arXiv subjects

Saad Nabeebaccus

Publications and source records attributed to Saad Nabeebaccus.

At least 19 recordsLinked to original sources

arXiv2026

Exclusive photoproduction of a di-meson pair with large invariant mass

The exclusive photoproduction of a pair of light mesons is studied within the framework of collinear factorisation. The amplitude factorises into a process-dependent perturbatively calculable hard part, a generalised parton distribution (GPD) and two distribution amplitudes (DAs). We focus on the production of any combination of $ρ$ and $π$ mesons (of any charge and polarisation) that do not involve neutral $C=+$ exchanges with the nucleon. This gives a total of 26 distinct channels, which are sensitive to quark GPDs only. We calculate the amplitude for this family of di-meson processes at leading order in the strong coupling constant $α_s$ and at leading twist, in a fully-automated way. Depending on the choice of mesons in the final state, some of these processes are sensitive to chiral-odd (helicity-flip) GPDs. Particular attention is given to the treatment of poles in the 3-dimensional convolution integral of the momentum fractions connecting the hard part with the different non-perturbative components. These poles are regularised by the usual Feynman $i ε$ factors, but lead to numerical instabilities if not dealt with properly. We also discuss in detail the construction of the phase space. Importantly, we propose a resolution for the inconsistency of the kinematics of the hard part of the process, where hadron masses and other soft scales are neglected, with the rest of the process. As a proof of concept, we explicitly evaluate the cross section, for a subset of processes whose amplitudes have been constructed, at energies typical of the CLAS12 experiment at JLab. Our results indicate that exclusive di-meson photoproduction processes have very good statistics, which can be a factor of up to a hundred more than the exclusive photon-meson photoproduction process. Therefore, the family of processes that we study here represents a great opportunity for GPD extraction.

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arXiv2025

Hard exclusive photoproduction of photon-meson pairs: pseudoscalar channels $π$, $η$ and $η'$

We investigate the hard exclusive photoproduction of photon-meson pairs at leading-twist and leading-order in perturbative QCD, and focus on pseudoscalar mesons $\text{M} \in \{π^\pm, π^0, η, η'\}$. Compact analytical expressions are obtained for the amplitudes involving quark generalized parton distributions, with the two-gluon components of the $η$ and $η'$ distribution amplitudes included. The numerical analysis is performed in the moderate-$ξ$ region, where valence-quark GPDs are expected to be important. In this region, we find a strong impact of the pion-pole term in $γπ^\pm$ production, and a non-negligible effect for neutral mesons. We also observe a marked dependence of $γη'$ photoproduction on two-gluon contributions. This process offers enhanced sensitivity to the shape of the GPDs already at leading-order, while the tested dependence on the meson distribution amplitude and the renormalization scale introduces further theoretical uncertainties, the latter emphasizing the need for next-to-leading-order corrections. Our results provide a concise analytical framework and a numerical baseline for future studies.

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arXiv2025

Uncharted Logarithmic Structures in QCD Transverse-Energy Flow

We investigate the QCD transverse-energy ($E_T$) flow distribution within an azimuthal region of phase space, defined by an angular interval $Δϕ$ on the plane transverse to a chosen jet axis. Vetoes on the resulting $E_T$ are widely employed at the LHC to isolate missing transverse momentum in final states with invisible particles. We show that this observable has logarithmic structures never seen before in QCD calculations. Notably, its description involves the resummation of non-global and coherence-violating logarithmic contributions that are more singular than any reported to date. We analyze its all-orders behavior, providing analytical resummations at next-to-double-logarithmic accuracy for $e^+e^-$ and $pp$ collisions, and numerical resummations at leading-logarithmic accuracy in the Veneziano limit for $pp$ collisions. We further present a computation of the leading coherence-violating correction for $pp$ collisions. The intricate structure of vetoes in azimuthal gaps reveals new aspects of QCD dynamics and offers a novel probe of collinear-factorization breaking at the LHC.

