Skip to main content

Directions for BSM Physics after the First Run of the LHC

Final Report Summary - BSMAFTERLHC8 (Directions for BSM Physics after the First Run of the LHC)

The main objective of the Project BSMafterLHC8 was to gain a better understanding of what could lie beyond the Standard Model (SM) of Particle Physics – our currently best description of nature at smallest length scales – such as to help guiding us to a more fundamental theory.

To this end, several avenues were explored. On the one hand, crucial predictions of extensions of the SM were derived and confronted with experimental data, in particular from the CERN Large Hadron Collider (LHC). Beyond that, new completions of the SM were built, featuring aesthetically attractive features (like simplicity and unification of different aspects), but also solving concrete problems of known models. Finally, new strategies to extract parameters of a 'model independent' effective field theory (EFT) extension of the SM – parametrizing nature in the most general way in agreement with low energy observations – were proposed. Prospects how accurately such parameters could be extracted at the LHC were derived and the validity of such extractions was discussed comprehensively. In detail, the following work was performed.

New Composite Higgs Models

Directly at the beginning of the project, I developed a new incarnation of the lepton sector in composite Higgs models (where the Higgs boson – associated to the generation of masses for elementary particles – is a bound state of some new strong interaction), which was not present in the literature before [1]. This ameliorated significantly an issue with such setups, namely the tension between the lightness of the found Higgs boson and the simultaneous experimental absence of expected light top-quark partners. The model requires only two composite multiplets and is motivated from the scale of neutrino masses. In fact, I pointed out a connection between the seesaw mechanism to generate neutrino masses and a moderate compositeness of charged leptons, allowing to lift the top-partner masses for a light Higgs boson, in agreement with LHC results. I thus showed that the absence of light partners does not yet threaten the idea of a natural and minimal composite Higgs.
I also investigated predictions for Higgs Physics at the LHC in the context of this analysis, delivering for example several new analytic results for the production of the Higgs boson in gluon fusion, summing infinite towers of excitations in closed form, and considering various modifications of the quark sector of the setup, which also can be different from known models.

Flavor Physics in Composite Models

Since my new model [1] promised a very interesting new phenomenology in the lepton-flavor sector (the sector of the electron and its heavier brothers), I scrutinized this in detail. I found an intriguing possibility to protect it from too large flavor-changing neutral current effects and pointed out that an anomaly seen by the LHCb experiment in testing the universality of lepton interactions in B meson decays could in fact be explained in that setup [2]. I also demonstrated that other constraints, like on flavor non-universality in charged current interactions, could be met.

Model-Independent Survey of Double Higgs Production and Properties of the Higgs Potential

I provided a comprehensive analysis of the impact of higher dimensional operators in the EFT framework on the production of Higgs-pairs at the LHC, which offers unique possibilities to get a handle on physics beyond the Standard Model [3]. My analysis delivered for example for the first time the expected constraints on the |H|^6 operator, taking into account all other potentially relevant contributions of new physics to double Higgs production in the EFT.
I also showed the viability of a special new scenario, not entertained before, where the |H|^2 term could deviate by ~100% from the Standard-Model expectation [4]. It turned out that even its absence would be viable phenomenologically, which is linked to the lightness of the Higgs boson. I also examined consequences for cosmology, finding that the latter scenario would allow for electroweak baryogenesis, which is not possible within the Standard Model [4].
Finally, I teamed up with experimentalists to enhance the sensitivity of my analysis of Higgs-pair production in [3] by studying distributions and defining characteristic benchmarks to be searched for with high priority at the LHC [5], and we provided an useful analytical parametrization of the cross section [6].

New Method to Extract the Charm-Quark Yukawa Coupling

In my project, I also studied prospects to constrain the Higgs couplings to light fermions. In particular, in the PRL article [7], I proposed a new method to measure the Higgs coupling to the charm quark, which will be helpful to understand the hierarchical structure of the fermion(-mass) sector.

Scrutinizing the Properties of a New Scalar Resonance

Models with an additional (pseudo-)scalar singlet with a mass of several hundred GeV represent a well motivated class of extensions of the SM, including composite Higgs scenarios, supersymmetry, Coleman-Weinberg models, models addressing the strong CP problem, as well as generic Higgs portal setups. I analyzed indirect constraints on properties of such a scalar resonance, motivated in particular by a bump in the di-photon invariant mass spectrum seen in CMS and ATLAS data in 2015 [8]. I examined correlations of such a signal with precision observables and flavor anomalies. I also showed how calculability and unitarity severely limit possible perturbative realizations of a sizable excess as well as how they could be tested indirectly in the Drell-Yan process at the LHC [9].

Constraining Higgs Portal Interactions

I also surveyed the possibility to constrain 'portal' interactions between the Higgs boson and a (generic) new scalar singlet, by measuring associated production of the scalar with the Higgs at the LHC [10]. Moreover, I demonstrated how the corresponding final state could be used to gain insights on the production mode of such a scalar.

Validity of the Effective Field Theory Approach

I analyzed in detail the validity of the EFT approach to SM precision tests, where new physics effects are parametrized 'model independently' by higher dimensional operators consisting of SM fields, suppressed by the scale of new physics. I pointed out the importance of a power counting scheme in accessing the error due to the truncation of the EFT, and gave concrete examples of how a consistent extraction of a bound could be obtained – demonstrating the interplay between the experimental accuracy and the EFT error [11].

Finally, the research carried out during the project lead to a large number of contributions to working group reports, such as the Les Houches 2015: Physics at TeV colliders report [12], and in particular to three different contributions to the Handbook of LHC Higgs Cross Sections: 4. Deciphering the Nature of the Higgs Sector [13].

In summary, the results following from the Marie Curie Research Project lead to advances and new insights in several fields of high energy physics, helping to improve our understanding of nature at the most fundamental level.