c

Volume 74B, number 3

PIfYSICS LETTERS

10 April 1978

A PARTIAL WAVE ANALYSIS OF THE (~,01r+n - ) SYSTEM PRODUCED IN K - p ~ ~,0n+n-n AT 10 GeV/c W. BEUSCH and A. BIRMAN 1 CERN, European Organization ]'or Nuclear Research, Geneva, Switzerland

E. K(JNIGS, G. OTTER, G. RANSONE, H. SCHLLJTTER and H. WIECZOREK IlL Physikalisches b~stitut der R WTH Aachen, Germany

and B. AEBISCHER, L. FLURI 2 , K. FREUDENREICH, F.X. GENTIT 3 A. NAKKASYAN 4 and J. PERNEGR Laboratorium for Hochenergiephysik, ETH Zftrich, Switzerland Received 20 January 1978

A partial wave analysis of the non-diffractively produced (~0 rr+n-) system has been performed. The system was produced in the reaction K - p -, K°Tr÷Tr-n at 10 GeV/c, measured in the CERN Omega spectrometer. Besides the well-known K*(1420) resonance, we find good evidence for the production of Q2(1400) and some indication for Q1(1290) production in JP= 1 ÷. In addition we clearly observe a b u m p in the 1800 MeV region, the properties of which are discussed.

Evidence for structures in the (KTrTr)-systemhas been found in several experiments (for a review of K*-spectroscopy, see ref. [I]). Most of the results arise from diffractive channels, having large cross sections even at high energies. In this paper we present results from the charge exchange reaction (1): K - p ~o n+Tr- n at 10 GeV/c, which is particularly well suited for studying resonance production in the (K°Tr+rr- ) system, since diffractive background is absent. The experiment has been performed with the onrega-spectrometer at the CERN PS. The apparatus, shown in fig. 1, has been described in ref. [2]. The spectrometer consists of a large aperture superconducting magnet yielding a field of 1.8 T. Particles are detected by a system of optical spark chambers placed inside the magnet. The chamber system cont 2 3 4

Present Present Present Present

282

address: address: address: address:

Technion, Israel Institute of Technology, Haifa. Ecole Polytechnique, Palaiseau, France. CEN, Sacley, France. EPFL, Lausanne, Switzerland.

5m

,

×=0

L

-~rn i

i

Y

Fig. 1. Experimental set up. Plan view on the CERN Omega spectrometer including the (2-prong + V °)-trigger layout.

sists of 8 large spark chamber modules (10 gaps each) arranged perpendicularly to the beam, plus 8 small spark chambers (8 gaps each) arranged parallel to the beam, i.e. 4 on each side of the cylindrical 30 cm long H 2 target. The chambers are viewed from above by

Volume 74B, number 3

PHYSICS LETTERS

,600

(:3 d-~.

/ -400

1.w0

~,

/

[]

g

13

uncorrected data

[~ corrected for

acceptance

"/'q

.......

1.6

, .....

1.9

"i

.

2.2

.

.

.

2'.5

M ~o It* rt" ( GeV/c21 Fig. 2. (K°rr%r-) effective mass spectrum for reaction (1) with

and without (hatched distribution) acceptance correction. The solid curve corresponds to the overall fit, consisting of a cubic polynomial for the background plus a sum of two Breit-Wigner functions for the K*(1420) and the K*(1800) peaks. The fitted background is represented by the dashed

line. TV cameras of the plumbicon type. The measurement accuracy for a pair of sparks is -+0.5 mm in space after all corrections. We used an unseparated negative beam containing 98.2% 7r-, 1.5% K - and 0,3% ~. The momentum and direction of each beam particle were measured by a scintillator hodoscope and multiwire proportional chambers. The measurement accuracy was +-0.2% for the momentum and -+0.15 mrad for the direction. Beam particles were identified with 3 Cerenkov counters, the beam flux was limited to 3 X 105 particles per burst. Tire central value of the beam momentum was (10.01 -+0.03) GeV/c. The trigger was designed to select the 2-prong + V0-topology. The two prongs coming from the vertex inside the target were detected in a multiwire proportional chamber MWPC 1 about 5 cm downstream of the target. This chamber had an active surface of 19 X 19 cm 2 and 1 mm spacing of horizontal and vertical wires. The second chamber MWPC 2 was 80 cm downstream o f the target it was 147 cm wide, 96 cm high

