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Multiplicity and angular distribution of charged pions emitted following n annihilation on Fe at low energy
Nuclear Physics North-Holland
A516 (1990) 662-672
MULTIPLICITY
AND
EMIlTED
ANGULAR
DISTRIBUTION
OF CHARGED
FOLLOWING ii ANNIHILATION LOW ENERGY M. AGNELLO,
F. IAZZI
PIONS
ON Fe AT
and B. MINETTI
Dipartimento di Fisica de1 Politecnico, Torino, Italy and INFN, Sezione di Torino, Italy L. CUGUSI,
M.P. MACCIOTTA, A. MASONI, S. SERCI and M.T. SERGI
G. PUDDU,
Dipartimento di Scienze Fisiche dell’ Universita’, Cagliari, Italy and INFN, Sezione di Cagliari, Italy T. BRESSANI
and S. MARCELLO
Diparrimento di Fisica Sperimenrale dell’ Universita’, Torino, Italy and INFN, Sezione di Torino, Italy M. MORANDIN INFN, Sezione di Padova, Italy Received 5 February 1990 (Revised 9 April 1990) Abstract: The mean multiplicity and the angular distribution of the charged pions produced in the ti annihilation on Fe in the range from 12 to 140 MeV is presented. The experimental technique and methods of analysis are described. A comparison with the p results is performed, together with a discussion of some possible pion-nucleus interaction mechanisms in the above ii energy range.
E
NUCLEAR
REACTIONS
56Fe (n-bar, X), E = 12-140 MeV; multiplicity, u( 0).
measured
charged
pion
1. Introduction In the last few years the opportunities to study the antinucleon interaction with both nucleons and nuclei grew for two main parallel reasons: antiproton beams became available in many laboratories (AGS, KEK, CERN) and new ways have been explored for detecting antinucleons, which are different from the traditional bubble chamber detectors, for example streamer chambers ‘), scintillator arrays *), limited streamer tubes ‘). Also the technique for obtaining antineutron beams has been recently developed 4Y5)and some results concerning the ii total and annihilation 0375-9474/90/$03.50
@ 1990 - Elsevier
Science
Publishers
B.V. (North-Holland)
M. Agnello et al. / Multiplicity and angular distribution
cross section 6-x) and charge exchange have been produced. Concerning the ii-nucleus interaction, to investigate annihilation
the nuclear reaction
properties
has been
(CEX)
663
2*9-‘3) on both nucleons
the idea of using the antinucleon became
regarded
recently
as a source
very attractive of many
pions,
and nuclei as a probe 14) since the which
may
interact with the nucleus and give rise to different processes like evaporation of nucleons, production of strange mesons, particular cascade features. The theoretical models based on different approaches [intranuclear cascade 14), statistical model “)I have been developed mainly for p-nucleus annihilation, but the above-mentioned improvements in producing and detecting antineutrons gave rise to contributions to this kind of investigations also in the n-nucleus annihilation channel. The experimental technique used to obtain the present data is described in sect. 2, while in sects. 3 and 4 the data are reported and discussed, respectively. 2. Experimental
apparatus and techniques
The experiment ANTIN (PS178) was carried out in the S-branch of the LEAR facility at CERN. Antineutrons were produced by means of the pp+ iin charge exchange reaction (CEX) on a 15 cm long LH2 target. More details are given in ref. ‘). The A detector had a structure shown in fig. 1 and was composed of 10 modules ‘): each module consisted of a scintillator plane, 1 cm thick, a Fe plate 1 cm thick and 3
4
5
6
6
IL imited Streamer Tubes SCintillator Iron Converter
Fig. 1. Schematic
picture
of the n-detector.
9
10
M. Agnello et al. / Multiplicity and angular distribution
664
1 plane of limited streamer tube (LST) with readout of the strips along the x- and y-directions. The ii’s were identified by means of their annihilation products (mainly charged
r’s).
in the plane
The signals
of the r-tracks
on the LST’s were seen from a top view
(y, z) and a side view in the plane
crosses
correspond
permit
the reconstruction
(x, z) as shown
to the fired strips in the modules. of the annihilation
of points
vertex and also the identification
the tracks in both views: unfortunately it looks evident that there to associate each track in one view to the corresponding one (methods based on equal track length cannot be applied due inefficiencies in the tubes), so that a direct spatial reconstruction the LST hits is not possible. The information that can be inferred from the observation is: (i) the coordinates of the annihilation vertex,
Fig. 2. A typical ii-annihilation the hits on the LST, the dark
in fig. 2, where the
These two systems
of
is no general way in the other view to possible local of the tracks from
of the two LST patterns
event as appears in the graphic online monitor: the crosses strips represent the iron plate and the grey strips represent scintillators, seen from side and top view.
