Time resolution of the Atlas Tile calorimeter modules has been measured using the test beam data. The resolution depends on an energy deposited in a given cell and is equal to about 1.5 $ns$ at $E$ = 1 $GeV$, 270 $ps$ at $E$ = 25 $GeV$ (high gain), 700 $ps$ at $E$ = 25 $GeV$ (low gain) and 170 $ps$ at $E$ = 150 $GeV$. These values have to be compared to the time of flight of relativistic particles to reach first samples of Tile calorimeter (from 8.3 $ns$ to 20 $ns$). Time of Flight measurement using Tile Calorimeter can be used to identify, and combined with momentum measurements by Atlas inner detector, to measure the mass of exotic heavy stable particles. The results are compared to previous analysis using the Atlas muon spectrometercite{KRAAN2}

Leitner, R

Shmakova, V V

Tas, P

CERN Document Server

Time resolution of the ATLAS Tile calorimeterand its performance for a measurement of heavystable particlesR. Leitner1 , V.V. Shmakova2 , P. Tas11 - Institute of Particle and Nuclear Physics, Faculty of Mathematics and Physics,Charles University in Prague, Prague, Czech Republic2 - Joint Institute for Nuclear Research, Dubna, Russian FederationAbstractTime resolution of the ATLAS Tile calorimeter modules has been measuredusing the test beam data. The resolution depends on the energy deposited in agiven cell and is equal to about 1.5 ns at E = 1 GeV , 270 ps at E = 25 GeV(high gain), 700 ps at E = 25 GeV (low gain) and 170 ps at E = 150 GeV .These values have to be compared to the time of flight of relativistic particles toreach the first samples of Tile calorimeter (from 8.3 ns to 20 ns). Time-Of-Flightmeasurements using Tile Calorimeter can be used to identify and, combined withmomentum measurements by the ATLAS inner detector, measure the mass ofexotic heavy stable particles. The results are compared to a previous analysisusing the ATLAS muon spectrometer[1].1 ATLAS Tile calorimeterThe ATLAS [?] Tile Calorimeter [3] is a large hadronic sampling calorimeterwhich makes use of steel as the absorber material and scintillating plates readout by wavelength shifting (WLS) fibers as the active medium. The new featureof its design is the orientation of the scintillating tiles which are placed in planeperpendicular to the colliding beams and are staggered in depth.The Tile Calorimeter consists of a cylindrical structure with an inner radius of2280 mm and an outer radius of 4230 mm. It is subdivided into a 5640 mm longcentral barrel and two 2910 mm extended barrels. The scintillating tiles lie in ther − φ (radius-azimuthal angle) plane and span the width of the module in the φdirection. WLS fibers running radially collect the light from the tiles along theirtwo open edges. Readout cells are then defined by grouping together a set of fibersinto a photomultiplier (PMT), to obtain a three dimensional segmentation. Intotal there is 4928 readout cells. Radially, the calorimeter is segmented into threelayers, approximately 1.4, 3.9 and 1.8 interaction lengths thick at pseudorapidityη = 0; the ∆ηx ∆φ segmentation is 0.1x0.1.Each cell of the Tile Calorimeter is read-out by two PMTs in order to provideredundancy in the light read-out and improve spatial uniformity. The conversion1ATL-TILECAL-PUB-2007-00205 April 2007of the light signal from the fiber bundles to electrical charge is done by thephotomultipliers in the PMT blocks.The output of the PMT block is a shaped electronic signal which is subse-quently digitized by electronics inside the superdrawer (see Fig. 1). Two linearoutputs (high and low gain) are used for each channel, differing in gain by afactor of 64.2 Tile calorimetry time measurementFor the purpose of this analysis, test beam data of Tile calorimeter modules takenin 2002 and 2003 have been used. Tile calorimeter production modules have beenirradiated by 180 GeV beam of negative pions at different pseudorapidities. Thebeam contained also muons, its contamination by electrons was very low. Nospecial requirements have been imposed on the particle trajectory. For eachPMT the time and energy are evaluated with the fit method [4], i.e. as the peakposition and height of the shaped signal fit to the digitized pulse samples.For a given cell (readout by two PMTs called even and odd) and a giveninterval of energies deposited in that cell the distribution of ∆t = (te − to)/2 isplotted and its sigma is estimated by a gaussian fit. For relativistic particles thetime of flight varies from approximately 8.3 ns for the cell A-1 to about 20 ns forthe cell D-8 of the extended barrel. A particle that moves slower than relativisticparticles and which interacts electromagnetically and/or