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CERN | Geneva | Switzerland

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Long-standing discrepancy put to rest

This morning at the European Physics Society conference in Stockholm, the LHCb experiment operating at the Large Hadron Collider (LHC) CERN brought one more argument to put to rest a long-standing discrepancy that had kept theorists puzzled for nearly two decades.

LHCb presented the most precise measurement to date of the b baryon lifetime. A baryon is a family of composite particles made of three quarks.  For example, protons and neutrons are made of a combination of u and d quarks.  What makes b baryons so special is that they contain a b quark, a much heavier type of quark. Composite particles containing b quarks like B mesons (made of a b and either a u or d quarks) and b baryons are unstable, meaning they have a short lifetime. About one picosecond after being created, they break down into smaller particles.

In theory, both B mesons and b baryons should have approximately the same lifetime. But in the 1990’s, when CERN operated with its previous accelerator called LEP (Large Electron Positron collider), all experiments measured a systematically shorter lifetime for b baryons than B mesons as can be seen on the plot below. Although the LEP experimental errors were quite large, the general trend of lower values was very puzzling since all four experiments (ALEPH, DELPHI, OPAL and L3) were working independently. Lb_lifetime_comparison

The various b baryon lifetime measurements over time from the oldest results at the bottom to the three latest results from the LHC experiments at the top. The measured value has now shifted toward a value of 1.5 picoseconds, as measured for the B mesons.

This prompted theorists to re-examine their calculations and to look for overlooked effects that could explain the difference. Despite all efforts, it was nearly impossible to reconcile the measured b baryon lifetime (somewhere between 1.1 to 1.3 picosecond) with the B meson lifetime at around 1.5 ps.

Nearly a decade later, D0 and CDF, the two experiments from another accelerator, the Tevatron near Chicago, started closing the gap. It took another decade for the LHC experiments to show that in fact, there is no large difference between b baryon and B meson lifetimes.

Already, earlier this year, ATLAS and CMS both reported values in line with the B meson lifetime. With this latest and most precise result from the LHCb experiment, there is now enough evidence to close the case on this two-decade-old discrepancy. LHCb measured the b baryon lifetime to be 1.482 ± 0.018 ± 0.012 ps. Hence, both lifetimes are now measured close to 1.5 picosecond and LHCb calculated their ratio to be 0.976 ± 0.012 ± 0.006, very close to unity as theoretically expected.

One possible explanation is that all LEP experiments were affected by a common but unknown systematic shift or simply, some statistical fluctuation (i.e. bad luck). The exact cause might never be found but at least, the problem is solved. This is a great achievement for theorists who can now rest assured that their calculations were right after all.

Pauline Gagnon

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14 Responses to “Long-standing discrepancy put to rest”

  1. somewhere in the story b-meson and b-baryon are confused, but i do get the point.

    • CERN says:

      Thanks for alerting me but unfortunately, I reread it and still do not see where I might have confused the two. Could you tell me where you think it got mixed up and I will gladly fix it? Thanks, Pauline

  2. sorry, yes!
    1. …all experiments measured a systematically shorter lifetime for b baryons than B mesons as can be seen on the plot below.
    2. The measured value has now shifted toward a value of 1.5 picoseconds, as measured for the B mesons.
    3. Despite all efforts, it was nearly impossible to reconcile the measured b baryon lifetime (somewhere between 1.1 to 1.3 picosecond) with the B meson lifetime at around 1.5 ps.

    consistant till now, but here it changes:
    3. LHCb measured the b baryon lifetime to be 1.482 ± 0.018 ± 0.012 ps. The ratio to the B meson lifetime is 0.976 ± 0.012 ± 0.006, very close to one as theoretically expected.

    • CERN says:

      Ah, I see what you mean. It is correct but ambiguous.

      The ratio (of 1.482 for b baryons) and (1.5 for B mesons) is not close to one (the ratio is close to one, not the lifetime). I will correct to make it clear.

      Thanks again, Pauline

  3. Ramsdonck Charles says:

    why must both type (b-mesons and b-baryons) have the same desintegration time?

    • Seth Zenz says:

      You expect them to be approximately the same because they both contain a b-quark, and they both decay when that b-quark decays. Then there are more detailed corrections associated with the other quarks that are present, but they’re not large enough to account for the (former) experimental discrepancy.

    • CERN says:

      Thanks Senz for providing an alternative way to look at it. Pauline

  4. Interesting but what i mean is from the last sentence you may conclude that the lifetime of b baryon is about 1.5 and for the B meson around 1 picosecond.

    This is in conflict with sentences 1, 2 and 3 in above comment from July 22, 2013 at 1:35 am.

    KR and thanks for all effort, Egbert

    • CERN says:

      Hello Egbert,

      the two particles lifetimes should be very similar simply because it all comes from the b quark itself. The fact that there is one more spectator quark in a b baryon should not have much effect. This is what was puzzling theorists working on b quark physics. The third quark should have had very little effect at all if any.

      Pauline

    • CERN says:

      Sorry if this last sentence was ambiguous. I have now corrected it. Let me just restate that clearly: B meson lifetime was always measured around 1.5 picosecond. And now, so is the lifetime for b baryons (before, it was smaller). Was is now nearly equal to one is the ratio of these two lifetimes.

      I hope this helps. Cheers, Pauline

  5. Alexey says:

    “Despite all efforts, it was nearly impossible to reconcile the measured b baryon lifetime (somewhere between 1.1 to 1.3 picosecond) with the B meson lifetime at around 1.5 ps.”

    I don’t think this is correct. It is already stated in all HFAG papers that theory predictions with alpha_s^2 and 1/m^4 corrections can accommodate even old D0 results. Unfortunately, theory predictions for the ratio of Lambda_b and B-meson lifetimes have several non-perturvbative parameters that are poorly known both at 1/m_b^3 and higher orders in 1/m_b.

    Also, all larger results (from LHCb, CMS and CDF) are form the same J/psi Lambda channel — it would be nice to see confirmation with semileptonic or Lambda_c pi channels as well.

    • CERN says:

      Thanks Alexei for the precision, even though you are getting way more technical that we usually get here. Not that it means I do not want it to be accurate though. You may very well be right that new theoretical corrections help bridge the gap there but my understand is that this was still seen as a discrepancy.

      But indeed, the fact that the higher results are all obtained in the J/psi Lambda channel and the lower ones in the semileptonic channel is something to ponder about and it has been mentioned elsewhere as well. Maybe the debate is not closed after all!

      Cheers, Pauline

  6. The experiment effort is correct, but there is lack of real but not only mathematical ideas. First the so called quarks don’t exist in reality – this is only theoretically point which cannot explain the structure of the nuclear particles. The correct answers of these problems lies in the USM nuclear particles http://www.kanevuniverse.com and more You can see in https://www.facebook.com/home.php where are discussed some space connection related to the nuclear particles.

    • CERN says:

      Hello Georgi,

      unfortunately, this is not the place to discuss your own theories. If you want the physics community to discuss your ideas, you shold submit your papers to the physics arXiv.

      Cheers, Pauline

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