Nuclear Experiment·nucl-ex

Thu·Jul 23, 2026Thursday·July 23, 2026

4 papers—2 primary·2 cross-listed

01
Experimental determination of the Dalitz plot for positronium decay using the J-PET detection system
Magdalena Skurzok🇵🇱 · Steven D. Bass🇵🇱 · Kamila Kasperska🇵🇱 · Ermias Beyene🇵🇱 · Neha Chug🇵🇱 · Catalina Curceanu🇮🇹 · Eryk Czerwinski🇵🇱 · Manish Das🇵🇱 · Marek Gorgol🇵🇱 · Sharareh Jalali🇵🇱 · Bozena Jasinska🇵🇱 · Krzysztof Kacprzak🇵🇱 and 28 other authors
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-ex0 citations
02
Measurement of the $γ γ \to τ τ$ cross section and constraints on the anomalous magnetic moment of the $τ$ lepton in ultraperipheral PbPb collisions at $s_{NN}$ = 5.02 TeV
CMS Collaboration
The production of $τ$ lepton pairs via photon-photon fusion, $γ γ \to τ τ$ , is studied in ultraperipheral lead-lead collisions at a nucleon-nucleon center-of-mass energy of 5.02 TeV. The dataset, collected by the CMS experiment in 2018, corresponds to an integrated luminosity of 1.70 nb $^{- 1}$ . Four different $τ^{+} τ^{-}$ decay final states are analyzed. A simultaneous likelihood fit to the measured lepton transverse momentum ( $p_{T}$ ) distributions, which incorporates information from both spectral shape and normalization, is used to constrain the anomalous magnetic moment of the $τ$ lepton, $a_{τ}$ , and to extract the cross section of the process. The measured 95% CL interval for $a_{τ}$ is $-$ 0.039 $<$ $a_{τ}$ $<$ 0.032. The fiducial cross section, $σ_{γ γ \to τ τ}^{fid}$ = 555 $_{- 14}^{+ 60}$ $μ$ b for tau leptons with $p_{T}^{τ}$ $>$ 1 GeV and pseudorapidity $∣ η^{τ} ∣$ $<$ 3, is the most precise measurement for this process at the LHC to date and is in agreement with next-to-leading-order quantum electrodynamics predictions.
nucl-exhep-ex1 citation
03
Nucleon unpolarized second Mellin moments using lattice QCD ensembles with physical quark masses and in the continuum limit
Constantia Alexandrou🇨🇾 · Simone Bacchio🇨🇾 · Jacob Finkenrath🇩🇪 · Christos Iona🇨🇾 · Giannis Koutsou🇨🇾 · Christian Kummer🇨🇾 · Yan Li🇨🇾 · Bhavna Prasad🇨🇾 · Gregoris Spanoudes🇨🇾
We compute the matrix elements of the energy-momentum tensor of the nucleon using four ensembles of twisted mass clover-improved fermions with the up, down, strange and charm quark masses tuned to approximately their physical values. The four ensembles have similar physical volume and lattice spacings $a = 0.080$ ~fm, $0.068$ ~fm, $0.057$ ~fm, and $0.049$ fm, allowing us to take the continuum limit directly at the physical pion mass point. We compute both connected and disconnected quark contributions as well as gluon contributions. All renormalization functions, including the mixing of the quark singlet with the gluon, are determined non-perturbatively. We extract the gravitational form factors in the continuum limit at $Q^{2} = 0$ and evaluate the contribution of quarks and gluons to the momentum and angular momentum of the proton. Using the values of the intrinsic quark spin computed using the same gauge ensembles we also determine the orbital angular momentum for each quark flavor.
↳ hep-lathep-exhep-phnucl-ex+11 citation
04
Fermionic pairs, from the surface to the bulk
Sandra Brandstetter🇩🇪 · Carl Heintze🇩🇪 · Fabian Brauneis · Stephanie M. Reimann · Georg Bruun · Maciej Gałka🇩🇪 · Selim Jochim🇩🇪
Fermion pairing underlies collective quantum phenomena across widely different forms of matter. In extended systems such as ultracold Fermi gases, pairing is commonly understood through the BCS--BEC crossover, where the pair size evolves from large, overlapping Cooper pairs to tightly bound dimers. In finite systems such as atomic nuclei, superconducting grains and quantum dots, however, the same pairing tendency competes with confinement, shell filling and spatial inhomogeneity, making the microscopic structure of pairs much harder to access. Here, we image pair correlations in a finite, tunable system of few fermionic atoms with single-particle resolution and full counting statistics. We observe that confinement and shell structure re-organize pairing in real space: In the weakly interacting, confinement-dominated regime, closed-shell configurations suppress correlations in the high-density trap center. Pairing is mainly observed toward the low-density surface. Open-shell systems, however, support substantially stronger central pairing. Already for surprisingly small systems, increasing either interaction strength or particle number restores a locally bulk-like Cooper-pair profile in the trap center, whereas the edge retains dimer-like correlations. By resolving where pairs form and how their character changes from localized dimers to overlapping Cooper pairs, our measurements provide a microscopic view of pairing in finite fermionic matter and connect the physics of mesoscopic cold atoms to pairing phenomena in nuclei and superconducting nanostructures.
↳ cond-mat.quant-gasnucl-exnucl-thquant-ph0 citations

Affiliations

first authorsco-authorsvia INSPIRE

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

Measurement of the γγ→ττ cross section and constraints on the anomalous magnetic moment of the τ lepton in ultraperipheral PbPb collisions at sNN​​ = 5.02 TeV

Nucleon unpolarized second Mellin moments using lattice QCD ensembles with physical quark masses and in the continuum limit

Fermionic pairs, from the surface to the bulk

Affiliations

Measurement of the $γ γ \to τ τ$ cross section and constraints on the anomalous magnetic moment of the $τ$ lepton in ultraperipheral PbPb collisions at $s_{NN}$ = 5.02 TeV