1. October 20-21, 2005 “Towards the Future” Workshop Andrew Smith mini- High Altitude Water Cherenkov experiment e   145 meters 4 meters  Andrew Smith University of Maryland

2. October 20-21, 2005 “Towards the Future” Workshop Andrew Smith What is miniHAWC? Milagro – Existing water Cherenkov all-sky gamma-ray observatory. HAWC – A “science” driven effort to construct an all-sky observatory with point source sensitivity of the Whipple 10m. miniHAWC – Demonstrate HAWC technology at low cost with Milagro PMTs/instrumentation.

3. October 20-21, 2005 “Towards the Future” Workshop Andrew Smith What is the Sensitivity of miniHAWC? Answer: ~15x Milagro. 1y  ~60mCrab source at 5  Interpretation 1: Designers of Milagro are Stupid. Interpretation 2: Designers of miniHAWC are Smart. Designers of Milagro = Designers of miniHAWC Interpretation 3: Designers of Milagro/miniHAWC were naive, but have wised up.

4. October 20-21, 2005 “Towards the Future” Workshop Andrew Smith Critical Variables Size – Bigger is better until you reach sqrt(A) regime. Photocathode Density – More is better until you detect all the particles. Altitude – Higher is better until you can’t breathe. Diminishing returns. Tools: PMTs (8” Hamamatsu), Water, black and white materials.

5. October 20-21, 2005 “Towards the Future” Workshop Andrew Smith Detector Layout Milagro: 450 PMT (25x18) shallow (1.4m) layer 273 PMT (19x13) deep (5.5m) layer 175 PMT outriggers Instrumented Area: ~40,000m 2 PMT spacing: 2.8m Shallow Area:3500m 2 Deep Area:2200m 2 HAWC: 5625 or 11250 PMTs (75x75x1,2) Single layer at 4m depth or 2 layers at Milagro depths Instrumented Area: 90,000m 2 PMT spacing: 4.0m Shallow Area:90,000m 2 Deep Area:90,000m 2 miniHAWC: 841 PMTs (29x29) 5.0m spacing Single layer with 4m depth Instrumented Area: 90,000m 2 PMT spacing: 4.0m Shallow Area:90,000m 2 Deep Area:90,000m 2

6. October 20-21, 2005 “Towards the Future” Workshop Andrew Smith Equipment Milagro DAQ: –898 8” Hamamatsu PMTs –Single data/HV cable ~150m length –Custom front end boards. Analog to level crossing conversion. (Amplitude through time over threshold.) –FASTBUS TDC –VME – FASTBUS interface with VME readout –2000Hz maximum readout with ave multiplicity ~20-30%

7. October 20-21, 2005 “Towards the Future” Workshop Andrew Smith How do you make an EAS array more sensitive to gamma-ray sources? Energy Threshold –Altitude –Big, hermetic, sensitive Angular Resolution –Big  Lever Arm gamma/hadron Separation –Must detect penetrating particles Sensitivity increase is the product of the improvement made in each category.

8. October 20-21, 2005 “Towards the Future” Workshop Andrew Smith 4500m 2600m Difference between 2600m (Milagro) and 4500m (Tibet): ~ 6x number of particles ~ 2x lower energy threshold Altitude

9. October 20-21, 2005 “Towards the Future” Workshop Andrew Smith Hadron induced cosmic ray showers contain 5-20x more energy in penetrating  +/- and hadrons than EM particles. High P t hadronic interactions lead to wide lateral distributions. Need mass! Need large Area! Muon/hadron Detection HAWC miniHAWC Milagro

10. October 20-21, 2005 “Towards the Future” Workshop Andrew Smith High Altitude Detector w/o  /hadron separation ARGO: ~6000m 2 RPC detector. Reported at ICRC expect sensitivity of 8-13  /year on the Crab. Milagro currently achieves ~8  /year on the Crab.

11. October 20-21, 2005 “Towards the Future” Workshop Andrew Smith Curtains A high altitude version of Milagro would trigger at >10kHz. Need to control spurious triggers due to single muons. Install curtains to optically isolate the the PMTs.

12. October 20-21, 2005 “Towards the Future” Workshop Andrew Smith Simulation Strategy Use Milagro Simulation/Reconstruction software. Use observed Milagro crab signal to anchor simulations to reality.  Shared systematics with Milagro. Use new  /hadron discrimination variable for HAWC/miniHAWC that excludes the core location. C Milagro = (nPMTs above 2 PE)/(Max “muon layer” hit) C miniHAWC = (nPMTs above 2 PE)/(Max “muon layer” hit > 20m from core) As an illustration, consider 2 trigger threshold: 50 PMTs, 200 PMTs.

