When we look out into the universe, we can see a cosmic background of microwaves in all directions. It’s the thermal remnant of the big bang, but it also serves as a wall beyond which we cannot see. Until now.

The early universe was extremely hot. So hot that any light emitted by particle interactions couldn’t travel far, much like light bouncing around the interior of our Sun. But by the time the universe was about 380,000 years old, its expansion cooled the universe to about 3,000 K, which is about the same as the surface temperature of a cool star. At that point the hydrogen and helium in the universe finally cooled enough to became transparent to light. The light of the universe could finally travel freely for billions of years. As the universe continued to expand, it cooled to the cosmic microwave background (CMB) we see today.

Since the background is from a time when the universe was 380,000 years old, we have no way of directly observing the earliest period of the universe. This doesn’t mean we can’t see anything. The CMB we observe isn’t perfectly uniform, but rather has small fluctuations in temperature. The scale at which these fluctuations occur is determined by various factors, including the amount of matter, dark matter and dark energy in the universe.  All the observations we’ve made of the CMB is consistent with the standard cosmological model, known as the LCDM model.

While light can’t be seen beyond the CMB wall, neutrinos could in principle. Since neutrinos only weakly interact with other matter, it could travel freely through the universe from the moment it formed. If we could observe these cosmic neutrinos we’d be able to see the universe when it was only a second old. Unfortunately, these primordial neutrinos would be far too cold and have too little energy to be seen anytime soon. From the big bang model they would be about 2 K, which is much less energy than current neutrino detectors can observe.

Observed fluctuations compared to neutrino number models. Credit: Brent Follin, et al.

But neutrinos also have an indirect effect on the fluctuations of the cosmic microwave background. Although they don’t interact much with other matter, they do interact slightly, and this can shift the fluctuations very slightly. In a recent paper, a team looked at CMB data from the Planck satellite, and they found just such a shift. The team compared this shift with different cosmological models where the number of neutrinos varied from 1 to 5. While we’ve long thought that there are just 3 different types of neutrinos, some speculative models predict more. What they found, pretty conclusively, is that the fluctuations agree with the 3-neutrino model.

So now we know cosmologically that there are just 3 types of neutrinos. The standard model of particle physics and the standard model of cosmology agree. Given how radically different the regimes of these models are, that’s pretty amazing indeed.

Paper: Brent Follin, et al. First Detection of the Acoustic Oscillation Phase Shift Expected from the Cosmic Neutrino Background. Phys. Rev. Lett. 115, 091301 (2015)

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Neutrinos are being detected from outer space, and that’s kind of a big deal.

Neutrinos are an elementary particle somewhat related to electrons. But unlike electrons, which have an electric charge and a measurable mass, neutrinos have no charge and a tiny difficult to measure mass. These elusive particles only interact through the weak force, and so they’re extremely difficult to detect. One way to detect them is by using large tanks of water and waiting for a neutrino to collide with a water molecule. Neutrinos strike the molecule with such energy that a small amount of light is produced, which we can detect. One of the biggest challenges comes from the fact that other energetic particles can collide with water molecules to produce light, so we shield the detectors by building them in abandoned mines, or burying them under the ice in Antarctica.

Because neutrinos are so difficult to detect, they typically come from three main sources: those produced in particle accelerators, those produced from fusion in the Sun’s core, and those produced by cosmic rays striking our atmosphere. None of these neutrinos are produced outside our solar system. The only time cosmic neutrinos were detected was during the 1987a supernova in the Large Magellanic Cloud. Supernovae produce tremendous quantities of neutrinos, and this caused small neutrino spikes at three separate neutrino observatories, which confirmed they were extra-solar neutrinos. We haven’t detected any cosmic neutrinos since.

But now the IceCube Collaboration in Antarctica has announced the discovery of cosmic neutrinos. The IceCube detector is much more sensitive than the detectors we had in 1987, so it’s not only able to detect neutrinos, it’s able to determine both the energy levels of different neutrino events and the direction from which they originate. Out of 35,000 neutrino detections over the course of 2 years, they found 21 that had very high energy levels. Higher than those typically produced by cosmic rays. They also came from the direction of the northern hemisphere, which meant they traveled through the Earth to reach the detector. Both of these facts point to the neutrinos originating from outside the solar system.

