- Email: [email protected]
dx relativistic rise
Nuclear Instruments and Methods in Physics Research A 433 (1999) 533}541
Complementary particle ID: transition radiation and dE/dx relativistic rise B. Dolgoshein Moscow Engineering and Physics Institute, 115409 Moscow, Russia
Abstract The Particle IDenti"cation (PID) methods using Transition Radiation (TR) and Relativistic Rise (RR) of the ionization losses are discussed as a complementary methods to a RICH technique. The comparative possibilities and limitations of these PID are considered. For the TR the modern high granularity detectors are described (ATLAS Transition Radiation Straw Tracker } TRT } as an example). The comparison is given between widely used Truncated Mean (TM technique) and a new approach based on Time over Threshold (ToT) measurements, which is more preferable due to the simpli"cation of readout electronics and possibility of the fast triggering. 1999 Elsevier Science B.V. All rights reserved.
1. Introduction There are similarities and at the same time large di!erences between RICH technique and dE/dx } TR techniques in the "eld of the particle identi"cation. The similarity of these PID methods in highenergy experiments is that all of them can provide valuable nondestructive information for the particle identi"cation, which is both complementary and supplementary, for instance, to calorimetric measurements. On the other hand there is a principal di!erence between RICH technique which is based on the particle velocity-dependent parameters (Cherenkov angle) measurements, whereas the measurements of the ionization losses and TR X-rays give the
E-mail address: [email protected] (B. Dolgoshein)
information about the particle Lorentz-factor c"E/mc (E } total energy, m } mass of the particle.) This means that the areas of the possible application of these methods, for instance, the energy intervals for hadron}hadron and electron} hadron identi"cation, where particle ID can be implemented more e!ectively, are di!erent. From this point of view, dE/dx and TR methods are complementary to the RICH technique.
2. dE/dx and TR-measurements: the general concept and features The particle identi"cation by dE/dx and Transition Radiation detection both are based on the measurements of the particle energy deposition in the multisample gaseous detectors. In case of dE/dx this is the ionization collision of the relativistic particle directly in the gas; the typical energy deposition is few keV/1 cm of the gas. In case of TR
0168-9002/99/$ - see front matter 1999 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 8 - 9 0 0 2 ( 9 9 ) 0 0 3 0 1 - 0
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B. Dolgoshein / Nuclear Instruments and Methods in Physics Research A 433 (1999) 533}541
Fig. 1. The hit energy distribution for 30 GeV electrons (ATLAS TRT data).
this is the absorption of TR X-rays in the gas, generated by the particle, which crosses many boundaries between two materials (multifoil or "ber radiator usually) just in front of the detector [1]. The typical energy of the absorbed X-rays is 5}30 keV and the number of the photons is quite small: 0.1}0.2 TR detected photons per cm of the optimized detector [1]. Because the TR X-rays are emitted very sharply forward relative to the particle direction (h"1/c) it is practically impossible to separate the energy deposition in the detector gas originated from dE/dx and TR. Therefore, these particle energy losses both are detected together. Figs. 1 and 2 show the typical spectra of the energy deposition in the proportional straw tubes of the ATLAS Transition Radiation Tracker (TRT) [2] for the electrons (without and with TR radiator) and for the pions with energy 1 and 20 GeV (dE/dx relativistic rise). The thickness of the detector gas for dE/dx and TR detection is usually quite small (4 mm straws in the case of ATLAS TRT). One can see the clear contribution of the TR signal (Fig. 1) and dE/dx relativistic rise (Fig. 2) in the energy deposition spectrum. Because of the impossibility to separate dE/dx and TR signals (which are measured together) any multisample gas detector inter-
Fig. 2. The hit energy distribution for pions, dE/dx relativistic rise (ATLAS TRT, MC).
