Scientists at the Large Hadron Collider triumphantly announced the discovery of the Higgs boson back in the summer of 2012. Nicknamed “the God particle,” it was the last new undiscovered particle predicted by the backbone theory of particle physics.
Since then, physicists have found a whole lot of, well, nothing. The Higgs high hasn’t carried through the past decade, and no groundbreaking discoveries have appeared since 2012. New York Times science reporter Dennis Overbye called this silence “ominous.”
But ahead lies a whole frontier of grand unsolved mysteries, including why there’s more matter than antimatter in the universe, what the true identity of dark matter and dark energy is, or how the strange, ultra-weak neutrino particles ended up so ghostly. For many, it’s an exciting time, with lots of new ideas and upcoming experiments to test them.
“With all of these projects, the prospect for discovery is very real,” JoAnne Hewitt, SLAC National Accelerator Laboratory chief research officer, told Gizmodo. More new experiments means a higher potential to solve some of these mysteries – or at least uncover clues.
Photo: Katrin Tritium Neutrino Experiment
One of the most abundant particles in the universe is also among the most difficult to study: The neutrino is often called “ghostly” because of how weakly it interacts with regular matter. Scientists now know that the particle comes in three separate flavours and three separate mass states – but the mass states don’t cleanly correspond to the flavours, and each flavour is a combination of the three mass states (blame quantum mechanics for the weirdness). Scientists hope to find out what these masses’ values are and the order they appear in when they combine to make up each flavour. Experiments like KATRIN in Germany are working on measuring those masses; KATRIN will be taking data for at least the next five years.
“Sometimes our job is to set up the environment for the successes of the future.”
Neutrinos’ mass weirdness comes with a strange side effect: as they travel through space, they seem to oscillate between flavours. Experiments that will attempt to understand this oscillation are the Jiangmen Underground Neutrino Observatory in China, scheduled to begin taking data on neutrinos emitted from nearby nuclear power plants next year, and Super-Kamiokande, which has long been taking observations in Japan. The United States has begun construction on its own large neutrino beam and corresponding detector, called the long-baseline neutrino facility (LBNF) and Deep Underground Neutrino Experiment (DUNE), which will be located in the states of Illinois and South Dakota, respectively. The internationally funded $1.5 billion (£1.2 billion) LBNF/DUNE experiment is scheduled to come online in 2024 and to be completely operational by 2027. Other experiments are dedicated to cracking these neutrino mysteries as well, like PROSPECT at Oak Ridge National Laboratory in Tennessee and the Short-Baseline Neutrino Program at Fermilab in Illinois.
Neutrinos are interesting for more than just their innate properties. The IceCube neutrino observatory buried underground at the South Pole measures neutrinos that travel through the Earth. IceCube only recently cracked the mystery of where ultra-high-energy cosmic ray particles came from. Construction of an underwater neutrino telescope, which will hunt for similar neutrinos from the Mediterranean Sea and is called KM3NeT, could begin as early as 2025.
Wide Field Infrared Survey Telescope rendering (Graphic: NASA)
Looking deep into the skies
The way our universe looks and evolves is determined by three things: ordinary matter, mysterious dark matter, and the even more mysterious dark energy. This means that particle physics doesn’t just happen on Earth; fundamental physical properties could leave their mark on the very structure of the cosmos. Experiments that are closer to telescopes than particle detectors may provide answers to unexplained physics mysteries.
Perhaps the biggest question is the nature of dark energy, the mysterious energy that permeates the cosmos and is driving the universe’s expansion to accelerate. The Dark Energy Spectroscopic Instrument (DESI) survey only just began this past year, and the US National Science Foundation/Department Of Energy’s $500 million (£386 million) Large Synoptic Survey Telescope in Chile is expected to come online in 2020 and begin its own survey in 2022. The Wide Field Infrared Survey Telescope, a space-based telescope, will launch in the mid 2020s, provided that deadlines are met and the US Congress doesn’t cut the $3.2 billion (£2.5 billion) project’s funding due to cost and schedule overruns from the upcoming James Webb Space Telescope. Experiments like these will survey the universe by observing lots and lots of galaxies to try to understand whether dark energy is caused by particles, an innate feature of the universe’s fabric, the result of unaccounted-for maths in our present-day theories of gravity, or something else.
Dark matter-hunting telescopes are probing other anomalies, like a mysterious drop in the number of high-energy electrons that reach Earth, a strange pattern in the signal from distant hydrogen, an antimatter excess from space, and extra gamma rays coming from the centre of our galaxy. Dark matter is often thrown around as a potential explanation for these observations, but there are other potential causes, such as pulsing, spinning neutron stars or simply that our theories are wrong. All of these anomalies might get explanations, or at least more data to help understand them, in the coming decade.
“Collider-based experiments collide things at high energy and look what comes out, but these processes have been happening for the duration of the universe,” Rebecca Leane, an MIT postdoctoral researcher, told Gizmodo. “For example, you have high-energy events that make cosmic rays that collide with everything in space. We can use these to try and find new particles and see if there’s something different happening from the physics of what we understand.”
