Measurement of the neutron and the proton F2 structure function ratio

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Volume 249, n u m b e r 2 PHYSICS LETTERS B 18 October 1990

Measurement

of the neutron and the proton F2 structure function ratio

N e w Muon Collaboration ( N M C )

D. Allasia a, p. Amaudruz b, M. Arneodo a, A. Arvidson c, B. Badelek d, G. Baum e,

J. Beaufays f,t, I.G. Bird g, M. Botje b, W.J. Burger b, C. Broggini h, W. Briickner g, A. Briill i, J. Ciborowski f R. Crittenden J, R. van Dantzig f H. D6bbeling g,2, j. Domingo b, j. Drinkard k, A. Dzierba J, H. Engelien i, M.I. Ferrero a, L. Fluri h, p. Grafstrom c,3, D. Greiner i,

P. Gretillat h, W. Giinther i, E. Hagberg ~, D. von Harrach g, M. van der Heijden f, C. Heusch k, Q. Ingram b, A. Jacholkowska j,4, K. Janson ~, M. de Jong f E.M. KabuB g, R. Kaiser i, T. Ketel f F. Klein ~, B. Korzen m, U. Kriiner m, S. Kullander ~, U. Landgraf i, F. Lettenstr/Sm ~,

T. Lindqvist c, G.K. Mallot ~, C. Mariotti a, G. van Middelkoop f, Y. Mizuno g, J. Nassalski ", D. Nowotny g, N. Pavel m,5, H. Peschel m, C. Peroni a, B. Povh g,o, R. Rieger ~, K. Rith g, K. R6hrich ~, E. Rondio d, L. Ropelewski d, A. Sandacz n, C. Scholz g, R. Schumacher b,7, U. Sennhauser b, F. Sever e,8,9, T.A. Shibata n, M. Siebler e, A. Simon g, A. Staiano a, G. Taylor p, lo, M. Treichel g,11, J.L. Vuilleumier h, T. Walcher ~, K. Welch j and R. Windmolders q

Universiti~ di Torino and lNFN, 1-10125 Turin, Italy b Paul Scherrer Institute, CH-5234 Villigen, Switzerland

University o f Uppsala, S- 75121 Uppsala, Sweden d University o f Warsaw, PL-00-681 Warsaw, Poland ~2

Universit~it Bielefeld, D-4800 Bielefeld, F R G 13 f NIKHEF-K, NL- 1009 A J A m s t e r d a m , The Netherlands

Max-Planck Institute, D-6900 Heidelberg, FRG

h Universitk de Neuchfttel, CH-2000 Neuchfttel, Switzerland i universitiit Freiburg, D-7800 Freiburg, F R G IJ

J Indiana University, Bloomington, I N 4 7 4 0 5 , USA k University o f California, Santa Cruz, CA 95064, USA

Universiti~t Mainz, D-6500 Mainz, F R G t3 m Universiti~t Wuppertal, D-5600 Wuppertal, F R G ~3

Institute for Nuclear Studies, PL-O0-681 Warsaw, Poland ~5 o Universitiit Heidelberg, D-6900 Heidelberg, F R G 13 P Oxford University, Oxford, UK

q Universit( de l'Etat Mons, B- 7000 Mons, Belgium

i Present address: Trasys, Brussels, Belgium.

2 Present address: PSI, CH-5234 Villigen, Switzerland.

3 Present address: CERN, CH-1211 Geneva, Switzerland.

4 Present address: Laboratoire de l'Acc616rateur Lin6aire, Universit6 de Paris-SUd, F-91405 Orsay, France.

Present address: DESY, D-2000 Hamburg, FRG.

6 Present address: MPI f'tir Neurologische Forschung, Cologne FRG.

7 Present address: Carnegie Mellon University, Pittsburgh, PA- 15213 USA.

s O n leave of absence from Jozef Stefan Institut, YU-61111 Ljubljana, Yugoslavia.

9 Present address: D P h N Saclay, F-91191 Gif-sur-Yvette, France.

,o Present address: University o f Melbourne, Parkville, Victo- ria, 3052-Australia.

