MATT STRASSLER
Of Particular Significance

Tag: proton

The Standard Model More Deeply: How the Proton is Greater than the Sum of its Parts

The mass of a single proton, often said to be made of three quarks, is almost 1 GeV/c2. To be more precise, a proton’s mass is 0.938 GeV/c2, while that of a neutron is 0.939 GeV/c2.

But the masses of up and down quarks, found in protons and neutrons, are each much less than 0.01 GeV/c2. In short, the mass of each quark is less than one percent of a proton’s or neutron’s mass. If a proton were really made from three quarks, then there would seem to be a huge mismatch.

(Here and below, by “mass” I mean “rest mass” — an object’s intrinsic mass, which does not change with speed. It is sometimes called “invariant mass”. [Particle physicists usually just call it “mass”, though.])

Part of the explanation for the apparent discrepancy is that a proton or neutron is, in fact, made from far more than just three quarks. In its interior, one would find many gluons and a variety of quarks and anti-quarks. However, that doesn’t resolve the issue.

  • Gluons, like photons, have zero rest mass, so they don’t help at all, naively speaking.
  • The typical number of quarks and anti-quarks inside a proton, while more than three, is too small to add up to the proton’s full mass;

And thus one cannot explain the proton or neutron’s large mass as simply the sum of the masses of the objects inside it. The discrepancy remains.

Moreover, as can be verified using either strong theoretical arguments in analogous systems or direct numerical simulations, protons and neutrons would still have a substantial mass even if the quarks and anti-quarks they contain had none at all! Mass — from no mass.

Clearly, then, the solution to the puzzle lies elsewhere.

(more…)
Picture of POSTED BY Matt Strassler

POSTED BY Matt Strassler

ON July 22, 2024

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Celebrating the Standard Model: The Twins We’re Made Of

At the core of every atom lies its nucleus, where protons and neutrons are found. As their names suggest, these two subatomic particles are profoundly different.

  • Protons carry positive electric charge, and can attract negatively-charged electrons, making atoms possible.
  • Neutrons have no electric charge and are thus electrically neutral, hence their name; they have no impact on the electrons in atoms.

The distinctions extend to their magnetic effects. Both protons and neutrons have a “magnetic moment,” meaning that in a magnetic field, they will point like compasses. But neutrons point in the opposite direction from protons, and less agressively.

Nevertheless, the proton and neutron have almost identical masses, differing by less than two tenths of a percent! If we ignored their electric and magnetic effects, they’d almost be twins. Why are they so different in some ways and so similar in others? What does it reflect about nature?

(more…)
Picture of POSTED BY Matt Strassler

POSTED BY Matt Strassler

ON May 29, 2024

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Details Added to Monday’s Article

A couple of days ago, I posted an article describing how the size of a quantum object, such as a proton or electron, can be measured. This isn’t obvious. For example, scientists say that an electron spreads out and is wave-like, and yet that it has no size. This apparent contradiction needs resolution. While I addressed this puzzle in the book‘s chapter 17, I didn’t do so in detail, and so I wrote this article to fill in the gaps.

Now, in response to a reader’s question, I’ve added a section to the end of the article, entitled “Estimating the Object’s Size From Its Excited States”. There I explain in more detail how one goes from simple measurements, which confirm that a proton’s size isn’t zero, to an actual estimate of a proton’s size. The discussion is a little more technical than the rest of the article; you will probably need first-year physics to follow it. But I hope that some readers will find it useful!

Picture of POSTED BY Matt Strassler

POSTED BY Matt Strassler

ON May 1, 2024

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How Do You Measure a Quantum Object’s Size?

Quantum physics is certainly confusing.

  • On the one hand, electrons are wave-like and can be quite spread out; in fact, as I’ve emphasized in my book and in a recent blog post, a stationary electron is a spread-out standing wave. (I’ve even argued that these “elementary particles” should really be called “wavicles” [a term from the 1920s].)
  • On the other hand, scientists say that electrons have no size — or at least, if they have a size, it’s too small to be measured using current technology. They are often described as “point particles.”

How can both these things be true?

