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.
What are the names of different neutrino interaction types with increasing energy?
Coherent (<50MeV, interaction with whole nucleus).
Elastic (individual electrons resolved)
Quasi-elastic (individual nucleons resolved, single lepton produced [CC interaction])
Resonance (Interaction with nucleon, often producing resonant meson [NC interaction])
Deep Inelastic Scattering DIS (nucleus breaks apart with many hadronic products)
What is the energy range of Very High Energy neutrinos?
What are their sources?
What detectors do we use?
TeV - PeV
Astrophysical (all flavours)
Resulting leptons are very high energy - travel for kilometres: we must use natural formations
–> ICECUBE
What is the energy range of High Energy neutrinos?
What are their sources?
What detectors do we use?
100MeV - GeV
Accelerator sources mainly (muon flavour mainly).
Large variety of detector technologies. SuperK NoVA, MINOS, muBooNE
What is the energy range of Low Energy neutrinos?
What are their sources?
What detectors do we use?
1-100MeV
Reactors, solar, supernova, geo.
Inverse beta decay is the only efficient detection mechanism (normal CC produces only low energy lepton).
*use liquid scintillator
e.g: Borexino, SNO+
Why is inverse beta decay not a useful detection mechanism for solar/supernova neutrinos?
Need electron antineutrinos for IBD, solar/supernova produce mainly electron neutrinos.
What is the energy range of Very Low Energy neutrinos?
What are their sources?
What detectors do we use?
~MeV
Old supernovae, CNB (~eV).
No detection so far, despite being the largest flux of all energies.
What are the 4 main types of neutrino detector?
Give a benefit to each.
[water] Cherenkov (cheap and scalable)
Liquid [argon] (very granular)
Trackers (granular - outdated)
Liquid scintillator (good energy resolution, inc. at low energy)
Give some examples of Cherenkov neutrino detectors.
All Kamiokande.
SNO and SNO+.
What are some downsides of Cherenkov detectors?
Very poor granularity and ID efficiency.
Heavy particles (e.g: proton) not detected as will not be superluminal.
Why would water-based liquid scintillators be good?
Allow Cherenkov light detection in addition to scintillation.
What are noble element detectors?
Why are noble elements used?
Time Projection Chambers
-transparent to their own scintillation light
~free electrons so long drift distance
How does a Time Projection Chamber work?
Light is scintillated and electrons ionized.
Light gives “t_0” (trigger).
Electrons drift (due to applied voltage) and are detected.
t_0 and drift time can be used to reconstruct 3D image of track.
Outline the DUNE experiment.
Fermilab beam, SURF detector, 1300km apart (long baseline).
Near detector to understand flux and cancel interaction sys. uncertainty.
Liquid Argon TPC. Sensitive to electron and anti-electron appearance (and muon disapearance).
Sensitive to mass ordering and CP questions.
Which unanswered questions can be investigated using neutrino oscillations?
Mass ordering
Maximal theta_23
Delta_CP
>3 neutrinos?
Why would solving the mass ordering problem be useful?
Simplify oscillation predictions (by a factor of 2).
-> Help optimise experiments and analysis.
Constrain GUTs
Guide neutrinoless double beta decay experiments.
How can we solve the mass ordering problem?
What experiments aim to do this?
Use matter effect with long baselines. GeV neutrinos at least 1000km baseline.
Muon and electron oscillation.
e.g: DUNE, HyperK
How can we measure delta_CP for the PMNS matrix?
For normal particles we would compare interaction/decay/production of particle and antiparticle.
We cannot do this for neutrinos, so compare oscillation of neutrinos/antineutrinos instead.
Long baseline, good energy resolution, knowing mass ordering will help.
How can we investigate whether delta_23 is maximal?
Why is this important?
Long-baseline muon flavour disappearance channel.
Maximal value implies some underlying symmetry.
If PMNS matrix is non-unitary, there must be additional neutrinos.
What is the idea behind sterile neutrinos?
How would we discover them?
What kind of experiments aim to do this?
There are hints of a mass difference of a few eV (i.e: sterile neutrinos much heavier).
–> SHORT baseline experiments (m -> km).
LSND, MiniBooNE, SAGE, GALLEX, SBN(future)
What are the two ways in which we could hope to determine absolute neutrino mass?
Investigating the “end-point” of beta-decay electron energy spectrum.
Using cosmology (CMB power spectrum).
How can we use beta-decay to determine absolute neutrino mass?
Investigate end-point (maximal electron momentum), we need precision on the eV scale!!
-use simplest beta-decay possible: tritium.
-measure electron energy with a massive spectrometer to get eV level precision
KATRIN, Project8(future)
What is the issue with using Cosmology to attempt to find neutrino absolute mass?
There are many “degenerate” parameters that we are unsure of, that would affect the CMB power spectrum in the same way as neutrino mass.
However, these measurements based on cosmology still give us the tightest limit on absolute neutrino masses.
How can we attempt to work out whether neutrinos are majorana particles?
Using neutrinoless double beta decays (already very rare).
The potential for the two neutrinos to annihilate if they are majorana.
