Do protons have intrinsic charm? New evidence suggests yes

Host: Benjamin Thompson

Welcome back to the Nature Podcast. This week: looking for intrinsic charm inside protons.

Host: Nick Petrić Howe

And the latest from the Nature Briefing. I’m Nick Petrić Howe.

Host: Benjamin Thompson

And I’m Benjamin Thompson.

[Jingle]

Host: Benjamin Thompson

Protons sit inside the nucleus of every single atom, and they’re what physicists smash into each other at the Large Hadron Collider. You might have heard that protons are made up of three quarks – two ‘up’ quarks and one ‘down’ quark. But, as is so often the case in physics, it’s actually rather more complicated than that. Quantum physics says that alongside these three quarks could be a whole bunch of others, popping fleetingly in and out of existence all the time. These extra quarks – not just up and down but other types too – can affect the proton’s mass. And one really weird fact is that quantum physics says that even the heaviest types of quarks could be present – including those heavier than the whole proton itself. Although it’s theoretically possible, whether heavy quarks actually do pop into existence inside protons or not has been up for debate for about 40 years because, while physicists have good theories to describe what’s going on inside a proton, in practice, its precise structure is just too complex to calculate or to determine experimentally. But understanding the proton’s insides is really important for things like searching for new physics. This week in Nature, a team have taken a new look for evidence of heavy quarks hiding within the proton, by combing through data stretching back 35 years. To find out more, reporter Lizzie Gibney spoke to one of the team, Stefano Forte from the University of Milan in Italy, and started by asking him what quark in particular they were looking for.

Interviewee: Stefano Forte

So, there are six kinds of quarks in nature, three are lighter than the proton and three are heavier. Now, one would think that only the lighter quarks are inside the proton, but actually, the laws of quantum physics allow also for the heavier quarks to be inside the proton. And so, the question we're after is, is the lightest of these heavier quarks, the charm quark whose mass is about one-and-a-half times the proton’s mass, also present inside the proton?

Interviewer: Lizzie Gibney

So, how do you actually go about trying to find the charm quark then? What kind of data can you use?

Interviewee: Stefano Forte

In general, we smash something on the proton. And what we do is we look at the debris of the collision and try to infer and to reconstruct what the proton structure was that could have produced this result. And what we can smash on the proton is photons, electrons, muons, neutrinos or other protons.

Interviewer: Lizzie Gibney

So, your analysis is looking at loads of different experimental results that already existed out there. How far back do they go?

Interviewee: Stefano Forte

So, the oldest experimental results that we include are actually very old. They're from the 80s, and they have to do with collisions of electrons against protons. And the most recent experiments are collisions of protons against other protons from the Large Hadron Collider of CERN. And in between, there is 40 years of experimentation. We use about 5,000 data points from at least 40 different experiments.

Interviewer: Lizzie Gibney

Okay, so then, what do you do with that data? How from those collision debris do you go about trying to figure out what's going on and what particles there are inside the proton?

Interviewee: Stefano Forte

So, we have a theory, which is very well-established, that describes the interactions of the constituents of the protons, the theory of strong interactions. Now, using that theory, given the structure of the proton, we can calculate what happens in a proton-proton collision. So, we can calculate the final state of any of these experiments. Now, we have to invert the procedure. We start from what people see, and then we have to undo the calculation in order to indirectly infer what the proton structure was in the first place.

Interviewer: Lizzie Gibney

And by structure we mean the probability of finding each different quark within the proton, with some given momentum, energy or mass, and you get a kind of set of probability distributions, and how did you go about finding that?

Interviewee: Stefano Forte

So, the original aspect of our analysis is that we use machine-learning tools to do that. That means that, unlike other possible approaches, we do not have to make a specific assumption or assume a specific model of the proton structure. Rather, the machine-learning tool proceeds like a pattern-recognition tool in order to deduce the structure of the proton that would produce the observed data.

Interviewer: Lizzie Gibney

So, it's a kind of like a best fit of what structure of the proton works best to produce all of these different collision outcomes that you've seen.

Interviewee: Stefano Forte

That’s exactly right. So, what the machine-learning tool does for you is not assume a single model. So, you'll get a probability distribution of best fits, of which one is the best, best fit, which is the most likely of this probability distribution of this cloud of best fits, each of which would correspond to different possible underlying models.

Interviewer: Lizzie Gibney

And so, we've then found our best guess at the structure of the proton in all of these high-energy collisions. Does that tell us about everyday protons?

