Nuclear Theory·nucl-th

Tuesday·July 25, 2017

18 papers—11 primary·7 cross-listed

12
Rigorous constraints on the matrix elements of the energy-momentum tensor
Peter Lowdon🇺🇸 · Kelly Yu-Ju Chiu🇺🇸 · Stanley J. Brodsky🇺🇸
The structure of the matrix elements of the energy-momentum tensor play an important role in determining the properties of the form factors $A (q^{2})$ , $B (q^{2})$ and $C (q^{2})$ which appear in the Lorentz covariant decomposition of the matrix elements. In this paper we apply a rigorous frame-independent distributional-matching approach to the matrix elements of the Poincaré generators in order to derive constraints on these form factors as $q \to 0$ . In contrast to the literature, we explicitly demonstrate that the vanishing of the anomalous gravitomagnetic moment $B (0)$ and the condition $A (0) = 1$ are independent of one another, and that these constraints are not related to the specific properties or conservation of the individual Poincaré generators themselves, but are in fact a consequence of the physical on-shell requirement of the states in the matrix elements and the manner in which these states transform under Poincaré transformations.
↳ hep-thhep-phnucl-thPLB(2017)·44 citations
13
Different Realizations of Cooper-Frye Sampling with Conservation Laws
C. Schwarz🇩🇪 · D. Oliinychenko🇩🇪 · L.-G. Pang🇩🇪 · S. Ryu🇩🇪 · H. Petersen🇩🇪
Approaches based on viscous hydrodynamics for the hot and dense stage and hadronic transport for the final dilute rescattering stage are successfully applied to the dynamic description of heavy ion reactions at high beam energies. One crucial step in such hybrid approaches is the so called particlization, the transition between the hydrodynamic description to microscopic degrees of freedom. For this purpose, individual particles are sampled on the Cooper-Frye hypersurface. In this work, 4 different realizations of sampling algorithms are compared, where three of them incorporate global conservation laws of quantum numbers in each event. The algorithms are compared within two types of scenarios: simple "box" hypersurface consisting of only one static cell and a typical particlization hypersurface for Au+Au collisions at $s_{NN} = 200$ GeV. For all algorithms the mean multiplicities (or particle spectra) remain unaffected by global conservation laws in the case of large volumes. In contrast, the fluctuations of the particle numbers are affected considerably. The fluctuations of the newly developed SPREW algorithm based on exponential weights and the recently suggested SER algorithm based on ensemble rejection are smaller than without conservation laws and agree with the expectation from the canonical ensemble. The previously applied mode sampling algorithm produces dramatically larger fluctuations, than it is expected in the corresponding microcanonical ensemble, and therefore should be avoided in fluctuation studies. This study might be of interest for investigations of particle fluctuations and correlations, e.g. the suggested signatures for a phase transition or a critical endpoint, in hybrid approaches that are affected by global conservation laws.
↳ hep-phnucl-thJ.Phys.G(2018)·17 citations
14
Holographic estimate of the meson cloud contribution to nucleon axial form factor
G. Ramalho🇧🇷
We use light-front Holography to estimate the valence quark and the meson cloud contributions to the nucleon axial form factor. The free couplings of the holographic model are determined by the empirical data and by the information extracted from lattice QCD. The holographic model provides a good description of the empirical data when we consider a meson cloud mixture of about 30\% in the physical nucleon state. The estimate of the valence quark contribution to the nucleon axial form factor compares well with the lattice QCD data for small pion masses. Our estimate of the meson cloud contribution to the nucleon axial form factor has a slower falloff with the square momentum transfer compared to typical estimates from quark models with meson cloud dressing.
↳ hep-phhep-exhep-latnucl-ex+1PRD(2018)·12 citations
15
$ψ (2 S)$ versus $J / ψ$ suppression in proton-nucleus collisions from factorization violating soft color exchanges
Yan-Qing Ma🇨🇳 · Raju Venugopalan🇺🇸 · Kazuhiro Watanabe🇨🇳 · Hong-Fei Zhang🇨🇳
