The Polarised Valence Quark Distribution from semi-inclusive DIS

Transcription

1 EUROPEAN ORGANIZATION FOR NUCLEAR RESEARCH CERN PH EP/ July 27 The Polarise Valence Quark Distribution from semi-inclusive DIS The COMPASS Collaboration Abstract The semi-inclusive ifference asymmetry A h+ h for harons of opposite charge has been measure by the COMPASS eperiment at CERN. The ata were collecte in the years using a 16 GeV polarise muon beam scattere off a large polarise 6 LiD target an cover the range.6 < <.7 an 1 < Q 2 < (GeV/c) 2. In leaing orer QCD (LO) the asymmetry A h+ h measures the valence quark polarisation an provies an evaluation of the first moment of u v + v which is foun to be equal to.4±.7 (stat.)±.5 (syst.) over the measure range of at Q 2 = (GeV/c) 2. When combine with the first moment of g1 previously measure on the same ata, this result favours a non-symmetric polarisation of light quarks u = at a confience level of two stanar eviations, in contrast to the often assume symmetric scenario u = = s = s. (To be Submitte to Physics Letters B)

2 The COMPASS Collaboration M. Alekseev 29), V.Yu. Aleakhin 8), Yu. Aleanrov 18), G.D. Aleeev 8), A. Amoroso 29), A. Arbuzov 8), B. Bae lek 3), F. Balestra 29), J. Ball 25), J. Barth 4), G. Baum 1), Y. Befer 25), C. Bernet 25), R. Bertini 29), M. Bettinelli 19), R. Birsa 28), J. Bisplinghoff 3), P. Boralo 15,a), F. Braamante 28), A. Bravar 16), A. Bressan 28), G. Brona 3), E. Burtin 25), M.P. Bussa 29), A. Chapiro 27), M. Chiosso 29), A. Cicuttin 27), M. Colantoni 29,b), S. Costa 29,+), M.L. Crespo 27), S. Dalla Torre 28), T. Dafni 25), S. Das 7), S.S. Dasgupta 6), R. De Masi 2), N. Deek 19), O.Yu. Denisov 29,c), L. Dhara 7), V. Diaz 27), A.M. Dinkelbach 2), S.V. Donskov 24), V.A. Dorofeev 24), N. Doshita 21), V. Duic 28), W. Dünnweber 19), P.D. Eversheim 3), W. Eyrich 9), M. Fabro 28), M. Faessler 19), V. Falaleev 11), A. Ferrero 29), L. Ferrero 29), M. Finger 22), M. Finger jr. 8), H. Fischer ), C. Franco 15), J. Franz ), J.M. Frierich 2), V. Frolov 29,c), R. Garfagnini 29), F. Gautheron 1), O.P. Gavrichtchouk 8), R. Gaza 3), S. Gerassimov 18,2), R. Geyer 19), M. Giorgi 28), B. Gobbo 28), S. Goertz 2,4), A.M. Gorin 24), S. Grabmüller 2), O.A. Grajek 3), A. Grasso 29), B. Grube 2), R. Gushterski 8), A. Guskov 8), F. Haas 2), J. Hannappel 4), D. von Harrach 16), T. Hasegawa 17), J. Heckmann 2), S. Heicke ), F.H. Heinsius ), R. Hermann 16), C. Heß 2), F. Hinterberger 3), M. von Hoenberg ), N. Horikawa 21,), S. Horikawa 21), N. Hose 25), C. Ilgner 19), A.I. Ioukaev 8), S. Ishimoto 21), O. Ivanov 8), Yu. Ivanshin 8), T. Iwata 21,32), R. Jahn 3), A. Janata 8), P. Jasinski 16), R. Joosten 3), N.I. Jouravlev 8), E. Kabuß 16), D. Kang ), B. Ketzer 2), G.V. Khaustov 24), Yu.A. Khokhlov 24), Yu. Kisselev 1,2), F. Klein 4), K. Klimaszewski 3), S. Koblitz 16), J.H. Koivuniemi 13), V.N. Kolosov 24), E.V. Komissarov 8), K. Kono 21), K. Königsmann ), I. Konorov 18,2), V.F. Konstantinov 24), A.S. Korentchenko 8), A. Korzenev 16,c), A.M. Kotzinian 8,29), N.A. Koutchinski 8), O. Kouznetsov 8,25), A. Kral 23), N.P. Kravchuk 8), Z.V. Kroumchtein 8), R. Kuhn 2), F. Kunne 25), K. Kurek 3), M.E. Laygin 24), M. Lamanna 11,28), J.M. Le Goff 25), A.A. Lenev 24), A. Lehmann 9), J. Lichtenstat 26), T. Liska 23), I. Luwig ), A. Maggiora 29), M. Maggiora 29), A. Magnon 25), G.K. Mallot 11), A. Mann 2), C. Marchan 25), J. Marroncle 25), A. Martin 28), J. Marzec 31), F. Massmann 3), T. Matsua 17), A.N. Maimov 8), W. Meyer 2), A. Mielech 28,3), Yu.V. Mikhailov 24), M.A. Moinester 26), A. Mutter,16), A. Nagaytsev 8), T. Nagel 2), O. Nähle 3), J. Nassalski 3), S. Neliba 23), F. Nerling ), S. Neubert 2), D.P. Neyret 25), V.I. Nikolaenko 24), K. Nikolaev 8), A.G. Olshevsky 8), M. Ostrick 4), A. Paee 31), P. Pagano 28), S. Panebianco 25), R. Panknin 4), D. Panzieri 29,b), S. Paul 2), B. Pawlukiewicz-Kaminska 3), D.V. Peshekhonov 8), V.D. Peshekhonov 8), G. Piragino 29), S. Platchkov 25), J. Pochozalla 16), J. Polak 14), V.A. Polyakov 24), J. Pretz 4), S. Procureur 25), C. Quintans 15), J.