Any developments on the origin of mass by QCD?

In summary: MeV.Overall, the results of these QCD-based mass calculations are consistent with the Standard Model predictions and with the experimental measurements to date.In summary, the weight of the world is quantum chromodynamics.
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Kattenbach
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Below I refer to the following article:

The Weight of the World Is Quantum Chromodynamics
Andreas S. Kronfeld
Science
21 November 2008
Vol.: 322, Issue 5905 - pp. 1198-1199


I've been trying to find more information on the subject but it's been hard for a Google peasant like myself.

Just the other day I was talking to someone about the mass produced by the Higgs field and got it confused in my memory, so I went back and looked for this article and it clarified my mistake, I was confusing the Higgs generated mass with the virtual mass from QCD due to Quark Gluon interaction.

But still, that made me wonder if the findings from this article on Science are still holding strong or what else had advanced in the subject after so many years and the LHC going full force.

This is the article on Science: https://science.sciencemag.org/content/322/5905/1198
This is the pdf version of the article available for free: http://karman3.elte.hu/janosi/pdf_pub_H/sci08qcd-cikk.pdf
 
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In the Science article is the statement, "Nucleons, in turn, are composed of particles called quarks and gluons, and physicists have long believed that the nucleon’s mass comes from the complicated way in which gluons bind the quarks to each other, according to the laws of quantum chromodynamics (QCD)."

On this page, https://www.symmetrymagazine.org/article/where-does-mass-come-from (2016), is the statement: "The Higgs field gives mass to fundamental particles—the electrons, quarks and other building blocks that cannot be broken into smaller parts. But these still only account for a tiny proportion of the universe’s mass.

The rest comes from protons and neutrons, which get almost all their mass from the strong nuclear force. These particles are each made up of three quarks moving at breakneck speeds that are bound together by gluons, the particles that carry the strong force. The energy of this interaction between quarks and gluons is what gives protons and neutrons their mass."

Also, see the comment: " “As it turns out, the down quarks interact more strongly with the Higgs [field], so they have a bit more mass,” says Andreas Kronfeld, a theoretical physicist at Fermilab. This is why the tiny difference between proton and neutron mass exists."

See also - QCD and the Origin of Mass (https://www.kitp.ucsb.edu/activities/akronfeld15)

Frank Wilczek (2012) notes on Origins of Mass: http://web.mit.edu/8.701/www/Lecture Notes/8.701originsOfMass04FA13.pdf

Additional historical note:"On the history of the strong interaction" by H. Leutwyler
https://arxiv.org/pdf/1211.6777.pdf
 
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The Higgs discovery might be somewhat recent but it was expected to be there for decades. Older books are still fine, they just won't tell you the mass of the Higgs boson and some related properties.
 
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Thats the first explanation for the slight mass difference of protons and neutrons Iv heard, are there any other theories?
 
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Thats great thanks. I thought the neutron being slightly less mass may give rise to the 'fundemental' electron in some way but I guess the math will give way first! Just divide everything by 137 and see what happens lol.
 
  • #7
Kattenbach said:
But still, that made me wonder if the findings from this article on Science are still holding strong or what else had advanced in the subject after so many years and the LHC going full force.

This is still holding strong. (High energy physicist Matt Strassler has another good mid-level multipart description from 2013 at his regularly updated webpage that he has seen no need to update since then since the relevant science hasn't changed.)

QCD Hadron Mass Calculations Continue To Work Well, Albeit To Low Precision

It is possible to compute the mass of the proton and neutron from QCD first principles to a precision greater than 1% but less than 0.1%, that is consistent with experiment.

Similarly, the difference between the rest mass of the neutron and the rest mass of a proton is 1.293,332,2(4) MeV. There are two predominant factors that drive the difference in mass between protons and neutrons in the Standard Model - one is the mass difference between the up quark and the down quark, and the other is a quantum electrodynamics (QED) contribution. Otherwise, given symmetry considerations, the proton and the neutron would have the same mass in the Standard Model, since the strength of the strong force interactions of an up quark and the strength of the strong force interactions of a down quark are the same in Standard Model QCD.

The QED contribution reflects the difference electromagnetic charges of the valence quarks in protons and neutrons, respectively, which in turn impacts the mass-energy contribution of the electromagnetic and strong force fields within each nucleon to the total mass of the nucleon. The QED contribution to the difference between the proton mass and neutron mass has been computed to ± 0.11 MeV precision using QED and this is one of several methods (which have produced consistent results) that has been used to in turn determine the difference in mass between the up quark and the down quark to the same precision since for all practical purposes the masses of the proton and neutron are known exactly.

