Diffractive Exchange, Leading Baryons


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Structure of Diffractive Exchange

    About 10 percent of the deep inelastic ep scattering is due to diffractive processes where no quantum numbers are exchanged. Signature for such processes is for example a region of the polar angle theta which is free of hadron production, a so called rapidity gap. The aim of analyzing diffractive events is to understand these processes within the framework of QCD. Owing to the color charge of quarks and gluons, QCD demands exchange of at least two quarks or gluons. 
The figure shows the lepton-proton structure function for diffractive events as a function of the virtuality Q2 of the photon. The observable beta denotes the momentum of the scattered quark measured relative to a colour neutral quark- gluon state which was emitted by the proton. At small fractional energy x_IP of this colour neutral state relative to the proton beam energy this state is called pomeron.  
    The measured Q2 dependence of the structure function is consistent with flat. At the large quark momentum shown, beta=0.4, the structure function looks different from that of a hadron, for example that of the proton.

    The dependence of a structure function on the virtuality Q2 of the probing photon shows the contributions of quarks and gluons and is explained by the QCD evolution equations:

    d F_2/d ln Q^2 ~ P_qq f_q + P_qg f_g + P_qgamma

    The terms P_ij f_j denote the convolution of a splitting function P_ij with a parton density f_j. For example P_qg gives the probability of a gluon splitting into a quark-antiquark pair, and f_g denotes the gluon density. The third term of the equation is relevant for the case of the photon structure function only. 

Comparison with Proton Structure:

The proton structure function at x=0.4 decreases as Q2 increases. The probability to find a parton in the proton at large x decreases with increasing resolving power Q2. 

Comparison with Photon Structure:

The photon structure function at x=0.4 increases with Q2: the third term P_qgamma in the above equation denotes the splitting of the photon into a quark-antiquark pair. This splitting function is to first order independent of Q2 and generates the characteristic logarithmic increase of the photon structure function with Q2. 

    The flat shape of the structure function for diffractive processes implies that the exchanged object cannot be a hadron consisting of valence quarks. The magnitude of the measured structure function excludes diffraction to be caused by photon exchange. A consistent interpretation of the data is given by a gluon dominated object. While in the case of the photon structure function the splitting of a photon into a quark-antiquark pair is analysed, the measurement of the structure function for diffractive exchange gives information on the gluon splitting into a quark-antiquark pair and therefore gives information on the structure of the gluon (second term on the right hand side of the above equation). 

    The large gluon density of the exchanged object of diffractive processes was confirmed in numerous measurements of the hadronic final state. 

Structure Function with a Leading Baryon

    The cross section of deep inelastic lepton-proton scattering was measured with the additional requirement of a highly energetic proton or neutron. They were detected in the H1 proton spectrometer and the neutron calorimeter respectively. Both devices are close to the outgoing proton beam line. 
The figure shows the measurements of the ep structure function with a proton or neutron of fractional energy z relative to the proton beam energy. In the context of Regge phenomenology these measurements can be interpreted by effective particle exchange. The measured cross section for neutrons can be explained by positively charged pion exchange alone. 
The figure shows the interpretation of the neutron data in terms of the pion structure function. These data give for the first time access to parton momenta in the pion as small as 10^-3. 

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H1 webmaster on 19/05/98