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Register a new userObservation of an Exotic S = -2, Q = -2 Baryon Resonance in Proton-Proton Collisions at the CERN SPS
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NA49 Collaboration
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Results of resonance searches in the Xi- pi-, Xi- pi+, antiXi+ pi- and antiXi+ pi+ invariant mass spectra in proton-proton collisions at sqrt{s}=17.2 GeV are presented. Evidence is shown for the existence of a narrow Xi- pi- baryon resonance with mass of 1.862+/-0.002 GeV/c^2 and width below the detector resolution of about 0.018 GeV/c^2. The significance is estimated to be 4.0 sigma. This state is a candidate for the hypothetical exotic Xi_(3/2)^-- baryon with S = -2, I = 3/2 and a quark content of (d s d s ubar). At the same mass a peak is observed in the Xi- pi+ spectrum which is a candidate for the Xi_(3/2)^0 member of this isospin quartet with a quark content of (d s u s dbar). The corresponding antibaryon spectra also show enhancements at the same invariant mass.
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Pentaquark states have been extensively investigated theoretically in the context of the constituent quark model. In this paper results of an experimental search for pentaquarks in the Ximpim, Ximpip, Xippim and Xippip invariant mass spectra in proton-proton interactions at $sqrt{s}$=17.3~GeV are presented. Previous possible evidence from the NA49 collaboration of the existence of a narrow Ximpim baryon resonance in p+p interactions is not confirmed with almost 10 times greater event statistics. The search was performed using the NASixtyOne detector which reuses the main components of the NA49 apparatus. No signal was observed with either the selection cuts of NA49 or newly optimised cuts.
The $e^{+}p$ and $e^{-}p$ scattering data recorded at HERA during the recent years offer the possibility to study electroweak effects in $ep$ interactions apparent at high momentum transfers, $Q^{2}$, and to reveal information on the proton parton densities at large values of the Bjorken scaling variable x. From the neutral current cross section measurements, H1 and ZEUS extract the generalized structure function $x{cal F}_{3}$, which can be related to the valence quark content of the proton. Individual quark densities are extracted by a global fit to the H1 neutral and charged current $e^{+}p$ and $e^{-}p$ data. The new results show the sensitivity of high $Q^{2}$ $ep$ data to the structure of the proton and indicate what to expect from a 1 fb$^{-1}$ data sample to be taken by H1 and ZEUS until 2006 at the upgraded HERA collider. Future perspectives concerning the investigation of electroweak effects and their utilization to extract the parton content of the proton are shortly discussed.
We report on baryon resonance production and decay in proton-proton collisions at a kinetic energy of $3.5$ GeV based on data measured with HADES. The exclusive channels $pp rightarrow nppi^{+}$ and $pp rightarrow pppi^{0}$ as well as $pp rightarrow ppe^{+}e^{-}$ are studied simultaneously for the first time. The invariant masses and angular distributions of the pion-nucleon systems were studied and compared to simulations based on a resonance model ansatz assuming saturation of the pion production by an incoherent sum of baryonic resonances (R) with masses $<2~$ GeV/$c^2$. A very good description of the one-pion production is achieved allowing for an estimate of individual baryon-resonance production-cross-sections which are used as input to calculate the dielectron yields from $Rrightarrow pe^+e^-$ decays. Two models of the resonance decays into dielectrons are examined assuming a point-like $RN gamma^*$ coupling and the dominance of the $rho$ meson. The results of model calculations are compared to data from the exclusive $ppe^{+}e^{-}$ channel by means of the dielectron and $pe^+e^-$ invariant mass distributions.
The cross section for inclusive multipion production in the pp->ppX reaction was measured at COSY-ANKE at four beam energies, 0.8, 1.1, 1.4, and 2.0 GeV, for low excitation energy in the final pp system, such that the diproton quasi-particle is in the 1S0 state. At the three higher energies the missing mass Mx spectra show a strong enhancement at low Mx, corresponding to an ABC effect that moves steadily to larger values as the energy is increased. Despite the missing-mass structure looking very different at 0.8 GeV, the variation with Mx and beam energy are consistent with two-pion production being mediated through the excitation of two Delta(1232) isobars, coupled to S-- and D-- states of the initial pp system.
Systematic differences in the the protons charge radius, as determined by ordinary atoms and muonic atoms, have caused a resurgence of interest in elastic lepton scattering measurements. The protons charge radius, defined as the slope of the charge form factor at Q$^2$=0, does not depend on the probe. Any difference in the apparent size of the proton, when determined from ordinary versus muonic hydrogen, could point to new physics or need for the higher order corrections. While recent measurements seem to now be in agreement, there is to date no high precision elastic scattering data with both electrons and positrons. A high precision proton radius measurement could be performed in Hall B at Jefferson Lab with a positron beam and the calorimeter based setup of the PRad experiment. This measurement could also be extended to deuterons where a similar discrepancy has been observed between the muonic and electronic determination of deuteron charge radius. A new, high precision measurement with positrons, when viewed alongside electron scattering measurements and the forthcoming MUSE muon scattering measurement, could help provide new insights into the origins of the proton radius puzzle, and also provide new experimental constraints on radiative correction calculations.
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