In particular (and this may not be the latest paper on point), a June 18, 2014 paper estimates that differences in electromagnetic field strength between the proton and neutron account for 1.04 +/- 0.11 MeV of the proton-neutron mass difference using lattice QCD methods; the proton gets more of its mass from its electromagnetic field than the neutron does. (An April 11, 2014 power point description of this paper is also available.) The same paper estimates, using this calculation, that the difference between the up quark mass and the down quark mass is 2.33 +/- 0.11 MeV, which was the most precise estimate of the up quark-down quark mass difference as of 2014. This paper replicated the result of another paper released in pre-print form by an independent group of lattice QCD investigators just two days earlier on June 16, 2014, using a moderately different approach, so the result can be considered quite reliable. Both results exploit the Coleman-Glashow relation (which dates to at least 1982 or earlier) which argues that the sum of the mass differences in three different pairs of charged and neutral hadrons (one of which is the proton and neutron pair) with carefully chosen combinations of properties that cancel out due to symmetries, should equal zero. This hypothesis is confirmed experimentally true to the current limits of experimental precision (something that a http://www.researchgate.net/publication/222556523_On_the_miracle_of_the_ColemanGlashow_and_other_baryon_mass_formulas called a "miracle", but which flows quite naturally from the quark model of QCD and symmetry considerations).

The precision of first principles calculations of the masses of hadrons (i.e. composite particles bound by the strong force which are basically made up of color charged quarks and gluons) that are heavier than the proton and neutron is better, typically on the order of 0.1%, and is consistent with experimental measurements.

The greater precision in determining the masses of heavier hadrons is, in part, because in a heavier hadron, much more of the total mass comes from the quark masses that have been determined fairly precisely with lattice QCD methods with data sets involving all of the kinds of hadrons that have the quark in question as a valence quark, and proportionately less of the total mass comes from QCD in which it is harder to determine precisely, and has to be calculated on a case by case basis (apart from some shortcut methods like sum rule calculations like utilization of the Coleman-Glashow relation). The masses of the heavier hadrons aren't measured to the same exquisite 32 parts per trillion precision as the proton mass but these masses have still been measured to greater precision than QCD calculations of what these masses should be (typically to at least parts per several thousand to ten thousand, and often better for hadrons that are most commonly seen at colliders).

I review the high energy physics arXiv preprints almost every day and see at least several new accurate hadron mass calculations done using QCD every month; dozens a year, and the track record of the accuracy of these calculations is quite good (although different approaches to operationalizing QCD such as light front method, chiral methods, sum rules, etc. can sometimes differ appreciably and there is a fair amount of art and expertise that goes into knowing the strengths and flaws of each method in order to choose the best one to estimate the mass of a particular hadron).

One of the latest LHC related developments is that there have been confirmed experimental observations of tetraquarks and pentaquarks (i.e. hadrons with four and five valence quarks respectively as opposed to the three valence quarks of an ordinary baryon and the two valence quarks of an ordinary meson). QCD phenomenologists have successfully generalized the QCD techniques developed to calculated the masses of mesons and baryons to calculate the masses of tetraquarks and pentaquarks, confirming the conclusion that the theoretical basis in QCD of the source of hadron mass is robust and correct, rather than merely being, for example, the product of overfitting a finite data set (there are roughly two hundred possible meson ground states and about two hundred and sixteen possible baryon ground states, most of which, in both cases, have been observed).

Another area of continued work in QCD phenomenology which is older but still fairly vigorous is the computation of the masses of excited states of hadrons (especially mesons) from first principles using QCD. These calculations are also consistently confirmed by experimental observations at the LHC although this too involves a certain amount of art and experience in the process of choosing the best method for operationalizing QCD for a particular excited hadron, and this also involves more art and experience on the experimental side to correctly classified an observed resonance in experimental results as the correct excited state of the correct hadron. When a new resonance is observed at the LHC, there is frequently a burst of half a dozen to a dozen or more papers over the course of the next few months to a couple of years or more debating which possible hadron ground state or excited state it corresponds to given its observed quantum numbers, mass, width (i.e. mean lifetime), and decay patterns.

Almost No High Energy Physicists Seriously Doubt That Standard Model QCD Is Correct

QCD calculations are far less precise than QED and weak force calculation in the Standard Model, and yet, dozens of new papers are released each week proposing one kind of beyond the Standard Model physics or another.

But there is almost no one in the high energy physics community out there who is proposing that the plain old Standard Model QCD (including the way it is used to determine hadron mass) is flawed or needs modification. I can't even think of a single notable proposal to modify it (in contrast, for example, with multiple proposals to rethink the Standard Model Higg mechanism which new LHC data continue to disfavor for the most part).

QCD reliably explains the portion of hadron masses not attributable to either valence quark masses or QED fields, pretty much without exception (in all cases where we understand the structure of the hadrons in question well enough to apply QCD to calculate their masses).

Also, as the examples of the muon g-2 anomaly, the recent outlier CDF measurement of the W boson mass, the superluminal neutrino false alarm at the Opera experiment, and the apparent violation of lepton universality seen in semi-leptonic B meson decays illustrate, in most of high energy physics, any time is an experimental result that is hard to explain using the unmodified Standard Model, there is a rush of papers proposing every pet theory of New Physics under the sun to explain it.

But, in QCD, even though there are a fair number of scalar meson and axial vector meson resonances for which it is hard to come up with a straight forward and obvious explanation for their structure, that have been a persistent thorn in the side of HEP researchers for more than forty years, almost all of the papers attempting to understand these unsolved problems seek to explain them purely with plain vanilla Standard Model QCD without resorting to New Physics.

