Proceedings Series


Vol. 16 (2023), No. 5, 47 Articles

Diffraction and Low-x 2022

Corigliano Calabro, Italy; 24–30 September, 2022

Front Matter


Group photo


all authors

M. Capua, L. Delle Rose, S. Fazio, R. Fiore, E. Meoni, A. Nava, A. Papa, M. Rossi, C Royon, E. Tassi

Preface


Measurements of Multijet Event Shapes and Photon Production with the ATLAS Detector

abstract

The study of jets and photons production in proton–proton (\(pp\)) collisions is a fundamental part of the ATLAS experimental physics program. In this contribution, measurements of new event-shape jet observables and inclusive isolated-photon production using LHC \(pp\) collision data at \(\sqrt s=13\) TeV with the ATLAS detector will be discussed. The results are unfolded for detector effects and compared to the state-of-the-art theoretical predictions.


Model-independent Odderon Results Based on TOTEM data on Elastic Proton–Proton Scattering at 8 TeV

abstract

We complete the model-independent analysis of the scaling properties of the differential cross section of elastic proton–proton cross sections, including new TOTEM data published in 2022 at \(\sqrt {s} = 8\) TeV. We separate the signal and background regions with a new gating method. In the signal region, we find that the statistical significance of the Odderon exchange from the combined 7.0 and 8.0 TeV \(pp\) data of TOTEM and the 1.96 TeV \(p\bar {p}\) data of D0 is at least 7.32\(\sigma \). In the background region, the scaling functions of elastic proton–proton data at 7 and 8 TeV, and that of elastic proton–antiproton scattering data at 1.96 TeV agree with a statistical significance not larger than 1.93\(\sigma \).


Energy Evolution Equation for Elastic Scattering Amplitudes of Hadrons in \(b\)-space

abstract

Basing our study on the Regge field theory, we formulate and solve numerically an energy evolution equation for the complex elastic scattering amplitude. The equation is a complex version of the Fisher–Kolmogorov–Petrovsky–Piscounov equation in the impact parameter space. We compare the resulting amplitudes with the experimental data, numerically demonstrate the existence of the fixed points, and discuss the recently discovered scaling.


The Low Theorem for Diffractive Bremsstrahlung and the Soft Photon Puzzle

abstract

The anomalous excess of small-\(k_\mathrm {T}\) photons radiated along with multi-hadron production has been challenging the physics community for over four decades, but no solution has been proposed so far. We argue that the problem is rooted in the comparison with incorrect calculations, based on the so-called “bremsstrahlung model” (BM). It is believed to be an extension of the Low theorem from the \(2\to 2+\gamma \) process to radiative multi-particle production \(2\to n+\gamma \), where either initial or final charged hadrons participate in radiation. We demonstrate that this breaks down unitarity of the \(S\)-matrix, thus contradicting the optical theorem.


Modeling Photon Radiation in Soft Hadronic Collisions

abstract

Soft hadronic collisions with multiple production of (anti)quarks accompanied by soft photon radiation are described in terms of higher Fock states of the colliding hadrons, which contain a photon component as well. The Fock state distribution functions are shaped with the Quark–Gluon String Model. Photon radiation by quarks is described within the color-dipole phenomenology. The results of calculations are in good accord with available data in a wide range of transverse momenta of the photons.


Graniitti: Towards a Deep Learning-enhanced Monte Carlo Event Generator for High-energy Diffraction

abstract

We introduce Graniitti, a new Monte Carlo event generator designed especially to solve the enigma of glueballs at the LHC. We discuss the available physics processes, compare the simulations against STAR data from RHIC, and span ambitious future directions towards the first diffractive event generator with a deep learning-enhanced computational engine.


