Y2 Neutrino Physics (spring term 2017)

Transcription

1 Y2 Neutrino Physics (spring term 2017) Lecture 5 Discoveries of the leptons Dr E Goudzovski eg@hep.ph.bham.ac.uk

2 Previous lecture In 1940s, nuclear reactors became the first powerful continuous artificial anti-neutrino sources (production rate: ~10 20 s 1 GW 1 ; typical energy: few MeV). Anti-neutrinos are detectable via the inverse beta decay (IBD) reaction, with a threshold of E th =1.8MeV. We have defined the reaction cross-section and mean free path, and found how they are related: For MeV-energy neutrinos, the interaction cross-section is tiny (~10 43 cm 2 ), free path in matter is astronomical (light years). Design of the reactor anti-neutrino experiment: the expected detection rate is ~0.01/hour/kg. 1

3 This lecture Discovery of the electron antineutrino: the Cowan Reines experiment (1956). Discoveries of the second generation leptons. Discoveries of the third generation leptons. Reading list: C. Sutton. Spaceship neutrino. Chapters 3, 4. N. Solomey. The elusive neutrino. Chapters 4, 5, 9. 2

4 IBD 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 ~200 s) ( ~10 s for Cd, Gd-doped targets) A possible detector type: scintillation detector Mean free path of a MeV photon in water: 10 cm. Interaction of a MeV photon: mainly Compton scattering ( e e ). Scintillation: fast (~1 ns) isotropic luminescence produced by absorption of ionising radiation a real-time experiment 3

5 Cowan Reines experiment (Savannah River nuclear power plant, South Carolina, US, ) Experimental setup Pb shielding 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. 4

6 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. Typical time resolution: <1 ns. Light absorption in (quartz) input window Wavelength, nm Insufficient photon energy for photoelectric effect 5

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

8 (S) (C) Top triad accidentals The discovery of e Reines et al., Phys. Rev. 117 (1960) 159 Counting the prompt+delayed coincidences: (S) Signal region: 0.75 s< t<7 s; (C) Control region: 11 s< t<25 s; subtraction of accidental counts Cross-check: run 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 = ( ) hr 1 ; Bottom triad: F = ( ) hr 1. compatible after correcting for the distance to reactor Further cross-checks: double Cd concentration in target or remove Cd; dissolve 64 Cu ( + emitter) in target; etc. 7

9 Cross-section measurement Reines et al., Phys. Rev. 117 (1960) 159 Cowan Reines IBD cross-section measurement: Our expectation for MeV neutrinos, assuming weak interaction in low-energy regime (see lecture 4): A remarkable agreement! The neutrino was discovered in Nobel Prize awarded in

10 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 (the heavy electron ) decays into electron (or positron); = s; c = 660 m. 1947: pion (the second lightest hadron) quark content: + = ud; = ud; decays into muon; = s; c = 7.8 m; short lifetime: discovered at high altitudes. Relativistic time dilation: mean free path in lab frame is enhanced by the Lorentz-factor 9

11 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) kink : an undetected neutrino 3-body decay: 2-body hypothesis ruled out by the continuous positron spectrum A possibility of producing neutrino beams at accelerators 10

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

13 The discovery of 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: / =0.012 Detect inverse beta decay in the spark chamber: e.g. Identify the lepton type (e or ). Results: shielding e,? 29 muon tracks identified: Trigger counter (trigger synchronized with proton delivery) Veto counters No electron tracks identified: the reaction WAS NOT OBSERVED e and demonstrated to be different particles: Nobel Prize 1988 Spark chamber: ~10 tons of Al. 12

14 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) Muon tracks are visible Q: How to identify e/ in a spark chamber? A: Muons are ~200 times heavier: smaller energy loss due to bremsstrahlung. Muons travel large distances and leave straight tracks. 13

15 images Photographs of the muon tracks produced in interactions taken by the Brookhaven experiment in

16 The discovery of the lepton Tau-lepton production was discovered at the SPEAR e + e collider at SLAC (California) in 1975 (threshold CM energy: 2m =3.55 GeV): undetected: c = 87 m undetected Experimental statement: opposite sign e -pair and at least 2 missing particles. NB: e + e and + pairs can be produced by e + e scattering. 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

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

18 Are there further neutrino flavours? The LEP e + e collider at CERN: Operation: Circumference: 27 km. Four large detectors. E CM up to 209 GeV. e + A Z 0 factory. Z 0 e ~17M Z ~1/E 2 ~36k W + W OPAL electromagnetic calorimeter endcap : 1132 Pb glass blocks (25 kg each) 17

19 Total Z 0 decay rate: Z 0 decay rate measurement Z 0 q Z 0 e +, +, + Z 0 x q e,, x Measured Standard Model expectation: GeV invisible part Measured number of generations: N = ( ). However there is still room for heavy (m >M Z /2) or sterile neutrinos; (sterile = no weak interactions). 18

20 Summary The six known leptons were discovered in Electron (e ): in cathode rays. J.J. Thompson, Cambridge, Positron (e + ) and muon ( ): in cosmic rays. C. Anderson, Caltech (US), 1932, Electron antineutrino ( e ): produced at a nuclear reactor. C. Cowan & F. Reines, South Carolina (US), Muon neutrino ( ): produced at a proton accelerator. L. Lederman, M. Schwartz, J. Steinberger, Brookhaven laboratory, New York (US), Tau lepton ( ): produced at an e + e collider. M.L. Perl et al., SLAC, California (US), Tau neutrino ( ): produced at a proton accelerator. DONUT experiment, FNAL, Illinois (US), There is no conclusive experimental evidence for further generations of leptons or quarks. 19