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Experimental determination of Parton Distributions

With HERA currently in its second stage of operation, it is possible to assess the potential precision limits of HERA data and to estimate the potential impact of the measurements which are expected at HERA-II, in particular with respect to the PDF uncertainties. Precision limits of the structure function analyses at HERA are examined in [1]. Since large amounts of luminosity are already collected, the systematic uncertainty becomes most important. A detailed study of error sources with particular emphasis on correlated errors for the upcoming precision analysis of the inclusive DIS cross section at low Q2 using 2000 data taken by the H1 experiment is presented. A new tool, based on the ratio of cross sections measured by different reconstruction methods, is developed and its ability to qualify and unfold various correlated error sources is demonstrated. An important issue is the consistency of the HERA data. In section 3, the H1 and ZEUS published PDF analyses are compared, including a discussion of the different treatments of correlated systematic uncertainties. Differences in the data sets and the analyses are investigated by putting the H1 data set through both PDF analyses and by putting the ZEUS and H1 data sets through the same (ZEUS) analysis, separately. Also, the HERA averaged data set (section 4) is put through the ZEUS PDF analysis and the result is compared to that obtained when putting the ZEUS and H1 data sets through this analysis together, using both the Offset and Hessian methods of treating correlated systematic uncertainties. The HERA experimental data can not only be cross checked with respect to each other but also combined into one common dataset, as discussed in section 4. In this respect, a method to combine measurements of the structure functions performed by several experiments in a common kinematic domain is presented. This method generalises the standard averaging procedure by taking into account point-to-point correlations which are introduced by the systematic uncertainties of the measurements. The method is applied to the neutral and charged current DIS cross section data published by the H1 and ZEUS collaborations. The averaging improves in particular the accuracy due to the cross calibration of the H1 and ZEUS measurements. The flavour decomposition of the light quark sea is discussed in [2]. For low x and thus low Q2 domain at HERA only measurement of the photon exchange induced structure functions F2 and FL is possible, which is insufficient to disentangle individual quark flavours. A general strategy in this case is to assume flavour symmetry of the sea. [2] considers PDF uncertainties if this assumption is released. These uncertainties can be significantly reduced if HERA would run in deuteron-electron collision mode. The impact of projected HERA-II data on PDFs is estimated in section 7. In particular, next-toleading order (NLO) QCD predictions for inclusive jet cross sections at the LHC centre-of-mass energy are presented using the estimated PDFs. A further important measurement which could improve understanding of the gluon density at low x and, at the same time, provide consistency checks of the low Q2 QCD evolution is the measurement of the longitudinal structure function FL. Perspectives of this measurement are examined in section 5, while the impact of this measurement is also estimated in section 7. Further improvements for consistently including final-state observables in global QCD analyses are discussed in section 8. There, a method for “a posteriori” inclusion of PDFs, whereby the Monte Carlo run calculates a grid (in x and Q) of cross section weights that can subsequently be combined with an arbitrary PDF. The procedure is numerically equivalent to using an interpolated form of the PDF. The