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.

10 responses to “LHCb’s Result From November Appears Confirmed by CDF

  1. Pingback: New CP Violation Results! « Whiskey…Tango…Foxtrot?

  2. Pingback: New CP Violation Results! « Whiskey…Tango…Foxtrot?

  3. Pingback: New CP Violation Results! « Whiskey…Tango…Foxtrot?

  4. Matt: “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.”

    Among all blog reports on this LHCb result, the above passage is the only one having the relevancy to the importance of this result. Thanks Matt.

  5. Harry Bostock

    So if this is evidence of new particles what particles would they be and what would be their properties?

  6. CP stands for Charge-Parity, right?
    If weak interaction does violate weak charge conservation (It is called weak isospin, am I right?), then such violations could also happen in eletric and chromo charge too?
    Thanks for your time, Matt.

  7. 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.

    I am not sure how to read this — surely `how to do’ a calculation in the standard model is well understood?
    Let ,me rephrase that — which sort of calculations (I guess those involving quarks are the suspect ones) are not understood?

    • I hope this question has an answer a layman can understand. I’d guess it’s contributions that have been known to be difficult to calculate but previously believed to be very small.

    • We might know what the formula is, but the calculations are extremely difficult. Think multi-dimension PDEs. My understanding (I’m not a physicist) is that there are heuristics that can simplify things, but it’s not always clear that the heuristics are correct.

  8. Pingback: Opgelet mensen, ΔA = -0,62 ± 0,23%

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