Future Hadron Colliders Research Articles

Erratum: Improved Monte Carlo Glauber predictions at present and future nuclear colliders [Phys. Rev. C 97 , 054910 (2018)

We present the results of an improved Monte Carlo Glauber (MCG) model of relevance for collisions involving nuclei at center-of-mass energies of BNL RHIC ($\sqrt{s_{\rm NN}}=0.2$ TeV), CERN LHC ($\sqrt{s_{\rm NN}}=2.76$-$8.8$ TeV), and proposed future hadron colliders ($\sqrt{s_{\rm NN}}\approx 10$-$63$ TeV). The inelastic pp cross sections as a function of $\sqrt{s_{\rm NN}}$ are obtained from a precise data-driven parametrization that exploits the many available measurements at LHC collision energies. We describe the nuclear transverse profile with two separated 2-parameter Fermi distributions for protons and neutrons to account for their different densities close to the nuclear periphery. Furthermore, we model the nucleon degrees of freedom inside the nucleus through a lattice with a minimum nodal separation, combined with a "recentering and reweighting" procedure, that overcomes some limitations of previous MCG approaches. The nuclear overlap function, number of participant nucleons and binary nucleon-nucleon collisions, participant eccentricity and triangularity, overlap area and average path length are presented in intervals of percentile centrality for lead-lead (PbPb) and proton-lead (pPb) collisions at all collision energies. We demonstrate for collisions at $\sqrt{s_{\rm NN}}=5.02$ TeV that the central values of the Glauber quantities change by up to 7%, in a few bins of reaction centrality, due to the improvements implemented, though typically remain within the previously assigned systematic uncertainties, while their associated uncertainties are generally smaller (mostly below 5%) at all centralities than for earlier calculations. Tables for all quantities versus centrality at present and foreseen collision energies involving Pb nuclei, as well as for collisions of XeXe at $\sqrt{s_{\rm NN}}=5.44$, and AuAu and CuCu at $\sqrt{s_{\rm NN}}=0.2$ TeV, are provided.

Bound-state dark matter with Majorana neutrinos

We propose a simple scenario in which dark matter (DM) emerges as a stable neutral hadronic thermal relic, its stability following from an exact U(1)D symmetry. Neutrinos pick up radiatively induced Majorana masses from the exchange of colored DM constituents. There is a common origin for both dark matter and neutrino mass, with a lower bound for neutrinoless double beta decay. Direct DM searches at nuclear recoil experiments will test the proposal, which may also lead to other phenomenological signals at future hadron collider and lepton flavor violation experiments.

Probing new electroweak states via precision measurements at the LHC and future colliders

Several new physics scenarios, motivated e.g. by dark matter, feature new electroweakly charged states where the lightest particle in the multiplet is stable and neutral. In such cases direct searches at LHC are notoriously difficult, while electroweak precision tests both at hadron and lepton colliders offer the possibility to indirectly probe those states. In this work, we assess the sensitivity of the high-luminosity phase of the LHC on new electroweak multiplets via the modification of neutral and charged Drell-Yan processes, and compare the reach of future hadron and lepton colliders presently under consideration.

Axion-like particles at future colliders

Axion-like particles (ALPs) are pseudo Nambu–Goldstone bosons of spontaneously broken global symmetries in high-energy extensions of the Standard Model (SM). This makes them a prime target for future experiments aiming to discover new physics which addresses some of the open questions of the SM. While future high-precision experiments can discover ALPs with masses well below the GeV scale, heavier ALPs can be searched for at future high-energy lepton and hadron colliders. We discuss the reach of the different proposed colliders, focusing on resonant ALP production, ALP production in the decay of heavy SM resonances, and associate ALP production with photons, Z bosons or Higgs bosons. We consider the leading effective operators mediating interactions between the ALP and SM particles and discuss search strategies for ALPs decaying promptly as well as ALPs with delayed decays. Projections for the high-luminosity run of the LHC and its high-energy upgrade, CLIC, the future e^+e^- ring-colliders CEPC and FCC-ee, the future pp colliders SPPC and FCC-hh, and for the MATHUSLA surface array are presented. We further discuss the constraining power of future measurements of electroweak precision parameters on the relevant ALP couplings.