We argue that the large suppression of the $ψ (2 S)$ inclusive cross-section relative to the $J / ψ$ inclusive cross-section in proton-nucleus (p+A) collisions can be attributed to factorization breaking effects in the formation of quarkonium. These factorization breaking effects arise from soft color exchanges between charm-anticharm pairs undergoing hadronization and comoving partons that are long-lived on time scales of quarkonium formation. We compute the short distance pair production of heavy quarks in the Color Glass Condensate (CGC) effective field theory and employ an improved Color Evaporation model (ICEM) to describe their hadronization into quarkonium at large distances. The combined CGC+ICEM model provides a quantitative description of $J / ψ$ and $ψ (2 S)$ data in proton-proton (p+p) collisions from both RHIC and the LHC. Factorization breaking effects in hadronization, due to additional parton comovers in the nucleus, are introduced heuristically by imposing a cutoff $Λ$ , representing the momentum kick from soft color exchanges, in the ICEM model. Such soft exchanges have no perceptible effect on $J / ψ$ suppression in p+A collisions. In contrast, the interplay of the physics of these soft exchanges at large distances, with the physics of semi-hard rescattering at short distances, causes a significant additional suppression of $ψ (2 S)$ yields relative to that of the $J / ψ$ . A good fit of all RHIC and LHC $J / ψ$ and $ψ (2 S)$ data, for transverse momenta $P_{⊥} \leq 5$ GeV in p+p and p+A collisions, is obtained for $Λ \sim 10$ MeV.
↳ hep-phhep-exnucl-exnucl-thPRC(2018)·58 citations
16
Sudakov suppression of jets in QCD media
Yacine Mehtar-Tani🇺🇸 · Konrad Tywoniuk🇨🇭
We compute modifications to the jet spectrum in the presence of a dense medium. We show that in the large- $N_{c}$ approximation and at leading logarithmic accuracy the jet nuclear modification factor factorizes into a quenching factor associated to the total jet color charge and a Sudakov suppression factor which accounts for the energy loss of jet substructure fluctuations. This factor, called the jet collimator, implements the fact that subjets, that are not resolved by the medium, lose energy coherently as a single color charge, whereas resolved large angle fluctuations suffer more quenching. For comparison, we show that neglecting color coherence results in a stronger suppression of the jet nuclear modification factor.
↳ hep-phnucl-thPRD(2018)·82 citations
17
Resonances in positron scattering on a supercritical nucleus and spontaneous production of $e^{+} e^{-}$ pairs
S.I. Godunov🇷🇺 · B. Machet🇫🇷 · M.I. Vysotsky🇷🇺
We re-examine the physics of supercritical nuclei, specially focusing on the scattering phase $δ_{ϰ}$ and its dependence on the energy $ε$ of the diving electronic level, for which we give both exact and approximate formulas. The Coulomb potential $Z α / r$ is rounded to the constant $Z α / R$ for $r < R$ . We confirm the resonant behavior of $δ_{ϰ}$ that we investigate in details. In addition to solving the Dirac equation for an electron, we solve it for a positron, in the field of the same nucleus. This clarifies the interpretation of the resonances. Our results are compared with claims made in previous works.
↳ hep-phnucl-exnucl-thEPJC(2017)·24 citations
18
Excited Hadrons and Quark-Hadron Duality
E. Ruiz Arriola🇪🇸 · P. Masjuan🇪🇸 · W. Broniowski🇵🇱
We review how Quark-Hadron Duality (QHD) for (u,d) flavors at high energies and in the scaling regime suggests a radial and angular behaviour of mesonic and baryonic resonance masses of the Regge form $M_{n J}^{2} = μ^{2} n + β^{2} J + M_{0}^{2}$ . The radial mass dependence is asymptotically consistent with a common two-body dynamics for mesons and baryons in terms of the quark-antiquark ( $q q ˉ)$ and quark-diquark ( $q D$ ) degrees of freedom, respectively. This formula is validated phenomenologically within an uncertainty determined by half the width of the resonances, $Δ M_{n J}^{2} \sim Γ_{n J} M_{n J}$ . With this error prescription we find from the non-strange PDG hadrons different radial slopes $μ_{q ˉ q}^{2} = 1.34 (4) GeV^{2}$ and $μ_{q D}^{2} = 0.75 (3) GeV^{2}$ , but similar angular slopes $β_{q ˉ q}^{2} \sim β_{q D}^{2} \sim 1.15 GeV^{2}$ .
↳ hep-phnucl-thActa Phys.Polon.Supp.(2017)·4 citations

Affiliations

first authorsco-authorsvia INSPIRE

Rigorous constraints on the matrix elements of the energy-momentum tensor

Different Realizations of Cooper-Frye Sampling with Conservation Laws

Holographic estimate of the meson cloud contribution to nucleon axial form factor

ψ(2S) versus J/ψ suppression in proton-nucleus collisions from factorization violating soft color exchanges

Sudakov suppression of jets in QCD media

Resonances in positron scattering on a supercritical nucleus and spontaneous production of e+e− pairs

Excited Hadrons and Quark-Hadron Duality

Affiliations

$ψ (2 S)$ versus $J / ψ$ suppression in proton-nucleus collisions from factorization violating soft color exchanges

Resonances in positron scattering on a supercritical nucleus and spontaneous production of $e^{+} e^{-}$ pairs