-F. Rajotte 19), S. Ramos 15,a), V. Rapatsky 8), G. Reicherz 2), A. Richter 9),F. Robinet 25), E. Rocco 28,29), E. Ronio 3), A.M. Rozhestvensky 8), D.I. Ryabchikov 24), V.D. Samoylenko 24), A. Sanacz 3), H. Santos 15), M.G. Sapozhnikov 8), S. Sarkar 7), I.A. Savin 8), P. Schiavon 28), C. Schill ), L. Schmitt 2), P. Schönmeier 9), W. Schröer 9), O.Yu. Shevchenko 8), H.-W. Siebert 12,16), L. Silva 15), L. Sinha 7), A.N. Sissakian 8), M. Slunecka 8), G.I. Smirnov 8), S. Sosio 29), F. Sozzi 28), A. Srnka 5), F. Stinzing 9), M. Stolarski 3,), V.P. Sugonyaev 24), M. Sulc 14), R. Sulej 31), V.V. Tchalishev 8), S. Tessaro 28), F. Tessarotto 28), A. Teufel 9), L.G. Tkatchev 8), G. Venugopal 3), M. Virius 23), N.V. Vlassov 8), A. Vossen ), R. Webb 9), E. Weise 3), Q. Weitzel 2), R. Winmolers 4), S. Wirth 9), W. Wiślicki 3), K. Zaremba 31), M. Zavertyaev 18), E. Zemlyanichkina 8), J. Zhao 16), R. Ziegler 3), an A. Zvyagin 19)

3 1) Universität Bielefel, Fakultät für Physik, 3351 Bielefel, Germany e) 2) Universität Bochum, Institut für Eperimentalphysik, 4478 Bochum, Germany e) 3) Universität Bonn, Helmholtz-Institut für Strahlen- un Kernphysik, Bonn, Germany e) 4) Universität Bonn, Physikalisches Institut, Bonn, Germany e) 5) Institute of Scientific Instruments, AS CR, Brno, Czech Republic f) 6) Burwan University, Burwan 7134, Inia h) 7) Matrivani Institute of Eperimental Research & Eucation, Calcutta-7 3, Inia i) 8) Joint Institute for Nuclear Research, Dubna, Moscow region, Russia 9) Universität Erlangen Nürnberg, Physikalisches Institut, 954 Erlangen, Germany e) ) Universität Freiburg, Physikalisches Institut, 794 Freiburg, Germany e) 11) CERN, 1211 Geneva 23, Switzerlan 12) Universität Heielberg, Physikalisches Institut, 6912 Heielberg, Germany e) 13) Helsinki University of Technology, Low Temperature Laboratory, 215 HUT, Finlan an University of Helsinki, Helsinki Institute of Physics, 14 Helsinki, Finlan 14) Technical University in Liberec, Liberec, Czech Republic f) 15) LIP, 49 Lisbon, Portugal g) 16) Universität Mainz, Institut für Kernphysik, 5599 Mainz, Germany e) 17) University of Miyazaki, Miyazaki , Japan j) 18) Lebeev Physical Institute, Moscow, Russia 19) Luwig-Maimilians-Universität München, Department für Physik, 8799 Munich, Germany e) 2) Technische Universität München, Physik Department, Garching, Germany e) 21) Nagoya University, 464 Nagoya, Japan j) 22) Charles University, Faculty of Mathematics an Physics, 18 Prague, Czech Republic f) 23) Czech Technical University in Prague, Prague, Czech Republic f) 24) State Research Center of the Russian Feeration, Institute for High Energy Physics, Protvino, Russia 25) CEA DAPNIA/SPhN Saclay, Gif-sur-Yvette, France 26) Tel Aviv University, School of Physics an Astronomy, Tel Aviv, Israel k) 27) ICTP INFN MLab Laboratory, 3414 Trieste, Italy 28) INFN Trieste an University of Trieste, Department of Physics, Trieste, Italy 29) INFN Turin an University of Turin, Physics Department, 125 Turin, Italy 3) So ltan Institute for Nuclear Stuies an Warsaw University, -681 Warsaw, Polan l) 31) Warsaw University of Technology, Institute of Raioelectronics, -665 Warsaw, Polan m) 32) Yamagata University, Yamagata, Japan j) +) Decease a) Also at IST, Universiae Técnica e Lisboa, Lisbon, Portugal b) Also at University of East Piemont, 15 Alessanria, Italy c) On leave of absence