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Register a new userMeasurement of the charged-pion polarisability at COMPASS
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Alexey Guskov
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The electric (${alpha}_{pi}$) and the magnetic (${beta}_{pi}$) polarisabilities are fundamental properties of the pion characterising the rigidity of its internal structure. They have been precisely measured at the COMPASS experiment at CERN with a ${pi}^{-}$ beam of 190~GeV/c assuming ${alpha}_{pi}+{beta}_{pi}=0$. Muons of the same momentum were used for controlling of systematic effects. The obtained result ${alpha}_{pi}=-{beta}_{pi}=(2.0pm 0.6_{stat.}pm 0.7_{syst.})times 10^{-4} fm^3$ is in agreement with the prediction of the Chiral Perturbation Theory.
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The COMPASS collaboration at CERN has investigated pion Compton scattering, $pi^-gammarightarrow pi^-gamma$, at centre-of-mass energy below 3.5 pion masses. The process is embedded in the reaction $pi^-mathrm{Ni}rightarrowpi^-gamma;mathrm{Ni}$, which is initiated by 190,GeV pions impinging on a nickel target. The exchange of quasi-real photons is selected by isolating the sharp Coulomb peak observed at smallest momentum transfers, $Q^2<0.0015$,(GeV/$c$)$^2$. From a sample of 63,000 events the pion electric polarisability is determined to be $alpha_pi = (,2.0 pm 0.6_{mbox{scriptsize stat}} pm 0.7_{mbox{scriptsize syst}},) times 10^{-4},mbox{fm}^3$ under the assumption $alpha_pi=-beta_pi$, which relates the electric and magnetic dipole polarisabilities. It is the most precise measurement of this fundamental low-energy parameter of strong interaction, that has been addressed since long by various methods with conflicting outcomes. While this result is in tension with previous dedicated measurements, it is found in agreement with the expectation from chiral perturbation theory. An additional measurement replacing pions by muons, for which the cross-section behavior is unambigiously known, was performed for an independent estimate of the systematic uncertainty.
The fixed-target MIPP experiment, Fermilab E907, was designed to measure the production of hadrons from the collisions of hadrons of momenta ranging from 5 to 120 GeV/c on a variety of nuclei. These data will generally improve the simulation of particle detectors and predictions of particle beam fluxes at accelerators. The spectrometer momentum resolution is between 3 and 4%, and particle identification is performed for particles ranging between 0.3 and 80 GeV/c using $dE/dx$, time-of-flight and Cherenkov radiation measurements. MIPP collected $1.42 times10^6$ events of 120 GeV Main Injector protons striking a target used in the NuMI facility at Fermilab. The data have been analyzed and we present here charged pion yields per proton-on-target determined in bins of longitudinal and transverse momentum between 0.5 and 80 GeV/c, with combined statistical and systematic relative uncertainties between 5 and 10%.
We report on the first cross section measurements for charged current coherent pion production by neutrinos and antineutrinos on argon. These measurements are performed using the ArgoNeuT detector exposed to the NuMI beam at Fermilab. The cross sections are measured to be $2.6^{+1.2}_{-1.0}(stat)^{+0.3}_{-0.4}(syst) times 10^{-38} textrm{cm}^{2}/textrm{Ar}$ for neutrinos at a mean energy of $9.6$ GeV and $5.5^{+2.6}_{-2.1}(stat)^{+0.6}_{-0.7}(syst) times 10^{-39} textrm{cm}^{2}/textrm{Ar}$ for antineutrinos at a mean energy of $3.6$ GeV.
The PHENIX experiment at the Relativistic Heavy Ion Collider has measured the longitudinal double spin asymmetries, $A_{LL}$, for charged pions at midrapidity ($|eta|<0.35$) in longitudinally polarized $p$$+$$p$ collisions at $sqrt{s}=510$ GeV. These measurements are sensitive to the gluon spin contribution to the total spin of the proton in the parton momentum fraction $x$ range between 0.04 and 0.09. One can infer the sign of the gluon polarization from the ordering of pion asymmetries with charge alone. The asymmetries are found to be consistent with global quantum-chromodynamics fits of deep-inelastic scattering and data at $sqrt{s}=200$ GeV, which show a nonzero positive contribution of gluon spin to the proton spin.
Using a custom 3 v{C}erenkov-ring fitter, we report cross sections for $ u_mu$-induced charged-current single $pi^0$ production on mineral oil (chtwo) from a sample of 5810 candidate events with 57% signal purity over an energy range of $0.5-2.0$GeV. This includes measurements of the absolute total cross section as a function of neutrino energy, and flux-averaged differential cross sections measured in terms of $Q^2$, $mu^-$ kinematics, and $pi^0$ kinematics. The sample yields a flux-averaged total cross section of $(9.2pm0.3_{stat.}pm1.5_{syst.})times10^{-39}$cm$^2$/CH$_2$ at mean neutrino energy of 0.965GeV.
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