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Parity violation in deep inelastic scattering at JLab 6 GeV

TL;DR: The parity-violating asymmetry in e-2H deep inelastic scattering (DIS) can be used to extract the weak neutral-current coupling constants C 2q as mentioned in this paper.
Abstract: The parity-violating asymmetry in e-2H deep inelastic scattering (DIS) can be used to extract the weak neutral-current coupling constants C 2q . A measurement of this asymmetry at two Q 2 values is planned at Jefferson Lab. Results from this experiment will provide a value of 2C 2u - C 2d to a precision of ±0.03, a factor of eight improvement over our current knowledge. If all hadronic effects can be understood, this result will provide information on possible extensions of the Standard Model, complementary to other experiments dedicated to new physics searches. Presented here are the physics motivation, experimental setup, potential hadronic effects and their implications, and the future of PV DIS at Jefferson Lab.

Summary (3 min read)

1 Introduction

  • A measurement of the parity-violating (PV) asymmetry of e−2H deep inelastic scattering (DIS) is planned [1] at the Thomas Jefferson National Accelerator Facility (JLab) in Virginia, USA.
  • The high Q2 measurement will be used to extract the effective weak coupling constants (2C2u − C2d).
  • This result will improve the current knowledge on this quantity by a factor of eight.

2.1 PV DIS Asymmetry

  • In electron scattering, the weak neutral current can be accessed by measuring a parity-violating asymmetry caused by the interference term between weak and electromagnetic scattering amplitudes [10].
  • The scattering amplitude, M, for the scattering process is a product of the current for the electron with the photon or the Z0 propagator and the hadron current:.
  • Following this formalism, the parity-violating asymmetry for scattering longitudinally polarized electrons from an unpolarized isoscalar target such as deuterium, assuming isospin symmetry, is given by [10,12].
  • At JLab energies both Eqs. (3) and (4) need to be modified to take into account terms proportional to ν2/Q2.
  • These coupling constants, along with the Weinberg angle θW itself, are fundamental parameters of the Standard Model.

2.2 Exploring New Physics Beyond the Standard Model from PV DIS

  • There also exist strong conceptual reasons (e.g., the so-called high-energy desert from Mweak ≈ 250 GeV up to the Planck scale MP ≈ 2.4 × 1018 GeV) to believe that the Standard Model is only a piece of some larger framework [14].
  • This strongly suggests additional physics not included in the Standard Model or that one or more of the experiments has significantly understated its uncertainties [15,16].
  • Most recently, the SLAC E-158 [4] experiment used the asymmetry in Møller scattering to determine a precise value of sin2 θW that is consistent with the Standard Model prediction.
  • A large axial quark coupling could cause the NuTeV effect, but cannot be seen in C1q.
  • The “mass scale” for which PV DIS is sensitive to physics beyond the Standard Model would be [20].

3.1 Overview

  • The floor plan for their experimental setup is shown in Fig.
  • The scattered electrons will be detected by the two standard Hall A High Resolution Spectrometers (HRS).
  • In the following the authors will describe some details of the experimental setup, focusing on two major upgrades required by the measurement.

3.2 Beam Line and the Compton Polarimetry Upgrade

  • To reduce the heat impact on the target, the beam will be circularly rastered such that the beam spot size at the target is ∼ 4mm in diameter.
  • The helicity-dependent asymmetry of the electron beam will be controlled by a DAQ specially developed in Hall A for parity-violation experiments [7].
  • A luminosity monitor will be used downstream of the target to monitor helicity-dependent target-density fluctuations and other possible false asymmetries.
  • The current Compton polarimetry in Hall A measures the asymmetry of Compton scattering between the electron beam and a high-power IR laser (1 kW, 1064 nm) achieved by a Fabry-Perot cavity.
  • The current systematic uncertainty of the Compton polarimeter is about 1.9% for a 6 GeV beam.

3.3 Spectrometers and the DAQ Upgrade

  • The authors will use the standard Hall A HRSs [21] to detect the scattered electrons.
  • For each HRS the effective solidangle acceptance for an extended target is 5.4 msr and the momentum acceptance is ±4.5%.
  • The HRS central momentum can be calculated from the dipole field magnitude to 5 × 10−4 and the central angle can be determined to ±0.2 mrad, giving an uncertainty of ±0.004/Q2 on 2C2u− Particle identification (PID) in each HRS will be done with a CO2 C̆erenkov detector and a double-layered lead-glass shower detector.
  • An on-board processor (FPGA) will analyze the digitized data and perform the PID.
  • The authors plan to build both systems such that the FADC-based DAQ can be crosschecked with the regular HRS counting DAQ at a low rate (<4 kHz) and with the scaler-based DAQ at a high rate (up to 1 MHz).

