[Primexd] [EXTERNAL] Re: Photoproduction test of the solution to the KLOE–MAMI η→π⁰γγ discrepancy.

[Gan, Liping]

Dear Yaroslav,

In the figure 3 of your paper, the nucleon acts as a spectator in  the left two diagrams (SM part). Therefore, the invariant mass of  final state particles (such as pi0 and 2 gammas) should be the mass of the initial decaying meson. The right two diagrams (V_B part) involves interactions with nucleon, as a result, the reconstructed  invariant mass of  final state particles (pi0, 2 gammas) may not be the same as the initial meson.

For all experimental analysis (including A2 and AGS papers), one alyway requires that the invariant mass of  final state particles (pi0, 2 gammas) matches the mass of the initial decaying meson. The V_B may contribute to the experimental result  due to non-perfect experimental resolutions. Therefore, the V_B comes in the game only partially. In other word, the new physics contribution may depend on the experimental sensitivities.

Best,
Liping


________________________________
From: Alexander Somov <somov at jlab.org>
Sent: Tuesday, November 18, 2025 5:11 PM
To: Yaroslav Balytskyi <hr6998 at wayne.edu>; Gan, Liping <ganl at uncw.edu>; Dave Mack <mack at jlab.org>
Subject: Re: Photoproduction test of the solution to the KLOE–MAMI η→π⁰γγ discrepancy.


[This email originated from outside of UNCW]

Dear Yaroslav,

Thanks a lot for the information. Interesting, I'll take a look.

Cheers,
                 Sasha

________________________________
From: Yaroslav Balytskyi <hr6998 at wayne.edu>
Sent: Tuesday, November 18, 2025 4:08 PM
To: ganl at uncw.edu <ganl at uncw.edu>; Alexander Somov <somov at jlab.org>; Dave Mack <mack at jlab.org>
Subject: [EXTERNAL] Photoproduction test of the solution to the KLOE–MAMI η→π⁰γγ discrepancy.

   Dear All!

   As you probably know, the recent KLOE measurement of the η →π⁰γγ decay: https://urldefense.proofpoint.com/v2/url?u=https-3A__arxiv.org_abs_2505.09285&d=DwIGaQ&c=CJqEzB1piLOyyvZjb8YUQw&r=lU05Rh9aCuulfMNDTEUwllfR5wGoS__-zKJvNoD4iqU&m=nkaBBXP3bbVSFL1NBIB6pz1sIICSda-vodfqlcl7d0y65wSEHdOgctj9L0OBmJbw&s=NgJOCHbE4wrNeVSRbpXsZ3eOnZZM0QV-pLdQvGpeXS8&e= <https://urldefense.proofpoint.com/v2/url?u=https-3A__arxiv.org_abs_2505.09285&d=DwMGaQ&c=CJqEzB1piLOyyvZjb8YUQw&r=UEJkvUcBvE4mBa6l1pQs5Q&m=y2yvftm61UrsjzrRwHxHz2jA59FM1oc6q_jEfPjqFG8je0Yi5KeuhmPytHwI52p1&s=0n0_ahtSxYfU_jAyg4MvEF469KMUgpAOmyzdw9AYjfc&e=>, finds the result twice smaller than the current world average.

   The PDG - KLOE difference is around 5.5 σ.

   I propose a solution to this discrepancy via a new nucleon-triggered vector VB: https://urldefense.proofpoint.com/v2/url?u=https-3A__arxiv.org_abs_2507.09436&d=DwIGaQ&c=CJqEzB1piLOyyvZjb8YUQw&r=lU05Rh9aCuulfMNDTEUwllfR5wGoS__-zKJvNoD4iqU&m=nkaBBXP3bbVSFL1NBIB6pz1sIICSda-vodfqlcl7d0y65wSEHdOgctj9L0OBmJbw&s=K527-N179xsN7DbwJWOf4SxpjBCNEfVvKZEIhMNMD5U&e= <https://urldefense.proofpoint.com/v2/url?u=https-3A__arxiv.org_abs_2507.09436&d=DwMGaQ&c=CJqEzB1piLOyyvZjb8YUQw&r=UEJkvUcBvE4mBa6l1pQs5Q&m=y2yvftm61UrsjzrRwHxHz2jA59FM1oc6q_jEfPjqFG8je0Yi5KeuhmPytHwI52p1&s=w1upi2cN0KhyD0LDVquxGs2jr48WXLRlI0pI15ohEr0&e=>

   If the KLOE-MAMI discrepancy is real (not experimental systematics), the same mechanism will inevitably manifest itself in the 2 -> 3 photoproduction via:

   γ + p → VB ​+ π/η/η' + p

   There can be three options:

   1) VB invisible (search via missing mass)

   2) VB decays to an invisible dark pion and the visible photon (tagging on the extra photon)

   3) VB decays to visible dark pion and visible photon (visible ALP particle, visible SM pseudoscalar, visible photon).

   As far as I understand, GlueX already measures 2 -> 3 reactions γp → ω π0p and γp →ω ηp, and therefore should be in an excellent position to confirm or exclude this scenario with a dedicated analysis.

  Please see the arXiv files I'm referring to and the illustration "Photoproduction_VB_nucl.pdf" attached.

  If you are interested, I can provide more details.

  BR, YB


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