**1. Introduction**

The High Energy Physics (HEP) [1] – often called Particle Physics – is one of the research areas where the accomplishment of scientific results is inconceivable without the infrastructure for distributed computing, the Computing Grid. The HEP is a branch of Physics that studies properties of elementary subatomic constituents of matter. It goes beyond protons and neutrons to study particles which existed only a fraction of a second after the Big Bang and quarks and gluons in the so-called Quark Gluon Plasma (QGP) [2]. These studies are based on experiments with particles colliding at very high energies, at speeds almost equal to the speed of light.

The world's leading Particle Physics research laboratory is CERN [3], the European Center for Nuclear and Particle Physics near Geneva, Switzerland. The CERN latest particle accelerator (see Fig. 1), the Large Hadron Collider (LHC) [4], installed in a 27 km long tunnel located about 100 m underground and crossing the Swiss - French border, uses counter-rotating beams of protons or lead ions (Pb) to collide at 4 points, inside large particle detectors: ALICE [5], ATLAS [6], CMS [7] and LHCb [8]. There are also another two smaller experiments, TOTEM [9] and LHCf [10]. These are much smaller in size and are designed to focus on

Fig. 1. LHC@CERN

in bunches, in counter-rotating beams. According to the original design proposal, there should be 2808 bunches per a beam. Each bunch of protons contains 1011 protons. the design beam energy is 7 TeV and the design luminosity is 1034 cm−2s−1. The bunch crossing rate

Grid Computing in High Energy Physics Experiments 183

However, the new phenomena looked for by the scientists appear at a rate of 10−<sup>5</sup> Hz. So the physicists must analyze 10<sup>13</sup> collision events/sec to have a chance to discover a New Physics phenomenon. At present, the machine has not yet reached the full number of bunches per beam and is operating at half of the originally proposed energy, but the luminosity is getting rapidly to the goal value. The LHC team has been increasing the number of bunches gradually reaching 1380 bunches/beam at the time of writing. The full beam energy will be reached in 2014, after one year of a technical stop to arrange for this increase. The machine has already beaten some world records which we will mention in section 5. Let us just mention the one concerning the stored energy: at the end of 2010, the energy stored in the accelerator ring was about 28 MegaJoules (MJ). At the target full intensity, this energy will reach about 130 MJ

In any case, the volume of data necessary to analyze to discover New Physics was already in the original proposal estimated to be about 15 PetaBytes (PB, 1PB=1 million GB) per data taking year. The number of the processor cores, CPUs, needed to process this amount of data was estimated to be about 200 thousands. And here, the concept of a distributed data management infrastructure comes into the scenario, because there is no single computing center within the LHC community/collaboration to offer such massive computing resources, even not CERN. Therefore in 2002, the concept of the Worldwide LHC Computing Grid (WLCG) [12] was launched to build a distributed computing infrastructure to provide the

In the present chapter, we give a short overview of the Grid computing for the experiments at the LHC and the basics of the mission of the WLCG. Since we are members of the ALICE collaboration, we will also describe some specific features of the ALICE distributed computing

In section 2, we will describe the architecture of the WLCG, which consist of an agreed set of services and applications running on the Grid infrastructures provided by the WLCG partners. In section 3, we will mention some of the middleware services provided by the WLCG which are used for the data access, processing, transfer and storage. Although WLCG depends on the underlying Internet - computer and communications networks, it is the special kind of software, so-called middleware, that enables the user to access computers distributed over the network. It is called "middleware" because it sits between the operating systems of the computers and the Physics applications that solve particular problems. In section 4, the Computing model of the ALICE experiment will be briefly described. It provides guide lines for the implementation and deployment of the ALICE software and computing infrastructure over the resources within the ALICE Grid and includes planning/estimates of the amount of needed computing resources. Section 5 will be devoted to the ALICE-specific Grid services and the ALICE Grid middleware AliEn. It is a set of tools and services which represents an implementation of the ALICE distributed computing environment integrated in the WLCG environment. In section 6, an overview will be given of the experience and performance of the WLCG project and also of the ALICE Grid project in particular during the real LHC data taking. The continuous operation of the LHC started in November 2009.

is 40 MegaHz and the proton collisions rate 107 <sup>−</sup> <sup>10</sup><sup>9</sup> Hz.

production and analysis environments for the LHC experiments.

which is an equivalent of 80 kg of TNT.

environment.

