High Energy Hadronic Collisions Research Articles

Event topology and global observables in heavy-ion collisions at the Large Hadron Collider

Particle production and event topology are very strongly correlated in high-energy hadronic and nuclear collisions. Event topology is decided by the underlying particle production dynamics and medium effects. Transverse spherocity is an event shape observable, which has been used in pp and heavy-ion collisions to separate the events based on their geometrical shapes. It has the unique capability to distinguish between jetty and isotropic events. In this work, we have implemented transverse spherocity in Pb–Pb collisions at sqrt{s_text{NN}} = 5.02 TeV using A Multi-Phase Transport Model (AMPT). While awaiting for experimental explorations, we perform a feasibility study of transverse spherocity dependence of some of the global observables in heavy-ion collisions at the Large Hadron Collider energies. These global observables include the Bjorken energy density (varepsilon _text{Bj}), squared speed of sound (c_text{s}^2) in the medium and the kinetic freeze-out properties for different collision centralities. The present study reveals about the usefulness of event topology dependent measurements in heavy-ion collisions.

Charged Lepton Flavor Violation at the High-Energy Colliders: Neutrino Mass Relevant Particles

We summarize the potential charged lepton flavor violation (LFV) from neutrino mass relevant models, for instance the seesaw mechanisms. In particular, we study, in a model-dependent way, the LFV signals at the high-energy hadron and lepton colliders originating from the beyond standard model (BSM) neutral scalar H, doubly charged scalar H±±, heavy neutrino N, heavy WR boson, and the Z′ boson. For the neutral scalar, doubly charged scalar and Z′ boson, the LFV signals originate from the (effective) LFV couplings of these particles to the charged leptons, while for the heavy neutrino N and WR boson, the LFV effects are from flavor mixing in the neutrino sector. We consider current limits on these BSM particles and estimate their prospects at future high-energy hadron and lepton colliders.

Radiative natural supersymmetry emergent from the string landscape

In string theory with flux compactifications, anthropic selection for structure formation from a discretuum of vacuum energy values provides at present our only understanding of the tiny yet positive value of the cosmological constant. We apply similar reasoning to a toy model of the multiverse restricted to vacua with the MSSM as the low energy effective theory. Here, one expects a statistical selection favoring large soft SUSY breaking terms leading to a derived value of the weak scale in each pocket universe (with appropriate electroweak symmetry breaking) which differs from the weak scale as measured in our universe. In contrast, the SUSY preserving μ parameter is selected uniformly on a log scale as is consistent with the distribution of SM fermion masses: this favors smaller values of μ. An anthropic selection of the weak scale to within a factor of a few of our measured value — in order to produce complex nuclei as we know them (atomic principle) — provides statistical predictions for Higgs and sparticle masses in accord with LHC measurements. The statistical selection then more often leads to (radiatively-driven) natural SUSY models over the Standard Model or finely-tuned SUSY models such as mSUGRA/CMSSM, split, mini-split, spread, high scale or PeV SUSY. The predicted Higgs and superparticle spectra might be testable at HL-LHC or ILC via higgsino pair production but is certainly testable at higher energy hadron colliders with sqrt{s} ∼ 30–100 TeV.

Combined-function optics for circular high-energy hadron colliders

The design of future circular high-energy hadron colliders is based on the achievement of challenging magnetic fields, needed to keep the hadron beams orbiting along the ring circumference. The strength of the dipolar magnetic field is a function of the machine radius, beam energy, and of the fraction of the ring circumference that can be filled with dipoles. In this paper, we propose to use a combined-function periodic cell to maximise the filling factor of the dipole magnets. The optical properties of the proposed periodic structure are discussed in detail together with the design of the superconducting magnets needed to implement the proposed approach.

A very high energy hadron collider on the Moon

The long-term prospect of building a hadron collider around the circumference of a great circle of the Moon is sketched. A circular collider on the Moon (CCM) of ∼11 000 km in circumference could reach a proton–proton center-of-mass collision energy of 14 PeV—a thousand times higher than the Large Hadron Collider at CERN—optimistically assuming a dipole magnetic field of 20 T. Several aspects of such a project are presented, including siting, construction, availability of necessary materials on the Moon, and powering, as well as a discussion of future studies and further information needed to determine the more concrete feasibility of each. Machine parameters and vacuum requirements are explored, and an injection scheme is delineated. Other unknowns are set down. Due to the strong interest from multiple organizations in establishing a permanent Moon presence, a CCM could be the (next-to-) next-to-next-generation discovery machine for high-energy particle physics and a natural successor to next-generation machines, such as the proposed future circular collider at CERN or a super proton–proton collider in China, and other future machines, such as a collider in the Sea, in the Gulf of Mexico. A CCM would serve as an important stepping stone toward a Planck-scale collider sited in our Solar System.

