What Are Neutrinos?
Neutrinos were first predicted by Wolfgang Pauli in 1930, who said "I have postulated a particle that can not be detected". Neutrinos were not detected until 1956 by Clyde Cowan et. al. who were awarded the Nobel Prize in 1995 (yes 1995, 49 years later). Neutrinos are some of the most abundant and yet elusive (ghostlike) items in particle physics. Incredibly lightweight as well as charge-less, they zip around the universe at near the speed of light and they do not interact with other particles.
There are three types of neutrinos - the electron neutrino, the muon neutrino and the tau neutrino. All are classified as leptons, the family of particles that includes electrons. Neutrinos do not feel the strong force that holds protons and neutrons together in an atomic nucleus. Because they do not have any electrical charge, neutrinos also do not feel the electromagnetic force. Only the force of gravity and the weak nuclear force (the force responsible for radioactive decay) interact with the three "known" neutrino versions. There may be other varieties not yet discovered. Some physicists speculate that there might be a fourth type of neutrino, a sterile version that does not feel the weak nuclear force, but this has not been confirmed at this time. (See right-handed neutrinos below.)
Neutrinos have been around since the beginning of the universe. The neutrinos left over from the Big Bang are the most abundant particles in the universe. This remnant neutrino density is estimated to be 100 neutrinos per cubic centimeter at an effective temperature of 2 degrees Kelvin. The background temperature for neutrinos is lower than that for the microwave background (2.7K) because the point in time that neutrinos became transparent came earlier.
Almost all neutrinos pass right through the earth without ever interacting with an earthly atom. Just as you read this, trillions of them have passed through your body. About 65 billion "solar" neutrinos per second pass through every square centimeter of the earth that is perpendicular to the sun.
The photo to he left above, is looking down the barrel of the NuMI (Neutrinos from the Main Injector) horn at Fermilab which fires protons that degrade into neutrinos (Image Caltech). A detector in the Soudan Mine in north-eastern Minnesota, MINOS (grey octagon in the left center of the photo to the left) picks up neutrinos that are shot from Fermilab in Illinois 450 miles away, directly through the earth (no tunnel, see the sketch below). Subterranean laboratories are ideal for neutrino detectors. Most neutrino facilities are in old converted mines. Thick layers of rock are perfect shields to block miscellaneous other types of particles, mostly cosmic rays from the sun that do not penetrate very deeply.
MINOS received the last neutrinos from the NuMI beam on the 30th of April, 2012. The system has now been upgraded to MINOS PLUS, which produces significantly higher energy. As of December, 2013 no new results have been published. Top
Neutrinos have a unique property in that as they travel some distance at near the speed of light they transform themselves into other neutrino flavors. 100% of the particles leave Fermilab as a pure beam of muon neutrinos. After about 100 kilometers, 50% of the neutrinos will have transformed themselves into tau neutrinos. At about 250 kilometers, most of the neutrinos will now be the tau type, with just a small percentage the electron type. See the illustration to the left. The exact distance of transformation depends on their initial energy. They then oscillate back and forth as they travel further distances.
The Standard Model of particle physics assumes that neutrinos are massless and therefore could not change flavor. However, if neutrinos do have mass, they can change flavor and "oscillate" between flavors as has been observed. Therefore neutrinos must have some mass even if it is extremely tiny. While it is generally now accepted that neutrinos have mass, at this time the masses of the three individual neutrinos have not been established.
However, in February 2014 scientists from the Universities of Manchester and Nottingham, studying the recent Planck satellite data, estimate that the mass of the three neutrinos combined is .320 eV (electron volts). A total mass for all three neutrino types of .320 eV is less than a millionth of the mass of an electron (.511 MeV). It is interesting that particle physicists are now using astronomical data to calculate the mass of small particles. These calculations do rely on some of the assumptions in the Standard Model of Cosmology as being correct. Specifically, that the universe has a flat geometry (pretty good data for this) and that dark energy can be described by the cosmological constant (not conclusively proven). These assumptions are not guaranteed, so stay tuned.
