The correct identification of charged hadrons plays a crucial role in flavor-physics measurements. The final detector configurations at the proposed Future Circular Collider are yet to be determined and this study aims to contribute to this discussion by benchmarking the particle-identification (PID) performance of the proposed CLD and IDEA detectors using fully simulated events. At present, neither detector proposal includes dedicated PID systems, relying instead on information from the tracking subsystems. We estimate the expected level of contamination due to misidentified charged hadrons for <math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><mi>b</mi></math>-flavor tagging, rare <math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><mi>b</mi><mo stretchy="false">→</mo><mi>s</mi></math> transitions, and <math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><mi>s</mi></math>-jet tagging. The PID information provided by silicon trackers, namely time-of-flight and energy-deposit measurements, leads to significant background suppression with high signal efficiency for the low-momentum hadrons considered for same-side <math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><mi>b</mi></math>-flavor tagging. In order to improve the contamination in rare decays where momenta are in the medium range, only good timing resolution of 30 ps and below can yield an improvement of one order of magnitude below the level achieved by kinematic criteria alone. Light-quark jet-flavor tagging requires identification of particles with very large momentum, which is not possible using only time-of-flight or energy-deposit information in silicon. Access to the number of clusters in a drift-chamber setup, as proposed for the IDEA detector, however, results in strong background suppression in every case. This suppression can be further improved in some scenarios by time-of-flight resolution of 30–50 ps or better. The PID quality generally exhibits only a small dependence on the cluster-counting efficiency. Whether dedicated PID detectors could further enhance flavor-physics sensitivity should be the subject of future study.

Anja Beck

Eluned Smith

Flavor-physics benchmarks for tracker-based particle identification at the FCC-ee

We explore the fundamental but untested possibility that the photon is a continuous spin particle (CSP) with a small but nonzero spin Casimir <math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><mi>ρ</mi></math>. When <math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><mi>ρ</mi><mo>≠</mo><mn>0</mn></math>, the familiar polarization modes of the photon transform nontrivially under Lorentz boosts, leading to deviations from familiar QED. Surprisingly, these deviations are strongest at low energy, but smoothly vanish in the <math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><mi>ρ</mi><mo stretchy="false">→</mo><mn>0</mn></math> limit. In this letter, we compute corrections to the hydrogen 21-cm transition rate, which is expected to be particularly sensitive given the small hyperfine energy splitting <math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><mi>ω</mi></math>. We find deviations from <math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><mi>QED</mi><mo>∝</mo><msup><mi>ρ</mi><mn>2</mn></msup><msup><mi>α</mi><mn>2</mn></msup><mo>/</mo><msup><mi>ω</mi><mn>2</mn></msup></math> at leading order, suggesting experimental constraints <math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><mi>ρ</mi><mo>≲</mo><mn>1</mn><mtext> </mtext><mtext> </mtext><mi>meV</mi></math>. Building on this work, we expect that a range of other atomic, molecular, or condensed matter systems could be used to provide even more stringent tests of <math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><mi>ρ</mi></math> in electromagnetic interactions.

Aidan Reilly

Alessandro Russo

Philip Schuster

Natalia Toro

Hydrogen 21-cm constraints on the photon’s spin scale

Using the coefficient-frame freedom of the <math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><mrow><mi mathvariant="script">O</mi><mo stretchy="false">(</mo><msup><mrow><mi>α</mi></mrow><mrow><mo>′</mo></mrow></msup><mo stretchy="false">)</mo></mrow></math> heterotic metric-dilaton effective action and incorporating it into Kaluza-Klein reduction with different reduction Ansätze and maximally symmetric internal spaces, we derive two distinct reduced five-dimensional scalar-tensor branches: a Lovelock-Horndeski-type theory and, for a special coefficient choice, an Einstein-dilaton-Gauss-Bonnet theory. We present a novel derivation of the full holographic conformal anomaly for these two reduced five-dimensional scalar-tensor theories. In the Lovelock-Horndeski case, we also construct exact asymptotically anti–de Sitter (AdS) solutions with linear-dilaton profiles and establish a holographic a-theorem. Our findings show that AdS/CFT remains sound in the presence of nonminimal scalars and higher-curvature terms, with string corrections fixing the conformal anomaly and the dual RG flow.