II Present address: Universit6 de Neuchfitel, CH-2000 Neuchfi- tel, Switzerland.

t2 Supported in part by CPBP.01.09.

t3 Supported by Bundesministerium f'tir Forschung and Technologie.

~4 Supported in part by F O M and NWO.

is Supported in part by CPBP.01.06.

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Received 17 July 1990

The ratio of the structure function F~/F~ (x) has been measured in deep inelastic scattering of 274 GeV muons on hydrogen and deuterium targets exposed simultaneously to the beam. The results were obtained from 0.3 (0.6) million events from hydrogen (deuterium) in the range 0.004 < x < 0.8 and l < Qz< 190 GeV 2. At x < 0.25 both the statistical and the systematic error is below 2%. Implications for parton distributions and for the trw/az production cross section ratio in Pl) collisions are discussed.

When compared to other results obtained at lower energies, the data indicate a Q2 dependence of the ratio.

In the p a r t o n p i c t u r e the structure f u n c t i o n ratio F~/F~ is sensitive to the r a t i o o f the up a n d d o w n q u a r k d i s t r i b u t i o n s . Therefore, F~/F~ puts strong c o n s t r a i n t s on the f l a v o u r d e c o m p o s i t i o n o f the structure f u n c t i o n s [ 1-7 ]. T h e p a r t o n d i s t r i b u t i o n s , especially in the low x region, are widely used to cal- culate h a r d scattering cross sections in pO, p p a n d ep collisions. H o w e v e r , the large s y s t e m a t i c errors in the p r e v i o u s m e a s u r e m e n t s o f the F~/F~ ratio l e a d to large u n c e r t a i n t i e s for the p r e d i c t i o n s [ 8 ].

P r e v i o u s results on the ratio were o b t a i n e d f r o m the m e a s u r e m e n t s with electron b e a m s at SLAC [ 9,10 ] a n d with the high energy m u o n b e a m at C E R N by the E M C [11] a n d the B C D M S [12]

C o l l a b o r a t i o n s .

T h e N e w M u o n C o l l a b o r a t i o n ( C E R N - N A 3 7 ) has p e r f o r m e d a m e a s u r e m e n t o f d e e p inelastic m u o n scattering on h y d r o g e n a n d d e u t e r i u m s i m u l t a n e - ously. F r o m these m e a s u r e m e n t s the x - d e p e n d e n c e o f the n e u t r o n to p r o t o n structure f u n c t i o n ratio F~/F~ has b e e n d e r i v e d . T h e results c o v e r the kine- m a t i c range o f x = 0 . 0 0 4 - 0 . 8 a n d Q 2 = 1 - 5 G e V 2 for the lowest a n d l 0 - 1 9 0 G e V 2 for the highest x value.

H e r e x = Q2/2Mu is the Bjorken scaling variable, M is the p r o t o n m a s s a n d - Q 2 a n d u are v i r t u a l p h o t o n m a s s s q u a r e d a n d energy respectively. C o m p a r e d to p r e v i o u s m e a s u r e m e n t s the p r e s e n t d a t a e x t e n d to lower values o f x a n d have s m a l l e r s y s t e m a t i c errors.

T h e e x p e r i m e n t was p e r f o r m e d at the m u o n b e a m line M2 o f the SPS at C E R N . The average i n c i d e n t m u o n m o m e n t u m was 274 G e V with an R M S s p r e a d o f 11 G e V a n d the i n t e g r a t e d b e a m i n t e n s i t y was 2.4 X 10 '2 muons. A n u p g r a d e d version [ 13,14 ] o f the E M C s p e c t r o m e t e r [ 15 ], shown in fig. 1, was used.

In a d d i t i o n to the s t a n d a r d trigger ( T 1 ) , which ac- cepts m u o n s at scattering angles larger t h a n 10 m r a d , a small-angle trigger ( T 2 ) which extends the acceptance d o w n to 5 m r a d was i m p l e m e n t e d . It selected

t a r g e t - p o i n t i n g c o i n c i d e n c e s o f three h o d o s c o p e planes ( H 1 ', H 3 ' , H 4 ' ) c o m p o s e d o f horizontal strips with a w i d t h o f 1 cm. The central strips were ex- c l u d e d leading to a m i n i m u m vertical m u o n scattering angle o f 5 m r a d . D u e to the small h o r i z o n t a l ex- t e n s i o n (50 c m for H 4 ' ), o n l y events with small horizontal b e n d i n g in the magnet were accepted. This r e m o v e d events with y = u/E, > 0.6 at the trigger level.

