Z. Physik C, Particles and Fields 7, 17-20 (1980) Zeitschdft P a r t i c l e s for Physik C

and F Ids �9 by Springer-Verlag 1980

Strangeness Exchange in the Photoproduction of K + A(1520) Between 2.8 and 4.8 GeV

(LAMP2 Group)

D. P. Barber 1, J. B. Dainton 2, L. C. Y. Lee 3, R. Marshall 1'4, J. C. Thompson ~, and D. T. Williams

Daresbury Laboratory, Daresbury, Warrington WA4 4AD, UK

T. J. Brodbeck s, G. Frost 6, D. Newton, G. N. Patrick, G. F. Pearce 1, and T. Sloan

University of Lancaster, Lancaster A1 4YB, UK

G. R. Brookes, W. J. Haynes4, and P. B. Wilkes ~

University of Sheffield, Sheffield $3 7RH, UK

Received 18 August 1980

Abstract. A tagged photon beam (2.8< E~<4.8 GeV) and multiparticle spectrometer have been used to study the photoproduction in hydrogen of K+A(1520). Precise values for the mass and width of theA(1520) are given. The total cross-section is found to fall with increasing photon energy like (6.5 • (2"1 _+0.2) gb.

da The differential cross section dr- indicates peripheral

forward production and exhibits no evidence for shrinkage when compared with higher energy data. The A(1520) spin density matrix shows that K exchange alone cannot account for the production mechanism. The reaction is found to resemble the process 7P--* K + A(1115) in all measurable respects.

Associated production of hadrons by photons has not been extensively studied to date. Measurements exist of K § photoproduction in which the recoil missing

Present mailin 9 addresses: 1 DESY, Notkestrasse 85, D-2000 Hamburg 52, Federal Republic of Germany 2 Department of Natural Philosophy, University of Glasgow, Glasgow G12 8QQ, UK 3 University of London Computing Centre, 20 Guilford St. London WC1N 1DZ, UK 4 Rutherford Laboratory, Chilton, Didcot, Oxon, OXll OQX, UK 5 CERN, CH-1211 Geneva 23, Switzerland 6 Systems Programming Ltd, 12 14 Windmill St. London W1P 1HF, UK 7 Daresbury Laboratory, Daresbury, Warrington WA4 4AD, UK

mass spectrum reveals a series of peaks which can be assigned to different associated production reactions of K § and hyperons [1]. The energy dependence of the total production cross-sections are broadly in agree- ment wit]h the expectations of strange meson exchange. In this paper we report a measurement of the process

7p-+g+A(1520) (1)

in which the exclusive channel

~p--, K + K - p (2)

has been isolated. Reaction (2) was studied in this experiment by

measuring all three final state particles in a large aperture magnet spectrometer [2] and by measuring the incoming photon energy in a high resolution tagging system. Events from reaction (2) could there- fore be identified and extracted from the data by means of a 4C kinematic fit to the reaction hypothesis without the need for additional particle identification.

The experiment was carried out at the 5GeV electron synchrotron N I N A at Daresbury using the LAMP 2 apparatus, details of which have already been published [2, 3]. The 5 GeV electron beam from the synchrotron was converted in a tagging system into a bremsstrahlung photon beam with a tagged energy range of 2.8 4.8 GeV and with an energy resolution which varied between 6 and 11 MeV across the energy range covered. The photon beam was collimated and focussed (by means of the electron beam focussing) to a 1 cm spot (95 % of flux) at a 30 cm long liquid hydrogen

0170-9739/80/0007/0017/$01.00

18 D . P . Barber et al. : St rangeness Exchange in the P h o t o p r o d u c t i o n of K+A(1520)

69 E

�9 "0 d) .r=

"5 $ .13 E= z

800

7 0 0

600 -

500 -

400 -

300 -

200 -

100-

0-

(b)

1

i

I

1.44 1.48 1.52 1.56 1.60 1.64

mK- p (G eV/c 2 )

