The reaction $\gamma p \to \pi^0 \pi^0 p$ has been measured using the TAPS BaF$_2$ calorimeter at the tagged photon facility of the Mainz Microtron accelerator. Chiral perturbation theory (ChPT) predicts that close to threshold this channel is significantly enhanced compared to double pion final states with charged pions. In contrast to other reaction channels, the lower order tree terms are strongly suppressed in 2$\pi^0$ photoproduction. The consequence is the dominance of pion loops in the 2$\pi^0$ channel close to threshold - a result that opens new prospects for the test of ChPT and in particular its inherent loop terms. The present measurement is the first which is sensitive enough for a conclusive comparison with the ChPT calculation and is in agreement with its prediction. The data also show good agreement with a calculation in the unitary chiral approach.
Total cross section measurement. Statistical errors only.
Differential cross sections of the reactions $(\gamma,\pi^\circ\pi^\circ)$ and $(\gamma,\pi^\circ\pi^++\pi^\circ\pi^-)$ have been measured for several nuclei ($^1$H,$^{12}$C, and $^{\rm nat}$Pb) at an incident-photon energy of $E_{\gamma}$=400-460 MeV at the tagged-photon facility at MAMI-B using the TAPS spectrometer. A significant nuclear-mass dependence of the $\pi\pi$ invariant-mass distribution is found in the $\pi^\circ\pi^\circ$ channel. This dependence is not observed in the $\pi^\circ\pi^{+/-}$ channel and is consistent with an in-medium modification of the $\pi\pi$ interaction in the $I$=$J$=0 channel. The data are compared to $\pi$-induced measurements and to calculations within a chiral-unitary approach.
Differential cross section for PI0PI0 production with a proton target. Errors are statistical only. Note that the data given in this table are sightly different (newer) than the data points presented in the paper.
Differential cross section for PI0PI0 production with a Carbon target. Errors are statistical only.
Differential cross section for PI0PI0 production with a Lead target. Errors are statistical only.
The total cross sections for the three γp → Nππ reactions have been measured for photon energies from 400 to 800 MeV. The γ p → p π 0 π 0 and γ p → n π + π 0 cross sections have never been measured before while the γ p → p π + π − results are much improved compared to earlier data. These measurements were performed with the large acceptance hadronic detector DAPHNE, at the tagged photon beam facility of the MAMI microtron in Mainz.
No description provided.
No description provided.
No description provided.
Differential cross sections for quasi-free Compton scattering from the proton and neutron bound in the deuteron have been measured using the Glasgow/Mainz tagging spectrometer at the Mainz MAMI accelerator together with the Mainz 48 cm $\oslash$ $\times$ 64 cm NaI(Tl) photon detector and the G\"ottingen SENECA recoil detector. The data cover photon energies ranging from 200 MeV to 400 MeV at $\theta^{LAB}_\gamma=136.2^\circ$. Liquid deuterium and hydrogen targets allowed direct comparison of free and quasi-free scattering from the proton. The neutron detection efficiency of the SENECA detector was measured via the reaction $p(\gamma,\pi^+ n)$. The "free" proton Compton scattering cross sections extracted from the bound proton data are in reasonable agreement with those for the free proton which gives confidence in the method to extract the differential cross section for free scattering from quasi-free data. Differential cross sections on the free neutron have been extracted and the difference of the electromagnetic polarizabilities of the neutron have been obtained to be $\alpha-\beta= 9.8\pm 3.6(stat){}^{2.1}_1.1(syst)\pm 2.2(model)$ in units $10^{-4}fm^3$. In combination with the polarizability sum $\alpha +\beta=15.2\pm 0.5$ deduced from photoabsorption data, the neutron electric and magnetic polarizabilities, $\alpha_n=12.5\pm 1.8(stat){}^{+1.1}_{-0.6}\pm 1.1(model)$ and $\beta_n=2.7\mp 1.8(stat){}^{+0.6}_{-1.1}(syst)\mp 1.1(model)$ are obtained. The backward spin polarizability of the neutron was determined to be $\gamma^{(n)}_\pi=(58.6\pm 4.0)\times 10^{-4}fm^4$.
Energy dependence of the free-proton differential cross section.
Energy dependence of the quasi-free proton differential cross section.
Energy dependence of the triple differential cross section w.r.t. the scattered proton.
