Search Results (31 total)

The repeated passage of a coasting ion beam of a storage ring through a thin target induces a shift in the revolution frequency due to the energy loss in the target. Since the frequency shift is proportional to the beam-target overlap, its measurement offers the possibility of determining the target thickness and hence the corresponding luminosity in an experiment. This effect has been investigated with an internal proton beam of energy 2.65 GeV at the COSY-Jülich accelerator using the ANKE spectrometer and a hydrogen cluster-jet target. Possible sources of error, especially those arising from the influence of residual gas in the ring, were carefully studied, resulting in an accuracy of better than 5%. The luminosity determined in this way was used, in conjunction with measurements in the ANKE forward detector, to determine the cross section for elastic proton-proton scattering. The result is compared to published data as well as to the predictions of a phase shift solution. The practicability and the limitations of the energy-loss method are discussed.

The Chao matrix formalism allows analytic calculations of a beam’s polarization behavior inside a spin resonance. We recently tested its prediction of polarization oscillations occurring in a stored beam of polarized particles near a spin resonance. Using a 1.85  GeV/c polarized deuteron beam stored in the COoler SYnchrotron, we swept a new rf solenoid’s frequency rather rapidly through 400 Hz during 100 ms, while varying the distance between the sweep’s end frequency and the central frequency of an rf-induced spin resonance. Our measurements of the deuteron’s polarization near and inside the resonance agree with the Chao formalism’s predicted oscillations.

The differential and total cross sections for kaon pair production in the pp→ppK⁺K^- reaction have been measured at three beam energies of 2.65, 2.70, and 2.83 GeV using the ANKE magnetic spectrometer at the COSY-Jülich accelerator. These near-threshold data are separated into pairs arising from the decay of the ϕ-meson and the remainder. For the non-ϕ selection, the ratio of the differential cross sections in terms of the K^-p and K⁺p invariant masses is strongly peaked toward low masses. This effect can be described quantitatively by using a simple ansatz for the K^-p final state interaction, where it is seen that the data are sensitive to the magnitude of an effective K^-p scattering length. When allowance is made for a small number of ϕ events where the K^- rescatters from the proton, the ϕ region is equally well described at all three energies. A very similar phenomenon is discovered in the ratio of the cross sections as functions of the K^-pp and K⁺pp invariant masses and the identical final state interaction model is also very successful here. The world data on the energy dependence of the non-ϕ total cross section is also reproduced, except possibly for the results closest to threshold.

Stored beams of polarized protons, electrons, or deuterons can be spin flipped by sweeping an rf dipole’s or solenoid’s frequency through an rf spin resonance. Fitting such data to the modified Froissart-Stora equation’s spin resonance strength E_FS gave very large deviations from the ^*E_Bdl obtained from each rf magnet’s ∫B_rmsdl. We recently varied an rf dipole’s frequency sweep range Δf, and the momentum spread Δp/p and betatron tune ν_y of stored 1.85  GeV/c polarized deuterons. We found a sharp constructive interference when ν_y was near an intrinsic spin resonance. Moreover, over large Δf and Δp/p ranges, E_FS was about 7 times smaller than the predicted ^*E_Bdl.

We recently started testing Chao’s proposed new matrix formalism for describing the spin dynamics due to a single spin resonance. The Chao formalism is probably the first fundamental improvement of the Froissart-Stora equation in that it allows analytic calculations of the beam polarization’s behavior inside a resonance. We tested the Chao formalism using a 1.85  GeV/c polarized deuteron beam stored in COSY, by sweeping an rf dipole’s frequency through 200 Hz, while varying the distance from the sweep’s end frequency to an rf-induced spin resonance’s central frequency. Since the Froissart-Stora equation itself can make no prediction inside a resonance, we compared our experimental data with the predictions of the Chao formalism and those of an empirical two-fluid model based on the Froissart-Stora equation. The data strongly favor the Chao formalism.

