Probing the valence quark region of nucleons with Z bosons at LHCb Tianqi Li on behalf of the LHCb collaboration

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Probing the valence quark region of nucleons with Z bosons

at LHCb∗

Tianqi Li, on behalf of the LHCb collaboration

Guangdong Provincial Key Laboratory of Nuclear Science, Guangdong-Hong

Kong Joint Laboratory of Quantum Matter, Institute of Quantum Matter, South

China Normal University, Guangzhou, China

Received October 4, 2022

Because of its forward coverage, LHCb can probe the valence quark dis-

tributions of protons and nuclei at small and large Bjorken-x with high pre-

cision. This proceeding presents new LHCb measurements of Z boson pro-

duction in association with charm jet in the forward region of proton-proton

collisions and Z boson production in proton-lead collisions. Z+charm jet

production could be sensitive to a valence-like intrinsic-charm component

in the proton wave function. The measurements of Z production in proton-

lead collisions provide new constraints on the partonic structure of nucleons

inside nuclei. Comparisons between the results and calculations with par-

ton distribution functions are also discussed.

1. Introduction

The LHCb detector [1, 2], optimized for studying heavy ﬂavour physics

in proton-proton (pp) collisions at the LHC, is a single-arm forward spec-

trometer covering the pseudorapidity range 2 < η < 5, providing a high mo-

mentum resolution down to very low transverse momentum (pT), excellent

reconstruction and particle identiﬁcation. Due to the forward acceptance,

LHCb results are highly complementary to the other LHC experiments.

LHCb participated successfully in the recording of proton-lead data sam-

ples in 2013, thus becoming a member of the LHC heavy ion program to

provide unique datasets for the heavy ion physics studies. As shown in

Fig. 1, LHCb provides data that can constrain nuclear parton distribution

functions (nPDFs) in both low-x and high-x regions. The measurements of

Z boson production at LHCb can provide constraints at high momentum

transfer (Q2). Furthermore, the LHCb measurements of Z+charm jet pro-

∗Presented at 2022 Quark Matter conference

(1)

arXiv:2210.00783v1 [hep-ex] 3 Oct 2022

2template printed on October 4, 2022

duction provide information on the quark PDFs at large-x region relevant

to studies of intrinsic charm.

Fig. 1. Kinematic regions corresponding to the acceptance of the four LHC exper-

iments in p-Pb collisions.

In this article, we report on recent results of the Z bosons produced in

association with charm in the forward region [3] and the Z boson production

in the proton-lead collisions [4] measured by the LHCb collaboration.

2. Probe intrinsic charm: Zc production in proton-proton

collisions

The possibility that the proton wave function may contain an intrinsic

charm (IC) component, in addition to the c-quark content due to pertur-

bative gluon radiation, has been an interesting topic in recent years (see

Ref. [5] for a review). Light front QCD calculations (Ref. [6, 7]) predict

the presence of the non-perturbative IC can manifest in valence-like charm

content in the parton distribution function, whereas, if the charm is entirely

perturbative, the charm PDF should rapidly decrease at large Bjorken-x.

The goal is to use a high-precision measurement of an observable with direct

sensitivity to large-x charm to probe IC in proton, where Q is large enough

that hadronic and nuclear eﬀects are negligible.

Ref. [8] proposes probing IC by studying events containing a Z boson

and a charm jet, Zc, in the forward region of pp collisions at the LHC. Since

Zc production is inherently at large Q, and the electroweak bosons do not

participate in the strong interaction, hadronic eﬀects are small. Fig. 2 shows

gc→Zc scattering, a leading-order Zc production mechanism where one of

the initial partons should have large x in the forward rapidity region. Thus

Zc production serves as a direct probe of IC in valence quark region.

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ProbingthevalencequarkregionofnucleonswithZbosonsatLHCbTianqiLi,onbehalfoftheLHCbcollaborationGuangdongProvincialKeyLaboratoryofNuclearScience,Guangdong-HongKongJointLaboratoryofQuantumMatter,InstituteofQuantumMatter,SouthChinaNormalUniversity,Guangzhou,ChinaReceivedOctober4,2022Becauseofitsforward...

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Probing the valence quark region of nucleons with Z bosons at LHCb Tianqi Li on behalf of the LHCb collaboration

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