Design Luminosity Research Articles

Operation of the Belle II imaging Time-Of-Propagation (iTOP) detector before and after the first long shutdown

The iTOP detector is a Cherenkov detector specialized on particle identification at Belle II. The SuperKEKB accelerator collides electrons and positrons with a design luminosity of 6⋅10351cm2s. In order to exploit the high collision rate Belle II has a trigger rate of up to 30 kHz. The iTOP detector uses quartz bars as the source of Cherenkov photons. The photons are reflected inside the bars until they hit photomultipliers installed at one end. The spatial distribution and precise arrival times of the detected photons are used to reconstruct the Cherenkov angle and particle flight time. To achieve a good pion–kaon separation the photon arrival times have to be measured with a resolution of 100 ps. Microchannel plate photomultipliers together with dedicated high-speed electronics for 2.7 GSa/s waveform sampling in 8192 channels are used to achieve this requirement. After four years of operation, the experiment entered its first long shutdown phase in 2022. Aging components of iTOP were exchanged and the detector was prepared for data-taking at increasing luminosities and backgrounds after the long shutdown. In this talk the design of the iTOP detector will be shown and experience and results from operation will be discussed together with an outlook on future running conditions.

Photon-odderon interference in exclusive χc charmonium production at the Electron-Ion Collider

Exclusive C=+1 scalar, axial-vector, and tensor quarkonium production in high-energy electron-proton scattering requires a C-odd t-channel exchange of a photon or a three-gluon ladder. We derive the expressions for the corresponding amplitudes. The relative phase of the photon vs three-gluon exchange amplitudes is determined by the sign of the light-front matrix element of the eikonal color current operator dabcJ+aJ+bJ+c at moderate x, and is not affected by small-x QCD evolution. Model calculations predict constructive interference, which is particularly strong for momentum transfer |t|∼1 GeV2 where the cross section for χcJ production exceeds that for pure photon exchange by up to a factor of 4. Exclusive χcJ electroproduction at the high luminosity Electron-Ion Collider occurs with well measurable rates and measurements of these processes should find evidence for the perturbative odderon exchange. We also compute the total electroproduction cross section as a function of energy and provide first estimates of the number of χcJ events per month at the Electron-Ion Collider design luminosity. Published by the American Physical Society 2024

The ATLAS experiment at the CERN Large Hadron Collider: a description of the detector configuration for Run 3

The ATLAS detector is installed in its experimental cavern at Point 1 of the CERN Large Hadron Collider. During Run 2 of the LHC, a luminosity of ℒ = 2 × 1034 cm-2 s-1 was routinely achieved at the start of fills, twice the design luminosity. For Run 3, accelerator improvements, notably luminosity levelling, allow sustained running at an instantaneous luminosity of ℒ = 2 × 1034 cm-2 s-1, with an average of up to 60 interactions per bunch crossing. The ATLAS detector has been upgraded to recover Run 1 single-lepton trigger thresholds while operating comfortably under Run 3 sustained pileup conditions. A fourth pixel layer 3.3 cm from the beam axis was added before Run 2 to improve vertex reconstruction and b-tagging performance. New Liquid Argon Calorimeter digital trigger electronics, with corresponding upgrades to the Trigger and Data Acquisition system, take advantage of a factor of 10 finer granularity to improve triggering on electrons, photons, taus, and hadronic signatures through increased pileup rejection. The inner muon endcap wheels were replaced by New Small Wheels with Micromegas and small-strip Thin Gap Chamber detectors, providing both precision tracking and Level-1 Muon trigger functionality. Trigger coverage of the inner barrel muon layer near one endcap region was augmented with modules integrating new thin-gap resistive plate chambers and smaller-diameter drift-tube chambers. Tile Calorimeter scintillation counters were added to improve electron energy resolution and background rejection. Upgrades to Minimum Bias Trigger Scintillators and Forward Detectors improve luminosity monitoring and enable total proton-proton cross section, diffractive physics, and heavy ion measurements. These upgrades are all compatible with operation in the much harsher environment anticipated after the High-Luminosity upgrade of the LHC and are the first steps towards preparing ATLAS for the High-Luminosity upgrade of the LHC. This paper describes the Run 3 configuration of the ATLAS detector.

