Calculation of Channel Sizes

The BSC single channel rate measurements made during the VdM scan in June 2011 (see section 5) showed that the scintillator tiles used in the BSC saturated at luminosities in the order of 10³² cm²s⁻¹. This, according to simulations [134], equates to a flux of 10⁴cm⁻²s⁻¹ at the BSC 1 location. The approximate flux of charged hadrons from collisions and backgrounds are shown as a function of radial distance in figure 9.4. Table 9.1 gives the expected collision (background) flux for luminosities ranging from 10²⁸- 10³⁴ cm⁻²s⁻¹ based on the simulations at

Design of the BHC 161 R = 40 cm from the beam. Also shown are the estimated particle rates for the current BSC tile sizes (∼500 cm²) and for a hypothetical tile sizes of 100 cm² and 2.5 cm² sized tile for the detector upgrade.

Table 9.1: The channel sizes of the upgrade can be estimated based on the luminosity limit of the current system and the expected flux at∼40 cm radius,

L = 10³⁴cm⁻²s⁻¹ and √

A scintillator tile size of 100 cm² at a larger radius could provide the acceptance required for beam background monitoring whilst keeping the collision induced signal down to a manageable rate. The following sections look in detail at the minimum bias and beam background event influence in the BSC plane.

9.2.1 Minimum Bias MC information

CMSSW simulations using minimum bias events were carried out on a MC dataset tuned to event pile-up of ∼10, as experienced during the Summer 2011 running.

The analysis counts all particles passing through the HF front face plane, where the BSC is located, from approximately 20,000 events. The counts were taken from the reconstructed simulation of the HF calorimeter response. It should be noted that as the radius of the HFη rings increases, so too does the area coverage of each ring. This is however not true for the outer most ring (η ring 1) which is smaller due to mechanical constraints. See figure 2.5(a). For this reason, the data from the outer ring has been omitted. Figure 9.5 shows the hit rates for each HFη ring , normalised for pile-up. As expected, the flux of particles falls away at higher radii (smallerη) decreasing to 0.5 - 1 MHz at |η| = 3 resulting in an average of 1 hit/cm² every 2 - 4 bunch crossings at the 50 ns bunch spacing expected in 2012.

2.8 3 3.2 3.4 3.6 3.8 4 4.2 4.4 4.6 4.8 η

Figure 9.5: The expected rate of detecting minimum-bias events with a 10×10 cm tile in various η locations on the HF front face. The inner tiles of the present system are located at 4.26< |η|<4.76. The scintillators for the upgrade are to be located at ∼ 2.9 - 3.3 to prevent saturation at the expected

L= 10³³ cm⁻²s⁻¹ in 2012.

The data shown in figure 9.5(a) includes all particles at all energies. Not all particles will cause the scintillator tiles to respond. Low energy photons (<5 MeV) and neutrons for example, will barely interact with the scintillator. After applying a cut on particle type and energy, the effective flux through the scintillator tiles is slightly reduced, as shown in figure 9.5(b). Figure 9.6 shows the kinetic energies of the particles which are incident on the HF calorimeters between the pseudo-rapidity ranges of 2.85< |η| <3.31 and would therefore pass through the region of interest for the BSC upgrade. Most of the signals induced into the BHC from collisions will be due to protons, charged pions and, indirectly from high energy

Design of the BHC 163 photons. Thus, any hits to the tiles by particles stemming from the I.P are very likely to cause a signal.

Figure 9.6: MC simulations of the energy of the minimum bias particles in-cident on the HF between 2.85< |η|<3.31 per event. Pythia_6_Tune_ Z2 at

7TeV (Pile-up = 10) MC data used for this simulation.

9.2.2 Beam Halo Simulations

Simulations of the beam backgrounds were made with FLUKA [136]. The sources of beam backgrounds are inelastic and elastic interactions with residual gas nuclei inside the beam pipe, beam halo originating from cleaning inefficiencies where scattered protons are not absorbed by the collimators, and particles stemming from the collisions at neighbouring interaction points [137]. The quantities of beam gas vary, depending on the quality of the vacuum in the long-straight sections near CMS. The presence of beam halo is inevitable. As well as contaminating the data and impeding offline analysis, excessive beam halo rates can cause the Cathode Strip Chamber detectors of CMS to power trip. It also suggests poor beam dynamics with an increased likelihood of a total beam loss.

Figure 9.7 shows the results of the CMS beam halo simulations, where beam 1 halo particles, traveling from +Z to -Z have been tracked through the entire CMS detector. Only the muons of the beam halo are able to penetrate through the HF (+Z) to hit the BSC +Z tiles, shown as the green points in left of figure 9.7.

As they pass through, the halo particles interact with the materials of the CMS

X [cm]

Figure 9.7: FLUKA driven background simulations processed in CMSSW show the particle spectra that hits the BSC scintillator tiles. The plots show the result of Beam 1 (+Z to -Z) halo particles interacting with the +Z BSC1 tiles (left) and the -Z BSC1 tile (right). Onlyµ^± are able to penetrate through the material to interact with the +Z BSC tiles. As they traverse, many electrons are produced

through collisions which then interact with the -Z BSC tiles.

detector, causing showers of electrons to hit the opposite BSC detector (-Z). Many of the original muons also interact with the BSC -Z with a time-of-flight of∼73 ns.

The situation is the same for the beam 2 halo simulations. The outgoing beam background particles interacting with BHC detector will be accompanied by the collision products. At the expected interaction pile-up rates (>10) in 2012, the outgoing side of the detector will see particles from every collision, meaning there is little information to be gained by the Time-of-Flight approach used in the BSC system.

If beam halo is present, it is likely to accompany every bunch passing through CMS, including non-colliding bunches. Using the knowledge of the timing of the incoming bunches relative to the outgoing electron showers and collision products, it is possible to measure the beam halo during 2012pprunning. This is explained in section 9.3.5.

Design of the BHC 165