hadronically will givethe response delayed with respect to that of relativistic particles. In case of nocorrelations between the two time measurements te and to, the value of σ∆t is thebest estimator of σt of a variable t = (te + to)/2. In ATLAS this variable t couldserve as a measurement of the time of flight of particles with electromagneticand/or hadronic interactions. Examples of ∆t distributions are plotted on Fig. 2and Fig. 3 for high gain and low gain respectively.On Fig. 4 time resolutions for low gain and high gain of the same cell arecompared for two different test beam runs. More distributions [10] showing acomparison of resolution curves for different cells in the same module are inFig. 5 and Fig. 6. The resolution has been parametrized by a quadratic sum ofstatistical (proportional to 1/√E) and noise (proportional to 1/E) terms (E isin GeV ):σt (E) =p0√E⊕ p1E=√√√√( p0√E)2+(p1E)2(1)As indicated on Fig. 4-6, the resolution curves are quite similar for differentcells, moreover in the ATLAS environment these curves could eventually be ob-tained for each Tile cell. For further analysis we took the parametrization of thetime resolution for the cell A-6 (see Fig. 5) with following values of parameters:2High Gain p0 = 1.45 ns ·GeV 1/2 , p1 = 0.38 ns ·GeVLow Gain p0 = 1.7 ns ·GeV 1/2 , p1 = 17.2 ns ·GeV .3 Detection of heavy stable particles with Tilecalorimeter and Inner detectorThere are different theories predicting an existence of heavy particles stableenough to be detected prior to their decay. For recent references see e.g. [5].A possibility to measure such particles in ATLAS using time of flight measure-ment performed by the muon system has been thoroughly discussed in [1].In this paper we discuss a possibility to detect such particles and measure theirmass using the time measurement performed by the Tile calorimeter combinedwith the momentum measurement using ATLAS inner detector (ID).The velocity of a particle that deposits energy in the Tile calorimeter cell ilocated at a distance Li from the interaction piont could be measured as follows:βtile =Li/cttile,i−trel,i+Li/cwhere for a given cell i the variable ttile,i =te+to2is the time measured in agiven event, while trel,i = 〈 te+to2 〉 is the mean value of the time measured over allevents in a given cell. The latter is the time of flight of the relativistic particle.The precision in measurement of β is given by a formula:σβ = β2σttile⊕σtrel⊕ttile−trelLi/cσLi/cLi/cDominant contribution to the velocity uncertainty is due to the first term.The precision of the mean value over many events σtrel is certainly much better.Also the third term contributes very little to the overall velocity resolution.In general the particle deposits the energy in couple of Tile calorimeter cells.The particle’s velocity could be evaluated for each cell and the measurementscould be combined with weights proportional to 1/σ2β.When the particle’s momentum p and velocity β are measured, the mass ofthe particle could be evaluated as follows:M =pβγ= p√1− β2β(2)The mass resolution is then given:σM = ∂pMσp ⊕ ∂βMσβ =√(∂pMσp)2 + (∂βMσβ)2 (3)To estimate the mass resolution, the following assumptions about momentumand velocity resolutions and particles energy losses in the Tile calorimeter havebeen made:3• To get a conservative estimate we assume that particle’s velocity is mea-sured by the closest to the ATLAS interaction point Tile calorimeter cellA-1. Then βtile =8.3nsttileand consequently σβtile = β2 σt(Etile)8.3ns, where σt (Etile)is given by formula (1) with quadratically added constant time resolutionterm of 70 ps. This term is caused by the time spread of the primary in-teraction due to finite lengths of the LHC proton bunches. The primaryinteraction could appear anytime within a time window of Lbunch30cmns−1, i.e. thetime of the primary interaction has a dispersion of Lbunch30cmns−11√12which equalsto 70ps for Lbunch = 7cm.• Purely hadronic losses of heavy stable particles have been parametrizedusing results of [6]. For 1 m of iron they are roughly equal to 4% and 1.2%of kinetic energy for the mass M=100 and 600 GeV respectively[6]. Becausethe thickness of the Tile calorimeter is more than 1.5 m, the values of theslopes have been rescaled accordingly.• Ionization losses in TileCal have been parametrized as ∆Eioniz = 2.2GeV/β2using a measured value of the most probable muon losses (2.2 GeV ) in theTile calorimeter at small pseudorapidities. Energy losses up to kinetic en-ergy of the particle have been allowed. The particles with the mass of 200(1000) GeV and a momentum smaller than about 80 (260) GeV are stoppedat the Tile calorimeter.