Fermi telescope closes in on source of cosmic rays

February 16, 2010 | 10:48 am

Supernova Remnant video showing specific remnants and their appearance at different wavelengths of electromagnetic radiation.

New images from the Fermi Gamma-ray Space Telescope show where supernova remnants emit radiation a billion times more energetic than visible light. The images bring astronomers a step closer to understanding the source of some of the universe’s most energetic particles–cosmic rays.

Cosmic rays consist mainly of protons that move through space at nearly the speed of light. In their journey across the galaxy, the particles are deflected by magnetic fields. This scrambles their paths and masks their origins.

“Understanding the sources of cosmic rays is one of Fermi’s key goals,” said Stefan Funk, an astrophysicist at the Kavli Institute for Particle Astrophysics and Cosmology (KIPAC), jointly located at SLAC National Accelerator Laboratory and Stanford University, Calif.

When cosmic rays collide with interstellar gas, they produce gamma rays.

“Fermi now allows us to compare emission from remnants of different ages and in different environments,” Funk added. He presented the findings Monday at the American Physical Society meeting in Washington, D.C.

Fermi’s Large Area Telescope (LAT) mapped billion-electronvolt (GeV) gamma rays from three middle-aged supernova remnants–known as W51C, W44, and IC 443–that were never before resolved at these energies.

(The energy of visible light is between 2 and 3 electronvolts.) Each remnant is the expanding debris of a massive star that blew up between 4000 and 30,000 years ago.

In addition, Fermi’s LAT also spied GeV gamma rays from Cassiopeia A (Cas A), a supernova remnant only 330 years old. Ground-based observatories, which detect gamma rays thousands of times more energetic than the LAT was designed to see, have previously detected Cas A.

“Older remnants are extremely bright in GeV gamma rays, but relatively faint at higher energies. Younger remnants show a different behavior,” explained Yasunobu Uchiyama, a Panofsky Fellow at SLAC. “Perhaps the highest-energy cosmic rays have left older remnants, and Fermi sees emission from trapped particles at lower energies.”

In 1949, the Fermi telescope’s namesake, physicist Enrico Fermi, suggested that the highest-energy cosmic rays were accelerated in the magnetic fields of gas clouds. In the decades that followed, astronomers showed that supernova remnants are the galaxy’s best candidate sites for this process.

Young supernova remnants seem to possess both stronger magnetic fields and the highest-energy cosmic rays. Stronger fields can keep the highest-energy particles in the remnant’s shock wave long enough to speed them to the energies observed.

The Fermi observations show GeV gamma rays coming from places where the remnants are known to be interacting with cold, dense gas clouds.

“We think that protons accelerated in the remnant are colliding with gas atoms, causing the gamma-ray emission,” Funk said. An alternative explanation is that fast-moving electrons emit gamma rays as they fly past the nuclei of gas atoms. “For now, we can’t distinguish between these possibilities, but we expect that further observations with Fermi will help us to do so,” he added.

Either way, these observations validate the notion that supernova remnants act as enormous accelerators for cosmic particles.

“How fitting it is that Fermi seems to be confirming the bold idea advanced over 60 years ago by the scientist after whom it was named,” noted Roger Blandford, director of KIPAC.

by Frank Reddy

Guest author

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A dam interesting experiment

February 15, 2010 | 4:13 pm

Sometimes the most intriguing ideas you hear in science are offhand comments or anecdotes thrown out by a conference speaker. The American Physical Society meeting in Washington, DC, is chock full of fascinating talks. Not all of it is what you would traditionally call news, but there are plenty of great stories.

One session talked about searches for ways in which Newton’s law of gravity might break down at short length scales. It’s a topic we’ve covered in symmetry before. (See the sidebar to a feature on searches for extra dimensions.) Stephan Schlamminger of the University of Washington gave seven reasons to justify why we should look for deviations from Newton’s law and a set of experimental results, but I want to focus on one of the less important stories because it’s, well, a good story.

