Very forward energy distributions and jet production observed with CASTOR in CMS

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1 Very forward energy distributions and jet roduction observed with CASOR in Universiteit Antweren he CASOR calorimeter at the exeriment at LHC rovides very forward (.6 < η <.) energy measurements. Jets in this regime can be used as very sensitive robes of the low-x arton dynamics in the roton. Measurements of deosited energy in this region can be used to model cosmic ray induced extensive air showers at ultra-high energies. he sectra of jets in CASOR and the total, electromagnetic and hadronic energy deositions in CASOR are measured and corrected to article level, and comared to various relevant Monte Carlo models. PoS(DIS6)9 XXIV International Worksho on Dee-Inelastic Scattering and Related Subjects 5 Aril, 6 DESY Hamburg, Germany Seaker. c Coyright owned by the author(s) under the terms of the Creative Commons Attribution-NonCommercial-NoDerivatives 4. International License (CC BY-NC-ND 4.). htt://os.sissa.it/

2 Very forward measurements with CASOR. Introduction he majority of rocesses taking lace in high energy roton-roton interactions are QCDmediated arton-arton scatterings. hese rocesses create a huge background for all rocesses and searches studies at the LHC and must be modelled recisely to searate such events from ossible new hysics events. QCD rocesses can be described to great recision with DGLAP art evolution equations and collinear factorization [,,, 4. Due to aroximations made it is exected that the redictions of this method will break down at very low x values, but no recise conditions for this breakdown can be given. Events with low x are characterized by very forward (high η), low- jets. he erformance of Monte Carlo models in the very forward regime is a very imortant benchmark for studies in the field of astroarticle hysics due to the very forward boost of the outgoing energy. his environment is very similar to collisions of ultra-high energy cosmic articles with stationary atmosheric articles (atomic nuclei) which result in extensive air showers [5. hese showers contain muons through the decay of mesons coming from the initial collision, but the roduction mechanism of these muons is a major mystery in the field [6.. Exerimental setu he central feature of the Comact Muon Solenoid () aaratus is a suerconducting solenoid of 6 m internal diameter, roviding a magnetic field of.8. Within the field volume are a silicon ixel and stri tracker, a lead tungstate crystal electromagnetic calorimeter (ECAL), and a brass/scintillator hadron calorimeter (HCAL). Muons are measured in gas-ionization detectors embedded in the steel return yoke. Extensive forward calorimetry comlements the coverage rovided by the barrel and endca detectors. he central detectors of are comlemented by calorimeters in the forward direction, which are all relying on the roduction of Cherenkov hotons by charged articles in quartz. he "hadron forward" (HF) calorimeters cover the seudoraidity interval < η < 5. and are using quartz fibers embedded in a steel absorber. A more extensive descrition can be found in [7. In the very-forward direction there is the CASOR calorimeter located at a distance of 4. m from the interaction oint at a radial distance from the LHC beam of about 4 to 5 cm. his corresonds to a seudoraidty coverage of 6.6 < η < 5.. CASOR is a samling calorimeter using layers of fused silica quartz lates and tungsten absorbers. CASOR is segmented in 4 longitudinal and 6 azimuthal channels. he first two front channels corresond to a combined deth of X and are used as the electromagnetic section, while the full deth of the calorimeter amounts to λ I. he data of CASOR are reconstructed in 6 towers, each summing u the 4 longitudinal channels at the same azimuthal location. owers are zero suressed with 65 MeV N channel by considering the noise level in each of the N channel channels used to construct the tower. As a last ste, towers are clustered into jets with the anti-k algorithm [8 using a radius arameter R =.5. Because of the very forward osition of CASOR, it does not have an η-segmentation: all jets are assigned a default η =.9 value, which corresonds to the value center of the calorimeter. CASOR can distinguish between deosits of electromagnetic energy and deosits of hadronic energy by measuring it in different modules. PoS(DIS6)9

