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  • richardmitnick 11:26 am on May 5, 2019 Permalink | Reply
    Tags: 'Where Does A Proton’s Mass Come From?', 99.8% of the proton’s mass comes from gluons, , Antiquarks, Asymptotic freedom: the particles that mediate this force are known as gluons., , , , , Gluons, , , , , , The production of Higgs bosons is dominated by gluon-gluon collisions at the LHC, , The strong interaction is the most powerful interaction in the entire known Universe.   

    From Ethan Siegel: “Ask Ethan: ‘Where Does A Proton’s Mass Come From?'” 

    From Ethan Siegel
    May 4, 2019

    The three valence quarks of a proton contribute to its spin, but so do the gluons, sea quarks and antiquarks, and orbital angular momentum as well. The electrostatic repulsion and the attractive strong nuclear force, in tandem, are what give the proton its size, and the properties of quark mixing are required to explain the suite of free and composite particles in our Universe. (APS/ALAN STONEBRAKER)

    The whole should equal the sum of its parts, but doesn’t. Here’s why.

    The whole is equal to the sum of its constituent parts. That’s how everything works, from galaxies to planets to cities to molecules to atoms. If you take all the components of any system and look at them individually, you can clearly see how they all fit together to add up to the entire system, with nothing missing and nothing left over. The total amount you have is equal to the amounts of all the different parts of it added together.

    So why isn’t that the case for the proton? It’s made of three quarks, but if you add up the quark masses, they not only don’t equal the proton’s mass, they don’t come close. This is the puzzle that Barry Duffey wants us to address, asking:

    “What’s happening inside protons? Why does [its] mass so greatly exceed the combined masses of its constituent quarks and gluons?”

    In order to find out, we have to take a deep look inside.

    The composition of the human body, by atomic number and by mass. The whole of our bodies is equal to the sum of its parts, until you get down to an extremely fundamental level. At that point, we can see that we’re actually more than the sum of our constituent components. (ED UTHMAN, M.D., VIA WEB2.AIRMAIL.NET/UTHMAN (L); WIKIMEDIA COMMONS USER ZHAOCAROL (R))

    There’s a hint that comes just from looking at your own body. If you were to divide yourself up into smaller and smaller bits, you’d find — in terms of mass — the whole was equal to the sum of its parts. Your body’s bones, fat, muscles and organs sum up to an entire human being. Breaking those down further, into cells, still allows you to add them up and recover the same mass you have today.

    Cells can be divided into organelles, organelles are composed of individual molecules, molecules are made of atoms; at each stage, the mass of the whole is no different than that of its parts. But when you break atoms into protons, neutrons and electrons, something interesting happens. At that level, there’s a tiny but noticeable discrepancy: the individual protons, neutrons and electrons are off by right around 1% from an entire human. The difference is real.

    From macroscopic scales down to subatomic ones, the sizes of the fundamental particles play only a small role in determining the sizes of composite structures. Whether the building blocks are truly fundamental and/or point-like particles is still not known. (MAGDALENA KOWALSKA / CERN / ISOLDE TEAM)


    Like all known organisms, human beings are carbon-based life forms. Carbon atoms are made up of six protons and six neutrons, but if you look at the mass of a carbon atom, it’s approximately 0.8% lighter than the sum of the individual component particles that make it up. The culprit here is nuclear binding energy; when you have atomic nuclei bound together, their total mass is smaller than the mass of the protons and neutrons that comprise them.

    The way carbon is formed is through the nuclear fusion of hydrogen into helium and then helium into carbon; the energy released is what powers most types of stars in both their normal and red giant phases. That “lost mass” is where the energy powering stars comes from, thanks to Einstein’s E = mc². As stars burn through their fuel, they produce more tightly-bound nuclei, releasing the energy difference as radiation.

    In between the 2nd and 3rd brightest stars of the constellation Lyra, the blue giant stars Sheliak and Sulafat, the Ring Nebula shines prominently in the night skies. Throughout all phases of a star’s life, including the giant phase, nuclear fusion powers them, with the nuclei becoming more tightly bound and the energy emitted as radiation coming from the transformation of mass into energy via E = mc². (NASA, ESA, DIGITIZED SKY SURVEY 2)

    NASA/ESA Hubble Telescope

    ESO Online Digitized Sky Survey Telescopes

    Caltech Palomar Samuel Oschin 48 inch Telescope, located in San Diego County, California, United States, altitude 1,712 m (5,617 ft)

    Australian Astronomical Observatory, Siding Spring Observatory, near Coonabarabran, New South Wales, Australia, 1.2m UK Schmidt Telescope, Altitude 1,165 m (3,822 ft)

    From http://archive.eso.org/dss/dss

    This is how most types of binding energy work: the reason it’s harder to pull apart multiple things that are bound together is because they released energy when they were joined, and you have to put energy in to free them again. That’s why it’s such a puzzling fact that when you take a look at the particles that make up the proton — the up, up, and down quarks at the heart of them — their combined masses are only 0.2% of the mass of the proton as a whole. But the puzzle has a solution that’s rooted in the nature of the strong force itself.

