1 of 43

Antiproton Annihilation

Studies

Viktoria Kraxberger

Internal SMI Seminar 27.01.2023

2 of 43

2

4

Assessment of Timepix4

3

Detector Design

2

Annihilation Studies Project

Antiproton-Nucleus Annihilation

1

3 of 43

3

4

Assessment of Timepix4

3

Detector Design

2

Annihilation Studies Project

Antiproton-Nucleus Annihilation

1

4 of 43

Antimatter Factory

@ CERN

Antiproton Decelerator�ELENA

Study antimatter through

Spectroscopy
Gravitational behavior
Magnetic moments
Exotic nuclear phenomena

4

Detection through p/H annihilation

_

5 of 43

Antiproton-Nucleus (p̅A) Annihilation

Most experiments detect annihilation products, not antimatter directly!

Variety of possible reaction channels

Annihilation mechanism not well established

5

MIP

HIP

6 of 43

Simulations don’t match reality

Monte Carlo simulations for detector design and event reconstruction

Current models don’t perform well at low energies�For ~ 100 keV antiprotons on silver nuclei:

6

Aghion, S. et al., Journal of Instrumentation (2017)

7 of 43

7

4

Assessment of Timepix4

3

Detector Design

2

Annihilation Studies Project

Antiproton-Nucleus Annihilation

1

8 of 43

p̅A Annihilation Studies

250 eV antiprotons annihilating on thin target foils of 1-2 μm

~ 15 different nuclei

Detector covering large solid angle

Particle identification
Total multiplicities
Kinetic energies
Angular distribution

8

9 of 43

Experimental Setup

Secondary experiment at ASACUSA-CUSP

9

10 of 43

Detection scheme

Cube-like geometry of pixel detectors around target foil

Time of Arrival (ToA) and Time over Threshold (ToT) in each pixel

Calibrations to get energy deposit�

Particle ID by energy deposit and cluster shape

Test beams with known particles + energies�

Annihilation vertex reconstruction

Reconstruct particle tracks
Extrapolate to get vertex

10

11 of 43

11

4

Assessment of Timepix4

3

Detector Design

2

Annihilation Studies Project

Antiproton-Nucleus Annihilation

1

12 of 43

Detector Design

Initial idea:�Combine existing 4 Timepix3 chips with 5 new Timepix4 chips

BUT

Complicates DAQ as two systems �would need to be combined
Different time resolutions
Timepix3 shows volcano effect �If the deposited energy in a pixel is > 500 keV, �a random value is assigned

12

Timepix4 - available in summer

55 μm pixel size

512 x 448 pixels

Time resolution < 200 ps

Timepix3

55 μm pixel size

256 x 256 pixels

Time resolution ~ 1.6 ns

https://indico.cern.ch/event/1121147/attachments/2389692/4084909/Xavi_DS_11Feb2022.pdf

13 of 43

Detector Design

Current best concept:�2 + 5 new Timepix4 chips

13

covers ~ 2.9 π sr

including PCB boards:

ongoing discussions with �NIKHEF for PCB design + construction

beam with 1 cm diameter is able to enter

14 of 43

Exploring Geant4 Physics Models

Only Fritiof model still available, �has different options for electromagnetic processes

Geant4 group suggested EMZ model for low energies� … but “low energy” for Geant4 means 100 Mev

Goal: “Reach stability within one model”

14

15 of 43

Simulation Results

Do the antiprotons actually annihilate on the foil?� Geant4:“No, not all of them…” Fluka: “Yes, absolutely all of them!”

15

16 of 43

Geometries

For PCB extending on one side of chip:

For PCB extending on all sides of chip:

16

“narrow”

“wide”

17 of 43

Multiplicities

Comparing two�geometries

Detection ratio of �~ 70 %

17

FLUKA

Geant4 - only annihilation events

18 of 43

18

4

Assessment of Timepix4

3

Detector Design

2

Annihilation Studies Project

Antiproton-Nucleus Annihilation

1

19 of 43

Test Beam Campaign @VERA

Timepix4v1 with 300 µm Si and Spidr4v0 (DAQ System)
100 Hz - 1 kHz event rate
p beams of 1.2 - 4 MeV
¹⁴C beam of 10.8 MeV

19

20 of 43

Energy Calibration

Testpulse measurements for charges from 0.3 - 22 ke (~ 1 - 80 keV)