Both triggers covered the small x region ( x < 0.4 ), the T2 events having s m a l l e r Q2 a n d v. H i g h e r values o f x were o b t a i n e d by the T 1 trigger only.

The l o n g i t u d i n a l vertex resolution, being inversely p r o p o r t i o n a l to the scattering angle, was i m p r o v e d by i n t r o d u c i n g a small ( 14 c m d i a m e t e r ) 8 plane p r o - p o r t i o n a l c h a m b e r (POB) with 1 m m p i t c h a n d b y u p g r a d i n g the p r o p o r t i o n a l c h a m b e r PV1 f r o m a 4 m m to a 2 m m p i t c h (see fig. 1 ). In o r d e r to h a n d l e higher trigger rates, a b e a m spill buffering system was used allowing to buffer up to 1000 events d u r i n g the 2 s spill. T h e 12 s interval b e t w e e n spills was used for event b u i l d i n g a n d t a p e writing.

In the free space (50 m ) b e h i n d the e x p e r i m e n t a b e a m m o m e n t u m c a l i b r a t i o n s p e c t r o m e t e r , consist- ing o f a p r e c i s i o n d i p o l e a n d p r o p o r t i o n a l c h a m b e r s with 1 m m pitch, was installed. It allowed a m o m e n - t u m d e t e r m i n a t i o n with a 0.2% accuracy.

T h e target system c o n s i s t e d o f two sets o f target pairs which were alternately e x p o s e d to the b e a m which h a d h o r i z o n t a l a n d vertical d i m e n s i o n s o f 1.3 c m a n d 1.0 c m R M S respectively. Each target cell was m a d e out o f a 10 c m d i a m e t e r , 3 m long m y l a r cell filled with l i q u i d H2 or D2. T h e cell was c o n t a i n e d in a h a r d p a p e r v a c u u m c o n t a i n e r o f 30 c m d i a m e t e r . T h e two target pairs were i d e n t i c a l except for the se- q u e n c e o f the target m a t e r i a l s (see fig. 1 ). By a lateral t r a n s p o r t m e c h a n i s m e i t h e r set could be m o v e d into the b e a m . F r e q u e n t exchange o f the two sets ( t y p i -

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Volume 249, number 2 PHYSICS LETTERS B 18 October 1990 NHE S P E C T R O M E T E R (TOP VIEW)

BMS Vl

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H2

Beam momentum station Veto counters

Beam hodoscopes Proportional chambers Forward spectrometer magnet Drift chambers

Large angle trigger hodoscopes Sma([ angle trigger hodoscopes Hadron calorimeter

Iron absorbers

Y Not to scale

I I I I I

0123~5 X ( ~

Fig. 1. The New Muon Collaboration spectrometer. The beam momentum calibration spectrometer located downstream of the apparatus is not shown.

cally twice an h o u r ) m i n i m i s e d the effects o f any t i m e d e p e n d e n t d e t e c t o r response.

Fig. 2, showing the l o n g i t u d i n a l d i s t r i b u t i o n o f the r e c o n s t r u c t e d i n t e r a c t i o n vertices after k i n e m a t i c cuts, d e m o n s t r a t e s the excellent s e p a r a t i o n o f the events f r o m d i f f e r e n t targets. It can be seen t h a t the s p e c t r o m e t e r a c c e p t a n c e varies strongly w i t h the vertex p o s i t i o n . H o w e v e r , in the p r o d u c t o f the counting rates, (ND/NH)upst . . . . a n d (ND/NH) a . . . . t . . . . b o t h the a c c e p t a n c e a n d the flux cancel. Therefore, the cross section r a t i o was o b t a i n e d for each (x, Q2) b i n f r o m

m e a s

O'p NH upst .. . . NH d .. . . t . . . .