    The way quarks bind into protons is fundamentally different from all the other forces and interactions we know of. Instead of the force getting stronger when objects get closer, like the gravitational, electric, or magnetic forces, the attractive force goes down to zero when quarks get arbitrarily close. And instead of the force getting weaker when objects get farther away, the force pulling quarks back together gets stronger the farther away they get.

    The internal structure of a proton, with quarks, gluons, and quark spin shown. The nuclear force acts like a spring, with negligible force when unstretched but large, attractive forces when stretched to large distances. (BROOKHAVEN NATIONAL LABORATORY)

    This property of the strong nuclear force is known as asymptotic freedom, and the particles that mediate this force are known as gluons. Somehow, the energy binding the proton together, responsible for the other 99.8% of the proton’s mass, comes from these gluons. The whole of matter, somehow, weighs much, much more than the sum of its parts.

    This might sound like an impossibility at first, as the gluons themselves are massless particles. But you can think of the forces they give rise to as springs: asymptoting to zero when the springs are unstretched, but becoming very large the greater the amount of stretching. In fact, the amount of energy between two quarks whose distance gets too large can become so great that it’s as though additional quark/antiquark pairs exist inside the proton: sea quarks.

    When two protons collide, it isn’t just the quarks making them up that can collide, but the sea quarks, gluons, and beyond that, field interactions. All can provide insights into the spin of the individual components, and allow us to create potentially new particles if high enough energies and luminosities are reached. (CERN / CMS COLLABORATION)

    Those of you familiar with quantum field theory might have the urge to dismiss the gluons and the sea quarks as just being virtual particles: calculational tools used to arrive at the right result. But that’s not true at all, and we’ve demonstrated that with high-energy collisions between either two protons or a proton and another particle, like an electron or photon.

    The collisions performed at the Large Hadron Collider at CERN are perhaps the greatest test of all for the internal structure of the proton. When two protons collide at these ultra-high energies, most of them simply pass by one another, failing to interact. But when two internal, point-like particles collide, we can reconstruct exactly what it was that smashed together by looking at the debris that comes out.

    A Higgs boson event as seen in the Compact Muon Solenoid detector at the Large Hadron Collider. This spectacular collision is 15 orders of magnitude below the Planck energy, but it’s the precision measurements of the detector that allow us to reconstruct what happened back at (and near) the collision point. Theoretically, the Higgs gives mass to the fundamental particles; however, the proton’s mass is not due to the mass of the quarks and gluons that compose it. (CERN / CMS COLLABORATION)

    Under 10% of the collisions occur between two quarks; the overwhelming majority are gluon-gluon collisions, with quark-gluon collisions making up the remainder. Moreover, not every quark-quark collision in protons occurs between either up or down quarks; sometimes a heavier quark is involved.

    Although it might make us uncomfortable, these experiments teach us an important lesson: the particles that we use to model the internal structure of protons are real. In fact, the discovery of the Higgs boson itself was only possible because of this, as the production of Higgs bosons is dominated by gluon-gluon collisions at the LHC. If all we had were the three valence quarks to rely on, we would have seen different rates of production of the Higgs than we did.

    Before the mass of the Higgs boson was known, we could still calculate the expected production rates of Higgs bosons from proton-proton collisions at the LHC. The top channel is clearly production by gluon-gluon collisions. I (E. Siegel) have added the yellow highlighted region to indicate where the Higgs boson was discovered. (CMS COLLABORATION (DORIGO, TOMMASO FOR THE COLLABORATION) ARXIV:0910.3489)

    As always, though, there’s still plenty more to learn. We presently have a solid model of the average gluon density inside a proton, but if we want to know where the gluons are actually more likely to be located, that requires more experimental data, as well as better models to compare the data against. Recent advances by theorists Björn Schenke and Heikki Mäntysaari may be able to provide those much needed models. As Mäntysaari detailed:

    “It is very accurately known how large the average gluon density is inside a proton. What is not known is exactly where the gluons are located inside the proton. We model the gluons as located around the three [valence] quarks. Then we control the amount of fluctuations represented in the model by setting how large the gluon clouds are, and how far apart they are from each other. […] The more fluctuations we have, the more likely this process [producing a J/ψ meson] is to happen.”

    A schematic of the world’s first electron-ion collider (EIC). Adding an electron ring (red) to the Relativistic Heavy Ion Collider (RHIC) at Brookhaven would create the eRHIC: a proposed deep inelastic scattering experiment that could improve our knowledge of the internal structure of the proton significantly. (BROOKHAVEN NATIONAL LABORATORY-CAD ERHIC GROUP)

    The combination of this new theoretical model and the ever-improving LHC data will better enable scientists to understand the internal, fundamental structure of protons, neutrons and nuclei in general, and hence to understand where the mass of the known objects in the Universe comes from. From an experimental point of view, the greatest boon would be a next-generation electron-ion collider, which would enable us to perform deep inelastic scattering experiments to reveal the internal makeup of these particles as never before.

