LHC Beam Research Articles

Effect of Ground Motion Introduced by HL-LHC CE Work on LHC Beam Operation

Effect of Ground Motion Introduced by HL-LHC CE Work on LHC Beam Operation

Single-transverse-spin asymmetries in exclusive photo-production of J/ψ in ultra-peripheral collisions in the fixed-target mode at the LHC and in the collider mode at RHIC

We investigate the potentialities offered by the study of J/ψ exclusive photo-production in ultra-peripheral collisions at a fixed-target experiment using the proton and lead LHC beams (generically denoted as AFTER@LHC) on hydrogen targets and at RHIC in the collider mode. We compare the expected cross sections in both set-ups. Studying Single-Transverse-Spin Asymmetries (AN) in such a process provides a direct path to the proton Generalised Parton Distribution (GPD) Eg(x,ξ,t). We evaluate the expected precision on AN for realistic conditions with the LHCb detector in pH↑ and PbH↑ collisions. We also discuss prospects with polarised deuterium and helium targets in the case of AFTER@LHC.

Heat load profile estimates on LHC beam screens by thermal transient analysis

The LHC beams are producing significant dynamic heat loads on the LHC cryogenic system. These heat loads are deposited on beam screens, where they must be properly extracted with dedicated cooling loops between 4.6 K and 20 K. Since 2015, unexpected beam-induced heat loads are observed in specific locations of the machine and their origin is still not completely understood. In order to improve our understanding on the heat load origin and on the spread between them on different areas of the accelerator, the thermal transients occurring after the beam dumps have been analyzed to reconstruct the heat load profiles of the beam screens. Following a description of this specific issue, the paper presents the methodology used for the measurements and estimates of the heat load profiles and its validation against some experimental data and dynamic simulations.

Study on heat transfer between the High-Luminosity LHC beam screens and the He II-cooled beam tube

The new beam screens for the inner triplets in the HL-LHC are designed to intercept heat loads of up to 25 Wm−1 between 60 K and 80 K. The screens need to be mechanically supported by the beam tube at 1.9 K while limiting heat transfer to the same tube to less than 0.5 W m−1. Measurements for the thermal validation of the beam screens were carried out on a dedicated test stand at the Central Cryogenic Laboratory at CERN. This paper describes the first measurement campaign of a full-scale D1-type beam screen mock-up: several parameters such as base temperature and heating power were varied, and results include heat transfer mechanisms within the beam screen and more importantly to the 1.9 K beam tube that forms the physical boundary between beam screen and He II-cooled magnets.

Some proposed fixed target experiments with the LHC beams

The physics opportunities offered by using the multi-TeV LHC beams for a fixed target experiment have been widely discussed in recent years. This mode is convenient to investigate rare processes of particle production and polarization phenomena because the expected luminosity exceeds the luminosity of the collider. The main physical goals of these experiments are: i) investigations of the large-xgluon, antiquark and heavy quark content in the nucleon and nucleus; ii) investigations of the dynamics and spin of quarks and gluons inside nucleus; iii) studies of the ion-ion collisions between SPS and RHIC energies towards large rapidities. With the LHC lead beam energy scan on a fixed target it would be possible to investigate the energy range up to 72 GeV to search for the critical point for the phase transition to the Quark Gluon Plasma (QGP).

CMS Computing Resources: Meeting the demands of the high-luminosity LHC physics program

The high-luminosity program has seen numerous extrapolations of its needed computing resources that each indicate the need for substantial changes if the desired HL-LHC physics program is to be supported within the current level of computing resource budgets. Drivers include large increases in event complexity (leading to increased processing time and analysis data size) and trigger rates needed (5-10 fold increases) for the HL-LHC program. The CMS experiment has recently undertaken an effort to merge the ideas behind short-term and long-term resource models in order to make easier and more reliable extrapolations to future needs. Near term computing resource estimation requirements depend on numerous parameters: LHC uptime and beam intensities; detector and online trigger performance; software performance; analysis data requirements; data access, management, and retention policies; site characteristics; and network performance. Longer term modeling is affected by the same characteristics, but with much larger uncertainties that must be considered to understand the most interesting handles for increasing the "physics per computing dollar" of the HL-LHC. In this presentation, we discuss the current status of long term modeling of the CMS computing resource needs for HL-LHC with emphasis on techniques for extrapolations, uncertainty quantification, and model results. We illustrate potential ways that high-luminosity CMS could accomplish its desired physics program within today's computing budgets.

