There really is something out there, bending the light from distant galaxies:

Galaxies, and galaxy clusters, appear to be surrounded by clouds of something invisible which interacts gravitationally. It apparently also causes the rotation of galaxies to deviate from the simple prediction of Newton’s laws assuming that only the visible matter in galaxies is present.

This “dark matter” makes up nearly a quarter of the mass/energy density of the universe, whereas “light matter” (stars, interstellar dust and atoms) make up only a few percent.

Our prejudice is that dark matter has a particle nature. Theories of particle physics beyond the Standard Model these days offer many possibilities for dark matter candidate particles, many of which could be detected via their weak interactions with ordinary matter. Here we mean weak in the sense of interacting by exchanging a W or Z boson with the ordinary matter particles. The common thread among the particles proposed for dark matter is that they are weakly interacting and massive (of the order of hundreds of times the proton mass - one to several hundred GeV). Particles in this general class of dark matter candidates are called WIMPs.

If WIMPs are really there, there are only a few per cubic meter in our galaxy, and most fly straight through ordinary matter without interacting. But, if they can and do interact weakly, occaisionally they could transfer enough energy to a nucleus of ordinary matter that it would be detected, if our equipment is sensitive enough. Also, there is hope that WIMP dark matter particles could be created in the high energy collisions of the Tevatron at Fermilab or soon at the LHC at CERN, and detected indirectly by the fact that they carry away apparently missing energy.

Experimentally, now, in the area of direct detection there is a race on between two main competing technologies for observing the feeble signal of WIMP interactions. Until a few days ago, the most sensitive search for WIMPs was that of the XENON10 collaboration, using a detector of liquid and gaseous xenon as the target for the WIMPs. The detector is kept deep underground at the Gran Sasso Laboratory in Italy. But at the Dark Matter 08 meeting in Marina del Rey last week, the lead in this search was recaptured by the Cryogenic Dark Matter Search collaboration. They use solid crystalline silicon and germanium detectors, cooled to liquid helium temperatures, to sense the nuclear recoil from WIMP interactions, taking extraordinary measures (as all these experiments do) to avoid false signals from cosmic rays, natural radioactivity, and stray neutrons. They did a blind search, making carefully controlled predictions of the number of WIMP signal events they expected to see, and then “opened the box” earlier this month. They expected 0.6 background events, give or take about half an event. The result: nothing. No signal from WIMPs or backgorund or anything!

To “measure nothing” is usually a great experimental challenge. You do have to convince the world that you would have seen something if it had been there, that your apparatus isn’t just mute for some other reason. CDMS have done a great job convincing the world of this, I’d say, and their result is nearly a factor of three more constraining than the previous Xenon 10 result. They show it in this plot of WIMP interaction strength versus WIMP mass:

What the plot is saying is that assuming that all dark matter is a WIMP of a certain mass, there is less than a 10% chance that, if the spin-independent cross section had some value greater than that indicated by the heavy black line, they would have seen no events in the detector. This can be turned around to say that at the 90% confidence level if such WIMPs exist, they must have even feebler interaction strengths than that indicated by the heavy black line.

These exclusion limits are starting to cut deep into the theoretically favored regions indicated on the plot…and the fact that there really are zero events at this stage means that any sort of five sigma discovery could be a long way away.

In the mean time, there are larger xenon based experiments planned, including a larger version of the Gran Sasso one and the LUX experiment, to be based at the nascent deep underground facility DUSEL.

And, perhaps as early as later this year, the LHC experiments CMS and ATLAS will begin to get a glimpse of the results of protons “colliding in anger” as a colleague of mine likes to put it. The Tevatron has seen no hint of WIMP production/decay yet, and so with seven times more energy, the LHC maybe able to produce the heavy particles that decay down to lighter ones which may include WIMPs such as the neutralinos predicted by supersymmetry.