Incorporated test pulse mode:�(Known) charge can be injected in test capacitor by external voltage pulse

Surrogate function used to fit the data

ToT(E) = a ᐧ E + b - c / (E - t)

Non-linear in low energies
Linear in higher energies

20

21 of 43

Per Pixel Energies

No volcano effect! (Tpx3 had cutoff ~ 500 keV)

21

22 of 43

Clustering

Clustering algorithm on data matrix, depends on position and time of arrival

22

Cluster diameters ~ 9 px

Most energy in center 1 px

Cluster diameters ~ 11 px

Most energy in center 4 px

Cluster diameters ~ 13 px

Most energy in center 9 px

23 of 43

Cluster Energies

Pixel energy within each cluster is summed up

23

24 of 43

Cluster Energies

Discrepancy to measured cluster energy grows with beam energy:

24

25 of 43

Calibration for Higher Energies

For Timepix:�(1st generation)

25

Sommer, M. et al., NIM A (2022)

26 of 43

Better Calibration Methods

Test pulses only go up to ~ 80 keV

Alternatives:

X ray/Gamma sources�Take into account only 1 pixel “clusters”�
Calibration method using alpha source �Use calibrated low energy pixels to compare�energy difference with high energy pixels

26

http://cds.cern.ch/record/2686194/

Sommer, M. et al., NIM A (2022)

27 of 43

Outlook

Digitise Simulation Data

Develop code for track reconstruction

Gather more data for known particles + energies

Refine energy calibration
Develop code for particle identification

Timepix4 available this summer

Before: finalise detector geometry
Energy Calibration for each chip
Detector construction
Measure annihilations in beam time 2024

Still measure backscattering with adapted Tpx3 setup?

27

28 of 43

28

29 of 43

3D Track Reconstruction - Bergmann, B. et al, EPJ C (2017)

Used Tpx3 similar to time projection chamber�500 μm thick Si sensor at 130 V bias�120 GeV/c pion beam
Works only for particles penetrating �the sensor completely (then path length is known)
Drift time:�
Compare difference in drift time along particle trajectory (ΔToA)
z-resolution of 50.4 μm could be achieved

29

30 of 43

Timepix Detector

30

	Timepix3	Timepix4
Pixel Size	55 x 55 μm	55 x 55 μm
Pixel Arrangement	256 x 256	512 x 448
Area	14.08 x 14.08 mm²	28.16 x 24.64 mm²
ToT energy resolution	< 2 keV	< 1 keV
ToA time resolution	1.56 ns	195 ps

https://indico.cern.ch/event/1121147/attachments/2389692/4084909/Xavi_DS_11Feb2022.pdf

4 of them used in 2018

Will be available this summer

31 of 43

Average Multiplicities - Aegis

31

Aghion, S. et al., Journal of Instrumentation (2017)

32 of 43

32

5

Assessment of Timepix4

4

Detector Design

3

Annihilation Studies

Antiproton-Nucleus Annihilation

2

1

Why do we study antimatter?

33 of 43

33

5

Assessment of Timepix4

4

Detector Design

3

Annihilation Studies

Antiproton-Nucleus Annihilation

2

1

Why do we study antimatter?

34 of 43

Antimatter Asymmetry

Standard Model of Particle Physics�and Big Bang Theory:

Matter and Antimatter �were produced equally

Over-abundance of matter means:

Some symmetry must be broken!

34

“

”

35 of 43

Antimatter Asymmetry

Sakharov Criteria

Baryon number violating process
Out of thermal equilibrium
C and CP symmetry is broken �to prefer this process

35

?

Already in the 1960s Sakharov proposed three conditions that must have been fulfilled at an early stage of the universe to explain the existing baryon asymmetry. There must be a process which violates the baryon number - this means that more matter particles come out of it than go into it. This process must happen at a time where there is no thermal equilibrium, otherwise it would also happen in the other direction. And the C and CP symmetry must be broken to make this process the preferred one. I will explain these symmetries in a second, but just know that C and CP symmetries were already found to be broken, but the extent of the “broken-ness” is not enough to explain the asymmetry.

So maybe not only CP, but also CPT is broken?

These letters stand for Charge conjugation, parity transformation and time reversal. For some time it was thought that after each individual of these transformations the laws of physics should still be the same. C turns a particle into its antiparticle, P inverts the spacial coordinates and T the time coordinate.

