What is IceCube?
Why a fourth article on neutrinos?
Originally, I intended to publish only three articles about neutrinos at the present time, namely;
Why are neutrinos so difficult to detect?
How do you detect neutrinos?
How do you detect neutrino oscillations?
But, just as the third article was put on the blog, it was announced, on the 6th October 2026, that Francis Halzen had been awarded the Nobel prize in physics for 2026 for "his decisive contributions to the IceCube Neutrino Observatory and the discovery of high-energy astrophysical neutrinos". With such an accolade, I felt that a fourth article dedicated to Halzen's amazing neutrino observatory was called for.
What is the IceCube Laboratory?
It runs experiments to detect neutrinos coming from beyond our solar system and even beyond the Milky Way. Some are detected after they have travelled through the earth, practically unhindered. A tiny number are then detected in a cubic kilometre of ice, located at the south pole. Because neutrinos travel vast distances in straight lines without deviation, IceCube can even determine which direction some of them came from in space. This allows other optical observatories to examine the source.
IceCube is the first gigaton particle detector.
Some facts;
All neutrino detectors have to be huge to catch just a handful of the neutrinos that pass through every second. Neutrinos hardly interact with normal matter and so, are difficult to detect. If you are unfamiliar with this, the first article on neutrinos on this blog will explain all.
Neutrinos are detected indirectly by the effect that the charged particles produced in the rare neutrino interactions create. These high-speed charged particles, moving faster than light moves in the detection medium, produce Cherenkov light that can be detected by photomultipliers. If this is unfamiliar to you, the second article on neutrinos on this blog will explain all, with some excellent AI generated images.
We looked at solar neutrinos in the first three articles listed above, with energies ranging from below 1MeV to a maximum of about 18 MeV. With IceCube, detecting neutrinos from vast distances in outer space, that energy scale rockets up to a billion MeV!
Structure of IceCube
If you look at the image at the top of the article, you'll see strings of photomultipliers dangling into the ice at the south pole. 86 bore holes were drilled vertically into the ice to a depth of 2,500 metres. Strings of 60 Digital Optical Modules (DOMs) were then lowered into each bore hole and frozen into place ranging from a depth of 1,450 metres (Highest DOM) to 2,450 metres (Lowest DOM). Each DOM consists of a photomultiplier, which is capable of detecting flashes of light. The outputs from all the DOMs are wired up to a central receiving station at the top of the ice, to collect all the data produced, all the time that an experiment is running.
Viewed from above, the 86 bore-holes are arranged in a hexagonal pattern with each bore hole separated from the next by 125 metres. This results in the first ever gigaton neutrino detector! There is also a more dense collection of detectors in the centre known as "Deep-Core".
Simplistic diagrams of the "side view" and the "top view" of IceCube are shown below.
The DOMs in the outer strings are vertically separated by 17 metres and in the deep-core, they are separated by 7 metres.
The outer strings can detect neutrinos from 100 GeV upwards, while the more dense inner, deep-core, can detect neutrinos between 10 and 100 GeV. In fact, data is analysed from both sources to determine what is being detected, as some neutrinos may trigger flashes in both the outer strings and the deep-core, some may only trigger flashes in deep-core and some only in the outer strings. All this data helps to identify neutrinos.
There is considerable background Cherenkov light generated by muons created in cosmic ray bombardment of atoms in the upper atmosphere (about 2,500 "downward" muon events per second). The earth shields this background produced in the northern hemisphere, so that neutrinos detected coming into the DOM array from the bottom section are more likely to be genuine cosmic neutrino events. The background flashes produced by atmospheric muons in the southern hemisphere, immediately above IceCube, produce flashes in the top of the array at very specific energies and so can be filtered from the data.
The highest energy neutrino event recorded by IceCube so far, where the neutrino deposited its entire energy payload inside IceCube, was 6.05 petaelectronvolts (PeV). By comparison, the LHC can produce a maximum collision energy between protons of 13.6 TeV, which is only 0.0136 PeV!
