A new collider can teach us about the origin of matter

The Universe is a remarkable place, and we’ve made tremendous leaps in the 20th and 21st century towards understanding it better than ever. We know enormous amounts about the fundamental particles that make up our Universe, as well as the laws and rules that govern not only those particles, but the space that they inhabit (and the space in between them) as well. And we have a cosmic origin story for our Universe as well: the inflationary hot Big Bang, which allows us to trace our cosmic history back some 13.8 billion years into the past. Through this understanding, we can paint a coherent picture of what our Universe was like way back in its earliest stages, and how those initial conditions enabled our cosmos to grow up and become the way it is today.

But despite all that we’ve learned about reality — and what composes it — at a fundamental level, there are still some puzzles that remain unsolved.

• Why do the known particles have the masses that they do, as opposed to any other value?
• How did our Universe, and practically everything in it, come to be made of matter and not antimatter?
• What are dark matter and dark energy, and how do they evolve over time?
• And is there some type of unification that exists beyond mere electroweak unification; is there a grand unified theory, a theory of everything, or a theory of quantum gravity out there to discover?

While many who study physics focus on dark matter and dark energy, those are regimes where astrophysics, rather than particle physics, may wind up providing our best clues forward. But the matter vs. antimatter question may be the most compelling puzzle when it comes to the question of building a new particle collider. Here’s why.

Deep underground, this tunnel is part of interior workings of the Large Hadron Collider (LHC), where protons pass each other at 299,792,455 m/s while circulating in opposite directions: just 3 m/s shy of the speed of light. Particle accelerators like the LHC consist of sections of accelerating cavities, where electric fields are applied to speed up the particles inside, as well as ring-bending portions, where magnetic fields are applied to direct the fast-moving particles toward either the next accelerating cavity or a collision point.

To understand what the motivation is to build a new particle collider, it’s important to look at the Large Hadron Collider: both its capabilities and limitations. The Large Hadron Collider does three things very well:

• it collides large “bunches” of protons very frequently and rapidly (what particle physicists call high luminosity),
• at a very high center-of-mass energy of ~14 TeV (or 14 Tera-electron-volts) by creating collision points where a clockwise-circulating and counterclockwise-circulating beam of protons intersect,
• and does an outstanding job of tracking the “daughter particles” (i.e., the particles produced in the aftermath of the collision) that result from any such collision event.

This has allowed us to produce enormous numbers of exotic, short-lived particles and to study them.

Detectors such as CMS and ATLAS, built around two “beam crossing” collision points, are outstanding tools for detecting the shortest-lived, highest-mass particles that can get created: top quarks, Higgs bosons, and W-and-Z bosons. In fact, the Large Hadron Collider has given us our best-ever determination of:

The observed Higgs decay channels vs. the Standard Model agreement, with the full suite of Run 1 data from ATLAS and CMS included. The agreement is astounding, and yet frustrating at the same time, as no evidence for either a second Higgs boson or for a non-Standard Model Higgs boson has yet arisen.

The Higgs boson is often called the “final piece” of the Standard Model puzzle; it was the last holdout and the final predicted Standard Model particle ever to be discovered: at the Large Hadron Collider, in fact, around 15 years ago. There’s an enormous amount that we’ve learned about the Higgs boson from our studies at the Large Hadron Collider, including that it decays in precisely the way that the Standard Model predicts: with no deviations. The lack of additional Higgs bosons — predicted by many extensions to the Standard Model — or of any novel, unexpected particles at the electroweak scale, for that matter, seems to point away from new physics at collider-accessible energies.

But there is a fascinating and important property that has to do with that scale that has not been measured, and that cannot be measured by the Large Hadron Collider at all: what type of phase transition occurs at the electroweak scale. We often use the term spontaneous symmetry breaking in physics to talk about these phase transitions: where the state of affairs transitions from a symmetric state, where all directions or locations look the same for a quantum field, to a broken-symmetry state, where you suddenly find yourself down in a “valley” and can’t get out, rather than in the prior symmetric state. We often draw a picture of a sombrero-like structure (or the bottom of a wine bottle), like the one below, to illustrate this.

When a symmetry is restored (yellow ball at the top), everything is symmetric, and there is no preferred state. When the symmetry is broken at lower energies (blue ball, bottom), the same freedom, of all directions being the same, is no longer present. In the case of the electroweak (or Higgs) symmetry, when it breaks, there’s a spontaneous process that occurs, giving mass to the particles in the Universe.

You might ask what this type of symmetry breaking has to do with the question of why there’s more matter than antimatter in the Universe, as it isn’t completely obvious, especially at a first glance. After all, this puzzle — the baryogenesis puzzle — has been under consideration since the middle of the 20th century, and we have made remarkably little progress on it. But determining the shape of the Higgs potential, and the type of phase transition that occurs during electroweak symmetry breaking, can teach us more about how our Universe came to be filled with matter (and not an equal amount of antimatter) than any other foreseeable avenue.

