HAWC Design and Performance

HAWC Review - December 2007

HAWCHAWC Design and PerformanceDesign and Performance

Andrew Smith/Vlasios VasileiouAndrew Smith/Vlasios Vasileiou

University of MarylandUniversity of Maryland


100 MeV photons shown

Detector Layout - As SimulatedDetector Layout - As Simulated

Through-going Muon


Performance and Design - OutlinePerformance and Design - Outline

• Design OptimizationDesign Optimization– Why water Cherenkov?– Optical isolation?– Why Tanks?– Design optimization. depth, radius, spacing…

• Detector SimulationDetector Simulation– Corsika,GEANT4,materials,PMT response

• HAWC PerformanceHAWC Performance– Effective Area– Angular Resolution– Gamma-Hadron Separation– Energy Resolution– Gamma-ray sensitivity

• Potential for “re-optimization” Potential for “re-optimization”


Detector Optimization - Why Water Cherenkov? Detector Optimization - Why Water Cherenkov?

• CoverageCoverage

– Water is an inexpensive detector medium that is at least an order of magnitude cheaper than particle detectors (RPCs, scintillator…).

• Gamma-Ray ConversionGamma-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.

• CalorimetryCalorimetry

– 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.


Detector Optimization - Optical IsolationDetector Optimization - Optical Isolation

• TimingTiming– 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.Better core/energy resolution for large events.

Shower

plan

e

Shower particles

Cherenkov Photons


Why Tanks?Why Tanks?• Tanks are less expensive than a pond with either a building or floating cover over it.Tanks are less expensive than a pond with either a building or floating cover over it.• Tanks have no single point failure.Tanks have no single point failure.• Servicing can be done without shutting down the detector.Servicing can be done without shutting down the detector.• Tanks allow us to start building and operating the detector more quickly for virtually all Tanks allow us to start building and operating the detector more quickly for virtually all

reasonable funding profiles.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: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 are a proven technology (Milagro and Auger …)• Tanks allow a flexible and expandable array arrangement.Tanks allow a flexible and expandable array arrangement.


Detector Optimization - Depth/WidthDetector Optimization - Depth/Width• Depth: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:

– Width is determined

roughly from depth and

photon yield.

– For a 4m depth, a

~2.5m radius is optimal.

100 MeV gamma-rays


Overview of the simulation chainOverview of the simulation chain

SimulationSimulationExtended Air Shower simulation with CorsikaDetector simulation with GEANT4

Preparing the MC data for analysisPreparing the MC data for analysisAdd noisePMT non-uniformity effectsElectronics simulation

AnalysisAnalysis


Extended Air Shower Simulation with CORSIKAExtended Air Shower Simulation with CORSIKA

• Physics simulationPhysics 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


Extended Air Shower Simulation with CORSIKAExtended Air Shower Simulation with CORSIKA

• 5GeV – 500TeV kinetic energy/nucleon5GeV – 500TeV kinetic energy/nucleon• Proton + Helium primariesProton + Helium primaries• Use the same simulated showers for both Milagro and Use the same simulated showers for both Milagro and

HAWC HAWC – Saving the data at both altitudes


Simulation of the detector with GEANT4Simulation of the detector with GEANT4

• C++ Simulation Toolkit from CERNC++ Simulation Toolkit from CERN• Written for the needs of the LHCWritten for the needs of the LHC• Powerful, transparent and easily extendablePowerful, transparent and easily extendable

– We've debugged and modified it to match our simulation needs (speed + accuracy).

• Physics simulation of GEANT4 overall the best Physics simulation of GEANT4 overall the best available in HEP. available in HEP.


Simulation of the detector with GEANT4Simulation of the detector with GEANT4

• CORSIKA's EAS particles reaching the detector altitude are CORSIKA's EAS particles reaching the detector altitude are injected in the GEANT4 simulation model.injected in the GEANT4 simulation model.

• Very detailed simulationVery 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.


Detailed detector simulationDetailed detector simulation

• PMT modelPMT 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.


Analysis – Post processing of the MC dataAnalysis – Post processing of the MC data

• Add noiseAdd 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 effectsApply 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 simulationElectronics simulation– Time over threshold and trigger


AnalysisAnalysis

• MC data are now in the same format as the real dataMC data are now in the same format as the real data• Apply the same analysis as in the real data.Apply the same analysis as in the real data.

– Same reconstruction algorithms, gamma-hadron separation etc.


