SEMI-INCLUSIVE PION ELECTROPRODUCTION WITH CLAS Mikhail Osipenkoa 0 INFN, sez. di Genova, 16146 Genova, Italy, 1 SINP, Moscow State University, 119991 Moscow, Russia 0 Abstract. Measurementofthesemi-inclusiveπ+ electroproduction offtheproton, 2 performedwithCLASdetectoratJeffersonLab,hasbeenpresented. Theobtained n fully-differential cross sections, including the azimuthal angle between hadronic a andleptonic planes, φ,allowedustoseparate the φ-dependent terms. While,the J φ-independent part of the cross section was found to be in good agreement with currentfragmentationpQCDcalculations. 3 1 The semi-inclusive electroproduction of hadrons is an important tool for ] studyingthe nucleonstructureinthe perturbativeQuantumChromodynamics x (pQCD) framework at medium energies. Indeed, the detection of a hadron e - produced by the struck quark or by nucleon spectator fragments provides an p information about the orbital momentum of the quark in the initial state. e Meanwhile, the undetected hadronic system allows to apply the optical theo- h [ rem, reducing the energy necessary for the convergence towards basic pQCD processes. 2 CLAShasmeasuredthesemi-inclusiveelectroproductionofπ+ ontheproton v 9 atthebeamenergyof6GeV.Themeasurementspanoverawide,continuous5- 0 dimensionaldomain,whichallowsforadetailedstudyofφandpT behaviors. In 3 this article we will focus onone particular aspect of the obtained results. This 5 aspect deals with the comparison of the data, integrated in φ and transverse . 2 momentum pT, expressed in terms of the structure function H2: 1 9 d3σ 4πα2 M2x2y2 :0 dxdQ2dz = xQ4 "xy2H1(x,z,Q2)+ 1−y− Q2 H2(x,z,Q2)# , (1) v (cid:16) (cid:17) i X withpQCDcalculations[4]. Thesecalculationsdescribethestructurefunction r H2 as the convolution of the parton density function f(x,Q2) obtained in in- a clusive processes and the parton fragmentation function Dh(z,Q2) measured in e+e− collisions: H2(x,z,Q2)= e2ixfi(x,Q2)⊗Dih(z,Q2) , (2) i X wherethe sumrunsoverquarkflavorsi andei isthe chargeofithflavorquark (we neglect the gluon contribution here). Inparticular,weareinterestedinthedifferencebetweendata-theorycompar- isonsmadeusingthestructurefunctionH2 andstructurefunctionratioH2/F2. The latter ratio,where the F2 is the inclusive structure function, represents in ae-mail: [email protected] DIS limit the widely used multiplicity observable, and can be calculated in pQCD as following: H2(x,z,Q2) ie2ixfi(x,Q2)⊗Dih(z,Q2) = . (3) F2(x,Q2) P ie2ixfi(x,Q2) Giventhe relativelylow beamenergyoPf JeffersonLabone mayexpect a mani- festationofvisibledeviationsfrompQCDcalculationsatthelow-Q2 endofthe covered interval. Such deviations should indicate the contribution of higher twists in the semi-inclusive electroproduction. The detailed calculations of these higher twists are not available due to their complexity. However, one may phenomenologically divide them in two types: Initial State Interactions (ISI)ofthecurrentquarkandFinalStateInteractions(FSI)ofstruckquarkor produced hadron. If the dominant contribution to the total higher twist term would be due to ISI, one could expect a partial cancellation of them in H2/F2 ratio. Hence, it is possible that H2/F2 ratio agrees with pQCD calculations better that the H2 structure function alone. In Fig. 1 comparisons of the measured structure function H2 and H2/F2 ratiotoLOandNLOpQCDcalculationsareshownfortwovaluesofz. Asone can see, the NLO calculations describe very well the data at z =0.45 for both observables, while at z =0.11 some deviation in the Q2-slope is evident. This deviation can be due to the higher twist contribution. z=0.11 z=0.45 1 1 2 H -1 -1 10 10 10 10 2 F / 2 H 1 1 2 2.5 3 3.5 2 2.5 3 3.5 Q2 GeV2 Q2 GeV2 Figure1: TheQ2-evolutionofthedataonstructurefunctionH2andratioH2/F2atx=0.34 and two values of z, in comparison to pQCD calculations: LO - solid line, NLO - dashed line. ThecalculationsuseCTEQ5partondistributions[2],andKretzerfragmentationfunc- tions[3]. Theerrorbarsgivestatisticalandsystematicuncertaintiescombinedinquadrature. In Fig. 2 the same comparison is shown for the ratio of the data over NLO calculations. In the amplified scale the deviation of the low-z data from the expected NLO evolution can be quantified. Both H2 and H2/F2 comparisons showthedeviationrisingwithQ2from-10%upto40%. Thedifferencebetween H2 and H2/F2 ratios results in a few percent overall shift, well below the systematic uncertainties of the measurement (about 15% in average) and the theory. z=0.11 z=0.45 1.4 1.4 O L1.2 1.2 N / 1 1 2 H0.8 0.8 0.6 0.6 O1.4 1.4 L N1.2 1.2 / 2 1 1 F /0.8 0.8 2 H 0.6 0.6 2 2.5 3 3.5 2 2.5 3 3.5 Q2 GeV2 Q2 GeV2 Figure2: SameasFig.1exceptwiththeratioofH2andH2/F2datatotheNLOcalculations. The dashed line indicates the unity. The inner error bars (mostly smaller than the symbol size)givestatistical uncertainties. Summarizing, the use of H2 structure function or H2/F2 ratio in the com- parison of the experimental data to pQCD calculations is equivalent in JLab energy domain. If the deviations of the data from NLO pQCD calculations, observed at low-z, are due to the higher twist contribution, they are probably related to the FSI mechanism. However, given the low-z values at which the difference is observed, an alternative explanation due to the mixing between current and target fragmentation evolutions is more likely. The pQCD calcu- lations described above are due to the current fragmentation only. The target fragmentationmechanism[5],expectedtoplayroleinthelow-z domain,isstill poorly established. In particular, the corresponding partonic functions, frac- ture functions for the pions are completely unknown. This encourages further pQCD studies of semi-inclusive reactions at Jefferson Lab. Acknowledgments AuthorwouldliketoexpresshisgratitudetoProf. A.Kataevforkindinvitation and Prof.B.S.Ishkhanov for the help with the present proceeding. References [1] M.Osipenko et al., Phys.Rev. D 80, 032004(2009). [2] H.L.Lai et al., Eur.Phys.J. C 12, 375 (2000). [3] S.Kretzer, et al., Phys.Rev. D 62, 054001(2000). [4] W.Furmanski and R.Petronzio, Z.Phys. C 11, 293 (1982). [5] L.Trentadue and G.Veneziano, Phys.Lett. B 323, 201 (1994).