High-energy Proton-proton Collisions Research Articles

On the breakdown of eikonal approximation and survival of Reggeization in presence of dimension-5 Higgs-gluon coupling

We consider the one-loop effective vertex for the interaction of a gluon with a Reggeized gluon and a Higgs boson in the infinite-top-mass limit, which is described by a dimension-5 non-renormalizable operator. This vertex enters the calculation of differential cross sections for the forward inclusive production of a Higgs boson in high-energy proton-proton collisions, possibly in association with a backward jet or identified hadron, in a framework where next-to-leading logarithms of the energy are resummed to all orders. The effective vertex is extracted from the high-energy behavior of two-to-two amplitudes for the Higgs production in parton-parton collisions and relies on the validity of the Regge form for these amplitudes. We find that the usual eikonal approximation (Gribov prescription) for the Regge limit and the known region-expansion technique in this limit lead to an incomplete result for the amplitude. The discrepancy is traced back to the non-renormalizable nature of the involved operator. However, the Regge limit of the exact QCD amplitude agrees with the Regge-pole exchange form at one loop, nontrivially supporting the Reggeization hypothesis.

Impact of the f-Reggeon exchanges on the observables of the single diffractive dissociation of nucleon at ultrahigh energies

Single diffractive dissociation (SDD) of nucleon in high-energy proton-proton and proton-antiproton collisions is considered in terms of a simple two-Reggeon model with nonlinear Regge trajectories. It is demonstrated that the f-Reggeon impact on the corresponding cross-sections is not negligible up to the LHC energies. As well, it is shown that the account of the f-Reggeon exchanges allows to describe the elastic diffractive scattering (EDS) and SDD of nucleons at ultrahigh energies in the framework of a unified phenomenological scheme. The predictive value of the proposed model is verified.

Central exclusive diffractive production of a single photon in high-energy proton-proton collisions within the tensor-Pomeron approach

We discuss central-exclusive production (CEP) of photons via different fusion processes in the reaction $pp \to pp \gamma$ at high energies, available at RHIC and LHC, within the tensor-pomeron model. We consider two types of processes, the photoproduction contribution via the photon-pomeron and photon-reggeon fusion reactions, and the purely diffractive contribution via the reggeon-pomeron and odderon-pomeron fusion reactions. We present predictions for the measurements of photons at midrapidity, $|{\rm y}| < 2.5$, and at relatively low transverse momentum, $0.1~{\rm GeV} < k_{\perp} < 1~{\rm GeV}$. To check the main results of our study the measurement of the outgoing protons is not necessary. This is of relevance, e.g., for the present version of the ALICE detector at the LHC. Several differential distributions, for instance, in ${\rm y}$, $k_{\perp}$ and $\omega$, the rapidity, the absolute value of the transverse momentum, and the energy of the photon, respectively, are presented. We show that the photoproduction is an important process in the kinematic region specified above. There it gives a much larger cross section than diffractive bremsstrahlung. This is remarkable as the CEP cross section is of order $\alpha_{\rm em}^{3}$ whereas the bremsstrahlung one is only of order $\alpha_{\rm em}$. On the other hand, the soft-photon bremsstrahlung where the basic $pp \to pp$ reaction is due to strong interaction diffraction is more important than CEP in the forward rapidity range, $|{\rm y}| > 4$, and/or at very low $k_{\perp}$. We leave it as a challenge for the planned ALICE 3 experiment at the LHC to study these two contributions to soft photon production in $pp$ collisions. This could shed new light on the so called ``soft photon puzzle'' in hadron-hadron collisions.

Exclusive J/ψ and ϒ production in high-energy pp and p -Pb collisions

We present a formalism for determining the cross section for exclusive heavy vector meson production ($J/\ensuremath{\psi},\mathrm{\ensuremath{\Upsilon}}$) as a function of rapidity, in both high-energy proton-proton and proton-heavy ion collisions, at next-to-leading order in QCD. We compare and contrast the production in $pp$ and $p$-Pb collisions and show how data for these processes can give information on the low-$x$ gluon distribution of the proton and heavy ions at a range of different scales.

