Testing the Standard Model using Parity Violating Deep Inelastic
Scattering
The electroweak Standard Model describes the unification of the
electromagnetic and weak interactions. The Standard Model has been
stringently tested by many high energy experiments, which have good, but
not perfect agreement with Standard Model predictions. An opportunity
exists to use Jefferson Laboratory (JLab)’s polarized electron beam to
probe the Standard Model at lower energies by measuring parity violation
in deep inelastic scattering, at both the currently available energy of
6 GeV or after the future 12 GeV upgrade.
One of the triumphs of
physics in the 1970’s was the theoretical unification of the
electromagnetic and weak interactions into what is now known as the
"Standard Model". The 1979 Nobel Prize in Physics was given to S.
Glashow, S. Weinberg and A. Salam for this work. One of the
basic parameters of the Standard Model is sin2θW,
which represents the relative coupling strength of the weak and
electromagnetic forces. The Standard Model predicts that the value sin2θW
will vary (or run) as a function of Q2, the energy squared at
which it is probed. Measurement of this "running" provides a strict test
of the Standard Model, as illustrated in figure 1. At an energy
equivalent to the mass of the Z-boson (Q2= MZ2),
sin2θW is well measured; but at Q2 < MZ2,
only a few measurements exist. Recently, two measurements have been
made at masses below the Z-boson mass. The first of these, by the
NuTeV
experiment, used neutrino and antineutrino deep inelastic scattering at
Fermilab found a three standard
deviation (3σ) disagreement with the
Standard Model.
And this deviation could increase to 3.8 σ if the recent
BNL-E865 measurement
of K+e3 branching
ratio
(which is 6% higher than the PDG value)
is used in the analysis.
The Stanford Linear Accelerator experiment E-158
measured sin2θW using parity violation in
Möller Scattering. The E-158 results, while only 1.1
standard deviations away from Standard Model predictions, had a pull in
the same direction as the NuTeV measurement as shown in figure 1.
Parity
violation in polarized electron deep inelastic scattering (PV-DIS) is
another way to probe sin2θW. The asymmetry in
PV-DIS is linearly dependent on sin2θW and
relatively large (Ad ˜ 10-4 Q2 with
Q2 in GeV2), making it
experimentally quite accessible. Historically, DIS parity violation from
a deuterium target was first observed by Prescott
et al. during SLAC
E122 in the
mid-1970 and was used to establish the Weinberg-Salam model as the
electroweak Standard Model.
A preliminary measurement of PV-DIS at JLab
with a 6 GeV beam (E05-007),
led by members of the Medium Energy Physics
Group at Physics Division at Argonne, has been approved at JLab
and plans for a
definitive measurement with an 11 GeV beam are one element of the
justification for the planned upgrade of the CEBAF accelerator at
JLab. The scientific goal of both 6 and 11 GeV
programs is to test the
Standard Model at relatively low energy scales.
In addtion to the Weinberg angle, the combination of
electron-quark coupling constants, 2C2u-C2d with
C2q=gVe·gAq,
will be extracted from Ad and is expected to improve the current
knowledge on this quantity by a factor of eight upon full completion of
E05-007, as shown by the red band on
the right panel of figure 2.
In order to reach the scientific goal of E05-007, two instrumental
upgrades are needed: 1) The laser currently being used by the Compton
polarimeter in Hall A of JLab will be upgraded from IR to a green
(523nm) laser and the Fabry-Perot cavity will be upgraded to
provide twice the laser power as currently achieved. Such upgrade will
provide a 1% precision for the electron beam polarization; 2) A new
Flash ADC (FADC)-based data aquisition system (DAQ) will be built to
handle the high event rate (up to ~1MHz) on a counting basis.
We are currently working on both upgrades. If they can be completed on time,
the experiment E05-007 will be schedule to run in 2008.
Milestones:
- FY2005 — Preliminary tests will be performed to determine the
DAQ design;
- FY2006 — Prototype of the FADC DAQ will be built and tested;
- FY2006 — The Fabry-Perot cavity prototype will be setup in optics
laboratory with the 523 nm laser;
- FY2007 — Full FADC DAQ will be built;
- FY2007 — Install system in Jefferson
Laboratory Hall A beamline and perform calibration tests of the system;
- FY2007/8 — Hope to get E05-007 running;
- FY2012(?) — 12 GeV upgrade of JLab completed.
Webmaster: webmaster @ mep.phy.anl.gov
One of the triumphs of
physics in the 1970’s was the theoretical unification of the
electromagnetic and weak interactions into what is now known as the
"Standard Model". The 1979 Nobel Prize in Physics was given to S.
Glashow, S. Weinberg and A. Salam for this work. One of the
basic parameters of the Standard Model is sin2θW,
which represents the relative coupling strength of the weak and
electromagnetic forces. The Standard Model predicts that the value sin2θW
will vary (or run) as a function of Q2, the energy squared at
which it is probed. Measurement of this "running" provides a strict test
of the Standard Model, as illustrated in figure 1. At an energy
equivalent to the mass of the Z-boson (Q2= MZ2),
sin2θW is well measured; but at Q2 < MZ2,
only a few measurements exist. Recently, two measurements have been
made at masses below the Z-boson mass. The first of these, by the
NuTeV
experiment, used neutrino and antineutrino deep inelastic scattering at
Fermilab found a three standard
deviation (3σ) disagreement with the
Standard Model.
And this deviation could increase to 3.8 σ if the recent
BNL-E865 measurement
of K+e3 branching
ratio
(which is 6% higher than the PDG value)
is used in the analysis.
The Stanford Linear Accelerator experiment E-158
measured sin2θW using parity violation in
Möller Scattering. The E-158 results, while only 1.1
standard deviations away from Standard Model predictions, had a pull in
the same direction as the NuTeV measurement as shown in figure 1.
