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The T2K Collaboration (2020), Constraint on the Matter-Antimatter Symmetry-Violating Phase in Neutrino Oscillations, Nature, 580, 339–344.

Below are a selection of the best student summary questions. You can also download the rest of the answers as a PDF.

 What is the significance of the T2K experiment?

The significance of the T2K experiment is to find profound evidence that could explain the matter antimatter asymmetry problem. It does this through looking for CP violation, the violation in symmetry in the mathematical operators charge conjugation (turning a particle into its antiparticle) and parity (turning a system into its mirror image), both of which would be conserved if matter and antimatter were perfectly symmetrical. Although CP violation has been found a few times in the past, with the Wu experiment and the decay problem of neutral kaons, this experiment is truly significant because the previous projects have concerned broken symmetry in quarks, and these findings do not show enough of a violation to account for the existence of the universe today. This experiment is the first attempt of finding CP violation in leptons, which could help support the theoretical idea of leptogenesis – a class of scenarios where the baryon asymmetry of the universe is produced from a lepton asymmetry in the primordial era. If the finding of a different rate of flavour change of a neutrino to an antineutrino were to be observed to a significance level of 5 sigma and above, we could then confidently confirm the idea of leptogenesis and perhaps answer the decades-old question: Why is there more matter than antimatter?

Jared

The history of neutrinos is filled with Nobel prizes – what makes them such a fascinating subject do you think?

Neutrinos are everywhere: supernovae, nuclear reactors, remnants of the big bang and bananas. Neutrinos are also ghostly, coasting through colossal chunks of matter effortlessly so much so that 100 trillion pass through you each passing second unnoticeably. Neutrinos are mystifying; counterintuitive quantum mechanical laws dictate they can transform from one type to another throughout their solitary journey’s. The burgeoning field of neutrino physics has offered countless breakthroughs in the past century, allowing astronomers glimpses at catastrophic super novaes and solving the existential crises created by the current framework modern physics stands on. A combination of these factors makes neutrinos such a captivating field to work in and learn about.

James

Describe, as clearly and simply as you can, what the T2K experiment does and what it is looking for.

The T2K studies the oscillations of neutrinos between their three different flavours: electron, muon and tau. First, it was focussed on just proving that these oscillations existed but now it's focused on comparing the oscillations of neutrinos with antineutrinos, hunting for CP violation. To do this a beam of muon neutrinos, or antineutrinos is generated in Tokai before being sent to the Super Kamiokande detector in Kamioka (Tokai to Kamioka is where the experiment gets its name from). The Super Kamiokande is a massive tank lined with photomultiplier tubes filled with water. When a neutrino interacts with the water it will form a charged particle, an electron in the case of the electron neutrino and a muon in the case of the muon neutrino. When they are produced these charged particles cause Cherenkov radiation, a flash of light due to the particle moving faster than the speed of light in water, which the detectors pick up. The rings of radiation produced are different for electrons and muons, allowing researchers to classify events as caused by an electron neutrino or muon neutrino. Finally, researchers can compare the number of each event with the expected number if neutrinos had no CP violation and with the expected number with maximal CP violation. This process allows researchers to constrain the value for the amount of CP violation by neutrinos and antineutrinos. They are interested in finding the amount of CP violation to better understand the imbalance between matter and antimatter that we see in the universe.

Molly

 

Natalie Wolchover (2020), Neutrino asymmetry passes critical threshold can be accessed at this link.

Comprehension questions:

1. Are antineutrinos the ‘mirror image twins’ of neutrinos?

A mirror flips spatial components of an object. An antiparticle is typically more than merely an upside-down particle, instead all of the quantum numbers of a particle are flipped e.g. a electron has a charge of -1 and a positron has a charge of +1. However, as neutrinos are such an odd sort of particle, there are very few differences between a neutrino and an antineutrino. The have opposite lepton number - a quantum number which is conserved in interactions and is in some sense just an accounting mechanism. The only other difference is that they have opposite chirality – you can think of this as the ‘handedness’ of a particle – and this naturally flips in a mirror. So antineutrinos, owing to the oddness of neutrinos themselves, seem very much the mirror twin of the neutrino. But in the truest sense, they are not, and we must consider them the CP opposites of one another.

Prof. Gary Barker’s answer: “The important point is that neutrinos and antineutrinos are not really just mirror images of each other but “CP opposites” i.e. all internal quantum numbers are reversed (the `charges) and all spatial properties are mirrored (e.g. momentum). Note that a spin vector does not change sign in a mirror but momentum does and so e.g. a left-handed state will change into a right-handed state under parity. Couple this with the C -operator changing the sign of all the charges and you have changed the quantum mechanical description of a neutrino into an antineutrino.”

