Charged hadron multiplicities at the HERMES experiment

The HERMES collaboration has measured charge-separated pion and kaon multiplicities in semi-inclusive deep inelastic scattering using a 27.6 GeV electron or positron beam scattering off a hydrogen or deuterium target. The results are presented as a function of the Bjorken variable x _B , the negative squared four-momentum transfer Q ², the hadron fractional energy z and it’s transverse momentum P _{h ⊥}. These data will be very useful to understand the quark-fragmentation process in deep-inelastic hadron electro-production and will serve as crucial input in the understanding of spin asymmetries in polarized semi-inclusive deep-inelastic scattering.     

Semi-inclusive π− cross sections in pp interactions up to 300 GeV/c

Semi-inclusive π^? cross sections in 200 and 300 GeV/c pp interactions are presented and compared with lower energy data. In the target fragmentation region the data are compared to the hypothesis of semi-inclusive KNO scaling. The energy and charge multiplicity dependences of the semi-inclusive cross sections in the central region are studied and a simple parametrization of the semi-inclusive density function is described.     

pp Interactions at 303 GeV/c: Production angle distribution

We have measured the projected production angles of all charged particles in more than 1600 pp interactions at 303 GeV/c. We have obtained semi-inclusive distributions of pseudo-rapidity and dispersion of pseudo-rapidity event-by-event. The mean dispersion is found to be approximately constant for charged multiplicity, n_ch, from 6 to 16.     

Inclusive photon and π0 production inK + p interactions at 70 GeV/c

Inclusive and semi-inclusive cross sections and distributions of γ's and π⁰'s inK ⁺ p interactions at 70 GeV/c are presented. The results are compared to other experiments and to the Lund model for low-p _T hadron collisions.     

Semiinclusive two-particle correlations and independent cluster emission at ISR energies

Semiinclusive two-particle correlations are calculated in the independent cluster emission model. The resulting semi-inclusive correlation functions ?₂ⁿ(y₁ = y₂ = 0) agree rather than well experimental data, esp. with respect to the multiplicity dependence. The average number of charged particles per cluster found from comparison with semiinclusive data is 2.5 charged particles per cluster, a value found earlier also from analysis of inclusive correlations.     

Scaling of semi-inclusive events in <Emphasis Type="Italic">pp</Emphasis> interactions

The normalized single-particle semi-inclusive double-differential spectrum of π^t- mesons from pp interactions at 6.6–400 GeV/c and the relative concentration of π⁰ and K _S ⁰ mesons in such events of fixed multiplicity of π^? mesons are completely determined by specifying any feature of this spectrum—for example, 〈y²〉 _n or 〈E〉 _n . Therefore, a two-parameter sample of semi-inclusive events that depends on the energy and the multiplicity reduces to a one-parameter sample.     

Symmetry aspects of the pion leptoproduction and the upper limit of the Levelt-Mulders asymmetry

We examined the symmetry aspect of the semi-inclusive one-pion production in the deep inelastic scattering of a lepton beam off an unpolarized nucleon target, with an emphasis on the positivity restrictions on the corresponding structure functions. In combination with the Callan-Gross-type relation between two twist-two structure functions W ₁ and W ₂, we derived an upper bound on the Levelt-Mulders asymmetry, which occurs when the lepton beam is longitudinally polarized.     

Analysis of the semi-inclusivep T distributions at the CERNp\bar p collider based on the two-temperature model

A two-temperature model is proposed for the semi-inclusive transverse momentum (p _T ) distributions at the CERN \(p\bar p\) collider. Our model accounts well characteristic multiplicity dependence of thep _T distributions. Energy dependence of thep _T distributions is investigated in comparison with the data at ISR energies. High temperature state becomes more important at high energy and multiplicity regions. The ratio between high and normal temperature is almost independent of energy and multiplicity.     

On the bin-wise and strip integrals of density function in high energy multiparticle production

It is shown through theoretical analysis and Monte Carlo simulation that when the translational invariance of semi-inclusive density function is destroyed the bin-wise and strip integrals of this function are inequivalent. The different experimental results about the anomalous scaling behaviour of factorial moment in the variables (y, p_⊥, φ) and that of strip integral in the variableQ ² is discussed briefly.     

Study of TMDs with polarized beam and/or targets

A Fourier analysis of single- and double-spin azimuthal asymmetries measured at HERMES in semi-inclusive deep-inelastic scattering of unpolarized or longitudinally polarized leptons off tranversely polarized protons is presented for pions and charged kaons. The extracted amplitudes can be interpreted as convolutions of transverse momentum-dependent distribution and fragmentation functions. In particular, the measured double-spin asymmetries are sensitive to the poorly known worm-gear quark distribution g _1T ^⊥      

Scaling in semi-inclusive experiments

In bubble chamber experiments (in contrast to counter experiments) the semi-inclusive experiment a+b→c+(n−1) charged + anything neutral is more natural than the corresponding inclusive experiment a+b→c+anything. We argue that the cross sections for semi-inclusive experiments have very neat scaling properties.     

