How the LHC will make the top quark old news

May 11, 2009 | 10:27 am

When the Large Hadron Collider begins collisions, the top quark won’t have as much of a role as the star of cutting-edge physics discoveries but more as a means to ensure the machine is working properly. “There’s the old saw: one man’s discovery is another man’s calibration,” said the University of Nebraska’s Ken Bloom at a press conference at the American Physical Society meeting in Denver, Colorado, last week. “Expect a lot of papers on top quark physics in the first few years of operation.”

The discovery of the top quark in 1995 marks a crowning achievement of the Fermilab’s Tevatron. Once the LHC has first collisions it will become just the second machine capable of producing the top quark, which is currently the most massive subatomic particle ever observed.

Bloom explained that while top quark decays are now well understood, they remain very complicated and pose challenging analyses. Observing such a complicated process and comparing the results with current understanding will calibrate the LHC.

“If you can show that you have things working in top quark events, that means you can trust things well enough to start to look for new physics,” said Bloom. “And I really think you have to establish that you can do the Standard Model physics before you can credibly start looking for new physics.”

Physicists at that panel also confirmed the LHC’s plans to continue operating through the 2009-2010 winter. This will be the first winter that the LHC does not shut down to save on energy costs.

“It’s really fabulous,” says Bloom of the extended run. “We’re really going to need that to shake down what we’re doing.”

Calla Cofield

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Gamma-ray bursts may last longer than previously thought

May 5, 2009 | 10:09 am

Gamma-ray bursts, the most powerful explosions in the universe since the big bang, are thought to last mere seconds or a few short minutes. But new data from the Fermi Gamma-ray Space Telescope show at least some of them have much more staying power.

In March, FGST’s Large Area Telescope, or LAT–an incredibly sensitive gamma-ray and particle detector assembled and operated by SLAC National Accelerator Laboratory–spotted high-energy gamma rays from two separate bursts lasting many minutes after they occurred. Such burst durations have been observed only once before. In 1994, NASA’s EGRET instrument picked up gamma rays 1.5 hours after a blast.

“With only one observation, you never know how often something happens,” said SLAC physicist Roger Blandford, who works on the FGST project. “Now these delayed gamma rays are beginning to look like a common phenomenon.”

Gamma-ray bursts are mysterious. Astronomers have proposed that they occur when massive stars run out of nuclear fuel and collapse into black holes, releasing intense jets of radiation. The collision of two neutron stars orbiting in a binary system is another possible source. FGST’s observations could help scientists tease out the actual cause or causes.

The new data set is “an important constraint on the nature of these explosions,” Blandford said. “The source of bursts such as these must remain active for a relatively long time. This means that certain explanations are not viable.”

Neutron-star collisions may fall into this category, according to Blandford. Scientists believe that when these collapsed stellar remnants smash into each other, it’s all over within a few minutes. Gamma-ray bursts could result, but they wouldn’t last very long.

One potential burst source that would fit FGST’s data: the formation of a black hole with an accretion disc of gas and dust spinning around it. As the black hole’s immense gravity pulls this material in and compresses it, high-energy electromagnetic radiation such as gamma rays could be emitted.

“Accretion discs could provide fuel for an hour or more,” Blandford said.

The new bursts occurred March 23 and March 28. FGST picked them up with the LAT and its other instrument, the Gamma-ray Burst Monitor. The LAT then tracked the blasts long after the initial fireworks ended–the first time the instrument was directed to stare rather than scan in response to an event.

“We got those things right in our sights and stared at them for hours,” said SLAC physicist Jim Chiang, who helped analyze the data. “We knew we had to confirm the EGRET event. We had to chase that down.”

The LAT’s measurements, announced over the weekend at the American Physical Society meeting in Denver, Colorado indicated that the first burst, named GRB090323, probably lasted at least half an hour. The second, GRB090328A, continued for 15 minutes or more.

Finding radiation emanating from a burst for so long is not new. But all previous instruments–with the exception of EGRET–picked up “afterglows” of lower-energy frequencies, such as X-rays and ultraviolet light.

All gamma-ray bursts recorded thus far have occurred very far away, outside our galaxy. The two long-lasting blasts are no different. GRB090323 had a measured redshift of 3.57, which corresponds to a distance of 11.9 billion light years. GRB090328A’s redshift was 0.736, translating to 6.5 billion light years away. That’s just as well: a nearby burst aimed directly at Earth could cause mass extinctions.

