Perugia, Italy, 28 June — Q July 1993 OCR Output

Particles and Nuclei, XIII International Conference,

Review talk at the

from new data is discussed.

structure functions is presented and an apparent discrepancy in the physics conclusionsdence of nuclear effects is briefly discussed. The experimental status of spin dependentand neutrino data supports the universality of the parton distributions. The A depenpling constant oi, and the gluon distribution were determined. Comparison of muonneutrino scattering is reviewed. In QCD analyses down to ac = 0.008 the strong couThe current status of nucleon structure functions as measured in charged lepton and

Abstract

Puaeeiszs

iiiilnilniliiinliirniuxiliilli

Inst. fiir Kernphysik, Univ. Mainz, Germany

and

CERN, Geneva, Switzerland

Gerhard K. Mallot

Deep Inelastic Lepton Scattering

g in we o VL18 January 1994

CERN—PPE/94-06

EUROPEAN ORGANIZATION FOR NUCLEAR RESEARCH

O Jl

x < 0.15 scaling violations of F2 are dominated by the gluon distribution :rG, which has OCR Output0.006 and first results from HERA reach almost down to xr = 10”4 at high Q2. Forextended our knowledge of the proton and deuteron structure functions down to x =

Recent high precision data [8] from the New Muon Collaboration (NMC) havetests of PQCD.implies the understanding of scaling violations, which in itself provides one of the cleanestrates at future hadron colliders. The determination of parton distributions from DIS dataprecise parton distributions are also needed, particularly at low x, to predict the eventcouplings c;, E;. Apart from providing fundamental information on the nucleon structure,where the (anti)quark distributions (Qi), qi are weighted by process and flavour dependent

e=1

(3)F(¤=» Q2) = Z [Ciqi(x> Q2) + ©<Z(¤>» Q2) l,

of universal parton distributionsnucleon structure, leads naturally to an interpretation of the structure functions in termsbative QCD (PQCD) and a soft non-perturbative part, containing the information on the

For DIS the factorisation of the cross section into a hard part, described by perturthe c.m. energy squared, respectively.and Q2. The variables y = 1/ / E and s = (P + lc)2 denote the relative energy transfer andwhere the structure functions F} only depend on the Bjorken scaling variable x = Q2 / 2M 1/

(2)= ¤>y2F{’” +(1— y)F{’” i 21 - ¤>F’»2 I/,17 2 (2% gf; [ 2 (§2]

(1)éé}; = 2 ig? [=¤y2Ff + (1 — y)Ff2 Z t

given bying (Fig. 1) from an unpolarised target are

der,(Z = e,;1) and v,v deep inelastic scatterThe ieehehe hee ehehhee hehheh Figure iz Deep mam scattering in first

Parton Model

representing a major success of the Quarkhas been verified [7] with high precision,in the nucleon and not involving sea quarks[6], counting the number of valence quarkshand the Gross—Llewellyn Smith sum rulethe Gottfried sum rule. [5] On the otherquark sea which leads to a violation [4] ofsurprise is the flavour asymmetry of the

sea seems to play a key role. The latestunderstanding of the nucleon spin structure, in which the polarisation of the strangenucleons are different [1], the violation [2] of the Ellis—Jaffe sum rule [3] questions ourture. After the discovery that the cross sections for scattering from free and boundever, a series of surprises demonstrate our still poor understanding of the hadron strucon the internal structure of nucleons originates from the analysis of DIS data. Howlight, point-like constituents inside the nucleon and since then most of our knowledge

OCR OutputDeep inelastic lepton scattering (DIS) experiments provided the first evidence for

and antiquark distributions, and due to its non-singlet structure allows an unambiguous OCR Outputparticular the parity violating structure function :rF3 measures the difference of quarkFNAL [16] provide also information on the flavour dependence of quark distributions. In

Structure function data obtained in (anti)neutrino scattering at CERN [15] andwith the ones of the two high Q2 data sets and extrapolations were not unambiguous.The SLAC data could not resolve this problem because their Q2 range hardly overlapsremained after an overall normalisation shift and a re—evaluation [14] of the EMC data.CERN muon experiments of the EMC [11] and the BCDMS Collaboration [12] whichA major problem was the zv dependent discrepancy [13] between the results from the twolast two decades investigated the nucleon structure functions in the range 0.07 § :1: § 0.75.

Deep inelastic scattering experiments at SLAC [10] and CERN [11, 12] have in the

2 New Measurements of Structure Functions

collaborations from their data.

the E142 Collaboration, respectively. Different physics conclusions were drawn by the twothe deuteron and the neutron were reported by the Spin Muon Collaboration (SMC) and

In the very much debated field of spin dependent structure functions new data onData at very low rc were presented by the E665 Collaboration at this conference. [9]detailed and systematic study from NMC at low x provides rich input for model testing.understanding is also essential for the comparison of neutrino and charged lepton data, asection ratio measured with very high accuracy by NMC. For the nuclear effects, whose

