Measurement of ΔσL in pp scattering between 200 and 583 MeV

Nuclear Physics @ North-Holland

A431 (1984) 637-668 Publishing Company

MEASUREMENT OF AaL IN pp SCATTERING BETWEEN 200 AND 583 MeV ‘* , J. BYSTRICKY’, J. DEREGEL*, PH. DROMPT’, E: APRILE-GIBONI C. EISENEGGER’, J.M. FONTAINE’, E. HEER’, R. HESS’, S. JACCARD4, F. LEHAR’, W.R. LEO’**, S. MANG04, S. MORENZONI’, Y. ONEL’, F. PERROT3, D. RAPIN’, J. VRZAL2*** and J. YONNET’ ’ DPNC, Univ. of Geneva, Switzerland 2 DPhPE, CEN-Saclay, France 3 DPhNf ME, CEN-Saclay, France 4 SZN, Villigen, Switzerland 5 LX, Caen, France Received 24 January 1984 (Revised 13 April 1984) Abstract: The main structure around m = 2.15 GeV first observed by the Argonne group in the spindependent total cross section Au, is confirmed in the energy range available at SIN. A simultaneous study of the scattered particles at small angles has been carried out with success and gave the spin-correlation parameter AOOkk for the pp elastic scattering and for the reaction pp+ r+d. The contribution of the 3-body reactions to this spin-dependent total cross section has been deduced and found to be lower than the contribution of the r+d reaction even at 583 MeV. E

NUCLEAR

STRUCTURE

pp scattering;

measured

Au, cross sections.

1. Introduction In 1977 indications of a prominent structure in the proton-proton spin-dependent total cross section were observed by the Argonne group who interpreted it as direct evidence for the existence of dibaryon resonances ‘). Resonances of 2.15 and 2.22 GeV total mass were found corresponding respectively to ‘D2 and 3F3 states. This structure in the spin-dependent total cross section AaL was also predicted by ‘the phase-shiit analyses ‘). For recent reviews in this field see refs. 3T4). It was thus very important to verify by independent measurements the structure observed in do,, to extend the data to the low-energy region and to look for other possible structures. A short experiment was run at SIN in the summer of 1980 at 206, 286, 421, 425, 451, 475, 500, 520, 540 and 583 MeV and preliminary results * Present address:

High Energy Physics Lab., Harvard University, Cambridge MA 02138, USA. ** Present address: Inst. de Genie Atomique, EPFL, PHB, 1015 Lausanne, Switzerland. *** Present address: Nuclear Center, Charles University, Praha, Czechoslovakia. 637

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were reported ‘). We present here our final results together with the analysis of a second experiment performed simultaneously on the A OOkkparameters for pp elastic scattering at small angle and for the reaction pp+ rd. These data together with existing measurements of Au, at TRIUMF (1980) 6), LAMPF (1981) ‘) and Saclay (1982, 1983) “) and the measurements of Au, at TRIUMF (1980) 6), Saclay (1981, 1982) ‘) and LAMPF (1982) lo) contribute mainly to the determination of the scattering amplitudes in the forward direction and the calculation of the forward dispersion relations I’) for pp scattering. Dibaryon resonances remain an open question. The scattering matrix M for the elastic proton-proton scattering is described by five complex amplitudes I*). At forward angles two amplitudes progressively disappear and, at 0 = 0”, the scattering is described by the amplitudes a, b, c, d connected by the relation ~(0’) - b(0”) = ~(0’) +d(O”). The total cross section in general depends on the beam polarization PB and on the target polarization PT [refs. 13,‘4)]: %(%

PT) = oo,to,+ (+,,tot& * pT+ q,,,w,

x k)(PT x k) .

(1)

The measured spin-dependent total cross sections Aa, using transversally polarized beam and target, or Am= using longitudinally polarized beam and target, are related to eq. (1) by -Aor = a,,,( tt ) - GA ti ) = 2a,,m, , -AuI_ = u,,,(++)

-

~,,d--++--1 = 2Cu1.m + uq,,) .

(2)

The optical theorem relates these cross sections to the imaginary part of the scattering amplitudes at 0 = 0”: u,,~,, = (2&/p*)

Im (a + b)

U l,tot

Im

=

@WP*)

(c+

,

4,

-Au,=(4?rh/p*)Im(c-d),

(3)

where p* is the proton momentum in the c.m. system. The real part of these amplitudes at small angles may be determined with the measurement of the Coulomb-nuclear interference. The unpolarized differential cross section is expressed as 15) da/da

=

[du/Wcou, +[d~/W,,,,

+[d~ldfG,,

.

(4)

Since the dominant Coulomb amplitudes a cou1-- b cou1--f Coul are real, the Coulombnuclear interference term (CNI) is given by

[du/W

CNI

=fcoul

Re

(hucl

+

bnucJ

(9

and allows the determination of the real part of the nuclear amplitude (unucr+ bnucl), i.e. of the same amplitude that enters in the optical theorem for the unpolarized

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E. Aprile-Giboni et al. / Measurement of AuL

total cross section mO,tot.The situation is similar for the spin-dependent total cross sections. The spin-correlation parameters AOOss(Aoow,) for transversally (longitudinally) polarized beam and target are expressed as A ooSSda/dL?=Re(a*d)cos Aookkda/d0

@+Re (b*c)-Im(d*e)sin

8,

= -Re (a*d) cos 8 +Re (b*c) +Im (d*e) sin 8.

(6)

At small angles the Coulomb effect is taken into account and the Coulomb-nuclear interference has a strong influence. With approximation cos (0) = 1, the corresponding terms are

[&om d~/d~lCNI =fcoulRe (cnucI - L,,J .

(7)

The pure Coulomb terms give a negligible contribution to the spin-correlation parameters. Again the nuclear amplitudes appearing in eq. (7) are exactly those appearing in eq. (3). Thus in principle a suitable experimental set-up may allow the simultaneous measurements of the spin-dependent total cross section and of the Coulomb-nuclear interference term of the corresponding spin-correlation parameter at small angle, i.e. the complete determination of the relevant amplitude at e = 0”. 2. General principles of the experiment A simple and precise way of measuring the total cross section of protons is to determine the transmission of a proton beam through the target material by counting the beam intensity in a rear counter F (see fig. 1). With the same layout and an additional small beam veto counter V, the differential cross section can also be measured simultaneously using an acquisition system allowing the registration of the scattered particles and their individual times of flight (TOF). Both experiments were performed in the present work using longitudinally polarized beam and target. Beam

counter

Rear

counter

F

BEAM_

Fig. I. Schematic

layout

for a transmission

experiment.

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The second experiment has thus measured the AOOkkparameter for the elastic small-angle scattering and in the reaction pp+ rrd. The spin-dependent total cross section for the reaction pp+ NNp was then estimated from the results of these two experiments and from the knowledge of the pp elastic contribution measured elsewhere.

