Quantum Monte Carlo and exact diagonalization study of a dynamic Hubbard model

A one-dimensional model of electrons locally coupled to spin-1/2 degrees of freedom is studied by numerical techniques. The model is one in the class of $dynamic$ $Hubbard$ $models$ that describe the relaxation of an atomic orbital upon double electron occupancy due to electron-electron interactions. We study the parameter regime where pairing occurs in this model by exact diagonalization of small clusters. World line quantum Monte Carlo simulations support the results of exact diagonalization for larger systems and show that kinetic energy is lowered when pairing occurs. The qualitative physics of this model and others in its class, obtained through approximate analytic calculations, is that superconductivity occurs through hole undressing even in parameter regimes where the effective on-site interaction is strongly repulsive. Our numerical results confirm the expected qualitative behavior, and show that pairing will occur in a substantially larger parameter regime than predicted by the approximate low energy effective Hamiltonian.Comment: Some changes made in response to referees comments. To be published in Phys.Rev.

Proton decay and light sterile neutrinos

Within the standard model, non-renormalizable operators at dimension six ($d=6$) violate baryon and lepton number by one unit and thus lead to proton decay. Here, we point out that the proton decay mode with a charged pion and missing energy can be a characteristic signature of $d=6$ operators containing a light sterile neutrino, if it is not accompanied by the standard $\pi^0e^+$ final state. We discuss this effect first at the level of effective operators and then provide a concrete model with new physics at the TeV scale, in which the lightness of the active neutrinos and the stability of the proton are related.Comment: 7 pages, 2 figures, published versio

Proposal for a Supersymmetric Standard Model

The fact that neutrinos are massive suggests that the minimal supersymmetric standard model (MSSM) might be extended in order to include three gauge-singlet neutrino superfields with Yukawa couplings of the type $H_2 L \nu^c$. We propose to use these superfields to solve the $\mu$ problem of the MSSM without having to introduce an extra singlet superfield as in the case of the next-to-MSSM (NMSSM). In particular, terms of the type $\nu^c H_1 H_2$ in the superpotential may carry out this task spontaneously through sneutrino vacuum expectation values. In addition, terms of the type $(\nu^c)^3$ avoid the presence of axions and generate effective Majorana masses for neutrinos at the electroweak scale. On the other hand, these terms break lepton number and R-parity explicitly implying that the phenomenology of this model is very different from the one of the MSSM or NMSSM. For example, the usual neutralinos are now mixed with the neutrinos. For Dirac masses of the latter of order $10^{-4}$ GeV, eigenvalues reproducing the correct scale of neutrino masses are obtained.Comment: 9 pages, latex, title modified. Final version published in PR

Invisible Higgs Boson Decays in Spontaneously Broken R-Parity

The Higgs boson may decay mainly to an invisible mode characterized by missing energy, instead of the Standard Model channels. This is a generic feature of many models where neutrino masses arise from the spontaneous breaking of ungauged lepton number at relatively low scales, such as spontaneously broken R-parity models. Taking these models as framework, we reanalyze this striking suggestion in view of the recent data on neutrino oscillations that indicate non-zero neutrino masses. We show that, despite the smallness of neutrino masses, the Higgs boson can decay mainly to the invisible Goldstone boson associated to the spontaneous breaking of lepton number. This requires a gauge singlet superfield coupling to the electroweak doublet Higgses, as in the Next to Minimal Supersymmetric Standard Model (NMSSM) scenario for solving the $\mu$-problem. The search for invisibly decaying Higgs bosons should be taken into account in the planning of future accelerators, such as the Large Hadron Collider and the Next Linear Collider.Comment: 24 pages, 10 figures; typos corrected, published versio

Founding brothers: Leland, buck, and cappon and the formation of the archives profession (session 404)

This session on archives history examines the role of three individuals-Waldo G. Leland (1879-1966), Solon J. Buck (1884-1962), and Lester J. Cappon (1900-1981)-in the formation of the archives profession in the United States in the first three-quarters of the twentieth century. These "founding brothers" published extensively, but they also created and maintained personal manuscript collections that reflect how they viewed themselves and how they wanted to be remembered. Four archivists/historians track through the lenses of the papers of the "founding brothers" the emergence of professional history to the beginnings of public history with their alliance and tension with archival science as a distinct profession