For the first time a shadowing signal was seen in the deuteron to proton crossChicago-Fermilab-Rochester (CCFR) Collaboration.determination of the strange sea distribution were recently reported by the Columbiabeen inferred with high precision from the NMC data. Improved QCD analyses and a

sets including data at m > 0.5 not shown here.is a phenomenological QCD inspired parametrisation (solid line) fitted to all three databars represent the quadratic sum of statistical and systematic uncertainties. Also shownFigure 2: The F; data from NMC compared to those from BCDMS and SLAC. The error

Q1 (GeV 1) 0’(0ev=)10 100

¤ BCDMS

A SLAC <"‘·°)1-0.50O NMC +

deuteron(x1.c) I 0.1+ ¤ -o.¤7o

(x1.0):-0.15

0.5 / · +(x 1.2)x · 0.050

(xyz)¤-0.275

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1-0.025

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(xs.2)•_ ¤·o.11

¤ - omzs3 } (x:.2) / +(117.5)

.-0.09deuteron ¤ -0.00s • NMC I / (x4.0)

the NMC data on the parton distributions comes from the observed increase of F2 for OCR OutputApart from the clarification of the EMC/BCDMS ambiguity the main impact of

and 200 GeV incident muon energies will fill this gap.Q2 apparent between the 90 GeV and the 280 GeV data. Data taken in 1989 at 120 GeVbe reproduced by Monte Carlo simulations. This exclusion is reflected in the small gap inEMC analysis but excluded from the NMC analysis because these inefficiencies could notlikely to be due to correlated inefficiencies in a large drift chamber system used in theare larger than those from the EMC as shown in Fig. 3. These systematic differences areexcellent agreement is found for the three data sets. At low x the F2 data from the NMC

data overlap with the kinematic domains of both the BCDMS and the SLAC data and anpositive scaling violations in the new low x region. Due to their large Q2 range the NMCtowards low 127 covering 0.006 $ IE $ 0.6 and 0.5 < Q2 < 55 GeV2 and exhibit strongThe data, shown for the deuteron in Fig. 2, extend the measured :1: range by one decadeand constructed by the EMC and then upgraded by the NMC with several new detectors.targets exposed simultaneously to the beam. The spectrometer was originally designedenergies of 90 GeV and 280 GeV with two three meters long liquid hydrogen and deuteriumCERN became available. The experiments were performed in 1986-1987 at incident muon

Recently, new proton and deuteron structure function data [8] from the NMC at

2.1 Results from the New Muon Collaboration

and with that of the QCD parton model.muon experiments represent a stringent test of the universality of parton distributionsfunctions as obtained from (anti)neutrino scattering with the results from electron anddetermination of the strong coupling constant as. The comparison of the F2 structure

of statistical and systematic errors.

data at x > 0.07.The error bars represent the quadratic sumdistributions based on the SLAC/BCDMSand NMC at Q2 = 5GeV2 and 20GeVcompared to F2’s calculated from partonfrom SLAC, EMC (rc—cvaIuatcd), BCDMS

Figure 3: Comparison of F§l(:c,Q2) data Figure 4: F§’(:v,Q2 = 5GeV2) from NMC

0.¤1o.1 1 0.10.¤1 o.1

EMC—NA2 (recnnf) cRv

. MT-$1acnus

was-so°·2suc

{ NMCNMC0.2l- •

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¢ G

+‘. G0.4 + + *

Org Oz = 5 GEV 2F,°

Oz=5GeV' Oz=2OGeV'

from charged lepton and neutrino scattering respectively measures the mean-square quark OCR Outputtwo processes should differ due to different mass thresholds. The ratio of the F2°s derivedit has been argued [20] that the strange and charmed sea quark distributions seen by thecharged lepton scattering are given by the same universal parton distributions. However,

In the QPM the structure functions measured in (anti)neutrino scattering and in2.3 Comparison of Muon and Neutrino F2 Data and the Strange Sea

at low x. [19]expected and precise data will become available to search for novel QCD effects expectedQ2. For 1993 an increase in luminosity at HERA by at least an order of magnitude isarithmic behaviour of PQCD and at sc = 0.03 extrapolate well to the NMC data at lowerscaling violations are within the statistical and systematic errors consistent with the logfurther strong increase of F2 towards smaller values of x at Q2 2 15 GeV2. The observednary results from the H1 [17] and ZEUS [18] Collaborations (Figs. 5,6) show an excitingregime of 0.0003 $ I S 0.01 and 5 < Q2 < 5000 GeV2 has been opened up. The prelimi

With the first F; structure function data reported from HERA a new kinematic

2.2 First Results from HERA

extrapolated into the unmeasured region at low 1, see Fig. 4.parton distributions underestimated the amount of sea quarks and gluons when they wereused parton distributions based on the BCDMS and SLAC data. Most phenomenologicalx < 0.07, where the NMC data are up to 30% higher than the extrapolations of commonly

included in the error bars.BCDMS and NMC at the same Q2

is not included. Also shown are data from all normalisation uncertainty of 7% is noterrors. The normalisation uncertainty of 8% ment with the GLAP equations. The overquadratic sum of statistical and systematic to the lower Q2 of the NMC data in agreeresent statistical errors, the outer ones the Q2. For x = 0.032 these data evolve down

oration for several x bins as a function oftion for Q2 = 15 GcV2. Inner error bars repFigure 5: F; data from the H1 Collabora Figure 6: F; data from the ZEUS Collab

