1. Beam-plug and shielding studies related to HCAL and M2 Robert Paluch, Burkhard Schmidt November 25, 2014 1

2. Outline M2 beam plug: – Modifications to the beam-plug around M2 – Impact on performance – Reminder: Low energy background not considered in these studies, done with nominal thresholds. HCAL beam plug: – Clearance towards the beam pipe around the HCAL beam plug. – Impact on performance on suggested modifications. Overall performance of modified or additional shielding. 2

3. Simulation results Simulation parameters and configuration: – Gauss version v42r2 – detector description dddb-20120831 – detector condition sim-20121025-vc-md100 – beam configuration file Beam3500GeV-md100-MC11-nu2-50ns.py – beam energy 7 TeV – luminosity per coll. bunch 2.47 x 10 29 /cm 2 /s – total luminosity 3.13 10 32 /cm 2 /s  Still problems in using the official beam configuration file for 7 TeV, and 25ns bunch spacing and a ν-value of 7.6 The average track multiplicities/cm 2 /s refer always to particles leaving up to 4 hits in a chamber, depending on their angle (no double counting of hits coming from the same track). 3

4. Track multiplicities with the layout we had in Run I for the M2 beam-plug 675211984132375392 1509915163 1884816677 5525950483955493 The track multiplicities vary by a factor three, depending on the chamber position. (The uncertainty on the values given is about 2%.) The chambers left and right from the beam-plug have the highest rates (effect of the magnet), as expected: – Left/right: top/bottom: corners: The chamber closer to HCAL have a 20% higher rate: – Front row chambers: back row chambers: A-sideC-side 4

5. Effect of adding a plate of 2cm iron in front of M2 R1/R2 -3%6%-3%7% 6% 13% -7% -4% -11%-7%-15%-2% The changes in track multiplicities are larger than expected. However, the overall reduction is small (~1%). The rate goes down in the front row chambers, as expected : Front row chambers: -2% ; back row chamber: +0%  The variations in the back row chambers are very large, from +13% to -15%. This is not understood! The idea has not been followed up. A-sideC-side 5

6. Piece cut out Small Openings ~12cm ~3cm ~9cm M2 beam plug 6

7. Description of beam-plug and beam-pipe in the simulation  Besides the outer dimension of the plug (which is important!), we are basically back for Run II where the MC description has been for Run I.  We learned about the measures necessary to take in order to modify an irradiated piece of iron. To be kept in mind for the future. 7

8. Effect of filling the openings in the M2 beam-plug 1.5%-4.6%8.1%-4.1% -1.7% 0.8% -1.7% -3.5% -3.6%-2.0%9.4%-0.2% The changes in track multiplicities are small, as expected.  Overall reduction < 1%. The chamber by chamber variations are however larger than expected. The reason is not really understood. We might need to run with higher statistics (for time being 4000 events).  Anyhow, the hardware change made to the beam-plug should be beneficial. A-sideC-side 8

9. Studies on the HCAL beam-plug 9

10. HCAL beam plug with beam-pipe when the calorimeters are closed 10 Large clearance between beam-pipe beam plug for both the HCAL and ECAL beam-plugs. The reason is mainly that space had been foreseen for the bake-out instrumentation, which is however not permanently installed. The beam-plug under M2 goes much closer to the bake-out instrumentation and the beam-pipe.

11. HCAL beam plug with beam-pipe when the calorimeters are closed 11 Clearance of 25mm - 45mm between beam-pipe beam plug for both the HCAL and ECAL beam-plugs. The HCAL beam-plug has three parts (although it has been molded in one piece). The part close to M2 would need to be modified (in material and in radius). 30mm ~85 mm 45 mm 1. 2. 3.

12. Summary of simulation results of Oct. 9 Focus has been recently on last part of the plug in front of M2. The 2 nd Muon station is not sufficiently protected from background hits – insufficient thickness (λ i ) of HCAL – showers in the material close to the beam-pipe Changes under consideration to reduce the occupancy in M2R1: – Change of material from steel (or lead) to tungsten for HCAL and M2 beam-plug – Additional shielding behind HCAL (replace PMT and CW bases by shielding) – Max. possible reduction: -55% -22% (M2 plug) -25%-43% HCAL plug: - 15-20%

13. Geometry of HCAL beam plug bellows of the beam-pipe bake out instrumentation Beam pipe flange M2 plug 13 Bake out instrumentation (in orange) clearly visible. For time being the possibility to reduce the radius of the last part of the HCAL beam-plug (light blue) is therefore limited

14. Simulation results Effect on M2R1: Comparison of realistic tungsten shielding behind HCAL, with no add. shielding – Overall reduction: Top/bottom: ; Left/right: Corners: Front rows: ; Back rows:  Huge gain in the left/right chambers with the highest rate -54.5% -52.1%-46.0% -62.6% -59.8% -69.4% -65.5% -46.3%-56.8%-48.4%-44.5% 14

15. Conclusions Effect of openings in M2 beam-plug is minor compared to the right outer dimensions of the plug. Still, the openings have been closed for Run II with steel plates. A reduction of close to 60% can be obtained with a tungsten plug under M2 and part of HCAL, and additional realistic W- shielding behind HCAL. The effect of a modified HCAL beam plug with reduced inner radius is large also on the other region (this was not the case previously. We will finalize the study in the coming weeks. 15 Region M2 plug tungsten M2+add.shield tungsten M2+HCAL plug + add.shield (W) M2R1-22%-43%-59% M2R2-6%-12%-25% M3R1-4%-5%-23% M3R2-1%-6%-14%

16. Backup 16

17. HCAL layout 20cm 17

18. HCAL layout The last 16cm of HCAL (along z), where the PMTs and the CWs are housed, are easily accessible and can easily be equipped with additional shielding. The 20cm (along z) where the wavelength- shifting fibres are bundled, are rather difficult to access, due to the end plate. It should be possible to enlarge the area of shielding along x to two HCAL cells (26cm), hence covering all of M2R1 along x. In y no changes are envisaged. 18

19. Shielding behind HCAL PID TDR versionRealistic version 19