c pp elastic scattering at large momentum transfers

Nuclear Physics B185 (1981) 1- 19 © North-Holland Publishing Company

MEASUREMENT

OF T H E P O L A R I Z A T I O N P A R A M E T E R IN 241 G e V / c PP

ELASTIC SCATTERING AT LARGE MOMENTUM

TRANSFERS

J. ANTILLE I, L. DICK and M. WERLEN 2 CERN, Geneva, Switzerland

A. GONIDEC, K. KURODA, A. MICHALOWICZ and D. PERRET-GALLIX 2 LAPP, BP 909, 74017 A nne¢y-le- Vieux, France

D.G. CRABB 3, P. KYBERD and G.L. SALMON Nuclear Physics Laboratory. Oxford Universi(v, Oxford, UK

Received 7 November 1980 (Final version received 16 February 1981)

A measurement of the polarization parameter P0 in pp elastic scattering has been made at 24 GeV/c over the range of momentum transfer squared 0.7
1. Introduction In the p a s t few years much a t t e n t i o n has been p a i d to the effects of spin in n u c l e o n - n u c l e o n i n t e r a c t i o n s and, in particular, p p elastic scattering has been well studied. T h e m e a s u r e m e n t s of the p o l a r i z a t i o n p a r a m e t e r P0 a n d various o t h e r spin p a r a m e t e r s such as ANN a n d ALL have revealed a rich a n d varied structure. This has h e l p e d to p u t limits on the five c o m p l e x a m p l i t u d e s a n d has i n d i c a t e d which are i m p o r t a n t for p a r t i c u l a r theoretical models. W i t h o u t a c o m p l e t e set of m e a s u r e m e n t it is i m p o s s i b l e to d e t e r m i n e uniquely the b e h a v i o u r of the a m p l i t u d e s , but the available spin d a t a has m a d e it possible to u n d e r s t a n d the m a i n features of the i n t e r a c t i o n so that in the small angle scattering region (It] < 1.0 ( G e V / c ) 2) the d a t a is r e a s o n a b l y well d e s c r i b e d by several m o d e l s [1-3]. T h e p u r p o s e of the present e x p e r i m e n t was to measure the p o l a r i z a t i o n p a r a m e t e r in a region of four m o m e n t u m transfer squared ( 1 . 0 < It I < 2.0 ( G e V / c ) 2 ) , where d a t a was scarce, a n d which had received little theoretical attention. A l s o it a i m e d to I Present address: IPN, 1015 Dorigny, Switzerland. : Prescnl address: SLAC, PO Box 4349, Stanford, Ca. 94305, USA. 3 Present address: Randall LaboratoW, University of Michigan, Ann Arbor, MI. 48109, USA.

I

2

J. Antille et al. / Polarizationparameter

extend the measurements to as high a momentum transfer as possible, where the dynamics of the interaction might be simpler, with the underlying thought that the data might signal the onset of a region where the scattering takes place between the constituents of the protons. Studying the effects of spin on pp scattering in this region could provide information on the spin of the constituents. The experiment [6, 7] used a polarized target to measure the polarization parameter in pp elastic scattering at 24 G e V / c . The experiment ran in a high intensity proton beam at the CERN PS and covered the four-momentum transfer range 0.7 < Ill< 5.0 ( G e V / c ) 2. The high intensity beam and a large acceptance spectrometer allowed measurements around it -- 5 ( G e V / c ) 2 where the cross section is only 5 nb.

2. Experimental apparatus and procedures The layout of the experiment is shown in fig. 1. A 24 G e V / c beam from the CERN PS was incident on a polarized proton target. The forward scattered proton was detected by scintillation counters S~, S2, and S3 and its position measured by scintillation counter hodoscopes H 5 and H 6. The (~erenkov counters C I and C 2 rejected pions and low energy protons. The recoil proton was detected by scintillation counters Ri and R 2 and its momentum measured using the spectrometer magnet and hodoscopes H I to H 4. The spectrometer magnet was moved to successive positions according to the It I range being measured. 2.1. THE BEAM

The 24 GeV,/c secondary beam C 9 was derived from a 24 G e V / c extracted beam at the CERN PS and used dispersive optics and collimators to reduce the primary

/It=-1 I

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Fig. I. Layout of the experiment.