3.4 Expected Results

  • The asymmetry Ad will be extracted from the measured raw counting asymmetry as Ad = Araw Pbeam + ∆ARCEM (13) where Pbeam is the beam polarization.
  • Provided that all uncertainties related to hadronic structure are well under control, one can then extract 2C2u − C2d from Ad using Eq.(3).
  • The expected total uncertainty in 2C2u − C2d is shown in Fig. 2 along with existing world data.
  • For illustration, current knowledge of C1q’s and their expected results from the Qweak experiments are also shown.
  • The authors expect to run E05007 in early 2009 and first results may become available within one year of its completion.

4 Interpretation of the Data and Discussions on Hadronic Effects

  • While the authors can extract 2C2u−C2d from the measured asymmetry, there are uncertainties coming from hadronic physics which may complicate the interpretation of this asymmetry in terms of Standard Model parameters.
  • This section will describe some of these interesting issues.
  • 1 Uncertainty from Parton Distributions and RLT.

4.2 Higher-Twist Effects

  • Among all hadronic effects that could contribute to PV electron-scattering observables, higher-twist (HT) effects are expected to be the most probable for kinematics at JLab.
  • These models were developed primarily during and after the previous PV DIS experiment in the 1970’s.
  • On the other hand, HT provides information on nonperturbative aspects of the strong interaction, which by itself has its own value.
  • By contrast, the prospects for observing HT contributions in PV-DIS are relatively uncomplicated because the QCD higher order terms are expected to cancel in the asymmetry.
  • If the NuTeV deviation from the Standard Model is fully due to the HT, then this would imply a contribution to their expected results much above their uncertainties.

5 Future PV DIS Program at JLab

  • By measuring the PV DIS asymmetry on the proton, the authors can extract the unpolarized PDF ratio d/u at large x without contamination of the deuteron nuclear (EMC) effect and thus provide a clean test on various predictions of this ratio from e.g. pQCD and relativistic constituent quark models.
  • By comparing the PV DIS asymmetry on the deuteron with that on the proton, the authors will be sensitive to charge-symmetry violation which implies that the u(d) distributions in the proton differ from the d(u) distributions in the neutron.

6 Summary

  • Two major instrumentational upgrades are required by this experiment and both are being developed at JLab.
  • This result will help to extract C3q from high-energy data, and has the potential to reveal possible new physics beyond the Standard Model.
  • The additional point at 1.1 GeV2 will help to investigate the non-perturbative hadronic physics contribution to PV DIS asymmetry and may provide the first precision observation of this effect.
  • The author would like to thank the PAVI06 organizers for their invitation to the beautiful Milos Island.
  • The Jefferson Science Associates, LLC operates the Thomas Jefferson National Accelerator Facility for the United States Department of Energy under contract DEAC05-06OR23177.

