Nobel Prize in Physics 1995 Awarded to Fred Reines "for pioneering experimental contributions to lepton physics" Physics 116 Session 41 Neutrinos Reines & Cowan at work, 1956 Dec 8, 2011 Email: ph116@u.washington.edu
Announcements • Final exam: Monday 12/12, 2:30-4:20 pm • Same length/format as previous exams (but you can have 2 hrs) • Kyle Armour is away this week; see TAs in study center • JW will have extra office hours Thu-Fri this week: • 12:45-1:15pm before class, • 2:30-3pm after class (my office B303 PAB, or B305 conf room next door) • Practice questions and formula pages posted, review tomorrow
Announcements PHYS 248: A new general-education physics course you might be interested in…
4 Today Lecture Schedule (to end of term)
The “Standard Model” of Particle Physics Last time… Now let’s look at those leptons… Basic ingredients of matter are the Fundamental fundamental forces are particles: quarks and mediated by leptons photons, gluons, 6 quarks Z’s and W’s 6 leptons These types of particles are + their antiparticles called 'bosons' (Symmetry!) Leptons These types of particles are called (after 'fermions' Satrendyanath Bose) (after Enrico Fermi) (from http://www.fnal.gov) 5
I work on 2 projects in Japan, studying physics of neutrinos: • Super-Kamiokande (since 1995) • Multiple physics goals: – Study interactions of high energy neutrinos from earth’s atmosphere – Watch for evidence of proton decay (> 10 33 yr half-life !) But Super-K contains 10 33 protons… • – Watch for neutrinos from a supernova – Neutrino astrophysics • Look for distant galaxies emitting beams of neutrinos – Far detector for T2K… • T2K (since 2006) • neutrino “oscillations” studies – Generate a beam of muon neutrinos with particle accelerator – Sample the beam to check its properties (“near detector”) – Send it through the Earth 300 km (takes about 0.001 sec) – See if particles come out still muon-flavored • Count how many change flavors (using the “far detector”) 6
Q: What are neutrinos? • Neutrinos = subatomic particles with: Symbol: ν – no electric charge – (almost) no mass (Greek letter nu) – only “weak force” interactions with matter That doesn't sound very interesting! But… – neutrinos are made in (almost) every radioactive decay – neutrinos are as abundant as photons in the Universe • Several hundred per cm 3 everywhere in the Universe – even though they are nearly massless, they make up a significant proportion of the mass in the Universe! • You are emitting ~ 40,000 neutrinos/sec right now ( 40 K decays) • Neutrinos can penetrate the entire Earth (or Sun) without blinking – maybe we can study earth's core with neutrinos? – astronomical window into places we can't see with light 7
Q: Where do neutrinos come from? • Radioactive decays = 'weak nuclear force' in action – Example: beta decay of neutron lepton number = conserved • 'beta ray' = old term for electron physical property (new kind of 'charge') that only leptons have neutron (lepton number = 0) proton (lepton number = 0) electron (lepton number = +1) anti- ν (lepton number = -1) (must be anti to conserve lepton #) – another example: muon decay µ - (lepton number = +1) electron (lepton number = +1) ν (lepton number = +1) anti- ν (lepton number = -1) 8
Q: Who said we need them? • Wolfgang Pauli, 1930~ 33 beta decays of tritium (H 3 ) Electron energies observed. Energy released is 18 keV. But usually electron carries away Pauli (with Heisenberg and Fermi) much less!! – If β -decay of nuclei produces only 2 particles (electron and daughter nucleus), it does not seem to conserve momentum! • Emitted electrons can have any energy up to maximum allowed by conservation of energy (E MAX = [parent mass - daughter mass]* c 2 ) • Pauli: There must be a neutral, ~ massless 3rd particle emitted – Fermi suggested the name 'neutrino' = little neutral one "I've done a terrible thing - I've invented a particle that can't be detected!" - Pauli …but he was wrong! 9
