The material budget of the LHCb detector at the time of the DC 06 data challenge is estimated

Needham, M

Ruf, T

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LHCb note 2007-025Estimation of the material budget ofthe LHCb detectorM. Needham, T. RufCERNMarch 20, 2007AbstractThe material budget of the LHCb detector at the time of the DC06 data challenge is estimated.1 IntroductionThe detectors located upstream of RICH2 contain a sizable amount of ma-terial. At the time of the re-optimization TDR [1] this constituted around40% of a radiation length. Search windows in pattern recognition algorithmsand the momentum resolution in the detector were found to be dominatedby the multiple scattering that this material caused. In addition, a sizablefraction of photons converted before the start of RICH2. For DC ’06 [2]several sub-detectors have implemented more realistic descriptions of the de-tector geometry [3, 4]. Therefore, it is important to update the estimatesof the material budget so that the impact of any increase on the detectorperformance can be judged.To estimate the radiation length between two points a python script is used.The GaudiPython module provides access to the TransportSvc and to theLHCb detector description. Hence, a material scan of the full LHCb geom-etry description can be performed. For these studies, the LHCb acceptanceis divided into 200× 200 bins in pseudorapidity (η) and azimuthal angle (φ).For each bin a test particle is traced from the origin through the elements ofthe detector and the radiation length estimated. The scan has been used toestimate the material before the first measured point, before the start of thespectrometer magnet and up to the end of the tracking system. For com-pleteness the radiation length in regions corresponding to each sub-detectoris given in Section 3.2 ResultsThe resolution on the track impact parameter has a significant contributiondue to the material before the first measured point. Fig. 1 shows the radiationlength seen by a particle before the first measured point. The material ofthe RF foil can clearly be seen. Averaging over 1.8 < η < 4.8 this amountsto 2.7 % of a radiation length. This is similar to the value found in previousstudies.In Fig. 2 the radiation length up to z= 270 cm, the start of the LHCb magnet,is plotted as a function of η and φ. Fig. 3 shows the radiation length as afunction of η. It can be seen that:• For η between 1.9 and 3.5 a particle sees just over 30 % of a radiation2-150 -100 -50 0 50 100 150η1.522.533.544.5500.0050.010.0150.020.0250.030.0350.040.0450.05φFigure 1: Radiation Length versus η and φ before the first measured point.Note the truncated scale.length.• The material of the detector frames and other dead material outsidethe acceptance gives a large increase in the amount of material belowη = 1.9.• Between η = 3.6 and 4.3 a particle sees 40−50 % of a radiation length.The increase compared to low η is due to the interface section betweenthe VELO vacuum tank and the beam-pipe.• A prominent spike can be seen at η = 4.3. This is due to the 25 mradsection of the beam-pipe.At the time of the reoptimization TDR the material between 1.9 and 3.5amounted to just under 30 % of a radiation length. The increase that isobserved is due to the change in the thickness of the VELO sensors from200 to 300 µm and the changes to the TT geometry description discussed in[4, 5]. Compared to the reoptimization TDR a 4 % increase in the amount ofmaterial above η = 4.5 is also seen. This is mainly attributed to the increasein the amount of material in the TT station around the beam-pipe. Finally,a scan was made to see what fraction of an interaction length this radiationlength corresponds to. On average a particle see 12 % of an interactionlength.Fig. 4 and 5 show the material integrated up to the end of the LHCb trackingsystem, z = 930 cm. It can be seen that a particle sees around 50 − 70 %3Frames and dead material outside acceptanceVELO overlapregion25 mrad coneFigure 2: Radiation Length as a function of η and φ integrated up to z =270 cm. Note that the radiation length scale is truncated.