Publications

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Searches for new physics that couple with third generation fermions

Valeria D'Amante, on behalf of the CMS collaboration

SciPost Phys. Proc. 17, 015 (2025) · published 23 July 2025 | · pdf

The $\tau$ lepton, with a mass of $1776.86 ± 0.12$ MeV, is unique in its ability to decay into hadrons and a neutrino. Approximately one-third of $\tau$ decays produce an electron or a muon and two neutrinos, denoted as $\tau_e$ and $\tau_\mu$. The remaining decays, mainly involving hadrons and a tau neutrino, are denoted as $\tau_h$. At the CERN LHC, searches involving $\tau$ leptons are crucial for studying the decay of Higgs bosons to $\tau$ pairs, probing Yukawa couplings, and CP properties of the Higgs. These measurements support Standard Model (SM) tests and searches for Beyond Standard Model (BSM) physics, including new or heavy Higgs bosons, leptoquarks, supersymmetric particles, or gauge bosons. The $\tau$ lepton polarization in Z boson decays is also significant for probing the SM. Despite its potential as a portal to new physics, the $\tau$ lepton's decay products, especially neutrinos and hadrons, make its reconstruction and identification challenging at the LHC. At the CMS experiment, neutrinos contribute to Missing Transverse Energy (MET), and hadronically decaying $\tau$ leptons are often misidentified as jets, complicating the separation of $\tau$-involved processes from background processes.
Precision $e^-$ beam polarimetry at an $e^+e^-$ B factory using tau-pair events

Caleb Miller

SciPost Phys. Proc. 17, 011 (2025) · published 23 July 2025 | · pdf

A novel technique for measuring the average beam polarization in $e^+e^-$ collisions, known as Tau Polarimetry, has been developed by the BABAR collaboration. This technique makes use of the relationship between the polarization of the beams, the resulting polarization of the $\tau^+\tau^-$ produced, and the kinematics of the decay products. Using this technique BABAR has measured the average $e^-$ beam polarization of PEP-II to be 0.0034 $± 0.0024_{stat} ± 0.0029_{sys}$. This technique is expected to be used at Belle II after a proposed polarization upgrade as the most precise method with which beam polarization can be determined.
Non-topological edge-localized Yu-Shiba-Rusinov states in CrBr$_3$/NbSe$_2$ heterostructures

Jan P. Cuperus, Daniel Vanmaekelbergh, Ingmar Swart

SciPost Phys. 18, 141 (2025) · published 28 April 2025 | · pdf

Topological superconductivity is predicted to emerge in certain magnet-superconductor hybrid systems. Here, we revisit the heterostructure of insulating monolayer CrBr$_{3}$ and NbSe$_{2}$, for which different conclusions on the presence of topological superconductivity have been reported. Using low-temperature scanning tunneling microscopy and (shot noise) spectroscopy, we find that the superconducting gap well inside the CrBr$_{3}$ islands is not affected by magnetism. At the island edges, we observe Yu-Shiba-Rusinov (YSR) states at a variety of in-gap positions, including zero energy. The absence of topological superconductivity is verified by extensive dI/dV measurements at the CrBr$_{3}$ island edges. Our results ask for a more detailed understanding of the interaction between magnetic insulators and superconductors.
Exotic structures in exotic nuclei

F. Miguel Marqués

SciPost Phys. Proc. 3, 001 (2020) · published 24 February 2020 | · pdf

Nuclei with proton and neutron numbers away from stability are known as exotic, and provide stringent tests of nuclear models, mainly developed for the description of stable nuclei. However, only the most extreme combinations lead to literally exotic structures, with sometimes unexpected properties. In this contribution we review some selected examples around the neutron dripline of very light elements: neutron halo structures with embedded substructures in the boron chain; the emission of neutron pairs in $^{16}$Be; and the existence of multi-neutron resonances in `superheavy' hydrogen isotopes.
Measurement of the presence of $a_1(1420)$ and $\omega(782)$ in $\tau^{-}\to \pi^{-}\pi^{+}\pi^{-}\nu_{\tau}$ at Belle

Andrei Rabusov, Daniel Greenwald, Stephan Paul

SciPost Phys. Proc. 17, 008 (2025) · published 23 July 2025 | · pdf

We present preliminary results of a partial-wave analysis of $\tau^-\to\pi^-\pi^-\pi^+\nu_\tau$ using data from the Belle experiment at the KEKB $\mathrm{e}^+\mathrm{e}^-$ collider. We validate our model with a model-independent analysis. We see the $\mathrm{a}_1(1420)$ and a G-parity-violating $1^-[\omega(782)\pi]_\mathrm{P}$ wave in tauon decays. Our results will improve models used in simulation studies necessary for measuring the electric and magnetic dipole moments and Michel parameters of the $\tau$.
Electric and magnetic tau dipole moments revisited

