Making sense of multivariate community responses in global change experiments

Ecological communities are being impacted by global change worldwide. Experiments are a powerful tool to understand how global change will impact communities by comparing control and treatment replicates. Communities consist of multiple species, and their associated abundances make multivariate methods an effective approach to study community compositional differences between control and treated replicates. Dissimilarity metrics are a commonly employed multivariate measure of compositional differences; however, while highly informative, dissimilarity metrics do not elucidate the specific ways in which communities differ. Integrating two multivariate methods, dissimilarity metrics and rank abundance curves (RACs), have the potential to detect complex differences based on dissimilarity metrics and detail the how these differences came about through differences in richness, evenness, species ranks, or species identity. Here we use a database of 106 global change experiments located in herbaceous ecosystems and explore how patterns of ordinations based on dissimilarity metrics relate to RAC-based differences. We find that combining dissimilarity metrics alongside RAC-based measures clarifies how global change treatments are altering communities. We find that when there is no difference in community composition (no distance between centroids of control and treated replicates), there are rarely differences in species ranks or species identities and more often differences in richness or evenness alone. In contrast, when there are differences between centroids of control and treated replicates, this is most often associated with differences in ranks either alone or co-occurring with differences in richness, evenness, or species identities. We suggest that integrating these two multivariate measures of community composition results in a deeper understanding of how global change impacts communities

Eta meson rescattering effects in the p + 6Li --> eta + 7Be reaction near threshold

The p + 6Li --> eta + 7Be reaction has been investigated with an emphasis on the eta meson and 7Be interaction in the final state. Considering the 6Li and 7Be nuclei to be alpha-d and alpha-3He clusters respectively, the reaction is modelled to proceed via the p + d [alpha] --> 3He [\alpha] + eta reaction with the alpha remaining a spectator. The eta meson interacts with 7Be via multiple scatterings on the 3He and alpha clusters inside 7Be. The individual eta-3He and eta-alpha scatterings are evaluated using few body equations for the eta-3N and eta-4N systems with a coupled channel eta-N interaction as an input. Calculations including four low-lying states of 7Be lead to a double hump structure in the total cross section corresponding to the $L = 1 (J = (1/2)^-, (3/2)^-)$ and $L = 3 (J = (5/2)^-, (7/2)^-)$ angular momentum states. The humps arise due to the off-shell rescattering of the eta meson on the 7Be nucleus in the final state.Comment: New results and references adde

Second-Order Eikonal Corrections for A(e,e'p)

The first-order eikonal approximation is frequently adopted in interpreting the results of $A(e,e'p)$ measurements. Glauber calculations, for example, typically adopt the first-order eikonal approximation. We present an extension of the relativistic eikonal approach to $A(e,e'p)$ which accounts for second-order eikonal corrections. The numerical calculations are performed within the relativistic optical model eikonal approximation. The nuclear transparency results indicate that the effect of the second-order eikonal corrections is rather modest, even at $Q^{2} \approx 0.2$ (GeV/c)$^2$. The same applies to polarization observables, left-right asymmetries, and differential cross sections at low missing momenta. At high missing momenta, however, the second-order eikonal corrections are significant and bring the calculations in closer agreement with the data and/or the exact results from models adopting partial-wave expansions.Comment: 16 pages, 4 figures, 1 table, submitted to Phys. Lett.

Electroproduction of the Λ(1520) Hyperon

The reaction e→pe′K+Λ(1520) with Λ(1520)→p′K− was studied at electron beam energies of 4.05, 4.25, and 4.46 GeV, using the CLAS detector at the Thomas Jefferson National Accelerator Facility. The cos θK+, φK+, Q2, and W dependencies of Λ(1520) electroproduction are presented for the kinematic region 0.9 \u3c Q2 \u3c 2.4 GeV2 and 1.95 \u3c W \u3c 2.65 GeV. Also, the Q2 dependence of the Λ(1520) decay angular distribution is presented for the first time. The cosθK+ angular distributions suggest t-channel diagrams dominate the production process. Fits to the Λ(1520) t-channel helicity frame decay angular distributions indicate the mz = ± ½ parentage accounts for about 60% of the total yield, which suggests this reaction has a significant contribution from t-channel processes with either K+ exchange or longitudinal coupling to an exchanged K*. The Q2 dependence of the Λ(1520) production cross section is the same as that observed for Λ(1116) photo- and electroproduction

First Measurement of the Double Spin Asymmetry in \u3csup\u3e→\u3c/sup\u3ee\u3csup\u3e→\u3c/sup\u3ep → e ‘π⁺ in the Resonance Region

The double spin asymmetry in the →e→p → e\u27 π+n reaction has been measured for the first time in the resonance region for four-momentum transfer Q2 = 0.35-1.5 GeV2. Data were taken at Jefferson Lab with the CLAS detector using a 2.6 GeV polarized electron beam incident on a polarized solid NH3 target. Comparison with predictions of phenomenological models shows strong sensitivity to resonance contributions. Helicity-1/2 transitions are found to be dominant in the second and third resonance regions. The measured asymmetry is consistent with a faster rise with Q2 of the helicity asymmetry A1 for the F15(1680) resonance than expected from the analysis of the unpolarized data

