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Transcript
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
1
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.
3
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...
4
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
5
Fundamental Physics. May 3rd 2006
weak Force Physics
Adapted From Smith and Lewin 1990.
6
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)
7
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)






8
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).
9
Fundamental Physics. May 3rd 2006
Spark eroded
Nb foil
feedback coils
Interferometer
Sphere
-Plane
Piezo
Float
10
Fundamental Physics. May 3rd 2006
Interferometer development for SSTB
11
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
12
Fundamental Physics. May 3rd 2006
Birmingham Interferometer:
First prototype (40x70x25mm).
13
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.
14
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
15
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.
16
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?
17
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.
18
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.
19
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.
20
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
21
Fundamental Physics. May 3rd 2006
Source/Test mass manufacture at RAL
150mm
deep
400mm
pitch, 50%
fill.
Al mandrill
22
Fundamental Physics. May 3rd 2006
Source/Test mass manufacture at RAL
Electroplate with Au. Cover Al relief.
23
Fundamental Physics. May 3rd 2006
Source/Test mass manufacture at RAL
Skim off the top layer to uncover Al.
24
Fundamental Physics. May 3rd 2006
Source/Test mass manufacture at RAL
Sputter coat Au to thickness of 3mm
25
Fundamental Physics. May 3rd 2006
Source/Test mass manufacture at RAL
Dissolve Al mandrill.
26
Fundamental Physics. May 3rd 2006
Al mandrill
Au plating
prior to
skimming
Courtesy of Peter Huggard, RAL.
27
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.
28
Fundamental Physics. May 3rd 2006
Parametrisation of violation of
inverse square law
Gm1m2
1  er / l 
Vni  
r
29
Fundamental Physics. May 3rd 2006
Possible signals
moduli
Dilaton
Radion
Vacuum
energy
scenario
2 compact
extra dimensions
30
Fundamental Physics. May 3rd 2006
Potential upper limits
31
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.
32
Fundamental Physics. May 3rd 2006
Acknowledgements
•
•
•
•
PPARC
EPSRC
BAE
Leverhulme
33