On one level, this discovery simply confirms the existence of astrophysical neutrinos, which we’ve known should exist for quite some time. But the fact that we now have the sensitivity to detect these things is pretty amazing.

Paper: M. G. Aartsen et al. (IceCube Collaboration) Evidence for Astrophysical Muon Neutrinos from the Northern Sky with IceCube. Phys. Rev. Lett. 115, 081102 (2015)

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Neutrinos are rather odd particles. They interact only weakly with matter, come in three different types or flavors, and while they have mass they don’t have definite mass. They can also change between different types of neutrinos, a process known as neutrino oscillation. Much of the evidence for neutrino oscillation has been indirect, but new research has made a direct observation of the effect.

Neutrino oscillation was first proposed in the 1960s to explain what was known as the solar neutrino problem. That is, the number of solar neutrinos we detected on Earth was about a third the expected number given theoretical reaction rates. It wasn’t until the late 1990s that we obtained evidence of neutrino oscillation. This was done by creating a beam of neutrinos with a particle accelerator such as the ones at Fermilab or CERN, and beaming these neutrinos through the Earth to an underground neutrino detector some distance away. By controlling the energy of the neutrino beam we can predict the number of expected detections expected both with or without neutrino oscillation. Experiments have consistently been in agreement with the oscillation model.

But one criticism of these experiments is that they are indirect evidence of neutrino oscillation. Basically they involved sending a particular number of a particular type of neutrino such as electron or muon neutrinos, and then detecting the drop in neutrinos at the underground site. According to the oscillation model, this drop is due to the fact that a portion of the neutrinos have metamorphosed into other neutrino types that aren’t detected. It would be better to actually detect the neutrinos that have changed rather than simply inferring their change. That’s exactly what’s been done in this new work.

A group observed a beam of muon neutrinos from CERN and detected the tau neutrinos some of them transformed into. The results clearly show that muon neutrinos can transform into tau neutrinos. A similar group working with the Super-Kamiokande neutrino detector observed electron neutrinos from a muon neutrino beam. So we now have directed evidence of neutrino oscillation.

Paper: N. Agafonova, et al. Observation of tau neutrino appearance in the CNGS beam with the OPERA experiment. Prog. Theor. Exp. Phys. 101C01 doi: 10.1093/ptep/ptu132 (2014)

Paper: K. Abe et al. Observation of Electron Neutrino Appearance in a Muon Neutrino Beam. Phys. Rev. Lett. 112, 061802 (2014)

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The Sun is so intensely bright that it’s difficult to look at (and you shouldn’t try). When observing the Sun with scientific instruments, we often use filters to diminish the light so that we can observe surface features of the Sun in detail, such as sunspots and the churning of granules due to convection near the surface. But how do we study the interior of the Sun?

One way is through neutrinos generated in the Sun’s core. Unlike light, which can take 20,000 to 150,000 years to travel from the Sun’s core to its surface, neutrinos leave the Sun soon after they are produced. We’ve been able to detect solar neutrinos since the 1960s, but these were neutrinos due to secondary reactions in the core. More recently we’ve been able to observe neutrinos from the principle fusion mechanism known as the pp-chain. From these observations we know the rate at which fusion occurs in the Sun, as well as its central pressure, temperature and density.

Between the core and surface things get a bit more tricky. Surrounding the core is a radiative zone, where the heat of the core moves toward the surface mainly through photon radiation. Surrounding that is a convection zone, where stellar material churns in a cycle. Heated by the interior, the material rises toward the surface. It then cools and sinks toward the interior where the process happens all over again. We know of these levels through helioseismology, which is the study of sound waves traveling through the Sun’s interior. While light takes thousands of years to travel from the Sun’s core to its surface, the solar interior is relatively transparent to acoustic waves, which means they can travel through the Sun at the speed of sound.