    But there’s another theoretical approach that can take us even farther into the realm of understanding where the proton’s mass comes from: Lattice QCD.

    A better understanding of the internal structure of a proton, including how the “sea” quarks and gluons are distributed, has been achieved through both experimental improvements and new theoretical developments in tandem. (BROOKHAVEN NATIONAL LABORATORY)

    The difficult part with the quantum field theory that describes the strong force — quantum chromodynamics (QCD) — is that the standard approach we take to doing calculations is no good. Typically, we’d look at the effects of particle couplings: the charged quarks exchange a gluon and that mediates the force. They could exchange gluons in a way that creates a particle-antiparticle pair or an additional gluon, and that should be a correction to a simple one-gluon exchange. They could create additional pairs or gluons, which would be higher-order corrections.

    We call this approach taking a perturbative expansion in quantum field theory, with the idea that calculating higher and higher-order contributions will give us a more accurate result.

    Today, Feynman diagrams are used in calculating every fundamental interaction spanning the strong, weak, and electromagnetic forces, including in high-energy and low-temperature/condensed conditions. But this approach, which relies on a perturbative expansion, is only of limited utility for the strong interactions, as this approach diverges, rather than converges, when you add more and more loops for QCD.(DE CARVALHO, VANUILDO S. ET AL. NUCL.PHYS. B875 (2013) 738–756)

    Richard Feynman © Open University

    But this approach, which works so well for quantum electrodynamics (QED), fails spectacularly for QCD. The strong force works differently, and so these corrections get very large very quickly. Adding more terms, instead of converging towards the correct answer, diverges and takes you away from it. Fortunately, there is another way to approach the problem: non-perturbatively, using a technique called Lattice QCD.

    By treating space and time as a grid (or lattice of points) rather than a continuum, where the lattice is arbitrarily large and the spacing is arbitrarily small, you overcome this problem in a clever way. Whereas in standard, perturbative QCD, the continuous nature of space means that you lose the ability to calculate interaction strengths at small distances, the lattice approach means there’s a cutoff at the size of the lattice spacing. Quarks exist at the intersections of grid lines; gluons exist along the links connecting grid points.

    As your computing power increases, you can make the lattice spacing smaller, which improves your calculational accuracy. Over the past three decades, this technique has led to an explosion of solid predictions, including the masses of light nuclei and the reaction rates of fusion under specific temperature and energy conditions. The mass of the proton, from first principles, can now be theoretically predicted to within 2%.

    As computational power and Lattice QCD techniques have improved over time, so has the accuracy to which various quantities about the proton, such as its component spin contributions, can be computed. By reducing the lattice spacing size, which can be done simply by raising the computational power employed, we can better predict the mass of not only the proton, but of all the baryons and mesons. (LABORATOIRE DE PHYSIQUE DE CLERMONT / ETM COLLABORATION)

    It’s true that the individual quarks, whose masses are determined by their coupling to the Higgs boson, cannot even account for 1% of the mass of the proton. Rather, it’s the strong force, described by the interactions between quarks and the gluons that mediate them, that are responsible for practically all of it.

    The strong nuclear force is the most powerful interaction in the entire known Universe. When you go inside a particle like the proton, it’s so powerful that it — not the mass of the proton’s constituent particles — is primarily responsible for the total energy (and therefore mass) of the normal matter in our Universe. Quarks may be point-like, but the proton is huge by comparison: 8.4 × 10^-16 m in diameter. Confining its component particles, which the binding energy of the strong force does, is what’s responsible for 99.8% of the proton’s mass.

    See the full article here .


    Please help promote STEM in your local schools.

    Stem Education Coalition

    “Starts With A Bang! is a blog/video blog about cosmology, physics, astronomy, and anything else I find interesting enough to write about. I am a firm believer that the highest good in life is learning, and the greatest evil is willful ignorance. The goal of everything on this site is to help inform you about our world, how we came to be here, and to understand how it all works. As I write these pages for you, I hope to not only explain to you what we know, think, and believe, but how we know it, and why we draw the conclusions we do. It is my hope that you find this interesting, informative, and accessible,” says Ethan

  • richardmitnick 11:31 am on December 21, 2018 Permalink | Reply
    Tags: , , , Gluons, , , , Relativistic Heavy Ion Collider (RHIC), Theory Paper Offers Alternate Explanation for Particle Patterns   

    From Brookhaven National Lab: “Theory Paper Offers Alternate Explanation for Particle Patterns” 

    From Brookhaven National Lab

    December 19, 2018
    Karen McNulty Walsh

    Quantum mechanical interactions among gluons may trigger patterns that mimic formation of quark-gluon plasma in small-particle collisions at RHIC.

