- Email: [email protected]
Overview of the LHCb calorimeters
ARTICLE IN PRESS Nuclear Instruments and Methods in Physics Research A 617 (2010) 40–44
Contents lists available at ScienceDirect
Nuclear Instruments and Methods in Physics Research A journal homepage: www.elsevier.com/locate/nima
Overview of the LHCb calorimeters$ F. Machefert a,c,, A. Martens b,c a b c
CERN, Geneva, Switzerland Univ. Paris-Sud, Laboratoire de l’Acce´le´rateur Line´aire UMR8607, Orsay F-91405, France CNRS, Orsay F-91405, France
a r t i c l e in f o
a b s t r a c t
Available online 7 October 2009
LHCb, one of the four LHC experiments, is dedicated to the study of CP violation and rare decays in the B meson sector. It aims at completing the understanding of the quark flavor physics and at revealing signs of new physics beyond the standard model. The goal of the LHCb calorimeter is twofold. On the one hand, the calorimeter system has to provide a fast response for the first level trigger (L0) on the nature of the meson decay. Thus, the scintillator pad detector and the preshower provide a good g/ charged particle and electron/p0 separation and the electromagnetic and hadronic calorimeters give a fast transverse energy determination. On the other hand, it provides offline precision measurements and particle identification. The calorimeter system consists of four sub-detectors. They are described in the first section, emphasising the technical choices and the similarities among those components. The second part concerns the monitoring and calibration tools and procedures that will be applied to have a satisfactory running of the detector. & 2009 Elsevier B.V. All rights reserved.
Keywords: Particle physics LHCb CP-violation Calorimetry
1. Introduction LHCb, one of the four particle physics experiments at the Large Hadron Collider at CERN, will perform studies of CP-symmetry violation and rare decays of B hadrons. It is a single arm spectrometer with a forward angular coverage motivated by the fact that at high energies both quarks from the bb- pairs are predominantly produced at small angles with respect to the beam. At the nominal luminosity, L ¼ 2 1032 cm2 s1 ; 1012 bb- pairs should be produced at the experiment interaction point in a year of data taking (107 s). LHCb [1,2] consists of a magnet, a vertex locator, a tracking system, two ring imaging Cherenkov detectors, a calorimeter and a muon system. The main purpose of the LHCb calorimeters [3] is the selection and identification of hadrons, electrons and photons and the measurement of their energies and directions, both at the first trigger level and for the offline reconstruction. Four subdetectors are associated to perform such identification: a scintillating pad detector (SPD) and a preshower (PS) allow to tag charged particles and to determine their electromagnetic nature; they are followed by an electromagnetic (ECAL) and a hadronic (HCAL) calorimeter. The calorimeter system is used at the first level trigger (L0) of LHCb by providing high transverse energy electron, photon, neutral pion and hadron candidates. The response of the calorimeter system has to match the accelerator
$
This document is a LHCb collaborative contribution.
Corresponding author at: CERN, Geneva, Switzerland.
E-mail address: [email protected] (F. Machefert). 0168-9002/$ - see front matter & 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.nima.2009.09.123
frequency and provides a measurement at 40 MHz. The data are pipelined in the front-end electronics waiting for the L0 decision that combines the information from the calorimeter, the muon chambers and the pile up veto. Finally, the data are read out and sent to the CPU farm of the High Level Trigger (HLT) of LHCb at an average rate of 1 MHz.
