[Paper Review] Measuring Mass and Cross Section Parameters at a Focus Point Region

R. Gray, J. Alexander+7 more|arXiv (Cornell University)|Jul 1, 2005

Particle physics theoretical and experimental studies3 references3 citations

TL;DR

This study investigates precision measurements of supersymmetric (SUSY) particle masses and cross sections at a 500 GeV linear collider in the mSUGRA focus point region, where only light neutralinos and charginos are accessible. Using the 2-jet + 2-lepton + missing energy signature from $\tilde{\chi}^0_2\tilde{\chi}^0_3$ pair production, the authors demonstrate that the LSP mass can be measured to within 1.0 GeV, and mass differences $M_{\tilde{\chi}^0_2} - M_{\tilde{\chi}^0_1}$ and $M_{\tilde{\chi}^0_3} - M_{\tilde{\chi}^0_1}$ can be determined with uncertainties of 0.3 GeV and 0.2 GeV, respectively, using invariant mass and energy distribution fits.

ABSTRACT

The purpose of this study is to determine the experimental uncertainties in measuring mass and cross section parameters of SUSY particles at a 500 GeV Linear Collider. In this study SUSY is a point in the focus point region of mSUGRA parameter space that is compatible with WMAP constraints on dark matter relic density. At this study point the masses of the squarks and sleptons are very heavy, and the only SUSY particles accessible at the Linear Collider would be the three lightest neutralinos, and the two lightest charginos: nino1, nino2, nino3, cino1+, cino2+, where nino1 is the lightest supersymmetric particle (LSP). The charginos or neutralinos may be pair produced, and the subsequent decay cascades to the LSP allow us to measure the SUSY couplings and mass spectrum. We find that by looking for the signature 2 jets plus 2 leptons plus missing energy we can determine the mass of the LSP to within 1 GeV uncertainty and that the mass differences of nino2 and nino3 with the LSP mass can be determined to better than 0.5 GeV.

Motivation & Objective

• To determine the experimental uncertainties in measuring SUSY particle masses and cross sections at a 500 GeV linear collider in the mSUGRA focus point region.
• To assess whether the collider can measure parameters precise enough to test if the LSP relic density matches WMAP constraints on dark matter.
• To evaluate the feasibility of distinguishing the correct sign combination of neutralino mass parameters using invariant mass and energy distributions.
• To quantify the precision of LSP mass and mass difference measurements using the 2-jet + 2-lepton + missing energy signature.

Proposed method

• Simulated events for $\tilde{\chi}^0_2\tilde{\chi}^0_3$ pair production were generated using Isajet 7.69 with 95% polarized electron beams on unpolarized positrons.
• Background processes (e.g., $t\bar{t}$, $W^+W^-$, $Z\bar{Z}$) were generated using WHiZaRD 1.22 and included initial state radiation effects.
• Detector simulation was performed using LCD Root FastMC, with lepton identification based on isolation and momentum thresholds (p > 10 GeV), and jet reconstruction via the Durham algorithm with ycut = 0.004.
• Invariant mass distributions of the two leptons were used to extract mass differences, with fits to signal and background distributions using unbinned log-likelihood methods.
• Energy distributions of the two leptons were numerically integrated to extract the LSP mass, with constraints from previously measured mass differences.
• Sign combinations of neutralino mass parameters were tested via likelihood fits to distinguish the correct (+,-) combination from incorrect (-,-) and (+,+) cases.

Experimental results

Research questions

• RQ1 Can a 500 GeV linear collider measure the LSP mass with sufficient precision to test its consistency with WMAP dark matter constraints?
• RQ2 What is the experimental uncertainty in measuring the mass differences $M_{\tilde{\chi}^0_2} - M_{\tilde{\chi}^0_1}$ and $M_{\tilde{\chi}^0_3} - M_{\tilde{\chi}^0_1}$ in the focus point region?
• RQ3 Can the correct sign combination of neutralino mass parameters be distinguished from incorrect ones using invariant mass and energy distributions?
• RQ4 How precisely can the production cross sections of $\tilde{\chi}^0_2\tilde{\chi}^0_3$ pairs be measured using the 2-jet + 2-lepton + missing energy signature?
• RQ5 To what extent do spin correlations and initial state radiation affect the reconstruction of lepton energy and invariant mass distributions?

Key findings

• The LSP mass ($\tilde{\chi}^0_1$) was measured to be 108.3 GeV ± 1.0 GeV, consistent with the generator-level value of 107.7 GeV.
• The mass difference $M_{\tilde{\chi}^0_2} - M_{\tilde{\chi}^0_1}$ was determined to be 58.8 GeV ± 0.3 GeV, compared to the generator-level value of 58.6 GeV.
• The mass difference $M_{\tilde{\chi}^0_3} - M_{\tilde{\chi}^0_1}$ was measured as 82.25 GeV ± 0.2 GeV, matching the generator-level value of 82.3 GeV.
• The correct sign combination of the neutralino mass parameters (+,-) was distinguished from incorrect combinations with a significance of at least 13 standard deviations.
• Production cross sections for $\tilde{\chi}^0_2\tilde{\chi}^0_3$ pairs were measured to within 4% uncertainty using 500 fb⁻¹ of data (250 fb⁻¹ left-polarized and 250 fb⁻¹ right-polarized).
• The invariant mass distribution of the two leptons showed clear edges at 58.6 GeV and 82.3 GeV, corresponding to $\tilde{\chi}^0_2$ and $\tilde{\chi}^0_3$ masses, respectively, confirming the signal's visibility.