    Raju Venugopalan and Mark Mace, two members of a collaboration that maintains quantum mechanical interactions among gluons are the dominant factor creating particle flow patterns observed in collisions of small projectiles with gold nuclei at the Relativistic Heavy Ion Collider (RHIC).

    A group of physicists analyzing the patterns of particles emerging from collisions of small projectiles with large nuclei at the Relativistic Heavy Ion Collider (RHIC) say these patterns are triggered by quantum mechanical interactions among gluons, the glue-like particles that hold together the building blocks of the projectiles and nuclei. This explanation differs from that given by physicists running the PHENIX experiment at RHIC—a U.S. Department of Energy Office of Science user facility for nuclear physics research at DOE’s Brookhaven National Laboratory. The PHENIX collaboration describes the patterns as a telltale sign that the small particles are creating tiny drops of quark-gluon plasma, a soup of visible matter’s fundamental building blocks.

    The scientific debate has set the stage for discussions that will take place among experimentalists and theorists in early 2019.

    “This back-and-forth process of comparison between measurements, predictions, and explanations is an essential step on the path to new discoveries—as the RHIC program has demonstrated throughout its successful 18 years of operation,” said Berndt Mueller, Brookhaven’s Associate Laboratory Director for Nuclear and Particle Physics, who has convened the special workshop for experimentalists and theorists, which will take place at Rice University in Houston, March 15-17, 2019.

    The data come from collisions between small projectiles (single protons, two-particle deuterons, and three-particle helium-3 nuclei) with large gold nuclei “targets” moving in the opposite direction at nearly the speed of light at RHIC. The PHENIX team tracked particles produced in these collisions and detected distinct correlations among particles emerging in elliptical and triangular patterns. Their measurements were in good agreement with particle patterns predicted by models describing the hydrodynamic behavior of a nearly perfect fluid quark-gluon plasma (QGP), which relate these patterns to the initial geometric shapes of the projectiles (for details, see this press release and the associated paper published in Nature Physics).

    But former Stony Brook University (SBU) Ph.D. student Mark Mace, his advisor Raju Venugopalan of Brookhaven Lab and an adjunct professor at SBU, and their collaborators question the PHENIX interpretation, attributing the observed particle patterns instead to quantum mechanical interactions among gluons. They present their interpretation of the results at RHIC and also results from collisions of protons with lead ions at Europe’s Large Hadron Collider in two papers published recently in Physical Review Letters and Physics Letters B, respectively, showing that their model also finds good agreement with the data.

    Gluons’ quantum interactions

    Gluons are the force carriers that bind quarks—the fundamental building blocks of visible matter—to form protons, neutrons, and therefore the nuclei of atoms. When these composite particles are accelerated to high energy, the gluons are postulated to proliferate and dominate their internal structure. These fast-moving “walls” of gluons—sometimes called a “color glass condensate,” named for the “color” charge carried by the gluons—play an important role in the early stages of interaction when a collision takes place.

    “The concept of the color glass condensate helped us understand how the many quarks and gluons that make up large nuclei such as gold become the quark-gluon plasma when these particles collide at RHIC,” Venugopalan said. Models that assume a dominant role of color glass condensate as the initial state of matter in these collisions, with hydrodynamics playing a larger role in the final state, extract the viscosity of the QGP as near the lower limit allowed for a theoretical ideal fluid. Indeed, this is the property that led to the characterization of RHIC’s QGP as a nearly “perfect” liquid.

    But as the number of particles involved in a collision decreases, Venugopalan said, the contribution from hydrodynamics should get smaller too.

    “In large collision systems, such as gold-gold, the interacting coherent gluons in the color glass initial state decay into particle-like gluons that have time to scatter strongly amongst each other to form the hydrodynamic QGP fluid—before the particles stream off to the detectors,” Venugopalan said.

    But at the level of just a few quarks and gluons interacting, as when smaller particles collide with gold nuclei, the system has less time to build up the hydrodynamic response.

    “In this case, the gluons produced after the decay of the color glass do not have time to rescatter before streaming off to the detectors,” he said. “So what the detectors pick up are the multiparticle quantum correlations of the initial state alone.”

    Among these well-known quantum correlations are the effects of the electric color charges and fields generated by the gluons in the nucleus, which can give a small particle strongly directed kicks when it collides with a larger nucleus, Venugopalan said. According to the analysis the team presents in the two published papers, the distribution of these deflections aligns well with the particle flow patterns measured by PHENIX. That lends support to the idea that these quirky quantum interactions among gluons are sufficient to produce the particle flow patterns observed in the small systems without the formation of QGP.

    Such shifts to quantum quirkiness at the small scale are not uncommon, Venugopalan said.

