Physics 116. Dec 8, Session 41 Neutrinos.

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1 Nobel Prize in Physics 1995 Awarded to Fred Reines "for pioneering experimental contributions to lepton physics" Physics 116 Reines & Cowan at work, 1956 Session 41 Neutrinos Dec 8,

2 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

3 Announcements PHYS 248: A new general-education physics course you might be interested in

4 Lecture Schedule (to end of term) Today 4

5 The Standard Model of Particle Physics Last time Basic ingredients of matter are the fundamental particles: quarks and leptons 6 quarks 6 leptons + their antiparticles (Symmetry!) These types of particles are called 'fermions' (after Enrico Fermi) Now let s look at those leptons Leptons Fundamental forces are mediated by photons, gluons, Z s and W s These types of particles are called 'bosons' (after Satrendyanath Bose) (from 5

6 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 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 sec) See if particles come out still muon-flavored Count how many change flavors (using the far detector ) 6

7 Q: What are neutrinos? Neutrinos = subatomic particles with: no electric charge (almost) no mass 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 Symbol: ν (Greek letter nu) 7

8 Q: Where do neutrinos come from? Radioactive decays = 'weak nuclear force' in action Example: beta decay of neutron 'beta ray' = old term for electron neutron (lepton number = 0) proton (lepton number = 0) lepton number = conserved physical property (new kind of 'charge') that only leptons have electron (lepton number = +1) another example: muon decay anti-ν (lepton number = -1) (must be anti to conserve lepton #) µ - (lepton number = +1) electron (lepton number = +1) ν (lepton number = +1) anti-ν (lepton number = -1) 8

9 Q: Who said we need them? Wolfgang Pauli, 1930~33 Electron energies observed. Energy released is 18 kev. But usually electron carries away much less!! beta decays of tritium (H 3 ) Pauli (with Heisenberg and Fermi) 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

10 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

11 Super-Kamiokande and T2K, in Japan Toyama SK Tokai 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

12 Super-Kamiokande Linac cave Tank Outer Detector Electronics Huts 40m tall Inner Detector Entrance 2 km Control Room Water System Mt. Ikeno US-Japan collaboration (~100 physicists) 1000 m of rock overhead to block cosmic ray particles 50,000 ton ring-imaging water Cherenkov detector Inner Detector: 11,146 phototubes, 20 diameter 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

13 Just how big is Super-K? Checking photomultiplier tubes by boat as the tank fills (1996)

14 View into Super-K from tank top: an application of the photoelectric effect Each photomultiplier tube is 20 inches in diameter!

15 Cherenkov light in water: applying ph116 optics Neutrino interacts in a nucleus in the water (oxygen or hydrogen) Produces a charged muon or electron, which carries an electromagnetic field Muon travels at v ~ c, but light travels at v=c/n ~ ¾ c in water Muon is going faster than its fields can travel in water: "shock wave" builds up Cherenkov light is emitted, in characteristic 42 o rings around the particle direction Cherenkov 'rings' are fuzzy for electrons and sharp for muons electrons scatter in the water heavier muons travel in straight paths until stopped light waves water (n=1.33) ν µ v c light rays (v=0.75c)

16 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 Electron Neutrino Event MUON Neutrino Event

17 June 5, 1998: Press clippings

18 Super K: underground neutrino observatory Kamioka Tokyo N Tokai J. Wilkes, UW Physics 18

19 Q: How do you make a neutrino beam? GPS GPS provides time synchronization accurate to ~50 nanoseconds GPS Near Detectors beam monitors target, magnets beam monitors pions proton beam Super-Kamiokande 100m decay pipe (180m of earth) beam monitors (300 km of earth) T2K (Tokai to Kamioka) Started data-taking 2010 JPARC 30 GeV proton accelerator

20 T2K beam Neutrino beamline At the J-PARC lab (in Tokai): Pacific ocean 30 GeV high intensity proton accelerator Near detectors Godzilla waded ashore here in Godzilla 2000!! proton beam aimed at SK, makes neutrino beam Near detectors sample the beam J. Wilkes, UW Physics Super-K 295 km Fukushima 75 km Tokyo 100 km 20

21 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 (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 neutrinos, count how many e neutrinos appear after travelling 300 km (t ~ 1 microsecond, by our clock) each mass is a mixture of flavors Super-K discovered this spacing: first proof that nu s have mass. We still don t know if order of mass states is normal or inverted. 21

22 What about faster than light neutrinos in the news? OPERA experiment at CERN is very similar to T2K: neutrino beam goes to a highway tunnel in Italy (Gran Sasso): same physics goals In November, they reported neutrinos arriving a tiny bit sooner than expected - see for example Claim neutrinos arrive 60 nanoseconds early - travel time at speed c should be 2.4 millisec, so difference is about 1 part in 100,000 Few physicists believe this is really due to violation of Einstein s special relativity: too many other measurements to explain away! Most likely: OPERA missed some item in their calculation of time delays between when particles actually pass through detectors, and when electronic signal is recorded by data system T2K is doing a very careful study to respond (UW students lead this)! We ve already spotted many tiny inconsistencies in our logbooks 22

23 Next: Is there even further substructure? Do quarks and leptons have smaller things inside them? Much current debate on this topic! Could all the particles be different states of a more basic entity? "String theory" suggests so. Universe is actually 11-dimensional (!?) All but 3 space dimensions are folded up inside strings Particles correspond to different vibrational modes The Fabric of the Cosmos (now on PBS) by Brian Greene, describes this view One difficulty: totally inaccessible for experimental tests! Planck Scale, meters, requires solar-system sized accelerator! we need new ideas... 23

24 What s the Nature of the Vacuum? What s going on when there is nothing there? Quantum Mechanics tells us the vacuum must be a turmoil of continuous production and annihilation of particle-antiparticle pairs: E=mc 2 in action Uncertainty also says you can violate energy conservation temporarily: E t ~ h [ borrowed energy] x [time of loan ] ~ Planck's constant (very tiny number) Vacuum! antielectron (positron) borrowed energy returned energy pair creation electron annihilation This happens all around us, all the time, in empty space. What impact does this sea of virtual particles have on the expansion of the Universe? Is this related to Dark Energy? 24

25 Quiz question Neutrinos are fundamental particles that A. Are massless B. Are composed of 3 quarks C. Interact only via the weak nuclear force, the force responsible for radioactive decays D. Are theoretically predicted but have never been observed 25