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WHAT ARE LEPTONS?

During the 19th and 20th centuries, physicists began to probe deep into the nature of matter and energy. In so doing, they quickly realized that the rules which govern them become increasingly blurry the deeper one goes. Whereas the predominant theory used to be that all matter was made up of indivisible atoms, scientists began to realize that atoms are themselves composed of even smaller particles.

From these investigations, the Standard Model of Particle Physics was born. According to this model, all matter in the Universe is composed of two kinds of particles: hadrons – from which Large Hadron Collider (LHC) gets its name – and leptons. Where hadrons are composed of other elementary particles (quarks, anti-quarks, etc), leptons are elementary particles that exist on their own.

Definition:

The word lepton comes from the Greek leptos, which means “small”, “fine”, or “thin”. The first recorded use of the word was by physicist Leon Rosenfeld in his book Nuclear Forces (1948). In the book, he attributed the use of the word to a suggestion made by Danish chemist and physicist Prof. Christian Moller.

The Standard Model of Particle Physics, showing all known elementary particles. Credit: Wikipedia Commons/MissMJ/PBS NOVA/Fermilab/Particle Data Group

The term was chosen to refer to particles of small mass, since the only known leptons in Rosenfeld’s time were muons. These elementary particles are over 200 times more massive than electrons, but have only about one-ninth the the mass of a proton. Along with quarks, leptons are the basic building blocks of matter, and are therefore seen as “elementary particles”.

Types of Leptons:

According to the Standard Model, there are six different types of leptons. These include the Electron, the Muon, and Tau particles, as well as their associated neutrinos (i.e. electron neutrino, muon neutrino, and tau neutrino). Each of these has a negaive charge and a distinct mass. Electrons are the lightest, with a mass of 0.000511 gigaelectronvolts (GeV), while Muons have a mass of 0.1066 Gev and Tau particles (the heaviest) have a mass of 1.777 Gev.

The different varieties of the elementary particles are commonly called “flavors”. While each of the three lepton flavors are different and distinct (in terms of their interactions with other particles), they are not immutable. A neutrino can change its flavor, a process which is known as “neutrino flavor oscillation”.

This can take a number of forms, which include solar neutrino, atmospheric neutrino, nuclear reactor, or beam oscillations. In all observed cases, the oscillations were confirmed by what appeared to be a deficit in the number of neutrinos being created. One observed cause has to do with “muon decay” (see below), a process where muons change their flavor to become electron neutrinos  or  tau neutrinos – depending on the circumstances.

Muons, a type of lepton, shown being produced by the Large Hadron Collider. Credit: CERN

In addition, all three leptons and their neutrinos have an associated antiparticle (antilepton). For each, the antileptons have an identical mass, but all of the other properties are reversed. These pairings consist of the electron/positron, muon/antimuon, tau/antitau, electron neutrino/electron antineutrino, muon neutrino/muan antinuetrino, and tau neutrino/tau antineutrino.

The present Standard Model assumes that there are no more than three types (aka. “generations”) of leptons with their associated neutrinos in existence. This accords with experimental evidence that attempts to model the process of nucleosynthesis after the Big Bang, where the existence of more than three leptons would have affected the abundance of helium in the early Universe.

Properties:

All leptons possess a negative charge. They also possess an intrinsic rotation in the form of their spin, which means that electrons with an electric charge – i.e. “charged leptons” – will generate magnetic fields. They are able to interact with other matter only though weak electromagnetic forces. Ultimately, their charge determines the strength of these interactions, as well as the strength of their electric field and how they react to external electrical or magnetic fields.

None are capable of interacting with matter via strong forces, however. In the Standard Model, each lepton starts out with no intrinsic mass. Charged leptons obtain an effective mass through interactions with the Higgs field, while neutrinos either remain massless or have only very small masses.

History of Study:

The first lepton to be identified was the electron, which was discovered by British physicist J.J. Thomson and his colleagues in 1897 using a series of cathode ray tube experiments. The next discoveries came during the 1930s, which would lead to the creation of a new classification for weakly-interacting particles that were similar to electrons.