Design and performance of the CMS High Granularity Calorimeter Level 1 trigger

The high luminosity (HL) LHC will pose significant detector challenges for radiation tolerance and event pileup, especially for forward calorimetry, and this will provide a benchmark for future hadron colliders. The CMS experiment has chosen a novel high granularity calorimeter (HGCAL) for the forward region as part of its planned Phase 2 upgrade for the HL-LHC. Based largely on silicon sensors, the HGCAL features unprecedented transverse and longitudinal readout segmentation, which will be exploited in the upgraded Level 1 (L1) trigger system. Together with the tracking information, which will also be available at L1, this will open the possibility of pioneering particle-flow-based techniques in the L1 trigger. The high channel granularity results in around one million trigger channels in total and so presents a significant challenge in terms of data manipulation and processing for the trigger, to be compared with the 2000 channels in the endcaps of the current detector. In addition, the high luminosity will result in an average of 140 interactions per bunch crossing that give a huge background rate in the forward region and these will need to be efficiently rejected by the trigger algorithms. Furthermore, 3-dimensional reconstruction of the HGCAL clusters, which will be used for particle-flow, in events with high hit rates is also a complex computational problem for the trigger, unprecedented with the 2-dimensional reconstruction in the current CMS calorimeter trigger. The status of the trigger architecture and design, as well as the concepts for the algorithms needed in order to tackle these major issues and their impact on trigger object performance, are presented here.

Probing electroweakly interacting massive particles with Drell–Yan process at 100 TeV hadron colliders

There are many models beyond the standard model which include electroweakly interacting massive particles (EWIMPs), often in the context of the dark matter. In this paper, we study the indirect search of EWIMPs using a precise measurement of the Drell–Yan cross sections at future 100TeV hadron colliders. It is revealed that this search strategy is suitable in particular for Higgsino and that the Higgsino mass up to about 1.3TeV will be covered at 95% C.L. irrespective of the chargino and neutralino mass difference. We also show that the study of the Drell–Yan process provides important and independent information about every kind of EWIMP in addition to Higgsino.

Collider probes of singlet fermionic dark matter scenarios for the Fermi gamma-ray excess

We investigate the collider signatures of three benchmark points in the singlet fermionic dark matter model motivated by the Fermi gamma-ray excess at the Galactic center. The benchmark points are classified according to the final state of the main dark annihilation process: a pair of b quarks, Higgs bosons and new scalar bosons. Since the dark sector in the model communicates with the visible sector through a mixing between the new scalar and the Higgs boson, collider observables related to the measurements of Higgs boson properties and searches for the new scalar boson are essential to probe the benchmark scenarios at colliders. In this paper, we consider four collider observables: (1) Higgs signal strength (essentially hZZ coupling), (2) triple Higgs coupling, (3) exotic Higgs decay, and (4) direct production of the new scalar particle. We find that the benchmark points have distinctive collider signatures that can be well tested by the interplay of future lepton and hadron colliders.

Cooling rate for microbunched electron cooling without amplification

The Microbunched Electron Cooling (MBEC) proposed by D. Ratner is a promising cooling technique that can find applications in future hadron and electron-ion colliders. In this paper, we develop a new framework for the study of MBEC which is based on the analysis of the dynamics of microscopic 1D fluctuations in the electron and hadron beams during their interaction and propagation through the system. Within this framework, we derive an analytical formula for the cooling rate and benchmark it against 1D computer simulations with a agreement between the analytical and numerical results. We then calculate the expecting cooling time for a set of parameters of the proposed electron-ion collider eRHIC in a simple cooling system with one chicane in the electron channel. While the cooling rate in this system turns out to be insufficient to counteract the intra-beam scattering in the proton beam, we discuss how the electron signal can be amplified by two orders of magnitude through the use of plasma effects in the beam.

