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

Berg, G.

Betker, A.

Bowyer, T.

Cameron, J.

Jacobs, W.

Keith, C.

Nann, H.

Peterson, T.

Pitts, W.K.

Shao, J.

Spraker, M.

Szymanski, J.

Vigdor, S.

Warman, K.

English

IUScholarWorks

PIONIUM PRODUCTION IN THE COOLER 
A. Betker, T. Bowyer, J. Cameron, G. Berg, W. Jacobs, C. Keith, H. Nann, 
T. Peterson, J. Shao, M. Spraker, J.  Szymanski, S. Vigdor, and K. Warman 
Indiana Universi ty  Cyclotron Facility, Bloomington,  Indiana 4 74 08 
W.K. Pitts 
Universi ty  of Louisville, Louisville, K e n t u c k y  40292 
CE49 is an experimental search for the production of atomic states in the a+a- system 
("pionium"), utilizing the pd + 3He X reaction very near the 2a production threshold. 
Pionium, whose production has been inferred from observation of correlated pion pairs in 
high-energy proton-nucleus reactions,' would provide a unique laboratory for the study 
of the a - a strong interaction at essentially zero relative energy. In particular, if one 
could produce a clean tagged sample of pionium, then measurement of its decay branching 
ratio to 27 would provide a model-independent determination2 of the a - a scattering- 
length combination laI=o - ar=2 1. These scattering lengths represent fundament a1 test s 
of any model of low-energy QCD incorporating chiral symmetry breaking, such as Chiral 
Perturbation Theory. 
During the past year we have made significant progress in understanding the magnetic 
channel used in the Cooler T-section for 3He detection, in probing relevant background 
rates, and in commissioning in-vacuum scintillators used to detect (and to veto for pionium 
production) low-energy a' deflected in the 6' magnet in the T-section. These advances 
will allow us to place a significant upper limit on the pionium production cross section. 
We have tuned up the magnetic channel during each of our runs by detecting the full 
kinematic cone for 3He's accompanying single a' production within 1 MeV of its production 
threshold. Here, the production cross section is large ( N  1 pb), the acceptance nearly loo%, 
and the background very small. In early CE49 runs, we found that the optimum settings 
of the channel magnets for the single ao production changed dramatically from run to 
run. This problem has recently been traced to intermittent shorts in the coils for both 
of the dipole magnets at the start of the detection channel. With these shorts repaired, 
we were able to adjust the channel magnets to optimize the 3He acceptance in our most 
recent production run (June 1995). The resulting spatial distribution of 3He's on the 
drift chamber at the entrance to the channel is shown in Fig. 1. We see very clearly the 
surface of the kinematic cone at an energy a 0.8 MeV above threshold, marked by the 
yield enhancement associated with the peak in the c.m.-to-lab conversion Jacobian for 
pd + 3He no. Under the conditions used in acquiring the data in Fig. 1, the channel 
acceptance appears to be limited slightly at only a few locations (especially near the top 
and bottom) on this cone, presumably by the gap size in the second of three quadrupole 
magnets in the channel. When the optimum magnet fields were scaled appropriately to 
detect 3He's associated with r+a- production, the acceptance of the channel was indeed 
observed to peak at the desired momentum. 
The singles 3He rate within the kinematic cone region shown in Fig. 1 falls to a level 
corresponding to a total background cross section 100 pb below the  single ao production 
I " ' I " ' I " ' I  ' " 1  1 " :  
-  ~ d + ~ H e  no
- ~?=199.4 MeV 
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Figure 1. Distribution of 3 ~ e ' s  from pd + 3 ~ e  ?ro at a bombarding energy (199.4 MeV) 
0.8 MeV above threshold on the drift chamber at the entrance to the magnetic channel. 
The circular spot indicates the extent of the 3He kinematic cone in the laboratory, while 
the enhancement of event rate around the circumference of the circle corresponds to the 
peak in the Jacobian at the lab maximum angle (- 1.0'). Spots around the circumference 
where the enhancement is weak reflect acceptance limits of the channel. 
threshold. In principle, conditions should be comparably clean just below the 27r0 thresh- 
old, since when the channel is properly tuned, there should be no acceptance for the open 
two-body channels (3 He no, 3He y), and negligible cross section times acceptance for the 
open electromagnetic reactions pd - 3He e+e-, 3He yy, 3He p+p-. However, we have 
observed appreciable background at these energies (< 415 MeV), consisting of 4He as well 
as m3He production, at rates dependent on the tuning of the Cooler beam. The 4He must 
arise, and the background 3He presumably as well, from reactions induced on material 
outside the D2 gas jet target, either by primary beam halo or by particles first scattered 
by the D2 jet. Under the best beam conditions we have achieved, the background 3He 
rate corresponds to a total cross section -1 nb, making it difficult, for example, to observe 
clear, statistically convincing evidence for the pd 4 3He ?ro ?ro reaction by comparing 3He 
singles spectra 1 MeV above and 1 MeV below threshold. 
In order to reduce the background rates, without having yet to understand their origin 
