FPP2 (Second half) Flashcards
What was the initial indication of the existence of neutrinos?
Beta-decays result in a beta particle with a wide energy spectrum. (whereas alpha-decay has specific momentum, as we expect for a two-body decay)
Suggests an additional decay product: a neutral fermion with very small mass.
“I have done a terrible thing, I have postulated a particle that cannot be detected”
What mechanism did the initial detection of neutrinos (anti-electron) use?
What improvements were made to facilitate this detection?
Neutrinos from nearby nuclear reactor (~MeV) interact with water detector in “inverse beta decay” (p + nu -> n + e). PMT’s detect photons from positron annihilation and neutron capture.
Improvements:
Detector size increase.
Lead shielding to reduce cosmic ray and other radioactive decay background.
Cadmium doped water to absorb neutrons - reduce time gap between annihilation gamma and capture gamma.
Coincidence timing data used to reduce other background.
What mechanism did the initial discovery of muon neutrinos use?
Accelerated protons collided with fixed target. Resulting mesons (pions) decay into muon neutrinos (~100MeV) that are selected using iron shielding.
Charged current interaction with fixed target producing muon.
Spark chamber used to identify muon.
How can we produce tau neutrinos? (very difficult)
High energy collider required as taus must also be produced.
Use a magnetic field to attempt to remove light mesons that will not produce taus.
Additionally use a muon detector as a veto.
Why are tau neutrinos so difficult to detect?
Tau decays are difficult to identify. (very high spatial resolution required to identify secondary vertex in muon decay channel).
–> use emulsion detectors.
How do we know there are 3 flavours of neutrino?
Via the measured value of Z boson width (LEP).
Z width = sum of width from all decay modes + N * neutrino decay width
N measured to be 3
What chirality of neutrinos exist?
LH neutrinos (opposite spin to momentum)
RH antineutrinos (aligned spin and momentum)
What was the first experimental indication that antineutrinos are always RH?
The Wu experiment
Beta-decay of Cobalt-60
How did the Goldhaber experiment confirm that there are no RH neutrinos?
Using the decay of a man-made radioactive isotope (europium) that decays after electron capture producing a neutrino.
The excited daughter nucleus produces a photon which has the same helicity as the neutrino helicity (as daughter spin is opposite to neutrino spin).
The iron magnet that is used to align the incident electron spin also acts as a polarising filter for the photon, “measuring” its helicity.
What is the solar neutrino problem?
1/3 of the predicted solar electron neutrinos are detected.
The neutrino oscillation that this eventually implied was a massive issue as neutrinos must then have mass. The mechanism by which neutrinos acquire mass is unknown, it cannot be the Yukawa mechanism as there are not both LH and RH neutrinos.
Which experiment initially found the solar neutrino problem?
How did this work?
The Ray Davis experiment.
neutrino interacts with chlorine-37 in tank (inverse beta-decay), resulting in radioactive isotope (Ar-37) and an electron.
Radioactive isotope is found by pumping all the liquid continuously past a germanium counter.
What was the key observation of Super-Kamiokande?
Why was this such a big issue?
Super-K studied atmospheric neutrinos (muon neutrinos produced from cosmic ray interaction), and showed that neutrinos from above are well described by theory but neutrinos from below fall short.
This implies neutrino oscillation –> neutrinos have mass where previously they were thought to be massless. This is especially a problem as they cannot gain mass via the Higgs mechanism.
Which experiment confirmed the solar neutrino problem?
SNO experiment measured both charged current and neutral current interactions. (nu + n -> p + e) and (nu + n/p -> n/p + nu).
Neutral current interactions detected the predicted number of neutrinos (as they are neutrino flavour-independent), charged current detected 1/3 the predicted value.
*used heavy water (twice the cross-section) left over from the Canadian nuclear program.
How did the Kamland experiment confirm neutrino oscillation?
Measured anti-electron neutrinos from nuclear reactors different distances away from the detector.
This directly showed the oscillatory behaviour with length.
What concept is the basis of neutrino mixing formalism?
Suggest that neutrino flavour eigenstates are not eigenstates of the hamiltonian. Suggest hamiltonian “mass” eigenstates and relate the two bases with a PMNS matrix.
Unlike other SM particles (quarks), neutrino mass eigenstates do not couple to any interaction.
