Study of the radiation damage effect on the Inner Detector

UC Berkeley, August 3rd 2018

Supervisor: Frédéric Derue

Precise measurement of the top quark mass using charmed mesons in the final state

Technical supervisor: Ben Nachman

Jad Zahreddine (LPNHE - Paris)

Pixel detector in ATLAS

The pixel detector is the innermost part of the ATLAS detectorlarge flux of particles

• 3 layers already existed since Run1‣ B-layer, Layer 1, Layer 2‣ 50 x 400 μm2

‣ Depth = 250 μm

• 1 layer added for Run 2: IBL‣ 50 x 250 μm2

‣ Depth = 200 μm‣ 3.3 cm away from the beam pipe

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Radiation damage

Large flux of particles high dose of radiation on the pixels

Radiation damage introduces defects in the sensor bulk

‣ Charge collection efficiency decreases‣ Hit efficiency decreases‣ Spatial resolution of the track position worsens

Increases the probability of charge trapping in the sensor

Decreases the collected charge

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Deforms the E-field

First steps and validation

• Migration of the radiation damage code from release 22 to release 21‣ Tested different releases within the release 21‣ Migration easier than we first expected

• Need to validate the outputs of the code: we firstly focused on the collected charge by the sensor after the passage of a charged particle

After irradiation, a phenomenon occurs: charge trapping

It is mainly caused by the creation of intermediate states in the gap where the charges get trapped

Expectation: collect less charges, hence a charge distribution shifted to the left smaller values in terms of dE/dx

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Validation: Spatial resolution

Unbiased residual X gives an idea on the spatial resolution of the track position in the direction with the best resolution (φ-direction)

IBL and Layer 2 have the worst “resolution” since we rely on them to extrapolate the tracks

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Useful information

φ

z

Track depth in a pixel

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Cluster sizes tan θL = minimal cluster size

Fit Function:

Validation: Lorentz angle

For the same value of the bias voltage, the Lorentz angle increases with the fluence

mobility increases, E-field strength decreases

For the same value of the fluence, the Lorentz angle decreases with the bias voltage

E-field strength increases, B-field strength decreases

Good agreement between data and MC

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New method in ATLAS: E-field validation

Yet, we still need to validate directly an important input: E-field.

New idea that has never been done before in ATLAS !

CMS suggested that we could do so by measuring the dependence that the Lorentz angle has on depth. In other words, rely on the geometry of the clusters and its link with the mobility of the charges and the Lorentz angle

from B. Nachman’s talk

Link between the E-field and geometry/mobility:

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This is feasible because: 1) The Lorentz angle and the charge trapping are independent (to a good approximation) 2) The depth dependent E-field affects the Lorentz angle in a known way

Depth measurement

Problem: if we consider all the modules, we get a smeared distribution because of a very bad resolution for the central modules, effect of multiple scattering

Need to make some cuts:pT,trk > 20 GeV : get rid of the multiple scattering effectMake sure to consider only the far right/left modules* (also a nice way to consider high values of cluster sizes better probe the depth)

Due to lack of statistics when we make the cuts, we use particle gun events where we shoot muons at high angles and high pT,trk

* η-module = {-6, -5, 4, 5} 9

depthtrack = (xi - x0)tan (θ)

where: xi : position of a single pixel belonging to the cluster x0 : predicted track position in local coordinates θ : incidence angle

Take the absolute value of the depth, round it around the second decimal

For each range of depth, plot the transverse cluster size vs. the incidence angle and take the minimum value of the transverse cluster size.

Depth [mm] [0, 0.03] [0.03, 0.05] [0.05, 0.07] [0.07, 0.1] [0.1, 0.2]

(Very) preliminary results

10However, still encountering problems while fitting the distribution …

Reminder:

Next Steps

* Modeling the Mobility and Lorentz angle for the ATLAS Pixel Detector

(2) To get μ = μ(E) from the theory*:

(1) To get μ = μ(z) from the simulation:

E = E(z)

with νs : saturation velocity (μm/ns)β : temperature exponent Ec : critical E-field (kV/cm)

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Conclusion

Qualification task started with the idea of migrating the radiation damage code from R22 to R21

Helping with the validation of the digitizer model

Working on a new way to measure the E-field in-situ, hopefully will fix the problems soon

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Back up

Analysis: Top quark mass measurement

• mtop using charmed mesons ATL-COM-PHYS-2018-261

Brief overview• Jet fragmentation studies

ATL-COM-PHYS-2018-261 • Projection of the

top quark mass (Yellow report) ATL-COM-PHYS-2018-964 ATL-COM-PHYS-2018-965

Qualification task: Impact of the radiation damage on the Inner Detector: IBL (Insertable b-layer)

Benchmarks

Benchmark 0 1 2 3 4 5 6

(1 MeV neq/cm2)/V F V F V F V F V F V F V F V

IBL 0 80 0 80 1 80 2 80 2 150 5 350 8.7 400

b-layer 0 150 0.7 150 1.2 150 1.7 150 1.7 350 3.1 350 4.6 400

Layer 1 0 150 0.3 150 0.5 150 0.7 150 0.7 250 1.3 250 2.1 250

Layer 2 0 150 0.2 150 0.3 150 0.4 150 0.4 150 0.8 150 1.3 150

Validation: Spatial resolution

The value of the resolution can be improved by making cuts on the pT of the tracks and considering only the central modules

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Unbiased residual X gives an idea on the spatial resolution of the track position in the direction with the best resolution (φ-direction)

Depth dependencies

Depth should indeed go from -0.2 to 0.2 mm

• We are indeed calculating the depth using local coordinates evaluated at the level of the pixels

• z = 0 corresponds to the plane of the pixels rather than the center of the module

z=0

z=0

So we actually have the correct definition of the depth, some tests also showed that we could also take the 3 far right/left modules not just the last two (as long as | Localθ | > 0.5) 19