Forward and diffractive physics is part of the LHC program and it covers a wide range of physics subjects, including low-x QCD, diffraction dissociation, underlying event and multiple interactions characteristics. The new experimental opportunities and large rapidity coverage of the detectors in the forward region allow these measurements to be made in as yet unexplored regions. In particular, the soft diffractive processes at the LHC, are important for understanding non-perturbative QCD effects and they also constitute a significant fraction of the total pp cross-section. Therefore, the measurement of the main characteristics of diffractive interactions are essential to improve our understanding of pp collisions. However, the modeling of diffraction is still mainly generator dependent and there is no unique, agreed upon, experimental definition of diffraction. In this project, we consider the pseudorapidity acceptance in the forward region of the ATLAS and CMS detectors at the LHC and study the methods to select soft diffractive dissociation at 7 TeV pp collisions. We also look for variables to help distinguish single and double diffractive dissociated events in particular using the very forward calorimeters. The results show that in the limited detector rapidity acceptance |eta| < 5.2, it is possible to select diffractive dissociated events by requiring a large rapidity gap in the event, however, it is not possible to make an unambiguous distinction between single and double diffractive dissociated events with a low diffractive mass. Therefore, it is preferable to measure the rapidity gap cross-section for all types of process, without attempting to correct for single diffractive dissociated events. As a second part of the project, we study minimum bias and underlying event distributions in three-jet events. We first identify new transverse, towards and away regions in phi for the events which have well-balanced three jets. Later, we look at the (transverse) energy flow vs |eta| in that regions including forward rapidities. The Rivet analysis package together with various event generators is used throughout the analyses in this project. The studies are summarized in this preprint: Methods to Select Soft Diffraction Dissociation at the LHC, here.
Sercan Sen
August, 2010 to December, 2010