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First years of running of the LHCb calorimeter system
Nuclear Instruments and Methods in Physics Research A 787 (2015) 373–375
Contents lists available at ScienceDirect
Nuclear Instruments and Methods in Physics Research A journal homepage: www.elsevier.com/locate/nima
First years of running of the LHCb calorimeter system Frédéric Machefert LAL, Univ Paris-Sud, CNRS/IN2P3, Orsay, France
On behalf of the LHCb experiment art ic l e i nf o
a b s t r a c t
Available online 4 February 2015
The LHCb detector and its calorimeter system have been taking data from 2010 up to beginning of 2013 1 and stored an integrated luminosity larger than L ¼ 3 fb . A first assessment of this period for the calorimeters can be made. The detectors are briefly described altogether with the beam conditions during those three years. The energy calibration methods are explained and some results on the ageing of the modules or of the photomultipliers are given. Finally, the performances will be illustrated with some first physics results. & 2015 The Author. Published by Elsevier B.V. All rights reserved.
Keywords: Particle physics LHCb CP-violation Rare decays Calorimetry
1. Introduction LHCb [1] is a detector installed about 100 m underground, on the ring of the Large Hadron Collider, at CERN (Geneva). It aims at studying the origin of the matter–antimatter asymmetry in the universe and at discovering new physics through some potential indirect manifestations (enhancement of some rare decays, anomaly of important parameters of the standard model, etc.). The bquark sector is very favorable for those studies and both the energy at LHC and the geometry of LHCb, covering a region in rapidity 2 r η r5, permit to produce a large quantity of hadrons containing such a quark. LHCb is made of several sub-detectors whose purpose is to reconstruct the event produced by proton–proton collisions at LHC. The beam crossing rate reached about 20 MHz, each event producing a large number of particles. The sub-detectors aim at reconstructing the tracks from charged particles and their momenta, at identifying those particles or at determining their energy.
2. The LHCb calorimeter system The calorimeter system [2] of LHCb is among the last systems crossed by the particles (Fig. 1). It is made of 4 parts: the scintillating pad detector (SPD), a preshower (PS), the electromagnetic (ECAL) and hadronic calorimeter (HCAL). The SPD is a wall of scintillator tiles. A wavelength shifting fibre (WLS) inserted in a groove inside the tile propagates the light up to a multianode photomultiplier (MAPMT). The signal from the SPD permits to separate charged particles from neutral ones (essentially charged pions and electrons from photons). After the SPD, the particles originating from the interaction point cross a 2:5X 0 lead absorber. http://dx.doi.org/10.1016/j.nima.2015.01.097 0168-9002/& 2015 The Author. Published by Elsevier B.V. All rights reserved.
Photons and electrons have a large probability to start showering at this stage. The PS, that follows the absorber measures the beginning of the shower and produces a signal, separating photons and electrons from pions. The PS is very similar to the SPD, although their electronics are different. The former gives a measurement of the energy deposited on 10 bits and the latter returns a 1 bit value based on the comparison of the measurement with a tunable threshold. The ECAL is a shashlik detector 25X 0 and 1:1λI thick, made of 66 layers of lead and scintillator. The light produced is absorbed by WLS fibres and converted into an electric pulse with photomultipliers (PMT). The light collected is large, 3000 photoelectrons (pe) per GeV, and the signal is readout on 12 bits (as for the HCAL, the two calorimeters sharing the same electronics). The pffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi resolution of the ECAL is 10%= EðGeVÞ 0:9%. The hadrons which are not absorbed by the ECAL continue to the HCAL, which is made of 26 modules built from interleaved layers of iron and scintillator. The light is captured and propagated with WLS fibres to PMT as for the ECAL. The thickness of the HCAL is 5:6λI , the HCAL producing 20 times less p light than the ECAL. The final resolution of the HCAL is ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi ð697 5Þ%= EðGeVÞ ð0 7 2Þ% as measured, like for the ECAL, in test beam. Apart from the HCAL, the three other sub-detectors contain 6016 cells each, with a pseudo-projective geometry, the size of the cells increasing with the lower occupancy, from 4 4 cm2 close to the beam pipe, to 12 12 at the edge of the detectors. The HCAL has about 1400 cells with only 2 different granularities. For convenience, each sub-detector may be open by halves. The high collision rate at the LHCb interaction point requires a powerful trigger that selects the interesting events from topological and kinematic criteria. The trigger is divided in two stages. The first level (L0) is a hardware stage based exclusively on the calorimeter and muon system information with an output rate of 1 MHz. The second stage is a software trigger using the information of all the
374
F. Machefert / Nuclear Instruments and Methods in Physics Research A 787 (2015) 373–375
Fig. 1. (a) The LHCb detector geometry with the calorimeter in dark. (b) The ECAL alone, with its gantry where is localized the front-end electronics. The wall shape of the ECAL is obvious, the beam pipe crossing the center of the detector. The supporting chariots are indicated and permit to open the detector by halves.
