Improving quark flavor separation with forward and production at LHCb
We quantify the constraints on the flavour separation between the quarks and antiquarks in the proton provided by the recent forward weak gauge boson production data from the LHCb experiment at and 8 TeV. Performed in the framework of the NNPDF3.1 global analysis, this study highlights the key role that the LHCb and data have in achieving a robust quark flavor separation in the large- region, including the strange and charm quarks. We demonstrate how the LHCb measurements lead to improved determinations of the the up and down quark PDFs in the region , with an uncertainty reduction that can be as large as a factor 2. We also show how the LHCb forward measurements severely restrict the size of the fitted charm PDF at large , imposing stringent constraints on non-perturbative models for the charm content of the nucleon.
Improving quark flavor separation with forward and production at LHCb
Juan Rojo††thanks: Speaker. ††thanks: On behalf of the NNPDF Collaboration.
Department of Physics and Astronomy, VU University, De Boelelaan 1081, 1081 HV Amsterdam,
and Nikhef Theory Group, Science Park 105, 1098 XG Amsterdam, The Netherlands.
The recent NNPDF3.1 global analysis  is the latest release from the NNPDF collaboration. It is based on the NNPDF3.0 framework  with two main improvements. On the one hand, a large number of new precision collider measurements have been added, including some that for the first time are included in a global fit such as the transverse momentum  and the differential distributions in top-quark pair production . On the other hand, the charm PDF is now treated on an equal footing as compared of the light quark PDFs , that is, in NNPDF3.1 is parametrized with an artificial neural network and then fitted to the data. This improved treatment of the charm PDF has several benefits, among which that of reducing the dependence of high-scale observables to the value of the charm quark mass .
Concerning the new experimental data, a wealth of recent electroweak gauge boson production measurements has been included in NNPDF3.1. These datasets provide useful information on the separation between different quark flavours, including strange and charm, as well as between quarks and antiquarks. In this respect, the forward weak boson production measurements presented by the LHCb collaboration are particularly interesting to access the large- region , where PDF uncertainties are the largest. NNPDF3.1 includes the complete LHCb and 8 TeV measurements of inclusive and production in the forward region in the muon channel [7, 8], which supersede all previous measurements in the same final state. These new measurements complement earlier LHCb data already included in NNPDF3.0 [9, 10]. In this contribution, we explore the impact of the LHCb data on the NNPDF3.1 analysis and show how sizable constraints are derived at large- for all the light quark PDFs and for the charm PDF. In turn, this leads to reduced theoretical uncertainties for the production of new massive BSM resonances, which are dominated currently by the large- PDF uncertainties .
Let us also mention that another set of recent LHCb measurements that have revealed an unexpected usefulness for PDF studies is forward meson production [12, 13]. As demonstrated in , the combination of LHCb charm production data at and 13 TeV leads to a reduction of the PDF uncertainties on the gluon at by up to an order of magnitude.
Forward weak boson production at LHCb. In Fig. 1 we show the kinematical coverage in the plane of the and production data from LHCb included in NNPDF3.1, compared with the corresponding coverage of production at ATLAS, CMS and the Tevatron, as well as with that of the HERA inclusive structure function data. For the Drell-Yan data, the values of for each data bin are approximated assuming leading order kinematics, namely , with and the invariant mass and the lepton rapidity of each bin. We observe that the LHCb data spans a wider and complementary range in as compared from the ATLAS and CMS measurements, and in particular ensure an improved coverage of the large- region.
In order to illustrate the impact of the LHCb data, in Fig. 2 we show the up quark and down quark PDFs at GeV, comparing the results of the NNPDF3.1 NNLO fit with those of the same fit without any LHCb data. We show the PDF ratios normalized to the central value of NNPDF3.1 and the relative PDF uncertainties. From this comparison, we see that the LHCb data has a significant impact in NNPDF3.1, both in terms of shifting the central value of the large- quarks, where the LHCb data prefer larger values, and in terms of reducing the PDF uncertainties. In the case of , the LHCb data reduce the PDF uncertainties by almost a factor 2 for .
Next, in Fig. 3 we show the quark-quark PDF luminosity and its relative uncertainty for the NNPDF3.1 fits with and without the LHCb data. We find that the LHCb data prefers harder values in the TeV region, and also that it leads to an important reduction of the PDF uncertainties in the same region.
In order to quantify the improvement in the description of the LHCb data achieved in the NNPDF3.1 analysis, in Table 1 we show the values for the individual LHCb datasets included in NNPDF3.1 (as well as their total) for three NNLO different fits: NNPDF3.1, NNPDF3.1 no LHCb, and NNPDF3.0. The numbers in brackets indicate those datasets not included in the corresponding fits. We see that NNPDF3.1 achieves a good description of the four LHCb datasets, markedly improving on the NNPDF3.0 predictions for the new 7 and 8 TeV measurements. We have also verified that the use of NNLO theory is crucial to achieve satisfactory values.
|Dataset||NNPDF3.1||NNPDF3.1 no LHCb||NNPDF3.0|
|LHCb 940 pb||1.4||1.3|
|LHCb 2 fb||1.1||1.2|
|LHCb 7 TeV||1.7|
|LHCb 8 TeV||1.4|
To further illustrate the improved agreement between theory and data, in Fig. 4 we show the comparison between the NNPDF3.1 and NNPDF3.0 NNLO predictions and the LHCb data on the lepton rapidity distribution in production at 8 TeV in the muon final state, normalized to the central value of the experimental data. We observe that not only the agreement is improved at the level of central values, specially in the forward region, but also that the PDF uncertainties for individual bins are reduced by up to a factor two in NNPDF3.1. In the same figure we also show the corresponding comparison for the rapidity distributuon, now in terms of the absolute cross-section. Also in this case we observe how the central values of the NNPDF3.1 prediction move closer to the data as compared to the NNPDF3.0 ones. It is also worth emphasizing that for all the LHCb datasets the agreement between NNLO theory and data is markedly better than at NLO.
Constraining the charm content of the proton. In the NNPDF3.1 global analysis, the charm PDF is parametrized at the input scale , with being the charm threshold, and then determined from experimental data in the same way as the light quark PDFs. It can be shown that the forward production data from LHCb provide, in addition to constrains on the light quark PDFs, also useful information on the charm content of the proton. This sensitivity can be understood from the fact that forward gauge boson production depends on charm PDF via partonic subprocesses such as and .
In Fig. 5 we show the charm PDF at GeV and its absolute uncertainty for the fits with and without LHCb data. We observe how the LHCb measurements lead to a suppressed at large-, as well as to a reduction of the associated PDF uncertainties. These results indicate that stringent constraints on models of the non-perturbative charm content of the proton can be provided by the LHCb data.
The impact of the LHCb data on the charm PDF can also be gauged by computing , the average momentum fraction carried by charm quarks in the proton, for the NNPDF3.1 fits with and without the LHCb data. When LHCb data is excluded we find that, for GeV, , while in the baseline NNPDF3.1 fit we have instead . This is a consequence that, as shown in Fig. 5, both the central value is reduced and the PDF error decreases. It is also interesting to note that, after the addition of the EMC charm data (which is not part of the NNPDF3.1 dataset), one finds , a central value consistent with the one preferred by LHCb. These results confirm the importance of the forward LHCb data for our understanding of the charm content of the nucleon.
Acknowledgments. We are grateful to S. Farry, P. Ilten and R. McNulty for discussions about the LHCb and data. This work has been supported by the ERC Starting Grant “PDF4BSM”.
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