CMS-PAS-HIN-18-019 | ||

First measurement of the forward rapidity gap distribution in pPb collisions at ${\sqrt {\smash [b]{s_{_{\mathrm {NN}}}}}}= $ 8.16 TeV | ||

CMS Collaboration | ||

June 2020 | ||

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Abstract:
We present, for the first time at LHC energies, the forward rapidity gap spectra from proton-lead collisions for both pomeron-lead and pomeron-proton topologies. The analysis is performed over 10.4 units of pseudorapidity at a center-of-mass energy of ${\sqrt {\smash [b]{s_{_{\mathrm {NN}}}}}}= $ 8.16 TeV, i.e. almost 300 times higher than previous measurements of diffractive production in proton-nucleus collisions. For the pomeron-lead topology the EPOS-LHC predictions are a factor of two below the unfolded data but the model does give a reasonable description of the shape of the spectrum. For the pomeron-proton topology the EPOS-LHC, QGSJET II and HIJING generator predictions are all at least a factor of five below the data. This effect can be explained by a significant contribution of ultra-peripheral photoproduction events mimicking the signature of diffractive processes. The obtained data may be of significant help in understanding the high energy limit of QCD and modeling cosmic ray air showers.
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Links:
CDS record (PDF) ;
inSPIRE record ;
CADI line (restricted) ;
These preliminary results are superseded in this paper, Submitted to PRD.The superseded preliminary plots can be found here. |

Figures | |

png pdf |
Figure 1:
Topologies of pPb events with large rapidity gaps for $\mathbb{P}$Pb (left) and $\mathbb{P}$p or ${\gamma}$p (right). The blue and red cones indicate the products of diffractive dissociation for the lead ion and proton respectively. The regions free of final state particles are marked with green arrows. It is possible for ${\gamma}$Pb interactions to mimic the topology on the left but these are much suppressed compared to the ${\gamma}$p case. |

png pdf |
Figure 1-a:
Topologies of pPb events with large rapidity gaps for $\mathbb{P}$Pb (left) and $\mathbb{P}$p or ${\gamma}$p (right). The blue and red cones indicate the products of diffractive dissociation for the lead ion and proton respectively. The regions free of final state particles are marked with green arrows. It is possible for ${\gamma}$Pb interactions to mimic the topology on the left but these are much suppressed compared to the ${\gamma}$p case. |

png pdf |
Figure 1-b:
Topologies of pPb events with large rapidity gaps for $\mathbb{P}$Pb (left) and $\mathbb{P}$p or ${\gamma}$p (right). The blue and red cones indicate the products of diffractive dissociation for the lead ion and proton respectively. The regions free of final state particles are marked with green arrows. It is possible for ${\gamma}$Pb interactions to mimic the topology on the left but these are much suppressed compared to the ${\gamma}$p case. |

png pdf |
Figure 1-c:
Topologies of pPb events with large rapidity gaps for $\mathbb{P}$Pb (left) and $\mathbb{P}$p or ${\gamma}$p (right). The blue and red cones indicate the products of diffractive dissociation for the lead ion and proton respectively. The regions free of final state particles are marked with green arrows. It is possible for ${\gamma}$Pb interactions to mimic the topology on the left but these are much suppressed compared to the ${\gamma}$p case. |

png pdf |
Figure 1-d:
Topologies of pPb events with large rapidity gaps for $\mathbb{P}$Pb (left) and $\mathbb{P}$p or ${\gamma}$p (right). The blue and red cones indicate the products of diffractive dissociation for the lead ion and proton respectively. The regions free of final state particles are marked with green arrows. It is possible for ${\gamma}$Pb interactions to mimic the topology on the left but these are much suppressed compared to the ${\gamma}$p case. |

png pdf |
Figure 2:
Reconstruction level $\mathrm{d}\sigma/\mathrm{d}\Delta \eta^{F}$ spectra for events with $\mathbb{P}$Pb (left) and $\mathbb{P}$p+${\gamma}$p (right) topologies where only information within $ |\eta| < $ 3 is used. Also shown are the predictions of EPOS-LHC (blue) and HIJING (green). For the $\mathbb{P}$Pb case (left) the ${\Delta \eta ^F}$ is measured from $\eta = $ 3, while for the $\mathbb{P}$p+${\gamma}$p case (right) ${\Delta \eta ^F}$ is measured from $\eta = -$3 for the pPb data sample. The statistical and systematic errors are added in quadrature. The Monte Carlo spectra are normalized to the total visible cross section of the data. The bottom panels show the ratio of Monte Carlo predictions to data. |