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d. Antille et al. / Polarization parameter TABLE l Characteristics of the C, beam

Intensity Momentum Resolution & P / P Horizontal divergence Vertical divergence Beam dimensions at the polarised target Spill time

~ 109 ppp 24 GeV/c 0.5% +_ 1.2 mrad _+ 0.13 mrad 7 × 6 mm2 500 ms

intensity of 3 × 10 ~1 protons per pulse to 10 9 ppp. The extracted beam was focussed on a stainless steel production target 8 cm long and 1.5 cm diameter. The beam, transmitted at 0 °, was then transported a distance of 51 m by standard beam elements to the polarized target. The details of the beam are given in table 1. The beam intensity was measured by an absolute ion chamber while the beam position and profile was measured by multiwire ionization chambers. 2.2. POLARIZED TARGET AND RADIATION DAMAGE T h e p o l a r i z e d p r o t o n target a s s e m b l y was a s t a n d a r d C E R N 3 H e c r y o s t a t o p e r a t ing at " K a n d u s i n g p r o p a n e d i o l ( C 3 H 8 0 2 ) as a target m a t e r i a l . T h i s s y s t e m a n d its o p e r a t i o n h a v e b e e n d e s c r i b e d p r e v i o u s l y [4]. 100 90 8O

°

o

70 ae

.~

60

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50

mr"

0

with beam scanning

•

without beam scanning

I

/,0

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30

Integrated

I 2 beam

intensity

I 3

( 1013 p r o t o n s )

Fig. 2. The decrease of the target polarization as a function of the amount of beam passing through the target, with and without beam scanning.

4

J. Antille et al. / Polarizationpararneter

In this particular target, the propanediol was contained in a rectangular copper cavity of dimensions 20 X 16 m m and 42 m m long rather than the normal cylindrical cavity (14 m m diameter, 42 m m long). This shape of cavity was used to allow the proton beam to be scanned over the target area in order to reduce the effects of radiation damage. A previous publication [5] discussed the problems of radiation damage in this target using a cylindrical cavity and pointed out that with a non-uniform irradiation the polarization measured by an N M R coil is not necessarily that seen by the beam and a correction has to be applied. Using the larger cavity and scanning the beam progressively over a rectangular area, allowed a more uniform irradiation of the target material, so that the polarization measured by the N M R was more nearly that seen by the beam. This method also lengthened the target lifetime so that target material changes were less frequent. Two N M R coils, each 5 mm diameter were used, one at each end of the cavity and offset from the central axis and from each other. In this way, different regions along the target were sampled as a check that the target was being uniformly irradiated. Initial polarizations, before irradiation, were in the range 80 to 90%. A comparison of the rate of radiation damage with and without beam scanning is shown in fig. 2. The beam scanning was accomplished by two small steering magnets which directed the beam at 35 different points on the face of the target. Five beam pulses were directed at a particular point before moving on to the next. The target material was changed every five days or when the polarization dropped to 50%. 2.3. THE SPECTROMETER AND DETECTION SYSTEM As shown in fig. 1, the spectrometer system used scintillation trigger counters and counter hodoscopes together with m o m e n t u m analysis on the recoil arm. (~erenkov counters in the forward arm improved the rejection of inelastic events. In the recoil arm the large spectrometer magnet had pole tips of 100 cm X 100 cm with a 50 cm gap. The central field was 10 kG. The trigger counters R 1 and R 2 defined a recoil particle while the counters A I and A 2 vetoed particles whose trajectories were not in the good field region of the magnet. The particle time of flight was measured between R 1 and R 2. Because R 2 covered an area approximately one metre square, it was broken up into a matrix of 3 X 3 counters each 30 cm X 30 cm so that the problems of transit times and pulse-height variations which occur in large scintillators were greatly reduced. Using a matrix also allowed some rough position information to be obtained. The particle trajectory was measured with hodoscopes H I to H 4, H 4 being used to define the vertical position. With this system the m o m e n t u m of the recoil particle was measured with a precision of about 4%. The t range covered at a particular magnet position was --~ 1 ( G e V / c ) 2 and the magnet had to be moved to 5 different positions in order to cover the complete t range of the experiment. The precision on measuring t ranged from 5% at Itl = 1 ( G e V / c ) 2 to 15% at It] = 5 ( G e V / c ) 2.

