GLASTcasts: Whet your appetite for tomorrow's launch

June 10, 2008 | 12:49 pm

After several delays, it looks like the Gamma-ray Large Area Space Telescope will finally launch tomorrow morning between 8:45 and 10:45 a.m. PDT. You can watch the launch on NASA TV via streaming video; pre-launch coverage starts at 6:45 a.m. It’s a time of elation and high anxiety for hundreds of people who have worked on the spacecraft, which will explore the last remaining gap in the spectrum of light reaching us from space–the highest-energy rays created by the most violent processes we know about, from supermassive black holes to pulsars.

The mission is a joint project of NASA and the Department of Energy, and Stanford Linear Accelerator Center, a DOE lab operated by Stanford University, was in charge of developing its main instrument, the Large Area Telescope. Housed in a cube 1.8 meters on a side–the combined width of 15 CD cases–it’s essentially a particle detector whose 880,000 silicon strips lie in wait to capture high-energy gamma rays. Laboratories around the world crafted parts of the telescope and shipped them for assembly at SLAC, which will also process the data from the telescope.

For a taste of what the mission has in store and the excitement surrounding the launch, check out these brief videos–GLASTcasts, NASA calls them. SLAC has more info here, and NASA’s site is here.

Glennda Chui

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Extreme particle acceleration with lasers (APS April 2008)

April 17, 2008 | 3:47 am

The APS April meeting is over but I still have a few more stories to post and I’ll get them up over the next week.

The leading edge of particle acceleration technology in use today is made of superconducting cavities that shape radiofrequency waves for particles to surf to higher energies. However, that technology is limited because the cavities themselves, made of superconducting niobium, have a maximum strength of rf waves that they can sustain.

Machines are now enormous as they strive for the energy frontier that physicists want to explore. There is, however, a limit to the size of big machines, particularly for financial and space reasons. That size limit is the motivation for exploring the alternative techniques being tested now. One of these is plasma wakefield acceleration. It’s a promising approach and each year there are great new advances, some of which were reported here today. A topic we haven’t really talked about in symmetry is an alternative wakefield approach using lasers.

Wim Leemans of Lawrence Berkeley National Laboratory talked about a project called BELLA that could potentially match the energy of the proposed 20 kilometer International Linear Collider in a few hundred meters instead. The technology is years away from working so that comparison is really just to give an idea of how much greater the acceleration is than conventional superconducting technology. However, if laser wakefield (or plasma wakefield) acceleration is successful, it would open the way to creating compact particle accelerators for medical applications and other uses.

The idea is to create a narrow channel in sapphire with a low density gas inside. A laser is piped down the channel and pushes a bunch of electrons, getting them to high energies. So far, the technique has been demonstrated to work to do some acceleration but hasn’t achieved the really high energies needed to be competitive with energy frontier machines like the Tevatron (for now) and the LHC (later this year).

It’s exciting stuff and also necessary if particle accelerators are to increase the energy frontier much beyond the LHC and ILC targets.

See all posts from the American Physical Society April 2008 conference here.

David Harris

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The hunt for rare materials (APS April 2008)

April 15, 2008 | 9:38 am

Before you build an experiment to find a rare particle, you need to find the rare material to make the detectors! The hunt for these materials is getting more challenging every year as the needs of science experiments, high-tech industries, and other disciplines turn what used to be everyday stuff into prized commodities.

I have been coming across a lot of tales at the APS conference about the difficulty of procuring the right materials for science experiments and have asked around on some of the topics. The common aspect to most of the searches for materials is the desire to obtain stuff that isn’t contaminated by or with some form of radiation. Combine that with the increasing demand by industry for many elements that were previously used primarily by science, and the entire market is changing, and causing scientists to get more creative about finding new supplies.

Xenon in demand

Some of the promising dark matter searches and neutrino experiments use noble gases as the detection medium. Dark matter experiments have had great success with xenon, but it is quite expensive, and now xenon is in huge demand from other places so the price has risen rapidly. It turns out that some semiconductor manufacturers have discovered that creating their chips in an atmosphere of xenon can be quite beneficial. The semiconductor industry is so big that it can suck up the world’s production of xenon pretty quickly, thereby inflating the prices. When a particle physics experiment needs 10kg, 100kg, perhaps a tonne of xenon, it can quickly get beyond the budget of science experiments.

