Primordial 7Li Research Articles

Slow 7Be and the primordial 7Li abundance

The cross sections of neutron-induced nuclear reactions are commonly large at low energies. Given this feature, the 7Be(n, p)7Li process with slow 7Be is studied in connection with the cosmological lithium problem. An idea is tested whether slow (non-Maxwellian) 7Be born in the radiative capture 3He(α, γ)7Be reaction can boost the 7Be(n, p)7Li(p, α)α mechanism of 7Li burn-up in the early universe, reducing the amount of primordial 7Li. The rate of this sequential process is examined and its effect on the 7Li abundance is clarified. In the absence of 7Be thermalization in the primordial plasma, the above mechanism can decrease 7Li/H by several percent, leaving unchanged the abundances of D and 4He which are consistent with observations. However, taking into account the 7Be thermalization due to scattering off electrons, positrons, nuclei, and photons radically suppresses the effect. It is found that the typical thermalization time of 7Be is less than 10−5 s, while the fraction of non-Maxwellian particles in the 7Be component is at most 2 × 10−7. Under this condition, the corresponding impact on 7Li/H is estimated to be no more than 10−5%.

Implications of the non-observation of 6Li in halo stars for the primordial 7Li problem

The primordial Lithium Problem is intimately connected to the assumption that the 7Li abundance observed in metal-poor halo stars is unchanged from its primordial value, which lies significantly below the predictions of standard big-bang nucleosynthesis. Two key lines of evidence have argued that these stars have not significantly depleted their initial (mostly primordial) 7Li: i) the lack of dispersion in Li abundance measurements at low metallicity (and high surface temperature); and ii) the detection of the more fragile 6Li isotope in at least two halo stars. The purported 6Li detections were in good agreement with predictions from cosmic-ray nucleosynthesis which is responsible for the origin of 6Li. This concordance left little room for 6Li depletion, and the apparent 6Li survival implied that 7Li largely evaded destruction, because stellar interiors destroy 6Li more vigorously then than 7Li. Recent (re)-observations of halo stars challenge the evidence against 7Li depletion: i) lithium elemental abundances now show significant dispersion, and ii) sensitive 6Li searches now reveal only upper limits to the 6Li/7Li ratio. We discuss the consequences of these 6Li non-detections on the primordial 7Li Problem, Galactic cosmic-ray nucleosynthesis, and the question of differential depletion of Li in stars. The tight new 6Li upper limits generally fall far below the predictions of cosmic-ray nucleosynthesis, implying that substantial 6Li depletion has occurred — by factors up to 50. We show that in stars with 6Li limits and thus lower bounds on 6Li depletion, an equal amount of 7Li depletion is more than sufficient to resolve the primordial 7Li Problem. This picture is consistent with well-studied stellar models in which 7Li is less depleted than 6Li, and strengthen the case that the Lithium Problem has an astrophysical solution. We conclude by suggesting future observations that could test these ideas.

Experimental studies on astrophysical reactions at the low-energy RI beam separator CRIB

Experimental studies on astrophysical reactions involving radioactive isotopes (RI) often accompany technical challenges. Studies on such nuclear reactions have been conducted at the low-energy RI beam separator CRIB, operated by Center for Nuclear Study, the University of Tokyo. We discuss two cases of astrophysical reaction studies at CRIB; one is for the 7Be+n reactions which may affect the primordial 7Li abundance in the Big-Bang nucleosynthesis, and the other is for the 22Mg(α, p) reaction relevantin X-raybursts.

Study of the contribution of the 7Be(d, p) reaction to the 7Li problem in the Big-Bang Nucleosynthesis

Our research goal is to measure the cross section of the 7Be(d, p) reaction to find a solution for the cosmological the 7Li problem, which is an overestimation of the primordial 7Li abundance in the standard Big-Bang nucleosynthesis (BBN) model. We developed a method to produce an unstable 7Be target to measure the reaction in normal kinematics in a high-resolution measurement. We performed an experiment to produce the 7Be target and also to measure the cross section of the 7Be(d, p)8Be reaction with the produced 7Be target at the tandem facility, Kobe University. A 2.36 MeV proton beam irradiated a Li target to transmute 7Li particles to 7Be particles via the 7Li(p, n)7Be reaction. We produced 2.6×1013 7Be particles in the target after two days’ proton irradiation. After the target production, the beam ion was changed to deuterons with energies of 0.6, 1.0, and 1.6 MeV to measure the 7Be(d, p) reaction. The outgoing protons were measured by two ΔE-E silicon telescopes placed at 30° and 45°. We report the preliminary results of this experiment, including the 7Be(d, p) cross section.