J. Antille et al. / Polarization parameter I

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--. r -

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Angular correlation Ae Fig. 3. Angular correlation distributions without cuts for It] = 1.7 __+0.1. The two curves show the effect of including (~erenkov counter C 2 in the trigger.

In the f o r w a r d a r m a trigger was o b t a i n e d from S,, S 2 a n d S 3 a n d the p o s i t i o n of the particle m e a s u r e d with h o d o s c o p e s H 5 a n d H 6. (~erenkov c o u n t e r C I was a c o n v e n t i o n a l t h r e s h o l d c o u n t e r using nitrogen gas at a pressure to veto pions. (~erenkov c o u n t e r C 2 was a high resolution threshold counter, also using nitrogen gas, set to detect p r o t o n s with m o m e n t u m consistent with having c o m e from elastic scatters a n d thus to reject p r o t o n s from inelastic collisions. T h e c o u n t e r has b e e n d e s c r i b e d in detail elsewhere [6, 7]. It was c a p a b l e of a m o m e n t u m r e s o l u t i o n of 6% a n d the effect of i n c l u d i n g it in the trigger is shown in fig. 3. T h e trigger c o n d i t i o n for a g o o d event was a scatter in the f o r w a r d arm, a recoil p a r t i c l e within the a c c e p t a n c e of the m a g n e t a n d a c o i n c i d e n c e b e t w e e n the two arms. 2.4. THE MONITORS

A number of monitors were used for checks on the beam and to normalize the data with target spin up to that with target spin down. The beam intensity was measured by a thin parallel plate ionisation chamber which h a d b e e n c a l i b r a t e d against an i r r a d i a t e d a l u m i n u m foil. It was also used as a

J. Antille et al. / Polarization parameter

normalization monitor. The beam profile and position were measured in both horizontal and vertical planes at two positions by multiwire ionization chambers with 1 mm wire spacing. Two, three scintillation counter telescopes M~ and M 2 viewed the polarized target in the plane of polarization. They were sensitive to the number of protons hitting the target and the target mass but insensitive to the polarization direction. A small magnetic spectrometer B123bending vertically provided a further monitor. It accepted 300 MeV/c protons scattered backwards of 90 ° from the heavy nuclei in the target and was therefore insensitive to the polarization direction.

2.5. ELECTRONICSAND DATAACQUISITION The forward arm (S l -$2-$3.C 1.C2) and the recoil arm (R 1.R2.(A 1 + A2) ) were brought together to form the final R. S trigger. Each hodoscope produced a signal indicating that there had been at least one hit in that hodoscope: the trigger was put into a coincidence with these signals to produce a CANDIDATE signal. A CANDIDATE signal was defined as an event with at least one particle in each of a specified number of hodoscopes. The CANDIDATE signal interrupted the on-line computer and started the data acquisition phase. During this phase the hodoscope data were encoded in a specially designed parallel input register. All other information, such as time of flight, pulse height, target polarization and beam position was registered in a series of CAMAC modules. The on-line computer system used three NOVA computers in series. The first computer, a NOVA 2 read in all the experimental information, decoded the hodoscope data blocks and performed various checks. Then the data block was sent to the second computer, a NOVA 840 for writing to tape and subsequent analysis. The last computer, a second NOVA 2, was used, together with the 840 for on-line analysis to produce various histograms, efficiencies and performance figures. This enabled the experiment and apparatus performance to be closely monitored.

3. Data analysis The off-line analysis was performed on the CERN CDC 7600 and about 4 million events were analysed. The first stage was to reconstruct the particle trajectories for each event, deriving the corresponding scattering angles, recoil particle momentum, coplanarity and the interaction point in the target. Another set of variables was calculated by assuming that the scattering was elastic, and then using the measured forward production angle to calculate the predicted four momentum transfer squared, the recoil momentum and recoil production angle. These variables were written to a summary tape (DST) for further analysis.

