Particle Physics

What Have We Here?

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Well, every now and again an experiment reports a result that forces scientists to go back to a long-established principle, to check whether it needs revision, extension or adjustment, or perhaps even replacement. Most times it eventually turns out that the experiment is wrong, though often in some subtle and non-obvious way, and the principle survives. But of course there are the rare occasions when it is the other way round. So a scientist must go into such a situation with an open, though skeptical, mind.

Is there a more famous principle from 20th century physics than Einstein’s principle that nature has a speed limit? We call this the “speed of light”.

We call it that. However, let’s be a bit careful.

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Supernovas and Neutrinos

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Supernovas are some of nature’s most common and powerful nuclear bombs.  They are also among the most useful for particle physicists and astrophysicists alike.   [The last quarter of this post, on the OPERA experiment’s claim of light traveling slower than neutrinos, has been updated a couple of times, in obvious italicized remarks, in response to some very useful comments.]

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Some Comments on Theory and Experiment at the LHC

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In the next couple of days I hope to update a post I put up a short time ago, one that, as of today, 8/29/11, still holds true.  The issue that I addressed in that post was : What does the Large Hadron Collider (LHC) currently have to say about Supersymmetry?  I took a slightly polemical point of view, but you can look at the links in the post to longer, more pedagogical articles to see where the point of view comes from.

The problem with trying to answer a question like this one — What do LHC results imply for Theory X — is that it is ill-posed. An experiment searches for a phenomenon — not a theory. If a theory always predicts a certain phenomenon, then an experiment can search for that phenomenon, and, in finding it or not, give a definitive thumbs-up or -down to the theory. But often a theory has many versions, and although it will have a general set of predictions — supersymmetry, for instance, predicts superpartner particles — its details can look quite different to an experiment, depending on the version. [For instance, in supersymmetry, whether or not one sees the classic supersymmetry signature depends on the masses of the superpartner particles, on whether there are extra types of particles that are not required by the theory but might just be there anyway, etc.] So any given experimental result is just one important but incomplete piece of information, one that constrains some versions of the theory but not others. Typically, to entirely rule out a general theory like supersymmetry requires a large number of experimental searches for many different phenomena.

Conversely, though, many different theories may predict the same phenomenon. So an experiment that looks for but fails to find a certain phenomenon doesn’t just rule out just one version of one theory. It typically rules out several versions of many different types of theories.

And if it does find that phenomenon, it does not tell you which of those different types of theories it comes from. Remember that, when the LHC makes its first discovery! You’ll likely see claims from physicists in the press about what the discovery means that aren’t really merited by the data.

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Nature is Full of Surprises

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Ok, hurricane over — and I was in a spot where it happens to have been particularly mild. Power was never lost. South of here, north of here, east of here, it is a different story. Nature deals in chance.

As I prepare some more particle physics posts, it seems a good moment to ask a more general scientific question: What are the chances that there would be a rare significant east-coast earthquake and a relatively rare New England hurricane affecting the New York area within five days of each other? Or more generally: What are the chances of two rare natural disasters affecting the same place within such a short time?

You will notice, if you are reading carefully, that the second question is not at all a rephrasing of the first. The two queries are utterly different, and they have entirely different answers.

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Winds are Rising

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UPDATE: LHCb reports precise measurement of B_s meson decaying to  J/Psi meson + Phi meson.  Agrees beautifully with the Standard Model.  Fantastic measurement!!  But disappointing — no sign of any new phenomena.

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Well, the hurricane now arriving on the east coast of the United States has forced significant changes in many people’s plans, mine included.  Certain big chores simply had to get done in preparation for the storm’s arrival. Consequently the post that I had hoped to write analyzing the results from Mumbai on the searches for the Higgs particle, and for more speculative phenomena, simply hasn’t been composed yet. But since Irene the Giant is expected to impact the New York area for about 24 hours, starting late tomorrow afternoon, I should have plenty of time to catch up while it’s on its way in.  Whether I’ll be able to finish the post, or post it when it’s finished, will depend on when the power goes out, and for how long.

[p.s. apologies to those who have asked questions that have gone unanswered; I’ll get to those during the storm as well.]

Meanwhile, there are  some “flavor physics” results being presented in Mumbai right now. Flavor specifically refers here to the behavior of bottom quarks and of charm quarks, as they behave inside of hadrons, and as they decay to other quarks lighter than themselves. These behaviors are predicted in considerable detail by the Standard Model of particle physics, and it is rather easy for some new phenomena, involving some new heavy particles, to come along and screw up the Standard Model prediction. In fact there are several measurements, from various experiments including the Tevatron and also the so-called B-factories [which make lots and lots of bottom quarks and anti-quarks] that don’t currently agree with the Standard Model very well.  

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The Latest Word on the Higgs from the Mumbai Conference

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Restructured the post: My preliminary discussion is first, the updates from the talks are now at the end.  The take-away message from the LHC talks: The most interesting point here is that ATLAS and CMS [crudely] taken together exclude all regions for the Standard Model Higgs particle except below 145 GeV, the range 288-296 GeV, and … Read more

Current LHC Data and Supersymmetry; Is Supersymmetry in Trouble?

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Ok, today I’ve posted the article you’ve been waiting for: What does the LHC have to say, so far, about supersymmetry? [Here’s an article about supersymmetry and what it predicts, and another about standard ways to look for signs of it at the Large Hadron Collider, under certain assumptions; if you want to review the known particles … Read more

New Experimental Result Searching for Hidden Particles

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As someone who has spent several years thinking hard about how to detect “hidden particles” — ones that are not affected by the three forces of the Standard Model, the electromagnetic force and the strong and weak nuclear forces — I am pleased to see the result that just appeared from the APEX experiment, at the … Read more

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