c

Nuclear Physics B131 (1977) 386-398 © North-Holland Publishing Company

STUDY OF THE REACTION ~p -+ ~ o p AT 9.1 GeV/c R. STENBACKA, R. CARLSSON, G. EKSPONG, S.O. HOLMGREN, S. NILSSON and Ch. WALCK University o f Stockholm, Sweden P. GREGORY, P. MASON, H. MUIRHEAD and G. WARREN University o f Liverpool, England Received 30 March 1977 (Revised 1 September 1977)

The reaction p-p ~ p-pTr+Tr-~r0 has been studied at 9.1 GeV/c. The cross section is 2.04 -* 0.04 mb based on 3062 events. The cross section for p-p ~ ~-copis 0.17 -+0.02 rob. The Plab dependence of this cross section is Plab -0"9 +-0.2. This rules out a double Regge pole exchange model with a pomeron trajectory.

1. Introduction A n t i p r o t o n - p r o t o n interactions at 9.1 GeV/c have been studied in a LiverpoolStockholm experiment [1]. The total sample consists of 55 729 events. In this paper some aspects of the reaction ~p -+ ~pTr+Tr-Tr°

3062 events

will be studied. The main ambiguous hypotheses were between ~pn+Tr-lr° and ~pTr+rr-Tr°

366 hypotheses (~ and 7r- and/or p and 7r+ exchanged);

and pffTr+rr-Tr-

333 hypotheses;

and 27r+21r-rr °

128 hypotheses;

and ~nTr+~r+Tr-

43 hypotheses.

Ambiguous hypotheses were weighted inversely proportionally to the number of hypotheses. The effects of the ambiguous events on the results in the following 386

387

R. Stenbacka et aL /~p-~fi~op

. sections were investigated in detail. They were found to be unimportant. The cross section for gp ~ ~-p=+Tr-g ° was determined to be [2] o = 2.04 -+ 0.04 m b . In subsequent sections the reaction ~-p ~ ~cop and the associated (~co), (cop) systems will be studied (sects. 2 and 3). Additional experimental features for gp ~ g p n + n - n ° are presented in sect. 4 and the results are discussed in sect. 5.

2. Production of (o(783)

The mass o f 7r+Tr-Tr° is shown in fig. 1. A pronounced production of 60(783) is observed. The co is defined by the cut 0.73 < M(~+Tr-~ °) < 0.83 GeV/c 2

354 events,

(1)

120 II0 15p~ pp ~+1~- 11:"

100

3062 events

90 O 80 O 70 6o

:r

"8 50 o Z4e 3c 2G i0

o16

o18

1',o

C2

;.~

M (T~* T[-T~° ) GeV/c 2

Fig. l. Mass of Ir+rr-rrO.

i'.6

i'.8

2'.o

388

R. S t e n b a c k a e t al. / -pp -~ ~eop

and the adjacent regions by 0.60 < MOr+Tr-Tr°) < 0.73 GeV/c 2 0.83
384 events,

(2)

GeV/c 2 .

In order to estimate the background under the peak the distribution of events is plotted as a function of the normalized matrix element squared for the co decay [p+ X p_l 2

A = 3/'1 ~112

(3)

m2)2

"~t,~ ~r13n --

in fig. 2. p_+ are the momenta of the charged pions in the 3zr rest frame, M37r is the mass of the 37r system and m~r is the pion mass. The decay distribution is zero at the boundary of the Dalitz plot (A = 0) and maximum at the centre (A = 1). The distribution has been fitted by a straight line and the background, assumed to be constant, can then be estimated to be 47 + 1%. This can be checked in a similar plot for the adjacent regions (defined in (2)) which gives 94% background. Both the mass plot and the decay distribution thus give a clear ~(783) signal.

¢,G

40

OJ-region

adjacent region

354events

normalized to 354events

~.3G

30 o ii <

20 o o Z

• 0.2

i

i

0.4

/

0.6

i

i

0.8

i

i

1.0

A

,

0.2

i

,

i

0.4

. . . . 0.6 0.8

A

Fig. 2. Normalized matrix e l e m e n t squared for the ~o decay, A, [p+ X p _ [2 A=~3el t,,f2 _ m 2 ) 2 ' (a) ~ region, (b) adjacent region, (normalized to 354 events).