I muse from time to time on the possibility that dark matter may really only interact with “light matter” gravitationally. Perhaps it is composed of an entirely separate sector which does not interact with light matter. It may be several or many particles which interact amongst themselves, forming structures we can only speculate about. If our only probe of dark matter is gravity, there will be a very long road ahead to understand it. I hope the cosmologists among the readers here can offer us more hope that indeed, there are strong reasons to believe dark matter interacts weakly…

Pretty soon, on March 9, we’ll all change our clocks one hour forward to change from standard to daylight-savings time. An absolutely pure misnomer, daylight savings time is nevertheless, to my mind, the greatest success story of mass psychological control there ever has been. Just imagine if the government put out some sort of strongly worded encouragement that everyone needs to get up an hour earlier, starting Monday, and should continue to do so for the next eight months, so as to save energy and have a little more time in the evening when it’s light out. I imagine not many people would comply.

But, what they do instead is to say, “okay, starting early Sunday morning, it will suddenly be an hour later on your clock for the next eight months!” And, magically, just about everyone complies…it’s breathtaking, actually.

But what time is it really? This week, on February 29 we have a Leap Day, a once-every-four-years event. Actually, it’s not once every four years; we skip it every hundred years, except we don’t skip it every four hundred years. That is, 2000 was a leap year, but 1900, 1800, 1700 etc. were not. We’ve been doing this since 1582 when Pope Gregory introduced the new calendar to keep Easter from drifting, slowly but surely, away from the spring equinox, that magical moment when the earth’s axis makes an angle of 90 degrees to the line connecting the center of the sun with the center of the earth. On the spring equinox, the length of day and night are equal everywhere on the earth (at the poles the sun remains on the horizon all day). In the Gregorian calendar, the average calendar year is 365.2425 days, because this is the average number of days from one spring equinox to the next.

If you ask the average person on the street, though, just what “one year” means, though, they’ll most likely say “it’s the amount of time it takes the earth to go around the sun”. What they are probably thinking is that the imaginary line mentioned above from the earth to the sun sweeps out a full circle in one year; this is called a sidereal year: the time it takes for the sun to appear in the same place against the backdrop of the fixed stars. They’d be close, but no cigar: the earth’s axis, which is tilted at about 23.5 degrees to the plane of the earth’s orbit, is actually not fixed in its direction in space. The earth rotates on an axis which precesses, similar to that of a spinning top with one point fixed. It takes 26,000 years to go al the way around and come back roughly where it was. And so, in fact, the calendar year (also called the “tropical year”) is about 20 minutes shorter than the sidereal year. (You can easily calculate this yourself: it’s 1/26000 of a year!)

Our Gregorian calendar will keep the spring equinox quite close to March 21 or so, but eventually our familiar winter constellations like Orion will eventually become summer ones, and the North Star will appear to move in ever-widening circles about the celestial north pole.

If you dig further into all this you quickly see that the day, which I am sure you’ll find people to tell you is “24 hours” has a similarly ambiguous definition. The sidereal day, the time it takes the earth to spin once on its axis, is 23 hours, 56 min, 4.1 sec., just less than the solar day of 24 hours. Not surprisingly, after you think about it, this difference is close to 1/365 of a day, since as the earth goes around the sun, one full orbit is in effect a day. (If the earth were not rotating, then there would be one solar day per year.)

But is a day even exactly 24 hours? It turns out that for various reasons, the earth’s rotation speeds up and slows down over long periods. The second was redefined in 1967 to be the time it takes for 9,192,631,770 periods of the radiation corresponding to the transition between the two hyperfine levels of the ground state of the cesium 133 atom. This is the basis for our measurement of time in the SI system of units. As a result, we no longer define the second as 1/86400 of a solar day, and we need to add in a leap second every so often to account for the fact that the earth’s rotation is slowing by about 2.3 milliseconds per day per century, and we chose the year 1900 as the reference for the second. This means we are now accumulating one leap second every 430 days or so. We’ve added leap seconds at about that rate since the first one in 1972 (always at midnight on New Year’s Eve).

So now we have International Atomic Time (TAI) with no leap seconds, and Coordinated Universal Time (UTC) which have drifted apart by 33 seconds since 1972. We need both systems, since we want a simple way to calculate accurate time differences without having to take into account leap seconds, but we want noon to stay when the sun is high in the sky. The US Navy is on it, don’t worry.

