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Beyond the SM : New Physics

Despite its many successes in describing and predicting numerous phenomena, there are several reasons suggesting that the SM cannot provide the ultimate description of particle physics. In addition to its theoretical caveats, the SM cannot account for three important observations :

  • massive neutrinos and leptonic mixing,
  • the baryon asymmetry of the Universe (BAU)
  • nor can it provide a candidate for the most abundant form of matter in the Universe, dark matter (DM).

Furthermore, there are currently several apparent tensions – typically at the 3 \sigma level – between the SM and experimental data (for instance the anomalous magnetic moment of the muon). The research carried in the Theory Pole revolves around three major axes :

  • direct searches for NP (at the LHC, and other future high-energy facilities) ;
  • indirect searches – looking for NP signatures emerging in heavy flavour decays, rare lepton processes (at low and high energies) ;
  • the role of sterile fermions ;
  • DM studies, focusing on dark matter detection and the viability of NP fields as DM candidates.

All these different approaches follow a common strategy, deeply rooted in phenomenological studies, in close contact with experimental data, with the goal of constraining and ultimately identifying or falsifying a given NP model.


Direct searches for NP carried at the Theory Pole have been focused on a well motivated class of SM extensions – supersymmetry (SUSY), in particular in the framework of the Next-to-Minimal Supersymmetric Standard Model (NMSSM), which corresponds to an extension of the MSSM via an additional single superfield. This leads to an extended Higgs sector, containing 3 scalars, 2 pseudoscalars and 2 charged states ; the neutralino sector is also enlarged due the presence of a singlino.

Compared to the MSSM, the NMSSM is theoretically more appealing, as it allows one to address issues such as the \mu-problem, and renders the fine-tuning possibly associated with a 126 GeV Higgs boson mass less severe. The new states and couplings will contribute non-negligibly to a number of observables : flavour (in particular B-physics), dark matter (new candidates, detection prospects), and new collider signatures.

The NMSSM can lead to new, exciting scenarios at the LHC : multi-Higgs, Higgs-to-Higgs decays, modified SUSY chains (difficult detection and new signatures). The unique features of the NMSSM call upon a re-interpretation of the ATLAS and CMS results for SUSY searches (leading to less stringent results on the SUSY parameter space, in particular allowing for a SUSY spectrum still below the TeV). Non-standard search channels have been proposed to identify the two lightest Higgs bosons, and possibly disentangle the NMSSM from other SM extension.

Recent publications :

  • U. Ellwanger and A. M. Teixeira, “Excessive Higgs pair production with little MET from squarks and gluinos in the NMSSM”, arXiv:1412.6394 [hep-ph].
  • U. Ellwanger and A. M. Teixeira, “NMSSM with a singlino LSP : possible challenges for searches for supersymmetry at the LHC”, JHEP 1410 (2014) 113 [arXiv:1406.7221 [hep-ph]].
  • D. Das, U. Ellwanger and A. M. Teixeira, “NMSDECAY : A Fortran Code for Supersymmetric Particle Decays in the Next-to-Minimal Supersymmetric Standard Model”, Comput. Phys. Commun. 183 (2012) 774 [arXiv:1106.5633 [hep-ph]].
  • D. Das, U. Ellwanger and A. M. Teixeira, “Modified Signals for Supersymmetry in the NMSSM with a Singlino-like LSP”, JHEP 1204 (2012) 067 [arXiv:1202.5244 [hep-ph]].
  • D. Das, U. Ellwanger and A. M. Teixeira, “LHC constraints on M_{1/2} and m_0 in the semi-constrained NMSSM”, JHEP 1304 (2013) 117 [arXiv:1301.7584 [hep-ph]].
  • U. Ellwanger, C. Hugonie and A. M. Teixeira, “The Next-to-Minimal Supersymmetric Standard Model”, Phys. Rept. 496 (2010) 1 [arXiv:0910.1785 [hep-ph]].

