Download LIGHT PSEUDOSCALAR BOSONS, PVLAS AND DOUBLE PULSAR …

THEORETICAL ASPECTS
Marco Roncadelli, INFN – Pavia
(Italy)
OUTLINE
1.
2.
3.
4.
5.
QED VACUUM EFFECTS
AXION-LIKE PARTICLES
ASTROPHYSICAL CONSTRAINTS
IMPLICATIONS OF PVLAS
DOUBLE PULSAR J0737-3039
QED VACUUM EFFECTS
Consider a photon beam in vacuo with k
along the z-axis. External B field present
(only x, y components of B relevant).
CLASSICALLY beam propagation
UNAFFECTED by B.
QUANTUM corrections change the
situation, since virtual fermion exchange
produces a photon-photon interaction.
At one-loop,
proceeds via a box
diagram and at low-energy is described by
Presently a photon can be replaced by B ….
From above lagrangian propagation
eigenstates are polarization states
,
(with respect to the B-k plane). Because
amplitude for
depends on initial
photon polarization, two conclusions
follow.
•
….
,
propagate with
different velocities ….
….
BIREFRINGENCE
.
•
….
do not split but
split into
two
…. selective photon absorption ….
DICHROISM
.
Suppose now the photon beam is at the beginning
LINEARLY polarized.
• Owing to birefringence alone an ELLIPTICAL
polarization shows up;
• Due to dichroism alone polarization ROTATEs,
since
decreases and
increases.
Net effect: ELLIPTICITY with ellipse’s major
axis ROTATED.
In 1979 E. Iacopini and E. Zavattini
proposed to measure the B-induced
vacuum birefringence with a PVLAS-like
apparatus.
N.B. Dichroism down by
.
AXION-LIKE PARTICLES
In 1977 R. Peccei and H. Quinn proposed a
solution of the strong CP problem based
on an additional global U(1) symmetry.
Soon after S. Weinberg and F. Wilczek
realized that the PQ symmetry is
spontaneously broken, thereby giving rise
to a Goldstone boson: the Axion. Actually
QCD instanton effects break the PQ
symmetry also explicitly, so that the axion
gets a mass
where
is the scale at which the PQ
symmetry is spontaneously broken.
Originally it was supposed that
but it was soon realized that resulting
axion is experimentally RULED OUT.
WAY OUT:
…. INVISIBLE axion
models.
Yukawa couplings of axion  to quarks
produce an effective 2 photon coupling at
one-loop of the form
with
where k = O(1) is model-dependent. So
the axion obeys the mass-coupling relation
N.B. Invisible axions excellent candidates for
cold nonbaryonic DARK MATTER
candidates for
.
In 1983 Sikivie pointed out that invisible
axions can be DETECTED owing to their 2
photon coupling. Axion-photon conversion
analogous to neutrino OSCILLATIONS but
here a nonvanishing transverse B is
necessary to account for spin mismatch.
Transition probability energy-independent for
oscillation wavenumber dominated by
photon-axion mixing term. As long as
photon energy is much larger than m –
WKB approximation – the SECOND-order
propagation equation for a monochromatic
beam reduces to a FIRST-order one.
Sikivie considered 3 observational strategies
(2 based on a tunable resonant cavity).
• Axion ELIOSCOPE …. detection of
SOLAR axions.
• Axion HALOSCOPE …. detection of
DARK MATTER axions.
• Regeneration experiment …. shining light
through a wall.
In 1986 L. Maiani, R. Petronzio and E.
Zavattini realized that axions can be
searched for by a PVLAS-like experiment.
Consider again a photon beam LINEARLY
polarized initially which propagates in a
magnetized vacuum. MIX with the axion
while
do NOT (pseudoscalar coupling).
• Virtual axion exchange affects propagation
of but not of .... BIREFRINGENCE.
• Real axion photo-production depletes the
polarization mode only ….DICHROISM.
M and m UNIQUELY determined in terms of
the beam ellipticity and rotation angle.
N.B. MPZ strategy INDEPENDENT of actual
PRESENCE of axions in the laboratory.
AXION-LIKE PARTICLES (ALPs) are
present in many extensions of the SM and
are described by either
or
