Core physics This apple tree at Woolsthorpe Manor is believed to have been the inspiration for Isaac Newton. (Courtesy: Bs0u10e01/CC BY-SA 4.0)
Physicists in the UK have drawn up plans for an International Year of Classical Physics (IYC) in 2027 – exactly three centuries after the death of Isaac Newton. Following successful international years devoted to astronomy (2009), light (2015) and quantum science (2025), they want more recognition for a branch of physics that underpins much of everyday life.
A bright green Flower of Kent apple has now been picked as the official IYC logo in tribute to Newton, who is seen as the “father of classical physics”. Newton, who died in 1727, famously developed our understanding of gravity – one of the fundamental forces of nature – after watching an apple fall from a tree of that variety in his home town of Woolsthorpe, Lincolnshire, in 1666.
“Gravity is central to classical physics and contributes an estimated $270bn to the global economy,” says Crispin McIntosh-Smith, chief classical physicist at the University of Lincoln. “Whether it’s rockets escaping Earth’s pull or skiing down a mountain slope, gravity is loads more important than quantum physics.”
McIntosh-Smith, who also works in cosmology having developed the Cosmic Crisp theory of the universe during his PhD, will now be leading attempts to get endorsement for IYC from the United Nations. He is set to take a 10-strong delegation from Bramley, Surrey, to Paris later this month.
An official gala launch ceremony is being pencilled in for the Travelodge in Grantham, which is the closest hotel to Newton’s birthplace. A parallel scientific workshop will take place in the grounds of Woolsthorpe Manor, with a plenary lecture from TV physicist Brian Cox. Evening entertainment will feature a jazz band.
Numerous outreach events are planned for the year, including the world’s largest demonstration of a wooden block on a ramp balanced by a crate on a pulley. It will involve schoolchildren pouring Golden Delicious apples into the crate to illustrate Newton’s laws of motion. Physicists will also be attempting to break the record for the tallest tower of stacked Braeburn apples.
But there is envy from those behind the 2025 International Year of Quantum Science and Technology. “Of course, classical physics is important but we fear this year will peel attention away from the game-changing impact of quantum physics,” says Anne Oyd from the start-up firm Qrunch, who insists she will only play a cameo role in events. “I believe the impact of classical physics is over-hyped.”
A new artificial intelligence/machine learning method rapidly and accurately characterizes binary neutron star mergers based on the gravitational wave signature they produce. Though the method has not yet been tested on new mergers happening “live”, it could enable astronomers to make quicker estimates of properties such as the location of mergers and the masses of the neutron stars. This information, in turn, could make it possible for telescopes to target and observe the electromagnetic signals that accompany such mergers.
When massive objects such as black holes and neutron stars collide and merge, they emit ripples in spacetime known as gravitational waves (GWs). In 2015 scientists on Earth began observing these ripples using kilometre-scale interferometers that measure the minuscule expansion and contraction of space–time that occurs when a gravitational wave passes through our planet. These interferometers are located in the US, Italy and Japan and are known collectively as the LVK observatories after their initials: the Laser Interferometer GW Observatory (LIGO), the Virgo GW Interferometer (Virgo) and the Kamioka GW Detector (KAGRA).
When two neutron stars in a binary pair merge, they emit electromagnetic waves as well as GWs. While both types of wave travel at the speed of light, certain poorly understood processes that occur within and around the merging pair cause the electromagnetic signal to be slightly delayed. This means that the LVK observatories can detect the GW signal coming from a binary neutron star (BNS) merger seconds, or even minutes, before its electromagnetic counterpart arrives. Being able to identify GWs quickly and accurately therefore increases the chances of detecting other signals from the same event.
This is no easy task, however. GW signals are long and complex, and the main technique currently used to interpret them, Bayesian inference, is slow. While faster alternatives exist, they often make algorithmic approximations that negatively affect their accuracy.
Trained with millions of GW simulations
Physicists led by Maximilian Dax of the Max Planck Institute for Intelligent Systems in Tübingen, Germany have now developed a machine learning (ML) framework that accurately characterizes and localizes BNS mergers within a second of a GW being detected, without resorting to such approximations. To do this, they trained a deep neural network model with millions of GW simulations.
