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| CMS-PAS-SMP-19-006 | ||
| Study of hard color singlet exchange in dijet events with proton-proton collisions at $\sqrt{s}= $ 13 TeV | ||
| CMS and TOTEM Collaborations | ||
| April 2020 | ||
| Abstract: This note presents a study of proton-proton collision events where the two leading jets are separated by a large pseudorapidity interval devoid of particle activity, known as jet-gap-jet events. Both jets have transverse momentum $p_\text{T, jet} > $ 40 GeV and pseudorapidity 1.4 $ < |\eta_\text{jet}| < $ 4.7, with $\eta_\text{jet-1}\times \eta_\text{jet-2} < $ 0. The analysis is based on data collected by the CMS experiment in proton-proton collisions during a low luminosity, high-$\beta^*$ run in 2015 at $\sqrt{s} = $ 13 TeV, with an integrated luminosity of 0.66 pb$^{-1}$. The number of charged particles detected with transverse momentum $p_\text{T} > $ 200 MeV in the fixed pseudorapidity interval $-1 < \eta < 1$ between the jets is used to discriminate jet-gap-jet events from color exchange dijet events. The fraction of jet-gap-jet events to all dijet events with similar kinematics, $f_\text{CSE}$, is presented as a function of the pseudorapidity difference between the leading two jets, the transverse momentum of the subleading jet, and the azimuthal angle separation between the leading two jets. The results are compared to previous measurements and to perturbative quantum chromodynamics predictions. In addition, the note presents the first study of jet-gap-jet events with a leading proton, interpreted as a proton-gap-jet-gap-jet topology, using a subsample of events collected by the CMS and TOTEM experiments with an integrated luminosity of 0.40 pb$^{-1}$. The leading protons are detected with the Roman pot detectors of the TOTEM experiment. The ratio $f_\text{CSE}$ in this sample is found to be 2.91 $\pm$ 0.70 (stat) $^{+ 1.02}_{- 0.94}$ (syst) times larger than that for inclusive dijet production for dijets with similar kinematics. | ||
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These preliminary results are superseded in this paper, Accepted by PRD. The superseded preliminary plots can be found here. |
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| Figures | |
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Figure 1:
(Left) Schematic diagram of $t$-channel two-gluon exchange in pp collisions, which yields the jet-gap-jet signature reconstructed in the CMS detector. The lines adjacent to the protons represent the proton breakup. (Right) Jet-gap-jet event signature in the $\eta $-$\phi $ plane. The filled circles represent final-state particles. The filled area between the jets denotes the fixed pseudorapidity region $|\eta | < $ 1 devoid of charged particle tracks. |
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Figure 1-a:
(Left) Schematic diagram of $t$-channel two-gluon exchange in pp collisions, which yields the jet-gap-jet signature reconstructed in the CMS detector. The lines adjacent to the protons represent the proton breakup. (Right) Jet-gap-jet event signature in the $\eta $-$\phi $ plane. The filled circles represent final-state particles. The filled area between the jets denotes the fixed pseudorapidity region $|\eta | < $ 1 devoid of charged particle tracks. |
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Figure 1-b:
(Left) Schematic diagram of $t$-channel two-gluon exchange in pp collisions, which yields the jet-gap-jet signature reconstructed in the CMS detector. The lines adjacent to the protons represent the proton breakup. (Right) Jet-gap-jet event signature in the $\eta $-$\phi $ plane. The filled circles represent final-state particles. The filled area between the jets denotes the fixed pseudorapidity region $|\eta | < $ 1 devoid of charged particle tracks. |
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Figure 2:
(Left) Schematic diagram of the production of jet-gap-jet event with a leading proton in pp collisions. The jet-gap-jet signature is observed in the CMS detector, while the leading proton is detected with the forward proton spectrometer of the TOTEM experiment. (Right) Proton-gap-jet-gap-jet event signature in the $\eta $-$\phi $ plane. The filled circles represent final-state particles. The filled areas denote the central gap region $|\eta | < $ 1 where the charged particle track multiplicity is measured, and the forward rapidity gap which is inferred from the forward proton detection. |
