Exclusive Rho 0 and Phi production in deep inelastic muon scattering

(1)

and F- Ls

9 Springer-Verlag 1988

Exclusive pO and production in deep inelastic muon scattering

E u r o p e a n M u o n C o l l a b o r a t i o n

J. A s h m a n 13, B. B a d e l e k 16a, G . B a u m l s b , J. B e a u f a y s 2, C.P. Bee s, C. B e n c h o u k 9, I . G . B i r d 6c, S.C. B r o w n sd, M . C . C a p u t o 18, H . W . K . C h e u n g 11, j . C h i m a 12 e, j . C i b o r o w s k i 1 6 a, R . W . Clifft 12, G . C o i g n e t 7, F . C o m b l e y 13, G . C o u r t s, G . D ' A g o s t i n i 9, J. D r e e s 17, M . D i i r e n 1, N . D y c e 6, A . W . E d w a r d s 17f, M . E d w a r d s 12, T. E r n s t 3, M . I . F e r r e r o 14, j . F o s t e r 13 g, D . F r a n c i s 8, E. G a b a t h u l e r 8, j. G a j e w s k i 4, R. G a m e t 8, N . G e d d e s 12,

V. G i b s o n 11i, J. G i l l i e s 11, P. G r a f s t r 6 m

TM,

K . H a m a c h e r 17, D . v. H a r r a c h 5, P. H a y m a n s, C . A . H e u s c h 2j, J.R. H o l t 8, V . W . H u g h e s 18, A . J a c h o l k o w s k a 2 k, F. J a n a t a 41, G . J a n c s o 2 m, T. J o n e s s, E . M . K a b u s s 3 c, B. K o r z e n 17, U . K r i i n e r 17, S. K u l l a n d e r 15, U . L a n d g r a f 3, D. L a n s k e 1, F. L e t t e n s t r 6 m 15, T. L i n d q v i s t 15, J. L o k e n 11, M . M a t t h e w s 8, y . M i z u n o 5, K . M 6 n i g 17, F . M o n t a n e t 9 i, j . N a s s a l s k i 16 n, E. N a g y 7 m, P . R . N o r t o n 12, G . O a k h a m 12~ R. O p p e n h e i m l s p , A . M . O s b o r n e 2, V. P a p a v a s s i l i o u i s , N . Pave117,

C. P e r o n i 14, H . P e s c h e l 17, R. P i e g a i a 18, B. P i e t r z y k 9, U . P i e t r z y k 17 q, B. P 6 n s g e n 4, B. P o v h 5, p. R e n t o n 11, P. R i b a r i c s 7 m, K . R i t h 3 c, E. R o n d i o 16 a, L. R o p e l e w s k i 16 a D. S a l m o n 13, A . S a n d a c z 16 n, M~ S c h e e r 1, H. S c h i e m a n n 4, K . P . S c h u l e r 18, K . S c h u l t z e 1, T - A . S h i b a t a 5, T. S l o a n 6, A . S t a i a n o 5 r H . E . S t i e r 3, j . S t o c k 3, M . S t u d t 4, G . N . T a y l o r 11 s, J.C. T h o m p s o n 12, j . T o t h 7 m, S. W h e e l e r 13, L. U r b a n 7 m, T. W a l c h e r 5 t,

W . S . C . W i l l i a m s 11, S.J. W i m p e n n y s u, R . W i n d m o l d e r s 1 o, W . J . W o m e r s l e y 11 v 1 III. Physikalisches Inst. A, Physikzentrum, D-5100 Aachen, Federal Republic of Germany 2 CERN, CH-1211 Geneva 23, Switzerland

3 Fakult/it fiJr Physik, Universit~it, D-7800 Freiburg, Federal Republic of Germany

4 II. Institut fiir Experimentalphysik, Universit/it, D-2000 Hamburg, Federal Republic of Germany Max-Planck Institut fiir Kernphysik, D-6900 Heidelberg, Federal Republic of Germany 6 Department of Physics, University, Lancaster LA1 4YB, England

7 Laboratoire d'Annecy de Physique des Particules, IN2P3, F-74019 Annecy-le-Vieux, France 8 Department of Physics, University, Liverpool L69 3BX, England