Once trained, the neural network can take fresh GW data as input and predict corresponding properties of the merging BNSs – for example, their masses, locations and spins – based on its training dataset. Crucially, this neural network output includes a sky map. This map, Dax explains, provides a fast and accurate estimate for where the BNS is located.
The new work built on the group’s previous studies, which used ML systems to analyse GWs from binary black hole (BBH) mergers. “Fast inference is more important for BNS mergers, however,” Dax says, “to allow for quick searches for the aforementioned electromagnetic counterparts, which are not emitted by BBH mergers.”
The researchers, who report their work in Nature, hope their method will help astronomers to observe electromagnetic counterparts for BNS mergers more often and detect them earlier – that is, closer to when the merger occurs. Being able to do this could reveal important information on the underlying processes that occur during these events. “It could also serve as a blueprint for dealing with the increased GW signal duration that we will encounter in the next generation of GW detectors,” Dax says. “This could help address a critical challenge in future GW data analysis.”
So far, the team has focused on data from current GW detectors (LIGO and Virgo) and has only briefly explored next-generation ones. They now plan to apply their method to these new GW detectors in more depth.
Waseem completed his DPhil in physics at the University of Oxford in the UK, where he worked on applied process-relational philosophy and employed string diagrams to study interpretations of quantum theory, constructor theory, wave-based logic, quantum computing and natural language processing. At Oxford, Waseem continues to teach mathematics and physics at Magdalen College, the Mathematical Institute, and the Department of Computer Science.
Waseem has played a key role in organizing the Lahore Science Mela, the largest annual science festival in Pakistan. He also co-founded Spectra, an online magazine dedicated to training popular-science writers in Pakistan. For his work popularizing science he received the 2021 Diana Award, was highly commended at the 2021 SEPnet Public Engagement Awards, and won an impact award in 2024 from Oxford’s Mathematical, Physical and Life Sciences (MPLS) division.
What skills do you use every day in your job?
I’m a theoretical physicist, so if you’re thinking about what I do every day, I use chalk and a blackboard, and maybe a pen and paper. However, for theoretical physics, I believe the most important skill is creativity, and the ability to dream and imagine.
What do you like best and least about your job?
That’s a difficult one because I’ve only been in this job for a few weeks. What I like about my job is the academic freedom and the opportunity to work on both education and research. My role is divided 50/50, so 50% of the time I’m thinking about the structure of natural languages like English and Urdu, and how to use quantum computers for natural language processing. The other half is spent using our diagrammatic formalism called “quantum picturalism” to make quantum physics accessible to everyone in the world. So, I think that’s the best part. On the other hand, when you have a lot of smart people together in the same room or building, there can be interpersonal issues. So, the worst part of my job is dealing with those conflicts.
What do you know today, that you wish you knew when you were starting out in your career?
It’s a cynical view, but I think scientists are not always very rational or fair in their dealings with other people and their work. If I could go back and give myself one piece of advice, it would be that sometimes even rational and smart people make naive mistakes. It’s good to recognize that, at the end of the day, we are all human.
Disabled people in science must be recognized and given better support to help reverse the numbers of such people dropping out of science. That is the conclusion of a new report released today by the National Association of Disabled Staff Networks (NADSN). It also calls for funders to stop supporting institutions that have toxic research cultures and for a change in equality law to recognize the impact of discrimination on disabled people including neurodivergent people.
About 22% of working-age adults in the UK are disabled. Yet it is estimated that only 6.4% of people in science have a disability, falling to just 4% for senior academic positions. What’s more, barely 1% of research grant applications to UK Research and Innovation – the umbrella organization for the UK’s main funding councils – are from researchers who disclose being disabled. Disabled researchers who do win grants receive less than half the amount compared to non-disabled researchers.
NADSN is an umbrella organization for disabled staff networks, with a focus on higher education. It includes the STEMM Action Group, which was founded in 2020 and consists of nine people at universities across the UK who work in science and have lived experience of disability, chronic illness or neurodivergence. The group develops recommendations to funding bodies, learned societies and higher-education institutions to address barriers faced by those who are marginalised due to disability.
In 2021 the group published a “problem statement” that identified issues facing disabled people in science. They range from digital problems, such as the need for accessible fonts in reports and presentations, to physical concerns such as needing access ramps for people in wheelchairs or automatic doors to open heavy fire doors. Other issues include the need for adjustable desks in offices and wheelchair accessible labs.