η1.5 2 2.5 3 3.5 4 4.50X1Figure 3: Radiation Length versus η integrated up to z = 270 cm. Thedotted line corresponds to 30 % of a radiation length.of a radiation length up to the end of the tracking system. In addition, thematerial of the beam-pipe supports together with the cooling rods and cablesof the Inner Tracker can clearly be seen. Finally, it should be noted that aparticle sees 20 % of an absorption length before the end of the trackingsystem.4Beampipe supportsIT Cable ConnectorsIT coolingIT Cables25 mrad coneFigure 4: Radiation length as a function of η and φ integrated up to z =930 cm. Note that the radiation length scale is truncated.η1.5 2 2.5 3 3.5 4 4.5 50X1Figure 5: Radiation Length versus η integrated up to z = 930 cm. Thedotted line corresponds to 50 % of a radiation length.3 Scans as a function of zFor completeness scans have been made to estimate the material in each ofthe sub-detectors in the tracking system. The z ranges for the six scans madeare summarized in Table 1 together with the radiation length in that regionaveraged over φ and for 2.0 < η < 4.8. Fig. 6 shows the results of the scansas a function of η and φ. Fig. 7 shows the shows the results of the scansaveraged over φ as a function of η.5Region zmin/cm zmax/cm X0/%VELO 0 83 16.2VELO-RICH1 interface 83.0 97.8 6.8RICH1 97.8 225 9.5TT 225 275 5.1Magnet 275 760 5.3T 760 930 17.8Table 1: Average radiation length, X0, in different regions of the detector.4 Missing materialUnfortunately there is still some material missing from the Monte Carlo. Forexample:• The VELO constraint system [7].• The diamond based radiation monitor located immediately in front ofthe TT station [8].• The supports for the TT half modules located directly above and belowthe beam-pipe.• The section of the IT cooling pipes closest to the detector box [3].• The beam-pipe approach system of the IT [9].• The additional material due to the changes to the IT cabling.Compared to the increase in the X0 seen in going from the reoptimizationTDR to DC 06 it is expected that the effect of these changes will be small.5 SummaryIn this note the material in the LHCb detector at the time of the DC 06data challenge has been estimated. Compared to the reoptimization TDRthe material in the detector has increased. On average a particle sees 60%of a radiation length up to RICH2, compared to 40− 50% at the time of thereoptimization TDR. This additional material will lead to increased multiple6-150 -100 -50 0 50 100 150η1.522.533.544.5500.050.10.150.20.250.3VELO-150 -100 -50 0 50 100 150η1.522.533.544.5500.050.10.150.20.250.3VELO2RICH1-150 -100 -50 0 50 100 150η1.522.533.544.5500.050.10.150.20.250.3RICH1-150 -100 -50 0 50 100 150η1.522.533.544.5500.050.10.150.20.250.3TT-150 -100 -50 0 50 100 150η1.522.533.544.5500.050.10.150.20.250.3Magnet-150 -100 -50 0 50 100 150η1.522.533.544.5500.050.10.150.20.250.3TFigure 6: Radiation length as a function of η and φ integrated over variousregions in z.scattering which will complicate the pattern recognition and degrade themomentum resolution. The size of the latter effect can easily be estimated.As shown in [6] the momentum resolution scales with√X0. Therefore, a 20% increase in the amount of material will degrade the momentum resolutionby 10%.7η1.5 2 2.5 3 3.5 4 4.50X00.10.20.30.40.50.60.70.80.91VELOη1.5 2 2.5 3 3.5 4 4.5 50X00.10.20.30.40.50.60.70.80.91VELO2RICH1η1.5 2 2.5 3 3.5 4 4.50X00.10.20.30.40.50.60.70.80.91RICH1η1.5 2 2.5 3 3.5 4 4.50X00.10.20.30.40.50.60.70.80.91TTη1.5 2 2.5 3 3.5 4 4.50X00.10.20.30.40.50.60.70.80.91Magnetη1.5 2 2.5 3 3.5 4 4.50X00.10.20.30.40.50.60.70.80.91TFigure 7: Radiation Length as a function of η averaged over φ for variousregions in z.References[1] The LHCb Collaboration. Reoptimized Detector Design and Perfor-mance. CERN/LHCC LHCC-2003-030.[2] Gauss v25r7, Boole v12r10, Brunel v30r14, XmlDDDB v30r14.[3] A. Perrin and K. Vervink. The Inner Tracker detector description andits implementation in the XMl database. LHCb-Note 2006-018.8[4] M. Needham and D. Volyanskyy. Updated geometry description for theLHCb Trigger Tracker. LHCb-Note 2006-032.[5] A. Wenger M. Needham. Material Budget Calculation for the LHCb TTstation. LHCb-Note 2056-020.[6] R. Hierck et. al. Particle Interactions in the LHCb detector. LHCb-Note2001-090.[7] M. John. Constraint system discussion. Minutes of the VELO Meeting,19th January 2007.[8] C. Llgner. The LHCb Beam Conditions Monitor. Minutes of the LHCbMeeting, 15th September 2006.[9] P. Fauland. IT ladder production. Minutes of the Silicon Tracker Meet-ing, 17th October 2006.9

Estimation of the material budget of the LHCb detector

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