The Belle II experiment: Status and prospects

The Belle II experiment at the SuperKEKB energy asymmetric e^{+}e^{-}e+e− collider is a substantial upgrade of the B factory facility at the Japanese KEK laboratory. The design luminosity of the machine is 6\times10^{35}6×1035cm^{-2}s^{-1}cm−2s−1 and the Belle II experiment aims to record 50 ab^{-1}ab−1 of data, a factor of 50 more than its predecessor. With this data set, Belle II will be able to measure the Cabibbo-Kobayashi-Maskawa (CKM) matrix, the matrix elements and their phases, with unprecedented precision and explore flavor physics with BB, charmed mesons, and \tauτ leptons. Belle II has also a unique capability to search for low-mass dark matter and low-mass mediators. In this paper, we will review the status of the Belle II detector, SuperKEKB accelerator and the prospects for physics at Belle II.

GEM detectors for the CMS endcap muon system: status of three new detector stations

The High-Luminosity LHC (HL-LHC, or Phase-2 LHC) will deliver proton-proton collisions at 5–7.5 times the nominal LHC luminosity, with an expected number of 140–200 pp-interactions per bunch crossing (Pile-up or PU). To maintain the performance of muon triggering and reconstruction under high background, the forward part of the Muon Spectrometer of the CMS experiment will be upgraded with Gas Electron Multipliers (GEM) and improved Resistive Plate Chambers (iRPC) detectors [1,2]. A first GEM station (GE1/1) was installed during Long Shutdown 2 (LS2, 2019–2021), a 2nd station (GE2/1) of Triple-GEM detectors will be installed in winter 2023–24 and 2024–25, while a new 6-layer station (ME0) will be installed in the third Long Shutdown (LS3, 2026–2028). GE11 is considered an early Phase-2 upgrade as it will reduce the pT threshold by combining GEM and Cathode Strip Chamber (CSC) hits in the forward muon system at twice the LHC design luminosity (ℒ = 2 · 1034 cm-2 s-1, 50 PU). After a successful start of Run-3 in 2022, with almost 40 fb-1 collected, the commissioning of the GE1/1 detector is nearly complete. Most chambers are operated stably with an efficiency in excess of 95%, next being the demonstration of the combined CSC-GEM trigger in 2023. The lessons learnt with the first large-area GEM station have lead to improvements in detector and electronics design for the Phase 2 detectors GE2/1 and ME0. This proceeding will discuss the progress made since last MPGD Conference (MPGD 2019) [3], discussing the commissioning and early performance of GE1/1; the design improvements and start of construction of GE2/1; and the R&D currently ongoing for ME0.

Recent results from Belle II

The Belle II experiment at the SuperKEKB energy-asymmetric e+e− collider is a substantial upgrade of the B factory facility at the Japanese KEK laboratory. The design luminosity of the machine is 6⋅1035cm−2s−1 and the Belle II experiment aims to ultimately record 50ab−1 of data, a factor of 50 more than its predecessor. With this data set, Belle II will be able to measure the Cabibbo-Kobayashi-Maskawa (CKM) matrix elements with unprecedented precision and explore flavor physics with B and D mesons, and τ leptons. Belle II has also a unique capability to search for low-mass dark matter and low-mass mediators. In this work, I will review the latest results from Belle II.

Summary of numerical noise studies for Electron-Ion Collider strong-strong beam-beam simulation

The Electron-Ion Collider (EIC) presently under construction at Brookhaven National Laboratory will collide polarized high energy electron beams with hadron beams with design luminosities up to 1 × 1034cm−2s−1 in center mass energy range of 20-140 GeV. We studied the planned electron-proton collisions using a Particle-In-Cell (PIC) based Poisson solver in strong-strong beam-beam simulation. We observed a much larger proton emittance growth rate than in weak-strong simulation. To understand the numerical noise and its impact on strong-strong simulation results, we carried out extensive studies to identify all possible causes for artificial emittance growth and quantify their contributions. In this article, we summarize our study activities and findings. This work will help us better understand the simulated emittance growth and the limits of the PIC based strong-strong beam-beam simulation.

The report of machine studies related to the vertical beam size blow-up in SuperKEKB LER

In the low energy ring (LER) for positrons in the SuperKEKB, a vertical beam size blow-up was observed when the bunch current was approximately 1 mA. If a beam size blow-up occurs, the design luminosity cannot be achieved. Therefore, beam size blow-ups must be pre-vented. According to calculations, the bunch current threshold of the transverse mode coupling instability (TMCI) is 2 mA or more, and the observed value is 50% or smaller. Ordinary TMCI cannot explain this vertical beam size blow-up. This paper shows that the cause of the vertical beam size blow-up can be determined by analyzing factors such as beam oscillation. The study results showed that the vertical beam size blow-up in the LER was caused by a -1 mode instability.

