Transcript Any Light Particle Search - ALPS experiment

Any Light Particle Search II.
R. Bähre1, N. Bastidon2, B. Döbrich3, J. Dreyling-Eschweiler3, S. Gharazyan3, R. Hodajerdi1,2,3,
D. Horns2, F. Januschek3, E.-A. Knabbe3, N. Kuzkova1,3, A. Lindner3, J. Põld3, A. Ringwald3, M.
Schott4, J. E. von Seggern3, R. Stromhagen3, D. Trines3, C. Weinsheimer4, B. Willke1
1: Albert-Einstein-Institute Hannover
2: Universität Hamburg
3: Deutsches Elektronen-Synchrotron
4: Johannes Gutenberg-Universität Mainz
Light-shining-througha-wall
Looking for WISPs
One of the most exciting
quests in particle physics
is the search for new
particles beyond the
standard model.
Extensions
of
the
standard model predict
not only new particles
with masses above the
electroweak scale (about
100 GeV), like SUSY
particles, also so-called
WISPs (very Weakly
Interacting
Sub-eV
Particles).
Hints for axions and ALPs Present and future ALPS II
The ALPS Collaboration started its first “Light Shining through
a Wall” experiment to search for photon oscillations into WISPs
in 2007. Results were published in 2009 and 2010. The ALPS I
experiment at DESY set the world-wide best laboratory limits for
WISPs in 2010, improving previous results by a factor of 10. After
its completion the ALPS collaboration decided to continue looking
for WISPs by designing the ALPS II experiment for probing
further into regions where there are strong astrophysical hints for
their existence.
NASA
Such new particles arise
naturally
in
many
extensions
of
the
Standard Model and might
also explain observations
that are not accounted for
within the particle physics
known today.
The principle of a light-shining-through a wall experiment
Light, typically from a strong laser, is shone into a magnetic field.
Experiments particularly apt to look for WISPs with photons are of
the "light-shining-through-a-wall" type. Laser photons can be
converted into a WISP in front of a light-blocking barrier
(generation region) and reconverted into photons behind that
barrier (regeneration region). Depending on the particle type,
these conversion processes are induced by magnetic fields or
happen by kinetic mixing. The most sensitive LSW laboratory
setup thus far is the first stage of the Any Light Particle Search
(ALPS I) concluded in 2010. With major upgrades in magnetic
length, laser power and the detection system, the proposed
ALPS II experiment aims at improving the sensitivity by a few
orders of magnitude for the different WISPs.
Courtesy of Manuel Meyer
There are many hints for the existence of axions and axion-like
particles: theoretical (most elegant solution to the strong CP
problem), them being a good dark matter candidate and several
astrophysical observations. TeV photons may “hide” as ALPs.
It is therefore an important and fundamental question whether
any of these light particles exists. WISP scenarios gain support
by recent astrophysical studies like the TeV-photon emission of
active galactic nuclei or the properties of white dwarf stars, which
hint at the existence of ALPs. Hence it is time to experimentally
search for WISPs!
WISPs produced by laser light as well as reconverted photons
originating from these WISPs have laser-like properties. This
allows to:
 Guide them through long and narrow tubes inside accelerator
dipole magnets;
 To exploit resonance effects by setting up optical resonators.
TeV transparency
One astrophysical hint pertains to the propagation of cosmic
gamma rays with TeV energies. Even if no absorbing matter
blocks the way of these high energy photons, absorption must be
expected as the gamma rays deplete through electron-positron
pair production through interaction with extragalactic background
light. However, the observed energy spectra do not seem to
match the absorption feature inferred from this argument. Axionlike particles could provide a resolution to this puzzle. Here, the
anomalous transparency can be explained if photons convert
into ALPs in astrophysical magnetic fields. The ALPs then
travel unhindered due to their weak coupling to normal matter.
Close to the solar neighborhood, ALPs could then be reconverted
to high-energy photons.
Stages of the experiment
Conceptual design
Optical setup of the ALPS II
The most famous WISP candidate is the axion, which has been
introduced to explain the smallness of CP violation in QCD and
which turned out to also be a prime candidate for a constituent of
the dark matter in the universe. Similarly axion like particles
(ALPs), light spin 1 particles called "hidden sector photons" or
light minicharged particles seem to occur naturally in realistic
embeddings of the standard model into string theory.
Sensitivity of ALPS II
The sensitivity gains compared to ALPS-I are achieved by
increasing the magnetic length, introducing a regeneration cavity
and an improved detector system.
ALPS II is planned to be realized in two stages:
 ALPS-IIa: with two 10m long production and regeneration
cavities, without HERA superconducting dipole magnets;
 ALPS-IIc: with two 100m long cavities using magnets.
