Study of high-pT charged particle suppression in PbPb compared to pp collisions at sqrt(sNN)=2.76 TeV (1202.2554v2)

Abstract: The transverse momentum spectra of charged particles have been measured in pp and PbPb collisions at sqrt(sNN) = 2.76 TeV by the CMS experiment at the LHC. In the transverse momentum range pt = 5-10 GeV/c, the charged particle yield in the most central PbPb collisions is suppressed by up to a factor of 5 compared to the pp yield scaled by the number of incoherent nucleon-nucleon collisions. At higher pt, this suppression is significantly reduced, approaching roughly a factor of 2 for particles with pt in the range pt=40-100 GeV/c.

Study of High-$\mathbf{p_T}$ Charged Particle Suppression in Pb-Pb Compared to pp Collisions at $\sqrt{s_{NN}} = 2.76$ TeV

This paper presents detailed measurements of charged particles produced in proton-proton (\Pp\Pp) and lead-lead (PbPb) collisions at a center-of-mass energy per nucleon pair of $\sqrt{s_{NN}} = 2.76$ TeV. The Compact Muon Solenoid (CMS) experiment at the Large Hadron Collider (LHC) conducted this paper, aiming to understand the suppression of high transverse momentum ($p_T$) particles in heavy-ion collisions compared to proton-proton collisions.

Methodology and Analysis

The paper uses data collected by the CMS experiment and includes comprehensive measurements of charged particle yields as a function of their transverse momentum and collision centrality. The primary metric employed is the nuclear modification factor $\RAA$, defined as the ratio of the yield of charged particles in heavy-ion collisions to the yield in \Pp\Pp collisions, scaled by the number of binary nucleon-nucleon collisions. To account for geometrical differences and multiple interactions, the analysis employed the Glauber model to calculate $\TAA$, the nuclear overlap function which represents the effective integrated luminosity per PbPb collision.

Data from two different collision centralities were considered: central (high-density region) and peripheral (low-density region), allowing the paper to evaluate how suppression patterns evolve with collision geometry.

Key Findings

1. Suppression Factor: In the transverse momentum range $p_T = 5$–10 GeV/$c$, the yield of charged particles in the most central PbPb collisions shows a suppression of up to a factor of seven compared to the scaled \Pp\Pp yield. This suppression decreases at higher $p_T$, reducing significantly to about a factor of two in the range $p_T = 40$–100 GeV/$c$.
2. Energy Loss Mechanisms: The results confirm prior observations from RHIC where high-$p_T$ particle production was significantly suppressed in central collisions. This paper extends the transverse momentum range of previous studies, providing more precise measurements.
3. Comparative Analysis: By contrasting $\RAA$ with $\RCP$, the paper also highlights the importance of underlying collision geometry and density in understanding particle suppression dynamics. $\RCP$ compares spectra from different collision centralities while removing direct dependence on \Pp\Pp data.

Implications

The findings from this paper are critical for advancing the understanding of parton energy loss mechanisms in dense mediums, such as those produced in heavy-ion collisions. This contributes significantly to the paper of the quark-gluon plasma and yields insights into the behavior of matter under extreme conditions.

The reduction in suppression at higher $p_T$ implies potential pathways for energy dissipation and highlights theoretical models' sensitivity to quenching parameters. These results provide significant constraints for refining these models, contributing to a clearer picture of the underlying processes in ultra-relativistic heavy-ion collisions.

Future Directions

Further research should focus on extending measurements to even higher $p_T$ values, improving the precision of particle yield suppression quantification. Investigating the interplay between geometrical properties and partonic energy loss will continue to be a key area to explore within both experimental and theoretical frameworks. This paper forms a foundational component for future comparison with planned studies at the increased LHC collision energy of $\sqrt{s_{NN}} = 5.5$ TeV, which will deepen our understanding of medium-induced modifications to hard scatterings.

In conclusion, the suppression measurements of high-$p_T$ charged particles in PbPb collisions serve as a significant probe of the dynamic properties of the hot, dense medium produced in heavy-ion collisions at LHC energies.