Could the X17 resonance, if it is even real, be an electromagnetically bound light quark-light antiquark meson?
This explanation is much more attractive than a new fundamental particle, as it wouldn't involve beyond the Standard Model physics, and would instead involve a low energy electromagnetically bound up-antiup or down-antidown pair of quarks.
It has to be electromagnetically bound, rather than strong force bound, because a neutral light quark-antiquark pair bound by the strong force, i.e. a neutral pion, has a mass of about 135 MeV, mostly due to the binding energy of the gluons confining them in a hadron.
This said, this theory has a big problem.
Why aren't the light quarks confined in a QCD bound hadronic state?
The only times quarks are not in QCD bound hadronic states that have so far been observed are shortly after top quarks form (because they almost always decay before they can hadronize, although we just learned in 2025 that in rare cases a top anti-top quark pair can form toponium in a QCD bound state the persists very briefly) and in quark-gluon plasma at temperatures corresponding to about 1-2 GeV (i.e. 11-23 trillion Kelvin).
The invariant mass spectrum of e+e− pairs produced in high-energy Pb-emulsion collisions at 160 A GeV at CERN SPS exhibits a complex structure of many resonances resting on top of a broad enhancement at invariant masses below 50 MeV, with the prominent resonance at 19 ±1 MeV providing independent support for the hypothetical X17 particle.
We show that this complex structure may be coherently described as signatures for the neutral color-singlet qq¯ quark matter in both its deconfined and confined phases. That is, the broad enhancement may arise from thermal annihilation of QED(U(1))-deconfined quarks and antiquarks into e+e− pairs at the phase transition temperature Tc(QED), theoretically estimated to be 4.75 ± 1.2 MeV from the transitional equilibrium condition. The observed 3±1 and 7±1 MeV resonances may correspond to the QED(U(1))-deconfined dd¯ and uu¯ Coulomb bound states near their quark rest masses, respectively, whereas the observed 19 ± 1 MeV resonance may correspond to the QED(U(1))-confined isoscalar QED meson.
The approximate agreement between the theoretical and the experimental spectrum suggests that both QED(U(1))-confined and QED(U(1))-deconfined neutral color-singlet qq¯ quark matter may have been produced in these high-energy Pb-emulsion collisions. We propose future experiments to confirm or refute these findings.
Cheuk-Yin Wong, "Possible Evidence for Neutral Color-Singlet qq¯ Quark Matter from High-Energy Pb-Emulsion Collisions" arXiv:2604.23473 (April 25, 2026) (21 pages).
Some conjectures
What would work without breaking the rules of the Standard Model, however, is if the 3 and 7 MeVs were light quark-antiquark pairs that were produced and immediately annihilated before they could hadronize, and if the 19 MeV resonance was an electromagnetically bound positron-electron state (i.e. positronium). Positronium has a ground state mass of 1.022 MeV (twice the 0.511 MeV mass of an electron or positron), however, with excited states varying in mass by single digit eV amounts per state, which wouldn't generate a single resonance at 17-19 MeV.
Another possibility is that the observed 3 ± 1 MeV resonances may correspond to the QED(U(1))-deconfined uu¯ Coulomb bound state near its quark rest masses, that the 7 ± 1 MeV resonances correspond to the QED(U(1))-deconfined dd¯ Coulomb bound state and also to uu¯uu¯ Coulomb bound state near their respective quark rest masses, and that the observed 19 ± 1 MeV resonance may correspond to the QED(U(1))-deconfined dd¯dd¯ Coulomb bound state.
The light quark masses, according to the Particle Data Group (admittedly at the 1-2 GeV energy scale and not the low single digit to tens of MeVs energy scale) is as follows:
The rest mass of four d-quarks is 18.8 MeV, which is right where the resonance is observed.
In this hypothesis, these resonances fail to hadronize because the e+e− pairs that produced one or two light quark-antiquark pairs didn't have enough mass-energy to form a 135 MeV neutral pion, so they instead formed one or two deconfined quark-antiquark pairs that quickly annihilate again because the system had enough energy to create the quarks, but not enough energy to create the bound system of quarks and gluons necessary to form a pion. This has the virtue, again, of not requiring any BSM fundamental particles or new forces.
A four quark solution requires angular momentum that wouldn't normally be present in a simple e+e− pair, but if there were two e+e− pairs in close proximity, both with only modest kinetic energy, which is plausible in the context of the complex overall environment of the high-energy Pb-emulsion collisions generating the data here, or the interactions of the full fledged multi-nucleon atoms present in other contexts where there are claimed sightings of the X17 resonance, a coincidence of two low energy e+e− pairs would be expected with some calculable frequency.
