Positrons are electron antiparticles. They are mass-produced at SuperKEKB B-Factory (SuperKEKB) and blasted into electrons at world-record brightness. Physicists examine the mysteries of matter and antimatter imbalance, as well as evidence of additional exotic particles outside the standard model, by analysing the hundreds of decay patterns of B mesons and anti B mesons in these encounters. Increasing the positron intensity is one of the important factors in this experiment to enhance the collision rate.
Positrons may be created by hitting a target made of a heavy metal, such as tungsten, with high-current, high-energy electrons. However, the target generates an equivalent amount of electrons, which are concurrently trapped by electric and magnetic forces in the positron capture section, which is positioned behind the target. Magnetic forces separate positrons from electrons shortly after the capture section.
In the capture section, it is very difficult to detect positrons and electrons independently at the same time. There are three reasons why measuring in the catch portion is problematic. First, there is virtually no room to place any beam monitors; second, the target is near to the instrumentation devices in the radiation environment; and third, the time delay between positrons and electrons is extremely small since they pass almost simultaneously through the capture section.
Prof. Tsuyoshi Suwada of KEK led a team that successfully placed a new kind of beam monitor into the SuperKEKB positron source. “The plan is to employ a wideband beam monitor and a basic rod antenna,” Suwada explains. “This concept is widely used in radio-frequency wave detection methods. For the first time, KEK successfully experimented with the use of charged particle beams in high-energy accelerators, such as electron and positron beams. It turns out that in the capture section, an electron (or positron) beam obviously precedes a positron (or electron) beam by some time interval in time domain.”
“Interestingly, what we observed in testing is that the time gap between electrons and positrons intricately variates in the range of 20 to 280 ps on average, and their travelling order is interchanged depending on the operating state of the capture section. The electrons with negative signal polarity clearly precede the positrons with plus signal polarity at the capture phase of 0 deg, and the time delay is 137 ps. The positrons with plus signal polarity clearly precede the electrons with negative signal polarity at the capture phase of 180 degrees, and the time interval is 140 ps.
“It turns out that the time gap between electrons and positrons varies delicately in time domain, and the travelling order is switched at capture phases of 50 and 230 degrees,” Suwada continues. When applied to the SuperKEKB, the improved positron capture efficiency allowed the SuperKEKB increase its world-record brightness.
“We think that this novel beam monitor might be used in future B-factories and e+e- linear colliders,” Suwada concludes. This study was completed with the assistance of Dr. Muhammad Abdul Rehman of KEK (present affiliation is IHEP).
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