Neutrino Physics Pedro Ochoa May 15th 2006

I. Historical Background James Chadwick I. Historical Background Radioactive beta decay as understood in the twenties: like for example in Observed electron (positron) spectrum Do you see any problems with this picture? Energy conservation ! (also) Recoil of proton not always opposite to electron (also) Spin seemed non-conserved YES !

Do you know why they were not named neutrons after all? Dear Radioactive Ladies and Gentlemen, As the bearer of these lines, to whom I graciously ask you to listen, will explain to you in more detail, how because of the "wrong" statistics of the N and Li6 nuclei and the continuous beta spectrum, I have hit upon a desperate remedy to save the "exchange theorem" of statistics and the law of conservation of energy. Namely, the possibility that there could exist in the nuclei electrically neutral particles, that I wish to call neutrons, which have spin 1/2 and obey the exclusion principle and which further differ from light quanta in that they do not travel with the velocity of light. The mass of the neutrons should be of the same order of magnitude as the electron mass and in any event not larger than 0.01 proton masses. The continuous beta spectrum would then become understandable by the assumption that in beta decay a neutron is emitted in addition to the electron such that the sum of the energies of the neutron and the electron is constant... Wolfgang Pauli I agree that my remedy could seem incredible because one should have seen those neutrons very earlier if they really exist. But only the one who dare can win and the difficult situation, due to the continuous structure of the beta spectrum, is lighted by a remark of my honored predecessor, Mr Debye, who told me recently in Bruxelles: "Oh, It's well better not to think to this at all, like new taxes". From now on, every solution to the issue must be discussed. Thus, dear radioactive people, look and judge. Unfortunately, I cannot appear in Tubingen personally since I am indispensable here in Zurich because of a ball on the night of 6/7 December. With my best regards to you, and also to Mr Back. Your humble servant . W. Pauli Note: In 1933 Pauli recognized the possibility of neutrinos having zero mass. Do you know why they were not named neutrons after all?

In 1934, Hans Bethe and Rudolf Peierls showed that the cross-section (related to the interaction probability) between neutrinos and matter should be extremely small…. BILLIONS of time smaller than that of an electron. Most people thought this “neutrino” was never to be observed… Never say never ! In 1953-56, Frederick Reines and Clyde Cowan made the first observation of electron antineutrinos.  How? Because of tiny cross-section, need very abundant flux of neutrinos and/or large detector: Nuclear bomb Nuclear plant 2 choices; go near a: They chose the nuclear plant of Hanford, Washington (and later on Savannah river, SC)

2 things happen after a neutrino interacts in the detector: The detection of a gamma after 5µs of the detection of the initial gamma pair provided a unique signature for antineutrino events. F. Reines got the Nobel Prize in 1995 for his contributions to neutrino physics.

In 34/40 interactions, they got a muon ! A question remained: Are the neutrinos associated with the electron (i.e. from beta decay) different than the ones associated with the muon (i.e. pion decay)? In modern terms: ? Earlier failed attempts to observe the reaction suggested that even if the weak coupling appeared to be universal, the two neutrino species were different. L. Lederman, M. Schwartz and J. Steinberger (Nobel Prize 1988), along with other collaborators answered this question, by showing that muons leave nice tracks Beam made mostly of goes, but does not go! In 34/40 interactions, they got a muon ! Schematic of the experimental apparatus used at the Alternating Gradient Synchrotron at BNL

Schematic of the DONUT beam at Fermilab It wasn’t until 2000 that the DONUT collaboration reported the observation of the tau neutrino: Observed in their detector (5 interactions!) Schematic of the DONUT beam at Fermilab This concept for making a neutrino beam is very similar to NuMI, the beam aimed at MINOS.

But not everything added up ! Since 1969 a physicist named Ray Davis tried to catch a few electron neutrinos from the sun every year through the reaction (Argon is a radioactive noble gas with half life ~35 days) 600 tons of chlorine expectation based on solar model Only ~1/2 of the expected neutrinos were found !!! Later, GALLEX, SAGE and KAMIOKANDE reported similar results. Either the solar model was wrong or…. (see next slide)

II. Neutrino oscillations Underlying principle: weak eigenstates mass eigenstates The oscillation probability is given by: where E[GeV], L[km], [ ], and

Do you understand this “mixing” concept? Let’s see what this gives for the 2 flavor model (see board & next slide).

We have: If then We obtain: where Do these oscillations happen for real? We’ll try to answer this question…

cosmic rays (protons mostly) strike earth from all directions But before answering let’s have a word on cosmic rays… Neutrinos produced by: cosmic rays (protons mostly) strike earth from all directions Note that:

1TeV proton shower on Chicago Movie time ! 1TeV proton shower on Chicago http://astro.uchicago.edu/cosmus/projects/aires/

The Super-Kamiokande Experiment So cosmic rays give us a practically isotropic flux of muon neutrinos at the earth’s surface ! The Super-K experiment uses those neutrinos to study neutrino oscillations:

Two examples of events at SK: Muon like event Electron like event

What they observed (1998): expected observed best fit

The interpretation: Observation of oscillations! Such that: at 90% confidence level.

The SNO Experiment 1kton of heavy water Neutral current interaction (through Z) Sensitive to Charged current interaction (through W) Sensitive to In 2001 the SNO collaboration announced that they observed: 1) ~1/3 of the electron neutrinos expected according to the solar model 2) ~exact flux of all types of neutrinos expected according to the model.  The electron neutrinos are also changing flavor !

The MINOS Experiment Fermilab, IL Soudan, MN Far detector NUMI beam 735 km NUMI beam & Near detector Far detector 250 200 150 100 50 Far detector NUMI beam # of CC events Measures the oscillated energy spectrum 120 GeV protons from the Main Injector Near detector Measures the unoscillated energy spectrum 0 10 20

How do you make a beam of neutrinos? Hadrons decay into neutrinos (and other stuff) Focus positively charged particles non-neutrino stuff gets absorbed

484 steel/scintillator planes 1 kton mass 3.8x4.8x15m The two detectors: Far Detector Near Detector Veto Shield Coil 5.4 kton mass, 8x8x30m 484 steel/scintillator planes 1 kton mass 3.8x4.8x15m 282 steel and 153 scintillator planes

completely consistent with: What MINOS has seen (2006): completely consistent with: E (GeV)

MINOS confirmed the hypothesis of oscillations and will make a 10% measurement of : The future for MINOS 2006 results