An Introduction to LArTPC work:

by Laura Jeanty

The Liquid Argon Time Projection Chamber (LArTPC) is a fine-grained bubble chamber detector which aims to improve upon current detectors by increasing the range and precision of cross section measurements in neutrino scattering experiments. A better understanding of these events will help address remaining questions about how neutrinos fit into the Standard Model and beyond: are there more than two independent neutrino oscillations? Does the neutrino have a magnetic moment, and if so, can we better constrain its upper limit? What is the contribution of the strange quark to proton spin?

To examine these questions, the detector will work in the FINeSSE project at Fermilab and is designed to replace the first phase Vertex detector in order to look at lower energy events in greater detail, with fewer statistical and systematic errors. Our group has set out to build a prototype in order to understand how the detector works and what its full potential is. The following summary looks at the physics questions motivating the FINeSSE project and the LArTPC detector and discusses our plans and goals for the prototype.

Physics goals

For a long time, the question of whether or not neutrinos have mass remained unanswered. If they do have mass, however, then quantum mechanics allows a mixing of the mass eigenstates, and this mixing permits a single neutrino to switch between its component mass eigenstates as its wavefunction propagates. Such fluctuations are called oscillations, and they are key to measuring neutrino mass because they only occur if the three neutrino flavors - electron, muon, and tao - have non-zero and distinct (from each other) masses. The mass eigenstates of neutrinos are considered the pure eigenstates, while the three flavors - electron, muon, and tao - are considered weak eigenstates, and are a combination of different mass eigenstates. As the neutrino switches between different relative combinations of its mass eigenstates, it changes flavor. In 1998, neutrino oscillations were observed in muon neutrinos produced by cosmic ray interactions with the atmosphere. Solar electron neutrinos have also been measured at lower rates than theoretically predicted, indicating that the so-called disappearance is a product of oscillations into other neutrino flavors which the detector was not designed to see.

A third oscillation signal was seen in the LSND experiment, one not consistent with observations of solar and cosmic ray neutrinos. Since three particles can only have two independent mass differences, the third signal does not agree with the standard theory of three neutrino flavors. Such an oscillation would lie outside the physics Standard Model and may require the introduction of at least one “sterile” neutrino which has mass but does not interact via the weak force. MiniBooNE is designed to test the LSND signal by looking for oscillations from muon to electron neutrinos at higher energies and with a longer baseline than LSND. MiniBooNE will both look for muon neutrino disappearance and electron neutrino appearance in a primarily muon neutrino beam produced at Fermilab.

In order to measure muon neutrino disappearance in the beam, it crucial to have an accurate count of the original muon neutrino flux. The proposed FINeSSE detector will be able to serve that role by monitoring neutrino and proton interactions in the beam in order to make a quantitative observation of the relationship between the number of neutrinos present and their energy. The probability of an oscillation between two neutrino flavors is a function of two fundamental parameters, the mass differences squared between the two flavors and their mixing angle, and of two experimental quantities, the distance over which the neutrino propagates L, and the neutrino energy E: P = sin^2(2*theta)sin^2(1.27*delta(m)^2*L / E) Because the probability of oscillation is a function of neutrino energy, the shape of the graph of neutrino number verse energy will change after oscillation. Comparing the initial plot, as measured by FINeSSE, against the final plot, as measured by MiniBooNE, will allow greater sensitivity to muon neutrino disappearance than a simple comparison between initial number of total (expected) neutrinos and the observed quantity. While the first phase Vertex detector will provide a good measurement of the relationship between the number of neutrinos and their energy, LArTPC can improve on this sensitivity by detecting lower energy neutrinos, reducing the background in free proton collisions, and providing greater discretionary capability between neutrino/proton and neutrino/neutron scattering events.

Non-Zero Magnetic Moment

The confirmation of neutrino mass raises the possibility of a non-zero magnetic moment despite a total neutral charge. A non-zero magnetic moment has important consequences for astrophysics and for understanding how neutrino mass fits into the standard model, since different theories - SUSY, extra-dimensions - predict different values of the magnetic moment. Current experiments place an upper limit on the magnetic moment at 1.5 * 10-10 times the magnetic moment of the electron (µB). Standard model predictions put the magnetic moment on the order of 10-19 µB, while theories of extra dimensions put it at 10-11 µB. Thus, increasing the sensitivity of detection by a few orders of magnitude will be of great importance in understanding beyond-the-standard-model physics.