Measurements of the analyzing power for the p→p→ppη reaction have been performed at excess energies of Q=10 and 36 MeV. The determined analyzing power is essentially consistent with zero, implying dominance of the s wave at both excess energies. The angular dependence of the analyzing power, combined with the isospin dependence of the total cross section for the η meson production in nucleon-nucleon collisions, reveal that the excitation of the nucleon to the S₁₁(1535) resonance is predominantly due to the exchange of the π meson between the colliding nucleons.

The proton and deuteron analyzing powers and ten of the possible 12 spin correlation coefficients have been measured for p+d elastic scattering at proton bombarding energies of 135 and 200 MeV. The results are compared with Faddeev calculations using two different NN potentials. The qualitative features of the extensive data set on the spin dependence in p+d elastic scattering over a wide range of angles presented here are remarkably well explained by two-nucleon force predictions without inclusion of a three-nucleon force. The remaining discrepancies are, in general, not alleviated when theoretical three-nucleon forces are included in the calculations.

The vector (P_z) and tensor (P_zz) polarizations of a deuteron beam have been measured using elastic deuteron–carbon scattering at 75.6 MeV and deuteron-proton scattering at 270 MeV. After acceleration to 1170 MeV inside the COSY storage ring, the polarizations of the deuterons were remeasured by studying the analyzing powers of a variety of nuclear reactions. For this purpose a hydrogen cluster target was employed at the ANKE magnetic spectrometer, which is situated at an internal target position in the ring. The overall precisions obtained were about 4% for both P_z and P_zz. Though all the measurements were consistent with the absence of depolarization during acceleration, only an upper limit of about 6% could be placed on such an effect.

We recently analyzed all available data on spin-flipping stored beams of polarized protons, electrons, and deuterons. Fitting the modified Froissart-Stora equation to the measured polarization data after crossing an rf-induced spin resonance, we found 10–20-fold deviations from the depolarizing resonance strength equations used for many years. The polarization was typically manipulated by linearly sweeping the frequency of an rf dipole or rf solenoid through an rf-induced spin resonance; spin-flip efficiencies of up to 99.9% were obtained. The Lorentz invariance of an rf dipole’s transverse ∫Bdl and the weak energy dependence of its spin resonance strength E together imply that even a small rf dipole should allow efficient spin flipping in 100 GeV or even TeV storage rings; thus, it is important to understand these large deviations. Therefore, we recently studied the resonance strength deviations experimentally by varying the size and vertical betatron tune of a 2.1  GeV/c polarized proton beam stored in COSY. We found no dependence of E on beam size, but we did find almost 100-fold enhancements when the rf spin resonance was near an intrinsic spin resonance.

It seems improper to publish a theoretical Comment about an experimental Article without mentioning the Article’s referenced theoretical paper, which derived the equation that the Comment claims is incorrect. Our experimental collaboration declines to offer any opinion about which factor of 2 is correct.

We recently studied the spin manipulation of 1.85  GeV/c vertically polarized deuterons stored in the COSY cooler synchrotron. We adiabatically swept an rf dipole’s frequency through an rf-induced spin resonance and observed its effect on the deuterons’ vector and tensor polarizations. After optimizing the resonance crossing rate and maximizing the rf dipole’s voltage, we measured spin-flip efficiencies of 97±1% and 98.5±0.3% in two separate runs. We also confirmed at higher energy the striking behavior of the spin-1 tensor polarization recently found at IUCF.

A measurement of the analyzing power A_y of the p→d→(pp)+n reaction was carried out at the ANKE spectrometer at COSY at beam energies of 0.5 and 0.8 GeV by detection of a fast forward proton pair of small excitation energy E_pp<3  MeV. The S-wave dominance in the fast diproton is experimentally demonstrated in this reaction. While at T_p=0.8  GeV the measured analyzing power almost vanishes, it rises to nearly unity at T_p=0.5  GeV for neutrons emitted at θ_n^c.m.=167 °. The results are compared with a model taking into account one-nucleon exchange, single scattering, and Δ(1232) excitation in the intermediate state. The model describes fairly well the unpolarized cross section obtained earlier and the analyzing power at 0.8 GeV; it fails to reproduce A_y at 0.5 GeV.

