Forward Proton Research Articles

Search for high-mass exclusive diphoton production with tagged protons in proton-proton collisions at s=13 TeV

A search is presented for high-mass exclusive diphoton production via photon-photon fusion in proton-proton collisions at s=13 TeV in events where both protons survive the interaction. The analysis utilizes data corresponding to an integrated luminosity of 103 fb−1 collected in 2016–2018 with the central CMS detector and the CMS and TOTEM precision proton spectrometer (PPS). Events that have two photons with high transverse momenta (pTγ>100 GeV), back-to-back in azimuth, and with a large diphoton invariant mass (mγγ>350 GeV) are selected. To remove the dominant inclusive diphoton backgrounds, the kinematic properties of the protons detected in PPS are required to match those of the central diphoton system. Only events having opposite-side forward protons detected with a fractional momentum loss between 0.035 and 0.15 (0.18) for the detectors on the negative (positive) side of CMS are considered. One exclusive diphoton candidate is observed for an expected background of 1.1 events. Limits at 95% confidence level are derived for the four-photon anomalous coupling parameters |ζ1|<0.073 TeV−4 and |ζ2|<0.15 TeV−4, using an effective field theory. Additionally, upper limits are placed on the production of axionlike particles with coupling strength to photons f−1 that varies from 0.03 TeV−1 to 1 TeV−1 over the mass range from 500 to 2000 GeV. © 2024 CERN, for the CMS and TOTEMs Collaboration 2024 CERN

Search for central exclusive production of top quark pairs in proton-proton collisions at s\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$ \\sqrt{s} $$\\end{document} = 13 TeV with tagged protons

A search for the central exclusive production of top quark-antiquark pairs (tt¯\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$ \ extrm{t}\\overline{\ extrm{t}} $$\\end{document}) is performed for the first time using proton-tagged events in proton-proton collisions at the LHC at a centre-of-mass energy of 13 TeV. The data correspond to an integrated luminosity of 29.4 fb−1. The tt¯\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$ \ extrm{t}\\overline{\ extrm{t}} $$\\end{document} decay products are reconstructed using the central CMS detector, while forward protons are measured in the CMS-TOTEM precision proton spectrometer. An observed (expected) upper bound on the production cross section of 0.59 (1.14) pb is set at 95% confidence level, for collisions of protons with fractional momentum losses between 2 and 20%.

Performance of the ATLAS forward proton Time-of-Flight detector in Run 2

We present performance studies of the Time-of-Flight (ToF) subdetector of the ATLAS Forward Proton (AFP) detector at the LHC. Efficiencies and resolutions are measured using high-statistics data samples collected at low and moderate pile-up in 2017, the first year when the detectors were installed on both sides of the interaction region. While low efficiencies are observed, of the order of a few percent, the resolutions of the two ToF detectors measured individually are 21 ps and 28 ps, yielding an expected resolution of the longitudinal position of the interaction, z vtx, in the central ATLAS detector of 5.3 ± 0.6 mm. This is in agreement with the observed width of the distribution of the difference between z vtx, measured independently by the central ATLAS tracker and by the ToF detector, of 6.0 ± 2.0 mm.

Recent ATLAS results on forward physics and diffraction

The ATLAS detector at the LHC is equipped with dedicated systems designed for the detection of forward protons produced in diffractive and photon-induced processes. These detectors significantly extend the ATLAS physics reach. Recent measurements performed using these forward proton detectors: elastic proton-proton scattering, exclusive charged pion-pair production, and two-photon production of lepton pairs are presented.

Forward proton measurements with ATLAS

The ALFA subdetector is designed to measure elastic proton-proton scattering from which the total cross section and \rhoρ-parameter are determined. In 2016, special runs at a center-of-mass energy of \sqrt{s}s = 13 TeV and with \beta^\starβ⋆ = 2.5 km were recorded.

Mechanism of Proton Transfer through the D1-E65/D2-E312 Gate during Photosynthetic Water Oxidation.

In photosystem II, the D1-E65/D2-E312 dyad in the Cl-1 channel has been proposed to play a pivotal role in proton transfer during water oxidation. However, the precise mechanism remains elusive. Here, the proton transfer mechanism within the Cl-1 channel was investigated using quantum mechanics/molecular mechanics calculations. The molecular vibration of the E65/E312 dyad and its deuteration effect revealed that the recently suggested stepwise proton transfer, i.e., initial proton release from the dyad followed by slow reprotonation, does not occur in the Cl-1 channel. Instead, proton transfer is proposed to take place via a conformational change at the E65/E312 dyad, acting as a gate. In its closed form, a proton is trapped within the dyad, preventing forward proton transfer. This closed form converts into the open form, where protonated D1-E65 provides a hydrogen bond to the water network, thereby facilitating fast Grotthuss-type proton transfer.