Thermonuclear Reaction Rates and Primordial Nucleosynthesis

Abstract Assuming the best numerical value for the cosmic baryonic density and the existence of three neutrino flavors, standard Big Bang nucleosynthesis is a parameter-free model. It is important to assess if the observed primordial abundances can be reproduced by simulations. Numerous studies have shown that the simulations overpredict the primordial 7Li abundance by a factor of compared to the observations. The discrepancy may be caused by unknown systematics in 7Li observations, poorly understood depletion of lithium in stars, errors in thermonuclear rates that take part in the lithium and beryllium synthesis, or physics beyond the standard model. Here, we focus on the likelihood of a nuclear physics solution. The status of the key nuclear reaction rates is summarized. Big Bang nucleosynthesis simulations are performed with the most recent reaction rates, and the uncertainties of the predicted abundances are established using a Monte Carlo technique. Correlations between abundances and reaction rates are investigated based on the metric of mutual information. The rates of four reactions impact the primordial 7Li abundance: 3He(α,γ)7Be, d(p,γ)3He, 7Be(d,p)2α, and 7Be(n,p)7Li. We employ a genetic algorithm to search for simultaneous rate changes in these four reactions that may account for all observed primordial abundances. When the search is performed for reaction rate ranges that are much wider than recently reported uncertainties, no acceptable solutions are found. Based on the currently available evidence, we conclude that it is highly unlikely for the cosmological lithium problem to have a nuclear physics solution.

The Interaction of Neutrons with 7Be at BBN Temperatures: Lack of Standard Nuclear Solution to the “Primordial 7Li Problem”

We report the first measurement of alpha-particles from the interaction of neutrons with 7Be at “temperatures” of Big Bang Nucleosynthesis (BBN). We measured the Maxwellian averaged cross sections (MACS), with neutron beams produced by the LiLiT at the SARAF in Israel (with kT = 49.5 keV hence 0.57 GK). In addition, we measured the cross section of the 7Be(n,p) reaction, which is in excellent agreement with the recent measurement of the n_TOF collaboration, further substantiating our method as a demonstration of “proof of principle”. The cross section for the 7Be(n,ga) and the 7Be(n,a) reaction measured in the “BBN window” is considerably smaller than compiled by Wagoner in 1969 and used today in Big Bang Nucleosynthesis (BBN). We also rule out a hitherto unknown resonance in 8Be at the BBN window, that was conjectured as a possible standard nuclear physics solution to the “Primordial 7Li Problem”. Together with previous results, we deduce a new Wagoner-like Rate for the destruction of 7Be by neutrons which is based on all current measured data. We conclude the lack of a standard nuclear solution to the “Primordial 7Li Problem”. Our upper limit on the cross sections for the high energy alpha-particles is in agreement with recent measurement of the n_TOF collaboration, but it is considerably smaller than the p-wave extrapolation of the Kyoto collaboration. We measured the alpha-particles from the 7Be(n,gi)8Be*(3.03 MeV) reaction, which is considerably larger than a previous s-wave estimate. Hence, in contrast, we conclude s-wave dominance at BBN energies, as would be expected due to the broad (122 keV) low lying 2” state at En = 10 keV.

Reply to Comment by D. Schumann et al. on The interaction of neutrons with 7Be at BBN temperatures: Lack of Standard Nuclear Physics Solution to the “Primordial 7Li Problem”

Reply to Comment by D. Schumann <i>et al.</i> on The interaction of neutrons with ⁷Be at BBN temperatures: Lack of Standard Nuclear Physics Solution to the “Primordial ⁷Li Problem”

