Tag Archives: fermion

Credit: CERN.

Scientists confirm that Higgs boson is coupled to bulky cousin in new physics breakthrough

Credit: CERN.

Credit: CERN.

When scientists operating the world’s most powerful particle accelerator confirmed the existence of the Higgs boson, the discovery was heralded as a landmark achievement in particle physics. This boson is a pretty big deal — it’s the particle associated with a quantum field that is supposed to give particles their masses. Without this field, there would be no atoms, there would be no matter, there would be no us. With the Higgs boson confirmed, physicists performed the most important validation yet of the Standard Model — the theoretical framework for our current understanding of the fundamental particles and forces of nature.

However, the 2013 achievement did not answer all our questions relating to the Higgs field and how the Higgs boson behaves. But there is progress, and according to a recent statement released by the European Organization for Nuclear Research (CERN), the scientific organization that operates the LHC, a new experiment is filling in the blanks by revealing how the Higgs particle fits into the delicate ecosystem of particles.

“We know that the Higgs interacts with massive force-carrying particles, like the W boson, because that’s how we originally discovered it,” said scientist Patty McBride from the U.S. Department of Energy’s Fermi National Accelerator Laboratory, which supports the research of hundreds of U.S. scientists on the Compact Muon Solenoid (CMS) experiment.

“Now we’re trying to understand its relationship with fermions.”

There are two types of elementary particles — that is, particles that either doesn’t have a substructure or have one we haven’t discovered yet. These particles are split up into categories, two of which being fermions and bosons. Fermions follow Fermi–Dirac statistic and bosons follow Bose-Einstein statistic. Another way to look at this is that fermions are particles that have half-integer spin, whereas bosons are particles with integer spin.

The electron is a fermion, for instance. Bosons, such as the photon, carry energy — they’re the physical manifestation of forces that glue fermions together.

Earlier in 2014, researchers working with the CMS experiment showed that the Higgs boson has a relationship with fermions by measuring the rate at which they decay into tau leptons — the heavier cousin of the electron. Later, evidence surfaced of the Higgs boson decaying into bottom quarks.

Now, two experiments — the Compact Muon Solenoid (CMS) and A Toroidal LHC Apparatus (ATLAS) — found that there’s also a relationship between the Higgs and the top quark (discovered in 1995), the latter being three million times more massive than an electron.

“The relationship between the Higgs and the top quark is particularly interesting because the top quark is the most massive particle ever discovered,” McBride said. “As the ‘giver of mass,’ the Higgs boson should be enormously fond of the top quark.”

The new experiments confirm theoretical predictions, finding that in very rare situations Higgs bosons are produced simultaneously with top quarks. In yet another experiment that confirms the Standard Model, the results have a statistical significance of 5.2 sigma, which is above the 5 sigma threshold physicists require. In other words, there’s just a 1-in-3.5-million chance that the observations scientists recorded were due to random chance.

“Higgs boson production is rare – but Higgs production with top quarks is rarest of them all, amounting to only about 1 percent of the Higgs boson events produced at the LHC,” said Chris Neu, a physicist at the University of Virginia who worked on this analysis.

“A top quark decays almost exclusively into a bottom quark and a W boson,” Neu said. “The Higgs boson, on the other hand, has a rich spectrum of decay modes, including decays to pairs of bottom quarks, W bosons, tau leptons, photons and several others. This leads to a wide variety of signatures in events with two top quarks and a Higgs boson. We pursued each of these and combined the results to produce our final analysis.”

The results published in the journal Physical Review Letters will help physicists learn more about the behavior of the Higgs boson and how it might also interact with other particles we haven’t discovered yet, like dark matter. It’s remarkable how much particle physics has progressed in the last two decades. At the end of 2018, the LHC will shut down for two years for refurbishment and upgrades and then return better than ever, operating without delays through 2030.

Who knows what kind of achievements await thereafter?

