278 research outputs found
Interplay of the Chiral and Large N_c Limits in pi N Scattering
Light-quark hadronic physics admits two useful systematic expansions, the
chiral and 1/N_c expansions. Their respective limits do not commute, making
such cases where both expansions may be considered to be especially
interesting. We first study pi N scattering lengths, showing that (as expected
for such soft-pion quantities) the chiral expansion converges more rapidly than
the 1/N_c expansion, although the latter nevertheless continues to hold. We
also study the Adler-Weisberger and Goldberger-Miyazawa-Oehme sum rules of pi N
scattering, finding that both fail if the large N_c limit is taken prior to the
chiral limit.Comment: 10 pages, ReVTe
Residual interaction effects on deeply bound pionic states in Sn and Pb isotopes
We have studied the residual interaction effects theoretically on the deeply
bound pionic states in Pb and Sn isotopes. We need to evaluate the residual
interaction effects carefully in order to deduce the nuclear medium effects for
pion properties, which are believed to provide valuable information on nuclear
chiral dynamics. The s- and p-wave interactions are used for the
pion-nucleon residual interactions. We show that the complex energy shifts are
around [(10-20)+i(2-7)]keV for 1s states in Sn, which should be taken into
account in the analyses of the high precision data of deeply bound pionic
states in Sn isotopes.Comment: REVTEX4, 6 pages, 5 tables, Submitted to Phys. Rev. C, Some
explanations are added in Version
Quasinormal distributions and expansion at the mode
The Gram-Charlier series of type A is discussed in terms of deviants which are related to moments in a way similar to the way Hermite polynomials are related to the powers. Distribution functions are also expressed in terms of the mode and moments (cumulants or deviants), which are useful expansions when the distributions are approximately normal. It is shown that such expansions as well as the Gram-Charlier series are valid asymptotically for discrete distributions defined on the semiinfinite interval [0, ∞].Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/45129/1/10955_2005_Article_BF01010217.pd
Relativistic two-pion exchange nucleon-nucleon potential: configuration space
We have recently performed a relativistic chiral expansion of the
two-pion exchange potential, and here we explore its configuration space
content. Interactions are determined by three families of diagrams, two of
which involve just and , whereas the third one depends on
empirical coefficients fixed by subthreshold data. In this sense, the
calculation has no adjusted parameters and gives rise to predictions, which are
tested against phenomenological potentials. The dynamical structure of the
eight leading non-relativistic components of the interaction is investigated
and, in most cases, found to be clearly dominated by a well defined class of
diagrams. In particular, the central isovector and spin-orbit, spin-spin, and
tensor isoscalar terms are almost completely fixed by just and .
The convergence of the chiral series in powers of the ratio (pion mass/nucleon
mass) is studied as a function of the internucleon distance and, for 1 fm,
found to be adequate for most components of the potential. An important
exception is the dominant central isoscalar term, where the convergence is
evident only for 2.5 fm. Finally, we compare the spatial behavior of the
functions that enter the relativistic and heavy baryon formulations of the
interaction and find that, in the region of physical interest, they differ by
about 5%.Comment: 27 pages, 33 figure
Elastic scattering of low energy pions by nuclei and the in-medium isovector pi N amplitude
Measurements of elastic scattering of 21.5 MeV pi+ and pi- by Si, Ca, Ni and
Zr were made using a single arm magnetic spectrometer. Absolute calibration was
made by parallel measurements of Coulomb scattering of muons. Parameters of a
pion-nucleus optical potential were obtained from fits to all eight angular
distributions put together. The `anomalous' s-wave repulsion known from pionic
atoms is clearly observed and could be removed by introducing a
chiral-motivated density dependence of the isovector scattering amplitude,
which also greatly improved the fits to the data. The empirical energy
dependence of the isoscalar amplitude also improves the fits to the data but,
contrary to what is found with pionic atoms, on its own is incapable of
removing the anomaly.Comment: 20 pages, 5 figures, 5 tables. V2 added details on
