We are about to learn a lot more about the most elusive of cosmic particles.
Researchers at the CERN laboratory in Switzerland announced
that they have observed and generated in the laboratory a highly energetic form
of radiation called high-energy neutrino radiation. Their accomplishment is
without precedent, and it will significantly improve the scientific community’s
understanding of some of the most energetic and destructive environments in the
cosmos.
The rarest particles
In nature, high-energy neutrinos are created only in
exceptional circumstances. These include colliding neutron stars, gamma ray
bursts, and pulsars. They also occur in the strong magnetic fields generated
when black holes absorb nearby stars. Such cosmic events are among the rarest
and most spectacular occurrences in the Universe.
Lower-energy neutrino radiation has been observed for over
half a century. Low-energy neutrinos emit from nuclear reactions, like those occurring
in the Sun or a nuclear reactor. Solar and reactor neutrinos can have less than
one-millionth of the energy carried by highly energetic ones created in the
cosmos.
Scientists can also generate neutrinos using particle beams
like the ones at the Fermi National Accelerator Laboratory, or Fermilab,
located just outside Chicago. Fermilab’s beams are the most intense in the
world. They are about 1,000 times more energetic than those created in the Sun
or in nuclear reactors, yet they still fall well short of the energy carried by
some neutrinos created in space.
High-energy neutrinos from space have been detected before,
but they are extremely rare, and their detection is at the whim of cosmic
events. After all, neutron stars do not collide on just any day. Researchers
wanting to study very-high-energy neutrinos are left to wait until a
high-energy event occurs somewhere in the Universe.
Patience has a cosmic limit
Thankfully, scientists are quite patient, and they have
built equipment that can identify high-energy cosmic neutrinos when they do
occur. Very large detectors are needed for the task — for example the
Super-Kamiokande detector in Japan, which is a tank containing 50,000 tons of
ultrapure water, or the IceCube Neutrino Observatory, which uses a cubic
kilometer of Antarctic ice.
The detectors must be so large because neutrinos interact
very weakly. For example, about 10 trillion trillion (1025) neutrinos from the
sun pass through the Super-Kamiokande tank every day, yet only thirty of those
neutrinos interact with the detector and can be observed.
It is clear, then, that for scientists wanting to study
energetic neutrinos, it is not ideal to wait for them to be generated somewhere
in space. It would be far better to create very-high-energy neutrinos on Earth,
and then point a beam of those neutrinos at a waiting detector. And that is
exactly what researchers now have done.
The most powerful particle accelerator in the world is
called the Large Hadron Collider, and it is located at the CERN laboratory on
the French-Swiss border. The Collider was built to bash very-high-energy beams
of protons together in hopes of creating, and then detecting, a particle called
the Higgs boson, which is the origin of the mass of matter’s smallest building
blocks. The discovery of the Higgs boson was announced on July 4, 2012.
While the Higgs boson was the Collider’s primary objective,
the detectors arrayed around the accelerator were designed to be very
versatile. Over the years, independent teams used it to make many measurements
of the laws of nature at the highest accessible energies. Indeed, since the
Collider began operating, more than 3,000 scientific papers have been published
using the data generated by the accelerator.
High-energy discoveries
One set of researchers took advantage of the unprecedented
energy of the facility’s beams to investigate how to create and detect
very-high-energy neutrinos. These scientists built what is called FASER, or
ForwArd Search ExpeRiment. A detector was placed very near the LHC beams —
about 480 meters from a location where beams of protons collide.
At this location, FASER could see the most energetic
particles created in the collisions, making it an ideal detector to search for
extremely high-energy neutrinos. At the Moriond 2023 Electroweak Conference in
LaThuile, Italy, FASER scientists announced that they had observed these
particles.
The particles carried as many as a couple thousand times the
energy of neutrinos generated using other particle accelerators. Scientists
will be able to use this data to better understand high-energy neutrinos from
space. This new knowledge will in turn help astronomers gain a much better
understanding of exactly what happens, for example, when neutron stars collide.
Thus, this recent work will shed light on some of the most spectacular and
rarest of cosmic phenomena.
This is just the beginning. Since the LHC will continue to
run for a couple of decades more — including a planned upgrade to the rate at
which its beams collide — researchers will continue to uncover and reveal the
behavior of very-high-energy neutrinos.
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