Current data on ultrahigh-energy (UHE) cosmic rays suggest they are predominantly made of heavy nuclei. This indicates that, in the absence of a subdominant proton component, the flux of neutrinos produced from cosmic ray collisions on the cosmic microwave background is small and hard to observe. Motivated by the recent extremely high-energy muon event reported by KM3NeT, we explore the possibility of enhancing the energy-flux of cosmogenic neutrinos through nuclear photodisintegration in the presence of new physics. Specifically, we speculate that UHE neutrons may oscillate into a new state, dark (or mirror) neutron <math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><mrow><msup><mrow><mi>n</mi></mrow><mrow><mo>′</mo></mrow></msup></mrow></math> that in turn decays injecting large amount of energy to neutrinos, <math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><mi>n</mi><mo stretchy="false">→</mo><msup><mi>n</mi><mo>′</mo></msup><mo stretchy="false">→</mo><msub><mi>ν</mi><mi mathsize="small">UHE</mi></msub></math>. While this mechanism does not explain the tension between the KM3NeT event and null results from IceCube, it reconciles the experimental preference for a heavier cosmic ray composition with a large diffuse cosmogenic flux of UHE neutrinos.

Brazil

Gustavo F.S. Alves

United States

Matheus Hostert

Maxim Pospelov

Neutron portal to ultrahigh-energy neutrinos

Axionlike particles (ALPs) are well-motivated examples of light, weakly coupled particles in theories beyond the Standard Model. In this work, we study long-lived ALPs coupled exclusively to leptons in the mass range between <math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><mn>2</mn><msub><mi>m</mi><mi>e</mi></msub></math> and <math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><msub><mi>m</mi><mi>τ</mi></msub><mo>−</mo><msub><mi>m</mi><mi>e</mi></msub></math>. For anarchic flavor structure the leptophilic ALP production in tau decays or from ALP-tau bremsstrahlung is enhanced thanks to derivative couplings of the ALP and can surpass production from electron and muon channels, especially for ALPs heavier than <math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><msub><mi>m</mi><mi>μ</mi></msub></math>. Using past data from high-energy fixed-target experiments such as CHARM and BEBC we place new constraints on the ALP decay constant <math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><msub><mi>f</mi><mi>a</mi></msub></math>, reaching scales as high as <math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><mi mathvariant="script">O</mi><mo stretchy="false">(</mo><msup><mn>10</mn><mn>8</mn></msup><mo stretchy="false">)</mo><mtext> </mtext><mtext> </mtext><mi>GeV</mi></math> in lepton-flavor-violating channels and <math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><mrow><msub><mrow><mi>f</mi></mrow><mrow><mi>a</mi></mrow></msub><mo>∼</mo><mi mathvariant="script">O</mi><mo stretchy="false">(</mo><msup><mrow><mn>10</mn></mrow><mrow><mn>2</mn></mrow></msup><mo stretchy="false">)</mo><mtext> </mtext><mtext> </mtext><mi>GeV</mi></mrow></math> in lepton-flavor-conserving ones. We also present projections for the event-rate sensitivity of current and future detectors to ALPs produced at the Fermilab Main Injector, the CERN SPS, and in the forward direction of the LHC. We show that SHiP will be sensitive to <math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><msub><mi>f</mi><mi>a</mi></msub></math> values that are over an order of magnitude above the existing constraints.

CERN

Yohei Ema

Patrick J. Fox

Tony Menzo

Anupam Ray

Jure Zupan

Long-lived axionlike particles from tau decays

We entertain the possibility that transient astrophysical sources can produce a flux of dark particles that induce ultra-high-energy signatures at neutrino telescopes such as IceCube and KM3NeT. We construct scenarios where such “dark flux” can produce meta-stable dark particles inside the Earth that subsequently decay to muons, inducing through-going tracks in large-volume neutrino detectors. We consider such a scenario in light of the 𝒪(70) PeV ultra-high-energy muon observed by KM3NeT and argue that because of its location in the sky and the strong geometrical dependence of the signal, such events would not necessarily have been observed by IceCube. Our model relies on the upscattering of a new particle $\textit{X}$ onto new metastable particles that decay to dimuons with decay lengths of 𝒪(100) km. This scenario can explain the observation by KM3NeT without being in conflict with the IceCube data.