Polarized antiprotons can be produced in a storage ring by spin-dependent interaction in a purely electron-polarized hydrogen gas target. The polarizing process is based on spin transfer from the polarized electrons of the target atoms to the orbiting antiprotons. After spin filtering for about two beam lifetimes at energies T≈40–170  MeV using a dedicated large acceptance ring, the antiproton beam polarization would reach P=0.2–0.4. Polarized antiprotons would open new and unique research opportunities for spin-physics experiments in p̅ p interactions.

We recently used a new ferrite rf dipole to study spin flipping of a 2.1  GeV/c vertically polarized proton beam stored in the COSY Cooler Synchrotron in Jülich, Germany. We swept the rf dipole’s frequency through an rf-induced spin resonance to flip the beam’s polarization direction. After determining the resonance’s frequency, we varied the frequency range, frequency ramp time, and number of flips. At the rf dipole’s maximum strength and optimum frequency range and ramp time, we measured a spin-flip efficiency of 99.92±0.04%. This result, along with a similar 0.49  GeV/c IUCF result, indicates that, due to the Lorentz invariance of an rf dipole’s transverse ∫Bdl and the weak energy dependence of its spin-resonance strength, an only 35% stronger rf dipole should allow efficient spin flipping in the 100 GeV BNL RHIC Collider or even the 7 TeV CERN Large Hadron Collider.

We have measured three axial polarization observables in d→p→ breakup with a polarized 270 MeV deuteron beam on a polarized proton target. Axial observables are zero by parity conservation in elastic scattering but can be easily observed in the breakup channel at the present energy. Based on a symmetry argument, the sensitivity of these observables to the three-nucleon force might be enhanced. Calculations without three-nucleon force are in fair agreement with our measurement, indicating that the expected sensitivity of axial observables to the three-nucleon force is not confirmed. Including a three-nucleon force in the calculation does not improve the agreement with the data.

We recently studied spin flipping of a 1.94  GeV/c vertically polarized proton beam at COSY in Jülich, Germany. We swept an rf-dipole’s frequency through an rf-induced spin resonance to flip the beam’s polarization direction. After determining the resonance’s frequency, we varied the dipole’s strength, frequency range, and frequency ramp time. At the rf-dipole’s maximum strength, and optimum frequency range and ramp time, we measured a spin-flip efficiency of 99.3%±0.1%. This result indicates that an rf dipole may allow efficient spin flipping in high energy proton rings.

We have measured the vector and tensor polarization of an atomic deuterium target as a function of the target density. The polarized deuterium was produced in an atomic beam source and injected into a storage cell. For this experiment, the atomic beam source was operated without rf transitions, in order to avoid complications from the unknown efficiency of these transitions. In this mode, the atomic beam is vector and tensor polarized and both polarizations can be measured simultaneously. We used a 1.2-cm-diam and 27-cm-long storage cell, which yielded an average target density between 3 and 9×10¹¹ at/cm³. We find that the tensor polarization decreases with increasing target density while the vector polarization remains constant. The data are in quantitative agreement with the calculated effect of spin exchange between deuterium atoms at low field.

A first measurement of longitudinal as well as transverse spin correlation coefficients for the reaction p→p→→pnπ⁺ is made using a polarized proton target and a polarized proton beam. We report kinematically complete measurements for this reaction at 325-, 350-, 375-, and 400-MeV beam energies. The spin correlation coefficients A_xx+A_yy, A_xx-A_yy, A_zz, and A_xz and the analyzing power A_y, as well as angular distributions for σ(θ_π) and the polarization observables A_ij(θ_π), are extracted. Partial wave cross sections for dominant transition channels are obtained from a partial-wave analysis that included transitions with final-state angular momenta of l<~1. The measurements of the p→p→→pnπ⁺ polarization observables are compared with the predictions from the Jülich meson exchange model. The agreement is very good at 325 MeV, but it deteriorates increasingly for the higher energies. At all energies agreement with the model is better than for the reaction p→p→→ppπ⁰.