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arXiv2025

Exclusive photoproduction of a $π^0γ$ pair in the saturation framework

We consider the exclusive photoproduction of a $π^0 γ$ pair with large invariant mass, as a promising channel to study the effects of gluon saturation. It has recently been demonstrated that this process is incompatible with a collinear factorization approach in terms of generalized parton distributions (GPDs) at the leading twist. In such a situation, a (generalized) $k_T$-dependent factorization at small $x$ is a valid alternative approach. We perform this calculation using the shockwave formalism, which resums multiple gluon exchanges between the projectile and the dense nuclear target. We find that the polarized amplitude changes sign as a function of back-to-back transverse momentum $|\vec{p}_t|$ of the pion-photon pair, resulting in a dip-like structure in the fully differential cross section as a function of $|\vec{p}_t|$.

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arXiv2025

Exclusive photoproduction of a di-meson pair with large invariant mass

We consider the exclusive photoproduction of a di-meson pair with large invariant mass, $γN \rightarrow N' M_1M_2$, in the framework of collinear factorisation. The mesons considered $M_1$ and $M_2$ are either pions or rho mesons, charged or neutral. We consider the kinematic regime characterised by a large invariant mass of the two-meson system, and a small deflection of the nucleon in the centre-of-mass frame. In this kinematic domain, the amplitude factorises into a perturbative hard part and non-perturbative parts described by Generalised Parton Distributions (GPDs) and Distribution Amplitudes (DAs). We automate the calculation of the fully differential cross section at leading twist and leading order, and we present some numerical results at JLab 12 GeV kinematics. This class of processes provides yet more exclusive $2 \to 3$ channels that can be used to extract GPDs.

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arXiv2024

Accessing Generalized Parton Distributions through $2 \to 3$ exclusive processes

We review our results on a new class of $2 \to 3$ exclusive processes, as a probe of both chiral-even and chiral-odd quark GPDs. We consider the exclusive photoproduction of a photon-meson pair, in the kinematics where the pair has a large invariant mass, described in the collinear factorization framework. We cover the whole kinematical range from medium energies in fixed target experiments to very large energies of colliders, by considering the experimental conditions of JLab 12-GeV, COMPASS, future EIC and LHC (in ultra-peripheral collisions) cases. Our analysis covers neutral and charged rho-mesons, as well as charged pions. The case of the rho-meson, depending on its polarization, provides access to either chiral-even or chiral-odd GPDs, at leading twist. We find that the order of magnitude of the obtained cross sections are sufficiently large for a dedicated experimental analysis to be performed, especially at JLab. Furthermore, we compute the linear photon beam polarization asymmetry, which we find to be sizeable, in the case of a longitudinally polarized $ρ$-meson or of a charged pion. These predictions are obtained for both asymptotic distribution amplitude (DA) and holographic DA.

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arXiv2024

Breakdown of collinear factorisation in the photoproduction of a $ π^{0}γ$ pair with large invariant mass

We identify a $ 2 \to 3 $ exclusive process, where collinear factorisation is broken, namely the exclusive photoproduction of a $ π^{0}γ$ pair with large invariant mass. This occurs because the process suffers from gluon exchanges trapped in the Glauber region. Using an explicit example, we show that the Glauber gluon, which is exchanged between a collinear spectator parton from the nucleon sector and a soft spectator parton from the outgoing pion, has both of its lightcone plus and minus components pinched. Therefore, it cannot be deformed to collinear/soft regions, as is often the case for processes that do factorise. We further confirm the leading power behaviour of the identified Glauber region, highlighting that this is the case although it relies on extracting a soft parton from the outgoing pion. We stress that the Glauber pinch for this process is of the leading power, due to the possibility of having two-gluon exchanges between the collinear nucleon sector and hard partonic scattering sub-process. In fact, the Glauber gluon that we identify is one of these two active gluons, and therefore, its effects are observed already at leading order. A direct consequence of our work is that collinear factorisation breaks in the same way for other $ 2 \to 3 $ exclusive processes, where two-gluon exchanges in the $ t $-channel are possible, like in the exclusive production of a photon pair from $ π^{0} N $ collisions. However, we highlight that in cases where such two-gluon exchanges do not exist, like in the exclusive $ π^{\pm}γ$ photoproduction, the Glauber exchanges that we discuss here do not occur, and hence they do not suffer from factorisation breaking effects.

