Subscribe by RSSCOMMISSIONING AND DETECTOR PERFORMANCE GROUPS (DPG)
The period since the last CMS week has witnessed the start of the LHC as a 'physics' machine. The excitement of the first collisions at 7 TeV on March 30 will be remembered for a long time!
The preparation for the event was meticulous. The LHC was pushed to deliver non-colliding stable beams prior to the real collisions, which allowed CMS teams to use the few beam gas interactions within the length of the pixel detector to verify that the beams would indeed collide once both beams were circulating and the separation bump was collapsed. In passing, this exercise allowed us to catch some last minute features of the system, which could have affected our performance on the day!
The first collisions were detected practically simultaneously around the ring. Within tens of minutes not only event displays, but also some physics distributions were delivered to the audience of the press conference.
A plan of work had been carefully defined on how to use the first collisions following the first hour of excitement. It mainly consisted of using the early luminosity to perform systematic latency scans of the readout/trigger of our subdetectors. This implied a little sacrifice of luminosity, but the detector trigger and readout timing had to be tuned as early as possible to make sure that future performance would be optimal.
Also the luminosity delivered in the early runs would soon become negligible given that peak luminosity was at least 4 orders of magnitude below what has been forecast in 2010.
The delay scans were carried out and within a few of weeks the final timing corrections (integrating the knowledge accumulated form cosmics, beam splashes and delay scans) were deployed by pixel, silicon tracker, ECAL, HCAL and CSC.
The subsequent period has seen -as foreseen- an LHC which has been changing all the time in terms of bunch intensities and configurations. The luminosity is typically delivered at night or during weekends with normal working hours dedicated most of the time to commissioning the accelerator and striving to increase the luminosity.
The preparation for the event was meticulous. The LHC was pushed to deliver non-colliding stable beams prior to the real collisions, which allowed CMS teams to use the few beam gas interactions within the length of the pixel detector to verify that the beams would indeed collide once both beams were circulating and the separation bump was collapsed. In passing, this exercise allowed us to catch some last minute features of the system, which could have affected our performance on the day!
The first collisions were detected practically simultaneously around the ring. Within tens of minutes not only event displays, but also some physics distributions were delivered to the audience of the press conference.
A plan of work had been carefully defined on how to use the first collisions following the first hour of excitement. It mainly consisted of using the early luminosity to perform systematic latency scans of the readout/trigger of our subdetectors. This implied a little sacrifice of luminosity, but the detector trigger and readout timing had to be tuned as early as possible to make sure that future performance would be optimal.
Also the luminosity delivered in the early runs would soon become negligible given that peak luminosity was at least 4 orders of magnitude below what has been forecast in 2010.
The delay scans were carried out and within a few of weeks the final timing corrections (integrating the knowledge accumulated form cosmics, beam splashes and delay scans) were deployed by pixel, silicon tracker, ECAL, HCAL and CSC.
The subsequent period has seen -as foreseen- an LHC which has been changing all the time in terms of bunch intensities and configurations. The luminosity is typically delivered at night or during weekends with normal working hours dedicated most of the time to commissioning the accelerator and striving to increase the luminosity.
Fig.1: Minimum bias trigger rates in the first physics fill at 7 TeV on March 30.
The record luminosities reached to date are beyond 2 1029 Hz/cm2.
More than 90% of the luminosity delivered has been collected. The time in between physics fills has been used to continue the investigations of problems that have led to the loss of data.
One major problem has been induced by events with particles running along the barrel pixel detector and hitting long rows of pixels: the load on the front end buffers has been such that it could induced loss of synchronization given the time it takes to 'flush' the buffers. The Pixel team has attacked the problem painstakingly, with several versions of firmware deployed which have progressively increased the robustness of the system, while mitigating actions have been deployed on the trigger side to reduce the impact of such (relatively rare) events.
More than 90% of the luminosity delivered has been collected. The time in between physics fills has been used to continue the investigations of problems that have led to the loss of data.
