Asymmetries -vs- Differences in xsecs

[virtual_image]

Hi, I was asked today about the difference between asymmetries and 'differences in the corresponding cross sections'. Can anyone help me with this? Also, what are the key measurements needed to calculate asymmetries (BSA, BCA, TTSA etc) and what measurements are needed to calculate the differences in the cross-sections?

e.g. F1, F2, electric charge etc

I'd be interested in comparing the measurements needed for each to see where they differ

 

[humanino]

Hi, I was asked today about the difference between asymmetries and 'differences in the corresponding cross sections'. Can anyone help me with this?

I don't know if what I'm going to tell you is trivial : the all point of asymetries as compared with cross section is that efficiencies and acceptances for detection cancel in the ratio, making asymetries so much easier to measure. Except for that, there is a common misconception that the denominator is not important, or constant in each single bin, but it is often not the case, and knowledge of cross section difference is more valuable than just asymetries.

 

[virtual_image]

Thanks,

the discussion I was having was about the two differing approaches that different physicists use. It was brought up that some use the asymmetries and others use the cross section differences.

What I was wondering was what are the benefits of using each. For example, in the BCA case (say), are there less things needed to be calculate to work out the asymmetries or cross sectional differences?

It was suggested I make a table of all quantities required to compare both methods. e.g do both require F1 and/or F2 to be extracted? which CFF's etc

 

[humanino]

What I was wondering was what are the benefits of using each. For example, in the BCA case (say), are there less things needed to be calculate to work out the asymmetries or cross sectional differences?

If you want to calculate asymetries, you mostly just make the ratio of the number of counts. If you want to calculate the cross section, you have to worry about the efficiencies, acceptances, and all normalizations that can appear. It is very clear that if you know the numerator and the denominator separately, you know more than if you just know the ratio. This is not even specific to physics.

So it all depends on what you want to do exactly. And it is here it gets technical.

It was suggested I make a table of all quantities required to compare both methods. e.g do both require F1 and/or F2 to be extracted? which CFF's etc

I guess you are talking about Dirac and Pauli form factors F1 and F2. They are usually determined from a Rosenbluth separation of electric and magnetic Sachs FFs. This amounts to measure the cross section at fixed angle and different Q2.

But what are CFF's ? Compton Form Factors ?

 

[virtual_image]

Yeah Compton Form Factors

Cheers for the reply

 

[humanino]

Compton Form Factors

Compton Form Factors appear in various contexts however, so it is difficult to tell you more without further information.

Assuming you are working on DVCS for instance, I can tell you that extracting GPDs from asymetries is not an easy task. At least in a model independent manner.