Search results for "Nucleon"
showing 10 items of 1041 documents
Improved unitarized Heavy Baryon Chiral Perturbation Theory for π N scattering
2000
We show how the unitarized description of pion nucleon scattering within Heavy Baryon Chiral Perturbation Theory can be considerably improved, by a suitable reordering of the expansion over the nucleon mass. Within this framework, the $\Delta$ resonance and its associated pole can be recovered from the chiral parameters obtained from low-energy determinations. In addition, we can obtain a good description of the six $S$ and $P$ wave phase shifts in terms of chiral parameters with a natural size and compatible with the Resonance Saturation Hypothesis.
Radiative pion capture by a nucleon
2000
The differential cross sections for $\pi^- p \to \gamma n$ and $\pi^+ n \to \gamma p$ are computed up to $O(p^3)$ in heavy baryon chiral perturbation theory (HBChPT). The expressions at $O(p)$ and $O(p^2)$ have no free parameters. There are three unknown parameters at $O(p^3)$, low energy constants of the HBChPT Lagrangian, which are determined by fitting to experimental data. Two acceptable fits are obtained, which can be separated by comparing with earlier dispersion relation calculations of the inverse process. Expressions for the multipoles, with emphasis on the p-wave multipoles, are obtained and evaluated at threshold. Generally the results obtained from the best of the two fits are i…
Burkhardt-Cottingham sum rule and forward spin polarizabilities in Heavy Baryon Chiral Perturbation Theory
2002
We study spin-dependent sum rules for forward virtual Compton scattering(VVCS) off the nucleon in heavy baryon chiral perturbation theory at order $O(p^4)$. We show how these sum rules can be evaluated from low energy expansions (in the virtual photon energy) of the forward VVCS amplitudes. We study in particular the Burkhardt -Cottingham sum rule in HBChPT and higher terms in the low energy expansion, which can be related to the generalized forward spin polarizabilities of the nucleon. The dependence of these observables on the photon virtuality $Q^2$ can be accessed, at small and intermediate $Q^2$ values, from existing and forthcoming data at Jefferson Lab.
Baryon decuplet in the chiral dynamics of Λ hyperons in nuclear matter
2007
6 pages, 5 figures.-- PACS nrs.: 21.80.+a; 21.65.+f; 13.75.Ev; 24.10.Cn.-- ISI Article Identifier: 000245332200063.-- ArXiv pre-print available at: http://arxiv.org/abs/nucl-th/0611082
Improved bounds on SUSY accompanied neutrinoless double beta decay
1998
Neutrinoless double beta decay induced by light Majorana neutrino exchange between two decaying nucleons with squark/slepton exchange inside one and W exchange inside the other nucleon (so-called vector-scalar exchange) gives stringent limits on R-parity violating interactions. We have extended previous work by including the tensor contribution to the transition rate. We discuss the improved limits on trilinear \Rp -MSSM couplings imposed by the current experimental limit on the $\znbb$ decay half-life of $^{76}$Ge.
On the SUSY Accompanied Neutrino Exchange Mechanism of Neutrinoless Double Beta Decay
1995
The neutrinoless double beta decay ($\znbb$) induced by light Majorana neutrino exchange between decaying nucleons, accompanied by the squark exchange inside one nucleon, recently discussed by Babu and Mohapatra, is carefully analyzed both from the particle and nuclear physics sides. New nuclear matrix elements relevant to this mechanism are calculated. We extend the analysis to include mixing of light neutrinos with heavy and "sterile" neutrinos. It introduces another supersymmetric (SUSY) contribution to $\znbb$. We discuss constraints on the \rp MSSM parameters imposed by the current experimental limit on $\znbb$ decay half-life of $^{76}$Ge.
THE PARTON DISTRIBUTIONS IN NUCLEONS: A QUARK MODEL ANALYSIS
1995
We use a laboratory frame description based on quark model wave functions to study the parton distributions in the nucleon. The present approach incorporates two major improvements, namely, it has the correct support and the renormalization group evolution is carried out to next-to-leading order. We obtain initially the parton distributions arising from the Isgur-Karl wave function. The failure of the latter to reproduce, even approximately, the data motivates us to analyze different scenarios, i.e. additional high momentum components and nonvanishing gluon distributions at the initial scale. We conclude that in order to understand data at various scales complex models simultaneously, it i…
Isoscalar Roper(N1440*)excitation in thepp→ppπ0reaction close to threshold
1999
A new mechanism for the $p\stackrel{\ensuremath{\rightarrow}}{p}\mathrm{pp}{\ensuremath{\pi}}^{0}$ reaction close to threshold is suggested coming from the isoscalar excitation of the Roper and its decay into $N(\ensuremath{\pi}\ensuremath{\pi}{)}_{s\ensuremath{-}\mathrm{w}\mathrm{a}\mathrm{v}\mathrm{e}},$ with one of the ${\ensuremath{\pi}}^{0}$ emitted and the other one reabsorbed on the second nucleon. We show that this new mechanism can lead to important interference with other mechanisms suggested so far.
Scalar isoscalar pion pairs in nuclei and theA(π,ππ)Xreaction
1999
The reaction A(pi,pi pi)X has been studied at low energies, paying particular attention to the interaction of the two final pions in the scalar isoscalar (I=J=0) channel. We have developed a microscopic model for the pion production, and then implemented the two pion final state interaction by using the results of a non-perturbative unitary coupled-channels method based in the standard chiral Lagrangians. The resulting model, describes well the reaction on the nucleon for all different isospin channels. Finally, we have considered the reaction in nuclei. Our calculation takes into account Fermi motion, Pauli blocking, pion absorption, and also the strong modification of the pi-pi interactio…
Centrality dependence of the pseudorapidity density distribution for charged particles in Pb–Pb collisions at sNN=2.76 TeV
2017
We present the charged-particle pseudorapidity density in Pb-Pb collisions at $\sqrt{s_{\mathrm{NN}}}=5.02\,\mathrm{Te\kern-.25exV}$ in centrality classes measured by ALICE. The measurement covers a wide pseudorapidity range from $-3.5$ to $5$, which is sufficient for reliable estimates of the total number of charged particles produced in the collisions. For the most central (0-5%) collisions we find $21\,400\pm 1\,300$ while for the most peripheral (80-90%) we find $230\pm 38$. This corresponds to an increase of $(27\pm4)\%$ over the results at $\sqrt{s_{\mathrm{NN}}}=2.76\,\mathrm{Te\kern-.25exV}$ previously reported by ALICE. The energy dependence of the total number of charged particles…