by Lidiya SMIRNOVA, Dr. Sc. (Phys. & Math.), Professor of the Physics Department, Lomonosov Moscow State University, senior research scientist of the Skobeltsyn Institute of Nuclear Physics
The first cycle of the operation of a particle accelerator known as the Large Hadron Collider (LHC) is coming to a close. The preparatory stage of this project took over twenty years, with unique particle detectors developed. The dramatic period of the collider's startup and the first results of measurements are now in the past. For more than three years LHC and the ATLAS, CMS, ALICE, LHCb* detectors operated on colliding beam lines of accelerated particles, and experimentalists collected information on what took place at superhigh energies of interacting particles. Next, new discoveries followed, and new particles and physical phenomena were discovered. The evidence thus obtained confirmed predictions of the Standard Model comprising a totality of modern concepts on elementary particles and their interactions.
The opening of the International Conference on High-Energy Physics in July of 2012 in Melbourne (Australia) compelled great attention of all world information agencies. The statement of the European Laboratory of Elementary Particles (CERN**, Geneva,
* See: L. Smirnova, "The 21st Century Megaproject", Science in Russia, No. 5, 2009; L. Smirnova, "Start of the Large Hadron Collider", Science in Russia, No. 5, 2010.--Ed.
** CERN, European Center for Nuclear Research.--Ed.
Switzerland), where LHC is operating, on the detection of a new particle, the Higgs boson, the target of LHC search, was in the breaking news. Two major experiments at the ATLAS and CMS detectors registered simultaneously this particle's signal. Its mass is 126 GeV, and it is observed in several types of decay. The strongest signal is registered in those cases, when the particle decays into two photons. The signal occurs also in case of its decay into four light leptons. These may be four
electrons, four muons or two different pairs: one of electrons, and the other of muons plus a pair of leptons with a neutrino.
DISCOVERY OF THE HIGGS BOSON
In the 20th century physicists got to the root of what matter is like. First in cosmic rays and then at accelerators, they observed a large number of particles and elucidated the laws of their formation and decay. Such processes are described by the Standard Model based on the quantum field theory. As components of the material world quarks and leptons are elementary particles in this model. They interact by exchange of gluons and vector bosons. The mass of each of them differs dramatically. But the Standard Model cannot explain how the mass of elementary particles forms and why they differ so much.
In 1964 a British physicist, Peter Higgs, published a paper in which he suggested a simple formative mechanism for particle masses. He postulated the existence of a quantum field to which a particle with a zero electrical charge and spin should correspond. This hypothetic particle was called a Higgs boson*. Nothing was known about its mass. Since the 1970s scientists have discovered three heavy quarks** (c, b and t)***, vector bosons with a spin equal to unit value and with electric charges equal to +1 and -1 (W+, W-) and a zero charge (Z 0). However, they failed to detect a particle which could be iden-
* Boson is a particle with a whole spin; spin is an intrinsic angular momentum of particle.--Auth.
** See: P. Yermolov, Ye. Shabalina, "Heavy Quarks: Search Goes On", Science in Russia, No. 3, 2001.--Ed.
*** C, b, t--"charmed", "beautiful" and "top" quark, respectively.--Ed.
tified with the Higgs boson. Physicists suggested other models to explain the particle mass but the answer to the validity of a particular model could be obtained only through experiment. So, the prime objective of the Large Hadron Collider was to identify the presence of the Higgs boson or its absence.
In more than twenty years physicists of 40 countries including Russian scientists created an accelerator and detectors to register collisions of protons and nuclei. Among them was the author of the present article who, for many years, had been heading a group in the Research Institute of Nuclear Physics of Lomonosov Moscow State University in the ATLAS experiment. It is hard to express agitation and joy of that moment when the collider started accelerating particles and the first data were obtained. An immense amount of new measurements in the ATLAS, CMS, ALICE and LHCb experiments has already brought the elementary particle physics to a new higher level. The discovery of a particle corresponding to the Higgs boson of the Standard Model is a triumph in the cognition of the physical reality of the world.
Two major ATLAS and CMS experiments resulted simultaneously in the discovery of the new particle. Different detectors, different observation techniques and different teams of physicists produced the same result. To this end scientists had to combine all data obtained in experiments during 2011 and up to June of 2012. The discovery came off! It is very important that experimentalists succeeded in excluding the possibility of the Standard Model having a Higgs boson with another mass in a very wide range of values up to 600 GeV.