These findings are the latest in a growing list of accomplishments for FGST. Since its launch last June, the telescope has already documented the most powerful gamma-ray burst ever seen and picked up an intriguing excess of cosmic electrons which is a possible signal of dark-matter annihilations. FGST will continue to sweep the gamma-ray sky through at least 2013, searching for signs of dark matter and clues about the most extreme events in the universe.

by Michael Wall

Symmetry Intern

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Finding 1 atom in 10,000,000,000,000,000,000,000,000,000

May 3, 2009 | 12:57 pm

Physicists are used to dealing with rare events and very small quantities, but rarely do they tackle a challenge of the kind facing the Enriched Xenon Observatory, or EXO. To find what they’re looking for, not only will they try to find a rare event, but to be sure they will need to find a single barium atom in the 10 ton bath of liquid xenon–10²⁸ atoms.

The EXO collaboration, involving SLAC National Accelerator Laboratory, Stanford University, and many other partners, is looking for a never-before-observed process called neutrinoless double beta decay. In their case, this means watching for an isotope of xenon decaying into barium, giving off two electrons (the double beta decay), but without giving out any neutrinos. A beta decay process gives off one neutrino, so how could this even be possible? It only works if the neutrino is its own antiparticle, so that the two beta decays each have a neutrino which essentially cancel each other out, like matter and antimatter annihilating. And the possibility that process exists is the reason for the experiment.

If neutrinoless double beta decay is observed, it means the neutrino must be its own antiparticle, a key unknown in the study of neutrinos. If the neutrino is indeed its own antiparticle, it has all kinds of implications for the structure of the Standard Model and the relationships between the fundamental particles.

The EXO experiment

EXO-200, the first version of the experiment, involves 200 kg of enriched xenon in an ultra-clean tank with light and electrical sensors that would detect the decay of xenon into barium. Steven Herrin of SLAC referred during the American Physical Society meeting in Denver, Colorado to the xenon as “the largest non-weapons based isotopically enriched stockpile in the world of any element.”

Herrin said that the whole experiment is to be transported from Stanford University in northern California to the Department of Energy’s Waste Isolation Pilot Plant in Carlsbad, New Mexico in June or July. The experiment is being installed 2000 feet underground in clean rooms at WIPP, a salt bed well suited to radioactive shielding. A technical run will begin in the late summer or autumn this year, with scientific data taking starting before the end of 2009.

A later version of EXO would include perhaps 10 times as much xenon and would be correspondingly more sensitive to the very rare neutrinoless double beta decay, if it exists at all.

Avoiding false alarms

Detecting a rare process like neutrinoless double beta decay is particularly challenging because it involves looking for a signal that occurs rarely but happens in an environment prone to generating false signals, called background. The very low-level but inherent radioactive decay of the experiment’s equipment, combined with the chance of random cosmic rays hitting the apparatus all could give rise to signals that could be confused for the signal physicists are seeking.

The experiment is being placed underground to help avoid the effects of cosmic rays, and the materials from which the apparatus is made are all specially prepared to avoid the low-level radiation that essentially all materials naturally emit.

But even cutting down the background to as low a level as possible isn’t ideal and experience with other related experiments trying to directly detect dark matter have shown that the major issue is dealing with these backgrounds.

The EXO team has an ambitious plan to try to make the backgrounds completely irrelevant, with no chance of them causing a false alarm. It requires finding the barium ion that is created when the xenon decays.

As graduate student Brian Mong from Colorado State University understated, “Detecting a single barium ion in 10²⁸ xenon atoms is a unique challenge.”

If the barium ion can be found and its creation matched to the time of the xenon emitting its two electrons, physicists can be sure that double beta decay really happened. But where do they begin?

Finding the needle

Mong described his graduate work on a process for detecting the barium ion. His is one of three different approaches being developed to try to find the barium. None of these will be ready for the EXO-200 experiment, but collaborators hope one of them will work well enough to implement in a larger-scale version of EXO.

Mong’s approach is to feed an optical fiber into the liquid xenon and, using a small cryogenic system at the end of the fiber, freezing a sample of the xenon, which might include the barium ion. The barium ion would be trapped at the end of the fiber, where its existence could be confirmed.

