There is a rich program of forward physics measurements within the CMS
Collaboration covering a wide range of topics. In many cases there is a
connection to quantities and effects relevant for very high energy cosmic ray
interaction. Some of the recent measurements in the fields of exclusive final
states, low-pT inclusive and diffractive cross sections, underlying event,
multi parton interactions, double parton scattering, final state particle
correlations and minimum bias results are briefly summarized here.Comment: ISVHECRI 2018 Nagoy

Ulrich, Ralf

EPJ Web of Conferences

English

arXiv

There is a rich program of forward physics measurements within the CMS Collaboration covering a wide range of topics. In many cases there is a connection to quantities and effects relevant for very high energy cosmic ray interactions. Some of the recent measurements in the fields of exclusive final states, low-pT inclusive and diffractive cross sections, underlying event, multiparton interactions, double parton scattering, final state particle correlations and minimum bias results are briefly summarized here

Ulrich Ralf

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EPJ Web of Conferences 208, 05002 (2019) https://doi.org/10.1051/epjconf/201920805002ISVHECRI 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/).Review of recent forward physics results from the CMS experimentRalf Ulrich1,∗ for the CMS Collaboration1Institute for Nuclear Physics, Karlsruhe Institute of Technology (KIT), 76131 KarlsruheAbstract. There is a rich program of forward physics measurements within the CMS Collaboration covering awide range of topics. In many cases there is a connection to quantities and effects relevant for very high energycosmic ray interactions. Some of the recent measurements in the fields of exclusive final states, low-pT inclusiveand diffractive cross sections, underlying event, multiparton interactions, double parton scattering, final stateparticle correlations and minimum bias results are briefly summarized here.1 IntroductionIn this proceedings some of the recent results of the for-ward physics activity of the CMS Collaboration are sum-marized. It is interesting that these measurements are notjust limited to small (forward) angles, but in particularcover also low-pT, exclusive final states and in particu-lar minimum bias results. The type of measurements dis-cussed here cover up to 14 orders of magnitude in crosssection, ranging from very rare exclusive W+W− pair pro-duction in proton-proton collisions at√s =8 TeV with across section on the order of O(10 fb) up to the inclusivecross section in proton-lead collisions at√sNN =5.02 TeVon the order of O(barn).Some of the measurements described have a relationto very high energy cosmic ray interactions and can be ofuse to further improve the understanding of the modellingof those.2 CMS and its forward instrumentationThe CMS Collaboration benefits from a powerful generalpurpose particle experiment, with good forward instru-mentation using dedicated detectors.At the heart of the CMS detector is a superconduct-ing solenoid of 6m internal diameter, providing a strongmagnetic field of 3.8T. The data used for this paper weretaken in June 2015 during a period without magnetic field.Within the CMS magnet volume are an inner silicon pixeland strip tracker that measure charged particles in therange |η| < 2.5, a homogeneous lead tungstate crystalelectromagnetic calorimeter, and a brass and scintillatorhadron calorimeter. The corresponding endcap detectorsinstrument the pseudorapidity range up to |η|  3 withtracking and calorimetry. Forward Cherenkov calorime-ters extend the coverage beyond |η|  3. Muons are mea-sured in gas-ionization detectors embedded in the steel re-turn yoke.