Yesterday, 4th of July was one of the remarkable achievement of the mankind has done over the century. I don't know how to describe the discovery of this particle. It was postulated to give mass to the SU(2) gauge bosons, and that mechanism also explained the mass of the fermions in the Standard Model via Yukawa couplings by a couple of Physicists in the 70s and took almost 40 years to find that particle. While studying (in fact I am still) QFT I always wonder if there could be another mechanism for electroweak symmetry breaking or let say not break the symmetry but somehow make them acquire mass by a different mechanism. For those who seek more theoretical explanation could look at this link.
Let me say couple things about Higgs particle. To capture the Higgs particle or the particles which it decays to took around 50 years (Higgs mechanism proposed around 1964). The reason was due to high background comes from other SM decays which ends up the same particles at the end of the decay thus the Higgs particle couldn't be extracted. I should mention that the production rate is also low compared to other SM processes so it needs high statistics to extract the signal and claim a discovery. Thanks to the detectors which designed to let it seen in and the people who investigated extensively by Monte Carlo studies. In the picture below you can see a real picture of the collision event where Higgs decays to 2 photons. This is one of the channels which Higgs could be extracted at high efficiency from the background. Timestamp of the event is also seen at the top left corner of the picture. The bending lines represent the charged particle deflected due to 4 Tesla magnetic field. Green bars show the energy deposited in electromagnetic calorimeter (ECAL) by the two photons. Since there is not much activity in the barrel region besides these two photons, they are reconstructed at high efficiency. There is some activity at the end caps but could be ignored since new physics or resonances tend to decay at barrel region, and these bars are, most likely, from the underlying activity.
 |
| In this figure, you can see Higgs->2 Photon decay. |
I think one of the beautiful plots from yesterday's talk was the one where you can see the diphoton invariant mass distribution and the very nice peak around $126GeV$ which indicates a resonance and a boson-like particle. This particle was one of the biggest pieces of the SM unresolved, and finally, it is found and fitted to the picture. But I am afraid this is not the end, in fact, this is just the beginning. There are still many open questions not answered by the SM but beyond the SM theories and these models and theories need to be tested as well. CMS and ATLAS will look at that those BSM signals with growing effort. One question asked by many at CERN is what kind of Higgs is that, for example, SUSY predicts there are 5 Higgs like particles at EW scale and could this resonance is one of these.
 |
| This figure shows the invariant Diphoton mass distribution for the selected events. |
Another decay channel of higgs particle is 4 lepton channel. Higgs could be extracted from the background if you have a high efficient electron or muon identification. In the plot below you can see the invariant mass distribution from different channels like $Z+X \rightarrow 4 lepton$, Z$\gamma \rightarrow 4 lepton$ and $higgs \rightarrow 4lepton$. The red line shows a peak around $126GeV$, and it is the Higgs particle.
 | |
|
This signal seen here is also recognized by ATLAS collaboration, and the significance of the discovery is around $5\sigma$ in both experiment which means it is likely to happen such a peak at one out of 1.744 Million by statistical fluctuation. In everyday words, it means it is almost impossible to be just a statistical fluctuation and happen to be a peak at that energy.
I have just stumble upon on a good explanation of the Higgs Mechanism in PhDComics. You could even watch it.
I have just stumble upon on a good explanation of the Higgs Mechanism in PhDComics. You could even watch it.