Non-factorization of the semi-inclusive neutrino-proton cross section

In a neutrino-proton experiment performed with the bubble chamber BEBC, which was exposed to the wide-band neutrino beam of the CERN SPS, the semi-inclusive reaction νp→μ^?h^± + X is studied where h^± is a hadron of a chosen type. Non-factorization of the semi-inclusive cross section is investigated in terms of double moments with respect to Bjorken-x and the energy fraction z and comparisons are made with predictions of perturbative QCD in next-to-leading order.     

Flavor dependent azimuthal asymmetries in unpolarized semi-inclusive DIS at HERMES

The azimuthal cos? _h and cos2? _h modulations of the distribution of hadrons produced in unpolarized semi-inclusive deep-inelastic scattering of electrons and positrons off hydrogen and deuterium targets have been measured in the hermes experiment. For the first time these modulations were determined in a 4-Dimensional kinematic space for positively and negatively charged pions and kaons separately, as well as for unidentified hadrons. These azimuthal dependences are sensitive to the transverse motion and polarization of the quarks within the nucleon via, e.g., the Cahn, Boer-Mulders and Collins effects.     

The spin structure of the nucleon

An introduction to deep inelastic scattering experiments investigating the spin structure of the proton and the neutron is given. Discussed are measurements with longitudinally polarized targets to determine the spin structure functiong ₁ and sum rules and to do pQCD analyses with inclusive data, while a flavour separation and first extractions of the gluon polarization are performed with semi-inclusive data.     

Scaling of semi-inclusive cross sections with respect to longitudinal and transverse momenta

The semi-inclusive one-particle transverse and longitudinal momenta spectra of π^± mesons have been studied in the centre-of-mass system for π^?p interactions at 40 GeV/c. These data are compared with similar data from pp interactions at 300 GeV/c and lower energies. Distributions in the variables p_T/〈p_T〉 and p_L/〈p_L〉 reveal scaling properties, which are found to be substantially independent of the nature of the projectile, of the incident energy and of the prong multiplicity. These spectra are reasonably described by universal functions.     

Inclusive and semi-inclusive ϱ0 production in π−p interactions at 147 GeV/c

We present data on inclusive and semi-inclusive ?⁰ production in 147 GeV/c π^?p interactions. We find a total cross section of 7.3 ± 1.3 mb. Most of this cross section is found in the lower topology events (? 10 prongs), and in the central and forward rapidity regions. The P_T² dependence of ?⁰ production, 〈:n>_?0 per event, and the ?⁰/π⁺ ratios are also discussed.     

On a possible node in the Sivers and Qiu-Sterman functions

The possibility of a node in the x dependence of the Sivers and Qiu-Sterman functions is discussed in light of its importance for the experimental check of the overall sign change of the Sivers effect between semi-inclusive DIS and the Drell-Yan process. An x-dependent version of the Ehrnsperger-Schäfer-Greiner-Mankiewicz relation between the Qiu-Sterman function and a twist-3 part of g₂ is presented, which naturally suggests a node in the Qiu-Sterman function. This relation could be checked experimentally as well and could provide qualitative information on the gluonic field strength inside the proton. Satisfying the Burkardt sum rule by means of a node is briefly discussed and the importance of modelling the Sivers function including its full Wilson line is pointed out.     

Spin dependent parton densities

We propose a set of polarised-beam polarisedtarget experiments to determine the complete set of spin dependent parton densities. The polarised valence quark densities are determined through semi-inclusive hadroproduction in polarised muon-proton collisions. This, together with present data then fixes the polarised sea quark densities. Polarised gluon densities are measurable through asymmetries in large,p _T direct photon production in polarisedp-p collisions. An independent measurement of polarised sea quark densities comes from direct double photon production in polarisedp-p collisions at rapidities close to zero. We illustrate these results with numerical estimates.     

Nuclear stopping power in αα anddd collisions

Inelasticαα anddd collisions were studied at a centre-of-mass energy √S _NN=31.2 GeV per nucleon-nucleon collision, using the Split-Field Magnet (SFM) detector at the CERN ISR. In this paper we show the inclusive and semi-inclusive rapidity distributions of protons, compare them with predictions of the Lund model, and calculate the average rapidity loss for participant protons. From the negative particles we calculate the inelasticity of the interaction, the average energy per particle, and the degree of isotropy of the produced hadrons.     

Higher-twist effects in deep-inelastic scattering

Non-scaling, non-factorizing 1/Q² contributions to cross sections are derived for semi-inclusive deep-inelastic scattering, ?N → ?′πX. These higher twist terms are dominant at large z = p_π · p_N/Q · p_N. They provide unusual (1 ? y) terms in the cross section. as well as asymmetries in the azimuthal angle dependence. Calculations are also presented for the quark to pion fragmentation function D_π(z, Q²).