LCWS 2019

Abstracts

The CLIC Physics Potential

• Speaker: Wolfgang Kilian
• Status: invited
• Abstract: not needed
• Slides

Inert Doublet Model signatures at Future e+e- Colliders

• Speakers: Aleksander Filip Zarnecki
• Track: #1, Physics at e+e- colliders, models of new physics, tests of the Standard Model through global fits
• Status: accepted as talk
• Abstract: The Inert Doublet Model (IDM) is one of the simplest extensions of the Standard Model (SM), providing a dark matter candidate. It is a two Higgs doublet model with a discrete Z2 symmetry, that prevents the scalars of the second doublet (inert scalars) from coupling to the SM fermions and makes the lightest of them stable. We study a large statistics of IDM scenarios, which are consistent with current constraints on direct detection, including the most recent bounds from XENON1T experiment and relic density of dark matter, as well as with all collider and low-energy limits. We propose a set of benchmark points with different kinematic features, that promise detectable signals at future e+e- colliders. Two inert scalar pair-production processes are considered, e+e- --> H+H- and e+e- --> AH, followed by decays of H+/- and A into the final states which include the lightest and stable neutral scalar dark matter candidate H. Significance of the expected observations is studied for different benchmark models and different running scenarios, for center of mass energies from 250 GeV up to 3 TeV. For low mass scenarios, high signal observation significance can be obtained for the signal signatures with two muons or an electron and a muon in the final state. For high mass scenarios, which are only accessible at high energy stages of CLIC, the significance is too low for the leptonic signature and the semi-leptonic final state has to be used as the discovery channel. Numerical results presented for this channel are based on the fast simulation of the CLIC detector response with the DELPHES package.
• Slides

Sensitivity to new physics scenarios in invisible Higgs boson decays at CLIC

• Speakers: Aleksander Filip Zarnecki
• Track: #1, Physics at e+e- colliders, models of new physics, tests of the Standard Model through global fits
• Status: accepted as talk
• Abstract: The Compact Linear Collider (CLIC) is a proposed TeV -scale high-luminosity electron-positron collider, an attractive option for the next large facility at CERN. Already the first CLIC stage, with energy of 380 GeV, will allow us to study the Higgs boson properties with very high precision. These measurements can also result in direct or indirect discovery of "new physics", Beyond the Standard Model (BSM) phenomena, which could help us to understand the nature of dark matter (DM). Higgs boson decays with emission of invisible DM particles can be the only way to observe "new physics" effects at low energy scales and establish connection between Standard Model (SM) and BSM sectors. We studied the possibility of measuring invisible Higgs boson decays with experiment at CLIC running at 380 GeV and 1.5 TeV. The analysis is based on the WHIZARD event generation and fast simulation of CLIC detector response with DELPHES. The approach consisting of a two step analysis was used to optimize separation between signal and background processes. First, a set of preselection cuts was applied to remove background events not consistent with expected signal signature; then, the multivariate analysis methods were applied to optimise significance of observations. We estimated the expected limits on the invisible decays of the 125 GeV Higgs boson, as well as the cross section limits for production of an additional neutral Higgs scalar, assuming its invisible decays, as a function of its mass. Extracted model-independent branching ratio and cross section limits were then interpreted in the framework of the vector-fermion dark matter model to set limits on the mixing angle between the SM-like Higss boson and the new scalar of the "dark sector".
• Slides

Multiple production of weak bosons in e+e- collisions

• Speakers: Wolfgang Kilian
• Track: #1, Physics at e+e- colliders, models of new physics, tests of the Standard Model through global fits
• Status: invited
• Abstract: An e+e- collider with a c.m. energy greater than 250 GeV will be capable of producing more than two massive bosons (W, Z, and Higgs) in an elementary process. We give an overview on the expected phenomenology for different energy ranges: below 800 GeV where multiple thresholds open up, beyond 1 TeV where a large set of multi-boson interactions can be studied in detail, and the asymptotic range beyond 10 TeV where multi-boson final states approach the form of electroweak jets. We discuss questions about the physics of those processes that can be addressed at a linear collider in the context of our current theoretical understanding, and in relation to data samples that eventually will be collected at the HL-LHC.
• Slides

Extracting the top-quark mass and Yukawa coupling from the threshold scan at CLIC

