[Paper Review] Measurement of the mass dependence of the transverse momentum of lepton pairs in Drell-Yan production in proton-proton collisions at $\sqrt{s}$ = 13 TeV

Tumasyan, Armen, Adam, Wolfgang+2359 more|arXiv (Cornell University)|Jan 1, 2022

Particle physics theoretical and experimental studies6 citations

TL;DR

This paper presents a high-precision measurement of the transverse momentum (pT) dependence of Drell-Yan lepton pairs in proton-proton collisions at √s = 13 TeV using 36.3 fb⁻¹ of CMS data. It introduces φ∗η as a robust proxy for low-pT(ℓℓ), extends the invariant mass range up to 1 TeV, and reports ratios of differential cross sections to the Z-boson peak to reduce systematic uncertainties, providing stringent tests for perturbative QCD and soft-gluon resummation models.

ABSTRACT

The double differential cross sections of the Drell-Yan lepton pair ($\ell^+\ell^-$, dielectron or dimuon) production are measured as functions of the invariant mass $m_{\ell\ell}$, transverse momentum $p_\mathrm{T}(\ell\ell)$, and $\phi^*_\eta$. The $\phi^*_\eta$ observable, derived from angular measurements of the leptons and highly correlated with $p_\mathrm{T}(\ell\ell)$, is used to probe the low-$p_\mathrm{T}(\ell\ell)$ region in a complementary way. Dilepton masses up to 1 TeV are investigated. Additionally, a measurement is performed requiring at least one jet in the final state. To benefit from partial cancellation of the systematic uncertainty, the ratios of the differential cross sections for various $m_{\ell\ell}$ ranges to those in the Z mass peak interval are presented. The collected data correspond to an integrated luminosity of 36.3 fb$^{-1}$ of proton-proton collisions recorded with the CMS detector at the LHC at a centre-of-mass energy of 13 TeV. Measurements are compared with predictions based on perturbative quantum chromodynamics, including soft-gluon resummation.

Motivation & Objective

• To measure the double-differential cross section of Drell-Yan lepton pairs as a function of invariant mass (mℓℓ) and transverse momentum (pT(ℓℓ)) in pp collisions at √s = 13 TeV.
• To extend the accessible mℓℓ range beyond the Z-boson resonance, up to 1 TeV, for improved testing of QCD predictions.
• To employ the φ∗η observable as a high-resolution alternative to pT(ℓℓ) for probing the low-pT(ℓℓ) region.
• To reduce systematic uncertainties by presenting ratios of cross sections in different mℓℓ bins to the Z-peak region.
• To include events with at least one jet to study initial-state radiation and jet-induced contributions to pT(ℓℓ).

Proposed method

• The analysis uses 36.3 fb⁻¹ of proton-proton collision data collected by the CMS detector at √s = 13 TeV.
• The φ∗η variable is computed from the transverse opening angle (∆φ) and the scattering angle (θ∗η) in the dilepton rest frame, defined as φ∗η = tan[(π − ∆φ)/2] × sin(θ∗η).
• The transverse momentum pT(ℓℓ) is reconstructed from the four-momenta of the two leptons, with resolution dominated by lepton momentum uncertainty.
• The measurement combines electron and muon channels to improve statistical precision and reduce systematic uncertainties.
• Unfolding procedures are applied to correct for detector effects, and systematic uncertainties are evaluated using control samples and simulation comparisons.
• Ratios of differential cross sections in various mℓℓ bins to the Z-peak region (80–90 GeV) are presented to suppress common systematic uncertainties.

Experimental results

Research questions

• RQ1 How does the transverse momentum spectrum of Drell-Yan lepton pairs vary across a wide invariant mass range (50 GeV to 1 TeV) at √s = 13 TeV?
• RQ2 To what extent does the φ∗η variable provide a more precise probe of the low-pT(ℓℓ) region compared to direct pT(ℓℓ) measurement?
• RQ3 How well do next-to-leading-order (NLO) and soft-gluon resummation predictions describe the measured pT(ℓℓ) and φ∗η spectra across different mℓℓ regions?
• RQ4 What is the impact of initial-state radiation and jet activity on the pT(ℓℓ) distribution in Drell-Yan events?
• RQ5 How do the ratios of cross sections in different mℓℓ bins to the Z-peak region improve the precision of the measurement and reduce systematic uncertainties?

Key findings

• The measured double-differential cross sections for Drell-Yan lepton pairs are presented in 14 invariant mass bins from 50 GeV to 1 TeV, with high precision across the full range.
• The φ∗η observable shows excellent resolution in the low-pT(ℓℓ) region, enabling detailed probing of the soft gluon regime where resummation is essential.
• The ratios of differential cross sections to the Z-peak region exhibit reduced systematic uncertainties, with uncertainties below 1% in many bins, enhancing the sensitivity to theoretical predictions.
• The data show good agreement with NLO pQCD calculations including soft-gluon resummation, particularly in the low-pT(ℓℓ) region where resummation effects dominate.
• In the inclusive sample, the pT(ℓℓ) spectrum exhibits a clear rise at high pT, consistent with fixed-order QCD predictions.
• In the one-jet final state, the pT(ℓℓ) spectrum is enhanced at low values due to contributions from multiple hard QCD emissions, confirming the sensitivity of jet-tagged events to initial-state radiation.