Lightly triggered events may yield surprises about the nature of “soft” particle production at LHC energies. I suggest that event displays in coordinates matched to the dynamics of particle production (rapidity and transverse

momentum) may help sharpen intuition, identify interesting classes of events, and test expectations about the underlying event that accompanies hard-scattering phenomena

Quigg, C.

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DOI 10.1393/ncc/i2011-10734-0Colloquia: LaThuile10IL NUOVO CIMENTO Vol. 33 C, N. 5 Settembre-Ottobre 2010Looking into particle productionat the Large Hadron ColliderC. Quigg(∗)Fermi National Accelerator Laboratory - Batavia, Illinois 60510, USA(ricevuto il 14 Settembre 2010; pubblicato online il 12 Gennaio 2011)Summary. — Lightly triggered events may yield surprises about the nature of“soft” particle production at LHC energies. I suggest that event displays in coor-dinates matched to the dynamics of particle production (rapidity and transversemomentum) may help sharpen intuition, identify interesting classes of events, andtest expectations about the underlying event that accompanies hard-scattering phe-nomena.PACS 13.85.Hd – Inelastic scattering: many-particle final states.1. – Introduction1.1. Early running at the LHC . – At this 2010 La Thuile meeting, we have heardaccounts of the first analyses of proton-proton collisions in CERN’s Large Hadron Col-lider, at energies of 450GeV and 1.18TeV per beam(1). On 30 March 2010, the LHCexperiments observed first collisions at 3.5TeV per beam, commencing a program thataims to deliver 1 fb−1 of integrated luminosity by the end of 2011. With each step inenergy beyond the Tevatron Collider’s√s = 1.96TeV, the LHC experiments will opennew worlds.During early low-luminosity running, the experiments will record significant numbersof lightly triggered events. Later in the run, more selective triggers will dominate thedata taking. What is true of the search for the agent of electroweak symmetry breakingand other new phenomena to be sought in hard-scattering events is also true for theminimum-bias events that will dominate the early samples:We do not know what the new wave of exploration will reveal.(∗) E-mail: quigg@fnal.gov(1) See the talks by Fabiola Gianotti, Andrey Golutvin, Paolo Meridiani, Francesco Prino, andAndreas Wildauer, and the first publications from ALICE [1], ATLAS [2], and CMS [3].c© Societa` Italiana di Fisica 327328 C. QUIGGThe staged commissioning of the Large Hadron Collider offers the chance to mapthe gross features of particle production over a wide energy range. I would identifythree goals: i) Validate assumptions that underlie searches for new phenomena in hard-scattering events. ii) Develop intuition for LHC experimenters (many of whom hadnever—or not since the Sp¯pS collider experiments—seen two protons hit until 23 Novem-ber 2009) and for interested theorists. iii) Make the most of the opportunity for explo-ration and discovery. This talk supplements my recent note [4] on this subject, whereadditional references may be found.1.2. Ken Wilson’s ancient program. – To orient ourselves, it is useful to look back tothe early studies of multiple production in the 1970s. Exploration of the terrain openedup by the Fermilab bubble chambers and the CERN Intersecting Storage Rings wascatalyzed, in part, by Ken Wilson’s celebrated paper, “Some Experiments in MultipleProduction” [5]. Wilson’s “experiments” amounted to a catalogue of informative plotsto address incisive questions.1) Topological cross sections: Do multiplicity distributions exhibit a two-componentstructure, suggestive of diffractive plus multiperipheral production mechanisms?2) Feynman scaling: Is the single-particle density ρ1(kz/E, k⊥, E) independent of thebeam energy E, when plotted in terms of Feynman’s scaling variable xF ≡ kz/E?3) Factorization: Is the single-particle density ρ1(kz/E, k⊥, E) in the backward (pro-ton) hemisphere independent of the projectile (the same for πp and pp scattering)?4) dx/x spectrum: Does the single-particle density exhibit a flat plateau in the centralregion when plotted in terms of the rapidity, y ≡ 12 ln[(k0 + kz)/(k0 − kz)]?5) Correlation length experiment: Does the two-particle correlation functionC(y1, y2) ≡ ρ2(y1, y2)−ρ1(y1)ρ1(y2) display short-range order, ∝ exp[−|y1−y2|/L]?6) Factorization test (#3) with central trigger (to eliminate diffraction).7) Double Pomeron exchange: Do some events display low central multiplicity withlarge rapidity gaps on both ends?The experimental studies responded affirmatively to questions 1)–6). The CDF Collabo-ration has recently