A search for massive neutral bosons in orthopositronium decay

We have searched for an exotic decay of orthopositronium into a single photon and a short-lived neutral boson in the hitherto unexplored mass region above 900 ${\rm keV}/{\it c}^{2}$, by noting that this decay is one of few remaining candidates which could explain the discrepancy of the orthopositronium decay-rate. A high-resolution measurement of the associated photon energy spectrum was carried out with a germanium detector to search for a sharp peak from this two-body decay. Our negative result provides the upper-limits of\mbox{ }$2.0 \times 10^{-4}$ on the branching ratio of such a decay in the mass region from 847 to 1013 ${\rm keV}/{\it c}^{2}$, and excludes the possibility of this decay mode explaining the discrepancy in the orthopositronium decay-rate.Comment: a LaTeX file (text 7 pages) and a uuencoded gz-compressed PostScript file (text 7 pages + figures 4 pages

Coupled-Cluster Approach to Electron Correlations in the Two-Dimensional Hubbard Model

We have studied electron correlations in the doped two-dimensional (2D) Hubbard model by using the coupled-cluster method (CCM) to investigate whether or not the method can be applied to correct the independent particle approximations actually used in ab-initio band calculations. The double excitation version of the CCM, implemented using the approximate coupled pair (ACP) method, account for most of the correlation energies of the 2D Hubbard model in the weak ($U/t \simeq 1$) and the intermediate $U/t$ regions ($U/t \simeq 4$). The error is always less than 1% there. The ACP approximation gets less accurate for large $U/t$ ($U/t \simeq 8$) and/or near half-filling. Further incorporation of electron correlation effects is necessary in this region. The accuracy does not depend on the system size and the gap between the lowest unoccupied level and the highest occupied level due to the finite size effect. Hence, the CCM may be favorably applied to ab-initio band calculations on metals as well as semiconductors and insulators.Comment: RevTeX3.0, 4 pages, 4 figure

Vertex operator approach for form factors of Belavin's (Z/nZ)(Z/nZ)-symmetric model

Belavin's $(\mathbb{Z}/n\mathbb{Z})$-symmetric model is considered on the basis of bosonization of vertex operators in the $A^{(1)}_{n-1}$ model and vertex-face transformation. Free field representations of nonlocal tail operators are constructed for off diagonal matrix elements with respect to the ground state sectors. As a result, integral formulae for form factors of any local operators in the $(\mathbb{Z}/n\mathbb{Z})$-symmetric model can be obtained, in principle.Comment: 24 pages, 4 figures, published in J. Phys. A: Math. Theor. 43 (2010) 085202. For the next thirty days from Feb 5 2010, the full text of the article will be completely free to access through our 'This Month's Papers' service (www.iop.org/journals/thismonth), helping you to benefit from maximum visibilit

First test of O(α2)O(\alpha ^{2}) correction of the orthopositronium decay rate

Positronium is an ideal system for the research of the bound state QED. New precise measurement of orthopositronium decay rate has been performed with an accuracy of 150 ppm. This result is consistent with the last three results and also the 2nd order correction. The result combined with the last three is 7.0401$\pm0.0007\mu \mathrm{sec}^{-1}$ (100 ppm), which is consistent with the 2nd order correction and differs from the 1st order calculation by 2.6$\sigma$ It is the first test to validate the 2nd order correction.Comment: will be submitted to Phys. Lett.

Free field approach to diagonalization of boundary transfer matrix : recent advances

We diagonalize infinitely many commuting operators $T_B(z)$. We call these operators $T_B(z)$ the boundary transfer matrix associated with the quantum group and the elliptic quantum group. The boundary transfer matrix is related to the solvable model with a boundary. When we diagonalize the boundary transfer matrix, we can calculate the correlation functions for the solvable model with a boundary. We review the free field approach to diagonalization of the boundary transfer matrix $T_B(z)$ associated with $U_q(A_2^{(2)})$ and $U_{q,p}(\hat{sl_N})$. We construct the free field realizations of the eigenvectors of the boundary transfer matrix $T_B(z)$. This paper includes new unpublished formula of the eigenvector for $U_q(A_2^{(2)})$. It is thought that this diagonalization method can be extended to more general quantum group $U_q(g)$ and elliptic quantum group $U_{q,p}(g)$.Comment: To appear in Group 28 : Group Theoretical Method in Physic

Vertex operator approach for correlation functions of Belavin's (Z/nZ)-symmetric model

Belavin's $(\mathbb{Z}/n\mathbb{Z})$-symmetric model is considered on the basis of bosonization of vertex operators in the $A^{(1)}_{n-1}$ model and vertex-face transformation. The corner transfer matrix (CTM) Hamiltonian of $(\mathbb{Z}/n\mathbb{Z})$-symmetric model and tail operators are expressed in terms of bosonized vertex operators in the $A^{(1)}_{n-1}$ model. Correlation functions of $(\mathbb{Z}/n\mathbb{Z})$-symmetric model can be obtained by using these objects, in principle. In particular, we calculate spontaneous polarization, which reproduces the result by myselves in 1993.Comment: For the next thirty days the full text of this article is available at http://stacks.iop.org/1751-8121/42/16521

Observation of the decay mode K_L -> pi^+ pi^- e^+ e^-

We report on results of an experimental search for the K_L -> pi^+ pi^- e^+ e^- decay mode. We found 13.5 +- 4.0 events and determined its branching ratio to be (4.4 +- 1.3(stat) +- 0.5(syst))*10^{-7}. The result agrees well with the theoretical prediction.Comment: 9 pages, 6 eps-figures, LaTeX2e, graphicx package, submitted to Physics Letters