2.1. Au,_ EXPERIMENT

Let M be the number of protons incident on the target, detected by the beam counters Sl and S2, and N(R) the number of particles in coincidence with Sl * S2 detected by the rear transmission detector F in the forward lab solid angle R. The ratio N(fl)/M is related to the partial cross section a(0) by [N(0)/M]=E(l-D)exp(-na(0)),

(8)

where n is the number of the scatterers/cm’ in the target, E is the efficiency of the F-counter and D is the background absorption due to the other scatterers in the beam line such as air, target windows, S2 counter, etc. For the measurement of the spin-dependent total cross section, the ratio N( 0)/M depends on the beam and target polarizations and will be referred to below as [N(R)/M],r where the indices B and T specify the polarization orientation of the beam and of the target, i.e. B = -or +, T = + or =+. The exponent ~(0) of eq. (8) has then to be replaced by

nH[u,,tot(H, el +inel) -;PJJ+~u~(H, +nc[u,,,(C,

I0

R [da(H, el)/dR +dc(H,

inel)/dQ]

dR

el +inel, a)] el+inel) -

n [da (C, el)/dfl

+da(C, inel)/d0]

dQ] .

(9)

The symbols H and C refer respectively to the hydrogen and the carbon (or background) atoms of the target. An azimuthal symmetry around the beam axis is assumed for the detection apparatus so that any effect produced by a residual transverse polarization of the beam will vanish. A detailed study of this effect has also been performed at SATURNE showing that the influence of the transverse polarization components is negligible. The beam polarization was flipped every 30 set, while the target polarization was reversed only once or twice at each energy. Combining the measurements with a fixed target orientation, it can be assumed that the detector efficiency E and the background absorption D remain unchanged for the both beam polarizations. Thus

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these factors cancel themselves in the ratio RT i

3 [Nf-J)IW+T = exp (+ n,ij,P,Aa,( * [N~)IKl4.

(10)

0))

with da‘(O)

= AaL(H, el +inel, 0)

n =Aa,+2

I0

C[do(H, el)/d~]AOOkk(el)lnuc, da

n

-2 I 0

Uda(H, el)/d@l&,,(el)]o,,

da

n

+2

[[da(H, inel)/dR]A,,,(inel)]

da.

(11)

I 0

When the solid angle R covered by the rear detector F is reduced to zero, the first and the last integrals are assumed to extrapolate to zero quadratically or linearly. The second integral takes into account the Coulomb-nuclear interference. This is a correction which is best estimated from the PSA and which should be performed before the reduction to R = 0. In the present experiment, the exponent of eq. (10) was always less than 0.001 and we may write to a good approximation

jj p

(1-R

T

HBT

(a)

da

L

.

(12)

Again this result is largely independent of the rear detector efficiency. Defining the quantities NOT(a), MOT as the averaged values over the two beam polarizations, we have &?-(0)

= No,(fl)(l

f Vr(fi)) ,

%T

= MO741 f 6%)

(13)

with

Eq. (12) may then be simplified up to a relative accuracy better than 0.001 as

where the target polarization approximation by

is kept fixed. The statistical error S is given to a good

(16)

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2.2. AookkEXPERIMENT The number of particles scattered from the thin polarized AOj at a mean lab angle 13~= 0, is expressed as

+nH

I

+nc

[da(H,

inel)/dL?][l

[da(C,

el +inel)/d0]

target in the solid angle

+ P&A,,&inel)]

dL!

da

.

(17)

Af2,

At small angles the scattering intensity IBr(ej) is dominated by the carbon scatters since [da(C)/d0]>[da(H)/dR]. Carbon scatterings can only be separated from the hydrogen scatterings with the use of a high resolution spectrometer. Again the asymmetry parameters PH and PC do not appear explicitly in the above formula since the detection apparatus is axial symmetric around the beam. The large carbon background could, however, be subtracted by forming the difference between the two beam polarization measurements,

AIT

= MOT

Ldej) M

-+T

L74ej)

M

-T

(18)

1

or (19) with

F(&oM, 0,)3

{I I

ARj

+

[da 0-Ael)ld~lAoodel) dfl [da (H, inel)/dO]Aookk(inel)

da

.

64

The statistical

errors are given by

(20) The inelastic term refers to the three reactions pp? r+d, pp + r+pn and pp + ~‘pp. In the present experiment the time-of-flight was used to separate the elastic scattering from these contributions. In addition, a copper absorber and another counter were set immediately behind the transmission counter in order to absorb all the deuterons. The reaction pp+ rrd is the dominant inelastic scattering not only because it has the largest total cross section below 500 MeV but also because all the deuterons are emitted inside of a small forward cone in the laboratory. For example, at zinc= 500 MeV the maximum scattering angle of the deuteron in the laboratory is about

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E. Aprile-Giboni et al. / Measurement of Au,

10”. The differential cross section for the rd reaction is relatively well known ‘*) and the Aookk(pp + rd) parameter can be evaluated from eq. (19). Since this AOOkk parameter has already been measured at five different energies in a fully separate experiment 19), a comparison between these two measurements gives an important cross-check of the factor (n,&&) which normalizes also the Aa, experiment as shown by eq. (15). For pi,, = 401 and 425 MeV, the transmission counter has covered almost completely the deuteron cone and the whole AOOkk(pp+ rd) angular distribution was accessible. The corresponding spin-dependent total cross section can then be obtained by an integration: 4sr

Aa,(pp + rd) = -2

I0

[do(pp +

~WdWk,c,~~(pp + d

da.

(21)

For the higher energies, only a fraction of the angular distribution was measured since the possibility to observe the whole distribution was unfortunately understood after the measurements completion. The reaction pp + NNr is not well known but has recently received renewed attention 2o-23). In the present experiment, the solid angle covered by the detectors is small and the reaction can only be treated as a background. We consider here essentially the scattered protons since the probability that the pion enters in the transmission counters is very small. For the protons, the Aookk parameter was assumed to be constant over the whole angular range under consideration so that the related difference TOF spectrum was just proportional to the phase-space. 2.3. AU, FOR THE REACTION

pp+NNlr

The spin-dependent total cross section Aa,(pp + NNr) can be deduced from the present measurements and from the Ac,(pp-el) computed by the integration of the spin correlation AOOkk data of ref. 24), using a formula analogous to eq. (21):

AaL(pp + NNm) = Aa,(pp-tot)

- Aa,(pp-el) - AaL(pp + md) .

(22)

In fact a better accuracy is gained by introducing the measured data for Ao,(tot) and A,,,,(pp-el) into the phase-shift analysis and then using the PSA fitted values for these quantities in eq. (22). 3. Experimental set-up The experiment

was performed in the PM1 area of SIN. The polarized target and the beam line have been described in detail in the context of a different experiment 25). The vertically polarized proton beam was produced at an atomic source and accelerated to 595 MeV. The polarization was monitored at the maximum energy by a beam polarimeter using a target of CH2 and had remained fairly constant to