Interviewee: Stefano Forte

So, as I mentioned, we have a very well-established theory which allows us to compute how the structure of the proton changes, according to the environment. So, given knowledge of the structure of the proton in a high-energy collision, we can calculate the structure of the proton at rest, as it would be if the proton were sitting there on your desk, so to speak. Now, this can be done both ways. It's a completely deterministic process. So, given the structure of the proton at high energy, we can calculate the one of the proton at rest and conversely.

Interviewer: Lizzie Gibney

So, you're kind of running backwards or your equations to figure out what would have happened in the proton at rest.

Interviewee: Stefano Forte

That's right, that's exactly it.

Interviewer: Lizzie Gibney

So, when you found the best fit for the proton structure, you found that the charm quark does appear to be there.

Interviewee: Stefano Forte

So, you have a chance, which is small but not negligible, of finding a charm quark in the proton, and when you do find one, it so happens that that charm quark is typically carrying about half of the proton mass. This is quantum physics, so everything is probabilistic.

Interviewer: Lizzie Gibney

So, the charm is really massive, heavier than the proton itself. But because there's only a small chance that it exists, those two factors kind of balance each other out and mean that we can have the charm quark inside the proton. That's cool. And this charm quark being inside an everyday proton is something that you call ‘intrinsic charm’, which is just a phrase I love.

Interviewee: Stefano Forte

Yeah, intrinsic means that the charm is still there when we're looking at the proton at rest. I mean, it's not some by-product of the high-energy collision, so that's why we call it intrinsic.

Interviewer: Lizzie Gibney

And this is the first time that anyone's found that kind of evidence then, right, that the proton has any kind of amount of charm quark in it at all?

Interviewee: Stefano Forte

Well, I will say that this is the first time that we have solid evidence. There were previous claims, but all of them have remained controversial, mostly because these claims were based on specific models. So, what people did was try to make an assumption on how things would change if we added a small charm proton component. But then the same effect could have been produced by something else.

Interviewer: Lizzie Gibney

And so, you've called this evidence, but not a discovery. Why is that?

Interviewee: Stefano Forte

As a general convention, in physics, we talk about discovery when the significance of what we see reaches what we call 5-sigma. What we find instead is what we call 3-sigma, actually, we find slightly more than 3-sigma, but let's say it's a chance of about 1 in 1,000 of not being true. 5-sigma, which is the discovery, it's 1 in 3 million. So, we cannot yet claim discovery, but we can claim evidence.

Interviewer: Lizzie Gibney

How might physicists go about using this finding? Why is it important to know whether the proton has this intrinsic charm, as you call it?

Interviewee: Stefano Forte

So, first of all, the structure of the proton is needed in order to calculate anything that is observed at the LHC. So, for example, when the Higgs boson was discovered 10 years ago, the structure of the proton was an input to the calculation that was then, by comparing to the data, used to claim discovery of the Higgs boson. These days, the searches for physics have become a precision exercise. We have not observed new particles and it is unlikely that the LHC has enough energy to actually produce a new particle. So, the way we can see evidence of, say, dark matter is going to be indirect, meaning small deviations between the current theory where there is no particle corresponding to dark matter, and what actually happens in nature. Now, if you are after small deviations, you need a completely accurate description of what the current theory predicts and the structure of the proton is an integral part of that.

Interviewer: Lizzie Gibney

And I guess it's just something that is good to know. I mean, we are all made of protons as well, right?

Interviewee: Stefano Forte

That's absolutely right. I mean, we are all made of protons and neutrons, but no one is able to actually calculate the structure of the proton from first principles. In principle, it should be possible but it's just too difficult. So, on top of the interest for the search for new physics at the LHC, an accurate determination of the structure of a proton and discovering that there is charm inside the proton is something that gives us hints on this very difficult calculation that is beyond the current state of the art.

Interviewer: Lizzie Gibney

Finally then, you mentioned that some other experiments, their results have been controversial in the past. Do you think that your findings are going to be controversial at all?

Interviewee: Stefano Forte

That's a very good question. That's possible because some people say that machine learning is a black box – you don't really know what's going in. And of course, I mean, we are aware of that and a large part of what we do consists of trying to validate the result of our procedure. And on the other hand also, the result touches a nerve and sometimes, I mean, there are people who debated for years, and when you come and say, ‘Look, it is like this,’ I'm sure that some people will be very happy and some other people are going to frown and say, ‘Are you really sure,’ and try and raise all possible kinds of objections. But this is science, so they are very welcome to raise their objections and we do our best to answer to them.

Host: Benjamin Thompson

That was Stefano Forte from the University of Milan talking with Lizzie Gibney. To find out more about the proton’s charmed life, check out a link to the paper in the show notes.

