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title: The renormalisation scale in SPheno
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permalink: /The_renormalisation_scale_in_SPheno/
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---
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[Category:SPheno](/Category:SPheno "wikilink")
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General
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-------
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The renormalisation scale at which SPheno performs the loop calculations to the mass spectrum can be defined in [SPheno.m](/SPheno.m "wikilink") via
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RenormalizationScale=...
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As argument either a fixed value can be given or a function of parameters of the considered model or of calculated tree-level masses. In addition, if a high-scale SPheno version is considered, an approximative scale must be given via
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RenormalizationScaleFirstGuess=...
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This choice for the scale is used only in the first iteration.
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### Example
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1. In the MSSM, it is common to use the mean of the stop masses are scale. If one can be sure that the two stops are the lightest and heaviest of the six up squarks are, the brief definition
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RenormalizationScale = MSu[1]*MSu[6];
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is possible. For the general case, the long form
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RenormalizationScale = Sqrt[(mq2[3, 3] + (vu^2*conj[Yu[3, 3]]*Yu[3, 3])/2)*(mu2[3, 3] + (vu^2*conj[Yu[3, 3]]*Yu[3, 3])/2)
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-((vd*\[Mu]*conj[Yu[3, 3]] - vu*conj[T[Yu][3, 3]])*(vd*conj[\[Mu]]*Yu[3, 3] - vu*T[Yu][3, 3]))/2];
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is the safe choice.
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2. For a first evaluation of the RGEs, before any mass has been calculated and SPheno hasn’t any glue about the renormalization scale, an approximated scale as function of the CMSSM parameters can be defined via
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RenormalizationScaleFirstGuess = m0^2 + 4 m12^2;
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Fixing the scale via the Les Houches file
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-----------------------------------------
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Independent of the settings for `RenormalizationScale` there are two possibilities to fix the scale via the Les Houches input file of SPheno
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1. **SPA conventions**: If SPA conventions are turned on via
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Block SPhenoInput # SPheno specific input
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...
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2 0 # SPA conventions
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the scale is always put to 1 TeV.
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2. **Fixed value**: One can fix the numerical value of the renormalisation scale via
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Block MODSEL #
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...
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12 VALUE # Renormalisation scale
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where `VALUE` is an integer or real.
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See also
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========
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= |
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\ No newline at end of file |