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Figure 2-a:
(Left) Schematic diagram of the production of jet-gap-jet event with a leading proton in pp collisions. The jet-gap-jet signature is observed in the CMS detector, while the leading proton is detected with the forward proton spectrometer of the TOTEM experiment. (Right) Proton-gap-jet-gap-jet event signature in the $\eta $-$\phi $ plane. The filled circles represent final-state particles. The filled areas denote the central gap region $|\eta | < $ 1 where the charged particle track multiplicity is measured, and the forward rapidity gap which is inferred from the forward proton detection. |
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Figure 2-b:
(Left) Schematic diagram of the production of jet-gap-jet event with a leading proton in pp collisions. The jet-gap-jet signature is observed in the CMS detector, while the leading proton is detected with the forward proton spectrometer of the TOTEM experiment. (Right) Proton-gap-jet-gap-jet event signature in the $\eta $-$\phi $ plane. The filled circles represent final-state particles. The filled areas denote the central gap region $|\eta | < $ 1 where the charged particle track multiplicity is measured, and the forward rapidity gap which is inferred from the forward proton detection. |
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Figure 3:
Side-view of detectors configuration during the 2015 CMS-TOTEM combined run. The horizontal dashed line indicates the beamline. The CMS detector is denoted by the filled circle in the center. The leading proton(s) are transported via the accelerator magnetic fields (blue rectangles), eventually passing through the silicon detectors housed in the Roman pots (black rectangles) of the TOTEM experiment. Sector 45 and sector 56 are located towards the positive and negative pseudorapidities in the CMS coordinate system, respectively. |
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Figure 4:
Distributions of the ratio of subleading jet transverse momentum to leading jet transverse momentum $p_\text {T, jet-2}/p_\text {T, jet-1}$ (left), azimuthal angle separation between leading two jets $\Delta \phi _\text {jj}$(right), and number of additional jets $N_\text {extra-jets}$ with $p_\text {T,extra-jet} > $ 15 GeV (bottom), for jet-gap-jet candidates with $N_\text {Tracks} = $ 0 in $|\eta | < $ 1 (black) and the color exchange dijet candidates $N_\text {Tracks} \geq $ 3 in $|\eta | < $ 1 (red). The distributions are normalized to unity. |
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Figure 4-a:
Distributions of the ratio of subleading jet transverse momentum to leading jet transverse momentum $p_\text {T, jet-2}/p_\text {T, jet-1}$ (left), azimuthal angle separation between leading two jets $\Delta \phi _\text {jj}$(right), and number of additional jets $N_\text {extra-jets}$ with $p_\text {T,extra-jet} > $ 15 GeV (bottom), for jet-gap-jet candidates with $N_\text {Tracks} = $ 0 in $|\eta | < $ 1 (black) and the color exchange dijet candidates $N_\text {Tracks} \geq $ 3 in $|\eta | < $ 1 (red). The distributions are normalized to unity. |
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Figure 4-b:
Distributions of the ratio of subleading jet transverse momentum to leading jet transverse momentum $p_\text {T, jet-2}/p_\text {T, jet-1}$ (left), azimuthal angle separation between leading two jets $\Delta \phi _\text {jj}$(right), and number of additional jets $N_\text {extra-jets}$ with $p_\text {T,extra-jet} > $ 15 GeV (bottom), for jet-gap-jet candidates with $N_\text {Tracks} = $ 0 in $|\eta | < $ 1 (black) and the color exchange dijet candidates $N_\text {Tracks} \geq $ 3 in $|\eta | < $ 1 (red). The distributions are normalized to unity. |
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Figure 4-c:
Distributions of the ratio of subleading jet transverse momentum to leading jet transverse momentum $p_\text {T, jet-2}/p_\text {T, jet-1}$ (left), azimuthal angle separation between leading two jets $\Delta \phi _\text {jj}$(right), and number of additional jets $N_\text {extra-jets}$ with $p_\text {T,extra-jet} > $ 15 GeV (bottom), for jet-gap-jet candidates with $N_\text {Tracks} = $ 0 in $|\eta | < $ 1 (black) and the color exchange dijet candidates $N_\text {Tracks} \geq $ 3 in $|\eta | < $ 1 (red). The distributions are normalized to unity. |
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Figure 5:
