While LHC is searching for many new particles the Higgs particle is the most famous candidate. Its discovery would proof a theory that was put up in the mid-1960's describing a mechanism that gives mass to all particles of the standard model. One can derive from symmetries that all elementary particles were massless immediately after the big bang. The theory suspects that a viscous medium called the Higgs field spread out through our universe a trillionth of a second after the big bang. Since that time particles have masses different from zero: The more they interact with this medium, i.e the larger their masses are, the more force is needed in order to accelerate them inside this medium. To proof the higgs field it has to be excited in some way, like creating little curls in a fluid or gas. These curls are the quanta of the Higgs field excitations and called Higgs particles. They are massive and extremely short-lived. Before they can reach the detectors they decay into other particles so that they can only be found on the basis of their decay products. Into which other particles a Higgs particle decays, predictably depends on its mass. But this value is as yet unknown. That is why physicists are looking for various signals that belong to all possible decay modes of the Higgs particle. In the diagram below that is based on theoretical calculations and contains recent experimental results from ATLAS and CMS, the fractions (y axis) of the most important decay processes of the Higgs particle are shown. Additionally, you can see shaded areas inside the diagram. These areas of theoretically possible Higgs masses have been either recently excluded after the analyses from ATLAS and CMS at the LHC (with 95% confidence) or were excluded by LHC's predecessor, the LEP experiment, 10 years ago. The influence of the Higgs mass (x axis) on these fractions becomes clear as well.



Pay attention to the blue, dashed line (indicated with WW). Since it is impossible to proof the existence of the Higgs particle by observing excesses in events with pairs of bottom and anti-bottom quarks (due to the vast background), this line tells us: The decay of the Higgs particle into two W particles is the most provable decay mode in the complete and still allowed mass range. These W's have opposite electric charges since the Higgs particle is electric neutral.

Here is the problem: If a Higgs particle decays into two W's it will look like the production of two oppositely electric charged W's. This production process is allowed by the Standard Model and has nothing to do with the Higgs production (see right Feynman graph below). It is even worse: The latter process (production of two W's without participation of the Higgs) is far more frequent (depending on the mass 4-10 times). But how can you then distinguish between these two processes? Well, just by looking at event displays only you cannot! But with the help of additional physics quantities (that you have to learn in order to understand how it works) we can better distinguish between those processes. Particle physicists call this process: increasing of the signal to background ratio.

We even want to specialize in the search for the Higgs in the WW decay mode. Both W particles decay independently from each other following the laws of the Standard Model. A single W particle can decay into a pair of either quark and anti-quark or lepton and anti-lepton. We want to have a look at this decay mode, where both W particles will decay into a pair of lepton and anti-lepton, excluding tau leptons due to their complicated identification. Physicists are calling this decay mode H→WW→lνlν or WW→lνlν short, where l stands for an electron, muon, positron or anti-muon and ν for neutrinos.

To increase the signal to background ratio in the chosen decay mode we will concentrate on the angle between our two detectable leptons in the plane at right angle to the beam pipe. This angle is called opening angle. By taking spin relations of the produced particles into account we expect to find the Higgs events mainly at angles less than 90 degrees while Standard Model WW events appear in the whole angle range in which they prefer to appear at angles greater than 90 degrees. This can be shown in histograms very well.

Here are two more Feynman diagrams showing the production and decay of the Higgs particle and one background event (in this case: production of a heavy top quark pair).

And now, you can find this particle – let's go to the measurements!

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