At the CERN laboratory today, there’s an ongoing report to the CERN council that oversees the lab, and this includes talks from the Large Hadron Collider [LHC] accelerator operators, and from the experimentalists who built and operate the detectors (ATLAS, CMS, LHCb, ALICE, TOTEM) that are designed to detect and interpret the debris from the LHC’s proton-proton collisions. Among the results being presented today are some measurements of the properties of the Higgs-like particle whose discovery was announced in July, including ones that were notably missing from the HCP conference presentations in Kyoto last month.
Here are some highlights, to be fleshed out in more detail later, if warranted.
LHC accelerator operations report:
An excellent year. In the best week the LHC produced 1.35 inverse femtobarn (fb) of data for both ATLAS and CMS; as of December 5, LHC produced 23.2 inverse fb for the year per experiment (note each experiment will have somewhat less recorded, due to normal losses), slightly above the target for the year.
Biggest problems: beam instability (much bigger problem in 2012 than 2011); stray high-energy particles affecting electronics in the tunnel; dust falling out of the beampipe into the beam, potentially a significant problem for 2015.
After the 2013-2014 shutdown, what will be the likely running conditions in 2015? The current intention is to go to 25 nanoseconds between collisions (the design) rather than the 50 nanoseconds used in 2012, and to start at 13 TeV per collision.
ATLAS: New two-photon and four-lepton measurements; new spin and parity measurements.
Two photons: new categories of events added, with either 1 lepton or of two jets at low invariant mass, characteristic of production of a Higgs with a Z or W.
This channel now shows 6.1 standard deviation significance (3.3 expected) by itself: discovery of a Higgs-like particle in a single decay mode. Mass 126.6±0.3±0.7 GeV/c²
Signal remains high: 1.8 ± 0.3 + 0.29 – 0.21 times the Standard Model expectation. (But note the expectation depends somewhat on the assumed mass of the Higgs;
not sure yet which mass was taken here. 126.6 GeV was assumed.)
Four leptons: 4.1 standard deviations, signal strength 1.3 ± 0.4 times Standard Model expectation. Mass 123.5±0.9+0.4-0.2 GeV/c² — notably lower than two photons.
The two mass measurements are 3 GeV apart (surprising but not impossible given the amount of data; or perhaps there is a technical problem somewhere, though I’m sure they looked very, very hard for one). They are compatible only at 2.7 standard deviations. Combined mass: 125.2 +- 0.3 +- 0.6 GeV/c²
Spin (from photons): spin 2 disfavored at the 91% level; compatible with spin 0.
Spin (from leptons): spin 2 disfavored only at the 85% level; compatible with spin 0.
Parity (from leptons): Exclusion of odd-parity spin-0 particle at 99%.
First limit on Higgs decay to Z + photon: although still ~20 times the Standard Model expectation, this limit is good enough to rule out various non-standard interpretations of the Higgs-like particle.
No update of Higgs decay to two photons. This is too bad. It’s somewhat exciting that ATLAS’s result on Higgs decay to two photons remains somewhat high compared to expectations. But the excess is still not yet 3 standard deviations. Deviations of this size do come and go. And we don’t have confirmation from CMS. So the situation remains tantalizing but unfortunately not yet very convincing. We may not learn anything more from CMS or ATLAS til March, when they have analyzed the full 2012 data set.
I did not catch anything new from LHCb or ALICE; generally I haven’t had time to cover ALICE’s research program here. TOTEM, a special purpose detector for measuring things I haven’t discussed on this website, is still getting rolling.