Precision experiments
Electroweak precision experiments
•
•
•
•
practical tools
scientific:
 test of theoretical models, existing
laws of physics
 confirm and/or constrain models
 potential to discover (interactions,
particles, ...)
measures in the distant past
precision measurements: what do they provide?
precision experiments part of large facilities
precision experiments with neutrons
proton decay measurements
muon decay measurements
neutron decay measurements
 lifetime experiment
 correlation parameters between neutron
and decay products
neutron electric dipole moment experiments
precision experiments: measurement tools
measures: a practical tool
define a length on the basis of a common feature
~ 3400 BC
1 cubit
Royal cubit stick
Giza pyramids
sides built on the basis of the cubit
to a precision of 0.05%!!!
precision experiments: measurements
measurements: to add to academic interests
deduce earth curvature by angle of sunlight
250 BC - Eratosthenes:
• In Syene ~5000 stadia south of Alexandria
sunlight shining directly down well shafts
• in Alexandria light measured to be at 7 angle
• ~5000 × 360/7 = 252,000 stadia (of the order of
40,000 km) - (cf 40,030 km)
precision experiments: particle physics
Scientific precision experiments: testing the limits of our description
and understanding of nature
particle physics:
 masses and lifetimes of particles
(quarks, leptons, hadrons, ...)
 matrix elements of transitions
(CKM, PMNS, nuclear trs, ...)
 forces and couplings in reaction
processes (GF, , ... )
 signals of rare events, breaking
of laws and symmetries, ...
goes hand-in-hand with ever more
precision calculations
• proton lifetime
• neutron lifetime (Vud)
• neutron decay
• neutrinoless -decay
precision experiments: proton decay
Standard Model describes the change of quark colour and flavour
and lepton conversion through gauge bosons g, W±, Z0
d
u
decay rate as function of energy T, coupling constant G:
νe
s
0 d
u
e-
Λ
u
d
u
  KGF2 T 5
gW2
GF
 2
2 MW
( Baryon number B and lepton number L conserved )
p
precision experiments: proton decay
GUT mechanisms
in models quarks and leptons incorporated into common families (e.g. e+ with d):
• interaction with new gauge bosons (X, Y)
• masses MX ~ 1015 GeV, coupling gU ~ 1/42
new allowed processes:
p → π0 + e+
p
{
e+
d
u
u
X
p
e+
-
{
Y
d
u
u
-u
u
}
-u
u
0
( Baryon number B and lepton number L NOT conserved )
B  L  B   Li
}
0
i

gU4 M 5p
M
4
X
 p  1  1029 yr
into specific channel:
 p e   1032 yr

0
precision experiments: proton decay
Super kamiokande: neutrino oscillation experiment
11,200 PMTs
detecting e and 
50,000 tonnes of ultra-pure water, 1000m
underground in the Kamioka Mine
   
(100 km < L < 10,000 km)
neutrino flavour states mix, neutrino’s are massive
Super kamiokande: use data to look for proton decay events
precision experiments: proton decay
p → π0 + e+
106 event triggers per day:
• background from cosmic rays
• flashing PMT’s
• radioactivities
18816 surviving events:
analyse all data to look for electron signals:
•
•
•
•
in the correct energy range
total invariant mass per event determined
in the correct momentum range
from the correct part of detector
 / B pe   8.2 1033 yr (90%CL)

0
precision measurement constraining GUT’s
precision experiments: lepton g-2
gq

S
2m

S  ,g  2
2
• 1927 Dirac intrinsic angular momentum and magnetic moment
of electron quantified
• measurements of g factors pushed further development of QED
• May and November 1947 electron g factor measurement different
from 2: g factor anomaly ae
• Formulation of QED with first order radiative correction
ae 

 0.00116
2
g 2
 0.00119(5)
2
six orders of magnitude improvement in precision expts
and theoretical calculation
testing the Standard Model to its limits, discovery of new
interactions beyond SM
precision experiments: lepton g-2
Muon g-2 experiment Brookhaven
target
protons
pions
muons
detectors
muon decay to electron
• 24 GeV proton focused on nickel target generates pions
• pions decay to polarised muons and are injected in
storage ring
• decay electrons emerge preferentially in direction of
muon spin
• detect those electrons with high enough energy to be in
the direction of the muon motion
detecting a signal of the muon spin in forward direction
signal oscillates with spin precession frequency of muon
precision experiments: lepton g-2
Brookhaven National Lab: 3 GeV muons stored in 14 m dia. ring in 1.45 T field
• muon has orbital motion in magnetic field
at cyclotron frequency ωC
• spin has precession frequency ωS
• relative precession of S with respect to velocity
of muon: ωS- ωC
 D   S  c  
a
g 2
eB
c  a
2
mc
g 2
2
direct relationship between ωD and a
g eB
eB
eB
S 
 (1   )
c 
2 mc
mc
mc
precision experiments: lepton g-2
precision experiments: lepton g-2
first signs of deviation of 2.6σ from
Standard Model description?
March 2001 PRL
not quite... error in experimental analysis code
SM
11659100
six orders of magnitude improvement in precision expts
opening a window to beyond SM physics phenomena
11659150
Experiment
11659200
a 10
10
11659250
precision experiments: neutron decay
neutron beta decay experiment:
• Standard Model precision measurements
• precision tests on unitarity of the CKM matrix
• cosmological significance
2
2
2
Vud  Vus  Vub  1
n  p  e  e  782 keV
neutron decay probability, function of particles
momenta, spin, correlation coefficients
 