in detail, we have concentrated in our most recent run on measuring the cross section for 
free 7r+7r- pair production 1 MeV above threshold (i.e., at 431.5 MeV). For this purpose 
we searched for coincidences between 3He detected in the magnetic channel and charged 
pions detected in one or two of the five plastic scintillators installed to ve to  charged pions 
in an eventual pionium search. The layout of three of the veto scintillators within the 
6' magnet vacuum box is shown in Fig. 2. Light is transported from these counters via 
Figure 2. Layout of the plastic scintillators installed in the Cooler vacuum to detect 
low-energy charged pions near the 6" magnet poletips, before the pions enter the intense 
field region. In addition to the three (cross-hatched) scintillators pictured, two others are 
arranged further downstream, and to the left and right of those shown, in order to detect 
n+ and n- (or their daughter muons) that get bent (by >90°) through the magnet. 
optical feedthroughs to photomultipliers outside the Cooler vacuum. The three scintillators 
pictured detect charged particles emitted from the target toward the upper or lower poletips 
of the 6" magnet, or low-rigidity negatively charged particles bent sharply toward beam 
right in the 6" magnet. In addition, a scintillator placed outside the vacuum on the beam- 
right side of the 6" vacuum box detects n- (or their daughter p - )  emerging through a thin 
(0.004" stainless steel) exit window, while one placed alongside a poletip spacer on beam 
left detects low-energy T+. Within 1 MeV of the pd + 3 ~ e  T*T- threshold, the pions 
are sufficiently confined in energy (3 5 T F ~  5 12.5 MeV) and angle ( O F b  5 16") that the 
simulated efficiency for detecting at least one of them in any of the five scintillators is 98%. 
The simulated efficiency for detecting both pions is 70%. 
The coincidence requirement cleans up the 3He spectrum substantially, as illustrated 
in Fig. 3. The left panel shows the AE-E particle identification spectrum acquired with Si 
and Ge detectors at the rear of the channel for all particles that fire scintillators at both the 
entrance and exit of the channel. The dominant groups correspond to --lo0 MeV protons 
and -50 MeV deuterons (from the pd + Ndn continuum) that fall within the channel's 
acceptance, but 3He and *He groups are also seen. The 3He's here arise predominantly 
from background reactions not induced on D2 and from pd + 3He .lrOnO (for which the 
energy was about 16 MeV above threshold). Only about 10% of the 3He's correspond to 
n+n- production. The right panel shows the same spectrum, now selecting only 3He's and 
gating by a prompt coincidence with one or more of the charged-pion scintillators. One now 
sees a very clear bunching of events over a narrow energy interval, precisely that expected 
for the 3He n+n- channel 1 MeV above its threshold. When we lowered the bombarding 
energy below ~ + n -  threshold, only a flat background, at a level --30 pb within the peak 
region, remained. However, the 3He n+n- coincidence signal itself in Fig. 3(b) corresponds 
to a total cross section times acceptance of only 4 3 0  pb. The data are currently being 
I " ' I " ' I " ' I " ' I " ' ~ " ' ~ " ' ~ " ' I "  
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. . .  l , , , k , , , 1 # , , 1 , , , 1 , , , 1 , , , 1 , , , 1 , , , 1 , -  
;i------p 
Figure 3. Particle identification spectra obtained at a bombarding energy of 431.5 MeV 
with Si (AE) and Ge (E) detectors at the exit of the magnetic channel. All particles 
that fire scintillators at the entrance and exit of the channel are included in (a), where 
the indicated two-dimensional window encloses 3He's. Spectrum (b) includes only those 
particles within the 3He gate that fire one or more of the charged-pion scintillators in 
prompt coincidence with the channel detectors. The window here indicates the energy 
range expected for pd 4 3He ?r+?r- events 1.0 MeV above threshold. 
analyzed to extract information about the experimental acceptance (e.g., from the ratio of 
coincidence rates for two pion scintillators vs. one pion scintillator firing). 
Pionium events and the detected pd -+ 3He ?r+?r- coincidences are, to very good 
approximation, mutually exclusive, since pionium decays to neutral particles. However, it 
is reasonable to guess that the pionium production cross section is considerably smaller 
than that for free ?r+n- pairs, since the relevant phase space for pionium is much smaller. 
We hope to extract from our recently acquired data a meaningful experimental upper limit 
on the pionium cross section, by comparing the 3He singles rates, vetoed by the pion scin- 
tillators, in the relevant kinematic region above and below threshold. It appears probable 
that this limit will be sufficiently small to make an eventual coincidence measurement of 
the yy decay of pionium of marginal feasibility with the pd -t 3He (?r+~-),~,, production 
reaction. Alternative production reactions are under consideration. 
1. L.G. Afanasyev, et al., Phys. Lett. B 255, 146 (1991); and Phys. Lett. B 308, 200 
(1993). 
2. A. Betker, et al., IUCF Sci. and Tech. Rep., May 1993 - April 1994, p. 53. 


Pionium Production in the Cooler

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Pionium Production in the Cooler

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