Here’s why. In every interaction, collision, and experiment we’ve ever performed, including at the highest energies of all (using both collider and cosmic rays), a few quantities have always been conserved. Some of them are things like energy, momentum, and angular momentum, which our theories tell us should always be conserved. But others are things like baryon number (the number of protons, neutrons, and other particles made up of three quarks, minus the number of anti-baryons) and lepton number, have always been observed to be conserved, but cannot be conserved at all times and under all conditions in an absolute sense. We know our Universe is made of matter and not antimatter, and this is a big puzzle.

X-rays, shown here in pink, are created when the gas within two colliding galaxy clusters heats up: both from the circumcluster and the intracluster mediums. The fact that no gamma-rays, or signatures of matter-antimatter annihilation, appear in these events, or in any large-scale regions of space, teach us that all of the stars, galaxies, galaxy clusters, and filaments of the cosmic web that we see are composed of matter and not antimatter. The reason behind how this came to be is still unknown.

It’s easy to imagine that there could be antimatter stars, antimatter galaxies, antimatter galaxy clusters, or antimatter-dominated sections of the cosmic web. However, observations rule this out in every location we’ve ever looked; our Universe is made of matter, as matter-antimatter annihilation would show itself at the interfaces: wherever a matter-dominated region interacted with an antimatter-dominated one.

That implies that, at some point in the Universe’s ancient past, a matter-antimatter asymmetry — or a dominance of matter over antimatter — must have emerged early on: before stars and galaxies formed, before neutral atoms formed, or even before the light elements were forged in the nuclear furnace of the hot Big Bang. How this could have, or even must have, occurred was worked out back in the 1960s by Soviet physicist Andrei Sakharov, who noted that as long as the following three conditions were met:

1. thermodynamically out-of-equilibrium conditions,
2. a sufficiently large amount of both C-violation and CP-violation,
3. and the existence of baryon non-conserving interactions,

that a matter-antimatter asymmetry could be created from a previously symmetric state. A straightforward example of this occurs in grand unified theories, where unstable X-and-Y bosons (and their antiparticle counterparts) decay, but with different branching ratios (i.e., percentages of the decays that take one pathway versus another) for the particle vs. antiparticle species.

If you admit the existence of novel particles (such as the X and Y here) with antiparticle counterparts, they must conserve CPT, but not necessarily C, P, T, or CP by themselves. If CP is violated, the decay pathways — or the percentage of particles decaying one way versus another — can be different for particles compared to antiparticles, resulting in a net production of matter over antimatter if the conditions are right. Although this illustrates the scenario of GUT baryogenesis, the Standard Model alone can produce baryon number violation through sphaleron interactions.

If you imagine creating 100 X particles, 100 Y particles, 100 anti-X particles, and 100 anti-Y particles, and they decay as illustrated above, you’ll wind up with an excess of:

• 8 more up quarks than anti-up quarks,
• 4 more down quarks than anti-down quarks,
• and 4 more electrons than positrons (anti-electrons),

for a net gain of four protons and four electrons, even though originally the Universe was symmetric between particles and antiparticles. This is a possible pathway for how the matter-antimatter imbalance came to be in our Universe: through a novel, relic species that decays away, admitting both baryon-number violation and possessing C-violation and CP-violation, creating an excess of baryons over anti-baryons.

Of course, we aren’t certain that this is the scenario by which it actually occurred in our Universe. GUT baryogenesis is an example of a theoretical scenario — one that’s plausible but that lacks support, and that has never been sufficiently tested — where some new, undiscovered physics in the very early Universe creates an asymmetry between baryons and anti-baryons. But there’s a catch: even if we create a baryon asymmetry early on, and allow matter to dominate over antimatter in some sense, that asymmetry has to survive electroweak symmetry breaking. And that poses both opportunities and challenges that, as explorers of the nature of reality, we’re compelled to look into more deeply.

Explanatory diagram showing how symmetry breaking works. At a high enough energy level, a ball settled in the center (lowest point), and the result has symmetry. At lower energy levels, the center becomes unstable, the ball rolls to a lower point – but in doing so, it settles on an (arbitrary) position and the result is that symmetry is broken – the resulting position is not symmetrical.

What you see, above, is another example of spontaneous symmetry breaking: one where the shape of the potential itself changes as the conditions of the Universe change. At the left, the Universe is in a symmetric state, and all directions look equivalent. Then, in the middle, the shape of the potential has changed, and where the blue ball was (where the white ball is now) is known as a point of unstable equilibrium: it can exist there temporarily, but the slightest change, blip, or fluctuation will cause the ball to roll either to the left or to the right. Finally, the right-most panel shows the ball after the symmetry has broken, with our “field” winding up in a new, true minimum.

This is what physicists call a second-order phase transition. If the electroweak symmetry breaks in precisely this fashion, then there are no baryon-violating interactions that can occur, and any initial asymmetry in the baryon sector i.e., any excess of quark-containing particles over antiquark-containing particles will persist. If something like GUT baryogenesis occurs early on, this is the type of transition that’s required to maintain it. However, as the phrase “second-order phase transition” might seem to indicate, that’s not the only possibility for the type of phase transition that could occur. There’s also the possibility of a first-order phase transition, where those transitions look very different from second-order ones.