Nuclear Effects in Coherent Photoproduction of Heavy Quarkonia

abstract

Coherent photoproduction of heavy quarkonia on nuclear targets is studied within the QCD color dipole formalism including several main phenomena: (i) The correlation between the impact parameter of a collision \(\vec b\) and dipole orientation \(\vec r\); (ii) The higher-twist nuclear shadowing related to the \(\bar QQ\) Fock state of the photon; (iii) The leading-twist gluon shadowing corresponding to higher Fock components of the photon containing gluons; (iv) Reduced effects of quantum coherence in a popular Balitsky–Kovchegov equation compared to calculations, which are frequently presented in the literature. Our calculations of differential cross sections are in good agreement with the recent ALICE data on charmonium production in ultra-peripheral nuclear collisions. We present also predictions for coherent photoproduction of other quarkonium states (\(\psip (2S)\), \({\mit \Upsilon }(1S)\), and \({\mit \Upsilon }^{\,\prime }(2S)\)) that can be verified by future measurements at the LHC.


Precision Small Scattering Angle Measurements of Proton–Proton and Proton–Nucleus Analyzing Powers at the RHIC Hydrogen Jet Polarimeter

abstract

At RHIC, the hydrogen jet target polarimeter (HJET) is used to measure proton beam polarization with the accuracy of \(\sigma _P^\text {syst}/P\lesssim 0.5\%\) by counting low-energy (1–10 MeV) recoil protons in left–right symmetric detectors. The HJET performance also allowed us to precisely measure \(pp\) and \(pA\) (where \(A\) is any ion stored at RHIC) analyzing powers in the CNI region. The results of the measurements are discussed.


Multiple Pion Pair Production in a Regge-based Model

abstract

Central diffractive event topologies at the LHC energies can be identified by two different approaches. First, the forward scattered protons can be measured in Roman pots. Second, a veto on hadronic activity away from midrapidity can be imposed to define a double-gap topology. Such a double-gap topology trigger has been implemented by the ALICE Collaboration in Run 1 and Run 2 of the LHC. The analysis of these events allows to determine the charged-particle multiplicity within the acceptance. The excellent particle identification capabilities of ALICE allows to study two-track events both in the pion and kaon sector. Events with measured charged particle multiplicity larger than two can arise from multiple pair production. A Regge-based approach for modeling such multiple pair production is presented.


The ReBB Model at 8 TeV: Odderon Exchange is not a Probability, but a Certainty

abstract

The Real Extended Bialas–Bzdak (ReBB) model study is extended to the 8 TeV \(pp\) TOTEM elastic differential cross-section data. The analysis shows that the ReBB model describes the \(pp\) and \(p\bar {p}\) differential cross-section data in the limited 0.37 GeV\(^2\) \(\leq -t \leq 1.2\) GeV\(^2\) and 1.96 TeV \(\leq \sqrt {s} \leq 8\) TeV kinematic region, in a statistically acceptable manner. In this kinematic region, a greater than 30\(\sigma \) model-dependent Odderon signal is observed by comparing the \(pp\) and ReBB extrapolated \(p\bar {p}\) differential cross sections. Thus, in practical terms, within the framework of the ReBB model, the Odderon exchange is not a probability, but a certainty.


Forward Proton Measurements with ATLAS

abstract

In special runs of the LHC, elastic-scattering events were recorded using both ATLAS and the forward subdetector ALFA, thanks to which two new measurements are presented here. The first is exclusive pion-pair production, measured at \(\sqrt {s} = 7\) TeV, using 80 \(\mu \)b\(^{-1}\) of low-luminosity data. A cross-section measurement is performed in two fiducial regions. The results of \(4.8 \pm 1.0(\mathrm {stat.})^{+0.3}_{-0.2}(\mathrm {syst.})\) \(\mu \)b and \(9 \pm 6(\mathrm {stat.}) ^{+1}_{-1}(\mathrm {syst.})\) \(\mu \)b are compared with theoretical models to provide a demonstration of the feasibility of this kind of measurement. The second is an elastic cross section, measured differentially in the Mandelstam \(t\) variable at \(\sqrt {s} = 13\) TeV using 340 \(\mu \)b\(^{-1}\) of low-luminosity data. From a fit to \(\mathrm {d}\sigma /\mathrm {d}t\), the total cross section, the \(\rho \)-parameter, and parameters of the nuclear slope are also determined. The results are \(\sigma _{\mathrm {tot}}(pp \rightarrow X) = 104.7 \pm 1.1\) mb, \(\rho = 0.098 \pm 0.011\). The energy evolution of \(\sigma _{\mathrm {tot}}\) and \(\rho \), connected through dispersion relations, is compared to several models. Furthermore, the total inelastic cross section is determined from the difference of the total and elastic cross section, and the ratio of the elastic to the total cross section is calculated.


