Experimental Collider Physics
The experimental collider physics group explores elementary particles and fundamental forces in nature using data from the ATLAS experiment at CERN’s Large Hadron Collider (LHC). The main activities of the group are development of instrumentation, data collection, and physics analysis of the collision data; as well as the design and development of future colliders, namely the Future Circular Collider (FCC) at CERN.
The first proton-proton collisions at the LHC happened in 2009, and the first physics data came early 2010. Since then, the LHC has completed two data-taking runs: Run-1 (2010-2012) with 7 and 8 TeV collision energies and Run-2 (2015-2018) at 13 TeV. Run-3 started in 2022, with an energy of 13.6 TeV, and will continue until the end of 2025.
With the data collected until now, we are sensitive to a variety of new physics models, and can probe properties of elementary particles and fundamental interactions at a precision previously inaccessible.
In Uppsala we are especially interested in investigating if the Higgs boson is as the Standard Model predicts, and explore its connection with top quarks, dark matter candidates, and other hypothetical new particles.
We are currently preparing for the high-luminosity upgrade of the LHC (HL-LHC) by developing new semiconductor technology for tracking. Beyond the HL-LHC, we are involved in the Future Circular Collider (FCC) at CERN, a two-stage project that could push both the intensity and energy frontiers to the next level.
The group welcomes new PhD students and postdocs to join its research program at ATLAS. Our research lines are described below.
Higgs boson pair production
The Standard Model predicts the existence of a scalar particle, the Higgs boson, which was discovered by the ATLAS and CMS Collaborations at the LHC in 2012. Since this scientific breakthrough, the properties of the Higgs boson have been measured with increasing accuracy and, so far, are found to agree very well with the Standard Model predictions. Nevertheless, the structure of the Higgs field as well as the exact shape of its associated energy potential remain postulated, and they have not yet been fully probed experimentally. In the Standard Model, the Higgs field Φ is described by a doublet of complex scalar fields and the Higgs potential takes the form V(Φ) = -μ2Φ2+λΦ4. While the vacuum expectation value of the Higgs field, where the potential is minimal, has been measured experimentally to be 246 GeV, the Higgs self-coupling parameter λ that determines the shape of the potential away from the minimum has never been measured.
The experimental signature of the Higgs self-coupling at the LHC is the simultaneous production of two Higgs bosons (HH). In the leading gluon-fusion production mode, these can emerge from either a loop of quarks or a single virtual Higgs boson that subsequently decays into two Higgs bosons, the latter being a direct manifestation of the Higgs self-coupling.
The cross-section for Higgs boson pair production at the LHC is three orders of magnitude smaller than for single Higgs boson production. Therefore, one does not expect to have evidence for this process before the high luminosity upgrade of LHC (HL-LHC). On the other hand, if Higgs boson pairs were to be observed in the current (Run-2 and Run-3) dataset of the LHC, this would be a clear indication of new physics in the Higgs sector. Modifications of the shape of the Higgs potential, and thereby of the Higgs boson self-coupling, with respect to the Standard Model prediction may indeed enhance the HH production cross-section, as well as modify the kinematical properties of the HH system. Many theories beyond the Standard Model anticipate deviations from the predicted value of λ and/or introduce new resonances that may decay into a pair of Higgs bosons. In the framework of Effective Field Theories (EFTs), new physics beyond the reach of the LHC can be explored in a model-independent manner by looking for low-energy deviations from the Standard Model predictions while ignoring the substructure and degrees of freedom at higher energies.
Figure from Nature 607, 41-47 (2022)
The ATLAS group in Uppsala is pursuing searches for Higgs boson pairs in the final state with two b-quarks and two tau-leptons. Based on the ATLAS Run-2 dataset, this channel currently provides the most stringent exclusion limit on the Higgs boson pair production cross-section, at 3.3 times the Standard Model prediction, among all the individual data analyses performed at the LHC. Our research group also contributes to the statistical combination of ATLAS searches for Higgs boson pairs as well as to their interpretation in terms of the Higgs self-coupling and in EFT frameworks, in order to probe new physics in the Higgs sector beyond the Standard Model. The analysis of the ATLAS Run-3 data is on-going in order to further enhance sensitivity to HH production, again based on the final state with two b-quarks and two tau-leptons.
