Ten months after a defective solder joint stopped in its tracks the most ambicious project ever for physicists across the world, tens of mexican scientists prepare the last details of their participation in the Large Hadron Collider (LHC), the instrument located in the limits of Switzerland and France built by the European Organization for Nuclear Research (CERN).

And even though it's a megaproject that involves thousands of scientists from dozens of countries, the mexican contribution is really important.

Gerardo Herrera Corral, from Centro de Investigación y Estudios Avanzados (Instituto Politécnico Nacional), has worked in the mexican projects with CERN practically since they began 14 years ago, and at present has been living since more than a year ago at Geneva.

At first the mexican presence in CERN was more limited, he recalls. “But in 2001 the World Bank released a project called Millennium Science Initiative, which gave substantial support to science groups that presented interesting projects”.

Today there are mexican teams working in two of the monumental machines installed in the LHC's main ring, a complex with a 27-kilometer diameter through which will circulate, almost at the speed of light, in opposite directions, two proton beams.

The LHC will deflect the proton beams to make them cross each other in four locations of the ring, locations where four multidetectors have been built, huge machines with names rather curious and arcane: ATLAS, CMS, ALICE, and LHCb.

According to theoretical models, two protons colliding head on travelling almost at the speed of light will produce in the collision or interaction point conditions of pressure and temperature paramount to the ones that existed in the universe... before matter was born!

But those conditions will last an infinitesimal fraction of a second, and the interaction products will quickly return to normal conditions, not before leaving a trace of what happened in the detectors built around the interaction zones.

Herrera Corral said that the ALICE detector, whose name means A Large Ion Collider Experiment, contains 18 detectors. "México designed, built and is operating two of those 18 subsystems".

One of the mexican groups, the one that has as its main actor the Cinvestav (with UNAM and the universities of Puebla and Sinaloa), is responsible for the instrument called ACORDE, ALICE Cosmic Ray Detector, a delicate array of detectors built atop the three upper facets of the ALICE magnet.

“Earth is being bombed all the time by cosmic rays”, explained the physicist from IPN, “and it is necessary for detectors to have a system that allows to veto or block this radiation”.

The instruments installed in the LHC will focus on the products of collisions between protons or ions, and if for instance muons or other particles coming from outer space enter the detector, they introduce noise. ACORDE's role is to eliminate that noise.

But of course there's always the possibility of capturing the information about cosmic rays to study some very rare events called "muon bundles", for instance.

The other work group, which includes people from UNAM and Cinvestav, designed, built and installed a detector called V0a, defined by Herrera Corral as a high-speed detector, a disk located very close to the interaction point in ALICE.

“It's a very fast detector, able to make in 25 thousand-millionths of a second a decision about the interaction that just happened", he said. The detector is part of the fundamental triggering system, which will allow physicists to gather valuable data from the more than eight thousand interactions per second that ALICE will see.

Just like groups from other countries, the mexican ones are now responsible for the operation of their instruments, and this entails a constant presence, particularly so because these systems will not operate alone.

In fact, said the physicist, "in this moment there are six or seven mexicans at work in the cosmic ray detector, because next tuesday it will begin sending signals to other devices in the detector".

The instrument called V0a is also crucial: it sends a very fast signal to an enormously powerful electronic equipment that, in a practical sense, will usually be asleep, waiting for a snappy signal from V0a or other instruments to awaken its electronic circuits in order to register an event, before going back to sleep mode.

"The whole experiment will start this year", said Herrera Corral. "In a few months the tests will begin; next weekend we'll have the first injection, but (proton) collisions will begin around october".

There's also mexicans (from BUAP, Cinvestav, Iberoamericana, UASLP) present in another of the large instruments called CMS or Compact Muon Solenoid. Mexican physicists worked there in the definition and construction of a tracks and vertex detector which will be vital in case the instrument does catch the so-called God particle.

What about data, daddy?

• ALICE will gather information from up to eight thousand collisions per secund. If you add all the instruments around the LHC, the volume of generated data soon becomes impossible to handle.
• To face this problem, detectors follow the actions of triggers that discard, in fractions of a microsecond, events that are uninteresting to focus on the promising ones.
• Besides, the project built the LHC Computing Grid, a structure that ties tens of thousands of computers across the world in a multilevel system. In México, Cinvestav will operate a Tier Two node.