This explanation would still be ground breaking, as it would represent a third circumstance, previously unknown and not predicted, where quarks are (briefly) deconfined. But it would be far less radical than most of the alternative explanations.
5 comments:
Cheuk-Yin Wong
Physics Division, Oak Ridge National Laboratory
Verified email at ornl.gov
Advisor:
John Archibald Wheeler
present
Oak Ridge
1966
PHD, Princeton U.
UNDERGRADUATE, Princeton U.
https://inspirehep.net/authors/983243
Princeton U is top tier physics Edward Witten
"The invariant mass spectrum of pairs produced in high-energy Pb-emulsion collisions at 160 A GeV at CERN SPS exhibits a complex structure of many resonances resting on top of a broad enhancement at invariant masses below 50 MeV, with the prominent resonance at 19 1 MeV providing independent support for the hypothetical X17 particle."
[Submitted on 24 Apr 2026]
A Flavor Specific Chiral Framework for Explaining the ATOMKI Anomaly
Aditya Batra, F. R. Joaquim, Hemant Prajapati, Rahul Srivastava
Recent anomalies in nuclear transitions observed by the ATOMKI collaboration suggest the existence of a new boson with a mass of MeV. A theoretically consistent interpretation requires a framework that not only matches the kinematics but also reproduces the observed decay rates while satisfying stringent experimental constraints. Among various possibilities, an axial-vector or mixed vector-axial-vector mediator emerges as the most viable candidate. However, getting such couplings for a light gauge boson is highly non trivial task. In this work, we construct a gauged chiral, flavor specific extensions of the Standard Model where the associated boson acts as the MeV particle. By employing a two Higgs doublet framework, we generate the necessary non-vanishing axial-vector couplings while ensuring gauge anomaly cancellation and consistent fermion mass generation. Focusing on the and signals, we show that in this model the viable parameter space to resolve the ATOMKI anomalies is also consistent with a diverse set of experimental constraints, including atomic parity violation, beam dump experiments, meson decays, and neutrino nucleus and neutrino electron scatterings. Our results demonstrate that this framework offers a theoretically sound and phenomenologically robust solution to the ATOMKI anomaly.
Comments: 34 pages, 5 figures, 8 Tables
Subjects: High Energy Physics - Phenomenology (hep-ph)
Cite as: arXiv:2604.22278 [hep-ph]
why x17 is hard to qualify
[Submitted on 15 Apr 2026]
Cornering MeV-GeV Axions and Dark Photons with LDMX
Sarah Gaiser, Alessandro Russo, Philip Schuster
Axion-like particles (ALPs), the QCD axion, and dark photons in the MeV-GeV mass range are motivated by various dark matter models and the strong CP problem, and are ubiquitous in extensions of the Standard Model. A long-standing blind spot for experimental searches is the sub-100 MeV mass range, where the particle lifetime is too long to be constrained by prompt-decay collider searches yet too short to be reached by beam-dump experiments. We investigate and estimate the sensitivity of the Light Dark Matter eXperiment (LDMX) to such axions and dark photons, motivated by the clean environment in which these particles can be produced and by the near-target tracking capabilities of LDMX. With reasonable charged track and momentum reconstruction capabilities, we find that LDMX could close much of this low-mass blind spot for axions and dark photons.
Comments: 9 pages, 5 figures
Subjects: High Energy Physics - Phenomenology (hep-ph); High Energy Physics - Experiment (hep-ex); High Energy Physics - Theory (hep-th)
Cite as: arXiv:2604.14285 [hep-ph]
"A long-standing blind spot for experimental searches is the sub-100 MeV mass range, where the particle lifetime is too long to be constrained by prompt-decay collider searches yet too short to be reached by beam-dump experiments. "
experiment
"The invariant mass spectrum of pairs produced in high-energy Pb-emulsion collisions at 160 A GeV at CERN SPS exhibits a complex structure of many resonances resting on top of a broad enhancement at invariant masses below 50 MeV, with the prominent resonance at 19 1 MeV providing independent support for the hypothetical X17 particle."
Cheuk-Yin Wong, "Possible Evidence for Neutral Color-Singlet qq¯ Quark Matter from High-Energy Pb-Emulsion Collisions" arXiv:2604.23473 (April 25, 2026) (21 pages).