9 Centre de Physique des Particules, Facult6 des Sciences de Luminy, F-13288 Marseille, France lo Facult6 des Sciences, Universit6 de L'Etat B-7000 Mons, Belgium

11 Nuclear Physics Laboratory, University, Oxford OX1 3RH, England 12 Rutherford-Appleton Laboratory, Chilton, Didcot O X l l 0QX, England

3 Department of Physics, University, Sheffield 537 RH, England 14 Istituto di Fisica, UniversitY, 1-10100 Torino, Italy

15 Gustav Werners Institut, University, S-75121 Uppsala, Sweden

16 Physics Institute, University of Warsaw, and Institute for Nuclear Studies, PL-00681 Warsaw, Poland 17 Fachbereich Physik, Universit~it, D-5600 Wuppertal, Federal Republic of Germany

8 Physics Department, Yale University, New Haven, CT 06520 USA Received 4 January 1988; in revised form 15 February 1988

a University of Warsaw, Warsaw, Poland, partly supported by CPBP-01.06

b Permanent address: University, Bielefeld, FRG c Now at MPI fiir Kernphysik, Heidelberg, FRG d Now at TESA S.A., Renens, Switzerland

~ Now at British Telecom, Ipswich, UK f Now at Jet, Joint Undertaking, Abingdon, UK g Now at University of Manchester, UK h NOW at RAL, Chilton, UK

i Now at CERN, Geneva, Switzerland

J Permanent address: University of California, Santa Cruz, USA

k NOW at L.A.L., Universit6 de Paris Sud, Orsay, France

J Now at Beiersdorf A.G., Hamburg, F R G

m Permanent address: Central Research Institute for Physics of the Hungarian Academy of Science, Budapest, Hungary

n Institute for Nuclear Studies, Warsaw, Poland, partly supported by CPBP-01.09

o Now at NRC, Ottawa, Canada P Now at AT&T, Naperville, Illinois, USA

q Now at MPI fiir Neurologische Forschung, K61n, FRG r Now at INFN, Torino, Italy

s Now at University of Melbourne, Victoria, Australia t Now at University of Mainz, Mainz, FRG

u Now at University of California, Riverside, CA 9252, USA v Now at University of Florida, Gainsville, USA

(2)

Abstract. D a t a are presented on exclusive p0 and q~

production in deep inelastic m u o n scattering from a target consisting mainly of nitrogen. The ratio of the total cross sections for p0 and ~b production is found to be 9: (1.6 ___ 0.4) at (Q z) = 7.5 GeV z, consistent with the SU(3) prediction of 9:2. The t dependence for exclusive pO p r o d u c t i o n is found to become shallover as Q2 increases and, for large Qa, the t dependence is typical of that for a hard scattering process. Fur- thermore, the ratio of the cross sections for coherent:

incoherent p r o d u c t i o n from nitrogen is found to decrease rapidly with Q2. Such behaviour indicates that even for exclusive vector meson production the virtual p h o t o n behaves predominantly as an electromagnetic probe.

Introduction

Exclusive lepto-production, e (or #) N ~ e (or #)

VN,

and p h o t o p r o d u c t i o n (Q2=0) ( y N ~

VN)

of vector mesons, V, has been studied previously in m a n y different experiments [-1]. These studies have been made mainly at rather low incident p h o t o n energies, v, and f o u r - m o m e n t u m transfer squared,

Q2,

(typically v <

20 GeV, Q2 < 3 GeVZ). These data indicate that the p h o t o n has hadron-like properties which may be described by the vector dominance model (VDM) [1].

In real p h o t o p r o d u c t i o n (Q2=0) the process is observed to be mainly diffractive and the helicity of the pO is nearly the same as that of the incident p h o t o n in the s channel helicity frame, i.e. helicity is conserved in this frame. In contrast, a measurement performed at larger values of

QZ

( 2 < Q Z < 2 0 GeV 2) [2] using a hydrogen target showed that the p0 mesons were produced dominantly in a helicity zero state. Com- parison of the measured cross sections with those found at lower

Q2 [-7]

shows that the production is mainly from transversely polarised virtual photons (i.e. helicity _ 1). This indicates that, at larger values of

