TWIST – the TRIUMF Weak Interaction Symmetry Test Designed to achieve ~ 0.01% in the shape of the decay spectrum Several data sets of 10 9 events each A precision test of the weak interaction in the Standard Model A precision study of the + decay spectrum NL Rodning – University of Alberta [email protected]
TWIST – the TRIUMF Weak Interaction Symmetry Test. Designed to achieve ~ 0.01% in the shape of the decay spectrum Several data sets of 10 9 events each A precision test of the weak interaction in the Standard Model. A precision study of the m + decay spectrum. - PowerPoint PPT Presentation
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TWIST – the TRIUMF Weak Interaction Symmetry Test
Designed to achieve ~ 0.01% in the shape of the decay spectrum
Several data sets of 109 events eachA precision test of the weak
TRIUMF Willy Andersson Curtis Ballard Yuri Davydov Jaap Doornbos Wayne Faszer Dave Gill Peter Gumplinger Richard Helmer Robert Henderson John Macdonald Glen Marshall Art Olin David Ottewell Robert Openshaw Jean-Michel Poutissou Renee Poutissou Grant Sheffer Dennis Wright
Presently at SLAC
Alberta Andrei Gaponenko Peter Green Peter Kitching Rob MacDonald Maher Quraan Nathan Rodning John Schaapman Jan Soukup Glen Stinson British Columbia Blair Jamieson Doug Maas Mike HasinoffNorthern British Columbia Elie Korkmaz Tracy PorcelliMontreal Pierre Depommier
Regina Ted Mathie Roman TacikSaskatchewan Bill ShinTexas A&M Carl Gagliardi John Hardy Jim Musser Robert Tribble Maxim VasilievValparaiso Don Koetke Robert Manweiler Paul Nord Shirvel StanislausKIAE (Russia) Arkadi Khruchinsky Vladimir Selivanov Vladimir Torokhov
TWIST - Personnel
Allows for possible
- scalar
- vector
- tensor
interactions of right-handed and left-handed leptons
… The most general interaction, which does not presuppose the W
2
,,
,,||~ jei
LRji
TVSij egrate
5
5
ar Pseudoscal
Vector Axial
Tensor
Vector
Scalar
TWIST Motivation – testing the Standard Model
55.0||
424.0||
125.0||
066.0||
SLL
SRL
SLR
SRR
g
g
g
g
110.0||
060.0||
033.0||
VRL
VLR
VRR
g
g
g
0||
122.0||
036.0||
0||
TLL
TRL
TLR
TRR
g
g
g
g
96.0|| VLLg
e±
±
Actual knowledge of couplings-
Plenty of room for a surprise
e+ angle (radians)
e+ Reduced Energy
Decay distribution
The general interaction can be expanded in terms of the Michel
parameters
)34(
3
21)cos(
13)34(
3
233~ 2 xxP
xx
xxxxrate o
The above decay distribution is modified by radiative corrections, required to second order
These are being calculated by Andrzej Czarnecki and Andrej Arbuzov, at the University of Alberta
T
RLSRL
TRL
SRL
TLR
SLR
TLR
SLR
TRL
TLR
VRL
VLR
gggggggg
gggg
****
2222
ReReReRe43
||||2||||43
43
The Michel Parameters - The parameter largely determines the shape of the positron energy spectrum
The effect of large deviations in on the shape of the energy spectrum. The effect shown is roughly 500 times the TWIST sensitivity
****** 66Re21 T
RLSRL
VLR
TLR
SLR
VRL
SLL
VRR
SRR
VLL gggggggggg
The Michel Parameters - The parameter makes a subtle correction to the shape of the positron energy spectrum
The effect of large deviations in on the shape of the energy spectrum. The effect shown is roughly 500 times the TWIST sensitivity
As well, the Fermi coupling constant has a significant dependence on
)(41
119215
32
mmmG
eF 00025.0
50
F
F
G
G Which is roughly 30x larger than the total quoted uncertainty in GF
****
2222222
Re21
||2||8||2||4||2||21
||21
1
TLR
SLR
TLR
SLR
TRL
SRL
TRL
SRL
TLR
TRL
VLR
VRL
VRR
SLR
SRR
gggggggg
ggggggg
The Michel Parameters -
The effect of large deviations in on the energy-integrated angular distribution. The effect shown is roughly 500 times the TWIST sensitivity
The parameter determines the asymmetry in the energy-integrated spectrum.
The Michel Parameters – The parameter determines the energy dependence of the asymmetry in the spectrum.
The effect of large deviations in on the energy-integrated angular distribution. The effect shown is roughly 500 times the TWIST sensitivity
****22
22222
Re||4||2
||||||2||||43
43
TLR
SLR
TLR
SLR
TRL
SRL
TRL
SRL
TLR
TRL
VLR
VRL
VRR
SLR
SRR
gggggggggg
ggggg
The muon lifetime
The Michel parameter
The Michel parameter
The outgoing positron polarization
The rate of absorption of e from muon decay
A test of (V-A)Because the coupling constants often enter as sums of positive definite terms, it is possible to test (V-A) with far less than the 19 experiments one might expect for the determination of the 19 free parameters.