"forward particles". These are particles that just brush past each other as the beams collide, rather than meeting head-on.

The energy of the protons is currently 3.5 TeV (1 Tera eV= 1 million MeV) and that of the Pb ions is 1.38 TeV, so the collision energies are 7 TeV for the protons and 2.76 TeV for the Pb ions.

The phrase often used to summarize the mission of the LHC is, that with the LHC we are going back in time very close to the Big Bang, as close as about 10−<sup>10</sup> seconds. In terms of length it represents about 10−<sup>16</sup> cm (compared to the dimensions of the Universe of about 1028 cm). At this scale, the matter existed in a form of a "soup" made of the quarks and gluons, the Quark Gluon Plasma. The quarks are objects protons and neutrons are made of, so the LHC represents in a sense a huge extremely complicated microscope enabling the study of the most basic elements of matter.

There are several major questions which scientists hope to get answered with the help of the LHC.


From the experiments analyzing the data from the LHC collisions, ATLAS and CMS are the largest. They were nominally designed to look for the Higgs boson but in fact these are general purpose detectors for the study of all kinds of Physics phenomena at the LHC energy range. The ALICE detector is a dedicated heavy ions detector to study the properties of the Quark Gluon Plasma formed in the collisions of lead ions at the LHC energies. The LHCb is much smaller detector and its mission is to study the asymmetry between matter and antimatter. Although all these experiments are designed for Particle Physics research, the scientific programs they follow actually cross a border between Particle Physics, Astrophysics and Cosmology.

Now, where does the Computing Grid show up in this scientific set-up? The LHC is the world's largest particle accelerator. The protons and lead ions are injected into the accelerator 2 Will-be-set-by-IN-TECH

"forward particles". These are particles that just brush past each other as the beams collide,

The energy of the protons is currently 3.5 TeV (1 Tera eV= 1 million MeV) and that of the Pb ions is 1.38 TeV, so the collision energies are 7 TeV for the protons and 2.76 TeV for the Pb ions. The phrase often used to summarize the mission of the LHC is, that with the LHC we are going back in time very close to the Big Bang, as close as about 10−<sup>10</sup> seconds. In terms of length it represents about 10−<sup>16</sup> cm (compared to the dimensions of the Universe of about 1028 cm). At this scale, the matter existed in a form of a "soup" made of the quarks and gluons, the Quark Gluon Plasma. The quarks are objects protons and neutrons are made of, so the LHC represents in a sense a huge extremely complicated microscope enabling the study of the most

There are several major questions which scientists hope to get answered with the help of the

• What is the origin of mass, why do elementary particles have some weight? And why do some particles have no mass at all? At present, we have no established answers to these questions. The theory offering a widely accepted explanation, the Standard Model [11], assumes the existence of a so-called Higgs boson, a key particle undiscovered so far, although it was first hypothesized in 1964. One of the basic tasks of the LHC is to bring an

• Where did all the anti-matter disappear? We are living in the World where everything is made of matter. We suppose that at the start of the Universe, equal amounts of matter and antimatter were produced in the Big Bang. But during the early stages of the Universe, an un-known deviation or in-equilibrium must have happened, resulting in the fact that in

• What are the basic properties of the Quark-Gluon Plasma, the state of the matter existing for a tiny period of time after the Big Bang? Originally, we thought it would behave like a plasma, but the latest scientific results including those delivered by the LHC suggest that

• What is the universe made of? At the moment, the particles that we understand create only 4 % of the universe. The rest is believed to be made out of dark matter and dark energy. The LHC experiments will look for supersymmetric particles, which would confirm a likely

From the experiments analyzing the data from the LHC collisions, ATLAS and CMS are the largest. They were nominally designed to look for the Higgs boson but in fact these are general purpose detectors for the study of all kinds of Physics phenomena at the LHC energy range. The ALICE detector is a dedicated heavy ions detector to study the properties of the Quark Gluon Plasma formed in the collisions of lead ions at the LHC energies. The LHCb is much smaller detector and its mission is to study the asymmetry between matter and antimatter. Although all these experiments are designed for Particle Physics research, the scientific programs they follow actually cross a border between Particle Physics, Astrophysics

Now, where does the Computing Grid show up in this scientific set-up? The LHC is the world's largest particle accelerator. The protons and lead ions are injected into the accelerator

established statement concerning the existence of the Higgs boson.

it behaves like a perfect liquid [2], which is somewhat surprising for us.

our world today hardly any antimatter is left.

hypothesis for the creation of dark matter.

and Cosmology.

rather than meeting head-on.

basic elements of matter.

