See also: CMS DP-2020/011

Characterizing background in terms of trends vs LHC instantaneous luminosity and of spatial distributions, helps to understand its sources and to devise mitigation measures to prevent detector early ageing, especially in view of the Luminosity increase expected at High Luminosity LHC.

During LHC Run2, the background in the CMS Drift Tubes (DT) was monitored both by means of online anode current measurements and offline analysis.

Currents show a linear dependence on Instantaneous Luminosity, whose slope depends, in turn, on the applied High Voltage and on the chamber position within CMS.

The observed position and direction of offline reconstructed track segments supports the interpretation of background being made of two different components: one coming directly from the interaction point, the other coming from outside the detector, through a so called “neutron gas” setting up in the cavern.

Trend of Currents versus LHC Instantaneous Luminosity

Currents drawn by the DT anode wires are good estimators of background.
Since they are expected to depend on LHC Instantaneous Luminosity, they were measured on-line as functions of Luminosity.
All DT chambers showed a linear trend, whose slopes depend on the chamber position and High Voltage Setting.
This plot shows the change of current slope when lowering High voltage, for one of the most exposed chambers.


HV Setting Evolution

The features of background spatial distribution within the DT detector were clearly observed already at the beginning of LHC operation (see results at LHC Run 1)
In order to prevent early detector ageing caused by charge accumulation, the Anode High Voltage in the most exposed chambers was progressively reduced, in 2017 and 2018, starting from the nominal value of 3600 V (applied until 2016), according to the sketch below. The loss of efficiency caused by HV reduction was checked and found to be negligible.


Slopes of current trends vs LHC Luminosity

Being the trends of currents vs LHC Luminosity linear in all chambers, the slopes obtained by fitting these trends can be used to characterize the background.
The plot shows the slope of current vs LHC Lumi., chamber by chamber.
Observed slopes were corrected for different HV applied (see p.2) and all referred to the value of 3550 V.

The plot confirms the known background spatial distribution (highest in the MB1 chambers of external wheels and in MB4 chambers of top sectors).
It also shows the effect of prototype shields that had been installed above top MB4 chambers of wheel -2 and +2.


Background track segments reconstructed in the DT detector

In the offline analysis, a background DT segment was defined as any track segment that was reconstructed within DT chambers which, in the considered event, weren’t crossed by any reconstructed muon:

  • for muons reconstructed with tracker information, the inner track was extrapolated to the DT
  • for muons reconstructed using only information from muon detectors, the presence was checked of DT segments associated to them

As expected, candidate background segments have on average less associated hits w.r.t. signal segments (background segments with > 6 associated hits were proved to come mostly from other bunch crossings)


Distribution of the number of hits associated to signal and background segments, reconstructed in all DT chambers using a data sample of events selected to contain a Z→µµ decay

DT background segment direction and position: MB1 station

Background segments reconstructed in the MB1 station were propagated in the z-R plane in order to observe their position and direction.
(The detector layout was superimposed to the plot in order to ease its interpretation.)

The vast majority of them point to the calorimeter gaps and originate from conversions in the solenoid or in the iron yoke.


DT background segment direction and position: stations 1 to 3

Maximum background density in MB3 is ~1/50 of that in MB1.

A component is visible of track segments pointing to the interaction region, most of which passing near the gaps between wheels. These are signal segments that failed to be associated to a muon track: either because the extrapolation of the muon track passed through a gap, or because they were out-of-time (i.e. generated at different bunch crossings).

Maximum background density in the MB2 station is ~1/8 of that in MB1.
It shows both components visible in MB1 and MB3.


DT background segment direction and position: station 4

Background segments reconstructed in the MB4 station were propagated in the R-φ plane in order to observe their position and direction.
(The detector layout was superimposed to the plot in order to ease its interpretation.)

The vast majority of them concentrate in the top sectors and come from outside the detector.

Their density is lower in Sector 4 (Φ = 90o) where shields were installed over Wheel+2 and Wheel-2.

This figure is consistent with the hypothesis of a neutron gas setting up within the cavern space.

Approval13012020Fig7.png -- FrancescaCavallo - 2020-02-07

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Topic revision: r2 - 2020-02-14 - FrancescaCavallo
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