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SPIN PHYSICS AT LEAR
F . Bradamante
To cite this version:
F . Bradamante. SPIN PHYSICS AT LEAR. Journal de Physique Colloques, 1985, 46 (C2), pp.C2-
421-C2-425. �10.1051/jphyscol:1985248�. �jpa-00224563�
SPIN PHYSICS AT LEAR F. Bradamante
Istituto di Vis-Lea dell'Vniversita, Trieste, Italy
e
Sezione di Trieste dell'INFN, 34127 Trieste, Italy
Résumé - L'anneau de stockage des antiprotons à basse énergie (LEAR) a commen- cé à fonctionner en août 1984. On présente ici les caractéristiques du fonc- tionnement de la machine et des renseignements sur les premières mesures de la polarization qui ont été effectuées dans le système pp.
Abstract - The low energy antiproton ring (LEAR)is operating at CERN since August 1983. The operation of the machine is reviewed, and a status report is given of the experiments measuring polarization parameters and spin-spin correlation parameters in the pp system.
I - INTRODUCTION
LEAR, the Low Energy Antiproton Ring at CERN, has started operation in August 1983, and is providing antiprotons (p's) beams of an extraordinary quality to a vast community of physicists. The polarization physics which was planned at LEAR has already been reviewed in this series of Conferences four years ago in Lausanne by K.Kilian '.
Generally speaking, no polarization data exist for pp reactions below 1000 MeV/c momentum, so the present experimental program is a first generation spin physics program, namely the measurement of the Polarization and Spin Correlation Parameters of a few simple exclusive channels.
All the physics program as'well as the machine aspects and possible future options have been discussed in detail in May 1982 in Erice, and proceedings of that Workshop are available '. In the past two years no new ideas have been put forward, probably beacause the people were too much involved in constructing and running in the machine on one side, installing and debugging the experiments on the other side. In particular no further work has been done on the various suggestions which had been made to polarize the p's LEAR beam or on the use of an internal polarized atomic hydrogen jet . While referring the interested reader to the quoted literature, I shall just concentrate on the performing of the machine and on the experiments which are now running. No polarization data are yet available, but some measurements have already been done and results should be available before the end of the year.
II - THE LEAR FACILITY
LEAR is a small strong-focussing synchrotron, housed in the South Hall of the CERN- PS 4 ) . It uses the whole of the p's machinery which recently entered into operation at CERN. Bunches of 2 to 4 x lo" p's are extracted every 75 minutes from the AA (Antiproton Accumulator) and transferred to the PS, where they are decelerated to 600 MeV/c and finally injected into LEAR. The quoted figures match the filling rate of the AA, and they refer to a mode of operation in which LEAR is the single p's user, which has been the situation since the running-in of the machine, in August 1983
(in 1985 it should be possible to operate in the same periods both the SppS Collider and LEAR).
After injection in LEAR, the beam is first cooled with a stochastic cooling system, then accelerated or decelerated to the desired momentum (in the range from 100 to 2000 MeV/c), eventually cooled a second time and finally extracted by means of an ultraslow stochastic ejection system^) . The result is a pure p's beam, with i- 3 1 Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1985248
C2-422 JOURNAL DE PHYSIQUE
and % 5 .rr (mm.mrad) horizontal and vertical emittances respectively, momentum dispersion less than 0.1%, and of reasonably constant intensity over a period of one hour, which respresents a unique facility in the world.
In the first year of operation, from August 1983 to August 1984, the machine has provided p's at four different momenta, 300, 600, 1500 and 200 MeV/c, for a total of about 750 spills. The intensity typically has been (1.5
-
2) x lo5 5's /sec, rising to a maximum of % 3 x 10~p's/sec during the best runs. Some factors are still missing when looking at the design figures, but taking into account the very short running in and MD time the overall performance of the machine has to be considered a great achievement.Two splitter magnets divide the extracted beam into three lines, and each line is further subdivided into two lines by means of switching magnets, for a total of six very crowded experimental areas, where 16 experiments are installed, 8 of which of considerable size. Normally, 3 experiments run simultaneously, each receiving a beam of % lo5 p's/sec.
I11
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POLARIZATION EXPERIMENTSOut of 16 experiments, only one, ~ ~ 1 7 2 ~ ) (the SING Collaboration) is specifically designed to investigate the spin dependence of the p interaction in a few exclusive channe and uses a polarized target and two polarimeters. A second experiment, PS185 "ystudies hyperon-antihyperon production, and consequently can get information on the spin structure of those reactions from the weak decay distributions.
Experiment PS172 hasas major goal the search of s-channel resonances in pp scattering, by measuring in small momentum steps (s 50 MeV/c) in the range 400 to 1000 MeV/c, the differential cross-section and the polarization parameter of the two body annihilation channels Pp + II n+ and pp + K-K+, which provide a useful filter to the quantum numbers of the resonances. The approach to the problem is standard, and has proved its fruitfulness in the range 1000 to 2000 MeV/c 8) where several resonances have been identified by phase-shift analysis9). Further information on the s-channel resonances will be obtained by measuring also the elastic channel, from 500 MeV/c to the largest available momentum, again in small momentum steps: the complex structure of this channel suggests to measure also the polarization transfer parameters D, or Dt, depending on the possibility of analyzing the polarization of the scattered p's.
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A complete study of the elastic reaction requires the use of a polarized p's beam, a polarized proton target and possibly a polarimeter to analyze the polarization of the final state proton (or antiproton). An important part of the program of experiment PSI72 is devoted to the attempt to polarize a p's beam by scattering through a piece of carbon, and cocversely to verify if a Carbon Polarimeter can act as polarization analyzer for p's. Again, no data are available from previous measurements with conventional beams.
