Neutrinos: Detecting the Invisible! The MINOS Neutrino Oscillation Experiment Andy Blake, Cambridge University

Current “Standard Model” of elementary particles and their interactions: All matter is composed of quarks and leptons. Four fundamental forces. – Electromagnetic force. – Weak nuclear force. – Strong nuclear force. – (Gravity). Quarks interact through electromagnetic, weak and strong forces. Charged leptons interact through electromagnetic and weak force. Neutrinos interact through weak force only. Particle masses generated by Higgs mechanism (?)

Properties of Neutrinos Three “flavours” of neutrino: – electron, muon, tau. Known properties: – Spin = ½ (i.e. fermions). – Charge = 0 (i.e. no electromagnetic interactions). – Colour = 0 (i.e. no strong interactions). Neutrino mass is known to be very small! – direct limit on e mass is <1eV/c2 (from studies of tritium beta decay) – 1 eV/c2 ≈ 500,000 times lighter than mass of electron. – 1 eV/c2 ≈ 0.000000000000000000000000000000001 g ! Challenge to physicists is to measure this mass. – Need the most sensitive set of weighing scales ever created!

The universe is filled with a dense flux of “relic neutrinos” created in the big bang – this makes neutrinos the most abundant known form of matter!

atmospheric neutrinos solar neutrinos supernova neutrinos geo-neutrinos atmospheric neutrinos reactor neutrinos

Each of you emits 300 million neutrinos per day!

 But neutrinos are weakly interacting – the probability that a neutrino will interact anywhere in the entire earth is 1 in 1,000,000 !

Detecting Neutrinos Neutrino Type Typical Energy Neutrino energies and interaction rates Note units! Neutrino Type Typical Energy Interactions per kT-Yr Solar (electron neutrinos) 0.1 - 10 MeV ~10,000 Reactor 1 - 5 MeV ~1,000 Atmospheric (electron/muon neutrinos) 0.1 - 100 GeV ~100 Compare with surface cosmic ray flux of ~10,000 m-2 s-1 “This makes looking for a needle in a haystack seem easy!” (John Bahcall, neutrino physicist)

Detecting Neutrinos Need a very intense flux of neutrinos ! Need a very massive neutrino detector ! Build the detector deep underground to shield it from surface radiation. Cannot observe neutrinos directly but can detect the particles produced by their interactions.

Borexino IMB Homestake MINOS SNO KamLAND Super-Kamiokande

The Super Kamiokande Neutrino Detector Located at depth of 1,000m beneath Mt. Ikenoyama in Japan. Cylindrical tank of ultra-pure water. – 40m high x 40m diameter. – total detection mass of 50 kT. Neutrinos interact with nuclei in water to produce charged leptons. – Electron neutrinos produce electrons. – Muon neutrinos produce muons. The charged leptons emit cones of “Cherenkov” light along their path. – Equivalent to sonic boom (the charged particles are travelling faster than the speed of light in water). Detector is instrumented by array of 13,000 photo-multiplier tubes. – Convert light into electrical signals. – Use PMT hits to map out progression of Cherenkov cone through detector. – Measure energy/direction of lepton, infer energy/direction of neutrino.

Neutrino Interaction in Super-K Detector.

Atmospheric Neutrinos The earth is continually bombarded by a stream of cosmic particles. – mostly protons and helium nuclei. Cosmic rays interact in atmosphere to produce secondary particles. – mostly pions () and kaons (κ). Decay chain of /κ produces e/. primary cosmic rays /κ  e  e ~20 km p/He p / He + N → X + ± / κ± ± / κ± → ± +  () ± → e± + e (e) +  () Neutrinos travel right through the earth, and a tiny fraction interact in Super-K. – down-going neutrinos travel ~20 km. – up-going neutrinos travel ~13,000 km. Expect following flux ratios: R / Re ~ 2 and Rup / Rdown ~ 1 Atmospheric neutrino production

Atmospheric Neutrino Oscillations electrons muons Super-K measured the angular distribution of the atmospheric electron and muon neutrinos. – Most atmospheric neutrinos travel straight through the earth but a tiny fraction interact in Super-K. – since cosmic rays are isotropic, expect atmospheric neutrinos to be isotropic as well. Electron neutrinos consistent with isotropic expectation. Muon neutrinos exhibit clear up-down asymmetry. – deficit of up-going neutrinos, which travel ~100 times further than down-going neutrinos. Up-going muon neutrinos have “oscillated” into tau neutrinos. – evidence for neutrino mass! null oscillations cos Θ = -1 up-going cos Θ = +1 down-going nm  nt oscillations

Masatoshi Koshiba receives Nobel Prize Nobel Prize for Physics 2002 Masatoshi Koshiba receives Nobel Prize

Neutrino Oscillations Neutrinos undergo spontaneous transitions between flavours. – These “oscillations” have been observed in atmospheric neutrinos, solar neutrinos, and reactor neutrinos. Compelling evidence! Neutrino oscillations imply that neutrinos have mass! – The neutrino flavour states are actually just quantum mechanical mixtures of a set of neutrino mass states. – Can determine neutrino mass by measuring neutrino oscillations. Neutrino oscillations are a purely quantum mechanical effect. – The effect is so weak that it is observed over macroscopic distances. “Flavour States” “Mixing Matrix” “Mass States”

Neutrino Oscillations A beam of neutrinos, initially produced as muon neutrinos, will oscillate into tau neutrinos and then back again! – The rate of oscillations is dependent on the neutrino mass.    probability propagation distance Wavelength of oscillations:  = 2.47 E / m2 (  = wavelength (km) ; E = energy (GeV) ; m2 = mass splitting (eV2) ).