Radioactive argon

So why not turn to a cheaper noble gas like argon? Argon is incredibly cheap. But argon from the atmosphere (the most plentiful supply) has a lot of argon-39, which is radioactive, due to the effects of cosmic rays, a big problem for experiments that rely on cutting down background sources of radiation so that they know any signals they are seeing come from the things they are looking for, like dark matter particles or neutrinos. Purifying argon to remove the radioactive isotope is extremely difficult and so it is expensive. Why not look for argon somewhere the cosmic rays don’t reach, like natural gas from underground, where it is also plentiful? Well, you can get argon there but it has been contaminated by radioactive elements in the Earth. So either way, you lose out. Argon is cheap and plentiful but it becomes as expensive as xenon when you have to purify it.

Searching for sunken lead

Even if you have a material that has low radioactivity, you still want to shield it from other external sources of radiation like cosmic rays, radon underground, or even the natural radioactivity of humans nearby, which is some cases can be enough to cause a measurable effect. Lead is a great shield for some forms of radiation but the lead from mines is contaminated with uranium and thorium. The contamination takes the form of lead-210. It has a half-life of 22 years so the radioactive component will die away but it takes time. Centuries-old lead, however, is perfect. But where to find it?

Some of the preferred options for purely lack-of-radioactivity reasons: ancient Roman ruins, sunken ships, and the lead from old stained-glass windows in churches. The lead in all of these is perfect to use, but it obviously incurs a significant cultural cost. Fortunately, some of the sunken ships contain ingots of Roman lead, so using it doesn’t cause any harm to anybody and cultural authorities in the past have given permission to use those ingots. The supply of this lead is limited so we can expect to see a second-hand trade in lead among scientific experiments as the found supply dwindles.

Demand for helium rising

Back to noble gases, helium is also in short supply. Despite being the second-most abundant element in the universe, there is not enough to go around to let kids inflate as many balloons as they want. Prices for balloon gas have spiked and many stores no longer stock it. The world can probably live without helium balloons but will they be prepared to say no more MRIs? When it comes to healthcare, people definitely want to have testing equipment available. MRIs have superconducting magnets inside them and they need to be cooled with liquid helium. The number of MRI machines and demand for their use is increasing, partly due to some ill-advised promotions by companies for full-body scans. Some physicists are looking at ways of creating MRI machines that don’t use liquid helium as the coolant for the magnets, so there is a promising way out of that bind.

Helium is used in many other places also, especially in the electronics industry. It is used to make flat-panel TVs and computer monitors, and in the production of computer chips and optical fibers. In total, demand for helium is up 80% in the past 20 years. Prices for helium have gone up 15-30% each year in recent years.

The list of materials in demand goes on; it seems that the resource struggle the whole world is facing has an impact on basic science research. Fortunately, the ingenuity of scientists generally finds a way around the problem, but this is one more challenge in the conduct of an experiment.

See all posts from the American Physical Society April 2008 conference here.

David Harris

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What can we expect from the LHC? (APS April 2008)

April 14, 2008 | 1:37 pm

In a press conference this morning, Abe Seiden of the University of California, Santa Cruz, showed a great timeline that plots the amount of data to be collected at the Large Hadron Collider against time, and then pointed out where physicists expect to make certain discoveries if nature has those discoveries waiting to be made. See the graphic below.

In summary, here are the potential milestones with my comments on each:

2009: Supersymmetry–if the appropriate energy scale is 1TeV

2009/2010: Higgs particle–if it is around 200 GeV in mass.

2010/2011: Higgs particle–if it is around 120 GeV in mass. (The lower energy is harder to see because at that energy, it would decay with the key signature involving photons. However, other decays also have similar photons so you need better statistics to tell the difference. A Higgs at higher energy would probably decay primarily into W bosons, with very obvious characteristic jets of particles coming out of the collision.)

