Tag: The Standard model

Physicists continue to study Neutrinos, the ghost elementary particle, hoping it will lead them to Physics beyond the Standard Model. The KATRIN experiment has now given us our best estimate for the maximum mass of a neutrino.      

The Standard Model of particle physics has several problems. For one thing it simply doesn’t contain gravity in any way. Another problem is the masses of all the particles. For example the muon resembles an electron in every respect except its mass, which is 206.84 times that of its cousin. The standard model can’t simply doesn’t explain that ratio or any of the other mass ratios. In fact the whole concept of generations, particles like the electron, muon and tau that behave in the same fashion except for their mass, is a complete mystery at present. Perhaps the biggest problem with the Standard Model however is that it works so well that we have very few clues pointing toward a more comprehensive theory that will answer our questions.

The Standard Model of Particle Physics tells us so much, but not everything and WE WANT IT ALL!! (Credit: ScienceAlert)

That’s part of the reason why physicists are so busy studying the particle known as the neutrino. These ghost particles have mystified physicists ever since their existence was first predicted by the theoretician Wolfgang Pauli. Pauli proposed the neutrino to explain some discrepancies in the type of radiation known as beta decay.

Wolfgang Pauli was one of the founding fathers of Quantum Mechanics, his exclusion principle is the reason why two objects cannot occupy the same space at the same time! (Credit: Facebook)

Now in beta decay a neutron splits into a proton and an electron. In the process conservation of the electric charge works out, a neutron is neutral while the positive proton and negative electron still add up to zero. The energy of the proton and electron did not always come out the same however, a violation of conservation of energy. And the spins of the particles were just all wrong as well, again violating conservation of angular momentum.

When a neutron decays into a proton the emitted electron is called beta radiation. In order to conserve energy and spin there has to be another particle emitted as well. This is Pauli’s neutrino. (Credit: ww2.ph.ed.ac.uk)

What Pauli proposed was that a third particle, both electrically neutral and with zero rest mass, was emitted at the same time and the experimentalists just hadn’t detected it yet. At first Pauli called his particle the neutron but when the bigger, massive neutron was discovered by James Chadwick, Enrico Fermi then suggested Pauli’s neutral particle be called the neutrino, which is Italian for ‘little neutral one’. Well it took more than twenty years but eventually Pauli’s neutrino was discovered in 1956, and in fact physicists now know that there are three different types of neutrino, one each complimenting the electron, the muon and the tau particles. Again why each generation of electron like particle should have its own neutrino is simply not explained in the Standard Model.

By placing a large detector (r) next to a nuclear reactor Cowan and Reines (l) succeeded in discovering the elusive neutrino. (Credit: CERN Indico)

Now neutrinos interact very rarely with other particles, it’s estimated that a neutrino could fly through a light-year of solid lead and still have a 50-50 chance of coming out the other side. At the same time neutrinos are generated in large amounts in nuclear reactions, such as the fusion reaction that powers our own Sun and the other stars. Solar physicists therefore wanted to try to capture as many solar neutrinos as they could hoping to learn about the interior of the Sun from them.

The Fusion reactions that power the Sun release a lot of neutrinos. If detected here on Earth they can tell us much about what’s going on inside the center of our star. (Credit: Forbes)

Instead they learned more about neutrinos. The first neutrino telescope was built deep beneath the Earth’s surface at the Homestake Mine in South Dakota in order to eliminate contamination from cosmic rays. What the telescope found was that the number of neutrinos coming from the Sun was exactly one-third the expected number. After wondering for some time if something was wrong with their theories of solar fusion, or maybe something was actually wrong with the Sun the physicists eventually found that the three types of neutrino oscillate, that is they change from one type to another over time. The neutrinos generated in the Sun are all electron neutrinos but by the time they reach Earth two-thirds have changed to muon or tau neutrinos.

The idea is crazy and the math is really hard. Could that be why physicists are so interested in neutrino oscillations? (Credit: www-he.scphys.kyoto-u.ac.jp)

Which means that neutrinos must have a rest mass because particles with zero rest mass move at the speed of light and according to Einstein’s theory of relativity time does not pass for anything moving at the speed of light. So the questions now are, just what is the mass of a neutrino and can we learn a clue from that about the masses of all the particles.

According to Relativity the time interval between two events differs for two observers moving relative to each other. For a particle moving at the speed of light time actually stands still! (Credit: Pinterest)

That’s the purpose of the KATRIN experiment at Karlsruhe Institute of Technology in Germany. KATRIN is trying to measure the mass of a neutrino by making the most precise measurements ever of beta decay, the original interaction for which Pauli first proposed the neutrino. Think about it, if the energy of a neutron gets shared in varying amounts between a proton, electron and a neutrino the minimum amount of energy the neutrino can get is its rest mass. So if you measure thousands or better still millions of neutron decays the maximum energy of the proton and electron taken together and subtracted from that of the neutron, is the rest mass of the neutrino. Easier said than done, remember we’re talking about sub-atomic particles here and previous experiments have already concluded that the neutrino rest mass is less than 1/100,000th that of the electron.