∗e-mail: ralf.m.ulrich@kit.eduThe hadron forward (HF) calorimeters cover the region2.9 < |η| < 5.2 and consist of 2×432 readout towers, eachcontaining a long and a short quartz fiber embedded withina steel absorber running parallel to the beam. The longfibers run the entire depth of the HF calorimeter (165cm,or approximately 10 interaction length), while the shortfibers start at a depth of 22cm from the front of the de-tector. The response of each tower is determined from thesum of signal in the corresponding long and short fiber.The very forward angles on one side of CMS (−6.6 <η < −5.2) are covered by the CASTOR calorimeter. It has16 azimuthal towers, each built from 14 longitudinal mod-ules. The 2 front modules form the electromagnetic sec-tion, and the 12 rear modules form the hadronic section.The calorimeter is made of stacks of tungsten and quartzplates, read out by PMTs, in two half-cylindrical mechani-cal structures, and is placed around the beam pipe at a dis-tance of −14.4 m away from the nominal interaction point.The overall longitudinal depth of both CASTOR and HFcorresponds to 10 hadronic interaction lengths.A more detailed description of the CMS detector canbe found in Ref. [1].3 Exclusive final statesThe exclusive production of a particular final state in ahadron-hadron collision is only possible if the exchangeparticle is colour neutral, thus, either a colour neutralhadronic state (i.e. a pomeron), a photon, or a combina-tion of both.In particular the collisions where a pomeron is in-volved offer the exciting opportunity to directly probethe low-x part of the gluon distribution in the hadrons.The most prominent of such measurements are the ex-clusive production of vector mesons. CMS has mea-sured exclusive Υ production in proton-lead collisions at√sNN=5.02 TeV. In this system the lead nucleus acts as thesource for the photons, while the proton for the pomerons.One result of this measurement is the photon-proton cross2EPJ Web of Conferences 208, 05002 (2019) https://doi.org/10.1051/epjconf/201920805002ISVHECRI 2018Figure 1. Measurement of the photon-proton cross section asfunction of photon energy (from Ref. [2]). This result is part ofthe exclusive Υ measurement by CMS in proton-lead collisionsat√sNN = 5.02 TeV.Figure 2. The distribution of the invariant π+π− mass distributionin (semi-) exclusive production, see Ref. [3], measured in proton-proton collisions at 7 TeV.section as a function of the photon energy over a muchwider range than previously possible, see Fig. 1.The (semi-)exclusive production of π+π− in proton-proton collisions at 7 TeV is a much more complex en-vironment. It contains a continuum distribution, and res-onances on top of that (see Fig. 2). The main productionmechanism is via diffraction, but with a transition to theregime where one or both of the protons dissociate andproduce further particles. These data can reveal informa-tion on the probability of proton dissociation in diffractiveinteractions.Furthermore, in inelastic collisions where jets are pro-duced it is possible that the two jets are pushed appartfrom each other by a color singlet exchange (CSE). TheFigure 3. The fraction of colour singlet exchange in events withtwo jets, see Ref. [4], as function of the jet-separation ∆η j j.Figure 4. The detector-level inelastic cross section in proton-proton collisions measured for ξ > 10−6 and for ξx > 10−7 orξy > 10−6, see Ref. [5]signature of this is studied in jet-gap-jet measurements inproton-proton collisions at 7 TeV, see Fig. 3. These dataare useful to search for the signature of BFKL effects.Another type of collision is where only photons are ex-changed. At a hadron collider this can provide an incred-ibly clean environment for very rare processes. Namely,these is exclusive pair production where e.g. W+W− are apowerful probe for anomalous quartic gauge coupling andphysics beyond the standard model [7]. If this can be com-bined with the simultaneous observation of the two outgo-ing proton projectiles the kinematics of the protons can becorrelated with the central exclusive system opening up awindow for searches for new physics. The CMS-TOTEMProton Precision Spectrometer (CT-PPS) is built for thispurpose [8].3EPJ Web of Conferences 208, 05002 (2019) https://doi.org/10.1051/epjconf/201920805002ISVHECRI 2018Figure 1. Measurement of the photon-proton cross section asfunction of photon energy (from Ref. [2]). This result is part ofthe exclusive Υ measurement by CMS in proton-lead collisionsat√sNN = 5.02 TeV.Figure 2. The distribution of the invariant π+π− mass distributionin (semi-) exclusive production, see Ref. [3], measured in proton-proton collisions at 7 TeV.section as a function of the photon energy over a muchwider range than previously possible, see Fig. 1.The (semi-)exclusive production of π+π− in proton-proton collisions at 7 TeV is a much more complex en-vironment. It contains a