Tag Archives: Standard Model

The Anomaly

Is the smallest, lightest, most ghostly particle about to upend our understanding of the universe? Recently, the ephemeral neutrino has begun to give up some of its secrets. Beginning in 1998 the neutrino experiments at Super-Kamiokande and Sudbury Neutrino Observatory showed for the first time that neutrinos oscillate with one of three flavors. In 2015, two physicists were awarded the Nobel prize for this discovery, which also proved that neutrinos must have mass. More recently, a small anomaly at the Super-Kamiokande detector has surfaced, which, is hoped, could shed light on why the universe is constructed primarily from matter and not anti-matter.

From Quanta:

The anomaly, detected by the T2K experiment, is not yet pronounced enough to be sure of, but it and the findings of two related experiments “are all pointing in the same direction,” said Hirohisa Tanaka of the University of Toronto, a member of the T2K team who presented the result to a packed audience in London earlier this month.

“A full proof will take more time,” said Werner Rodejohann, a neutrino specialist at the Max Planck Institute for Nuclear Physics in Heidelberg who was not involved in the experiments, “but my and many others’ feeling is that there is something real here.”

The long-standing puzzle to be solved is why we and everything we see is matter-made. More to the point, why does anything — matter or antimatter — exist at all? The reigning laws of particle physics, known as the Standard Model, treat matter and antimatter nearly equivalently, respecting (with one known exception) so-called charge-parity, or “CP,” symmetry: For every particle decay that produces, say, a negatively charged electron, the mirror-image decay yielding a positively charged antielectron occurs at the same rate. But this cannot be the whole story. If equal amounts of matter and antimatter were produced during the Big Bang, equal amounts should have existed shortly thereafter. And since matter and antimatter annihilate upon contact, such a situation would have led to the wholesale destruction of both, resulting in an empty cosmos.

Somehow, significantly more matter than antimatter must have been created, such that a matter surplus survived the annihilation and now holds sway. The question is, what CP-violating process beyond the Standard Model favored the production of matter over antimatter?

Many physicists suspect that the answer lies with neutrinos — ultra-elusive, omnipresent particles that pass unfelt through your body by the trillions each second.

Read the entire article here.

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95.5 Percent is Made Up and It’s Dark


Physicists and astronomers observe the very small and the very big. Although they are focused on very different areas of scientific endeavor and discovery, they tend to agree on one key observation: 95.5 of the cosmos is currently invisible to us. That is, only around 4.5 percent of our physical universe is made up of matter or energy that we can see or sense directly through experimental interaction. The rest, well, it’s all dark — so-called dark matter and dark energy. But nobody really knows what or how or why. Effectively, despite tremendous progress in our understanding of our world, we are still in a global “Dark Age”.

From the New Scientist:

TO OUR eyes, stars define the universe. To cosmologists they are just a dusting of glitter, an insignificant decoration on the true face of space. Far outweighing ordinary stars and gas are two elusive entities: dark matter and dark energy. We don’t know what they are… except that they appear to be almost everything.

These twin apparitions might be enough to give us pause, and make us wonder whether all is right with the model universe we have spent the past century so carefully constructing. And they are not the only thing. Our standard cosmology also says that space was stretched into shape just a split second after the big bang by a third dark and unknown entity called the inflaton field. That might imply the existence of a multiverse of countless other universes hidden from our view, most of them unimaginably alien – just to make models of our own universe work.

Are these weighty phantoms too great a burden for our observations to bear – a wholesale return of conjecture out of a trifling investment of fact, as Mark Twain put it?

The physical foundation of our standard cosmology is Einstein’s general theory of relativity. Einstein began with a simple observation: that any object’s gravitational mass is exactly equal to its resistance to accelerationMovie Camera, or inertial mass. From that he deduced equations that showed how space is warped by mass and motion, and how we see that bending as gravity. Apples fall to Earth because Earth’s mass bends space-time.

