Physicists measured something new in the radioactive decay of neutrons.

The experiment inspired theorists; future ones could reveal new physics.

A physics experiment performed at the National Institute of Standards and Technology (NIST) has enhanced scientists’ understanding of how free neutrons decay into other particles. The work provides the first measurement of the energy spectrum of photons, or particles of light, that are released in the otherwise extensively measured process known as neutron beta decay. The details of this decay process are important because, for example, they help to explain the observed amounts of hydrogen and other light atoms created just after the Big Bang.

Published in Physical Review Letters, the findings confirm physicists’ big-picture understanding of the way particles and forces work together in the universe—an understanding known as the Standard Model. The work has stimulated new theoretical activity in quantum electrodynamics (QED), the modern theory of how matter interacts with light. The team’s approach could also help search for new physics that lies beyond the Standard Model.

Neutrons are well known as one of the three kinds of particles that form atoms. Present in all atoms except the most common form of hydrogen, neutrons together with protons form the atomic nucleus. However, “free” neutrons not bound within a nucleus decay in about 15 minutes on average. Most frequently, a neutron transforms through the beta decay process into a proton, an electron, a photon, and the antimatter version of the neutrino, an abundant but elusive particle that rarely interacts with matter.

The photons from beta decay are what the research team wanted to explore. These photons have a range of possible energies predicted by QED, which has worked very well as a theory for decades. But no one had actually checked this aspect of QED with high precision.

“We weren’t expecting to see anything unusual,” said NIST physicist Jeff Nico, “but we wanted to test QED’s predictions very precisely in a way no one has done before.”

Nico and his colleagues, who represent nine research institutions, performed their measurements at the NIST Center for Neutron Research (NCNR). It produces an intense beam of slow-moving neutrons whose photon emissions can be detected with the same setup used for earlier precision measurements of the neutron’s lifetime.

The team measured two aspects of neutron decay: the energy spectrum of the photons, and also its branching ratio, which can provide information on how frequently the decays were accompanied by photons above a specific energy. The results of this effort gave them a branching ratio measurement more than twice as accurate as the previous value, and the first measurement of the energy spectrum.

“Everything we found was consistent with the predominant QED calculations,” Nico said. “We got quite a good match with theory on the energy spectrum, and we reduced the uncertainty in the branching ratio.”

According to Nico, the results provided specific information that theoretical physicists are already using to further develop QED to provide more detailed descriptions of neutron beta decay.

The results serve as a needed check on the Standard Model, said Nico, and validates the team’s experimental approach as a way to go beyond it. With better detectors, the approach could be used to search for so-called “right-handed” neutrinos, which have not yet been detected in nature, and potential time-reversal symmetry violations, which could explain why there is much more matter than antimatter in the universe.

Paper: M.J. Bales, R. Alarcon, C.D. Bass, E.J. Beise, H. Breuer, J. Byrne, T.E. Chupp, K.J. Coakley, R.L. Cooper, M.S. Dewey, S. Gardner, T.R. Gentile, D. He, H.P. Mumm, J.S. Nico, B. O'Neill, A.K. Thompson and F.E. Wietfeldt (RDK II Collaboration). Precision measurement of the radiative beta decay of the free neutron. Physical Review Letters. June 14, 2016, DOI: 10.1103/PhysRevLett.116.242501

NIST

Weighing the lightest particle

Neutrinos are everywhere. Every second, 100 trillion of them pass through your body unnoticed, hardly ever interacting. Though exceedingly abundant, they are the lightest particles of matter, and physicists around the world are attempting the difficult challenge of measuring their mass.   

For a long time, physicists thought neutrinos were massless. This belief was overturned by the discovery that neutrinos oscillate between three flavors: electron, muon and tau. This happens because each flavor contains a mixture of three mass types, neutrino-1, neutrino-2 and neutrino-3, which travel at slightly different speeds.

According to the measurements taken so far, neutrinos must weigh less than 2 electronvolts (a minute fraction of the mass of the tiny electron, which weighs 511,000 electronvolts). A new generation of experiments is attempting to lower this limit—and possibly even identify the actual mass of this elusive particle.