One-loop electroweak Sudakov logarithms: a revisitation and automation

In this work we revisit the algorithm of Denner and Pozzorini for the calculation of one-loop electroweak Sudakov logarithms and we automate it in the MadGraph5_aMC-@NLO framework. We adapt the formulas for modern calculations, keeping light-quarks and photons strictly massless and dealing with infrared divergences via dimensional regularisation. We improve the approximation by taking into account additional logarithms that are angular dependent. We prove that an imaginary term has been previously omitted and we show that it cannot be in general neglected for 2 → n processes with n > 2. We extend the algorithm to NLO EW corrections to squared matrix-elements that involve also QCD corrections on top of subleading LO terms. Furthermore, we discuss the usage of this algorithm for approximating physical observables and cross sections. We propose a new approach in which the QED component is consistently removed and we show how it can be superior to the commonly used approaches. The relevance of all the novelties introduced in this work is corroborated by numerical results obtained for several processes in a completely automated way. We thoroughly compare exact NLO EW corrections and their Sudakov approximations both at the amplitude level and for physical observables in high-energy hadronic collisions.

Momentum and multiplicity dependence of strangeness and nuclei production

Prominent phenomena observed in high-energy hadronic collisions, such as the strangeness enhancement in small collision systems with increasing multiplicity and the production of loosely bound states in collisions where extreme temperatures are reached, are still mysterious and at the center of the experimental programs of several scientific Collaborations. In this contribution, recent experimental results from the ALICE and STAR Collaborations on these two fronts are presented and discussed.

Production of light nuclei in small collision systems with ALICE

The energy densities reached in high-energy hadronic collisions at the LHC allow significant production of light (anti)nuclei. Their production yields have been measured as a function of pT and charged-particle multiplicity in different collision systems and at different center-of-mass energies by ALICE. One of the most interesting results obtained from such a large variety of experimental data is that the dominant production mechanism of light (anti)nuclei seems to depend solely on the event charged-particle multiplicity. Evidence for this comes from the continuous evolution of the deuteron15 to-proton and 3He-to-proton ratios with the event multiplicity across different collision systems and energies. The characterization of the light nuclei production mechanism is complemented by measurements of their production yields in jets and in the underlying event. In this paper, recent results on light nuclei production in small collision systems are shown and discussed in the context of the statistical hadronization and coalescence models. In addition, recent results on the deuteron production in jets and new preliminary results on its production in the underlying event measured in pp collisions at √S = 13 TeV are discussed.

Photostimulated desorption performance of the future circular hadron collider beam screen

Synchrotron radiation (SR) originated at superconducting bending magnets is known to be at the origin of several beam detrimental effects related to vacuum instabilities. One of the major challenges in the design of the vacuum beam pipes of high-energy hadron colliders is the SR coping strategy. In the case of the future circular hadron collider (FCC-hh), a Cu-coated beam screen (BS) operating in the range of 40--60 K has been designed with the aim of protecting the superconducting magnet cold bores from direct synchrotron irradiation. In order to experimentally study the FCC-hh BS vacuum and cryogenic performance, two sample prototypes were manufactured and installed in the beam screen test-bench experiment (BESTEX) at the Karlsruhe Research Accelerator (KARA) at the Karlsruhe Institute of Technology (KIT). The emitted SR has a critical energy of 6.2 keV, very similar to the 4.6 keV of FCC-hh. Irradiation at both room (RT) and cryogenic (77 K) temperatures showed a significant reduction of the molecular photostimulated desorption yields ($\ensuremath{\eta}$) of the FCC-hh beam screen compared to those of Cu samples. A first approximation of $\ensuremath{\eta}$ and its evolution with the photon dose accumulated on the FCC-hh BS prototype at 77 K allows to estimate that a machine conditioning period of $\ensuremath{\sim}1.2$ months would be needed to reduce the photostimulated molecular density at the necessary levels to ensure a 100 h beam lifetime at nominal FCC-hh operation.

On new physics contributions to the Higgs decay to Z\u03b3

We explore models of new physics that can give rise to large (100% or more) enhancements to the rate of Higgs decay to Zγ while still being consistent with other measurements. We show that this is impossible in simple models with one additional multiplet and also in well motivated models such as the MSSM and folded SUSY. We do find models with several multiplets that carry electroweak charge where such an enhancement is possible, but they require destructive interference effects. We also show that kinematic measurements in Higgs decay to four leptons can be sensitive to such models. Finally we explore the sensitivity of four lepton measurements to supersymmetric models and find that while the measurement is difficult with the high luminosity LHC, it may be possible with a future high energy hadron collider.

