Tag Archives: gluons

The Proton’s Spin

Protons are the work horses of chemistry. Their numbers determine which element you are talking about, and their positive charge determines how many electrons will form a cloud around them to facilitate all manner of chemical reactions.

For decades we thought that protons were absolutely fundamental particles along with neutrons and electrons, but then came the quantum revolution of the 1920s and the escalating quest to understand what their actual physical properties were. Through experimentation, we found that protons all had exactly the same mass to many decimal places. They all had exactly +1.0000 unit of charge, also to many decimal places. But they also possessed an entirely new physical quantity found only in atomic-scale physics. This quantity was called ‘spin’ but had nothing to do with the motion of a top about its axis, although paradoxically it could nonetheless be interpreted in that way.

Quantum spin, unlike the continuous spinning of a top, comes only in integer units like 0, 1, 2, etc, or in half-integer units like ½, 3/2, 5/2 etc. Physicists soon discovered that fundamental particles like photons ( the carriers of light energy) only had a quantum spin of exactly 1.0, while protons, neutrons, neutrinos and electrons had exactly ½ unit of spin. The former kinds of particles were called bosons while the latter were given the name fermions. Composite particles made up from these elementary bosons and fermions could have other spin values, but only what arises from adding, in the proper way, the elementary spins of their constituents.

By the 1960s, experiments had begun to show that protons were not actually fundamental particles at all, nor were neutrons for that matter. Theoretical models that built-up protons and neutrons and many other known particles called mesons and baryons soon led to the idea of the quark. For protons and neutrons, you needed three quarks, while for the mesons you only needed two of which one would be a quark and the other an anti-quark. The mathematics were impressive and elegant, and this system of quarks soon became the favored model for all particles that interacted through the strong nuclear force, itself produced by the exchanges of particles called gluons. Also in this scheme, quarks would be spin-1/2 fermions and the gluons would be spin-1 bosons much like the photons which carry light energy.

All seemed to be going great by the 1970s and 1980s. The quark model flourished, and many new subtle phenomena were uncovered through the application of what became the Standard Model of physics. But there was a fly in the ointment.

At first the explanation for how a proton could have a spin of ½ while at the same time being composed of three quarks, each also spin-1/2 particles, was pretty well settled. Because a proton consisted of two identical ‘up’ quarks and one ‘down’ quark, it was entirely reasonable that the two up quarks would have equal and opposite spin canceling each other out, leaving behind the down quark to carry the protons ½ unit of spin. Similarly for the neutron, its two down quarks combined to have a net-zero spin leaving the single up quark to carry the ½ unit of spin for the neutron.

The Proton Spin Crisis

All seemed to be well until experiments in 1987 at the European Muon Collaboration actually used carefully prepared beams of particles called muons to probe the interior of protons and double-check the way the quark spins were oriented with the protons spin. What they found was startling. Not more than 25% of the proton’s spin was generated by the quarks at all. The remaining 75% of what defines the spin of a proton had to come from some other source!

When you look at the mass of a proton compared to the masses of the three constituent quarks you discover something very fascinating. The masses of the quarks only account for about 1% of the mass of the entire proton. Instead, thanks to Einstein’s E=mc2, it is the stress energy of the gluon fields inside the proton that contribute the missing 99%. The mass that you read on the bathroom scale is only 1% contributed by the mass of your elementary quarks in grams, but 99% by the invisible energy(mass) of the gluon fields that occupy nuclear space!

Now for proton spin, the only other things rattling around inside the intense fields in the interior of a proton were the gluons holding the quarks together, and an ephemeral sea of quark-antiquark pairs that momentarily appeared and disappeared in the vacuum of space found there. This sea of vacuum or ‘virtual’ particles is absolutely required by modern quantum physics, and although we can never detect their comings and goings by any direct observation, we can detect their influence on nearby elementary particles.

In 2014, experiments at the Relativistic Heavy Ion Collider (RHIC) at Brookhaven, New York collided polarized protons together and physicists think they have found a large part of the remainder of the protons spin. Perhaps 40% to 50% seems to be contributed by the gluons themselves. This still leave about 25% in some other source. Meanwhile, other experiments by MIT physicists determined that any anti-quarks produced inside a proton among the virtual quark sea contribute very little to the over-all spin of the proton.

The bottom line today seems to be what this table shows:

Quark spin……………………….………..25%
Gluon spin…………………………………40-50%
Orbital angular momentum……..25% to 35%

When the experimental constraints are added up, we still do not have a precise measure of how the various proton constituents add up to give the universally constant spin of 1/2 to a proton that is observed for all protons to many decimal places.

