Dave Mack (TJNAF) Workshop on Hadronic Physics in China and Opportunities with 12 GeV Jlab

The Moeller PV Experiment:the ultimate scattering sector determination of the weak

mixing angle at low energies

Dave Mack (TJNAF) Workshop on Hadronic Physics in China and

Opportunities with 12 GeV JlabAugust 1, 2009Lanzhou, China

Qweak evolves

Interactions of Electrons The well understood interactions of point-like electrons, and the high intensity and quality of modern electron beams, make them ideal for studying the charge and magnetization distributions in nuclear matter.

Because of the different isospin coupling of the γ and Z0, parity violating electron scattering provides an additional window on flavor.

In precision measurements of Standard Model-suppressed observables, the large mass of the Z0 even brings potential new physics at TeV-scales within reach.

2Moeller PV Experiment at 12 GeV

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

Leptonic measurements

Semi-leptonic measurements


sin2θW at Low EnergyFrom Qw = A + Bsin2θW, one can derive

Measurement Mag. Factor F Δsin2θW/sin2θW Δsin2θW

0.5% Qw(Cs) 1.4 0.7% 0.0016

13.1% Qw(e) 0.041 0.5% 0.0013

2.5% Qw(e) 0.041 0.1% 0.00025(on par with Z pole)

4% Qw(p) 0.078 0.3% 0.00072

with error magnification factor

Qwe is the most attractive way to measure sin2θW at low energies due to the error

demagnification caused by 1-4sin2ΘW suppression, and the lack of significant hadronic dilutions.


Comparing Low and High Energies: the running of sin2θW

Electroweak radiative corrections shift the effective neutral weak

couplings. To access the effective coupling, data must be corrected for highly process-dependent electroweak box diagrams like

The shift from γ-Z mixing is energy-dependent but universal: a property of the vacuum. If we include this effect in the effective coupling, it will cause sin2θW to “run”.

The EM coupling α runs due to a similar γγ mixing diagram. It is 1/137 at low energy but about 1/128.5 approaching the Z pole due to reduced screening.

I. Levine et al., PRL 78 (1997) 424.


Scale Dependence of sin2θW The red curve is a SM prediction which includes γ-Z mixing in addition to the tree-level exchange.

With the normalization defined by Z-pole measurements,

Like the case of α, the e-e coupling gets stronger with increasing energy due to reduced screening.

Eventually the e-e coupling gets weaker due to increased screening by W+W- pairs.


Z pole

Low and High Energy Data on sin2θW

• Cs APV and SLAC E158 verify the predicted running. •The NuTeV result (not shown) is clouded by hadronic ambiguities.


Cs APV centroid and error recently updated by Porsev et al., PRL 102, 181601 (2009).

Constraints on new physics have moved to higher energy.

Detailed boundaries for allowed phase space in new physics models is in flux.

With Future Weak Charge Data

• Qweak is projected to have a still smaller uncertainty than Cs APV, but is primarily a low energy search for new PV interactions of light quarks.

•The Moeller experiment is projected to have a much smaller uncertainty still.


The Qweak Experiment

Energy Scale of an Indirect Search • 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))

where Λ is the mass and g is the coupling. A new physics “pull” ΔQ can then be related to the mass to coupling ratio:

which reaches the TeV scale for a few percent weak charge measurements of the electron or proton. This is well above present colliders and complementary to the LHC.

(Without the 1-4sin2ΘW suppression, the accuracy would have to be several times 0.1% to reach the TeV-scale.)

e-e and e-quark Compositeness

Experiment/Proposal Compositeness (LL)

e-q e-e

(TeV) (TeV)

Colliders

(LEP2, CDF, Hera)

2.5-3.7 2.2-2.4

0.5% Qw(Cs)exists!

28 ---

13.1% Qw(e)exists!

--- 13

2.5% Qw(e) --- 29

4% Qw(p)under construction

28 ----

Low energy values scaled from Ramsey-Musolf, PRC 60 (1999), 015501

Collider limits from Erler and Langacker, hep-ph/0407097 v1 8 July 2004


The combination of Cs APV plus the Jlab Qweak and Moeller experiments will raise compositeness limits on electrons, u-quarks, and d-quarks to ~28 TeV.

SUSY SensitivitiesRPV (tree-level): allowed pulls to ~6σ RPC (loop-level): allowed pulls to 3σ

Qwe is complementary

to other searchs:

• EDM’s require CP violation, PVES does not.

• Direct production of a pair of supersymmetric particles could be beyond LHC reach. Theory and Experiment bands 95% CL

Updated contours courtesy of Shufang Su (U. Arizona)

The neutralino is unstable in RPV SUSY so cannot be a candidate for dark matter.


A. Kurylov et al., PRD 68, (2003) 035008

Relative Qw(e) shift

RelativeQw(p) Shift

Moeller Scattering

Moeller PV Experiment at 12 GeV 12

The PV asymmetry is proportional to Qwe with no hadronic dilutions.

This is a purely leptonic process, thus highly interpretable.

Qwe

JLab Moeller Experiment Parameters


APV = 35.6 ppb

E = 11 GeV

E’ = 1.8-8.8 GeV

Θlab = 0.230-1.10

150 cm LH2 target

153 GHz rate

5040 hours

∆sin2θW = ± 0.00026(stat) ± 0.00013(sys)

Apparatus Overview


Given the small lab scattering angles and relatively high E’, it was a significant challenge to fit the spectrometer inside the endstation.