2. Design of the LHCb calorimeter system 2.1. The scintillating pad detector and the preshower The four calorimeters are wall-like structures divided into two halves which may be open and fully taken out of the acceptance. The first calorimeters seen by the particles incoming from the interaction point are the scintillating pad detector and the preshower. Their design is very similar and consists of two scintillating vertical planes made of 6016 pads. A 2.5 radiation length lead sheet is sandwiched between the two sub-detectors. This lead converter allows to initiate the electromagnetic showers so that electrons and photons deposit a sizable amount of energy in the PS. Charged particles leave in the SPD a minimum ionising particle (mip) signal which is detected while photons do not interact. Combining the SPD and the PS information with the cluster position reconstruction of the ECAL gives a determination of the nature of the electromagnetic particle interacting with the calorimeter system. This technique is used offline but also at the first level trigger of LHCb to tag high transverse momentum
ARTICLE IN PRESS F. Machefert, A. Martens / Nuclear Instruments and Methods in Physics Research A 617 (2010) 40–44
41
Table 1 The requirements to the LHCb calorimeter system. Sub-detector
SPD/PS
ECAL
HCAL
Number of channels Lateral size (m2 ) Longitudinal depth Basic requirement
6016 each 6:2 7:6
6016 6:3 7:8
1488 6:8 8:4
180 mm 2:5X 0 0:1lI 20/30 photo-electrons per MIP
Dynamic range
0–100 MIPs—1 bit (SPD), 10 bits (PS)
25X 0 1:1lI pffiffiffi 10%= E 1:5% ðE in GeVÞ 0–10 GeV ET
5:6lI pffiffiffi 80%= E 10% ðE in GeVÞ 0–10 GeV ET
hadron, electron, photon and pion candidates, characterising a high mass B meson decay. Table 1 gives the main requirements to the LHCb calorimeter system. The PS and SPD have a segmentation that varies with respect to the distance to the beam pipe, the cell sizes matching the ECAL cell size in order to make a projective system pointing to the LHC beam collision. The SPD/PS cells are scintillator pads grooved and holding an helicoidal wavelength shifting fibre (WLS). The light is propagated by clear fibres to multi-anode photo-multipliers (MAPMT) located in boxes above and below the SPD/PS walls and containing the very front-end electronics in charge of the shaping and for the SPD only, also of the sampling of the signal. The SPD very front-end provides to the front-end electronics a binary information corresponding to the amplification and integration of the charges collected from the MAPMT. After pedestal subtraction and spill-over correction, the signal is compared with a threshold loaded by the slow control of the experiment. The output of this comparison is sent to the front-end electronics on a differential line. The PS has an energy range of 100 mips. The signal of its MAPMT is also shaped and integrated on the very front-end. But unlike the SPD, the differential analog output is sent on 27 m long twisted pairs to the PS front-end boards housing a 10 bit ADC where the pedestal, spill-over and integrator gain corrections are applied. Both the SPD and PS rely on a similar technique to measure the MAPMT signal and based on two parallel interleaved integrators running at 20 MHz per channel, one being read out and reset while the other is integrating the pulse. The overall performances of the SPD and PS system have been determined during the commissioning of the detectors that is taking place since a couple of years and are fully satisfactory. The noise is estimated to be of the order of 3 mV for the former (a mip producing 100 mV in average), the channel offset being distributed around 70 mV with a spread of 70 mV. The noise of the PS is reduced to 1.2 ADC count (a mip corresponding to 10 ADC). Its pedestal is centred at 140 ADC counts with a maximum of 300 saving the expected dynamic range. The SPD and PS detectors are built around a very front-end and a front-end which are located from 20 to 30 m apart. This leads to stringent timing constrains on the design and to the integration of degrees of freedom to compensate for the cable lengths and to accurately sample the signals at the level of the front-end boards. The corresponding parameters have been intensively worked out during the commissioning to make a robust system. 2.2. The electromagnetic and hadronic calorimeters The ECAL and HCAL are two wall-like calorimeters with a variable segmentation that fit the particle multiplicity. They have the same electronics, the main difference between those detectors is in the design of the modules. The ECAL is a shashlik system, each module consisting of 66 layers of scintillator (4 mm) and lead (2 mm) corresponding to one, four and nine cells of 12 12; 6 6 and 4 4 cm2 , respectively, in the outer, middle and inner areas