One major problem has been induced by events with particles running along the barrel pixel detector and hitting long rows of pixels: the load on the front end buffers has been such that it could induced loss of synchronization given the time it takes to 'flush' the buffers. The Pixel team has attacked the problem painstakingly, with several versions of firmware deployed which have progressively increased the robustness of the system, while mitigating actions have been deployed on the trigger side to reduce the impact of such (relatively rare) events.
Fig.2: Details of CMS up-time since March 30.
To be noted is also the discovery of features in our network configuration and of certain DB queries, which could affect our performance, especially when configuring and starting runs. This type of issue has probably caused most of the dead-time (which has been nevertheless below 10%). As a consequence, we expect to improve performance in the future runs and now the goal is to reach an average of more than 95% recorded luminosity.
As detailed below, major progress has been made in understanding other beam-related effects, such as isolated crystal signals in ECAL and/or background events in general.
The situation with respect to the fraction of detector usable for physics is shown in the table below. It should be noted that the impact on the corresponding physics object efficiency both for trigger and reconstruction is typically much smaller!
As detailed below, major progress has been made in understanding other beam-related effects, such as isolated crystal signals in ECAL and/or background events in general.
The situation with respect to the fraction of detector usable for physics is shown in the table below. It should be noted that the impact on the corresponding physics object efficiency both for trigger and reconstruction is typically much smaller!
Table 1: Active fraction of detector channels usable for physics.
Detector Performance Groups
L1 Trigger
At the LHC startup, CMS collected data using the BPTX zero-bias and BSC minimum-bias triggers. Using this data, L1 Trigger DPG concentrated on verifying the trigger synchronization of the remaining triggers: e/γ, jet, muon, energy sums, and technical triggers. This allowed the full menu of triggers to be enabled, with confidence that the tracking detectors would be readout in time with collisions.
Since then, work has focused on measuring and understanding the performance of the basic object triggers. These studies include performance criteria such as resolution and efficiency with respect to offline measured objects, as well as detailed technical studies at the trigger hardware level.
Plots showing the trigger synchronization, as well as efficiency turn-on curves for e/γ and endcap muons have been approved for showing outside CMS, and similar plots for jets, energy sums and barrel muons are under preparation. Studies starting now include energy corrections for e/γ and jet triggers, and validation of e/γ isolation criteria.
In support of the performance studies, a suite of prompt analysis tools has been under continuous development and improvement during the past 6 months.
Tracker
The Tracker has been operating successfully and with high performance. In the pixel sub-detector 98.3% of the channels are operational and 98.1% in the strip sub-detector. Offline operations during data-taking are now reaching a routine pace with the automation of most of the calibration tasks. Data quality monitoring and certification are performed by the offline shifters and shift leaders with the occasional help of experts. The pixel sub-detector is seen to be a well calibrated and understood detector. A good agreement between data and M-C is obtained for most distributions.
The strip sub-detector operates as foreseen in deconvolution mode, and nominal S/N is observed. Since this readout mode has an impact on the local reconstruction due to the charge collection time, a measurement of the corrections needed has been done. This removes the biases observed when using the Lorentz-Angle measured with cosmic tracks, and allows combination of cosmic and collision data in order to produce an improved aligned geometry. Such a new alignment has been delivered for the reprocessing in view of the ICHEP conference.
The initial geometry used for prompt reconstruction of 2010 collisions was based on the 2.2M cosmic events collected in February 2010. This geometry was obtained with a combined method running the local method (HIP) on top of the global method (Millepede). While it allowed determination of the position of the barrel modules in the measurement coordinates with a few μm resolution, it nevertheless had a limited precision in the forward detectors. By combining these cosmic tracks with the collision tracks, a substantial improvement in the forward regions is observed. This also confirms the stability of the tracker since the winter shutdown within the current accuracy of the alignment.
