Dielectron production in proton-proton and proton-lead collisions at $\sqrt{s_{\rm{NN}}}$ = 5.02 TeV

This paper presents the first measurements of dielectron production in proton-proton and proton-lead collisions at $\sqrt{s_{\rm{NN}}}$ = 5.02 TeV using the ALICE detector, determining charm and beauty cross sections in pp collisions and finding that the pPb results are consistent with expectations without nuclear matter effects while providing the first direct comparison of these systems via the nuclear modification factor $R_{\rm{pPb}}$.

The Gist

Imagine the Large Hadron Collider (LHC) as the world's most powerful particle accelerator, a giant circular racetrack where scientists smash tiny particles together at near-light speeds to see what happens. The ALICE experiment is like a high-speed, 360-degree camera system sitting on this track, designed to take pictures of the debris.

This specific paper is about a new set of photos ALICE took, but instead of smashing heavy lead nuclei together (which creates a "soup" of matter called the Quark-Gluon Plasma), they smashed protons (the building blocks of atoms) into other protons, and protons into lead nuclei. They did this at a specific energy level (5.02 TeV), which is like setting the accelerator to a very precise, high gear.

Here is the breakdown of what they did and found, using some everyday analogies:

1. The Goal: Finding the "Ghost" in the Machine

Scientists are trying to understand a state of matter called the Quark-Gluon Plasma (QGP). Think of normal matter (like a proton) as a tightly packed suitcase. If you heat it up enough, the suitcase melts, and the clothes (quarks and gluons) float around freely in a hot, dense soup. This is the QGP.

To see if this soup exists, scientists look for "messengers" that don't get stopped by the soup. Electrons and positrons (anti-electrons) are these messengers. They are like ghosts that can walk right through a crowded room without bumping into anyone. When these ghosts appear in pairs (called dielectrons), they carry a secret message about the temperature and conditions of the "soup" at the exact moment they were born.

2. The Challenge: The "Noise" Problem

The problem is that these "ghost" pairs are very rare. Most of the time, when particles smash, they create a huge amount of "noise"—lots of other particles that look like the ghosts but aren't. It's like trying to hear a single violin solo in a stadium full of people shouting.

The scientists had to figure out how to separate the signal (the real ghost pairs from the hot soup or heavy particles) from the background noise (pairs created by common, boring particle decays).

3. The Experiment: Two Different Scenarios

They ran two types of races:

• Proton-Proton (pp): Two small, lightweight cars crashing. This is the "control group." We know exactly what happens here because there's no "soup" created. It's just a clean crash.
• Proton-Lead (p-Pb): A small car crashing into a giant, heavy truck. Theoretically, this might create a tiny drop of the hot soup, or it might just be a messy crash with some "cold" nuclear effects (like the truck's bumper denting the car).

4. The "Cocktail" Recipe

To know what to expect, the scientists built a theoretical "cocktail." Imagine you are making a smoothie. You know exactly how much banana, strawberry, and milk you put in.

• The Ingredients: They calculated how many electron pairs should come from known sources (like the decay of light particles, similar to how a fruit smoothie has specific flavors).
• The Heavy Flavor: They also accounted for "heavy" particles (Charm and Beauty quarks). These are like the heavy, dense fruits in the smoothie. They are harder to predict, so the scientists used two different "recipe books" (computer models called PYTHIA and POWHEG) to guess how much of these heavy fruits would be in the mix.

5. The Findings: What the Data Said

A. The Proton-Proton Crash (The Control)
When they smashed protons into protons, the data matched their "cocktail" recipe almost perfectly.

• The Result: They successfully measured how often "Charm" and "Beauty" quarks are produced. It's like finally getting a precise count of how many heavy fruits are in the smoothie. This helps them calibrate their recipe books for future experiments.

B. The Proton-Lead Crash (The Mystery)
When they smashed protons into lead, they compared the results to the Proton-Proton results, scaled up to account for the bigger target (the lead nucleus).

• The Result: Surprisingly, the Proton-Lead crash looked almost exactly like the Proton-Proton crash, just with more particles because the lead nucleus is bigger.
• The "Cold" vs. "Hot" Debate:
  • Cold Nuclear Matter (CNM): This is like the lead nucleus acting as a "shadow," blocking some particles from being made. The data showed this shadow effect is very small at the center of the collision.
  • Hot Medium (The Soup): Some theories suggested that even in a small crash like Proton-Lead, a tiny drop of the hot soup might form, creating extra ghost pairs (thermal radiation).
  • The Verdict: The data didn't show a clear "extra" signal. The results were consistent with the idea that no hot soup was formed, or if it was, it was so small that the "cold shadow" effects canceled it out. It's like trying to detect a single drop of hot water in a cold bucket; the instruments weren't quite sensitive enough to say for sure if the drop was there or not.

6. Why This Matters

This paper is a crucial step in the scientific detective story.

