Events

MLIT Director talked about searching for new physics

On 17 – 24 February 2025, the Presidium of the Russian Academy of Sciences in Moscow hosted the "Physics of Fundamental Interactions" session-conference dedicated to the 70th anniversary of the birth of the outstanding Russian theoretical physicist, Academician Valery Rubakov. The event was organized by the Russian Academy of Sciences, the National Research Nuclear University MEPhI, and the RAS Institute for Nuclear Research.

The program of the Physics of Fundamental Interactions conference was designed for five days and embraced eight sections devoted to the main theoretical and experimental issues of fundamental interactions physics, from particle astrophysics and cosmic rays, gravity and cosmology to neutrino physics and fundamental nuclear physics. In total, over 50 reports out of 330 presented at the conference were delivered by JINR representatives.

One of the moderators of the Physics beyond the Standard Model section was Director of the JINR Meshcheryakov Laboratory of Information Technologies Sergei Shmatov.

In his talk, he spoke about new approaches in the search for new physics in the CMS (Compact Muon Solenoid) experiment at the Large Hadron Collider (LHC) at CERN:

“In the LHC experiments, the collision of proton beam bunches occurs with a frequency of up to 30 MHz, moreover, the effect of superposition of events, when several proton-proton collisions occur during one intersection of two bunches, increases the frequency of proton interactions up to 50 times (in the third stage of LHC data-taking). Such an intensity of interactions generates data flows of up to several petabytes per second, even taking into account the fact that not all information is accepted by physical facilities, the total annual data volume can exceed several exabytes. The storage, processing and analysis of such a data volume is, surely, not feasible at present. To reduce the information flow to an acceptable value during data-taking in real time, data is filtered. Events are selected using a system of a trigger with a sequential structure of several levels, at each of which the parameters of physical objects and the event as a whole are assessed, and their values are compared with predefined threshold values. This enables to reduce the data flow frequency to 100 Hz -1 kHz, which corresponds to ~100-1000 MB/s.

With this traditional approach, there often arises the question whether the very existence of predefined threshold parameter values introduces some uncertainty, when the physics we are interested in remains beyond the vision of scientists, being outside the narrowed parameter space. Surely, this problem can be solved by reducing (or zeroing out) these threshold values, however, this would result in a critical increase in the volume of incoming data up to exabytes per year and the impossibility of further operations with data.

For this reason, two system approaches (Fig. 1) to search for new physics at relaxed trigger threshold values are proposed. In the first case, data is processed “on the fly” using online algorithms and a fairly rough estimate of the characteristics, the usual subsequent offline reconstruction is not performed. Nevertheless, it is possible to record some indications of unusual phenomena and then configure a detailed analysis specifically for this area. In this case, the scouting data stream increases to ~5 kHz instead of the traditional ~0.2-1 kHz, and the volume of written data even drops to ~40 MB/s due to the reduced format of events written to disk.

The second mode involves parking data, however, data-taking is performed over a short period of time. Data is then analyzed using more accurate offline reconstruction. This strategy allows for enhanced accuracy in measuring physical processes in the low mass region. Nevertheless, processing such data streams (~3 kHz, ~2000 MB/s) entails enormous computing powers, potentially exceeding the capabilities of the experiment’s computer infrastructure. Therefore, such datasets are transferred unprocessed to tape drives and stored in raw form until additional resources are available, for example, between data-taking periods.

Fig. 1. CMS experiment data-taking scenarios.

Another interesting direction of the development of search analyses in the LHC experiments is the registration of events with topologies unconventional for collider experiments. From the very beginning of its existence, the CMS experiment focused on the registration of events in which the intersection region of beams of colliding protons was considered as the starting point of tracks of charged particles or hadron jets (interaction vertices), i.e., the entire reconstruction is based on binding tracks to this nominal interaction point (IP).

