Status and Plan for the Central Preshower

D. Alton, A. Askew, P. Baringer, D. Coppage, J. Gardner, A. Magerkurth,
Y. Maravin, J. Qian

Abstract

The D0 central preshower detector is designed to improve electron and
photon identification and to restore some of the electromagnetic energy
resolution, which is otherwise degraded by the presence of the solenoid.
This document will give a brief description of the detector, describe
its current operating capabilities and the plans for how to operate it
in the long term. Specifically this document discusses why the CPS group
plans on changing the operating gain at the end of the Oct. 2003
shutdown.

1\) Introduction

The central preshower (CPS) is situated between the solenoid and the
central calorimeter. It is constructed from a lead radiator outside the
solenoid and triangular scintillator strips arranged in three
cylindrical layers surrounding that. The light from the scintillator is
wavelength shifted by fibers that are inserted in the center of the
triangular scintillator, this fiber is then coupled to a clear
waveguide. The waveguide carries the light to VLPC cassettes (described
in more detail elsewhere) that transform that light into an electrical
signal that is read out of AFE boards. Details of its construction can
be found in D0 Notes 3014, 3647, and 3765. One change from note 3014,
which people often miss is that the idea of tapered lead was never
implemented. Another major change between the detector designed and
implemented is that the AFE boards used only have one electronics
channel per detector channel, while the design called for 2 readout
channels with different gains. This would have allowed a larger dynamic
range.

The central preshower has been fully instrumented and operating since
Apr. 17, 2002. It has run in two different modes and probably will run
in a third soon. The different modes are changes to the electronics
gain, which basically means operating with different dynamic ranges. The
first transition in gain occurred Nov. 1 2002, and we plan to change the
gain again on Nov. 22 2003. We believe this change will be the last
change of its kind unless the collaboration decides to pursue creation
of AFE II's, which is still under study. This document is being created
and released now, to give all of D0 the chance to comment on these plans
before they go into effect. To allow for educated comments, the document
will include a section that describes the current status and
capabilities of the detector. Following that we will describe what we
believe the right way to operate the detector is and why.

2\) Current status

In order to understand the status it helps to recognize the goals at
design. The CPS had two basic goals: to aid in electromagnetic (EM)
identification, and to recover electromagnetic energy resolution, which
was lost by introduction of the solenoid and other material in the
tracking volume. Electromagnetic identification can be further broken
down into: fake electron rejection, a soft electron tag for b-jet
identification, and separation of π^0^ from γ. The energy resolution has
a more continual nature, in that we don't have the dynamic range to
improve the resolution everywhere. What energies coupled to the
saturation point (and the electronics gain).

This section will discuss the current alignment, reconstruction
efficiency, calibration, and energy studies.

2a) Alignment

The alignment is in good shape, but can still be improved. The expected
spatial resolution for the gain we are operating is 1.5 mrad in phi (2.5
mm in z) for 50 GeV electrons. In electrons from Z's, we find a
resolution of 1.58 mrad (3.2 mm). There is some evidence for a
misalignment, and Don Coppage has agreed to try to improve this. Note
that Monte Carlo studies indicate reducing the gain ought to improve
this resolution to about 1.0 mrad (1.8 mm) for high pt (50 GeV)
electrons, however if there is a misalignment much of that improvement
will not be realized.

2b) Reconstruction Efficiency

![](media/image1.png){width="6.0in" height="4.5in"}

**Figure 1** - **test the efficiency for electrons to pass a cut based
on the CPS discriminate,**

for a sample of Z candidates (black) and QCD-EM objects (red).

Studies have indicated that the CPS is about 97% efficient for EM
objects. It is hard to define efficiency here, because as one opens up
the window on a spatial match one allows fake matches, so to demonstrate
the usefulness John Gardner has constructed a discriminate based on CPS
variables. He uses default reconstruction, which is described in D0 Note
4014. He used a sample of Z candidates as the signal and a sample of
QCD-EM events as the background. The results are shown in Figure 1.

2c) Calibration

The calibration of the CPS was driven by results presented in D0 Notes
3535 and 3572 these results explained that the cps needed channel by
channel calibrations that were good to approximately 10%. The basic idea
was that energy would be defined:

Energy = (ADC- \<Pedestal\>)/MIP

Where ADC is the read out ADC value, \<Pedestal\> is the mean ADC value
of the pedestal, and MIP is the separation between the mean of the
Pedestal and the most probable value of the minimum ionizing particle
peak.

