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| CMS-PAS-SMP-21-002 | ||
| Measurement of the Drell-Yan forward-backward asymmetry at high dilepton masses in proton-proton collisions at $\sqrt{s} = $ 13 TeV | ||
| CMS Collaboration | ||
| October 2021 | ||
| Abstract: A measurement of the forward-backward asymmetry of oppositely charged lepton pairs (dielectron and dimuon) produced by the Drell-Yan process in proton-proton collisions is presented. The data sample corresponds to an integrated luminosity of 138 fb$^{-1}$ collected with the CMS detector at the center-of-mass energy of 13 TeV. The asymmetry is measured for lepton pair masses larger than 170 GeV and compared with standard model predictions. No statistically significant deviations from standard model predictions are observed and the measurements are then used to set limits on the presence of additional gauge bosons. | ||
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Links:
CDS record (PDF) ;
CADI line (restricted) ;
These preliminary results are superseded in this paper, Submitted to JHEP. The superseded preliminary plots can be found here. |
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| Figures | |
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Figure 1:
The invariant mass distribution of e$\mu$ events observed in data (black dots with statistical uncertainties) and expected backgrounds (stacked histograms). The lower panel shows the ratio of the data to the expectation. The gray band represents the total uncertainty on the predicted yield. |
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Figure 2:
A comparison of the data and expected signal and background distributions in $m$ (top row), $ {\cos\theta _\mathrm {r}} $ (middle row) and dilepton rapidity (bottom row). The left column shows the $\mu \mu $ channel and the right column the ee channel. The black points with error bars represent the data and their statistical uncertainties, whereas the combined signal and background expectation is shown as stacked histograms. The lower panel shows the ratio of the data to the expectation. The gray band represents the total uncertainty on the predicted yield. |
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Figure 2-a:
A comparison of the data and expected signal and background distributions in $m$ (top row), $ {\cos\theta _\mathrm {r}} $ (middle row) and dilepton rapidity (bottom row). The left column shows the $\mu \mu $ channel and the right column the ee channel. The black points with error bars represent the data and their statistical uncertainties, whereas the combined signal and background expectation is shown as stacked histograms. The lower panel shows the ratio of the data to the expectation. The gray band represents the total uncertainty on the predicted yield. |
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Figure 2-b:
A comparison of the data and expected signal and background distributions in $m$ (top row), $ {\cos\theta _\mathrm {r}} $ (middle row) and dilepton rapidity (bottom row). The left column shows the $\mu \mu $ channel and the right column the ee channel. The black points with error bars represent the data and their statistical uncertainties, whereas the combined signal and background expectation is shown as stacked histograms. The lower panel shows the ratio of the data to the expectation. The gray band represents the total uncertainty on the predicted yield. |
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Figure 2-c:
A comparison of the data and expected signal and background distributions in $m$ (top row), $ {\cos\theta _\mathrm {r}} $ (middle row) and dilepton rapidity (bottom row). The left column shows the $\mu \mu $ channel and the right column the ee channel. The black points with error bars represent the data and their statistical uncertainties, whereas the combined signal and background expectation is shown as stacked histograms. The lower panel shows the ratio of the data to the expectation. The gray band represents the total uncertainty on the predicted yield. |
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Figure 2-d:
A comparison of the data and expected signal and background distributions in $m$ (top row), $ {\cos\theta _\mathrm {r}} $ (middle row) and dilepton rapidity (bottom row). The left column shows the $\mu \mu $ channel and the right column the ee channel. The black points with error bars represent the data and their statistical uncertainties, whereas the combined signal and background expectation is shown as stacked histograms. The lower panel shows the ratio of the data to the expectation. The gray band represents the total uncertainty on the predicted yield. |
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Figure 2-e:
A comparison of the data and expected signal and background distributions in $m$ (top row), $ {\cos\theta _\mathrm {r}} $ (middle row) and dilepton rapidity (bottom row). The left column shows the $\mu \mu $ channel and the right column the ee channel. The black points with error bars represent the data and their statistical uncertainties, whereas the combined signal and background expectation is shown as stacked histograms. The lower panel shows the ratio of the data to the expectation. The gray band represents the total uncertainty on the predicted yield. |
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Figure 2-f:
A comparison of the data and expected signal and background distributions in $m$ (top row), $ {\cos\theta _\mathrm {r}} $ (middle row) and dilepton rapidity (bottom row). The left column shows the $\mu \mu $ channel and the right column the ee channel. The black points with error bars represent the data and their statistical uncertainties, whereas the combined signal and background expectation is shown as stacked histograms. The lower panel shows the ratio of the data to the expectation. The gray band represents the total uncertainty on the predicted yield. |
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Figure 3:
Measurement of the DY forward-backward asymmetry as a function the dilepton mass as compared to MC predictions. The green line is the predicted value for ${A_\text {FB}}$ from aMC@NLO and the shaded green region the uncertainty on the predicted value. The red, blue, and black points and error bars represent the dielectron, dimuon and combined measurements respectively. Errors bars on the measurements include statistical and systematic errors. |
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Figure 4:
Measurement of the difference in forward-backward asymmetry between the dimuon and dielectron channels. The green line is drawn at zero, the predicted value for $\Delta {A_\text {FB}} $ assuming lepton flavor universality. The black points and error bars represent the measurements of $\Delta {A_\text {FB}} $ in different mass bins. The blue line and shaded light blue region represent the inclusive measurement of $\Delta {A_\text {FB}} $ and corresponding uncertainty. The errors bars on the measurements and the shaded region include statistical and systematic errors. |
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Figure 5:
Exclusion limits at 95% CL on the coupling $\kappa _\mathrm {L}$ for a Z' in the sequential standard model as a function of the Z' mass. The expected (observed) limit is shown by the dashed (solid) line. The inner and outer shaded areas around the expected limits show the 68% and 95% CL intervals, respectively. The dashed blue line shows $\kappa _{L} = $ 1 which corresponds to a Z' with exactly the same couplings as the SM Z boson. |
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Figure 6:
The postfit distributions in the mass bin 170-200 GeV. The left column is the $\mu \mu $ channel, and the right column the ee channel. The contribution of the $\tau \tau $ background is not visible on the scale of these plots and has been omitted. The 2D templates follow the binning defined in Section xxxxx but have been presented in one dimension, where the first eight bins correspond the eight $ {c_\mathrm {r}} $ bins of the first $y$ bin, the second eight bins the second $y$ bin, the next six bins the third $y$ bin and the final six bins the last $y$ bin. The bottom panel in each figure shows the ratio between the number of events observed in data and the best fit value. The gray shaded region shows the total uncertainty on the best fit result and vertical error bars represent statistical uncertainties in the data. |
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Figure 6-a:
The postfit distributions in the mass bin 170-200 GeV. The left column is the $\mu \mu $ channel, and the right column the ee channel. The contribution of the $\tau \tau $ background is not visible on the scale of these plots and has been omitted. The 2D templates follow the binning defined in Section xxxxx but have been presented in one dimension, where the first eight bins correspond the eight $ {c_\mathrm {r}} $ bins of the first $y$ bin, the second eight bins the second $y$ bin, the next six bins the third $y$ bin and the final six bins the last $y$ bin. The bottom panel in each figure shows the ratio between the number of events observed in data and the best fit value. The gray shaded region shows the total uncertainty on the best fit result and vertical error bars represent statistical uncertainties in the data. |
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Figure 6-b:
The postfit distributions in the mass bin 170-200 GeV. The left column is the $\mu \mu $ channel, and the right column the ee channel. The contribution of the $\tau \tau $ background is not visible on the scale of these plots and has been omitted. The 2D templates follow the binning defined in Section xxxxx but have been presented in one dimension, where the first eight bins correspond the eight $ {c_\mathrm {r}} $ bins of the first $y$ bin, the second eight bins the second $y$ bin, the next six bins the third $y$ bin and the final six bins the last $y$ bin. The bottom panel in each figure shows the ratio between the number of events observed in data and the best fit value. The gray shaded region shows the total uncertainty on the best fit result and vertical error bars represent statistical uncertainties in the data. |
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Figure 7:
The postfit distributions in the mass bin 200-250 GeV. The left column is the $\mu \mu $ channel, and the right column the ee channel. The contribution of the $\tau \tau $ background is not visible on the scale of these plots and has been omitted. The 2D templates follow the binning defined in Section xxxxx but have been presented in one dimension, where the first eight bins correspond the eight $ {c_\mathrm {r}} $ bins of the first $y$ bin, the second eight bins the second $y$ bin, the next six bins the third $y$ bin and the final six bins the last $y$ bin. The bottom panel in each figure shows the ratio between the number of events observed in data and the best fit value. The gray shaded region shows the total uncertainty on the best fit result and vertical error bars represent statistical uncertainties in the data. |
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Figure 7-a:
The postfit distributions in the mass bin 200-250 GeV. The left column is the $\mu \mu $ channel, and the right column the ee channel. The contribution of the $\tau \tau $ background is not visible on the scale of these plots and has been omitted. The 2D templates follow the binning defined in Section xxxxx but have been presented in one dimension, where the first eight bins correspond the eight $ {c_\mathrm {r}} $ bins of the first $y$ bin, the second eight bins the second $y$ bin, the next six bins the third $y$ bin and the final six bins the last $y$ bin. The bottom panel in each figure shows the ratio between the number of events observed in data and the best fit value. The gray shaded region shows the total uncertainty on the best fit result and vertical error bars represent statistical uncertainties in the data. |
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Figure 7-b:
The postfit distributions in the mass bin 200-250 GeV. The left column is the $\mu \mu $ channel, and the right column the ee channel. The contribution of the $\tau \tau $ background is not visible on the scale of these plots and has been omitted. The 2D templates follow the binning defined in Section xxxxx but have been presented in one dimension, where the first eight bins correspond the eight $ {c_\mathrm {r}} $ bins of the first $y$ bin, the second eight bins the second $y$ bin, the next six bins the third $y$ bin and the final six bins the last $y$ bin. The bottom panel in each figure shows the ratio between the number of events observed in data and the best fit value. The gray shaded region shows the total uncertainty on the best fit result and vertical error bars represent statistical uncertainties in the data. |
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Figure 8:
The postfit distributions in the mass bin 250-320 GeV. The left column is the $\mu \mu $ channel, and the right column the ee channel. The contribution of the $\tau \tau $ background is not visible on the scale of these plots and has been omitted. The 2D templates follow the binning defined in Section xxxxx but have been presented in one dimension, where the first eight bins correspond the eight $ {c_\mathrm {r}} $ bins of the first $y$ bin, the second eight bins the second $y$ bin, the next six bins the third $y$ bin and the final six bins the last $y$ bin. The bottom panel in each figure shows the ratio between the number of events observed in data and the best fit value. The gray shaded region shows the total uncertainty on the best fit result and vertical error bars represent statistical uncertainties in the data. |
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Figure 8-a:
The postfit distributions in the mass bin 250-320 GeV. The left column is the $\mu \mu $ channel, and the right column the ee channel. The contribution of the $\tau \tau $ background is not visible on the scale of these plots and has been omitted. The 2D templates follow the binning defined in Section xxxxx but have been presented in one dimension, where the first eight bins correspond the eight $ {c_\mathrm {r}} $ bins of the first $y$ bin, the second eight bins the second $y$ bin, the next six bins the third $y$ bin and the final six bins the last $y$ bin. The bottom panel in each figure shows the ratio between the number of events observed in data and the best fit value. The gray shaded region shows the total uncertainty on the best fit result and vertical error bars represent statistical uncertainties in the data. |