J. Antille et al. / Polarizationparameter

7

3.1. DISTRIBUTIONSAND ELASTIC EVENTS The program which analysed events on the DST classified them according to various criteria and accumulated them by momentum transfer interval in histograms. For extracting the elastic signal four distributions were used: (a) the interaction point x 0 in the target, (b) the coplanarity, A~, (c) the recoil momentum correlation Apt ----pr (measured)-p~ (calculated), (d) the recoil angle correlation A0~ = 0r (measured) - Or (calculated). The distributions for the setting 0.6 < Itl < 1.6 ( G e V / c ) 2 are shown in fig. 4. In fig. 4a the target dimensions are clearly defined and one can also see events which come from the cryostat walls. In the other distributions the elastic signal is clearly visible above the background. In the Ap~ plots, A p r / p r ~ 0.04 and is constant for different t regions. The background is higher for negative values of pr corresponding to inelastic background. The elastic peak in the A0~ and A~ distributions is about 1.2 ° and 1.8 ° wide, respectively, and is compatible with the experimental resolution. 3.2. BACKGROUNDS Two distinct processes can contribute to the background under the elastic peak: (a) quasi-elastic scatters off the bound protons of the carbon and oxygen in the polarized target; (b) inelastic reactions from both the bound and free protons. Since there are approximately 4 times as many bound protons as free protons in the target, there is the possibility of a large contribution to the elastic signal. In addition, the bound protons are not polarized and so the background of quasi-elastic and inelastic scatters will tend to dilute any asymmetry coming from purely elastic scattering. On the other hand, inelastic background from the polarized protons can give rise to false asymmetries since it has been determined that N* production can give rise to large asymmetries [9, 10]. In order to study the source of background in detail a Monte Carlo program was written to simulate the experiment. The Monte Carlo was tested by comparing its prediction for various distributions with the experimental ones. There was good agreement as shown in fig. 5, in which cuts on the vertex, recoil momentum and coplanarity have been made to purify the sample of elastic events. 3.2.1. Quasi-elastic background. The contribution of quasi-elastic scatters to the elastic events depends strongly on the Fermi-momentum distribution of the nucleons in the carbon and oxygen of the target. The larger the mean value of the Fermi momentum, the more the kinematic correlations are changed so that eventually a large fraction of the quasi-elastic events are not accepted by the spectrometer. Thus, on the A0 plot, the quasi-elastic scatters would tend to be smeared out under the elastic peak. The Monte Carlo simulation was used to confirm that the events

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J. Antille et al. / Polarizationparameter

400 3O0 C

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Fig. 4. Event distributions for 0.6 < }t I < 1.6 ( G e V / c ) 2. The arrows show where the cuts were to be applied. (a) the interaction point along the length of the polarized target, x and y are the limits of the target and w and z are the cryostat walls, (b) recoil momentum correlation, Apf, (c) coplanarity, Aep, and (d) angular correlation A0r.

J. Antille et al. / Polarization parameter

Monte Carlo events

Real events

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(GeV/c) 2.

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J. Antille et al. / Polarizationparameter

outside the elastic peak were due to quasi-elastic scattering and that a smooth extrapolation could be made under the elastic peak. In the Monte Carlo the Fermi-momentum distribution d N / d p was described by the Hulth6n distribution [11] where dN

p2

dp

(p2 + az)Z(p2 +f12) 2

and where the values of a -- 0.14 and fl = 0.5 were determined from the experimental distributions of van Rossum et al. [8]. The results of the Monte Carlo were compared with the angular correlation distribution after cuts on vertex, coplanarity and recoil momentum. Fig. 6 shows the results for the t range 0.6 < It[ < 1.6 ( G e V / c ) z. There is good agreement between predicted and experimental distributions. Similar results were obtained for the other t regions. 3.2.2. Inelastic background. At 24 G e V / c the cross sections for inelastic reactions of the type p + p ~ p + N* have been measured [12] and found in some cases to be comparable to that of pp elastic scattering. In addition, at this energy, the kinematics for N* production are not too different from that of elastic scattering. Thus, it was possible for the elastic signal in this experiment to have been contaminated by such inelastic events. Using the Monte Carlo the following reactions were generated: (a) pp ~ Nfo~w~dp, (b) pp ~ pNbackward, (C) pp ~ N ' N * . The N*(1688) has the largest cross section and was considered first.

elastic signal

150 I

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Fig. 6. Angular correlation distributions for background events obtained from the experiment and for a Monte Carlo simulation of quasi-elastic events from the bound protons in the target material.