Well, to clarify this, let’s look at objects that do have an intrinsic size, such as protons and neutrons. How are their sizes actually determined? While this question is addressed in the book’s Chapter 17 (see Figure 40 and surrounding text, and footnote 2), I didn’t go into much detail there.

To supplement what’s in the book, I have written a webpage that outlines two classic methods that are used to measure the intrinsic size of a proton, or of any ultra-microscopic object.

When these same methods are used on electrons, one finds no evidence of any finite size, and so one concludes that their intrinsic size (if any) is too small to measure.

Picture of POSTED BY Matt Strassler

POSTED BY Matt Strassler

ON April 29, 2024

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W boson mass too high? Charm quarks in the proton? There’s a (worrisome) link.

Two of the most widely reported stories of the year in particle physics,

both depend crucially on our understanding of the fine details of the proton, as established to high precision by the NNPDF collaboration itself.  This large group of first-rate scientists starts with lots of data, collected over many years and in many experiments, which can give insight into the proton’s contents. Then, with a careful statistical analysis, they try to extract from the data a precision picture of the proton’s internal makeup (encoded in what is known as “Parton Distribution Functions” — that’s the PDF in NNPDF).  

NNPDF are by no means the first group to do this; it’s been a scientific task for decades, and without it, data from proton colliders like the Large Hadron Collider couldn’t be interpreted.   Crucially, the NNPDF group argues they have the best and most modern methods for the job  — NN stands for “neural network”, so it has to be good, right? 😉 — and that they carry it out at higher precision than anyone has ever done  before.

But what if they’re wrong? Or at least, what if the uncertainties on their picture of the proton are larger than they say?  If the uncertainties were double what NNPDF believes they are, then the claim of excess charm quark/anti-quark pairs in the proton — just barely above detection at 3 standard deviations — would be nullified, at least for now.  And even the claim of the W boson mass being different from the theoretical prediction,  which was argued to be a 7 standard deviation detection, far above “discovery” level, is in some question. In that mass measurement, the largest single source of systematic uncertainty is from the parton distribution functions.  A mere doubling of this uncertainty would reduce the discrepancy to 5 standard deviations, still quite large.  But given the thorny difficulty of the W mass measurement, any backing off from the result would certainly make people more nervous about it… and they are already nervous as it stands. (Some related discussion of these worries appeared in print here, with an additional concern here.)

In short, a great deal, both current and future, rides on whether the NNPDF group’s uncertainties are as small as they think they are.  How confident can we be?

(more…)
Picture of POSTED BY Matt Strassler

POSTED BY Matt Strassler

ON November 4, 2022

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An Extraordinarily Productive Visit to Fermilab

Fermilab’s main building at sunset. [Credit: M. Strassler]

This has been an exceptional few days, and I’ve had no time to breathe, much less blog. In pre-covid days, visits to the laboratories at CERN or Fermilab were always jam-packed with meetings, both planned and spontaneous, and with professional talks by experts visiting the labs. But many things changed during the pandemic. The vitality of labs like Fermilab and CERN depends on their many visitors, and so it is going to take time for them to recover from the isolation and work-from-home culture that covid-19 imposed on them.

My visit, organized by the LHC Physics Center [LPC], the US organizing center for the CMS experiment at the Large Hadron Collider [LHC], is my first trip to Fermilab since before 2020. I feared finding a skeleton crew, with many people working from home, and far fewer people traveling to Fermilab from other institutions. There is some truth in it; the place is a quieter than it was pre-2020. But nevertheless, the quality of the Fermilab staff and the visitors passing through has not declined. It is fair to say that in every meeting I’ve had and every presentation I have attended — and yesterday I started at 7:30 and ended at 4 without a single break — I have learned something new and important.

Today I’ll just give you a flavor of what I’ve learned; each one of these topics deserves a blog post all its own.

