arXiv subjects

Steven D. Bass

Publications and source records attributed to Steven D. Bass.

At least 19 recordsLinked to original sources

arXiv2026

Experimental determination of the Dalitz plot for positronium decay using the J-PET detection system

We present the first measurements of the Dalitz plot for ortho-positronium annihilation to three photons. Our measurements, accurate to about 3% statistical and 2-3% systematic uncertainty in angular representation over almost the entire available phase space, were performed using the Jagiellonian Positron Emission Tomograph (J-PET) based on organic scintillator strips. Until now, the Dalitz plot for the three-body positronium decay has been poorly explored. The new measurements presented here are consistent with both the leading-order and next-to-leading order QED predictions for the Dalitz plot.

nucl-ex↗

arXiv2026

Emergent Gauge Symmetries in Particle Physics and Cosmology

Where do gauge symmetries come from? These lectures develop the idea that the Standard Model might be emergent, with its gauge symmetries dissolving in some phase transition deep in the ultraviolet. The (meta-)stability of the Higgs vacuum may be pointing to some new critical phenomena at very high energy scales, with the Higgs connecting physics at LHC laboratory energies to that in the deep ultraviolet. In the emergence scenario, the dark energy scale comes out similar to the size of light Majorana neutrino masses. These two quantities appear at the same order in a low energy expansion in inverse powers of the scale of emergence, about $10^{16}$ GeV. Dark matter candidates include axions and phonon like excitations of degrees of freedom above the scale of emergence. Possible tests of these ideas involve neutrinos as well as gravitational-waves-related signals from the early Universe, which are sensitive to physics at very high energy scales.

hep-ph↗

arXiv2026

Particle Physics and Gravitational Waves as complementary windows on the Universe

Particle physics and gravitational waves provide complementary probes of the deep structure of the Universe. Gravitational waves from the mergers of neutron stars and black holes are sensitive to the structure of dense quark matter and to different dark matter scenarios. Measurements of stochastic gravitational waves backgrounds can teach us about possible first order phase transitions in the early Universe, including providing sensitivity to the TeV scale which is of key interest to future particle collider experiments. Gravitational waves measurements will also give new probes of the evolution and expansion of the Universe, complementary to measurements with electromagnetic radiation. This Perspectives article explores the physics synergies between the science opportunities provided by next generation gravitational waves measurements and particle physics experiments. Gravitational waves can also probe deep into the early Universe reaching physics much above possible collider energies if the signals can be detected.

astro-ph.CO↗

arXiv2025

Cosmology with an emergent Standard Model

We discuss new ideas that the Standard Model might be emergent with connection to electroweak vacuum stability and related consequences for cosmology. In this scenario, the gauge symmetries and particles of the Standard Model would be ``born'' in some phase transition at a large scale about $10^{16}$ GeV with the Standard Model parameters constrained by the requirement of vacuum stability.Emergent gauge symmetries are well known in condensed matter physics. Perhaps the Standard Model might also be emergent. In this case the particles would be the stable long range excitations of degrees of freedom that operate above the scale of emergence. The dark energy scale comes out with similar size to the tiny masses of light Majorana neutrinos. The emergence scenario comes with interesting constraints on possible dark matter structure. New physics at energy scales around $10^{16}$ GeV might be explored through its effects in the neutrino mass matrix plus using high frequency gravitational waves and polarisation observables in the cosmic microwave background.

hep-ph↗

arXiv2025

Gluons in the $η'$ and in nucleon resonances

We discuss the role of gluon dynamics in $η'$ physics and in nucleon resonances where excitations of gluonic potentials may also be important. Interesting phenomenology includes a possible narrow near threshold resonance in $η'$ photoproduction and whether the parity doublets observed in the higher mass nucleon resonance spectrum might be hinting at a possible second minimum in the confinement potential corresponding to supercritical confinement.

hep-ph↗

arXiv2024

Positronium Physics and Biomedical Applications

Positronium is the simplest bound state, built of an electron and a positron. Studies of positronium in vacuum and its decays in medium tell us about Quantum Electrodynamics, QED, and about the structure of matter and biological processes of living organisms at the nanoscale, respectively. Spectroscopic measurements constrain our understanding of QED bound state theory. Searches for rare decays and measurements of the effect of gravitation on positronium are used to look for new physics phenomena. In biological materials positronium decays are sensitive to the inter- and intra-molecular structure and to the metabolism of living organisms ranging from single cells to human beings. This leads to new ideas of positronium imaging in medicine using the fact that during positron emission tomography (PET) as much as 40% of positron annihilation occurs through the production of positronium atoms inside the patient's body. A new generation of the high sensitivity and multi-photon total-body PET systems opens perspectives for clinical applications of positronium as a biomarker of tissue pathology and the degree of tissue oxidation.