Q2,

s-channel helicity is no longer conserved. Fur- thermore, as

Q2

increases the dependence of the differential cross section on the 4 - m o m e n t u m transfer, t, from the p h o t o n to the pO falls less steeply. F o r Q 2 > 5 GeV 2 the t dependence becomes inconsistent with diffractive production mechanisms. These properties suggest that for large

Q2

exclusive vector meson production becomes a hard scattering process and it seems no longer appropriate to use the V D M to describe such processes (for

Q2>

2 GeV2). M o r e o v e r the simple p r o p a g a t o r behaviour of the exclusive pO and

JAk

cross section which is well established by many experiments [-1, 2, 7-9] and supported by the

V D M [1] and p h o t o n gluon fusion models [18]

points to a single production mechanism in the whole Q2 range, showing up as a hard scattering process at large Q2 and reproducing the V D M behaviour to- wards the p h o t o - p r o d u c t i o n regime.

In this paper, new data are presented which test further these ideas. The data were taken using #+

beams of 120 and 200 GeV incident energy, scattering from an a m m o n i a target, so that most of the events occur off nitrogen nuclei. Both coherent and incoherent production of exclusive pO measons have been studied and the total yield of exclusive ~b mesons has been measured.

Experimental procedure

The experiment was performed in the M 2 m u o n beam at the C E R N SPS using the E M C forward spectrometer to detect the scattered m u o n and the fast forward produced hadrons. Figure 1 shows a schematic dia- gram of the apparatus. The spectrometer and the analysis procedures were similar to those described in [2-4] with the following differences. The drift chambers upstream of the magnet in the original spectrometer [4] operated in a high b a c k g r o u n d environ- ment and were subject to substantial efficiency corrections. Here they have been replaced by the multiwire proportional chambers labelled PV1 and P V 2 in Fig. 1, allowing data to be taken efficiently at incident m u o n intensities up to 4 x 107 muons per SPS pulse, which is a factor 3 higher than previously. The multiwire proportional chambers P 4 and P 5 (Fig. 1) were also installed to cover the central regions of the drift chambers W 4 and W5. The latter chambers tended to become inefficient in this region after prolonged exposure to radiation due to the deposition of silicon compounds on the sense wires. In addition, the small multiwire proportional chambers labelled POA, P O D and POE in Fig. 1 were installed to cover the dead- ened chamber areas in the beam region. The apparatus had good efficiency for charged hadrons of mo- menta greater than ,-~ 5 GeV.

The target was an 80 cm long polarised target, the main purpose of which was to study the spin dependence of the p r o t o n structure function. It consisted mainly of a m m o n i a with a small admixture of helium ( ~ 10% by weight). The mean atomic weight of the target was 10.8. Other thinner targets, located a b o u t 1 m downstream (Fig. 1) were also present during most of the data taking. Only events originating from the polarised target (summed over the different spin alignments) are used in the present analysis. The data were taken in three experimental runs, two at 200 GeV and one at 120 GeV incident m u o n energy,

(3)

P4.A P/*B P/.,s / P5A

\ / /PsB

I vls v3 v21 v2 wl w2 I / / ' . P S C H3H H39

,,,,,,,,,, \!o \/

~,s

v, ,v,3,

\Nil I I ~o~G~, I ,,~/ll...--F IIlllllll,~lll,lln I111

~. n rT~a v 0r~r-T~n,,,,nuu ] ,ll.,.~-z~., I I.I],1 II I, IIIII c21

IIIlUI, II

E\\~IIIIIB]I II

'

~ ,I

__..r - ~ ' ' ' " OI;;llTil'"llBnlln ' ' ' ' ' ~ _ - / l [ I I I_/i III I, I I"l','l'l' II-~-~ I 'l U I1...lln

/ / / / / / / / / / 4 ,,wPc,l I / IIII II II ,11 II

I P~'~ P2k'~!E W4. 37!3

^{W 7 /}

IK~ BHA BHB' BHB POB POE PV2 POD H1H H1V \

y L-~ Fe W/+B WSB H/, HS

,, ,, ,, I I ,, 0 1 2 3 L 5 m

x

Fig. l. The apparatus. Key: H=scintillator trigger hodoscope, V = v e t o hodoscope, B H = b e a m hodoscope, P=multiwire proportional chamber, W = drift chamber, FSM = forward spectrometer magnet C2 = Cerenkov counter, H2 = calorimeter

