Photoproduction of charmed F mesons at γ energies of 20–70 GeV

Volume 100B, number 1

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19 March 1981

PHOTOPRODUCTION OF CHARMED F MESONS AT ¥ ENERGIES OF 2 0 - 7 0 GeV Bonn a _ CERN b _ Ecole Polytechnique c _ Glasgow d _ Lancaster e _ Manchester f - Orsay g Paris VI h _ Paris VII i _ Rutherford J - Sheffield k Collaboration D. ASTON J, M. ATKINSON J, R. BAILEY k, A.H. BALL f , B. BOUQUET g, G.R. BROOKES k, J. BR(JRING a, p.j. BUSSEY d, D. CLARKE J, A.B. CLEGG e, B. D'ALMAGNE g, G. DE ROSNY e, B. DIEKMANN a, A. DONNACHIE f, M. DRAPER d, B. DREVILLON c, I.P. DUERDOTH f, J.-P. DUFEY b, R.J. ELLISON f, D. EZRA f, P. FELLER a, A. FERRER g, P.J. FLYNN e, F. FRIESE a, W. GALBRAITH k, R. GEORGE h, S.D.M. GILL f, M. GOLDBERG h, S. GOODMAN h, M. GORSKI g,~, W. GRAVES c, B. GROSSETI~TE i, P.G. HAMPSON f, K. HEINLOTH a, R.E. HUGHES-JONES f, J.S. HUTTON J, M. IBBOTSON f, M. JUNG a, S. KATSANEVAS c, M.A.R. KEMP J, F. KOVACS i, B.R. KUMAR J, G.D. LAFFERTY e, J.B. LANE f, J.-M. LI~VY h, V. LIEBENAU a, j. LITTJ, G. LONDON h, D. MERCER f, J.V. MORRIS J, K. MULLER a, D. NEWTON e, E. PAUL a, p. PETROFF g, Y. PONS i, C. RAINE k, F. RICHARD g, R. RICHTER a, J.H.C. ROBERTS f, P. ROUDEAU g, A. ROUGI~ c, M. RUMPF c, M. SENt~ h, j. SIX g, I.O. SKILLICORN d, J.C. SLEEMAN d, K.M. SMITH d, C. STEINHAUER a, K.M. STORR e, R.T. THOMPSON f, D. TREILLE b, Ch. De La VAISSII~RE h, H. VIDEAU c, I. VIDEAU c, A.P. WAITE f, A. WlJANGCO c, W. WOJCIK g,2, J.-p. WUTHRICK c and T.P. YIOU h Received 7 October 1980

Evidence is presented for inclusive photoproduction of F ± mesons in three decay modes, r~"+, r/~r±~r+~r- and r/~r±Ir+~r+~r-n-. The average mass of the F -+is found to be 2.020 ± 0.010 GeV.

F mesons contain both charm and strangeness, and are generally expected to decay into systems containing a pair of K mesons or an r/[1]. Evidence for the F meson decaying into ~/zr± has been found in e+e annihilation [2]. In a previous paper [3], we reported on the photoproduction of charmed D mesons. In this paper we report on a systematic search for F observed as narrow width signals, i.e. having a width compatible with the experimental resolution, in (r/mr) ± mass spectra. The experiment was carried out using the Omega Spectrometer at the CERN SPS [4,5]. A tagged photon i On leave of absence from University of Warsaw, Warsaw, Poland. 2 On leave of absence from Institute for Nuclear Research, Warsaw, Poland.

facility provided a beam intensity of 2 X 105 photons/ burst over an energy range 2 0 - 7 0 GeV. Gamma rays from interactions in the 67 cm 10ng liquid hydrogen target were measured in a photon detector consisting of an active converter, made up of 42 blocks of lead glass of thickness 3 radiation lengths, followed by a position detector of 792 scintillation counters and then an array of 343 lead glass blocks, each of area 140 X 140 m m 2, and depth 470 m m (18.5 radiation lengths). This photon detector subtended a solid angle of 0.4 sr about the forward direction and measured the photon direction to an accuracy ~ +0.4 mrad. The data presented here come from two triggers, T1 and T2. In T1, the spectrometer was triggered by requiring at least four charged particles at a plane 1.5 m downstream of the centre of the target. The integrated luminosity for T1 was 60 events/nb. In T2 we required

0 0 3 1 - 9 1 6 3 / 8 1 / 0 0 0 0 - 0 0 0 0 / $ 02.50 © North-Holland Publishing Company

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3001 > o

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Fig. 1. (a) "r~'mass for Trigger 1 data (see text for the "t selection); (b) mr+n- mass for Trigger 1 data. The curves are obtained from polynomial plus Breit-Wigner fits. a secondary photon with transverse m o m e n t u m greater than 800 MeV, together with at least one charged particle leaving the target. The integrated luminosity of T2 was 50 events/nb. The ~7mesons were detected via their decay into two photons. To reduce combinatorial background un-

