Baur, Sebastian

CMS Collaboration

Ulrich, Ralf

English

Baur, S.

Ulrich, R.

KITopen

PoS(ICRC2019)188Measurements of the very-forward energy in ppcollisions at the LHC and constraints for cosmic rayair showersSebastian Baur∗IIHE, Université Libre de BruxellesE-mail: sebastian.baur@ulb.ac.beRalf UlrichIKP, Karlsruhe Institute of TechnologyE-mail: ralf.ulrich@kit.edufor the CMS collaborationThe very-forward energy production in hadron collisions is of great importance for the under-standing of ultra-high energy cosmic ray air showers. The CASTOR calorimeter of CMS islocated at −6.6 < η < −5.2 in a unique phase-space for this purpose; the characteristics of sec-ondaries in this phase-space of hadron collisions have a determining impact on the formation ofair shower cascades. We present the latest results of CASTOR including the first direct measure-ment of the ratio of electromagnetic to hadronic energy in the forward phase-space at the LHC.Possible implications of these results for cosmic ray physics are outlined.36th International Cosmic Ray Conference -ICRC2019-July 24th - August 1st, 2019Madison, WI, U.S.A.∗Speaker.c© Copyright owned by the author(s) under the terms of the Creative CommonsAttribution-NonCommercial-NoDerivatives 4.0 International License (CC BY-NC-ND 4.0). http://pos.sissa.it/PoS(ICRC2019)188Very-forward energy measurements with CMS Sebastian Baur1. IntroductionParticle production at forward rapidities in high energy hadron collisions has crucial impact onthe development of extensive air showers in the atmosphere [1]. The underlying production mech-anisms, however, can – to a large extent – not be accurately described by calculations from firstprinciples. The most relevant processes, especially multiparton interactions (MPI) and beam rem-nant fragmentations, are therefore modelled phenomenologically in Monte Carlo event generators.Free parameters in these models are tuned to experimental data [2, 3].A large variety of measurements at hadron colliders is therefore necessary to ensure a bestpossible understanding of the particle production in air showers. Data from the CERN LHC hasproven essential in reducing model-related uncertainties in the prediction of air showers at ultra-high energies [4, 5].The CASTOR calorimeter of the CMS detector is ideally suited to contribute relevant con-straints for very-forward particle production and subsequently for the understanding of air showerdevelopments. In this contribution, we review measurements performed with proton-proton colli-sions at a center-of-mass energy of 13 TeV at the CERN LHC and outline possible implications forhadronic event generators used for air shower simulations.2. The CASTOR calorimeter of CMSThe CASTOR calorimeter of the CMS experiment is a sampling calorimeter composed oflayers of fused silica quartz plates and tungsten absorbers. It is located on the negative side of CMSand covers the pseudorapidity region−6.6< η <−5.2. CASTOR is composed of two subsections:the two channels closest to the interaction region have a combined depth of 20 radiation lengthsand form the electromagnetic section. The remaining 12 channels constitute the hadronic section.The full depth of the calorimeter amounts to 10 hadronic interaction lengths. The energy depositedin the electromagnetic section is dominated by the energy loss of electrons and photons. SinceCASTOR is located about 14 m away from the interaction region, neutral pions decay before theyreach the detector and contribute to the energy measured in the electromagnetic section. Due to itsdesign, CASTOR can not detect single particles, but measures the combined energy of all particlesarriving at the detector. It