We're cautiously optimistic about our new findings.
Dark matter, microscopic black holes and hidden dimensions were just some of the possibilities. But aside from the spectacular discovery of the Higgs boson, the project has failed to yield any clues as to what might lie beyond the standard model of particle physics, our current best theory of the micro-cosmos.
So our new paper from LHCb, one of the four giant LHC experiments, is likely to set physicists' hearts beating just a little faster. After analysing trillions of collisions produced over the last decade, we may be seeing evidence of something altogether new – potentially the carrier of a brand new force of nature.
But the excitement is tempered by extreme caution. The standard model has withstood every experimental test thrown at it since it was assembled in the 1970s, so to claim that we're finally seeing something it can't explain requires extraordinary evidence.
The standard model describes nature on the smallest of scales, comprising fundamental particles known as leptons (such as electrons) and quarks (which can come together to form heavier particles such as protons and neutrons) and the forces they interact with.
There are many different kinds of quarks, some of which are unstable and can decay into other particles. The new result relates to an experimental anomaly that was first hinted at in 2014, when LHCb physicists spotted "beauty" quarks decaying in unexpected ways.
Specifically, beauty quarks appeared to be decaying into leptons called "muons" less often than they decayed into electrons. This is strange because the muon is in essence a carbon-copy of the electron, identical in every way except that it's around 200 times heavier.
You would expect beauty quarks to decay into muons just as often as they do to electrons. The only way these decays could happen at different rates is if some never-before-seen particles were getting involved in the decay and tipping the scales against muons.
While the 2014 result was intriguing, it wasn't precise enough to draw a firm conclusion. Since then, a number of other anomalies have appeared in related processes. They have all individually been too subtle for researchers to be confident that they were genuine signs of new physics, but tantalisingly, they all seemed to be pointing in a similar direction.
The big question was whether these anomalies would get stronger as more data was analysed or melt away into nothing. In 2019, LHCb performed the same measurement of beauty quark decay again but with extra data taken in 2015 and 2016. But things weren't much clearer than they'd been five years earlier.
Today's result doubles the existing dataset by adding the sample recorded in 2017 and 2018. To avoid accidentally introducing biases, the data was analysed "blind" – the scientists couldn't see the result until all the procedures used in the measurement had been tested and reviewed.
Mitesh Patel, a particle physicist at Imperial College London and one of the leaders of the experiment, described the excitement he felt when the moment came to look at the result. "I was actually shaking", he said, "I realised this was probably the most exciting thing I've done in my 20 years in particle physics."
When the result came up on the screen, the anomaly was still there – around 85 muon decays for every 100 electron decays, but with a smaller uncertainty than before.
What will excite many physicists is that the uncertainty of the result is now over "three sigma" – scientists' way of saying that there is only around a one in a thousand chance that the result is a random fluke of the data. Conventionally, particle physicists call anything over three sigma "evidence". However, we are still a long way from a confirmed "discovery" or "observation" – that would require five sigma.
Theorists have shown it is possible to explain this anomaly (and others) by recognising the existence of brand new particles that are influencing the ways in which the quarks decays. One possibility is a fundamental particle called a "Z prime" – in essence a carrier of a brand new force of nature. This force would be extremely weak, which is why we haven't seen any signs of it until now, and would interact with electrons and muons differently.
Another option is the hypothetical "leptoquark" – a particle that has the unique ability to decay to quarks and leptons simultaneously and could be part of a larger puzzle that explains why we see the particles that we do in nature.
Interpreting the findings
So have we finally seen evidence of new physics? Well, maybe, maybe not. We do a lot of measurements at the LHC, so you might expect at least some of them to fall this far from the standard model. And we can never totally discount the possibility that there's some bias in our experiment that we haven't properly accounted for, even though this result has been checked extraordinarily thoroughly. Ultimately, the picture will only become clearer with more data. LHCb is currently undergoing a major upgrade to dramatically increase the rate it can record collisions.
Even if the anomaly persists, it will probably only be fully accepted once an independent experiment confirms the results. One exciting possibility is that we might be able to detect the new particles responsible for the effect being created directly in the collisions at the LHC. Meanwhile, the Belle II experiment in Japan should be able to make similar measurements.
What then, could this mean for the future of fundamental physics? If what we are seeing is really the harbinger of some new fundamental particles then it will finally be the breakthrough that physicists have been yearning for for decades.
We will have finally seen a part of the larger picture that lies beyond the standard model, which ultimately could allow us to unravel any number of established mysteries. These include the nature of the invisible dark matter that fills the universe, or the nature of the Higgs boson. It could even help theorists unify the fundamental particles and forces. Or, perhaps best of all, it could be pointing at something we have never even considered.