GEM Detectors for the Upgrade of the CMS Muon Spectrometer

The Large Hadron Collider (LHC) will be upgraded in several phases to allow a significant expansion of its physics program. We are nearing the end of Long Shutdown 2 (LS2, 2019-2021), after which the LHC will initiate its third running period (Run-3) expected to last three years. In Run-3 LHC will collide protons with an instantaneous luminosity of 2-3×1034cm−2s−1 exceeding its design luminosity by a factor of 2-3, allowing the CMS experiment to collect an integrated luminosity of approximately 100 fb−1/year. A subsequent upgrade in 2025-27 (Long Shutdown 3 - LS3) will increase the luminosity up to 5×1034cm−2s−1. The CMS muon system must enable a physics program that maintains sensitivity for electroweak measurements and for Beyond the Standard Model searches, similar to what was achieved in Run-1 and Run-2. To cope with the corresponding increase in trigger rates and to provide additional coordinate measurements in the high background environment, the installation of additional sets of muon detectors, referred to as GE1/1, GE2/1 and ME0, that use Gas Electron Multiplier (GEM) technology, has been approved. The GE1/1 chambers have already been installed in spring-summer 2020 and are currently being commissioned and tested with cosmic rays. The construction of GE2/1 will start in 2021, while for ME0 the final R&D is in progress and construction is foreseen in 2023. We present an overview of the Muon Spectrometer upgrade using the GEM technology, the performance of the GE1/1 chambers during Quality Control tests and in cosmic ray tests, and the design of the GE2/1 and ME0 chambers.

Problems and considerations about the injection philosophy and timing structure for CEPC

This paper describes the injection philosophy and the design of timing and filling scheme for the high luminosity Circular Electron and Positron Collider (CEPC) scheme under different energy modes. It is found that the RF frequency choice in Conceptual Design Report (CDR) cannot meet the injection requirements for the bunch number at the Z pole. A modified scheme was proposed to support the design luminosity, which basically meets our current design requirements and retains more flexibility for a future high-luminosity upgrade.

Dark Z at the International Linear Collider

A dark Z is a massive Abelian gauge boson which is coupled to the Standard Model through both kinetic and mass mixing with the electroweak sector. We study the phenomenology of the dark Z at an energy-frontier electron-positron collider, such as the proposed International Linear Collider (ILC). We show that precision electroweak constraints and the current bounds from the hadron colliders allow a dark Z that is kinematically accessible at the ILC with 250 GeV or 500 GeV center-of-mass energy. Further, the reach of the ILC searches for a dark Z significantly exceeds the expected reach of the high luminosity LHC. If a signal consistent with a dark Z is discovered, it would motivate a dedicated run of the ILC at the center-of-mass energy matching the dark Z mass. We demonstrate that a short one month run at design luminosity could measure the dark Z chiral couplings to fermions with percent precision. This measurement can be used to discriminate between competing theoretical models of the resonance: for example, the dark Z can be distinguished from a dark photon with purely kinetic mixing.

Measurements of beam backgrounds in SuperKEKB Phase 2

The high design luminosity of the SuperKEKB electron–positron collider will result in challenging levels of beam-induced backgrounds in the interaction region. Understanding and mitigating these backgrounds is critical to the success of the Belle II experiment. We report on the first background measurements performed after roll-in of the Belle II detector, a period known as SuperKEKB Phase 2, utilizing both the BEAST II system of dedicated background detectors and the Belle II detector itself. We also report on first revisions to the background simulation made in response to our findings. Backgrounds measured include contributions from synchrotron radiation, beam-gas, Touschek, and injection backgrounds. At the end of Phase 2, single-beam backgrounds originating from the 4 GeV positron Low Energy Ring (LER) agree reasonably well with simulation, while backgrounds from the 7 GeV electron High Energy Ring (HER) are approximately one order of magnitude higher than simulation. We extrapolate these backgrounds forward and conclude it is safe to install the Belle II vertex detector.

