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| CMS-FSQ-12-001 ; CERN-EP-2017-141 | ||
| Study of dijet events with a large rapidity gap between the two leading jets in pp collisions at $\sqrt{s}= $ 7 TeV | ||
| CMS Collaboration | ||
| 2 October 2017 | ||
| Eur. Phys. J. C 78 (2018) 242 [Erratum] | ||
|
Abstract:
Events with no charged particles produced between the two leading jets are studied in proton-proton collisions at $\sqrt{s}= $ 7 TeV. The jets were required to have transverse momentum ${p_{\mathrm{T}}}^{\text{jet}} > $ 40 GeV and pseudorapidity 1.5 $ < |{\eta^{\text{jet}}}| < $ 4.7, and to have values of $\eta^{\text{jet}}$ with opposite signs. The data used for this study were collected with the CMS detector during low-luminosity running at the LHC, and correspond to an integrated luminosity of 8 pb$^{-1}$. Events with no charged particles with ${p_{\mathrm{T}}} > $ 0.2 GeV in the interval $-1 < \eta < 1$ between the jets are observed in excess of calculations that assume no color-singlet exchange. The fraction of events with such a rapidity gap, amounting to 0.5-1% of the selected dijet sample, is measured as a function of the ${p_{\mathrm{T}}}$ of the second-leading jet and of the rapidity separation between the jets. The data are compared to previous measurements at Tevatron, and to perturbative quantum chromodynamics calculations based on the Balitsky-Fadin-Kuraev-Lipatov evolution equations, including different modelings of the non-perturbative gap survival probability. We dedicate this paper to the memory of our colleague and friend Sasha Proskuryakov, who started this analysis but passed away before it was completed. His contribution to the study of diffractive processes at CMS is invaluable. | ||
| Links: e-print arXiv:1710.02586 [hep-ex] (PDF) ; CDS record ; inSPIRE record ; HepData record ; CADI line (restricted) ; | ||
| Figures | |
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Figure 1:
Schematic diagram of a dijet event with a rapidity gap between the jets (jet-gap-jet event). The gap is defined as the absence of charged particle tracks above a certain $ {p_{\mathrm {T}}} $ threshold. |
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Figure 2:
Schematic picture of a jet-gap-jet event in the $\varphi $ vs. $\eta $ plane. The circles indicate the two jets reconstructed on each side of the detector, while the dots represent the remaining hadronic activity in the event. The shaded area corresponds to the region of the potential rapidity gap, in which the charged-particle multiplicity is measured (the so-called gap region). |
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Figure 3:
Distribution, uncorrected for detector effects, of the number of central tracks between the two leading jets in events with $ {p_{\mathrm {T}}} ^\text {jet2} = $ 40-60 (top left), 60-100 (top right), and 100-200 (bottom) GeV, compared to predictions of PYTHIA 6 (inclusive dijets) and HERWIG 6 (CSE jet-gap-jet events). The PYTHIA 6 and HERWIG 6 samples are normalized to the number of events measured for $N_\text {tracks} > $ 3 and $N_\text {tracks}= $ 0, respectively. Beneath each plot the ratio of the data yield to the sum of the normalized HERWIG 6 and PYTHIA 6 predictions is shown. The vertical error bars indicate the statistical uncertainty. |
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Figure 3-a:
Distribution, uncorrected for detector effects, of the number of central tracks between the two leading jets in events with $ {p_{\mathrm {T}}} ^\text {jet2} = $ 40-60 (top left), 60-100 (top right), and 100-200 (bottom) GeV, compared to predictions of PYTHIA 6 (inclusive dijets) and HERWIG 6 (CSE jet-gap-jet events). The PYTHIA 6 and HERWIG 6 samples are normalized to the number of events measured for $N_\text {tracks} > $ 3 and $N_\text {tracks}= $ 0, respectively. Beneath each plot the ratio of the data yield to the sum of the normalized HERWIG 6 and PYTHIA 6 predictions is shown. The vertical error bars indicate the statistical uncertainty. |
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Figure 3-b:
Distribution, uncorrected for detector effects, of the number of central tracks between the two leading jets in events with $ {p_{\mathrm {T}}} ^\text {jet2} = $ 40-60 (top left), 60-100 (top right), and 100-200 (bottom) GeV, compared to predictions of PYTHIA 6 (inclusive dijets) and HERWIG 6 (CSE jet-gap-jet events). The PYTHIA 6 and HERWIG 6 samples are normalized to the number of events measured for $N_\text {tracks} > $ 3 and $N_\text {tracks}= $ 0, respectively. Beneath each plot the ratio of the data yield to the sum of the normalized HERWIG 6 and PYTHIA 6 predictions is shown. The vertical error bars indicate the statistical uncertainty. |
