Um Neutrinos? Sam Zeller Columbia University

Transcription

1 Um Neutrinos? Sam Zeller Columbia University what is particle physics anyway? how a particle physicist views the world how do neutrinos fit in? - a little bit of history - they can be full of surprises what do I do? the MiniBooNE neutrino experiment - what do you mean neutrinos oscillate?!

2 Particle Physics what do particle physicists do? investigate the fundamental structure of matter what is the world made of and what holds it together? how do we do that? essentially, we take things apart why? it s been successful what have we found out? we ve discovered > 200 particles, but only a few are fundamental (simple & structureless not made of anything smaller) simplicity is astounding small number of basic constituent components that make up everyday matter

3 Particles: Consider the Atom ordinary matter is made up of atoms - is the atom fundamental? No! electric force is what holds atoms together - electrons (-) are negatively charged dense, positively charged core nucleus (of course, none of this is drawn to scale) the nucleus is composite too! - contains protons (+) & neutrons (0) + + which raises another question

4 Particles, Particles, Particles if protons are (+) and squeezed into a tiny nucleus, why don t they repel?. ADDITIONAL FORCE: only operates at short distances called the STRONG force (don t call it this for nothing overwhelms repulsive EM force) 3 constituents of atom: electron, proton, & neutron are these all equivalently fundamental? + + no, protons and neutrons are composite electrons are not protons and neutrons are made up of QUARKS

5 Particles, Particles, Particles before quarks: now with quarks: pu c t quarks feel the STRONG force nd s b electrons and their two heavier cousins do not e µ τ is this the whole picture? no

6 Particles, Particles, Particles certain radioactive decays revealed the existence of additional, elusive particles ν e e ν µ ν τ µ τ they have NO electric charge these new particles hardly interact with other particles (no strong force, no charge) if they have a mass, it s very, very tiny these are the NEUTRINOS each pairs with an electrically charged partner

7 Periodic Table of Particle Physics just like chemists categorize atoms periodic table: u d c s t b ν e ν µ ν τ NEUTRINOS particle physicists have standard model of elementary particles e µ τ simple model explains what the world is & what holds it together

8 The Birth of the Neutrino when radioactivity was first being explored, some scientists began working on nuclear breakup nuclear beta decay was observed to cause one element to change into another (heavy elements can decay into simpler things) in this process, a radioactive nucleus changes its charge by one unit & becomes the nucleus of another element by emitting a β particle (i.e. a speedy electron) expect all β s to have the same energy: Y Y' β expect β s emitted with a discrete energy E number of β s E but that s NOT what we get! shows unexpected results energy

9 The Birth of the Neutrino this graph, or spectrum, was at the heart of the β-decay puzzle: energy is not being conserved?! in most cases, some of the energy appeared to be lost! Y and sometimes not, by a lot! Y' β BIG PROBLEM energy crisis number of β s sometimes energy appears to be conserved energy

10 The Birth of the Neutrino 1930: Wolfgang Pauli came up with a solution to this energy crisis. new, but elusive, subatomic particle carries away the missing energy Y β Y' ν Wolfgang Pauli zero electric charge zero mass very difficult to detect energy conserved (which is why hadn t been seen before) but introduced a brand new particle ν (very BOLD thing to do)

11 The Birth of the Neutrino Enrico Fermi a little later, in developing his famous theory of β decay Enrico Fermi renames this new particle the NEUTRINO ( little neutral one ) but for ~30 years, the neutrino is not seen Why not?

12 What s With These Neutrinos? remember, neutrinos don t have electric charge neutrinos interact only weakly - how weak? ν ν - ν has a good chance of traveling thru 200 earths without interacting - ν interacts 100,000,000,000 x less often than quarks! so to find neutrinos, need a lot of neutrinos and lot of detector

13 Reines and Cowen Come Up With a Plan surely neutrinos must exist, someone had to demonstrate this! 1950 s Clyde Cowan, Jr. Fred Reines situation changed with the advent of very intense sources of ν s going to try to detect neutrinos from a nuclear reactor

14 Discovery of the Neutrino So Cowan and Reines concocted a plan to detect neutrinos using stacked photon-catchers Savannah River nuclear reactor neutrino target photon catcher 10 ton detector installed in basement of reactor building

15 Discovery of the Neutrino and of course, they need to see a clear footprint in this detector that only a neutrino could leave inverse beta decay ν e + neutrino target photon catcher p n

16 Discovery of the Neutrino ν e + e - neutrino target photon catcher p n

17 Discovery of the Neutrino evidence for ν: 2 flashes of light separated in time (delayed coincidence) ν e + neutrino target photon catcher p n

18 Discovery of the Neutrino they collected data for ~ a year recording flashes of light produced by impact of neutrinos from the nearby reactor Wolfgang Pauli Cowan W E S T E R N U N I O N Dear Professor Pauli, June 14, 1956 We are happy to inform you that we have definitely detected neutrinos... Fred Reines Clyde Cowan ghostly particle (ν) had become a tangible reality this ground breaking experiment changed the role the ν was to play in physics won the Nobel Prize for detection of the ν e (1995) Reines ν not just the by-product of β decay but would be used to expand our understanding of the subatomic world

19 Modern Neutrino Experiments today s neutrino experiments, still need a copious source of neutrinos and a large scale detector ν beam it s a good thing baseball is a long-attention span sport, because remember, neutrinos rarely interact fewer than 1/trillion will get caught (ν pitcher ) ν detector (ν catcher )

