Oscar Stl osirfu.se

1
Detection of Ultra High-Energy Cosmic Particles
with the Use of Radio and Radar Methods
December 7, 2005 Oscar StålPhysics in Space
ProgrammeSwedish Institute of Space Physics,
Uppsala Dept. of Astronomy and Space Physics,
Uppsala University Supervisor Bo Thidé

Oscar Stål os_at_irfu.se
IRFU seminarUppsala, 7/12/2005
2
Outline

• Presentation of objectives
• Introductory high-energy cosmic particle physics
• Radar studies of EAS ionisation columns
• Lunar satellite detection of Askaryan radio
  pulses
• Discussion and outlook

Oscar Stål os_at_irfu.se
IRFU seminarUppsala, 7/12/2005
3
Background and framework

• LOIS radio sensor network and deep space radar
  projectInitiated by Bo Thidé, collaboration with
  LOFAR (Netherlands)
• Provides new methods to study fundamental physics
  in space Can this facility or these methods be
  of use in astroparticle physics?
• If not usable, at least it is necessary to
  quantify what radio/radar background to expect
  from UHE cosmic particles
• Ultimate goal is low-frequency array on the Moon
  LIFE (LURBO)

Oscar Stål os_at_irfu.se
IRFU seminarUppsala, 7/12/2005
4
Objectives

• Our objectives for this diploma work have been
  two-fold
• To determine, by approximate analytical methods,
  the radar cross section of the ionisation
  columns created by Extensive Air Showers
• To investigate the feasibility of using a lunar
  satellite for in situ detection of Askaryan
  radio pulses from cosmic particle interactions
  with the lunar regolith

Oscar Stål os_at_irfu.se
IRFU seminarUppsala, 7/12/2005
5
Outline

• Presentation of objectives
• Introductory high-energy cosmic particle physics
• Radar studies of EAS ionisation columns
• Lunar satellite detection of Askaryan radio
  pulses
• Discussion and outlook

Oscar Stål os_at_irfu.se
IRFU seminarUppsala, 7/12/2005
6
Cosmic rays

• Charged particles p, ?
• Steep decrease of flux with energy? E-2.7
  E lt 1015 eV? E-3.1 E gt 1015 eV
• Flux extends to the highest energy ever observed
  (UHE)
• Isotropic flux, B obfuscation
• Sources of UHE particles not yet determined

Oscar Stål os_at_irfu.se
IRFU seminarUppsala, 7/12/2005
7
Extensive Air Showers (EAS)

• Cosmic particle interacts with constituentsof
  the atmosphere
• Shower of secondary particles generated
• Hadronic, muonic and EM components
• Transverse scale given by Molière radius,90 of
  the energy contained within rM
• For air, rM 70 m at sea level andincreasing
  with altitude to severalhundred metres

Oscar Stål os_at_irfu.se
IRFU seminarUppsala, 7/12/2005
8
Detection of UHE cosmic rays

• Air shower arraysHaverah Park, AGASA, KASCADE,
  Pierre AugerDetects shower particles at ground
  level
• Fluorescence telescopesFly's eye, HiRes Fly's
  EyeExcellent energy resolution, sensitivity Low
  duty cycle
• Radio methodsPioneered in the 60's, now
  LOPESCoherent geosynchrotron emissionHighly
  polarised short radio pulses

Falcke et al., Nature 435 (2005)

Oscar Stål os_at_irfu.se
IRFU seminarUppsala, 7/12/2005
9
The Greisen-Zatsepin-Kuzmin (GZK) cutoff

• For E gt 1019.5 eV, cosmic rays interact with CMB
  photons
• Pion photoproduction p ? ? p0 ? p p ?? ?
  ? ?n etc.
• Intergalactic medium no longer transparent over
  Mpc scales
• Still, cosmic rays have been observed beyond
  this cutoff
• A most interesting questionin astroparticle
  physics is why?