Here we see a negatively charged particle with linear momentum and angular momentum, defining its handedness. The charge conjugation turns this into a positive particle - its antiparticle, with the momenta staying the same. This symmetry is broken by the weak force because neutrinos only exist left-handed while antineutrinos were only found right-handed. Since parity symmetry was also already found to be broken next people thought well then the combination CP must be a fundamental symmetry - here it turns a righthanded particle into its lefthanded antiparticle with opposite linear momentum. But also this was found to be untrue, leaving only CPT as a fundamental symmetry which has not yet been disproven.

36 of 43

36

5

Assessment of Timepix4

4

Detector Design

3

Annihilation Studies

Antiproton-Nucleus Annihilation

2

1

Why do we study antimatter?

37 of 43

37

5

Assessment of Timepix4

4

Detector Design

3

Annihilation Studies

Antiproton-Nucleus Annihilation

2

1

Why do we study antimatter?

38 of 43

38

5

Assessment of Timepix4

4

Detector Design

3

Annihilation Studies

Antiproton-Nucleus Annihilation

2

1

Why do we study antimatter?

39 of 43

39

5

Assessment of Timepix4

4

Detector Design

3

Annihilation Studies

Antiproton-Nucleus Annihilation

2

1

Why do we study antimatter?

40 of 43

Antimatter Asymmetry

Sakharov Criteria

Baryon number violating process
Out of thermal equilibrium
C and CP symmetry is broken �to prefer this process

Is it actually CPT breaking?

Charge conjugation ѱ(t, x) ➝ ѱ*(t, x)
Parity transformation ѱ(t, x) ➝ ѱ(t, -x)
Time reversal ѱ(t, x) ➝ ѱ(-t, x)

40

Already in the 1960s Sakharov proposed three conditions that must have been fulfilled at an early stage of the universe to explain the existing baryon asymmetry. There must be a process which violates the baryon number - this means that more matter particles come out of it than go into it. This process must happen at a time where there is no thermal equilibrium, otherwise it would also happen in the other direction. And the C and CP symmetry must be broken to make this process the preferred one. I will explain these symmetries in a second, but just know that C and CP symmetries were already found to be broken, but the extent of the “broken-ness” is not enough to explain the asymmetry.

So maybe not only CP, but also CPT is broken?

These letters stand for Charge conjugation, parity transformation and time reversal. For some time it was thought that after each individual of these transformations the laws of physics should still be the same. C turns a particle into its antiparticle, P inverts the spacial coordinates and T the time coordinate.

Here we see a negatively charged particle with linear momentum and angular momentum, defining its handedness. The charge conjugation turns this into a positive particle - its antiparticle, with the momenta staying the same. This symmetry is broken by the weak force because neutrinos only exist left-handed while antineutrinos were only found right-handed. Since parity symmetry was also already found to be broken next people thought well then the combination CP must be a fundamental symmetry - here it turns a righthanded particle into its lefthanded antiparticle with opposite linear momentum. But also this was found to be untrue, leaving only CPT as a fundamental symmetry which has not yet been disproven.

41 of 43

Exploring Geant4 Physics Models

Comparing ways to set step size of 100 nm (= 10 steps inside foil)

41

Right now I am still working on simulations, where I actually came upon another still unanswered question - will these very low energy antiprotons actually annihilate?

Geant4 says something like nooo, not really… I did simulations with all the different foils we will want to use and as you can see for most of them, only about a third of the simulated particles are actually annihilating and not just being reflected. This is not at all what we expected, and FLUKA simulations are currently made to compare the results, which for now look like they should all annihilate there. But since this was never done, we actually cannot say which one is more correct!

The good thing about this is - even if it is the case that the antiprotons are more like to be reflected than to annihilate we will still be able to see this in the backplane of our detector. So we are hopefully able to settle which simulation programme is the most correct and we are also in contact with a theory group from Paris and the Geant4 people who will include our data in a new low energy physics model for their software.

42 of 43

Simulation Results - Geant4

Comparing two geometries - 2.9 π sr (narrow) and 2.2 π sr (wide)

42

43 of 43

Simulation Results - FLUKA

Comparing two geometries - 2.9 π sr (narrow) and 2.2 π sr (wide)

43