Two stories about Particle Physics, and the Particles in one may Surprise You.

A freshman physics class in any college or university will for the most part deal with the movement and behavior of particles, that is objects with easily measurable and long lasting quantities like size, shape, mass, position etc, etc. As someone who grew up in a baseball family I freely admit that whenever I encounter the word ‘particle’ the first thought in my brain is something very like a baseball.

I’ll be the first to admit it, growing up in baseball family and becoming a physicist I do tend to think of elementary particles as little tiny baseballs! (Credit: The Conversation)

In that freshman physics class the students will learn about such other particle quantities as velocity, acceleration, and momentum while discussing collisions between particles, the forces between particles and so on. In other words freshman physics talks a lot about particles.

Collisions are rather complicated problems that really follow a few simple rules so they are a big topic in most freshman physics classes. (Credit: HyperPhysics Concepts)

Higher level physics is pretty much the same. Experimental physics is usually conducted at High Energy Particle Accelerators like the Large Hadron Collider at CERN (A Hadron is a kind of sub-atomic particle) or deep underground where ‘Ghost Particles’ called Neutrinos are captured in huge vats of water. Today I’ll be discussing some recent studies in particle physics, the first concerning those ghost particles the neutrinos while the second concerns objects you might consider it strange to call particles, human beings.

For over 100 years now physicists have been studying collisions between elementary particles in order to learn more about them. (Credit: ResearchGate)

Neutrinos have fascinated physicists ever since Wolfgang Pauli first predicted their existence in order to ‘balance the books’ in beta radiation decay. Originally thought to be a single kind of particle with no electric charge and either no or very little rest mass (Neutrino = little neutral one in Italian) they have only gotten stranger as we’ve learned more about them. We now know that there are at least three distinct types, or generations of neutrino, the electron, muon and tau each named for the type of electron like particle they are generated with. Unlike other generations however, up, charm and top quarks for example, the three types of neutrino can oscillate from one type into the other into the other. This implies that neutrinos must have some rest mass, perhaps one millionth that of an electron.

Neutrinos make up three of the twelve different types of Fermions, that is particles that follow the rule of ‘No two identical particles in the same quantum state’. (Credit: PhysicsMasterClasses.org)

Like other Fermions each type of neutrino also has an anti-particle, or does it? You see in general anti-particles have the exact same mass but the opposite electric charge of their species of particle. However, since neutrinos have no electric charge there is a real possibility that neutrinos may be their own anti-particle and that would certainly be new physics.

First ever evidence for the existence of anti-matter, a photograph of a ‘positive’ electron by Carl Anderson. (Credit: Phys.org)

Now neutrinos are very difficult to study, they only rarely, very rarely interact with other kinds of particles. It’s estimated that the neutrinos generated in the Sun by fusion could go through a light year of solid lead and half would still come out the other side. Physicists who study neutrinos usually need sources of trillions of the particles in order to catch just a few.

In order to study neutrinos physicists have to build large detectors like this one at the Sudbury neutrino observatory. (Credit: Atlas Obscura)

Experimentalists decided that their best chance for determining if neutrinos were their own anti-particle was to use a very rare type of radioactive decay called a double beta decay that occurs in the isotope 76 of the element germanium, atomic number 32. Double beta decay happens when the nucleus emits two electrons and two anti-neutrinos at the same time while jumping up two spots in the periodic table to selenium, atomic number 34.

Double beta decay should happen as pictured on the left with two electrons and two neutrinos being emitted. But if the neutrino is its own anti-particle then the decay on the right is possible with only two electrons being emitted. (Credit: Quantum Diaries)

If the neutrino is its own antiparticle however then the two anti-neutrinos could annihilate each other and only two electrons would come out, electrons, the easiest particle to detect and measure. So what the experimentalists would have to do is measure the mass / energy of the germanium nucleus before the decay, and the mass / energy of the selenium nucleus after the decay and if the two electrons got all of the energy difference then there were no neutrinos and the neutrino is its own anti-particle.

