arXiv subjects

P. Aguar-Bartolomé

Publications and source records attributed to P. Aguar-Bartolomé.

15 recordsLinked to original sources

arXiv2020

Measurement of the beam-helicity asymmetry in photoproduction of $π^{0}η$ pairs on carbon, aluminum, and lead

The beam-helicity asymmetry was measured, for the first time, in photoproduction of $π^{0}η$ pairs on carbon, aluminum, and lead, with the A2 experimental setup at MAMI. The results are compared to an earlier measurement on a free proton and to the corresponding theoretical calculations. The Mainz model is used to predict the beam-helicity asymmetry for the nuclear targets. The present results indicate that the photoproduction mechanism for $π^{0}η$ pairs on nuclei is similar to photoproduction on a free nucleon. This process is dominated by the $D_{33}$ partial wave with the $ηΔ(1232)$ intermediate state.

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arXiv2018

Experimental study of the $γp\rightarrow K^0Σ^+$, $γn\rightarrow K^0Λ$, and $γn\rightarrow K^0 Σ^0$ reactions at the Mainz Microtron

This work measured $dσ/dΩ$ for neutral kaon photoproduction reactions from threshold up to a c.m.\ energy of 1855MeV, focussing specifically on the $γp\rightarrow K^0Σ^+$, $γn\rightarrow K^0Λ$, and $γn\rightarrow K^0 Σ^0$ reactions. Our results for $γn\rightarrow K^0 Σ^0$ are the first-ever measurements for that reaction. These data will provide insight into the properties of $N^*$ resonances and, in particular, will lead to an improved knowledge about those states that couple only weakly to the $πN$ channel. Integrated cross sections were extracted by fitting the differential cross sections for each reaction as a series of Legendre polynomials and our results are compared with prior experimental results and theoretical predictions.

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arXiv2018

High-statistics measurement of the eta->3pi^0 decay at the Mainz Microtron

The largest, at the moment, statistics of 7x10^6 eta->3pi^0 decays, based on 6.2x10^7 eta mesons produced in the gamma p -> eta p reaction, has been accumulated by the A2 Collaboration at the Mainz Microtron, MAMI. It allowed a detailed study of the eta->3pi^0 dynamics beyond its conventional parametrization with just the quadratic slope parameter alpha and enabled, for the first time, a measurement of the second-order term and a better understanding of the cusp structure in the neutral decay. The present data are also compared to recent theoretical calculations that predict a nonlinear dependence along the quadratic distance from the Dalitz-plot center.

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arXiv2018

Study of the $γp\to π^0ηp$ reaction with the A2 setup at MAMI

The data available from the A2 Collaboration at MAMI were analyzed to select the $γp\to π^0ηp$ reaction on an event-by-event basis, which allows for partial-wave analyses of three-body final states to obtain more reliable results, compared to fits to measured distributions. These data provide the world's best statistical accuracy in the energy range from threshold to $E_γ=1.45$ GeV, allowing a finer energy binning in the measurement of all observables needed for understanding the reaction dynamics. The results obtained for the measured observables are compared to existing models, and the impact from the new data is checked by the fit with the revised Mainz model.

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arXiv2017

Measurement of the omega -> pi^0 e^+e^- and eta -> e^+e^-g Dalitz decays with the A2 setup at MAMI

The Dalitz decays eta -> e^+e^-g and omega -> pi^0 e^+e^- have been measured in the g p -> eta p and g p -> omega p reactions, respectively, with the A2 tagged-photon facility at the Mainz Microtron, MAMI. The value obtained for the slope parameter of the electromagnetic transition form factor of eta, Lambda^{-2}_eta=(1.97+/-0.11_tot) GeV^{-2}, is in good agreement with previous measurements of the eta -> e^+e^-g and eta -> mu^+mu^-g decays. The uncertainty obtained in the value of Lambda^{-2}_eta is lower than in previous results based on the eta -> e^+e^-g decay. The value obtained for the omega slope parameter, Lambda^{-2}_omega_pi^0 = (1.99+/-0.21_tot) GeV^{-2}, is somewhat lower than previous measurements based on omega -> pi^0 mu^+mu^-, but the results for the omega transition form factor are in better agreement with theoretical calculations, compared to earlier experiments.

