Illuminating the charged leptons in the proton Victor P. Gonçalves1 2 3and Daniel E. Martins3 4y 1Institut für Theoretische Physik Westfälische Wilhelms-Universität Münster

2025-05-08 0 0 467.66KB 10 页 10玖币

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Illuminating the charged leptons in the proton

Victor P. Gonçalves1, 2, 3, ∗and Daniel E. Martins3, 4, †

1Institut für Theoretische Physik, Westfälische Wilhelms-Universität Münster,

Wilhelm-Klemm-Straße 9, D-48149 Münster, Germany

2Institute of Modern Physics, Chinese Academy of Sciences, Lanzhou 730000, China

3Institute of Physics and Mathematics, Federal University of Pelotas,

Postal Code 354, 96010-900, Pelotas, RS, Brazil

4The Henryk Niewodniczanski Institute of Nuclear Physics (IFJ) - Polish Academy of Sciences (PAN),

31-342, Krakow, Poland

The description of the structure of proton is fundamental in order to describe the standard

model processes at the LHC as well as for the searching of New Physics. Quantum ﬂuctu-

ations imply the presence of photons and leptons inside the proton, which admit a parton

distribution function (PDF). Although the lepton PDFs are expected to be small, its pres-

ence opens new production mechanisms. In order to explore the lepton - induced processes

at the LHC, a precise determination of the leptonic content of the proton is needed. In this

paper we propose to constrain the content of charged leptons inside the proton through the

study of the QED Compton scattering in ultraperipheral proton - nucleus collisions at the

LHC. We estimate the total cross sections and associated distributions considering diﬀerent

models for the lepton PDFs and distinct lepton ﬂavours. We demonstrate that a future ex-

perimental analysis of this process is feasible and that it can be used to constrain the content

of electrons, muons and taus inside the proton.

PACS numbers:

Keywords: Leptons, QED Compton Scattering, Ultraperipheral Collisions

I. INTRODUCTION

One of the main goals of Particle Physics is to achieve a deeper knowledge of the proton structure,

which is fundamental to describe standard model (SM) processes and to identify possible signals of

New Physics in hadronic collisions at the Large Hadron Collider (LHC) [1–3]. Our view of the proton

structure has been largely improved with the successful operation of the DESY ep collider HERA

during the period between the years of 1992 and 2007, which has observed the striking rise of the

proton structure function F2(x, Q2)for small values of the Bjorken - xvariable (≤10−2) and a ﬁxed

∗Electronic address: barros@ufpel.edu.br

†Electronic address: daniel.ernani@ifj.edu.pl

arXiv:2210.02827v2 [hep-ph] 26 Apr 2023

photon virtuality Q2. Such behaviour was interpreted in terms of the increasing with the energy of

the gluon and sea quark densities inside the proton [4]. The advent of the LHC and the computation

of higher - order QCD and electroweak (EW) corrections for hadronic processes has motivated a huge

progress in the determination of the parton distribution functions (PDFs) of the proton over the last

years (For a recent review see, e.g. Ref. [3]). Recent results indicate the presence of an intrinsic charm

component in the proton wave function [5], as well as a photon and a lepton content in the proton

(For recent results see, e.g. Refs. [6–11]). In particular, the presence of leptons in the initial state of

hadronic collisions opens the possibility of study lepton - lepton and lepton - quark subprocesses at

the LHC [12–14], with all combinations of charge and ﬂavours, enlarging the scope of the LHC also

for a lepton – lepton (quark) collider. Such possibilities allow us to study new production channels,

which can improve our understanding of the standard model and that can be used to search for Beyond

- the - Standard - Model (BSM) Physics. However, in order to derive realistic predictions for these

processes, a precise knowledge of the leptonic densities in the proton is fundamental.

Photons and leptons inside the proton can arise from quantum ﬂuctuations, with photons being

generated by the photon - quark splitting process q→qγ and charged leptons by the photon splitting

process γ→l+l−. In order to derive the photon and lepton PDF sets, one can implement such processes

in the Dokshitzer - Gribov - Altarelli - Lipatov - Parisi (DGLAP) evolution equations [15] and perform

a global ﬁt of the existing data. However, such an alternative is still not viable due to the limited

sensitivity of the LHC data for the photon and lepton initiated subprocesses. Another possibility is to

compute these PDFs for a given scale on the base of a theoretically motivated model ansatz and derive

the PDFs for other scales by solving the QED - corrected DGLAP equations, as performed e.g. in

Refs. [16] and [6] for the photon and lepton cases, respectively (See also Refs. [16–21]). Finally, a more

precise determination of the photon and lepton PDFs in the proton can be performed using the LUX

method, proposed originally for the photon case in Refs. [22, 23] and extended for charged leptons

in Ref. [9], where these PDFs are computed using only information from electron - proton scattering

data. Such formalism was recently applied by the CTEQ - TEA [11] and MSHT [10] groups to derive

new sets of parton distributions including the QED corrections to the DGLAP evolution. Although

such a method has improved the accuracy of the predictions, the associated uncertainties are still non

- negligible, which motivates the proposition of an alternative to probe the lepton PDFs in the proton

using a physical process that can be measured at the LHC.

The information about the charged leptons inside the proton can be directly accessed using a

photon as a probe of the hadronic structure. Such interactions naturally will occur in electron - proton

collisions at the EIC and LHeC [24, 25]. In contrast, at the LHC, this electromagnetic process is, in

principle, very diﬃcult to separate in typical hadronic collisions. The suppression of the photon and

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IlluminatingthechargedleptonsintheprotonVictorP.Gonçalves1,2,3,andDanielE.Martins3,4,y1InstitutfürTheoretischePhysik,WestfälischeWilhelms-UniversitätMünster,Wilhelm-Klemm-Straÿe9,D-48149Münster,Germany2InstituteofModernPhysics,ChineseAcademyofSciences,Lanzhou730000,China3InstituteofPhysicsandMathem...

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Illuminating the charged leptons in the proton Victor P. Gonçalves1 2 3and Daniel E. Martins3 4y 1Institut für Theoretische Physik Westfälische Wilhelms-Universität Münster

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