Nuclear Theory·nucl-th

Thursday·August 8, 2019

10 papers—3 primary·7 cross-listed

04
[Submitted on 6 Aug 2019] (cross-list from hep-ph)
Further tests of lepton flavour universality from the charged lepton energy distribution in $b \to c$ semileptonic decays: The case of $Λ_{b} \to Λ_{c} ℓ ν ˉ_{ℓ}$
Neus Penalva🇪🇸 · Eliecer Hernández🇪🇸 · Juan Nieves🇪🇸
In a general framework, valid for any $H \to H^{'} ℓ^{-} ν ˉ_{ℓ}$ semileptonic decay, we analyze the $d^{2} Γ/ (d ω d cos θ_{ℓ})$ and $d^{2} Γ/ (d ω d E_{ℓ})$ distributions, with $ω$ being the product of the hadron four-velocities, $θ_{ℓ}$ the angle made by the three-momenta of the charged lepton and the final hadron in the $W^{-}$ center of mass frame and $E_{ℓ}$ the charged lepton energy in the decaying hadron rest frame. Within the Standard Model (SM), $d^{2} Γ/ (d ω d E_{ℓ}) \propto (c_{0} (ω) + c_{1} (ω) E_{ℓ} / M + c_{2} (ω) E_{ℓ}^{2} / M^{2})$ , with $M$ the initial hadron mass. We find that $c_{2} (ω)$ is independent of the lepton flavor and thus it is an ideal candidate to look for lepton flavor universality (LFU) violations. We also find a correlation between the $a_{2} (ω)$ structure function, that governs the $(cos θ_{ℓ})^{2}$ dependence of $d^{2} Γ/ (d ω d cos θ_{ℓ})$ , and $c_{2} (ω)$ . Apart from trivial kinematical and mass factors, the ratio of $a_{2} (ω) / c_{2} (ω)$ is a universal function that can be measured in any semileptonic decay, involving not only $b \to c$ transitions. These two SM predictions can be used as new tests in the present search for signatures of LFU violations. We also generalize the formalism to account for some new physics (NP) terms. Finally, in order to illustrate our findings, we apply our general framework to the $Λ_{b} \to Λ_{c} ℓ ν ˉ_{ℓ}$ decay. We show that a measurement of $c_{2}$ (or $a_{2}$ ) for $τ$ decay would not only be a direct measurement of the possible existence of NP, but it would also allow to distinguish from NP fits to $b \to c τ ν ˉ_{τ}$ anomalies in the meson sector, that otherwise give the same total and differential $d Γ/ d ω$ widths.
Comments:
We show how NP scenarios predicting fully compatible $R_{D}$ , $R_{D^{*}}$ and $R_{Λ_{c}}$ ratios give rise to results for an observable, extracted from the charged lepton energy distribution in the $Λ_{b} \to Λ_{c} ℓ ν ˉ_{ℓ}$ decay, which are separated by 5 $σ$ . Version accepted for publication in Phys. Rev. D
Subjects:
High Energy Physics — Phenomenology (hep-ph); High Energy Physics — Experiment (hep-ex); Nuclear Theory (nucl-th)
arXiv:
1908.02328 [pdf]
PRD(2019)·36 citations
05
[Submitted on 7 Aug 2019] (cross-list from hep-ph)
Valence-quark distributions of pions and kaons in a nuclear medium
Parada T. P. Hutauruk🇰🇷 · J. J. Cobos-Martinez🇰🇷 · Yongseok Oh🇰🇷 · K. Tsushima🇰🇷
In-medium valence-quark distributions of $π^{+}$ and $K^{+}$ mesons in symmetric nuclear matter are studied by combining the Nambu--Jona-Lasinio model and the quark-meson coupling model. The in-medium properties of the current quarks, which are used as inputs for studying the in-medium pion and kaon properties in the Nambu--Jona-Lasinio model, are calculated within the quark-meson coupling model. The light-quark condensates, light-quark dynamical masses, pion and kaon decay constants, and pion- and kaon-quark coupling constants are found to decrease as nuclear density increases. The obtained valence quark distributions in vacuum for both the $π^{+}$ and $K^{+}$ could reasonably describe the available experimental data over a wide range of Bjorken- $x$ . The in-medium valence $u$ -quark distribution in the $π^{+}$ at $Q^{2} = 16 \mbox G e V^{2}$ is found to be almost unchanged compared to the in-vacuum case. However, the in-medium to in-vacuum ratios of both the valence $u$ -quark and valence $s$ -quark distributions of the $K^{+}$ meson at $Q^{2} = 16 \mbox G e V^{2}$ increase with nuclear matter density, but show different $x$ -dependence. Namely, the ratio for the valence $u$ -quark distribution increases with $x$ , while that for the valence $s$ quark decreases with $x$ . These features are enhanced at higher density regions.
Comments:
12 pages, REVTeX
Subjects:
High Energy Physics — Phenomenology (hep-ph); Nuclear Theory (nucl-th)
arXiv:
1908.02406 [pdf]
PRD(2019)·24 citations
06
[Submitted on 7 Aug 2019] (cross-list from hep-lat)
Numerical exploration of three relativistic particles in a finite volume including two-particle resonances and bound states
Fernando Romero-López🇪🇸 · Stephen R. Sharpe🇺🇸 · Tyler D. Blanton🇺🇸 · Raúl A. Briceño🇺🇸 · Maxwell T. Hansen🇨🇭
