The MINOS Neutrino Oscillation Experiment

1. Detecting the Invisible! The MINOS Neutrino Oscillation Experiment Andy Blake, Cambridge University

2. SCIENCE MARCHES ON …

3. 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 (?)

4. 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 (and weirdest) weighing scales ever created!

5. 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 in the universe!

6. geo-neutrinos solar neutrinos supernova neutrinos reactor neutrinos atmospheric neutrinos

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

8. Detecting Neutrinos Note units! Neutrino energies and interaction rates 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)

9. 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.

10. Neutrino Interaction in Super-K Detector.

11. primary cosmic rays p/He /κ  ~20 km e  e  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/. 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 ~ 2andRup / Rdown ~ 1 Atmospheric neutrino production

12. Atmospheric Neutrino Oscillations electrons muons • Super-K measured the angular • distribution of the atmospheric • electron and muon neutrinos. • – 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. null oscillations cos Θ= -1 up-going cos Θ= +1 down-going nm nt oscillations

13. Neutrino Oscillations • Neutrinos undergo spontaneous transitions between flavours. • – this is a purely quantum mechanical phenomenon. • – neutrinos oscillate between different flavours.    probability propagation distance • Wavelength of oscillations:  = 2.47 E / m2 (  = wavelength (km) ; E = energy (GeV) ;m2 = mass splitting (eV2) ).

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

15. 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

16. THE MINOS COLLABORATION Argonne • Athens • Benedictine • Brookhaven • Caltech • Cambridge • Campinas • Fermilab College de France • Harvard • IIT • Indiana • ITEP-Moscow • Lebedev • LivermoreMinnesota-Twin Cities • Minnesota-Duluth • Oxford • Pittsburgh • Protvino • RutherfordSao Paulo • South Carolina • Stanford • Sussex • Texas A&M •Texas-Austin • Tufts UCL • Western Washington • William & Mary • Wisconsin

17. Soudan Mine, Minnesota 735 km Fermi Laboratory, Chicago

18. Fermi Laboratory

19. 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 Neutrino Beamline Main Injector p p

20. The Neutrino Beam protons p+ n The NuMI beam (“Neutrinos at 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 sec-1.

21. Record Breaker! Guinness Book of World Records 2007. (page 150)

22. MINOS Near Detector • The MINOS near detector • measures the spectrum of • neutrinos before oscillations. • Total detection mass of 1 kT. • – 4m high x 5m wide x 17m long. • Composed of 280 interleaved • planes 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. 

23. Near Detector Neutrinos steel scintillator   Nuclear Scattering

24. Near Detector Neutrinos U vs Z beam V vs Z U vs V TIME PROFILE

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

26. MINOS Far Detector

27. MINOS Far Detector • The MINOS far detector • measures the spectrum of • neutrinos after oscillations. • Total detection mass of 5.4 kT. • – 8m high x 8m wide x 30m long. • Composed of 485 interleaved • planes of magnetized steel • and plastic scintillator. • – Magnetic field focuses charged • particles into the detector. • – Also enables their momentum • to be determined from the • curvature of their tracks. • Neutrino interactions identified • by their coincidence with the • beam spill times. • – Both near and far detectors • equipped with GPS receivers.

28. Far Detector Neutrinos beam neutrinos! cosmic muons 10s

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

30. 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. spectrum ratio nm spectrum Unoscillated Oscillated Simulation Simulation

31. Neutrino Oscillations Results from 1st year of MINOS experiment observed nm spectrum observed spectrum ratio • Difference in mass between muon and tau neutrinos is: + 0.44 m2 = 2.74 x 10-3 eV2 – 0.26

32. Why is this important? • 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.

33. A-Level dissertation (1996) PhD Thesis (2005)