“Many of these physical issues tend to be afterthoughts in the planning process,” says Francesca Doddato, a physicist from Lancaster University, who co-wrote the latest report. “But at that point they are much harder, and more costly, to implement.”
We need to have this big paradigm shift in terms of how we see disability inclusion
Francesca Doddato
Workplace attitudes and cultures can also be a big problem for disabled people in science, some 62% of whom report having been bullied and harassed compared to 43% of all scientists. “Unfortunately, in research and academia there is generally a toxic culture in which you are expected to be hyper productive, move all over the world, and have a focus on quantity over quality in terms of research output,” says Doddato. “This, coupled with society-wide attitudes towards disabilities, means that many disabled people struggle to get promoted and drop out of science.”
The action group spent the past four years compiling their latest report – Towards a fully inclusive environment for disabled people in STEMM – to present solutions to these issues. They hope it will raise awareness of the inequity and discrimination experienced by disabled people in science and to highlight the benefits of having an inclusive environment.
The report identifies three main areas that will have to be reformed to make science fully inclusive for disabled scientists: enabling inclusive cultures and practices; enhancing accessible physical and digital environments; and accessible and proactive funding.
In the short term, it calls on people to recognize the challenges and barriers facing disabled researchers and to improve work-based training for managers. “One of the best things is just being willing to listen and ask what can I do to help?” notes Doddato. “Being an ally is vitally important.”
Doddato says that sharing meeting agendas and documents ahead of time, ensuring that documents are presented in accessible formats, or acknowledging that tasks such as getting around campus can take longer are some aspects that can be useful.“All of these little things can really go a long way in shifting those attitudes and being an ally, and those things they don’t need policies that people need to be willing to listen and be willing to change.”
Medium- and long-term goals in the report involve holding organisations responsible for their working practice polices and to stop promoting and funding toxic research cultures. “We hope that report encourages funding bodies to put pressure on institutions if they are demonstrating toxicity and being discriminatory,” adds Doddato. The report also calls for a change to equality law to recognize the impact of intersectional discrimination, although it admits that this will be a “large undertaking” and will be the subject of a further NADSN report.
Doddato adds that disabled people’s voices need to be hear “loud and clear” as part of any changes. “What we are trying to address with the report is to push universities, research institutions and societies to stop only talking about doing something and actually implement change,” says Doddato. “We need to have a big paradigm shift in terms of how we see disability inclusion. It’s time for change.”
Neutron-activated gold Novel activation imaging technique enables real-time visualization of gold nanoparticles in the body without the use of external tracers. (Courtesy: Nanase Koshikawa from Waseda University)
Gold nanoparticles are promising vehicles for targeted delivery of cancer drugs, offering biocompatibility plus a tendency to accumulate in tumours. To fully exploit their potential, it’s essential to be able to track the movement of these nanoparticles in the body. To date, however, methods for directly visualizing their pharmacokinetics have not yet been established. Aiming to address this shortfall, researchers in Japan are using neutron-activated gold radioisotopes to image nanoparticle distribution in vivo.
The team, headed up by Nanase Koshikawa and Jun Kataoka from Waseda University, are investigating the use of radioactive gold nanoparticles based on 198Au, which they create by irradiating stable gold (197Au) with low-energy neutrons. The radioisotope 198Au has a half-life of 2.7 days and emits 412 keV gamma rays, enabling a technique known as activation imaging.
“Our motivation was to visualize gold nanoparticles without labelling them with tracers,” explains Koshikawa. “Radioactivation allows gold nanoparticles themselves to become detectable from outside the body. We used neutron activation because it does not change the atomic number, ensuring the chemical properties of gold nanoparticles remain unchanged.”
In vivo studies
The researchers – also from Osaka University and Kyoto University – synthesized 198Au-based nanoparticles and injected them into tumours in four mice. They used a hybrid Compton camera (HCC) to detect the emitted 412 keV gamma rays and determine the in vivo nanoparticle distribution, on the day of injection and three and five days later.
The HCC, which incorporates two pixelated scintillators, a scatterer with a central pinhole, and an absorber, can detect radiation with energies from tens of keV to nearly 1 MeV. For X-rays and low-energy gamma rays, the scatterer enables pinhole-mode imaging. For gamma rays over 200 keV, the device functions as a Compton camera.