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2 Xiaochao Zheng for the JLab E05-007 Collaboration: Parity Violation in Deep Inelastic Scattering at JLab 6 GeV
EPJ manuscript No.
(will be inserted by the editor)
Parity Violation in Deep Inelastic Scattering at JLab 6 GeV
Xiaochao Zheng
1
for the JLab E05-007 Collaboration
Lab for Nuclear Science, Massachusetts Institute of Technology, Cambridge, MA 02139, USA
Received: date / Revised version: date
Abstract. The parity-violating asymmetry in e
2
H deep inelastic scattering (DIS) can be used to extract
the weak neutral-current coupling constants C
2q
. A measurement of this asymmetry at two Q
2
values is
planned at Jefferson Lab. Results from this experiment will provide a value of 2C
2u
C
2d
to a precision of
±0.03, a factor of eight improvement over our current knowledge. If all hadronic effects can be understood,
this result will provide information on possible extensions of the Standard Model, complementary to other
experiments dedicated to new physics searches. Presented here are the physics motivation, experimental
setup, potential hadronic effects and their implications, and t he future of PV DIS at Jefferson Lab.
PACS. 13.60.-r Photon and charged-lepton interactions with hadrons 12.15.-y Electroweak interactions
12.15.Mm Neutral currents 12.60.-i Models beyond th e standard model
1 Introduction
A measurement of the parity-violating (PV) asymmetry
of e
2
H deep inelastic scattering (DIS) is planned [1] at
the Thomas Jefferson National Accelerator Facility (JLab)
in Virginia, USA. The measurement will be performed a t
two Q
2
values of 1.1 and 1.9 GeV
2
and a fixed x = 0.3.
The high Q
2
measurement will b e used to extract the ef-
fective weak coupling constants (2C
2u
C
2d
). The low
Q
2
measurement may provide the first observation of the
hadronic higher-twist contribution to the PV DIS asym-
metry, which at JLab energies is the most likely source
of a large deviatio n from the Standard Model prediction.
Using the Sta ndard Model value of 2C
1u
C
1d
which will
be tested by combining the results from Cs atomic par-
ity violation (APV) and the future Qweak experiment, the
exp ected uncertainty o n (2 C
2u
C
2d
) is ±0.03. This result
will improve the current knowledge on this quantity by a
factor of eight. It will help to extract the couplings C
3q
from high-energy data, and might reveal possible physics
beyond the Standard Model.
Results from this experiment (E05-007) will provide
an important guidance for the future DIS-parity program,
of which the ultimate goals are two-fold: by choosing a
kinematics where hadronic effects are negligible, we can
extract sin
2
θ
W
and study possible extensions of the Stan-
dard Model to a high precision. By choosing kinematics
where hadronic e ffects are e xpected to be large, various in-
teresting phenomena can be studied, such as higher-twist
effects, charge-symmetry violation (CSV), and the parton
distribution function ratio d/u. These results may also
Present address: Department of Physics, University of Vir-
ginia, Charlottesville, VA 22904, US A
have implications for the interpretation of existing neu-
trino sc attering data.
2 Parity Violation in Deep Inelastic
Scattering off a Deuterium Target
Historically, PV DIS was one of the first tests of the Stan-
dard Model and an early measurement of the PV DIS
asymmetry off a deuterium target [2,3] ser ved to e stablish
the value of sin
2
θ
W
at sin
2
θ
W
1/4. Since this ground-
breaking experiment, parity violation has become an im-
portant tool not only for probing the Standard Model [4–6]
but also for probing the structure of the nucleon [7–9].
2.1 PV DIS Asymmetry
In e le c tron scattering, the weak neutral current can be ac -
cessed by measuring a parity-violating asymmetry ca us ed
by the interference term between weak and electromag-
netic scattering amplitudes [10]. The scattering amplitude,
M, for the scattering process is a product of the current
for the electron with the photon or the Z
0
propagator
and the hadron current: M
γ
= j
µ
1/q
2
J
µ
and M
Z
=
j
µ
1/M
2
Z
J
µ
. The cross section for scattering right- and
left-handed electrons off an unpolarized target is propor-
tional to the square of the total amplitudes: σ
l,r
(M
γ
+
M
l,r
Z
)
2
, where only a longitudinally polarized electron beam
was considered and M
r
Z
and M
l
Z
represent the incident
right- a nd left-handed electrons, respectively. The parity-
violating as ymmetry may be expressed as [10]
A
LR
σ
r
σ
l
σ
r
+ σ
l
M
r
Z
M
l
Z
M
γ
. (1)

