Validation and performance of the CMS Barrel Muon Drift ...



Validation and performance of the cms barrel muon drift chambers with cosmic rays*

A.T. MENEGUZZO AND F. GONELLA

Dipartimento di Fisica ‘G.Galileo’, Università degli studi di Padova

& Istituto Nazionale di Fisica Nucleare, sezione di Padova

8 ,Via Marzolo Padova, 35100, Italia. E-mail: anna.meneguzzo@pd.infn.it

The production of the 250 muon barrel Drift Tube chambers [1] for the CMS experiment at the LHC has started in 2001. In each assembly site the produced chambers equipped with the final front-end electronics and the Low and High Voltage boards are checked in a cosmic rays facility. While the wire position is measured and recorded at construction time, all electrical connections and the quality and performance of the chambers and of the local trigger devices can only be checked with real tracks. The methods used and the behavior of the first 22 chambers built in the Legnaro (Padova) INFN National Lab (LNL) are reported and compared with Test Beam results. The mean noise level at nominal condition is 20Hz/m/wire, the R.M.S. 5Hz/m/wire. The detection efficiency of each layer is above 98%. The resolution is 200-250μm per layer. The wires with a noise above 2 S.D. is less than 0.1% of the total.. Due to HV problems the number of wire not connected is less then 0.1 % of channels. The wire position accuracy is better then 70μm..

Introduction: the Barrel Muon system of CMS

The first aim of the Large Hadron Collider (LHC) is the final test of the Standard Model with the discovery of the Higgs boson. For mass greater then ~130 Gev, the cleanest signature is the decay with four leptons in the final state. Nevertheless, searching for events having rates of less then 10-2 Hz with an overall rate of ~1011 Hz (at the highest machine luminosity), puts very tight condition on the detector performance. For CMS, a detector was designed [1] to serve for the offline muon track reconstruction (providing correct charge assignment up to Tev region) and for the first level trigger selection with a fast muon identification and accurate online transverse momentum measurement. Moreover, the time between two beam collisions (BCO) being 25ns, i.e. much shorter than the time needed to form and process the electronic signals in the detector, also the parent BCO of each muon must be identified. These tasks require a time resolution much smaller than 25ns, a track resolution of ~100μm (~250μm for single points) and an angular resolution ~1mrad.

The CMS muon detection system [1,2] relies in the barrel part on Drift Tube chambers (DT). In this region the expected particle rate is 10Hz/cm2 and the longitudinal and radial components of the stray magnetic field in the gas do not exceed 0.4 and 0.8T respectively. The system consists of 4 concentric shells of DT chambers made (except the outer made of 2) of 3 independent units, called SuperLayers (SL). Each SL is composed of 4 layers of drift tubes, with all wires parallel. Odd layers inside a SL are staggered by a half-cell width with respect to the even layers for left right ambiguity resolution. The two external SLs, called Φ1 and Φ2, have wires parallel to the beam and to the magnetic field thus measuring the muon trajectory in the CMS bending plane. The third SL, called θ, has wires perpendicular to the Φ SLs. The drift tube, with 42x13mm2 cross

section, is shown in fig.1. The electric field is shaped by a central Anode wire, two strips on the I-beams and two Strips on the near wall. The tube is filled with a mixture of Ar(85%)CO2(15%). The time distribution (fig.2) yields the drift velocity from the maximum drift time and the linearity from its flatness.

Figure 1. Cell with equipotential and drift lines.

Performances of the cell and of the Local Trigger as a function of track angle and Magnetic field have been checked in Test Beam runs and results published [3].

After being assembled and before being moved to CERN, all chambers are tested to check electronic noise rate, detection efficiency, uniformity of behaviour, time resolution, and wire, layer and SL position accuracy with cosmic rays.

Figure 2. Drift time distribution N(t) with uniform particle flux and and dN(t)/dt. The mean drift velocity in the cell is 55 μm/ ns (~20500 μm / 372ns).

MB3 Chamber production at LNL and cosmic rays set up.

The production of MB3 chambers [1,4] (the ones in the third shell of CMS muon barrel) began in 2001 at the Legnaro INFN Laboratories. The three SLs are made separately, equipped with the final front-end electronics and final high-voltage and low-voltage boards and cables and then glued with a honeycomb panel to form a chamber. The chamber, placed horizontally, is filled with an Ar(85%)-CO2(15%) gas mixture. As soon as the measured oxygen concentration O2 in the gas exiting the chamber is below 200ppm, the gas flow is regulated in such a way that an absolute pressure of 1020 mbar is kept constant with a gas flow of ~0.2 l/min, corresponding to about a full gas exchange every 3 days. The standard high voltage values of the cell electrodes are:Vwire=3700V, Vstrip=1800V, VIbeam=1200V. At these voltages, the wire gain is on the order of 3:5 105which is about the same value as in our previous runs.

All measurements are performed by using cosmic rays crossing the chamber and triggered by a system (as shown in figure 3) of four double plastic scintillators placed below the chamber. The final trigger time accuracy is not better than 3ns. Event by event the drift times of all channels above the nominal threshold of 4.5 mV are recorded with TDCs. As soon 1 million cosmic-ray events are collected (in a couple of hours), data analysis is performed and wrong features of the chamber (if any) are quickly identified and localized from the time distribution in each cell, the efficiency, the Mean Time between hits in aligned staggered cells. Particularly relevant are the measurements of the wire, layer and SL positions and the check that systematic shifts were not introduced in the construction phase [4].