is, however, possible to separate the energy in the two subsections andtherefore study the energy of electrons and photons independently from the one of hadrons. A moredetailed description of the CMS detector and the CASTOR calorimeter is given in Refs. [6, 7, 8].3. Energy measurements with CASTORDuring the startup period of the LHC Run 2 in 2015, CASTOR took data of proton-protoncollisions at the current record center-of-mass energy of 13 TeV. In this period, LHC was operatingwith very small beam intensities and therefore provided very pure conditions to measure very-forward particles without interference from additional pileup collisions. The solenoid magnet ofCMS was turned off during this period, which reduced the impact of stray fields on the calorimeter,increasing the experimental resolution compared to previous data taking periods.The data are compared to predictions of the relevant interaction models used to describe highenergy hadronic collisions like PYTHIA8 [9, 10, 11] and HERWIG7 [12, 13], as well as those1PoS(ICRC2019)188Very-forward energy measurements with CMS Sebastian BaurFigure 1: Energy density at particle level as a function of pseudorapidity at 13 TeV for compared to pre-dictions of various event generators. The grey band shows the total systematic uncertainty correlated acrosspseudorapidity bins. The bottom panel shows the models with respect to the data [19].commonly used for the simulation of cosmic ray air showers, such as EPOS [14, 5], QGSJETII [15],and SIBYLL [16, 17, 18].All measurements discussed in this contribution share a common event selection, which istuned to be as inclusive as possible and – at the same time – to reproduce the detector coverage.On detector level, a minimal energy deposition of 5 GeV is required in the Hadron Forward (HF)calorimeters of CMS, which cover the range 3.15 < |η |< 5.2. On the particle level, this selectionis reproduced by separating the final state particles into two subsystems with respect to the largestrapidity gap. The ratio of the larger mass divided by the squared center-of-mass energy is theproton momentum loss ξ , which is required to be ξ > 10−6, see Ref.[19] for details. Diffractiveevents with very low masses in the diffractive system(s) are therefore excluded in these analyses.3.1 Pseudorapidity-dependent forward energy densityThe average energy density per collision and unit in pseurorapidity is measured as a functionof the pseudorapidity using the CASTOR and the Hadron Forward (HF) calorimeter of CMS. Thecovered range extends from 3.15< |η |< 6.6 [19]. Fig. 1 shows the results as published in Ref. [19].The most accurate predictions are provided by PYTHIA8 with tune CUETP8M1. The otherinteraction models show overall good agreement with the data, slightly overestimating the data inparts of the studied pseudorapidity region. The relative change of the energy with η , however,is not well described by all models, which exhibit a too strong increase in the region covered bythe HF calorimeter within 3.15 < |η |< 5.20. For PYTHIA8 CUETP8S1, the tune uncertainties are2PoS(ICRC2019)188Very-forward energy measurements with CMS Sebastian Baurb/GeV)µ/dE (σd2−101−10110210310DataTotal uncertaintyModel uncertaintyEPOS LHCEPOS 1.99Sibyll 2.3Sibyll 2.1QGSJet II.04QGSJet II.03 (13 TeV)-1bµ0.34 CMS-6 >10ξ < -5.2 , η-6.6 < Total energy (GeV)0 1000 2000 3000 4000 5000 MC/Data012b/GeV)µ/dE (σd2−101−10110210310DataTotal uncertaintyModel uncertaintyPYTHIA8 CUETP8M1PYTHIA8 CUETP8M1, no MPIPYTHIA8 CUETP8M1, pt0Ref=1.5PYTHIA8 CUETP8M1, pt0Ref=3.0PYTHIA8 4C+MBR (13 TeV)-1bµ0.34 CMS-6 >10ξ < -5.2 , η-6.6 < Total energy (GeV)0 1000 2000 3000 4000 5000 MC/Data012b/GeV)µ/dE (σd020406080100120140DataTotal