gluons – Close Read

‘Weak charge’ measurement holds up SM prediction

Various dark matter detectors around the world, massive particle accelerators and colliders, powerful telescopes on the ground and in space all have their distinct agendas but ultimately what unites them is humankind’s quest to understand what the hell this universe is on about. There are unanswered questions in every branch of scientific endeavour that will keep us busy for millennia to come.

Among them, physics seems to be sufferingly uniquely, as it stumbles even as we speak through a ‘nightmare scenario’: the most sensitive measurements we have made of the physical reality around us, at the largest and smallest scales, don’t agree with what physicists have been able to work out on paper. Something’s gotta give – but scientists don’t know where or how they will find their answers.

The Qweak experiment at the Jefferson Lab, Virginia, is one of scores of experiments around the world trying to find a way out of the nightmare scenario. And Qweak is doing that by studying how the rate at which electrons scatter off a proton is affected by the electrons’ polarisation (a.k.a. spin polarisation: whether the spin of each electron is “left” or “right”).

Unlike instruments like the Large Hadron Collider, which are very big, operate at much higher energies, are expensive and are used to look for new particles hiding in spacetime, Qweak and others like it make ultra-precise measurements of known values, in effect studying the effects of particles both known and unknown on natural phenomena.

And if these experiments are able to find that these values deviate at some level from that predicted by the theory, physicists will have the break they’re looking for. For example, if Qweak is the one to break new ground, then physicists will have reason to suspect that the two nuclear forces of nature, simply called strong and weak, hold some secrets.

However, Qweak’s latest – and possibly its last – results don’t break new ground. In fact, they assert that the current theory of particle physics is correct, the same theory that physicists are trying to break free of.

Most of us are familiar with protons and electrons: they’re subatomic particles, carry positive and negative charges resp., and are the stuff of one chapter of high-school physics. What students of science find out quite later is that electrons are fundamental particles – they’re not made up of smaller particles – but protons are not. Protons are made up of quarks and gluons.

Interactions between electrons and quarks/gluons is mediated by two fundamental forces: the electromagnetic and the weak nuclear. The electromagnetic force is much stronger than the aptly named weak nuclear force. On the other hand, it is agnostic to the electron’s polarisation while the weak nuclear force is sensitive to it. In fact, the weak nuclear force is known to respond differently to left- and right-handed particles.

When electrons are bombarded at protons, the electrons are scattered off. Scientists at measure how often this happens and at what angle, together with the electrons’ polarisation – and try to find correlations between the two sets of data.

An illustration showing the expected outcomes when left- and right-handed electrons, visualised as mirror-images of each other, scatter off of a proton. Credit: doi:10.1038/s41586-018-0096-0

At Qweak, the electrons were accelerated to 1.16 GeV and bombarded at a tank of liquid hydrogen. A detector positioned near the tank picked up on electrons scattered at angles between 5.8º and 11.6º. By finely tuning different aspects of this setup, the scientists were able to up the measurement precision to 10 parts per billion.

For example, they were able to achieve a detection rate of 7 billion per second, a target luminosity of 1.7 x 10³⁹ cm^-2 s^-1 and provide a polarised beam of electrons at 180 µA – all considered high for an experiment of this kind.

The scientists were looking for patterns in the detector data that would tell them something about the proton’s weak charge: the strength with which it interacts with electrons via the weak nuclear force. (Its notation is Q_weak, hence the experiment’s name.)

At Qweak, they’re doing this by studying how the electrons are scattered versus their polarisation. The Standard Model (SM) of particle physics, the theory that physicists work with to understand the behaviour of elementary particles, predicts that the number of left- and right-handed electrons scattered should differ by one for every 10 million interactions. If this number is found to be bigger or smaller than usual when measured in the wild, then the Standard Model will be in trouble – much to physicists’ delight.

SM’s corresponding value for the proton’s weak charge is 0.0708. At Qweak, the value was measured to be 0.0719 ± 0.0045, i.e. between 0.0674 and 0.0764, completely agreeing with the SM prediction. Something’s gotta give – but it’s not going to be the proton’s weak charge for now.

Paper: Precision measurement of the weak charge of the proton

Featured image credit: Pexels/Unsplash.