The nuclear polarization of H₂ molecules formed by recombination of polarized H atoms on a Cu surface was measured as a function of external magnetic field and of temperature of the surface. The proton polarization of the molecules was determined by scattering of a longitudinally polarized 203-MeV proton beam in the Indiana University Cyclotron Facility storage ring. The nuclear polarization of the molecules, relative to the polarization of the atoms before recombination, increased from near zero in a weak magnetic field to 0.42±0.02 in a 0.66 T field. A simple model of the relaxation accounts quantitatively for the observations.

K⁺-meson production in pA ( A  =  C,Cu,Au) collisions has been studied using the ANKE spectrometer at an internal target position of the COSY-Jülich accelerator. The complete momentum spectrum of kaons emitted at forward angles, ϑ≤12°, has been measured for a beam energy of T_p  =  1.0 GeV, far below the free NN threshold of 1.58 GeV. The spectrum does not follow a thermal distribution at low kaon momenta and the larger momenta reflect a high degree of collectivity in the target nucleus.

In a kinematically complete experiment we have measured the two analyzing powers and the five spin correlation coefficients of the reaction p→p→→ppπ⁰ as a function of all five parameters of the three-body final state for bombarding energies between 325 and 400 MeV. The data are in disagreement with the theoretical predictions available at this time. Below 400 MeV, fewer than a dozen complex partial-wave amplitudes are likely to be significant, and it is expected that the present experimental information constrains these amplitudes. We also describe the formalism for an expansion of the spin observables into a complete set of angular functions and use this to completely characterize the polarization information obtainable from reactions with polarized spin-1/2 collision partners and a three-body final state.

The spin correlation coefficent combinations A_xx+A_yy and A_xx-A_yy, the spin correlation coefficients A_xz and A_zz, and the analyzing power were measured for p→p→→dπ⁺ between center-of-mass angles 25°<~θ<~65° at beam energies of 350.5, 375.0, and 400.0 MeV. The experiment was carried out with a polarized internal target and a stored, polarized beam. Nonvertical beam polarization needed for the measurement of A_zz was obtained by the use of solenoidal spin rotators. Near threshold, only a few partial waves contribute, and pion s and p waves dominate with a possible small admixture of d waves. Certain combinations of the observables reported here are a direct measure of these d waves. The d-wave contributions are found to be negligible even at 400.0 MeV.

A polarized proton beam with a large longitudinal polarization component of 0.545±0.005 (96% of the total polarization) was prepared in a storage ring (IUCF–Cooler). This was achieved by means of spin precession solenoids in two of the six straight sections of the ring. A polarized hydrogen storage cell target internal to the ring was used to measure the longitudinal spin correlation coefficient A_zz in pp elastic scattering over the laboratory angular range 5.5°–43.5° (θ_c.m.=11.5°–90°) with statistical errors of typically 0.025. The absolute normalization was determined to an accuracy of 2.0% by use of the identity A_yy-A_xx-A_zz≡1 at θ_c.m.=90°. The identity also allows a reduction of the scale factor uncertainty of the previously published analyzing powers and spin correlation coefficients. The results are compared to recent pp partial wave analyses and NN potential models.

We report a measurement of the spin-dependent total cross section ratios Δσ_T/σ_tot and Δσ_L/σ_tot of the pp→ppπ⁰ reaction between 325 and 400 MeV. The experiment was carried out with a polarized internal target in a storage ring. Nonvertical beam polarization was obtained by the use of solenoidal spin rotators. Near threshold, the knowledge of both spin-dependent total cross sections is sufficient to deduce the strength of certain participating partial waves, free of any model.