As the methods of helioseismology have gotten more sophisticated, we’ve been able to determine some of the characteristics of the convection flow, and what we’ve found is that it’s much more turbulent than originally supposed. This means that while our surface and deep interior models are pretty good, our mid-range models aren’t. This isn’t particularly surprising, since the complex transition between the radiative and convective regions is notoriously difficult to model.

But what’s amazing is that we can use sound waves to actually test these models. With methods such as neutrino physics and helioseismology, we can really see the complex beauty of the Sun’s interior.

Paper: Laurent Gizona & Aaron C. Bircha. Helioseismology challenges models of solar convection. PNAS, vol. 109, no. 30, 11896–11897 (2012).

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A new paper proposes that neutrinos may be tachyons. This is being presented in the press as a claim that neutrinos move faster than light, but that’s not the focus of the paper. Instead the paper argues that the electron neutrino could have an imaginary mass.Tachyons are hypothetical particles proposed in the context of special relativity. One aspect of relativity is usually stated as “nothing can move faster than light,” but technically what relativity says is that nothing can be accelerated to the speed of light. So if a particle is initially moving slower than light, it can never travel faster than light. But what if there were particles initially traveling faster than light? According to relativity they could never slow down to light speed. Hence the idea of tachyons.

If you assume tachyons exist, then special relativity also requires that they have imaginary mass. Tachyons were originally proposed before we had a solid understanding of quantum theory. The mathematics of quantum theory is now very clear, so it is relatively straightforward to plug an imaginary mass into the equations to see what happens. When you do that, it looks like a description of a faster-than-light particle, but when you look closely what you find is that the “particle” aspect of the quantum field actually travels slower than light. So what started as an idea for particles to move faster than light turns out to be something that moves slower than light.

Hypothetical quantum particles with imaginary mass are often referred to as tachyons, even though they wouldn’t travel faster than light. Hypothetical particles that actually move faster than light are also called tachyons, but they would violate both special relativity and particle physics. Strangely, the author claims the two are the same, and that imaginary mass neutrinos would in principle travel faster than light, which just isn’t the case.

But the main focus of the paper is a demonstration that an electron neutrino with imaginary mass is consistent with current observations, including cosmic expansion and the cosmic microwave background. While that seems to be true, the data is also consistent with neutrinos having regular positive mass. Another weakness of the paper is that the author focuses only on electron neutrino mass, when we know experimentally that the different flavors of neutrinos have indefinite masses. So it doesn’t make much sense to talk about the mass of one type of neutrino.

Overall the paper is a fairly weak argument for a rather fringe idea. Just because one aspect of a model can be made to fit one set of data, that isn’t very convincing. Models need to account for a range of evidence, and the evidence points to neutrinos having regular masses like other particles.

Paper: Robert Ehrlich. Six observations consistent with the electron neutrino being a tachyon with mass: m2νe=−0.11±0.016eV2. arXiv:1408.2804 [physics.gen-ph] arxiv.org/abs/1408.2804

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In earlier posts about dark matter, I’ve written about how neutrinos would seem to be a good candidate, but there simply aren’t enough of them to account for all of dark matter. As far as we can tell, there are three types (flavors) of neutrinos, and we know the upper limit of their mass from the distribution of galaxies in the universe. So the three known neutrino flavors can’t be the solution to dark matter.

But there has been speculation that a fourth type of neutrino could exist, known as a sterile neutrino. Sterile neutrinos wouldn’t interact through the weak nuclear force the way regular neutrinos do, but instead only interact with things gravitationally (hence sterile). If these neutrinos had a much greater mass than regular neutrinos, then it could be an answer to the dark matter problem. But the catch is that their gravitational interactions would have interacted gravitationally with matter in the early universe, and this would affect the fluctuation patterns in the cosmic microwave background. So in principle we should see their effect in CMB fluctuations.

