Nucleon PDF separation with the collider and fixed-target data
We consider the impact of the recent data obtained by the LHC, Tevatron, and fixed-target experiments on the nucleon quark distributions with a particular focus on disentangling different quark species. An improved determination of the poorly known strange sea distribution is obtained due to including data from the neutrino-induced deep-inelastic scattering experiments NOMAD and CHORUS. The impact of the associated production data by CMS and ATLAS on the strange sea determination is also studied and a comparison with earlier results based on the collider data is discussed. Finally, the recent LHC and Tevatron data on the charged lepton asymmetry are compared to the NNLO ABM predictions and the potential of this input in improving the non-strange sea distributions is evaluated.
keywords:Parton distributions, strangeness, hadron colliders
00 \journalnameNuclear Physics B Proceedings Supplement \runauthAlekhin et al. \jidnuphbp \jnltitlelogoNuclear Physics B Proceedings Supplement
The nucleon quark distributions are important ingredients of the high-energy hadron collision phenomenology, particularly at large Bjorken , since these are the quarks, which dominate the partonic content of the nucleon for these kinematics. The fixed-target inclusive deep-inelastic scattering (DIS) data, which play a central role in the determination of the parton distribution functions (PDFs), provide a constraint only on the linear combinations of the quark distribution.
Therefore, the global PDF fits are commonly based on a combination of the DIS data with ones for the Drell-Yan (DY) process, which yield a supplementary constraint on the PDFs. At that the strange sea distribution can be independently determined from the data on the -quark production in neutrino-nucleon DIS probing the strange sea by the charged-current transition . If the -quark produced is detected by its semi-leptonic decay into a muon, this process can be triggered by two muons in the final states. This signature was used by the NuTeV/CCFR experiments Goncharov:2001qe () at Tevatron and the NOMAD experiment Samoylov:2013xoa () at CERN. Alternatively, the events with a -quark in the final state can be selected by the hadronic decays of charmed hadrons using the emulsion technique, like in the CHORUS experiment KayisTopaksu:2011mx (). The NuTeV/CCFR data Goncharov:2001qe () have been employed to constrain the strange sea in the ABM PDF fit Alekhin:2008mb (); Alekhin:2012ig (); Alekhin:2013nda (). In the present proceedings we describe how the recent NOMAD Samoylov:2013xoa () and CHORUS KayisTopaksu:2011mx () data can further improve the strange sea determination. We also discuss the associated () production in the proton-proton collisions measured by the CMS Chatrchyan:2013uja () and ATLAS Aad:2014xca () experiments at the LHC. Similarly to the case of neutrino-induced DIS, this process receives essential contributions from the charged-current transition . Therefore, it can be also used to pin down the strangeness contribution to the nucleon. Finally, we check the recent data on the charged lepton asymmetry by the CMS Chatrchyan:2013mza () and D0 Abazov:2013rja () experiments measured in (anti-)proton-proton collisions w.r.t. the ABM12 PDFs Alekhin:2013nda (). Since the measurement gives a constraint allowing to separate the - and -quark distributions this comparison demonstrates a further trend in determination of the non-strange quark PDFs.
2 Strange sea improvement
The NOMAD experiment collected about 15K dimuon events in the neutrino-nucleon DIS Samoylov:2013xoa () exceeding the total statistics of earlier NuTeV/CCFR samples Goncharov:2001qe () by a factor of 1.5. Furthermore the NOMAD beam energy is essentially lower than the NuTeV/CCFR one that allows to probe the strange sea in the kinematic region inaccessible for NuTeV/CCFR, up to . The strange sea distribution can be extracted from the dimuon data provided the semi-leptonic branching ratio of the produced charmed hadrons is known. Meanwhile, their production rate essentially depends on the beam energy therefore an effective semi-leptonic branching ratio where is the relative production rate of hadron and is its semi-leptonic branching ratio, also depends on . We take into account this dependence employing an empirical parameterization of Ref. Samoylov:2013xoa ()
with the parameters constrained by the existing data on Ushida:1988rt (); KayisTopaksu:2005je (). The values of and obtained in the present analysis are in agreement with the ones obtained by NOMAD Samoylov:2013xoa (). With these values the parameterization Eq. (1) flattens in the beam energy range of NuTeV/CCFR and is consistent with the value of used in the earlier ABM12 fit Alekhin:2013nda ().