physics.med-ph↗

arXiv2024

The cosmological constant and scale hierarchies with emergent gauge symmetries

Motivated by the stability of the electroweak Higgs vacuum we consider the possibility that the Standard Model might work up to large scales between about $10^{10}$ GeV and close to the Planck scale. A plausible scenario is an emergent Standard Model with gauge symmetries originating in some topological like phase transition deep in the ultraviolet. In this case the cosmological constant scale and neutrino masses should be of similar size, suppressed by factor of the large scale of emergence. The key physics involves a subtle interplay of Poincaré invariance, mass generation and renormalisation group invariance. The Higgs mass would be environmentally selected in connection with vacuum stability. Consequences for dark matter scenarios are discussed.

hep-ph↗

arXiv2024

Quantum sensing for particle physics

Quantum sensing is a rapidly growing approach to probe fundamental physics and explore new phase space for possible new physics with precision and highly sensitive measurements in our quest to understand the deep structure of matter and its interactions. This field uses properties of quantum mechanics in the detectors to go beyond traditional measurement techniques. Key particle physics topics where quantum sensing can play a vital role include neutrino properties, tests of fundamental symmetries (Lorentz invariance and the equivalence principle as well as searches for electric dipole moments and possible variations in fundamental constants), the search for dark matter and testing ideas about the nature of dark energy. Interesting new sensor technologies include atom interferometry, optomechanical devices, and atomic and nuclear clocks including with entanglement. This Perspective explores the opportunities for these technologies in future particle physics experiments, opening new windows on the structure of the Universe.

quant-ph↗

arXiv2024

Spinning protons and gluons in the $η'$

The proton spin puzzle has inspired a vast programme of experiments and theoretical work challenging our understanding of QCD and its role in the structure of hadrons.The proton's internal spin structure is connected to chiral symmetry and, through gluon degrees of freedom in the flavour singlet channel, to the physics of the $η'$ meson. Why do quarks contribute just about one third of the proton's spin? Why are $η'$ mesons and their interactions so sensitive to gluonic degrees of freedom? We review the status of these topics and some key observables for forthcoming experiments.

hep-ph↗

arXiv2024

Discrete symmetries tested at 10$^{-4}$ precision using linear polarization of photons from positronium annihilations

Discrete symmetries play an important role in particle physics with violation of CP connected to the matter-antimatter imbalance in the Universe. We report the most precise test of P, T and CP invariance in decays of ortho-positronium, performed with methodology involving polarization of photons from these decays. Positronium, the simplest bound state of an electron and positron, is of recent interest with discrepancies reported between measured hyperfine energy structure and theory at the level of $10^{-4}$ signaling a need for better understanding of the positronium system at this level. We test discrete symmetries using photon polarizations determined via Compton scattering in the dedicated J-PET tomograph on an event-by-event basis and without the need to control the spin of the positronium with an external magnetic field, in contrast to previous experiments. Our result is consistent with QED expectations at the level of 0.0007 and one standard deviation.

hep-ph↗

arXiv2023

The Gerasimov-Drell-Hearn sum-rule with nuclear targets

Hadron properties are modified when the hadron is embedded in a nuclear medium. Here we discuss the Gerasimov-Drell-Hearn, GDH, sum rule for polarised photoproduction from a polarised nucleon within a polarised nuclear target. Strong enhancement is expected with the suppression of the proton and nucleon resonance masses and enhancement of the proton's anomalous magnetic moment in medium. This could be tested in polarised photoproduction experiments with interesting targets being polarised deuterons, $^3$He, $^6$Li and $^7$Li. The largest contribution to the GDH sum rule comes from the $Δ$ resonance excitation. In existing data with polarised deuterons and $^3$He the $Δ$ excitation is shifted to slightly lower energy when compared to model predictions where the $Δ$ is treated with its free mass.

nucl-th↗

arXiv2021

The $η$- and $η'$-nucleus interactions and the search for $η$, $η'$- mesic states

The isoscalar $η$ and $η'$ mesons are special in QCD, being linked both to chiral symmetry and to non-perturbative glue associated with the axial anomaly. The properties of these mesons in medium are sensitive to how these dynamics work in the nuclear environment. In contrast to pionic and kaonic atoms which are mainly bound by the Coulomb interaction with some corrections due to the strong force (Coulomb assisted binding), the $η$ and $η'$ as neutral mesons can only be bound by the strong interaction. Is this interaction strong enough? This topic has inspired a vigorous program of experiments, conducted in close contact with theory, One has to determine the complex $η$, $η'$-nucleus potential. Does the real part $V$ provide a sufficiently deep potential? Is the imaginary part $W$ small enough to allow for narrow states that can more easily be detected experimentally, i.e. $|W| \ll |V|$? The $η'$ effective mass is observed to be suppressed by $\approx -40$ MeV at nuclear matter density. Bound state searches are ongoing. This article gives an overview of the status of knowledge in this field together with an outlook to future experiments.