Longitudinal Z-boson polarization and the Higgs boson production cross section at the Large Hadron Collider

Charged lepton pairs are produced copiously in high-energy hadron collisions via electroweak gauge boson exchange, and are one of the most precisely measured final states in proton-proton collisions at the Large Hadron Collider (LHC). We propose that measurements of lepton angular distributions can be used to improve the accuracy of theoretical predictions for Higgs boson production cross sections at the LHC. To this end, we exploit the sensitivity of the lepton angular coefficient associated with the longitudinal Z-boson polarization to the parton density function (PDF) for gluons resolved from the incoming protons, in order to constrain the Higgs boson cross section from gluon fusion processes. By a detailed numerical analysis using the open-source platform xFitter, we find that high-statistics determinations of the longitudinally polarized angular coefficient at the LHC Run III and high-luminosity HL-LHC improve the PDF systematic uncertainties of the Higgs boson cross section predictions by 50% over a broad range of Higgs boson rapidities.

Renaissance of Nuclear Physics at the LHC

High-energy Large Hadron Collider (LHC) experiments provide a unique laboratory for nuclear and hadron physics studies that have a wide breadth of possible applications to astrophysics. This articl...

Double-crystal measurements at the CERN SPS

The UA9 setup, installed in the Super Proton Synchrotron (SPS) at CERN, was exploited for a proof of principle of the double-crystal scenario, proposed to measure the electric and the magnetic moments of short-lived baryons in a high-energy hadron collider, such as the Large Hadron Collider (LHC). Linear and angular actuators were used to position the crystals and establish the required beam configuration. Timepix detectors and high-sensitivity Beam Loss Monitors were exploited to observe the deflected beams. Linear and angular scans allowed exploring the particle interactions with the two crystals and recording their efficiency. The measured values of the beam trajectories, profiles and of the channeling efficiency agree with the results of a Monte-Carlo simulation.

Loss of Transverse Landau Damping by Diffusion in High-Energy Hadron Colliders

Loss of Transverse Landau Damping by Diffusion in High-Energy Hadron Colliders

Higher-order corrections for tW production at high-energy hadron colliders

We discuss cross sections for tW production in proton-proton collisions at the LHC and at higher-energy colliders with energies of up to 100 TeV. We find that, remarkably, the soft-gluon corrections are numerically dominant even at very high collider energies. We present results with soft-gluon corrections at approximate NNLO and approximate N3LO matched to complete NLO results. These higher-order corrections are large and need to be included for better theoretical accuracy and smaller scale dependence. Total cross sections as well as top-quark and W-boson transverse-momentum and rapidity distributions are presented using various recent sets of parton distribution functions.

Impact of betatron collimation losses in the High-Energy Large Hadron Collider

In response to recommendations in the 2013 update of the European Strategy for Particle Physics, a conceptual design effort for an energy upgrade of the Large Hadron Collider (LHC) at CERN, the so-called high-energy LHC (HE-LHC), was launched as part of the Future Circular Collider study. The HE-LHC machine, which is meant to use 16 T magnet technologies in the existing LHC tunnel, would provide proton collisions at a center-of-mass energy of 27 TeV ($\ensuremath{\sim}2\ifmmode\times\else\texttimes\fi{}\mathrm{LHC}$) with a total stored energy of 1.34 GJ ($\ensuremath{\sim}4\ifmmode\times\else\texttimes\fi{}\mathrm{LHC}$) per beam. By adapting the LHC collimation system, a first layout of the HE-LHC's betatron cleaning insertion was conceived, with the requirement to sustain---for at least 10 seconds---the impact of about 1.86 MW, corresponding to a beam lifetime of 12 minutes, without inducing any magnet quench nor any damage to other accelerator components. In this article, we evaluate the power deposition on the collimation insertion for proton beam operation in the HE-LHC machine at top energy, by means of particle tracking and interaction calculations. The beam loss effects on the warm elements as well as on the superconducting dispersion suppressor magnets are assessed through a three-step simulation approach. In particular, for the proposed future high-energy LHC, we demonstrate the necessity of adding local collimators in the dispersion suppressor, and we uncover the harmful consequences of a potential removal of the beam line ``dogleg'' in the collimation insertion.

Simultaneous extraction of fragmentation functions of light charged hadrons with mass corrections

Achieving the highest possible precision for theoretical predictions at the present and future high-energy lepton and hadron colliders requires a precise determination of fragmentation functions (FFs) of light and heavy charged hadrons from a global QCD analysis with great accuracy. We describe a simultaneous determination of unpolarized FFs of charged pions, charged kaons and protons/antiprotons from single-inclusive hadron production in electron-positron annihilation (SIA) data at next-to-leading order and next-to-next-to-leading order accuracy in perturbative QCD. A new set of FFs, called SGKS20, is presented. We include data for identified light charged hadrons (${\ensuremath{\pi}}^{\ifmmode\pm\else\textpm\fi{}}$, ${K}^{\ifmmode\pm\else\textpm\fi{}}$ and $p/\overline{p}$) as well as for unidentified light charged hadrons, ${h}^{\ifmmode\pm\else\textpm\fi{}}$ and show that these data have a significant impact on both size and uncertainties of the fragmentation functions. We examine the inclusion of higher-order perturbative QCD corrections and finite-mass effects. We compare the new SGKS20 FFs with other recent FFs available in the literature and find in general reasonable agreement, but also important differences for some parton species. We show that theoretical predictions obtained from our new FFs are in very good agreement with the analyzed SIA data, especially at small values of $z$. The SGKS20 FF sets presented in this work are available via the LHAPDF interface.

