Nuclear Theory·nucl-th

Wednesday·August 11, 2021

5 papers—1 primary·4 cross-listed

01
Angular momentum removal by neutron and $γ$ -ray emissions during fission fragment decays
I. Stetcu · A.E. Lovell · P. Talou · T. Kawano · S. Marin · S.A. Pozzi · A. Bulgac
We investigate the angular momentum removal from fission fragments (FFs) through neutron and $γ$ -ray emission, where we find that about half the neutrons are emitted with angular momenta $\geq 1.5ℏ$ and that the change in angular momentum after the emission of neutrons and statistical $γ$ rays is significant, contradicting usual assumptions. Per fission event, in our simulations, the neutron and statistical $γ$ -ray emissions change the spin of the fragment by 3.5 -- 5~ $ℏ$ , with a large standard deviation comparable to the average value. Such wide angular momentum removal distributions can hide any underlying correlations in the fission fragment initial spin values. Within our model, we reproduce data on spin measurements from discrete transitions after neutron emissions, especially in the case of light FFs. The agreement further improves for the heavy fragments if one removes from the analysis the events that would produce isomeric states. Finally, we show that while in our model the initial FF spins do not follow a saw-tooth like behavior observed in recent measurements, the average FF spin computed after neutron and statistical $γ$ emissions exhibits a shape that resembles a saw tooth. This suggests that the average FF spin measured after statistical emissions is not necessarily connected with the scission mechanism as previously implied.
nucl-thnucl-exPRL(2021)·45 citations
02
Holographic charm and bottom pentaquarks I: Mass spectra with spin effects
Yizhuang Liu🇵🇱 · Maciej A. Nowak🇵🇱 · Ismail Zahed🇺🇸
We revisit the three non-strange pentaquarks $[\frac{1}{2} \frac{1}{2}^{-}]_{S = 0, 1}$ and $[\frac{1}{2} \frac{3}{2}^{-}]_{S = 1}$ predicted using the holographic dual description, where chiral and heavy quark symmetry are manifest in the triple limit of a large number of colors, large quark mass and strong $^{'}$ t Hooft gauge coupling. In the heavy quark limit, the pentaquarks with internal heavy quark spin $S$ are all degenerate. The holographic pentaquarks are dual to an instanton bound to heavy mesons in bulk, without the shortcomings related to the nature of the interaction and the choice of the hard core inherent to the molecular constructions. We explicitly derive the spin-spin and spin-orbit couplings arising from next to leading order in the heavy quark mass, and lift totally the internal spin degeneray, in fair agreement with the newly reported charmed pentaquarks from LHCb. New charm and bottom pentaquark states are predicted.
↳ hep-phhep-thnucl-exnucl-thPRD(2021)·13 citations
03
Advancement of Photospheric Radius Expansion and Clocked Type-I X-Ray Burst Models with the New $^{22}$ Mg $(α, p)^{25}$ Al Reaction Rate Determined at Gamow Energy
J. Hu · H. Yamaguchi · Y. H. Lam · A. Heger · D. Kahl · A. M. Jacobs · Z. Johnston · S. W. Xu · N. T. Zhang · S. B. Ma · L. H. Ru · E. Q. Liu and 25 other authors
We report the first (in)elastic scattering measurement of $^{25} Al + p$ with the capability to select and measure in a broad energy range the proton resonances in $^{26}$ Si contributing to the $^{22}$ Mg $(α, p)$ reaction at type I x-ray burst energies. We measured spin-parities of four resonances above the $α$ threshold of $^{26}$ Si that are found to strongly impact the $^{22}$ Mg $(α, p)$ rate. The new rate advances a state-of-the-art model to remarkably reproduce light curves of the GS 1826 $-$ 24 clocked burster with mean deviation $< 9$ % and permits us to discover a strong correlation between the He abundance in the accreting envelope of photospheric radius expansion burster and the dominance of $^{22}$ Mg $(α, p)$ branch.