Previously the WMAP observations of the CMB were inconclusive. The data agreed with the 3-neutrino model, but the effect of a sterile neutrino couldn’t be excluded. But the latest Planck results now confirm that there is no fourth neutrino. The  standard model of particle physics remains successful. This is good news for the standard model, but it eliminates one more dark matter candidate from the list.

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Yesterday I talked about the weirdness of neutrinos, specifically that there three types of neutrinos (known as flavors), and they can oscillate between different flavors due to the quantum fuzziness of their masses.  If you go back and read that post, you’ll find its a pretty bizarre model that seems to assume a great deal just to solve what is known as the solar neutrino problem. So how could we possibly know that such a model is correct?

The short answer is that our neutrino detectors got better, but the more detailed answer is a nice demonstration of how evidence builds to support even strange models such as neutrino oscillation.

The earliest direct detection of neutrinos in the 1950s used a nuclear reactor to produce neutrinos from radioactive decay. The detector consisted of tanks of water, so that high energy neutrinos would strike protons in the water creating a positron and neutron. The positron then collided with other material to create two gamma rays, which could be detected. This experiment demonstrated that neutrinos were real, but wasn’t sensitive enough to gather detailed information about them.

The first observation of solar neutrinos was done by the Homestake experiment in the late 1960s. This was performed in a gold mine in South Dakota, and involved a large tank of perchloroethylene (commonly used in dry cleaning). Since it was buried deep underground, it was shielded from things such as cosmic rays. When neutrinos from the Sun struck chlorine in the perchloroethylene, it caused it to decay into a radioactive isotope of argon. By analyzing the amount of argon produced, the level of solar neutrinos could be determined. It was this experiment that first found that the level of solar neutrinos was a third the expected result.

The Homestake experiment (and similar experiments done later) could only detect electron neutrinos, but by the 1990s the Sudbury Neutrino Obervatory (SNO) was created to detect all flavors of neutrinos. The SNO detector was also buried in a mine, this time in Sudbury, Canada. This experiment used a large tank of heavy water, which is water made with deuterium instead of hydrogen. Deuterium is an isotope of hydrogen that contains a proton and neutron, rather than just a proton. When a neutrino of any flavor strikes the deuterium, one of two things can happen. Electron neutrinos can cause the neutron to become a proton and electron. Neutrinos of any flavor can cause the proton and neutron in the deuterium to split, created hydrogen and a free neutron. So the SNO can detect both the total number of neutrinos and the number of electron neutrinos. It was this experiment that confirmed that the Sun did, in fact, produce the expected number of neutrinos.

So this told us that our predicted level of neutrinos was correct, and that only a third of the neutrinos reaching Earth were electron neutrinos. But how do we know that neutrinos actually change flavor? If the Sun produced neutrinos in equal amounts, that would match the data as well.

The answer was found by another observatory, the Super-Kamiokande detector in Japan. This experiment is also buried in a mine to shield from cosmic rays, and involves a large tank of pure water surrounded by photomultiplier tubes. The detection method is similar to the Homestake experiment, but with vastly greater sensitivity. Super-K is so sensitive that it can distinguish between electron and muon neutrinos. It can also determine the direction of origin of these neutrinos. Because of this, Super-K could perform an ingenious experiment.

In addition to being produced by the Sun, neutrinos are also produced by cosmic rays striking the Earth’s atmosphere. When cosmic rays strike the atmosphere, they produce a cascade of energetic particles, including electrons, muons, and their corresponding neutrinos. According to the standard model, the number of muon neutrinos produced should be double that of electron neutrinos. What Super-K found was that the number of muon neutrinos was on average only about 28% greater than the number of electron neutrinos. But because Super-K can identify the direction of the neutrinos, it can distinguish between ones produced by cosmic rays directly over the mine, and ones produced on the opposite side of the Earth. When they looked at the highest energy neutrinos on both sides of the Earth, the Super-K team found that when they came from directly overhead, muon neutrinos were double the electron neutrinos. When they came from the opposite side of the Earth, they were an even mix of electron and muon neutrinos.