2. Why is this measurement so important?

Because it’s one of the purported mechanisms that led to the dominance of matter over antimatter. From our current understanding of the laws of physics, pure energy can turn into mass if the mass produced comes in the form of equal amounts of matter and antimatter. The downside of this is that the antimatter and matter can then mutually annihilate to form pure energy again. There must have been a mechanism that tipped the balance ever so slightly in the favour of mass to allow the universe to exist in its current state today.

3. What is the difference between ‘evidence’ and a ‘discovery’?

It’s to do with the level of statistical significance within your data. If there is only a 0.3% chance of your signal being due to chance, then you have evidence. If there’s a 0.00006% of the signal being due to chance, you are much more certain and can therefore claim a discovery. These limits are not, though, rigorous limits but guidelines.

4. Why are further experiments necessary?

As more data is needed to claim a discovery, to ensure that the ‘evidence’ was definitely not due to chance. Note that the sensitivity of a measurement can be improved by taking more data with the same experiment – so using T2K for longer - or designing a more precise experiment or (even better) doing both, which is what the next generation of neutrino oscillation projects will do (e.g. DUNE).

5. Why do we not notice the ‘trillions [of neutrinos] each second” that pass through our bodies?

Because they don’t strongly interact with matter. Neutrinos only interact by the weak interaction (which has a relatively weak coupling). Neutrinos do not carry any electric or strong charge.

6. Describe what is being shown by the diagram in “A Window on the Asymmetric Universe”.

The symmetry side shows oscillations in both neutrino and antineutrino flavours happening in the same time period – the oscillations are symmetrical. In an asymmetric scenario, on the other hand, neutrinos and antineutrinos oscillate between flavours at different rates.

In the box ‘Primordial Universe’ the difference in decay rates is suggested as a reason for the dominance of matter in the universe today. Primordial, supermassive neutrinos and antineutrinos may also have oscillated at different rates. Remembering that they are also annihilating if neutrino meets an equally flavoured antineutrino, in the second step of the diagram, we start to see an imbalance occur.

7. Looking at the numbers of electron neutrinos and electron antineutrinos detected, explain why this finding is only ‘evidence’ rather than a ‘discovery’.

Whilst the data certainly shows a deviation from the expected numbers in a symmetrical universe, the numbers remain small so the difference between measured and expected is not that large – they found 12 more electron neutrinos and 5 fewer electron antineutrinos than expected. Whilst this is significant in showing there is an imbalance, there is still some margin for error – a couple of electron antineutrinos more or less has a significant difference on the result. With more data, the discrepancies will become clearer.

8. How are charge conjugation and parity symmetry linked to the discussion of the ‘handedness’ of the neutrinos?

When viewed through a charge conjugation mirror, a neutrino becomes an antineutrino. Charge conjugation is the mathematical operation that can convert a neutrino wavefunction into an antineutrino wavefunction.

When viewed through a parity mirror, a left handed particle becomes a right handed particle.

Combining both together, a left handed neutrino viewed through a CP symmetry mirror will turn into a right handed antineutrino.

 

Silvia Pascoli and Jessica Turner (2020), Matter-antimatter symmetry violated can be accessed at this link.

Comprehension questions:

1. What are fermions?

They are spin half fundamental particles. The fermions can be split into two families: quarks and leptons. There are six members of each of these families (so 12 matter fermions). There are antiparticle counterparts to all of them (so 12 antimatter fermions).

2. What are leptons?

One of the branches of fermions. Unlike the quark branch, leptons don’t take part in the strong interaction. They interact via the electromagnetic and weak interactions (as well as the gravitational interaction). There are 6 matter leptons: electron, muon and tau all have a charge of -1; the electron neutrino, muon neutrino and tau neutrino are electrically neutral and have a much smaller mass. All of these matter leptons have antimatter counterparts.

3. What is leptogenesis?

The umbrella term given to the physical process that led to the dominance of leptons over antileptons. In the bible, Genesis tells the story of creation (genesis is the Greek word for beginning), so leptogenesis is the idea behind the creation of leptons (instead of antileptons).

4. What is meant by the neutrino flavour being described as a quantum superposition of the different mass states?

Schrodinger’s cat describes the hypothetical situation in which a cat trapped in a box with a sealed vial of poison that is opened via a random, quantum mechanical process. As you don’t know whether the cat is alive or dead, it is considered to be a superposition of both. When you make a measurement (when you lift the lid), this superposition collapses into one state or the other. The same is true of neutrinos – they live as a mixture of different states and can collapse into either one when measured.

