cern.ch

Measurement of the forward-backward asymmetry in $Z/\gamma^{\ast} \rightarrow \mu^{+}\mu^{-}$ decays and determination of the effective weak mixing angle

[to restricted-access page]

Information

Tools

Contact

Abstract

The forward-backward charge asymmetry for the process $q\bar{q} \rightarrow Z/\gamma^{\ast} \rightarrow \mu^{+}\mu^{-}$ is measured as a function of the invariant mass of the dimuon system. Measurements are performed using proton proton collision data collected with the LHCb detector at $\sqrt{s} = 7$ and 8\tev, corresponding to integrated luminosities of $1$fb$^{-1}$ and $2$fb$^{-1}$ respectively. Within the Standard Model the results constrain the effective electroweak mixing angle to be $$sin^{2}\theta_{W}^{eff} = 0.23142 \pm 0.00073 \pm 0.00052 \pm 0.00056 $$ where the first uncertainty is statistical, the second systematic and the third theoretical. This result is in agreement with the current world average, and is one of the most precise determinations at hadron colliders to date.

Figures and captions

Dimuon invariant mass distribution for data, simulated signal and background processes for (left) { $\sqrt{s}$ $=$ 7 TeV} and (right) { $\sqrt{s}$ $=$ 8 TeV}.

 AfterC[..].pdf [11 KiB]
HiDef png [364 KiB]
Thumbnail [106 KiB]
*.C file
AfterCommonHits_ZMass_Paper_rebin2011_new.pdf
AfterC[..].pdf [11 KiB]
HiDef png [365 KiB]
Thumbnail [106 KiB]
*.C file
AfterCommonHits_ZMass_Paper_rebin2012_new.pdf

The measurements of $ A_\text{FB}$ as a function of the dimuon invariant mass for data compared to SM predictions for (left) {$\sqrt{s} = 7$ TeV} and (right) {$\sqrt{s} = 8$ TeV}. The SM predictions are calculated using Powheg interfaced with Pythia for parton showering with the world average value for $ {\sin}^{2}\theta_{\mathrm{W}}^{\mathrm{eff}} =0.2315$ [27]. The data include both statistical and systematic uncertainties, and the SM predictions include the theoretical uncertainties described in Sec. 5.

 Paper_[..].pdf [6 KiB]
HiDef png [241 KiB]
Thumbnail [63 KiB]
*.C file
Paper_afb_statsyst2011_theo.pdf
Paper_[..].pdf [6 KiB]
HiDef png [239 KiB]
Thumbnail [63 KiB]
*.C file
Paper_afb_statsyst2012_theo.pdf

Difference between the $\chi^2$ and the minimum $\chi^2$ obtained by comparing the final $ A_\text{FB}(m_{\mu\mu})$ measurements in data to $ A_\text{FB}^{\text{pred}}$ calculated using values of $ {\sin}^{2}\theta_{\mathrm{W}}^{\mathrm{eff}}$ ranging from 0.22 to 0.24, indicated by the crosses on the plot. A quadratic fit is used to determine the minimum value for $ {\sin}^{2}\theta_{\mathrm{W}}^{\mathrm{eff}}$ and the corresponding uncertainty, and is shown for the different centre-of-mass energies and the combination. The black dashed horizontal line corresponds to one unit of $\chi^2$ from the minimum and the intersecting $ {\sin}^{2}\theta_{\mathrm{W}}^{\mathrm{eff}}$ for the combination are indicated by the vertical red dashed lines.

 chi_pl[..].pdf [6 KiB]
HiDef png [232 KiB]
Thumbnail [72 KiB]
*.C file
chi_plot_all.pdf

A comparison of the $ {\sin}^{2}\theta_{\mathrm{W}}^{\mathrm{eff}}$ measurement at LHCb and other experiments. The combined LEP and SLD measurement is indicated by the vertical yellow band.

 stw_co[..].pdf [5 KiB]
HiDef png [217 KiB]
Thumbnail [65 KiB]
*.C file
stw_comp_fullref_final.pdf

Animated gif made out of all figures.

 PAPER-2015-039.gif
Thumbnail
thumbnail_PAPER-2015-039.gif

Tables and captions

Weighted average of the absolute systematic uncertainties for $ A_\text{FB}$ , for different sources, given separately for $\sqrt{s}$ $=$ 7 and 8 TeV.

 Table_1.pdf [33 KiB]
HiDef png [65 KiB]
Thumbnail [10 KiB]
tex code 		Table_1.pdf

Values for $ A_\text{FB}$ with the statistical and positive and negative systematic uncertainties for { $\sqrt{s}$ $= 7$ TeV} data. The theoretical uncertainties presented in this table, corresponding to the PDF, scale and FSR uncertainties described in Sec. 5, affect only the predictions of $ A_\text{FB}$ and the $ {\sin}^{2}\theta_{\mathrm{W}}^{\mathrm{eff}}$ determination, and do not apply to the uncertainties on the measured $ A_\text{FB}$ .

 Table_2.pdf [55 KiB]
HiDef png [166 KiB]
Thumbnail [26 KiB]
tex code 		Table_2.pdf

Values for $ A_\text{FB}$ with the statistical and positive and negative systematic uncertainties for { $\sqrt{s}$ $= 8$ TeV} data. The theoretical uncertainties presented in this table, corresponding to the PDF, scale and FSR uncertainties described in Sec. 5, affect only the predictions of $ A_\text{FB}$ and the $ {\sin}^{2}\theta_{\mathrm{W}}^{\mathrm{eff}}$ determination, and do not apply to the uncertainties on the measured $ A_\text{FB}$ .

 Table_3.pdf [55 KiB]
HiDef png [156 KiB]
Thumbnail [26 KiB]
tex code 		Table_3.pdf

Weighted average of the absolute systematic uncertainties for $ A_\text{FB}^{\text{pred}}$ , for the different sources of theoretical uncertainty. The value quoted for the PDF uncertainty corresponds to the $68\%$ confidence range, while for the others the maximum and minimum shifts are given. The correlations among the invariant mass bins are not taken into account.

 Table_4.pdf [55 KiB]
HiDef png [79 KiB]
Thumbnail [12 KiB]
tex code 		Table_4.pdf

Supplementary Material [file]

Supplementary material full pdf

 supple[..].pdf [209 KiB]
 supplementary.pdf
Table5.pdf [18 KiB]
HiDef png [161 KiB]
Thumbnail [23 KiB]
*C file
Table5.pdf
Table6.pdf [22 KiB]
HiDef png [142 KiB]
Thumbnail [20 KiB]
*C file
Table6.pdf
Table7.pdf [22 KiB]
HiDef png [145 KiB]
Thumbnail [21 KiB]
*C file
Table7.pdf

Created on 20 April 2019.Citation count from INSPIRE on 24 April 2019.