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Can somebody help me out? I'm reading about formulas for cross sections for spin1 particles but I don't understand the delta functions, in calculating the 2particle pahse space psi

For example the interaction; A+B -> C+D has the formula;

psi= (2pi)^2 delta(Pa+Pb-Pc-Pd) d3Pc d3Pd / 4EcEd

then all the books say the same;

can somebody explain this to me?

the left side delta is zero everywhere pa+pb =not pc+pd.

the integral over the delta function d3Pd is;

1 for pd<pa+pb-pc and

0 for pd>pa+pb-pc

? how do I use this?

For example the interaction; A+B -> C+D has the formula;

psi= (2pi)^2 delta(Pa+Pb-Pc-Pd) d3Pc d3Pd / 4EcEd

then all the books say the same;

**delta(Pa+Pb-Pc-Pd) d3Pd = delta (Ea+Eb-Ec-Ed)**can somebody explain this to me?

the left side delta is zero everywhere pa+pb =not pc+pd.

the integral over the delta function d3Pd is;

1 for pd<pa+pb-pc and

0 for pd>pa+pb-pc

? how do I use this?

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