10 April 1978

and had 2 mm spacing of the horizontal and vertical wires. MWPC 2 responded to the two tracks of the V 0 decay in addition to the two prongs from the production vertex (see fig. 1). 260 000 triggers were recorded within 10 days of running time. Pattern recognition and geometrical event reconstruction were performed by the off-line program ROMEO, the kinematical fitting by the program KOMEGA [3]. After removing of background we obtained 5741 good events o f reaction (I), out of which 3935 events had a (KrrTr) mass below 2 GeV and t'pn < 0.8 GeV 2, corresponding to a sensitivity of about 30 events/~b in this region. The geometrical acceptance of the apparatus was calculated using bubble chamber events at the same energy * 1. The (K0zr+rr - ) effective mass distribution for raw and acceptance corrected data is shown in fig. 2. Besides a structure in the 1400 MeV mass region, we observe an additional peak around 1800 MeV, which we denote by K*(1800). Tire (KTr~) mass spectrum in fig. 2 can be described by a sum of two Breit Wigner functions (M t = 1428-+ 7 MeV, P l = 118-+ 10MeV a n d M 2 = 1812 -+ 28 MeV, ['2 = 181 -+24 MeV) above a mass dependent background ,2 [4}. A partial wave analysis (PWA) of the (K%r+rr - ) system has been performed using the Illinois PWA program [5], adapted to our reaction. The (KTrTr) system was described in terms of states (JPm ~ljn}, where JPm gives the spin-parity of the (KTr~r) system and its third component in the Gottfried Jackson frame; ~ denotes the naturality of the exchange, I gives the relative angular momentum between the dimeson system with s p i n / a n d the remaining meson, and n indicates the various dimeson couplings. The details of the method are described in ref. [61. The observed experimental distributions o f the kinematical variables were affected by the limited trigger acceptance of the spectrometer. To take this fact into account we have included the acceptance into the fit process. The fit likelihood was normalized to the experimentally populated region o f the phase ,1 10 GeV/c K-p exposure of the Aachen-Berlin-CERNLondon-Vienna Collaboration. We are grateful to our colleagues for allowing us to use a data summary tape for acceptance calculations. 4-2 As shown later, the (1400) mass enhancement consists, however, of several spin-parity states. 283

Volume 74B, number 3

0-

PHYSICS LETTERS

++++++++

10 April 1978

I÷0+S(K'E

2+

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o

200-

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Fig. 3. Intensities of the various JP-states as function of the (K°n+n-) mass in the region 1.0 GeV < MK~rn< 2.0 GeV in 100 MeV intervals. The Breit-Wigner curves which are drawn in the 2+ and 3- distributions correspond to M1 = 1428 MeV, Fx = 118 MeV andM2 = 1812 MeV, F2 = 181 MeV, respectively. space. This was done by a Monte Carlo integration (tracing the generated events through a simulated trigger). A more detailed description of the procedure can be found in ref. [7]. In fig. 3 we present the intensities of the various spin-parity states (summed over decay modes and helicities) of the (g,0n+Tr-) system. The PWA fits have been carried out in 100 MeV (Knn) mass intervals up to a mass of 2.0 GeV. In this mass region N* production is of minor importance. Fits including or excluding the A+ region give the same results within statistical errors. In the data presented, no N* cuts are applied. Due to our limited statistics the results concerning the relative phases of the various states are not conclusive. Below a (Knn) mass of 1400 MeV, the 1+ state is dominant. In diffractive (Knn) systems, produced at high energy, evidence for two 1+ resonances has been 284

1.0

1:4

£8 MK°Tt ÷ n- [GEM]

Fig. 4. Intensities of the JP = l + wave as a function of the (K.°lr%r-) mass. The interference between (Kp) and (Kn) is given in the distribution "Int. (Kp, K~r)". found. Brandenburg et al. observe both the Q1(1290) and the Q2(1400) resonances [81, while Otter et al. find only a clear Q1 and some evidence for Q2 [9]. In a non-diffractive (KTrn) system, produced at low energy, the behaviour of the 1+ wave shows also some indication of Q2 production [10]. Since several cuts to remove N* contributions have to be applied to these data, their interpretation is complicated. All these results show that Q1 decays dominantly into Kp and that the main decay mode of Q2 is K*Tr. Both decays are observed to be in S-wave. In fig. 4 we show the decomposition of the JPm = 1+0 state of our data into the contributing dimeson couplings K*Tr, Kp and Kn (the states with rn 4=0 are found to be of minor importance). The (K*zr) mode has a pronounced peak around 1400 MeV. Since an enhancement produced merely by Deck effect should peak at a lower mass, we interprete this peak at 1400 MeV as evidence for the production of the Q2 resonance.

Volume 74B, number 3

PttYSICS LETTERS

10 April 1978

Table 1 Density matrix forJ P = 2+ in the region 1.3 GeV < MKrr,r < 1.55 GeV. The trace of the density matrix was normalized to 1. 02+1+

