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The OPERA experiment has now presented its results, suggesting that a high-energy neutrino beam has traveled 730 kilometers at a speed just a bit faster than the speed of light.  It is clear the experiment was done very carefully.  Many cross-checks were performed.  No questions were asked for which the speaker did not have at least a reasonable answer.

Some preliminary comments on the experiment (none of which is entirely well-informed, so caution…)

• They have to measure times and distances to an accuracy of 1 part in a few hundred thousand. This is hard, not impossible, and they have worked with metrology experts to carry these measurements out.
• The timing measurement is not direct; it has to be made in a statistical fashion. The proton beam pulses that make the neutrino beam pulses [read more about making neutrino beams here] are not sharp spikes in time, but are distributed in time over ten thousand nanoseconds. (Recall the measured early arrival of the neutrinos is only 60 nanoseconds.) And so one cannot measure, for each arriving neutrino, how long it took to travel. Instead one has to measure the properties of the proton beam pulses carefully, infer the properties of the neutrino pulses, measure the timing of the many arriving neutrinos, and work backwards to figure out how much time on average it took for the neutrinos to arrive. This sounds tricky. [Thanks to Ryan Rohm for calling my attention to this a few days ago; however, see his comment below.] That said, the experimenters do show some evidence that their technique works.  But this could be a weak point.
• I am a bit concerned about the way in which statistical and systematic errors are combined. The theory for statistical errors is well-defined; one assumes random fluctuations. In combining two statistical errors E1 and E2, one says that the overall error is the square root of E1-squared + E2-squared.  This is called “adding errors in quadrature.”  But systematic errors are much less well-defined, and it is not clear you should combine them in quadrature, or combine them with statistical errors in quadrature. The OPERA experiment combines all errors in quadrature, and says they have a measurement at 6 standard deviations away from the speed of light. If you instead combined systematic errors linearly with statistical errors (E1+E2 instead of as above) you would get 4 standard deviations. If you combined all the systematic errors with each other linearly, and then with the statistical error linearly, you would get 2 standard deviations (though that is surely too conservative). All this is to say that this result is not yet so significant that different and more conservative treatments of the uncertainties would all give a completely convincing result. This is just something to keep in mind when evaluating such an exceptional claim; we need exceptional confidence.

Now, some brief comments on the theoretical implications, in addition to what I said in Tuesday’s and Thursday’s posts.

You may have heard some people say that neutrinos traveling faster than light would mean that Einstein’s theory is completely wrong and implies that instantaneous communication and even time travel would be possible. Balderdash! this is loose and illogical thinking. If Einstein’s theory were exactly correct AND neutrinos could travel faster than light, then this would follow. But if neutrinos travel faster than light, then Einstein’s theory is wrong at least in some part, and until you know exactly how it needs to be modified, you can draw no such conclusions.

As I emphasized in yesterday’s post, Einstein’s principles should be divided for current purposes into two parts:

1. There is a universal speed limit.
2. Light travels at this speed limit.

Observing that some neutrinos travel faster than light could mean there is no speed limit at all, or simply that light does not travel at the speed limit. And there are much more complex logical possibilities. Some of these would require only rather small (though still revolutionary) adjustments to current theoretical physics. Others would be more disruptive to current thinking. But it is certainly not true (as some physicists have said in public fora) that it requires going back to the drawing board as far as theoretical physics and Einsteinian relativity are concerned. It will depend on which of the various logical possibilities is actually operating. Hyperventilating about the impending collapse of existing theoretical physics is a tad inappropriate at this time.

In fact, over the years quite a few theorists (including very mainstream and well-respected scientists at major universities) have considered the possibility that Einstein’s principles might be slightly violated, and some of that work from the 1990s (and probably earlier [?]) suggested that studying neutrino properties would be a good way to look for signs of such violations. So it is not as though these issues have never come up before in theoretical physics… though I think it fair to say that everyone has viewed this exciting possibility as a long-shot.  (And most of us probably still do, until this experiment is confirmed.)

There is certainly more to say about the theoretical situation, but I am still learning about what the experts already know.

After a few days of preparation, my colloquium on the Large Hadron Collider appears to be ready for presentation at Harvard today.  Typically in a colloquium one discusses some details of one’s own research, but there’s so much to say about the primary physics motivation behind the LHC (clarifying the nature of the Higgs field), the challenges of doing science at such a machine, and the flood of current results from the ATLAS and CMS experiments that I’ll be lucky if I manage to squeeze any of it in. Almost all of my LHC research these days involves trying to make sure that the experimental program is comprehensive and efficient, and isn’t missing anything.  So there’s not much point in trying to explain it to a broad audience of physicists without going through the details  of the LHC first… and doing that easily takes up the whole available hour.

Meanwhile, following up on this post from last week , I have put up a new webpage describing a very interesting class of searches at the Large Hadron Collider that are aimed at finding models which violate the first or second of the assumptions of classic supersymmetry, and produce a new very long-lived particle that is electrically charged and/or is affected by the strong nuclear force.   The methods used actually don’t depend on supersymmetry at all, and can find (or exclude) any similar long-lived particle from any new source.  (The new page accompanies a previous webpage describing a class of searches that are aimed at models that violate the second of the three assumptions of classic supersymmetry, and that therefore often produce photons or Z particles.  More pages of this type are on the way.)

