The study of dilepton production (e.g. e+e−, μ+μ−) is crucial in high energy physics as it provides a direct way to probe the electroweak interaction. However, these experiments face a significant challenge in the form of a combinatorial background. This background arises from the combination of tracks that do not originate from the same physical vertex, making it difficult to extract meaningful results from the data. In this article, we describe a new method developed to estimate the combinatorial background in dilepton experiments, particularly in the SeaQuest spectrometer.
SeaQuest is a high-energy physics experiment designed to study the production of dilepton pairs in proton-proton collisions. It comprises several detectors including x- and y-measuring hodoscopes and drift chambers, wire chambers, and proportional tubes, each serving a specific purpose. The trigger for an event in SeaQuest is determined by a field-programmable gate array (FPGA) that looks for likely opposite-sign track pairs based on simulations.
In SeaQuest, the relevant quantity for determining the combinatorial background is the station-1 drift chamber occupancy, which is largely influenced by the tremendous variation in proton beam bunch sizes delivered to the target. The number and momentum distribution of tracks in the data depend strongly on the D1 occupancy, and hence the events must be sorted by this quantity. The researchers have developed a method to estimate the combinatorial background in dilepton experiments where the signal-to-background ratio in the data stream is low, and the events can be sorted into classes containing the same track distributions.
The method involves mixing tracks from different events, such that each track is only combined with tracks from one event. The two tracks forming a dimuon must satisfy the trigger condition for both normal and mixed events. The normalization factor for the mixed distribution is obtained by ensuring that the signal-to-background ratio of tracks is very small in the data stream, as required in Ref. The events are sorted into similar groups, in this case, according to the D1 chamber occupancy.
The researchers have included 91 true signal pairs in the analysis and have shown that the method they have developed has the correct normalization. The computed distribution can be directly subtracted from the total yields to recover the signal yields.
The method developed by the researchers provides a way to estimate the combinatorial background in dilepton experiments, particularly in the SeaQuest spectrometer. It takes into account the unique aspects of the SeaQuest data and ensures the correct normalization of the mixed distribution. The ability to accurately estimate the combinatorial background is crucial in extracting meaningful results from dilepton experiments and furthering our understanding of the electroweak interaction.
Pate, S.F., Pun, A., Hossain, M.F., Nagai, K., Aidala, C.A., Fassi, L.E., Hague, T.J., Lorenzon, W., Nakano, K., Kinney, E.R., Reimer, P.E., Scott, M.B., & Towell, R. (2023). Estimation of Combinatoric Background in SeaQuest using an Event-Mixing Method.