defined by the distance of the cells to the beam pipe. The HCAL is made of 26 modules of iron and scintillator tiles whose light is also transported by WLS fibres and readout by 1488 photo-multipliers dividing the detector in cells of 26:2 26:2 or 13:1 13:1 cm2 in the outer and inner regions, respectively, the previously defined middle zone being merged here with the inner one. WLS fibres cross longitudinally the modules of the ECAL and HCAL to collect and propagate the scintillator light to photomultipliers powered by Cockcroft–Walton bases. Both, the ECAL and HCAL PM high voltages are adjusted so that the measurement is directly performed in transverse energy, which is the most relevant quantity to trigger on. The ECAL and HCAL photo-multiplier pulses are clipped in order to be fully contained in 25 ns (and prevent any further spillover) and sent to the front-end electronics located on the calorimeter platform. Unlike the SPD and PS, here the number of photo-electrons is such that the statistics loss due to the clipping is irrelevant. On the front-end board, the signal is shared in two paths: the first one feeds the negative input of a differential buffer, the second one feeds the positive one after a 25 ns delay. The integrator collects the output of the buffer and presents a tension increasing up to a plateau 4 ns wide corresponding to the integration of the first path. The delayed integration of the second path permits to perform a noiseless reset of the integrator. The clock of the 12 bit ADC of each channel is linked to a robot clock allowing to adjust the phase of the sampling with the timing of the plateau which may depend on the high voltage applied on the PM, the time of flight of the particles incoming from the interaction point and the cable length spread. An overall dispersion of up to 6 ns is foreseen which may be fully corrected. The trigger is based on the existence of high transverse energy deposits. The front-end electronics identify them by summing the ET converted from 12 to 8 bits on 2 2 cell towers. This procedure requires data exchanges between front-end boards in a single crate, through the backplane and sometimes also in between different crates with cables. At a first stage, the determination of the list of high ET candidates is done per half crate (roughly 3.5% of the surface of the ECAL). The location of the electromagnetic calorimeter deposits is sent to the SPD and PS system that provides the corresponding and necessary information to determine its nature. In parallel, 4 HCAL candidates are formed from the HCAL and ECAL cell signal sums. Only one of each of the types photon, electron, neutral pion and up to 4 hadron candidates emerge at this first stage. The following step is performed in the counting room, in the safe area of the LHCb cavern and merge the previous data to identify the highest ET candidates for the full calorimeter system. Moreover, the SPD provides its total multiplicity to refine the trigger decision. Like the SPD and PS, the electromagnetic and hadronic calorimeters have been intensively commissioned since a few years. RAM patterns were loaded in the digital electronics, pulse tests have been injected right at the level of the input signal cables or LED pulses were used (see below). This permitted to control the detector cable signal connections and to check the functioning of the full electronics chain. Although the LHCb detector geometry is
ARTICLE IN PRESS 42
F. Machefert, A. Martens / Nuclear Instruments and Methods in Physics Research A 617 (2010) 40–44
Fig. 1. Cosmic event recorded in the LHCb cavern. The information of the four subdetectors have been superimposed showing clearly the muon trajectory.
not adapted for this, the last commissioning stage was mostly dedicated to cosmic runs (see Fig. 1). The PMT gain and thresholds of the ECAL and HCAL have been tuned in order to trigger on almost horizontal muons. The rate was below 1 Hz but allowed to store several millions of events in a couple of months and to study, for example, the calorimeter system timing and channel mapping.
3. Calibration and monitoring 3.1. The scintillating pad detector and the preshower Each SPD and PS cell is equipped at its centre with a LED. The corresponding pulse varying from channel to channel, it will be used only for monitoring and not for detector calibration that ought to be done with particles. The LED monitoring system is efficient to rapidly detect dead channels and to perform a rough time alignment. Moreover, any variation of the cell response will allow to detect a MAPMT gain variation. The foreseen calibration procedures of the SPD and PS rely on the same strategy but the two detector electronics and measurements being different, the technique used are adapted. The SPD is a binary detector and the calibration is done by scanning the threshold over a range large enough to contain the mip peak. Below this peak, the averaged signal per event is 1 and above it is 0. The transition occurs at the peak position which can be easily determined by derivating the detector output averaged on a sample of events and with respect to the threshold value. The minimum of the derivative corresponds to the mip position. After the fit, the chosen position of the threshold giving the best signal over noise separation is 0.7 mip (see Fig. 2). The PS calibration uses also the mip to correct the nonuniformities of the detector and the gains channel per channel. The output of the PS being a value on 10 bits, the full energy curve can be obtained and the mip peak is positioned at 10 ADC counts by correcting, at the level of the front-end boards, the gain of each channel of the MAPMT. Apart from the LED system, the monitoring will also be done by reconstructing events online on a specific data stream and at a frequency of 5 Hz. This sample will give a rapid measurement of the occupancy of the detectors, of the trigger rate of the PS, of the energy correction to be added on top of the ECAL measurement or of the channel crosstalk.
Fig. 2. Energy deposited in the SPD and estimated from a threshold scan (Monte Carlo simulations). The expected position of the threshold is 0.7 mip and is indicated on the plot by a vertical line.