19 March 1981

der the r/we eliminated photons that could be associated to make rr0's. We consider first the T1 data. For this trigger the background remaining after the 7r0's were removed was due to low energy 7's from non-reconstructed rr0's. To reduce this background we rejected photons with energy below 2 GeV; this rejects a negligible number o f r/'s. The 7's from ~ decay are likely to have larger PT than 3"s from non-reconstructed zr°'s. We find that a suitable cut to exploit this feature is that the sum o f the p2T of the two 7's forming an 7/candidate be greater than The relevant part o f the resulting 3'7 mass spectrum, fig. la shows an r? peak at 552 MeV with an FWHM of 72 MeV. In order to improve the resolution, 3"r mass combinations were selected between 500 and 600 MeV and a 1C fit was performed imposing the r/mass. The r/Tr+zr- inclusive mass spectrum of fig. lb shows the r~'(958); its width agrees with our calculated mass resolution (see table 1). We have plotted inclusive mass distributions for charge +1 states r/(mrr)-* (m = 1, 3, 5) and for the corresponding subsamples r/'(nTr) ± (n = 1,3). Evidence for F is found in r/rr±rr+Tr- (fig. 2a) and in r~Tr-+rr+rr+Tr-rr- (fig. 2c). There is no indication that the signal appears more strongly in one charge state than in the other. The r/Tr-+Tr+rr+Tr-rr- signal originates, for the most part, from 7/'rr+-rr+Tr- (fig. 3a); there is no significant signal for F on the r/'rr+- mass spectrum (fig. 3b). The widths of the F peaks are consistent with the experimental resolutions (table 1) and the masses agree with the mass of F +- published b y the DORIS group [2]. Assuming a polynomial shape for the background, and counting events in an interval equal to the expected FWHM, the significance o f the signals is about 40 for ~(3zr) and 30 for r/(5rr). No significant F signal is observed in ~Trre (fig. 2b).

m2/2.

Table 1 Characteristics of the observed signals a).

rl ~ "r3' r~' ~ ~r+zrF ~ r/~r F ~ r131r F ~ ~51r

Expected width FWHM (MeV)

Observed width FWHM (MeV)

Observed mass (GeV)

14 75 50 40

72 ± 4 17 +- 6 108 ± 31 38 +-24 48 +- 34

0.552 0.960 2.047 2.021 2.008

a) The observed width of the r/is used to normalize the errors. 92

+-0.001 +-0.003 ± 0.025 ± 0.015 -+0.020

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Fig. 2. Inclusive ~/(mr) mass spectra: (a) r/~r±rr%r-; Trigger 1. Co) r~n±; Trigger 1. (c) r~lr±lr%r%r-lr-; Trigger I. (d) r/Tr±, Trigger 2. The curves are obtained from polynomial plus B r e i t Wigner fits.

We now consider the T2 data. This trigger only has good acceptance for the r/Tr± channel, since this twobody decay of the F in an S-wave naturally produces 20E

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19 March 1981

a large fraction of r/'s with high transverse momentum. The absence of an F signal in r/rr± with the T1 trigger could indicate a suppression coming from the multiplicity requirement in that trigger. To check this possibility we have analyzed the T2 data using only events with a forward charged particle multiplicity less than or equal to four. The r/mesons were selected using the high PT photon plus any photon with energy greater than 500 MeV. The 3'7 mass spectrum (not shown) is similar to that in fig. 1. As before, the 3'3' pairs were fitted to the r/mass. To reduce combinatorial background in r/rr± mass combinations, we eliminated charged pions with PT < 300 MeV. This selection would reject only a small fraction of F mesons decaying into r/ft. With an additional cut requiring the total visible mass to be > 4 . 0 GeV, we obtain the spectrum on fig. 2d showing evidence for F. The statistical significance of this signal is 4.5o. Within statistics, the F signal appears equally in both F ÷ and F - . The mass and width are given in table 1. Estimation of cross section is clearly model dependent. Assuming that the F's are produced in pairs and that the pair carries most of the incident photon energy, the acceptances are essentially independent of the details of the production mechanism and vary smoothly with r/mr mass. The efficiency, as defined by the product of the over-all acceptance and the r/decay branching ratio into 3'% is given in table 2, which also gives the values of the product B'o, where o is the cross section for FF- production for the three observed decay modes. One can notice that the upper limit on r/•r ± with trigger T1 is not incompatible with the value obtained with trigger T2, so that we cannot draw a definite conclusion on any multiplicity bias coming from T1. Averaging the three modes, we find for the mass Table 2 Inclusive F F photoproduction cross section a).

~->~200

Mode

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a) The efficiencies include the branching ratios for r/in 3'3' and 72' in rt%r-r/. Upper limits are calculated at the three standard deviation level.

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M F = 2.020 + 0.010 G e V , where the error is statistical only. We estimate that the systematic errors are less than 20 MeV. We are grateful to the Omega group at CERN for their help in running the spectrometer and providing on-line and off-line software. The work o f the technical support staff in our home institutions has been invaluable. We thank SRC (UK), the BMFT (W. Germany) and the IN2P3 (France) for their financial support.

References [1 ] See, for instance, C. Quigg and J.L. Rosner, Phys. Rev. D17 (1978) 239.

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[21 R. Brandelik et al., Phys. Lett. 70B (1977) 132; 80B (1979) 412. [31 D. Aston et al., Phys. Lett. 94B (1980) 113. [4] For details of the Omega Spectrometer and the tagging system, see D. Aston et al., Nucl. Phys. B166 (1980) 1. [51 F. Bourgeois, H. Grote and J.-C. Lassalle, Pattern recognition methods for Omega and SFM spark chamber experiments, CERN/DD [DH/101 ; H. Grote, M. Hansroul, J.-C. Lassalle and P. Zanella, Identification of digitized particle trajectories, Proc. Intern. Computing Symp. (Davos, 1973) (North-Holland, Amsterdam, 1974) p. 413.