Chromodynamics: Gluons are just gonzo

One of the more fascinating bits of high-energy physics is the branch of physics called quantum chromodynamics (QCD). Don’t let the big name throw you off: it deals with a bunch of elementary particles that have a property called colour charge. And one of these particles creates a mess of this branch of physics because of its colour charge – so much so that it participates in the story that it is trying to shape. What could be more gonzo than this? Hunter S. Thompson would have been proud.

Like electrons have electric charge, particles studied by QCD have a colour charge. It doesn’t correspond to a colour of any kind; it’s just a funky name.

(Richard Feynman wrote about this naming convention in his book, QED: The Strange Theory of Light and Matter (pp. 163, 1985): “The idiot physicists, unable to come up with any wonderful Greek words anymore, call this type of polarization by the unfortunate name of ‘color,’ which has nothing to do with color in the normal sense.”)

The fascinating thing about these QCD particles is that they exhibit a property called colour confinement. It means that all particles with colour charge can’t ever be isolated. They’re always to be found only in pairs or bigger clumps. They can be isolated in theory if the clumps are heated to the Hagedorn temperature: 1,000 billion billion billion K. But the bigness of this number has ensured that this temperature has remained theoretical. They can also be isolated in a quark-gluon plasma, a superhot, superdense state of matter that has been creating fleetingly in particle physics experiments like the Large Hadron Collider. The particles in this plasma quickly collapse to form bigger particles, restoring colour confinement.

There are two kinds of particles that are colour-confined: quarks and gluons. Quarks come together to form bigger particles called mesons and baryons. The aptly named gluons are the particles that ‘glue’ the quarks together.

The force that acts between quarks and gluons is called the strong nuclear force. But this is misleading. The gluons actually mediate the strong nuclear force. A physicist would say that when two quarks exchange gluons, the quarks are being acted on by the strong nuclear force.

Because protons and neutrons are also made up of quarks and gluons, the strong nuclear force holds the nucleus together in all the atoms in the universe. Breaking this force releases enormous amounts of energy – like in the nuclear fission that powers atomic bombs and the nuclear fusion that powers the Sun. In fact, 99% of a proton’s mass comes from the energy of the strong nuclear force. The quarks contribute the remaining 1%; gluons are massless.

When you pull two quarks apart, you’d think the force between them will reduce. It doesn’t; it actually increases. This is very counterintuitive. For example, the gravitational force exerted by Earth drops off the farther you get away from it. The electromagnetic force between an electron and a proton decreases the more they move apart. But it’s only with the strong nuclear force that the force between two particles on which the force is acting actually increases as they move apart. Frank Wilczek called this a “self-reinforcing, runaway process”. This behaviour of the force is what makes colour confinement possible.

However, in 1973, Wilczek, David Gross and David Politzer found that the strong nuclear force increases in strength only up to a certain distance – around 1 fermi (0.000000000000001 metres, slightly larger than the diameter of a proton). If the quarks are separated by more than a fermi, the force between them falls off drastically, but not completely. This is called asymptotic freedom: the freedom from the force beyond some distance drops off asymptotically towards zero. Gross, Politzer and Wilczek won the Nobel Prize for physics in 2004 for their work.

In the parlance of particle physics, what makes asymptotic freedom possible is the fact that gluons emit other gluons. How else would you explain the strong nuclear force becoming stronger as the quarks move apart – if not for the gluons that the quarks are exchanging becoming more numerous as the distance increases?

This is the crazy phenomenon that you’re fighting against when you’re trying to set off a nuclear bomb. This is also the crazy phenomenon that will one day lead to the Sun’s death.

The first question anyone would ask now is – doesn’t asymptotic freedom violate the law of conservation of energy?

The answer lies in the nothingness all around us.

The vacuum of deep space in the universe is not really a vacuum. It’s got some energy of itself, which astrophysicists call ‘dark energy’. This energy manifests itself in the form of virtual particles: particles that pop in and out of existence, living for far shorter than a second before dissipating into energy. When a charged particle pops into being, its charge attracts other particles of opposite charge towards itself and repels particles of the same charge away. This is high-school physics.

But when a charged gluon pops into being, something strange happens. An electron has one kind of charge, the positive/negative electric charge. But a gluon contains a ‘colour’ charge and an ‘anti-colour’ charge, each of which can take one of three values. So the virtual gluon will attract other virtual gluons depending on their colour charges and intensify the colour charge field around it, and also change its colour according to whichever particles are present. If this had been an electron, its electric charge and the opposite charge of the particle it attracted would cancel the field out.