sub-detectors and running in a large computer farm with more than 300 thousand tasks in parallel. The calorimeter is a key ingredient of the trigger and especially the L0 imposes stringent constraints on the speed and precision of the sub-detectors. The design of the LHCb calorimeter system based on 4 complementary sub-detectors permits to get a rapid measurement of the energy of the particles absorbed, a precise determination of their impact position and a good indication on the type of the particles. This is obtained offline for the physics analysis, combining the calorimeter data with the other sub-detectors of the experiment, and already at the trigger level.
3. Running conditions The running conditions of LHCb varied a lot from 2010 to 2013. 1 38 pb have been stored in 2010 at the start-up of the accelerator. But the increase of the number of bunches on the accelerator ring and 1 1 the focusing of the beam permitted to reach 1:1 fb and 2:2 pb in pffiffi pffiffi 2011 ( s ¼ 7 TeV) and 2012 ( s ¼ 8 TeV) respectively. In 2012, the instantaneous luminosity was twice the one foreseen during the design of the experiment reaching L ¼ 4 1032 cm 2 s 1 . This was possible thanks to the confidence of the collaboration in the performances of the detector and used a new technique called luminosity levelling. It consists in keeping the instantaneous luminosity constant during a fill of the machine by slightly shifting the two beam axis with respect to each other. The shift is larger at the beginning of the fill, when the proton multiplicity of the bunches is large and is progressively reduced. The action on the machine magnets is automatically triggered by the instantaneous luminosity measurement provided by LHCb. From the beginning, the operation efficiency of the experiment is excellent, reaching more than 94% on average, the fraction of data tagged as of “good” quality being 98%. During the three years of data taking, more specifically, the calorimeter system had a rate of good channel larger than 99% on average.
4. Calibration and experience The calibration method differs depending on the sub-detector. Here, issues related to the energy calibration are discussed. The SPD is a 1-bit detector and is difficult to calibrate. The method
Fig. 2. Light yield degradation over 44 HCAL cells and for four rows. The row 0 is the closest to the beam pipe, row numbers increasing with the distance to the beam.
used to determine the energy threshold consists in extrapolating tracks from the LHCb tracking system up to the SPD and measuring the efficiency of the SPD for those tracks. In order to reach 95%, the threshold should be set at 0.5 minimum ionizing particle (mip). A scan of the efficiency with respect to the energy threshold applied on the electronics is performed. Taking into account the expected shape of the efficiency that is based on the convolution of a Landau distribution (for the energy loss) with a Gaussian function (for the fluctuations of the number of photoelectrons at the photocathode), it is possible to reach a 5% precision on the mip position. The resolution and dynamic of the PS are such that an optimal mip position is at 10 ADC counts. As for the SPD, the extrapolation of the tracks to the PS is used, although, here, no scan is necessary. A precision of 5% is also reached. The calibration of the ECAL is the most requiring one and is done in several steps. Let's mention that the cells have been calibrated with cosmics prior to the installation of the modules. An averaging of the cell response to particles is done first to extract a set of corrections. A precision of a few percent is obtained at this level. The fine calibration is done through an iterative procedure based on the reconstruction of the neutral pion mass distribution. A large statistics sample is
F. Machefert / Nuclear Instruments and Methods in Physics Research A 787 (2015) 373–375
375
Fig. 3. Reconstruction of the b-meson invariant masses in the decay channels (a) B0 -K⋆ γ and (b) Bs -ϕγ. The resolution, related mainly to the photon reconstruction, is approximately 90 MeV=c2 .