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arXiv2024

Complete 1-loop study of exclusive $ J/ψ$ and $ Υ$ photoproduction with full GPD evolution

We discuss the exclusive photoproduction of a heavy vector quarkonium, namely $J/ψ$ and $Υ$ at 1-loop in $α_s$. In collinear factorisation (CF), the amplitude for such a process is obtained by the convolution of a hard partonic sub-amplitude, with a universal generalised parton distribution (GPD). For the first time, we perform a complete calculation at 1-loop including full leading-log (LL) GPD evolution. We first demonstrate the huge instability of the cross section at high energies when the factorisation scale $μ_F$ is varied. This instability has been reported previously in the literature, and occurs due to the large logarithms generated by the huge difference between the hard scale of the process, which is the mass of the heavy quarkonium here, and the centre-of-mass energy of the process. This problem was also reported in inclusive heavy vector quarkonium photoproduction. We show that this issue can be resolved by resumming these large logarithms using high-energy factorisation (HEF), by performing a matching with the result in CF using the doubly logarithmic approximation (DLA) in order to be consistent with the fixed order evolution of GPDs. Finally, we show that the cross sections obtained from such a matching, besides being free from the previously mentioned factorisation scale variation instabilities, are consistent with the H1 data for $J/ψ$ production and with the ZEUS data for $Υ$ production.

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arXiv2024

Breakdown of collinear factorization in the exclusive photoproduction of a $ π^{0}γ$ pair with large invariant mass

We study the exclusive photoproduction of a $ π^{0}γ$ pair with large invariant mass $ M^{2}_{γπ} $, which is sensitive to the exchange of either two quarks or two gluons in the $ t $-channel. In this paper, we show that the process involving two-gluon exchanges does not factorize in the Bjorken limit at the leading twist. This can be explicitly demonstrated by the fact that there exist diagrams, which contribute at the leading twist, for which Glauber gluons are trapped, due to the pinching of the contour integration of both the plus and minus component of the Glauber gluon momentum. For the same reason, $π^0$-nucleon scattering to two photons also suffers from the same issue. On the other hand, we stress that there are no issues with respect to collinear factorization for the quark channels. By considering an analysis of all potential reduced diagrams of leading pinch-singular surfaces, we argue that the quark channel is safe from Glauber pinches, and therefore, a collinear factorization in that case follows through without any problems. This means that processes where gluon exchanges are forbidden, such as the exclusive photoproduction of $ π^{\pm}γ$ and $ ρ^{0,\,\pm} γ$, are unaffected by the factorization breaking effects we point out in this paper.

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arXiv2024

Evidence of collinear factorization breaking due to collinear-to-soft Glauber exchanges for a $2 \to 3$ exclusive process at leading twist

We exhibit an exclusive process, namely the photoproduction of a $π^{0}γ$ pair with large invariant mass, which violates collinear factorization. We explicitly demonstrate that this is due to the fact that there exists diagrams with gluon exchange in $t$ channel, contributing at the leading power, for which Glauber gluons are trapped. This is caused by the pinching of the contour integration of both the plus and minus light-cone components of the Glauber gluon momentum. We argue that this leads to the observed ``endpoint-like'' divergence of the convolution integral at leading order and leading power when collinear factorization is naïvely assumed.