Indirect searches for NP – looking for deviations between observation and the predictions of the SM for a given observable - are highly complementary to direct ones. Not only can they lead to the exclusion of NP scenarios, but are also instrumental in pointing the way to specific, testable models. Among them, flavour transitions, such as rare heavy meson decays, are excellent probes of NP : only occurring at higher order in the SM, they are very sensitive to the presence of virtual new particles in the loops ; moreover, there is at present abundant experimental data, with very small uncertainties, offering the possibility to search for deviations from the SM’s predictions.

The B-meson sector constitutes by itself a very challenging theoretical problem, involving very different physics scales, from \Lambda_{QCD} to the NP scale, which can potentially be above the TeV. An effective field theory approach was systematically adopted in the studies carried out, keeping the light quarks and the gluons as dynamical degrees of freedom and integrating out any NP particles, as well as the Z and W bosons and the top quark.

The research done at the Theory Pole focuses on B_s \to \mu \mu and B \to K^* \ell \ell, as well as the B \to X_s \gamma decay.Extensive work has been done regarding the angular distributions of the B \to K^* \mu^+ \mu^- decay, an exclusive mode under good theoretical control. To further minimise hadronic uncertainties, optimised observables were constructed. Discrepancies were found between LHCb measurements and SM predictions for some of these observables, and numerous SUSY models were investigated to probe if they could account for the observed tension.

Recent publications :

  • F. Mahmoudi, S. Neshatpour and J. Orloff, “Supersymmetric constraints from B_s \to \mu \mu and B \to K^* \ell \ell observables”, JHEP 1208 (2012) 092 [arXiv:1205.1845 [hep-ph]].
  • F. Mahmoudi, S. Neshatpour and J. Virto, “B \to K^* \mu^+ \mu^- optimised observables in the MSSM”, Eur. Phys. J. C 74 (2014) 6, 2927 [arXiv:1401.2145 [hep-ph]].

On the lepton sector, neutrino oscillations provided the first evidence of physics beyond the SM (in which neutrinos are massless by construction, and hence there is no leptonic mixing). Minimal ad-hoc extensions of the SM can be constructed where neutrino masses and mixings (i.e. violation of neutral lepton flavour) are incorporated by hand. However, in these ad-hoc extensions, rare charged lepton flavour violating (cLFV) transitions have negligible rates.

This implies that the observation of any such transition indisputably signals the presence of NP states ; in order to unveil the underlying NP model (or falsify candidates), studies relying on the synergy of low- and high-energy cLFV observables were carried. The supersymmetrisation of seesaw mechanisms for neutrino mass generation was explored, identifying relevant collider signatures in correlation with low-energy cLFV observables.

In particular, flavour violation was addressed in neutralino decay chains \chi^0_2 \to \ell_i \ell_j \chi^0_1, studying new edges in dilepton mass distributions and slepton mass splittings. The correlation with numerous low-energy observables such as \ell_i \to \ell_j \gamma, \ell_i \to 3 \ell_j, \mu - e conversion in Nuclei, or \Delta r_K (kaon decays) allows to devise strategies to favour or exclude regimes of the type I and III SUSY seesaw.

Other studies aimed at identifying "golden channels" to probe the SUSY seesaw at a future Linear Collider (e^- e^- \to \mu^- \mu^- + E_{miss}).

Recent publications :

  • A. Abada, A. J. R. Figueiredo, J. C. Romão and A. M. Teixeira, “Interplay of LFV and slepton mass splittings at the LHC as a probe of the SUSY seesaw”, JHEP 1010 (2010) 104 [arXiv:1007.4833 [hep-ph]].
  • A. Abada, A. J. R. Figueiredo, J. C. Romão and A. M. Teixeira, “Probing the supersymmetric type III seesaw : LFV at low-energies and at the LHC”, JHEP 1108 (2011) 099 [arXiv:1104.3962 [hep-ph]].
  • R. M. Fonseca, J. C. Romão and A. M. Teixeira, “Revisiting the \Gamma(K \to e \nu) / \Gamma(K \to \mu \nu) ratio in Supersymmetric Unified Models”, Eur. Phys. J. C 72 (2012) 2228 [arXiv:1205.1411 [hep-ph]].
  • A. J. R. Figueiredo and A. M. Teixeira, ”Slepton mass splittings and cLFV in the SUSY seesaw in the light of recent experimental results”, JHEP 1401 (2014) 015 [arXiv:1309.7951 [hep-ph], arXiv:1309.7951].
  • A. Abada, A. J. R. Figueiredo, J. C. Romão and A. M. Teixeira, “Lepton flavour violation : physics potential of a Linear Collider”, JHEP 1208 (2012) 138 [arXiv:1206.2306 [hep-ph]].