The team is still unsure how they will trap the barium ion. The barium ion could be pushed toward the fiber using electric fields, or perhaps the fiber could be inserted on a probe into the middle of the xenon at the place identified by the flashes of light that will accompany the xenon decay. That is a problem for later and Mong is currently concentrating on getting the identification to work.

The basic principle for identification is that laser light of a specific frequency tuned to the properties of barium will be shone into the optical fiber. The light would hit the barium ion, if present, and the barium would absorb the light and then fluoresce, or re-emit some of it back toward the fiber. The light going back into the fiber could be detected and the existence of the barium confirmed.

Mong has a prototype of the equipment built and to test it, he is using a fluorescent dye called rhodamine 6G. He diluted some rhodamine many times over until there should have been about 100 molecules in the test tank. The fiber and laser system was able to very clearly identify the presence of the molecules showing the technique works in principle.

In what might be the closest physics has ever come to homeopathy, the next step was to dilute the sample even further until he expected to have only one molecule in the test tank. In a series of tests, he was able to extract a signal corresponding to the presence of a molecule, while tests with samples containing no rhodamine came up empty as expected.

Although the technique is not foolproof yet–there were a couple or results that didn’t work which Mong fears might have come from two samples being confused–it shows promise for actually finding that elusive barium ion when xenon decays. The technique will be re-tested over the summer with many more samples.

Finding a single barium ion in the pool of xenon would be one of the most difficult detections of a single atom yet performed, but the payoff is enormous. It would bypass the problem of backgrounds and the kind of controversy that they have caused in related dark matter experiments.

Are the neutrino and antineutrino the same?

Even with detection of barium operating well, the physicists will still need to see what nature has in store for them. There is no doubt that barium will be created, as the xenon isotope being used will definitely decay.

But whether it decays only emitting neutrinos, or whether it also decays with no neutrinos must be disentangled. The trouble is that the neutrinos are invisible to the detector.

All that physicists have to work with is the energies of the electrons that are emitted during the decay of xenon to barium. Instruments will be able to determine the energies of the electrons with about one percent precision.

There is a maximum amount of energy that the electrons could carry away from the decay. But some of the energy would go into neutrinos if they are created. That means the energy spectrum of the electrons should be spread out over a wide range depending on just how much energy the neutrinos take with them.

If there is neutrinoless double beta decay, all of the energy goes into the electrons and that should form a spike of detections at the maximum energy. Physicists are confident that their equipment will be sensitive enough to resolve whether that spike exists.

If it does, physicists will know definitively that the neutrino is its own antiparticle. If the experiment has been running long enough and no such spike appears, physicists can be quite confident that the neutrino and the antineutrino are different particles. The longer the experiment runs, the more confident they will be.

Either way, with the system working and a little time, it should provide a key piece of the puzzle surrounding neutrinos, perhaps the least understood particles physicists have ever detected.

David Harris

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The stuff you can do with accelerators

May 2, 2009 | 5:59 pm

There are 15,000-17,000 particle accelerators in the world. Yes, those numbers are correct! So where are they all?

A summary table in a document from the International Linear Collider collaboration discussing technology benefits of accelerators (PDF) includes the following counts of accelerators for various uses:

>7500 radiotherapy
>7000 ion implanters, surface modification
>1500 industrial processing and research
~1000 research including biomedical research
~200 medical radioisotope production
~120 high-energy accelerators (>1 GeV)
~50 synchrotron radiation sources

In a presentation at the American Physical Society meeting in Denver, Colorado, Murray Gibson from Argonne National Laboratory gave a quick overview of the broad range of applications for just a small subset of these accelerators: the synchrotron radiation sources and neutron sources.

He concentrated on showing the interplay between X-rays, electrons, and neutrons as probes of the very small, showing how they each allow complementary views of atoms, molecules, and materials. They each offer different levels of penetration into materials and views on different size and time scales.

Gibson gave a whirlwind tour of applications of the X-ray synchrotron source at Argonne National Laboratory, the Advanced Photon Source. Similar applications are explored at synchrotrons at SLAC National Accelerator Laboratory, Brookhaven National Laboratory, and Lawrence Berkeley National Laboratory, and three others in the United States and dozens of other synchrotrons around the world.