processed and a calibration coefficient is extracted for each cell in order to move the π0 mass closer to the theoretical one. The processing is performed again after applying a first set of corrections and up to the convergence of the 6016 coefficients. This is done after a few steps. But the method requires such a large 1 sample that it is done for 200 pb typical, i.e. every month. The percent precision is reached on the energy scale. Another correction is done with electrons and compares the energy measurement from the tracking system with the sum of the PS and ECAL 1 contributions. This is done more regularly, every 40 pb . The calibration of the HCAL uses an external tool based on a radioactive 137 Cs source. The source travels through an hydraulic system inserted into the modules of each HCAL half. Dedicated integrators measure the response of the module to the source flux. The absolute normalization precision obtained is 10%, and the inter-cell calibration 4%. This calibration can be done only during the technical stops which occur regularly on the month basis on the accelerator. A LED system is used continuously during data taking, to monitor the HCAL cell gains. Detector ageing was observed on the HCAL optical system. This 1 was expected and appears already after 3 fb of data taking. This affects mainly the scintillator and the plastic tiles which become less transparent. This is proportional to the particle flux and is correlated with the detector occupancy, the innermost cells exhibited the effect. The PMT ageing, is also observed. It is related to the integrated current and thus on the position (the PMT gain of the cells depends on the position, the measurement being done in transverse energy for the trigger) and the cell size. Those combined ageing effects pushed the collaboration to regularly revise the high-voltage settings of the PMT in order to compensate and to have a constant trigger efficiency for hadronic channels. Fig. 2 shows the HCAL light yield measured with respect to the distance to the beam pipe and according to the integrated luminosity. The ageing effect is localized but quite pronounced close to the beam. 5. Performances In most of the analysis, the calorimeter information is combined with the information from the tracking system, the ring imaging Cherenkov detectors (RICH) and the muons system. As an example, and by itself, the calorimeter permits to select electrons with an efficiency of 90% and a purity of 5%. Those numbers reach 97% and 2% respectively after combination. The selection is based
on likelihood difference estimation for the signal and background hypothesis, the likelihoods being determined from data distributions. The characteristics of the ECAL allow to reconstruct resolved neutral pions with two identified clusters up to Et r 2:5 GeV with a resolution of 8 MeV=c2 . However, neutral pions whose photons cannot be separated may still be identified with a resolution only slightly degraded. The calorimeter system of LHCb is used in different key measurements of LHCb. The b-γ radiative decay mode are among those golden channels, as they are good candidates to identify new physics at LHC. This is related to the fact that high mass virtual particles may contribute to the process. The observables that may diverge from the Standard Model predictions are the branching ratios of the radiative decays of the CP asymmetry (CP relates a process to the corresponding one after applying the charge and parity transformations). Fig. 3 shows the LHCb reconstruction of the B0 and Bs candidate masses in the decays B0 -K⋆ γ and Bs -ϕγ. Those analyses led to the world best measurements of the branching ratio BRðBs -ϕγÞ ¼ ð3:5 7 0:4Þ 10 5 and ACP ðB0 -K⋆ γÞ ¼ ð0:8 7 1:7 7 0:9Þ% CP asymmetry [3]. 6. Conclusion The operation efficiency of LHCb and its calorimeter system have been excellent during the first three years of data taking. The 1 first effects of ageing are observed, after more than 3 fb , on the scintillator tiles and PMT. The project of upgrade of the LHCb experiment is on track and the calorimeter system is part of it [4], the main improvement being a full readout of the calorimeter system at 40 MHz.
Acknowledgments The author wishes to thank the organisers of the NDIP2014 conference for their hospitality during the conference and the members of LHCb for their help. References [1] The LHCb Collaboration, Journal of Instrumentation 3 (2008) S08005. [2] S. Amato, et al., LHCb Calorimeters. Technical Design Report, CERN-LHCC-2000036, 2000. [3] The LHCb Collaboration, Nuclear Physics B 867 (2013) 1. [4] The LHCb Collaboration, LHCb PID Upgrade TDR, CERN-LHCC-2013-022, 2000.