    “Classical systems like billiard balls obey well-defined trajectories when they collide with each other because there are a sufficient number of particles that make up the billiard balls, causing them to behave in aggregate,” he said. “But at the subatomic level, the quantum nature of particles is far less intuitive. Quantum particles have properties that are wavelike and can create patterns that are more like that of colliding waves. The wave-like nature of gluons creates interference patterns that cannot be mimicked by classical billiard ball physics.”

    “How many such subatomic gluons does it take for them to stop exhibiting quantum weirdness and start obeying the classical laws of hydrodynamics? It’s a fascinating question. And what can we can learn about the nature of other forms of strongly interacting matter from this transition between quantum and classical physics?”

    The answers might be relevant to understanding what happens in ultracold atomic gases—and may even hold lessons for quantum information science and fundamental issues governing the construction of quantum computers, Venugopalan said.

    “In all of these systems, classical physics breaks down,” he noted. “If we can figure out the particle number or collision energy or other control variables that determine where the quantum interactions become more important, that may point to the more nuanced kinds of predictions we should be looking at in future experiments.”

    The nuclear physics theory work and the operation of RHIC at Brookhaven Lab are supported by the DOE Office of Science.

    Collaborators on this work include: Mark Mace (now a post-doc at the University of Jyväskylä), Vladimir V. Skokov (RIKEN-BNL Research Center at Brookhaven Lab and North Carolina State University), and Prithwish Tribedy (Brookhaven Lab).

    See the full article here .


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    BNL RHIC Campus

    BNL/RHIC Star Detector


    One of ten national laboratories overseen and primarily funded by the Office of Science of the U.S. Department of Energy (DOE), Brookhaven National Laboratory conducts research in the physical, biomedical, and environmental sciences, as well as in energy technologies and national security. Brookhaven Lab also builds and operates major scientific facilities available to university, industry and government researchers. The Laboratory’s almost 3,000 scientists, engineers, and support staff are joined each year by more than 5,000 visiting researchers from around the world. Brookhaven is operated and managed for DOE’s Office of Science by Brookhaven Science Associates, a limited-liability company founded by Stony Brook University, the largest academic user of Laboratory facilities, and Battelle, a nonprofit, applied science and technology organization.

  • richardmitnick 11:34 am on November 9, 2017 Permalink | Reply
    Tags: , But what is matter exactly, Einstein: m = E/c2. This is the great insight (not E = mc2), Frank Wilczek, Gluons, Higgs field, , , , Physics Has Demoted Mass, , Quarks are quantum wave-particles   

    From Nautilus: “Physics Has Demoted Mass” 



    November 9, 2017
    Jim Baggott

    You’re sitting here, reading this article. Maybe it’s a hard copy, or an e-book on a tablet computer or e-reader. It doesn’t matter. Whatever you’re reading it on, we can be reasonably sure it’s made of some kind of stuff: paper, card, plastic, perhaps containing tiny metal electronic things on printed circuit boards. Whatever it is, we call it matter or material substance. It has a characteristic property that we call solidity. It has mass.

    But what is matter, exactly? Imagine a cube of ice, measuring a little over one inch (or 2.7 centimeters) in length. Imagine holding this cube of ice in the palm of your hand. It is cold, and a little slippery. It weighs hardly anything at all, yet we know it weighs something.

    Let’s make our question a little more focused. What is this cube of ice made of? And, an important secondary question: What is responsible for its mass?

    Credit below

    To understand what a cube of ice is made of, we need to draw on the learning acquired by the chemists. Building on a long tradition established by the alchemists, these scientists distinguished between different chemical elements, such as hydrogen, carbon, and oxygen. Research on the relative weights of these elements and the combining volumes of gases led John Dalton and Louis Gay-Lussac to the conclusion that different chemical elements consist of atoms with different weights which combine according to a set of rules involving whole numbers of atoms.

    The mystery of the combining volumes of hydrogen and oxygen gas to produce water was resolved when it was realized that hydrogen and oxygen are both diatomic gases, H2 and O2. Water is then a compound consisting of two hydrogen atoms and one oxygen atom, H2O.

    This partly answers our first question. Our cube of ice consists of molecules of H2O organized in a regular array. We can also make a start on our second question. Avogadro’s law states that a mole of chemical substance will contain about 6 × 10^23 discrete “particles.” Now, we can interpret a mole of substance simply as its molecular weight scaled up to gram quantities. Hydrogen (in the form of H2) has a relative molecular weight of 2, implying that each hydrogen atom has a relative atomic weight of 1. Oxygen (O2) has a relative molecular weight of 32, implying that each oxygen atom has a relative atomic weight of 16. Water (H2O) therefore has a relative molecular weight of 2 × 1 + 16 = 18.