Host: Nick Petrić Howe

Coming up, we'll be hearing why taxing mental tasks can make us feel tired. That’s in the Briefing chat. Right now, though, it’s Dan Fox with the Research Highlights.

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Dan Fox

Sea sponges are renowned filter feeders, straining tens of thousands of litres of water through their bodies every day to collect food. But they can also hoover up particles that clog their internal filter systems. Luckily, though, they have a way to rid themselves of this accumulated mucus – sneezing. To understand this behaviour, researchers filmed samples of a Caribbean tube sponge. They observed mucus oozing out of small pores on the sponges’ surface that are mainly used to suck in water. The substance then moved along the animal's exterior on what the authors term ‘mucus highways’, towards junctions where the debris accumulated. Every few hours, the sponges contracted, which released this debris into the water. The team analysed previously recorded time-lapse footage of another species of sponge and witnessed a similar sneezing behaviour, meaning that this method of clearing their filters might be widespread among sponges. Ride the mucus highway to that research in Current Biology.

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Dan Fox

You might have heard the term ‘crocodile tears’ to describe an insincere expression of emotion. But perhaps bonobo tears would be more accurate, as researchers have found that the chimpanzee-like ape may voluntarily control their emotional expressions in order to solicit consolation from its peers. The team of researchers observed rescued bonobos for hundreds of hours, and over the course of 144 separate conflicts, they noticed a clear pattern. Victims of a conflict were more likely to receive consolation when they produced cute, babyish signals. These include throwing a tantrum and making a ‘pout face’. They also observed that the length of time these distress signals were produced grew longer as the size of the audience of spectator bonobos increased. Read that research in full in Philosophical Transactions of the Royal Society B.

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Host: Benjamin Thompson

Finally on the show, it's time for the Briefing chat, where we discuss a couple of articles that have been featured in the Nature Briefing. And Nick, do you know what, I think I'll go first this week, and I've got a story that I read about in Discover Magazine, and it's all about evolution, and specifically why evolution seems to love crabs.

Host: Nick Petrić Howe

When you say evolution seems to love crabs, what do you mean by that?

Host: Benjamin Thompson

Well, if I asked you to sort of draw a crab on a piece of paper, I'm pretty sure you would draw what I'm thinking of right now, right? This body plan that's wide and flat, and crabs tend to have their abdomens tucked under this kind of hard shell, right? Now, this body plan seems to have evolved independently so often that it actually has its own name, which is carcinization, and this is convergent evolution of crustaceans from a non-crab form to a crab-like form.

Host: Nick Petrić Howe

Right, so this carcinization, this becoming more crab-like, it's happened a bunch of times.

Host: Benjamin Thompson

Yeah, so, generally speaking, crabs can be kind of split into two subgroups, I guess. They're the true crabs and the false crabs. Okay, now, there's a number of anatomical differences there. But what's weird is that this carcinization has happened independently in both groups, I think, five or more times, researchers have suggested, even though they're separated by millions of years of evolution anyway, and it's resulted in animals that look like crabs, right? And in one sort of particularly striking example in the false crab group, okay, is king crabs, right? These enormous animals that obviously look like crabs, right? It's thought that they evolved from hermit crabs, okay, which kind of wear their shells on their back, so quite a striking change. But again, here's some carcinization towards a crab-like body plan.

Host: Nick Petrić Howe

So, there seems to be some sort of advantage to being kind of crabby.

Host: Benjamin Thompson

It is very unclear, Nick, I will say, why this is going on. Why does evolution love crabs so much, right? So, some theories are maybe this body shape is advantageous to avoiding predators, for example, crawling into gaps. Maybe it's more stable for standing up and walking and moving from the sea to the land, or something like that. Maybe there's a link between the body parts. If evolution favours a smaller, curled-under abdomen, then perhaps you have to have a flatter body type to kind of account for that. But nobody really knows. And what's going to confuse things even more is that the process has gone in reverse. Decarcinization seems to have happened quite a lot as well, which has led away from the crab body plan, and it's led to some very strange animals. So, the frog crab, for example, as its name suggests, it's a crab that kind of looks a bit like a frog, and there's a few of these things and it's all very strange. But despite their similar body structures, crabs are hugely diverse. They can be huge, they can be tiny, they can burrow, they can be parasites. There's so much going on.

Host: Nick Petrić Howe

Wow, so it seems like evolution loves crabs and also doesn't like things to be crabs, which seems like a really hard problem to understand. What can scientists learn from this conundrum?

Host: Benjamin Thompson

Yeah, I mean, it is a very tricky one, and evolution is, of course, incredibly complex. But researchers seem to think that there must be some advantage to this carcinization, right? But what it seems to have done is really give researchers a chance to study convergent evolution, so evolution finding the same answer from two different directions, if you see what I mean, and over enormous evolutionary timespans.