0* (sew;10 102 103

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{ Zeus cms 10

flavour symmetric sea. The updated value reported at this conference [30] for Q2 = 4 GeV2 OCR Outputthat the Gottfried sum [5] Sq; is smaller than 1/3, the value expected in the QPM for a

An important impact on the parton distributions had the observation [4] of the NMC2.4 Flavour Asymmetry of the Quark Sea

distribution analyses. [28, 29]Recently, the flavour dependence of the quark sea has also been addressed in global partonassuming its validity as has been determined from the higher order corrections to it. [27]Smith sum rule for f F3 da: has been verified very accurately [7] at Q2 = 3GeV2 andof the Gottfried [4] and Ellis-Jaffe [2] sum rules. On the other hand the Gross-Llewellynon the flavour dependence of sea quark distributions and thus linked to the violations

The question of the strange sea is embedded in the ongoing more general discussionscattering at low ar: are still under investigation.discrepancy, like a possible difference [26] of R(x, Q2) = aL/UT for neutrino and muonaccount for the observed difference. Other experimental and theoretical sources of thethe CCFR and NMC data at low x, since a 2-3 times larger strange sea is needed toThis seems to rule out the strange sea correction as the origin of the discrepancy betweenand Ii = 0.44 :l: 0.06 in agreement with previous determinations from other experiments.yields a similar shape for the strange sea and its non—strange counterpart (04 = ——0.3i0.7)However, in a recent NLO analysis of the same data [25] including gluon fusion diagrams12 and the d distributions and to carry less than half their average momentum (uc = 0.37).In a leading order analysis [24] the strange sea was found to be softer (or = 2.50) than the

rc _ Q 2 f xs(:v) dx

depicted in Fig. 8. The strange sea was parametrised in the form

from neutrino induced charm production tagged by its opposite-sign dimuon signature,is important. This distribution was determined by the CCFR Collaboration independentlyas shown in Fig. 7. In this region the correction due to the strange sea distribution (Eq. 4)two .0: bins (sc = 0.015 and x = 0.045) the CCFR data lay up to 20% above the NMC datafor nuclear effects, and the NMC and BCDMS deuterium data. However, for the lowest

For z > 0.1 a good agreement is found between the CCFR iron F2 data, correctedperfect agreement with the QPM prediction.CCFR F;§’F° data a mean-square quark charge [23] of (1.00 ;t 0.03) · 5/18e2 is found inand 1 < Q2 < 50 GeV2 using a 690 ton iron target. From the BCDMS Ffd data and the(anti)neutrino energies of 30 < E., < 600 GeV in a kinematic range of 0.015 < zc < 0.65the CCF R Collaboration at F N AL. Their experiments E744 and E770 were performed atkinematic region of interest. Presently the most precise neutrino data [16] are those fromdata. This correction is well known from muon [21] and electron [22] experiments in themust be taken into account when neutrino data are compared to charged lepton deuterium:1: z 0.01. Since neutrino experiments use large nuclear (iron) targets, nuclear effects in F2correction due to the asymmetry of strange and charmed quarks amounts to about 10% atwhere (5) s and (E) c denote the strange and charmed (anti)quark distributions. The

F;}"" 18 5 q + Q_—1 — (4)F;N,2{§(5-[-.§)—(E+C)

charge (ef + eg)/2 = 5/18

experiment (E866) aiming at very high statistical accuracy was approved at FNAL. [36] OCR Outputfrom proton and deuteron targets was measured are expected later this year. A similarruled out. First results from the N A51 experiment at CERN [35] in which DY production0.04 < x < 0.27 and certain models for the :1: dependence of this asymmetry could bethe E772 experiment at FNAL an upper limit on the d-zi asymmetry was inferred forproton and deuteron targets. In an analysis of carbon and tungsten DY data [34] fromDY data from two nuclear targets with different neutron excess. [33] The best choice arethe target nucleon. Information on the ratio d(a:) /&(a;) can thus be obtained by comparing

Drell·Yan (DY) lepton-pair production is sensitive to the antiquark distribution ofthat the d sea is larger than the G sea remains unchanged.and F; is a small effect, which is discussed in Sect. 3.3. The original physics conclusionFf at low sc. The modification of the radiative corrections to F;/F; due to the new Fgdata. The re-evaluation [30, 32] became necessary due to the above discussed increase ofEarlier, a slightly lower value of SG = 0.240 i 0.016 was reported [4] based on the same

(7)T1. p F; - FQ = 2F; .[311 F?/F5is determined from the deuteron structure function Ff and the accurately known ratiocorresponding to f(§(& - d) dx = -0.113 j; 0.024. In the NMC analysis the Gottfried sum

3 3 0(6)S'G=/10 xFP — F " 1 1 - Q;i2dx=-+g/(1E—d)dx=0.258:l:0.018,

1S

x bins. At higher at the two data sets agree.O1(GevI) :1: = 0.045, together with the closest NMC

est two m bins of CCFR, x = 0.015 andand strange sea effects. Shown are the lowthe CCFR F{"'° data. corrected for nuclearFigure 7: Comparison of the NMC Ffa and°° +

