e2ePV Motivation and Conceptual Design

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e2ePV Motivation and Conceptual Design

• Dave Mack (TJNAF)
• Not in China
• July 28, 2006

• Motivations
• Conceptual design
• Summary

2
Standard 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

• 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.

3
Existing/Future Determinations of sin2?W at Low
Energy
From Qw A Bsin2?W, one can derive
with error magnification factor
Due to error demagnification and the lack of
hadronic dilutions, Qw(e) is the best way to
determine sin2?W in a low energy scattering
experiment. This means it also has strong
sensitivity to new e-e interactions. (But
provides no information about new e-q
interactions.)
4
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 ?-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.
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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.
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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.
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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
• 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)
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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 ?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
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So, 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.
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Benchmarks

• 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!
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Parameters

• 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.
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Acceptance

• 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

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12 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
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Optics 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
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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.

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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
• 25-50 bins/octant, fully instrumented

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Position 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.
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PSICs 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)
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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.
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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 ? 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.)

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Systematic 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.

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2nd Order Sensitivities

• New beamline diagnostics?
• More frequent reversal of half-wave plate?
• g-2 precession?
• transverse polarization leakage?

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Errors
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.)
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Interpretability

• 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
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Hurdles

• 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 )

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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 needs to take place.
• 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.

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Subsystems
Lots of responsibilities 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

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Summary

• 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.

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extras
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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 JLabs high luminosity in a
lengthy run.

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

(ie, 32 weeks at 75 efficiency)
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Another 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.