The LHCb experiment at CERN

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LHCb is one of the four major experiments at the Large Hadron Collider, the biggest and most powerful particle accelerator ever built, currently in operation at CERN. A key objective of the experiment is to study the mechanisms that produced, in the early Universe, a small asymmetry between matter and antimatter, originally produced in equal amounts in the Big Bang. Today it is believed that such mechanisms are precisely those responsible for the fact that our present Universe (made of planets, stars, galaxies) is composed almost entirely of matter.

 

Bigbang Antimateria

 

It is known, in fact, that if particles of matter and antimatter come in contact with each other they annihilate, converting their mass into electromagnetic radiation. Therefore, if in nature matter and antimatter behave in exactly the same way, they would be completely annihilated immediately after the Big Bang, leaving a Universe made ​​only of radiation.

 

A small asymmetry in the behavior of matter and antimatter, known as CP violation, could be behind the slight imbalance in favor of matter in the early Universe, causing the latter to be not completely annihilated, and thus making it possible to form the Universe in which we live.

CP violation was observed for the first time in 1964 in weak interaction processes in experiments with kaons at Brookhaven National Laboratory, yielding the Nobel Prize for Physics in 1980. More recently it has also been observed in other experiments, including the B-factories BaBar (SLAC) and Belle (KEK), in different processes involving the B-meson, a composite particle containing the b (bottom) quark. These discoveries have proven that CP violation is a universal phenomenon in processes regulated by weak interactions. There is as yet no experimental evidence of CP violation in strong and electromagnetic interactions.

 

The Standard Model of particles allows for the possibility of CP violation through the so-called CKM mechanism, which describes the probability that a quark is transformed from one type (flavor) to another (e.g. from bottom to charm). However, the amount of CP violation allowed by the Standard Model is too small to explain the asymmetry between matter and antimatter created in the early Universe. The nature must therefore have provided other particles (not yet observed) and other mechanisms capable of generating the amount of CP violation needed to explain the Universe in which we live. It is thus a priority of today physics to look for signs of new physics beyond the Standard Model.

 

Modello Standard

 

The search for new physics beyond the Standard Model is at the basis of the construction of the Large Hadron Collider (LHC). The LHC is located inside a 27 km long and 100 m deep tunnel in the vicinity of the France-Switzerland border. Collisions between very high energy proton beams (up to 7 TeV per beam) circulating in opposite directions take place at four points of the ring, in correspondence to the four main LHC experiments: ATLAS , CMS , ALICE and LHCb.

 

LHC1 LHC2

 

The LHCb experiment is mainly dedicated to the study of the physics of b (beauty) and in particular to the measure of the parameters of CP violation as well as rare decays and phenomena associated to mesons and baryons with c and b quarks. The huge amount of B-mesons produced in LHCb (thousands of times more than those produced in B-factories) allows to study the proprtites of these mesons with unprecendented precision.

 

 

Being unstable particles, B-mesons are not anymore present in the Universe. However, they had to be quite abundant soon after the Big Bang. Once generated in laboratory (e.g. in proton-proton collisions), they live for a very short time () before decaying into lighter particles. From the comparison between the decays of B-mesons (containing a b-quark) and those of the corresponding anti-particles, (containing an anti-b quark), one can learn a lot about the mechanisms that allow to distinguish between matter and  antimatter in nature.

 

The LHCb detector, specifically designed to reveal the decay products of B-mesons, is a forward spectrometer, optimized to cover relatively small angles (<17 °) with respect to the direction of the colliding beams, where the B-mesons are more likely to be produced. It consists of a series of detectors systems of different type, positioned in sequence beyond the interaction point. A sub-detector particularly important for the experiment is the VELO (Vertex Locator), a silicon strip detector located only 8 mm away from the proton beams, aimed at the detection of traces of charged particles near the interaction point. This detector allows to determine with high precision the point (vertex) where the B-mesons are produced. The tracks of charged particles produced in the decay of B-mesons are detected by a tracking system consisting of silicon strip detectors and gas detectors (straw tubes) immersed in the magnetic field generated by a 4.2 Tm magnetic dipole. Other very important sub-detectors are those dedicated to the particle identification, and in particular to the discrimination among different types of hadrons (pions, kaons, protons, etc). These are the RICH detectors, whose operation principle rely on a particular phenomenon called Cherenkov light emission. Other important detectors are those dedicated to measure the energy of particles: the electromagnetic and the hadronic calorimeters, and those dedicated to the detection of muons (particles similar to electrons, but 200 times heavier), constituting the muon system.

 

 

LHCb1

 

The figure above shows a scheme of the LHCb spectrometer and its various sub-detectors. The figure on the left shows a typical event recorded in LHCb during the data taking. Clearly visible are: the tracks of charged particles bent by the magnetic field, the Cherenkov light produced in the RICH (magenta), the traces of two muons (purple) in the muon system and the energy released in the calorimeters (shown in the form of red and blue histograms).

 

LHCb2

 

The LHCb Collaboration consists of more than 700 physicists from about 50 different Institutions from 15 countries. The Department of Physics and Earth Sciences of University of Ferrara is actively involved in the experiment, along with the Ferrara Section of the National Institute of Nuclear Physics (INFN). The Ferrara group is involved in many activities that include detector construction and development, computing and data analysis:

 

Hardware activities:

  • Construction, installation and commissioning of the muon system;
  • Upgrade of the RICH detector, test and characterization of light detectors (multi-anod photomultipliers) and development of the dedicated readout electronics.

Software activities:

  • Computing (GRID, data production, accounting, database);
  • Development of  triggers based on GPU technology;
  • Data analysis: study of CP violation in  meson decays, development of flavor tagging algorithms, study of semi-leptonic decays of the B-meson, measurement of branching ratios and Dalitz plot analysis.

 

Find more info in:

http://lhcb-public.web.cern.ch/lhcb-public /  (LHCb experiment)

http://home.web.cern.ch/ (CERN)

 

LHCb3

 

Composition of the Ferrara group (staff)

 

University of Ferrara:                         INFN:

Prof. Roberto Calabrese                    Dr. Concezio Bozzi

Prof.ssa Eleonora Luppi                     Dr. Stefania Vecchi

Dr. Luca Tomassetti                           Dr. Wander Baldini

Dr. Massimiliano Fiorini                    Dr. Angelo Cotta Ramusino

Dr. Luciano L. Pappalardo                 Dr. Mirco Andreotti