Another axis being pursued by the Theory Pole is the study of sterile neutrinos appearing in SM extensions, in particular of their impact for different observables, and of strategies to indirectly search for these states.

Sterile neutrinos are a common feature of several extensions of the SM aiming at accounting for neutrino masses and mixings. Moreover, their existence is motivated by reactor experiments (anomalies) and certain cosmological observations. These states only interact with active neutrinos (and the Higgs), but their non-negligible mixing might modify leptonic charged currents leading to abundant phenomena such as violation of lepton flavour universality, enhanced cLFV, rare meson decays, among others.

Sterile neutrinos can also have a non-negligible impact on flavour conserving observables (as is the case of anomalous magnetic moments), and contribute to neutrinoless double beta decay. We are also addressing indirect searches for these states at a future circular collider, running in the electron-positron mode (FCC-ee), by styudying very rare lepton flavour violating Z decays.

Recent publications :

  • A. Abada, D. Das, A. M. Teixeira, A. Vicente and C. Weiland, “Tree-level lepton universality violation in the presence of sterile neutrinos : impact for R_K and R_\pi", JHEP 1302 (2013) 048 [arXiv:1211.3052 [hep-ph]].
  • A. Abada, A. M. Teixeira, A. Vicente and C. Weiland, “Sterile neutrinos in leptonic and semileptonic decays”, JHEP 1402 (2014) 091 [arXiv:1311.2830 [hep-ph]].
  • A. Abada, V. De Romeri and A. M. Teixeira, “Effect of steriles states on lepton magnetic moments and neutrinoless double beta decay”, JHEP 1409 (2014) 074 [arXiv:1406.6978 [hep-ph]].
  • A. Abada, V. De Romeri, S. Monteil, J. Orloff and A. M. Teixeira, “Indirect searches for sterile neutrinos at a high-luminosity Z-factory”, arXiv:1412.6322 [hep-ph].


Providing viable dark matter candidates (non-luminous, only weakly and gravitationally interacting particles) is paramount to many SM extensions. The strong experimental commitment has led to the accumulation of a vast array of data (bounds from direct and indirect detection, relic abundance), which in turn allows to constrain extensions of the SM. Dark matter studies carried in the Theory Group revolved around prospects of NP candidates, mostly of the WIMP (weakly interacting massive particles) category, regarding indirect detection (investigating products of DM annihilation), direct detection (addressing the interaction of the candidates with SM particles in detectors) and finally on cosmological and astrophysical models.

Indirect DM detection studies are currently focused on the extragalactic gamma-ray background (GRB), in respect to which abundant data has been collected by the Fermi-LAT satellite. If the GRB is indeed due to DM annihilation, then its intensity can provide conservative bounds on the annihilation cross section ; the anisotropies in the gamma-ray sky can also constitute an important DM signature. The angular power spectrum intensity of the gamma-ray flux is studied in Milky Way-like galaxies (pure DM simulations and taking into account contribution of main haloes and satellites).

Recent publications :

  • F. Calore, V. De Romeri, M. Di Mauro, F. Donato, J. Herpich, A. V. Macció and L. Maccione, “Gamma-ray anisotropies from dark matter in the Milky Way : the role of the radial distribution”, Mon. Not. Roy. Astron. Soc. 442 (2014) 1151 [arXiv:1402.0512 [astro-ph.HE]].