layered by TR radiator is actually dE/dx#TR combined detector. The necessity to use the multisample gas detector is dictated by: E large #uctuations of the dE/dx losses. We need to average many separate measurements for one particle; E small yield of TR X-rays. The thickness of the TR radiator is limited due to absorption of soft TR X-rays in the radiator material, therefore many radiator}detector sets are needed. It can be seen from Figs. 1 and 2 that most of the energetic part of the energy deposition spectrum (more than 1}2 keV for dE/dx relativistic rise and 5 keV for TR) is most sensitive to the Lorentzfactor. Moreover [1,3], the dE/dx relativistic rise itself and the TR X-rays detection e$ciency are higher for high Z gases (Xe gas is the best). There are a few di!erent methods for dE/dx and TR measurements: 1. Total energy deposition method. 1. 䡩 The Maximum Likelihood method (¸) uses for the analysis the Likelihood function ¸
"log = /(= #= ) G
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where =(1, 2) is the probability that the detector response was caused by particle 1, 2 for each out of detectors (samples) used for dE/dx and TR measurements. The Maximum Likelihood method is the most powerful but it needs a slow readout electronics (ADCs), di$cult for fast triggering and is used usually for the o!-line analysis. 1. 䡩 Truncated Mean (TM) method is most widely used. The cut at the 30}40% of the highest values of the energy deposition results in the signi"cant reduction of the dE/dx Landau tail (delta rays). Thus it gives rise to a better separation of the energy deposition spectra from two di!erent particles, averaged for many (n) detectors (samples): L E " (E !max(E ))/(n!1). 2+ G G G The implementation of the TM method is also relatively slow and complicates the fast triggering. Furthermore, TM method rejects the most energetic part of the energy deposition spectrum, which is most sensitive to the particle Lorentz factor. 2. Cluster counting method (N) N-method uses the count of the ionization clusters above the certain threshold locally (2}3 mm) deposited in the gas. It results in a signi"cant reduction of the dE/dx #uctuations and gives rise to a Poisson-like (binomial) distribution. The number of the clusters (large energy transfers) is most sensitive to the dE/dx relativistic rise [3]. The typical thresholds for dE/dx and TR measurements are about 1 keV and 5}6 keV correspondingly [1,7] (compare with Figs. 1 and 2). The electronics for N-method (discriminators) is more simple, cheap and fast and can therefore be built into the fast triggering more easily. Moreover, the combine dE/dx#TR particle ID can be easily implemented by counting the clusters for two di!erent thresholds [7]. 3. As an example of the usage of the N-method Fig. 3 shows the cluster distributions for the electrons and pions obtained for the Transition Radiation Detector in E715 experiment at
Fig. 3. The cluster distribution for electrons and pions, E715 TRD [4].
FNAL [4]. The excellent pion separation factor of 1500 for very high electron e$ciency of 99.5% has been obtained for TRD length of 3.6 m. 4. Time over Threshold (ToT) method (see talk by V. Bashkirov for details [5]). Time over Threshold method is a kind of modi"cation of a cluster counting method; its operational principle is illustrated in Fig. 4. The implementation of ToT method is even more simple, than N-method, especially for fast triggering. At the same time the ToT identi"cation power is not worse (or even better) compared to the TM one. Fig. 5 gives a comparison of the TM and ToT methods for the same sample of data [6,7], obtained for the detector with 360 Xe gas slices 0.5 mm each using FADC. It can be seen that this method works most e$ciently for a pion}electron separation at 1 GeV/c. The Cluster Counting and ToT methods allow to realize very easily and fast the combined `twodimensionala particle ID by dE/dx and TR simultaneously. Fig. 6 displays the comparison of such a pion}electron separation for the di!erent methods for the momentum interval of 0.5}5 GeV/c. These
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Fig. 4. The Time over Threshold (ToT) concept.
results were obtained for ZEUS straw TRD prototype [7]. It can be seen that ToT method at least is as powerful as TM method. The PID by dE/dx relativistic rise and TR are complementary for each other in sense of the Lorentz-factor intervals available for hadron}hadron (pion}kaon for instance) and electron}hadron ID. The Lorentz-factor dependence of the ionization relativistic rise [8] and TR yield [4] are shown in Figs. 7 and 8. The Lorentz-factor intervals for these PID methods are indeed complementary: E for dE/dx relativistic rise } from 10 to 400; E for Transition Radiation from !500 to 3000. Based on these numbers possible momentum intervals are: dE/dx for pion}kaon separation (1}5) GeV/c for electron}pion separation (0.5}10) GeV/c
TR (150}400) GeV/c; (1.5}150) GeV/c.
It can be seen that the electron}hadron ID is available for much wider momentum interval especially in case of the TR. The electron ID performance of any TR detector depends critically on the quality of the radiator and the e!ective length of the detector (radiator#Xray absorber) traversed [1]. It also depends on the particle energy since the TR production rises rapidly for energies above 0.5 GeV and reaches saturation for most detectors around 2 GeV; whereas the probability for charge pions to produce high-energy delta-rays and to start the TR radiation also rises substantially as the pion energy increases from few GeV to about 150 GeV. Most widely used TR radiator is the polypropylene foils of 15}20 lm thick with a 200}300 lm gap in between [1], which is the best practical radiator. As an alternative the polypropylene/polyethylene "bers, oriented perpendicularly to the particle direction is also used [2], which provides the TR-yield of about 85}90% of that of a perfectly regular foil radiator. The summary of known experimental results for the TRD electron ID performance for the
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Fig. 5. The comparison of the Truncated Mean (TM) and Time over Threshold (ToT) methods of the dE/dx losses analysis [6,7].