The Bottom Line

The bottom line is that "the findings from this article on Science are still holding strong", notwithstanding advances in the subject after many years of the LHC and other HEP physics experiments around the world going full force. While there may be a stray speculative paper asking questions, like the one cited above, the way that QCD gives rise to mass is considered a solved problem of high energy physics in the general case (even if there are particular possible hadrons for which precision calculations, which are extremely difficult, have yet to be made).

Indeed, by discovering the Higgs boson in a paper released in 2012, and confirming with every published result in the subsequent decade that the observed Higgs boson has properties perfectly consistent with the minimal Standard Model Higgs boson pre-2012, which is the predominant non-QCD source of mass in the universe, the findings in the Science article cited about the sources of mass of ordinary stuff in the universe (putting aside issues of dark matter and dark energy that aren't well understood in a consensus fashion) have grown more confident, and the room for alternative explanations consistent with observations has narrowed greatly.

The Limiting Factor Is Computational Capacity, Not Insufficient Experimental Data

It is also worth pointing out that an underlying assumption of the question, which is that new experimental results the like new data from the LHC are the main factor driving our understanding of how QCD gives rise to mass, really isn't quite right.

There has never been a time in the era since quarks were discovered that experimental measurements of hadron masses have not been significantly more precise than QCD calculations of what those masses should be.

For example, the experimentally measured mass of the proton has been determined "with a precision of 32 parts-per trillion." See Fabian Heiße, et al., "High-precision measurement of the proton's atomic mass" (June 21, 2017). But the first principles calculation of the proton mass using QCD has a precision of less than one part-per thousand, which is about eight orders of magnitude (a factor of ten million) less precise.

We could never do another experimental measurement of a hadron mass and we would still have more than enough experimental data to confirm or rule out the accuracy of any QCD hadron mass calculation that will be possible for the foreseeable future.

Also, half a century after quarks were discovered, we still have never been able to observe a quark other than a top quark that is not confined in a hadron in a context in which it is possible to measure the mass of a free quark directly (quark confinement can break down at extremely high energies where quark-gluon plasma forms, and the LHC has briefly created tiny amounts of quark-gluon plasma in its run, but it isn't feasible to measure the masses of deconfined quarks in quark-gluon plasma). So, extracting the quark mass component and the QCD gluonic contribution to hadron masses (including the strength of the strong force coupling constant which is a factor in basically every single QCD calculation) each requires detailed QCD calculations.

The practical limiting factor to calculating hadron masses using QCD and to using precise calculations made in this manner to compare to experimental results and thereby test to accuracy of the nearly universally presumed to be true plain vanilla Standard Model QCD orthodoxy in order to test the theory, is our practical ability to do these very cumbersome calculations that can take months or years with multiple state of the art supercomputers (that could do comparable QED or weak force calculations in a manner of minutes or hours) under supervision from teams of elite high energy physicists, is computational capacity.

So, really, the key to making a break though or finding new developments is Moore's Law regarding the rate at which computer power has grown which has held since 1975, the emerging "quantum leap" in computational capacity associated with quantum computing that is just starting to become something more than vaporware in the last few years, and the efforts of "amplitudologists" (i.e. physicists who figure out how to do the kind of calculations necessary to do Standard Model physics calculations which involves an immense number of terms each of which must be solved individually using the calculus of complex numbers in four dimensions unless you kind find a way to make terms cancel out of streamline the computation from this brute force approach) to come up with streamlined ways that more efficiently calculate quantities that we now tackle with brute force computation.

Historically, it has taken decades to make each significant digit of improvement in the accuracy of QCD calculations. But to really kick the tires properly and confirm that our theoretical understanding is accurately aligned with reality, we really need to be making precision calculations that have perhaps three to six more significant digits, at least, and each new significant digit of precision in QCD calculations takes exponentially more computational effort to produce.
 
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1. What is QCD?

QCD stands for Quantum Chromodynamics, which is a theory that describes the interactions between quarks and gluons, the fundamental particles that make up protons, neutrons, and other hadrons.

2. How does QCD relate to the origin of mass?

QCD is one of the fundamental theories in particle physics that helps explain the origin of mass. According to the theory, the mass of particles comes from the energy of the strong nuclear force that binds quarks together in protons and neutrons.

3. What developments have been made in understanding the origin of mass by QCD?

In recent years, scientists have made significant progress in understanding the origin of mass by QCD. This includes the discovery of the Higgs boson, which is responsible for giving mass to particles, as well as advancements in theoretical calculations and experiments to test QCD predictions.

4. How does QCD explain the mass of different particles?

QCD explains the mass of different particles by considering the mass of their constituent quarks and the energy of the strong nuclear force that holds them together. The mass of a particle is a combination of these two factors, with the strong force playing a larger role in determining the mass of heavier particles.

5. Are there any remaining mysteries about the origin of mass by QCD?

While significant progress has been made, there are still some mysteries surrounding the origin of mass by QCD. For example, the exact mechanism by which the Higgs boson gives mass to particles is still not fully understood. Scientists continue to conduct research and experiments to further our understanding of this fundamental aspect of particle physics.

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