Looking Forward: Photon-induced Processes with Tagged Protons at the CMS Experiment

abstract

Photon-induced processes can be used as a sensitive probe into new physics searches and can be studied using exclusive processes. The study of these processes may lead to unprecedented sensitivities of quartic anomalous couplings between photons and \(W\) and \(Z\) bosons, and new physics searches. By tagging the leading proton from the hard interaction, the Precision Proton Spectrometer (PPS) provides an increased sensitivity to select exclusive processes. PPS is designed to operate in standard high-luminosity runs at the LHC to perform measurements of e.g. the quartic gauge couplings and search for rare exclusive processes. The first results obtained with PPS and the status of the ongoing program are discussed.


Exclusive Production of Vector Mesons at the LHCb

abstract

Studies of the central exclusive production (CEP) of vector mesons in \(pp\) collisions at the LHCb are reported. A search is performed for the CEP of charmonium pairs using data corresponding to an integrated luminosity of 3 fb\(^{-1}\) collected at centre-of-mass energies of 7 and 8 TeV. Pairs of \(J/\psi \,J/\psi \) and \(J/\psi \psi (2S)\) are observed in the absence of any other activity inside the LHCb acceptance and the measured cross sections are consistent with theoretical expectations. A study of CEP of \({\mit \Upsilon }(nS)\) states based on the same data is presented. The \({\mit \Upsilon }(1S)\) cross section is measured as a function of rapidity and is found to be in good agreement with the Standard Model predictions. The CEP of \(J/\psi \) and \(\psi (2S)\) mesons is measured at \(\sqrt {s}=13\) TeV and backgrounds are significantly reduced compared to previous measurements through the use of forward shower counters.


Photon–photon Transition Form Factors of Axial Vector Quarkonia in a Light Front Approach

abstract

We present a detailed study of transition form factors for axial vector meson production via the two-photon fusion process \(\gamma ^* \gamma ^* \to 1^{++}\), with space-like virtual photons in the initial state and a \(P\)-wave axial vector quarkonium in the final state. In this analysis, we employ the formalism of light front quarkonium wave functions obtained from a solution of the Schrödinger equation for a selection of interquark potentials for the \(Q \bar Q\) interaction.


Quarkonia Production in Ultra-peripheral PbPb Collisions at the LHCb

abstract

Measurements of coherent charmonium production cross sections together with their ratio in ultra-peripheral PbPb collisions are studied at a nucleon–nucleon centre-of-mass energy of 5.02 TeV. The differential cross-sections are measured as a function of rapidity and transverse momentum, separately. The photo-production of \(J/\psi \) mesons at low transverse momentum is studied in peripheral PbPb collisions and coherent \(J/\psi \) production in hadronic collisions is confirmed. These latest results significantly improve previous measurements and are compared with some theoretical predictions.


all authors

S. Nabeebaccus, S. Wallon, G. Duplančić, K. Passek-Kumerički, B. Pire, L. Szymanowski

Accessing GPDs Through the Exclusive Photoproduction of a Photon–Meson Pair with a Large Invariant Mass

abstract

We study the exclusive photoproduction of a photon–meson pair with a large invariant mass, working in the QCD factorisation framework. Explicitly, we consider a \( \rho \)-meson or a charged \( \pi \) in the final state. This process gives access to chiral-even GPDs as well as chiral-odd GPDs. We focus here on the chiral-even sector. The computation is performed at the leading order and leading twist. We discuss the prospects of measuring them in various experiments such as the JLab 12-GeV, COMPASS, future EIC, and LHC (in ultraperipheral collisions). In particular, the high center-of-mass energies available at collider experiments can be used to probe GPDs at small skewness \( \xi \). We also compute the polarisation asymmetries with respect to the incoming photon. The results for an alternative distribution amplitude (‘holographic’ form) are also compared with the predictions obtained with an asymptotic distribution amplitude.