png pdf |
Figure 3:
The number of high purity tracks (left), their ${p_{\mathrm {T}}}$ distributions (middle) and the total energy of all PF candidates (right) in the first $\eta $ bin after a gap of ${\Delta \eta ^F} = $ 4.5 for events with the $\mathbb{P}$p+${\gamma}$p topology. Also shown are the corresponding distributions for the EPOS-LHC and HIJING generators. |

png pdf |
Figure 4:
Unfolded diffraction enhanced $\mathrm{d}\sigma/\mathrm{d}\Delta \eta^{F}$ spectra compared to hadron level predictions of the EPOS-LHC, HIJING and QGSJET-II generators. The data are corrected for the contribution from events with undetectable energy in the HF calorimeter adjacent to the rapidity gap. The corrections are obtained using the EPOS-LHC MC samples. For the pPb data sample, in the $\mathbb{P}$Pb case (left) the rapidity gap, ${\Delta \eta ^F}$, is measured from $\eta = $ 3 and no particles are present within 3 $ < \eta < $ 5.19, while for the $\mathbb{P}$p+${\gamma}$p case (right) the rapidity gap is measured from $\eta = -$3 and no particles are present within $-$5.19 $< \eta < -$3. The statistical and systematic uncertainties are added in quadrature. The gray band shows the resulting uncertainty excluding the error introduced with the correction for the undetectable energy in the HF calorimeter, while the yellow band accounts for all uncertainty sources. The bottom panels show the ratio of the three generators to data. |

png pdf |
Figure 5:
Reconstruction level $\mathrm{d}\sigma/\mathrm{d}\Delta \eta^{F}$ spectra obtained for the central acceptance, $ |\eta| < $ 3, for the $\mathbb{P}$Pb (left) and $\mathbb{P}$p+${\gamma}$p (right) topologies and compared to the corresponding EPOS-LHC predictions. The EPOS-LHC predictions are broken down into the non-diffractive (ND) in red, central diffractive (CD) in green, single diffractive (SD) in yellow and double diffractive (DD) in purple components, shown as stacked contributions. |

png pdf |
Figure 6:
Unfolded diffractive enhanced $\mathrm{d}\sigma/\mathrm{d}\Delta \eta^{F}$ spectra for the $\mathbb{P}$Pb (left) and $\mathbb{P}$p+${\gamma}$p (right) topologies compared to the EPOS-LHC predictions. The EPOS-LHC predictions are broken down into the non-diffractive (ND) in red, central diffractive (CD) in green, single diffractive (SD) in yellow and double diffractive (DD) in purple components, shown as stacked contributions. |

png pdf |
Figure 7:
Unfolded diffractive enhanced $\mathrm{d}\sigma/\mathrm{d}\Delta \eta^{F}$ spectra for the $\mathbb{P}$Pb (left) and $\mathbb{P}$p+${\gamma}$p (right) topologies compared to the QGSJET-II predictions. The QGSJET-II predictions are broken down into the non-diffractive (ND) in red, central diffractive (CD) in green, single diffractive (SD) in yellow and double diffractive (DD) in purple components, shown as stacked contributions. |

png pdf |
Figure 8:
Top: Reconstruction level diffraction enhanced $\mathrm{d}\sigma/\mathrm{d}\Delta \eta^{F}$ spectrum corrected for the contribution from events with undetectable energy in the HF calorimeter adjacent to the rapidity gap. The statistical and systematic uncertainties are added in quadrature. The gray band shows the resulting uncertainty excluding the error introduced with the correction for the undetectable energy in the HF calorimeter, while the yellow band accounts for all uncertainty sources. The distribution is shown together with the spectrum obtained with events satisfying the ZDC veto requirement $E_{\mathrm {ZDC-}} < $ 1 TeV which selects only the events without lead nuclear break up. No correction for HF undetectable energy is applied to this distribution. The statistical and systematic uncertainties are added in quadrature. Bottom: A fraction of events selected with the ZDC veto requirement as a function of the rapidity gap size. |

Summary |

For the first time, forward rapidity gap spectra $\mathrm{d}\sigma/\mathrm{d}\Delta \eta^{F}$ from proton-lead collisions at the energy of ${\sqrt {\smash [b]{s_{_{\mathrm {NN}}}}}} = $ 8.16 TeV have been measured for both pomeron-lead and pomeron-proton topologies. For the $\mathbb{P}$Pb topology, where the photon-exchange contribution is expected to be negligible, EPOS-LHC is about a factor of two and qgsjet ii a factor of 4 below the data. However for both of these generators the shape of the $\mathrm{d}\sigma/\mathrm{d}\Delta \eta^{F}$ spectrum is similar to that of the data. The spectrum from the HIJING generator falls rapidity at large ${\Delta\eta^F}$ in contradiction to the data. For the $\mathbb{P}$+$\gamma$p topology all the generators are more than a factor of 5 below the data. This suggests a very strong contribution from $\gamma$p interactions which are not yet present in these event generators. These data may be of significant help in understanding the high energy limit of QCD and modelling cosmic ray air showers. |

