We’re One Step Closer to Knowing Why There’s More Matter Than Antimatter in the Universe

The T2K experiment just released its first batch of data, which could help answer why there is an apparent imbalance of matter and antimatter in the Universe.

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Physicists Maybe, Just Maybe, Confirm the Possible Discovery of 5th Force of Nature

The discovery of a possible fifth fundamental force could change our understanding of the universe. Credit: ESA/Hubble/NASA/Judy Schmidt

For some time, physicists have understood that all known phenomena in the Universe are governed by four fundamental forces. These include weak nuclear force, strong nuclear force, electromagnetism and gravity. Whereas the first three forces of are all part of the Standard Model of particle physics, and can be explained through quantum mechanics, our understanding of gravity is dependent upon Einstein’s Theory of Relativity.

Understanding how these four forces fit together has been the aim of theoretical physics for decades, which in turn has led to the development of multiple theories that attempt to reconcile them (i.e. Super String Theory, Quantum Gravity, Grand Unified Theory, etc). However, their efforts may be complicated (or helped) thanks to new research that suggests there might just be a fifth force at work.

In a paper that was recently published in the journal Physical Review Letters, a research team from the University of California, Irvine explain how recent particle physics experiments may have yielded evidence of a new type of boson. This boson apparently does not behave as other bosons do, and may be an indication that there is yet another force of nature out there governing fundamental interactions.

As Jonathan Feng, a professor of physics & astronomy at UCI and one of the lead authors on the paper, said:

“If true, it’s revolutionary. For decades, we’ve known of four fundamental forces: gravitation, electromagnetism, and the strong and weak nuclear forces. If confirmed by further experiments, this discovery of a possible fifth force would completely change our understanding of the universe, with consequences for the unification of forces and dark matter.”

The efforts that led to this potential discovery began back in 2015, when the UCI team came across a study from a group of experimental nuclear physicists from the Hungarian Academy of Sciences Institute for Nuclear Research. At the time, these physicists were looking into a radioactive decay anomaly that hinted at the existence of a light particle that was 30 times heavier than an electron.

In a paper describing their research, lead researcher Attila Krasznahorka and his colleagues claimed that what they were observing might be the creation of “dark photons”. In short, they believed that they might have at last found evidence of Dark Matter, the mysterious, invisible mass that makes up about 85% of the Universe’s mass.

This report was largely overlooked at the time, but gained widespread attention earlier this year when Prof. Feng and his research team found it and began assessing its conclusions. But after studying the Hungarian teams results and comparing them to previous experiments, they concluded that the experimental evidence did not support the existence of dark photons.

Instead, they proposed that the discovery could indicate the possible presence of a fifth fundamental force of nature. These findings were published in arXiv in April, which was followed-up by a paper titled “Particle Physics Models for the 17 MeV Anomaly in Beryllium Nuclear Decays“, which was published in PRL this past Friday.

Essentially, the UCI team argue that instead of a dark photon, what the Hungarian research team might have witnessed was the creation of a previously undiscovered boson – which they have named the “protophobic X boson”. Whereas other bosons interact with electrons and protons, this hypothetical boson interacts with only electrons and neutrons, and only at an extremely limited range.

This limited interaction is believed to be the reason why the particle has remained unknown until now, and why the adjectives “photobic” and “X” are added to the name. “There’s no other boson that we’ve observed that has this same characteristic,” said Timothy Tait, a professor of physics & astronomy at UCI and the co-author of the paper. “Sometimes we also just call it the ‘X boson,’ where ‘X’ means unknown.”

If such a particle does exist, the possibilities for research breakthroughs could be endless. Feng hopes it could be joined with the three other forces governing particle interactions (electromagnetic, strong and weak nuclear forces) as a larger, more fundamental force. Feng also speculated that this possible discovery could point to the existence of a “dark sector” of our universe, which is governed by its own matter and forces.

“It’s possible that these two sectors talk to each other and interact with one another through somewhat veiled but fundamental interactions,” he said. “This dark sector force may manifest itself as this protophopic force we’re seeing as a result of the Hungarian experiment. In a broader sense, it fits in with our original research to understand the nature of dark matter.”

If this should prove to be the case, then physicists may be closer to figuring out the existence of dark matter (and maybe even dark energy), two of the greatest mysteries in modern astrophysics. What’s more, it could aid researchers in the search for physics beyond the Standard Model – something the researchers at CERN have been preoccupied with since the discovery of the Higgs Boson in 2012.

But as Feng notes, we need to confirm the existence of this particle through further experiments before we get all excited by its implications:

“The particle is not very heavy, and laboratories have had the energies required to make it since the ’50s and ’60s. But the reason it’s been hard to find is that its interactions are very feeble. That said, because the new particle is so light, there are many experimental groups working in small labs around the world that can follow up the initial claims, now that they know where to look.”

As the recent case involving CERN – where LHC teams were forced to announce that they had not discovered two new particles – demonstrates, it is important not to count our chickens before they are roosted. As always, cautious optimism is the best approach to potential new findings.

Further Reading: University of California, Irvine

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Beyond WIMPs: Exploring Alternative Theories Of Dark Matter

Image from Dark Universe, showing the distribution of dark matter in the universe. Credit: AMNH

The standard model of cosmology tells us that only 4.9% of the Universe is composed of ordinary matter (i.e. that which we can see), while the remainder consists of 26.8% dark matter and 68.3% dark energy. As the names would suggest, we cannot see them, so their existence has had to be inferred based on theoretical models, observations of the large-scale structure of the Universe, and its apparent gravitational effects on visible matter.

