Title: Outlook for an Improved Moeller Experiment at 12 GeV JLab e2ePV
1Outlook for an Improved Moeller Experiment at 12
GeV JLab (e2ePV)
- Dave Mack (TJNAF)
- PAVI06
- May 16, 2006
- Physics motivations
- sin2?W
- new physics and LHC
- Conceptual design
- Summary
2Standard Model sin2?W
3Standard Model sin2?W Determination at High Energy
- The Standard Model value of sin2?W is dominated
by two high precision measurements at the Z pole
(one leptonic, one semi-leptonic) which are
inconsistent.
leptonic
semi-leptonic
LEPWG hep-ex/0509008
- Error demagnification and the lack of significant
hadronic dilutions makes Qw(e) perhaps the most
attractive way to measure sin2?W at low
energies. - Combining the JLab 12 GeV upgrade with the
shoulders of giants (SLAC E158 experience), it
may be possible to make the ultimate low energy
measurement of sin2?W at low energies with an
error better than - 0.0003. - (ultimate at least in the scattering sector!)
4Existing/Future Determinations of sin2?W at Low
Energy
From Qw A Bsin2?W, one can derive
with error magnification factor
The ability of the 2.5 Qw(e) and 4 Qw(p)
measurements to discern small shifts in sin2?W
means that also have strong sensitivity to new
e-e physics and to new e-quark physics,
respectively (the latter as 2u1d).
5The Running of sin2?W
- Experiments at different energy scales, and on
different - targets, have non-universal EW radiative
corrections. To - get something universal for apples-to-apples
comparisons - One regresses out the Z-Z and W-W boxes
- (eg, the latter appears one way e-p
interactions but crossed in e-e- scattering) - One regresses out the ?-Z boxes
- (eg, these corrections involve hadronic
structure in ep but not in ee scattering) - leaving only the tree-level Z0 exchange
and ?-Z mixing, the latter containing significant
corrections from universal (but scale-dependent)
properties of the vacuum. - When sin2?W is measured over a large range
of energies, this allow one to keep track of how
well the SM accounts for all the cool stuff in
the vacuum (virtual ee- pairs, pp- pairs, WW-
pairs, t-tbar pairs, etc.)
The real story is a more complicated due to
factors of sin2?W(Q) in the EW radiative
corrections. Global fits incorporate these
properly.
6Scale Dependence of the Weak Mixing Angle
Normalization is defined by Z-pole measurements.
Away from Z pole, the red curve is a SM
prediction which includes ?-Z mixing in addition
to the tree-level exchange.
Some progress has been made by Cs APV and
particularly by SLAC E158 in testing the running.
The NuTeV result is apparently clouded by
hadronic ambiguities.
7Non-Scattering Techniques
- Non-scattering techniques could
conceivably make comparable or superior sin2?W
measurements within the next generation. Isotope
ratios are a suggested means to cancel
uncertainties in many-electron wave functions. - APV Isotope Ratios
- D. DeMille, PRL 74, 4165 (1995)
- V.A. Dzuba et al, ZP D1, 243(1986)
- PV Magnetic Susceptibility Ratios in Dysprosium
Fluoride crystals - Mukhamedjanov and Suchkov, cond-mat/0511003
v2 8Jan 2006 - All isotope ratios suffer from an error
magnification of order N/?N, necesitating
measurements an order of magnitude more precise
than the Cs APV measurement by Wood et al. - But quanta are cheap in non-scattering
experiments! - The interpretability of isotope ratio
measurements is ultimately limited by how well
one can predict relative changes in the neutron
radius, presumably for the largest possible range
of even-N nuclei. - JLab 208Pb radius measurements will help
calibrate models. - Given the resources that will be required
to make a -0.00025 measurements of sin2 ?W in
ee scattering at low energies, anything better
will have to come from the table-top. - Lots of relevant talks Wednesday morning!
8new physics searches
9Direct Searches for New Physics
- Extra neutral vector bosons appear in many
SM extensions. Call it a Z. - If you have the CM energy and luminosity, direct
searches are ideal. - Limitations statistics, backgrounds, and
branching ratio to e e- - but a large resonance bump should be
unambiguous confirmation.
PPbar ?e e- X
Z0
No bump.
At 95 confidence level, the worlds collection
of pair data constrains Z-primes with SM-like
couplings to 800 GeV.