, * Um (anti)quark sea.4+ 3 •‘ 4 + __°_m production is sensitive to the strange

Figure 8: (Ar1ti)neutrin0 induced charm

c W4.z;g;A 75

A CCPR

• NMC

deutercn

to the data in an iterative procedure. In addition to the determination of quark and gluon OCR Outputparameters of these distributions are then determined by comparing the QCD calculation:cqNS(x,Q§), xqSI(2r:,Q§), and xG(;1:,Q§) are parametrised at a fixed Q2 = Q5 and the

In a QCD analysis of structure function data the unknown x dependences ofevolution equations.decomposed into a flavour singlet and a flavour non-singlet part obeying the accordingquark distributions of all flavours, any linear combination of quark distributions can besinglet quark distribution qSI(z, Q2), which is defined as the sum of all quark and antifunctions and thus introducing additional experimental uncertainties. Using the flavourpartly overruled by the necessity to combine different data sets to obtain these structurebutions. The advantage of not being coupled to the a priori unknown- gluon distribution isfunctions mF3 and F; — F1? correspond to such non-singlet combinations of quark distributions qNs(x, Q2)) the contribution from the gluon distribution cancels. The structure[37] However, in the evolution of quark distribution differences (flavour non-singlet districoupled and described quantitatively by the Gribov-Lipatov-Altarelli—Parisi equations.for such calculations. In general the Q2 evolutions of quark and gluon distributions arecc dependence of quark and gluon distributions at some value Q3 are needed as inputspair creation, is a firm prediction of QCD. The strong coupling constant oz,(Q2) and thenot be calculated in PQCD their Q2 evolution which is due to gluon radiation and qc]of the nucleon or nucleus. While the x dependence of quark and gluon distributions cangiven by those of the quark and gluon momentum distributions, :1:q(:c, Q2) and :cG'(:c, Q2),

In the QCD parton model the :v and Q2 dependences of the structure functions are3 QCD Analysis of Structure Function Data

errors.

the 90 and 280 GeV data sets found in the fit is 1.8% and well within the normalisation

of the higher twist contribution one obtains the dashed line. The relative normalisation ofFigure 9: The solid line shows the QCD fit to the NMC proton F2 data. After subtraction

O' (sew)10 o' (csv)TOO

proton

/%¤.¤ L NMC D Zim

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C However, varying cx, and C within reasonable limits results in a modification of OCR Outputand systematic uncertainties except those arising from or, and higher twist contributionszG is large has been investigated. The error bands shown in Fig. 10 include normalisationcompared with previous results, and for the first time the region 0.008 f zv f 0.07, wherevalue [30], see Sect. 2.4. The gluon distribution was determined with an improved precisionof SG = 0.243;l:0.030 for the Gottfried sum can be inferred in agreement with the updatedFig. 10a-c. From the non-singlet distribution xqNS which corresponds to 3(F; —F{‘) a valueQ2 = 1 GeV2. The parton distributions obtained from this fit for Q2 = 7 GeV2 are shown indata. As shown in Fig. 9 the QCD fit describes the data very well down to the lowestonly be improved in a forthcoming combined analysis of the NMC, BCDMS and SLACexperiments. [44] Due to the restricted range in Q2 of a single experiment this value can

In a first analysis o4,(M§) was fixed at 0.113, the average value obtained in DISthis extrapolation.

from Ref. [38] and extrapolated linear to x < 0.07. The results were not very sensitive toturbative Q2 evolution a twist—four component was added like in Eq. 8, which was takenobtained in QCD [43] with the fitted gluon distribution yields similar results. To the perbe small [42] and new data have been presented on this conference [30]. Using RQGD ascal parametrisation [41]. The difference of R for the proton and the deuteron is known tothe longitudinal to transverse absorption cross sections was taken from a phenomenologirenormalisation group equations and including flavour thresholds. The ratio R(:c, Q2) offor the proton and the deuteron. The “running” of cx,(Q2) was calculated from the NLCrenormalisation and factorisation schemes and the same gluon distribution was assumedand the deuteron. The next-to-leading order calculations were carried out in the MSQ3 = 7GeV2, the average Q2 of the data, and simultaneously evolved for the proton

The distributions :vqd(x, Q2), ;z:qNs(x, Q2), and xG(x, Q2) were parametrised atqcan be determined reliably.NS

proton and deuteron data both the singlet and non-singlet quark distributions, qd andcontains both singlet and non-singlet contributions. Hence, in a simultaneous analysis ofdistributions does not change its Q2 evolution significantly. The proton quark distributioncontribution to qd, which is proportional to the difference of the charmed and strange quarkthe deuteron quark distribution q2 is almost a pure flavour singlet. The small non-singlet0.008 f :1: $ 0.5 and 1 < Q2 < 50 GeV2 was published. [39, 40] Due to isospin symmetry

