After one quite frantic November, I emerged victorious two weeks ago from the delivery of a 78-pages, 49-thousand-word review titled "Hadron Collider Searches for Diboson Resonances". The article, which will be published in the prestigious "Progress in Particle and Nuclear Physics", an Elsevier journal with an impact factor above 11 (compare with Physics Letters B, IF=4.8, or Physical Review Letters, IF=8.5, to see why it's relevant), is currently in peer review, but that does not mean that I cannot make a short summary of its contents here.
"Dibosons" do not exist in a proper sense - the word is a shorthand for "boson pairs". Physicists are keen to use similar abbreviations, along with diphotons, dijets, dihiggs, dielectrons, dimuons, dildos - no, scratch the last one. Boson pairs are relevant because they can be a tell-tale signature of new particles we can produce in LHC collisions, so there is a distinct focus on events that contains a decay signal of two simultaneously produced W bosons, or two Z bosons, or one W and one Z, or two photons... 

And I could go on. In fact we can exactly define the list of elementary bosons as such:

1 - the photon, the quantum of electromagnetic radiation. Known to be emitted and absorbed by anything that possesses electric charge, the photon is exactly massless, travels at its own speed (pun intended - I mean it travels at the speed of light, which is made of photons!), and is quite a striking signature in proton-proton collisions of high energy, especially if it itself is produced with very large energy. 

2 - the W boson, the mediator of charged-current weak interactions. W bosons come in two varieties depending on the electric charge they possess: the W+ and the W-. The W boson mass has been measured with high precision by the Tevatron, LEP II, and LHC experiments: we know its mass to be 80379 MeV, give or take only 12 MeV! W bosons are essential in the new physics searches at the LHC, as they are always produced in the decay of top quarks, and also frequently the result of Higgs boson decays h-->WW.

3 - The Z boson, the mediator of neutral-current weak interactions, is the neutral brother of the W's. The Z weighs 91187.6 +- 2.1 MeV, a number extracted in the nineties at LEP in electron-positron lineshape measurements. The Z is also a frequent product of h-->ZZ decays, and can also be the messenger of new physics processes.

4 - The gluon, of course. The gluon, the carrier of strong interactions and the particle responsible for the binding of hadrons and nuclei, is massless as the photon, but unlike the electromagnetic counterpart it comes in 8 varieties, as it carries the charge of the field it mediates. Gluons cannot be found as individual particles, as they obey the "infrared slavery" of QCD. In other words, if you pulled one out of a proton (by kicking it off with another proton, e.g.), it would "dress up" by connecting to other quarks or gluons, such that the total colour charge of each of the produced particles would be zero. This is the phenomenon we call "jet", and in fact gluons are experimentally seen as hadronic jets of particles emitted at high energy in the same direction (within some angular spread).

5 - Aaand the Higgs boson, discovered in 2012 at the LHC, is the only elementary scalar boson we know - all others mentioned above are vector particles, meaning that they carry one unit of spin; the Higgs has no spin instead. The Higgs boson has a mass of 125 GeV, give or take some 0.3 GeV, and has a host of possible visible decay modes, including the mentioned WW, ZZ decays, as well as photon pairs, bottom-quark pairs, tau-lepton pairs, and even gluon pairs (a dreadfully hard one to distinguish - I fear it will never be done).

That makes it 5. Five bosons (six if you consider W+ and W- as separate entities, and 13 if you insist that differently-coloured gluons are counted separately, but that would be rather meaningless). So how many meaningful combinations can you cook up of pairs of these particles ? 25, right ? Well, no. You should remove equal combinations, as the order here does not matter - hence e.g. WZ and ZW are really the same thing. It's not 25 but 5+4+3+2+1, or 15. Or is it ?

Yes and no. While the fantasy of theoretical physicists is unlimited, few of them have proposed that there exist new particles decaying into dibosons made of by a gluon and another elementary boson that isn't also a gluon. They may exist, but they are not sought for with great impetus by experiments -partly because gluons are hard to distinguish from quarks, maybe, but partly also because there is a dearth of fashionable models predicting such final states.

Something similar can be said for the time being of the Higgs-photon combination, although here there may be ideas... For now we'll leave it out.

So we can have the following combinations that make sense to look for and study:

A) pairs of weak bosons: WW; WZ; ZZ.
B) pairs including one Higgs boson: WH; ZH; HH.
C) pairs including photons: Wγ, Zγ, γγ;
D) pairs of gluons: gg

As you see, there are 10 interesting combinations. However, my review article would have been A LOT easier to write if I had had to consider just ten different searches. The problem is that all of these elementary bosons, except the gluon, give rise to different experimental signatures, so that when you make a search for e.g. WH pairs you have to reckon with literally dozens of different possible final states!

E.g. say you set out to search for resonances that decay to a WH pair. What kind of signature will you look for ? An event with a W and a H signal, you will be quick to answer. But wait - the W signal can be an electron and a neutrino, or a muon and a neutrino, or a tau lepton and a neutrino, or two hadronic jets. And the H signal may be two W bosons, or two Z bosons, or two photons, or two hadronic jets, or two tau leptons... And the two W bosons I mentioned in the previous sentence themselves produce a host of different possible final states! 

Is there a solution to this mess ? Well, not really - you just have to consider dozens of different categories of events, and decide whether to analyze them independently, similarly, or together. If you lump everything together you lose sensitivity, but your life is limited, so you make choices. The final result is that CMS and ATLAS have produced almost a hundred scientific articles this far where they search for resonances in some final state. That made my job very hard in the attempt of summarizing all these interesting analyses, each of which - I am certain - involved quite a bit of cunning and imaginativeness.

I think this is enough for an introduction - in the next post I will get to the dirty work of discussing some of the new physics that we may unearth by looking for boson pairs!

Part 2 is here.

Tommaso Dorigo is an experimental particle physicist, who works for the INFN at the University of Padova, and collaborates with the CMS experiment at the CERN LHC. He coordinates the European network AMVA4NewPhysics as well as research in accelerator-based physics for INFN-Padova, and is an editor of the journal Reviews in Physics. In 2016 Dorigo published the book “Anomaly! Collider physics and the quest for new phenomena at Fermilab”. You can get a copy of the book on Amazon.