Experiment Atlas

ATLAS is a particle physics experiment at the Large Hadron Collider at CERN that is searching for new discoveries in the head-on collisions of protons of extraordinarily high energy. ATLAS studies the basic forces that shape our Universe since the beginning of time.

Scientists and students from the Institute of Particle and Nuclear Physics are participating in analyses of a wide variety of physics processes and also contribute to a smooth operation of the experiment. Our field of study includes measurements of Higgs boson production, top and beauty physics, heavy ion physics, search for particles beyond the Standard Model, forward physics. We have also contributed to a construction of the detector (inner detector, hadronic calorimeter, and forward detector Alfa) and we take part in the upgrade of the detector.

Contacts:

The Higgs Boson

The Higgs boson is an elementary particle required by the Standard Model to explain the origin of particle masses. It was first observed in 2012 at the LHC (Large Hadron Collider) at CERN; the theoretical prediction by François Englert and Peter Higgs was recognised with the Nobel Prize in Physics in 2013. The Higgs boson is extremely short-lived and decays immediately after being produced, so it is observed only through its decay products in the ATLAS detector. Among several possible decays, the H → ττ channel is especially important because it probes the Higgs coupling to fermions.

Htautau decay candidate (Image: ATLAS Collaboration/CERN)

Htautau decay candidate (Image: ATLAS Collaboration/CERN)

Our group has been continuously involved in H → ττ analyses from LHC Run 1 through Run 2 and into the ongoing Run 3 programme. We focus primarily on precision measurements of Higgs-boson production cross sections, and—to a lesser extent—on searches for effects beyond the Standard Model, such as charge–parity (CP) symmetry violation in Higgs production and lepton-flavour-violating (LFV) Higgs decays. Both CPV in Higgs production and LFV decays are forbidden in the Standard Model; any observation would be a clear sign of new physics. Our current work covers all τ-lepton decay channels.

We also contribute essential supporting studies, including the development of data-driven methods to estimate backgrounds in which quark- or gluon-initiated jets are misidentified as τ leptons (“fake τ”), a major background for Higgs and other analyses involving τ leptons. We have also developed expertise in using Machine Learning (Artificial Intelligence) techniques for various tasks in Higgs boson analyses. These include improving signal–background discrimination and optimising reconstruction algorithms.

Publications:


B-physics

B-physics studies hadrons that contain a b (bottom) quark, the second heaviest of the six quarks. Quarks, together with leptons, form fundamental building blocks of all known matter in the Universe. The b-quark cannot be found in nature around us.

Powerful colliders such as the LHC are needed to create hadrons containing a b-quark. Furthermore, these B-hadrons decay rapidly into less exotic and more stable particles. The mean lifetime of B-hadrons is measured in picoseconds; a time so short that particles flying almost at the speed of light travel only a few millimeters before decaying. Thanks to its highly precise tracking detectors – developed in part by members of our department – the ATLAS detector is capable of identifying these decays.

Measurement of the B<sup>0</sup> meson lifetime (Image: ATLAS Collaboration/CERN)

Reconstructed decay B0 → J/ψK*0 (Image: ATLAS Collaboration/CERN)

The Standard Model of particles and interactions is a very successful theory, which has so far managed to satisfactorily explain all the observed phenomena in the world of elementary particles. Despite its success (or because of it), physicists are trying to find its weak points.

Such a discovery would open doors to new development in the field and would help theorists to better understand the universe and formulate new, more complete theories. Testing of the Standard Model and search for New Physics is one of the main goals of the B-physics.

Measurement of the CP violating phase in the Bs decay

Measurement of the CP-violating phase in Bs → J/ψφ

This is done, for example, by studying the lifetime of the B-mesons, or studying decays of Bs mesons to pairs of muons and a K*0 meson, or by measuring the CP-violating phase in the decay of Bs into J/ψ and φ mesons.

So far, experiments ATLAS, CMS, and LHCb have not observed any deviation from expectations of the Standard Model. However, planned operation of the LHC in the next decade promises a significant increase in collected data statistics, which will lead to a great improvement in the current experimental precision and will enable future measurements.

Contacts:

Publications:

Search for particles beyond the Standard Model

One of the main goals of contemporary particle physics is to discover new phenomena beyond the Standard Model, which successfully describes known elementary particles and their interactions, but does not provide answers to fundamental questions – such as the origin of dark matter or the hierarchy of particle masses.

Our group is involved in the search for hypothetical heavy charged particles with relatively long lifetimes, which can leave a characteristic signal in the form of strong ionization in the inner detector of the ATLAS experiment. Due to their large mass, such particles move more slowly, which can be measured using the flight time in other detectors of this experiment.

