CMS-HIN-18-011

CMS-HIN-18-011 ; CERN-EP-2021-071
Azimuthal Correlations within Exclusive Dijets with Large Momentum Transfer in Photon-Lead Collisions
CMS Collaboration
29 April 2022
Phys. Rev. Lett. 131 (2023) 051901
Abstract: The structure of nucleons is multidimensional and depends on the transverse momenta, spatial geometry, and polarization of the constituent partons. Such a structure can be studied using high-energy photons produced in ultraperipheral heavy-ion collisions. The first measurement of the azimuthal angular correlations of exclusively produced events with two jets in photon-lead interactions at large momentum transfer is presented, a process that is considered to be sensitive to the underlying nuclear gluon polarization. This study uses a data sample of ultraperipheral lead-lead collisions at $ \sqrt{\smash[b]{s_{_{\mathrm{NN}}}}} = $ 5.02 TeV, corresponding to an integrated luminosity of 0.38 nb$^{-1}$, collected with the CMS experiment at the LHC. The measured second harmonic of the correlation between the sum and difference of the two jet transverse momentum vectors is found to be positive, and rising, as the dijet transverse momentum increases. A well-tuned model that has been successful at describing a wide range of proton scattering data from the HERA experiments fails to describe the observed correlations, suggesting the presence of gluon polarization effects.
Links: e-print arXiv:2205.00045 [hep-ex] (PDF) ; CDS record ; inSPIRE record ; HepData record ; CADI line (restricted) ;

Figures & Tables	Summary	Additional Figures	References	CMS Publications

Figures
png pdf	Figure 1: Magnitudes of the vector sum ($ Q_{\mathrm{T}} $) and vector difference ($ P_{\mathrm{T}} $) of the two jets (left). The dashed blue line illustrates the $ Q_{\mathrm{T}} < $ 25 GeV requirement. Invariant mass (center) and rapidity (right) of the dijet candidates after all selection requirements. The lines show the RAPGAP MC generated events including detector resolution effects. The statistical uncertainties are covered by the symbol size.
png pdf	Figure 2: The unfolded 1 $ /{N_{\text{events}}} \,\mathrm{d} N/\mathrm{d}\Phi $ distribution (left) and the unfolded $ \langle \cos(2\Phi) \rangle $ values as a function of $ Q_{\mathrm{T}} $ (right). The corresponding distributions from the RAPGAP simulation at the generator level (blue lines) and theoretical calculation by Hatta \textitet al. $ \mbox{et al.} $ [20] for $ \langle \cos(2\Phi) \rangle $ (red dashed lines) are also shown for $ Q_{\mathrm{T}} < $ 15 GeV, consistent with the back-to-back limit in their calculation. The dijet events are found predominantly in the forward direction, with $ {0 < \eta_\text{jet} < 2.4} $. Both the statistical (error bars) and systematic (green boxes) uncertainties are shown.
png pdf	Figure 2-a: The unfolded 1 $ /{N_{\text{events}}} \,\mathrm{d} N/\mathrm{d}\Phi $ distribution. The corresponding distributions from the RAPGAP simulation at the generator level (blue lines) and theoretical calculation by Hatta et al. [20] for $ \langle \cos(2\Phi) \rangle $ (red dashed lines) are also shown for $ Q_{\mathrm{T}} < $ 15 GeV, consistent with the back-to-back limit in their calculation. The dijet events are found predominantly in the forward direction, with $ {0 < \eta_\text{jet} < 2.4} $. Both the statistical (error bars) and systematic (green boxes) uncertainties are shown.
png pdf	Figure 2-b: The unfolded $ \langle \cos(2\Phi) \rangle $ values as a function of $ Q_{\mathrm{T}} $. The corresponding distributions from the RAPGAP simulation at the generator level (blue lines) and theoretical calculation by Hatta et al. [20] for $ \langle \cos(2\Phi) \rangle $ (red dashed lines) are also shown for $ Q_{\mathrm{T}} < $ 15 GeV, consistent with the back-to-back limit in their calculation. The dijet events are found predominantly in the forward direction, with $ {0 < \eta_\text{jet} < 2.4} $. Both the statistical (error bars) and systematic (green boxes) uncertainties are shown.

Tables
png pdf	Table of $ \langle \cos(2\Phi) \rangle $ systematic uncertainties (absolute values). The individual components are discussed in the text. :

Summary

In summary, the exclusive production of two jets in photon-lead interactions with a large rapidity gap has been studied for the first time. The dijet events are characterized by a large momentum transfer. The second harmonic of the angular correlation between the sum and difference of the two jet transverse momenta is found to be positive and rising, with dijet transverse momentum in the measured range 0–25 GeV. An a posteriori calculation that includes the effect of soft gluons from final-state radiation describes the average magnitude of the correlations, but does not rise with the magnitude of the dijet total momentum $ Q_\text{T} $ to the extent found with the CMS data. A model [41] that has been successful at describing a wide range of proton scattering data from the HERA experiments overestimates the strength of the correlations, suggesting the presence of gluon polarization effects. This experiment calls for new theoretical calculations to quantify the theoretical implications and opens a new direction for probing the high-energy limit of strong interactions.

Additional Figures
png pdf	Additional Figure 1: Difference in pseudorapidity between the most backward jet in an event and the most backward track defining the backward rapidity gap, $\eta _{\text {jet}}^{B} - \eta _{\text {track}}^{B}$, shown for data and reconstructed rapgap MC events. The dashed line with the arrow corresponds to the selection applied in the analysis that this difference should be less than 1.
png pdf	Additional Figure 2: Backward rapidity gap, $\Delta \eta ^{B}$, defined as the difference between the minimum pseudorapidity of any high-purity charged track having $ {p_{\mathrm {T}}} > $ 0.2 GeV and the lower limit of the tracker shown for data and reconstructed rapgap MC events. The dashed line with the arrow corresponds to the selection $\Delta \eta ^{B} > $ 1.2 applied in the analysis.
png pdf	Additional Figure 3: Correlation of the magnitudes of the vector sum ($Q_{\mathrm {T}}$) and vector difference ($P_{\mathrm {T}}$) of the two jets for the events that pass the selection criteria.
png pdf	Additional Figure 4: The $\mathrm{d}N/\mathrm{d}\Phi $ distribution shown for the mixed events obtained from data and compared to the rapgap MC prediction after full simulation and reconstruction (blue line). Both the statistical (error bars) and systematic (magenta boxes) uncertainties are shown. Dashed histograms show the toy-MC distribution including the effect of detector smearing.
png pdf	Additional Figure 5: The second harmonic, $\langle \cos(2\Phi) \rangle $, as a function of $Q_{\mathrm {T}}$ shown for the mixed events obtained from data and compared to the rapgap MC prediction after full simulation and reconstruction (blue line). Both the statistical (error bars) and systematic (magenta boxes) uncertainties are shown. Dashed histograms show the toy-MC distribution including the effect of detector smearing.

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