Contents lists available at ScienceDirect
Nuclear Instruments and Methods in Physics Research A journal homepage: www.elsevier.com/locate/nima
First years of running of the LHCb calorimeter system Frédéric Machefert LAL, Univ Paris-Sud, CNRS/IN2P3, Orsay, France
On behalf of the LHCb experiment art ic l e i nf o
a b s t r a c t
Available online 4 February 2015
The LHCb detector and its calorimeter system have been taking data from 2010 up to beginning of 2013 1 and stored an integrated luminosity larger than L ¼ 3 fb . A first assessment of this period for the calorimeters can be made. The detectors are briefly described altogether with the beam conditions during those three years. The energy calibration methods are explained and some results on the ageing of the modules or of the photomultipliers are given. Finally, the performances will be illustrated with some first physics results. & 2015 The Author. Published by Elsevier B.V. All rights reserved.
Keywords: Particle physics LHCb CP-violation Rare decays Calorimetry
1. Introduction LHCb [1] is a detector installed about 100 m underground, on the ring of the Large Hadron Collider, at CERN (Geneva). It aims at studying the origin of the matter–antimatter asymmetry in the universe and at discovering new physics through some potential indirect manifestations (enhancement of some rare decays, anomaly of important parameters of the standard model, etc.). The bquark sector is very favorable for those studies and both the energy at LHC and the geometry of LHCb, covering a region in rapidity 2 r η r5, permit to produce a large quantity of hadrons containing such a quark. LHCb is made of several sub-detectors whose purpose is to reconstruct the event produced by proton–proton collisions at LHC. The beam crossing rate reached about 20 MHz, each event producing a large number of particles. The sub-detectors aim at reconstructing the tracks from charged particles and their momenta, at identifying those particles or at determining their energy.
2. The LHCb calorimeter system The calorimeter system [2] of LHCb is among the last systems crossed by the particles (Fig. 1). It is made of 4 parts: the scintillating pad detector (SPD), a preshower (PS), the electromagnetic (ECAL) and hadronic calorimeter (HCAL). The SPD is a wall of scintillator tiles. A wavelength shifting fibre (WLS) inserted in a groove inside the tile propagates the light up to a multianode photomultiplier (MAPMT). The signal from the SPD permits to separate charged particles from neutral ones (essentially charged pions and electrons from photons). After the SPD, the particles originating from the interaction point cross a 2:5X 0 lead absorber. http://dx.doi.org/10.1016/j.nima.2015.01.097 0168-9002/& 2015 The Author. Published by Elsevier B.V. All rights reserved.
Photons and electrons have a large probability to start showering at this stage. The PS, that follows the absorber measures the beginning of the shower and produces a signal, separating photons and electrons from pions. The PS is very similar to the SPD, although their electronics are different. The former gives a measurement of the energy deposited on 10 bits and the latter returns a 1 bit value based on the comparison of the measurement with a tunable threshold. The ECAL is a shashlik detector 25X 0 and 1:1λI thick, made of 66 layers of lead and scintillator. The light produced is absorbed by WLS fibres and converted into an electric pulse with photomultipliers (PMT). The light collected is large, 3000 photoelectrons (pe) per GeV, and the signal is readout on 12 bits (as for the HCAL, the two calorimeters sharing the same electronics). The pffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi resolution of the ECAL is 10%= EðGeVÞ 0:9%. The hadrons which are not absorbed by the ECAL continue to the HCAL, which is made of 26 modules built from interleaved layers of iron and scintillator. The light is captured and propagated with WLS fibres to PMT as for the ECAL. The thickness of the HCAL is 5:6λI , the HCAL producing 20 times less p light than the ECAL. The final resolution of the HCAL is ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi ð697 5Þ%= EðGeVÞ ð0 7 2Þ% as measured, like for the ECAL, in test beam. Apart from the HCAL, the three other sub-detectors contain 6016 cells each, with a pseudo-projective geometry, the size of the cells increasing with the lower occupancy, from 4 4 cm2 close to the beam pipe, to 12 12 at the edge of the detectors. The HCAL has about 1400 cells with only 2 different granularities. For convenience, each sub-detector may be open by halves. The high collision rate at the LHCb interaction point requires a powerful trigger that selects the interesting events from topological and kinematic criteria. The trigger is divided in two stages. The first level (L0) is a hardware stage based exclusively on the calorimeter and muon system information with an output rate of 1 MHz. The second stage is a software trigger using the information of all the
374
F. Machefert / Nuclear Instruments and Methods in Physics Research A 787 (2015) 373–375
Fig. 1. (a) The LHCb detector geometry with the calorimeter in dark. (b) The ECAL alone, with its gantry where is localized the front-end electronics. The wall shape of the ECAL is obvious, the beam pipe crossing the center of the detector. The supporting chariots are indicated and permit to open the detector by halves.