3.2. The electromagnetic and hadronic calorimeters The electromagnetic and hadronic calorimeters are also equipped with a fully dedicated monitoring system based on LED. The pulse request is sent by the Time and Fast Control system of LHCb (TFC) which is in charge of providing to the front-end electronics altogether the commands and the triggers. The calibration events pass through the same acquisition path as the physics ones. It is foreseen to use not only the time between machine runs but also the gaps in the LHC beam collision structure in order to generate such events. The detectors are equipped with monitoring boxes containing several LED whose pulse is adjustable in amplitude and timing by 1ns steps. This allows to measure the full integration curve of the front-end electronics. Every LED is connected to a bundle of clear fibres going each to a single photomultiplier. A fibre of the bundle also illuminates a pin photo-diode to trace any fluctuation. The full electronics chain, from the photo-multiplier to the acquisition can be tested with this technique and any gain variation may be followed. The HCAL has also been designed with an in situ calibration system made from a 10 mCi cesium source which is usually stored in a lead box above the calorimeter platform. When a calibration of the HCAL is requested, a water flow pushes in a pipe the source that goes through all the modules of the HCAL. The detector response during the trip of the source permits to calibrate it. All the modules have undergone such a calibration in order to be certified for the installation and comparative calibrations with the Cs source and a 50 GeV pion beam showed a coincidence of the two methods to a precision of 2–3%. As was already mentioned, each channel of the ECAL and HCAL has to be time aligned with the arrival of the signal taking into account the PMT HV, the time of flight of the particles from the interaction point to the cells and any difference in the cable length. As a whole, the time spread should be less than 6 ns and can be corrected by the front-end electronics. This will be ultimately done with the first collisions, by putting the detector in a specific configuration where several triggers are forced right before and after a first level trigger yes-decision. A properly time aligned channel should have no signal in the sample preceding and following this L0. Moreover, shifting by half a period a channel from its perfect timing should lead to an equal signal in
ARTICLE IN PRESS F. Machefert, A. Martens / Nuclear Instruments and Methods in Physics Research A 617 (2010) 40–44
χ2/ndf 112.0 / 119 P1 89.91± P2 0.1351± P3 0.8310E-02± P4 3.565± 442.8± P5
250
200
43
3.671 0.3893E-03 0.4307E-03 2.193 16.58
MC
a.u.
150
100
50
0
0
0.05
0.1
0.15
0.2
0.25
0.3
2000 35.96 / 21 χ2/ndf P1 0.2368E+05± 232.9 P2 0.9931± 0.8496E-03 P3 0.8499E-01± 0.1171E-02
a.u
1500 1000 500 0 0.3
0.4
0.5
0.6
0.7
0.8 0.9 Outer section
1
1.1
1.2
1.3 E/P
0.8 0.9 Middle section
1
1.1
1.2
1.3 E/P
0.8 0.9 Inner section
1
1.1
2000 χ2/ndf 21.85 / 15 P1 0.1589E+05± 171.4 P2 1.005± 0.6905E-03 P3 0.5953E-01± 0.8805E-03
a.u
1500 1000 500 0 0.3 1500
a.u
1000
0.4
0.5
0.6
0.7
χ2/ndf 10.16 / 7 P1 7510.± 159.5 P2 1.005± 0.8033E-03 P3 0.3887E-01± 0.1139E-02
500 0 0.3
0.4
0.5
0.6
0.7
1.2
1.3 E/P
Fig. 3. Absolute ECAL calibration with neutral pions (top) and electrons (bottom). The reconstructed neutral pion mass width is expected to be of the order of 10 MeV and should permit to correct for the channel gains. The bottom plot shows the absolute calibration expected from E/p for the three ECAL regions, the impulsion being given by the tracking system.
adjacent samples. An asymmetry quantify the sharing of signal between these samples and permit to determine precisely any departure from the correct time alignment. The ECAL calibration will mainly be done with particles. The foreseen strategy consists first in performing a rough calibration to an accuracy of 10%. This was done before the installation of the modules and with cosmics. A parallel technique used to get this
precision relies on the LED pulse width measurement and the statistical fluctuation of the signal of the photo-multipliers. Going from 10% to less than 5% will be done by recording the energy flow at the level of each cell and smoothing the integrated deposits on the ECAL surface. Local linearity and detector left–right/top–bottom symmetry assumptions are used to smooth the energy response of the detector. The third stage permitting to
ARTICLE IN PRESS 44
F. Machefert, A. Martens / Nuclear Instruments and Methods in Physics Research A 617 (2010) 40–44
go down to 1% on an absolute scale consists in reconstructing the p0 mass and performing electron E/p measurements (see Fig. 3). Those last methods require a larger statistics than the previous one, the E/p calibration being highly dependent from the efficiency of the tracking system.
on cosmics which permitted to check the mapping and response of the detector and to perform a rough time analysis of the channels. Those triggers have also been largely used for the commissioning of the other sub-detectors. This past experience makes us confident that the LHCb calorimeter is ready for triggering and data taking on the LHC proton–proton collisions.
