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The Physics of the ALICE EMCal

T.Awes9, H.Bichsel13, R. Bellwied14, M.Cherney3, V.Cianciolo9, T. M. Cormier14,J.Cramer13, Y.Efremenko9, M. Elnmir14, A.Enokizono7, D.Ferenc1, V.Ghazikhanian2

Y.Gorbunov3, M. Heffner7, H.Huang2, T.Humanic10, D. Keane5, J.Klay8, S.Klein6

I.Kotov10, P.Jacobs6, M.Lisa10, E.Lorenz1, S. Margetis5 B. Mayes4, J. Newby8,B.Nilsen10, G.Odyniec6, A.Pavlinov14, V.Petrov14, L.Pinsky4, D.Prindle13, C.Pruneau14,J.Putschke6, K.Read9, J.Riso14, H.G.Ritter6, I.Sakrejda6, R.Scharenberg11, J.Seger3,D.Silvermyr9, R.Soltz7, S.Sorensen12 P.Stankus9, J.Symons6, S.Trentalange2,M.VanLeeuwen6, S.Voloshin14, F.Wang11, G.Westfall8, C.Whitten2, G.Young9

ALICE-USA Collaboration December 2005

1. University of California Davis2. University of California Los Angeles3. Creighton University4. University of Houston5. Kent State university (Observer Institution)6. Lawrence Berkley National Laboratory7. Lawrence Livermore National Laboratory8. Michigan State University9. Oak Ridge National Laboratory10. Ohio State University11. Purdue University12. University of Tennessee13. University of Washington, Seattle14. Wayne State University

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The proposed ALICE Electromagnetic Calorimeter (EMCal) adds significant newphysics scope to the ALICE experiment, in particular for the study of medium–inducedmodification to jets as a probe of dense matter (“jet quenching”). The EMCal in ALICEenables the most extensive exploration possible at the LHC of jet fragmentation in heavyion collisions. The major capabilities of the EMCal are as follows:

The EMCal provides an efficient and unbiased high ET jet trigger over a broadenergy range even in the most complex heavy ion collisions. It also triggersefficiently on high pT photons and electrons.

The EMCAL, together with the excellent tracking and particle identificationcapabilities of ALICE, enables detailed exploration of the medium-inducedmodification of jet fragmentation over a broad kinematic range, from the hardestjet fragments of high ET jets to very soft fragments.

The EMCAL provides good direct photon detection, enabling precise explorationof the fragmentation of the jet recoiling from the photon over a broad kinematicrange.

The EMCal provides good hadron rejection for efficient measurement of high pT

electrons, enabling study of heavy quark jets over a broad kinematic range.

This document gives an overview of the physics of jet quenching, the current status ofexperimental studies in this area at the Relativistic Heavy Ion Collider (RHIC), and therole of jet quenching in the LHC heavy ion program. We then discuss the physicscapabilities of the EMCal in more detail.

1.1 Jet quenching as a probe of QCD matter

High energy partons interact and lose energy in colored matter, in analogy for QCD to thefamiliar energy loss in matter of objects carrying electric charge1. This situation isrealized with jet production in high energy hadronic collisions, where a hard scatteredparton propagates through the surrounding matter generated in the collision beforefragmenting into a jet. Jets thereby provide a self-generated tomographic probe of themedium produced in high energy nuclear collisions (“jet quenching”).

Bjorken1 calculated the thermally-averaged elastic momentum transfer with the mediumand showed that the total energy loss E of the hard parton depends on the temperature ofthe medium as T2. Later calculations concluded that elastic energy loss generates onlysmall effects in practice, but that energy loss via medium-induced gluon radiation (gluonbremsstrahlung) could generate significant modification of jet fragmentation and that themagnitude of the radiative energy loss is sensitive to the color charge density of themedium2. The effects of medium-induced radiative energy loss have since beencalculated in various theoretical frameworks: multiple soft scattering3 (BDMPS), fewhard scatterings4 (GLV), twist expansion5, and a path integral approach6.

In the case of multiple soft scattering the medium is characterized by a transportcoefficient /ˆ 2q , where is the average momentum kick of a gluon interacting in

the medium and is its mean free path. The gluon radiation spectrum is suppressed

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relative to the Bethe-Heitler spectrum due to coherence effects, leading to quadratic pathlength dependence of the total medium-induced radiated energy7 8 2ˆLqE smedium .

Longitudinal expansion of the medium reduces the length dependence toLEmedium , while finite partonic energy truncates the induced radiation spectrum and

results in energy-dependent energy loss. The induced radiation spectrum differs in detailbetween the BDMPS and GLV approaches, but their total energy loss is similar forcomparable medium properties8. For realistic initial conditions the energy loss in bothapproaches is typically carried by a moderate number (~3) of radiated gluons, eachcarrying moderate energy (~1 GeV)8. Recent reexamination of elastic energy lossindicates that it may in fact not be negligible in the experimentally accessible kinematicrange9.

These theoretical considerations suggest that jet-related measurements provide uniquetools to probe the QCD matter generated in high energy nuclear collisions. High Q2

processes occur only between constituents of the incoming nuclei, and the formation timefor hard scattering products is ~1/Q~1/ET

jet. High ET jets are therefore formed early (<<1 fm/c) and probe the medium at the hottest, densest phase of its evolution. Theintegrated energy loss is therefore sensitive to the color charge (gluon) density at thisearly time. Radiative energy loss corresponds to softening and broadening of jetfragmentation, and detailed study of jet modification in dense matter can probe both themechanisms underlying energy loss and the response of the medium to interactions withthe jet.

1.2 The discovery of jet quenching at RHIC

Measurements of jet quenching in high energy nuclear collisions are challenging. Highbackground multiplicities can obscure the details of the fragmentation, making full jetreconstruction with good energy resolution difficult, and less direct methods musttherefore be found. The inclusive high pT hadron spectrum is the most easily accessiblejet-related observable in nuclear collisions, since jet fragmentation is expected todominate the yield above pT~few GeV/c. Softening of the partonic spectrum due toenergy loss in matter is therefore reflected in the suppression of inclusive hadron yields athigh pT. However, the inclusive hadron spectrum is dominated by relatively low energyjets (i.e. with relatively high production cross section) that happen to fragment hard intoleading hadrons that carry a large fraction of their energy (“trigger bias”). For pT~10GeV/c at RHIC (s=200 GeV) this fraction is <z>=<pT/Ejet>~0.6-0.7, compared to<z>~0.2-0.3 for the highest energy particle in a jet from an unbiased jet population.

