3/22/2002 M.A. Huang

George W.S. Hou & M.A. Huang

Center for Cosmology and Particle AstrophysicsDepartment of Physics, National Taiwan University

3/22/2002 M.A. Huang

Contents

• A new type of detector for Neutrino • Neutrino conversion inside mountain• Potential site at Hawaii, Big Island• Acceptance and flux sensitivity• Sky coverage

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Why neutrino telescope?• JLC schedule delay

– 1997 proposed to build in 2001– 2nd ACFA statement 2001, expected

construction time as early as 2005, finish time ~ 2009, well beyond CosPA schedule!

– No need to continue original plan “BPC prototype”.

• Dark matter detector prototype: Finished!

• Great potential for neutrino astrophysics.

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Neutrinos from Universe CR interact with matter or

photons and produce neutrino through pions decay.– CR + X e 2 e

• Cosmological sources: WB and MPR limit

• Galactic CR + ISM galactic

– UHECR + CMB p + GZK

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Conventional detectors

• Shield from CR and atmospheric muons.– Underground, under-sea, or under-ice.

• Very large target volume = detection volume

• Difficult to expand target volume, maximum energy ~ 1015 eV.

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UHECR detectors

• UHECR detector such as Auger array could also detect neutrino induced air showers

• Conversion efficiency in atmosphere is small and the energy threshold is high ~ 1018 eV.

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Window of opportunityConventional detector UHECR detector?

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Alternative approach• Use mountain as target and shield.

• Use atmosphere as calorimeter, measured air shower initiated by the decay/interaction of .

• Advantage– Lower cost

– Larger acceptance

• Disadvantage– Limited by site, same problem

as any experiment.

– Limited field of view

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Detection mechanism

• High energy interact inside mountain, produce lepton via charge current interaction. + X e/ + X’– e will shower in very short distance, will pass through valley without interaction could decay in the valley, produce shower and being detected.

• Detector similar to -ray imaging Chrenkov telescope.

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Site selection• The cross-section of target mountain should be as large as

possible

• The valley should be as wide as few 10s km.– Shower maximum ~ 500 -700 gm/cm2, for atmosphere at 1-3 km

altitude, this corresponds to 4.5km to 7.8 km.

– Proper distance for to decay.

• Because of optical detection, the atmosphere should be dry and less cloudy.

• Night sky should be dark and free from artificial lights.

• It is preferred if the galactic center is visible.

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Hawaii big island• Astronomer’s dream site

– Good weather

– Less artificial light

• Mt. Hualalai provide a good view of Mt. Loa and situated in the dryer west side of island.

• Mt. Loa provide long base line, ~ 90 km wide and 4 km high.

Mauna Loa

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Field of view of telescope• Azimuth angle: from

south to east.• Zenith angle: from

86.9º to 91.5º min=86.9º: from

detector to top of

Mauna Loa, < min sky is visible.

max=91.5º: line of sight tangent to Earth,

> max skimming through Earth first.

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From to detectable signal

Efficiency of convert to in mountain, then decay and being detected.

= P1× ( 0L P2(x) P3(L-x) dx/ ) × Pd

P1: survive in atmosphere, P1 = exp{-Xatm/ }

P2: survive in rock, P2 = exp{-Xrock/ }

dx/ : convert to

P3: survive the rest of rock, P3 = exp{-(L-Xrock)/ }

Pd: detection probability

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P1: Survivor probability in atmosphere

P1 = exp{-Xatm/ }• Xatm : atmospheric depth

– Linsley’s atmosphere model from Aires

– Consider the curvature and ellipsoid shape of the Earth.

• Zenith angle changes with position

• 1/ = NA ×N)

• Interaction probability = 1- P1

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interaction cross-section• 1/ = NA × ×N)

: neutrino current cross-

section,

+ N + X : rock density = 2.65 g/cm3

= × c × T

(E /1015 eV) ×48.92 m

• E = (1-y) E where y is fraction of energy carry out by interacting nucleon, y=¼, So E = ¾ E

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P : Conversion efficiency in mountain

• When energy loss is ignored, P can be calculated analytically.

//

0

11/

0

/)(/

LL

L xL

L xLx

ee

dxee

dxeeP

»

P /P E1.4

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Optimal thickness

• Most of the effective interaction occur several decay length inside mountain.

log

0/

m

LL

LL

L

ee

L

P m

m

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Energy loss of tau High energy tau loss

energy quickly, tau surviving probability decrease much quicker.

βxeExE

Edx

dE

0

0

)(

xExexP /)(

Example of of ¾1018 eV in rock.

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Effect of energy loss• Reduce range of tau,

increase acceptance• Increase fluctuation of

tau energy, energy resolution become worse.

Blue : No dE/dX Red: dE/dX

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Pd: Detection probability

= 0.83 : Branching ratio of decay to detectable channels ( ) ~ 0.17,

undetectable

• Decay probability of in distance d, from mountain to detector.

Edd eP /

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Acceptance and Event Rate

R (E) = A E) (E)– R: event rate [s–1 ]

– A: acceptance = area solid angle [cm2 sr ]

E) : cosmic neutrino flux [cm –2 s –1 sr –1 ] (E) : neutrino conversion efficiency

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Effective solid angle• Effective solid angle is

Cerenkov light cone• Because lateral

distribution, air shower light cone is extended to c ~ 5 º

sr 0.0239

cos12

sin0

2

0

c

c

dd

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Effective area• Effective area: area

where tau decay and initiate shower.– On average, tau decay at

one decay length ( ) pass

mountain. : solid angle of each pixel

– D: distance from detector to

mountain surface

dEDEaFOV

2)()(

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Acceptance• Acceptance : Include

Mauna Loa and Mauna Kea 1.72 - 0.3 km2 sr

(1014 to 1018 eV)

• Consider: ( shower)

conversion efficiency

– Energy loss of

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Sensitivity • Assuming sensitivity

is the flux which produce 0.3 events/year per half decade of energy.

• Chance to explore MPR limits and set similar upper limit as AMANDA-B10 at higher energy.

• Nearby point source could be detected.

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Run time• Optical detector operate in

moonless and cloudless night.

• The moonless nights from 12/2003 to 12/2007 are shown, ~5200 hours, ~20%.– In realistic case, the run time

should be deducted by some fraction when weather is cloudy or foggy.

– Normally, use 10% as duty time.

Source code come from HiRes group

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Sky coverage :

• Consider:– FOV of Hualalai site (looking at Mauna Kea and Mauna Loa)– Run time 12/2003 to 12/2007; 20% duty time

Galactic center is visible!

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Conclusion - 1• The optimal range for detecting by conversion in

mountain/Earth is 1015 to 1018 eV, – Conversion efficiencies are high and energy resolutions

are reasonable.

– Gap between conventional detectors and UHECR detectors.

– This uniqueness make this project attractive!

• Great chance to initiate the first experiment of this technique.

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Conclusion - 2• Hualalai on the Big Island of Hawaii is a great site.

– Good weather

– Large acceptance ~ 1 km2 sr

– Reach similar sensitivity as AMANDA-B10.

– Galactic center is visible

• Potential increase of acceptance– Add Earth skimming events below horizon (>91.5º)

– Add fluorescent mode

– Add sea-skimming events• Looking at the west of Hualalai• Could be more noisy due to reflection from waves.

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Technical challenges

• Acceptance is limited by the site!

• A compact detector would need low-noisy and high gain electronics.

• Short signal pulse (~ ns), extremely low event rate (~1/year)– Potentially many background signals – Need multiple coincidence trigger