Measurement of Antiproton Production in p-He Collisions at sqrt[s_{NN}]=110 GeV.

The cross section for prompt antiproton production in collisions of protons with an energy of 6.5TeV incident on helium nuclei at rest is measured with the LHCb experiment from a data set corresponding to an integrated luminosity of 0.5 nb^{-1}. The target is provided by injecting helium gas into the LHC beam line at the LHCb interaction point. The reported results, covering antiproton momenta between 12 and 110 GeV/c, represent the first direct determination of the antiproton production cross section in p-He collisions, and impact the interpretation of recent results on antiproton cosmic rays from space-borne experiments.

Conception and design of a cooling system for the LHC injection kicker magnets

The CERN Large Hadron Collider is equipped with two fast single-turn injection kicker systems that deflect the incoming particle beam onto the accelerator’s orbit. The high intensity LHC beam, circulating for many hours, can cause considerable heating of the injection kicker magnets. Numerical models have been developed and validated to study the beam induced heating and thermal behaviour of these magnets. The models are used to predict the power deposition and temperatures for various operation scenarios. According to predictions, heating issues are expected for High Luminosity LHC operation with high intensity beams unless appropriate measures are taken. Cooling of the magnet yokes is complicated as the magnets are in vacuum and are pulsed at high voltage. A description of the evolution of the studies is presented, culminating with the conception and design of a cooling system which is thoroughly described, from the conception process to the final proposed solution.

Overview of the CMS beam loss monitoring system (BCML) and diamond detectors surface modification

Abstract In CMS a beam loss monitoring system (BCML) is in place to protect the CMS Tracker from potentially damaging, high intensity beam loss events. Above a pre-defined detector current the LHC beams are automatically dumped. Detectors for this system require high radiation tolerance for stability over time, sufficient signal over noise at a given particle rate and a linear response up to the abort threshold. Poly-crystalline (pCVD) diamond sensors are used as detectors. Additionally sapphire based and diamond-on-iridium prototype sensors were installed during 2017 to investigate more radiation tolerant detectors. This report discusses the BCML system, as well as promising ways of solving problems with adhesion to the detector’s plates by using the diamond surface modification with doped diamond sublayers.

Safe Disposal of the LHC Beam without Beam Dump: Method and Experimental Verification

In the extremely unlikely event of a non-working beam dumping system in the LHC, the 360 MJ of stored beam energy can be dissipated in the collimation system as a last mitigation measure. In such a situation, it is important to reduce the stored beam energy both quickly and at the same time as smoothly as possible in order to limit the risk of trips of critical systems, to avoid quenches of superconducting magnets (which would lead to changes of the beam trajectory and damage to the accelerator) and ultimately damage to the collimators themselves. Detailed steps and parameters have been developed and validated during two dedicated experiments with beam in the LHC. This paper summarizes the key aspects in view of the preparation of such a procedure for operational use, which will allow for the safe disposal of the full LHC beam by the operation crews.

Measurement of Z \u2192 \u03c4+\u03c4\u2212 production in proton-proton collisions at sqrt{mathrm{s}}=8 TeV

A measurement of Z → τ+τ− production cross-section is presented using data, corresponding to an integrated luminosity of 2 fb−1, from pp collisions at sqrt{s}=8 TeV collected by the LHCb experiment. The τ+τ− candidates are reconstructed in final states with the first tau lepton decaying leptonically, and the second decaying either leptonically or to one or three charged hadrons. The production cross-section is measured for Z bosons with invariant mass between 60 and 120 GeV/c2, which decay to tau leptons with transverse momenta greater than 20 GeV/c and pseudorapidities between 2.0 and 4.5. The cross-section is determined to be {sigma}_{pp}{{}_{to Zto {tau}^{+}}}_{tau^{-}}=95.8 pm 2.1 pm 4.6 pm 0.2 pm 1.1 pb, where the first uncertainty is statistical, the second is systematic, the third is due to the LHC beam energy uncertainty, and the fourth to the integrated luminosity uncertainty. This result is compatible with NNLO Standard model predictions. The ratio of the cross-sections for Z → τ+τ− to Z → μ+μ− (Z → e+e−), determined to be 1.01 ± 0.05 (1.02 ± 0.06), is consistent with the lepton-universality hypothesis in Z decays.