Everything goes like for the axion case .... in
a PVLAS-like experiment both m and M of
an ALP can be DETERMINED by
measuring both ellipticity and rotation
angle of a laser beam with initial linear
polarization.
N.B. I assume ALP to be pseudoscalar.
ASTROPHYSICAL CONSTRAINTS
THEORETICAL BOUND – Thermal photons
produced in central regions of stars can
become ALPs in the fluctuating EM field of
stellar plasma. In main-sequence stars this
occurs via Primakoff scattering off ions.
The ALPs escape …. star looses energy
…. central temperature increases ….
observed properties change. Agreement
between standard stellar models and
observations
demands that unwanted ALP effects have
to be sufficiently suppressed.
• Sun ….
.
• Red giants ….
CAST EXPERIMENT – “Blind magnetic
telescope” is pointed toward the Sun:
detection of KeV-energy photons would
the signal for axions coming from the Sun.
Remarkably CAST yields
for
.
IMPLICATIONS OF PVLAS
ASSUME that PVLAS has detected an ALP
with
and
.
N.B. Alternatives are possible!
• A look back at m-M relation …. this ALP is
NOT the axion.
• Theoretical astrophysical bound as well as
CAST bound VIOLATED by 5 orders of
magnitudes.
MORAL
• A NEW PARTICLE has been discovered.
• NEW PHYSICS at low-energy MUST
exist to make the 2 photon coupling
MUCH WEAKER in stellar environment
than in laboratory.
Some possibilities have been explored
based on plasma effects in paraphoton
models and a sub-KeV phase transition.
“Sic stantibus rebus”…. INDEPENDENT
CHECKS of PVLAS claim look
COMPELLING.
• Experiments similar to PVLAS.
• Photon regeneration experiments.
• Astrophysical effects with
UNSUPPRESSED 2 photon coupling.
DOUBLE PULSAR J0737-3039
Discovered in 2003.
• Orbital period T = 2.45 h.
• Rotation periods P(A) = 23 ms, P(B) =
2.8 s.
• Inclination of orbital plane i = 90.29 deg ….
it is seen almost EDGE-ON.
Focus on emission from A.
Pulsar B has DIPOLAR magnetic field
on the surface.
• LARGE impact parameter …. NOTHING
interesting happens.
• SMALL impact parameter …. beam from A
traverses magnetosphere of B ….
photon-ALP conversion IMPORTANT
depending on m, M.
TWO effects are expected.
• Production of real ALPs …. periodic
attenuation of photon beam which
depends on T, P(B).
N.B. Analog of DICHROISM in PVLAS
experiment.
• Exchange of virtual ALPs …. periodic
LENSING which depends on T, P(B).
N.B. Analog of BIREFRINGENCE in PVLAS
experiment.
Here I consider only attenuation effect (A.
Dupays, C. Rizzo, M. R., G. F. Bignami,
Phys. Rev.Lett. 95 211302 (2005)).
We work within WKB approximation and
solve numerically the first-order
propagation equation for an
UNPOLARIZED, monochromatic beam
travelling in the dipolar B produced by
pulsar B. Resulting transition probability as
a function of beam frequency is
N.B. Effect relevant ABOVE 10 MeV ….
remarkable result.
• J0737-3039 is expected to be a gammaray SOURCE.
• Interaction of photon beam with plasma in
magnetosphere of B is NEGLIGIBLE.
• WKB approximation JUSTIFIED.
INTUITIVE explanation assuming B constant
3
4


4

10
km.
B

1
.
6

10
G
i.e.
for min
• Mixing effects important for mixing angle in
photon-ALP system of order 1 …. OK with
THRESHOLD behaviour.
• Transition probability becomes energyindependent for oscillation wavenumber
dominated by photon-ALP mixing term ….
OK with FLAT behaviour.
TEMPORAL behaviour best described by
TRANSMISSION = 1 – P. We find beam
attenuation up to 50 % as
This effect turns out to be OBSERVABLE
with GLAST.
For example, ABSENCE of attenuation A at
10 % level yields the exclusion plot
This attenuation requires 100 counts
during observation time. For 2 weeks
 A  2 10  / cm / s
7
2
in agreement with expectations and about
1000 times LARGER than GLAST
sensitivity threshold for point sources.