Using non-tagged bremsstrahlung produced by a 130 MeV–20 μA c.w. electron beam of MAMI A quasi-free Compton scattering by the neutron was investigated via the 2 H( γ , γ 'n) 1 H reaction for lab scattering angles of θ γ = 90° and 135°. The energy spectrum and angular distribution of recoiling neutrons were measured via time of flight and a plastic-scintillator hodoscope, respectively. Double-differential cross sections for quasi-free scattering by the neutron were determined on an absolute scale by normalizing to the Compton cross section of the proton. By comparing the experimental double-differential cross sections with predictions the electric polarizability of the neutron was determined, leading to α n = (10.7 −10.7 +3.3 ) × 10 −4 fm 3 . Thus, the upper limit of α n is further reduced as compared to our previous result, but the lower limit is still consistent with zero.
No description provided.
The reaction gamma p -> p pi0 gamma' has been measured with the Crystal Ball / TAPS detectors using the energy-tagged photon beam at the electron accelerator facility MAMI-B. Energy and angular differential cross sections for the emitted photon gamma' and angular differential cross sections for the pi0 have been determined with high statistics in the energy range of the Delta+(1232) resonance. Cross sections and the ratio of the cross section to the non-radiative process gamma p -> p pi0 are compared to theoretical reaction models, having the anomalous magnetic moment kappa_Delta+ as free parameter. As the shape of the experimental distributions is not reproduced in detail by the model calculations, currently no extraction of kappa_Delta+ is feasible.
Total cross section for the background reaction GAMMA P --> P PI0.
Total cross section for the background reaction GAMMA P --> P PI0 PI0.
Angular distribution of the PI0 in the reaction GAMMA P --> P PI0 at beam energy 400 MeV. Inclusive measurement where only the PI0 decay photons are detected.
This paper describes measurements of the semileptonic weak-neutral-current reactions νpμ→νpμ and ν¯pμ→ν¯pμ which yield the absolute differential cross sections dσ(νpμ)dQ2 and dσ(ν¯pμ)dQ2. The weak-neutral-current parameter, sin2θW, is determined to be sin2θW=0.220±0.016(stat.)(syst.)−0.031+0.023.
No description provided.
No description provided.
No description provided.
The production of the ω meson by virtual photons has been measured as a function of Q2 and W. The ratio of ω production to ρ0 production is independent of Q2 in contrast to indications at lower values of W.
No description provided.
The cross section for production of ρ0 mesons has been measured in the reaction e+p→e+p+π++π−. The cross section is presented as a function of W, the c.m. energy of the virtual-photon-proton system, and -Q2, the square of the virtual-photon mass. The vector-dominance model is not able to describe the dependence of the cross section on the parameters Q2 and W. The slope parameter B describing the scattering of the proton exhibits a significant variation with Q2.
No description provided.
The cross section for the fully constrained reaction ep→eppp¯ has been determined as a function of the electroproduction variables Q2 and s, in the range 0.7<Q2<3 GeV2 and 8<s<16 GeV2. Evidence is presented and cross-section estimates are made for resonances in the final-state p¯p system.
The cross section for virtual photon are derived from E- P cross section bydividing on the virtual-photon flux factor.
The cross section for virtual photon are derived from E- P cross section bydividing on the virtual-photon flux factor.
Total and differential cross sections for νμe→νμe and ν¯μe→ν¯μe are measured. Values for the model-independent neutral-current couplings of the electron are found to be gV=−0.107±0.035(stat)±0.028(syst) and gA=−0.514±0.023(stat)±0.028(syst). The electroweak mixing parameter sin2θW is determined to be 0.195±0.018(stat)±0.013(syst). Limits are set for the charge radius and magnetic moment of the neutrino as (〈r2〉)<0.24×10−32 cm2 and fμ<0.85×10−9 Bohr magnetons, respectively.
No description provided.
No description provided.
No description provided.
Measurements of the semileptonic weak-neutral-current reactions νμp→νμp and ν¯μp→ν¯μp are presented. The experiment was performed using a 170-metric-ton high-resolution target detector in the BNL wide-band neutrino beam. High-statistics samples yield the absolute differential cross sections dσ(νμp)/dQ2 and dσ(ν¯μp)/dQ2. A measurement of the axial-vector form factor GA(Q2) is also presented. The results are in good agreement with the standard model SU(2)×U(1). The weak-neutral-current parameter sin2thetaW is determined to be sin2θW=0.220±0.016(stat)−0.031+0.023(syst).
Errors contain both statistics and systematics, except for additional overall normalisation error given above. Neutrino energy is 0 to 5 GeV with peak at 0.8 Gev.