A major challenge of the ALPS II optical design is the
stabilization of both optical cavities to ensure a decent overlap
between the optical modes. This is achieved by actively
controlling production and regeneration cavity length and
alignment in a Pound-Drever-Hall (PDH) and Differential
Wavefront Sensing (DWS) scheme.
Schematic of the ALPS-II injection stage including the production
cavity (PC):
The ALPS experiment utilizes a light-shining-through-a-wall
setup. Strong light fields are send towards a wall that is opaque
to photons but transparent to WISPs due to their vanishing
interaction with ordinary matter. WISPs, which exhibit coupling to
a photon field, can thus be produced in front of the wall and be
reconverted afterwards and consequently be detected by a
photon detector.
ALPS-II experiment is going to be conducted in two steps with
increasing requirements reaching its full sensitivity in ALPS-IIc.
Schematic of the ALPS-II regeneration cavity (RC) including control
loop:
Intermediate step of ALPS-IIa:
Parameters of the long cavity
 20 m length;
 Two 750 ppm curved mirrors (250 m ROC);
 Finesse 4100 and power buildup of 1300.
To avoid disturbance of the single photon detector with spurious photons from optical readout of the regeneration cavity mode, an auxiliary
green beam obtained via second harmonic generation from the infrared production field is fed into the regeneration cavity. The green
light is then separated from the infrared signal field prior to detection. A production probability for 1064 nm from 532 nm photons of less
than 10-21 photons is to be achieved. The total lateral and angular beam shift introduced to the 1064 nm beam by optical components
between two cavities on the central breadboard has to be smaller than 1 mm and 10mrad, respectively, because any beam shift is not
seen by particles traversing the wall and hence reproduced light would not match the RC Eigenmode.
Advantages of the long cavity
 More stable: G-factor 0,85 (for 10 m flat/curved: G-factor 0,96);
 Two mirrors from the same coating run;
 Impedance matched cavity.
Laser system
35 W Laser system
An end-pumped laser design was
chosen to achieve a well defined
Gaussian mode and an efficient
amplification with excellent beam
quality. An efficient amplification
of a Nd:YAG single-frequency,
fundamental mode laser with
Nd:YVO4 as amplifier material is
feasible. With a 2 W NPRO as
seed laser source output power
levels of more than 35 W were
achieved. If the amplifier was
seeded with higher power levels,
for example with one of the
currently used gravitational wave
laser systems (10 W to 20 W)
output power levels up to 65 W
could be realized.
Present status of the
experiment
ALPS II in the HERA tunnel
ALPS-IIc in the HERA tunnel
The final stage of the ALPS II will be realized with full length
cavities and straightened HERA dipoles. The picture shows a
straight section of the HERA tunnel equipped with HERA dipoles.
The middle part, accommodating the central breadboard including
the “wall” is highlighted.
Using a 532 nm green laser which was aligned inside the beam
pipe a central position of a laser beam which will go through the PC
and RC mirrors was set. The results proved that it will be possible
to achieve a power buildup 40000 with 0,025 m available
effective mirror diameter.
Power buildup
The main goal of the optical design of ALPS-II is to make the
electro-magnetic field provided by the laser beam on one side of
the wall as large as possible and to detect a possibly regenerated
field on the other side with a very high sensitivity. On the side in
front of the wall, the ALPS-II production cavity can increase the
optical power of the light beam directed towards the wall by a
factor of 5000 compared to the power of the injected laser. Behind
the wall, the regeneration cavity increases the production
probability with which photons are created from the axion field by
a factor of 40000.
Calculated dependence of the power buildup vs effective mirror
diameter to achieve a buildup 40000:
ALPS II schedule
Closure of the LINAC tunnel of the European
XFEL project under construction at DESY
Straightening magnets
To increase the sensitivity for the detection of axion-like
particles, the ALPS-II collaboration plans to set up optical
cavities both on the production and the regeneration side of the
experiment with a power buildup of 5000 and 40000,
respectively, and magnet strings of superconducting HERA
dipoles as long as possible, as the sensitivity for the detection
of axion-like particles scales with the product of magnetic field
strength B and magnetic length L.
The maximal length is determined by the aperture of the beam
tube, because clipping losses of the laser light are to be avoided.
Now the inner diameter of the vacuum pipe in the
superconducting HERA dipole is 55 mm, which is more than
sufficient for an installation of 20 HERA dipole magnets for ALPS
II. However, due to the curvature of the dipoles built for HERA,
the free horizontal aperture is reduced to ~35 mm. Such an
aperture would limit ALPS II to just 8 magnets.
https://ALPS.desy.de