MMHT PDFs: Updates and Outlook

Abstract

We present the latest results of studies within the MMHT PDF framework. We discuss the impact of the most recent ATLAS 7 TeV jet data, demonstrating that while a good fit can be achieved for individual jet rapidity bins, it is not possible to achieve a good description of the data when all bins are fitted. We examine the role that the experimental correlated systematic uncertainties play in this, and demonstrate that by simply decorrelating no more than two sources of error between rapidity bins a remarkably improved description of the data can be achieved. We then study the impact of NNLO corrections, showing that a mild decrease in the fit quality is produced. We also present the results of including new LHC W , Z, W + c and tt data on the MMHT14 PDF set, showing that a marked decrease in the s + s uncertainty is in particular achieved. Finally, some discussion of the latest work towards the inclusion of the photon PDF within the MMHT framework is presented.

The impact of LHC jet data on the MMHT PDF fit at NNLO

“…On the other hand, for the low jet radius, and choice, some slight deterioration in the fit quality is observed. We note that in [34] a deterioration in the fit quality to the ATLAS data when going to NNLO was reported; however, here it was precisely these choices of scale and jet radius that were taken. Following the more detailed study in this work, we can see that this effect is not in general present.…”

“…Before considering the general impact of the jet data on the NNLO fit, some care is needed when dealing with the ATLAS data [23]. The discussion follows closely that given in [34], but for completeness we present a summary below. For illustration, we will work at NLO only, using as a baseline PDF the MMHT14 set [3] including the HERA I + II combined data [2], that is, as presented in [35].…”

“…However, for the in-situ statistical uncertainty the correlations are particularly well determined [37], and therefore we omit this from the investigation below. In fact, as shown in [34], decorrelating this source of uncertainty has a less significant impact on the quality of the data description in comparison to the other two sources.…”

Parton distributions from LHC, HERA, Tevatron and fixed target data: MSHT20 PDFs

“…An analysis of the effect of these was quickly produced [10], and it was concluded that while the improvement in PDFs was significant it was hardly dramatic, and a full update was not required. A small amount of new LHC data was also considered in [11], and then increasing amounts of new data and accompanying procedural improvements appeared in [12][13][14][15][16][17]. To begin with changes and improvements in the PDFs were incremental [10,11], with predictions for new data sets all good, and only relatively minor changes being required.…”

“…However, with increasing amounts of new data some problems appeared and changes in procedure and more significant changes in PDFs were required. In roughly chronological order the most striking of these were: difficulties in fitting some jet data [18], studied in [13,15], associated with correlated uncertainties (similar issues later also appearing in differential top data [19]); a degree of tension between the strange quark required to best fit new precise ATLAS W , Z data [20] and older dimuon structure function data studied in [14,17]; and a general need for the PDFs to have a more flexible parameterisation, particularly for the valence quarks andū,d difference in order to best fit a variety of new data [17]. Overall, this has resulted in a cumulative change compared to the MMHT14 PDFs mainly in the flavour sector, i.e.…”