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Figure 8-b:
The postfit distributions in the mass bin 250-320 GeV. The left column is the $\mu \mu $ channel, and the right column the ee channel. The contribution of the $\tau \tau $ background is not visible on the scale of these plots and has been omitted. The 2D templates follow the binning defined in Section xxxxx but have been presented in one dimension, where the first eight bins correspond the eight $ {c_\mathrm {r}} $ bins of the first $y$ bin, the second eight bins the second $y$ bin, the next six bins the third $y$ bin and the final six bins the last $y$ bin. The bottom panel in each figure shows the ratio between the number of events observed in data and the best fit value. The gray shaded region shows the total uncertainty on the best fit result and vertical error bars represent statistical uncertainties in the data. |
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Figure 9:
The postfit distributions in the mass bin 320-510 GeV. The left column is the $\mu \mu $ channel, and the right column the ee channel. The contribution of the $\tau \tau $ background is not visible on the scale of these plots and has been omitted. The 2D templates follow the binning defined in Section xxxxx but have been presented in one dimension, where the first eight bins correspond the eight $ {c_\mathrm {r}} $ bins of the first $y$ bin, the second eight bins the second $y$ bin, the next six bins the third $y$ bin and the final six bins the last $y$ bin. The bottom panel in each figure shows the ratio between the number of events observed in data and the best fit value. The gray shaded region shows the total uncertainty on the best fit result and vertical error bars represent statistical uncertainties in the data. |
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Figure 9-a:
The postfit distributions in the mass bin 320-510 GeV. The left column is the $\mu \mu $ channel, and the right column the ee channel. The contribution of the $\tau \tau $ background is not visible on the scale of these plots and has been omitted. The 2D templates follow the binning defined in Section xxxxx but have been presented in one dimension, where the first eight bins correspond the eight $ {c_\mathrm {r}} $ bins of the first $y$ bin, the second eight bins the second $y$ bin, the next six bins the third $y$ bin and the final six bins the last $y$ bin. The bottom panel in each figure shows the ratio between the number of events observed in data and the best fit value. The gray shaded region shows the total uncertainty on the best fit result and vertical error bars represent statistical uncertainties in the data. |
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Figure 9-b:
The postfit distributions in the mass bin 320-510 GeV. The left column is the $\mu \mu $ channel, and the right column the ee channel. The contribution of the $\tau \tau $ background is not visible on the scale of these plots and has been omitted. The 2D templates follow the binning defined in Section xxxxx but have been presented in one dimension, where the first eight bins correspond the eight $ {c_\mathrm {r}} $ bins of the first $y$ bin, the second eight bins the second $y$ bin, the next six bins the third $y$ bin and the final six bins the last $y$ bin. The bottom panel in each figure shows the ratio between the number of events observed in data and the best fit value. The gray shaded region shows the total uncertainty on the best fit result and vertical error bars represent statistical uncertainties in the data. |
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Figure 10:
The postfit distributions in the mass bin 510-700 GeV. The left column is the $\mu \mu $ channel, and the right column the ee channel. The contribution of the $\tau \tau $ background is not visible on the scale of these plots and has been omitted. The 2D templates follow the binning defined in Section xxxxx but have been presented in one dimension, where the first eight bins correspond the eight $ {c_\mathrm {r}} $ bins of the first $y$ bin, the second eight bins the second $y$ bin, the next six bins the third $y$ bin and the final six bins the last $y$ bin. The bottom panel in each figure shows the ratio between the number of events observed in data and the best fit value. The gray shaded region shows the total uncertainty on the best fit result and vertical error bars represent statistical uncertainties in the data. |
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Figure 10-a:
The postfit distributions in the mass bin 510-700 GeV. The left column is the $\mu \mu $ channel, and the right column the ee channel. The contribution of the $\tau \tau $ background is not visible on the scale of these plots and has been omitted. The 2D templates follow the binning defined in Section xxxxx but have been presented in one dimension, where the first eight bins correspond the eight $ {c_\mathrm {r}} $ bins of the first $y$ bin, the second eight bins the second $y$ bin, the next six bins the third $y$ bin and the final six bins the last $y$ bin. The bottom panel in each figure shows the ratio between the number of events observed in data and the best fit value. The gray shaded region shows the total uncertainty on the best fit result and vertical error bars represent statistical uncertainties in the data. |
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Figure 10-b:
The postfit distributions in the mass bin 510-700 GeV. The left column is the $\mu \mu $ channel, and the right column the ee channel. The contribution of the $\tau \tau $ background is not visible on the scale of these plots and has been omitted. The 2D templates follow the binning defined in Section xxxxx but have been presented in one dimension, where the first eight bins correspond the eight $ {c_\mathrm {r}} $ bins of the first $y$ bin, the second eight bins the second $y$ bin, the next six bins the third $y$ bin and the final six bins the last $y$ bin. The bottom panel in each figure shows the ratio between the number of events observed in data and the best fit value. The gray shaded region shows the total uncertainty on the best fit result and vertical error bars represent statistical uncertainties in the data. |
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Figure 11:
The postfit distributions in the mass bin 700-1000 GeV. The left column is the $\mu \mu $ channel, and the right column the ee channel. The contribution of the $\tau \tau $ background is not visible on the scale of these plots and has been omitted. The 2D templates follow the binning defined in Section xxxxx but have been presented in one dimension, where the first eight bins correspond the eight $ {c_\mathrm {r}} $ bins of the first $y$ bin, the second eight bins the second $y$ bin, the next six bins the third $y$ bin and the final six bins the last $y$ bin. The bottom panel in each figure shows the ratio between the number of events observed in data and the best fit value. The gray shaded region shows the total uncertainty on the best fit result and vertical error bars represent statistical uncertainties in the data. |
png pdf |
Figure 11-a:
The postfit distributions in the mass bin 700-1000 GeV. The left column is the $\mu \mu $ channel, and the right column the ee channel. The contribution of the $\tau \tau $ background is not visible on the scale of these plots and has been omitted. The 2D templates follow the binning defined in Section xxxxx but have been presented in one dimension, where the first eight bins correspond the eight $ {c_\mathrm {r}} $ bins of the first $y$ bin, the second eight bins the second $y$ bin, the next six bins the third $y$ bin and the final six bins the last $y$ bin. The bottom panel in each figure shows the ratio between the number of events observed in data and the best fit value. The gray shaded region shows the total uncertainty on the best fit result and vertical error bars represent statistical uncertainties in the data. |
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Figure 11-b:
The postfit distributions in the mass bin 700-1000 GeV. The left column is the $\mu \mu $ channel, and the right column the ee channel. The contribution of the $\tau \tau $ background is not visible on the scale of these plots and has been omitted. The 2D templates follow the binning defined in Section xxxxx but have been presented in one dimension, where the first eight bins correspond the eight $ {c_\mathrm {r}} $ bins of the first $y$ bin, the second eight bins the second $y$ bin, the next six bins the third $y$ bin and the final six bins the last $y$ bin. The bottom panel in each figure shows the ratio between the number of events observed in data and the best fit value. The gray shaded region shows the total uncertainty on the best fit result and vertical error bars represent statistical uncertainties in the data. |
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Figure 12:
The postfit distributions in the mass bin $\geq $ 1000 GeV. The left column is the $\mu \mu $ channel, and the right column the ee channel. The contribution of the $\tau \tau $ background is not visible on the scale of these plots and has been omitted. The 2D templates follow the binning defined in Section xxxxx but have been presented in one dimension, where the first eight bins correspond the eight $ {c_\mathrm {r}} $ bins of the first $y$ bin, the second eight bins the second $y$ bin, the next six bins the third $y$ bin and the final six bins the last $y$ bin. The bottom panel in each figure shows the ratio between the number of events observed in data and the best fit value. The gray shaded region shows the total uncertainty on the best fit result and vertical error bars represent statistical uncertainties in the data. |
