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

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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 $W\to\mu\nu$ decay and muons with a transverse momentum, $p_{\rm T}$, larger than 20 GeV in the pseudorapidity range $2.0<\eta<4.5$. The partons are reconstructed as jets with $p_{\rm T} > 20$ GeV and $2.2 < \eta < 4.2$. 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 $Z\to\mu\mu$ decay. All results are in agreement with Standard Model predictions.

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Distributions of $ p_{\rm T}$ (\mu)/ $p_{\rm T} (j_\mu)$ with fits overlaid from (top) $\sqrt{s}=7\mathrm{\,TeV} $ and (bottom) $8\mathrm{\,TeV} $ data for (left) $\mu^+$ and (right) $\mu^-$.

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(top left) Two-dimensional SV-tag BDT distribution and (top right) fit for events in the subsample with $ p_{\rm T} (\mu)/ p_{\rm T} (j_\mu) > 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.

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(top left) Two-dimensional SV-tag BDT distribution and (top right) fit for events in the subsample with $0.5 < p_{\rm T} (\mu)/ p_{\rm T} (j_\mu) < 0.6$, 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.

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Projections of simultaneous fits of (left) $M_{\rm cor}$ and (right) SV track multiplicity for the SV-tagged subsample with $ {\rm BDT}(bc|udsg) > 0.2$ and $ p_{\rm T} (\mu)/ p_{\rm T} (j_\mu) > 0.9$. The highest $M_{\rm cor}$ bin includes candidates with $M_{\rm cor} > 10\mathrm{\,GeV} $. Combined data for $\sqrt{s}=7$ and 8 TeV for both muon charges are shown.

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Fits to $ p_{\rm T}$ (\mu)/ $p_{\rm T} (j_\mu)$ distributions for $b$-tagged data samples for $\sqrt{s} = 7$ and 8 TeV.

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Fits to $ p_{\rm T}$ (\mu)/ $p_{\rm T} (j_\mu)$ distributions for $c$-tagged data samples for $\sqrt{s} = 7$ and 8 TeV.

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Fits to the $ p_{\rm T} (\mathrm{SV})/ p_{\rm T} (j)$ distributions in (left) $7\mathrm{\,TeV} $ and (right) $8\mathrm{\,TeV} $ data for candidates with $ p_{\rm T} (\mu)/ p_{\rm T} (j_\mu) > 0.9$ and $ {\rm BDT}(bc|udsg) > 0.2$.

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Tables and captions

Summary of signal yields. The two $Zj$ yields denote the charge of the muon on which the trigger requirement is made. The $Zj$ yields given are the numbers of candidates observed, while the $W$ boson yields are obtained from fits. The yield due to top quark production is subtracted in these results.

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Systematic uncertainties. Relative uncertainties are given for cross section ratios and absolute uncertainties for charge asymmetries.

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Summary of the results and SM predictions. For each measurement the first uncertainty is statistical, while the second is systematic. All results are reported within a fiducial region that requires a jet with $ p_{\rm T} > 20\mathrm{\,GeV} $ in the pseudorapidity range $2.2 < \eta < 4.2$, a muon with $ p_{\rm T} > 20\mathrm{\,GeV} $ in the pseudorapidity range $2.0 < \eta < 4.5$, $ p_{\rm T} (\mu+j) > 20\mathrm{\,GeV} $, and $\Delta R(\mu, j) > 0.5$. For $Z+$jet events both muons must fulfill the muon requirements and $60 < M(\mu\mu) < 120\mathrm{\,GeV} $; the $Z+$jet fiducial region does not require $ p_{\rm T} (\mu+j) > 20\mathrm{\,GeV} $.

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Created on 09 December 2018.Citation count from INSPIRE on 18 December 2018.