The Standard Model (SM) of particle physics has been intensively tested experimentally over the past few decades and accurately fits the measurements. The latest major success of the SM is the discovery of the Higgs boson at the LHC in 2012. However, in the SM all is not clear. Many of the properties of the interactions and particles cannot be explained by the SM, such as the number of particle families or the differences in scale of their masses. Moreover, the SM has many free parameters to be determined experimentally, indicating that it is a theory that is only effective at low energy. More importantly, the SM can explain neither dark matter nor dark energy. While theorists are building new models to fill in these gaps, the experimentalists are trying to highlight physics beyond the SM, or ”new physics”, on the one hand by carrying out high-precision measurements to detect inconsistencies in the SM, and on the other by directly looking for new particles.
In 2012, both the ATLAS and CMS collaborations observed a new boson with a mass of approximately 125 GeV whose properties are at present compatible with those of the SM Higgs boson. The analyses of data in the diphoton final state (one of the two most sensitives modes) leading to this discovery probed an invariant mass range extending from 110 to 150 GeV. However, physics beyond the SM (BSM) can also provide a Higgs boson that is compatible with the observed 125 GeV boson. The extended parameter space of several BSM models, for example generalized models containing two Higgs doublets (2HDM) and the next-to-minimal supersymmetric model (NMSSM) gives rise to a rich and interesting phenomenology, including the presence of additional Higgs bosons, some of which could have masses below 125 GeV. Such models provide good motivation for extending searches for Higgs bosons to masses as far below mH = 110 GeV as possible.
Another mystery in the SM is the strong CP problem: why does the quantum chromodynamic (QCD) Lagrangian conserve CP symmetry, to within extraordinarily strict experimental limits, in absence of a correspondent fundamental symmetry? A possible way to make it natural, as introduced by Peccei and Quinn in 1977, is to add an extra global symmetry into the theory, the U(1) symmetry, that is spontaneously broken at some high energy fa. Such a symmetry leads in turn to the prediction of a new light pseudo-scalar particle: the axion, coupling to photons and gluons. A light axion (with a mass ma below the eV) could solve the strong CP problem. More generally, ”axion-like particles” (ALP) appear in any theory with a spontaneously broken global symmetry and can be searched for at particle colliders.
It is thus of primary importance to look for light resonances at the LHC and it is encouraged by many theorists. The X -> gamma gamma decay channel provides a clean final-state topology that allows the mass of a potential new resonance to be reconstructed with high precision. Present published direct searches at LHC in the diphoton decay channel cover a mass range down to about 65 to 70 GeV. Interesting limits could also be achieved in the lower mass range down to about 10 to 20 GeV. For example in ref , a conservative re-interpretation of inclusive diphoton cross section at LHC allows to put limits in this still unexplored region. Extending the mass range in the diphoton decay channel down to as lower masses as possible is what proposed in this thesis. Such a search is specially ambitious because of the difficulty to trigger on the signal. There is already a thesis about this on ATLAS, although the result is still not yet public.
This is the right moment to perform this search at LHC. Run 3 should start in the first half of 2022 and there should be about 133 fb-1 of new data available for the thesis. This very low mass analysis (below 70 GeV) has never been pursued in CMS. The triggers in run 2 were not optimised for this search, their pT thresholds were too high to have a good efficiency. In run 3, there is a possibility to adapt the trigger to gain in efficiency. Also, the run 2 data could still be used to set limits using the available boosted events. Since no limit at all is available at the moment in this mass region, even a mild one would be of interest. The thesis will be divided into 4 parts:
- During the 6 first months and before the start of run 3, the student will work on optimising a trigger for run 3 to gain in efficiency for this very low mass search. This means lowering the photons pT thresholds without taking too much bandwidth.
- Then, during the following year, the student will carry out and optimise the analysis of run 2 data.
- During the next 6 months, once the data in run 3 are recorded with a better suited trigger, the analysis of run 3 data will be carried out as well end eventually combined with the results from run 2.
- Finally, the last six months will be used to write the thesis.
The IRFU CMS group has a great expertise in photon energy measurement, as it has been involved in the ECAL construction and design and has a leading role in its calibration. There were recently several important responsibilities in the group relating to ECAL calibration (ECAL detector performance group conveners, electron/photon physics object group convener, ...). The IRFU CMS group did play an important role in H -> gamma gamma nalysis at 13 TeV. Two members of the saclay group were conveners of the H -> gamma gamma group during run 2. The group is also in close contact with the main authors of the current low mass analysis, based in Lyon (IP2I, IN2P3). The student will greatly benefit from group’s knowledge to lead these studies.