Displaced searches for light vector bosons at Belle II

With a design luminosity of 50 ab−1 and detectors with tracking capabilities extending beyond 1 m, the Belle II experiment is the perfect laboratory for the search of particles that couple weakly to the Standard Model and have a characteristic decay length of a few centimetres and more. We show that for models of dark photons and other light vector bosons, Belle II will be successful in probing regions of parameter space which are as of now unexplored by any experiment. In addition, for models where the vector boson couples sub-dominantly to the electron and quarks as compared to muons, e.g. in the Lμ−Lτ model, Belle II will probe regions of mass and couplings compatible with the anomalous magnetic moment of muon. We discuss these results and derive the projected sensitivity of Belle II for a handful of other models. Finally, even with the currently accumulated data, ∼ 200 fb−1, Belle II should be able to cover regions of parameter space pertaining to the X(17) boson postulated to solve the ATOMKI anomaly.

Mechanical and thermal design of the target neutral beam absorber for the High-Luminosity LHC upgrade

The High-Luminosity target neutral beam absorber (TAXN) is the radiation absorber of the neutral particle debris generated at the beam collisions carried out in the ATLAS and CMS detectors at the Large Hadron Collider (LHC). The new absorber will protect the superconducting magnets from this radiation to prevent their quenching at a design luminosity of $7.5\ifmmode\times\else\texttimes\fi{}{10}^{34}\text{ }\text{ }{\mathrm{cm}}^{\ensuremath{-}2}\text{ }{\mathrm{s}}^{\ensuremath{-}1}$. This article describes the mechanical design of the TAXN assembly and the recombination chamber together with the thermal computations performed for the water-cooling and bakeout system prepared for the engineering design review held in January, 2021. It takes into account the internal integration of the zero degree calorimeter and beam rate of neutrals luminosity detector as well as their physics case overview. In addition, it provides the beam aperture breakdown and the heat deposition expected in the absorber.

Heavy neutrino searches via same-sign lepton pairs at a Higgs boson factory

This paper investigates the $e^-e^+\to Zh_1$ sensitivity for Higgs boson's rare decay into heavy neutrinos $h_1 \to NN$ at the proposed electron-positron collider, with the focus on multi-lepton final states that contain same-sign lepton pairs. $h_1 \to NN$ decay can derive from Higgs boson's mixing with new physics scalar(s) that is complementary to the contribution from active-sterile neutrino mixings. We consider the scenario with a singlet scalar which gives the heavy neutrino mass and has a small mixing with the SM Higgs boson. We analyze the semileptonic, fully leptonic and mixed $NN$ decay scenarios, and categorize the signal on the number of leptons in the final state: $ \ell^\pm \ell^\pm$ with at least 3 jets, $\ell^\pm \ell^\pm \ell$ with at least 2 jets, and $e^\pm e^\pm \mu^\mp \mu^\mp$ plus with at least 1 jet, each containing one or two same-sign dilepton system(s). Selection cuts are optimized for the presence of the associated $Z$-boson, which leads to additional backgrounds at the $e^-e^+$ collider. The Standard Model background channels are systematically analyzed. Sensitivity limits on $h_1 \rightarrow NN$ branching fractions are derived for signals with 2-4 final leptons assuming the heavy neutrino masses are between 10 and 60 GeV. With 240 GeV center-of-mass energy and 5.6 ab$^{-1}$ design luminosity, $h_1 \rightarrow NN$ branching fraction can be probed to $2\times 10^{-4}$ in $2\ell$ and $3\ell$ channels, and $6\times 10^{-4}$ in the $4\ell$ channel at $95\%$ confidence level. $3\ell, 4\ell$ channels expect one or fewer background event, and their sensitivities saturate the statistic limit at 5.6 ab$^{-1}$ luminosity. A same-sign trilepton ($\ell^\pm \ell^\pm \ell^\pm$) signal in the $3\ell$ channel is also discussed. Our search strategy provides an approach to discovering the singlet scalar and exploring the origin of neutrino masses at future $e^-e^+$ colliders.

Dark Sector first results at Belle II

Understanding the nature of dark matter is one of the most exciting challenges in fundamental physics nowadays, requiring the synergy of different search techniques, as well as theoretical inputs. An interesting opportunity for the investigation of dark matter is the one offered by the B-factories. The Belle II experiment, located at the interaction point of the SuperKEKB asymmetric energy e + e − collider, is a new generation B-factory experiment operating at the Japanese KEK laboratory. With a machine design luminosity of 6 × 1035 cm−2 s −1, Belle II aims to record 50 ab−1 of data within the next decade. Thanks to this large data-sample and by using dedicated triggers, Belle II is expected to explore dark sector candidates with unprecedented sensitivity in the mass range up to 10 GeV/c 2. During 2018, the experiment concluded a commissioning run, collecting a data-sample corresponding to an integrated luminosity of about 0.5 fb−1, while main operations started on March 2019 with an almost complete detector. So far the experiment collected an integrated luminosity of ∼ 90 fb−1. With these data-sets Belle II has already shown the possibility to search for a large variety of dark sector candidates in the GeV mass range. This paper reviews the status of the dark sector searches performed at the Belle II experiment, with a focus on the first obtained results and the discovery potential with the data-set available in the short term.