    It so happens that our cube of ice weighs about 18 grams, which means that it represents a mole of water, more or less. According to Avogadro’s law it must therefore contain about 6 × 10^23 molecules of H2O. This would appear to provide a definitive answer to our second question. The mass of the cube of ice derives from the mass of the hydrogen and oxygen atoms present in 6 × 10^23 molecules of H2O.

    But, of course, we can go further. We learned from J.J. Thompson, Ernest Rutherford, and Niels Bohr and many other physicists in the early 20th century that all atoms consist of a heavy, central nucleus surrounded by light, orbiting electrons. We subsequently learned that the central nucleus consists of protons and neutrons. The number of protons in the nucleus determines the chemical identity of the element: A hydrogen atom has one proton, an oxygen atom has eight (this is called the atomic number). But the total mass or weight of the nucleus is determined by the total number of protons and neutrons in the nucleus.

    Hydrogen still has only one (its nucleus consists of a single proton—no neutrons). The most common isotope of oxygen has—guess what?—16 (eight protons and eight neutrons). It’s obviously no coincidence that these proton and neutron counts are the same as the relative atomic weights I quoted above.

    If we ignore the light electrons, then we would be tempted to claim that the mass of the cube of ice resides in all the protons and neutrons in the nuclei of its hydrogen and oxygen atoms. Each molecule of H2O contributes 10 protons and eight neutrons, so if there are 6 × 10^23 molecules in the cube and we ignore the small difference in mass between a proton and a neutron, we conclude that the cube contains in total about 18 times this figure, or 108 × 10^23 protons and neutrons.

    So far, so good. But we’re not quite done yet. We now know that protons and neutrons are not elementary particles. They consist of quarks. A proton contains two up quarks and a down quark, a neutron two down quarks and an up quark. And the color force binding the quarks together inside these larger particles is carried by massless gluons.

    Okay, so surely we just keep going. If once again we approximate the masses of the up and down quarks as the same we just multiply by three and turn 108 × 10^23 protons and neutrons into 324 × 10^23 up and down quarks. We conclude that this is where all the mass resides. Yes?

    No. This is where our naïve atomic preconceptions unravel. We can look up the masses of the up and down quarks on the Particle Data Group website. The up and down quarks are so light that their masses can’t be measured precisely and only ranges are quoted. The following are all reported in units of MeV/c2. In these units the mass of the up quark is given as 2.3 with a range from 1.8 to 3.0. The down quark is a little heavier, 4.8, with a range from 4.5 to 5.3. Compare these with the mass of the electron, about 0.51 measured in the same units.

    Now comes the shock. In the same units of MeV/c2 the proton mass is 938.3, the neutron 939.6. The combination of two up quarks and a down quark gives us only 9.4, or just 1 percent of the mass of the proton. The combination of two down quarks and an up quark gives us only 11.9, or just 1.3 percent of the mass of the neutron. About 99 percent of the masses of the proton and neutron seem to be unaccounted for. What’s gone wrong?

    To answer this question, we need to recognize what we’re dealing with. Quarks are not self-contained “particles” of the kind that the Greeks or the mechanical philosophers might have imagined. They are quantum wave-particles; fundamental vibrations or fluctuations of elementary quantum fields. The up and down quarks are only a few times heavier than the electron, and we’ve demonstrated the electron’s wave-particle nature in countless laboratory experiments. We need to prepare ourselves for some odd, if not downright bizarre behavior.

    And let’s not forget the massless gluons. Or special relativity, and E = mc2. Or the difference between “bare” and “dressed” mass. And, last but not least, let’s not forget the role of the Higgs field in the “origin” of the mass of all elementary particles. To try to understand what’s going on inside a proton or neutron we need to reach for quantum chromodynamics, the quantum field theory of the color force between quarks.

    icedmocha / Shutterstock

    Quarks and gluons possess color “charge.” Just what is this, exactly? We have no way of really knowing. We do know that color is a property of quarks and gluons and there are three types, which physicists have chosen to call red, green, and blue. But, just as nobody has ever “seen” an isolated quark or gluon, so more or less by definition nobody has ever seen a naked color charge. In fact, quantum chromodynamics (QCD) suggests that if a color charge could be exposed like this it would have a near-infinite energy. Aristotle’s maxim was that “nature abhors a vacuum.” Today we might say: “nature abhors a naked color charge.”

    So, what would happen if we could somehow create an isolated quark with a naked color charge? Its energy would go up through the roof, more than enough to conjure virtual gluons out of “empty” space. Just as the electron moving through its own self-generated electromagnetic field gathers a covering of virtual photons, so the exposed quark gathers a covering of virtual gluons. Unlike photons, the gluons themselves carry color charge and they are able to reduce the energy by, in part, masking the exposed color charge. Think of it this way: The naked quark is acutely embarrassed, and it quickly dresses itself with a covering of gluons.

