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Measuring slepton spin at the LHC

A. J. Barr

TL;DR

This work introduces a boost-invariant angular observable, $\cos^*_{ll} \cos\theta^*_{ll}$, derived from the dilepton rapidity difference to determine slepton spin at the LHC without knowing the parton-parton center-of-mass frame. Through Monte Carlo studies of SUSY test points and detector simulations, the authors demonstrate that the slepton production angular distribution can be distinguished from phase-space and from UED predictions, with $\sim$100–300 fb$^{-1}$ of data sufficient for several MSSM points. While some regions (e.g., SPS2, SPS6, SPS4) pose challenges due to cross-sections or small mass gaps, the method generally provides a robust, model-discriminating tool for sleptons and complements existing spin-determination strategies. The work emphasizes data-driven background control and systematic studies to enable a practical, LHC-era spin measurement.

Abstract

A new method is presented for measuring the spin of selectrons and smuons at the Large Hadron Collider (LHC), using an angular variable which is sensitive to the polar angle in direct slepton pair production. This variable is invariant under boosts along the beam axis, so it can be used at the LHC despite the fact that the longitudinal boost of the centre-of-mass frame cannot be determined. Monte Carlo simulations demonstrate that, using this method, the LHC can distinguish between the supersymmetric production angular distribution and phase space, or between supersymmetry and the production angular distribution of universal extra dimensions. An integrated luminosity of about 100 to 300 inverse fb provides sufficient statistics to measure the slepton spin for points which had left-handed slepton masses in the range 202 to 338 GeV, and right-handed sleptons in the range 143 to 252 GeV. Good sensitivity was found in the `bulk' and `stau co-annihilation' regions of the cMSSM favoured by cosmological relic density measurements. Various systematic uncertainties are investigated, and some methods for reducing them are discussed.

Measuring slepton spin at the LHC

TL;DR

This work introduces a boost-invariant angular observable, , derived from the dilepton rapidity difference to determine slepton spin at the LHC without knowing the parton-parton center-of-mass frame. Through Monte Carlo studies of SUSY test points and detector simulations, the authors demonstrate that the slepton production angular distribution can be distinguished from phase-space and from UED predictions, with 100–300 fb of data sufficient for several MSSM points. While some regions (e.g., SPS2, SPS6, SPS4) pose challenges due to cross-sections or small mass gaps, the method generally provides a robust, model-discriminating tool for sleptons and complements existing spin-determination strategies. The work emphasizes data-driven background control and systematic studies to enable a practical, LHC-era spin measurement.

Abstract

A new method is presented for measuring the spin of selectrons and smuons at the Large Hadron Collider (LHC), using an angular variable which is sensitive to the polar angle in direct slepton pair production. This variable is invariant under boosts along the beam axis, so it can be used at the LHC despite the fact that the longitudinal boost of the centre-of-mass frame cannot be determined. Monte Carlo simulations demonstrate that, using this method, the LHC can distinguish between the supersymmetric production angular distribution and phase space, or between supersymmetry and the production angular distribution of universal extra dimensions. An integrated luminosity of about 100 to 300 inverse fb provides sufficient statistics to measure the slepton spin for points which had left-handed slepton masses in the range 202 to 338 GeV, and right-handed sleptons in the range 143 to 252 GeV. Good sensitivity was found in the `bulk' and `stau co-annihilation' regions of the cMSSM favoured by cosmological relic density measurements. Various systematic uncertainties are investigated, and some methods for reducing them are discussed.

Paper Structure

This paper contains 14 sections, 11 equations, 1 figure, 1 table.

Table of Contents

  1. Introduction
  2. Angular distributions, and θ^*_ll$\cos\theta^*_{ll}$
  3. Monte Carlo simulation of test points
  4. Event selection
  5. Results
  6. Systematic uncertainties
  7. Lepton efficiency, acceptance and charge identification
  8. Standard Model background determination
  9. SUSY backgrounds
  10. SUSY spectrum
  11. Migrations at cut boundaries
  12. Migrations between bins
  13. Summary of systemic uncertainties
  14. Conclusions

Figures (1)

  • Figure 1: As for fig. \ref{['fig:s5comparison']} but for Snowmass points (a)SPS1a, (b)SPS1b(c)SPS3 and (d)SPS5. The integrated luminosity simulated in these plots is 200 ${\mathrm{fb}^{-1}}$ for SPS1b and SPS 3, and 300 ${\mathrm{fb}^{-1}}$ for SPS 1a and SPS 5.