This multiplication is what leads to the build up of energy when we’re talking about asymptotic freedom.

Physicists refer to the three values of the colour charge as blue, green and red. (This is more idiocy – you might as well call them ‘baboon’, ‘lion’ and ‘giraffe’.) If a blue quark, a green quark and a red quark come together to form a hadron (a class of particles that includes protons and neutrons), then the hadron will have a colour charge of ‘white’, becoming colour-neutral. Anti-quarks have anti-colour charges: antiblue, antigreen, antired. When a red quark and an antired anti-quark meet, they will annihilate each other – but not so when a red quark and an antiblue anti-quark meet.

Gluons complicate this picture further because, in experiments, physicists have found that gluons behave as if they have both colour and anti-colour. In physical terms, this doesn’t make much sense, but they do in mathematical terms (which we won’t get into). Let’s say a proton is made of one red quark, one blue quark and one green quark. The quarks are held together by gluons, which also have a colour charge. So when two quarks exchange a gluon, the colours of the quarks change. If a blue quark emits a blue-antigreen gluon, the quark turns green whereas the quark that receives the gluon will turn blue. Ultimately, if the proton is ‘white’ overall, then the three quarks inside are responsible for maintaining that whiteness. This is the law of conservation of colour charge.

Gluons emit gluons because of their colour charges. When quarks exchange gluons, the quarks’ colour charges also change. In effect, the gluons are responsible for quarks getting their colours. And because the gluons participate in the evolution of the force that they also mediate, they’re just gonzo: they can interact with themselves to give rise to new particles.

A gluon can split up into two gluons or into a quark-antiquark pair. Say a quark and an antiquark are joined together. If you try to pull them apart by supplying some energy, the gluon between them will ‘swallow’ that energy and split up into one antiquark and one quark, giving rise to two quark-antiquark pairs (and also preserving colour-confinement). If you supply even more energy, more quark-antiquark pairs will be generated.

For these reasons, the strong nuclear force is called a ‘colour force’: it manifests in the movement of colour charge between quarks.

In an atomic nucleus, say there is one proton and one neutron. Each particle is made up of three quarks. The quarks in the proton and the quarks in the neutron interact with each other because they are close enough to be colour-confined: the proton-quarks’ gluons and the neutron-quarks’ gluons interact with each other. So the nucleus is effectively one ball of quarks and gluons. However, one nucleus doesn’t interact with that of a nearby atom in the same way because they’re too far apart for gluons to be exchanged.

Clearly, this is quite complicated – not just for you and me but also for scientists, and for supercomputers that perform these calculations for large experiments in which billions of protons are smashed into each other to see how the particles interact. Imagine: there are six types, or ‘flavours’, of quarks, each carrying one of three colour charges. Then there is the one gluon that can carry one of nine combinations of colour-anticolour charges.

The Wire
September 20, 2017

Featured image credit: Alexas_Fotos/pixabay.

Assuming this universe…

Accomplished physicists I have met or spoken with in the last four months professed little agreement over which parts of physics were set-in-stone and which parts simply largely-corroborated hypotheses. Here are some of them, with a short description of the dispute.

Bosons – Could be an emergent phenomenon arising out of fermion-fermion interaction; current definition could be a local encapsulation of special fermionic properties
Colour-confinement – ‘Tis held that gluons, mediators of the colour force, cannot exist in isolation nor outside the hadrons (that are composed of quarks held together by gluons); while experimental proof of the energy required to pull a quark free being much greater than the energy to pull a quark-antiquark pair out of vacuum exists, denial of confinement hasn’t yet been conclusively refuted (ref: lattice formulation of string theory)
Massive gluons – A Millennium Prize problem
Gravity – Again, could be an emergent phenomenon arising out of energy-corrections of hidden, underlying quantum fields
Compactified extra-dimensions & string theory – There are still many who dispute the “magical” mathematical framework that string theory provides because it is a perturbative theory (i.e., background-dependent); a non-perturbative definition would make its currently divergent approximations convergent

If you ever get the opportunity to listen to a physicist ruminate on the philosophy of nature, don’t miss it. What lay-people would daily dispute is the macro-physical implications of a quantum world; the result is the all-important subjective clarification that lets us think better. What physicists dispute is the constitution of the quantum world itself; the result is the more objective phenomenological implications for everyone everywhere. We could use both debates.