In this work, we use an extension of the quantization condition, given in Ref. [1], to numerically explore the finite-volume spectrum of three relativistic particles, in the case that two-particle subsets are either resonant or bound. The original form of the relativistic three-particle quantization condition was derived under a technical assumption on the two-particle K matrix that required the absence of two-particle bound states or narrow two-particle resonances. Here we describe how this restriction can be lifted in a simple way using the freedom in the definition of the K-matrix-like quantity that enters the quantization condition. With this in hand, we extend previous numerical studies of the quantization condition to explore the finite-volume signature for a variety of two- and three-particle interactions. We determine the spectrum for parameters such that the system contains both dimers (two-particle bound states) and one or more trimers (in which all three particles are bound), and also for cases where the two-particle subchannel is resonant. We also show how the quantization condition provides a tool for determining infinite-volume dimer-particle scattering amplitudes for energies below the dimer breakup. We illustrate this for a series of examples, including one that parallels physical deuteron-nucleon scattering. All calculations presented here are restricted to the case of three identical scalar particles.
Comments:
42 pages, 16 figures, CERN-TH-2019-129, JLAB-THY-19-3011. Minor clarification and updated references
Subjects:
High Energy Physics — Lattice (hep-lat); Statistical Mechanics (cond-mat.stat-mech); High Energy Physics — Phenomenology (hep-ph); Nuclear Theory (nucl-th); Atomic Physics (physics.atom-ph)
arXiv:
1908.02411 [pdf]
JHEP(2019)·102 citations
07
[Submitted on 7 Aug 2019] (cross-list from cond-mat.quant-gas)
In-medium bound states of two bosonic impurities in a one-dimensional Fermi gas
D. Huber · H.-W. Hammer · A. G. Volosniev
We investigate the ground-state energy of a one-dimensional Fermi gas with two bosonic impurities. We consider spinless fermions with no fermion-fermion interactions. The fermion-impurity and impurity-impurity interactions are modelled with Dirac delta functions. First, we study the case where impurity and fermions have equal masses, and the impurity-impurity two-body interaction is identical to the fermion-impurity interaction, such that the system is solvable with the Bethe ansatz. For attractive interactions, we find that the energy of the impurity-impurity subsystem is below the energy of the bound state that exists without the Fermi gas. We interpret this as a manifestation of attractive boson-boson interactions induced by the fermionic medium, and refer to the impurity-impurity subsystem as an in-medium bound state. For repulsive interactions, we find no in-medium bound states. Second, we construct an effective model to describe these interactions, and compare its predictions to the exact solution. We use this effective model to study non-integrable systems with unequal masses and/or potentials. We discuss parameter regimes for which impurity-impurity attraction induced by the Fermi gas can lead to the formation of in-medium bound states made of bosons that repel each other in the absence of the Fermi gas.
Comments:
version accepted for publication in Physical Review Research
Subjects:
Quantum Gases (cond-mat.quant-gas); Strongly Correlated Electrons (cond-mat.str-el); Nuclear Theory (nucl-th)
arXiv:
1908.02483 [pdf]
PRResearch(2019)·17 citations
08
[Submitted on 7 Aug 2019] (cross-list from nucl-ex)
Spectroscopy and excited-state $g$ ~factors in weakly collective $^{111}$ Cd: confronting collective and microscopic models
B. J. Coombes · A. E. Stuchbery · A. Blazhev · H. Grawe · M. W. Reed · A. Akber · J. T. H. Dowie · M. S. M. Gerathy · T. J. Gray · T. Kibédi · A. J. Mitchell · T. Palazzo
The even cadmium isotopes near the neutron midshell have long been considered good examples of vibrational nuclei. However, the vibrational nature of these nuclei has been questioned based on E2 transition rates that are not consistent with vibrational excitations. In the neighbouring odd-mass nuclei, the g factors of the low-excitation collective states have been shown to be more consistent with a deformed rotational core than a vibrational core. Beyond the comparison of vibrational versus rotational models, recent advances in computational power have made shell-model calculations feasible for Cd isotopes, which may give insights into the emergence and nature of collectivity in the Cd isotopes. Collective excitations in the A ~ 100 region were studied through magnetic moments and electromagnetic transitions in 111Cd. The spectroscopy of 111Cd has been studied following Coulomb excitation. Angular correlation measurements, transient-field g-factor measurements and lifetime measurements by the Doppler-broadened line shape method were performed. The structure of the nucleus was explored in relation to particle-vibration versus particle-rotor interpretations. Large-scale shell-model calculations were performed with the SR88MHJM Hamiltonian. Excited-state g factors have been measured, spin assignments examined and lifetimes determined. Attention was given to the reported $5/ 2^{+}$ 753-keV and $3/ 2^{+}$ 755-keV states. The $3/ 2^{+}$ 755-keV level was not observed; evidence is presented that the reported $3/ 2^{+}$ state was a misidentification of the $5/ 2^{+}$ 753-keV state. It is shown that the g factors and level structure of 111Cd are not readily explained by the particle-vibration model. A particle-rotor approach has both successes and limitations. The shell-model approach successfully reproduces much of the known low-excitation structure in 111Cd.
