The Collider Detector at Fermilab recorded high-energy particle collisions from 1985 to 2011. (Fermilab Photo)
A decade ago, physicists wondered whether the discovery of the Higgs boson at Europe’s Large Hadron Collider would point to a new frontier beyond the Standard Model of subatomic particles. So far, that’s not been the case — but a new measurement of a different kind of boson at a different particle collider might do the trick.
That’s the upshot of fresh findings from the Collider Detector at Fermilab, or CDF, one of the main experiments that made use of the Tevatron particle collider at the U.S. Department of Energy’s Fermilab in Illinois. It’s not yet time to throw out the physics textbooks, but scientists around the world are scratching their heads over the CDF team’s newly reported value for the mass of the W boson.
For decades, various physicists have theorized that even the slightest changes in the fundamental laws of nature would make it impossible for life to exist. This idea, also known as the “Fine-Tuned Universe” argument, suggests that the occurrence of life in the Universe is very sensitive to the values of certain fundamental physics. Alter any of these values (as the logic goes), and life would not exist, meaning we must be very fortunate to be here!
But can this really be the case, or is it possible that life can emerge under different physical constants, and we just don’t know it? This question was recently tackled by Luke A. Barnes, a postdoctoral researcher at the Sidney Institute for Astronomy (SIA) in Australia. In his recent book, A Fortunate Universe: Life in a Finely Tuned Cosmos, he and Sydney astrophysics professor Geraint F. Lewis argued that a fine-tuned Universe makes sense from a physics standpoint.
The Muon g-2 experiment at the Fermi National Accelerator Laboratory (Fermilab). Credit: Reidar Hahn/Fermilab
Since the long-awaited detection of the Higgs Boson in 2012, particle physicists have been probing deeper into the subatomic realm in the hope of investigating beyond the Standard Model of Particle Physics. In so doing, they hope to confirm the existence of previously unknown particles and the existence of exotic physics, as well as learning more about how the Universe began.
At the Fermi National Accelerator Laboratory (aka. Fermilab), researchers have been conducting the Muon g-2 experiment, which recently announced the results of their first run. Thanks to the unprecedented precision of their instruments, the Fermilab team found that muons in their experiment did not behave in a way that is consistent with the Standard Model, resolving a discrepancy that has existed for decades.
SCGSR Awardee Jacob Zettlemoyer, Indiana University Bloomington, led data analysis and worked with ORNL’s Mike Febbraro on coatings, shown under blue light, to shift argon light to visible wavelengths to boost detection. Credit: Rex Tayloe/Indiana University
The neutrino is a confounding little particle that is believed to have played a major role in the evolution of our Universe. They also possess very little mass, have no charge, and interact with other particles only through the weak nuclear force and gravity. As such, finding evidence of their interactions is extremely difficult and requires advanced facilities that are shielded to prevent interference.
One such facility is the Oak Ridge National Laboratory (ORNL) where an international team of researchers are conducting the COHERENT particle physics experiment. Recently, researchers at COHERENT achieved a major breakthrough when they found the first evidence of a new kind of neutrino interaction, which effectively demonstrates a process known as coherent elastic neutrino-nuclear scattering (CEvNS).
Minute vibrating strings found in string theory are not the only ones that are of interest to physicists. The Standard Model of particle physics provides for a theory regarding a different type of string – this one is a string of very sparse gas strung over very long distances. In fact, the standard model predicts that a large percentage of “baryonic matter” (i.e. the type that makes up everything we can see and interact with) would be contained in these filaments. And now for the first time, scientists led by a team at the University of Bonn in Germany have detected one of these super long strings of gas.
Molecular clouds scattered by an intermediate black hole show very wide velocity dispersion in this artist’s impression. This scenario well explains the observational features of a peculiar molecular cloud CO-0.40-0.22. Credit: Keio University
Wormholes are a popular feature in science fiction, the means through which spacecraft can achieve faster-than-light (FTL) travel and instantaneously move from one point in spacetime to another. And while the General Theory of Relativity forbids the existence of “traversable wormholes”, recent research has shown that they are actually possible within the domain of quantum physics.
The only downsides are that they would actually take longer to traverse than normal space and/or likely be microscopic. In a new study performed by a pair of Ivy League scientists, the existence of physics beyond the Standard Model could mean that there are wormholes out there that are not only large enough to be traversable, but entirely safe for human travelers looking to get from point A to point B.
The NUmI (Neutrinos from the Main Injector) horn at Fermilab, which fires protons that degrade into neutrinos. (Image: Caltech)
The Standard Model of Particle Physics is one of science’s most impressive feats. It’s a rigorous, precise effort to understand and describe three of the four fundamental forces of the Universe: the electromagnetic force, the strong nuclear force, and the weak nuclear force. Gravity is absent because so far, fitting it into the Standard Model has been extremely challenging.
But there are some holes in the Standard Model, and one of them involves the mass of the neutrino.