J. Antille et al. / Polarization parameter

1I

The results of the calculation were: (i) Contamination by reactions (b) and (c) were negligible compared to (a) because of kinematic rejection. (ii) For reaction (a), the decay channel N * ~ ~r + n was negligible because the (~erenkov system rejected 80% of the ~r + and those which remained had a momentum < 3.5 G e V / c and were not accepted by the spectrometer. (iii) By contrast, the decay channel N * ~ 7r°p for reaction (a), had a small contribution to the elastic peak. About 84% of the protons from this channel were rejected by the Cerenkov counter C 2. The effects of acceptance and cuts reduced the contamination still further. An angular correlation plot of the remaining events showed them to be grouped about 1½° from the elastic peak and to contaminate the elastic signal at worst to a level of about 1%. The contribution from other N* states was estimated to be at the level of 1%. Thus the upper limit on the contamination of the elastic signal by all N* production was 2%. In conclusion, the background under the elastic peak is described quite well by quasi-elastic scattering and the contamination of the elastic peak from inelastic events was negligible. 3.3. B A C K G R O U N D S U B T R A C T I O N

The asymmetry in elastic scattering is obtained from the expression

A ( t ) - - N+ ( t ) - - CN_ (t) N+ (t) + CN_ (t) ' where N+ (t) and N_ (t) are the number of elastic scattering events obtained for each polarization sign of the target, and where C is a factor to normalize N_ to the same integrated flux of incident beam as N+. However, the effect of the background on the measured asymmetry can be seen if we use the peak in the angular correlation distribution and note that N+ = N~_ -- Nb+ ,

N_ = N'_ -- N b-

where N~_ and N'_ are the total number of events obtained with each polarization sign and N f f is the background. Since N ~ are due entirely to quasi-elastic scattering and are independent of the polarization sign, Nb+ = C N b . Then

A(t) = N'+ (t) - CN" (t) 1 N'+ (t)-~CN'__ (t) 1 - F ' where F represents the importance of the background level and is given by F = N~- ( t ) + C N b (t) N'+ (t) + CN'_ ( t ) "

12

J. Antille et al. / Polarization parameter

The background for each setting was estimated by fitting the tails of the angular correlation distribution with a polynomial function of order 4 to 6 and interpolating under the elastic peak to find the background to be subtracted. Since the events in the quasi-elastic background have very similar kinematical properties to the elastic events, they should be affected in the same way by any change in the efficiency of the apparatus. In addition, one setting of the apparatus might use several loads of target material and any load to load variations (in amount of target material for instance) would affect both types of event in the same way. As a check at each setting some data was taken with a carbon target and compared with the estimates of quasi-elastic scattering obtained from the Monte Carlo and with the polynomial fit. There was good agreement within the rather poor errors on the carbon data. 3.4. NORMALIZATION The normalization of the number of events for target polarization up (N+) to those for target polarization down (N_) was achieved by using the two sets of monitors M~ + M 2 and Bl23 and the asymmetries obtained for each were equal to within 0.1%. For the measurements at large momentum transfer, the number of quasi-elastic events outside the elastic peak in the tails of the coplanarity and angular correlation distributions was available as a third polarization insensitive monitor. This internal monitor had the advantage of tracking changes in the spectrometer detection efficiency, and asymmetries calculated using it showed a similar scatter to those derived from the external monitors M~ + M 2 and B123.

4. Results

The polarization parameter P0 for each t bin was calculated from the numbers of elastic events (N+ ,N_ ) for each polarization sign. N+ and N were found by subtracting the background events Nb+ and N b- from the total number of events N~_ and N'_ in the angular correlation distribution, as discussed in subsect. 3.3. The numbers of events are given by N_+ --- N o (1 -+- P0(Pf~ ) cos~), where No are the number of events which would be recorded for an unpolarized target and ( P ~ ) is the average target polarization for each orientation. ~ is the polar angle of the scattering plane with respect to the polarization axis. In this experiment 0.998 < cosq~ < 1 and was therefore neglected. Then

u+ - c u _ V° -- ( P ~ )N+ +C(P~. )N_ ' since No+ and No- are normalized together by No+ =

CNo-.

