(n) 100
10-
10
-2
*(n-)
* (nK0)
(nA) " (nv o)
100
101
102
A
Fig. 1. The A dependence of the total yields of K0, A, V0, and n-. The lines are the result of the exponential fit.
(n) 10°
10-
10-
y* < 0 y* > 0
Dy* < 0 • y* > 0 i "i i 11_
100
10
102
A
R(K 0/n -) 10-1
100
101
102
A
Fig. 2. The A dependence of the yields of V0 and n- in the forward (y* > 0) and backward (y* < 0) hemispheres. The lines are the result of the exponential fit.
number of the accepted neutral strange particles (V°) was 110 out of which 46(64) had the biggest probability to be identified as K°(A). The corresponding average multiplicities corrected for the decay losses are {nVo) = (7.11 ± 0.68) x 10-2, {nKo) = (4.29 ± ± 0.63) x 10-2, {nA) = (2.82 ± 0.35) x 10-2.
For further analysis the whole event sample was subdivided using several topological and kinemati-cal criteria [4] into three subsamples: the cascade subsample BS with a sign of intranuclear secondary interactions, the quasiproton Bp and quasineutron Bn subsamples for which no sign of secondary interactions was observed. The corresponding weighted event numbers for BS, Bp, and Bn subsamples are 3654, 1653, and 1646, respectively. About 40% of subsample Bp is contributed by interactions with free hydrogen. Weighting the quasiproton events with a
Fig. 3. The A dependence of the ratio R(K0/n ). The lines are the result of the exponential fit.
factor of 0.6 one can compose a quasinucleon sub-sample BN = Bn + 0.6Bp and a pure nuclear sub-sample BA = BS + BN having an effective atomic weight Aegw 21 (the definition of Aeg for the composite target will be done below in Section 4). In the next sections we use the data obtained for subsamples BN and BA, as well as the published data [2] on neutrino—freon interactions (with Aeg w 45), in order to extract the A dependence of the yields of the neutral strange particles and to infer an information about the nuclear-medium influence on their production. We have verified that the mean values of the kinematical variables (quoted above) are consistent in all three data sets (at Aeff = 1, 21, and 45).
3. THE A DEPENDENCE OF THE MEAN MULTIPLICITIES AND INCLUSIVE SPECTRA
The mean multiplicities {nKo), {nA), and, for comparison, {nn-) for n- mesons, as well as the ratio R(K0/n-) = {nKo)/{nn-) are presented in Table 1 for the quasinucleon and nuclear subsamples and for neutrino—freon interactions (Aeg w 45) [2].
It should be noted that the quoted values of {nKo)n = 0.030 ± 0.007 and {na)N = 0.018 ± 0.004 for the quasinucleon subsample do not contradict the available data around Ev ~ 10 GeV obtained for vp interactions [5, 6]. The A dependence of the
data presented in Table 1 is approximated as (see Fig. 1). The fitted values of the slope parame ter (3 are: (3Ko = 0.225 ± 0.070, (3a = 0.147 ± 0.069, and (3n- = 0.068 ± 0.007. Similarly, for the combined
data on the neutral strange particles (V0 = K0 + + A) one gets (3Vo = 0.196 ± 0.049 (Fig. 2) which significantly exceeds that for n- mesons. The ratio R(V°/n-) = {nVo)/{nn-) being equal to 0.087 ± ± 0.015 for the quasinucleon interactions increases up to 0.116 ± 0.012 (i.e., by a factor of about 1.3) at Aeff w 21 and up to 0.143 ± 0.012 (i.e., by a factor of about 1.6) at Aeff w 45. Hence, the production of the neutral strange particles is influenced by the nuclear medium stronger than that for pions. A similar