from JINR Dubna ) Also at Chubu University, Kasugai, Aichi, Japan e) Supporte by the German Bunesministerium für Bilung un Forschung f) Suppporte by Czech Republic MEYS grants ME492 an LA242 g) Supporte by the Portuguese FCT - Funação para a Ciência e Tecnologia grants POCTI/FNU/4951/22 an POCTI/FNU/5192/23 h) Supporte by DST-FIST II grants, Govt. of Inia i) Supporte by the Shailabala Biswas Eucation Trust j) Supporte by the Ministry of Eucation, Culture, Sports, Science an Technology, Japan; Daikou Founation an Yamaa Founation k) Supporte by the Israel Science Founation, foune by the Israel Acaemy of Sciences an Humanities l) Supporte by KBN grant nr 621/E-78/SPUB-M/CERN/P-3/DZ 298 2, nr 621/E-78/SPB/CERN/P- 3/DWM 576/236, an by MNII reasearch funs for m) Supporte by KBN grant nr 134/E-365/SPUB-M/CERN/P-3/DZ299/2 1

4 The COMPASS eperiment at CERN has recently publishe an evaluation of the euteron spin-epenent structure function g1 () in the DIS region, base on measurements of the spin asymmetries observe in the scattering of 16 GeV longituinally polarise muons on a longituinally polarise 6 LiD target [1]. These measurements provie an accurate evaluation of the first moment of g 1 for the average nucleon N in an isoscalar target g1 N = (gp 1 + gn 1 )/2 Γ N 1 (Q2 = (GeV/c) 2 ) = 1 g N 1 (, Q2 = (GeV/c) 2 ) =.51 ±.3 (stat.) ±.6 (syst.) (1) from which the first moment of the strange quark istribution can be etracte if the value of the octet matri element (a 8 = 3 F D) is taken from semi-leptonic hyperon ecays. 1) At LO in QCD s + s = 3 Γ N a 8 =.9 ±.1 (stat.) ±.2 (syst.) (2) at Q 2 = (GeV/c) 2. Since quarks an antiquarks of the same flavour equally contribute to g 1, inclusive ata o not allow to separate valence an sea contributions to the nucleon spin. We present here aitional information on the contribution of the nucleon constituents to its spin, base on semi-inclusive spin asymmetries measure on the same ata as those use in Ref. [1]. The semi-inclusive spin asymmetries for positive an negative harons h + an h are efine by A h+ = σh+ σ h+ σh+ + σh+, A h = σh σ h σh + σh, (3) where the arrows inicate the relative beam an target spin orientations. The ata use in the present analysis were collecte by the COMPASS collaboration at CERN uring the years The event selection requires a reconstructe interaction verte efine by the incoming an scattere muons an locate insie one of the two target cells [2]. The energy of the beam muon is require to be in the interval 14 < E µ < 18 GeV an its etrapolate trajectory is require to cross entirely the two cells in orer to equalise the flues seen by each of them. DIS events are selecte by cuts on the photon virtuality (Q 2 > 1 (GeV/c) 2 ) an on the fractional energy of the virtual photon (.1 < y <.9). The harons use in the analysis are require to originate from the interaction verte an to be prouce in the current fragmentation region. The latter requirement is satisfie by selecting harons with fractional energy z >.2. In aition an upper limit z <.85 is impose in orer to suppress harons from eclusive iffractive processes an to avoi contamination from wrongly ientifie muons. The resulting sample contains 3 an 25 million of positive an negative harons, respectively. The haron ientification provie by the RICH etector is not use in the present analysis. The target spins are reverse at regular intervals of 8 hours uring the ata taking. The spin asymmetries are obtaine from the numbers of harons collecte from each target cell uring consecutive perios before an after reversal of the target spins, following the same proceure as for inclusive asymmetries [3]. They are liste in Table 1 an also shown in Fig. 1 as a function of, in