c

Nuclear Physics B137 (1978) 276-282 © North-Holland Publishing Company

A STUDY OF THE REACTION ~p "-* n - n + AT 10 GeV/c A. BERGLUND 1), T. BURAN, P.J. CARLSON 1), C.J.S. DAMERELL 2), I. ENDO 3), A.R. GILLMAN 2), V. GRACCO 4), R.J. HOMER 5), M.J. HOTCHKISS 2), A. LUNDBY, M. MACRI 4), B.N. R A T C L I F F 6), A. SANTRONI a), T. TSO 7), F. WlCKENS 2) and J.A. WILSON s) CERN, Geneva, Switzerland Received 31 January 1978

The reaction ~p -~ ~r-~r+ has been studied at 10.1 GeV/c in the - t interval from 0.15 to 1.5 (GeV/c) 2. A line-reversal comparison with backward elastic scattering ~r+p--, p~r+ shows good agreement for - t > 0.3 (GeV/c)2.

In this paper we present new data at 10.1 GeV/c on the two-body reaction ~p -*. n-zr + .

(I)

In addition there is a suggestion of a signal corresponding to the reaction ffp -* n - p + .

(2)

Earlier measurements, all at lower energies, have been reported for the 2~r channel at 3 and 4 GeV/c [1], 5 GeV/c [2], 6 GeV/c [3], 6.2 GeV/c [4], and 8 GeV/c [5]. Measurements in the s-channel region below 3 GeV/c have also been repqrted [ 6 - 1 1 ]. No measurements have to our knowledge been reported on the zr-p + channel, which is experimentally more difficult than the charge-reversed zr+p - channel for which several measurements exist [5,12,13 ]. In addition to studying the energy dependence of these baryon-exchange reactions, it is o f particular interest to make line-reversal comparisons [14]. The reaction ~p ~ zr-zr+ * is the line-reversal partner of backward elastic scattering, lr+p ~ plr +. 1) University of Stockholm, Sweden. Now at CERN, Geneva, Switzerland. 2) Rutherford Laboratory, Chilton, Didcot, Oxon, England. 3) CERN. Present address: Phys. Dept., Univ. of Hiroshima, Japan. 4) Istituto di Fisica dell'UniversitA, INFN, Sezione di Genova, Italy. 5) Physics Dept., University of Birmingham, England. 6) Rutherford Laboratory. Present address: SLAC, Stanford, Calif., USA. 7) Rutherford Laboratory. Present address: Brookhaven Nat. Lab., Upton, New York, USA. * We use the convention that the small momentum transfer squared, denoted t, is between the incident particle and the first mentioned product. 276

277

A. Berglund et al. / ffp ~ ~r-~r+

Target

T1

,/

-~

T2

C1"~

M1

~

M2

~

C2

~---~-t

w6

- 0.2

w7

Fig. 1. Schematic experimental layout. Not shown in the figure are three beam scintillation counters, three beam (~erenkov counters and the beam hodoscopes measuring the direction of the incident beam. Scintillation counters are A, a beam anticoincidence counter, and the hodoscopes T1 and T2. W1 - W 7 are wire spark chambers and M1 and M2 two large aperture spectrometer magnets. C1 and C2 are ~erenkov counters. Two typical trajectories, for - t = 0.2 and - t = 1 (GeV/c)2 are shown. UBL is the undeflected beam line.

The experiment was performed at the CERN proton synchrotron. The data on the annihilation reactions (1) and (2) were taken simultaneously with data on the hypercharge-exchange reaction [ 15]. The experimental layout is shown schematically in fig. 1. The incident beam of momentum 10.1 GeV/c had a A p / p of +-1.5%. Three threshold (~erenkov counters in the beam permitted identification of all hadrons of which 0.7% were antiprotons. The incident m o m e n t u m was measured to -+0.3% b y a scintillation counter hodoscope, placed at a m o m e n t u m dispersed image. F o r the experiment reported here, only a forward arm was used. This forward spectrometer contained a pressurized Cerenkov counter C1, for the negative beam set to count 7r's, a scintillation counter hodoscope T1, a beam anticoincidence counter A, two bending magnets with an integrated field of 3 Tm, a further scintillation counter hodoscope T2 and an atmospheric pressure multicell Cerenkov counter C2 set to count lr's above 3 GeV/c. The direction of the incident particle was measured with beam hodoscopes, the trajectory o f the forward going particle was measured by spark chambers W1 - WT, equipped with a capacitive readout system. The scattering angle was measured to +0.8 mrad and the m o m e n t u m resolution was -+0.5%, folding together the precision on the incident and forward tracks. The trigger for the annihilation reaction was the coincidence ~,- T1 • T2 • C1 • C2. The trigger rate per incident ff was about 10 -3. The data from Cerenkov counters, scintillation counters and spark chambers was read by an on-line computer and stored for further analysis on magnetic tape. To ensure good rejection against unwanted particles, pulse height information was recorded from all t~erenkov counters and also from the beam scintillators. The apparatus accepted events in the t-range from - 0 . 1 1 to - 2 . 5 (GeV/c) z. The acceptance, calculated b y a Monte Carlo method, varied from a maximum o f 15% for - t = 0.15 (GeV/c) 2 falling to 9% at - t = 1.5 (GeV/c) z .

