WG7: Experimental signatures of new physics


Maurizio Pierini, Chris Rogan

Mandate/Job description:

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Group activities:

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  1. A parametric/fast simulation of displaced vertices and Heavy Stable Charged Particles: many interesting signatures of new physics at FCCee involve the reconstruction of long-living neutral and charged particles. This kind of signatures usually involve the detailed simulation of detector effects with GEANT. On the other hand, examples exist in literature of parametric simulations of signal efficiency, as for instance for the HSCP search by CMS. It would be extremely usefult to define a baseline of FCCee detector performance for these signatures, based on the extrapolation from LEP and LHC detectors and eventually on the comparison with the GEANT simulation of the FCCee detector concept. 

  2. Kinematic variables for New Physics: given the progresses made in the recent years for New Physics searches at the LHC (e.g. in the domain of SUSY searches), the strategy for NP searches at an electron-positron collider could be revised, combining what we learned at the LHC with the advantage of knowing the center-of-mass frame. 

  3. SUSY searches in LHC blind spots: while the LHC extended the searches for R-parity conserving SUSY to the TeV scale, a few blind spots were left behind. Running at different energies (Z, WW, ZH, tt), the FCCee physics program offer the opportunity of exploring these blind spots in detail, taking advantage of the clean environment. A few examples follow

    1. Ewikinos in compressed spectra: EWkino production at the LHC is a rare process. Typical signatures are boson (W, Z, H, photon) pairs. If the mass difference between the produced sparticles and the lightest SUSY particle is smaller than ~ 100 GeV, off-shell bosons are produced, which are difficult to reconstruct (soft particles) and discriminate from the corresponding Standard Model background. These difficulties translate in weak bounds from the LHC, particularly for light Ewkinos. This unexplored portion of the parameter space could be explored with the FCCee detector, particularly at the WW and ZH thresholds, looking for an excess of events with large missing mass. A further improvement could be obtained developing new kinematic variables.

    2. Squarks in compressed spectra: while squarks are abundantly produced at the LHC (due to the large cross section), it is difficult to probe compressed squark-neutralino spectra, due to the difficulty in triggering events with soft jets. Instead, events with two soft jets + missing energy could be probed at the FCCee. Particularly interesting is the case of light top squarks, where the use of a charm tag could help in the identification of stop -> charm LSP decays.

    3. Stealthy stops: the LHC analyses have put stringent bounds on top squark pair production but they lack sensitivity to models in which the mass separation between the top squark and the lightest SUSY particle is close to the top mass. While this scenario is accessible through the cascade decays of gluinos, the gluino might be heavy enough to escape the direct detection at the LHC. The final state (t-tbar+MET) would be difficult to discriminate from the t-tbar background. Instead, the clean environment of the FCCee could allow to exploit the different properties (fermion vs boson pair production) in order to enhance the discrimination against Standard Model top pairs. This topic is of specific interest for the physics case of the run at the t-tbar threshold.

  4. Dark Matter production: Dark matter candidates (e.g. the lightest SUSY particle) could be produced in pair at the FCCee. One could detect these events from the initial state radiation of a standard model particle at large momentum (e.g., photon, a Z, or a Higgs). Similarly to the case of the monojet analyses at the LHC, dedicated searches should be put in place. Dark Matter scenarios could be explored, different than those probed at the LHC. The assessment of the sensitivity and the comparison to the LHC and underground detection experiment should be addressed. 

  5. Exotic signatures: many exotic scenarios (e.g. right-handed neutrinos, dark photons, etc) predict the possibility of producing exotic signatures in the FCCee collider. An assessment of the physics potential in these scenarios is needed, for a strong physics case to be built. In view of dedicated experiments being proposed to search for New Physics in these scenario, the complementarity of the FCCee physics reach should be addressed. Maintaining the sensitivity to these signatures has a direct implication on the detector design and could be used as a benchmark to define the detector geometry and the choice of technology. 

  6. Rare decays of the Higgs boson: with 2 million Higgs boson events to be produced, FCCee has the potential to probe exotic decays of the Higgs bosons, predicted in many scenarios of physics beyond the SM. Assuming a  minimum need of ten signal events to be detected, with typical reconstruction efficiency of 10%, BR as small as 5E-5 could be probed. Following the example of http://exotichiggs.physics.sunysb.edu/web/wopr/ the sensitivity of FCCee to exotic final states (e.g. 4b, 4photons, 4taus, 2b+missing energy, etc) should be determined.

  7. Sensitivity to Dark Photon scenarios: FCCee could probe dark photons from direct production and final-state radiation, e.g. in the decays of Z bosons. 

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