Associated photoproduction of a charmed meson and a charmed baryon

r~.V.TT~.R~ AL ~UOVO CIMV.~TO VOL. 30, N. 6 7 Febbraio 1981

Associated Photoproduction of a Charmed Meson and a Charmed Baryon.

(PHOTON-]~IULSION COLLABORATIOn)

A. FIoR~NO, It. Gv.SSAROT,~ and A. QUAR~I~I-VIGNUDELLI

I s t i tu to d i F i s i c a dell' Unive~sith - Bologna, I t a l i a I s t i tu to Naz iona le di F i s i ea Nuclea te - Sezione d i .Bologna, l t a l i a

G. VA~ DEICHA~E GI~E

G e m - Geneva, Swi t zer land

t. ~VL CARTACCI, B. CONFOI~TO, t. CONTI, M. G. DAGLIANA, G. DI CAPORIACCO and G. PARRINI

I s t i tu to d i F i s i ca dell' Universi t~ - E&enze , I t a l i a I s t i tu to Naz ionale di F i s i ca Nuclea te . Sezione di Firenze , I t a l i a

~V~. DAMERI, G. DIA~IBRINI-PALAEZI, B. OSCULATI, C. ~ERSANO, ~r SANNINO and G. TOiVIASINI

I s t i tu to d i .Fisiea dell' Univevsi th - Genova, I t a l i a I s t i tu to Naz ionale di F i s i c a Nuclea te - Sezione d i Genova, I t a l i a

R. LLOSA

Madrid (JEN)

M. I. ADAMOVICH, Y. A. ALEXANDROV, M. M. CIrERNYAu S. P. KHARLA~IOV, V. Ca-. LARINOVA, G. I. ORLOVA, ~. G. PER]~SADKO, K. ~/[. I:~OMANOVSKAYA and ~1. I. TR:ETYAKOVA

Moscow

J. LoRY, C. ~v.TO~, D. SCHUN~, TSAI CHV and B. WILLOT

P a r i s ( L P N H E )

]5. BRAvo, A. RuIz and E. VILLA~

Santander

J. ~r BOLTA and E. HIGON

Valenc ia

166

ASSOCIA~I~D PIIOTOPRODUCTION OF A CHARMED )I]~SON AND A CHARIIII~D BARYON

(O~EGA-PHo~o~ COLLAbORAtIOn)

K. HEINLOTH and A. SCHL(iSSER

Bon~

J.-P. DtrFEY, G. D. LAFF]~RTY and J.-C. LASSALLW

CEI~N - Geneva, Switzerland

P. J. BuSsEY, G. N. PATRICK and C. RAI~E

Glasgow

T. BRODBEC~:, D. NEWTON and K. M. STORR

]Sa neaster

R. J. ELLISON, R. E. ttlYGHES-JON]~S, M. IBBOTSO~, J. B. LA~w, D. M]~RC~R and A. Y. WAITE

Manchester

)/~. ATKINSON, M. DAVlgNPORT, J. S. HVTTON, B. R. KUMAR and J. V. Mom~Is

1luther]oral

G. R. BROOKES, W. GALBRAITH and R. 3{cCLATCH]~Y

She]]ietd

(ricevuto il 15 Dieembre 1980)

167

In this letter we give a detailed description of an event in which associated photoproduction of a charmed meson an4 a charmed baryon has been observed. The event was recorded in the course of an experiment using nuclear emulsions (*) exposed r the CERN super proton synchrotron (SPS) tagged-photon beam and coupled to the Omega prime spectrometer (experiment WA58). Single-emulsion pellicles of size (20 • 5) cm ~ and 600 ~m thickness were automatically placed, one at a time, at an angle of 5 ~ to the beam axis, and exposed to a dose of about 106 tagged photons with energies between 20 and 70 GeV. Secondary particles from photon interactions were detected and analysed by the spectrometer. The events recorded by the spectrometer are recon- structed by the CERN geometry program TRIDEN T (1), and a search is then made for the event in the emulsion around the predicted vertex co-ordinates. This method

(*) The BR2-type emulsion were manufactured by the State Research Institute for Photochemical Projects (GCPP), Moscow, USSI~. (1) J, C. LASS~tL~E et aL: Internal report CERN-DD/EE[79-2 (1979).

