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Reducing Beam-Related Background on Forward Physics Detectors Using Crystal Collimation at the Large Hadron Collider*

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Reducing Beam-Related Background on Forward Physics Detectors Using Crystal Collimation at the Large Hadron Collider* PHYSICAL REVIEW APPLIED 14, 064066 (2020) Reducing Beam-Related Background on Forward Physics Detectors Using Crystal Collimation at the Large Hadron Collider* D. Mirarchi ,1,2,† V. Avati,3,2 R. Bruce,2 M. Butcher,2 M. D’Andrea ,2,4 M. Di Castro,2 M. Deile ,2 B. Dziedzic ,5 K. Hiller,6 S. Jakobsen ,2 J. Kašpar,7,2 K. Korcyl ,5 I. Lamas ,2 A. Masi,2 A. Mereghetti,2 H. Garcia Morales ,8,2,‡ Y. Gavrikov,9,2 S. Redaelli,2 B. Salvachua Ferrando,2 P. Serrano,2 M. Solfaroli Camillocci,2 and N. Turini10 1 The University of Manchester, Manchester M13 9PL, United Kingdom 2 CERN, European Organization for Nuclear Research, Geneva 23 CH-1211, Switzerland 3 AGH University of Science and Technology, Kraków, Poland 4 Università degli Studi di Padova, Padova, Italy 5 Institute of Nuclear Physics Polish Academy of Sciences, Krakow PL-31342, Poland 6 Deutsches Elektronen-Synchrotron, Hamburg 22607, Germany 7 Institute of Physics of the Academy of Sciences of the Czech Republic, Prague, Czech Republic 8 Royal Holloway University of London, Egham Hill, Egham TW20 0EX, United Kingdom 9 Petersburg Nuclear Physics Institute in National Research Centre “Kurchatov Institute” (PNPI), Gatchina, Russia 10 Università degli Studi di Siena and Gruppo Collegato INFN di Siena, Siena, Italy (Received 2 June 2020; revised 6 October 2020; accepted 2 December 2020; published 23 December 2020) Beam collimation in high-energy colliders is customarily carried out by means of massive amorphous absorbers surrounding the circulating beam. Several studies were performed in the last decades to establish an innovative collimation technique that relies on particle deflection by means of channeling between crys- talline planes of a bent crystal. We report the operational use of crystal collimation in the Large Hadron Collider that was achieved during a special high-β∗ physics run with low-intensity beams, represent- ing a milestone for both accelerator and high-energy physics that could pave the way for new synergies in the near future. The deployment of this scheme was steered and motivated by machine-simulation studies, which were then confirmed experimentally using data provided by the experiments thanks to a sensitivity not accessible with the ring instrumentation. The evidence of beam-related experimental back- ground reduction, improved data quality, and faster halo removal with respect to amorphous collimators is obtained using bent crystals as the primary collimation stage. A detailed description of preparatory studies and operational performance is reported, together with a comparison between experimental results and theoretical expectations. DOI: 10.1103/PhysRevApplied.14.064066 I. INTRODUCTION energy of 6.5 TeV. Apart from this standard operation, typ- ically using beam intensities of a few 1014 protons, special The CERN Large Hadron Collider (LHC) routinely runs are sometimes carried out. For example, the forward stores and collides high-intensity proton beams [1]. physics community required collisions at low energy. In Throughout run 2 (2015–2018), it was operated at a top October 2018 such a run was carried out at 450 GeV injection energy, using low-intensity beams (up to about 11 * 6 × 10 protons per beam) and a special optics with a high Work Supported by the HL-LHC Project. ∗ †[email protected] β (the optical function at the interaction point) that was ∗ ‡Now also at University of Oxford, Oxford OX1 2JD, United much larger than in standard operations. In particular, a β Kingdom. down to 25 cm was used for high-luminosity production at 6.5 TeV in run 2 [2], while a β∗ in the range of 50–100 m Published by the American Physical Society under the terms of the Creative Commons Attribution 4.0 International license. Fur- (reported in Table I) was used for this special physics run ther distribution of this work must maintain attribution to the at 450 GeV. author(s) and the published article’s title, journal citation, and The physics motivations of this run were the measure- DOI. ment of the elastic cross section and the extrapolation of its 2331-7019/20/14(6)/064066(20) 064066-1 Published by the American Physical Society D. MIRARCHI et al. PHYS. REV. APPLIED 14, 064066 (2020) ∗ TABLE I. LHC operational parameters during the high-β B7L1 A7L1 ALFA A7R1 B7R1 physics run at 450 GeV in October 2018. 