Available Locations

The BHC has been designed with the aim to monitor the beam background in CMS, ignoring the activity from collisions that currently overwhelm the back-ground rates. The current system monitors the backback-ground exceedingly well up to the point of collisions. Figure 9.1 shows the longitudinal view of CMS and the various subdetectors that run along the beam pipe. On the left is the inner pixel tracker (barrel and endcaps) as well as the Beams Condition Monitors, BCM1L and BCM1F (0 m - 3.1 m). Moving right, along the beam pipe, there is a cone (1) through the middle of the electromagnetic calorimeter (ECAL) and Hadronic Calorimeter (HCAL) endcaps providing a space of∼4.3 m in front of the TOTEM T1 detector (7.4 m). This region is very difficult to access and provide services to. Behind the TOTEM T1 subdetector is the current BSC1 location (10.9 m)

Design of the BHC 157 and the HF calorimeter (2). The rear of the calorimeter ends at 13.8 m (3) after which TOTEM T2 and CASTOR (one side only) are situated. Beyond this point (∼15 m) is the rotating shielding (4), two large iron structures designed to provide some protection to CMS from beam losses in the long straight section leading to CMS.

1 2 3 4

Figure 9.1: The longitudinal view of CMS. As well as the effects of radiation flux and magnetic fields, there are many mechanical issues to consider when

planning a detector upgrade.

Additional to the mechanical constraints, consideration must be given to the time difference between incoming beam background particles and outgoing collision products if they are to be differentiated by their timing. At 25 bunch spacing, the beams cross at the I.P and every ^25ns·c₂ = 3.75 m in z. At these locations, it is impossible to differentiate the incoming (background) and outgoing (collision fragments + background) beams in terms of timing. As one moves away from these

‘nodes’, the time difference increases to a maximum of ^25ns₂ = 12.5 ns. Figure 9.2 shows (red dashed line) the time difference between incoming and outgoing particles alongz. Also shown are theexclusion zonesdue to mechanical restrictions and timing restrictions based on a minimum detector time resolution of 4 ns.^∗ With these restrictions, there is very limited space to place a new detector.

After considering all possibilities, including placing the detector beyond the TAS in the Long Straight Sections, it was decided to stay with the current location of 10.86 m on the HF front face to reduce the cost and allow a system to be installed during the 2011 winter shutdown.

∗The original BSC time resolution was 3 ns. This is expected to be matched in the upgrade and the figure of 4 ns allows for some safety margin.

Figure 9.2: ∆t and mechanical restrictions for the BHC location within the CMS cavern.

9.1.1 Radiation Environment

The radiation flux in the region of the BHC detector is the most intense in CMS. It has been calculated that the HF calorimeter will absorb 10 MGy of dose within 10 years of operation [27] mainly from charged hadrons and photons. The combined flux of all particles in the forward region is of the order of 10⁷cm⁻²s⁻¹. This includes thermal neutrons, photons, muons and charged hadrons of all energies [134]. The map of the expected particle flux atL= 10³⁴ and√

s= 7 TeV is shown in figure 9.3.

9.1.2 Considerations of Particle Flux

To serve as a beam background monitor, the detector must be able to distinguish between the signals generated from the charged particles stemming from the col-lision and those entering CMS as beam background. As figure 9.4 and table 9.1 both show, the ratio of collision flux to background flux is ∼10⁵ meaning that either the upgrade must be able to cope with the high collision rates and select the background signals based on the timing (by gating the signals), or it must be insensitive to the direction of the collision particles and sensitive only to the direction of background particles. The first is highly dependent on the sizes of the active area of the detector channels and their radial distance from the beam pipe where collision product rates are greatest. If they are too large, each channel will be hit by collision products every bunch crossing and may not be able to recover

Design of the BHC 159

Figure 9.3: The radiation flux from all particles in the CMS cavern.

in time to measure the background associated with non-colliding bunches. Making the areas smaller will result in more channels and a more expensive sub-detector or a reduction in φ coverage and acceptance. The second option is more technically challenging and will require the exploitation of the Cěrenkov light cone to provide the directional sensitivity.

After several discussions with several CMS institutes and BRM members, two concepts emerged from several proposals. The first was a Cěrenkov detection method that uses the directional sensitivity to separate the detection of incoming background particles and the outgoing collision products. The second is a plastic scintillator gated system which uses the arrival time of the signals to discriminate between background and collision products. For the 2012 proton physics run, it was decided to pursue the simplest option of the time gated method using plastic scintillator tiles similar to those used in the BSC detector. This choice meant that the read-out system of the original BSC could be re-used with minimal reconfigu-ration and allow the original system to continue its use as a Minimum Bias trigger and luminosity monitor during the 2012 H.I runs.

2]

Figure 9.4: The predicted signal rates from charged hadrons from (a) collisions and (b) beam background. The charged particle rates per channel increase with

the active area of the channel and decreases rapidly with radial distance.