uncertaintyModel uncertaintyEPOS LHCEPOS 1.99Sibyll 2.3Sibyll 2.1QGSJet II.04QGSJet II.03 (13 TeV)-1bµ0.34 CMSSupplementary-6 >10ξ < -5.2 , η-6.6 < Total energy (GeV)0 200 400 600 800 1000 MC/Data012b/GeV)µ/dE (σd020406080100120140DataTotal uncertaintyModel uncertaintyPYTHIA8 CUETP8M1PYTHIA8 CUETP8M1, no MPIPYTHIA8 CUETP8M1, pt0Ref=1.5PYTHIA8 CUETP8M1, pt0Ref=3.0PYTHIA8 4C+MBR (13 TeV)-1bµ0.34 CMSSupplementary-6 >10ξ < -5.2 , η-6.6 < Total energy (GeV)0 200 400 600 800 1000 MC/Data012Figure 2: Differential cross section as a function of the total energy in −6.6 < η < −5.2. The left panelshows the data compared to MC event generators mostly developed for cosmic ray induced air showers,and the right panel to different PYTHIA8 tunes. The bottom panel shows the same data with linear scaleat low energies. The yellow band indicates the total uncertainty of the measurement, the orange band themodel-uncertainty due to the unfolding [20].illustrated by a red band in the right panels of Fig. 1. These uncertainties are smaller than the spreadof the different models. Therefore, inherent differences of these models are resolved. Furthermore,it is observed, that the modelling of MPI is essential to describe the data.3.2 Forward energy spectraThe detailed energy distribution within the CASTOR acceptance is studied in more detail inRef. [20]. The results in terms of the differential cross sections as a function of energy are shownin Figs. 2 and 3. This measurement is performed as a function of the total energy (Fig. 2), as wellas of the electromagnetic and hadronic components (Fig.3) individually. This represents the firstmeasurement in which the electromagnetic and hadronic energies are investigated separately withthe CASTOR calorimeter.3PoS(ICRC2019)188Very-forward energy measurements with CMS Sebastian Baurb/GeV)µ/dE (σd2−101−10110210310DataTotal uncertaintyModel uncertaintyEPOS LHCEPOS 1.99Sibyll 2.3Sibyll 2.1QGSJet II.04QGSJet II.03 (13 TeV)-1bµ0.34 CMS-6 >10ξ < -5.2 , η-6.6 < Electromagnetic energy (GeV)0 500 1000 1500 2000 MC/Data012b/GeV)µ/dE (σd6−105−104−103−102−101−10110210310410510DataTotal uncertaintyModel uncertaintyEPOS LHCEPOS 1.99Sibyll 2.3Sibyll 2.1QGSJet II.04QGSJet II.03 (13 TeV)-1bµ0.34 CMS-6 >10ξ < -5.2 , η-6.6 < Hadronic energy (GeV)0 1000 2000 3000 4000 5000 MC/Data012Figure 3: Differential cross section as a function of the electromagnetic energy (left) and the hadronicenergy (right). The yellow band indicates the total uncertainty of the measurement, the orange band themodel-uncertainty due to the unfolding [20].The importance of MPI modelling is demonstrated by changing of the parameter prefT,0 ofPYTHIA8. The spectral shape changes significantly with changed parameters. This emphasizesthe fact that the provided data can be of great value for future parameter tunes. Furthermore, theoverall collision elasticity and the amount of diffraction influences the spectral shape at low ener-gies. It can, for example, be observed that the fraction of events with little energy is significantlyoverestimated by SIBYLL 2.3 compared to the data (see bottom panel of Fig. 2). This can be a hintof a too large elasticity in the model. The separation into the electromagnetic and hadronic contri-bution provides additional constraints for the models. While the electromagnetic energy spectrumis well described by the latest generation of interaction models, the hadronic energy is slightlyoverestimated at the edge of the uncertainties.3.3 Forward energy in correlation to the central particle multiplicityThe most recent measurement studies the average energy deposited in CASTOR in correla-tion to the charged particle multiplicity in the central acceptance