sub-detectors and running in a large computer farm with more than 300 thousand tasks in parallel. The calorimeter is a key ingredient of the trigger and especially the L0 imposes stringent constraints on the speed and precision of the sub-detectors. The design of the LHCb calorimeter system based on 4 complementary sub-detectors permits to get a rapid measurement of the energy of the particles absorbed, a precise determination of their impact position and a good indication on the type of the particles. This is obtained offline for the physics analysis, combining the calorimeter data with the other sub-detectors of the experiment, and already at the trigger level.
3. Running conditions The running conditions of LHCb varied a lot from 2010 to 2013. 1 38 pb have been stored in 2010 at the start-up of the accelerator. But the increase of the number of bunches on the accelerator ring and 1 1 the focusing of the beam permitted to reach 1:1 fb and 2:2 pb in pffiffi pffiffi 2011 ( s ¼ 7 TeV) and 2012 ( s ¼ 8 TeV) respectively. In 2012, the instantaneous luminosity was twice the one foreseen during the design of the experiment reaching L ¼ 4 1032 cm 2 s 1 . This was possible thanks to the confidence of the collaboration in the performances of the detector and used a new technique called luminosity levelling. It consists in keeping the instantaneous luminosity constant during a fill of the machine by slightly shifting the two beam axis with respect to each other. The shift is larger at the beginning of the fill, when the proton multiplicity of the bunches is large and is progressively reduced. The action on the machine magnets is automatically triggered by the instantaneous luminosity measurement provided by LHCb. From the beginning, the operation efficiency of the experiment is excellent, reaching more than 94% on average, the fraction of data tagged as of “good” quality being 98%. During the three years of data taking, more specifically, the calorimeter system had a rate of good channel larger than 99% on average.
4. Calibration and experience The calibration method differs depending on the sub-detector. Here, issues related to the energy calibration are discussed. The SPD is a 1-bit detector and is difficult to calibrate. The method
Fig. 2. Light yield degradation over 44 HCAL cells and for four rows. The row 0 is the closest to the beam pipe, row numbers increasing with the distance to the beam.
used to determine the energy threshold consists in extrapolating tracks from the LHCb tracking system up to the SPD and measuring the efficiency of the SPD for those tracks. In order to reach 95%, the threshold should be set at 0.5 minimum ionizing particle (mip). A scan of the efficiency with respect to the energy threshold applied on the electronics is performed. Taking into account the expected shape of the efficiency that is based on the convolution of a Landau distribution (for the energy loss) with a Gaussian function (for the fluctuations of the number of photoelectrons at the photocathode), it is possible to reach a 5% precision on the mip position. The resolution and dynamic of the PS are such that an optimal mip position is at 10 ADC counts. As for the SPD, the extrapolation of the tracks to the PS is used, although, here, no scan is necessary. A precision of 5% is also reached. The calibration of the ECAL is the most requiring one and is done in several steps. Let's mention that the cells have been calibrated with cosmics prior to the installation of the modules. An averaging of the cell response to particles is done first to extract a set of corrections. A precision of a few percent is obtained at this level. The fine calibration is done through an iterative procedure based on the reconstruction of the neutral pion mass distribution. A large statistics sample is
F. Machefert / Nuclear Instruments and Methods in Physics Research A 787 (2015) 373–375
375
Fig. 3. Reconstruction of the b-meson invariant masses in the decay channels (a) B0 -K⋆ γ and (b) Bs -ϕγ. The resolution, related mainly to the photon reconstruction, is approximately 90 MeV=c2 .