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the value p, = 0.80 during the whole experiment. A copper degrader was used to lower the energy. The longitudinal polarization was produced by a spin precession using a superconducting solenoid and a final beam deflection of roughly 32”. A small residual component of a transverse polarization was present at all energies except at Tin, = 500 MeV but it had no effect since it is integrated out. The polarization was changed every 30 set in the sequence +, t, 0. The unpolarized beam was used for checking purposes. The good microstructure of SIN allowed us to work with a typical beam intensity at the PPT of about M = 5 x lo5 p/set. The beam intensity was very stable during all the experiment. The polarized target was built at SIN by S. Mango according to the present techniques of the art. It consisted of frozen beads of butanol CH3(CH,)&H,0H immersed in liquid 3He at a temperature of 0.5 K which was contained in a thin-walled c,opper cylinder of diameter 1.8 cm and length I = 2.4 cm. This sample was placed in a 2.5 T magnetic field set longitudinally with an accuracy of kO.5” and polarized by the method of dynamic nuclear orientation. The position of the target sample was checked using X-ray pictures. The polarization was monitored continuously by an NMR technique and calibrated using the natural polarization signal. The typical polarization was PT=0.50. At the end of the experiment, the target material was carefully weighted in a nitrogen atmosphere. The estimated error in this weight measurement introduces an uncertainty in the hydrogen content of the target of AnH/nH = 3%. Fig. 2 shows a schematic drawing of the experimental layout. The counters Sl (diameter 20 mm, thickness e = 3 mm and distance from the target z = -2460 mm) and S2 (diameter 10 mm, e = 1.8 mm and z = -210 mm) detected the incident protons. The particles in the beam halo were suppressed by the anticoincidence of the counter AH1 which had a circular hole in its center (internal diameter 19 mm, external diameter 50 mm, e = 10 mm and z = -2500 mm). The particles passing

CU

Fig. 2. Schematic layout of the present experiment.

F4

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645

through the polarized target PPT were detected by the transmission counters consisting of the trigger counters Fl and F2 (diameter 300 mm, e = 4 mm and z = 1071 and 1105 mm) and the counters assembly defining the angular bins called “gray-code detector” or GC (eight counters of diameter 300 mm and e = 4 mm, sandwiched between Fl and F2). A beam veto counter F3 (diameter 60 mm and e = 4 mm) was placed immediately in front of Fl. A copper absorber was placed behind the F2 counter and the particles crossing this absorber were detected by an end-counter F4 (diameter 400 mm, e = 10 mm and z = 1350 mm). The thickness of the absorber was taken as the full range of the deuterons produced in the reaction pp+ 7rd and was varied with the beam energy. All the counters were made of plastic scintillator NE1 10. axial sym?ry Fl A0

Al

BO 61 co DO

Cl Dl

F2 0

Plexi

m

Fig. 3. Sideview

Scintillator

of the gray code detector.

The gray-code detector is an assembly of four pairs of thin circular counter 26S27). Each counter is made of plastic scintillation rings alternating with rings of plastic light guide. They all have the same outer diameter of 30 cm. The counters were centered on a common axis and mounted in a compact assembly sandwiched between the trigger counters Fl and F2 of the same outer diameter. Fig. 3 shows a side view of this detector. The first counter in each pair, referred to as AO, BO, CO and DO, divided the GC area in fifteen angular bins corresponding to the fifteen coding possibilities with four detectors. The gray code has the interesting property of changing by one single bit only in passing from one bin to the next. The second counter of each pair called Al, Bl, Cl and Dl is the respective complementary counter so that when a particle is passing through the detector, the correct response always has exactly four counters with a signal. Thus the GC has a high rejection for accidentals. Table 1 gives explicitly these expected responses as a function of increasing scattering angles or of increasing bins.

4.

Electronics and data acquisition

The electronics was controlled by a CAMAC system and operated simultaneously the Aa, and AOOkk experiments. A PDP 1l/20 computer handled the data acquisition.

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TABLE 1

Correct Code Bin 9,

0

1 2 3 4 5 6 7

A0

BO

CO

DO

Al

0 1 1 0 0 1 1 0

0 0 1 1 1 1 0 0

0 0 0 0 1 1 1 1

0 0 0 0 0 0 0 0

1 0 0 1 1 0 0

1

responses Anticode Bin 0, Bl Cl 1 1 0 0 0 0 1 1

1 1 1 1 0 0 0 0

of the gray-code

detector Anticode

Code Bin 0, Dl 1 1 1 1 1 1 1 1

A0 8 9 10 11 12 13 14 15

BO

CO

DO

Al

Bl

Cl

Dl

00111100 1 0

1

1

0

1

0

0

11110000 01111000 0 1 1 1 1 0 0 0

0 0 0 0

1 1 1 1

1 0 0 1

0 0 1 1

1 1 1 1

0 0 0 0

The bin 0, = 0 is unphysical.

Extensive tests of this electronics were performed prior the data-taking using a fast pulse generator or directly with the PM1 “scattered” beam. A detailed description of the Aa, electronics can be found in ref. *“) and only a short survey will be given in the following. All modules defining the trigger and all scalers could run at 100 MHz while the r.f. structure of the SIN beam is 50 MHz so that the electronics was very independent of the beam intensity. Fig. 4a shows the electronics defining the incident beam and which was made using modules developed by the DPhPE laboratory of Saclay. The coincidence Sl resulted from the signals Sl, (direct), Sl, (veto) and AH1 (veto). The signals SIL were short pulses of 5.0 nsec issued from low threshold discriminator. The threshold of the Sin discriminator was set just above the mean of the linear pulse amplitudes given by the Sl counter. When two or more particles crossed simultaneously the counter, the received pulse was quite greater than this threshold and the firing of the Sin discriminator resulted in the veto of the Sl coincidence output. This electronics Counters

disc

Fig. 4a. Electronics

defining

the incident

beam.

E. Aprile-Giboni

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647

of Au,

Case Input

Sl,

Output

Coinc.Sl

i I I I I I

A

B

j

Output

! /I

I I 1 i I / 1 I I

Coinc.

I I / I I

C

S2 +

D

i-

I

Sl-after

_I r

A I / S2-

I / I ’ I I I /

after

the cllppmg

I I I /

t’

I I I I I I

i

;

the cllpping

E

Fig. 4b. Timing diagram of the electronics.

‘RIGGE .AM.FI.F

computer

reset

Fig. 4c. Electronics

diagram of the A,,,,

experiment.

suppression of accidentals is a safe way to treat the problem of multi beam-particles coming in the same 0.5 nsec r.f. burst or pile-up events, since the measurement of accidental rates using delay cables will not prevent the occurrence of possible and dangerous spikes in the data. Without the above electronic precaution and in absence of spikes, the actual accidental rate would cause a renormalization of about 1% relative of our Aa, values, a problem of minor importance. The output of the Sl coincidence had a width of 30 nsec, i.e. larger than the 20 nsec time between two r.f. bursts of the SIN beam. This pulse was then clipped to 5 nsec using a short cable before its entrance in the coincidence Sl . S2 module. Thus if two beam

