DISCOVERHEP

The LHC delivers an enormous number of proton-proton collisions to the different experiments, but these collisions typically correspond to lower-energy physics processes (black line). It is not possible to record every collision, as this would produce an impossibly large amount of data for the experiments to process and store. Instead, the experiments preferentially select, or trigger, on rare occurrences. Typically, this means that the experiments record all of the high-energy collisions, and only a small number of low-energy collisions (blue area). In contrast, by focusing on the collisions which happen to have been recorded together with the triggering process, data is collected across the spectrum; this data is most relevant at low energy (red area). The exact point at which the blue and red lines cross is constantly evolving with the LHC collisions.

The cylindrical structure of the detector can be unwrapped in terms of its azimuthal coordinate and the pseudorapidity, where the latter is a transformed version of the polar coordinate. The amount of energy deposited in a given geometric region of the detector can then be summed, where the most common result is the production of two regions of significant energy deposition per collision, inferred to originate from two particles. Due to the selections applied as per the figure to the left, there is typically one collision that is very high energy in each recorded event, and the others are lower in energy. However, these lower energy collisions are numerous, and may be the key to discovering new physics.

Discovering New Science in the Noise from LHC Experiments

Deep underground, at the CERN facility beneath the French-Swiss border, is the renowned Large Hadron Collider (LHC), including CMS and ATLAS, two immense machines designed to detect new particles, behaviours and laws in physics that complement and stretch the Standard Model. When in action, they can create more data than is reasonable to analyse.

“The LHC collides protons at an astounding rate, there are more than 2 billion collisions occurring per second. Every one of those collisions may be producing some interesting new particle, but we can’t possibly store everything, it’s simply too much data!” explained Schramm.

Finding needles in the haystack

In 2012, researchers working with the LHC discovered the Higgs boson particle, a fundamental particle that gives mass to other fundamental particles like electrons and quarks. It was a breakthrough discovery at the time, only made possible by breaking boundaries on technological, and experimental limits.

Whilst the results were what was hoped for and expected, the team on the Data

Interpretation Strategy for COmplete Vertex Event Reconstruction in High Energy Physics (DISCOVERHEP) project is revisiting the raw data that was not being scrutinised from the experiments. The data they will analyse is specifically the low-energy regime in the noise, the huge amount of data that has already been recorded but has been ‘ignored’ to date – which they theorise, has the potential to harbour more scientific discoveries that were not being searched for originally.

“We are colliding roughly 60 collisions every 25 ns at the LHC. Each of those sets of 60 collisions is referred to as an event, or one snapshot of the detector, just like taking a picture with a very complex camera. In this event, we further discard all of the collisions except one, which is deemed to be the one interesting high-energy collision. The other 59 collisions are ignored, even though they are part of that same picture. It’s like taking a group picture, only to look at the tallest person; it may be where your eyes go first, but there are lots of others around who have their own stories to tell.

“My project flips this choice around: why not discard the highest-energy collision and look at all the low-energy collisions? This

provides a huge dataset, which can be used to look for the rare production of new particles.” This novel search strategy could power a new era of discoveries in physics.

Matter matters

Whilst the Standard Model is the best proposed ‘model of matter’, describing what particles make things and how those particles interact, there remain open questions and missing pieces to the scientific puzzle.

“The Standard Model is essentially a list of the particles that we know exist, and a list of how those particles can interact with each other,” said Steven Schramm. “Together, this gives us the building blocks that make up the world around us. While it is a fantastic tool, and it describes the world around us in our daily lives, the Standard Model is unable to describe the vast majority of our universe. All of the particles that we know about represent only five per cent of the universe, with the rest being labelled as ‘dark matter’ and ‘dark energy’ because we don’t yet know exactly what they are. We know with certainty that there is something else out there, but the Standard Model does not provide any answers.”

A typical day of proton-proton collisions at the future High-Luminosity LHC, where there are 200 simultaneous proton-proton collisions delivered the ATLAS and CMS Experiments, leading to a very complex picture of what has taken place. A single collision is highlighted in the insert, where cyan shows the charged particles directly originating from the collision, yellow lines indicate charged particles which come from the decays of particles from the same collision, and pink lines indicate charged particles that come from the other simultaneous collisions. The current LHC “only” produces roughly 60 simultaneous collisions, but this number increases regularly.