However, the record event recorded so far by IceCube is over an estimated 10 PeV! The neutrino underwent an interaction in the ice below the array of detectors and only a portion of its total energy was detected in the IceCube DOMs. Computer modelling and AI indicated its total estimated energy at just over 10 PeV.
But the world record for an ultra-high-energy neutrino goes to KM3Net, a detector under the Mediterranean sea, that measured a single neutrino event at a staggering 220 PeV!
Somewhere out there, beyond the solar system and even beyond the Milky Way, there are some big cosmic accelerators!
High-Energy neutrino Interactions in IceCube
IceCube detects the blue Cherenkov light produced by charged particles created when a neutrino interacts with a proton or a neutron in the ice (remember ice is frozen H2O - two proton targets and lots of neutron targets in each oxygen nucleus). There are only two main interactions involved in protons and neutrons, but the ultra-high-energies involved, actually smash nucleons apart.
Charged Current Interactions
Ultra-high-energy neutrinos can interact with valence quarks or sea quarks inside both protons and neutrons, undergoing charged-current (CC) interactions mediated by a W boson. These interactions produce a charged lepton and a hadronic shower, destroying the target nucleon. (if you are not familiar with "sea quarks" there is an article on this blog "How many quarks are in a proton).
Neutral Current Interactions
High-energy NC interactions can also smash apart protons or neutrons, creating hadronic cascades from the shattered nucleon, resulting in showers of light created by charged particles. However, the inbound neutrino does not change into a charged lepton. High-energy impact NC interactions are studied in IceCube, in which there is a massive transfer of energy to the quarks in a nucleon, mediated by a Z0 boson and the identity of the neutrino creating the mayhem remains hidden.
The CC interactions outnumber the NC interactions at IceCube by about 3 to 1.
IceCube as a Muon-Neutrino Observatory
If you look at the image at the top of the article, you'll see a muon-neutrino entering IceCube from the bottom left moving to the top right. It undergoes a CC interaction with a neutron in an oxygen nucleus and the neutrino turns into a muon. The charged muon heads off at an angle to the path of the original muon-neutrino, represented by the angle between the yellow and red direction indicators. This angular relationship is shown in the following graph for muon-neutrino energies.
For ultra-high-speed muon-neutrinos, this angle can be very close to 0 degrees. In other words, the muon actually travels in the same direction as its former self when it was still a muon-neutrino.
So, IceCube can tell which part of the sky, the muon-neutrino came from.
Computers are used to track the muon from the moment it starts creating Cherenkov cones for as long as it remains in the field of the detectors. From the data, information about the neutrino's origin in space can be relayed to observatories all over the planet, to check what the source may be.
Identifying the Source of Cosmic Rays
The origin of cosmic rays, mainly very high-speed protons coming from sources in space way beyond our solar system, has long been a mystery. As protons are positively charged, they are not reliable pointers to where they came from as they will have been deflected by strong magnetic fields throughout their travels. But neutrinos travel in straight lines from source to earth and are also produced in vast numbers where cosmic rays originate.
When IceCube detects a high-speed neutrino and is able to determine the point in the sky from which it came, it sends an alert to conventional telescope facilities on earth and in orbit. These observatories, then point to the designated direction in the sky and, using electromagnetic wave detectors attempt to identify the source of the high speed neutrino. See the 300 TeV neutrino detected in 2017, listed in the IceCube achievements topic as TXS 0506+056, for details.
Although IceCube has identified many high-speed neutrinos, their exact origin has proved quite difficult to determine and only a small number have been successful so far.
How Does IceCube Detect each type of Neutrino?
IceCube can indirectly detect the interactions created by all three types of ultra-high-energy neutrino. The signatures of the interactions are different and summarised in the following AI generated diagrams for the most common CC interactions.
NOTE: As ultra-high-energy neutrinos can interact with valence quarks or sea quarks in a proton or a neutron, the reactions depicted below show "N" for a nucleon (can be a proton or a neutron) and "X" represents the final hadronic state, when the struck quark and the remaining fragments of the nucleon develop into a hadronic shower. In each case, the neutrino becomes its heavier, charged, lepton partner.