278

A. Berglund et al. / ffp ~ lr-lr + I

I

I

I

I

I

I

I

I

I

I

I

15 2

m13

v

10 LO T--

/

d

/ m

/

> UJ

/ o

~

--0.4

I

I i

I

Missing

mass

0.2

0.8 squared

1.4

I

2.0

(GeV/e2)2

Fig. 2. The missing-mass squared distribution. The full curve is a fit to a ~r peak centered at MM 2 = 0.02 (GeV/c2) 2, a p peak with a width Fp = 0.15 GeV/c 2 and background, dashed in the figure.

A total number of 22 000 triggers were accumulated, most of which have a missingmass squared (MM 2) greater than 1 GeV 2. These were passed through an analysis chain where the following cuts were applied: (a) Beam scintillator and beam (~erenkov pulse height cuts selected events with an incident ~ with no other beam particle within the sensitive time of the detectors. (b) An important background to the reactionffp ~ lr- + ... is the much more frequent reaction~p ~ f f + .... where the forward emitted ~ for some reason, e.g. 8-ray production, gives light in the ~erenkov counters C 1 and C2. A combination of pulse height cuts in C1 and C2, and the requirement that the tracks seen in W 4 - W7 should intersect the illuminated C2 mirror, completely eliminated this background. The MM2 distribution is displayed in fig. 2. Before drawing conclusions, we emphasize: (i) The MM2 scale was calibrated using the elastic channel~p ~ p . The resolution in MM2 is ---0.1 (GeV/c2) 2. (ii) The region MM2 < 0.8 (GeV/c2) 2 is kinematically accessible only to the reaction~p ~ rr- + ... (an incident lr- or K - not'rejected by the beam ~erenkov counters would give a MM2 t> 0.8 (GeV/c2)2). (iii) The three-body background starts at 0.08 (GeV/c2) 2. The shape of the back-

A. Berglund et aL / lip ~ ~r-Tr+

279

ground that we use is the same as that observed in other experiments [5,11,12,17,21]. (iv) There are no events with MM 2 < - 0 . 4 (GeV/c2) 2 indicating negligible background from spurious tracks. The peak centred at a missing-mass squared (MM 2) of 0.05 (GeV/c2) 2 is clearly due to the two-body reaction ~p -* rr+rr- . No other reaction can give rise to such a peak. The three-body background starts at MM 2 = 0.08 (GeV/c2) 2. The bump in the distribution for MM 2 = 0.5 (GeV/c2) ~ suggests some p production, ~p ~ n - p +. We have fitted the missing-mass distribution to a sum of a background, a Gaussian 7r- peak and a relativistic Breit-Wigner folded with the Gaussian resolution function for the p peak. The background was taken as linear in MM 2 but constrained near the threshold as indicated by the dashed line in fig. 2. The p width was kept fixed at I"o = 0.15 GeV/c 2. The full curve in fig. 2 gives the result o f the fit. The p is centred at M 2 = 0.50 + 0.06 *. In view of the large background we do not give any angular distribution. The cross section for reaction (2) in the - t range from 0.15 to 1.5 (GeV/c) 2 is estimated at 0.2 -+ 0.1/ab. We define the 7rrt channel, reaction (1), as the events with - 0 . 2 < MM 2 < 0.2 (GeV/c2) 2. The differential cross sections are tabulated in table 1 and plotted in fig. 3 together with data at lower energies [2,4,6]. The errors given in table 1 and shown in fig. 3 are statistical. Events with higher MM 2 show the same shape of the - t distribution. The absolute value of the differential cross sections depends critically on the shape of the background. The shape shown in fig. 2 is the same as that used in other similar experiments. Extrapolating the straight line for MM 2 > 1.2 (fig. 2) down to MM 2 = - 0 . 4 (GeV/c2) 2 gives a maximum possible background of about 40%. We estimate that the absolute value of the differential cross section is known to +25%. Our data is consistent with the existence of a shallow dip at - t = 0.35 (GeV/c2) 2 [3]. For larger values of I tl, the differential cross section drops in a similar way to the data at 5 and 6.2 GeV/c [2,4]. Fig. 3 includes a representation of the data on rr+p ~ prt + at 9.85 GeV/c [24], scaled according to the line-reversal relations [14]. The line-reversal comparison indicates good agreement, with the exception o f the narrow dip in the lr+p -* prr + data, which seems not to be present in our ~p ~ rr-rr + data. Our t resolution is +0.02 (GeV/c) 2. Using the variable t' = t - tmin would not change this conclusion, since at our energy, the curve in fig. 3 would move by only 0.07 (GeV/c) 2. We note that at lower energies the line-reversal comparison fails completely

