MadGraph5_aMC@NLO

the error of cross section too large

Asked by karo

Hi,

     I was calcuating photon +jets process.
   p p > a j @1
   p p > a j j @2
   p p > a j j j @3

   I also have matched and set related parameter xqcut and qcut. I also set pta>20, etaa<2.5, delta(a,j)>0.3. After matched, the result is 1.337e+05+-1.4e+03 pb. The order of the error was so large. I didn't know whether the result was reliable. Or for this process, I must notice other phisical question which I have not known.

Thanks,
Karo

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Olivier Mattelaer (olivier-mattelaer) said : 		#1

No clearly the result is not reliable.

This is usually linked to a uncut divergencies ( or one which is not cut enough).
looking at the distribution might help you to see where/if this happen.

Cheers,

Olivier

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karo (karo-t) said : 		#2

Hi Oliver,

    Generally speaking, what value the error should be is normal in MG5. For example, the central value is e+06 pb order, what the error should be.

Thanks,
Karo

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Olivier Mattelaer (olivier-mattelaer) said : 		#3

Hi Karo,

looks like I sadi something stupid yesterday.
Your error is 10^-3 times the value so the statistical error is under-control.
(Yesterday I miss read your error) If you want smaller statistical error, this is just a matter of requesting more events.
Of course you expect at a point (if this is not already the case) the systematics error to be larger than the statistical error.

Cheers,

Olivier

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