2012: Extra dimensions of space–if the energy scale is 9 TeV

2012: Compositeness–if quarks are actually composite particles instead of being fundamental and that composite nature reveals itself on an energy scale of 40 TeV.

2017: Supersymmetry–if the appropriate energy scale is 3 TeV.

2019: Z‘–if there is a new type of force that comes into play around the 6 TeV energy scale. If it does, the particle that communicates the force is represented by the temporary name Z‘ in analogy with the Z that transmits the weak force.

This timeline is of course dependent on the LHC starting up according to the current plan. The director-general of CERN recently made a statement that said CERN plans to have the LHC cooled down by mid-June with first beam injection two months later. The world is waiting excitedly!

See all posts from the American Physical Society April 2008 conference here.

David Harris

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The extreme deficit of physics undergraduates (APS April 2008)

April 13, 2008 | 4:00 pm

I’ve just been to a session about undergraduate education in physics. Ted Hodapp from the APS talked about an APS/AAPT statement that calls for a doubling of the number of physics undergrad students. (I can’t find the formal statement online–I only read it on a screen in the presentation.)

Hodapp presented a string of evidence that shows just how serious the dearth of physics undergraduates is. Because there was so much information, I’ll just post a couple of dot points about it.

• The nuclear power industry will soon be suffering a shortage of qualified physicists to work for them. About 33 new power plants have been approved in the United States and will be starting up from 2010. That industry needs people with good science/math/problem solving abilities and physics graduates are an obvious choice.
• The medical physics industry employs about 3200 physicists, and have about 300 new jobs each year more than the current capacity for people with undergrad physics degrees. 78% of those people work in radiation oncology, and 16% in medical imaging.
• The growth of occupations requiring science and engineering undergraduate degrees has much higher growth than the civilian labor force but S&E enrollments are not growing anywhere near that fast.
• School principals rated physics and maths teachers about the hardest to recruit along with special needs teachers, primarily due to a shortage of qualified people.
• Math and computer science have about 70,000 undergraduate degrees granted each year, life science about 260,000. Physics has a mere 5000.
• Unemployment for physics graduates is very low, and for physics PhDs is an all-time low of 2.5%
• There is a need for US citizens with advanced physics degrees to work in classified areas. Hodapp says that Cherry Murray called the lack of US citizens with advanced degrees as “a national crisis.”
• The Rising Above the Gathering Storm report, the America COMPETES act, and the Tapping America’s Potential (PDF) report all call for large increases in science, technology, engineering, and mathematics graduates.

All in all, it paints a picture of a serious undersupply of physics graduates for teaching, technical, and other skilled roles in US society, and the promise of a good job market in future years.

See all posts from the American Physical Society April 2008 conference here.

David Harris

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The charming case of X(3872) (APS April 2008)

April 13, 2008 | 1:28 pm

Most of the media attention in particle physics goes to the high-energy frontier. The big news stories tend to be discovery of new particles like the top quark and the likely discovery of the Higgs at the Large Hadron Collider.

However, there is a lot of lower-energy particle physics research going on and the whole field is getting very interesting. At the APS meeting this morning, Eric Swanson of the University of Pittsburgh gave a nice summary of what he calls “the new charmonia.” These are particles observed in collider experiments that have an energy suggesting they are contain charm and anti-charm quarks.

Swanson showed a table of particles that have been observed where he rated a dozen particles in terms of robustness of results and of interest to him. Sitting in the corner of extremely robust and very high interest is the particle called the X(3872). The 3872 refers to the energy of the particle which is measured as 3871.2 MeV and the X refers to some other technical properties of the particle but also indicates that it isn’t yet well understood.

The X(3872) is established as a real signal without any reasonable doubt. But what is it? The energy of the particle and the ways in which it decays suggest a few possibilities. It could be a tetraquark: a particle consisting of four quarks. It could also be a signal that doesn’t indicate an actual stable particle as such but the energy at which certain particle processes take place. Perhaps the most interesting case, and the one that Swanson favors is that this is a “molecule” of D⁰ and anti-D⁰ mesons. (A D⁰ is a combination of a charm and an anti-up quark.) These two particles would be bound together just like two hydrogen atoms can bind together to make a gaseous hydrogen molecule.