• σpid (pid) = 3.6 · 10−4 p2id ⊕ 3.6 · 10−2 (all values in GeV ) - approximateprecision of the momentum measurement with the ATLAS inner detectorat the investigated momentum region [8];• σpµ (pµ) = 1.3 · 10−4 p2µ - approximate precision of the momentum measure-ment with the ATLAS muon spectrometer at the region of interest [9];• σβµ (βµ) = 0.067β2µ - anticipated precision of the velocity measurement usingthe muon system [1];On Fig. 7,8 and 9 the mass resolution σM/M for three different masses (200,500 and 1000 GeV ) of particles are plotted as a function of a momentum atpseudorapidity close to η =0. The resolution of Tile+ID is compared with thatof the muon system.4 Discussion of results and conclusionsFor particles of low mass the results of Tile+ID are compatible with an anticipatedprecision of the muon spectrometer, for the case of hadronic losses it is evenbetter. For masses around 500 GeV Tile+ID precision is worse than that of themuon spectrometer, but it still could improve the muon measurements. For the4highest masses the Tile+ID precision is significantly worse than that of the muonspectrometer, however with Tile calorimeter being closer to the interaction point,a possible range of measurements of the muon spectrometer can be extended. Thisfeature is indicated on Fig. 7-9 by limits which correspond to a limit of β > 0.5required for a correct reconstruction of the track in muon system [1] and to alimit of β > 8.3ns/(8.3+ 25)ns = 0.25 ensuring the signal in the first cells of theTile is seen within a correct bunch crossing.In overlapping regions, the Tile+ID and muon measurements can be combinedin order to improve the resolution. This is true for charged exotic particlesmeasured both by Tile and muon spectrometer.For the case of hadronically interacting particles, there are following possibil-ities in addition:• In case of neutral particles with hadron interactions, the Tile calorimetertime of flight measurement can identify these particles giving, however, onlylower limit of their mass.• In the case of charged particles which are converted to neutral within Tiles,only Tile measurements will be available.• In the case when particle is neutral, in Tile it interacts hadronically and con-verts to charged particle, muon measurements of its mass can be improvedusing the time of flight measurements performed by the Tile calorimeter.An important question arises what deterioration of the time measurement isexpected due to the pile-up effect. On Fig. 10,11 and 12 the contributions to themass resolution due to momentum and time precisions are plot. For the particleswith mass of 200 GeV, the Tile calorimeter time uncertainty dominates the errors,while the dominant contribution to the mass resolution for particles with masslarger than 500 GeV is due to uncertainty in the momentum measurement of theID.The results obtained in this analysis are encouraging and indicate a very goodpotential of ATLAS to measure heavy stable exotic particles with electromagneticand/or hadronic interactions.AcknowledgementAuthors are indebted A. Kraan and her colleagues for their work on the AT-LAS muon spectrometer performance for R-hadrons searches. These works servedas an inspiration for current analysis. The results, of course, would not be possibleto obtain without an excellent work of the ATLAS Tile and TDAQ communitiesand CERN SPS accelerator group during the test beams.5References[1] A.C. Kraan, J.B. Hansen, P. Nevski, Discovery potential of R-hadronswith the ATLAS detector, SN- ATLAS-2005-053, Nov 2005. 26pp. e-PrintArchive: hep-ex/0511014[2] ATLAS Collaboration, ATLAS Technical Proposal, reportCERN/LHCC/94–43.[3] F. Ariztizabal et al., Nucl. Instrum. Methods A349 (1994) 384.E. Berger et al., report CERN/LHCC 95–44.ATLAS Collaboration, Tile Calorimeter TDR, CERN/LHCC 96-42.[4] T. Davidek, R. Teuscher private communicationsR. Teuscher: Methods of energy reconstruction in Tilecal, talk at Tile-cal Analysis meeting (http://agenda.cern.ch/fullAgenda.php?ida=a031558),23.6.2003, CERN, Geneva[5] M. Fairbairn, A.C. Kraan, D.A. Milstead, T. Sjostrand, P. Skands, T. Sloan,Stable massive particles at colliders, Phys.Rept.438:1-63,2007[6] A.C. Kraan, Interactions Of Heavy Stable Hadronizing Particles,Eur.Phys.J.C37 (2004) 91-104.[7] A.C. Kraan, Inetractions and Detection of R-hadrons, PhD Thesis, Copen-hagen Univ., Copenhagen 2004.[8] ATLAS coll., ATLAS Inner Detector Technical Design Report, Vol. 1,CERN/LHCC 97-16, CERN, 1997.[9] ATLAS coll., ATLAS Muon Spectrometer Technical Design Report,CERN/LHCC 97-22, CERN, 1997.