Similar CC interactions occur for ultra-high-energy anti-neutrinos, where the resultant charged lepton is always the anti-lepton.
In all ultra-high-energy CC interactions, the net charge of all the hadronic components and the final lepton equals the charge of the original nucleon, either +1 (for a target proton) or 0 (for a target neutron).
In the case of the Glashow resonance, the W- boson, created when the electron-anti-neutrino and electron fuse, decays into hadrons and an electron, such that the net charge will be -1.
Each of the three neutrino types (electron-, muon- and tau-neutrino) produce different light patterns or signatures, in the Antarctic ice, picked up by the IceCube detectors. This results in a substantial volume of data, including spurious background data produced from atmospheric events.
Computers are used to analyse the light flashes found in the mass of electronic data created by the DOMs, hunting for the tell-tale signatures, at the energy ranges identified and described in the above diagrams.
You can see how the indirect detection of ultra-high-energy muon-neutrinos is so important in that the unique muon trail can be used to determine where the original muon neutrino came from in the sky.
The Glashow resonance is special in that an electron-anti-neutrino fuses with an electron in the ice to become a W- boson. The in-bound neutrino must have the exact resonance energy for this very rare interaction to occur.
What have Been IceCubes Main Achievements?
2011
The IceCube neutrino observatory went live for the first time.
2013 - First detections from beyond our solar system
IceCube made the first ever detection of an ultra-high-energy neutrino coming from beyond our solar system. IceCube showed that ultra-high-speed neutrinos are present throughout deep space. Because they rarely interact with baryonic matter and because they carry no charge, they are unaffected by magnetic fields and can travel vast distances at very close to light speed, in perfectly straight lines. Despite their tiny rest-masses, they can have huge energy (rest-mass + kinetic energy). This discovery opened up the possibility of high-energy-neutrino observatories.
2017 - Earth's opacity to ultra-high-energy neutrinos
IceCube showed that earth actually absorbs ultra-high-energy neutrinos, whereas lower energy neutrinos pass through, hardly interacting at all. Ultra-high-energy neutrinos undergo both CC and NC interactions with the protons and neutrons of the earth.
The cross section for interactions with neutrinos below 40 TeV is tiny. As neutrinos climb above the 40 TeV threshold, the cross section for interactions starts to increase exponentially due to quantum effects and ultra-high-energy neutrinos smash into quark seas or valence quarks in hadrons. CC interactions in the earth always destroy the neutrino and NC interactions deflect the neutrino and lessen its energy. The overall result of this opacity is that many ultra-high-energy neutrinos are blocked by the earth, but not all of them.
The following graph shows the standard model transmission curves for neutrinos passing through the diameter of the earth, demonstrating the planetary filtering effect.
2017-2018 - 300 TeV neutrino detected from blazar TXS 0506+056
IceCube detected a 300TeV neutrino and through co-operation with observatories around the globe, traced its origin to a blazar, known as TXS 0506+056. This is a super-massive black hole at the centre of an active galaxy. The black hole has a high-speed, spinning-accretion disk surrounding it, coupled to an immensely powerful magnetic field. Periodically, particle jets are launched along the rotational axis/poles of the system, emerging from the inner edge of the disk and the black hole's magnetosphere. If the spinning system is pointing in the direction of earth, then any high-energy neutrinos produced may reach earth as they travel in straight lines. (These blazars are so far away that they pose no threat to our planet).
Automated alerts sent from IceCube caused telescopes across the globe to home in on the source of the neutrino and observe the source using gamma rays, X-rays and radio waves. This was the first time that a deep space object of interest was identified by both matter and electromagnetic-radiation emissions.
2021 - detection of 6.3 PeV neutrino
IceCube detected an astonishingly high energy neutrino at 6.3 petaelectronvolts. The event occurred when the in-bound electron-anti-neutrino collided with an electron in the ice and created a W- boson. This created confirmation of the standard model of particle physics at an energy scale thousands of times higher than can be achieved in the LHC.
