Baur, Sebastian

Itow, Y.

Menjo, H.

Pattison, B.

Sako, T.

English

CMS Collaboration

KITopen

EPJ Web of Conferences 208, 05005 (2019) https://doi.org/10.1051/epjconf/201920805005ISVHECRI 2018© The Authors, published by EDP Sciences. This is an open access article distributed under the terms of the Creative Commons Attribution License 4.0 (http://creativecommons.org/licenses/by/4.0/).Measurements of very-forward energy with the CASTOR calorimeter of CMSSebastian Baur1,∗ for the CMS Collaboration1Institute for Nuclear Physics, Karlsruhe Institute of Technology (KIT), 76131 KarlsruheAbstract. The very-forward energy production in hadron collisions is of paramount importance for the un-derstanding of ultra-high energy cosmic ray air showers. The CASTOR calorimeter of CMS is located at−6.6 < η < −5.2 in the phase-space where the peak of energy is deposited at LHC. The composition andcharacteristics of the particles in this phase-space have a determining impact on the formation of air showercascades. An overview of various energy measurements performed with CASTOR is reported and possibleimplications for cosmic ray physics are outlined.1 IntroductionParticle production at forward rapidities in high energyhadronic collisions is to a large extent not accurately de-scribed by calculations from first principles. The rele-vant processes, especially multiparton interactions (MPI)and the fragmentation of the beam remnants, are modelledphenomenologically in Monte Carlo event generators withparameters tuned from data [1, 2]. A good understandingof forward particle production is important to accuratelysimulate the interactions of cosmic rays in the upper at-mosphere as well as the subsequent development of exten-sive air showers [3]. In addition, the production of forwardcharged hadrons has a direct impact on the total number ofair-shower muons at the ground, whose model predictionsstill show a deficiency compared to the data [4].2 The CASTOR calorimeterThe CASTOR calorimeter of the CMS experiment is aunique detector at the LHC and allows to perform dedi-cated measurements to address these questions. The CAS-TOR calorimeter is a sampling calorimeter composed oflayers of fused silica quartz plates and tungsten absorbers.It is located only on the negative side of CMS and cov-ers the pseudorapidity region −6.6 < η < −5.2. Thetwo front channels of each azimuthal segment (so calledtower), have a combined depth of 20 radiation lengths andform the electromagnetic section of each tower. The en-ergy recorded in these channels is dominated by energydeposits of electrons and photons, which include photonsfrom neutral pion decays. The remaining 12 channels ofeach tower constitute the hadronic section. The full depthof a tower amounts to 10 hadronic interaction lengths.A more detailed description of the CMS detector can befound in Ref. [5]. A detailed description of the CASTORcalorimeter is given in Refs. [6, 7].∗e-mail: Sebastian.Baur@kit.edu3 Energy measurements with CASTORCASTOR took data at many different centre-of-mass ener-gies and beam configurations at the LHC. Here, we presenthighlighted measurements of the energy seen by CAS-TOR in proton-proton collisions during LHC Run 1 andRun 2 at√s = 0.9, 2.76, 7 and 13 TeV. The data are com-pared to predictions of various hadronic interaction mod-els, especially those relevant for the simulation of cosmicray air showers, such as epos [8, 9], QGSJetII [10], andSibyll [11, 12].3.1 Underlying event at√s = 0.9, 2.76 and 7 TeVIn Fig. 1, the ratio of the forward energy density, dE/dη,for events with a charged-particle jet produced at centralpseudorapidity (|ηjet| < 2) relative to the forward energydensity for inclusive events is shown. This forward en-ergy density ratio is measured as a function of the centraljet transverse momentum, pT, at three different centre-of-mass energies (√s = 0.9, 2.76, and 7 TeV). The data arecorrected for detector effects and compared to various dif-ferent hadronic event generators [13].It is observed that the evolution of the forward energydensity behaves significantly different at different centre-of-mass energies. At√s = 0.9 GeV, the energy seen byCASTOR decreases as the central jet pT increases. Thisis due to the fact that the energy is dominated by the frag-mentation of the beam remnant and that more energy istaken from the remnant as the central activity increases.With increasing centre-of-mass energy, the contribution ofMPI becomes more and more important to the energy inthe CASTOR acceptance. The typical behaviour of theunderlying event is seen at√s = 7 TeV, where a sharprise, followed by a plateau region is observed. If MPI isturned off in Pythia6 [14], the forward energy becomes in-dependent of the central jet pT, which is ruled out by thedata. The most studied event generators perform well indescribing the general features of the data, although nonedescribes all features at once.2EPJ Web of Conferences 208, 05005 (2019) https://doi.org/10.1051/epjconf/201920805005ISVHECRI 2018 (GeV/c)TLeading charged jet p5 10 15 20 25)η/dincl)/(dEη/dhard(dE0.60.811.21.41.61.82 = 0.9 TeVs < -5.2ηCMS    -6.6 < | < 2jetηLeading charged jet |DataPYTHIA6 D6TPYTHIA6 Z2*PYTHIA6 Z2* no MPIPYTHIA8 4CHERWIG++ 2.55 10 15 20 25MC/data0.80.911.11.2 (GeV/c)TLeading charged jet p5 10 15 20 25)η/dincl)/(dEη/dhard(dE0.60.811.21.41.61.82 = 2.76 TeVs5 10 15 20 25MC/data0.80.911.11. (GeV/c)TLeading charged jet p5 10 15 20 25)η/dincl)/(dEη/dhard(dE0.60.811.21.41.61.82 = 7 TeVs (GeV/c)TLeading charged jet p5 10 15 20 25MC/data0.80.911.11. (GeV/c)TLeading charged jet p5 10 15 20 25)η/dincl)/(dEη/dhard(dE0.60.811.21.41.61.82 = 0.9 TeVs < -5.2ηCMS    -6.6 < | < 2jetηLeading charged jet |DataEPOS 1.99QGSJETII-03SIBYLL 2.1CASCADE 2DIPSY5 10 15 20 25MC/data0.80.911.11.2 (GeV/c)TLeading charged jet p5 10 15 20 25)η/dincl)/(dEη/dhard(dE0.60.811.21.41.61.82 = 2.76 TeVs5 10 15 20 25MC/data0.80.911.11. (GeV/c)TLeading charged jet p5 10 15 20 25)η/dincl)/(dEη/dhard(dE0.60.811.21.41.61.82 = 7 TeVs (GeV/c)TLeading charged jet p5 10 15 20 25MC/data0.80.911.11.Figure 1. Ratio of the energy deposited in the pseudorapidity range −6.6 < η < −5.2 for events with a charged-particle jet with |ηjet| < 2with respect to the energy in inclusive events, as a function of the jet transverse momentum pT for√s = 0.9 (left), 2.76 (middle), and7 TeV (right). Data are compared to the Pythia and herwig++ MC models (top) as well as to models used for air shower physics(bottom). Error bars indicate the statistical uncertainty on the data points, while the grey band represents the statistical and systematicuncertainties added in quadrature [13].3.2 Forward energy density at√s = 13 TeVAt√s = 13 TeV, the forward energy density is measuredas a function of the pseudorapidity using CASTOR and theHadron Forward (HF) calorimeter of CMS in the range3.15 < |η| < 6.6 [15]. The energy density is measuredfor different classes of inelastic collisions. Fig. 2 showsthe energy density for the inclusive-inelastic event selec-tion, which requires a minimal fractional proton momen-tum loss ξ > 10−6, see Ref.