These could maybe be detected by looking at the end point of double beta decay electron spectra.
What is the theoretical benefit of majorana neutrinos?
We get RH neutrinos for free, so the Higgs mechanism (yukawa coupling) can provide neutrino mass (without having to make up any sterile neutrinos).
What is the approximate half life of a double beta decay?
What about neutrinoless?
10^20 years
Neutrinoless is constrained to be at least 10^5 times longer.
What do we need experimentally to try and detect neutrinoless double beta decays?
Extremely good energy resolution (albeit at a very specific energy).
Extremely low background.
Scalability.
What kind of detector is NEXT and what is it aiming to detect?
Aiming to detect neutrinoless double beta decays.
High-pressure gas Xenon TPC
How may Barium tagging provide a hope for detecting neutrinoless double beta decays?
TPC uses scintillation light and ionization electrons. If Barium (daughter nucleus) is also detected this could be used to remove all background!
What is the evidence for dark matter?
(chronological)
Viriel theorem applied to galaxy size/luminosity.
Rotation velocity of spiral galaxies implies halo.
Gravitational lensing.
Bullet cluster (galactic collision).
CMB power spectrum.
What percentage of the energy of the universe is Dark Matter?
25%
What are three (albeit outdated) DM hypotheses?
Astrophysical objects (No, CMB means DM in early universe).
Modified gravity (No current theory).
DM particle (accounts for CMB signal and galactic distribution).
What are the properties of DM?
“Cold” (non-relativistic from CMB data).
Non-baryonic (obviously).
Massive, stable (present in early and current universe).
Neutral (no EM interaction).
Weakly interacting.
What does an indirect detection of a WIMP mean?
What experiments aim to do this?
What is a key experimental challenge?
Detect the products of DM annihilation / interaction with matter (cosmic).
ICECUBE.
We have to understand astrophysics extremely well to rule out background sources.
What does a direct detection of a WIMP mean?
What experiments aim to do this?
What is a key experimental challenge?
Detect WIMP interaction with matter within a detector.
DEAP, DarkSide.
Nucleus/electron recoil at the KeV scale -> need very low energy threshold detector.
What does a collider detection of a WIMP mean?
What experiments aim to do this?
What is a key experimental challenge?
Produce WIMPs from annihilation of matter, detect via missing energy.
LHC
Highly model dependent, may need extremely high energy if DM mass is very large.
What are the three things we want to exploit to make a direct WIMP recoil detection?
Charge (of ionized nucleus / electron).
Light (From scintillation of charged particles).
Phonons/Heat (due to recoil in crystal/bulk)
What are some categories of background we have to deal with when trying to detect DM?
Internal radioactivity (of detector components/medium).
External radioactivity (e.g: Radon).
Cosmogenic (cosmic rays and the products of their interactions).
Neutrino floor (coherent scattering can fake DM signal).
How might we (maybe) be able to reduce background due to the “neutrino floor”?
Exploit directionality: veto events that come from the sun.
Why do we expect annual modulation of dark matter detection?
Dark matter “wind” varies due to the orbit of the earth and our movement through the galaxy.
What are some non-WIMP candidates for DM?
Sterile neutrinos.
Dark “sector” particles like dark photons.
Axions.
What are two main sources of background for electron neutrino appearance experiments?
How can these be dealt with?
Pions(0) produced from NC (and CC) interactions decay to two photons that can fake an EM shower - fake an electron neutrino CC interaction. [need high resolution detector to distinguish these]
The muon neutrino beam will inevitably have some electron neutrinos due to inefficiencies in the production mechanism. These cannot be removed, but using a near detector can help mitigate these systematic uncertainties.
What is (was) the “WIMP Miracle”?
the lightest particle of
Supersymmetry (the neutralino) had all the required characteristics to be a
dark matter candidate.
~1 TeV
Explain the role of the PMNS matrix.
Flavour eigenstates = PMNS * Mass eigenstates
What does the angle gamma refer to?
Gamma = arg(- **check wall note)
What do we want when choosing a medium for a noble element detector?
-“high” boiling point as we need liquid.
-high density to give large interaction rate.
-lots of ionization electrons (large dE/dx).
-lots of scintillation light
How were neutrinos initially discovered?
Nuclear reactor electron antineutrinos.
IBD
Positron annihilation and neutron capture both produce light which is detected by PMT’s.
Describe a neutrinoless double beta decay Feynman diagram.
*check wall notes
Quickly run through the neutrino energy ranges for:
Atmospheric
Accelerator
Reactor
Solar
Supernova
Atmospheric: 0.1GeV - TeV
Accelerator: 1 - 100GeV
Reactor: ~1GeV
Solar: ~10MeV
Supernova: 1 - 100MeV
What is the nature of the DM flux modulation expected?
Sinusoidal modulation
Max in summer
Min in winter
(annoying because of lots of sources of systematic error which could also follow these modulations)
What kind of neutrinos form the majority of the neutrino background to DM experiments?
Mainly solar neutrinos as these interact coherently with nuclei, producing the same nuclear recoil signal as DM.
Directionality must be exploited to mitigate this background.