using a trigger corresponding to a scattering angle 0, > 0.75 ~

Data analysis and results

The data were passed through a chain of analysis programmes in which pattern recognition, track and vertex reconstruction were carried out. Events containing a reconstructed scattered m u o n which was consistent with having produced the trigger and a pair of hadrons of opposite charges were selected for further analysis. Events were rejected if the ratio of the energies deposited in the electromagnetic part of the calorimeter (H2, Fig. 1) to the sum of the electromagnetic and hadronic parts was greater than 0.8 for either n o n - m u o n track. In this way, electron-positron pairs were removed from the data sample. The loss of h a d r o n tracks due to this procedure was measured from the data by extrapolating the distribution of the measured ratios into the rejected region. The loss was found to be small (~-,6%).

In the analysis presented here, further details of which are given in [5], the standard variables em- ployed in deep inelastic scattering were used. These are Q2, v and Wthe total energy in the virtual p h o t o n - p r o t o n centre of mass system. The data were selected with Q 2 > I G e V 2 and y < 0 . 9 where

y=v/E

and E is the energy of the incident muon. F o r the compari- son of the pO and ~b yields the 200 GeV data were used and restricted to the ranges 2 < Q 2 < 2 5 GeV 2, 3 6 < W 2 < 2 8 0 GeV 2 and the scattered m u o n angle

was required to be greater than 10 mrad. These cuts exclude the regions where the apparatus acceptance was small or varied rapidly or where Q E D radiative corrections are large.

T o select exclusive events containing a single hadron pair, the total energy of the pair was required to be greater than 0.92v. As shown in [2], there is only a small residual contamination of events containing an extra undetected h a d r o n with these selec- tion criteria. Figure 2 shows the invariant mass distributions of the hadrons treated as either K + K - or n+ ~ - pairs. Clear peaks are seen at the pO mass in the n + n - distribution and at the ~b mass in the K + K - distribution. In these distributions events falling in the pO mass region ( r n p - ~ / 2 ) treated as ~+ n - pairs were excluded from the K + K - spectrum. Simi- larly events falling in the ~b mass region ( m , - ~ )

< mKK < (too + ~ ) when treated as K + K - pairs were excluded from the n+ n - mass spectrum. Here mp, m~, ~ and ~ are the masses and widths of the pO and q~ mesons, respectively. Only a small n u m b e r of events satisfied both conditions simultaneously and the correction for the loss of these events was included in the acceptance calculation.

The acceptance of the apparatus was computed by M o n t e Carlo simulation in which exclusive pO and events were generated and weighted according to parameterisations of the data in [2]. The total numbers of pO and q~ mesons were determined by applying acceptance corrections to the mass distributions (Fig. 2) and by fitting a p-wave Breit-Wigner distribution together with a smooth b a c k g r o u n d func-

(4)

5

>

oJ

b

3

[ I I I I

a)

1.0 1,1 1.2 1.3

M KK (GeV)

l t~

32 28 24 20 16 12 8 4 0

i I I I ~ I i

025

,

,$,

^n,,n,I

0.75 125 ¹⁷⁵

M . . (GeV)

Fig. 2a, b. Invariant mass spectra of oppositely charged pairs of hadrons of total energy larger than 0.92v, treated as a K § pairs b n + pairs

tion to each. The corrected total n u m b e r of elastic pO and ~b mesons were found to be 5 3 8 + 4 0 a n d 94 + 24, respectively, (200 G e V data). F r o m these yields the ratio of the cross sections is

a(V*p~p~ 9

a(y*p~q~p) 1.6___0.4

at a m e a n value of 0 2 of 7.5 G e V z. The estimated systematic error of ,-, 10% is m u c h smaller t h a n the quoted statistical error. The observed ratio is consistent with the value of 9:2 expected from SU(3) flavour symmetry. However, it is larger t h a n the value of 9 :(0.65 + 0.03) obtained f r o m the m e a s u r e m e n t in real p h o t o n p r o d u c t i o n (02 = 0) at similar incident p h o t o n energies [6].