3 Very forward measurements with CASOR he resonse of the detector is simulated using the GEAN4 framework [9 and is used to correct from measured sectra to article level sectra. he Monte Carlo models used during the analyses are PYHIA8 (version 8.) [ with tune CUEP8M [ and MBR [ model as well as EPOS-LHC [.. analysis For the measurement of the sectra./nb of collected data were used. hese were collected at the beginning of the second LHC run when the magnet was turned off.. Energy measurement was selected by monitoring activity in the HF calorimeters. he influence of electronic noise in CASOR is minimized by requiring at least one tower in either of the HF calorimeters (on negative or ositive side of the interaction oint) above 5 GeV: this reduces sensitivity to noise to less than %. he total energy measured by CASOR is taken as the sum of all calorimeter towers above the noise threshold (Sec. ). he sum of energy of towers in the two frontmost modules is the electromagnetic fraction of energy deosited in CASOR, towers in the twelve backmost sum u to form the hadronic fraction of the energy. he energy sectra are corrected to stable article level, where stable means a lifetime cτ > cm. Stable articles are assigned to system X or system Y, with these systems lying on oosite sides of the largest raidity ga in the event. A variable ξ is then defined as: ξ X = MX /s (.) ξ Y = MY /s (.) ξ = max(ξ X,ξ Y ). (.) Events with ξ > are selected. energy sectra are corrected to stable article level with calibration to counter the noncomensation of CASOR and are unfolded with the d Agostini iterative rocedure with early stoing [4. PoS(DIS6)9. Jet energy sectrum o correct for the non-comensating nature of the CASOR calorimeter, jet energies were calibrated. he calibration factors were determined from Monte Carlo simulations that incororate the calorimeter resonse: essentially the jet energies are scaled u with aroximately 4%. In a following ste the measured and calibrated jet energy sectrum was unfolded. o this end, a resonse matrix was constructed in Monte Carlo with reconstructed CASOR jets and generator level jets (anti-k with radius arameter R =.5). Jets are matched by looking at the most energetic detector level jet and matching this to the most energetic generator level jet with a maximum difference in azimuthal angle φ max, and iterating these stes until no more matches can be made. Each jet cannot be matched more than once. Non-matched generator level and detector level jets ("misses" and "fakes") imose a model deendence on the result of the unfolding rocedure: their influence can be reduced by setting φ max =.5 and considering only generator level jets in.6 <

4 Very forward measurements with CASOR η <. in the matching rocedure. At > GeV the fractions of misses and fakes are both below %, and for > 5 GeV these values are less than %. he measured sectrum was unfolded with the d Agostini iterative rocedure with early stoing [4. It was found that 8 iterations the unfolding bias is small enough, and Poisson-induced statistical fluctuations do not yet contribute to the result. 4. Uncertainties Several sources of systematic uncertainties were found and taken into account. CASOR energy scale. CASOR is cross-calibrated via a measurement of energy in < η < 5 which is extraolated with a large set of Monte Carlo models to 5. < η < 6.6. his is the largest uncertainty in both measurements. Luminosity. deendence. he model deendence is evaluated by unfolding the data with three different models (PYHIA8 (CUEPM), PYHIA8 (MBR) and EPOS). he deence ranges between % and 5%. uncertainty. he true osition of CASOR is known with a recision of mm, and it can vary u to cm over the eriod during whihc data is taken. Changes in osition change the true η accetance of the calorimeter, and have an influence of % to %. 4. Results 4. Energy measurement Figures, and show the resectively the total, electromagnetic and hadronic energy sectra after calibration and unfolding. he sectra are comared to cosmic ray Monte Carlo models (left anel) and PYHIA and HERWIG (right anel). 4. Jet energy sectrum Figures 4 and 5 show the final unfolded sectrum of the data comared to several Monte Carlo model redictions. It is clear that the uncertainty on the CASOR energy scale has a large influence on the final uncertainty of the differential cross section but is significantly smaller when considering the yield er event. At low values the influence of MPI on the cross section is clearly visible. PoS(DIS6)9 5. Conclusions In both measurements the CASOR energy scale is large and increases at higher energies. None of the models describes the shae of all observations. QGSJEII.4 and SIBYLL. describe data best overall while PYHIA8 4C+MBR and HERWIG++ UE-EE-5C erform worst. he electromagnetic energy sectrum is described best by all models. he influence of multi-arton interactions and the underlying event are clear. he jet energy sectra corresond with the the data due to the large uncertainty. PYHIA tends to redict a harder sectrum, QGSJEII.4 and EPOS indicate a slightly sofer sectrum. Overall EPOS yields the best agreement.

5 Very forward measurements with CASOR /N evt.6 < η < -5.5, ξ > 4.5 µb otal uncertainty Sibyll. Sibyll. EPOS.99 EPOS LHC QGSJet II. QGSJet II.4 s= ev (B=) /N evt.6 < η < -5.5, ξ > 4.5 µb otal uncertainty PYHIA8 CUEP8M PYHIA8 CUEP8M, MPI off PYHIA8 CUEP8M, tref =.5 PYHIA8 CUEP8M, tref =. PYHIA8 4C+MBR Herwig++.7 UE-EE-5C s= ev (B=) Ratio MC/ /N evt 4 5 otal Energy [GeV Ratio MC/ 4 5 otal Energy [GeV Figure : Normalized total energy sectrum in the accetance of CASOR for events with ξ >. Ratio MC/.6 < η < -5.5, ξ > 4.5 µb otal uncertainty Sibyll. Sibyll. EPOS.99 EPOS LHC QGSJet II. QGSJet II.4 s= ev (B=) /N evt Ratio MC/.6 < η < -5.5, ξ > 4.5 µb otal uncertainty PYHIA8 CUEP8M PYHIA8 CUEP8M, MPI off PYHIA8 CUEP8M, tref =.5 PYHIA8 CUEP8M, tref =. PYHIA8 4C+MBR Herwig++.7 UE-EE-5C s= ev (B=) PoS(DIS6)9 5 5 Electromagnetic Energy [GeV 5 5 Electromagnetic Energy [GeV Figure : Normalized electromagnetic energy sectrum in the accetance of CASOR for events with ξ >. References [ V. N. Gribov and L. N. Liatov, Dee inelastic e scattering in erturbation theory, Sov. J. Nucl. Phys. 5 (97) 48. [ L. N. Liatov, he arton model and erturbation theory, Sov. J. Nucl. Phys. (975) 94. [ G. Altarelli and G. Parisi, Asymtotic freedom in arton language, Nucl. Phys. B6 (977) 98. [4 Y. L. Dokshitzer, Calculation of the structure functions for dee inelastic scattering and e + e annihilation by erturbation theory in quantum chromodynamics, Sov. Phys. JEP 46 (977) 64. 4