represent the fired
M. Agneh
et al. / ~u~t~pl~cityand angular disrribufion
665
(ii) the number of prongs in each view, (iii) the projected tracks of the prongs in the (x, z) and (y, z) plane. The ii energy was determined by measuring the time of flight (t.o.f.) spent by the ii along the path from the production in the CEX target to the annihilation point. The criteria listed below have been followed in case of mismatch in the info~atio~ from the two different views: (1) the events with different values of the vertex z-coordinate as reconstructed in the (x, z> and (y, z) plane were discarded, (2) in the events where the number of tracks seemed different in the two views [see fig. 21, the top view has been selected as more reliable since the average hit multiplicity in the (x, z) tubes is smaller than in the (y, z) tubes ‘). Of course some distortions in the prong multiplicity evaluation and in the track distribution can arise from the application of these criteria; other sources of distortion are: (a) the trigger logic chosen. for data taking (based on suitable patterns of fired scintillators in the ii detector), (b) the finite dimensions of the detector: some tracks originating in an annihilation near the detector edges can leave the useful volume without giving any hit on the LST, (c) y-rays from yr* decay: if they convert near the annihilation point, the electron shower can simulate a rr* track; energetic protons too could (rarely) simulate a pion prong. 3. Data analysis About 7200 ii annihilation events distributed in 9 energy intervals of 20 MeV from 12 to 140 MeV have been visually examined: automatic routines that have been developed for the hit analysis and were useful for other tasks 5,8) like the vertex reconstruction, were in fact unreliable for this purpose. For each energy the distribution of the number of tracks per event, referred to as multiplicity in the following, is reported in histograms looking like that in fig. 3. The finite size of the detector, spurious events like cosmic rays, and misleadingly visualized patterns (electron showers look sometimes like a prong) can distort the multiplicity distribution. To correct for the first two distortion sources, the accepted events should contain a pattern of hits on the tubes showing at least two clusters of fired tubes on a LST plane at a suitable distance in at least 3 consecutive planes: this requirement corresponds to select events with some pairs of tracks (each containing 3 or more points) forming an angle greater than 20” if they intersect inside the detector; or forming smaller angles if the tracks are intersecting outside the detector. By applying this software cut, the statistics of the events decreases to 40% of the total and the O-prong and l-prong channels of the multiplicity distributions are drastically reduced; nevertheless they are not zero since in a few cases (< 1% of the
666
M. Agnello et al. / Multiplicity and angular distribution
1
2
3
4
5
6
>
of ii at 140 MeV. The error bars indicate only the Fig. 3. Charged pion multiplicity N,, * for annihilation statistical incertitudes: no correction has been applied for the observer misinterpretation (r”) and the trigger constraints (see text).
total track track pion tracks
number of the accepted events) the fired tubes pattern showed a clean first and 1 or 2 shorter tracks (typically containing 3 points) intersecting the first inside the detector: they were interpreted like an annihilation with a charged plus 1 or 2 tracks due to y’s (satisfying the selection criteria) showers, both containing only 3 points each and intersecting inside the detector: they were
interpreted as y’s from 7r” converted after a few Fe planes. The expected fraction of these two channels (0- and l-prong) should be of the same order of the pp annihilation, i.e. around 7-8% of the total rr* multiplicity 14): the value obtained in this case is much lower (less than one order of magnitude) and therefore negligible, thus confirming that the apparatus was unsuitable to detect both 0- and l-prong events. To correct the biases in the visualized multiplicity distributions due to the abovementioned software cuts and to the observer effects, we used a detailed Monte Carlo program that simulated the ii-annihilation in the detector, taking into account all the instrumental effects (geometrical and/or electronic inefficiencies of the apparatus, spurious signals, cluster size) and using as input the pion (charged and neutral) multiplicity measured in the pp annihilation and parametrized by Iljinov et al. 14). In order to adapt the distribution to the iip distributions we assigned the charge to each pion taking into account the ratio between the number of charged and neutral pions and finally correcting for conservation. From a comparison between the initial charged pion distribution to as “true”) with that one reconstructed (fig. 4b) we observed that of the “true” multiplicity was shifted by -0.35 f 0.14 relative to the Comparing fig. 4a with fig. 3 (experimental measurements on ii-Fe) the two distributions are rather similar and then that presumably
the total
charge
(fig. 4a, referred the mean value observed value. we observe that also the “true”
h4. Agnello et al. ,I M~~ti~~ic~ty and angular distribution
667
cm40
30
20
10
0
0
1
2345678 3-r m~ltipl~c~t~
50
s 40
30
20
10 0 012345678 37 multiplicity
Fig. 4. Pion multiplicity distributions for simulated ii annihilations at 140 MeV: (a) simulated T* (white columns) and T*, 71’ (shaded columns) distribution, (b) observed ‘in*distribution.