The researchers reconstructed the 412 keV gamma signals into images, using an energy window of 412±30 keV. With the HCC located 5 cm from the animals’ abdomens, the spatial resolution was 7.9 mm, roughly comparable to the tumour size on the day of injection (7.7 x 11 mm).
In vivo distribution Images of 198Au nanoparticles in the bodies of two mice obtained with the HCC on the day of administration. (Courtesy: CC BY 4.0/Appl. Phys. Lett. 10.1063/5.0251048)
Overlaying the images onto photographs of the mice revealed that the nanoparticles accumulated in both the tumour and liver. In mice 1 and 2, high pixel values were observed primarily in the tumour, while mice 3 and 4 also had high pixel values in the liver region.
After imaging, the mice were euthanized and the team used a gamma counter to measure the radioactivity of each organ. The measured activity concentrations were consistent with the imaging results: mice 1 and 2 had higher nanoparticle concentrations in the tumour than the liver, and mice 3 and 4 had higher concentrations in the liver.
Tracking drug distribution
Next, Koshikawa and colleagues used the 198Au nanoparticles to label astatine-211 (211At), a promising alpha-emitting drug. They note that although 211At emits 79 keV X-rays, allowing in vivo visualization, its short half-life of just 7.2 h precludes its use for long-term tracking of drug pharmacokinetics.
The researchers injected the 211At-labelled nanoparticles into three tumour-bearing mice and used the HCC to simultaneously image 211At and 198Au, on the day of injection and one or two days later. Comparing energy spectra recorded just after injection with those two days later showed that the 211At peak at 79 keV significantly decreased in height owing to its decay, while the 412 keV 198Au peak maintained its height.
The team reconstructed images using energy windows of 79±10 and 412±30 keV, for pinhole- and Compton-mode reconstruction, respectively. In these experiments, the HCC was placed 10 cm from the mouse, giving a spatial resolution of 16 mm – larger than the initial tumour size and insufficient to clearly distinguish tumours from small organs. Nevertheless, the researchers point out that the rough distribution of the drug was still observable.
On the day of injection, the drug distribution could be visualized using both the 211At and 198Au signals. Two days later, imaging using 211At was no longer possible. In contrast, the distribution of the drug could still be observed via the 412 keV gamma rays.
With further development, the technique may prove suitable for future clinical use. “We assume that the gamma ray exposure dose would be comparable to that of clinical imaging techniques using X-rays or gamma rays, such as SPECT and PET, and that activation imaging is not harmful to humans,” Koshikawa says.
Activation imaging could also be applied to more than just gold nanoparticles. “We are currently working on radioactivation of platinum-based anticancer drugs to enable their visualization from outside the body,” Koshikawa tells Physics World. “Additionally, we are developing new detectors to image radioactive drugs with higher spatial resolution.”
Edinburgh researchers filmed ants and the sequence of movements they do when picking up seeds and other things. They then used this to build a robot gripper.
The device consists of two aluminium plates that each contain four rows of “hairs” made from thermoplastic polyurethane.
The hairs are 20 mm long and 1 mm in diameter, protruding in a v-shape. This allowing the hairs to surround circular objects, which can be particularly difficult to grasp and hold onto using parallel plates.
In tests picking up 30 different household items including a jam jar and shampoo bottle (see video), adding hairs to the gripper increased the prototype’s grasp success rate from 64% to 90%.
The researchers think that such a device could be used in environmental clean-up as well as in construction and agriculture.
Barbara Webb from the University of Edinburgh, who led the research, says the work is “just the first step”.
“Now we can see how [ants’] antennae, front legs and jaws combine to sense, manipulate, grasp and move objects – for instance, we’ve discovered how much ants rely on their front legs to get objects in position,” she adds. “This will inform further development of our technology.”
Researchers at the EMBL in Germany have dramatically reduced the time required to create images using Brillouin microscopy, making it possible to study the viscoelastic properties of biological samples far more quickly and with less damage than ever before. Their new technique can image samples with a field of view of roughly 10 000 pixels at a speed of 0.1 Hz – a 1000-fold improvement in speed and throughput compared to standard confocal techniques.
Mechanical properties such as the elasticity and viscosity of biological cells are closely tied to their function. These properties also play critical roles in processes such as embryo and tissue development and can even dictate how diseases such as cancer evolve. Measuring these properties is therefore important, but it is not easy since most existing techniques to do so are invasive and thus inherently disruptive to the systems being imaged.
