CMS-PAS-HIN-18-019

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

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.
Links: CDS record (PDF) ; inSPIRE record ; CADI line (restricted) ; These preliminary results are superseded in this paper, Submitted to PRD. The superseded preliminary plots can be found here.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	Summary	References	CMS Publications

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-12-005 1503.08689
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-14-001 1706.04965
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-11-001 1405.6569
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-12-009 1605.06966
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-16-007 1902.01339
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-13-009 1809.11080
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-16-012 1810.04602
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-15-015 1611.01664
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-18-001 1910.00079
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