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Physics at the
Intensity Frontier

Particle physics at the intensity frontier involves probing new physics (NP) by increasing the the experiment’s luminosity rather than it’s energy scale.

The intensity frontier could provide signs of NP in two ways. The first one is measuring SM processes for which theoretical predictions with uncertainties well under control exist: observing a significant discrepancy between the experimental measurement and the prediction would be the sign of NP. This technique is often applied to study processes which are mediated at leading order by loop diagrams. In such diagrams, yet undiscovered particles, with masses beyond the energy of the collisions, could intervene, modifying the rates and the properties of the decay respect to the SM predictions. These measurements need to be extremely precise, so they require a large quantity of data. The second way is searching for processes which are hugely suppressed or forbidden in the SM, and therefore a measurement automatically signifies NP. This could either probe (effective) couplings which do not exist in the SM, or particles at scales much below the energy frontier but which have not been seen so far due to the fact that they are very weakly interacting with SM particles. Some examples are lepton flavour violating decays, axion searches or neutrinoless double beta decay.

From the experimental point of view, the challenge with the intensity frontier is to collect a large and pure enough data sample in order to obtain evidence of NP interactions. Historically the French community has been very active in this domain, participating to the conception and realization, as well as to the analysis of the collected data, of very successful experiments like, for example NA48 and BaBar. The focus of the French community today is on the LHCb experiment, dedicated to flavour physics and currently challenging the SM predictions with many precise measurements; its scope extends well beyond the realm of B physics. Worldwide, several other experiments currently search for NP using high-intensity facilities (notably NA62, MEG), some will start their data taking soon (for example Belle II) and other are in the preparatory phase (for example SHIP and COMET).

From the theory side, it is crucial to have the description of the processes in the SM under control. For example, hadronic effects need to be evaluated precisely using various advanced tools, like lattice calculations, effective field theory, sum rules. Given the need to compare the theoretical predictions with the experimental measurements, the interplay between theory and experiment in this field is essential.

Working Groups

Rare, radiative and semi-leptonic B decays; Charm and Kaon Physics
Conveners: Christina Agapopoulou, Jacopo Cerasoli, Nazila Mahmoudi, Olcyr Sumensari
This WG broadly collects flavour-changing neutral-current decays, whether in the beauty, charm, or strange sector. FCNCs are well-known probes of new physics, because of the concurrence of several suppression mechanisms within the SM. For many of these modes one can take advantage of precise theoretical predictions and experimentally clean observables. The large dataset collected by the LHCb experiment is currently showing a coherent, persistent pattern of deviations with respect to the SM expectations in semileptonic b → s transitions, which clearly deserves further attention on both theory and experimental sides. The WG includes also kaons and charmed mesons, that have marked the origins of flavour physics. Renewed interest in the analysis of their decays is emerging, not only because they provide strong and complementary tests of new effects, but also because of novel experimental opportunities, as well as unprecedented lattice-QCD capabilities.
Non-perturbative aspects
Conveners: Aoife Bharucha, Elisabeth Niel, Méril Reboud
This WG includes aspects such as heavy-flavour production, spectroscopy, and the understanding and measurement of form factors. Not only do these subjects offer reference frameworks to test QCD predictions, but they provide crucial input needed for other measurements and for the interpretation of new-physics sensitive channels. Besides, the existence of exotic bound states of quarks such as tetra- and pentaquarks has now been established but they are not yet fully understood.
Interplay of quark and lepton flavour
Conveners: Vitalii Lisovskyi, Ana Teixeira
Flavour violation in the charged lepton sector is a clear sign of new physics by itself, and many experiments are directly searching for it. Besides, the deviations from the SM observed in lepton universality tests of beauty-hadron decays calls for joint consideration of the quark and lepton sectors, where measurements are combined and theory aspects are developed concurrently.
Light new particles
Conveners: Mark Goodsell, Christopher Smith
Light new particles are commonplace in SM extensions, e.g. as (near-)Goldstone bosons of hidden global symmetries. In such setup, they are actually messengers from high-energy physics. Besides, there is no fundamental reason why such particles should couple universally across the generations, or for their couplings to be flavour-diagonal. And besides, the strongest constraints, that come from astrophysics, occur for coupling to first-generation matter. Hence such particles may be as heavy as a MeV-GeV, and couple non-negligibly to heavier generations. All these considerations make flavour-physics experiments very suitable to probe this sort of new physics. The WG will likewise cover the vast landscape of non-flavour experiments also dedicated to the search of light new particles.
Discrete symmetries
Conveners: Pierre Delahaye, Vincent Tisserand
Discrete symmetries such as parity and CP are broken in the SM. Broken discrete symmetries not only warrant interesting phenomenology — some of them are even necessary conditions for baryogenesis. CP violation in the quark sector has meanwhile provided a set of excruciating tests of the SM, through the measurement of the parameters of the CKM matrix at the B factories. Since these parameters — determined from decays "immune" to NP — are fundamental constants for any prediction, their determination with improved precision will be an important aspect of this WG. LHCb and Belle II will actually be able to provide substantial new insights on the existing channels, and will be capable of additional ones, e.g. in Bs mesons and in b baryons. Other crucial, and complementary, tests of CP violation are provided by EDMs, whose experimental landscape is flourishing, with great opportunities for interdisciplinarity, and with France involved in a central way.
Future experiments
Conveners: Benjamin Audurier, Andrew Morris
Experimental planning is crucial, especially at a time where future upgrades of the LHCb experiment as well as new experiments are being proposed. This GDR aims at playing a role in identifying priorities for the involvement of the French community. France is for example one of the founding countries of CERN, one of its largest contributors, and one of the most active communities within its premises.


If you are interested in joining, proposing a workshop or for any other information about the GDR-InF, please contact Giulio Dujany (IN2P3) or Diego Guadagnoli (INP).

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