Search for Heavy Stable Charged Particles at $\sqrt{s}$ = 13 TeV Utilizing a Multivariate Approach

Heavy stable charged particles (HSCPs) have been searched for at the Large Hadron Collider since its initial data taking in 2010. The search for heavy stable charged particles provide a means of directly probing the new physics realm, as they produce a detector signature unlike any particle discovered to date. The goal of this research is to investigate an idea that was introduced in the later stages of 2010-2012 data taking period. Rather than utilizing the current tight selection on the calculated particle mass the hypothesis is that by incorporating a multivariate approach, specif- ically an artificial neural network, the remaining selection criteria could be loosened allowing for a greater signal acceptance while maintaining acceptable background rejection via the multivariate discriminator from the artificial neural network. The increase in signal acceptance and retention or increase in background rejection increases the discovery potential for HSCPs and as a secondary objective calculates improved limits on the HSCP signal models. The multivariate approach was developed and tested using 2.5 fb$^{−1}$ of 2015 data at √s = 13 TeV based on both the past tracker-only and Tracker+TOF HSCP analyses. The multivariate analyses were able to produce improved upper cross section limits on both expected and observed cross sections compared to the past 2015 results. The lower mass limits produced by the multivariate analyses are also improved, but the improvement was found to be less than 5% higher compared to the past 2015 HSCP search results. A final comparison of the multivariate approach to past HSCP searches was conducted on 12.9 fb$^{−1}$ of 2016 data at √s = 13 TeV. No statistically significant excess of data over background pre- diction is observed. Therefore no evidence of HSCPs is claimed. The tracker-only (Tracker+TOF) multivariate analysis produced lower mass limits of 1870 (1820) GeV for gluinos with 10% R- hadrons produced neutral, 1260 (1210) GeV for stops, 680 (670) GeV for decay-product staus, 340 (370) GeV for directly pair-produced staus, 720 (750) GeV for modified Drell-Yan |Q| = 1e, and 700 (900) GeV for modified Drell-Yan |Q| = 2e. Overall, the multivariate approach produced improved lower mass limits compared to the results from the past 2016 HSCP search. The overall improvements in cross section and mass limits using the multivariate approach produced the best limits on HSCPs to date. Furthermore the multivariate approach is shown as a viable method of searching for HSCPs. With the HSCP search covering a broad range of beyond xxi the Standard Model physics, the lack of evidence and subsequent limits produced place important restrictions on the theoretical models that predict HSCPs.