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First measurement of $Z \to \mu^+\mu^-$ angular coefficients in the forward region of $pp$ collisions at $\sqrt{s} = 13$ TeV

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Abstract

The first study of the angular distribution of $\mu^+ \mu^-$ pairs produced in the forward rapidity region via the Drell-Yan reaction $pp \rightarrow \gamma^{*}/Z +X \rightarrow l^+ l^- + X$ is presented, using data collected with the LHCb detector at a centre-of-mass energy of 13TeV, corresponding to an integrated luminosity of 5.1 $\rm{fb}^{-1}$. The coefficients of the five leading terms in the angular distribution are determined as a function of the dimuon transverse momentum and rapidity. The results are compared to various theoretical predictions of the $Z$-boson production mechanism and can also be used to probe transverse-momentum-dependent parton distributions within the proton.

Figures and captions

Comparison of the measured angular coefficients with different predictions, as a function of the $ Z $ -boson $ p_{\mathrm{T}}$ , in the rapidity region of $ Z $ -boson $y^Z>2$ and $75<M_{\mu\mu}<105\text{ Ge V /}c^2 $. The total uncertainty (shown in the figure) is dominated by the statistical component. The theoretical predictions correspond to the same $ Z $ -boson $ p_{\mathrm{T}}$ bins as data and the $ p_{\mathrm{T}}$ shifts of the theoretical markers in all plots are for visualization purposes only. {DYTurbo} and ResBos predictions include the theoretical uncertainties.

Combin[..].pdf [22 KiB]
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Combined_Ai_Normal.pdf

Comparison of the measured angular coefficient $A_{2}$ with different predictions, as a function of the $ Z $ -boson $ p_{\mathrm{T}}$ , in the rapidity region of $ Z $ -boson $y^Z>2$ and $50<M_{\mu\mu}<75\text{ Ge V /}c^2 $ (left), $50<M_{\mu\mu}<75\text{ Ge V /}c^2 $ (middle) and $105<M_{\mu\mu}<120\text{ Ge V /}c^2 $ (right). The total uncertainty (shown in the figure) is dominated by the statistical component. The horizontal positions of the theoretical predictions within the bins are adjusted to increase visibility. {DYTurbo} and ResBos predictions include the theoretical uncertainties.

Combin[..].pdf [17 KiB]
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Combined_lowmass.pdf

Comparison of the measured angular coefficients with ResBos predictions as a function of the $y^Z$ in the $75<M_{\mu\mu}<105\text{ Ge V /}c^2 $ mass region. The total uncertainty (shown in the figure) is dominated by the statistical component. ResBos predictions include the theoretical uncertainties.

Combin[..].pdf [20 KiB]
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Combined_Ai_Y.pdf

Animated gif made out of all figures.

PAPER-2021-048.gif
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thumbnail_PAPER-2021-048.gif

Tables and captions

The number of selected data candidate events in different $ Z $ boson $ p_{\mathrm{T}}$ and rapidity intervals in the mass region $50<M_{\mu\mu}<120\text{ Ge V /}c^2 $.

Table_1.pdf [52 KiB]
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Table_1.pdf

Summary of measured angular coefficients and regularised uncertainties for $A_{i}$ and $A_{0}-A_2$, in intervals of $ Z $ -boson $ p_{\mathrm{T}}$ , in the region of $ Z $ -boson $y>2$ and $75<M_{\mu\mu}<105\text{ Ge V /}c^2 $. The total systematic uncertainty is shown with the breakdown into its underlying components. Entries marked with `-' indicate that the uncertainty is below 0.0001.

Table_2.pdf [63 KiB]
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Table_2.pdf

Summary of measured angular coefficients and regularised uncertainties for $A_{i}$ and $A_{0}-A_2$, in intervals of $ Z $ -boson rapidity, in the region of $ Z $ -boson $ p_{\mathrm{T}} <100 \text{ Ge V /}c $ and $75<M_{\mu\mu}<105\text{ Ge V /}c^2 $. The total systematic uncertainty is shown with the breakdown into its underlying components. Entries marked with `-' indicate that the uncertainty is below 0.0001.

Table_3.pdf [67 KiB]
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Table_3.pdf

Summary of measured angular coefficients and regularised uncertainties for $A_{2}$, in intervals of $ Z $ -boson $ p_{\mathrm{T}}$ , in the region of $ Z $ -boson $y>2$ and different mass ranges. The total systematic uncertainty is shown with the breakdown into its underlying components. Entries marked with `-' indicate that the uncertainty is below 0.0001.

Table_4.pdf [67 KiB]
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Table_4.pdf

Created on 20 April 2024.