Leading hadron suppression is measured by comparing the inclusive yields for nuclearcollisions and more elementary collisions via the ratio

TppAA

TAAAA dpdT

dpdNR

,

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where dNAA/dpT is the inclusive pT spectrum in A+A collisions, dpp/dpT is thedifferential cross section measured in p+p collisions, and TAA is a geometric factorcalculated using Glauber theory that accounts for the equivalent number of binarynucleon-nucleon collisions in an A+A collision. RAA is normalized to unity in the absenceof nuclear effects, so that hadron suppression in nuclear collisions corresponds to RAA<1.

Figure 1 shows RAA measured at RHIC for and unidentified charged hadrons in 200GeV central Au+Au and p+p collisions. For pT>5 GeV/c, hadron production in Au+Au issuppressed by a factor 5 relative to p+p. In contrast, direct photon production is seen notto be suppressed. Since jet and photon production are both hard processes with similarscaling of production cross section and initial spatial distributions in nuclear collisions,the absence of suppression for colorless photons demonstrates that the high pT hadronsuppression is in fact due to final state interactions of the colored parton with themedium. Similar conclusions were drawn from the comparison to RAA for d+Aucollisions10, which likewise is not suppressed at high pT but rather exhibits anenhancement due to initial state multiple scattering (“Cronin effect”). The large hadronsuppression for central Au+Au is qualitatively different from that expected fromconventional nuclear effects, including shadowing. Figure 1 also shows comparison to apQCD-based energy loss calculation using the GLV framework, which requires an initialgluon density dNg/dy~1100 to reproduce the magnitude of the suppression. This densityis about 30 times that of cold nuclear matter. The hadron suppression measurements andcomparison to theory in Figure 1 are clear evidence for jet quenching in very densematter at RHIC.

Figure 1: RAA for 0, charged hadrons, and direct photons in central Au+Au compared to p+pcollisions at RHIC11.

Similar calculations to that shown in Figure 1, but within BDMPS framework, find that abroad range of transport coefficient 155~ˆ q GeV2/fm can adequately describe theinclusive hadron suppression12. This poor discrimination results from strong partonicenergy loss biasing the observed jets to those generated on the surface and headed

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outward, which thereby avoid the high density region and suffer smaller than averageenergy loss. Once the core of the reaction volume is sufficiently opaque, the surface-biased population has little sensitivity to the magnitude of opacity. A more detailed probeof energy loss requires more sensitive observables.

Figure 2 utilizes the back-to-back nature of dijets for an additional probe of jetquenching. The left panel shows the event-averaged distribution of azimuthal interval between a high pT charged hadron trigger and other high pT tracks in the event (pT

trigger>4GeV/c, pT

associated>2 GeV/c), chosen to bias towards jet fragments and against theproducts of soft processes. Combinatorial background has been subtracted. The near-sidepeak (~0) is characteristic of hadrons drawn from the fragmentation of a single jet,with similar strength for p+p, d+Au and central Au+Au collisions. The recoil peak(~) is characteristic of hadrons drawn from back-to-back dijets. It exhibits similarcorrelation strength in p+p and d+Au collisions, but shows a dramatic suppression incentral Au+Au collisions. Since the trigger population is biased towards outward-goingsurfaced-generated jets, the recoil is biased towards those jets with longer than averagepath length in the medium. The strong suppression of the high pT recoil yield in centralAu+Au collisions but not p+p and d+Au is additional clear evidence of energy loss of therecoiling parton in dense matter.

Figure 2: Dihadron azimuthal correlations with a high pT trigger13 14.

The jet energy and momentum are not lost, however, and if the fragmentation is softeneda statistical correlation should exist between a high pT trigger and lower pT hadrons.Figure 2, right panel, shows a similar analysis to the left panel but with the threshold forthe associated hadron lowered to 150 MeV/c to attempt to recover the fragments of thesoftened jet. A recoil correlation is indeed seen which is stronger and broader in Au+Authan in p+p and d+Au. The curve shows the function cos(), which roughly describesthe soft recoil distribution and is the expected shape if there are no dynamical correlationsbeyond simple momentum conservation. If the trigger population is dominated by jetfragments, the broadening of the correlation relative to p+p indicates that the recoil jethas interacted with the medium and its energy may have been equilibrated. More recentdiscussion has centered on the possible existence of additional structure in the low pT

azimuthal correlation, expected for instance from shock phenomena due to finite speed ofsound in the medium15. While these analyses have significant systematic uncertainties

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due to large background subtraction, especially at low pT, they suggest that details of theinteraction of the jet with the medium and the response of the medium to the depositedenergy may be measurable. However, distinguishing various underlying mechanisms willrequire analysis with higher order correlations and large datasets for statisticallysignificant results.

Figure 3 shows another striking discovery at RHIC, pertaining to the particle compositionat pT of a few GeV/c in nuclear collisions. The left panel shows RCP

* for identifiedprotons and pions in 200 GeV Au+Au collisions. While 0 exhibit a strong suppression incentral collisions, the proton yield is not suppressed, scaling rather as the number ofbinary collisions. The right panel shows the yield ratio of /K0

s measured to higher pT.The enhancement of baryon relative to meson yield in central Au+Au collisions islikewise evident here but is seen to have strong pT dependence, reverting at high pT to thep+p ratio that is also characteristic of jets measured in e+e- collisions. The baryonenhancement is evidently confined to the “intermediate pT” interval pT~2-5 GeV/c.Measurements at intermediate pT of baryon and meson elliptic flow in non-centralcollisions show a striking scaling with valence quark number, while dihadron correlationmeasurements in this region exhibit jet fragmentation-like features. These phenomenacan be accommodated in models in which hadron production at intermediate pT is due tocoalescence of constituent quarks which originate both from fragmenting jets and from athermalized and flowing bulk medium. These models suggest that the novel effects seenat intermediate pT in nuclear collisions arise from the interplay between hard and softprocesses and may provide a new window into the dynamics of the medium and theprocesses leading to hadronization.

