PERFORMANCE DEGRADATION IMPACT OF
POSTPONING THE DT ELECTRONICS
REPLACEMENT
1.
DT ELECTRONICS REPLACEMENT MOTIVATION
As already described in the Phase 2 Technical Proposal [1], the motivation to replace the
DT electronics is various fold, with two aspects being critical: radiation tolerance of some
components and aging.
1.1.
Radiation
Radiation tests were performed at the PSI in 2003 in several of the Minicrate boards. It was
seen that two components were particularly weak to the total dose received, the UART device,
responsible of the backup link of the Minicrate (broken at 100 Gy) and the Altera MAX7000 CPLD
EPM7128S (broken at 100 Gy). This CPLD is used in the CCB board of the Minicrate, which is in
charge of the control and monitoring of the full electronics. A failure in this component
compromises the operation of the full chamber.
The failure mode was related with the total dose accumulated, ending up in destroying the
configuration memory of the CPLD, with no possibility of recovery. The result of these tests is
confirmed by independent radiation tests performed by other groups [2], although in their case the
CPLD broke at even lower levels (7.66 Gy).
The radiation expected in the DT detector during Phase 2 has been extrapolated from
simulations and is foreseen to be up to 10 Gy. Considering the uncertainty on the value at which the
CPLDs have died, and leaving some safety margin, we do not consider safe to run the present
electronics under HL-LHC conditions. It is to be noted that once the CCB is dead, no possibility of
repair takes place until next long shutdown, when the CMS wheels are opened.
1.2.
Aging
In this text, only the aging of the Minicrates is discussed. Aging of the DT chambers has
been studied elsewhere and is considered to have small impact in the overall CMS performance.
Aging of a complex electronics system such as the Minicrate is a serious concern for
Phase 2 since the ASICs employed were produced in the late 90´s and the boards manufactured in
the early 2000´s. The system will be 25 years old, operating intensively since production through
validation, commissioning and running.
Electronics modules reliability follows a bathtub curve with a high rate at the beginning due
to infant mortality of the devices with latent failures, a constant failure rate during the normal
operation and an accelerated high rate when the system approaches to its wear-out period [3]. There
are no valid models in the literature to determine the moment in time when the electronics wear-out
starts nor at which speed. Entering into the end of life of a system is detected by the increase of
failure rates with respect to earlier periods. Operation over the first years of LHC (Run I + LS1)
shows roughly 3% channel loss with no clear acceleration in failure rate. Note however that a large
fraction of the Minicrates (40 %) had to be accessed during LS1 to repair the above mentioned 3 %,
so the maintenance of the system being efficient already requires a large access time to the wheels
during shutdowns. We do expect that the access time to the wheels during LS3 to maintain the aged
Minicrates will be equivalent to the required for installation of the upgraded electronics, or even
larger.
Considering that the design, evaluation and production of the new DT electronics will take
roughly 8 years, we consider extremely risky to wait until we start to see evidences of wear-out. As
a comparison, complex electronic systems such as computers follow a 5-year replacement policy.
Summarizing, delaying the replacement, beyond the date we can realistically build and install it
(LS3), is a serious risk to the overall CMS performance that the DT detector is not willing to take.
Figure 1: Drawing of the “bathtub” curve that represents the failure rate in an electronics system versus
time.
A degraded scenario in the DT system to study performance degradation should take into
account both contributions (radiation + aging) which translate into full chambers dead spread
around the detector. Following the geometrically dose distribution for the first, and in a random way
for the second.
2.
MUON DETECTOR PERFORMANCE DEGRADATION
The muon system was built with large redundancy to be tolerant to various sources of
inefficiencies. Apart from having a complementary trigger detector (RPCs), the DT itself relies on
four layers of chambers to cover cracks between wheels and sectors plus 8 detection layers per
chamber in the phi coordinate to insure a good segment reconstruction, even in the presence of
electronic noise or intense background. As a consequence of that, the DT system is pretty immune
to dead cells or layers inside each chamber (which is the expected aging mode for the drift tube
chambers themselves) and moderately immune to the loss of few chambers.
