IN 1973, physicist Steven Weinberg gave a talk in Aix-en-Provence, France. It was there, according to Weinberg, that he first used the term “standard model” to describe the nascent description of the fundamental constituents of the universe and their interactions. Fifty years on, the standard model of particle physics is a stunningly accurate picture of what everything is made of and how it all works to produce reality.
Practically everything, anyway. Because although the 50th anniversary is well worth celebrating, it is impossible to ignore the fact that the theory is incomplete. It doesn’t explain gravity, or why we have so much matter in the universe and so little antimatter. And it says nothing about so-called dark matter and dark energy, postulated to explain why the cosmos behaves in certain ways.
This is why physicists are casting around for clues that could lead us to a better theory. But which, if any, will deliver an upgrade to the standard model? How do we find the deluxe version? We let six of today’s leading physicists explain how they think we will finally discover a more complete picture of reality.
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Collisions at the energy frontier
Jon Butterworth
University College London
It is always risky to bet against the standard model of particle physics. Historically, most people who have done so have lost. But over the next decade and a half, the Large Hadron Collider (LHC) will continue smashing protons together and examining the messy aftermath. And it is there, within the details of these collisions, that I will be looking most closely for evidence of new physics beyond the standard model.
I work on what we call the “energy frontier”, where concentrating a lot of energy into colliding subatomic particles can give us access to new physics. This works in two related ways. Firstly, if there is a new, heavy particle out there, then we might be able to make it if we have enough energy. Secondly, the highest-energy colliders are, in a sense, the highest-resolution microscopes. As we get to higher energies, the resolution with which we can probe the structure of matter increases.
We have pretty much maxed out on the energy we can get to, but we still plan to record around 10 billion more collisions. And more is better. Determining whether we have some physics beyond the standard model is like trying to determine whether a dice is fair or not. Six rolls of the dice will tell you next to nothing, but 6 million will give you a good idea.
A plethora of different things can happen when we collide protons at the LHC. Multiple Higgs bosons, W and Z bosons, very high-energy photons and jets of hadrons arising from scattered quarks and gluons can all be produced. So far, we know the distribution of these is about that expected of the standard model. But in some cases, that is a very approximate “about”.
Some important types of collision – for example, the production of pairs of Higgs bosons – indicated by the standard model are so rare that they haven’t yet been seen. Many ideas for physics beyond the standard model make predictions for things that could easily hide under that “about”. Over the next several years, we will make measurements that could, and I hope will, flush them out.
This effort requires a couple of things to happen. We experimentalists need to make more measurements to quantify how well the standard model is really doing, and they must be as independent of the theory as possible. We can’t avoid making theoretical assumptions sometimes, we just have to minimise their impact.
On the theory side, the predictions need to increase in precision. We don’t directly measure the Higgs mass, for example, but infer it from particles produced in a collision. Making more precise predictions, which can be compared directly against experiments, means we can make better inferences about the underlying processes.
Both of these are already happening. I look forward in the next few years to heated exchanges about the level of agreement, or disagreement, between theory and experiment along the lines of recent discussions about the anomalous “magnetic moment” of the muon (see “Muons behaving oddly”), for example.
Even if everything we measure agrees with the standard model, that will still be important. It will mean we have established that physics beyond the standard model lies far above the Higgs mass and hence the capabilities of current particle colliders. That may not help us much in understanding dark matter or other questions the standard model leaves unanswered. But we don’t get to choose nature, all we can do is explore it to the best of our ability.
Hunting cosmological chameleons
Clare Burrage
University of Nottingham, UK
THE standard model fails to account for 95 per cent of the contents of the universe. Cosmologists split this unknown portion of the cosmos into 27 per cent dark matter, which clumps together under gravity, and 68 per cent dark energy, which causes the expansion of the universe to accelerate. We have a plethora of theories for what dark matter could be. Dark energy, on the other hand, remains more mysterious. I have chosen to look for a new force, carried by a new particle, that could explain dark energy.
We expect particles associated with the acceleration of the expansion of the universe to have two properties. Firstly, they should be very light, and secondly, they should mediate a “fifth” force – beyond those of gravity, electromagnetism and the strong and weak forces – across cosmological distances. Many precise experiments have looked for light particles and long-range fifth forces without seeing them.
Over the past few decades, however, we have realised the environment . In particular, if a particle that transmits the fifth force can change its mass as the density of its environment changes, the force can only act over short distances in dense regions, but over long ranges in less dense ones.
The ability to change their properties to evade detection has earned these proposed particles the name “chameleons”. This doesn’t mean they are impossible to detect, we just have to carefully design our experiments.
Some objects can be so small that the chameleon doesn’t have enough room inside it to change its mass, so the behaviour of the force also doesn’t change. Because of this, if we drop a tennis ball and an atom side by side, we would expect the atom to fall faster than the ball if this fifth force exists.
Dropping individual atoms is hard to do, but, with new experimental techniques, we can measure how atoms fall extremely precisely. We cool clouds of atoms to make them as still as possible. Then, we shine a laser at them to excite an electron orbiting the nucleus of an atom from one energy level to another. When this atom absorbs a photon from the laser beam, an electron is excited, but the atom also starts moving.
