Kate HusbandCambridge University, UK

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

Supervisor: Eike Middell

Signals

• IceCube looks for high-energy astrophysical • E.g. from supernova remnants• Also detects atmospheric ’s

• Primary cosmic rays produce muons which is the main background to IceCube.

IceCube

• 1km3 high energy, telescope at S Pole

• Contains DOMs with PMT which record amp. & time of photons

• Ice contains dust particles and air bubbles so photons scatter

Events: Cascade-like

• 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

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

Multi- Vs. Single-Peak

• If triggered, DOM records continuous waveform

1. 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)

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

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 ~0.3, tracks ~0.

B. LineFit Velocity

• Fits a straight line through triggered DOMs

• Var(time) is not a good cut variable

• Charge weighting doesn’t increase separation

C: Cumulative Charge

• Cumulative sum of charge of each DOM over time

• Take time at x% of total charge to get tx

• Single-peak

• ~t75 found to give largest separation

D: Charge Propagation

• Decay of charge-energy ratio different

• Total charge ≈energy and centre of gravity ≈ interaction vertex

• Single-peak

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

Optimization

• New filter: MCD+ToI (sp): 73% e

• Previous filter: vel+ToI (mp): 71.7% e (smaller detector)

Energy Efficiency

• New filter is slightly better at low energies

• Blue: vel +ToI (mp), Red: MCD+ToI (sp)

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.