Berntzon, Lisa

Wigmans, Richard

CERN Document Server

The DREAM Project – Results and PlansRequest for SPS Beam TimeThe DREAM Collaboration(Submitted November 15, 2006)Spokesman: Richard Wigmans, Texas Tech University, U.S.A.Testbeam contact: Lisa Berntzon, CERN11 IntroductionDREAM started in 2002 as a generic detector R&D project, intended to explore(and, if possible, eliminate) the obstacles that prevent calorimetric detection ofhadrons with a comparable level of precision as we have grown accustomed tofor electrons and photons. The initial collaboration, consisting of fewer than 10physicists, built a prototype detector (DREAM =Dual REAdout Module) that wassuccessfully tested at the SPS in 2003 and 2004. The excellent results obtained inthese tests generated a lot of interest, and the collaboration has now expanded to7 institutions, 3 from the USA, 4 from Italy.After the initial studies, in which the effects of the dominating source of fluc-tuations on the calorimeter performance were successfully eliminated, the collab-oration is now focusing on the remaining effects, which have risen to prominenceas a result: Sampling fluctuations, signal quantum statistics and nuclear breakupeffects. By reducing these effects as much as possible in a systematic, sequen-tial manner, we expect to be able to approach the theoretical limits for hadroniccalorimeter performance (e.g., energy resolution 15%/√E).Even though DREAM is in essence still a generic R&D project, some of thenew collaboration members have of course practical applications in mind. Theseapplications include a detector for an experiment at a future Linear e+e− Colliderin the TeV energy range, and an upgrade of the CMS calorimeter system.The physics program of a Linear e+e− Collider requires excellent hadronicenergy resolution. It has been estimated that to distinguish between hadronicallydecaying W and Z bosons, a resolution better than 4% will be needed. Thisrequires detectors that are considerably better than the ones used in experiments atLEP, the Tevatron and the LHC. One of the proto-detectors studied for the ILC (theso-called 4th Concept) is based on the DREAM approach to calorimetry. Severalproponents of this concept are also members of the DREAM collaboration.2 The DREAM approach to ultimate calorimetryThe energy resolution of calorimeters is determined by fluctuations. If one wantsto improve that resolution significantly, then one has to address the dominat-ing source of these fluctuations. In almost all calorimeters (i.e. the ones withe/h 6= 1.0), fluctuations in the electromagnetic shower fraction (fem) dominatethe energy resolution for hadrons and jets. These fluctuations, and their energy-dependent characteristics, are also responsible for other undesirable calorimeter2characteristics, in particular hadronic signal non-linearity and a non-Gaussian re-sponse function. There are two possible approaches to eliminate (the effects of)these fluctuations [1]: By designing the calorimeter such that the response to emand non-em energy deposit is the same (compensation, e/h = 1.0), or by measur-ing fem event by event. The DREAM project follows the latter approach.Calorimeters based on Cˇerenkov light as the signal source are, for all practi-cal purposes, only responding to the em fraction of hadronic showers [2]. Thisis because the electrons/positrons through which the energy is deposited in theem shower component are relativistic down to energies of only 200 keV. On theother hand, most of the non-em energy in hadron showers is deposited by non-relativistic protons generated in nuclear reactions [1]. However, in other types ofactive media (scintillator, LAr) such protons do generate signals. The DREAMdetector uses two active media, hence the name (dual-readout): Scintillating fibersmeasure dE/dx, while clear fibers measure the Cˇerenkov light generated in theshower development. By comparing the two signals, fem can be measured eventby event, and the total shower energy can be reconstructed using the known e/hvalue(s) of the calorimeter.Figure 1: Cˇerenkov signal distributions for 100 GeV pi−. Shown are all events (top) and samplesselected on the basis of their electromagnetic shower content (bottom).3The DREAM calorimeter, as well as many results obtained in beam tests ofthis device, have been described in detail in a number of papers [3]. In the fol-lowing, we only illustrate that this principle works very well. Figure 1 shows theCˇerenkov signal distribution for 100 GeV pi− showers (top diagram), as well as thesignal distributions for event samples selected for 3 bins of the em shower fraction(bottom diagram). The larger the value of fem, the larger the calorimeter signal.The overall signal distribution (top) is evidently a superposition of many narrowdistributions such as the ones in the bottom diagram. By using the measured valueof fem, the total signal distribution can be transformed into a narrow one, with thecorrect central value, i.e. the signal one would find for pure em showers of thenominal energy. This is illustrated in Figure 2, which concerns the signal distri-Figure 2: Cˇerenkov signal distributions for 200 GeV multi-particle events. Shown are the rawdata (a), and the signal distributions obtained after application of the corrections based on themeasured em shower content, with (c) or without (b) using knowledge about the total “jet” energy.butions from 200 GeV multiparticle events (reaction products from an upstreamtarget, intended to mimick jets). The raw Cˇerenkov signal distribution (Figure2a) shows the usual characteristics: Asymmetric, broad and a central value thatis much too small (133 GeV). After applying the correction method based onevent-by-event measurements of fem, this distribution is transformed into the one4shown in Figure 2b, which is almost perfectly symmetric, much more narrow, andcentered around approximately