[Paper Review] Measurement of the forward charged particle pseudorapidity density in pp collisions at sqrt(s) = 8 TeV using a displaced interaction point

The TOTEM Collaboration, G. Antchev+77 more|arXiv (Cornell University)|Nov 18, 2014

High-Energy Particle Collisions Research1 references4 citations

TL;DR

This paper presents a measurement of forward charged particle pseudorapidity density in pp collisions at √s = 8 TeV using a displaced interaction point at 11.25 m from the nominal IP. The TOTEM experiment measured dNch/dη = 5.11 ± 0.73 at η = 3.95 and 1.81 ± 0.56 at η = -6.925, with predictions from Pythia 8, Sibyll 2.1, EPOS, and QGSJet II-04 within systematic uncertainties, providing critical constraints for forward physics and cosmic ray models.

ABSTRACT

The pseudorapidity density of charged particles dN(ch)/deta is measured by the TOTEM experiment in pp collisions at sqrt(s) = 8 TeV within the range 3.9 < eta < 4.7 and -6.95 < eta < -6.9. Data were collected in a low intensity LHC run with collisions occurring at a distance of 11.25 m from the nominal interaction point. The data sample is expected to include 96-97\% of the inelastic proton-proton interactions. The measurement reported here considers charged particles with p_T > 0 MeV/c, produced in inelastic interactions with at least one charged particle in -7 < eta < -6 or 3.7 < eta <4.8 . The dN(ch)/deta has been found to decrease with |eta|, from 5.11 +- 0.73 at eta = 3.95 to 1.81 +- 0.56 at eta= - 6.925. Several MC generators are compared to the data and are found to be within the systematic uncertainty of the measurement.

Motivation & Objective

• To measure the forward charged particle pseudorapidity density in pp collisions at √s = 8 TeV in the range 3.9 < η < 4.7 and -6.95 < η < -6.9.
• To constrain non-perturbative QCD models and Monte Carlo event generators by providing precise data in the forward region.
• To reduce systematic uncertainties in forward particle production by using a displaced interaction point to access extended pseudorapidity coverage.
• To improve understanding of diffractive and beam remnant effects in high-energy proton-proton collisions.
• To support the interpretation of high-energy cosmic ray air showers by providing data on forward particle production.

Proposed method

• Data were collected using the TOTEM T2 detector during a low-intensity LHC run with collisions displaced 11.25 m from the nominal interaction point.
• A minimum bias trigger was used to select inelastic pp collisions with at least one charged particle in either -7 < η < -6 or 3.7 < η < 4.8.
• The pseudorapidity density dNch/dη was measured for charged particles with transverse momentum pT > 0 MeV/c.
• Systematic uncertainties were evaluated, including an η-uncorrelated uncertainty of ση = 0.05 due to resolution and misalignment effects.
• Measurements were corrected to include particles with pT down to 0 MeV/c, ensuring completeness of the inelastic sample.
• Predictions from MC generators (Pythia 8, Sibyll 2.1, EPOS, QGSJet II-04) were compared to the data to assess model performance.

Experimental results

Research questions

• RQ1 What is the charged particle pseudorapidity density dNch/dη in pp collisions at √s = 8 TeV in the forward region (3.9 < η < 4.7 and -6.95 < η < -6.9)?
• RQ2 How do different Monte Carlo event generators (Pythia 8, Sibyll 2.1, EPOS, QGSJet II-04) describe the measured dNch/dη in the forward region?
• RQ3 To what extent do systematic uncertainties, particularly from pseudorapidity resolution and misalignment, affect the measurement?
• RQ4 How does the displaced interaction point configuration enable access to a pseudorapidity range beyond the nominal T2 detector coverage?
• RQ5 What is the consistency of the data with models that include diffractive and beam remnant contributions in the forward region?

Key findings

• The pseudorapidity density dNch/dη was measured to be 5.11 ± 0.73 at η = 3.95 in the positive pseudorapidity region.
• At η = -6.925 in the negative pseudorapidity region, dNch/dη was measured to be 1.81 ± 0.56, showing a significant decrease with increasing |η|.
• The measurement is consistent with predictions from Pythia 8 (tune 4C), Sibyll 2.1, EPOS (tune LHC), and QGSJet II-04, within systematic uncertainties.
• Sibyll 2.1 and EPOS predictions systematically underestimate and overestimate the data by about 6–10% and 15–30%, respectively.
• The statistical uncertainty is negligible, and the dominant uncertainty is systematic, with η-uncorrelated uncertainty estimated at ση = 0.05.
• The data sample includes 96–97% of inelastic proton-proton interactions, ensuring high completeness for the inelastic sample.