Noninvasive LHC transverse beam size measurement using inelastic beam-gas interactions

The beam-gas vertex (BGV) detector is an innovative instrument measuring noninvasively the transverse beam size in the Large Hadron Collider (LHC) using reconstructed tracks from beam-gas interactions. The BGV detector was installed in 2016 as part of the R&D for the High-Luminosity LHC project. It allows beam size measurements throughout the LHC acceleration cycle with high-intensity physics beams. A precision better than 2% with an integration time of less than 30 s is obtained on the average beam size measured, while the transverse size of individual proton bunches is measured with a resolution of 5% within 5 min. Particles emerging from beam-gas interactions in a specially developed gas volume along the beam direction are recorded by two tracking stations made of scintillating fibers. A scintillator trigger system selects, on-line, events with tracks originating from the interaction region. All the detector elements are located outside the beam vacuum pipe to simplify the design and minimize interference with the accelerated particle beam. The beam size measurement results presented here are based on the correlation between tracks originating from the same beam-gas interaction vertex.

Figure 1

A schematic drawing of the BGV detector.

Figure 2

Simulated longitudinal pressure profile along the BGV vacuum sector.

Figure 3

Trigger system performance.

Figure 4

The BGV $L0$ trigger rate per bunch for an LHC fill with 13 bunches (circles) and an independent measurement of the bunch populations made with the LHC BCT system (rectangles, with values referring to the scale on the right).

Figure 5

Left: Signal formation in a SciFi module. Scintillation photons are detected by several SciFi fibers, triggering pixels of the SiPM. Only the sum of signals from the 104 pixels within a SiPM channel is read out. A weighted average is used to measure the exact coordinate of a cluster. Right: Illustration of the clustering algorithm and thresholds.

Figure 6

Cluster sum distribution in the two detection planes of a four-layer BGV SciFi module. The measurement was performed with the nominal SiPM overvoltage $\mathrm{\Delta }V=3.5\mathrm{V}$.

Figure 7

Performance of the track reconstruction algorithm.

Figure 8

Two tracks from the same vertex (red lines), with their corresponding $d$ and projected azimuth angle. The arrows indicate the direction of particles emerging from the beam-gas interaction vertex through the BGV detector.

Figure 9

Estimation of the transverse beam spot position with the BGV detector. Red lines are the result of a fit with Eq. (3) estimating the beam spot position $(x_v,y_v)$.

Figure 10

IP correlation vs $\cos ({\varphi }_1-{\varphi }_2)$ (a) and $\cos ({\varphi }_1+{\varphi }_2)$ (b). From the two line slopes and Eqs. (6) and (7), one gets for the transverse beam size ${\sigma }_x=660\mu \mathrm{m}$ and ${\sigma }_y=492\mu \mathrm{m}$.

Figure 11

Measured (blue points) and MC simulated (red points) SciFi cluster distribution along a SciFi module.

Figure 12

The distribution of the point of closest approach of reconstructed tracks along the $z$ axis (beam direction), for real data and MC simulation predictions.

Figure 13

Reconstructed beam size ${\sigma }^{\mathrm{rec}}$ plotted against the corresponding MC generated beam size ${\sigma }^{\mathrm{MC}}$. The red line is a quadratic fit to the data. The two green squares are beam size estimations for two additional MC samples that were not used in the line fit.

Figure 14

Distribution of independent beam size measurements for two integration times. Black lines are fits with a normal distribution, whose width reflects the statistical error of the BGV beam size measurement.

Figure 15

Transverse beam size measurements for the BGV (blue squares) and BSRT extrapolated to the BGV location (red circles) for three different beam energies.

Figure 16

BGV measurement of transverse beam size during the LHC acceleration phase. The expected beam size evolution given by Eq. (9) is also drawn. For clarity, the symbols used per data point are larger than the error bars of the measurements.

Figure 17

BGV and WS beam size measurements comparison for individual LHC proton bunches. Data were taken during a special calibration run at 6.5 TeV.