From kroll@physik.uni-wuppertal.de Tue Jan 4 13:33:07 2005 Date: Tue, 4 Jan 2005 12:23:41 +0100 (CET) From: Peter Kroll To: xiaochao@jlab.org Subject: PR 05-007 Dear Xiaochao, I have read your proposal and thing that this is an important attempt to measure some of the fundamental parameters of the standard model. My concern is the estimate of size of the higher twist effects and its influence on the extraction of SM parameters. It is clear that as yet there is no theory of higher-twist effects. All estimates are based on often very old models. Thus, there is a potential danger in the interpretation of your results. In my opinion it would be helpful if you could clearly identify a higher-twist effect in your experiment, for instance by seing its 1/Q^2 dependence. If I understand your proposal correctly, you don't expect to see a higher-twist effect at Q^2=1.9 GeV^2. Thus, you are left with a measurement of it at only one value of Q^2. A question of minor importance: what is the definition of u_V(x) and u_sea(x) below (13)? Best regards, Peter Kroll *********************************************************************** Peter Kroll Tel: +49 202 4392620 Fachbereich Physik FAX: +49 202 4393860 Universitaet Wuppertal E-mail: kroll@physik.uni-wuppertal.de Gaussstrasse 20 D-42097 Wuppertal **************************************************************************