At the same time, there are a number of physical scenarios (theories with dark matter axions, extended supersymmetric scenarios, baryogenesis models, etc.) that predict the production of long-lived particles (LLPs), which after their production at the beam intersection point can fly for a long time without decaying. Thus, the decay point, the vertex, is very strongly shifted from the proton interaction point. This shift can be several millimeters or hundreds of meters. If the decay vertex lies in the volume of the setup, then one can try to reconstruct such an event, but without reference to the IP and even without involving information from detector systems located closest to the IP, for example, an LLP can fly through the entire tracker without decaying and, thus, its reconstruction can rely only on signals from the calorimeter system and/or the muon system. There are more than ten configurations of such topologies, the physical objects that form events can be leptons, photons, jets, form various combinations, and the decay vertices can lie in almost any detector system.

Fig. 2. Topologies of events during the production of long-lived particles.

There may be cases when LLP lifetimes are so long that they decay beyond the ATLAS or CMS physical facilities. To search for such phenomena, so-called satellite experiments are proposed, when large facilities are surrounded by new detectors located from the IP at a distance of up to several hundred meters. Due to such an arrangement, the decay base length increases significantly and, as a result, LLPs with long lifetimes become available for investigation. One example of such facilities is the FASER (ForwArd Search ExpeRiment) project, which is already being implemented near the ATLAS experiment. This facility is located almost 500 m from the IP and aimed at searching for dark photons, axions and other hypothetical particles predicted by theories beyond the SM. This experiment has already obtained new interesting experimental results, namely, high-energy electron neutrinos were observed for the first time in collider experiments. In addition, there is under discussion a new project, MATHUSLA (Massive Timing Hodoscope for Ultra Stable neutraL pArticles), which envisages the construction of a whole football-sized field of scintillator detectors near the SX5 point at CERN, where the CMS facility is located underground. In terms of the possibility of registering completely new signals, this is a highly promising direction, which is of great interest to the JINR group in CMS.

The third new direction in the search for signals of physics beyond the SM, which emerged during the implementation of the LHC experiments’ research program, is associated with the Higgs boson. As known, one of the main goals of these experiments was to detect the Higgs boson and further investigate its properties. More than 10 years have passed since the epochal discovery of 2012, during which physicists have collected sufficient statistics to make fairly precise measurements of the characteristics of this object, namely, the mass is measured with an accuracy of 0.1%, the width, coupling constants, the spin, and parity are defined, and most of the decay channels and birth mechanisms of the Higgs boson are experimentally confirmed. The systematic accumulation of knowledge allowed physicists to turn the Higgs boson into a tool for searching for new physics.

Fig. 3. Possible ways to search for new physics using the Higgs boson.

First of all, new physics should be expected to manifest itself as contributions to processes of rare or exotic Higgs boson decays, such as decays into light leptons (muons and electrons), invisible decays, i.e., decays into particles that are not registered in the detector (for example, neutrinos in the SM or dark matter candidate particles), decays that occur with lepton-flavor violation (LFV), etc. Moreover, it is possible to try to detect such decays of both the already discovered Higgs boson of the SM and scalar states from the extended Higgs sector.

Another way is to use the Higgs boson to directly search for exotic particles predicted by a variety of theoretical scenarios. If the Higgs boson mass is greater than the expected masses of hypothetical particles, then the decay of the Higgs boson into these hypothetical particles is searched for on the basis of their decay into SM particles. Massive hypothetical particles themselves can decay into Higgs bosons, which are then registered in the standard way.

Thus, to the traditional directions in the search for signals of physics beyond the SM, such as the study of supersymmetry (SUSY), the search for non-supersymmetric signals of physics Beyond the Standard Model (BSM) and the search for SUSY and BSM signals in channels with third-generation quarks (Beyond two Generations, B2G), studies with the Higgs boson have been added.”

As Sergei Shmatov noted, scientists from the Meshcheryakov Laboratory of Information Technologies actively participate in this work to search for new physics together with colleagues from the Veksler and Baldin Laboratory of High Energy Physics and the Bogoliubov Laboratory of Theoretical Physics, as well as with colleagues from the RDMS CMS association (a collaboration between CMS of Russia and the JINR Member States