Q: How were they first ‘seen’? • Fred Reines and Clyde Cowan, 1956 ν source: initially, nuclear reactor in Hanford, WA (later they – moved to more powerful Savannah River reactor in South Carolina) Nobel Prize in Physics 1995 Awarded to Fred Reines "for pioneering experimental contributions to lepton physics" – Detector: water with CdCl 2 – inverse beta decay: ν + → + + p n e Observed light flashes from e + annihilation followed by decay of neutron 10
Super-Kamiokande and T2K, in Japan Toyama Toyama Tokai Tokai SK SK SK Super-Kamiokande Underground Neutrino Observatory In Mozumi mine of Kamioka Mining Co, near Toyama City • Detects natural (solar, atmospheric) and beam (T2K) neutrinos • T2K (Tokai to Kamiokande) long baseline experiment Neutrino beam is generated and sampled at Tokai (particle physics • lab, near Tokyo) Beam goes through the earth to Super-K, 300 km away •
Super-Kamiokande Electronics • US-Japan collaboration Huts Entrance • (~ 100 physicists) Linac 2 km Control cave • 1000 m of rock overhead to Room block cosmic ray particles Tank • 50,000 ton ring-imaging 40m tall Water water Cherenkov detector System Inner • Inner Detector: 11,146 Outer Detector Mt. Ikeno phototubes, 20” diameter Detector • Outer Detector: 1,885 phototubes, 8” diameter • 50,000 cubic meters of ultra-pure water • Neutrino interactions make charged particles in water • Began operation in April, 1996 • Published first evidence for neutrino mass in June, 1998 • Typically records about 15 neutrino events per second
Just how big is Super-K? • Checking photomultiplier tubes by boat as the tank fills (1996)
View into Super-K from tank top: an application of the photoelectric effect • Each photomultiplier tube is 20 inches in diameter!
Cherenkov light in water: applying ph116 optics • Neutrino interacts in a nucleus in the water (oxygen or hydrogen) light waves • Produces a charged muon or electron, which carries an electromagnetic field • Muon travels at v ~ c, but light travels water (n= 1.33) at v= c/n ~ ¾ c in water • Muon is going faster than its fields can ν µ travel in water: "shock wave" builds up • Cherenkov light is emitted, in characteristic 42 o rings around the particle direction v ≈ c • Cherenkov 'rings' are fuzzy for electrons and sharp for muons – electrons scatter in the water light rays (v= 0.75c) – heavier muons travel in straight paths until stopped
Neutrino “events”: ν e and ν µ Map of phototubes: imagine a soup can, cut open and unfolded to show the inside: Electrons scatter in water and produce fuzzy Cherenkov rings; Muons travel in straight lines and produce sharp rings Outer Detector Inner Detector MUON Electron Neutrino Neutrino Event Event
June 5, 1998: Press clippings…
Super ‐ K: underground neutrino observatory Kamioka Tokyo N Tokai J. Wilkes, UW Physics 18
Q: How do you make a neutrino beam? GPS provides time synchronization accurate to ~50 nanoseconds GPS GPS Near Detectors beam beam target, monitors monitors magnets proton beam pions Super-Kamiokande 100m decay pipe beam monitors (180m of earth) (300 km of earth) JPARC 30 GeV proton accelerator T2K (Tokai to Kamioka) Started data-taking 2010
T2K beam Neutrino beamline Pacific At the J-PARC lab ocean (in Tokai): Godzilla waded • 30 GeV high Near ashore intensity proton detectors here in accelerator Godzilla 2000 !! • proton beam aimed at SK, makes neutrino beam Tokyo 100 km • “Near detectors” sample the beam Super-K 295 km Fukushima 75 km J. Wilkes, UW Physics 20
What are we looking for? Interference effects! • Neutrino oscillations = quantum wave effects visible on a macroscopic scale • Neutrinos have 2 sets of properties: flavor (electron, muon or tau) and mass each mass is a mixture of flavors (m1, m2 or m3) • Each flavor state is a mixture of mass states, and vice-versa • Mass states have different wavelengths (rest energy, momentum ~ λ ) – So the different mass states making up a muon neutrino interfere! – We may observe a few electron neutrinos, after some time/distance • Plan: Generate beam of muon Super-K discovered this spacing: neutrinos, count how many e neutrinos first proof that nu’s have mass. appear after travelling 300 km (t ~ 1 We still don’t know if order of mass microsecond, by our clock) states is “normal” or “inverted”. 21
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