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particles were 20 nsec apart, only the first particle was counted (fig. 4b, case A). If these two particles were 40 nsec apart, or more, they were counted both (case B). This system guaranteed that the output pulses of the coincidence Sl were at least 40 nsec apart, a time long enough to set a clean busy/gate signal following the AOOkk trigger or when the beam polarization was flipped. With a beam intensity of 5 x lo5 p/set, the occurrence probability of two incident particles distant exactly of 20 nsec is less than 1% . This system also guaranteed the emptiness of the r.f. burst preceding immediately a burst associated with a trigger. This is easily seen in fig. 4b. The coincidence between the dashed Sl pulse of case A and the S2 pulse of case D is prevented, while the dashed Sl pulse of case B will produce with the S2 pulse of case D a Sl - S2 coincidence output. Thus it will greatly reduce the possible background generated around the experimental set-up by preceding particles or by their decay products like P-decay. Thus roughly speaking, the incident beam was defined by BEAM = SIL - S2, - %i, * D, - AHl. The trigger was produced by the coincidence between this signal and the two rear counters Fl and F2, i.e. TRIGGER(AaL) = BEAM * Fl - (F2A+F2B). This controlled the processing of the GC counters into 15 bins which was done in a special electronics box 29). The output signals of this box were accumulated in the external scalers. About 60 scalers were implemented allowing an extensive survey of the experiment. The AOOkkelectronics is shown in fig. 4c. Its purpose was to implement the TOF system and to compact the information of an event into a single 16-bit word. In this way it was possible to register about lo7 events at each energy. The BEAM signals of this box were accumulated in the external scalers. About 60 scalers were the counter F3 since only the scattered events are useful for the Aookk measurement. TRIGGER(A,,& = BEAM * Fl . (F2A+F2B) - F3. The trigger set an internal busy which prevented the acceptance of subsequent events, paralysed the normalization BEAM * Fl . F3 and BEAM - F3, BEAM - Fl a F2, scalers BEAM, BEAM - Fl - F2 * F3, and latched the information of the GC counters. The trigger also generated the gate for the TOF measurement, gave later the signal for storing the event into a fast buffer and reset the internal busy. Fig. 5 presents a simplified diagram of the acquisition sequence between the two experiments. Following a change of the beam polarization, all the scalers were reset and the gates of the Aa, and AOOkkexperiments were opened. The computer waited until 200 events were accumulated in the A OOkkmemory (CAMAC fast buffer). It then proceeded to read this buffer and reopen the Aookk gate. This microsequence repeated itself until the end of a 30 set measurement period for the Aa, experiment. At this point the computer emptied the A OOkkbuffer again and read all the AaL scalers. The beam polarization was then changed and a new measurement sequence started. The main characteristics of these two modes of acquisition were thus: Au, experiment

(i) fast acquisition by the 100 MHz scalers and very small dead-time, (ii) integrated information,

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[

beam polaruation

DOWN

UP

I LAM-Aq

ioFF; / ON!

II

II

u

’

gate-Aookk\OFF l-l

) ON’

;

1n

nnm

n

nr

2G~e”is

I

I

I

Fig. 5. Timing diagram of the acquisition

sequences.

(iii) the data are partly analysed since only the correct response of the GC counters is accumulated; AOOkkexperiment: (i) event per event acquisition, relatively slow, (ii) TOF information, (iii) information from F4 signal indicating the scattered particle had crossed the absorber, (iv) full information on the response of the GC counters. 5. Data analysis

A number of problems were encountered during the data analysis. Among them were: (i) the summation of the runs taken with opposite target polarization, (ii) the response of the gray-code detector and the influence of the finite geometry of the apparatus, (iii) the TOF spectra, (iv) the differential cross sections and the influence of the Coulomb-nuclear interference, and (v) the extrapolation of AaL to 0”. These problems are discussed below. 5.1. RUNS

WITH

OPPOSITE

TARGET

POLARIZATION

Data were accumulated with the both orientations of the target polarization at all energies. For the AOOkk experiment, the difference TOF spectra AIT were summed prior to the analysis because of the lack of statistics. Eqs. (18) and (19) become then, respectively, A~~(ej)=AL(ej)+A~-(Bj),

(23)

Al,(ej)=[M,~~(T=~)P,(T=~)+M,,~~(T=t)P,(T=t)] x (nHF(&ti,

0,).

(24)

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P-,( T = +) and Pr( T = -z-) are the values of the target polarization for the measurements with positive or negative orientation of the polarized target. r’,(T) is already an averaged value of the beam polarization

ii,< T) = O.S[P,(B = -+, T) + P,(B = +T)]

.

(25)

Defining the mean value of I’,& as

and, with MZ = Mh + MO,, we obtain finally

(27) All the AOOkkmeasurements were anaiysed using eq. (27). The same procedure was also adopted for the final analysis of the Aa, experiment in order to keep an internal consistency between the two experiments. Consequently eq. (12) was rewritten as (1=[M,(l-~)~M,,(l-~~)l/~~ = -(PPPfnn)&_(Ln) The product (PJQtH)

(28) 1

(29)

is identical to the one appearing in eq. (27).

5.2. RESPONSE OF THE GRAY-CODE

DETECTOR

Corrections for the finite size of the apparatus and for the response of the GC detector were computed using a Monte Carlo program. This program took into account the location and the dimension of the beam defining counters, the beam phase-space, the magnetic field of the polarized target, the dimension and the materials of the target, the differential cross sections for the studied pp-elastic and pp-inelastic reactions, the decay of the pion for the reaction pp -+ rrd, the geometry and the response of the GC detector, and the absorption of the protons or the deuterons in this detector. Multiple scattering along the path of the particles was also included in the calculations. The response of the gray-code detector was studied with a great care. The correct responses are given in the table 1 and all the other responses correspond to a false answer. The events may be sorted in the 16 bins defined by the direct counters or by the other 16 bins defined by the anticounters. Thus each event will be placed in a 16 X 16 matrix and the correct responses of the GC detector correspond to the events located in the matrix diagonal. An analysis of the full matrix was made at 583 MeV with the raw data collected for the A ookkexperiment 30). Most of the bad events were located just off-diagonal where the answers of the direct counters and

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TABLE

The fitted values

for the inefficiencies counters

and the accidentals at 583 MeV

of the individual

CC

Counter

A0

Al

BO

Bl

CO

Cl

DO

Dl

inefficiency [%] accidental [%]

0.2 0.2

0.2 0.2

0.7 0.4

0.3 0.2

0.3 0.2

0.3 0.1

1.8 0.2

0.6 0.6

of the anticounters differ only by one bin. These events come from a single counter inefficiency or from a rescattering in the immediate vicinity of the border of a bin and could be interpreted as an accidental signal or an inefficiency in one of the individual counters. Table 2 gives the values found for each counter. Although rare, events were observed in which signals were present in almost all the GC counters (probability 10P4) or in which no or very few signals were present (probability 3 x lo-‘). The corresponding TOF spectra have helped to sort these cases. The first category was most likely generated by a y-ray shower and presented a fast TOF. The other category was due to an accidental trigger from the counters Fl or F2 leading to a uniformly distributed TOF spectrum. Only diagonal events were used for the final analysis, Monte Carlo being applied to supply the off-diagonal missing events. The global Monte Carlo corrections Cd were defined as ICBO;n( ej)

=

C’,IEi(

ej)

(30)

,

where ITT:: (0,) is the number of events measured in the diagonal of the matrix for the studied reaction (Y(a = pp-el or pp + r’d) and Iz;,m( 0,) is the number of events generated at the target and having their true scattering angle inside the corresponding bin 0,. Table 3 gives the corrections Ci,,_,, used for elastic pp scattering at 583 MeV as an example. These corrections result mainly from geometry effects, i.e. from small solid angle variations in the definition of the bining, caused by the finite thickness of the GC detector. To illustrate this point, table 3 gives also the geometry corrections only which were calculated for a straight line beam scattered on a punctual target and with the assumption that, if a scattered particle is crossing the border of a bin at the position z between the two counters defining this bin limit, it will be counted TABLE

The Monte-Carlo

bin 0,

Cj,,,, geom

corrections

C’p,,,

3

for the diagonal events from pp elastic defined by eq. (30)

scattering

at 583 MeV as

5

6

7

8

9

10

11

12

13

14

15

1.06 1.08

0.96 0.99

1.36 1.31

0.84 0.89

1.16 1.15

0.94 0.98

1.46 1.36

0.84 0.90

1.24 1.25

0.94 0.97

2.10 1.97

Corrections

“geom”

took account

only of geometry

effects (see text).

