10 Years After the Higgs Boson, What’s the Next Big Thing for Physics?

The Compact Muon Solenoid (CMS) detector in a tunnel of the Large Hadron Collider. Photo: VALENTIN FLAURAUD/AFP (Getty Images) On July 4, 2012, scientists at CERN confirmed the observation of the Higgs boson, an elementary particle first proposed in the 1960s. The boson’s discovery was a momentous occasion, as it meant physicists were a step closer to probing the field associated with the boson, which gives particles mass.
But since 2012, particle physics hasn’t had another seismic event . Important discoveries have been made—measurements were taken of the muon’s behavior in a magnetic field , the W boson’s mass was more precisely measured , and new particles have been discovered —but nothing as jaw-dropping as the Higgs confirmation. But we’re not pessimistic: There are many fascinating experiments currently underway that may provide the next big leap in our understanding of the subatomic universe. So we asked several physicists about where they think that breakthrough may happen. The below responses have been condensed and lightly edited for clarity. Physicist at Rice University and contributor to the CMS experiment at CERN The next big thing in physics will be a better understanding of dark matter. A number of facilities will turn on and allow us to explore the nature of dark matter significantly better than has been achieved to date. For example, the High Luminosity-LHC will increase by an order of magnitude the number of Higgs bosons that we have to study, and we will be able to study their properties with tremendous precision. That in turn will give us a new window through which to explore the dark matter that pervades the universe, as any deviations from Standard Model predictions will point us in the direction of the new physics involved. Other new facilities, such as the Cosmic Microwave Background Stage 4 (CMB-S4), will operate in a similar time frame. It will be possible to combine the results from these different facilities to paint our best picture yet of the dark matter that pervades the universe. Theoretical cosmologist at the University of Chicago Here are five possibilities, at least as good as the Higgs.
1) Discovery of the dark matter particle. We have an airtight case that there is 5x more matter than atoms (in any form) can account for (> 50 sigma). We have good candidates—the lightest supersymmetry particle and the axion—and experiments with the capability of making a discovery. The dark matter problem has been with us almost 100 years and is ripe to be solved. When it is we will close out a mystery, discover a new form of matter, and open a new door to studying the first microsecond of the Universe. What more could you ask for!
2) Discovery of the signature of inflation-produced gravitational waves in the polarization of the Cosmic Microwave Background. If the “B-mode” polarization signature is discovered and confirmed, this would tell us when inflation took place as well as being the oldest relic in cosmology. (If detected, these gravitational waves would have been produced when the Universe was 10^-36 sec old.) It is not an easy task, but the experiments/experimenters are up to it: the signal is a the nanoKelvin level in the CMB (whose temperature is 2.76 K).
3) Confirmation that the Hubble discrepancy is real. Namely, that the expansion rate directly measured today is not equal to that measured at 400,000 years (cosmic microwave background measurements) and extrapolated forward using our current cosmological paradigm (Lambda CDM). Both measurements can be correct if something is missing from Lambda CDM.
4) The discovery of supersymmetry at CERN. A whole new world of particles and the first big home run for superstring theory.
5) Something unexpected at the Laser Interferometer Gravitational-Wave Observatory (LIGO). As we know and like to say, it is the unexpected discovery at a new facility like LIGO or telescope or accelerator that is the most transformational. LIGO has been a fantastic success, but all the events it has discovered were the ones predicted: coalescences of two black holes, two neutron stars, and a black hole and a neutron star. How about a surprise? (e.g., like pulsars or Quasars of the mid 1960s)
I won’t even mention signs of life elsewhere (e.g., Venus, a moon of Jupiter or Saturn, or in the atmosphere of an exoplanet). This is going to happen, the only question is when and where. Particle physicist at the University of Hamburg and a contributor to the CMS and FCC-ee collaborations So this is also a sort of challenging situation that we’re in, that we weren’t in when we were dealing with the Standard Model Higgs boson. With the Standard Model Higgs boson, you basically had a nice jigsaw puzzle and you were missing this one piece. You sort of knew the shape of the piece, and then you looked in the box and you found the shape of the piece and you put it in. What we have now is a box full of 3D or possibly 2D puzzle pieces. You’re not really sure. And they just said, ‘yeah, there should be something there. Have fun.’
According to the Standard Model, how often the Higgs boson interacts or falls apart—these two things are interchangeable for particle physicists—that sort of depends on the mass of the other particle of the Higgs, for that matter. That means that you can predict (if you know the mass of all these particles) how often they should be made. When you make a Higgs boson, often the Higgs boson should make those particles. And this is the kind of stuff that we’ve been checking out for the last year: seeing that the Higgs boson decays to Z bosons, seeing that the Higgs boson decays to W bosons, seeing that it decays to Tau leptons, to B quarks, if it then it interacts with top quarks. Recently that it can decay to muons—those kinds of things are all tests of internal consistency of the Standard Model in the hope that we find something that is inconsistent, that will guide us to see where where the Standard Model starts breaking.