However, it will be hardly possible to answer all at once so many questions pertaining to the nature and properties of the new particle. It is necessary to determine all possible scenarios of its decay and check on the consistency of theoretical predictions concerning its properties. A much greater amount of new information has to be collected. Therefore, a long search still lies ahead; but the newly made discovery shows that experimentalists are on the right truck.
OTHER NEW PARTICLES AND RARE DECAYS
Observation of a putative Higgs boson in LHC experiments was not the first discovery of the particle at the collider. Experimentalists first observed the new particle at the ATLAS detector in December of 2011. Naturally, this event was not comparable to the discovery of the Higgs boson and did not excite as much physicists because they referred it to the already known family
of mesons. The new element consists of a b-quark and a b-antiquark (designated as χb(3P)), its mass being 10.53 GeV; and it has a high orbital moment.
In the CMS experiment one observe a new particle referred to an excited state of the heavy baryon Ξb*0, which includes the b-quark. The mass of the observed baryon is equal to 5.9 GeV, with the measuring accuracy being only fractions of percent (~0.2 percent).
The high-precision measuring accuracy is indicative of a search for rare decays of B-mesons. They include also the b-quark. The probability of B-meson decay into two muons predicted by the Standard Model is 3.2•10-9 and 10-10 depending on the nature (type) of an antiquark paired with the b-quark. Therefore, to prove or disprove the validity of such predictions is one of the priorities of LHC. The prompt attainment of the prediction level of the Standard Model is an impressive result of the experiments. The LHCb experiment directed to a high-precision measuring of B-hadrons is crucial in this analysis. It is followed by the close results of the CMS and ATLAS experiments. It is safe to say there is no significant excess of decay probabilities. Combining the results of the LHCb, CMS and ATLAS experiments, it will be possible to reach the level of these predictions using the data obtained in 2012. This will eliminate a considerable part of scenarios of a minimal expansion of the Standard Model.
NEW OBSERVATIONS OVER KNOWN PARTICLES
The prompt obtaining of the desired results is indicative of the unique characteristics of LHC experiments. As early as 2011 dozens of scientific publications discussed the data obtained by the ATLAS and CMS detectors. The powerful LHC computer system and the well-developed analysis methods facilitated their processing despite the multicomponent structure of detectors and large information flows. Therefore, we can state for certain that particle physics has attained a new high.
The high energy of proton interactions is a distinctive feature of the conducted experiments. This is important, though the collider did not reach yet the rated capacity of proton beam line collisions as high as 14 TeV. Another distinctive feature: the high luminous emittance (rate) of collisions (encounters). As early as May of 2011 LHC became a world leader in luminous emittance exceeding the achievement at the Tevatron accelerator in the Fermi National Accelerator Laboratory (Fermilab, USA). In 2012 basic data were obtained at radiation 610•1033 cm-2c-1. The next step will be to attain the rated value of 1034 cm-2c-1 and more. What this means is that information reaches specialists very fast and in great volume.
It should be noted that the main result of LHC experiments is confirmation of the Standard Model predictions for a wide range of processes. For example, at proton
Spectrum of effective masses of four leptons (muons and electrons) in which the Higgs boson signal was observed in the ATLAS experiment. The signal is indicated by blue.
beam interaction energy 7 TeV, cross-sections of production of heavy quarks c, b, t and vector bosons W and Z were measured. They correlate well with calculations of the perturbation theory of quantum chromodynamics, and their accuracy is well above those conducted before. For instance, a discrepancy between the experiment and the theory for the production probability of b-quarks was detected at Tevatron and remained for long a topical problem for analysis. Moreover, the development of new analytical methods for the follow-up of b-quarks orders and creation of their computer algorithms for modeling at LHC provided for consistency between new measurements and theoretical computations.
