The Origin Of Matter's Mass

I can't believe it. I'm reporting on Ethan Siegel's article two days in a row! The last one yesterday was a doozy, wasn't it? :)

This one is a bit different and interesting. The first part of the article describes our understanding of where mass comes from for matter. I want to highlight this because it clarify one very important misconception that many people have, especially the general public. After all the brouhaha surrounding the Higgs and its discovery, a lot of people seem to think that all the masses of every particle and entity can be explained using the Higgs. This is clearly false as stated in the article.

Yet if we take a look at the proton (made of two up and one down quark) and the neutron (made of one up and two down quarks), a puzzle emerges. The three quarks within a proton or neutron, even when you add them all up, comprise less than 0.2% of the known masses of these composite particles. The gluons themselves are massless, while the electrons are less than 0.06% of a proton's mass. The whole of matter, somehow, weighs much, much more than the sum of its parts.

The Higgs may be responsible for the rest mass of these fundamental constituents of matter, but the whole of a single atom is nearly 100 times heavier than the sum of everything known to make it up. The reason has to do with a force that's very counterintuitive to us: the strong nuclear force. Instead of one type of charge (like gravity, which is always attractive) or two types (the "+" and "-" charges of electromagnetism), the strong force has three color charges (red, green and blue), where the sum of all three charges is colorless.

So while we may use the Higgs to point to the origin of  mass in, say, leptons, for hadrons/partons, this is not sufficient. The strong force itself contributes a significant amount to the origin of mass for these particles. The so-called "God Particles" are not that godly, because it can't do and explain everything.

The other interesting part of the article is that he included a "live blog" of the talk by Phiala Shanahan at occurred yesterday at the Perimeter Institute, related to this topic. So you may want to read through the transcript and see if you get anything new.

Zz.

RHIC Sees Another First

The quark-gluon plasma created at Brookhaven's Relativistic Heavy Ion Collider (RHIC) continues to produce a rich body of information. They have now announced that the quark-gluon plasma has produced the most rapidly-spinning fluid ever produced.

Collisions with heavy ions—typically gold or lead—put lots of protons and neutrons in a small volume with lots of energy. Under these conditions, the neat boundaries of those particles break down. For a brief instant, quarks and gluons mingle freely, creating a quark-gluon plasma. This state of matter has not been seen since an instant after the Big Bang, and it has plenty of unusual properties. "It has all sorts of superlatives," Ohio State physicist Mike Lisa told Ars. "It is the most easily flowing fluid in nature. It's highly explosive, much more than a supernova. It's hotter than any fluid that's known in nature."
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We can now add another superlative to the quark-gluon plasma's list of "mosts:" it can be the most rapidly spinning fluid we know of. Much of the study of the material has focused on the results of two heavy ions smacking each other head-on, since that puts the most energy into the resulting debris, and these collisions spit the most particles out. But in many collisions, the two ions don't hit each other head-on—they strike a more glancing blow.

It is a fascinating article, and you may read the significance of this study, especially in relation to how it informs us on certain aspect of QCD symmetry.

But if you know me, I never fail to try to point something out that is more general in nature, and something that the general public should take note of. I like this statement in the article very much, and I'd like to highlight it here:

But a logical "should" doesn't always equal a "does," so it's important to confirm that the resulting material is actually spinning. And that's a rather large technical challenge when you're talking about a glob of material roughly the same size as an atomic nucleus.

This is what truly distinguish science with other aspects of our lives. There are many instances, especially in politics, social policies, etc., where certain assertions are made and appear to be "obvious" or "logical", and yet, these are simply statements made without any valid evidence to support it. I can think of many ("Illegal immigrants taking away jobs", or "gay marriages undermines traditional marriages", etc...etc). Yet, no matter how "logical" these may appear to be, they are simply statements that are devoid of evidence to support them. Still, whenever they are uttered, many in the public accept them as FACTS or valid, without seeking or requiring evidence to support them. One may believe that "A should cause B", but DOES IT REALLY?

Luckily, this is NOT how it is done in science. No matter how obvious it is, or how verified something is, there are always new boundaries to push and a retesting of the ideas, even ones that are known to be true under certain conditions. And a set of experimental evidence is the ONLY standard that will settle and verify any assertion and statements.

This is why everyone should learn science, not just for the material, but to understand the methodology and technique. It is too bad they don't require politicians to have such skills.

Zz.

The Emergence Of QCD

This is a nice review of how QCD came about and its role in the Standard Model, written by Nobel Laureates David Gross and Franck Wilczek.

Zz.