The Linac4 accelerator (Photo: Robert Hradil, Monika Majer/ProStudio22.ch)
The LHC gets an upgrade
Particle physics’ hallmark experiment, the Large Hadron Collider in Geneva, Switzerland, may have produced one of most important particle physics finds of the century, but physicists hope to squeeze more out of the machine yet. The Long Shutdown 2 will keep the machine offline until 2021 for maintenance work, after which it will begin running again at similar or slightly higher energies until 2023. It will then receive a major upgrade, planned for completion in 2026.
The billion-dollar upgrade, called the High Luminosity-LHC, could increase the number of collisions the machine produces per second by as much as 10 times by 2030, we’ve reported. Each collision has the tiny potential of producing new particles or violations to the laws of physics as presently theorised. Increasing the collision rate, or luminosity, increases the statistics available for physicists’ search and gives them more precise values for things like particles’ masses or how often they decay into other particles. It’s kind of like each of the particle collisions is its own experiment with a potential outcome, but the experiment finding something and not finding something look so similar that you need to run it a billion times to know whether the outcome happened more than you’d expect. Now, the LHC can do even more of these mini experiments in the same amount of time. It’s pretty nuts.
“The LHC is a marathon, not a sprint, and we have to run it for a long time.”
More statistics will allow physicists to see if theoretical particle decays exist as predicted – if they don’t, it opens up new lines of inquiry. One such rare decay spotted by the LHCb detector seems to vary from what current theories predict, and more statistics will determine if this variance is actually statistically significant, meaning highly unlikely to have appeared by chance and potentially a sign of unknown particles or physical forces. Scientists will also attempt to produce as many Higgs bosons as they can, studying its many properties and comparing them to the Standard Model, looking for deviations.
“The LHC is a marathon, not a sprint, and we have to run it for a long time,” James Beacham, particle physicist with the ATLAS Experiment at the Large Hadron Collider at CERN, told Gizmodo. “Something that may only show up with a tiny bump may finally get a chance to peep through the noise. That’s what the next 10 years of the LHC is all about.”
Scientists are hoping to squeeze the LHC for all that it’s worth, sifting through data in new ways or building new detectors that can access physics theories that the old detectors can’t. For example, the LHC’s detectors today hunt for particles that leave a signature in the few meters just outside of the initial collision. But what if, instead, some particles drift all the way out of the detector before they’re actually detectable? Mostly plastic detectors outside of the main LHC detectors like MoEDAL and MilliQan hope to find these longer-lived particles. One proposal, MATHUSLA, calls for an aeroplane hangar-sized, air-filled chamber above ground, far from the collision points, meant to trap any of these potential wanderers.
Others are thinking about ways to look at and collect LHC data differently. In one recent analysis, the LHCb detector took a closer look for particles called dark photons by tweaking the system that determines whether data should be thrown out or not. There is still plenty of LHC data to weed through. With all the new data from the HL-LHC, it’s all sure to keep physicists busy.
Waiting for something to happen
One such particle yet to be detected at the LHC is one to explain the mysterious source of mass that we call dark matter – perhaps five times more mass than regular matter. What this dark matter is made of, scientists still don’t know. Until recently, the most popular theory was “WIMPS,” or weakly interacting massive particles. Right now, experiments around the world are lying in wait, buried deep underground at various locations, hoping that one of these WIMPs will interact with their sensitive detecting medium and generate a visible signal.
For now, these experiments haven’t found anything; or more accurately, they’ve found a lot of nothing. They’ve ruled out potential WIMP candidates. But finding nothing, while valuable, is certainly less exciting than finding something. During the next decade, these experiments will receive upgrades to improve their sensitivity. The XENON-nT experiment, an enormous vat of liquid xenon deep underground in Italy, is in the midst of being brought online and will begin its own search soon. Another huge vat of xenon in South Dakota, LZ, will begin its own search in 2020. Another experiment composed of super-sensitive, ultra-cold semiconductor detectors called SuperCDMS SNOLAB will begin taking data in Ontario in the early 2020s.
Nor are WIMPs the only dark matter candidates that scientists are hunting for. An alternative, super-low-mass particles called axions might instead appear in experiments, a lot like slowly tuned radios waiting to hear a telltale signal. Perhaps they’re blasting at Earth from the Sun.
Giant underground detectors are good for some less mysterious physics as well. Since they’re basically just extremely sensitive particle detectors, scientists employ them to measure incredibly rare radioactive decay events. One they especially hope to see is called neutrinoless double beta decay, in which two neutrons from an atom’s nucleus simultaneously decay into protons, each spitting out an electron and a neutrino... which then meets the other neutrino and annihilates. If this kind of reaction existed, it would prove that neutrinos are their own antiparticle, indirectly bolstering another theory of the early universe that explains why there’s more matter than antimatter. There are other dedicated experiments hunting for this reaction, such as the LEGEND-200 experiment, scheduled to start taking data in 2021.