Where did the energy go?

Neutrinos were first proposed by the Austrian-born theoretical physicist Wolfgang Pauli to resolve a problem with beta decay. In the process of beta decay, a neutron in an unstable nucleus transforms into a proton while emitting an electron. Something about this process was especially puzzling to scientists. During the decay, some energy seemed to go missing, breaking the well-established law of energy conservation.

Pauli suggested that the disappearing energy was slipping away in the form of another particle. This particle was later dubbed the neutrino, or “little neutral one,” by the Italian physicist Enrico Fermi.

Scientists are now applying the principle of energy conservation to direct neutrino mass experiments. By very precisely measuring the energy of electrons released during the decay of unstable atoms, physicists can deduce the mass of neutrinos.

“The heavier the neutrino is, the less energy is left over to be carried by the electron,” says Boris Kayser, a theoretical physicist at Fermilab. “So there is a maximum energy that an electron can have when a neutrino is emitted.”

These experiments are considered direct because they rely on fewer assumptions than other neutrino mass investigations. For example, physicists measure mass indirectly by observing neutrinos’ imprints on other visible things such as galaxy clustering.

Detecting the kinks

Of the direct neutrino mass experiments, KATRIN, which is based at the Karlsrule Institute for Technology in Germany, is the closest to beginning its search.

“If everything works as planned, I think we'll have very beautiful results in 2017,” says Guido Drexlin, a physicist at KIT and co-spokesperson for KATRIN.

KATRIN plans to measure the energy of the electrons released from the decay of the radioactive isotope tritium. It will do so by using a giant tank tuned to a precise voltage that allows only electrons above a specific energy to pass through to the detector at the other side. Physicists can use this information to plot the rate of decays at any given energy.

The mass of a neutrino will cause a disturbance in the shape of this graph. Each neutrino mass type should create its own kink. KATRIN, with a peak sensitivity of 0.2 electronvolts (a factor 100 better than previous experiments) will look for a “broad kink” that physicists can use to calculate average neutrino mass.

Another tritium experiment, Project 8, is attempting a completely different method to measure neutrino mass. The experimenters plan to detect the energy of each individual electron ejected from a beta decay by measuring the frequency of its spiraling motion in a magnetic field. Though still in the early stages, it has the potential to go beyond KATRIN’s sensitivity, giving physicists high hopes for its future.

“KATRIN is the furthest along—it will come out with guns blazing,” says Joseph Formaggio, a physicist at MIT and Project 8 co-spokesperson. “But if they see a signal, the first thing people are going to want to know is whether the kink they see is real. And we can come in and do another experiment with a completely different method.”

Cold capture

Others are looking for these telltale kinks using a completely different element, holmium, which decays through a process called electron capture. In these events, an electron in an unstable atom combines with a proton, turning it into a neutron while releasing a neutrino.

Physicists are measuring the very small amount of energy released in this decay by enclosing the holmium source in microscopic detectors that are operated at very low temperatures (typically below minus 459.2 degrees Fahrenheit). Each holmium decay leads to a tiny increase of the detector’s temperature (about 1/1000 degrees Fahrenheit).

“To lower the limit on the electron neutrino mass, you need a good thermometer that can measure these very small changes of temperature with high precision,” says Loredana Gastaldo, a Heidelberg University physicist and spokesperson for the ECHo experiment.  

There are currently three holmium experiments, ECHo and HOLMES in Europe and NuMECs in the US, which are in various stages of testing their detectors and producing isotopes of holmium.

The holmium and tritium experiments will help lower the limit on how heavy neutrinos can be, but it may be that none will be able to definitively determine their mass. It will likely require a combination of both direct and indirect neutrino mass experiments to provide scientists with the answers they seek—or, physicists might even find completely unexpected results.

“Don't bet on neutrinos,” Formaggio says. “They’re kind of unpredictable.”

Symmetry Magazine

Is the neutrino its own antiparticle?

