Background Study in 4th-Generation Quark Search

Contact: Michael G. Wilson <>

Context and Motivation

The standard model of particle physics comprises three generations of quarks and leptons. However, a fourth quark generation, or similar fermions manifesting new symmetries or extra dimensions, are not excluded by existing measurements. Such particles are well motivated by theory. Recent measurements of CP violation in the decays of B mesons, which may be inconsistent with standard-model sources, are suggestive of a fourth-generation up-type quark, u4. If it exists, a fourth generation would be closely tied to the source of electroweak symmetry breaking, possibly the Higgs boson. Moreover, some theories attempting to explain electroweak symmetry breaking or the large difference between the Planck and electroweak scales involve new fermions that mix quantum mechanically with the standard-model quarks and decay similarly to a fourth generation.

There is a striking experimental signature from a fourth-generation down-type quark (d4), or a similarly decaying fermion (B, T5/3), that may be observable with the first ATLAS run, but which is just out of reach of current experiments. The signature is two high-momentum, same charge leptons (e or μ) in an event with many jets, characterisic of fermion-pair decays to t quarks and W bosons,
QQ → (tW−/+)(tW+/−).
The sources of this signature within the standard model are rare, and the analysis of this decay mode has recently proven to be the most sensitive direct search for 4th-generation particles. It is thus ideal for searching for new phenomena with the first ATLAS data.

Project Description

One of the largest sources of background in this search is from t-quark pairs decaying to a final state with two opposite-charge leptons. In a small fraction of these events, the charge of one of the leptons is not reconstructed correctly, leading to a fake same-charge dilepton event.

This project is primarily to estimate the rate of this background for events with electrons and determine an electron selection that optimizes the signal sensitivity. The student will learn about electron identification, including the aspects of calorimetry and charged-particle tracking most relevant for this analysis, and will gain familiarity with ATLAS data and analysis tools. This project may extend in several directions. Since the charge-misidentification rate for electrons is largely determined by the amount of material in the silicon-based tracking detectors, a study of the material distribution would be natural. Another analysis path would be to compare the background rates for electrons and muons to arrive at a method to cross-check the background rates.