Xiaochao Zheng for the JLab E05-007 Collaboration: Parity Violation in Deep Inelastic Scattering at JLab 6 GeV 3
Thus, measuring the parity-violating asymmetry gives ac-
cess to the weak neutr al current in a ratio of amplitudes
rather than the square of this ratio, grea tly enhancing its
relative contribution. The size of the asymmetry can be
estimated based on the ratio of the propagato rs:
A
LR
Q
2
M
2
Z
120 ppm at hQ
2
i = 1 GeV
2
(2)
with M
Z
= 91.2 GeV [11].
Following this formalism, the parity-violating asymme-
try for scattering longitudinally p olarized electrons from
an unpolarized isoscalar target such as deuterium, assum-
ing isospin symmetry, is given by [10,12]
A
d
=
σ
L
σ
R
σ
L
+ σ
R
=
3G
F
Q
2
πα2
2
×
(2C
1u
C
1d
) [1 + R
s
] + Y (2C
2u
C
2d
)R
v
5 + R
s
. (3)
Here, the kinematic variable Y is defined as
Y =
1 (1 y)
2
1 + (1 y)
2
+ f(y, R
LT
)
(4)
with y = ν/E and ν = E E
is the energy lost by
an incident electron of ener gy E scattering to an elec -
tron of energy E
. The ratio R
LT
(x, Q
2
) = σ
L
T
0.2
measures the vir tua l photon-absorption cross- section ratio
from long itudinally and transversely polarized photons,
and f (y, R
LT
) is a function describing the effect of lon-
gitudinally polarized photons to the asymmetry and van-
ishes when R
LT
= 0. In a work by Blumlein et al. [13]
it was derived that f (y, R
LT
) = y
2
R
LT
/(1 + R
LT
) when
ν
2
Q
2
. At JLab energ ie s both Eq s. (3) and (4) need
to be modified to take into account terms proportional
to ν
2
/Q
2
. The ratios R
s
and R
v
depend on the parton
distribution functions:
R
s
(x) =
s(x) + ¯s(x)
u(x) + ¯u(x) + d(x) +
¯
d(x)
and (5)
R
v
(x) =
u
v
(x) + d
v
(x)
u(x) + ¯u(x) + d(x) +
¯
d(x)
, (6)
where we have neglected co ntributions from the charm
quark. The weak couplings C
1q
and C
2q
with q u, d are
defined as:
C
1u
= g
e
A
g
u
V
=
1
2
+
4
3
sin
2
(θ
W
), (7)
C
1d
= g
e
A
g
d
V
=
1
2
2
3
sin
2
(θ
W
), (8)
C
2u
= g
e
V
g
u
A
=
1
2
+ 2 sin
2
(θ
W
), (9)
C
2d
= g
e
V
g
d
A
=
1
2
2 s in
2
(θ
W
), (10)
where the second equality is valid at the tree-level of the
Standard Model. T hese coupling constants, along with the
Weinberg angle θ
W
itself, are fundamental parameters of
the Standard Model.
In an approximation of moderately large x where sea
quark contributions vanish, R
v
1 and R
s
0. The
uncertainty in 2C
2u
C
2d
extracted from measured asym-
metry is then approximately:
δ(2C
2u
C
2d
)
2C
2u
C
2d
δA
d
A
d
1
1
Y
2C
1u
C
1d
2C
2u
C
2d
(11)
2.2 Exploring New Physics Beyond the Standard
Model from PV DIS
Although ther e exists a large amount of data confirm-
ing the elec troweak sector of the Standard Model at the
level of a few parts per thousand, there also exist strong
conceptual reasons (e.g., the so -called high-energy desert
from M
weak
250 GeV up to the Planck scale M
P
2.4 × 10
18
GeV) to believe that the Standard Model is
only a piece of some large r framework [14]. This frame-
work should provide answers to the conceptual puzzles of
the Standard Model; but must a lso leave the SU(3)
C
×
SU(2)
L
× U(1)
Y
symmetry of the Standard Model intact
at M
weak
25 0 GeV. Hence, there e xists intense interest
in the search for physics beyond the Standard Model.
The value of sin
2
θ
W
at the Z-pole (Q
2
= M
2
Z
) is mea-
sured to remarkable precision, sin
2
θ
W
[M
Z
]
MS
= 0.23120±
0.00015 [11]; however, a careful c omparison of measure-
ments involving pure ly leptonic and semi-leptonic elec-
troweak currents shows a la rge inconsistency. This strongly
suggests additional physics not included in the Standard
Model or that one or more of the e xperiments has sig-
nificantly understated its uncertainties [15,16]. Below the
Z- pole , there are only three precise measurements: Atomic
parity violation (APV) in Cs atoms [5] yields a result
which, while in agree ment with Standard Model predic-
tions, has somewhat large uncertainties, and a difficult
theoretical calculation is neces sary to extract sin
2
θ
W
from
the meas ured asymmetry. The NuTeV experiment at Fer -
milab measured sin
2
θ
W
through a careful comparison of
neutrino a nd anti-neutrino deep inelastic scattering (DIS).
Their result is approximately three standard deviations
from Standard Model predictions [18]; however, the NuTeV
result is not without considerable controversy. Most re-
cently, the SLAC E-158 [4] experiment used the asymme-
try in Møller scattering to determine a precis e value of
sin
2
θ
W
that is consistent with the Standard Model pre-
diction. A fourth measurement, the Qweak experiment [6],
is planned at JLab, and will determine sin
2
θ
W
to 0.3% by
measuring the weak charge of the proton.
Among various experimental efforts to search for new
physics, PV DIS involves the exchange of a Z
0
between
electrons and quarks a nd thus is sensitive to physical pro-
cesses that might not be seen in purely leptonic observ-
ables, such as the precision A
LR
at SLC and A
l
F B
at LEP.
The recent NuTeV [18] result o n sin
2
θ
W
at low Q
2
in-
volves a particular set of semi-leptonic charged and neutral
current reactions and disagrees with the Standard Model
prediction by three standard deviations. A precision mea-
surement of DIS-Parity will provide a clea n semi-leptonic
observable to the world data below the Z-pole and will

