Introduction

This twiki presents measurements of the hit rates in the Monitored Drift Tube(MDTs) of the ATLAS Muon Spectrometer in 2016-2017 operation. The aim of this study is to understand the evaluation of rates with the luminosity of the LHC in the MDT detector of Muon Spectrometer and further to compare the measurements obtained from real data and the Monte Carlo (MC) predictions. The hit rates are considered as a function of detector region, instantaneous luminosity, and transverse distance from the beampipe. The hit rates show linear behavior as a function of the instantaneous luminosity, indicating good performance.

The majority of hits in the MDTs are due to cavern background, not muons. * Cavern background* is a mixture of prompt particles from a recent proton collisions and long-lived particles accumulated from many collisions before and after the collision of interest. The most abundant particles in the cavern background are low energy photons, neutrons, and electrons, among other charged particles.

Hurdles in perdicting Muon Hit Rate

1) While prompt particles created in proton-proton collisions are recorded over the first few hundred nanoseconds by ATLAS, the muon spectrometer also receives significant amount of hits from the long lived,low energy neutrons and photons after the initial collision. Low energy neutrons has large elastic scattering cross-section. The neutron scatter multiple times in the detector, effectively forming a long lived neutron gas in the detector cavern. These neutrons can excite atoms in the detector and the ATLAS cavern. The excited atoms emit photons when they fall back to their ground states. Late photons can also be created through other methods. These neutrons and photons create a significant amount of late background hits in the muon spectrometer. Collectively these backgrounds hit compose of what is called cavern background . Given a long delay between collision and cavern background hits, collisions that occur a thousands bunches prior to trigger can still contribute hits to the current event and therefore it is important to estimate it.

2) MDT detector has a very long drift time i.e upto 900ns with a even longer live recording window of 2500ns. Therefore the MDT can record hits from up to 50 separate collisions into one event, even if hits are created by prompt particles

Samples

The data used for this analysis is from proton proton collisions provided by LHC and recorded by ATLAS detector in 2016 with center of mass energy 13 TeV and the spacing between the proton bunches is 25 ns.The data are required to be collected during the stable beam conditions and all ATLAS sub-systems are required to be operational and fit for physics analysis obtained through the good run lists i.e.

data16_13TeV.periodAllYear_DetStatus-v88-pro20-21_DQDefects-00-02-04_PHYS_StandardGRL_All_Good_25ns.xml

• Collisions events are selected if they pass a
  • zero-bias trigger to study a generic sample of the proton-proton collision. In this, events are taken from the physics_ZeroBias stream that contains random triggers in coincidence with the colliding bunches.
  • express_express stream, selecting the events taken by L1_RD0_empty, a random trigger in coincidence with the empty bunches.

Different runs are selected from the full span of operation of ATLAS detector during 2016 , with different characteristics, such as colliding bunches, average and peak luminosity .The instantaneous luminosity delivered by the LHC increases through out the year. The peak luminosity obtained during 2016 data taking is 13.8 x 10^{33} cm^{-2}s^{-1}. Different runs used in this analysis with their corresponding number of bunches.

• Simulated Samples : The private simulation sample have been considered based on Geant4 taken from MuonUpgradeSamples
  • group.perf-muons.MB_with_cavernBkg.mu40.run2.ESD.1a.v7_EXT0/ : Minimum bias events with the additional of cavern background. µ= 40 and nCav=25
  • user.mcorradi.Zmumu_with_cavernBkg.mu40.run2.ESD.v1a.t2_EXT0/ : Zmumu events with the additional of cavern background µ= 40 and nCav=25

• Along with it many more samples were tested with different µ pileup and nCav cavern background events
  • mc15_14TeV.147807.PowhegPythia8_AU2CT10_Zmumu.recon.AOD.e1564_s2988_s3000_r9899, µ=200 and nCav = 25
  • mc15_14TeV.147807.PowhegPythia8_AU2CT10_Zmumu.recon.AOD.e1564_s2988_s3000_r10280, µ=140 and nCav = 16
  • many more in pipeline

Hit Rate Calculation

The instantaneous rate [Hz] in a given detector element of MDT is defined as

where

• is the total number of hits recorded in a given detector element.

• is the total number of events collected for each run.

• is the time for which the detector element is active and recording data around the collision of interest. For MDTs it is taken to be 1300 ns; a long time window as compare to other detectors like TGC's have a livetime of 50-75 ns. So therefore in MDT s many hits are recorede from incident particles arising from proton collisions long before and after the collision which triggered the event.

As different detector elements in MDTs have different surfaces, the local flux; is calculated for each detector element as the ratio between the instantaneous luminosity and detector active area. Detector active area is obtained by summing up the active area of each detector element.

Results

The flux in for different regions of MDT chamber for the run 311287 selected with zero-bias trigger is shown in below in barrel and endcap regions . It can be seen that the rate grows with the increasing η and is independent of the azimuth angle φ for various chambers. Moreover the overall rate is higher is seen in larger sectors as compare to small sectors.

Τhe summary of hit rate in [Hz/cm^{2}] for different detector element in MDT chambers. The reported hit rate includes all φ sectors for the given η station. The region farthest from the interaction point , such as MDT BOL and BOS chambers, record the smallest hit rate.The regions closet to the beampipe, such as MDT EIL and MDT EIS chambers , record the largest hit rate and results are consistent with the old studies.

Rate from the Zero Bias and Empty Bunches

Rate ZeroBais ( )

where:

• : the instantaneous rate as defined
• : number of colliding bunches as shown in Table \ref{table:run}
• : total number of colliding bunches (3543)

The following plot shows that the average rate assuming all the rate is coming from colliding bunches. the as a function of the instantaneous luminosity. It shows that the number of hits per event versus instantaneous luminosity lie on the same line for the same bunch structure whereas runs with different bunch structures have different slopes. The runs with less number of colliding bunches corresponds to lesser rate.

Rate Empty Bunches ( )

Below shows the component of rate due to collision in empty bunches . As expected the runs with lower number of colliding bunches have proportionally a larger contribution from empty bunches.

shown as a function of luminosity , a linear fit is also shown. The linear relation between rate and luminosity is verified to a good level of approximation. The zoom in histogram is also shown for the runs with the lesser number of colliding bunches. showing the intercept of line is compatible with zero less than three sigma.

Documentation

Code : The various codes and Macros and plotting scripts used for the evaluation of hit rate in MDT is documented with instructions at CavernBackgroundMacros

Internal Note : The studies has been documented in form of ATLAS internal note InternalNote

-- MonikaMittal - 2018-03-16