the correct energy value (190 GeV). It should beemphasized that the value of fem was uniquely determined on the basis of the ratioof the two measured signals (the so-called Q/S method1), no other informationwas used. Because of the relatively small detector size (1200 kg), this result isdominated by fluctuations in (lateral) leakage. We have demonstrated that, by us-ing knowledge of the total shower energy, this effect could be eliminated and thesignal distribution improved to the one shown in Figure 2c.3 Current activities for further improvementThe beam tests of the DREAM detector have shown that, simply by using theratio of the Cˇerenkov and scintillation signals, all detrimental effects of fluctua-tions in the em shower fraction could be eliminated: Hadronic signal linearity wasrestored, deviations from E−1/2 scaling in the hadronic energy resolution wereeliminated, a Gaussian response function was obtained and, most importantly, thehadronic energy scale was the same as the electromagnetic one, so that the entireinstrument could be calibrated with electrons [3].The elimination of (the effects of) this dominant source of fluctuations meansthat other types of fluctuations now dominate the detector performance. Furtherimprovements may be obtained by concentrating on these. Three types of fluctua-tions currently dominate and limit the energy resolution of the DREAM calorime-ter:• Leakage fluctuations• Fluctuations in Cˇerenkov light yield• Sampling fluctuationsThe first source can be eliminated by making the detector sufficiently large. Thetested instrument had an effective radius of only 0.8 λint. Side leakage amounted,on average, to about 10% of the shower energy, and fluctuations in this fractionplayed a dominant role (Figure 2). The small Cˇerenkov light yield (8 photoelec-trons per GeV) contributed more than 35%/√E to the measured hadronic energyresolution. Sampling fluctuations were measured to contribute ∼ 15%/√E to the1The symbol Q refers to the quartz fibers that measured the Cˇerenkov light.5electromagnetic resolution of the detector, and may thus be estimated to contributeabout twice as much to the hadronic energy resolution.There is absolutely no reason why the DREAM principles should be limitedto fiber calorimeters. In particular, they could be applied to homogeneous detec-tors, provided that a way is found to distinguish the Cˇerenkov and scintillationlight produced by such a detector. If successful, this approach could eliminate atonce both the effects of sampling fluctuations and the effects of fluctuations in theCˇerenkov light yield to the hadronic energy resolution.Figure 3: Left-right asymmetry measured for cosmic rays traversing a PbWO4 crystal, as afunction of the orientation of this crystal. The curves represent the results of calculations for afixed ratio of the numbers of Cˇerenkov and scintillation photons produced in this process.To that end, we have started a series of studies with crystals, and in particularPbWO4. This material has the advantage of producing relatively little scintillationlight, while the high effective Z value promises a substantial Cˇerenkov light yield.Moreover, it has the great advantage of being readily available. Both CMS and6ALICE are using large numbers of PbWO4 crystals for their em calorimeters. Themeasurements described below were carried out with crystals generously madeavailable to us by ALICE.We have measured the ratios of the two types of signals for cosmic rays, whichtraversed a crystal that was read out from both ends. By changing the orientationof the crystal, the acceptance for (directional) Cˇerenkov light was varied, and bymeasuring the left/right asymmetry of the total signal as a function of the angleθ, we were able to establish that 15-20% of the photons were actually generatedby the Cˇerenkov mechanism (Figure 3). This result was corroborated by measur-ing the time structure of the signals. The signals from the PMT that “saw” theFigure 4: Time structure of cosmic ray events. Shown are the pulse shapes for the signalsmeasured in the 2 PMTs reading out the PbWO4 crystal, as well as the difference between thesetwo pulse shapes. The pulses represent the sum of 11 randomly chosen events that generatedsignals in the most probable region of the Landau distribution. The crystal is oriented as shown.7Cˇerenkov component exhibited a clear fast component that was absent in the sig-nals from the other PMT which, because of the crystal orientation, only detectedscintillation light (Figure 4).In a recent beam test2, we have studied PbWO4 crystals with high energy parti-cle beams. Preliminary results confirm the cosmic ray results described above. Wehave also studied hadronic shower development in a calorimeter system consist-ing of an em section made of PbWO4 crystals, backed up by the DREAMmodule.The purpose of this test was to see if and to what extent it is possible to measurethe em shower fraction event by event on the basis of the crystal signals, and ifthis information can be used to improve the calorimeter performance for hadrondetection, in the same way as with DREAM in stand-alone mode. Analysis ofthese data has just started, and no results are available at the present time.It would be a major achievement if the PbWO4 signals could indeed be usedfor the purpose described above. This is because this crystal is in many waysfar from ideal. An ideal crystal would generate similar fractions of scintillationand Cˇerenkov light, would scintillate primarily in the λ > 500 nm region, andthe decay time of the scintillation light would be sufficiently long to distinguishit easily from the prompt Cˇerenkov component. None of these conditions weremet in the PbWO4 crystals used for our studies. As mentioned above, the mostattractive feature of these crystals was that they existed and were available for ourtests. Any success obtained with