1. Calibration: By proving they can accurately measure the "background noise" in Proton-Proton collisions, they have a perfect baseline.
2. The Baseline: Now, when they look at heavy lead-lead collisions (where a huge soup is definitely created), they can subtract this baseline to see the "extra" signal from the soup with much higher confidence.
3. Future Tech: The paper mentions that the ALICE detector is getting a massive upgrade (like upgrading from a standard camera to a 4K super-slow-motion camera). This will allow them to see these "ghost" pairs with much greater precision in the future, potentially finally spotting that tiny drop of hot soup in the small collisions.

In a nutshell: The scientists took a very careful look at particle crashes to make sure they understand the "normal" background noise. They found that in small crashes (Proton-Lead), the physics looks very much like simple Proton-Proton crashes, with no obvious sign of a new "hot soup" forming yet. But now that they have their measurements down pat, they are ready to look harder with better tools.

Technical Summary

1. Problem and Motivation

The primary goal of the ALICE experiment at the Large Hadron Collider (LHC) is to study the Quark-Gluon Plasma (QGP), a state of deconfined matter formed in ultra-relativistic heavy-ion collisions. Dileptons (specifically dielectrons, $e^+e^-$) are ideal probes for this study because they do not interact via the strong force, carrying information about the medium's properties at the time of their emission.

• The Challenge: In the intermediate-mass region (IMR, $1.1 < m_{ee} < 2.7$ GeV/$c^2$), the dielectron spectrum is dominated by correlated pairs from open heavy-flavor hadron decays (charm and beauty). To isolate potential signals of the QGP (such as thermal radiation) or modifications due to Cold Nuclear Matter (CNM), one must precisely understand the "vacuum" baseline of heavy-flavor production.
• The Gap: While dielectron measurements exist at $\sqrt{s} = 7$ and $13$ TeV in $pp$ collisions and at lower energies in $d$–Au collisions, there was a lack of precise measurements at $\sqrt{s_{NN}} = 5.02$ TeV in both $pp$ and $p$–Pb systems. Furthermore, previous LHC measurements in $p$–Pb collisions relied on theoretical models for the heavy-flavor baseline, introducing model dependence.
• Objective: This paper presents the first measurements of dielectron production in $pp$ and $p$–Pb collisions at $\sqrt{s_{NN}} = 5.02$ TeV. The goals are to:
  1. Extract charm ($c\bar{c}$) and beauty ($b\bar{b}$) production cross-sections in $pp$ collisions.
  2. Compare $p$–Pb results directly to $pp$ results at the same energy to search for CNM effects (e.g., shadowing) or final-state effects (e.g., thermal radiation from a small hot medium).

2. Methodology

Data Samples:

• Collision Systems: Proton–proton ($pp$) and proton–lead ($p$–Pb).
• Energy: $\sqrt{s_{NN}} = 5.02$ TeV.
• Data Taking: $p$–Pb data collected in 2016; $pp$ data collected in 2017.
• Integrated Luminosity: $19.93 \pm 0.4$ nb$^{-1}$ ($pp$) and $299 \pm 11$ $\mu$b$^{-1}$ ($p$–Pb).
• Kinematic Range: Midrapidity ($|\eta_e| < 0.8$), pair transverse momentum ($p_{T,ee} < 8$ GeV/$c$), and invariant mass ($m_{ee} < 3.5$ GeV/$c^2$).

Detector and Reconstruction:

• Detectors: Inner Tracking System (ITS), Time Projection Chamber (TPC), and Time-Of-Flight (TOF).
• Track Selection: Electrons selected with $0.2 < p_{T,e} < 10$ GeV/$c$. Requirements include minimum space points in TPC and hits in ITS to reject secondary tracks.
• Particle Identification (PID): Combined $dE/dx$ in TPC and time-of-flight in TOF. Hadron contamination is suppressed to $<4\%$ on average.
• Signal Extraction:
  • Opposite-Sign (OS) vs. Same-Sign (SS): The signal is extracted by subtracting the combinatorial background estimated from same-sign pairs ($S = OS - R_{acc} \cdot SS$).
  • Photon Conversion Rejection: A cut on the opening angle relative to the magnetic field ($\phi_V$) is applied for low masses ($m_{ee} < 0.14$ GeV/$c^2$) to reject $e^+e^-$ pairs from photon conversions.