At this point, it is possible for the atom to be in a superposition of two states at the same time, one in which it absorbed the photon and is moving, and the other where it didn’t and isn’t. By pulsing the laser, we can spatially separate these two states and then recombine them. In between the pulses, the atoms fall freely under the influence of gravity and the hypothetical fifth force. We if the dark energy force is causing atoms to fall faster than we would expect.
So far, experiments haven’t seen any evidence of new forces, constraining our models. Within the next few years, I hope we will detect chameleon particles or rule out this model altogether. Either way, we will be closer to understanding a mysterious chunk of the universe that can’t be described by the standard model.
Quantum sensors
Surjeet Rajendran
Johns Hopkins University, Maryland
PARTICLE colliders have had a revolutionary impact on our understanding of the universe, but these wonderful machines aren’t the right tool to search for a specific kind of new physics: particles that interact weakly with the electrons or protons being smashed together.
Finding these so-called weakly coupled particles is an important avenue to explore, since much of our observational evidence for physics beyond the standard model is “dark” physics – relating to dark matter and dark energy. This “darkness” means they don’t interact much with the electrons, protons, neutrons and photons we use in experiments.
To detect weakly interacting particles, not only do sensors need to measure minuscule effects, but they must also be immune to noise. The art of creating technologies that solve these problems is called precision sensing. Explosive developments in this field over the past two decades have led to astonishingly accurate sensors by exploiting the wonders of quantum mechanics.
To create an accurate sensor, we need a near-perfect yardstick to use for the measurements. Consider the amazing fact that there are 1080 hydrogen atoms in the observable universe and every one is identical. A quantum sensor exploits this fact to create robust yardsticks that enable high-precision sensing.
An example is the use of synchronised atomic clocks, which “tick” based on the behaviour of electrons within certain atoms, to search for dark matter.
A grand scientific success of precision sensing is the LIGO experiment. This uses devices called interferometers to search for gravitational waves, ripples in space-time. Interferometers contain waves, usually of light, which interfere to create patterns that can be studied to reveal forces or fields that have moved through the device. The discoveries of gravitational waves by the LIGO collaboration could be a harbinger of the kind of physics we may go on to discover using other quantum sensors.
Physics is an experimental science, so we aren’t going to know what is there without experimental exploration. This is a frustrating fact for the theorist who might want to discover the theory of everything from their armchair. But, sobering as this is, I see it as a call to action: there is new physics out there and our job is to find it.
Rethinking time
Emily Adlam
Chapman University, California
WE TEND to assume that time works in a linear way. Even the standard model is traditional when it comes to how we think of time: some “input” evolves into an “output”. Yet many other parts of modern physics, such as Albert Einstein’s equations of general relativity, aren’t a good fit with this simple time-evolution picture. This is why I believe that, to find physics beyond the standard model, we should move away from the time-evolution paradigm.
There are a few approaches that might replace it. One possibility is , where the future plays some role in “producing” the past. Another is , in which the whole of history co-exists in some way.
We live in a universe in which entropy, a measure of disorder, increases over time. This suggests the universe must have begun in a very special kind of state with very low entropy. Within the standard model, it seems the only prospect for explaining this is to imagine a process producing a large number of possible universes, or multiverses. Since we find ourselves in this universe, the argument goes, we must be in one of the universes with a low-entropy initial state. But if we don’t start from a time-evolution picture, there may be a way to explain the special initial state without imagining other universes.
The block universe approach also seems like a promising route to one of the largest problems that the standard model can’t solve – unifying gravity and quantum mechanics – since our best theory of gravity, general relativity, seems most at home in a block universe picture.
Excitingly, we may soon have new experimental data. Recently, there have been proposals for experiments testing, for example, . One experiment involves putting two masses into quantum superpositions where they are effectively in two different positions at once, then seeing if it results in quantum entanglement. If entanglement is created in that experiment, this probably indicates that gravity itself has quantum properties.
This kind of result would certainly represent a significant departure from the intuitive time-evolution picture used in the standard model. My hope is that these experiments will provide a direct window into the nature of time in our universe.
Muons behaving oddly
Alex Keshavarzi
The University of Manchester, UK
For the past six years, a team of scientists, myself included, has been searching for new physics using an unfamiliar fundamental particle. It turns out the muon, a heavier cousin of the electron, is very useful for searching for new physics outside of the standard model.
Our experiment, based at the Fermi National Accelerator Laboratory (Fermilab), Illinois, measures a property called the muon’s “magnetic moment”. In a magnetic field, muons can interact with a sea of virtual particles that pop in and out of existence. Some might be particles we know of, but unknown particles could pop up, too.
These interactions cause the muons to wobble, or “precess”: the more interactions the muon experiences, the faster it wobbles. Measuring this wobble can indirectly tell us how many virtual particles the muon has interacted with, and the number of forces through which it has done so. By comparing this measurement with the standard model prediction, we can see whether the muon has interacted with new particles or forces.