1.0 1.2 1.4 1.6 1.8 2.0 inK+ K- (GeV/c 2)

(c)

(a)

1.4 1.6 1.8 20 2.2 2.4 2.6

mK+ p (GeV/c 2)

1400 1200 1000 800

600 400 200

0

600

500 400 300 200 100

0

Fig. 1. a The K + K - i nva r i an t mass spec- t rum for al l events m a k i n g a 4C fit to reac t ion (2), as defined in the text. b The K - p inva r i an t mass spec t rum for events selected as in a above, c The exotic K+p inva r i an t mass spec t rum

target with a cell diameter of 2.5cm. Immediately downstream of the hydrogen target was a large aper- ture magnetic spectrometer [2] for the detection and momentum analysis of charged particle tracks. A 480 channel lead glass detector used for the detection of photons and electrons was situated at the exit aperture of the spectrometer [3].

The trigger for the whole exposure was either one charged particle which completely traversed the spec- trometer or a deposition of energy in the lead glass of more than 500MeV, or both, in coincidence with a tagged photon which failed to trigger a downstream beam counter (because it had interacted).

The data presented here are based on a total of 10 7

triggers which contained ~8 .5x106 good events where at least one charged or neutral particle could be identified in the pattern recognition or lead glass reconstruction programmes. A subset of events was selected from the total sample of good events on the criterion that they contained two positive and one negatively charged particle originating from a com- mon vertex. This subset amounted to 127,000 events.

These 3-prong, net charge + 1 candidates were subjected to 4C kinematic fits to the following hypotheses :

7p--,K+K p, (2)

--~z+Tr-p, (3)

~ Kzcp , (4)

--+ ~pp , (5)

where particle masses were different, all combinations were tried, consistent with the measured sign of the charge on the track. The kinematic fit took into account all measurement errors and correlations and

the effects of multiple scattering and dE/dx losses in the target. The latter two effects were of the same size as the tagged photon errors and were crucial in properly understanding the behaviour of the fitting procedure. The 137 events which fitted the hypothesis (5) have been the subject of of a separate analysis [4].

A total of 653 events fitted the hypothesis of reaction (2) with a probability of greater than 1%. None of these 653 events could be simultaneously fitted to the hypotheses of reactions (3), (4) or (5), a consequence of the good photon resolution and the power of the four energy and momentum constraints. Although this property is in itself sufficient to convince us that the reaction (2) was correctly identified, a further check was made by exploiting the known resonance parameters of the q~(1019) and the A(1520). Both peaks were observed to have the correct masses and widths (Fig. 1); the results of a fit to theA(1520) line shape being given below. The Z 2 probability distribution for all 653 events was flat, the customary spike n e a r P ( )~2) - - -0 being well contained within the interval 0<p(z2)<0.01. Moreover, since both peaks were narrow, roughly 10 % of the events in the peaks cofild be identified in the sample with Z 2 probability between 1 Too and 10 %.

The data were then corrected for effects due to acceptance, experimental dead time, e+e - normali- sation, track reconstruction efficiency, interactions of secondary tracks in the target [2] and K -+ decay [63. The resulting invariant mass spectra for the three combinations K + K , K - p , and K+p are shown in Fig. 1. Clear ~b andA(1520) signals can be seen in the K + K and K - p channels respectively, whereas the (exotic) K+p channel exhibits no noticeable structure.

Although the statistical sample of A(1520) events is modest by hadronic beam standards, the background

D. P. Barber et al. : Strangeness Exchange in the Photoproduct ion of K+A(1520) 19

Fig. 2. a The total cross-section for the process ~p-*K+A(1520) measured in this experiment. Superimposed is the combined fit to our data and the measurements at higher energy by Boyarski et al. h The

de differential cross-section - - for the photo-

dt production of K+A(1520) averaged over the incident photon energy range (2.8-4.8 GeV)

2 do- of this experiment, e E.~ d t measured by

this experiment and the higher energy data of Boyarski et al.