Triangle singularity in the production of Tcc+(3875) and a soft pion

The double-charm tetraquark meson $T_{cc}^+(3875)$ can be produced in high-energy proton-proton collisions by the creation of the charm mesons $D^{*+} D^0$ at short distances followed by their binding into $T_{cc}^+$. The $T_{cc}^+$ can also be produced by the creation of $D^{*+} D^{*+}$ at short distances followed by their rescattering into $T_{cc}^+ \pi^+$. A charm-meson triangle singularity produces a narrow peak in the $T_{cc}^+ \pi^+$ invariant mass distribution 6.1 MeV above the threshold with a width of about 1 MeV. Well beyond the peak, the differential cross section decreases with the invariant kinetic energy $E$ of $T_{cc}^+ \pi^+$ as $E^{-1/2}$. The fraction of $T_{cc}^+$ that are accompanied by $\pi^+$ with $E< m_\pi$ is estimated to be roughly 3%. The fraction of $T_{cc}^+$ events with $T_{cc}^+ \pi^+$ in the narrow peak from the triangle singularity could be comparable.

Soft-photon radiation in high-energy proton-proton collisions within the tensor-Pomeron approach: Bremsstrahlung

We discuss diffractive processes in proton-proton collisions at small momentum transfers without and with photon radiation. We consider the soft exclusive reactions $pp\ensuremath{\rightarrow}pp$, $p\overline{p}\ensuremath{\rightarrow}p\overline{p}$, and $pp\ensuremath{\rightarrow}pp\ensuremath{\gamma}$ within the tensor-Pomeron and vector-odderon approach. We compare our results with the data for $pp$ and $p\overline{p}$ total cross sections, for the ratio of real to imaginary parts of the forward scattering amplitude, and for the elastic $pp$ cross sections, especially those from TOTEM. To describe the low-energy data more accurately, the secondary reggeons must be included. We write down the amplitudes for the photon bremsstrahlung in high-energy proton-proton collisions using the tensor-Pomeron model. These results are relevant for the c.m. energies presently available at the Relativistic Heavy Ion Collider and at the LHC. We present predictions for the proposed measurements of soft photons with the planned future upgrade of the ALICE experiment at the LHC. We investigate the limits of applicability of the soft-photon approximation (SPA) based on Low's theorem. The corresponding SPA results are compared to those obtained from our complete model. The regions of phase space are given quantitatively where SPA and our complete tensor-Pomeron results are close to each other. As an example, let ${k}_{\ensuremath{\perp}}$, $\mathrm{y}$, and $\ensuremath{\omega}$, be the absolute value of the transverse momentum, the rapidity, and the energy of the photon, respectively, in the overall c.m. system. For the region $1\text{ }\text{ }\mathrm{MeV}&lt;{k}_{\ensuremath{\perp}}&lt;100\text{ }\text{ }\mathrm{MeV}$ and $3.5&lt;|\mathrm{y}|&lt;5.0$, we find that the SPA Ansatz with only the pole terms $\ensuremath{\propto}{\ensuremath{\omega}}^{\ensuremath{-}1}$ agrees at the percent level with our complete model result up to $\ensuremath{\omega}\ensuremath{\cong}2\text{ }\text{ }\mathrm{GeV}$.

Hadron production with collinearly-improved unintegrated gluon distributions in high energy proton-proton collisions * *Supported by the National Natural Science Foundation of China (12165004, 12061141008, 11975005), Guizhou Provincial Science and Technology Projects ([2019]5103, [2019]5653), the Education Department of Guizhou Province (KY[2021]131), the National Key Research and Development Program of China (2018YFE0104700, 2018YFE01014800)

The collinearly-improved Balitsky-Kovchegov (ciBK) equation evolved unintegrated gluon distribution (UGD) is used for the first time to study hadron production in high energy proton-proton collisions in order to improve the predictive power of the Color Glass Condensate effective theory. We show that the ciBK equation evolved UGD provides a relatively better description of LHC data on the transverse momentum and integrated multiplicity distributions of charged hadron and neutral pion production for several collision energies compared with the running coupling Balitsky-Kovchegov (rcBK) equation evolved UGD. This is because the ciBK evolved UGD has a sharper transverse momentum distribution than the rcBK UGD. The impact of running coupling prescriptions on hadron production is studied, and it is found that the parent dipole and smallest dipole running coupling prescriptions provide similar depictions of the data. Moreover, the scale dependence of the fragmentation function is investigated by taking three typical values of scale. We find that the differences resulting from the scale dependence of the fragmentation function can be fully absorbed into the normalization factor, which lumps higher order corrections.

