Scientists at CERN's Large Hadron Collider (LHC) are finding novel ways to search for new particles. Elusive, long-lived particles could be decaying into other particles away from the LHC collision point - leaving an unusual signature in a detector. The ATLAS collaboration has broadened its extensive search programme to look for these unconventional collision events. In the process, they've drastically improved the limits on new massive long-lived particles decaying into particles called leptons.
Long-lived particles are a feature of the Standard Model, although for relatively low-mass particles only. Massive long-lived particles can occur in theories of new physics beyond the Standard Model. A theory that encompasses new long-lived particles in some of its manifestations is supersymmetry (SUSY). SUSY predicts that each particle of the Standard Model has a "superpartner" particle, which differs from its corresponding particle in a quantum property known as spin. In its new study, the ATLAS collaboration looked for the superpartners of the electron, muon and tau lepton, called "sleptons" ("selectron", "smuon" and "stau", respectively).
The typical search for new physics with ATLAS data is oriented towards new particles that would decay instantaneously, the way heavy Standard Model particles do and also most new physics particles are expected to do. For their new search, ATLAS physicists had to develop new methods of identifying particles in order to increase the likelihood of discovering long-lived particles.
Because the particles created by the decay of a long-lived particle would appear away from the collision point, unusual background sources can arise: photons misidentified as electrons, muons that are mismeasured, and poorly measured cosmic-ray muons. Cosmic-ray muons come from high-energy particles colliding with Earth's atmosphere and can traverse the more than 90 metres of rock above the ATLAS detector, as well as the detector itself. Since they do not necessarily pass through the detector near the collision point, they can appear as if originating from a long-lived particle decay. ATLAS physicists have developed techniques not only for reducing but also for estimating the effects of these background sources.
The ATLAS collaboration found no evidence of long-lived particles in its search, but it was able to set limits on the mass and lifetime of long-lived sleptons decaying to Standard Model leptons inside the detector. For the slepton lifetime that the new search is most sensitive to (around 0.1 nanoseconds, corresponding to a flight length of about 30 centimetres), the researchers excluded selectrons and smuons up to a mass of around 700 GeV, and staus up to around 350 GeV. The previous best limits on these long-lived particles were around 90 GeV and came from the experiments on the Large Electron-Positron Collider (LEP) - CERN's predecessor to the LHC - more than 20 years ago. The new result was able not only to meet LEP's best limits but also to surpass them.
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