[15] for details.The considered models provide a reasonable descrip-tion of the measured energy flow. The best description ofthe data is provided by the Pythia8 tune CUETP8M1 [16].However, the evolution of the energy with η is not well de-scribed by all models, especially in the region of the HFcoverage at 3.15 < |η| < 5.20. The spread of the modelpredictions is larger than the tuning uncertainties, illus-trated by a red band given for Pythia8 CUETP8S1, thusinherent model differences are resolved.3.3 Forward energy spectra at√s = 13 TeVFor the same class of events with ξ > 10−6, the energydistribution within the CASTOR acceptance is studied inmore detail. The differential cross section as a functionof energy in proton-proton collisions at a centre-of-massenergy of 13 TeV is shown in Figs. 3 and 4. Here forthe first time, the possibility of CASTOR to separate elec-tromagnetic and hadronic energy depositions is used [17].Therefore, the measurement is performed as a function ofthe total energy deposited in CASTOR (Fig. 3), as wellas of the electromagnetic (energy of electrons and pho-tons only) and hadronic (energy of charged and neutralhadrons) components (Fig.4).The spectra are sensitive to the modelling of multipar-ton interactions, which is demonstrated by changes of theparameter prefT,0 of Pythia8. Furthermore, the overall col-lision elasticity and the amount of diffraction influencesthe spectra shape at low energies. It can, for example,be observed that the fraction of events with little energyis significantly overestimated by Sibyll 2.3 compared tothe data. This can be a hint of a too large elasticity inthe model. The separation into the electromagnetic andhadronic contribution provides additional constraints forthe models. While the electromagnetic energy spectrum iswell described by the latest generation of interaction mod-els, the hadronic energy is slightly overestimated at theedge of the uncertainties. The data suggest that there istherefore no room to increase the number of air showermuons by increasing the hadronic energy in proton-protoncollisions.3EPJ Web of Conferences 208, 05005 (2019) https://doi.org/10.1051/epjconf/201920805005ISVHECRI 2018 (GeV/c)TLeading charged jet p5 10 15 20 25)η/dincl)/(dEη/dhard(dE0.60.811.21.41.61.82 = 0.9 TeVs < -5.2ηCMS    -6.6 < | < 2jetηLeading charged jet |DataPYTHIA6 D6TPYTHIA6 Z2*PYTHIA6 Z2* no MPIPYTHIA8 4CHERWIG++ 2.55 10 15 20 25MC/data0.80.911.11.2 (GeV/c)TLeading charged jet p5 10 15 20 25)η/dincl)/(dEη/dhard(dE0.60.811.21.41.61.82 = 2.76 TeVs5 10 15 20 25MC/data0.80.911.11. (GeV/c)TLeading charged jet p5 10 15 20 25)η/dincl)/(dEη/dhard(dE0.60.811.21.41.61.82 = 7 TeVs (GeV/c)TLeading charged jet p5 10 15 20 25MC/data0.80.911.11. (GeV/c)TLeading charged jet p5 10 15 20 25)η/dincl)/(dEη/dhard(dE0.60.811.21.41.61.82 = 0.9 TeVs < -5.2ηCMS    -6.6 < | < 2jetηLeading charged jet |DataEPOS 1.99QGSJETII-03SIBYLL 2.1CASCADE 2DIPSY5 10 15 20 25MC/data0.80.911.11.2 (GeV/c)TLeading charged jet p5 10 15 20 25)η/dincl)/(dEη/dhard(dE0.60.811.21.41.61.82 = 2.76 TeVs5 10 15 20 25MC/data0.80.911.11. (GeV/c)TLeading charged jet p5 10 15 20 25)η/dincl)/(dEη/dhard(dE0.60.811.21.41.61.82 = 7 TeVs (GeV/c)TLeading charged jet p5 10 15 20 25MC/data0.80.911.11.Figure 1. Ratio of the energy deposited in the pseudorapidity range −6.6 < η < −5.2 for events with a charged-particle jet with |ηjet| < 2with respect to the energy in inclusive events, as a function of the jet transverse momentum pT for√s = 0.9 (left), 2.76 (middle), and7 TeV (right). Data are compared to the Pythia and herwig++ MC models (top) as well as to models used for air shower physics(bottom). Error bars indicate the statistical uncertainty on the data points, while the grey band represents the statistical and systematicuncertainties added in quadrature [13].3.2 Forward energy density at√s = 13 TeVAt√s = 13 TeV, the forward energy density is measuredas a function of the pseudorapidity using CASTOR and theHadron Forward (HF) calorimeter of CMS in the range3.15 < |η| < 6.6 [15]. The energy density is measuredfor different classes of inelastic collisions. Fig. 2 showsthe energy density for the inclusive-inelastic event selec-tion, which requires a minimal fractional proton momen-tum loss ξ > 10−6, see Ref.