Preliminary results on the performance of the CMS Drift Tubes at LHC in 2017

The performance of Drift Tubes was studied with a subset of 2017 data and, where appropriate, compared to last year. Results are presented for Local Trigger performance, Hit Efficiency and Time Resolution.

See also: CMS DP-2017/047

TRIGGER PERFORMANCE

The DT Local Trigger efficiency was computed as described in [1].

The main difference in the trigger configuration w.r.t. last year is the implementation of the TwinMux SuperPrimitives which exploit RPC time information and therefore allow for a more accurate BX assignment [2].

[1] CMS DP-2016/046
https://twiki.cern.ch/twiki/bin/view/CMSPublic/MuonDPGPublic160729
[2] CMS-DP 2016/074
https://twiki.cern.ch/twiki/bin/view/CMSPublic/MuonDPGPublic161124

RESULTS

DT Local Trigger efficiency, station by station, as a function of η of the reconstructed muon
2017 results are superimposed on 2016 ones.
The efficiency gain observed w.r.t. last year is mostly due to the implementation of the TwinMux SuperPrimitives that exploit RPC time information to improve the BX assignment.

DT Local Trigger Efficiency, station by station, vs pT of the reconstructed muon
The pT range shown here is wider than explored in the past [1].
The efficiency slightly decreases with increasing momentum: this result is consistent with past measurements at test beam [2].

[1] CMS DP-2016/046
https://twiki.cern.ch/twiki/bin/view/CMSPublic/MuonDPGPublic160729
[2] CMS NOTE 2006/072. NIM 579, 951-960
http://www.sciencedirect.com/science/article/pii/S0168900207012168?via%3Dihub

Out-Of-Time fake primities, station by station, vs pT of the reconstructed muon, 2017 data
The plot shows the fraction of DT primitives at BX=0 and associated to good muon tracks, that are preceded by another primitive in the same chamber.
The reduction of the Out-Of-Time fraction w.r.t. to 2015 [1] is mostly due to the implementation of the TwinMux SuperPrimitives.

[1] CMS DP-2015/060
https://twiki.cern.ch/twiki/bin/view/CMSPublic/DTDPGResults201120154

DT Local Trigger Efficiency, station by station, as a function of pT of the reconstructed muon
2017 results are superimposed on 2016 ones.
The efficiency gain observed w.r.t. last year is mostly due to the implementation of the TwinMux SuperPrimitives that exploit RPC time information to improve the BX assignment.

DT Local Trigger Efficiency in 2017, shown chamber by chamber
The two chambers with lower efficiency are affected by hardware problems, one of which cannot be fixed before LS2.

DT Local Trigger Efficiency, in 2017, shown station by station, as a function of η and φ of the reconstructed muon
The effect of “chimney” chambers with reduced acceptance in YB-1/Sec3 and YB+2/Sec4 is clearly visible in the first station.
The effect of the two MB3 chambers with hardware faults is also visible in this plot.

HIT EFFICIENCY

METHOD

The DT efficiency to detect a single hit was defined and measured as the ratio between the number of detected and expected hits. The position of expected hits was determined using sets of well reconstructed track segments: - at least 7 or at least 3 hits were required to be associated to a segment, in the φ and θ view respectively - the segment itself was required to cross the chamber with an inclination lower than 45 degrees. No requirement was applied to the layer under study. To avoid any bias, segments crossing known dead cells were rejected.
The intersection of such a high quality track segment with the layer under study determined the position, therefore the cell, where a hit was expected. The cell was considered efficient if a hit was found within it.

EXPECTED CHANGES

In 2017 the High Voltage was lowered by 50 V, from 3600 to 3550 V, in the DT chambers most exposed to radiation background, in order to preserve them from ageing. These chambers are:
- the MB1 chambers of Wheels – 2 and +2, that are exposed to the background produced by Pile-Up and coming from the interaction region
- the MB4 chambers of sectors 3, 4 and 5 ( the so called “MB4-Top” chambers) that are exposed to background produced by neutron gas outside the detector.
The lower HV is expected to produce a slight loss of hit efficiency: this was already observed in the last period of 2016, when the HV was lowered in the MB4-Top of Wheel -2.

RESULTS

DT Hit Efficiency distribution, 1 entry per chamber, computed as described in the text
Final results were reweighted to take dead cells into account. 2017 results are superimposed on 2016 ones.
The slightly reduced efficiency is due to some chambers having, this year, a lower High Voltage, in order to preserve them from radiation ageing.

Hit Efficiency trend for the Φ layers of the MB2 chambers, wheel by wheel
Dead cells were excluded from the efficiency computation.
MB2 chambers are kept at the same HV in 2017 as in 2016: consistently, their efficiency is constant in time.

Hit Efficiency Trend for the ϕ layers of MB1 chambers
Dead cells were excluded from the efficiency computation.
A 0.5% drop is observed, in 2017, in Wheels -2 and +2: this is due to lowering the High Voltage by 50 V, in order to preserve them from radiation ageing.

Hit Efficiency Trend for MB4 chambers of Sectors 3, 4 and 5 (“MB4-Top”)
Dead cells were excluded from the efficiency computation.
A drop ranging from 1% (in the external Wheels) to 3% (in Wheel 0) is observed in 2017: this is due to lowering the High Voltage by 50 V, in order to preserve them from radiation ageing. The effect is larger at low η (Wheel 0) where the mean muon path within the DT cell is shorter.
The HV of the MB4-Top in Wheel -2 had already been brought to the present value in the last period of 2016, as can be seen in the plot.

DT Hit Efficiency distribution, 1 entry per chamber. Chambers with lower voltage are superimposed on the total distribution
Dead cells were not taken into account.

It should be noticed that the modest drop of hit efficiency observed in these chambers causes no observable loss of segment reconstruction efficiency (as shown in the next plot).

Distribution of the number of hits associated to segments fitted in MB1 chambers of Wheels ±2 and used for muon reconstruction, normalized to the number of crossing muons.
Data with lower HV (2017) are superimposed on data with nominal HV (2016). As expected, the fraction of best quality segments (12 associated hits) slightly decreased in 2017.
On the other hand the number of segments is unchanged, as shown by the ratio between the integrals of the two distributions.

TIME RESOLUTION

The DT time resolution was measured as described in:

CMS DP-2016/046
https://twiki.cern.ch/twiki/bin/view/CMSPublic/MuonDPGPublic160729

RESULTS

The present value of DT time resolution, derived from the width of central peak, is 1.57 ns: it is expected to improve with the final calibration, needed in particular for the chambers at lower HV.

DT time distribution for all reconstructed muons with pT>10GeV: the excellent DT time sensitivity makes the Out-Of-Time peaks clearly visible.

DT time distribution for reconstructed muons matched to a tracker track: this matching suppresses the Out-Of-Time background.

-- FrancescaCavallo - 2017-11-06