Results of UFO dynamics studies with beam in the LHC

Micrometer sized particles entering the LHC beam (the so called Unidentified Falling Objects or UFOs) are a known cause of localized beam losses since the beginning of high intensity beam operation, however the origin of these particles is not fully known. Their effect limits LHC availability by causing premature dumps due to excessive beam losses and occasionally even magnet quenches. This could become an important limitation for future accelerators such as the High Luminosity upgrade of the LHC (HL-LHC) and the Future Circular Collider (FCC). The dynamics of these UFOs was investigated in two dedicated experiments. In the first experiment, it was shown that the transverse movements of these particles can be studied by observing bunch-by-bunch losses from bunches with different horizontal and vertical emittances. In the second experiment, UFO-like events around the 16L2 interconnect in the LHC, which has seen intense UFO activity in 2017, were studied with the above method. This paper summarizes the results of both experiments.

A high angular resolution silicon microstrip telescope for crystal channeling studies

A charged particle telescope has been deployed for data taking at high rates in a CERN beam line using protons and other particles. The apparatus has a baseline of approximately 10 m in each arm, and achieves an angular resolution of 5.4 μrad using 400 GeV/c protons. The electronic readout and data acquisition system is based on that developed for the CMS Tracker, and provides almost deadtime-free operation at trigger rates of up to about 10 kHz.The telescope was developed to characterize crystals used in channeling experiments with a primary objective to validate them for use in a future LHC beam collimation system. The telescope has also been used for other studies of fundamental phenomena associated with the channeling process.The telescope is described, and its measured performance, referring to results from channeling studies, including recent measurements in heavy ion beams.

Energy-Frontier Lepton-Hadron Collisions at CERN: The LHeC and the FCC-eh

Lepton-hadron colliders that use a proton or nucleus beam of current and future hadron colliders and let it collide with an electron beam from a newly built electron accelerator bring attractive physics programs which are strong and complementary to the hadron collider physics. Machine development for Energy Recovery LINAC and physics performance studies of such electron-hadron colliders, specifically the LHeC that uses the existing LHC beam and FCC-eh that is an option of Future Circular Collider program, are ongoing and reviewed in this article.

Physics perspectives with AFTER@LHC (A Fixed Target ExpeRiment at LHC)

AFTER@LHC is an ambitious fixed-target project in order to address open questions in the domain of proton and neutron spins, Quark Gluon Plasma and high-x physics, at the highest energy ever reached in the fixed-target mode. Indeed, thanks to the highly energetic 7 TeV proton and 2.76 A.TeV lead LHC beams, center-of-mass energies as large as [see formula in PDF] = 115 GeV in pp/pA and [see formula in PDF] = 72 GeV in AA can be reached, corresponding to an uncharted energy domain between SPS and RHIC. We report two main ways of performing fixed-target collisions at the LHC, both allowing for the usage of one of the existing LHC experiments. In these proceedings, after discussing the projected luminosities considered for one year of data taking at the LHC, we will present a selection of projections for light and heavy-flavour production.

Strangeness freeze-out: role of system size and missing resonances

The conventional approach to treat strangeness freezeout has been to consider a unified freezeout scheme where strangeness freezes out along with the nonstrange hadrons (1CFO), with or without an additional parameter accounting for out-of-equilibrium strangeness production (γS). Several alternate scenarios have been formulated lately. Here, we will focus on flavor dependent freezeout with early freezeout of strangeness (2CFO) in comparison to 1CFO and its variants with respect to the roles played by the system size and missing resonances predicted by different theoretical approaches but yet to be seen in experiments. In contrast to the performance of 1CFO with/without γS that is insensitive to system size, 2CFO exhibits a clear system size dependence-while for Pb+Pb the χ2/NDF is around 0-2, for smaller system size in p+Pb and p+p, the χ2/NDF> 5 and larger than 1CFO+γS. This clearly shows a system size dependence of the preference for the freezeout scheme, while 2CFO is preferred in Pb+Pb, 1CFO+γS is preferred in p+Pb and p+p. We have further investigated the role of the missing resonances on strangeness freezeout across SPS to LHC beam energies.