all authors

A.D. Bolognino, M.M.A. Mohammed, A. Papa, F.G. Celiberto, M. Fucilla, D.Yu. Ivanov

High-energy Signals from Heavy-flavor Physics

abstract

Working in the hybrid high-energy/collinear factorization, where the next-to-leading resummation of energy logarithms is combined with collinear parton densities and fragmentation functions, we study observables sensitive to high-energy dynamics in the context of heavy-flavor physics.


Effects of the Sudakov Form Factor in the Golec-Biernat–Wüsthoff Saturation Model

abstract

We incorporate the Sudakov form factor into the Golec-Biernat–Wüsthoff and Bartels–Golec-Biernat–Wüsthoff saturation models. The parameters are fitted to the HERA data. Both the models show considerable improvements in the fit quality.


Glasma Fluctuations at Finite Proper Times

abstract

By describing the initial stage of heavy-ion collisions in terms of freely-evolving classical fields, we calculate the energy density of one- and two-point correlation functions at finite proper times. With this approach, we are able to effectively resum the high-momentum terms of a power series expansion in proper time, thus making it convergent. Our results provide analytical insight into the dynamics of the Glasma phase.


Diffractive Structure Function in the Dipole Picture

abstract

We calculate the contribution from the \(q\bar {q}g\) component of a virtual photon state to the small-\(x\) diffractive cross section in deep inelastic scattering in the saturation regime. The obtained cross section is finite by itself and a part of the full next-to-leading order result. We perform the calculation in exact kinematics in the eikonal limit, and show that the previously known high virtuality \(Q^2\) and large invariant mass \(M_X^2\) results for the structure functions can be extracted. We furthermore discuss the steps required to obtain the full next-to-leading order result.


Diffractive Di-hadron Production at NLO Within the Shockwave Formalism

abstract

We compute the next-leading-order cross sections for diffractive electro- or photoproduction of a pair of hadrons with large \(p_{\mathrm {T}}\), out of a nucleus or a nucleon. A hybrid factorization is used, mixing collinear and small-\(x\) factorizations, more precisely the shockwave formalism. We demonstrate the cancellation of divergences and extract the finite parts of the differential cross section in general kinematics.


\(J/\psi \) Production in High-multiplicity \(pp\) and \(pA\) Collisions

abstract

Using the Color Glass Condensate (CGC) effective theory and non-relativi-tic QCD (NRQCD) factorization, we provide predictions of \(J/\psi \) polarization parameters in high-multiplicity proton–proton (\(pp\)) and proton–nucleus (\(pA\)) collisions. Energies and rapidity ranges are chosen to be accessible at the Large Hadron Collider (LHC). We predict a weak polarization of \(J/\psi \) that additionally decreases with growing event activities. Small difference between \(pp\) and \(pA\) collisions is observed.


Production of Forward Charm and Neutrinos, and Unintegrated Gluon Distributions at Very Small \(x\)

