[Paper Review] A direct probe of the intrinsic charm content of the proton

T. Boettcher, P. Ilten+1 more|DSpace@MIT (Massachusetts Institute of Technology)|Dec 21, 2015

Particle physics theoretical and experimental studies7 citations

TL;DR

This paper proposes a direct probe of intrinsic charm (IC) in the proton via precise measurement of $Z$-boson plus charm-jet ($Zc$) production in the forward region at the LHCb experiment. Using NLO calculations and simulations, it demonstrates that LHCb can detect valence-like IC at $\langle x\rangle_{\rm IC} \gtrsim 0.3\%$ and sea-like IC at $\langle x\rangle_{\rm IC} \gtrsim 1\%$ in Runs 2 and 3, offering a sensitive test of non-perturbative charm components and their impact on Higgs production.

ABSTRACT

Measurement of $Z$ bosons produced in association with charm jets $(Zc)$ in proton-proton collisions in the forward region provides a direct probe of a potential non-perturbative (intrinsic) charm component in the proton wave function. We provide a detailed study of the potential to measure $Zc$ production at the LHCb experiment in Runs 2 and 3 of the LHC. The sensitivity to valence-like (sea-like) intrinsic charm is predicted to be $\langle x angle_{ m IC} \gtrsim 0.3\%(1\%)$. The impact of intrinsic charm on Higgs production at the LHC, including $Hc$, is also discussed in detail.

Motivation & Objective

• To provide a direct experimental probe of non-perturbative intrinsic charm (IC) in the proton wave function using high-precision $Zc$ production measurements.
• To assess the sensitivity of the LHCb experiment in Runs 2 and 3 to valence-like and sea-like IC components at large momentum fractions ($x$).
• To quantify the impact of IC on Higgs boson production cross sections, particularly $Hc$ production, and to constrain the charm Yukawa coupling.
• To test DGLAP evolution for charm quarks from low-$Q$ DIS to the electroweak scale, independent of IC discovery.
• To extend the method to $Wc$ production for probing large-$x$ strange quark PDFs via charge asymmetry measurements.

Proposed method

• Use next-to-leading order (NLO) calculations with the CT14 NNLO PDF set and PowhegBox matrix elements to model $Zc$ production in $pp$ collisions.
• Perform cross-checks using aMC@NLO and different showering methods (Powheg and FxFx) to ensure theoretical robustness.
• Simulate detector response using Pythia for parton showering, hadronization, and decay via EvtGen, with Photos for QED radiation.
• Focus on the $Z \to \mu\mu$ decay channel with $60 < m(\mu\mu) < 120\,\text{GeV}$ to reduce background and improve signal purity.
• Define the ratio $Z^{c}_{j} \equiv \sigma(Zc)/\sigma(Zj)$ as the key observable to reduce experimental and theoretical uncertainties.
• Analyze the forward region ($|\eta| > 2$) to maximize sensitivity to large-$x$ charm quarks, leveraging LHCb’s superior forward tracking and $c$-jet tagging.

Experimental results

Research questions

• RQ1 Can $Zc$ production in the forward region of LHCb provide a direct and sensitive probe of intrinsic charm in the proton?
• RQ2 What is the minimum intrinsic charm component (in terms of $\langle x\rangle_{\rm IC}$) that LHCb can detect in Runs 2 and 3?
• RQ3 How does intrinsic charm affect the cross section for Higgs boson production, particularly $Hc$ production?
• RQ4 To what extent does intrinsic charm alter the DGLAP evolution of charm quarks from low-$Q$ DIS to high-$Q$ scales?
• RQ5 Can $Wc$ production at LHCb be used to probe large-$x$ strange quark PDFs via charge asymmetry?

Key findings

• LHCb is predicted to achieve sensitivity to valence-like intrinsic charm with $\langle x\rangle_{\rm IC} \gtrsim 0.3\%$ in Runs 2 and 3.
• Sensitivity to sea-like intrinsic charm is expected to reach $\langle x\rangle_{\rm IC} \gtrsim 1\%$.
• The impact of intrinsic charm on $Hc$ production is comparable to a $\approx 25\%$ increase in the charm Yukawa coupling, making $Hc$ a sensitive probe of IC.
• For the SEA2 IC model, $\sigma(Hc)$ with $Y_c = 0$ matches the no-IC case with $Y_c \approx 0.7\,Y_c^{\rm SM}$, indicating strong interplay between IC and Yukawa coupling.
• Even in the absence of IC, the $Zc$ measurement will provide a critical test of DGLAP evolution for charm quarks from low-$Q$ DIS to the electroweak scale.
• The $Z^{c}_{j}$ ratio is less sensitive to experimental and theoretical uncertainties than $\sigma(Zc)$, making it ideal for IC sensitivity.