Efforts have been spent in understanding the beam-related background and its impact on the pixel detector. It has been confirmed that its nature is most probably secondaries from beam-gas collisions. Methods have been developed to efficiently reject contamination from these events in collisions. Further work will be needed to better assess the impact of pileup of beam-gas and collision events as the beam current increases. The tracker DPG is also strengthening its contacts with the POG and PAG groups through involvement in the various task forces. This is of particular importance to make the best use of the detector and to properly evaluate systematics, for example from material budget and alignment uncertainties.
ECAL
The ECAL detector performance is now being tuned on 7 TeV collision data. Data taking is generally smooth and the ECAL trigger is fully enabled. Focus is now on optimizing the data taking efficiency and reliability. Part of this effort is a dedicated investigation into the small number of channels that are currently masked in the readout or the trigger and to potentially recover some of them for data taking.
The general performance of ECAL, demonstrating excellent understanding of the single channel response, the noise, the zero-suppression and selective readout scheme as well as the trigger, has been presented at the CALOR2010 conference in Beijing.
The calibration of the ECAL is making rapid progress, exploiting a data set of more than 10 million reconstructed π0 decays. The AlCa procedure for the 0 calibration is in full operation. The inter-calibration of individual channels is reaching a precision of 1% in the region around η = 0. The energy scale is studied using π0 decays, η decays as well as the phi-invariance of minimum bias events. The energy scale in barrel is uniform within 1% and agrees well with MC expectations on the same level of precision. Calibrating the endcap is progressing as well, but will require more statistics and careful investigation of systematic effects.
First J/φ and Z decays to electrons have been reconstructed and will be used to further study the detector performance. The preshower detector has performed an in-situ calibration and an in-situ alignment which significantly improve the sub-detector and CMS performance. A wide range of physics analyses, now being prepared for the summer conferences, demonstrates the superb quality of the ECAL detector on a daily basis.
Handling of the ECAL anomalous signals is continuously being improved and tuned to the needs of the physics analysis. In a joint effort with the respective Physics Object Groups and Physics Analysis Groups, procedures and tools are being put in place to ensure that there is no impact on the physics output of the CMS detector.
HCAL
The HCAL detector data taking proceeded smoothly during the last month with an effective fraction of active/readout channels of 99.2%.
Attention has been focused in the study of the noise events, which might have an impact on physics studies. A CMS task force (ASCTF) was established to provide recommendation for how CMS should treat the anomalous signals from ECAL and HCAL. Within the HCAL DPG, a working group was formed to provide optimal recommendations to clean up the anomalous HCAL signals, which are due to charged particles producing Cerenkov light in the windows of the HF PMTs. Such signals were first observed in test beam data and are seen to occur with collision data.
Filters have since been developed, using the HF pulse time and shape, to effectively identify and remove such anomalous signals in HF. The set of filter recommendations developed by the HCAL DPG was adopted by the ASCTF.
The most powerful filters utilize the pulse shape of signals in HF. This was only possible after the HCAL channel phase was adjusted using collision data at 7TeV. The pulse shape filter requires access to digi information and taking advantage of this filter requires data to be processed using CMSSW37x.
Events with high MET have been scanned and we see some events which appear to have residual noise in HF. Such events can occur when multiple PMTS are hit or if there is some noise overlapping with energy deposits from jet. A tighter timing requirement may help in identifying this residual noise.
Special triggers developed for the HCAL calibration are active and collecting calibration data. These include triggering to collecting non-zero suppressed events in order to determine the phi symmetry and isolated charged particles to determine response corrections. Response corrections will be determined using isolated charged particles with momenta of 40 - 60 GeV. The collection of this sample will require extended running, with an integrated luminosity of several inverse picobarns. The calibration procedures are currently being exercised using lower momenta tracks.