13

J. A ntille et al. / Polarization parameter

T h e sign o f P0 is in a c c o r d w i t h t h e s p e c i f i c a t i o n s o f the Bfile c o n v e n t i o n . It s h o u l d a l s o b e n o t e d t h a t t h e A n n A r b o r c o n v e n t i o n [13] d e f i n e s t h e a s y m m e t r y p a r a m e t e r m e a s u r e d w i t h a p o l a r i z e d t a r g e t as the a n a l y s i n g p o w e r A. 4.1. POLARIZATION PARAMETERS T h e results are t a b u l a t e d in t a b l e 2 a n d p l o t t e d in fig. 7. F o r c o m p l e t e n e s s , p r e v i o u s l y p u b l i s h e d v a l u e s at l o w t [14] w h i c h u s e d a d i f f e r e n t s p e c t r o m e t e r are also t a b u l a t e d a n d p l o t t e d . It s h o u l d b e p o i n t e d o u t t h a t in the p r e v i o u s p u b l i c a t i o n the q u o t e d e r r o r o n t h e v a l u e at - t : 0.9 was i n c o r r e c t a n d s h o u l d h a v e r e a d P0 : 0.022 _ 0.018. F u r t h e r r u n n i n g w i t h t h e s m a l l t s p e c t r o m e t e r at - t = 0.9 g a v e a v a l u e o f Po = - 0 . 0 1 8 _ 0.01 w h i c h c o m b i n e s to give the v a l u e listed in t a b l e 2 of - 0 . 0 0 9 +__ 0.009 w h i c h is in g o o d a g r e e m e n t w i t h the v a l u e f r o m this e x p e r i m e n t .

TABLE 2

P0 for pp elastic scattering at 24 GeV/c

It l (GeV/c) 2

Range of I t l in acceptance A t (GeV/c)

P0

0.74 0.86 1.07 1.28 1.49 1.68 1.91 2.09 2.28 2.46 2.69 2.97 3.33 3.84 4.23 4.70

0.7-0.8 0.8-1.0 1.0-1.2 1.2-1.4 1.4-1.6 1.6-1.8 1.8-2.0 2.0-2.2 2.2-2.4 2.4- 2.6 2.6-2.8 2.8-3.2 3.2-3.6 3.6-4.0 4.0-4.5 4.5-5.0

- 0,002 -4- 0.002 - 0.002 ± 0.003 - 0.012 ± 0.008 0.036 -+'0.014 0.029 -+-0.021 0.049 ± 0.025 0.063 -- 0.020 0.039 0.015 0.019 ---+0.016 0.043 ± 0.022 0.032 ± 0.024 0.011 + 0.020 - 0.078 ±0.068 - 0.163 ± 0.065 - 0.025 ±0.041 0.017 ± 0.067

The results for Po from ref. [14], including the revised value for It I : 0.9 0.10 0.20 0.30 0.40 0.50 0.60 0.70 0.80 0.90

0.033 0.047 0.058 0.066 0.074 0.081 0.086 0.093 0.098

0.036 __--_0.003 0.028 -+- 0.004 0.032 ± 0.003 0.032 ± 0.006 0.014 ± 0.008 0.009 ± 0.004 0.003 __+_0.007 0.005 + 0.009 -- 0.009 __--_0.009

14

J. Antille et al. / Polarization parameter I

I

I

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Fig. 7. The results for the polarization parameter versus t. Data at low It I from ref. [14] are included.

