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Fundamental Physics. May 3rd 2006
Precision tests of gravity: Particle physics at
the low energy Frontier.
Clive Speake
G.Hammond, A. Matthews, F.Pena, S. Aston, E.Rocco.
Gravitation Group, University of Birmingham.
• Motivation
• Brief overview of laboratory tests of gravitation
• Work at University of Birmingham
• Summary
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Fundamental Physics. May 3rd 2006
Motivation
• Standard Model of Particle Physics successfully
describes Electro-weak and Strong interactions up
to ~102 GeV.
•
Standard Model of Cosmology (founded on
classical General Relativity) successfully ‘explains’
observations of the Universe from a second or so
after ‘Big-Bang’.
BUT...
1 q1q2
Vem 

4 0 r
e
2
n1n2
Vem 
 c 
4 0 c
r
m1m2
Vg  GN
r
2
mf
n1n2
Vg  2  c 
Mp
r
2
Fundamental Physics. May 3rd 2006
But...
• Gravitation cannot be renormalised like the other
quantum interactions as there is no mf in nature.
•
The natural scale for a quantum theory of gravity
is the Planck scale: Mpc2~1019GeV. What happens
between the Electro-Weak scale and the Planck
scale (16 orders of energy)? Hierarchy problem.
• We need new symmetries eg Supersymmetry,
Peccei-Quinn symmetry, but we have no direct
evidence for these.
• Cosmology needs Dark Matter but we have not
observed it yet.
• We require the majority of the mass/energy density
of the Universe to consist of a zero-point
fluctuation vacuum energy: Dark Energy.
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Fundamental Physics. May 3rd 2006
Motivation
• Recent attempts at solving these problems suggest
the possibility of new macroscopic forces.
• New gauge symmetries and conserved quantities
lead to new forces eg axion, new forces coupling to
conserved charges B, B-L.
• String theories predict a number of phenomena:
macroscopic compactified dimensions, dilaton,
moduli and others...
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Fundamental Physics. May 3rd 2006
Generic form of new
interactions
• Assume a Yukawa-type potential:
T
• with
q1q2 r / l
Vni  c  g
e
r
2
l   / mb c
• l~1 mm for mbc2~0.2 meV
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Fundamental Physics. May 3rd 2006
weak Force Physics
Adapted From Smith and Lewin 1990.
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Fundamental Physics. May 3rd 2006
Tests of gravitation
• Equivalence Principle.
• Searches for G-dot.
• Macroscopic forces coupling to intrinsic
spin: search for axion-like particles,
search for cosmic spin fields, breakdown
of Lorentz invariance.
• Inverse square law/ Casimir force.
For a review see Gundlach New J. Phys. 7 205 (2005)
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Fundamental Physics. May 3rd 2006
Superconducting Torsion Balance
Birmingham Instrument
in Casimir mode (1998-Present)
Based on Meissner effect zero stiffness suspension utilising Niobium
Temperature of 4.2K
Lift capacity  600g
Superconducting magnetic torque feedback.
We will eventually utilise a novel homodyne interferometric readout
MkI Noise 10-13Nm/Hz
Rev. Sci. Instrum. 75, 955 (2004)
Cavendish Balance (1798-Present)






