Proceedings of the Erice Workshop:A new baseline for the hybrid, asymmetric, linear Higgs factory HALHF

The HALHF collaboration has discussed a new baseline for the project, taking into account comments from the accelerator community on various aspects of the original design. In particular, these concerned the practicality of the dual-purpose linac to accelerate both colliding positron bunches and the drive beams required for the plasma linac. In addition, many other aspects of the project were also considered; the discussion and conclusions are documented in this paper. Finally, a new baseline is outlined that has been optimised and addresses several weaknesses in the original design, has higher luminosity, reduced centre-of-mass energy boost and additional features such as positron polarization as well as electron polarization. Although HALHF has become longer and more expensive, it remains significantly smaller and cheaper than other mature Higgs factory designs currently under discussion

HALHF: a hybrid, asymmetric, linear Higgs factory using plasma- and RF-based acceleration

HALHF is a hybrid linear collider that uses electron-driven plasma-wakefield acceleration to accelerate electrons to high energy while using radio-frequency cavity technology to accelerate positrons. The most cost-effective solution collides low-energy positrons with high-energy electrons, producing a boost to the final state in the electron direction with $\gamma= 1.67$. The current HALHF baseline design produces a luminosity comparable to that of the baseline ILC but with a greatly reduced construction and carbon footprint and hence much lower cost than the mature linear-collider designs ILC and CLIC. Costs for HALHF are evaluated, together with that for the approximate 15-year R\&D programme necessary to realise HALHF. Time scales and cost for the R\&D are estimated. Upgrade paths for HALHF technology from a 250~GeV Higgs factory, through 380 and 550~GeV, up to 10~TeV are sketched

The CALICE AHCAL: a highly granular SiPM-on-tile hadron calorimeter prototype

The Analogue Hadron Calorimeter (AHCal) of the CALICE collaboration is a technological prototype for future linear collider detectors, addressing scalability, integration and engineering challenges imposed by the experimental environment. It is based on the SiPM-on-tile technology, where the active layers of the calorimeter are formed by 3 × 3 cm$^{2}$ plastic scintillator tiles placed on top of SiPMs mounted on readout boards that also house SPIROC-2E front-end ASICs. A large prototype with 22 000 channels has been constructed using techniques suitable for mass production and automatic assembly. The calorimeter successfully took muon, electron and pion data at the CERN-SPS. Smaller setups for dedicated measurements were also tested later at the DESY-II facility. Main features of the alternative KlauS ASIC and megatile scintillator designs are also presented

The SiPM-on-tile system of the CMS High Granularity Calorimeter Upgrade (HGCAL)

For the CMS High-Granularity Calorimeter (HGCAL) for HL-LHC, scintillator tiles, readout with individual on-tile silicon photomultipliers (SiPMs), will be used where the radiation levels are expected to be less than 5 x 10^13 n/cm^2. The scintillator tiles will be mounted on highly-integrated “tileboards” (typical area 30 x 30 cm^2) that host up to 108 tiles and their SiPMs, as well as front-end electronics, control and powering components. A dedicated LED system will be implemented to monitor stability effects. We present recent developments for the HGCAL scintillator material and SiPMs, including quantification of the scintillator and SiPM radiation-damage impact, modeling of SiPM noise and its evolution with time, SiPM production testing and quality control plans, and tests of tileboards in laboratories and beam-tests

Measurement of differential cross sections and of the Higgs boson mass in Higgs boson decays to bosons using the ATLAS detector

Despite small branching fractions, the $H \to \gamma\gamma$ and $H \to ZZ^* \to 4\ell$ Higgs boson decays provide clean and well reconstructed final states, allowing for precise measurements of the Higgs boson properties. These proceedings present measurements of total and differential fiducial cross sections in the $H \to \gamma\gamma$ and $H \to ZZ^* \to 4\ell$ decay channels, and a mass measurement in the $H \to ZZ^* \to 4\ell$ decay channel. The total fiducial cross section is measured to be $\sigma^{\gamma\gamma}_{\text{fid}} = 65.2 \pm 7.1$ fb in the $H \to \gamma\gamma$ channel and $\sigma^{4\ell}_{\text{fid}} = 3.28 \pm 0.32$ fb in the $H \to ZZ^* \to 4\ell$ channel, in agreement with the Standard Model predictions. Differential cross-section measurements are reported for Higgs boson production- and decay-related observables. The Higgs boson transverse momentum differential cross-section distributions are used to constrain the charm Yukawa coupling modifier (95% confidence level interval on $\kappa_c$ of $[-12, 11]$ in the $H \to 4\ell$ analysis and of $[-19, 24]$ in the $H \to \gamma\gamma$ analysis). Other differential distributions allow constraints on pseudo-observables and effective field theory coefficients. The Higgs boson mass in the $H \to ZZ^* \to 4\ell$ decay channel is measured to be $m_H = 124.92 \pm 0.19 \, \text{(stat.)}^{+0.09}_{-0.06} \, \text{(sys.) }$ GeV. All the results are derived using 139 fb$^{-1}$ of $\sqrt{s} = 13$ TeV proton-proton collisions collected with the ATLAS detector during the Run 2 of the LHC