Our team is involved primarily in the precise time calibration of the Tile Calorimeter using muons, which behave as minimum ionizing particles in the detector. Due to such precise calibration, we are able to very accurately reconstruct the relativistic velocity β for selected events. With the knowledge of β and momentum, we then determine the mass of the hypothetical particle, which is key to its possible identification. We have published the results of the analysis of data from Run 2 and are now continuing to work on new data from Run 3, where we expect further improvements in the sensitivity of these searches.

Contacts:

Publications:

[1] ATLAS Collaboration, Search for long-lived charged particles using large specific ionisation loss and time of flight in 140 fb-1 of pp collisions at √s=13 TeV with the ATLAS detector, JHEP 07 (2025) 140

[2] ATLAS Collaboration, Search for heavy, long-lived, charged particles with large ionisation energy loss in pp collisions at √s=13 TeV using the ATLAS experiment and the full Run 2 dataset, JHEP 06 (2023) 158

Tile Calorimeter

Calorimeters are used in particle physics to measure the energy and direction of flight of particles, both charged and neutral. A primary high-energy particle entering the calorimeter interacts with its material and produces secondary particles.

If these have sufficient energy, they produce further particles in subsequent interactions, but the energy of the secondary particles produced in this way decreases rapidly.  The resulting shower of particles is eventually absorbed by the calorimeter. The signal from the secondary particles is measured in the active parts of the calorimeter, and its total sum is directly proportional to the original energy of the primary particle.

The ATLAS experiment uses several calorimeters. The hadronic calorimeter TileCal is located in the central part and is constructed from alternating layers of absorber (iron) and active medium (plastic scintillator). Charged particles generate scintillation light in the active medium, which is measured by photomultipliers.

TileCal is equipped with several calibration systems that monitor signal propagation at different stages. The corresponding calibration constants are then used to determine the energy of the primary particles from the measured signal; we also measure the time phase of the signal.

Researchers and students from the Institute of Particle and Nuclear Physics have been actively involved in the international TileCal group since its inception. We focus primarily on the energy calibration of the calorimeter, time measurement and time calibration, muon response measurement, quality control of acquired data, and development of the relevant software. At the same time, we contribute to the operation and analysis of data in the ongoing Run 3 (2022-2026), continuing our activities from Run 1 (2008-2012) [1] and Run 2 (2015-2018) [2].

Contacts:

Publications:

[1] ATLAS Collaboration, Operation and performance of the ATLAS Tile Calorimeter in Run 1, Eur. Phys. J C78 (2018) 987

[2] ATLAS Collaboration, Operation and performance of the ATLAS tile calorimeter in LHC Run 2, Eur. Phys. J C84 (2024) 1313

Tile Calorimeter as installed in the ATLAS experimental hall

Basic concept of the Tile Calorimeter.

ATLAS Inner Tracker 

The Inner Detector (ID) of the ATLAS experiment is a cylindrical detector with end-cap disks that directly surrounds one of the particle collision points at the Large Hadron Collider (LHC) at CERN. The future upgrade of the LHC, High-Luminosity LHC, will increase the instantaneous luminosity by a factor of ten.

Consequently, the current Inner Detector will be completely replaced by a new all-silicon Inner Tracker (ITk) [1], composed of pixel and strip sensors. The ITk will provide significantly improved spatial resolution over a wider angular range while maintaining performance in a much more demanding radiation environment.

Our research group has been involved in the development and testing of strip modules for the Silicon Strip Detector (SCT), which is part of the current ATLAS Inner Detector, as well as its upgrade.

Simulated layout of the ATLAS ITk detector, with the inner pixel sub-detector surrounded by the strip sub-detector [1].

Within the ITk project, we have contributed to the design, production, and testing of prototype silicon strip modules [2, 3]. From 2025 onwards, the serial production of ITk modules is gradually ramping up, with our laboratory responsible for the assembly and testing of approximately 600 out of a total of about 7000 silicon strip modules forming the end-cap section of the ITk.

Each module consists of a silicon strip sensor equipped with radiation-hard readout electronics, including hybrids and powerboards. The production takes place in a specially designed ISO 7 cleanroom with controlled temperature and humidity. Our responsibilities include testing sensors and readout electronics, assembly of new modules, and comprehensive quality control.

Contacts: 

Publications: 

[1] ATLAS collaboration, Technical Design Report for the ATLAS Inner Tracker Strip Detector, CERN-LHCC-2017-005, CERN, Geneva (2017) 

[2] J.-H. Arling et all, Test beam measurements and computer simulations of the ATLAS ITk R2 silicon strip detector, 2025 JINST 20 P07033 

[3] Richard Salami et al, “Quality concerns caused by quality control — deformation of silicon strip detector modules in”, thermal cycling tests”, 2025 JINST 20 P03004 

Other useful links