A Why does the 2gnnlo prediction in small diphoton transverse momentum diverge?

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As i know the qt-subtraction formalism is a method to cancel IR divergences, for example in small diphoton transverse momentum . nevertheless I can't understand why the prediction of 2gnnlo in https://arxiv.org/abs/1301.4536 diverges in region of small diphoton transverse momentum (qt) .
thank you in advance.
 
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are you referring to Fig.1b?
 
ChrisVer said:
are you referring to Fig.1b?
yes
 
So, I am not sure I understand your question though... 2gNNLO prediction seems to be far off the data by 2-3 stds in the whole diphoton pT spectrum? At least compared to SHERPA
 
I think i understand the answer. qt-subtraction formalism is like phase "phase space slicing" formalism suitable for reaction like 1+2->3+4+5 ,that was explained in http://citeseerx.ist.psu.edu/viewdoc/download?doi=10.1.1.344.8809&rep=rep1&type=pdf where 3 and 4 have high pt and 5 can be soft or collinear. whereas singularity related to qt->0 originated from 1+2->3+4 processes, consequently for this region of phase space we need soft gluon resummation.
 
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