So, should we be excited? Yes, results like this don't come around very often, the hunt is definitely on. But we should be cautious and humble too; extraordinary claims require extraordinary evidence. Only time and hard work will tell if we have finally seen the first glimmer of what lies beyond our current understanding of particle physics.
Harry Cliff, Particle physicist, University of Cambridge; Konstantinos Alexandros Petridis, Senior lecturer in Particle Physics, University of Bristol, and Paula Alvarez Cartelle, Lecturer of Particle Physics, University of Cambridge
Researchers propose a new method that could definitively prove the existence of dark matter.
- Scientists identified a data signature for dark matter that can potentially be detected by experiments.
- The effect they found is a daily "diurnal modulation" in the scattering of particles.
- Dark matter has not yet been detected experimentally.
Dark matter, a type of matter that is predicted to make up around 27 percent of the known universe, has never been detected experimentally. Now a team of astrophysicists and cosmologists think they found a clue that may lead them to finally detect the elusive material, so hard to find because it does not absorb, reflect, or emit light.
The existence of dark matter has so far been predicted by inference from its gravitational effects on the motion of the stars and galaxies rather than direct observation. No existing technologies can pick it out. This has led researchers at the Shanghai Jiao Tong University and the Purple Mountain Observatory of the Chinese Academy of Sciences to identify characteristic dark matter signatures that would be easier to detect.
Their new paper proposes a new type of effect that relates to the so-called "sub-GeV dark matter" which is boosted by cosmic rays. Looking for this effect can potentially allow direct detection of dark matter using nuclear recoil techniques.
The diurnal effect of accelerated dark matter rays. Credit: Ge et al.
The research team included Shao-Feng Ge and Qiang Yuan, who explained that their approach is to look for a prominent signature of accelerated dark matter particles that come from the galaxy's center, where dark matter and cosmic rays are at high density. They found that these particles have a "diurnal modulation" – a scattering pattern that is linked to the time of day. At periods when the Galaxy Center faces the side of the planet that's opposite the location of the detector, the Earth shadows a large amount of these particles. At other times, they come in as a signal with "higher recoil energy."
"The conventional diurnal effect is only for slow moving (nonrelativistic) DM particles in our galaxy (so-called standard DM halo)," Ge and Yuan said to Phys.org. "The effect is negligibly small either from direct experimental constraints, or due to the detection threshold. For light DM particles, on the other hand, the DM-nucleus interaction is much less constrained, which leaves room for strong diurnal modulation."
Researchers Ning Zhou and Jianglai Liu, who were also involved in the study, said in an interview that the signature they are proposing could be "a smoking gun of cosmic ray boosted dark matter detection".
The researchers plan next to look for the signature in previously gathered data, as well as in underground dark matter experiments.
They are also encouraging scientists around the world to look for this signature in their data.
Check out the new paper "Diurnal Effect of Sub-GeV Dark Matter Boosted by Cosmic Rays" published in Physical Review Letters.
A new study found the possible reason why some dwarf galaxies appear to not have dark matter.
- A new paper presents a possible reason for why some dwarf galaxies appear to be missing dark matter.
- The researchers at the University of California, Riverside ran cosmological simulations to find the answers.
- They discovered some galaxies were stripped of dark matter through extreme tidal loss.
Astronomers discovered that extreme tidal loss may be a possible explanation for why some galaxies seem to have no dark matter, a mystery type of matter that's supposed to take up to 27 percent of the universe, according to NASA. Dark energy takes up another 68 percent, creating a repulsive force that speeds up the universe's expansion. Neither has been directly seen so far but rather inferred through their effects on space.
The team from the University of California, Riverside, found anomalies in some smaller galaxies, known as "dwarf galaxies" (containing up to a billion stars, compared to the Milky Way's 200-400 billion). Some appear to have no dark matter at all. This is despite the fact that they were formed in galaxies that were teeming with dark matter previously. What is the explanation for this phenomenon, which muddies our understanding of dark matter?
The scientists used a cosmological simulation called Illustris on dark-matter-free galaxies DF2 and DF4. They wanted to understand how similar space objects would evolve and what might have happened that led them to lose dark matter. The simulation could create galaxies, with evolving stars, supernovas, and growing and merging black holes. Within the simulation, the researchers found "dwarf galaxies" similar to DF2 and DF4 which lost over 90 percent of their dark matter through the process of tidal stripping, in which material is stripped from the galaxy by galactic tidal forces.
The study's first author was the physics and astronomy graduate student Jessica Doppel, while the co-author Laura Sales, an associate professor of physics and astronomy, was Doppel's graduate advisor.
"Interestingly, the same mechanism of tidal stripping is able to explain other properties of dwarfs like DF2 and DF4 — for example, the fact that they are 'ultradiffuse' galaxies," said Sales. "Our simulations suggest a combined solution to both the structure of these dwarfs and their low dark matter content. Possibly, extreme tidal mass loss in otherwise normal dwarf galaxies is how ultradiffuse objects are formed."