Except for that one-hour daylight “savings” time…

Craig Barrett, the chairman of Intel Corporation, doesn’t mince words. In a scathing op-ed piece in the SF Chronicle, he describes the high-tech industry view of the dreadful state of funding for basic science research in 2008, due to the last-minute earmarking by the congressional appropriations committees.

“At a time when the rest of the world is increasing its emphasis on math and science education (the most recent international tests - NAEP and PISA - show U.S. kids to be below average) and increasing their budgets for basic engineering and physical science research, Congress is telling the world these areas are not important to our future.”

Mr. Barrett, thank you!

My mother-in-law, a fellow word-play lover, turned me on to the Eggcorn Database, so I thought I’d try to strum up support for what they’re doing to catalog the ongoing daily damage to (or enhancement of…you decide) the English language.

The database has nearly 600 instances of common expressions in English which, via a homonym or near-homonym substitute (in lame man’s terms, a word that sounds the same), result in really oddly apropos but totally novel (and hilarious) coinages.

For example, you might think you have free reign to use English how you see fit, but for that you’d need to be a monarch rather than simply riding on horseback. I am not trying to ferment trouble here. On the contrary, once you’ve mastered a few of these you might just pass the SAT with flying collars.

All tolled you wouldn’t think there are so many of these eggcorns around, and truth be told, for all intensive purposes you might not recognize all that many when you do come across them. You mine as well grin and bare it. (Or is it grim and bear it?) We all have to bare the brunt of this assault!

In the mist of all these eggcorns, all this pigeon English, where can we get any piece of mind? (Okay that last one is not in the database…yet.) As you pour over the New York Times, see if you can spot any (I found one on line the other day, and didn’t spill a drop).

Well I hope I’ve peaked your interest, or at least wetted your appetite. I’ll sieze and desist now. After all I am a ten year professor and should have better things to be doing, at least if I want to stay gamefully employed.

We started to get a naked-eye view of Comet 17P/Holmes last week, after it underwent a transformation in which it apparently ejected a huge cloud of gas, much of which is CN and C2, and NH2 molecules, and which are fluorescing. Unless you knew it, you would think it’s another star in the constellation Perseus, about half way between Cassiopeia and the Pleiades in the northeast sky in the evening. With no moon now and clear skies in Northern California, I got this shot with my little Canon Rebel on a tripod (with a 300 mm Tamron lens, 10 sec exposure at f/4.5):

Looking at it with binoculars, it it hard to resist the urge to try to focus it! But it’s just a big fuzzy ball…

The comet is on its way out of the inner solar system, having made perihelion last spring. The gas cloud has expanded to 70% of the diameter of the sun.

The Higgs boson..the “God Particle” (ick…sorry Leon). The Holy Grail of particle physics. (Um, also there’s that dark matter stuff but who knows what it might be…)

Where to begin? The Higgs boson is one thing, or many, there, or not, waiting to be discovered, if we can. But what is it? Why is it? Does it exist at all?

We know a lot, now, after decades of experiments at the great accelerators. Working backwards: the Tevatron at Fermilab, LEP 1 and 2 at CERN, KEK-B at KEK in Japan, PEP2 at SLAC, CESR at Cornell, the SLC at SLAC, the SppS at CERN, HERA at DESY, Tristan at KEK, PEP at SLAC, PETRA at DESY. But it’s all coming down, sooner or later, to the Big One: the LHC at CERN. This machine will, in all likelihood, answer the question: what is the origin of electroweak symmetry breaking? That is, why do the carriers of the weak nuclear force, the W and Z bosons, have mass (and large mass at that) and the photon does not have mass at all? If we are really lucky we may start to get an answer to the question: why do fundamental particles have masses at all, and the rather peculiar masses they do have? And even if we know all this, so what? What then?

That was a lot of jargon and acronyms for one paragraph…so we’ll start there. Firstly, the word “electroweak”. We believe that all of the particles we know about interact “electroweakly” in the sense that they all partake of the weak nuclear force and, if they have electric charge, interact electromagnetically as well. We believe that the electromagnetic and weak nuclear forces are manifestations of a single underlying force of nature. The word “electromagnetic” should start to give you the flavor of the enterprise. Electricity=magnetism has been with us for nearly 150 years, first unified in the form of the “classical” electromagnetic relations of James Clerk Maxwell in 1861. This feat gave humanity its first taste of victory over matter, energy, space, and time, and propelled us headlong into the modern age. Surely one could unify electromagnetism with the other obvious force of nature, gravity, the universal nature of which was established in the late 1600’s by Isaac Newton.

