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| CMS-PAS-HIN-18-011 | ||
| Angular correlations in exclusive dijet photoproduction in ultra-peripheral PbPb collisions at ${\sqrt {\smash [b]{s_{_{\mathrm {NN}}}}}} = $ 5.02 TeV | ||
| CMS Collaboration | ||
| July 2020 | ||
| Abstract: Exclusive dijet photoproduction has been measured in ultra-peripheral lead-lead (PbPb) collisions at ${\sqrt {\smash [b]{s_{_{\mathrm {NN}}}}}} = $ 5.02 TeV. The analysis is based on a data sample corresponding to an integrated luminosity of 0.38 nb$^{-1}$ collected by the CMS Collaboration. For each dijet, the transverse momentum vectors of the leading and subleading jets are measured and their vector sum and vector difference determined. The azimuthal angle between the vector sum and vector difference defines an angle $\phi$. The distribution of $\phi$ and, in particular, the second Fourier harmonic $ {<} \cos(2\phi) {>} $ is measured. The dependence of $ {<} \cos(2\phi) {>} $ on the sum of the jet momentum vectors provides the first azimuthal anisotropy measurement related to exclusive dijet production. | ||
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Links:
CDS record (PDF) ;
inSPIRE record ;
CADI line (restricted) ;
These preliminary results are superseded in this paper, Submitted to PRL. |
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| Figures | |
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Figure 1:
Distributions of the maximum distance in pseudorapidity between the most backward jet and a track in an event (left) and the backward rapidity gap (right) shown for data and reconstructed RAPGAP MC (see text for details). |
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Figure 1-a:
Distribution of |
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Figure 1-b:
Distribution of the backward rapidity gap shown for data and reconstructed RAPGAP MC (see text for details). |
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Figure 2:
Magnitudes of the vector sum ($Q_{\mathrm{T}}$) and vector difference ($P_{\mathrm{T}}$) of the two jets (top left) and their correlation (top right) for the events that pass the selection criteria. The diagonal line illustrates the $P_{\mathrm{T}} > Q_{\mathrm{T}}$ requirement. Invariant mass (bottom left) and rapidity (bottom right) of the dijet candidates after further $Q_{\mathrm{T}}$ and $P_{\mathrm{T}}$ requirements. The lines show the prediction of the RAPGAP MC. |
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Figure 2-a:
Magnitudes of the vector sum ($Q_{\mathrm{T}}$) and vector difference ($P_{\mathrm{T}}$) of the two jets. The lines show the prediction of the RAPGAP MC. |
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Figure 2-b:
Correlation 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. The diagonal line illustrates the $P_{\mathrm{T}} > Q_{\mathrm{T}}$ requirement. |
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Figure 2-c:
Invariant mass of the dijet candidates after further $Q_{\mathrm{T}}$ and $P_{\mathrm{T}}$ requirements. The lines show the prediction of the RAPGAP MC. |
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Figure 2-d:
Rapidity of the dijet candidates after further $Q_{\mathrm{T}}$ and $P_{\mathrm{T}}$ requirements. The lines show the prediction of the RAPGAP MC. |
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Figure 3:
The mixed event distributions obtained from data and compared to the RAPGAP prediction after full simulation and reconstruction (blue line). The left panel shows the $\phi $ distribution while the right panel shows the second harmonic of this distribution as a function of $Q_{\mathrm{T}}$. 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 (see text for details). |
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Figure 4:
The left panel shows the $\phi $ distribution while the right panel shows the second harmonic of this distribution for the data, as a function of $Q_{\mathrm{T}}$. The corresponding distributions from RAPGAP after full simulation and reconstruction are also shown (blue lines). Both the statistical (error bars) and systematic (green boxes) uncertainties are shown. |
| Tables | |
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Table 1:
$ {<} \cos(2\phi) {>} $ systematic uncertainties. |
| Summary |
| In summary, for the first time, angular correlations present in exclusive dijet photoproduction have been studied using ultra-peripheral lead-lead collisions at ${\sqrt {\smash [b]{s_{_{\mathrm {NN}}}}}} = $ 5.02 TeV. The second moment of the angular distribution, $ {<} \cos(2\phi) {>} $, where $\phi$ is the angle between the vector sum and vector difference of the leading and subleading jet transverse momenta, has been measured as a function of the momentum sum of the two jets. This analysis amounts to the first, yet essential, step towards the extraction of the Wigner or Husimi gluon distributions, which are believed to be the most fundamental gluon distributions. It also introduces new techniques for the analysis of jet angular correlations in exclusive dijet events at colliders. |
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