Why is the DUNE project important to physics?
Neutrinos, which are the focus of the DUNE experiment, were generated in huge numbers just after the big bang and are therefore the key to understanding the origin and following evolution of the universe. DUNE will not only observe neutrinos created at the Fermilab but also neutrinos emerging from exploding stars (supernovae) allowing us to peer inside and even potentially witness the creation of a black hole. DUNE could also help scientists form relation between the stability of matter and the Grand Unification of forces (A theory pioneered by Albert Einstein who searched for a single grand unifying theory that would tie together all of physics), if rare subatomic interactions predicted by theoretical physicists are detected.
The initial motivation for constructing the DUNE experiment started with an anomaly; Over two decades ago, the LSND neutrino experiment at Los Alamos National Laboratory produced an unexpected result that would challenge what scientists believed they knew about neutrinos. The prominent theory about the anomaly’s caused was that it was the previously unpredicted footprint of a new type of particle, known as the sterile neutrino. Other experiments have been conducted in order to search for the existence of this elusive new particle for example the IceCube experiment, who’s main purpose was to detect very high-energy neutrinos propelled toward Earth by explosive events in space but it just happened to be in the right position to also study low-mass sterile neutrinos.
DUNE will also require 70,000 tonnes of liquid argon for its detector making it the largest ever experiment of its kind and some scientists believe the importance of the goals of DUNE could rival that of the Higgs particle discovery at CERN in 2012. In 2015 a group of physicists were awarded with a nobel prize for proving the existence of neutrino oscillations and it was hailed as compelling experimental evidence for the incompleteness of the Standard Model as a description of nature, possibly leading to exciting new models of the universe.
What are Neutrinos?
Neutrinos are subatomic particles that are very peculiar due to being so elusive despite being the most abundant particles in the universe. Neutrinos come in three “flavors”: electron neutrinos, muon neutrinos, and tau neutrinos which respectively associated with the electron, muon, and tau (more about flavours later!). Each neutrino also has a corresponding antiparticle, called an antineutrino, which also has no electric charge and half-integer spin (Read more about spin and other properties of subatomic particles here). Neutrinos have no electric charge so they’re unaffected by the electromagnetic force, unlike other well known particles such as the proton and neutron. They are also are unaffected by the strong force (one of four fundamental forces that overcomes the nature of like-charged particles to repel each other and holds atomic nuclei together). They do experience the weak force (which is the 3rd fundamental force) however this does not aid in detection as this is very short range force and therefore particles have to be extremely close together in order to feel it. Finally, they are affected gravity, but just barely, as neutrinos are almost massless (they were first discovered they were assumed to be massless, as the standard model predicts, and it was only recently that they were proven to have mass through neutrino oscillations, for which the 2015 Nobel prize was awarded to Takaaki Kajita and Arthur B. McDonald ). Essentially neutrinos are barely affected by anything else in the universe, making them extremely difficult to detect. Take a look at this video to understand more about why neutrinos are so elusive and what they are.
The origin of the neutrino dates back to 1914 when J. Chadwick first demonstrated that the beta−decay(a type of radioactive decay which results in the nucleus receiving an extra proton and an electron being released) energy spectrum of a radioactive element was continuous, which seemed to imply energy and momentum were being lost in the decay – or in other words energy was not being conserved! However this conundrum was resolved when in 1930 W.Pauli postulated an electrically neutral particle which would account for the energy and momentum that was being lost. He dubbed this particle the neutron, however 2 years later when Chadwick discovered a larger, neutral and strongly interacting particle that was similar to the proton, it made more sense to call this the neutron. E.Fermi aided in the renaming of Pauli’s ghostly particle, proposing to add the suffix -ino (which mean “small” in Italian) to part of the original name, making it a neutrino, as it was possible the particle was massless.
Due to its extremely small or non-existent mass and its elusive nature Pauli was not expecting the neutrino to ever be observed. However, approximately 20 years after its postulation F. Reines and C.L. Cowan Jr., set up an experiment at the Savannah River nuclear reactor in South Carolina to demonstrate that neutrinos produced in the reactor from beta decay occasionally interacted with protons in the detector medium. Each reaction of these neutrinos would result in a neutron and a positron(an anti-electron) which could be detected and would be unambiguous proof of the existence of the neutrino. In June 1956, only two years before Pauli’s death, Reines and Cowan sent an unexpected telegram to Pauli, informing him of the neutrinos discovery.
What Do “Neutrino Oscillations” Entail?
Neutrinos come in three “flavours”; electron, muon and tau. Neutrino oscillation is the phenomenon in which neutrinos can change from one flavour to another. The observation of this was critical to neutrino science as if all three neutrinos flavours had a mass of zero (as was initially thought), or even the same arbitrary mass, this would not be allowed. Therefore, it was shown that neutrinos had mass due to the observation of neutrino oscillations, with muon neutrino oscillations being discovered in 1998 at the Super-Kamiokande in Japan and electron and tau neutrino oscillations being found in 2001/2002 at the Sudbury Neutrino Observatory.
In order to observe neutrino oscillations the neutrinos must be allowed to travel for a large distance through earth’s surface as the mantle acts as an amplifying medium for neutrino oscillations. This is because as they travel they are affected by electrons in the Earth’s atoms, through subatomic forces, which increases the likelyhood of these mutating particles to transform in to a different flavour.
What Exactly is the DUNE Experiment?
The DUNE experiment is supported by the LBNF, which is a new 60–120 GeV (which is approximately the kinetic energy of a flying mosquito! ) beamline at Fermilab that is able to produce either a high-energy beam of muon neutrinos or antineutrinos. One of the goals of the experiment is to study neutrino oscillations which has driven the large scale design of the experiment (as the large distance the neutrinos travel across the experiment allows them oscillate) and determine whether or not neutrinos are their own antiparticles (particles that are their own antiparticle are known as Majorana particles).