The errors are a combination of statistical and monitor normalization'Lerrors. In addition, there is a negligible overall systematic uncertainty due to the uncertainty of 4% in the knowledge of the absolute target polarization. 4.2. DISCUSSION

The data are shown in fig. 8 together with data from other experiments at energies from 7.9 to 300 G e V / c [5, 15-20]. The overall structure is qualitatively similar to that of the lower energy data but closer examination shows some interesting features. In the region ]t] = 0.7 to 1.2 ( G e V / c ) z the polarization is zero or slightly negative, indicating that the double zero structure of lower energy is changing to a single zero followed by negative polarizations and a further single zero as the polarization climbs back to the second peak. Traditionally, such changes in the polarization have signalled changes in the cross section and this change seems to be associated with the deepening of the dip in the cross section around ]t] = 1.5 ( G e V / c ) 2. This change means also that the rise to the second peak is delayed compared to lower energies. One noticeable feature of this data is that the value of the polarization at the second peak is considerably suppressed compared to lower energies, being at a level of about 5%. The values of the polarization at It] = 1.7 ( G e V / c ) 2, which is where the peak occurs, are plotted in fig. 9. At low energy it is approximately constant with a value of 17% then drops to 5% at 24 G e V / c . It should be remarked that the fall is hinted at by the 17.5 G e V / c data; although the individual errors are large the variation of P0 with t is quite smooth giving a peak value of 10%. Above the second peak there is a dip around It I = 3.6 ( G e V / c ) 2 where P0 goes down to about - 16%.

15

J. A ntille et al. / Polarization parameter

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Fig. 8. Comparison of the data from this experiment with other experiments at various energies.

This is the first time negative values of polarization have been seen in large-t pp scattering. There are a number of models which fit the data below i t [ = 1.0 ( G e V / c ) 2 and several have been applied with some success to the highest energy data [2, 23, 24]. However, the application of these models to Itl > 1.5 ( G e V / c ) 2 has met with little success. A few years ago purely geometric models were in vogue and gave reasonable parametrizations of pp elastic scattering over a large ttl range up to 12 GeV/c. However, in a geometrical model np polarization must be essentially the same as pp. It was clearly demonstrated that this was not so [30, 31]. In addition, geometrical models always gave polarization values > 0. More recently, constituent scattering models have been invoked to explain spin-spin correlation measurements at high It I [25, 26]. These models predict P0 = 0 but it is not clear at what region of t or s these models are applicable.

16

/

J. Antille et al. i

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Fig. 9. The polarization parameter at fixed [t I = 1.7 ( G e V / c ) 2 as a function of Plab.

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Fig. 10. The predictions of two theoretical models compared with the data.

I

J. Antille et al. / Polarization parameter

17

Two models by Bourrely et al. [1, 24] have been used to calculate polarizations at fairly large It] in nucleon-nucleon scattering and have made predictions for the polarization in 24 G e V / c pp elastic scattering. One is an eikonal model which combines the Regge pole and geometrical approaches and has been refined over the past few years so that a reasonable description of nucleon-nucleon scattering has been obtained. The prediction for 24 G e V / c is shown in fig. 10. It is reasonable up to Itl = 1.5 (GeV/c) 2 and does predict the suppression of the second peak. A newer model of pp scattering is an improved version of the impact picture [27] and incorporates spin through the idea of matter currents [28]. This model has been applied to pp scattering at high energies and describes the data reasonably well. The prediction for 24 G e V / c is shown in fig. 10 and is not too different from the eikona i prediction except for a shift in t. Neither model shows a dip around It I= 3.6 but both show negative values of P0 in this region. Finally, an interesting extension of the idea of correlations between cross section and polarization is shown in fig. 11. Here the spin-averaged differential cross section I

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Pl =~ Pi ~Or (s)138"3 Fig. 11. The pure spin state cross sections plotted with the spin-averaged cross section at very high energy and with a curve representing the polarization data for this experiment against a geometrical scaling variable p 2

18

J. A ntille et al. / Polarization parameter

at very high energy [29] and pure spin differential cross sections [17] are shown together with a curve representing the polarization data of this experiment all plotted against a geometrical scaling variable p~_. The relation between the polarization and spin-averaged cross section was discussed earlier while the relation between the two cross sections has been discussed elsewhere [17]. The point here is that changes in the slope of the spin-dependent cross section are correlated with changes in the structure of the polarization. In particular, the negative dip in polarization at p~_ ~ 3.2 occurs at the point where the spin cross sections diverge; in the original paper [17] this is where the value of the spin correlation parameter ANN rises dramatically to 60%. This has been interpreted as the onset of a hard scattering region which is dominated by the scattering of the nucleon constituents. If so, it is intriguing to speculate that polarization data may still have a role to play in signalling the onset of such a regime.