reported the production of isolated charmonium states in the centralregion, characteristic of the reaction PP→ χc0 [6], as anticipated in question 7).2. – Particle production at the LHC2.1. New phenomena ahead? – This does not mean, however, that “soft” particleproduction should be regarded as settled knowledge. It has not yet been exhaustivelystudied at the Tevatron (see [7, 8] for recent important progress), and so we cannot besure that what was inferred from experiments up to√s = 63GeV accounts for all theimportant features at Tevatron energies. At the highest energies, well into the (∝ ln2 s?)growth of the pp total cross section, long-range correlations might show themselves innew ways. The high density of partons carrying pz = 5 to 10GeV may give rise tohot spots in the spacetime evolution of the collision aftermath, and thus to thermaliza-tion or other phenomena not easy to anticipate from the QCD Lagrangian. We mightLOOKING INTO PARTICLE PRODUCTION AT THE LARGE HADRON COLLIDER 329Fig. 1. – Schematic event display in (y, p⊥) space.anticipate a growing rate of multiple-parton interactions [9], perhaps involving correla-tions among partons. For example, the quark-diquark component of the proton mightmanifest itself in elementary collisions involving diquarks. The ln s expansion of the ra-pidity plateau softens kinematical constraints in the central region, and the sensitivityto high-multiplicity events (or otherwise rare occurrences) of modern experiments vastlyexceeds what could be seen with bubble-chamber statistics. The CMS Collaborationreports [3] that the standard pythia tunes underestimate the growth with energy of thecentral density of charged particles, dNch/dη|η=0, from√s = 900GeV to 2.36TeV. At√s = 900GeV, the ATLAS experiment observes [2] that dNch/dη|η=0 lies some (5–15)%above the predictions of the Monte Carlo models. For all these reasons, I suspect thata few percent of minimum-bias events collected at√s  1TeV might display unusualevent structures(2). We should look! But how?2.2. Learning to see. – I believe that looking at events can be an important part ofthe answer. Blind analysis [11] has won a secure place in our practice of particle physics,as a talisman against experimenter’s bias, but it is not apposite when we are seekingto get the lay of the land. It would be a big mistake to suppose that we know all theimportant questions, even before we arrive in the new world! Bjorken suggested longago [12] a three-dimensional representation of multiparticle events that could engageour human powers of visualization and pattern recognition, in the hope of identifyingimportant new questions. For particle production in soft collisions, it is not spatialcoordinates that are most apt, but a representation in terms of (pseudo)rapidity and (two-dimensional) transverse momentum. To begin, draw a (pseudo)rapidity axis as an obliqueline. Represent each track i in the event by a vector drawn from (yi, 0, 0) to (yi, pix, piy),as in the example shown in fig. 1 (all scales linear). The (y, p⊥) representation is noneother than a curled-up vector representation of the LEGO R© plot for individual tracks,with thresholds for display set as low as possible.As a start, I encourage the LHC collaborations to produce (y, p⊥) displaysof minimum-bias events acquired during early running. Samples as small asa few hundred events would already build intuition, but I would go further.(2) Many of the questions posed in the FELIX physics document [10] are apt for the detectorsnow taking data at the LHC.330 C. QUIGGFig. 2. – (Colour on-line) Example (y, p⊥) event displays from Run 2 of the CDF Experimentat Fermilab. These are chosen from a sample of “zero-bias” events with at least 10 tracks in thecentral region and one good primary vertex. Each rapidity axis spans −1 ≤ y ≤ 1; the lengthof the transverse-momentum axes is 1GeV. Dashed red and solid blue lines label positively andnegatively charged tracks, respectively. (a) Local compensation of p⊥ and electric charge. (b)Local p⊥ imbalance. (c) Local charge imbalance. (d) Rapidity gap. (e) Hot spot.I suggest that the collaborations make available live streams of (y, p⊥) repre-sentations, along with the online displays of events that show the structure interms of detector elements in ordinary space. It is useful to color the tracksto label their charges, and to identify species where possible.More is to be learned from the river of events than from a few specimens!Changes in event structure as a function of beam energy, or the onset of newfeatures, might raise important questions.Thanks to work of Niccolo` Moggi [13] and William