Tag Archives: #Tevatron

LHCb’s Result From November Appears Confirmed by CDF

Back in November, I described a surprising result from the LHCb experiment at the Large Hadron Collider [LHC] concerning “CP violation” in the decays of particles called “D mesons” (which are hadrons that contain an unpaired charm quark or an unpaired charm antiquark) at a level much larger than expected by theorists.  Rather than rehashing the explanation for what that was all about, I’m going to point you to what I wrote in November.

There’s news this week that the CDF experiment at the now-closed Tevatron has updated its result for a measurement of the same quantity, using the full CDF data set.  And they now find a very similar result to what LHCb found.   This is indicated on the slide shown below, taken from the talk given by Angelo Di Canto at the La Thuile conference (but edited by me to fix a big typo; I hope he does not mind.)  You see that while LHCb found a CP-violating asymmetry of -0.82%, with statistical and systematic uncertainties of 0.21% and 0.11%, CDF finds -0.62%, with almost identical uncertainties — a little closer to zero, but still well away from it.

A slide from the CDF presentation on its measurement of CP violation in D meson decays (with an edit by me to fix a glaring typo.) The CDF result is in orange at the top; the LHCb result is in black just below it. In the figure, the LHCb result is in blue, the CDF result in orange, and the traditional expectation for the Standard Model is very close to the point (0,0), the isolated red dot at dead center.

This lends support to LHCb’s result, and putting the two results (and a couple of other weaker ones) together makes their combination discrepant from zero by about 3.8 standard deviations.  That’s great, but not as great as it would have been if what theorists thought a few years ago was still considered reliable.  Back then, the relevant experts (and I should emphasize I am not one of them) would have told you that they were pretty darn certain that the Standard Model [the equations we use to describe the known particles and forces] could not produce CP violation of this type, and any observation of a non-zero signal would imply the existence of previously unknown particles.  But the experts  have been backing away from this point of view recently, worrying that maybe they know less about how to calculate this in the Standard Model than they used to think.  If we’re to be sure this is really a sign of new particles in nature, and not just a sign that theorists have trouble predicting this quantity, we’re going to need additional evidence from another quarter.  And so far, we haven’t got any.

The Tevatron Comes to an End

[If you are a layperson interested in the faster-than-light neutrino claim, and you haven’t yet looked at my recent “open-space’’ post and the list of excellent questions laypeople have asked in the comments, you definitely should.  And ask your own if you want. That post also gives an organized list of links to my main posts on the neutrino experiment.]

Today is a sad day in American particle physics.  It is the day that Fermilab will shut down the Tevatron, once the world’s leading particle accelerator, which discovered and measured the mass and other properties of the top quark (the sixth discovered, and by far the heaviest), tested the Standard Model of particle physics in very great detail (confirming that everything I wrote in this post about the known particles is correct to the available precision of the experiment), and looked hard for the Higgs particle before being overtaken by the Large Hadron Collider.   Continue reading