In the late 1960s, several experiments found that the number of electron neutrinos arriving from the sun was between one third and one half the number that was predicted by the Standard Solar Model, a mathematical model of the physical properties of the sun which has proved to be quite accurate. This discrepancy, which became known as the "solar neutrino problem" remained unsolved for about thirty years. In 1998, experiments showed that solar and atmospheric neutrinos did change flavors. This resolved the solar neutrino problem because some of the electron neutrinos produced in the sun had changed into other flavors which the earlier experiments did not detect. Top
The Quantum Mechanics Model of particle physics predicted that the neutrino would have zero mass. Numerous experiments have proven that the opposite is true. Neutrinos do have masses. However, neutrinos have minuscule masses compared to their lepton counterparts. One of the ways to accommodate their mass theoretically is to introduce a "hypothetical" new neutrino flavor called the right-handed neutrino. Handedness is a sub-variant of electrical charge that determines whether a particle feels the weak interaction. A particle must be left-handed to feel the weak force. The three known types of neutrinos are left-handed.
The "hypothetical" right-handed particle (sometimes called the sterile neutrino) would be more mysterious than their left-handed experimentally proven brothers. Because right-handed neutrinos would not feel the weak interaction, their mass would not depend upon the Higgs Field (see the Higgs Field). Instead, their mass would probably emerge from a completely different very high energy mechanism most likely making the right-handed neutrino extremely heavy. However, keep in mind that a right-handed neutrino has never been observed. But, right-handed anti-neutrinos are real and have been observed. Top
Supernovas And Neutrinos
During the bulk of a star's life, about 99% of the energy produced is in the form of light and about 1% as neutrinos. For a supernova, almost all of the energy produced is in the form of neutrinos. It is now thought that neutrinos are an integral part of the explosion mechanism for supernovas and in the production of heavy elements created during the explosion.
Neutrinos also have an important feature in that they escape the exploding star before the light emerges. So detecting a burst of neutrinos from a supernova would allow optical astronomers to study the "turn on" of a supernova long before it would normally be observed.
There exists a network of neutrino detectors around the world that belongs to SNEWS (Supernova Early Warning System) and, in the event that a burst of neutrinos is detected, SNEWS will alert the astronomy community (and the the amateur astronomer community as well). Top
How Are Neutrinos Studied?
Neutrinos are not easy to detect, so how are they studied? There are three main ways:
Scientists can deliberately produce neutrinos by firing protons from an accelerator at pieces of graphite which then emit specific types of neutrinos. This type is illustrated above with the NuMI accelerator at Fermilab. Accelerator experiments have the advantage of being able to examine either neutrinos or antineutrinos. The intense beams of these accelerator particles increase the chance for a neutrino interaction in the detectors. In addition, accelerators produce neutrinos that have higher energy than those from nuclear reactors and the sun.
Scientists can also investigate neutrinos made by nuclear reactors that normally generate electric power. These reactors produce electron-flavor antineutrinos. Experiments to study antineutrinos from this type of source require the construction of a particle detector near the nuclear power plant which yields valuable information about antineutrino interactions with matter.
Finally, scientists can detect neutrinos that occur naturally throughout the universe. Examples are those produced by nuclear reactions in the sun, or from collisions from outer space cosmic particles, or from explosions of supernovas. Stars like our sun produce electron-flavor neutrinos, while cosmic particles and supernovas produce all three neutrino flavors and their antineutrino counterparts.
Since neutrinos themselves are invisible to detectors, scientists must take an indirect approach: They record the charged particles and flashes of light created when a neutrino hits an atom, and thus "infer" the neutrino’s presence. Because the tiny neutrinos interact with matter so rarely, the only way to detect them is to put lots of matter in their path. Top
The NOvA Detector
The NOvA collaboration is made up of 208 scientists from 38 institutions in the United States, Brazil, the Czech Republic, Greece, India, Russia and the United Kingdom. The experiment receives funding from the US Department of Energy, the National Science Foundation and other funding agencies. The NOvA experiment is scheduled to run for six years starting in mid 2014 until about 2020.
Although neutrinos are everywhere and are passing through us in unimaginable quantities every second, they are very, very difficult to detect. That is why researchers use particle accelerators to create extremely intense beams and huge detectors to try and capture a rare collision with another material particle.
