Patricia Vahle,
University College London
Main Injector Neutrino Oscillation Search
The Elusive Neutrino
Making a neutrino beam
MINOS is a neutrino experiment poised to provide a precision measurement of the parameters
governing the mixing among neutrino flavors. One of the fundamental particles of nature, the
neutrino comes in 3 “flavors”, called the electron neutrino, the muon neutrino, and the tau
neutrino. In the Standard Model of particle physics, each of these neutrinos is masseless; however,
new developments indicate that these particles do in fact have mass and each has a different mass.
As a consequence of the difference in masses, neutrinos will change flavor as they propagate, a
phenomenon called neutrino oscillation.
Neutrinos have no charge and interact with other particles only
weakly; in fact, a neutrino can travel through one light year of
lead without interacting. To study neutrino properties, physicists
need a lot of neutrinos and a big detector.
The neutrinos detected in the MINOS experiment are produced in the Neutrinos at the Main Injector
(NuMI) beam line at Fermi National Accelerator Laboratory. First, energetic protons are fired into a
graphite target, creating pions. These pions are focused into a beam using magnetic horns. The pions
then decay into muons and neutrinos. Undecayed pions and muons are stopped in rock absorbers, and the
neutrinos stream through the rock towards the detectors.
The probability that a muon neutrino changes into a tau neutrino is dependent on the energy of the
neutrino and the distance it travels. For a fixed propagation distance, the probability of changing
flavor swings up and down as a function of neutrino energy, hence the name neutrino oscillation.
By measuring the probability that a neutrino will change flavor at different energies, MINOS will
determine the two parameters in the oscillation probability, namely the difference in mass squared
between two neutrinos, Δm2 , and the mixing amplitude, sin2(2θ).
νμ survival probability, Δm2=0.002 eV2
Expected energy
distribution, no oscillations
Expected energy
distribution,
with oscillations
The MINOS Detectors
MINOS measures the properties of neutrinos in two detectors, first, in
the Near Detector, close to the neutrino production point and again in
the Far Detector, 735 km away. One then compares the measurements
taken at the two detectors to see how the neutrinos have changed.
The Far Detector
Depth of dip-sin2(2θ)
Veto Shield
Position of dip-Δm2
Energy (GeV)
Dip-signature of oscillations
Beam
Me
NuMI Target
NuMI Focusing Horn
PMT Boxes
Courtesy J. Meyer
Scintillator
2.54 cm Fe
Steel
Extruded
PS scint.
4.1 x 1 cm
5.9 cm
WLS fiber
U V planes
+/- 450
Clear
Fiber cables
Detector Technology
Both MINOS detectors are magnetized calorimeters,
made of sheets of steel, with arrays of plastic
scintillator strips sandwiched in between. Neutrinos
interact in the steel, producing many daughter
particles. These resulting particles traverse the
scintillator and produce light. This light is collected on
fibers embedded in the scintillator, then routed out of
the detector via fiber optics where it is incident on
photomultiplier tubes. The signals from the phototube
indicate how much light was produced and where it
was produced.
The MINOS Near Detector is located at Fermilab, 1
km downstream of the neutrino production target.
The Near Detector will record billions of neutrino
interactions per year. The picture below shows the
traces of several neutrino interactions in the near
detector.
The MINOS Far Detector is located in the Soudan
Underground Laboratory in northern Minnesota. Located
half a mile underground to shield it from cosmic rays, the
Far Detector is 8 m tall and nearly 30 m long. Weighing in
at 5400 tons, the Far Detector has almost half the mass of
the HMS Belfast. The detector was brought down from
the surface in pieces, then assembled underground; like
building a ship in a bottle.
The Near Detector
PMT+FEE Racks
Scint. Modules
Multi-anode PMT
A front end view of a beam neutrino in the Far
Detector. The long curving track is the trail left
by a muon, bending in the magnetic field. In the
absence of oscillations, about 1 beam neutrino
will interact in the Far detector every 4 hours
Coil Hole
Collaborators
MINOS is an international collaboration made up
of 175 physicists in 32 institutions across 6 nations
Beam
What do neutrinos look like?