Back in 2004, a measurement of the muon’s magnetic moment at the Brookhaven National Laboratory, New York, found the muon’s magnetic moment was larger than the standard model prediction. It had a precision of 0.5 parts per million, as precise as measuring the length of a stadium to the width of a human hair. But higher precision was needed to prove new physics.
Part of the Brookhaven experiment was moved more than 4800 kilometres to Fermilab and the Muon g-2 experiment was born. Our first result in 2021 confirmed the Brookhaven finding, and in August 2023, we released another consistent result with a precision of 0.19 parts per million: the most precise measurement ever made at a particle accelerator.
If there is new physics beyond the standard model, our measurement is precise enough to confirm it for the first time. But that rests on the standard model prediction itself, which has evolved since 2021 to have different results depending on which method is used to calculate it. Some align more with our experiment and therefore suggest there is no new physics. The rest, enticingly, are beyond the threshold needed to claim that first ever discovery of new physics.
An international collaboration of theoretical physicists known as the Muon g-2 Theory Initiative, of which I am also a member, is working very hard to understand and resolve these differences, hopefully within the next few years. Whatever the outcome, however, the Muon g-2 experiment will continue to release new results at even higher precision, with our last batch of data now ready to be analysed. I am excited to see whether our final result, expected in the next couple of years, could irrefutably confirm physics beyond the standard model for the first time.
A new paradigm
Matt Strassler
Harvard University
In THE 19th century, scientists understood most waves as we understand them today: rhythmic disturbances of a medium, like sound waves through air. Once they realised light was a wave, it seemed obvious it would have a medium, too, which they called the luminiferous aether. Earth’s motion through the aether was expected to cause light’s speed to change slightly depending on its direction of motion, just as sound’s speed does for moving observers.
In an experiment in 1887, physicists Albert Michelson and Edward Morley found no such dependence. This “null result” puzzled physicists until Albert Einstein proposed an explanation in 1905: that the speed of light never changes.
Ever since then, particle physics has been driven by a sequence of puzzles and clues that has led scientists from one discovery to the next (see “A brief history of the standard model, our theory of almost everything“). Unfortunately, this trend seems to have ended in 2012 with the discovery of the Higgs boson. I view the absence of further discoveries at the Large Hadron Collider (LHC) as potentially among the most important null results in the history of physics, comparable with the Michelson-Morley measurement.
Just as the aether result violated simple, general reasoning about waves, the LHC’s null result also goes against certain basic reasoning about quantum fields. Quantum field theory seems to imply, and experiments seem to confirm, that it is highly unusual to have a Higgs field and Higgs boson that exist entirely on their own. There ought to be a whole set of additional fields and particles, of which the Higgs field and Higgs boson are just the first. Some of them should be observable at the LHC.
So far, the collider has seen nothing of the kind. While its search is far from over, many physicists like me have been wondering if the reasoning is misguided. Perhaps, as with Michelson-Morley, we might be seeing the first signs of a paradigm breaking down.
Calculations concerning the Higgs field rely on three assumptions: quantum field theory is valid; it can be used without having to account for phenomena at energies vastly larger than those probed directly by the LHC; and events in the universe’s distant past don’t affect the calculations. Although no one has a reasonable alternative to quantum field theory yet, it behoves us to question the other assumptions.
One option involves hidden symmetries relating traits of both known and unknown particles. These could lead to “magic zeroes”, causing effects that would destabilise a lone Higgs field to be much smaller than expected. Or quantum gravity may impose unknown constraints on quantum field theories. It might be that only a few quantum field theories are consistent with gravity, and in those that resemble the standard model, the LHC’s null result is automatic.
Another possibility is that the universe’s history has left us with a world where this null result was necessary for our existence. The simplest version of this idea suggests our universe is a mostly uninhabitable multiverse, in which the rare habitable places are described by very unusual quantum field theories. While similar reasoning potentially explains why the cosmological constant that drives the universe’s accelerating expansion is very small (see “Hunting cosmological chameleons”), it fails here, merely replacing one puzzle with another.
Another idea is that the universe passed through many phase transitions, at each stage being described by a more and more unusual quantum field theory. When it reached the most unusual phase, the transitions stopped, leaving us with a surprising universe. Yet this proposal, which was made explicit through an idea known as the “relaxion”, has difficulty solving the cosmological constant problem.
All of these ideas seem rather desperate, indicative of a moment when the concepts underlying particle physics may need to be rethought. If so, particle physicists face a tremendous challenge. The path forwards is unclear, making it difficult to guess which experimental efforts are worthwhile. Perhaps classic particle physics methods are no longer the answer, but what to replace or enhance them with is anyone’s guess.
Physicists of the 1890s faced such obstacles, too. But they were lucky. Cathode rays and radioactivity opened up entirely new classes of experiments that could reveal the structure of atoms and the properties of particles moving at near light speed. Whether today’s physicists will be so lucky remains entirely unclear.
This article is part of a special series on the standard model, in which we explore:
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