The NuMI Beam. The Fermilab accelerator produces the most intense neutrino beam in the world. The beam travels a distance of 500 miles from Fermilab close to Chicago, to Ash River near the Minnesota-Canadian border, in less than three microseconds. The beam travels straight through the earth (no tunnel) about 6 miles deep at its maximum depth, at slightly less than the speed of light.
Billions of neutrinos will be sent through the earth every two seconds as part of the NuMI beam. Once the experiment is fully operational, scientists will catch only a few neutrinos each day and only about 5,000 total during its 6 years of operation. The neutrino beam, like a beam of light from a flashlight, gradually spreads out as it travels. The width of the NuMI beam at Fermilab starts at about six feet and grows to several miles by the time it reaches the far detector in Minnesota.
Good question. How can physicists steer a beam of neutrinos when neutrinos have no charge and are not affected by electric or magnetic fields?
Fermilab creates a beam of neutrinos by first smashing protons from the accelerator into a graphite target which unleashes a variety of particles including short-lived pions (pīe'-äns) and kaons. Kaons soon decay into more pions. Both pions and kaons are charged particles. Scientists use magnets to focus the pion particles into a beam. These pions travel a short distance (about 200 meters) through a "decay pipe." See the illustration to the left. As the pions pass through the decay pipe, a high fraction of them decay into neutrinos that continue on in the same direction forming the neutrino beam, while the other particles are filtered out. Of the three known neutrino types, the beam produced in this manner consists of mostly muon neutrinos.
Fermilab started producing a new NuMI beam of neutrinos in September, 2013 after 16 months of work by about 300 people to upgrade the lab’s accelerator complex. The original Fermilab accelerator delivered 400 kilowatts of power to the beam. Part of the NOvA project included an upgrade to the accelerator which now allows it to produce 700 kilowatts of power.
The Detectors. There are two detectors. One is in the Fermilab complex, the "near" detector which weighs about 300 tons, to sample the beam that is being sent out. The second detector is in Ash River, and weighs 14,000 tons, the "far" detector. It will be operated by the University of Minnesota.
The far detector is located slightly off the center line of the neutrino beam coming from Fermilab. At this slightly off-axis location, scientists expect a large flux of neutrinos at an energy level of 2 GeV (2 billion electron volts) the energy at which oscillations from muon neutrinos to electron neutrinos is expected to be at a maximum.
The detectors are made up of cells of extruded, highly reflective plastic PVC filled with a liquid scintillator, a liquid that releases a burst of light emitting charged particles when a neutron collision occurs. As these particles come to rest in the detector, their energy is collected by fibers attached to light sensitive sensors, photo-detectors. Using the pattern of light seen by the photo-detectors, scientists can determine what kind of neutrino caused the interaction and what its energy was.
The size of the far detector at Ash River is gigantic. The panel blocks are 51 feet high, 51 feet wide and 7 feet deep. (See the photo at the left and notice the size of the man in the lower left hand corner.) The full detector when complete will house 28 panel blocks and be about 200 feet long.
The individual sensing tubes are assembled at the University of Minnesota in Minneapolis-Saint Paul and strung with particle detecting fibers by working college students. The tubes are then shipped to Ash River for final assembly into panel blocks. Each individual block is constructed while lying flat on a pivoter. When complete, the 190 ton blocks are lifted upright by the pivoter mechanism until they are vertical and are slowly moved down the tracks to their final position. There they are fitted with electronics and filled with the liquid scintillator. Each finished block will now weigh about 500 tons. Each panel block takes crews working two shifts a day, about four weeks to complete.
The first block was finished in February 2014 complete with all electronics, etc. and was able to detect a batch of neutrinos sent from Fermilab. The whole detector will be finished in the summer of 2014. See the illustration below of the detector build in process. What an enormous project! See the interesting Youtube NOvA Video describing the build process.
NOvA Goals. The NOvA experiment is designed to address three fundamental issues in neutrino physics:
Scientists know that neutrinos oscillate, changing from one type neutrino to another, and have seen oscillations of muon neutrinos to tau neutrinos. However, scientists have not observed muon neutrinos oscillating into electron neutrinos which is a major goal of NOvA.
