QCD analysis of CMS W + charm measurements at LHC with \sqrt s = 7TeV and implications for the strange PDF

We calculate cross-sections and cross-section ratios of a charm quark production in association with a $W$ gauge boson at next-to-leading order QCD using MadGraph and CT10NNLO, CT14NNLO, and MSTW2008NNLO PDFs. We compare the results with measurements from the CMS detector at the LHC at a center-of-mass energy of 7TeV. Moreover, we calculate absolute and normalized differential cross-sections as well as differential cross-section ratios as a function of the lepton pseudorapidity from the $W$ boson decay. The correlation between the CT14NNLO PDFs and predictions for $W+$ charm data are studied as well. Furthermore, by employing the error PDF updating method proposed by the CTEQ-TEA group, we update CT14NNLO PDFs, and analyze the impact of CMS 7TeV $W+$ charm production data to the original CT14NNLO PDFs. By comparison of the $g(x,Q)$, $s(x,Q)$, $u(x,Q)$, $d(x,Q)$, $\bar u(x,Q)$, and $\bar d(x,Q)$ PDFs at $Q=1.3$ GeV and $Q = 100$ GeV for the CT14NNLO and CT14NNLO+Wc, we see that the error band of the $s(x,Q)$ PDF is reduced in the region $x<0.4$, and the error band of $g(x,Q)$ PDF is also slightly reduced in the region $0.01


I. INTRODUCTION
In the standard model (SM), the associated W + charm production in hadron collisions is described at leading order (LO) in perturbative quantum chromodynamics (QCD) by g + q → W − + c, (q = d, s, b) and g +q → W + +c, (q =d,s,b). Although the dquark parton distribution function (PDF) is large in the proton, the processes g + d → W − + c and g +d → W + +c contribute only about 10% [1] to the total W + charm production rate, because it is suppressed by the small quark-mixing Cabibbo-Kobayashi-Maskawa (CKM) matrix element [2] |V cd | and |Vcd|. The major contribution to the total W + charm production rate is due to strange quark-gluon fusion g + s → W − + c, and g +s → W + +c. The contribution from g + b → W − + c and g +b → W + +c is also heavily suppressed by the quark mixing matrix elements (|V cb |, |Vcb|) and the b-quark PDF. The W + charm production cross-section is therefore particularly sensitive to the proton g(x, Q) and s(x, Q) PDFs [3] and to the magnitude of the CKM matrix element V cs , where x is the momentum fraction of the proton carried by the s-quark, and Q is the hard scale. The study cited in Ref. [4] calculated the W + charm production at LO and next-to-leading order (NLO) in QCD, and found that the factorization and renormalization scale uncertainty in the NLO calculation is about 20%. Ref. [5] explored the strangeness degrees of freedom in the parton structure of the nucleon within the global analysis framework, and showed that the precise determination of the s(x, Q) PDF affects the W + charm cross-section. The s(x, Q) PDF has been determined by neutrino-nucleon deep inelastic scattering experiments at momentum transfer squared Q 2 = 10 GeV, and momentum fraction x ∼ 0.1 [6,7]. The Tevatron CDF [8] and D0 [9] experiments have measured the cross-section for charm quark produced in association with W bosons, using muon tagging of the charm-quark jet. The ATLAS collaboration [10] measured the total cross-section, differential cross-section as a function of the pseudorapidity of the lepton from the W boson decay, and the cross-section ratio of the production of a W boson in association with a single charm quark at √ s = 7 TeV. The CMS experiment measured [11] total cross-sections (σ(W − + c), σ(W + +c)), absolute and normalized differential cross-sections as a function of the absolute value of the pseudorapidity of the lepton from the W boson decay, and the cross-section ratio R c = σ(W + +c)/σ(W − +c) at a center of mass energy 7TeV for the fiducial region defined, namely p j T > 25 GeV, |η j | < 2.5, |η l | < 2.1, p l T > 25 GeV, for W → µν µ , p l T > 35 GeV, for W → µν µ and W → eν e .
There are two different transverse momentum cuts for the charged lepton in the final state.
