This research was sponsored by the National Science Foundation Grant NSF PHY-931478

Fields, D.E.

Meyer, H.O.

Nann, H.

Pancella, P.

Pate, S.

Pollock, R.E.

Rinckel, T.

Ross, A.

Sperisen, F.

von Przewoski, B.

English

IUScholarWorks

PION PRODUCTION 
TOTAL CROSS SECTION FOR p+p -+ p+p+aO CLOSE TO THRESHOLD 
H.O. Meyer, D. E. Fields, H. Nann, P. Pancella, S. Pate, R. E. Pollock, 
B. v. Przewoski, T. Rinckel, A. Ross, and F. Sperisen 
Indiana University Cyclotron Facility, Bloomington, Indiana 4 74 08 
During the year covered by this report, the first nuclear physics experiment with 
the Indiana Cooler (CEO1) has been brought to a conclusion and an account of it has 
been published.' The measurement has resulted in pp+ppaO total cross section data of 
unprecedented precision at energies much closer to threshold than those covered by previous 
e ~ ~ e r i m e n t s . ~ ' ~  Its results are shown in Fig. 1. 
Pion production in the NN system is of fundamental importance. The threshold region 
is particularly interesting, since at sufficiently low energy the cross section is dominated 
by the single partial wave 3Po -+ ( 'So~)O. In addition, theoretical arguments limit the 
possible mechanisms for pion production. Thus, near-threshold production provides a 
crucial testing ground for models of non-resonant pion production. 
The energy dependence of the total NN+NNr  cross section, close to threshold, is 
expected to be determined by the density of final states. In addition, if the outgoing nu- 
cleons are in a S1 (T=O) or a ' So (T=l) state, the NN final-state interaction is important. 
The CEOl data' are consistent with an energy dependence predicted from the phase-space 
factor and a final-state interaction expressed in terms of an effective range expansion which 
includes the Coulomb interaction, as shown by the curve in Fig. 1. This calculation has 
been normalized to fit the data. 
I " " I " "  
PP'PP"~ 
CEOl 
- 
Figure 1. Total cross section for 
pp+ppaO divided by 112 as a func- 
tion of 11 (7 is the dimensionless 
maximum pion momentum; it is 
simply related to the bombarding 
energy). Data from two previous 
experiment s2'3 are indicated by 
squares and horizontal bars. The 
solid dots are the results' of the 
first Cooler experiment (CEO1). 
The curve is described in the text. 
The energy region covered in CEOl (282.5 MeV - 325.0 MeV) contains the thresholds 
for the channels pp+dn+ (Tp=287.5 MeV) and pp+pnn+ ( ~ ~ = 2 9 2 . 3  MeV). The fact that 
the cross section measured by CEOl at 7=0.289 (T,=292.4 MeV) is somewhat low, raised 
the suspicion that this could be caused by the coupling between channels. It was thus 
decided to follow up on CEOl and to repeat the pp-+ppno experiment in smaller energy 
steps and with improved statistics. The new experiment (labelled CE23) was carried out 
during 6 days in December 1990. 
The target consisted of a gas jet of 5 . 1 0 ~ ~  hydrogen molecules/second, emerging from 
a 0.11 mm diameter nozzle, cooled to 40 OK. It was the same as for CEO1, except for larger 
openings in the differential pumping apertures, needed to accommodate the increased beta 
functions ,8,=1.84 m, ,By=2.04 m (CEO1: ,8,=0.51 m, ,8,=1.0'3 m) at the G section waist. 
The change in beta functions was caused by the use of a new tune designed to yield a larger 
machine acceptance. The target thickness was about 7.10'~ atoms/cm2. About 40% of 
the total target thickness was uniformly distributed along k8.2 cm of the beam path on 
either side of the jet. The target was mounted in the downstream position of the CEOl 
target box. 
Except for some minor improvements, the detector system was the same as for CEO1. 
The detector positions are variable relative to the target. Three settings with angular 
acceptances of 11.6') 15.6" and 20.4") respectively, were used, corresponding to three 
ranges of bombarding energies. As explained elsewhere,' the detector setup was capable 
of measuring the direction and the energy of outgoing charged particles falling within 
its acceptance. In addition, recoil protons from pp scattering were detected by position- 
sensitive silicon detectors mounted in the midplane on either side of the target. The 
concurrent observation of pp scattering provided the normalization needed to determine 
an absolute p p t p p x o  cross section (see below). 
The trigger for a ppn" event was based on the observed scintillator pattern. This 
was also true for pp scattering events for which, in addition, a coincidence with one of the 
position-sensitive detectors was required. 
Data were acquired in so-called cycles. Each cycle lasted about, 30 s and consisted 
of injection of 45 MeV protons from dissociating a 90 MeV H: beam on a thin carbon 
stripper foil, acceleration to the final energy, data acquisition while cooling (8-17 s), and 
restoration of initial conditions. The high voltage to the wire chambers and the flow of gas 
through the target nozzle were turned on only during the data acquisition phase. During 
data taking, the RF cavity was run at a constant frequency, whose value, together with 
the known ring circumference, determined the beam energy. 
During the course of the CE23 experiment, about 100 individual runs at 31 different 
bombarding energies between 278 and 325 MeV were carried out. Stored beam currents 
ranged from 50 to 250 PA, resulting in luminosities, averaged over the whole duration of 