pion multiplicity on Fe is not so different with respect to the fiN one. Then we applied the same correction term For the shift from the measured to the “true” charged multiplicity. The average charged pion multiplicities (N,,-) from the observed curve have been corrected by the above shift and reported in table 1 as a function of the energy. Statistical errors are reported in the table, while the systematic error has been estimated to be *0.17 for all the points including the contribution from the annihilations in the scintillator of the detector. The errors on the energy are due to the ii time-of-flight incertitude. In order to obtain the spatial angular dist~bution of charged pions, we examined about 2000 ~-annihiIation at the energies 37 MeV and 140 NeV by measuring the angle t$, between each pion track and the ii-direction both projected in the top view:
M. Agnelh
668
et al. / Multiplicity and angular distribution TABLE 1
Charged
pion mean multipIicity
as a function
of the ii-energy
Tii [ MeV]
(N,*)
ll.Pzt3.2 23.2k3.3 32.7 * 5.1 36.7 ;t 6.7 45.o*s.9 76.6 * 2.7 91.6rt8.2 115.5i66.8 140.0* 5.5
2.85 * 0.24 2.67 i 0.24 2.82 f 0.38 2.80 j: 0.23 2.79 f 0.24 2.90& 0.30 3.09*0.49 2.91 f 0.24 2.85 kO.21
the other view was neglected
since it is generally
impossible
to match correctly
the
planar projections in order to reconstruct every track in three dimensions, as said before. The top view has been preferred since the projected tracks are cleaner than the side view ones, for the above-mentioned reasons. Antineutrons having momentum perpendicular to the detector planes (A& ==*4O) were selected, in order to simplify
the relationship
between
the projected
and spatial
angles
of the emitted
pions. The distribution of the spatial angle B between each pion direction and the ii-direction has been calculated by means of a polynomial fit of the observed cos 8, distribution and by transforming the obtained parameters into the corresponding
h
+I 2 0.03-
.
Tji=
37
V : G
Fig. 5. Angular distribution of the charged pions from the annihilation of il at 37 MeV. White squares: this experiment; black lozenges, black squares and white lozenges: simulated distribution from mechanisms 1,2 and 3 respectively (see text, sect. 4). All the distributions are normalized to 1. Black lozenges and black squares fully overlap.
M. Agnello et al. / Multiplicity and angular distribution
h
Tyy=
+I
r
:
0.03
I
140 MeV
-
669
L
z
$ g \
0
-1
cos ( cl,? 1 Fig. 6. Angular distribution of the charged pions from the annihilation of h at 140 MeV. White squares: this experiment; black lozenges, black squares and white lozenges: simulated distribution from mechanisms 1,2 and 3 respectively (see text, sect. 4). All the distributions are normalized to 1. Black lozenges and black squares are nearly coincident.
ones in the reference system (cos 0, +), assuming that the spatial direction distribution is uniform in the azimutal 4 angle. The method gives quite good results in the range of n-momentum 100-600 MeV/ c. In fact, by comparing the simulated spatial distributions with those inferred by the projected ones with the above-mentioned reconstruction method, we estimated an error of less than *7%. We used also the simulated distributions in order to evaluate the total error due also to the observation of the projected tracks (absorbed or distorted by the visualization apparatus), and we found finally a total error of less than
*15%
on all points
of the reconstructed
distributions
shown
in figs. 5 and 6.
4. Discussion and conclusions The reconstructed cos 8 distributions (in the laboratory frame) for 37 and 140 MeV ii’s are reported in figs. 5 and 6. They show a similar behavior for both momenta: an evident increase of the number of tracks in the forward direction, a slight depletion around the cos 8 = 0 direction and a slight raise in the backward direction. The depletion is more remarkable in the 37 MeV curve. Such a forward peaked behavior recalls the rr momentum distribution for a free NN annihilation, without any further interaction of the outgoing pions with the nuclei. On the other hand, the mean values of the multiplicity (somehow lower than the elementary RN annihilation) seem to indicate that some absorption mechanism occurs inside the nucleus.
670
M. Agnello et al. / Multiplicity
and angular distribution
Three different simulations based on different assumptions order to get a first rough check of such a mechanism: (1) elementary
ii+ N+ MT annihilation
without
have been
rrN interaction
done,
in
(figs. 5 and 6,
black lozenges); this mechanism has been simulated following the phase-space momentum distribution for the emitted pions. The target nucleon N is a proton or a neutron with a probability p = Z/A or l-Z/A, respectively, with Z = 26 and A = 56 (Fe nucleus). The n* multiplicities are chosen as in ref. 14). (2) elementary annihilation ii+ N +=mr, with AJ3 formation (figs. 5 and 6, white squares); this mechanism has been simulated as at the point (l), but including the formation of the AJ3 resonance for all pions whose momentum lies between 200 and 400 MeV/c (zero-range hypothesis for A,, formation ‘“)); about f of the pions interact with nucleons producing a AS3. (3) annihilation onto coherent residual nucleus ii + AFe + rnr + (A-1)X (figs. 5 and 6, white lozenges); this mechanism has been simulated with the phase-space distribution of the m final pions and the (A - 1) nucleus, with the same hypothesis of multiplicity as in (1) and (2). This kind of reaction should represent a coherent interaction of the incoming ri and the whole target nucleus, through multiple TN and NN interaction, producing a final nucleus (A - 1) in some highly excited state (this excitation energy has not been taken into account, since it is negligible for the phase-space simulation purposes). Other annihilation mechanisms involving less TN and NN interactions are possible (and more probable) but this case can be assumed as quite representative from the point of view of the direction distribution, since a greater number of outgoing particles (nucleons) contribute to a flatter angular distribution. From figs. 5 and 6, one can see that, first, the A,, formarion does not affect appreciably the angular distribution of the elementary annihilation and, second, the experimental points are closer to the distribution obtained from the mechanisms (1) and (2): they agree quite well in the forward region, while some discrepancy can be seen in the range -1 < cos 19< 0 (although this region is the most affected by the local detector inefficiency). In any case the experimental curve is far from the flat behavior of the coherent mechanism simulation for both n momenta: one could deduce that (a) a free ii+ N + mn- mechanism is dominant in the ii-Fe annihilation and (b) a partial (i.e. 5) rfi interaction via A,, resonance is not in conflict with the angular distribution of the emitted pions. These remarks have already been pointed out in the literature 17) where sophisticated models (of the INC family) were developed to explain the n-nucleus interaction: they agree with a picture in which the main part of the absorption occurs via r + N + A, A + N -+ NN” steps. Looking at figs. 5 and 6 in more detail, the depletion that can be seen in the -1 < cos 0 < 0 region (more evident at 280 MeV/c) has no corresponding trend in any simulated curve: although this region is critical due to the detector inefficiency, nevertheless some confidence could be given to these points since a similar behavior
M. Agnello et al. / Multiplicity and angular distribution
has been different
observed apparatus.
