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Alexey Petrov | WSU | USA

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LHCb reports observation of CP-violation in charm. Welcome New Physics? Or not?

One of CERN’s collaborations, the LHCb, has reported observation of direct CP-violation in the decays of charmed mesons at the Hadronic Collider Physics Symposium 2011 (HCP 2011) in Paris today. This is a fantastic news! While I am not at HCP 2011, kind folks at LHCb let me know about this fantastic measurement — since charm physics is my specialty.

So, what are we talking about here?

First things first. CP (or Charge Parity) is a set of (discrete) transformations performed on a theory’s Lagrangian — a function that describes what particles we have in a theory and how they interact. If your Lagrangian is symmetric  under this transformation, then particles and antiparticles — matter and antimatter — have the same properties. If not — interactions of matter particles are different from interactions of antimatter particles.  This possible difference is a crucial property of a theory because, according to three Sakharov criteria, the Universe could evolve in what we see around us only if matter and antimatter have different interaction properties. Otherwise, at best, we’d have big chanks of antimatter floating around — or at worst would not not exist at all.

This is why many huge experiments built to study CP violation. Big national labs’ flagship experiments were designed to search and study CP-violation (BaBar at SLAC, Belle at KEK, LHCb at CERN), with hopes to see glimpses of New Physics that could explain matter-antimatter asymmetry in the Universe. This new result from LHCb can in principle provide one.

LHCb experiment

So, what did LHCb see? The reported analysis looks at the difference of a difference — i.e. a difference of CP-violating asymmetries in kaons and pions. The CP-violating asymmetry is defined as the difference between decay widths (roughly speaking, decay probabilities) of a neutral D-meson to decay into a final state, say positive K-meson and a negative K-meson and the same quantity for the D-anti-particle to decay to the same final state. This quantity is also defined for the final state of two pions — and it is CP-violating!

The structure of this CP-violating asymmetry, aCP, is not that simple. Because D0 is a neutral particle it can, in principle, mix with its antiparticle (see here) — and this antiparticle can also decay into the same final state! This process can be also CP-violating (this type of CP-violation is called indirect CP-violation). So the result would depend on both types of CP-violation!

Moreover, experimentally, the asymmetries like this are not easy to measure — there are experimental systematics associated with D-production asymmetries, difference of interactions of positive and negative kaons with matter, etc. For this reason, experimentalists at LHCb decided to report the difference of CP-violating asymmetries, in which many of those effects, like productions asymmetries, would cancel. So, here is the result:

ΔaCP = -0.82 ± 0.21 (stat) ±0.11 (syst)%

In other words, this quantity is 3.5 sigmas away from being zero. The first question that one should ask is whether this quantity is consistent with previous measurements. The biggest question, however, is whether this quantity is consistent with Standard Model expectations.

There is a bunch of previous measurements available for aCP (KK) and aCP (ππ) separately. The thing is that

aCP (KK) = – aCP (ππ)

or approximately so. So by subtracting those quantities we not only subtract the experimental uncertainties, but also enhance the signal! However, looking at the table on page 6 of the talk, one can immediately realize that this measurement is at least consistent with the previous ones.

Is it a sign of something beyond the Standard Model? This one is hard to answer. I usually put an upper bound on the SM value (that is, absolute value) of asymmetries like aCP (KK) at 0.1% — which would make ΔaCP to be about 0.2%. Is it consistent with LHCb findings? Maybe. The size of this asymmetry is notoriously difficult to estimate due to hadronic effects. Maybe it is a sign of New Physics — this could be an exciting conclusion, as we have never seen CP-violation in up-quark sector.

It is interesting that the first “big” result from LHC comes in the realm of charm physics, not Higgs searches. Moreover, all “big” results in the last decade were from the experiments searching for New Physics indirectly, in the “intensity frontier” (this is lingo of US Department of Energy) — with most of them coming from charm physics. Maybe at the very least LHC-b should be renamed as LHC-c?

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12 Responses to “LHCb reports observation of CP-violation in charm. Welcome New Physics? Or not?”

  1. Xezlec says:

    What is it about charm that makes it so special?

    • Alexey says:

      Well, every quark is special in its own way. But, indeed, charm has some interesting things that, at least at the moment, sets it apart from other quarks:

      1. It is the only up-type quark where we, so far, have seen CP-violation — provided that this result from LHCb is not a fluctuation. Top results are not that precise yet to see direct CP violation. And, as mfb pointed out below, it is the only up-type quark whose neutral bound states can experience meson-antimeson mixing.

      2. CP-violation asymmetries and mixing amplitudes for charm are small in the Standard Model, so there is a chance to see glimpses of New Physics.

      3. Charm physics experiments usually have a lot of statistics.

      Calculations with charm quarks, on the other hand, are pain…

  2. mfb says:

    Well, one thing is that charm quarks are the only quarks of the up-sector where you can do such measurements. The top decays before it hadronizes, and the up-quark has no lighter quark to form a meson (and neutral pions are their own antiparticles).

    For calculations, the charm sector is tricky: The charm is not light (where you can use m_quark<>lambda_QCD gives another way to calculate stuff). To make things even more complicated, the dominant contribution to mixing is not the simple box diagram (like for B-mesons), but long-range contributions with some intermediate mesons in between.

  3. Old Wolf says:

    Charm physics is also the sector for X(3872), something discovered 6 years ago that the Standard Model can’t explain, but doesn’t seem to have gotten a lot of press..

  4. mfb says:

    I would say “it is unclear whether the Standard Model can explain it”.
    Maybe better mass measurements and analyses of the decays can help there in the future…

    • Alexey says:

      Indeed. And lots of theory work :-).

      One correction though — mass measurements would not help it here, but measurements of CP-violation asymmetries in other modes, including charged D-mesons, definitely would. And, once again, lots of theoretical work…

    • mfb says:

      The mass would help at the idea of a DD* bound state.
      I think CP violation in the charm system will get a bit more attention in the future :).

  5. [...] “LHCb has evidence of new physics! Maybe.“, Resonaances. 2. Alexey Petrov, “LHCb reports observation of CP-violation in charm. Welcome New Physics? Or not?“, Quantum Diaries. 3. Anna Phan, “A New Surprising Result!“, Quantum Diaries. 4. [...]

  6. Xezlec says:

    Thanks for the responses. It sounds like much of the charm’s charm might come from being in just the right position, light enough to exist for a while before decaying, but still noticeably separate from the first quark generation.

  7. [...] four full-packed days filled with many aspects of physics related to charmed quark. As I reported earlier, many exciting recent results are associated with charm [...]

  8. [...] neutrinos, but if it holds up it’s big stuff. You can read about it at Résonaances or Quantum Diaries, or look at the talk recently given at the Hadronic Collider Physics Symposium 2011 in Paris. [...]

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