Who would have thought that such an important number as ‘1/2’ arises from combining a number of messy phenomena that themselves seem imprecise!

Check back here on Tuesday, May 30 for my next topic!

Glueballs anyone?

Today, physicists are both excited and disturbed by how well the Standard Model is behaving, even at the enormous energies provided by the CERN Large Hadron Collider. There seems to be no sign of the expected supersymmetry property that would show the way to the next-generation version of the Standard Model: Call it V2.0. But there is another ‘back door’ way to uncover its deficiencies. You see, even the tests for how the Standard Model itself works are incomplete, even after the dramatic 2012 discovery of the Higgs Boson! To see how this backdoor test works, we need a bit of history.

Glueballs found in a quark-soup (Credit: Alex Dzierba, Curtis Meyer and Eric Swanson)

Over fifty years ago in 1964, physicists Murray Gell-Mann at Caltech and George Zweig at CERN came up with the idea of the quark as a response to the bewildering number of elementary particles that were being discovered at the huge “atom smasher” labs sprouting up all over the world. Basically, you only needed three kinds of elementary quarks, called “up,” “down” and “strange.” Combining these in threes, you get the heavy particles called baryons, such as the proton and neutron. Combining them in twos, with one quark and one anti-quark, you get the medium-weight particles called the mesons. In my previous blog, I discussed how things are going with testing the quark model and identifying all of the ‘missing’ particles that this model predicts.

In addition to quarks, the Standard Model details how the strong nuclear force is created to hold these quarks together inside the particles of matter we actually see, such as protons and neutrons. To do this, quarks must exchange force-carrying particles called gluons, which ‘glue’ the quarks together in to groups of twos and threes. Gluons are second-cousins to the photons that transmit the electromagnetic force, but they have several important differences. Like photons, they carry no mass, however unlike photons that carry no electric charge, gluons carry what physicist call color-charge. Quarks can be either ‘red’, ‘blue’ or ‘green’, as well as anti-red, anti-green and anti-blue. That means that quarks have to have complex color charges like (red, anti-blue) etc. Because the gluons carry color charge, unlike photons which do not interact with each other, gluons can interact with each other very strongly through their complicated color-charges. The end result is that, under some circumstances, you can have a ball of gluons that resemble a temporarily-stable particle before they dissipate. Physicists call these glueballs…of course!

Searching for Glueballs.

Glueballs are one of the most novel, and key predictions of the Standard Model, so not surprisingly there has been a decades-long search for these waifs among the trillions of other particles that are also routinely created in modern particle accelerator labs around the world.

Example of glueball decay into pi mesons.

Glueballs are not expected to live very long, and because they carry no electrical charge they are perfectly neutral particles. When these pseudo-particles decay, they do so in a spray of other particles called mesons. Because glueballs consist of one gluon and one anti-gluon, they have no net color charge. From the various theoretical considerations, there are 15 basic glueball types that differ in what physicists term parity and angular momentum. Other massless particles of the same general type also include gravitons and Higgs bosons, but these are easily distinguished from glueball states due to their mass (glueballs should be between 1 and 5GeV) and other fundamental properties. The most promising glueball candidates are as follows:

Scalar candidates: f0(600), f0(980), f0(1370), f0(1500), f0(1710), f0(1790)
Pseudoscalar candidates: η(1405), X(1835), X(2120), X(2370), X(2500)
Tensor candidates: fJ(2220), f2(2340)

By 2015, the f-zero(1500) and f-zero(1710) had become the prime glueball candidates. The properties of glueball states can be calculated from the Standard Model, although this is a complex undertaking because glueballs interact with nearby quarks and other free gluons very strongly and all these factors have to be considered.

On October 15, 2015 there was a much-ballyhooed announcement that physicists had at last discovered the glueball particle. The articles cited Professor Anton Rebhan and Frederic Brünner from TU Wien (Vienna) as having completed these calculations, concluding that the f-zero(1710) was the best candidate consistent with experimental measurements and its predicted mass. More rigorous experimental work to define the properties and exact decays of this particle are, even now, going on at the CERN Large Hadron Collider and elsewhere.

So, between the missing particles I described in my previous blog, and glueballs, there are many things about the Standard Model that still need to be tested. But even with these predictions confirmed, physicists are still not ‘happy campers’ when it comes to this grand theory of matter and forces. Beyond these missing particles, we still need to have a deeper understanding of why some things are the way they are, and not something different.

Check back here on Wednesday, April 5 for my next topic!