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arXiv2014

Scaling properties of light-cluster production

We show that ratios of light-particle energy spectra display scaling properties that can be accu- rately described by effective local chemical potentials. This demonstrates the equivalence of t/3He and n/p spectral ratios and provides an essential test of theoretical predictions of isotopically resolved light-particle spectra. In addition, this approach allows direct comparisons of many theoretical n/p spectral ratios to experiments where charged-particle spectra but not neutron spectra are accurately measured. Such experiments may provide much more quantitative constraints on the density and momentum dependence of the symmetry energy.

nucl-ex↗
arXiv2014

Light WIMPs And Equivalent Neutrinos

Very light WIMPs (chi), thermal relics that annihilate late in the early Universe, change the energy and entropy densities at BBN and at recombination. BBN, in combination with the CMB, can remove some of the degeneracies among light WIMPs and equivalent neutrinos, constraining the existence and properties of each. Depending on the nature of the light WIMP (Majorana or Dirac fermion, real or complex scalar) the joint BBN + CMB analyses set lower bounds to m_chi in the range 0.5 - 5 MeV (m_chi/m_e > 1 - 10), and they identify best fit values for m_chi in the range 5 - 10 MeV. The joint BBN + CMB analysis finds a best fit value for the number of equivalent neutrinos, Delta N_nu ~ 0.65, nearly independent of the nature of the WIMP. In the absence of a light WIMP (m_chi > 20 MeV), N_eff = 3.05(1 + Delta N_nu /3). In this case, there is excellent agreement between BBN and the CMB, but the joint fit reveals Delta N_nu = 0.40+-0.17, disfavoring standard big bang nucleosynthesis (SBBN) (Delta N_nu = 0) at ~ 2.4 sigma, as well as a sterile neutrino (Delta N_nu = 1) at ~ 3.5 sigma. The best BBN + CMB joint fit disfavors the absence of dark radiation (Delta N_nu = 0 at ~ 95% confidence), while allowing for the presence of a sterile neutrino (Delta N_nu = 1 at ~ 1 sigma). For all cases considered here, the lithium problem persists. These results, presented at the TAUP 2013 Conference, are based on Nollett & Steigman (2013).

astro-ph.CO↗
arXiv2014

Level density and gamma-ray strength function in the odd-odd 238Np

The level density and gamma-ray strength function in the quasi-continuum of 238Np has been measured using the Oslo method. The level density function follows closely the constant-temperature level density formula and reaches 43 million levels per MeV at Sn = 5.488 MeV of excitation energy. The gamma-ray strength function displays a two-humped resonance at low-energy as also seen in previous investigations of Th, Pa and U isotopes. The structure is interpreted as the scissors resonance and has an average centroid of wSR = 2.26(5) MeV and a total strength of BSR = 10.8(12)m2N, which is in excellent agreement with sum-rule estimates. The scissors resonance is shown to have an impact on the 237Np(n; g)238Np cross section.

nucl-ex↗
arXiv2014

Hall A Annual Report 2013

Report over the experimental activities in Hall A at Thomas Jefferson National Accelerator Facility during 2013.

nucl-ex↗
arXiv2014

Phenomenology of the polarized cross-sections of the rho meson leptoproduction at high energy

We present a model for the polarized cross-sections of the hard diffractive leptoproduction of rho meson in the high energy limit. Our model is based on the light-cone collinear factorization of the virtual photon to rho meson impact factor when using the impact factor representation of the helicity amplitudes of the rho meson leptoproduction. This gauge invariant treatment when expressed in impact parameter space, leads to the factorization on one hand of the color dipole scattering amplitude and on the other hand of the distribution amplitudes of the rho meson up to twist 2 and 3. We show that the results of this approach are in good agreement with HERA data for virtualities above ~5 GeV^2.

hep-ph↗
arXiv2014

Nucleon - Nucleon Interactions at Short Distances

Despite the progress made in understanding the NN interactions at long distances based on effective field theories, the understanding of the dynamics of short range NN interactions remains as elusive as ever. One of the most fascinating properties of short range interaction is its repulsive nature which is responsible for the stability of strongly interacting matter. The relevant distances, $\le 0.5$ fm, in this case are such that one expects the onset of quark-gluon degrees of freedom with interaction being dominated by QCD dynamics. We review the current status of the understanding of the QCD dynamics of NN interactions at short distances, highlight outstanding questions and outline the theoretical foundation of QCD description of hard NN processes. We present examples of how the study of the hard elastic NN interaction can reveal the symmetry structure of valence quark component of the nucleon wave function and how the onset of pQCD regime is correlated with the onset of color transparency phenomena in hard $pp$ scattering in the nuclear medium. The discussions show how the new experimental facilities can help to advance the knowledge about the QCD nature of nuclear forces at short distances.

nucl-th↗
arXiv2014

The characteristics of a low background germanium gamma ray spectrometer at China JinPing underground Laboratory

