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HAWC Design and Performance

HAWC Design and Performance. Andrew Smith/Vlasios Vasileiou University of Maryland. 100 MeV  photons shown. Through-going Muon. Detector Layout - As Simulated. Performance and Design - Outline. Design Optimization Why water Cherenkov? Optical isolation? Why Tanks?

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HAWC Design and Performance

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  1. HAWCDesign and Performance Andrew Smith/Vlasios Vasileiou University of Maryland HAWC Review - December 2007

  2. 100 MeV photons shown Through-going Muon Detector Layout - As Simulated HAWC Review - December 2007

  3. Performance and Design - Outline • Design Optimization • Why water Cherenkov? • Optical isolation? • Why Tanks? • Design optimization. depth, radius, spacing… • Detector Simulation • Corsika,GEANT4,materials,PMT response • HAWC Performance • Effective Area • Angular Resolution • Gamma-Hadron Separation • Energy Resolution • Gamma-ray sensitivity • Potential for “re-optimization” HAWC Review - December 2007

  4. Detector Optimization - Why Water Cherenkov? • Coverage • Water is an inexpensive detector medium that is at least an order of magnitude cheaper than particle detectors (RPCs, scintillator…). • Gamma-Ray Conversion • ~80-90% of the energy in an EM shower is carried by gamma-rays. • Water detectors efficiently convert gamma-rays (Pair-production and Compton scattering) for detection. • Calorimetry • Measurement of deposited energy is critical to gamma/hadron separation. • Muon vs electron discrimination. • Hadron identification. • High pT hadronic interaction lead to large energy depositions outside the core region. HAWC Review - December 2007

  5. Detector Optimization - Optical Isolation • Timing • Shower front photon timing is critical to optimizing the angular resolution. • Non-locally produced Cherenkov photons are delayed compared to locally produced photons. • Better core/energy resolution for large events. Shower plane Shower particles Cherenkov Photons HAWC Review - December 2007

  6. Why Tanks? • Tanks are less expensive than a pond with either a building or floating cover over it. • Tanks have no single point failure. • Servicing can be done without shutting down the detector. • Tanks allow us to start building and operating the detector more quickly for virtually all reasonable funding profiles. • With tanks, we can begin development and debugging operations with a small number of tanks deployed in the first year • HAWC should be larger and more sensitive than Milagro in less than two years. • With a pond, we cannot start taking data until the pond and building/cover are complete (which might require several years of funding) and the pond is filled with water. • Water filling is simplified: • We would have to fill the pond all at once requiring a 100ml of water in ~6 months • The tanks require ~20% less total water. • Tanks can be filled incrementally over the several years of deployment reducing the amount of water we need per year by approximately a factor of 7-10. • Tanks are a proven technology (Milagro and Auger …) • Tanks allow a flexible and expandable array arrangement. HAWC Review - December 2007

  7. 100 MeV photons shown Detector Optimization - Depth/Width • Depth: • Shallower: Poorer calorimeter, but more PEs/GeV. • PMT must be far from site of Cherenkov photon production. • Increasing depth further reduced photon yield. • Optimal depth: ~3.5-4.5m. We choose 4m. • Width: • Width is determined roughly from depth and photon yield. • For a 4m depth, a ~2.5m radius is optimal. 100 MeV gamma-rays HAWC Review - December 2007

  8. Overview of the simulation chain • Simulation • Extended Air Shower simulation with Corsika • Detector simulation with GEANT4 • Preparing the MC data for analysis • Add noise • PMT non-uniformity effects • Electronics simulation • Analysis HAWC Review - December 2007

  9. Extended Air Shower Simulation with CORSIKA • Physics simulation • EGS4 for EM interactions • FLUKA for low energy hadronic interactions (E<80 GeV)‏ • Best package available • NeXus for higher energy hadronic interactions • Theory + experiment (H1 and Zeuss) driven • Alternative theory-driven QGSJETII, produced similar results HAWC Review - December 2007