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arXiv2017

Measurement of the pi^0 -> e^+e^-gamma Dalitz decay at the Mainz Microtron

The Dalitz decay pi^0 -> e^+e^-gamma has been measured in the gamma p -> pi^0 p reaction with the A2 tagged-photon facility at the Mainz Microtron, MAMI. The value obtained for the slope parameter of the pi^0 electromagnetic transition form factor, a_pi = 0.030+/-0.010_tot, is in agreement with existing measurements of this decay and with recent theoretical calculations. The uncertainty obtained in the value of a_pi is lower than in previous results based on the pi^0 -> e^+e^-gamma decay.

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arXiv2014

Quasifree photoproduction of $η$ mesons off protons and neutrons

Differential and total cross sections for the quasifree reactions $γp\rightarrowηp$ and $γn\rightarrowηn$ have been determined at the MAMI-C electron accelerator using a liquid deuterium target. Photons were produced via bremsstrahlung from the 1.5 GeV incident electron beam and energy-tagged with the Glasgow photon tagger. Decay photons of the neutral decay modes $η\rightarrow 2γ$ and $η\rightarrow 3π^0 \rightarrow 6γ$ and coincident recoil nucleons were detected in a combined setup of the Crystal Ball and the TAPS calorimeters. The $η$-production cross sections were measured in coincidence with recoil protons, recoil neutrons, and in an inclusive mode without a condition on recoil nucleons, which allowed a check of the internal consistency of the data. The effects from nuclear Fermi motion were removed by a kinematic reconstruction of the final-state invariant mass and possible nuclear effects on the quasifree cross section were investigated by a comparison of free and quasifree proton data. The results, which represent a significant improvement in statistical quality compared to previous measurements, agree with the known neutron-to-proton cross-section ratio in the peak of the $S_{11}(1535)$ resonance and confirm a peak in the neutron cross section, which is absent for the proton, at a center-of-mass energy $W = (1670\pm 5)$ MeV with an intrinsic width of $Γ\approx 30$ MeV.

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arXiv2014

A new measurement of the rare decay eta -> pi^0 gamma gamma with the Crystal Ball/TAPS detectors at the Mainz Microtron

A new measurement of the rare, doubly radiative decay eta->pi^0 gamma gamma was conducted with the Crystal Ball and TAPS multiphoton spectrometers together with the photon tagging facility at the Mainz Microtron MAMI. New data on the dependence of the partial decay width, Gamma(eta->pi^0 gamma gamma), on the two-photon invariant mass squared, m^2(gamma gamma), as well as a new, more precise value for the decay width, Gamma(eta->pi^0 gamma gamma) = (0.33+/-0.03_tot) eV, are based on analysis of 1.2 x 10^3 eta->pi^0 gamma gamma decays from a total of 6 x 10^7 eta mesons produced in the gamma p -> eta p reaction. The present results for dGamma(eta->pi^0 gamma gamma)/dm^2(gamma gamma) are in good agreement with previous measurements and recent theoretical calculations for this dependence.

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arXiv2014

Measurement of the g p -> K^0 Sigma^+ reaction with the Crystal Ball/TAPS detectors at the Mainz Microtron

The g p -> K^0 Sigma^+ reaction has been measured from threshold to Eg=1.45 GeV (W_cm=1.9 GeV) using the Crystal Ball and TAPS multiphoton spectrometers together with the photon tagging facility at the Mainz Microtron MAMI. In the present experiment, this reaction was searched for in the 3pi^0 p final state, by assuming K^0_S -> pi^0 pi^0 and Sigma^+ -> pi^0 p. The experimental results include total and differential cross sections as well as the polarization of the recoil hyperon. The new data significantly improve empirical knowledge about the g p -> K^0 Sigma^+ reaction in the measured energy range. The results are compared to previous measurements and model predictions. It is demonstrated that adding the present g p -> K^0 Sigma^+ results to existing data allowed a better description of this reaction with various models.

