Cosmic rays. Artwork of high energy particles and radiation from a star in space (cosmic rays) … [+]
Getty
The earth is constantly hit by cosmic rays from space. Most of them are low-energy protons that come from our sun; but some others are high-energy monsters that come from the jets of black holes, supernovae, colliding stars, and other cosmic disasters. Researchers use observations of these high-energy particles to better understand rare astronomical phenomena. However, to gain an accurate understanding, scientists need to make the same particles in the laboratory to better untangle the story they tell us. Researchers at CERN are starting to generate very high energy neutrinos and record their behavior when they interact. This research uses sophisticated tools to tell us a lot about this interaction.
The creation of high energy particles is not new. Scientists have been making particle beams for a century, with every new particle accelerator facility generating ever higher energy beams from particles with an impressive number of particles to study. However, an interesting particle of cosmic rays cannot be accelerated – the neutrino.
Neutrinos are most common in nuclear reactions. They are particles of very low mass – at least 500,000 times lighter than the known electron, and a subatomic featherweight themselves. They also interact extremely rarely. While the likelihood of a neutrino interacting in a detector increases with energy, neutrinos from the world’s largest nuclear reactor – the sun – would have to pass five light years of solid lead to have a fifty percent chance of interacting. As a result, almost none of them will stop at a realistic detector that scientists can build.
Fermilab is America’s flagship particle physics laboratory and a premier facility in the field … [+]
Fermilab / R. Rooster
But almost no one is not the same. For example, researchers at the Fermilab shoot neutrino beams at highly developed particle detectors. For example, in a Fermilab detector called MINOS, around one neutrino interacts for every ten billion that pass through it. That is a small fraction, but while the Fermilab facility was in operation, around 430 trillion neutrinos shot at the MINOS detector every day. That means that their daily catch of neutrino interactions was around 50,000.
However, the neutrinos examined in laboratories like Fermilab are relatively low in energy compared to the most energetic neutrinos that come from the cosmos. For example, the neutrinos examined in the MINOS experiment have an average energy of around six billion electron volts. In contrast, the highest energy cosmic neutrino ever seen has an energy of about a thousand trillion electron volts of energy, more than 150,000 times higher. Now, most cosmic neutrinos do not have the same energy as this record breaking one, but many are still much larger than those observed in most accelerator-based studies. (To give some perspective, neutrinos from the Sun only have hundreds of thousands or a small number of millions of electron volts of energy.)
That’s a problem because it means researchers can take in detailed data at lower energies, but have to project what they see onto much higher energy. It’s like someone taking a foot-length ruler, determining the direction it is pointing, and then projecting it nearly thirty miles away. It could well be that small uncertainties or errors in low-energy measurements spread to large uncertainties in higher energies. Therefore, it is important to find new ways for scientists to measure the behavior of neutrinos with higher energy than commonly available. To do this, they have to use a new device.
The Large Hadron Collider (LHC) is a particle accelerator on the French-Swiss border. It accelerates two proton beams to a world record energy of 6.5 trillion electron volts each. It then collides these rays in the center of four different detectors. It has had many successes, such as testing accepted physical theories at higher energies than before and discovering the Higgs boson in 2021, but it also means researchers can use this incredible facility to look for very high-energy neutrinos.
Now it is not difficult to produce high-energy neutrinos at the LHC. Scientists at the facility have been doing this since it was commissioned in 2010. However, these neutrinos were not neutrino beams, but sprayed in random directions. In addition, while the detectors are fantastic for what they were designed for, they are simply too small to detect neutrinos directly.
The FASER neutrino detector (yellow detector in the trench) will be able to examine very high energies … [+]
FIBER / CERN
Scientists have therefore developed the ForwArd Search Experiment for Neutrinos, better known by the strained acronym “FIBERS”. FIBER is a relatively small experiment, located about 480 meters from a place where the proton beams collide, and hugs the LHC’s beam tube tightly. Given the size of the detector and the great distance from the collision point, FASERn will examine extremely high-energy neutrinos with an average energy of around one trillion electron volts, which is around 200 times higher than the average neutrinos at MINOS. It is a far cry from the highest energy cosmic neutrinos, but many cosmic ray neutrinos are in the energy range that FIBERS can measure.
In a recent paper, the FASERn collaboration used a pilot data run to look for high-energy neutrinos and reported that they had seen six of them. This was just an attempt at exploration. Their calculations predict that they will capture around 10,000 in the period 2022-2024.
Observing neutrinos is important for astronomical studies because neutrinos are not affected by the magnetic fields that penetrate intergalactic space – they basically move in almost straight lines. Therefore, experiments designed to capture high-energy cosmic neutrinos, such as the IceCube detector, can determine the direction from which these high-energy neutrinos are coming. IceCube is a detector that uses one cubic kilometer of Antarctic ice to search for high-energy cosmic neutrinos. It looks for neutrinos with energies above 6 trillion electron volts of energy, not much above the energy range of the neutrinos examined by FASERs.
Astronomers can then compare the direction of the neutrinos with observations with telescopes to find out which cosmic phenomenon emits such high-energy neutrinos. Whatever the source, from hypernovae to supermassive black holes devouring entire stars to gamma-ray bursts which have been the brightest thing in the universe since the Big Bang, anything that creates such high-energy neutrinos is incredibly interesting. And the new FIBER facility is an important step forward in our ability to understand the universe around us.
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