these crystals could be strongly improved withdedicated crystals specifically developed for this type of application. Developingsuch dedicated new types of crystals is one of the projects we have undertaken inthe context of DREAM.4 Toward ultimate calorimetryIf we assume that the dual-readout principles can be as efficiently applied in homo-geneous detectors as in the original DREAM calorimeter, then the contributions ofsignal quantum fluctuations and sampling fluctuations to the hadronic energy res-olution can be made negligibly small. In that case, the resolution of a sufficientlylarge detector will become dominated by nuclear breakup effects. Fluctuations inthe fraction of the total energy needed to release protons, neutrons and heaviernuclear fragments in the nuclear reactions initiated by the shower particles lead tofluctuations in visible energy, and thus to fluctuations in the calorimeter response.2CERN SPS, October 29 - November 4, 20068It has been demonstrated that measurement of the total kinetic energy carried byneutrons generated in the shower development is a powerful tool for reducing theeffects of these fluctuations, especially in high-Z absorber materials where mostof the nucleons released in the nuclear reactions are indeed neutrons [1].Measuring the signal contributions from shower neutrons event by event is an-other important objective of the DREAMCollaboration. Two different approachesare considered to achieve this:1. By equipping the DREAM detector with a third type of fibers, which isspecifically intended for this purpose. One could either use a material thatis specifically sensitive to neutrons, or a hydrogen-free scintillating fiber. Inthe latter case, the neutron contribution could be deduced from the differ-ence between the signals from the plastic and the hydrogen-free scintillatingfibers.2. By measuring the time structure of the scintillator signals. The neutron con-tribution should manifest itself as a tail with a characteristic time constant.This tail is more dominant in the calorimeter towers that detect the showerhalo than in the tower located on the shower axis.In our recent beam tests, exploratory measurements of the time structure and pos-sible differences between on-axis and off-axis towers have been performed. Theresults of these measurements will determine our strategy in this respect.In summary, we have established that the dual-readout approach combinesthe advantages of compensating calorimetry with a reasonable amount of designflexibility. Since there is no limitation on the sampling fraction, the dominatingfactors that limited the energy resolution of compensating calorimeters (SPACAL,ZEUS) to∼ 30%/√E can be eliminated, and the theoretical resolution limit of∼15%/√E seems to be within reach. Dual-readout detectors thus hold the promiseof high-quality calorimetry for all types of particles, with an instrument that canbe calibrated with electrons.5 Test beam requestsAs indicated above, the DREAM project is at present primarily a generic detectorR&D project. The next steps to be taken will depend on the outcome of the teststhat were recently performed. It is therefore too early to describe our plans forthe 2007 test beam campaign in great detail. However, the road map is clear: We9want to gain a detailed understanding of the various factors that limit the hadroniccalorimeter performance, and of the possibilities for reducing/eliminating the ef-fects of these factors. Once this understanding is complete, we will want to builda full-scale calorimeter to put it to the test. Such a full-scale model will no doubtbe a prototype for a Linear Collider experiment.We would like to request two weeks of SPS test beam in 2007. The programcurrently foreseen for these tests includes:• Measurements of the scintillation/Cˇerenkov characteristics of new types ofcrystals that are currently being developed• Measurements of hadronic shower development in a calorimeter systemconsisting of a crystal em section backed up by the DREAMhadronic calori-meter• Detailed measurements of the time structure of hadronic signals in all 19DREAM scintillator channels, in an effort to determine the contribution ofneutrons to these signals event by event. As demonstrated in our recenttests, modern FADCs allow nanosecond sampling of this time structure.In the past years, we have built the infrastructure needed for these tests in theH4A beam area (remotely controlled support table, cables/cable trays, trackinghodoscopes, etc. ) Given the increased emphasis on measurement of the timestructure of the signals, we may decide to digitize the signals at the front end,rather than transporting them through special air-core cables to the counting room,as was done up to now. This would require a few days of additional installationtime.The beams available in the H4 area, and the existing auxiliary equipment formeasuring the beam characteristics, are adequate for our purpose. We are plan-ning to use electrons in the momentum range 10 - 200 GeV/c, and hadrons in themomentum range 20 - 300 GeV/c. The purity of the beams will be determinedwith our own equipment.We would also like to say that, once we move to the stage of full-scale proto-typing, we will want to reconsider the experimental area. H4A is in several waysnot very practical, for example in terms of accessibility. However, for our 2007program it is adequate.10References[1] R. Wigmans, Calorimetry, Energy Measurement in Particle Physics, In-ternational Series of Monographs on Physics, Vol. 107, Oxford UniversityPress (2000).[2] N. Akchurin et al. , Nucl. Instr. and Meth. A399 (1997) 202.[3] N.Akchurin et al. , Nucl. Instr. and Meth. A533 (2004) 305; A536 (2005)29; A537 (2005) 537; A548 (2005) 336; A550 (2005) 185.11

The DREAM Project - Results and Plans Request for SPS Beam Time

Abstract

Similar works

Full text

Available Versions

CERN Document Server