In a relatively low-gravity environment such as Earth, general relativity’s effects look very like those predicted by Newton’s earlier theory, which treats gravity as a force that travels instantaneously between objects. With stronger gravitational fields, however, the predictions diverge considerably. One extra prediction of general relativity is that large accelerating masses send out tiny ripples in the weave of space-time called gravitational waves. While these waves have never yet been observed directly, a pair of dense stars called pulsars, discovered in 1974, are spiralling in towards each other just as they should if they are losing energy by emitting gravitational waves.

Gravity is the dominant force of nature on cosmic scales, so general relativity is our best tool for modelling how the universe as a whole moves and behaves. But its equations are fiendishly complicated, with a frightening array of levers to pull. If you then give them a complex input, such as the details of the real universe’s messy distribution of mass and energy, they become effectively impossible to solve. To make a working cosmological model, we make simplifying assumptions.

The main assumption, called the Copernican principle, is that we are not in a special place. The cosmos should look pretty much the same everywhere – as indeed it seems to, with stuff distributed pretty evenly when we look at large enough scales. This means there’s just one number to put into Einstein’s equations: the universal density of matter.

Einstein’s own first pared-down model universe, which he filled with an inert dust of uniform density, turned up a cosmos that contracted under its own gravity. He saw that as a problem, and circumvented it by adding a new term into the equations by which empty space itself gains a constant energy density. Its gravity turns out to be repulsive, so adding the right amount of this “cosmological constant” ensured the universe neither expanded nor contracted. When observations in the 1920s showed it was actually expanding, Einstein described this move as his greatest blunder.

It was left to others to apply the equations of relativity to an expanding universe. They arrived at a model cosmos that grows from an initial point of unimaginable density, and whose expansion is gradually slowed down by matter’s gravity.

This was the birth of big bang cosmology. Back then, the main question was whether the expansion would ever come to a halt. The answer seemed to be no; there was just too little matter for gravity to rein in the fleeing galaxies. The universe would coast outwards forever.

Then the cosmic spectres began to materialise. The first emissary of darkness put a foot in the door as long ago as the 1930s, but was only fully seen in the late 1970s when astronomers found that galaxies are spinning too fast. The gravity of the visible matter would be too weak to hold these galaxies together according to general relativity, or indeed plain old Newtonian physics. Astronomers concluded that there must be a lot of invisible matter to provide extra gravitational glue.

The existence of dark matter is backed up by other lines of evidence, such as how groups of galaxies move, and the way they bend light on its way to us. It is also needed to pull things together to begin galaxy-building in the first place. Overall, there seems to be about five times as much dark matter as visible gas and stars.

Dark matter’s identity is unknown. It seems to be something beyond the standard model of particle physics, and despite our best efforts we have yet to see or create a dark matter particle on Earth (see “Trouble with physics: Smashing into a dead end”). But it changed cosmology’s standard model only slightly: its gravitational effect in general relativity is identical to that of ordinary matter, and even such an abundance of gravitating stuff is too little to halt the universe’s expansion.

The second form of darkness required a more profound change. In the 1990s, astronomers traced the expansion of the universe more precisely than ever before, using measurements of explosions called type 1a supernovae. They showed that the cosmic expansion is accelerating. It seems some repulsive force, acting throughout the universe, is now comprehensively trouncing matter’s attractive gravity.

This could be Einstein’s cosmological constant resurrected, an energy in the vacuum that generates a repulsive force, although particle physics struggles to explain why space should have the rather small implied energy density. So imaginative theorists have devised other ideas, including energy fields created by as-yet-unseen particles, and forces from beyond the visible universe or emanating from other dimensions.

Whatever it might be, dark energy seems real enough. The cosmic microwave background radiation, released when the first atoms formed just 370,000 years after the big bang, bears a faint pattern of hotter and cooler spots that reveals where the young cosmos was a little more or less dense. The typical spot sizes can be used to work out to what extent space as a whole is warped by the matter and motions within it. It appears to be almost exactly flat, meaning all these bending influences must cancel out. This, again, requires some extra, repulsive energy to balance the bending due to expansion and the gravity of matter. A similar story is told by the pattern of galaxies in space.