n
0
V
Table 1. The mean multiplicities (nKo}, (nA}, (nn-} and the ratio R(K0/n ) at different Aeff
Aeff (nKo} (nA} (nn- } R(K 0/n-)
1 0.030 ± 0.007 0.018 ± 0.004 0.55 ± 0.01 0.055 ± 0.013
21 0.044 ± 0.006 0.030 ± 0.004 0.63 ± 0.01 0.070 ± 0.011
45 0.071 ± 0.008 0.031 ± 0.004 0.72 ± 0.01 0.099 ± 0.011
Table 2. The mean multiplicities (nVo} and (nn-} at y* > 0 and y* < 0
Aeff y* > 0 y* < 0
(nVo } (nn- } (nVo } (nn- }
1 21 45 0.024 ± 0.006 0.030 ± 0.005 0.042 ± 0.005 0.328 ± 0.011 0.308 ± 0.007 0.024 ± 0.005 0.044 ± 0.005 0.060 ± 0.006 0.224 ± 0.009 0.323 ± 0.008
Table 3. The A dependence of the experimental and calculated multiplicity gains for K0, A, and n
Aeff 5exp 5K0 5th 5K0 5Axp 5Ah 5exp n 5th n
21 0.014 ± 0.007 0.009 ± 0.002 0.012 ± 0.004 0.008 ± 0.002 0.087 ± 0.010 0.108 ± 0.023
45 0.041 ± 0.011 0.012 ± 0.002 0.013 ± 0.004 0.010 ± 0.002 0.168 ± 0.016 0.146 ± 0.031
Table 4. The A dependence of the experimental and calculated (at an increased string tension, see text) multiplicity gains for K0, A, and n-
Aeff 5exp 5K0 5th 5K0 5Axp 5Ah 5exp n 5th n
21 0.014 ± 0.007 0.020 ± 0.004 0.012 ± 0.004 0.015 ± 0.003 0.087 ± 0.010 0.112 ± 0.024
45 0.041 ± 0.011 0.025 ± 0.005 0.013 ± 0.004 0.019 ± 0.003 0.168 ± 0.016 0.151 ± 0.032
pattern was observed recently [7] in deep-inelastic neutrino—nucleus scattering (at W > 2 GeV and Q2 > 1 (GeV/c)2). As it was shown in [7], the V0 multiplicity gain 5Vo = (nVo }A — (nVo }N could be qualitatively explained in the framework of a model incorporating the secondary intranuclear interactions of produced pions, nN ^ V0X, the role of which turns out to be relatively more prominent, than that for secondary interactions nN ^ n-N which results in a n- multiplicity gain 5n- = (nn-}A — (nn- }N (see also [8]). In the next section a similar model predictions will be compared with the multiplicity gains 5Ko, 5a, and 5n- extracted from the data of Table 1.
It is expected that the particles produced in sec-
ondary interactions occupy predominantly the backward hemisphere in the hadronic c.m.s. (i.e., the region of y* < 0, y* being the particle rapidity in that system). This expectation is verified by the data on (nVo} and (nn-} for the both hemispheres (Table 2).
The data of Table 2 were approximated by an exponential dependence (Fig. 2) resulting in f3Vo(y* > > 0) = 0.147 ± 0.074, ¡3Vo (y* < 0) = 0.240 ± 0.065, 3n- (y* > 0) = —0.021 ± 0.013, and 3n- (y* < 0) = = 0.120 ± 0.016. As it is seen from Table 2 and Fig. 2, the nuclear effects induce a significant rise of the V0 multiplicity in the backward hemisphere and, to a less extent, in the forward hemisphere. On the contrary, the nuclear medium acts as an attenuator for the n- yield in the forward hemisphere, while the A
(1/N)dn/dy* 0.04 r
0.03 0.02 0.01 0
(1/N)dn/dy* 0.04 r
♦
-1
(1/N)dn/dy* 0.3-
0.2 0.1 0
* *
«b
-2
0.03 0.02 0.01
0 -1 0
(dn/dy*)/(dn/dy**-)
0.12
0.08
0.04 0
y*
»0
-1
Fig. 4. The rapidity distributions of Ko (a), A (b), n (c) and the ratio Ko/n (d) in the quasinucleon (o) and nuclear (•) subsamples.
