Neutrinos & Weak Interactions

Transcription

1 Neutrinos & Weak Interactions Lecture 3 Discoveries of the leptons; neutrino mass Evgueni Goudzovski School of Physics and Astronomy University of Birmingham, United Kingdom eg@hep.ph.bham.ac.uk

2 Designing a neutrino detector Reactor antineutrino production rate per unit of thermal power: F ν / P th ~ s 1 GW 1. Power output of a typical reactor: P th ~ 1 GW, therefore F ν ~ s 1. Let s place a detector at a distance L=10m from the reactor core. Antineutrino flux at the detector: dφ/dt = F ν / (4πL 2 ) ~ cm 2 s 1. Detector active mass: m det = 100 kg. Rate of IBD interactions in the detector: F int (m det /(2m p ))σ(dφ/dt) cm cm 2 s 1 = 0.03 s 1. ~2 interactions / minute Most reactor antineutrinos are below IBD threshold. Also, some protons are bound in nuclei (80% for H 2 O). The detector is not 100% efficient. Rate of detected interactions: ~ few interactions / hour 1

3 Inverse beta decay signature: prompt signal from the positron annihilation + delayed signal from the neutron capture Positron detection: via annihilation IBD detection principle Neutron detection: via thermalization & capture, e.g. e p p (typical capture time τ~200 µs) (τ~10µs for Cd, Gd-doped targets) A possible detector type: scintillation detector Scintillation: fast (~1ns) isotropic luminescence produced by absorption of ionising radiation A real-time experiment 2

4 Cowan Reines experiment (Savannah River nuclear power plant, South Carolina, US, ) Experimental setup Pb shielding (~20cm) Antineutrino interaction event Top triad MeV Bottom triad MeV Liquid scintillator detectors (each equipped with 110 photomutipliers) Thin H 2 O+CdCl 2 target tanks (0.2m 3 each). Cd/H atomic ratio = 1%. Reines et al., Phys. Rev. 117 (1960) 159 Prompt signal: MeV photons. Delayed signal: n capture on Cd, ~8 MeV. Both signals: coincidence in two detectors. 3

5 Photomultipliers Detectors of visible/uv light Typical quantum efficiency: two modern Hamamatsu PMTs R9880U-110 R7400U-03 Photocathode: photoelectric effect Dynodes: secondary emission Typical operating voltage: 1000 V. Typical number of amplification stages: 10. Typical gain: ~10 6. Light absorption in (quartz) input window Wavelength, nm Photon energy not sufficient for photoelectric effect 4

6 First neutrino oscillograms PROMPT Reines et al., Phys. Rev. 117 (1960) 159 DELAYED Signal in the top triad: t=2.5µs Signal in the bottom triad: t=13.5µs 5

7 (S) (C) Top triad accidentals The discovery of ν e Reines et al., Phys. Rev. 117 (1960) 159 Counting the prompt+delayed coincidences: (S) Signal region: 0.75µs< t<7µs; (C) Control region: 11µs< t<25µs; subtraction of accidental counts Cross-check: runs with the reactor switched off. Accidental background: Background/Signal 25%; Mostly non reactor associated. (S) Bottom triad (C) Counting rates after background subtraction: Top triad: F = (1.69±0.17) hr 1 ; Bottom triad: F = (1.24±0.12) hr 1. compatible after correcting for the distance to reactor Cross-checks: double Cd concentration in target or remove Cd; dissolve 64 Cu (β + emitter) in target; etc. 6

8 Cross-section measurement Reines et al., Phys. Rev. 117 (1960) 159 Cowan Reines IBD cross-section measurement: Our expectation for MeV neutrinos, assuming they interact via the weak force only (see lecture 2): A remarkable agreement! The neutrino was discovered in Nobel Prize awarded in

9 Discoveries in the cosmic rays Particles known by 1937: proton, neutron, electron, positron (the neutrino was proposed in 1930 to explain the beta decay spectrum) Particles not present in ordinary matter and decaying by the weak interaction discovered in cosmic rays: 1937: muon µ ± (the heavy electron ) decays into electron (or positron); τ = s; cτ = 660 m. 1947: pion π ± (the 2 nd lightest meson) quark content: π + = ud; π = ud; decays into muon; τ = s; cτ = 7.8 m; short lifetime: discovered at high altitudes. Relativistic time dilation: mean free path in lab frame is enhanced by the Lorentz-factor γ 8

10 The pion decay chain e ± kink The decay chain observed in photographic emulsions exposed at Pic du Midi (2,877 m) in the French Pyrenees: (Powell et al., Bristol University, 1947; Nobel Prize 1950) µ ± π ± kink : an undetected neutrino 3-body decay: 2-body hypothesis ruled out by the continuous positron spectrum A possibility to produce neutrino beams at accelerators 9

11 First accelerator neutrinos (1950s) protons target π ± µ ± ν (diverging beam) Are the ν produced together with muons identical to the ν produced together with electrons (e.g. in a reactor)? Neutrino interaction (IBD) cross-section: Accelerator-produced (GeV) ν s are ~10 5 times more likely to interact than reactor ones Interaction probability in 2.25m thick Al block (used as the first detector): density of relevant nucleons Production rates required for an experiment: (high intensity) The first accelerator proton beam of such intensity became available in the Brookhaven lab (US) in the early 1960s 10