All of this leaves us with a precise recipe for the universe. The average density of ordinary matter in space is 0.426 yoctograms per cubic metre (a yoctogram is 10-24 grams, and 0.426 of one equates to about 250 protons), making up 4.5 per cent of the total energy density of the universe. Dark matter makes up 22.5 per cent, and dark energy 73 per cent (see diagram). Our model of a big-bang universe based on general relativity fits our observations very nicely – as long as we are happy to make 95.5 per cent of it up.

Arguably, we must invent even more than that. To explain why the universe looks so extraordinarily uniform in all directions, today’s consensus cosmology contains a third exotic element. When the universe was just 10-36 seconds old, an overwhelming force took over. Called the inflaton field, it was repulsive like dark energy, but far more powerful, causing the universe to expand explosively by a factor of more than 1025, flattening space and smoothing out any gross irregularities.

When this period of inflation ended, the inflaton field transformed into matter and radiation. Quantum fluctuations in the field became slight variations in density, which eventually became the spots in the cosmic microwave background, and today’s galaxies. Again, this fantastic story seems to fit the observational facts. And again it comes with conceptual baggage. Inflation is no trouble for general relativity – mathematically it just requires an add-on term identical to the cosmological constant. But at one time this inflaton field must have made up 100 per cent of the contents of the universe, and its origin poses as much of a puzzle as either dark matter or dark energy. What’s more, once inflation has started it proves tricky to stop: it goes on to create a further legion of universes divorced from our own. For some cosmologists, the apparent prediction of this multiverse is an urgent reason to revisit the underlying assumptions of our standard cosmology (see “Trouble with physics: Time to rethink cosmic inflation?”).

The model faces a few observational niggles, too. The big bang makes much more lithium-7 in theory than the universe contains in practice. The model does not explain the possible alignment in some features in the cosmic background radiation, or why galaxies along certain lines of sight seem biased to spin left-handedly. A newly discovered supergalactic structure 4 billion light years long calls into question the assumption that the universe is smooth on large scales.

Read the entire story here.

Image: Petrarch, who first conceived the idea of a European “Dark Age”, by Andrea di Bartolo di Bargilla, c1450. Courtesy of Galleria degli Uffizi, Florence, Italy / Wikipedia.

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The Large Hadron Collider is So Yesterday

CERN’s Large Hadron Collider (LHC) smashed countless particles into one another to reveal the Higgs Boson. A great achievement for all concerned. Yet what of the still remaining “big questions” of physics, and how will we find the answers?

From Wired:

The current era of particle physics is over. When scientists at CERN  announced last July that they had found the Higgs boson — which is responsible for giving all other particles their mass — they uncovered the final missing piece in the framework that accounts for the interactions of all known particles and forces, a theory known as the Standard Model.

And that’s a good thing, right? Maybe not.

The prized Higgs particle, physicists assumed, would help steer them toward better theories, ones that fix the problems known to plague the Standard Model. Instead, it has thrown the field  into a confusing situation.

“We’re sitting on a puzzle that is difficult to explain,” said particle physicist Maria Spiropulu of Caltech, who works on one of the LHC’s main Higgs-finding experiments, CMS.

It may sound strange, but physicists were hoping, maybe even expecting, that the Higgs would not turn out to be like they predicted it would be. At the very least, scientists hoped the properties of the Higgs would be different enough from those predicted under the Standard Model that they could show researchers how to build new models. But the Higgs’ mass  proved stubbornly normal, almost exactly in the place the Standard Model said it would be.

To make matters worse, scientists had hoped to find evidence for other strange particles. These could have pointed in the direction of theories beyond the Standard Model, such as the current favorite  supersymmetry, which posits the existence of a heavy doppelganger to all the known subatomic bits like electrons, quarks, and photons.

