Nuclear Theory·nucl-th

Friday·December 4, 2020

19 papers—9 primary·10 cross-listed

10
Jet radiation in a longitudinally expanding medium
Paul Caucal🇫🇷 · Edmond Iancu🇫🇷 · Gregory Soyez🇫🇷
In a series of previous papers, we have presented a new approach, based on perturbative QCD, for the evolution of a jet in a dense quark-gluon plasma. In the original formulation, the plasma was assumed to be homogeneous and static. In this work, we extend our description and its Monte Carlo implementation to a plasma obeying Bjorken longitudinal expansion. Our key observation is that the factorisation between vacuum-like and medium-induced emissions, derived in the static case, still holds for an expanding medium, albeit with modified rates for medium-induced emissions and transverse momentum broadening, and with a modified phase-space for vacuum-like emissions. We highlight a scaling relation valid for the energy spectrum of medium-induced emissions, through which the case of an expanding medium is mapped onto an effective static medium. We find that scaling violations due to vacuum-like emissions and transverse momentum broadening are numerically small. Our new predictions for the nuclear modification factor for jets $R_{AA}$ , the in-medium fragmentation functions, and substructure distributions are very similar to our previous estimates for a static medium, maintaining the overall good qualitative agreement with existing LHC measurements. In the case of $R_{AA}$ , we find that the agreement with the data is significantly improved at large transverse momenta $p_{T} ≳ 500$ GeV after including the effects of the nuclear parton distribution functions.
↳ hep-phnucl-thJHEP(2021)·37 citations
11
Gluon distributions and their applications to Ioffe-time distributions
Raza Sabbir Sufian🇺🇸 · Tianbo Liu🇺🇸 · Arpon Paul🇮🇹
We investigate unpolarized and polarized gluon distributions and their applications to the Ioffe-time distributions, which are related to lattice QCD calculations of parton distribution functions. Guided by the counting rules based on the perturbative QCD at large momentum fraction $x$ and the color coherence of gluon couplings at small $x$ , we parametrize gluon distributions in the helicity basis. By fitting the unpolarized gluon distribution, the inferred polarized gluon distribution from our parametrization agrees with the one from global analysis. A simultaneous fit to both unpolarized and polarized gluon distributions is also performed to explore the model uncertainty. The agreement with the global analysis supports the $(1 - x)$ power suppression of the helicity-antialigned distribution relative to the helicity-aligned distribution. The corresponding Ioffe-time distributions and their asymptotic expansions are calculated from the gluon distributions. Our results of the Ioffe-time distributions can provide guidance to the extrapolation of lattice QCD data to the region lacking precise gluonic matrix elements. Therefore, they can help regulate the ill-posed inverse problem associated with extracting the gluon distributions from discrete data from first-principle calculations, which are available in a limited range of the nucleon momentum and the spatial separation between the gluonic currents. Given various limitations in obtaining lattice QCD data at large Ioffe time, phenomenological approaches can provide important complementary information to extract the gluon distributions in the entire $x$ region. The possibility of investigating higher-twist effects and other systematic uncertainties in the contemporary first-principle calculations of parton distributions from phenomenologically well-determined Ioffe-time distributions in the large Ioffe-time region is also discussed.
↳ hep-phhep-latnucl-thPRD(2021)·25 citations
12
Arbitrarily large neutron amplification in subcritical nuclear reactors
Antoine Tilloy
In a subcritical reactor, each neutron produces only keff < 1 neutron per generation (asymptotically and on average), and thus the neutron population decreases exponentially in the absence of external source. The chain reaction is thus easy to stop, making such reactors inherently stable. Interestingly, and contrary to common wisdom, there is no relation between keff, the external source intensity, and the output power of the reactor. In this paper, I present various possible strategies to exploit this fact, and apply them to the design of a rudimentary multi-layer system that allows to reach an arbitrarily large number of fissions per source neutron, while keeping keff < 1 fixed (I used keff=0.97). The behavior of the amplifier is verified with simple Monte-Carlo transport simulations. The proposal admittedly brings complications and subtleties that need to be studied further, but, if developed, could allow to drive subcritical reactors with faint neutron sources, e.g. radioisotope based.
↳ physics.ins-detnucl-thphysics.app-phPhys.Rev.Applied(2021)·0 citations
13
Electroweak axial structure functions and improved extraction of the $V_{u d}$ CKM matrix element
K. Shiells🇨🇦 · P.G. Blunden🇨🇦 · W. Melnitchouk🇺🇸