4 Xiaochao Zheng for the JLab E05-007 Collaboration: Parity Violation in Deep Inelastic Scattering at JLab 6 GeV
Right HRS
Left HRS
LD Target
Polarimeter
Compton
Moller
Polarimeter
Raster
BCM BPM
ARC eP
2
Luminosity
Monitor
Fig. 1. Hall A floor plan for the p roposed measurement.
provide essential clues as to the source of these discrepan-
cies.
The values of C
1q
have been determined to a reason-
able prec ision [17]. However, our present knowledge of C
2q
is poo r: δ(2C
2u
C
2d
) = ±0.24. This also affect the ex-
traction of C
3q
g
e
A
g
q
A
from neutrino scattering data. A
precision measurement of C
2q
is highly desirable to explore
possible extensions of the Standard Model. PV DIS is a
semi-leptonic process and is sensitive to the C
2q
’s, there-
fore it is complementary to other Standard Model test ex -
periments including the Qweak exper iment which stud-
ies semi-leptonic processes but is only sensitive to C
1q
’s.
For example, a la rge axial quark coupling could cause the
NuTeV effect, but canno t be seen in C
1q
. Quark and lepton
compositeness is accessible only through C
2q
but no t C
1q
if a particular symmetry, SU(12), is respected. A preci-
sion PV DIS measurement will significantly strengthen the
constraints o n these possible extensions to the Standard
Model. Possible new physics that PV DIS may be sensi-
tive to are Z
Searches, quark and lepton compositeness,
lepto quarks a nd supersymmetry. We are currently work-
ing with theorists on an updated list of new physics limits
achievable from the measurement described here [19,20].
The “mass s c ale” for which PV DIS is sensitive to physics
beyond the Standard Model would be [20]
Λ
g
=
1
q
2
2G
F
δ(2C
2u
C
2d
)
1.0 TeV. (12)
3 Experimental Se tup
3.1 Ov erview
The floor plan for our experimental setup is shown in
Fig. 1. We will us e an 85 µA polarized beam and a 25 cm
long liquid deuterium target. The scattered elec trons will
be detected by the two standard Hall A High Resolution
Spectr ometers (HRS). In the following we will describe
some details of the ex perimental setup, focusing on two
major upgrades required by the measurement.
3.2 Beam Line and the Compton Polarimetry Upgrade
We plan to use a 6.0 GeV 85 µA beam with an 85% polar-
ization. To reduce the heat impact o n the targe t, the beam
will be circularly rastered such that the beam spot size at
the target is 4mm in diameter. The b e am energy can
be meas ured to ∆E/E = 2 × 10
4
using either the ARC
or eP devices [21]. The helicity-dependent asymmetry of
the electro n b e am will be controlled by a DAQ specially
developed in Hall A for parity-violation experiments [7]. A
luminosity monitor will be used downstream of the target
to monitor helicity-dependent target-density fluctuations
and other possible false asymmetries.
We need 1% precision in the beam polarization mea-
surement in order to achieve an acceptable systematic un-
certainty on the final results. T he current Compton po-
larimetry in Hall A measures the asymmetry of Compton
scattering between the electro n beam and a high-power
IR laser (1 kW, 1064 nm) achieved by a Fabry- Perot cav-
ity. The current systematic uncertainty of the Compton
polarimeter is about 1.9% for a 6 GeV beam. In order
to achieve a 6 1% precision, we will upgrade the current
IR laser to a green laser (532 nm), and replace the c ur-
rent 600 µm micro-strips used in the electron detector by
300 µm strips. With these upgrades the total systema tic
uncertainty is expected to be reduced to 6 0.9%. These
upgrades are presently being carried out at JLab and we
exp ect to install and commissio n the new Compton po-
larimeter by late 2007.
3.3 Spectrometers and the DAQ Upgrade
We will use the standard Hall A HRSs [21] to detect
the scattered electrons. For each HRS the effective solid-
angle acceptance for an extended target is 5.4 msr and
the momentum acceptance is ±4.5%. The HRS central
momentum can be c alculated from the dipole field mag-
nitude to 5 × 10
4
and the central angle can be deter-
mined to ±0.2 mrad, giving an uncertainty of ±0.004/Q
2
on 2C
2u
C
2d
where Q
2
is in GeV
2
. Particle identification
(PID) in each HRS will be done with a CO
2
˘
Cerenkov de-
tector and a double-layered lead-glass shower detector. We
exp ect to achieve a pion rejection factor of 10
4
at an
electron efficiency of 99%.
Because of the need to separate the pion background
we must use a counting method instead of an integrating
DAQ. The detecto r signals we will use include those fr om
the two PID detectors and scintillators (for crude dir e c -
tional information). To process this information we are
considering a Flash ADC (FADC) -based DAQ presently
being designed by the Fast Electronics Group at JLab.
This design will allow for the possibility of counting ex-
periments at approximately 1 MHz with a low and pre-
cisely measurable dead time, e.g. a 1% dead time mea-
sured w ith a 0.3% absolute accuracy. The FADC fills an
on-board memory at 250 MHz with 4 µs latency (buffer
size). An on-board processor (FPGA) will analyze the dig-
itized data and perform the PID. The DAQ will be flexible
enough to accommodate a variety of experiments. A first
version of the FADC is expected to be ready by 2007.
As an alternative, a scaler-based DAQ using fast NIM
electronics is also being considere d. We plan to build both