The first discovery was made by Austrian-Swiss physicist Wolfgang Pauli in 1930, who proposed the existence of the electron neutrino in order to resolve the ways in which beta decay contradicted the Conservation of Energy law, and Newton’s Laws of Motion (specifically the Conservation of Momentum and Conservation of Angular Momentum).

The positron and muan were discovered by Carl D. Anders in 1932 and 1936, respectively. Due to the mass of the muon, it was initially mistook for a meson. But due to its behavior (which resembled that of an electron) and the fact that it did not undergo strong interaction, the muon was reclassified. Along with the electron and the electron neutrino, it became part of a new group of particles known as “leptons”.

In 1962, a team of American physicists – consisting of Leon M. Lederman, Melvin Schwartz, and Jack Steinberger – were able to detect of interactions by the muon neutrino, thus showing that more than one type of neutrino existed. At the same time, theoretical physicists postulated the existence of many other flavors of neutrinos, which would eventually be confirmed experimentally.

The tau particle followed in the 1970s, thanks to experiments conducted by Nobel-Prize winning physicist Martin Lewis Perl and his colleagues at the SLAC National Accelerator Laboratory. Evidence of its associated neutrino followed thanks to the study of tau decay, which showed missing energy and momentum analogous to the missing energy and momentum caused by the beta decay of electrons.

In 2000, the tau neutrino was directly observed thanks to the Direct Observation of the NU Tau (DONUT) experiment at Fermilab. This would be the last particle of the Standard Model to be observed until 2012, when CERN announced that it had detected a particle that was likely the long-sought-after Higgs Boson.

Today, there are some particle physicists who believe that there are leptons still waiting to be found. These “fourth generation” particles, if they are indeed real, would exist beyond the Standard Model of particle physics, and would likely interact with matter in even more exotic ways.

We have written many interesting articles about Leptons and subatomic particles here at Universe Today. Here’s What are Subatomic Particles?, What are Baryons?, First Collisions of the LHC, Two New Subatomic Particles Found, and Physicists Maybe, Just Maybe, Confirm the Possible Discovery of 5th Force of Nature.

For more informationm, SLAC’s Virtual Visitor Center has a good introduction to Leptons and be sure to check out the Particle Data Group (PDG) Review of Particle Physics.

Astronomy Cast also has episodes on the topic. Here’s Episode 106: The Search for the Theory of Everything, and Episode 393: The Standard Model – Leptons & Quarks.

Russian Progress Cargo Ship Launch Failure Deals Setback to ISS

Ken Kremer द्वारा

The Progress 65 cargo spaceship launched on time Thursday morning, Dec. 1, 2016 from the Baikonur Cosmodrome in Kazakhstan but failed to reach orbit minutes later. Credit: NASA TV

KENNEDY SPACE CENTER, FL - An unmanned Russian Progress resupply ship bound for the International Space Station (ISS) was lost shortly after launch from the Baikonur Cosmodrome in Kazakhstan on Thursday when its Soyuz booster suffered a catastrophic anomaly.

The Russian launch failure deals somewhat of a setback to the ever ongoing efforts by all the space station partners to keep the orbiting outpost well stocked with critical supplies of food and provisions for the multinational six person crew and science experiments to carry out the research activities for which the station was assembled.

The Soyuz rocket failed in flight around six and a half minutes after what had been an otherwise flawless nighttime liftoff from the Baikonur Cosmodrome at 9:51 a.m. EST (8:51 p.m. Baikonur time), Thursday, Dec. 1.

“The Russian space agency Roscosmos has confirmed a Progress cargo resupply spacecraft bound for the International Space Station and her six person crew has lost shortly after launch,” said NASA.

Per protocol, the Russian space agency Roscosmos has formed a state commission to investigate the accident, seek out the root cause and implement measures to prevent such failures in the future.

“The first few minutes of flight were normal, but Russian flight controllers reported telemetry data indicating a problem during third stage operation. The Russians have formed a State Commission and are the source for details on the specific failure cause.”