The MINOS Experiment (“Main Injector Neutrino Oscillation Search”) The first of a new generation of neutrino experiments. Objective is to determine the neutrino mass through a precision study of neutrino oscillations. Manufacture accelerator beam of muon neutrinos. Measure muon neutrino spectrum before/after oscillations. – near detector constructed adjacent to neutrino source. – far detector constructed hundreds of miles away. – search for muon neutrino disappearance in far detector. Near Detector Far Detector Neutrino Source   730 km

735 km The MINOS Experiment Accelerator beam of muon neutrinos produced by NuMI facility at Fermilab. Near Detector at Fermilab to measure spectrum and composition of beam. Far Detector at Soudan mine to study neutrino disappearance in beam. Soudan Mine, Minnesota 735 km Fermi Laboratory, Chicago

THE MINOS COLLABORATION Argonne • Athens • Benedictine • Brookhaven • Caltech • Cambridge • Campinas Fermilab • Harvard • IIT • Indiana • Minnesota Duluth • Minnesota Twin Cities • Oxford Pittsburgh • Rutherford • Sao Paulo • South Carolina • Stanford • Sussex • Texas A&M Texas Austin • Tufts • UCL • Warsaw • William & Mary • Wisconsin

Fermi Laboratory

CDF D0 The Tevatron The CDF Detector The D0 Detector

Main Injector p p p p Booster NuMI Beam Line Proton beam generated from H- Source. – Ramped to 400 MeV in linear accelerator. – Ramped to 8 GeV in “Booster” ring. – Ramped to 120 GeV in “Main Injector” ring. Protons kicked into neutrino beam line. Booster p p NuMI Beam Line Main Injector p p

The NuMI Beam protons p+ n The NuMI beam (“Neutrinos from the Main Injector”) 1.5 km Direct protons onto 50g segmented graphite target. – produces an intense flux of secondary pions and kaons. Focus +/κ+ into tight beam using magnetic focusing. – requires two 200kA parabolic electromagnets (act as lenses). Direct +/κ+ into 675m evacuated decay pipe. – need to point the beam 3 degrees into earth to reach Soudan! – +/κ+ decay in pipe to produce +/ (and 1% e+/e). Absorb  in 200m rock to leave pure neutrino beam. – produce ~1 neutrino for each proton on target. # – proton beam intensity is 1013 s-1.

Guinness Book of World Records 2007. (page 150)

The MINOS Detectors Far Detector Near Detector 1 kT mass coil coil Near Detector Far Detector Functionally Identical Detectors 1 kT mass 1 km from target 282 steel planes 153 scintillator planes 100m underground 5.4 kT mass 735 km from target 486 steel planes 484 scintillator planes 700m underground steel and scintillator sampling calorimeters. Magnetized steel (B ~1.3T). GPS time-stamping for synchronization.

 MINOS Near Detector The MINOS near detector measures the spectrum of neutrinos before oscillations. Neutrino beam is pulsed. – Several interactions per pulse! Detector composed of many layers of magnetized steel and plastic scintillator. – Neutrinos interact in steel to produce charged particles. – Charged particles induce light emissions in scintillator. Scintillator planes divided into many thin strips. – Light in each strip measured using photo multiplier tubes. – Use scintillator hits to piece together particle tracks from neutrino interactions. 

MINOS Neutrinos steel scintillator   Nuclear Scattering

MINOS Far Detector shaft MINOS Soudan mine Expected interaction rate is ~1/day. Far detector is constructed 700m under the ground to shield against surface cosmic radiation. MINOS Soudan 2/CDMS II shaft Photo by Jerry Meier

Far Detector Neutrinos Online event display: http://farweb.minos-soudan.org/events/

Neutrino Oscillations Use observed near detector energy spectrum to predict far detector energy spectrum in the absence of neutrino oscillations. If oscillations have occurred, expect to see an energy-dependent deficit at the far detector relative to this prediction. nm spectrum spectrum ratio Unoscillated Oscillated Simulation Simulation

Neutrino Oscillations observed spectrum ratio Results after 2½ years of MINOS running: Phys. Rev. Lett. 101, 131802 (2008) observed nm spectrum observed spectrum ratio Difference between masses of muon and tau neutrinos is: m2 = ( 2.43 ± 0.13 ) x 10-3 eV2

Neutrinos in the Early Universe One of the great unsolved problems in physics is the imbalance between matter and anti-matter in the universe. – Physicists believe that the universe was created with equal amounts of matter and anti-matter. – The matter and anti-matter annihilated (this is the origin of the cosmic microwave background that we observe today). – However, some matter was left over! Neutrinos provide a possible mechanism for this imbalance. – The Standard Model allows an asymmetry between neutrinos and anti-neutrinos. – This asymmetry would have been at work in the early seconds of the universe. – The extent of this asymmetry can be measured by studying oscillations. The MINOS experiment is a step along the road to solving this puzzle.

Andy’s A-Level Dissertation (1996) “Neutrino oscillations!” Andy’s PhD Thesis (2005) “Solar deficit” “Atmospheric anomaly” “Neutrino oscillations!”