Instead, they were disappointed by being right. So how do we get out of this mess? More data!

Over the next few years, experimentalists will be churning out new results, which may be able to answer questions about dark matter, the properties of neutrinos, the nature of the Higgs, and perhaps what the next era of physics will look like. Here we take a look at the experiments that you should be paying attention to. These are the ones scientists are the most excited about because they might just form the next cracks in modern physics.

The Large Hadron Collider isn’t smashing protons right now. Instead, engineers are installing upgrades to help it search at even higher energies. The machine may be closed for business until 2015 but the massive amounts of data it has already collected is still wide open. The two main Higgs-searching experiments, ATLAS and CMS, could have plenty of surprises in store.

“We looked for the low-hanging fruit,” said particle physicist David Miller of the University of Chicago, who works on ATLAS. “All that we found was the Higgs, and now we’re going back for the harder stuff.”

What kind of other stuff might be lurking in the data? Nobody knows for sure but the collaborations will spend the next two years combing through the data they collected in 2011 and 2012, when the Higgs was found. Scientists are hoping to see hints of other, more exotic particles, such as those predicted under a theory known as supersymmetry. They will also start to understand the Higgs better.

See, scientists don’t have some sort of red bell that goes “ding” every time their detector finds a Higgs boson. In fact, ATLAS and CMS can’t actually see the Higgs at all. What they look for instead are the different particles that the Higgs decays into. The easiest-to-detect channels include when the Higgs decays to things like a quark and an anti-quark or two photons. What scientists are now trying to find out is exactly what percent of the time it decays to various different particle combinations, which will help them further pin down its properties.

It’s also possible that, with careful analysis, physicists would add up the percentages for each of the different decays and notice that they haven’t quite gotten to 100. There might be just a tiny remainder, indicating that the Higgs is decaying to particles that the detectors can’t see.

“We call that invisible decay,” said particle physicist Maria Spiropulu. The reason that might be exciting is that the Higgs could be turning into something really strange, like a dark matter particle.

We know from cosmological observations that dark matter has mass and, because the Higgs gives rise to mass, it probably has to somehow interact with dark matter. So the LHC data could tell scientists just how strong the connection is between the Higgs and dark matter. If found, these invisible decays could open up a whole new world of exploration.

“It’s fashionable to call it the ‘dark matter portal’ right now,” said Spiropulu.

NOVA and T2K
Neutrinos are oddballs in the Standard Model. They are tiny, nearly massless, and barely like interacting with any other members of the subatomic zoo. Historically, they have been the subject of  many surprising results and the future will probably reveal them to be even stranger. Physicists are currently trying to figure out some of their properties, which remain open questions.

“A very nice feature of these open questions is we know they all have answers that are accessible in the next round of experiments,” said physicist Maury Goodman of Argonne National Laboratory.

The US-based NOvA experiment will hopefully pin down some neutrino characteristics, in particular their masses. There are three types of neutrinos: electron, muon, and tau. We know that they have a very tiny mass — at least 10 billion times smaller than an electron — but we don’t know exactly what it is nor which of the three different types is heaviest or lightest.

NOvA will attempt to figure out this mass hierarchy by shooting a beam of neutrinos from Fermilab near Chicago 810 kilometers away to a detector in Ash River, Minnesota. A similar experiment in Japan called T2K is also sending neutrinos across 295 kilometers. As they pass through the Earth, neutrinos oscillate between their three different types. By comparing how the neutrinos look when they are first shot out versus how they appear at the distant detector, NOvA and T2K will be able to determine their properties with high precision.

T2K has been running for a couple years while NOvA is expected to begin taking data in 2014 and will run six years. Scientists hope that they will help answer some of the last remaining questions about neutrinos.

Read the entire article here.

Image: A simulation of the decay of a Higgs boson in a linear collider detector. Courtesy of Norman Graf / CERN.

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