a run, between 5 lo2' and 3 . lo3' ~ m - ~ s - l .  During a total time of 52 h (CEO1: 110 h) 
for production runs, a total integrated luminosity of about 170 nb-' (CEO1: 16 nb-l) was 
achieved. This represents about a factor of 20 improvement over the CEOl performance. 
Some of the machine parameters are not yet sufficiently well controlled to guarantee 
consistent optimum performance. This explains the large difference between the smallest 
and largest luminosity seen. In Fig. 2 some relevant parameters are shown as a function of 
the run number, i.e., the time through the experiment. They include the luminosity (from 
the pp coincidence rate), the beam current (measured with a beam position monitor), the 
beam energy spread (obtained from the time between the RF and the arrival scattered 
protons in the first scintillator element of the detector stack), and the beam lifetime (from 
the time dependence of the event rate). 
Considerable effort went into the offline analysis of the data. The main improvements 
included refined matching criteria of the reconstructed track with the firing scintillator 
elements, recovery of abnormal firing patterns of the wire chambers, and a tracking al- 
gorithm that enhanced the angular resolution of the detector system and improved the 
missing-mass resolution by about a factor of two. 
For an internal target in a stored beam it is difficult to measure the beam current and 
the target thickness with sufficient accuracy to derive a luminosity. Instead, the integrated 
luminosity was obtained from the number of pp scattering events, observed concurrently 
r u n  
Fzgure 2. The luminosity, the stored beam current, the beam energy spread, and the beam 
lifetime recorded during the duration of the CE23 experiment. 
with pion production, using pp scattering cross sections reported in the literature. For this 
purpose, the luminosity was derived from the factor needed to scale a model calculation of 
the distribution of scattering centers along the beam axis to the corresponding measured 
distribution. As a check, a second method was devised to derive the luminosity. It made 
use of a run with a diffuse target (gas flowing into the target chamber, no jet) to assess the 
contribution from the target gas outside the jet. Comparison of the two methods led to 
the discovery of an error in the analysis of the CEOl data. The published data1 must be 
- 
- - 
corrected downwards by about a standard deviation. This correction has been applied to 
the CEOl data shown in Fig. 1. With this error eliminated, the two methods to determine 
the luminosity agreed within a few percent. 
- 
The detectors must accommodate the beam pipe, resulting in a dead region around 
the beam axis. A Monte-Carlo simulation of the detector was used to calculate the fraction 
ppnO events that were not seen because of this. For the lowest energy with a given detector 
setting the required correction could be as much as a factor of two, but it was known to 
within a few percent. Other adjustments due to systematic effects are small (less than 
10% total). Measurements at the same energy and with the same detector geometry were 
found to be internally consistent and were thus combined. 
The resulting total cross sections are shown in Fig. 3. Different symbols (crosses, 
circles, and diamonds) are used to identify the three different geometries. The errors 
Figure 3. Total cross section for pp--+pp.rro divided by 112 as a function of 11 (same as 
Fig. 1). Instead of the CEOl data, the results from the follow-up experiment CE23 are 
shown (crosses, circles, and diamonds). The curves in Figs. 1 and 3 are identical. 
shown include counting statistics, the uncertainty of the luminosity, and errors associated 
with the adjustments mentioned above. An overall normalization error of about 5% due 
to the uncertainty of the pp scattering cross section is not included. The data with large 
errors (squares and horizontal bars) are from two previous e ~ ~ e r i m e n t s . ~ ' ~  The curve shown 
represents the energy dependence of the total cross section deduced from the phase space 
factor and the final-st ate interaction between the out going protons. In this calculation, 
the matrix element is assumed to be energy-independent and its magnitude is adjusted to 
fit the data. The curves in Figs. 1 and 3 are identical. 
The stated goal of this experiment was to investigate the possibility of coupling be- 
tween pion production channels in the NN system. On the level of about 5% = deviation 
from a smooth energy dependence (or 'cusp') of the pp+pp.rrO total cross section was 
observed. A full report of this experiment for publication is currently in preparation. 
1. H.O. Meyer, M.A. Ross, R.E. Pollock, A. Berdoz, F. Dohrmann, J.E. Goodwin, M.G. 
Minty, H. Nann, P.V. Pancella, S.F. Pate, B. v. Przewoski, T. Rinckel, and F. Sperisen, 
Phys. Rev. Lett. 65, 2846 (1990). 
2. A.F. Dunaitsev and Yu.D. Prokoshkin, Zh. Eksp. Teor. Fiz. 36, 1656 (1959) [Sov. 
Phys. JETP 9, 1179 (1959)l. 
3. S. Stanislaus, D. Horvath, D.F. Measday, and A.J. Noble, Phys. Rev. C41, R1913 
(1990). 


Total Cross Section for p+p → p+p+pi0 Close to Threshold

https://scholarworks.iu.edu/dspace/bitstream/2022/655/1/iucf1990-1991_47-51.pdf

Total Cross Section for p+p → p+p+pi0 Close to Threshold

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