Figure 3: baryon enhancement at intermediate pT from PHENIX16 (left) and STAR17.

1.3 Jet quenching: current status

A recent, high statistics analysis18 extends the correlation study in Figure 2, left panel, tomuch higher pT, beyond intermediate pT region and into the region where jetfragmentation in vacuum is expected to dominate. The recoil jet is now seen clearly but at

* RCP is similar in definition to RAA, but with the reference spectrum taken from peripheral A+A collisionsrather than p+p collisions. RCP is sometimes preferred to RAA for systematic or statistical reasons.

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a suppressed rate, raising the possibility of a precise differential measurement of theenergy loss which could set an upper limit on the energy density. Combined withmultiplicity measurements that reflect entropy production, this measurement could set alower bound on the number of degrees of freedom in the medium, providing a newapproach to observing deconfinement19.

Heavy quark energy loss at moderate momentum (i.e. moderate velocity) is expected tobe reduced relative to that for light quarks due to the suppression of gluon radiation in theforward cone <mQ/EQ (“dead cone effect”)20. Additional effects modify thisexpectation21 but the general trend of lower energy loss for massive quarks is expected topersist. Explicit reconstruction of charmed mesons is not yet achievable in central heavyion collisions at RHIC, but a first look at heavy quark energy loss via RAA of the high pT

non-photonic electron spectrum (dominated by semi-leptonic decay of c and b) showssurprisingly similar suppression to that of light hadrons (Figure 1). There is currently avigorous debate in the theory community whether these measurements can be fullyaccounted for by radiative energy loss, or whether a significant elastic scatteringcontribution is needed.

Jet-related measurements in nuclear collisions at RHIC have revealed a rich set ofphenomena that probe the medium generated in heavy ion collisions. Jet studies haveshown that the medium is opaque, with density many times that of cold nuclear matter.The medium appears to absorb the energy lost by the jet, and its response to this energydeposition can potentially also be studied. The current results are largely qualitative,however, due to the biased nature of the probes and the complexity of interpreting themat the modest pT measured thus far. The dominant theoretical paradigm holds thatmedium-induced gluon radiation is the main energy loss mechanism, though the presentdata do not provide sharp discrimination of radiative from elastic mechanisms.

New, more differential and precise measurements may enable a quantitative measurementof the opacity and provide tests of the energy loss mechanism such as the parton massand color charge dependence. Higher order multi-hadron correlations will discriminatevarious mechanisms contributing to the two-particle correlation distributions discussedabove. Comparison to theoretical calculations in a regime where perturbative approachesare most reliable will allow quantitative measurement of key properties of the mediumand give new insight into the mechanisms underlying jet quenching.

1.4 Jet quenching measurements at the LHC

The matter generated in heavy ion collisions at the LHC is expected to be much hotterand denser initially than at RHIC. Particle production will be dominated by hardscattering of partons at low enough x to be in the saturation regime, and the initialconditions of LHC heavy ion collisions may be calculable ab initio by controlledtheoretical methods. The fireball lifetime is expected to be much longer than at RHIC,with its dynamics dominated by partonic degrees of freedom. Jet-related measurementswill play as central a role in the study of QCD matter at the LHC as they have at RHIC,perhaps with even stronger medium-induced modifications to jet structure due to the

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higher initial density. We expect that comparison of jet measurements at RHIC and theLHC facilities will provide unique and important cross-calibrations, leading to deeperunderstanding of the matter created at both energy regimes.

Full understanding of jets as a probe of dense matter requires the study of the evolution ofjet fragmentation over a broad jet energy range, from the moderate energy jets studied atRHIC where quenching effects are seen to be strong, to jets of 100 GeV and beyond,where the fraction of jet energy lost due to interactions in matter expected to be small.The factor 30 increase in collision energy at the LHC relative to RHIC corresponds to ahuge increase in kinematic and statistical reach for hard probes. Figure 4 shows annualyields for various hard processes in ALICE, for minimum bias Pb+Pb collisions atnominal luminosity (1 Pb+Pb year = 106 seconds, L=51026 cm-1s-1). Simple binarycollision scaling (~A2) from calculated p+p cross-sections has been applied, with nonuclear effects taken into account. Within the ALICE EMCal acceptance the annual jetyields are large: 107 per year for ET>50 GeV (~10 Hz) and 6105 per year for ET>100GeV.

The annual yields of hard processes are expected to be similar for runs with lighter ionbeams to those for Pb+Pb. The second column of Table 1 (page 13) shows the maximumluminosity anticipated at the beginning of an LHC fill for various systems. The quantityin square brackets is the luminosity scaled by A2 to obtain the “p+p equivalentluminosity”, which is proportional to the rate of hard processes in the absence of nucleareffects. The variation in p+p equivalent luminosity for various collision systems is abouta factor 2, indicating that Figure 4 is also approximately valid for lighter nuclei. The p+pluminosity is limited in ALICE due to pileup effects in the slow detectors. However, thep+p data-taking period at 14 TeV will be an order of magnitude longer than the heavy ionperiod (107 s vs 106 s) and the cross sections are larger at higher s†, making the annualyields of hard processes in ALICE similar or greater than those in Figure 4 for p+p.Reality factors such as trigger efficiencies and beam lifetimes will cause additionalvariation in recorded yield per collision system, but the above simple scaling estimatesshow that a robust and efficient experimental program is possible in which hard processescan be studied with comparable kinematic reach in systems of widely varying mass andinitial state nuclear effects.

The extended kinematic and statistical reach will enable the study of qualitatively new jetobservables at the LHC. As discussed above, leading particle inclusive spectra andcorrelation studies at RHIC sample a jet population that is highly biased in terms of bothfragmentation and geometric origin. In contrast, for the high energy jet population seenby ALICE, more complete and less biased jet reconstruction over the heavy ionbackground will be achievable on an event-wise basis. Since partonic energy losscorresponds to the modification of jet fragmentation, this unbiased jet population willsample the full spectrum of energy loss and provide more precise probes of the mattergenerated in the collision.

† The need for p+p running at 5.5 TeV, for heavy ion reference data at a common s, is under discussion.