5. Describe what is shown in Figure 1.

This diagram aims to show the that as the mass states of the neutrino change over time in different ways (the three wave like structures), then at different points, the neutrino is composed of different ‘portions’ of each mass state. If a measurement were to be taken, the proportions of the mass states at that given moment may lead the neutrino to be seen as a flavour different to its original one.

The figure further demonstrates that the oscillation for neutrinos and antineutrinos may be different and thus break CP symmetry. In the figure, the antineutrinos evolve at different times to the neutrinos, despite them being in a mirror (which is meant to show the hypothetical CP symmetry that we would naively expect).

Prof. Gary Barker’s answer: “This is a rather difficult situation to paraphrase. Flavour states are those that take part in the weak interaction i.e. the ones we measure with T2K. You can consider each flavour state to be composed of 3 `mass states’ which are states of definite energy (and hence mass). As the flavour states propagate, the mass states get out of phase with each other because they propagate at different velocities due to their different masses. This different mixture of mass states, at any particular future time, corresponds to a particular mixture of the 3 flavour states and so, on measurement, it is possible that a flavour state other than the flavour state you started with, is the result of the measurement.”

6. What is unique about this particular finding?

It is the first time that a significant difference has been seen between the oscillation rate involving neutrinos c.f. anti-neutrinos.

7. How is the neutrino beam produced in Tokai?

High energy protons are directed at a graphite target. In the collision, pions and kaons are produced – these are examples of mesons, particles composed of a particle and an antiparticle (of different flavours so annihilation doesn’t instantly occur). The pions and kaons eventually decay, via the weak force, leading to the formation of neutrinos (or antineutrinos).

8. What do the neutrinos travel through to get to the Kamioka observatory?

The neutrino beam is simply directed through the earth. As neutrinos barely interact with matter, they don’t need to be sent down an evacuated tunnel like we would traditionally expect of a particle physics experiment.

9. In 9 years of data, we still haven’t got enough information to warrant a discovery – why is this?

Because neutrinos interact so infrequently that not enough events have been detected so far. Particle physics relies on huge numbers of detections so that we can have some statistical certainty in what has happened. Seeing one neutrino oscillation proves that they happen, but we need to measure lots to understand how often they happen and therefore if there is a difference between neutrino oscillations and antineutrino oscillations.

Dennis Overbye (2020), Why the universe produced something rather than nothing can be accessed at this link.

1. The path to this experiment seems to be littered with Nobel prizes – what have they been won for?

Andrei Sakharov, a nuclear physicist by day, won the Nobel Peace Prize for his work on human rights and his opposition to the abuse of power.

Tsung-Dao Lee and Chen Ning Yang won the Nobel prize in Physics for suggesting the existence of parity violation (later detected by Chien-Shiung Wu).

James Cronin and Val Fitch won the Nobel prize in physics for demonstrating that CP symmetry could be violated when looking at kaon decays.

Frederick Reines won the Nobel prize in physics for first detecting neutrinos. He shared his prize with Martin Perl who first detected the tau neutrino.

Leon M. Lederman, Melvin Schwartz and Jack Steinberger won the Nobel prize in physics for detecting the muon neutrino.

Takaaki Kajita and Arthur McDonald won the Nobel prize in physics for the discovery of neutrino oscillations.

2. Why is this experiment alone not sufficient?

Andrei Sakharov set three conditions that must be fulfilled to explain the genesis of the universe: CP symmetry violation is one, but he also required baryon number violation and for interactions to occur out of thermal equilibrium. This paper only provides further evidence of CP violation by demonstrating it for leptons.

3. Why are we continuing to look for CP violation in leptons when it’s been seen in kaons and b mesons?

The amount of CP violation seen in B mesons and kaons is not large enough to explain the level of matter dominance over antimatter. Also, further experiments in different situations are always useful in physics to allow us to understand where the limits of our theories lie.

4. Discuss the merit of describing neutrinos as “the most tiny quantity of reality ever imagined by a human being”.

Neutrinos are incredibly small in terms of their mass. For a long term it was thought that they had no mass at all. Equally, their lack of interaction makes them seem even smaller. Thinking of them as ‘imagined’ is also an interesting idea as neutrinos are certainly a physical object – but by considering them as something almost plucked from a thought, it gives them an even weaker grip on reality.

5. Why were neutrinos first suggested?

They were suggested as a mechanism to conserve energy in radioactive decays. Measurements showed Wolfgang Pauli that energy seemed to disappear in beta decays so he suggested this additional particle as the carrier of this energy.