[15] for details.The considered models provide a reasonable descrip-tion of the measured energy flow. The best description ofthe data is provided by the Pythia8 tune CUETP8M1 [16].However, the evolution of the energy with η is not well de-scribed by all models, especially in the region of the HFcoverage at 3.15 < |η| < 5.20. The spread of the modelpredictions is larger than the tuning uncertainties, illus-trated by a red band given for Pythia8 CUETP8S1, thusinherent model differences are resolved.3.3 Forward energy spectra at√s = 13 TeVFor the same class of events with ξ > 10−6, the energydistribution within the CASTOR acceptance is studied inmore detail. The differential cross section as a functionof energy in proton-proton collisions at a centre-of-massenergy of 13 TeV is shown in Figs. 3 and 4. Here forthe first time, the possibility of CASTOR to separate elec-tromagnetic and hadronic energy depositions is used [17].Therefore, the measurement is performed as a function ofthe total energy deposited in CASTOR (Fig. 3), as wellas of the electromagnetic (energy of electrons and pho-tons only) and hadronic (energy of charged and neutralhadrons) components (Fig.4).The spectra are sensitive to the modelling of multipar-ton interactions, which is demonstrated by changes of theparameter prefT,0 of Pythia8. Furthermore, the overall col-lision elasticity and the amount of diffraction influencesthe spectra shape at low energies. It can, for example,be observed that the fraction of events with little energyis significantly overestimated by Sibyll 2.3 compared tothe data. This can be a hint of a too large elasticity inthe model. The separation into the electromagnetic andhadronic contribution provides additional constraints forthe models. While the electromagnetic energy spectrum iswell described by the latest generation of interaction mod-els, the hadronic energy is slightly overestimated at theedge of the uncertainties. The data suggest that there istherefore no room to increase the number of air showermuons by increasing the hadronic energy in proton-protoncollisions.|η|3.5 4 4.5 5 5.5 6 6.5| (GeV)ηdE/d|210310 (13 TeV)-10.06 nbCMSPreliminaryDataPythia8 MonashEPOS-LHCQGSJET IIB = 0 TINEL|η|3.5 4 4.5 5 5.5 6 6.5| (GeV)ηdE/d|210310 (13 TeV)-10.06 nbCMSPreliminaryDataPythia8 CUETP8M1Pythia8 CUETP8M1+MBRPythia8 CUETP8S1B = 0 TINEL|η|3.5 4 4.5 5 5.5 6 6.5[MC]|ηd|dE[DATA] / |ηd|dE0.70.80.911.11.21.31.41.5  (13 TeV)-10.06 nbCMSPreliminaryB = 0 TINELPythia8 MonashEPOS-LHCQGSJET IITotal Exp. Unc.|η|3.5 4 4.5 5 5.5 6 6.5[MC]|ηd|dE[DATA] / |ηd|dE0.70.80.911.11.21.31.41.5  (13 TeV)-10.06 nbCMSPreliminaryB = 0 TINELPythia8 CUETP8M1Pythia8 CUETP8M1+MBRPythia8 CUETP8S1Total Exp. Unc.Figure 2. Energy density at particle level as a function of pseudorapidity at 13 TeV for inclusive-inelastic events compared to pre-dictions of various event generators. The grey band shows the total systematic uncertainty correlated across pseudorapidity bins. Thebottom panel shows the ratio of the data to model predictions [15].b/GeV)µ/dE (σd2−101−10110210310DataTotal uncertaintyModel uncertaintyEPOS LHCEPOS 1.99Sibyll 2.3Sibyll 2.1QGSJet II.04QGSJet