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Figure 4: Hard process yields in EMCal and ALICE accetpance for one LHC year of 5.5 TeV Pb+Pb.Vertical arrows show reduction yield due to reduction in acceptance for finite size jet trigger patch.

Guidance for the pT scale of hadron production arising from medium-induced radiationcan be obtained from a recent calculation which incorporates medium effects into theModfied Leading Log Approximation (MLLA) parton shower framework22. Figure 5, leftpanel, shows the well-known “humped-backed” hadron multiplicity distribution, plottedas a function of the scaling variable =-log(phadron/Ejet). The MLLA formalism accuratelydescribes this distribution for jet fragmentation in vacuum, as seen from comparison toe+e- data. Also shown is its expected modification in medium, with suppression of hardfragments (low ) and a marked enhancement for the softest fragments (high ). Themedium-induced excess persists up to pT~5 GeV/c for jets with ET~100 GeV and above,which should be readily observable above background even in the highest multiplicityevents.

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Figure 5. Left: MLLA calculations of the single inclusive hadron distribution as function of =-log(phadron/Ejet) for vacuum fragmentation compared to e+e- data and for medium-modified jets22.Right: gluon multiplicity distribution in jet cone of radius C as function of transverse momentum kT

relative to jet direction23.

Radiative energy loss should both soften and broaden the jet structure. Figure 5, rightpanel, shows the gluon multiplicity distribution within jet cone R=C as a function ofmomentum kT

‡ perpendicular to the jet direction23. The left sub-panels show thedistribution for vacuum fragmentation, together with the broadening due to interaction inthe medium. A significant medium-induced enhancement is seen at kT~few GeV/c,calculated both in the GLV (“single hard”) and BDMPS (“multiple soft”) frameworks ofenergy loss. The enhancement persists when the gluon distribution is truncated below Ecut

(right sub-panels). This calculation suggests that measurements of hadrons with pT of afew GeV/c will be very sensitive to energy loss effects, even for very high energy jets.

The rich phenomenology of jet measurements at RHIC, together with theoreticalpredictions of significant medium-induced softening and broadening of jetstructure, underline the importance of measuring the full momentum spectrum ofjet fragments, down to pT~1 GeV and below, in order to understand fully the energyloss process of partons in matter and the response of the medium to the lost energy.While calorimetery is essential to provide triggering and complete coverage of jetstructure, jet measurements in heavy ion collisions also require robust tracking inthe high multiplicity environment together with detailed particle identificationcapabilities.

Potentially the most precise investigation of jet quenching can be made by utilizing thecoincidence measurement of a jet recoiling from a gauge boson (or Z). The colorlessboson does not interact with the medium and therefore provides a clean record of themomentum transfer in the hard interaction, enabling measurement of the truefragmentation function of the recoiling jet27. Figure 4 shows that the +jet rate isstatistically robust for pT < 40 GeV/c in heavy ion collisions. The / ratio (the key

‡ This momentum component is more commonly denoted jT.

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experimental parameter) is however less than 10% in this region for p+p and peripheralPb+Pb, making the measurement challenging, though suppression raises it by a factor~5 in central Pb+Pb24. QCD fragmentation photons may dominate the prompt photonyield up to 50 GeV or higher25, and the use of isolation cuts in heavy ion events is understudy to control this background. The +jet measurement, while difficult, is important forthe jet physics program. The Z+jet channel is background-free but suffers from smallcross section and will be statistically marginal for all LHC experiments at nominal Pb+Pbluminosity (~1000 Z()+1 jet events per year for pT>40 GeV, integrated over all )26.

ALICE will run with p+p luminosity <1031 cm-2s-1 in order to limit TPC pileup. Thequestion then arises whether jet-related measurements of p+p collisions in ALICE arecompetitive with ATLAS and CMS, which will necessarily run at much higher p+pluminosities. ALICE with the EMCal may in fact occupy a unique niche in the LHC p+pprogram, precisely due to the limited luminosity at ALICE. The p+p bunch crossinginterval of 20 nsec and minimum bias cross section of ~100 mbarn correspond to anaverage of 20 independent interactions per bunch crossing at design luminosity 1034 cm-

2s-1, with the event pileup largely obscuring the soft part of a triggered event. ATLAS andCMS will record p+p data at lower luminosity and may run with low magnetic fields toaid in low pT tracking, and we do not wish to imply that they will not contribute in thisarea, but it appears to us that ALICE with the EMCal is optimally configured for suchmeasurements.

1.5 Physics performance of the EMCalThe proposed EMCal brings new capabilities to the ALICE experiment for the study ofjet quenching. It provides high pT triggers for photons and electrons, with greaterkinematic reach than the PHOS due to greater acceptance, and supplies a jet trigger viaenergy sums over large phase space areas. Combined with the ALICE tracking detectors,it provides an almost complete measurement of jet energy and characterization of jetfragmentation. It augments and extends the electron identification capabilities of ALICE.In this section we discuss the EMCal performance for the four main aspects of its physicsprogram: jet trigger, jet reconstruction, direct photon measurements, and high pT electronidentification. We consider the full spectrum of collision systems, from p+p to centralPb+Pb.

To set the context for the measurements, a HIJING model calculation of background incentral Pb+Pb collisions predicts that 20% of EMCal towers will contain at least 100MeV of background energy, while 8% will contain at least 200 MeV.

1.5.1 Jet trigger

The maximum Pb+Pb interaction rate is ~4-8 kHz, while minimum bias Pb+Pb eventscan be recorded by ALICE at ~100 Hz. In order to achieve hard process trigger ratesbelow ~10 Hz, an overall rejection of 400-800 is therefore needed. The EMCal utilizesthe same front-end electronics as the ALICE PHOS calorimeter29 and thus will providesimilar () and electron triggers. Measurent of high pT , and electrons are of broadinterest and these EMCal triggers will be allocated significant DAQ bandwith.

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A high pT trigger also provides a bias towards jets. However, 10 Hz trigger raterequires threshold pT~20 GeV/c (see Figure 4), generating significant fragmentation biasover a broad energy range. The resulting jet sample will be dominated by relatively lowET jets that fragment hard into a single leading pion. A more refined selection of high ET

jets requires a jet trigger which sums energy over a finite area of phase space.

