[Paper Review] Transverse momentum resummation at small x for the Tevatron and LHC

Stefan Berge, Pavel Nadolsky+2 more|CERN Bulletin|Oct 27, 2004

Particle physics theoretical and experimental studies4 citations

TL;DR

This paper investigates transverse momentum (qT) resummation at small x in Drell-Yan-like processes at the Tevatron and LHC, proposing that small-x parton density broadening in impact-parameter space modifies qT distributions. It shows that such broadening could shift the W boson mass by 10–20 MeV in the central region and over 50 MeV in forward regions at the Tevatron, and lead to a significantly harder qT spectrum at the LHC, challenging existing predictions and requiring revised analysis of W and Higgs boson measurements.

ABSTRACT

Analysis of semi-inclusive DIS hadroproduction suggests broadening of transverse momentum distributions at $x$ below a few $10^{-3}$, which can be modeled in the Collins-Soper-Sterman formalism by a modification of impact-parameter-dependent parton densities. We discuss the consequences of such a modification for the production of electroweak bosons at hadron-hadron colliders. If substantial small-$x$ broadening is observed in forward Z boson production in the Tevatron Run-2, it will strongly affect predicted transverse momentum distributions for W, Z, and Higgs boson production at the Large Hadron Collider.

Motivation & Objective

• To assess the impact of small-x parton density broadening on transverse momentum (qT) distributions in electroweak boson production at hadron colliders.
• To evaluate how such broadening, modeled via modified impact-parameter-dependent parton densities, affects qT resummation in the Collins-Soper-Sterman formalism.
• To quantify the implications for W and Z boson mass measurements using transverse momentum and transverse mass methods at the Tevatron and LHC.
• To examine the feasibility of observing qT broadening in forward Z boson production at the Tevatron Run-2 and its implications for LHC physics.
• To assess whether current LHC analysis strategies for Higgs boson and W/Z boson signals need revision due to non-uniform qT broadening in signal and background processes.

Proposed method

• Adapts the Collins-Soper-Sterman (CSS) formalism to include x-dependent modifications in impact-parameter (b) space parton densities, modeling small-x broadening effects.
• Uses the Drell-Yan process as a framework to study qT distributions, incorporating resummation of large logarithms in qT/Q and 1/x.
• Applies the Ciafaloni-Catani-Fiorani-Marchesini (CCFM) equation to simultaneously resum qT and 1/x logarithms in the small-x, low-qT regime.
• Evaluates detector acceptance and kinematic coverage at the Tevatron (CDF, DØ) and LHC (ATLAS, CMS, TOTEM) to determine feasibility of observing qT broadening in forward regions.
• Performs numerical simulations to estimate shifts in W boson mass measurements due to qT broadening, using the pTe method and transverse mass reconstruction.
• Compares qT spectra from qg/gq and gg scattering processes with dominant q q̄ annihilation, showing enhanced qT at small x.

Experimental results

Research questions

• RQ1 How does small-x parton density broadening in impact-parameter space affect transverse momentum distributions of W and Z bosons at the Tevatron and LHC?
• RQ2 To what extent does qT broadening at small x alter the predicted W boson mass when measured via transverse momentum or transverse mass methods?
• RQ3 Can qT broadening in forward Z boson production at the Tevatron be observed with current detector acceptances, and what are the implications for LHC physics?
• RQ4 How does the inclusion of qg and gg scattering contributions at small x modify the qT spectrum compared to standard q q̄ annihilation?
• RQ5 What revisions to LHC analysis strategies are needed for W and Higgs boson measurements to account for non-uniform qT broadening in signal and background processes?

Key findings

• Small-x broadening in impact-parameter space can lead to a 10–20 MeV shift in the W boson mass measured via the pTe method in the central region (|ye| < 1) at the Tevatron.
• In the forward region (|ye| > 1), the W boson mass shift due to qT broadening may exceed 50 MeV, significantly exceeding standard theoretical uncertainties.
• At the LHC, the predicted qT distribution for W bosons becomes substantially harder due to small-x broadening, potentially exceeding other theoretical uncertainties.
• The effect of qT broadening is more pronounced in forward regions and depends on the boson type and charge, affecting both signal and background processes differently.
• Selection cuts in the Higgs boson γγ decay channel may need to be reconsidered to account for non-uniform qT broadening in signal and background distributions.
• The CDF detector at the Tevatron has limited capability to identify forward Z bosons due to lack of forward tracking, but a sample with one central and one forward electron may still reveal small-x broadening with sufficient statistics.