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arXiv2024

Exclusive photoproduction of a photon-meson pair: A new class of observables to probe GPDs

We discuss the exclusive photoproduction of a photon-meson pair with large $ p_{T} $ as a channel to probe generalised parton distributions (GPDs). Like other $ 2\to3 $ exclusive processes, this channel allows us to better study the $ x $-dependence of GPDs, in contrast to $ 2\to2 $ processes such as Deeply Virtual Compton Scattering (DVCS) which give only "moment-type" information. Moreover, it also gives the possibility to access, at leading twist, the chiral-odd GPDs, which are still completely unknown experimentally. In the first part of these proceedings, we present the computation of the amplitude at leading order and leading twist, for charged pions, and rho mesons of any charge and polarisation. These processes are sensitive to quark GPDs only. We also discuss the possibility of measuring such processes at various experiments, namely JLab, COMPASS, future EIC and LHC in ultra-peripheral collisions. In particular, in collider experiments, the dependence of GPDs at small skewness $ (<10^{-3}) $ can be extracted. For the second part, we report on our recent work on the exclusive photoproduction of $ π^{0}γ$ pair, which, in addition to a quark GPD channel, also has a contribution from gluon GPDs. In the latter case, we demonstrate that a collinear factorisation of the process fails, due to the presence of a Glauber pinch. Our findings also imply that other similar processes, like $ π^{0} N \to γγN $ also suffer from the same issue. On the other hand, we stress that the processes discussed earlier, which are sensitive to the quark GPD channels only, are safe from these factorisation breaking effects.

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arXiv2023

Resolving Charged Hadrons in QED -- Gauge Invariant Interpolating Operators

Standard interpolating operators for charged mesons, e.g. $J_{B} = \bar b i γ_5 u$ for $B^-$, are not gauge invariant in QED and therefore problematic for perturbative methods. We propose a gauge invariant interpolating operator by adding an auxiliary charged scalar $Φ_B$, ${\cal J}_{B}^{(0)} = J_B \, Φ_B$, which reproduces all the universal soft and collinear logs. The modified LSZ-factor is shown to be infrared finite which is a necessary condition for validating the approach. At ${\cal O}(α)$, this is equivalent to a specific Dirac dressing of charged operators. A generalisation thereof, using iterated integrals, establishes the equivalence to all orders and provides a transparent alternative viewpoint. The method is discussed by the example of the leptonic decay $B^- \to \ell^- \bar ν$ for which a numerical study is to follow. The formalism itself is valid for any spin, flavour and set of final states (e.g. $B^- \to π^0 \ell^- \bar ν$).

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arXiv2023

Probing chiral-even and chiral-odd leading twist quark generalised parton distributions through the exclusive photoproduction of a $ γρ$ pair

We extend our studies of a new class of $2 \to 3$ exclusive processes using the collinear factorisation framework by considering the exclusive photoproduction of a $γ\,ρ$ pair, in the kinematics where the pair has a large invariant mass, {and the outgoing $ ρ$-meson has a sufficiently large transverse momentum to not resonate with the nucleon}. We cover the whole kinematical range from medium energies in fixed target experiments to very large energies of colliders, by considering the experimental conditions of JLab~12-GeV, COMPASS, future EIC and LHC (in ultra-peripheral collisions) cases. One of the main interest in studying the present process is that it provides access to both chiral-even and chiral-odd GPDs, depending on the polarisation of the outgoing $ρ$-meson, both at leading twist. Our analysis covers both neutral and charged $ ρ$-mesons. We find that the order of magnitude of the obtained cross sections are sufficiently large for a dedicated experimental analysis to be performed, especially at JLab, for both longitudinally and transversely polarised $ρ$. Furthermore, we compute the linear photon beam polarisation asymmetry {which is} sizeable for a longitudinally polarised meson. These predictions are obtained for both asymptotic distribution amplitude (DA) and the \textit{holographic} DA.

