Volume 29B, number 2 PHYSICS LETTERS 14 April 1969
OBSERVATION OF THE p-p+ DECAY MODE OF THE no MESON
B.D. HYAMS, W.KOCH *, D.C. POTTER ** CERN. Geneva, Switzerland
L. VON LINDERN, E. LORENZ, G. LUTJENS, U. STIERLIN and P. WEILHAMMER Max-Planck-Institut ftir Physik und Astrophysik! Munich. Gemany
Received 6 March 1969
The rare decay mode of the 71 meson into two muons has been observed. The branching ratio for this decay mode is calculated using production cross-sections from other experiments, and the result is compared with theoretical predictions.
We report here on some of the results ob- tained from a spark chamber experiment per- formed at the CERN Proton 11.2 GeV/c n--meson beam actions
71-p - Cc-p+n
Synchrotron in an measuring the re-
77-p - p-p+ + (2 2 particles).
A peak observed in the invariant n-p+ mass at 550 MeV is attributed to the decay ~0 - c(-p+. The branching ratio of this decay mode is ob- tained by using no production cross-sections from other experiments.
The experimental arrangement (shown in
* Now at the Max-Planck-Institut fur Physik und Astrophysik, Munich, Germany.
** Now at the Physics Department, University of Penn- sylvania, Philadelphia, USA.
Et A 5.1
fig. 1) consists of two magnetic spectrometers
with optical thin-foil spark chambers to measure the momenta and the direction of beam and sec-
ondaries. Muons were selected by a total of 2 m of iron filter Fl, and two counter arrays Al and A2. To trigger on reaction (1) and (2), we de- manded a coincidence ABCD, indicating an in- teracting beam particle, together with a signal from two not adjacent counters in each of the two arrays. The 40 cm thick lead filter F2 be- hind the spark chamber S5 was necessary in or- der to reduce the background of p pairs, which have their origin in 7~ pairs decaying between the target and Fl. The non-interacting beam parti- cles passed through a 3.5 x 4 cm2 hole in F2. A more detailed description of the experimental set-up and the data-handling is given else- where [ 11.
From 36 000 photographs, about 5 200 events of types (1) or (2) were found and were measured
1 2 3m c-=~-~:*erCa Aa_ AZ_ x,, x,,
Fig. 1. Schematic diagram of the experimental set-up,
RCUFLTFJ?Fl A2 I_,
Volume 29B, number 2 PHYSICS LETTERS 14 April 1969
14 EVENTS/O.01 GeV
12 RESOLUTION -
il3 04 05 06 07 08 09 1.0 GeV INVARIANT MASS O_-P)
Fig. 2. Invariant mass spectrum of p pairs with energy Ep-c(+ 2 10.7 GeV and /tl CO.4 (G~V/C)~. Unweighted data.
by hand. In fig. 2 we plot the invariant mass dis- does not contribute significantly, because most tribution of those CL airs with energy
of the mass error is due to the error in deter- E 2 10.7 GeV/c and I 1 =S 0.4 (GeV/c)2. A Monte mining the particle direction using only one Carlo calculation shows that this energy-cut re- eight-gap spark chamber, S5. duces the number of ,U pairs arising from n de- In fig. 2 one observes a broad enhancement in cays in flight to less than 3% of the data in fig. 2. the region of the p and w mesons! and a narrow In the calculation, the number of 71 pairs pro- peak with a width corresponding to our mass duced and the penetration rate of pions through resolution at the mass of the no meson. To our the filter F2 were taken from measurements knowledge this is the first evidence for the rare with the same apparatus. decay mode no + p - p+.
The invariant mass resolution of the spec- trometer was determined by measuring 2n de- cays of the KY meson in the same apparatus with the filters Fl and F2 taken out, and treating these events in the same way as the p pairs. The resulting distribution in fig. 3 shows a mass resolution of A&? = *17 MeV at the KY mass. In the case of the p pairs, the additional error coming from the energy loss straggling in F2
From the observed number of p pairs in the ~0 peak we calculate the cross-sections for the reactions (3) and (4):
a-p -lip,_,+ + (2 2 particles)
(4 with Ep-p+ 2 10.7 GeV.
J 0.3 0.4 0.5 06 0.7 0.8 GeV
INVARIANT MASS (H-H)
Fig. 3. Invariant mass spectrum of KY decays. These events are measured in the same way as the /J pairs.
and compare these to the production cross-sec- tions for the qo in the channels (5) and (6)
n-p +qon (5)
n-P - no + (a 2 particles) with EvO a 10.7 GeV.
(6) The geometrical acceptance of our apparatus for p pairs at the q mass has been calculated by a Monte Carlo method. For reaction (3) we used in this calculation the measured  differential cross-section of reaction . For reaction  we assumed that the qo is produced only with the N*O (1238), and used the da/dt dependence ob- served  in the process n+p - noN*++ (1238).
In order to select reaction (3) from the data in fig. 2, it was necessary that no other particle coming from the n-pair vertex should be visible in the spark chamber S5, and no signal was re-
Volume 29B. number 2 PHYSICS LETTERS 14 April 1969
corded from the lead sandwich counter arrange- ment El-El2 surrounding the H2 target. In addi- tion, a suitable cut in the missing was intro- duced. The resulting cross-section for reaction (3) is then
u(7i -p -+ q n ) = 416 + 164) pb. 1p-cL+
For reactions (3) + (4) one finds
[a-p - 17O L/-L-CL+
+ (1 or more particles)] =
= (677 * 217) pb
based on 27 events in the no mass region, of which 9 events are attributed to background. The cross-sections refer to the intervals E a 10.7 GeV/c and /t / SO.4 (GeV/c)2. The quoted errors include errors in statistics, in electronics- and analysis-efficiency, and in the estimate of the background.