A low background germanium gamma ray spectrometer, GeTHU, has been installed at China JinPing underground Laboratory. The integral background count rate between 40 and 2700 keV was 0.6 cpm, and the origin was studied by Monte Carlo simulation. Detection limits and efficiencies were calculated for selected gamma peaks. Boric acid and silica sand samples were measured and 137Cs contamination was found in boric acid. GeTHU will be mainly used to measure environmental samples and screen materials in dark matter experiments.

physics.ins-det↗
arXiv2014

Jet quenching effects on the direct, elliptic, and triangular flow at RHIC

In this paper we investigate how the energy and momentum deposited by partonic dijets in the quark-gluon plasma may affect the direct, elliptic and triangular flow of low (and intermediate) $p_T$ hadrons in central Au+Au collisions at RHIC. The dijets are modeled as external sources in the energy-momentum conservation equations for hydrodynamics, which are solved on an event-by-event basis within the ideal fluid approximation. We focus our investigation at mid-rapidity and solve the hydrodynamic equations imposing boost invariance. Differential anisotropic flow coefficients for $p_T \gtrsim 1$ GeV are found to be significantly enhanced if the dijets deposit on average more than 12 GeV in the QGP (or more than 6 GeV per jet). Because this jet-induced extra anisotropic flow is not related to the fluctuations of the initial geometry of the collision, the correlation between the $v_2$ and $v_3$ coefficients and their corresponding eccentricities is considerably weakened. In addition, we argue that the extra amount of direct flow induced by dijets may be quantified by comparing the azimuthal dependence of dihadron correlations in dijet events with the corresponding quantity obtained in events without dijets. This comparison could be used to give a rough estimate of the magnitude of the effective coupling between the jets and the medium.

nucl-th↗
arXiv2014

Properties of resonant states in 18Ne relevant to key 14O(alpha,p)17F breakout reaction in type I x-ray bursts

The $^{14}$O($α$,$p$)$^{17}$F reaction is one of the key reactions involved in the breakout from the hot-CNO cycle to the rp-process in type I x-ray bursts. The resonant properties in the compound nucleus $^{18}$Ne have been investigated through resonant elastic scattering of $^{17}$F+$p$. The radioactive $^{17}$F beam was separated by the CNS Radioactive Ion Beam separator (CRIB) and bombarded a thick H$_2$ gas target at 3.6 MeV/nucleon. The recoiling light particles were measured by using three $Δ$E-E silicon telescopes at laboratory angles of $θ$$_{lab}$$\approx$3$^\circ$, 10$^\circ$ and 18$^\circ$, respectively. Five resonances at $E_{x}$=6.15, 6.28, 6.35, 6.85, and 7.05 MeV were observed in the excitation functions. Based on an $R$-matrix analysis, $J^π$=1$^-$ was firmly assigned to the 6.15-MeV state. This state dominates the thermonuclear $^{14}$O($α$,$p$)$^{17}$F rate below 1 GK. We have also confirmed the existence and spin-parities of three states between 6.1 and 6.4 MeV. As well, a possible new excited state in $^{18}$Ne was observed at $E_{x}$=6.85$\pm$0.11 MeV and tentatively assigned as $J$=0. This state could be the analog state of the 6.880 MeV (0$^{-}$) level in the mirror nucleus $^{18}$O, or a bandhead state (0$^+$) of the six-particle four-hole (6$p$-4$h$) band. A new thermonuclear rate of the $^{14}$O($α$,$p$)$^{17}$F reaction has been determined, and its astrophysical impact has been examined within the framework of one-zone x-ray burst postprocessing calculations.

nucl-ex↗
arXiv2014

Initial state geometry and fluctuations in Au+Au, Cu+Au and U+U collisions at RHIC

We study within the IP-Glasma and two-component MC-Glauber models the effects of initial state geometry and fluctuations on multiplicities and eccentricities for several collision species at the Relativistic Heavy Ion Collider (RHIC). These include copper-gold (Cu+Au), gold-gold (Au+Au), and uranium-uranium (U+U) collisions. The multiplicity densities per participant pair are very similar in all systems studied. Ellipticities vary strongly between collision systems, most significantly for central collisions, while fluctuation driven odd moments vary little between systems. Event-by-event distributions of eccentricities in mid-central collisions are wider in Cu+Au relative to Au+Au and U+U systems. An anti-correlation between multiplicity and eccentricity is observed in ultra central U+U collisions which is weaker in the IP-Glasma model than the two-component MC-Glauber model. In ultra central Au+Au collisions the two models predict opposite signs for the slope of this correlation. Measurements of elliptic flow as a function of multiplicity in such central events can therefore be used to discriminate between models with qualitatively different particle production mechanisms.

nucl-th↗
arXiv2014

Exclusive Processes: Theory Introduction

We review the recent developments on the theoretical description of exclusive processes at medium and asymptotical energies. These are illustrated based on a few examples.