Recently, a QCD analysis of the NMC proton and deuteron F2 data in the range3.1 QCD Analysis of the NMC F2(x,Q2) Data

determination of cx,(M3) = 0.113 :1: 0.005.negative for x < 0.3. The excellent agreement with the QCD calculation led to a preciseC were found to be large and positive for x > 0.5 and Q2 < 5GeV2 and small andwas chosen, where F{‘T(x, Q2) denotes the perturbative leading twist part. The values of

(8)TF2(<¤» Q2) = Ff(¢.Q2) · {1+ C'(¢)/Q2

account for twist four contributions. A parametrisation of the formQ2. This analysis clearly showed the necessity to include terms proportional to 1 / Q2 to[12] and the re—evaluated SLAC data [10] covers 0.07 < rr < 0.75 and a large range in

A common QCD analysis [38] of the BCDMS proton and deuteron F2(:2:, Q2) data

(10)a,(7 GeV2) = 0.264 :1: 0.018(stat.) :t 0.073(syst.). OCR Outputyielding

In a second QCD analysis the NMC treated cx,(Q§) as a free parameter in the fitshown in the insert of Fig. 10c.range of 1.25 § Q2 $ 48 GeV2 the gluon momentum fraction evolves from 0.40 to 0.48 asfraction of 0.55 is attributed to the quarks and of 0.45 to the gluons. Within the coveredmomentum sum rule in the analysis as described below. At Q2 = 7GeV2 a momentumonly a small fraction for the region below sc = 0.008. Therefore it is justified to impose theIn the measured region the NMC finds about 95% of the nucleon’s momentum leaving

(9)x q(x= Q2) + G(w» Q2)? dw = 1[ [1 SIcan be checked

the singlet quark distribution xqsl which corresponds to §F§, the momentum sum ruleQCD analysis [38], and CDHSW neutrino results [15]. From the gluon distribution andprevious determinations at higher x from J /1,/¤ muo·production [45, 46], the SLAC-BCDMSlow x a modified parametrisation [40] had to be used. The gluon distribution agrees withparametrisation for xG was found to be not flexible enough to describe this behaviour atfor Q2 § 3GeV2 and x § 0.05 it drops towards low x. Since the conventional (1 —— 2:)"in Fig. 11. At Q2 2 7GeV2 the gluon distribution is rising with decreasing 2:, whereasxG just within the shown error bands. The full z and Q2 dependence of a:G(z:, Q2) is shown

0,10.01 tribution in the Q2 range ofthe data.in (c) shows the evolution of the gluon disties due to cx, and higher twist. The insert

tematic errors. Not included are uncertain1 P Oz=7 Ge\/ZThe error bands contain statistical and sysgluon distribution a:G, all at Q2 = 7GeV

0-O! 0.1 x I deuteron quark distribution xqd and (0) the0*-1.25 0ev* n0n—sing1et quark distribution zqNS (b) the

and deuteron F2 data. Shown are (a) thein the QCD analysis of the NMC proton

ST \ ¤’=4& c¤v’ Figure 10: Parton distributions obtained

(c)

0.01 0.010.1

0.5

0,2

0,:

Qz=7 GeV'0,4 L O2=7 Gevz

tb)(al

2.5__ 0.5

10 OCR Output

[39].Q2 = 5 GeV2 the gluon distribution is in agreement with the one from the NMC analysisfrom other DIS experiments [38, 39] than with the present LEP average [47]. Compared atx data still under discussion. The value for or. is rather low and agrees better with those

Only data with Q2 > 15 GeV2 were included in this fit which effectively excluded the low

(12)ry = 4.52;; :l: 1.2.

(11)cx,(M§) = 0.113 zh 0.002(stat.) :1: 0.003(syst.)

ry in the gluon distribution zG(:v, 5GeV2) = A(1 — x)" were determinedthe CCFR data. In a simultaneous analysis of F2 and xF3 data both as and the exponentviolations at x < 0.2 reported earlier from the CDHSW experiment [15] were not seen inthe QCD parton model is valid also in heavy nuclei like iron. The larger positive scalingin F2"(x,Q2) and xFg’(:c, Q2) are well reproduced by PQCD, see Fig. 13, indicating thathave been presented recently by the CCFR Collaboration. The observed scaling violations

Improved preliminary QCD analyses of the F; and xF$’ struction function data [48]3.2 QCD analysis of the CCFR F2 and :1:F3 data

different from earlier determinations at higher x, where xG is less important.dominates at low x. Thus the determination of oz., from this sc region is methodicallycalculation. The contribution of a:G to the evolution is indicated by the shaded area andFig. 12 where the logarithmic slopes d ln F2 / d ln Q2 are compared to those from the QCDthis analysis. The good description of the NMC data by PQCD is also demonstrated inthe correlation of the two quantities the momentum sum rule (Eq. 9) was imposed in

For z $ 0.1 the Q2 evolution depends strongly on both cy, and :1:G. To minimiselatter two results will remain in future.

