Nuclear Theory·nucl-th

Wednesday·June 23, 2021

10 papers—3 primary·7 cross-listed

04
The photon PDF within the CT18 global analysis
Keping Xie🇺🇸 · T. J. Hobbs🇺🇸 · Tie-Jiun Hou🇨🇳 · Carl Schmidt🇺🇸 · Mengshi Yan🇨🇳 · C.-P. Yuan🇺🇸
Building upon the most recent CT18 global fit, we present a new calculation of the photon content of the proton based on an application of the LUX formalism. In this work, we explore two principal variations of the LUX ansatz. In one approach, which we designate "CT18lux," the photon PDF is calculated directly using the LUX formula for all scales, $μ$ . In an alternative realization, "CT18qed," we instead initialize the photon PDF in terms of the LUX formulation at a lower scale, $μ \sim μ_{0}$ , and evolve to higher scales with a combined QED+QCD kernel at $O (α), O (α α_{s})$ and $O (α^{2})$ . While we find these two approaches generally agree, especially at intermediate $x$ ( $1 0^{- 3} ≲ x ≲ 0.3$ ), we discuss some moderate discrepancies that can occur toward the end-point regions at very high or low $x$ . We also study effects that follow from variations of the inputs to the LUX calculation originating outside the pure deeply-inelastic scattering (DIS) region, including from elastic form factors and other contributions to the photon PDF. Finally, we investigate the phenomenological implications of these photon PDFs for the LHC, including high-mass Drell-Yan, vector-boson pair, top-quark pair, and Higgs associated with vector-boson production.
↳ hep-phhep-exnucl-thPRD(2022)·55 citations
05
Improved neutron lifetime measurement with UCN $τ$
F. M. Gonzalez · E. M. Fries · C. Cude-Woods · T. Bailey · M. Blatnik · L. J. Broussard · N. B. Callahan · J. H. Choi · S. M. Clayton · S. A. Currie · M. Dawid · E. B. Dees and 33 other authors
We report an improved measurement of the free neutron lifetime $τ_{n}$ using the UCN $τ$ apparatus at the Los Alamos Neutron Science Center. We counted a total of approximately $38 \times 1 0^{6}$ surviving ultracold neutrons (UCN) after storing in UCN $τ$ 's magneto-gravitational trap over two data acquisition campaigns in 2017 and 2018. We extract $τ_{n}$ from three blinded, independent analyses by both pairing long and short storage-time runs to find a set of replicate $τ_{n}$ measurements and by performing a global likelihood fit to all data while self-consistently incorporating the $β$ -decay lifetime. Both techniques achieve consistent results and find a value $τ_{n} = 877.75 \pm 0.2 8_{stat} + 0.22/ - 0.1 6_{syst}$ ~s. With this sensitivity, neutron lifetime experiments now directly address the impact of recent refinements in our understanding of the standard model for neutron decay.
↳ nucl-exhep-exnucl-thPRL(2021)·194 citations
06
Spectral function of fermions in a highly occupied non-Abelian plasma
Kirill Boguslavski🇦🇹 · Tuomas Lappi🇫🇮 · Mark Mace🇫🇮 · Sören Schlichting🇩🇪
We develop a method to obtain fermion spectral functions non-perturbatively in a non-Abelian gauge theory with high occupation numbers of gauge fields. After recovering the free field case, we extract the spectral function of fermions in a highly occupied non-Abelian plasma close to its non-thermal fixed point, i.e., in a self-similar regime of the non-equilibrium dynamics. We find good agreement with hard loop perturbation theory for medium-induced masses, dispersion relations and quasiparticle residues. We also extract the full momentum dependence of the damping rate of the collective excitations.
↳ hep-phhep-latnucl-thPLB(2022)·19 citations
07
The strong CP problem solved by itself due to long-distance vacuum effects
Y. Nakamura🇯🇵 · G. Schierholz🇩🇪