The performance of the DT system can be studied in terms of:


impact on the Level 1 muon trigger
impact on the muon reconstruction
In both cases, since we should foresee a tracking trigger during HL-LHC, what is critical is
to maintain the muon identification capability through all stages. It is to be noted, though, that for
very high transverse momentum (>200 Gev), the pt resolution should improve when making use of
the DT resolution information as can be seen in figure 2.
Figure 2: Resolution of the transverse momentum as a function of the generated p t for standalone, tracker and
global muon reconstruction [4].
2.1
DT level 1 muon trigger efficiency loss and rate increase
With an aged DT electronics, the impact on the level 1 muon trigger performance implies
both a lower efficiency and a higher trigger rate. In particular, a higher trigger rate of mis-measured
low pt muons, which get promoted above threshold.
Efficiency itself at a given pt threshold will be moderately impacted by the loss of DT
Minicrates (i.e. full DT chambers). In figure 3 the turn on curve for muon candidates above 20 GeV
is shown in the present (red) and aged (blue) scenario. The aged scenario has been simulated by
assuming inefficient a 31.2 % of the DT chambers distributed in the detector with higher probability
for the areas with larger radiation (Annex A).
As can be seen the efficiency in the plateau is approximately 20% smaller than in the
present system, which may be recovered with the inclusion of the RPC information (no aged
scenario is available for that).
Figure 3: Turn on curve of the DT trigger for a 20 cut of 20 GeV. This study has been done with a
run 2 Drell-Yan sample with pile up of 50.
Looser turn on curves translate into longer tails in the pt measurement capability of the
system, with low pt muons being mismeasured as high pt ones, and thus, increasing significantly the
trigger rate. This increase in rate will be dangerous not only for the standalone muon trigger which
will need higher thresholds for a give trigger rate, sacrificing physics output, but will also
complicate the matching with the tracker by having a much larger number of candidates. Moreover,
using RPC only information in this case, implies using a much coarser resolution for tracker
matching, having also an important impact in the required tracking trigger thresholds, and, possibly,
matching efficiency.
2.2
Importance of a level 1 standalone muon trigger
Maintaining a single muon hardware trigger independent of the tracker information is
fundamental for some of the exotica searches of long lived particles decaying in the detector far
away from the interaction point. As can be seen in figure 4, without a tracker independent low
threshold muon trigger, CMS will be extremely inefficient when the point of decay moves away
few centimeters from the interaction point.
Figure 4: Muon detection efficiency for a dark photon of 50 mm lifetime versus the distance from the decay
spot to the interaction point measured in the transverse plane. The blue curve is the efficiency of the
standalone muon while the red curve represents the muon measured with the level 1 tracker.
2.3
Importance of a standalone muon reconstruction
In general terms, there are two types of muon reconstruction scenarios depending on the
physics case: the ones used by B-physics where soft muons are required and only one segment (i.e.
hits in only one chamber) is needed in the muon detector (with more than one segment required
there is an intrinsic and undesired cut in transverse momentum) and the ones used in electroweak
analysis where tight muons are required and two segments (i.e. hits in at least two chambers) are
needed to insure that the track corresponds to a muon.
Even if the requirements for muon identification are not very stringent (one or two
segments), it should be noted that in Phase 2, reconstruction algorithms may be relaxed to cope with
the overall CMS aged detector, and at the same time, algorithms may need to be more restrictive to
cope with increased luminosity and pile-up. Therefore, as a starting point, it would be good to
assume that the possible improvements in the reconstruction algorithms will be compensated by the
increased difficulty and at the end, the muon identification requirements will be similar. Therefore,
the two segments per muon, which is nowadays used for the tight muon selection and for most of
the physics studies (such as the Higgs or the Z prime searches), will still be valid in Phase 2.