13. October 20-21, 2005 “Towards the Future” Workshop Andrew Smith  = ~0.4 deg  = ~0.25 deg Angular Resolution  /hadron Separation Cut: nTop/cxPE>5.0 Eff  = 34% Eff CR= 3% Cut: nTop/cxPE>5.0 Eff  = 56% Eff CR= 1.5%

14. October 20-21, 2005 “Towards the Future” Workshop Andrew Smith Triggering with Curtains Multiplicity trigger at ~80 PMTs gives same trigger rate as Milagro at 50 PMTs Much higher Gamma area.

15. October 20-21, 2005 “Towards the Future” Workshop Andrew Smith Effective Area Detector size

16. October 20-21, 2005 “Towards the Future” Workshop Andrew Smith Energy (Crab Spectrum, nTop/cxPE>5.0.,  <30 O ) Significance from Crab Transit (~5 hr) 4  Crab signif/year80  5  point source sensitivity reach ~60mCrab of 1 year survey Energy Resolution~30% above median Angular Resoultion0.25 O -0.40 O S/B (hard cuts) ~ 1:1 for Crab Typical day 20 excess on 25 bkg Q(Milagro -> miniHAWC) = 15! Single layer doesn't limit sensitivity miniHAWC Sensitivity

17. October 20-21, 2005 “Towards the Future” Workshop Andrew Smith Curtains Test in Milagro

18. October 20-21, 2005 “Towards the Future” Workshop Andrew Smith Site –High Altitude. –Power –Internet –Don’t need darkness or good weather… –YBJ very interested. –Chinese don’t have money for site prep. (ARGO) –Investigating a site in Mexico.

19. October 20-21, 2005 “Towards the Future” Workshop Andrew Smith Cost - Detector Elements Pond (0.2-2.0 M$) Black Liner Material (~100k$ @ $1/m 2 ) Pump/Recirculation System. (~$200k$) PMTs – Reuse with base and encapsulation w/ new connectors. Cables – Purchase new. (~100$k) Front End Electronics – keep as is. TDC and DAQ –2kHz DAQ keep current electronics –Faster  VME TDCs (~200$k) Online computing – A few computers can reconstruct in real time. (~10$k) Building – Assemble functional DAQ in a trailer and ship to site. (~200$k) Internet Access – Live with slow internet if necessary. Need prompt alert capability.

20. October 20-21, 2005 “Towards the Future” Workshop Andrew Smith Issues – The Good/Bad Simple Analysis- Event weighting could increase sensitivity further. In Milagro Q=1.6. Can reconstruct showers down to 20 PMTs if we could trigger at 6kHz. Site. Calibration. Noise. Milagro in 1.4y  ~8  /y

21. October 20-21, 2005 “Towards the Future” Workshop Andrew Smith Summary 15x Sensitvity increase over Milagro ~3x from Altitude,Area ~3x from  /hadron separation ~1.5x from Angular resolution ~60mCrab sensitivity (5  in 1year) Mostly proven technology –Leverage $1.5M investment in Milagro equipment –Could construct rapidly if site available.

22. October 20-21, 2005 “Towards the Future” Workshop Andrew Smith An Ideal Air Shower Gamma-Ray Detector: 1) Large Physical Area Collection Area Contain Core Sample Lateral Tails 2) High Efficiency for 20 MeV  Continuous Detector Efficient   e Converter 3) Calorimetry Hadron/Muon Identification 4) Altitude ~40gm/cm 2 increase in altitude  double ground level particles. Lowers threshold.

23. October 20-21, 2005 “Towards the Future” Workshop Andrew Smith 1) Large Physical Area (Diagram Showing Shower Curvature) Core position reconstruction required for accurate angle reconstruction. --> Core must be contained within detector --> Effective area <~ Physical Area --> 10 4 to 10 5 m 2 detector required to rival area of IACTs --> Long lever arm for angle reconstruction. Shower front is curved. Without core position: Pointing error dominated by systematics ==>  PSF >~ 0.7 O

24. October 20-21, 2005 “Towards the Future” Workshop Andrew Smith 2) High Efficiency for detection of 20MeV Gamma-Rays In extended air showers  's out number e +/- by 5-10 to 1. Mean energy of EAS  's is ~20 MeV. ( not strongly correlated with primary VHE  energy.) Plot showing spectrum of showering particles Plot Showing de/dx for electrons.