This is exactly the result predicted by neutrino oscillation. Neutrinos originating from overhead only travel through a few thousand meters of Earth before reaching the detector, and therefore have little time to oscillate into other neutrinos. The ones originating on the far side of the Earth travel through more than 12,000 kilometers of rock to reach the detector, and by then have smeared out into equal amounts of neutrino flavors due to neutrino oscillation.

So as strange as the neutrino model is, it really works. We can prove it. There are still things we don’t know about neutrinos. For example, we know there are three types of neutrino mass, but we don’t know what their values are. We have an idea of their upper limit, but we lack the precision to pin them down. And we’d really like to know, because the results will have cosmological implications.

But we’ve come a long way in neutrino physics in the last 60 years, from their first detection to the confirmation of their complex behavior. And that’s quite amazing when you think about it.

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Neutrinos are perhaps the most enigmatic particles in the universe. They were first discovered in the 1950s as a product of radioactive decay, but they are also produced in nuclear fusion reactions. As a result, copious amounts of neutrinos are produced in the Sun through the pp-chain and CNO nuclear fusion processes in the core of our star. This makes the Sun a perfect candidate for doing neutrino astronomy. But when we first starting observing solar neutrinos in the 1960s, revealed mystery known as the solar neutrino problem. The solution to this problem wasn’t proven until the late 1990s, and it demonstrated that neutrinos are far more strange than we had imagined.

The solar neutrino problem arose from the fact that the amount of neutrinos we observed from the Sun was about a third the expected amount. That meant either our understanding of nuclear fusion in Sun was very wrong, or something strange was going on with neutrinos. Around the same time we started measuring solar neutrinos, it was found the the electron had two sister particles known as the muon and tauon, (together known as leptons) and that each of these had a corresponding neutrino. This meant that there were three types (or flavors) of neutrinos. With three times the expected neutrinos, and one third the measured neutrinos, it looked suspiciously like the two were connected.

A few things were known right off the bat. The early solar neutrino detectors could only detect electron neutrinos, so if the Sun was producing the predicted amount of neutrinos but in equal amounts of the three flavors, that would solve the mystery. But the Sun couldn’t be producing all three neutrino types, because the nuclear reactions in the Sun’s core only produce electron neutrinos. The obvious solution is to look for a way for some neutrinos to be converted from the electron type to the other types, but according to the well established standard model of particle physics, neutrinos should be massless. As a result, they would move at the speed of light, and there would be no way for them to change flavors.

Of course if neutrinos have mass, then they could change flavors. But it turns out that neutrino mass isn’t the simple kind of mass we’re used to dealing with. In the standard model, neutrinos are governed by the electroweak force, which is a unification of the electromagnetic force of charges and magnets, and the weak nuclear force which governs radioactive decay. The electroweak model is a quantum theory, and so things like the uncertainty principle come into play. As a result, you can either measure a neutrino’s mass, or its flavor, but not both. This means we can say that neutrinos have mass, but we can never say that the electron flavor has a particular mass.

Because of this quantum fuzziness between mass and flavor, we’re always limited to knowing one or the other. According to the model there are three mass types (mass eigenstates) and three flavors (flavor eigenstates) of neutrinos. If we know the flavor of a neutrino (electron, muon, tauon), then that flavor is a superposition (quantum mixture) of the three mass types. If we know the mass of a neutrino, then it is a superposition of the three flavors. What distinguishes an electron neutrino from, say, a muon neutrino is their mixture of the different mass types. Each flavor of neutrino is a specific superposition of the different mass eigenstates.

Neutrino oscillation changes the probability of an electron neutrino being observed as another flavor. Credit: Wikipedia

So how do neutrinos with “fuzzy” quantum mass solve the solar neutrino problem? It turns out that each mass eigenstate has a slightly different speed. So if an electron neutrino is produced in a nuclear reaction, its superposition of mass states will gradually shift because of the different speeds. In quantum theory, each mass state has a different wavelength, so their waves start to interfere as they shift. This effect is known as neutrino oscillation. So as an electron neutrino travels across the universe, it oscillates between the other flavors, and the chance of it being observed as a muon or tauon neutrino rises and falls.