hep-ph↗

arXiv2021

Emergent gauge symmetries -- making symmetry as well as breaking it

Gauge symmetries play an essential role in determining the interactions of particle physics. Where do they come from? Might the gauge symmetries of the Standard Model unify in the ultraviolet or might they be emergent in the infrared, below some large scale close to the Planck scale? Emergent gauge symmetries are important in quantum many-body systems in quantum phases associated with long range entanglement and topological order, e.g., they arise in high temperature superconductors, with string-net condensation and in the A-phase of superfluid He-3. String-nets and superfluid He-3 exhibit emergent properties similar to the building blocks of particle physics. Emergent gauge symmetries also play an important role in simulations of quantum field theories. This article discusses recent thinking on possible emergent gauge symmetries in particle physics, commenting also on Higgs phenomena and the vacuum energy or cosmological constant puzzle in emergent gauge systems.

hep-ph↗

arXiv2021

The Higgs boson -- its implications and prospects for future discoveries

The Higgs boson, a fundamental scalar, was discovered at CERN in 2012 with mass 125 GeV, a mass that turned out to be a remarkable choice of Nature. In the Standard Model of particle physics, the Higgs boson is closely linked to the mechanism that gives mass to the W and Z gauge bosons that mediate the weak interactions and to the charged fermions. Following discovery of the Higgs boson, present measurements at the Large Hadron Collider are focused on testing the Higgs boson's couplings to other elementary particles, precision measurements of the Higgs boson's properties and initial investigation of the Higgs boson's self-interaction and shape of the Higgs potential. With the Higgs boson mass of 125 GeV the vacuum sits very close to the border of stable and metastable, which may be a hint to deeper physics beyond the Standard Model. The Higgs potential also plays an important role in ideas about the cosmological constant or dark energy that drives the accelerating expansion of the Universe, the mysterious dark matter that comprises about 80% of the matter component in the Universe, as well as a possible phase transition in the early Universe that might be responsible for baryogenesis. Detailed study of the Higgs boson is at the centre of the recent European Strategy for Particle Physics update. Here we review the present status of this physics and discuss the new insights expected from present and future experiments.

hep-ph↗

arXiv2020

Gamow-Teller transitions and the spin EMC effect: the Bjorken sum-rule in medium

Gamow-Teller transitions in nuclei tell us that the nucleon's axial charge $g_A^{(3)}$ is quenched in large nuclei by about 20%. This result tells us that the spin structure of the nucleon is modified in nuclei and disfavours models of the medium dependence of parton structure based only on nucleon short range correlations in nuclei. For polarized photoproduction the Gerasimov-Drell-Hearn integral is expected to be strongly enhanced in medium.

hep-ph↗

arXiv2020

The cosmological constant and Higgs mass with emergent gauge symmetries

We discuss the Higgs mass and cosmological constant in the context of an emergent Standard Model, where the gauge symmetries "dissolve" in the extreme ultraviolet. In this scenario the cosmological constant scale is suppressed by power of the large scale of emergence and expected to be of similar size to neutrino masses. Cosmology constraints then give an anthropic upper bound on the Higgs mass.

hep-ph↗

arXiv2020

Vacuum energy with mass generation and Higgs bosons

We discuss the Higgs mass and cosmological constant hierarchy puzzles with emphasis on the interplay of Poincare invariance, mass generation and renormalization group invariance. A plausible explanation involves an emergent Standard Model with the cosmological constant scale suppressed by power of the large scale of emergence. In this scenario the cosmological constant scale and neutrino masses should be of similar size.

hep-ph↗

arXiv2020

Emergent Gauge Symmetries and Particle Physics

Hadron properties and interactions are emergent from QCD. Atomic and condensed matter physics are emergent from QED. Could the local gauge symmetries of particle physics also be emergent? We give an introduction to this question and recent ideas connecting it to the (meta)stability of the Standard Model Higgs vacuum. With an emergent Standard Model the gauge symmetries would "dissolve" in the ultraviolet. This scenario differs from unification models which exhibit maximum symmetry in the extreme ultraviolet. With emergence, new global symmetry violations would appear in higher dimensional operators.

hep-ph↗