  10. Extended Air Shower Simulation with CORSIKA • 5GeV – 500TeV kinetic energy/nucleon • Proton + Helium primaries • Use the same simulated showers for both Milagro and HAWC • Saving the data at both altitudes HAWC Review - December 2007

  11. Simulation of the detector with GEANT4 • C++ Simulation Toolkit from CERN • Written for the needs of the LHC • Powerful, transparent and easily extendable • We've debugged and modified it to match our simulation needs (speed + accuracy). • Physics simulation of GEANT4 overall the best available in HEP. HAWC Review - December 2007

  12. Simulation of the detector with GEANT4 • CORSIKA's EAS particles reaching the detector altitude are injected in the GEANT4 simulation model. • Very detailed simulation • Surface reflectivities to Cherenkov photons • From theory or experimental measurements • Water properties • Attenuation length and angular distribution function by our measurements • We extended GEANT4 physics to include forward-scattering of optical photons in the water. HAWC Review - December 2007

  13. Detailed detector simulation • PMT model • Full optical simulation on the PMTs • Reflections/refractions/absorptions are fully simulated for all parts of the PMT • Using the complex refractive index of the photocathode material to calculate photocathode absorptivity vs. energy and incidence angle. HAWC Review - December 2007

  14. Analysis – Post processing of the MC data • Add noise • Uncorrelated single-pe noise • Radioactivity from the water, glass, light leaks etc • Cosmic-ray “noise” • Overlay small fragments of simulated showers • Apply photocathode non-uniformity effects • We found that the PMT Gain and detection efficiency are considerably reduced for illumination near the equator of the photocathode (vs. illumination near the center). • Discard photoelectrons based on the position-dependent detection efficiency • Assign a pulse height based on the position-dependent gain • Electronics simulation • Time over threshold and trigger HAWC Review - December 2007

  15. Analysis • MC data are now in the same format as the real data • Apply the same analysis as in the real data. • Same reconstruction algorithms, gamma-hadron separation etc. HAWC Review - December 2007

  16. Simulation performance - Milagro • gamma-hadron discrimination (see left plot)‏ • Angular distribution function (see right plot)‏ • MC gamma-ray rate from the Crab agrees to a factor of 10% with the measurements • MC cosmic-ray rate is 60% of the one predicted by balloon experiments (BESS, ATIC, JACEE)‏ • Various distributions in excellent or very good agreement Profile of the γ-ray signal from the Crab. Black points are for data and red for simulation of hadronic showers. HAWC Review - December 2007

  17. Conclusion • We have a very detailed simulation for Milagro. • Milagro's simulation predictions are in excellent agreement with the experimental results. • The HAWC simulation based on the verified components of the Milagro simulation. • For that reason we believe HAWC's simulation is also accurate. HAWC Review - December 2007

  18. Detector Layout • Rows of close packed tanks with aisles for access and cabling. • Counting house at center to minimize cable runs. HAWC Review - December 2007

  19. Simulation Strategy • Use same simulation as Milagro • HAWC is made up of the same materials as Milagro (PMTs, water, surfaces, baffles…) • Use same Corsika shower database (2 elevations). • Anchor Simulation to Milagro. • Compute gamma-ray and proton rates relative to Milagro simulation. • Compute HAWC predicted rates based on observed rates in Milagro. Gamma-ray rates are normalized to the Crab and CR backgrounds are normalized to the observed background rate in Milagro (Higher than predicted rate). • Compute sensitivity for crab-like source spectrum (2.82x10-11 x E-2.62 cm-2TeV-1s-1). • Milagro (with standard analysis) observes 9 /sqrt(yr). • Compute Q (Milagro --> HAWC) and optimize design to maximize Q. HAWC Review - December 2007

  20. Effective Area • “Threshold” of HAWC is lower by a factor of ~3.5. • At HAWC elevation 5x more energy reaches the ground. • HAWC has a lower PMT density than Milagro (Higher particle density threshold.) • HAWC has much higher area (~50x) below 1 TeV. • With gamma/hadron cut applied, HAWC has ~100x the area of Milagro at low energy. • ~100m2 effective area at 100 GeV. HAWC Review - December 2007