It has been shown, for example, that a rigorous test of the (V-A) postulate can be made by measuring:
TWIST will contribute two of these five required measurements
For example, a measurement of and provides a model independent test of five coupling constants set to zero in the standard model
The muon decay rate can be written as
Where Q describes the decay of a left- or right-handed muon into a left- or right-handed positron
RLRL
,,
Q
22
222
222
22
||||4
1Q
||3||||4
1Q
||3||||4
1Q
||||4
1Q
VRR
SRRRR
TRL
VRL
SRLRL
TLR
VLR
SLRLR
VLL
SLLLL
gg
ggg
ggg
gg
Dependence of the decay on Chirality
Coupling to right-handed muons The decay rate of right-handed muons into either right- or left-handed electrons is given by the sum
This combination of couplings happens to be equal to a combination of Michel parameters, so that
A determination of and provides a model-independent test for the existence of right-handed weak couplings to muons. Q 0 indicates a violation of the Standard Model, and the existence of right-handed couplings for muons Q = 0 indicates that right-handed couplings to the muon do not exist
22222 ||||41
||3||||41 V
RRSRR
TLR
VLR
SLRLRRR ggggg QQQR
916
31
121
RQ
Region consistent with no right-handed weak couplings to the muon
Existing limits on right-handed currents in muon decay
916
31
121
RQ
Existing limits- Discovery region
TWIST preliminary
Anticipated TWIST sensitivity to right-handed currents in
muon decay
916
31
121
RQ
TWIST final
Consider Left/Right Symmetric extensions to the Standard Model
1L
R
W
W
m
m
)sin()cos(
)sin()cos(
21
21
MMeM
MMM
iW
W
R
L
Consider a model with two weak bosons with the mass eigenstates M1 and M2
Parity violation at low energy is presumably due to
In general, the models may include a CP violating phase (), and a left/right mixing parameter
Left/Right Symmetric extensions to the Standard Model
Expect a non zero gRR and gLR
If tensor and scaler couplings are excluded (as unnecessary) from these extensions, then-
2
RLV
RR mmg1 V
RLVLR gg
22 ||2||4
3 VLR
VLL gg
0
222 ||||2||4
3 VRR
VLR
VLL ggg
222 2|| VLR
VRR
VLL ggg
For Left/Right Symmetric extensions
is sensitive to the Left/Right mixing
to the mixing and to the WRmass
and are unchanged by Left/Right extensions with manifest symmetry
A measurement of and determines the WR mass and its mixing
04
3
23
4
221
214
3
4
24
2
R
L
R
L
m
m
m
m
2
RLV
RR mmg1 V
RLVLR ggFor
(left/right Mixing)
WR Mass (GeV)
Left/Right Mixing constraints – Existing limits
Mixing consistent with accepted value of
WR mass consistent with D0 direct search – without assuming manifest left/right symmetry
D0 (and CDF) limit assuming that VR
ud = VLud without CP
violation
D0 Sensitivity to assumptions for the right-handed CKM matrix 200 GeV
Strovink exclusion assuming cos(R)/cos(R) = 1 and no CP violation
(left/right Mixing)
WR Mass (GeV)
Left/Right Mixing constraints – Existing limits
Strovink exclusion assuming cos(R)/cos(R) = 0
WR Mass (GeV)
(left/right Mixing)
Left/Right Mixing constraints – Anticipated
TWIST Sensitivity
Anticipated TWIST result
Anticipated TWIST sensitivity due only to the P measurement
Manifest l/r symmetry
Discovery potential
Testing SUSY
R-parity violating SUSY leads to the following deviations in the parameters at tree level
2
16
3 2
2322311
~24 mGF
where
4
02
2
4
3
2
So that
Deviations in bigger than 1% would show up with deviations in and bigger than 0.01% - with unchanged. Speculative, but a rather specific prediction.
The Experiment
Highly polarized muons enter the spectrometer one at a time
Unbiased trigger on muon entering system
Data sets of 109 muon decay events are obtained in roughly two weeks
The experiment is systematics limited. The high data rate is essential for systematics studies
The large acceptance of the device makes it possible essentially to make measurements of the Michel parameters under differing conditions – therefore improving the reliability of the result.
TWIST - BeamlineThe role of the beamline is to deliver highly polarized “surface muons” to the spectrometer
Pion decay at rest produces polarized muons
Polarized Muon Source
µ++
Back to back to conserve linear momentum
The has zero angular momentum
=> no angular momentum in the final system
TWIST channel resolution allows for the selection of muons produced within 25 microns of the surface of the production target
Depolarization: 0.0002 per 25 microns of target material
Protons incident on graphite produce numerous pions, some of which stop and decay into
muons
TWIST – RF Cuts
Backgrounds (extrapolated from higher momentum)
Cloud Muonspolarization ~ -0.3flux ~ 1%
Flight time through beamline
+
e+
Flight time through the channel show the time that pions are stopped in the pion production target prior to their decay
RF period ~ 1.5 pion lifetimes makes the TRIUMF beam perfect for TWIST
Anticipated Michel Spectrum in TWIST
Distributions in cos() and reduced energy (x). The fiducial volume will be cut at roughly
x > 0.30.5 < |cos()| <
0.95
Drift Plane GeometryThe chambers are
built to high precisionWires are placed to
within about three microns of their nominal position
Average deviation in wire position much less than three microns
Specification on the average wire spacing is exceeded by a factor of 30. Average deviation in wire positions is 2
Scatter in the individual wire placements is ~ 20 microns, a factor of 7 better than specification.
