First time evidence of pronounced plateaus right above the Coulomb barrier in 8Li + 4He fusion

We investigate unprecedented experimental information on the fusion reaction induced by the radioactive projectile 8 Li on a 4 He gas target, at center-of-mass energies between 0.6 and 5 MeV. The main issue is the tendency of the dimensionless fusion cross section σfπƛ2 to form well visible plateaus alternated to steep rises. This finding is likely to be the most genuine consequence of the discrete nature of the intervening angular momenta observed so far in fusion reactions right above the Coulomb barrier. A partial-wave analysis, exclusively based on a pure quantal penetration fusion model and sensitive to the interaction potential, identifies a remarkably low-height barrier

Design of a Lambda system for population transfer in superconducting nanocircuits

The implementation of a Lambda scheme in superconducting artificial atoms could allow detec- tion of stimulated Raman adiabatic passage (STIRAP) and other quantum manipulations in the microwave regime. However symmetries which on one hand protect the system against decoherence, yield selection rules which may cancel coupling to the pump external drive. The tradeoff between efficient coupling and decoherence due to broad-band colored Noise (BBCN), which is often the main source of decoherence is addressed, in the class of nanodevices based on the Cooper pair box (CPB) design. We study transfer efficiency by STIRAP, showing that substantial efficiency is achieved for off-symmetric bias only in the charge-phase regime. We find a number of results uniquely due to non-Markovianity of BBCN, namely: (a) the efficiency for STIRAP depends essentially on noise channels in the trapped subspace; (b) low-frequency fluctuations can be analyzed and represented as fictitious correlated fluctuations of the detunings of the external drives; (c) a simple figure of merit for design and operating prescriptions allowing the observation of STIRAP is proposed. The emerging physical picture also applies to other classes of coherent nanodevices subject to BBCN.Comment: 14 pages, 11 figure

Big Bang nucleosynthesis revisited via Trojan Horse Method measurements

Nuclear reaction rates are among the most important input for understanding the primordial nucleosynthesis and therefore for a quantitative description of the early Universe. An up-to-date compilation of direct cross sections of 2H(d,p)3H, 2H(d,n)3He, 7Li(p,alpha)4He and 3He(d,p)4He reactions is given. These are among the most uncertain cross sections used and input for Big Bang nucleosynthesis calculations. Their measurements through the Trojan Horse Method (THM) are also reviewed and compared with direct data. The reaction rates and the corresponding recommended errors in this work were used as input for primordial nucleosynthesis calculations to evaluate their impact on the 2H, 3,4He and 7Li primordial abundances, which are then compared with observations.Comment: 22 pages, 7 figures, accepted for publication in The Astrophysical Journa

Relaxation phenomena in classical and quantum systems

et al.Relaxation phenomena in three different classical and quantum systems are investigated. First, the role of multiplicative and additive noise in a classical metastable system is analyzed. The mean lifetime of the metastable state shows a nonmonotonic behavior with a maximum as a function of both the additive and multiplicative noise intensities. In the second system, the simultaneous action of thermal and non-Gaussian noise on the dynamics of an overdamped point Josephson junction is studied. The effect of a Lévy noise generated by a Cauchy-Lorentz distribution on the mean lifetime of the superconductive metastable state, in the presence of a periodic driving, is investigated. We find resonant activation and noise enhanced stability in the presence of Lévy noise. Finally, the time evolution of a quantum particle moving in a metastable potential and interacting with a thermal reservoir is analyzed. Within the Caldeira-Legget model and the Feynman-Vernon functional approach, we obtain the time evolution of the population distributions in the position eigenstates of the particle, for different values of the thermal bath coupling strength.This work was partially supported by MIUR (Ministero dell’Istruzione dell’Universita‘ e della Ricerca), CNISM (Consorzio Nazionale Interuniversitario per le Scienze Fisiche della Materia) and the Russian Foundation for Basic Research (project 11-02-01418). This work was also partially supported by the EU through Grant No. PITN-GA-2009-234970 and the Joint Italian Japanese Laboratory on “Quantum Technologies” of the Italian Ministry of Foreign Affairs.Peer Reviewe

Effect of High-energy Resonances on the 18O(p, α)15N Reaction Rate at AGB and Post-AGB Relevant Temperatures

The 18O(p, ?)15N reaction is of great importance in several astrophysical scenarios, as it influences the production of key isotopes such as 19F, 18O, and 15N. Fluorine is synthesized in the intershell region of asymptotic giant branch (AGB) stars, together with s-elements, by ? radiative capture on 15N, which in turn is produced in the 18O proton-induced destruction. Peculiar 18O abundances are observed in R-Coronae Borealis stars, having 16O/18O 1, hundreds of times smaller than the galactic value. Finally, there is no definite explanation of the 14N/15N ratio in pre-solar grains formed in the outer layers of AGB stars. Again, such an isotopic ratio is influenced by the 18O(p, ?)15N reaction. In this work, a high accuracy 18O(p, ?)15N reaction rate is proposed, based on the simultaneous fit of direct measurements and of the results of a new Trojan Horse experiment. Indeed, current determinations are uncertain because of the poor knowledge of the resonance parameters of key levels of 19F. In particular, we have focused on the study of the broad 660?keV 1/2+ resonance corresponding to the 8.65?MeV level of 19F. Since ? ~ 100-300?keV, it determines the low-energy tail of the resonant contribution to the cross section and dominates the cross section at higher energies. Here, we provide a reaction rate that is a factor of two larger above T ~ 0.5 109?K based on our new improved determination of its resonance parameters, which could strongly influence present-day astrophysical model predictions