In heavy ion collisions, the interaction rate is sufficiently low that all interactions can besampled by the Level 1 (L1) trigger, which has a latency of ~6 s. The EMCal canpotentially provide an L1 jet trigger on this timescale. However, since the EMCal isprimarily sensitive to electromagnetic energy, the least-biased jet trigger will utilize theALICE High Level Trigger (HLT) to combine the electromagnetic energy measurementof the EMCal with charged particle measurements from the tracking detectors. The L1rejection needed to match the HLT input bandwidth is about a factor 10 for Pb+Pb, whichsets the performance criterion for the EMCal L1 jet trigger.

In p+p collisions, the ALICE L0 minimum bias trigger is in fact somewhat biased and theEMCal must issue a jet trigger at L0. The PHOS electronics allows a fast analog sum of2x2 towers to provide an L0 trigger on the timescale of ~600-800 ns. A threshold on thissum of about 1 GeV may provide sufficient event rejection with good jet efficiency, sothat the L1 jet trigger can be run on the surviving events. This issue has not yet beenstudied, however.

Figure 6; Level 1 jet trigger bias for p+p collisions (PYTHIA) for various patch energy thresholds.Features at ET<50 are artifacts due to a threshold in event generation.

A candidate L1 jet trigger algorithm sweeps a square patch of dimensions x=sxsover the EMCal and finds the location of the patch with the highest integrated EMCalenergy (ET

max). Figure 6 shows the distribution of jet trigger cross section in p+p

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collisions (PYTHIA) for a 0.21x0.21 patch and various lower bounds on ETmax. For jets in

p+p collisions below ET~100 GeV, ETmax threshold larger than ~10 GeV generates

significant bias.

Table 1: Trigger enhancement for rare processes due to EMCal trigger relative to TPC+minbiastrigger, for various collision systems and for trigger patch of dimensions x=sxs. Lmax is themaximum luminosity expected at the beginning of a fill. Numbers in square brackets in secondcolumn indicate the “p+p equivalent luminosity” obtained by scaling the ion luminosity by A2.

Trigger enhancement(x=sxs)

Lmax

[ p+p equivalent](1027 cm-2s-1)

interactionrate (Hz)

Max rateto tape(Hz) s=0 s=0.4 s=0.8

Pb+Pb 1.0 [4104] 8K 100 14 10 6

Ar+Ar 60 [1105] 130K 500 44 31 21

O+O 200 [5104] 220K 500 75 53 35

p+p 5103 [5103] 200K 500 68 48 32

Table 1 shows the potential gain in event statistics due to the EMCal L1 trigger,calculated based on considerations of acceptance, luminosity and DAQ bandwidth. Thetable compares the rate to tape of EMCal-triggered observables with a simple interactiontrigger (minimum bias) and equivalent observables using only charged tracks in the TPC.The Trigger Enhancement is defined as

sacc

sacceffLE

TPC

EMCtrigmbAA

trig

tapetoratemax

where efftrig~0.6, taking into account trigger efficiency (~80%) and livetime(~80%). TheALICE DAQ incorporates dynamical scaledown for common triggers (minimum bias andcentral collision triggers), ensuring high livetime for rare triggers. The acceptance of theEMCal relative to the TPC is nominally 25%, which is reduced for finite trigger patchsize s due to edge effects. Loss in luminosity due to pileup rejection in the slow detectorshas not been incorporated into the estimate.

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Figure 7: jet patch response in Pb+Pb events to background-only and background+50 GeV jetevents.

For the /0 trigger (s=0) the enhancement is large relative to untriggered charged pionmeasurements, while for finite jet patches the enhancement is reduced by up to a factor 2.It should be noted however that charged jets are substantially less complete than full jetmeasurements incorporating the EMCal, thus even limited enhancement factors arevaluable since the resulting measurement is more robust.

Figure 7 shows the ETmax cross section (jet patch 0.21x0.21) for otherwise unbiased

central (b=0-2 fm) and peripheral (b=8-10 fm) Pb+Pb collisions, and for the same eventswith 50-60 GeV PYTHIA jets superimposed. (50 GeV appears to be the lower limit forefficient jet triggering in heavy ion events.) The filled area in each figure shows 80% ofthe jet yield, i.e. its lower edge indicates the ET cut necessary for 80% jet efficiency.Background fluctuations are seen to be significant relative to the instrinsic fluctuations ofthe jet, both for central and for peripheral collisions. The overall level of background isstrongly centrality-dependent, as expected, meaning that the ET

max threshold must varywith centrality (by a factor 2 in this calculation) for centrality-independent jet triggerefficiency.

Figure 8 shows the L1 jet trigger efficiency as a function of jet energy for different patchsizes and for central and peripheral Pb+Pb collisions. Trigger thresholds were set toreduce the L1 output datarate by a factor 10 in order to match the HLT input rate.Efficiency for peripheral collisions exceeds 80%, while it is significantly poorer for 50GeV jets in central collisions. It is also seen that the 0.3x0.3 patch has poorer efficiency,reflecting the more rapid growth in background fluctuations than jet energy signal forlarger patches. Similar studies of trigger efficiency have been carried out withmodification of the jet fragmentation due to energy loss models calculated using theParton Quenching Model30 and the event generator PYQUEN31, as implemented in theALICE simulation framework32. In general the quenching models soften and broaden thefragmentation, and reduce the trigger efficiency below about 70 GeV. Above this region,the trigger appears to be efficient and insensitive to the details of the fragmentation.

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Figure 8 L1 jet trigger efficiency for Pb+Pb events.

There conclusions are however dependent on the currently available models for signal(PYTHIA), background (HIJING), and jet quenching. The quenching models arephenomenological, with modification of the angular distribution of jet fragments that isnot strongly motivated theoretically. This study should therefore be regarded as only aqualitative indication that quenching could have significant influence on the triggerefficiencies. The lessons to be taken from this discussion of trigger performance are thatL1 rejection required to match the HLT input bandwidth is likely achievable, but that thetrigger architecture must be flexible and enable a large range in jet trigger patch size.Monte Carlo tools are at present limited, and establishment of the real triggerperformance awaits the measurement of Pb+Pb collisions at the LHC.

