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 Deep Underground Neutrino Experiment (DUNE) Ethan Holland Noah Levine Eric Lu Brian Morris Aaron Szabo Abstract The Deep Underground Neutrino Experiment (DUNE), formerly known as the Long Baseline Neutrino Experiment (LBNE), is a planned experiment to test certain properties of fundamental particles called neutrinos. It will take place between two facilities: a neutrino production facility in Illinois, which will use the existing accelerator complex of Fermilab along with some new structures that remain to be built; and a detector in South Dakota, currently under construction. The experiment's primary goal is to investigate the process of neutrino oscillation, in which one of the basic properties of a neutrino, its “flavor,” changes during flight. DUNE’s results will help answer several open questions in particle physics, such as whether neutrinos violate charge­parity (CP) symmetry, what the correct ordering of the neutrino masses is, and why there is more matter than antimatter in the universe. DUNE will also expand the search for proton decay – an experimentally unprecedented phenomenon whose discovery would have major implications for the development of a Grand Unified Theory (GUT) of physics – among other investigations of physical phenomena that defy explanation by the Standard Model of particle physics. History of Neutrino Experiments Neutrinos were first theorized to exist by Wolfgang Pauli in 1931, in order to preserve conservation of energy for beta emission. Enrico Fermi named the particle “neutrino,” meaning “little neutral one” in Italian, to distinguish it from the larger neutral neutron. However, neutrinos were not observed until 1956, when Clyde L. Cowan and Frederick Reines discovered the particle after detecting gamma rays from inverse beta decay, for which Reines received the 1995 Nobel Prize. Because they are leptons, neutrinos do not interact via the strong force, and because they are neutral, they do not interact via the electromagnetic force. Since neutrinos only interact via the weak force (and, due to their nonzero mass, the extremely weak gravitational force), they are extremely difficult to detect. The neutrino was first detected in the Cowan­Reines experiment in 1956. Cowan and Reines placed two tanks of water next to a nuclear reactor, a powerful source of neutrinos. Some neutrinos that passed through the tanks interacted with protons in the tanks and produced positrons and neutrons. The positrons and electrons of atoms in the tank then annihilated each other, emitting gamma rays. The tanks of water were surrounded by tanks filled with scintillating materials, which experience photoluminescence in the presence of ionizing radiation. To provide an extra layer of certainty and confirm that the gamma radiation was being caused by neutrinos, the scientists also placed cadmium chloride in the tanks. Cadmium absorbs neutrons and then emits gamma rays. Upon detecting radiation, presumably from a positron­electron interaction, the scientists expected to detect a second ray from a cadmium atom's absorption of a neutron within 5 microseconds (The Reines­Cowan Experiment: Detecting the Poltergeist, 1997). All fermions have half­integer spin, unlike bosons, which have integer spin. Because of this, all neutrinos have spin ½. Since leptons do not have color, they are not affected by the strong nuclear force. Leptons do come in types called “flavors:” electron, muon, tau, electron neutrino, muon neutrino, and tau neutrino. Each flavor has an associated antiparticle. Neutrinos are created during radioactive decay, which is caused by the weak force. The image below is a Feynman diagram for beta minus decay. In the diagram, a down quark (labeled “d”) turns into an up quark (“u”), causing a neutron to decay into a proton. The down quark emits a W boson, which turns into an electron and an electron antineutrino. In beta plus decay, a positron and an electron neutrino are instead created. Neutrinos are produced in nuclear reactions in the Sun via the proton­proton chain (a fusion reaction), in supernovae, in cosmic ray collisions, and most notably in the Big Bang. Feynman diagram of beta minus decay (“Beta Negative Decay”) Of particular relevance to DUNE is the phenomenon of neutrino oscillation, in which neutrinos change flavor during flight. The Homestake experiment, conducted by astrophysicist Raymond Davis, Jr. in the late 1960s, provided the first evidence that the oscillation was taking place. The experiment’s purpose was to compare calculations made