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Volume 63B, number 3

PHYSICS LETTERS

2 August 1976

O B S E R V A T I O N O F T H E K * ( 1 7 8 0 ) IN T H E R E A C T I O N K+p ~ K os Ir+ p AT 10 G e V / c R. BALDI, T. BOEHRINGER, P.A. DORSAZ, V. HUNGERBUHLER, M.N. KIENZLE-FOCACCI, M. MARTIN, A. MERMOUD, C. NEF and P. SIEGRIST University of Geneva, Switzerland Received 31 March 1976 An analysis of the K~r-systemin the mass region of the K*(1780), based on a sample of 46000 Ks°n÷ final states, is presented. Evidence for a relatively narrow width, T"~ 100 MeV, and for the spin parity assignment JP= 3- is found. We present results of an analysis of the KTr-system in the mass region 1500
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Experimental set-up and data selection. The present data have been taken with a two-arm spectrometer using counters and multiwire proportional chambers (fig. 1). Incident particles in the unseparated beam are identified by four threshold Cerenkov counters. Incident momentum and direction are measured by scintillation hodoscopes. A recoil detector, consisting of 6 MWPC planes, time-of-flight and range counters, measures direction and momentum of the final state proton at polar laboratory angles between 39 and 70 degrees, and with an azimuthal acceptance/x@/27r ~ 15%. The forward arm consists of 8 MWPC planes, to record the trajectories of the other final state particles. The absence of a magnetic momentum analysis allows to cover a large forward solid angle (+22.5 ° with respect to the

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Volume 63B, number 3

PHYSICS LETTERS

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beam). A detailed description of the spectrometer is given elsewhere [ 1]. Events of the type K+p ~ K°lr+p with K ° ~ rr+rrare identified first by requiring a seen K °-decay vertex, at least 27 mm downstream from the production vertex. We then use momentum conservation to calculate the momenta of the forward tracks. Events which satisfy the constraints of overall energy balance and K °-mass within wide limits, are then processed by a kinematical 2C-fit, and accepted if P(× 2) > 5%. For the remaining events we require that the forward tracks have momentum p > 300 MeV/c. This cut eliminates events suffering from large multiple scattering in the target. In addition, it removes completely a small background (2%) from the K+p ~ K°A ++ reaction, which is already suppressed by geometry and the requirement of a single

2 August 1976

track in the proton detector. The amount of background in the final sample of fitted events can be estimated from the distribution of unfitted K°-mass. Interpolation of the tails of this distribution under the K°-mass peak gives a background of ~2%. This is also consistent with an independent estimate based on the shape of the p(×2) distribution. The spectrometer acceptance has been calculated by Monte-Carlo simulation as a function of K~r-mass and decay angles, taking into account geometry and chamber efficeincy, the decay and absorption of the particles, and the momentum cut on the forward particles. The acceptance has been averaged over the t-interval used, assuming do/d t ~ e 5 t;it is insensitive to the value of the slope of do/dt. It is a smooth function of mass and decay angles. For a mass of 1800 MeV and an isotropic decay, the acceptance of the forward arm is 42%, and the variation over the mass region 1.5-2.0 GeV is a factor of 2. Regions close to cos 0 = +-1 are depopulated due to chamber inefficiency in the beam region and wide angle acceptance. The Monte Carlo calculation has been checked by generating samples of events with the same angular distribution in the KTr rest frame as the real data and comparing the K ° decay and lifetime distributions. There is good agreement with the observed spectra. Fig. 2 shows the observed (histogram) and acceptance corrected (points) K°rr+-mass spectrum in the region from 1.5-2.2 GeV *~ . Above the tail of the K*(1420), production of the K*(1780) is evident in the acceptance corrected spectrum. The correction is larger in the K*(1780) region than in the neighbouring regions. This effect is due to pronounced peaks of the KTr-decay angular distribution in the forward and backward direction. Mass and width o f the K*(1780). To obtain mass and width of the K*(1780), we first determine the shape of the background. We fit simultaneously the mass intervals 1100-1580 MeV and 1980-2200 MeV, using a non-relativistic Breit-Wigner shape for the K*(1420) plus a third degree polynomial. The background is then kept fixed and a fit of the whole mass region is done, adding a Breit-Wigner shape for the K*(1780). The resulting fit, shown in fig. 2 (full line), gives the parameters , t A constant factor of 6, due to the azimuthal acceptance of the proton detector, has not been included in the correction.