Fig. 7. The shape of the dE/dx relativistic rise [8].
polypropylene foils/"bers or Li-foil radiators is shown in Fig. 9 as a function of the TRD lengths for a compact (length less than 1 m) TRDs. The majority of the data is extracted from Ref. [1] with addition of some recently published results [7,9}11]. The pion rejection factors obtained at di!erent experiments agree satisfactorily over 2.5 orders of magnitude within a wide range of the total TRD length and for di!erent radiators.
All results for PID by TRD described above were performed using the Xe-contained gas proportional chambers as a TR X-rays detectors. One interesting possibility was recently proposed [12] to use the Secondary Electron Emission (SEE) initiated by TR X-rays in thin (200 nm) CsI `photocathodea for TR detection. Such a TRD comprises a many sets of foil radiators followed by CsI-based SEE low pressure gas detectors. Its main advantages
Fig. 6. &Two-dimensional' electron}hadron particle ID for the di!erent methods of the analysis [7].
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Unfortunately, due to the low TR X-rays detection e$ciency in a very thin CsI photocathode the total number of the radiator}detector sets should be very large for a good TR performance of such a SEE TRD. As a result the SEE TRD cannot compete as a particle identi"er with Xe-based gas detectors for the same TRD length. For instance, for SEE TRD length of 1 m the pion rejection factor is expected at the level of about 5% only (compare with Fig. 9).
3. ATLAS Transition Radiation Tracker (TRT)
Fig. 8. The detected number of the TR photons for di!erent Lorentz factors [4].
Fig. 9. Pion rejection capabilities of various compact TRDs (length less than 1 m) [1]. Closed and open symbols are referred in Ref. [1], numbers are recent publications [7,9}11].
compared to presently employed Xe-"lled TR detectors could be the time resolution (in the range of a few nanoseconds), good spatial accuracy, high rate capability and very low occupancy due to a very low minimum ionizing particle e$ciency.
ATLAS Transition Radiation Tracker (TRT) has been developed in the framework of the RD-6 Program for the ATLAS experiment at LHC [2,13 and references therein]. The main detecting element of the TRT is straw drift tube. The straw chambers operating as drift tubes are well known in experimental high-energy and nuclear physics [14]. TRT straw tubes are cylindrical drift tubes with thin multi-layer walls made of Kapton, aluminum and carbon-loaded Kapton. The straws are typically 40}150 cm long and 4 mm in diameter. They are operated with 70 : 20 : 10% Xe : CF : CO gas mix ture with a gas gain of 2.5*10. This mixture combines advantages of e$cient TR X-rays absorption, short drift time and stability with respect to discharges. The drift time information from the straws is used for the purposes of track position reconstruction. This allows to obtain the spatial resolution of about 130 lm for each straw. The role of the TRT in ATLAS is E Robust pattern recognition (40 hits per track). E Momentum measurement together with other ATLAS trackers. E Fast and e$cient level-2 trigger for: E 1. high p leptons at high luminosity; 2 E 2. B-physics at low luminosity. E Electron ID with (or without) electromagnetic calorimeter: E 1. high p inclusive electrons with a jet rejec2 tion by factor of 10; E 2. rejection of photon conversions and Dalitz decays; E 3. tagging of soft electrons in b-jets;
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Fig. 10. Fraction of the TR hits on electron track from b-quark decay (shaded) and charged particles in gluon jets [2].
E 4. extraction of a signal from J/tPe>e\; E 5. veto against electrons for HPc!c search. For the illustration of the TRT performance in B-physics Fig. 10 shows the tagging of soft electrons capability as a fraction of TR hits on electron tracks compared to a charge particles in gluon jets [2]. The ATLAS TRT [2] (see Fig. 11, where the main TRT parameters are shown) is located just in front of the electromagnetic calorimeter and consists of two parts } barrel and end-caps. The TRT provides tracking and particle ID over the rapidity range "g"(2.5. The orientation of the straws chosen to make optimal use of the 2 T axial magnetic "eld, so that particles traverse the straws as close to normal incidence as possible. The barrel TRT with axially oriented straws has 18 modules arranged in 3 layers. Each wire in the barrel TRT is split at the center, giving two readout channels per straw, halving the occupancy. The end-cap TRT with radially oriented straws has 18 modules in each hemisphere. The total amount of the material in the TRT from inner support structure to the end of active detector volume varies from 0.1X to a maximum 0.21X as a function of the rapidity. Of this the material of the straws is 0.04X and the material of the radiator is 0.04}0.10X .