![](media/image2.wmf){width="6.040157480314961in"
height="3.7299212598425195in"}

Figure 2 - the average pedestal value for 64 channels, and the RMS of
those 64 channels over an eight month time span. Each line represents a
known change. The first line is swapping AFE boards to implement cryo
modifications, the second is when sequencer firmware changed, the third
is new clockgen code, the fourth was an attempt to implemented 264
readout, and the last was correctly implementing 264 readout.

The pedestal mean is extracted on a channel by channel basis from the
calibration for the whole central fiber tracker (CFT) system. The
pedestal mean is very constant over long time scales.

Figure 2 shows the mean pedestal for 64 channels, and the RMS of the
mean of those channels over an eight month time period in 2002. The
vertical lines represent known changes in either the hardware or
firmware of the AFE board. One can see that except for these known
changes the pedestal is constant over months. However the pedestal does
change on short timescales; specifically it has a crossing dependence.
As most people know the Tevatron has 36 filled crossings per turn, these
crossings are divided in to 3 super-bunches each with 12 filled
crossings. The pedestal in the CFT system varies as a function of these
12 crossings. Figure 3 shows the pedestal deviation as a function of
crossing from its nominal value for an individual channel.

![](media/image3.png){width="5.75in" height="3.9298611111111112in"}

Figure 3 - The pedestal deviation from nominal as a function of tick,
for a specific CPS channel.

The pedestal splits, by about 5 ADC for the odd numbered crossings, and
is slightly (about 1 ADC) above nominal for the first crossing. The
behavior of the first crossing wouldn't deserve comment except that this
effect is larger in the tracking portion of the system. Neither of these
problems are serious for most uses of the CPS, because EM objects leave
10's of MIP's in the detector and these effects are at the level of half
a MIP or less; however they do complicate the identification and
measurement of the MIP signal. Additionally this could be a significant
bias in the energy measurement, if all the pedestals split up or down
uniformly. There are studies underway to remove this effect.

Measurement of the MIP peak is complicated by the way the pedestal
issues cited above cause the threshold to be set higher than originally
anticipated. Previous studies (D0 Notes 3765 and 3572) indicated that
the zero suppression threshold would be around 1 photoelectron or
roughly of 1/10^th^ of a MIP, however we find that we need to operate
with the threshold more like a third of a MIP. A threshold this high
will influence the measurement of the MIP peak value. Additionally the
thresholds are not very uniform. Due to these issues it became clear
that one could not simply look at isolated singlets to perform the MIP
calibration. Instead, we found we needed to look at isolated singlets
which were matched to a track.

![](media/image4.png){width="5.99375in" height="4.470833333333333in"}

Figure 4 - shows the extracted MIP distribution for two channels which
have specific types of behavior.

The MIP distribution is shown in Figure 4 for 2 channels. The black
distribution is the energy defined above, and the red has been corrected
for path length as determined by the track eta. (Recall that tracks that
pass through the scintillator at an angle give off more light than those
with normal incidence.) These channels are chosen to represent two types
of anomaly. Specifically note that the channel on the right has a very
hard left hand threshold, while the one on the left shows a bit of the
pedestal leaking in. These differences are correlated to channels on a
specific chip of the AFE boards. It turns out that it is difficult to
automate fitting of the left hand channels, while it is easy to automate
fitting channels like the right hand channel, however there is a large
uncertainty in the fit of the right hand channel due to the possibility
that the threshold is significantly cutting into the MIP peak. For a
handful of chips these two channels were fit as well as possible by
hand, and their ratios were taken. It is believed the two sets ought to
have the same light yield on average; however the channels on the right
measured a mean 1.11 ± 0.07 times more ADC then those of the left. While
this is statistically negligible it gives us some feeling for the
systematic uncertainty associated with these fits. Another measure of
the systematic uncertainty is to refit the same set of data. The initial
calibrations were extracted from 100,000 events in physics run 167,663,
the same technique was used to extract a value for the MIP after
applying the initial calibration. The sample used had a large overlap
with the original sample. In principle this should result in every
channel giving exactly 1. The distribution of values is found in Figure
5a.

![](media/image5.png){width="2.873611111111111in"
height="2.352777777777778in"}![](media/image6.png){width="2.873611111111111in"
height="2.352777777777778in"}

Figure 5 - 5a shows the distribution of re-extracted MIP values in the
same sample as they were originally extracted. Figure 5b shows
difference in MIP extracted from run 167663, and those extracted from
175054, divided by the value measured in 175057.