When p l T > 25 GeV, we only consider the muon decay channel(W → µν µ ) for W boson; both muon(W → µν µ ) and electron(W → eν e ) decay channels for W boson are considered, when p l T > 35 GeV. This study is organized as follows: in Section II, we present our results for various latest PDF sets and compare these with the CMS measurements of the total cross-section, absolute and normalized differential cross-sections and ratios, as well as the correlation between the CT14NNLO PDFs and predictions for W + charm data. In Section III, we discuss the impact of the CMS W + charm production 7TeV data to the CT14NNLO PDFs. In Section IV, we draw our conclusions.

II. RESULTS
In this section, we present a detailed numerical study of the pp → W + c + X process at the LHC at a center-of-mass energy of 7TeV at NLO order QCD using the Monte-Carlo numerical calculation program MadGraph [12] with CT10NNLO [13], CT14NNLO [14], and MSTW2008NNLO [15] PDFs. PDF uncertainties on the theoretical predictions are given at 68% confidence level (C.L.). We calculate the total cross-section, differential (absolute and normalized) cross-sections, and the cross-section ratio R c = σ(W + +c)/σ(W − + c) with the W → lν decay (where l = µ or e). In our study, we use the same kinematical cuts as the CMS detector at the LHC at a center-of-mass energy of 7TeV [11], that are given in section I; both the factorization and renormalization scales are set to the value of the W boson mass µ R = µ F = m w ; charm quark mass is considered and is set to 1.550 GeV; strong interaction coupling α s is set to 0.118, and for electro-weak parameters, the W boson mass is set to 80.385 GeV; Fermi coupling is set to as 1.166 × 10 −5 GeV −2 ; related CKM matrix elements are set to as V cd = 0.225 and V cs = 0.974; the mass of charged light leptons is considered and set to as m e = 0.511 MeV and m µ = 105.658 MeV. At LO, the Feynman diagrams for the hard scattering processes of the W + charm production pp → W + c + X are shown in Fig.1. The main contribution for the cross-sections of W + charm production comes from strange quark and gluon scattering, the down-quark contribution is strongly Cabibbo  The total cross-sections σ(W + +c) and σ(W − + c) of the production of a W boson in association with a charm quark in pp collisions at √ s = 7 TeV at NLO QCD are summarized in Table I. PDF uncertainties are at 68% confidence level (C.L.), that are obtained from the error sets of the CT10NNLO, CT14NNLO, and MSTW2008NNLO. The experimental measurements from CMS collaboration at the LHC at a center-of-mass energy of 7TeV [11] are also included in this    Table II. The strange quark contributes the most to this W + charm production.
With regard to the parametrization of the strange-quark content of the proton, CTEQ-TEA and MSTW2008 PDF groups make different assumptions in their global fits. In CT10NNLO and CT14NNLO, the strange is parameterized symmetrically s =s, and in MSTW2008, it is parameterized asymmetrically, s −s = 0. Hence corresponding theoretical predictions differ accordingly. For MSTW2008, the production of W + + c is slightly larger than the W + +c, as expected because of the s −s asymmetry. Because of the dominance of the d quark over thed -quark in the proton, the production of W − + c is larger than W + +c. The numbers in bracket correspond to p l T > 35 GeV. B. Absolute and normalized differential cross-section The absolute and normalized differential cross-sections are obtained by MadGraph using the same setup as the CMS collaboration at the LHC and a center-of-mass energy of 7TeV.