uncertainties,extended discussion. To appear in PR
Precision spectroscopy of pionic 1s states of Sn nuclei and evidence for partial restoration of chiral symmetry in the nuclear medium
Deeply bound 1s states of in Sn were preferentially
observed using the Sn(,He) pion-transfer reaction under the recoil-free
condition. The 1s binding energies and widths were precisely determined, and
were used to deduce the isovector parameter of the s-wave pion-nucleus
potential to be . The observed enhancement
of over the free value ()
indicates a reduction of the chiral order parameter, , at the normal nuclear density, .Comment: 4 pages including 3 postscript figures, RevTeX 4 with multirow.sty,
submitted to Physical Review Letter
Cyclooxygenase-2 overexpression correlates with tumour recurrence, especially haematogenous metastasis, of colorectal cancer
Epidemiological studies have demonstrated that nonsteroidal anti-inflammatory drugs (NSAIDs), known to inhibit cyclooxygenase (COX), reduce the risk of colorectal cancer. COX is a key enzyme in prostaglandin biosynthesis, and two isoforms of COX, COX-1 and COX-2, have been identified. Recently COX-2 has been reported to frequently overexpress in colorectal neoplasms and to play a role in colorectal tumorigenesis and tumour progression. In this study, using immunohistochemistry, we examined COX-2 expression in advanced human colorectal cancer and its correlation with clinicopathological features. COX-2 expression was observed mainly in the cytoplasm of cancer cells in all the specimens examined, but some stromal cells and endothelial cells were also stained. According to the grade of COX-2 expression of the cancer cells, patients were divided into high- and low-COX-2 expression groups. High-COX-2 expression significantly correlated with tumour recurrence, especially haematogenous metastasis. These results suggest that a selective COX-2 inhibitor can be a novel class of therapeutic agents not only for tumorigenesis but also for haematogenous metastasis of cololectal cancer. To our knowledge, this is the first report on the correlation between COX-2 overexpression and recurrence of colorectal cancer. © 2000 Cancer Research Campaig
Chiral corrections to kaon-nucleon scattering lengths
We calculate the threshold T-matrices of kaon-nucleon and antikaon-nucleon
scattering to one loop order in SU(3) heavy baryon chiral perturbation theory.
To that order the complex-valued isospin-1 threshold T-matrix can be
successfully predicted from the isospin-0 and 1 threshold T-matrices. As
expected perturbation theory fails to explain the isospin-0 threshold
T-matrix which is completely dominated by the nearby subthreshold
-resonance. Cancelations of large terms of second and third
chiral order are observed as they seem to be typical for SU(3) baryon chiral
perturbation theory calculations. We also give the kaon and eta loop
corrections to the scattering lengths and we investigate
scattering to one-loop order. The second order s-wave low-energy constants are
all of natural size and do not exceed 1 GeV in magnitude.Comment: 8 pages, 2 figures, published in Phys. Rev. C64, 045204 (2001),
corrections of numerical prefactors in Eqs.(10,11,12
Charge-Independence Breaking in the Two-Pion-Exchange Nucleon-Nucleon Force
Charge-independence breaking due to the pion-mass difference in the (chiral)
two-pion-exchange nucleon-nucleon force is investigated. A general argument
based on symmetries is presented that relates the charge-symmetric part of that
force to the proton-proton case. The static potential linear in that mass
difference is worked out as an explicit example by means of Feynman diagrams,
and this confirms the general argument.Comment: 10 pages, latex, 1 figure -- epsfig.sty required -- To appear in
Phys. Rev.
Equivalence of Nonstatic Two-Pion-Exchange Nucleon-Nucleon Potentials
Off-shell aspects of the one-pion-exchange potential and their relationship
to different forms of the nonstatic (subleading-order) chiral two-pion-exchange
nucleon-nucleon potential are discussed. Various types of off-shell behavior
are categorized and numerous examples are given. Recently derived potentials
based on chiral approaches are supplemented by a rather general form of the
two-pion-exchange potential derived using old-fashioned methods. The latter is
closely related to a general form of one-pion-exchange relativistic corrections
and nonstatic two-pion-exchange three-nucleon forces developed long ago.Comment: 16 pages, latex -- Phys. Rev. C (to appear) -- Published versio
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