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### ZHS and EP theory

C. W. James, Columbus, Ohio, Feb 23rd, 2012

Step 1: Liénard-Weichert Potentials

- Begin with Maxwell’s equations
- Add a single (monopolar) particle as a source
- Allow for finite light propagation speed
- Use Lorentz gauge

Step 2:

- Apply to get:

Nearfield Term

Energy/area as R-4

Energy decreases with distance

Radiation term

Energy/area as R-2

Energy transport to infinity

“Accelerating charged particles radiate”

Get rid of this

(no Frank-Tamm VC)

Toy experiments

- Take a straight particle track:
- Place an observer in x-z plane
- Calculate emission via…
- Endpoints
- ZHS (single track)
- ZHS (very many sub-tracks)

*

*

R

What do we expect to see?

- Afanasiev, Kartavenko, Stepanovsky J Phys D, 32 (1999)

Vavilov-Cherenkov radiation important

Bremsstrahlung from endpoints dominates

Bremsstrahlung from endpoints dominates

Far-field, far from theta_C

- Endpoints, ZHS agree perfectly.
- No ZHS track sub-division needed (1m source at 1 km unresolved)

1000 m

Low-frequency-limit

- Endpoints reduce to:
- ZHS low-phase limit:

- Tends towards a constant term at low frequencies

- Tends towards zero at low frequencies

Difference in the near-field

1 m

- Observer much closer to track start than track end
- Endpoints accounts for this, ZHS can not

Why? Endpoints: Clearly, an observer in the nearfield should see a monopolar component to the pulse Important for:

- ZHS formula:
- Accounts for distance difference in phase, but not magnitude
- true no matter how tracks are subdivided

- Distance affects both magnitude and phase

- [total net change in potential]

- Lunar Cherenkov? No! (very far field)
- Important for air-showers? Perhaps (REAS3 vsZHAires).
- Important for dense media?...

What about near the Cherenkov angle?

- Endpoint formulation:
- In ZHS:

- Result can be arbitrarily large (it blows up)

- Result is always finite (more sensible)

Behaviour near the Cherenkov angle

- Endpoints produce a larger contribution (can be arbitrarily large)

1000 m

Why do endpoints blow up? Result: potentially infinite field This should not be unexpected! This is small consolation.

- Endpoints allow:
- Infinitely small acceleration zone
- Infinitely small source particle
- Infinitely small detector
- [time-domain only] constant refractive index

- Very common to see infinities in the literature
- This is why textbooks always derive the total radiated power and not the field strengths.

What happens in the near-field in the Cherenkov regime?

OR:

When good techniques go bad

C. W. James, Columbus, Ohio, Feb 23rd 2012

Spectrum: n=2

- Now we see differences…

Time-domain (no band limit)

- Time-domain output (ZHS vs EP) (n=2):
- Large contribution from ZHS NOT in endpoints!
- Could this be a ‘true’ Vavilov-Cherenkov emission? (or a numerical artefact?)

(note different y-axis scales)

Quick check: in vacuum

- We do not expect and Cherenkov shock
- But we do expect two bremsstrahlung shocks…
- I do not understand this ZHS behaviour

1cm from the vacuum track

- Large ZHS pulse… in a vacuum.
- This is not V-C radiation!
- It is a numerical artefact OR a static term.

Summary from toy experiments What we see:

- Theoretical expectation:
- EP theory models only bremsstrahlung
- Handles near-field
- Breaks down near theta_C

- ZHS models only bremsstrahlung + far-field approx
- Breaks down in near-field
- Handles theta_C

- EP theory models only bremsstrahlung

- EP theory matches expectation
- ZHS: some strange results…
- Produces phantom Vavilov-Cherenkov-like pulse
- Somehow misses bremsstrahlung

Main conclusion

- Neither endpoints nor ZHS get it completely right

Far-field

Near-field

EP & ZHS agree

(probably correct)

EP theory is better

(probably correct)

Far from θC

ZHS crazy

EP misses VC (main)

(probably both crap)

ZHS is better

(probably not correct)

Near θC

Philosophical aside

- What about smooth particle motion?
- Radiation is emitted constantly
- Limit (description -> perfection) [inf points]:
- Endpoints have contributions equal-and-opposite sides of the Cherenkov angle
- Divergences are expected to cancel
- Hence tendency towards ZHS treatment in REAS3

What does the ZHS formula produce And yet… We do not know what ZHS produces at the Cherenkov angle

- ZHS formula approximates:
- This approximation can not be made near the Cherenkov angle
- Same approximation as Tamm (1939)
- Shown to exclude Frank-Tamm Cherenkov

- ZHS formula produces something sensible.
- Endpoints do not.

Is the divergence physical? Then yes! Divergence/magnification at the Cherenkov angle does NOT necessarily mean Vavilov-Cherenkov radiation! Q: Why do we often see total radiated power calculated, but not the fields? A: Because this can hide nasty divergences (integrate away this divergence over finite spatial angles)

- If:
- n is constant
- The acceleration event is truly instantaneous
- The particle and detector are both infinitely small

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