5. Conclusions Polarization data for pp elastic scattering over the range 0.7 < It] < 5.0 ( G e V / c ) 2 has been presented and shows some new features. The double zero region around - t = 0.7 ( G e V / c ) 2 seems to be changing to the negative dip structure observed at higher energies, the polarization at the second peak is much smaller than at lower energies, and a negative dip has appeared at around - t = 3.6 ( G e V / c ) 2. Theoretically a qualitative description of the data has been achieved and the suppression of the second peak has been anticipated. However, in general the data spans a theoretically neglected region above the region where Regge pole models can be successful and probably below the region where hard scattering models can be applied. We are indebted to M. Borghini, T. Niinikoski and J.M. Rieubland for the efficient operation of the polarized target and to the C E R N PS staff for setting up the special beam-line. We are also indebted J. Bibby, W. Huta, A. Kupferschmid and A. Looten for their technical support.

References [1] C. Bourrely et al., Nucl. Phys. B117 (1976) 95 [2] G. Kane and A. Seidl, Rev. Mod. Phys. 48 (1976) 309 [3] E.L. Berger et al., Phys. Rev. DI7 (1978) 2971 [4] P. Roubeau et al., Nucl. Instr. 82 (1970) 323 [5] D.G. Aschman et al., Nucl. Phys. B125 (1977) 349 [6] J. AntiUe, Thesis, University of Lausanne (1979) [7] D. Perret-Gallix, Thesis, University of Paris-Orsay (1979) [8] M. Fujisaki et al., AIP Conf. Proc., ed. G. Thomas 51 (1978) 478,; I. van Rossum et al, private communication [9] A. Wicklund et al., AIP Conf. Proc., ed: M. Marshak, 35 (1976) 198

J. A ntille et al. / Polarization parameter

[10] [11] [12] [13] [14] [15] [16] [17]

[18] [19] [20] [21] [22] [23] [24] [25] [26] [27] [28] [29] [30] [31]

19

T. del Prete and M. Valdata, Nuovo Cim. 13A (1973) 976 A. Fridman, Fortschr. Phys. 23 (1975) 243 J. Allaby et al., Nucl. Phys. B52 (1973) 316 High energy polarized proton beams, AIP Conf. Proc. 42, ed. A.D. Krisch and A.J. Salthouse (1978) D.G. Crabb et al., Nucl. Phys. BI21 (1977) 231 M. Borghini et al., Phys. Lett. 36B (1971) 501; 31B (1970) 405; Errata and new data tables CERN preprint (January, 1972) G.W. Bryant et al., Phys. Rev. DI3 (1976) 1 K. Abe et al., Phys. Lett. 63B (1976) 239: H . E Miettinen et al., Phys. Rev. DI6 (1977) 549; J.R. O'Fallon et al., Phys. Rev. Lett. 39 (1977) 733; D.G. Crabb et al., Phys. Rev. Lett. 41 (1978) 1257 A. Gaidot et al., Phys. Lett. 61B (1976) 103 J.H. Snyder et al., Phys. Rev. Lett. 41 (1978) 781 G. Fidecaro et al., Nucl. Phys. B173 (1980) 513 J.H. Parry et al., Phys. Rev. D8 (1978) 45 G.W. Abshire et al., Phys. Rev. Lett. 32 (1974) 1261 A.C. Irving, Nucl. Phys. B101 (1975) 263 C. Bourrely et al., Phys. Rev. DI9 (1979) 3249 G.R. Farrar et al., Phys. Rev. D20 (1979) 202 S.J. Brodsky et al., Phys. Rev. D20 (1979) 2278 H. Cheng et al., Phys. Lett. 44B (1973) 97 T. Chou and C.N. Yang, Nucl. Phys. B107 (1976) 1 H. de Kerret et al., Phys. Lett. 62B (1976) 363 R. Diebold et al., Phys. Rev. Lett. 35 (1975) 632 S.L. Kramer et al., Phys. Rev. D17 (1978) 1709