Some of the synchrotron lightsource examples Gibson mentioned are:

Energy applications
- Better burning: ANL is trying to watch in real time the combustion of fuel to improve the efficiency of engines.
- Artificial solar cells: by understanding how photosynthesis and other natural processes work, new materials that convert solar energy into electrical energy might be developed.

Climate research
- Carbon capture: by understanding how sea animals capture carbon, sequestration techniques might be better developed.
- Free radicals: understanding atmospheric chemistry will help remove damaging free-radicals from the atmosphere.

Biology
- Drug development: companies use synchrotron beamlines to develop better structures for new medications.
- Protein structures: more than 1000 protein structures are solved each year just at the Advanced Photon Source.
- Diseases: structural and dynamical information contributes to understanding the mechanisms of diseases and conditions ranging across viruses that attack cancer, autism, dental conditions, and obesity.

Infrastructure
- Metal fatigue: understanding how metals fatigue on a molecular level would allow engineers to prevent events such as bridge collapses.
- Oxide scales: knowing more about the structure of oxides in metals could save the US hydrogen industry vast amounts of money each year.

This list just touches on a few areas explored at these facilities, but shows the range of applications for accelerator-driven X-ray lightsources.

Neutron sources are complementary to X-ray sources in many cases. They typically start with a proton accelerator which bombards a target that then emits neutrons. (The process is called neutron spallation.) That beam of neutrons is used as the probe for looking into larger thicknesses of material, and is particularly good for examining magnetic materials, as neutrons are sensitive to magnetic effects where X-rays are not.

Studies with neutrons cover almost as broad a range of topics as X-ray studies. Here are some of them:
- Hydrogen storage and fuel cells, with use of deuterium to identify structures
- Exploring the possibility of welding instead of rivets in airplane construction
- Finding the positions of oxygen atoms in the structure of high-temperature superconductors
- Finding precise positions of hydrogen atoms in protein structures, including some protein dynamics
- Understanding how industrial palladium catalysts are “poisoned” by adsorbates over time
- Understanding high-pressure environments like the Earth’s interior
- Developing higher-density digital storage, including spintronics

Imaging structures and dynamics with X-rays, neutrons, and electrons touches on nearly every area of science. The facilities, all driven by accelerators, are allowing scientists to understand matter and materials on a molecular level in a way never before possible.

David Harris

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Making 3D images of the proton

May 2, 2009 | 2:03 pm

The proton is a surprisingly complicated object. Far from the two up quarks and one down quark you might have heard make up the proton, it is actually a seething sea of quark pairs and gluons that surround the “bare” up and down quarks. In fact, 99 percent of the mass of the proton comes from the “sea”. Only one percent comes from the bare quarks.

Kent Paschke of the University of Virginia, speaking at the American Physical Society meeting in Denver, Colorado called it an “exciting QCD vacuum bubbling with quark-antiquark pairs.” Quantum Chromodynamics, or QCD, is the theory of the strong force, which governs the interactions of quarks and gluons.

So what does the proton really look like and how can physicists actually observe what happens on the inside? The answer relies on using high-energy particle accelerators, some tricky investigative work, and a lot of data.

The basic technique for looking inside a proton is electron scattering. An accelerator boosts electrons to high energies, such as the 6 GeV energies achieved by the CEBAF experiment at Thomas Jefferson National Accelerator Facility, soon to be upgraded to 11 GeV. Those electrons are slammed into a target atom, and the remnant particles are observed, measured, and analyzed.

In practice, the data that comes out is a series of number that contribute to the “nuclear form factors”, which physicists tend to look at as lines on a graph. But Paschke said, “It’s the closest thing we can do to take a picture of a subatomic object.”

The guts of a proton

The picture that can be pieced together from this data is quite rich. It reveals that the quarks and gluons aren’t evenly distributed through the proton, have some orbital motion within the proton, and even reveal the contributions of more than just up and down quarks-strange quarks play an important role in the structure of the proton.

One of the fairly recent innovations in experimental nuclear physics is the use of polarized electron beams and polarization detectors. In this setup, the electrons have their spins preferentially aligned in a certain direction. Because the spin is a magnetic property of an electron, it can be used to tease apart the differences between the electric and magnetic properties of the proton.