Non-destructive, label- and contact-free
In recent years, Brillouin microscopy has emerged as a non-destructive, label- and contact-free optical spectroscopy method for probing the viscoelastic properties of biological samples with high resolution in three dimensions. It relies on Brillouin scattering, which occurs when light interacts with the phonons (or collective vibrational modes) that are present in all matter. This interaction produces two additional peaks, known as Stokes and anti-Stokes Brillouin peaks, in the spectrum of the scattered light. The position of these peaks (the Brillouin shift) and their linewidth (the Brillouin width) are related to the elastic and viscous properties, respectively, of the sample.
The downside is that standard Brillouin microscopy approaches analyse just one point in a sample at a time. Because the scattering signal from a single point is weak, imaging speeds are slow, yielding long light exposure times that can damage photosensitive components within biological cells.
“Light sheet” Brillouin imaging
To overcome this problem, EMBL researchers led by Robert Prevedel began exploring ways to speed up the rate at which Brillouin microscopy can acquire two- and three-dimensional images. In the early days of their project, they were only able to visualize one pixel at a time. With typical measurement times of tens to hundreds of milliseconds for a single data point, it therefore took several minutes, or even hours, to obtain two-dimensional images of 50–250 square pixels.
In 2022, however, they succeeded in expanding the field of view to include an entire spatial line — that is, acquiring image data from more than 100 points in parallel. In their latest work, which they describe in Nature Photonics, they extended the technique further to allow them to view roughly 10 000 pixels in parallel over the full plane of a sample. They then used the new approach to study mechanical changes in live zebrafish larvae.
“This advance enables much faster Brillouin imaging, and in terms of microscopy, allows us to perform ‘light sheet’ Brillouin imaging,” says Prevedel. “In short, we are able to ‘under-sample’ the spectral output, which leads to around 1000 fewer individual measurements than normally needed.”
Towards a more widespread use of Brillouin microscopy
Prevedel and colleagues hope their result will lead to more widespread use of Brillouin microscopy, particularly for photosensitive biological samples. “We wanted to speed-up Brillouin imaging to make it a much more useful technique in the life sciences, yet keep overall light dosages low. We succeeded in both aspects,” he tells Physics World.
Looking ahead, the researchers plan to further optimize the design of their approach and merge it with microscopes that enable more robust and straightforward imaging. “We then want to start applying it to various real-world biological structures and so help shed more light on the role mechanical properties play in biological processes,” Prevedel says.
FLASH irradiation, an emerging cancer treatment that delivers radiation at ultrahigh dose rates, has been shown to significantly reduce acute skin toxicity in laboratory mice compared with conventional radiotherapy. Having demonstrated this effect using proton-based FLASH treatments, researchers from Aarhus University in Denmark have now repeated their investigations using electron-based FLASH (eFLASH).
Reporting their findings in Radiotherapy and Oncology, the researchers note a “remarkable similarity” between eFLASH and proton FLASH with respect to acute skin sparing.
Principal investigator Brita Singers Sørensen and colleagues quantified the dose–response modification of eFLASH irradiation for acute skin toxicity and late fibrotic toxicity in mice, using similar experimental designs to those previously employed for their proton FLASH study. This enabled the researchers to make direct quantitative comparisons of acute skin response between electrons and protons. They also compared the effectiveness of the two modalities to determine whether radiobiological differences were observed.
Over four months, the team examined 197 female mice across five irradiation experiments. After being weighed, earmarked and given an ID number, each mouse was randomized to receive either eFLASH irradiation (average dose rate of 233 Gy/s) or conventional electron radiotherapy (average dose rate of 0.162 Gy/s) at various doses.
For the treatment, two unanaesthetized mice (one from each group) were restrained in a jig with their right legs placed in a water bath and irradiated by a horizontal 16 MeV electron beam. The animals were placed on opposite sides of the field centre and irradiated simultaneously, with their legs at a 3.2 cm water-equivalent depth, corresponding to the dose maximum.
The researchers used a diamond detector to measure the absolute dose at the target position in the water bath and assumed that the mouse foot target received the same dose. The resulting foot doses were 19.2–57.6 Gy for eFLASH treatments and 19.4–43.7 Gy for conventional radiotherapy, chosen to cover the entire range of acute skin response.