Back to the Lithium Plateau with the [Fe/H] &lt; −6 Star J0023+0307∗

Abstract We present an analysis of the Ultraviolet and Visual Echelle Spectrograph (UVES) high-resolution spectroscopic observations at the 8.2 m Very Large Telescope of J0023+0307, a main-sequence extremely iron-poor dwarf star. We are unable to detect iron lines in the spectrum but derive [Fe/H] &lt; −6.1 from the Ca ii resonance lines assuming [Ca/Fe] ≥ 0.40. The chemical abundance pattern of J0023+0307, with very low [Fe/Mg] and [Ca/Mg] abundance ratios but relatively high absolute Mg and Si abundances, suggests J0023+0307 is a second generation star formed from a molecular cloud polluted by only one supernova in which the fallback mechanism played a role. We measure a carbon abundance of A(C) = 6.2 that places J0023+0307 on the low band in the A(C)–[Fe/H] diagram, suggesting no contamination from a binary companion. This star is also unique having a lithium abundance A(Li) = 2.02 ± 0.08, close to the level of the lithium plateau, in contrast with lower Li determinations or upper limits in all other extremely iron-poor stars. The upper envelope of the lithium abundances in unevolved stars spanning more than three orders of magnitude in metallicity (−6 &lt; [Fe/H] &lt; −2.5) defines a nearly constant value. We argue that it is unlikely that such uniformity is the result of depletion processes in stars from a significantly higher initial Li abundance, but suggests instead a lower primordial production, pointing to new physics such as decaying massive particles, varying fundamental constants, or nuclear resonances, that could have affected the primordial 7Li production.

On the relative velocity distribution for general statistics and an application to big-bang nucleosynthesis under Tsallis statistics

The distribution function of the relative velocity in a two-body reaction of nonrelativistic uncorrelated particles is derived for general cases of given distribution functions of single particle velocities. The distribution function is then used in calculations of thermonuclear reaction rates. As an example, we take the Tsallis non-Maxwellian distribution, and show that the distribution function of the relative velocity is different from the Tsallis distribution. We identify an inconsistency in previous studies of nuclear reaction rates within Tsallis statistics, and derive revised nuclear reaction rates. Utilizing the revised rates, accurate results of big bang nucleosynthesis are obtained for the Tsallis statistics. For this application it is more difficult to reduce the primordial 7Li abundance while keeping other nuclear abundances within the observational constraints. A small deviation from a Maxwell-Boltzmann distribution can increase the D abundance and slightly reduce 7Li abundance. Although it is impossible to realize a 7Li abundance at the level observed in metal-poor stars, a significant decrease is possible while maintaining a consistency with the observed D abundance.

Impact of the 7Be(α, γ)11C Reaction on the Primordial Abundance of 7Li

Abstract We calculate the radiative capture cross section for 7Be(α, γ)11C and its reaction rate of relevance for the Big Bang nucleosynthesis (BBN). The impact of this reaction on the primordial 7Li abundance is revised including narrow and broad resonances in the pertinent energy region. Our calculations show that it is unlikely that very low energy resonances in 11C of relevance for the BBN would emerge within a two-body potential model. Based on our results and a comparison with previous theoretical and experimental analyses, we conclude that the impact of this reaction on the so-called “cosmological lithium puzzle” is completely irrelevant.

Search for resonant states in 10C and 11C and their impact on the primordial 7Li abundance

The cosmological 7Li problem arises from the significant discrepancy of about a factor 3 between the predicted primordial 7Li abundance and the observed one. The main process for the production of 7Li during Big-Bang nucleosynthesis is the decay of 7Be. Many key nuclear reactions involved in the production and destruction of 7Be were investigated in attempt to explain the 7Li deficit but none of them led to successful conclusions. However, some authors suggested recently the possibility that the destruction of 7Be by 3He and 4He may reconcile the predictions and observations if missing resonant states in the compound nuclei 10C and 11C exist. Hence, a search of these missing resonant states in 10C and 11C was investigated at the Orsay Tandem-Alto facility through 10B(3He,t)10C and 11B(3He,t)11C charge-exchange reactions respectively. After a short overview of the cosmological 7Li problem from a nuclear physics point of view, a description of the Orsay experiment will be given as well as the obtained results and their impact on the 7Li problem.

Review on effects of long-lived negatively charged massive particles on Big Bang Nucleosynthesis

We review important reactions in the Big Bang Nucleosynthesis (BBN) model involving a long-lived negatively charged massive particle, [Formula: see text], which is much heavier than nucleons. This model can explain the observed 7Li abundances of metal-poor stars, and predicts a primordial 9Be abundance that is larger than the standard BBN prediction. In the BBN epoch, nuclei recombine with the [Formula: see text] particle. Because of the heavy [Formula: see text] mass, the atomic size of bound states [Formula: see text] is as small as the nuclear size. The nonresonant recombination rates are then dominated by the [Formula: see text]-wave [Formula: see text] transition for 7Li and [Formula: see text]Be. The 7Be destruction occurs via a recombination with the [Formula: see text] followed by a proton capture, and the primordial 7Li abundance is reduced. Also, the 9Be production occurs via the recombination of 7Li and [Formula: see text] followed by deuteron capture. The initial abundance and the lifetime of the [Formula: see text] particles are constrained from a BBN reaction network calculation. We derived parameter region for the 7Li reduction allowed in supersymmetric or Kaluza–Klein (KK) models. We find that either the selectron, smuon, KK electron or KK muon could be candidates for the [Formula: see text] with [Formula: see text] TeV, while the stau and KK tau cannot.

