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<p>The rare hyperon decay Σ<sup>+</sup> → <i>pℓ</i><sup>+</sup><i>ℓ</i><sup>−</sup> is a flavour-changing neutral current process mediated by an <i>s</i> → <i>d</i> transition that occurs only at loop level within the Standard Model. Consequently, this decay is highly suppressed, making it a promising avenue for probing potential new physics. While phenomenological calculations have made important progress in predicting the decay amplitude, there remains a four-fold ambiguity in the relevant transition form factors that prevents a unique prediction for the branching fraction and angular observables. Fully resolving this ambiguity requires a first-principles Standard-Model calculation, and the recent observation of this process using LHCb Run 2 data reinforces the timeliness of such a calculation. In this work, we present the first lattice-QCD calculation of this decay, performed using a 2+1-flavour domain-wall fermion ensemble with a pion mass of 340 Mev. At a small baryon source-sink separation, we observe the emergence of a signal in the relevant baryonic four-point functions. This allows us to determine the positive-parity form factors for the rare hyperon decays from first-principles, albeit with large statistical and systematic uncertainties.</p>

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Exploratory calculation of the rare hyperon decay Σ+ → pℓ+ℓ− from lattice QCD

  • 期刊论文

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  • 发表日期：

    2025年7月2日

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  • Felix Erben,
  • Vera Gülpers,
  • Maxwell T. Hansen,
  • Raoul Hodgson,
  • Antonin Portelli

摘要

The rare hyperon decay Σ+ → pℓ+ℓ− is a flavour-changing neutral current process mediated by an s → d transition that occurs only at loop level within the Standard Model. Consequently, this decay is highly suppressed, making it a promising avenue for probing potential new physics. While phenomenological calculations have made important progress in predicting the decay amplitude, there remains a four-fold ambiguity in the relevant transition form factors that prevents a unique prediction for the branching fraction and angular observables. Fully resolving this ambiguity requires a first-principles Standard-Model calculation, and the recent observation of this process using LHCb Run 2 data reinforces the timeliness of such a calculation. In this work, we present the first lattice-QCD calculation of this decay, performed using a 2+1-flavour domain-wall fermion ensemble with a pion mass of 340 Mev. At a small baryon source-sink separation, we observe the emergence of a signal in the relevant baryonic four-point functions. This allows us to determine the positive-parity form factors for the rare hyperon decays from first-principles, albeit with large statistical and systematic uncertainties.