Determination of the relative branching ratios for pp→π+π− and pp→K+K−

Physics Letters B 267 ( 1991 ) 154-158 North-Holland

PHYSICS LETTERS B

D e t e r m i n a t i o n o f the relative branching ratios for p x+x - and ppK+K CPLEAR Collaboration R. Adler ", A. Angelopoulos b, A. Apostolakis b, E. Aslanides c,d, G. Backenstoss a, C.P. Bee e, J. Bennet e, P. Bloch ~, Ch. Bula f, C. Burgun g, P. Carlson h, j. Carvalho i, M. Chardalas J, S. Charalambous 5, H. Cobbaert i, S. Dedoussis J, M. Dejardin g, J. Derre g, M. Dodgson °, J.C. Dousse k, j. Duclos g, A. Ealet d, B. Eckart a, L. Faravel k, p. Fassnacht d, J.L. Faure g,1, C. Felder ~, R. Ferreira-Marques i, W. Fetscher ~, M. Fidecaro c, A. Filip~i~ m, D. Francis c, J.R. Fry ~, C. Fuglesang h, E. Gabathuler ~, R. Garnet ~'~, D. Garreta g, T. Geralis d, H.-J. Gerber ~, A. Go n, p. Gumplinger ~,2, C. Guyot g, P.F. Harrison e,3, p.j. Hayman ~, W.G. Heyes ~, R.W. Hollander o, H.U. Johner k, K. Jon-And h, K. Jansson h, A. Kerek h, J. Kern k, P.-R. Kettle f, C. Kochowski g, P. Kokkas P'q, E. Kossionides P, R. Kreuger o, T. Lawry n, R. Le Gac r, E. Machado c, p. Maley e, N. Manthos q, G. Marel ~'q, P. Marotte r, M. Miku~. m, j. Miller n, F. Montanet d, T. Nakada r, A. Onofre i, B. Pagels ~, T. Paradelis P, P. Pavlopoulos ~, F. Pelucchi d, j. Pinto da Cunha i, A. Policarpo i, H. Postma o, R. Rickenbach a, B.L. Roberts n, E. Rozaki b, T. Ruf ~, L. Sacks e, L. Sakeliou b, p. Sanders ~, C. Santoni ~, K. Sarigiannis b,p, L.A. Schaller k, A. Schopper c, P. Schune ~, U. Seljak m, S. Szilagyi h, L. Tauscher a, C. Thibault r, F. Touchard r, C. Touramanis PJ, F. Triantis q, D.A. Troester ~,4, E. Van Beveren i, M. Van den Putte o, C.W.E. Van Eijk o, G. Varner n, S. Vlachos ~, D. Warner ~, P. Weber ~, C. Witzig ~,5, C. Yeche g, D. Zavrtanik ,1 and D. Zimmerman n a b c d e f g h

University of Basle, Basle, Switzerland University of Athens, Athens, Greece. CERN, CH-1211 Geneva 23, Switzerland CPPM, CNRS/IN2P3, F-13288 Marseille, France University of Liverpool, Liverpool, UK PauI-Scherrer-lnstitut (PSI), CH-5232 Villigen, Switzerland DPhPe, CENSaclay, F-91191 Gifisur-Yvette, France MSI, S-104 05 Stockholm, Sweden i L I P , P-IO00 Lisbon, Portugal and University o f Coimbra, Coimbra, Portugal J University of Thessaloniki, Thessaloniki, Greece k University of Fribourg, Fribourg, Switzerland ETH-IMP, CH-5232 Villigen, Switzerland m J. Stefan Institute and Department o f Physics, University o f Ljubljana, YU-61111 Ljubljana, Yugoslavia n Boston University, Boston, USA o Technical University of Delft, Delft, The Netherlands P Demokritos INP, GR-15 310 Athens, Greece q University of loannina, GR-4 51 I0 loannina, Greece r CSNSM, CNRS/IN2P3, F-91405 Orsay, France Received 3 July 1991

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The ratio of the branching fractions for pl~--,K+K- and ptb~x+x- was determined with the CPLEAR detector, by stopping antiprotons in a gaseous hydrogen target at 15 bar pressure. It was found to be BR(K÷K - )/BR(x+x - ) =0.205 +0.016. The fraction of P-wave annihilation at rest at this target density was deduced to be (38 + 9)%.