Accelerating polarized protons to 22 GeV/c at the Brookhaven Alternating Gradient Synchro- tron required both extensive hardware modifications and a difficult commissioning process. We had to overcome 45 strong depolarizing resonances to maintain polarization up to 22 GeV/c in this strong-focusing synchrotron. At 18.5 GeV/c we measured the analyzing power A and the spin-spin correlation parameter Ann in large- P⊥2 proton-proton elastic scattering, using the polarized proton beam and a polarized proton target. We also obtained a high-precision measurement of A at P⊥2=0.3 (GeV/c)2 at 13.3 GeV/c. At 18.5 GeV/c we found that Ann=(-2±16)% at P⊥2=4.7 (GeV/c)2, where it was about 60% near 12 GeV at the Argonne Zero Gradient Synchrotron. This sharp change suggests that spin-spin forces may have a strong and unexpected energy dependence at high P⊥2.
No description provided.
2.2 GeV point taken from Brown et al., PR D31(85) 3017.
No description provided.
The weak coupling constants of the electron, gVe and gAe, are determined from measurements of the total and differential cross sections for the reaction νμe→νμe. The data also place limits of interest on the magnitudes of a neutrino charge radius and a possible neutrino magnetic dipole moment.
Neutrino beam 0 - 5 GeV.
This paper reports the results of an experiment measuring the parameters of various electroproduction reactions for a range in the electroproduction variables 0.7<Q2<4 GeV2 and 2<W2<16 GeV2. This report is limited to nondiffractive exclusive channels, with detailed results regarding the πΔ final states, statistically limited results for KΛ final states, and upper limits on the production of a number of event topologies containing a single unseen neutral particle.
No description provided.
No description provided.
No description provided.
We have measured exclusive ρ0, ω, and φ meson electroproduction at the Cornell Wilson Synchrotron. The final ρ0 data sample included 4637 four-constraint e+p→e+π++π−+p events, with incident energy E=11.5 GeV and electroproduction variables Q2 and W in the region 0.7<Q2<4 GeV2 and 1.9<W<4 GeV. We find that the width of the forward ρ0 diffraction peak increases rapidly as the lifetime of the intermediate hadron states decreases below cΔτ=1 fm and that the peak is wider for longitudinal ρ0 than it is for transverse ρ0. The longitudinal-transverse cross-section ratio Rp=σLσT, obtained assuming s-channel helicity conservation, becomes constant at high Q2. At fixed W the diffractive vector-meson-dominance (VMD) model reproduces the Q2 dependence of our cross section, σ=(σT+εσL), but is is not able to account for the rapid decrease in the cross section with increasing W we observe. We find that σωσρ depends on W but is independent of Q2 for 0.7<Q2<3 GeV2 and 2.2<W<3.7 GeV. However, σω is substantially larger than the diffractive VMD cross section. Our results for σφ are consistent with the Q2 dependence of the diffractive VMD model for 0.8<Q2<4 GeV2 and 2<W<3.7 GeV, but this model again fails to predict the W dependence we observe.
FOUR CHANNEL FIT TO TWO PION PRODUCTION ASSUMING NO INTERFERENCE.
DEPENDENCE OF TOTAL, LONGITUDINAL (L) AND TRANSVERSE (U) DIFFERENTIAL CROSS SECTIONS ON C*DELTA(TAU), THE FORMATION TIME FOR VIRTUAL INTERMEDIATE HADRON STATES. DELTA(TAU) IS 1/DELTA(E) WHERE DELTA(E) IS E(RF=LAB,P=3) - NU = SQRT(NU**2 + Q2 + M(RHO)**2) - NU.
No description provided.
We have measured ρ0, ω (combined) and ϕ electroproduction over a range of virtual-photon four-momentum Q2 from 0.4 to 2.2 GeV2 and for photon energies ν from 2.7 to 8.6 GeV. We find that the slope of the t (momentum transfer) dependence of the ρ0 and ω forward peak decreases with increasing Q2 to less than half of the photoproduction slope.
The cross section for virtual photon are derived from E- P cross section bydividing on the virtual-photon flux factor.