FINeSSE will look at this question by studying neutrino-electron scattering. In the standard model, neutrinos scatter with electrons only through the weak force, but if neutrinos have a non-zero magnetic moment, they can - through an intermediate muon/W boson decay and recombination - emit a gamma ray which can interact with an electron and create scattering events. Such events are quite rare and have very small cross sections. Still, if they occur, then they will create an excess of electron scattering events over those predicted by weak interactions alone. The excess will be particularly noticeable at low electron kinetic energies. Indeed, if scattering on account of the magnetic moment does occur, there will be a shape change in the graph of scattering events verse electron energy, since the magnetic moment scattering is much more likely at low energies. LArTPC will have the advantage of being able to study lots of events at low electron kinetic energies.

At the same time that FINeSSE works in conjunction with MiniBooNE to look for muon neutrino disappearance and measure the neutrino magnetic moment, it will, on its own, examine another fundamental question in nuclear physics: what is the contribution of the strange quark to the spin of the proton?

The strange quark contribution to the proton spin can be determined from the ratio of neutral current neutrino/proton scattering to charged current neutrino/neutron scattering. To do this, an experiment must have an ability to finely distinguish between the two types of scattering, a unique feature of the LArTPC detector which increases the resolution of such scattering events. Additionally, the ability to observe these interactions at lower kinetic energies will greatly reduce the statistical (and/or systematic??) errors in present measurements.

The Liquid Argon Time Projection Chamber (LArTPC) detector

The LArTPC is a modern adaptation of the bubble chamber technology originally used to observe low energy neutrino interactions. In the current FINeSSE proposal,the upstream Vertex detector, designed to measure proton energy and angle and to track muons, is a matrix of WLS fibers submerged in a cubic volume of liquid scintillator. Using the time of arrival of the scintillation light at the end of a fiber, a particle’s path can be accurately reconstructed.

The LArTPC detector differs from the traditional sci-bath detector in that it collects both light and charge. The light collection works in a similar fashion to current detectors. A small percentage of the neutrinos entering the detector will collide with an argon atom, imparting enough energy to knock out a proton or a neutron. If it is a proton, the particle will ionize other atoms as it passes through the argon volume. The ionization electrons will loose energy as they drift through the liquid argon in the form of scintillation photons. This scintillation light travels to the edge of the detector, where each photon is absorbed by the face of a photomultiplier tube (PMT). On the reverse side of the glass in each PMT is a photo cathode, and as the photon is absorbed a photo electron is emitted. The electron falls into an electron amplification chamber consisting of a dynode chain and the resulting current can be recorded and analyzed. Additional light comes from from Cerenkov radiation if the scattered particle is traveling faster than its light cone, and detailed reconstruction must be done to sort out which light came from which source (Cerenkov radiation arrives as a cone on many PMTs, while scintillation light tends to be more linear.)

The unique component of the LArTPC detector is that the ionization electrons themselves are also recorded. After drifting through the argon volume, the are collected and imaged by wire chamber planes at the edge of the detector. A major technical challenge of these detectors is to purify the argon enough to prevent the capture of free electrons as they drift several meters to the edge of the detector.

If the particle knocked out by the original neutrino is a neutron, detection is more difficult because it does not itself ionize atoms as it moves through the argon volume. However, it does have the potential to knock out protons, which will in turn behave like the protons scattered by neutrinos, only with slightly lower energies. A neutron can be seen in these detectors by observing the scintillation light and ionization charge produced by the lower-energy protons it scatters. Of course, because the neutrinos themselves will knock out neutrons and protons with a range of energies, higher energy neutrons may produce second-generation protons that look similar to protons scattered by a neutrino in a lower energy event. Accurate reconstruction of these events will be a challenge of the LArTPC detector.


The main advantage of the LArTPC detector is that it provides a fine-grained image of low-energy neutrino scattering events. Using both charge and light information, events can be reconstructed in greater detail than with light alone.

Although there may be a few high energy neutrino/neutron scattering events that will be difficult to distinguish from low energy neutrino/proton events, the LArTPC detector will greatly improve on the current resolution of scattering events, and thus will greatly reduce (though not eliminate) the error in reconstruction and allow for greater identification of neutron events. Clarifying the neutrino/neutron signal and the neutrino/proton signal will provide a cleaner background neutrino oscillations and will help address the strange quark contribution to proton spin.

The proposed FINeSSE vertex detector can track muons and protons down to kinetic energies of 100 MeV. LArTPC hopes to be able to easily identify electrons above the 5 MeV threshold; indeed, electrons down to 150 keV may produce ionization charge, but at such low energies, it may be very difficult to distinguish signal from noise. One of the main advantages of LArTPC is its ability to track such low energy electrons, essential for understanding neutrino/electron scattering events and in looking for the neutrino magnetic moment.

An additional benefit in using the LArTPC detector is that argon has no free protons, which will reduce uncertainty in proton scattering present in the scibath Vertex detector.