of CMS [21]. For this purpose, amodified tracking algorithm is used in order to compensate for the missing magnetic field. Straightlines are reconstructed with the CMS pixel tracker. For this analysis, the detector-level observ-ables are not corrected to the particle level. A forward-folding technique is used instead in orderto compare the data to various model predictions. More details on this method are described inRef. [21]. The results are shown in Figs. 4 and 5. A clear correlation of central particle multiplicityand very-forward energy is found, following the typical shape of the underlying event: the for-ward energy increases with increasing particle multiplicity. Most of the interaction models predictthe average energies accurately within uncertainties. The dominant systematic uncertainty is theCASTOR energy scale which can be eliminated when only the relative increase with multiplicity isstudied (see bottom panel of Fig. 4). The data indicate a softer rise than predicted by most models.Both versions of SIBYLL, however, describe the shape very accurately.4PoS(ICRC2019)188Very-forward energy measurements with CMS Sebastian Baur050010001500 (GeV)〉 totreco E〈DataSyst. uncertaintyPYTHIA8 CUETP8M1PYTHIA8 4C+MBREPOS LHCSibyll 2.1 (13 TeV)-10.22 nbCMS < -5.2η-6.6 < 0 50 100 150|<2)η (|tracksN0.511.5 MC/data050010001500 (GeV)〉 totreco E〈DataSyst. uncertaintySibyll 2.3cQGSJetII.04PYTHIA8 CP5Herwig 7.1 (13 TeV)-10.22 nbCMS < -5.2η-6.6 < 0 50 100 150|<2)η (|tracksN0.511.5 MC/data0246〉<10) tracks(Ntotreco E〈 / 〉 totreco E〈DataSyst. uncertaintyPYTHIA8 CUETP8M1PYTHIA8 4C+MBREPOS LHCSibyll 2.1 (13 TeV)-10.22 nbCMS < -5.2η-6.6 < 0 50 100 150|<2)η (|tracksN0.511.5 MC/data0246〉<10) tracks(Ntotreco E〈 / 〉 totreco E〈DataSyst. uncertaintySibyll 2.3cQGSJetII.04PYTHIA8 CP5Herwig 7.1 (13 TeV)-10.22 nbCMS < -5.2η-6.6 < 0 50 100 150|<2)η (|tracksN0.511.5 MC/dataFigure 4: Average energy reconstructed in the CASTOR calorimeter as a function of the number of recon-structed tracks within |η | < 2 (top panel) and normalised to that in the first bin (Nch < 10) (bottom panel).The data are shown as black markers and the corresponding systematic uncertainties with a gray band, thebands associated with the model predictions illustrate the model uncertainty. [21]Also here, by using the segmentation of CASTOR, the electromagnetic and hadronic energiesare studied independently as well. The ratio of the average electromagnetic and hadronic energyis shown in Fig. 5 as a representative example. In accordance to what is observed in the energyspectra discussed above, the models tend to overestimate the hadronic energy with respect to theelectromagnetic and therefore exhibit a lower ratio compared to the data. Most of the models stillagree with the measurement within uncertainties. The largest disagreement, not covered by theuncertainties, in found for PYTHIA8 tune CP5 and SIBYLL2.3C.Regarding the still unsolved deficit of muons in air shower simulations [22, 23], the data pro-vide important constraints. Increasing the production of forward hadrons and the energy they carry,could be an effective way to increase the amount of energy available for muon production. The data,however, indicate that there is no room to increase the hadronic energy in forward direction.5PoS(ICRC2019)188Very-forward energy measurements with CMS Sebastian Baur00.511.52〉 hadreco E〈 / 〉 em reco E〈DataSyst. uncertaintyPYTHIA8 CUETP8M1PYTHIA8 4C+MBREPOS LHCSibyll 2.1 (13 TeV)-10.22 nbCMS < -5.2η-6.6 < 0 50 100 150|<2)η (|tracksN0.511.5 MC/data00.511.52〉 hadreco E〈 / 〉 em reco E〈DataSyst. uncertaintySibyll 2.3cQGSJetII.04PYTHIA8 CP5Herwig 7.1 (13 TeV)-10.22 nbCMS < -5.2η-6.6 < 0 50 100 150|<2)η (|tracksN0.511.5 MC/dataFigure 5: Ratio of the average electromagnetic and hadronic energies reconstructed in the CASTORcalorimeter as a function of the number of reconstructed tracks within |η | < 2. The data are shown asblack markers and the corresponding systematic uncertainties with a gray band, the bands associated withthe model predictions illustrate the model uncertainty. [21]4. SummaryThe CASTOR calorimeter of CMS is a unique detector in the forward phase space. A seriesof measurements was performed with proton-proton collisions of√s = 13 TeV, resulting in de-tailed descriptions of the energy density and distribution in the forward phase space. The energyis measured inclusively as well as in correlation to the particle multiplicity at central rapidities.Furthermore, the first measurements separating the contribution of electromagnetic and hadronicenergy in this phase space were performed. The presented results, therefore, represent benchmarktests for hadronic event generators – in particular those used to describe extensive air showers. Mostimportantly, the presented data show good sensitivity to the modelling of multiparton interactionsin hadronic event generators. The dedicated measurement of the electromagnetic and hadronicenergy allows, in addition, to constrain the amount of energy carried by hadrons. This results instrong implications for the modelling of muon production in extensive air showers. Overall, thepresented data provide crucial and unique input for future model improvements and an improvedunderstanding of air shower characteristics.References[1] R. Ulrich, R. Engel, and M. Unger, Phys. Rev. D 83 (2011) 054026.[2] R. Corke and T. Sjostrand, JHEP 03 (2011) 032.[3] P. Skands, S. Carrazza, and J. Rojo, Eur. Phys. J. C 74 (2014) 3024.[4] D. d’Enterria, R. Engel, T. Pierog, S. Ostapchenko, and K. Werner, Astropart. Phys. 35 (2011) 98.[5] T. Pierog, I. Karpenko, J. M. Katzy, E. Yatsenko, and K. Werner, Phys. Rev. C 92 (2015) 034906.[6] CMS Collaboration, JINST 3 (2008) S08004.6PoS(ICRC2019)188Very-forward energy measurements with CMS Sebastian Baur[7] V. Andreev et al., Eur. Phys. J. C 67 (2010) 601.[8] CMS-CASTOR Collaboration, P. Göttlicher, Nucl. Instrum. Meth. A 623 (2010) 225.[9] T. Sjöstrand et al., Comput. Phys. Commun. 191 (2015) 159.[10] CMS Collaboration, Eur. Phys. J. C 76 (2016) 155.[11] CMS Collaboration, arXiv:1903.12179.[12] J. Bellm et al., Eur. Phys. J. C 76 (2016) 196.[13] S. Gieseke, F. Loshaj, and P. Kirchgaeßer, Eur. Phys. J. C 77 (2017) 156.[14] K. Werner, B. Guiot, I. Karpenko, and T. Pierog, Phys. Rev. C 89 (2014) 064903.[15] S. Ostapchenko, Phys. Rev. D 83 (2011) 014018.[16] E.-J. Ahn, R. Engel, T. K. Gaisser, P. Lipari, and T. Stanev, Phys. Rev. D80 (2009) 094003.[17] F. Riehn, R. Engel, A. Fedynitch, T. K. Gaisser, and T. Stanev, PoS(ICRC2015)558 (2016).[18] F. Riehn, H. P. Dembinski, R. Engel, A. Fedynitch, T. K. Gaisser, and T. Stanev,PoS(ICRC2017)301 (2018).[19] CMS Collaboration, Eur. Phys. J. C 79 (2019) 391.[20] CMS Collaboration, JHEP 08 (2017) 046.[21] CMS Collaboration, arXiv:1908.01750.[22] Pierre Auger Collaboration, Phys. Rev. D 91 (2015) 032003. Erratum: Phys. Rev. D 91 (2015)059901.[23] Pierre Auger Collaboration, Phys. Rev. Lett. 117 (2016) 192001.7

Measurements of the very-forward energy in pp collisions at the LHC and constraints for cosmic ray air showers

Measurements of the very-forward energy in ppcollisions at the LHC and constraints for cosmic rayair showers

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Measurements of the very-forward energy in pp collisions at the LHC and constraints for cosmic ray air showers

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