This week another paper emerged from the internal approval process of ATLAS, blinking into the light of day. (ATLAS is the experimental collaboration at the LHC of which I am a member.) This one has a lengthy title, "Measurement of the electroweak production of dijets in association with a Z-boson and distributions sensitive to vector boson fusion in proton-proton collisions at sqrt(s) = 8 TeV using the ATLAS detector", appropriate for an extensive, detailed study of a rare and interesting process, picking up where a previous paper by CMS left off.



The Z is a vector boson, one of the carriers of the weak force. It is a little like the photon, which carries the electromagnetic force, except that the Z has mass. A lot of mass, for a fundamental particle. At 91 GeV/c² it is nearly 100 times heavier than a proton, and after the top quark and the Higgs boson, it is the third heaviest fundamental particle in the Standard Model.

There are many ways to produce Z bosons and hadronic jets at the LHC. The Feynman diagrams at the top show two examples. Both of these are relatively rare, because they are electroweak processes. They involve the radiation of a vector boson - W or Z - off the incoming quarks, and that is an unusual thing to happen. Much more unusual than, for example, a quark and an antiquark annihilating and producing a Z, and radiating a couple of gluons (to make the jets) while they do so.



The ATLAS paper measures Z and jet production, and picks out electroweak processes, showing, with more than 5𝜎 confidence, that they are really taking place. The most interesting process is the left hand diagram (a), where you can see two W bosons, each radiated off a quark in a proton, fusing together to make a Z. This is interesting because such fusion diagrams can also produce the Higgs, and in fact according to the Standard Model a significant fraction of the Higgs bosons we have seen are produced this way. Also, these diagrams are closely related to vector boson scattering processes, where the Higgs definitely plays a role. Those processes were the reason we knew that some new physics had to show up at the LHC, even if the Higgs wasn't there (they were the subject of the first paper I wrote on LHC physics, for that reason).



The ATLAS paper also contains measurements of a range of distributions in these kind of events. These measurement are important for building confidence in our understanding of the Standard Model in this regime of energy, where the weak and electromagnetic forces have in some sense become unified. One of the tools used to make the Standard Model predictions is a Monte Carlo generator called Sherpa. Another good thing on the arXiv this week was a paper by some of the Sherpa authors examining the uncertainties in such predictions.







We really must not obsess over citations. Bad. Very bad. But I can't resist checking them every now and then, and was interested to see that the ATLAS Higgs boson discovery paper (2189 citations since August 2012) has just this week overtaken the paper describing the ATLAS detector (2188 citations since September 2008). And it shows no sign of slowing down, as people digest the new knowledge and write papers about it.



And finally, Stephen Hawking put a short paper out this week, where he discussed the "event horizons" of black holes and postulated that in fact they don't exist. That is, massive objects that look a lot like black holes do still happen when matter collapses into a small volume, but the event horizon from behind which not even light can escape is replaced by an "apparent horizon" behind which light has to wait for a while but can eventually escape. I think. Best to watch this excellent Sixty Symbols video with Tony Padilla and Ed Copeland if you want to know more.



Jon Butterworth’s book, Smashing Physics, is out on 22 May. Order it now!