presented on this conference. It remains to be seen if the apparent discrepancy of the[44] cz,(M§) = 0.113 :’c 0.002 and with the LEP average [47] o4,(M§—) = 0.123 :b 0.003This corresponds to c¤,(M§) = 0.117fgjg[é in agreement with the average from DIS data

other values of Q2analysis. The distribution was parametrised at Q2 = 7GeV2 and then evolved for theFigure 11: The x and Q2 dependence of the gluon distribution from the NMC QCD

0.1

0.0110iQ

[ 6 sxg(x)

11 OCR Output

[52] 0}*2,/cg/d, = 0.113 i 0.08. The gluon distribution enters via the photon-gluon fusionnow supported by the cross section ratio for inelastic J / z/2 production from tin to carbonwith increasing A points to a with A increasing momentum fraction of the gluons. This is

The observation that the momentum sum obtained from F2 ratios [21] decreaseseffects become very small.A, whereas in for ac > 0.25, B is again negative. At cc = 0.055 and x = 0.25 average nuclear0.055 < a: < 0.25, 5 is positive leading to an enhancement of the cross sections for largerrA decreases with increasing A and nuclear effects become smaller with increasing sc. Forof the logarithmic slopes B shown in Fig. 16. In the shadowing region x < 0.055 the ratiois remarkable. Therefore the coarse features of the A dependence can be discussed in termsstructure in detail, particularly not for light nuclei, the description of the overall behaviourrA = 0:+ B ln A. Although such a parametrisation cannot be expected to follow the nuclearof nuclear effects in DIS emerged. It can be parametrised by a simple logarithmic functionearlier results from the NMC [21, 51] and SLAC [22] a detailed picture of the A dependencerA = Ff/F5 for Be, Al, Fe, and Sn to Carbon were measured for 0.01 < 2: < 1. Includingnumber A by the NMC [50] is shown in Fig. 15. The preliminary structure function ratios

A high—precision systematic study of the dependence of nuclear effects on the massobserved for x < 0.02 and no strong Q2 dependence was found.of :v M 10"‘*, where Q2 is about 0.1 GeV2, see Fig. 14. Saturation of the shadowing isFf/F; structure function ratios for the nuclei C, Ca, and Pb down to very small values

The E665 Collaboration has reported on this conference [9] new results for thewill be assumed to be independent of A.NMC found the difference RC°— RC to be compatible with zero [42]. In the following Rfunction ratio, provided R(a:,Q2) is the same for both nuclei. In a recent analysis theThe ratio of deep inelastic cross sections of different nuclei is equal to the F2 structure

Nuclear effects will here only be discussed briefly. For a recent review see Ref. [49].

3.3 New Data on Nuclear Effects

calculation without higher twist. The shaded area represents the contribution from gluons.QCD fits. Only statistical errors are shown. The dashed curve corresponds to the QCDFigure 12: Logarithmic slopes dln F2 / dln Q2 of the NMC data compared to those of the

*°=° xG=O

x %%

2X , Fr:2 0.2Qu,

deuteronp|'OtOn

F`z=Fzu(1 +H(x)/Oz) Fz=FzU(1NMC· I b 0,5b3;

12 OCR Output

together with a gaseous 3He target which to a good approximation is a polarised neutrondeuterated butanol target. The E142 experiment uses polarised electrons of 19-26 GeVEMC experiment, polarised muons of 100 GeV were scattered from a polarised solid stateSLAC, respectively. In the SMC experiment which is basically an improved version of thereported by the Spin Muon Collaboration (SMC) at CERN and the E142 Collaboration at

Early this year, the first data on gl for the deuteron [58] and the neutron [59] wereprovides a stringent test of QCD.a precise determination its difference l`§’ - PQ, whose value was predicted by Bjorken [56],As = -0.19 i 0.06. Apart from the interest in the individual first moments of gf and g{‘be cancelled by the negative contribution of the sea, particularly by the strange quarkston spin AE = 0.12 zh 0.17. The positive contribution of the valence quarks appears to

result leads to a negligible contribution of the quark and antiquark spins to the prooretical papers, for reviews see Refs. [54] and [55]. In the QPM interpretation the EMCsequences on the internal spin structure of the proton were discussed in numerous thethe first moment F1 = fg g1(z) dx of the spin dependent structure function gf, the con

Since the EMC reported the violation [2] of the Ellis-Jaffe (EJ) sum rule [3] for4 V Polarised Deep Inelstic Scattering

effect in the deuteron which has been predicted earlier [53].almost 1: independent value of F;/F; = 0.96. This can be interpreted as a 2% shadowingunity. New data from the NMC [30] reaching down to rr x l0'3 show for az < 0.01 anlow x (Fig. 4), results in a small drop of the ratio for 1: < 0.01 and causes it to stay belowcorrections. A re-evaluation [30, 32] of FQ/F; using the new F2 data which are larger atdirectly. The absolute size of F2 enters into this determination of F; / F { only via radiativewere obtained from an experiment measuring the deuteron to proton cross section ratioratio F;/F; is expected to approach unity as seen in the NMC data [31]. The data

For as —> 0 where the sea quarks dominate, the neutron to proton structure functionprocess into the J / rb production cross sections.

fits.

dln F2/d ln Q2. Only the statistical errors are shown. The solid lines represent the QCDFigure 13: Logarithmic slopes of the CCFR neutrino data (a) dln :1cF3/d ln Q2 and (b)

(b)(a)

-04 0 0.2 0.4 0.6 0.8 -0* o ¤.z 0.4 ¤.e o.¤

-0.3-0.3

q' > 1¤ G•V'q' > 15 c•v‘.5 -0.2 v F w•> an c•v*-0 2 I. v' > 10 c•v'

5 -0.1-0.1

Au ¤ 210 4- 28 I(•V

0.00.0

—-· Non-s•ngl•L QCD·FitO}!