Searching for the W\u03b3 decay of a charged Higgs boson

We study the prospects for charged Higgs boson searches in the Wγ decay channel. This loop-induced decay channel can be important if the charged Higgs is fermiophobic, particularly when its mass is below the WZ threshold. We identify useful kinematic observables and evaluate the future Large Hadron Collider sensitivity to this channel using the custodial-fiveplet charged Higgs in the Georgi-Machacek model as a fermiophobic benchmark. We show that the LHC with 300 fb−1 of data at 14 TeV will be able to exclude charged Higgs masses below about 130 GeV for almost any value of the SU(2)L-triplet vacuum expectation value in the model, and masses up to 200 GeV and beyond when the triplet vacuum expectation value is very small. We describe the signal simulation tools created for this analysis, which have been made publicly available.

Cornering colored coannihilation

In thermal dark matter models, allowing the dark matter candidate to coannihilate with another particle can considerably loosen the relic density constraints on the dark matter mass. In particular, introducing a single strongly interacting coannihilation partner in a dark matter model can bring the upper bound on the dark sector energy scale from a few TeV up to about 10 TeV. While these energies are outside the LHC reach, a large part of the parameter space for such coannihilating models can be explored by future hadron colliders. In this context, it is essential to determine whether the current bounds on dark matter simplified models also hold in non-minimal scenarios. In this paper, we study extended models that include multiple coannihilation partners. We show that the relic density bounds on the dark matter mass in these scenarios are stronger than for the minimal models in most of the parameter space and that weakening these bounds requires sizable interactions between the different species of coannihilation partners. Furthermore, we discuss how these new interactions as well as the additional particles in the models can lead to stronger collider bounds, notably in jets plus missing transverse energy searches. This study serves as a vital ingredient towards the determination of the highest possible energy scale for thermal dark matter models.

Probing the effects of dimension-eight operators describing anomalous neutral triple gauge boson interactions at FCC-hh

The effects of dimension-eight operators giving rise to anomalous neutral triple gauge boson interactions of Zγγ and ZγZ vertices in pp→l−l+γ and pp→νν¯γ are investigated at 100 TeV center of mass energy of future circular hadron collider (FCC-hh). The transverse momentum of photon, invariant mass of l−l+γ and angular distribution of charged lepton in the rest frame of l−l+ and Missing Energy Transverse (MET) are considered in the analysis. The realistic detector effects are also included with Delphes simulation. Sensitivity limits obtained at 95% C.L. for CB˜W/Λ4 and CBB/Λ4 couplings are [−0.52;0.52]([−0.40;0.40]) TeV−4, [−0.43;0.43]([−0.33;0.33]) TeV−4 in the dilepton+photon channel and [−0.11;0.11]([−0.084;0.084]) TeV−4, [−0.092;0.092]([−0.072;0.072]) TeV−4 in the MET+photon channel with Lint=1 (3) ab−1, respectively.

DAMPE excess from decaying right-handed neutrino dark matter

The flux of high-energy cosmic-ray electrons plus positrons recently measured by the DArk Matter Particle Explorer (DAMPE) exhibits a tentative peak excess at an energy of around 1.4 TeV. In this paper, we consider the minimal gauged U(1)[Formula: see text] model with a right-handed neutrino (RHN) dark matter (DM) and interpret the DAMPE peak with a late-time decay of the RHN DM into [Formula: see text]. We find that a DM lifetime [Formula: see text] can fit the DAMPE peak with a DM mass [Formula: see text]. This favored lifetime is close to the current bound on it by Fermi-LAT, our decaying RHN DM can be tested, once the measurement of cosmic gamma ray flux is improved. The RHN DM communicates with the Standard Model particles through the U(1)[Formula: see text] gauge boson ([Formula: see text] boson), and its thermal relic abundance is controlled by only three free parameters: [Formula: see text], the U(1)[Formula: see text] gauge coupling [Formula: see text], and the [Formula: see text] boson mass [Formula: see text]. For [Formula: see text], the rest of the parameters are restricted to be [Formula: see text] and [Formula: see text], in order to reproduce the observed DM relic density and to avoid the Landau pole for running [Formula: see text] below the Planck scale. This allowed region will be tested by the search for a [Formula: see text] boson resonance at the future Large Hadron Collider.