pe  p

 


 pe
p
p e  p
me

dW  1  b
a
 Pn A
B
D

Ee
Ee E
E
Ee E
 Ee




precision experiments: neutron decay parameters
neutron beta decay experiment:
• correlation coefficients between particles spin and momenta
• coupling constants
   1
A  2
1  32
1  2
a
1  32
correlation electron and
anti-neutrino momentum
correlation electron
momentum – neutron spin
GA

GV
ratio axial-vector /
vector coupling constant
free neutron decay
lifetime
K
n 
f nGV2 1  32


2 3 7
K 5 4
me c
GV  Vud GF
from muon decay
precision experiments: neutron correlation parameter experiments
measurement of λ
the “A” experiment: correlation electron
momentum – neutron spin
• polarised neutrons
• electron detection with respect to
neutron spin direction
precision experiments: neutron correlation parameter experiments
Spectra for both spin states
2002: result:
2006: result:
A = -0.1189(8)  = -1.2739(19)
A = -0.1198(5)  = -1.2762(13)
testing the CKM matrix of Standard Model
B. Maerkisch, PERKEO III : Neutron Decay Measurements
precision experiments: neutron correlation parameter experiments
measurement of λ
the “a” experiment: correlation electron-neutrino momentum
proton energy spectrum depends on a
p
• neutrons (unpolarised)
• proton detection, energy measurement
e
neutrons energy ~ meV, energy release ~MeV
n
n
e
p
e-
proton energy depends on angle between
electron and anti-neutrino
e-
precision experiments: neutron correlation parameter experiments
measurement of proton energy spectrum
• Penning trap
• proton detection, energy measurement
cold neutrons pass through volume
between two electrodes, kept in a
magnetic field
decay protons trapped and orbit
around magnetic field lines
open trap by lowering voltage on
gate electrode
repeat sequence for mirror voltages
ranging 0V to 800 V
precision experiments: neutron correlation parameter experiments
measurement of decay proton integrated
energy spectrum
fit curve to energy spectrum as function
of a:
a = -0.1054 ± 0.0055, λ = 1.271 ± 0.018
no competition for A measurement
but independent method
precision experiments: neutron lifetime experiments
the neutron lifetime experiment:
• neutrons (of cold or ultra-cold energy)
• detect decay products or detect surviving neutrons
• precision tests on unitarity of
the CKM matrix
• cosmological significance
experiment at NIST - USA:
• beam of cold neutrons
• neutrons pass through penning trap
• decay protons recorded
precision experiments: neutron lifetime experiments
the neutron lifetime experiment:
NIST
superconducting magnet 3T
high voltage (27 kV) cage for
proton acceleration
incoming neutron beam
solid-state
charged particle detector
precision experiments: neutron lifetime experiments
• need to know neutron flux to very high precision
• need to know trap volume to high accuracy
the neutron lifetime experiment:
NIST
• need to know efficiency of detectors to high accuracy
• need to collect many events for statistical precision
neutron flux monitor: n + 6Li→3H + 
• ρ = (39.30 ± 0.10) µg/cm2 6Li density
• σ = (941.0 ± 1.3) b absorption cross section at 2200 m/s
• Ω/4π = 0.004196 ± 0.1% fractional solid angle detector
τn = 885.5 ± 3.4 s.
precision experiments: neutron lifetime experiments
experiment at ILL:
•
•
•
•
the neutron lifetime experiment:
stored ultra-cold neutrons
ultra-cold neutrons guided into storage chambers
seal chamber and store neutrons for a period T
open chamber to neutron detector and count remaining neutrons
repeat cycle for different storage periods T
two storage chamber
configurations: different surface
exposure
UCN
detector
precision experiments: neutron lifetime experiments
• need to know neutron flux stability
the neutron lifetime experiment:
stored ultra-cold neutrons
• need to know neutron loss mechanism during storage
• need to collect many events for statistical accuracy
• different detection efficiencies for two
chamber configurations ± 0.36 s
• uncertainty in shape of chamber
• statistical uncertainty
precision experiments: neutron lifetime experiments
experiment at ILL:
•
•
•
•
the neutron lifetime experiment:
stored ultra-cold neutrons
ultra-cold neutrons guided into storage chambers
seal chamber and store neutrons for a period T
open chamber to neutron detector and count remaining neutrons
repeat cycle for different storage periods T and different energies
precision experiments: neutron lifetime experiments
precision experiments: neutron lifetime experiments
the neutron lifetime experiment:
stored ultra-cold neutrons
latest result too far off to be included in
average, now additional measurement:
• polarised ultra-cold neutrons guided
into storage chambers
• seal chamber and store neutrons for a
period T
• open chamber to neutron detector and
count remaining neutrons
• repeat cycle for different storage
periods T
precision experiments: neutron lifetime experiments
measurements / error bars incompatible, to be continued...
precision experiments: neutron lifetime experiments
the neutron lifetime experiment:
stored ultra-cold neutrons
Vud from neutron and nuclear beta decay
Vud
2
4908.7  1.9 s

