Measurements of the reaction K−n→π−Λ in the c.m.s. energy range 1750 MeV to 2000 MeV

Measurements of the reaction K−n→π−Λ in the c.m.s. energy range 1750 MeV to 2000 MeV

Nuclear Physics B153 (1979) 485-492 0 North-Holland Publishing Company MEASUREMENTS OF THE REACTION K-n + n- A IN THE C.M.S. ENERGY RANGE 1750 MeV TO...

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Nuclear Physics B153 (1979) 485-492 0 North-Holland Publishing Company

MEASUREMENTS OF THE REACTION K-n + n- A IN THE C.M.S. ENERGY RANGE 1750 MeV TO 2000 MeV M.J. CORDEN, G.F. COX, A. DARTNELL, D.P. KELSEY, I.R. KENYON, J.A. STUBBS and P.M. WATKINS Department

of Physics, University of Birmingham,

UK

Received 22 January 1979

The angular distribution and polarization for the reaction K-n + n-l\ have been measured in the c.m.s. energy range 1750 MeV to 2000 MeV by means of a bubble chamber experiment, producing on average 500 events of this type per 10 MeV energy interval. The data are compared with the predictions of a recent partial-wave analysis of this reaction.

1. Introduction

It is perhaps surprising

that there are no well established

IX resonances

in the ener-

and the E( 19 15) [ 11. Furthermore, recent analyses have produced conflicting results for suggested I: states (in particular the E( 1940)) with mass between 1915 MeV/c* and 2000 MeV/c* [ 11. In the present paper, data of high statistical quality are presented for the pure I= 1 reaction K-n + n-A in the energy range between 1750 MeV and 2000 MeV. It is hoped that these data will help to clarify the Z spectrum in this energy region. The work is part of a systematic study of K-ninteractions between 1750 MeV and 2200 MeV. Results for the reaction K-n + n-l\ at 1.45 GeV/c and 1.65 GeV/c [2] and at 1.78 GeV/c [3] have already been published. The experimental procedure and methods of analysis are very similar to those described in [3] so only differences will be fully explained here. gy region between

2. Experimental

the Z( 1765)

details

The results presented here are derived from two exposures of the CERN 2m liquid deuterium bubble chamber to K- beams of momenta 1.13 GeV/c and 1.32 GeV/c. These exposures consisted of 220 000 pictures and 110 000 pictures respectively. Except where otherwise mentioned, events from the two exposures were combined and treated similarly. 485

486

M.J. Cordrn er al. / K-.n +

FA

Scannmg cr~terra arru kmematic fittmg were both as descrrbed m [3]. The first measurerrrent and the re-measurement of faiied events wtle both carried our on the Brrrnmgharn HPD. The datasummary tape containedover 9000 fits to the reactron K-d -+ rr-Ap, followed by A + pn-, where pS IS a specraror proton either seen or unseen.

The same selectron criteria as m [3] welt: ustcl except that a productron kinematic fit probability of greater tkran 1% was requned. . 3.1. Resolution

vertex

o_fAlZ? ambiguity

In addition to the procedure described m sect. 3 of [3], the ambiguity between the reaction of interest here and the reacrron K-d + 71-.Z”pS was further resolved by examining the distribution of cos /3, where /3 1s the angle between the productron plane normal and the decay y in the X0 rest frame. Instead of berng flat, as wouid be expecred for genuine Eo7s, for ambiguous events this distribution was found to be peaked at small values of cos /3. Ambrguous events were accepted as A events if they satisfied -0.5 < cos /3< 0.5 or passed the criterion described in [3]. 3.2. Weigh ring The weighting procedures described in subsect. 3.1 of [J] were applied. The azimuthal loss due to a small projected angle between the incoming and outgoing negative tracks was not so severe at these lower momenta. Car-rection factors as a function of cos 8, where 13is the c.m.s. scattering angle between the K- and rr-, and for seen and unseen spectators are shown in table 1. These factors were determined

Table 1 Azimuthal hss currwrion factors Beam momentum

1.13 GeV/c - ._ Seen spectators

0.7 < cos 0.9 < c0s -____-a)

N0tusd.

e < e <

0.9 1.0

1.0 1.09 --.___~_._-..