Modeling of charged-particle multiplicity and transverse-momentum distributions in pp collisions using a DNN

A machine learning technique is used to fit multiplicity distributions in high energy proton-proton collisions and applied to make predictions for collisions at higher energies. The method is tested with Monte Carlo event generators. Charged-particle multiplicity and transverse-momentum distributions within different pseudorapidity intervals in proton-proton collisions were simulated using the PYTHIA event generator for center of mass energies sqrt{s}= 0.9, 2.36, 2.76, 5, 7, 8, 13 TeV for model training and validation and at 10, 20, 27, 50, 100 and 150 TeV for model predictions. Comparisons are made in order to ensure the model reproduces the relation between input variables and output distributions for the charged particle multiplicity and transverse-momentum. The multiplicity and transverse-momentum distributions are described and predicted very well, not only in the case of the trained but also in the case of untrained energy values. The study proposes a way to predict multiplicity distributions at a new energy by extrapolating the information inherent in the lower energy data. Using real data instead of Monte Carlo, as measured at the LHC, the technique has the potential to project the multiplicity distributions for different intervals at very high collision energies, e.g. 27 TeV or 100 TeV for the upgraded HE-LHC and FCC-hh respectively, using only data collected at the LHC, i.e. at center of mass energies from 0.9 up to 13 TeV.

Exploring the Σ+p interaction by measurements of the correlation function

The spin-dependent components of the fundamental hyperon-nucleon interactions are largely unknown. We show that under reasonable assumptions, a measurement of the Σ+p correlation function in high-energy proton-proton or heavy-ion collisions combined with cross section data allows to separate the singlet and the triplet S-wave contributions.

Signs of Phase Transition in High Energy Proton-Proton Collisions

The present work aims to study the possible states of matter and the location of phase boundaries between hadronic gas and the quark-gluon plasma QGP. The boundary at the hadron freeze-out is also considered. Proton-proton collisions at a wide range of center of mass energies are used to examine the phase transition (entropy-temperature) diagram. Local thermodynamic equilibrium is assumed at different intervals of rapidity space. The entropy of the system is expressed in terms of the multiplicity of hadron production in each interval. However, the local temperature is estimated using the average transverse momentum. The values of the critical temperatures are found at the boundaries of the phases with a quite clear description of the states.

Triangle Singularity in the Production of T+cc(3875) and a Soft Pion at Hadron Colliders

If the double-charm tetraquark meson T+cc(3875) is a loosely bound molecule, it can be produced in high-energy proton-proton collisions by the creation of the charm mesons D∗+D0 at short distances followed by their binding into T+cc. It can also be produced by the creation of D∗+D∗+ at short distances followed by their rescattering into T+ccπ+ through a charm-meson triangle loop. A charm-meson triangle singularity produces a narrow peak in the T+ccπ+ invariant mass distribution 6.1 MeV above the threshold with a width of about 1 MeV. The fraction of T+cc that are accompanied by π+ with E &lt; mπ is estimated to be roughly 3%. The fraction of T+cc events with T+ccπ+ in the narrow peak from the triangle singularity could be comparable.

Femtoscopy of the J/ψ-nucleon interaction

I discuss the prospects of using femtoscopy in high-energy protonproton and heavy-ion collisions to learn about the low-energy J/ψ-nucleon interaction. Femtoscopy is a technique that makes it possible to obtain spatiotemporal information on particle production sources at the femtometer scale through measurements of two-hadron momentum correlation functions. These correlation functions also provide information on low-energy hadron-hadron forces as final-state effects. In particular, such correlation functions give access to the forward scattering amplitude. One can express the forward amplitude as the product of the J/ψ chromopolarizability and the nucleon’s average chromoelectric gluon distribution, the latter being relevant to the problem of the origin of the nucleon mass. I will present the results of a recent study using the information on the J/ψ-nucleon interaction from lattice QCD simulations to compute J/ψ-nucleon correlation functions. The calculated correlation functions show clear sensitivity to the final-state interaction. I conclude discussing open issues regarding the use of the effective range expansion formula to fit experimental data for small scattering lengths and large effective range parameters.