Fig. 11. ATLAS Transition Radiation Tracker (TRT) conceptual design [2].
The straw layers are evently spaced. This gives a low correlation between pile-up hits when searching for high p tracks. Thus the large number of 2 points per track makes the pattern recognition robust and create a "ne grain TRD structure of the radiator (CH foils for end-cap and CH "bers for a barrel) interlayered by the straws; this provides better conditions for TR particle ID [1]. The robustness of the particle ID is still good enough despite the sizable (20% on average) expected single channel occupancy at the luminosity of 10 cm\ s\. The TRT performance as a particle identi"cation device in the high multiplicity environment has been measured in the test beam with the TRT prototype (Fig. 12) [13]. Here the dependence of the hadron rejection is shown as a function of the electron e$ciency for di!erent occupancies up to 32%. It can be seen that misidenti"cation of a pion as an electron increases for high occupancy. Nevertheless, for 90% electron e$ciency a hadron rejection factor of 10}100 is achievable even for highest expected TRT occupancies. The pion rejection for ATLAS TRT for low occupancy (luminosity of 10 cm\ s\) is shown in Fig. 13 [2] as a function of
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4. Summary and conclusions Coming back to the comparison between di!erent PID techniques we conclude that PID by dE/dx relativistic rise and TR is well developed and can be successfully used in certain areas of the applications complementary and supplementary to the RICH detectors. This is illustrated by Figs. 13 and 14. These "gures show di!erent areas of the most e!ective application of the di!erent PID techniques in terms of the lengths of given PID detector, which is needed in order to achieve 3 sigma level of two particle separation as a function of the momentum. As it can be seen from Fig. 14
Fig. 12. The hadron rejection as a function of the electron e$ciency for di!erent occupancies.
Fig. 14. Pion}kaon separation by di!erent PID methods: the length of the detectors needed for 3 sigma separation.
Fig. 13. Pion e$ciency as a function of pseudorapidity for the "xed electron e$ciency of 90%.
the pseudorapidity for two values of p "2 and 20 2 GeV/c. Due to relativistic rise in the dE/dx losses for pions the rejection is as expected, slightly better for p "2 GeV/c than for 20 GeV/c, especially in 2 the end-cap region. The rejection factors of 15}1000 can be achieved depending on the momentum and rapidity. For slightly lower electron e$ciency of 85% the rejection which can be achieved would improve by a factor of about two.
Fig. 15. The same as Fig. 14 for electron}pion separation.
B. Dolgoshein / Nuclear Instruments and Methods in Physics Research A 433 (1999) 533}541
(pion}kaon separation) RICH's look more powerful among other techniques in a wide range of the momentum (1}100 GeV/c). For highest momentum (100}500 GeV/c) TR particle ID may compete successfully with the RICH detectors, although the length of the TRD needed is large enough (up to few meters). Much more favorable possibilities for the application of dE/dx#TR particle ID can be seen for the electron}pion separation (Fig. 15). Here very compact TRDs with a length not more then 0.5 m give most powerful PID in a very wide momentum range of 0.5}100 GeV/c. `Purea dE/dx particle ID which does not need the TR radiator (and therefore the detector can be much shorter compared to combined dE/dx#TR detector) is a better technique for a momentum range below of about 2 GeV/c.
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References [1] B. Dolgoshein, Nucl. Instr. and Meth. A 326 (1993) 434. [2] ATLAS Inner Detector Technical Design Report, CERN/ LHCC/97-16, April 1997. [3] V. Ermilova et al., Nucl. Instr. and Meth. 145 (1977) 555. [4] A. Denisov et al., preprint Fermilab-Conf-84/134E, 1984. [5] V. Bashkirov, Nucl. Instr. and Meth. A 433 (1999) 560. [6] E. O'Brien et al., IEEE NS-40 (1993) 153. [7] V. Bashkirov, Ph.D. Thesis, MEPHI, Moscow, 1997. [8] B. Lasiuk, Nucl. Instr. and Meth. A 409 (1998) 402. [9] N. Terinuma et al., Nucl. Instr. and Meth. A 323 (1992) 471. [10] E. O'Brien et al., Nucl. Phys. A 566 (1994) 615c. [11] R. Kaizer, Ph.D. Thesis, Simon Fraser University, 1997. [12] R. Chechik et al., Nucl. Phys. B 44 (1995) 364. [13] B. Dolgoshein, Nucl. Instr. and Meth. A 368 (1995) 239. [14] For a review see: W.H. Toki, preprint SLAC-PUB 5232, 1990.
