372596221

15

RIKEN Accel. Próg. Rep. 24 (1990)

IN-l-6. Negative Pion Production in Subthreshold Nuclear Collisions

T. Suzuki, N. Inabe, T. Nakagawa, K. Yoshida, and I. Tanihata [NUCLEAR REACTION, ¹⁴N(C,n“)X, measured o„-(£'); deduced slope factor.)

Inclusive jz~ spectra have been measured for ¹⁴N+C collisions at 67A, 80A and 135/ł MeV. Experimental details can be found in Ref. 1. Lorentz-invariant cross sections, (l//>)d^o/di^r2cL£‘, depend exponentially upon the pion energy in the nucleon-nucleon c.m. frame with a inverse slope T₀ as indicated in Fig. 1. In Fig. 2 we plot the slope factors T₀, for ji“ produced at 0_c.m.=9O°, over a wide rangę of the mass and beam energy.³⁾ The slope factor T₀ decreases monotonously as the beam energy decreases down to around 100/1 MeV. However, the behavior seems to change below 100/1 MeV, where T₀ appears to flatten out down to 25/1 MeV. Although a slight decrease in the slope factor with beam energy is indeed observed within a error bar, energy distributions below 100/1 MeV are less steep than expected from the monotonous decrease in the slope factor down to 100/1 MeV. A recent BUU calculation which includes both the A—>N7i; channel and the taił portion of the momentum distribution originating from short rangę correlations reproduces above behavior.⁴⁾

50    100    500    1000

beam energy (MeV/u)

Fig. 2. Slope factor, T₍₎, for jt~ spectra at 0_c>m> = 90°, plotted as a function of beam energy for a variety of targets and projectiles, at beam energies between 41 and 800/4 MeV. For comparison, the apparent temperaturę deduced from jt⁽⁾ transverse momentum spectrum, (do/ dpi), assuming the isotropic emission with d³o/d/>³ ^ exp(—T^/To) in the frame where the pion source is at rest, are also plotted. The ji^- data are from Refs. 1,2, and the present study. The ji° data are taken from Ref. 2.

T_n^cm(MeV)

Fig. 1. Lorentz-invariant cross sections, (l//>)d²o/d/2d£, for inclusive tc production in ¹⁴N + C collision, at ener-gies indicated. OnJy statistical errors are shown in the plot. T₍₎ is the slope factor when the cross sections are parameterized by exp(—T_n/T₀).

References

1)    T. Suzuki et al.: Phys. Lett. B, (1991), in press.

2)    G.R. Young et al.: Phys. Rev., C33, 742 (1986); J. Stachel et al.: ibid., p. 1420.

3)    I. Tanihata et al.: Phys. Lett., 87B, 349 (1979); S. Nagamiya et al.: Phys. Rev., 24, 971 (1981); Phys. Rev. Lett., 48, 1780 (1982); T. Johansson et al.: ibid., 48, 732 (1982); V. Bernard et al.: Nuci. Phys., A422, 511 (1984); J. Miller et al.: Phys. Rev. Lett., 58, 2408 (1987); ibid., 59, 519 (1987).

4)    K. Niita et al.: Nuci. Phys., (1991), in press.