In August 1995 a Letter of Intent was submitted for the LHCb experiment, the world’s first dedicated B-physics experiment exploiting the unique potential of high-energy hadron-hadron collisions. This year, the LHCb collaboration has marked the 30th anniversary of this event. Click the cartoon (by Adrien Miqueu) for a higher-resolution version.
The bound state of a beauty quark b and anti-quark b, the Υ meson, was discovered in 1977 at Fermilab. For many years afterwards, the study of hadrons containing beauty quarks was dominated by experiments at e+e– colliders, which included DORIS, CESR, VEPP and LEP. So-called “fixed-target” experiments at hadron accelerators, in which beauty hadrons were produced by colliding accelerated protons with stationary objects were very limited in their scope, as the production rate of beauty particles was tiny compared to the production rate of other particles. At hadron colliders, however, the beauty particle production rate is much higher thanks to the higher available energy. The Fermilab collider experiments CDF and D0 at the Tevatron, although not designed specifically for studies of beauty particles, took advantage of this opportunity and started to obtain interesting results in the 1990s. The BaBar and Belle experiments at the e+e– collider beauty factories were approved in 1993 and began collecting data in 2000. The B-physics experiment HERA-B at the HERA electron-proton collider at DESY was taking data between 1999 and 2003. Beauty particles were produced from the collisions of the HERA proton beam with a set of thin wire targets.
Clearly, the high proton-proton collision energy at the LHC would result in a high production rate of beauty particles. But how could high precision experiments be conducted in such a challenging environment? At the LHC workshop in Evian, in 1992, three B-physics experiments were proposed:
- COBEX, a forward spectrometer using LHC proton-proton collisions, as the majority of beauty particles are produced around the direction of colliding protons;
- LHB, using protons extracted from the LHC ring and running as a fixed-target experiment;
- GAJET, injecting gas into the LHC beam pipe, running as a fixed-target experiment, similarly to the LHCb SMOG setup.
In June 1994, the LHC Committee decided not to approve any of the three individual experiments. Instead, they requested that the interested parties formed a new collaboration to propose a single new experiment based on the proton-proton collider concept, exploiting the large bb production cross section with an appropriate trigger strategy. Given the wealth of B physics results obtained by the e+e– B-factory and Tevatron experiments, adopting the collider mode was a posteriori the correct choice, as insufficient beauty hadrons would have been produced at the LHC in a fixed-target setup.
The LHCb Letter of Intent was submitted in 1995 and the experiment was approved in 1998. The experiment’s design was re-optimised in 2003, during which many significant improvements were made. The tracking stations in the magnet were removed to reduce secondary particle interactions, and all of the first tracking stations were made in silicon technology. Particle interactions in the LHC beampipe were reduced by installing a beryllium pipe. Finally, technological advances enabled the entire LHCb detector to be read out at a rate of 1MHz, thereby improving the selection process of beauty particle decays in a large computing farm.
But what could the name and the logo of the experiment be? Alternative names were used 30 years ago: “LHC-B”, “LHCB” or “LHCb”. The final name was selected in 1997 as a result of a vote together with the design of the logo, and officially announced at the web page. Here the “Cb” is transformed into the mirror image of “CP” which is then “violated” by a red bar, referring to “CP violation”, i.e., one of the most important themes of LHCb research.
“The ideas conceived 30 years ago have been highly successful, yielding excellent physics results beyond the original goal,” says Tatsuya Nakada, the first LHCb spokesperson. As shown on this web site, beauty hadron distributions can be measured with background levels as low as those obtained at e+e– colliders, while the data are collected at a much higher rate. Furthermore, the production of all beauty mesons and baryons is observed at LHCb, in contrast to B-factory experiments, which are limited to studying light beauty meson decays only.
The members of the LHCb collaboration are constantly upgrading the LHCb detector, taking advantage of the most recent technological developments. The installation of the new Upgrade I detector was completed in April 2023. Enhancements will be made to the RICH detectors, the electromagnetic calorimeter and the online system during the upcoming long shutdown. Work is currently ongoing on a second major upgrade of the LHCb detector, the LHCb Upgrade II.
Read more about the history of the LHCb experiment and its physics results in the presentations at the LHCb20-fest.