Exclusive Electroproduction of ᵠ Mesons at 4.2 GeV

We studied the exclusive reaction ep → e\u27 p\u27 ᵠ using the ᵠ →K+K- decay mode. The data were collected using a 4.2 GeV incident electron beam and the CEBAF Large Acceptance Spectrometer (CLAS) at the Thomas Jefferson National Accelerator Facility. Our experiment covers the range in Q2 from 0.7 to 2.2 GeV2and W from 2.0 to 2.6 GeV. Taken together with all previous data, we find a consistent picture of ᵠ production on the proton. Our measurement shows the expected decrease of the t slope with the vector-meson formation time c Δ t below 2 fm. At c Δ t = 0.6 fm, we measure bɸ = 2.27 ± 0.42 GeV-2. The cross section dependence on W as W0.2 ±0.1 at Q2, 1.3 GeV2 was determined by comparison with ɸ production at HERA after correcting for threshold effects. This is the same dependence as observed in photoproduction

Corrected Article: Exclusive Electroproduction of ᵠ Mesons at 4.2 GeV [Physical. Rev. C 63, 065205, (2001)]

We studied the exclusive reaction ep → e\u27 p\u27 ᵠ using the ᵠ →K+K- decay mode. The data were collected using a 4.2 GeV incident electron beam and the CEBAF Large Acceptance Spectrometer (CLAS) at the Thomas Jefferson National Accelerator Facility. Our experiment covers the range in Q2 from 0.7 to 2.2 GeV2and W from 2.0 to 2.6 GeV. Taken together with all previous data, we find a consistent picture of ᵠ production on the proton. Our measurement shows the expected decrease of the t slope with the vector-meson formation time c Δ t below 2 fm. At c Δ t = 0.6 fm, we measure bɸ = 2.27 ± 0.42 GeV-2. The cross section dependence on W as W0.2 ±0.1 at Q2, 1.3 GeV2 was determined by comparison with ɸ production at HERA after correcting for threshold effects. This is the same dependence as observed in photoproduction

ep → ep π⁰ Reaction Studied in the Δ(1232) Mass Region Using Polarization Asymmetries

Measurements of the angular distributions of target and double-spin asymmetries for the Δ+(1232) in the exclusive channel → p(→e,e\u27p)π0 obtained at the Jefferson Lab in the Q2 range from 0.5 to 1.5 GeV2/c2 are presented. Results of the asymmetries are compared with the unitary isobar model [D. Drechsel , Nucl. Phys. A645, 145 (1999)], dynamical models [T. Sato and T. S. Lee, Phys. Rev. C 54, 2660 (1996); S. S. Kamalov , Phys. Lett. B 27, 522 (2001)], and the effective Lagrangian theory [R. M. Davidson , Phys. Rev. D 43, 71 (1991)]. Sensitivity to the different models was observed, particularly in relation to the description of background terms on which the target asymmetry depends significantly

Measurement of Inclusive Spin Structure Functions of the Deuteron

We report the results of a new measurement of spin structure functions of the deuteron in the region of moderate momentum transfer [Q2 = 0.27-1.3 (GeV/c)2] and final hadronic state mass in the nucleon resonance region (W=1.08-2.0 GeV). We scattered a 2.5 GeV polarized continuous electron beam at Jefferson Lab off a dynamically polarized cryogenic solid state target (15ND3) and detected the scattered electrons with the CEBAF large acceptance spectrometer. From our data, we extract the longitudinal double spin asymmetry A∥ and the spin structure function g1d. Our data are generally in reasonable agreement with existing data from SLAC where they overlap, and they represent a substantial improvement in statistical precision. We compare our results with expectations for resonance asymmetries and extrapolated deep inelastic scaling results. Finally, we evaluate the first moment of the structure function g1d and study its approach to both the deep inelastic limit at large Q2 and to the Gerasimov-Drell-Hearn sum rule at the real photon limit (Q2→ 0). We find that the first moment varies rapidly in the Q2 range of our experiment and crosses zero at Q2 between 0.5 and 0.8 (GeV/c)2, indicating the importance of the Delta resonance at these momentum transfers

Measurement of the Deuteron Structure Function F₂ in the Resonance Region and Evaluation of its Moments

Inclusive electron scattering off the deuteron has been measured to extract the deuteron structure function F2 with the CEBAF Large Acceptance Spectrometer (CLAS) at the Thomas Jefferson National Accelerator Facility. The measurement covers the entire resonance region from the quasielastic peak up to the invariant mass of the final-state hadronic system W similar or equal to 2.7 GeV with four-momentum transfers Q2 from 0.4 to 6 (GeV/c)2. These data are complementary to previous measurements of the proton structure function F2 and cover a similar two-dimensional region of Q2 and Bjorken variable x. Determination of the deuteron F2 over a large x interval including the quasielastic peak as a function of Q2, together with the other world data, permit a direct evaluation of the structure function moments for the first time. By fitting the Q2 evolution of these moments with an OPE-based twist expansion we have obtained a separation of the leading twist and higher twist terms. The observed Q2 behavior of the higher twist contribution suggests a partial cancelation of different higher twists entering into the expansion with opposite signs. This cancelation, found also in the proton moments, is a manifestation of the duality phenomenon in the F2 structure function