arXiv subjects

Reinhard Schumacher

Publications and source records attributed to Reinhard Schumacher.

11 recordsLinked to original sources

arXiv2018

Observation of $γp \to Λ\barΛ p$ with GlueX at Jefferson Lab

For the first time, baryon-antibaryon photoproduction in the reaction $γp \to Λ\barΛ p$ has been observed at photon energies from threshold near 4.9 GeV to 11.6 GeV. The measurements are in progress with the GlueX spectrometer in Hall D at Jefferson Lab. We describe here the apparatus and methods used to make these measurements and outline the physics goals of the work. Some of the newly-seen reaction phenomenology is presented.

hep-ex↗

arXiv2013

Measurement of Excited Hyperons in Photoproduction at CLAS

Measurement results of photoproduced excited hyperon states using the CLAS detector at Jefferson Lab are shown. The invariant mass distribution of the Lambda(1405) has recently been shown to be different for each of the three Sigma pi channels that it decays to, showing that there is prominent interference between the isospin I=0 and I=1 isospin amplitudes. Measurements of the differential and total cross sections of the three hyperons Lambda(1405), Sigma0(1385), and Lambda(1520) are presented and compared. Prospects of future studies using a 12 GeV beam with the GlueX detector are briefly given.

nucl-ex↗

arXiv2013

Differential Photoproduction Cross Sections of the Sigma0(1385), Lambda(1405), and Lambda(1520)

We report the exclusive photoproduction cross sections for the Sigma(1385), Lambda(1405), and Lambda(1520) in the reactions gamma + p -> K+ + Y* using the CLAS detector for energies from near the respective production thresholds up to a center-of-mass energy W of 2.85 GeV. The differential cross sections are integrated to give the total exclusive cross sections for each hyperon. Comparisons are made to current theoretical models based on the effective Lagrangian approach and fitted to previous data. The accuracy of these models is seen to vary widely. The cross sections for the Lambda(1405) region are strikingly different for the Sigma+pi-, Sigma0 pi0, and Sigma- pi+ decay channels, indicating the effect of isospin interference, especially at W values close to the threshold.

nucl-ex↗

arXiv2013

Measurement of the Sigma pi photoproduction line shapes near the Lambda(1405)

The reaction gamma + p -> K+ + Sigma + pi was used to determine the invariant mass distributions or "line shapes" of the Sigma+ pi-, Sigma- pi+ and Sigma0 pi0 final states, from threshold at 1328 MeV/c^2 through the mass range of the Lambda(1405) and the Lambda(1520). The measurements were made with the CLAS system at Jefferson Lab using tagged real photons, for center-of-mass energies 1.95 < W < 2.85 GeV. The three mass distributions differ strongly in the vicinity of the I=0 Λ(1405), indicating the presence of substantial I=1 strength in the reaction. Background contributions to the data from the Sigma0(1385) and from K^* Sigma production were studied and shown to have negligible influence. To separate the isospin amplitudes, Breit-Wigner model fits were made that included channel-coupling distortions due to the NKbar threshold. A best fit to all the data was obtained after including a phenomenological I=1, J^P = 1/2^- amplitude with a centroid at 1394\pm20 MeV/c^2 and a second I=1 amplitude at 1413\pm10 MeV/c^2. The centroid of the I=0 Lambda(1405) strength was found at the Sigma pi threshold, with the observed shape determined largely by channel-coupling, leading to an apparent overall peak near 1405 MeV/c^2.

nucl-ex↗

arXiv2011

Properties of the Lambda(1405) Measured at CLAS

The nature of the Lambda(1405), and its place in the baryon spectrum has remained uncertain for decades. Theoretical studies have shown that it may possess strong dynamical components which are not seen in other well-known baryons. Using the CLAS detector system in Hall B at Jefferson Lab, we have measured the photoproduction reaction gamma+p->K+ Lambda(1405) with high statistics and over different Sigma pi decay channels. The reconstructed invariant mass distribution (lineshape) has been measured, as well as the differential cross sections for the Lambda(1405), Sigma(1385), and Lambda(1520). Our analysis method is discussed and our near-final results for the Lambda(1405) lineshape and differential cross section are presented.

nucl-ex↗

arXiv2011

Electroproduction of $Λ$(1405)