Somewhat surprisingly, the electric and magnetic properties of the proton are significantly different. Paschke commented that this has led to a “rethinking of the nucleon wavefunction”, the complete 3D quantum description of the proton.

A set of measurements from particle physics and nuclear physics labs around the world have shown that there seems to be orbital motion of the proton’s parts. This orbital angular momentum is also critical in describing contributions of other quarks to the proton. Pairs of strange and antistrange quarks popping up temporarily in the “sea” might not survive long but they can leave a mark.

Strange effects in the proton sea

Researchers can fire electrons at a proton and see them collide with the up and down quarks, known as the valence quarks, sitting inside. But how can experimenters observe the sea? There is a lot of it but it is so ephemeral that it is difficult track down.

One property of the sea saves the day, however. The sea is dominated by up, down, and strange quarks because they are the lightest quarks, although other particles can pop out of the quantum vacuum temporarily. So measuring strangeness is a way to really get at measuring the sea’s behavior, independent of the up and down valence quarks.

This pushes the investigation one step back to figuring out how to observe strangeness inside a proton. The trick here is not to use the strong force, but to take advantage of a feature of the weak force-parity violation-in which particles can display a handedness, and the right hand behaves subtly differently to the left.

Electrons also see the weak force, so when an electron is fired at a proton the electron can interact on occasion with strange quarks in the sea through processes involving a Z boson. This interaction happens in something like one in each million interactions. It’s a small effect but enough to be measured.

Trying to map the strangeness in the proton has become a major goal of nuclear physics research because, Paschke said, “evidence of certain types of strange contributions to the proton would be the first unambiguous failure of the naïve quark model.”

At the moment, measurements are consistent with there being no critical contribution of strangeness to the quark, but the precision of the experiments is increasing fast, based on experiments at places including Jefferson Lab, the MIT Bates Linear Accelerator Laboratory, and at the University of Mainz, Germany. JLab’s G0 experiment expects to have new data on this topic in the next few weeks.

Putting protons (and neutrons) together

The same kinds of physics that is key to understanding the structure of the proton comes into play when examining how protons and neutrons combine to form atomic nuclei. The long-time textbook model of nuclear structure involves nucleons (either protons or neutrons) filling concentric shells, just like electrons form shells around a nucleus in an atom, with full shells corresponding to the most stable nuclei. The basic shell model, however, assumes that the nucleons are essentially independent.

When scientists examine the distribution of protons inside a nucleus, they discover that they only make up about 60-70 percent of the amount predicted by the basic shell models, and not in the right places. Since this was discovered, physicists have guessed that the difference between the models comes from interactions between the protons and neutrons. Those interactions are fiendishly difficult to measure.

A clever technique to observe those interactions involves firing electrons at a nucleus and looking for protons that are kicked out with too much energy to have come from the incoming electron. If a proton were to interact with another nucleon as the electron slams into them, the exiting proton could steal some of the other nucleon’s energy to create these “fast” protons.

When the experiment is formed, physicists observe protons that come out at a stepped set of energies corresponding to the electron interacting with a single proton, but also with either two or three nucleons.

This is very exciting to nuclear physicists because, as Paschke said, “these long-sought clear experimental signatures open the door to detailed testing of nuclear models.”

The observation of these short-range interactions inside the nucleus go a long way to closing the gap between experiments and the naïve shell model, but not all the way. Paschke said he expects that the remaining difference comes from some longer-range interactions among the nucleons and more effects from multiple nucleons interacting with each other.

Nuclear physicists are making great progress in cracking open the proton and mapping its insides. As they gain a lever point in the proton, they are planning to use protons as systems to study as much as they can about the strong force, in a way complementary to particle physics experiments. For example, they hope to look for glueballs-collections of gluons that don’t have any quark contributions, and which might reside temporarily inside protons or nuclei. They see this as the purest type of system to study the strong force.

Far from the up+up+down quark model that students have seen for decades, the proton is revealing itself to not only be a complex creature with many more mysteries to reveal, but also a testing ground for understanding one of the fundamental forces of nature.

David Harris

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Pulsars or dark matter might be the source of high-energy cosmic electrons

May 2, 2009 | 10:00 am

Something in our galactic neighborhood seems to be producing large numbers of high-energy electrons, according to new data gathered by the Fermi Gamma-ray Space Telescope. The electrons could be coming from nearby pulsars-or they could be a longed-for signal of dark matter, the elusive, invisible material thought to make up nearly a quarter of the universe.

