Table of Contents

• Summary
• Details

Summary

• HEM region: contained in: −3.0 ≺ 𝜂 < −1.3, −1.57 < 𝜙 < −0.87
• Due to the power interruptions generated by the false fire alarms on Saturday June 30, 2018, negative endcap HCAL sectors HEM15 and HEM16 could not operate until the end of 2018 run.
  • First regular physics run afffected is 319077

Details

• HEM stands for HE minus. The negative endcap HCAL sectors HE minus 15 and 16 were no longer functional starting from June 30, 2018 until the end of 2018 run.
• This has resulted in the loss of readout of dPhi = 40 degree on HE minus side. The region contained in −3.0 ≺ 𝜂 < −1.3, −1.57 < 𝜙 < −0.87. This corresponds to a 3% of the total HCAL (HB+HE) coverage.
• The problem can only be solved by opening the endcap (minus side) in December 2018.
• The details of the failure:
  • The damage occurred during low voltage power up procedure after an interlock due to a fire alarm
  • The power supply unit that controlled HEM15/16 (and only those) incurred in an extremely rare and intermittent failure mode
  • A voltage transient that reached 22 V was generated
    • More than twice as much as the front-end operational voltage of 10V and the HW limit of 11 V
  • This extremely rare fatal failure mode was never observed before
    • Escaped extensive burn-in and post-installation power cycling tests
    • After PS extraction, 2 weeks of testing were needed to expose this failure mode on a test stand
    • We have also verified that this PS transient does damage the front-end on a test stand
• We have implemented an additional safety system to mitigate the risk of further damage
  • The incident is being followed up with the power supply company to eliminate the risk of similar behavior from other power supplies
• The impact of the HEM15-16 loss has been investigated in a variety of physics analyses
  • Trigger rates are OK, impact on physics analyses is generally small and mitigation options in reconstruction code have been implemented and are being further studied

Reference:

• https://indico.cern.ch/event/980266/contributions/4129523/attachments/2152470/3629691/201130-HEM%20issue.pdf
• https://indico.cern.ch/event/754760/contributions/3127685/attachments/1722906/2782166/1_Borras.pdf
• EGamma: https://indico.cern.ch/event/732976/contributions/3095244/attachments/1698817/2735133/egammaHEM1516FakeRates.pdf