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Measurement of the forward charged particle pseudorapidity density in pp collisions at sqrt{s} = 7 TeV with the TOTEM experiment

G. Antchev, P. Aspell, I. Atanassov, V. Avati, J. Baechler, V. Berardi, M. Berretti, E. Bossini, M. Bozzo, P. Brogi, E. Brücken, A. Buzzo, F. S. Cafagna, M. Calicchio, M. G. Catanesi, C. Covault, M. Csanád, T. Csörgő, M. Deile, K Eggert, V. Eremin, R. Ferretti, F. Ferro, A. Fiergolski, F. Garcia, S. Giani, V. Greco, L. Grzanka, J. Heino, T. Hilden, M. R. Intonti, J. Kašpar, J. Kopal, V. Kundrát, K. Kurvinen, S. Lami, G. Latino, R. Lauhakangas, T. Leszko, E. Lippmaa, M. Lokajíček, M. Lo Vetere, F. Lucas Rodríguez, M. Macrí, L. Magaletti, T. Mäki, A. Mercadante, N. Minafra, S. Minutoli, F. Nemes, H. Niewiadomski, E. Oliveri, F. Oljemark, R. Orava, M. Oriunno, K. Österberg, P. Palazzi, J. Procházka, M. Quinto, E. Radermacher, E. Radicioni, F. Ravotti, E. Robutti, L. Ropelewski, G. Ruggiero, H. Saarikko, A. Santroni, A. Scribano, W. Snoeys, J. Sziklai, C. Taylor, N. Turini, V. Vacek, M. Vitek, J. Welti, J. Whitmore

TL;DR

This study reports the first forward-region measurement of the charged-particle pseudorapidity density in pp collisions at √s = 7 TeV, using the TOTEM T2 detector to cover 5.3<|eta|<6.4. By employing a ZImpact-based primary/secondary separation and detailed detector alignment and efficiency corrections, the authors extract dN_ch/deta across four T2 quarters and combine them with careful treatment of systematic uncertainties. The results show a decreasing dN_ch/deta with increasing |eta|, from 3.84 to 2.38 (statistic-only values) with substantial systematic contributions, and they reveal that contemporary MC generators do not fully reproduce the forward data. The measurement provides essential constraints for forward hadron production models and diffractive processes, with implications for cosmic-ray physics and the interpretation of forward showers in collider experiments.

Abstract

The TOTEM experiment has measured the charged particle pseudorapidity density dN_{ch}/deta in pp collisions at sqrt{s} = 7 TeV for 5.3<|eta|<6.4 in events with at least one charged particle with transverse momentum above 40 MeV/c in this pseudorapidity range. This extends the analogous measurement performed by the other LHC experiments to the previously unexplored forward eta region. The measurement refers to more than 99% of non-diffractive processes and to single and double diffractive processes with diffractive masses above ~3.4 GeV/c^2, corresponding to about 95% of the total inelastic cross-section. The dN_{ch}/deta has been found to decrease with |eta|, from 3.84 pm 0.01(stat) pm 0.37(syst) at |eta| = 5.375 to 2.38 pm 0.01(stat) pm 0.21(syst) at |eta| = 6.375. Several MC generators have been compared to data; none of them has been found to fully describe the measurement.

Measurement of the forward charged particle pseudorapidity density in pp collisions at sqrt{s} = 7 TeV with the TOTEM experiment

TL;DR

This study reports the first forward-region measurement of the charged-particle pseudorapidity density in pp collisions at √s = 7 TeV, using the TOTEM T2 detector to cover 5.3<|eta|<6.4. By employing a ZImpact-based primary/secondary separation and detailed detector alignment and efficiency corrections, the authors extract dN_ch/deta across four T2 quarters and combine them with careful treatment of systematic uncertainties. The results show a decreasing dN_ch/deta with increasing |eta|, from 3.84 to 2.38 (statistic-only values) with substantial systematic contributions, and they reveal that contemporary MC generators do not fully reproduce the forward data. The measurement provides essential constraints for forward hadron production models and diffractive processes, with implications for cosmic-ray physics and the interpretation of forward showers in collider experiments.

Abstract

The TOTEM experiment has measured the charged particle pseudorapidity density dN_{ch}/deta in pp collisions at sqrt{s} = 7 TeV for 5.3<|eta|<6.4 in events with at least one charged particle with transverse momentum above 40 MeV/c in this pseudorapidity range. This extends the analogous measurement performed by the other LHC experiments to the previously unexplored forward eta region. The measurement refers to more than 99% of non-diffractive processes and to single and double diffractive processes with diffractive masses above ~3.4 GeV/c^2, corresponding to about 95% of the total inelastic cross-section. The dN_{ch}/deta has been found to decrease with |eta|, from 3.84 pm 0.01(stat) pm 0.37(syst) at |eta| = 5.375 to 2.38 pm 0.01(stat) pm 0.21(syst) at |eta| = 6.375. Several MC generators have been compared to data; none of them has been found to fully describe the measurement.

Paper Structure

This paper contains 9 sections, 1 equation, 3 figures, 2 tables.

Table of Contents

  1. Introduction
  2. Experimental apparatus
  3. Detector simulation
  4. Track reconstruction and alignment
  5. Data and MC sample
  6. Analysis procedure
  7. Systematic uncertainties
  8. Results
  9. Conclusions

Figures (3)

  • Figure 1: Definition of the track ZImpact parameter. The description is discussed in the text.
  • Figure 2: ZImpact parameter distribution for the data tracks reconstructed in one T2 quarter in the $5.35<\eta<5.4$ range. The reported $\chi^2$/ndf refers to the global (double Gaussian + exponential) fit, performed in the range from -15 m up to 9 m. The blue-solid curve represents the exponential component due to secondaries, while the red-dashed curve is the double Gaussian component mainly related to primary tracks. The condition Z$_{0}\, \cdot$ sign($\eta$) $<13.5\,$m has been required, reducing the amount of secondary tracks by about 60%.
  • Figure 3: Charged particle pseudorapidity density distribution. The experimental points (black squares) represent the average of the four T2 quarters, with the error bars including both statistical and systematic error. Red triangles, blue circles and orange diamonds show, respectively, the Phojet, Pythia8 and Sherpa predictions for charged particles with $P_{T}>\,$40 MeV/c in events where at least one charged particle is generated in the $5.3<|\eta|<6.5$ range.