Searching for low energy collisions

The project team, made up of five researchers, grasped the opportunity to investigate lowenergy collisions at high rates from data supplied by LHC.

“It is entirely possible that new physics is hiding at lower energy, but there are never any guarantees when looking for the unknown. There are reasons to believe that dark matter may lie in this energy regime, and there are other theories of new physics that could be present at low energy. This is why we are following a more generic approach: rather than relying on a specific interpretation of what new physics may look like, we are conducting searches that are as generic as possible; if we see something, we will then have to figure out what it is.”

DISCOVERHEP

Turning noise into data: a discovery strategy for new weakly-interacting physics

Project Objectives

The project involves looking at the dataset delivered by the Large Hadron Collider, and recorded by ATLAS, in a new way. Rather than focusing on the primary collision that caused the event to be recorded, that collision is discarded; all of the other simultaneous collisions, typically discarded as noise, are studied instead. This provides an enormous dataset of hadronic interactions, which may be the key to discovering rare new physics processes.

Project Funding

This article is part of a project that has received funding from the European Research Council (ERC) under the European Union’s Horizon 2020 research and innovation programme (Grant agreement No. 948254) and from the Swiss National Science Foundation (SNSF) under Eccellenza grant number PCEFP2_194658.

Project Participants

• Antti Pirttikoski

• Carlos Moreno Martinez

• Mário Alves Cardoso

• Vilius Čepaitis

forward at what any discoveries may lead to, solving the biggest scientific mysteries normally translates to innovations that benefit us, whilst broadening our knowledge.

“The gap between discovering new physical laws and a subsequent application may be long, but human ingenuity is strong. Discovering electromagnetism has led to our modern world, the weak nuclear force plays a significant role in radiation therapy, and the strong nuclear force will hopefully lead to clean energy through nuclear fusion. It is difficult to predict what the discovery of a new force would imply. Similarly, dark matter is five times more abundant in the universe than normal matter: understanding more about what it is may have profound implications that we cannot begin to guess at right now.”

Contact Details

Project Coordinator, Professor Steven Schramm

Department of Nuclear and Corpuscular Physics

University of Geneva

T: +41 22 379 63 68

E: steven.schramm@unige.ch

W:https://www.unige.ch/dpnc/en/members/ actual-members/s/steven-schramm/

Prof. Schramm received his PhD from the University of Toronto in 2015. He subsequently joined the University of Geneva as a post-doctoral researcher, and after obtaining an ERC Starting Grant and SNSF Eccellenza Professorial Fellowship, he became a professor in 2021. His research involves searches for new physics in high-energy particle collisions at the Large Hadron Collider, with an emphasis on hadronic interactions, machine learning, and advanced data analysis.

DISCOVERHEP is embracing a completely new approach to looking at data, which itself presents practical challenges. The team use Machine Learning to improve their understanding of the data and speed up complex calculations. It can detect the unexpected, through techniques such as anomaly detection or complex classification algorithms. It helps mitigate inherent biases about what new physics might look like. This highly exploratory approach does not presume or assume answers. Looking further

The team have managed to perform the processing of the full dataset and is subsequently able to demonstrate that it is suitable for the intended purposes: searching for low-energy weakly-interacting hadronic physics. The exciting next steps involve searching to find out whether there is evidence of new physics.

“We know that there is more to the Universe, and the key may be hiding in the LHC data, in a direction we just haven’t thought of yet.”

It is entirely possible that new physics is hiding at lower energy, but there are never any guarantees when looking for the unknown.

Prof. Steven Schramm

ATLAS Experiment © 2019 CERN

Steven Schramm and his team of researchers on the EU-funded DISCOVERHEP project are conducting an analysis of previously ignored ‘noise’ data of experiments with the Large Hadron Collider (LHC), searching for evidence of new physics in low-energy interactions.