BSC as a Luminosity Monitor

During the low-luminosity phases of the LHC operation, including the Heavy Ion runs, there was the desire to have an independent method of monitoring the lumi-nosity to act as a cross check to the HF based system. Throughout most of 2010 pp running and subsequent Heavy Ion runs, the BSC was capable of providing another independent measurement by using the minimum bias triggers. However, in its current form, this capability was limited due to the fact that there were

Luminosity Studies 83 only 16 channels at each end of CMS (BSC1) and rates from the large and sen-sitive scintillator tiles quickly saturated. Above some value of luminosity, signals were produced in every channel in every bunch crossing. Before saturation, the event rate in the BSC detector increased with luminosity. Assuming a detection efficiency of _det, the rate of events detected per second is given by;

R_events =PN_bbf_orbdet (5.10)

whereR_eventsis the minimum bias trigger rate in Hz,N_bbis the number of colliding bunches,f_orbis the LHC orbit frequency andP is the average number ofppcollision events occurring simultaneously, known as the pile-up fraction (0≤ P ≤1).

The event rate,R is also related to the luminosity by;

< R_events >=kσ_pp L_inst (5.11) where < R_events> is the mean number of events, assuming Poisson statistics, σ_pp is the pp minimum bias cross-section^†,L_inst is the instantaneous luminosity per second and k is the correction factor accounting for the detector efficiency and geometrical acceptance.

Therefore, by monitoring the rate of the minimum bias trigger and applying a suitable factor, it is possible to obtain a relative luminosity value from the BSC, provided that the average number of collisions within a single bunch crossing (the pile-up) is ≤1.

Minimum bias trigger rates and CMS instantaneous luminosity were compared for the months of August - October 2010 to obtain a relationship between the minimum bias trigger and the luminosity during this early phase of pp running.

It should be noted that the luminosity was increased throughout September and October bringing the instantaneous luminosity beyond the design luminosity of the BSC, L ≈ 10³²cm⁻²s⁻¹ [40]. Figures 5.1(a), 5.1(b) and 5.1(c) show the normalized BSC Minimum Bias rates (Red) and Instantaneous luminosity (Blue) for August, September and October respectively.

The signals from the BSC were used to monitor the luminosity online during the Heavy Ion (H.I) runs, temporarily replacing the HF Zero Counting method. The

†The elastic and many of the diffractive collisions will not be detected by the BSC due to kinematic considerations. The minimum bias trigger is fired mainly by non-diffractive and double diffractive events. See chapter 6

BSC channels did not reach saturation due to the lower minimum bias rates during these runs. The minimum bias trigger rates of the BSC closely followed the CMS instantaneous luminosity in September (Linst <30µb⁻¹s⁻¹). A single, empirically derived conversion factor (≈71000µb), required to convert the BSC minimum bias trigger count rates (Hz) to an instantaneous luminosity measurement (µb⁻¹s⁻¹) was sufficient. In late September, there was the first significant increase in lumi-nosity (L_inst = 3.5 × 10³¹cm⁻²s⁻¹ = 35µb⁻¹s⁻¹), when it was observed that the BSC minimum bias trigger was triggering at a rate ∼ 10 − 20% lower than expected. Inspection showed that the increase in bunch intensity lead to excessive signal amplitudes from the front-end PMTs. Such large signals caused oscilla-tions to be produced by the tube base electronics (typically by the dynode bypass capacitors) with amplitudes capable of crossing the discriminator thresholds (see figure 3.7), causing secondary firing of some triggers. When these oscillations oc-curred within the 20 ns output pulse width of the discriminators, it had the effect of prolonging the output pulse, effectively increasing the dead-time of the corre-sponding channel. Adjustments to the discriminator thresholds and high voltages were made to accommodate these increased signal amplitudes which, together with a new conversion factor, restored the linearity of the BSC minimum bias triggers with the instantaneous luminosity values, as shown in figure 5.1(b).

Through 28 September to 18 October, the luminosity in CMS was increased from L_inst = 30µb⁻¹s⁻¹ (3×10³¹cm⁻²s⁻¹) to L_inst = 180µb⁻¹s⁻¹ (1.8×10³²cm⁻²s⁻¹) pushing beyond the limit of the BSC performance as a luminosity monitor without adjusting the sensitivity and triggering efficiency. Figure 5.1(c) shows the point at which the BSC minimum bias trigger rate saturated at ∼40µb⁻¹s⁻¹. The HF based luminosity calculations, explained in [67] are not susceptible to saturation due to pile up until at least L= 5×10³⁴cm⁻²s⁻¹ (50 nb⁻¹s⁻¹). However, the HF luminosity monitor does not function well for low luminosities which is where the BSC excels.

Summary

The BSC detector operated as a relative luminosity monitor during the low lumi-nosity phases of the LHC. During these phases, in which the BSC tiles were able to operate at ≈95% efficiency without suffering from excessive signal heights or

Luminosity Studies 85 saturation, it was possible to convert the minimum bias trigger rates into lumi-nosity values by a single, empirical conversion factor.

As the luminosities and particle flux increased, the signal amplitudes from each channel became excessively large, causing signal ringing and overlapping pulses on the arrival at the readout electronics. This had the effect of increasing the dead-time of the trigger system, reducing the efficiency of counting the minimum bias rates. The effect could be corrected for by fine tuning thresholds of the first level of discriminators in the readout and tuning the high voltages to the PMTs to reduce the signal heights and prevent excessive pulse amplitudes due to the increase in particle flux.

Further increases in luminosity during October signaled the limitation of the BSC sub-detector as a luminosity monitor. Because of the simplicity of the design, the BSC did not provide adequate timing or topological information to help dis-tinguish and account for multiple events occurring simultaneously. The larger number of particle interactions with the BSC tiles pushed the BSC front end to its operational limits. Even after reducing the high voltage and single channel efficiency, the minimum bias trigger rates reached a plateau when the luminosity reached ∼40µb⁻¹s⁻¹.

However, it had been shown that, with the aid of VdM scans, the BSC was capable of providing online luminosity information throughout the 2010 and 2011 H.I runs.

The functionality of a H.I and low pp luminosity monitor was one consideration when determining the requirements of the upgrade system.

(a)

(b)

(c)

Figure 5.1: Trigger and luminosity data throughout (a) August, (b) September and (c) October 2010. BSC luminosity monitoring is shown in red. The official

CMS off-line luminosity measurements are shown in blue.

Chapter 6

Forward Physics

This thesis investigates the performance of the BSC in terms of its triggering efficiency, particularly for minimum bias events, and subsequently, the capabilities for such a detec-tor to perform as an online luminosity monidetec-tor should such a device be required by the BSC upgrade. Chapter 7 also describes the measurement of the inelastic cross-section for pp collisions at √

s= 7 TeV. All of these require some understanding of the theory and mechanisms behind high energy hadronic interactions. This chapter provides an overview of the Standard Model followed by the current theory of elastic, inelastic and diffractive hadronic interactions.