This image shows a visual representation of one of the highest-energy neutrino detections superimposed on a view of the IceCube Lab at the South Pole. Credit: IceCube Collaboration
At the Amundsen–Scott South Pole Station in Antarctica lies the IceCube Neutrino Observatory – a facility dedicated to the study of elementary particles known as neutrino. This array consists of 5,160 spherical optical sensors – Digital Optical Modules (DOMs) – buried within a cubic kilometer of clear ice. At present, this observatory is the largest neutrino detector in the world and has spent the past seven years studying how these particles behave and interact.
The most recent study released by the IceCube collaboration, with the assistance of physicists from Pennsylvania State University, has measured the Earth’s ability to block neutrinos for the first time. Consistent with the Standard Model of Particle Physics, they determined that while trillions of neutrinos pass through Earth (and us) on a regular basis, some are occasionally stopped by it.
The IceCube Neutrino Observatory at the South Pole. Credit: Emanuel Jacobi/NSF
Back in 2013, the first detections of high-energy neutrinos were made by IceCube collaboration. These neutrinos – which were believed to be astrophysical in origin – were in the peta-electron volt range, making them the highest energy neutrinos discovered to date. IceCube searches for signs of these interactions by looking for Cherenkov radiation, which is produced after fast-moving charged particles are slowed down by interacting with normal matter.
By detecting neutrinos that interact with the clear ice, the IceCube instruments were able to estimate the energy and direction of travel of the neutrinos. Despite these detections, however, the mystery remained as to whether or not any kind of matter could stop a neutrino as it journeyed through space. In accordance with the Standard Model of Particle Physics, this is something that should happen on occasion.
After observing interactions at IceCube for a year, the science team found that the neutrinos that had to travel the farthest through Earth were less likely to reach the detector. As Doug Cowen, a professor of physics and astronomy/astrophysics at Penn State, explained in a Penn State press release:
“This achievement is important because it shows, for the first time, that very-high-energy neutrinos can be absorbed by something – in this case, the Earth. We knew that lower-energy neutrinos pass through just about anything, but although we had expected higher-energy neutrinos to be different, no previous experiments had been able to demonstrate convincingly that higher-energy neutrinos could be stopped by anything.”
The Icetop Tank, the neutrino detectors at the heart of the IceCube Neutrino Observatory. Credit: Dan Hubert
The existence of neutrinos was first proposed in 1930 by theoretical physicist Wolfgang Pauli, who postulated their existence as a way of explaining beta decay in terms of the conservation of energy law. They are so-named because they are electrically neutral, and only interact with matter very weakly – i.e. through the weak subatomic force and gravity. Because of this, neutrinos pass through normal matter on a regular basis.
Whereas neutrinos are produced regularly by stars and nuclear reactors here on Earth, the first neutrinos were formed during the Big Bang. The study of their interaction with normal matter can therefore tell us much about how the Universe evolved over the course of billions of years. Many scientists anticipate that the study of neutrinos will indicate the existence of new physics, ones which go beyond the Standard Model.
Because of this, the science team was somewhat surprised (and perhaps disappointed) with their results. As Francis Halzen – the principal investigator for the IceCube Neutrino Observatory and a professor of physics at the University of Wisconsin-Madison – explained:
“Understanding how neutrinos interact is key to the operation of IceCube. We were of course hoping for some new physics to appear, but we unfortunately find that the Standard Model, as usual, withstands the test.
Looking down one of IceCube’s detector bore holes. Credit: IceCube Collaboration/NSF
For the most part, the neutrinos selected for this study were more than one million times more energetic than those that are produced by our Sun or nuclear power plants. The analysis also included some that were astrophysical in nature – i.e. produced beyond Earth’s atmosphere – and may have been accelerated towards Earth by supermassive black holes (SMBHs).
Darren Grant, a professor of physics at the University of Alberta, is also the spokesperson for the IceCube Collaboration. As he indicated, this latest interaction study opens doors for future neutrino research. “Neutrinos have quite a well-earned reputation of surprising us with their behavior,” he said. “It is incredibly exciting to see this first measurement and the potential it holds for future precision tests.”
This study not only provided the first measurement of the Earth’s absorption of neutrinos, it also offers opportunities for geophysical researchers who are hoping to use neutrinos to explore Earth’s interior. Given that Earth is capable of stopping some of the billions of high-energy particles that routinely pass through it, scientists could develop a method for studying the Earth’s inner and outer core, placing more accurate constraints on their sizes and densities.
It also shows that the IceCube Observatory is capable of reaching beyond its original purpose, which was particle physics research and the study of neutrinos. As this latest study clearly shows, it is capable of contributing to planetary science research and nuclear physics as well. Physicists also hope to use the full 86-string IceCube array to conduct a multi-year analysis, examining even higher ranges of neutrino energies.
This event display shows “Bert,” one of two neutrino events discovered at IceCube whose energies exceeded one petaelectronvolt (PeV). Credit: Berkeley Labs.
As James Whitmore – the program director in the National Science Foundation’s (NSF) physics division (which provides support for IceCube) – indicated, this could allow them to truly search for physics that go beyond the Standard Model.