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Volume 63B, number 3

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2 August 1976

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Volume 63B, number 3

PHYSICS LETTERS

150

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M = 1783+-6 MeV,

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The errors are statistical; the systematic error of the mass scale, estimated from the position of the K*(890) and K*(1420), is negligible. The fitted number of K*(1780) events corresponds to a cross section of Ao = 0.9+_0.3 tab (systematic error included) within the t-interval 0.05 < It l < 1.0 (GeV/c) 2 . The slope*2 of the differential cross section de/d t eat, for all events in the mass band 1700 KOrt+

Spin-parity determination. We have used a moment analysis of the K~r-decay angular distribution to determine the spin of the K*(1780). Using parity conservation in the production process, the angular distribution of the 7r+ in the Gottfried-Jackson system is written as

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2 August 1976

where the spherical harmonics moments (Re YL M) are functions OfMK,~and t. We determine the moments as functions Of MKTr, in the t-interval 0.2 < [tl < 0.8 (GeV/c) 2 from a fit of I ( ~ ) to the data. Fig. 3 shows the unnormalized, acceptance corrected, moments N < Re yM), where N is the number of produced events in each MK,r mass bin. This fit includes the terms L~<6 with M = 0,2. Several other sets L, M have been explored. The higher moments L = 7,8 vanish within statistical errors; the largest fluctuations in single mass bins are about 2 standard deviations. Also the moments with M = 3,4 are consistent with zero. Moments with M = 1 are not required by the data to obtain a good fit. However, the finite acceptance of the spectrometer introduces correlations between the M= 1 and M=2 moments, such that we cannot exclude some contribution from M= 1 terms. A significant signal at the mass of the K*(1780) is seen in all moments with M=2 up to L=6. A BreitWigner fit to the moment N(Re y2) gives values in agreement with the fit to the mass spectrum M= 1779-+ 11MeV, 1"= 135-+22 MeV. Together with the absence of significant L = 7, 8 moments, this is evidence for a JP= 3 - resonance. Furthermore, the shape of N(Re Y5M) is consistent with the interference of a spin-2 K*(1420) with a spin-3 K*(1780). To estimate the significance of the spin 3 component, we plot the uncorrect cos 0-projection in the GottfriedJackson frame for events in the mass band 1700 < MKn < 1900 MeV. We use the moments resulting from fits with and without L = 5,6 terms, indicative of spin 3, to generate the Monte-Carlo distributions shown as curves in fig. 4. A X2-comparison in the interval Icos 0J < 0.9, where the acceptance is >10% throughout, gives ×2 = 114.4/17 degrees of freedom for the fit with Lmax = 4, which is therefore clearly rejected. Allowing for M= 1 terms does not improve the fit (×2 = 115.1/17 d.f.). This rules out J < 2. Addition of a spin 3 component, Lmax = 6, however, gives a good description of the data, ×2 = 22.5/17 d.f. (C.L. 16%). In conclusion, we have observed clear evidence for a K*(1780) resonance with a width of ~ 100 MeV and spin parity JP= 3-. In comparison to other recent experiments, our value of the width is in good agreement with that found in K*(1780)-production in the non-charge-exchange channel [5], but differs from the braod ( P > 2 0 0 MeV) structure seen in the charge-exchange channel [6]. 347

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We gratefully acknowledge the hospitality and technical assistance o f CERN, as well as the loan o f equipment by the EP Division. We thank the Fonds National Suisse for support of the project, and the technicians of the University o f Geneva for expert assistance.

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2 August 1976

References [1] R. Baldi et al., Large acceptance spectrometer for simple event topologies, paper submitted to the EPS Conf. Palermo (1975). [2] D.D. Carmony et al., Phys. Rev. Lett. 27 (1971) 1160. [3] A. Firestone et al., Phys. Lett. 36B (1971) 513. [4] M. Aguilar-Benitez et al., Phys. Rev. Lett. 30 (1973) 672. [5] M. Spiro et al., Phys. Lett. 60B (1976) 389. [6] G.W. Brandenburg et al., Phys. Lett. 60B (1976) 478.