Bernhard Spaan, an exceptional particle physicist and a wonderful colleague, unexpectedly passed away on 9 December, much too early at the age of 61.
Bernhard studied physics at the University of Dortmund, obtaining his diploma thesis in 1985 working on the ARGUS experiment at DESY’s electron–positron collider DORIS. Together with CLEO at Cornell, ARGUS was the first experiment dedicated to heavy-flavour physics, which became the central theme of Bernhard’s research work for the following 36 years. Progressing from ARGUS and CLEO to the higher-statistics experiments BaBar and ultimately LHCb, for which he made early contributions, he was one of the pioneering leaders in the next generation of heavy-flavour experiments at both electron–positron and hadron colliders.
While working on tau–lepton decays at ARGUS for his doctorate, Bernhard led a study of tau decays to five charged pions and a tau neutrino, which resulted in the world’s best upper limit for the tau-neutrino mass at the time. He also pioneered a new method of reconstructing the pseudo mass of the tau lepton by approximating the tau direction with the direction of the hadronic system. This method led to a new tau-lepton mass, which was an important ingredient to resolve the long-standing deviation from lepton universality as derived from the measurements of the tau lifetime, mass and leptonic branching fraction.
In 1993 Bernhard joined McGill University in Montreal, where he contributed to CLEO operation, data-taking and analysis, and was brought into contact with the formative stages of an asymmetric electron–positron B-factory at SLAC. He was an author of the BaBar letter of intent in 1994 and remained a leading member of the collaboration for the two following decades.
Bernhard saw the unique potential of a dedicated B experiment at the LHC and joined the LHCb collaboration
In 1996 Bernhard started a professorship at Dresden where, together with Klaus Schubert, he built a strong German BaBar participation including involvement in the construction and operation of the calorimeter. At that time, BaBar was pioneering the use of distributed computing resources for data-processing. As one of the proponents of this approach, Bernhard played a crucial role in the German contribution via the computing centre at Karlsruhe, later “GridKa”. Building on the success of the electron–positron B-factories, Bernhard saw the unique potential of a dedicated B experiment at the LHC and joined the LHCb collaboration in 1998.
Bernhard’s scientific journey came full circle when he accepted a professorship at Dortmund University in 2004, which he used to significantly grow his LHCb participation. The Dortmund group is one of LHCb’s largest, with a long list of graduate students and main research topics including the determination of the CKM angles β and γ governing CP violation in rare B decays. In parallel with LHC Run 1 and 2 data-taking, Bernhard investigated the possibility of using scintillating fibres for a novel tracking detector capable of operating at much larger luminosities. In all phases of the “SciFi” detector, which was recently installed ahead of LHC Run 3, he supported the project with his ideas, his energy and the commitment of his group.
Bernhard was an outstanding experimental physicist whose many contributions shaped the field of experimental heavy-flavour physics. He was also a great communicator. His ability to resolve conflicts and to find compromises brought many additional tasks to Bernhard, whether as dean of the Dortmund faculty, chair of the national committee for particle physics, member of R-ECFA or chair of the LHCb collaboration board. When help was needed, Bernhard never said “no”.
We have lost a tremendous colleague and a dear friend who will be sorely missed not only by us, but the wider field.