comparison with the SMC [4, 5] an HERMES [6] results. The results from the three eperiments are consistent. The COMPASS results show a large gain in statistical precision with respect to SMC, especially in the low region ( <.4), while at larger the COMPASS errors are comparable to those of HERMES. The systematic errors, shown by the bans at the bottom of the figure, result from ifferent sources. The uncertainty on the various factors entering in the asymmetry calculation (beam an target polarisation, epolarisation factor an ilution factor) leas to a relative error of 8% on the asymmetry when combine in quarature. The uncertainty ue to raiative corrections is smaller than in the inclusive case ue to the selection of haronic 1) At the precision of the eperiment the value of Γ N 1 is unchange when the evolution of the measure values g 1( i, Q 2 i ) to a common Q 2 is one at LO or at NLO in QCD. 2

5 h+ A h- A COMPASS HERMES SMC COMPASS HERMES SMC Figure 1: Haron asymmetries A h+ (left) an A h (right) measure by COMPASS, SMC [5] an HERMES [6] eperiments. The bans at the bottom of the figures show the systematic errors of the COMPASS measurements. Q 2 A h+ A h (GeV/c) ±.12 ±.6.2 ±.12 ± ±.8 ±.4.8 ±.8 ±.4.81 ±.138 ± ±.7 ±.3.9 ±.7 ±.4.7 ±.67 ± ±.11 ±.5.14 ±.12 ±.6.27 ±.77 ± ±.15 ±.8.12 ±.16 ±.8.7 ±.9 ± ±.14 ±.7.25 ±.16 ±.8.6 ±.76 ± ±.16 ±.9.33 ±.18 ± ±.7 ± ±.24 ± ±.28 ±.16.7 ±.87 ± ±.37 ± ±.45 ±.25.9 ±.121 ± ±.44 ±.29.9 ±.54 ± ±.13 ± ±.81 ± ±.1 ± ±.217 ± ±.121 ± ±.15 ± ±.291 ±.186 A h+ h Table 1: Values of A h+, Ah an A h+ h with their statistical an systematical errors as a function of with the corresponing average value of Q 2. events an oes not ecee 3 in any bin. The presence of possible false asymmetries ue to time-epenent apparatus effects has been stuie in the same way as for the inclusive asymmetries: the ata sample has been ivie into a large number of subsamples, each of them collecte in a small time interval. The observe ispersion of the asymmetries obtaine for these subsamples has been foun compatible with the value epecte from their statistical error. This allows to set an upper limit for this type of false asymmetries at about half of the statistical error. Asymmetries, obtaine with ifferent settings of the microwave frequency use for ynamic nuclear polarisation of the target, have also been compare an i not reveal any systematic ifference. In orer to avoi possible (, z) correlate acceptance effects, the asymmetries have also been calculate in three ifferent intervals of z for each bin of. No significant z epenence is observe an the weighte averages in each bin of are consistent with the quote values. In the present analysis we use the ifference asymmetry which is efine as the spin asymmetry for the ifference of the cross sections for positive an negative harons: A h+ h = (σh+ (σ h+ σh ) (σh+ σh σh ) + (σh+ ) σh ). (4) 3

6 The ifference asymmetry approach for the etraction of helicity istributions was introuce in [7] an further iscusse in [8]. For the first time it was use by SMC [4]. In LO QCD, uner the assumption of isospin symmetry an charge conjugation symmetry, fragmentation functions cancel out from A h+ h. In aition, in the case of an isoscalar target the ifference asymmetries for pions an kaons are both equal to the valence quark polarisation A h+ h N = A π+ π N = A K+ K N = u v + v u v + v, (5) where we introuce the valence quark istributions q v = q q. Since kaons contribute to the asymmetry in the same way as pions, the use of haron ientification is not neee, allowing to reuce the statistical errors. It is worth noting that the ifference asymmetry for (anti)protons,, has the same value uner slightly more restrictive assumptions. At higher