But no. Gravity has stubbornly resisted unification to this day. Perhaps more on that later. Meanwhile, the late 1800’s and early 1900’s saw an incredible unfolding of unexpected events: the establishment of an apparently absolutely empty vacuum (no “ether” to serve as host to electromangetic waves), the undeniable ultimate universal speed limit namely that of light. Then in rather quick succession came the quantum, special relativity, the atom, the nucleus, and finally the dawning of the quantum mechanical description of our world.

By 1930 the quantum revolution was complete, and we could set about the usual human pattern of (intellectual) colonization, militarization, exploitation, and capitalization. All the modern technology we now enjoy derives principally on our ability to first understand and then technologize the quantum world.

Continue reading ‘Higgs 101′

Last January, in my blogger-virginity-losing post to CV, and a follow-up post, I wrote about the experience of “opening the box” on a never-before-seen sample of data from the CDF experiment at Fermilab and being perhaps the first human to see what nature had to tell CDF in the search for the Higgs boson predicted by supersymmetry with the then-current data sample. What we’d seen was a small excess that might have in fact been the first experimental glimpse of a Higgs boson with a mass of around 160 GeV, or 170 times the mass of a proton. Or, it could have been a statistical fluctuation, or an artifact of the detector or analysis. It was excitng, but as scientists we had to keep our heads on straight.

There is basically nothing we can do about a statistical fluctuation - you get what you get. What keeps us awake at night is the prospect that we had made a mistake, or overlooked some detail. And so for months now, we (and when I say “we” I mostly mean Anton Anastassov, a postdoc at Rutgers, and my student Cris Cuenca) worked very hard to make sure that we hadn’t missed anything.

As far as we could tell, we hadn’t missed any problems, and so by late summer we decided to “open the box” again on a sample with 1.8 times more data (but containing the original sample). So it was not totally new data, but a sample with 80% more statistics. Was the bump still there? Would we see an even more significant excess?

We already kind of knew, given that the D0 experiment had not seen a similar excess, that we might not see the bump. So finally, after we had dotted the i’s and crossed the t’s, we took a look and there it was:

Gone! The data points all fell within an error bar or so of every bin…no excess, no bump, no Higgs… I am sure you are thinking “no tickets to Stockholm” too. Were we suprised? No. Once you’ve been working in this field awhile you realize that this is what happens with two-standard-deviation effects very often: they go away with more data. If you want all the gory details you can find them here.

If you do go back and read the original posts, you’ll find that we assumed that a statistical fluctuation was one very possible explanation. Unless we had auxiliary information that said that there should be a Higgs at that mass with that production rate, etc., it was much more likely than not to have been a statistical fluctuation. And in the end that is what it was…even with a probability of 1 in 50 or so. It happens.

So the quest for this beast continues. Mother Nature is a big fat tease!

Now, gentle readers, one thing we’ve learned is that among you are many science journalists who use blogs as a means to catch wind of breaking news. You all have to have an angle, and a story to tell (sell). In the past, our field has been treated to stories of the ilk “300 Physicists Fail to Find Supersymmetry” with the subtitle “Study Illustrates the Risks of Big Science”. (New York Times, 1993). I sincerely hope that’s not your angle here, science writers! Our favorite put-down of that is to ask whether there should have been an 1888 story titled “Physicists Fail to Find Ether in Vacuum” about the Michelson and Morley null result. But okay, we’re nerds.

Of all the stories that appeared this past year about the quest for the Higgs I think that the one that got it right was Dennis Overbye’s in the New York Times. He captured the true spirit of this hunt without hitting false notes about blogging and science, or trying to make it look like some sort of last-chance desperate ploy by an accelerator nearing the end of its useful life, or trying to foment some non-existent controversy. I challenge you journalists out there to tell it like it is: this is a great human adventure, with all the twists and turns any good adventure has. And someday, maybe soon, if not at the Tevatron then at the LHC, there it will be…but will it be what we expect?

