Search results for "Proton"
showing 10 items of 5886 documents
The RMS charge radius of the proton and Zemach moments
2010
On the basis of recent precise measurements of the electric form factor of the proton, the Zemach moments, needed as input parameters for the determination of the proton rms radius from the measurement of the Lamb shift in muonic hydrogen, are calculated. It turns out that the new moments give an uncertainty as large as the presently stated error of the recent Lamb shift measurement of Pohl et al.. De Rujula's idea of a large Zemach moment in order to reconcile the five standard deviation discrepancy between the muonic Lamb shift determination and the result of electronic experiments is shown to be in clear contradiction with experiment. Alternative explanations are touched upon.
Mesonic and binding contributions to the nuclear Drell-Yan process
1997
We have evaluated the Drell-Yan cross section in nuclei paying special attention to the meson cloud contribution from pion and rho-meson, for which an accurate calculation using the meson nuclear spectral functions is used. Similarly, the nucleonic contribution is evaluated in terms of a relativistic nucleon spectral function. Fair agreement with experiment is found for different nuclei and the results show a sizeable contribution from the renormalized meson cloud. In order to reproduce the experiment a novel element is introduced, consisting of a gradual energy loss of the incoming proton in its pass through the nucleus which produces a strong A dependence at x_1 large.
Generalized polarizabilities of the proton in a constituent quark model revisited
2001
We study low-energy virtual Compton scattering off the proton within the framework of a nonrelativistic constituent quark model. The Compton tensor is divided into two separately gauge-invariant contributions. The first consists of the groundstate propagation in the direct and crossed channels together with an appropriately chosen term to satisfy gauge invariance. The residual part contains the relevant structure information characterized by the so-called generalized polarizabilities. We discuss two different schemes to obtain the generalized polarizabilities from the residual term. Explicit predictions for the generalized polarizabilities are presented for the Isgur-Karl model. Our results…
ϕ meson width in the medium from proton induced ϕ production in nuclei
2004
We perform calculations for the production of $\phi$ mesons in nuclei at energies just above threshold and study the $A$ dependence of the cross section. We use results for the $\phi$ selfenergy in the medium obtained within a chiral unitary approach. We find a strong $A$ dependence which is tied to the distortion of the incident proton and to the absorption of the $\phi$ in its way out of the nucleus. The effect of this latter process reduces the cross section in about a factor two in heavy nuclei proving that the $A$ dependence of the cross section bears valuable information on the $\phi$ width in the nuclear medium. Calculations are done for energies which are accessible in an experiment…
Energy and system size dependence of subnucleonic fluctuations
2018
The energy evolution of the fluctuating proton structure is studied by solving the JIMWLK renormalization group equation. The initial condition at moderate $x$ is obtained by fitting the charm reduced cross section data from HERA, requiring that the proton size remains compatible with the diffractive vector meson production measurements. Additionally, we show that the nucleon shape fluctuations are visible in exclusive vector meson production off nuclei.
No evidence for medium effects in the12C(e, e'p)11Bg.s. reaction
1995
The12C(e, e'p)11B g.s reaction has been measured in parallel and anti-parallel kinematics over ap m range of −120<p m<120 MeV/c at a centralT p of 90 MeV. In contrast to previous measurements at NIKHEF, both kinematics can be described without enhancement of the transverse form factor of the bound proton (η=0.98(5)). The spectroscopic factor for this transition as determined with the new Mainz data (S α =1.54(5)) is 22% smaller than the NIKHEF value.
The minimal adjoint-SU (5) x Z(4) GUT model
2013
An extension of the adjoint SU (5) model with a flavour symmetry based on the Z(4) group is investigated. The Z(4) symmetry is introduced with the aim of leading the up-and down-quark mass matrices to the Nearest-Neighbour-Interaction form. As a consequence of the discrete symmetry embedded in the SU (5) gauge group, the charged lepton mass matrix also gets the same form. Within this model, light neutrinos get their masses through type-I, type-III and one-loop radiative seesaw mechanisms, implemented, respectively, via a singlet, a triplet and an octet from the adjoint fermionic 24 fields. It is demonstrated that the neutrino phenomenology forces the introduction of at least three 24 fermio…
Spontaneous proton decay and the origin of Peccei-Quinn symmetry
2019
We propose a new interpretation of Peccei-Quinn symmetry within the Standard Model, identifying it with the axial $B + L$ symmetry i.e. $U(1)_{PQ} \equiv U(1)_{\gamma_5(B+L)}$. This new interpretation retains all the attractive features of Peccei-Quinn solution to strong CP problem but in addition also leads to several other new and interesting consequences. Owing to the identification $U(1)_{PQ} \equiv U(1)_{\gamma_5(B+L)}$ the axion also behaves like Majoron inducing small seesaw masses for neutrinos after spontaneous symmetry breaking. Another novel feature of this identification is the phenomenon of spontaneous (and also chiral) proton decay with its decay rate associated with the axion…
Evidence of the same multiparticle production mechanism in p−p collisions as in e+e− annihilation
1980
The split-field magnet spectrometer at the CERN intersecting storage rings was used to measure, in p-p collisions at square root s=62 GeV, the inclusive momentum distribution of the charged particles produced in the same hemisphere as the leading proton (x>0.4). A new scaling variable was introduced in order to take into account baryon-number conservation effects in p-p interactions. It is shown that distributions in this variable are in good agreement with the momentum distribution of the hadrons produced in e/sup +/e/sup -/ annihilation. The results suggest that the multiparticle production mechanism in p-p collisions is the same as in e/sup +/e/sup -/ provided that the effects of baryon-…
Proton-antiproton annihilation into ηηπ-observation of a scalar resonance decaying into ηη
1992
Abstract The results of a measurement of pp annihilation at rest into ηηπ 0 are presented. Assuming a resonance dominated model for the annihilation process an isoscalar J PC =0 ++ resonance decaying into ηη is observed with a mass of 1560±25 MeV/ c 2 and a width of 245±50 MeV/ c 2 .