References | ||||

1 |
V. N. Gribov | Possible Asymptotic Behavior of Elastic Scattering | JEPTL 41 (1961) 667 | |

2 |
G. F. Chew and S. C. Frautschi | Principle of Equivalence for All Strongly Interacting Particles Within the S Matrix Framework | PRL 7 (1961) 394 | |

3 |
F. E. Low | A Model of the Bare Pomeron | PRD 12 (1975) 163 | |

4 |
S. Nussinov | Colored Quark Version of Some Hadronic Puzzles | PRL 34 (1975) 1286 | |

5 |
V. S. Fadin, E. A. Kuraev, and L. N. Lipatov | On the Pomeranchuk Singularity in Asymptotically Free Theories | PLB 60 (1975) 50 | |

6 |
ATLAS Collaboration |
Rapidity gap cross sections measured with the ATLAS detector in $ pp $ collisions at $ \sqrt{s}= $ 7 TeV | EPJC 72 (2012) 1926 | 1201.2808 |

7 |
CMS Collaboration |
Measurement of diffraction dissociation cross sections in pp collisions at $ \sqrt{s} = $ 7 TeV | PRD 92 (2015) 012003 | CMS-FSQ-12-0051503.08689 |

8 |
TOTEM Collaboration | First measurement of the total proton-proton cross section at the LHC energy of $ \sqrt{s} = $ 7 TeV | EPL 96 (2011), no. 2, 21002 | 1110.1395 |

9 |
TOTEM Collaboration | Measurement of proton-proton elastic scattering and total cross-section at S**(1/2) = 7-TeV | EPL 101 (2013) 21002 | |

10 |
TOTEM Collaboration | First measurement of elastic, inelastic and total cross-section at $ \sqrt{s}= $ 13 TeV by TOTEM and overview of cross-section data at LHC energies | EPJC 79 (2019) 103 | 1712.06153 |

11 |
A. B. Kaidalov | Diffractive Production Mechanisms | PR 50 (1979) 157 | |

12 |
A. Donnachie and P. V. Landshoff | Elastic Scattering and Diffraction Dissociation | NPB 244 (1984) 322 | |

13 |
LHC Forward Physics Working Group Collaboration | LHC Forward Physics | JPG 43 (2016) 110201 | 1611.05079 |

14 |
V. N. Gribov | Glauber corrections and the interaction between high-energy hadrons and nuclei | Sov. Phys. JETP 29 (1969) 483 | |

15 |
B. Z. Kopeliovich, L. I. Lapidus, and A. B. Zamolodchikov | Dynamics of Color in Hadron Diffraction on Nuclei | JEPTL 33 (1981) 595.[Pisma Zh. Eksp. Teor. Fiz.33,612(1981)] | |

16 |
HELIOS Collaboration | Diffraction dissociation of nuclei in 450-GeV/c proton - nucleus collisions | Z. Phys. C 49 (1991) 355 | |

17 |
EHS/NA22 Collaboration | Reactions with leading hadrons in meson - proton interactions at 250-GeV/c | Z. Phys. C 75 (1997) 229 | |

18 |
A. B. Kaidalov, V. A. Khoze, A. D. Martin, and M. G. Ryskin | Diffraction of protons and nuclei at high-energies | Acta Phys. Polon. B 34 (2003) 3163 | hep-ph/0303111 |

19 |
B. Z. Kopeliovich, I. K. Potashnikova, and I. Schmidt | Large rapidity gap processes in proton-nucleus collisions | PRC 73 (Mar, 2006) 034901 | hep-ph/0508277 |

20 |
L. Frankfurt and M. Strikman | Novel QCD phenomena in pA collisions at LHC | in 2nd Workshop on Hard Probes in Heavy Ion Collisions at the LHC: 2nd Plenary Meeting, Geneva 2002 | hep-ph/0210088 |

21 |
V. Guzey and M. Strikman | Proton-nucleus scattering and cross section fluctuations at RHIC and LHC | PLB 633 (2006) 245 | hep-ph/0505088 |

