More is different: 50 years of nuclear BCS

Many of the concepts which are at the basis of the development associated with a quantitative treatment of the variety of phenomena associated with the spontaneous breaking of gauge symmetry in nuclei have been instrumental in connection with novel studies of soft matter, namely of protein evolution and protein folding. Although the route to these subjects and associated development does not necessarily imply the nuclear physics connection, such a connection has proven qualitatively and quantitatively inspiring. In particular to model protein evolution in terms of the alignment of quasispins displaying twenty different projections, one for each of the twenty amino acids occurring in nature, and the associated symmetry breaking in information (sequence) space. Emergent properties of the corresponding phase transition are domain walls which stabilize local elementary structures (LES), few groups of 10-20 aminoacids which become structured already in the denatured state provide the molecular recognition directing protein folding. In fact, their docking is closely related to the transition state of the process. While the two-step, yes or no, folding process, does not provide direct information concerning LES, one can force LES from virtual to become real, observable final state entities. Getting again inspiration from the nuclear case (virtual processes contributing to pair correlations can be forced to become real with the help of a probe which itself changes particle number by two), one would expect that to make real virtual LES, that is segments of the protein which already at an early stage of the folding process flicker in and out of their native conformation, one needs a probe which itself displays a similar behaviour. Peptides displaying a sequence identical to LES are such probes.Comment: Contribution to the Volume 50 years of Nuclear BCS edited by World Scientifi

Study of the transition from pairing vibrational to pairing rotational regimes between magic numbers N=50 and N=82, with two-nucleon transfer

Absolute values of two-particle transfer cross sections along the Sn-isotopic chain from closed shell to closed shell (100Sn,132Sn) are calculated taking properly into account nuclear correlations, as well as the successive, simultaneous and non-orthogonality contributions to the differential cross sections. The results are compared with systematic, homogeneous bombarding conditions (p, t) data. The observed agreement, almost within statistical errors and without free parameters, testify to the fact that theory is able to be quantitative in its predictions

Direct observation of the glue pairing the halo of the nucleus 11Li

With the help of a unified description of the nuclear structure and of the direct reaction mechanism we show that a recent 1H(11Li,9Li)3H experiment provides, for the first time in nuclear physics, direct evidence of phonon mediated pairing.Comment: 9 pages, 4 figures. Major change

Reading the three-dimensional structure of a protein from its amino acid sequence

While all the information required for the folding of a protein is contained in its amino acid sequence, one has not yet learnt how to extract this information so as to predict the detailed, biological active, three-dimensional structure of a protein whose sequence is known. This situation is not particularly satisfactory, in keeping with the fact that while linear sequencing of the amino acids specifying a protein is relatively simple to carry out, the determination of the folded-native-conformation can only be done by an elaborate X-ray diffraction analysis performed on crystals of the protein or, if the protein is very small, by nuclear magnetic resonance techniques. Using insight obtained from lattice model simulations of the folding of small proteins (fewer than 100 residues), in particular of the fact that this phenomenon is essentially controlled by conserved contacts among strongly interacting amino acids, which also stabilize local elementary structures formed early in the folding process and leading to the (post-critical) folding core when they assemble together, we have worked out a successful strategy for reading the three-dimensional structure of a notional protein from its amino acid sequence.Comment: misprints eliminated and small mistakes correcte

Sensitivity to multi-phonon excitations in heavy-ion fusion reactions

Measured cross sections for the fusion of {64}Ni with {64}Ni, {74}Ge, and {100}Mo targets are analyzed in a coupled-channels approach. The data for the {64}Ni target above 0.1 mb are reproduced by including couplings to the low-lying 2^+ and 3^- states and the mutual and two-phonon excitations of these states. The calculations become more challenging as the fusing nuclei become softer and heavier, and excitations to multi-phonon states start to play an increasingly important role. Thus it is necessary to include up to four-phonon excitations in order to reproduce the data for the {64}Ni+{74}Ge system. Similar calculations for {64}Ni+{100}Mo, and also for the symmetric {74}Ge+{74}Ge system, show large discrepancies with the data. Possible ways to improve the calculations are discussed.Comment: 24 pages, 7 figures, 4 table

Difference between stable and exotic nuclei: medium polarization effects

The bare NN-potential, parametrized so as to reproduce the nuclear phase shifts leads to a sizable Cooper pair binding energy in nuclei along the stability valley. It is a much debated matter whether this value accounts for the "empirical" value of the pairing gap or whether a similarly important contribution arises from the exchange of collective vibrations between Cooper pair partners. In keeping with the fact that two-particle transfer reactions are the specific probe of pairing in nuclei, and that exotic halo nuclei like 11Li are extremely polarizable, we find that the recent studied reaction, namely 11Li+p -> 9Li+t, provides direct evidence of phonon mediated pairing in nuclei