The ratio o f the branching fractions for the reactions pl)--,n+lt - and p ~ - - , K + K - , B R ( K + K - ) / B R ( n + n - ), was d e t e r m i n e d at the Low-Energy Antiproton Ring ( L E A R ) at C E R N , in the f r a m e w o r k o f the e x p e r i m e n t PS195 ( C P L E A R ) [ 1 ]. This exp e r i m e n t is designed to study CP v io la ti o n in the decay o f the neutral K meson, whose strangeness at the time o f p r o d u c t i o n is m e a s u r e d by selecting, a m o n g the m a n y other channels, the annihilations o f antii Present address: DPhN/STEN, Saclay, France. 2 Presentaddress: TRIUMF, Vancouver, BC, Canada V6T 2A3. 3 Present address: Queen Mary and Westfield College, University of London, London E! 4NS, UK. 4 Present address: SBS, Basle, Switzerland. 5 Presentaddress: Brookhaven National Laboratory, Upton, NY 11973, USA.

protons with protons at rest into K + ~ - I ~ ° and K - n + K °. By making use o f some o f the features o f the C P L E A R detector during the running-in phase, the final states rc+Tt- and K + K - could be identified exclusively. Th e branching ratios for these channels p r o v i d e i n f o r m a t i o n on annihilation dynamics in general and, m o r e specifically, can be used to extract the fraction o f P-wave annihilation at rest at the target density used in the experiment, thus allowing, in principle, a check o f models for the de-excitation o f protonium. Th e detector is shown in fig. 1. It has cylindrical geometry, with the lb b e a m defining the z-axis, and is built inside a solenoid o f 1 m radius, 3.6 m length, with a field o f 0.44 T. The 200 M e V / c l)'S are stopped in the target, which is at the centre o f the detector and

e.m. calorimeter (6,5 LR, 18 Layers) PIDs Streamer tubes Drift chambers ( D C I - D [ 6 ) MWPCs ( P I Z I - P [ 2 ) H2 t a r g e t (15 bar)

Fig. 1. Transverse view of the CPLEAR detector. The outer diameter of the electromagnetic calorimeter is 2 m and defines the scale. For details see text. A typical signature for a n+~t- event is shown. A hit on the wire chambers is marked by a ×, in the streamer tubes by a hatched circle. The hit PID scintillators are marked by their hatched areas, the hit and fired ~erenkov detectors by their black areas. 2

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consists of a laminated Kevlar sphere (7 cm radius, 800 Ixm wall thickness) filled with gaseous hydrogen at 15 bar pressure. The beam enters the target through a window of 11 m m diameter and 120 lxm thickness. The stopping depth of the lb'S extends over 25 m m ( F W H M ) in the hydrogen gas. Tracking is done by means of two multiwire proportional chambers, followed by six drift chambers ( D C 1 - D C 6 ) and two layers of streamer tubes. The z-coordinates are determined using cathode-strip readout from the proportional and drift chambers, and the time difference obtained from the upstream and downstream signals of the streamer tubes. Particle identification ( P I D ) is provided by a Cerenkov counter, sandwiched between two plastic scintillators, S1 and $2 (PID). A more complete description of the detector can be found in ref. [ 2 ]. The present analysis uses data taken in December 1989 with a minimum-bias trigger, requiring at least one hit in the scintillator S 1. In this run only the cathode strips of the drift chambers DC1, DC2, and DC6 were read out, with reduced efficiency, and only digitally. Efficiencies and dead-time were not monitored thoroughly during this run. Hence absolute branching fractions could not be obtained with high accuracy. Further details can be found in ref. [ 3 ]. The particular type of events under investigation are expected to exhibit back-to-back features, if the p's had really come to rest. Therefore, in a first step, the following obvious and rather loose requirements were imposed: (i) the event should have two tracks only, from oppositely charged particles; (ii) the z-coordinates of the two tracks (z = 0 corresponds to the centre of the detector), as provided by the DC6 cathode strips or streamer tubes, sum up to Izl+z21 <20 cm; (iii) the angle 4'12 between the two tracks in the transverse plane, evaluated at the point of closest approach of each track to the z-axis, is I qh21 >/170 °. This selection reduced the original data sample of 1.7 × 106 events to one of 19 577 back-to-back events. This sample contains only geometrically defined backto-back events, where the curvature of the two tracks may, however, not be the same. For true two-body annihilations, the two tracks correspond to the same momentum and are contiguous parts of the same circle in the transverse plane. A further selection of two156