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arXiv2014

A new determination of the eta transition form factor in the Dalitz decay eta -> e^+ e^- gamma with the Crystal Ball/TAPS detectors at the Mainz Microtron

The Dalitz decay eta -> e^+ e^- gamma has been measured in the gamma p -> eta p reaction with the Crystal Ball and TAPS multiphoton spectrometers, together with the photon tagging facility at the Mainz Microtron MAMI. The experimental statistic used in this work is one order of magnitude greater than in any previous measurement of eta -> e^+ e^- gamma. The value obtained for the slope parameter 1/Lambda^2 of the eta transition form factor, 1/Lambda^2 = (1.95 +/- 0.15_stat +/- 0.10_syst) [1/GeV^2], is in good agreement with recent measurements conducted in eta -> e^+ e^- gamma and eta -> mu^+ mu^- gamma decays, as well as with recent form-factor calculations. The uncertainty obtained in the value of 1/Lambda^2 is lower compared to results from previous measurements of the eta -> e^+ e^- gamma decay.

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arXiv2013

Measurement of the $^{1}H$($\vecγ$, $\vec{p}$)$π^{0}$ reaction using a novel nucleon spin polarimeter

We report the first large-acceptance measurement of polarization transfer from a polarized photon beam to a recoiling nucleon, pioneering a novel polarimetry technique with wide application to future nuclear and hadronic physics experiments. The commissioning measurement of polarization transfer in the $^{1}H$($\vecγ$,$\vec{p}$)$π^{0}$ reaction in the range $0.4<E_γ<1.4$ GeV is highly selective regarding the basic parameterizations used in partial wave analyses to extract the nucleon excitation spectrum. The new data strongly favor the recently proposed Chew-Mandelstam formalism.

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arXiv2012

Determination of the η mass with the Crystal Ball at MAMI-B

A new precise determination of the η meson mass is presented. It is based on a measurement of the threshold for the γp -> pη reaction using the tagger focal-plane microscope detector at the MAMI-B facility in Mainz. The tagger microscope has a higher energy resolution than the standard tagging spectrometer and, hence, allowed an improvement in the accuracy compared to the previous η mass measurement at MAMI-B. The result mη = (547.851 +- 0.031 stat. +- 0.062 syst.) MeV agrees very well with the precise values of the NA48, KLOE and CLEO collaborations and deviates by about 5σ from the smaller, but also very precise value obtained by the GEM collaboration at COSY.

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arXiv2012

Study of the gamma p --> pi0 pi0 p reaction with the Crystal Ball/TAPS at the Mainz

The gamma p --> pi0 pi0 p reaction has been measured from threshold to 1.4 GeV using the Crystal Ball and TAPS photon spectrometers together with the photon tagging facility at the Mainz Microtron. The experimental results include total and differential cross sections as well as specific angular distributions, which were used to extract partial-wave amplitudes. In particular, the energy region below the D13(1520) resonance was studied.

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arXiv2010

First measurement of the circular beam asymmetry in the gamma p --> pi0 eta p reaction

The circular photon asymmetry for pi0 eta photoproduction on the proton was measured for the first time at the tagged photon facility of the MAMI C accelerator using the Crystal Ball/TAPS photon spectrometer. The experimental results are interpreted within a phenomenological isobar model that confirms the dominant role of the Delta(1700)D33 resonance. The measured asymmetry allows us to identify small contributions from positive-parity resonances via interference terms with the dominant D33 amplitude.

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arXiv2010

Radiative pi0 photoproduction on protons in the Delta+(1232) region

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.

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