4. Conclusion The LHCb calorimeter system has been optimised to provide at 40 MHz decisive data for the first level trigger of the experiment and to allow a good offline reconstruction and identification of the electrons, photons, neutral pions and hadrons. The technical choices are such that the four sub-detectors rely on common technologies based on scintillators, clear and wavelength shifting fibres and photo-multipliers. They partly share their electronics or at least have similar electronics architectures. This makes the LHCb calorimeter an homogeneous system which is particularly important as the trigger part of the device makes the subdetectors dependent from each other. The LHCb calorimeter has been already intensively used during the commissioning stage of the experiment. It was used to trigger
Acknowledgements We would like to thank the organisers of the 11th Pisa meeting on advanced detectors for their hospitality during the conference and the members of LHCb for their help.
References [1] LHCB Technical proposal, CERN-LHCC 98-4. [2] The LHCb Detector at the LHC, the LHCb collaboration, Journal of Instrumentation (JINST) 3 (2008) S08005. [3] LHCb Calorimeters Technical Design Report, CERN/LHCC 2000-0036.
Contents lists available at ScienceDirect
Nuclear Instruments and Methods in Physics Research A journal homepage: www.elsevier.com/locate/nima
Overview of the LHCb calorimeters$ F. Machefert a,c,, A. Martens b,c a b c
CERN, Geneva, Switzerland Univ. Paris-Sud, Laboratoire de l’Acce´le´rateur Line´aire UMR8607, Orsay F-91405, France CNRS, Orsay F-91405, France
a r t i c l e in f o
a b s t r a c t
Available online 7 October 2009
LHCb, one of the four LHC experiments, is dedicated to the study of CP violation and rare decays in the B meson sector. It aims at completing the understanding of the quark flavor physics and at revealing signs of new physics beyond the standard model. The goal of the LHCb calorimeter is twofold. On the one hand, the calorimeter system has to provide a fast response for the first level trigger (L0) on the nature of the meson decay. Thus, the scintillator pad detector and the preshower provide a good g/ charged particle and electron/p0 separation and the electromagnetic and hadronic calorimeters give a fast transverse energy determination. On the other hand, it provides offline precision measurements and particle identification. The calorimeter system consists of four sub-detectors. They are described in the first section, emphasising the technical choices and the similarities among those components. The second part concerns the monitoring and calibration tools and procedures that will be applied to have a satisfactory running of the detector. & 2009 Elsevier B.V. All rights reserved.
Keywords: Particle physics LHCb CP-violation Calorimetry
1. Introduction LHCb, one of the four particle physics experiments at the Large Hadron Collider at CERN, will perform studies of CP-symmetry violation and rare decays of B hadrons. It is a single arm spectrometer with a forward angular coverage motivated by the fact that at high energies both quarks from the bb- pairs are predominantly produced at small angles with respect to the beam. At the nominal luminosity, L ¼ 2 1032 cm2 s1 ; 1012 bb- pairs should be produced at the experiment interaction point in a year of data taking (107 s). LHCb [1,2] consists of a magnet, a vertex locator, a tracking system, two ring imaging Cherenkov detectors, a calorimeter and a muon system. The main purpose of the LHCb calorimeters [3] is the selection and identification of hadrons, electrons and photons and the measurement of their energies and directions, both at the first trigger level and for the offline reconstruction. Four subdetectors are associated to perform such identification: a scintillating pad detector (SPD) and a preshower (PS) allow to tag charged particles and to determine their electromagnetic nature; they are followed by an electromagnetic (ECAL) and a hadronic (HCAL) calorimeter. The calorimeter system is used at the first level trigger (L0) of LHCb by providing high transverse energy electron, photon, neutral pion and hadron candidates. The response of the calorimeter system has to match the accelerator
$
This document is a LHCb collaborative contribution.
Corresponding author at: CERN, Geneva, Switzerland.
E-mail address: [email protected] (F. Machefert). 0168-9002/$ - see front matter & 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.nima.2009.09.123
frequency and provides a measurement at 40 MHz. The data are pipelined in the front-end electronics waiting for the L0 decision that combines the information from the calorimeter, the muon chambers and the pile up veto. Finally, the data are read out and sent to the CPU farm of the High Level Trigger (HLT) of LHCb at an average rate of 1 MHz.
