Diversifying to stay healthy
Although the eyes of the world are almost all on the LHC and its four large experiments, there are hundreds of scientists working on experiments installed at the smaller accelerators. Below is a short tour of a less visible face of CERN that is bursting with vitality and new ideas.
Besides the six experiments installed at the LHC, CERN's scientific programme includes some 17 other experiments installed at the PS, SPS and AD, plus several others that use the ISOLDE facility. A large community of scientists uses these beam lines to run experiments on a wide variety of physics topics that span from searches for new physics, nuclear physics, and studies of rare particle decays to the interaction of cosmic rays with the atmosphere.
Although it may not appear so at first sight, each of these non-LHC experiments, however small, has a lot in common with its big brothers: they are challenging in their physics goals, they are carried out by international collaborations, and they use cutting-edge technology.
Among the most recently approved by the Research Board is AEGIS, currently the only experiment in the world aiming at directly measuring the gravitational interaction of antimatter. It was approved in December and will be carried out by about 50 scientists from some 20 institutions around the world. AEGIS will use the beam of antiprotons from the AD and will precisely measure the gravity-induced deflection on a beam of antihydrogen atoms. "The methodologies involved require a number of new developments", says Michael Doser, AEGIS spokesperson. "For example, in order to obtain the planned 1% sensitivity, it is required to cool the antiprotons down to 100 mK - more than ten times lower than the current state-of-the-art experiments". If differences in the deflection of antihydrogen atoms in flight were observed by AEGIS, theorists could be forced to review their theories on gravitation. "A generic feature of attempts to quantize and unify gravity with the other forces of nature is the appearance of partners to the normal spin-two graviton", explains Doser, "If AEGIS should measure a different value of the acceleration of gravity for antimatter, these hypothetical new gravi-scalar and gravi-vector components might be to blame".
In the field of experimental physics, not all new science has to be done at the high-energy frontier. "Rare decays are extremely sensitive to the presence of new particles", confirms Augusto Ceccucci, NA62 spokesperson. NA62 is the continuation of NA48, NA48/1, and NA48/2 whose results have remarkably improved our knowledge of the properties of kaons. NA62 will be carried out by a new collaboration that includes about 160 members from some 25 institutes. It will also have a new set-up that will include new detectors, among which a newly conceived "GigaTracker" will be installed in the beam-pipe to precisely tag and track the particles before they decay. "NA62 will study with unprecedented precision the rare decays of K particles", continues Ceccucci. "Such decays are the only ones where the interactions between strange quarks and down quarks can be accurately studied. Thanks to the sensitivity of our future experimental set-up, we will be able to measure deviations from the precise Standard Model predictions, thus giving indications of the possible existence of new particles or physics processes".
When it comes to versatility, CERN's beam lines prove to be some of the most unique tools in the world. A good example is the M2 beam line from SPS, which can supply both high-energy polarized muon beams and high-energy hadron (mostly pions and protons) beams. Both beams are used by the COMPASS experiment to study the inner structure of hadronic matter, like protons and neutrons. Although, naively, the proton can be considered composed of one "down" quark and two "up" quarks, in reality it turns out to be a much more complex object. In particular, previous experiments - such as the European Muon Collaboration experiment installed at the same M2 beam line - revealed that strange quarks contribute to the spin of the proton, and suggested that the total quark spin could account for a small fraction of the total spin of the proton. "Back in the late 1980s, some theorists argued that a large gluon polarization could hide the contribution from the quark spins", explains Gerhard Mallot, COMPASS co-spokesperson. "However, in 2006 our experiment proved that this factor is not large enough and that new sources must be researched". Since then, COMPASS has continued to be at the forefront of precise measurements of QCD - the theory that describes the interactions of quarks and gluons. In 2008 COMPASS started spectroscopy measurements with hadron beams focusing on more exotic states of matter, formed by quarks and gluons. A detailed characterization and possibly the discovery of new states will test predictions of QCD, in particular lattice calculations. "We have recently submitted a Letter of Intent to the SPSC for precision measurements on the transverse and longitudinal spin structure of the nucleon", confirms Mallot. "In the Letter, we explain that an upgraded set-up of COMPASS, could allow the collaboration to tackle two new central issues in our understanding of the nucleon structure, namely transverse spin effects and generalised parton distribution functions."