Instabilities Driven by Proton Temperature Anisotropy in the Presence of Alpha Particles: Implications for Proton-temperature-anisotropy Constraint in the Solar Wind

In situ measurements reveal that proton temperature anisotropy is ubiquitous in the solar wind. Various plasma instabilities have been proposed to regulate the distribution of the proton temperature anisotropy in the solar wind; detailed constraint processes are still unclear. In this paper, we study the effects of alpha beams on both the forward and backward proton temperature anisotropy instabilities at parallel and oblique propagation with the Vlasov theory, and compare the theoretical results with the Wind observation. As the alpha-beam drift velocity v α /v A increases, the growth rates of forward Alfvén/ion-cyclotron (FA/IC) and backward magnetosonic/whistler (BM/W) instabilities increase, those of backward Alfvén/ion-cyclotron (BA/IC) and forward magnetosonic/whistler (FM/W) instabilities decrease, and those of the mirror and forward Alfvén wave (FAW) instabilities are nearly constant. In particular, there are different constraining mechanisms on the distribution of proton temperature anisotropy for different values of the alpha-beam drift velocity. The proton temperature anisotropy instability together with the alpha beam can provide a potential explanation for the distribution of the proton temperature anisotropy in the solar wind.

Search for an axion-like particle with forward proton scattering in association with photon pairs at ATLAS

A search for forward proton scattering in association with light-by-light scattering mediated by an axion-like particle is presented, using the ATLAS Forward Proton spectrometer to detect scattered protons and the central ATLAS detector to detect pairs of outgoing photons. Proton-proton collision data recorded in 2017 at a centre-of-mass energy of sqrt{s} = 13 TeV were analysed, corresponding to an integrated luminosity of 14.6 fb−1. A total of 441 candidate events were selected. A search was made for a narrow resonance in the diphoton mass distribution, corresponding to an axion-like particle (ALP) with mass in the range 150–1600 GeV. No excess is observed above a smooth background. Upper limits on the production cross section of a narrow resonance are set as a function of the mass, and are interpreted as upper limits on the ALP production coupling constant, assuming 100% decay branching ratio into a photon pair. The inferred upper limit on the coupling constant is in the range 0.04–0.09 TeV−1 at 95% confidence level.

Search for high-mass exclusive γγ → WW and γγ → ZZ production in proton-proton collisions at sqrt{s} = 13 TeV

A search is performed for exclusive high-mass γγ → WW and γγ → ZZ production in proton-proton collisions using intact forward protons reconstructed in near-beam detectors, with both weak bosons decaying into boosted and merged jets. The analysis is based on a sample of proton-proton collisions collected by the CMS and TOTEM experiments at sqrt{s} = 13 TeV, corresponding to an integrated luminosity of 100 fb−1. No excess above the standard model background prediction is observed, and upper limits are set on the pp → pWWp and pp → pZZp cross sections in a fiducial region defined by the diboson invariant mass m(VV) > 1 TeV (with V = W, Z) and proton fractional momentum loss 0.04 < ξ < 0.20. The results are interpreted as new limits on dimension-6 and dimension-8 anomalous quartic gauge couplings.

Ion acceleration with few-cycle relativistic laser pulses from foil targets

Ion acceleration resulting from the interaction of 11 fs laser pulses of 35 mJ energy with ultrahigh contrast (<10−10) and 1019 W cm−2 peak intensity with foil targets made of various materials and thicknesses at normal (0°) and 45° laser incidence is investigated. The maximum energy of the protons reached ∼1.4 MeV accelerated in the laser propagation direction and ∼1.2 MeV in the opposite direction from a formvar target. The energy conversion efficiency from the laser to the proton beam is estimated to be as high as ∼1.4% at 45° laser incidence using a 51 nm thick target. The high laser contrast indicates the predominance of vacuum heating via Brunel’s effect as an absorption mechanism involving a tiny pre-plasma at the target front. The experimental results are in reasonable agreement with theoretical estimates, where proton acceleration from the target front side in the backward direction is well explained by the Coulomb explosion of a charged cavity formed in a tiny pre-plasma, while forward proton acceleration is likely to be a two-step process: protons are first accelerated in the target front-side cavity and then further boosted in energy through the target back side via the target normal sheath acceleration (TNSA) mechanism.

