Parton Distributions

I present an overview of some current topics in the measurement of Parton Distribution Functions.Comment: 13 pages, 9 figures. Plenary talk presented at the XIII International Workshop on Deep Inelastic Scattering (DIS 2005), Madison WI USA, April 27--May 1, 200

Improving the Measurement of the Top Quark Mass

Two possible ways to improve the mass resolution for observing hadronic top quark decay $t \to bW \to 3 jets$ are studied: (1) using fixed cones in the rest frames of the $t$ and $W$ to define the decay jets, instead of the traditional cones in the rest frame of the detector; and (2) using the jet angles in the top rest frame to measure $m_t/m_W$. By Monte Carlo simulation, the second method is found to give a useful improvement in the mass resolution. It can be combined with the usual invariant mass method to get an even better mass measurement. The improved resolution can be used to make a more accurate determination of the top quark mass, and to improve the discrimination between $t \bar t$ events and background for studies of the production mechanism.Comment: Revised and expanded. New and better method introduced. Some conclusions changed. 17 pages, RevTeX, 4 uuencoded figure

PDF uncertainties: A strong test of goodness of fit to multiple data sets

We present a new criterion for the goodness of global fits. It involves an exploration of the variation of \chi^2 for subsets of data.Comment: 4 pages, 1 figure. To appear in Proceedings of 9th International Workshop on Deep Inelastic Scattering and QCD (DIS 2001), Bologna, Italy, 27 Apr-1 May 200

Data set diagonalization in a global fit

The analysis of data sometimes requires fitting many free parameters in a theory to a large number of data points. Questions naturally arise about the compatibility of specific subsets of the data, such as those from a particular experiment or those based on a particular technique, with the rest of the data. Questions also arise about which theory parameters are determined by specific subsets of the data. I present a method to answer both of these kinds of questions. The method is illustrated by applications to recent work on measuring parton distribution functions.Comment: Published versio

Experimental consistency in parton distribution fitting

The recently developed "Data Set Diagonalization" method (DSD) is applied to measure compatibility of the data sets that are used to determine parton distribution functions (PDFs). Discrepancies among the experiments are found to be somewhat larger than is predicted by propagating the published experimental errors according to Gaussian statistics. The results support a tolerance criterion of $\Delta\chi^2 \approx 10$ to estimate the 90% confidence range for PDF uncertainties. No basis is found in the data sets for the much larger $\Delta\chi^2$ values that are in current use; though it will be necessary to retain those larger values until improved methods can be developed to take account of systematic errors in applying the theory. The DSD method also measures how much influence each experiment has on the global fit, and identifies experiments that show significant tension with respect to the others. The method is used to explore the contribution from muon scattering experiments, which are found to exhibit the largest discrepancies in the current fit.Comment: 30 pages; 7 figure

Stability of NLO Global Analysis and Implications for Hadron Collider Physics

The phenomenology of Standard Model and New Physics at hadron colliders depends critically on results from global QCD analysis for parton distribution functions (PDFs). The accuracy of the standard next-to-leading-order (NLO) global analysis, nominally a few percent, is generally well matched to the expected experimental precision. However, serious questions have been raised recently about the stability of the NLO analysis with respect to certain inputs, including the choice of kinematic cuts on the data sets and the parametrization of the gluon distribution. In this paper, we investigate this stability issue systematically within the CTEQ framework. We find that both the PDFs and their physical predictions are stable, well within the few percent level. Further, we have applied the Lagrange Multiplier method to explore the stability of the predicted cross sections for W production at the Tevatron and the LHC, since W production is often proposed as a standard candle for these colliders. We find the NLO predictions on sigma_W to be stable well within their previously-estimated uncertainty ranges.Comment: 24 pages, 11 figures. Minor changes in response to JHEP referee repor