Xiaochao Zheng for the JLab E05-007 Collaboration: Parity Violation in Deep Inelastic Scattering at JLab 6 GeV 5
APV−Ti
APV−Cs
Bates
Qweak
PDG/SLAC
SAMPLE
JLab E05−007 (Expected)
Fig. 2. The effective couplings C
1q
(left) and C
2q
(right) . The
future Qweak experiment combined with the APV-Cs result
will provide the most precise data and the best Standard Model
test on C
1q
. For C
2q
, the SAMPLE result for C
2u
C
2d
at
Q
2
= 0.1 GeV
2
[22] and the current PDG value for 2C
2u
C
2d
are shown. Assuming the SM prediction of 2C
1u
C
1d
, the
value of 2C
2u
C
2d
can be determined from this experiment
to (2C
2u
C
2d
) = ±0.03.
systems such that the FADC-based DAQ can be cross-
checked with the regular HRS counting DAQ at a low
rate (<4 kHz) and with the scaler-based DAQ at a high
rate (up to 1 MHz).
3.4 Expected Results
The asymmetry A
d
will be extracted from the measured
raw counting asymmetry as
A
d
=
A
raw
P
beam
+ ∆A
RC
EM
(13)
where P
beam
is the beam polarization. A
RC
EM
is the elec-
tromagnetic radiative co rrection a nd can be calculated to
0.2% (relative to A
d
). Pr ovided that all uncertainties re-
lated to hadronic structure are well under control, one can
then extract 2C
2u
C
2d
from A
d
using Eq.(3). The ex-
pected total uncertainty in 2C
2u
C
2d
is shown in Fig. 2
along with existing world data. For illustration, current
knowledge of C
1q
’s and their expected results fr om the
Qweak experiments ar e also shown. We expect to run E0 5-
007 in early 2009 and first results may become available
within one year of its completion.
4 Interpretation of the Data and Discussions
on Hadronic Effects
While we can extract 2C
2u
C
2d
from the measured asym-
metry, there are uncertainties coming from hadronic physics
which may complicate the interpretation of this asymme-
try in terms of Standard Model parameters. This section
will describe some of these interesting issues.
4.1 Uncert ainty from Parton Distributions and R
LT
The uncertainties due to R
s
and R
v
in Eq. (3) can be
estimated using CTEQ [23,24] and MRST [25 ,26] PDF
parameterizations. We find that our c urrent knowledge of
PDFs gives an uncertainty at the level of δ(2C
2u
C
2d
) 6
±0.0025. The ratio R
LT
= σ
L
T
is taken from a global
fit, R1998 [27]. Propagation of the uncertainty from the
this fit yields an uncertainty of δ(2C
2u
C
2d
) 6 ±0.0017.
4.2 Higher-Twist Effects
Among all hadronic effects that could c ontribute to PV
electron-scattering observables, higher-twist (HT) effects
are expected to be the most probable for kinematics at
JLab. Here higher-twist effects refer to the fa ct that the
strong interactions between the quarks become observable
at low Q
2
and the process cannot be described by the γ(Z)
exchange between the electron and a single quark (the
leading-twist process). For electromagnetic scattering pro-
cesses, these interactions introduce a scaling violation to
the structure functions for Q
2
< 1 GeV
2
that is stronger
than the ln(Q
2
)-dependence of the DGLAP equations of
perturbative QCD (pQCD). For PV e
2
H scattering, HT
effects start from twist-four terms which diminish as 1/Q
2
.
Hence, observation of any Q
2
-dependent deviation from
the expected asymmetry would strongly imply a contri-
bution from HT.
The theor y for HT is not well established. In a naive
picture, HT is expected to vanish in the asymmetry A
d
be-
cause the strong coupling between the struck quark and
other spectator quarks (the origin of HT) should not de-
pend on what type of the gauge boson (γ or Z
0
) is probing
the quark, hence the HT contributions cancel in the ratio.
In less hand-waving picture s, most of existing HT mod-
els show that the HT contribution to A
d
is below 1%/Q
2
level (see e.g. Ref. [1]), which is acceptable fo r interpret-
ing our exp ected data in terms of Standard Mo del tests.
However, these models were developed primarily during
and after the previous PV DIS experiment in the 1970’s.
A modern calculation of the HT from Q CD is needed and
we hope such calculation will bec ome available in the nex t
1-2 year(s) [19].
On the other hand, HT provides information on non-
perturbative aspects of the strong interaction, which by
itself has its own value. So fa r first-hand information on
HT comes primarily fr om experimental data, e.g. by ob-
serving the non-ln Q
2
dependence of DIS structur e func-
tions. However, such an extraction is complicated by the
determination of the leading-twist term, which ha s a large
uncertainty from the truncation of higher order terms in
α
S
. By contrast, the prospects for observing HT contribu-
tions in PV-DIS ar e relatively uncomplicated because the
QCD higher o rder terms are expected to cancel in the
asymmetry. Thus our PV DIS experiment will provide
valuable insights into the study of the non-perturbative
side of strong interactions.
One interesting remark is that it has been shown that
although the NuTeV measurement was performed at hQ
2
i
=20 GeV
2
, the HT contribution to the typically measured
Paschos-Wolfenstein (P-W) ratio could be of the same
magnitude as that of the PV DIS observable at Q
2
2 GeV
2
[28]. If the NuTeV deviation fr om the Standard