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Complementary particle ID: transition radiation and dE/dx relativistic rise B. Dolgoshein Moscow Engineering and Physics Institute, 115409 Moscow, Russia
Abstract The Particle IDenti"cation (PID) methods using Transition Radiation (TR) and Relativistic Rise (RR) of the ionization losses are discussed as a complementary methods to a RICH technique. The comparative possibilities and limitations of these PID are considered. For the TR the modern high granularity detectors are described (ATLAS Transition Radiation Straw Tracker } TRT } as an example). The comparison is given between widely used Truncated Mean (TM technique) and a new approach based on Time over Threshold (ToT) measurements, which is more preferable due to the simpli"cation of readout electronics and possibility of the fast triggering. 1999 Elsevier Science B.V. All rights reserved.
1. Introduction There are similarities and at the same time large di!erences between RICH technique and dE/dx } TR techniques in the "eld of the particle identi"cation. The similarity of these PID methods in highenergy experiments is that all of them can provide valuable nondestructive information for the particle identi"cation, which is both complementary and supplementary, for instance, to calorimetric measurements. On the other hand there is a principal di!erence between RICH technique which is based on the particle velocity-dependent parameters (Cherenkov angle) measurements, whereas the measurements of the ionization losses and TR X-rays give the
E-mail address: [email protected] (B. Dolgoshein)
information about the particle Lorentz-factor c"E/mc (E } total energy, m } mass of the particle.) This means that the areas of the possible application of these methods, for instance, the energy intervals for hadron}hadron and electron} hadron identi"cation, where particle ID can be implemented more e!ectively, are di!erent. From this point of view, dE/dx and TR methods are complementary to the RICH technique.
2. dE/dx and TR-measurements: the general concept and features The particle identi"cation by dE/dx and Transition Radiation detection both are based on the measurements of the particle energy deposition in the multisample gaseous detectors. In case of dE/dx this is the ionization collision of the relativistic particle directly in the gas; the typical energy deposition is few keV/1 cm of the gas. In case of TR
0168-9002/99/$ - see front matter 1999 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 8 - 9 0 0 2 ( 9 9 ) 0 0 3 0 1 - 0
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Fig. 1. The hit energy distribution for 30 GeV electrons (ATLAS TRT data).
this is the absorption of TR X-rays in the gas, generated by the particle, which crosses many boundaries between two materials (multifoil or "ber radiator usually) just in front of the detector [1]. The typical energy of the absorbed X-rays is 5}30 keV and the number of the photons is quite small: 0.1}0.2 TR detected photons per cm of the optimized detector [1]. Because the TR X-rays are emitted very sharply forward relative to the particle direction (h"1/c) it is practically impossible to separate the energy deposition in the detector gas originated from dE/dx and TR. Therefore, these particle energy losses both are detected together. Figs. 1 and 2 show the typical spectra of the energy deposition in the proportional straw tubes of the ATLAS Transition Radiation Tracker (TRT) [2] for the electrons (without and with TR radiator) and for the pions with energy 1 and 20 GeV (dE/dx relativistic rise). The thickness of the detector gas for dE/dx and TR detection is usually quite small (4 mm straws in the case of ATLAS TRT). One can see the clear contribution of the TR signal (Fig. 1) and dE/dx relativistic rise (Fig. 2) in the energy deposition spectrum. Because of the impossibility to separate dE/dx and TR signals (which are measured together) any multisample gas detector inter-
Fig. 2. The hit energy distribution for pions, dE/dx relativistic rise (ATLAS TRT, MC).