In many physical instances, you can find yourself trapped in a local, false minimum, unable to reach the lowest-energy state, which is known as the true minimum. Whether you receive a kick to hurdle the barrier, which can occur classically, or whether you take the purely quantum mechanical path of quantum tunneling, going from one state to another is always possible so long as no fundamental conservation laws are violated. This is an example of a first-order phase transition, rather than a smooth (second-order) transition without any false minima.

A first-order phase transition describes a transition akin to the boiling of water. When water boils, it transitions from a liquid state to a gaseous state, where rather than increase in temperature, it involves latent heat: the heat of vaporization. Heat pumped into the system goes into changing its phase, not changing its temperature.

If the electroweak phase transition is second-order, it “smoothly transitions” from a symmetric state to a broken symmetry state without changing the net number of baryons or leptons in the Universe. But if the transition is first-order instead, the Universe will develop “bubbles” where the transition occurs first, the same way boiling water develops “bubbles” within the water around nucleation points, rather than all transitioning from liquid to gas at once.

If we have a first-order breaking of the electroweak symmetry, the region outside the bubbles allows for what are known as sphaleron interactions: where you can create a net number of baryons and leptons, but you must conserve the difference, or the net number of baryons minus the net number of leptons. (E.g., you can create a proton if you also create an electron, conserving baryon minus lepton number.) Although the Standard Model, alone, doesn’t have enough CP-violation to account for the observed baryon number in the Universe (and predicts a second-order electroweak phase transition), either:

• extensions to the Standard Model at the electroweak scale,
• extensions above the electroweak scale, such as Higgs doublet models, supersymmetry, or extra dimensions,
• or an early period of leptogenesis, creating (perhaps) a neutrino asymmetry, could lead to sphalerons converting that into a baryon asymmetry,

could all result in the net creation of baryons and a first-order transition, as the electroweak transition’s “bubbles” expand to eventually occupy all of space.

When the electroweak symmetry (the symmetry that corresponds to the Higgs field) breaks, the combination of CP-violation and baryon number violation can create a matter/antimatter asymmetry where there was none before, owing to the effect of sphaleron interactions working on, for example, a neutrino excess. This can only occur, however, if the electroweak phase transition is first-order, rather than the second-order transition predicted by the Standard Model alone.

This brings us up to the present day. We know that, according to all of the indications that we have from experiment (including at the Large Hadron Collider), we see only indications that the Standard Model is correct, with no significant deviations from it seen in the full suite of data collected. And yet, we know that our Universe is full of matter (and not antimatter), and that somehow it came to be this way. The big question then becomes, “is there a way to learn more about how it got to be this way?”

And the answer is yes. We can study electroweak symmetry breaking by studying the Higgs boson and other aspects of physics at (and just above) the electroweak scale, and can test a wide variety of scenarios against one another by performing exactly these kinds of experiments.

And that’s exactly why a new collider is needed. We need a new collider to create large numbers of Higgs bosons through a variety of different channels, and measure their properties more precisely than we ever could at the Large Hadron Collider. We need a new collider to rise above the energies required by the electroweak phase transition and create a restored-symmetry state, and to probe the aftermath of that. And we need a new collider for all the different ways it will allow us to study the electroweak phase transition, with the goal of determining whether it’s first-order or second-order, with all the implications that come along with it for understanding how our Universe came to be a matter-dominated place.

The cross-section for producing a Higgs boson (plus other particles) through various production channels at a lepton-antilepton collider. The enormous yellow peak corresponds to producing a Z-boson and a Higgs boson together: the most prolific production channel, which peaks at 216.3 GeV of energy: the Higgs boson mass plus the Z-boson mass.

The nature of the electroweak phase transition, and whether it’s first-order or second-order, is arguably the biggest puzzle remaining to solve concerning the Higgs. We can learn whether it’s purely the Standard Model Higgs, or whether it mixes with other particles with the same quantum numbers, even at much greater energies. We can learn whether the production ratios are exactly as anticipated, or whether there are hints of new physics. And, if we determine that it’s either first-order or second-order, we can learn what viable options remain (and which options are ruled out) for creating the matter-antimatter asymmetry we presently observe our Universe to have.

As a bonus, if the electroweak phase transition does turn out to be a first-order phase transition, it brings up an astonishing possibility: if we build a relativistic heavy ion collider whose energies far exceed the electroweak scale, we can create a quark-gluon plasma that may wind up directly creating an excess of baryons over antibaryons. Baryon number violation has never been observed in a particle physics experiment; we can only assume that it must have occurred at some point in the Universe’s history. But a new particle collider will certainly teach us more about how this matter-antimatter asymmetry could have (and couldn’t have) come to be, while simultaneously giving us a chance at a big prize: to discover exactly how it did occur, if nature is kind. The only question is whether we’ll build the machines needed to find out.