Title: Recent Measurement of Flavor Asymmetry of Antiquarks in the Proton by Drell–Yan Experiment SeaQuest at Fermilab

A measurement of the flavor asymmetry of the antiquarks ($$\bar{d}$$ and $$\bar{u}$$) in the proton is described in this thesis. The proton consists of three valence quarks, sea quarks, and gluons. Antiquarks in the proton are sea quarks. They are generated from the gluon splitting: g → q + $$\bar{q}$$. According to QCD (Quantum Chromodynamics), the gluon splitting is independent of quark flavor. It suggests that the amounts of $$\bar{d}$$ and $$\bar{u}$$ should be the same in the proton. However, the NMC experiment at CERN found that the amount of $$\bar{d}$$ is larger than that of $$\bar{u}$$ in the proton using the deep inelastic scattering in 1991. This result is obtained for $$\bar{d}$$ and $$\bar{u}$$ integrated over Bjorken x. Bjorken x is the fraction of the momentum of the parton to that of the proton. The NA51 experiment (x ~ 0.2) at CERN and E866/NuSea experiment (0.015 < x < 0.35) at Fermilab measured the flavor asymmetry of the antiquarks ($$\bar{d}$$/$$\bar{u}$$) in the proton as a function of x using Drell–Yan process. The experiments reported that the flavor symmetry is broken over all measured x values. Understanding the flavor asymmetry of the antiquarks in the proton is a challenge of the QCD. The theo- retical investigation from the first principle of QCD such as lattice QCD calculation is important. In addition, the QCD effective models and hadron models such as the meson cloud model can also be tested with the flavor asymmetry of antiquarks. From the experimental side, it is important to measure with higher accuracy and in a wider x range. The SeaQuest (E906) experiment measures $$\bar{d}$$/$$\bar{u}$$ at large x (0.15 < x < 0.45) accurately to understand its behavior. The SeaQuest experiment is a Drell–Yan experiment at Fermi National Accelerator Laboratory (Fermilab). In the Drell–Yan process of proton-proton reaction, an antiquark in a proton and a quark in another proton annihilate and create a virtual photon, which then decays into a muon pair (q$$\bar{q}$$ → γ* → µ⁺µ^-). The SeaQuest experiment uses a 120 GeV proton beam extracted from Fermilab’s Main Injector. The proton beam interacts with hydrogen and deuterium targets. The SeaQuest spectrometer detects the muon pairs from the Drell–Yan process. The $$\bar{d}$$/$$\bar{u}$$ ratio at 0.1 < x < 0.58 is extracted from the number of detected Drell–Yan muon pairs. After the detector construction, commissioning run and detector upgrade, the SeaQuest experiment started the physics data acquisition from 2013. We finished so far three periods of physics data acquisition. The fourth period is in progress. The detector construction, detector performance evaluation, data taking and data analysis for the flavor asymmetry of the antiquarks $$\bar{d}$$/$$\bar{u}$$ in the proton are my contribution to SeaQuest. The cross section ratio of Drell–Yan process in p- p and p-d reactions is obtained from dimuon yields. In the experiment with high beam intensity, it is important to control the tracking efficiency of charged particles through the magnetic spectrometer. The tracking efficiency depends on the chamber occupancy, and the appropriate method for the correction is important. The chamber occupancy is the number of hits in drift chambers. A new method of the correction for the tracking efficiency is developed based on the occupancy, and applied to the data. This method reflects the real response of the drift chambers. Therefore, the systematic error is well controlled by this method. The flavor asymmetry of antiquarks is obtained at 0.1 < x < 0.58. At 0.1 < x < 0.45, the result is $$\bar{d}$$/$$\bar{u}$$ > 1. The result at 0.1 < x < 0.24 agrees with the E866 result. The result at x > 0.24, however, disagrees with the E866 result. The result at 0.45 < x < 0 the statistical errors. u¯ results extracted from experiments are used to investigate the validity of the theoretical models. The present experimental result provides the data points in wide x region. It is useful for understanding the proton structure in the light of QCD and effective hadron models. The present result has a practical application as well. Antiquark distributions are important as inputs to simulations of hadron reactions such as W± production in various experiments. The new knowledge on antiquark distributions helps to improve the precision of the simulations.