The CHORUS experiment collected 2013 events with the charmed hadrons produced in the neutrino-nucleon DIS using the emulsion detection technique KayisTopaksu:2011mx (). This is much smaller than the statistics of the dimuon experiments. However, due to the charm-decay vertex reconstruction, the CHORUS data are not sensitive to the corresponding branching ratios and provide in this way a complementary constraint on the strange sea.
The strange sea distribution obtained in the updated version of the ABM12 fit where the NOMAD and CHORUS data included is given in Fig. 1 as a ratio quantifying the suppression of the strange sea w.r.t. the non-strange one.
It is roughly constant in a wide range of and consistent with the value obtained earlier in our analysis of the NuTeV/CCFR dimuon data. However, the error in the updated determination of the strange sea at is improved up to a factor 2, mainly due to the impact of the NOMAD data Samoylov:2013xoa () (cf. more details in Ref. Alekhin:2014sya ()). The CHORUS data KayisTopaksu:2011mx () somewhat overshoot the fit and therefore pull the strange sea up.
The same is valid for the CMS and ATLAS data on production Chatrchyan:2013uja (); Aad:2014xca (), cf. Figs. 2, 3, although in this case the tension can be at least partially explained by the impact of the still missing NNLO QCD corrections to this process. Note, the ATLAS data are in a good agreement with the predictions based on a variant of the ABM12 fit with only CHORUS and CMS data are used to constrain the strange sea, while the CMS ones are also described very well in this variant of fit. This demonstrates a good agreement between these three samples, which all together prefer enhanced strange sea. To check the upper margin of allowed by this data subset we consider the variant of the fit with the NuTeV/CCFR and NOMAD data dropped and only the CHORUS, CMS, and ATLAS data used to constrain the strange sea.
In this case the value of at is enhanced as compared to the determination based on the combination of the NuTeV/CCFR, NOMAD, and CHORUS data. However the difference is within and it does not exceed 20% at maximum.
At the same time, the strange sea obtained in the ATLAS PDF analysis Aad:2012sb () is comparable with the non-strange one, cf. Fig. 1. The analysis of Ref. Aad:2012sb () is based on the ATLAS data on the inclusive production of the - and -bosons taken in combination with the HERA inclusive DIS data, while all fixed-target DIS and DY data are not considered. However, the result of the ATLAS analysis is in disagreement with the data by the FNAL-E-866 experiment on the ratio of the proton and deuteron DY-process cross sections Towell:2001nh (), which are commonly used to constrain the sea iso-spin asymmetry in global PDF fits. Indeed, the value of extracted from the FNAL-E-866 data in the Born approximation is positive for the whole range of and in the ATLAS analysis Aad:2012sb () negative values are preferred. This means, the enhancement of the strange sea observed by ATLAS is obtained at the expense of a suppression of the -quark PDF. Note, that negative values of were also observed in the NNPDF fit Ball:2012cx () based on collider data only and a corresponding strange sea enhancement appears there, too. Meanwhile, the discrepancy between the ATLAS results and the FNAL-E-866 data is not dramatic and amounts to about only. Moreover, the ATLAS result on is in agreement with ours within the errors, cf. Fig. 1. On the other hand, the ATLAS data on the - and -boson production Aad:2011dm () are well described by the PDF set with the suppressed strange sea. In particular, in the updated variant of the ABM12 fit with the NOMAD and CHORUS data included we get a value of for 30 data points in the combined ATLAS - and -data set. In summary, these findings rather point at an insufficient statistical potential of the data used in the ATLAS analysis of Ref. Aad:2012sb () in disentangling the quark PDFs than at a real strange sea enhancement. In this context it is also worth noting that in the analysis of Ref. Chatrchyan:2013mza () based on the combination of the CMS and HERA data the strange sea suppression at was observed, cf. Fig. 1.