I F, Data F, NL0 QCD·FilI F, DanO xi`, Bat;

0 2

13 OCR Output

The structure functions gl are determined from the measured spin dependentBjorken sum.large at low Q2, were discussed by Ellis [64] on this conference with emphasis on theuncertainties, are neglected. Possible effects, including higher twist which may becomementally yet and which are expected to be small [63] compared to present experimental

In the following discussion scaling violations in gl, which were not seen experi

to the Bjorken sum rule to 1.5 standard deviations.

Q2 = 2 GeV2, were not taken into account and this would have reduced the discrepancyvalue. However, second and third order PQCD corrections [62], which amount to 5% atand E142 results has been reported by E142 to be in disagreement with the theoreticalin good agreement with the theoretical prediction whereas the one obtained from EMCdraw firm conclusions. The combination of EMC and SMC data yields a value which isSMC data to the E142 result. The precision on the Bjorken sum is not yet sufficient toand increases to 3.3 standard deviations for the comparison of the combined EMC and

1. The difference in AE from the SMC and E142 amounts to 1.9 standard deviations

reports agreement with the EJ sum rule for the neutron and AE = 0.57 :1; 0.11, see Tab.fraction AE of the nucleon spin carried by quark spins, whereas the E142 Collaborationthe deuteron, like the EMC for the proton, a violation of the EJ sum rule and thus a small

Contradicting physics conclusions were drawn by the two groups. The SMC finds forPQCD corrections. On the other hand the E142 data are statistically much more precise.Therefore their interpretation is less affected by higher twist [60, 61] and higher orderdata reach to lower x than the E142 data and have a higher average Q2 for each az bin.nuclei and cover different kinematical ranges. Due to the high muon energies, the SMCtarget. The two experiments are complementary in the sense that they use different target

shaded area represents the systematic uncertainty.Figure 14: Cross section ratios JA/ad for C, Ca, and Pb as a function of log(:c). The

l-¤910(X¤gl-3.2 -2.8 -2.4 -2.0 -1.6 -1.2 -0.8

0.50

0.75 .¤»-¤•-¢-•- [b'cpb/0D 1

¤ Esss Xa (A- 1:11) PRL 68 (1992) 326s1.25 O E665 Preliminary

0.50

0.75

aca/6D 1 . r-•-•···—•—¤·•·¤ '*¤"¤ NMc z Phys. c 5141991)

1.25 • E665 Preliminary

0.50

0.75

cc/00 1 , ¤D N ys. 1(1991) .5

1.25 ° E?°32’2L"""°£’

14 OCR Output

from the combined I`? is in agreement with that from the EMC and SMC, i.e. the dataextrapolation and its assumed high accuracy. The physics conclusion which can be drawnThe resulting larger error is due to the errors of the data points, which replace the E142the E142 extrapolation. Thus one obtains I`? = -0.05 zb 0.03 instead of -0.022 i 0.011.(-0.007) is obtained from the SMC / EMC data (extrapolation) compared to -0.006 fromdata dominate the integral, but for 0.006 < x < 0.03 (ac < 0.006) a contribution of -0.03proton data discarding the E142 extrapolation. In the region of overlap the precise E142

This is reflected in a simple combined analysis for I`? of the neutron, deuteron, andE142 extrapolation indicating that g?(z < 0.03) is more negative than assumed.data on a point by point basis, all three additional data points at low (B fall below theAlthough the extrapolation used by E142 is compatible with the combined SMC/ EMCmeasured region 0.006 < x < 0.03 and to the corresponding extrapolation for x < 0.006.data obtained from a combination of the EMC proton and the SMC deuteron data in thebecomes valid. In Fig. 18 the extrapolation of E142 for :cg?(:z —+ 0) is compared to thewith -0.5 < oz < 0. However, it is unclear at which value of x such a behaviour of glimportant. It is common practice to assume for 1: —-> 0 a Regge-type form gl o< :z:‘and for x —> 1. Due to the factor 1/zz in Eq. 13 the extrapolation to as = 0 is particularly

The evaluation of the first moment I`] involves extrapolations of g1(x) for 2: —-> 0experimental problems can not be responsible for the different results.the conflicting physics conclusions do not originate from the measured region. Thereforeshown in Fig. 17 the data sets are in good agreement in the region of overlap and thuscomparison, and the A? data were converted to A? using the EMC proton data. AsA?, A? and A?. Here the deuteron as an “average nucleon” has been chosen for theproton, deuteron and neutron data is best performed on the directly measured quantities,using the structure functions F2 and R known from unpolarised DIS. A comparison of the

$(1+ R(¢» Q ))

virtual-photon asymmetries A1

ratios).X-)

10*2 10 " ; E139 (open circles, obtained from Ff/F;0.9 from the NMC (full circles) and from SLAC

ratios rA = F?/Fg for Be, Al, Fe, and Sn1.1 Sn/C Figure 15: Preliminary structure function0.9

+ _B = drA/d 1nA as a function of sc.