1.0

R. Stenbacka et al. / pp ~ - p ~ p

389

adjacent

"-q u - r e g i o n o

it

eo I#l

\

region

normatized to 354 events

~1 lOO

d

ii

~80 u

~

60

~ 60

m

ffl ¢-

c

o

~ 40 "6 0

z

>40 "6 o

20

z

0

-1.0 COS

÷I.0

2o

-1.0

. . . .

()

'

'

'

' .1.0

COSOCM

~cM

Fig. 3. Angular distribution cos 0cm o f the co, (a) in the w band, and (b) in the adjacent regions (normalized to 354 events).

The cross section can be estimated to be [2] o(fip -+ ~cop) = 0.17 -+ 0.02 mb

(4)

for the charged decay mode w -+ rr+zr-n °. This value is the mean of an estimate based on the above method and a fit of the mass distribution with a Gaussian w

\

~)p--~ 5uJp 354 events

~8

3 :E6

/J 6

8 10 M2(~p) (GeV)2/c 4

Fig. 4. Dalitz plot for p-p ~ p-cop.

R. Stenbacka et aL / ~p ~ ~top

390

2°°t

loot

~

~p-~lSup t f~,pp 354 events

60

~4o ~

30

~ 20

3 "6

l

i

-ols

-0.7

-o15 -o3

i

-o.i

t (Geric) 2

Fig. 5. The f o u r - m o m e n t u m transfer s q u a r e d d i f f e r e n t i a l cross s e c t i o n for p-p --, ~ o p , t for p ~ p (or p ~ p).

peak and polynomial background. There is also evidence in fig. 1 for ~ production corresponding to a cross section of o(~p -+ ~r/p) ~ 20/ab

(5)

for the charged decay mode (assuming all events with mass <0.58 to be ~/). The cross section of 0.17 -+ 0.02 mb [2] for ~p -+ gcop together with the cross

looJ

m-region 100

3 ~ events

o ,,

80

c~ ii 80 I (D

u

611

u

6C

~

4O

g

4o

'*6 o z

217

o

Z a

. -i.o

.

.

. 0 cos e

.

.

.

. .1.0

.

.

.

adjacent region normalized to 354 events

,

-1.0

,

=

0

. . . . °t.0

cos O

Fig. 6. A n g l e o f t h e n o r m a l t o t h e d e c a y p l a n e w i t h r e s p e c t to the i n c o m i n g p a r t i c l e : (a) for e v e n t s in the ~ region and (b) for e v e n t s in the a d j a c e n t regions ( n o r m a l i z e d to 354 events).

R. Stenbacka et al. / ~p ~ p w p

391

section of 0.257 -+ 0.019 nab at 5.7 GeV/c [4] and 0.120 -+ 0.025 mb at 12 GeV/c [16] makes it possible to estimate the Plab dependence of the cross section cr(gp -+ ~cop) ~x Plab --O-9+-O'2 .

(6)

Some features of the production of the co are shown in figs. 3 - 6 . The angular distribution, cos Ocm,of the co and m(Tr+Tr-rr°) of the adjacent regions in the ~p c.m.s. are shown in fig. 3. They are both very peaked forward and backward. At 5.7 GeV/c [3,4] the background is less peaked. The coefficient a in the angular distribution dN d(cos 0era )

(7)

oc 1 + a COS20cm

has been obtained from a fit and it is a = 15 +- 5 (co region),

a = 12 ± 4 (adjacent).

(8)

Another illustration of the forward and backward peaking is given in the Dalitz plot for ~cop in fig. 4. The co preferably forms small masses with the nucleon (antinucleon). The four-momentum transfer squared differential cross section, fig. 5, do - - cc e ~t dt for the incident and outgoing antiproton (proton) has the slope b = 2.0 -+ 0.3(GeV/c2) -2 ,

-0.7 < t < -0.05.

