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Measurement of the $D_s^+ - D_s^-$ production asymmetry in 7 TeV pp collisions

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Abstract

Heavy quark production in 7 TeV centre-of-mass energy pp collisions at the LHC is not necessarily flavour symmetric. The production asymmetry, A_P, between Ds+ and Ds- mesons is studied using the \phi\pi(+/-) decay mode in a data sample of 1.0/fb collected with the LHCb detector. The difference between \pi+ and \pi- detection efficiencies is determined using the ratios of fully reconstructed to partially reconstructed D*(+/-) decays. The overall production asymmetry in the Ds rapidity region 2.0 to 4.5 with transverse momentum larger than 2 GeV is measured to be A_P=(-0.33 +/- 0.22 +/- 0.10)%. This result can constrain models of heavy flavour production.

Figures and captions

Production of $c\overline{c}$ quark pairs in a $pp$ collision via gluons.

Ds-thy.pdf [58 KiB]
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Ds-thy.pdf

Distributions of mass differences in partial reconstruction for (a) RS $m(\pi ^+ _{s} K ^- \pi ^+\pi ^- )-m( K ^- \pi ^+\pi ^- )$ and (b) WS $m(\pi ^+ _{s} K ^+ \pi ^+\pi ^- )-m( K ^+ \pi ^+\pi ^- )$ candidates, for magnet up data. The (green) dotted line shows the signal, the (red) dashed line the background, and the (blue) solid line the total. The fit shapes are defined in Appendix A.

mag_0_[..].pdf [87 KiB]
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mag_0_phi_total_p_bin_0_pi_plus_part.pdf

Distributions of the mass difference $\Delta m_{\text{full}}$ for (a) RS and (b) WS events using magnet up data. The (green) dotted line shows the signal, the (red) dashed line the background, and the (blue) solid line the total. The fit shapes are defined in Appendix B.

mag_0_[..].pdf [67 KiB]
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mag_0_phi_total_p_bin_0_pi_plus_full.pdf

Distribution of fully reconstructed signal candidates for magnet up data as a function of pion (a) $p$ and (b) $p_{\rm T}$ .

Contro[..].pdf [115 KiB]
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Control_sample_p-pt.pdf

Relative detection efficiency in bins of detected pion momentum: (red) circles represent data taken with magnet polarity up and (blue) squares show data taken with magnet polarity down. Only statistical errors are shown.

relati[..].pdf [39 KiB]
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relative_efficiency.pdf

Relative efficiency averaged over magnet up and magnet down samples versus pion $p$ and $p_{T}$.

p_slic[..].pdf [56 KiB]
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p_slice_all.pdf

Azimuthal angle distribution of $\epsilon(\pi^+)/\epsilon(\pi^-)$ for magnet up data (red circles) and magnet down data (blue squares), and their average (black diamonds) for (a) pion momentum $2<p<20$ GeV and (b) $p>20$ GeV.

effici[..].pdf [43 KiB]
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efficiency_ratio_vs_Phi.pdf

Invariant mass distributions for (a) $ K ^+ K ^- \pi ^+ $ and (b) $ K ^- K ^+ \pi ^- $ candidates, when $m(K^+K^-)$ is within $\pm$20 MeV of the $\phi$ mass, for the dataset taken with magnet polarity down. The shaded areas represent signal, the dashed line the background and the solid curve the total.

Ds_mas[..].pdf [46 KiB]
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Ds_mass_fit_magdown_TOSonPhi.pdf

(a) $ D ^{\pm}_ s $ rapidity distribution (b) $ D ^{\pm}_ s $ $p_{T}$ distribution for background subtracted magnet up data. The statistical uncertainty on the number of events in each bin is smaller than the line thickness.

Ds_Pt_[..].pdf [71 KiB]
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Ds_Pt_Rapidity_TosOnPhi.pdf

Observed production asymmetry $A_{\rm P}$ as a function of (a) y, and (b) $p_{\rm T}$ . The errors shown are statistical only.

Ap_total.pdf [77 KiB]
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Ap_total.pdf

Animated gif made out of all figures.

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Tables and captions

Event yields for partial and full reconstruction.

Table_1.pdf [52 KiB]
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Table_1.pdf

Fitted numbers of $D_s^{\pm}$ events for both magnet up and down data.

Table_2.pdf [49 KiB]
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Table_2.pdf

$A_{\rm P}$ (%) shown as a function of both $y$ and $p_{\rm T}$ .

Table_3.pdf [34 KiB]
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Table_3.pdf

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