  • One Fermilab postdoc explained a new and very powerful technique for looking for long-lived particles at CMS, using parts of the CMS detector in a novel, creative way. Because it’s possible that the Higgs boson (or top quark, Z boson, W boson, bottom quark, or some unknown particle) can sometimes decay to a long-lived particle, which travels a macroscopic distance before decaying to a spray of other particles, this is an important scientific target. It’s one the existing LHC experiments weren’t really designed to study, but with a wide range of creative developments, they’ve developed an impressive range of techniques for doing so.
  • Another has a strategy for looking for certain decays of the Higgs boson that would be extremely difficult to find using standard techniques. Specifically, decays in which only hadrons are produced are very difficult to observe; hadrons are so abundant in collisions at the LHC that this is a signal drowned in background. But there is a possible way around this if the Higgs boson is kicked hard enough sideways in the collision.
  • A third is digging very deep into the challenging subject of low-energy muons and electrons. Particles with energy below 5 GeV become increasingly difficult to observe, for a whole host of reasons. But again, there can be decays of the Higgs boson (or other known particles) which would predominantly show themselves in these low-energy, difficult-to-identify muons or electrons. So this is a frontier where new ground needs to be broken.
  • A visiting expert taught me more about the technical meaning of “intrinsic charm”, which was widely over-reported as meaning that “there are charm quarks in the proton”. Understanding precisely what this means is quite subtle, even for a theoretical physics expert, and I’m still not in a position where I can explain it to you properly — though I did discuss it a closely related issue carefully. Moreover, he questions whether the story is actually correct — it depends on a claim of statistical errors being small enough, but he has doubts, and some evidence to support his doubts. (The same doubts, incidentally, potentially affect whether the difference of the W boson from the Standard Model prediction is really as significant as has been claimed.) In my opinion, it is not yet certain that there really is “intrinsic” charm in the proton. You can definitely expect another blog post about this!
  • Another visiting expert pointed out that in some limited but interesting cases, there could be very slowly-moving particles captured not only in the core of the Earth but also floating near its surface, a possible target for underground experiments that are sensitive to extremely low energy collisions of unknown particles with atoms.
  • Then there are the applications of machine learning in particle physics, which are increasingly being used in the complex environment of the LHC to make certain basic techniques of particle physics much more efficient. I heard about several very different examples, at least one of which (involving the identification of jets from bottom quarks) has already proven particularly successful.
  • A visiting CMS experimentalist pointed out to me that in a search through LHC data that she’d been involved in for many years, there are two surprising collisions observed with an extraordinary amount of energy, and very unusual (but similar) characteristics. It’s hard to quantify how unusual they are, but hopefully we will soon hear about a similar search at ATLAS, which could add or subtract weight from this observation. In any case, upcoming data from Run 3 will give us enough information, within a year or two, to see if this hint is actually of something real.
  • If these events aren’t a fluke and represent something real and new, then one of the local theorists at Fermilab is the fellow to talk to; back in 2018, when only one of these events had been observed, he and a couple of others thought through what the options are to explain where it might have come from. The options are unusual and would certainly be surprising to most theorists, but he convinced me that they’re not inconsistent with theoretical reasoning or with other data, so we should keep an open mind.
  • Yet another visiting theorist taught me about the possibility of non-linearities in quantum physics. Steven Weinberg tried to consider this possibility some time ago, but it turned out his approach violated causality; but now, inspired by old ideas of Joe Polchinski, there’s a new proposal to try this in another way. I’m grateful for that 45 minute conversation, at the end of which I felt pretty confident that I understood how the idea works. Now I can go off and think about it. When I understand its implications in some very simple settings (the only way I ever deeply understand anything), I’ll explain it to you.
  • Oh, and on top of this, I gave a talk on Tuesday, about powerful and sweeping strategies for searches in LHC data that haven’t yet been done, but ought to be, in my opinion. My ideas about this are 10-15 years old, but I have stronger arguments now that rely on Open Data. That of course led to a variety of follow-up conversations.


The visit’s not over; I’ve got one more day to try to drink from this fire-hose.

Picture of POSTED BY Matt Strassler

POSTED BY Matt Strassler

ON October 20, 2022

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Celebrating the Standard Model: The Electric Charges of Quarks

A post for general readers who’ve heard of quarks; if you haven’t, try reading here:

The universe has six types of quarks, some of which are found in protons and neutrons, and thus throughout all ordinary material. For no good reasons, we call them up, down, strange, charm, bottom and top. Today and tomorrow I want to show you how we know their electric charges, even though we can’t measure them directly. The only math we’ll need is addition, subtraction, and fractions.