Luminosity optimization and leveling in the Super Proton–Proton Collider

The design luminosity and the beam energy scale in the future Super Proton–Proton Collider (SPPC) are unprecedented. In this article, different luminosity and leveling scenarios are studied to see if the luminosity design goal is feasible. The luminosity in a collider can be optimized by a group of machine parameters including beam emittance, number of bunches, bunch population, beta functions at the interaction point (IP), colliding mode (such as head-on or crossing, round or flat optics) and their changes during the colliding process. The beam–beam effects, event pileup and machine turnaround time have direct impact on the luminosity. Different measures or scenarios by taking the above factors are studied. When crab cavities are not available, the flat optics is studied to recover the luminosity lost due to a large crossing angle. The luminosity patterns during the physics run and averaged luminosities are given for different luminosity schemes. One can compare the performance potential and the technical difficulties among the schemes. The difference between the round optics and flat optics in the case of no crab cavities is presented. SPPC has a very high performance in the averaged luminosity. Flexible luminosity optimization and leveling schemes are available to enhance the physics performance. The flat optics as a backup plan in the case of no crab cavities does help to improve the luminosity compared with the round optics.

Beam background study for the Belle II Silicon Vertex Detector

The Belle II experiment aims to accumulate 50ab−1 of e+e− collision data at the SuperKEKB asymmetric energy collider (Tsukuba, Japan). The first physics data using all Belle II detectors were taken in spring 2019.In the vast physics program of the Belle II experiment, the vertex detector plays a crucial role for the determination of the B-meson decay vertices. It consists of two inner layers of pixelated silicon detectors and four outer layers of double-sided silicon strip detectors (SVD).To achieve a design luminosity of 8×1035cm−2s−1, 40 times higher than the recorded luminosity of its predecessor, the SuperKEKB collider squeezes the beams to a vertical size of 50 nm (“nano-beam scheme”) and doubles the beam currents.Therefore, the detectors are required to tolerate intense beam induced background due to the very high luminosity. During the 2019 spring run we measured the occupancy rate in the SVD to estimate the level of the beam induced background. With the low initial luminosity, the observed beam induced background mostly originated from Touschek processes and beam-gas scattering within individual beams. Since these different background contributions depend differently on accelerator conditions, such as the beam current, beam size and pressure, they can be disentangled. We estimate the background rate of each contribution and compare them with simulated ones. The results enable us to predict the background levels at increased beam currents and luminosity in the coming years. They also hint at background mitigation measures for running at higher luminosity. In this proceeding we present the results of our study of the beam induced background in the SVD and the prospects for future operation.

Status of the High-Voltage Electron-Cooling System of the NICA Collider

In order to achieve the design luminosity in the NICA collider, it is planned to equip it with both stochastic and electron cooling. The electron-cooling system (ECS) for 2.5 MeV consists of two coolers which cool both ion beams simultaneously. The Budker Institute of Nuclear Physics (BINP SB RAS) already built and commissioned an electron cooling system for the NICA booster, and now the high-voltage ECS for the collider is being developed. This paper describes the basic elements of the system and the status of the cooler development.

Dark Sector Physics with Belle II

The Belle II experiment at the SuperKEKB energy-asymmetric e + e − collider is a substantial upgrade of the KEKB facility at the Japanese KEK laboratory. The design luminosity of the machine is 8 · 1035 cm−2s−1 and the Belle II experiment aims to record 50 ab−1 of data, a factor of 50 more than its predecessor. From February to July 2018, the machine has completed a commissioning run, where about 0.5 fb−1 of data have been collected. Regular operation of SuperKEKB has started in March 2019: the machine has achieved a peak luminosity of 1034 cm−2s−1, and Belle II has recorded a data sample of about 6.5 fb−1. Already this early data set with specifically designed triggers offers the possibility to search for a large variety of dark sector particles in the GeV mass range complementary to searches at LHC and dedicated low energy experiments. The Belle II dark matter analyses will benefit from more data in the process of being accumulated. This talk will review the state of the dark sector searches at Belle II with a focus on the discovery potential of the early data, and show the first results.