continuum distribution, and res-onances on top of that (see Fig. 2). The main productionmechanism is via diffraction, but with a transition to theregime where one or both of the protons dissociate andproduce further particles. These data can reveal informa-tion on the probability of proton dissociation in diffractiveinteractions.Furthermore, in inelastic collisions where jets are pro-duced it is possible that the two jets are pushed appartfrom each other by a color singlet exchange (CSE). TheFigure 3. The fraction of colour singlet exchange in events withtwo jets, see Ref. [4], as function of the jet-separation ∆η j j.Figure 4. The detector-level inelastic cross section in proton-proton collisions measured for ξ > 10−6 and for ξx > 10−7 orξy > 10−6, see Ref. [5]signature of this is studied in jet-gap-jet measurements inproton-proton collisions at 7 TeV, see Fig. 3. These dataare useful to search for the signature of BFKL effects.Another type of collision is where only photons are ex-changed. At a hadron collider this can provide an incred-ibly clean environment for very rare processes. Namely,these is exclusive pair production where e.g. W+W− are apowerful probe for anomalous quartic gauge coupling andphysics beyond the standard model [7]. If this can be com-bined with the simultaneous observation of the two outgo-ing proton projectiles the kinematics of the protons can becorrelated with the central exclusive system opening up awindow for searches for new physics. The CMS-TOTEMProton Precision Spectrometer (CT-PPS) is built for thispurpose [8].Figure 5. The shapes of the distributions of the number of events as function of the proton rapidity loss as determined by CMScalorimeters ξcalx . In (a1) all events with forward rapidity gap are shown, in (b1) all of those events with no signal in CASTOR, and in(c1) those events that were flagged by CASTOR. See Ref. [6].4 Inclusive cross sections, and diffractionOne of the most important parameter concerning the de-velopment of extensive air shower cascades is the inelasticcross section for particle production. The inelastic crosssection in proton-proton collision has been measured at13 TeV [5]. This analysis is published in two acceptanceregions, where the difference is in including the CASTORvery forward calorimeter, see Fig. 4. It is very interestingto see that both measured values are considerably smallerthan all model predictions. The CMS measurement atξ > 10−6 is consistent with the corresponding measure-ment by ATLAS. A smaller inelastic cross section will leadto a deeper penetration of air shower cascades.In the determination of the inelastic cross section, themost difficult problem is to understand the impact of low-mass diffraction. The amount of cross section lost dueto detector acceptance is a relatively poorly constrainedquantity. In Fig. 5 the measured proton momentum loss,ξ, distributions in proton-proton collisions at 7 TeV areshown with different event selections. It can be seenthat the separation of single- and double-diffraction is ex-tremely difficult with typical LHC detectors. In CMS, onlythe inclusion of the CASTOR calorimeter helps to get abetter handle on it.5 Measurements of inclusive distributionsThe measurement of differential inclusive jet pT spectraover very wide pT ranges, shown in Fig. 6, is a very strongtest of model predictions. It has been shown that the shapeof the parton distribution functions is sensitive to such dis-tributions. In particular the gluon distribution could beimproved considering high-precision inclusive jet spectra.The very low-x gluon parton distribution becomes more-and-more important when the centre-of-mass energy in-creases, thus, it is relevant in very high energy cosmic rayinteractions.In cosmic ray air showers nuclear effects are anotheraspect that requires precise modelling. The parton distri-bution functions in nuclear interactions are subject to im-portant further corrections, like shadowing or saturation.Figure 6. Differential cross sections of inclusive jet productionas function of jet pT, see Ref. [9]. The filled markers are mea-sured by all CMS Calorimeters with η < 3, while the open mark-ers by the HF calorimeters.This will also affect multiparton interactions and the un-derlying event. For this reason it is very important thatCMS measured the very forward inclusive