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This seems wrong to me. I know the LHC records total energy in and compares total energy out. Wouldn't any long lived particles would "appear" as areas of mis

by cdumler 10y ago

This seems wrong to me. I know the LHC records total energy in and compares total energy out. Wouldn't any long lived particles would "appear" as areas of missing energy?

ISL 10y ago
Yes, unless they appear as something you think you understand. In the specific case of this article, they're interested in particles that look a lot like a muon. The signature of a muon in a particle detector is that it leaves a charged track in the inner tracker, passes through the substantial mass of the electromagnetic calorimeter without saying much, and then goes pow in the muon detector (usually alternating plates of steel and detector). A long-lived particle could be quite similar; a charged track, a lot of nothing, and a decay in the muon tracker. Any particle that might roughly replicate the muon's signature might have hidden from existing searches for new physics. The proposal in the article is to cover the muon detector with higher-resolution trackers. If the "muon"'s decay doesn't conform to the standard plan, higher resolution would make it apparent. I'm not enough of an expert to know how sensitive this method is in comparison to other ways of hunting long-lived particles, but I have taken a class or two from one of the authors. If Lubatti's pitching the idea, it's worth thinking about.
- mattheww 10y ago
  As you can imagine, the long-lived particles can have "any" lifetime. Since the lifetime is unknown, it makes sense to probe many lifetimes. The experiment being suggested by Lubatti and his collaborators probes lifetimes on the order of 10^5 - 10^7 m. There aren't strong constraints on this range, which can appear as missing energy in CMS or ATLAS (or not). CMS, ATLAS, and LHCb are giving constraints at 10^-6 to 10^2 m. Interestingly, the different experiments are probing mostly different ranges. To correct something in the grandparent (?) post, the LHC experiments do not know the incoming energy. Even though the beam energy is 6.5 TeV on 6.5 TeV, the collisions are only a fraction of that. We only know that the energy has to balance in the perpendicular direction. However, if a collision produces two invisible particles that balance each other, it would appear that there is no missing energy. In many of models, pair production of new particles is preferred, so if the particles are long-lived, they can be hard to find.
- cshimmin 10y ago
  Well said but a few details are a bit off. The alternating plates of metal and detector is actually the hadronic calorimeter, which the muons also go straight through (but which stop almost everything else). The muon detectors are outside of that, and are just various types of spark chambers that get zapped as the muon passes through them. Generally the muons neither stop nor decay in the detector.
  - ISL 10y ago
    Ah, true story. That makes more sense. Thanks!

by cdumler 10y ago

This seems wrong to me. I know the LHC records total energy in and compares total energy out. Wouldn't any long lived particles would "appear" as areas of missing energy?

ISL 10y ago

Yes, unless they appear as something you think you understand. In the specific case of this article, they're interested in particles that look a lot like a muon. The signature of a muon in a particle detector is that it leaves a charged track in the inner tracker, passes through the substantial mass of the electromagnetic calorimeter without saying much, and then goes pow in the muon detector (usually alternating plates of steel and detector). A long-lived particle could be quite similar; a charged track, a lot of nothing, and a decay in the muon tracker. Any particle that might roughly replicate the muon's signature might have hidden from existing searches for new physics. The proposal in the article is to cover the muon detector with higher-resolution trackers. If the "muon"'s decay doesn't conform to the standard plan, higher resolution would make it apparent. I'm not enough of an expert to know how sensitive this method is in comparison to other ways of hunting long-lived particles, but I have taken a class or two from one of the authors. If Lubatti's pitching the idea, it's worth thinking about.

mattheww 10y ago
As you can imagine, the long-lived particles can have "any" lifetime. Since the lifetime is unknown, it makes sense to probe many lifetimes. The experiment being suggested by Lubatti and his collaborators probes lifetimes on the order of 10^5 - 10^7 m. There aren't strong constraints on this range, which can appear as missing energy in CMS or ATLAS (or not). CMS, ATLAS, and LHCb are giving constraints at 10^-6 to 10^2 m. Interestingly, the different experiments are probing mostly different ranges. To correct something in the grandparent (?) post, the LHC experiments do not know the incoming energy. Even though the beam energy is 6.5 TeV on 6.5 TeV, the collisions are only a fraction of that. We only know that the energy has to balance in the perpendicular direction. However, if a collision produces two invisible particles that balance each other, it would appear that there is no missing energy. In many of models, pair production of new particles is preferred, so if the particles are long-lived, they can be hard to find.
cshimmin 10y ago
Well said but a few details are a bit off. The alternating plates of metal and detector is actually the hadronic calorimeter, which the muons also go straight through (but which stop almost everything else). The muon detectors are outside of that, and are just various types of spark chambers that get zapped as the muon passes through them. Generally the muons neither stop nor decay in the detector.
- ISL 10y ago
  Ah, true story. That makes more sense. Thanks!