The SiPM-on-tile system of the CMS HGCAL

For the CMS High-Granularity Calorimeter (HGCAL) for HL-LHC, scintillator tiles, readout with individual on-tile silicon photomultipliers (SiPMs), will be used where the radiation levels are expected to be less than 5 × 1013 neq/cm2. The scintillator tiles will be mounted on highly-integrated “tile-boards” (typical area 30 × 30 cm2) that host up to 108 tiles and their SiPMs, as well as front-end electronics, control and powering components. A dedicated LED system will be implemented to monitor stability e↵ects. We present recent developments for the HGCAL scintillator material and SiPMs, including quantification of the scintillator and SiPM radiation-damage impact, modelling of SiPM noise and its evolution with time, SiPM production testing and quality control plans, and tests of tile-boards in laboratories and beam tests

Intercalibration des couches du calorimètre électromagnétique d'ATLAS et mesure de couplages CP impairs du boson de Higgs dans son canal de désintégration en quatre leptons avec les données du Run 2 au LHC

After the Higgs boson discovery at the LHC in 2012, interest turned to Higgs boson property measurements to refine the tests of the Standard Model and probe for new physics. One of its key properties is its spin-parity (CP), predicted to be 0+ in the Standard Model. Analyses of data collected during the Run 1 of the LHC rejected all pure spin-parity (CP) state other than 0+. However mixed CP states are still possible, and would indicate CP violation in the Higgs sector.The first part of this thesis focuses on the ATLAS electromagnetic calorimeter calibration, needed to reach a permil level on electron and photon energy resolution which are of prime importance for Higgs boson studies. One step of the calibration sequence consists of the layer intercalibration of the electromagnetic calorimeter, needed to correct residual electronics miscalibration and cross-talk effects. The Run 1 method has proven to be unreliable for the pileup levels in Run 2 and a new method was developed, ensuring a precise control on the systematic uncertainties.The second part of this thesis puts emphasis on the Higgs boson to vector boson CP-odd couplings, with the Higgs boson decaying to four leptons. This channel, despite low statistics, provides a clean signature and a signal-to-noise ratio over two, allowing for a precise determination of the Higgs boson properties. The vector boson fusion production channel offers the best sensitivity to CP effects thanks to its two characteristic tagging jets in the final state. The contamination from the gluon fusion production mode with additional jets is reduced using neural networks. To unambiguously distinguish yet unknown CP-even from possible CP-odd effects, a variable whose shape asymmetry only depends on CP-odd effects is built. This observable is based on the matrix element computation, maximally using the kinematic information available from Higgs boson and associated jets. Results are interpreted in a context of effective field theory, and the statistical precision on the tCzz Wilson coefficient is estimated to [-0.80, 0.80] at the 68% confidence level.Après la découverte du boson de Higgs en 2012 au LHC, l'intérêt s'est porté sur l'étude de ses propriétés pour vérifier le Modèle Standard et pour sonder la nouvelle physique. L'une de ses propriétés fondamentales est sa spin-parité (CP), dont le Modèle Standard prédit la valeur 0+. Les analyses menées sur les données récoltées au Run 1 du LHC ont rejeté toutes les hypothèses d'état pur de spin-parité autre que cette valeur. Cependant des états mixtes de CP sont toujours possibles, ce qui indiquerait une violation de symmétrie CP dans le secteur du Higgs.La première partie de cette thèse se concentre sur la calibration du calorimètre électromagnétique d'ATLAS permettant d'atteindre une résolution de l'ordre du pour mille sur l'énergie des électrons et photons, primordiaux dans les analyses du boson de Higgs. Une des étapes est l'inter-calibration des couches du calorimètre électromagnétique, corrigeant des effets résiduels de calibration électronique et de diaphonie (cross-talk). La méthode établie au Run 1 a montré ses limites devant les niveaux d'empilement mesurés au Run 2, et une nouvelle analyse a été alors dévéloppée, assurant le contrôle précis des incertitudes systématiques.La deuxième partie de cette thèse porte sur la mesure des couplages CP-impairs du boson de Higgs aux boson vecteurs, étudié dans le canal de désintégration du boson de Higgs en quatre leptons. Malgré une faible statistique, ce canal offre une signature propre et un rapport signal sur bruit de plus de deux, permettant l'analyse précise des propriétés du boson de Higgs. Le mode de production par fusion de bosons vecteurs offre la meilleure sensibilité aux effets de CP grâce à la présence de deux jets dans l'état final. La pollution venant du mode de production par fusion de gluon avec des jets additionels est réduite grâce à l'utilisation de réseaux neuronaux. Pour distinguer de manière univoque les effets CP-impair d'éventuels effets CP-pair encore inconnus, une nouvelle variable est construite dont l'asymétrie de forme dépend uniquement d'effets CP-impairs. Composée d'éléments de matrice, cette variable utilise les informations cinématiques du boson de Higgs et des jets de manière maximale. Les résultats sont interprétés en termes de théorie effective, et la sensibilité statistique à 68% de confiance sur le coefficient de Wilson tCzz est estimée à [-0.80, 0.80]