Iran

Yasaman Farzan

Astrophysical flux of dark particles as a solution to the KM3NeT and IceCube tension over KM3-230213A

The Standard Model (SM) of particle physics provides a very successful description of fundamental particles and their interactions but it is incomplete, as neutrino masses, dark matter and the baryon asymmetry of the Universe indicate. In addition, the origin of masses and of the approximate fundamental symmetries call out for deeper explanations. The quest for a New SM Theory, that extends the SM to a more general theory, is ongoing. For decades the main focus has been on the TeV scale, but despite an impressive theoretical and experimental effort, no hints of new physics at such scale has been found in experiments.

Spain

Asli Abdullahi

Czechia

Francesco Costa

Italy

Andrea Giovanni De Marchi

Alessandro Granelli

Poland

Jaime Hoefken-Zink

Michele Lucente

Elina Merkel

Jacopo Nava

Silvia Pascoli

Salvador Rosauro-Alcaraz

Filippo Sala

From oversimplified to overlooked: The case for exploring rich dark sectors

Muon colliders provide an exciting new direction to expand the energy frontier of particle physics. We point out a new use of these facilities for neutrino and beyond the standard model physics using their main detectors. Muon decays along the accelerator rings create an intense and highly collimated neutrino beam that crosses a thin slice of the kt-scale detector. As a result, it would induce an unprecedented number of neutrino interactions, with <math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><mrow><mi mathvariant="script">O</mi><mo stretchy="false">(</mo><msup><mrow><mn>10</mn></mrow><mrow><mn>4</mn></mrow></msup><mo stretchy="false">)</mo></mrow></math> events per second for a 10 TeV <math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><mrow><msup><mrow><mi>μ</mi></mrow><mrow><mo>+</mo></mrow></msup><msup><mrow><mi>μ</mi></mrow><mrow><mo>−</mo></mrow></msup></mrow></math> collider. These interactions are highly energetic and possess a distinct timing signature and a large transverse displacement. We discuss promising applications of these events for instrumentation, electroweak, and beyond-the-standard model physics. For instance, a subpercent measurement of the neutrino-electron scattering rate enables new precision measurements of the Weak angle and a novel detection of the neutrino charge radius.

Luc Bojorquez-Lopez

Carlos A. Argüelles

Zhen Liu

Muon Colliders and the Neutrino Slice

The sterile neutrino interpretation of the LSND and MiniBooNE neutrino anomalies is currently being tested at three liquid argon detectors: MicroBooNE, SBND, and ICARUS. It has been argued that a degeneracy between <math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><mrow><msub><mrow><mi>ν</mi></mrow><mrow><mi>μ</mi></mrow></msub><mo stretchy="false">→</mo><msub><mrow><mi>ν</mi></mrow><mrow><mi>e</mi></mrow></msub></mrow></math> and <math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><mrow><msub><mrow><mi>ν</mi></mrow><mrow><mi>e</mi></mrow></msub><mo stretchy="false">→</mo><msub><mrow><mi>ν</mi></mrow><mrow><mi>e</mi></mrow></msub></mrow></math> oscillations significantly degrades their sensitivity to sterile neutrinos. Through an independent study, we show two methods to eliminate this concern. First, we resolve this degeneracy by including external constraints on <math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><msub><mi>ν</mi><mi>e</mi></msub></math> disappearance from the PROSPECT reactor experiment. Second, by properly analyzing the full three-dimensional parameter space, we demonstrate that the stronger-than-sensitivity exclusion from MicroBooNE alone already covers the entire <math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><mrow><mn>2</mn><mi>σ</mi></mrow></math> preferred regions of MiniBooNE at the level of <math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><mrow><mn>2</mn><mo>−</mo><mn>3</mn><mi>σ</mi></mrow></math>. We show that upcoming searches at SBND and ICARUS can improve on this beyond the <math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><mn>4</mn><mi>σ</mi></math> level, thereby providing a rigorous test of short-baseline anomalies.