The online DQM is being refined in response to the operational experience gained. A key input to Run Certification requires access to the voltage settings from the Detector and Control System (DCS). The information resides in an online database and the software to propagate this information to the offline database is being tested. The DCS information will be used as part of the automated Run Certification in order to ensure that HCAL is operating at the target voltage settings.
Two abstracts have been submitted to ICHEP. One paper describes the Isolated Charge Particle response and a second paper describes the commissioning and performance of the HCAL.
Triggers to collect calibration data have been implemented in the online trigger menu and are working as expected. However, it will require time to accumulate sufficient statistics of isolated charged particles before any response corrections can be determined. We do not expect to apply a response correction for the ICHEP dataset. The calibration techniques are being tested with lower momenta tracks and are performing well.
DT
The 7 TeV LHC operations have offered the first sizable sample of collision tracks for the barrel muon system.
The DT DPG group concentrated its work on preliminary tests of the synchronization, calibration and local reconstruction of the Drift Tubes. Proper synchronization of the DT local trigger is essential to perform unambiguous BX assignment and optimize overall barrel muon trigger performance.
Methods to set local trigger fine synchronization with a precision ~1ns were developed and tested during commissioning with cosmic rays. As soon as the first 7 TeV data was available, coarse synchronization of the system was tested to ensure all chambers were delivering triggers with the same latency (i.e. the BX distribution for collision trigger for every chamber was peaked at the same bunch crossing). Computation of fine phase correction is currently ongoing, based on comparison with respect to timing information coming from local reconstruction.
The calibration of the DT system will need integrated luminosity of an inverse picobarn in order to be fully exploited. The calibration procedure involves determining the time pedestal of the signal coming from the chamber, which represents the latency with respect to the L1 trigger. The DTs are currently running with an educated guess of the time pedestal, whose validity was verified using the first data. The distributions of the residuals between the hits in the chambers and the reconstructed segments have been studied, and no statistically significant biases have been found.
The local reconstruction code has been tested extensively during the CRAFT data taking. In the recent months it has been refined with collision muons. A cut on the longitudinal plane was present in the collision reconstruction code to reject cosmic muons, but this led to a large number of segments with azimuthal information only. This problem has been corrected. The distribution of the number of hits used in the reconstructed segments has also been checked and found to be reasonable when quality selection criteria were applied.
RPC
The main activities in the RPC DPG have focused on the analysis of the data collected and on improving the tools for detector monitoring.
In order to protect CMS operations against increase of fake trigger rates due to noisy channels, the RPC operations started in a conservative way, masking all the channels exceeding a noise rate of 3KHz. With those strong requirements the total number of masked and dead channels in the system was around 3.5%.
A detailed analysis of the noise is in progress, in order to reduce the rate by increasing the threshold or to unmask channels that do not degrade the fake trigger rate. After careful analysis the fraction of dead and masked channels was reduced successfully to 1.2%. Work is in progress to understand the impact of this unmasking on the Trigger and Reconstruction performance.
The largest effort was on the organization for software release validation and Data certification. Tools for monitoring of the noise and for certification of the run quality through the DQM have been improved. The software validation modules have been integrated in the central validation suite and are now ready to be run for each new release.
A common skim for the RPC analysis has been defined that selects good muons from the Minimum Bias sample. The skim makes use of the JSON files to select good runs from the point of view of the muon system and is analyzed by several people to study RPC efficiencies, make overall comparison between data and M-C on standard RPC Reco variables, improve the synchronization, and study trigger performance.
Although the statistics are still not enough to evaluate the efficiency roll by roll, overall efficiencies for wheel/disks have been produced and are in reasonable agreement with results coming from cosmic data (around 93% for the barrel and 90% for the endcaps). The working point of the endcaps is under study to improve the performance of the system at the level of the barrel.
