RICH meeting provides a wealth of information

The well established ring imaging Cherenkov (RICH) technique measures the Cherenkov angle via direct imaging of photons emitted through the Cherenkov effect. It is mainly used in high-energy and astroparticle physics experiments to identify charged particles over an impressive range in momentum, from a few hundred mega-electron-volts/c up to several hundred giga-electron-volts/c. The performance of the technique has yet to be matched by competing technologies, especially when the physics objectives require excellent particle identification.

In 1993, Eugenio Nappi of Bari and Tom Ypsilantis of Collège de France began a series of international workshops to provide a forum for reviewing the most significant developments and new perspectives on this powerful technique. From 30 November to 5 December 2004, the beautiful resort of Playa del Carmen on the Yucatan Peninsula in Mexico hosted the fifth in the series. Following on from the first workshop in Bari, Italy, and subsequent meetings in Uppsala, Ein Gedi and Pylos, this was the first foray to the other side of the Atlantic.

RICH2004 was dedicated to the centenary of the birth of Pavel Cherenkov in July 1904, who discovered the effect through which charged particles travelling faster than light in a medium emit a characteristic radiation. To honour him, the local organizing committee – headed by two seasoned RICH practitioners in Mexico, Jurgen Engelfried from the University in San Luis Potosi (UASLP) and Guy Paic from the Instituto de Ciencias Nucleares of the National University in Mexico City (UNAM) – invited Cherenkov’s daughter, Elena Cherenkova, to the meeting. A physicist herself, she captured the attention of the audience by recollecting her father through unpublished photographs and anecdotes. Boris Govorkov, a long-time collaborator of Cherenkov, was also invited to talk about the history of the discovery of Cherenkov radiation.

The workshop itself consisted of topical sessions on Cherenkov-light imaging applications and related technological issues. It attracted some 100 participants from around the world and the large number of abstracts received proved that this field is still very fertile and open to innovative ideas, about both the basic configuration of detectors and the technology used in their construction. While all the submitted contributions were very interesting, the organizers had to make a selection to allow time for extensive discussions between talks. In addition to the 10 invited talks, 55 other papers were accepted for oral presentations, while the remainder were conveyed in the poster session.

The main advances of the past few years played a central role in the workshop. These include the imaging of Cherenkov photons totally reflected in quartz bars (the basic principle of the Detection of Internally Reflected Cherenkov light [DIRC] technique adopted in the BaBar experiment at the Stanford Linear Accelerator Center [SLAC]); the development and applications of photocathodes made of thin films of caesium iodide (CsI); and the current availability of multi-anode photomultipliers (MAPMTs) and large-area hybrid photon detectors (HPDs).

Jochen Schwiening of SLAC gave an overview of the current DIRC layout for BaBar, while in a contribution to the poster session Jerry Va’vra, also from SLAC, described the possibility of enhancing the detector’s performance by adding a focusing system. The design and construction of gaseous photodetectors based on large-area CsI photocathodes, which work in reflective mode with electron extraction in CH₄ at atmospheric pressure, have been mastered. This was shown by Abraham Gallas of CERN, Silvia Dalla Torre of Trieste and Mauro Iodice of Rome, who reported on applications in the ALICE and COMPASS detectors at CERN and in experiments in Hall A at the Jefferson Laboratory, respectively. Herbert Hoedlmoser of CERN also reviewed preliminary results from irradiation tests on CsI photocathodes.

Although gaseous photon detectors remain the most effective solution for large detector areas in relatively low-rate experiments, the improvements in the technology of multichannel vacuum-based photon detectors have created the possibility of using the Cherenkov-light imaging technique in applications that were unthinkable only a few years ago. One example is measuring how long Cherenkov photons take to propagate in long quartz bars (the time-of-propagation or TOP counter), as Toru Iijima from Nagoya discussed. The two RICHs being constructed for the LHCb experiment at CERN are the most exacting examples of this “new generation” and several talks covered their challenging design.

