This brief review is dedicated to the memory of Maxim V. Polyakov and his pioneering contributions to pentaquark physics. We focus on his seminal 1997 work with Diakonov and Petrov that predicted the ${\mit \Theta }^+$ pentaquark, a breakthrough that initiated an intense period of research in hadron physics. The field faced a significant setback when the CLAS Collaboration at Jefferson Lab reported null results in 2006, leading to a dramatic decline in light-pentaquark research. Nevertheless, Maxim maintained his scientific conviction, supported by continued positive signals from the DIANA and LEPS collaborations. Through recent experimental findings on the ${\mit \Theta }^+$ and the nucleon-like resonance $N^*(1685)$, we examine how Polyakov’s theoretical insights, particularly the prediction of a narrow width (${\mit \Gamma } \approx 0.5$–$1.0$ MeV), remain relevant to our understanding of the ${\mit \Theta }^+$ light pentaquark. Abstract Published by the Jagiellonian University 2025 authors

South Korea

H.-Ch. Kim

Pentaquarks and Maxim V. Polyakov

WIMP-type dark matter may have additional interactions that break baryon number, leading to induced nucleon decays which are subject to direct experimental constraints from proton decay experiments. In this work, we analyze the possibility of continuous baryon destruction, deriving strong limits from the dark matter accumulating inside old neutron stars, as such a process leads to excess heat generation. We construct the simplest particle dark matter model that breaks the baryon and lepton numbers separately but conserves <math><mi>B</mi><mo>−</mo><mi>L</mi></math>. Virtual exchange by DM particles in this model results in dinucleon decay via <math><mi>n</mi><mi>n</mi><mo>→</mo><mi>n</mi><mover><mi>ν</mi><mo>¯</mo></mover></math> and <math><mi>n</mi><mi>p</mi><mo>→</mo><mi>n</mi><msup><mi>e</mi><mo>+</mo></msup></math> processes.

United States

Yohei Ema

Robert McGehee

Maxim Pospelov

Anupam Ray

Dark matter catalyzed baryon destruction

We investigate the <math><mi>W</mi></math> boson’s exotic decay channel, <math><mi>W</mi><mo>→</mo><mo>ℓ</mo><mo>ℓ</mo><mo>ℓ</mo><mi>ν</mi></math>, at the LHC. Although the four-body final states suppress the decay branching ratio, the large production of <math><mi>W</mi></math> bosons makes detecting and precisely measuring this decay probability entirely feasible. Our simulation study indicates that this tiny branching ratio can be measured with subpercent precision at the HL-LHC. This decay channel can also constrain Standard Model extensions. Using the <math><msub><mi>L</mi><mi>μ</mi></msub><mo>−</mo><msub><mi>L</mi><mi>τ</mi></msub></math> model as a benchmark, we find that the current bound on the gauge coupling for <math><msup><mi>Z</mi><mo>′</mo></msup></math> mass in the range of [4, 75] GeV can significantly improve.