Layer Intercalibration of the ATLAS Electromagnetic Calorimeter and CP-odd Higgs Boson Couplings Measurements in the Four-Lepton Decay Channel with Run 2 Data of the LHC

After the Higgs boson discovery at the LHC in 2012, interest turned to Higgs boson property measurements to refine the tests of the Standard Model and probe for new physics. One of its key properties is its spin-parity (CP), predicted to be $0^+$ in the Standard Model. Analyses of data collected during the Run 1 of the LHC rejected all pure spin-parity state other than $0^+$. However mixed CP states are still possible, and would indicate CP violation in the Higgs sector. The first part of this thesis focuses on the ATLAS electromagnetic calorimeter calibration, needed to reach a permil level on electron and photon energy resolution which are of prime importance for Higgs boson studies. One step of the calibration sequence consists of the layer intercalibration of the electromagnetic calorimeter, needed to correct residual electronics miscalibration and cross-talk effects. The Run 1 method has proven to be unreliable for the pileup levels in Run 2 and a new method was developed, ensuring a precise control on the systematic uncertainties. The second part of this thesis puts emphasis on the Higgs boson to vector boson CP-odd couplings, with the Higgs boson decaying to four leptons. This channel, despite low statistics, provides a clean signature and a signal-to-noise ratio over two, allowing for a precise determination of the Higgs boson properties. The vector boson fusion production channel offers the best sensitivity to CP effects thanks to its two characteristic tagging jets in the final state. The contamination from the gluon fusion production mode with additional jets is reduced using neural networks. To unambiguously distinguish yet unknown CP-even from possible CP-odd effects, a variable whose shape asymmetry only depends on CP-odd effects is built. This observable is based on the matrix element computation, maximally using the kinematic information available from Higgs boson and associated jets. Results are interpreted in a context of effective field theory, and the statistical precision on the $\tilde{c}_{zz}$ Wilson coefficient is estimated to $[-0.80, 0.80]$ at the 68% confidence level

Physics Performance and Detector Requirements at an Asymmetric Higgs Factory

Recently, a concept for a Hybrid Asymmetric Linear Higgs Factory (HALHF) has been proposed, where a center-of-mass energy of 250 GeV is reached by colliding a plasma-wakefield accelerated electron beam of 500 GeV with a conventionally accelerated positron beam of about 30 GeV. While clearly facing R&D challenges, this concept bears the potential to be significantly cheaper than any other proposed Higgs Factory, comparable in cost e.g. to the EIC. The asymmetric design changes the requirements on the detector at such a facility, which needs to be adapted to forward-boosted event topologies as well as different distributions of beam-beam backgrounds. This contribution will give a first assessment of the impact of the accelerator design on the physics prospects in terms of some flagship measurements of Higgs factories, and how a detector would need to be adjusted from a typical symmetric Higgs factory design

Physics Performance and Detector Requirements at an Asymmetric Higgs Factory

Recently, a concept for a Hybrid Asymmetric Linear Higgs Factory (HALHF) has been proposed, where a center-of-mass energy of 250 GeV is reached by colliding a plasma-wakefield accelerated electron beam of 500 GeV with a conventionally accelerated positron beam of about 30 GeV. While clearly facing R&D challenges, this concept bears the potential to be significantly cheaper than any other proposed Higgs Factory, comparable in cost e.g. to the EIC. The asymmetric design changes the requirements on the detector at such a facility, which needs to be adapted to forward-boosted event topologies as well as different distributions of beam-beam backgrounds. This contribution will give a first assessment of the impact of the accelerator design on the physics prospects in terms of some flagship measurements of Higgs factories, and how a detector would need to be adjusted from a typical symmetric Higgs factory design