In parallel with the industrial production of HPDs and MAPMTs, the development of custom designs has recently evolved considerably. A partnership with one major industrial manufacturer has been established to develop multi-anode hybrid avalanche photodiodes and photodevices based on the combination of a micro-channel plate and micromegas, as reported by Takayuki Sumiyoshi of KEK and Va’vra, respectively.

Besides the CsI-based RICHs mentioned above, Vladimir Peskov from Paris also discussed novel gaseous detectors. In the same vein, Fabio Sauli of CERN reviewed advances in the gas electron-multiplier (GEM) technique, which enables high-performance detectors to be built that are essentially discharge-free and have very high gains. Amos Breskin of the Weizmann Institute reviewed the important results obtained in detecting single photons with a multi-GEM counter, filled with CF₄ or Ar/CH₄, which operates smoothly up to a gain as high as 10⁵. These studies were the basis for the development of the “hadron-blind” Cherenkov detector under construction for the Pioneering High-Energy Nuclear Interaction Experiment (PHENIX) at the Brookhaven National Laboratory (BNL), as reported by Itzak Tserruya, also of the Weizmann Institute.

The trend of operating RICH detectors in the visible range improves performance because the chromatic aberration is less than with detectors working in the far-ultraviolet region; it also implies a larger choice of materials for the radiator, such as silica aerogel. Several speakers discussed the outstanding improvement of the optical characteristics of this amazing material, made possible by the work of the group of Alexander Danyliuk and Alexei Onuchin in Novosibirsk and of the Japanese company Matsushita.

The high transparency of aerogel nowadays and the possibility of producing tiles made of layers with different refractive indices enable more compact detector designs based on proximity focusing geometry. Such a design is envisaged for the upgrade of the Belle experiment at KEK and, in threshold mode, for heavy-ion experiments, as reported by Peter Krizan of Ljubliana and Paic, respectively. On technological aspects, Veljko Radeka of BNL reported on perspectives for front-end and read-out electronics, and Olav Ullaland of CERN discussed the design of fluid systems.

A discussion about the performance of operating detection systems included overviews of the RICH for the HERA-B experiment at DESY, from Marko Staric of Ljubljana; the triethylamine RICH in the CLEO III experiment at Cornell, from Radia Sia of Syracuse; and the dual-radiator RICH of the HERMES detector at DESY, from Harold Jackson of Argonne. Forthcoming RICH detectors in fixed-target and collider experiments took centre stage halfway through the workshop with reports on RICH2 for COMPASS at CERN from Fulvio Tessarotto of Trieste, and the RICHs for the Charged Kaons at the Main Injector (CKM) and B Physics at the Tevatron (BTeV) experiments at Fermilab from Peter Cooper of Fermilab and Tomasz Skwarnicki of Syracuse, respectively. The impressive results obtained with RICH detectors, especially in charge-parity violation in B-physics experiments, were reviewed by Blair Ratcliff of SLAC.

A full day of the workshop was devoted to astroparticle physics applications, beginning with overviews from Greg Hallewell of Marseille and Alan Watson of Leeds. They made it clear that the new generation of experiments under construction in astrophysics will be the most challenging designs ever attempted.

The consensus of the workshop is that nowadays, with the exception of the next generation of experiments at linear colliders, all experiments need and plan for particle identification at ever-higher momenta and therefore, for the most part, rely on RICH detectors. This was the key message of the talk by Nappi at the end of the meeting. The high quality of the talks and the enjoyable location made this event a great success and RICH practitioners are very much looking forward to the sixth in the series, which will be held in Trieste in autumn 2007.

• RICH2004 was sponsored by several Mexican institutions including the Centro de Investigación y de Estudios Avanzados (CINVESTAV), the Consejo Nacional de Ciencia y Tecnología (CONACyT), UNAM and UASLP; CERN; the National Science Foundation (NSF); the Centro LatinoAmericano de Fisica (CLAF); SLAC; and Fermilab.