10 July, 1998
To: The Director General
Subject: Recommendations of the LHC committee
Evaluation of the LHCb Technical Proposal and Recommendation
to the Research Board
The LHCC has completed the scientific, technical, and cost review of the LHCb Technical Proposal (LHCC 98-4/P4), and the subsequent documentation, including the plan with milestones for further development of the project.
1 B Physics at the LHC
Because of the large cross section for the production of particles containing the b quark in pp collisions at 14 TeV which from present models is likely to be in the range of 500mbarns, the LHC provides a unique opportunity to explore rare processes in B Physics. Among these are precision studies of CP violation complimentary to those that will be carried on at the new B-Factories and rare decays. The two general purpose detectors, ATLAS and CMS have capabilities in studying these areas but a dedicated detector with both excellent vertex reconstruction capability and good particle identification would provide the precision needed in determining the exact nature of the CP violation mechanism. The detector proposed by the LHCb collaboration in their technical proposal provides this capability.
2 Evaluation of the LHCb Technical Proposal
The principal features of the LHCb detector are excellent vertex resolution with separated vertex trigger capability at the second level of the trigger, a high resolution forward spectrometer with acceptance from approximately 10 mrad to 300 (250) mrad in the bending (non-bending) plane and good particle identification. The particle identification is by means of calorimetry (electrons, gammas and hadrons) penetration through material (muons) and fast RICH counters (charged kaons, pions, and protons up to 150 GeV/c). The trigger and data acquisition system are key to the success of this type of detector and the design outlined in the technical proposal and detailed in subsequent discussions with the collaboration provides a robust and economical solution. It uses a four level trigger to minimize the length of the front end pipelines, avoids large and complex data switches and employs developments carried out by the two general purpose detectors where possible. The large number of "pixels" at low cost required for the photon detectors for the RICH counters represent the main development effort compared to existing technology. The collaboration has a well defined R&D effort following two possible paths to demonstrate this technology. They have a very high probability of success and there is an existing commercial product which could be used but at a factor of two higher cost.
The LHCC believes that LHCb has reached the staue at which the development, tests, and engineering which remain can be monitored through the achievement of the agreed milestones.
In the follow paragraphs, the proposed detector systems are discussed in more detail.
Vertex Detector, Tracking System and the Muon Detector:
The vertex detector system consists of a silicon strip detector configured in an r-F geometry and a pile-up veto counter. The main purpose of the vertex detector is to provide precise information on the production and decay vertices of the B-mesons. The pile-up veto suppresses the events with more than one pp interaction in the same bunchcrossing. The proposed LHCb vertex detector consists of 17 stations, each having radial (~) and circular (r) sensitive strips. The detector has been optimized for an efficient angular coverage with an expected impact parameter resolution of about 40 [tm. Experience with existing silicon detectors indicates that this requirement can be met in LHCb. Tests of a prototype are being carried out now. The tests of the radiation resistance of the silicon detector are planned for August and September 1998.
The inner and outer track chambers in the spectrometer compliment the vertex detector. For the inner track chambers a MSGC/GEM solution has been chosen, whilst for the outer tracker a straw tube like technology is proposed. The main purpose of these detectors is the reconstruction of the momentum of the charged particles produced in B-meson decays as well as the reconstruction of the track direction for the RICH. An expected precision for the momentum measurement of delta p/p = 0.3% is limited by the multiple scattering in the material of the detector. Robust track finding is obtained by redundancy in the present design. The requirements and technology are very close to that used in the HERA-B detector, which provides a good test for the LHCb experiment. Major tasks of the R&D programme in the LHCb tracker are the choice of gas mixture, the material for the outer tracker, and the optimization of GEM technology for the inner tracker. Prototypes will be built and tested during 1998-99.
The LHCb Muon system is placed downstream of the calorimeter and consists of 5 layers of track chambers interleaved with iron absorber plates. The technologies chosen for the muon detector planes are Multigap Resistive Plate Chambers (MRPQ for most of the coverage and Cathode Pad Chambers (CPQ for regions which are subjected to high rates. The CPCs are similar to those used in ATLAS and CMS. The performance of the muon detector evaluated using Monte Carlo calculations meets the requirements of the experiment. Studies of MRPC and CPC prototypes in the test beam are scheduled for 1998-99.