The vacuum of quantum chromodynamics has an incredibly rich structure at the nonperturbative level, which is intimately connected with the topology of gauge fields, and put to a test by the strong CP problem. We investigate the long-distance properties of the theory in the presence of the topological $θ$ term. This is done on the lattice, using the gradient flow to isolate the long-distance modes in the functional integral measure and tracing it over successive length scales. The key point is that the vacuum splits into disconnected topological sectors with markedly different physical characteristics, which gives rise to a nontrivial behavior depending on $θ$ . We find that the color fields produced by quarks and gluons are screened, and confinement is lost, for bare vacuum angles $∣ θ ∣ > 0$ , thus providing a natural solution of the strong CP problem. The renormalized vacuum angle $θ$ is found to flow to zero in the infrared limit, leading to a self-consistent solution within QCD.
↳ hep-phhep-lathep-thnucl-thNPB(2023)·40 citations
08
Model dependence of the polarization asymmetries in weak pion production off the nucleon
Krzysztof M. Graczyk🇵🇱 · Beata E. Kowal🇵🇱
The work presents the studies of the polarization observables in the single pion production (SPP) induced by the interaction of the muon neutrino (antineutrino) with nucleons. The SPP cross-sections and spin asymmetries are computed within two phenomenological models. One is basing on the nonlinear sigma model \cite{Hernandez:2007qq}; the other has origin in linear sigma model~\cite{Fogli:1979cz}. Firstly, we show that the final nucleon polarization and target spin asymmetries are good observables to obtain information about the $C_{5}^{A}$ axial form factor. Secondly, we demonstrate that the nucleon polarization and the target spin asymmetries are sensitive to the relative phase between resonance and nonresonance amplitudes. We conclude that the polarization of the final nucleon and the target spin asymmetry are promising observables for testing SPP models, including studies of the axial content of $Δ (1232)$ resonance and unitarization procedures.
↳ hep-phhep-exnucl-exnucl-thPRD(2021)·6 citations
09
Beyond the relaxation time approximation
Grzegorz Wilk🇵🇱 · Zbigniew Włodarczyk🇵🇱
The relaxation time approximation (RTA) is a well known method of describing the time evolution of a statistical ensemble by linking distributions of the variables of interest at different stages of their temporal evolution. We show that if all the distributions occurring in the RTA have the same functional form of a quasi-power Tsallis distribution the time evolution of which depends on the time evolution of its control parameter, nonextensivity $q (t)$ , then it is more convenient to consider only the time evolution of this control parameter.
↳ cond-mat.stat-mechhep-phnucl-thEPJA(2021)·4 citations
10
Doubly Heavy Tetraquark Resonant States
Qi Meng🇨🇳 · Masayasu Harada🇯🇵 · Emiko Hiyama🇯🇵 · Atsushi Hosaka🇯🇵 · Makoto Oka🇯🇵
Spectrum of the doubly heavy tetraquarks, $bb q ˉ q ˉ$ , is studied in a constituent quark model. Four-body problem is solved in a variational method where the real scaling technique is used to identify resonant states above the fall-apart decay thresholds. In addition to the two bound states that were reported in the previous study we have found several narrow resonant states above the $B B^{*}$ and $B^{*} B^{*}$ thresholds. Their structures are studied and are interpreted by the quark dynamics. A narrow resonance with spin-parity $J^{P} = 1^{+}$ is found to be a mixed state of a compact tetraquark and a $B^{*} B^{*}$ scattering state. This is driven by a strong color Coulombic attraction between the $bb$ quarks. Negative-parity excited resonances with $J^{P} = 0^{-}$ , $1^{-}$ and $2^{-}$ form a triplet under the heavy-quark spin symmetry. It turns out that they share a similar structure to the $λ$ -mode of a singly heavy baryon as a result of the strongly attractive correlation for the doubly heavy diquark.
↳ hep-phnucl-thPLB(2022)·40 citations

Affiliations

first authorsco-authorsvia INSPIRE

The photon PDF within the CT18 global analysis

Improved neutron lifetime measurement with UCNτ

Spectral function of fermions in a highly occupied non-Abelian plasma

The strong CP problem solved by itself due to long-distance vacuum effects

Model dependence of the polarization asymmetries in weak pion production off the nucleon

Beyond the relaxation time approximation

Doubly Heavy Tetraquark Resonant States

Affiliations

Improved neutron lifetime measurement with UCN $τ$