LEP EWWB hep-ex/0511027 Nov 2005)
10New Contact Interactions
- The sensitivity to new physics Mass/Coupling
ratios can be estimated - by adding a new contact term to the
electron-quark Lagrangian - (Erler et al. PRD 68, 016006 (2003))
This was derived for Qw(p), but the general
lesson is that any few measurement of a
suppressed weak-scale quantity is sensitive to
physics at the multi-TeV scale, well above
present colliders and complementary to LHC.
11 Misc. Model Sensitivities (non-SUSY)
scaled from R-Musolf, PRC 60 (1999), 015501
Collider limis from Erler and Langacker,
hep-ph/0407097 v1 8 July 2004
One has to be careful taking model-dependent
sensitivities too seriously. The listed E6 Z
models dont couple to up-quarks, so d-quark rich
targets are favored. However, for these
particular models, a 2.5 Qw(e) measurement looks
appealing, in fact irreplaceable as an e-e
compositeness test.
12Qw(e) at 12 GeV SUSY Sensitivities
No dark matter candidate (decayed)
- RPV (tree-level) SUSY
- would tightly constrain
-
- RPC (loop-level) SUSY
- one of the few low energy measurements
capable of placing significant constraints -
- Qw(e) complementarity wrt other RPC SUSY
searches -
- EDMs require CP violation,
- direct production of a pair of
supersymmetric particles could be above LHC
reach, - leaving precision measurements like
(g-2)muon and Qw(e). -
Theory and Experiment bands 95 CL
Contours courtesy of Shufang Su (U. Arizona)
13The Impending LHC Revolution
PAVI08? LHC collaborations need only a trickle
of data to quickly discover or exclude a Z with
mass below 2 TeV. Of course, it will take time
to get their calibrations and analysis going.
(Each additional increase in mass range ?M
1 TeV costs another order of magnitude in
integrated luminosity.) PAVI10? Depending on
how well things go, they could be announcing
discovery or exclusion for Z masses up to 3-4
TeV. For 4-5 TeV, the pace must slow to a
crawl.
F. Ruggiero seminar, 8th ICFA, Daegu, 2005
14Implications of Contact Interaction Formula for
Low Energy
Experimentalists
- If an experiment is limited by systematic errors,
a factor of 2 increase in mass scale requires x4
reduction in the systematic error. (eg, atom
trap beta decay) - If an experiment is limited by statistical
errors, a factor of 2 increase in mass scale
requires x16 improvement in statistical figure of
merit. ( eg, rare kaon decay ) - Experimentally, a factor of 2 increase
in mass sensitivity for a mature low energy
experimental program can take a generation. - Potential pulls are proportional to (g/?)2
- (We are 4 times less sensitive to masses of
2 TeV than 1 TeV.) - When the energy frontier moves
beyond the several TeV-scale, our discovery
potential will be reduced to an increasingly
smaller model space with relatively large
couplings, g. - For the time being, the low energy electron PV
program has new physics discovery potential. But
that window will soon become a keyhole. -
15Post 100 fb-1 at LHC With Decreasing Discovery
Potential, Whats our Niche?
- New neutral particles which may be observed
at LHC wont come with name tags. The mass will
be known, and the product of cross section and
branching ratio - BR(X0?l l-) x s
- But what about the spin? The couplings to light
quarks and leptons? - The answers to these questions will be required
for - IDENTIFICATION.
- The common wisdom is that our niche is to
determine/bound the couplings to light quarks and
leptons. But there are ways to do this at LHC.
(eg, M. Dittmar et al. PLB 583 (2004) 111-120) - Still, our role in the identification
process could be important.
16So, Why a Moeller PV Measurement In 2012?
- Assume the SM, and make a measurement of sin2?W
at low energies to help resolve the apparent
discrepancies in the precision sin2?W database. -
- Compare our result to the Z pole value of sin2?W
which has been extrapolated to low energy, as a
constraint on new physics (with either discovery
or identification potential).
Its impossible to predict the context an
improved Moeller experiment would occupy in the
year 2012. Well certainly know more about
sin2?W, and but we should expect at least one
revolution before then. In most scenarios, an
improved Moeller measurement will be extremely
interesting.
17 18Benchmarks
- The E158 error bar on Qw(e) was about 13.
- An important new low energy sin2?W measurement
- could be achieved with one half the E158 error
bar - (or 6.5).