Citations
More filters
Book ChapterDOI
01 Jan 2007
TL;DR: In this article, a PV-DIS measurement using the baseline spectrometers that will exist as part of the 12 GeV JLab upgrade is presented, where the asymmetry from parity violation is large (A PV ≈ 10−4 Q 2 ).
Abstract: The couplings of leptons to quarks are fundamental parameters of the electroweak interaction. Within the framework of the Standard Model, these couplings can be related to sin2 θ W . Parity violation (PV) in deep inelastic scattering (DIS) is proportional to these couplings and hence sensitive to sin2 θ W . PV-DIS, first measured at SLAC in the mid-1970’s, was used to establish the Standard Model. The high quality and intensity of the upgraded 11 GeV CEBAF beam at Jefferson Laboratory will make it an ideal tool for PV studies. In DIS the asymmetry from parity violation is large (A PV ≈ 10−4 Q 2), allowing precise measurements with modest beam time. This talk will explore a PV-DIS measurement which can be made using the baseline spectrometers that will exist as part of the 12 GeV JLab upgrade.
Book ChapterDOI
01 Jan 2007
TL;DR: The RES-Parity experiment as mentioned in this paper was proposed to search for evidence of quark-hadron duality and resonance structure with parity violation in the resonance region in the low-Q 2, low-W domain.
Abstract: Parity violating electron scattering has become a well established tool which has been used, for example, to probe the Standard Model and the strange-quark contribution to the nucleon. While much of this work has focused on elastic scattering, the RES-Parity experiment, which has been proposed to take place at Jefferson Laboratory, would focus on inelastic scattering in the low-Q 2, low-W domain. RES-Parity would search for evidence of quark-hadron duality and resonance structure with parity violation in the resonance region. In terms of parity violation, this region is essentially unexplored, but the interpretation of other high-precision electron scattering experiments will rely on a reasonable understanding of scattering at lower energy and low-W through the effects of radiative corrections. RES-Parity would also study nuclear effects with the weak current. Because of the intrinsic broad band energy spectrum of neutrino beams, neutrino experiments are necessarily dependent on an untested, implicit assumption that these effects are identical to electromagnetic nuclear effects. RES-Parity is a relatively straight forward experiment. With a large expected asymmetry (≈ 0.5 × 10−4) these studies may be completed with in a relatively brief period.
References
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Journal ArticleDOI
TL;DR: The Particle Data Group's biennial review as discussed by the authors summarizes much of Particle Physics using data from previous editions plus new measurements from papers, and evaluate and average measured properties of gauge bosons leptons quarks mesons and baryons.