Modeling of nonlinear effects due to head-on beam-beam interactions

The beam-beam interaction is one of the most severe limitations on the performance of circular colliders, as it is an unavoidable strong nonlinear effect. As one aspires for greater luminosity in future colliders, one will simultaneously achieve stronger beam-beam interactions. We study the limitations caused by strong incoherent head-on beam-beam interactions, using a new code (cabin) that calculates on a graphics processing unit (GPU), allowing for a detailed description of the long-term particle trajectories in 6D phase space. The evolution of the beam emittance and beam intensity has been monitored to study the impact quantitatively, while frequency map analysis has been performed to understand the impact qualitatively. Results from cabin have shown good quantitative agreement with dedicated experiments in the Large Hadron Collider (LHC). For large beam-beam tune shifts, alternatives to the LHC tunes have been found to improve the beam quality. Schemes devised to cancel beam-beam driven resonances, by use of specific intermediate phase advances between the interaction points, work very well with zero crossing angle, and the accuracy required is achievable. Due to lack of symmetry, these schemes have an almost negligible impact with a significant crossing angle. The hourglass effect has been found to reduce the detrimental effects caused by the chromaticity and vice versa. The optimal level of the hourglass effect has been achieved when ${\ensuremath{\beta}}^{*}=1.5{\ensuremath{\sigma}}_{s}$. The ultimate parameters of the Future Circular Hadron Collider (FCC-hh) seem within reach, in absence of residual odd resonances.

Top quark FCNCs in extended Higgs sectors

The large number of top quarks produced at the LHC and possible future hadron colliders allows to study rare decays of this particle. In many well motivated models of new physics, for example in non-minimal composite-Higgs models, the existence of scalar singlets can induce new flavor-violating top decays surpassing the Higgs contribution by orders of magnitude. We study the discovery prospects of rare top decays within such models and develop new search strategies to test these interactions in top pair-produced events at the LHC. We demonstrate that scales as large as 10–50 TeV can be probed. Improvements by factors of sim 1.5 and sim 3 can be obtained at sqrt{s} = 27 TeV and sqrt{s} = 100 TeV colliders respectively.

Constraining Gluonic Quartic Gauge Coupling Operators with gg→γγ.

Gluon-gluon to photon-photon scattering gg→γγ offers to the LHC experiments a uniquely powerful probe of dimension-8 operators in the standard model effective field theory that are quadratic in both the electromagnetic and gluonic field-strength tensors, such as would appear in the Born-Infeld extension of the standard model. We use 13-TeV ATLAS data on the production of isolated photon pairs to set lower limits on the scales of dimension-8 operators M≳1 TeV and discuss the prospective sensitivities of possible future hadron colliders.

Higgs boson pair production at future hadron colliders: From kinematics to dynamics

The measurement of the triple Higgs coupling is a key benchmark for the LHC and future colliders. It directly probes the Higgs potential and its fundamental properties in connection to new physics beyond the Standard Model. There exist two phase space regions with an enhanced sensitivity to the Higgs self-coupling, the Higgs pair production threshold and an intermediate top pair threshold. We show how the invariant mass distribution of the Higgs pair offers a systematic way to extract the Higgs self-coupling, focusing on the leading channel $pp\to hh+X\to b\bar b\ \gamma\gamma+X$. We utilize new features of the signal events at higher energies and estimate the potential of a high-energy upgrade of the LHC and a future hadron collider with realistic simulations. We find that the high-energy upgrade of the LHC to 27 TeV would reach a 5$\sigma$ observation with an integrated luminosity of 2.5 ab$^{-1}$. It would have the potential to reach 15% (30%) accuracy at the 68% (95%) confidence level to determine the SM Higgs boson self-coupling. A future 100 TeV collider could improve the self-coupling measurement to better than 5% (10%) at the 68% (95%) confidence level.