This also intersects with my most recent post in this series on the Standard Model, which explained how we know that each type of quark comes in three “colors”, or versions — each one a type of strong nuclear charge akin to electric charge.

Today we’ll review the usual lore that you can find in any book or on any website, but we’ll see that there’s a big loophole in the lore that we need to close. Tomorrow we’ll use a clever method to close that loophole and verify the lore is really true.

The Lore for Protons and Neutrons

Physicists usually define electric charge so that

  • the proton has electric charge +1
  • the electron has charge -1,
  • the neutron has charge 0 (i.e. electrically neutral, hence its name).

[Throughout the remainder of this post, I’ll abbreviate “electric charge” as simply “charge“.]

As for the six types of quarks, the lore is that their charges are [using notation that “Qu” means “electric charge of the u quark“]:

  • Up, Charm, Top (u,c,t): Qu = Qc = Qt = 2/3
  • Down, Strange, Bottom (d,s,b): Qd = Qs = Qb = -1/3

But how do we know this?

(more…)
Picture of POSTED BY Matt Strassler

POSTED BY Matt Strassler

ON August 25, 2022

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Celebrating the Standard Model: Atoms, Quarks and the Strong Nuclear Force

For the general reader:

Last week I showed you, without any technicalities, how to recognize the elementary forces of nature in the pattern of particle masses and lifetimes. This week we’ll start seeing what we can extract just from the particles’ masses alone… and what we cannot. Today we’ll focus on quarks and the strong nuclear force.

A key factor in nature, which plays an enormous role in everyday life, is the mass of a typical atom. [Note: on this website, “mass” always means “rest mass”, which does not increase with a particle’s speed.] This in turn arises mainly from the masses of protons and neutrons, which are about equal, and tiny: about 0.00000000000000000000000000167 kg (or 0.00000000000000000000000000368 pounds). Since those numbers are crazy-small, physicists use a different measure; we say the mass is about 1 GeV/c2, and more precisely, 0.938 GeV/c2. In any case, it’s tiny on human scales, but it’s some definite quantity, the same for every proton in nature. Where does this mass come from; what natural processes determine it?

You may have heard the simplistic remark that “a proton is made of three quarks” (two up quarks and a down quark), which would suggest these quarks have mass of about 1/3 of a proton, or about 0.313 GeV/c2. But something’s clearly amiss. If you look at websites and other sources about particle physics, they all agree that up and down quark masses are less than 0.01 GeV/c2; these days they usually say the up quark has mass of 0.002 GeV/c2 and the down quark has 0.005 GeV/c2. So if the proton were simply made of three quarks, it would naively have a mass of less than 1% of its actual mass.

What’s going on? A first little clue is that different sources sometimes quote different numbers for the quark masses. There are six types of quarks; from smallest mass to largest, they are up, down, strange (u,d,s, the three light quarks), charm, bottom (c,b, the two somewhat heavy quarks) and top (t, the super-heavy quark.) [Their names, by the way, are historical accidents and don’t mean anything.] But some websites say the up quark mass is 0.003 instead of 0.002 GeV/c2, a 50% discrepancy; the bottom quark’s mass is variously listed as 4.1 GeV/c2, 4.5 GeV/c2, and so forth. This is in contrast to, say, the electron’s mass; you’ll never see websites that disagree about that.

The origin of all these discrepancies is that quarks (and anti-quarks and gluons) are affected by the strong nuclear force, unlike electrons, Higgs bosons, and all the other known elementary particles. The strong forces that quarks undergo make everything about them less clear and certain. Among numerous manifestations, the most dramatic is that quarks (and anti-quarks and gluons) are never observed in isolation. Instead they’re always found in special combinations, called “hadrons“. A proton is an example, but there are many more. And the strong nuclear force can have a big effect on their masses.

The Modern Proton and the Masses of Quarks

A proton, in fact, is not simply made from three quarks, the way a hydrogen atom is simply made from a proton and an electron. As I described in this article, it’s vastly more complex; it’s made from three quarks plus lots of gluons plus lots of pairs of other quarks and anti-quarks. So the simple intuition we get from atoms does not apply to hadrons like the proton.

