Study of $W$ boson production in association with beauty and charm

TL;DR

This study measures W+jet production in the forward region using LHCb data at 7 and 8 TeV, extracting W+b and W+c components via SV-based heavy-flavor tagging and data-driven templates. By reporting cross-section ratios (σ(Wb)/σ(Wj), σ(Wc)/σ(Wj), σ(W^+j)/σ(Zj), σ(W^-j)/σ(Zj)) and charge asymmetries (A(Wb), A(Wc)) within a tight fiducial region, the analysis tests perturbative QCD and constrains proton PDFs, including strange and bottom content. All results agree with SM predictions, supporting gluon-splitting as the dominant W+b mechanism in the forward region and indicating sizable strange-quark contributions to W+c, with mild tensions in A(Wc) relative to certain PDFs. The measurements also validate the forward-region SM description and provide input for PDF refinements and heavy-flavor dynamics in proton-proton collisions.

Abstract

The associated production of a $W$ boson with a jet originating from either a light parton or heavy-flavor quark is studied in the forward region using proton-proton collisions. The analysis uses data corresponding to integrated luminosities of 1.0 and $2.0\,{\rm fb}^{-1}$ collected with the LHCb detector at center-of-mass energies of 7 and 8 TeV, respectively. The $W$ bosons are reconstructed using the $W\toμν$ decay and muons with a transverse momentum, $p_{\rm T}$, larger than 20 GeV in the pseudorapidity range $2.0<η<4.5$. The partons are reconstructed as jets with $p_{\rm T} > 20$ GeV and $2.2 < η< 4.2$. The sum of the muon and jet momenta must satisfy $p_{\rm T} > 20$ GeV. The fraction of $W+$jet events that originate from beauty and charm quarks is measured, along with the charge asymmetries of the $W\!+\!b$ and $W\!+\!c$ production cross-sections. The ratio of the $W+$jet to $Z+$jet production cross-sections is also measured using the $Z\toμμ$ decay. All results are in agreement with Standard Model predictions.

Figures (7)

• Figure 1: Distributions of $\hbox{$p_{\rm T}$}\xspace(\mu)/\hbox{$p_{\rm T}$}\xspace(j_\mu)$ with fits overlaid from (top) $\sqrt{s}=7~T eV\xspace\xspace$ and (bottom) $8~T eV\xspace\xspace$ data for (left) $\mu^+$ and (right) $\mu^-$.
• Figure 2: (top left) Two-dimensional SV-tag BDT distribution and (top right) fit for events in the subsample with $\hbox{$p_{\rm T}$}\xspace(\mu)/\hbox{$p_{\rm T}$}\xspace(j_\mu)\xspace > 0.9$, projected onto the (bottom left) ${\rm BDT}(bc|udsg)$ and (bottom right) ${\rm BDT}(b|c)$ axes. Combined data for $\sqrt{s}=7$ and 8 TeV for both muon charges are shown.
• Figure 3: (top left) Two-dimensional SV-tag BDT distribution and (top right) fit for events in the subsample with $0.5 < \hbox{$p_{\rm T}$}\xspace(\mu)/\hbox{$p_{\rm T}$}\xspace(j_\mu)\xspace < 0.6$, projected onto the (bottom left) ${\rm BDT}(bc|udsg)$ and (bottom right) ${\rm BDT}(b|c)$ axes. Combined data for $\sqrt{s}=7$ and 8 TeV for both muon charges are shown.
• Figure 4: Projections of simultaneous fits of (left) $M_{\rm cor}$ and (right) SV track multiplicity for the SV-tagged subsample with ${\rm BDT}(bc|udsg)\xspace > 0.2$ and $\hbox{$p_{\rm T}$}\xspace(\mu)/\hbox{$p_{\rm T}$}\xspace(j_\mu)\xspace > 0.9$. The highest $M_{\rm cor}$ bin includes candidates with $M_{\rm cor} > 10\mathrm{\,Ge V}\xspace$. Combined data for $\sqrt{s}=7$ and 8 TeV for both muon charges are shown.
• Figure 5: Fits to $\hbox{$p_{\rm T}$}\xspace(\mu)/\hbox{$p_{\rm T}$}\xspace(j_\mu)$ distributions for $b$-tagged data samples for $\sqrt{s} = 7$ and 8 TeV.