abstract

In this presentation, we discussed production of charm quarks and antiquarks, \(D\) mesons, and neutrinos/antineutrinos in forward and very forward directions. Gluon–gluon fusions, intrinsic charm, and recombination mechanisms were included. The gluon–gluon contribution, as well as the intrinsic charm contribution, were calculated in the \(k_{\mathrm {t}}\) and hybrid factorization, while the recombination contribution in the standard collinear approach. Different unintegrated gluon distributions from the literature were used. We compared the results of our calculations for \(D\) mesons with the LHCb data for different rapidity intervals. The best description was achieved for the Martin–Ryskin–Watt (MRW) uPDF. We presented also energy distributions for forward electron, muon, and tau neutrinos to be measured at the LHC by the currently operating FASER\(\nu \) experiment, as well as by future experiments such as FASER\(\nu 2\) or FLArE, proposed very recently by the Forward Physics Facility project. At very forward directions, the intrinsic charm and recombination contributions become very important. We presented also neutrino energy fluxes including the above-mentioned processes. For electron and muon neutrinos, intrinsic charm and recombination mechanisms lead to similar production rates and their separation seems rather impossible. On the other hand, for \(\nu _{\tau } + {\bar \nu }_{\tau }\) neutrino flux, the recombination is further reduced making the measurement of the IC contribution very attractive.


Mechanisms of Fully Heavy Tetraquark Production in Proton–Proton Collisions

abstract

We discuss the mechanism of production of the \(X(6900)\) state discovered in the \(J/\psi J/\psi \) channel by the LHCb Collaboration. Both single-parton scattering (SPS) and double-parton scattering (DPS) mechanisms are discussed. The cross section for the \(c \bar c c \bar c\) system is calculated in the \(k_{\mathrm {t}}\)-factorization approach. We present distributions in invariant mass of the four quark system. The DPS contribution is almost two orders of magnitude larger than the SPS one. Imposing a mass window around the resonance position, we calculate the corresponding distribution in \(p_{{\mathrm {t}},4c}\) — the potential tetraquark transverse momentum. The cross section for the \(J/\psi J/\psi \) continuum is calculated. As for the signal, we include SPS (box diagrams) and DPS contributions. They are of similar size. We calculate in addition the \(g^* g^* \to X(6900)\) mechanism in the \(k_{\mathrm {t}}\)-factorization approach. Then the \(0^+\) assignment is preferred over the \(0^-\) one.


Signature of Accidental Symmetry Breaking in Two-particle Correlations within the CGC

abstract

This work is devoted to the computation of the double inclusive gluon production cross section in a proton–nucleus collision within the framework of the Color Glass Condensate (CGC). Given the nature of the scattering, it is fitting to consider the dense-dilute particle production via going beyond the glasma graph approximation given its correspondence to the dilute–dilute limit of the CGC approach. More specifically, starting with the most general expression for the scattering amplitude of the process, not only we do re-obtain the piece encompassing the independent production of the two gluons from separate colour charge densities in the projectile wave function, but also we shed light on the picture encapsulating a correlation rooted in the incoming wave function. This procedure enables us to calculate the anisotropic second and third flow coefficients for both configurations, thereby revealing for the first time the odd azimuthal anisotropy harmonics at leading order in the eikonal expansion associated with the quantum correction embodying the original correlation, that is to say, a breaking of the accidental symmetry in the two-particle scenario.


The QCD Shockwave Approach at NLO: Towards Precision Physics in Gluonic Saturation

abstract

We review on the recent developments of the QCD shockwave approach at the Next-to-Leading Order. The general method of this effective action is sketched and illustrated by the case of diffractive processes.


New Constraints on PDFs with CMS Data

abstract

Recent results on inclusive jet production and production of a \(W\) boson in association with a charm quark by the CMS Collaboration are presented in this contribution. The impact of these measurements on proton PDFs is also discussed.


Small-\(x\) Resummation in PDF Fits and Future Prospects

abstract

I review recent progress in the determination of PDFs with the inclusion of small-\(x\) resummation, and its impact in precision phenomenology, and discuss future prospects.


Pseudo- and Quasi-PDFs in the BFKL Approximation

abstract

I will present the formalism one may use to study the behavior of the Ioffe-time distribution at large distances and show that the pseudo-PDF and quasi-PDF are very different in this regime. Using light-ray operators, I will also show that the higher twist corrections of the quasi-PDF come in not as inverse powers of \(P\) but as inverse powers of \(x_B P\).