Simulation performance - MilagroSimulation performance - Milagro gamma-hadron discrimination (see left plot)gamma-hadron discrimination (see left plot) Angular distribution function (see right plot)Angular distribution function (see right plot) MC gamma-ray rate from the Crab agrees to a factor of 10% with the MC gamma-ray rate from the Crab agrees to a factor of 10% with the

measurementsmeasurements MC cosmic-ray rate is 60% of the one predicted by balloon experiments (BESS, MC cosmic-ray rate is 60% of the one predicted by balloon experiments (BESS,

ATIC, JACEE)ATIC, JACEE) Various distributions in excellent or very good agreementVarious 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.


ConclusionConclusion

• We have a very detailed simulation for Milagro.We have a very detailed simulation for Milagro.• Milagro's simulation predictions are in excellent Milagro's simulation predictions are in excellent

agreement with the experimental results. agreement with the experimental results. • The HAWC simulation based on the verified The HAWC simulation based on the verified

components of the Milagro simulation. components of the Milagro simulation. • For that reason we believe HAWC's simulation is also For that reason we believe HAWC's simulation is also

accurate.accurate.


Detector LayoutDetector Layout

• Rows of close packed Rows of close packed tanks with aisles for tanks with aisles for access and cabling.access and cabling.

• Counting house at Counting house at center to minimize center to minimize cable runs.cable runs.


Simulation StrategySimulation Strategy

• Use same simulation as MilagroUse 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.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 (2.82x10Compute sensitivity for crab-like source spectrum (2.82x10 -11-11 x x EE-2.62-2.62 cm cm-2-2TeVTeV-1-1ss-1-1).).– Milagro (with standard analysis) observes 9 /sqrt(yr).– Compute Q (Milagro --> HAWC) and optimize design to

maximize Q.


Effective AreaEffective Area• ““Threshold” of HAWC is lower by a Threshold” of HAWC is lower by a

factor of ~3.5.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) HAWC has much higher area (~50x) below 1 TeV.below 1 TeV.– With gamma/hadron cut applied,

HAWC has ~100x the area of Milagro at low energy.

• ~100m~100m22 effective area at 100 GeV. effective area at 100 GeV.


Energy Threshold - Zenith AngleEnergy Threshold - Zenith Angle

Energy required to trigger HAWC

Atmospheric depth depends on zenith angle

0.5 sr viewable at < 23o





Longitudinal Shower ProfileLongitudinal Shower Profile

From http://www.ast.leeds.ac.uk/~fs/photon-showers.html

Fixed first interaction elevation: 30km

HAWC elevation: 4.1km

1 TeV

gamm

a-ray shower Longitudinal P

rofile

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.

10km


Energy at Observation level vs First Interaction HeightEnergy at Observation level vs First Interaction Height

Energy reaching ground level (4.1km) vs. first interaction height for 100 GeV vertical showers.

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.

First interaction elevation distribution


Intrinsic Capabilities - Effective AreaIntrinsic Capabilities - Effective Area

Characteristic threshold

Power Law: A~E2.6

Eff Area ≈ Detector Area

Cascade profiles all have the same slope past shower max

€

ln(E / Eo) = −N ln(1.65)

The probability the a VHE gamma ray will penetrate N radiation lengths before interacting is(Pair production cross-section)

€

P = exp(−9

7N)

Combining the 2 expressions gives

P(E) ~ (E/Eo)-2.6

Observe: Shower energy attenuates by a factor of 1.65 with each radiatino length

EAS detectors below characteristic threshold have a power law effective area that extends down in energy.


Angular ResolutionAngular Resolution

• At similar energies, HAWC’s angular resolution is ~1.5x better than Milagro.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 At the highest energies systematic error in core and angle reconstruction limit resolution. Improvement is expected.resolution. Improvement is expected.

• Resolution here is sigma for a 2-d Gaussian.Resolution here is sigma for a 2-d Gaussian.

Resolution at 10 TeV


Intrinsic Capabilities - Angular ResolutionIntrinsic Capabilities - Angular Resolution

(GeV)

Optimal Angular Resolution

HAWC (<30o)

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.

1 TeV shower 5 TeV shower

Primary particle energy for small zenith angle

300 GeV shower


Gamma-Hadron Separation TechniqueGamma-Hadron Separation Technique

Gam

mas

Pro

ton

s

Size of Milagro deep layer Energy Distribution at ground level

Size of HAWC

• 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.