 n 1  32


n = (878.5  0.7st  0.3syst) s
“Gravitrap” result
n = (885.7  0.7) s
world average
Perkeo result:
A0 = -0.1189(7)
 = -1.2739(19)
=GA/GV
precision experiments: neutron electric dipole moment
+
-
T reversal
dn S
-
-
dn S
electric dipole moment dn
Electric Dipole Moment:
neutron is electrically neutral
If average positions of positive and
negative charges do not coincide:
+
spin S
+
-
P transform.
S
dn
+
dn S
P & T violation
EDM dn
CPT conservation  CP violation
CP violation in Standard Model generates very small neutron EDM
Beyond the Standard Model contributions tend to be much bigger
neutron a very good system to look for CP violation beyond the Standard Model
nEDM: measurement principle
Compare the precession frequency for parallel fields:
Experiments:
Measurement of Larmor precession
frequency of polarised neutrons in a
magnetic & electric field
 = E/h = [-2B0n - 2Edn]/h
to the precession frequency for anti-parallel fields

 (d n ) 
2ET N
: polarisation product
E: electric field
T: observation time
N: number of neutrons
 = E/h = [-2B0n + 2Edn]/h
The difference is proportional to dn and E:
h( - ) = 4E dn
nEDM: measurement principle
1.
“Spin up”
neutron...
2.
Apply /2
spin
flip pulse...
4.
Free
precession
...
Second /2
spin
flip pulse.
Ramsey Resonance Curve
24000
1/Ts
22000
neutron spin up count
3.
20000
18000
16000
14000
12000
10000
working points
29.7
resonance frequency
29.8
29.9
30.0
applied frequency [Hz]
30.1
nEDM at ILL: scheme used
Four-layer mu-metal shield
High voltage lead
Coil for 10 mG
magnetic field
Quartz insulating
cylinder
Upper
electrode
Main storage
cell
Hg u.v.
lamp
PMT to
detect Hg
u.v. light
Vacuum wall
Mercury
prepolarising
cell
RF coil to flip spins
Magnet
UCN polarising foil
UCN detector
Hg u.v. lamp
S
N
UCN guide
changeover
Ultracold
neutrons
(UCN)
nEDM at ILL: set-up room temperature experiment
nEDM at ILL: normalised frequency measurement
29.9295
Neutron resonant frequency (Hz)
7.7890
29.9290
7.7888
29.9285
29.9280
10
-10
T
7.7886
29.9275
29.9270
7.7884
29.9265
7.7882
29.9260
0
5
10
Run duration (hours)
15
20
nEDM at ILL: performance room temperature experiment
100
 d n  3 10 26 ecm
neutron EDM [10
-25
e cm]
80
60
40
20
0
-20
-40
-60
1300
1400
1500
1600
run number
1700
1800
1900
nEDM: experiment vs theory
Progress at ~ order of magnitude per decade
Standard Model out of reach
Severe constraints on e.g. Super Symmetry
+e
1 cm
-e
Neutron EDM upper limit [ecm]
Experiment
Theory
10-19
10-19
10-20
10-20
10-21
10-21
10-22
10-22
10-23
10-23
10-24
10-24
10-25
10-25
10-26
10-26
10-27
10-27
10-28
1960
1980
2000
year of publication
10-28
10-29
10-30
dn = 1 ecm
10-31
|dn|< 3 x 10-26 ecm
10-32
10-33
10-34
10-35
precision experiments
we have seen:
precision measurements examples
 neutron electric dipole moment experiments
 neutron lifetime & correlation experiment
 anomalous g-factor (g-2)
 decay experiments (p, double beta)
these can:
current precision
experiments:
 test of theoretical models,
existing laws of physics
 confirm and/or constrain models
 potential to discover
(interactions, particles, ...)
mostly indirect measurements
a very powerful tool to probe theories
and their limits
revealing signatures of new physics