Higher-point Functions in \({\cal N}=4\) SYM from an Integrable System

abstract

We review how structure constants in \({\cal N}=4\) super Yang–Mills theory in four dimensions can be computed using an integrable system. Then we present our generalisation of the formalism to higher-point functions of gauge-invariant composite operators of the model. We conclude by listing achievements and future directions.


New Developments in \(\mathcal {N}=2\) Supersymmetric Gauge Theories: from Integrability to Black Holes

abstract

We explain how new physical results in 4D \(\mathcal {N}=2\) supersymmetric gauge theories can be found by connecting them to 2D quantum integrable models. In particular, we set up an identification between the basic mathematical and physical objects of the two kinds of theories (the \(Q\) or \(Y\) and \(T\) functions of integrability and the two periods of the gauge theories) and then, we derive a stream of concepts and mathematical identities between them. Moreover, we use this new correspondence to prove, understand, and possibly generalise a recent application of gauge theories to black holes perturbation theory. From this, several new insights follow into black holes physics, especially a new powerful way of computing quasinormal mode frequencies (the Thermodynamic Bethe Ansatz nonlinear integral equation), characterising the gravitational wave signal (in the ringdown phase of black hole merging). For simplicity and limits of space, we restrict the discussion to the simplest case of the Liouville integrable model/pure SU(2) gauge theory/D3 brane gravitation background triad.


Cross-section Ratio of the Charmonia States \(\psi (2S)\) and \(J/\psi (1S)\) in Exclusive Photoproduction at HERA

abstract

The exclusive photoproduction reactions \(\gamma ^* p \to J/\psi (1S) p\) and \(\gamma ^* p \to \psi (2S) p\) have been measured with the ZEUS detector at the HERA collider. The used data sample corresponds to an integrated luminosity of 373 pb\(^{-1}\) collected during the HERA-II running period. The analysis was performed in the kinematic range of \(30 \lt W \lt 180\) GeV, \(Q^2 \lt 1\) GeV\(^2\), and \(|t| \lt 1\) GeV\(^2\), where \(W\) is the photon–proton centre-of-mass energy, \(Q^2\) is the virtuality of the photon (\(\gamma ^*\)), and \(t\) is the squared four-momentum transfer at the proton vertex. The following decay channels were investigated: \(J/\psi (1S) \to \mu ^+ \mu ^-\), \(\psi (2S) \to \mu ^+ \mu ^-\), and \(\psi (2S) \to J/\psi (1S) \pi ^+ \pi ^-\) with subsequent decay \(J/\psi (1S) \to \mu ^+ \mu ^-\).


Single, Double, and Central Diffractive Dissociation

abstract

Recent results on proton diffractive dissociation are updated and summarized with emphasis on the upgraded LHC kinematics. The high missing mass background is modified to meet recent developments in theory and experiment. In the differential cross section of single diffractive dissociation, a structure (dip-bump) is predicted around \(t=-1.1\) GeV\(^2\).


Measurement of Mass Dependence of the Transverse Momentum of Lepton Pairs in Drell–Yan Production at \(\sqrt {s}=13\) TeV

abstract

A measurement of the differential Drell–Yan cross section in the dielectron and dimuon channel is presented with respect to the transverse momentum of the dilepton pair \(p_{\mathrm {T}}(\ell \ell )\) in the several dilepton mass ranges from 50 GeV to 1 TeV. The result is based on the proton–proton collision data at \(\sqrt {s}=13\) TeV recorded with the CMS detector at the LHC. The corresponding integrated luminosity is 36.3 fb\(^{-1}\). Additional measurements are also performed including the cross section as a function of \(\varphi _{\eta }^{*}\) which is correlated to \(p_{\mathrm {T}}(\ell \ell )\), the cross section with at least one jet in the final state, and the cross-section ratio with respect to the result in the \(Z\) mass peak region. Various comparisons to the latest theoretical predictions are presented with different approaches based on quantum chromodynamics including soft-gluon resummation.


