J. Sowinski

The STAR endcap electromagnetic calorimeter

The STAR endcap electromagnetic calorimeter will provide full azimuthal coverage for high-pT photons, electrons and electromagnetically decaying mesons over the pseudorapidity range 1.086⩽η⩽2.00. It includes a scintillating-strip shower-maximum detector to provide π0/γ discrimination and preshower and postshower layers to aid in distinguishing between electrons and charged hadrons. The triggering capabilities and coverage it offers are crucial for much of the spin physics program to be carried out in polarized proton–proton collisions.

Polarizing a Stored Proton Beam by Spin-Flip?

We discuss polarizing a proton beam in a storage ring, either by selective removal or by spin flip of the stored ions. Prompted by recent, conflicting calculations, we have carried out a measurement of the spin-flip cross section in low-energy electron–proton scattering. The experiment uses the cooling electron beam at COSY as an electron target. The measured cross sections are too small for making spin flip a viable tool in polarizing a stored beam. This invalidates a recent proposal to use co-moving polarized positrons to polarize a stored antiproton beam.

Test of a windowless storage cell target in a proton storage ring

Abstract Stored ion beams offer the possibility to use polarized internal targets that consist of a source of polarized atoms in conjunction with a long, narrow, windowless target cell to enhance the target thickness. In this paper, we discuss the effect of such a cell on the performance of the storage ring, based on measurements carried out with the Indiana Cooler. A prototype target cell was constructed and was operated with a controlled flow of H 2 target gas in a beam of stored protons. Detection of protons scattered at angles from 4° to 15° in coincidence with the associated recoil particles was used to identify elastic scattering. The results show that the presence of a 25 cm long target cell with a rectangular opening of 6.4 mm by 7.9 mm is compatible with operation of the Cooler ring. This demonstrates the feasibility of an important component of experiments with stored, polarized beams and carrier-free polarized, internal targets.

Spin-dependent scattering of polarized protons from a polarized 3He internal gas target

We describe the first experiment to use a polarized internal gas target and polarized beam in a storage ring. A laser optically pumped polarized 3He internal gas target has been used with circulating beams of 197–414 MeV polarized protons to carry out an extensive set of measurements of spin dependent scattering. A large acceptance non-magnetic detector system consisting of wire-chambers, scintillators and microstrip detectors was used to detect protons, neutrons, deuterons, and 3He nuclei from the beam-target interaction. It is demonstrated that these techniques result in low backgrounds (< 1%) due to scattering from species other than the polarized target gas and allow detection of low energy recoiling nuclei. Specific issues such as interfacing the experiment to the storage ring and monitoring the luminosity and polarizations are discussed in detail.

THE HERMES POLARIZED 3HE INTERNAL GAS TARGET

Abstract The HERMES experiment is investigating the spin structure of the proton and neutron via deep-inelastic scattering of polarized positrons from polarized nuclear targets. The polarized positrons are provided by the HERA positron storage ring at DESY, Hamburg, Germany. The targets are pure internal gas targets. Data acquisition began in 1995, utilizing a polarized 3 He internal gas target to study the spin structure of the neutron. The target gas was polarized using the metastability-exchange optical-pumping technique and then injected into a cryogenically cooled target cell. The target was designed to operate with either longitudinal or transverse directions of polarization. Operating conditions included polarizations of up to 54% and target thicknesses of 1×10 15 nucleons/cm 2 . In this paper the HERMES polarized 3 He internal gas target is described in detail.

High precision measurement of the analyzing powers for d-p elastic scattering at Ed = 10 MeV

Abstract Measurements of the analyzing powers iT11, T20, T21 and T22 are presented for d -p elastic scattering at Ed = 10 MeV. The uncertainties in the measured analyzing powers are typically ± 0.0003. The results are compared with the predictions of Faddeev calculations which employ a separable representation of the Paris nucleon-nucleon potential.

On the determination of the deuteron asymptotic D- to S-state wave function ratio by the pole-extrapolation method: Truncation errors☆

Abstract The pole-extrapolation method for determining the asymptotic D-state to S-state wave function ratio of the deuteron is discussed. It is concluded that the method is subject to systematic errors of 10–20% or more, which result from truncation of the polynomial series used to extrapolate the d-p elastic scattering measurements. We see no clear-cut evidence that the polynomial series is converging.

A test of charge symmetry in n-p scattering at En = 183 MeV

Abstract Measurements of the spin-dependent left-right asymmetries for n-p elastic scattering at E n = 183 MeV have been obtained by scattering polarized neutrons from polarized protons. If charge symmetry holds in the n-p system, the two analyzing powers A n and A p must be equal. Our preliminary experimental result for the average value of A n −A p in the range 82.2° θ cm ≤ 116.1° is (32.1 ± 6.1 ± 6) × 10 −4 where the first error is the statistical uncertainty and the second is the systematic uncertainty. This result provides clear evidence of charge symmetry breaking in the nuclear force, and agrees well with meson-exchange predictions based on the Bonn nucleon-nucleon potential.

Measurement of a complete set of spin observables in the 2H(p,n)2p reaction at 200 MeV☆

Abstract A complete set of spin observables was measured for the 2 H(p,n)2p reaction at 200 MeV at the Indiana University Cyclotron Facility (IUCF) with the high intensity polarized ion source (HIPIOS) and the Indiana Neutron Polarization Facility (INPOL). The incident proton beam was delivered successively in each of the three spin states, normal ( N ), sideways ( S ), and longitudinal ( L ). Two large-volume neutron polarimeters placed along the 0° and 24° beam lines measured the polarizations of the emitted neutrons in each of the three spin states. The measured cross sections and spin observables are compared with three-body Faddeev calculations, which generally agree with the data. The ratio of the longitudinal to transverse response is suppressed rather than enhanced, as predicted by some models.

FEW-BODY PHYSICS WITH POLARIZED BEAMS AND TARGETS AT IUCF

An experiment utilizing a polarized 3He target and other 3-nucleon experiments at the Indiana University Cyclotron Facility are discussed. The 3He(p, pN) reaction has been used to investigate the spin dependent momentum distributions of protons and neutrons in 3He. It was found that with large momentum transfer the PWIA is valid, thus allowing extraction of the distributions directly from the data. A similar experiment is currently under preparation to investigate the spin structure of the deuteron via the 2H(p, pN) reaction. Other experiments will study n-d and p-d breakup spin observables in final state interaction kinematics where intermediate energy Faddeev calculations predict large 3-body force effects.