China

Yifan Fei

Peiran Li

Zhen Liu

Kun-Feng Lyu

<math><mi>W</mi></math>-boson exotic decay into three charged leptons at the LHC

Current discrepancy between the measurement and the prediction of the muon anomalous magnetic moment can be resolved in the presence of a long-range force created by ordinary atoms acting on the muon spin via axial-vector and/or pseudoscalar coupling, and requiring a tiny <math><mi>O</mi><mo>(</mo><msup><mn>10</mn><mrow><mo>−</mo><mn>13</mn></mrow></msup><mtext> </mtext><mtext> </mtext><mi>eV</mi><mo>)</mo></math> spin energy splitting between muon state polarized in the vertical direction. We suggest that an extension of the muon spin resonance (<math><mi>μ</mi><mi>SR</mi></math>) experiments can provide a definitive test of this class of models. We also derive indirect constraints on the strength of the muon spin force by considering the muon-loop-induced interactions between nuclear spin and external directions. The limits on the muon spin force extracted from the comparison of <math><mrow><mmultiscripts><mrow><mi>Hg</mi></mrow><mprescripts></mprescripts><none></none><mrow><mn>199</mn></mrow></mmultiscripts></mrow><mo>/</mo><mrow><mmultiscripts><mrow><mi>Hg</mi></mrow><mprescripts></mprescripts><none></none><mrow><mn>201</mn></mrow></mmultiscripts></mrow></math> and <math><mrow><mmultiscripts><mrow><mi>Xe</mi></mrow><mprescripts></mprescripts><none></none><mrow><mn>129</mn></mrow></mmultiscripts></mrow><mo>/</mo><mrow><mmultiscripts><mrow><mi>Xe</mi></mrow><mprescripts></mprescripts><none></none><mrow><mn>131</mn></mrow></mmultiscripts></mrow></math> spin precession are strong for the pseudoscalar coupling but are significantly relaxed for the axial-vector one. These limits suffer from significant model uncertainties, poorly known proton/neutron spin content of these nuclei, and therefore do not exclude the possibility of a muon spin force relevant for the muon <math><mi>g</mi><mo>−</mo><mn>2</mn></math>.

Ting Gao

Muon spin force

The search for charged-lepton flavor violation in muon capture on nuclei is a powerful probe of heavy new physics. A smoking gun signal for <math><mi>μ</mi><mo>→</mo><mi>e</mi></math> conversion is a monochromatic electron with energy almost equal to the muon mass. We show that light new physics can mimic this signature and that it can also lead to electrons above the <math><mi>μ</mi><mo>→</mo><mi>e</mi></math> signal peak. A concrete example of such light new physics is <math><msup><mi>μ</mi><mo>−</mo></msup></math>-nucleon annihilation into a light dark sector, which can produce an energetic <math><msup><mi>e</mi><mo>−</mo></msup></math> as well as <math><msup><mi>e</mi><mo>+</mo></msup><msup><mi>e</mi><mo>−</mo></msup></math> by-products. Due to the size of the muon mass, the exotic muon capture process can be kinematically allowed, while the otherwise stringent constraints, e.g., from proton decay, are kinematically forbidden. We also discuss other relevant constraints, including those from the stability of nuclei and muon capture in the interior of neutron stars.

Patrick J. Fox

Matheus Hostert

Tony Menzo

Jure Zupan

Muon-induced baryon number violation

We revisit the chiral properties of nucleon interpolating currents, and show that of the two leading order currents <math><msub><mi>j</mi><mn>1</mn></msub></math> and <math><msub><mi>j</mi><mn>2</mn></msub></math>, only two linear combinations <math><msub><mi>j</mi><mn>1</mn></msub><mo>±</mo><msub><mi>j</mi><mn>2</mn></msub></math> transform covariantly under the anomalous <math><mi>U</mi><mo>(</mo><mn>1</mn><msub><mo>)</mo><mi>A</mi></msub></math> symmetry. As a result, calculations of quantities which vanish by symmetry in the chiral limit may produce unphysical results if carried out with different linear combinations of the currents. This includes observables such as electric dipole moments, induced by the quantum chromodynamics (QCD) parameter <math><mi>θ</mi></math>, and the <math><mi>θ</mi></math>-dependence of the nucleon mass. For completeness, we also exhibit the leading order results for nucleon electric dipole moments (<math><msub><mi>d</mi><mrow><mi>n</mi><mo>,</mo><mi>p</mi></mrow></msub></math>) induced by <math><mi>θ</mi></math>, and the nucleon magnetic moments (<math><msub><mi>μ</mi><mrow><mi>n</mi><mo>,</mo><mi>p</mi></mrow></msub></math>), when calculated using QCD sum rules for both the covariant choices of the nucleon interpolating current. The results in each channel, conveniently expressed as the ratios, <math><msub><mi>d</mi><mrow><mi>n</mi><mo>,</mo><mi>p</mi></mrow></msub><mo>/</mo><msub><mi>μ</mi><mrow><mi>n</mi><mo>,</mo><mi>p</mi></mrow></msub></math>, are numerically consistent, and reflect the required physical dependence on <math><mi>θ</mi></math>.