jet cross sec-tions in proton-lead collisions at√sNN = 5.02 TeV usingdata of the CASTOR calorimeter, see Fig. 7. These datahighlights the large importance of nuclear effects for veryhigh energy cosmic ray interactions.6 Underlying Event, multipartoninteractions, double parton scatteringThe measurements of the underlying event is a typicalexperimental method to assess the accuracy of QCD ef-fects in event generation that are very difficult to be de-termined from first principles. This includes initial- andfinal-state radiation, multiparton interactions, and projec-tile fragmentation. The corresponding measurement inproton-proton collisions at 13 TeV is shown in Fig. 8. TheEPOS LHC model has some problems to describe the datain the region around jet-pT of 20 GeV.4EPJ Web of Conferences 208, 05002 (2019) https://doi.org/10.1051/epjconf/201920805002ISVHECRI 2018Figure 7. The ratio between proton-lead by lead-proton ratesof very forward jets measured in CASTOR as function of theobserved jet energy, see Ref. [10].Figure 8. The average number of charged particles as functionof the leading jet pT in proton-proton collisions at 13 TeV, seeRef. [11]. Shown here is the “transMIN” region, which is trans-verse to the leading jet and in the hemisphere with less activity.While typical underlying event measurements are per-formed relative to the highest jet pT, CMS has also mea-sured it in reference the production of a Z boson [12]. Thisis shown in Fig. 9.Another way to study the internal partonic structure inhadron collisions is to search for double parton scattering.In Fig. 10 the measurement of same-sign WW productionin proton-proton collisions at 8 TeV [13] is shown. Thebackgrounds in such a measurement are numerous, thus,Figure 9. The average number of charged particles as function ofcentral Z pT, see Ref. [12], in proton-proton collision of differentcentre-of-mass energies.Figure 10. Output of a boosted decision tree for same-sign WWevents in proton-proton collisions at 8 TeV. All relevant signalcontributions are outlined. See Ref. [13].only a multivariate analysis is able to extract the quantityof interest.7 CorrelationsIt is an important discovery at the LHC that secondary par-ticle production in proton-proton collisions contains a newtype of correlations that were not expected. This was alsoobserved in proton-proton collisions at 13 TeV [15]. Such5EPJ Web of Conferences 208, 05002 (2019) https://doi.org/10.1051/epjconf/201920805002ISVHECRI 2018Figure 7. The ratio between proton-lead by lead-proton ratesof very forward jets measured in CASTOR as function of theobserved jet energy, see Ref. [10].Figure 8. The average number of charged particles as functionof the leading jet pT in proton-proton collisions at 13 TeV, seeRef. [11]. Shown here is the “transMIN” region, which is trans-verse to the leading jet and in the hemisphere with less activity.While typical underlying event measurements are per-formed relative to the highest jet pT, CMS has also mea-sured it in reference the production of a Z boson [12]. Thisis shown in Fig. 9.Another way to study the internal partonic structure inhadron collisions is to search for double parton scattering.In Fig. 10 the measurement of same-sign WW productionin proton-proton collisions at 8 TeV [13] is shown. Thebackgrounds in such a measurement are numerous, thus,Figure 9. The average number of charged particles as function ofcentral Z pT, see Ref. [12], in proton-proton collision of differentcentre-of-mass energies.Figure 10. Output of a boosted decision tree for same-sign WWevents in proton-proton collisions at 8 TeV. All relevant signalcontributions are outlined. See Ref. [13].only a multivariate analysis is able to extract the quantityof interest.7 CorrelationsIt is an important discovery at the LHC that secondary par-ticle production in proton-proton collisions contains a newtype of correlations that were not expected. This was alsoobserved in proton-proton collisions at 13 TeV [15]. SuchFigure 11. Geometry of the particle emission region measured in different collisions systems at different energies, see Ref. [14]. Shownare the parameters longitudinal size RL (left) and transverse size RT (right) as function of number of tracks, Ntrk.two-particle correlations were only known from heavy-ion collisions before. One possible conclusion from