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Outlook for an Improved Moeller Experiment at 12 GeV JLab (e2ePV). Dave Mack (TJNAF) PAVI06 May 16, 2006. Physics motivations sin 2 θ W new physics and LHC Conceptual design Summary. Standard Model sin 2 θ W. Standard Model sin 2 θ W Determination at High Energy.

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outlook for an improved moeller experiment at 12 gev jlab e2epv

Outlook for an Improved Moeller Experiment at 12 GeV JLab (e2ePV)

Dave Mack (TJNAF)


May 16, 2006

  • Physics motivations


new physics and LHC

  • Conceptual design
  • Summary
standard model sin 2 w determination at high energy
Standard Model sin2θW Determination at High Energy
  • The Standard Model value of sin2θW is dominated by two high precision measurements at the Z pole (one leptonic, one semi-leptonic) which are inconsistent.



LEPWG hep-ex/0509008

  • Error demagnification and the lack of significant hadronic dilutions makes Qw(e) perhaps the most attractive way to measure sin2θW 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 sin2θW at low energies with an error better than +- 0.0003.

(ultimate at least in the scattering sector!)

existing future determinations of sin 2 w at low energy
Existing/Future Determinations of sin2θW at Low Energy

From Qw = A + Bsin2θW, 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 sin2θW means that also have strong sensitivity to new e-e physics and to new e-quark physics, respectively (the latter as 2u+1d).

the running of sin 2 w
The Running of sin2θW

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 γ-Z boxes

(eg, these corrections involve hadronic structure in e+p but not in e+e scattering)

… leaving only the tree-level Z0 exchange and γ-Z mixing, the latter containing significant corrections from universal (but scale-dependent) properties of the vacuum.

When sin2θW 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 e+e- pairs, π+π- pairs, W+W- pairs, t-tbar pairs, etc.)

The real story is a more complicated due to factors of sin2θW(Q) in the EW radiative corrections. Global fits incorporate these properly.

scale dependence of the weak mixing angle
Scale 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 γ-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.

non scattering techniques
Non-Scattering Techniques

Non-scattering techniques could conceivably make comparable or superior sin2θW 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/ΔN, 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 θW in e+e scattering at low energies, anything better will have to come from the table-top.

Lots of relevant talks Wednesday morning!

direct searches for new physics
Direct Searches for New Physics

PPbar e+ e- X

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.


No bump.

At 95% confidence level, the world’s collection of pair data constrains Z-primes with SM-like couplings to > 800 GeV.

LEP EWWB hep-ex/0511027 Nov 2005)

new contact interactions
New 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.

misc model sensitivities non susy
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 don’t 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.

q w e at 12 gev susy sensitivities
Qw(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:

EDM’s 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)

the impending lhc revolution
The 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 ΔM = 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

implications of contact interaction formula for low energy experimentalists
Implications 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/Λ)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.
post 100 fb 1 at lhc with decreasing discovery potential what s our niche
Post 100 fb-1 at LHC: With Decreasing Discovery Potential, What’s our Niche?

New neutral particles which may be observed at LHC won’t come with name tags. The mass will be known, and the product of cross section and branching ratio:

BR(X0l+ + l-) x σ

But what about the spin? The couplings to light quarks and leptons?

The answers to these questions will be required for


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.

so why a moeller pv measurement in 2012
So, Why a Moeller PV Measurement In 2012+?
  • Assume the SM, and make a measurement of sin2θW at low energies to help resolve the apparent discrepancies in the precision sin2θW database.
  • Compare our result to the Z pole value of sin2θW which has been extrapolated to low energy, as a constraint on new physics (with either discovery or identification potential).

It’s impossible to predict the context an improved Moeller experiment would occupy in the year 2012+. We’ll certainly know more about sin2θW, and but we should expect at least one revolution before then.

In most scenarios, an improved Moeller measurement will be extremely interesting.

  • The E158 error bar on Qw(e) was about 13%.
  • An important new low energy sin2θW 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 (Λ/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).


figure of merit
Figure 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 JLab’s high luminosity in a lengthy run.

  • E = 12 GeV
  • I = 100 μA
  • L = 150 cm
  • 4000 hours

(ie, 32 weeks at 75% efficiency)

  • E = 12 GeV
  • E’ = 3-6 GeV
  • Θ = .5°-.9°
  • APV = -40 ppb

For reference, the Qw(p) experiment asymmetry is currently projected to be about -260 ppb.

Clearly, we won’t understand the deep doo we’re stepping into until JLab’s 3rd generation of experiments like Qw(p) and PREX are complete.