Here is one of the best ideas I’ve heard in a long time - thanks to Matt Searle for passing this on to me!

Computers often do the same thing over and over again. Microprocessors have become amazingly fast, but since they are general purpose, they are not as fast as dedicated circuits which just do one operation, but do it blazingly fast. Field-programmable gate arrays (FPGAs) have been used for over two decades for dedicated operations in high-speed electronics, and now Prof. Frank Vahid and his Ph.D. student Roman Lysecky at UC Riverside have married the FPGA to the microprocessor to create “warp speed” computing.

The idea, like many great ideas, is simple: when a computer program finds that it is executing the same instructions repeatedly, and these can be done faster in an FPGA, the program automatically moves that code section to an on-board FPGA, which will run that section up to a 1000 times faster than the microprocessor.

Lysecky’s dissertation on warp computing won the 2006 “Dissertation of the Year” prize at the European Design and Automation Association.

This is so obviously a great idea, and will speed up computing in so many circumstances that I expect we’ll see it in commercial systems very rapidly. This could be a huge breakthrough…

This may not be the world’s most pressing problem, but it’s one that has started to become more and more odd to me as time has gone by. On the surface, you can easily understand how it has arisen, but the more you consider it the more you wonder just how other people do see the world around them.

We are saturated with video imagery now, in our homes, in shopping malls, airports, and on line. It’s only going to increase with time. So, I ask, why can’t we get the friggin’ aspect ratio correct anyway on all this video?

Up until the last few years, just about all video in the US (and Japan and elsewhere) was displayed in a 4:3 format called NTSC. (Europe adopted different standards, PAL/SECAM, but never mind that for now.) With good old TV, the NTSC format had a resolution of 486 horizontal lines. Translate that into a digital video screen and you’d need 648×486 pixels.

Typical computer screens in the early days had resolutions on that order, but quickly got better. Before long, “XGA” became very common, with 1024×768 pixels. This neatly utilized 10 bits for the horizontal and retained the 4:3 aspect ratio. Cool: all you had to do was translate the video NTSC signal into a digital signal and you could use a digitial monitor!

I’ll skip all the details, but in the past decade we’ve seen an explosion of flat-screen plasma and LCD monitors, which are almost all a wider 16:9 format. This allows them to accomodate the HDTV format, which is designed to have such a ratio, with resolution such as 1920×1080. That’s an aspect ratio of 1.78, quite a bit wider than the old 1.33, but not nearly as big as modern Hollywood films (2.35!).

The problem is that a lot of television (broadcast and cable) is still transmitted in 4:3 ratio, but then displayed on a wide-format 16:9 screen. The designers of the 16:9 monitors have built in the choice (often the default) of simply stretching the 4:3 image to fit the 16:9 screen. The result, as I am sure you have all seen, is that faces and bodies are distorted horizontally by a factor of 4:3 or 1.333. Everyone looks 33% wider, unless of course they are laying down, in which case they look 33% taller. The alternative is to set the monitor to display the 4:3 image so that it uses up all the vertical part of the display but not the horizontal. That seems to bother some people even more than the distorted faces - why buy a big expensive wide screen display and then not use it all?

My own strong preference is to never distort the aspect ratio of image, no matter how big a TV you have. It just looks really stupid to do that. But here we come to the firtst odd thing that I have noticed: some people apparently don’t even notice that there is a distortion! I first encountered this in an electronics store, where the salesperson swore up and down he could see no difference. I’ve asked some random people in various places if they could see the distortion, and around a third claimed not to. You have to wonder how the human visual processing system works…are people who don’t see the distortion internally correcting for this automatically or something?

Even more bizarre is the video you can get online from news outlets like CNN. It’s hard to believe, but they acutally put the distortion into their online video, even though there is absolutely no reason to do so, except perhaps to make it look like it does when distorted on displays you see in public, or in upscale hotel rooms. They are intentionally distorting the image! WTF ? I find this totally baffling.