0

0

0

2+0 02+0 -

2+0 02+ 1 _ J =

0.23 +-0.08

0

2+1 02+0_

2+1 - 1 02+1 _ /

0.69 -+0.10 0.01 + 0.07 + i(0.21 -+0.24)~ L \

0.08 -+0.08

In both the (Kp) and (Krr) states we observe a clear peak in the mass interval 1200 to 1300 MeV, corresponding to the QI mass region. However, the strong destructive interference between the (Kp) and (Krr) modes weakens the interpretation of the observed peaks as being due to the Q1 resonance. The 1 - state is dominantly produced by unnatural parity exchange. It shows a rather smooth behaviour with (Krrrr) mass having a broad maximum around 1600 MeV (see fig. 3). This coincides with the region where the production of K*(1650) is expected [11 ]. The 2 + wave, shown in fig. 3, is identified with the K*(1420) resonance. The solid line is a Breit-Wigner curve for K*(1420) (M = 1428 MeV, P = 118 MeV), corresponding to a production cross section of 11 ~b. As seen in the figure, the data are not very well described by the fitted Breit-Wigner curve. This is not due to systematical errors of our apparatus, since the mass of the K 0 (from the V0 decay), as well as the mass of the K*(890) have the correct values. The density matrix elements for the 2 + state, given in table 1, are in good agreement with corresponding values of the reaction K - p -+ K*(1420)n at the same energy, with K*(1420)decaying into ( K - u +) [12]. For the (Ko) to (K%r) branching ratio (BR) we obtain BR = 0.21 + 0.08. The 3 - state is present in the higher (KTrTr)mass region from 1600 to 2000 MeV. However, we emphasize, that it is possible to replace the 3 - state by 2 + in this mass region, since the density matrix elements for the 3 - wave are such that all (D/=6)-moments are small [7]. Due to our limited statistics it is not possible

/

to include both the 2 + and the 3 - waves simultaneously in the PWA. The data are described equivalently by the two ambiguous solutions. The following arguments leas us to prefer tile 3 - solution, as given in fig. 3. Indications for the production of various resonances in tile (Krr) and (KrrTr) mass spectrum around 1800 MeV have been claimed recently (see ref. [131 and references therein), but only the K*(1780)with spinparity 3 - is well established (see e.g. refs. [11,14,15]). From (Krr)phase shift analyses [11,15], this resonance is known to be rather inelastic. Since it has been observed in the reaction K - p -+ K-rr+n at 13 GeV/c [16], it is also expected to be produced in tile (K0rr+Tr- ) system of our reaction (1). In our PWA the 3 - wave is well described by a Breit-Wigner resonance curve with M = 1812 +- 28 MeV and F = 181 -+ 24 MeV (see fig. 3). We therefore believe that the 3 - state can be identified with K*(1780). The production cross section is found to be (16 +- 3)pb. The 3 - wave makes up the entire enhancement in the (KTrrr)mass spectrum around 1800 MeV (see fig. 2). Table 2 gives the 3 - density matrix elements. For the (Kp) to (K%r) branching ratio of K*(1780) we obtain BR = 0.9 -+ 0.3. Other decay modes (K*(1420)rr, Kt) are not observed, which is in agreement with the results from ref. [17].

We are grateful to the Omega technical staff and to all other people who have contributed to the success of this experiment. We thank Dr. G. Rudolph for fruitful discussions.

Table 2 Density matrix for JP = 3- in the region 1.6 GeV < MKrrrr < 1.9 GeV. The trace of the density matrix was normalized to 1. 3 -1+ 03-1+

0

0

0

3 -0 03 -0 -

3 -0 03 -1 -

0

3 -1 03-0-

/

3 -1 0 3 - 1 --/

0.26 -+ 0.09 =

0.45 -+0.10

"-0,12 -+0.06 + i(0.04 -+0,18)] 0.29 + 0.09

/ 285

Volume 74B, number 3

PHYSICS LETTERS

References [1 ] D.W.G.S. Leith, SLAC-preprint, SLAC-PUB-1980 (1977). [2] O. Gildemeister, Intern. Conf. on Instrumentation for high-energy physics (Frascati, 1973). [3] F. Bourgeois et al., CERN DD/DH/70-13 (1970) ROMEO, KOMEGA, CERN Program Library. [4] E. K6nigs et al., Aachen preprint, Aachen IIIB[2-77 (1977). [5] G. Ascoli et al., Phys. Rev. Lett. 25 (1970) 962. [6] J.D. Hansen et al., Nucl. Phys. B81 (1974) 403. [7] E. K6nigs, Thesis, Aachen (1977) unpublished.

286

[8] [9] [10] [11 ] [12] [13] [14} [15] [16] [17]

10 April 1978

G.W. Brandenburg et al., Phys. Rev. Lett. 36 (1976) 703. G. Otter et al., Nucl. Phys. B106 (1976) 77. J.S.M. Vergeest, Phys. Lett. 62B (1976) 471. P. Estabrooks et al., SLAC preprint, SLAC-PUB-2004 (1977). P. Lauscher et al., Nucl. Phys. B86 (1976) 77. H. Gr~issler et al., Nucl. Phys. B125 (1977) 189. R. Baldi et al., Phys. Lett. 63B (1976) 344. M.G. Bowler et al., Nucl. Phys. B126 (1977) 31. G.W. Brandenburg et al., Phys. Lett. 60B (1976) 478. D.D. Carmony et al., Phys. Rev. D16 (1977) 1251.