↳ nucl-exastro-ph.HEnucl-thPRL(2021)·22 citations
04
Single-Logarithmic Corrections to Small- $x$ Helicity Evolution
Yossathorn Tawabutr🇺🇸
The small- $x$ quark helicity evolution equations at double-logarithmic order, with the kernel $\sim α_{s} ln^{2} (1/ x)$ , have been derived previously. In this work, we derive the single-logarithmic corrections to the equations, to order $α_{s} ln (1/ x)$ of the evolution kernel. The new equations include the effects of the running coupling and the unpolarized small- $x$ evolution, both of which are parametrically significant at single-logarithmic order. The large- $N_{c}$ and large- $N_{c} & N_{f}$ approximations to the equation are computed. (Here, $N_{c}$ and $N_{f}$ are the numbers of quark colors and flavors, respectively.) Their solutions will provide more precise estimates of the quark helicity distribution at small $x$ , contributing to the resolution of the proton spin puzzle.
↳ hep-phhep-exnucl-exnucl-thSciPost Phys.Proc.(2022)·4 citations
05
Subleading contributions to the decay width of the $T_{cc}^{+}$ tetraquark
Mao-Jun Yan🇨🇳 · Manuel Pavon Valderrama🇨🇳
Recently the LHCb collaboration has announced the discovery of the $T_{cc}^{+}$ tetraquark. Being merely a few hundred $keV$ below the $D^{*+} D^{0}$ threshold, the $T_{cc}^{+}$ is expected to have a molecular component, for which there is a good separation of scales that can be exploited to make reasonably accurate theoretical predictions about this tetraquark. Independently of its nature, the most important decay channels will be $D^{+} D^{0} π^{0}$ , $D^{0} D^{0} π^{+}$ and $D^{+} D^{0} γ$ . Its closeness to threshold suggests that the mass and particularly the width of the $T_{cc}^{+}$ tetraquark depend on the resonance profile. While the standard Breit-Wigner parametrization generates a $T_{cc}^{+}$ that is too broad for current theoretical calculations to reproduce, a three-body unitarized Breit-Wigner shape reveals instead a decay width ( $Γ_{pole} = 48 \pm 2_{- 12}^{+ 0} keV$ ) consistent with theoretical expectations. Here we consider subleading order contributions to the decay amplitude, which though having at most a moderate impact in the width still indicate potentially significant differences with the experimental width that can be exploited to disentangle the nature of the $T_{cc}^{+}$ . Concrete calculations yield $Γ^{LO} = 49 \pm 16 keV$ and $Γ^{NLO} = 5 8_{- 6}^{+ 7} keV$ , though we expect further corrections to the $NLO$ decay widths from asymptotic normalization effects. We find that a detailed comparison of the $NLO$ total and partial decay widths with experiment suggests the existence of a small (but distinguishable from zero) non-molecular component of the $T_{cc}^{+}$ .
↳ hep-phhep-exnucl-thPRD(2022)·79 citations

Affiliations

first authorsco-authorsvia INSPIRE

Angular momentum removal by neutron and γ-ray emissions during fission fragment decays

Holographic charm and bottom pentaquarks I: Mass spectra with spin effects

Advancement of Photospheric Radius Expansion and Clocked Type-I X-Ray Burst Models with the New 22Mg(α,p)25Al Reaction Rate Determined at Gamow Energy

Single-Logarithmic Corrections to Small-x Helicity Evolution

Subleading contributions to the decay width of the Tcc+​ tetraquark

Affiliations

Angular momentum removal by neutron and $γ$ -ray emissions during fission fragment decays

Advancement of Photospheric Radius Expansion and Clocked Type-I X-Ray Burst Models with the New $^{22}$ Mg $(α, p)^{25}$ Al Reaction Rate Determined at Gamow Energy

Single-Logarithmic Corrections to Small- $x$ Helicity Evolution

Subleading contributions to the decay width of the $T_{cc}^{+}$ tetraquark