3,025 citations

Journal ArticleDOI
TL;DR: In this paper, a critical review of the current status of cosmological nucleosynthesis is given, where the baryon-to-photon ratio, ε, corresponding to the inferred primordial abundances of helium-4 and lithium-7 is presently 2σ below the value implied by the abundance of deuterium.

2,806 citations

Journal ArticleDOI
TL;DR: In this article, the NuTeV Collaboration has extracted the electroweak parameter sin 2 θ W from the measurement of the ratios of neutral current to charged current v and ν cross sections.
Abstract: The NuTeV Collaboration has extracted the electroweak parameter sin 2 θ W from the measurement of the ratios of neutral current to charged current v and ν cross sections. Our value, sin 2 θ W ( o n - s h e l l ) = 0.2277 ′0.0013(stat) ′ 0.0009(syst), is 3 standard deviations above the standard model prediction. We also present a model independent analysis of the same data in terms of neutral-current quark couplings.

462 citations

Journal ArticleDOI
TL;DR: In this paper, parity violating asymmetries in the inelastic scattering of longitudinally polarized electrons from deuterium and hydrogen were measured, and the asymmetry is (−9.5 × 10−5)Q2 with statistical and systematic uncertainties each about 10%.

455 citations

Journal ArticleDOI
TL;DR: In this article, the uncertainties on observables arising from the errors on the experimental data that are fitted in the global MRST2001 parton analysis were determined by diagonalizing the error matrix and producing sets of partons suitable for use within the framework of linear propagation of errors.
Abstract: We determine the uncertainties on observables arising from the errors on the experimental data that are fitted in the global MRST2001 parton analysis. By diagonalizing the error matrix we produce sets of partons suitable for use within the framework of linear propagation of errors, which is the most convenient method for calculating the uncertainties. Despite the potential limitations of this approach we find that it can be made to work well in practice. This is confirmed by our alternative approach of using the more rigorous Lagrange multiplier method to determine the errors on physical quantities directly. As particular examples we determine the uncertainties on the predictions of the charged-current deep-inelastic structure functions, on the cross-sections for W production and for Higgs boson production via gluon-gluon fusion at the Tevatron and the LHC, on the ratio of W- to W+ production at the LHC and on the moments of the non-singlet quark distributions. We discuss the corresponding uncertainties on the parton distributions in the relevant x,Q2 domains. Finally, we briefly look at uncertainties related to the fit procedure, stressing their importance and using $\sigma_W$ , $\sigma_H$ and extractions of $\alpha_S(M_Z^2)$ as examples. As a by-product of this last point we present a slightly updated set of parton distributions, MRST2002.

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Frequently Asked Questions (1)
Q1. What are the contributions mentioned in the paper "Parity violation in deep inelastic scattering at jlab 6 gev" ?

Results from this experiment will provide a value of 2C2u −C2d to a precision of ±0. If all hadronic effects can be understood, this result will provide information on possible extensions of the Standard Model, complementary to other experiments dedicated to new physics searches. Presented here are the physics motivation, experimental setup, potential hadronic effects and their implications, and the future of PV DIS at Jefferson Lab.