Blogging ICHEP 2010

A collective forum about the 35th edition of
the International Conference on High Energy Physics (Paris, July 2010)

Saturday, July 24, 2010

Heavy Long-lived Particles at the LHC

Yesterday at ICHEP CMS and ATLAS presented their searches of long-lived charged particles. Most of the particles we deal with at the LHC decay almost immediately, basically right at the collision point, and one needs to reconstruct them from their decay product such as electrons, photons, muons or jets. However, it's conceivable that there exist new particles whose lifetime is 100 nanoseconds or more, in which case they traverse the entire detector before decaying (recall that the speed of light is one foot per nanosecond ;-). If these particles have an electric or color charge, they will leave a mark of their passage. Long-lived charged particles arise in many extensions of the standard model. The best known example is gauge mediated supersymmetry with a high scale of supersymmery breaking. In this case the lightest (and stable) supersymmetric particle is typically the gravitino who itself is uncharged. But the next-to-lightest supersymmetric particle may be charged (it could be stau, for example) and couples very weakly to the gravitino, in which case it lives very long for particle physics (or even human) standards. Supersymmetry is just one example; there are tons of less or more motivated models that predict long-lived particles.

So what does such a long lived particle looks like? It slashes through the detector all the way to the muon chambers, leaving an energetic track in the tracker but little energy deposition in the calorimeters. In other words, it looks much like a muon, and in most case it will be identified as one by online systems. But it can be distinguished from a muon by looking more closely at its traces. First of all, we expect it to be heavy (100 GeV or more, otherwise LEP would have seen it before), and therefore the track will have a large transverse momentum. Another consequence of the large mass is that the majority of these particels are produced with small velocities (where small here means a fraction of the speed of light, say 0.3 -0.7c). The Bethe-Bloch equation tells us that slow moving particles will lose energy faster when passing through matter. Thus, one can find heavy long-lived charged particles by looking for an excess of tracks with high pT and high dE/dx (one can also look at the time-of-flight, or the number of hits in the tracker). CMS, who seems more advanced in this search than ATLAS, already presented the first limits based on 200 nb-1 of data. The limits are not competitive with those of the Tevatron yet, but this will change soon.
Actually, long-lived charged particle may lose so much energy when passing through the detector that they stop altogether, and then decay after a while.This can readily happen for strongly interacting particles, such as long-lived gluinos that arise for example in split supersymmetry. Since in this case the particle does not reach the muon system, the search is more challenging and also more spectacular. Namely, one searches for localized energy bursts in the calorimeter during the time when there is no collisions, e.g. between beam crossings or when the accelerator is switched off altogether. CMS has already first limits on these events, and in this case they are already better than the Tevatron limits in a part of the parameter space.
It's reassuring that from day one the LHC plunges into out-of-the-box searches that some call exotic. My hunch is that, if there's any new physics at the TeV scale at all, it will take some unexpected form that will require non-standard techniques to discover. And the added value is that in these less explored corners of particle phenomenology interesting results and non-trivial limits can be obtained relatively fast, even during the first year of the LHC running.

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