Scientists do not know the masses of the different types of neutrino nor do they know the neutrino mass hierarchy – that is, which kind of neutrino is the lightest and which is the heaviest. Scientists "think" that neutrinos get their mass through a different process than other particles (i.e. not the Higgs Field). Therefore they cannot be sure that neutrino masses follow the same mass pattern as other particles.
If the NOvA experiment discovers that muon antineutrinos oscillate at a different rate than muon neutrinos, scientists will know the charge-parity symmetry (see the next paragraph) between neutrinos and antineutrinos is broken, which will mean matter and antimatter can behave differently. This might be a clue as to why the universe has more matter than antimatter.
Physicists once theorized that nothing would change about the laws of physics if every particle were replaced with its antiparticle. This is called charge-parity symmetry. But it turns out that matter and antimatter are not exactly mirror images, and this could explain why they exist in unbalanced quantities. Breaking charge-parity symmetry is called CP violation.
If antineutrinos do not follow the same pattern as neutrinos when they change from one flavor to another, this is a signal of CP violation. The same mechanism that could cause neutrinos and antineutrinos to oscillate differently could have implications for the mechanism that would have led to an abundance of matter over antimatter in the early universe. Physicists theorize that the Big Bang created equal amounts of matter and antimatter. When corresponding particles of matter and antimatter meet, they annihilate one another. However antimatter, for the most part, has mysteriously vanished while matter remains. Top
Long-Baseline Neutrino Experiment (LBNE)
The Long Baseline Neutrino Experiment (LBNE) is a "proposed" system with neutrinos produced at Fermilab near Chicago traveling underground to a detector at the Homestake Mine at Sanford Lab in Lead, South Dakota. Fermilab would fire an intense beam of trillions of neutrinos from a modified production facility used in the NOvA experiment (above). The beam would travel a distance of 810 mi1es (1,300 kilo metres) to an upgraded far detector. The system would also have a local detector at Fermillab to sample the beam as it leaves the decay pipe (shown above in the NuMI system).
The US has committed $1 billion to the LBNE development. A total of $1.5 billion is necessary for the total project. The UK has announced that they will help fund the project and nine British universities will be involved. Collaborators will also come from India, Italy, and Japan. Other potential collaborators are welcome. LBNE is planned to be operational by 2022. The goal is to study neutrino oscillations and perhaps determine whether neutrinos are their own anti-particles (Majorana particles, see the next section below).
The beam will exit the main proton accelerator, go up a small 50 foot hill and then be repositioned down into the ground towards the Sanford Lab. The accelerated protons will then smash into a graphite target and the particles that emerge from this collision will go through a 670 foot decay tunnel which will generate neutrinos that are about 100 feet under the earth's surface on the exact same path towards the Sanford Lab. See the illustration below.
The path of the neutrinos will take them 19 miles (30 kilometers) underground at its deepest point. See the illustration above. The proposed "main" detector ideally will be 4850 feet underground. (If there are not enough funds for an underground facility, a back up plan is to have the detector on the surface.) The deep underground detector would better shield the detector from the cosmic rays that constantly bombard the earth. The deeper location would also greatly increase the detector's capability to identify the rare reaction of neutrinos with other particles.
Most of the neutrinos sent from Fermilab will begin as the muon type. The experiment will measure these particles as they transform into other particles and pass through the far detector in South Dakota. The 810 miles distance is an ideal distance to detect the transformation from muon neutrinos into electron neutrinos. The detector will be about 60 feet wide and 60 feet high and about 100 feet long. See the detector illustration at the left. The 10 kiloton detector will be filled with super cold liquid argon, which is similar to liquid helium but heavier. Cryogenic equipment will cool the liquid argon to about minus 300 degrees Fahrenheit.
As the rare neutrino interacts with an argon atom, the collision will create some ionized electrons. These new electrons will interact with custom designed internal wire planes buried in the cold liquid argon. Three planes of wires allow three dimensional reconstruction of the electron's track. The wires will then transmit the collision information to computers for storage and analysis. LBNE participating scientists will have access to the information to study the neutrino behavior.