(more…)
Picture of POSTED BY Matt Strassler

POSTED BY Matt Strassler

ON July 14, 2022

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If It Holds Up, What Might BICEP2’s Discovery Mean?

Well, yesterday was quite a day, and I’m still sifting through the consequences.

First things first.  As with all major claims of discovery, considerable caution is advised until the BICEP2 measurement has been verified by some other experiment.   Moreover, even if the measurement is correct, one should not assume that the interpretation in terms of gravitational waves and inflation is correct; this requires more study and further confirmation.

The media is assuming BICEP2’s measurement is correct, and that the interpretation in terms of inflation is correct, but leading scientists are not so quick to rush to judgment, and are thinking things through carefully.  Scientists are cautious not just because they’re trained to be thoughtful and careful but also because they’ve seen many claims of discovery withdrawn or discredited; discoveries are made when humans go where no one has previously gone, with technology that no one has previously used — and surprises, mistakes, and misinterpretations happen often.

But in this post, I’m going to assume assume assume that BICEP2’s results are correct, or essentially correct, and are being correctly interpreted.  Let’s assume that [here’s a primer on yesterday’s result that defines these terms]

  • they really have detected “B-mode polarization” in the “CMB” [Cosmic Microwave Background, the photons (particles of light) that are the ancient, cool glow leftover from the Hot Big Bang]
  • that this B-mode polarization really is a sign of gravitational waves generated during a brief but dramatic period of cosmic inflation that immediately preceded the Hot Big Bang,

Then — IF BICEP2’s results were basically right and were being correctly interpreted concerning inflation — what would be the implications?

Well… Wow…  They’d really be quite amazing. (more…)

Picture of POSTED BY Matt Strassler

POSTED BY Matt Strassler

ON March 18, 2014

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Dog Brains and Fishing Line: 2 Fun Articles

Nothing about quantum physics today, but … wait, everything is made using quantum physics…

—

Could you imagine getting a dog to sit absolutely still, while fully awake and listening to voices, for as much as 8 minutes? Researchers trained dogs to do it, then put them in an MRI [Magnetic Resonance Imaging] machine to obtain remarkable studies of how dogs’ brains react to human voices and other emotional forms of human expression. [MRI is all about magnetic fields, protons, spin, and resonance; particle physics!! more on that another time, perhaps.]   The authors claim this is the first study of its type to compare human brains to those of a non-primate species. Here’s something from the scientific article’s abstract:

…We presented dogs and humans with the same set of vocal and non-vocal stimuli to search for functionally analogous voice-sensitive cortical regions. We demonstrate that voice areas exist in dogs and that they show a similar pattern to anterior temporal voice areas in humans. Our findings also reveal that sensitivity to vocal emotional valence cues engages similarly located non-primary auditory regions in dogs and humans… 

So it seems, as dog owners have long suspected, that we’re not just imagining that our best friends are aware of our moods; they really are similar to us in some important ways.

Here’s a BBC article: http://www.bbc.co.uk/news/science-environment-26276660

—

Once you’re done with that, would you like to build up your muscles?  No exercise needed, just call the University of Texas at Dallas.  They’ve found that “ordinary fishing line and sewing thread can be cheaply converted to powerful artificial muscles.  The new muscles can lift 100 times more weight and generate 100 times higher mechanical power than a human muscle of the same length and weight… The muscles are powered thermally by temperature changes, which can be produced electrically, by the absorption of light or by the chemical reaction of fuels.”  [Quantum Physics = cool!!] The quotation above is from an interesting press release from the university, reporting the research which was just published in the journal Science.  I recommend the press release because it mentions several interesting possible applications, including robotics technology  and clothing that adjusts to temperature.  Here’s also a nice article by Anna Kuchment (who’s on Twitter here):

http://www.utdallas.edu/news/2014/2/21-28701_Researchers-Create-Powerful-Muscles-From-Fishing-L_story-wide.html

—

Though evolution left us with many wonderful abilities, it does seem that, year by year, humans are becoming less and less practically useful.   But at least our dogs will comfort us in our obsolescence.

 

Picture of POSTED BY Matt Strassler

POSTED BY Matt Strassler

ON February 21, 2014

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