A Search for Neutralino Dark Matter usingGamma-rays

Mani Tripathi, UCLA, Feb23, 2006.Mani Tripathi, UCLA, Feb23, 2006.

A Search for Neutralino Dark Matter A Search for Neutralino Dark Matter usingGamma-raysusingGamma-rays

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.

Mani Tripathi, ICHEP, Moscow, July 27, 2006 .

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/

http://ucdcms.ucdavis.edu/solar2/


Draco and Dark MatterDwarf 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.

Grillmair et al 1997


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 2, where ~ r - 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.

Neutralino Mass


CACTUS: Using the Solar 2 Heliostat ArrayCACTUS: Using the Solar 2 Heliostat ArrayLocated 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 - 83,000 m2.


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

80 channel PMT camera and 160 Heliostat Field

80 channel PMT camera and 160 Heliostat Field


Data Acquisition Using Multi-hit TDCs

rise = 3.2

ns

fall = 1.7

ns

200 showers

Time of Arrival (ns)

Mapping of multiple heliostats on one PMT allows for ~250 effective pixels.

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.


Typical events with a fitted paraboloid wavefront overlaid on the hits. Partial rejection of cosmic rays is inherent.

Event Shape Analysis


Reconstruction of Shower Centroid on the Ground

Simulations: Reconstructed vs Generated Radial Position of Shower Centroid

Data: Reconstructed Positions of Shower Centroid on the ground for

Energy ~100-200 GeV.


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.

Some showers suffer from wrong centroid determination.

50 GeV


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nal

R

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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.


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.

Calibrations using the Crab Nebula


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 a significance of 13

CRAB: The observed CACTUS Excess in Total Pulse-Height.

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).

Energy70 500 2000

Num

ber

of

Events


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

Flu

x (G

eV-1 m

-2 s

-1)

Energy (GeV)CELESTE: Piron et al 2002

Preliminary

70 500 2000


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 (~ 0.15o) measured in the data.

ON-Crab

ON-OFF

Simulation

OFF-Crab


CACTUS Observations of Draco

Variable degrees in RA

~1

de

g in

De

c

0.5

deg

in D

ec

1 deg in RA

•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

Number of Channels in Coincidence

Single Event

One 4o Scan

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.

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f Tri

ggeri

ng


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 balancing technique applied to a typical Crab data set.

Before

After

The trigger rate is stabilized for the 28 min observing period.

Noise Immunity for Crab


Sum of 65 scans.Stabilized rate from Draco scans.


2.7 Hz

Draco Scans after a > 100 GeV cut

Preliminary

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.


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

CACTUS Limits

Density Profiles Space Probed by CACTUS

-Blue curve : modified SIS

-Red curve: SIS with a 1 au core

-Pink curve: NFW profile

CACTUS Preliminary

Preliminary

Upper Limits on Draco Flux obtained from 5 ON/OFF pairs.


Summary

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.

1. We have measured the Crab spectrum in the region above EGRET and below ACTs.

2. We have observed no excess from the direction of Draco above ~ 70 GeV.

3. The upper limit on the Flux is ~400 milli-Crab for energies in the range 70-1000 GeV.

CACTUS Preliminary

Documents

A Search for Neutralino Dark Matter usingGamma-rays