[Paper Review] The beam-gas method for luminosity measurement at LHCb

P. Hopchev|arXiv (Cornell University)|May 24, 2010

Particle physics theoretical and experimental studies3 citations

TL;DR

This paper presents the beam-gas method for absolute luminosity measurement at LHCb, using vertex reconstruction of beam-gas interactions in the VELO detector to determine beam parameters such as position, width, and crossing angle. The method achieved a relative precision of 15% on luminosity in 2009 data, demonstrating feasibility for future high-precision luminosity calibration at the LHC.

ABSTRACT

The high resolution of the LHCb vertex detector makes it possible to perform precise measurements of the vertex positions of beam-gas and beam-beam interactions. With these measurements beam parameters such as width and position can be measured. A novel method for determining the absolute luminosity at the LHC using the directly measured beam parameters is presented. The data taken in 2009 is used to illustrate the procedure.

Motivation & Objective

• To develop a novel method for absolute luminosity measurement at LHCb using beam-gas interactions.
• To utilize the high-resolution vertex reconstruction of the VELO detector to measure beam parameters such as position, width, and crossing angle.
• To reduce systematic uncertainties in luminosity determination by directly measuring beam overlap and intensities.
• To calibrate reference cross-sections and lumi counters using a primary luminosity scale derived from beam-gas data.

Proposed method

• Reconstruct vertex positions of beam-gas interactions using the LHCb VELO detector, which has high spatial resolution and close proximity to the beam pipe.
• Measure beam parameters (width, position, slope, crossing angle) from the spatial distribution of beam-gas vertices in the x-z and y-z planes.
• Use the beam overlap integral from Eq. (1), incorporating measured beam densities and bunch intensities, to calculate luminosity.
• Apply deconvolution techniques to correct measured beam sizes for vertex resolution effects, improving accuracy of beam profile determination.
• Combine beam intensity measurements from accelerator instrumentation with reconstructed beam geometry to compute absolute luminosity.
• Validate the method using 2009 LHC data, where beam conditions were less stable and VELO was partially retracted, to assess systematic uncertainties.

Experimental results

Research questions

• RQ1 Can beam-gas vertex reconstruction in the VELO detector provide a reliable and precise measurement of beam parameters essential for luminosity determination at LHCb?
• RQ2 What is the achievable precision of the beam-gas method for absolute luminosity measurement using 2009 data?
• RQ3 How do vertex resolution and detector geometry affect the accuracy of beam size and overlap integral measurements in the beam-gas method?
• RQ4 To what extent can beam-gas interactions be used to calibrate luminosity counters and reference cross-sections in the absence of standard model processes?
• RQ5 How do beam adjustments (e.g., mini-scans) affect the agreement between predicted and measured luminous region shapes and positions?

Key findings

• The beam-gas method achieved a relative precision of 15% on absolute luminosity in 2009 data, with contributions of ~10% from beam overlap measurement and ~15% from beam intensity uncertainty.
• Beam-gas vertex reconstruction enabled measurement of beam slopes, widths, and crossing angles, with the observed crossing angle in the x-z plane matching the expected value due to the LHCb dipole magnet.
• The vertex resolution in the VELO had a small but non-negligible impact on the determination of beam sizes, particularly in the x-direction due to partial retraction of the detector in 2009.
• Deconvolution of vertex resolution from measured beam sizes improved the accuracy of beam profile reconstruction, with the corrected beam size shown in yellow in Fig. 4.
• The method demonstrated feasibility for luminosity calibration even under suboptimal conditions, such as the partially retracted VELO configuration in 2009.
• Future improvements in beam intensity measurement and increased data volume in 2010 are expected to yield significantly more competitive precision on luminosity determination.