652

E. Aprile-Giboni et al. / Measurement

of AuL

in the upper bin if zd the middle z, of the code counter, or in the lower bin if z 3 the middle z, of the anticode counter, and as an inefficiency or a border effect if z, < z < z,. The reference bins are defined for an infinitely thin GC detector located at the upstream entrance of the physical GC detector.

5.3. TIME-OF-FLIGHT

MEASUREMENTS

The measurement of the time of flight was a necessary condition in the AOOkk experiment to separate the individual contributions of the different reactions. To illustrate the expected situation, fig. 6 shows the TOF over a path length of 1 m computed from the kinematics for pp scattering. At a fixed lab. scattering angle et., one will first observe the fast peak produced by the elastically scattered protons. The next peak will correspond to the deuterons of the reaction pp+ rd emitted in the forward hemisphere of the c.m. system. The deuterons emitted in backward hemisphere will appear in a third late peak. Fig. 7 shows such TOF spectrum measured in a previous experiment 31)using an unpolarized proton beam of 455 MeV scattered by a liquid hydrogen target at &_= 7.25”. The logarithmic scale reduces the importance of the elastic proton peak and emphasizes the 3-body background.

TIME

OF FLIGHT over IO =Imeter

t

8 TOF [ nsec]

6 5 O*

! 2p” I /

@cm-+ 19’ i i ’ PP-PP

4--

3o0400 500 ‘600

i ’

MeV

PP-PP

I

,J-I

0

L

5 range

of

the

t

1

1 I 10

I

I

t

I

@lab

15

CG

counter

Fig. 6. A comparison of the proton and of the deuteron time-of-digit different incident proton energies.

over a I m path versus 8, for

E. Apde-Gihi

500

550

et al / Measurement

600

Time of flight -Channct

ofAu,

650

653

700

number

Fig. 7. A TOF spectrum for one-track events scattered at 8,= 7.25” at 445 MeV [ref. 3’)]. Notice the ~oga~thmic scale of the verticaf axis.

This picture is totally modified when using a polarized target. The hydrogen feature will appear clearly only in the difference spectrum AI= as indicated by eqs. (19) and (24). Due to the lack of statistics we had to compact further this information in four angular bins only, corresponding to the GC-counter bins 0, = (5 +6+7), (8 +9 +lO), (11 + 12) and (13 +14). Figs. 8 and9showthesefourdi~erence spectra A&( 0,)at 401 MeV and 425 MeV. Similar spectra were obtained at other energies. It should be obvious from eq, (19) that the area under each peak are proportional to the product of the differential cross sections and of the corresponding bkL parameters. In each spectrum the first peak was found negative indicating that the sign of Aookk(pp-el) is opposite to the sign of AOOkk(pp+ ?rd). The first peak has also larger error bars as expected from the subtraction of large peaks produced essentially by the protons scattered from the carbon atoms of the target (see eq. (20)).

E. Aprile-Giboni et al. / Measurement of Au,

654

>

P

1b I-

E. Aprile-Giboni et 01./ Measurement of Au,

655

656

E. A~~~e-Gi~~~ et aL / ~ea~~ment

of &rL

To evaluate properly these spectra, all the peaks were fitted with gaussian distributions. The position tr, and the width or, for the proton peak were easily found from the fit of the direct spectra. The position of the deuteron peaks relative to the proton peak were computed from kinematics and from the calibration of the TDC unit. This calibration was done by an iterative procedure. It was first considered as a free parameter at each energy. The mean value averaged over all the energies was then assumed in the final fits. The width of the forward and backward deuteron peaks crfi and ai were found lo-15% smaller than cp and slowly varying of the incident beam energy. For a fixed beam energy, the two-deuteron peak had, however, the same width od which remained constant for the four angular bins. Only at low energy the width of the backward deuteron peak in the (13 -I-14) angular bin was increased by the natural fluctuation of the deuteron velocity generated by the vicinity of the jacobian peak. The three-body reactions pp-, pnv+ and pp-, pp~’ also contribute as a background to these difference spectra Alx( 6’). The pion has a negligible inlhrence since it is most of the time faster than the protons of the elastic scattering and had a very small phase-space covered by the GC detector. The main contribution comes from the slow protons. Because of lack of knowledge the AOOkk(pp+NNr) parameter was assumed constant over the range of the experiment, so that the TOF difference spectra, shapes and relative amplitudes, were estimated from a Monte Carlo phasespace program without taking into account any polarization effect. Only a single free parameter PNN~, unique for the four Al, spectra, was introduced at each beam energy to allow an overall normalization. The 15 free parameters entering in the final fits of the four Al, spectra at each energy were thus the amplitudes of the proton and of the deuterons peaks, 4( ap, ai, ai), the width of the deuteron peaks a, and ai{ 13 + 14), and the normalization PNN,, of the three-body reaction. The area S and its statistical error of each peak were computed from the fit results and the Monte-Carlo corrections were applied according to eq. (30). The AOOkk parameters were then obtained directly from these values. For pp elastic scattering we have, for example,

Aoodpp-4

ej> =

Szod-e*( ej 1

_

~_E(Pr3P,n,)

1

(31)

[da( H, el)/dL2] d0

The differential cross section values were taken from the PSA for the elastic reaction and from Aebischer et al. 3’) and Weddigen et al. Ia) for the reaction pp+ Td. 5.4. DISTRIBUTION

OF AuL(0)

Most of the incident beam particles passed unscattered through the polarized target and hit the CC detector in one of the first 3 bins. A subsequent large-angle

E. Aprile-Giboni

et

al. / Measurement of AoL

657

scattering on the carbon atoms of the counter was then likely to result in a falsely coded answer. This apparent inefficiency could have affected the AaL experiment if the beam position was slightly changing with the polarization orientation. Notice data since the unscattered events that this effect could not be studied using the A OOkk were vetoed by the counter F3 which covered the first three bins of the GC detector. To elude this problem, the number of particles NB,(fij) falling inside the solid angle 0j was evaluated as the difference between the triggers number NBT(.Ooc) and the number of particles scattered outside of L?j: (32) The large values of the Monte Carlo corrections C&, have very little influence of the final values of Acrb For example, at ri,,, = 583 MeV, the use of the geometry corrections instead of the Monte Carlo corrections produce a change of less than 0.05 mb and without any correction (all Cj, = 1.0) the change is of 0.2 mb. The integration of the Al, (+) TOF spectra for the A OOkk experiment shoufd Clearly give the same results as the corresponding scalers of the AmL experiment, but with reduced statistics. We have verified at all energies that both evaluations gave indeed the same result. In addition, a scaler BEAM - F3 was available in the AOOkk experiment. The counter F3 was located just in front of the GC detector, covered the first bins 1+2 +3 and allowed thus a direct and independent measuremnt of N&L&). These values were found to be in good agreement with the corresponding results of eq. (32). The Aookk experiment gives also a detailed insight into the behavior of Aa,(f2) as a function of the solid angle R which was a somewhat controversial matter between the AaL measurements of other groups 32). The TOF difference spectra presented in figs. 8 and 9 show clearly that the reaction pp+ rrd is of major importance for this AaL behavior. Notice first that the differential cross section for the reaction pp + rrd is strongly anisotropic in its cm. system, for example, at T,,, = 583 MeV [ref. 3’)], [dcr(pp+ rrd),‘dfi] = [11.6+49.4 cos’ (0) - 13.0 cos4(8)]/32r.