Detection of particles of dark matter from the spectrum of secondary particles in high-energy proton-proton collisions in a thermodynamic model

In a simple hydrodynamic model, the transverse momentum distributions are found for A hyperons formed in pp collisions at ultrarelativistic energies. The calculated spectra are compared with the experimental data obtained for various colliders in a wide range of proton collision energies, including the data from experiments at the Large Hadron Collider. An interpretation of the transverse momentum spectra of soft photons in pp collisions is proposed, taking into account the X17 boson with a mass of 17 MeV - a new particle, a possible candidate for the role of dark matter particles. A tube model is proposed on the basis of combining two-dimensional quantum chromodynamics and quantum electrodynamics. An interpretation is proposed for the detection of a 38 MeV boson in the spectra of photons emitted in the reactions of protons with carbon nuclei at an incident proton momentum of 5.5 GeV/c. The X38 boson with a mass of 38 MeV has a mass close to the boson mass obtained by us, equal to 35 MeV for an electromagnetic tube. This new particle was discovered in experiments carried out recently in Dubna for the reactionp p + C→2γ + X. To interpret the obtained experimental data on the spectra of emitted photons depending on their mass, it is proposed to use the formulas obtained for massive particles, setting the mass of a boson decaying into two photons equal to 38 MeV. It was proposed to consider bosons X17 and X38 as particles of dark matter.

The REINFORCE citizen-science project and the search for new long-lived particles at the LHC

The REINFORCE project engages and supports citizens to cooperate with researchers and actively contribute to the development of new knowledge for the needs of science and society. The overall aim is to bridge the gap between them, and reinforce society’s science capital. Within this context, the demonstrator titled “New Particle Search at CERN” will engage citizen-scientists in searches for new elementary particles produced in the high-energy proton-proton collisions at the LHC. To make this possible, the demonstrator adopts a three-stage architecture. The first two stages use simulated data from the ATLAS detector to train citizens, but also to allow for a quantitative assessment of their performance and a comparison with machine learning algorithms. The third stage, on the other hand, uses real data from the ATLAS Open-Data subset, providing two research paths: (a) study of Higgs boson decays to two photons, one of which could be converted to an electron-positron pair by interaction with detector material, and (b) search for yet undiscovered long-lived particles predicted by certain theories Beyond-the-Standard-Model.

Fully-heavy structures in the invariant mass spectrum of J/ψψ(3686), J/ψψ(3770), ψ(3686)ψ(3686), and J/ψϒ(1S) at hadron colliders

In this work, we predict the line shape of potential fully-heavy structures existed in the invariant mass spectrum of J/ψψ(3686), J/ψψ(3770), ψ(3686)ψ(3686), and J/ψϒ(1S) at high energy proton-proton collisions based on a dynamical rescattering picture, whose verification in experiments should be helpful to further clarify the dominant role in producing fully-charm structure X(6900) recently observed by LHCb. The above final states of vector heavy quarkonia can be experimentally reconstructed more effectively by a μ+μ− pair in the muon detector compared with QQ¯ meson with other quantum numbers. Furthermore, the corresponding peak mass positions of each of predicted fully-heavy structures are also given. Our theoretical studies here could provide some valuable information for the future measurement proposals of LHCb and CMS, especially based on the accumulated data after completing Run III of LHC in the near future.