(9)

(10)

A spin alignment of the co with respect to the incoming antiproton (proton) was found at 5.7 GeV/c [3,4]. We compute the angle P i n c ( P + )~ P - )

cos 0 = iPincll(p+ × P-)[ ,

(11)

where p are the three-momenta in the 37r rest system. Pinc is the antiproton momentum ifM~-~o Mwp.The angle 0 is the angle of the normal to the decay plane with respect to the incoming particle. The distribution of this angle is shown in fig. 6a, for the events in the co band and in fig. 6b, for the events in the adjacent regions. The fitted coefficients in the cos20 distribution are (cf. eq. (7)), a =0.1 ± 0 . 1 , t

a = - 0 . 3 +- 0.1 ,

co region, background.

As the percentage of background is known in the co region, see sect. 2, we can deduce the asymmetry coefficient of the true co events. It turns out to be r

a = 0.5 -+ 0.2 ,

co events.

We conclude there is a 2.5 s.d. evidence for co alignment at 9.1 GeV/c.

(12)

392

R. Stenbacka et al. / pp --, ~top

3. The (~to, top) system The properties of the ~w (wp) system will be studied in this section. The Dalitz plot, fig. 4, showed a peaking at low masses. The mass plot of ~co (cop) is shown in fig. 7a. The full histogram contains all combinations. The least mass combinations are cross-hatched. These events consist o f co events (53%) and background. The background has been estimated from the interpolated plot of the adjacent regions and the channels pff~r+n-n - (~nn+n+n - ) with an co cut in the 3n mass. The two estimates agree and a mean has been used. The average background, properly norrealized, has been subtracted from the cross-hatched histogram, fig. 7a, and the resulting histogram, fig. 7b, now represents the best estimate at the pco mass distribution free from background. In order to compare with an expected distribution we use a cylindrical phase space (c.p.s.) with the slope 2.0 (see (10)) for the ~cop t distribution. This c.p.s, is shown as the solid line in fig. 7a. It should describe the pw mass distribution in fig. 7b. A X2 test gives x2/NDF = 45/14 i.e. a confidence limit o f < 1 0 -3. We take this as evidence for an enhancement of the pw mass at ~ 1800 MeV/c 2. We have fitted an s-wave Breit-Wigner curve and the c.p.s, background and obtained the

i

rn{rc*rt-~*) in w-region 354 events

5C

I

I

i

i

b

30

4C

% (.9

I

~20

~o t/I

"6 20

°°"

0 Z

l0

I

1.8

2.0

2.2

2/.

2.6

2.8

M(15to, top) GeV/c 2

3.0

32

1.8

I

2.0

2.2

2.4

2.6

M(iSu~,uJp) GeV/c z

Fig. 7. Mass of p-to (top) in the (a) to region and (b) subtracted p-~ mass plot. Solid line, fit of Breit-Wigner and cylindrical phase space. Dotted line, cylindrical phase space.

R. Stenbaeka et al. / ~p -~ -ficop

393

1.0 a)

0.8

b)

0.6 0.4 0.2 0.0 -0.2 -0.4 -0.6 -08 -1.0

1'8

24

,'8

2'2

1.0 0.8

c) .< Y~>

d) o

0.6 0.4 0.2

{+ {+{ F-

0.0 -0.2 -0.z, -0.6 -0.8 -I.0

i

1.8

2.0

2.2

2.4

1.8

2.0

2.2

2.4

M(~co, ~ p ) GeV/c 2

Fig. 8. Moments of the spherical harmonics (y~n) as functions of the ~co (cop) mass in tile ~o band, (a) ( Y~l>, (b) ( Y~2>, (c) (1~3) and (d) (I~4).

mass and width of the gco (cop) object in fig. 7b, M(~co, cop) = 1780 -+ 20 MeV/c 2 ,

P = 145 + 70 GeV/c 2 .