The non-LHC experiments are yet another sign of CERN's scientific vitality and uniqueness. Some of them will be presented at the New Opportunities in the Physics Landscape at CERN workshop that will be held from 10-13 May; an event not to be missed by the whole scientific community.
Further information:
http://aegis.web.cern.ch/aegis/
http://na48.web.cern.ch/NA48/NA48-3/
Cycles and cogs
In addition to preparing beams for the LHC, the entire CERN accelerator chain is working tirelessly to supply different types of particles to the experiments that are installed on dozens of beam lines in the various experiment halls.
It all starts from the proton source, followed by the Linac. To allow each accelerator to feed the next one in the chain and to keep the secondary beam lines supplied, the PS Booster, PS and SPS work in cycles in which the beam is pulsed and reaches different specified energy levels. Thus, the PS is capable of running some twenty different cycles: in addition to the cycles for particles destined for the LHC, at an energy of 25 GeV, there are others which supply the SPS for the purposes of the fixed-target programme and CNGS, at an energy of 13 GeV; n_TOF, at 19 GeV; the lines of the East Hall, at 23 GeV; and the AD beam, at 25 GeV.
All these cycles are synchronised by the CERN Control Centre (CCC), which also manages the operations of extraction and injection. Django Manglunki, of the Operations Group in the BE Department, explains: "If you want to make sure the right particle bunch is injected at just the right energy, all of the machines have to be in phase and function like cogs in a machine. The final synchronisation is done at the radio frequency level. But for each cycle there are hundreds of parameters to be programmed into the machine."
Setting up an accelerator for a complete running cycle may take a few days or several weeks. This year, however, the PS Booster commissioning was particularly impressive, as the operation was completed in just one day. Klaus Hanke, chief of the PS Booster & ISOLDE section in the Operations Group, recalls: "The machine received its first proton beam from the Linac on Tuesday, 31 March, and the next day the first beam was ready for ISOLDE. On Friday 3 April the first beam was successfully sent to the PS, ahead of schedule. Since then, the work of setting up the PS Booster has continued, as the machine is required to supply downstream parts of the accelerator chain with a whole range of beams possessing very different characteristics: from very low-intensity beams for LHC commissioning to very high intensity beams for users such as ISOLDE and CNGS. The smooth start-up is the result of a team effort by operators, engineers and physicists who are all experts in different areas of accelerator physics and technology and complement each other perfectly."
The new landscape
The workshop on “New Opportunities in the Physics Landscape at CERN” was announced in February and the attention it received can be measured by the large number of submitted abstracts, which stood at 95 by the April 6 deadline.
The subjects are widely spread, ranging from nuclear physics measurements at ISOLDE, to particle physics at PS and measurements with antiprotons in AD to the wide range of SPS measurements with different kind of beams.
Neutrino physics will be addressed only marginally during this workshop: a dedicated event will be organized on 1-3 October 2009 to cover this subject.
The workshop will include a session dedicated to the discussion of the CERN accelerator complex as well as reports on the status and outlook of the various topics in other laboratories.
The workshop foresees only plenary sessions of variable length, with rapporteur talks to present all the submitted contributions, which cannot be presented individually (all the abstracts will be published). Every session will include time for discussion. The sessions will take place in the Main Auditorium.
Since the workshop is the starting point of a longer process, we expect the formation of dedicated working groups to focus further on new ideas.
Up to now there are more than 250 registered participants and we are looking forward to the presentation of new ideas and to a lively discussion.