The Katrin experiment seeks to measure the mass of a neutrino by measuring the maximum energy that the electron emitted in beta decay can have. The rest has to be the neutrino’s mass. (Credit: DW)

Let me take a moment here to mention the units by which particle physicists measure mass. Remembering Einstein’s most famous equation E=Mc2 physicists like to turn that equation around to get m=E/c2. So to describe the mass of elementary particles physicists use a measure of energy known as the electron-volt, the energy an electron will gain by accelerating across one volt of electrical potential, and divide it by c2 getting eV/c2 or kilo eV/c2 (Kev/c2) or Million eV/c2 (Mev/c2) or even GeV/c2, a billion eV.

Particle physicists measure energy in terms of the ‘electron-volt’, the energy an electron would gain by accelerating across one volt of electric potential. (Credit: Slideshare)

Now neutrons are themselves hard to handle, being neutral you can’t use an electric field to control them. So the KATRIN experiment uses the heavy isotope of hydrogen called Tritium, whose nuclei consists of one proton and two neutrons. Tritium is a well studied beta decay source and as a gas it is much easier to handle than a free neutron would be. Also the proton that results when the neutron decays remains in the nucleus, transforming it to a nucleus of helium-3. That means that the only thing you really have to measure is the energy of the produced electron.

The KATRIN detector being wheeled through the streets of Karlsruhe Germany. The size of the detector gives you some idea just how difficult experiments in particle physics can be to perform. (Credit: Symmetry Magazine)

Nevertheless it’s still a difficult task, which is why the KATRIN experiment is an enormous instrument 70m in length, much of which is the main spectrometer for measuring the electron’s energy. For the experiment the tritium gas of cooled down to a temperature of 30K (-247ºC) in order to minimize thermal motion and an set of 24 superconducting magnets are used to collimate the emitted electrons into the spectrometer.

While KATRIN is still continuing to collect data an analysis of the measurements gathered by the end of 2019 has achieved a milestone, at the 90% confidence level the rest mass of a neutrino is less than 0.8 eV/c2. An elementary particle with a rest mass that is less than 1eV would have been a shocking result just a few decades ago and in a sense a rest mass of around one-millionth that of the electron, or even less, only deepens the mystery of elementary particle masses.

The KATRIN team announcing their latest results, that neutrinos have a mass that is less than 1 electron volt! (Credit: CEA/Irfu)

Still the results of KATRIN are reality and the only way to get beyond the standard model is to gather more facts that don’t fit in the model. Who knows, maybe right now some grad student in some university somewhere is reading the article published in Nature Physics by the KATRIN collaboration and is thinking to themselves, ‘hey, wait a minute… that actually makes sense’! After all, that’s how it started with Pauli, and Einstein, and Bohr and all those others who built the standard model.

Axions, the elementary particle that ought to exist, but do they?

The sub-atomic physics that I was taught in high school was pretty simple. Atoms were made up of Neutrons, Protons and Electrons. The Neutrons and Protons stayed in the atom’s nucleus, and are given the name nucleons for that reason while the Electrons orbited around the nucleus. We also learned that the Protons had a positive charge, the Electrons a negative charge while the Neutrons were electrically neutral.

The Bohr model of an atom of Nitrogen. This is about as sophisticated as high school science classes will get. (Credit: SlidePlayer)

That was about all you’d learn in class, if you wanted to learn any more you’d have to do outside reading on your own, of which I did plenty. It was from books like George Gamow’s “Thirty Years that shook Physics” that I learned about other particles like the neutrino, muon, pion, Lepton and Delta particles. (Although the science may be rather outdated, I still highly recommend Gamow’s book as a history of Quantum Mechanics!!!) Oh, and I also learned that every one of those particles had an anti-particle, identical in every way to its partner except having the opposite electrical charge.

Thirty Years that Shook Physics by George Gamow. Highly Recommended!!! (Credit: Flickr)

But even as I was attending high school physicists were digging deeper. In fact it was in 1964 that physicist Murray Gell-Mann proposed the quark theory of nucleons. Gell-Mann’s idea was that the Proton and Neutron were composed of three smaller particles called quarks, two up quarks and a down quark made a proton while a neutron was two downs and an up. At the same time the lambda and delta particles were also composed of three quarks but for these unusual particles one of the quarks was a strange quark, a name given to the particle to indicate how little physicists understood it at the time. In Gell-Mann’s theory the pion was also composed of quarks but they were made of a quark anti-quark pair. Meanwhile the electron, muon and neutrino were not made of quarks, they remained elementary, fundamental particles that cannot be decomposed into smaller pieces.

