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CERN | Geneva | Switzerland

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Charm and beauty: LHCb has it all!

This week, the LHCb experiment reported an anomaly they have just observed in the decays of charmed mesons. Could this be the tip of the iceberg for new physics?

As I reported before, the main goal of the LHCb experiment is to use heavy quarks (called beauty and charm quarks) to make precise measurements in the hope of detecting small deviations from the Standard Model, the theoretical framework that has been guiding particle physicists for a few decades. But it has a few known shortcomings that make us think new physics should be discovered soon.

This is a great model that allows theorists to make very accurate predictions. So far, every single one of them has proven to be true but if we were to find a flaw, it would be like discovering the secret passage guiding us further into our investigation of how matter works.

The results presented this week by the LHCb experiment hint in this direction, although as usual, further checks are needed. The scientists involved were looking at decays of charmed mesons denoted D0, particles made of one charm quark c (with electric charge +2/3) and one antiquark u (-2/3).  The D0 is therefore electrically neutral and can decay into a pair of kaons, K+K- (mesons containing an s quark) or pions π+π-(mesons made of light quarks, u or d).

But one could also make a charmed antimeson with one antiquark c (-2/3) and one quark u (+2/3). This is the meson antimatter. These antimesons also decay into pairs of kaons or pions, K+K- and π+π-.

How can one know if he or she is dealing with a charmed meson or antimeson since both decay to the same final products? One way is to “tag” the charmed mesons when they are created. For this measurement, the LHCb team selected excited charmed mesons D*+ and D*-, which decayed to a positively charged pion and a D0 charmed meson, or into a negative pion when charmed antimesons are created. The pion charge gives it away.

What LHCb measured is the difference between how often charmed mesons and how often charmed antimesons decay into K+K-. The same measurement was repeated with D0 Þ π+π- as final decay products. The idea is to see if there is a difference in the behavior of matter (the mesons) and antimatter (the antimesons), what we call charge-parity (or CP) violation.

Then they looked at the difference of the difference between the K+K- and π+π- channels. The advantage is that many potential experimental biases cancel out, while a true signal would remain, as the CP violation need not be the same in the two channels.

Our Standard Model of particle physics predicts that this difference should be very small, of the order of 0.01% to 0.1% (the theoretical uncertainly is fairly large here). LHCb measures −0.82 ± 0.21 (statistical uncertainty) ± 0.11 (systematic uncertainty)%, a 3.5 standard deviation away from zero. In other words, if every source of experimental uncertainty has been properly accounted for, there is only a 0.05% chance this is due to chance. Other experiments had detected a hint of this effect before, but with much less precision, so this in itself is an accomplishment.

Does this mean the Standard Model has been proven wrong? Not yet. We need to see if this effect will remains once the group has a chance to analyze all of the 2011 data, which should be completed by March next year. Only 60% were used for this analysis.

As Mat Charles, one of the four physicists directly involved in this analysis told me: “This could be the hint that something interesting is going on. Very much worth pursuing”. They plan to add a different channel, tagging the D0 mesons and antimesons using B mesons, those containing the beauty quark, instead of D*, just to be sure this is true. Let’s hope they’ll be lucky. Such a discovery would be a great step forward.

Pauline Gagnon

To be alerted of new postings, follow me on Twitter: @GagnonPauline or sign-up on this mailing list to receive and e-mail notification.

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14 Responses to “Charm and beauty: LHCb has it all!”

  1. Bernie Shwayder says:

    As a “civilian” non-scientist fascinated by particle physics, it seems to me that we we are on an edge that’s about to collapse. Doubt about the Higgs particle, possible dissolution of string theory and now this “charming surprise” sounds like a science about to enter a whole new dimension, maybe in more ways than one. What an exciting time!

  2. Katy Washburne says:

    Is it possible that Einstein’s theory was correct for his time, but perhaps since his theory, we are moving at a greater speed, therefore excelarating everything around us and changing the math so to speak?

  3. Rudy says:

    Could you elaborate the numbers and the minus sign before 0,82 please?

    Are the D-Mesons decaying faster than the D-Antimesons (or vice versa)?
    And did LHCb found, that they decay faster than it was predicted by the standard model?

    • Pauline Gagnon says:

      Hello Rudy,

      sorry for the delay. In fact, it is not clear yet what the minus sign means. It is still too early to tell.

      As explained in the blog, the quantity measure is the difference between two differences. This represents the difference between the CP asymmetry for kaons and pions. If this difference is negative, it can mean several things:
      1. the asymmetry for kaons is negative (that is, charmed mesons decay more often into K+K- than antimesons.
      2 the asymmetry for pions is positive (that is, charmed mesons decay into π+π- less often than antimesons).
      One would need to measure each rate separately to find out. But this will be more difficult because, when you take the difference, some common errors calncel out. So LHCb will need more data to be able to make accurate measurement of these 2 quantities separately.

      What is important at this point is that they measured something different from zero, that is the two rates are not close to being equal as was expected by the Standard Model.

      Note also that is is not how fast mesons and antimesons decay that was measured in this case, but how often they do so. How fast is another type of measurement, a lifetime measurement. This is something else that will also be studied.

      I hope this helps. Pauline

    • Rudy says:

      Thanks a lot, Pauline, this was very helpful and interesting. I am looking forward to new results in the future.

  4. driftrat says:

    Neutrino mass is measured theorectically as a term – as mass squared…typically a negatine number reflecting a FTL velocity. Can we imply a primary universal FTL terraine?

    the notions ‘nothing’ and ‘infinty’ see equal to me in that each reflects the limits of our comprehension and not that of our universes.

  5. [...] neutrinos, search results of higgs boson at LHC and rumors about mass of higgs around 120 Gev, result from LHCb and the hint of beyond standard model physics made this year quite exciting. As a student i am just [...]

  6. driftrat says:

    could tacyons imply ,when gaining on light speed after losing energy…that our quantum world is a reflection of a’universal speed/field’ slowed down?

  7. driftrat says:

    “The only reason for time is so that everything doesn’t happen at once”. Einstein . A faster than light (FTL) terrain as world would be perceived relative to our light speed world observation as a non-locality/superposition…that is everything would appear to be occurring in different places at the same time. When observing a rotating wagon wheel the image appears to go backwards as a result of the backing-up and blending in to the observers terrain time reference. By analogy when the spoke image just begins to reverse rotation the wheel is reflecting a true virtual non-locality/superposition. ie; any point can be at a different place at the same time.

  8. driftrat says:

    “Einstein’s statement is more tautological than true”
    yes it is without question that the E-man made the statement as a jest however, an element of truth is always within the base of genuine humor and this original whimsical corrrelation came from the mind of the E-man.

  9. [...] Charm and beauty: LHCb has it all! (quantumdiaries.org) [...]

  10. Craig Conley says:

    Perhaps there are only two forces. The Electroweak and the Strong Nuclear. One Radiative and one Attractive. The residual attraction of a single proton, even after falling off according to the Inverse Square Law, at 100,000 kilometers could be measured with a sensitive enough instrument. Granted it’s be a decimal point followed by a gazillion zeros, but at the end of that astring there’d of necessity be a ’1′. Multiply that times the number of protons (and neutrons) in a body the size of the Earth and I’ll bet you’ll find “Gravity”.

  11. Craig Conley says:

    That would make GUTS a simple duality…I’d name it ART (for Attraction/Radiation Theory).

  12. Craig Conley says:

    Two forces, two quarks (up & down – the others are just heavier versions), two leptons (electron & neutrino – again, the others are just heavier versions)…a duality. ART. Toodles, Craig.

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