Ohana Benevides Rodrigues

Kevin J. Kelly

Bryce Littlejohn

Pedro A.N. Machado

Ibrahim Safa

Tao Zhou

Toward a Robust Confirmation or Refutation of the Sterile-Neutrino Explanation of Short-Baseline Anomalies

The MiniBooNE low-energy excess stands as an unexplained anomaly in short-baseline neutrino oscillation experiments. It has been shown that it can be explained in the context of dark sector models. Here, we provide an overview of the possible new-physics solutions based on electron, photon, and dilepton final states. We systematically discuss the various production mechanisms for dark particles in neutrino-nucleus scattering. Our main result is a comprehensive fit to the MiniBooNE energy spectrum in the parameter space of dark neutrino models, where short-lived heavy neutral leptons are produced in neutrino interactions and decay to <math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><msup><mi>e</mi><mo>+</mo></msup><msup><mi>e</mi><mo>−</mo></msup></math> pairs inside the detector. For the first time, other experiments will be able to directly confirm or rule out dark neutrino interpretations of the MiniBooNE low-energy excess.

Asli M. Abdullahi

Jaime Hoefken Zink

Belgium

Daniele Massaro

Panorama of new-physics explanations to the MiniBooNE excess

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.

Muon-induced baryon number violation

Long-standing anomalous experimental results from short-baseline neutrino experiments have persisted for decades. These results, when interpreted with one or more light sterile neutrinos, are inconsistent with numerous null results experimentally. However, if the sterile neutrino decays en route to the detector, this can mimic <math><mrow><msub><mrow><mi>ν</mi></mrow><mrow><mi>μ</mi></mrow></msub><mo>→</mo><msub><mrow><mi>ν</mi></mrow><mrow><mi>e</mi></mrow></msub></mrow></math> oscillation signals while avoiding many of these external constraints. We revisit this solution to the MiniBooNE and LSND puzzles in view of new data from the MicroBooNE experiment at Fermilab. Using MicroBooNE’s liquid-argon time-projection chamber search for an excess of <math><msub><mi>ν</mi><mi>e</mi></msub></math> in the Booster beam, we derive new limits in two models’ parameter spaces of interest; where the sterile neutrino decays (I) via mixing with the active neutrinos, or (II) via higher-dimensional operators. We also provide an updated, comprehensive fit to the MiniBooNE neutrino- and antineutrino-beam data, including appearance (<math><msub><mi>ν</mi><mi>e</mi></msub></math>) and disappearance (<math><msub><mi>ν</mi><mi>μ</mi></msub></math>) channels. Despite alleviating the tension with muon neutrino disappearance experiments, we find that the latest MicroBooNE analysis rules out the decaying sterile neutrino solution in a large portion of the parameter space at more than 99% CL.

Decaying sterile neutrinos at short baselines

We study the exotic muon decays with five charged tracks in the final state. First, we investigate the Standard Model rate for $\textit{μ}$ → 3$\textit{e}$ 2$\textit{e}$ 2$\textit{ν}$ ( $$ \mathcal{B} $$ = 4$\textit{.}$0 × 10 ) and find that the Mu3e experiment should have tens to hundreds of signal events per 10 $\textit{μ}$ decays, depending on the signal selection strategy. We then turn to a neutrinoless $\textit{μ}$ → 3$\textit{e}$ 2$\textit{e}$ decay that may arise in new-physics models with lepton-flavor-violating effective operators involving a dark Higgs $\textit{h}$ . Following its production in $\textit{μ}$ → $\textit{e}$ $\textit{h}$ decays, the dark Higgs can undergo a decay cascade to two $\textit{e}$ $\textit{e}$ pairs through two dark photons, $\textit{h}$ $\textit{→ γ}$ $\textit{γ}$ $\textit{→}$ 2($\textit{e}$ $\textit{e}$ ). We show that a $\textit{μ}$ $\textit{→}$ 3$\textit{e}$ 2$\textit{e}$ search at the Mu3e experiment, with potential sensitivity to the branching ratio at the $$ \mathcal{O} $$ (10 ) level or below, can explore new regions of parameter space and new physics scales as high as Λ ∼ 10 GeV.

Canada

New physics in multi-electron muon decays