in p - Ne annihilation A transparency
at 607 MeV/c
of nuclear
671
[ref. ‘“)I with
matter to pions emitted
a totally
at an energy
higher than that corresponding to the As3 seems to be a satisfactory explanation “) which is in agreement with the frame of pions non-interacting with nucleons above the Ax3 resonance. Concerning the mean r* multiplicity (table l), one can observe that: (a) the prong multiplicity distributions at the various n-energies look very similar in shape to each other and to the elementary RN multiplicity distribution 14) (this last comparison holds only for multiplicities ~2) (b) no correlation with the ii-momentum is evident. (c) the (constant) value (-2.85) is slightly lower than that one of the elementary pp annihilation at rest [ref. ‘*) and experiments quoted therein]. On the other hand, it is remarkable that such a value is generally larger than that obtained in pexperiments with different nuclei. Also for this measurement the large error bars don’t allow a very detailed analysis, but the independence of the produced n’s upon the incident energy in the iiannihilations seems evident as well as it seems confirmed that the absorption mechanism (if it exists) affects very slightly the pion emission from the Fe nucleus. This last statement is supported by the good agreement of the mean multiplicity value (~2.85) in all the present energy range with the prediction of the INC model in ref. I’), among whose hypotheses also a not large contribution of the absorption mechanism is included. Concluding, we present a complete set of the n* multiplicity measurements of the ii-annihilation in Fe in the range, 12-140 MeV together with the charged pion angular distribution at two n-energies. Both measurements indicate a negligible dependence of the T-residual nucleus interaction upon the ii-energy and are consistent with models assuming that the rr-absorption mechanism inside the nucleus is small and that the formation of the A33 resonance plays the main role in this absorption. Previous results on the Fe cross sections “) indicated a dominant surface mechanism
of the iiN annihilation
and this information
too seems consistent
with
the above picture of the rr-nucleus interaction. The present data are affected by quite large error bars and are limited to two observables (multiplicity and angular distribution of the emitted charged pions): both limitations are due to the non-dedicated detector apparatus, which was designed for other purposes 19). By means of a detector of totally different design like that one of the PS201 experiment at LEAR (CERN) 20) the multiplicity measurements could include the neutral pions, and the angular distributions could include the pion momentum spectra for several nuclear targets. These goals are included in the physics program of the experiment: hopefully from those data more precise and quantitative information about the amount of the rr-nucleus interaction and the role of the A33 in the ii-nucleus annihilation should be deduced.
M. Agnello
672
The authors the manuscript enlightening
thank and
et al. / Multiplicity and angular distribution
Prof. J. Cugnon Prof.
R. Baldini
(UniversitC Ferroli
de Likge) for a critical
(Laboratori
Nazionali
reading
Frascati)
of for
suggestions.