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arXiv2022

Accessing GPDs through the exclusive photoproduction of a photon-meson pair with a large invariant mass

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 $ ρ$-meson or a charged $ π$ 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 leading order and leading twist. We discuss the prospects of measuring them in various experiments such as JLab 12-GeV, COMPASS, future EIC and LHC (in ultraperipheral collisions). In particular, the high centre of mass energies available at collider experiments can be used to probe GPDs at small skewness $ ξ$. 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.

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arXiv2022

Accessing chiral-even quark generalised parton distributions in the exclusive photoproduction of a $ γπ^{\pm} $ pair with large invariant mass in both fixed-target and collider experiments

We compute the exclusive photoproduction of a $γ\,π^\pm$ pair using the collinear factorisation framework, in the kinematic regime where the pair has a large invariant mass. This exclusive channel presents a new avenue for the investigation of GPDs. It is particularly interesting as the high centre of mass energies available at future experiments will allow the study of GPDs at small skewness $ ξ$. We compute the scattering amplitude of the process, at leading twist and leading order in $α_s$, which is used to estimate its cross-section and linear polarisation asymmetries with respect to the incoming photon, for JLab~12-GeV, COMPASS, future EIC and LHC (in ultra-peripheral collisions) kinematics. We find that the order of magnitude of estimates are sufficiently large for a dedicated experimental analysis to be performed, especially at JLab. We also compare the results from an asymptotic distribution amplitude (DA) to those using a recently proposed \textit{holographic} DA.

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arXiv2022

QED in $\bar B \to \bar K \ell^+\ell^-$ LFU ratios: Theory versus Experiment, a Monte Carlo Study

Using analytic results obtained in a meson effective theory, that includes all infrared sensitive logs, we build a dedicated Monte Carlo framework to describe QED corrections in $\bar B \to \bar K \ell^+\ell^-$ for a generic form factor. For the neutral mode $\bar B^0 \to \bar K^0 \ell^+\ell^-$, we perform a detailed numerical comparison versus those obtained with the general-purpose photon-shower tool PHOTOS. The comparison indicates a good agreement, at the few per-mil level, when focusing on the rare mode only. In addition, our framework allows us to investigate the impact of the charmonium resonances. Interference effects, not described by PHOTOS in the experimental analysis, are found to be small in the dilepton invariant mass region $q^2 < 6 \textrm{GeV}^2$, which is used to determine $R_{K^{(*)}}$. Using a semi-analytic framework we assess the full, rare and resonant, mode. Based thereupon, we discuss strategies to check the subtraction of the resonant mode, which has a sizeable impact at $q^2 \approx 6 \textrm{GeV}^2$ in the electron mode.

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arXiv2022

Accessing GPDs through the exclusive photoproduction of a $ γ$-meson pair

We consider the exclusive photo-production of a photon-meson pair with a large invariant mass, working in the QCD factorisation framework. Explicitly, we consider a $ ρ$-meson or a charged $ π$ in the final state. This process gives access both to chiral-even GPDs and chiral-odd GPDs, which are not well-known experimentally, especially the latter ones. The computation is performed at leading order and leading twist. We discuss the prospects of measuring them in experiments, focusing on the kinematics at the JLab 12-GeV experiment, and pPb ultra-peripheral collisions at LHC. In particular, the latter gives access to the small $ ξ$ regime of GPDs.

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arXiv2022

On the $ R_{K} $ Theory Error

To quantify the theory error on $R_K$, essentially means to quantify the uncertainty due to QED corrections since the latter breaks lepton flavour universality through the lepton masses. Since experiment uses photon shower programs, e.g. \texttt{PHOTOS}, to capture QED effects, assessing the uncertainty involves investigating effects not captured by the specific use of these tools. This includes structure-dependent corrections, potentially large non-logarithmic terms and charmonium resonances entering the lower bin by migration of radiation. We are able to close in on these loopholes. For example, using gauge invariance, we show that structure-dependent QED corrections do not lead to additional (sizeable) hard-collinear logs of the form ${\cal O}(α) \ln m_\ell/m_B$. Hence, from the theory point of view $R_K$ is a safe observable.

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