CMS-PAS-FTR-17-001

CMS-PAS-FTR-17-001
A proposal for the measurement of the weak mixing angle at the HL-LHC
CMS Collaboration
December 2017

Abstract: A proposal is presented for measuring the weak mixing angle using the forward-backward asymmetry of Drell-Yan dimuon events in pp collisions at $\sqrt{s} =$ 14 TeV with the CMS detector at the high luminosity LHC (HL-LHC). In addition to the increased luminosity, the upgraded part of the muon system extends the pseudorapidity coverage of the CMS experiment to $\|\eta\| <$ 2.8 for muons. Since the measurement has higher sensitivity in this pseudorapidity region, both the statistical and systematic uncertainties will be significantly reduced. To estimate the increased potential for this measurement we use a Monte Carlo data sample of pp events corresponding to a luminosity of 3000 fb $^{-1}$ and compare to the recent CMS measurements at $\sqrt{s} =$ 8 TeV.
Links: CDS record (PDF) ; inSPIRE record ; CADI line (restricted) ;

Figures & Tables	Summary	References	CMS Publications

Figures
png pdf	Figure 1: Forward-backward asymmetry distribution, ${A_\text {FB}} (M_{\mu \mu},Y_{\mu \mu})$ , in dimuon events at $\sqrt {s} =$ 8 TeV and 14 TeV. The distributions are made with POWHEG event generator using NNPDF3.0 PDFs and interfaced with PYTHIA 8 for parton-showering, QED final-state radiation (FSR) and hadronization. Following acceptance selections are applied to the generated muons after FSR: $\|\eta \| <$ 2.4 (or $\|\eta \| <$ 2.8), ${p_{\mathrm {T}}} ^\mathrm {lead} >$ 25 GeV, ${p_{\mathrm {T}}} ^\mathrm {trail} >$ 15 GeV. The error bars represent the statistical uncertainties for the integrated luminosities corresponding to 19 fb $^{-1}$ at $\sqrt {s} =$ 8 TeV and 3000 fb $^{-1}$ at $\sqrt {s} =$ 14 TeV.
png pdf	Figure 2: Forward-backward asymmetry distribution, ${A_\text {FB}} (M_{\mu \mu},Y_{\mu \mu})$ , in dimuon events at $\sqrt {s} =$ 14 TeV. The distributions are made with POWHEG event generator using NNPDF3.0 PDFs and interfaced with PYTHIA 8 for parton-showering, QED final-state radiation (FSR) and hadronization. Following acceptance selections are applied to the generated muons after FSR: $\|\eta \| <$ 2.8, ${p_{\mathrm {T}}} ^\mathrm {lead} >$ 25 GeV, ${p_{\mathrm {T}}} ^\mathrm {trail} >$ 15 GeV. The solid lines in the bottom panel correspond to six variations of ${\sin^2\theta ^{\text {lept}}_{\text {eff}}}$ around the central value: $\pm$ 0.0004, $\pm$ 0.0008, and $\pm$ 0.0012. The shaded band shows the standard deviation over the 100 NNPDF3.0 replicas.
png pdf	Figure 3: Projected statistical, nominal PDF and constrained PDF uncertainties in ${\sin^2\theta ^{\text {lept}}_{\text {eff}}}$ extracted by fitting ${A_\text {FB}} (m_{\mu \mu},y_{\mu \mu})$ distributions at $\sqrt {s} =$ 14 TeV with different values of integrated luminosities and for $\|\eta \| <$ 2.4 and $\|\eta \| <$ 2.8 acceptance selections for the muons. The nominal NNPDF3.0 uncertainty is calculated as a standard deviation of the extracted ${\sin^2\theta ^{\text {lept}}_{\text {eff}}}$ over the 100 NNPDF3.0 replicas. To calculate the constrained NNPDF3.0 uncertainty, each replica is weighted by $\exp(-\chi ^2_{\mathrm {min}}/2)$ , where $\chi ^2_{\mathrm {min}}$ is the best-fit $\chi ^2$ obtained with this replica.

Tables
png pdf	Table 1: Statistical, nominal NNPDF3.0, and constrained NNPDF3.0 uncertainties of extracted ${\sin^2\theta ^{\text {lept}}_{\text {eff}}}$ at 14 TeV for muon acceptance of $\|\eta \| <$ 2.4 and $\|\eta \| <$ 2.8 and for the different values of integrated luminosity. For comparison, results of the 8 TeV estimate of this analysis are compared to the results obtained from 8 TeV measurement [1].

Summary

We presented prospects for precision

${\sin^2\theta^{\text{lept}}_{\text{eff}}}$ measurement with the upgraded CMS detector at the high-luminosity LHC. We find that extending the lepton acceptance from

$|\eta| <$ 2.4 to 2.8 decreases the statistical uncertainties by about 30% and PDF uncertainties by about 20%. We also find that starting from about 1000 fb

$^{-1}$ , a single measurement would already have a negligible statistical uncertainty and the PDF uncertainty could be constrained to improve the precision of

${\sin^2\theta^{\text{lept}}_{\text{eff}}}$ measurement.

References
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