[10] V.V. Shmakova, Diploma thesis, Moscow State University, 2007 (in Russian)6Figure 1: Shaped Tile signal and digitized samples.7e308Entries  1600Mean    10.02RMS     1.163Integral    16008 8.5 9 9.5 10 10.5 11 11.5 12024681012141618E2+E5, cut 22, 83 h308Entries  1600Mean   -0.4358RMS    0.4847Integral    1599 / ndf 2χ 50.32 / 61Constant  2.28± 69.04 Mean      0.0115± -0.4438 Sigma     0.0096± 0.4485 -5 -4 -3 -2 -1 0 1 2 3 4 501020304050607080dT pion  8-12 GeV HGFigure 2: Left - Energy distribution in the interval 8-12 GeV. Right - Distributionof ∆t = (te − to)/2 in the range from -5 ns to 5 ns and gaussian fit with σ∆t =0.45 ns.8Figure 3: Left plots - Energy distribution in the interval 100-110 GeV and 110-120 GeV for cell A-1. Right plots - Distribution of ∆t = (te − to)/2 in the rangefrom -5 ns to 5 ns and gaussian fits with σ∆t = 0.230 and 0.216 ns respectively.9Energy [GeV]0 5 10 15 20 25Resolution [ns]0.30.40.50.60.70.80.911.1 / ndf 2χ 0.1641 / 4p0        0.0522± 1.405 p1        1.251± 0.6687 Time resolution High GainA-1 r0200222.rootEnergy [GeV]20 40 60 80 100 120Resolution [ns]0.20.30.40.50.6 / ndf 2χ 6.185 / 8p0        0.1511±  1.66 p1        1.174± 15.89 Time resolution Low GainEnergy [GeV]0 5 10 15 20 25Resolution [ns]0.30.40.50.60.70.80.911.1 / ndf 2χ 1.434 / 4p0        0.05523± 1.401 p1        0.9472± 0.9984 A-1 r0340433.rootTime resolution High GainEnergy [GeV]20 40 60 80 100 120Resolution [ns]0.20.30.40.50.60.7 / ndf 2χ 1.933 / 8p0        0.1677± 1.452 p1        1.079± 16.67 Time resolution Low GainFigure 4: Time resolution in the first Tile calorimeter sampling cell A-1 for twodifferent Tile calorimeter modules.10Figure 5: Time resolution in different A cells (A-1 (the best resolution), A-3,A-6(with data points)).11Figure 6: Time resolution in cells at different depths (A-6(with data points),B-6(the best resolution), D-3).12200 400 600 800 1000 1200p@GeVD0.20.40.60.81ΣMMΜFigure 7: Mass resolution σM/M for exotic heavy stable charged particle withmass M=200 GeV as a function of its momentum. The curve with µ is forthe muon spectrometer, the other three curves are for the Tile calorimeter andID combined measurement. Dashed, dotted and solid lines correspond to onlyionization energy losses, purely hadronic and ionization plus hadronic energylosses of the particle respectively. Thick parts of all curves indicate the regionwhere β > 0.5 for muon sepctrometer and β > 0.25 for the Tile calorimeter (seedetails in the text).13200 400 600 800 1000 1200p@GeVD0.20.40.60.81ΣMMΜFigure 8: Mass resolution σM/M for exotic heavy stable charged particle withmass M=500 GeV as a function of its momentum. The curve with µ is forthe muon spectrometer, the other three curves are for the Tile calorimeter andID combined measurement. Dashed, dotted and solid lines correspond to onlyionization energy losses, purely hadronic and ionization plus hadronic energylosses of the particle respectively. Thick parts of all curves indicate the regionwhere β > 0.5 for muon sepctrometer and β > 0.25 for the Tile calorimeter (seedetails in the text).14200 400 600 800 1000 1200p@GeVD0.20.40.60.81ΣMMΜFigure 9: Mass resolution σM/M for exotic heavy stable charged particle withmass M=1000 GeV as a function of its momentum. The curve with µ is forthe muon spectrometer, the other three curves are for the Tile calorimeter andID combined measurement. Dashed, dotted and solid lines correspond to onlyionization energy losses, purely hadronic and ionization plus hadronic energylosses of the particle respectively. Thick parts of all curves indicate the regionwhere β > 0.5 for muon sepctrometer and β > 0.25 for the Tile calorimeter (seedetails in the text).15200 400 600 800 1000 1200p@GeVD0.20.40.60.81ΣMMFigure 10: Mass resolution σM/M for heavy R-hadron with mass M=200 GeVand both hadronic and ionization interactions are shown by the solid line andthe contributions from momentum and time measurements are shown by dashedand dotted lines respectively.16200 400 600 800 1000 1200p@GeVD0.20.40.60.81ΣMMFigure 11: Mass resolution σM/M for heavy R-hadron with mass M=500 GeVand both hadronic and ionization interactions are shown by the solid line andthe contributions from momentum and time measurements are shown by dashedand dotted lines respectively.17200 400 600 800 1000 1200p@GeVD0.20.40.60.81ΣMMFigure 12: Mass resolution σM/M for heavy R-hadron with mass M=1000 GeVand both hadronic and ionization interactions are shown by the solid line andthe contributions from momentum and time measurements are shown by dashedand dotted lines respectively.18

Time resolution of the Atlas Tile calorimeter and its performance for a measurement of heavy stable particles

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