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where x, the ratio o f the m o l a r v o l u m e s o f the D 2 a n d H2, was d e t e r m i n e d as 0.8664 ( 6 ) . The u n c e r t a i n t i e s in the a c c e p t a n c e a n d flux n o r m a l i s a t i o n were the i m p o r t a n t sources o f s y s t e m a t i c errors in p r e v i o u s e x p e r i m e n t s [ 11,12 ].

D a t a were t a k e n in three SPS p e r i o d s o f 17 d a y s each d u r i n g the years 1986 a n d 1987. T h i s represents a b o u t h a l f o f the total a c c u m u l a t e d statistics at 274 GeV. T h e d e s c r i p t i o n o f the event r e c o n s t r u c t i o n can be f o u n d in refs. [ 14,15 ]. K i n e m a t i c cuts were ap- p l i e d to r e m o v e events with large r a d i a t i v e corrections, p o o r k i n e m a t i c resolution or b a c k g r o u n d f r o m h a d r o n i c decays. T h e following cuts were used:

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Fig. 2. The distribution of the reconstructed vertex position along the beam for the analysed T1 events. Full symbols are for the runs with ( D 2 ) u p s t r e a m - ( H 2 ) d . . . . t r e a m target set while open symbols are for the complementary target set. The drawing on the top shows positions of the targets and detectors seen in the distribution.

p ~ > 3 0 G e V , x > 0 . 0 0 4 , f o r T l a n d T 2 , 0 > 1 0 m r a d , u > 1 0 G e V , f o r T 1 , 0 > 5 m r a d , u > 1 5 G e V , f o r T 2 ,

where p~ a n d 0 are the scattered m u o n l a b o r a t o r y m o m e n t u m a n d the scattering angle, respectively.

These cuts i m p l y Q 2 > 1 G e V 2 a n d y < 0 . 8 9 for E , = 274 GeV. N o cuts were a p p l i e d to exclude d a t a close to the edges at the acceptance. T h i s is j u s t i f i e d since the s p e c t r o m e t e r acceptance cancels in the cross section r a t i o ( 1 ). T h e total n u m b e r o f events which p a s s e d the cuts was 222 000 (447 0 0 0 ) f r o m H ( D ) , for T 1 a n d 80 000 ( 172 0 0 0 ) f r o m H ( D ) for T2.

T h e ratio o f the o n e - p h o t o n cross section a~ ~ ~alp ~ was c a l c u l a t e d f r o m ( 1 ) after weighing each event

with the c o r r e s p o n d i n g r a d i a t i v e c o r r e c t i o n factor a~v/a . . . . . T h e r a d i a t i v e c o r r e c t i o n factors for the H a n d D targets were calculated b y using the m e t h o d o f M o a n d Tsai [ 16 ]. T h e calculations require the ab- solute structure functions F ~ a n d F 2 d as an input.

Both FE'S were d e t e r m i n e d f r o m a fit to electron a n d m u o n scattering d a t a d o w n to the t h r e s h o l d for reso- nance p r o d u c t i o n . W i t h i n o u r k i n e m a t i c range, how- ever, we d e t e r m i n e d F ~ f r o m the m e a s u r e d ratio a n d the F 2 d in an i t e r a t i v e procedure. T h e result o f the i t e r a t i o n s was f o u n d to be insensitive to the starting value o f F d / F ~. F o r T 1 events the average r a d i a t i v e c o r r e c t i o n factor ranged f r o m 0.64 ( 0 . 7 0 ) for H ( D ) events at x - - 0.007 to m o r e t h a n 0.90 for x > 0.1. F o r T2 events the c o r r e c t i o n factor was a b o u t 0.80 at the lowest x. T h e resulting effective c o r r e c t i o n on the ratio was a b o u t 12% at x = 0 . 0 0 7 a n d was less t h a n 1%

for x > 0.1. The a s s u m e d 7% u n c e r t a i n t y in the abso- lute n o r m a l i s a t i o n o f F a gives rise, t h r o u g h the ra- d i a t i v e corrections, to a systematic e r r o r on the ratio o f 1.3% at the smallest x a n d to less t h a n 0.1% at x > 0.06. T h e a s s u m e d u n c e r t a i n t i e s in the suppres- sion [ 9 ] o f the quasielastic electric ( 2 0 % ) a n d mag- netic (50%) form factors in d e u t e r i u m contribute 1%

a n d 1.4% respectively, to the systematic e r r o r on the ratio at the smallest x a n d less t h a n 0.1% at x > 0.06.

