Take, as an example, the Higgs boson finding:
But the same is found in many other particle physics detector graphs...
**Why is the shape of the background a decaying exponential? **
At least when energy is on the horizontal axis.
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asked Feb 4, 2015 at 19:23
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$\begingroup$ Are you asking specifically when collision energy is plotted on the horizontal axis? Because other experiments could have time on the horizontal axis depending on what you're looking at. $\endgroup$– BMSCommented Feb 4, 2015 at 19:27
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$\begingroup$ Yes, sorry maybe I should have specified that $\endgroup$– SuperCiociaCommented Feb 4, 2015 at 19:31
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$\begingroup$ Please add that info to the question, as most won't read these comments. $\endgroup$– BMSCommented Feb 4, 2015 at 19:40
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It is not an exponential, it is the result of the available phase space. The higher the mass the smaller the probability from the phase space of n particles sharing the available energy that two particles will have a large invariant mass.
The background depends on the energies and experiments. One uses for the phase space Monte Carlo simulations that in the case of LHC and the Higgs include gluon jets and quark jets simulations. To get an idea of how the background looks in full see this presentation, page 17. The photon decay that you show in your question is a cleaner channel , but the kinematics is similar.
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$\begingroup$ Thanks, the experiment I have in mind is actually LHCb. Is that presentation still relevant? $\endgroup$ Commented Feb 5, 2015 at 9:51
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$\begingroup$ I just found the presentation to show how the background appears. Yes it is relevant. In the higgs plot above they have cut off the low and the high masses so it looks like an exponential, but it is not. It is a complicated function of phasespace variables. $\endgroup$– anna vCommented Feb 5, 2015 at 10:28