16a PI-[OTON-EItIULSION COLLAD01tATION AND 0MEGA-PHOTON COLLABORATION

is similar to the one used in a previous experiment (WA45), where a ~0-meson was observed (~).

The even~ in question, produced by a 26 GeV photon, was found at a distance of 4 m m from the predicted position. At the primary vertex 0 there are seven black and five minimum-ionization tracks {fig. 1). After (50 ~= 1) ~m, one of the minimum- ionization tracks (No. 4) undergoes a deflection of about 10 ~ (secondary vertex 0')

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without ionization change and without visible recoil. This deflected track was not detected by the spectrometer, possibly because it interacted in the plexiglas support of the emulsion. O~ the remaining four minJmum-ioIfization tracks, only the one emitted at a small angle to %he beam axis (No. 3) was detected by the spectrometer (see table I).

A third vertex (denoted as 0") is seen in the emulsion at a distance o~ (124 ~= 1) vm from the primary one. It consists of four minimum-ionization tracks (No. 6.1-6.4), which were all detected by the spectrometer, and there is no visible recoil. Table I illus- trates the quality of the track matching. The total momentum of the four particles is ~s ~ (8.990 J= 0.008) GeV/c, and the transverse momentum with respect to the line connecting the 0" vertex to the primary one is PT = (0.04 =L 0.06) GeV/e, compatible

(3) M.I. ADAMOVlCE, Y. A. i]~XANDROV, J. M. BO~TA, L. BRAVO, A. hl. CARTACOI, B. CONFORTO, ~. 0ONTI, ~/L I~. CHERNYAVSKI, IV[. ~. DAGLIANi, ~. DA~VIERI, ~. DIAI~IBRINI-PADAZZI, G. DI CAPO" RIACOO, /~. FIORINO, R. GEBS/iROLI, ]~. IIIGON, S. D. KHARLAMO'~, V. ~. LIRIONOVA, R. LLOSA, S. LORY, O. IV[ETON, B. OSCITLATI, G. PAItRINI, i. QUARENI, I~, SANNINO, D. SCHUNE, Gi TO~s 1El. I. Tt%ETYAKOVi, T. OHU, G. VANDERHAEGHE, E. VILLARj ]~. ~r D. ASTON, ~ . ~_TKINSON~ 1%. BAILEY, A. BALL, G. B. BROOEES, J. BRORING, P. J. BUSSEI r, D. CLARKE, G. DE ROSNY, B. DIEK- MANN, B. DRE~/IDLON, IV[. DRIPER, I. P. DU]~RDOTH, J.-P. DUFEY~ I%. J. ELLISON, D. EZRA, P. FELLER, P. S. ~LYNN, I~. FRIESE, R. GEORGE, S. D. ~. GILL, 2r GOLDBERG, W. GRAVES, B. ~ROSSET~T]~, iP. G. I:[AMPSON, S. ]~UTTON, IV[. ~BBOTSON, i~. ]~. ~[..JONES, l~. SUNG, S. KATSANEVAS, i~. i. ~. KEMP, F. ~OVACS, ~. ~U~ClAR, ~. D. LAFFERTY, S. B. LINE, S.-~I. LEV~[~ V. LIEBENA~, I. L~TT, (~. LONDON, D. I~RCER, K. iVIOLLER, ]). NEWTON, E. PAUL, P, IDRTRO~F, Y. lOONS, C. I~AINE, F. RICHARD, R. I:~ICHTER, S. ~. ]~. ROBERTS, P. ROUDEA~7, A. I~OUG]~, ]VL RUMPF, J. ~LE~4s C. STEINHAUER, E. I~1. STORR, R. 5. T~OMPSON, D. TREILLE, CH. DE Li VAISSI~RE, H. VIDI~AU, L VIDEAZr, A. P. WAITE, 2L. ~VIJANGCO, T%V. ~ZOJCIE, J.-P. WUTHR][CE and T. P. YIOU: Phys. Left. J~, 89, 4Z[ (1980).

