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

Bent, R.

Blomgren, J.

Brown, J.

Chen, F.-J.

Hardie, G.

Heimberg, P.

Homolka, J.

Jacobsen, E.

Kurepin, A.

Nann, H.

Pancella, P.

Rehm, K.E.

Rinckel, T.

Saber, M.

Schneider, R.

Segel, R.

Sun, C.

Yu, Z.

Zhuravlev, A.

English

IUScholarWorks

PION PRODUCTION 
STUDIES OF ( p , ~ )  AND (p,27r) REACTIONS ON THE 
INDIANA COOLER RING USING RECOIL-ION DETECTION 
F.-J. Chen, P. Heimberg, R. Senel, - and 2. Yu 
Nor thwes t e rn  Univers i ty ,  Evans ton ,  I l l inois  60201  
J .  Brown and E. Jacobsen 
P r i n c e t o n  Univers i ty ,  Pr ince ton ,  N e w  Je r sey  0854 0 
J.  Homolka and R. Schneider 
Phys ik  Depar tmen t ,  Technische Univers i ta t  M u n c h e n ,  G e r m a n y  
A. Kurepin and A. Zhuravlev 
I n s t i t u t e  for Nuc lear  Research, Moscow,  R u s s i a  
G. Hardie and P. Pancella 
W e s t e r n  Mich igan  Univers i ty ,  Kalamazoo ,  Mich igan  4 9 0 0 8  
K.E. Rehm 
Argonne  Na t iona l  Laboratory, Argonne ,  I l l inois  60499  
R. Bent, J .  Blomgren, H. Nann, T. Rinckel, M. Saber, and C. Sun 
I n d i a n a  Univers i ty  Cyc lo t ron  Facili ty,  B looming ton ,  I n d i a n a  4 74 0 8  
A method has been developed for studying high momentum transfer reactions on the 
Cooler ring using recoil-ion detection. The system employs the dipole magnet in the Cooler 
ring, which bends the primary beam by 6", to sweep the recoil ions into a detection system 
consisting of a parallel grid avalanche counter (PGAC) that measures the (x, y) position, 
a proportional counter (PC) that measures the energy loss dE/dx and y position, and a 
silicon microstrip detector that measures the total energy and x position of the recoil ions. 
With this method, cross sections for single and double pion production in different isospin 
channels are measured simultaneously with the same target and detection system. 
The two-dimensional AE(PC) vs. E(S1) spectrum gives nuclear charge separation. 
The 6" magnet gives a measurement of the rigidity R = p/Q = Mv/Q of the recoil ion, 
where p is its momentum, Q its atomic charge, M its atomic mass, and v its velocity. 
The momentum per unit charge, p/Q, and the emission angle, 9, of the recoil ions are 
determined from the two (x, y)  positions by ray-tracing back through the 6" magnet to the 
target position. By dividing p/Q by the recoil ion velocity v ,  determined from the TOF 
between the PGAC and the Si detector, M/Q is determined. The recoil-ion charge and 
mass are given by Q = 2E/Rv and M = 2E/v2. Taking advantage of the fact that the 
charge is quantized, two other determinations of the mass are obtained: M2 = QR/v and 
M3 = Q2 R2/2E. 
1 2 ~ ( p ,  7r*)13c at 166 MeV, Q=6 
\ Detector 
0 0 o o r  
a 0 
0 0 . - .:F- / 
0 --• 
u / r 
/ 
- / 
Figure 1. Laboratory angle of emission of the recoil ions vs. their magnetic rigidity as 
determined by backward ray tracing for the reaction 12C(p,~+)13C at Tp = 166 MeV. 
The first experiments on the 12C(p,.rr) and l2C(p,2.rr) reactions were carried out in 
November-December, 1992 and February, 1993 at bombarding energies of 166, 200, 250, 
290, 330, and 350 MeV using carbon fiber and foil skimmer targets. Luminosities from 
5 x to a few x lo2' cm-2 s were achieved. 
Figure 1 is a plot of the angle of emission ( B l a b )  of the recoil ions vs. their magnetic 
rigidity (p/Q) as determined by backward ray tracing for 13C recoils in the Q=6 charge 
state from the 12C(p,n+)13C reaction at 166 MeV bombarding energy. The recoils are 
confined to a 7' forward cone about the beam axis and fall completely inside the detector 
Figure 2. Laboratory angle of emission of recoil ions vs. their magnetic rigidity as deter- 
mined by backward ray tracing for the reactions '' C(p,.lr+)'3 C and l2 C(p,.lr+.lr0)'3 C at TP 
= 330 MeV. 
acceptance at this low bombarding energy, which is only 18 MeV above threshold. The 