layered by TR radiator is actually dE/dx#TR combined detector. The necessity to use the multisample gas detector is dictated by: E large #uctuations of the dE/dx losses. We need to average many separate measurements for one particle; E small yield of TR X-rays. The thickness of the TR radiator is limited due to absorption of soft TR X-rays in the radiator material, therefore many radiator}detector sets are needed. It can be seen from Figs. 1 and 2 that most of the energetic part of the energy deposition spectrum (more than 1}2 keV for dE/dx relativistic rise and 5 keV for TR) is most sensitive to the Lorentzfactor. Moreover [1,3], the dE/dx relativistic rise itself and the TR X-rays detection e$ciency are higher for high Z gases (Xe gas is the best). There are a few di!erent methods for dE/dx and TR measurements: 1. Total energy deposition method. 1. 䡩 The Maximum Likelihood method (¸) uses for the analysis the Likelihood function ¸
"log = /(= #= ) G
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where =(1, 2) is the probability that the detector response was caused by particle 1, 2 for each out of detectors (samples) used for dE/dx and TR measurements. The Maximum Likelihood method is the most powerful but it needs a slow readout electronics (ADCs), di$cult for fast triggering and is used usually for the o!-line analysis. 1. 䡩 Truncated Mean (TM) method is most widely used. The cut at the 30}40% of the highest values of the energy deposition results in the signi"cant reduction of the dE/dx Landau tail (delta rays). Thus it gives rise to a better separation of the energy deposition spectra from two di!erent particles, averaged for many (n) detectors (samples): L E " (E !max(E ))/(n!1). 2+ G G G The implementation of the TM method is also relatively slow and complicates the fast triggering. Furthermore, TM method rejects the most energetic part of the energy deposition spectrum, which is most sensitive to the particle Lorentz factor. 2. Cluster counting method (N) N-method uses the count of the ionization clusters above the certain threshold locally (2}3 mm) deposited in the gas. It results in a signi"cant reduction of the dE/dx #uctuations and gives rise to a Poisson-like (binomial) distribution. The number of the clusters (large energy transfers) is most sensitive to the dE/dx relativistic rise [3]. The typical thresholds for dE/dx and TR measurements are about 1 keV and 5}6 keV correspondingly [1,7] (compare with Figs. 1 and 2). The electronics for N-method (discriminators) is more simple, cheap and fast and can therefore be built into the fast triggering more easily. Moreover, the combine dE/dx#TR particle ID can be easily implemented by counting the clusters for two di!erent thresholds [7]. 3. As an example of the usage of the N-method Fig. 3 shows the cluster distributions for the electrons and pions obtained for the Transition Radiation Detector in E715 experiment at
Fig. 3. The cluster distribution for electrons and pions, E715 TRD [4].
FNAL [4]. The excellent pion separation factor of 1500 for very high electron e$ciency of 99.5% has been obtained for TRD length of 3.6 m. 4. Time over Threshold (ToT) method (see talk by V. Bashkirov for details [5]). Time over Threshold method is a kind of modi"cation of a cluster counting method; its operational principle is illustrated in Fig. 4. The implementation of ToT method is even more simple, than N-method, especially for fast triggering. At the same time the ToT identi"cation power is not worse (or even better) compared to the TM one. Fig. 5 gives a comparison of the TM and ToT methods for the same sample of data [6,7], obtained for the detector with 360 Xe gas slices 0.5 mm each using FADC. It can be seen that this method works most e$ciently for a pion}electron separation at 1 GeV/c. The Cluster Counting and ToT methods allow to realize very easily and fast the combined `twodimensionala particle ID by dE/dx and TR simultaneously. Fig. 6 displays the comparison of such a pion}electron separation for the di!erent methods for the momentum interval of 0.5}5 GeV/c. These
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Fig. 4. The Time over Threshold (ToT) concept.
results were obtained for ZEUS straw TRD prototype [7]. It can be seen that ToT method at least is as powerful as TM method. The PID by dE/dx relativistic rise and TR are complementary for each other in sense of the Lorentz-factor intervals available for hadron}hadron (pion}kaon for instance) and electron}hadron ID. The Lorentz-factor dependence of the ionization relativistic rise [8] and TR yield [4] are shown in Figs. 7 and 8. The Lorentz-factor intervals for these PID methods are indeed complementary: E for dE/dx relativistic rise } from 10 to 400; E for Transition Radiation from !500 to 3000. Based on these numbers possible momentum intervals are: dE/dx for pion}kaon separation (1}5) GeV/c for electron}pion separation (0.5}10) GeV/c
TR (150}400) GeV/c; (1.5}150) GeV/c.
It can be seen that the electron}hadron ID is available for much wider momentum interval especially in case of the TR. The electron ID performance of any TR detector depends critically on the quality of the radiator and the e!ective length of the detector (radiator#Xray absorber) traversed [1]. It also depends on the particle energy since the TR production rises rapidly for energies above 0.5 GeV and reaches saturation for most detectors around 2 GeV; whereas the probability for charge pions to produce high-energy delta-rays and to start the TR radiation also rises substantially as the pion energy increases from few GeV to about 150 GeV. Most widely used TR radiator is the polypropylene foils of 15}20 lm thick with a 200}300 lm gap in between [1], which is the best practical radiator. As an alternative the polypropylene/polyethylene "bers, oriented perpendicularly to the particle direction is also used [2], which provides the TR-yield of about 85}90% of that of a perfectly regular foil radiator. The summary of known experimental results for the TRD electron ID performance for the
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Fig. 5. The comparison of the Truncated Mean (TM) and Time over Threshold (ToT) methods of the dE/dx losses analysis [6,7].