This is a resonance interaction, created when a high-energy electron-neutrino of the correct energy collides and fuses with an electron to form a W- boson. This unusual resonance was first predicted by Sheldon Glashow in the 1960s.
The W boson decays into a shower of quarks and anti-quarks that form multiple hadronic cascades, which generate a shower of light across hundreds of metres in the ice, to be picked up by the photomultipliers.
2023 - neutrino image of Milky Way
Scientists used IceCube data created over a ten year period, to develop the first neutrino image of the plane of the Milky Way, eliminating the glow of particle collisions seen with optical devices.
2025-2026 Major Upgrade
IceCube has undergone a major upgrade, which considerably lowers the threshold energy at which it can detect neutrinos and will improve its ability to pinpoint the direction from which detected neutrinos originate.
Final Takeaways
IceCube is the first gigaton neutrino observatory and is located in the Antarctic.
IceCube consists of an outer layer of detectors and an inner deep-core.
IceCube uses the earth to shield the detectors from unwanted muons created in the upper atmosphere in the norther hemisphere.
The data has to be filtered to remove light produced by muons created in the upper atmosphere immediately above IceCube in the southern hemisphere. These muons only produce Cherenkov cones in the upper part of the detector array.
IceCube has detected ultra-high-energy neutrinos in the PeV energy range and, in some cases, has been able to pinpoint the direction from which they came. Neutrinos are the lightest of all fundamental particles by a long way. The energy of the ultra-high-energy neutrinos consists of a minute amount of rest-mass and a colossal amount of kinetic energy, produced by something like the particle jets thrown out from super-massive, black-hole, accretion disks.
What other types of high energy accelerators may exist out there I wonder?
IceCube has confirmed the capability to detect ultra-high-energy neutrinos coming from cosmic accelerators beyond the Milky Way, far more powerful than the LHC.
All three types of high energy neutrino (electron- muon- and tau-neutrino) are detected by IceCube.
Electron-neutrino interactions produce a negatively charged electron and a shower of particles that create a localised, roughly spherical ball of light called a "cascade". Ultra-high-energy electron-neutrinos, at the correct energy, can also produce "Glashow resonances", proposed back in the 1960s.
Muon-neutrino interactions produce a negatively charged muon that travels a long distance through the ice, creating a lengthy, straight "track" of light from the characteristic Cherenkov cones.
Tau-neutrinos interactions produce a tau particle that typically decays and creates a distinctive single or double-cone signature in the ice.
See diagrams from "How does IceCube detect each type of neutrino" in the main body of the article for AI generated pictorial representations of these interactions.
In all CC and NC interactions, regardless of energy and structure of hadronic cascade, net charge is always preserved across the interaction.
Francis Halzen
Won the 2026 Nobel prize in physics;
"for decisive contributions to the IceCube Neutrino Observatory and the discovery of high-energy neutrinos of astrophysical origin."
In 1988, Professor Halzen came up with the idea of using the pure ice of the Antarctic as a giant particle detector. This was met with skepticism, as many though that the ice would not act as good transparent medium for the blue Cherenkov light produced by neutrino interactions.
In the 1990s, Halzen served as the Principal Investigator for the AMANDA (Antarctic Muon and Neutrino Detector Array) project. This was the the "proof-of-concept" needed for Halzen's proposed neutrino observatory. AMANDA proved that Halzen's physics model was correct and resulted in the ability to scale up the dimensions of AMANDA to those of IceCube.
In 2001, Halzen became the primary scientific lead and principal investigator throughout the design phase of IceCube, its construction and operation. He remains scientific lead and principal investigator to this day.
While Halzen provided the original concept and leadership, he frequently points out that building IceCube was a monumental engineering achievement, only made possible by an international community of some 400 scientists, engineers and technicians.
Halzen's vision has made neutrino astronomy a reality.
© 2026 Kieron Conway - All rights reserved.
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That's definitely it on neutrinos for now! However, I'm quite sure there will more to write about in the future. Currently, the blog has the following articles;
Why are neutrinos so difficult to detect?
How do you detect neutrinos?
How do you detect neutrino oscillations?
What is IceCube
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