We have measured ep, eπ+, and eπ− coincidences for scattered electrons in the range Q2=0.4 to 2.2 GeV2 and W=2 to 4 GeV. We find (a) that vector-meson production decreases with Q2 more rapidly than does the total virtual-photon-plus-proton cross section, more rapidly even than the prediction of simple vector dominance, (b) that the slope of the t distribution in ρ and ω production becomes flatter with increasing Q2 and seems to be at least approximately a function of the single variable xρ=(Q2+mρ2)2Mν, (c) that the fraction of final states containing a proton decreases with increasing Q2, (d) that in the central region of longitudinal momenta the inclusive π+ yield seems to increase relative to the π− yield as Q2 increases, and (e) that the average transverse momentum of π− is greater than of π+ in the central region of longitudinal momenta.
No description provided.
The data with (C=Q=RHO+OMEGA) are obtained by excluding the contribution from RHO and OMEGA production.
No description provided.
The total photoabsorption cross section for 4He has been measured for the first time over a wide photon energy range (200 MeV<~Eγ<~800 MeV). By using the large acceptance detector DAPHNE at the tagged photon facility of the MAMI microtron in Mainz high precision results with small systematic errors were obtained. This measurement shows that 4He has a behavior similar to heavy nuclei and, in particular, a reduction of the cross section with respect to the lighter nuclei is found for Eγ>600 MeV.
No description provided.
Differential and total cross sections for the photoproduction of neutral pions from the proton have been measured for incident photon energies from 140–270 MeV, using the photon spectrometer TAPS at the tagged photon beam of the 855 MeV Mainz Microtron. The energy dependence of the s- and p-wave multipoles close to threshold was deduced from a multipole fit and a multipole analysis. The extracted s-wave amplitude E 0+ at threshold is found to be significantly smaller than the prediction of the classical low energy theorems, but is in reasonable agreement with the chiral perturbation theory.
No description provided.
Total and differential cross sections for photoproduction of η mesons from 12 C, 40 Ca, 93 Nb, and nat Pb have been obtained up to 790 MeV incident photon energy at the Mainz Microtron (MAMI) with the TAPS spectrometer. The absorption cross section σ ηN abs = (30 ± 2.5 ± 6)mb of η mesons in nuclear matter and the absorption length λ η = (2.0 ± 0.2 ± 0.4) fm are extracted. No significant depletion of the S 11 (1535) strength in the η photoproduction on nuclei is observed.
THE TOTAL SIG WAS PARAMETRIZED BY A**POWER.
The first measurement of incoherent η-photoproduction from the deuteron in the threshold region is reported. The experiment was carried out at the MAMI accelerator with the TAPS spectrometer. Total and differential inclusive cross sections have been obtained between 627 and 790 MeV. It is found that the reaction is completely dominated by the incoherent part. An upper limit for coherent η-photoproduction on the deuteron is deduced, which is substantially lower than the result from an earlier measurement. The incoherent cross section is reproduced in a participant-spectator approach under the assumption of an energy-independent ratio between the neutron and proton cross sections. Best agreement is found for the ratio σ n σ p ≈ 2 3 . The implications for the isospin components of the electromagnetic excitation of the S 11 (1535) resonance are discussed.
The helicity amplitudes A(1/2) = <S11|j(em)|nucleon> are measured.
None
SIG(Q=N) and SIG(Q=P) ratio is extracted from the data of scattering on HE4.
The photoproduction of eta-mesons from 2H and 4He has been studied for energies close to the production thresholds. The experiments were carried out with the tagged photon beam of the Mainz MAMI accelerator. The eta-mesons were detected via their two photon decays with the electromagnetic calorimeter TAPS. Total cross sections, angular and momentum distributions of the eta-mesons have been determined for both reactions. The total cross sections in the threshold region show a large enhancement over the predictions of a participant - spectator model, indicating significant final state interaction effects. The results are compared to recent model calculations taking into account nucleon-nucleon and nucleon-eta final state interaction effects on different levels of sophistication.
Total inclusive photoproduction cross sections. Statistical errors only.
Angular distributions of ETA mesons from the Deuterium target in the photon-nucleus cm system. Statistical errors only.
Angular distributions of ETA mesons from the Deuterium target in the photon-nucleus cm system. Statistical errors only.
The photon asymmetry in the reaction p(\vec{\gamma},\pi^{0})p close to threshold has been measured for the first time with the photon spectrometer TAPS using linearly polarized photons from the tagged-photon facility at the Mainz Microtron MAMI. The total and differential cross sections were also measured simultaneously with the photon asymmetry. This allowed determination of the S-wave and all three P-wave amplitudes. The low-energy theorems based on the parameter-free third-order calculations of heavy-baryon chiral perturbation theory for P1 and P2 agree with the experimental values.
Polarized photon beam.
Forum