Besides Sales and Doppel, the study involved Julio F. Navarro from the University of Victoria in Canada, Mario G. Abadi and Felipe Ramos-Almendares of the National University of Córdoba in Argentina, Eric W. Peng of Peking University in China, and Elisa Toloba of the University of the Pacific in California.
Laura Sales (seated, left) and her research group of students, including Jessica Doppel (seated, right).
Credit: UCR/Stan Lim
Sales's team is currently collaborating with the Max Planck Institute for Astrophysics in Germany to improve the simulations with more advanced physics and a resolution that's 16 times better than the Illustris they used on this study.
Check out the new paper, published in the Monthly Notices of the Royal Astronomical Society.
Scientists with the the Dark Energy Spectroscopic Instrument (DESI) Legacy Imaging Surveys spent six years creating a detailed map of more than 1 billion galaxies.
- An international team of scientists created the world's largest astronomical map in an effort to better understand dark energy.
- Dark energy is the force that's thought to be driving the expansion of the universe.
- The ultimate goal of the team is to develop a three-dimensional map of the universe, which could help scientists unravel the mysteries of dark energy.
The universe is constantly expanding: Galaxies are hurtling away from each other—and from our own—at ever-increasing speeds. But why? In 1998, scientists theorized that a repulsive force called dark energy is causing the universe to expand, though much remains unknown about this mysterious, invisible energy, which makes up 70 percent of our universe.
To better understand dark energy, an international team of scientists has spent years creating the largest map ever of the sky. The scale of the map is enormous, covering half of the visible sky and comprising more than 1 billion galaxies, depicted through some 10 trillion pixels. That's equivalent to 833,000 high-resolution smartphone photos.
It's the product of the Dark Energy Spectroscopic Instrument (DESI) Legacy Imaging Surveys, a collaborative effort involving the Kitt Peak National Observatory and Cerro Tololo Inter-American Observatory (CTIO), and images from NASA's Wide-field Infrared Survey Explorer (WISE) mission. You can view the two-dimensional interactive map on the Legacy Survey Sky Viewer website.
CosmoView Episode 18: Giant Map of the Sky Sets Stage for Ambitious DESI Survey
"For millennia humans have used maps to understand and navigate our world and put ourselves in context: we rely on maps to show us where we are, where we came from, and where we're going. Astronomical maps continue this tradition on a vast scale," the National Science Foundation's NOIRLab, which is involved in the project, wrote in a blog post.
"They locate us within the cosmos and tell the story of the history and fate of the Universe: it will expand forever, the expansion currently accelerating because of an unknown quantity called dark energy. Astronomical maps may help explain what this dark energy is and why it exists. Capitalizing on that possibility requires an unprecedented map — one that charts faint galaxies more uniformly and over a larger area of sky than ever before."
Credit: NASA/STSci/Ann Feild
The vast amounts of data collected by the DESI Legacy Imaging Surveys has already led to significant scientific discoveries, including some of the coolest brown dwarfs ever discovered, active black holes in galaxies, and light-bending gravitational lenses, discovered through machine-learning algorithms.
But the new map is only the first stage in DESI's main goal: Creating a three-dimensional map of the universe. Over the next five years, scientists with DESI will use the data to measure the spectra of 35 million galaxies and 2.4 million quasars in the map. (Spectra is the intensity of light emitted over a range of energies.)
DESI/Legacy Survey Sky Viewer
By determining these galaxies' positions and distance from Earth, the team will be able to plot them in three dimensions, and potentially help scientists learn more about arguably the biggest puzzle in cosmology.
"Capturing the spectra of so many galaxies so quickly requires a high degree of automation," wrote NOIRLab. "DESI — equipped with an array of 5000 swiveling, automated robots, each toting a thin fiber-optic cable that can point at individual galaxies — is designed to measure the spectra of 5000 galaxies at a time. The results will ultimately provide new insights into the mysterious dark energy that is driving the Universe's accelerating expansion."
Dr. Katie Mack explains what dark energy is and two ways it could one day destroy the universe.
- The universe is expanding faster and faster. Whether this acceleration will end in a Big Rip or will reverse and contract into a Big Crunch is not yet understood, and neither is the invisible force causing that expansion: dark energy.
- Physicist Dr. Katie Mack explains the difference between dark matter, dark energy, and phantom dark energy, and shares what scientists think the mysterious force is, its effect on space, and how, billions of years from now, it could cause peak cosmic destruction.
- The Big Rip seems more probable than a Big Crunch at this point in time, but scientists still have much to learn before they can determine the ultimate fate of the universe. "If we figure out what [dark energy is] doing, if we figure out what it's made of, how it's going to change in the future, then we will have a much better idea for how the universe will end," says Mack.