Tianhao Wu

Michael Stone

Holographic conformal anomaly and a-theorem in 5D scalar-tensor theories from heterotic strings

We develop a systematic Hamiltonian formulation of minimally doubled lattice fermions in (<math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><mrow><mn>3</mn><mo>+</mo><mn>1</mn></mrow></math>) dimensions, derive their nodal structures (structures of zeros), and classify their symmetry patterns for both four-component Dirac and two-component Weyl constructions. Motivated by recent single-Weyl proposals based on Bogoliubov-de Gennes representation, we argue that the corresponding single-Weyl Hamiltonians are obtained from the minimal-doubling Hamiltonians supplemented by an appropriate species-splitting mass term, and we reexamine the non-on-site symmetry protecting the physical Weyl node in terms of a Ginsparg-Wilson-type relation. We then construct a one-parameter family of deformations that preserves all the symmetries and demonstrate that, once the parameter exceeds a critical value, additional Weyl nodes emerge and the system exits the single-node regime. This indicates that in interacting theories radiative corrections can generate symmetry-allowed counterterms, so maintaining the desired single-Weyl phase generically requires “moderate” parameter tuning.

Tatsuhiro Misumi

Minimal-doubling and single-Weyl fermion Hamiltonians

We study the <math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><mi>N</mi><mo>=</mo><mn>3</mn></math> case of the <math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><mi>C</mi><msup><mi>P</mi><mrow><mi>N</mi><mo>−</mo><mn>1</mn></mrow></msup></math> model, which is a field theory of <math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><mi>N</mi></math> complex scalars in <math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><mn>3</mn><mi>d</mi></math> coupled to an Abelian gauge field with <math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><mi>S</mi><mi>U</mi><mo stretchy="false">(</mo><mi>N</mi><mo stretchy="false">)</mo><mo>×</mo><mi>U</mi><mo stretchy="false">(</mo><mn>1</mn><mo stretchy="false">)</mo></math> global symmetry. Recent evidence suggests the <math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><mi>N</mi><mo>=</mo><mn>2</mn></math> theory is not critical, which makes the <math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><mi>N</mi><mo>=</mo><mn>3</mn></math> theory the simplest possibility of deconfined quantum criticality. We apply the conformal bootstrap to correlators of charge <math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><mi>q</mi><mo>=</mo><mn>0</mn></math>, 1, 2 scalar operators under the <math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><mi>U</mi><mo stretchy="false">(</mo><mn>1</mn><mo stretchy="false">)</mo></math> symmetry, which gives us access also to <math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><mi>q</mi><mo>=</mo><mn>3</mn></math>, 4 operators. After imposing that only the lowest <math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><mi>q</mi><mo>=</mo><mn>0</mn></math>, 1, 2 scalar operators are relevant, we find that the bootstrap bounds are saturated by the large <math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><mi>N</mi></math> prediction for <math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><mi>q</mi><mo>=</mo><mn>1</mn></math>, 2, 3, 4 scalar monopole operator scaling dimensions, which were shown earlier to be accurate even for small <math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><mi>N</mi></math>, as well as a lattice prediction for the <math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><mi>q</mi><mo>=</mo><mn>0</mn></math> non-monopole scalar operator. We also predict the scaling dimensions of the lowest spinning monopole operators, which we match to the large charge prediction for spinning operators. This suggests that the critical <math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><mi>C</mi><msup><mi>P</mi><mn>2</mn></msup></math> model is described by this bootstrap bound.

Shai M. Chester

Alessandro Piazza

Marten Reehorst

Ning Su

Bootstrapping the simplest deconfined quantum critical point

We investigate the static scaling behavior of the chiral condensate near the two-flavor critical point within the framework of the soft-wall AdS/QCD. The scaling functions are extracted from the chiral order parameters and are found to precisely match those obtained through mean-field calculations. Additionally, it is also checked that the scaling functions are independent of the specific construction of the holographic model. Furthermore, we develop the formalism for calculating the chiral susceptibility and demonstrate that the pseudocritical temperatures obey the scaling law for moderate quark masses. It is shown that the temperature scaling could be comparable with those obtained from Dyson-Schwinger equations and lattice simulations. While the soft-wall AdS/QCD framework predicts mean-field critical exponents that are universal, the critical coefficients and the crossover dynamics are model dependent and provide quantitative constraints for phenomenological model building. These findings could help improve the effectiveness of the soft-wall AdS/QCD.