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body final states was therefore done, by fitting the two tracks by only one circle in the transverse plane and removing events with large Z2 in the track fit. This leads to a reduction of background and to a substantial improvement of the momentum resolution by more than a factor of 4 with respect to the single-track resolution. Fig. 2 shows the momentum distribution obtained by this "merged" fit. The peaks for K + K and n + n - are clearly visible. In a final step the remaining background was further reduced by the following sharp "back-to-back" requirements: (i) the angle between the two tracks in the transverse plane satisfies I qh2 ] >i 178 °, corresponding to a cut at about 3a, hence accepting more than 99% of the candidate events, (ii) at least three z-values per track are available, (iii) the angle between the two tracks in the z-direction is close to 180 ° through the condition IPz( 1 ) / PT ( 1 ) + Pz ( 2 ) / P T (2) I < 0.20, where PT and Pz are the transverse and longitudinal components of the momentum. Fig. 3 shows the momentum distribution of the events fulfilling these requirements. The peaks for K + K and n + n - are well separated, and only a small background remains. This momentum distribution was fitted by the function

~ . 800 >

I

7xTT

-~ 700 600

o-500

~400 hi 30Q 200 100 0

400

800

12O0

momentum [MeV/c] Fig. 2. Momentum distribution when the two tracks are fitted as one track ("merged fit"). The K+K - and n ÷ n - peaks appear at the expected momenta of 797 MeV/c and 927 MeV/c, respectively. The broad enhancement at low momenta is due to background events.

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acc(K+K - ) x = a c c ( x + _ ) = 0 . 8 5 6 + 0.015star + 0.01 lsyst • ~'e 200

~

~r3

The systematic error has two contributions. The first one, estimated to be 0.005, arises from the choice o f the cut parameters for the back-to-back selection, and from the non-gaussian tails o f the K + K - a n d n + n m o m e n t u m distributions. The second one stems from uncertainties in the simulation, which were estim a t e d to a m o u n t to 1%. Thus the final result is

- 160 13_

120 LJ

80 40

600

800 1000 momentum [MeV/cl

Fig. 3. Momentum distribution obtained with the final cuts.

P ( P ) =PBG(P) + P K ( P ) + P,~(P) ,

where P s G ( P ) , PK(P), a n d P ~ ( p ) describe the background, the K + K - , a n d the n + n - peaks, respectively. The best p a r a m e t r i z a t i o n was f o u n d to be a linear function for the b a c k g r o u n d (which was justified by a M o n t e Carlo simulation o f different background channels, such as n -+n ~ n o or n -+n :~n°n °, whose distributions extend up to the m o m e n t a u n d e r investigation), and gaussians for the K + K - a n d n + n peaks. The n u m b e r o f K + K - a n d n + n - events are then given by the area u n d e r the gaussian peaks, whose positions coincided with the expected mom e n t a within the errors, a n d whose widths a ( p ) / p were about 3%, in agreement with M o n t e Carlo simulations. A total o f N K + K - = 4 1 7 + 2 5 a n d N~+~_ = 2377 + 51 events were found, which result in a relative yield o f

NK+K

= 0.1754 -----0.011 stat +- 0 " 0 0 7 s y s t • R,~w - - N~+=-

BR(K+K - ) R=

BR(n+n

_)

Rraw = 0 . 2 0 5 + 0 . 0 1 6 -

x

where the statistical a n d systematic errors have been a d d e d quadratically. Previous experiments measured R to be 0.161 + 0.009 in gaseous hydrogen at N T P [ 5 ] a n d 0.327 + 0.008 [ 6 ] in liquid hydrogen. The target densities in these two experiments differ by a factor o f almost 1000. The present m e a s u r e m e n t provides a value at an i n t e r m e d i a t e density a n d confirms the rise o f R with increasing target density. This is because, at high target density, annihilation from a t o m i c P-states become less probable than annihilation from a t o m i c Sstates due to Stark-effect i n d u c e d mixing. The A S T E R I X Collaboration at L E A R has obt a i n e d the partial widths y~= F f f F for plb a n n i h i l a t i o n into K + K - a n d n + n - for initial S- a n d P-states separately [5]: y s ( K + K - ) = (1.08+_0.05)X 10 -3 , yp(K+K - ) = (0.287 _+0.051 ) × 1 0 - 3 , a n d ys (r~+~ - ) -- (3.19 _+ 0 . 2 0 ) X 10-3, yv(rC+~ - ) = (4.81 +- 0.49) × 10 -3. O u r result for R m a y be used to determine, with the help o f these partial widths, the fraction o f P-wave a n n i h i l a t i o n f p in gaseous hydrogen at a pressure o f 15 b a r through fp = [ T s ( K + K - ) - R y s ( ~ + ~ - ) ]