  21. Energy Threshold - Zenith Angle Atmospheric depth depends on zenith angle 0.5 sr viewable at < 23o 1.0 sr viewable at  < 31o 1.5 sr viewable at  < 40o 2.0 sr viewable at  < 47o Energy required to trigger HAWC HAWC Review - December 2007

  22. Longitudinal Shower Profile Fixed first interaction elevation: 30km • Prior to shower maximum: • Exponential growth in particle. • Energy --> particle creation (pair,brems.) • After shower maximum: • Exponential decay in particle number. • Particle energies fall below ECritical (Compton >Pair). • Particle spectrum is independent of elevation. • Energy deposited in atmosphere through ionization. • For a 1 TeV shower, 100 GeV reaches HAWC observation level. 1 TeV gamma-ray shower Longitudinal Profile HAWC elevation: 4.1km 10km From http://www.ast.leeds.ac.uk/~fs/photon-showers.html HAWC Review - December 2007

  23. Energy at Observation level vs First Interaction Height First Interaction height is an excellent predictor of energy reaching the ground. Benefit: Deep fluctuations allow for the detection of showers below the nominal detector “threshold”. Liability: Inability to distinguish between low-energy deeply-penetrating showers and high-energy shallowly-penetrating showers limits energy resolution. Energy reaching ground level (4.1km) vs. first interaction height for 100 GeV vertical showers. First interaction elevation distribution HAWC Review - December 2007

  24. P(E) ~ (E/Eo)-2.6 Cascade profiles all have the same slope past shower max Power Law: A~E2.6 Eff Area ≈ Detector Area Characteristic threshold Intrinsic Capabilities - Effective Area Observe: Shower energy attenuates by a factor of 1.65 with each radiatino length The probability the a VHE gamma ray will penetrate N radiation lengths before interacting is (Pair production cross-section) EAS detectors below characteristic threshold have a power law effective area that extends down in energy. Combining the 2 expressions gives HAWC Review - December 2007

  25. Angular Resolution • At similar energies, HAWC’s angular resolution is ~1.5x better than Milagro. • At the highest energies systematic error in core and angle reconstruction limit resolution. Improvement is expected. • Resolution here is sigma for a 2-d Gaussian. Resolution at 10 TeV HAWC Review - December 2007

  26. Intrinsic Capabilities - Angular Resolution The angular resolution of an EAS detector can never be better than the momentum of the particles reaching the observation level. Optimal Angular Resolution = Space angle difference between the primary gamma and the vector sum of the momentum of the particles that reach the observation level. Rather than plotting resolution for primary gamma-ray energy, we plot it for total energy reaching the ground (Eground). This variable is a better predictor of angular resolution. Primary particle energy for small zenith angle 300 GeV shower 5 TeV shower 1 TeV shower Optimal Angular Resolution HAWC (<30o) (GeV) HAWC Review - December 2007

  27. Gamma-Hadron Separation Technique Gammas Size of HAWC Protons Size of Milagro deep layer Energy Distribution at ground level • Proton showers (with high PT hadronic interactions) contain high-energy muons, hadrons and multiple EM clumps. • Large energy depositions outside the core region indicate hadron-like showers. HAWC Review - December 2007

  28. Gamma/Hadron Separation • Gamma/Hadron Parameter: C = nHit/cxPE • nHit = number of hits in the detector • cxPE = largest hit (in PEs) >30m from shower core Gammas C = 12.0 C = 16.3 C = 7.5 C = 9.7 Protons C = 0.6 C = 0.6 C = 3.2 C = 1.6 HAWC Review - December 2007

  29. Gamma/Hadron Separation • Efficacy of gamma/hadron separation improves with energy. • Background rejection: Q>5 for E> 5 TeV. • Limited by ability to simulate high energy hadrons. gammas gammas gammas hadrons hadrons hadrons HAWC Review - December 2007

  30. Gamma/Hadron Separation Efficiency for rejecting hadron when retaining 50% of gamma-rays. Comparison of Milagro and HAWC Milagro HAWC rejects ~10x the background compared to Milagro at the same energy. HAWC HAWC Review - December 2007