Without any neutrinos conservation of energy would require that all of the energy has to go into the emitted electrons and that is something we can measure, barely. (Credit: Nitty Gritty Science)

Easier said than done. The experiment to search for neutrinoless double beta decay (0νββ) was carried out using 30 kg of germanium that had been enriched to 88% isotope 76, in nature isotope 76 makes up 7.75% of all germanium. The germanium was then surrounded by detectors to both find and measure the energy of any electrons that were emitted. In order to reduce as much as possible interference from other radioactive decays the experiment was conducted a kilometer and a half beneath the Earth’s surface at the Sanford Underground Research Facility in South Dakota, an old abandoned gold mine. After ten years of operation the collaboration of scientists that runs the experiment has just released its results, no neutrinoless double beta decays were observed by the experiment which sets new limits on the possibility that a neutrino could be its own anti-particle.

Part of the Neutrinoless Double Beta Decay experiment at the Sanford Underground Research Facility. (Credit: Sciencesprings)

Of course just because you haven’t found something doesn’t mean that it doesn’t exist, it could mean you just haven’t looked hard enough. So the physicists who ran the experiment are now planning a new experiment that will employ 1,000 kg of germanium in their search to learn more about the ‘Ghost Particle’.

Here’s a book I read as a teenager that really helped spur my interest in Physics. Even today physicists are fascinated by the neutrino. (Credit: AbeBooks)

Human beings are not generally thought of as ‘particles’ but in many ways can be treated as such. After all we each have a definite size and shape as well as mass so there are many circumstances where the involuntary motion of a person is exactly like any particle. That’s why we can use ‘crash test dummies’ as substitutes for real people in automobile safety testing because during the conditions of a crash the humans inside the car are really just particles.

If you think about it, crash test dummies are designed to replicate the ‘particle’ aspects of human beings while eliminating everything else. (Credit: CNBC)

There are even times when the voluntary motions of humans can be studied as particles and something of the laws governing that behavior learned. One well known example of this is when two groups of people, traveling in opposite directions have to move through each other, such as when two groups of pedestrians are trying to cross to opposite sides of the same street using the same crosswalk and have to go through each other.

In many ways large numbers of human beings each going there own way are very much like an ensemble of particles interacting with each other. (Credit: NAIOP Blog)

What has been observed in such circumstances is that the two groups will break up into a series of ‘lanes’ that will interleave with the lanes of the opposing group. These lanes then move past each other in what is technically known as two component flow. This type of phenomenon in general has been given the name ‘active matter’ and has been observed in many different kinds of animals from flocks of birds to schools of fish.

Thanks to modern computers even very complicated problems, like the crowd pictured here, can be analyzed and order found in all of the apparent chaos. (Credit: Science News)
Scientists have found that in order to form a school, fish only have to obey a few simple rules. (Credit: HuffPost)

While the development of lanes in groups of people moving in opposite directions has been studied for several decades no rigorous mathematical model of the behavior had been published. Until now, for a new paper in the journal Science by mathematicians at the University of Bath in the UK has presented a kinetic description of lane nucleation, in other words equations describing how lanes form and behave. The model was in fact based upon Albert Einstein’s description of ‘Brownian Motion’ of pollen grains in a solution.

As described by Albert Einstein, Brownian Motion was the first direct evidence for the existence of atoms. (Credit: Toppr)

The mathematicians have even teamed up with experimentalists at the Department of Human Motor Behavior at the Academy of Physical Education in Katowice in Poland to test their model. One of the experiments was set up in King’s Cross Station in London where groups of volunteers moved through different gates and obstacles. The movement of the volunteers was video recorded and in every case order arose out of chaos allowing the recordings to be compared to the model’s predictions.

The milling crowds at King’s Cross Station in London proved to be the perfect experiment for studying the ‘particle nature’ of human traffic. (Credit: iStock)

So it seems that the idea of a particle, so useful in physics, can also be applied to the study of living creatures, even we humans.

The CUORE Experiment in Italy releases its first Results. The Search for New Physics.

Perhaps the most sophisticated, sensitive experiment even attempted is the Cryogenic Underground Observatory for Rare Events or CUORE now underway at Laboratori Nazionali del San Grasso in Italy. To order to give you an idea of just what lengths the scientists have gone to in order to achieve such sensitivity the researchers boast of having build “the coldest cubic meter of space in the known Universe!”