orer in QCD the ifference asymmetries still etermine the valence quark polarisation without any assumption on the sea an gluon ensities [8]. Fragmentation functions no longer cancel out but their effect is epecte to be small [9]. The relation between the ifference asymmetries of Eq. (4) an the single haron asymmetries of Eq. (3) is A p p N A h+ h = 1 1 r (Ah+ ra h ), with r = σh σ h+ + σh + σh+ = σh. (6) σh+ The ratio of cross sections for negative an positive harons, r, epens on the event kinematics an is obtaine as the prouct of the corresponing ratio of the number of observe harons N /N + by the ratio of the geometrical acceptances a + /a : r = σh σ h+ = N N + a+ a. (7) Figure 2 (left) shows the ratio of the number of negative to positive harons which ecreases with increasing. This ratio is subject to acceptance corrections because positive an negative harons, prouce at the same angle, cross ifferent regions of the spectrometer. To this en LEPTO generate Monte Carlo events have been processe through the program simulating the COMPASS spectrometer performance [2] an reconstructe in the same way as the ata. The acceptances a + an a are inee foun to be ifferent: the ratio a /a + which is about 1. at low, increases for >.1 reaching 1.12 in the highest bin. The correcte cross section ratio σ h /σ h+ is also shown in Fig. 2. The resulting values of the ifference asymmetry A h+ h as a function of are shown in Fig. 2 (right) an liste with their statistical an systematic errors in Table 1. The statistical correlation between A h+ an A h which is approimately.2 over the measure range of, is taken into account in the evaluation of the error of A h+ h. As can be seen from Eq. (6), a singularity appears when the cross section ratio becomes close to one, leaing to infinite statistical errors. For this reason, we iscar the lowest bin use in the inclusive g 1 analysis [1] an take =.6 as lower limit for the present analysis. The increase of A h+ h for >.1 illustrates the increasing polarisation of valence quarks carrying a larger fraction of the nucleon momentum. The polarise valence quark istribution u v + v is obtaine by multiplying A h+ h by the unpolarise valence istribution of MRST4 at LO []. Here two corrections are applie, one accounting for the fact that although R(, Q 2 ) = at LO, the unpolarise parton istribution functions (pfs) originate from F 2 s in which R = σ L /σ T was ifferent from zero [11], the other one accounting for euteron D-state contribution (ω D =.5 ±.1 [12]): 4 u v + v = (u v + v ) MRST (1 + R)(1 1.5ω D ) Ah+ h. (8)

7 1 h+ - h- A N /N unmeasure σ h- /σ h Figure 2: Left: The ratio σ h /σ h+ before (triangles) an after acceptance corrections (circles). Right: The ifference asymmetry, A h+ h, for unientifie harons of opposite charges, as a function of at the Q 2 of each measure point. The LO parameterisation of the DNS fit[13] has been use to evolve all values of u v + v to a common Q 2 fie at (GeV/c) 2. The DNS analysis inclues all DIS g 1 ata prior to COMPASS, the partial COMPASS ata on g 1 from Ref. [3] as well as the SIDIS ata from SMC [5] an HERMES [6]. Two parameterisations of polarise pfs are provie at LO, corresponing to two ifferent choices of fragmentation functions, KRE [14] an KKP [15]. We have checke that the epenence of the ratio σ h /σ h+ (Fig. 2) is fairly well reprouce by the LO MRST4 pfs an the KKP fragmentation functions whereas the KRE parameterisation leas to a much weaker epenence. For this reason we choose the fit with the KKP parameterisation. The resulting values are shown in Fig. 3 (left). The DNS fit, also shown in the figure, is basically efine by the SMC an HERMES semi-inclusive asymmetries. Its goo agreement with