Charged-particle multiplicity dependence of charm-baryon-to-meson ratio in high-energy proton-proton collisions

We propose that the charged-particle multiplicity dependence of the charm-baryon-to-meson ratio observed in high-energy pp collisions can be explained by canonical treatment of quantum charges in the statistical hadronization model (SHM). Taking the full particle listings of PDG complemented by additional charm-baryon states from relativistic quark model predictions, we evaluate the canonical partition function and the charm-hadron chemical factors that measure the canonical suppression arising from the requirement of strict conservation of quantum charges. We demonstrate that, while charm number conservation induces common suppression on the production of both charm-baryons and -mesons, baryon (strangeness) number conservation causes further suppression on charm-baryons (charm-strange mesons) relative to nonstrange charm-mesons, thereby resulting in a decreasing Λc/D0 (Ds/D0) ratio toward smaller multiplicity events. The charm-hadron thermal densities thus computed are then used as pertinent weights to perform charm-quark fragmentation simulations yielding pT-dependent Λc/D0 and Ds/D0 ratios at varying multiplicities in fair agreement with ALICE measurements.

Recoil-free azimuthal angle for precision boson-jet correlation

The azimuthal decorrelation between a vector boson and a jet is an essential hard probe in high energy proton-proton and heavy-ion collisions. We overcome intrinsic limitations of previous studies by using a recoil-free axis, achieving unprecedented next-to-next-to-leading logarithmic accuracy with small nonperturbative corrections. This choice of axis also makes the observable robust in the presence of a large background. Furthermore, the azimuthal angle distribution is minimally changed when determined using only charged particle tracks, which offer superior angular resolution for precise measurements. Our effective field theory includes full jet dynamics, and we find contributions from linearly-polarized gluon transverse momentum distributions in the initial and final state.

The proton structure via double parton scattering

In this talk we present the results of the investigation on the so called double parton distribution functions (dPDFs), accessible quantities in high energy proton-proton and proton nucleus collisions, in double parton scattering processes (DPS). These new and almost unknown distributions encode information on how partons inside a proton are correlated among each other and represent a new tool to explore the three dimensional partonic structure of hadrons. In the present contribution, results of the calculations of dPFDs are presented also including phenomenological investigations on the impact of double correlations in experimental observables, showing how the latter could be observed in the next LHC run. In addition we discuss how present information on experimental observables could be related to the transverse proton structure.

Comparison of improved TMD and CGC frameworks in forward quark dijet production

For studying small-x gluon saturation in forward dijet production in high-energy dilute-dense collisions, the improved TMD (ITMD) factorization formula was recently proposed. In the Color Glass Condensate (CGC) framework, it represents the leading term of an expansion in inverse powers of the hard scale. It contains the leading-twist TMD factorization formula relevant for small gluon’s transverse momentum kt, but also incorporates an all-order resummation of kinematical twists, resulting in a proper matching to high-energy factorization at large kt. In this paper, we evaluate the accuracy of the ITMD formula quantitatively, for the case of quark dijet production in high-energy proton-proton(p+p) and proton-nucleus (p+A) collisions at LHC energies. We do so by comparing the quark-antiquark azimuthal angle ∆ϕ distribution to that obtained with the CGC formula. For a dijet with each quark momentum pt much larger than the target saturation scale, Qs, the ITMD formula is a good approximation to the CGC formula in a wide range of azimuthal angle. It becomes less accurate as the jet pt’s are lowered, as expected, due to the presence of genuine higher-twists contributions in the CGC framework, which represent multi-body scattering effects absent in the ITMD formula. We find that, as the hard jet momenta are lowered, the accuracy of ITMD start by deteriorating at small angles, in the high-energy-factorization regime, while in the TMD regime near ∆ϕ = π, very low values of pt are needed to see differences between the CGC and the ITMD formula. In addition, the genuine twists corrections to ITMD become visible for higher values of pt in p + A collisions, compared to p+p collisions, signaling that they are enhanced by the target saturation scale.

Model-Independent Femtoscopic Lévy Imaging for Elastic Proton-Proton Scattering

A model independent Levy expansion method is introduced to describe nearly Levy shaped squared moduli of Fourier transforms. We apply this method to precisely characterize the most recent elastic scattering data of proton-proton collisions at $$\sqrt s = 13$$ TeV. The results reveal a substructure of protons, which is found to be significantly larger and darker in high energy proton-proton collisions at the TeV scale, as compared to a rather faint and apparently overlooked substructure, that we also identify in elastic proton-proton scattering at the ISR energy range of $$\sqrt s = 23{\kern 1pt} - {\kern 1pt} 62$$ GeV.