[Clascomment] OPT-IN: Differential cross sections and spin density matrix elements for the reaction gamma p --> p omega

[Marco Ripani]

Hi Mike,
thanks now I think I got it. And indeed I had overlooked that in the 
second term the uncertainties were first summed, then squared.

Cheers
Marco

Mike Williams wrote:
>
> Hi Marco,
>
> To fully understand the bkgd method, one would have to read the JINST 
> paper (which is in the "minor revision" stage now and should be 
> published by the end of the summer...hopefully).  In Eq.(7), the 1st 
> term (summing Q_i^2) is the statistical error and the second term (sum 
> sigma_Q_i)^2 is the Q-factor uncertainties assuming 100% correlation 
> (since they're added up, then squared...that's 100% correlated).
>
> To understand the stat error, consider a simple example where all of 
> the Q's are the same.  In that case, sum Q^2 = Q^2 N (where N is the 
> number of events).  Thus, sigma = Q sqrt(N).  If Q=1 (all signal), the 
> you'd get a stat error of sqrt(N) as expected.  For Q!=1, you need to 
> remember that the stat error is on the total event sample (not the 
> number of signal). Thus, e.g. Q=1/2, the stat error is 0.5 sqrt(N) for 
> this simple example.
>
> Cheers,
>
> Mike
>
> On Fri, 31 Jul 2009, Marco Ripani wrote:
>
>> Hi,
>> I don\'t understand formula (7) in the paper. Why do you add Q_i^2 to 
>> the yield uncertainty and in what sense does it represent 100 % 
>> correlation ? Sorry, I must be missing something here.
>>
>> Thanks
>> Marco
>>

-- 

Marco Ripani
Senior Scientist, Istituto Nazionale di Fisica Nucleare - Genova
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