    This isn’t enough, however. The energy is high enough to produce not only virtual particles (like a kind of background noise or hiss), but elementary particles, too. In the scramble to cover the exposed color charge, an anti-quark is produced which pairs with the naked quark to form a meson. A quark is never—but never—seen without a chaperone.

    But this still doesn’t do it. To cover the color charge completely we would need to put the anti-quark in precisely the same place at precisely the same time as the quark. Heisenberg’s uncertainty principle won’t let nature pin down the quark and anti-quark in this way. Remember that a precise position implies an infinite momentum, and a precise rate of change of energy with time implies an infinite energy. Nature has no choice but to settle for a compromise. It can’t cover the color charge completely but it can mask it with the anti-quark and the virtual gluons. The energy is at least reduced to a manageable level.

    This kind of thing also goes on inside the proton and neutron. Within the confines of their host particles, the three quarks rattle around relatively freely. But, once again, their color charges must be covered, or at least the energy of the exposed charges must be reduced. Each quark produces a blizzard of virtual gluons that pass back and forth between them, together with quark–anti-quark pairs. Physicists sometimes call the three quarks that make up a proton or a neutron “valence” quarks, as there’s enough energy inside these particles for a further sea of quark–anti-quark pairs to form. The valence quarks are not the only quarks inside these particles.

    What this means is that the mass of the proton and neutron can be traced largely to the energy of the gluons and the sea of quark–anti-quark pairs that are conjured from the color field.

    How do we know? Well, it must be admitted that it is actually really rather difficult to perform calculations using QCD. The color force is extremely strong, and the corresponding energies of color-force interactions are therefore very high. Remember that the gluons also carry color charge, so everything interacts with everything else. Virtually anything can happen, and keeping track of all the possible virtual and elementary-particle permutations is very demanding.

    This means that although the equations of QCD can be written down in a relatively straightforward manner, they cannot be solved analytically, on paper. Also, the mathematical sleight-of-hand used so successfully in QED no longer applies—because the energies of the interactions are so high we can’t apply the techniques of renormalization. Physicists have had no choice but to solve the equations on a computer instead.

    Considerable progress was made with a version of QCD called “QCD-lite.” This version considered only massless gluons and up and down quarks, and further assumed that the quarks themselves are also massless (so, literally, “lite”). Calculations based on these approximations yielded a proton mass that was found to be just 10 percent lighter than the measured value.

    Let’s stop to think about that for a bit. A simplified version of QCD in which we assume that no particles have mass to start with nevertheless predicts a mass for the proton that is 90 percent right. The conclusion is quite startling. Most of the mass of the proton comes from the energy of the interactions of its constituent quarks and gluons.

    John Wheeler used the phrase “mass without mass” to describe the effects of superpositions of gravitational waves which could concentrate and localize energy such that a black hole is created. If this were to happen, it would mean that a black hole—the ultimate manifestation of super-high-density matter—had been created not from the matter in a collapsing star but from fluctuations in spacetime. What Wheeler really meant was that this would be a case of creating a black hole (mass) from gravitational energy.

    But Wheeler’s phrase is more than appropriate here. Frank Wilczek, one of the architects of QCD, used it in connection with his discussion of the results of the QCD-lite calculations. If much of the mass of a proton and neutron comes from the energy of interactions taking place inside these particles, then this is indeed “mass without mass,” meaning that we get the behavior we tend to ascribe to mass without the need for mass as a property.

    Does this sound familiar? Recall that in Einstein’s seminal addendum to his 1905 paper on special relativity the equation he derived is actually m = E/c2. This is the great insight (not E = mc2). And Einstein was surely prescient when he wrote: “the mass of a body is a measure of its energy content.”[1] Indeed, it is. In his book The Lightness of Being, Wilczek wrote:[2]

    “If the body is a human body, whose mass overwhelmingly arises from the protons and neutrons it contains, the answer is now clear and decisive. The inertia of that body, with 95 percent accuracy, is its energy content.”

    In the fission of a U-235 nucleus, some of the energy of the color fields inside its protons and neutrons is released, with potentially explosive consequences. In the proton–proton chain involving the fusion of four protons, the conversion of two up quarks into two down quarks, forming two neutrons in the process, results in the release of a little excess energy from its color fields. Mass does not convert to energy. Energy is instead passed from one kind of quantum field to another.

    Where does this leave us? We’ve certainly come a long way since the ancient Greek atomists speculated about the nature of material substance, 2,500 years ago. But for much of this time we’ve held to the conviction that matter is a fundamental part of our physical universe. We’ve been convinced that it is matter that has energy. And, although matter may be reducible to microscopic constituents, for a long time we believed that these would still be recognizable as matter—they would still possess the primary quality of mass.