Fig. 7. The shape of the dE/dx relativistic rise [8].
polypropylene foils/"bers or Li-foil radiators is shown in Fig. 9 as a function of the TRD lengths for a compact (length less than 1 m) TRDs. The majority of the data is extracted from Ref. [1] with addition of some recently published results [7,9}11]. The pion rejection factors obtained at di!erent experiments agree satisfactorily over 2.5 orders of magnitude within a wide range of the total TRD length and for di!erent radiators.
All results for PID by TRD described above were performed using the Xe-contained gas proportional chambers as a TR X-rays detectors. One interesting possibility was recently proposed [12] to use the Secondary Electron Emission (SEE) initiated by TR X-rays in thin (200 nm) CsI `photocathodea for TR detection. Such a TRD comprises a many sets of foil radiators followed by CsI-based SEE low pressure gas detectors. Its main advantages
Fig. 6. &Two-dimensional' electron}hadron particle ID for the di!erent methods of the analysis [7].
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Unfortunately, due to the low TR X-rays detection e$ciency in a very thin CsI photocathode the total number of the radiator}detector sets should be very large for a good TR performance of such a SEE TRD. As a result the SEE TRD cannot compete as a particle identi"er with Xe-based gas detectors for the same TRD length. For instance, for SEE TRD length of 1 m the pion rejection factor is expected at the level of about 5% only (compare with Fig. 9).
3. ATLAS Transition Radiation Tracker (TRT)
Fig. 8. The detected number of the TR photons for di!erent Lorentz factors [4].
Fig. 9. Pion rejection capabilities of various compact TRDs (length less than 1 m) [1]. Closed and open symbols are referred in Ref. [1], numbers are recent publications [7,9}11].
compared to presently employed Xe-"lled TR detectors could be the time resolution (in the range of a few nanoseconds), good spatial accuracy, high rate capability and very low occupancy due to a very low minimum ionizing particle e$ciency.
ATLAS Transition Radiation Tracker (TRT) has been developed in the framework of the RD-6 Program for the ATLAS experiment at LHC [2,13 and references therein]. The main detecting element of the TRT is straw drift tube. The straw chambers operating as drift tubes are well known in experimental high-energy and nuclear physics [14]. TRT straw tubes are cylindrical drift tubes with thin multi-layer walls made of Kapton, aluminum and carbon-loaded Kapton. The straws are typically 40}150 cm long and 4 mm in diameter. They are operated with 70 : 20 : 10% Xe : CF : CO gas mix ture with a gas gain of 2.5*10. This mixture combines advantages of e$cient TR X-rays absorption, short drift time and stability with respect to discharges. The drift time information from the straws is used for the purposes of track position reconstruction. This allows to obtain the spatial resolution of about 130 lm for each straw. The role of the TRT in ATLAS is E Robust pattern recognition (40 hits per track). E Momentum measurement together with other ATLAS trackers. E Fast and e$cient level-2 trigger for: E 1. high p leptons at high luminosity; 2 E 2. B-physics at low luminosity. E Electron ID with (or without) electromagnetic calorimeter: E 1. high p inclusive electrons with a jet rejec2 tion by factor of 10; E 2. rejection of photon conversions and Dalitz decays; E 3. tagging of soft electrons in b-jets;
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Fig. 10. Fraction of the TR hits on electron track from b-quark decay (shaded) and charged particles in gluon jets [2].
E 4. extraction of a signal from J/tPe>e\; E 5. veto against electrons for HPc!c search. For the illustration of the TRT performance in B-physics Fig. 10 shows the tagging of soft electrons capability as a fraction of TR hits on electron tracks compared to a charge particles in gluon jets [2]. The ATLAS TRT [2] (see Fig. 11, where the main TRT parameters are shown) is located just in front of the electromagnetic calorimeter and consists of two parts } barrel and end-caps. The TRT provides tracking and particle ID over the rapidity range "g"(2.5. The orientation of the straws chosen to make optimal use of the 2 T axial magnetic "eld, so that particles traverse the straws as close to normal incidence as possible. The barrel TRT with axially oriented straws has 18 modules arranged in 3 layers. Each wire in the barrel TRT is split at the center, giving two readout channels per straw, halving the occupancy. The end-cap TRT with radially oriented straws has 18 modules in each hemisphere. The total amount of the material in the TRT from inner support structure to the end of active detector volume varies from 0.1X to a maximum 0.21X as a function of the rapidity. Of this the material of the straws is 0.04X and the material of the radiator is 0.04}0.10X .