- But still only on par with
projected 4 Qw(p). - A factor of 2 increase in new e-e physics reach
(?/g) - requires a new measurement with one quarter the
E158 - error bar (or 3.25).
- To have in some very vague and subjective sense
- a new physics - impact on par with a 0.5 Qw(Cs), or a 4 Qw(p).
Ouch!
19Figure of Merit
The only way to reduce the statistical part of
the error to the required level, while
compensating for the reduction from 48 GeV to 12
GeV, is to utilize JLabs high luminosity in a
lengthy run.
- E 12 GeV
- I 100 µA
- L 150 cm
- 4000 hours
(ie, 32 weeks at 75 efficiency)
20Parameters
- E 12 GeV
- E 3-6 GeV
- T .5-.9
- APV -40 ppb
For reference, the Qw(p) experiment asymmetry is
currently projected to be about -260
ppb. Clearly, we wont understand the deep doo
were stepping into until JLabs 3rd generation
of experiments like Qw(p) and PREX are complete.
2112 GeV Experiment Overview
Worlds highest power LH2 target Scattered
electrons drifted to Q2-defining collimator
Moeller-focusing, resistive spectrometer
Position Sensitive Ion Chamber (PSIC) detectors
Fits in endstation A or C
22Optics Concept
-
- For ?CM 900-1200 (or 3-6 GeV/c),
- Drift scattered electrons to Q2 defining
collimator - Bend angle collimation must block 1-bounce
backgrounds - Drift electrons to the detector
- Hardware focus electrons with the momentum vs
angle correlation of Moeller electrons
The reference design is based on an iron-free,
resistive torus because a 1/R field profile is a
natural way to produce a ?scatt -dependent
hardware focus. We will optimize for e-e
focus, but by tilting the focal plane, we get a
reasonable e-p focus for free.
E158 scanner
e-e
e2ePV focal plane
e-beam
e-p
e-e
Radiation tails
e-p
0cm
70cm
50cm
23Toroidal Spectrometer Modeling
- Hardware-focusing toroidal spectrometers
with external target have some unusual
properties, including - Nonlinear focal plane (effective length is a
function of angle and momentum) - strong azimuthal defocusing at the incoming field
boundary. - Little progress can be made without
tracking rays thru a field map.
- A few important things established so far
- The field integrals required for a 2-bounce
system are consistent with a resistive magnet
design. (no SC magnets means low cost and high
reliability!!) - A good radial e-e focus is possible (at least on
the mid-plane between coils) despite the long
cryotarget and large momentum bite. - To do
- Shape incoming field boundary to control
azimuthal defocusing, - Design trim coils to allow for imperfections in
alignment and fabrication errors.
24Background Suppression
Backgrounds minimized by good design choices
learned from E158 running experience, Hall C
running experience, and Qw(p) toroidal
spectrometer simulations
- No bending of the degraded beam
- Two bounce system
- Focus e-e signal to O(1cm) radial
- Transport neutrals in vacuum
- Detectors
-
- 25-50 bins/octant, fully instrumented
-
25Position Sensitive Ion Chambers (PSICs)
- Fused silica-based Cerenkov detectors are
expensive/difficult to sculpt to match the shape
of a crude hardware focus. - An ion chamber with an optimized preradiator is
very promising - a clever E158 implementation had good time
response, good linearity, low susceptibility to
dielectric breakdown. - Ion chambers are intrinsically rad-hard with the
signal size determined by geometry and pressure. - By partitioning the anode into strips, it is
possible to make detectors with radial
resolutions of - Cost will be dominated by the electronics.
We found poor energy resolution in simulations
which placed a pre-radiator in front of fused
silica at E 1 GeV due to the low number of
e- e. The signal in ion chambers is derived
from the much more plentiful photons, so the
energy resolution should be better.
26PSICs Minimum Position Resolution
- Simulation
- Ee 4.5 GeV
- 1.9 cm W (5.4 X0)
- (shower max!)
- 10 cm, 1 atm He gas
- Minimum position resolution is a few mm (
rMoliere) - Need to control point to point variations in the
gas column
M. Gericke (U. Manitoba)
27e2ePV Target Requirements
- Target cooling power requirements are about 2.4
times more aggressive than Qw(p) - (? 5 kWatt target)
- Qw(p) already plans to increase helicity reversal
to nearly 300Hz in order to freeze density
fluctuations. - Qw(p) target groups are now using finite element
analysis codes to upgrade existing G0 design. - Astonishly, sufficient cooling power is available
on site, though more refrigeration would allow
more flexibility in scheduling.
