First LHC results from 7 TeV data presented by the LHCb experiment

Figure 1: Response of the Hybrid Photon Detectors in RICH1 to laser light.
Figure 1: Response of the Hybrid Photon Detectors in RICH1 to laser light.

Physicists from the School of Physics & Astronomy have announced the first results from data produced by the Large Hadron Collider (LHC). This comes just weeks after the LHC started operating at a new beam energy of a record 7 trillion electron volts (TeV).

Speaking at the RICH10 conference, the University of Edinburgh’s Prof. Franz Muheim presented the first results from the Ring Imaging Cherenkov (RICH) Detectors of the LHCb Experiment with data taken at the LHC.

The RICH detectors are built to distinguish between different types of charged particles known as pions, kaons and protons. Their proper identification is crucial for measuring differences between particles and antiparticles. The detectors were built by a collaboration of 10 institutes from the UK, Italy and CERN.

The results represent an immense achievement – and a lot of hard work – over the last decade by the LHCb collaboration.

The Edinburgh team has made major contributions to the LHCb RICH detectors. Also to the development of distributed analyses techniques for the large LHC dataset on the Grid, including using ScotGrid and ECDF. The group has pioneered new ideas for searching for new physics beyond the Standard Model and have now commenced analysing LHCb data.

Prof. Franz Muheim says: “We are all thrilled to observe such an excellent RICH detector performance from day one.

“The LHCb experiment is located in the forward region, close to the beam pipe of a hadron collider, which is a very harsh environment and this makes these initial results a great achievement. We are all looking forward to the large data rates that the LHC will produce as it gradually increases the collision rate over the year."

“With the LHCb experiment at the LHC, we will explore uncharted territory and hopefully find something new, possibly unexpected."

“This is a fantastic time for science. Particle physics is one of the disciplines that excites school kids and motivates them to study physics, and we need a more scientifically literate workforce to tackle the challenges of the 21th century.”

The LHCb group in the School of Physics & Astronomy comprises: Prof. Franz Muheim, Prof. Pete Clarke, Prof. Steve Playfer, Dr. Stephan Eisenhardt, Dr. Greig Cowan, Dr. Yuehong Xie and Ph.D Students Rob Currie, Gemma Fardell, Conor Fitzpatrick, Young Min Kim, Ailsa Sparkes, Nick Styles, Ross Young and technician Andrew Main.

The results were presented at the 7th International Workshop on Ring Imaging Cherenkov detectors (RICH 2010), which runs from 3–7 May in Cassis , South of France. RICH2010 focuses on the present state of the art and the future developments in Cherenkov light imaging techniques for applications in High Energy Physics, Nuclear Physics and Astroparticle Physics.

See below for an explanation of the LHCb experiment and some first-hand accounts of the University’s involvement in LHCb and the RICH detectors.

The Hybrid Photon Detectors

Here some of the members of the LHCb group in the School of Physics & Astronomy describe their involvement with the experiment.

Ross Young

Ross Young is a PhD student who, at the RICH2010 conference, gave a presentation on results from operating the Hybrid Photon Detectors (HPDs) in the LHCb RICH counters.

 “I was very fortunate that as part of my PhD studies I started a placement at CERN in October 2008, just after the installation of the HPDs into the RICH1 detector. Part of my work has been with the performance studies of HPDs - the light sensitive detectors of the RICH system.

“During the installation, columns of HPDs were mounted side-by-side to form 'bee-hive' arrays of HPDs (see Figure 2, below). Since then, we have been testing the performance of these arrays under a variety of different environments.

“As an example, we shine a continuous-wave laser on to the arrays to test the HPD response to light. Looking at the pixel hitmap of RICH1 (see Figure 1, above) you can notice, for every HPD under such illumination from laser, the projection of the circular photocathode image onto the pixel chip, in addition to other features such as shadowing. As well as carrying out analysis of the results, I have had the opportunity to carry out the runs myself in the control room.”

