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

Summaries and answers now available

You can also download the Cornell notes template for this paper (which includes the same questions) as a Word Document or a PDF. Teachers, feel free to download this and forward it on to your students.

Last week we mentioned the idea of a review article – an article which brings together a whole series of research and provides some form of commentary on it. When writing a review article, the author performs what we call a literature review – they read as much as they can about the topic, hunting down different references, to form a complete picture. This is the process that PhD students typically start with when they begin their studies to become accustomed to their research area.

This week, we’re going to have a two-week deadline, so that you can perform a mini literature survey. This won’t be a traditional literature review as you won’t be reading papers to find the majority of the information, but useful web pages and videos. We’re not going to ask you to hunt for links yourself – we’ll provide them – but you might want to look further if something doesn’t make sense or you suddenly want to delve into more detail. We’re not going to ask you to write one giant piece either, we’re going to use our Cornell notes style and give detailed descriptions of the different aspects of our topic.

Our topic is going to be the very recent results from a particle physics experiment called T2K in Japan. It’s another huge collaboration of scientists from all over the globe looking to understand one of the most significant questions in modern-day physics – why did the universe come to be filled with matter? Our current understanding of the laws of physics has quite a big hole in them when it comes to the creation of everything around us – the big hole being that our laws say that the universe should really be empty.

Our aim this week is to:

• Perform a review to understand what each of the basic ideas behind this experiment are below – you can use your own knowledge or links and videos that you find, but we have given some links too.
• Armed with this knowledge, we’re going to read three popular articles about the experiment to give us a rounded view of what’s going on.
• Finally, we’re going to take some glimpses at the paper that has been published on this work. This will only be glimpses as this paper is not the most readable to non-experts. In truth, I don’t understand 75% of it, so I’ve asked for lots of help from the Particle Physics department at Warwick. We’re going to focus on specific bits of the paper that we should be able to understand.

Before we get started, I just want you to think about how far you’ve come during Journal Club. How would you have felt about reading part of a current particle physics paper? In fact, here’s the link now, read the abstract on the webpage and then download the PDF and just flick through the pages very quickly. Is it as intimidating as it would have been if I’d given this to you in week one? It probably doesn’t look like a stroll in the park – there are some big formulae and some odd-looking graphs – but hopefully, you can see that it’s not as intimidating any more.

We’re going to start by looking at some of the general ideas from particle physics that we need to know. We’ve provided some ideas for answers and some useful links to get you started.

The standard model – useful link; useful video playlist
Matter-antimatter asymmetry – useful link
Charge conjugation	A mathematical operation that converts a particle into its antiparticle. If I apply charge conjugation to an electron, then the result is its antiparticle – the positron. If I then apply charge conjugation to the positron, I return to the electron.
Parity – useful link	Parity is another mathematical operator that turns a system into its mirror image. If I have a particle moving to the right in the real world, then the parity operator ‘flips it’ so that it is moving left – so that it is now moving as if ‘in a mirrored world’ compared to the original. We would tend to think that the laws of physics stays the same whether or not we ‘flip the coordinates’ and live in the mirrored world or not. This is called parity symmetry, or P-symmetry – a ball will fall in the same way if you throw it to the left or to the right. Parity symmetry applies at the quantum level too (sometimes…) and any interaction that is via the electromagnetic or strong nuclear interaction will look the same whether or not the system is viewed normally or in the flipped mirrored world.
Spin and the ‘handedness’ of particles – the helicity section of this link is useful.	Fundamental particles have a property called spin. It is analogous to a much larger body spinning. If you imagine a particle moving out of this page, the particle could be spinning in a clockwise or anticlockwise direction as we look at it. Take your hands and point your thumbs towards your face – the fingers of you left hand are curled in a clockwise direction and the fingers of your right hand are curled in an anticlockwise direction. We use this convention to define the handedness of the particle. If the particle is moving towards you and spinning anticlockwise, it is a right-handed particle. If the particle is moving towards you and spinning clockwise, it is a left-handed particle. If we look at the particle in a mirror, such that it is moving in the opposite direction, then it’s handedness will change (in a similar way to you seeming to be the opposite handedness when you look at yourself in a mirror).
Parity violation – useful link
CP Symmetry (Charge-Parity symmetry) – useful link	A combined symmetry which is invariant for the majority of fundamental interactions.
CP Violation – useful link

Now we have this idea that the imbalance that we see in the universe (the dominance of matter over antimatter) may have been caused by some asymmetry in the laws of physics – matter and antimatter may behave ever so slightly differently.

We’re now going to look at the specific particles that are measured by the T2K experiment – neutrinos. Remember, from a quantum mechanical point of view (the laws that apply to the smallest of particles) particles can behave as waves. We describe their behaviour with a wavefunction which encapsulates all of the information about the state of the particle. Specifically, the square of the wavefunction gives us the probability of finding that particular particle at a particular point at a particular time. This wavefunction changes or evolves over time and that is going to be crucial for this experiment.

Neutrinos – useful link; useful long panel discussion
Neutrino oscillations – useful link one; useful link two; very useful video (particularly towards the end)
T2K – useful link one; useful video
Cherenkov radiation

You might hear articles talk about the ‘mixing angle’ of neutrinos – this isn’t a physical angle at all, but a way of quantifying the probability of a neutrino swapping flavours.

With our mini literature review complete, we’re in a much better position to read our popular articles. We’ll provide some comprehension questions for each one. We also have some example answers for the more difficult questions which Professor Gary Barker has kindly agreed to give. He is part of the T2K team.

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?

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?

 

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

 

4. Why are further experiments necessary?

 

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

 

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

 

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

 

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

 

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

Comprehension questions:

1. What are fermions?

 

2. What are leptons?

 

3. What is leptogenesis?

 

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

 

5. Describe what is shown in Figure 1.

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?

 

7. How is the neutrino beam produced in Tokai?

 

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

 

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

 

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?

 

2. Why is this experiment alone not sufficient?

 

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

 

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

 

5. Why were neutrinos first suggested?

 

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

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?

 

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

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 two 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.

SUMMARY QUESTIONS

What is the significance of the T2K experiment?

 

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

 

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

 

FURTHER READING

If, after all that, you still want something more to read then you’ll just have to apply to start a physics degree because even I’m worn out at this point. Equally, you could look at our new Summer Plans - links to books, videos, podcasts that should keep you busy for weeks!

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.