The initial beam power will be 1.2 MW and this is just the first step for LBNF and the beam is being designed to be upgradable to at least 2.4 MW, which will facilitate further discoveries. The DUNE experiment is projected to be completed in 2022.
Neutrinos created by the LBNF beamline will travel 800 miles (1,300 km) and be intercepted by the huge neutrino detector at the Sanford Lab. Surprisingly, no tunnel is needed for the neutrinos to travel this far as these ghostly particles pass easily through soil and rock as they only rarely interact with matter. In fact the neutrinos could travel for a billion miles (or km) of rock, on average, before coming to stop!
The experiment is actually comprises of two detectors, with a near detector at Fermilab , Illinois and a far detector at the Sanford Underground Research Facility, South Dakota which will observe the neutrinos produced at Fermilab. An intense beam of trillions of neutrinos will be fired over a distance of 1,300 km through the Earth’s crust from a production facility near fermilab, which gives ample distance for the neutrino flavours to oscillate. These neutrinos then reach a large volume (multi-kiloton) of liquefied argon, which is a noble gas, at the far detector.
The far detector consists of four 10 kton liquid-argon time projection chambers (or LAr-TPCs) which are very large with each being approximately 62 × 15 × 14 m. Using LAr-TPCs allows 3D bubble-chamber-like imaging of neutrino interactions (or proton decay) in the vast volume of the detector. Visualising the tracks of neutrino interactions is important in determining the type of particle being seen, as, under the influence of a magnetic field, charged particles to travel in helical paths with the radii of these paths being determined by their charge-to-mass ratios and velocities. Their radius of curvature is also used to calculate their momentum, as the higher the kinetic energy, the shallower the curvature of the path.
In order to determine the constraints on the experimental uncertainties, the near detector at the Fermilab site will observe the unoscillated neutrino beam. In the context of neutrino physics, the near-detector rate of events will be incredible as it will detect trillions of neutrino interactions. The typical event rate for most neutrino experiments is less than one day. These high event rates will enable a diverse and world-leading neutrino-physics programme.
How do you create neutrinos?
In the DUNE project, physicists will use one of Fermilab’s existing particle accelerators, known as the Main Injector, to generate neutrinos and it has been in operation since 2004. It accelerates protons using very strong magnets and electric fields and then hurls them into a piece of graphite or similar material where they collide with atoms in the material and produce daughter particles. The particles that emerge from these collisions generate neutrinos which will then travel 800 miles to the Stanford detector. This short video is an excellent summary of the neutrino-generating process.
How does the far detector work?
LAr-TPCs, which are the main component of the far detector, are a type of Time Projection Chamber (where the “TPC” part comes from) and it is a type of particle detector that uses a combination of electric fields and magnetic fields together with a volume of gas or liquid that can be ionised to perform a 3-D reconstruction of particle trajectories or interactions. The LAr-TPCs consist of a steel frame and layers of thin wires in different orientations. The basic principle is that of an ionisation chamber, in which
incident charged particles ionise (where a charged particle “knocks out” one or more electrons of atoms in a medium, creating ions) a volume of gas/liquid where the resultant positive ions move towards a cathode (the wires) and the dissociated electrons move towards the anode (the frame). This generates an ionization current which can be measured by an electrometer circuit, which is capable of measuring very small output currents in the regions of femtoamperes (which is a billionth of a millionth of an ampere) to picoamperes (One millionth of on millionth of an ampere).
The medium to be ionised chosen to fill for the LAr-TPCs is liquid argon (where “LA” part comes from) as it is a noble gas, meaning it has closed electron shells. This means that the dissociated electrons will not rejoin atoms in the medium as they travel towards the anode as there are no free positions for them in the electron shells. Argon also releases a number of scintillation photons (also known as light) when it is interacted with by a charged particle and these photons are proportional to the energy deposited in the argon by the interacting particle. Liquid argon is also relatively cheap, making filling the large APA structures more economically feasible. Additionally, liquid argon is approximately 1000x more dense than the gas used in the first TPC design (Read more about the origin of the TPC here), which results in an increased chance of particles interacting within the detector by a factor of 1000x. As the particles are so unfathomably small, this is very important to consider otherwise we wouldn’t observe any interactions, as the particles would miss each other!
This design of a ionisation chamber initially appears to be flawed however as the essential ingredient, a charged particle, seems to be missing – neutrinos aren’t charged! However neutrinos interact with some forms of matter and create charged particles such as muons, and it is these that can be detected. Therefore neutrinos are inherently elusive as they can only be inferred as opposed to being directly detected. The image below shows a simplified image of how the detector at LHC in Switzerland is not able to directly detect neutrinos although it is able to detect other particles.
Below is a video illustrating how neutrinos are detected in LArTPCs
How does the near detector work?
The near detector, which is being designed by collaborators in India, is comprised of a straw tube tracking detector and an electromagnetic calorimeter placed inside of a 0.4 T dipole magnet(which is 400x stronger than the average fridge magnet!) which enables the tracking of particles.
A straw tube tracking detector is a type of gaseous ionisation detector and the basic principle of detection is the same as the ionisation chamber described above. It is composed of a hollow tube with a wire running alone the centre and a gas which becomes ionised when a particle passes through. Similar to above, a potential difference is maintained between the wire and the walls of the tube, in order to attract the ions and electrons in different directions. This difference in charge produces a current, which indicates that a particle entered the detector medium.
An electromagnetic calorimeter is an experimental apparatus used in order to measure the energy of particles entering the detector, which will enable scientists to further study the behaviour and properties of these particles.
Are there other neutrino detectors?
Yes! There are several neutrino detectors around the world, take a lot at the image below to find out more about them.