And they’re going to need it; the CUORE team are looking for subatomic events so rare that they happen once or twice a year in 100 kilograms worth of atoms. The specific reaction that the CUORE team is studying is the extremely rare double beta decay, they’re trying to see whether or not two neutrinos are produced, as the Standard Model of Elementary Particles requires.

Let me take a step back and describe single beta decay first. Back in the early 20th century physicists found three distinct types of radiation, alpha, beta and gamma rays. Beta radiation was found to occur when a neutron broke up into a proton and an electron, the electron is the high energy beta particle. Problem was that some of the energy of the neutron went missing, an apparent violation of the law of conservation of energy. It was the physicist Wolfgang Pauli who suggested in 1935 that there was another particle as well, a neutral particle with little or no mass that would be very difficult to detect. The image below shows a Feynman diagram of the beta decay process (The W particle in the middle is the boson that carries the weak nuclear interaction). Detecting neutrinos turned out to be so difficult that in fact it took experimentalists 25 years to finally prove that the neutrino was real.

Feynman Diagram of single Beta decay

Single neutrino decay happens quite often, in fact a free neutron, one not in a nucleus will undergo beta decay with a half life of about 12 minutes. (See my post of 4Mar2017). Double beta decay, where two neutrons simultaneously decay to two protons and two electrons, is far rarer and was only proven to exist in 1987.

Now it’s been suggested that double beta decay might not produce any neutrinos! This would require the neutrino to be its own anti-particle so that they would annihilate each other. Such a reaction would be a violation of conservation of lepton number, a key element of the Standard Model of elementary particles. So physicists are very interested in the possibility of neutrinoless double beta decay. The image below shows the Feynman diagrams for double beta decay with and without neutrinos.

Double Beta decay, left with neutrinos, right neutrinoless

There are several reasons why physicists are so interested in neutrinoless double beta decay. One is that it would indicate a possible channel to explain why there is more matter in our Universe than anti-matter while at the same time it could also enable us to measure the tiny rest mass of the neutrino.

Now as I said double beta decay is very rare. You need to observe all of the atoms in kilograms of a material that is capable of double beta decay in order to see one or two a year! And then you have to measure the total energy of both of the electrons to make certain that they got it all, with no neutrinos the electrons get all of the kinetic energy generated.

The experiment the CUORE team has developed uses a device known as a bolometer that will actually measure the heat generated by a single subatomic event. There are 988 total bolometers composed of crystals made from the chemical TeO2 where Te is the element whose isotope Te130 is capable of double beta decay. It is in order to measure the tiny amount of energy released by the double beta decay that all 988 bolometers have to be maintained at the unbelievably cold temperature of 10mK (That’s 10 thousandths of a degree above absolute zero Kelvin), the coldest place in the known Universe. The image below shows the detector ‘towers’ ready to be installed in the cold chamber.

CUORE Detectors before installation (Credit: Meteoweb.eu)

Before I forget I need to mention that in order to prevent radiation from outside, primarily cosmic rays, interfering with the measurements the detectors are first wrapped in lead shielding and then the entire experiment is buried deep underground.

The CUORE collaboration, which consists of over 150 scientists from around the world, have just released the results of the first year of the experiment and so far it looks like the standard model still stands. The CUORE team puts the half-life of a neutrinoless double beta decay at greater than 1.5 x 10^25 years. That doesn’t mean that neutrinoless double beta decay never happens, you can never prove something never happens, it means on average you’ll have to wait 15,000,000,000,000,000,000,000,000 years to see a nucleus of Te130 produce a neutrinoless double beta decay.

The CUORE experiment will continue to gather data, looking not only for neutrinoless double beta decay but also for possible signs of minute interactions between the material in the detectors and hypothetical Dark Matter particles called ‘WIMPS’, Weakly Interacting Massive Particles.

Another thing I like about CUORE however is that it is a search for new physics at low energy; it is an experiment that doesn’t need the huge particle accelerators like those at CERN or Fermilab. I hope CUORE does find new physics of some kind and I’ll let you know when it does.