hep-ph↗
arXiv2014

Phenomenology of $ϕ$ photoproduction from recent CLAS data at Jefferson Lab

We comment on the important phenomenological aspects of the recent high-statistics and wide-angle coverage $ϕ$ photoproduction data from CLAS at Jefferson Lab. The most prominent feature is a localized structure at a center-of-mass (c.m) energy $\sqrt{s} \sim 2.2$ GeV that is not expected in a simple $t$-channel Pomeron-exchange model. The structure exists only at the forward production angles that almost rules out any resonance contribution. Strong rescattering effects between the $p ϕ$ and $K^+Λ(1520)$ channels could be possible explanations. The analyses of both charged- ($ϕ\to K^+ K^-$) and neutral- ($ϕ\to K^0_S K^0_L$) $K\bar{K}$ decay modes of the $ϕ$, that show some minor differences, can be illuminating in this respect. We also comment on the angular structure of the Pomeron-parton coupling as borne out in the polarization data where the often-asumed $s$-channel helicity conservation is seen to be broken.

hep-ex↗
arXiv2014

Coulomb effects in isobaric cold fission from reactions 233U(nth,f), 235U(nth,f), 239Pu(nth,f) and 252Cf(sf)

The Coulomb effect hypothesis, formerly used to interpret fluctuations in the curve of maximal total kinetic energy as a function of light fragment mass in reactions 233U(nth,f), 235U(nth,f) and 239Pu(nth,f), is confirmed in high kinetic energy as well as in low excitation energy windows, respectively. Data from reactions 233U(nth,f), 235U(nth,f), 239Pu(nth,f) and 252Cf(sf) show that, between two isobaric fragmentations with similar Q-values, the more asymmetric charge split reaches the higher value of total kinetic energy. Moreover, in isobaric charge splits with different Q-values, similar preference for asymmetrical fragmentations is observed in low excitation energy windows.

nucl-ex↗
arXiv2014

Dynamically-generated baryon resonances with heavy flavor

We study baryon resonances with heavy flavor in a molecular approach, thus as dynamically generated by meson-baryon scattering. This is accomplished by using a unitary coupled-channel model taking, as bare interaction, the extension to four flavors and spin symmetry of the Weinberg-Tomozawa interaction potential. A special attention is paid to the inclusion of heavy-quark spin symmetry and to the studies of the generated baryon resonances which complete the heavy-quark spin multiplets. We reproduce few charmed and strange baryon resonances found experimentally, that is, Λ_c(2595), Λ_c(2625), Ξ_c(2790), Ξ_c(2815), and make predictions for more states. The bottom-flavored Λ_b(5912) and Λ_b(5920) states, found by the LHCb collaboration, are also obtained by our model, thus these resonances can be interpreted as molecular states. We also study Ξ_b resonances, which belong to the same SU(3) x HQSS multiplets as the observed Λ_b particles. Finally we analyze hidden-charm baryon resonances. In this sector, we predict seven N-like and five Δ-like states with masses around 4 GeV, most of them as bound states, and compare them with predictions from other models. The predicted states can be searched for in future experiments that involve studies of heavy-flavor physics, e.g. PANDA/FAIR, where charm physics will be analyzed.

hep-ph↗
arXiv2014

Complementarity of Neutrinoless Double Beta Decay and Cosmology

Neutrinoless double beta decay experiments constrain one combination of neutrino parameters, while cosmic surveys constrain another. This complementarity opens up an exciting range of possibilities. If neutrinos are Majorana particles, and the neutrino masses follow an inverted hierarchy, then the upcoming sets of both experiments will detect signals. The combined constraints will pin down not only the neutrino masses but also constrain one of the Majorana phases. If the hierarchy is normal, then a beta decay detection with the upcoming generation of experiments is unlikely, but cosmic surveys could constrain the sum of the masses to be relatively heavy, thereby producing a lower bound for the neutrinoless double beta decay rate, and therefore an argument for a next generation beta decay experiment. In this case as well, a combination of the phases will be constrained.

astro-ph.CO↗
arXiv2014

Effects of medium modification of pion production threshold in heavy ion collisions and the nuclear symmetry energy

Using the relativistic Vlasov-Uehling-Uhlenbeck (RVUU) equation based on mean fields from the nonlinear relativistic models, we study the effect of medium modification of pion production threshold on the total pion yield and the $π^-/π^+$ ratio in Au+Au collisions. We find that the in-medium threshold effect enhances both the total pion yield and the $π^-/π^+$ ratio, compared to those without this effect. Furthermore, including the medium modification of the pion production threshold in asymmetric nuclear matter leads to a larger $π^-/π^+$ ratio for the $NLρδ$ model with a stiffer symmetry energy than the $NLρ$ model with a softer symmetry energy, opposite to that found without the in-medium threshold effect. Experimental data from the FOPI Collaboration are reproduced after including a density-dependent cross section for $Δ$ baryon production from nucleon-nucleon collisions, which suppresses the total pion yield but hardly changes the $π^-/π^+$ ratio. The large errors in the experimentally measured $π^-/π^+$ ratio prevent, however, the distinction between the predictions from the $NLρ$ and $NLρδ$ models.

nucl-th↗