The operations of RPC PAC trigger hardware and online software have gone smoothly. Minor updates of the firmware and online software were made recently. During last months the main focus was on synchronization of the RPC hits. The synchronization parameters were updated, first based on analysis of the splash data, and then secondly, based on the collision data. After these corrections, the number of the hits not assigned to the proper BX is < 0.5%. The number of muon candidates generated in incorrect BX (mostly BX +1) is about 0.3%. First studies are ongoing to look at the muon identification efficiency of Pattern Comparator algorithm and to better understand impact of chamber efficiency (~90-93%) on trigger results.
CSC
The CSCs have settled into standard data collection operation over the past months, although only recently have the collision rates been high enough to start collecting muon-triggered events. The general performance of the CSC system is stable, and the fraction of dead channels is slightly above 1%.
CSC has been approved for 'unattended operation' and thus in principle the CSC system can be fully monitored by the central P5 operations crew.
There is still a dearth of collision muons for detailed chamber efficiency studies, although statistics are slowly accumulating. Progress continues in adjusting the timing of the trigger and readout using both collision and halo muons.
L1 Trigger
At the LHC startup, CMS collected data using the BPTX zero-bias and BSC minimum-bias triggers. Using this data, L1 Trigger DPG concentrated on verifying the trigger synchronization of the remaining triggers: e/γ, jet, muon, energy sums, and technical triggers. This allowed the full menu of triggers to be enabled, with confidence that the tracking detectors would be readout in time with collisions.
Since then, work has focused on measuring and understanding the performance of the basic object triggers. These studies include performance criteria such as resolution and efficiency with respect to offline measured objects, as well as detailed technical studies at the trigger hardware level.
Plots showing the trigger synchronization, as well as efficiency turn-on curves for e/γ and endcap muons have been approved for showing outside CMS, and similar plots for jets, energy sums and barrel muons are under preparation. Studies starting now include energy corrections for e/γ and jet triggers, and validation of e/γ isolation criteria.
In support of the performance studies, a suite of prompt analysis tools has been under continuous development and improvement during the past 6 months.
Tracker
The Tracker has been operating successfully and with high performance. In the pixel sub-detector 98.3% of the channels are operational and 98.1% in the strip sub-detector. Offline operations during data-taking are now reaching a routine pace with the automation of most of the calibration tasks. Data quality monitoring and certification are performed by the offline shifters and shift leaders with the occasional help of experts. The pixel sub-detector is seen to be a well calibrated and understood detector. A good agreement between data and M-C is obtained for most distributions.
The strip sub-detector operates as foreseen in deconvolution mode, and nominal S/N is observed. Since this readout mode has an impact on the local reconstruction due to the charge collection time, a measurement of the corrections needed has been done. This removes the biases observed when using the Lorentz-Angle measured with cosmic tracks, and allows combination of cosmic and collision data in order to produce an improved aligned geometry. Such a new alignment has been delivered for the reprocessing in view of the ICHEP conference.
The initial geometry used for prompt reconstruction of 2010 collisions was based on the 2.2M cosmic events collected in February 2010. This geometry was obtained with a combined method running the local method (HIP) on top of the global method (Millepede). While it allowed determination of the position of the barrel modules in the measurement coordinates with a few μm resolution, it nevertheless had a limited precision in the forward detectors. By combining these cosmic tracks with the collision tracks, a substantial improvement in the forward regions is observed. This also confirms the stability of the tracker since the winter shutdown within the current accuracy of the alignment.
Efforts have been spent in understanding the beam-related background and its impact on the pixel detector. It has been confirmed that its nature is most probably secondaries from beam-gas collisions. Methods have been developed to efficiently reject contamination from these events in collisions. Further work will be needed to better assess the impact of pileup of beam-gas and collision events as the beam current increases. The tracker DPG is also strengthening its contacts with the POG and PAG groups through involvement in the various task forces. This is of particular importance to make the best use of the detector and to properly evaluate systematics, for example from material budget and alignment uncertainties.