FLASH confers skin protection
To evaluate the animals’ response to irradiation, the researchers assessed acute skin damage daily from seven to 28 days post-irradiation using an established assay. They weighed the mice weekly, and one of three observers blinded to previous grades and treatment regimens assessed skin toxicity. Photographs were taken whenever possible. Skin damage was also graded using an automated deep-learning model, generating a dose–response curve independent of observer assessments.
The researchers also assessed radiation-induced fibrosis in the leg joint, biweekly from weeks nine to 52 post-irradiation. They defined radiation-induced fibrosis as a permanent reduction of leg extensibility by 75% or more in the irradiated leg compared with the untreated left leg.
To assess the tissue-sparing effect of eFLASH, the researchers used dose–response curves to derive TD50 – the toxic dose eliciting a skin response in 50% of mice. They then determined a dose modification factor (DMF), defined as the ratio of eFLASH TD50 to conventional TD50. A DMF larger than one suggests that eFLASH reduces toxicity.
The eFLASH treatments had a DMF of 1.45–1.54 – in other words, a 45–54% higher dose was needed to cause comparable skin toxicity to that caused by conventional radiotherapy. “The DMF indicated a considerable acute skin sparing effect of eFLASH irradiation,” the team explain. Radiation-induced fibrosis was also reduced using eFLASH, with a DMF of 1.15.
Reducing skin damage Dose-response curves for acute skin toxicity (left) and fibrotic toxicity (right) for conventional electron radiotherapy and electron FLASH treatments. (Courtesy: CC BY 4.0/adapted from Radiother. Oncol. 10.1016/j.radonc.2025.110796)
For DMF-based equivalent doses, the development of skin toxicity over time was similar for eFLASH and conventional treatments, throughout the dose groups. This supports the hypothesis that eFLASH modifies the dose–response rather than causing a changed biological mechanism. The team also suggests that the difference in DMF seen for fibrotic response and acute skin damage suggests that FLASH sparing depends on tissue type and might be specific to acute and late-responding tissue.
Similar skin damage between electrons and protons
Sørensen and colleagues compared their findings to previous studies of normal-tissue damage from proton irradiation, both in the entrance plateau and using the spread-out Bragg peak (SOBP). DMF values for electrons (1.45–1.54) were similar to those of transmission protons (1.44–1.50) and slightly higher than for SOBP protons (1.35–1.40). “Despite dose rate and pulse structure differences, the response to electron irradiation showed substantial similarity to transmission and SOBP damage,” they write.
Although the average eFLASH dose rate (233 Gy/s) was higher than that of the proton studies (80 and 60 Gy/s), it did not appear to influence the biological response. This supports the hypothesis that beyond a certain dose rate threshold, the tissue-sparing effect of FLASH does not increase notably.
The researchers point out that previous studies also found biological similarities in the FLASH effect for electrons and protons, with this latest work adding data on similar comparable and quantifiable effects. They add, however, that “based on the data of this study alone, we cannot say that the biological response is identical, nor that the electron and proton irradiation elicit the same biological mechanisms for DNA damage and repair. This data only suggests a similar biological response in the skin.”
New data from the NOvA experiment at Fermilab in the US contain no evidence for so-called “sterile” neutrinos, in line with results from most – though not all – other neutrino detectors to date. As well as being consistent with previous experiments, the finding aligns with standard theoretical models of neutrino oscillation, in which three active types, or flavours, of neutrino convert into each other. The result also sets more stringent limits on how much an additional sterile type of neutrino could affect the others.
“The global picture on sterile neutrinos is still very murky, with a number of experiments reporting anomalous results that could be attributed to sterile neutrinos on one hand and a number of null results on the other,” says NOvA team member Adam Lister of the University of Wisconsin, Madison, US. “Generally, these anomalous results imply we should see large amounts of sterile-driven neutrino disappearance at NOvA, but this is not consistent with our observations.”
Neutrinos were first proposed in 1930 by Wolfgang Pauli as a way to account for missing energy and spin in the beta decay of nuclei. They were observed in the laboratory in 1956, and we now know that they come in (at least) three flavours: electron, muon and tau. We also know that these three flavours oscillate, changing from one to another as they travel through space, and that this oscillation means they are not massless (as was initially thought).