Effect of Low-frequency Noise on Adiabatic Passage in a Superconducting Nanocircuit

Recent experiments have demonstrated coherent phenomena in three-level systems based on superconducting nanocircuits. This opens the possibility to detect Stimulated Raman Adiabatic Passage (STIRAP) in artificial atoms. Low-fequency noise (often 1/f) is one of the main sources of decoherence in these systems, and we study its effect on the transfer e±ciency. We propose a way to analyze low frequency fluctuations in terms of fictitious correlated fluctuations of external parameters. We discuss a specific implementation, namely the Quantronium setup of a Cooper-pair box, showing that optimizing the trade-off between efficient coupling and protection against noise may allow us to observe coherent population transfer in this nanodevice

nuclear physics and its role for describing the early universe

Big Bang Nucleosynthesis (BBN) requires several nuclear physics inputs and nuclear reaction rates. An up-to-date compilation of direct cross sections of [Formula: see text], [Formula: see text]He and [Formula: see text]He reactions is given, being these ones among the most uncertain bare-nucleus cross sections. An intense experimental effort has been carried on in the last decade to apply the Trojan Horse Method (THM) to study reactions of relevance for the BBN and measure their astrophysical S(E)-factor. The reaction rates and the relative error for the four reactions of interest are then numerically calculated in the temperature ranges of relevance for BBN [Formula: see text]. These value were then used as input physics for primordial nucleosynthesis calculations in order to evaluate their impact on the calculated primordial abundances and isotopical composition for H, He and Li. New results on the [Formula: see text]He reaction rate were also taken into account.These were compared with the observational primordial abundance estimates in different astrophysical sites. Reactions to be studied in perspective will also be discussed

Astrophysical SS factor for the 15N(p,γ)16O{}^{15}{\rm N}(p,\gamma){}^{16}{\rm O} reaction from RR-matrix analysis and asymptotic normalization coefficient for 16O→15N+p{}^{16}{\rm O} \to {}^{15}{\rm N} + p. Is any fit acceptable?

The $^{15}{\rm N}(p,\gamma)^{16}{\rm O}$ reaction provides a path from the CN cycle to the CNO bi-cycle and CNO tri-cycle. The measured astrophysical factor for this reaction is dominated by resonant capture through two strong $J^{\pi}=1^{-}$ resonances at $E_{R}= 312$ and 962 keV and direct capture to the ground state. Recently, a new measurement of the astrophysical factor for the $^{15}{\rm N}(p,\gamma)^{16}{\rm O}$ reaction has been published [P. J. LeBlanc {\it et al.}, Phys. Rev. {\bf C 82}, 055804 (2010)]. The analysis has been done using the $R$-matrix approach with unconstrained variation of all parameters including the asymptotic normalization coefficient (ANC). The best fit has been obtained for the square of the ANC $C^{2}= 539.2$ fm${}^{-1}$, which exceeds the previously measured value by a factor of $\approx 3$. Here we present a new $R$-matrix analysis of the Notre Dame-LUNA data with the fixed within the experimental uncertainties square of the ANC $C^{2}=200.34$ fm${}^{-1}$. Rather than varying the ANC we add the contribution from a background resonance that effectively takes into account contributions from higher levels. Altogether we present 8 fits, five unconstrained and three constrained. In all the fits the ANC is fixed at the previously determined experimental value $C^{2}=200.34$ fm${}^{-1}$. For the unconstrained fit with the boundary condition $B_{c}=S_{c}(E_{2})$, where $E_{2}$ is the energy of the second level, we get $S(0)=39.0 \pm 1.1$ keVb and normalized ${\tilde \chi}^{2}=1.84$, i.e. the result which is similar to [P. J. LeBlanc {\it et al.}, Phys. Rev. {\bf C 82}, 055804 (2010)]. From all our fits we get the range $33.1 \leq S(0) \leq 40.1$ keVb which overlaps with the result of [P. J. LeBlanc {\it et al.}, Phys. Rev. {\bf C 82}, 055804 (2010)]. We address also physical interpretation of the fitting parameters.Comment: Submitted to PR

Nuclear astrophysics and resonant reactions: Exploring the threshold region with the Trojan Horse Method

Resonant reactions play an important role in astrophysics as they might significantly enhance the cross section with respect to the direct reaction contribution and alter the nucleosynthetic flow. Moreover, resonances bear information about states in the intermediate compound nucleus formed in the reaction. However, nuclear reactions in stars take place at energies well below [Formula: see text] MeV and the Coulomb barrier, exponentially suppressing the cross section, and the electron screening effect, due to the shielding of nuclear charges by atomic electrons, make it very difficult to provide accurate input data for astrophysics. Therefore, indirect methods have been introduced; in particular, we will focus on the Trojan Horse Method. We will briefly discuss the theory behind the method, to make clear its domain of applicability, the advantages and the drawbacks, and two recent cases will be shortly reviewed: the [Formula: see text] reaction, which is an important fluorine destruction channel in the proton-rich outer layers of asymptotic giant branch (AGB) stars, and the [Formula: see text] reactions, which play a critical role in astrophysics to understand stellar burning scenarios in carbon-rich environments