Automatised data quality monitoring of the LHCb Vertex Locator

The LHCb Vertex Locator (VELO) is a silicon strip semiconductor detector operating at just 8mm distance to the LHC beams. Its 172,000 strips are read at a frequency of 1.1 MHz and processed by off-detector FPGAs followed by a PC cluster that reduces the event rate to about 10 kHz. During the second run of the LHC, which lasts from 2015 until 2018, the detector performance will undergo continued change due to radiation damage effects. This necessitates a detailed monitoring of the data quality to avoid adverse effects on the physics analysis performance. The VELO monitoring infrastructure has been re-designed compared to the first run of the LHC when it was based on manual checks. The new system is based around an automatic analysis framework, which monitors the performance of new data as well as long-term trends and using dedicated algorithms flags issues whenever they arise. The new analysis framework then analyses the plots that are produced by these algorithms. One of its tasks is to perform custom comparisons between the newly processed data and that from reference runs. The most-likely scenario in which this analysis would identify an issue is the parameters of the readout electronics no longer being optimal and requiring retuning. The data of the monitoring plots can be reduced further, e.g. by evaluating averages, and these quantities are input to long-term trending. This is used to detect slow variation of quantities, which are not detectable by the comparison of two nearby runs. Such gradual change is what is expected due to radiation damage effects. It is essential to detect these changes early such that measures can be taken, e.g. adjustments of the operating voltage, to prevent any impact on the quality of high-level quantities and thus on physics analyses. The plots as well as the analysis results and trends are made available through graphical user interfaces (GUIs). These GUIs are dynamically configured by a single configuration that determines the choice and arrangement of plots and trends and ensures a common look and feel.

New high resolution measurements of open and hidden charm production in proton-nucleus collisions at [formula omitted] with LHCb

Open and hidden charm production in nucleus-nucleus collisions is considered as a key probe of Quark Gluon Plasma (QGP) formation. In the search of specific QGP effects, proton-nucleus collisions are used as the reference as they account for the corresponding Cold Nuclear Matter (CNM) effects. The LHCb experiment, thanks to its System for Measuring Overlap with Gas (SMOG) can be operated in a fixed target mode with the LHC beams, at an intermediate center-of-mass energy between nominal SPS and RHIC energies. In 2015, for the first time, reactions of incident LHC proton beams on noble gas targets have been recorded by the LHCb experiment at a center-of-mass energy of 110 GeV and within the center-of-mass rapidity range −2.77<y⁎<−0.2. The first high resolution measurements on open and hidden charm production obtained under these conditions are presented.

Bunched-beam Schottky monitoring in the LHC

The LHC Schottky beam diagnostic system has the potential to measure beam parameters such as tune, chromaticity and momentum spread in a non-invasive way. It does so by detecting beam induced signals at 4.8 GHz, and extracting the characteristic modulation caused by incoherent motion of particles within a bunch. The system was commissioned in 2011 and provided satisfactory beam-parameter measurements during LHC Run I for lead-ions. However, for protons its usability was substantially limited due to strong interfering common-mode signals. The system was upgraded with optimized traveling-wave pick-ups and an improved microwave signal path. Design and operational aspects for the complete system are shown, and the results from measurements with LHC beams in Run II are presented and discussed.

Fixed target project AFTER at the LHC beams for heavy ion and hadron physics

High intensity proton and lead ion beams at the LHC collider allow one to use the beam halo by placing a fixed target or a bent crystal for beam extraction. The particle energy in this case is just half that at the RHIC collider, but the luminosity exceeds the collider luminosity many times. It is also possible to install a polarized target in the extracted beam. The project AFTER is aimed at investigation of rare processes, polarization phenomena, determination of the parameters required for analysis of cosmic rays and neutrino astrophysics, detailed investigation of quarkonia production and suppression depending on the phase transition of matter to the quark-gluon phase.