Position of the LHC Beam Spot in the CMS Interaction Point

Results are published in DP Note DP-2022/034


In this note the position of the LHC Beam Spot in the CMS Interaction Point is reported as a function of time, starting from the beginning of the LHC operations in 2010 to mid 2022.


Figure in PNG format (click on plot to get PDF) Description
The beam spot (BS) represents a 3-D profile of the region where the LHC beams collide in the CMS detector.
The figure shows the X and Y position of the beam spot centre as a function of time. The positions are computed in the CMS reference frame, with the origin in the centre of the detector [1]. These parameters are determined by fitting the track impact parameter distribution as a function of the azimuthal angle [1]. The fit is performed for each luminosity section (corresponding to a period of 23.31 s). Subsequently, the results are averaged over each month of data taking.
The figure shows values obtained with data produced in proton-proton collisions and collected in 2010-2011 (7 TeV center-of-mass energy), 2012 (8 TeV), 2015-2018 (13 TeV), 2021 (900 GeV) and 2022 (13.6 TeV).
The upwards drift of the Y position from 2010 to 2016 is caused by a natural movement of the LHC tunnel with respect to the CMS cavern.
At the end of 2016 CMS asked LHC to move the beam by 1.5 mm using magnetic correctors near the interaction point and by additional 0.3 mm at the end of 2017.
During the LHC Long Shutdown 2 the LSS5 section of the LHC tunnel was realigned to adjust the Y position of the beam in CMS as the shift was beyond the capabilities of the optical power of the magnets.
The change in the X position is instead caused by a mechanical movement of the magnets in the horizontal plane.
1. CMS Collaboration, “Description and performance of track and primary-vertex reconstruction with the CMS tracker”, JINST 9 (2014) P10009

-- DavideZuolo - 2022-09-01

Topic attachments
I Attachment History Action Size Date Who Comment
PDFpdf cms_beamspotXY_vs_time_2022_Approval_v6.pdf r1 manage 21.8 K 2022-09-02 - 15:55 DavideZuolo PDF version of the plot
PNGpng cms_beamspotXY_vs_time_2022_Approval_v6.png r1 manage 75.9 K 2022-09-02 - 15:57 DavideZuolo PNG version of the plot
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Topic revision: r11 - 2022-09-19 - DavideZuolo
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