Around the Laboratories

The HERA-B detector, using an internal target in HERA's ring.

DESY HERA-B, fourth HERA experiment

he phenomenon of CP violation T (the subtle disregard of physics for invariance under simultaneous particle-antiparticle and left-right reversal, observed so far only in decays of neutral kaons) is one of the very fundamental issues in . The availability of neutral B mesons - particles composed of a heavy beauty and a light down quark - provides a fresh arena for observing CP violation, opening up new tests of theories on the origin of this mysterious effect. CP violation in B decays is now the target of a number of major initiatives all around the world. One approach, which emerged about two years ago, is to use DESY's HERA proton ring as a eventually hit the beam pipe or the netic calorimeter and a muon detec­ source of B mesons, operated as a collimator system. In the HERA-B tor system. The challenge resides in fixed target accelerator by installing setup, thin wires positioned inside the the huge number of particles entering an internal target inside the machine machine vacuum serve as a target the detector - over 109 per second. In vacuum. (HERA, with two separate for these halo and provide many aspects, the requirements of rings, was designed to provide well-localized, high-intensity source. the HERA-B detector correspond to -proton collisions.) A group Since CP violating decays of B those of detectors at LHC, and centred around the old ARGUS mesons are rare, HERA-B aims for HERA-B construction necessarily collaboration at DESY's DORIS ring very large interaction rates of about exploits the extensive R&D carried submitted a Letter of Intent for "An 30 to 40 MHz, corresponding to 3 to out towards LHC. Experiment to Study CP Violation in 4 interactions in each proton bunch A particular challenge is the trigger the B System Using an Internal crossing the target. Prototype halo system, which has to locate among Target at the HERA Proton Ring", targets have been tested over the 10 million bunch crossings per followed in 1994 by a detailed pro­ last two years, and have demon­ second those 100 or so most likely to posal. This June, the DESY directo­ strated that sufficient rates can be contain interesting B meson decays. rate approved the experiment on the reached. The HERA-B group has chosen to condition that the collaboration finds At first glance, the proposed HERA- concentrate on B decays with a sufficient resources to build the B detector looks like a normal fixed particularly clean signature, namely detector in a timely fashion. target detector, with a silicon vertex those producing a J/psi meson Using the HERA proton ring for a detector, followed by a magnetic decaying into a pair. This fixed-target experiment is made spectrometer with a dipole magnet channel has the additional advantage possible by the fact that a stored and tracking chambers, a ring that theoretical predictions of CP beam is always accompanied by halo imaging Cerenkov counter (RICH) for asymmetries are particularly clean particles which diffuse outward and particle identification, an electromag­ and model- independent.

10 CERN Courier, October 1994

Around the Laboratories

Test run of an internal target in the proton ring of DESY's HERA collider. Top: distance between the target wire and the beam, in units of the beam width, as a function of time. Bottom: mean number of protons interacting in the wire target per bunch crossing. The HERA- B experiment aims for four interactions per crossing at design current, consistent with the rate of about two interactions per crossing reached in these tests at 30% of the design bunch current.

With less frequent need to inject, accumulated luminosity should also build up faster than before.

HERA luminosity 1992-94

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Rapid growth in HERA proton-electron luminosity at DESY following the introduction of positrons in the collider's 'electron' ring.

The present schedule calls for the +49-40-89 94 4306; e-mail: installation of first detector prototypes schwarza @ x4u2.desy.de and the spectrometer magnet in CORNELL HERA in the winter shutdown 1995- More penguins in 6, with a complete detector available HERA gets better for the 1998 run. With a bit of luck, upstate New York after just one year of running HERA- B could see evidence for CP violation ollowing the introduction of major physics result last year in the B system, in addition to a F positrons in its 'electron' ring, A (June 1993, page 1) was the broad programme of other topics in luminosity at the HERA proton- sighting by the CLEO collaboration heavy- quark physics. electron collider, DESY, , working at Cornell's CESR electron- The HERA-B proponents are now has attained 4.7 x 1030 cm2 s1, 31% positron collider of rare events where seeking to enlarge their collaboration. of the design figure of 1.5 x 1031, and a B particle (containing the fifth - An "Open Collaboration Meeting" at is still improving. 'beauty' quark) decays into a particle DESY from 4-6 October is a major With , filling had to start containing a strange quark. milestone towards the formation of with very low currents (2.5 mA) due These 'penguin' processes the final collaboration. The collabora­ to deleterious effects (probably due cannot be mediated by the single tion will submit a technical design to electrons absorbed by residual quark transformations of ordinary report by the end of the year which impurity ions). With positrons, rela­ weak decays, needing instead should provide the basis for a final tively immune to positive ions, the two such quark transformations approval early in 1995. For further current has already given 32 mA - coupled back-to-back. Proceeding information: A.S. Schwarz, DESY- 55% of the design value, while through this secondary route, F15, Notkestr.85, D- 22603 Ham­ protons are now at 35% of the design 'penguin' processes can play a burg, Phone:+49-40-89 98 2524; fax: current with 55 mA. major role in the delicate CP violation

12 CERN Courier, October 1994