On a cosmic scale, the distance between the Sun and Earth is fairly small, so there isn’t much time for electron neutrinos to mix with the other types. But when neutrinos travel through matter, another oscillation effect comes into play known as the MSW effect. When neutrinos travel through matter, their speeds shift, similar to the way light shifts through glass due to its index of refraction. The shift is different for each flavor, and this accelerates the mixing of the neutrinos (similar to the way a prism can spread out the colors of light). By the time neutrinos reach the surface of the Sun, they are mixed to equal amounts of the three flavors. As a result, only about a third of the neutrinos that reach Earth are electron neutrinos, which explains why early neutrino detectors saw a third the expected amount.

And thus, the solar neutrino problem is solved.

Of course you might argue that this is a pretty convoluted model just to explain solar neutrinos. Bold claims require bold evidence, so what makes us so confident that flavor-changing neutrinos with fuzzy masses really is the solution? We’ll look at the answer to that question next time.

The post Mixing It Up appeared first on One Universe at a Time.

Our Sun (like other stars) is powered by nuclear reactions within its core. Part of the way we know this is through the observation of solar neutrinos. When solar neutrinos were first observed, the levels observed were less than predicted by about a factor of three, which came to be known as the solar neutrino problem. Since then we’ve come to understand that neutrinos have mass, and can change between flavors (electron, muon, and tauon), which solves the solar neutrino problem. Our neutrino detectors are good enough that we can now produce neutrino images of the Sun, such as the image above, and the rates of neutrino emission are in good agreement with solar core models. 

Despite all that success, there has been one aspect of solar neutrinos that we’ve wanted to observe, but haven’t, and that is neutrinos from the Sun’s most common nuclear interaction. Nuclear fusion in the Sun is a complex process where hydrogen protons are fused into higher elements. The start of that process is the fusion of two protons, which is known as the proton-proton chain or pp-chain. The pp-chain is the most common reaction in the Sun’s core, and it provides about 99% of the energy produced by the Sun.  The problem is that the neutrinos produced in this reaction have rather low energies, which makes them very difficult to detect. The solar neutrinos we’ve observed so far have been from higher order reactions such as boron decay.

Now a new paper in Nature presents the first direct detection of pp-neutrinos from the solar core. The results are pretty impressive, since the signal of pp-neutrinos can be masked by various sources of noise, including some from the detector itself. The team was able to isolate noise and focus their detector on a narrow energy range sensitive only to pp-neutrinos. They not only detected pp-neutrinos, but were able to confirm the rate of pp-chain reactions in agreement with solar core models.

They were also able to demonstrate that pp-neutrinos change flavor at a slightly lower rate than higher energy boron neutrinos. This implies that not only do neutrinos change flavor while moving through matter (such as during their journey out of the Sun’s core) but that neutrino flavor changing is somehow dependent upon neutrino energy.  Overall this is a big result. It strongly confirms our solar fusion model, and has enough sensitivity that future data could fine tune our understanding of nuclear reactions in the solar core. It could also provide data on the mechanism of neutrino flavor changing, which is still not entirely understood.

And for fans of the electric universe, this new work hammers the last nail in the coffin of the electric Sun model. Proton-proton reaction rates depend upon the pressure, temperature and density of hydrogen nuclei. The energy and level of pp-neutrinos clearly shows that 99% of the Sun’s energy is produced in the core, and not in the corona as EU fans claim.

Paper: G. Bellini, et al. Neutrinos from the primary proton–proton fusion process in the Sun. Nature 512, 383–386 (2014)

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The Soudan Iron Mine in Northern Minnesota is home to several experiments in particle physics and cosmology. I’ve written about one of the projects there, known as the Cryogenic Dark Matter Search (CDMS). Another experiment is Main Injector Neutrino Oscillation Search (MINOS), which detects muon neutrinos produced at Fermilab in Northern Illinois. MINOS is about 48 feet long, and contains 6000 tons of steel layered between scintillators. The entire detector had to be lowered down a narrow mine shaft piece by piece and then assembled on site.

