Recent results on open heavy flavor production ($pp$, $p$Pb, PbPb) from LHCb

This paper presents recent LHCb results on open heavy flavor production in $pp$, $p$Pb, and PbPb collisions, highlighting how these measurements probe QCD medium transport properties, cold nuclear matter effects, and hadronization mechanisms across a unique energy range.

The Gist

Imagine the Large Hadron Collider (LHC) as a massive, high-speed racetrack where scientists smash particles together to see what happens when the universe's most fundamental building blocks collide. Usually, these collisions happen head-on between two beams of protons. But the LHCb experiment is like a special camera positioned at a specific angle, looking down the track to catch particles flying off to the side. This unique view lets them see things other cameras miss.

This paper is a report card from the LHCb team, detailing what they learned about "heavy flavor" particles. Think of heavy quarks (like charm and bottom quarks) as the "heavyweights" of the particle world. Because they are so massive, they are born in the very first split-second of a collision and then have to travel through the chaotic "soup" created by the crash. By watching how these heavyweights move and change, scientists can learn about the properties of that soup and how particles stick together to form matter.

Here is a breakdown of their latest findings, explained simply:

1. The "Fixed-Target" Trick: Shooting at a Wall

Usually, the LHC smashes two beams together. But LHCb has a clever trick called SMOG (System for Measuring the Overlap with Gas). Imagine a proton beam as a high-speed train. Instead of smashing it into another train, they inject a cloud of gas (like Neon or Argon) right in front of the train. The protons smash into the gas atoms.

• Why do this? It allows them to study collisions at energy levels that are "in-between"—higher than old experiments but lower than the biggest crashes. It's like testing a car's engine at a speed you can't get on a normal highway but isn't quite top speed yet.
• The Neon Test: They smashed protons into Neon gas. They looked at how often a specific heavy particle (called a $D^0$ meson) was made. They compared their results to different "recipe books" (theoretical models). Some recipes guessed right, while others (like the FONLL and PHSD models) got the speed distribution wrong, even if they got the direction right. This suggests we need better recipes to understand how these particles form.

2. The "Crowded Room" Effect: More Traffic, More Baryons

In normal proton-proton collisions, scientists looked at the ratio of two specific heavy particles: a "b-baryon" ($\Lambda_b^0$) and a "B-meson" ($B^0$). Think of these as two different types of heavy vehicles.

• The Discovery: They found that in collisions where the "traffic" (multiplicity) is very heavy—meaning a lot of other particles are created at the same time—the ratio of these heavy vehicles changes.
• The Analogy: Imagine a quiet street where you mostly see sedans ($B^0$). But if you go to a massive, chaotic festival with thousands of people, suddenly you see a lot more trucks ($\Lambda_b^0$). The data showed that as the "crowd" gets bigger, the number of trucks increases significantly.
• The Theory: A model called EPOS4HQ, which includes a "coalescence" mechanism (a fancy way of saying particles sticking together like magnets in a crowded room), predicted this behavior perfectly. This helps explain how heavy quarks decide what kind of particle to become when the environment gets crowded.

3. The "Nuclear Shadow" and "Refractive Index"

The team also smashed protons into Lead nuclei (heavy atoms). This is like shooting a bullet at a dense brick wall instead of a single brick.

• Forward vs. Backward: They looked at the particles flying in the direction of the proton beam (forward) and the direction of the lead beam (backward).
• The Findings:
  • Forward: The results matched the predictions of how the "shadow" of the lead nucleus affects the proton. It's like looking through a slightly foggy window; the light (particles) is dimmed in a predictable way.
  • Backward: Here, the results were surprising. The particles were dimmer than the "foggy window" theory predicted. This suggests there might be other effects happening inside the nucleus that we don't fully understand yet.
• The Strangeness Ratio: They also looked at the ratio of two types of heavy particles ($D_s^+$ vs $D^+$). They found that as the "density" of the collision (how many particles are packed in) increases, this ratio goes up. This pattern was the same whether they looked forward or backward. It's like finding that the ratio of red cars to blue cars on a highway depends only on how busy the highway is, not on which direction you are driving.

4. What's Next? Upgrading the Camera

The paper concludes by looking at the future (Run 3). The LHCb team is upgrading their "camera" and their "gas injection" system.

• Better Gas: They are installing a new gas cell (SMOG2) that can hold much more gas, making the "fixed-target" experiments much brighter and more powerful.
• Better Vision: They are upgrading their tracking detectors to have higher "granularity" (like going from a standard definition TV to 4K). This will allow them to see the center of the most violent collisions (Lead-Lead) with much greater clarity, potentially seeing details they couldn't see before.

In Summary:
This paper is about using a unique camera angle and a clever gas-injection trick to study how heavy particles behave in different environments—from empty space to crowded collisions to dense nuclear walls. The results confirm some theories about how particles stick together in crowds but also reveal that our understanding of how heavy particles interact with dense nuclear matter is still incomplete, pointing the way for future, sharper observations.