The electroproduction of $K^+ Λ$ (1405) was studied by analyzing the E1F data set collected in Hall B at Jefferson Lab. The analysis utilized the decay channel $Σ^+ π^-$ of the $Λ$ (1405) and $p π^0$ of the $Σ^+$. Simulations of background, $Λ$ (1405) and $Λ$ (1520) production according to PDG values were performed by using standard CLAS analysis tools adapted for the E1F run. Fits of the acceptance-corrected simulations were made to the acceptance-corrected data to determine contributions from signal and background processes. The line shape of $Λ$ (1405) varies with the four momentum transfer, $Q^2$, and does not match the line shape based on PDG resonance parameters. It corresponds approximately to predictions of a recent two-pole meson-baryon picture of this state.

nucl-ex↗

arXiv2009

Properties of the Lambda(1405) Measured at CLAS

Using the CLAS detector system in Hall B at Jefferson Lab, we have done a high statistics measurement of the photoproduction of K+ Lambda(1405) using a proton target. The reconstructed invariant mass seen in the various Sigma pi decay modes has been measured, as well as the differential cross section dsigma/dt. The nature of the Lambda(1405) with its peculiar mass distribution is not well understood, and studies have shown that it may posess strong dynamical components, which could be extracted from our data. Various aspects of our analysis are discussed, as well as future prospects to further enhance the precision of the results.

nucl-ex↗

arXiv2009

Hyperon Photo and Electro Production at CLAS

Developments in strangeness photo- and electro- production off the proton, as investigated using the CLAS system in Hall B at Jefferson Lab, are discussed in this paper. By measuring sufficient spin observables one can decompose the reaction mechanism into elementary amplitudes. We discuss progress toward this end in recent data from CLAS, including cross sections and spin observables. We next discuss new results on the mass distribution of the $Λ(1405)$, which shows signs of being a composite meson-baryon object of mixed isospin. The work on other hyperons such as the $Ξ$ resonances will be mentioned, and future prospects outlined.

nucl-ex↗

arXiv2008

Polarization in Hyperon Photo- and Electro- Production

Multiple polarization observables must be measured to access the amplitude structure of pseudoscalar meson photoproduction off the proton. The hyperon-producing reactions are especially attractive to study, since the weak decays allow straightforward measurement of the induced and recoil polarization observables. In this paper we emphasize $γ+ p \to K^+ + Λ$, discussing recent measurements of $C_x$, $C_z$, and $P$ for this reaction. An empirical constraint on the helicity amplitudes is obtained. A simplified model involving spin-flip and spin non-flip amplitudes is presented. Finally, a semi-classical model of how the polarization may arise is presented.

nucl-ex↗

arXiv2006

Polarization of Hyperons in Elementary Photoproduction

Recent measurements using the CLAS detector at Jefferson Lab of the reactions $\vecγ+ p \to K^+ + \vecΛ$ and $\vecγ+ p \to K^+ + \vecΣ^0$ have been used to extract the spin transfer coefficients $C_x$ and $C_z$ for the first time. These observables quantify the degree of the photon circular polarization that is transferred to the recoiling hyperons in the scattering plane. The unexpected result is that $Λ$ hyperons are produced ``100% polarized'' as seen when combining $C_x$ and $C_z$ with the induced transverse polarization, $P$. Furthermore, $C_x$ and $C_z$ seem to be linearly related. This paper discusses the experimental results and offers a hypothesis which can explain these observations. We show how the produced strange quark can be subject to a pure spin-orbit type of interaction which preserves its state of polarization throughout the hadronization process.

nucl-ex↗

arXiv2006

Measurements of C_x and C_z for K^+Lambda and K^+Sigma^o Photoproduction

The CLAS collaboration has recently completed first measurements of the double polarization observables C_x and C_z for the reactions K^+Lambda and K^+Sigma^o photoproduction. C_x and C_z are the beam-recoil polarization asymmetries measuring the polarization transfer from incoming circularly polarized photons to the outgoing hyperons along two directions in the reaction's production plane. The Lambda is found to be nearly maximally polarized along the direction of the incident photon's polarization for forward-going kaons. Polarization transfer to the Sigma^o is different from the Lambda case.

nucl-ex↗