I have left the field of particle physics for computational physics (quantum mechanics in many-particle systems), but I still follow what happens at CERN and the LHC. Especially the blog Résonaances is a good source for updates.
Latest news is that the LHCb detector has not detected any anomaly in the Bs-Bsbar mixing. Bs-mesons are heavy mesons made up of a bottom-quark and a strange-quark. One matter and the other one anti-matter (the only possible way to combine two quarks due to colour-charge restrictions). These mesons however will oscillate between two states. Essentially the quarks swap who is the matter and who is the anti-matter particle through an exchange of virtual top-quarks (mostly) and W-bosons. Current physics predicts that this mixing violates conservation of charge/parity (CP), however so-called new physics—essentially what the LHC was built to find—predicts a larger violation. This has not been found. Which is disappointing. Why the need to find “new physics”? Well, because the Standard Model is incomplete. It doesn’t explain all the phenomena we observe—like dark matter for instance—so we need to figure out what’s missing from the theory.
This is also of course the case for the infamous Higgs. The last particle predicted by the Standard Model that not yet has been discovered. Not that it is far behind the rest. The top-quark wasn’t confirmed until 1995.
The problem with the Higgs is that the theoretical model (electroweak theory) of the Higgs has two unknown parameters. For this reason we don’t quite know where to find the Higgs (essentially how heavy it is). However we have a fairly good idea of how it will behave depending on how heavy it is, so we can look for signs of its presence along the mass axis in the data. The other challenge is that the Higgs resonance is in most cases so weak that it drowns in background “noise” from other more common processes. Or in other words. Many other particles do the same thing as the Higgs. Like the Z-boson. So how do you tell which did what? Well, that is the challenge.
The latest news from ATLAS and CMS is that they have excluded the Higgs from 145 to 466 GeV. The old exclusion was a lower bound of 115 GeV from back when the LEP accelerator was running at CERN and from Tevatron in the US. Tevatron also gave us an exclusion range in the 150-ish to just over 180 GeV range. The new limits now leaves us with the 115-145 GeV window. The Higgs is running out of places to hide … if it exists at all.
The relevant posts from Résonaances:
- LHCb says: no Bs anomaly
- Higgs won’t come out of the closet, part I, part II