Figure 3 a shows this result together with previous m e a s u r e m e n t s of the ratio of the cross sections for elastic pO to elastic ~b p r o d u c t i o n as a function of Q2. Figure 3 b shows the cross sections for elastic pO p h o t o - p r o d u c t i o n on h y d r o g e n [2, 7] a n d elastic J/~

p h o t o - p r o d u c t i o n obtained on iron [8] as a function of Q2. It can be seen that these cross sections b e c o m e almost equal at high Q2. Figure 3c shows the same d a t a presented as a ratio of the cross sections where the ratio was calculated by interpolating between the J/~ points (Fig. 3b). The s m o o t h curves in Fig. 3 a and c show the predictions of the following simple model. Firstly, it is assumed that in the limit of large

Q2

(Q2 ~ m~, where ml is the mass of the vector m e s o n i) the ratio av:ai a p p r o a c h e s the value expected f r o m flavour s y m m e t r y [1] i.e. 9:2 for ~b a n d 9:8 for JAb.

Secondly, in the intermediate Q2 range it is assumed that the cross section follows the f o r m 1/(m:~ + 02)2, a b e h a v i o u r which is well established for the 02 dependence of the pO and JAI, cross sections [1, 2, 7-9].

With these assumptions the ratio of the cross sections will vary as

+ Q2)2 R = a i = c i mv

ap 9 (m{ + 02) 2

with c~ = 2 for ~b and 8 for J/~p. Both curves without free p a r a m e t e r s give a reasonable representation of the d a t a in Fig. 3 a and c. This indicates that the production mechanisms for the elastic vector mesons pO, qS, and J/O are similar in the whole Q2 range with the differences in the p r o d u c t i o n cross-sections being determined by the different m e s o n masses which are related to the q u a r k masses.

Figure 4 shows the yields for elastic p0 p r o d u c t i o n as function of t' = [ t -- tmin 1, where t is the f o u r - m o m e n - t u m transfer squared between the virtual p h o t o n and the pO m e s o n a n d tmin is the kinematic m i n i m u m value. The acceptance of the a p p a r a t u s as a function of t' is essentially flat so that no correction is neces- sary. The peaks at small values of t' correspond to coherent p r o d u c t i o n from the nitrogen in the target, smeared by experimental resolution. The lines show fits of the f o r m e -be to the data for t ' > 0 . 2 G e V 2, which excludes the coherent region. The slopes of the fitted lines decrease as Q2 increases consistent with the b e h a v i o u r of the hydrogen data described in [2].

(5)

0 . 8

0 . 6

e

t"'q 0 . 4

0 . 2

O.

104

103

JEl

{3..

> 102

t

r-~

1 0

/~H= CHIO 150 GeV 9 # N H = EMC

ll]J

10 - 1

f

- - ( ( Q ' + m . ' ) / ( Q ' + B , ' ) ) '

I I I I I

0 2 4 6 8 10

(2 z (GeV 2)

4 '~ 4 +

I I [ 1 1 1 1 [ 1

9 EMC pO 120 GeV 9 EMC pO 2 0 0 / 2 8 0 Ge\

o CHIO po 150 GeV o CHIO p0 100 GeV 9 EMC J / q 2 5 0 GeV

b)

, t' t

+

I I I I llllJ J I I,

1 0

O, z (GeV 2)

10 -1

lo

oo-

0 ~, 1 0 - 2

9 EMC 120 GeV 9 EMC 2 0 0 / 2 8 0 GeV o CHIO 150 GeV [] CHIO 100 GeM

- - ( ( Q ' + m , / . ' ) / ( Q ' + m , ' ) ) '

10 - 3 [ I I I I I I I

O. 5. 10. 15. 20,

Q.Z (GeV 2 )

Fig. 3. a The ratio 9ao/2a" as a function of Q2, including data from I-6] and I-7]. b The photoproduction cross sections for pO [2, 7] and J/ip [8] as a function of Q2. The J/~b data is from Table 1 of [8] averaged over the v range 120-180 GeV. e The ratio 9as/r from the data in b, as a function of Q2. The smooth curves in a and c are the predictions of the simple model described in the text

The measured slope parameters, b, show reasonable agreement within the errors with the h y d r o g e n data, as s h o w n in Fig. 5. At large

Q2

the measured slopes, b,-~l-2 GeV -2, are smaller than those expected for diffractive processes (b-,, 3-4 GeV-2) and so are typi- cal o f a hard scattering process.

