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Development of an Online Filter for Cascade-like Events in IceCube

This project focuses on the development of an online filter for the selection of cascade-like events in IceCube, a high-energy neutrino telescope located at the South Pole. The aim is to improve efficiency by introducing new single-peak cut variables such as t75 and MCD (mean on new axis weighted with distance). The proposed filter achieves a signal efficiency of 73.0% for a 1% background rejection, with slightly better performance at low energies compared to the existing filter.

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Development of an Online Filter for Cascade-like Events in IceCube

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  1. Kate HusbandCambridge University, UK Development of an Online Filter for Selection of Cascade-like Events in IceCube Supervisor: Eike Middell

  2. Signals • IceCube looks for high-energy astrophysical n • E.g. from supernova remnants • Also detects atmospheric n’s • Primary cosmic rays produce muons which is the main background to IceCube.

  3. IceCube • 1km3 high energy, n telescope at S Pole • Contains DOMs with PMT which record amp. & time of photons • Ice contains dust particles and air bubbles so photons scatter

  4. Events: Cascade-like • n interact through weak force only • Quarks produce cascade of Cherenkov photons • Photon intensity decays rapidly • Muons can also knock out e- resulting in em cascade

  5. Events: Track-like • Muons move close to c • Don’t lose much energy in producing Cherenkov light • Only arrive from above detector as decay quickly • Hence long straight downwards tracks

  6. Multi- Vs. Single-Peak • If triggered, DOM records continuous waveform • Try and fit pulses of different times and amp. into waveform = MULTI-PEAK (mp) OR 2. Find no. of photons from total amplitude and give all photons the same time = SINGLE-PEAK (sp)

  7. AIMs • Online filter must separate multiple background events from signal within constraints. • Constraints: limited bandwidth & CPU power • Aim to improve efficiency by looking at possible new single-peak cut variables

  8. A. ToI • Each DOM is given a mass proportional to amount of charge it records • Centre of gravity of distribution is used as approx. to interaction vertex • Good cut variable as spherical n ~0.3, tracks ~0.

  9. B. LineFit Velocity • Fits a straight line through triggered DOMs • Var(time) is not a good cut variable

  10. Charge weighting doesn’t increase separation

  11. C: Cumulative Charge • Cumulative sum of charge of each DOM over time • Take time at x% of total charge to get tx • Single-peak

  12. ~t75 found to give largest separation

  13. D: Charge Propagation • Decay of charge-energy ratio different • Total charge ≈energy and centre of gravity ≈ interaction vertex • Single-peak

  14. Mean on new axis for each event weighted with distance is new variable: MCD

  15. Optimization • Previous filter: vel+ToI (mp): 71.7% ne (smaller detector) • New filter: MCD+ToI (sp): 73% ne

  16. Energy Efficiency • New filter is slightly better at low energies • Blue: vel +ToI (mp), Red:MCD+ToI (sp)

  17. Summary • LineFit velocity cannot be replaced by variance of time • New single-peak cut variables: t75 and MCD • New filter suggested with MCD>61.4, ToI>0.14 passing 73.0% sig. for 1% bkd. • New filter has slightly better low energy efficiency.

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