Search

In Fig. 1c of this Article, the x-axis scale was inadvertently duplicated from Fig. 1b. The original Article has been corrected online.

The fundamental building blocks of the proton, quarks and gluons, have been known for decades. However, we still have an incomplete theoretical and experimental understanding of how these particles and their dynamics give rise to the quantum bound state of the proton and its physical properties, such as for example its spin. The two up and the single down quarks that comprise the proton in the simplest picture account only for a few percent of the proton mass, the bulk of which is in the form of quark kinetic and potential energy and gluon energy from the strong force. An essential feature of this force, as described by quantum chromodynamics, is its ability to create matter-antimatter quark pairs inside the proton that exist only for a very short time. Their fleeting existence makes the antimatter quarks within protons difficult to study, but their existence is discernible in reactions where a matter-antimatter quark pair annihilates. In this picture of quark-antiquark creation by the strong force, the probability distributions as a function of momentum for the presence of up and down antimatter quarks should be nearly identical, since their masses are quite similar and small compared to the mass of the proton. In the present manuscript, we show evidence from muon pair production measurements that these distributions are significantly different, with more abundant down antimatter quarks than up antimatter quarks over a wide range of momentum. These results revive interest in several proposed mechanisms as the origin of this antimatter asymmetry in the proton that had been disfavored by the previous results and point to the future measurements that can distinguish between these mechanisms., 10 pages, 5 figures, 5 tables; Matches published version

Author(s): Aidala, CA; Arrington, JR; Ayuso, C; Bowen, BM; Bowen, ML; Bowling, KL; Brown, AW; Brown, CN; Byrd, R; Carlisle, RE; Chang, T; Chang, WC; Chen, A; Chen, JY; Christian, DC; Chu, X; Dannowitz, BP; Daugherity, M; Diefenthaler, M; Dove, J; Durandet, C; El Fassi, L; Erdos, E; Fox, DM; Geesaman, DF; Gilman, R; Goto, Y; Guo, L; Guo, R; Hague, T; Hicks, CR; Holt, RJ; Isenhower, D; Jiang, X; Katich, JM; Kerns, BM; Kinney, ER; Kitts, ND; Klein, A; Kleinjan, D; Kras, J; Kudo, Y; Lin, PJ; Liu, D; Liu, K; Liu, MX; Lorenzon, W; Makins, NCR; Martinez, JD; McClellan, RE; McDonald, B; McGaughey, PL; McNease, SE; Medeiros, MM; Miller, B; Miller, AJ; Miyasaka, S; Miyachi, Y; Mooney, IA; Morton, DH; Nadim, B; Nagai, K; Nakahara, K; Nakano, K; Nara, S; Obata, S; Peng, JC; Prasad, S; Puckett, AJR; Ramson, BJ; Raymond, RS; Reimer, PE; Rubin, JG; Salinas, R; Sanftl, F; Sawada, S; Sawada, T; Scott, MBC; Selensky, LE; Shibata, TA; Shiu, S; Su, D; Tadepalli, AS; Teo, M; Tice, BG | Abstract: The SeaQuest spectrometer at Fermilab was designed to detect oppositely-charged pairs of muons (dimuons) produced by interactions between a 120 GeV proton beam and liquid hydrogen, liquid deuterium and solid nuclear targets. The primary physics program uses the Drell–Yan process to probe antiquark distributions in the target nucleon. The spectrometer consists of a target system, two dipole magnets and four detector stations. The upstream magnet is a closed-aperture solid iron magnet which also serves as the beam dump, while the second magnet is an open aperture magnet. Each of the detector stations consists of scintillator hodoscopes and a high-resolution tracking device. The FPGA-based trigger compares the hodoscope signals to a set of pre-programmed roads to determine if the event contains oppositely-signed, high-mass muon pairs.