ECAL
The ECAL detector performance is now being tuned on 7 TeV collision data. Data taking is generally smooth and the ECAL trigger is fully enabled. Focus is now on optimizing the data taking efficiency and reliability. Part of this effort is a dedicated investigation into the small number of channels that are currently masked in the readout or the trigger and to potentially recover some of them for data taking.
The general performance of ECAL, demonstrating excellent understanding of the single channel response, the noise, the zero-suppression and selective readout scheme as well as the trigger, has been presented at the CALOR2010 conference in Beijing.
The calibration of the ECAL is making rapid progress, exploiting a data set of more than 10 million reconstructed π0 decays. The AlCa procedure for the 0 calibration is in full operation. The inter-calibration of individual channels is reaching a precision of 1% in the region around η = 0. The energy scale is studied using π0 decays, η decays as well as the phi-invariance of minimum bias events. The energy scale in barrel is uniform within 1% and agrees well with MC expectations on the same level of precision. Calibrating the endcap is progressing as well, but will require more statistics and careful investigation of systematic effects.
First J/φ and Z decays to electrons have been reconstructed and will be used to further study the detector performance. The preshower detector has performed an in-situ calibration and an in-situ alignment which significantly improve the sub-detector and CMS performance. A wide range of physics analyses, now being prepared for the summer conferences, demonstrates the superb quality of the ECAL detector on a daily basis.
Handling of the ECAL anomalous signals is continuously being improved and tuned to the needs of the physics analysis. In a joint effort with the respective Physics Object Groups and Physics Analysis Groups, procedures and tools are being put in place to ensure that there is no impact on the physics output of the CMS detector.
HCAL
The HCAL detector data taking proceeded smoothly during the last month with an effective fraction of active/readout channels of 99.2%.
Attention has been focused in the study of the noise events, which might have an impact on physics studies. A CMS task force (ASCTF) was established to provide recommendation for how CMS should treat the anomalous signals from ECAL and HCAL. Within the HCAL DPG, a working group was formed to provide optimal recommendations to clean up the anomalous HCAL signals, which are due to charged particles producing Cerenkov light in the windows of the HF PMTs. Such signals were first observed in test beam data and are seen to occur with collision data.
Filters have since been developed, using the HF pulse time and shape, to effectively identify and remove such anomalous signals in HF. The set of filter recommendations developed by the HCAL DPG was adopted by the ASCTF.
The most powerful filters utilize the pulse shape of signals in HF. This was only possible after the HCAL channel phase was adjusted using collision data at 7TeV. The pulse shape filter requires access to digi information and taking advantage of this filter requires data to be processed using CMSSW37x.
Events with high MET have been scanned and we see some events which appear to have residual noise in HF. Such events can occur when multiple PMTS are hit or if there is some noise overlapping with energy deposits from jet. A tighter timing requirement may help in identifying this residual noise.
Special triggers developed for the HCAL calibration are active and collecting calibration data. These include triggering to collecting non-zero suppressed events in order to determine the phi symmetry and isolated charged particles to determine response corrections. Response corrections will be determined using isolated charged particles with momenta of 40 - 60 GeV. The collection of this sample will require extended running, with an integrated luminosity of several inverse picobarns. The calibration procedures are currently being exercised using lower momenta tracks.
The online DQM is being refined in response to the operational experience gained. A key input to Run Certification requires access to the voltage settings from the Detector and Control System (DCS). The information resides in an online database and the software to propagate this information to the offline database is being tested. The DCS information will be used as part of the automated Run Certification in order to ensure that HCAL is operating at the target voltage settings.
Two abstracts have been submitted to ICHEP. One paper describes the Isolated Charge Particle response and a second paper describes the commissioning and performance of the HCAL.
Triggers to collect calibration data have been implemented in the online trigger menu and are working as expected. However, it will require time to accumulate sufficient statistics of isolated charged particles before any response corrections can be determined. We do not expect to apply a response correction for the ICHEP dataset. The calibration techniques are being tested with lower momenta tracks and are performing well.
