Of Particular Significance

Winds are Rising

POSTED BY Matt Strassler

POSTED BY Matt Strassler

ON 08/27/2011

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.


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.   (An example — not the best one as other experiments don’t see it— is here.)  So we’re all very interested to hear updates from both those experiments and from the LHC — in particular the LHCb experiment that was specifically designed to be very good at these types of measurements — to find out which of these discrepancies become more striking, and which ones begin to fade away.

These types of measurements are more indirect than searches for the Higgs particle.  Rather than producing a new heavy particle in a proton-proton collision, the idea here is to look for a subtle quantum mechanical effect, involving a well-known particle, that is only indirectly sensitive to the existence of heavy unknown particles.  What’s good about this method is that by studying processes at relatively low energy — around the mass-energy (E = m c-squared) of the bottom quark, 5 GeV or so, we can potentially be sensitive to the presence of new particles whose mass-energies might reach a TeV or so …  particles that we may not have produced yet, or identified yet, at the LHC. What’s bad about it is that it is a blunt instrument. If we see something, we’ll know we have something new, because the well-known particle isn’t behaving as the Standard Model predicts — but we won’t have a clear idea what’s causing the effect.  One should view indirect and direct measurements as complementary.  When you produce some new particle directly, you have to know more or less what to look for, but when you find it, you’ve got it in your clutches; whereas an indirect measurement casts a very wide net, but unfortunately catches vague fish.

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8 Responses

    1. Hmm… That’s complicated. It doesn’t really relate, in the sense that the two issues are largely independent.

      First, supersymmetry certainly doesn’t guarantee an effect on the LHCb measurement. Depending on the masses of the superpartners, and the way some of them mix with each other, there may or may not be an effect. So if LHCb had seen something amiss, it would be evidence for new physics (not necessarily supersymmetry), but the fact that it doesn’t is certainly not evidence against new physics.

      Meanwhile, the first two of the three assumptions that I keep emphasizing typically would not enter into the prediction for the LHCb measurement at all, or at best in a very indirect sense. The third assumption does affect the calculation but isn’t in any clear way correlated with it.

      To give you a better answer I’d really need to write a couple of posts about it… which I certainly would do right away if LHCb had seen something, but since they didn’t, I’m inclined to move that down the priority list…

  1. Nice post again 🙂

    I just love Prof. Strassler`s clear style of writing and explaining; be warned that I will spend more time here as I can and sometimes come up with questions … ;-P

    I wish all the people affected by it that the storm strikes not too hard and that the collision with it gets not too violent :-/

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