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1. Unleashing the full power of LHCb to probe stealth new physics

   Borsato, M; Cid Vidal, X; Tsai, Y; Vázquez Sierra, C; Zurita, J; Alonso-Álvarez, G; Boyarsky, A; Brea Rodríguez, A; Buarque Franzosi, D; Cacciapaglia, G; et al (February 2022, Reports on Progress in Physics)

   Abstract In this paper, we describe the potential of the LHCb experiment to detect stealth physics. This refers to dynamics beyond the standard model that would elude searches that focus on energetic objects or precision measurements of known processes. Stealth signatures include long-lived particles and light resonances that are produced very rarely or together with overwhelming backgrounds. We will discuss why LHCb is equipped to discover this kind of physics at the Large Hadron Collider and provide examples of well-motivated theoretical models that can be probed with great detail at the experiment. 

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2. Predicting the Magnetic Fields of a Stealth CME Detected by Parker Solar Probe at 0.5 au

   https://doi.org/10.3847/1538-4357/ac25f4  

   Palmerio, Erika; Kay, Christina; Al-Haddad, Nada; Lynch, Benjamin J.; Yu, Wenyuan; Stevens, Michael L.; Pal, Sanchita; Lee, Christina O. (October 2021, The Astrophysical Journal)

   Abstract Stealth coronal mass ejections (CMEs) are eruptions from the Sun that are not associated with appreciable low-coronal signatures. Because they often cannot be linked to a well-defined source region on the Sun, analysis of their initial magnetic configuration and eruption dynamics is particularly problematic. In this article, we address this issue by undertaking the first attempt at predicting the magnetic fields of a stealth CME that erupted in 2020 June from the Earth-facing Sun. We estimate its source region with the aid of off-limb observations from a secondary viewpoint and photospheric magnetic field extrapolations. We then employ the Open Solar Physics Rapid Ensemble Information modeling suite to evaluate its early evolution and forward model its magnetic fields up to Parker Solar Probe, which detected the CME in situ at a heliocentric distance of 0.5 au. We compare our hindcast prediction with in situ measurements and a set of flux-rope reconstructions, obtaining encouraging agreement on arrival time, spacecraft-crossing location, and magnetic field profiles. This work represents a first step toward reliable understanding and forecasting of the magnetic configuration of stealth CMEs and slow streamer-blowout events. 

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3. FASER’s Electromagnetic Calorimeter Test Beam Studies

   https://doi.org/10.3390/instruments6030031  

   Cavanagh, Charlotte (September 2022, Instruments)

   FASER, or the Forward Search Experiment, is a new experiment at CERN designed to complement the LHC’s ongoing physics program, extending its discovery potential to light and weakly interacting particles that may be produced copiously at the LHC in the far-forward region. New particles targeted by FASER, such as long-lived dark photons or axion-like particles, are characterised by a signature with two oppositely charged tracks or two photons in the multi-TeV range that emanate from a common vertex inside the detector. The full detector was successfully installed in March 2021 in an LHC side tunnel 480 m downstream from the interaction point in the ATLAS detector. FASER is planned to be operational for LHC Run 3. The experiment is composed of a silicon-strip tracking-based spectrometer using three dipole magnets with a 20 cm aperture, supplemented by four scintillator stations and an electromagnetic calorimeter. The FASER electromagnetic calorimeter is constructed from four spare LHCb calorimeter modules. The modules are of the Shashlik type with interleaved scintillator and lead plates that result in 25 radiation lengths and 1% energy resolution for TeV electromagnetic showers. In 2021, a test beam campaign was carried out using one of the CERN SPS beam lines to set up the calibration of the FASER calorimeter system in preparation for physics data taking. The relative calorimeter response to electrons with energies between 10 and 300 GeV, as well as high energy muons and pions, has been measured under various high voltage settings and beam positions. The measured calorimeter resolution, energy calibration, and particle identification capabilities are presented. 

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4. Effect of the high-level trigger for detecting long-lived particles at LHCb

   https://doi.org/10.3389/fdata.2022.1008737  

   Calefice, Lukas; Hennequin, Arthur; Henry, Louis; Jashal, Brij; Mendoza, Diego; Oyanguren, Arantza; Sanderswood, Izaac; Sierra, Carlos Vázquez; Zhuo, Jiahui (November 2022, Frontiers in Big Data)

   Long-lived particles (LLPs) show up in many extensions of the Standard Model, but they are challenging to search for with current detectors, due to their very displaced vertices. This study evaluated the ability of the trigger algorithms used in the Large Hadron Collider beauty (LHCb) experiment to detect long-lived particles and attempted to adapt them to enhance the sensitivity of this experiment to undiscovered long-lived particles. A model with a Higgs portal to a dark sector is tested, and the sensitivity reach is discussed. In the LHCb tracking system, the farthest tracking station from the collision point is the scintillating fiber tracker, the SciFi detector. One of the challenges in the track reconstruction is to deal with the large amount of and combinatorics of hits in the LHCb detector. A dedicated algorithm has been developed to cope with the large data output. When fully implemented, this algorithm would greatly increase the available statistics for any long-lived particle search in the forward region and would additionally improve the sensitivity of analyses dealing with Standard Model particles of large lifetime, such as K S 0 or Λ 0 hadrons. 

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5. Long-lived particle reconstruction downstream of the LHCb magnet

   https://doi.org/10.1140/epjc/s10052-024-13686-6  

   Aaij, R; Abdelmotteleb, A_S W; Beteta, C Abellan; Abudinén, F; Ackernley, T; Adefisoye, A A; Adeva, B; Adinolfi, M; Adlarson, P; Agapopoulou, C; et al (January 2025, The European Physical Journal C)

   Abstract Charged-particle trajectories are usually reconstructed with the LHCb detector using combined information from the tracking devices placed upstream and downstream of the 4 T m dipole magnet. Trajectories reconstructed using only information from the tracker downstream of the dipole magnet, which are referred to as T tracks, have not been used for physics analysis to date. The challenges of the reconstruction of long-lived particles with T tracks for physics use are discussed and solutions are proposed. The feasibility and the tracking performance are studied using samples of long-lived$${\Lambda }$$ $\Lambda$and$$K_S^0$$ $K_S^0$hadrons decaying between 6.0 and 7.6 m downstream of the proton–proton collision point, thereby traversing most of the magnetic field region and providing maximal sensitivity to magnetic and electric dipole moments. The reconstruction can be expanded upstream to about 2.5 m for use in direct searches of exotic long-lived particles. The data used in this analysis have been recorded between 2015 and 2018 and correspond to an integrated luminosity of 6 $$\hbox {fb}^{-1}$$ ${\text{fb}}^{-1}$. The results obtained demonstrate the possibility to further extend the decay volume and the physics reach of the LHCb experiment. 

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