(33)

The lab differential cross section for the “forward” or for the “backward” deuterons turns out however to be fairly constant as the result of the jacobian transformation. The corresponding integrals over the solid angle R have thus an almost linear dependence in R which is in strong contrast with the behavior of the individual terms 1, cos’ (0) and cos4 (@) of eq. (33). Fig. 10 presents these different integrals at Ti:,,,= 583 MeV. The situation remains similar even when the beam and the target are polarized. This is also illustrated in fig. 10 where the corresponding curves were computed for Ps = &= 1, using the measured spin-correlation data I’) and by summing the contribution of the forward and backward deuterons. Thus the evaluation of AcL by a linear extrapolation of AmL(Ln) at a= 0 may eventually present some difficulties, but a parabolic extrapolation seems quite adequate as already suggested by Bugg et al. 6).

658

E. Apogee-Gi~ni 1..

.

.

of hq

et al. f’ ~eas~r~eni

I

”

,

Tlnc

PP-PI’0

,

.

= 583

1

,

MeV I

I’ 8’

I’

t’

TOT.

1+.-l.

TOT.

10.0)

.

I’

I’ I’

3.0

,’ GRW-CODE

1’

--)

t‘ I’ #’

2.5

#’

,’ _,

-j

? f ,D :: ” v)

2.0,’ ,’ <‘ I’ 1.5-

#’ I’

I’

FORY.

I’ ,

#’ ,’

,’

TOT.

IO, I+.

OJ +J

,’ fJRCKY. cos*(

10.01

FORW.)

cos’ (FORW cos 0.00

(FORW.)

) 3

0.05

Fig. 10. Deuteron intensity of the pp+ nd reaction falling inside the lab solid angle R,,, computed for different longitudinal polarization of the beam and of the target (Ps, Pr). FORW and BACK refer to the deuterons emitted in the forward hemisphere or in the backward hemisphere, respectively. “~0s” refers to the integration of the individual terms of the differential cross section (eq. (33)).

The Coulomb-nuclear interference in pp elastic scattering also makes an important contribution to the Aa,_ behavior at small angles, when &, s 3” or flL Q 0.01 sr. This effect is responsible for the enhancement of the AOokk (pp-el) parameter at small angle shown in fig. 13, and the ACTS distribution has to be corrected. These CNI corrections were estimated by switching on and off the Coulomb scattering in the Saclay-Geneva PSA 33). Table 4 gives these corrections. They have been subtracted, according to eq. (1 l), in our final values of AaL presented in fig. 11, i.e. prior to performing the extrapolation of 0 to zero. Their importance increases at lower energy and also when L? approaches zero where the Coulomb amplitude f is dominating but the nuclear amplitudes remain different from zero (see eq. (7)).

E. Aprile-Giboni

ofAaL

et al. / Measurement

659

TABLE 4 Coulomb-nuclear

interference

corrections

applied

to Ao,(R,,,)

expressed

in mb

T,,, [Meal Bin 0,

3 4 5 6 7 8 9 IO 11 12 13 14 15

203

285

401

425

451

475

500

520

542

583

3.68 3.22 2.92 2.64 2.46 2.22 2.06 1.90 1.80 1.62 1.52 1.40 1.36

2.20 1.90 1.68 1.48 1.38 1.20 1.10 1.00 0.90 0.80 0.74 0.66 0.62

1.48 1.24 1.08 0.92 0.84 0.72 0.64 0.54 0.50 0.42 0.38 0.32 0.30

1.40 1.16 1.02 0.86 0.78 0.66 0.58 0.50 0.46 0.38 0.34 0.28 0.28

1.20 1.00 0.88 0.74 0.66 0.56 0.50 0.42 0.36 0.34 0.28 0.24 0.20

1.06 0.88 0.76 0.64 0.58 0.50 0.42 0.36 0.32 0.28 0.24 0.20 0.20

0.96 0.78 0.66 0.56 0.50 0.42 0.36 0.32 0.28 0.22 0.20 0.16 0.16

1.00 0.72 0.62 0.52 0.46 0.38 0.34 0.28 0.26 0.20 0.16 0.14 0.12

0.80 0.66 0.56 0.48 0.42 0.34 0.30 0.26 0.22 0.18 0.16 0.12 0.10

0.70 0.56 0.48 0.40 0.36 0.28 0.24 0.20 0.18 0.14 0.12 0.10 0.08

Another little problem may have appeared for the backward deuteron at small angle, where the associated pion or its decay muon may also hit the transmission counter. In that case, two particles simultaneously hit the counter at different places and a false counting could have occurred. This effect was taken into account using the Monte Carlo program. At all energies this correction was about 0.1 mb or smaller.

6. Results and discussion The results for the spin-dependent total cross section are given in table 5 with the statistical errors only. These have been obtained from a parabolic fit of the AeL(i2) values corrected for the Coulomb-nuclear interference. These data have expe~ment after the integration of the been compared with the results of the A oDkk TABLE

Results

expressed

5

in mb of the spin-dependent

total cross section

AaL

206

286

401

425

451

475

500

520

542

583

meas. +/-

-27.85 1.50

-27.85 0.90

-20.65 0.45

-16.80 0.45

-14.70 0.35

-12.80 0.40

-10.05 0.45

-11.50 0.50

-10.90 0.50

-10.45 0.60

PSA +/-

-28.62 0.32

-27.68 0.23

-19.03 0.21

-16.80 0.16

-14.52 0.16

-12.60 0.14

-11.17 0.14

-10.39 0.14

-10.01 0.16

-10.46 0.20

The errors are statistical only. PSA are the prediction of the Saclay-Geneva after the introduction of the present experimental values of Auk

phase-shift

analysis

660

E, Aprile-Giboni et al. / Measurement of Av, I.

SIN-DSIGHR-L

35.0

I

X

5.0

0 Q A X

583 MeV 540t.teV 520 MeV 500 WV 425 MeV

0 l

X . .

451 425 401 286 206

MaV MeV MeV MeV MeV

o.oso

Omega- Lab

Fig. 1t. Measured dist~butions of Aq(fl,& after the subtraction of the Coulomb-nuclear interference. The curves are parabolic fits used for the extrapolation to (I,,, + 0 and for the determination of the Aa, cross sections.