Almost every particle has an antimatter counterpart: a particle with the same mass but opposite charge, among other qualities.

This seems to be true of neutrinos, tiny particles that are constantly streaming through us. Judging by the particles released when a neutrino interacts with other matter, scientists can tell when they’ve caught a neutrino versus an antineutrino.

But certain characteristics of neutrinos and antineutrinos make scientists wonder: Are they one and the same? Are neutrinos their own antiparticles?

This isn’t unheard of. Gluons and even Higgs bosons are thought to be their own antiparticles. But if scientists discover neutrinos are their own antiparticles, it could be a clue as to where they get their tiny masses—and whether they played a part in the existence of our matter-dominated universe.
Dirac versus Majorana

The idea of the antiparticle came about in 1928 when British physicist Paul Dirac developed what became known as the Dirac equation. His work sought to explain what happened when electrons moved at close to the speed of light. But his calculations resulted in a strange requirement: that electrons sometimes have negative energy.

“When Dirac wrote down his equation, that’s when he learned antiparticles exist,” says André de Gouvêa, a theoretical physicist and professor at Northwestern University. “Antiparticles are a consequence of his equation.”

Physicist Carl Anderson discovered the antimatter partner of the electron that Dirac foresaw in 1932. He called it the positron—a particle like an electron but with a positive charge.

Dirac predicted that, in addition to having opposite charges, antimatter partners should have opposite handedness as well.

A particle is considered right-handed if its spin is in the same direction as its motion. It is considered left-handed if its spin is in the opposite direction.

Dirac’s equation allowed for neutrinos and anti-neutrinos to be different particles, and, as a result, four types of neutrino were possible: left- and right-handed neutrinos and left- and right-handed antineutrinos. But if the neutrinos had no mass, as scientists thought at the time, only left-handed neutrinos and right-handed antineutrinos needed to exist.

In 1937, Italian physicist Ettore Majorana debuted another theory: Neutrinos and antineutrinos are actually the same thing. The Majorana equation described neutrinos that, if they happened to have mass after all, could turn into antineutrinos and then back into neutrinos again. 

The matter-antimatter imbalance

 

Whether neutrino masses were zero remained a mystery until 1998, when the Super-Kamiokande and SNO experiments found they do indeed have very small masses—an achievement recognized with the 2015 Nobel Prize for Physics. Since then, experiments have cropped up across Asia, Europe and North America searching for hints that the neutrino is its own antiparticle.

The key to finding this evidence is something called lepton number conservation. Scientists consider it a fundamental law of nature that lepton number is conserved, meaning that the number of leptons and anti-leptons involved in an interaction should remain the same before and after the interaction occurs.

Scientists think that, just after the big bang, the universe should have contained equal amounts of matter and antimatter. The two types of particles should have interacted, gradually canceling one another until nothing but energy was left behind. Somehow, that’s not what happened.

Finding out that lepton number is not conserved would open up a loophole that would allow for the current imbalance between matter and antimatter. And neutrino interactions could be the place to find that loophole.

Neutrinoless double-beta decay

 

Scientists are looking for lepton number violation in a process called double beta decay, says SLAC theorist Alexander Friedland, who specializes in the study of neutrinos.

In its common form, double beta decay is a process in which a nucleus decays into a different nucleus and emits two electrons and two antineutrinos. This balances leptonic matter and antimatter both before and after the decay process, so it conserves lepton number.

If neutrinos are their own antiparticles, it’s possible that the antineutrinos emitted during double beta decay could annihilate one another and disappear, violating lepton number conservation. This is called neutrinoless double beta decay.

Such a process would favor matter over antimatter, creating an imbalance.

“Theoretically it would cause a profound revolution in our understanding of where particles get their mass,” Friedland says. “It would also tell us there has to be some new physics at very, very high energy scales—that there is something new in addition to the Standard Model we know and love.”

It’s possible that neutrinos and antineutrinos are different, and that there are two neutrino and anti-neutrino states, as called for in Dirac’s equation. The two missing states could be so elusive that physicists have yet to spot them.

