Paulo Afonso Maxwell Chertok Carly Kopecki Juan Lizarazo Peter Marleau John Stilley Melinda Sweany Mani Tripathi Senior Engineer: Britt Holbrook Junior Specialist: Tiffany Landry. Technical Assistants: John Linn, Joe Trad. A Search for Neutralino Dark Matter usingGamma-rays
Outline Draco as a source for Gamma-rays from Neutralino annihilations. CACTUS Experiment: Technique and Resolutions. Calibrations with the Crab. Draco Observations: Flux Limits Status and Future http://ucdcms.ucdavis.edu/solar2/ .
Grillmair et al 1997 Draco and Dark Matter Dwarf Spheroidal galaxy ~ 82kpc away Odenkirchen et al 2001 Total Luminosity ~2 x 105 Lsolar Rave et al 2003 M/L ~ 92 +- 28 Solar Units Angular size ~ 0.5o Optically faint (mag ~11) => an ideal candidate for observations using Air Cherenkov Telescopes. .
Neutralinos: Previous attempts at understanding Draco. Neutralino annihilations will result in hadronic jets that will contain gammas from neutral pion decays. The rate will depend on r2, where r~ r -g is the density profile of the neutralinos. Tyler (2002) obtained exclusion contours using EGRET data in the region of Draco. A survey by Whipple(2004) did not yield any excess for energies above ~300 GeV. <sn> Neutralino Mass .
CACTUS: Using the Solar 2 Heliostat Array Located 15 miles outside Barstow, CA Over 1,900 42m2 heliostats. The largest array in the world. We have ~160 heliostats in the FOV of our camera. Total mirror area ~6,700 m2. Collection (ground) area ~ 40,000 m2 Effective (100% eff) area for E>200 GeV ~ 61,000 m2 - 83,000m2. .
80 channel PMT camera and 160 Heliostat Field Part of the field being used. 160 heliostats* available. 116-144 used in this campaign. *limited by aperture CACTUS is capable of collecting nearly the entire Cherenkov light pool .
Data Acquisition Using Multi-hit TDCs srise = 3.2 ns sfall = 1.7 ns time-of-arrival resolution ~0.5 ns. Excellent S/N ratio for Cherenkov wavefront detection. Pulse heights are measured with a time-over-threshold technique. 200showers Time of Arrival (ns) Mapping of multiple heliostats on one PMT allows for ~250 effective pixels. .
Event Shape Analysis Typical events with a fitted paraboloid wavefront overlaid on the hits. Partial rejection of cosmic rays is inherent. .
Reconstruction of Shower Centroid on the Ground Data: Reconstructed Positions of Shower Centroid on the ground for Energy ~100-200 GeV. Simulations: Reconstructed vs Generated Radial Position of Shower Centroid .
Energy Resolution Optimized via the paraboloid fit Pulse height is corrected for accepted fraction of the shower after fitting the shower centroid on the ground. 50 GeV Some showers suffer from wrong centroid determination. .
Energy Resolution A resolution in the 20-30% range is achieved at low energies. Saturation effects begin to show up at energies above 500 GeV. Fractional Resolution .
Calibrations using the Crab Nebula Standard Candle for High Energy gamma-rays. We rely on our own measurements to validate the simulations. The Crab spectrum has not been very well established in the 10-100 GeV region. Cross-check at high energies with recent new results. .
CRAB: The observed CACTUS Excess in Total Pulse-Height. Data are recorded in pairs of 28 min ON-source (heliostats track the Crab) and OFF-source (heliostats revert and track a point 30 mins away from the Crab). We require >7 channels in a 13 ns window for the event to be triggered. This 28 min sample from the Crab represents an excess rate of 42/min and asignificance of 13s Number of Events 70 500 2000 Energy .
CRAB: Efficiency and Effective Area Events were generated spread uniformly over a radius of 180m with an input spectrum of k*E-2.4 Simulations indicate an effective area >50,000 m2 for energy > ~200 GeV. Areaeff = 61,000 - 1.1 x 106 E -0.73 m2 .
CACTUS Measurement of the Crab Differential Flux CACTUS Fit dF/dE ~ 3.39x 10-3 E –2.29 (GeV-1 m-2 s-1) Errors: Systematic error in energy not shown ~10% in Flux from effective area Flux (GeV-1 m-2 s-1) Preliminary CELESTE: Piron et al 2002 70 500 2000 Energy (GeV) .
ON-Crab OFF-Crab ON-OFF Simulation Q2 Angular Resolution for the Crab A planar fit to the shower wave-front yields the incidence angle. The Crab is a point source and the excess is peaked at zero cone-angle. Simulations reproduce the resolution in Q2 (~ 0.15o) measured in the data. .
Variable degrees in RA 0.5 deg in Dec ~1 deg in Dec 1 deg in RA CACTUS Observations of Draco • 12 nights observation period in July 2005. • 12 ON/OFF pairs (5 analyzed) • Drift scans: +-1, +- 2 and +-3 in physical degrees. • The raw rates recorded by CACTUS displayed variability with RA. • We have now completed a first pass analysis of all the data using the spherical wave-front fitting technique and improved stabilization of trigger against fluctuations in night sky background.. .
Calculation of Trigger Probability Per Event Single Event For each event, the noise conditions preceeding the trigger is sampled for 1 us. A trigger turn-on probability distribution for each event is calculated. For example, the distribution shown indicates that this event would have triggered if there were 6 signal hits. This is the effective threshold for this event. A curve is fit to the turn-on probability. Demand that each event have > 50% probability of triggering. An overlap of such curves is shown for one scan. Probability of Triggering One 4o Scan Number of Channels in Coincidence .
Effective Thresholds The data are cleaned up by remving rate “spikes” after cuts based on background noise. The cut is 1 sigma from the mean of each data set. .
Noise Immunity for Crab Noise balancing technique applied to a typical Crab data set. Before After The trigger rate is stabilized for the 28 min observing period. .
Draco Scans after a > 100 GeV cut The total live-time (On Draco) for this data set, after correcting for trigger suppression etc is ~5 hours. The background rate from cosmic rays is 2.7 Hz per 0.2 bin in R.A. Preliminary 2.7 Hz .
Draco: Analysis of 5 ON/OFF pairs The significance of (ON-OFF) rates normalized to the square-root. Total live Time for the sum of 5 runs is ~95 mins. The lack of excess is converted into upper limit on Flux. Preliminary .
Setting Flux Limits: Effective Area. Simulated effective area for Draco observing angles is higher than for Crab due to higher number of heliostats used in the July run. Areaeff = 83,000 – 2.3 x 106 E - 0.79 m2 .
CACTUS Limits Preliminary Upper Limits on Draco Flux obtained from 5 ON/OFF pairs. Density Profiles Space Probed by CACTUS • -Blue curve : modified SIS • Red curve: SIS with a 1 au core • -Pink curve: NFW profile CACTUS Preliminary .
Summary • We have measured the Crab spectrum in the region above EGRET and below ACTs. • We have observed no excess from the direction of Draco above ~ 70 GeV. • The upper limit on the Flux is ~400 milli-Crab for energies in the range 70-1000 GeV. CACTUS Preliminary • FUTURE • Hardware Upgrades: • Improve electronics for low energy response. Lower threshold to ~30 GeV. • Outrigger mini-cameras for improved cosmic ray rejection. • Future dSph Observations: • Draco, Leo II and Sextans. .