• Speakers: Kacper Nowak (remote), Aleksander Filip Zarnecki
• Track: #2, Analysis and reconstruction of e+e- processes, event simulation, physics performance
• Status: accepted as talk
• Abstract: The Compact Linear Collider (CLIC) is a concept for a next-generation accelerator at CERN, colliding electrons and positrons at energies up to several TeV. One of the main goals at the initial CLIC running stage is the measurement of the top-quark mass and width in a scan of the beam energy through the pair production threshold. However, the shape of the threshold cross section depends not only on the top-quark mass but also on other model parameters as the top-quark width, top Yukawa coupling and the strong coupling constant. We study the expected precision of the top-quark mass determination from the threshold scan. We use the most general fit approach with all relevant model parameters taken into account. In addition, we take the normalisation uncertainty into account as well as the expected constraints on the top-quark mass, top Yukawa coupling and strong coupling constant from earlier experiments. We demonstrate that even in the most general approach the top-quark mass can be extracted with statistical precision of the order of 20 to 30 MeV. Additional improvement is expected if the running scenario is optimized. We also address the feasibility of the top Yukawa coupling determination from the threshold scan.
• Slides

Conformal Tracking for all-silicon trackers at future electron-positron colliders

• Speakers: Frank Richard Simon
• Track: #2, Analysis and reconstruction of e+e- processes, event simulation, physics performance
• Status: invited
• Abstract: Conformal tracking is an innovative and comprehensive pattern recognition technique adopted for the detector designed for CLIC (CLICdet). It uses a cellular network track finding performed in a conformally-mapped plane. It is particularly well-suited for light-weight silicon systems with high position resolution, such as the next generation of tracking detectors designed for future electron-positron colliders. Being based exclusively on the spatial coordinates of the hits, the conformal tracking is very flexible and can easily be adapted to different detector designs and beam conditions. In this talk, the performance of this algorithm developed and validated with simulated data of the CLICdet tracker is presented in terms of tracking efficiency, track parameters resolutions and robustness against the presence of beam-induced backgrounds expected at 3 TeV CLIC.
• Slides

Long-lived particle reconstruction at CLIC

• Speakers: Ulrike Schnoor (remote)
• Track: #2, Analysis and reconstruction of e+e- processes, event simulation, physics performance
• Status: invited
• Abstract: Various new physics scenarios motivate the search for long-lived new particles. Charged long-lived particles could be produced in electron-positron collisions, traveling through the detector and leaving hits before decaying within the vertex and tracking system to an invisible new particle and a very soft pion. CLIC at a centre-of-mass energy of 3 TeV has the potential to discover the thermal Higgsino, a dark matter candidate with a mass of ~1.1 TeV which has a few-millimiter life-time. The clean environment and the low material budget of the CLIC detector make it suitable to search for "stub track" signatures originating from such particles. The performance of the conformal tracking algorithm in CLIC has been studied for the reconstruction of the stub tracks and the soft pions. A definition of stub tracks with a high efficiency for thermal Higgsinos has been identified and the rejection of fake stub tracks from background has been studied based on full detector simulation with beam-induced background at 3 TeV CLIC.
• Slides

Resistive wall effects in the CLIC BDS

• Speakers: Dominik Arominski (remote)
• Track: #3, Detector R&D for e+e- Colliders
• Status: accepted as talk
• Abstract: Resistive wall wakefields in the Beam Delivery System (BDS) can cause severe multi-bunch effects, leading to beam quality and luminosity losses. The resistive wall effects depend on the beam pipe apertures and materials, which have to be optimised to limit the impact on the beam. The resistive wall impact on the beams in the CLIC BDS at 380 GeV and 3 TeV is studied using the PyHEADTAIL software. Different apertures, which take into account the quadrupole magnet pole field limitations and collimation depths, as well as steel and copper beam pipes, are simulated to optimise the BDS. To speed up the convergence for an optimised design, the evolution of the radii of normalised phase-space along the accelerator is used to find the regions especially sensitive to wakefields. For each layout, the luminosity performance of beams impacted by wakefields is simulated in Guinea-Pig. It was found, that some of the initial designs, based on a previous resistive wall study, provide insufficient beam stability and lead to non-monotonic beam behaviour. A detailed study is presented and optimised designs are proposed.
• Slides

Mitigation of occupancies in the CLIC HCal and MuonID endcaps

• Speakers: Dominik Arominski (remote)
• Track: #3, Detector R&D for e+e- Colliders
• Status: accepted as talk
• Abstract: Alarge number of unwanted particles would be produced in electron-positron collisions in the future Compact Linear Collider. These particles, incoherent electron-positron pairs, and gg->hadrons deposit substantial amounts of energy per bunch train into certain detector regions and cause high occupancies in the endcaps of the Hadronic Calorimeter (HCal) and Muon Identification System (MuonID). The backgrounds are simulated using Guinea-Pig for two CLIC energy stages: 380 GeV and 3 TeV. The produced particles are embedded in full detector simulations in DD4hep/lcgeo detector model. The occupancies are analysed, and mitigation methods of varying the detector granularity and introducing shielding are studied. A two-granularity design with a lead shield is proposed for the HCal and a steel shield for the MuonID, which together bring the occupancies back to a manageable level.
• Slides