1.32 GeV/c ~.._ __ ____--.__

Unseen spectators

Seen spectators

Unseen specrators

1.05 1.09a)

1.05 1.06

1.16 1.18”)

sqarately for the two exposules. OWIU~ to trle prubiem described m the next subsection the iac~~s for ~IISL-C~sp,ccLator evGllu with cus 0 > 0.9 wre nut m fact UXd.

Even atter the weighrings already descrmed, a cunrparison of the arquiar dntributious tor eveucs with SCHI speczaruls alid for events with unseen spectalurs at cumparabie euergles indicated a 10~s of evCllis with UIMX~ spectators when cos 8 > 0.9. The difficulty in WXXI~the ~IW~LIL.LIU~ WI Lex when tllele IS no visible spectatur arld the angle of staller IS very stall, apparently p~~duccs a residual scarming aud/or p~oce~ug loss ullculreiated with aLuulrth. Smce the two angular distribut~urls agree well fur cus: 8 < 0.9 they wsie s~~lrply added m this region. However the total numuer of evenIs with cws 0 > 0.9 was estirrla& by multiplying tile number of such evcrlts with sdcn specta~rs by (U + s)/s whole u(s) 1s file toral welghhted nuluLer of events with uuaeG:n (seen) specrators in the angular region cos 8 < 0.9. The e11ur on the numLtz of sacn events was increased appropriately.

4. Experrmenial results After se~c~us anu weightings the two exposures were found to give conslslent results in the overlap region. Therefore the events frorn the two exposures were curnbmed alld divided into twelve c.m.s. enargy bms each contamirlg about 600 events on average. lIerails of the bin limits and numbers of events are shown in table 2.

Table 2 Derails of c.m.s. enwgy bins Bin limits

Mean energy

(MeV)

(McV)

No. of events after all selections

1750-1800 1800-1825 1825-1840 1840-1850 1850-1860 1860-1875 1875-1890 1890-1908 1908-1922 1922-1940 1940-1965 1965-1990

1781 1814 1833 1845 1855 1867 1882 1899 1915 1931 1951 1976

499 706 789 785 801 708 445 393 502 684 431 153

Weighted no. of events

590.1 863.3 992.0 995.3 1010.5 881.8 555.8 487.8 654.5 899.6 532.9 183.3

94* 111 + 99 f 97 f

90*

92* 39*

1867 1882 1899 1915

1931

1951 1976

11 17

12

9 11 12 13

20 f 10 41* 8 67* 9 57 f 11 76 f 10 10 12 13 17

14 11 12 15 13

27* 11 59 f 20

66 + 16

35 + 42 f. 45* 68 f

118 f 63 f 41 f 36 + 39+

-108 -95

-87

-57 -84 -87 -78

13 16 16 22

16 13 14 19 16

f 15 + 25

+ 20

f * + +

24* -39 f -28 f -57* -62 f

AtlAo A&o AJA (All ratios and erros have been multiplied by 100)

for K-n + Z-A

1781 1814 1833 1845 1855

(Mev)

c.m.s. energy

Table 3 Coefficient ratios AL/A,

-97 -39

-58

-68 -79 -73 -I4

15 18 18 23

f 17 + 28

f 22

+ f * f

53 f 17 -1 f 15 -28 of 16 -52*21 -29 + 18

A&o

?r 20 * 17 f 17 f 22 f 19

-49 -70

-22

f 20 A 33

+_24

* 17 f 20 o* 21 -16 f 26

-19 -37

-9 -19 -10 -12 14

AslAo

19 16 17 19 17

8 i 20 32 + 34

8 + 20

+_17 f 20 2 * 22 18 f 22

-15 -42

13 f 30 f -18 f -24 f o+

A&o

17 19 20 20

21 18 17 19 17

14 + 20 -19 f 37

10 f 18

8+ -10 + -2 f 19 f

45 f -13 f -85 6a 7*

A&o

>’