The mine is located near Tower-Soudan, which happens to be close to where family live. So yesterday I was able to visit the physics lab, located more than 2000 feet underground. The mine has daily tours during the Summer, so if you find yourself in the Northwoods of Minnesota, it’s worth making a visit.

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Radioactive decay is where the atom of an unstable element can decay into another type of element, releasing energy in the process. One process by which this can occur is known as beta decay. When beta decay occurs an element such as caesium decays to barium. This process releases an electron, first known as a beta particle, hence the term beta decay.

But there was something rather strange about beta decay. The mass of a barium atom is less than the mass of a caesium atom, and Einstein had demonstrated that mass can be transformed to energy and vice versa. So the amount of energy released during the decay should simply depend on difference in atomic masses of the two elements. This meant the energy given to the electron should always be the same. But experiments demonstrated that the electron could have a wide range of energies.

What was going on here? One proposal, made by Niels Bohr, was that conservation of energy only held true on average, so that variations in the electron energies were due to some quantum fluctuation. However Wolfgang Pauli proposed the existence of an additional particle. Thus beta decay must emit both an electron and a small uncharged particle we now call the neutrino. Pauli proposed calling it the neutron, but between the time he proposed the idea and the time it was accepted the particle we know as the neutron was discovered. So Enrico Fermi proposed the name neutrino, which is Italian for “little neutral one”.

Pauli proposed the neutrino in 1930, but its existence wasn’t solidly confirmed until 1956. Neutrinos interact very weakly with other particles, so it took a nuclear reactor to produce enough neutrinos to be detected in the lab. Modern neutrino observatories, sensitive enough to observe solar and astrophysical neutrinos, are large arrays of detectors. One example is the Super-Kamiokande detector seen above. The three technicians on the left give you an idea of scale.

Neutrinos are not only produced in radioactive decay, they are also a byproduct of nuclear fusion. Thus the Sun produces neutrinos in its core due to the fusion of hydrogen. These solar neutrinos were detected in the mid 1960s, but it was found that the number of neutrinos was a third the expected amount. This result was repeatedly confirmed, and it became known as the solar neutrino problem.

The solar neutrino problem meant either our understanding of the temperature and pressure within the Sun was significantly off, or our understanding of particle physics was wrong. Since our understanding of particle physics was based upon laboratory experiments, it was thought that our Solar model must be wrong. There were several attempts to reformulate solar fusion models to account for the smaller neutrino rate, going so far as to propose that fusion in the Sun’s core had shut down temporarily. But over time these alternatives became increasingly untenable. Observational data supported traditional solar fusion models, therefore our understanding of particle physics must be wrong.

The physics of fundamental particles (electrons, neutrinos, quarks, etc) is described by what is known as the Standard Model. This model unified our understanding of electromagnetism and the nuclear forces known as the strong and weak. It described all known elementary particles, and even predicted new particles that were eventually observed. By the 1970s it was established as the theory of fundamental particles.

In the 1960s, it was found the the electron had two sister particles known as the muon and tauon, (together known as leptons) and that each of these had a corresponding neutrino. This meant that there were three types (or flavors) of neutrinos. According to the standard model, these neutrinos should be massless. This would mean that each flavor of neutrino only interacted with its corresponding lepton. The reason for this is that massless particles travel at the speed of light. They therefore don’t experience time, and can’t change their state. So a muon (which has mass) can decay into an electron, but a muon neutrino cannot change into an electron neutrino.

In 1968 Bruno Pontecorvo proposed that neutrinos had a small mass. This proposal would change the standard model slightly, but leave the overall predictions largely unchanged. The mass of the neutrinos couldn’t be large, otherwise they would have already been observed. But even the tiniest amount of mass would mean neutrinos could not move at the speed of light, and could therefore change flavors. This flavor-changing effect is known as neutrino oscillation.