245.7 m 237.4 m distance from IP1 237.4 m 245.7 m B6L5 D6L5 TOTEM D6R5 B6R5 Crossing Separation 220.0 m 213.0 m distance from IP5 213.0 m 220.0 m IP β∗ Beam 1 (m) β∗ Beam 2 (m) angle (μrad) (mm) (plane) (plane) (plane) (plane) 1 53 (H), 98 (V) 53 (H), 101 (V) 0 0 XRP XRP XRP XRP 2 10 (H), 10 (V) 10 (H), 10 (V) 170 (V) 3.5 (H) IP[1/5] 5 76 (H), 97 (V) 63 (H), 95 (V) 0 0 Beam 1 Beam 2 8 10 (H), 10 (V) 10 (H), 10 (V) −170 (H) −3.5 (V) FIG. 1. Illustrative layout of the ALFA and TOTEM stations ∗ around√ IP1 and IP5, respectively, used in this high-β physics run at s = 900 GeV. nuclear part to the four-momentum transfer squared t = 0, together with the evaluation of the real-to-imaginary ratio of the forward hadronic amplitude (i.e., the ρ parameter) in some Tevatron collider stores in the 2010’s, showing a by accessing the Coulomb-nuclear interference region [3]. twofold reduction of beam losses in detector regions [6,7]. The smaller the |t|, the smaller the scattering angle, However, this technique was not regularly used after. The and the closer the trajectory of the elastically scattered added value of the results in this paper is showing that a proton will be to the circulating beam envelope. Thus, ded- reduction by more than a factor 10 of beam-related exper- icated detectors housed in movable Roman pots (XRPs) imental backgrounds is achievable using crystals, while and transversally placed very close to the edge of the their operation does not cause any overhead thanks to circulating beam are used for this data taking. √ the significant technological improvements made along the A recent determination of the ρ parameter at s = 13 years. TeV revealed values significantly lower than predicted by The detectors used for forward physics experiments are many theoretical and phenomenological models that typi- based on tracking detectors housed in XRPs and designed cally neglect the crossing-odd t-channel exchange of col- to reconstruct tracks of forward protons emerging from orless three-gluon compound states [3]. On the other hand, elastic and diffractive scattering events at the interaction there are models that take the crossing-odd exchanges into point (IP). The XRPs consist of movable stations where account and are in agreement with total cross-section√ mea- the detector is placed inside a secondary vacuum vessel, surements at various energies and the ρ value at s = 13 called pot, which is transversally moved into the primary TeV. However, these models give different predictions for vacuum of the machine [8]. Two sets of XRP stations are the ρ parameter at lower energies. This motivates√ the spe- installed on the outgoing beams downstream of IP1 and cific setup of the LHC that provides collision at s = 900 IP5, which are operated by the ATLAS (ALFA) [9]and GeV, in order to collect a data set that could allow one TOTEM [10] collaborations, respectively. Different XRPs to discriminate between models that try to explain these with dedicated settings are used depending on the physics recent results [3]. goal of the data taking. Only vertical XRP stations (each The preparation of this run poses several challenges with a top and a bottom jaw) are used for this specific from the accelerator physics point of view, requiring the physics run, in total four stations for each IP, as shown in design of dedicated beam optics [4] and collimation lay- Fig. 1. outs and settings [5], to allow probing values of t as The results reported in this paper show the matu- small as possible. On the other hand, machine-protection rity reached by this powerful and innovative beam- constraints are relaxed with respect to standard physics manipulation technique that relies on bent crystal, of which operations, given the very low circulating intensity and a historical review can be found in Ref. [11]. Refer- beam energy needed. One of the main limitations of a con- ence crystal positions are established prior to the physics ventional collimation approach is the induced background run, during which a fully automated crystal handling is on XRPs, which is linked to its intrinsic nature. Multi- deployed. Even though this special run is much shorter ple Coulomb scattering in amorphous collimators is used than the standard physics run at the LHC, the crystal- to increase the transverse particle amplitude in a standard collimation scheme shows stable performance, while a multiturn cleaning process, while the use of bent crystals realignment of some collimators is required for the stan- as a primary collimation stage promises a faster cleaning dard scheme. Moreover, the evidence of reduction of process and reduced multiturn halo population. Thus, the the beam-related background rate at the experiments is relaxed machine-protection constraints, combined with the obtained using crystals with respect to a standard colli- peculiar needs of this special physics run, provide the ideal mation approach. The solid operational and experimental conditions for the operational deployment of crystal colli- performance obtained, enhance the possibility to use bent mation in the LHC. Crystal collimation was already used crystals in future applications at high-energy colliders that 064066-2 REDUCING BEAM-RELATED BACKGROUND..
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