Why study neutrinos?
Neutrino interactions come in two types, charged current
(CC) and neutral current (NC). A muon neutrino, CC
interaction, is characterized by a the presences of a muon
along with the remains of a broken nucleus. The muon
shows up in the detector as a track, and the remnant of the
nucleus shows up as a shower of hits near the beginning of
the muon track. The energy of the muon is determined
either by how far it travels in the detector or by how much it
bends in the magnetic field, while the shower energy is
determined from the amount of light produced in the
scintillator. The energy of the original neutrino is the sum of
the muon and shower energy. The signature of an NC
interaction is the presence of a shower without the muon.
The primary MINOS measurement amounts to looking for a
deficit of muon neutrino CC interactions in the Far Detector
relative to what is measured in the Near Detector.
νμ CC candidate
E=3.7 GeV
Courtesy Fermilab Visual Media Services
NC candidate
E=4.5 GeV
Neutrinos are a fundamental constituent of matter, perhaps the most
abundant particle in the universe, yet our understanding of the properties of
the neutrino lags far behind our knowledge of the other elementary particles.
Non-zero neutrino mass gives an indication of physics beyond the Standard
Model of particle physics and has ramifications on the evolution of the
universe. Beyond the questions surrounding inner workings of our cosmos,
the neutrino’s power of penetration opens an alternate window into the
furthest reaches of our universe, our sun, and our earth.
CalDet
To learn how the big MINOS detectors
respond to interactions of different
particles with different energies, a small
version of the detectors was built and
tested in a beam of particles at CERN.
While too small to efficiently detect
neutrinos, the Calibration Detector, or
CalDet, allowed for the study of pions,
protons, electrons and muons in a
MINOS-like detector.
2 GeV
electron
2 GeV
pion
2 GeV
proton
2 GeV
muon
The CDF Detector
ATLAS
Other Exciting HEP Projects at UCL
While MINOS, and other oscillation experiments, have sensitivity to differences in neutrino masses 100,000 times smaller than
the electron mass, they only measure differences in masses, not the actual mass of the neutrino. Other experiments, such as
NEMO, and the future SuperNEMO, both being pursued at UCL, aim to measure the absolute value of the neutrino mass. By
2008 NEMO will achieve sensitivy to neutrino masses down to 0.2eV (that’s about 0.00000000000000000000000000000000001
grams). Another neutrino project at UCL, ACORNE, proposes to detect ultra-high energy cosmic neutrinos acoustically. Such
neutrinos deposit so much energy in the target medium that they actually make audible clicks when they hit. The fact that
neutrinos only interact weakly means they travel through the universe almost undisturbed. If these ultra-high energy neutrinos
can be detected, they could provide a unique insight into cosmology and astrophysics.
Beyond neutrino physics, UCL physicists play a large role in the CDF experiment. Ten years ago, the CDF+D0 experiments
discovered the top quark, the heaviest of the predicted quarks. Still taking data at the world’s highest energy collider, CDF is
making precision measurements of the top quark mass as well as the W boson mass and cross sections. Such precision
measurements provide valuable tests of the Standard Model and constrain new physics models beyond. CDF continues to search
for the Higgs particle and strives to provide insight on why the universe is dominated by matter rather than antimatter. UCL is
also involved in another on-going experiment, studying the particle reactions in the ZEUS detector. Using the high energy
electron-proton beam at the HERA accelerator facility, ZEUS measures electroweak phenomena and extends our understanding
of the strong force
Looking to the future, UCL is an active participant in ATLAS, one of the two general purpose detectors that will record the
collisions of protons at the Large Hadron Collider at CERN. While the neutrino experiments try to measure the neutrino mass,
ATLAS will probe the very origin of mass by searching for the Higgs Boson. When the LHC starts running in 2007, ATLAS will
shed light on the fundamental questions of particle physics, including the existence of extra dimensions, supersymmetry, and the
nature of dark matter. Planning for the longer term, UCL physicists are also involved in the design of next generation
accelerators and detectors such as the International Linear Collider.
The NEMO Detector
ZEUS
ILC