References F. Bale&a et al., Nucl. Instr. Meth. A257 (1987) 114 W. Bruekner et al., Phys. Lett. Bl58 (1985) 180 T. Bressani ef al., I.E.E.E. Trans. Nucl. Science, NS-32 (1985) 733. T. Brando et al., Nucl. Instr. Meth. 180 (1981) 461 L. Cugusi et al., Nucl. Instr. Meth. A270 (1988) 354 T. Armstrong et al., Phys. Rev. D36 (1987) 659 B. Gunderson et al., Phys. Rev. D23 (1981) 587 M. Agnello er al., Europhys. Lett. 7(l) (1988) 13 T. Bressani et al., Europhys. Lett. 2 (8) (1986) 587 K. Nakamura et al., Phys. Rev. C31 (1985) 1853 T. Tsuboyama et al., Phys. Rev. D9 (1983) 2135 W. Brueckner et al., Phys. Lett. B169 (1986) 302 K. Nakamura et al., Phys. Rev. Lett. 53 (1984) 885 A.S. Iljinov et al., Nucl. Phys. A382 (1982) 378 J. Cugnon et al., Europhys. Lett. 4 (5) (1987) 535 J. Rafelski, Phys. Lett. B91 (1980) 281 J. Cugnon et al., Nucl. Phys. A500 (1989) 701 F. Balestra et al., Nucl. Phys. A491 (1989) 541 T. Bressani et al., in Fundamental interaction in low energy systems, ed P. Dalpiaz, and G. Torelli (Plenum, New York, 1985) p. 329 20) R. Armenteros et al., Obelix Proposal, CERN report/PSCC/86-4 (1986) 1) 2) 3) 4) 5) 6) 7) 8) 9) 10) 11) 12) 13) 14) 15) 16) 17) 18) 19)
G. Fiorentini
A516 (1990) 662-672
MULTIPLICITY
AND
EMIlTED
ANGULAR
DISTRIBUTION
OF CHARGED
FOLLOWING ii ANNIHILATION LOW ENERGY M. AGNELLO,
F. IAZZI
PIONS
ON Fe AT
and B. MINETTI
Dipartimento di Fisica de1 Politecnico, Torino, Italy and INFN, Sezione di Torino, Italy L. CUGUSI,
M.P. MACCIOTTA, A. MASONI, S. SERCI and M.T. SERGI
G. PUDDU,
Dipartimento di Scienze Fisiche dell’ Universita’, Cagliari, Italy and INFN, Sezione di Cagliari, Italy T. BRESSANI
and S. MARCELLO
Diparrimento di Fisica Sperimenrale dell’ Universita’, Torino, Italy and INFN, Sezione di Torino, Italy M. MORANDIN INFN, Sezione di Padova, Italy Received 5 February 1990 (Revised 9 April 1990) Abstract: The mean multiplicity and the angular distribution of the charged pions produced in the ti annihilation on Fe in the range from 12 to 140 MeV is presented. The experimental technique and methods of analysis are described. A comparison with the p results is performed, together with a discussion of some possible pion-nucleus interaction mechanisms in the above ii energy range.
E
NUCLEAR
REACTIONS
56Fe (n-bar, X), E = 12-140 MeV; multiplicity, u( 0).
measured
charged
pion
1. Introduction In the last few years the opportunities to study the antinucleon interaction with both nucleons and nuclei grew for two main parallel reasons: antiproton beams became available in many laboratories (AGS, KEK, CERN) and new ways have been explored for detecting antinucleons, which are different from the traditional bubble chamber detectors, for example streamer chambers ‘), scintillator arrays *), limited streamer tubes ‘). Also the technique for obtaining antineutron beams has been recently developed 4Y5)and some results concerning the ii total and annihilation 0375-9474/90/$03.50
@ 1990 - Elsevier
Science
Publishers
B.V. (North-Holland)
M. Agnello et al. / Multiplicity and angular distribution
cross section 6-x) and charge exchange have been produced. Concerning the ii-nucleus interaction, to investigate annihilation
the nuclear reaction
properties
has been
(CEX)
663
2*9-‘3) on both nucleons
the idea of using the antinucleon became
regarded
recently
as a source
very attractive of many
pions,
and nuclei as a probe 14) since the which
may
interact with the nucleus and give rise to different processes like evaporation of nucleons, production of strange mesons, particular cascade features. The theoretical models based on different approaches [intranuclear cascade 14), statistical model “)I have been developed mainly for p-nucleus annihilation, but the above-mentioned improvements in producing and detecting antineutrons gave rise to contributions to this kind of investigations also in the n-nucleus annihilation channel. The experimental technique used to obtain the present data is described in sect. 2, while in sects. 3 and 4 the data are reported and discussed, respectively. 2. Experimental
apparatus and techniques
The experiment ANTIN (PS178) was carried out in the S-branch of the LEAR facility at CERN. Antineutrons were produced by means of the pp+ iin charge exchange reaction (CEX) on a 15 cm long LH2 target. More details are given in ref. ‘). The A detector had a structure shown in fig. 1 and was composed of 10 modules ‘): each module consisted of a scintillator plane, 1 cm thick, a Fe plate 1 cm thick and 3
4
5
6
6
IL imited Streamer Tubes SCintillator Iron Converter
Fig. 1. Schematic
picture
of the n-detector.