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17~ PHOTON-E~ULSION COLLABORATION AND OMEGA-P~OTON COLLABORATION

with zero. Because each of the four particles has a momentum below the ~erenkov threshold for mass identification, the effective mass of the four particles has been computed for different combinations of mass assignments. In one case it turns out to be very close to the mass of the D-meson, i.e. when the kaon mass is assigned to track 6.2 and the pion mass to the other three, giving m 6 (T:-K+u-r: +) = (1.847 • 0.007) GeV/c ~ (*) The hypothesis of a D o decay appears then very likely. From its total momentum and decay length, one derives a proper decay time t~0 = (0.86 =h 0.01)- 10 -18 s.

In order to find the second charmed particle produced in the primary photohadronic interaction, the secondary vertex 0' and the particles detected by the spectrometer, but not seen in the emulsion, were examined. Assuming that tracks 7 and 8 are, respec- tively, a proton and a negative pion, their effective mass is m~,s= (1.115• GeV/c 2, i.e. that of a A ~ This A ~ can also be associated with the secondary vertex 0 ~, which is very near to the primary one. Moreover , its line of flight is coplanar with tracks 4 and 4.1 within the errors: [~4• 0.007 4-0.010. This strongly suggests tha t the vertex 0' corresponds to a two-body decay. The momentum of the A ~ is ~A0 =(2.967 • 0.003)GeV/e, that Of the outgoing charged particle is then calculated to be P , . I= (3;96 4-0.24) GeV/c, and that of the decaying particle 24 = (6.73 4-0.23) GeV/c. Under the assumption that particle 4.1 is a pion, the effective mass of the: decaying particle is rod= (2.33 4- 0.05)GeV/e ~, which is consistent with the mass of the A +- baryon (8). We can, therefore, assume that the vertex 0' results from th e decay of a A + into A~ +. The proper decay time of this A + is found to be tA + ~ (0.57 4- 0.02)" 10 -1S s.

The effective masses, computed by associating either of the above charmed particle with the charged particle from the primary interaction which was detected by the spectrometer (No. 3), are inconsistent with any known charmed states with higher mass.

We conclude that the event described can be interpreted as the associated photoproduction of a charmed meson and a charmed baryon. I t is the first completely recon- structed example that has been observed. The charmed meson, identified as a D", has a proper decay time t~~ = (0.86 4- 0.01). 10 -~3 s, a n d the charmed baryon, identified as a A~ +, has a proper decay time tA+r = (0.57 4- 0.02). 10 -las.

We acknowledge the invaluable help of the Mechanical Assistance Groups of CERN EP Division and of Messrs. F. CONFO~TI and C. RICCI throughout the experiment. We are very much indebted to the Omega technical staff for their essential con- t r ibut ion during the run, We wish to express our thanks to Dr. C. LVBO~ILOV and his Group at Dubna as well as to the staff of the CERN Emulsion Treatment Group for the successful processing of the emulsions. Our gratitude is also due to the scanning teams of the Photon Emulsion Collaboration for their enthusiasm and efficiency.

The 0mega-Photon Collaboration thanks the Science Research Council (UK) and the Bundesministerium flit Forschung und Teohnologie (FRG) for their support.

(*) The mass values and the errors quoted are the present estimates from TRIDENT.

(s) PARTICLE DATA GROUP, REVIEW OF PARTICLE PROPERTIES: ROY. MO•. Phys., 52, 105 (1980).

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Associated photoproduction of a charmed meson and a charmed baryon