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 Solar Fusion Cross Sections for the pp Chain and CNO Cycle
 (INT Workshop January 21 - 23, 2009)

  Reported by W. Haxton
  Date posted April 7, 2009

The 21-23 January 2009 workshop brought together 42 participants - experimentalists and theorists - to consider the nuclear microphysics important to the standard solar model and to red giants. The workshop was highly international, reflecting the important contributions being made by facilities like Gran Sasso's LUNA and Japan's RIKEN.

The workshop was intended as an update to an earlier one, held in 1997. The main purpose of the meeting was to critically evaluate the state of experimental data important to key nuclear astrophysics cross sections, recommend "best value" cross sections, estimate current uncertainties, and discuss future needs, including new facilities that could speed progress. The organization was by working groups, divided according to cross section or technique, and led by designated discussion leaders (see Fig. 1).

The end product of this meeting will be a Reviews of Modern Physics article updating the nuclear physics of the pp chain and CNO cycle. Prior to the meeting each working group had identified key papers and archived these on the workshop's web site. During the meeting these papers were reviewed and discussed, and needed work identified. Work will continue off line, until consensus is reached and the various chapters for the RMP article are written.

One motivation for holding the workshop now is the importance of precision in interpreting current and future solar neutrino experiments. For example, the 8B neutrino spectrum - the subject tackled by Working Group 4 - is used in the interpretation of results from SNO and Super-Kamiokande. Currently these collaborations employ slightly different spectra. Similarly, Borexino is currently running (Fig. 2), and a deeper version of this experiment, SNO+, is expected to be one of the first experiments mounted in SNOLab. These experiments will check the MSW mechanism, which predicts that the survival probability for low-energy neutrinos is greater than that of the high-energy 8B neutrinos. They may be able to isolate the neutrino flux from the Sun's weak CN cycle, which would in turn directly constrain the metalicity of the Sun's core.

One of the questions where additional consensus was sought concerns indirect methods for measuring S-factors. In the case of asymptotic normalization coefficients, it was argued that in cases where elastic scattering is measured to constrain optical potentials, and other checks are made to demonstrate the reaction is peripheral and 1-step (DWBA), then reliability at the level of 10% is achievable. Continuum couplings destroy the relation between cross section and ANC, and have been argued to be absent theoretically only in one case.

In the case of Coulomb dissociation, there was wide agreement that the GSI-2 results provide the best data set. Summerer showed that Descouvemont 2004 fits their data better than other parametrizations.

Because of ongoing discussions about model dependence in the comparison of direct and indirect methods, the decision was made to analyze direct and indirect data sets for reactions like 7Be(p,γ) separately, and to rely on the direct data for the S17(0) recommendation.

The ERNA collaboration data on 3He+4He were presented: the preprint was released just prior to the workshop. The new results disagree significantly with older measurements made at high energy. One preliminary new activation point measured at Madrid with the Weizmann apparatus seems to confirm the high energy behavior seen by ERNA. At low energy the ERNA results are in good agreement with Seattle group measurements, both of which lie somewhat above Weizmann and LUNA measurements.

There was great interest in new underground facilities, given the marvelous results obtained by LUNA for reactions like 14N(p,γ).


Working Group Working Group Leaders
1. S11 and Related Theory
    Key Papers
Jiunn-Wei Chen, National Taiwan Univ.
Kuniharu Kubodera, Univ. South Carolina
2. 3He+3He S-factor
    Key Papers
Uwe Greife, Colorado School of Mines
Paolo Prati, INFN-Genova
3. p + 7Be and p+d S-factors
    Key Papers
Gianluca Imbriani, Univ. Napoli
Michael Hass, Weizmann Institute
4. Neutrino spectra
    Key Papers
Stuart Freedman, Univ. California Berkeley
Alejandro Garcia, Univ. Washington
5. 14N(p,γ) S-factor
    Key Papers
Heide Costantini, INFN-Genova
Hanns-Peter Trautvetter, Ruhr-U. Bochum
6. Other CNO S-factors
    Key Papers
Peter Parker, Yale Univ.
R. G. Hamish Robertson, Univ. Washington
7. 3He+4He S-factor
    Key Papers
Kurt Snover, Univ. Washington
Frank Strieder, Ruhr-U. Bochum
8. hep S-factor
   Key Papers
Laura Marcucci, Univ. of Pisa
Tae-Sun Park, Sungkyunkwan Univ.
9. 7Be, pep, CNO electron capture
    Key Papers
Andrei Gruzinov, New York Univ.
R. G. Hamish Robertson, Univ. Washington
10. Theoretical issues
       Key Papers
Wick Haxton, Univ. Washington
Stefan Typel, GSI
11. Validating new techniques:
           Trojan horse/Coulomb
       Key Papers
Eric Adelberger, Univ. Washington
Barry Davids, TRIUMF
Filomena Nunes, Michigan State Univ.
12. Future facilities
        Key Papers
Daniela Leitner, Lawrence Berkeley Lab
Michael Wiescher, Notre Dame Univ.

FIG. 1 The working group organization for the workshop.

FIG. 2 The Borexino detector, now operating in Gran Sasso.