6 Very forward measurements with CASOR /N evt.6 < η < -5.5, ξ > 4.5 µb otal uncertainty Sibyll. Sibyll. EPOS.99 EPOS LHC QGSJet II. QGSJet II.4 s= ev (B=) /N evt.6 < η < -5.5, ξ > 4.5 µb otal uncertainty PYHIA8 CUEP8M PYHIA8 CUEP8M, MPI off PYHIA8 CUEP8M, tref =.5 PYHIA8 CUEP8M, tref =. PYHIA8 4C+MBR Herwig++.7 UE-EE-5C s= ev (B=) Ratio MC/ 4 5 Hadronic Energy [GeV Ratio MC/ 4 5 Hadronic Energy [GeV Figure : Normalized hadronic energy sectrum in the accetance of CASOR for events with ξ >. [µb/gev dσ/d MC / 5 4 Lumi. nb ( ev) PYHIA8 CUEP8M PYHIA8 CUEP8M (no MPI) PYHIA8 CUEP8M CEQ6. PYHIA8 CUEP8M HeraPDF (R=.5) (.6 < η < -5.) anti-k t [GeV [/GeV dn/d /N jet MC /.5.5. nb ( ev) PYHIA8 CUEP8M PYHIA8 CUEP8M (no MPI) PYHIA8 CUEP8M CEQ6. PYHIA8 CUEP8M HeraPDF (R=.5) (.6 < η < -5.) anti-k t [GeV PoS(DIS6)9 Figure 4: Final unfolded differential jet- sectrum in CASOR comared to different model redictions. Left anel: the cross section. Right anel: the jet yield er event. [5 R. Ulrich, R. Engel, and M. Unger, Hadronic Multiarticle Production at Ultra-High Energies and Extensive Air Showers, Phys. Rev. D8 () 546, [doi:./physrevd.8.546,arxiv:.4. [6 Pierre Auger Collaboration, Muons in air showers at the Pierre Auger Observatory: Mean number in highly inclined events, Phys. Rev. D9 (5), no.,, [doi://physrevd.9.599,./physrevd.9.,arxiv:48.4. [Erratum: Phys. Rev. D9, no.5, 599 (5) [7 Collaboration, he exeriment at the CERN LHC, JINS (8) S84, [doi:.88/748-//8/s84. 5

7 Very forward measurements with CASOR [µb/gev dσ/d MC /. nb ( ev) 5 4 Lumi EPOS-LHC QGSJetII.4 PYHIA6 Z* PYHIA8 CUEP8M PYHIA8 Monash PYHIA8 MBR anti-k t (R=.5) (.6 < η < -5.) [GeV [/GeV dn/d /N jet MC / nb ( ev) EPOS-LHC QGSJetII.4 PYHIA6 Z* PYHIA8 CUEP8M PYHIA8 Monash PYHIA8 MBR anti-k t (R=.5) (.6 < η < -5.) [GeV Figure 5: Final unfolded differential jet- sectrum in CASOR comared to different model redictions. Left anel: the cross section. Right anel: the jet yield er event. [8 M. Cacciari, G. P. Salam, and G. Soyez, he anti-k t jet clustering algorithm, JHEP 4 (8) 6, [doi:.88/678/8/4/6,arxiv:8.89. [9 S. Agostinelli et al., Geant4a simulation toolkitâăi, Nuclear Instruments and Methods in Physics Research Section A: Accelerators, Sectrometers, Detectors and Associated Equiment 56 (), no., 5 -, [doi:htt://dx.doi.org/.6/s68-9()68-8. [. Sjstrand et al., An Introduction to PYHIA 8., Comut. Phys. Commun. 9 (5) 5977, [doi:.6/j.cc.5..4,arxiv:4.. [ Collaboration, Event generator tunes obtained from underlying event and multiarton scattering measurements, Eur. Phys. J. C76 (6), no., 55, [doi:.4/ejc/s5-988-x,arxiv:5.85. [ R. Ciesielski and K. Goulianos, MBR Monte Carlo Simulation in PYHIA8, PoS ICHEP (), [arxiv: [. Pierog et al., EPOS LHC: est of collective hadronization with data measured at the CERN Large Hadron Collider, Phys. Rev. C9 (5), no., 496, [doi:./physrevc.9.496,arxiv:6.. [4 G. D Agostini, A Multidimensional unfolding method based on Bayes theorem, Nucl. Instrum. Meth. A6 (995) , [doi:.6/68-9(95)74-x. PoS(DIS6)9 6