Figure 3. LNL cosmic rays set up with two MB3 chambers. Below the chambers, the 4 double PMs facing the scintillator are shown. The cables of the two Φ SLs of the upper chamber are on the right. On the left the cables of the θ SL of the other chamber. On the left the first full Local Trigger and Readout system used in the bunched Test Beam at Cern.

Noise

The number of hits inside the recorded time window in random trigger determines the noise rate of a cell. Fig.4 shows on the left, the noise rate,

Figure 4. Left: Noise in Hz/m of each wire in a MB3 chamber; right: noise, noise mean in a layer and RMS in a layer in Hz/m of wire length for all the first 22 MB3 chambers.

channel by channel, for the all layers in the SLs of a chamber. The average noise rate is ~20Hz/m/channel (the cosmic rays rate of ~9Hz/m/channel is included). The noise of all channels of the first 22 chambers, the mean channel noise value in the layers and their spread in the layer are shown on the plots of fig.4 at left.

Efficiency.

The measurement of the efficiency is performed using the reconstruction of cosmic-rays track segments in each superlayer. We look for tracks with at least three hits in different layers and check if a fourth hit is present in the fourth layer within a fixed window of half a cell around the track extrapolation. The efficiency of normal cell is well above 98%; the 1-2% inefficiency is due to the I-beams as shown in fig.4. We diagnostic bad HV connections from the presence of higher inefficiency [4].

Figure 5. On the left: cell efficiency in the layers of a SL vs cell number; on the right: efficiency as a function of the track position in the cell (I-beams are at zero and at 42 mm).

Drift Velocity and wire position uniformity and accuracy.

As shown in fig.6, for every track located in the same semi-column of a SL, the measured quantities MT234 and MT123 (defined below) must be the same if the drift time measured is linear to the track position in the middle plane of each layer and if the cell dimensions and the wire position are constant. Even more their value yields the drift time velocity in the cell. From the MeanTime distribution of the tracks in

Figure 6.Left: SL view with 2 crossing tracks; on the right: Mean Time (MT) definition

in each semi-column we checks :the wire position accuracy in a SL from the width of the distribution of the mean of the MT in each semi column (usually around 1.7 ns that is ~ 70μm) and the resolution and resolution uniformity on the cell along the layer and in the SL from the RMS of the fitted MT values (usually around 6 ns with 3ns due to the trigger , resulting in 250 μm in each point (see fig,.7).

[pic]

Figure7. RMS (a) and the Mean (b) of the MT as a function of position in the cell. The wire is at –2.1 cm. In (c) the MT Mean as a function of position in the SL. One bin is 2.1 cm i.e. one semi column. In (d) and in (e) the mean of the RMS in a layer (projection of plot (a )) and the RMS of the MT mean distribution in a SL for the first 22 MB3 chambers.

Even more, if the MTs on the semi columns at right of the wires of a layer (MTR), differ systematically from the MTs computed on the semi column at left of the wires, the quantity MTR-MTL measures the error on the staggering between layers.

MT analysis is fast and many results do not depend on t0 and on the drift velocity. The mean MT and the RMS of all the produced chambers are plotted in fig 7. The wire position accuracy is better then 70 μm and the resolution better then 250μm in all chambers. Correction to the signal propagation along the wire has been applied.

Test at LNL of the Local First Level Trigger device.

The method of computing the MT that is to look at aligned hits in time space is the kernel of the algorithm of the muon Local First Level Trigger system [5] which will reconstruct at LHC the best two tracks in a chamber and will identify the parent Bunch Crossing. The first assembled Local Trigger System has been tested using the cosmic rays set up (as shown in fig. 2) and triggering only with cosmic rays in phase with the internal clock of the system. That allowed checking accurately the Local Trigger performances on all chambers. The preliminary results, confirmed in a dedicated test beam run at CERN, show excellent performances on efficiency and track parameter measurements.

Conclusions.

All chambers produced for the Muon Barrel system of the CMS experiment, near half the total, have been checked with their final High and Low Voltage cards and their final FE electronics. Their performances and construction accuracy are well within the required characteristics. The performance of their trigger capability with track parameter and bunch crossing identification has been successfully tested with the first final First Level Local Trigger system.

References

1. CMS collaboration, Compact Muon Solenoid Technical Proposal, LHCC94-38; CMS collaboration, Muon Technical Design Report, CERN LHCC97-32.

2. F. Gasparini et al., Nucl. Instrum. And Meth. A 336 (1993), 91-97.

3. M.Aguilar-Benitez et al., Nucl. Instrum. and Meth. A 480 (2002), 658-669, CMS Note 1998/06,CMS Note 2001/041, CMS Note 2001/051.

4. CMS Note 2003-017 and references therein.

5. CMS,collaboration, Level-1 Trigger Technical Design Report, CERN LHCC 2000-038. CMS Note 2001/051.

* This work is supported by INFN, sezione di Padova ,Italy.

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