The LHCb calorimeter system identifies electrons, photons and hadrons and measures their energy and position. This information is needed both for triggering and for the off-line analysis. The system is subdivided into three sections: preshower (PS), electromagnetic (ECAL) and hadronic calorimeter (HCAL). The active material for the three sections is scintillator coupled to wavelength-shifting fibers readout by fast photodetectors. The preshower detector is made with a sandwich of lead and scintillator tiles; the ECAL is a Shashlik type lead calorimeter, and the HCAL is a scintillator tile/iron calorimeter. The chosen technologies are well proven and match the requirements of a fast, cheap and robust device, with relatively easy construction and no special needs for R&D.
The main purpose of the ECAL is the identification of electrons and pions at the trigger level. This is crucial for the experiment and it is accomplished by providing a high-pt isolated-particle trigger signal, with a rejection power against minimum bias events of more than a factor of 30. The preshower detector (PS) is needed to reach the required e/pion discrimination. From 5 to 50 GeV the electron efficiency of the PS ranges from 90 to 99.8% with a pion contamination of less than 10%. The readout of the PS is performed either with PMT's or, possibly, by Avalanche Photodiodes (APD). The latter would have the advantage of higher quantum efficiency and compactness. R&D is ongoing, since the main limitation of the API)s is given by the relatively small
quantity of light from the tile scintillator. The basic idea is to improve the matching between the spectral response of the photodetector and the scintillator/WLS.
The physics requirements for the ECAL define its performance requirements. The response time is short (below 25 ns). The transverse granularity of ECAL matches the steep variation of the density of particles hitting its surface (more than a factor 100 from the center to the edge). The energy resolution, about (10%/sqrt(E) + 1.5%), is sufficient to ensure e/p separation at the level of 100/1 and with adequate p0 reconstruction capability. The innermost part of the ECAL must withstand a radiation dose of 0.4 Mrad/year. Tests on prototypes show that no significant degradation of the performance is observed for a dose equivalent to 10 years of LHCb operation. The total depth of ECAL amounts to 25 X0 with a total of about 6000 readout channels.
The HCAL technology is similar to the one of the ATLAS hadronic calorimeter. The design is adequate to the main task of the detector, namely the measurement of the pt of single hadrons, with sufficient two-hadron separation, to be used at the trigger level. Optimization of the HCAL design is under way. The technical proposal design is 7.5 l in thickness and contains about 3000 channels.
PMT tubes will be used for the ECAL and HCAL read-out. The front end electronics has a shaping time short compared to the 25 ns bunch crossing and of a 12 bit dynamic range (from 50 MeV to 200 GeV in the inner region and from 12.5 MeV to 50 GeV in the outer region). Events are digitized at 40 N1Hz and stored in a 128-deep pipeline buffer with 3.2 microsecond latency. Different approaches are possible for the analog shaping and for the implementation of the front end electronics. One option is the digital FERNE system.
The overall design of the calorimeter system matches the LHCb requirements. It guarantees the needed trigger selectivity with a robust and reliable design. Further design optimization is expected as well as results from further R&D and beam tests.
The RICH counter system consists of two counters. RICH I has two radiators with different indices of refraction and a common optical system and photon detection plane. Aerogel with an index of n = 1.03 provides sensitivity to slow Ks while the C4FIO gas radiator provides good separation for intermediate energy particles. RICH II which is placed behind the spectrometer magnet uses CF4 as the radiator and long focal length optics to provide separation ofps and Ks up to 150 GeV/c. The Cherenkov rings in the counters are imaged on photon detector planes with a total of 480,000 pixels for both counters. For efficient detection of the rings, the active photocathode area must be at least 80% of the total area. A significant R&D effort both at CERN and in industry is making excellent progress towards developing an economic solution satisfying these requirements based on hybrid photodiodes (HPDs). Multianode photomultiplier tubes combined with a short focal length lens array to collect and focus the Cherenkov light on the anodes are a possible but currently expensive alternate solution.
Prototypes of both counters equipped with 65 element commercial HPI)s have been successfully tested in test beams and have demonstrated the needed angular resolution for the Cherenkov rings. The photon detection efficiency has been as expected in these tests.
In parallel with the beam test programme, the problem of pattern recognition of the rings in the high multiplicity environment has been receiving considerable attention. A solution has been found which will match available computing resources.
Trigger & DAQ:
Events containing B particles can be distinguished from others at LHC on the basis of secondary vertices and particles with relatively high transverse momentum. However, the rate of interactions in LHCb requires a complex trigger with four levels in order not only to separate out B events from background but also to trigger on events which can be reconstructed and which contain interesting decay modes. The high level-0 trigger rate means that the data acquisition system must cope with high bandwidths of more than 4 Gbytes/sec. Thus the trigger/DAQ requirements for LHCb are significantly more stringent than for other experiments, in particular ATLAS and CMS.