In calculating the r a d i a t i v e c o r r e c t i o n s we have as- s u m e d the value o f R, the ratio o f the l o n g i t u d i n a l to transverse p o l a r i s e d v i r t u a l - p h o t o n - n u c l e o n a b s o r p - t i o n cross section, to be the s a m e for H a n d D targets.

F o r x > 0. I, this a s s u m p t i o n is consistent with m e a - s u r e m e n t s [ 17 ]. As a consequence we o b t a i n F'~/F~ _ _ ~ l ' y / ~ 1 " / _{- - O d} _{l o p} _• ( 2 ) The F~/F~ ratio is d e f i n e d as

F'~/ F~ = F ~ / F~ - 1 (3)

w i t h o u t any c o r r e c t i o n for possible b i n d i n g effects in the d e u t e r i u m nucleus. In p a r t i c u l a r , we d i d not cor- rect for the s m e a r i n g effect due to F e r m i m o t i o n . Current models [ 18 ] give insignificant corrections for x < 0 . 6 .

The effects o f the l i m i t e d s p e c t r o m e t e r resolution in x a n d Q2 ( s m e a r i n g effects) on the r a t i o F~/F~

were d e t e r m i n e d b y a M o n t e Carlo s i m u l a t i o n o f the e x p e r i m e n t . The s i m u l a t e d events were p a s s e d t h r o u g h the s a m e chain o f r e c o n s t r u c t i o n p r o g r a m s as the real data. F o r T1 events the s m e a r i n g correc-

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Volume 249, number 2 PHYSICS LETTERS B 18 October 1990 t i o n factor, a v e r a g e d o v e r Q2, changes f r o m 1.002 to

0.995 for x v a r y i n g b e t w e e n 0.007 a n d 0.55. F o r T2 events the biggest smearing corrections factor is f o u n d to be 1.016 at x = 0.175. Fig. 2 shows the s e p a r a t i o n o f different target m a t e r i a l s seen in the b e a m . T h e a s s o c i a t i o n o f events to the wrong target was esti- m a t e d f r o m an e x t r a p o l a t i o n o f the tails o f the v e r t e x d i s t r i b u t i o n . T h e largest c o r r e c t i o n was 1%, at small x. The c o r r e c t i o n f r o m a 3% H D a d m i x t u r e in l i q u i d d e u t e r i u m is 0.3% ( 2 % ) at small ( l a r g e ) x.

T h e r a t i o s F'~/F~ were c a l c u l a t e d in x a n d Q2 b i n s for each SPS p e r i o d s e p a r a t e l y a n d t h e y were f o u n d to be m u t u a l l y consistent. T h e results f r o m different p e r i o d s a n d d i f f e r e n t Q2 b i n s were m e r g e d t a k i n g their g e o m e t r i c a l average [ 19 ].

T h e final results a n d t h e i r errors are given in table 1 s e p a r a t e l y for each trigger. T h e weighted a r i t h m e t i c average o f the results f r o m b o t h triggers is also shown in table 1 a n d fig. 3. T h e d o m i n a n t c o n t r i b u t i o n to the s y s t e m a t i c e r r o r at small x c o m e s f r o m the uncer- t a i n t i e s in the r a d i a t i v e c o r r e c t i o n s for d e u t e r i u m , whereas at high x it is due m a i n l y to the u n c e r t a i n t i e s in the b e a m ( 0 . 2 % ) a n d s c a t t e r e d m u o n (0.15%) m o m e n t u m . T h e s y s t e m a t i c errors due to vertex position smearing, H2 a n d D2 d e n s i t y a n d to target position d e p e n d e n t r e c o n s t r u c t i o n efficiency ( d u e to b a c k g r o u n d hits in the c h a m b e r s ) c o n t r i b u t e in total to less t h a n 0.5%. T h e total s y s t e m a t i c e r r o r has been c a l c u l a t e d b y a d d i n g i n d i v i d u a l c o n t r i b u t i o n s in quadrature. The systematic error is seen to range f r o m

2% at small x to 0.4% at x = 0 . 1 a n d to 8% at high x.

