The production of $b$ ($b$ bar) quarks I have a question, please if there anyone helps me to understarnd 
how we distinguish the production pair of quarks $b$ ($b$ bar) it comes from a QCD background or comes from higgs decay?
 A: If you are asking whether a given pair in an event with the mass within the width of the Higgs ,   can be identified as coming from the Higgs, or from the QCD background, the answer is no, it is not possible. These productions are controlled by  quantum mechanical interactions,  and it is only the probabilities that can be predicted, not individual events.
It is a laborious analysis having to take into account a lot of contributions from other channels, in order to be able to see  an excess of signal that can be attributed to Higgs going to b bbar.:


Figure 1: Distribution of mbb in the (W→ℓν)(H→bb) search channel. The signal is shown in red, the different backgrounds in various colours. The data are shown as points with error bars. (Image: ATLAS Collaboration/CERN)

Even after subtracting some backgrounds:


Figure 2: Distribution of mbb from all search channels combined after subtraction of all backgrounds except for WZ and ZZ production. The data (points with error bars) are compared to the expectations from the production of WZ and ZZ (in grey) and of WH and ZH (in red). (Image: ATLAS Collaboration/CERN)

it is not a stand out signal, because of the large QCD backgrounds.
