Tuning Pythia8,1: The Monash 2013 Tune

1. Tuning Pythia 8.1: The Monash 2013 Tune
The Role of Parton Distributions
Based on arXiv:1404.5630 with P. Skands and S. Carrazza
Juan Rojo
STFC Rutherford Fellow
University of Oxford
CMS Physics Comparisons and Generator Tunes
CERN, 09.05.2014
Juan Rojo CERN, 09/05/2014

2. 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
Juan Rojo CERN, 09/05/2014

3. 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
Juan Rojo CERN, 09/05/2014

4. 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
Juan Rojo CERN, 09/05/2014

5. 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
Juan Rojo CERN, 09/05/2014
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

6. 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
Juan Rojo CERN, 09/05/2014

7. 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
Juan Rojo CERN, 09/05/2014

8. Juan Rojo CERN, 09/05/2014
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

9. 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
Juan Rojo CERN, 09/05/2014

10. 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
Juan Rojo CERN, 09/05/2014

11. Juan Rojo CERN, 09/05/2014
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

12. Juan Rojo CERN, 09/05/2014
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

13. 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
Juan Rojo CERN, 09/05/2014