Measurements of quarks and vector bosons are connected with registration of a great number of particle jets (cascades), i.e. energy bursts in very narrow spans of solid angles. Their emergence is caused by hard collisions of partons, i.e. structural elements of initiating protons. Therefore it is important to identify the type of a jet and the nature of a quark that forms this jet. For this purpose high-precision measurements of particle track coordinates are used near the point of protons primary collision. This results in a reliable isolation of jets formed by c- and b-quarks. The jets thus formed allow to go further and isolate decays of heavier t-quarks. In their turn, the reconstructed vector bosons can be add-on decay products of t-quarks. Against the background of the known processes a search is on for new resonance states with large masses; they are attainable due to the high energy of initiating particles. Thus far such states have not been identified, though the search boundaries have been extended greatly.
The physical procedures at LHC have also revealed new phenomena in detecting of correlation effects in the structure of colliding protons. It has been found that for a correct description of rigorous processes in these collisions it is necessary to take into account both solitary and binary collisions of quarks and gluons. True, these correlations do not exhaust the entire class of their multiple interactions. More complex combinations are taken account of through phenomenological models (also with account of process characteristics). This consideration is especially important for modeling jets of particles, attendant to hard collisions of quarks and gluons.
New measurements of production probabilities of heavy particles at LHC are also important because earlier measurements at Tevatron took place in collisions of protons and antiprotons, i.e. with an equal number of quarks and antiquarks. There are more quarks than anti-quarks in proton collisions. The difference is shown through a prevalence of particles with a positive electrical charge over negatively charged particles.
The probability of pair production of t-quarks was measured at energy 7 TeV, and its value agreed with the expected one. Next, it is necessary to determine polarization of t-quarks and see whether resonances take place in their production. Of special interest are processes in which one t-quark is produced without its paired t-antiquark. There are three types of diagrams to cause the production of a single t-quark. For many years attempts were made to detect similar processes at Tevatron as well. But since the initial energy was too low, the probability of these processes was very low, too. Certain results were obtained even there. Anyway, the reliable measurements of cross-sections of single t-quarks produced at LHC are a major success of experimentalists. The measured probabilities are less than l/10th of the cross-section of t-quarks pair production. These data, together with the results of detailed measurements of vector bosons W and Z production, bring the research of elementary interactions to a qualitatively higher level. This fact will stimulate further studies of complex diagrams with three-boson vertices.
CHARACTERISTICS OF MATTER IN COLLISIONS OF HIGH-ENERGY LEAD NUCLEI
The complex structure of proton interactions is not comparable to multiple elementary processes in colli-
sions of accelerated lead nuclei. LHC accelerates them to an energy 2.76 TeV per nucleon. The ALICE, ATLAS and CMS detectors register all particles formed in nuclear collisions. Consequently, the nature of the processes under study requires different approaches and objectives. The main thing is to determine the nuclear environment characteristics; this environment is formed at high density and temperature as inferred from the nature of interactions of quarks, gluons and other particles with it. In other words, it is necessary to use statistical and thermodynamic methods for this purpose.
The ALICE detector is a leader in the study of nuclear collisions. This detector has unique possibilities in determining the nature of particles. Russia is represented by physicists of the National Research Center "Kur-chatov Institute". The experiment provides a detailed comparison of spectra of particles formed in nuclear collisions with different target parameters. It also identifies groups of peripheral faraway interactions between nuclear centers and groups with different centrality, up to head-on encounters. It becomes obvious that particles of different nature and different momenta interact differently with the nuclear environment. Significant correlations are detected in the emission of particles. An analogy between nuclear matter and a fluid of different viscosity is used in the description of the observed phenomena.
A demonstrative result was obtained in the ATLAS experiment at the end of 2010. It turned out that some jets produced in nuclear collisions had no compensating jet. They occur, as a rule, in pairs and go in opposite directions. Episodes with a single, not paired, are registered in nuclear collisions. In such a case a total compensation of energy momenta takes place. This phenomenon is explained by the specifics of the nuclear environment through which the jet passes. Perhaps this is a signature of nascent quark-gluon plasma searched for, in nuclear collisions. A follow-up analysis of these complex processes should give a conclusive answer.
To sum up, it may be said that the physical data so far obtained by LHC demonstrate a success of the project. The results comply with the Standard Model.
There is no evidence for supersymmetry or other models to enable a full understanding of the Standard Model and to clarify the interconnection between gravitation and the microworld. The Higgs boson characteristics are yet to be determined, and the search for new phenomena outside the Standard Model to be continued.
Research work at LHC is suspended till 2014 in preparation for its operation at energies twice as high, i.e. 14 TeV.
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