20 Modern Neutrino Beams Neutrinos come from: - reactions inside atomic nucleus - fusion (inside stars and the sun) - fission (nuclear reactor) - decay of certain subatomic particles - when fast-moving protons smack into matter - in the upper atmosphere (atmospheric ν s) - or at a particle accelerator near you making neutrinos requires making particles which decay into neutrinos which you accomplish by slamming protons into a chunk of matter ( and perhaps filtering out the leftovers) letting the parents decay

21 MiniBooNE rest of talk is about the neutrino experiment I m currently on our neutrinos are produced in just this way

22 MiniBooNE FNAL 8 GeV Booster p 50 m decay pipe decay region: π µν, K µν Neutrino Beam ν MiniBooNE detector absorber: stops particles that are not neutrinos

23 MiniBooNE Detector ν s detector built in 1999

24 MiniBooNE Detector tanks contains 250,000 gallons of mineral oil (neutrino target) - 44 tanker trucks worth! tons! lined w/ 1520 PHOTOTUBES (electronic eyes of the detector) Phototubes work like inverse light bulbs - produce an electrical signal whenever light strikes them - like Reines-Cowan, we re going to look for light as evidence for ν passage

25 MiniBooNE Detector so we have a beam of lots of neutrinos & a massive detector how are we going to know neutrinos have interacted inside? these phototubes are completely submerged in the mineral oil inside the sealed tank

26 Seeing the Invisible (ν) we can only catch a particle if the particle is electrically charged see a neutrino ONLY if it hits something and knocks out or produces a charged particle one way a neutrino can interact: run into something and change into its own charged partner : ν e + X e - + X ν µ + X µ - + X ν τ + X τ - + X can tag the reaction this way

27 Modern Neutrino Detectors way we see µ and e is through their production of Čerenkov light if a charged particle moves through a liquid at a speed faster than the speed of light in the liquid, the liquid will give off a shock wave burst of light (optic boom) called Čerenkov light light pulses are very dim - need exquisitely sensitive light detectors and - a very clear liquid to serve as ν target Same phenomenon which causes reactor core to glow brilliant blue color

28 Modern Neutrino Hunting long track (muon) track direction sharp-edged, sharp-edged, filled-in open ring ring light sensor array shockwaves short, scattering of light track (electron) (Čerenkov light) are conical in shape spread out from the fuzzy collision ring point, striking walls of detector as the cone-like wave strikes the wall, a circular ring of light appears different kinds of particles make different tracks and therefore make different kinds of rings but what do these particle signatures look like in a real detector?

29 What Do Real Events Look Like in the MiniBooNE Detector? muon filled in, sharp outer edge

30 What Do Real Events Look Like in the MiniBooNE Detector? electron diffuse, fuzzy ring shape and sharpness of rings tell us whether we re seeing a muon or an electron

31 Super-Kamiokande we are not the only experiment in this business muon (sharp outer edge) Super-Kamiokande electron (fuzzy ring) deep in Kamioka mine, Japan 50,000 tons ultra-pure water ~11,000 phototubes

32 Modern Neutrino Hunting the kind of neutrinos we shoot at the detector determines the particles we collect ν µ X µ - X ν e X e - X why do we care what type of neutrino we re detecting? - we re looking to see if neutrinos oscillate from one type into another

33 Neutrinos Oscillate?! Quantum mechanics presents us with a lot of phenomena that seem weird & non-intuitive - neutrino oscillations is one of them - ν s can shift their identity & transform into one another - particles can sometimes behave like waves Quantum mechanical state can be the sum of several states - let is suppose ν µ is sum of two different mass states (or matter waves ) - might seem odd, but is perfectly allowable - can generate the interference pattern we call neutrino oscillations

34 sometimes the waves are in-phase wave 1 ν µ ν µ wave 2 wave 1 + wave 2 sometimes they are out of phase ν µ -ness begins to fade

35 Neutrino Oscillations in much the same way as beats are formed through the interference of two waves with different frequencies - neutrinos waves can oscillate between types but only if they have different masses - this interference causes first the disappearance & then reappearance of the original ν type a neutrino can change its identity!

36 How Do You Weigh a Neutrino? oscillates ν flux neutrin-o-maker 100% ν µ 0% ν e neutrin -otaker 100% ν µ 0% ν e neutrin -otaker neutrin -otaker (100-x)% ν µ x% ν e 100% ν µ 0% ν e so if we move the detector some distance away, a fraction of the muon neutrinos can change to electron neutrinos this is what MiniBooNE is looking for: ν µ ν e (if ν s oscillating will demonstrate that (contrary to what we thought) ν s have mass)

37 MiniBooNE is Still Collecting Data so I can t tell you whether we re seeing if our accelerator-based neutrinos are oscillating or not 2002: two of the Nobel Prize winners were neutrino physicists Ray Davis: neutrinos from sun Masatoshi Koshiba: supernova neutrinos from two experiments which have observed ν oscillations

38 Neutrinos Have Been Full of Surprises and are still generating Nobel Prizes these are pretty simple things we are only recently learning first, we didn t even know that neutrinos existed do they exist? we didn t know they came in 3 different types (ν e, ν µ, ν τ ) how many are there? didn t know they could oscillate from one type to another neutrinos have mass! how much do they weigh?

39 But It s Not All About MiniBooNE MINERvA there are many neutrino experiments several of which have KARMEN unearthed some surprises along the way HOMESTAKE MINOS MiniBooNE Gargamelle LSND NuTeV KAMLAND CHARM II NOMAD and CHORUS K2K near DONUT CHOOZ SNO it s an exciting time to be a neutrino physicist! stay tuned Super-Kamiokande