9
10
M. Agnello et al. / Multiplicity and angular distribution
664
1 plane of limited streamer tube (LST) with readout of the strips along the x- and y-directions. The ii’s were identified by means of their annihilation products (mainly charged
r’s).
in the plane
The signals
of the r-tracks
on the LST’s were seen from a top view
(y, z) and a side view in the plane
crosses
correspond
permit
the reconstruction
(x, z) as shown
to the fired strips in the modules. of the annihilation
of points
vertex and also the identification
the tracks in both views: unfortunately it looks evident that there to associate each track in one view to the corresponding one (methods based on equal track length cannot be applied due inefficiencies in the tubes), so that a direct spatial reconstruction the LST hits is not possible. The information that can be inferred from the observation is: (i) the coordinates of the annihilation vertex,
Fig. 2. A typical ii-annihilation the hits on the LST, the dark
in fig. 2, where the
These two systems
of
is no general way in the other view to possible local of the tracks from
of the two LST patterns
event as appears in the graphic online monitor: the crosses strips represent the iron plate and the grey strips represent scintillators, seen from side and top view.
represent the fired
M. Agneh
et al. / ~u~t~pl~cityand angular disrribufion
665
(ii) the number of prongs in each view, (iii) the projected tracks of the prongs in the (x, z) and (y, z) plane. The ii energy was determined by measuring the time of flight (t.o.f.) spent by the ii along the path from the production in the CEX target to the annihilation point. The criteria listed below have been followed in case of mismatch in the info~atio~ from the two different views: (1) the events with different values of the vertex z-coordinate as reconstructed in the (x, z> and (y, z) plane were discarded, (2) in the events where the number of tracks seemed different in the two views [see fig. 21, the top view has been selected as more reliable since the average hit multiplicity in the (x, z) tubes is smaller than in the (y, z) tubes ‘). Of course some distortions in the prong multiplicity evaluation and in the track distribution can arise from the application of these criteria; other sources of distortion are: (a) the trigger logic chosen. for data taking (based on suitable patterns of fired scintillators in the ii detector), (b) the finite dimensions of the detector: some tracks originating in an annihilation near the detector edges can leave the useful volume without giving any hit on the LST, (c) y-rays from yr* decay: if they convert near the annihilation point, the electron shower can simulate a rr* track; energetic protons too could (rarely) simulate a pion prong. 3. Data analysis About 7200 ii annihilation events distributed in 9 energy intervals of 20 MeV from 12 to 140 MeV have been visually examined: automatic routines that have been developed for the hit analysis and were useful for other tasks 5,8) like the vertex reconstruction, were in fact unreliable for this purpose. For each energy the distribution of the number of tracks per event, referred to as multiplicity in the following, is reported in histograms looking like that in fig. 3. The finite size of the detector, spurious events like cosmic rays, and misleadingly visualized patterns (electron showers look sometimes like a prong) can distort the multiplicity distribution. To correct for the first two distortion sources, the accepted events should contain a pattern of hits on the tubes showing at least two clusters of fired tubes on a LST plane at a suitable distance in at least 3 consecutive planes: this requirement corresponds to select events with some pairs of tracks (each containing 3 or more points) forming an angle greater than 20” if they intersect inside the detector; or forming smaller angles if the tracks are intersecting outside the detector. By applying this software cut, the statistics of the events decreases to 40% of the total and the O-prong and l-prong channels of the multiplicity distributions are drastically reduced; nevertheless they are not zero since in a few cases (< 1% of the
666
M. Agnello et al. / Multiplicity and angular distribution
1
2
3
4
5
6
>
of ii at 140 MeV. The error bars indicate only the Fig. 3. Charged pion multiplicity N,, * for annihilation statistical incertitudes: no correction has been applied for the observer misinterpretation (r”) and the trigger constraints (see text).
total track track pion tracks
number of the accepted events) the fired tubes pattern showed a clean first and 1 or 2 shorter tracks (typically containing 3 points) intersecting the first inside the detector: they were interpreted like an annihilation with a charged plus 1 or 2 tracks due to y’s (satisfying the selection criteria) showers, both containing only 3 points each and intersecting inside the detector: they were
interpreted as y’s from 7r” converted after a few Fe planes. The expected fraction of these two channels (0- and l-prong) should be of the same order of the pp annihilation, i.e. around 7-8% of the total rr* multiplicity 14): the value obtained in this case is much lower (less than one order of magnitude) and therefore negligible, thus confirming that the apparatus was unsuitable to detect both 0- and l-prong events. To correct the biases in the visualized multiplicity distributions due to the abovementioned software cuts and to the observer effects, we used a detailed Monte Carlo program that simulated the ii-annihilation in the detector, taking into account all the instrumental effects (geometrical and/or electronic inefficiencies of the apparatus, spurious signals, cluster size) and using as input the pion (charged and neutral) multiplicity measured in the pp annihilation and parametrized by Iljinov et al. 14). In order to adapt the distribution to the iip distributions we assigned the charge to each pion taking into account the ratio between the number of charged and neutral pions and finally correcting for conservation. From a comparison between the initial charged pion distribution to as “true”) with that one reconstructed (fig. 4b) we observed that of the “true” multiplicity was shifted by -0.35 f 0.14 relative to the Comparing fig. 4a with fig. 3 (experimental measurements on ii-Fe) the two distributions are rather similar and then that presumably
the total
charge
(fig. 4a, referred the mean value observed value. we observe that also the “true”
h4. Agnello et al. ,I M~~ti~~ic~ty and angular distribution
667
cm40
30
20
10
0
0
1
2345678 3-r m~ltipl~c~t~
50
s 40
30
20
10 0 012345678 37 multiplicity
Fig. 4. Pion multiplicity distributions for simulated ii annihilations at 140 MeV: (a) simulated T* (white columns) and T*, 71’ (shaded columns) distribution, (b) observed ‘in*distribution.
