Outlook

1
e2ePV A Possible Evolutionary Path for Qweak
Dave Mack (TJNAF) Qweak Meeting at JLab July 27,
2006
2
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.)
3
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, E158 and Qweak
  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.

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.
5
12 GeV Moeller Experiment Background

• In Fall 2002, at perhaps the only restaurant in
  East Lansing with a tablecoth, the Dutch
  Godfather suggested looking into a Qweak(e)
  measurement at 12 GeV JLab. I told him I had
  (back of the envelope fashion), and it would be
  difficult to match SLAC E158s FOM.
• The Italian Godfather expressed disappointment at
  this news and suggested I look into it again.
  (visions of cement overshoes?) I did, and
  assuming a large integrated luminosity and a
  little better control of systematic errors than
  Qweak(p), it began to look very interesting.
• The West Coast Godfather of Fundamental
  Symmetries was interested but demanded higher
  profit margins. So assuming an even larger
  integrated luminosity (but not yet in the realm
  of science fiction) and that systematic problems
  were beaten to a pulp, the experiment looked
  extremely interesting.
• Meanwhile, we negotiated a truce with the 12 GeV
  CDR Godfather (an experiment is not a program)
  so we could invade his turf.

6
Progress

• A conceptual design for a new small angle torus
  was developed.
• We got into the 12 GeV CDR and the 12 GeV PAC
  liked the physics.
• Greg Smith studied available cooling power on
  site.
• Meanwhile, it became clear there werent enough
  DOE funds for an upgrade in Hall A. The Moeller
  experiment could provide a high-profile, NSERC
  and NSF-funded experiment which would occupy Hall
  A for several years.
• The Director routinely mentions the Moeller
  experiment as one of the more exciting things to
  do at 12 GeV.
• E158 completed their experiment, learned a lot
  about doing these measurements, and missed their
  uncertainty target by almost a factor of 2.
  (a lesson for Qweak!)
• Roy Holt (ANL) confirmed the projected
  statistical errors.
• Wim injected the idea of the Moeller experiment
  into the Canadian long range planning process,
  and has requested funding for an organizational
  meeting with theorists.
• and this was done without diverting significant
  resources from our highest
  priority the Qweak(p) experiment.

7
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)
8
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.
9
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
10
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.
11
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.)
12
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
13
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 )

14
e2ePV Collaboration Formation

• A formal organizational meeting of interested
  parties will take place after Wim hears about his
  NSERC proposal.
• The pressure is on because we have perhaps 1
  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.

15
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.
• 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.

16

• Extras

17
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

18
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!
19
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.
20
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)