the COMPASS values (χ 2 = 7.7 for 11 ata points) illustrates the consistency between the three eperiments. The sea contribution to the unpolarise structure function F 2 ecreases rapily with increasing an becomes smaller than.1 for >.3. Due to the positivity conitions q q an q q, the polarise sea contribution to the nucleon spin also becomes negligible in this region. In view of this, the evaluation of the valence spin istribution of Eq. (8) can be replace by a more accurate one obtaine from inclusive interactions, inee at LO u v + v = 36 5 g1 ( (1 1.5ω D ) 2( ū + ) ( s + s) ). (9) The values obtaine by taking only the first term on the r.h.s. for >.3 are also shown in Fig. 3. They agree very well with the DNS curve, which is base on previous eperiments where the same proceure ha been applie [5, 6]. The neglecte sea quark contributions are taken into account in the systematic error. The first moment of the polarise valence istribution, truncate to the measure range of Γ v ( min ) =.7 min ( u v () + v ()), () erive from the ifference asymmetry for <.3 an from g1 for.3 < <.7, is shown in Fig. 3 (right). Practically no epenence on the lower limit is observe for min <.3. We obtain for the full measure range of Γ v (.6 < <.7) =.4 ±.7 (stat.) ±.5 (syst.) (11) 5

8 -range Q 2 u v + v ū + (GeV/c) 2 Ep.Value DNS Ep.Value DNS SMC ±.21 ± ±.8 ±.6.9 HERMES ±.7 ± ±.4 ±.3.5 COMPASS ±.7 ± ±.7 ±.5. ±.4 ±.3 Table 2: Estimates of the first moments u v + v an ū+ from the SMC [5], HERMES [6], COMPASS ata an also from the DNS fit at LO [13] truncate to the range of each eperiment (lines 1 3). The SMC results were obtaine with the assumption of a SU(3) f symmetric sea: ū = = s. The last line shows the COMPASS results for the full range of. at Q 2 = (GeV/c) 2, with contributions of.26 ±.7 an.14 ±.1 for <.3 an >.3, respectively. It shoul be note that removing the factor (1 + R) in Eq. (8) woul increase the value of Γ v to.42 ±.8 ±.6. Our value of Γ v confirms the HERMES result obtaine at Q 2 = 2.5 (GeV/c) 2 over a smaller range of an is also consistent with the SMC result which has three times larger errors (Table 2). The factor (1 + R) was also use in the previous eperiments. The ifference between our measure value of Γ v (.6 < <.3) an the integral of g N 1 over the same range of gives a global measurement of the polarise sea. Inee, re-orering Eq. (9) we obtain.3.6 (( u + ) ( s + s) ) =.2 ±.3 (stat.) ±.2 (syst.), (12) where the correlation between inclusive an semi-inclusive asymmetries has been taken into account in the statistical error. This result is compatible with zero but also consistent with the strange quark contribution of Eq. (2) an a vanishing contribution from the light quarks. It shoul be kept in min that moments of sea quarks evaluate at LO have to be taken with caution because their values are small an thus comparable to the NLO corrections. The unmeasure contribution to Γ v for >.7 estimate from the LO DNS parameterisation of Ref. [13] is.4 at Q 2 = (GeV/c) 2. Its upper limit corresponing to the assumption A h+ h = 1 for >.7 is.7 accoring to the MRST4 parameterisation. The unmeasure low contribution to Γ v is epecte to be negligible since the integral shows not significant variation when its lower limit is varie between.6 an.2. We thus estimate the first moment Γ v ( < < 1) =.41 ±.7 (stat.) ±.5 (syst.). (13) The assumption of a fully flavour symmetric sea u = = s = s obviously leas to Γ v ( < < 1) = a 8. As shown in Fig. 3 (right), our eperimental value is two stanar eviations below the value of a 8 = 3 F D =.58 ±.3 erive from hyperon β ecays [16]. It has been suggeste that a value of the valence contribution Γ v smaller than a 8 (as epecte from the constituent quark moels) coul be a hint that a so far unmeasure part of the nucleon s spin resies at = [17]. An estimate of the light sea quark contribution to the nucleon spin can be obtaine by combining the values of Γ v (Eq. (13)), Γ N 1 (Eq. (1)) an a 8 u + = 3Γ N Γ v a 8 (14) an the result is foun to be zero (Table 2). Possible eviations from the nominal value of a 8 ue to SU(3) f symmetry violation in hyperon ecays are generally assume to be of the orer of % [18] an are inclue in the systematic error. The zero value of u + is in contrast with 6