In fig. 3 we also show the results f r o m the B C D M S m u o n b e a m e x p e r i m e n t [ 12 ] which covers a s i m i l a r range in Q2. O u r d a t a e x t e n d to s m a l l e r x a n d have significantly smaller s y s t e m a t i c errors. T h e y are in g o o d a g r e e m e n t with the B C D M S results in the region o f overlap. The present d a t a are also consistent with the early E M C results [ 11 ] ( n o t shown in the figure) w i t h i n t h e i r large s y s t e m a t i c errors. In this figure we also show the recently r e - e v a l u a t e d results from 1.6-20 G e V electron b e a m experiments at SLAC [ 17 ] which cover s m a l l e r Q2 (0.6 < Q2 < 30 G e V 2) a n d h a v e small s y s t e m a t i c errors. F r o m the o b s e r v e d difference b e t w e e n the present a n d the SLAC results for x = 0 . 1 5 - 0 . 3 5 we o b t a i n an average slope o f A ( F ~ / F ~ ) / A l n Q 2 = - O . O I 9 ( 4 ) . L e a d i n g o r d e r Q C D calculations m a d e using the f o r m a l i s m o f A b b o t et al. [ 20 ] predict slopes between 0 a n d - 0.01.

T h e p r e s e n t m e a s u r e m e n t s e x t e n d b e l o w x - - 0 . 0 3 to a region not c o v e r e d by p r e v i o u s e x p e r i m e n t s . In this region, d o m i n a t e d by sea partons, F ~ / F ~ de- p e n d s also on the residual valence p a r t o n d i s t r i b u - t i o n a n d on the a m o u n t o f a possible f l a v o u r sym- m e t r y b r e a k i n g in the sea. At x = 0 the q u a r k - p a r t o n m o d e l p r e d i c t s no c o n t r i b u t i o n f r o m valence p a r t o n s to the structure functions. T h e sea p a r t o n s o f different flavours are b e l i e v e d to have the s a m e coupling to the p o m e r o n which d o m i n a t e s the low x Regge be- h a v i o u r o f the v i r t u a l - p h o t o n - n u c l e o n cross section [21 ]. A t o u r lowest x p o i n t ( x = 0 . 0 0 7 , ( Q 2 ) = 2 . 6 Table 1

The ratio F~/F~ averaged over Q2.

x Trigger 1 Trigger 2 Both triggers

(Q2) F~/F~ trsu,,, asyst. (Q2)

(GeV) 2 (GeV) 2

F~/F~ o'~t~t, o',y~t. (Q2) F'~/F~ o,t~t, a~y~t.

(GeV) 2

0.007 3.1 0.990 0.021 0.023 2.0

0.015 5.6 0.970 0.016 0.014 3.3

0.030 9.2 0.935 0.014 0.010 4.2

0.050 14.5 0.920 0.017 0.005 4.9

0.080 18.9 0.857 0.014 0.003 5.5

0.125 23.9 0.803 0.015 0.003 7.0

0.175 27.4 0.709 0.017 0.003 8.2

0.250 31.3 0.697 0.015 0.003 10.2

0.350 35.5 0.572 0.019 0.004 13.6

0.450 36.3 0.562 0.027 0.006

0.550 36.4 0.529 0.037 0.011

0.700 33.5 0.292 0.036 0.024

0.986 0.022 0.023 2.6 0.988 0.015 0.023

0.979 0.021 0.014 4.8 0.973 0.013 0.014

0.943 0.019 0.010 7.9 0.938 0.011 0.010

0.909 0.024 0.005 11.4 0.917 0.014 0.005

0.863 0.021 0.003 14.9 0.858 0.011 0.003

0.825 0.028 0.004 20.3 0.808 0.013 0.003

0.879 0.042 0 . 0 0 4 24.8 0.732 0.016 0.003 0.700 0.044 0 . 0 0 7 29.0 0.698 0.014 0.003 0.723 0.096 0 . 0 1 2 34.6 0.578 0.019 0.004