NON-EXTENSIVE STATISTICS TO THE COSMOLOGICAL LITHIUM PROBLEM

ABSTRACT Big Bang nucleosynthesis (BBN) theory predicts the abundances of the light elements D, 3He, 4He, and 7Li produced in the early universe. The primordial abundances of D and 4He inferred from observational data are in good agreement with predictions, however, BBN theory overestimates the primordial 7Li abundance by about a factor of three. This is the so-called “cosmological lithium problem.” Solutions to this problem using conventional astrophysics and nuclear physics have not been successful over the past few decades, probably indicating the presence of new physics during the era of BBN. We have investigated the impact on BBN predictions of adopting a generalized distribution to describe the velocities of nucleons in the framework of Tsallis non-extensive statistics. This generalized velocity distribution is characterized by a parameter q, and reduces to the usually assumed Maxwell–Boltzmann distribution for q = 1. We find excellent agreement between predicted and observed primordial abundances of D, 4He, and 7Li for 1.069 ≤ q ≤ 1.082, suggesting a possible new solution to the cosmological lithium problem.

Breaking Be: a sterile neutrino solution to the cosmological lithium problem

The possibility that the so-called ``lithium problem'', i.e., the disagreement between the theoretical abundance predicted for primordial 7Li assuming standard nucleosynthesis and the value inferred from astrophysical measurements, can be solved through a non-thermal Big Bang Nucleosynthesis (BBN) mechanism has been investigated by several authors. In particular, it has been shown that the decay of a MeV-mass particle, like, e.g., a sterile neutrino, decaying after BBN not only solves the lithium problem, but also satisfies cosmological and laboratory bounds, making such a scenario worth to be investigated in further detail. In this paper, we constrain the parameters of the model with the combination of current data, including Planck 2015 measurements of temperature and polarization anisotropies of the Cosmic Microwave Background (CMB), FIRAS limits on CMB spectral distortions, astrophysical measurements of primordial abundances and laboratory constraints. We find that a sterile neutrino with mass MS = 4.35−0.17+0.13 MeV (at 95% c.l.), a decay time τS = 1.8−1.3+2.5 · 105 s (at 95% c.l.) and an initial density n̄S/n̄cmb = 1.7−0.6+3.5 · 10−4 (at 95% c.l.) in units of the number density of CMB photons, perfectly accounts for the difference between predicted and observed 7Li primordial abundance. This model also predicts an increase of the effective number of relativistic degrees of freedom at the time of CMB decoupling ΔNeffcmb ≡ Neffcmb −3.046 = 0.34−0.14+0.16 at 95% c.l.. The required abundance of sterile neutrinos is incompatible with the standard thermal history of the Universe, but could be realized in a low reheating temperature scenario. We also provide forecasts for future experiments finding that the combination of measurements from the COrE+ and PIXIE missions will allow to significantly reduce the permitted region for the sterile lifetime and density.

Search for new resonant states in10C and11C and their impact on the cosmological lithium problem

The observed primordial 7Li abundance in metal-poor halo stars is found to be lower than its Big-Bang nucleosynthesis (BBN) calculated value by a factor of approximately three. Some recent works suggested the possibility that this discrepancy originates from missing resonant reactions which would destroy the 7Be, parent of 7Li. The most promising candidate resonances which were found include a possibly missed 1- or 2- narrow state around 15 MeV in the compound nucleus 10C formed by 7Be+3He and a state close to 7.8 MeV in the compound nucleus 11C formed by 7Be+4He. In this work, we studied the high excitation energy region of 10C and the low excitation energy region in 11C via the reactions 10B(3He,t)10C and 11B(3He,t)11C, respectively, at the incident energy of 35 MeV. Our results for 10C do not support 7Be+3He as a possible solution for the 7Li problem. Concerning 11C results, the data show no new resonances in the excitation energy region of interest and this excludes 7Be+4He reaction channel as an explanation for the 7Li deficit.