by John N. Bahcall of the rate of neutrino emission of the Sun with experimental evidence. A 10,000 gallon tank of perchloroethylene, a fluid used in dry­cleaning, was placed 4,850 feet underground in Homestake Gold Mine, South Dakota. The depth was required to filter out non­neutrino forms of solar radiation. Upon interaction with neutrinos, neutrons within the chlorine atoms of the perchloroethylene transformed into protons, creating argon gas. Every few weeks, helium was bubbled through the tanks to capture the formed argon. The number of atoms of argon was used to calculate the Sun's rate of neutrino emission. Unfortunately, the experimental calculations discovered significantly fewer neutrinos than the theoretical calculations predicted (Solar Neutrino Experiments). The construction of the tank used in the Homestake Experiment (“Construction of the Homestake Mine Tank”) This discrepancy was not resolved until the experimental confirmation of neutrino oscillation in 2001 at the Sudbury Neutrino Observatory, for which Takaaki Tajita and Arthur B. McDonald won the 2015 Nobel Prize in Physics. Tajita worked at the Super Kamiokande and McDonald worked at the Sudbury Neutrino Observatory. Neutrino oscillation, a process by which neutrinos can change from one flavor (electron, muon, or tau) to another, explains the shortage of solar electron neutrinos in the Homestake experiment. Interestingly, the confirmation of neutrino oscillation settled a long­standing open question of whether neutrinos have mass: it demonstrates that they ​
must​
be massive particles, because the oscillation could not happen if they were not (see next section). The detector at Sudbury consisted of a 1,000 metric ton tank of heavy water, which contains an isotope of hydrogen with a neutron, surrounded by photomultiplier tubes (First Results from the Sudbury Neutrino Observatory Explain the Missing Solar Neutrinos and Reveal New Neutrino Properties). Two experiments were set up: in one, neutrinos converted the neutron in deuterium to a proton, and in the other, neutrinos dissociated the deuterium, splitting it up into a neutron and proton. Only electron neutrinos (those produced inside the Sun) had the energy to participate in the first reaction, whereas all types of neutrinos could participate in the second one. Because the Sun was the only source of neutrinos so far underground and fewer electron neutrinos were detected than all types of neutrinos, some of the electron neutrinos from the Sun must have transformed into either muon or tau neutrinos (Waller). The tank of heavy water surrounded by photomultiplier tubes in the Sudbury Neutrino Observatory (“View of the SNO detector after installation of the bottom PMT panels, but before cabling”) A new way that people are trying to detect neutrinos is with the IceCube Neutrino Observatory in Antarctica. The IceCube Neutrino Observatory is a cubic kilometer of ice with 5160 digital optical modules that can detect light (via photomultiplier tubes). A neutrino interacts with IceCube via a weak interaction. When a neutrino hits the nucleus of an atom, it can create an electron, a muon, or a tau. If the created particle is travelling faster than the speed of light in ice, it will emit Cherenkov radiation, a blue light emitted when a particles travels faster than the local speed of light. Based on the pattern of radiation emitted, researchers will be able to determine which of the three particles was formed, and will consequently know the flavor of the neutrino. The reason why the observatory is so big is to increase the chance that a neutrino interacts with the ice. The observatory detects around 275 neutrinos each day (​
IceCube Neutrino Observatory​
). Diagram of IceCube (​
IceCube Neutrino Observatory​
) DUNE’s Physics Program DUNE’s principal purpose is to investigate neutrino oscillation, in which the flavor of a neutrino changes in flight. DUNE’s results in this area will give more precise values for the parameters that govern this oscillation, help determine the correct ordering of the the three neutrino masses, and provide evidence for or against the neutrino’s violation of “charge conjugation parity (CP) symmetry” (discussed below). Results about CP violation may also help to answer the questions of whether the neutrino is a Majorana fermion – that is, whether it is its own antiparticle – and why there is more matter than antimatter in the modern universe. In addition, DUNE will investigate phenomena that push or break the boundaries of the Standard Model of particle physics, such as proton decay, a phenomenon that has never been experimentally detected but is predicted by many Grand Unified Theories (GUTs) of physics. Wave Function Collapse Before delving into the details of neutrino oscillation, it is necessary to go over the mathematical formalism behind quantum states and what happens when a quantum system is observed. At any point in time, a quantum system has some state, which is associated with a ​