9 ( u v + v ).5 From iff asymmetry From Inclusive g 1.4 DNS fit (without COMPASS) min.7 ( u v + v ).6 ( u= = s= s) ( u=- ).2.1 unmeasure min Figure 3: Left: Polarise valence quark istribution ( u v () + v ()) evolve to Q 2 = (GeV/c) 2 accoring to the DNS fit at LO [13]. The line shows the DNS fit which oes not inclue the present COMPASS ata. Three aitional points at high are obtaine from g1 [1]. Right: The integral of u v()+ v () over the range.6 < <.7 as the function of the low limit of integration min, evaluate at Q 2 = (GeV/c) 2. SIDIS ata are use in the interval.6 < <.3 an inclusive g1 ata from Ref. [1] in the interval.3 < <.7. the non-zero value obtaine for s + s (Eq. (2)) an suggests that u an, if ifferent from zero, must be of opposite sign. Previous estimates by SMC an HERMES, also given in Table 2, are compatible with this hypothesis. The DNS parameterisation fins a positive u an a negative, about equal in absolute value. Opposite signs of u an are also obtaine in the statistical moel of Ref. [19]. Forthcoming COMPASS ata on a proton target will provie separate eterminations of u an. In conclusion, we have etermine at LO QCD the polarise valence quark istribution from the ifference asymmetry for oppositely charge harons in DIS of muons on a polarise isoscalar target. Its first moment at Q 2 = (GeV/c) 2 over the measure range of (.6.7) is foun to be.4 ±.7 (stat.) ±.5 (syst.). This value isfavours at a two σ level the assumption of a flavour symmetric polarise sea an suggest that u an are most likely of opposite sign. Acknowlegements We gratefully acknowlege the support of the CERN management an staff an the skill an effort of the technicians of our collaborating institutes. Special thanks are ue to V. Anosov an V. Pesaro for their technical support uring the installation an the running of this eperiment. This work was mae possible by the financial support of our funing agencies. References [1] COMPASS Collaboration, V.Yu. Aleakhin et al., Phys. Lett. B 647 (27) 8. [2] COMPASS Collaboration, P. Abbon et al., Nucl. Instrum. Methos A 577 (27) 455. [3] COMPASS Collaboration, E.S. Ageev et al., Phys. Lett. B 612 (25) 154. [4] SMC Collaboration, B. Aeva et al., Phys. Lett. B 369 (1996) 93. [5] SMC Collaboration, B. Aeva et al., Phys. Lett. B 42 (1998) 18. [6] HERMES Collaboration, A. Airapetian et al., Phys. Rev. D 71 (25) 123. [7] L.L. Frankfurt et al., Phys. Lett. B 23 (1989) 141. [8] E. Christova, E. Leaer, Nucl. Phys. B 67 (21) 369. [9] A.N. Sissakian, O.Yu. Shevchenko, O.N. Ivanov, Phys. Rev. D 73 (26) [] A.D. Martin, W.J. Stirling, R.S. Thorne, Phys.Lett. B 636 (26) 259. [11] E143 Collaboration, K. Abe et al., Phys. Lett. B 452 (1999)

10 [12] R. Machleit et al., Phys. Rep. 149 (1987) 1. [13] D. e Florian, G.A. Navarro, R. Sassot, Phys. Rev. D 71 (25) [14] S. Kretzer, Phys. Rev. D 62 (2) 541. [15] B.A. Kniehl, G. Kramer, B. Potter, Nucl. Phys. B 582 (2) 514. [16] F.E. Close, R.G. Roberts, Phys. Lett. B 316 (1993) 165. [17] S. D. Bass, Eur. Phys. J. A5 (1999) 17; S. D. Bass, Rev. Mo. Phys. 77 (25) [18] E. Leaer, D. Stamenov, Phys. Rev. D 67 (23) [19] C. Bourrely, F. Buccella, J. Soffer, Eur. Phys. J. C 41 (25)