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Homework Help: Computing a differential from plotted data

  1. Apr 24, 2012 #1
    I have a differential that depends only on [itex]\cos{\theta}[/itex]

    [tex]\frac{\operatorname{d}\sigma}{\operatorname{d} \Omega} = f(\cos{\theta})[/tex]

    I am numerically solving this differential equation for [itex]\sigma[/itex], which physically is a cross section, for [itex]0 \leq \theta \leq \pi[/itex]. The differential contains a parameter [itex]s[/itex]. I am solving the differential for a given value of [itex]s[/itex], then incrementing this parameter and solving the differential again.

    I am solving the differential over a range of [itex]s[/itex] (3 to 200), in increments of 0.1 ([itex]s[/itex] is, for the curious, my particle accelerator energy in GeV).

    My problem is that I'm asked to now plot the transverse momentum

    [tex]p_{T} = \lvert p_{f} \rvert \sin{\theta}[/tex]

    I've been told that I am effectively trying to plot

    [tex]\frac{\operatorname{d} \sigma}{\operatorname{d} p_{T}}[/tex]

    and that I'll have to do some sort of histogram/binning to make my plot.

    I'm not sure what I should binning; the differentials, the final integrated cross section?

    So, in essence, my questions is how can I plot [itex]\frac{\operatorname{d} \sigma}{\operatorname{d} p_{T}}[/itex] given that I have numerical data for [itex]\frac{\operatorname{d}\sigma}{\operatorname{d} \Omega}[/itex] (and [itex]\sigma[/itex])?
    Last edited: Apr 24, 2012
  2. jcsd
  3. Apr 25, 2012 #2


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    Normally posts which don't use the homework template tend to be at low priority, so keep that in mind for the future... but anyway: [itex]\mathrm{d}\sigma/\mathrm{d}p_T[/itex] can be physically interpreted as the "amount" of cross section which falls into a certain (small) range of [itex]p_T[/itex], divided by the size of that range. When you are plotting this differential cross section, it makes sense to plot it versus [itex]p_T[/itex].
  4. Apr 25, 2012 #3

    Apologies, I had trouble fitting the question in to the format. I shall try harder next time :)

    Do you mean I should plot [itex]\operatorname{d}\sigma/\operatorname{d} p_T[/itex] against [itex]p_T[/itex]? I know I need to do that, my problem is trying to find the differential values. i.e. How could I figure out "the 'amount' of cross section which falls into a certain (small) range of [itex]p_T[/itex]"?

    Would I calculate a small range in [itex]p_T[/itex] and then see what values of [itex]\sigma[/itex] (the integrated cross section) have a value in this range?
  5. Apr 25, 2012 #4


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    Yeah, that sounds about right. The details vary somewhat depending on exactly what kind of data you have, so it's kind of hard for me to be specific without actually seeing your numbers (or inventing an example); unfortunately I don't have time for that right now but if you will still be working on this in a couple days, perhaps I can get back to it).
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