Fraction of ${\chi }_c$ Decays in Prompt $J/\psi$ Production Measured in $p\mathrm{Pb}$ Collisions at $\sqrt{s_{\mathrm{NN}}}=8.16\mathrm{TeV}$

The fraction of ${\chi }_{c1}$ and ${\chi }_{c2}$ decays in the prompt $J/\psi$ yield, $F_{{\chi }_c\to J/\psi }={\sigma }_{{\chi }_c\to J/\psi }/{\sigma }_{J/\psi }$, is measured by the LHCb detector in $p\mathrm{Pb}$ collisions at $\sqrt{s_{\mathrm{NN}}}=8.16\mathrm{TeV}$. The study covers the forward ($1.5<y^{*}<4.0$) and backward ($-5.0<y^{*}<-2.5$) rapidity regions, where $y^{*}$ is the $J/\psi$ rapidity in the nucleon-nucleon center-of-mass system. Forward and backward rapidity samples correspond to integrated luminosities of $13.6\pm 0.3$ and $20.8\pm 0.5{\mathrm{nb}}^{-1}$, respectively. The result is presented as a function of the $J/\psi$ transverse momentum $p_{\mathrm{T},J/\psi }$ in the range $1<p_{\mathrm{T},J/\psi }<20\mathrm{GeV}/c$. The $F_{{\chi }_c\to J/\psi }$ fraction at forward rapidity is compatible with the LHCb measurement performed in $pp$ collisions at $\sqrt{s}=7\mathrm{TeV}$, whereas the result at backward rapidity is $2.4\sigma$ larger than in the forward region for $1<p_{\mathrm{T},J/\psi }<3\mathrm{GeV}/c$. The increase of $F_{{\chi }_c\to J/\psi }$ at low $p_{\mathrm{T},J/\psi }$ at backward rapidity is compatible with the suppression of the $\psi (2S)$ contribution to the prompt $J/\psi$ yield. The lack of in-medium dissociation of ${\chi }_c$ states observed in this study sets an upper limit of 180 MeV on the free energy available in these $p\mathrm{Pb}$ collisions to dissociate or inhibit charmonium state formation.

Figure 1

(a) Difference in the invariant mass of the ${\mu }^{+}{\mu }^{-}\gamma$ and ${\mu }^{+}{\mu }^{-}$ combinations in the ${\chi }_c$ region. Here the fitted combinatorial background component has been subtracted from both the data points and the fitted curve. The gray band represents the correlated physics background component and the red band is the sum of the signal and correlated physics background components. The widths of the bands represent 68% C.L. (b) Invariant mass distribution of ${\mu }^{+}{\mu }^{-}$ pairs in the $J/\psi$ mass region along with the fitted function.

Figure 2

Fraction of ${\chi }_c$ decays in the prompt $J/\psi$ yield in $p\mathrm{Pb}$ and $pp$ collisions [29] as a function of $p_{\mathrm{T},J/\psi }$. The error bars show the statistical uncertainties. Boxes represent systematic uncertainties and the gray band represents the maximum uncertainties from ${\chi }_c$ and $J/\psi$ polarization effects.

Figure 3

Double ratio of the quarkonium state yield relative to its ground state or open heavy quark meson in $p\mathrm{Pb}$ to $pp$ collisions vs the binding energy of the given quarkonium state. The estimated freeze-out temperature in $p\mathrm{Pb}$ collisions [4, 5, 6] is indicated. The details of the measurement for each data point in the figure are given in Table 3.