by Lidiya SMIRNOVA, Dr. Sc. (Phys. & Math.), Skobeltsyn Research Institute of Nuclear Physics, Lomonosov Moscow State University
In late November 2009, the start of the Large Hadron Collider (LHC) was successfully accomplished at the European Center for Nuclear Research (CERN, Switzerland). Proton bunches within a pulse of 450 GeV/s were put into its ring, transported in two directions of circulation without additional acceleration and put together in collision areas. The detectors of interaction recorded the first collisions of particles at an energy of 900 GeV. In December 6-12 they were accelerated to the impulse value of 1.18 TeV/s. Since March 30, 2010, LHC is uninterruptedly working at an energy of proton collision of 7 TeV.
The LHC acceleration ring has four intersection points of oncoming proton beams, in which ATLAS, CMS, ALICE and LHCb* interaction detectors are constructed. The first two are of general purpose, and their basic task is search for Higgs boson and other new particles. ALICE is designed to find quark-gluon plasma. Its formation is expected at collisions of relativistic heavy nuclei, first of all of gold (Au), which will be also accelerated in the collider. LHCb is designed for stud-
* See: L. Smirnova, "The 21st Century Megaproject". Science in Russia, No. 5, 2009.-Ed.
ies of symmetry violation at B-meson decomposition and indirect observation of Higgs boson and other new particles in properties of such decompositions.
It should be noted that all these detectors were ready for operation by September 10, 2008, the date of the launch of the collider, but the heat removal system had failed. It took more than a year to eliminate the failure. Nonetheless, the detectors did not stand idle, they recorded particles of cosmic rays penetrating to them under the ground to a depth of about 100 m. The flux of such particles consists of muons, which slightly interact with the substance and can penetrate the soil and the detectors, thus allowing to register their tracks. Millions of such tracks were measured by ATLAS and CMS. The LHCb design differs radically from the central symmetric configuration of the first two and is not adapted for recording of vertically falling particles, but cosmic particle tracks were observed on it too. This ensured adjustment of all detectors and also the systems of data collection and analysis. Measurements of cosmic muon impulse spectra by these detectors are not sufficiently informative due to difficulties of registration of the soil structure over them. However, some information of scientific importance can be obtained, for example, from the measurement of the relation between the quantity of muons and positive and negative electric charges. Its value is determined by the composition of primary cosmic rays falling on the border of the Earth's atmosphere, and the obtained data are topical for the cosmic ray physics.
It is difficult to describe impatience of the participants of the experiments, who waited for a repeated launch of the collider and registration of proton collisions. ALICE demonstrated a higher degree of readiness as compared with other detectors. Some days after observation of the first collisions, on December 1,
Chart of location of detectors on the Large Hadron Collider.
2009, a group of specialists published a scientific paper on measurement of charged particle multiplicity in proton collisions at an energy of 900 GeV, and in February 2010, CMS detector on similar results at 900 GeV and 2.36 TeV. On March 15, 2010, ATLAS submitted its results on particle multiplicity in proton-proton (pp) interactions at 900 GeV based on measurements of 300,000 events. In November-December 2009, CMS detector recorded 105 pp collisions at 900 GeV and 15,000 at an energy of 2.36 TeV.
After a short winter break all experimentalists were to start a prolonged period of stable data collection. The energy problem required for the work of the accelerator was thoroughly discussed. To provide its stable operation it was decided to use 7 TeV, i.e. twice less as compared with the design energy. But even such level exceeds more than thrice the collision energy of the Tavatron collider at the Fermi National Laboratory (USA) and undoubtedly ensures an essential advance to an unknown area of interactions. The start of LHC for proton beam collisions with the energy of 7 TeV took place on March 30, 2010. Since that time systematic work of all detectors on recording and analysis of the incoming information is constantly under way.
The first publications of results of charged particle multiplicity, born on LHC in the course of proton interactions, refer to the sphere of energies reached earlier on accelerators. A quarter of a century ago, in 1986, in the UA5 experiment (CERN) these multiplicities were measured at collisions of protons with antiprotons at 900 GeV. The theoretical expectations of their difference for these two reactions make up shares of percentage. Now we have an opportunity to compare old and new measurements. It was performed on CMS and ALICE and showed matching of results. Operation of ATLAS is based on measurements of 300,000 proton interactions, and a comparison was carried out with predictions of different calculations according to the Monte Carlo simulation*. The more precise measurements showed that there existed divergences with predictions of the already available models. It turned out that the experimental values exceeded the design ones by 5-15 percent. Such differences are not new, but suggest further development of models to describe more precisely the known processes, which form backgrounds in the course of search for new physical phenomena.
The main amount of publications on LHC experiments at this stage is devoted to characteristics of the detectors. The possibility to observe rare processes depends on the precision of their operation. For the first time events in the center of detectors are registered, i.e. in the way as they were designed. All these results will be presented at international conferences of 2010.
INCREASE OF LHC EFFICIENCY
The foremost problem for the experimentalists at this stage is efficiency maximization in registration of proton interactions. It presupposes permanent readiness of the detectors for registration of signals, undoubtedly of a good quality. Teams of physicists on duty provide continuous control of operational characteristics and quality of incoming information. They are also in charge of coordinated operation of the detectors and the collider. The ATLAS efficiency in using a proton beam made up 98 percent in April 2010 as compared with 90 percent in November-December 2009. This leap forward became possible due to the adjustment of procedures of the start of the detector and termination of working sessions. But you never know, how many other such improvements are ahead!