We show that the process of Coherent Elastic neutrino (v) Nucleus Scattering (CEvNS) at nuclear reactor experiments has significant sensitivity to the so-called X17 particle, which has been invoked to explain the ATOMKI anomaly, wherein electron-positron pairs emerging from a nuclear transition of excited Be-8, He-4 and C-12 nuclei are studied.
Specifically, we fit CONUS+ and Dresden-II data and show that a robust statistical analysis renders these more compatible with the X17 hypothesis, in turn interfering with the Standard Model, than with that of the latter alone. The same stays true when also adding COHERENT data from pi+ decays at rest, singling out two regions of preferred couplings of the X17 to electron and muon neutrinos as well as nuclei.
Glimpses of the X17 from coherent elastic neutrino nucleus scattering
Johan Rathsman, Joakim Cederkäll, Yasar Hicyilmaz, Else Lytken, Stefano Moretti
Recent anomalies observed in nuclear transitions of , , and by the ATOMKI collaboration may hint at the existence of a vector boson with a mass around 17 MeV, referred to as X17. If it exists, this boson would also affect similar processes in particle physics, including the Dalitz decays of vector mesons. Recently, the BESIII collaboration measured the Dalitz decay for the first time and reported a excess over the theoretical prediction based on the vector meson dominance (VMD) model. This excess may be another signal of the X17. In this study, we investigate the possible effects of the X17 on the Dalitz decays , , and . The required hadronic form factors are calculated within the framework of our covariant confined quark model, without relying on heavy quark effective theory or the VMD model. We present predictions for the Dalitz decay widths and the ratios within the Standard Model and in several new physics scenarios involving modifications due to the X17. Our results are compared with other theoretical calculations.
Probing the ATOMKI X17 vector boson using Dalitz decays
Chien-Thang Tran, Mikhail A. Ivanov, Anh-Tuyet T. Nguyen
In order to develop the statistical model for proton–nucleus collisions at the
stage of expansion of the compound nuclear system, the adiabatic temperature change
and the correction to the Boltzmann distribution of the multiplicity of emitted secondary
particles are additionally taken into account. As a result, improved agreement with
experimental data is obtained compared to previous studies for the soft-photon spectrum
by transverse momentum in pp collisions at an incident proton momentum of 450 GeV/c
in order to isolate a clearly expressed X17 signal at around 17 MeV. An interpretation of
the detection of a boson with mass 38 MeV in the spectra of photons emitted in reactions
of protons with carbon nuclei at an incident proton momentum of 5.5 GeV/c is proposed.
Analyzing the spectra of ultra-high-energy cosmic rays (photons), confirmation of the
existence of new particles — the X17 and X38 bosons, with masses of 17 MeV and 38
MeV, respectively, has been found
On the Possible Detection of New Particles from Data on
Soft Photons in Collisions of Protons and Nuclei a
Composites of SM particles are an easier pill to swallow than new fundamental particles and forces.
In search of new neutral boson particles with masses below 100 MeV decaying into e+e− pairs, Jain
and Singh examined the tracks of e+e− pairs produced in central collisions of high-energy 207Pb nuclei
incident on the AgBr emulsion at the energy of 160 A GeV at CERN SPS [1]. Among their data sets,
they studied in particular the invariant mass spectrum of the e+e− pairs originating at a distance of
50 μm to 200 μm away from the primary Pb-emulsion collision vertex, corresponding to the search
for neutral boson particles which were produced in the Pb-emulsion collisions and subsequently
decayed into e+e− pairs with lifetimes of order 10−15s to 10−12s.
[1] P. L. Jain and G. Singh, Search for new particles decaying into electron pairs of mass below 100 MeV/c2 , J. Phys. G 34,
134
From the Pb-emulsion spectrum with many
resonances and an enhancement at many different energies, as we now know in Fig. 1, those earlier
observations of e+e− resonances at many different energies at {∼2, ∼9, ∼12, ∼17} MeV are likely
the natural consequences of the complexity of the e+e− spectrum as shown in Fig. 1. The energy
locations of these earlier reported neutral boson resonances coincide with the energy locations of the
boson resonances and the peak of the enhancement observed by Jain and Singh in Fig. 1. It was
not that a single resonance was mis-identified at many different energies, as it was often assumed
in the search for a single elementary particle using e+e− pairs.
what experiments would determine whether x17 if successful candidate will help determine whether Composites of SM particles hypothesis or new fundamental particles and forces?
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