A theoretical, diffractive m o d e l for the process has been d e v e l o p e d recently by D o n n a c h i e and Landshoff [16]. This m o d e l predicts steeper slope parameters, b, than observed at the highest values o f

Q2.

^The

observed shallow t slopes at the largest values of

QZ

are ascribed in [16] to the residual inelastic back-

ground. H o w e v e r , even if the a s s u m p t i o n is m a d e that this b a c k g r o u n d is flat in t, the c o m b i n a t i o n o f such a b a c k g r o u n d with the predicted t distribution of [16]

fails to represent the shape o f the data at the largest value of Q2 in Fig. 4.

The polarisation o f the pO in the s-channel helicity frame was investigated by measuring the density ma- trix element roo~ from the decay angular distribution in the pO rest frame [2]. This density matrix element can be identified with the probability to p r o d u c e a pO in a helicity zero state. The m e a n value o f the density matrix element (ro~ 4 ) was found to be 0.72

(6)

1 _< Q 2 _ < / , lO

r,l

(.9

Z -o i0 - I

lO - 2

--200 GeV

/, _<Q2<_ 7 7 < 0 2 - < 25

F-

I I ] I L I

0.75 1.5 O. 0.75 1.5 O. 0.75 1.5

t' [ G e V 2 ]

no

%

E = 120 t 1 <Q2<__ 4 lO

**1 ~k* f**

10 -1

~eV

4 <__ Qz-< 20

= - I

10 - 2 I I I I I I I

O. 0.75 1.5 O. 0.75 1.5

t ' [ G e v z]

Fig. 4. The event yields as a function of t'=lt--tmi, I in different Q2 intervals at each incident m u o n energy. The lines are the fits for t ' > 0 . 2 GeV 2

..m

I I I I I I

,, W> 6 GeV

\

I+t

i i i i , i

W > 6 GeV 9 120 GeV.

9 200 ,, ~tap EHE

9 280 - .x

/

fit lap CHIO

~, yp OHEGA

[] 120 GeV

| 200 5eVJla NH3,EH

+

] I I I ] I I I I L I I I L

5 10

0,2 (GeV 2)

Fig. 5. The slopes of the linear fits ( t ' > 0 . 2 GeV 2) s h o w n in Fig. 4 as a function of Q2, together with the data from I-2] and [7] a n d the point at Q2 = 0 from [-14]

i

c 3

t3 2

X 0, 2 = O(Copper) HcCet[an et at.

0 0,z= O(Carbon) Asbury e t a [ . EMC }

9 120 GeV , This expt, 9 200 GeV , EHI"

§

I I I f l I ~l

2 t, 6 8 10 12

0`2 GeV 2

it,

Fig. 6. The ratio of the total coherent to incoherent cross sections as a function of Q:. The points at Q2 = 0 come from [12, 13]. The s m o o t h curve shows the expected decrease in this ratio from the increase of tmi, with Q2

_+0.08 for t ' > 0 . 2 GeV 2 at ( Q 2 ) = 7 . 5 GeV 2. This is consistent with the measurement on hydrogen in [2]

and shows that the angular distribution varies almost as cos 20 in the s-channel frame. F o r t ' < 0.2 the angular distribution is approximately flat showing that it is a mixture of sin20 and cos20 dependences. This suggests that, for t' < 0.2, the cross section is a mixture of coherent (diffractive) and incoherent parts.

The straight line fits in Fig. 4 were extrapolated under the coherent peak in order to estimate the ratio of the coherent (0%0 to incoherent cross sections (o-i,r correcting for the effects of quasi-elastic sup- pression in the coherent region of t' using the calcula- tions of [17]. Figure 6 shows the ratio of acoh/tri, r for elastic p0 p r o d u c t i o n from nitrogen as a function of Q2. The points at Q2 = 0 were obtained by perform- ing similar extrapolations using the data from carbon and copper targets [12, 13]. The Q 2 = 0 point was chosen to be 2.0 from the data of McClellan et al.

[12] on copper. This value comes close to an optical model calculation [15]. However, the data of Asbury et al. [13] on carbon indicate a value which is a factor 2 higher than this.