For the B-physics scenario (where only one segment is required), the operation of the MB1
chambers is critical, since they are the ones that will provide most of the low p t segments. On the
other hand, they are the most affected by the increased radiation levels. For the B-physics case, an
MB1 dead will mean no event reconstructed, so the impact of a degraded DT scenario is pretty
obvious.
It is to be noted that the standalone reconstruction degradation in the HLT will be of
outermost importance, since in the expected degradation scenario where a full Minicrate dies, both
the trigger and readout systems will be dead in the same region, so no recovery is possible.
2.4
DT degraded: the Higgs case
The Higgs to four muons decay channel is one of the cleanest channels and it is good to
remind that a large fraction of the muons are central. As can be seen in figure 5, up to 75% of the
Higgs to 4 muons events contain at least one muon in the barrel region.
Figure 5: Fraction of Higgs to 4 u events according to the number of muons that fall in the barrel
region.
We have studied the effect of the DT degraded scenario on the Higgs reconstruction by
using two methods, (A) a full simulation where a set of chambers are killed and (B) a toy simulation
where the cracks between the wheels are included (although not the cracks between the sectors). We
have checked that the values obtained are consistent with a slightly higher inefficiency
(approximately 2%) in the full simulation exercise, as it is to be expected. However, the toy
simulation has allowed us to rapidly test different degraded scenarios.
As an example, the reconstructed mass of the Higgs boson is shown in figure 6. In red, 73
Minicrates have been killed. As can be seen in the legend, the Higgs reconstruction efficiency is
72.7% compared with the no degraded scenario, thus, 27.3% of the Higgs have been lost. This plot
has been produced using the full simulation in CMSSW with 2012 geometry (future geometries are
not changed for what concerns the barrel muons, so no difference is expected). All three curves use
same CMSSW version and same detector configuration except for the considered DT chambers
dead.
Figure 6: Full simulation of the Higgs mass reconstructed for three scenarios. Black with no DT
aging considered, blue with a 30% of the DT cells dead randomly distributed and red with a 30% of the
Minicrates dead.
As can be seen, the impact on the efficiency is clearly visible, why the Higgs mass
resolution appears unchanged. If the DT failures caused any spoiling of momentum resolution it
would be included in the results, however, we have shown several times that no losses in
momentum resolution are observable as a result of the considered DT failure scenarios (we used the
variable (pt_reco-pt_gen)/pt_gen), so it is reasonable that the mass resolution appears unchanged.
In addition, various other points (Table 1) have been calculated by using the toy simulation
in order to estimate the inefficiency that we would obtain for different degraded scenarios in DT.
The dead Minicrates have been distributed randomly event per event, not to introduce bias in the
selection done. Note that in reality this is not the case since the radiation map will introduce higher
probability for some of the Minicrates.
Table 1:Higgs reconstruction inefficiency for a given DT inefficiency calculated with the toy
simulation (B).
% dead DT
Minicrates
Higgs reconstruction
inefficiency
5%
10%
10%
15%
15%
18%
20%
22%
30%
37%
40%
2.5
53%
DT degraded: the Z’ case
We have also studied the Z’ reconstruction scenario for a 31.2% of the Minicrates dead
distributed according to the higher radiation areas. The simulation has been done using a Z’ sample
with pile up equal 20 at 13 TeV. The efficiency loss is shown in figure 7 versus the Z’ simulated
mass.
Figure 7: Simulation of the Z’ efficiency versus the dimuon mass for a scenario where 32.1% of the
DT Minicrates where dead.
3.
SUMMARY
This is an extremely clean scenario: harsher scenarios (i.e. pile-up as large as 140) will
worsen the performance further.
References
[1] Phase 2 Technical Proposal.
[2] CSC radiation tests
[3] Electronics reliability bathtub curve
[4] CMS Collaboration, “CMS Physics TDR: Volume 1, Detector Performance and Software”,
CERN/LHCC 2006-001 (2006)
Annex A: List of dead DT chambers in the DT muon trigger simulation
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