1.5.2 Jet reconstruction

As discussed in section 1.4, full jet reconstruction in heavy ion collisions has the potentialto provide an unbiased jet population, giving a much more complete view of the medium-induced modification of fragmentation than the current leading particle studies at RHIC.The copiously produced high ET jets at the LHC are clearly identifiable over the heavyion background, and indeed much of their energy can be recovered. However, it is stillchallenging to achieve good jet energy resolution for this population in the heavy ionenvironment. In this section we report our current understanding of jet reconstructionperformance in p+p and heavy ion events using the EMCal and ALICE tracking,identifying the improvements the EMCal brings to these measurements over trackingalone.

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For jets with ET~50-100 GeV measured in pp collisions, about 80% of the charged

track energy is contained in a cone of phase space radius R=(2+2)~0.2 33, while anarea of this size in central Pb+Pb collisions at 5.5 TeV may contain ~75 GeV fromuncorrelated processes34. The essential difficulty in correcting for this large backgroundarises from its fluctuations, in particular impact parameter fluctuations, statisticalfluctuations due to the finite number of tracks, and dynamical fluctuations due to lowerET jet production. The impact parameter fluctuations can largely be corrected using anevent-by-event estimate and subtraction of the background.

Durham/kT jet reconstruction algorithms35 assign all energy measured in the event to a jetand are not suitable for subtracting large background contributions. We therefore restrictdiscussion to simpler UA1-type cone algorithms36. Jet reconstruction is based onelectromagnetic energy measured by the EMCal and hadronic energy carried by chargedparticles measured in the tracking chambers, with hadronic energy deposition in theEMCal corrected on a track-wise basis37. The energy carried by unmeasured particles(primarily neutrons and K0

L) is estimated to be less than 10% and is corrected for onaverage.

Figure 9. Left: inclusive jet spectrum measured in 200 GeV p+p collisions by STAR38 (preliminary)compared to an NLO pQCD calculation. Right: relative difference of data and theory together withsystematic uncertainty due to jet energy scale.

This technique differs from the conventional approach to ET measurements in colliderdetectors, in which hadronic energy is measured by a hadronic calorimeter and correctionfor double counting of energy or unmeasured particles is not necessary. An approach totransverse energy measurements similar to that proposed here has been taken successfullyby the PHENIX39 and STAR37 collaborations. As an example, Figure 9 shows apreliminary STAR measurement38 of the inclusive jet spectrum in 200 GeV p+pcollisions using tracking and an EM calorimeter, compared to an NLO pQCD calculation.Agreement in inclusive jet yield between measurement and calculation is within theuncertainty due jet energy scale, which is dominated by the convolution of calibrationuncertainty and the steep pT spectrum41. A hadronic calorimeter would remove thesystematic uncertainties in this measurement that are due to the unmeasured hadrons, butthis effect is secondary. As discussed below, significant background suppression for jet

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measurements in heavy ion collisions is obtained from a track-wise pT cut, which cannotbe applied using a hadronic calorimeter.

For jet reconstruction in heavy ion events, two cuts are found to be important to reducethe overall background contribution while maintaining significant jet signal: a limit onthe jet cone radius R and a lower bound on the pT of tracks considered by the algorithm.These cuts are somewhat correlated, due to the well-known angle ordering in jetfragmentation. A study with the HIJING model shows that accepting only tracks withpT>2 GeV/c excludes 98% of the background tracks. Figure 10, left panel, shows thetransverse energy measured with the track cut pT>2 GeV/c applied within a cone ofradius R, for 50 and 100 GeV PYTHIA jets and for background generated by HIJING.For a 100 GeV jet, background ET exceeds the measured jet ET for R>0.4. Figure 10,right panel, shows the resulting energy resolution from the cone algorithm for 50 and 100GeV PYTHIA jets embedded into the HIJING background. A larger radius integratesmore jet signal, improving the resolution, while at the same time incorporating largerbackground signal and fluctuations which deteriorate the resolution. These effects offseteach other to a large extent with this model of signal and background, giving roughlyconstant resolution ~30% for ET~100 GeV jets with R>0.3.

Figure 10: Left: background and jet energy vs R; Right: jet energy resolution as a function of coneradius R. Only tracks with pT>2 GeV/c are included in both panels.

The large background in heavy ion events dictates the need for small jet cone radius R,but naïve application of the cone algorithm with small R will split jets in cases wherehard radiation during the fragmentation generates substructure in the jet. This results in amis-reconstruction of the jet and a significant underestimate of its energy. The effects areillustrated in Figure 11, which shows the reconstruction of 100-120 GeV PYTHIA-generated jets using small cone radius. Parameterized response of the EMCal andtracking is included§. The thrown jet is within the EMCal acceptance (excluding aboundary region to account for its finite size) and its energy is determined by thePYTHIA internal jet finder. For reference, curve (a) shows the reconstructed energy

§ For jet reconstruction the parameterized detector response was found to generate effectively the sameresults as a full Geant simulation and is practical for much higher statistics studies.

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distribution for large cone R=1.0, with the shift in the peak relative to the thrown energydemonstrating the jet energy lost due to unmeasured particles and the track cut pT>2GeV/c. Curve (b) shows the energy distribution for R=0.3 jets, with the additionalcondition that they be the only jet found in the EMCal acceptance (Njet=1). Curves (c)and (d) show the distribution of highest and second highest jet energy for the case thatmore than one R=0.3 jet is found in the EMCal, while curve (e) is their sum. Curve (f)represents the sum of (b) and (e), in other words the energy distribution of the sum allR=0.3 jets found in the acceptance. Its distribution is seen to follow closely that of thecone algorithm with much larger radius.

Figure 11 suggests that the optimal algorithm for jet reconstruction in heavy ion eventsmay be to apply small jet cone size to suppress background, but to resum all close-bysub-jets to counteract the effects of splitting. However, the study presented here shouldonly be considered as a first attempt at developing suitable algorithms. Fullunderstanding of the biases and resolutions of reconstruction algorithms will require acontinuous interplay of experiment and theory, most importantly when data becomeavailable.

Figure 11: jet energy reconstruction for 100-120 GeV PYTHIA jets. See text for details.