Measurement and QCD Analysis of Inclusive Jet Production in Deep Inelastic Scattering at HERA

abstract

A new measurement of inclusive jet cross sections in deep inelastic scattering using the ZEUS detector at the HERA collider is obtained. The data were taken at HERA 2 at a center-of-mass energy of 318 GeV and correspond to an integrated luminosity of 347 pb\(^{-1}\). The measured jet cross sections are compared to previous measurements as well as NNLO QCD theory predictions. The measurement is used in a QCD analysis at NNLO accuracy to perform a simultaneous determination of parton distribution functions of the proton and the strong coupling constant, resulting in a value of \(\alpha _{\mathrm {s}}(M_{Z}^2) = 0.1138 \pm 0.0014\) (exp/fit) \({}^{+0.0004}_{-0.0008}\) (model/param.) \({}^{+0.0008}_{-0.0007}\) (scale). A significantly improved accuracy is observed compared to similar measurements of the strong coupling constant.


BFKL Description of Jet-gap-jet Events at the LHC

abstract

We discuss recent measurements of jet–gap–jet events performed by the CMS Collaboration and we compare them with the BFKL NLL calculations implemented in the PYTHIA Monte Carlo. We show that the initial-state radiation in PYTHIA plays an important role in the gap definition and is found to be too large.


Probing Low-\(x\) Phenomena at the LHCb

abstract

The LHCb experiment is able to probe kinematic coverage at low Bjorken‑\(x\) down to \(10^{-5}\) or lower due to its forward rapidity coverage. In this contribution, studies of vector boson and hadron production in proton–lead collisions are presented. The \(Z\) boson events are used to probe the proton structure, while a relatively unknown low-\(x\) region is studied with charged and neutral hadron production. Comparisons to theoretical model calculations are also discussed.


Small-\(x\) Resummation in Coefficient Function for Differential Heavy-quarks Production

abstract

High-energy logarithmic corrections are enhanced when the ratio \(x=\frac {Q^2}{s}\) between the typical energy scale of a scattering process \(Q\) and the total center-of-mass energy available \(s\) is small. We discuss recent developments on their resummation in differential cross sections in rapidity, transverse momentum, and invariant mass, and their application to heavy-flavor production at the LHC.


Helix String Fragmentation and Charged Particle Correlations with ATLAS

abstract

Correlations between charged particles provide important insight into the hadronization process. The analysis of the momentum difference between charged hadrons in proton–proton, proton–lead, and lead–lead collisions at the LHC is performed by the ATLAS Collaboration in order to study the dynamics of hadron formation. The spectra of correlated hadron chains are explored and compared to the predictions based on the quantized fragmentation of a three-dimensional QCD helix string. This provides an alternative view of the two-particle correlation phenomenon typically attributed to the Bose–Einstein interference.


Next-to-soft Virtual Resummation for QCD Observables

abstract

We present a framework that resums threshold-enhanced logarithms, originating from soft-virtual and next-to-soft virtual (NSV) contributions in colour-singlet productions, to all orders in perturbation theory. The numerical impacts for these resummed predictions are discussed for the inclusive Drell–Yan di-lepton process up to next-to-next-to-leading logarithmic accuracy, restricting to only diagonal partonic channels.


Stabilizing BFKL via Heavy-flavor and NRQCD Fragmentation

abstract

We bring evidence that the recently discovered property of natural stability of the high-energy resummation is directly connected to the fragmentation mechanism of heavy hadrons. As a phenomenological support, we provide predictions for differential distributions sensitive to heavy-hadron tags, calculated at the next-to-leading logarithmic level of the hybrid high-energy/collinear factorization (NLL/NLO), as implemented in the JETHAD multimodular code. We show that the stabilizing mechanism is encoded in gluon channels of both heavy-flavor collinear fragmentation functions extracted from data and the ones evolved from a nonrelativistic QCD input.