But spotting evidence of neutrinoless double beta decay would be a sign that Majorana had the right idea instead—neutrinos and antineutrinos are the same.

“These are very difficult experiments,” de Gouvêa says. “They’re similar to dark matter experiments in the sense they have to be done in very quiet environments with very clean detectors and no radioactivity from anything except the nucleus you're trying to study."

Physicists are still evaluating their understanding of the elusive particles.

“There have been so many surprises coming out of neutrino physics,” says Reina Maruyama, a professor at Yale University associated with the CUORE neutrinoless double beta decay experiment. “I think it’s really exciting to think about what we don’t know.”

Fermilab/SLAC

Cold fermions keep distance from each other

Scientists at MPQ observe “Pauli blocking” between fermionic lithium atoms in an optical lattice

The microcosm is governed by its own laws – the rules of quantum mechanics. They describe, for instance, the statistical behaviour of elementary particles. Electrons and all the other constituents of atoms belong to the group of fermions that are never allowed to share identical quantum numbers. This “exclusion principle” was formulated by the Austrian physicist Wolfgang Pauli in 1925 in order to explain the structure and the stability of atoms. Today, quantum optical experiments provide methods to prove the rules that have been thought of and pressed into elegant mathematical equations in those days. In this regard, scientists in the Quantum Many-Body Division of Professor Immanuel Bloch at the Max Planck Institute of Quantum Optics have made a big step forward (PRL, 31 December 2015, DOI:10.1103/PhysRevLett.115.263001).

The team around project leader Dr. Christian Groß demonstrated the first direct observation of “Pauli blocking”, a consequence of the exclusion principle. To this end, they cooled a cloud of fermionic lithium-6 atoms down to extremely low temperatures and loaded the particles into an optical lattice. Since identical fermions are not allowed to occupy the same lattice site each atom is supposed to find its own place. This was exactly what was observed in the experiment with the help of a quantum gas microscope that images atoms with single-particle single-site resolution. “Our success is the result of adapting our cooling and imaging methods that were developed for bosons, to the needs of fermions”, Christian Groß explains. “Our work opens a new avenue for studying quantum correlations in fermionic quantum matter, or to get a better understanding of phenomena such as quantum magnetism and superconductivity.”

Quantum statistics distinguishes between two fundamentally different kinds of particles. There are, on the one hand, the “sociable” bosons which condense into a single quantum state at zero temperature. On the other hand, there are the “solitary” fermions for which multiple occupation of a single state is forbidden. In their work with cold quantum gases scientists have a free choice: particles with integer spins are bosons, whereas fermions are characterized by their half-integer spin. Hence, what kind of statistics, i.e. what ‘social behaviour’ the atoms obey depends on the total number of their electrons, protons, and neutrons. If the goal is to simulate the behaviour of electrons in a solid crystal with atoms in optical lattices, fermions are, of course, the better approximation. However, in most experiments so far scientists have used bosonic particles since it is, for several reasons much harder to bring fermions down to the low temperatures required.

Now, by combining efficient cooling methods with high precision detection the team around Dr. Christian Groß has taken this hurdle in an experiment with fermionic lithium-6 atoms. At first, the atoms are cooled down and caught in a dipole trap. By applying several light-fields the scientists obtain a single plane with a couple of hundred atoms. Now an optical lattice – created by interfering laser beams – is superimposed. This lattice defines the “crystal geometry”, i.e. the sites where the atoms are allowed to settle down.

The most important and new step, however, was the modification of the quantum gas microscope developed a couple of years ago in the Bloch group. The scientists applied a special cooling method, originally developed to cool ions, to fermions in an optical lattice. Here the quantum mechanical oscillation states of an atom in a lattice well are manipulated in such a way that the atoms are driven into the lowest state, accompanied with a lower temperature.