(1/N)dn/dz
10-
10-
10-
J_I_I_I_I
9 Y»
J_I_I_I_I
0 0.2 0.4 0.6 0.8 1.0 0 0.2 0.4 0.6 0.8 1.0
z
Fig. 5. The z distributions of Ko (a) and A (b) in the quasinucleon subsample ( * ) at Aeff « 21 (•) and Aeff « 45 (o).
dependence of (nn- (y* < 0)} is significantly weaker as compared to (nVo (y* < 0)}.
A more relevant (conventional) measure of the nuclear strangeness enhancement can be inferred from a comparison of the relative yield R(K°/n-) = = (nKo}/(nn-} in the quasinucleon and nuclear
interactions. The data on R(K0/n-) presented in Table 1 and Fig. 3 exhibit a noticeable A dependence which can be described by a slope parameter 3Ko/n- = 0.157 ± 0.070.
More detailed information concerning nuclear effects in the particle yield in different domains of the phase space can be inferred from following figures.
b
a
0
1
1
d
c
0
2
1
b
a
ß*0 0.4Г
0.2
ßA
0.3
0.2
0.1
-0.21—I—'—I-'—I——I —0 1 —1—1—1-1—1-1—1
0 0.2 0.4 0.6 0 0.2 0.4 0.6
b
a
0
0
z
z
Fig. 6. The Zmin dependence of the slope parameters 3Ko (a) and 3a (b).
(1/N)dn/dpT, (GeV/c)-2
Fig. 7. The pT distributions of K0 (a) and Л (b) in the quasinucleon subsample ( * ) at Aeff ~ 21 (•) and Aeff ~ 45 (o). The lines are the result of the exponential fit for the quasinucleon subsample (dashed line) and for Aeff ~ 21 (solid line).
Figure 4 shows the rapidity distributions. The distributions for the subsample BA are shifted towards lower values of y* as compared to those for the sub-sample BN (Fig. 4). The nuclear enhancement effect is more expressed at y* < —0.3, being less significant at the midrapidity (|y*| < 0.3). For the both (quasinucleon and nuclear) subsamples the ratio R(K0/п-) tends to increase with increasing y*, being systematically higher for the nuclear subsample. A faint indication is seen that the nuclear strangeness enhancement factor Ra(K0/n-)/Rn(K0/п-) is higher in the domain y* < —0.3 overlapping with the target fragmentation region.
The distributions on the kinematical variable z = = Eh/v (Eh being the energy of K0 or A) are plotted in Fig. 5. It is seen from Fig. 5a, that the nuclear enhancement effects for the K0 yield are significant at the low-z region (z < 0.2—0.3), while for the leading K0 (z > 0.4) there is a faint indication on a nuclear attenuation. These effects can be also seen from Fig. 6a, which shows the slope parameter (3Ko for K0 mesons acquiring z > zm;n. With increasing zm;n the nuclear enhancement regime ((Ko > 0) tends to be transformed to the attenuation one ((Ko < 0). Note, that the nuclear attenuation effects for charged kaons with z > 0.2 were observed recently at higher
Fig. 8. The A dependence of the total yields of K0 (o), A (•), and n (★). The solid and dashed lines are the model predictions and the uncertainty in the latter (see text).
energies (7 <v < 23 GeV) in the deep-inelastic scattering of positrons on nuclei [9]. The data on A are less conclusive (Figs. 5b and 6b). However, they indicate that the A yield at z < 0.2 is definitely higher for the heaviest target (Aeff ^ 45).
Figure 7 shows the squared-transverse-momen-tum distributions for K0 and A (pT being measured transverse to the current direction). The approximation by an exponential form exp(—bpT) results in bN (K0) = 4.0 ± 1.4 and bN (A) = 4.1 ± ± 1.2 (GeV/c)-2 for the qusinucleon subsam-ple and bA(K0) = 4.0 ± 0.8 and bA(A) = 3.3 ± ± 0.6 (GeV/c)-2 for the nuclear subsample. These values are consistent with those obtained for neutrino-freon interactio
Для дальнейшего прочтения статьи необходимо приобрести полный текст. Статьи высылаются в формате PDF на указанную при оплате почту. Время доставки составляет менее 10 минут. Стоимость одной статьи — 150 рублей.