Tuning Pythia8,1: The Monash 2013 Tune

Tuning Pythia 8.1: The Monash 2013 TuneThe Role of Parton Distributions

Based on arXiv:1404.5630 with P. Skands and S. Carrazza

Juan RojoSTFC Rutherford Fellow

University of Oxford

CMS Physics Comparisons and Generator TunesCERN, 09.05.2014

Juan Rojo CERN, 09/05/2014

Parton Distributions are an essential ingredient of Event Generation at the LHC

A crucial point is that, in Event Generators, PDFs are required to describe not only the perturbative physics but also the non-perturbative aspects from Underlying Event, Multiple Interactions etc

Leading order PDFs are particularly useful in these context, since the steep rise of the gluon at small-x allows to describe a wide variety of soft and semi-hard data keeping other parameters of the tune with reasonable values

NLO PDFs typically have flat or negative gluons at low scales and small-x, difficult to accommodate in Pythia tunes keeping all other parameters within natural values

The Monash 2013 tune of Pythia8 is based on NNPDF2.3LO as the default PDF set

In this talk, companion to Peter’s talk, I will present some of the PDF-related aspects of the new Monash 2013 tune, with enfasis on the differences between NNPDF2.3 and CTEQ6L, used in older Pythia tunes


LO PDFs are very different from NLO and NNLO PDFs, in particular for the gluon PDF

The reason for this is that in a LO fit, a steep rise of the small-x gluon is required to fit HERA data compensating the missing NLO corrections in the splitting functions and matrix elements

Note that at small-x, region relevant for the tuning of soft and semihard data, uncertainties are very large because of the absence of direct experimental constraints on the PDFs

Crucial to provide a robust, unbiased extrapolation of the gluon PDF in the small-x region


Various LO PDFs available for their use in Monte Carlo event generators

Some are vanilla LO PDFs, like NNPDF, others are modified LO (LO*) PDFs, like MRST07mod or CT09MC, for instance releasing momentum sum rule or using different running of �S(MZ)

Note strikingly different predictions for the gluon PDF both at small-x and large-x, which reflects the large theoretical uncertainties in LO PDF fits: direct impact on tuning studies


Various LO PDFs available for their use in Monte Carlo event generators

Some of them are vanilla LO PDFs, like NNPDF, others are modified LO PDFs, like MRST07mod and CT09MC, releasing the momentum sum rule or using a different running of �S(MZ)

In the NNPDF2.1LO study, it was found that the vanilla LO fit had essentially the same fit quality as LO* fits (without momentum sum rule, NLO running of �S(MZ))

We also found that the �2 in the LO was rather flat with respect to the choice of �S(MZ), with a slight preference for �S(MZ) = 0.130


Note at in LO fits, the flexible functional parametrization of NNPDF ensures that one achieves the best theoretical description of data when varying substantially the theory assumptions in the fit

For instance, CT/MSTW found that their �2 in the LO fits improved when removing the momentum sum rule, perhaps just because used too constrained PDF parametrization? Same for the NLO running of the strong coupling

See arxiv:1107.2652 for a complete discussion of LO vs LO* PDFs

The NNPDF2.3LO set is available as internal PDF set in Pythia8 (can be used without LHAPDF)

Two values of �S(MZ) available, and for completeness also the NLO and NNLO sets also provided

The NNPDF2.3 LO set includes QED corrections, so in particular can also be used to consistently simulate photon-induced contributions (for example for dilepton production), see arXiv:1308.0598

We use as baseline NNPDF2.3LO with �S(MZ)=0.130. As compared to CTEQ6L used in older tunes, the gluon is harder for x > 0.2 and x < 0.00003, and a bit softer in between

Note that at small-x NNPDF2.3LO has steepest gluon, important for correct description of forward data


The distribution of values of Bjorken-x probed in Minimum Bias events, including MPI, is directly sensitive to the shape of the gluon PDF

In the Monash 2013 tune, based on NNPDF2.3LO, we find a harder distribution at large and small-x as compared to previous tunes, based on CTEQ6L, consistently with the differences between the gluon PDFs



The distribution of values of Bjorken-x probed in Minimum Bias events, including MPI, is directly sensitive to the shape of the gluon PDF

In the Monash 2013 tune, based on NNPDF2.3LO, we find a harder distribution at large and small-x as compared to previous tunes, based on CTEQ6L, consistently with the differences between the gluon PDFs

It is also interesting to study the fraction of events which are initiated by gluons

The same trend as for the distribution of Bjorken-x is found

Note that for x < 0.1, between 80% and 90% of all MB events are gluon-initiated. This emphasizes the crucial role of a precision determination of the gluon PDF for these tuning studies


It is also interesting to study the fraction of events which are initiated by gluons

The same trend as for the distribution of Bjorken-x is found

Note that for x < 0.1, between 80% and 90% of all MB events are gluon-initiated. This emphasizes the crucial role of a precision determination of the gluon PDF for these tuning studies



The precise forward measurements from TOTEM and CMS are very important in the context of tuning Event Generators, since they provide direct access to the gluon PDF in a region with scarce constraints

The Monash2013 tune describes better the forward data, specially for TOTEM large rapidity data, where previous tunes clearly undershoot the data

This improved agreement in part can be explained by the NNPDF2.3 LO gluon being rather steeper at small-x than CTEQ6L


The precise forward measurements from TOTEM and CMS are very important in the context of tuning Event Generators, since they provide direct access to the gluon PDF in a region with scarce constraints

The Monash2013 tune describes better the forward data, specially for TOTEM large rapidity data, where previous tunes clearly undershoot the data

This improved agreement in part can be explained by the NNPDF2.3 LO gluon being rather steeper at small-x than CTEQ6L

Kinematics of 2 -> 2 scattering

Small pT and large y -> Probing the gluon PDF at very small-x

Leading Order PDFs are an essential ingredient for the tuning of Monte Carlo event generators, and in particular for the correct description of soft and semi-hard processes

These processes depend heavily on the shape and normalization of the gluon PDF in regions where experimental data is scarce: a robust and unbiased extrapolation of the gluon PDF in the small-x region is essential

The new Monash 2013 tune of Pythia8 uses NNPDF2.3LO as default PDF set

As compared to the previously used CTEQ6L, NNPDF2.3LO has a substantially steeper gluon at small-x and low scales, as well as a harder gluon for x > 0.2

A representative example of the advantages of using NNPDF2.3LO is the improved description of the CMS and TOTEM forward data, thanks partially to the steeper small-x gluon


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Tuning Pythia8,1: The Monash 2013 Tune