state vector​
that encapsulates all of the observable information about that system. This state vector is a member of a Hilbert space, a generalization of Euclidean space that extends calculus to any finite or infinite number of dimensions (technically, it is a real or complex vector space equipped with an inner product that is also a complete metric space). This Hilbert space can be represented as a function space, whose members are functions, in which case the state vector is called the ​
wave function​
of the system. The wave function representation of quantum states leads to things like the Schrödinger equation, which dictates how a system’s wave function evolves over time. An observable of a system is, true to its name, a physical quantity that can be measured or observed (the momentum of a single electron, for example). In the formalism, observables correspond to linear operators from the Hilbert space to itself (square matrices, essentially, except the Hilbert space can be infinite­dimensional, so this is not precisely correct) that are self­adjoint (for a finite number of dimensions, this just means that the matrix is Hermitian, or equal to its conjugate transpose). When a system is observed and an observable is measured, the system’s wave function (state vector) collapses to an ​
eigenstate​
of the observable: an eigenvector (in the Hilbert space) of the linear operator associated with the observable. The actual number resulting from the measurement (which might represent a position, a momentum, an energy, etc.) is the eigenvalue associated with the eigenvector to which the wave function collapsed (Bombelli, 2015). If |ᵱ〉 is the wave function before observation and |ᵱ​
i〉 is a particular eigenstate, then the probability of ​
collapsing to |ᵱ​
〉 is |ci|2 , where ci , a complex number, is the component of |ᵱ​
〉 in the eigenbasis i​
i​
representation of |ᵱ〉 (representation as a linear combination of the eigenvectors of the observable’s operator): The epiphenomenon of wave function collapse is really just a manifestation of a larger process in which the wave functions of a measurement device and a measured system overlap and begin to interact. Neutrino Oscillation Neutrino oscillation is a process by which neutrinos can change flavor (electron, muon, or tau) during flight. The Homestake experiment in the late 1960s offered the first hints of the phenomenon: a detector designed to count electron neutrinos emitted from the sun came back with one third of the number predicted from theory. Later experiments showed that other flavors accounted for the missing electron neutrinos, so the electron neutrinos must have changed flavor in flight. More recent research by the T2K project in Japan has revealed that all neutrinos can change flavor, regardless of their initial flavor (Amos, 2013). The theoretical explanation of the neutrino oscillation is based on the assumption that neutrinos have mass and the fact that each flavor does ​
not ​
have its own distinct mass. Given that distinct flavors correspond to distinct particles (electron neutrino, muon neutrino, or tau neutrino), one might think that each particle should have a well­defined mass – each muon neutrino weighs precisely this much, and so forth – but this is in fact not the case. Upon observation, a flavor measurement will cause a neutrino’s wave function to collapse to one of three flavor eigenstates (electron, muon, or tau), and a mass measurement will cause a collapse to one of three mass eigenstates (mass 1, mass 2, or mass 3). One might expect that each flavor eigenstate would correspond to one mass eigenstate (so that electron neutrinos always had mass 1, for instance), but instead the flavor eigenstates are superpositions (linear combinations) of the three mass eigenstates. Thus, a neutrino of a particular flavor has a probability to have any of three masses. The three mass eigenstates and the three flavor eigenstates each form a basis for the space of possible masses and flavors that a neutrino’s wave function could have. (The wave function, being a combination or superposition of several eigenstates, could have “multiple” masses or flavors, but