Neutrino oscillation would explain the solar neutrino problem. The neutrinos produced in the Sun’s core would all be electron neutrinos, but as they moved through the Sun to its surface they would oscillate between the various types. By the time they reached the Earth, the neutrinos would be an equal mix of each type, meaning that only a third would be electron neutrinos. Since only the electron neutrinos were measured, the neutrino number would be a third that predicted by the standard model.

It wasn’t until 1998 that neutrino oscillation was confirmed by the Super-Kamiokande detector seen below. Thus the solar neutrino problem was resolved. It also demonstrated that neutrinos do indeed have mass. Just how massive they are has yet to be precisely determined, but we have a general idea from cosmological observations.

Moments after the big bang, the initial electrons, photons and neutrinos of the universe were produced. This means the number of neutrinos in the universe is related to the number of photons. We can calculate the photon number by observing the cosmic microwave background, so we have a good idea of the number of neutrinos in the universe. If the neutrinos had a larger mass their gravity would affect the distribution of galaxies. Galaxy surveys see no such effect, which means the total mass of all three neutrino types can be no more than 0.23 eV, or less than 2 millionths the mass of an electron. Observations of neutrino oscillations also require that at least one flavor of neutrino has a mass of at least 0.04 eV.

Because neutrinos have mass and weakly interact with other particles, they are often brought up as a possible solution to dark matter. However neutrinos have such a small mass that they cannot be the solution to the dark matter problem. They might contribute in a minor way, but they aren’t the complete solution.

So while we’ve solved the mystery of one “little neutral” particle, there is another particle mystery waiting to be solved.

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The Guitar Nebula is a guitar-shaped nebula in the constellation Cepheus.  It is a faint nebula, about 6,500 light years away, but it is unusual because it is a bow shock nebula.  This particular bow shock nebula is produced by a pulsar moving about 800 kilometers per second through the surrounding interstellar media.  As the pulsar plows through the interstellar gas it disrupts it, much like a boat on a still pond produces waves.  The nebula was first discovered in 1993, and it is among the strongest evidence a phenomenon known as a pulsar kick.

Pulsars are neutron stars, formed when a large star explodes as a supernova.  Because of this, one would expect a pulsar to lie within the surrounding supernova remnant, and to move at the same relative speed.  But this is not the case with the Guitar Nebula.  It seems that something must have caused the pulsar to move at great speed relative to the remnant. Given it a kick, as it were, hence the term pulsar kick (or neutron star kick).  Given the mass of a neutron star (greater than that of our Sun) the only thing that could have provided such a kick would be the supernova itself.

Computational simulation of an asymmetrical supernova. Credit: J. Nordhaus, et al

Just how the supernova can kick the neutron star isn’t clear, but there are a couple of ideas.  One is that during the collapse of the progenitor star, the core becomes slightly off-center.  This causes the surrounding star to compress asymmetrically, which means the resulting supernova exerts a large force on the core, causing it to race off at great speed.  Another is that it is an electromagnetic effect.  If the magnetic field of a pulsar is off-center, the jets of energy produced by the pulsar would also be off-center, and thus would push the neutron star like a rocket.

There is a third, much more hypothetical possibility which involves the sterile neutrinos I mentioned yesterday.  The basic idea is that there is an asymmetry between regular neutrinos and sterile neutrinos.  Since intense nuclear interactions occur within a supernova, it is possible that both regular and sterile neutrinos could be produced.  But because of the asymmetry between the two, this would result in a kick being given to the resulting pulsar.  We still aren’t sure of the nature of sterile neutrinos (or even if they exist), but some theoretical papers show that it could solve the pulsar kick mystery.

At the moment, the asymmetrical supernova model is the most popular. It will take more study to determine if that is indeed the solution.

Paper: James M. Cordes, Roger W. Romani and Scott C. Lundgren. The Guitar nebula: a bow shock from a slow-spin, high-velocity neutron star. Nature 362, 133 – 135 (11 March 1993); doi:10.1038/362133a0

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