Some theoretical ideas would cause gravity to change on the scale of tens or hundreds of meters. When testing gravity, it usually helps to have a detector approximately the same size as the phenomenon you’re looking for. So where do you go to build an experiment on the scale of 100 meters where you can move large amounts of mass nearby and far away? Think on that for a moment before you read on…

One answer is the tunnel that hangs down inside most large dam walls. The walls typically have this tunnel so that the dam integrity can be monitored. But it’s also a long straight shaft of the right length, and the water level in the dam moves up and down, providing the movement of mass needed.

In one particular dam physicists hung a long wire with a mass on the end, attached the top to a very sensitive scale to weigh the mass and compared the weight with the water level over the course of hours and days. The weight of the hanging mass depends on the configuration of the mass near it. It won’t change much but enough to be detected and the weight is predictable from knowing the water level and various other details. If there is any variation in Newton’s Law over that distance, it should change the weight of the hanging mass just a little.

An older experiment did something similar but using gravimeters at different heights in a tower built for the experiment in a pumped storage reservoir in Germany’s Black Forest.

Over the course of the dam and reservoir measurements, the results perfectly matched predictions within the precision of the experiments, so there was no discovery of a deviation. Newton’s Law is safe for now but it makes you think differently about the mass all around you, especially next time you stand on top of a large dam wall.

David Harris

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Just how often are you hit by a neutrino?

February 15, 2010 | 3:17 pm

Just a little snippet of information I picked up at the American Physical Society meeting in Washington, DC, this weekend:

Chris Walter of Duke University says that the probability of a neutrino interacting with your body at some point in your life is about 1 in 4. So look around your friends–how many of them have been hit by a neutrino?

Keep in mind that neutrinos are flooding through you generally without interacting–about 100 trillion per second. That translates to something like 2.5 x 10²¹ neutrinos pass through you in your lifetime.

With a few simple estimates, we can convert that to say that the chance of a particular neutrino actually interacting with you is about 1 in 1 trillion trillion. That number is so extreme I can’t even really imagine it so I’ll just wonder for the rest of my life whether I’ve been one of the people lucky enough to be hit by a neutrino.

David Harris

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Do particle theorists have a blind spot?

February 14, 2010 | 5:05 pm

In a provocative section of a talk at the American Physical Society meeting in Washington, DC, yesterday, theorist Matthew Strassler from Rutgers University challenged particle theorists to not be too simple in their analyses. Most people would probably not claim that theoretical particle physics is too simple, but Strassler argued that nature is likely to be even more complicated than physicists expect. And if theorists only properly examine the simplest classes of models, where simple is a relative term, they might be led astray in interpreting future Large Hadron Collider data.

Physicists know that the Standard Model of particle physics is broken, but they don’t yet know how to fix it. The general approach is to augment the Standard Model with new particles, forces, or phenomena and see what those extended theories predict. Then when experimenters gather more data, they will be able to see which theories might best reflect reality.

As is natural, theorists start by examining the models that are the simplest extensions of the Standard Model. Those simplest extensions usually have the word minimal in their name, such as the Minimal Super Symmetric Model or the Minimal Technicolor Model.

“The preference for minimal models is unquestionably a cultural bias,” Strassler claimed. He said that many theorists argue that minimal is elegant which appears well-motivated and is therefore appreciated. He said that that line of argument works in converse also, where non-minimal models are considered inelegant, not well motivated, and therefore are unappreciated.

He gave several historical anecdotes of where minimalism has failed. When the muon, a heavy form of the electron, was discovered in 1936 and tipped off physicists to the existence of a second generation of leptons beyond the seemingly sufficient electron, Isidor Isaac Rabi asked in shock, “Who ordered that?”

Strassler gave other examples of non-minimalism such as the fact that neutrinos have non-zero mass and that there is dark energy, which he phrased in terms of the existence of a non-zero cosmological constant.