Charged particle multiplicity distribution in the fixed pseudorapidity region $|\eta | < $ 1 for charged particle tracks with ${p_{\mathrm {T}}} > $ 200 MeV for dijet events with 40 $ < p_\text {T, jet-2} < $ 50 GeV. Opposite side (OS) dijet events satisfy $\eta _\text {jet-1}\times \eta _\text {jet-2} < $ 0. Vertical bars, which represent statistical uncertainties, are smaller than the markers for most data points. Results from color exchange dijet background estimation based on the equal side (ES) dijet events and the negative binomial distribution (NBD) function fit are shown on the left and right panels, respectively. The NBD function is fit in 3 $ \leq N_\text {Tracks} \leq $ 35, and extrapolated down to $N_\text {Tracks} = $ 0. The vertical dashed line represents the jet-gap-jet signal region used in the analysis, $N_\text {Tracks} < $ 3. The fraction $f_\text {CSE}$ corresponds to the ratio of the excess of events at low multiplicities relative to the integrated number of events, as described in text. The dashed curve in $N_\text {Tracks} > $ 35 on the right panel is an extrapolation of the NBD fit. |
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Figure 5-a:
Charged particle multiplicity distribution in the fixed pseudorapidity region $|\eta | < $ 1 for charged particle tracks with ${p_{\mathrm {T}}} > $ 200 MeV for dijet events with 40 $ < p_\text {T, jet-2} < $ 50 GeV. Opposite side (OS) dijet events satisfy $\eta _\text {jet-1}\times \eta _\text {jet-2} < $ 0. Vertical bars, which represent statistical uncertainties, are smaller than the markers for most data points. Results from color exchange dijet background estimation based on the equal side (ES) dijet events and the negative binomial distribution (NBD) function fit are shown on the left and right panels, respectively. The NBD function is fit in 3 $ \leq N_\text {Tracks} \leq $ 35, and extrapolated down to $N_\text {Tracks} = $ 0. The vertical dashed line represents the jet-gap-jet signal region used in the analysis, $N_\text {Tracks} < $ 3. The fraction $f_\text {CSE}$ corresponds to the ratio of the excess of events at low multiplicities relative to the integrated number of events, as described in text. The dashed curve in $N_\text {Tracks} > $ 35 on the right panel is an extrapolation of the NBD fit. |
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Figure 5-b:
Charged particle multiplicity distribution in the fixed pseudorapidity region $|\eta | < $ 1 for charged particle tracks with ${p_{\mathrm {T}}} > $ 200 MeV for dijet events with 40 $ < p_\text {T, jet-2} < $ 50 GeV. Opposite side (OS) dijet events satisfy $\eta _\text {jet-1}\times \eta _\text {jet-2} < $ 0. Vertical bars, which represent statistical uncertainties, are smaller than the markers for most data points. Results from color exchange dijet background estimation based on the equal side (ES) dijet events and the negative binomial distribution (NBD) function fit are shown on the left and right panels, respectively. The NBD function is fit in 3 $ \leq N_\text {Tracks} \leq $ 35, and extrapolated down to $N_\text {Tracks} = $ 0. The vertical dashed line represents the jet-gap-jet signal region used in the analysis, $N_\text {Tracks} < $ 3. The fraction $f_\text {CSE}$ corresponds to the ratio of the excess of events at low multiplicities relative to the integrated number of events, as described in text. The dashed curve in $N_\text {Tracks} > $ 35 on the right panel is an extrapolation of the NBD fit. |
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Figure 6:
Distribution of $\xi _p(\text {PF}) - \xi _p(\text {RP})$ in sector 45 (left) and sector 56 (right) in data, where $\xi _p(\text {PF})$ and $\xi _p(\text {RP})$ denote the fractional momentum loss of the proton reconstructed with the particle-flow (PF) candidates of CMS and the Roman pots (RP) of TOTEM, respectively. Vertical bars indicate statistical uncertainties only. The estimated background contamination (beam background events) is represented by the filled histogram, and is estimated from the data, as described in text. No central pseudorapidity gap is required for this plot. The vertical dashed line represents the requirement applied in the analysis to remove most of the beam background contribution. |
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Figure 6-a:
Distribution of $\xi _p(\text {PF}) - \xi _p(\text {RP})$ in sector 45 (left) and sector 56 (right) in data, where $\xi _p(\text {PF})$ and $\xi _p(\text {RP})$ denote the fractional momentum loss of the proton reconstructed with the particle-flow (PF) candidates of CMS and the Roman pots (RP) of TOTEM, respectively. Vertical bars indicate statistical uncertainties only. The estimated background contamination (beam background events) is represented by the filled histogram, and is estimated from the data, as described in text. No central pseudorapidity gap is required for this plot. The vertical dashed line represents the requirement applied in the analysis to remove most of the beam background contribution. |
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Figure 6-b:
Distribution of $\xi _p(\text {PF}) - \xi _p(\text {RP})$ in sector 45 (left) and sector 56 (right) in data, where $\xi _p(\text {PF})$ and $\xi _p(\text {RP})$ denote the fractional momentum loss of the proton reconstructed with the particle-flow (PF) candidates of CMS and the Roman pots (RP) of TOTEM, respectively. Vertical bars indicate statistical uncertainties only. The estimated background contamination (beam background events) is represented by the filled histogram, and is estimated from the data, as described in text. No central pseudorapidity gap is required for this plot. The vertical dashed line represents the requirement applied in the analysis to remove most of the beam background contribution. |
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Figure 7:
Charged particle multiplicity distribution in the fixed pseudorapidity region $|\eta | < $ 1 after the dijet selection and proton selection. Opposite side (OS) dijet events satisfy $\eta _\text {jet-1}\times \eta _\text {jet-2} < $ 0. Vertical bars indicate statistical uncertainties. The filled histogram represents the residual beam related contamination. The contribution of proton-gap-jet-jet events which feature a central gap is modeled with the equal side (ES) dijet events (left) and with the negative binomial distribution (NBD) function fit (right), as described in text. The NBD function is fit in 2 $ \leq N_\text {Tracks} \leq $ 25, and extrapolated down to $N_\text {Tracks} = $ 0. The vertical dashed line represents the jet-gap-jet signal region used in the analysis, $N_\text {Tracks} < $ 2. An excess is observed in the lowest charged particle multiplicity bins, which corresponds to the presence of proton-gap-jet-gap-jet events in the sample. The dashed curve on the right panel represents an extrapolation of the NBD fit to $N_\text {Tracks} > $ 25. |
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Figure 7-a:
Charged particle multiplicity distribution in the fixed pseudorapidity region $|\eta | < $ 1 after the dijet selection and proton selection. Opposite side (OS) dijet events satisfy $\eta _\text {jet-1}\times \eta _\text {jet-2} < $ 0. Vertical bars indicate statistical uncertainties. The filled histogram represents the residual beam related contamination. The contribution of proton-gap-jet-jet events which feature a central gap is modeled with the equal side (ES) dijet events (left) and with the negative binomial distribution (NBD) function fit (right), as described in text. The NBD function is fit in 2 $ \leq N_\text {Tracks} \leq $ 25, and extrapolated down to $N_\text {Tracks} = $ 0. The vertical dashed line represents the jet-gap-jet signal region used in the analysis, $N_\text {Tracks} < $ 2. An excess is observed in the lowest charged particle multiplicity bins, which corresponds to the presence of proton-gap-jet-gap-jet events in the sample. The dashed curve on the right panel represents an extrapolation of the NBD fit to $N_\text {Tracks} > $ 25. |
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Figure 7-b:
Charged particle multiplicity distribution in the fixed pseudorapidity region $|\eta | < $ 1 after the dijet selection and proton selection. Opposite side (OS) dijet events satisfy $\eta _\text {jet-1}\times \eta _\text {jet-2} < $ 0. Vertical bars indicate statistical uncertainties. The filled histogram represents the residual beam related contamination. The contribution of proton-gap-jet-jet events which feature a central gap is modeled with the equal side (ES) dijet events (left) and with the negative binomial distribution (NBD) function fit (right), as described in text. The NBD function is fit in 2 $ \leq N_\text {Tracks} \leq $ 25, and extrapolated down to $N_\text {Tracks} = $ 0. The vertical dashed line represents the jet-gap-jet signal region used in the analysis, $N_\text {Tracks} < $ 2. An excess is observed in the lowest charged particle multiplicity bins, which corresponds to the presence of proton-gap-jet-gap-jet events in the sample. The dashed curve on the right panel represents an extrapolation of the NBD fit to $N_\text {Tracks} > $ 25. |