TOF spectra and an excellent internal consistency was found at all the energies of the Aookk experiment. Fig. I1 shows the values of AC,(~) and the fitted curves at all energies. A similar behavior is observed except at 286 MeV around the pionproduction threshold energy. The effect is not well understood. A possible technical explanation, although unlikely, may be that the beam energy is somewhat higher than 286 MeV and that the data contain some contamination from the reaction pp-, wd. Unfortunately, a TOF measurement was not available at this energy to verify this hypothesis, although the energy of the beam was carefully checked from the current in the beam magnets, from the value of the copper degrader and by performing diverse range curves. A typical error on the beam energy is zlz2MeV. Fig. 12 shows a comparison of the different ACT,total cross-section measurements with the results of the Saclay-Geneva PSA. A generally good agreement is found with the Argonne ‘) and LAMPF’) data and with the new Saclay data. A more severe discrepancy is observed with the TRIUMF results 6, which have absolute values systematically higher than the data of the other groups. The energy behavior

661

E. Aprile-Giboni et al. / Measurement of Au,

30.0-

- IOELTR-SICMR-Ll = a(=)-C(C)

Id

n THIS

EXP. o RRGONNE 1977 1980 ,, LDKPF 1901 l S!xL6Y 1982-3 .PSR SOCLRY-GENEVR - 0. R. lGREIN+KAOLLI

q TRIUKF

25.0 -

20.0 -

15.0-

lO.O-

5.0 -

0.0

1

0

Fig. 12. Comparison broken lines indicate

I

200

6

1

400

600

800

1

1000 Tine IMeVI

between the different measurements of AU, cross sections for pp scattering. The the error corridor given by the Saclay-Geneva PSA. The full line was obtained from the dispersion relations ‘I).

is well reproduced however. This normalization problem between the different experiments gives a good illustration of the difficulty in controlling the systematic errors coming essentially from the beam and the target polarization and from the hydrogen thickness in the target. In the present experiment these relative errors are respectively APB/ PB = 2%) APT/ PT = 5% and An,/ nH = 3%. This last error results only from the weight measurement (see sect. 3). Since the target material taken out of a liquid nitrogen bath will liquefy very quickly, it is difficult to perform systematic studies of other factors like packing effects of contraction (or dilatation) effects at low temperature. The main problem is the need to examine samples which are not necessarily identical to the sample effectively used in the experiment or which may not exactly reproduce the conditions of the experiment, for example the target temperature T = 0.5 K. For these reasons, we renounced to such little reliable studies. However, as explained above, a check of our internal normalization was possible with the results of the AOOkkparameters for the reaction pp + rd since this parameter has also been measured in a completely different dedicated experiment at 445, 496, 515, 538 and 578 MeV [ref. “)I. The renormalization of the present experiment to this other more precise experiment was found to be N = 0.99 f 0.03. We feel thus that the overall systematic uncertainty of the present experiment is possibly less than 5%. Fig. 12 illustrates also the most recent calculation of dispersion relations ‘I). The agreement with the data is satisfactory but another calculation 32) fits the

662

et al. / Measurement

E. Apde-Giboni

of Acr,

TRIUMF data better. One has thus to conclude that some flexibility is still allowed in these evaluations which weakens any conclusion on the existence of the dibaryons. We would like to stress that the lack of experimental data on the pp inelastic channels, especially on the 3-body reaction, also weakens any strong conclusion on the result of the PSA, pa~icularly on the behavior of the imaginary components of the phases 33). The Saclay-Geneva phase-shift analysis, which contains all the new results of the pp elastic program of SIN, fits satisfactorily the data presented here. TABLET

Results of the A,,, parameter

for pp eiastic scattering

5.8”

2.9” 401 425 451 475 500 520 542 583 The errors

0.32~0.13 0.35*0.11 0.17*0.10 0.28+0.14 0.2650.13 0.17*0.12 0.14*0.13 0.44*0.1s are statistical

at small angles

0.06*0.11 0.15*0.09 0.15LtO.08 0.11 rto.10 -0.04* 0.09 0.11 f 0.08 0.08*0.10 0.08*0.12

0.29ztO.11 0.17Ito.09 0.09 f 0.08 0.29~00.10 0.07 f 0.09 0.00 f 0.08 -0.05 f 0.09 0.11*0.11

6.8” 0.21 iO.09 0.23 f 0.07 0.06 f 0.06 0.19Lto.07 0.12rtO.07 O.lOztO.06 0.01 f 0.07 0.11 rtO.08

only.

The results of the Awkk parameter for the elastic pp scattering at small angles are given in table 6. The errors are stati$ical only and their large values are the consequence of the large cross section for the proton scattering on the carbon atoms of the target. It is the first time that this parameter has been measured at small angle and the data, shown in fig. 13, are found to be in good agreement with the prediction of the PSA. The effect of the coulomb-nuclear interference is clearly visible on this figure but the error bars of the data are too large to allow a detailed analysis of the structure as suggested in the introduction. The results of the Aookk parameter for the reaction pp+ ?rd are given in table 7 with their statistical errors. Since the position of the rear counter was kept fixed during the whole experiment, the center-of-mass solid angle subtended by this detector became smaller as a function of the incident energy and the corresponding angular distributions were thus only partly measured. Only at low energy, 401 and 425 MeV, are the data sufficiently completed to allow by an integration the evaluation of the specific spin-dependent total cross section Aa,(pp+ ad). These data are shown in fig. 14 together with the predictions by Niskanen 34). New predictions from other authors have recently become available. They are discussed in detail in connection with a dedicated experiment I’). As stated above the present data were found in excellent agreement with this other measurement.