He argued that physicists are entering the post-minimal era where the potentially sufficient minimal theories so far developed, especially in the 1990s, will not be enough to describe what we see at the LHC. Strassler argued for the exploration of other theoretical frameworks, such as his own hidden valleys framework, which includes a new set of light neutral particles and new forces. He also mentioned frameworks by others, including “quirks”, “unparticles”, and the “WIMPless miracle”, all of which are non-minimal. These models might be tested with Tevatron and LHC data or even with existing data from the BaBar and Belle experiments. “Light, neutral particles could be a blind spot in our field,” he said. Strassler argued that low energy experiments at the intensity frontier will be vital in advancing theoretical understanding.

While there is no question among theorists they are not exploring all of the possibilities, some theorists and journal editors at the conference seemed unpersuaded by the argument on practical grounds. Theorists only have so much time and they don’t really know which direction to go in making more complicated theories. They have too many choices open to them and no reliable way to decide which might be most fruitful. One editor of Physical Review Letters commented that the journal wouldn’t typically be interested in papers which head off in a random direction with an extension of theories beyond the minimal models unless they showed some particularly dramatic signature in a detector, such as a paper last year that included a model which had a new type of force represented by a heavy particle called the Z’ which would decay into a very clear signature of six leptons (such as electrons and muons).

With the LHC expected to switch back on in the next week or so, the data will begin to flow quite rapidly, and, as discussed by many other presenters at the conference, results will rapidly begin to winnow down the theories that have been proposed in the past decades. Perhaps the most salient aspect of Strassler’s warning would apply if data has ruled out most existing models. It would argue that theorists shouldn’t be too quick to think that what remains is actually a true reflection of reality, unless they have dug much deeper into some of the more complicated models. Only then might they be assured that they are getting closer to painting a true picture of how nature works at its most fundamental level.

David Harris

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CHAMPs take a chomp from SUSY

February 14, 2010 | 9:00 am

A whimsically named type of particle, the CHAMP, is taking a bite out of possible theories of supersymmetry.

CHArged Massive stable Particles are a theoretically conjectured type of long-lived, heavy particles that could be created in particle colliders and which would survive long enough to make it out of the detector before decaying. Todd Adams from Florida State University explained how they leave a fairly unusual signal because they penetrate a detector a long way, are slow moving, and are heavily ionizing, therefore potentially leaving strong signals in detectors. However, there is also a risk that the particles could be confused for either energetic muons or a type of particle called an “R-hadron”.

So far, there has been no evidence of the existence of CHAMPs at the Fermilab’s Tevatron, but the fact that they haven’t shown up allows physicists to place limits on how supersymmetry must appear if it is present in nature. The DZero experiment has looked for CHAMPs that masquerade as muons in the detector and the CDF experiment has looked for particles that mimic R-hadrons.

One CDF search reveals that the mass of the “stop”, the supersymmetric partner of the top quark, must be more than 249 GeV, and also places limits on how strongly it could interact with other particles, ruling out predictions in some supersymmetry models. That paper was published in 2009 in Physical Review Letters and described yesterday by Adams at the American Physical Society meeting in Washington, DC. The DZero search was used as a key part of a theoretical paper “Supersymmetry Without Prejudice” by Carola Berger and colleagues about how the signals of supersymmetry can lead to unusual and unexpected signatures. Adams described the CHAMP searches as good ways of exploring some of the theoretical predictions of SUSY theories because they can cover areas of supersymmetry predictions that other searches can’t interrogate.

David Harris

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Exotic particle of the day: the stringball

February 13, 2010 | 4:35 pm

Today at the American Physical Society meeting in Washington, DC, Ayana Arce from Duke University spoke about prospects for discovering new physics at the Large Hadron Collider. Among the many theoretically proposed exotica she discussed, the most fun name goes to the stringball. It looks like a mini black hole but it involves what she described as “a democratic decay to a large number of objects per event.” That statement takes a bit of unpacking to understand, but here’s how a stringball could behave in a snapshot:

Imagine a small black hole. The black hole evaporates away through Hawking radiation, getting smaller over time. String theory suggests that when the black hole gets to some critically small size, it transforms into highly excited vibrating string state, which can then disintegrate into a burst of massless radiation. That radiation would appear as jets of particles coming out in every direction in an LHC detector such as ATLAS or CMS.