Scientists find “Angel Particle” — a particle that’s its own antiparticle

Confirming a theory from the 1920s, Stanford physicists have found evidence of a particle that is its own antiparticle.

An anti-story

In 1928, British physicist Paul Dirac sent ripples through the world of science when he made a stunning prediction: every fundamental particle in the universe has an antiparticle — a particle with the same mass, but opposite charge. When matter and antimatter meet, they annihilate themselves, going out with a BANG. While his prediction was met with some skepticism, it didn’t take long for scientists to discover the first antimatter particle: the position. After the positron — the electron’s anti-particle — was confirmed, Dirac’s theory really took off.

But in 1937, things got even more bizarre. Another brilliant physicist, Ettore Majorana, said that some particles known as fermions can be their own antiparticles. A fermion can be an elementary particle, such as the electron, or it can be a composite particle, such as the proton. But how could such a particle be its own anti-particle? At a first glance, that doesn’t even seem to make sense, but many physicists supported Majorana’s theory.

So fermions were split into two categories: Dirac fermions and Majorana fermions. But the first was certainly a broader category than the second. In order for a particle to be its own anti-particle, it needed to not have a charge; otherwise, it just wouldn’t make sense. All of the Standard Model fermions except the neutrino have charges — so that means they’re Dirac fermions and have antiparticles. The only one whose nature was not settled was the neutrino. Well, it took a much longer time, but scientists believe they’ve finally found a Majorana fermion.

Cloud chamber photograph of the first positron ever observed. Credits: Carl D. Anderson.

A smoking gun

Now, a team of Stanford scientists carried out a series of experiments on exotic matter. Following a plan proposed by Shoucheng Zhang, professor of physics at Stanford, his colleagues and collaborators are confident they’ve found evidence of this phenomenon.

“Our team predicted exactly where to find the Majorana fermion and what to look for as its ‘smoking gun’ experimental signature,” said Zhang, a theoretical physicist and one of the senior authors of the research paper. “This discovery concludes one of the most intensive searches in fundamental physics, which spanned exactly 80 years.”

It’s not the first time this has been suggested. In 2016, researchers from the Fermi National Accelerator Laboratory suggested that the neutrino might be its own antiparticle. But this is the first time a clear indication of such particles has been found.

“It does seem to be a really clean observation of something new,” commented Frank Wilczek, a theoretical physicist and Nobel laureate at the Massachusetts Institute of Technology who was not involved in the study.

“It’s not fundamentally surprising, because physicists have thought for a long time that Majorana fermions could arise out of the types of materials used in this experiment. But they put together several elements that had never been put together before, and engineering things so this new kind of quantum particle can be observed in a clean, robust way is a real milestone.”

The Stanford experiments focused on quasiparticles — not truly particles, but particle-like excitations that arise out of the collective behavior of electrons in superconducting materials, which conduct electricity with 100 percent efficiency. As you’d imagine, the experiments were complex and laborious, but we can understand them by taking it step by step.

Creating quasiparticles

They stacked such a superconductor material on top of a topological insulator, a material which behaves as an insulator in its interior but allows the passing of current on its sides. With this material sandwich, they created a superconducting topological insulator — a material which is amazing at conducting current but only on its sides.

It was also tweaked to be magnetic, so that electrons would flow in one way on one side, and in the opposite way on the other side. When Zhang swept a magnet over it, he made the electrons slow down, stop, or even change direction. This wasn’t a linear, smooth process. As it so often happens in quantum mechanics, the process happened in abrupt steps — quanta. At some point in the process, the desired quasiparticles emerged, in pairs.

Zhang and his collaborators then observed these pairs split up, deflected out of the path of flow. They measured the flow of the individual quasiparticles that kept forging ahead. They behaved just like electrons, their movement being determined by the magnetic field. Their movement was also done in steps, but these steps were twice smaller than the electron steps. This was the smoking gun they were looking for. These were the Majorana fermions.

They named them the “angel particle,” a reference to the Angels & Demons book where a secret group plans to destroy the Vatican using a matter-antimatter bomb.