Figure 12 illustrates the role of the EMCal in jet reconstruction. The left panel shows thedistribution of reconstructed energy for a monochromatic sample of ET=100 GeV jets forthree different reconstruction algorithms: leading charged particle (LCP), charged-onlyjets in which the reconstruction uses only charged tracks in the TPC, and charged+EMjets in which the EMCal response is included in addition. The jet cone is R=0.4 and onlytracks with pT>2 GeV/c are included. The strong LCP bias is apparent, as is the poorresolution of charged-only jets which results from the large charged/neutral fluctuations

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in jet fragmentation. Inclusion of the EMCal recovers a large fraction of the energy,thereby reducing the bias substantially and improving the resolution.

Finite resolution will generate a different bias for the physical 1/pTn jet spectrum. In

general the spectrum will appear to harden for poorer resolution, since the fluctuation of alow ET jet to higher energy will dominate the cross section at fixed measured ET. Figure12, right panel, shows this effect for a physical jet spectrum (PYTHIA) and threedifferent jet reconstruction algorithms (R=0.4, track cut pT>2 GeV), in order of best toworst resolution: all particle energy, charged+EM energy, charged-only. In each case thecuts on reconstructed energy are chosen so that the most probable generated energy is100 GeV. The charged-only response has a markedly wider generated energydistribution, corresponding to poorer selectivity of the underlying partonic energy.

Figure 12. Left: distribution of reconstructed energy for monochromatic ET=100 GeV jetsreconstructed using leading charged particles, charged particle jets, and charged+EM jets. Jetreconstruction requires R=0.4, pT>2 GeV/c. Right: distribution of generated energy for variousreconstruction techniques, with physical jet pT spectrum and most probable genearated ET=100 GeVin each case.

Figure 13: Ratio of observed to generated energy for charged-only jets (left) and charged+EM jets(right) from physical pT spectrum. Circles are plotted against generated jet energy with vertical barsshowing rms of reconstructed energy, squares are plotted against observed energy. R=0.4 with trackcut pT>2 GeV/c in both cases.

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The effect of the bias is due to the physical spectrum is further illustrated in Figure 13,which shows the ratio of observed to generated energy for charged-only and charged+EMjets. The lower sets of points are plotted against generated energy and correspond to themean and rms of the reconstructed response for monochromatic input. When plottedagainst reconstructed energy (upper points) the cross section is redistributed and the biasdue to the physical spectrum is apparent, corresponding to a softer underlying partonicspectrum for fixed reconstructed energy. The shift in bias is reduced for charged+EM jetsrelative to charged jets because of improved resolution. Extraction of the underlyingpartonic spectrum from the measurement therefore has less sensitivity to the specificshape of the pT spectrum and to the resolution.

1.5.3 /discrimination and direct photons

Direct photons provide a critical calibration for jet probes of the matter produced inheavy ion collisions since the photon does not carry color charge and does not interactwith the medium. At RHIC the inclusive direct photon yield has provided a convincingcross-check of high pT hadron suppression measurements of partonic energy loss (Figure1), while the coincidence measurement of a direct photon with fragments of the recoilingjets is expected to provide the most precise measurement of the modified fragmentationfunction27. Direct photon production cross sections are calculable in collinear factorizedpQCD, where the leading processes are g+q+jet and q+qbar+jet. Significant directphoton rates are predicted in the ALICE acceptance up to pT~50 GeV/c (Figure 4).

The ALICE PHOS is the most highly granular calorimeter at the LHC, targeted atprecision measurement of direct photons and correlations. Its acceptance is however onlyabout 12% of the EMCal acceptance, with correspondingly limited kinematic reach(Figure 4). The EMCal will augment and extend the PHOS photon measurements throughits larger coverage. Note that the coincidence +jet measurement does not require fullreconstruction of the recoiling jet, but rather detailed study of event-averageddistributions of recoil jet fragments measured in tracking detectors; thus, the incompleteazimuthal coverage of ALICE PHOS and EMCal measurements do not fundamentallylimit their capabilities for this measurement.

Direct photon measurements are subject to large backgrounds from neutral meson decay(0, ). At low pT the decay photons generate separate calorimeter showers and mesonscan be reconstructed based on the two-photon invariant mass spectrum. At higher pT thedecay photon showers merge, and shower shape observables are needed to separate

from direct photon signals. Highly asymmetricdecays will mimic the direct photonsignal, thus the physical / yield ratio plays a crucial role in determining the practicalpT reach of a given measurement.

An additional background to direct (leading-order) photon production is hard QCDbremsstrahlung from a quark jet (“fragmentation photons”), which at the LHC maydominate the real photon yield up to pT~50 GeV/c25. Such photons should in principle beaccompanied by hadrons from the jet and may potentially be suppressed by means of anisolation cut. The application of isolation cuts in heavy ion events is under study and we

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do not discuss it further here. In this section we restrict our discussion to EMCalcapabilities for discriminating direct photons from , focusing on shower shapediscrimination at the highest measurable pT.

The best discrimination of one shower from two overlapping showers is obtained fromfitting an ellipse to the cluster, using logarithmic weighting of the tower energies. Theparameter characterizes the long axis of the ellipse and is the most sensitive cutvariable. For Pb+Pb collisions only towers with signal greater than 300 MeV areconsidered, to minimize the background contribution in central events, whereas for p+pthis cut can be reduced to 100 MeV. A full Geant simulation of the ALICE detector isincorporated, in particular to model the effects of conversions in material interior to theEMCal.

Figure 14, left panel, shows the input distributions of and (dashed lines) from pQCDcalculations24. Also shown are the distributions of the EMCal cluster energies (full lines).In both cases the reconstructed cluster spectrum is shifted to lower energy and suppresseddue to decays and conversions. The central panel shows the accepted fraction of and

clusters as a function of cluster energy for <0.55, which biases towards single showers.For photons the efficiency is close to unity and roughly independent of cluster energy.For at low pT the decay photons are reconstructed as separate clusters and all areaccepted by this method, though they can be reliably be discriminated from direct usingan invariant mass analysis. At high pT the decay photon showers are merged to the degreethat they cannot be distinguished from a single shower. The right panel, shows the

yield ratio vs. cluster pT, for all clusters and for clusters with <0.55. The inputdistribution for p+p is at the few percent level in the effective range24, with the showershape cut providing an enhancement in the signal up to pT~30 GeV/c.

