The following is an extract from our Lost in Space-Time newsletter. Each month, we hand over the keyboard to a physicist or two to tell you about fascinating ideas from their corner of the universe. You can sign up for Lost in Space-Time here.
Ten years ago on 4 July 2012, I was both delighted and astonished by the discovery being announced to the world. The CMS and ATLAS collaborations at CERN’s Large Hadron Collider were now certain – the Higgs boson was real. I was part of the CMS (Compact Muon Solenoid) team so knew the announcement was coming, but hearing the public statements confirm it was thrilling. Peter Higgs had hypothesised 50 years prior that the particle could be the last missing piece of the standard model of particle physics. After decades of hunting for it, we confirmed its existence and that there is a new force field, the Higgs field, permeating all of space. Through Higgs boson interactions with this Higgs field, elementary particles gain their observed masses. The more strongly they interact with the field, the larger their masses.
With this year’s decadal anniversary, I’ve been reflecting on where the Higgs boson will take us next. Although the standard model puzzle is now complete, we physicists believe that it is an incomplete description of the universe, a piece of a greater cosmic puzzle. The standard model describes how objects and particles interact with the electromagnetic, weak nuclear and strong nuclear forces, but we have not yet successfully included gravity in this picture. The standard model also says nothing about the dark matter that makes up 85 per cent of the matter in the universe or the dark energy that we believe is contributing to the universe’s accelerating expansion.
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The Higgs boson could be our best tool for cracking some of these puzzles. Thanks to high-energy experiments at CERN, it could be a compass for navigating through this uncharted territory of new physics. Assuming, that is, that things start to change.
The ATLAS and CMS experiments have been combing through vast amounts of high-energy data on the Higgs boson, but so far they haven’t revealed anything unexpected. We have measured many aspects of the Higgs boson, such as its mass, spin and couplings to other particles, all of which are in line with what the standard model predicts.
However, there is one important aspect of the Higgs boson for which there is still much to learn: the self-interaction of the Higgs boson. The standard model predicts that elementary particles derive their masses from their interactions with the Higgs field, and the Higgs boson is no different. It interacts with the Higgs field to gain its observed mass of 125 GeV/c2. The parameters which describe the Higgs boson’s self-interaction are predicted by the standard model, and any deviations from expectations would hint at new physics. One way we can probe the Higgs boson’s self-interaction is through a process called double Higgs production, or Higgs pair production, in which two Higgs bosons are produced in the same process.
Unfortunately, the rate at which the Large Hadron Collider can produce Higgs boson pairs according to the standard model is very, very small – about a thousand times smaller than the rate at which single Higgs bosons are produced. The rate is actually so small that we would not be able to produce enough Higgs pairs at the Large Hadron Collider to provide enough evidence for a discovery of double Higgs production.
Into another dimension
However, if the standard model is wrong, so too could be the rate at which these Higgs boson pairs are produced. One intriguing new beyond-the-standard-model scenario is that of extra dimensions of space-time. If they exist, the rate of double Higgs production could be enhanced, making it conceivable that we could measure the process at the Large Hadron Collider. We wouldn’t be aware of these extra dimensions in the same way an ant confined to a piece of paper only experiences two dimensions (left-right, forward-backward). It doesn’t realise that its two-dimensional piece of paper exists in a higher, three-dimensional space. We might discover we’re akin to ants walking on a three-dimensional plane in a higher dimensional universe.
These extra dimensions could explain the enormous differences in strength between gravity and the forces that make up the standard model (the electromagnetic, weak and strong forces). In the theory of warped extra dimensions, there are two “branes” – lower dimensional worlds embedded in a higher dimensional space. The first is our three-dimensional world, where we and the particles of the standard model exist. The second is where particles called gravitons that transmit the gravitational force are concentrated but not confined. Gravitons can also permeate the higher-dimensional “bulk” space between the two branes.
Gravity’s weakness may come from it being effectively diluted as it permeates throughout the extra dimensions. The warping of the higher dimensional space exacerbates the extremely large difference in energy scales.
Producing more double Higgs bosons at the Large Hadron Collider could also provide evidence of graviton particles. In theories of extra dimensions, the massive gravitons would decay to Higgs pairs, which would subsequently decay to standard model particles. The graviton could decay to other types of standard model particles, giving us additional ways to study this theoretical particle.
In addition to giving us insight into extra dimensions, double Higgs production may also provide a window into the eventual fate of our universe. First, let’s take a step back and understand the “vacuum” that makes up our universe. In quantum physics, the “vacuum” is the lowest energy state of a system but is not necessarily empty as we may typically imagine. In fact, from the viewpoint of quantum physics, the vacuum state of our universe is teeming with virtual particle-antiparticle pairs popping in and out of existence. Typically, unless there is a physical particle sitting at a particular point in space, the field associated with that particle has an average value, called the vacuum expectation value, of zero. However, that is not true for the Higgs field, which has a non-zero vacuum expectation value. This implies that the vacuum is full of Higgs bosons.
The non-zero vacuum expectation value of the Higgs boson is due to the unusual shape of the potential of the Higgs field. In physics, a potential function determines the evolution of a system. For example, the gravitational potential around a massive body determines how other massive objects around it will interact with it. The gravitational potential around a spherical mass will have the shape of a bowl, with the minimum at its centre. If you were to drop a marble in this potential, it would roll towards the centre. On the other hand, the Higgs potential function has an unusual shape more like the bottom of a wine bottle – if you were to drop a marble into it, the marble would roll down the sides, falling into the lowest point in a ring around the centre.
In our universe, the Higgs marble has come to rest at a particular spot, but that point may not be a stable one. We call this phenomena metastability and it has important implications for the eventual fate of our universe. It means that quantum fluctuations could cause our universe to tunnel into a different state, resulting in an unrecognisable world obeying completely different laws of physics. More precise measurements of the Higgs boson and other particles could help us ascertain exactly how stable our universe is.
Rather than closing a chapter in particle physics, the discovery of the Higgs boson has opened a new one, ushering in a renewed era of exploration at the highest energy frontier. In July of this year, Run 3 of the Large Hadron Collider began, with proton-proton collisions at a higher energy level than ever before. The data we gather from this run will allow us to further explore these exciting possibilities, as well as many other potential new physics scenarios. The Large Hadron Collider will be upgraded in a few years to the High-Luminosity Large Hadron Collider, which will collide protons at much higher rates, providing even more data in a shorter amount of time. This will allow us to finally measure Higgs pair production and the Higgs self-coupling, and hopefully better understand the destiny of our universe.
Toyoko Orimoto is an experimental particle physicist based at Northeastern University in Boston, Massachusetts, who works on fundamental questions about the universe.Â
