9/3/2023 0 Comments Tau lepton massIf the tau lepton is divisible, then its size is less than one-tenth of one quintillionth of a metre! The excellent prediction allowed physicists to exclude the existence of excited tau leptons if their production rate is larger than 1 in 100 trillion proton-proton collisions. The observed data (black points) show no excess over the background prediction. Figure 1 shows the background prediction (the coloured area) with its incredibly small uncertainty displayed by the hatched band. The main background processes are the production of a Z boson in association with jets, the production of top-quark pairs, and the production of events with many jets, with two misidentified as tau leptons. In the excited-tau hunt, the signal collisions would produce two Standard Model tau leptons and at least two jets. Here, artificial intelligence helps tremendously to distinguish hadronic taus from jets. Hadronically-decaying taus are difficult to measure because their detector signature is very similar to that of ‘jets’ (collimated sprays of hadrons). Two-thirds of taus decay to hadrons (particles made from quarks) and one-third of taus decay to electrons or muons. It has a short lifetime and decays before reaching the ATLAS detector. The Standard Model tau lepton is a tough particle to detect. The S_T variable is, loosely speaking, the amount of energy carried by the most energetic particles in each proton-proton collision. The considered signal is the excited tau lepton with mass 1.5 TeV. Figure 1: Data are compared with the expected event yields given just background processes (coloured area) or background plus signal processes (dashed red line). This leads to a judgement on the existence – or not – of the rare hypothetical particles. With this at hand, physicists can compare it with the number of observed collisions. These are considered as background, and precisely estimating their expected number is essential. However, there are many collisions where only Standard Model particles are created that look very similar. ![]() The ATLAS detector, acting like an oversized camera, has great potential to capture the handful of collisions – out of trillions – in which these rare particles could be produced. ![]() Tau leptons are the clueīoth types of hypothetical particles – the excited tau and the vector-like tau – would decay to a Standard Model tau lepton and some other particles. The LHC might also produce vector-like taus, particles with similar properties as the Standard Model tau lepton, except that it would have a spin of 1 rather than a spin of 1/2. If it is composite, then the LHC could produce its excited state: the excited tau lepton. One particle under investigation by the ATLAS experiment is the tau lepton, a heavier analogue of the electron. Not only can it produce new types of particles, it can reveal the structure – if any – of the known particles. It acts like a giant microscope with the most fantastic spatial differentiation in the world. ![]() The LHC is the perfect instrument to study the puzzle of matter. Could matter be broken down endlessly? Are the known elementary particles the true fundamental building blocks of matter or are they composite? Are there new elementary particles with similar properties as the existing known ones? Two recent searches by the ATLAS experiment shed light on these profound questions!
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