We present a comprehensive analysis of the $γ W$ interference radiative correction to the neutron $β$ -decay matrix element. Within a dispersion relations approach, we compute the axial-vector part of the $γ W$ box amplitude $□_{A}^{γ W}$ in terms of the isoscalar part of the $F_{3}^{γ W}$ interference structure function. Using the latest available phenomenology for $F_{3}^{γ W}$ from the nucleon elastic, resonance, deep-inelastic, and Regge regions, we find the real part of the box correction to be $□_{A}^{γ W} = 3.90 (9) \times 1 0^{- 3}$ . This improved correction gives a theoretical estimate of the CKM matrix element $∣ V_{u d} ∣^{2} = 0.94805 (26)$ , which represents a 4 $σ$ violation of unitarity.
↳ hep-phhep-exnucl-thPRD(2021)·99 citations
14
Phenomenological QCD equations of state for neutron star dynamics: Nuclear-2SC continuity and evolving effective couplings
Toru Kojo🇨🇳 · Defu Hou🇨🇳 · Jude Okafor🇨🇳 · Hajime Togashi🇯🇵
We delineate the quark-hadron continuity by constructing QCD equations of state for neutron star dynamics, covering the wide range of charge chemical potential ( $μ_{Q}$ ) and temperatures ( $T$ ). Based on the nuclear-2SC continuity scenario, we match equations of state for nuclear and two-flavor color-superconducting (2SC) quark matter, where the matching baryon density is $n_{B} ≃ 1.5 n_{0}$ ( $n_{0} ≃ 0.16 f m^{- 3}$ : nuclear saturation density). The effective vector and diquark couplings in a quark matter model evolve as functions of $n_{B}, μ_{Q}, T$ , whose low density values are constrained by the nuclear matter properties and neutron star radii, with the high density behavior by the two-solar mass ( $2 M_{⊙}$ ) constraint. With couplings dependent on $n_{B}$ , we examined how smooth the nuclear-2SC continuity can be, and found problems in matching nuclear and 2SC entropies at low temperatures. To proceed with the continuity scenario, we enforce smooth matching by making the couplings ( $n_{B}, μ_{Q}, T$ )-dependent. In effect, this adds phenomenological contributions which we call "X". After the phenomenological matching, we take the rest as our predictions. The 2SC and color-flavor-locked (CFL) phases computed with these evolving couplings are called 2SCX and CFLX. The CFLX appears around $n_{B} ≃ 2$ - $4 n_{0}$ and, in contrast to the conventional CFL, has non-negligible dependence on $(μ_{Q}, T)$ . To examine the astrophysical consequences of our modeling, we add charged leptons and neutrinos, and study the composition of matter for lepton fractions relevant for protoneutron stars and neutron star mergers. The abundance of neutrinos and thermal effects reduce the strangeness fraction and stiffen equations of state. For a neutrino trapped neutron star at $T ≃ 30$ MeV with a lepton fraction $Y_{L} ≃ 0.05$ , the mass is larger than its cold static counterpart by $\sim 0.1 M_{⊙}$ .
↳ astro-ph.HEhep-phhep-thnucl-thPRD(2021)·33 citations
15
QCD evolution of the gluon Sivers function in heavy flavor dijet production at the Electron-Ion Collider
Zhong-Bo Kang🇺🇸 · Jared Reiten🇺🇸 · Ding Yu Shao🇺🇸 · John Terry🇺🇸
Using Soft-Collinear Effective Theory, we develop the transverse-momentum-dependent factorization formalism for heavy flavor dijet production in polarized-proton-electron collisions. We consider heavy flavor mass corrections in the collinear-soft and jet functions, as well as the associated evolution equations. Using this formalism, we generate a prediction for the gluon Sivers asymmetry for charm and bottom dijet production at the future Electron-Ion Collider. Furthermore, we compare theoretical predictions with and without the inclusion of finite quark masses. We find that the heavy flavor mass effects can give sizable corrections to the predicted asymmetry.
↳ hep-phhep-exnucl-thJHEP(2021)·25 citations
16
Charge radii of exotic potassium isotopes challenge nuclear theory and the magic character of $N = 32$
Á. Koszorús🇧🇪 · X. F. Yang🇨🇳 · W. G. Jiang🇺🇸 · S. J. Novario🇺🇸 · S. W. Bai🇨🇳 · J. Billowes🇬🇧 · C. L. Binnersley🇬🇧 · M. L. Bissell🇬🇧 · T. E. Cocolios🇧🇪 · B. S. Cooper🇬🇧 · R. P. de Groote🇫🇮 · A. Ekström🇸🇪 and 17 other authors
Nuclear charge radii are sensitive probes of different aspects of the nucleon-nucleon interaction and the bulk properties of nuclear matter; thus, they provide a stringent test and challenge for nuclear theory. The calcium region has been of particular interest, as experimental evidence has suggested a new magic number at $N = 32$ [1-3], while the unexpectedly large increases in the charge radii [4,5] open new questions about the evolution of nuclear size in neutron-rich systems. By combining the collinear resonance ionization spectroscopy method with $β$ -decay detection, we were able to extend the charge radii measurement of potassium ( $Z = 19$ ) isotopes up to the exotic $^{52}$ K ( $t_{1/2}$ = 110 ms), produced in minute quantities. Our work provides the first charge radii measurement beyond $N = 32$ in the region, revealing no signature of the magic character at this neutron number. The results are interpreted with two state-of-the-art nuclear theories. For the first time, a long sequence of isotopes could be calculated with coupled-cluster calculations based on newly developed nuclear interactions. The strong increase in the charge radii beyond $N = 28$ is not well captured by these calculations, but is well reproduced by Fayans nuclear density functional theory, which, however, overestimates the odd-even staggering effect. These findings highlight our limited understanding on the nuclear size of neutron-rich systems, and expose pressing problems that are present in some of the best current models of nuclear theory.