II.03 (13 TeV)-1bµ0.34CMS-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)-1bµ0.34CMS-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)-1bµ0.34CMSSupplementary-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)-1bµ0.34CMSSupplementary-6 >10ξ < -5.2 , η-6.6 < Total energy (GeV)0 200 400 600 800 1000 MC/Data012Figure 3. Differential cross section as a function of the total energy in −6.6 < η < −5.2. The left panel shows the data compared toMC event generators mostly developed for cosmic ray induced air showers, and the right panel to different Pythia8 tunes. The bottompanel shows the same data with linear scale at low energies. The yellow band indicates the total uncertainty of the measurement, theorange band the model-uncertainty due to the unfolding [17].4EPJ Web of Conferences 208, 05005 (2019) https://doi.org/10.1051/epjconf/201920805005ISVHECRI 2018b/GeV)µ/dE (σd2−101−10110210310DataTotal uncertaintyModel uncertaintyEPOS LHCEPOS 1.99Sibyll 2.3Sibyll 2.1QGSJet II.04QGSJet II.03 (13 TeV)-1bµ0.34CMS-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)-1bµ0.34CMS-6 >10ξ < -5.2 , η-6.6 < Hadronic energy (GeV)0 1000 2000 3000 4000 5000 MC/Data012Figure 4. Differential cross section as a function of the electromagnetic energy (left) and the hadronic energy (right). The yellow bandindicates the total uncertainty of the measurement, the orange band the model-uncertainty due to the unfolding [17].4 SummaryThe CASTOR calorimeter of CMS is a unique detectorin the forward phase space. A series of measurementswas performed to study the energy density and distribu-tion in this acceptance in order to perform benchmark testson hadronic event generators with the special goal to im-prove the understanding of the development of extensiveair showers.Relative energy density as function of the central jetpT probes the transition between the remnant fragmen-tation and MPI dominated regime as the centre-of-massenergy increases. The forward energy density as a func-tion of pseudorapidity and the inclusive energy spectrain the CASTOR acceptance have good sensitivity to themodelling of multiparton interactions and are furthermoresensitive to diffraction. In addition, the first measure-ment separating the electromagnetic and hadronic energyin the same phase-space was performed. This constrainsthe modelling of muon production in extensive air show-ers.References[1] R. Corke and T. J. Sjöstrand, JHEP 03 032 (2011)[2] P. Skands, S. Carrazza, and J. Rojo, Eur. Phys. J. C 743024 (2014)[3] R. Ulrich, R. Engel, and M. Unger, Phys. Rev. D 83054026 (2011)[4] Pierre Auger Collaboration, Phys. Rev. D 91 032003(2015) [Erratum: Phys. Rev. D 91 059901 (2015)][5] CMS Collaboration, JINST 3 S08004 (2008)[6] V. Andreev et al., Eur. Phys. J. C 67 601 (2010)[7] CMS-CASTOR Collaboration, P. Göttlicher, Nucl. In-strum. Meth. A 623 225 (2010)[8] T. Pierog and K. Werner, Nucl. Phys. B - Proc. Suppl.196 102 (2009)[9] T. Pierog et al., Phys. Rev. C 92 034906 (2015)[10] S. Ostapchenko, Phys. Rev. D 83 014018 (2011)[11] E.-J. Ahn et al., Phys. Rev. D 80 094003 (2009)[12] F. Riehn et al., PoS (ICRC2015) 558 (2016)[13] CMS Collaboration, JHEP 04 (2013)[14] T. Sjöstrand, S. Mrenna, and P. Skands, JHEP 05 026(2006)[15] CMS Collaboration, CMS-PAS-FSQ-15-006, CDS2146007 (2016)[16] CMS Collaboration, Eur. Phys. J. C 76 155 (2016)[17] CMS Collaboration, JHEP 08 (2017)

Measurements of very-forward energy with the CASTOR calorimeter of CMS

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Measurements of very-forward energy with the CASTOR calorimeter of CMS

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