FGST’s Large Area Telescope, a collaboration between NASA, the US Department of Energy, and multiple international partners, has been scanning the skies for gamma rays and particles since its launch last summer. The LAT, which was assembled at the SLAC National Accelerator Laboratory in Menlo Park, California, measured a strikingly high number of electrons with energies between 100 billion and one trillion electronvolts­. It is not known from the LAT data alone if these electrons are coming from the distant background, or are the signal of a nearby source of high-energy particles.

“If these particles were emitted far away, they’d have lost a lot of their energy by the time they reached us,” said LAT collaborator Luca Baldini of the Istituto Nazionale di Fisica Nucleare in Pisa, Italy.

When combined with other recent results, the LAT finding provides compelling evidence that something close by is churning out high-energy particles. The European satellite PAMELA, for example, last fall reported detecting surprisingly large quantities of high-energy positrons, the antimatter counterparts of electrons.

“Between the PAMELA results and our results, it’s very hard to construct a conventional galactic cosmic-ray model” explaining these particle energies, said Elliott Bloom, a SLAC physicist who works on the LAT project. “You need relatively local sources of positrons and electrons.”

These local sources could be pulsars, rapidly rotating neutron stars that emit intense electromagnetic radiation, positrons, and electrons. Alternatively, they could be bits of dark matter annihilating when they crash into each other or decaying because they are unstable. Such annihilations and decays also release high-energy particles, theorists think.

Physicists infer the existence of dark matter-which doesn’t interact with any of the electromagnetic forces, making it invisible to our eyes and our instruments-from its gravitational effects on light and “normal matter” such as stars, planets and interstellar gas. Though studies suggest that dark matter is more than five times as abundant as normal matter, nobody has yet directly measured the strange material or characterized its nature. The LAT team isn’t claiming they have detected dark matter.

“Occam’s Razor says pulsars are the most prosaic, and therefore perhaps most likely, explanation,” Bloom said. “But dark matter is also a possibility. This is particle astrophysics at its most exciting, trying to track down what’s going on here.”

A few other projects have recently mapped the spread of electron energies in space. One, the ATIC collaboration, found an even larger number of high-energy electrons than LAT did. However, the balloon-based ATIC must deal with atmospheric interference, which the orbiting LAT doesn’t have to worry about. And LAT is a remarkably precise instrument.

“This measurement provides the definitive determination of the spectrum of electrons outside of Earth’s atmosphere,” said SLAC physicist Greg Madejski, another LAT team member. Without such an accurate spectrum, suppositions about pulsars, dark matter or any other source of high-energy particles are on shaky ground. The current measurement allows the Fermi LAT team to constrain astrophysical models but the young mission needs to collect further data to say definitively whether or not there is a signal due to dark matter.

The LAT measurements, presented in the opening plenary session at the American Physical Society meeting in Denver, Colorado on May 2 and published in the journal Physical Review Letters, are difficult to make. Luca Latronica of INFN, Pisa, Alex Moiseev of NASA’s Goddard Space Flight Center, and Stefano Profumo of the University of California, Santa Cruz, will present further details of the results and their interpretation on behalf of the Fermi LAT collaboration at the APS meeting on Monday, May 4.

For each electron that hits LAT’s detectors, 50 to 100 other charged particles, mainly protons, come through as well. “It’s like finding a needle in a haystack,” Baldini said. “It requires a lot of simulations, a lot of cross-checking and a lot of study about how electrons behave in the detector.”

The LAT team is currently trying to pin down where exactly the electrons are coming from. The possibilities are that some electrons are coming from local sources, such as pulsars, supernova remnants, or from dark matter particle annihilations. They’re hoping to correlate any significant departures from the background with positions of known pulsars. And the LAT team is extending measurements even further, to energies of a few trillion electronvolts, according to collaborator Igor Moskalenko of Stanford University.

“What we will see at higher energies can only come from local sources,” he said. “If there are cosmic ray sources nearby, we may be able to find them.”

By Mike Wall

Update: The scientific paper has now been published. You can get to it free by going to the viewpoint in APS’ online publication Physics and clicking through free. If you go direct to the paper, you will need to be a subscriber to access it.

Symmetry Intern

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