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50 GeV π^- in, nothing out: a sensitive probe of invisible η and η' decays with NA64h

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arxiv 2406.01990 v1 pith:W2L6VC4G submitted 2024-06-04 hep-ex hep-ph

50 GeV π^- in, nothing out: a sensitive probe of invisible η and η' decays with NA64h

Yu. M. Andreev , A. Antonov , M. A. Ayala Torres , D. Banerjee , B. Banto Oberhauser , J. Bernhard , P. Bisio , A. Celentano
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N. Charitonidis D. Cooke P. Crivelli E. Depero A. V. Dermenev S. V. Donskov R. R. Dusaev T. Enik V. N. Frolov S. V. Gertsenberger S. Girod S. N. Gninenko M. Hosgen V. A. Kachanov Y. Kambar A. E. Karneyeu G. D. Kekelidze B. Ketzer D. V. Kirpichnikov V. N. Kolosov V. A. Kramarenko L. V. Kravchuk N. V. Krasnikov S. V. Kuleshov V. E. Lyubovitskij A. Marini L. Marsicano V. A. Matveev R. Mena Fredes R. G. Mena Yanssen L. Molina Bueno M. Mongillo D. V. Peshekhonov V. A. Polyakov B. Radics K. M. Salamatin V. D. Samoylenko H. Sieber D. A. Shchukin O. Soto V. O. Tikhomirov I. V. Tlisova A. N. Toropin M. Tuzi P. V. Volkov V. Yu. Volkov I. V. Voronchikhin J. Zamora-Saa A. S. Zhevlakov
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classification hep-ex hep-ph
keywords invisibledecaysdarkeventslimitmesonsna64hneutral
verification ladder T0 review T1 audit T2 compute T3 formal T4 reserved
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We present the first results from a proof-of-concept search for dark sectors via invisible decays of pseudoscalar $\eta$ and $\eta'$ mesons in the NA64h experiment at the CERN SPS. Our novel technique uses the charge-exchange reaction of 50 GeV $\pi^-$ on nuclei of an active target as the source of neutral mesons. The $\eta, \eta' \to invisible$ events would exhibit themselves via a striking signature - the complete disappearance of the incoming beam energy in the detector. No evidence for such events has been found with $2.9\times10^{9}$ pions on target accumulated during one day of data taking. This allows us to set a stringent limit on the branching ratio ${\rm Br}(\eta' \to invisible) < 2.1 \times 10^{-4}$ improving the current bound by a factor of $\simeq3$. We also set a limit on ${\rm Br}(\eta \to invisible) < 1.1 \times 10^{-4}$ comparable with the existing one. These results demonstrate the great potential of our approach and provide clear guidance on how to enhance and extend the sensitivity for dark sector physics from future searches for invisible neutral meson decays.

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