1100

1000 -

9 0 0 -

800 -

.s 700 '

600 .

500-

400- #

300"

13 ~- 200-

100"

O-

(a)

2.9 3.1 3.3 3.5 3.7 3.9 4,1 4.3 4.5 4.7

Ey ( GeV )

i~ + �9 This experiment

+

(b) t

0 Boyarski etal

(c)

.1 .2 .3 .4 .5 .6 .7 .8 .9

- t (GeV/c) 2

.1.0

0.5

(.9

.0,2 ~"

-0.1 g l ~

10

5 z.

2 2Y

under the peak appears to be much smaller in photo- production. Moreover, since all the final state particles were measured and the A(1520) is a relatively nar row state with a small Q for the K p decay mode, our mass resolution was very good (3-4 MeV/c 2) and the mass and the width can be determined quite accurately. In particular, the measured width needs only a small correction for the effects of resolution. We fitted the mass region m from 1.45 to 1.60 GeV/c 2 to the sum of a relativistic Breit-Wigner plus a linear background as follows,

do- 2ram o F(m) d m - A (m 2 _ rnZ)2+ m2 F2(m ) + B + Cm,

where [5]

Q 2L + 1

F(m)=FoQ~L+I and L = 2 .

Q is the centre of mass m o m e n t u m in the K p system. The resulting fit parameters were:

m 0 = 1517.3 __ 1.5 MeV/c 2

F 0 = 16.3__3.3 MeV/c 2

z2/DF = 28/30.

The errors include the effects of changes in the form of the background parametrisat ion [e.g. B'(m--mthresh ) instead of B + Cm] and the width has been corrected for the finite resolution of the spectrometer. The best fit curve is shown in Fig. 1.

Events of reaction (1) were selected as those with K - p invariant mass between 1.49 and 1.55 GeV/c 2 and the total product ion cross-section o- of reaction (1) was then calculated as a function of incident pho ton energy after correcting for background under theA(1520) peak (_~25%). The result is shown in Fig. 2a where the

errors are purely statistical and the data have been corrected for the A(1520) branching ratio to K - p of 46.2% [71. The systematic uncertainty in normalisa- tion was estimated to be less than 5 % on the basis of the measured e§ - and re+re - yields. The total cross- section of reaction (1) falls smoothly across our pho ton energy range; a fit to the form ~=AE-~ x, where E~ is the pho ton energy, produced a value of x = 2.1 _+ 0.2. Alternatively, if we fit to the form G=As -x, the corresponding best estimate for the exponent is x = 1.9 _+0.2. Figure 2a also shows the best fit to both our data and those of Boyarski et al. [1] superimposed on the data. We note that this reaction exhibits the same energy dependence [8] as 7p~K+A(l l l5) and that the normalisat ion (we find A = 6.5 + 0.7 lab. GeV 2 in- cluding the data of [11) also matches that for 7P ~ K § product ion where A(K +A(1115)) = 6.7 + 0 . 4 g b . GeV 2 [11.

do- In Fig. 2b, we show ~ - for reaction (1) averaged

over our pho ton energy range. The reaction is ob- served to be peripheral, the dependence on t being well parametrised by exp(6.1_+2.0)t in the range of this experiment, -0 .2>t>-O.7(GeV/c) . In Fig. 2c, we

~2do- show 7 dt for both our data and the higher energy

data of Boyarski et al. [11. There is good agreement in the c o m m o n region of - 0.2 > t > - 0.7 (GeV/c) 2, in-

do- dicating no evidence for any shrinkage of d~"

Fur thermore, the shape and normalisat ion of the differential cross-section is the same E81 within the errors as the process 7p~K+A(1115).