The muon’s moment
Last year, we asked celebrity physicist Brian Cox about ghosts as well as what the most important outstanding mystery in physics was – and he pointed to the results of Fermilab’s experiment measuring something called the “muon g-2.” Muons are particles kind of like electrons, with the same electric charge but more massive. They also have a magnetic moment, meaning they react and twist in an applied magnetic field. A measure of this moment, called the g-factor, should be equal to two if not for an extra quantum piece. Scientists are using a 50-foot-wide electromagnet to try and measure this extra piece, the g-2.
As of now, the g-2 value seems to differ from the Standard Model of particle physics’ predictions. But wrapped in that extra piece is information on all of the other particles that can interact with the muon. That means that perhaps some new particle, or some new, unexplained behaviour, is causing the discrepancy. Differences between theory and experiment, if they hold up, are clues to the answers to the big questions of particle physics.
But the g-2 discrepancy is not statistically significant, yet. Researchers are working to measure the value with higher precision, which might make the discrepancy clearer – or get rid of it completely. The current run at Fermilab will go until 2020, and we’ll hopefully have a clearer answer soon after.
WP 14 of the HL-LHC project, TDIS pre-series (Photo: Brice, Maximilien; Ordan, Julien Marius)
Going even bigger
On top of the myriad particle physics projects in the works, physicists (and international collaborations) are considering even larger, next-generation colliders. A consortium in China plans to begin construction on a 100-kilometre (62-mile), $4.3 billion (£3.3 billion) particle collider called the Chinese Circular Electron Positron Collider as early as 2022. CERN has its own plans for an approximately $5.5 billion (£4.25 billion) 100-kilometre collider that would begin operation in the 2040s. Today’s colliders have not seen any evidence for new particles at the higher energy ranges of these proposed colliders, and some have expressed scepticism that these giant machines are worth building. But it’s science – it’s hard to know whether there actually is something new to be found in this higher-energy realm unless you actually build a collider big enough to explore it.
Other planned colliders would probe the frontiers of physics in different ways. An upgrade to a collider in the US would turn it into an electron-ion collider, one that would act like a microscope for the protons that make up atoms. Another collaboration hopes to see a International Linear Collider (ILC), a 30-to-50-km (20-to-30-mile) linear collider in Japan – Japan would need to pay around half of the $7 billion (£5.4 billion) price tag. CERN has plans for its own 11-to-50-km (7-31-mile) linear collider. Linear colliders can allow physicists to make ultra-accurate measurements of the mass of particles like the Higgs boson to look for new physics beyond the Standard Model of particle physics or perhaps reveal other potential particles.
But building these giant projects requires time, support, and commitment. In the case of the ILC, for example, Japan hasn’t fully thrown its support behind the experiment, as decision makers question whether it will be worth all that money.
Some say colliders like the FCC are simply too big and costly, when there are many other smaller experiments vying for funding from often-strapped science budgets. “If we had good reasons to think that there are particles to find in the energy range of this collider, then we should certainly go for it,” Sabine Hossenfelder, physicist at the Frankfurt Institute for Advanced Studies, told Gizmodo. “It’s possible that we build this thing and it measures constant for just one more digit...” That might be a worthy achievement to scientists, “but for people who have to pay the bill, maybe not so much.”
Of course, science requires taking major risks, and present economic models make it hard to justify experiments that don’t guarantee big discoveries – but the core goal of science isn’t profit.
“Sometimes our job is to set up the environment for the successes of the future,” Chanda Prescod-Weinstein, assistant professor in physics at the University of New Hampshire, told Gizmodo. “It’s a very anthropocentric perspective on science to think that it should be working out for us more.”
Maybe the future will require more radical thinking, or even taking on enormous experiments that might not reveal anything new. We can only hope that in the next 10 years, society will radically shift and people will value science enough to take on large experimental endeavours without worrying whether a Nobel Prize lies at the other end and without compromising smaller experiments. Probably not, but who knows?
Will it always be this way?
“I’m not expecting drastic to happen in the next five years, but that doesn’t make me feel bad, it just gives me time to learn more physics and come up with good or better ideas,” Seyda Ipek, postdoctoral researcher at the University of Californa, Irvine, told Gizmodo.
The next decade might bring us the most profound physics discoveries of the modern era – like the true nature of dark matter, how the universe came to look the way it does, and what its ultimate fate will be. Or perhaps governments and other funding agencies will spend billions of dollars and find nothing interesting. There aren’t obvious particles lying just beyond physicists’ reach, like the Higgs boson was during the last decade.
But there are opportunities to learn, opportunities to be wrong, and some incredible new technologies behind some of these experiments. Technological advances made with particle physics in mind can find other, important uses, such as in medicine, the internet, and even aerospace. Perhaps these experiments will lead to advances of their own, like new applications for quantum technology.
We’re humans, we ask questions, and we have the resources to answer these questions if we want. But no one said it was going to be easy.
Featured image: LSST Project/NSF/AURA