6. What is being described by Dr Reines when he mentions a “cat turning into a dog”?

He is trying to convey the strangeness of neutrinos changing their flavour during neutrino oscillations. It seems utterly remarkable that a subatomic particle can simply change what it is, without any interactions, as it simply lives its life.

Prof Gary Barker’s answer: “It is wrong to think of the neutrino changing type without any interaction having occurred. Each neutrino is a quantum superposition of 3 states (albeit changing in time) but until a measurement is made i.e. an interaction, there is no sense in which the neutrino could be considered to have spontaneously changed into another type. This is a subtle but important point because it is the essence of quantum mechanics.”

7. How are the neutrinos and antineutrinos detected in Kamioka?

The neutrinos are detected if they interact with the large amount of pure water stored underground at Kamioka. If an interaction occurs, the products of the interaction will be travelling quickly to conserve momentum. If they are travelling faster than the speed of light in water (but not the speed of light in air of course), then light is emitted in a cone-like shape – called Cherenkov radiation. The water tank is surrounded with sensitive light detectors which look for these rings of light.

8. How might the detector at Kamioka differentiate between neutrinos coming from Tokai and the trillions of other neutrinos around?

They’re looking at the light emitted to get a sense of which direction the neutrino came from – they know that ‘their neutrinos’ came from a specific direction. They also monitor the production of neutrinos (indirectly as they can’t measure the neutrinos themselves) at the production site and so know when to expect them.

Prof. Gary Barker’s answer: “The most important aspect of reliably measuring only neutrinos from the beam is that the beam is produced in very short bursts and so only interactions inside a well-defined time-window are recorded. This hugely reduces the background from cosmic rays which are anyway suppressed by the experiment being under more than a kilometer of rock.”

 

GLIMPSES AT THE MAIN PAPER

The main paper can be found here.

We should now be able to look through the introduction of the paper and understand a little more.

Prof Gary Barker: “Much of the introduction section of the paper can now be understood from the earlier studies. Note that there are 2 solutions/interpretations of the oscillation data according to the two different possibilities for the `mass hierarchy’ which alludes to whether neutrino mass state 3 has a mass that is higher or lower than the masses of states 1 and 2. This is currently unknown and so both possibilities must be considered but an experiment like DUNE will be able to quite easily determine from the data what the correct mass hierarchy is.”

We can attempt to understand Figure 1 as well.

• The top panel shows the neutrino events.
• The lower panel shows antineutrino events.
• The x-axis shows the energy of the neutrinos and the y-axis shows how many have been seen – so we have a spectrum where the experiment is counting the number of electron neutrinos seen with different energies (bearing in mind we started with muon neutrinos).
• The coloured histogram in each panel shows the scenario in which there is no CP violation.
• The measured events are shown as data points with the error bars. The error bars are large because T2K sees so few events.
• The data points, even taking into account the error bars, do not nicely fit the scenario in which there is no CP violation (i.e. the data points don’t fit the coloured bars).
• The parameter governing the amount of matter/antimatter symmetry breaking in neutrino oscillations, called δ_cp, can take a value from -180º to 180º. There are two dotted lines added on each panel to show the values of δ_cp for which CP violation is maximal. Looking carefully, you can see the data is more aligned to the δ_cp= -90º= -π/2 scenario than the δ_cp= 90º= π/2 scenario.
• For the first time, T2K has disfavoured almost half of the possible values at the 99.7% (3σ) confidence level, and is starting to reveal a basic property of neutrinos that has not been measured until now.
• Not only that, but the statistical agreement between the data and the extremal situation of δ_cp= -90º= -π/2 indicates that the amount of CP violation seen may be close to the maximum that could be seen, which would be extremely interesting.

Think back to when you glanced over this paper, have you understood more of it now? We don’t expect you to completely comprehend the paper, but hopefully you can see a better glimpse of it.

GENERAL INFORMATION

Remember, reading a paper isn't like reading a piece of fiction or a newspaper article. Don't get frustrated if it doesn't immediately make sense - you might need to do a little research of your own to understand some of the ideas. This article gives you an idea of how scientists read differently.

Each question refers to a specific part of the paper e.g. Page 2, Column 3 is written as (P2, C3).

Next week, we'll publish solutions to the questions and the best submitted summaries from students across the country.

NEXT WEEK

This week's Journal Club has a two week deadline and is the last one of this academic year. If you're still looking for physics to do, take a look at our Summer Plans page - suggested reading/videos/podcasts/activities selected by us and the rest of the academics at Warwick.