2. Design of the LHCb calorimeter system 2.1. The scintillating pad detector and the preshower The four calorimeters are wall-like structures divided into two halves which may be open and fully taken out of the acceptance. The first calorimeters seen by the particles incoming from the interaction point are the scintillating pad detector and the preshower. Their design is very similar and consists of two scintillating vertical planes made of 6016 pads. A 2.5 radiation length lead sheet is sandwiched between the two sub-detectors. This lead converter allows to initiate the electromagnetic showers so that electrons and photons deposit a sizable amount of energy in the PS. Charged particles leave in the SPD a minimum ionising particle (mip) signal which is detected while photons do not interact. Combining the SPD and the PS information with the cluster position reconstruction of the ECAL gives a determination of the nature of the electromagnetic particle interacting with the calorimeter system. This technique is used offline but also at the first level trigger of LHCb to tag high transverse momentum
ARTICLE IN PRESS F. Machefert, A. Martens / Nuclear Instruments and Methods in Physics Research A 617 (2010) 40–44
41
Table 1 The requirements to the LHCb calorimeter system. Sub-detector
SPD/PS
ECAL
HCAL
Number of channels Lateral size (m2 ) Longitudinal depth Basic requirement
6016 each 6:2 7:6
6016 6:3 7:8
1488 6:8 8:4
180 mm 2:5X 0 0:1lI 20/30 photo-electrons per MIP
Dynamic range
0–100 MIPs—1 bit (SPD), 10 bits (PS)
25X 0 1:1lI pffiffiffi 10%= E 1:5% ðE in GeVÞ 0–10 GeV ET
5:6lI pffiffiffi 80%= E 10% ðE in GeVÞ 0–10 GeV ET
hadron, electron, photon and pion candidates, characterising a high mass B meson decay. Table 1 gives the main requirements to the LHCb calorimeter system. The PS and SPD have a segmentation that varies with respect to the distance to the beam pipe, the cell sizes matching the ECAL cell size in order to make a projective system pointing to the LHC beam collision. The SPD/PS cells are scintillator pads grooved and holding an helicoidal wavelength shifting fibre (WLS). The light is propagated by clear fibres to multi-anode photo-multipliers (MAPMT) located in boxes above and below the SPD/PS walls and containing the very front-end electronics in charge of the shaping and for the SPD only, also of the sampling of the signal. The SPD very front-end provides to the front-end electronics a binary information corresponding to the amplification and integration of the charges collected from the MAPMT. After pedestal subtraction and spill-over correction, the signal is compared with a threshold loaded by the slow control of the experiment. The output of this comparison is sent to the front-end electronics on a differential line. The PS has an energy range of 100 mips. The signal of its MAPMT is also shaped and integrated on the very front-end. But unlike the SPD, the differential analog output is sent on 27 m long twisted pairs to the PS front-end boards housing a 10 bit ADC where the pedestal, spill-over and integrator gain corrections are applied. Both the SPD and PS rely on a similar technique to measure the MAPMT signal and based on two parallel interleaved integrators running at 20 MHz per channel, one being read out and reset while the other is integrating the pulse. The overall performances of the SPD and PS system have been determined during the commissioning of the detectors that is taking place since a couple of years and are fully satisfactory. The noise is estimated to be of the order of 3 mV for the former (a mip producing 100 mV in average), the channel offset being distributed around 70 mV with a spread of 70 mV. The noise of the PS is reduced to 1.2 ADC count (a mip corresponding to 10 ADC). Its pedestal is centred at 140 ADC counts with a maximum of 300 saving the expected dynamic range. The SPD and PS detectors are built around a very front-end and a front-end which are located from 20 to 30 m apart. This leads to stringent timing constrains on the design and to the integration of degrees of freedom to compensate for the cable lengths and to accurately sample the signals at the level of the front-end boards. The corresponding parameters have been intensively worked out during the commissioning to make a robust system. 2.2. The electromagnetic and hadronic calorimeters The ECAL and HCAL are two wall-like calorimeters with a variable segmentation that fit the particle multiplicity. They have the same electronics, the main difference between those detectors is in the design of the modules. The ECAL is a shashlik system, each module consisting of 66 layers of scintillator (4 mm) and lead (2 mm) corresponding to one, four and nine cells of 12 12; 6 6 and 4 4 cm2 , respectively, in the outer, middle and inner areas