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Figure 8:
Fraction of color singlet exchange dijet events, $f_\text {CSE}$, measured as a function of $\Delta \eta _\text {jj}$, $p_\text {T,jet-2}$, and $\Delta \phi _\text {jj}$ in pp collisions at $\sqrt {s} = $ 13 TeV. Vertical bars represent statistical uncertainties, while boxes represent the combination of statistical and systematic uncertainties in quadrature. The results are plotted at the mean values of $\Delta \eta _\text {jj}$, $p_\text {T, jet-2}$, and $\Delta \phi _\text {jj}$ in the bin. The results on $f_\text {CSE}$ versus $\Delta \eta _\text {jj}$ and $p_\text {T,jet-2}$ are integrated over $\Delta \phi _\text {jj}$. The solid curve corresponds to theoretical predictions by the Royon, Marquet, Kepka (RMK) model [70,71] with survival probability of $|\mathcal {S}|^2 = $ 10%. The hatched band represents the associated theoretical uncertainties. |
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Figure 8-a:
Fraction of color singlet exchange dijet events, $f_\text {CSE}$, measured as a function of $\Delta \eta _\text {jj}$, $p_\text {T,jet-2}$, and $\Delta \phi _\text {jj}$ in pp collisions at $\sqrt {s} = $ 13 TeV. Vertical bars represent statistical uncertainties, while boxes represent the combination of statistical and systematic uncertainties in quadrature. The results are plotted at the mean values of $\Delta \eta _\text {jj}$, $p_\text {T, jet-2}$, and $\Delta \phi _\text {jj}$ in the bin. The results on $f_\text {CSE}$ versus $\Delta \eta _\text {jj}$ and $p_\text {T,jet-2}$ are integrated over $\Delta \phi _\text {jj}$. The solid curve corresponds to theoretical predictions by the Royon, Marquet, Kepka (RMK) model [70,71] with survival probability of $|\mathcal {S}|^2 = $ 10%. The hatched band represents the associated theoretical uncertainties. |
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Figure 8-b:
Fraction of color singlet exchange dijet events, $f_\text {CSE}$, measured as a function of $\Delta \eta _\text {jj}$, $p_\text {T,jet-2}$, and $\Delta \phi _\text {jj}$ in pp collisions at $\sqrt {s} = $ 13 TeV. Vertical bars represent statistical uncertainties, while boxes represent the combination of statistical and systematic uncertainties in quadrature. The results are plotted at the mean values of $\Delta \eta _\text {jj}$, $p_\text {T, jet-2}$, and $\Delta \phi _\text {jj}$ in the bin. The results on $f_\text {CSE}$ versus $\Delta \eta _\text {jj}$ and $p_\text {T,jet-2}$ are integrated over $\Delta \phi _\text {jj}$. The solid curve corresponds to theoretical predictions by the Royon, Marquet, Kepka (RMK) model [70,71] with survival probability of $|\mathcal {S}|^2 = $ 10%. The hatched band represents the associated theoretical uncertainties. |
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Figure 8-c:
Fraction of color singlet exchange dijet events, $f_\text {CSE}$, measured as a function of $\Delta \eta _\text {jj}$, $p_\text {T,jet-2}$, and $\Delta \phi _\text {jj}$ in pp collisions at $\sqrt {s} = $ 13 TeV. Vertical bars represent statistical uncertainties, while boxes represent the combination of statistical and systematic uncertainties in quadrature. The results are plotted at the mean values of $\Delta \eta _\text {jj}$, $p_\text {T, jet-2}$, and $\Delta \phi _\text {jj}$ in the bin. The results on $f_\text {CSE}$ versus $\Delta \eta _\text {jj}$ and $p_\text {T,jet-2}$ are integrated over $\Delta \phi _\text {jj}$. The solid curve corresponds to theoretical predictions by the Royon, Marquet, Kepka (RMK) model [70,71] with survival probability of $|\mathcal {S}|^2 = $ 10%. The hatched band represents the associated theoretical uncertainties. |
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Figure 9:
Fraction of color singlet exchange dijet events, $f_\text {CSE}$, measured as a function of the subleading jet transverse momentum $p_\text {T,jet-2}$ by the D0 and CDF Collaborations [42,43,40] at $\sqrt {s} = $ 0.63 (red symbols) and 1.8 TeV (green symbols), and by the CMS Collaboration [44] at 7 TeV (magenta symbol) and the present results at 13 TeV (blue band). The central gap is defined by means of the particle activity in the fixed pseudorapidity interval $|\eta | < $ 1 in these measurements. |