In Fig.3 and Fig.4, we compare the absolute and normalized differential cross-sections in bins of lepton pseudo-rapidity with CMS measurements. The absolute and normalized differential cross-sections with PDF uncertainty at 68% C.L. are summarized in Table III     26.6 ± 3.8% ± 6.4%  We calculated total (σ(W − + c), σ(W + +c)) and differential (absolute and normalized) cross-sections independently under the same conditions in Subsections II A and II B. The CMS [11] collaboration introduced the charged cross-section ratio, The advantage of using this ratio is that many of the theoretical and experimental uncertainties can cancel. The comparison of the total cross-section ratio and differential cross-section ratio with PDF uncertainty at 68% C.L. with CMS data are shown in Fig.5 and Fig.6, the left column corresponds to p l T > 25 GeV, and right one is for p l T > 35 GeV. The total cross-section ratio and differential cross-section ratio are also summarized in Table V and Table VI. From tables V and VI, we see that the total cross-section ratio, differential cross-section ratio, and the associated PDF uncertainties are different for the CT10, CT14, and MSTW2008 PDF sets. These differences arise from the parametrization assumptions in each global analysis. For example, the CT10 and CT14 PDF sets assume s(x, Q) =s(x, Q), cross-section ratios almost exclusively are determined by the d −d asymmetry and with a very small PDF uncertainty. In contrast, the MSTW08 PDF set assumes asymmetric strangeness s(x, Q) −s(x, Q) = 0, that yields a larger PDF uncertainty in the prediction.   0.784 ± 6.4% ± 1.4%

D. Correlation between the CT14NNLO and predictions for W + charm data
One way to determine the sensitivity of a specific data point to some PDF f i (x, Q) at a given x and Q is to compute a correlation cosine between the theoretical prediction for this point and the PDFs of various flavors [16][17][18]. Therefore, we will study the correlations between CT14NNLO PDFs of various flavors at specific x and each data point of CMS 7TeV W + charm production with transverse momentum of the charged lepton from W boson decay at the p l T > 35 GeV region. However first we briefly provide the definition of the correlation cosine. If there are two variables X( a j ) and Y ( a j ) in the parameter space, where a j are the PDF parameters, then the correlation cosine can be expressed as: where ∇X and ∇Y are gradient of the variables X(a j ) and Y (a j ). For X(a i ), ith component of gradient vector is where X + (a j ) and X − (a j ) are computed from the two sets of PDFs along the positive and negative direction of the i-th eigenvector. The quantity cosφ characterizes whether the variables X and Y are correlated (cosφ ∼ 1), anti-correlated (cosφ ∼ −1) or not correlated (cosφ ∼ 0).  Fig.7. In each subfigure, the correlation between one of the PDF flavors with each data point is distinguished by different type of line. Solid, long-dashed-dotted, dotted, shortdashed, and short-dashed-dotted lines correspond to correlation of differential cross-section, differential cross-section ratio, normalized differential cross-section, total cross-section, and total cross-section ratio data respectively. As we discussed in Section II, differential crosssection, differential cross-section ratio and normalized differential cross-section data has included five data points that are measured by five rapidity bin ranges. The lines with darker color correspond to higher rapidity bin range. In the case of the total cross-section, differential cross-section and ratio, s(x, Q) PDF correlations are most significant (cos φ ∼ 1) at x from few times 10 −2 to few times 10 −1 , when Q = 1.3 GeV and Q = 100 GeV respectively. However at other x range, the s(x, Q) PDFs correlation is not very strong. There are no clear relations between the rapidity bin range and correlation cosine, however it can be seen from two the subfigures at the first row that each data point in various rapidity bins has a strong correlation with s(x, Q) PDF at the x-region mentioned above. There is other information for the normalized differential cross-section, which includes five data points that are partially correlated and partially anticorrelated, and represented with each flavor, as illustrated in each subfigure of Fig.7 with The study cited in Ref. [19] presented a software package, ePump (error PDF updating method package), that can be used to update or optimize a set of PDFs, including the best-fit PDF set and error PDFs, and to update any other set of observables. Furthermore, Ref. [20] and Ref. [21] cite interesting further studies using ePump. In this section, we use ePump to analyze the impact of CMS 7TeV W + charm production measurements on the CT14NNLO PDFs. To update CT14NNLO PDFs, we use the CMS 7TeV total cross-section(one data point), differential cross-section(five data points), total cross-section ratio(one data point) and differential cross-section ratio(five data points), as well as combined data sets and their NLO QCD predictions from MadGraph as ePump inputs. CT14NNLO+sig, CT14NNLO+dsig, CT14NNLO+R, CT14NNLO+dR, and CT14NNLO+Wc in Figs.8-9 are the ePump-updated PDFs by total cross-section data, differential cross-section data, total cross-section ratio data, differential cross-section ratio data, and combined CMS 7TeV W + charm data. The weight factor for each data is three in our ePump studies. A weight larger than one is equivalent to having more data points with the same experimental uncertainties or, alternatively, to reducing the experiment uncertainties by a factor of the square root of the weight. In the combined data, we excluded the normalized differential cross-section data to avoid double counting. After updating, the relative changes in CT14NNLO ensembles are best visualized by comparing their PDF error band and PDF ratio, in which ratio plot is obtained by dividing the error set and best fit of updated PDFs by the best fit of original CT14NNLO PDFs. In Figs At the LO, u quark does not contribute to the W + charm production cross-section, however it does so beyond the LO. In our ePump study, we employ the theoretical prediction of the W + charm production cross-section at NLO QCD. Therefore, we compare the ePumpupdated PDFs via CMS 7TeV W + charm data and CT14NNLO PDFs to see the impact on u(x, Q) andū(x, Q) PDFs in CT14NNLO for both Q = 1.3 GeV and Q = 100 GeV. We found that the central value and uncertainties of the ePump updated u(x, Q) andū(x, Q) are almost unchanged.