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The Higgs fine-tuning problem and vector-like quarks
The Higgs boson explains why the other elementary particles have masses, but its own mass is an enigma. The theoretical calculation involves quantum corrections which need to cancel at a precision of 1 to 1019 in order to arrive at the measured Higgs boson mass of about 125 GeV (approximately 125 times the mass of the proton). This is called the Higgs fine-tuning problem. Popular proposed solutions include the existence of new heavy particles “top partners”, from supersymmetric models, to stabilise the corrections. The top partners resemble the heaviest known elementary particle, the top quark, hence the name. In Uppsala, we are interested in an alternative approach, namely that the Higgs boson is not an elementary particle but instead composite. This could among other things give rise to top partners in the form of vector-like quarks with interesting decay patterns.
Vector-like quarks, if they exist, can be created in pairs or singly at the Large Hadron Collider, as indicated in the figure.
![Figure from arXiv:2406.09193 [https://arxiv.org/abs/2406.09193 ]](/images/200.36065822191e4e2203d57a27/1727161458159/VLQ_prod.png)
Figure from arXiv:2406.09193 [https://arxiv.org/abs/2406.09193 ]
The ATLAS and CMS experiment have search for vector-like quarks in many different possible decays, but all previous analyses consider only decays into known, existing particles. However, if vector-like quarks exist, they are very likely accompanied by other exotic particles, such as new heavy scalars (particles with spin 0), which could occur in the decay chains. This would open up completely new possible search channels, which are of interest in our pursuit to understand the nature of the Higgs boson.
In 2018-2023, the Uppsala team was part of the KAW-funded SHIFT (Solving the Higgs Fine-tuning problem with Top partners) project, which was a Sweden-wide theory-experiment collaboration with members from Stockholm University and Chalmers University as well as international collaborators. The current work in Uppsala builds on the expertise and the experience gained from this project.
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Dark matter searches and long-lived particles
The nature of dark matter is one of the most pressing questions of particle physics. Dark matter is prevalent in the Universe but other than that, we know very little about it. Since dark matter seems to be affected only by gravitational effects, we think it could have mass. And if it has mass, then it could be related to the Higgs boson, which couples to mass.
One of the research lines of the ATLAS group in Uppsala University goes into the Higgs sector to explore it as a portal of new physics at the LHC. One of our recent papers covered the first search for composite dark sector particles (dark mesons) decaying to Standard Model particles through Higgs interaction.
In the absence of clear signs of new physics, subtle experimental signatures take central stage. Long-lived particles (LLPs) are new particles with relatively long lifetimes that travel inside ATLAS some distance before decaying into complicated experimental signatures. Lifetime relates mainly to couplings and mass, and so these new particles are usually light or with feeble couplings, as the kind of particles that could arise from a dark sector connect to dark matter.
Data science methods, such as AI/ML can be crucial to reach the full potential of dark sector searches in ATLAS, and in general any searches for new physics at the LHC and beyond and the group is active in that area as well.
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Detector performance and operations
The Uppsala group is also involved in a variety of crucial tasks that ensure the smooth operation and maintenance of the ATLAS detector as well as efficient, high-quality data taking.
Some of the activities performed by the group include the monitoring of the data quality, luminosity measurements, and in different software and computing activities, as well as maintenance of the SCT detector.
Data quality
The ATLAS detector does not only collect data, it should collect the best data. During data-taking it is crucial to continuously monitor the data quality so that problems can be immediately corrected. Otherwise the data collected might be biased or bad in some other way.
At Uppsala University, we are involved in coordination of the data quality effort in ATLAS and we have a long-standing contribution to especially the ATLAS trigger monitoring.
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Luminosity
When the LHC is operating, it produces more than one billion proton-proton interactions every second. The total number of interactions in a given dataset, referred to as the luminosity, is a crucial input to all analyses in ATLAS and must be measured with very high precision. The uncertainty in the luminosity is often one of the main systematic uncertainties in precision Standard Model measurements. Searches for physics beyond the Standard Model also rely on an accurate luminosity measurement to predict the sensitivity to the signal and to estimate background yields.
ATLAS uses several detectors and algorithms to determine the luminosity. The absolute calibration of these algorithms is carried out in LHC runs with special beam conditions at low luminosity. Two of the main systematic effects on the luminosity measurement stem from the calibration transfer from the low-luminosity regime to the high-luminosity conditions typical of standard physics data taking, and changes in the calibration during data-taking across long periods of time.
In Uppsala we use data from several ATLAS systems, including the Inner detector and the calorimeter, to monitor these effects and to reduce the corresponding systematic effects. Our work contributed to the ATLAS Run-2 luminosity measurement which reached a record uncertainty of only 0.83%.