If that is true it would be first known case of this kind of matter. It opens up a whole host of interesting ideas about the chemistry of subatomic particles instead of a chemistry of atoms. It also really pushes physicists into a deeper understanding of their theories of quarks and the strong force that operates between them. The idea of particles being “merely” constituted of quarks is known to be an approximation that works at low energies, so there is bound to be a lot new to learn about how this matter behaves.

Swanson provocatively asked, “The constituent quark model must fail somewhere. Have we seen it?”

See all posts from the American Physical Society April 2008 conference here.

David Harris

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Pioneer spacecraft a step closer to being boring (APS April 2008)

April 13, 2008 | 9:51 am

For many years, scientists have known that the Pioneer spacecraft have not been exactly where they thought they should be. Each year the spacecraft falls behind where it should be by about 5000 km. The spacecraft seem to have been undergoing a very small acceleration toward the Sun and, so far, scientists haven’t been to explain it.

Explanations for this have ranged from the prosaic (heat is being radiated into space and providing an acceleration) to the speculative (gravity might not act the way we expect). One thing is certain. The debate about the cause of the Pioneer anomaly, as it is known, has been raging for years.

Newly released telemetry data, incorporating over 100 measured properties including the temperatures of many points on the spacecraft, have been released. At the APS April meeting in St. Louis, Slava Turyshev from the Jet Propulsion Laboratory described his group’s efforts to build a very detailed computer model of the spacecraft geometry and heat flow, and showed the comparison of the model to the new data.

Their model manages to match the measured temperatures of Pioneer to within 3 degrees Celsius at every measured point, which Turyshev seemed extremely pleased with. Having a good thermal model meant that the scientists could start to really ask whether thermal effects could account for the anomaly.

Indeed, when Turyshev’s team calculated the emissions from the Pioneer spacecraft, it found that heat is given off in some directions preferentially, enough to account for 28-36% of the anomalous acceleration.

So what does this result now mean? It weakens some previous claims that the thermal emissions weren’t significant, an argument that not many physicists really believed anyway. However, there is still two thirds of the anomaly to account for? Could there be mysterious physics hiding in the gaps? Turyshev thinks that there is a lot more to take into account such as whether the optical properties of the spacecraft have changed over time-perhaps there is a layer of dust on some surfaces now, for example.

In May, new data about the speed of the spacecraft will be released and that could further clarify the situation, or just add to the debate!

I find the fact that this argument has received so much attention quite amusing. After all, nobody is going to really believe that the laws of physics are different, based on interpretation of Pioneer’s flight. And the immense amounts of work that have to go into trying to model the system properly is quite incredible. Scientists need to dig out information from decades ago to try to get everything they need and there are a lot of uncertainties. Turyshev quipped, “It’s like being on CSI.”

The exercise is certainly improving scientists modeling skills, which could then be used for much more practical purposes like building structures or vehicles on earth. It could even be quite useful in future space missions, although the problem will always be much easier in the future as the engineers will have better data about any spacecraft they send up.

Perhaps the story just reflects human’s unending fascination with the exploration of space and a desire to be part of that exploration, in whatever form it can take.

See all posts from the American Physical Society April 2008 conference here.

Note: This post has been edited since it was first posted to correct the distance Pioneer is falling behind each year.

David Harris

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Can the Tevatron find the Higgs? (APS April 2008)

April 12, 2008 | 8:27 pm

In the past year, there has been a lot of discussion about whether the Tevatron collider at Fermilab can find the Higgs boson before the Large Hadron Collider can. There have been all kinds of claims, and even stories of hints of sightings. In a session today, Brian Winer from Ohio State University gave a very clear presentation about just how close the Tevatron is to potentially finding the Higgs.

Cutting to the chase: Can the Tevatron find the Higgs? Yes, if Nature is kind.

But first let me set the scene.

What do we know about the Higgs now?

