Article
Reference
Proton-proton elastic scattering analyzing power in the 2.16 to 2.28 GeV energy region
ARVIEUX, Jacques, et al.
Abstract
The angular dependence of the pp elastic scattering analyzing power was measured at SATURNE II with an unpolarized proton beam and the Saclay polarized proton target. The energy region in the vicinity of the accelerator depolarizing resonance Gγ = 6 at Tkin = 2.202 GeV was studied. Measurements were carried out at seven energies between 2.16 and 2.28 GeV from 17° to 55°CM. No significant anomaly was observed in the angular and energy dependence of the results presented, whereas the existing data sets differ in this energy range.
ARVIEUX, Jacques, et al . Proton-proton elastic scattering analyzing power in the 2.16 to 2.28 GeV energy region. Zeitschrift für Physik. C: Particles and Fields , 1997, vol. 76, no. 3, p.
465-468
DOI : 10.1007/s002880050568
Available at:
http://archive-ouverte.unige.ch/unige:112304
Disclaimer: layout of this document may differ from the published version.
1 / 1
1 Laboratoire National SATURNE, CNRS/IN2P3 et CEA/DSM, CE-Saclay, F-91191 Gif-sur-Yvette Cedex, France
2 CEA, DAPNIA, CE Saclay, F-91191 Gif sur Yvette Cedex, France
3 DPNC, University of Geneva, 24, quai Ernest-Ansermet, CH-1211 Geneva 4, Switzerland
4 C.E.N.B., Domaine du Haut-Vigneau, F-33170 Gradignan, France,
5 HEP Division, Argonne National Laboratory, Argonne, IL 60439, USA
Received: 15 May 1997
Abstract. The angular dependence of the pp elastic scat- tering analyzing power was measured at SATURNE II with an unpolarized proton beam and the Saclay polarized proton target. The energy region in the vicinity of the accelerator depolarizing resonance Gγ = 6 at Tkin = 2.202 GeV was studied. Measurements were carried out at seven energies between 2.16 and 2.28 GeV from 17◦ to 55◦CM. No sig- nificant anomaly was observed in the angular and energy dependence of the results presented, whereas the existing data sets differ in this energy range.
1 Introduction
The pp elastic scattering analyzing power Aooon was mea- sured at SATURNE II using an unpolarized proton beam and a polarized proton target. The aim of the experiment was to study the energy and angular dependence of Aooon around the depolarizing resonance Gγ = 6 of the accelerator. This powerful resonance occurs at the beam kinetic energy of 2.202 GeV. Measurements were carried out at 2.16, 2.18, 2.20, 2.22, 2.24, 2.26, and 2.28 GeV in the angular region from 17◦ to 55◦CM.
We compare the energy dependence at two fixed angles with the SATURNE II measurements from [1], BNL results at 1.63 and 2.24 GeV [2], LBL data at 1.70 [3], CERN data at 1.958 GeV [4], and with five KEK data points between 1.784 and 1.850 GeV [5]. All these data were measured with the proton beam energy having a relatively small uncertainty.
This was not the case for the ANL-ZGS data, used at 1.732, 1.967, 2.138, and 2.444 GeV from [6], at 2.205 GeV from [7], at 2.301 GeV from [8], and quasielastic pp data mea- sured with a deuterium target and a polarized proton beam at 2.205 GeV from [9, 10]. It should be noted that the ANL- ZGS was a weak-focussing accelerator with a momentum
∗Deceased
spread of±3.5% (ΔTkin±100M eV around 2.2 GeV), and that the kinetic energy at the target was determined by the currents of the beam line magnets. Therefore, the beam en- ergy may not have been well known, and possibly differed from one ZGS experiment to another.
The angular dependence is compared with the data from [2, 6, 7, 9, 10] and with predictions of a very recent phase shift analysis (PSA) [11]. In [12] it was observed that in the energy region under discussion the data measured before 1983 show a considerable difference in the absolute polar- ization values between different data sets. A common fit averaging the different sets suggested to normalize the data [3, 8] downward by 10%, 8%, and 8%, respectively. The data [6, 9] needed to be normalized upwards by 15% and 12%, based on the fit and a comparison of beam polariza- tions before and after acceleration in the ANL-ZGS. In the present paper the data are shown from original references, but the conclusions based on fits including the SATURNE II data are similar to those in [12].
