The Compact Muon Solenoid (CMS) detector in a tunnel at 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 discovery of the boson was an important occasion because it meant that physicists were one step closer to studying the field associated with the boson that gives particles mass.
But since 2012, particle physics has not had another seismic event. Important discoveries were made—measurements were made of the muon’s behavior in a magnetic field, the mass of the W boson was more precisely measured, and new particles were discovered—but nothing as startling as the confirmation of the Higgs.
But we are not pessimistic: there are many fascinating experiments underway that may provide the next big leap in our understanding of the subatomic universe. So we asked a few physicists where they think this breakthrough might happen. The answers below have been shortened and slightly edited for clarity.
A physicist at Rice University and a participant in the CMS experiment at CERN
The next big thing in physics will be a better understanding of dark matter. A number of facilities will come on board and allow us to probe the nature of dark matter far 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 we need to study, and we will be able to study their properties with enormous precision.
This in turn will give us a new window through which to study the dark matter that permeates the universe, as any deviation from the predictions of the Standard Model will point us in the direction of the new physics involved. Other new facilities, such as Cosmic Microwave Background Stage 4 (CMB-S4), will operate on a similar time frame. It will be possible to combine the results of these different facilities to paint our best picture yet of the dark matter that permeates the universe.
“The next big thing in physics will be a better understanding of dark matter.”
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 5 times more matter than atoms (in any form) can account for (> 50 sigma). We have good candidates – the lightest supersymmetric particle and the axion – and experiments with the ability to make discoveries. The dark matter problem has been with us for almost 100 years and is ripe for resolution. When it does, we will close a mystery, discover a new form of matter, and open a new door to the study of the first microsecond of the universe. What more could you ask for!
2) Detecting the signature of inflation-induced gravitational waves in the polarization of the cosmic microwave background. If the “B-mode” polarization signature is found and confirmed, it will tell us when inflation happened, and it will be the oldest relic in cosmology. (If detected, these gravitational waves would have been produced when the universe was 10^-36 seconds old.) It’s not an easy task, but experimenters/experimenters are up to it: the signal is a nanoKelvin level in the CMB (whose temperature is 2.76 K).
3) Confirmation that the Hubble discrepancy is real. Namely, that the expansion rate measured directly today is not equal to that measured 400,000 years ago (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, an unexpected discovery in a new facility like LIGO or a telescope or an accelerator is the most transformative. LIGO was a fantastic success, but all the events it detected were the predicted ones: the merger 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 from the mid-1960s)
I won’t even mention signs of life elsewhere (eg Venus, a moon of Jupiter or Saturn, or in the atmosphere of an exoplanet). It will happen, the question is when and where.
“…we will close a mystery, discover a new form of matter and open a new door to the study of the first microsecond of the universe. What more could you ask for!”
Particle physicist at the University of Hamburg and participant in the CMS and FCC-ee collaboration
So that’s also kind of a challenging situation that we’re in that we weren’t in when we were dealing with the standard model of the Higgs boson. With the standard model of the Higgs boson, you actually had a nice puzzle and you were missing that one piece. You kind of knew the shape of the piece and then you looked in the box, found the shape of the piece and put it inside. What we have now is a box full of 3D or possibly 2D puzzle pieces. You’re not too sure. And they just said, “Yeah, there’s got to be something there. Have fun.’
According to the Standard Model, how often the Higgs boson interacts or decays—the two things are interchangeable for particle physicists—depends somewhat on the mass of the other Higgs particle, for that matter. This means you can predict (if you know the mass of all those particles) how often they should be made. When you create a Higgs boson, often the Higgs boson must create these particles. And that’s the kind of thing we’ve been checking 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 is, then it interacts with the top quarks. Lately, that it can decay to muons — all of these things are tests of the internal consistency of the Standard Model in the hope that we’ll find something that’s inconsistent that will point us to where the Standard Model starts to break down.
There are some very exciting dark matter experiments coming online again. If they see something, [the LHC] can change our choice so we can check if we can reproduce this in a consistent way. And that’s because these particle detectors are really very good at this: once you know what you’re looking for, it’s very easy to find an algorithm to somehow isolate these particles.
I mean the xenon experiment and the LUX-Zeplin experiment. Both have been upgraded in recent years and are now coming back online. These experiments are big tanks of xenon (which is why they all have an X in their name) and they all hope that Earth is moving through dark matter and the experiment is standing on Earth, and that the dark matter will then Xenon atom and they can detect this an atom bouncing around.
The expectation that these kinds of experiments should produce something groundbreaking and Nobel Prize-winning every five years is unrealistic. It’s a long-term science where you have to plan things and you need huge data sets that are extremely difficult to analyze.
“With the standard model of the Higgs boson, you actually had a nice puzzle and you were missing that one piece… What we have now is a box full of 3D or possibly 2D puzzle pieces. You’re not too sure.
Particle physicist at Nikhef and participant in the LHCb experiment at CERN
We are currently preparing to restart the LHC with a brand new LHCb detector (called “LHCb Upgrade I”), so all the excitement is to get the new detector up and running, as well as the data processing chain, which is what I’m working on .
The main goal for us will be to determine the “flavor anomalies” in particles containing b quarks. I’m very excited that they show a discrepancy with the Standard Model: there seem to be too few b-quarks that transform into muon pairs compared to electrons. I started this study at LHCb 10 years ago, so I will be watching it very closely. The vast amount of data we will collect over the next 10 years will tell us.
If this is true, it requires a new natural force associated with (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, it 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 collaboration at the Moriond conference in March. These may be the first signs of the new particles causing the taste abnormalities. But experience shows that such irregularities disappear with more data. So let’s see.
If the LHC is too low energy to produce these new bosons, we need another machine. This could be the brute force of the Future Circular Collider (FCC) and its 100 km and energy 7 times greater than the LHC. Or a much smaller but more challenging muon collider. Depending on what’s causing the anomalies (still hoping they’ll survive verification with more data), the muon collider could be the perfect tool: if we have a problem with muons, let’s use muons to find out.
“There are some ‘bumps’ in the data shown by the ATLAS and CMS collaboration… These may be the first signs of the new particles causing taste abnormalities.”
Physicist at Texas A&M University and spokesperson for the CDF Collaboration
I see two major potential breakthroughs in physics over the next 10 years in physics. The first is that with the recent observation from the CDF experiment at Fermilab that the mass of the W-boson is 7 standard deviations short of expectations, there will be worldwide focus on this potential breakthrough in the Standard Model of particles…
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