First Measurement of Bose-Einstein Correlations in Proton-Proton Collisions at $\sqrt{s}=0.9$ and 2.36 TeV at the LHC

Bose-Einstein correlations have been measured using samples of proton-proton collisions at 0.9 and 2.36 TeV center-of-mass energies, recorded by the CMS experiment at the CERN Large Hadron Collider. The signal is observed in the form of an enhancement of pairs of same-sign charged particles with small relative four-momentum. The size of the correlated particle emission region is seen to increase significantly with the particle multiplicity of the event.

Figure 1

Ratios $R(Q)$ obtained with the opposite-hemisphere, same-charge reference samples for data (dots) and MC calculations with no BEC effect (crosses).

Figure 2

Double ratios $\mathcal{R}(Q)$ for the 0.9 TeV data, using the opposite-hemisphere, same-charge reference samples for combinations enriched, using a $dE/dx$ measurement, in pion-pion pairs (dots) and in pion-nonpion pairs (open circles), respectively.

Figure 3

Fits to the double ratios ${\mathcal{R}}^{\mathrm{comb}}(Q)$ with exponential (solid lines) and Gaussian (dashed lines) functions, for 0.9 TeV (top panel) and 2.36 TeV (bottom panel) data. The range $0.6<Q<0.9\mathrm{GeV}$ is excluded from the fits.

Figure 4

Values of the $\lambda$ (top panel) and $r$ (bottom panel) parameters as a function of the charged-particle multiplicity in the event for combined (dots) and opposite-hemisphere, same-charge (open circles) reference samples, at 0.9 TeV. The errors shown are statistical only. The points are placed on the horizontal scale at the average of the multiplicity distribution in the corresponding bin.