Host: Nick Petrić Howe

Wow, evolution never ceases to fascinate me. This is super cool, Ben. Thanks for telling me about it. For my story this week, I've been learning, well, I'm actually quite tired, to be honest, but I've been learning about why it is that thinking really hard might make you tired.

Host: Benjamin Thompson

Nick, I knew it.

Host: Nick Petrić Howe

I think we all know it to some extent. It's definitely a feeling that we all get. And when you do sort of physical exertion, it seems more obvious why you would be tired. But why, when you're not moving very much, maybe putting together a podcast, reading a paper, reading about evolution of crabs, why would that make you tired? And now, there's a new study in Current Biology that was reported on in Nature that will give us a bit of a clue as to why this happens.

Host: Benjamin Thompson

Yeah, I mean, I guess you're right. If you're in the gym working out, you can see that your muscles are being used and the exertion can lead to fatigue, right? But I guess your brain is a very, very different organ, so, yeah, what what's been going on to test why this might be the case then?

Host: Nick Petrić Howe

So, what I will say first is there have been some previous studies that have shown sort of differences in like heart rate and that sort of thing as well when you're doing sort of mental work, but these have been really subtle, so it's been hard to pin down exactly what's going on here. But in this new study, what they've done is they've done something called magnetic resonance spectroscopy, and this allows you to get an idea of what's going on inside the brain. And they got some people to do, basically, a very menial but taxing task for about six hours to see what's going on. And they found that glutamate started to accumulate in a region of the brain that is to do with sort of cognitive control, so controlling your impulses and that sort of thing.

Host: Benjamin Thompson

Right, so doing this task over a long period of time leads to an increase in this chemical, and that's what's making you tired?

Host: Nick Petrić Howe

Yeah, that’s what they think. So, glutamate is an important signalling molecule in the brain. They’re exactly sure what's going on here. But it can disrupt brain functions, so having a lot of it may be just reducing the brain function in that particular region if you've been thinking hard for a very long time. But what some researchers who were interviewed for this article point out is this is correlative. There's not necessarily like a causal link between, but it seems like there's definitely an association between lots of glutamate and people feeling very tired, especially because they did a follow up to when the people had done this sort of mentally taxing task, and found that people who were mentally exhausted, they would take a quick and easy reward as compared to something that might require a bit more effort from them.

Host: Benjamin Thompson

Just because they were exhausted and they were like, ‘Do you know what, I'll just take an easy win right now.’

Host: Nick Petrić Howe

That’s what it seems to be, yeah. So, there's a lot more work to be done to better understand this. But the technique is really promising because it can give you a sort of look inside the brain, and people can start to understand the sort of metabolic processes that might be going on to understand this. And that could be really useful for helping people who have like mentally taxing jobs, like flight control or something, where they need to be alert, they need to be paying attention, they can't be mentally tired. And this could help them understand like, ‘Okay, how much of a break do you need for this glutamate to go away? Like how much of an effect does sleep have?’ and that sort of thing. But of course, there need to be animal studies and things to understand the actual causal link of what's going on here.

Host: Benjamin Thompson

Well, Nick, that is a fascinating story and I think to take it all in, I probably best go for a quick lie down right now.

Host: Nick Petrić Howe

And listeners, if you're not lying down and you'd like to hear more about these stories, and also find out where you can sign up for the Nature Briefing to get more stories like them direct to your inbox, you can check out the show notes.

Host: Benjamin Thompson

And that's all for this week. But before we go, just time for a couple of shoutouts. Firstly, we've got a new video about researchers engineering bacteria to form a range of complex patterns and shapes. It's one to be seen for sure, so look out for a link to it in the show notes. And Nick, you've also been busy making a brand-new podcast series.

Host: Nick Petrić Howe

That's right. This week, I published a new show called Nature’s Take. It’s a roundtable show where I get some clever people from Nature and sit them down and dive deep into a topic. For the first one, we were talking about preprints. Now, numbers of these articles surged during the pandemic, and we chat about the effects of this and how they're changing the game in science publishing. If that sounds up your street, you can find that wherever you found this podcast.

Host: Benjamin Thompson

Well, Nick it is a fascinating discussion, and one that I thoroughly recommend that listeners check out. We'll be back next week with another regular edition of the Nature Podcast, but don't forget, you can keep in touch with us on Twitter before then. We're @NaturePodcast. Or on email – [email protected]. I'm Benjamin Thompson.

Host: Nick Petrić Howe

And I'm Nick Petrić Howe. Thanks for listening.