LHC.

in bunches, in counter-rotating beams. According to the original design proposal, there should be 2808 bunches per a beam. Each bunch of protons contains 1011 protons. the design beam energy is 7 TeV and the design luminosity is 1034 cm−2s−1. The bunch crossing rate is 40 MegaHz and the proton collisions rate 107 <sup>−</sup> <sup>10</sup><sup>9</sup> Hz.

However, the new phenomena looked for by the scientists appear at a rate of 10−<sup>5</sup> Hz. So the physicists must analyze 10<sup>13</sup> collision events/sec to have a chance to discover a New Physics phenomenon. At present, the machine has not yet reached the full number of bunches per beam and is operating at half of the originally proposed energy, but the luminosity is getting rapidly to the goal value. The LHC team has been increasing the number of bunches gradually reaching 1380 bunches/beam at the time of writing. The full beam energy will be reached in 2014, after one year of a technical stop to arrange for this increase. The machine has already beaten some world records which we will mention in section 5. Let us just mention the one concerning the stored energy: at the end of 2010, the energy stored in the accelerator ring was about 28 MegaJoules (MJ). At the target full intensity, this energy will reach about 130 MJ which is an equivalent of 80 kg of TNT.

In any case, the volume of data necessary to analyze to discover New Physics was already in the original proposal estimated to be about 15 PetaBytes (PB, 1PB=1 million GB) per data taking year. The number of the processor cores, CPUs, needed to process this amount of data was estimated to be about 200 thousands. And here, the concept of a distributed data management infrastructure comes into the scenario, because there is no single computing center within the LHC community/collaboration to offer such massive computing resources, even not CERN. Therefore in 2002, the concept of the Worldwide LHC Computing Grid (WLCG) [12] was launched to build a distributed computing infrastructure to provide the production and analysis environments for the LHC experiments.

In the present chapter, we give a short overview of the Grid computing for the experiments at the LHC and the basics of the mission of the WLCG. Since we are members of the ALICE collaboration, we will also describe some specific features of the ALICE distributed computing environment.

In section 2, we will describe the architecture of the WLCG, which consist of an agreed set of services and applications running on the Grid infrastructures provided by the WLCG partners. In section 3, we will mention some of the middleware services provided by the WLCG which are used for the data access, processing, transfer and storage. Although WLCG depends on the underlying Internet - computer and communications networks, it is the special kind of software, so-called middleware, that enables the user to access computers distributed over the network. It is called "middleware" because it sits between the operating systems of the computers and the Physics applications that solve particular problems. In section 4, the Computing model of the ALICE experiment will be briefly described. It provides guide lines for the implementation and deployment of the ALICE software and computing infrastructure over the resources within the ALICE Grid and includes planning/estimates of the amount of needed computing resources. Section 5 will be devoted to the ALICE-specific Grid services and the ALICE Grid middleware AliEn. It is a set of tools and services which represents an implementation of the ALICE distributed computing environment integrated in the WLCG environment. In section 6, an overview will be given of the experience and performance of the WLCG project and also of the ALICE Grid project in particular during the real LHC data taking. The continuous operation of the LHC started in November 2009. When the data started to flow from the detectors, the distributed data handling machinery was performing almost flawlessly as a result of many years of a gradual development, upgrades and stress-testing prior to the LHC startup. As a result of the astounding performance of WLCG, a significant number of people are doing analysis on the Grid, all the resources are being used up to the limits and the scientific papers are produced with an unprecedented speed within weeks after the data was recorded.

Section 7 contains a short summary and an outlook. This chapter is meant to be a short overview of the facts concerning the Grid computing for HEP experiments, in particular for the experiments at the CERN LHC. The first one and half a year of the LHC operations have shown that WLCG has built a true, well functioning distributed infrastructure and the LHC experiments have used it to rapidly deliver Physics results. The existing WLCG infrastructure has been and will be continuously developing into the future absorbing and giving rise to new technologies, like the advances in networking, storage systems, middleware services and inter-operability between Grids and Clouds.