2 Experimental set-up and data acquisition
Throughout this article we use the nucleon-nucleon four- index notation of the observables as in [13].
For any single scattering measurement with the beam and the target polarized along the normal ±n to the scattering plane:
dσ/dΩ= (dσ/dΩ)o[1 + (PB,n)Aoono+ (PT,n)Aooon
+(PB,n)(PT,n)Aoonn], (2.1) where (dσ/dΩ)o is the unpolarized differential cross sec- tion, and PB and PT are the beam and target polarization vectors, respectively. The observables dσ/dΩ, (dσ/dΩ)o, Aoono, Aooon, and Aoonn are functions of angle and en- ergy. The beam and target analyzing powersAoono(pp) and Aooon(pp) are identical due to the Pauli principle. They are
466
Fig. 1.Angular dependence ofAooon(pp) at the seven SATURNE II en- ergies. Solid curves are predictions of the energy dependent PSA [11]. The meaning of the symbols is:•.... present results,.... BNL [2], +.... ANL [6],.... ANL [7],◦.... ANL [9],.... ANL [10]
antisymmetric functions of CM scattering angle with re- spect to 90◦CM, whereas the spin correlation parameter Aoonn(pp) is a symmetric function.
From (2.1) it follows that any measurement with polar- ized beam and unpolarized target provides Aoono only; a measurement with an unpolarized beam and polarized target givesAooon.
The present measurements were carried out at SAT- URNE II using the Nucleon-Nucleon (NN) experimental set- up. This array is described in detail in [14]. It consisted of a two arm spectrometer with an analyzing magnet in one arm.
Each arm was equipped with single scintillation counters and counter hodoscopes selecting events with pairs of charged particles. These signals triggered eight multi-wire propor- tional chambers (MWPC’s) with three wire planes each. The recorded events were analyzed andppelastic scattering can- didates were selected.
The Saclay frozen spin polarized proton target (PPT), 35 mm thick, 40 mm long, and 49 mm high, contained pentanol- 1 doped by paramagnetic centers [15]. The typical positive polarization was +80%, while the negative one was−85%.
The target was working in the frozen spin mode and has a magnetic holding field of 0.33 Tesla. The relaxation time of the target, on average 25 days, was taken into account in the off-line data analysis. The polarized target was followed by a small unpolarized target positioned 16 cm downstream from the PPT center. The events from both targets were recorded by the same MWPC’s, but the triggers were target selective.
The unpolarized beam was obtained from the polarized ion source HYPERION. The radiofrequencies of the source were not applied and the correctors for depolarizing reso- nances of SATURNE II were switched off. Moreover, the Hyperion solenoid [16], which rotates low energy particle spins to the vertical direction, was not working. This tuning was important, since the ion source under normal working conditions may provide a non-negligible beam polarization for “unpolarized” states of the order of 6% [16]. Without the tuning this will introduce undesirable contributions to the beam analyzing power Aoono and to the spin correla- tion parameterAoonnin (2.1). Since the asymmetry with an
Fig. 2a,b. Energy dependence of Aooon(pp) at 11.6◦lab a and at 40.3◦CM b. The solid line is the fit to data (see text). The meaning of the symbols is:•.... present results,black squares.... SATURNE II data from [1],.... BNL [2],open squares.... LBL [3],×.... CERN [4],....
KEK [5], +.... ANL [6],.... ANL [7],.... ANL [8],◦.... ANL [9],....
ANL [10]
additional unpolarized target was measured simultaneously in the experiment, it was possible to check for the absence of the beam polarization from the measured data. At all energies we found the mean absolute value of the unpolar- ized target asymmetry to be very small (typically less than 0.002±0.004). From this we deduce that no corrections to the data were needed.
Compared to a measurement with a polarized beam, where the sign of the polarization changes every spill, this experiment has an extra random-like systematic error, as the target polarization was reversed typically every 2 or 3 days.
The dispersion of results from individual measurements was used to determine this overall systematic error.