will collapse to one definite mass or flavor upon observation.) The flavor eigenstates are linear combinations of the mass eigenstates, so there should be a linear transformation between the flavor eigenbasis and the mass eigenbasis. The linear transformation that relates the two eigenbases and thus describes the “mixing” of masses is the Pontecorvo­Maki­Nakagawa­Sakata (PMNS) matrix: (Here, ν e , ν μ , and ν τ are the flavor eigenstates, and ν 1 , ν 2 , and ν 3 are the mass eigenstates.) The PMNS is a 3 by 3 complex unitary matrix: in each row and column, the sum of the squared magnitudes of the elements is one. Unitarity is required because the square of a given component is the probability that a particle of a given flavor is observed with a given mass (and the three probabilities in each row and column must sum to one). If the PMNS matrix were the identity matrix, then each flavor would have a unique mass and neutrino oscillation would not occur. Experiments that aim to understand neutrino oscillation (including DUNE) seek to find values for the components of the PMNS matrix. They do this by parameterizing the matrix in terms of “mixing angles,” which quantify the amount of mixing between different masses and flavors, and a “CP­violating phase,” which quantifies the extent to which neutrinos violate “charge conjugation parity (CP) symmetry” (see next subsection). The fact that the three masses are different means that their phases advance at slightly different rates, thus creating an interference pattern in which the mass composition of a neutrino (really, of its wave function) changes as the particle moves. Mass is related to flavor by a change of basis using the PMNS matrix, so this means that the flavor composition also changes. Thus, at one point in its flight, a neutrino might be more likely to collapse to the electron flavor if observed, while at another, it might be more likely to collapse to tau. This oscillation in terms of the superposition of flavor eigenstates gives rise to the phenomenon of neutrino oscillation. The oscillation makes itself apparent only over long distances (such as that between the Earth and the sun). This oscillation also explains why neutrinos ​
must ​
have mass: if they did not, it would not occur (​
LBNF and DUNE​
, 2015). Majorana Fermions, CP Violation, and the Matter/Antimatter Imbalance A Majorana fermion (named after the Italian physicist Ettore Majorana, who hypothesized its existence in 1937) is a fermion that is its own antiparticle. A particle and its antiparticle must have opposite charges, which means that the only Majorana fermions that the Standard Model can possibly allow are the neutrinos: all of the other fundamental fermions are charged. It is still an open question as to whether neutrinos are Majorana fermions. If they are, then that would allow for phenomena such as neutrinoless double beta decay, in which two beta decay processes occur but the two antineutrinos that they generate, being effectively a neutrino and an antineutrino, annihilate each other (meaning no observable neutrinos or antineutrinos are produced, hence the title “neutrinoless”). Neutrinoless double beta decay is a process that favors matter over antimatter – it does not preserve the balance between matter and antimatter. Thus, if neutrinos ​
are​
Majorana fermions, that could explain the prevalence of matter over antimatter in the universe today (Brewster, 2016). On the other hand, DUNE’s investigation into the components of the PMNS matrix may be able to demonstrate that neutrinos and antineutrinos oscillate in different patterns. This would show that neutrinos are ​
not​
Majorana fermions, and furthermore that they violate CP (charge conjugation and parity) symmetry. CP symmetry asserts that the laws of physics would remain unchanged if every particle were replaced with its antiparticle and space were mirrored. CP symmetry has been shown to break under certain interactions with the weak force, but leptons (the category to which neutrinos belong) have never violated it. CP violation ​
can​
explain the asymmetry between matter and antimatter in the universe, but the known sources of CP violation in the Standard Model are too small to account for the size of the asymmetry. DUNE’s experimental determination of the parameters governing neutrino oscillation (in particular the “CP­violating phase” mentioned in the previous subsection) may be able to demonstrate that neutrinos do violate CP symmetry. This would be a major step forward in understanding the imbalance between matter and antimatter in the modern universe (​