Technical Summary

Technical Summary: Recent Results on Open Heavy Flavor Production from LHCb

Problem and Motivation
Heavy quarks are produced in the early stages of heavy-ion collisions due to their large mass and subsequently traverse the entire evolution of the QCD medium. Consequently, open heavy flavors serve as critical probes for understanding the transport properties of the medium and the mechanisms by which quarks neutralize their color charge to form hadrons. While perturbative QCD calculations based on the factorization theorem generally describe heavy-flavor production at hadron colliders, specific regimes remain under-explored. The LHCb experiment addresses this by utilizing its unique forward pseudorapidity coverage ($2 < \eta < 5$) to probe parton distribution functions (PDFs) at low Bjorken-$x$ ($10^{-5}$) and, via the System for Measuring the Overlap with Gas (SMOG) fixed-target program, to access high Bjorken-$x$ (the anti-shadowing region). This program covers an energy range between previous fixed-target experiments and top RHIC energies, providing a necessary baseline for interpreting PbPb measurements and studying cold nuclear matter (CNM) effects such as nuclear PDF modifications and energy loss.

Methodology
The paper presents a series of measurements utilizing data from the LHCb detector across $pp$, $p$Pb, and $p$Ne collision systems at various center-of-mass energies.

• Fixed-Target $p$Ne Collisions: $D^0$ meson production was measured using 2017 data at $\sqrt{s_{NN}} = 68.5$ GeV with an integrated luminosity of $21.7 \pm 1.4$ nb$^{-1}$. Differential cross-sections were analyzed as functions of rapidity ($y^*$) and transverse momentum ($p_T$).
• High-Multiplicity $pp$ Collisions: The $\Lambda_b^0/B^0$ cross-section ratio was measured in $pp$ collisions at $\sqrt{s} = 13$ TeV. The analysis categorized events by normalized multiplicity ($N_{\text{VELO}}^{\text{tracks}}$), defined by the number of charged tracks reconstructed in the Vertex Locator (VELO).
• $p$Pb Collisions: Prompt production of $D^+$ and $D_s^+$ mesons was measured in both forward (proton beam towards detector) and backward (lead beam towards detector) configurations at $\sqrt{s_{NN}} = 5.02$ TeV and $8.16$ TeV. Key observables included the nuclear modification factor ($R_{p\text{Pb}}$) and the $D_s^+/D^+$ cross-section ratio as a function of charged particle density ($dN_{ch}/d\eta$).

Key Results

1. $D^0$ Production in $p$Ne: The measured differential cross-sections for $D^0$ mesons were compared against theoretical models incorporating various CNM effects. The data are well-described by predictions both with and without intrinsic charm (IC) contributions (specifically the Vogt models), provided shadowing effects are included. Models including 1% IC and 10% recombination (MS) also align closely with the data. However, standard FONLL and PHSD calculations failed to reproduce the observed $p_T$ distribution, though they showed better agreement with rapidity distributions.
2. $\Lambda_b^0/B^0$ Ratio in $pp$: In high-multiplicity $pp$ collisions at 13 TeV, the $\Lambda_b^0/B^0$ cross-section ratio increases significantly with event multiplicity. In the lowest multiplicity bin, the ratio matches values observed in $e^+e^-$ collisions. Theoretical models incorporating a coalescence mechanism (EPOS4HQ) provide a more accurate description of the multiplicity-dependent trend than models without it or those relying solely on statistical hadronization.
3. Nuclear Modification in $p$Pb: At $\sqrt{s_{NN}} = 5.02$ TeV, the nuclear modification factor $R_{p\text{Pb}}$ for $D$ mesons in the forward rapidity region is consistent with nPDF and CGC calculations. In the backward region, the $R_{p\text{Pb}}$ for $D^+$ is observed to be lower than theoretical predictions.
4. $D_s^+/D^+$ Ratio and Multiplicity: At $\sqrt{s_{NN}} = 8.16$ TeV, the $D_s^+/D^+$ cross-section ratio increases with charged particle density ($dN_{ch}/d\eta$) across all $p_T$ intervals. This trend is consistent in both forward and backward collisions, indicating the ratio is independent of rapidity but strongly correlated with particle density. While EPOS4HQ calculations show some discrepancies with absolute data values, they successfully capture the multiplicity-dependent trends.

Significance and Outlook
These measurements provide new insights into nuclear structure and the mechanisms of heavy-quark hadronization, particularly regarding the role of multiplicity and CNM effects. The results highlight the importance of coalescence mechanisms in describing baryon-to-meson ratios in high-multiplicity environments and reveal discrepancies in theoretical descriptions of backward $p$Pb collisions.

Looking toward LHC Run 3, the paper notes that the fixed-target program has been upgraded with the SMOG2 gas storage cell, which increases local gas pressure by up to two orders of magnitude and enables parallel data-taking with collider mode, significantly enhancing luminosity. Furthermore, upgrades to the tracking detectors with higher granularity will allow the reconstruction of PbPb events with centrality pushed to 30%. These improvements are expected to greatly benefit future research in this domain.