(12)

The fit has a chi-squared/no, of degrees of freedom = 20/12. If the ~co (cop) enhancement is a resonance we can obtain information about its spin from the decay distributions. The m o m e n t s of the spherical harmonics

(I'7/> are shown as functions of the ~co (cop) mass in fig. 8a, for (Y~l), 8b, for (Y~2), 8c, for (Y~3) and 8d, for ( Y~4)The ( y ] z ) for events in the co bands are shown. The figure shows that (Y~t)~ 0

f o r / = 1, 2, 3 , 4

(13)

R. Stenbacka et al. / ~ p ~ ~ w p

394

for masses M(tSco, cop) = 1.80 - 1.90 GeV/c 2. In this case there is no difference between the co region and the adjacent region. We conclude that the above results indicate that an enhancement above cylindrical phase space decaying into ~co (cop) is formed. The average moments are consistent with a low spin ~ or 3. The isospin is I = ~. The cross section for the enhancement can be estimated to be ~p --+ N(1800)p + c.c.,

L co o = 25 -+ 5 / J b .

(14)

4. Influence of other channels in Op -+ ~pn + n - n

o

Some additional features of the ~pTr+lr-~T° system will be presented in this section as background information for the discussion of the ~cop system. The mass distributions for 7r+zr- and n+-n° are shown in figs. 9a and b. We find no evidence for strong p0 and p-+ production. A fit of a Breit-Wigner p-wave resonance plus a polynomial background, however, gives Opo = 60 +- 30 #b [2]. In the same way some evidence f o r p +- can be found. The experiment at 5.7 GeV/c [15] finds no evidence for pO while o(~p -+ ~ppeTr-+) = 78 +- 12/Jb. Thus the results are not entirely consistent, but a reasonable interpretation is that some p+-0 production occurs, but that the cross section is small <~ 100/lb. Another feature of the ~prr+~-lr ° system is shown in figs. 10a, and 10b, which give the masses for gzr- and wr +.

,20I

3062 events

(.9

~. I00 0

E a0 ,.6 60 0

z

40

20 i

0.4

i

0.6

0.8

1.0 1.2 M (r~*~ - ) GeV/c 2

I./.

1.5

Fig. 9a. Mass distribution o f n+;r - in pp ~ pp~+Tr-~ O.

1.6

R. Stenbacka et al. / ~p ~ ~ o p

395

220 200 pp--~ ~p ~÷ rc-]~ ° 3062 events

180

%

160

• , 140

~120 100 z 60 0

40 20 0.z,

0'.8

1'.0 1.2 M ( ~ t n : ° ) GeV/c 2

I.Z,

1.6

18

Fig. 9b. Mass distribution of ~±~0 in p-p ~ p-prr+n-~ O.

5OO

500 ~p-~p~*~-~° ~p--~ ~Sp~÷r;- n:°

3062 events

3062 events

/-00

aoc

(9 ~ 30C o

300 C~

oa

20¢

200 0

d

Z

Z

100i

,oo

L,__

,

1.0

1.2

i

1.4 1.6 1.8 2.0 M (151U) GeV/c 2

2.2

2.4

Fig. 10a. Mass distribution of p r r - in pp --, ~p~+~-TrO.

2.6

1.0

1__

1'.2 1'.4 1'.6 1'.8. 2'.0

2.2

2A

k4 (plZ*) GeV/c 2

Fig. lOb. Mass distribution ofp~r + in pp _, ~ p , + ~ - ~ O .

2.6

396

R. Stenbacka et al. / ~ p -~ ~cop

The cross section for Fp ~ A ++ (or A ++) inclusive is o=820-+40/1b [2]. Single and double production of A ++ and A ++ is frequent with the process Fp -> A++Tr°A++ dominating. This can be deduced from the mass plot of FTr-versus pTr+, fig. 11. An estimate of the cross section gives o(~p ~ A++Tr°A++) = 330 -+ 40/ab to be compared with o(~p ~ A++zr°A++)

=

530 -+ 40/ab at 5.7 GeV/c [4] .

The events with A p r o d u c t i o n and co p r o d u c t i o n do n o t overlap in phase space and the properties o f one system are therefore n o t reflections o f the properties o f the other.