LBNF and DUNE​
, 2015). Beyond the Standard Model In addition to specific work on understanding and quantifying the parameters behind neutrino oscillation, DUNE is more broadly equipped to search for many examples of physics Beyond the Standard Model (BSM) – phenomena that cannot be explained using the Standard Model alone. Neutrino oscillation is one such phenomenon. The oscillation ​
itself ​
is not incompatible with the Standard Model – quarks also mix flavors in the same way that neutrinos do, with a matrix analogous to the PMNS matrix, in a way that the Standard Model can explain. However, neutrinos’ much smaller masses and much larger mixing rates indicate that there may be different physics at play in neutrino oscillation. Quark oscillation and mass can be explained under the aegis of the Standard Model and traced back to the Higgs boson, whereas neutrino oscillation cannot (since the Standard Model holds that neutrinos have no mass). In addition, through its investigation of CP violation, DUNE will be able to test the three­flavor paradigm of neutrino interactions itself. DUNE will measure the extent to which neutrinos violate CP symmetry using two different metrics. These metrics are related to each other through the PMNS matrix – but the particular structure of the PMNS matrix depends on the assumption that there are only three neutrino flavors. If the experiment finds a discrepancy between the two metrics for CP violation, that will provide evidence that the three­neutrino paradigm is incomplete. This would imply that there are additional flavors of neutrinos: for example, sterile neutrinos, which theoretically interact ​
only​
via gravity, not via the weak force. Normal neutrinos, which interact via the weak force and gravity, are already difficult to detect – sterile neutrinos, if they exist, would be extremely elusive. DUNE will also search for proton decay (which lacks experimental evidence but is predicted by many Grand Unified Theories (GUTs)) and violation of CPT symmetry (charge conjugation, parity, ​
and​
time reversal – a symmetry that has never been experimentally broken before) (​
LBNF and DUNE​
, 2015). Fermilab Accelerator Complex The DUNE project will use the existing accelerator complex at Fermilab in Batavia, Illinois. The current Fermilab accelerator complex began operation in 2012 and was used by over 1,850 scientists in fiscal year 2014 for a variety of research projects many of which involved neutrinos (User Facilities: Fermilab Accelerator Complex, 2015). The accelerator consists of an ion source, a linear accelerator called the Linac, the Booster, the Main Injector, the Recycler Ring, and many connecting lines directing particles to different experiments (User Facilities: Fermilab Accelerator Complex, 2015). The complex is used for many experiments, but this paper focuses solely on the portion of the beam designated for DUNE. The process starts with a proton beam from the ion source, which is accelerated to high speeds by the Linac, Booster, and finally Main Injector. From the Main Injector, the protons are sent into the Neutrinos at the Main Injector line where they interact with a target material to form pions and kaons, which decay into neutrinos. Finally, the beam, now consisting of neutrinos, passes through a near detector and is embarks on its long journey to the far detector at the Sanford Underground Research Facility in South Dakota. Radio Frequency (RF) Cavities The Lorentz force on a point charge represents the combined effect of the electric and magnetic force created by an electromagnetic field. A particle of charge q moving at velocity ​
v through an electric field ​
E ​
and magnetic field ​
B ​
will experience a Lorentz force of F​
=q(​
E​
+​
v ​
x ​
B​
) (Barak, Harrison, & Watts, 2013). It is important to note the magnetic force, q(​
v ​
x B​
), is velocity dependent and furthermore will always be perpendicular to the velocity. This means that a magnetic field cannot increase the velocity of a particle in the direction of its current movement. However, magnetic fields can be used for steering and focusing beams. This is especially useful in ring shaped accelerators such as the Booster and Main Injector. On the other hand, the force due to the electric field, q​
E​
