[Paper Review] Study of fermion pair productions at the ILC with center-of-mass energy of 250 GeV

Hiroaki Yamashiro, K. Kawagoe+4 more|arXiv (Cornell University)|Jan 15, 2018

Particle physics theoretical and experimental studies7 references3 citations

TL;DR

This study investigates fermion pair production at the ILC's 250 GeV center-of-mass energy to probe new physics via precise measurements of $e^+e^-\to\ell^+\ell^-$ cross sections and angular distributions. Using full simulation with ILCSoft, WHIZARD, and Geant4-based Mokka, it demonstrates sensitivity to $Z^\prime$ bosons up to 4.0 TeV and WIMPs up to 330 GeV at 3$\sigma$ significance, significantly extending indirect discovery reach beyond direct detection limits.

ABSTRACT

Precise measurements of electroweak processes at the International Linear Collider (ILC) will provide unique opportunities to explore new physics beyond the Standard Model. Fermion pair productions are sensitive to a new contact interaction or a new heavy gauge boson by comparing cross section and angular distribution with expectations of the new physics models. In this proceedings we report a simulation study of fermion pair productions at a center-of-mass energy of 250 GeV, with a focus on lepton pairs, to demonstrate the potential of the first phase of the ILC.

Motivation & Objective

• To assess the potential of the ILC's 250 GeV phase to probe new physics through precision measurements of $e^+e^-\to\ell^+\ell^-$ processes.
• To evaluate the sensitivity of total and differential cross sections to new physics models such as $Z^\prime$ bosons and WIMPs.
• To quantify the discovery reach for $Z^\prime$ and WIMP models using simulated data with 2000 fb$^{-1}$ luminosity and realistic detector response.
• To compare the performance of different lepton channels ($e^+e^-$, $\mu^+\mu^-$, $\tau^+\tau^-$) in detecting deviations from the Standard Model.
• To explore the impact of beam polarization and background suppression on the sensitivity to new physics.

Proposed method

• Full simulation using ILCSoft v01-16-02-p1, with event generation via WHIZARD and PYTHIA for hadronization.
• Detector simulation with Mokka based on Geant4, using the ILD v1_05 geometry and PandoraPFA algorithm for particle flow reconstruction.
• Event reconstruction with Marlin processors, including tracking and cluster matching, and lepton tagging based on ECAL/HCAL energy deposits and track curvature.
• Application of kinematic and identification cuts: $E_{\rm sum} > 230$ GeV, $|\cos\theta| < 0.95$, and $E_{\rm tr} > 10$ GeV, with electron/muon identification via $E_{\rm ECAL}/(E_{\rm ECAL}+E_{\rm HCAL}) > 0.9$ and $< 0.5$, respectively.
• Statistical analysis using $\chi^2$-based probability tests to assess consistency with the SM and determine 3$\sigma$ discovery reach.
• Incorporation of event weights to scale partial Monte Carlo samples to full 2000 fb$^{-1}$ luminosity for H20 staging scenario.

Experimental results

Research questions

• RQ1 What is the sensitivity of ILC 250 GeV $e^+e^-$ collisions to $Z^\prime$ bosons in various models?
• RQ2 Can precision measurements of $e^+e^-\to\ell^+\ell^-$ reveal deviations from the Standard Model indicative of new physics?
• RQ3 What is the minimum $Z^\prime$ mass that can be excluded at 3$\sigma$ significance using differential cross section measurements?
• RQ4 How effective is the $e^+e^-\to\ell^+\ell^-$ process in probing WIMP dark matter candidates?
• RQ5 What is the combined discovery reach across $e^+e^-$, $\mu^+\mu^-$, and $\tau^+\tau^-$ final states?

Key findings

• The $Z^\prime$ model from the SSM (Singlet-Model) can be probed up to 2.8 TeV at 3$\sigma$ significance.
• The ALR $Z^\prime$ model has a 3$\sigma$ discovery reach of 4.0 TeV, the highest among the models studied.
• The $\psi$ $Z^\prime$ model has the lowest reach at 1.4 TeV due to weaker couplings.
• For WIMP searches, the higgsino and wino models are detectable up to 150 GeV, while the Minimal Dark Matter (MDM) model reaches 330 GeV at 3$\sigma$.
• The combined analysis of all three lepton channels significantly enhances sensitivity, enabling discovery reach beyond the beam energy.
• The study confirms that precision measurements of $e^+e^-\to\ell^+\ell^-$ at 250 GeV can probe new physics models with mass scales far exceeding direct production thresholds.