Canada

Adam Ritz

Chiral properties of the nucleon interpolating current and <math><mi>θ</mi></math>-dependent observables

Strongly-interacting dark matter can be accumulated in large quantities inside the Earth, and for dark matter particles in a few GeV mass range, it can exist in large quantities near the Earth’s surface. We investigate the constraints imposed on such dark matter properties by its upscattering by fast neutrons in nuclear reactors with subsequent scattering in nearby well-shielded dark matter detectors, schemes which are already used for searches of the coherent reactor neutrino scattering. We find that the existing experiments cover new parameter space on the spin-dependent interaction between dark matter and the nucleon. Similar experiments performed with research reactors, and lesser amount of shielding, may provide additional sensitivity to strongly-interacting dark matter.

Probing earth-bound dark matter with nuclear reactors

I discuss implications of rare species of particles that have larger than commonly assumed interaction and/or annihilation cross sections. Large fractions of the parameter space of such models remain unexplored. I point out a number of the most interesting consequences, such as large accumulation of such particles in the earth, direct annihilation inside large neutrino detectors, possibility to accelerate such particles using particle beams with subsequent detection using the direct detection experiments. I also discuss the de-excitations on nuclear isomers through a recoil on dark matter, as well as in-situ formation of the DM-nucleus bound states.

Rare species of dark matter

The simplest cosmologies motivate the consideration of dark matter subcomponents that interact significantly with normal matter. Moreover, such strongly coupled relics may have evaded detection to date if upon encountering the Earth they rapidly thermalize down to terrestrial temperatures, <math><mrow><msub><mrow><mi>T</mi></mrow><mrow><mo>⊕</mo></mrow></msub><mo>∼</mo><mn>300</mn><mtext> </mtext><mtext> </mtext><mi>K</mi><mo>∼</mo><mspace></mspace><mn>25</mn><mtext> </mtext><mtext> </mtext><mi>meV</mi></mrow></math>, well below the thresholds of most existing dark matter detectors. This shedding of kinetic energy implies a drastic enhancement to the local density, motivating the consideration of alternative detection techniques sensitive to a large density of slowly moving dark matter particles. In this work, we provide a rigorous semianalytic derivation of the terrestrial overdensities of strongly coupled relics, with a particular focus on millicharged particles (MCPs). We go beyond previous studies by incorporating improved estimates of the MCP-atomic scattering cross section, new contributions to the terrestrial density of sub-GeV relics that are independent of the Earth’s gravitational field, and local modifications that can arise due to the cryogenic environments of precision sensors. We also generalize our analysis in order to estimate the terrestrial density of thermalized MCPs that are produced from the collisions of high-energy cosmic rays and become bound by the Earth’s electric field.

Asher Berlin

Hangwan Liu

Harikrishnan Ramani

Terrestrial density of strongly-coupled relics

We study the exotic muon decays with five charged tracks in the final state. First, we investigate the Standard Model rate for μ + → 3e + 2e  −  2ν (  <math> <mi>B</mi> </math>  $$ \mathcal{B} $$  = 4.0 × 10 −10) and find that the Mu3e experiment should have tens to hundreds of signal events per 1015 μ + decays, depending on the signal selection strategy. We then turn to a neutrinoless μ + → 3e + 2e  −  decay that may arise in new-physics models with lepton-flavor-violating effective operators involving a dark Higgs h  d . Following its production in μ + → e + h  d  decays, the dark Higgs can undergo a decay cascade to two e + e  −  pairs through two dark photons, h  d  → γ  d  γ  d  → 2(e + e  − ). We show that a μ + → 3e + 2e  −  search at the Mu3e experiment, with potential sensitivity to the branching ratio at the   <math> <mi>O</mi> </math>  $$ \mathcal{O} $$ (10 −12) level or below, can explore new regions of parameter space and new physics scales as high as Λ ∼ 1015 GeV.