3 Constraints on the non-strange quark PDFs
Recent data on the muon asymmetry obtained by CMS Chatrchyan:2013mza () for the sample with an integrated luminosity of 4.7 1/fb are compared with the predictions based on the NNLO ABM12 PDFs Alekhin:2013nda () in Fig.5. The predicted central values and their uncertainties due to PDFs are computed with DYNNLO (version 1.3) Catani:2009sm () and FEWZ (version 3.1 Li:2012wna ()), respectively, because the former code shows a better numerical convergence, while the latter allows for a parallel computation with different PDF set members. The agreement between data and the predictions is in general quite good. For the data sample with the predictions do somewhat undershoot the data at the muon pseudo-rapidity since the ABM12 PDF were tuned to the earlier CMS data on the electron asymmetry Chatrchyan:2012xt (), which go lower than the muon ones, cf. Fig. 5. However, this tension is within the 1 PDF uncertainties. Furthermore, for the data sample with a tension does not show up. Therefore the ABM12 fit does not change significantly due to inclusion of the CMS data on the electron asymmetry. The values of obtained in the variants of this fit with the data at and included are 16 and 10, respectively, for 11 data points. The former value is somewhat worse than the ideal one due to the data fluctuations at going beyond the experimental errors quoted. The change in the ABM12 PDFs due to the CMS muon asymmetry data is within with the most significant impact observed for the -quark distribution. The uncertainty in the latter at is also somewhat improved, cf. Fig. 6.
In contrast with the CMS muon asymmetry and the related LHC data in general, the Tevatron data on the charged lepton asymmetry do not demonstrate a good agreement with the ABM12 predictions. In particular, recent D0 results on the muon asymmetry Abazov:2013rja () obtained with the integral luminosity of 7.3 1/fb significantly overshoot the ABM12 predictions at , cf. Fig. 7. A discrepancy is especially pronounced for the sample with the cut of . In this case the shape of the ABM12 predictions differ from the data even qualitatively. The agreement is improved in case the D0 data are added to the ABM12 fit. However, even in this case the profile of data at is not entirely reproduced and the value of is 40 for 10 data points. A better value of can be achieved in a model-independent fit with a 5-th order polynomial. However, in this case the fitted curve exhibits a step at , which evidently cannot be provided with a smooth PDF shape. The same step, although less pronounced, appears in the profile of the D0 sample with , cf. Fig. 7. It is worth noting that the MSTW08 PDFs being tuned to the earlier Tevatron data also demonstrate poor agreement with the D0 data of Ref. Abazov:2013rja (). This may point to a disagreement with the earlier Tevatron data. These issues should evidently be clarified before the recent D0 data can be included into our PDF fit.
In conclusion, we find that the ABM strange sea distribution can be essentially improved at by adding to the fit recent NOMAD and CHORUS data on the charm production in the DIS neutrino-nucleon scattering. The updated PDFs are in agreement with the ABM12 ones although the CHORUS data obtained on the emulsion target demonstrate a particular trend w.r.t. the results of dimuon experiments NuTev, CCFR, and NOMAD pulling strange sea somewhat up. The CMS and ATLAS data on the associated production in the proton-proton collisions, which are also sensitive to the strangeness content of nucleon, overshoot the ABM predictions, however, statistical significance of the discrepancy is inessential in view of the large data uncertainties and foreseen impact of the NNLO corrections to this process. From the variant of the ABM fit based on the CHORUS, CMS, and ATLAS data preferring enhanced strange sea we estimate an upper margin of such enhancement as 20%.
The recent CMS high-statistics data on the muon asymmetry are in a good agreement with the ABM12 NNLO predictions and allow further improvement in disentangling the - and -quark distributions. As a result, the uncertainty in ABM -quark distribution at can be reduced by adding the CMS sample to the fit. At the same time, the D0 data on the muon asymmetry, which can potentially further improve the quark PDF uncertainties at bigger values of , demonstrate poor agreement with the ABM12 predictions and cannot be employed in our analysis before clarification of these discrepancies.
Acknowledgments. We would to thank P. Marquard for careful reading of the text and valuable comments. This work has been supported in part by Helmholtz Gemeinschaft under contract VH-HA-101 (Alliance Physics at the Terascale), DFG Sonderforschungsbereich/Transregio 9 and by the European Commission through contract PITN-GA-2010-264564 (LHCPhenoNet). We thank the Galileo Galilei Institute for Theoretical Physica for the hospitality and the INFN for partial support during the completion of this work.
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