“IceCube was built to both explore the frontiers of physics and, in doing so, possibly challenge existing perceptions of the nature of universe. This new finding and others yet to come are in that spirit of scientific discovery.”
Ever since the discovery of the Higgs boson in 2012, physicists have been secure in the knowledge that the long journey to confirm the Standard Model was now complete. Since then, they have set their sets farther, hoping to find new physics that could resolve some of the deeper mysteries of the Universe – i.e. supersymmetry, a Theory of Everything (ToE), etc.
This, as well as studying how physics work at the highest energy levels (similar to those that existed during the Big Bang) is the current preoccupation of physicists. If they are successful, we might just come to understand how this massive thing known as the Universe works.
The Spallation Neutron Source, located at the Oak Ridge National Laboratory. Credit: neutrons.ornl.gov
Neutrinos are one of the fundamental particles that make up the Universe. Compared to other types of particles, they have very little mass, no charge, and only interact with others via the weak nuclear force and gravity. As such, finding evidence of their interactions is extremely difficult, requiring massive instruments located deep underground to shield them from any interference.
However, using the Spallation Neutron Source (SNS), a research facility located at the Oak Ridge National Laboratory (ORNL) – an international team of researchers recently made a historic discovery about neutrinos using an entirely different method. As part of the COHERENT experiment, these results confirm a prediction made 43 years ago and offers new possibilities for neutrino research.
The Standard Model of particle physics has been the predominant means of explaining what the basic building blocks of matter are and how they interact for decades. First proposed in the 1970s, the model claims that for every particle created, there is an anti-particle. As such, an enduring mystery posed by this model is why the Universe can exist if it is theoretically made up of equal parts of matter and antimatter.
This seeming disparity, known as the charge-parity (CP) violation, has been the subject of experiments for many years. But so far, no definitive demonstration has been made for this violation, or how so much matter can exist in the Universe without its counterpart. But thanks to new findings released by the international Tokai-to-Kamioka (T2K) collaboration, we may be one step closer to understanding why this disparity exists.
First observed in 1964, CP violation proposes that under certain conditions, the laws of charge-symmetry and parity-symmetry (aka. CP-symmetry) do not apply. These laws state that the physics governing a particle should be the same if it were interchanged with its antiparticle, while its spatial coordinates would be inverted. From this observation, one of the greatest cosmological mysteries emerged.
If the laws governing matter and antimatter are the same, then why is it that the Universe is so matter-dominated? Alternately, if matter and antimatter are fundamentally different, then how does this accord with our notions of symmetry? Answering these questions is not only important as far as our predominant cosmological theories go, they are also intrinsic to understanding how the weak interactions that govern particles work.
Established in June of 2011, the international T2K collaboration is the first experiment in the world dedicated to answering this mystery by studying neutrino and anti-neutrino oscillations. The experiment begins with high-intensity beams of muon neutrinos (or muon anti-neutrinos) being generated at the Japan Proton Accelerator Research Complex (J-PARC), which are then fired towards the Super-Kamiokande detector 295 km away.
This detector is currently one of the world’s largest and most sophisticated, dedicated to the detection and study of solar and atmospheric neutrinos. As neutrinos travel between the two facilities, they change “flavor” – going from muon neutrinos or anti-neutrinos to electron neutrinos or anti-neutrinos. In monitoring these neutrino and anti-neutrino beams, the experiment watches for different rates of oscillation.
This difference in oscillation would show that there is an imbalance between particles and antiparticles, and thus provide the first definitive evidence of CP violation for the first time. It would also indicate that there are physics beyond the Standard Model that scientists have yet to probe. This past April, the first data set produced by T2K was released, which provided some telling results.
The detected pattern of an electron neutrino candidate event observed by Super-Kamiokande. Credit: Kavli IMPU
As Mark Hartz, a T2K collaborator and the Kavli IPMU Project Assistant Professor, said in a recent press release:
“While the data sets are still too small to make a conclusive statement, we have seen a weak preference for large CP violation and we are excited to continue to collect data and make a more sensitive search for CP violation.”
These results, which were recently published in the Physical Review Letters, include all data runs from between January 2010 to May 2016. In total, this data comprised 7.482 x 1020 protons (in neutrino mode), which yielded 32 electron neutrino and 135 muon neutrino events, and 7.471×1020 protons (in antineutrino mode), which yielded 4 electron anti-neutrino and 66 muon neutrino events.
In other words, the first batch of data has provided some evidence for CP violation, and with a confidence interval of 90%. But this is just the beginning, and the experiment is expected to run for another ten years before wrapping up. “If we are lucky and the CP violation effect is large, we may expect 3 sigma evidence, or about 99.7% confidence level, for CP violation by 2026,” said Hartz.
If the experiment proves successful, physicists may finally be able to answer how it is that the early Universe didn’t annihilate itself. It is also likely help to reveal aspects of the Universe that particle physicists are anxious to get into! For it here that the answers to the deepest secrets of the Universe, like how all of its fundamental forces fit together, are likely to be found.