At the same time photons are scattered at the atoms, causing them to light up like tiny nano light bulbs that can each be observed individually. A high resolution microscope objective images the atoms all at once, such that a photographic snapshot of the atomic gas is taken. The measurements show a rather flat distribution in the centre of the trap, with one atom per lattice site. “It is important that this distribution arises as a result of quantum statistics – i.e. Pauli blocking – only,” Ahmed Omran points out, doctoral candidate at the experiment. “Identical fermions repel each other; there is no other interaction at work.”

Due to the periodic order of atoms in a solid state crystal the energy levels of the electrons split up into “bands” of closely neighboured states. If the highest so-called valence band is fully occupied the electrons cannot move, therefore the material is an insulator. The quantum state of the fermionic lithium-system generated in the experiment represents an analogue to that state: the Pauli principle leads to a fully occupied valence band, i.e. to a suppression of conductivity, which manifests itself by a strong suppression of particle number fluctuations detected with the quantum gas microscope.

The new technique offers a variety of new possibilities for further experiments with quantum many-body systems made of fermionic atoms. For instance, modifications of the method will make it possible to manipulate even single fermions in the systems, leading to even lower temperatures. There a new antiferromagnetic order is expected to emerge that should be detected and characterized with the quantum gas microscope. Antiferromagnetism is a widely discussed candidate for the explanation of fundamental phenomena in superconductivity. Olivia Meyer-Streng.

Max Planck Institute of Quantum Optics

A Particle Purely Made of Nuclear Force

Scientists at TU Wien (Vienna) have calculated that the meson f0(1710) could be a very special particle – the long-sought-after glueball, a particle composed of pure force.

 

For decades, scientists have been looking for so-called “glueballs”. Now it seems they have been found at last. A glueball is an exotic particle, made up entirely of gluons – the “sticky” particles that keep nuclear particles together. Glueballs are unstable and can only be detected indirectly, by analysing their decay. This decay process, however, is not yet fully understood.

Professor Anton Rebhan and Frederic Brünner from TU Wien (Vienna) have now employed a new theoretical approach to calculate glueball decay. Their results agree extremely well with data from particle accelerator experiments. This is strong evidence that a resonance called “f0(1710)”, which has been found in various experiments, is in fact the long-sought glueball. Further experimental results are to be expected in the next few months.

Forces are Particles too

Protons and neutrons consist of even smaller elementary particles called quarks. These quarks are bound together by strong nuclear force. “In particle physics, every force is mediated by a special kind of force particle, and the force particle of the strong nuclear force is the gluon”, says Anton Rebhan (TU Wien).

Gluons can be seen as more complicated versions of the photon. The massless photons are responsible for the forces of electromagnetism, while eight different kinds of gluons play a similar role for the strong nuclear force. However, there is one important difference: gluons themselves are subject to their own force, photons are not. This is why there are no bound states of photons, but a particle that consists only of bound gluons, of pure nuclear force, is in fact possible.

In 1972, shortly after the theory of quarks and gluons was formulated, the physicists Murray Gell-Mann and Harald Fritsch speculated about possible bound states of pure gluons (originally called “gluonium”, today the term “glueball” is used). Several particles have been found in particle accelerator experiments which are considered to be viable candidates for glueballs, but there has never been a scientific consensus on whether or not one of these signals could in fact be the mysterious particle made of pure force. Instead of a glueball, the signals found in the experiments could also be a combination of quarks and antiquarks. Glueballs are too short-lived to detect them directly. If they exist, they have to be identified by studying their decay.

Candidate f0(1710) decays strangely

“Unfortunately, the decay pattern of glueballs cannot be calculated rigorously”, says Anton Rebhan. Simplified model calculations have shown that there are two realistic candidates for glueballs: the mesons called f0(1500) and f0(1710). For a long time, the former was considered to be the most promising candidate. The latter has a higher mass, which agrees better with computer simulations, but when it decays, it produces many heavy quarks (the so-called “strange quarks”). To many particle scientists, this seemed implausible, because gluon interactions do not usually differentiate between heavier and lighter quarks. 