Oscar Stål os_at_irfu.se
IRFU seminarUppsala, 7/12/2005
10
Ultra high-energy neutrinos

• UHE neutrinos should be produced in the GZK
  process? ? ? ??? ? e ??e ?? ??
• More sources of UHE neutrinos suggested, but
  none confirmed
• No magnetic field influence
• Detection using optical, radio oracoustic
  methods
• Best limits on UHE neutrino fluxobtained using
  radio methods

Gorham et al., Phys. Rev. Lett. 93 (2004)

Oscar Stål os_at_irfu.se
IRFU seminarUppsala, 7/12/2005
11
Outline

• Presentation of objectives
• Introductory high-energy cosmic particle physics
• Radar studies of EAS ionisation columns
• Lunar satellite detection of Askaryan radio
  pulses
• Discussion and outlook

Oscar Stål os_at_irfu.se
IRFU seminarUppsala, 7/12/2005
12
Nishimura-Kamata-Greisen (NKG) model for EAS
Longitudinal development parametrised by shower
age slant depth Total
number of particles in the shower Transverse
distribution of shower particles

Oscar Stål os_at_irfu.se
IRFU seminarUppsala, 7/12/2005
13
Ionisation columns

• Atmospheric ionisation yield calculated from
  transverse particle density and ionisation
  parameters for air
• Ionisation over long distances, gt 10 km
• Evaporation timescale uncertain20 ?s 20 ms

Oscar Stål os_at_irfu.se
IRFU seminarUppsala, 7/12/2005
14
Ionisation columns

• Ionisation volume modelled as collisionless,
  cold, non-magnetised plasma with inhomogeneous
  density

Oscar Stål os_at_irfu.se
IRFU seminarUppsala, 7/12/2005
15
Radar cross section and scattering width

• The radar cross section m2 is the projected
  area of a perfectly reflecting sphere giving a
  reflected power equivalent to that of the real
  target
• In a two-dimensional problem, the cross section
  is replaced by the scattering width m

Cross section (RCS) Scattering width (SW)

Oscar Stål os_at_irfu.se
IRFU seminarUppsala, 7/12/2005
16
Inhomogeneous wave equations

• To determine the radar cross section of the EAS
  ionisation columns, the scattered E and B fields
  are required
• Wave equations for the spatially inhomogeneous
  medium derived from first principles (Maxwell)

Oscar Stål os_at_irfu.se
IRFU seminarUppsala, 7/12/2005
17
Separation of the wave equations

• For radially inhomogeneous n(x) it is possible to
  separate the wave equations in cylindrical
  coordinates
• The Ez and Bz component equations decouple for
  infinite cylinderShower aging is effectively
  neglected, we treat only maximum
• TM and TE mode scattering for normally incident
  wavem is the azimuthal quantum number

Oscar Stål os_at_irfu.se
IRFU seminarUppsala, 7/12/2005
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Scattering theory in cylindrical geometry
The Sommerfeld radiation condition for n 2
allows the total field to be written as incoming
plane wave scattered cylindrical
wave Asymptotic dependence of scattered wave
is chosen consistently with plane wave expansion
when there is no scatterer The phase shifts
?m contain all information about the scattering

Oscar Stål os_at_irfu.se
IRFU seminarUppsala, 7/12/2005
19
Phase-integral approximations

• How to obtain the phase-shifts? Usually by
  analytic matching if the inner solution is known.
  We use the phase-integral (WKB) method

The general second order linear ODE of a complex
z has approximate phase-integral solutions in
terms of where q(z) is generated
asymptotically from an arbitrary base-function,
for order 2N1 of approximation

Oscar Stål os_at_irfu.se
IRFU seminarUppsala, 7/12/2005
20
First order approximations to the phase-shifts

• Phase-shifts in phase-integral method obtained
  from the asymptotics of the solutions
• Connection formula used to cross the turning
  point from q2 lt 0 to q2 gt 0
• In first order approximation,it is possible to
  use a pathalong the real line

Oscar Stål os_at_irfu.se
IRFU seminarUppsala, 7/12/2005
21
Choice of base function

• The base function needs to be consistent with the
  physics, and this choice is in general a
  difficult problem. No generic method exists

From direct transformation of the radial
equations, we obtain for the TM mode
equation By using instead the modified base
function we reproduce the zero phase-shifts in
the no-scattering limit,which is desirable.
Corresponds to the Langer modification in a
spherical problem and was previously studied by
Berry et al.