Plane-to-plane Alignments
Tracking ResidualsResults to date; further improvements in alignments and T-zero are underway.
Drift chamber resolution meets spec
Drift Distance (cm)
Distribution of tracking residualsRMS tracking residuals as a function of drift distance in wire cells.
Efficiency – artificial random hit losses accurately identified as inefficiencies by
tracking
TDC hits were rejected at random at the event unpacking stage to mimic an inefficiency. The inefficiency is accurately identified by the track reconstruction code. Efficiency differences of ~ 0.1% are accurately identified. Plane efficiencies are ~99.8%.
Planar Drift Chambers
Energy lost along positron track in the TWIST spectrometer vs. 1/cos(). The curves are for successive sets of detector planes. The slope is proportional to the amount of material in the path of the positron prior to the reconstruction.
The intercept is related to our ability to calibrate the energy scale.
All material seen by the outgoing positrons is in a planar geometry Effects of interactions with the detector are proportional to 1/cos()
Energy loss, Multiple scattering, Hard scattering (kinks) including wire scatters
Calibration of the energy scale through End-Point Fits
The energy calibration is obtained from the data itself. The endpoint of the spectrum has a “sharp edge” at 52.83 MeV.
The edge is rounded by finite resolution and by radiative corrections. As well, the edge is reduced at forward angles (opposite the muon spin).
The forward and backward data can be calibrated to approximately 3.8 and 1.3 keV, respectively. The resultant contribution to the uncertainty in the extracted Michel parameters is typically on the order of 1 part in 10,000.
Opposite muon spin
Toward muon spin
Upcoming schedule107 muon decay events are in hand
“practice” analysis during January-March 2002
Field mapping: April 2002First physics beam: Summer 2002
Preliminary measurement of andBeamline and depolarization studies:
2002/2003Preliminary data on P
Final publications: 2005/2006
ConclusionsThe TWIST experiment is underway
Anticipate preliminary measurements at ~0.1% of:and (Data this summer)P (Data during the summer of 2003)
Final precision on and and P at ~ 0.02%
TWIST will explore significant new space where evidence may be found for physics beyond the standard model
For left/right symmetric models, TWIST has a mass reach which is comparable to - and which complements - that of the Tevatron
Blank slide
Wire Position Mapping - Reproducibility
Wire scans are reliable and reproducible
Blue – Initial wire position scanRed – Replacement of broken wireGreen – Rescan of wire positions
Drift Chamber Noise
The chambers are extremely quiet. Spurious hits in coincidence with a decay positron are roughly one hit in 50 events, and occur at random in the tracking volume so that they are readily ignored by the tracking software.
Cross Talk – No effect on
decay positron tracking
Width
TDC Time
Cross talk is at the 0.2% level.
Cuts on- short width- Adjacency to a
wider, “real” hitReduce tracking
“confusion” to a few extra hits per 105 events – these are eliminated as track outliers
Drift plane efficiency and Operating Point
All DC planes behave the same to a high degree of uniformity. Three planes did not function due to misconnected preamp output cables. In a physics run, these cables would have been reconnected before data taking (at the cost of one week), but having 94% of the planes operational at first startup was excellent for the commissioning run.
Muon stopping distributionWe were fortunate to see a 3% pressure change in 12 hours of running during the commissioning run.
We look at the ratio of counts in the PC’s upstream and downstream of the target. The sensitivity of the ratio determines our ability to measure the range of the muons and consequently their stopping distribution. Note that the data and Geant show the same sensitivity to atmospheric pressure.
106 events are suitable for the determination of the centroid of the stopping distribution to an accuracy equivalent to 66 g/cm2 equivalent to roughly a ¼ keV variation in the average positron energy loss.
A 3 centroid shift would correspond to approximately ¾ keV shift in the positron energy.
Wire Recovery
When a large muon pulse is found on a wire, does that wire recover for use in positron tracking?
Yes.
At right is a histogram of the time at which a positron hits a wire which was hit by a muon a few microseconds earlier. The fit gives an accurate value for the muon lifetime.
Note that hits within 1 s after the incident muon are not used, due to the drift time required for collection of charge from the muon hits.
We are investigating the much rarer occurrence of a positron passing through the same segment of a wire cell that a muon traveled through within several microseconds.
Muon lifetime: 2.20 0.05 sWire efficiency is independent of time since a muon hits a given wire