defined by the distance of the cells to the beam pipe. The HCAL is made of 26 modules of iron and scintillator tiles whose light is also transported by WLS fibres and readout by 1488 photo-multipliers dividing the detector in cells of 26:2 26:2 or 13:1 13:1 cm2 in the outer and inner regions, respectively, the previously defined middle zone being merged here with the inner one. WLS fibres cross longitudinally the modules of the ECAL and HCAL to collect and propagate the scintillator light to photomultipliers powered by Cockcroft–Walton bases. Both, the ECAL and HCAL PM high voltages are adjusted so that the measurement is directly performed in transverse energy, which is the most relevant quantity to trigger on. The ECAL and HCAL photo-multiplier pulses are clipped in order to be fully contained in 25 ns (and prevent any further spillover) and sent to the front-end electronics located on the calorimeter platform. Unlike the SPD and PS, here the number of photo-electrons is such that the statistics loss due to the clipping is irrelevant. On the front-end board, the signal is shared in two paths: the first one feeds the negative input of a differential buffer, the second one feeds the positive one after a 25 ns delay. The integrator collects the output of the buffer and presents a tension increasing up to a plateau 4 ns wide corresponding to the integration of the first path. The delayed integration of the second path permits to perform a noiseless reset of the integrator. The clock of the 12 bit ADC of each channel is linked to a robot clock allowing to adjust the phase of the sampling with the timing of the plateau which may depend on the high voltage applied on the PM, the time of flight of the particles incoming from the interaction point and the cable length spread. An overall dispersion of up to 6 ns is foreseen which may be fully corrected. The trigger is based on the existence of high transverse energy deposits. The front-end electronics identify them by summing the ET converted from 12 to 8 bits on 2 2 cell towers. This procedure requires data exchanges between front-end boards in a single crate, through the backplane and sometimes also in between different crates with cables. At a first stage, the determination of the list of high ET candidates is done per half crate (roughly 3.5% of the surface of the ECAL). The location of the electromagnetic calorimeter deposits is sent to the SPD and PS system that provides the corresponding and necessary information to determine its nature. In parallel, 4 HCAL candidates are formed from the HCAL and ECAL cell signal sums. Only one of each of the types photon, electron, neutral pion and up to 4 hadron candidates emerge at this first stage. The following step is performed in the counting room, in the safe area of the LHCb cavern and merge the previous data to identify the highest ET candidates for the full calorimeter system. Moreover, the SPD provides its total multiplicity to refine the trigger decision. Like the SPD and PS, the electromagnetic and hadronic calorimeters have been intensively commissioned since a few years. RAM patterns were loaded in the digital electronics, pulse tests have been injected right at the level of the input signal cables or LED pulses were used (see below). This permitted to control the detector cable signal connections and to check the functioning of the full electronics chain. Although the LHCb detector geometry is
ARTICLE IN PRESS 42
F. Machefert, A. Martens / Nuclear Instruments and Methods in Physics Research A 617 (2010) 40–44
Fig. 1. Cosmic event recorded in the LHCb cavern. The information of the four subdetectors have been superimposed showing clearly the muon trajectory.
not adapted for this, the last commissioning stage was mostly dedicated to cosmic runs (see Fig. 1). The PMT gain and thresholds of the ECAL and HCAL have been tuned in order to trigger on almost horizontal muons. The rate was below 1 Hz but allowed to store several millions of events in a couple of months and to study, for example, the calorimeter system timing and channel mapping.
3. Calibration and monitoring 3.1. The scintillating pad detector and the preshower Each SPD and PS cell is equipped at its centre with a LED. The corresponding pulse varying from channel to channel, it will be used only for monitoring and not for detector calibration that ought to be done with particles. The LED monitoring system is efficient to rapidly detect dead channels and to perform a rough time alignment. Moreover, any variation of the cell response will allow to detect a MAPMT gain variation. The foreseen calibration procedures of the SPD and PS rely on the same strategy but the two detector electronics and measurements being different, the technique used are adapted. The SPD is a binary detector and the calibration is done by scanning the threshold over a range large enough to contain the mip peak. Below this peak, the averaged signal per event is 1 and above it is 0. The transition occurs at the peak position which can be easily determined by derivating the detector output averaged on a sample of events and with respect to the threshold value. The minimum of the derivative corresponds to the mip position. After the fit, the chosen position of the threshold giving the best signal over noise separation is 0.7 mip (see Fig. 2). The PS calibration uses also the mip to correct the nonuniformities of the detector and the gains channel per channel. The output of the PS being a value on 10 bits, the full energy curve can be obtained and the mip peak is positioned at 10 ADC counts by correcting, at the level of the front-end boards, the gain of each channel of the MAPMT. Apart from the LED system, the monitoring will also be done by reconstructing events online on a specific data stream and at a frequency of 5 Hz. This sample will give a rapid measurement of the occupancy of the detectors, of the trigger rate of the PS, of the energy correction to be added on top of the ECAL measurement or of the channel crosstalk.
Fig. 2. Energy deposited in the SPD and estimated from a threshold scan (Monte Carlo simulations). The expected position of the threshold is 0.7 mip and is indicated on the plot by a vertical line.