Magnetic Spectrometer

Another challenge was to focus e+e e+e events well-separated from higher energy e+pe+X events.

Note the very different vertical and horizontal scales( 1 m in diameter but 30 m long)!


Resistive Toroidal Magnets


The downstream torus has a complex shape needed to produce the hardware focus.

The conceptual design shown here needs further optimization to improve the focus, and engineers need to verify that that is buildable.

(maybe plumbers too!)

Integrating Detectors


The detectors for PV data must be integrating due to the extremely high rate (153 GHz : more than one electron per channel per nsec).

Our reference design provides radial as well as azimuthal binning, and consists of fused silica radiators connected to PMTs via air lightguides.

Side viewFront view

Additional DetectorsPion detector – integrating detector which operates at high luminosity to measure the PV asymmetry of muons and charged pions after EM shower products are ranged out by lead shielding.

Tracking detector – event mode detector which operates only at low luminosity to measure the detector response, search for backgrounds, etc.

Luminosity monitor – integrating detector at small angles

Backgrounds

Mollers

e-p’s


Backgrounds in our reference design are 8%. With further optimization of the magnets, we hope to improve the focus and reduce this further.

Inelastics

Qwe Errors

Which would allow an error on sin2ΘW of about

+- 0.00029 = +- 0.00026(stat)

+- 0.00013(sys)

comparable to the best individual Z pole measurements.


Errors Value

Signal statistics 2.1%

Beam polarization 0.7%

Q2 0.5%

Backgrounds 0.5%

Total 2.3%

Getting InvolvedSome of the culture of parity violation experiments is different from that associated with parity-conserving experiments of cross sections and asymmetries. This can be a bit daunting, but skills in things like detectors, polarimetry, or simulations would allow a new group joining us to immediately make a big impact.

Good ways to get started include

•the Hall A PV program: HAPPExIII, DISparity, PREx (relatively short experiments which start running almost immediately) •the Hall C measurement of the weak charge of the proton: Qweak (a multi-year experiment now finishing construction whose size and schedule is a good match to groups interested in the Moeller experiment) Most recent version of the Qweak proposal (2007) is available at http://www.jlab.org/qweak


http://www.jlab.org/qweak

Some Contact PersonsAn experiment of this complexity has many subsystems, some of which are better suited for outside help. Here I have provided a non-comprehensive contact list which fits on a single slide:

Regarding Name Email addressIntegrating detectors Dave Mack (TJNAF) [email protected]

Tracking detectors David Armstrong (William and Mary) [email protected]

Spectrometer design Kent Paschke (U. of Virginia) [email protected]

Beamline Instrumentation Mark Pitt (VPI) [email protected]

Simulations Neven Simicevic (Louisian Tech) [email protected]

General questions Krishna Kumar (U. of Massachusetts) [email protected]

Hall A Group Leader Kees de Jager [email protected]


mailto:[email protected]







Summary

• With the 12 GeV upgrade, JLab will be well-positioned to perform a greatly improved Qw

e measurement.

• In the context of the SM, a 2.5% measurement of Qwe 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 were the measurement done today (realistically, we’ll be in a good position to help identify any new neutral bosons discovered at LHC).

• I’ve tried to overview the potential of this exciting experiment in Hall A at 12 GeV. The proposal can be downloaded at

http://www1.jlab.org/Ul/ul_office/experimentdb/view_experiment_detail.cfm?paperid=PR-09-005


http://www1.jlab.org/Ul/ul_office/experimentdb/view_experiment_detail.cfm?paperid=PR-09-005

Acknowledgements The organizers of this workshop for their invitation and the countless headaches they must have undergone.

The workshop support staff for making it all work.


Extras


Main Detector Requirements•Full coverage of e+ee+e locus

•Good coverage of e+pe+p including radiative tail

•Rad-hard

•Low excess noise (i.e., shower and electronic noise should be negligible)

•Radial segmentation for systematics (backgrounds)

•Azimuthal segmentation for systematics (e.g., parity-conserving cos(Φ) asymmetries, azimuthal defocusing, beam sensitivities, backgrounds, etc.)

Our collaboration will investigate several straightforward detector technologies which are consistent with our proposed budget . We will then choose the best option(s).

Interpretability• 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 has been published

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


Non-scattering Measurements Table-tops and laser quanta are relatively cheap. Atomic parity violation may one

day compete with our projected Moeller error bar on sin2θW.

• Isotope ratios cancel uncertainties in many-electron wave functions. See

D. DeMille, PRL 74, 4165 (1995)V.A. Dzuba et al, ZP D1, 243 (1986)

• It has been argued recently that the uncertainties in the neutron radius are highly correlated , thus also largely cancel in isotope ratios. See

B.A. Brown et al, PRC 79, 035501 (2009).

However, isotope ratios suffer from an error magnification of order N/ΔN, necessitating measurements an order of magnitude more precise than the Cs APV measurement by Wood et al. , precisions which have not yet been matched by other groups.


Figure of Merit

Despite Jlab’s lower beam energy (11 GeV vs 48 GeV at

SLAC) , we can improve on the E158 result by providing a

much higher integrated beam power.

The relative statistical error dA/A is minimized by the configuration which maximizes

FOM = dσ/dΩ*A2*Luminosity*time

hence the FOM is proportional to

E*Luminosity*time


For fixed target length, what matters is the integrated beam power: total Joules or Megawatt*Years.

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


Direct 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

No bump.

Z0

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)


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Dave Mack (TJNAF) Workshop on Hadronic Physics in China and Opportunities with 12 GeV Jlab