G. A. González-Sprinberg

SciPost Phys. Proc. 17, 004 (2025) · published 23 July 2025 | · pdf

Precise measurements of magnetic and electric dipole moments are important tests of the Standard Model and beyond Standard Model physics, particularly for the electron and the muon. However, the situation presents distinctive challenges when dealing with the tau lepton due to its very short lifetime and relatively high mass. Here, we review the theoretical predictions and experimental measurements of both the anomalous magnetic and electric dipole moments of the tau lepton.
Monte Carlos for tau lepton - Standard Model and new physics signatures

Zbigniew Was

SciPost Phys. Proc. 17, 002 (2025) · published 23 July 2025 | · pdf

One of the purposes of High Energy accelerator experiments is confrontation of theory and measurements in ever new realms. Any new agreement extends theory applicability domain, any discrepancy hints to unexplained, calling for better calculations or new, deeper theory. Often one has to search for small contributions over large Standard Model background. Multidimensional signatures and complex background subtraction cuts imply that Monte Carlo techniques are indispensable. My talk included description of: KKMC Monte Carlo for $e^+e^- \to \tau^+\tau^- (n\gamma)$ (with $\tau$ decays), generation of additional lepton pairs of SM and New Physics, and contributions from anomalous magnetic and electric dipole moments.
Applying Quantum Tomography to Hadronic Interactions

J. C. Martens, J. P. Ralston, D. Tapia Takaki

SciPost Phys. Proc. 8, 154 (2022) · published 14 July 2022 | · pdf

A proper description of inclusive reactions is expressed with density matrices. Quantum tomography reconstructs density matrices from experimental observables. We review recent work that applies quantum tomography to practical experimental data analysis. Almost all field-theoretic formalism and modeling used in a traditional approach is circumvented with great efficiency. Tomographically-determined density matrices can express information about quantum systems which cannot in principle be expressed with distributions defined by classical probability. Topics such as entanglement and von Neumann entropy can be accessed using the same natural language where they are defined. A deep relation exists between {\it separability}, as defined in quantum information science, and {\it factorization}, as defined in high energyphysics. Factorization acquires a non-perturbative definition when expressed in terms of a conditional form of separability. An example illustrates how to go from data for momentum 4-vectors to a density matrix while bypassing almost all the formalism of the Standard Model.
Universality of closed nested paths in two-dimensional percolation

Yu-Feng Song, Jesper Lykke Jacobsen, Bernard Nienhuis, Andrea Sportiello, Youjin Deng

SciPost Phys. 18, 057 (2025) · published 18 February 2025 | · pdf

Recent work on percolation in $d=2$ [J. Phys. A: Math. Theor. 55, 204002 (2015)] introduced an operator that gives a weight $k^{\ell}$ to configurations with $\ell$ 'nested paths' (NP), i.e.\ disjoint cycles surrounding the origin, if there exists a cluster that percolates to the boundary of a disc of radius $L$, and weight zero otherwise. It was found that $\mathbb{E}(k^{\ell}) \sim L^{-X_{NP}(k)}$, and a formula for $X_{NP}(k)$ was conjectured. Here we derive an exact result for $X_{NP}(k)$, valid for $k ≥ -1$, replacing the previous conjecture. We find that the probability distribution $\mathbb{P}_\ell (L)$ scales as $ L^{-1/4} (\ln L)^\ell [(1/\ell!) \Lambda^\ell]$ when $\ell ≥ 0$ and $L \gg 1$, with $\Lambda = 1/\sqrt{3} \pi$. Extensive simulations for various critical percolation models confirm our theoretical predictions and support the universality of the NP observables.
Hands-on introduction to randomized benchmarking

Ana Silva, Eliska Greplova

SciPost Phys. Lect. Notes 97 (2025) · published 21 July 2025 | · pdf

Randomized benchmarking techniques have been an essential tool for assessing the performance of contemporary quantum devices. The goal of this tutorial is to provide a pedagogical, self-contained, introduction to randomized benchmarking. With this intention, every chapter is also supplemented with an accompanying Python notebook, illustrating the essential steps of each protocol. In addition, we also introduce more recent trends in the field that bridge shadow tomography with randomized benchmarking, namely through the gate-set shadow protocol.

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