3.2. The electromagnetic and hadronic calorimeters The electromagnetic and hadronic calorimeters are also equipped with a fully dedicated monitoring system based on LED. The pulse request is sent by the Time and Fast Control system of LHCb (TFC) which is in charge of providing to the front-end electronics altogether the commands and the triggers. The calibration events pass through the same acquisition path as the physics ones. It is foreseen to use not only the time between machine runs but also the gaps in the LHC beam collision structure in order to generate such events. The detectors are equipped with monitoring boxes containing several LED whose pulse is adjustable in amplitude and timing by 1ns steps. This allows to measure the full integration curve of the front-end electronics. Every LED is connected to a bundle of clear fibres going each to a single photomultiplier. A fibre of the bundle also illuminates a pin photo-diode to trace any fluctuation. The full electronics chain, from the photo-multiplier to the acquisition can be tested with this technique and any gain variation may be followed. The HCAL has also been designed with an in situ calibration system made from a 10 mCi cesium source which is usually stored in a lead box above the calorimeter platform. When a calibration of the HCAL is requested, a water flow pushes in a pipe the source that goes through all the modules of the HCAL. The detector response during the trip of the source permits to calibrate it. All the modules have undergone such a calibration in order to be certified for the installation and comparative calibrations with the Cs source and a 50 GeV pion beam showed a coincidence of the two methods to a precision of 2–3%. As was already mentioned, each channel of the ECAL and HCAL has to be time aligned with the arrival of the signal taking into account the PMT HV, the time of flight of the particles from the interaction point to the cells and any difference in the cable length. As a whole, the time spread should be less than 6 ns and can be corrected by the front-end electronics. This will be ultimately done with the first collisions, by putting the detector in a specific configuration where several triggers are forced right before and after a first level trigger yes-decision. A properly time aligned channel should have no signal in the sample preceding and following this L0. Moreover, shifting by half a period a channel from its perfect timing should lead to an equal signal in
ARTICLE IN PRESS F. Machefert, A. Martens / Nuclear Instruments and Methods in Physics Research A 617 (2010) 40–44
χ2/ndf 112.0 / 119 P1 89.91± P2 0.1351± P3 0.8310E-02± P4 3.565± 442.8± P5
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Fig. 3. Absolute ECAL calibration with neutral pions (top) and electrons (bottom). The reconstructed neutral pion mass width is expected to be of the order of 10 MeV and should permit to correct for the channel gains. The bottom plot shows the absolute calibration expected from E/p for the three ECAL regions, the impulsion being given by the tracking system.
adjacent samples. An asymmetry quantify the sharing of signal between these samples and permit to determine precisely any departure from the correct time alignment. The ECAL calibration will mainly be done with particles. The foreseen strategy consists first in performing a rough calibration to an accuracy of 10%. This was done before the installation of the modules and with cosmics. A parallel technique used to get this
precision relies on the LED pulse width measurement and the statistical fluctuation of the signal of the photo-multipliers. Going from 10% to less than 5% will be done by recording the energy flow at the level of each cell and smoothing the integrated deposits on the ECAL surface. Local linearity and detector left–right/top–bottom symmetry assumptions are used to smooth the energy response of the detector. The third stage permitting to
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go down to 1% on an absolute scale consists in reconstructing the p0 mass and performing electron E/p measurements (see Fig. 3). Those last methods require a larger statistics than the previous one, the E/p calibration being highly dependent from the efficiency of the tracking system.
on cosmics which permitted to check the mapping and response of the detector and to perform a rough time analysis of the channels. Those triggers have also been largely used for the commissioning of the other sub-detectors. This past experience makes us confident that the LHCb calorimeter is ready for triggering and data taking on the LHC proton–proton collisions.
4. Conclusion The LHCb calorimeter system has been optimised to provide at 40 MHz decisive data for the first level trigger of the experiment and to allow a good offline reconstruction and identification of the electrons, photons, neutral pions and hadrons. The technical choices are such that the four sub-detectors rely on common technologies based on scintillators, clear and wavelength shifting fibres and photo-multipliers. They partly share their electronics or at least have similar electronics architectures. This makes the LHCb calorimeter an homogeneous system which is particularly important as the trigger part of the device makes the subdetectors dependent from each other. The LHCb calorimeter has been already intensively used during the commissioning stage of the experiment. It was used to trigger
Acknowledgements We would like to thank the organisers of the 11th Pisa meeting on advanced detectors for their hospitality during the conference and the members of LHCb for their help.
References [1] LHCB Technical proposal, CERN-LHCC 98-4. [2] The LHCb Detector at the LHC, the LHCb collaboration, Journal of Instrumentation (JINST) 3 (2008) S08005. [3] LHCb Calorimeters Technical Design Report, CERN/LHCC 2000-0036.