Time for a little insight into what it means to be a professor…

One of the most interesting and challenging tasks that academics, especially academic scientists, are invited to do as part of their careers is to talk about their work, and more generally their field, to an audience consisting of an entire academic department.   This kind of talk is called a “colloquium”, and most departments hold a colloquium weekly.  In a few days I’ll have the honor of giving one. (more…)

Following up on yesterday’s post, I have put up a webpage describing a class of searches that are aimed at models that violate one of the three assumptions of classic supersymmetry, and that therefore often produce photons or Z particles.  More pages of this type are on the way.

A sensitive outsider, watching and reading the press and the physics blogs carefully, might detect an unusual level of irrationality surrounding the topic of supersymmetry, both in terms of what the press is reporting and in terms of what scientists are saying. Not that this is intrinsically abnormal.  Although science is in the long run a largely rational process, scientists themselves are no more rational than anyone else.  Just because you are well-trained to employ rational thinking doesn’t mean you are trained (or could be trained!) to use it exclusively.  Indeed the best scientists, in my experience, tend to have a very individual (and often strikingly unusual) brew of abilities and quirks, with the abilities to think rationally and do mathematics joined to far less stereotypical talents and characteristics.  Some scientists are always passionately certain that their ideas are correct.  Occasionally they are right, though it is rare.  Others are by nature highly skeptical, dismissive of fancy ideas, or fatalistic. All this is to say that there is plenty of rationally-unjustifiable exuberance and scorn in science.  But the amount of it surrounding supersymmetry is a bit higher than usual.  What’s behind it? (more…)

It has been a somber weekend in New York. In a city of many millions, thousands of premature deaths in ninety minutes  left a deep scar.  Among my acquaintances, not everyone knew someone who died, but it seems almost everyone knew someone who knew someone who perished.  And for those of us who watched the towers being built, wandered for years in their shadows, and walked on the roof deck gaping at the astounding view, their destruction and absence from the skyline will be permanently unsettling, and motivating.  I have stood atop that which does not exist; my memory floats in the air.

Three nights ago I was far downtown, and the austere memorial that has graced Manhattan skies each September — a pair of vertical shafts of blue-white floodlight, recalling the towers — was illuminating the sky just overhead.  Several layers of thin clouds caught the light, creating pairs of soft-edged moons stacked one atop the next.  As the wind rushed the clouds along, each moon separately glowed and flickered, yielding a spectacle at once simple and enigmatic — a conjunction of artifice and nature, of the steady and the unpredictable.

There’s much to say about the aftermath of 9/11/01 and the consequences, as far as internal and external U.S. politics, U.S. society and values, and U.S. science in particular. But not right now.

In the next week (two weeks? three?), in between pushing two papers toward completion and preparing a colloquium (= a talk intended for an entire physics department) that I will give a few times this year, I will try to use my last week’s intensive study of the existing papers and talks from the Large Hadron Collider (LHC) to characterize what their measurements tell us so far about nature — and what they don’t tell us.  This isn’t easy, at the very least because [for very general reasons] one typically can’t rule out a class of theories (such as supersymmetry or extra dimensions or technicolor) through a single set of measurements — and conversely, a single measurement says something about many classes of theories.   Given this, should a discussion of LHC results be organized by measurements or by theories?  By empirical data from nature or by the  ideas of the human mind?  These practical questions have a principled side, and the answers illuminate more than we might initially expect.  For what is the best way to summarize our knowledge so that we actually know what we know?

I have updated the article laying out some of the implications of the search for the Higgs particle, as of September 2011.  In particular, I filled in the last two subjects in the table of contents:

1. The Standard Model Higgs particle
2. A Standard Model Higgs sector but with Other New Particles
3. The case of two Higgs particles
4. The case of five Higgs particles
5. Supersymmetry: Five Higgs particles along with superpartners [new!]
   
6. No Higgs particle (to speak of) [new!]
   

If you’d like to learn a bit about how Higgs particles work in supersymmetric theories, and about how a theory can have a Higgs field but no Higgs particle, you may enjoy taking a look at the new material.  Implications of the Higgs searches so far are still rather limited for these two cases, but the article describes what you should keep an eye on, and why.

Two full non-stop days of work at Canada’s Perimeter Institute, filled with  intensive study of the papers, notes and conference talks from the ATLAS and CMS experiments at the Large Hadron Collider, have left me slightly exhausted but pretty well up-to-date.   This higher level of knowledge will start to percolate into these pages almost immediately.

Meanwhile, a bit more insight into the CRESST dark matter situation (see yesterday’s post). There  have already been silly press articles.  The BBC article says

• Researchers at the Cresst experiment in Italy say they have spotted 67 events in their detectors that may be caused by dark matter particles called Wimps.

No, that’s not what they said.  (more…)

Well, just as if we didn’t have enough to keep track of in particle physics, along comes another claim that dark matter has been detected, by looking for extremely rare and faint signals of collisions of dark matter particles with atomic nuclei in a special type of crystal. (more…)