“The principle of a Ring Imaging Cherenkov detector is as follows:  when a highly relativistic particle traverses through a medium at a very large speed exceeding the speed of light in the radiator, Cherenkov light is emitted at an angle which depends only on the speed of the particle and the refractive index of the medium.

“The Cherenkov photons are focused by spherical mirrors onto a ring on the photon detector arrays. The RICH detectors have recorded the first Cherenkov photon rings from collisions of the LHC beams.

“Figure 3 (below) shows a typical event. Each orange point depicts a single photon recorded by the HPDs. The sets of blue rings correspond to different particle hypotheses. One observes that photons lying typically on the ring with the largest radius identifies them as coming from a pion. Cherenkov photons from kaons or protons would lie on the smaller circles.”

Stephan Eisenhardt

Dr. Stephan Eisenhardt is coordinator of the University’s photodetector laboratory and he has been leading the testing of the HPDs in Edinburgh.

“The fully operational Ring Imaging Cherenkov Detectors of LHCb are a great achievement. The HPDs are a new technology. From idea and concept via design and development to commissioning they are a brain child of LHCb physicists.

“The HPDs provide an unprecedented noise-free sensitivity to enable particle identification. Never before has a detector been built which provides particle identification over such a wide momentum range.

“New concepts had to be developed for many aspects of it. Yet it is working from day one at a very high efficiency. This is testimony of the spirit of dedication and collaboration of so many people."

Ailsa Sparkes and Conor Fitzpatrick

Ailsa Sparkes and Conor Fitzpatrick are currently on a placement at CERN and take part in the day-to-day running of the RICH detectors.

Ailsa: “It is extremely exciting to be contributing to such an important experiment as LHCb at this pivotal time in particle physics.

“I didn't expect to be given direct control over the RICH detectors so soon into arriving here at CERN and it's a great experience. The atmosphere at CERN makes it a fun and lively place to work with potentially ground-breaking discoveries just around the corner.”

Conor: “This is very exciting. As a RICH Piquet I am responsible for the safe and reliable running of these detectors, a role I share with a number of physicists from universities and institutes throughout the world.

“It is humbling to be part of such a dedicated collaborative effort, and to share in the continued successes of our work."

Young Min Kim

Young Min Kim will present a poster at the RICH2010 conference, and explain his simulation studies of the performance expected from improved RICH photon detectors which will allow the LHCb experiment to operate at a ten times greater collision rate in the second half of the decade.

"It is a great honour to see my work on the HPDs integrated with that of everyone else to give fruit to the LHCb RICH detector system. A mix of fascination and pride swims in me knowing I'm contributing to detective work using the smallest traces of light as our clues."

The LHCb Experiment

The LHCb experiment aims to understand the matter-antimatter asymmetry in the Universe.

It is a real puzzle why the observable Universe consists almost entirely of matter. When the Universe was created in the Big Bang the amounts of matter and antimatter were identical, so the question is: “Where has all the antimatter gone?”

The Standard Model of Particle Physics provides a partial answer. The BaBar experiment at SLAC, Stanford, observed a large asymmetry between the decay rate of a neutral B meson (a particle containing a b-quark), which oscillates (transmutates) at a high frequency into its antiparticle and back, compared to the corresponding decay rate of the antiparticle into the same final state.

This phenomenon is known as CP violation in the interference between mixing and decay of B mesons, but the observed magnitude of CP violation is too low by a large factor of 1000 millions to explain the observed asymmetry.

The University of Edinburgh is a member of the BaBar experiment and the physicists proposing this mechanism as the origin of CP violation within the Standard Model were recognised with the Physics Nobel prize in 2008. By recreating at the LHC the conditions that existed a millionth of a millionth of a second after the Big Bang, the LHCb experiment is aiming to shed light on the cause of this matter-antimatter asymmetry which is at the basis of our existence.