Gamma/Hadron SeparationGamma/Hadron Separation• Gamma/Hadron Parameter: C = nHit/cxPEGamma/Hadron Parameter: C = nHit/cxPE

– nHit = number of hits in the detector– cxPE = largest hit (in PEs) >30m from shower core

Gam

mas

Pro

ton

s

C = 12.0 C = 16.3 C = 7.5 C = 9.7

C = 0.6 C = 0.6 C = 3.2 C = 1.6


Gamma/Hadron SeparationGamma/Hadron Separation

• Efficacy of gamma/hadron separation improves with Efficacy of gamma/hadron separation improves with energy.energy.– Background rejection: Q>5 for E> 5 TeV.– Limited by ability to simulate high energy hadrons.

hadrons

gammas gammas gammas

hadrons hadrons


Gamma/Hadron SeparationGamma/Hadron Separation

Comparison of Milagro and HAWC

Efficiency for rejecting hadron when retaining 50% of gamma-rays.

Milagro

HAWC

HAWC rejects ~10x the background compared to Milagro at the same energy.


Energy ResolutionEnergy Resolution

• Energy resolution dominated by 2 sources: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%Resolution in energy measurement at ground level ~<25%


Intrinsic Capabilities - Energy ResolutionIntrinsic Capabilities - Energy Resolution

• Longitudinal Energy fluctuations dominate energy Longitudinal Energy fluctuations dominate energy resolutionresolution

75 GeV 600 GeV

9600 TeV 37 TeV

Energy resolution is well described by a log-normal distribution in the fraction of energy reaching the ground.

Energy resolution is well described by a log-normal distribution in the fraction of energy reaching the ground.

HAWC Threshold

@ 5 TeV : +70%/-44%

@ 20 TeV : +32%/-24%


Energy ResolutionEnergy Resolution

• HAWC and Milagro HAWC and Milagro energy resolution energy resolution compared.compared.

• Much better energy Much better energy resolution than resolution than Milagro. This is Milagro. This is mostly due to mostly due to increased elevation.increased elevation.

• Maybe able to Maybe able to measure shower age, measure shower age, improve energy improve energy resolution.resolution.


SensitivitySensitivity

• Crab-like SpectrumCrab-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 transitSingle 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 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.


SensitivitySensitivity

• Sensitivity of HAWC Sensitivity of HAWC compared to other compared to other gamma-ray telescopes.gamma-ray telescopes.

• Sensitivity is computed Sensitivity is computed for a crab-like point for a crab-like point source.source.

• 1 and 5 year all sky 1 and 5 year all sky HAWC sensitivity is HAWC sensitivity is compared to 50hr compared to 50hr exposure for ACTs.exposure for ACTs.


Sensitivity vs Source transit declinationSensitivity vs Source transit declination


Event Rates - Requirements for Technical DesignEvent Rates - Requirements for Technical Design

• Results presented assume a multiplicity trigger of 50 PMTs. 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).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 With optical isolation, simulations show that noise rates in

HAWC are similar to Milagro 30 kHz/PMT.HAWC are similar to Milagro 30 kHz/PMT.• Timing:Timing:

– Timing calibration <1ns absolute and relative.• Water ClarityWater Clarity

– >10m attenuation.


Expansion, Reconfiguration and RedeploymentExpansion, Reconfiguration and Redeployment

• Modular design permits us to expand, or redesign Modular design permits us to expand, or redesign and even redeploy.and even redeploy.

• Increase number of PMTs in each Tank from 1 to 3 Increase number of PMTs in each Tank from 1 to 3 will decrease the median energy from 1 TeV to about will decrease the median energy from 1 TeV to about 700 GeV.700 GeV.

• Expanding the array size will increase the maximum Expanding the array size will increase the maximum collection area, expand energy reach.collection area, expand energy reach.


High Energy Optimization High Energy Optimization Reconfigure with 1/2 size central core and ~3x larger peak area.Reconfigure with 1/2 size central core and ~3x larger peak area.

Increase sensitivity at the highest energies, probably at the expense of sensitivity at <10 TeV due to decreases gamma/hadron separation and higher threshold.

Std configuration

HE optimized configuration


Low Energy OptimizationLow Energy OptimizationDense central core reduces threshold from 1 TeV to 700 GeV.Dense 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

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.


SummarySummary

• The HAWC detector was simulated from end to The HAWC detector was simulated from end to end.end.

• Sensitivity estimation anchored to Milagro Sensitivity estimation anchored to Milagro observations giving.observations giving.

• We estimate that HAWC will have ~15x We estimate that HAWC will have ~15x Milagro’s sensitivity.Milagro’s sensitivity.

• Results are consistent with simple estimations.Results are consistent with simple estimations.• The design is sensible (not idealized).The design is sensible (not idealized).• The design is flexible.The design is flexible.

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HAWC Design and Performance