Analysis Strategy:

1. Hadronic Cocktail: A simulation-based "cocktail" of expected dielectrons from known light-flavor hadron decays ($\pi^0, \eta, \omega, \phi, J/\psi$, etc.) is constructed using measured hadron spectra.
2. Heavy-Flavor Extraction (in $pp$): The measured $pp$ spectrum in the IMR is fitted using templates from two event generators: PYTHIA 6 (Perugia 2011 tune) and POWHEG (NLO generator). The fit determines the normalization of the charm and beauty contributions.
3. Baseline for $p$–Pb: The heavy-flavor contribution in $p$–Pb is estimated by scaling the extracted $pp$ cross-sections by the atomic mass number of Lead ($A=208$), assuming binary collision scaling (no CNM effects initially).
4. Nuclear Modification Factor ($R_{pPb}$): Calculated as $R_{pPb} = \frac{1}{A} \frac{d\sigma_{pPb}}{dm_{ee}} / \frac{d\sigma_{pp}}{dm_{ee}}$. This allows a direct comparison to test for deviations from binary scaling.

3. Key Contributions

• First Direct Comparison at 5.02 TeV: This is the first time dielectron production in $pp$ and $p$–Pb collisions has been directly compared at the same center-of-mass energy at the LHC.
• Model-Independent Baseline: Unlike previous studies that relied on theoretical predictions for the heavy-flavor baseline in $p$–Pb, this analysis uses the measured $pp$ data (fitted with event generators) to define the reference. This significantly reduces model dependence.
• Updated Cross-Sections: The paper provides updated midrapidity cross-sections for $c\bar{c}$ and $b\bar{b}$ production, incorporating recent measurements of charm fragmentation fractions which affect the effective branching ratio to electrons.
• Comprehensive Uncertainty Analysis: Detailed systematic uncertainties are provided for tracking, PID, efficiency corrections, and the cocktail construction.

4. Results

A. Heavy-Flavor Cross-Sections in $pp$ Collisions:

• The dielectron continuum in $pp$ collisions is well-described by the sum of light-flavor decays and heavy-flavor decays.
• Cross-Sections: Extracted values for $d\sigma_{c\bar{c}}/dy|_{y=0}$ and $d\sigma_{b\bar{b}}/dy|_{y=0}$ were obtained using both PYTHIA 6 and POWHEG.
  • POWHEG results: $\sigma_{c\bar{c}} \approx 1299$ $\mu$b, $\sigma_{b\bar{b}} \approx 28$ $\mu$b.
  • PYTHIA 6 results: $\sigma_{c\bar{c}} \approx 900$ $\mu$b, $\sigma_{b\bar{b}} \approx 34$ $\mu$b.
• The results are consistent with Fixed-Order Next-to-Leading Log (FONLL) calculations, though the charm cross-section tends to lie on the upper edge of the theoretical uncertainty band.

B. Dielectron Spectra in $p$–Pb Collisions:

• The measured $p$–Pb spectra ($m_{ee}$ and $p_{T,ee}$) are in good agreement with the hadronic cocktail, where the heavy-flavor component is scaled by $A=208$ from the $pp$ measurement.
• No significant deviation from the vacuum expectation (binary scaling) is observed within current uncertainties.

C. Nuclear Modification Factor ($R_{pPb}$):

• Low Mass Region ($m_{ee} < 1.1$ GeV/$c^2$): $R_{pPb}$ is below unity. This is attributed to the different scaling behavior of light-flavor hadrons (which do not scale linearly with $A$ at low $p_T$) rather than heavy-flavor suppression.
• Intermediate Mass Region (IMR, $1.1 < m_{ee} < 2.7$ GeV/$c^2$): $R_{pPb}$ is consistent with unity within uncertainties.
  • This suggests that heavy-flavor production scales with the number of binary nucleon-nucleon collisions.
  • Calculations including CNM effects (using EPS09 nPDFs) predict a suppression at low $p_{T,ee}$, but the data does not show a significant suppression, implying CNM effects are small or compensated.
  • Calculations including thermal radiation from a hot medium (hadronic + partonic phases) tend to improve the description of the data in the IMR, particularly at low $p_{T,ee}$, though they slightly overestimate the Low Mass Region.

5. Significance

• Constraint on CNM Effects: The results indicate that Cold Nuclear Matter effects on open heavy-flavor production at midrapidity in $p$–Pb collisions are small compared to current experimental uncertainties. This validates the use of $p$–Pb collisions as a baseline for heavy-ion (Pb–Pb) studies.
• Thermal Radiation Search: While no definitive signal of thermal radiation was found, the data does not exclude the presence of an additional thermal source in high-multiplicity $p$–Pb collisions. The interplay between CNM suppression and thermal enhancement makes the interpretation complex.
• Future Prospects: The paper highlights that disentangling these effects (CNM vs. Thermal) requires separating prompt dielectrons from those originating from displaced heavy-flavor vertices. The upcoming ALICE upgrades (TPC, ITS, and new readout) will provide the necessary precision and statistics to perform this separation, potentially revealing the formation of a small hot medium in small collision systems.

In summary, this work establishes a robust, data-driven baseline for heavy-flavor production at LHC energies and provides the most stringent constraints to date on nuclear modifications of dielectron production in $p$–Pb collisions at $\sqrt{s_{NN}} = 5.02$ TeV.