, is not velocity dependent and therefore the force can be applied parallel to movement, increasing the velocity in the desired direction (Barak, Harrison, & Watts, 2013). Therefore, electromagnetic fields can be used for either of two purposes: to accelerate particles with an electric field or to steer particles using the magnetic field. Most accelerators, including the Fermilab accelerators, use hollow, electromagnetically resonant structures called RF Cavities that store oscillating radio waves (Barak, Harrison, & Watts, 2013). Fermilab has developed superconducting RF cavities that allow for maximum efficiency. Additionally, in order to keep energy losses to a minimum, the RF Cavities are tuned to a very narrow range of frequency. However, the Booster, which will be discussed in more detail later, requires RF frequency to increase as the beam energy increases. In order to accomplish this without losing energy, ferrite tuners are used to change the resonant frequency of the cavity (Fermilab, 2009). Since the radio waves oscillate in a sinusoidal manner in the RF cavity, the electric force points in the desired direction for half of the period and the wrong direction for the other half. It is therefore important to time the oscillation with the arrival of the particle beam such that the particles are accelerated rather than slowed down. The sinusoidal oscillation of the electric field also focuses the beam. Not all particles will arrive at the exact same time and may have different velocities. However, the particles that arrive on the leading edge of the pulse will receive less of the electric field and less acceleration, while the trailing particles will experience the peak of the field and greater acceleration (Barak, Harrison, & Watts, 2013). This means that any outliers will receive a different acceleration that will cause them to approach the mean, focusing the beam. Ion Source and Initial Acceleration All experiments using the accelerator complex begin with a 35 keV proton beam (Barak, Harrison, & Watts, 2013). The beam is in the form of short pulses rather than one continuous beam. It should be noted that not all pulses are actually used and that the accelerator complex hosts multiple experiments so only a portion of the pulses are designated for DUNE. The proton beam is first sent into the Low Energy Beam Transport (LEBT). A beam chopper at the end of the LEBT can either block the beam or let it pass at any point in time (Barak, Harrison, & Watts, 2013). This allows scientists to shorten the pulses to a desired length by letting the beam pass for a set amount of time and then blocking the beam. From the LEBT, the beam is accelerated to 750 keV and then sent to the Medium Energy Beam Transport (MEBT). A buncher in the MEBT slows down the particles at the leading end of the pulse and accelerates the particles at the trailing end of the pulse to maintain structure and pulse width. The MEBT feeds the pulse, now at 750 keV, into the Linac (Barak, Harrison, & Watts, 2013). Linac ​
The Linac accelerates the beam from 750 keV to 400 MeV through two subsections: a Low Energy Linac (LE Linac), which accelerates from 750 keV to 116.5 keV, and a High Energy Linac (HE Linac), which accelerates from 116.5 MeV to 400 MeV (Barak, Harrison, & Watts, 2013). The LE Linac is a Drift Tube Linac (DTL) and consists of 5 large RF cavities called tanks. DTLs have connecting tubes within large RF cavities to shield the particles from the unwanted sections of the sinusoid (Barak, Harrison, & Watts, 2013). The shielding allows for larger cavities where the beam is in the cavity for multiple radio wave oscillation periods. On the other hand, the HE Linac is a Side­Coupled Cavity Linac consisting of 7 main RF cavities and 3 low energy RF cavities. Side coupled linear accelerators alternate accelerating cells and coupling cells to provide acceleration. After the HE Linac, the 400 MeV beam is sent to the Booster. Booster In proton synchrotrons built before Fermilab’s, the proton beam was injected directly from the linear accelerator into the main accelerator. However, the original Fermilab Main Ring was designed to receive a much higher energy beam, so a linear accelerator capable of accelerating a proton beam to the desired energy would have to be nearly 4 miles long. Such an accelerator would obviously be impractical and expensive. It was decided that a “Booster” would be created to bridge the gap between the 400 MeV beam coming from the Linac and the 8 GeV desired injection energy. The Booster was designed as a ring, to minimize space, in which each proton makes multiple laps to further decrease the necessary circumference. Seventeen resonator cavities deliver radio frequency energy to the particles. A diagram of the Booster can be seen below. Diagram of the Booster (Fermilab, 2009) The first phase of acceleration