12 gev experiment overview
12 GeV Experiment Overview

World’s 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

optics concept
Optics Concept

For θCM= 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 θscatt -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


e2ePV focal plane




Radiation tails





toroidal spectrometer modeling
Toroidal 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.
background suppression
Background 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:
  • < 1 cm resolution (including Xtalk)
  • 25-50 bins/octant, fully instrumented
position sensitive ion chambers psic s
Position Sensitive Ion Chambers (PSIC’s)

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.

  • 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 < 1 cm.
  • Cost will be dominated by the electronics.
psic s minimum position resolution
PSIC’s: 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)

e2epv target requirements
e2ePV 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.

absolute calibrations
Absolute 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 γ+ e  hard γ +e’

are quite feasible.

(Not meaning to belittle someone else’s hard work. I’m not an expert on Compton polarimeters.)

2. Q2( = 4EE’ sin2(θ/2) )

E: arc energy measurement system will be recommissioned for 12 GeV

(Unfortunately, I am enough of an expert that I’m stuck with the job.)

θ: 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.)

systematic checks
Systematic Checks
  • Radial profile of yield and asymmetry of Signal+Bkg continuously measured with the main detector (PSIC) with <1 cm radial resolution.
  • 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 e+p 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 isn’t yet clear.

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 e+p which magnify the error to produce a 4% error on the proton’s weak charge. Those dilutions are not present in e+e.)

  • Only a few years ago, the interpretability of an improved low energy Moeller measurement was limited by the hadronic corrections in the γ-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 Δsin2θW = +- 0.00016.

This is only about ½ the projected experimental error.

Now reduced

  • 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 …)

e2epv collaboration formation
e2ePV 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.
  • We’ll 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.


Lot’s 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!

  • 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 θW , 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 R&D 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 JLab’s 3rd generation of integrating PV experiments.)

  • 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 haven’t been dealt with yet.
another fom
Another FOM
  • The previous FOM assumes a fixed beam current.
  • If one wishes to compare different facilities, each running at the lab’s highest energy*, a better FOM is

FOM’ = A2 x σ x Ibeam,

which is proportional to IbeamxEbeam orbeam power.

Given JLab’s maximum beam power of 1 MWatt versus SLAC’s maximum of 1.8 MWatt, real-world scheduling conflicts, and the higher price of electricity in California, it’s clear that the relevant FOM can’t be expressed in a cute formula: it’s the lab’s commitment.

*This increases the PV asymmetry while reducing the required target cooling power.


Considerations of resistive magnet strength, feasible momentum bites, and the desire to avoid double-counting lead to a similar conclusion as in E158:

θCM= 900-1200

E’ = 6-3 GeV

psic s ongoing r d
PSIC’s: Ongoing R&D
  • 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 Δ asymmetry during the Qw(p) experiment.

Potential longer term applications: as a modest position resolution detector for

Qw(0+0+) or Qw(e)

model dependent new physics 10 cent tour with a few susy examples
Model-dependent New Physics: 10 cent tour with a few SUSY examples

Example tree-level interactions

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???


Kurylov et al, PRD 68, 035008 (2003)

Example loop-level interaction


avoiding cs apv and other constraints
Avoiding 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.

pves caveats
PVES 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/Λ)

Hence our experiments are controls for things that don’t

violate parity, like a Z’ with purely axial couplings.

2 nd order sensitivities
2nd Order Sensitivities
  • New beamline diagnostics?
  • More frequent reversal of half-wave plate?
  • g-2 precession?
  • Logical place for transverse polarization?
post 100 fb 1 at lhc with decreasing discovery potential what s our niche1
Post 100 fb-1 at LHC: With Decreasing Discovery Potential, What’s our Niche?

New neutral particles which may be observed at LHC won’t come with name tags. The mass will be known, and the product of cross section and branching ratio:

BR(X0l+ + l-) x σ

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

(PDF’s imply slightly different shapes for rapidity distributions of q+qbarX0, which allow one in principle to deconvolute X0 rapidity distributions into u+ubar and d+dbar. The errors will be relatively large and anti-correlated, but perhaps good enough for ID.)

1 st order sensitivities ie corrections for false asymmetries from beam parameters
1st Order Sensitivities (ie, corrections for false asymmetries from beam parameters)
  • Sensitivities are the potential false asymmetries (normalized as error derivatives) due to changes in beam properties as the electron polarization is reversed.
  • In Qweak, sensitivities are dominated by
  • Rapid kinematic dependence of the cross section
  • Interaction of the defining collimator with the beam envelope.
  • Elastic radiative tail slopping off the edges of the detector.
  • Our ring of azimuthally symmetric detectors reduces some sensitivities by a factor of 100.
  •  linear errors cancel in opposite detectors,
  • parabolic errors don’t cancel but are 2nd order

Changes which would produce 6 ppb shift in the asymmetry (~ Astat):

Sensitivities determined by J. Birchall (U. Manitoba)