In Fig. 3, we show the distribution in cos0, the polar decay angle of A(1520)~K-p, measured in the t channel helicity frame (Gottfr ied-Jackson frame). The

20 D.P . Barber et al. : Strangeness Exchange in the Photoproduction of K+A(1520)

6

~ 5

~ 4 3

~ 2

1+3cos28

0 i i i

- 1 . 0 - . ; - ; -14 -.2 ,; . 2 : 4 .6 .8 1.0 c o s @

Fig. 3. The t channel helicity frame decay distribution as a function of cos0 for events in the A(1520) peak. Superimposed are the expected distributions if theA(1520) were produced only in the spin substates • ~ (i.e. 1 + 3 cos 20) and +_ ~ (i,e, sin z 0)

z-axis is taken to be ant• to the direction of the incident proton in the K p rest frame and the y-axis is taken to be the normal to the production plane; the K is taken as the decay indicator. Superimposed are the distributions expected if the A(1520) ( j e = ~ - ) is produced in spin substates _+ t (i.e. 1 + 3 cos20) and in the spin substates +23 (i.e. sin20). If K • exchange dominates the production mechanism, we expect only the former, and this is clearly inconsistent with the data.

If we assume the decay distribution expected for 23- decay of the K p to terms of the parent spin density matrix 02M2M', where M and M' are spin projection quantum numbers, namely

3 f . 2 0 t(cos0, qS) = 4~ 1r sm -l- ~ 1 1(31 -I- COS2 0)

2 sin 0co 2 4 then, by the method of moments, we find the density matrix elements presented in Table 1. The trace con- dition ~33+01~=~ has been assumed in this eval- uation. Simple K • exchange requires ~33 =0 which is inconsistent with the data by seven standard de- viations. It is again worth recalling that the process 7p--*K§ is dominated by natural parity ex- change (K*) and not by K exchange. This is expected,

Table 1. Elements of the A (1520) spin density matrix in the photo- production process ~2p~K+A(1520), evaluated assuming parity invariance in the production mechanism and the spin ~ trace condition.

~o3~ 0tt Re~931 R%~ 1

0.38+_0.05 0.12• --0.10+0.15 -0 .03+0.15

since the K* has a higher lying trajectory in the J- plane.

In conclusion, we have measured the cross-section for the photoproduction of K+A(1520). The total cross-section falls with photon energy like (6.5+_0.7) E.,.(2"l•176 gb across our measured range. There

da is no evidence for shrinkage of ~t between our

measurement at (E7) -~3 .6GeV and a higher energy measurement at 11 GeV. The spin polar•177 of the A(1520) demonstrates that the production mechanism is inconsistent with only simple K exchange in the t channel. The process is in all measurable respects identical to 7p~K§ it has the same total cross-section, the same s dependence, the same t dependence and the same spin polar•177

Acknowledgments. We thank our colleagues at Daresbury, Lancaster, and Sheffield who have assisted in carrying out this experiment. We are grateful to Drs. E. Gabathuler and A. M. Osborne for their help in the early stages of the LAMP 2 experiment and to Dr. J. K. Storrow for interesting comments. We should also like to thank Professors A. Ashmore, A. B. Clegg, and W. Galbraith for their encouragement and support.

References

1. A.M.Boyarski et al. : Phys. Lett. 34B, 547 (1971) 2. D.P, Barber et al.: Nucl. Instr. Meth. 155, 353 (1978) 3. D.P.Barber et al.: Nucl. Instr. Meth. 145, 453 (1977) 4. D.P.Barber et al. :Phys. Lett. 90B, 470 (1980) 5. J.D.Jackson: Nuovo Cimento 34, 1644 (1964) 6. D.P.Barber et al.:Phys. Lett. 79B, 150 (1978) 7. C.Bricman et al.: Phys. Lett. 75B, 1 (1978) 8. For a recent review of 7p-*K+A(lll5), see J. K. Storrow:

Electromagnetic interactions of hadrons, p. 307. Ed. A. Donnachie, G. Shaw. New York, London: Plenum Press 1978