The Proton Improvement Plan-II (PIP-II), a proposed new facility for Fermilab that would significantly increase the number of protons the Main Injector could supply, will provide increased intensity for LBNE's neutrino beam. The schedule for building PIP-II is expected to be later than that for LBNE, therefore LBNE will need to undergo an upgrade to a higher beam power part way through its operational lifetime. Top
Majorana Demonstrator - Neutrinoless Double Beta Decay
In a cavern almost a mile underground at the Sanford Underground Research Facility (SURF) in Lead, South Dakota, an experiment called the Majorana Demonstrator, will try to answer one of the most persistent questions in physics: are neutrinos their own antiparticles? A Majorana particle is its own antiparticle. Particles that were their own antiparticle were hypothesized by Italian Ettore Majorana in 1937. No elementary particles are known to be their own antiparticle. The detailed nature of the neutrino is still being investigated so it might just be a Majorana particle.
So far only the boson force carriers are their own antiparticles. Bosons facilitate the decay of heavy particles into lighter particles. The best way to test whether neutrinos are their own antiparticles would be to observe a a neutrinoless double beta decay. It has never been conclusively detected, and if it does occur, it is exceedingly rare.
Ordinary beta decay is a common type of radioactivity. An atomic nucleus changes into a different kind of element, a neighbor on the periodic table with lower mass. In the process the original particle emits a beta particle, an electron or positron, plus a neutrino or an antineutrino. For example, carbon-14 transforms to nitrogen-14 when one of its neutrons turns into a proton, emitting an electron and an antineutrino.
It was beta decay that led to the theory that there must be a particle like the neutrino since an electron alone could not account for all the energy lost in the decay. There had to be something else - so a neutrino was proposed.
Then there is the rare double beta decay, for example, when germanium 76 changes to selenium 76. (Shown at the above left.) Two neutrons change into protons while emitting two electrons and two antineutrinos, transforming the nucleus two places higher in the periodic table.
A neutrinoless double beta decay would "prove" that neutrinos are their own antiparticle. A diagram, to the left, of a neutrinoless double-beta decay shows a right-handed antineutrino emitted when a neutron decays. The antineutrino flips its handedness in flight and is absorbed by a second neutron, which also decays. Only a single antiparticle/particle is involved. How fast it can flip its handedness depends on its mass. The more massive, the easier the flip, and the more often this kind of decay would occur.
A neutrinoless double beta decay can be detected by the absence of two neutrinos being observed in the energy of the decay. However, this decay pattern is purely theoretical and has never been conclusively observed. To actually observe it under intense scrutiny is the goal of the Majorana Demonstrator program.
The Majorana Collaboration proposes to search for neutrinoless double beta decay using an array of germanium (Ge) crystals enriched in isotope 76Ge (76 is the total number of protons and neutrons in a specific isotope of Germanium). Of the candidate isotopes, 76Ge has some of the most favorable characteristics. Germanium diode detectors are a well established technology, and the detectors can work as both source and detector. See the Demonstrator illustration below.
If neutrinoless double-beta decay occurs, it is an extremely rare process. Current experimental results indicate that, in 76Ge, the lifetime of this decay is greater than 2 x 10^25 years per molecule, so you need many 76Ge molecules. To achieve sensitivity to such a rare decay, Majorana must have a very low background noise rate. The Demonstrator will be deployed deep underground in an ultra low background shielded environment in the Sanford Underground Research Facility. With a low enough noise background, Majorana will be able to easily separate the sharp spike of a "two electron event" from a broad smear of other energies.
The experiment’s detectors will be enriched germanium crystals that will be installed in strings inside cryostats made of ultra-pure copper. The Demonstrator will contain a total of 40 kilograms of germanium. The cryostats will keep the detectors cool to below 300 degrees F, which improves their performance as semiconductors. The project is slated to begin operations in late 2014.