Highest priority is to build and test the Qw(p)
target.
28Absolute Calibrations
- Beam Polarization
- This is the (systematic) achilles heel of
sub-GeV electron scattering experiments.
However, at 12 GeV, 1 absolute measurements with
Compton polarimeters -
- laser ? e ? hard ? e
- are quite feasible.
- (Not meaning to belittle someone elses hard
work. Im not an expert on Compton polarimeters.)
- 2. Q2 ( 4EE sin2(?/2) )
- E arc energy measurement system will
be recommissioned for 12 GeV - (Unfortunately, I am enough of an expert that Im
stuck with the job.) - ? absolute angles come from survey
- (The spectrometer itself does not require
precise absolute calibration of its field
integrals. The e-e and e-p elastic peaks will be
dominant features of the spectrum.)
29Systematic Checks
- Radial profile of yield and asymmetry of
SignalBkg continuously measured with the main
detector (PSIC) with - Offset-type errors (due to beam parameter false
asymmetries) monitored with small(er) angle
Moeller scattering in lumi monitors. - Scale-type errors (mostly Pbeam) monitored with
larger PV asymmetries ep and DIS. - Isolation from the reversal signal continuously
monitored with current sources in the
experimental area. - Event-mode operation would be useful, but the
feasibility of doing this in PSIC-type detectors
isnt yet clear.
30Errors
Which would allow an error of about -0.00025, on
par with the best Z pole measurements. Except
for more allowance for corrections these error
estimates are similar to those for Qw(p). (It
is the hadronic dilutions in ep which magnify
the error to produce a 4 error on the protons
weak charge. Those dilutions are not present in
ee.)
31Interpretability
- Only a few years ago, the interpretability of an
improved low energy Moeller measurement was
limited by the hadronic corrections in the ?-Z
mixing diagrams. - A dramatic improvement was published last year
- Erler and M.J. Ramsey-Musolf,
- PRD72, 073003 (2005).
- with a theory error on low energy ?sin2?W -
0.00016. - This is only about ½ the projected experimental
error.
Now reduced
32Hurdles
- 4th generation PV experiment infrastructure
- Successfully complete Qw(p) Runs I-II while
developing infrastructure for sub-ppb systematic
control at JLab. - Target coolant supply
- Available in principle. But
- Will FEL program be complete? Just a
scheduling issue? - New fridge needed?
- Funding
- New capital equipment cost should be low
compared to other recent PV scattering
experiments with custom large acceptance
detectors. - However, all US DOE s for 12 GeV upgrade
are committed, so US NSF or foreign sources of
funding are needed. - (As usual, the Canadians are ahead of
everyone )
33e2ePV Collaboration Formation
- A small working group (JLab, U. Manitoba, ANL,
LaTech), has existed for several years, examining
rates, a toroidal spectrometer concept, and the
physics case for the expected level of precision. - A formal organizational meeting of interested
parties will take place at JLab later this year,
possibly end of August. - Well then have one year to choose a reference
design and develop a Letter of Intent or
proposal based on this. - A resistive toroidal spectrometer is a concept
design, not yet a reference design. The
collaboration might come up with something even
better.
34Subsystems
Lots of reponsibilities to parcel out
- Spectrometer Magnet
- Target
- Detector
- Low noise pre-amps
- Low noise digitizers
- Polarimetry
- Beamline diagnostics (lumi monitors, spot size)
- Beam dithering
- DAQ parity and pulsed mode
- Slow Controls
- Data analysis
- Simulations, simulations, simulations
- more software, more software, more software
- Certainly makes one long for the table-top!
35Summary
- Following Qw(p) Run II and the 12 GeV upgrade,
JLab could potentially be well-positioned to
perform a greatly improved Qw(e) measurement. - In the context of the SM, a 2.5 measurement of
Qw(e) would provide critical input on sin2 ?W ,
with an error less than -0.0003, comparable in
impact to the best SLC and LEP measurements. - New e-e interactions would be constrained at TeV
scales, with significant discovery potential as
of today. - (But the LHC may soon turn our world upside
down, unless RPC SUSY rules and the thresholds
are out of their reach.) - There are years of RD ahead of us in terms of
target, spectrometer, detector, and beam
diagnostics, much of it synergistic with the
Qw(p) experiment effort. - (Best way to get seriously involved is to
join Qw(p) or PREX, which represent JLabs 3rd
generation of integrating PV experiments.)