Signatures of long-lived exotic strongly interacting massive particles on light element abundances through reactions triggered by the particles

A primordial 7Li abundance inferred from observations of metal-poor halo stars is a factor of about three smaller than predictions of standard big bang nucleosynthesis (BBN) model. Some particle models beyond the standard model include heavy long-lived colored particles with mass much larger than 1 GeV. They would be confined inside exotic heavy hadrons, i.e., strongly interacting massive particles. We have found possible reactions which destroy 7Be and 7Li in the scenario of BBN including a long-lived sub-strongly interacting massive particle (sub-SIMP, X). The destruction is associated with non radiative X captures of the nuclei, and it can be realized only if the interaction strength between an X and a nucleon is properly weaker than that between two nucleons. Binding energies of nuclei to an X and nuclear reaction rates associated with the X are estimated. We perform a network BBN calculation using the estimated rates, and suggest that the 7Li problem can be solved if long-lived sub-SIMPs have existed.

Observation of interstellar lithium in the low-metallicity Small Magellanic Cloud

The primordial abundances of light elements produced in the standard theory of Big Bang nucleosynthesis (BBN) depend only on the cosmic ratio of baryons to photons, a quantity inferred from observations of the microwave background. The predicted primordial (7)Li abundance is four times that measured in the atmospheres of Galactic halo stars. This discrepancy could be caused by modification of surface lithium abundances during the stars' lifetimes or by physics beyond the Standard Model that affects early nucleosynthesis. The lithium abundance of low-metallicity gas provides an alternative constraint on the primordial abundance and cosmic evolution of lithium that is not susceptible to the in situ modifications that may affect stellar atmospheres. Here we report observations of interstellar (7)Li in the low-metallicity gas of the Small Magellanic Cloud, a nearby galaxy with a quarter the Sun's metallicity. The present-day (7)Li abundance of the Small Magellanic Cloud is nearly equal to the BBN predictions, severely constraining the amount of possible subsequent enrichment of the gas by stellar and cosmic-ray nucleosynthesis. Our measurements can be reconciled with standard BBN with an extremely fine-tuned depletion of stellar Li with metallicity. They are also consistent with non-standard BBN.

Astrophysical S factor for the 4He(3He,γ)7Be reaction at medium energies

The astrophysical S factor for the 4He(3He,γ)7Be direct capture reaction plays a major role in the context of solar neutrino flux and primordial 7Li abundances that demand accurate information on the reaction. We report here our recent cross section measurements using the activation method in the region of ECM = 900-2800 keV, that aim to shed light on the discrepancies in the existing data and lead to a more accurate extrapolation of the S factor.

Galactic evolution of 7Li

AbstractLithium represents a key element in cosmology, as it is one of the few nuclei synthesized during the Big Bang. The primordial abundance of 7Li allows us to impose constraints on the primordial nucleosynthesis and on the baryon density of the universe. However, 7Li is not only produced during the Big Bang but also during galactic evolution: measures of stellar Li in our Galaxy suggest an almost constant Li abundance (the so-called Spite plateau) at low metallicities and a subsequent increase in the disk stars, leading to a Li abundance in Population I stars higher by a factor of ten than in Population II stars. This means that there must exist several possible stellar sources of 7Li: asymptotic giant branch stars, supernovae, novae, red giant stars. 7Li is also partly produced in spallation processes while 6Li is entirely produced by such processes. All of these sources have been included in galactic chemical evolution models and constraints have been derived on the primordial 7Li and its evolution, as well on stellar models. I will review these models and their results and what we have learned about 7Li evolution. Some still open problems, such as the disagreement between the primordial 7Li abundance as derived by WMAP and as measured in Population II stars, and the uncertainties about the main sources of stellar 7Li will be discussed.

Stellar and Primordial Nucleosynthesis ofBe7: Measurement ofHe3(α,γ)Be7

The 3He(alpha,gamma)7Be reaction presently represents the largest nuclear uncertainty in the predicted solar neutrino flux and has important implications on the big bang nucleosynthesis, i.e., the production of primordial 7Li. We present here the results of an experiment using the recoil separator ERNA (European Recoil separator for Nuclear Astrophysics) to detect directly the 7Be ejectiles. In addition, off-beam activation and coincidence gamma-ray measurements were performed at selected energies. At energies above 1 MeV a large discrepancy compared to previous results is observed both in the absolute value and in the energy dependence of the cross section. Based on the available data and models, a robust estimate of the cross section at the astrophysical relevant energies is proposed.