What is the form of neutrino mixing probability in the simplified case of a two-neutrino system?
sin squared (2theta) * sin squared (1.27 * square mass difference * baseline[km] / neutrino energy[GeV])
theta is mixing parameter
*square mass difference in GeV?
What is the energy and baseline range of atmospheric neutrinos?
What kind of neutrinos are these?
What is the L/E range?
What is the mass^2 sensitivity range?
Produced by cosmic rays –> pion decays –> muon neutrinos. (and anti)
L: 10km -> 12 000 km (other side of earth)
E: 0.1GeV -> 1TeV
L/E: 0.01 –> 10^5
m^2: 10^-5 –> 100
What is the energy and baseline range of nuclear reactor neutrinos?
What kind of neutrinos are these?
What is the L/E range?
What is the mass^2 sensitivity range?
Anti-electron neutrinos (beta decay).
L: 100km –> 1000km
E: ~ 1GeV
L/E: 100 –> 1000
m^2: 10^-3 –> 10^-2
What is the energy and baseline range of solar neutrinos?
What kind of neutrinos are these?
What is the L/E range?
What is the mass^2 sensitivity range?
Initially electron neutrinos.
L: 10^8km
E: 10 MeV (0.01GeV)
L/E: ~10^10
m^2: ~10^-10
What is the dominant neutrino transition amplitude?
Between muon and tau neutrinos.
What is the matter effect on neutrinos?
Enhances mixing (longer baseline -> more matter effect).
Introduces asymmetry as matter generally only contains electrons.
Allows the sign of 1-2 mass difference to be measured.
However it complicates the measurement of CP violation.
In what way do we usually separate the PMNS matrix?
Why do we do this?
Separate into product of three matrices. Matrices for (2,3), (1,3) and (1,2) mass eigenstates.
These matrices then relate to: atmospheric neutrinos (2,3), short baseline reactor neutrinos (1,3) and solar neutrinos (1,2).
The (1,3) matrix is given the CP-violating phase by convention.
What are four sources of neutrinos?
Give neutrino type and typical energy.
Atmospheric (mainly muon) [wide E range].
Reactor (mainly anti-electron) [1-100MeV]??
Solar (electron, muon, tau) [~10MeV]
Accelerator
Why is the choice of detector technology particularly important for neutrino experiments?
Detector can only be sensitive to a particular flavour for charged-current interactions.
Detector can only be sensitive to a particular energy range.
Why is the choice of baseline important for neutrino experiments?
(three considerations)
Considering the energy range, the baseline should be chosen to give maximum sensitivity to the parameter being measured.
The matter effect should be exploited.
Longer baseline is more difficult due to 1/r^2 dispersion.
What do the “appearance” and “disappearance” channels refer to?
Appearance is detection of oscillation from one neutrino flavour to another.
Disappearance is detection of neutrinos that have not oscillated.
Which PMNS matrix parameters have we measured so far?
All three mixing angles.
Both square mass difference magnitudes.
(1,2) mass difference sign.
What questions remain in the neutrino sector?
(give 6)
Absolute neutrino mass
Neutrino mass ordering - what is the sign of (2,3) mass difference.
Is the value of the (2,3) mixing parameter maximal?
What is the value of the CP-violating phase of the PMNS matrix? (is there CP violation in the neutrino sector)
Majorana neutrinos?
Are there more than 3 neutrinos (sterile neutrinos)?
What kind of experiments are needed for modern neutrino sector investigations?
Long-baseline neutrino oscillation experiments.
Why are charged-current (CC) neutrino interactions the best channel for detection?
Allows the neutrino flavour to be determined (same as the lepton).
Charged lepton is easy to detect.
What are the issues with detecting neutral-current (NC) neutrino interactions?
Neutrino is re-emitted so we cannot determine flavour.
Difficult to remove background signals.
Pions resultant from de-excitation of nucleon can fake muons - fake CC interaction.
How do we determine the flavour of an interacting neutrino in a CC interaction?
Electrons result in an EM shower.
Muons result in a muon track (straight).
Taus are difficult to detect, but a secondary vertex can be resolved with a highly granular detector (sub-mm).
What is the effect of neutrino energy on the interaction with target material?
The effective “target” changes.
Below 50MeV the neutrino “sees” the nucleus as whole (interacts coherently with the whole nucleus). Above these energies individual particles start to be resolved.
