Dear Dr. Bianchi and Dr. Kroll:
Thank you very much for reviewing PR05-007. Below please find our response
to your comments and questions:
(Comments/questions from Dr. Bianchi)
> SM ASSUMPTION : since the kinematics is slightly different from
> LOI-03-106 can you redo the simple calculation of the impact of
> using APV and Qweak uncertainties on C1u and C1d
> (instead of SM values) into the uncertainty for the
> extraction of 2C2u-C2d?
Using the expected uncertainty of the proton Qweak [which equals to
-2(C1u+C1d)]:
Delta (2C1u+C1d)/(2C1u+C1d) = \pm 4%
and the Cs-APV result Delta (-376C1u-422C1d) = \pm 0.48, one obtains
Delta (2C1u-C1d) = \pm 0.0055
The error propagation from (2C1u-C1d) to (2C2u-C2d) is an amplification by
a factor of 1/(Y*Rv), as can be seen from Eq. (23) and (25). Using the
values for Y and Rv from table 2, the Qweak values on C1q give an extra
uncertainty of
Delta (2C2u-C2d) = 0.0133 for Q^2=1.1 (GeV/c)^2,
and 0.0084 for Q^2=1.9 (GeV/c)^2.
Compared to the expected uncertainty in Table 4 where the SM values of C1q
are used, it is a negligible effect.
> JLAB vs BATES : in table 1 and fig. 1 at Q2=0.1 you show the result of
> BATES for 1*C2u-C2d with an accuracy of 0.057. In your proposal
> the accuracy on 2*C2u-C2d is 0.03 i.e. of about a factor 2 better.
> You claimed a factor 8 in improvement in the combination
> 2*C2u-C2d. Where this factor comes from? Why the combination
> 2*C2u-C2d is more important than the combination 1*C2u-C2d?
> According to your formula (3) and (4) C2u=-C2d=C2 \approx -0.03 so that
> 2*C2u-C2d=3*C2 while 1*C2u-C2d=2*C2, a part from Rad corrections.
> Why beyond SM effects would be different for the two combinations?
The factor of eight comes from a comparison to the previous SLAC data:
Delta (2C2u-C2d) = \pm 0.24. This is also the latest value published by
the particle data group. It is important to note that to determine the
value for C2u and C2d separately, one needs two different combinations.
Therefore the combination 1*C2u-C2d is as important as 2*C2u-C2d, but
cannot substitute each other.
Another thing one needs to pay attention to is that, while SAMPLE has
decreased the uncertainty in combination C2u-C2d, e-nucleon elastic
scattering experiments like SAMPLE (and Mainz 9Be) will always be limited
by the uncertainty coming from multi-quark hadronic corrections (the
anapole correction). In a sense SAMPLE does not directly measure e-quark
couplings, one has to assume they can be derived from e-N scattering, and
there is a significant systematic uncertainty coming from the anapole
correction as computed by Zhu et al, Phys. Rev. D 62 (2000) 033008. So the
e-2H DIS measurement is a more direct determination of C2u/C2d (assuming
the higher twist corrections are under control), or at least is
complementary to the SAMPLE result.
> DEADTIME : to use the counting method you plan to use FADC which are
> being developed for the 12 GeV upgrade. You wrote that a first version
> should be ready by 2007. This means that your proposed measurement will
> not run before 2008? If instead you will use the integration of the
> flux, how this is reflecting in your systematic error?
For the question, the proposed measurement cannot use flux integration
method because there is too much background (for example, one cannot
separate pions from electrons in the integrated signal).
It is true that the proposed measurement (if approved) cannot run until
the electronics is ready. However, from our discussion with people from
both the DAQ and the electronics group, the projection is that if there is
an approved and highly-rated experiment requiring the new FADC system, it
should not be a problem to have it ready in two years. So the earliest time
could be 2007. The advantage of using FADC is that it is what we will make
ultimately at JLab, has virtually zero deadtime, and will have more general
applications. Also, full information of events can be sampled at low rates
for later off-line analysis.
There is an alternative option to the FADC design -- the NIM/scaler-based
fast counting DAQ as proposed in LOI03-106: Signals from the scintillators,
the gas cherenkov and the lead glass counts are fed to a set of logic
units and the events are counted by either the electron or the pion scaler.