Block-Coil High-Field Dipoles Using Superconducting Cable-in-Conduit

Dipoles with 15–20 T operating field are required for some designs of future hadron colliders. Such dipoles require the use of high-field superconductors Nb3Sn, Bi-2212, and REBCO. Designs are presented for hybrid block-coil dipoles using superconducting cable-in-conduit (CICC). CICC conveys several benefits that optimize cost and performance at high field: cable-level stress management; compact, robust flared ends; and in-cable cryogen flow. CICC windings uniquely make possible a hybrid coil configuration in which subwindings of Bi-2212, Nb3Sn, and NbTi can be separately wound and heat treated, and then assembled and preloaded in a compact assembly.

The case for future hadron colliders from B \u2192 K(*)\u03bc+\u03bc\u2212 decays

Recent measurements in B → K(*)μ+μ− decays are somewhat discrepant with Standard Model predictions. They may be harbingers of new physics at an energy scale potentially accessible to direct discovery. We estimate the sensitivity of future hadron colliders to the possible new particles that may be responsible for the anomalies at tree-level: leptoquarks or Z′s. We consider luminosity upgrades for a 14 TeV LHC, a 33 TeV LHC, and a 100 TeV pp collider such as the FCC-hh. In the most conservative and pessimistic models, for narrow particles with perturbative couplings, Z′ masses up to 20 TeV and leptoquark masses up to 41 TeV may in principle explain the anomalies. Coverage of Z′ models is excellent: a 33 TeV 1 ab−1 LHC is expected to cover most of the parameter space up to 8 TeV in mass, whereas the 100 TeV FCC-hh with 10 ab−1 will cover all of it. A smaller portion of the leptoquark parameter space is covered by future colliders: for example, in a μ+μ−jj di-leptoquark search, a 100 TeV 10 ab−1 collider has a projected sensitivity up to leptoquark masses of 12 TeV (extendable to 21 TeV with a strong coupling for single leptoquark production).

Top-tagging at the energy frontier

At proposed future hadron colliders and in the coming years at the LHC, top quarks will be produced at genuinely multi-TeV energies. Top-tagging at such high energies forces us to confront several new issues in terms of detector capabilities and jet physics. Here, we explore these issues in the context of some simple JHU/CMS-type declustering algorithms and the N-subjettiness jet-shape variable tau_32. We first highlight the complementarity between the two tagging approaches at particle-level with respect to discriminating top-jets against gluons and quarks, using multivariate optimization scans. We then introduce a basic fast detector simulation, including electromagnetic calorimeter showering patterns determined from GEANT. We consider a number of tricks for processing the fast detector output back to an approximate particle-level picture. Re-optimizing the tagger parameters, we demonstrate that the inevitable losses in discrimination power at very high energies can typically be ameliorated. For example, percent-scale mistag rates might be maintained even in extreme cases where an entire top decay would sit inside of one hadronic calorimeter cell and tracking information is completely absent. We then study three novel physics effects that will come up in the multi-TeV energy regime: gluon radiation off of boosted top quarks, mistags originating from g -> tt, and mistags originating from q -> (W/Z)q collinear electroweak splittings with subsequent hadronic decays. The first effect, while nominally a nuisance, can actually be harnessed to slightly improve discrimination against gluons. The second effect can lead to effective O(1) enhancements of gluon mistag rates for tight working points. And the third effect, while conceptually interesting, we show to be of highly subleading importance at all energies.

Electroweak precision tests in high-energy diboson processes

A promising avenue to perform precision tests of the SM at the LHC is to measure differential cross-sections at high invariant mass, exploiting in this way the growth with the energy of the corrections induced by heavy new physics. We classify the leading growing-with-energy effects in longitudinal diboson and in associated Higgs production processes, showing that they can be encapsulated in four real “high-energy primary” parameters. We assess the reach on these parameters at the LHC and at future hadronic colliders, focusing in particular on the fully leptonic W Z channel that appears particularly promising. The reach is found to be superior to existing constraints by one order of magnitude, providing a test of the SM electroweak sector at the per-mille level, in competition with LEP bounds. Unlike LHC run-1 bounds, which only apply to new physics effects that are much larger than the SM in the high-energy tail of the distributions, the probe we study applies to a wider class of new physics scenarios where such large departures are not expected.