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Figure 12-a:
The postfit distributions in the mass bin $\geq $ 1000 GeV. The left column is the $\mu \mu $ channel, and the right column the ee channel. The contribution of the $\tau \tau $ background is not visible on the scale of these plots and has been omitted. The 2D templates follow the binning defined in Section xxxxx but have been presented in one dimension, where the first eight bins correspond the eight $ {c_\mathrm {r}} $ bins of the first $y$ bin, the second eight bins the second $y$ bin, the next six bins the third $y$ bin and the final six bins the last $y$ bin. The bottom panel in each figure shows the ratio between the number of events observed in data and the best fit value. The gray shaded region shows the total uncertainty on the best fit result and vertical error bars represent statistical uncertainties in the data. |
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Figure 12-b:
The postfit distributions in the mass bin $\geq $ 1000 GeV. The left column is the $\mu \mu $ channel, and the right column the ee channel. The contribution of the $\tau \tau $ background is not visible on the scale of these plots and has been omitted. The 2D templates follow the binning defined in Section xxxxx but have been presented in one dimension, where the first eight bins correspond the eight $ {c_\mathrm {r}} $ bins of the first $y$ bin, the second eight bins the second $y$ bin, the next six bins the third $y$ bin and the final six bins the last $y$ bin. The bottom panel in each figure shows the ratio between the number of events observed in data and the best fit value. The gray shaded region shows the total uncertainty on the best fit result and vertical error bars represent statistical uncertainties in the data. |
| Tables | |
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Table 1:
Results for the measurement of ${A_\text {FB}}$ from the maximum likelihood fit to data in different mass bins in the different channels. The first uncertainty listed with each measurement corresponds to the statistical uncertainty and the second the systematic uncertainty. |
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Table 2:
Results for the measurement of A$_0$ from the maximum likelihood fit to data in different mass bins in the different channels. The first uncertainty listed with each measurement corresponds to the statistical uncertainty and the second the systematic uncertainty. To help in the interpretation of these results, we also list the average dilepton $ {p_{\mathrm {T}}} $ of the data events in each mass bin. |
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Table 3:
Results for the measurement of $\Delta {A_\text {FB}} $ and $\Delta \mathrm {A}_0$ from the maximum likelihood fit to data in different mass bins as well as an inclusive measurement across all mass bins. The first uncertainty listed with each measurement corresponds to the statistical uncertainty and the second the systematic uncertainty. |
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Table 4:
The fraction of photon-induced background as compared to the total amount of DY signal plus photon-induced events ($\frac {N_{\gamma \gamma}}{N_{\gamma \gamma} + N_\mathrm {DY}}$) in the different mass bins. These numbers are averaged across the different years and channels. |
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Table 5:
A comparison of the magnitude of the different sources of systematic uncertainty for the measurement ${A_\text {FB}}$ when combining the electron and muon channels and for the measurement of $\Delta {A_\text {FB}} $. Results for the 170-200 GeV mass bin are shown because that is the mass bin in which the systematic uncertainty has the largest contribution to the total uncertainty; the results for other mass bins are similar. The contributions from different sources of systematic uncertainty are evaluated by running the fit while freezing different groups of nuisances and taking the quadrature difference between the resulting uncertainty and the full uncertainty. Results are reported as a fraction of the overall systematic uncertainty for the measurement of ${A_\text {FB}}$ and $\Delta {A_\text {FB}} $ respectively. |
| Summary |
| The CMS detector at the LHC has been used to measure the DY forward-backward asymmetry, ${A_\text{FB}}$, for muon and electron pairs with invariant mass above 170 GeV. The measurement is performed using 138 fb$^{-1}$ of proton-proton collision data at $\sqrt{s} = $ 13 TeV collected in Run 2 using a new template fitting approach. The measurements show good agreement with the standard model and limits are set on the existence additional gauge bosons. For a Z' boson in the canonical Sequential standard model, a lower limit on its mass is set at $m_{\mathrm{Z'}} > $ 4.4 TeV. |
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