3062 events

235 2.50 2.25 o (,9 29 7" i,~ 175 =r 1.5C

1.2E

6s

,.;o

,.'Ts 2'.o

z~5

215o z'75

M (p'~*) GeV/c 2

Fig. 11. Mass of ~zr- versus mass of p~r+ in ~p --* ppn+rr n O.

R. Stenbacka et al. / ~p ~ ~cop

397

5. Summary and discussion We summarize the main results for the reaction ~p ~ ~cop in the following. (i) The cross section for ~p ~ ~cop, co -+ n+lr-n°, at 9. l GeV/c is 0.17 -+ 0.02 rob. (ii) The co angular distribution is peaked in the c.m.s, with do/d cos 0cm) cc 1 + (15 -+ 5) cos20cm . (iii) The slope of the four-momentum squared distribution between p -+ p (or p -+ p) in ~p -+ ~cop is 2.0 + 0.3(GeV/e2) -2. (iv) The Pfa~ dependence of the cross section for ~p ~ ~cop is n = 0.9 -+ 0.2. (v) The spin aligmnent of the co with respect to the incoming direction is indicated. (vi) The ~co (cop) system forms a low-mass enhancement with mass 1780 -+ 20 GeV/c 2 and width 145 + 70 GeV/c 2. 3 (vii) The spin of the ~co (cop) is small, ~ or 3(viii) The cross section for the production of N!(1800) + c.c. is 25 -+ 5/lb. (ix) The cross sections for p0 and p-+ production are difficult to estimate. Our estimate is o o = 60 _+30/~b. (x) A-+~°and A ++ are abundantly produced in ~p ~ ~pn+n-n °. The cross section for ~p -+ A++n°A++ is estimated to be 330 -+ 40/lb. These processes do not, however, influence the analysis of ~p ~ ~cop. The production features of tip -+ ~cop can be compared with the results at 5.7 GeV/c [4]. The c.m.s, angular distribution of the co gets more peaked a = 15 -+ 5 at 9. l GeV/c compared to a = 9 -+ 2 at 5.7 GeV/e. We do not find evidence for a two-slope structure in the t' = t - tmax distribution (not shown) as was found at 5.7 GeV/c. do/d{ is similar to do/dt, fig. 5, but our statistics is limited. The spin alignment of the co is evident at 5.7 GeV/c and might also be present at 9.1 GeV/c. The cross sections at the two energies allow an estimate of the Plab dependence. A double Regge-pole exchange model gives [5] O-CO s 2 ~ m a x ( 0 ) - 2

,

where amax(0) is the highest lying trajectory. The result 2C~max(0) - 2 ~ - 1 gives amax(0) ~ 0.5 i.e. a meson trajectory. Thus the pomeron is ruled out as a possible trajectory. There is evidence for a possible ~co (cop) enhancement. It is found both at 5.7 and 9.1 GeV/c. It is probably diffractively produced as the cross section could be constant, o = 28 -+ 6/~b at 5.7 GeV/c to be compared with our result o = 25 -+ 5/~b. The quantum numbers of the N(1800) are then likely to be 1 + o r 3 - according to the Gribov-Morris0n rule for diffractively produced systems [6]. The N(1800) ~ pco has been observed in several experiments [ 7 - 1 4 ] . We refer to the paper by Lednicky [7] for a thorough discussion of the experimental results. Lednicky's conclusion is that J = ~ is ruled out by the co alignment and that