↳ nucl-exnucl-thphysics.atom-phNat.Phys.(2021)·162 citations
17
The path from lattice QCD to the short-distance contribution to $0 ν β β$ decay with a light Majorana neutrino
Zohreh Davoudi🇺🇸 · Saurabh V. Kadam🇺🇸
Neutrinoless double- $β$ ( $0 ν β β$ ) decay of certain atomic isotopes, if observed, will have significant implications for physics of neutrinos and models of physics beyond the Standard Model. In the simplest scenario, if the mass of the light neutrino of the Standard Model has a Majorana component, it can mediate the decay. Systematic theoretical studies of the decay rate in this scenario, through effective field theories matched to \emph{ab initio} nuclear many-body calculations, are needed to draw conclusions about the hierarchy of neutrino masses, and to plan the design of future experiments. However, a recently identified short-distance contribution at leading order in the effective field theory amplitude of the subprocess $nn \to pp (ee)$ remains unknown, and only lattice quantum chromodynamics (QCD) can directly and reliably determine the associated low-energy constant. While the numerical computations of the correlation function for this process are underway with lattice QCD, the connection to the physical amplitude, and hence this short-distance contribution, is missing. A complete framework that enables this complex matching is developed in this paper. The complications arising from Euclidean and finite-volume nature of the corresponding correlation function are fully resolved, and the value of the formalism is demonstrated through a simple example. The result of this work, therefore, fills the gap between first-principle studies of the $nn \to pp (ee)$ amplitude from lattice QCD and those from effective field theory, and can be readily employed in the ongoing lattice-QCD studies of this process.
↳ hep-lathep-phnucl-thPRL(2021)·51 citations
18
Qubit regularization of asymptotic freedom
Tanmoy Bhattacharya (1)🇺🇸 · Alexander J. Buser (2 and 1)🇺🇸 · Shailesh Chandrasekharan (3)🇺🇸 · Rajan Gupta (1)🇺🇸 · Hersh Singh (4 and 3) ((1) Los Alamos National Laboratory, Los Alamos, NM, USA, (2) Institute for Quantum Information and Matter, Caltech, Pasadena, CA, USA, (3) Department of Physics, Duke University, Durham, NC, USA, (4) Institute for Nuclear Theory, University of Washington, Seattle, WA, USA)🇺🇸
We provide strong evidence that the asymptotically free (1+1)-dimensional non-linear O(3) sigma model can be regularized using a quantum lattice Hamiltonian, referred to as the "Heisenberg-comb", that acts on a Hilbert space with only two qubits per spatial lattice site. The Heisenberg-comb consists of a spin-half anti-ferromagnetic Heisenberg-chain coupled anti-ferromagnetically to a second local spin-half particle at every lattice site. Using a world-line Monte Carlo method we show that the model reproduces the universal step-scaling function of the traditional model up to correlation lengths of 200,000 in lattice units and argue how the continuum limit could emerge. We provide a quantum circuit description of time-evolution of the model and argue that near-term quantum computers may suffice to demonstrate asymptotic freedom.
↳ hep-latcond-mat.str-elhep-thnucl-th+1PRL(2021)·68 citations
19
Large- $N_{c}$ and renormalization group constraints on parity-violating low-energy coefficients for three-derivative operators in pionless effective field theory
Son T. Nguyen🇺🇸 · Matthias R. Schindler🇺🇸 · Roxanne P. Springer🇺🇸 · Jared Vanasse🇺🇸
We extend from operators with one derivative to operators with three derivatives the analysis of two-body hadronic parity violation in a combined pionless effective field theory (EFT $_{π /}$ ) and large- $N_{c}$ expansion, where $N_{c}$ is the number of colors in quantum chromodynamics (QCD). In elastic scattering, these operators contribute to $S$ - $P$ and $P$ - $D$ wave transitions, with five operators and their accompanying low energy coefficients (LECs) characterizing the $S$ - $P$ transitions and six operators and LECs those in $P$ - $D$ transitions. We show that the large- $N_{c}$ analysis separates them into leading order in $N_{c}$ , next-to-leading order in $N_{c}$ , etc. Relationships among EFT $_{π /}$ LECs emerge in the large- $N_{c}$ expansion. We also discuss the renormalization scale dependence of these LECs. Our analysis can complement lattice QCD calculations and help prioritize future parity-violating experiments.
↳ hep-phnucl-thPRC(2021)·6 citations

Affiliations

first authorsco-authorsvia INSPIRE