Oscar Stål os_at_irfu.se
IRFU seminarUppsala, 7/12/2005
22
Scattering widths numerical results

• Numerical integration of the phase-shift formula
  using the NKG refractive index for horisontal EAS
  at maximum development
• Calculations of the scattering width for various
  shower altitudes, radar frequencies and primary
  particle energy

Oscar Stål os_at_irfu.se
IRFU seminarUppsala, 7/12/2005
23
Longitudinal length scale and 3D cross section

• Shower not infinitely long what is the
  longitudinal length scale?
• Scattering theory is formulated in the extreme
  far-field?
• Consider the first Fresnel zone, the longitudinal
  dimension is given by inverting the far-field
  condition
• Affects the range of the radar system, and
  thereby the sensitivity to UHE particle showers.
  No simulations performed on this

Oscar Stål os_at_irfu.se
IRFU seminarUppsala, 7/12/2005
24
Outline

• Presentation of objectives
• Introductory high-energy cosmic particle physics
• Radar studies of EAS ionisation columns
• Lunar satellite detection of Askaryan radio
  pulses
• Discussion and outlook

Oscar Stål os_at_irfu.se
IRFU seminarUppsala, 7/12/2005
25
Background

• Chandrayaan-1 Moon mission
• India's first mission to the Moon
• Scheduled for launch in 2007-08
• 100 km polar orbit
• ELVIS instrument proposal
• HF/VHF radio receiver of LOIS type
• Can it be used for detection of UHEcosmic
  particles?

Oscar Stål os_at_irfu.se
IRFU seminarUppsala, 7/12/2005
26
Detection principles
4. Rays refracted at interface

3. Coherent V-C radio emission
5. Detection by satellite or surface-based
aerials
2. First interaction, shower initiated
1. Neutrino enters the Regolith

Oscar Stål os_at_irfu.se
IRFU seminarUppsala, 7/12/2005
27
Radio emissions from particles in dense media

• Showers very localised in dense media, rM 10 cm
  
• Wavelengths longer than shower dimensions
  meansemission becomes coherent
• Ouput power scales quadratically with E,
  dominates optical output at UHE
• Radio transparent material required
• Ice, very dry rock, permafrost, giantunderground
  salt domes suggested

Oscar Stål os_at_irfu.se
IRFU seminarUppsala, 7/12/2005
28
Coherent Vavilov-Cerenkov emission

• Particle track is not infinite, so the output is
  smeared around the Cerenkov angle
• Coherent radiation for higher frequencies closer
  to the Cerenkov angle qC
• Long wavelengths means the shower radiates as a
  single particle, hence only the net charge
  contributes

v gt c/n
cos(qC)1/(nb)

Oscar Stål os_at_irfu.se
IRFU seminarUppsala, 7/12/2005
29
The Askaryan effect

• Neutral shower gives no emission
• The particles in an initially neutral shower will
  undergo scattering processes as they traverse the
  material...
• ... and acquire a negative charge excess of
  20-30
• This process is called the Askaryan effect

Compton scattering ? e-atom ? ?? ?? e-
Bhaba scattering e ?? e-atom ? e??? e-
Annihilation e ?? e-atom ? ?? ?? ?

Oscar Stål os_at_irfu.se
IRFU seminarUppsala, 7/12/2005
30
Radiation properties

• Zas-Halzen-Stanev (ZHS) Monte Carlo based on
  simulations of showers in dense media confirms
  the 20-30 charge excess

Radio emission at Cerenkov angle well
parametrised byn0 specific decoherence
frequency, 2.5 GHz for regolithThe angular
spread is given by a Gaussianwhere the width
is frequency dependent, decreasing with n
E. Zas, F. Halzen and T. Stanev, Phys. Rev. D 45
(1992)

Oscar Stål os_at_irfu.se
IRFU seminarUppsala, 7/12/2005
31
Radiation properties

Oscar Stål os_at_irfu.se
IRFU seminarUppsala, 7/12/2005
32
Experimental confirmation

• Experiment at SLAC using 3.6 tons Si target, GeV
  photons, total shower energy up to 1019 eV
• The ZHS simulation results confirmed to a factor
  of two, radiation coherent and linearly
  polarised

D. Saltzberg et al., Phys. Rev. Lett. 86
(2001) P. W. Gorham et al., Phys. Rev. D 72 (2005)

Oscar Stål os_at_irfu.se
IRFU seminarUppsala, 7/12/2005
33
The Moon as an UHE particle detector

• Moon opaque to neutrinos with E gt 1016 eV
• UHE cosmic rays of minor interest
• Target (upper 10 m) is regolith Si grains and
  tiny rocks
• Dielectric properties depends on TiO, FeO
  contaminants
• Radio transparency for n lt 1 GHzif 5
  contaminants assumed

