CERN LHCb 寻获40余年罕见衰变首个证据：每100亿个美粒子约一次，显著性3.2西格玛
欧洲核子研究中心（CERN）大型强子对撞机 LHCb 实验的物理学家在40多年搜寻后，首次获得一种极罕见粒子衰变的证据。该衰变被预言在约每100亿个美粒子中才发生约一次。
这一名为 Bd to mu mu 的衰变中，一个含美夸克的粒子转变为两个缪子。研究团队报告信号统计显著性为3.2西格玛，达到称为证据的水平，但仍需更多数据达到粒子物理学宣称发现所需的5西格玛门槛。曼彻斯特大学物理与天文学系主任 Chris Parkes 参与领导 LHCb 合作组及此次结果批准。
研究团队称，新测量与标准模型预言相符。升级后的 LHCb 顶点定位器距 LHC 束流仅5毫米，由英国团队主导建造；升级实验预计以最高5倍于此前的速率收集数据至2030年代初，后续升级还可能再提高10倍速率。
CERN LHCb Finds First Evidence After 40-Year Search for Rare Decay Occurring Once in About 10 Billion Beauty Particles, at 3.2 Sigma
Physicists at CERN have detected the first evidence, after more than 40 years of searching, of an extraordinarily rare particle decay predicted to occur only about once in every 10 billion beauty particles. The signal was recorded by the upgraded LHCb experiment at CERN's Large Hadron Collider.
In the decay, known as Bd to mu mu, a particle containing a beauty quark transforms into two muons, particles similar to electrons but heavier. The signal has a statistical significance of 3.2 sigma, strong enough to qualify as evidence of the decay, although more data will be needed to reach the five-sigma threshold required to claim a discovery in particle physics. Professor Chris Parkes, head of the Department of Physics and Astronomy at the University of Manchester, led the international LHCb collaboration through construction of the upgrade and led approval of the new result.
The new measurement matches the predictions of the Standard Model, the researchers said. The upgraded LHCb Vertex Locator, a silicon pixel detector built with leading UK involvement, sits just five millimetres, or about 0.2 inches, from the LHC beams. By collecting data at up to five times its previous rate, the upgraded experiment is expected to build a much larger dataset into the early 2030s, and a further planned upgrade could increase that rate by another factor of ten.
来源：欧洲核子研究中心
发布：GSM·美国（发布链接）
图片来源：图中是 CERN LHCb 实验的外径迹探测器部件，该实验记录了此次罕见衰变信号。来源：Wikimedia Commons, File:LHCb's outer tracker goes to GSI's PANDA experiment 02.jpg, Traczyk, Piotr, CC BY 4.0。
GSM Global · 本篇英文版 English edition of this story