398

R. S t e n b a c k a et al. / p p -+ ~cop

N ( 1 8 0 0 ) -+ p w is likely to be 3 - , higher spins being excluded by (Y~t) = 0. Our results support gl+ o r ~--. The N ( 1 8 0 0 ) -+ pco has also b e e n found in 7r-p ~ n - w p reactions [8]. In this case the s-dependence is s - n with n = 0.66 -+ 0.06. Events w i t h Itl < 0.15 ( G e V / c ) 2 were selected. The p r o d u c t i o n m e c h a n i s m in this case could be a m i x t u r e o f p o m e r o n and meson exchanges. However, one observes that the selection criteria m e a n that the energy d e p e n d e n c e is deduced on a fraction o f the cross section at different energies. That fraction might be energy dependent. We wish to t h a n k the operating crews o f the C E R N PS, the C E R N 2 m h y d r o g e n bubble chamber, the constructors o f the b e a m and our scanning and measuring staff The financial support o f the Science Research Council for P.G. and G.W. and o f the Swedish A t o m i c Research Council for the S t o c k h o l m group is gratefully acknowledged. We are greatly i n d e b t e d to H. G e n n o w and P. L u n d b o r g o f the 19 G e V / c pd e x p e r i m e n t for use o f their analysis programs and valuable discussions.

References [ 1 ] Ch. Walck, Review of recent results in the 9 GeV/c pp experiment, Proc. Int. Syrup. on p-p interactions, Loma-Koli, 1975, p. 65; P.S. Gregory, The separation of the annihilation and inelastic final states in pp reactions, Proc. Int. Symp. on pp interactions, Loma-Koli, 1975, p. 256. [2] P.S. Gregory, P. Johnson, P. Mason, H. Muirhead, G. Warren, G. Ekspong, S.O. Holmgren, S. Nilsson, R. Stenbacka and Ch. Walck, Nucl. Phys. B119 (1977) 60. [3] V. Alles-Borelli, B. French, A. Frisk and L. Michedja, Nuovo Cim. 46A (1966) 438. [41 H.W. Atherton, I. Bar-Nit, B. French, J. Skura, J. B6hm, J. Cvach, M. Jires, J. Deslak, V. Simak and J. Zacek, Nuovo Cim. 30A (1975) 505. [5] N.F. Bali, G.F. Chew and A. Pignotti, Phys. Rev. Lett. 19 (1967) 614. [6] D.R.O. Morrison, Phys. Rev. 165 (1968) 1699; V.N. Gribov, Yad. Fiz. 5 (1967) 197. [7] R. Lednicky, JINR, E2-8527, Dubna, 1975. [8] V. Davidson, A. Dzierba, A. Firestone, W. Ford, R. Gomez, F. Nagy, C. Peck, G. Rosenfeld, A. Sheng, H.A. Gordon, M. Habibi, I. Stumer, R. Ely, D. Grether, P. Oddone and Kwan-Wu Lai, Phys. Rev. Lett. 32 (1974) 855. [9] M.S. Milgram, A. Chao, T.F. Johnston, J.D. Prentice, N.R. Steenberg, T.S. Yoon, R. Morse B.Y. Oh and W.D. Walker, Nucl. Phys. B18 (1970) 1. [10] R.E. Juhala, R.A. Leacock, J.I. Rhode, J.B. Kopehnan, L. Marshall Libby and E. Urvater, Phys. Rev. 184 (1969) 1461. [ 11] P.J. Davis, M. Alston-Garnjost, A. Barbaro-Galtieri, S.M. Flatt6, J.M. Friedman, G.R. Lynch, M.S. Rabin, F.T. Solmitz and N.M. Uyeda, Nucl. Phys. B44 (1972) 344. [12] E. Colton and E. Gellert, Phys. Rev. D1 (1970) 1979. [13] D.M. Binnie, L. Camilleri, N.C. Debenham, A. Duane, D.A. Garbutt, J.R. Holmes, W.G. Jones, 1. Keyne, M. Lewis, I. Siotis, P.N. Upadhyay, I.F. Burton and J.G. McEwen, Phys. Rev. D8 (1973) 2789. [14] J.S. Danburg, M.A. Abolins, O.I. Dahl, D.W. Davies, P.L. Hoch, J. Kirk, D.H. Miller and R.K. Rader, Phys. Rev. D2 (1970) 2564. [ 15] Amsterdam-CERN-Nijmegen-Oxford Collaboration, Observation of a poe enhancement near threshold in K - p --" K-Tr+Tr-TrOp at 4.2 GeV/c, Conf. on baryon resonances, Oxford, July 1976. [16] G. Drews, Thesis, F1-71/7 (1971), Hamburg.