25. October 20-21, 2005 “Towards the Future” Workshop Andrew Smith HAWC layout 75x75 grid of 8” PMTs - in 2 layers (depth =2m,6m, separation = 4m) Angle reconstruction with top layer. Calorimetry with bottom layer Opaque “curtains” separate PMT cells. Eliminate “cross talk” between counter. Limits trigger rate compared to Milagro

26. October 20-21, 2005 “Towards the Future” Workshop Andrew Smith How does the HAWC design measure up? 1) Size: – 300mx300m = 90,000 m 2 2) Efficiency: Water acts a both conversion medium and radiator. ~1 PE/25 MeV 3) Calorimetry: Deep (~15Xo) PMT layer for Muon/Hadron rejection. 4) Altitude: Select optimal site.

27. October 20-21, 2005 “Towards the Future” Workshop Andrew Smith Simulation Strategy Simulate various HAWC geometries at 2 altitudes. Compare results with Milagro. (Milagro sensitivity verified by observations of the Crab.) 2 altitudes considered: 4500m ( ~Tibet Lab altitude) 5200m ( ~Atacama Plateau) 2 sets of cuts considered: Std - 50 PMT multiplicity cut Hard - 200 PMT multiplicity cut  = ~0.4 deg  = ~0.25 deg

28. October 20-21, 2005 “Towards the Future” Workshop Andrew Smith Gamma-Hadron Separation: Remove events with one or more large hits away from the core ---> muon/hadrons in lateral tails ---> “cxPE” is largest bottom layer hit with R core >10m ---> “nTop” is number of PMTs hit in top layer Cut: nTop/cxPE>10.0 Eff  = 40% Eff CR= 5% Cut nTop/cxPE>10.0 Eff g = 65% Eff CR= 2%

29. October 20-21, 2005 “Towards the Future” Workshop Andrew Smith Energy of reconstructed events (Crab Spectrum) nTop/cxPE>10.,  <30 O Altitude = 4500m Altitude = 5200m Threshold Lower at zenith.

30. October 20-21, 2005 “Towards the Future” Workshop Andrew Smith Point Source Sensitivity of HAWC: Altitude5200m4500m Crab Transit (~4 hr) 25  10  Median Energy Time to 5  10min60min 5  point source sensitivity reach 10mCrab25mCrab of 1 year survey Energy Resolution~30% above median ~1/4 sensitivity of HESS (40  /hr) with >1000x the exposure!

31. October 20-21, 2005 “Towards the Future” Workshop Andrew Smith miniHAWC: HAWC is a potentially large complex project. Consider a another possibility, the relocation of the Milagro apparatus to an optimized pond located at a high altitude site. Milagro owns ~900 8” Hamamatsu PMTs and a DAQ capable of 2000Hz readout. --> Single layer consisting of a 29x29 Grid of PMTs with 5m separation (150mx150m pond) and 4m depth. --> Utilize same reconstruction and  /hadron separation methods as HAWC. --> Simulation altitude 4500m.

32. October 20-21, 2005 “Towards the Future” Workshop Andrew Smith Summary: Current EAS arrays are not detecting/utilizing the “whole” shower. Huge improvements in  /hadron separation possible. Excellent angular resolution possible. miniHAWC/HAWC could survey the entire northern or southern sky to a sensitivity of ~60mCrab/10mCrab. (conservative estimates) Limiting detector to a single layer doesn't seem to reduce sensitivity. Could reduce cost of HAWC instrumentation by factor of ~2-3 Wide field of view and high duty cycle: Surveys Prompt VHE GRB emission? AGN monitor. Ideal for study of diffuse/extended sources. ???

33. October 20-21, 2005 “Towards the Future” Workshop Andrew Smith The Diffuse Galactic Plane in miniHAWC and HAWC Use Neutral H map to trace out VHE Gamma-Ray flux. Normalize to Milagro observed TeV diffuse emission from the Galactic plane.

34. October 20-21, 2005 “Towards the Future” Workshop Andrew Smith Conclusions Milagro and other air-shower arrays play an important and complementary roll in VHE astronomy. –Survey –GRBs –Extended/Diffuse Sources –Monitoring Variable Sources –Solar Activity Monitoring –Water Cherenkov Method has not been exploited efficently –Design improbments (Size, Altitude, …) lead to much better than sqrt(N) sensitivity improvents.

35. October 20-21, 2005 “Towards the Future” Workshop Andrew Smith

36. 1) Size: – 150m x 150m = 22,500 m**2 2) Efficiency: Water acts a both conversion medium and radiator (Cherenkov) ~1 PE/40 MeV 3) Calorimetry: Deep (~15Xo) PMT layer for Muon/Hadron rejection. 4) Altitude: Select an optimal site.