I do not have a crystal ball but it is nevertheless possible to sketch what can be expected from the Large Hadron Collider (LHC) experiments at CERN this year.
Right now, the accelerator is stopped for the annual maintenance shutdown. This is the opportunity to fix all problems that occurred during the past year both on the accelerator and the experiments. The detectors are opened and all accessible malfunctioning equipment is being repaired or replaced.
In the 27-km long LHC tunnel, surveyors are busy getting everything realigned to a high precision, while various repairs and maintenance operations are on their way. By early March, all magnets will have been cooled down again and prepared for operation.
The experimentalists are not only working on their detectors but also improving all aspects of their software: the detector simulations, event reconstruction algorithms, particle identification schemes and analysis techniques are all being revised.
By late March, the LHC will resume colliding protons with the goal of delivering about 16 inverse femtobarns of data, compared to 5 inverse femtobarns in 2011. This will enable the experiments to improve the precision of all measurements achieved so far, push all searches for new phenomena slightly further and explore areas not yet tackled. The hope is to discover particles associated with new physics revealing the existence of new phenomena. The CMS and ATLAS physicists are looking for dozens of hypothetical particles, the Higgs boson being the most publicized but only one of many.
When protons collide in the LHC accelerator, the energy released materializes in the form of massive but unstable particles. This is a consequence of the well-known equation E=mc2, which simply states that energy (represented by E) and mass (m) are equivalent, each one can change into the other. The symbol c2 represents the speed of light squared and acts like a conversion factor. This is why in particle physics we measure particle masses in units of energy like GeV (giga electronvolt) or TeV (tera electronvolt). One electronvolt is the energy acquired by an electron through a potential difference of one volt.
It is therefore easier to create lighter particles since less energy is required. Over the past few decades, we have already observed the lighter particles countless times in various experiments. So we know fairly well how many events containing them we should observe. We can tell when new particles are created when we see more events of a certain topology than what we expect from those well-known phenomena, which we refer to as the background.
We can claim that something additional and new is also occurring when we see an excess of events. Of course, the bigger the excess, the easier it is to claim something new is happening. This is the reason why we accumulate so many events, each one being a snap-shots of the debris coming out of a proton-proton collisions. We want to be sure the excess cannot be due to some random fluctuation.
Some of the particles we are looking for are expected to have a mass in the order of a few hundred GeV. This is the case for the Higgs boson and we already saw possible signs of its presence last year. If the observed excess continues to grow as we collect more data in 2012, it will be enough to claim the Higgs boson discovery beyond any doubt in 2012 or rule it out forever.
Other hypothetical particles may have masses as large as a few thousand GeV or equivalently, a few TeV. In 2011, the accelerator provided 7 TeV of energy at the collision point. The more energy the accelerator has, the higher the reach in masses, just like one cannot buy a 7000 CHF car with 5000 CHF. So to create a pair of particles with a mass of 3.5 TeV (or 3500 GeV), one needs to provide at least 7 TeV to produce them. But since some of the energy is shared among many particles, the effective limit is lower than the accelerator energy.
There are ongoing discussions right now to decide if the LHC will be operating at 8 TeV this year instead of 7 TeV as in 2011. The decision will be made in early February.
If CERN decides to operate at 8 TeV, the chances of finding very heavy particles will slightly increase, thanks to the extra energy available. This will be the case for searches for particles like the W’ or Z’, a heavier version of the well-known W and Z bosons. For these, collecting more data in 2012 will probably not be enough to push the current limits much farther. We will need to wait until the LHC reaches full energy at 13 or 14 TeV in 2015 to push these searches higher than in 2011 where limits have already been placed around 1 TeV.
For LHCb and ALICE, the main goal is not to find new particles. LHCb aims at making extremely precise measurements to see if there are any weak points in the current theoretical model, the Standard Model of particle physics. For this, more data will make a whole difference. Already in 2011, they saw the first signs of CP-violation involving charm quarks and hope to confirm this observation. This measurement could shed light on why matter overtook antimatter as the universe expanded after the Big Bang when matter and antimatter must have been created in equal amounts. They will also investigate new techniques and new channels.
Meanwhile, ALICE has just started analyzing the 2011 data taken in November with lead ion collisions. The hope is to better understand how the quark-gluon plasma formed right after the Big Bang. This year, a special run involving collisions of protons and lead ions should bring a new twist in this investigation.
Exploring new corners, testing new ideas, improving the errors on all measurements and most likely the final answer on the Higgs, that is what we are in with the LHC for in 2012. Let’s hope that in 2012 the oriental dragon, symbol of perseverance and success, will see our efforts bear fruit.
Pauline Gagnon
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