Here is a random example of some CNN video:

Now if we shrink it horizontally by 25% (3:4 of course!) then it looks like this:

If you cannot see the difference between these, I am baffled. And if you don’t prefer the one where the nice-looking mom’s face is not grossly distorted, then I am even more baffled!

There are perhaps deeper reasons for this effect. Somehow, perhaps CNN thinks that since people have been going around seeing distorted images all over the place, they’ve started to think that this is the new normal? Or perhaps it’s part of the vast media conspiracy aimed at making us feel good about being fat? It’s a fact that more and more obese people are appearing in advertisements…this makes good marketing sense in that people want to identify with the people they see in ads (or at least the advertisers want the target audience to do so) and since such a huge proportion of Americans are obese, adding an additional 33% to their video width is, well, just good marketing.

I would like to believe that this is all a phase, growing pains of our new digital culture. As video designers get better and the hardware gets more sophisticated, I hope that the distorted faces we see all around us will begin to look like the unfortunate by-product of the early phase of this technology. Some day we’ll look back on this and…cringe.

I got here to CERN a few days ago, and things are quieter than recently. It’s the start of vacation season and so the cafeteria is not so full and parking is plentiful. Lots of summer students are hanging around in the evening, and some of the meetings are sparsely attended.

In the CMS experiment, the folks who have labored long and hard to build the world’s biggest and most complex silicon-strip detector have earned a bit of vacation. As of about August 1, this billion-dollar baby will begin buttoning up for its journey underground into the heart of the experiment.

So I found myself alone in the room with this thing, in the Tracker Integration Facility, and it is hard not to hold it in awe. “It” comprises thousands of flat detectors, each a thin rectangle of silicon crystal with microscopic aluminum strips embedded in it. These strips sense the passage of charged particles like pions, muons, electrons, and so forth, passing the tiny charge they collect to custom-designed readout chips which send the data in digital form to an army of processors which gather all the information from a single 25 nanosecond “beam bunch crossing” into a nice tight wad for later processing.

If you unrolled and laid out all the silicon detectors in the CMS Tracker you could tile a tennis court. It’s mind bogglingly complex, the product of hundreds of people workng for a dozen years. But there it is, and you know what? It works. Its been put through its paces on the surface and it’s just about show time downstairs at LHC Point 5.

I call it a billion dollar baby but it’s hard to say what it really cost. That number takes into account at least some of the labor by engineers, technicians, physicists, and students all over the world, but probably not all of it.

With the testing of the tracker at an end and the transport not yet begun, I am here with my CMS pixel colleagues to take advantage of our last (and first, actually) chance at seeing if our detector will fit neatly inside the tracker.

We are working on the innermost detector in the CMS experiment, the pixels. Once you get so close to the collision that charged particles are millimeters apart, you can no longer use long strips to detect them but have to go to arrays of pixel sensors. Our detector may be a lot smaller than the tracker, but it has even more individual readout channels, around 45 million in total. Each pixel is 0.1 by 0.15 mm (100 by 150 microns) and is read out by a custom chip developed at PSI in Zurich, and bump bonded to the silicon sensors. (Enough jargon yet?) The data from pixels with charge above a preset threshold are sent out on a serial line, converted to an optical signal and digitized upon receipt in high speed electronics in an adjacent cavern underground, then sent up the data acquisition stream.

Anyway, when all this mumbo-jumbo is done we have a big set of three-dimensional space points along the trajectory of the same particles that have also passed through the tracker. But the pixel points allow us to see what happened very close to the primary proton-proton collision vertex. Combining the pixel hits along a track we can project back to the vertex with a resolution of about 10 microns. Given that the pixels are much larger than that this is quite a trick: we rely on the fact that particles split their signal among adjacent pixels, and we can use a sort of averaging trick to get much more precise than a single pixel width.

My task here, though is much more practical. We want to make sure that the pixel detectors we are building will fit inside the tracker, the two halves of the detector meshing neatly at the end of their grooves inside the tracker. Up until this point it’s been an engineering project, but now we have real hardware and we need to be sure that no parts will interfere mechanically with each other.

Stay tuned, and in a few days I will post some photos of that and tell you how it worked. Gulp!