The Standard Model of particle physics predicts that a Higgs boson is most likely to have a mass of 87 GeV. (Physicists like to express masses in terms of energies. If you want to convert to an actual mass number, divide the energy by the speed of light squared.) The nature of this prediction is that the Higgs won’t necessarily have that mass but it is likely to have it somewhere in a range centered on that number. There is roughly a 2/3 chance that the Higgs will have mass between 60 GeV and 123 GeV.

Experiments at the Large Electron Position collider at CERN showed that the Higgs does not exist at any mass lower than 114 GeV however, and the theory predicts that the mass will be less than 160 GeV with a 95% probability. So physicists are really trying to look in this region of 114 to 160 GeV.

How sensitive do experiments need to be to find the Higgs?

Experiments have different sensitivities to the Higgs depending on its mass. Physicists can estimate how close they are to observing the Higgs by comparing their measurements to theoretical predictions.

At the moment, the performance of experiments is expressed as a factor of how much better the measurements need to be to start to have a chance of being sure that any Higgs-like signal is really a Higgs boson. It is a game of statistics and it gets pretty complicated so I’ll just take the simplest possible path through this, warning that the full story is much more detailed.

The biggest problem to start with is that what a collider detector measures is a set of particles, none of which is specifically the particle you are looking for. The Higgs decays into certain sets of particles but other decays look very similar. So when your detector sees a set of particles, you need to make extra sure that you are looking at what you think you are looking at! You can predict how many of certain types of known particles you should see and then look for any excess if you are hunting a Higgs or some other unknown particle.

This “background” of decays from known particles must all be excluded from analysis before you can see the Higgs. The big problem is that the backgrounds are a factor of 100 billion times larger than the signal from the Higgs! Higgs would make up only tens of events out of the trillions measured.

Fortunately, detectors are very good at finding the presence of b and anti-b quarks (they create characteristic jets of particle from the collision). Higgs will tend to decay into b and anti-b quarks along with other particles so the bs can be used to identify most of the collisions you are interested in. In fact, this “b-tagging” can reduce the background by a factor of one billion.

But as Winer put it today, “the first factor of a billion is easy, the last factor of 100 is hard.”

How low can you go?

In trying to get this factor of 100 down to a factor of 1, a lot depends on the energy the Higgs has. If the Higgs has an energy at the bottom end of the range, about 115 GeV, and then you combine all the data from both the CDF and DZero experiments, you end up about a factor of 5 short of where you need to be to have a chance at the Higgs. But given that the Tevatron is hoping to collect 4-8 times more data than was used in this analysis, it potentially comes in range though it would still be a real stretch.

If the Higgs has a mass of 160 GeV, then the combined data already brings us to a factor of a mere 1.1 times where we need to be. In other words, a 10% improvement and “Game on!”

Of course, it is not as simple as collecting 10% more data. That would be too easy. That level is really the starting point for when your detectors and dataset have the statistical power to resolve Higgs hiding in the flood of collisions.

But if the Higgs really sits around that mass, then the Tevatron has a genuine chance of finding it soon. Winer said today, “As early as summer, we can start excluding masses around 160 GeV.” In other words, the data will be sensitive enough to say either that there is definitely no Higgs at that mass, or it will see signals that look like the Higgs and it will just be a matter of some more data to determine if they really are Higgs bosons.

Is this the Higgs?

And just to tease you a little more, this event could even be a Higgs.

It is event 6577 from run 196170 and has all the characteristics of a Higgs. It was found on April 5, 2005, at 8:09 a.m. Unfortunately, there is just no way to tell if it is truly a Higgs because the statistics don’t allow anybody to prove what this is. But there is some change that you are looking at the first identified Higgs boson!

The possibility of finding the Higgs at the Tevatron is real and the race is really on.

See all posts from the American Physical Society April 2008 conference here.

David Harris

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Dark matter discovered? (APS April 2008)

April 12, 2008 | 5:01 pm

Has dark matter been discovered? Rumors floating about suggest that the DAMA-LIBRA collaboration will announce that they have detected dark matter after repetition of their controversial experiment which released results in 2000.