Since it was first proposed, there have been no shortages of suggestions as to what Dark Matter particles look like. Not long ago, many scientists proposed that Dark Matter consists of Weakly-Interacting Massive Particles (WIMPs), which are about 100 times the mass of a proton but interact like neutrinos. However, all attempts to find WIMPs using colliders experiments have come up empty. As such, scientists have been exploring the idea lately that dark matter may be composed of something else entirely.

Current cosmological models tend to assume that the mass of dark matter is around 100 Gev (Giga-electrovolts), which corresponds to the mass scale of a lot of the other particles that interact via weak nuclear force. The existence of such a particle would be consistent with supersymmetric extensions of the Standard Model of particle physics. It is further believed that such particles would have been produced in the hot, dense, early Universe, with an a matter mass-density that has remained consistent to this day.

However, ongoing experimental efforts to detect WIMPs have failed to produce any concrete evidence of these particles. These have including searching for the products of WIMP annihilation (i.e. gamma rays, neutrinos and cosmic rays) in nearby galaxies and clusters, as well as direct detection experiments using supercolliders, like the CERN Large Hardon Collider (LHC) in Switzerland.

Because of this, many researcher teams have begun to consider looking beyond the WIMPs paradigm to find Dark Matter. One such team consists of a group of cosmologists from CERN and CP3-Origins in Denmark, who recently released a study indicating that Dark Matter could be much heavier and much less interacting than previously thought.

As Dr. McMullen Sandora, one of the research team members from CP-3 Origins, told Universe Today via email:

“We can’t rule out the WIMP scenario yet, but with each passing year it’s getting more and more suspect that we haven’t seen anything. In addition, the usual weak scale physics suffers from the hierarchy problem. That is, why all the particles we know about are so light, especially with respect to the natural scale of gravity, the Planck scale, which is about 1019 GeV. So, if dark matter were closer to the Planck scale, it wouldn’t be afflicted by the hierarchy problem, and this would also explain why we haven’t seen the signatures associated with WIMPs.”

Using a new model they call Planckian Interacting Dark Matter (PIDM), the team has been exploring the upper limit of mass of dark matter. Whereas WIMPs place the mass of dark matter at the upper limit of the electroweak scale, the Danish research team of Marthias Garny, McCullen Sandora and Martin S. Sloth proposed a particle with a mass near another natural scale entirely – the Planck Scale.

On the Planck Scale, a single unit of mass is equivalent to 2.17645 × 10-8 kg – roughly a microgram, or 1019 times greater than the mass of a proton. At this mass, every PIDM is essentially as heavy as a particle can be before it becomes a miniature black hole. The team also theorizes that these PIDM particles interact with ordinary matter only through gravitation and that large numbers of them formed in the very early Universe during the “reheating” epoch –  a period that occurred at the end the Inflationary Epoch, some 10-36 t0 10-33 or 10-32 seconds after the Big Bang.

This is epoch is so-named because, during Inflation, cosmic temperatures are believed to have dropped by a factor of 100,000 or so. When the inflation ended, the temperatures returned to their pre-inflationary temperature (an estimated 1027 K). At this point, the large potential energy of the inflation field decayed into Standard Model particles that filled the Universe, which would have included Dark Matter.

Naturally, this new theory comes with its share of implications for cosmologists. For example, for this model to work, the temperature of the reheating epoch would have to have been higher than is currently assumed. What’s more, a hotter reheating period would also result in the creation of more primordial gravitational waves, which would be visible in the Cosmic Microwave Background (CMB).

“Having such a high temperature tells us two interesting things about inflation,” says Sandora. “If dark matter turns out to be a PIDM: the first is that inflation happened at a very high energy, which in turn means that it was able to produce not just fluctuations in the temperature of the early universe, but also in spacetime itself, in the form of gravitational waves. Second, it tells us that the energy of inflation had to decay into matter extremely rapidly, because if it had taken too long the universe would have cooled to the point where it would not have been able to produce any PIDMs at all.”

The existence of these gravitational waves could be confirmed or ruled out by future studies involving Cosmic Microwave Background (CMB). This is exciting news, since the recent discovery of gravitational waves is expected to lead to renewed attempts to detect primordial waves that date back to the very creation of the Universe.

As Sandora explained, this presents a win-win scenario for scientists, as its means that this latest candidate for Dark Matter will be able to proven or disproven in the near future.

“[O]ur scenario makes a concrete prediction: we will see gravitational waves in the next generation of cosmic microwave background experiments.  Therefore, it’s a no-lose scenario: if we see them, that’s great, and if we don’t see them, we’ll know dark matter is not a PIDM, which will mean that we know it has to have some additional interactions with ordinary matter. And all this will happen within the next decade or so, which gives us plenty to look forward to.”

Ever since Jacobus Kapteyn first proposed the existence of Dark Matter in 1922, scientists have been searching for some direct evidence of its existence. And one by one, candidate particles – ranging from gravitinos and MACHOS to axions – have been proposed, weighed, and found wanting. If nothing else, it is good to know that this latest candidate particle’s existence can be proven or ruled out in the near future.

And if proven to be correct, we will have resolved one of the greatest cosmological mysteries of all time! A step closer to truly understanding the Universe and how its mysterious forces interact. Theory of Everything, here we come (or not)!

Further Reading: Physical Review Letters

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What’s Next for the Large Hadron Collider?

The world’s most powerful particle collider is waking up from a well-earned rest. After roughly two years of heavy maintenance, scientists have nearly doubled the power of the Large Hadron Collider (LHC) in preparation for its next run. Now, it’s being cooled to just 1.9 degrees above absolute zero. “We have unfinished business with understanding […]