To fulfill this experimental program the SIKG Collaboration disposes of three major pieces of equipment, which are depicted in Fig. 1. Fig. l(a) shows the beam line, which has been designed so as to transport a scattered beam to the experiment. Two bending magnets allow to wobble the beam extracted from LEAR and to have it impinge at an angle (up to % lo0) on a Carbon scatterer, locate! at Bo, a focal point of the line. The selection of the elastically scattered p's is done by the C2 beam line, which consists of two bending magnets, a field lens in between to correct for the chromatic aberration, and two quadruple triplets. The acceptance of the line is 2.3 x lo-' srad. Two different apparatuses are then used, depending on the particular measurement. Fig. l(b) shows the small-angle set-up, consisting of a liquid hydrogen target, 95 mm long, surrounded by a scintillation counters box, acting as veto against annihilation, followed by a Carbon Polarimeter.
Alternating measurements with the direct beam anh with the scattered beam, one can measure three double-scattering, asymmetries, i.e. AH.AC, AH.A', and ACAtc, where
%
and A are the polarization parameters for elastic scattering on protons andSix spills were used in May 1984, when LEAR was running at 1500 MeV/c, to tune the scattered beam and to take some test double scattering data. The "polarized"
beam intensity is shown in Fig. 2 as a function of production angle, for a reduced beam acceptance (6 x srad), and compared with a Glauber theory calculationlo).
In June 1984 some 60 spills were devoted to the measurements of AH, A, and A w c at 600 MeV/c. A total of about 106 double-scattering events have been collected (see Table I), which are presently being analyzed.
Fig.
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The apparatus used in experiment PS172:(a)-the C2 beam line, showing the positions of the LH2 target (F,) the Polarized Target (FZ).
(b) the small angle set-up.
and of
(c) the PT assembly for the two-body reactions.
C2-424 JOURNAL
DE
PHYSIQUEFig. l(c) shows finally the set-up which is used for the two-body reactions. A 30 mm- long polarized target is surrounded by cylindrical MWP chambers and two scintillation counters arrays. Cherenkov counters help the n-K separation, and.a Carbon Polarimeter measures the polarization of the recoil proton (or p). This experiment had beam time,
same 50 spillsl only+last month, when LEAR operated at 600 MeV/c. About 20,000 Gp + pp, 5000 pp + n .rr and 2000 pp + K+K- events are expected at 580 MeV/c momentum in the target, and about one fourth of these figures at 500 MeV/c.
Fig. 2
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The scattered beam intensity at 1500 ~eV/c.plsec.
Fig. 3
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The apparatus for Experiment PS185, showing an example of a 4 charge AA decay.Carbon Thickness 12
TABLE 1
Double scattering Statistics at 600 MeV/c Scattered beam ( 2 80)
Events with 5' < 0 < 15O 0.4 x 10 5 H
Events with 5O < 8 < 15' 1.4 x 10 5 Direct beam
Events with (OH> 5O) and
(ec>
)'5 8.4 x 10 5 1000-100:
8 I . . ,
.
. . . ..
Fig. 3 shows the apparatus used in experiment PS185, whose main objective is the study of the production mechanism for A71 pairs. Since symmetry considerations require the light (ud) diquark in the A to have zero spin, all the spin.information in the A is carried by the strange-quzrk. By measuring the polarization vectors and the s ~ i n correlations of the A ardA one gets detailed information on the spin of the ss quark pair. The acceptance of the-apparatus at threshold is such that in 41%
of the cases the four charged decay AA+ pp nr+*- is detected, so that not only the pclarization vectors are measured from the recorded decay tracks, but also the AA spin correlations. In May 1984 the experiment received about 100 spills, and measured AA production at 1509 and 1480 MeV/c. Analysis is in progress, and the expected number of four charged decays is 2000 to 4000, more than two orders of mag~itudes the world statistics at this low energy.
Int. Symp. on High-Energy Physics with Polarized Beams and Polarized Targets, Lausanne, 25 Sept.-1 Oct. 1980, Ed. C. Joseph and J. Soffer, Basel, BirkhHuser, 1981
2
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"Physics at LEAR with Low-Energy Cooled Antiprotons", GASTALDI U, and KLAPISCH R, Ed., Plenum Press (1984).3
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KILIAN K., and MOHL D., "Internal Hydrogen or solid targets and Polarization Experiments at LEAR", p. 701 of Ref. 2.4
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LEFEVRE P., "Construction of the LEAR facility: status Report"., p. 15 of Ref. 2.5
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HARDT W. and GIANNINI R. "Ultraslow extraction (status report)" p. 49 of Ref. 2.6
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BOS K., "Status Report on Experiment PS172" p. 427 of Ref. 2.BRADAMANTE F., A polarized
p
beam at LEAR" p. 455 of Ref. 2.RAPIN D., "Spin Effects in pp scattering" p. 447 of Ref. 2.
-
7
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EISENSTEIN R.A., "LEAR Experiment PS185:pp
+<
y, p. 477 of Ref. 2.SCHMITT H., "YN and
f
N Scattering at LEAR", p. 489 of Ref. 2.8
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EISENHANDLER E. et al., Nucl. Phys.B96
(1975) 109.9
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CARTER A.A. et al., Nucl. Phys. =7(1977) 202 and Phys. Letters67B
(1977) 117.10