The DArk MAtter (DAMA) experiment, situated in an underground laboratory in Gran Sasso, Italy, will announce the results of a new experiment using the same technology that led to the 2000 result. The collaboration has been very quiet for the past few years but will break silence on Wednesday, April 16, at the NO-VE neutrino oscillation workshop in Venice, Italy. Sources here at the APS meeting tell me that DAMA plans to claim they have again seen a signal for dark matter detection. People here don’t have more details at this time.

The DAMA result was controversial because it relies on looking for a small variation in a signal in a sodium iodide detector which has a lot of background noise. Other attempts since to see dark matter have not found anything with the properties DAMA claimed. Might the signal have been due to a systematic error? To see the signal, the detector needs to run for at least a year and look for variation over the year due to the motion of the Earth through the cosmic dark matter background. The quantity of data measured in this new experiment is as much as what led to the original claim of dark matter observation in 2000.

Whatever the DAMA-LIBRA collaboration says, physicists will not be entirely convinced of any claim for detection of dark matter until it is repeated in other types of experiments. However, prepare for another media storm if the announcement is indeed what the rumors are saying. I am certain the collaboration will be much more prepared to answer their challengers and will have a much tighter argument ready if they do indeed try to claim a result.

One of the reasons I’m telling this story is that it highlights another aspect of the value of conferences for physicists. They hear about all kinds of things that are going through the strength of personal connections instead of just having to wait for official publications. A lot of work gets done over a beer in the evenings, and a lot of news spreads through casual conversations.

See all posts from the American Physical Society April 2008 conference here.

David Harris

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The cosmic quantum bounce (APS April 2008)

April 12, 2008 | 2:57 pm

Was the big bang the beginning? There have been plenty of theories put forth that suggest there was something before the big bang, like the cyclic universe and ekpyrotic universe models. They all tend to involve some exotic physics like the collision of branes (multidimensional sheets of spacetime intersecting, nothing to do with zombies) or other ideas that would take a book rather than a short piece here to explain.

This morning I heard more about an interesting addition to the list of pre-big bang models that involves what the speaker Abhay Ashtekar, of Penn State University, called a “quantum bounce.” Ashtekar described some work in loop quantum gravity, one of the more promising alternatives to string theory as a possible theory of quantum gravity. The idea has been floating around for a while but he has made considerable progress in developing the model.

When physicists studying the big bang talk, they tend to run the universe in reverse and they talk about the evolution of the universe going backward so you hear descriptions about the universe getting closer to the big bang, but it is coming from this end of time, not the beginning. It takes a minute to adjust your head to, but it starts to make sense eventually.

Ashtekar talked about how within loop quantum gravity, the universe can look like a normal universe governed by Einstein’s general theory of relativity until very close to the big bang, at which point the quantum aspects of loop quantum gravity take over. In general relativity everything would become what is called a singularity at the point of the big bang. A singularity is a place where the mathematics turns, to use a technical term, “stupid.” There are infinities to deal with and the laws of physics break down.

But within loop quantum gravity, no singularity appears. Without having to invent any new rules, the theory transitions from being like the classical laws into the quantum laws and there is essentially a patch of spacetime, ruled by the laws of quantum geometry, that joins it all together and prevents a singularity appearing.

However, if you push this to its conclusion, you find that you come out the other side of the quantum geometry region with a bounce–the quantum bounce Ashtekar talked about. Then the universe turns back into something like a classical universe. This process can repeat many times over with the universe repeatedly undergoing these cosmic quantum bounces. During the transition, the laws of loop quantum gravity are well-behaved and deterministic (not random).

Furthermore, within the classes of models Ashtekar and his coworkers have investigated, the quantum bounce seems to happen in every case. The research comes with a lot of other technical results that were very interesting but that I don’t understand well enough to talk about here! This is all early work and you can’t take it as a given that this is the way the universe works.

It is, however, intriguing to consider a universe where, as Ashtekar said, “Physics does not end at the big bang.” Nor does it begin there.

See all posts from the American Physical Society April 2008 conference here.

David Harris

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