The threshold of these logic units can be easily adjusted and there will be
multiple e and pi scalers to count events using different particle
identification creteria. This scaler-based counting DAQ is easy to make and
will probablly be ready sooner than the FADC. The disadvantage is that only
event counts are recorded by scalers and no more information will be
available if one wants to study events off-line. Also it will bring extra
work to the electronics group and thus requires extra expenses (manpower
plus $50-100K), and it is highly customerized and will not have other uses
than the proposed measurement.
> PION CONTAMINATION : in the systematics you treat the possible pion
> contamination as a dilution of your asymmetry. I was wondering if
> possible larger effects may arise from a beam-spin asymmetry in the pion
> production in the case of mis-identification of an electron. Due to the
> large momentum setting of the spectrometers you will select pion at
> large z where the BSA is large if exact integration over the final state
> azimuthal distribution is not obtained (as it should be in principle in
> inclusive measurement).
There are three different processes that can contribute a single
beam-spin asymmetry (SSA) to pion events:
1) Existing data from HERMES and CLAS on the single beam-spin asymmetry of
pion production are from semi-inclusive processes (e,e'pi). In their case,
the beam and the scattered electron defines the scattering plane and the
pions are observed to have %-level asymmetries as a function of the final
state electron's azimuthal angle defined by the scattering plane. For the
proposed measurement, the pions come from inclusive (e,pi) process and as
far as there is no scattered electrons being detected in coincidence, the
electrons are integrated exactly over their azimuthal distributions.
Therefore there sould be no contribution from the beam single-spin
asymmetry to our measured asymmetry.
2) Parity violating SSA in (e,pi): this is suppressed by a few orders of
magnitude, as described in section 3.4 (page 20) of the proposal.
3) Parity-conserving SSA from two-photon-exchange: analyzing power is of
the order 10ppm and shows both in (e,e') and (e,pi'); further multiplied
by the beam transverse polarization (1%), this will cause a negligible
effect to the measured asymmetry.
On the other hand, one can check the pion beam SSA by tuning the beam to
be polarized completely in the vertical or in the horizontal direction
(transverse to the beam-line). This kind of transverse running has been
achieved during the 2004 running of HAPPEX-II.
Because the typical transverse polarization for a longitudinally-polarized
beam is 1-2%, and can reach 80% during the transverse-running; also pion
events will be used for the transverse running and are suppressed by a
factor of 10^3 for normal running, 1 hour of transverse running using the
pion events will be sufficient to limit the error on the measured asymmetry
Ad due to pion contamination to below 0.2% (negligible compared to the 2%
statical uncertainty). The beam time request for transverse running will
be one shift (8 calendar hours, including data taking and beam tuning) at
each Q^2 value. The total request is 12 PAC hours.
> HIGHER TWIST : you mention that "reasonable" expectation for HT are
> smaller than the precision of the measurement. If so, does it means
> that, with the proposed two Q2 values, very likely no 1/Q2 extraction
> will be possible? On other hand if you found large effects, how you can
> separate HT from deviations beyond the Standard Model
The higher twist (HT) effects are common concerns when performing e-2H PV
DIS measurements (The same concern was expressed by Dr. Kroll, message
attached below, and the theory group reviewers).
A 1/Q^2 extraction is possible by fitting to the measured asymmetries at
Q^2=1.1 and 1.9, the precision of the extraction on the HT coefficient
will be \pm 7.5%/Q^2.
To answer the 2nd question, this is why measurements at two different Q^2
are important. Deviation from the SM is likely to be Q^2 independent, and
the deviation from higher-twist effects is believed to have a 1/Q^2
dependence. Therefore if there are large deviations from the SM, one can
have an idea whether it comes from the HT or new physics by comparing the
results at two Q^2 points.
> BEAM TIME : you divided your beam time schedule in 2 phases. Suppose
> to use in phase I both spectrometers for the Q2=1.1 and to found a
> result in agreement with SM showing minimal HT. Then, do you think that
> the measurement at Q2=1.9 will be still necessary?
Because of the Q^2 dependence of the higher-twist effect, a comparison of
measurements at two different Q^2 points as proposed is important in
distringuishing SM deviation from HT effects. Assuming result at Q^2=1.1
agrees perfectly well with the SM, there might still be a chance that
there is non-zero contribution from HT and new physics, but they cancel
each other. Therefore measurement at Q^2=1.9 is as important as the lower
Q^2 measurement.
One also needs to be aware that due to the limit momentum range (up to 3.1
GeV/c) of the Right HRS, one cannot use it for the Q^2=1.1 measurement
(E'=3.6 GeV/c).