The L0 trigger relies strongly on high pt particles detected in the muon chambers and the calorimeter, together with a pile-up veto based on silicon planes which rejects events containing more than one interaction. The algorithms are relatively simple, but the high rate of interactions means that the hardware solution is challenging. LHCb is investigating a variety of solutions in parallel, at least one of which for each detector is "conventional". The collaboration has investigated possible variations in the Monte Carlo models of the background and the trigger performance appears to be robust aaainst such variations. The Ll trigger brings in the vertex detector information for the first time and identifies events with well-separated secondary vertices. Further confirmation of high pt track candidates is given at this stage. The vertex detector geometry is designed to allow simple implementation of the trigger, which operates by identifying two-track vertices. L2 refines the secondary vertex trigger by removing false vertices which predominantly originate from multiple scattering of low momentum tracks. Finally at L3 full or partial reconstruction of final states takes place, allowing the retention of B decays corresponding to particular decay modes of interest. For all levels of trigger, LHCb has demonstrated designs, schematics or algorithms which promise a performance capable of achieving the design goals of the experiment.
The high trigger rate and large bandwidth of information flowing through the system demands high performance from the LHCb DAQ and computing systems. The collaboration has taken part in the development of fast control systems for LHC and has investigated necessary modifications and additions for specific LHCb requirements. The collaboration is investigating a variety of hardware methods of data transfer, several of which appear to offer cost-effective and reliable solutions. A significant amount of development in this area will be required in the next few years. LHCb has adopted a policy of using solely Object Oriented techniques in its computing activities, and has embarked on a vigorous programme of training for its personnel. It has also produced well developed and convincing plans for the staff and time scales require to produce a working system. It already has in place a sizeable team of people working in this area, and has made good progress towards its goal.
The LHCb collaboration has developed a detector concept which makes use as much as possible of existing or soon to be developed technology and is well adapted to the expected characteristics of the physics under study. The needed developments in vertex detectors, tracking systems, the trigger and data acquisition system and the high granularity photon detectors are making excellent progress.
The Committee congratulates LHCb on its excellent work and considers that the collaboration deserves encouragement and the full support of the scientific community to proceed in its exciting and challenging task.
In order to ensure that LHCb can take data in 2005, it is essential that the process of the final design and construction of the detector be initiated now, together with systematic reviews of the various components of the detector as they proceed to final decisions on the individual subsystems in order to assure viability of the complete detector.
4 Future Work
Before proceeding to the final construction phase, each subsystem will be subject to a technical, financial, and effort review based on a Technical Design Report for each subsystem, which must include:
- Performance specifications
- Prototype results
- Engineering design
- Cost estimates and profile
- Schedule with milestones, including relationship to the overall project
- Production and Testing: Procedures and effort
- Maintenance and Repair: Procedures and access time
- Responsibilities for the construction, operation, and safety
- Resources and contingency
LHCb has established the milestones shown below which will lead to Subsystem Technical Design Reports
The status of the overall project will be reviewed on a regular basis.
LHCb Major Milestones:
Freeze Design and Submit TDR 10/99
Tender Out 12/99
Order Placed 4/00
Start Construction 7/00
Design Mechanics 4/00
Design Si Detector 6/00
Submit Final Radhard FE Chip 7/00
TDR Submission 4/01
Inner Tracking Detector
Freeze Chamber Parameters 9/00
Freeze Front End Design 6/01
Outer Tracking Detector
Choice of Cathode Material 1/00
Freeze Design 6/00
Choice of Photodetector 10/99
Complete Simulation 1/00
Complete Design 3/00
Choice of Technologies (Detector and Electronics) 1/00
Final Design 7/00
Basic Design Optimization Complete 7/99
Engineering Design Complete 4/00
Decision on Technologies for all LO Triggers 7/99
TDR Level 0/Level 1 1/02
Define Interface Technology (Control System) 1/00
Define Event Building Strategy 4/00
Finish Prototype l(Simulation, Reconstruction, & Analysis) 7/00
Finish Prototype 2/TDR 7/02
Simulated Data Challenge 7/04
5 Recommendations to the Research Board
The LHCC recommends the approval of the LHCb project, together with the plans, including milestones, leading to the Subsystem Technical Design Reports.