particle_physics (4)



Image Source: Axion particle spotted in solid-state crystal, Max Planck Society,



Topics: Cosmology, Dark Matter, Particle Physics, Quantum Mechanics, Standard Model

A team of physicists has made what might be the first-ever detection of an axion.

Axions are unconfirmed, hypothetical ultralight particles from beyond the Standard Model of particle physics, which describes the behavior of subatomic particles. Theoretical physicists first proposed the existence of axions in the 1970s in order to resolve problems in the math governing the strong force, which binds particles called quarks together. But axions have since become a popular explanation for dark matter, the mysterious substance that makes up 85% of the mass of the universe, yet emits no light.

If confirmed, it’s not yet certain whether these axions would, in fact, fix the asymmetries in the strong force. And they wouldn’t explain most of the missing mass in the universe, said Kai Martens, a physicist at the University of Tokyo who worked on the experiment. These axions, which appear to be streaming out of the sun, don’t act like the “cold dark matter” that physicists believe fills halos around galaxies. And they would be particles newly brought into being inside the sun, while the bulk of the cold dark matter out there appears to have existed unchanged for billions of years since the early universe.*

Still, it sure seems like there was a signal. It turned up in a dark underground tank of 3.5 tons (3.2 metric tons) of liquid xenon—the XENON1T experiment based at the Gran Sasso National Laboratory in Italy. At least two other physical effects could explain the XENON1T data. However, the researchers tested several theories and found that axions streaming out of our sun were the likeliest explanation for their results.

Physicists who weren’t involved in the experiment have not reviewed the data as of the announcement at 10 a.m. ET today (June 17). Reporters were briefed on the finding before the announcement, but data and paper on the find were not made available.

Live Science shared the XENON collaboration’s press release with two axion experts.

Physicists Announce Potential Dark Matter Breakthrough, Rafi Letzter, Live Science/Scientific American

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Figure 1.
Spin–orbit coupling can open a bulk bandgap in materials with inverted valence and conduction bands. That gap is complete in a topological insulator, but in a Weyl semimetal, the bands still touch at certain points. Both phases also host surface states not shown here. (Adapted from ref. 4, B. Yan and C. Felser.)


Topics: Modern Physics, Particle Physics, Quantum Mechanics

When Paul Dirac introduced his famous equation for relativistic fermions in 1928, he aimed to describe one well-known particle: the electron. Shortly thereafter, Hermann Weyl observed that the equation has a special solution when the mass is set to zero. The so-called Weyl fermions embodied by that solution would be charged, like electrons, but being massless, they would travel faster and with less energy dissipation. The particles would also be chiral, like neutrinos, with each one’s handedness depending on whether its spin is aligned or antialigned with its momentum. Those features make Weyl fermions appealing candidates for use in electronic and spintronic devices.

No such elementary particle has yet been found. However, in 2015 three groups of researchers identified the first Weyl semimetal (WSM), tantalum arsenide, which hosts quasiparticles—collective excitations of electrons—with the properties of Weyl fermions.1 A WSM must have a broken symmetry, and in TaAs, it’s inversion symmetry. Researchers, however, have continued searching for materials, particularly ferromagnetic materials, that instead rely on broken time-reversal symmetry. Tying a WSM crystal’s properties to magnetism, which can be adjusted using temperature changes or external fields, makes them potentially tunable.

Three new papers provide experimental evidence for magnetic WSMs. Yulin Chen’s team at Oxford University and Haim Beidenkopf’s team at the Weizmann Institute of Science, together with collaborators,2 presented studies of Co3Sn2S2, and Zahid Hasan’s group at Princeton University3 looked at Co2MnGa. The works identify important features in the electronic structures of both materials’ bulk and surface states.


Magnetic semimetals host massless quasiparticles, Christine Middleton, Physics Today

#P4TC: Weyl Fermions...July 27, 2015

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In Finnegan's Wake...

Murray Gell-Mann won the 1969 Nobel Prize in Physics.Credit: Santa Fe Institute


Topics: Nobel Laureate, Nobel Prize, Particle Physics, Quarks, Standard Model, Theoretical Physics

The Nobel Prize in Physics 1969 was awarded to Murray Gell-Mann "for his contributions and discoveries concerning the classification of elementary particles and their interactions."

The Nobel Prize in Physics 1969. Nobel Media AB 2019. Wed. 29 May 2019. < >

Murray Gell-Mann, one of the founders of modern particle physics, died on 24 May, aged 89. Gell-Mann’s most influential contribution was to propose the theory of quarks — fundamental particles that make up most ordinary matter.

To bring order to a plethora of recently discovered subatomic particles, in 1961 Gell-Mann proposed a set of rules based on symmetries in the fundamental forces of nature. The rules classified subatomic particles called hadrons into eight groups, a scheme he named the eightfold way in a reference to Buddhist philosophy.

In 1964, he realized that such rules would naturally arise if the particles were composed of two, three or more fundamental particles, held together by the strong nuclear force. (US–Russian physicist George Zweig came to the same conclusion independently in the same year.) Protons and neutrons, for example, would be made up of three of these more fundamental particles, which Gell-Man named quarks, inspired by a quote — “Three quarks for Muster Mark!” — from James Joyce’s 1939 novel Finnegan's Wake. [1]

Quarks and Leptons are the building blocks which build up matter, i.e., they are seen as the "elementary particles". In the present standard model, there are six "flavors" of quarks. They can successfully account for all known mesons and baryons (over 200). The most familiar baryons are the proton and neutron, which are each constructed from up and down quarks. Quarks are observed to occur only in combinations of two quarks (mesons), three quarks (baryons). There was a recent claim of observation of particles with five quarks (pentaquark), but further experimentation has not borne it out. [2]


1. Murray Gell-Mann, father of quarks, dies - US physicist was one of the chief architects of the standard model of particle physics. Davide Castelvecchi, Nature
2. Hyperphysics: Quarks

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Left, schematics of the apparatus (positron beam, collimators, SiN gratings and emulsion detector. A HpGe detector is used as beam monitor). Right, single-particle interference visibility as a function of the positron energy is in agreement with quantum mechanics (blue) and disagrees with classical physics (orange dashed). Courtesy: Politecnico di Milano


Topics: Antimatter, High Energy Physics, Particle Physics, Quantum Mechanics

Researchers in Italy and Switzerland have performed the first ever double-slit-like experiment on antimatter using a Talbot-Lau interferometer and a positron beam.

The classic double-slit experiment confirmed that light and matter have the characteristics of both waves and particles, a duality that was first put forward by de Broglie in 1923. This superposition principle is one of the main postulates of quantum mechanics and researchers have since been able to diffract and interfere matter waves of objects of increasing complexity – from electrons to neutrons and molecules.

The QUPLAS (QUantum Interferometry and Gravitation with Positrons and LAsers) collaboration, which includes researchers from the Politecnico di Milano L-NESS in Como, the Milan unit of the Istituto Nazionale di Fisica Nucleare (INFN), the Università degli Studi di Milano and the University of Bern, has now performed the first interference experiment on positrons – the antimatter equivalent of electrons.

“The experiment was first proposed for electrons by Albert Einstein and Richard Feynman as a thought experiment and realized by Merli, Missiroli and Pozzi in 1976 and more systematically by Tonomura and colleagues in 1989,” explains QUPLAS spokesman Marco Giammarchi of the INFN. “In this original experiment, which was voted by Physics World as the most beautiful experiment, the researchers demonstrated the specifically quantum effect of single particle interference, which – according to Feynman – is the central ‘mystery’ of quantum theory.”


Antimatter quantum interferometry makes its debut, Belle Dumé, Physics World

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