thiscould be that the underlying mechanisms in proton-proton,proton-lead and lead-lead collisions are the same and onlydepend on the maximum energy densities reached in col-lisions.Another option to look at correlations is to observeBose-Einstein correlations in the production of identicalsecondary particles. Due to interference effects, the proba-bility to observe identical particles as a function of particledirection is modified. These data will contain informationon the geometry of the emission region. In Fig. 11 someof these data are shown.8 Minimum bias resultsTo describe very high energy cosmic ray interactions alsominimum bias data is of particular importance. In cosmicray interactions there are typically no filters or any sortof event selection. Thus the particle production in typicalminimum bias collisions is what is most relevant. Herewe will focus only on a single examples of this type ofmeasurements.One of the directly relevant results for very high cos-mic ray interactions is the detailed measurements of thespectra of identified particles. In Fig. 12 some identifiedparticle spectra determined in proton-proton collisions at13 TeV are shown. There is obvious room also by theEPOS-LHC model for improvements.9 SummaryA brief overview of some of the most recent CMS forward-physics measurements has been reported. The physicscovered by those measurements covers a very wide andalso diverse range of fields. Several unique and very im-portant observations relevant for very high cosmic ray in-teractions are part of this.Figure 12. Measurement of the average pT of identified particlesversus the average number of charged tracks Ntracks in proton-proton collisions at 13 TeV, see Ref. [16].References[1] CMS Collaboration, JINST 3, S08004 (2008)[2] CMS Collaboration, Eur. Phys. J. 79, 277 (2018),arxiv:1809.11080 [hep-ex][3] CMS Collaboration, Submitted to: Phys. Rev. D(2017), arxiv:1706.08310 [hep-ex][4] CMS Collaboration, Eur. Phys. J. C78, 242 (2018),arxiv:1710.02586 [hep-ex][5] CMS Collaboration, JHEP 07, 161 (2018),arxiv:1802.02613 [hep-ex]6EPJ Web of Conferences 208, 05002 (2019) https://doi.org/10.1051/epjconf/201920805002ISVHECRI 2018[6] CMS Collaboration, Phys. Rev. D92, 012003 (2015),arxiv:1503.08689 [hep-ex][7] CMS Collaboration, JHEP 08, 119 (2016),arxiv:1604.04464 [hep-ex][8] CMS and TOTEM Collaborations, JHEP 07, 153(2018), arxiv:1803.04496 [hep-ex][9] CMS Collaboration, JHEP 03, 156 (2017),arxiv:1609.05331 [hep-ex][10] CMS Collaboration, Submitted to: JHEP (2018),arxiv:1812.01691 [hep-ex][11] CMS Collaboration, CMS-PAS-15-007 (2015)http://cds.cern.ch/record/2104473[hep-ex][12] CMS Collaboration, JHEP 07, 032 (2018),arxiv:1711.04299 [hep-ex][13] CMS Collaboration, JHEP 02, 032 (2018),arxiv:1712.02280 [hep-ex][14] A.M. Sirunyan et al. (CMS), Phys. Rev. C97, 064912(2018), arxiv:1712.07198 [hep-ex][15] V. Khachatryan et al. (CMS), Phys. Rev. Lett. 116,172302 (2016), arxiv:1510.03068 [hep-ex][16] A.M. Sirunyan et al. (CMS), Phys. Rev. D96, 112003(2017), arxiv:1706.10194 [hep-ex]

Review of recent forward physics results from the CMS experiment

CMS Collaboration

KITopen

There is a rich program of forward physics measurements within the CMS Collaboration covering a wide range of topics. In many cases there is a connection to quantities and effects relevant for very high energy cosmic ray interactions. Some of the recent measurements in the fields of exclusive final states, low-pT inclusive and diffractive cross sections, underlying event, multiparton interactions, double parton scattering, final state particle correlations and minimum bias results are briefly summarized here.There is a rich program of forward physics measurements within the CMS Collaboration covering a wide range of topics. In many cases there is a connection to quantities and effects relevant for very high energy cosmic ray interaction. Some of the recent measurements in the fields of exclusive final states, low-$\mathrm{p_T}$ inclusive and diffractive cross sections, underlying event, multi parton interactions, double parton scattering, final state particle correlations and minimum bias results are briefly summarized here

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Review of recent forward physics results from the CMS experiment

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