E. Aprile-Giboni

et al. / Measurement

d

of Au,

d

664

E. Aprile-Giboni P

AOOKK

P -pr+rl

et al. / Measurement

MeV

401

P

AOOKK

~~~:~

of Au,

1

.

0.4

I 0.6

.

425

nev

I 0.0

0.2

cos2(&J

parameter

of the A,,,,,,

0.4 cos2

0.6

0.8

&,I

for the pp+ ?rd reaction measured at 401 and 425 MeV. The curves are the theoretical predictions of Niskanen 34).

7

TABLE

Results

-PI+D

::_:1_;;q

0.2 Fig. 14. A,,,

P

parameter

for the reaction

pp+

?rd as a function

of the c.m. angle

0

401

e

23.7 -0.77 I (0.082)

37.6 -0.755 (0.057)

50.5 -0.75 1 (0.073)

62.8 -0.801 (0.088)

162.6 -0.772 (0.088)

152.2 -0.699 (0.054)

142.2 -0.593 (0.063)

132.2 -0.753 (0.069)

425

6

22.1 -0.778 (0.078)

34.9 -0.853 (0.043)

46.5 -0.827 (0.071)

57.3 -0.889 (0.084)

164.2 -0.720 (0.076)

155.0 -0.797 (0.040)

146.3 -0.740 (0.061)

138.1 -0.880 (0.064)

451

e

20.9 -0.923 (0.071)

32.8 -0.822 (0.048)

43.4 -0.817 (0.054)

52.7 -0.706 (0.065)

165.6 -0.793 (0.110)

157.1 -0.783 (0.046)

149.5 -0.858 (0.050)

142.4 -0.779 (0.052)

475

e

19.9 -0.814 (0.080)

31.2 -0.878 (0.057)

41.2 -0.678 (0.063)

49.8 -0.503 (0.070)

166.6 -0.899 (0.173)

158.8 -0.753 (0.053)

151.7 -0.750 (0.063)

147.4 -0.746 (0.065)

500

e

19.2 -0.847 (0.074)

29.9 -0.692 (0.046)

39.4 -0.704 (0.057)

47.5 -0.505 (0.062)

167.7 -0.803 (0.073)

160.1 -0.627 (0.036)

153.6 -0.599 (0.037)

147.8 -0.790 (0.061)

520

8

18.6 -0.858 (0.080)

29.1 -0.734 (0.064)

38.2 -0.682 (0.062)

46.0 -0.521 (0.070)

167.9 -0.847 (0.078)

161.0 -0.677 (0.045)

154.4 -0.841 (0.046)

149.4 -0.731 (0.080)

542

e

18.2 -0.630 (0.067)

28.3 -0.660 (0.047)

37.1 -0.609 (0.059)

44.5 -0.446 (0.064)

168.5 -0.625 (0.067)

161.9 -0.614 (0.040)

156.0 -0.599 (0.062)

150.9 -0.655 (0.050)

583

e

17.4 -0.607 (0.097)

27.1 -0.692 (0.077)

35.4 -0.842 (0.078)

42.4 -0.483 (0.094)

169.2 -0.600 (0.097)

163.2 -0.658 (0.068)

157.8 -0.532 (0.054)

153.2 -0.596 (0.073)

The errors,

given in parentheses,

are statistical

only.

E. Aprik-Giboni et al. / Measurement of AaL

665

The spin-dependent total cross section for the 3-body reactions pp + pp?~” plus pp+ pn7t+ can be evaluated by a subtraction from the AcrL(pp-tot) data as indicated by eq. (22). The Geneva members of our group have also extensively measured the spin-correlation parameter Aookk for the pp elastic scattering at large angles “) so that the corresponding cross section Aa,(pp-el) can easily be estimated. However, the use of the PSA values for A~~(pp-tot) and A~=(pp-elm is better for reducing the statistical fluctuation 33). The evaluation of the cont~bution from the reaction pp + nd requires a good normalization of the usual spin-independent differential cross section. This normalization was achieved here so that the total cross sections calculated by the integration of the differential cross section correspond to the values estimated from a best fit of the measured total cross sections taken from the compilation of Bystricky and Lehar et al 3s), The values of the AOOk*parameter are The different contributions to the spin-dependent total cross section Aa, expressed in mb T1°C 401 425 4.51 sM1 520 542 583

PP+PPa) -19.f%*o.17 -18.57*0.14 -17.35*0.20 -15.37*0.18 -14.77+=0.18 -14.26*0.18 - 13.75f 0.20

pp+=db)

pp+ NNlr “)

1.2_5*0.10 1.89+0.12 2.07*0.10 3.03*0.10 3.09*0.10 3.12*0.10 2.75*0.10

0.85 f 0.30 0.96 f 0.25 1.49 f 0.30 1.54+0.25 1.62 f 0.25 1.50*0.26 0.97 Ito.

“) The contribution from the pp elastic scattering computed from the Saclay-Geneva PSA after the introduction of the spin-correlation measurements 24). b, The contribution from the pp+ rrd reaction. The values of the A,,, parameter are taken from the present experiment for 401 and 425 MeY and from ref. 19) for the other energies. “) The contribution from the pp + NNw computed using eq. (22).

taken from the present experiment and from ref. 19). Table 8 gives the obtained results with their statistical errors. Fig. 15 shows the results for the spin-dependent total cross section Aal(pp 3 inelastics), A~,(pp -, rd) and the deduced values for the 3-body reaction Ao;(pp + NNrr). The broken lines are to guide the eye through the data. It is interesting to point out that the two inelastic reactions have a similar energy behavior peaking around 520 MeV in contrast to their spin-indpendent total cross sections indicated with specific lines on fig. 15. It is interesting to notice that the spin-dependent effects from the 3-body reactions are always smaller than those from the pp+ rrd reaction in the studied energy domain. For example at 583 MeV, this is in strong contrast with the spin-independent total cross sections where their ratio is c+&pp + NNrr)/o;,(pp-, rrd) = 2.1. Fig. 16 presents the same data for Ao,(pp + NN?r) together with recent theoretical predictions 21,38*39). The agreement

666

E. Aprile-Giboni

et al. / Measurement

DELTR-SIGMA-L

of Au,

INELASTICS

m-]i

6.0

TI~c.

MeV

Fig. 15. Contributions of the pp-inelastic reactions to the Aa,(tot) cross section (see table 8). Specific and pp -) NN?r (-) lines show the spin-independent total cross section for the pp + md (- +-) reactions which present very different energy behaviors. Broken lines are eye-guides through the data. DELTA-SIGMA-L

PP

--c

N N PI

C”“““““‘.“““““’ l THIS

YORK

---KLOET+SILBRR ---KONIG+KROLL . -RINRT 5.0 .z

-

E

4.0-

_A $ c3

3.0-

cn 2

2.0-

ii Q +

l.Or

Tim.

Fig. 16. Theoretical

predictions

for Aa,(pp+

NNv)

MeV

and comparison

with the data.

667

E. Aprile-Giboni et al. / Measurement of Au, is

quite bad and confirms

the Argonne

our doubts

group is of a different

requested. To summarize,

the main

on the interpretation opininon

structure

around

of the structure

4’). More theoretical

although

work is certainly

m = 2.15 GeV first observed

by the

Argonne group in the spin-dependent total cross section Au, is confirmed in the energy range available at SIN. A simultaneous study of the scattered particles at small angles has been carried out with success and gave the spin-correlation parameter AOOkk for the pp elastic scattering and for the reaction pp + rd. The contribution of the 3-body reactions to this spin-dependent total cross section has been deduced and found lower than the contribution of the rd reaction even at 583 MeV. Nevertheless, the existence of true dibaryons remains an open question and it seems clear that only detailed analyses based on the same explanation of many different experiments will be able to solve the problem. All the data presented here accumulated after 100 h exposure to the accelerated polarized proton beam. Since this operation represents a single-user mode of the SIN accelerator, it was impossible to run longer for an improvement of the statistics. We would like to thank the Swiss Institut for Nuclear Research for its invaluable technical assistance during the experiment and express our special gratitude to Professor J.P. Blaser, its director. We would like to thank R. Mermod, J.F. Detoeuf and J. Saudinos for their constant encouragements through this work and the technical staff of the University of Geneva and of the DPhPE CEN Saclay. We are indebted to W. Grein, R. Hausammann, P. Kroll, C. Lechanoine-Leluc, M. Lecher, L. Van Rossum and P. Wintemitz for helpful discussions. We are grateful to T. Siemiarczuk, J.E. Simmons and P. Veluard for their participation in the construction and in the test of the gray-code detector. This work was partly supported by the Swiss National Science Foundation and the Convention Intercantonale d’Enseignement du 3eme cycle de la Physique

en Suisse Romande.

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