pion multiplicity on Fe is not so different with respect to the fiN one. Then we applied the same correction term For the shift from the measured to the “true” charged multiplicity. The average charged pion multiplicities (N,,-) from the observed curve have been corrected by the above shift and reported in table 1 as a function of the energy. Statistical errors are reported in the table, while the systematic error has been estimated to be *0.17 for all the points including the contribution from the annihilations in the scintillator of the detector. The errors on the energy are due to the ii time-of-flight incertitude. In order to obtain the spatial angular dist~bution of charged pions, we examined about 2000 ~-annihiIation at the energies 37 MeV and 140 NeV by measuring the angle t$, between each pion track and the ii-direction both projected in the top view:
M. Agnelh
668
et al. / Multiplicity and angular distribution TABLE 1
Charged
pion mean multipIicity
as a function
of the ii-energy
Tii [ MeV]
(N,*)
ll.Pzt3.2 23.2k3.3 32.7 * 5.1 36.7 ;t 6.7 45.o*s.9 76.6 * 2.7 91.6rt8.2 115.5i66.8 140.0* 5.5
2.85 * 0.24 2.67 i 0.24 2.82 f 0.38 2.80 j: 0.23 2.79 f 0.24 2.90& 0.30 3.09*0.49 2.91 f 0.24 2.85 kO.21
the other view was neglected
since it is generally
impossible
to match correctly
the
planar projections in order to reconstruct every track in three dimensions, as said before. The top view has been preferred since the projected tracks are cleaner than the side view ones, for the above-mentioned reasons. Antineutrons having momentum perpendicular to the detector planes (A& ==*4O) were selected, in order to simplify
the relationship
between
the projected
and spatial
angles
of the emitted
pions. The distribution of the spatial angle B between each pion direction and the ii-direction has been calculated by means of a polynomial fit of the observed cos 8, distribution and by transforming the obtained parameters into the corresponding
h
+I 2 0.03-
.
Tji=
37
V : G
Fig. 5. Angular distribution of the charged pions from the annihilation of il at 37 MeV. White squares: this experiment; black lozenges, black squares and white lozenges: simulated distribution from mechanisms 1,2 and 3 respectively (see text, sect. 4). All the distributions are normalized to 1. Black lozenges and black squares fully overlap.
M. Agnello et al. / Multiplicity and angular distribution
h
Tyy=
+I
r
:
0.03
I
140 MeV
-
669
L
z
$ g \
0
-1
cos ( cl,? 1 Fig. 6. Angular distribution of the charged pions from the annihilation of h at 140 MeV. White squares: this experiment; black lozenges, black squares and white lozenges: simulated distribution from mechanisms 1,2 and 3 respectively (see text, sect. 4). All the distributions are normalized to 1. Black lozenges and black squares are nearly coincident.
ones in the reference system (cos 0, +), assuming that the spatial direction distribution is uniform in the azimutal 4 angle. The method gives quite good results in the range of n-momentum 100-600 MeV/ c. In fact, by comparing the simulated spatial distributions with those inferred by the projected ones with the above-mentioned reconstruction method, we estimated an error of less than *7%. We used also the simulated distributions in order to evaluate the total error due also to the observation of the projected tracks (absorbed or distorted by the visualization apparatus), and we found finally a total error of less than
*15%
on all points
of the reconstructed
distributions
shown
in figs. 5 and 6.
4. Discussion and conclusions The reconstructed cos 8 distributions (in the laboratory frame) for 37 and 140 MeV ii’s are reported in figs. 5 and 6. They show a similar behavior for both momenta: an evident increase of the number of tracks in the forward direction, a slight depletion around the cos 8 = 0 direction and a slight raise in the backward direction. The depletion is more remarkable in the 37 MeV curve. Such a forward peaked behavior recalls the rr momentum distribution for a free NN annihilation, without any further interaction of the outgoing pions with the nuclei. On the other hand, the mean values of the multiplicity (somehow lower than the elementary RN annihilation) seem to indicate that some absorption mechanism occurs inside the nucleus.