In Fig.10, we compared the s(x, Q) PDF from CT14NNLO, and ePump-updated s(x, Q) PDF from combined CMS 7TeV W + charm data with weights three amd ten. Fig. 10 shows that the s(x, Q) PDF error band greatly decrease at x < 0.4 for Q = 1.3 GeV and Q = 100 GeV when the weight factor is increased from three to ten. The best fit s( Using ePump [19], we updated CT14NNLO PDFs. One might want to know how the inclusion of the CMS 7TeV W + charm data in the global PDF fits would modify the prediction and uncertainties for any other set of observables including the original observables that were used for updating the CT14NNLO PDFs. ePump can also directly update predictions and uncertainties for any observables after including new data. In Fig.11, we compare the predictions from CT14NNLO PDFs and CT14NNLO+Wc PDFs, obtained by updating the CT14NNLO with CMS 7TeV W + charm data using ePump with the CMS 7TeV W + charm data. Fig.11 shows that the uncertainties decreased after the update, and the predicted central value is also closer to the data.

IV. CONCLUSIONS
In this study we calculated the total and differential cross-sections and cross-section ratios using the MadGraph up to O(α 2 s ) with a massive charm quark m c = 1.55 GeV for three NNLO PDF sets: MSTW2008, CT10, and CT14NNLO. Subsequently, we compared the experimental measurements of W + charm production at √ s = 7 TeV at LHC. In our calculation, we use the same kinematic cuts as experimental measurements: p jet T > 25 GeV, |η jet | < 2.5, and |η l | < 2.1 GeV, and two different transverse momentum cuts p l T > 25GeV for the W → µν channel and p l T > 35 GeV for the W → µν and W → eν channels. In our calculation, both the factorization and the renormalization scales are set to the value of the W boson mass, and α s (M Z ) is set to the central value provided by the respective PDF groups.
Our results are summarized in Tables I -VI and in Figures 2 -6, where the central value of the prediction and the PDF uncertainty are given. The theoretical predictions from various PDFs agree well with experimental measurements. However, there are some differences depending on the PDFs used in the calculations. For example, unlike the assumption in MSTW20018 NNLO PDFs, the CT10 and CT14 assume s =s in the proton, yielding to a total and differential cross-sections ratio dominated by the d −d asymmetry. The total and differential cross-sections are larger for the W − + c production than for W + + c, because the former process involves a d, whereas the latter involvesd (sea) antiquark. Hence, both total and differential cross-section ratios are smaller than 1.0. Fig. 7 shows that the observable from the CMS 7TeV W + charm production has a strong correlation with the strange(anti) quark PDFs, therefore these measurements also provide a direct constraint on the strange(anti) quark content of the proton.
Furthermore, using the ePump updating method, and CMS 7TeV W + charm production data at lepton transverse momentum p l T > 35 GeV, we find that these data sets mainly reduce the s(x, Q) PDF error band and increase magnitude of its best fit in the x < 0.4 region for both Q = 1.3 GeV and Q = 100 GeV. In Fig.11, we also compare the predictions from CT14NNLO PDFs and CT14NNLO+Wc PDFs.