Polarization of Inclusively Produced Hyperons

We report here polarization results from a series of Fermilab experiments from the years 1974 through 1980, with some preliminary data from a high polarization experiment completed in FebruaryP1982. The A polarization has a remarkably simple and interesting behavior when expressed as a function of x F and PT" The discovery of A polarization from 300 GeV inclusive proton-nucleon reactions I initiated a series of successful experiments at Fermilab which measured polarizations and magnetic moments for A, H~ H', Z',and Z" hyperons2"~ "~ The A data is by far the most extensive and well studied with over 107 events reconstucted to date at many production angles. Hence for the A's a detailed behavior of inclusive polarization with respect to the kinematic variables can be determined from the data. The experimental apparatus is described in detail in References 2 and 3. Briefly, it consists of a system of magnets to pitch the proton beam on target at a specific production angle, followed by a collimation system imbedded in a primary beam dump magnet which defines the secondary hyperon beam. The hyperons are detected by a conventional proportional chamber spectrometer. For the high PT experiment, the production angle, 8p, could be varied from 0 to +25 milliradians. Most of the data was taken with 8p set to +20, +12, +i0, and +6 milliradians, although only 20 0094-243X/83/950083-0753.00 Copyright 1983 American Institute of Physics 84 and 12 milliradian data is presented here. For all results given here the reaction is p + A --~ h + X where the target, A, is hydrogen, beryllium, copper, or lead, and h is the particular hyperon being studied. The behavior of A polarization as a function of PT (pT & 2 GeV) for fixed production angle, 8p, is now well known . The magnitude of the polarizatlon, IPI, increases monotonically with PT" This trend, in itself, is not very illuminating since the important scaling variable x_ is not fixed for constant 8p. To map out the polarization as a function of both PT and ~[f one needs to have many sets of data with each at a ferent production angle. A large sample of A's with 9 different values of 8p has already been studied and reported %6. This data and the preliminary results from the latest higher PT experiment, are compltetely consistent and support the observations given below. The magnitude of the A polarization , IPI , as a function of x for three ranges of p is shown in F . T Figure i. A linear fit to the data polnts cons~ained to pass through the origin is indicated with the determined slope. The behavior can be written IPI = C(P T) x F where C(PT), the slope, depends only on the transverse momentum. The same data as in Figure 1 is also shown in Figure 2, however the polarization is plotted as a function of PT for four bins of x_. The most . noteworthy feature is that IPl is independent of PT for ~ Z 1 GeV/c. In addition, the value of IP{ at which e polarization "saturates", i.e. ~T > 1 GeV/c, increases linearly with x F, as shown in Figure 3. The preliminary data points are indicated by the open circles and show the highest PT, 3.4 GeV/c, that can be attained with the present (incomplete) data sample. Once the data analysis is complete a measurement of polarization near 4 GeV/c will be possible. It must be pointed out that no distinction has been made between directly produced A's and the daughter A's from E o, Z0"or other resonance decays. The polarization of the Z0 is not yet known and hence the measured A polarization is diluted by some undetermined factor. This factor, proportional to the ratio of A and Z0 cross sections, should not have a strong PT dependence, hence daughter A contamination is not beleived to have much effect on the qualitative behavior described above. 85 The data from Reference 3 is also shown on Figures 1 & 2 to illustrate the similarity of the results. The ~0 data includes three production angles, and so offers the second best independent polarization sample for mapping out the x~ and PT trends. The charged hyperon pol~rizations show the same qualitative trend as the A's but since there is far less data taken at different production angles it is hard to draw the same conclusions as in the data. The results of the polarization analyses for ~-, Z +, Z', are shown in Figure 4, at fixed 8p instead of fixed x~. For comparison, the A polarization at 8p = 5 mllliradians is shown by the dashed line. The slgn is important: both Z § , and Zare polarized in the opposite sense relative to the A , H0, and H-. The data for both the Z" and Z" was obtained with only one value for the magnetic field in the precession magnet which implies that the sign of the polarization cannot be unambiguously determined without some other reference. For the Z*, the magnetic moment is known well enough to constrain the initial polarization 7 , and for the Zthe preliminary data from the Charged Hyperon Group at Fermilab provides the necessary constraint 8.9. One must remember, though, that polarization is not a universal feature of inclusive hyperon production. Data for ~'s produced from a proton beam shows no measurable signal at PT ~ 1 GeV/c and preliminary results at higher PT also show IP ~I consistent with zero up to PT ~ 2.2 GeV/c. Another interesting feature of inclusive polarization is that it appears to be independent of target material at Fermilab energies (/-s = 27 GeV). However, polarization data at lower energy (J~ = 5 GeV) seems to show a strong dependence on target material. There does, however, seem to be a strong dependence on target material at lower energies i~ Conclusion The previously analyzed data and the preliminary data from the higher p_ polarization experiment are consistent with each ot~er and the properties of inclusive polarization can be summarized with these important points: (i) For inclusive A polarization the x F behavior is striking and can be expressed as IPI = C (pT) x (2) A inclusive polarization becomes essentially independent of pT for pT > 1 GeV/c.