670
M. Agnello et al. / Multiplicity
and angular distribution
Three different simulations based on different assumptions order to get a first rough check of such a mechanism: (1) elementary
ii+ N+ MT annihilation
without
have been
rrN interaction
done,
in
(figs. 5 and 6,
black lozenges); this mechanism has been simulated following the phase-space momentum distribution for the emitted pions. The target nucleon N is a proton or a neutron with a probability p = Z/A or l-Z/A, respectively, with Z = 26 and A = 56 (Fe nucleus). The n* multiplicities are chosen as in ref. 14). (2) elementary annihilation ii+ N +=mr, with AJ3 formation (figs. 5 and 6, white squares); this mechanism has been simulated as at the point (l), but including the formation of the AJ3 resonance for all pions whose momentum lies between 200 and 400 MeV/c (zero-range hypothesis for A,, formation ‘“)); about f of the pions interact with nucleons producing a AS3. (3) annihilation onto coherent residual nucleus ii + AFe + rnr + (A-1)X (figs. 5 and 6, white lozenges); this mechanism has been simulated with the phase-space distribution of the m final pions and the (A - 1) nucleus, with the same hypothesis of multiplicity as in (1) and (2). This kind of reaction should represent a coherent interaction of the incoming ri and the whole target nucleus, through multiple TN and NN interaction, producing a final nucleus (A - 1) in some highly excited state (this excitation energy has not been taken into account, since it is negligible for the phase-space simulation purposes). Other annihilation mechanisms involving less TN and NN interactions are possible (and more probable) but this case can be assumed as quite representative from the point of view of the direction distribution, since a greater number of outgoing particles (nucleons) contribute to a flatter angular distribution. From figs. 5 and 6, one can see that, first, the A,, formarion does not affect appreciably the angular distribution of the elementary annihilation and, second, the experimental points are closer to the distribution obtained from the mechanisms (1) and (2): they agree quite well in the forward region, while some discrepancy can be seen in the range -1 < cos 19< 0 (although this region is the most affected by the local detector inefficiency). In any case the experimental curve is far from the flat behavior of the coherent mechanism simulation for both n momenta: one could deduce that (a) a free ii+ N + mn- mechanism is dominant in the ii-Fe annihilation and (b) a partial (i.e. 5) rfi interaction via A,, resonance is not in conflict with the angular distribution of the emitted pions. These remarks have already been pointed out in the literature 17) where sophisticated models (of the INC family) were developed to explain the n-nucleus interaction: they agree with a picture in which the main part of the absorption occurs via r + N + A, A + N -+ NN” steps. Looking at figs. 5 and 6 in more detail, the depletion that can be seen in the -1 < cos 0 < 0 region (more evident at 280 MeV/c) has no corresponding trend in any simulated curve: although this region is critical due to the detector inefficiency, nevertheless some confidence could be given to these points since a similar behavior
M. Agnello et al. / Multiplicity and angular distribution
has been different
observed apparatus.
in p - Ne annihilation A transparency
at 607 MeV/c
of nuclear
671
[ref. ‘“)I with
matter to pions emitted
a totally
at an energy
higher than that corresponding to the As3 seems to be a satisfactory explanation “) which is in agreement with the frame of pions non-interacting with nucleons above the Ax3 resonance. Concerning the mean r* multiplicity (table l), one can observe that: (a) the prong multiplicity distributions at the various n-energies look very similar in shape to each other and to the elementary RN multiplicity distribution 14) (this last comparison holds only for multiplicities ~2) (b) no correlation with the ii-momentum is evident. (c) the (constant) value (-2.85) is slightly lower than that one of the elementary pp annihilation at rest [ref. ‘*) and experiments quoted therein]. On the other hand, it is remarkable that such a value is generally larger than that obtained in pexperiments with different nuclei. Also for this measurement the large error bars don’t allow a very detailed analysis, but the independence of the produced n’s upon the incident energy in the iiannihilations seems evident as well as it seems confirmed that the absorption mechanism (if it exists) affects very slightly the pion emission from the Fe nucleus. This last statement is supported by the good agreement of the mean multiplicity value (~2.85) in all the present energy range with the prediction of the INC model in ref. I’), among whose hypotheses also a not large contribution of the absorption mechanism is included. Concluding, we present a complete set of the n* multiplicity measurements of the ii-annihilation in Fe in the range, 12-140 MeV together with the charged pion angular distribution at two n-energies. Both measurements indicate a negligible dependence of the T-residual nucleus interaction upon the ii-energy and are consistent with models assuming that the rr-absorption mechanism inside the nucleus is small and that the formation of the A33 resonance plays the main role in this absorption. Previous results on the Fe cross sections “) indicated a dominant surface mechanism
of the iiN annihilation
and this information
too seems consistent
with
the above picture of the rr-nucleus interaction. The present data are affected by quite large error bars and are limited to two observables (multiplicity and angular distribution of the emitted charged pions): both limitations are due to the non-dedicated detector apparatus, which was designed for other purposes 19). By means of a detector of totally different design like that one of the PS201 experiment at LEAR (CERN) 20) the multiplicity measurements could include the neutral pions, and the angular distributions could include the pion momentum spectra for several nuclear targets. These goals are included in the physics program of the experiment: hopefully from those data more precise and quantitative information about the amount of the rr-nucleus interaction and the role of the A33 in the ii-nucleus annihilation should be deduced.
M. Agnello
672
The authors the manuscript enlightening
thank and
et al. / Multiplicity and angular distribution
Prof. J. Cugnon Prof.
R. Baldini
(UniversitC Ferroli
de Likge) for a critical
(Laboratori
Nazionali
reading
Frascati)
of for
suggestions.
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