    Modern physics teaches us something rather different, and deeply counter-intuitive. As we worked our way ever inward—matter into atoms, atoms into sub-atomic particles, sub-atomic particles into quantum fields and forces—we lost sight of matter completely. Matter lost its tangibility. It lost its primacy as mass became a secondary quality, the result of interactions between intangible quantum fields. What we recognize as mass is a behavior of these quantum fields; it is not a property that belongs or is necessarily intrinsic to them.

    Despite the fact that our physical world is filled with hard and heavy things, it is instead the energy of quantum fields that reigns supreme. Mass becomes simply a physical manifestation of that energy, rather than the other way around.

    This is conceptually quite shocking, but at the same time extraordinarily appealing. The great unifying feature of the universe is the energy of quantum fields, not hard, impenetrable atoms. Perhaps this is not quite the dream that philosophers might have held fast to, but a dream nevertheless.


    1. Einstein, A. Does the inertia of a body depend upon its energy-content? Annalen der Physik 18 (1905).

    2. Wilczek, F. The Lightness of Being Basic Books, New York, NY (2008).

    Photocollage credits: Physicsworld.com; Thatree Thitivongvaroon / Getty Images

    See the full article here .

    Please help promote STEM in your local schools.

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    Welcome to Nautilus. We are delighted you joined us. We are here to tell you about science and its endless connections to our lives. Each month we choose a single topic. And each Thursday we publish a new chapter on that topic online. Each issue combines the sciences, culture and philosophy into a single story told by the world’s leading thinkers and writers. We follow the story wherever it leads us. Read our essays, investigative reports, and blogs. Fiction, too. Take in our games, videos, and graphic stories. Stop in for a minute, or an hour. Nautilus lets science spill over its usual borders. We are science, connected.

  • richardmitnick 4:04 pm on June 30, 2017 Permalink | Reply
    Tags: , , Gluons, , , What really hapens?   

    From Symmetry: “What’s really happening during an LHC collision?” 

    Symmetry Mag


    Sarah Charley

    It’s less of a collision and more of a symphony.

    Wow!! ATLAS collaboration.

    The Large Hadron Collider is definitely large.


    CERN/LHC Map

    CERN LHC Tunnel

    CERN LHC particles

    With a 17-mile circumference, it is the biggest collider on the planet. But the latter fraction of its name is a little misleading. That’s because what collides in the LHC are the tiny pieces inside the hadrons, not the hadrons themselves.

    Hadrons are composite particles made up of quarks and gluons.

    The quark structure of the proton 16 March 2006 Arpad Horvath

    The gluons carry the strong force, which enables the quarks to stick together and binds them into a single particle.


    The main fodder for the LHC are hadrons called protons. Protons are made up of three quarks and an indefinable number of gluons. (Protons in turn make up atoms, which are the building blocks of everything around us.)

    If a proton were enlarged to the size of a basketball, it would look empty. Just like atoms, protons are mostly empty space. The individual quarks and gluons inside are known to be extremely small, less than 1/10,000th the size of the entire proton.

    “The inside of a proton would look like the atmosphere around you,” says Richard Ruiz, a theorist at Durham University. “It’s a mixture of empty space and microscopic particles that, for all intents and purposes, have no physical volume.

    “But if you put those particles inside a balloon, you’ll see the balloon expand. Even though the internal particles are microscopic, they interact with each other and exert a force on their surroundings, inevitably producing something which does have an observable volume.”

    So how do you collide two objects that are effectively empty space? You can’t. But luckily, you don’t need a classical collision to unleash a particle’s full potential.

    In particle physics, the term “collide” can mean that two protons glide through each other, and their fundamental components pass so close together that they can talk to each other. If their voices are loud enough and resonate in just the right way, they can pluck deep hidden fields that will sing their own tune in response—by producing new particles.

    “It’s a lot like music,” Ruiz says. “The entire universe is a symphony of complex harmonies which call and respond to each other. We can easily produce the mid-range tones, which would be like photons and muons, but some of these notes are so high that they require a huge amount of energy and very precise conditions to resonate.”

    Space is permeated with dormant fields that can briefly pop a particle into existence when vibrated with the right amount of energy. These fields play important roles but almost always work behind the scenes. The Higgs field, for instance, is always interacting with other particles to help them gain mass. But a Higgs particle will only appear if the field is plucked with the right resonance.

    When protons meet during an LHC collision, they break apart and the quarks and gluons come spilling out. They interact and pull more quarks and gluons out of space, eventually forming a shower of fast-moving hadrons.

    This subatomic symbiosis is facilitated by the LHC and recorded by the experiment, but it’s not restricted to the laboratory environment; particles are also accelerated by cosmic sources such as supernova remnants. “This happens everywhere in the universe,” Ruiz says. “The LHC and its experiments are not special in that sense. They’re more like a big concert hall that provides the energy to pop open and record the symphony inside each proton.”

    See the full article here .

    Please help promote STEM in your local schools.

    STEM Icon

    Stem Education Coalition

    Symmetry is a joint Fermilab/SLAC publication.

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