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Figure 10:
Fraction of color singlet exchange dijet events, $f_\text {CSE}$, measured as a function of $\Delta \eta _\text {jj}$ by CMS at 7 TeV [44] and the present measurement at 13 TeV. The 7 TeV measurement was performed in three bins of the transverse momentum of the subleading jet $p_\text {T, jet-2} = $ 40-60, 60-100, 100-200 GeV, which are represented by the open circle, square, and cross symbols, respectively. |
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Figure 11:
Gap fraction, $f_\text {CSE}$, measured as a function of $\Delta \eta _\text {jj}$ and $p_\text {T, jet-2}$ in inclusive dijet event production (labeled CMS, represented by the circle marker) and in dijet events with a leading proton at 13 TeV (labeled CMS-TOTEM, represented by the cross marker). Vertical bars represent statistical uncertainties, while boxes represent the combination of statistical and systematic uncertainties in quadrature. The CMS-TOTEM results are plotted at the mean values of $\Delta \eta _\text {jj}$ and $p_\text {T, jet-2}$ in the bin. Most of the CMS-TOTEM events considered here have separations 3 $ < \Delta \eta _\text {jj} < $ 6.5, and transverse momenta 40 $ < p_\text {T, jet-2} < $ 100 GeV, as indicated in the figure. |
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Figure 11-a:
Gap fraction, $f_\text {CSE}$, measured as a function of $\Delta \eta _\text {jj}$ and $p_\text {T, jet-2}$ in inclusive dijet event production (labeled CMS, represented by the circle marker) and in dijet events with a leading proton at 13 TeV (labeled CMS-TOTEM, represented by the cross marker). Vertical bars represent statistical uncertainties, while boxes represent the combination of statistical and systematic uncertainties in quadrature. The CMS-TOTEM results are plotted at the mean values of $\Delta \eta _\text {jj}$ and $p_\text {T, jet-2}$ in the bin. Most of the CMS-TOTEM events considered here have separations 3 $ < \Delta \eta _\text {jj} < $ 6.5, and transverse momenta 40 $ < p_\text {T, jet-2} < $ 100 GeV, as indicated in the figure. |
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Figure 11-b:
Gap fraction, $f_\text {CSE}$, measured as a function of $\Delta \eta _\text {jj}$ and $p_\text {T, jet-2}$ in inclusive dijet event production (labeled CMS, represented by the circle marker) and in dijet events with a leading proton at 13 TeV (labeled CMS-TOTEM, represented by the cross marker). Vertical bars represent statistical uncertainties, while boxes represent the combination of statistical and systematic uncertainties in quadrature. The CMS-TOTEM results are plotted at the mean values of $\Delta \eta _\text {jj}$ and $p_\text {T, jet-2}$ in the bin. Most of the CMS-TOTEM events considered here have separations 3 $ < \Delta \eta _\text {jj} < $ 6.5, and transverse momenta 40 $ < p_\text {T, jet-2} < $ 100 GeV, as indicated in the figure. |
| Tables | |
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Table 1:
Relative systematic uncertainties in percentage for the measurements of $f_\text {CSE}$ in jet-gap-jet events and proton-gap-jet-gap-jet events. The jet-gap-jet results summarize the systematic uncertainties found in bins of the kinematic variables of interest $p_\text {T, jet-2}$, $\Delta \eta _\text {jj}$, and $\Delta \phi _\text {jj}$. When an uncertainty range is given, the range of values is representative of the variation found in the jet-gap-jet fraction in bins of the kinematic variables of interest. |
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Table 2:
Measured values of the jet-gap-jet fraction $f_\text {CSE}$ in bins of pseudorapidity difference between the leading two jets $\Delta \eta _\text {jj}$. The first column indicates the $\Delta \eta _\text {jj}$ intervals and the last column represents the measured fraction. The first and second uncertainties correspond to the statistical and systematic components, respectively. The results are for jets satisfying $p_\text {T, jet} > $ 40 GeV and 0 $ < \Delta \phi _\text {jj} < \pi $. The mean values of $\Delta \eta _\text {jj}$ in the bin are given in the middle column. |
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Table 3:
Measured values of the jet-gap-jet fraction $f_\text {CSE}$ in bins of the subleading jet transverse momentum $p_\text {T,jet-2}$. The first column indicates the $p_\text {T,jet-2}$ bin intervals and the last column represents the measured fraction. The first and second uncertainties correspond to the statistical and systematic components, respectively. The results are for jets satisfying 1.4 $ < |\eta _\text {jet}| < $ 4.7 and 0 $ < \Delta \phi _\text {jj} < \pi $. The mean values of $p_\text {T,jet-2}$ in the bin are given in the middle column. |
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Table 4:
Measured values of the jet-gap-jet fraction $f_\text {CSE}$ in bins of azimuthal angle difference between the leading two jets $\Delta \phi _\text {jj}$. The first column indicates the $\Delta \phi _\text {jj}$ bin intervals and the last column represents the measured fraction. The first and second uncertainties correspond to the statistical and systematic components, respectively. The results are for jets satisfying $p_\text {T, jet} > $ 40 GeV and 1.4 $ < |\eta _\text {jet} | < $ 4.7. The mean values of $\Delta \phi _\text {jj}$ in the bin are given in the middle column. |
| Summary |
|
Events with two leading jets separated by a large pseudorapidity gap have been studied in pp collisions at $\sqrt{s}=$ 13 TeV with the CMS detector. The "gap'' is determined by the absence of charged particles with ${p_{\mathrm{T}}} > $ 200 MeV in the pseudorapidity range $|\eta| < $ 1 produced in the collision. Each of the two leading jets has pseudorapidity values of 1.4 $ < |\eta_\text{jet}| < $ 4.7 and transverse momentum of $p_\text{T, jet} > $ 40 GeV, with $\eta_\text{jet-1} \times \eta_\text{jet-2} < $ 0. The pseudorapidity gap signature is indicative of an underlying hard color singlet exchange, which is described in terms of two-gluon exchange in perturbative quantum chromodynamics. Jet-gap-jet events appear as an excess of events over the expected charged particle multiplicity of color exchange dijet events at the lowest charged particle multiplicity counts. The fraction of jet-gap-jet events to events where the two jets have similar kinematics, $f_\text{CSE}$, has been measured as a function of the pseudorapidity difference between the leading two jets, $\Delta\eta_\text{jj} \equiv |\eta_\text{jet-1} - \eta_\text{jet-2}|$, the transverse momentum of the subleading jet, $p_\text{T, jet-2}$, and the azimuthal angle separation between the leading two jets, $\Delta\phi_\text{jj} \equiv |\phi_\text{jet-1} - \phi_\text{jet-2}|$. The fraction $f_\text{CSE}$ has values of 0.6-1%. It increases with $\Delta\eta_\text{jj}$, is only weakly dependent on $p_\text{T, jet-2}$, and increases as $\Delta\phi_\text{jj}$ approaches $\pi$. No significant difference in $f_\text{CSE}$ is observed comparing the present results at 13 TeV with those presented by the CMS Collaboration at 7 TeV. This is in contrast to the trend found at lower collision energies of 0.63 and 1.8 TeV by the D0 and CDF Collaborations, where a significant decrease with increasing energy was observed. The results are compared with calculations based on the Balitsky-Fadin-Kuraev-Lipatov framework with resummation of large logarithms of energy at next-to-leading logarithm accuracy, leading order impact factors, and a constant survival probability factor. The implementation by Royon, Marquet, and Kepka describes some features of the data, but is not able to simultaneously describe all aspects of the measurement. The present disagreement between theory and data provides guidance for further improvements on the perturbative and nonperturbative treatment for pseudorapidity gap formation and destruction mechanisms. Complementary to the jet-gap-jet study, a sample of dijet events with leading protons collected by the CMS and TOTEM experiments in 2015 is used to study jet-gap-jet events with leading protons, which correspond to proton-gap-jet-gap-jet topologies. This is the first study of this diffractive event topology. The gap fraction extracted in this sample is found to be 2.91 $\pm$ 0.70 (stat) $^{+ 1.02}_{- 0.94}$ (syst) times larger than that found in inclusive dijet production, pointing to a larger abundance of jets with a central gap in events with leading protons. This can be interpreted in terms of a lower spectator parton activity in events with leading protons, which decreases the likelihood of the central gap signature being spoiled. |
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