The smooth curve in Fig. 6 shows the expected kinematic effect due to the increase of t m i n ' ~ [ ( Q 2

+m2)/2v] 2 for the 200 GeV data. T o compute this curve the coherent differential cross section was assumed to decrease as e - 50t, as expected from the radi- us of the nitrogen nucleus. The data fall much faster with Q2 than expected from such kinematic effects.

(7)

Conclusions

E x c l u s i v e pO a n d q5 m e s o n p r o d u c t i o n h a s b e e n m e a - s u r e d i n d e e p i n e l a s t i c m u o n s c a t t e r i n g f r o m n i t r o g e n . T h e r a t i o o f t h e c r o s s s e c t i o n f o r e l a s t i c pO a n d q~

p r o d u c t i o n a t ( Q 2 ) = 7 . 5 G e V 2 is f o u n d t o b e 9 : ( 1 . 6 ___0.4). T h i s r e s u l t is c l o s e t o t h e S U ( 3 ) p r e d i c t i o n o f 9: 2. A n a n a l o g o u s b e h a v i o u r is s e e n in t h e c o m p a r - i s o n o f o u r e a r l i e r pO a n d J/~b d a t a . T h e t d e p e n d e n c e o f pO p r o d u c t i o n r a p i d l y b e c o m e s h a r d e r as Q2 i n - c r e a s e s a n d t h e c o h e r e n t p r o d u c t i o n f r o m a n u c l e a r t a r g e t d e c r e a s e s s t r o n g l y w i t h i n c r e a s i n g Q2. T h e s e o b s e r v a t i o n s i n d i c a t e t h a t e v e n in e l a s t i c v e c t o r m e - s o n p r o d u c t i o n t h e v i r t u a l p h o t o n a c t s p r e d o m i n a n t l y as a n e l e c t r o m a g n e t i c p r o b e . T h e s i m p l e p r o p a g a t o r b e h a v i o u r o f t h e c r o s s s e c t i o n p o i n t s t o a s i n g l e p r o - d u c t i o n m e c h a n i s m i n t h e w h o l e Q2 r a n g e .

References

1. For a review see, T. Bauer, R.D. Spital, D.R. Yennie, F.M. Pip- kin: Rev. Mod. Phys. 50 (1978) 261

2. EMC, J.J. Aubert et al.: Phys. Lett. 161B (1985) 203 3. EMC, J.J. Aubert et al.: Nucl. Phys. B259 (1985) 189

4. EMC, O.C. Allkofer et al.: Nucl. Inst. and Meth. 179 (1981) 445

5. K. M6nig: Diplom thesis, University of Wuppertal, WuD 86-10 (1986)

6. R.M. Egloff et al.: Phys. Rev. Lett. 43, 10 (1979) 657. For the evaluation of the dp/p ratio only the data in the energy range of the data presented here have been used and furthermore the ratio corrected for the revised ~b ~ K § K - branching ratio 7. CHIO, W.D. Shambroom et al.: Phys. Rev. D26 (1982) 1 8. EMC, J.J. Aubert et al.: Nucl. Phys. B213 (1983) 1 9. BFP, A.R. Clark et al.: Phys. Rev. Lett. 45 (1980) 2092 10. E i . Berger, D. Jones: Phys. Rev. D23 (1981) 1521 11. R. Baier, R. R/ickl: Nucl. Phys. B218 (1983) 289 12. G. McClellan et al.: Phys. Rev. Lett. 22 (1969) 377 13. J.G. Asbury et al.: Phys. Rev. Lett. 19 (1967) 865 14. D. Aston et al.: Nucl. Phys. B209 (1982) 56

15. For this calculation a simple spherical nucleus was assumed to perform the optical model integral. This is given in D.W.G.S.

Leith; Electromagnetic interactions of hadrons. A. Donnachie and G. Shaw eds, p. 419

16. A. Donnachie, P.V. Landshoff: Phys. Lett. 185B (1987) 403 17. J. Bailey et al.: Nucl. Phys. B151 (1979) 367

18. R.J.N. Phillips: XX Int. Conf. on High Energy Physics, Madison, Wisconsin 1980, L. Durand, L.G. Pondrom (eds.)