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Software and computing
Software and computing
As part of our contributions to the ATLAS collaboration, our group has provided advanced user support within the ATLAS Distributed Analysis Support Team.
We are currently developing a new, streamlined data format, PHYSLITE, for future use by ATLAS. PHYSLITE is intended to be the main data format for the HL-LHC serving about 80% of all physics analyses in ATLAS. In connection to this, we work on the development of the ATLAS columnar analysis, that runs on PHYSLITE files and streamlines data analysis.
In our research, we explore cutting-edge data science methods for data analysis and collection, including data compression using machine learning. Implementing and refining these selection algorithms is crucial for extracting the maximum amount of information from the data.
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GRID
GRID
The development of distributed computing GRID has been crucial to the success of the LHC program. This innovation has paved the way for distributed computing now available in commercial clouds and remains the flagship for scientific data processing. The GRID engine developed by the Nordic countries, known as ARC, is widely recognized and utilized in computing infrastructures worldwide. ARC is continuously evolving, and we are currently working on enhancing support for submitting computing jobs that require mixed CPU-GPU architectures.
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Instrumentation
The LHC is preparing to increase its luminosity by an order of magnitude in the coming years in what is called the High-Luminosity LHC (HL-LHC). The goal is to discover new physics processes and study known interactions with high precision. This ambitious upgrade necessitates a complete overhaul of the ATLAS experiment, with the most critical component being the replacement of the tracker.
Inner Tracker
The tracker is the detector system in ATLAS closest to the point where the protons collide.
The particles created in the proton-proton collisions travel through the tracker. Charged particles produce small signals in the detector layers of the tracker while passing through. The signals are collected using the tracker’s millions of readout channels. This information can be used to make out the path of the particle. Since the tracker is placed in a 2T magnetic field, the paths will be curved and will allow us to measure the track’s momentum. The spatial resolution of the tracker’s detecting elements is high, and it is possible to identify the decay vertices of a particle with high 3D precision. This allows us to identify particles by their lifetime.
The high interaction rate in ATLAS generates intense radiation that damages the detector elements. With time, radiation damage becomes severe and degrades the performance of the tracker. For this reason we are now preparing for the replacement of the current tracker towards the HL-LHC. The new tracker isa called Inner TracKer (ITK) Uppsala together with our Scandinavian collaborators in Lund, Copenhagen and Oslo is setting up a production line for new silicon strip detector modules that will be more radiation hard, faster and better performing than their predecessors.
In order to use modern production facilities we collaborate with a Swedish electronics company. Initial small scale R&D is done in-house, targeting a production method that can be ported to industry. Specialized interconnection methods, quality assurance and control will be done in academia.
A detector module for the End-Cap strip part of ITK.
Following the module presented in the figure, the module consists of a large segmented silicon sensor wire bonded to Application Specific Integrated Circuits (ASIC). They read out the minimal signal from the sensor and buffer the information until receiving a trigger(order) to send the data to the Data Acquisition System (DAC). There are two small ASICs in the module to take care of the data transfer between readout ASICs and the DAC. All ASICs are mounted on a low weight, high density flexible Printed Circuit Board (PCB). The final ITK will have a large number of detector modules of various sizes depending on the position in the tracker. Because of the size and complexity of the tracker the detector modules will be made in an international collaboration between Europe, America and Asia. The Scandinavian production line is focused on producing all hybrid types for all End Cap modules types for 50% of the total of end cap modules.
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Future Circular Collider at CERN
Our group is dedicated to addressing the fundamental questions that the Standard Model is not yet able to answer. Following the High-Luminosity Large Hadron Collider (HL-LHC), the European Strategy for Particle Physics has identified a Higgs factory as the next collider. This facility will allow us to conduct detailed studies of the Higgs boson, potentially uncovering new physics through precision measurements and direct observations in the less-explored, low-mass regions of phase space.
The group is involved the Future Circular Collider (FCC) at CERN, in all aspects: from accelerator and detector development and R&D, to physics.
Beyond the standard model
We have a leading role in prospective physics searches at FCC-ee that began connecting our interest in long-lived particles from ATLAS to FCC-ee. We identify three main physics questions suitable for the signature-driven analyses we are interested in: Heavy Neutral Leptons (HNLs), Axion-like particles (ALPs), and exotic Higgs boson decays.
We have made studies invetigating the potential sensitivity of each of these cases at FCC-ee at different levels and the work is ongoing to provide ultimate, realistic estimates of the discovery potential of FCC-ee.
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