  31. Energy Resolution • Energy resolution dominated by 2 sources: • Ability to measure energy at the ground level. • Fluctuations in the atmosphere. • Resolution in energy measurement at ground level ~<25% HAWC Review - December 2007

  32. HAWC Threshold Intrinsic Capabilities - Energy Resolution • Longitudinal Energy fluctuations dominate energy resolution Energy resolution is well described by a log-normal distribution in the fraction of energy reaching the ground. 75 GeV 600 GeV Energy resolution is well described by a log-normal distribution in the fraction of energy reaching the ground. 9600 TeV 37 TeV @ 5 TeV : +70%/-44% @ 20 TeV : +32%/-24% HAWC Review - December 2007

  33. Energy Resolution • HAWC and Milagro energy resolution compared. • Much better energy resolution than Milagro. This is mostly due to increased elevation. • Maybe able to measure shower age, improve energy resolution. HAWC Review - December 2007

  34. Sensitivity • Crab-like Spectrum • Compute baseline sensitivity for a crab-like spectrum. • Point source. • In general, HAWC is more sensitive to harder sources. Peak sensitivity: 5-20 TeV. • Single transit • We compute the sensitivity for a single source transit from horizon to horizon through the detector's field of view. • The baseline sensitivity is for a source transiting with a minimum zenith angle of 15 deg. • Improved sensitivity for sources that transit closer to zenith. • Crab Sensitivity • With Basic cuts: 75/year (4/day) • With event weighting: 120/year (6/day) • 42mCrab (18mCrab) sensigivity for 1 year (5 year) survey. HAWC Review - December 2007

  35. Sensitivity • Sensitivity of HAWC compared to other gamma-ray telescopes. • Sensitivity is computed for a crab-like point source. • 1 and 5 year all sky HAWC sensitivity is compared to 50hr exposure for ACTs. HAWC Review - December 2007

  36. Sensitivity vs Source transit declination HAWC Review - December 2007

  37. Event Rates - Requirements for Technical Design • Results presented assume a multiplicity trigger of 50 PMTs. • Trigger rate ~4x Milagro Rate = 6000 Hz. • Event multiplicity is smaller than Milagro: 10MB/s. • Want to extend reach to lower energy (Multiplicity 20-30). • DAQ may need to reach 9kHz • 15 MB/s. • Improved trigger may be needed (gamma/hadron sep. at trigger level) • With optical isolation, simulations show that noise rates in HAWC are similar to Milagro 30 kHz/PMT. • Timing: • Timing calibration <1ns absolute and relative. • Water Clarity • >10m attenuation. HAWC Review - December 2007

  38. Expansion, Reconfiguration and Redeployment • Modular design permits us to expand, or redesign and even redeploy. • Increase number of PMTs in each Tank from 1 to 3 will decrease the median energy from 1 TeV to about 700 GeV. • Expanding the array size will increase the maximum collection area, expand energy reach. HAWC Review - December 2007

  39. High Energy Optimization Reconfigure with 1/2 size central core and ~3x larger peak area. Std configuration HE optimized configuration Increase sensitivity at the highest energies, probably at the expense of sensitivity at <10 TeV due to decreases gamma/hadron separation and higher threshold. HAWC Review - December 2007

  40. Sensitivity increase at low energies (GRBs, distant AGN). Additional PMTs could be acquired by reducing the size of the HAWC detector or with additional support. Low Energy OptimizationDense central core reduces threshold from 1 TeV to 700 GeV. Dense central core: 3 PMTs/tank Increase photon yield to ~60 PEs/GeV. Standard outer region: 1 PMT/tank Standard photon yield ~20PEs/GeV HAWC Review - December 2007

  41. Summary • The HAWC detector was simulated from end to end. • Sensitivity estimation anchored to Milagro observations giving. • We estimate that HAWC will have ~15x Milagro’s sensitivity. • Results are consistent with simple estimations. • The design is sensible (not idealized). • The design is flexible. HAWC Review - December 2007

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