If stringballs exist, there is a good chance that the LHC could see them when it ramps up its energy and collects sufficient data. As a bonus, the characteristics of a stringball would be determined by any extra dimensions that might exist. It’s a long shot to discover, but it’s a curious example of the exotica that could be created by the Large Hadron Collider.

David Harris

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Searching ALL the Tevatron data for exotic physics

February 13, 2010 | 3:12 pm

The majority of published particle physics papers involve looking for a very specific elementary particle process. They conclude by either showing that it exists or constraining limits on how often that process could possibly occur. But is there a way to take advantage of a collider’s whole data set and seeing if it is consistent with an expansive theoretical model, such as the Standard Model of particle physics?

In a wide-ranging discussion about searches for new and exotic particles, forces, and phenomena at Fermilab’s Tevatron, Florida State University’s Todd Adams, described today at the American Physical Society meeting in Washington, DC, how physicists interrogate the entire collider data set. This is not actually new work, and they published a paper (preprint, Phys. Rev. D) about a year ago on this topic. However, it’s not the kind of analysis you usually see in particle physics and it makes for a nice way to look at the data.

Typically, the CDF and DZero collaborations at Fermilab’s Tevatron (and other colliders) publish papers with titles like “Measurement of the branching fraction Br(Bs -> Ds(*) Ds(*))”. In such a paper, physicists are looking at a very specific way that one type of particle decays into other specific particles. This is important work for testing the details of models, but it is easy to get overwhelmed by those details if you’re not a particle physicist.

The all-data studies break up the Tevatron data into sets of collision events which each have a specific final state. That is, for a particle proton-proton collision in the heart of one of the detectors, the outgoing particles must end up being, say, two electrons and two jets of particles; or a muon, two jets, and missing energy. It could also be one of many other possibilities. But this cuts down the full data set to a manageable size without assuming anything about how the particles got to that end state. Perhaps there were top quarks involved, or maybe W bosons. Or perhaps there are more exotic intermediate particles like Higgs bosons, or gravitons, or a Z’ representing an extra so-far-unknown fundamental force.

Then physicists take the best model they have of particle interactions, the Standard Model, and grind through the calculations of what all the possible events look like with that same final state. With the theoretical prediction in hand, and the experimental data crunched, physicists can see if they match up. If not, it might be sign of exotic physics.

The physicists use four different techniques to look for deviations from theory. First they look for an overall excess or deficit of end products. Second, they see if the shape of the data—that is how the number of events depends on energy—is consistent with theory. Third, they look for unexpected events at very high energies. Fourth, the go on a bump hunt—they see if there are extra events at well-defined, narrow energy ranges in among the regularly predicted collision events.

When physicists do these searches, they find that on a first cut, they see a whole bunch of discrepant events. In technical terms, they see events with more than 3 sigma deviation from the theory. That would usually be enough to claim evidence for a new particle process. Five sigma deviation is generally regarded as the cutoff for a new discovery. Some results from the analysis have deviations as much as 4.3 sigma—enticingly close to discovery.

However, the all-data approach is a different way of analyzing data and is akin to large scale data mining. When you do so many possible searches, you are likely to get some results randomly fluctuating almost to discovery level. But that is all they are: random fluctuations. There are extra statistical precautions you need to take when doing a large number of tests on a single data set. When you apply those precautions, the significance of these deviations drops down. What seemed like a 4.3 sigma deviation becomes a mere 2.7 sigma deviation. Interesting, and maybe even a hint of something, but nowhere near enough to claim evidence for a new physics process.

With the whole data set taken into account (as of about a year ago), there were no convincing signs of new physics, but the process demonstrated the possibilities of interrogating the data in a different way. In cases where there is no discovery of new physics, the data places tighter constraints on the existence of those processes. Adams commented that these techniques are beginning to place real limits on some of the theoretical predictions about particle processes that are interesting on cosmological scales, such as the properties of any extra dimensions and dark matter.

Whatever the power of these techniques, the story reflects an interesting approach to slicing up the huge data sets of particle physics and looking for what might lie beyond the theories that have served physicists so well for so many years.

David Harris

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