Despite the failure there was no immediate impact on the current Expedition 50 crew and life goes on.

“The spacecraft was not carrying any supplies critical for the United States Operating Segment (USOS) of the station,” NASA reported.

However the continued useful utilization of the million pound station is totally dependent on receiving a steady train of supplies from Earth - comprising Russian, US and Japanese cargo freighters launching multiple times per year.

The Progress 65 cargo freighter was jam packed with 2.6 tons of food, fuel, and supplies for the space station crew, including approximately 1,400 pounds of propellant, 112 pounds of oxygen, 925 pounds of water, and 2,750 pounds of spare parts, supplies and scientific experiment hardware.

The Russian Progress 62 spacecraft approaches the International Space Station on July 1, 2016. Credits: NASA

The Progress was carrying a few items from NASA but they are all replaceable, says NASA. The US items packed on board included spare parts for the station’s environmental control and life support system, research hardware, crew supplies and crew clothing.

Stay tuned here for Ken's continuing Earth and Planetary science and human spaceflight news.

Ken Kremer

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Researchers Find Biggest Exposed Fault on Earth

• 
  
  Geologists have for the first time seen and documented the Banda Detachment fault in eastern Indonesia and worked out how it formed.

MEDIA CONTACT

Dr. Jonathan Pownall
jonathan.pownall@anu.edu.au
61-261-258-099

CITATIONS

Geology

CHANNELS

Environmental Science, Geology, Earthquakes, Staff Picks, Tsunami, All Journal News

KEYWORDS

Newswise — Geologists have for the first time seen and documented the Banda Detachment fault in eastern Indonesia and worked out how it formed.

Lead researcher Dr Jonathan Pownall from The Australian National University (ANU) said the find will help researchers assess dangers of future tsunamis in the area, which is part of the Ring of Fire - an area around the Pacific Ocean basin known for earthquakes and volcanic eruptions.

"The abyss has been known for 90 years but until now no one has been able to explain how it got so deep," Dr Pownall said.

"Our research found that a 7 km-deep abyss beneath the Banda Sea off eastern Indonesia was formed by extension along what might be Earth's largest-identified exposed fault plane."

By analysing high-resolution maps of the Banda Sea floor, geologists from ANU and Royal Holloway University of London found the rocks flooring the seas are cut by hundreds of straight parallel scars.

These wounds show that a piece of crust bigger than Belgium or Tasmania must have been ripped apart by 120 km of extension along a low-angle crack, or detachment fault, to form the present-day ocean-floor depression.

Dr Pownall said this fault, the Banda Detachment, represents a rip in the ocean floor exposed over 60,000 square kilometres.

"The discovery will help explain how one of the Earth's deepest sea areas became so deep," he said.

Professor Gordon Lister also from the ANU Research School of Earth Sciences said this was the first time the fault has been seen and documented by researchers.

"We had made a good argument for the existence of this fault we named the Banda Detachment based on the bathymetry data and on knowledge of the regional geology," said Professor Lister.

Dr Pownall said he was on a boat journey in eastern Indonesia in July when he noticed the prominent landforms consistent with surface extensions of the fault line.

"I was stunned to see the hypothesised fault plane, this time not on a computer screen, but poking above the waves," said Dr Pownall.

He said rocks immediately below the fault include those brought up from the mantle.

"This demonstrates the extreme amount of extension that must have taken place as the oceanic crust was thinned, in some places to zero," he said.

Dr Pownall also said the discovery of the Banda Detachment fault would help assesses dangers of future tsunamis and earthquakes.

"In a region of extreme tsunami risk, knowledge of major faults such as the Banda Detachment, which could make big earthquakes when they slip, is fundamental to being able to properly assess tectonic hazards," he said.

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The research has been published in the journal GEOLOGY: http://geology.geoscienceworld.org/content/44/11/947.full?ijkey=r0IjGZXS8x9Ro&keytype=ref&siteid=gsgeology

Images are available at CloudStor: https://cloudstor.aarnet.edu.au/plus/index.php/s/OP6oeNqTHreVsOU

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