Resource Letter: Quantum Chromodynamics

I always like these "resource letter" in AJP. They are chokeful of useful and valuable references, and having them right at your fingertips in one document is extremely handy.

This is one such example, and this time, it is on QCD. Anyone just learning about QCD will find the references very handy. It cuts down on the time hunting for the majors papers in the various topic in QCD.

Zz.

QCD - The Source of Everyday Mass

Reported in this week's issue of Science, a new ab initio theoretical calculation using lattice QCD has produced a good agreement between the mass of various nucleons and other hadrons. This work was done by Durr et al.[1]

As reviewed in the Perspective by Andreas S. Kronfeld in the same issue of Science, this means that the source of our everyday mass lies in QCD.

Almost all of the mass (or weight) of the world we live in comes from atomic nuclei, which are composed of neutrons and protons (collectively called "nucleons"). 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). A challenge since the introduction of QCD has been to carry out an ab initio calculation of the nucleon's mass. On page 1224 of this issue, Dürr et al. (4) report the first such calculation that incorporates all of the needed physics, controls the numerical approximations, and presents a thorough error budget. Because these accurate calculations agree with laboratory measurements, we now know, rather than just believe, that the source of mass of everyday matter is QCD.

It is now up to the LHC to show that this premise is correct.

Dürr et al. start with QCD's defining equations and present a persuasive, complete, and direct demonstration that QCD generates the mass of the nucleon and of several other hadrons. These calculations teach us that even if the quark masses vanished, the nucleon mass would not change much, a phenomenon sometimes called "mass without mass" (19, 20). It then raises the question of the origin of the tiny up and down quark masses. The way nature generates these masses, and the even tinier electron mass, is the subject of the LHC, where physicists will explore whether the responsible mechanism is the Higgs boson or something more spectacular.

Edit: there's a coverage of this on Nature's daily news. {the link is open for free only for a limited time}

Zz.

[1] S. Dürr et al., Science v.322, p.1224 (2008).

Symmetry Breaking on a Supercomputer

This article is a month old, but better late than never.

One of the points that I've always tried to get across is that fields such as condensed matter physics, which many physicists who study "fundamental" issues look down upon, can and have made significant contribution to the fundamental issues in physics. This article on spontaneous symmetry breaking in lattice QCD is one such example.

Chiral symmetry distinguishes right-hand spinning quarks from left-handed and is exact only if the quarks move at c and are therefore massless. In 1961 Yoichiro Nambu and Giovanni Jona-Lasinio proposed the idea of SCSB, inspired by the Bardeen–Cooper–Schrieffer mechanism of superconductivity in which spin-up and spin-down electrons pair up and condense into a lower energy level. In QCD a quark and an antiquark pair up, leading to a vacuum full of condensed quark–antiquark pairs. The result is that chiral symmetry is broken, so that the quarks – and the particles they form – acquire masses.

This is not the first such case, and in fact, not the only time the Nambu et al. work has inspired a development in something fundamental. Peter Higgs made the same leap using Nambu's work and later on, took up Phil Anderson's work on Goldstone boson to come up with what we now called the Higgs mechanism.

"When I moved back to Edinburgh in October 1960 I was not sure where I was going next," he recalls. That all changed the following year when he read a paper by Yoichiro Nambu that based a theory of elementary particles on an analogy with the BCS theory of superconductivity. "This is where the idea of a spontaneously broken symmetry being the way in which the mass of particles could be generated first arose," says Higgs. "Although my name gets thrown around in this context, it was Nambu who showed how fermion masses would be generated in a way that was analogous to the formation of the energy gap in a superconductor."

There was, however, a problem with the Nambu approach. Although the spontaneous breaking of symmetry generated particles with mass, Jeffrey Goldstone, Salam and Steven Weinberg had shown that it also generated a particle known as a Goldstone boson that had no mass. This was bad news because no such particle was known to exist.

Once more help arrived from the condensed-matter community when, in 1963, Phil Anderson pointed out that the equivalent of a Goldstone boson in a superconductor could become massive due to its electromagnetic interactions. But did Anderson's argument apply in the relativistic case? No, said a paper by Walter Gilbert in an issue of Physical Review Letters that arrived in Edinburgh the middle of July. Yes, said Higgs, after thinking about it over the weekend.

There you go. These are clear proofs that many of the fundamental ideas and principles of world can come from a field of study that deals with materials and many-body interactions. So kids, you don't have to be string theorists, astrophysicists, or study high-energy physics to make significant contributions to our basic understanding of the universe.

Zz.