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Figure 4:
Transverse momentum distributions, uncorrected for detector effects, of the leading jet (left) and the second-leading jet (right) in three dijet samples with $ {p_{\mathrm {T}}} ^\text {jet2} = $ 40-60, 60-100, and 100-200 GeV (from top to bottom) after all selections, for events with no tracks reconstructed in the gap region $ {< \eta >} < $ 1, compared to predictions of PYTHIA 6 (inclusive dijets) and HERWIG 6 (CSE jet-gap-jet events), normalized as in Fig. xxxxx. The error bars indicate the statistical uncertainty. |
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Figure 4-a:
Transverse momentum distributions, uncorrected for detector effects, of the leading jet (left) and the second-leading jet (right) in three dijet samples with $ {p_{\mathrm {T}}} ^\text {jet2} = $ 40-60, 60-100, and 100-200 GeV (from top to bottom) after all selections, for events with no tracks reconstructed in the gap region $ {< \eta >} < $ 1, compared to predictions of PYTHIA 6 (inclusive dijets) and HERWIG 6 (CSE jet-gap-jet events), normalized as in Fig. xxxxx. The error bars indicate the statistical uncertainty. |
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Figure 4-b:
Transverse momentum distributions, uncorrected for detector effects, of the leading jet (left) and the second-leading jet (right) in three dijet samples with $ {p_{\mathrm {T}}} ^\text {jet2} = $ 40-60, 60-100, and 100-200 GeV (from top to bottom) after all selections, for events with no tracks reconstructed in the gap region $ {< \eta >} < $ 1, compared to predictions of PYTHIA 6 (inclusive dijets) and HERWIG 6 (CSE jet-gap-jet events), normalized as in Fig. xxxxx. The error bars indicate the statistical uncertainty. |
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Figure 4-c:
Transverse momentum distributions, uncorrected for detector effects, of the leading jet (left) and the second-leading jet (right) in three dijet samples with $ {p_{\mathrm {T}}} ^\text {jet2} = $ 40-60, 60-100, and 100-200 GeV (from top to bottom) after all selections, for events with no tracks reconstructed in the gap region $ {< \eta >} < $ 1, compared to predictions of PYTHIA 6 (inclusive dijets) and HERWIG 6 (CSE jet-gap-jet events), normalized as in Fig. xxxxx. The error bars indicate the statistical uncertainty. |
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Figure 4-d:
Transverse momentum distributions, uncorrected for detector effects, of the leading jet (left) and the second-leading jet (right) in three dijet samples with $ {p_{\mathrm {T}}} ^\text {jet2} = $ 40-60, 60-100, and 100-200 GeV (from top to bottom) after all selections, for events with no tracks reconstructed in the gap region $ {< \eta >} < $ 1, compared to predictions of PYTHIA 6 (inclusive dijets) and HERWIG 6 (CSE jet-gap-jet events), normalized as in Fig. xxxxx. The error bars indicate the statistical uncertainty. |
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Figure 4-e:
Transverse momentum distributions, uncorrected for detector effects, of the leading jet (left) and the second-leading jet (right) in three dijet samples with $ {p_{\mathrm {T}}} ^\text {jet2} = $ 40-60, 60-100, and 100-200 GeV (from top to bottom) after all selections, for events with no tracks reconstructed in the gap region $ {< \eta >} < $ 1, compared to predictions of PYTHIA 6 (inclusive dijets) and HERWIG 6 (CSE jet-gap-jet events), normalized as in Fig. xxxxx. The error bars indicate the statistical uncertainty. |
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Figure 4-f:
Transverse momentum distributions, uncorrected for detector effects, of the leading jet (left) and the second-leading jet (right) in three dijet samples with $ {p_{\mathrm {T}}} ^\text {jet2} = $ 40-60, 60-100, and 100-200 GeV (from top to bottom) after all selections, for events with no tracks reconstructed in the gap region $ {< \eta >} < $ 1, compared to predictions of PYTHIA 6 (inclusive dijets) and HERWIG 6 (CSE jet-gap-jet events), normalized as in Fig. xxxxx. The error bars indicate the statistical uncertainty. |
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Figure 5:
Distributions, uncorrected for detector effects, of the azimuthal angle $\Delta \varphi ^\mathrm {jet1,2}$ between the two leading jets (left) and the ratio $ {p_{\mathrm {T}}} ^\text {jet2}/ {p_{\mathrm {T}}} ^\mathrm {jet1}$ of the second-leading jet $ {p_{\mathrm {T}}} $ to the leading jet $ {p_{\mathrm {T}}} $ (right) for events after all selections, with no tracks ($N_\text {tracks} = $ 0, full circles) or more than three tracks ($N_\text {tracks} > $ 3, open circles) reconstructed in the $ {< \eta >} < 1$ region, compared with the MC predictions. The distributions are summed over the three $p^\text {jet2}_\mathrm {T}$ bins used in the analysis and normalized to unity for shape comparison. |
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Figure 6:
Distribution, uncorrected for detector effects, of the number of central tracks in opposite-side (OS) dijet events (black circles) with $ {p_{\mathrm {T}}} ^\text {jet2} = $ 40-60 (top), 60-100 (middle), and 100-200 GeV (bottom), plotted (left) together with the $N_\text {tracks}$ distribution of same-side (SS) dijet events (blue circles), and fitted to a NBD function (right). |
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Figure 6-a:
Distribution, uncorrected for detector effects, of the number of central tracks in opposite-side (OS) dijet events (black circles) with $ {p_{\mathrm {T}}} ^\text {jet2} = $ 40-60 (top), 60-100 (middle), and 100-200 GeV (bottom), plotted (left) together with the $N_\text {tracks}$ distribution of same-side (SS) dijet events (blue circles), and fitted to a NBD function (right). |
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Figure 6-b:
Distribution, uncorrected for detector effects, of the number of central tracks in opposite-side (OS) dijet events (black circles) with $ {p_{\mathrm {T}}} ^\text {jet2} = $ 40-60 (top), 60-100 (middle), and 100-200 GeV (bottom), plotted (left) together with the $N_\text {tracks}$ distribution of same-side (SS) dijet events (blue circles), and fitted to a NBD function (right). |
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Figure 6-c:
Distribution, uncorrected for detector effects, of the number of central tracks in opposite-side (OS) dijet events (black circles) with $ {p_{\mathrm {T}}} ^\text {jet2} = $ 40-60 (top), 60-100 (middle), and 100-200 GeV (bottom), plotted (left) together with the $N_\text {tracks}$ distribution of same-side (SS) dijet events (blue circles), and fitted to a NBD function (right). |
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Figure 6-d:
Distribution, uncorrected for detector effects, of the number of central tracks in opposite-side (OS) dijet events (black circles) with $ {p_{\mathrm {T}}} ^\text {jet2} = $ 40-60 (top), 60-100 (middle), and 100-200 GeV (bottom), plotted (left) together with the $N_\text {tracks}$ distribution of same-side (SS) dijet events (blue circles), and fitted to a NBD function (right). |
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Figure 6-e:
Distribution, uncorrected for detector effects, of the number of central tracks in opposite-side (OS) dijet events (black circles) with $ {p_{\mathrm {T}}} ^\text {jet2} = $ 40-60 (top), 60-100 (middle), and 100-200 GeV (bottom), plotted (left) together with the $N_\text {tracks}$ distribution of same-side (SS) dijet events (blue circles), and fitted to a NBD function (right). |
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Figure 6-f:
Distribution, uncorrected for detector effects, of the number of central tracks in opposite-side (OS) dijet events (black circles) with $ {p_{\mathrm {T}}} ^\text {jet2} = $ 40-60 (top), 60-100 (middle), and 100-200 GeV (bottom), plotted (left) together with the $N_\text {tracks}$ distribution of same-side (SS) dijet events (blue circles), and fitted to a NBD function (right). |
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Figure 7:
Background-subtracted central track multiplicity distributions, uncorrected for detector effects, in the three bins of $ {p_{\mathrm {T}}} ^\text {jet2}$, compared to the HERWIG 6 predictions without underlying event simulation ("no MPI"), normalized as in Fig. xxxxx. The background is estimated from the NBD fit to the data in the 3 $ \le N_\text {tracks} \le $ 35 range, extrapolated to the lowest multiplicity bins. |
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Figure 7-a:
Background-subtracted central track multiplicity distributions, uncorrected for detector effects, in the three bins of $ {p_{\mathrm {T}}} ^\text {jet2}$, compared to the HERWIG 6 predictions without underlying event simulation ("no MPI"), normalized as in Fig. xxxxx. The background is estimated from the NBD fit to the data in the 3 $ \le N_\text {tracks} \le $ 35 range, extrapolated to the lowest multiplicity bins. |
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Figure 7-b:
Background-subtracted central track multiplicity distributions, uncorrected for detector effects, in the three bins of $ {p_{\mathrm {T}}} ^\text {jet2}$, compared to the HERWIG 6 predictions without underlying event simulation ("no MPI"), normalized as in Fig. xxxxx. The background is estimated from the NBD fit to the data in the 3 $ \le N_\text {tracks} \le $ 35 range, extrapolated to the lowest multiplicity bins. |
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Figure 7-c:
Background-subtracted central track multiplicity distributions, uncorrected for detector effects, in the three bins of $ {p_{\mathrm {T}}} ^\text {jet2}$, compared to the HERWIG 6 predictions without underlying event simulation ("no MPI"), normalized as in Fig. xxxxx. The background is estimated from the NBD fit to the data in the 3 $ \le N_\text {tracks} \le $ 35 range, extrapolated to the lowest multiplicity bins. |
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Figure 8:
Fraction of dijet events with a central gap ($f_\mathrm {CSE}$) as a function of $ {p_{\mathrm {T}}} ^\text {jet2}$ at $\sqrt {s}=7$ TeV, compared to similar D0 [27] and CDF [29] results at $\sqrt {s}= $ 1.8 TeV. The details of the jet selections are given in the legend. The results are plotted at the mean value of $ {p_{\mathrm {T}}} ^\text {jet2}$ in the bin. The inner and outer error bars represent the statistical, and the statistical and systematic uncertainties added in quadrature, respectively. |
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Figure 9:
Fraction of dijet events with a central gap ($f_\mathrm {CSE}$) as a function of $ {p_{\mathrm {T}}} ^\text {jet2}$ at $\sqrt {s}= $ 7 TeV, compared to the predictions of the Mueller and Tang (MT) model [21], and of the Ekstedt, Enberg, and Ingelman (EEI) model [22,23] with three different treatments of the gap survival probability factor $ {< S >}^2$, as described in the text. The results are plotted at the mean value of $ {p_{\mathrm {T}}} ^\text {jet2}$ in the bin. The inner and outer error bars represent the statistical, and the statistical and systematic uncertainties added in quadrature, respectively. |
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Figure 10:
Fraction of dijet events with a central gap ($f_\mathrm {CSE}$) as a function of $\Delta \eta _\mathrm {jj}$ at $\sqrt {s}= $ 7 TeV in three different $ {p_{\mathrm {T}}} ^\text {jet2}$ ranges, compared to the predictions of the Mueller and Tang (MT) model [21], and of the Ekstedt, Enberg, and Ingelman (EEI) model [22,23] with three different treatments of the gap survival probability factor $ {< S >}^2$, as described in the text. The results are plotted at the mean value of $\Delta \eta _\mathrm {jj}$ in the bin. Inner and outer error bars correspond to the statistical, and the statistical and systematic uncertainties added in quadrature, respectively. |
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Figure 10-a:
Fraction of dijet events with a central gap ($f_\mathrm {CSE}$) as a function of $\Delta \eta _\mathrm {jj}$ at $\sqrt {s}= $ 7 TeV in three different $ {p_{\mathrm {T}}} ^\text {jet2}$ ranges, compared to the predictions of the Mueller and Tang (MT) model [21], and of the Ekstedt, Enberg, and Ingelman (EEI) model [22,23] with three different treatments of the gap survival probability factor $ {< S >}^2$, as described in the text. The results are plotted at the mean value of $\Delta \eta _\mathrm {jj}$ in the bin. Inner and outer error bars correspond to the statistical, and the statistical and systematic uncertainties added in quadrature, respectively. |
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Figure 10-b:
Fraction of dijet events with a central gap ($f_\mathrm {CSE}$) as a function of $\Delta \eta _\mathrm {jj}$ at $\sqrt {s}= $ 7 TeV in three different $ {p_{\mathrm {T}}} ^\text {jet2}$ ranges, compared to the predictions of the Mueller and Tang (MT) model [21], and of the Ekstedt, Enberg, and Ingelman (EEI) model [22,23] with three different treatments of the gap survival probability factor $ {< S >}^2$, as described in the text. The results are plotted at the mean value of $\Delta \eta _\mathrm {jj}$ in the bin. Inner and outer error bars correspond to the statistical, and the statistical and systematic uncertainties added in quadrature, respectively. |