DT
The 7 TeV LHC operations have offered the first sizable sample of collision tracks for the barrel muon system.
The DT DPG group concentrated its work on preliminary tests of the synchronization, calibration and local reconstruction of the Drift Tubes. Proper synchronization of the DT local trigger is essential to perform unambiguous BX assignment and optimize overall barrel muon trigger performance.
Methods to set local trigger fine synchronization with a precision ~1ns were developed and tested during commissioning with cosmic rays. As soon as the first 7 TeV data was available, coarse synchronization of the system was tested to ensure all chambers were delivering triggers with the same latency (i.e. the BX distribution for collision trigger for every chamber was peaked at the same bunch crossing). Computation of fine phase correction is currently ongoing, based on comparison with respect to timing information coming from local reconstruction.
The calibration of the DT system will need integrated luminosity of an inverse picobarn in order to be fully exploited. The calibration procedure involves determining the time pedestal of the signal coming from the chamber, which represents the latency with respect to the L1 trigger. The DTs are currently running with an educated guess of the time pedestal, whose validity was verified using the first data. The distributions of the residuals between the hits in the chambers and the reconstructed segments have been studied, and no statistically significant biases have been found.
The local reconstruction code has been tested extensively during the CRAFT data taking. In the recent months it has been refined with collision muons. A cut on the longitudinal plane was present in the collision reconstruction code to reject cosmic muons, but this led to a large number of segments with azimuthal information only. This problem has been corrected. The distribution of the number of hits used in the reconstructed segments has also been checked and found to be reasonable when quality selection criteria were applied.
RPC
The main activities in the RPC DPG have focused on the analysis of the data collected and on improving the tools for detector monitoring.
In order to protect CMS operations against increase of fake trigger rates due to noisy channels, the RPC operations started in a conservative way, masking all the channels exceeding a noise rate of 3KHz. With those strong requirements the total number of masked and dead channels in the system was around 3.5%.
A detailed analysis of the noise is in progress, in order to reduce the rate by increasing the threshold or to unmask channels that do not degrade the fake trigger rate. After careful analysis the fraction of dead and masked channels was reduced successfully to 1.2%. Work is in progress to understand the impact of this unmasking on the Trigger and Reconstruction performance.
The largest effort was on the organization for software release validation and Data certification. Tools for monitoring of the noise and for certification of the run quality through the DQM have been improved. The software validation modules have been integrated in the central validation suite and are now ready to be run for each new release.
A common skim for the RPC analysis has been defined that selects good muons from the Minimum Bias sample. The skim makes use of the JSON files to select good runs from the point of view of the muon system and is analyzed by several people to study RPC efficiencies, make overall comparison between data and M-C on standard RPC Reco variables, improve the synchronization, and study trigger performance.
Although the statistics are still not enough to evaluate the efficiency roll by roll, overall efficiencies for wheel/disks have been produced and are in reasonable agreement with results coming from cosmic data (around 93% for the barrel and 90% for the endcaps). The working point of the endcaps is under study to improve the performance of the system at the level of the barrel.
The operations of RPC PAC trigger hardware and online software have gone smoothly. Minor updates of the firmware and online software were made recently. During last months the main focus was on synchronization of the RPC hits. The synchronization parameters were updated, first based on analysis of the splash data, and then secondly, based on the collision data. After these corrections, the number of the hits not assigned to the proper BX is < 0.5%. The number of muon candidates generated in incorrect BX (mostly BX +1) is about 0.3%. First studies are ongoing to look at the muon identification efficiency of Pattern Comparator algorithm and to better understand impact of chamber efficiency (~90-93%) on trigger results.
CSC
The CSCs have settled into standard data collection operation over the past months, although only recently have the collision rates been high enough to start collecting muon-triggered events. The general performance of the CSC system is stable, and the fraction of dead channels is slightly above 1%.
CSC has been approved for 'unattended operation' and thus in principle the CSC system can be fully monitored by the central P5 operations crew.