Exclusive diffractive bremsstrahlung of one and two photons at forward rapidities: Possibilities for experimental studies in pp collisions at the LHC

We evaluate the cross section for diffractive bremsstrahlung of a single photon in the pp→ppγ reaction at high energies and at forward photon rapidities. Several differential distributions, for instance, in y, k⊥ and ω, the rapidity, the absolute value of the transverse momentum, and the energy of the photon, respectively, are presented. We compare the results for our standard approach, based on QFT and the tensor-pomeron model, with two versions of soft-photon-approximations, SPA1 and SPA2, where the radiative amplitudes contain only the leading terms proportional to ω−1. The SPA1, which does not have the correct energy-momentum relations, performs surprisingly well in the kinematic range considered. We discuss also azimuthal correlations between outgoing particles. The azimuthal distributions are not isotropic and are different for our standard model and SPAs. We discuss also the possibility of a measurement of two-photon-bremsstrahlung in the pp→ppγγ reaction. In our calculations we impose a cut on the relative energy loss (0.02<ξi<0.1, i=1,2) of the protons where measurements by the ATLAS Forward Proton (AFP) detectors are possible. The AFP requirement for both diffractively scattered protons and one forward photon (measured at LHCf) reduces the cross section for pp→ppγ almost to zero. On the other hand, much less cross-section reduction occurs for pp→ppγγ when photons are emitted in opposite sides of the ATLAS interaction point and can be measured by two different arms of LHCf. For the SPA1 ansatz we find σ(pp→ppγγ)≃0.03 nb at s=13 TeV and with the cuts 0.02<ξi<0.1, 8.5<y3<9, −9<y4<−8.5. Our predictions can be verified by ATLAS and LHCf combined experiments. We discuss also the role of the pp→ppπ0 background for single photon production.

Resistive plate chambers for precise measurement of high-momentum protons in short range correlations at R[formula omitted

The Reactions with Relativistic Radioactive Beams (R3B) collaboration of the Facility for Antiproton and Ion Research (FAIR) in Darmstadt, Germany, has constructed an experimental setup to perform fundamental studies of nuclear matter, using as a probe reactions with exotic nuclei at relativistic energies. Among the various detection systems, one of the most recent upgrades consists of the installation of a large area, around 2 m2, multi-gap Resistive Plate Chamber (RPC). The chamber is equipped with twelve 0.3mm gaps and readout by 30mm pitch strips, exhibiting a timing precision down to 50 ps and efficiencies above 98% for minimum ionizing particles in a previous characterization of the detector. The RPC was part of the setup of the FAIR Phase 0 experiment that focused on measuring for the first time, in spring 2022, nucleon–nucleon short-range correlations (SRC) inside an exotic nucleus (16C). The excellent timing precision of this detector will allow the measurement of the forward emitted proton momentum with a resolution of around 1%. In beam measurements show an RPC efficiency above 95% and a time precision better than 100 ps (including the contribution of a reference scintillator and the momentum spread of the particles) for forward emitted particles.

Neutron emission in ultraperipheral Pb-Pb collisions at sNN=5.02 TeV

In ultraperipheral collisions (UPCs) of relativistic nuclei without overlap of nuclear densities, the two nuclei are excited by the Lorentz-contracted Coulomb fields of their collision partners. In these UPCs, the typical nuclear excitation energy is below a few tens of MeV, and a small number of nucleons are emitted in electromagnetic dissociation (EMD) of primary nuclei, in contrast to complete nuclear fragmentation in hadronic interactions. The cross sections of emission of given numbers of neutrons in UPCs of $^{208}$Pb nuclei at $\sqrt{s_{\mathrm{NN}}}=5.02$ TeV were measured with the neutron zero degree calorimeters (ZDCs) of the ALICE detector at the LHC, exploiting a similar technique to that used in previous studies performed at $\sqrt{s_{\mathrm{NN}}}=2.76$ TeV. In addition, the cross sections for the exclusive emission of one, two, three, four, and five forward neutrons in the EMD, not accompanied by the emission of forward protons, and thus mostly corresponding to the production of $^{207,206,205,204,203}$Pb, respectively, were measured for the first time. The predictions from the available models describe the measured cross sections well. These cross sections can be used for evaluating the impact of secondary nuclei on the LHC components, in particular, on superconducting magnets, and also provide useful input for the design of the Future Circular Collider (FCC-hh).

Site-Averaged Ab Initio Kinetics: Importance Learning for Multistep Reactions on Amorphous Supports

Single-atom centers on amorphous supports include catalysts for polymerization, partial oxidation, metathesis, hydrogenolysis, and more. The disordered environment makes each site different, and the kinetics exponentially magnifies these differences to make ab initio site-averaged kinetics calculations extremely difficult. This work extends the importance learning algorithm for efficient and precise site-averaged kinetics estimates to ab initio calculations and multistep reaction mechanisms. Specifically, we calculate site-averaged proton transfer relaxation rates on an ensemble of cluster models representing Brønsted acid sites on silica-alumina. We include direct and water-assisted proton transfer pathways and simultaneously estimate the water adsorption and activation enthalpies for forward and backward proton transfers. We use density functional theory (DFT) to obtain a site-averaged rate, somewhat like a turnover frequency, for the proton transfer relaxation rate. Finally, we show that importance learning can provide orders-of-magnitude acceleration over standard sampling methods for site-averaged rate calculations in cases where the rate is dominated by a few highly active sites.