The production cross-section for reaction (5) is taken from the experiment of Guisan et al. . Interpolating their data one finds at 11 GeV/c u(ri -p qon _ yyn) = (6.7 f 0.8) pb for / 11 ~0.4 (GeV/c)2. For reaction (6) we ex- tracted the contribution, which comes from the 70 N*O (1238) final state. from a bubble chamber experiment  which measured the reaction n-p m+ 7i-p7i-+~;~ at 11 GeV/c. Correcting for the neutral decay mode of the N*o (1238)) which is also accepted in our apparatus, and for the dif- f erent decay modes of the no [ 51, one finds a cross-section of o[~-p m* goNo(1238) - ~. yyN*o(1238)] = (5.2 + 2.6) pb in the E- and t- intervals mentioned above. The given error comes from statistics only.
With these results we calculate the branching ratio of the 170 -+ ~_r-p+ decay combining chan- nels (3) and (4), and obtain
BY = (@&y;)+) = (5.9 f 2.2) x 10-5.
Channel (3) alone yields By = (6.2 5 2.5) x 10-5, Wehmann et al.  have published an upper
limit for the branching ratio
They find B .-C 2 X 10m5 *. which is compatible with our value.
* Wehmann et :11. quote their upper limit on the branching ratio to be twice the theoretical lower limit given by Cnllan and Treimnn  who find By - 1.1 x 10-S. correspondinff to B - 0.4 x 10m5. Their upper limit is therefore really nbout five times the theoretical lower bound.
The decay rate v - yp-p+ (all p-p+ invar- iant masses) is calculated [8,9] to be about 70 times higher than the rate ~0 - p-p+. Never- theless, the contribution of p pairs to our data from q - yl_~-p+ and also from other similar processes is small because of the energy cut at 10.7 GeV for the p pairs, and because of a bias in the acceptance against very small p-pair in- variant masses.
There have been several theoretical at- tempts [7-141 to calculate the branching ratio By. It was first pointed out by Geffen and Young [lo] that the transition to the physical state ~0 - yy is an order of cy faster than any other real physical state via which the final state p-p+ can be reached. Consequently, unitarity gives a value reliable to 10% for the imaginary part of the amplitude for lepton pair decay, leading to By 3 1.07 X 10-5.
If one accepts that all hadronic electromag- netic interactions are mediated by the vector mesons (vector dominance), then only pp, ww, I$@, and 04 can be invoked as intermediate states for the decays 77 - yy and 77 - p-p+. Quigg and Jackson  calculate B7, to be 1.13 X 10-5, 1.17 X 10-5, or 1.29 X 10-5, on the assumptions that only p, w, or @ contribute, respectively.
Choosing the SU3 coupling (pop0 + ww - 2@+)77o, assuming that the photon transforms as po + f w - ~3 @, and inserting vector-meson- photon coupling constants of the universal form em!/y~, they find By = 1.08 x 10-5. In order to ag+ee with this theoretical value, we would ex- pect to observe not more than four ~0 - pp de- cays in our experiment. This could not be dis- tinguished from the background of nine events in the ~0 region.
However if one leaves the relative contribu- tions of the three vector mesons in the coupling to the v as free parameters, then any value for By can be obtained in this model.
We would like to thank Professor S. Ratti for letting us have the bubble chamber data for the evaluation of the ~0 N *o cross-section. We thank D. Pellett for his help in the early stages of the experiment, and the Machine Division of the CERN Proton Synchrotron for their able assist- ance in all phases of the experiment. We are al- so grateful to Professor W. Heisenberg for his interest and continued support.
1. B. D. Hyams. W. Koch, D. Pellet. D. Potter: L. Von Lindern. E. Lorenz, G. Liitjens. U. Stierlin and P. Weilhammer, Phys. Letters 24B (1967) 634.
Volume 29B, number 2 PHYSICS LETTERS 14 April 1969
0. Guisan, J. Kirz, P. Sonderegger, A. V. Stirling, P. Borgeaud, C. Bruneton, F. Falk-Vairant, B. Amblard, C. Caversasio, J. P. Guillaud and M. Yvert, Phys. Letters 18 (1965) 200. Aachen-Berlin-CERN Collaboration, M. Aderholz et al., Nucl. Phys. BE (1968) 45. Genova-Hamburg-Milano-Saclay Collaboration, C. Caso et al., The Collaboration kindly let us use the Data Summary Tapes with all four-prong events (1C fit) from the 11 GeV/c 7r-p experiment. Particle Data Group, UCRL-8030 Pt. 1 Jan. 1969, Rev. A. Wehmann, F. Engels Jr., C. M. Hoffmann, P. G. Innocenti, R. Wilson, W. A. Blanpied, D. J. Drickey,
L. N. Hand and D. G. Stairs, Phys. Rev. Letters 20 (1968) 748.
7. C. G. Callan Jr. and S. B. Treiman, Phys. Rev. Letters 15 (1965) 316.
8. C. Jarlskog and H. Pilkuhn, Nucl. Phys. Bl (1967) 264.
9. C. Quigg and J. D. Jackson, Preprint, UCRL 18487, 1968.
10. D.A. Geffen and B. L. Young, Phys. Rev. Letters 15 (1965) 316.
11. S. D. Drell, Nuovo Cimento 11 (1959) 693. 12. S. M. Berman and D. A. Geffen, Nuovo Cimento 18
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