The fascinating thing about neutrinoless double beta decay is that it would violate one of the basic principles of the Standard Model, which is: all interactions should conserve the lepton number. Electrons and neutrinos are both leptons, so if an interaction produces two electrons, that’s a plus two. In the usual kind of double beta decay, two antineutrinos are also emitted with minus lepton numbers, which makes a minus two. The total lepton number is therefore conserved at zero. However, not so with neutrinoless double beta decay, which increases the lepton number from zero to two. Top
IceCube Observatory - Detecting Heavenly Neutrinos
The $275m US led IceCube Neutrino Observatory at the South Pole, operational since December 2010, has measured the highest energy neutrinos yet. See the IceCube photo at the left. Thousands of sensors have been lowered into a cubic kilometer of Antarctic ice, looking for flashes of blue light that are given off by the cascade of debris generated by a neutrino collision with an ice molecule. What makes IceCube different is that it is looking specifically for very high energy neutrinos. Distant high energy events in the outer universe produce neutrinos with a correspondingly high energy level. If these neutrinos can be studied, they can reveal information about their origins.
The IceCube Observatory consists of 86 cables, each up to 1.5 miles deep, suspended in vertical holes in the ice. Attached to each cable are dozens of photomultiplier tubes. See an actual IceCube hole, cable and photomultiplier to the left below
8,000 photomultipliers record the radiation given off by the secondary particles created when incoming neutrinos collide with hydrogen or oxygen nuclei inside the ice. Each photomultiplier tube contains its own data processing computer that provides some preliminary filtering and enables event signals to be synchronized to within two nanoseconds. These signals are then sent to a huge, local computing center (pictured above) that sits at the center of the array. The computing center does more processing before sending the data out to the rest of the world. Shown at the left below, is an artist's perception of the tubes hanging in the clear ice.
IceCube has had a hard time finding the neutrinos it has been looking for. The observatory records 2,700 cosmic rays per second, and a locally produced neutrino turns up every six minutes. The number of outer space signals has only been about ten neutrinos per year.
At high energies, a neutrino will tend to collide with an atom "outside" the cubic volume occupied by the detectors. The resulting high energy muon streaks off like a meteor through the ice, losing energy through radiation and the production of electrons, positrons and other particles. IceCube looks for these tracks and figures out where the neutrino came from. Some of these neutrinos have traveled through the entire earth before being detected.
Once you get to even higher energies, neutrinos that are detected are a result of collisions from "within" the detector's volume. Again the muon darts off causing havoc, but the event track begins from within the detector volume - a pattern that the researchers initially excluded. Neutrinos are now being spotted in the 1,000 TeV range (an absurd peta-electronvolts) roughly the energy equivalent of one million times a proton’s mass. (For comparison, the Large Hadron Collider at CERN will operate at about 14 TeV when the upgrade is complete).
High-energy events of outer space origin tend to create showers of secondary particles in the detector. Whereas lower energy atmospheric neutrinos tend to produce single muon tracks. See the photo to the left below of a very high energy collision nicknamed "Ernie". Ernie and two other neutrinos nicknamed "Bert" and "Big Bird" each had energy levels of about 1,000 TeV. Such interactions are so infrequent that IceCube researchers had to search for two years to find these three super energy neutrinos. During that same time span, the observatory also detected 34 neutrinos of somewhat lower energies - those above 30 TeV and below 1,000 TeV.
At these very high energies the neutrinos have quite a high probability of interacting with the ice molecules, so they are detected quite close to the surface of the ice. Even better if the tracks begin within the detector's volume, then the energy of the neutrinos can be calculated with a very high accuracy.
Because "cosmic rays" are actually high energy "charged" particles, they travel curved paths shaped by the magnetic fields they encounter as they travel through the universe. As a result, they do not preserve information about where they came from. Neutrinos are valuable because they rarely ever interact with other particles so their direction is not swayed by magnetic fields in the universe. Thus, their trajectories should point straight back to their source, which astronomers think could be a variety of intense events such as humongous black holes, gamma-ray bursts, supernovas or galaxies forming stars at furious rates.
"This is the first evidence we have of neutrinos that are not of local origin," says IceCube principal investigator Francis Halzen of the University of Wisconsin-Madison. "It opens up a new wavelength in astronomy, thanks to a different kind of particle."