Overview of ATLAS forward proton detectors for LHC Run 3 and plans for the HL-LHC

The status of the ATLAS Roman Pot detectors (AFP and ALFA) for LHC Run 3 after all refurbishments and improvements done during Long Shutdown 2 is discussed.

Effects of carboxyl- and amino-groups on the antioxidant activity of hydroxyanthraquinones with ESIPT property: A theoretical study

Although anthraquinones have aroused great scientific interest due to their excellent radical scavenging activities, there is seldom systematic theoretical research about their antioxidant mechanisms. Herein, the effect of electron-withdrawing and donating substituents (–COOH and –NH2) on the antioxidant activity of 1,8-Dihydroxyanthraquinone (DHAQ) and its structural analogues (DHAQ-COOH, DHAQ-2COOH, DHAQ-NH2, and DHAQ-2NH2) in acetonitrile (ACN) has been comprehensively studied using quantum chemistry methods. The significant parameters associated with three primary antioxidant mechanisms were unveiled. Besides, the constructed potential energy curves (PECs) were scanned, which have not only proved that the forward proton transfer processes are merely available in the excited state, but also directly confirm the possible stable configurations in the excited state. Subsequently, the ESIPT-antioxidant activity relationship has been established and based on which DHAQ-COOH was judged to be the most promising free radical scavenger upon the photoexcitation.

Exclusive doubly charged Higgs boson pair production in pp collisions at the LHC

The type II seesaw model predicts the existence of a doubly charged Higgs boson ($H ^{\pm\pm}$), which can be produced through the Drell - Yan process and photon -- induced interactions at hadronic colliders. In recent years, such reactions have been largely investigated in inclusive processes, where both incident protons breakup and a large number of particles is produced in addition to the doubly charged Higgs pair. In this paper, we investigate, for the first time, the possibility of searching for $H ^{\pm\pm}$ in exclusive processes, which are characterized by two rapidity gaps and two intact very forward protons in final state. We estimate the associated cross section making use of the exclusive nature of the final state, together with the use of timing information provided by forward proton detectors. Moreover, the background is estimated considering distinct amounts of pile-up. Our results indicate that the exclusive doubly charged Higgs pair production is a promising way to search for signatures of the type II seesaw and to obtain lower mass bounds on $H ^{\pm\pm}$.

New calculation of semiexclusive axionlike particle production at the LHC

We present a new calculation of axionlike particle (ALP) production in the semiexclusive photon-initiated (PI) channel, that is either with intact outgoing protons or rapidity gaps present in the final state, and with no color flow between the colliding LHC protons. This is the first full treatment of this process, accounting for both the possibility of proton dissociation and the survival factor probability of no additional proton-proton interactions, including its kinematic and process dependence. We present the expected event yields in the case that either one or both protons are required to be registered in the forward proton detectors installed in association with ATLAS and CMS. We find that this process can be sensitive to currently unprobed regions of ALP mass and ALP-photon coupling, in particular in the intermediate mass region. Our calculation is provided in the publicly available superchic 4 Monte Carlo (MC) generator, and can be passed to a general purpose MC for showering and hadronization of the final state.

Upgraded Cherenkov time-of-flight detector for the AFP project.

We present the results of our performance studies of the upgraded Cherenkov time-of-flight (ToF) detector for the AFP (ATLAS Forward Proton) project. The latest version consists of solid L-shaped fused silica bars, new customized ALD-coated micro-channel plate photomultipliers (MCP-PMTs) miniPlanacon XPM85112-S-R2D2 with an extended lifetime which operate at low gains (order of 103), and an updated construction. The improvements were aimed to increase the efficiency, the lifetime as well as the radiation hardness of the detector which has been designed to operate in high radiation areas (above 400 kGy/year). The detector was finally tested at the CERN-SPS test-beam facility (120 GeV π+ particles) in August 2021 prior to its installation at the Large Hadron Collider (LHC) at CERN. Measurements proved the detector kept its inner timing resolution of 20 ps despite the rather low gain of its photodetector and reduced optical throughput caused by inevitable changes in the detector geometry.

Searches for QCD instantons with forward proton tagging

We study the possibility to observe heavy (M_{textrm{inst}}> 60 GeV) QCD instantons at the LHC in events with one or two tagged leading protons including fast simulation of detector and pile-up effects. We show that the expected instanton signal in a single-tagged configuration is strongly affected by central detector and pile-up effects. For double-tagged approach, where larger integrated luminosities and hence larger pile-up contaminations need to be considered, the combinatorial background overwhelms the expected signal. We suggest that additional time information about tracks at central and forward rapidities would be crucial for potential improvements.