png pdf |
Figure 10-c:
Fraction of dijet events with a central gap ($f_\mathrm {CSE}$) as a function of $\Delta \eta _\mathrm {jj}$ at $\sqrt {s}= $ 7 TeV in three different $ {p_{\mathrm {T}}} ^\text {jet2}$ ranges, compared to the predictions of the Mueller and Tang (MT) model [21], and of the Ekstedt, Enberg, and Ingelman (EEI) model [22,23] with three different treatments of the gap survival probability factor $ {< S >}^2$, as described in the text. The results are plotted at the mean value of $\Delta \eta _\mathrm {jj}$ in the bin. Inner and outer error bars correspond to the statistical, and the statistical and systematic uncertainties added in quadrature, respectively. |
| Tables | |
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Table 1:
Percent systematic (individual, and total) and statistical uncertainties in the measurement of the CSE fraction in the three bins of $ {p_{\mathrm {T}}} ^\text {jet2}$. |
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Table 2:
Measured values of $f_\mathrm {CSE}$ as a function of $ {p_{\mathrm {T}}} ^\text {jet2}$. The first and second (asymmetric) uncertainties correspond to the statistical and systematic components, respectively. The mean values of $ {p_{\mathrm {T}}} ^\text {jet2}$ in the bin are also given. |
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Table 3:
Measured values of the fraction of dijet events with a central gap ($f_\mathrm {CSE}$) as a function of the pseudorapidity separation between the jets ($\Delta \eta _\mathrm {jj}$) in bins of $ {p_{\mathrm {T}}} ^\text {jet2}$. The columns in the table correspond to $ {p_{\mathrm {T}}} ^\text {jet2}$ bins and the rows to $\Delta \eta _\mathrm {jj}$ bins. The first and second (asymmetric) errors correspond to the statistical and systematic uncertainties, respectively. The mean values of $\Delta \eta _\mathrm {jj}$ in the bin are also given. |
| Summary |
|
Events with a large rapidity gap between the two leading jets have been measured for the first time at the LHC, for jets with transverse momentum ${p_{\mathrm{T}}}^{\text{jet}} > $ 40 GeV and pseudorapidity 1.5 $ < |{\eta^{\text{jet}}}| < $ 4.7, reconstructed in opposite ends of the detector. The number of dijet events with no particles with ${p_{\mathrm{T}}} > $ 0.2 GeV in the region $|{\eta}| < $ 1 is severely underestimated by PYTHIA6 (tune Z2*). HERWIG6 predictions, which include a contribution from color singlet exchange (CSE), based on the leading logarithmic Balitsky-Fadin-Kuraev-Lipatov (BFKL) evolution equations, are needed to reproduce the type of dijet topologies selected in our analysis. The fraction of selected dijet events with such a rapidity gap has been measured as a function of the second-leading jet transverse momentum (${p_{\mathrm{T}}}^\text{jet2}$) and as a function of the size of the pseudorapidity interval between the jets, $\Delta\eta_\mathrm{jj}$. The $f_\mathrm{CSE}$ fraction rises with ${p_{\mathrm{T}}}^\text{jet2}$ (from 0.6 to 1%) and with $\Delta\eta_\mathrm{jj}$ (from 0.3 to 1.2% for 40 $ < {p_{\mathrm{T}}}^\text{jet2} < $ 60 GeV, from 0.5 to 0.9% for 60 $ < {p_{\mathrm{T}}}^\text{jet2} < $ 100 GeV, and from 0.8 to 2% for 100 $ < {p_{\mathrm{T}}}^\text{jet2} < $ 200 GeV). The measured CSE fractions have been compared to the results of the D0 and CDF experiments at a center-of-mass energy of 1.8 TeV. A factor of two decrease of the CSE fraction measured at $\sqrt{s} = $ 7 TeV with respect to those at lower collision energies is observed. Such a behavior is consistent with the decrease seen in the Tevatron data when $\sqrt{s}$ rises from 0.63 to 1.8 TeV, and with theoretical expectations for the $\sqrt{s}$ dependence of the rapidity gap survival probability. The data are also compared to theoretical perturbative quantum chromodynamics calculations based on the BFKL evolution equations complemented with different estimates of the non-perturbative gap survival probability. The next-to-leading-logarithmic BFKL calculations of Ekstedt, Enberg and Ingelman, with three different implementations of the soft rescattering processes, describe many features of the data, but none of the implementations is able to simultaneously describe all the features of the measurement. |
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