36Acknowledgements
- Wim Van Oers and Michael Ramsey-Musolf, for
encouragement. - Jens Erler and Bill Marciano for recent
discussions on e2ePV motivation. - Roy Holt, for independently checking the rate and
asymmetry estimates. - Greg Smith, for his ongoing search for truth in
cryogenics. - Peter Bosted and Michael Woods for many important
experimental insights, most of which havent been
dealt with yet.
37 38Another FOM
- The previous FOM assumes a fixed beam current.
- If one wishes to compare different facilities,
each running at the labs highest energy, a
better FOM is - FOM A2 x s x Ibeam,
- which is proportional to IbeamxEbeam or beam
power. - Given JLabs maximum beam power of 1 MWatt
versus SLACs maximum of 1.8 MWatt, real-world
scheduling conflicts, and the higher price of
electricity in California, its clear that the
relevant FOM cant be expressed in a cute
formula its the labs commitment. -
- This increases the PV asymmetry while reducing
the required target cooling power.
39Acceptance
- Considerations of resistive magnet strength,
feasible momentum bites, and the desire to avoid
double-counting lead to a similar conclusion as
in E158 - ?CM 900-1200
- E 6-3 GeV
40PSICs Ongoing RD
- Continue simulations,
- build a working device,
- Beam test,
- search for unusual sources of excess noise (eg,
ion spallation) - Study sensitivity to soft backgrounds
- Event-mode capability?
Possible near-term application a cheap, coarse
position resolution detector to monitor the
large N? ? asymmetry during the Qw(p)
experiment. Potential longer term applications
as a modest position resolution detector for
Qw(0?0) or Qw(e)
41Model-dependent New Physics 10 cent tour with a
few SUSY examples
Example tree-level interactions
RPV SUSY
Kurylov et al, PRD 68, 035008 (2003)
Example loop-level interaction
RPC SUSY
- Qw(p), and perhaps all experiments, is most
sensitive to tree-level exchanges. - New conservation laws (here, R-parity) may
require associated production, thus loops. - New physics which appears only in loops is tough
to see magnitude is suppressed, energy for
production at a collider is higher (gotta make
two of em!). - If R-parity is conserved, lightest SUSY particle
cannot decay. Dark Matter???
42Avoiding Cs APV (and other) Constraints
- The 0.5 measurement of Qw(Cs) was a great
test of TeV-scale physics. -
- So who needs Qw(p), Qw(e), Yb ratios, etc?
- 1. We are attempting to mount experiments
of equal or greater sensitivity, complementary to
ongoing collider experiments. -
- Experiments like Qw(p) are needed to test other
isospin combinations. - Eg, a conspiracy of couplings could
have rendered the Cs experiment relatively
insensitive. - Important experiments like Cs APV require
confirmation it was a difficult experiment and
the theory corrections were more complex - than originally assumed.
-
43PVES Caveats
- APV or electron scattering PV measurements
are only sensitive to new physics which - couples to electrons and light quarks (except for
Qw(e)) - is parity violating (ie, both vector and axial
couplings), - yields an outgoing electron (ie, neutral
current) - has sufficient coupling-to-mass ratio (g/?)
- Hence our experiments are controls for
things that dont - violate parity, like a Z with purely axial
couplings.
442nd Order Sensitivities
- New beamline diagnostics?
- More frequent reversal of half-wave plate?
- g-2 precession?
- Logical place for transverse polarization?
45Post 100 fb-1 at LHC With Decreasing Discovery
Potential, Whats our Niche?
- New neutral particles which may be observed at
LHC wont come with name tags. The mass will be
known, and the product of cross section and
branching ratio - BR(X0?l l-) x s
- But what about the spin? The couplings to light
quarks and leptons? - The answers to these questions will be required
for IDENTIFICATION. - The common wisdom is that our niche is to
determine/bound the couplings to light quarks and
leptons. But there are ways to do this at LHC
(below). Still, our role could be important.
M. Dittmar et al. PLB 583 (2004) 111-120
(PDFs imply slightly different shapes for
rapidity distributions of qqbar?X0, which allow
one in principle to deconvolute X0 rapidity
distributions into uubar and ddbar. The errors
will be relatively large and anti-correlated, but
perhaps good enough for ID.)