12 The discovery of ν µ Lederman Schwartz Steinberger experiment, Brookhaven, 1962 Proton beam (15 GeV) Target (Be) First large scale particle experiment Photographic detection. Exposure: 8 months 25 good days. Detector ON for a total of 5.5 s. ~10 14 neutrinos through the detector. ~5000 spark chamber photographs taken. Method: π ± µ ± π ± µ ± ν τ π /τ µ =0.012 Detect inverse beta decay in the spark chamber: e.g. Identify the lepton type (e or µ). ν ν Results: shielding e,µ? 29 muon tracks identified: Trigger counter (trigger synchronized with proton delivery) Veto counters No electron tracks identified: the reaction WAS NOT OBSERVED ν e and ν µ demonstrated to be different particles: Nobel Prize 1988 Spark chamber: ~10 tons of Al. 11

13 Spark chambers Stack of metal plates, HV between pairs of plates. 10 tons of aluminium. The Brookhaven spark chamber Proton-antiproton collision seen by a spark chamber in a different experiment (at CERN) Cosmic rays tracks are visible Q: How to identify e/µ in a spark chamber? A: Muons are ~200 times heavier: smaller energy loss due to bremsttrahlung. Muons travel large distances and leave straight tracks. 12

14 images Photographs of the muon tracks produced in ν µ interactions taken by the Brookhaven experiment in

15 The discovery of the τ lepton The tau-lepton was discovered at the SPEAR e + e collider at SLAC (California) in 1975 (threshold energy: 3.55 GeV): undetected: cτ = 87 µm undetected Experimental statement: opposite sign eµ-pair and at least 2 missing particles. NB: e + e and µ + µ pairs can be produced by e + e scattering. τ is the only lepton massive enough to decay into hadrons (m=1.777 GeV) (by lepton universality, almost independent of daughter lepton type) Nobel Prize

16 The observation of ν τ Secondary beam production: (tungsten) Primary tau-neutrino source: [BR=5.6%] (~5% of all ν s are expected to be ν τ ) Detector type: Pb/emulsion sandwich + spectrometer ν τ postulated following τ discovery in 1975; directly observed by the FNAL E872 (DONUT) experiment in ; Mean τ free path: γcτ=2mm; decay into a single charged track: track with a kink 15

17 Summary: lepton discoveries The six known leptons were discovered in Electron (e ): in cathode rays. J.J. Thompson, Cambridge, Positron (e + ) and muon (µ ± ): in cosmic rays. C. Anderson, Caltech (US), 1932, Electron antineutrino (ν e ): produced at a nuclear reactor. C. Cowan & F. Reines, South Carolina (US), Muon neutrino (ν µ ): produced at a proton accelerator. L. Lederman, M. Schwartz, J. Steinberger, Brookhaven laboratory, New York (US), Tau lepton (τ ± ): produced at an e + e collider. M.L. Perl et al., SLAC, California (US), Tau neutrino (ν τ ): produced at a proton accelerator. DONUT experiment, FNAL, Illinois (US), There is no experimental evidence for further generations of leptons or quarks. 16

18 ν mass measurement: β decays Arbitrary units 210 Bi β-decay spectrum (τ 1/2 =5 days) Q: What is the smallest possible energy E min of a neutrino with a mass m ν? A: E 2 = p 2 +m 2, therefore E min = m ν Q: What part of the e energy spectrum is most sensitive to the neutrino mass? A: The endpoint at highest E e (massive ν takes more energy; less energy available for the electron) Electron kinetic energy, MeV Q: Which β-source is most suitable for E max measurement? High or low released energy E max? Long or short mean lifetime? A: Low released energy E max : more electrons are close to E max. Short lifetime: high specific activity; smaller and thinner samples. 17

19 Optimal β-emitter: tritium (pnn) (ppn) Advantages of tritium sources: Second lowest endpoint (E kin max =18.6 kev; ~60 times lower than 210 Bi). Relatively short lifetime (τ 1/2 12 years) and low atomic mass: high specific activity ( Bq/g). Small amount of 3 H required: reduced scattering of electrons in target. Simple electronic shell configuration: precise calculations of the final state spectrum. Another possibility: Rhenium sources ( 187 Re 187 Os + e + ν e ). The lowest endpoint: E kin max = 2.47 kev. However τ 1/2 ~10 10 years and Z=75: low specific activity. Rhenium cryogenic bolometer experiments (detector=source) are becoming competitive. 18

20 Endpoint vs neutrino mass Beta-decay: Electron energy endpoint (derived in lecture 2): Dependence of the energy endpoint on the neutrino mass: A very simple linear dependence: 19

21 3 H decay spectrum and sensitivity (m e = 511 kev 18.6 kev: non-relativistic electrons) E 0 E max m ν Kinetic energy, kev E e E max, ev 20

22 Recent experimental results Method: electrons run against a retarding electrostatic potential. Low-energy electrons do not reach the detector. 3 H sample Electron detector e e U kv U=0 No evidence for non-zero mass. Best experimental limit: m(ν e ) < % CL The Mainz β-decay experiment, Kraus et al., EPJC 40 (2005) 447 Counting rate [s 1 ] 3 H decay electron spectrum near endpoint Non-zero counting rate: background Electron kinetic energy, kev 21

23 Next generation experiment: KATRIN Vessel length: L=24m; Volume: 1400 m 3 ; High vacuum (~10 11 mbar); Low radioactivity (low cobalt content in steel); Expected resolution on ν mass: 0.2 ev/c 2. Data taking to start in Vessel delivery to Karlsruhe Institute of Technology (Germany) Installation work Photographs taken from 22

24 KATRIN setup KATRIN experiment at Karlsruhe Institute of Technology (Germany). Spectrometer vessel: Length: L=24m; Volume: 1400 m 3 ; High vacuum (~10 11 mbar); Low radioactivity (low cobalt content in steel); Low background (~0.01 Hz); ν mass resolution: 0.2 ev/c 2. 23