Figure 15 is similar to Figure 14 but for central Pb+Pb collisions. The minimum towerenergy is 300 MeV to suppress background, and the intrinsic ratio is increased by afactor 5 to account for the expected hadron suppression due to jet quenching. Note theincreased scale in the right panel for Pb+Pb relative to p+p. In the most favorable region,the measured direct photon population is about a third of all photon candidates.

For the granularity of the EMCal, the optimum momentum range for vs.

discrimination via shower shape is pT~10-30 GeV. This matches well the region of robuststatistical reach for the +jet measurement (Figure 4), though this measurement remainschallenging for the physics reasons outlined above.

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Figure 14: discrimination in the EMCal for p+p collisions, with minimum tower energy 100MeV. Left: pT spectra of , and their respective reconstructed cluster energies in the EMCal.Center: efficiency for 0 and with shower shape cut biasing towards . Right: yield ratio in p+pcollisions, with and without shower shape cut.

Figure 15: discrimination for central Pb+Pb collisions. This differs from Figure 14 by a highertower cut (300 MeV) and factor 5 enhancement in intrinsic ratio due to hadron suppression.

1.5.4 High pT electrons

Figure 4 shows significant inclusive electron yield in the ALICE acceptance up to pT~20GeV/c. Charm and bottom quarks fragment hard into heavy quark mesons, whose semi-leptonic decay (BR~10%) is the dominant source of primary electrons at high pT. High pT

electrons therefore provide a triggerable tag of heavy quark jets, with an estimated heavyquark jet energy range up to ET~50 GeV in the ALICE acceptance. Since light hadron-ledjets in this region are produced dominantly from gluon fragmentation, a clean sample ofhigh ET heavy quark jets (beyond the “dead cone” region for b quarks) will enablesensitive tests of partonic energy loss mechanisms by exploiting the difference in colorcharge of gluons and quarks (factor 9/4). The EMCal will expand the existing strongelectron measurement capabilities in ALICE, in particular by providing a trigger andgood hadron rejection at high pT. We present here a first, preliminary study of high pT

electrons measurements using the EMCal.

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Figure 16: Geant simulation of electron/hadron discrimination using EMCal and TPC. All ALICEmaterial included in simulation. Left: E/p for electrons (black) and pions (red); Right: charged pionrejection as a function of electron efficiency.

Misidentified hadrons are a significant background source to real electrons. Figure 16shows the discrimination of 20 GeV electrons from pions using energy measured in theEMCal compared to the momentum measured in the TPC, together with shower shapediscrimination. The left panel shows the E/p ratio for electrons (black) and pions (red),while the right panel shows pion rejection as a function of electron efficiency. A pionrejection factor 1000 is obtained for 80% electron efficiency. Figure 4 shows that at 20GeV the ratio of electrons to 0 is about 1/100, without taking into account hadronsuppression. The rejection from the EMCal and TPC shown in the figure will thereforeprovide S/B~10 for inclusive electron measurements.

Electron background due to conversions has not yet been estimated, though conversionsat radii larger than the TPC should be efficiently rejected due to absence of a matchingtrack. This may be identifiable in the High Level Trigger. The radiation length interior tothe TPC is ~6%. In a similar geometry to ALICE, the STAR collaboration hasdemonstrated good rejection of conversions in a silicon vertex detector by explicitreconstruction in the TPC of zero invariant mass pairs.

The calculation in Figure 16 does not incorporate uncorrelated background, whose effectshave not yet been studied. Shower shape discrimination will deteriorate in the presence ofbackground, but hadron suppression in central collisions will relax the required rejectionfactor for given S/B. Hadron rejection at lower pT requires additional study, especially inthe few GeV range where non-normal incidence due to the magnetic field spreads theshower significantly.

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1.6 Summary: the role of the ALICE EMCal in the LHC program

The observation of jet quenching and its application as a probe of QCD matter are amongthe most important discoveries at RHIC. The study of jets in dense matter will play ascentral a role in the LHC heavy ion program. It is crucial that the LHC experiments havethe capabilities both to exploit the huge kinematic range of jets made available by themachine and to measure jet structure and its medium-induced modification in detail. Thisdictates the need for efficient triggering but also for robust tracking and detailed particleidentification capabilities down to low pT, since much of the physics is carried by softfragments even for the highest energy jets.

The ALICE experiment, augmented by the EMCal, will meet these requirements well. Ascan be seen from Figure 4, the kinematic reach of the EMCal for jets and dijets is verybroad. It provides an efficient and potentially unbiased jet trigger which is limited onlyby the complexity of the physics of the collision and not by the instrument. The jetsample triggered and measured by the EMCal will be analyzed in detail using theoutstanding capabilities of ALICE for tracking and particle identification. Robust tests ofthe physics underlying energy loss will be made using quark jets tagged by heavy flavordecay, light hadron-led gluon jets, and +jet coincidences. The interaction of the jet withthe medium and the response of the medium will be studied at low and intermediate pT

for an broad range of jet kinematics. New and unexpected physics may emerge when anew energy regime opens up, and the ALICE experiment provides a unique range ofchannels and kinematics in which to look for novel phenomena.

The essential design parameters for the physics performance of the EMCal are its totalcoverage and tower granularity. The performance we have outlined in the foregoingsections shows that the EMCal has broad and robust capabilities. The overall acceptanceof the EMCal is large enough to carry out a strong jet physics program, while largereduction of the acceptance would have a significant negative impact on its kinematicreach and on its ability to reconstruct jets robustly. A reduction in granularity by a factortwo would have significant negative impact on the , direct gamma, and electronmeasurements.

The EMCal is targeted at detailed studies of jet fragmentation in heavy ion and p+pcollisions. It is not intended to compete with ATLAS and CMS in their areas of strengthfor Higgs and supersymmetry searches. It is our view that the EMCal makes ALICE thepremier experimental facility at the LHC for studying the full spectrum of jet physics inheavy ion collisions.

1 J. D. Bjorken, FERMILAB-Pub-82/59-THY

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