processed and a calibration coefficient is extracted for each cell in order to move the π0 mass closer to the theoretical one. The processing is performed again after applying a first set of corrections and up to the convergence of the 6016 coefficients. This is done after a few steps. But the method requires such a large 1 sample that it is done for 200 pb typical, i.e. every month. The percent precision is reached on the energy scale. Another correction is done with electrons and compares the energy measurement from the tracking system with the sum of the PS and ECAL 1 contributions. This is done more regularly, every 40 pb . The calibration of the HCAL uses an external tool based on a radioactive 137 Cs source. The source travels through an hydraulic system inserted into the modules of each HCAL half. Dedicated integrators measure the response of the module to the source flux. The absolute normalization precision obtained is 10%, and the inter-cell calibration 4%. This calibration can be done only during the technical stops which occur regularly on the month basis on the accelerator. A LED system is used continuously during data taking, to monitor the HCAL cell gains. Detector ageing was observed on the HCAL optical system. This 1 was expected and appears already after 3 fb of data taking. This affects mainly the scintillator and the plastic tiles which become less transparent. This is proportional to the particle flux and is correlated with the detector occupancy, the innermost cells exhibited the effect. The PMT ageing, is also observed. It is related to the integrated current and thus on the position (the PMT gain of the cells depends on the position, the measurement being done in transverse energy for the trigger) and the cell size. Those combined ageing effects pushed the collaboration to regularly revise the high-voltage settings of the PMT in order to compensate and to have a constant trigger efficiency for hadronic channels. Fig. 2 shows the HCAL light yield measured with respect to the distance to the beam pipe and according to the integrated luminosity. The ageing effect is localized but quite pronounced close to the beam. 5. Performances In most of the analysis, the calorimeter information is combined with the information from the tracking system, the ring imaging Cherenkov detectors (RICH) and the muons system. As an example, and by itself, the calorimeter permits to select electrons with an efficiency of 90% and a purity of 5%. Those numbers reach 97% and 2% respectively after combination. The selection is based
on likelihood difference estimation for the signal and background hypothesis, the likelihoods being determined from data distributions. The characteristics of the ECAL allow to reconstruct resolved neutral pions with two identified clusters up to Et r 2:5 GeV with a resolution of 8 MeV=c2 . However, neutral pions whose photons cannot be separated may still be identified with a resolution only slightly degraded. The calorimeter system of LHCb is used in different key measurements of LHCb. The b-γ radiative decay mode are among those golden channels, as they are good candidates to identify new physics at LHC. This is related to the fact that high mass virtual particles may contribute to the process. The observables that may diverge from the Standard Model predictions are the branching ratios of the radiative decays of the CP asymmetry (CP relates a process to the corresponding one after applying the charge and parity transformations). Fig. 3 shows the LHCb reconstruction of the B0 and Bs candidate masses in the decays B0 -K⋆ γ and Bs -ϕγ. Those analyses led to the world best measurements of the branching ratio BRðBs -ϕγÞ ¼ ð3:5 7 0:4Þ 10 5 and ACP ðB0 -K⋆ γÞ ¼ ð0:8 7 1:7 7 0:9Þ% CP asymmetry [3]. 6. Conclusion The operation efficiency of LHCb and its calorimeter system have been excellent during the first three years of data taking. The 1 first effects of ageing are observed, after more than 3 fb , on the scintillator tiles and PMT. The project of upgrade of the LHCb experiment is on track and the calorimeter system is part of it [4], the main improvement being a full readout of the calorimeter system at 40 MHz.
Acknowledgments The author wishes to thank the organisers of the NDIP2014 conference for their hospitality during the conference and the members of LHCb for their help. References [1] The LHCb Collaboration, Journal of Instrumentation 3 (2008) S08005. [2] S. Amato, et al., LHCb Calorimeters. Technical Design Report, CERN-LHCC-2000036, 2000. [3] The LHCb Collaboration, Nuclear Physics B 867 (2013) 1. [4] The LHCb Collaboration, LHCb PID Upgrade TDR, CERN-LHCC-2013-022, 2000.