Figure 5b shows the difference between the number extracted the second
time run 167663 was processed and the value extracted from run 175054 (5
months later). Note that this includes movement of electronics (AFE
boards), which occurs occasionally and has not been removed.

The issues above demonstrate that it will be very difficult and
conceivably impossible to achieve calibration errors of 10%. The bright
side of this is that the value of the calibration seems stable at the
same level of its initial uncertainty so that recalibration isn't
necessarily an important aspect. Given these concerns it was decided to
re-examine the need for 10% uncertainty; which will be addressed later.

![](media/image7.png){width="3.25in" height="2.216666666666667in"}

Figure 6 - 6 a and b are a sample of Z and J/ψ to electrons from the
data, 6a shows the average strip energy of the central strip in MeV
versus the reconstructed calorimeter pt, and 6b shows the CPS cluster
energy in GeV versus the reconstructed calorimeter pt.

2c) Energy measurement

Figure 6a and b show energy correlations between the calorimeter and the
central CPS strip and the calorimeter and the whole CPS cluster
respectively. The central strip is saturated at about 25 GeV, and that
the whole cluster becomes insensitive (due to saturation) by about 35
GeV. Due to this (and the meagerness of our J/ψ to electrons sample at
the time), attempts to improve the energy resolution have used Monte
Carlo (MC) samples.

This set of studies on EM energy resolution started with a sample of Z
to electron MC, where each strip of the CPS was made to saturate at 60
MeV. One can match reconstructed electrons to truth electrons with a 0.2
x 0.2 (eta x phi) box, and define:

**χ2 = Σ(α0\*Ecps + α1\*Eem1 + α2\*Eem2 + α3\*Eem3 + α4\*Eem4 + α5\*Efh
--Emc)**

When one minimizes with respect to the layer weights (**α**) one gets a
set of 6 linear equations for 6 unknowns and one can solve the matrix to
find the optimal layer weights. One can also do this ignoring the CPS
terms and this was done so that you can see how much it affects the
results to apply the layer weights to the same sample they were
extracted from. If one then fits a gaussian to the distribution of :

**(α0\*Ecps + α1\*Eem1 + α2\*Eem2 + α3\*Eem3 + α4\*Eem4 + α5\*Efh
--Emc)/E~mc~**

The results using the original layer weights (ie straight from the MC)
one finds a resolution of (3.34 ± 0.04)%, with layer weights extracted
without the CPS it is (3.42 ± 0.04)%, and including the CPS one gets
(3.01 ± 0.04)%. There is a relative improvement of 10%.

![](media/image9.wmf){width="5.78125in" height="3.46875in"}

Figure 7 - shows the strip saturation in GeV along the x axis, and the
energy resolution on the y axis, however the last two points are the
calorimeter only resolutions.

![](media/image10.wmf){width="3.25in" height="2.046527777777778in"}

Figure 8 - 8a shows how calibration uncertainty affects the energy
resolution for a saturation of 20 MeV. Note that the first 2 points are
calorimeter only. For the MC sample each strip was smeared by a gaussian
with a mean of the correct (original Monte Carlo) value and a sigma
which is the number along the x axis times the correct value. So when
the x axis is 1, the energy is smeared by 100%. Figure 8b shows the same
thing for a saturation of 60 MeV.

The results of the saturation study are shown in Figure 7, and indicate
that higher saturation is better for electrons from Z's. Single electron
MC was studied to see how things behave if saturation cutoffs larger
than 60 MeV are used, and the resolution flattens out. Further studies
also showed that one could achieve improvements of 15% if one used
larger clusters and allowed non-contiguous 'clusters'. Now we shall
examine how saturation and calibration affect that. The results from
smearing the calibrations are shown in Figure 8a and b. The results of
the saturation and smearing studies clearly indicate that it is better
to have a large uncertainty on the energy than to saturate. These
studies seem to indicate that a 50% uncertainty on the relative
calibrations is ok, but that is in conflict with note 3535. Further
investigation revealed that the original study smeared the whole CPS
energy by 10%, which is not representative of calibration uncertainties.

3\) Summary and Plans

In summary, the CPS is basically ready for all uses, except improving
the EM energy resolution for high pt EM objects. Nothing has been shown
for γ-π^0^ separation, studies have begun and there are no known
problems. It is felt that the improvement in EM resolution offered by
the CPS is nearly essential to a W mass analysis. We plan to change the
gain such that strips with 60 MeV saturate, which means that electrons
with pt's around 75 GeV will have saturated strips. Additionally
electrons with around 35 GeV pt will saturate if they have the highest
eta that still hit the CPS, however generally this type of electrons hit
the ICD region and are not used for analysis. This section will discuss
other options we have, what damage could be caused by this, and present
the detailed plan.