(Comments from Dr. Kroll)
> I have read your proposal and thing that this is an important attempt
> to measure some of the fundamental parameters of the standard model.
> My concern is the estimate of size of the higher twist effects and its
> influence on the extraction of SM parameters. It is clear that as yet
> there is no theory of higher-twist effects. All estimates are based on
> often very old models. Thus, there is a potential danger in the
> interpretation of your results. In my opinion it would be helpful if you
> could clearly identify a higher-twist effect in your experiment, for
> instance by seing its 1/Q^2 dependence. If I understand your proposal
> correctly, you don't expect to see a higher-twist effect at Q^2=1.9
> GeV^2. Thus, you are left with a measurement of it at only one value of
> Q^2.
If we believe in existing calculations, it is true that we do not expect to
see HT effects at Q^2=1.9 GeV^2 with the proposed precision. However, it is
not quite correct to say that we are left with measurement of the HT at only
one value of Q^2. Existing calculations also very small HT contribution to
1.1 GeV^2. On the other hand, if there is an unexpectedly large deviation
at 1.1 GeV^2 due to the HT, we will also see a deviation at 1.9 GeV^2, just
with smaller (half) size. As we stated in the answer to Dr. Bianchi's
question, the HT can be clearly identified by a 1/Q^2 fit to the measured Ad
at Q^2=1.1 and 1.9 GeV^2.
> A question of minor importance: what is the definition of u_V(x) and
> u_sea(x) below (13)?
u_V(x) and u_sea(x) are the valence and the sea u quark distributions,
respectively. While it is sometimes not clear how to define valence and sea
quarks theoretically, experimentalists usually assume u_sea(x) = bar u(x)
and subtract u_sea from the measured u(x) to get u_V(x). This was also used
in the proposal as some of the PDF fits do not give u_V and u_sea separately,
but only give u(x) and bar u(x).
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A few more remarks about the higher-twist effect:
(A) In addition to the calculations listed in the proposal, we have received
more calculations on the HT contribution to Ad by Dr. Giangi Sacco. His
Ph.D. thesis was focused on hadronic effects of PV-DIS and was under the
supervision of Dr. Michael Ramsey-Musolf. Dr.Sacco's calculations show that
the HT contribution to Ad is (0.2-0.4)% for both Q^2=1.1 GeV^2 (E'=3.66 GeV)
and Q^2=1.9 GeV^2 (E'=2.66 GeV). Attached to this email please find a plot
to show \Delta_Ad(HT)/Ad vs. E' for the proposed kinematics (top panel is
for Q^2=1.1 and bottom panel is for 1.9 GeV^2, blue, red and green curves
are results from three different MIT bag models).
(B) Just because there is no theory of HT effects being confirmed by data
yet and all estimates are based on old models (as commented by Dr. Kroll),
it is important to start DIS-parity measurement now and see if HT effects
are really present in PV DIS: How big are they? Do they behave like what
we expected? Do they affect our interpretation of the SM test and how?
Therefore, although the proposed measurement cannot provide a precise
measurement (to 1% level) of the HT, it is the best place we can start from
and will provide the first results of both the HT and the SM test from PV
DIS.
As for how to interprete the results at two Q^2's from the proposed
measurements, provided that there is no reliable calculation of the HT,
one possible strategy is the following:
(1) If large deviations are observed at both Q^2 measurements and appear
to be linear in 1/Q^2, then the deviation is likely to be coming from
HT effects. A 1/Q^2 fit will be preformed and the HT coefficient will be
extracted. Meanwhile, such results will have important impact on planning
the future 12 GeV program and other DIS analysis (as described in the
proposal).
(2) If large deviations are observed at both Q^2 measurements and appear
tohave the same size (Q^2-independent), then the deviation is likely to be
coming from new physics. A 1/Q^2 fit will be preformed and deviation from
the SM will be quantified. Again, the fact that the HT effects appear to
be negligible will have impacts on other DIS analysis.
(3) If no large deviation is observed at both Q^2 measurements, then it is
likely that the contribution from new physics, as well as from the HT, is
negligible.
I hope all your questions/comments are answered/addressed in this message.
Please do not hesitate to contact me should you have more questions.
Thank you much!
Best regards,
Xiaochao Zheng
(also for Paul Reimer and all collaborators).
Xiaochao Zheng
xiaochao@jlab.org
Medium Energy Group, Argonne National Lab
Stationed at: MS16B, 12000 Jefferson Ave, Newport News, VA 23606
Tel: (757)269-5923
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