The systematic error arises from the influence o f the limits o f the fit interval, a n d from possible deviations o f the b a c k g r o u n d from the linear p a r a m e t r i z a t i o n , especially u n d e r the K + K - peak. The ratio R,aw m u s t be corrected for the different acceptances for K + K - a n d n + n - events, arising mainly from the different lifetimes o f charged kaons a n d pions. The ratio o f the acceptances was o b t a i n e d from a simulation using G E A N T 3 [ 4 ] as

X { R [yp ( = + ~ - ) - ys ( ~ + ~ - ) ]

- [ y p ( K + K - ) - y s ( K + K - ) l} -1 We obtain fv = 0.379 + 0.039s,a, + 0.025sys, + 0.072ext. The statistical and systematic errors are due to the errors o f our measurement. The d o m i n a n t error (ext) is due to the errors o f the above absolute branching ratios [ 5 ]. Our value fv = 0.379 + 0.086 might be used 157

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for refining de-excitation models for p r o t o n i u m in targets o f different density. It is in agreement with the recent cascade calculations o f Reifenroether and K l e m p t [7] who use experimentally d e t e r m i n e d atomic parameters. C o m p a r i s o n with calculations by Borie and Leon [ 8 ] shows agreement with some o f their predictions. The error o f our value for fp is, however, still too large to allow for rigorous constraints. This work was s u p p o r t e d by the following funding institutions: the Swiss N a t i o n a l Science F o u n d a t i o n , the Swedish Natural Science Research Council, the French C N R S / I n s t i t u t N a t i o n a l de Physique Nucl6aire et de Physique des Particules, the French C o m m i s s a r i a t ~t l'Energie Atomique, the Science and Engineering Research Council o f the U K ( S E R C ) , the G r e e k General Secretariat o f Research a n d Technology, and N S R C (Nuclear Physics Institute) " D e mokritos", the Laboratorio de I n s t r u m e n t a c a o e Fisica Experimental de Particulos ( L I P ) a n d the University o f C o i m b r a (Portugal), the Netherlands F o u n d a t i o n for F u n d a m e n t a l Research on M a t t e r ( F O M ) , the US N a t i o n a l Science F o u n d a t i o n , a n d the Slovene Secretariat for Research a n d Technology.

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We also wish to thank the C E R N L E A R staff for their support a n d co-operation. Furthermore, we are ind e b t e d to the technical staff o f the collaborating institutes. We also acknowledge the useful discussions we had with C..Amsler and E. Klempt.

References [ 1] CPLEAR Collab., R. Adler et al., CPLEAR proposal, CERN/ PSCC/85-6 (1985). [ 2 ] R. Adler et al., LEAP 90, Proc. First Biennial Conf. on Low energy antiproton physics (Stockholm, 1990), eds. P. Carlson, A. Kerek and S. Szilagyi (World Scientific, Singapore, 1991 ) p. 414. [3] Ch. Witzig, Thesis, ETH Zurich (1990). [4] GEANT3, DD/EE 84-1, Data handling division (CERN, Geneva, 1987). [ 5 ] M. Doser et al., Nucl. Phys. A 486 ( 1988 ) 493. [6] A. Angelopoulos et al., Antiproton 86, Proc. 8th European Symp. on Nucleon-antinucleon interactions, eds. S. Charalambous, C. Papastefanou and P. Pavlopoulos (World Scientific, Singapore, 1987) p. 79; see also R. Armenteros and B. French, in: High energy physics, ed. E.H.S. Burhop, Vol. 4 (Academic Press, New York, 1969); C. Baltay et al., Phys. Rev. Lett. 15 (1965) 532. [ 7 ] G. Reifenroether and E. Klempt, Nucl. Phys. A 503 (1989) 885; Phys. Lett. B 245 (1990) 129. [ 8 ] E. Borie and M. Leon, Phys. Rev. A 21 (1980) 1460.