highest p / Q  events correspond to 180" pion emission and the lowest p / Q  events to 0" 
pion emission. The distribution of events on the ellipse gives the complete pion angular 
distribution. 
Figure 2 is a 6' vs. p / Q  plot at 330 MeV bombarding energy, which is about 40 MeV 
above the (p,27r) threshold. At this higher bombarding energy, the recoils from the ( p , ~ + )  
reaction are confined to a 19" cone about the beam axis. They fall on the large ellipse due 
to the two-body kinematics. The diagonal cuts in the upper and lower parts of the figure 
represent the detector acceptance. Recoils from the (p,.lr+no) reaction are kinematically 
constrained to fall inside the small ellipse and can fill this ellipse because of the three-body 
final state. Several candidates for 12C(p,?r+?r0)'3C events are seen. 
The background in the 27r region was estimated from similar data taken at 290 MeV 
bombarding energy, which is just below the (p,27r) threshold. This showed that most of 
the events in the (p,7r+7ro) region at 330 MeV probably are background, and thus we have 
been able, so far, to place only an upper limit on the (p,7r+7ro) cross section of about 15 nb 
at 330 MeV bombarding energy. 
PIONIUM PRODUCTION IN THE COOLER 
A. Betker, T. Bowyer, J .  Cameron, G. Berg, W. Jacobs, H. Nann, T. Peterson, 
J. Shao, M. Spraker, J. Szymanski, S. Vigdor, and K. Warman 
I nd iana  Univers i ty  Cyc lo t ron  Facili ty,  B loomington ,  I nd iana  4 74 08 
W. K. Pitts 
Univers i ty  of Louisville,  Louisville,  K e n t u c k y  4 0292 
Experiment CE-49 is intended to evaluate the rate and cleanliness with which we can 
produce in the IUCF Cooler a tagged sample of "pionium," i.e., atomic bound states of a 
7r+ and a 7r-. Such an atom has been predicted'-4 to have a relatively long lifetime ( r  
> 10-l5 s) for a system with an allowed strong decay (from the S-states to 7r07ro). To date, 
there is only indirect experimental evidence for its existence, inferred5 from the observed 
strength of 7r+7rW momentum correlations as a function of target thickness in 70 GeV p + 
Ta collisions. Pionium is worthy of fundamental study because it represents the lightest 
and, in some senses, simplest system of two interacting hadrons. More specifically, it allows 
study of the K - K strong interaction at essentially zero relative energy, an interaction 
that would vanish in the limit of perfect chiral symmetry. Precise measurements of the 
interaction at zero energy provide probes of dynamica l  chiral symmetry breaki11~~7~ (xSB), 
over and above the PCAC violation8 that arises inescapably from the non-zero physical 
mass of the pion. The extent of such dynamical xSB is parametrized in Chiral Perturbation 
Theory, but should ultimately be predictable in a more fundamental theory (i.e., non- 
perturbative QCD solved in the confinement limit). 
Our specific long-term interest is to use pionium decay to determine a linear com- 
bination (laT=0 - a ~ = 2  I) of the S-wave 7r - 7r scattering lengths to a precision f 5%. 
This information can be extracted directly from a f 10% measurement of the relative 
rates for the S-states of pionium to decay to 27 us. 27r0. For this purpose it is sufficient 
to measure the absolute 27 branching ratio B2,. For pionium produced in a thin Cooler 
target (where there is negligible probability of ionization of the recoiling atom induced by 
collisions with electrons), all other decay branches (except 27r0) can be reliably predicted to 
be several orders of magnitude weaker than 27, so that the ratio of interest is, to excellent 
approximat ion, given by 


Studies of (p,pi) and (p,2pi) Reactions on the Indiana Cooler Ring Using Recoil-Ion Detection

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Studies of (p,pi) and (p,2pi) Reactions on the Indiana Cooler Ring Using Recoil-Ion Detection

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