Hadronization of correlated gluon fields

Following an explicit example, we present the chain of steps required for an event-by-event description of hadron production in high energy hadronic and nuclear collisions. We start from incoming nuclei, described in the color glass condensate effective theory, whose collision creates the gluon fields of the glasma. Individual gluons are then sampled from the gluon fields' Husimi (smeared Wigner) distributions and clustered using a new spacetime based algorithm. Clusters are fed into the Herwig event generator, which performs the hadronization, conserving energy and momentum. We discuss the physical implications of smearing and problems with the quasiparticle picture for the studied processes. We compute spectra of charged hadrons and identified particles and their azimuthal momentum anisotropies, and address systematic uncertainties on observables, resulting from the general lack of detailed knowledge of the hadronization mechanism.

Emittance growth suppression with a multibunch feedback in high-energy hadron colliders: Numerical optimization of the gain and bandwidth

A transverse feedback system can effectively mitigate the emittance growth caused by injection oscillations and machine noise in hadron beams. However, as its action on the beam depends on beam position measurements of finite accuracy, it introduces additional noise on its own. The machine noise is in general strongest at low frequencies. Hence, the feedback is less needed at high frequencies. In this paper, two theories for the reduction of the machine noise induced emittance growth rate, with a bunch-by-bunch feedback, have been extended to a multibunch feedback. The extended theories show quantitative agreement with sophisticated macroparticle simulations. The emittance growth caused by the beam position measurement noise is numerically found to be only weakly dependent on the feedback's cutoff frequency, while it is strongly dependent on the single-bunch gain. The ultimate goal of this study is to find the optimal transverse feedback bandwidth and gain, determined by the minimization of the total emittance growth rate. The optimum depends on the ratio between the amplitudes of the beam position measurement error and the machine noise, the power spectrum of the machine noise, the response of the feedback filters, and the magnitude and details of the detuning. For the illustrative case of the Large Hadron Collier during collision in run 2, the optimum is found at the currently lowest possible cutoff frequency of 0.5 MHz, with a single-bunch damping time of approximately 270 turns. Using a chromaticity of 15 units, the minimal emittance growth rate at this cutoff frequency is 72% lower than with a bunch-by-bunch feedback. If the beam position measurement error can be reduced relative to the machine noise, the optimum will shift to larger single-bunch gains, or equivalently shorter single-bunch damping times.

Sensitivity Reach on Anomalous Higgs Couplings via Triphoton Production for the Post-LHC Circular High-energy Hadron Colliders

The potential of triphoton production to obtain limits on anomalous Higgs boson couplings at $H\gamma\gamma$ and $HZ\gamma$ vertices is studied in Standard Model Effective Field Theory (EFT) framework for the post-LHC circular high-energy hadron colliders: High Luminosity-LHC (HL-LHC), High Energy LHC (HE-LHC) and Low Energy FCC (LE-FCC) which are designed with standard configurations of 14 TeV/3 ab$^{-1}$, 27 TeV/15 ab$^{-1}$ and 37.5 TeV/15 ab$^{-1}$. Madgraph in which the effective Lagrangian of the SM EFT is implemented using FeynRules and UFO framework is used to generate both background and signal events. These events are then passed through Pythia 8 for parton showering and Delphes to include realistic detector effects. After optimizing cuts on kinematics of three photons as well as the reconstructed invariant mass of the two leading photons, invariant mass of three leading photon is used to obtain constraints on the Wilson coefficients of dimension-six operators. We report the result of two dimensional scan of $\bar{c}_{\gamma}$ and $\tilde{c}_{\gamma}$ couplings at 95\% confidence level and compare with LHC results. Our obtained limits without systematic error on $\bar{c}_{\gamma}$ ( $\tilde{c}_{\gamma}$) are $[-3.15;1.41]\times10^{-2}$ ($[-2.12;2.12]\times10^{-2}$), $[-1.21;0.78]\times10^{-2}$ ($[-0.98;0.98]\times10^{-2}$) and $[-0.89;0.66]\times10^{-2}$ ($[-0.77;0.77]\times10^{-2}$) for HL-LHC, HE-LHC and LE-FCC, respectively.