Subjects:
Nuclear Experiment (nucl-ex); Nuclear Theory (nucl-th)
arXiv:
1908.02485 [pdf]
PRC(2019)·14 citations
09
[Submitted on 7 Aug 2019] (cross-list from astro-ph.HE)
Formation and evaporation of strangelets during the merger of two compact stars
Niccolò Bucciantini (INAF, Dip. Fisica e Astronomia, INFN - Firenze)🇮🇹 · Alessandro Drago (Dip. Fisica e Scienze della Terra, INFN - Ferrara)🇮🇹 · Giuseppe Pagliara (Dip. Fisica e Scienze della Terra, INFN - Ferrara)🇮🇹 · Silvia Traversi (Dip. Fisica e Scienze della Terra, INFN - Ferrara)🇮🇹 · Andreas Bauswein (GSI, Darmstadt)🇩🇪
We study the partial fragmentation of a strange quark star into strangelets during the process of merger of two strange quark stars. We discuss the fate of the fragments considering their possible evaporation into nucleons. We show that only a rather small amount of large size strangelets, ejected from the spiral arms in the post-merger, survives a total evaporation into nucleons. In this way we demonstrate that: 1) the density of strangelets in the galaxy is too low to trigger the conversion of all neutron stars into strange quark stars and it allows the co-existence of both types of compact objects; 2) the probability of direct detection of a strangelet is negligible and therefore its nondetection is compatible with the strange quark matter hypothesis; 3) most of the matter ejected during and after the merger of two strange quark stars evaporates into nucleons and therefore it can generate a kilonova-like signal.
Comments:
10 pages, 7 figures, 1 table. Significantly expanded version in print on Physical Review D
Subjects:
High Energy Astrophysical Phenomena (astro-ph.HE); High Energy Physics — Phenomenology (hep-ph); Nuclear Theory (nucl-th)
arXiv:
1908.02501 [pdf]
PRD(2022)·29 citations
10
[Submitted on 7 Aug 2019] (cross-list from nucl-ex)
Cross Section for $γ n \to π^{0} n$ measured at Mainz/A2
W. J. Briscoe · M. Hadzimehmedovi · A. E. Kudryavtsev · V. V. Kulikov · M. A. Martemianov · I. I. Strakovsky · A. Svarc · V. E. Tarasov · R. L. Workman · S. Abt · P. Achenbach · C. S. Akondi and 80 other authors
The $γ n \to π^{0} n$ differential cross section evaluated for 27 energy bins span the photon-energy range 290-813 MeV (W = 1.195-1.553 GeV) and the pion c.m. polar production angles, ranging from 18 deg to 162 deg, making use of model-dependent nuclear corrections to extract pi0 production data on the neutron from measurements on the deuteron target. Additionally, the total photoabsorption cross section was measured. The tagged photon beam produced by the 883-MeV electron beam of the Mainz Microtron MAMI was used for the 0-meson production. Our accumulation of 3.6 x 10^6 $γ n \to π^{0} n$ events allowed a detailed study of the reaction dynamics. Our data are in reasonable agreement with previous A2 measurements and extend them to lower energies. The data are compared to predictions of previous SAID, MAID, and BnGa partial-wave analyses and to the latest SAID fit MA19 that included our data. Selected photon decay amplitudes $N^{*} \to γ n$ at the resonance poles are determined for the first time.
Comments:
16 pages, 12 figures, 3 tables
Subjects:
Nuclear Experiment (nucl-ex); High Energy Physics — Experiment (hep-ex); High Energy Physics — Phenomenology (hep-ph); Nuclear Theory (nucl-th)
arXiv:
1908.02730 [pdf]
PRC(2019)·39 citations

Affiliations

first authorsco-authorsvia INSPIRE

Further tests of lepton flavour universality from the charged lepton energy distribution in b→c semileptonic decays: The case of Λb​→Λc​ℓνˉℓ​

Valence-quark distributions of pions and kaons in a nuclear medium

Numerical exploration of three relativistic particles in a finite volume including two-particle resonances and bound states

In-medium bound states of two bosonic impurities in a one-dimensional Fermi gas

Spectroscopy and excited-state g~factors in weakly collective 111Cd: confronting collective and microscopic models

Formation and evaporation of strangelets during the merger of two compact stars

Cross Section for γn→π0n measured at Mainz/A2

Affiliations

Further tests of lepton flavour universality from the charged lepton energy distribution in $b \to c$ semileptonic decays: The case of $Λ_{b} \to Λ_{c} ℓ ν ˉ_{ℓ}$

Spectroscopy and excited-state $g$ ~factors in weakly collective $^{111}$ Cd: confronting collective and microscopic models

Cross Section for $γ n \to π^{0} n$ measured at Mainz/A2