The violation of mirror symmetry in the weak force provides a powerful tool to study the internal structure of the proton. Experimental results have been obtained that address the role of strange quarks in generating nuclear magnetism. The measurement reported here provides an unambiguous constraint on strange quark contributions to the proton's magnetic moment through the electron-proton weak interaction. We also report evidence for the existence of a parity-violating electromagnetic effect known as the anapole moment of the proton. The proton's anapole moment is not yet well understood theoretically, but it could have important implications for precision weak interaction studies in atomic systems such as cesium.
We report new precise H(e,e(')p)pi(0) measurements at the Delta(1232) resonance at Q(2)=0.127 (GeV/c)(2) obtained at the MIT-Bates out-of-plane scattering facility which are particularly sensitive to the transverse electric amplitude (E2) of the gamma(*)N-->Delta transition. The new data have been analyzed together with those of earlier measurements to yield precise quadrupole to dipole amplitude ratios: Re(E(3/2)(1+)/M(3/2)(1+))=(-2.3+/-0.3(stat+syst)+/-0.6(model))% and Re(S(3/2)(1+)/M(3/2)(1+))=(-6.1+/-0.2(stat+syst)+/-0.5(model))% for M(3/2)(1+)=(41.4+/-0.3(stat+syst)+/-0.4(model))(10(-3)/m(pi(+))). The derived amplitudes give credence to the conjecture of deformation in hadrons favoring, at low Q2, the dominance of mesonic effects.
It has been recently shown that radiotherapy at ultrahigh dose rates (>40 Gy/s, FLASH) has a potential advantage in sparing healthy organs compared to that at conventional dose rates. The purpose of this work is to show the feasibility of proton FLASH irradiation using a gantry-mounted synchrocyclotron as a first step toward implementing an experimental setup for preclinical studies. Methods: A clinical Mevion HYPERSCAN â synchrocyclotron was modified to deliver ultrahigh dose rates. Pulse widths of protons with 230 MeV energy were manipulated from 1 to 20 ls to deliver in conventional and ultrahigh dose rate. A boron carbide absorber was placed in the beam for range modulation. A Faraday cup was used to determine the number of protons per pulse at various dose rates. Dose rate was determined by the dose measured with a plane-parallel ionization chamber with respect to the actual delivery time. The integral depth dose (IDD) was measured with a Bragg ionization chamber. Monte Carlo simulation was performed in TOPAS as the secondary check for the measurements. Results: Maximum protons charge per pulse, measured with the Faraday cup, was 54.6 pC at 20 ls pulse width. The measured IDD agreed well with the Monte Carlo simulation. The average dose rate measured using the ionization chamber showed 101 Gy/s at the entrance and 216 Gy/s at the Bragg peak with a full width at half maximum field size of 1.2 cm. Conclusions: It is feasible to deliver protons at 100 and 200 Gy/s average dose rate at the plateau and the Bragg peak, respectively, in a small~1 cm 2 field using a gantry-mounted synchrocyclotron.
We report the first measurement of the vector analyzing power in inclusive transversely polarized elastic electron-proton scattering at Q 2 ϭ0.1 (GeV/c) 2 and large scattering angles. This quantity must vanish in the single virtual photon exchange, plane-wave impulse approximation for this reaction, and can therefore provide information on two photon exchange amplitudes for electromagnetic interactions with hadronic systems. The observable we have measured is driven by the imaginary part of the two photon exchange amplitude, the hadronic side of which is simply the Compton amplitude for the proton with two virtual photons. We find a small but nonzero value of AϭϪ15.4Ϯ5.4 ppm. The recent development and refinement of experimental methods for measurements of small ͑few parts per million, or ppm͒ parity violating effects in polarized electron scattering ͓1-3͔ provides a new technique for further studies of the electromagnetic structure of the proton. We have exploited these methods for the first time to measure the small vector analyzing power in the elastic scattering of 200 MeV electrons from the proton at large laboratory scattering angles (130°р р170°), corresponding to a four-momentum transfer squared of Q 2 ϭ0.1 (GeV/c) 2 . This parity conserving quantity is associated with transverse electron polarization, in contrast to the parity violating longitudinal ͑i.e., helicitydependent͒ asymmetry. It has been previously noted ͓4͔ that transverse polarization effects will be suppressed by the relativistic boost factor 1/␥. Nevertheless, as demonstrated here, the development of the technology to measure small parity violating asymmetries, along with the ability to produce transversely polarized electron beams at high energies, now renders these transverse polarization effects amenable to measurement.The vector analyzing power is a time-reversal odd observable that must vanish in first-order perturbation theory, and can only arise in leading order from the interference of twophoton exchange ͑second order͒ and single-photon exchange amplitudes. Our observation of this quantity therefore demonstrates the viability of a new technique to access the physics associated with the absorption of two virtual photons by a hadronic system. Thus, the study of vector analyzing powers provides another method to study processes in which two photons couple to the proton, i.e., the Compton amplitude, that is complementary to virtual Compton scattering ͑VCS͒, in which there is presently a great deal of interest as a means to further probe the structure of the proton ͓5͔. VCS involves the coupling of one virtual and one real photon to a hadronic system, but in practice includes Bethe-Heitler amplitudes, associated with radiation of a real photon from the electron, with which care must be taken for proper treatment to allow a correct interpretation of those measurements. In contrast, the two-photon exchange amplitude involves the coupling of two virtual photons to a hadronic system, and the vector analyzing power in elastic electron-proton...
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