1. 6. Preliminary Results from MINOS
High Energy Physics
1. The Neutrino
Neutrinos are amongst the most abundant particles in the universe and are mainly produced in the nuclear reactions that make stars shine. Every second billions of neutrinos pass through our bodies.
Neutrinos are tiny subatomic particles that carry no electric charge and only very rarely interact with matter. A neutrino could pass though a light-year of lead without interacting, so to study their properties, physicists need to detect a large number of them and so need a big detector.
Today we believe that there are 3 kinds (or flavours) of neutrino which, together with their cousins the electron, muon, and tau, constitute the lepton family in the Standard Model of Particle Physics.
It has long been thought that a neutrino born as a particular flavour can spontaneously change into a neutrino of another flavour as it travels through the universe. This phenomenon is known as neutrino oscillations.
The Standard Model assumes that neutrinos are massless but neutrino oscillations can only occur if neutrinos have mass. This mass has not been measured directly yet but experiments have placed limits at least 200,000 times smaller than the electron mass.
A unique opportunity for neutrino ‘astronomy’ arose in 1987 when neutrinos produced in supernova 1987A were observed by different neutrino experiments:
2. The MINOS Experiment
3. The NuMI Beam
MINOS is a long baseline experiment consisting of a beam of muon neutrinos () produced at Fermilab (near Chicago) whose path takes them through a Near Detector on site at Fermilab and a Far Detector 735km away at the Soudan mine (in Minnesota).
The main physics goal of MINOS is to confirm (or disprove) the phenomenon of neutrino oscillations and to precisely measure the parameters that govern them.
The probability for a  to remain as a  as it travels is given by:
The NuMI beamline takes 120GeV protons from the Main Injector at Fermilab and brings them to focus on a graphite target.
Some of the produced hadrons (mostly + and K+) are then focussed by 2 magnetic horns such that when they subsequently decay the resultant  are travelling along paths that take them through the Near and Far detectors (there is a small fraction of e in the neutrino beam).
Sections of absorber material and rock remove heavier particles leaving only neutrinos and this beam intercepts the Near Detector ~1km downstream from the target. 1 2
The experimental parameter L is the distance travelled by a neutrino (735km) whereas E is the energy of the neutrino - in this case set by the configuration of the Neutrinos at the Main Injector (NuMI) beam. 1 2
Unoscillated
Oscillated
nm-CC Spectrum Spectral Ratio
Monte Carlo
Theory and Monte Carlo simulations show that a ‘dip-like’ structure of the ratio is the signature for oscillations. The position of the dip is related m2 and its depth is related to sin22.
The distance between the target and horns can be changed so as to modify the neutrino energy spectrum incident on the detectors and alter the oscillation parameter space to which MINOS is sensitive.
Data taken with these different beamline configurations can be combined to provide a strong handle on the flux of neutrinos at the detectors.
The Near Detector measures the energy spectrum of charged current (CC)  events without oscillations and can be used to predict an un-oscillated  -CC energy spectrum at the Far Detector using Monte Carlo (MC) simulations. This predicted spectrum is compared to the observed spectrum at the Far Detector and a ratio taken. Deviations from unity indicate  disappearance.
5. MINOS Data
MINOS has been taking data using both the Near and the Far Detectors since March 2005 at a steadily growing rate (atmospheric neutrino data accumulation started even earlier using the Far Detector only).
4. The MINOS Detectors
The MINOS Near and Far Detectors are both fine grained tracking-sampling calorimeters that consist of large magnetized iron and scintillator planes. Alternate planes are mounted orthogonal to each other to allow for 3D particle tracking.
For each ‘bunch’ of protons delivered to NuMi there are many neutrino interactions occurring in the MINOS Near Detector. These are then ‘sliced’ into candidate neutrino events using timing and geometry.
Coil
Veto Shield
The Far Detector
The Near Detector ND FD
Weight
Size
Architecture
Electronics
Magnetic Field
Depth underground
1 kiloton
3.8m4.8m15m
282 steel and 153
scintillator planes
Fast QIE
1.2T
105m
5.4 kilotons
8m8m30m
484 steel/scintillator
planes VA
Over 700m!
These candidate events can then be analysed in order to find muon tracks that indicate muon neutrino interactions. The  CC event sample for the oscillation analysis is selected by using different topology and energy deposition signatures. nm
CC Event
NC Event UZ VZ
long m track + hadronic activity at vertex
short event, often diffuse
3.5m
1.8m ne
short, with typical EM shower profile
Some typical events are shown here. The  CC events are used in the main analysis, but other events also occur and could possibly be used in challenging future analyses.
The Near and Far Detectors are constructed to have identical performance.
Relative calibration of the detectors is achieved using cosmic ray muons and a light injection system.
Absolute calibration is achieved with data from a third calibration detector that was operated between 2001 and 2003 and stood in a test beam at CERN.
Calibration Detector
7. MINOS Future
6. Preliminary Results from MINOS
As the beam protons on target increase, the statistical sensitivity of the MINOS experiment will be able to better constrain the parameters m2 and sin22 and to test/rule out alternate models like neutrino decay.
MINOS also has sensitivity to sub-dominant  to e oscillations and should be able to place a limit on another neutrino mixing parameter: sin2213 by using the observed e events. The observation of e appearance in the Far Detector would be the first ever measurement of this mixing mode. In the absence of a signal, MINOS will be able to improve the current best limits on the mixing angle by a factor of 2-3.
MINOS has made a neutrino oscillation measurement using atmospheric neutrino events in the Far Detector and this will continue to improve in statistical sensitivity. The MINOS Far Detector is the only deep underground detector in the world to be magnetized, and as such could be used to compare muon neutrino and muon antineutrino oscillations and to search for CPT-violation. Maybe oscillations to sterile neutrinos could also be investigated.
MINOS announced its preliminary measurement of the oscillation parameters at the end of March 2006 at Fermilab. This measurement is already competitive with previous experiments.
-CC Spectrum:
Allowed Region in Oscillation Parameter Space:
Spectral Ratio:
The MINOS experiment is an international collaboration of almost 200 physicists and students from 6 countries and is mainly funded by the UK Particle Physics and Astronomy Research Council (PPARC) and the US Department of Energy (DoE).
Many sources of systematic error have been considered but the measurement is currently statistics limited. MINOS already has half as much additional data as the dataset used for this preliminary measurement!
Contacts:
hep.ucl.ac.uk
hep.ucl.ac.uk