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Fundamental Physics. May 3rd 2006
The Spherical Superconducting
Torsion Balance:
Cryogenic analogue of a spherical air-bearing
Levitation Bearing
Float
Hard
drawn Nb
wire.
Copper shell
0.2mm, coated
with Pb, (Nb).
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Fundamental Physics. May 3rd 2006
Spark eroded
Nb foil
feedback coils
Interferometer
Sphere
-Plane
Piezo
Float
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Fundamental Physics. May 3rd 2006
Interferometer development for SSTB
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Fundamental Physics. May 3rd 2006
Birmingham interferometer for LISA:
Schematic of first prototype.
PD1
Reference
Mirror
B
B
A1
Laser
Diode
A1,2 Polarising
P
Beamsplitter
B l/4 Plate
C Non-Polarising
Beamsplitter
D l/2 Plate
PD1,2,3 Photodiode
P Polariser
L1,2,3 Lens
C
A2
A2
Proof Mass
D
L2
L1
PD3
PD2
L3
Main
beamsplitter
Cat’s
eye
C&QG 2005
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Fundamental Physics. May 3rd 2006
Birmingham Interferometer:
First prototype (40x70x25mm).
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Fundamental Physics. May 3rd 2006
Birmingham interferometer: Performance.
Using a 664nm VCSEL with 60 nW of optical power on diodes. Shot noise limited above 20 Hz.
Nominally equal optical path lengths.
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Fundamental Physics. May 3rd 2006
Casimir’s Calculation
• Zero-point energy of modes between plates of dimension
L:
x
d
z
E( d )  2k ,k ,k
x
y
z
 x ,y ,z
n 2 2
2
2
 c k ,k ,k k x  k y  2
2
d
x
y
z
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Fundamental Physics. May 3rd 2006
Shortcomings of Casimir’s
analysis
• Thermal Correction
When
c
, corresponding to d=7mm at room temperature,
d
kT
.
thermal photons contribute to Casimir force.
•
How to model conductivity of real metals?
• Roughness correction
• Electrostatic forces due to patch-potentials.
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Fundamental Physics. May 3rd 2006
Reynaud and
Lambrecht et al
2001
• Conductivity, roughness, thin film and patch-potential corrections
are minimised by using larger spacings between conductors. But
force is smaller!
• The controversial thermal correction is minimised at larger
separations at 4K.
• Plasmons have larger effect at shorter spacing?
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Fundamental Physics. May 3rd 2006
Birmingham work
• Assuming sensitivity of Mk1 device (Hammond et al 2004), we
can resolve 0.5% of Casimir force at 4mm in 1 hour (R=10cm).
• Aim at ‘precision’ determination of Casimir force 0.1%.
• Crucial to damp parasitic modes of oscillation:
– horizontal and vertical translational modes damped using
copper-cored inductor in series with levitation bearing.
– Simple pendulum mode damped using copper disk attached to
the inside of float at its pole with superconducting
electromagnet.
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Fundamental Physics. May 3rd 2006
Experimental Tests of Newton’s law
• University of Washington
Torsion fibre
Optical lever
Source mass
Eot-wash website
Test mass
• Currently testing Newtonian gravity at 150mm.
• Aiming at 50mm.
• Employ conducting membrane as electrostatic shield between source and
test mass.
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Fundamental Physics. May 3rd 2006
Birmingham work in progress
• Push to shorter ranges by dispensing with the
electrostatic shield.
• Use transverse geometry to eliminate forces due to
long and short range electrostatic interactions and
Casimir force
 2c 
 lp 

E PP  
1   C n 

3
2

z
720 z



n



• Exploit novel features of Spherical Superconducting
torsion balance being developed at University of
Birmingham.
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Fundamental Physics. May 3rd 2006
Test of the inverse square law:
Basic concept
Modulated masses
Centre of
simple
pendulum
motion
coincides with
centre of
buoyancy.
Long range stick-slip piezo
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Fundamental Physics. May 3rd 2006
Source/Test mass manufacture at RAL
150mm
deep
400mm
pitch, 50%
fill.
Al mandrill
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Fundamental Physics. May 3rd 2006
Source/Test mass manufacture at RAL
Electroplate with Au. Cover Al relief.
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Fundamental Physics. May 3rd 2006
Source/Test mass manufacture at RAL
Skim off the top layer to uncover Al.
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Fundamental Physics. May 3rd 2006
Source/Test mass manufacture at RAL
Sputter coat Au to thickness of 3mm
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Fundamental Physics. May 3rd 2006
Source/Test mass manufacture at RAL
Dissolve Al mandrill.
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Fundamental Physics. May 3rd 2006
Al mandrill
Au plating
prior to
skimming
Courtesy of Peter Huggard, RAL.
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Fundamental Physics. May 3rd 2006
Current Status
• We have completed development of Mk2
SSTB with capacitative angular readout.
• Current sensitivity is limited by capacitive
sensor noise. This can be improved.
• Completion of cryogenic interferometer
is due in 2-3 months.
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Fundamental Physics. May 3rd 2006
Parametrisation of violation of
inverse square law
Gm1m2
1  er / l 
Vni  
r
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Fundamental Physics. May 3rd 2006
Possible signals
moduli
Dilaton
Radion
Vacuum
energy
scenario
2 compact
extra dimensions
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Fundamental Physics. May 3rd 2006
Potential upper limits
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Fundamental Physics. May 3rd 2006
Summary
• Ideas beyond the Standard Model of
Particle physics and, perhaps, also that
of Cosmology are needed to make sense
of gravity.
• Searches for new weak interactions are
complementary to direct searches for
new bosons in particle accelerators.
• Fundamental physics experiments in the
lab or, perhaps, space can contribute.
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Fundamental Physics. May 3rd 2006
Acknowledgements
•
•
•
•
PPARC
EPSRC
BAE
Leverhulme
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