There are a few very exciting dark matter experiments coming online again. If they see something, [the LHC] can change our selection so that we can check if we can also reproduce this in a consistent manner. And that’s because that’s really what these particle detectors are very good at: once you know what you’re looking for, it’s very easy to find an algorithm to sort of isolate these particles.
I’m referring to the Xenon experiment and the LUX-Zeplin experiment. Both of them have been upgraded over the last years and they’re now coming online again. These experiments are big tanks of xenon (which is why they all have X’s in their name), and all of them are hoping that the Earth is moving through dark matter and the experiment is standing on the Earth, and that dark matter will then the Xenon atom and they can detect that atom bouncing around.
The expectation that these kinds of experiments should produce something groundbreaking, Nobel Prize-winning every five years is unrealistic. This is long-term science where you need to plan things and you need huge datasets that are extremely difficult to analyze. Particle physicist at Nikhef and a contributor to the LHCb experiment at CERN Presently, we are preparing for the LHC restart with a brand-new LHCb detector (dubbed “LHCb Upgrade I”), so all the excitement is into getting the new detector to work, as well as the data processing chain, which is what I work on.
The main goal for us will be to pinpoint the “flavour anomalies” in particles containing b quarks. I am very excited that these exhibit a discrepancy with the Standard Model: There seem to be too few b quarks transforming into pairs of muons as compared to electrons. I started this study in LHCb 10 years ago, so will watch it very closely. The enormous amount of data we will be collecting in the next 10 years will tell us.
If this is true, it requires a new force of nature associated to (at least) one new boson. It could be a Z’ boson, similar to the known Z, or something completely different, like leptoquarks (or both). Either way, that would be a revolution in particle physics.
The next question is whether these new particles can be produced at the LHC. There are some “bumps” in the data shown by the ATLAS and CMS collaborations at the Moriond conference in March. These may be first signs of the new particles causing the flavour anomalies. But experience has shown that such bumps disappear with more data. So let’s see.
If the LHC is of too low energy to produce these new bosons, we need another machine. That could be the brute force of Future Circular Collider (FCC) and its 100km and energy 7 times larger than the LHC. Or a much smaller but more challenging muon collider. Depending on what causes the anomalies (still hoping they will survive scrutiny with more data), a muon collider may be the ideal tool: if we have a problem with muons, let’s use muons to find out. Physicist at Texas A&M University and a spokesperson for the CDF collaboration I see two big potential breakthroughs in physics over the next 10 years in physics. The first is that with the recent observation by the CDF experiment at Fermilab that the mass of the W-boson is 7 standard deviations away from expectations , there will be a worldwide focus on this potential break in the Standard Model of Particle Physics. This is exceedingly difficult measurement to make, but the leading competitors at the LHC, the ATLAS and CMS experiments, have incredibly powerful detectors and lots of data coming.
If the result is confirmed, and there is no change in the Standard Model prediction, then this must mean there is some new fundamental particle(s) or force(s) in nature that needs to be discovered and then understood. Ideally, any such discovery would provide a clue to understanding the dark matter that fills the universe.
For decades, physicists and astronomers have basically assumed that the dark matter is made up of fundamental particles. The next generation of dark matter experiments are coming online, and within the next 10 years are expected to have enough sensitivity to observe the individual dark matter particle interactions, if that’s how nature is (and the current best guesses that take into account cosmology are correct). If they don’t, then this would signal a fundamental shift in our guesses about the nature of dark matter and how it came to exist in our universe.
Either way, between these two fields, our understanding of the fundamental particles that fill the universe has a good chance of fundamentally changing within the next 10 years, or we will be looking to understand in very different ways, since nature is so stingy with her secrets.
It’s not clear if the LHC can discover dark matter. The HOPE is that it can produce dark matter particles (if they exist), but that requires that they can produced in collisions between protons. If so, we have a shot. Another possibility is that the LHC can produce particles that decay into dark matter particles. That was the hope of supersymmetry , but that hasn’t panned out. If they can produce them, then the hope is with lots of collisions, and great detectors, we could discover them. If they can’t produce dark matter particles, or it’s super rare… then they aren’t in the game. It’s an interesting experiment to do either way, but it’s exploring uncharted territory. Totally worth doing, but high-risk high reward. My personal guess is that they will be detected with a dedicated, deep underground detector. Since we are quite sure that the Milky Way is full of dark matter, I think it’s a pretty safe bet that if dark matter is a particle then it should be flowing through the Earth for free (just like neutrinos). Thus, the question is whether the dark matter detectors like CDMS or LZ are massive enough or sensitive enough to observe an interaction (again, assuming they interact fashion at all).