1.1 Fe/C Figure 16: Logarithmic slopes0.9

X%V ••¢•?1 10-110-2

1.1 CG/CQ smc E139-0.050.9• wc1 °é°

1.1 An/C

¤s1.¢z¤:¤0.9INMC

1.1 Be/C O •• Q.!

0.05

15 OCR Output

the E142 Collaboration from their results. Because the measured asymmetries from theneutron were reported. Contradicting physics conclusions were drawn by the SMC and

The first data on the spin dependent structure functions of the deuteron and theprovides a very clean case to test our quantitative understanding of shadowing.clear now also at low 2:. For the first time shadowing was observed in the deuteron. This

The phenomenology of nuclear effects on structure function ratios has become veryunderstanding of the spin structure of the nucleon.

for the comparison of muon and neutrino data. Its polarisation plays a key role in theimportant ingredient for the understanding of the nucleon structure and is also neededthe observation that the Gottfried sum rule is violated. The strange sea appears to be an

A general discussion of the flavour dependence of the quark sea was triggered by

collisions at LEP.

agreement, but their average value seems to be lower than the one obtained from e+e‘Determinations of the strong coupling constant os from scaling violations are in goodat 22 < 0.1 between the NMC and CCFR data and its clarification of utmost importance.

the universality of the parton distributions. However, a discrepancy of up to 20% remainsincluded. The excellent agreement of neutrino and muon data for x > 0.1 demonstrates

parton model. At Q2 < 10 GeV2 a parametrisation of twist—four contributions must bethose obtained in neutrino scattering from iron targets, are well described by the QCDresults can be expected in the coming years. Presently all scaling violations includingQ2. The first data from HERA show a further strong increase towards low x and excitingproton and deuteron F2 structure functions are now well known in a wide range of x and

Our knowledge of parton distributions made great progress in the last years. The

5 Conclusions

from it.

would mainly affect the E142 result at low Q2 and modify the physics conclusion drawnthat corrections due to higher twist [60, 61, 64] could be large. Again such a correctionimproves when the combined value for l`? is used. On the other hand it was estimatedspin. If higher twist contributions are small, also the agreement with the Bjorken sum ruledeviate from the EJ sum rule and the quark spins carry only a small fraction of the nucleon

makes use of the proton data.l`1(EJSR) was calculated using F / D = 0.575. The value given in the last row for I`? — I`?The statistical and theoretical errors were added in quadrature. The Ellis—Jaffe predictionTable 1: Summary of the results from the E80/E130—EMC, E142 and SMC experiments.

0.20:l;0.06 I 0.146:l;0.021I`? — I`?

As 0.19:l;0.06 I —0.01:l:0.06 I -0.21:}:0.08

AE 0.12:h0.17 I 0.57:t0.11 I 0.06:}:0.25

0.176:I:0.006 I —0.021i0.010 I 0.085:t0.005F1 (EJSR)0.126t0.018 I —0.022j:0.011 I 0.023:b0.025F1 (exp.)

10.7 2.0 4.0(Q2) (GeV2)Q2range(GeV2)| 1<Q2<70] 1<Q2<7 I 1<Q2<3Oz range 0.01 < .1: < 0.7 I 0.03 < x < 0.6 I 0.006 < x < 0.6

,Process Ep'if é°3He

Data proton[2, 57] I neutron[59] | deuteron]58]

16 OCR Output

(1992) 475.[10] L.W. Whitlow, E.M. Riordan, S. Dasu, S. Rock, and A. Bodek, Phys. Lett. B282

G.Y. Fang, these proceedings.[9]PPE/92—124.NMC, P. Amaudruz et al., Phys. Lett. B295 (1992) 159; and preprint CERN[8]CCFR Collab., W.C. Leung et al., preprint NEVIS 1460, 1992[7]D.J. Gross and C.H. Llewyllyn Smith, Nucl. Phys. B14 (1969) 33[6]K. Gottfried, Phys. Rev. Lett. 18 (1967) 1174.[5]

[4] NMC, P. Amaudruz et al., Phys. Rev. Lett. 66 (1991) 2712

1669.

[3] J. Ellis and R.L. Jaffe, Phys. Rev. D9 (1974) 1444; and Phys. Rev. D10 (1974)(1989) 1EMC, J. Ashman et al., Phys. Lett. B206 (1988) 1167; and Nucl. Phys. B328[2]EMC, J.J. Aubert et al., Phys. Lett. B 123 (1983) 275[il

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-c.os

vé O _$ lé + L I T T] + nmuzy_ ’°§»°_` 0.2 i5 é

¢ d(sMc1OA 0.os+ p (EMC+E80+E130)+n(El42)

0.6

17 OCR Output

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