36.3 0.562 0.027 0.006

36.4 0.529 0.037 0.011

33.5 0.292 0.036 0.024

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1.2 F n

2 F2 p 1.

0.8

0.6

0.4

0.2

i , 1 ~ , i , i , r , , I l , , i i i ,

• NMC; this expt.

o BCDMS o SLAC

e

*g

¢ =

o o

o

*0

SLAC

- - ' - - - I BCDMS

N M [ ~ - - ~

1 1 1 1 I , I I ,r ,II i i [ , i Ii

10 -2 10 -1

Fig. 3. The ratio F~/F~ (x) from this experiment compared to previous results from SLAC [ 17 ] and BCDMS [ 12 ]. The con- tours show the size of the systematic errors.

GeV, ( u ) _= 200 GeV) the structure function ratio is already consistent with one, indicating the expected approach to symmetry between u and d partons in the sea for x ~ 0 . It is interesting to note that the real photon ( Q 2 = 0 ) total cross section ratio ant°t/apt°t =

t O t t o t

(a~ °~ - a p ) / a p =0.898(15) at v = 16-18 GeV [22].

This significant deviation from unity was interpreted in terms of shadowing of real photons in deuterium [23]. Assuming no discontinuity between real and virtual photon cross sections the shadowing should also be present in virtual-photon-deuterium interac- tions at Q2==_ 0 and similar u. The fact that we do not observe shadowing could be due to either not reach- ing low enough x or being too high in Q2.

Parton distributions have been extracted [ 1 - 7 ] from simultaneous fits to hard scattering cross section data. The present high precision data at small x constrain such parameterisations in the region where valence and sea partons have comparable eontribu-

tions. The most recent fit [ 7 ], in which our preliminary data [24] were also included, correctly de- scribes the shape of the F'~/F~ ratio in terms of parton distributions; see fig. 4.

As indicated in refs. [7,8], precise data on F'~/F~

reduce the uncertainty in predictions for the aw/az production cross section ratio in Pl~ collisions. In par- ticular, at the Fermilab collider energy of ~ = 1800 GeV the predictions are sensitive to the d / u parton ratio at x = 0.05, which can be well constrained only by our data. Calculations [ 7 ] at this energy, based on our preliminary results [24 ], predict aw/az= 3.30 for three neutrino families.

To summarise, the ratio F'~/F~ (x) has been deter- mined with high statistical and systematic accuracy down to x = 0.007. The results put strong constraints on the prediction of the parton distributions in a region where the contribution from both sea and va-

1.2

F;

F p • t h i s e x p e r i m e n t

1.0 MRS p a r a m e t e r i s a f i o n

0.8

0.6

0.4 ~

+

0.2

i 01.2 i i i i i

0 0.4 0.6 0.8

x

Fig. 4. The ratio F'~/F~ (x) compared to the parameterisation (HMRSB) ofref. [7].

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Volume 249, number 2 PHYSICS LETTERS B 18 October 1990

l e n c e p a r t o n s is i m p o r t a n t . A t t h e s m a l l e s t x t h e

F~/F~

r a t i o is close to u n i t y ; t h e s h a d o w i n g o f v i r - t u a l p h o t o n s in d e u t e r i u m is n o t s e e n in t h e p r e s e n t k i n e m a t i c range. T h e c o m p a r i s o n o f o u r results w i t h t h o s e f r o m S L A C in t h e x - r a n g e o f 0 . 1 5 - 0 . 3 5 i n d i - cates a s t r o n g e r Q2 d e p e n d e n c e o f t h e r a t i o t h a n t h a t p r e d i c t e d b y l e a d i n g o r d e r p e r t u r b a t i v e Q C D .

W e w i s h to t h a n k t h e t e c h n i c a l s t a f f o f C E R N a n d o f t h e p a r t i c i p a t i n g i n s t i t u t e s f o r t h e i r i n v a l u a b l e c o n t r i b u t i o n s to t h e e x p e r i m e n t . T h e c o n t r i b u t i o n o f D r . J. Z m e s k a l in m e a s u r i n g t h e i s o t o p i c p u r i t y o f t h e d e u t e r i u m t a r g e t is g r a t e f u l l y a c k n o w l e d g e d .

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