Yoctosecond Metrology Through Hanbury Brown–Twiss Correlations from a Quark-Gluon Plasma

Expansion dynamics at the yoctosecond time scale affect the evolution of the quark gluon plasma (QGP) created in heavy ion collisions. We show how these dynamics are accessible through Hanbury Brown–Twiss (HBT) intensity interferometry of direct photons emitted from the interior of the QGP. A detector placed close to the beam axis is particularly sensitive to early polar momentum anisotropies of the QGP. Observing a modification of the HBT signal at the proposed FoCal detector of the LHC ALICE experiment would allow us to measure the isotropization time of the plasma and could provide first experimental evidence for photon double pulses at the yoctosecond time scale.

Figure 1

Concept of HBT detection. Two heavy ions collide with impact parameter $b$ and produce a QGP that rapidly expands at early times along the beam axis ($z$ axis). Photons are emitted from the QGP and arrive at a detector placed in the forward direction, like the proposed FoCal detector [24]. HBT correlations are obtained from photons that arrive simultaneously, whose momentum vectors are separated by an angle ${\theta }_{\mathrm{rel}}$.

Figure 2

HBT photon correlation for a collinear configuration. The detector is placed at a fixed angle $\theta =25°$ ($\eta \approx 1.5$) and the difference $\mathbf{q}={\mathbf{k}}^{\prime }-\mathbf{k}$ between collinear photon momenta ${\mathbf{k}}^{\prime }\parallel \mathbf{k}$ with $k=4\mathrm{GeV}/c$ ($k_T\approx 1.7\mathrm{GeV}/c$) is varied. The various isotropization time scales correspond to initial free-streaming expansion with intermediate polar momentum anisotropy starting from ${\tau }_{\mathrm{iso}}=2\mathrm{fm}/c$ (solid line) down to ${\tau }_{\mathrm{iso}}={\tau }_0$ (dotted line) which corresponds to an ideal hydrodynamic expansion. The impact parameter is chosen as $b=10\mathrm{fm}$ and the anisotropy model parameter as $\gamma =2$.

Figure 3

HBT photon correlation for a detector placed in the forward direction. One momentum vector is placed at 2° away from the beam axis ($\eta \approx 4$) and the other varied between 2° and 8° ($\eta \approx 4$ to $\eta \approx 2.7$, at the same azimuthal angle) at fixed momentum $k=25\mathrm{GeV}/c$. A distinct difference and change of shape of the HBT function with respect to the isotropization time of the quark-gluon-plasma is observed. The impact parameter is $b=12\mathrm{fm}$ and the anisotropy model parameter is $\gamma =2$.