Zhongzheng Zhang

Danning Li

Lang Yu

Zhibin Li

Xinyang Wang

Scaling functions in the soft-wall AdS/QCD models

Motivated by the observation of nonzero neutrino masses and the potential for discovering physics beyond the Standard Model, numerous experiments are actively searching for neutrinoless double beta <math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><mo stretchy="false">(</mo><mn>0</mn><mi>ν</mi><mi>β</mi><mi>β</mi><mo stretchy="false">)</mo></math> decay. In all of these searches, a substantial amount of data on two-neutrino double beta <math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><mo stretchy="false">(</mo><mn>2</mn><mi>ν</mi><mi>β</mi><mi>β</mi><mo stretchy="false">)</mo></math> decay has been collected. In this work, we explore the sensitivity of current and future double beta decay experiments to a massive Majoron-like scalar particle coupled to neutrinos and potentially dark sector fermions, and compare their reach to the relevant cosmological constraints. On- and off-shell production of such a scalar leads to characteristic distortions in the emitted electron spectrum. We investigate how these distortions manifest in current and future double beta decay experiments, deriving the sensitivity to such a scenario. We project the reach of future experiments which can probe scalar-neutrino couplings of <math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><mo stretchy="false">|</mo><msub><mi>a</mi><mi>ν</mi></msub><mo stretchy="false">|</mo><mo>≈</mo><mn>2</mn><mo>×</mo><msup><mn>10</mn><mrow><mo>−</mo><mn>6</mn></mrow></msup></math> for sub-MeV scalar particles and remain sensitive to off-shell production above the <math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><mi>Q</mi></math>-value of double beta isotopes.

Noor-Inès Boudjema

Frank F. Deppisch

Antonio Herrero-Brocal

Chayan Majumdar

Supriya Senapati

Probing dark sector particles coupling to neutrinos with double beta decay

This study examines the spectral functions of neutral (<math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><msub><mi>π</mi><mn>0</mn></msub></math>) and charged (<math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><msub><mi>π</mi><mo>±</mo></msub></math>) pions under a uniform magnetic field using the SU(2) Nambu–Jona-Lasinio model with the Ritus method. The analysis highlights the complex interplay of magnetic field effects, thermal influences, and chiral symmetry on meson properties in extreme QCD environments. For <math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><msub><mi>π</mi><mn>0</mn></msub></math>, whose properties are governed by the behavior of its constituent quarks, magnetic field-induced Landau levels lead to a multipeak structure in its spectral function, reflecting stable and resonance solutions that evolve with temperature, showing shifts and critical enhancements near chiral restoration. For <math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><msub><mi>π</mi><mo>±</mo></msub></math>, cross terms that come from the asymmetry between the constituent quarks introduce Landau cuts alongside unitary cuts, indicating damping effects, with decay widths narrowing at higher temperatures, suggesting increased stability.

Jie Mei

Rui Wen

Min Zhou

Shijun Mao

Mei Huang

Spectral function for pions in a magnetic field

We present an algorithm to include the correlation between the production and decay planes of gluons in a parton-shower simulation. The technique is based on identifying the charge currents responsible for the creation and annihilation of the vector field. It is applicable in both the hard-collinear and the soft wide-angle region. As a function of the number of particles, the algorithm scales linear in computing time and memory. We demonstrate agreement with fixed-order perturbative calculations in the relevant kinematical limits, and present a new observable that can be used to probe correlations beyond current-current interactions.

Stefan Höche

Mareen Hoppe

Daniel Reichelt

Simple algorithm for polarized parton evolution

We investigate abelian Chern-Simons gauge theory on a strip geometry with two spatial boundaries. In the presence of boundaries, gauge invariance is broken by boundary conditions, leading to physical edge excitations. By deriving the most general local boundary conditions consistent with power counting in the sense of Symanzik, we show that the bulk equations of motion determine the boundary degrees of freedom through a broken gauge Ward identity, yielding boundary Kac-Moody current algebras with opposite central charges on the two edges. The corresponding two-dimensional boundary actions are of Tomonaga-Luttinger type and describe chiral bosons propagating in opposite directions along the two boundaries. A consistency condition, interpreted as a holographic-like bulk-boundary matching, relates the Chern-Simons coupling constant and the boundary parameters to the physical edge velocities. Within this framework, the equality and opposite sign of the two velocities in a symmetric setup follow directly from the boundary structure rather than from model-dependent assumptions about confining potentials, and the velocities are independent of the strip width. Our analysis provides a fully field-theoretic realization of bulk-boundary correspondence in Chern-Simons theory with two boundaries, with direct applications to edge physics in quantum Hall systems and related topological/hydrodynamic settings.