is injection, capture, and bunching (Fermilab, 2009). First, magnets in the 400 MeV line decrease the lengthwise spread of the beam by negatively accelerating the early particles and accelerating the later particles. The beam is then allowed to travel around the Booster 5 to 10 times without acceleration, long enough that the beam loses its structure and the particles have relatively equal momenta (Fermilab, 2009). During this time, the group A RF stations and group B RF stations are out of phase so the electromagnetic waves have no effect on the beam. After the beam has lost its structure and all particles are traveling in a circular orbit, the group A and group B RF stations, which were previously out of phase, are brought into phase over a period of 200­300 nanoseconds at which point the beam is accelerated to 8GeV. As the velocity of each bunch increases and the period of revolution decreases, the frequency of the RF oscillation must be increased using ferrite tuners as mentioned earlier (Barak, Harrison, & Watts, 2013). Finally, after the batches have reached 8 GeV, they are extracted and sent through the MI8 line to the main injector. ​
Main Injector The main injector accelerates particles to 120 GeV before ejecting the particles designated for the DUNE project into the Neutrinos at the Main Injector (NuMI) line. The NuMI line sends the pulse to an underground area where the protons interact with a target material to create positively charged pions and kaons (Barak, Harrison, & Watts, 2013). The pions and kaons are then allowed to decay in a long decay channel. The resulting neutrino beam will consist of muon neutrinos produced through either a positively charged pion or a positively charged kaon decaying into a positively charged muon and a muon neutrino ( π+ → μ+ + vμ or K + → μ++ vμ ) (Popovic, et al.). However, the beam will also include a tiny number of electron neutrinos created through a positively charged kaon decaying into a positron, a neutral pion, and an electron neutrino ( K + → e+ + π0 + ve ) (Popovic, et al.). Though the beam will be over 99% muon neutrino, the small impurities do cause some problems, as it is difficult to know ​
exactly what percent of the beam consists of muon neutrinos and what percent consists of electron neutrinos (Popovic, et al.). This is especially problematic because the experiment concerns neutrino oscillation, so knowing the starting composition of the beam is of great importance. To solve this problem, a near particle detector will be constructed to determine the exact composition of the beam before it is sent on its 800 mile journey to the Sanford Underground Research Facility. Existing (blue) and future (orange) components of DUNE (Long­Baseline Neutrino Facility (LBNF)) Detectors The are two proposed detectors for the DUNE experiment: the near detector system (NDS) at Fermilab and the far detector (FD) in Sanford Lab, located in Homestake Mine. The main purpose of the NDS is to characterize the neutrino beam right as it leaves Fermilab in order to better understand the beam when it arrives at the FD (“Neutrino Detectors”). The NDS is needed to measure the variables and parameters of the beam before it reaches the FD, such as the energy of the neutrinos and the flavor composition of the beam (as discussed in the previous section). Once the beam reaches the FD, similar measurements will be made again. The differences will provide insights into neutrino oscillations. Near Detector System The near detector system is made up of two main detection systems: the fine­grained tracker (FGT) near detector system and the beamline measurement system. The FGT is made up of a straw­tube tracking detector (STT) and an electromagnetic calorimeter (ECAL). Both of these subsystems are contained within a 0.4 Tesla dipole magnet. Their specific purpose is to measure the neutrinos’ fluxes, cross sections, signal rates, and background rates. The center of the FGT is made up of the straw­tube tracking detector (STT). The straws are 1 cm diameter tubes whose walls are made up of layers of carbon and aluminum film, with a gold plated tungsten anode wire in the center of each tube. The straws will be put together in carbon fiber frames consisting of two layers of straws staggered by 5 mm. Each of these modules will contain 1344 straws. Modules are then grouped in pairs with the straws of each module aligned perpendicularly, with one set vertical (YY) and one set horizontal (XX), to create double modules. There will be 107,520 individual straws in the detector, divided among 80 modules. Schematic of the current fine­grained tracker design (Heavey, 2015) The straw tubes will be filled with a mixture of gases, either 70% Ar gas and 30% CO​