Fig. 11. ATLAS Transition Radiation Tracker (TRT) conceptual design [2].
The straw layers are evently spaced. This gives a low correlation between pile-up hits when searching for high p tracks. Thus the large number of 2 points per track makes the pattern recognition robust and create a "ne grain TRD structure of the radiator (CH foils for end-cap and CH "bers for a barrel) interlayered by the straws; this provides better conditions for TR particle ID [1]. The robustness of the particle ID is still good enough despite the sizable (20% on average) expected single channel occupancy at the luminosity of 10 cm\ s\. The TRT performance as a particle identi"cation device in the high multiplicity environment has been measured in the test beam with the TRT prototype (Fig. 12) [13]. Here the dependence of the hadron rejection is shown as a function of the electron e$ciency for di!erent occupancies up to 32%. It can be seen that misidenti"cation of a pion as an electron increases for high occupancy. Nevertheless, for 90% electron e$ciency a hadron rejection factor of 10}100 is achievable even for highest expected TRT occupancies. The pion rejection for ATLAS TRT for low occupancy (luminosity of 10 cm\ s\) is shown in Fig. 13 [2] as a function of
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B. Dolgoshein / Nuclear Instruments and Methods in Physics Research A 433 (1999) 533}541
4. Summary and conclusions Coming back to the comparison between di!erent PID techniques we conclude that PID by dE/dx relativistic rise and TR is well developed and can be successfully used in certain areas of the applications complementary and supplementary to the RICH detectors. This is illustrated by Figs. 13 and 14. These "gures show di!erent areas of the most e!ective application of the di!erent PID techniques in terms of the lengths of given PID detector, which is needed in order to achieve 3 sigma level of two particle separation as a function of the momentum. As it can be seen from Fig. 14
Fig. 12. The hadron rejection as a function of the electron e$ciency for di!erent occupancies.
Fig. 14. Pion}kaon separation by di!erent PID methods: the length of the detectors needed for 3 sigma separation.
Fig. 13. Pion e$ciency as a function of pseudorapidity for the "xed electron e$ciency of 90%.
the pseudorapidity for two values of p "2 and 20 2 GeV/c. Due to relativistic rise in the dE/dx losses for pions the rejection is as expected, slightly better for p "2 GeV/c than for 20 GeV/c, especially in 2 the end-cap region. The rejection factors of 15}1000 can be achieved depending on the momentum and rapidity. For slightly lower electron e$ciency of 85% the rejection which can be achieved would improve by a factor of about two.
Fig. 15. The same as Fig. 14 for electron}pion separation.
B. Dolgoshein / Nuclear Instruments and Methods in Physics Research A 433 (1999) 533}541
(pion}kaon separation) RICH's look more powerful among other techniques in a wide range of the momentum (1}100 GeV/c). For highest momentum (100}500 GeV/c) TR particle ID may compete successfully with the RICH detectors, although the length of the TRD needed is large enough (up to few meters). Much more favorable possibilities for the application of dE/dx#TR particle ID can be seen for the electron}pion separation (Fig. 15). Here very compact TRDs with a length not more then 0.5 m give most powerful PID in a very wide momentum range of 0.5}100 GeV/c. `Purea dE/dx particle ID which does not need the TR radiator (and therefore the detector can be much shorter compared to combined dE/dx#TR detector) is a better technique for a momentum range below of about 2 GeV/c.
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References [1] B. Dolgoshein, Nucl. Instr. and Meth. A 326 (1993) 434. [2] ATLAS Inner Detector Technical Design Report, CERN/ LHCC/97-16, April 1997. [3] V. Ermilova et al., Nucl. Instr. and Meth. 145 (1977) 555. [4] A. Denisov et al., preprint Fermilab-Conf-84/134E, 1984. [5] V. Bashkirov, Nucl. Instr. and Meth. A 433 (1999) 560. [6] E. O'Brien et al., IEEE NS-40 (1993) 153. [7] V. Bashkirov, Ph.D. Thesis, MEPHI, Moscow, 1997. [8] B. Lasiuk, Nucl. Instr. and Meth. A 409 (1998) 402. [9] N. Terinuma et al., Nucl. Instr. and Meth. A 323 (1992) 471. [10] E. O'Brien et al., Nucl. Phys. A 566 (1994) 615c. [11] R. Kaizer, Ph.D. Thesis, Simon Fraser University, 1997. [12] R. Chechik et al., Nucl. Phys. B 44 (1995) 364. [13] B. Dolgoshein, Nucl. Instr. and Meth. A 368 (1995) 239. [14] For a review see: W.H. Toki, preprint SLAC-PUB 5232, 1990.
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