There is still a dearth of collision muons for detailed chamber efficiency studies, although statistics are slowly accumulating. Progress continues in adjusting the timing of the trigger and readout using both collision and halo muons.
Fig.3: Effect of CSC trigger timing change.
The first iteration of a timing scan with collision muons has been successfully performed with the CSC trigger. The result is that approximately 99% of triggers from CSC are correctly timed in with CMS (see Figure). A set of corrections to account for time differences in electronic pathways at peripheral crate level has been derived and stored in the conditions database.
Another significant step forward has been the completion of the first track-based alignment for the CSC system, based on halo muons and photogrammetry. The original photogrammetry measurements have been shown to be consistent with measurements based on beam-halo muons, and vice versa, so that the photogrammetry can be used to fill in gaps where chambers do not overlap and the halo muons do not provide alignment values. This alignment has been approved and released for use in the ICHEP physics analyses.
Comparisons between minimum bias collisions data and simulated data continue and in general give very good agreement for quantities related to local reconstruction of muon tracks in the CSCs. We need to account for the readout conditions which - since the CSC system was designed to trigger on muons - require a trigger primitive before readout. This is a concern only when dealing with minimum bias data, in which muons are predominantly from π/K decays-in-flight, and hadron punch-through, which might not be expected to trigger the system. The fully-reconstructed muon tracks in the minimum bias collisions data are also well described by the simulation.
A number of 'performance plots' were approved by CMS for public presentation, and additional plots will be added as more data are accumulated. Where necessary, the reconstruction algorithms are being improved; particularly in handling of data from the ME1/1A chambers, which cover the highest rapidity region of the CSC acceptance from |η| = 2.1 to 2.4 and so are highly populated but have ganged strips in order to reduce the number of instrumented readout channels.
A very active topic is determination of the CSC L1 trigger efficiency, covering both trigger primitive generation and the CSC 'Track Finder' reconstruction. This work is intended to supplement tag-and-probe efficiency studies. There is a gradual but gratifying increase in statistics for the onia resonance channels decaying to muons, most of which are detected in the endcap CSC regions. The few candidate Z→μμ and W→μμ events which have been observed are whetting the appetite for future physics as LHC operation progresses.
Another significant step forward has been the completion of the first track-based alignment for the CSC system, based on halo muons and photogrammetry. The original photogrammetry measurements have been shown to be consistent with measurements based on beam-halo muons, and vice versa, so that the photogrammetry can be used to fill in gaps where chambers do not overlap and the halo muons do not provide alignment values. This alignment has been approved and released for use in the ICHEP physics analyses.
Comparisons between minimum bias collisions data and simulated data continue and in general give very good agreement for quantities related to local reconstruction of muon tracks in the CSCs. We need to account for the readout conditions which - since the CSC system was designed to trigger on muons - require a trigger primitive before readout. This is a concern only when dealing with minimum bias data, in which muons are predominantly from π/K decays-in-flight, and hadron punch-through, which might not be expected to trigger the system. The fully-reconstructed muon tracks in the minimum bias collisions data are also well described by the simulation.
A number of 'performance plots' were approved by CMS for public presentation, and additional plots will be added as more data are accumulated. Where necessary, the reconstruction algorithms are being improved; particularly in handling of data from the ME1/1A chambers, which cover the highest rapidity region of the CSC acceptance from |η| = 2.1 to 2.4 and so are highly populated but have ganged strips in order to reduce the number of instrumented readout channels.
A very active topic is determination of the CSC L1 trigger efficiency, covering both trigger primitive generation and the CSC 'Track Finder' reconstruction. This work is intended to supplement tag-and-probe efficiency studies. There is a gradual but gratifying increase in statistics for the onia resonance channels decaying to muons, most of which are detected in the endcap CSC regions. The few candidate Z→μμ and W→μμ events which have been observed are whetting the appetite for future physics as LHC operation progresses.