Olhoeft and Strangway, Plan. Sci. Lett. 24 (1975)

Oscar Stål os_at_irfu.se
IRFU seminarUppsala, 7/12/2005
34
Threshold energy

• Threshold determined from the ZHS parametrisation
  andthe minimum detectable signal
• Using Chandrayaan-1 and ELVIS parameters
• Altitude h 100 km Centre freq. n? 100
  MHz Bandwidth ??? 50 MHz Sensitivity Pmin
  -135 dBm/Hz

gt Threshold neutrino energy becomes 51019 eV

Oscar Stål os_at_irfu.se
IRFU seminarUppsala, 7/12/2005
35
Detection aperture

• Detection rate is given by the effective aperture
  (cross section), determined through simulations
  
• Sensitivity dependent on
• - Primary neutrino mixing and branching ratios
• - Primary energy and neutrino-nucleon cross
  section - Dielectric properties, attenuation
  in the regolith
• - Surface effects (refraction, reflection
  etc.)
• - Distance from surface to observation point
  (geometry)
• - Measurement frequency, bandwidth and minimum
  signal
• Monte Carlo sensitivity simulation implemented in
  Matlab
• Simulations performed for different primary
  neutrino energy

Oscar Stål os_at_irfu.se
IRFU seminarUppsala, 7/12/2005
36
Simulation results
2pAmoonNdetected/Ntotal

Oscar Stål os_at_irfu.se
IRFU seminarUppsala, 7/12/2005
37
Model dependent event rate

• Using model for minimum GZK neutrino flux (Engel
  et al., 2001)
• The event rate can then be determined from

gt 2.2 detectable events per year )

Oscar Stål os_at_irfu.se
IRFU seminarUppsala, 7/12/2005
38
Outline

• Presentation of objectives
• Introductory high-energy cosmic particle physics
• Radar studies of EAS ionisation columns
• Lunar satellite detection of Askaryan radio
  pulses
• Discussion and outlook

Oscar Stål os_at_irfu.se
IRFU seminarUppsala, 7/12/2005
39
Discussion, part 1

• We have presented how an EM scattering problem in
  cylindrical geometry can be conveniently treated
  by the phase-integral approximation
• The results have been applied to scattering of
  radio waves from EAS ionisation columns for
  determination of radar cross sections
• The physics of the EAS has been modelled in a
  simplistic manner, which might put restrictions
  on the applicability of our results from this
  aspect
• Applications of radio wave scattering in
  cylindrical geometry also exist for meteor
  trails, ionospheric striations and for lightning
  ionisation

Oscar Stål os_at_irfu.se
IRFU seminarUppsala, 7/12/2005
40
Discussion, part 2

• We have considered observation of coherent radio
  pulses from showers induced in the lunar regolith
  by UHE cosmic neutrinos
• A simple simulation program has been constructed
  for estimating the efficiency of a satellite
  experiment with this purpose
• This program offers generous possibilities for
  further variation of different experimental
  parameters
• Porting to a faster code (e.g. FORTRAN) is
  desirable before more extensive simulations can
  be performed
• We believe that optimisation might further
  increase the 2.2 events to something really
  useful, although 2.2 yr-1 is still competitive (!)

Oscar Stål os_at_irfu.se
IRFU seminarUppsala, 7/12/2005
41
THE ENDThank you for listening!
Special thanks to everyone who has supported me
during this workBo Thidé, IRF-UJan Bergman,
IRF-UGunnar Ingelman, THEP, UUJohn A. Adam, ODU
(Norfolk, VA)Fellow diploma students at IRF-U
and elsewhereAll members of the friendly staff
at IRF-U

Oscar Stål os_at_irfu.se
IRFU seminarUppsala, 7/12/2005
42
Surface effects

• Radio waves refracted at surface
• Beam becomes wider ? Better opening angle, but
  lower field strength
• Total internal reflection, qTIR ?/2 ?c
• TIR more important at high frequencies
• CR detection supressed since all rays are
  down-going
• Smooth surface or detailed topographic map

Vacuumn 1
??'
??
Regolithn ? 1.7

Oscar Stål os_at_irfu.se
IRFU seminarUppsala, 7/12/2005