; 3’ 4 1

s & 3 z P

E

182 15ill 15 * 18* 16 + -15 f

21 f 21 f 4* 18 f

1915 1931 1951 1976

14 12 16 25

11 12 15 16

11

-5 f 13 13 f 12

12 10 13 20

f 10 Ok 8 o* 10 25 t 16

-14

-31% 7 -445 8 -39 f 7 -2l* 8 -18 * 11 -34 * 11

-18+ 9 -27 f 9 -8* 9 -3 _+10 -11 f 13 -6+ 13 9* -5 f o* 0 f

-21* 10 -30 k 8

+ 12 f 10

-25 -20 6 7 6 7 8 10

9 7

-41* 8 -13 f 7 -8* 9 -11 f 14

-21% -17+ -19 f -12* -3Ok -34 i-

-18* -14 f

B&o B&o B&o M-40 (All ratios and errors have been multiplied by 100)

1833 1845 1855 1867 1882 1899

1781 1814

(Mev)

c.m.s. energy

Table 4 Coefficient ratios BL/A~ for K-n + n-12

-16* ll+ 18* 30*

-5* -17* -14 f -11 fr -ll* -272

-13 f -2*

B&o

8 6 8 13

6 6 6 6 8 9

8 6 5 5 5 6 7 8

o+ 4i 2* -4* -lO* -18i

-12? 7 -5* 6 7+ 8 -5+ 12

-2+ 3k

8 6

o+ 4+

5 5 5 5 7 7

7 5

-2k 6 7+ 6 82 7 9+ 11

6k -12 o+ -8~ 3* -4t

BI~Ao

B&‘o

SJ

2 ? 5=I

s i;. 9 z .%

E

490

,

d-i,’

I

tn

0

Ln

0

0

!’ , 4’

M.J. Cnr&n et al. /K-n

-e-i

+

n-A

491

M.J. Corden et

492

al. /K-n

--t n-A

As in [3] the angular distributions and polarizations were analysed in terms of the Legendre coefficient ratios AL/& and B,JAo for 1
5. Analysis and discussion of results A partial-wave analysis of the form described in [3] has been carried out using the present data only, apart from the inclusion of 11 values of Ae from [6]. It was found that the present data can be described extremely well (x2/NDF = 123/l 37) by a parametrization including only well-established resonances, namely the X( 1765) Z(2030) and Z( 19 15), and “linear” backgrounds. (The parameters of the first two resonances were held fixed at average values derived from [ 1 ] and the parameters of the 2(1915) were allowed to vary). Thus it seems that the present data alone are insufficient to require the existence of other C states in this energy region. To determine the partial-wave amplitudes accurately, it is probably necessary to analyse all available data over as wide an energy range as possible. Finally the data have been compared with the predictions of a recent partial-wave analysis of KN two-body reactions [4]. The predictions of this analysis are shown in figs. 1 and 2 and may be taken to represent existing data, derived mainly from K-p experiments. The agreement is in general good and comparison of our 168 measured A and B coefficient ratios with their predicted values yields a total x2 of 200.2 without any allowance for the uncertainty of the partial-wave analysis predictions. Also the data contain three “rogue” points (involving three different coefficients and three different energies) which are all more than 3 standard deviations from their predicted values. Possible systematic differences occur in the case of the ratios Aa/& and As/A,, between 1800 and 1900 MeV where the data lie consistently below the predicted curves. It is a pleasure to thank the scanning and measuring staff in the Film Analysis Unit at Birmingham University for their conscientious work and the Science Research Council for their financial support.

References [l] [2] [3] [4 ] [S] [6]

Particle Data Group, Phys. Lett. 75B (1978) 1: G.F. Cox et al., Nucl. Phys. B19 (1970) 61. M.J. Corden et al., Nucl. Phys. B104 (1976) 382. G.P. Gopal et al., Nucl. Phys. B119 (1977) 362. M.J. Corden et al., Nucl. Phys. B129 (1977) 253. B. Conforto et al., Nucl. Phys. B105 (1976) 189.