3 Results and discussion
Two sets of data were measured at each energy in two dif- ferent time periods and the present results are the averaged values from all measurements. The results are listed in Ta- ble 1 and are shown in Fig. 1 as black dots. The errors are statistical only. A systematic error of ±3% is attributed to the PPT polarization measurement, which may move the re- sults at one energy up or down together. This systematic error is not correlated at different energies. The random-like systematic errorΔ(Tkin) corresponds to a mean dispersion of data point values from individual runs.
At all energies we observe a decrease of the analyzing power values with increasing angle. The highest measured angles are close to the minimum of Aooon as explained in [17].
Our results at 2.16 GeV are compared with the data from [6]. At small angles the two sets differ considerably. This in-
41.0 0.227±0.019 0.232±0.019 0.215±0.016 0.202±0.020 43.0 0.192±0.020 0.181±0.018 0.237±0.016 0.131±0.022 45.0 0.178±0.023 0.164±0.021 0.147±0.019 0.086±0.025 47.0 0.183±0.026 0.132±0.023 0.168±0.020 0.128±0.026 49.0 0.121±0.028 0.113±0.025 0.132±0.022 0.076±0.028 51.0 0.098±0.029 0.143±0.026 0.085±0.024 0.088±0.030 53.0 0.121±0.029 0.139±0.027 0.145±0.024 0.137±0.030 54.3 0.123±0.061 0.191±0.063 0.132±0.059 0.114±0.079
41.0 0.206±0.018 0.197±0.018 0.246±0.016 43.0 0.227±0.019 0.202±0.019 0.217±0.016 45.0 0.158±0.022 0.221±0.021
46.0 0.211±0.019
47.0 0.133±0.023 0.179±0.024
49.0 0.181±0.026 0.168±0.025 0.132±0.022 51.0 0.111±0.028 0.146±0.027 0.085±0.024 52.9 0.152±0.027 0.174±0.028 0.145±0.024
54.3 0.175±0.059 0.132±0.059
consistency of the data of [6] was already mentioned in [12].
At 2.20 GeV our data are compared with theAoono=Aooon
data measured with ANL-ZGS polarized beam and with the PPT [7]. The quasi-elastic data from [9, 10], measured with the same polarized proton beam and a liquid deuterium tar- get, are also plotted. Three sets of the ANL data were mea- sured at the nominal energy of 2.205 GeV. This energy is very close to the depolarizing resonanceGγ = 6, which will affect the beam polarization at any accelerator. Nevertheless, we observe fairly good agreement of all results. The present results at 2.24 GeV are compared with the BNL data [2], measured at the same energy. Here we also observe good agreement.
At all energies the measured data are compared with predictions of the energy dependent PSA [11], where our new results were not introduced.
The energy dependence of the analyzing power data is illustrated in Figs. 2a, b at two fixed scattering angles: 11.6◦ in the laboratory system and at 40.25◦CM.
The energy dependence at 11.6◦labis plotted in Fig. 2a.
This angle was used by many polarimeters in the world. It corresponds to 33.7◦CM at 2.2 GeV. The linear fit to the present data as a function of the kinetic energyT (solid line) gives:
Aoono(11.6◦lab) = 0.50745−0.10768·T, (3.1) whereT is in GeV.
Three ANL data points from [6] were removed, since they contributed to the total χ2 value by 64%. These data need to be normalized upwards by 20% with respect to the present fit. This is in good agreement with the recommen- dation in [12].
Fig. 2b shows the results at 40.25◦CM. The linear fit to the data
Aoono(40.25◦CM) = 0.47850−0.11158·T, (3.2)
describes the existing points well.
4 Conclusions
We have presented the target analyzing power values at seven beam kinetic energies close to the depolarizing res- onance Gγ = 6. The data were compared with existing re- sults. In our angular region, below the minimum of the ana- lyzing power, we observe monotonically decreasing angular spectra with increasing energy. The present data consider- ably increase the database and will help to extend the future phase-shift analyses toward high energies.
Acknowledgements. We acknowledge the help of P.-A.Chamouard and of the accelerator operator crew. We thank C. Lechanoine-Leluc for helpful suggestions and I.I. Strakovsky for fruitfull discussions and for PSA predic- tions. This work was supported in part by the U.S. Department of Energy, Division of Nuclear Physics, Contract No. W-31-109-ENG-38 and by Swiss National Science Foundation.
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