INCREASE OF THE COLLIDER LUMINOSITY
At the early stage there were only two proton bunches in the collider ring. It is planned to increase their number to 16 by the end of June 2010. At the same time, the number of protons in the bunch will increase tenfold, from 1010 to 10.11 In that case, luminosity (collision frequency) will reach 10 cm-2s-1. The reality of expectations of such unique events with new information from them is determined by accumulated integral the luminosity in experiments. It depends on joint
* Process simulation by the Monte Carlo method plays a key role in assessment of events and analysis of reactions.-Auth.
operation time of the collider and detectors. The scheduled indicators of functioning of the collider allows, for example, the obtain an integral of luminosity of approximately 1 reciprocal picobarn (pb-1)* in the ATLAS experiment at the end of June 2010. It means that one event can be registered, whose probability corresponds to the section of 1 picobarn. It will happen in one of 1011 collisions of protons. The assessments demonstrate that the most optimistic (from the standpoint of observations) predictions of new models can be expected starting from an integral luminosity of about 10 pb-1. But the expectations can be more well-grounded, when the number of accumulated events is dozens and hundreds the times more. Consequently, the luminosity has to be further increased to 1033 cm-2 s-1 planned for the first stage of the collider operation. Then the expected unique events can become a reality. It is planned to carry out heavy nuclei acceleration in late 2010. It should be noted that such massive particles will get into the LHC ring for the first time.
Today scientists are preoccupied with the simulation of proton interactions. It is necessary to create new mathematical models of detectors corresponding to their current state and obtain quantitative predictions of physical reactions at the energy of 7 TeV. The debugging of trigger** programs is under way, designed to select events with special characteristics from the total flow of interactions. For the time being all interactions are registered, which is necessary for correct estimation of the efficiency and general characteristics of events.
It is necessary to introduce teams of specialists engaged in experiments on ATLAS and CMS detectors (2.5-3 thous. physicists each) working in different groups with specific tasks. A high level of coordination of their actions and internal evaluation of all results for each experiment provide efficiency and reliability of presentation of the results outside each experiment. Here, the credibility requirement competes with the ambition to maintain supremacy of one's own team. It is more difficult to determine in cases, when the data diverge from the model predictions, whether it is a result of imperfection of the procedure of registration and analysis or a really observed deviation of the experiment, requiring refinement of the current knowledge. The unique conditions created on the LHC for observation of rare physical processes will help more accurately see peculiarities of the already known particles and reactions, i.e. will allow to state an invention. The latter will happen without fail within the next two or three years. It can be Higgs boson or neutralino, which creates the dark mass of the Universe, which will change scientific knowledge about the structure of the Universe.
Already now we have the first registered physical objects connected with structural elements of the contemporary model of the physical world—Standard Model-on each detector. They have been submitted to scientific community, and they are carefully analyzed.
Decompositions of charged vector W+-bosons serve as such objects for ATLAS and CMS experiments. They were also discovered at CERN in 1983. Decompositions to muon/electron and neutrino have already been registered. These reactions with creation of muon and electron are similar in terms of the process physics, but they are observed differently. Muon tracks are measured in muon spectrometer, and electrons in electromagnetic calorimeter. Similarity of events lies in the fact that both muon track and electron signal depend on the presence of an uncompensated (unregistered) transverse pulse, which takes away neutrino leaving no traces in detectors. The number of measured W-bosons makes dozens and correlates with the expected probabilities of their creation. Observation of these particles is important for many reactions, in which creation of new particles is expected. For this purpose, it is necessary to register W-bosons in millions of events.
* Barn is a unit of the effective cross section of processes running in collision of atomic and nuclear particles. -Ed.
** Trigger is a switching device, which can keep a long time one of its two states of true equilibrium and switch step-wise on a signal from outside from one state to another. -Ed.
Event of W-boson creation in proton collisions at 7 TeV in ATLAS experiment. W-boson decomposition to positron and neutrino.
Event of W-boson creation in proton collisions at 7 TeV in ATLAS experiment. W-boson decomposition to muon and neutrino.
NEUTRAL PARTICLE DECOMPOSITIONS
Alongside with tracks of charged particles, neutral strange particles A, Ã, и K0s are already measured, and decompose within the internal detector. Of special note is observation of J/Ψ-meson, whose composition includes charmed quark c and antiquark ċ. The number of J/Ψ-mesons in each experiment does not exceed a hundred as yet. With an increase of their number, it is expected to select those particles, which have been created from decompositions of B-mesons. The first decomposition of B+-meson is registered in the LHCb detector. The heavier b-quark is present in the composition of B-mesons. In the course of studies of B-meson decompositions it is necessary to reveal new particles and investigate the fundamental properties of symmetry and, first of all, violations of the charge and space symmetry (CP-symmetry), which make themselves evident in decompositions of B-hadrons. According to Academician Andrei Sakharov (1921-1989), it is just these violations, which guaranteed the existence of the Universe, and we are a part of it.
It should be noted in conclusion that a high degree of readiness of LHC experiments for execution of the planned program of physical research infuses US with hope for outstanding discoveries in the first prolonged period of work (2010-2011).
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