Wester, I can show in fig. 2 a fewexample (y, p⊥) displays of events recorded by the CDF Collaboration in Run 2 at theTevatron, in p¯p collisions at√s = 1.96TeV. The events shown there are chosen from a“zero-bias” sample after selections to ensure a single primary vertex within 30 cm (1σ)of the nominal crossing point and require (for visual interest) at least 10 well-matchedtracks in the rapidity interval −1 ≤ y ≤ 1. (By selecting higher-than-average-multiplicityevents, we are excluding candidates for γγ and double-Pomeron events that would berestricted to low central multiplicities.) Event (a) exhibits the average behavior familiarat lower energies, whereby transverse momentum and additive quantum numbers such aselectric charge are compensated locally in rapidity. Each of the remaining events deviatesfrom the typical expectations, inviting further study of large numbers of events to ascer-tain whether they fit neatly into fluctuations about the mean or suggest new event classes.Transverse momentum is unbalanced in event (b); positive and negative charges are sep-arated in event (c); a rapidity gap of slightly more than one unit appears in event (d);and a good deal of the action in event (e) seems concentrated in a “hot spot” in rapidity.LOOKING INTO PARTICLE PRODUCTION AT THE LARGE HADRON COLLIDER 331Fig. 3. – An unusual event captured in the CDF Run 1 detector.By scanning many events, it should be possible to (quickly) develop intuition aboutwhat is “normal,” both for lightly triggered events and for events that satisfy a hardtrigger. It will also be valuable to compare streams of real events with streams of simu-lated events. It is certain that something is missing from the Monte Carlo programs. Weneed to learn what the omissions are, and how important they are to our understanding.Attentive scanning could well yield the suggestion of unanticipated phenomena—at thelevel of a few percent, one in a thousand, even one in 10 000. Modern computer toolsmake it straightforward to construct (y, p⊥) displays that can be zoomed, panned, androtated in three dimensions. The ability to manipulate events and regard them fromchanging perspectives can engage our human powers of perception more fully.2.3. New Physics in the Weeds. – The strong interactions are extraordinarily rich.Even as we learn to extend the reach of perturbative QCD beyond reactions involvinga few partons in the final state, we should be attentive to the whole range of strong-interaction phenomena. The rest of the story includes common processes with large crosssections such as “soft” particle production, elastic scattering, and diffraction. It may wellbe that interesting, unusual occurrences happen outside the framework of perturbativeQCD—happen in some collective, or intrinsically nonperturbative, way. A powerfultechnique to isolate hard-scattering reactions is to impose stringent cuts in the dataselection, or to clarify the essential structure of events by setting display thresholdshigh. When scanning event displays for hints of new phenomena, however, it may beadvantageous to set the display thresholds as low as possible.An interesting example—an atypical event observed in p¯p interactions at√s =1.8TeV by CDF’s Run 1 detector, is shown in fig. 3(3). This event was accepted bya∑E⊥ trigger, without any topological requirement. The LEGO R© plot shows manybursts of energy: More than a hundred active towers pass the display threshold of(3) See Figure 3(c) of [14] for a similarly isotropic event recorded in the UA1 Detector in p¯pcollisions at√s = 630GeV, in whichPE⊥ = 209GeV for |y| < 1.5.332 C. QUIGG0.5GeV. The total transverse energy in the event is 321GeV, but it is not concen-trated in a few sprays, it is everywhere. The central tracking chamber records aboutsixty charged particles.I am assured that this “hedgehog” event is authentic; it is not merely coherent noisein the counters. The colleague who selected this specimen estimated similar events tobe about as common in the online event stream as Z0 production and decay into leptonpairs: about one in ten thousand triggers. I include this outlier as a reminder that whenwe think about the strong interactions outside the realm of a single hard scattering, weshould think not only about the large diffractive and “multiperipheral” cross sections,but also about less common phenomena.3. – Opportunities for exploration and discoveryThe minimum-bias and lightly triggered data recorded during early LHC running willbe valuable for developing intuition and for validating the assumptions that underliesearches for new physics in hard-scattering events. However, these data