBooNE, the IceCube detector in Antarctica, and the Super-Kamiokande detector in Japan. Like PHASES, these experiments all focus on detecting the constituents of a neutrino to which it decays after hitting an atomic nucleus just right.
PHASE experiment – Compared to the resource requirements of its predecessors (e.g. 50,000 tons of water in the case of Super-Kamio jug), FASER is a rather meager experiment. The first iteration of FASER, which is cobbled together by remaining parts of CERN, weighs only 29 kg – about the weight of a medium-sized dog.
By 2022, however, the team plans to have a full-size version of PHASES in place that will weigh more than 2,400 pounds (1,100 kg) and generally be more powerful and sensitive than the smaller pilot version.
But regardless of size, PHASES work in the same way using technology reminiscent of film photography called emulsion detection to capture neutrinos:
- Sheets of lead and tungsten are squeezed together with layers of emulsions – a kind of light-sensitive goop
- Roaming neutrinos hurled by the LHC hit these plates and decay into new particles
- These particles then leave marks in the emulsion layers that physicists can evoke as a photo
- Based on the energies of these marks, scientists can tell which taste of neutrino is present – tau, muon or electron – and whether it was a neutrino or anti-neutrino.
What is a tau neutrino?
Neutrinos come in what physicists call “taste” – either tau, muon or electron. In essence, this title describes what the neutrino will break into when it hits an atomic nucleus. In addition to releasing a proton and anti-neutrino in a process called beta minus decay, neutrinos will also release either a muon, tau or electron.
Of these flavors, tau neutrinos have proven to be the rarest and most challenging to discover. Prior to the FASER experiment, only 10 tau neutrinos had ever been observed, and the first detection took place only 21 years ago at Fermilab. The detection of tau neutrinos at FASER’s initial runs means that many more tau observations may be right on the horizon.
What is an anti-neutrino?
In addition to distinguishing between different variants of neutrinos, PHASES will also be able to distinguish between neutrinos and anti-neutrinos hitting its detector. While an anti-neutrino may sound like a villainous form of the original neutrino, it is actually just a neutrino with an opposite charge. But because both neutrinos and anti-neutrinos are electromagnetically discharged, this opposite charge refers to the lepton number of the particles instead – a kind of quantum number used to describe the properties of subatomic particles.
If this sounds confusing, it’s because it is – and scientists themselves are not quite sure what exactly separates a neutrino from an anti-neutrino. However, studying more anti-neutrinos directly using experiments like PHASES will be a big step towards answering this question.
As of 2022, the FASER team says they expect to begin capturing 10,000 or more neutrino signals using their detector. This data can be a big step forward when it comes to answering existential questions in physics, like where matter comes from and why so much of it is dark.
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