or 2​
70% Xe gas and 30% CO​
, depending on what is to be detected. Energized particles passing 2​
through the detector will ionize the gas in the straws they pass through, causing electrons to flow from the anode wire to the walls of the tube, creating a measurable current. Particles will pass through multiple straws, allowing their progress to be tracked. Diagram of a double module in the straw­tube tracking detector (Heavey, 2015) The electromagnetic calorimeter (ECAL) will be divided into three parts, the forward, backward, and barrel ECALs. The purpose of the ECAL is to measure the energy of the particles passing through the detector based on their interactions with the magnetic field created by the magnet. The ECAL is made up of layers of lead sheets and scintillator layers. Energized particles will strike particles in the scintillator layers to create photons, which will be registered by photodetectors at the edges of the scintillator layers. The beamline measurement system (BLM) is designed to measure the secondary beam particles with an emphasis on measuring the muons leaving the decay tunnel. The processes that create the neutrinos in the beam also produce daughter muons, so data on the muons in the beam can provide information on the neutrinos in the beam as well. These measurement instruments are placed right after the hadron absorber after the decay tunnel. The purpose of the hadron absorber is to absorb any non­neutrino particles that remain in the beam after passing through the decay tunnel. Far Detector The far detector is a 40,000 ton Liquid Argon Time Projection Chamber (LArTCP). Time Projection Chamber (TCP) detectors were pioneered by David Nygren at Lawrence Berkeley Laboratory in the late 1970s. A TCP uses a chamber filled with gas in order to detect particles and an electric field to collected the electrons created in interactions between the incoming particles and the gas. The collector is designed to be position sensitive so that it is possible to determine the original positions of the interactions in 3 dimensions. In 1977, Carlo Rubbia developed the concept of a liquid argon TCP or LArTPC. The LArTPC design uses a very similar approach to Nygren design, but uses argon as the sensitive gas. Argon has many advantages as a sensitive medium. Argon is a noble gas; thus it does not affect the path of the electrons, which are generated by collisions between neutrinos and the argon atoms. The lack of interaction due to the complete outer shell of the argon atoms allows the electrons to drift towards the detector unperturbed. Argon is also very dense, increasing the probability of the sensitive gas interacting with weakly interacting particles like neutrinos. A basic model of a typical LArTPC (“RealSchematicTPC”) The detector consists of a negatively charged cathode plate and on the opposite side a more positively charged anode wire plane. The anode plane is typically made up of wires running in two directions, separated by approximately 1 cm. The two sets of wires intersect, allowing an interaction’s exact position in two dimensions to be determined. The collision also generates a photon. This photon interacts with the photomultipliers surrounding the chamber. Since the distances are relatively short, it is assumed that the time the photon is recorded is the same time as the interaction that created it. The third dimension can then be determined using the difference in time between the arrival of the photon and the arrival of the electron, allowing accurate three dimensional maps of particle interactions. The LArTPC at Sanford lab will consist of four separate chambers (cryostats), each containing 10,000 tons of liquid argon. Each cryostat is over six stories tall and longer than a football field (18 m high by 19 m wide by 66 m long). The entire complex is 1475 m underground to shield the detector from cosmic high energy particles. Configuration of the four cryostats (​
Neutrino Detectors​
, 2016) Design of the outer frame of one of the cryostats (​
The LBNF Cryostats and Cryogenics Systems​
, 2015) By comparing the muons measured at the the NDS and the neutrinos recorded at the FD, it is possible to determine the composition of the beam at both points and from the differences gain valuable insights into neutrino oscillations. Works Cited Amos, J. (2013, July). Neutrino 'flavour' flip confirmed. ​
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