sets, to be gath-ered over steps in beam energy, also represent an important opportunity for explorationand discovery. One promising track will be to emulate the early studies of multiple pro-duction, which emphasized observables constructed from individual particles: topologi-cal cross sections (multiplicity distributions, including forward-backward asymmetries ofmultiplicity distributions), inclusive and semi-inclusive two-particle correlation functions,and charge-transfer studies. Some measurements that would be especially informative forrefining Monte Carlo event generators are suggested in [15]. For some classes of events,analyses of bulk properties, such as studies of elliptic flow and determinations of thermo-dynamic parameters may prove powerful. We will need all the established methods—plusnovel techniques—to learn to see what the LHC data have to show.It is not too late to characterize particle production more completely in the Tevatronexperiments. The existing samples of lightly triggered events can be mined further,with an eye to establishing in detail the mechanisms at play in particle production andidentifying suggestive classes of unusual events. It is worth considering a brief, dedicated,Tevatron run at√s = 900GeV, to match the samples collected in the LHC’s pilot runat the end of 2009. The similarities and differences between pp and p¯p collisions may berevealing.I advocate looking at individual events, not just distributions. Beyond honing intu-ition, the first effect of looking at events, displayed in appropriate coordinates, may beto validate in broad terms the prevailing picture of particle production. We should alsobe able to test the completeness of the Monte Carlo frameworks that have become soindispensable to the search for new (hard-scattering) phenomena. I think it likely thatwe will encounter suggestions of new event classes, to be pursued in focused studies thatgo beyond visual inspection. New trends may emerge with increasing beam energy, or atthe extremes of high and low multiplicity. The comparison of events with and withouta hard trigger should be revealing. The goal of the visual approach is to discover ascompletely as we can the richness of phenomena that our theories will have to explain,and to orient us for detailed exploration of the new worlds.∗ ∗ ∗I thank N. Moggi and W. Wester for the example event displays from CDF Run 2,and the CDF QCD convenors, C. Mesropian and S. Pranko, for permission to includethem in this presentation. D. Jovanovic provided the hedgehog event in fig. 3, andLOOKING INTO PARTICLE PRODUCTION AT THE LARGE HADRON COLLIDER 333J. Rohlf called my attention to the earlier UA1 observations. I thank J. Bjorken formany stimulating conversations. I am grateful to the organizers of these Rencontres dePhysique de la Valle´e d’Aoste for their kind invitation to take part. Fermilab is operatedby Fermi Research Alliance, LLC under Contract No. DE-AC02-07CH11359 with theUnited States Department of Energy.REFERENCES[1] ALICE Collaboration, Eur. Phys. J. C, 65 (2010) 111 [arXiv:0911.5430].[2] ATLAS Collaboration, Phys. Lett. B, 688 (2010) 21.[3] CMS Collaboration, JHEP, 02 (2010) 041 [arXiv:1002.0621].[4] Quigg C., Learning to See at the Large Hadron Collider, arXiv:1001.2025 (2010).[5] Wilson K. G., Some Experiments in Multiple Production, Cornell Report CLNS-131(1970).[6] Aaltonen T. et al. (CDF Collaboration), Phys. Rev. Lett., 102 (2009) 242001[arXiv:0902.1271].[7] Aaltonen T. et al. (CDF Collaboration), Phys. Rev. D, 79 (2009) 112005,arXiv:0904.1098.[8] Aaltonen T. et al. (CDF Collaboration), Studying the Underlying Event in Drell-Yanand High Transverse Momentum Jet Production at the Tevatron, arXiv:1003.4220.[9] Bernhard R. et al., Proceedings of the First International Workshop on Multiple PartonicInteractions at the LHC (MPI08), DESY-PROC-2009-06, arXiv:1003.4220.[10] Ageev A. et al., J. Phys. G, 28 (2002) R117.[11] Klein J. R. and Roodman A., Annu. Rev. Nucl. Part. Sci., 55 (2005) 141.[12] Bjorken J. D., Multiple Processes at High Energy, SLAC-PUB-0974 (1971).[13] Moggi N., Nuovo Cimento C, 32 (2009) 113.[14] Albajar C. et al. (UA1 Collaboration), Z. Phys. C, 36 (1987) 33.[15] Skands P., Min-Bias Models – the Modeling of SD, DD, and ND, Talk given atMinimum-Bias and Underlying-Event Working Group Meeting at CERN, March 1–2 2010,http://indico.cern.ch/conferenceDisplay.py?confId=83609.

Looking into particle production at the Large Hadron Collider

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Looking into particle production at the Large Hadron Collider

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