Anton Rebhan and his PhD-student Frederic Brünner have now made a major step forward in solving this puzzle by trying a different approach. There are fundamental connections between quantum theories describing the behaviour of particles in our three dimensional world and certain kinds of gravitation theories in higher dimensional spaces. This means that certain quantum physical questions can be answered using tools from gravitational physics.

“Our calculations show that it is indeed possible for glueballs to decay predominantly into strange quarks”, says Anton Rebhan. Surprisingly, the calculated decay pattern into two lighter particles agrees extremely well with the decay pattern measured for f0(1710). In addition to that, other decays into more than two particles are possible. Their decay rates have been calculated too.

Further Data is Expected Soon

Up until now, these alternative glueball decays have not been measured, but within the next few months, two experiments at the Large Hadron Collider at CERN (TOTEM and LHCb) and one accelerator experiment in Beijing (BESIII) are expected to yield new data. “These results will be crucial for our theory”, says Anton Rebhan. “For these multi-particle processes, our theory predicts decay rates which are quite different from the predictions of other, simpler models. If the measurements agree with our calculations, this will be a remarkable success for our approach.” It would be overwhelming evidence for f0(1710) being a glueball. And in addition to that, it would once again show that higher dimensional gravity can be used to answer questions from particle physics – in a way it would be one more big success of Einstein’s theory of general relativity, which turns 100 years old next month.

Technische Universität Wien

The QUTIS Group creates a quantum simulator of impossible physics

The research group Quantum Technologies for Information Science (QUTIS) of the UPV/EHU-University of the Basque Country, led by the Ikerbasque professor Enrique Solano, in collaboration with an experimental group of the University of Tsinghua (Beijing, China) led by professor Kihwan Kim, has created a quantum simulator that is capable of creating unphysical phenomena in the atomic world, in other words, impossible physical phenomena.

The researchers in the two groups have succeeded in getting a trapped atom to imitate behaviours that contradict its own fundamental laws, thus taking elements of science fiction to the microscopic world.

"We have managed to get an atom to act as if it were infringing the nature of atomic systems, in other words, quantum physics and the theory of relativity. It is just like what happens in the theatre or in science fiction films in which the actors appear to display absurd behaviours that go against natural laws; in this case, the atoms are obliged to simulate absurd actions as if an actor in the theatre or in science fiction were involved," explained Prof Solano.

The results of this research have been published in the prestigious journal Nature Communications, in the article "Time reversal and charge conjugation in an embedding quantum simulator". The research team of the UPV/EHU's QUTIS group has been led by Prof Enrique Solano and has had the participation of Dr Lucas Lamata and Dr Jorge Casanova, currently at the University of Ulm, Germany.

In this experiment the researchers reproduced in the lab the theoretical proposal previously included in a previous piece of research led by the QUTIS group; it describes the possibility that a trapped atom can display behaviour that is incompatible with the fundamental laws of quantum physics. More specifically, we are talking about operations prohibited in microscopic physical systems, such as charge conjugation, which transforms a particle into an antiparticle, or time reversal, that reverses the direction of the time arrow.

To conduct the experiment it was necessary to use a charged atom trapped by means of electromagnetic fields under the action of an advanced laser system. We could describe symmetry operations of this type as prohibited ones, as they could only exist in a universe that is different from the one we know and governed by different laws. Yet in this experiment it has been possible to simulate the realisation of this set of impossible laws in an atomic system.

The UPV/EHU's QUTIS group is a world leader in quantum simulation and its influential theoretical proposals are often verified in the most advanced quantum technology laboratories. In this case, physical operations that are prohibited for the atomic world can be reproduced just as in science fiction, in other words, just as if they were taking place artificially in a quantum theatre.

Bibliographical reference

Xiang Zhang, Yangchao Shen, Junhua Zhang, Jorge Casanova, Lucas Lamata, Enrique Solano, Man-Hong Yung, Jing-Ning Zhang & Kihwan Kim. Time reversal and charge conjugation in an embedding quantum simulator. Nature Communications (2015),

http://dx.doi.org/10.1038/ncomms8917

University of the Basque Country

Nanotechnology World Association