Here’s a lengthy presentation of the process, if you really want to get a feel for what it entailed.

But this isn’t quite like the real thing, says Professor Giorgio Gratta, who also works at Stanford and studies such interactions. He believes that particles and quasiparticles are different beasts and we can learn different things from them. Gratta believes that the quest for understanding if the neutrino is its own antiparticle continues.

“The quasiparticles they observed are essentially excitations in a material that behave like Majorana particles,” Gratta said. “But they are not elementary particles and they are made in a very artificial way in a very specially prepared material. It’s very unlikely that they occur out in the universe, although who are we to say?”

“On the other hand, neutrinos are everywhere, and if they are found to be Majorana particles we would show that nature not only has made this kind of particles possible but, in fact, has literally filled the universe with them.”

In March 1938, Italian physicist Ettore Majorana disappeared under mysterious circumstances while going by ship from Palermo to Naples. We don’t really know what happened to him. Some said he wanted to escape Fascist Italy and lived in Venezuela. Others say he was killed. But if he could see this, no doubt, his smile would stretch from ear to ear.

Journal Reference: Vlad S. Pribiag — A twist on the Majorana fermion. DOI: 10.1126/science.aao0793

Organic topological insulators are made from a thin molecular sheet that resembles chicken wire and conducts electricity on its right edge - with the electrons carrying more information in the form of "up" spin.

Organic topological insulator demonstrated for first time

Researchers at University of Utah have recently demonstrated that it is indeed feasible to construct a topological insulator from organic compounds. Topological insulators are deemed very important by scientists because of their unique property of conducting electrons on their edges, while at the same time acting as an insulator on the inside. These capabilities make it an ideal component for quantum computing, key for unlocking some of the Universe’s well kept secrets and for pushing technology in a new age.

Organic topological insulators are made from a thin molecular sheet that resembles chicken wire and conducts electricity on its right edge - with the electrons carrying more information in the form of "up" spin.

Organic topological insulators are made from a thin molecular sheet that resembles chicken wire and conducts electricity on its right edge – with the electrons carrying more information in the form of “up” spin.

Insulators came in two types. The most common and well known are the “ordinary” insulators, where electrons fully occupy energy bands and thus keep electricity from flowing. One of the best ordinary insulators are considered diamonds. Now, topological insulators on the other hand are a lot more interesting. In these types of materials the spin-orbit interaction is so strong that the insulating energy gap is inverted — the states that should have been at high energy above the gap appear below the gap.  As a result, we have highly conducting metallic states on the surface, while the inside is completely insulated.

Feng Liu, professor and chair of materials science and engineering at University of Utah, lead a team of scientists that proved that organic topological insulators can be created after they performed theoretical calculations to predict the existence of an organic topological insulator using molecules with carbon-carbon bonds and carbon-metal bonds, called an organometallic compound.

“This is the first demonstration of the existence of topological insulators based on organic materials,” says Liu. “Our findings will broaden the scope and impact of these materials in various applications from spintronics to quantum computing.”

In order for a material to behave like a topological insulator it needs to shuttle information at light speed or in other words be able to transmit fermions – massless or weightless packets of light that conduct electricity as they move along the material surface. Dirac fermions have a property called spin, or angular momentum around the particle’s axis that behaves like a magnetic pole. So using dirac fermions, scientists can use another way of encoding information, either signaled by “clockwise” or “counterclockwise” according to the spin of the fermions.

“We have demonstrated a system with a special type of electron – a Dirac fermion – in which the spin motion can be manipulated to transmit information,” Liu says. “This is advantageous over traditional electronics because it’s faster and you don’t have to worry about heat dissipation.”

Organic topological insulators have been rather poorly studied compared to conventional topological insulators, however if finally synthesized, organic topological insulators could mean such materials could be created a lot cheaper, easing production.

Findings were reported in a paper published in the journal Nature Communications.

source: Uni Utah press release / image copyright University of Utah.