The Gell-Mann model of nucleon structure. Three quarks make a proton or neutron while a quark anti-quark pair make a pion! (Credit: Lumen Learning)

It took physicists more than 20 years to work out the ramifications of Gell-Mann’s theory but by the early 1990s they had a framework called ‘The Standard Model’ that was able to broadly describe the interactions between the particles that they saw in their high energy ‘atom smasher’ experiments. The final piece in the standard model was the discovery in 2012 of the Higgs boson, the particle that gives all other particles their mass.

Robert Higgs standing in front of a photograph of some of the equipment needed to finally discover his Higgs boson. (Credit: Science / How Stuff Works)

The standard model doesn’t answer all our questions however. For example while the Higgs boson does give other particles their mass we don’t understand why those particles have the mass they do. The up and down quarks have roughly the same mass, about 5 times that of an electron but other quarks have much larger masses. At the same time we know that the neutrino also has a mass but one that is so small that we haven’t been able to measure it accurately yet, it’s less than one millionth that of the electron. What sets all of the masses for these different particles, we just don’t know?

The masses, in millions of electron volts (MeV) of some elementary particles. What makes all of these masses what they are is completely unknown! (Credit: Semantic Scholar)

One of the problems not addressed by the standard model is that according to theory the neutron should possess a strong electric dipole, it should act like a strong positive and strong negative charges brought close together, a property that would be easily discovered. In order to solve this dilemma, known as the strong CP problem (for Charge Conjugation / Parity) in 1977 the physicists Roberto Peccei, Helen Quinn, Frank Wilczek and Steven Weinberg proposed a new particle called the axion. This new particle would have a very low mass, like the neutrino on the order of one millionth that of an electron, and hardly interact with other types of particles.

One interesting experiment that might discover the axion. (Credit: Universe-Review.ca)

Even while particle physicists were trying to make sense of the concept of the axion astrophysicists and cosmologists heard about the particle and realized that the axion, if it existed, could be a major component of Dark Matter. With its low mass the axion would have been created in enormous numbers during the original Big Bang, and since they hardly interact with other particles they would still exist. Could the axion be the dark matter that the astrophysicists were searching for?

Now predicting new particles is a risky business. If you’re right you’ll become famous like Wolfgang Pauli with the neutrino or Robert Higgs and his boson. On the other hand there are dozens of ‘predicted particles’ that have never been found. And it often takes decades for experimentalists to develop the technology needed to prove that a particle exists. Pauli predicted the neutrino in 1930 and it wasn’t proven to exist until 1956. Same for the Higgs boson, Robert Higgs wrote his original paper in 1964 but the particle was only officially discovered in 2012.

That discovery is what researchers at the Gran Sasso National Labouratory in Italy hope to accomplish with their XENON1T experiment. The experiment consists of a 3.2 metric ton tank of Xenon gas in what is known as a Time Projection Chamber. Photomultiplier tubes inside the tank detect the tiny flashes of light produced by the interactions and the entire apparatus was constructed deep within a mine beneath the Gran Sasso Mountain in order to shield the experiment from false signals due to cosmic rays.

Main Detector of the Xenon1T experiment being readied for installation. (Credit: LNGS-Infn)

After two years of operation the XENON1T team has now announced the first ever measured evidence for the existence of axions. At a news conference on June the 17th the XENON1T physicists presented their data showing an excess number of flashes in the low energy region. This was exactly the sort of signal that would be expected for interactions with axions produced in the interior of the Sun. According to the announcement the amount of data collected was sufficient for a 3.5 sigma confidence level in the discovery.

Latout of the Gran Sasso Labouratory, now the world’s largest underground physics labouratory. (Credit: Nature)

That 3.5-sigma level is the problem; statistically 3.5-sigma means that there is only a one in 10,000 chance that the excess flashes are simply a matter of luck. Like rolling a pair of dice and getting boxcars three times in a row, something that only happens very rarely, but it does happen. The physics community has agreed that in order to really announce a ‘Discovery’ an experiment must achieve a confidence level of 5-sigma, which means that there is only one chance in 3.5 million that the data is just a statistical fluke. 

Scientists express their ‘confidence level’ by using the normal of bell curve distribution. The larger the sigma value the more of the bell is contained, the more confident you are! (Credit: Dummies.com)

So what do experimental physicists do when their experiment looks like it’s found something but the data is too small to be certain? Build a bigger, more sensitive experiment of course. The scientists at XENON1T are already doing just that, upgrading their equipment to an 8 metric ton container of Xenon for a new 5-year run that should be able to cross the magic 5-sigma threshold.

So has the axion been found? Well some other physicists are already criticizing the whole setup; the same signal could be produced by the detector being contaminated by the isotope of hydrogen called tritium. It takes time to be certain so we’re all just going to have to wait. Making a discovery is what every scientist dreams of but as they all know, it’s more important to be right than to be first!