Significant discrepancies
Over the past few decades, physicists have used underground detectors to probe neutrino oscillation more deeply. A few of these detectors, including the LSND at Los Alamos National Laboratory, BEST in Russia, and Fermilab’s own MiniBooNE, have observed significant discrepancies between the number of neutrinos they detect and the number that mainstream theories predict.
One possible explanation for this excess, which appears in some extensions of the Standard Model of particle physics, is the existence of a fourth flavour of neutrino. Neutrinos of this “sterile” type do not interact with the other flavours via the weak nuclear force. Instead, they interact only via gravity.
Detecting sterile neutrinos would fundamentally change our understanding of particle physics. Indeed, some physicists think sterile neutrinos could be a candidate for dark matter – the mysterious substance that is thought to make up around 85% of the matter in the universe but has so far only made itself known through the gravitational force it exerts.
Near and far detectors
The NOvA experiment uses two liquid scintillator detectors to monitor a stream of neutrinos created by firing protons at a carbon target. The near detector is located at Fermilab, approximately 1 km from the target, while the far detector is 810 km away in northern Minnesota. In the new study, the team measured how many muon-type neutrinos survive the journey through the Earth’s crust from the near detector to the far one. The idea is that if fewer neutrinos survive than the conventional three-flavour oscillations picture predicts, some of them could have oscillated into sterile neutrinos.
The experimenters studied two different interactions between neutrinos and normal matter, says team member V Hewes of the University of Cincinnati, US. “We looked for both charged current muon neutrino and neutral current interactions, as a sterile neutrino would manifest differently in each,” Hewes explains. “We then compared our data across those samples in both detectors to simulations of neutrino oscillation models with and without the presence of a sterile neutrino.”
No excess of neutrinos seen
Writing in Physical Review Letters, the researchers state that they found no evidence of neutrinos oscillating into sterile neutrinos. What is more, introducing a fourth, sterile neutrino did not provide better agreement with the data than sticking with the standard model of three active neutrinos.
This result is in line with several previous experiments that looked for sterile neutrinos, including those performed at T2K, Daya Bay, RENO and MINOS+. However, Lister says it places much stricter constraints on active-sterile neutrino mixing than these earlier results. “We are really tightening the net on where sterile neutrinos could live, if they exist,” he tells Physics World.
The NOvA team now hopes to tighten the net further by reducing systematic uncertainties. “To that end, we are developing new data samples that will help us better understand the rate at which neutrinos interact with our detector and the composition of our beam,” says team member Adam Aurisano, also at the University of Cincinnati. “This will help us better distinguish between the potential imprint of sterile neutrinos and more mundane causes of differences between data and prediction.”
NOvA co-spokesperson Patricia Vahle, a physicist at the College of William & Mary in Virginia, US, sums up the results. “Neutrinos are full of surprises, so it is important to check when anomalies show up,” she says. “So far, we don’t see any signs of sterile neutrinos, but we still have some tricks up our sleeve to extend our reach.”
Last week I had the pleasure of attending the Global Physics Summit (GPS) in Anaheim California, where I rubbed shoulders with 15,000 fellow physicists. The best part of being there was chatting with lots of different people, and in this podcast I share two of those conversations.
First up is Chetan Nayak, who is a senior researcher at Microsoft’s Station Q quantum computing research centre here in California. In February, Nayak and colleagues claimed a breakthrough in the development of topological quantum bits (qubits) based on Majorana zero modes. In principle, such qubits could enable the development of practical quantum computers, but not all physicists were convinced, and the announcement remains controversial – despite further results presented by Nayak in a packed session at the GPS.
I caught up with Nayak after his talk and asked him about the challenges of achieving Microsoft’s goal of a superconductor-based topological qubit. That conversation is the first segment of today’s podcast.
Distinctive jumping technique
Up next, I chat with Atharva Lele about the physics of manu jumping, which is a competitive aquatic sport that originates from the Māori and Pasifika peoples of New Zealand. Jumpers are judged by the height of their splash when they enter the water, and the best competitors use a very distinctive technique.
Lele is an undergraduate student at the Georgia Institute of Technology in the US, and is part of team that analysed manu techniques in a series of clever experiments that included plunging robots. He explains how to make a winning manu jump while avoiding the pain of a belly flop.