3a) Other options

There are two basic ideas for other ways to deal with this problem.
These ideas are to treat the detector as a digital calorimeter (ie.
count strips) or model the energy lost by saturation and correct for the
effect. Both ideas have been briefly studied.

In the MC studies discussed above it was discovered that a better
improvement on energy resolution was possible by integrating all energy
in an eleven-strip window. (This technique will be more sensitive to
occupancy and overlaid min-bias events, and thorough studies should be
done to determine what the best technique is long term.) Using this
technique a relative improvement of 15% was achieved. That is to say the
resolution with the calorimeter and CPS is 15% better than the
resolution of just the calorimeter. Using the same 11 strip window but
counting strips rather than summing energy one achieves a resolution
which is 10% better than the calorimeter alone. Which gets one back to
the clustered energy result.

A simple model of the energy lost due to saturation being a linear
function of the energy in the nearest unsaturated strips worked well.
Using the MC we applied saturation, but kept track of how much energy
was lost by saturation, and extracted a slope and intercept from
correlations to the nearest unsaturated strips. If one corrects using
this model one finds that the energy is smeared by about 10%.

Both of these techniques provide reasonable alternatives to changing the
gain on the detector. They aren't as good, but they come close.

![](media/image12.png){width="5.991666666666666in"
height="4.139583333333333in"}

Figure 9 - Figure 9 shows the fraction of J/y electrons with 1 or more
strips per layer as a function of the strip threshold in GeV. The lowest
line has all three layers of the CPS having at least 2 strips. One can
see that a strip threshold above \~ 2 MeV cause noticeable inefficiency.

3b) The downside to changing.

There are a number of effects, which may cause problems if we change the
gain. First off, the energy now has a larger granularity. The MIP peak
would now be 6 ADC counts above the pedestal mean. So further
calibration at this gain will be basically impossible. The other worry
is that changing the gain may negatively impact things for the low
energy EM objects. The ways that could happen, is that the readout
threshold would effectively rise and reduce the efficiency for making
CPS clusters, or reduce the number of strips in those cluster which
would reduce the information in them.

Note that calibration issues above indicate that recalibration isn't
necessary. If someone wants to test the channel by channel calibration
at a later date some special runs would be required, but could occur
only every few months. The other worry concerning how many strips are in
a cluster is addressed by looking at J/ψ events. Figure 9 shows the
efficiency for the cluster attached to a J/ψ electron to have 1 or more
strips in each CPS layer. When a cluster has only 1 strip, its spatial
resolution isn't as good, so those have been shown. The current
threshold is around 1/3 of a MIP (or around 0.4 MeV), and we plan to
change the threshold by about a factor of 3, so in the worst case the
threshold would be around 1 MIP (or 1.2 MeV). This would mean around 10%
of clusters have only 1 strip for at least 1 layer, however the
efficiency for finding clusters would still be in the high 90's. This
study makes us comfortable that the change (in the worst case) only has
a small impact on the lower energy EM objects and conceivably has no
nearly zero impact.

3c) The Plan

While a reasonable fraction of the benefit of changing the gain can be
achieved by other means, all the studies indicate that the risks to the
lower energy physics is small, and so we still plan to change the gain.
The gain change has two components that can be addressed separately: The
overall change in dynamic range (which we have focused on), and the idea
that we can make the detector have a more uniform saturation value. The
uniform saturation value is being pursued to make analysis and
simulation of the detector simpler. Currently the experiment is in an
accelerator driven shutdown. This shutdown will end on or around Nov 11.
The plan is to come up from the shutdown in this new configuration where
each SVX that reads out the CPS is adjusted to try and give the best
dynamic range, and made to have as uniform a saturation point as
possible. The saturation point that will be aimed for is 60 MeV (at this
saturation it is believed less than 10% of electrons from Z's will have
a saturated strip). Currently there are no plans to recalibrate in the
future. The efforts of the CPS group will be focused on making data and
MC agree, and improving the software.

4\) Summary

The CPS is ready for use. The one thing that is less than optimal is
it's improvement in energy resolution for high pt electrons, but that
will be fixed after Nov. 11^th^ 2003. Please let us know if you feel we
have forgotten something, or if you feel our efforts should be devoted
elsewhere.