[2-4]. In figs. 4a, b we display, respectively, the energy dependence [1,3,5,6,11 ] at - t = 0.3 (GeV/c) 2 and the effective trajectory, using our data at 10 GeV/c and the data at 6 and 6.2 GeV/c [3,4]. * It is interesting to note that whenever the p is produced by baryon exchange, i.e. reactions like 22 in the particle data ~p Irp, 7rN~ Np, its mass is lower than the value M~2 = 0.58 (GeV/c) table [16]. The average value from some 10 different experiments inMo2 = 0.50 ± 0.02 (GeV/c2) 2 [5,12,13,17-23] in excellent agreement with the position of our p peak.

A. Berglund et al. / ~p ~ 7r-n +

280

Table 1 Differential cross sections for the reaction~p ~ n - n + at 10.1 GeV/c

-t

,xt

do/dt (nb/(GeV/c) 2)

0.05 0.08 0.16 0.16 0.40 0.50

510 310 210 340 180 46

((GeV/c) 2) 0.175 0.24 0.36 0.52 0.80 1.25

± ± ± ± ± ±

290 180 120 170 90 46

The quoted errors are statistical, representing one standard deviation. We estimate an overall normalization error of -+25% due to uncertainty in background subtraction. I

10

I

[

• 5.0 GeWc O 6.0 GeV/c ZE 6.2 GeV/c • 10.1 GeV/c

++ %

0.1

\\t\

I 0.0'

I 0.5

I 1.0

'\\ \ \ I 1.5

- t (GeV/c) 2

Fig. 3. The differential cross sections for ~ p ~ n - r t + as functions o f - t . Together with our data at 10 GeV/c we also show data at 5.6 and 6.2 GeV/c [2,3,4]. The dashed line represents data on 7t+p ~ plr + [24] at 9.85 GeV/c. The latter have first been scaled to s = 20.80 GeV 2 ~ p at I0.I GeV/e) using the scaling s -1-62+1"74t [24], then multiplied with the crossing factor ~[pcm(~rp)/Pcm(~p)] 2 = 0.551 [14].

281

A. Berglund et al. / ~p ~ 7r-Tr+ PDab GeV/c I I

3 I

2 f

4 I pp

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6 I

~

8 I

10 I

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= 0.3

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,

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\

10

/

t

\ \ h\

"o

\\\\\ \

0.1

1

I

5

I

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15

20

s GeV 2

Fig. 4. (a) The differential cross section for ~p ~ 7r-Tr+ at - t = 0.3 (GeV/c) 2 as a function of s, the total energy in the c.m.s, squared. Data from refs. [1,3,5,6,11] and from this experiment. The two straight lines are least-squares fits discussed in the text. (b) The effective trajectory calculated from our data at 10 GeV/c and from data at 6 and 6.2 GeV/c [3,4]. The two lines represent the A and N trajectories [16].

With the exception o f the data point at Plab = 3 GeV/c, the cross sections fit reasonably (X2 = 15 for 13 degrees of freedom) to the form do/dt ( - t = 0.3) = const, s - a with a = 4.7 + 0.2. This fit is shown as a dashed line in the figure. There is slight evidence for some flattening of the s dependence at large s. A fit to the data points for Plab/> 3 GeV/c to thesame form gives ot = 2.7 -+ 0.5. This fit is shown as a full line. We note that the higher energy points show an energy dependence characteristic of baryon exchange. The effective trajectory, shown in fig. 4b, is consistent with A8 or Na exchange.

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