22 |
R. Luna, A. Zepeda, C. A. Garcia Canal, and S. J. Sciutto | Influence of diffractive interactions on cosmic ray air showers | PRD 70 (2004) 114034 | hep-ph/0408303 |

23 |
X.-N. Wang and M. Gyulassy | HIJING: A Monte Carlo model for multiple jet production in pp, pA and AA collisions | PRD 44 (1991) 3501 | |

24 |
T. Pierog et al. | EPOS LHC: Test of collective hadronization with data measured at the CERN Large Hadron Collider | PRC 92 (2015) 034906 | 1306.0121 |

25 |
S. Ostapchenko | Monte Carlo treatment of hadronic interactions in enhanced Pomeron scheme: I. QGSJET-II model | PRD 83 (2011) 014018 | 1010.1869 |

26 |
H. J. Drescher et al. | Parton based Gribov-Regge theory | PR 350 (2001) 93 | hep-ph/0007198 |

27 |
CMS Collaboration |
The CMS experiment at the CERN LHC | JINST 3 (2008) S08004 | CMS-00-001 |

28 |
CMS Collaboration |
Particle-flow reconstruction and global event description with the CMS detector | JINST 12 (2017) P10003 | CMS-PRF-14-0011706.04965 |

29 |
CMS Collaboration |
CMS luminosity measurement using 2016 proton-nucleus collisions at nucleon-nucleon center-of-mass energy of 8.16 TeV | CMS-PAS-LUM-17-002 | CMS-PAS-LUM-17-002 |

30 |
CMS Collaboration |
Description and performance of track and primary-vertex reconstruction with the CMS tracker | JINST 9 (2014) P10009 | CMS-TRK-11-0011405.6569 |

31 |
A. J. Baltz | The Physics of Ultraperipheral Collisions at the LHC | PR 458 (2008) 1 | 0706.3356 |

32 |
CMS Collaboration |
Coherent $ J/\psi $ photoproduction in ultra-peripheral PbPb collisions at $ {\sqrt {\smash [b]{s_{_{\mathrm {NN}}}}}} = $ 2.76 TeV with the CMS experiment | PLB 772 (2017) 489 | CMS-HIN-12-0091605.06966 |

33 |
CMS Collaboration |
Measurement of exclusive $ \rho(770)^0 $ photoproduction in ultraperipheral pPb collisions at $ {\sqrt {\smash [b]{s_{_{\mathrm {NN}}}}}} = $ 5.02 TeV | EPJC 79 (2019) 702 | CMS-FSQ-16-0071902.01339 |

34 |
CMS Collaboration |
Measurement of exclusive $ \Upsilon $ photoproduction from protons in pPb collisions at $ {\sqrt {\smash [b]{s_{_{\mathrm {NN}}}}}} = $ 5.02 TeV | EPJC 79 (2019) 277 | CMS-FSQ-13-0091809.11080 |

35 |
CMS Collaboration |
Evidence for light-by-light scattering and searches for axion-like particles in ultraperipheral PbPb collisions at $ {\sqrt {\smash [b]{s_{_{\mathrm {NN}}}}}} = $ 5.02 TeV | PLB 797 (2019) 134826 | CMS-FSQ-16-0121810.04602 |

36 |
CMS Collaboration |
Charged-particle nuclear modification factors in PbPb and pPb collisions at $ {\sqrt {\smash [b]{s_{_{\mathrm {NN}}}}}}= $ 5.02 TeV | JHEP 04 (2017) 039 | CMS-HIN-15-0151611.01664 |

37 |
CMS Collaboration |
Calibration of the CMS hadron calorimeters using proton-proton collision data at $ \sqrt{s} = $ 13 TeV | JINST 15 (2020) P05002 | CMS-PRF-18-0011910.00079 |

38 |
G. D'Agostini | A Multidimensional unfolding method based on Bayes' theorem | NIMA 362 (1995) 487 | |

39 |
T. Adye | Unfolding algorithms and tests using RooUnfold | in Proceedings, PHYSTAT 2011 Workshop on Statistical Issues Related to Discovery Claims in Search Experiments and Unfolding, CERN,Geneva 2011 | 1105.1160 |

40 |
S. Schmitt | Data unfolding methods in high energy physics | in EPJ Web Conf. XIIth Quark Confinement and the Hadron Spectrum, volume 137 | 1611.01927 |

41 |
O. Sur\'anyi | Performance of the CMS Zero Degree Calorimeters in the 2016 pPb run | J. Phys. Conf. Ser. 1162 (2019) 012005 |

Compact Muon Solenoid LHC, CERN |