Mueller–Tang Jets in Next-to-leading BFKL

abstract

We provide first predictions for the cross section of Mueller–Tang jets at the LHC. Our calculation is based on a factorization formula of the BFKL type that represents exchanges of colour-singlet objects among the external particles. This formula resums to all perturbative orders a certain class of Feynman diagrams that are supposed to dominate the cross section in the Regge limit. Our explicit calculations at next-to-leading logarithmic order question the validity of such factorization when an IR-safe jet algorithm is used to reconstruct jets. We show the origin of such violation of factorization, and quantify its impact on LHC phenomenology.


QCD at a Forward Physics Facility at the High-Luminosity LHC

abstract

The Forward Physics Facility (FPF) is a proposal to build a new underground in the far-forward region of ATLAS to house a suite of experiments with groundbreaking new capabilities for many Standard Model studies and new physics searches. Although existing LHC detectors have great coverage of the central region, the production of particles in the far-forward direction is poorly constrained. In this regime, the measurement of the neutrino flux and spectrum will provide constraints on QCD that are complementary to those provided by other facilities. This will help validate and improve the underlying hadronic interaction models and multi-purpose event generators, and constrain the gluon PDF in the low-\(x\) region.


The Higgs Impact Factor at Next-to-leading Order

abstract

We compute at the next-to-leading order level the impact factor for the production of a forward Higgs boson from a colliding proton. Combined with other forward impact factors, it can be used to describe, at the next-to-leading logarithmic accuracy, processes in which two objects featuring large separation in rapidity are detected at the Large Hadron Collider (LHC). As well, combined with a proper definition of the unintegrated gluon distribution (UGD), it can be used to compute small-\(x\) corrections to the forward Higgs production.


BSM and SM Signals and Backgrounds in Far-forward Experiments at the LHC

abstract

Two far-forward experimental systems are currently taking data during Run 3 at the Large Hadron Collider (LHC): FASER + FASER\(\nu \) and SND@LHC. They are sensitive to some classes of beyond-the-Standard Model (BSM) particles, muons and neutrinos produced in the ATLAS interaction point (IP) and propagating for several hundred meters along the tangent to the accelerator beamline, up to the caverns where they are respectively located, in opposite directions with respect to the IP. Proposals are being prepared to extend these experiments to bigger ones during the HL-LHC phase. Building a Forward Physics Facility (FPF) capable of hosting a number of far-forward experiments characterized by different detection techniques, kinematical acceptance and purpose is a possibility also under discussion. In this contribution, I discuss some of the BSM and SM signals and backgrounds at the FPF, mainly focusing on QCD-related aspects.


A Parton Branching Algorithm with Transverse-momentum-dependent Splitting Functions

abstract

Parton branching methods underlie the Monte Carlo (MC) generators, being therefore of key importance for obtaining high-energy physics predictions. We construct a new parton branching algorithm which, for the first time, incorporates the off-shell, transverse-momentum-dependent (TMD) splitting functions, defined from the high-energy limit of partonic decay amplitudes. Based on these TMD splitting functions, we construct a new TMD Sudakov form factor. We present the first MC implementation of the algorithm for the evolution of the TMD and integrated parton distribution functions (PDFs). We use this implementation to evaluate small-\(x\) corrections to the distributions and verify the momentum sum rule. The presented study is a first step towards a full TMD MC generator covering the small-\(x\) phase space.


Next-to-Leading Order Virtual Correction to Higgs-induced DIS

abstract

We calculate the Next-to-Leading Order (NLO) virtual correction to the Higgs-induced DIS coefficient function in the infinite top-mass limit. Since we want to use this result in the framework of \(k_\mathrm {t}\)-factorization to resum small-\(x\) logarithms up to Next-to-Leading Logarithm (NLL), we work in light-cone gauge and we keep the incoming gluon off-shell. This choice raises many challenging points such as the presence of spurious singularities and a different definition for the UV-counterterms. This calculation is a necessary ingredient for the coefficient function that will be used to resum up to NLL small-\(x\) logarithms for this process.


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