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CERN | Geneva | Switzerland

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2012: the year of the dragon

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

To be alerted of new postings, follow me on Twitter: @GagnonPauline or sign-up on this mailing list to receive and e-mail notification.


 

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9 Responses to “2012: the year of the dragon”

  1. Dylan Williams says:

    If I read this a few more times I think I might be able to get my head round the inverse femtobarn !!
    Interesting update, thanks.

    • Pauline Gagnon says:

      Hello Dylan,

      you are very right. This is a cumbersome unit. The initial unit is the barn, which is a cross-section or how big a target appears to be. It comes from a bad joke that the unit is so big it looks like a barn to an incoming projectile like a proton or electron. In particle physics, common units are always just a fraction of a barn, like millibarn, or microbarn. Given that anything with such a large cross-section or in other words, probability of being hit, corresponds to phenomena we have seen so many times, we treat them as background now. Nowadays, we are studying the much rarer processes, like the probability to create new particles like the Higgs or supersymmetric particles. There the probability of getting an event is so small we are calculating in picobarn (one millionth of a millionth of a barn) or femtobarn (one billionth of a millionth barn)! These are so rare, we need to produce lots of collisions to have a chance to see one of these events. The quantity of data needed estimated in units of inverse femtobarn.

      So if a process is very rare, with a cross-section as small as a femtobarn, we need to produce a data sample equivalent to one inverse femtobarn to produce one of these events.

    • no_HEP_guy says:

      Hi Dylan,

      the inverse femtobarn thing is really simple. Inverse means just “one over”, i.e. 1 / unit of area.

      Assume you got a shotgun and aim at the door of a shed (ha ha).
      You fire a few rounds, and then you count: 100 bullet holes in 1 square meter of your target.
      That is 100 /m^2 collisions.

      Now, you keep shooting (as LHC keeps running), and after some time you count again. Now you count 100’000 bullet holes per m^2. That is a huge number, and as such it becomes inconvenient, so you change the area you refer the “bullet hole density” to:
      That is 1000 bullet holes per square decimeter, or 10 bullet holes per square mm.
      So, 100000/m^2 = 10/mm^2, 10 inverse square millimeters

      So, instead of counting up the number of bullet holes per (constant) area, you shrink the area, and remain within reasonably sized numbers.

      And 1/fbarn is really really _many_ bullet holes per square meter.

      R.

  2. raven says:

    Great post it was very informative, just one thing though…. that is not a dragon that is a dinosaur :)

  3. Ignacio Fernandez says:

    Thanks pauline , you have a fan , thank you teacher……soooo well explained , the future in Cern how nice.

  4. Jody Porter says:

    Thanks for the very informative post! I’m going to find you on twitter so I can keep up with what will surely be a very exciting year’s worth of news.

    Question: I teach physics in high school (second year…still learning) yet only recently have I become fascinated with quantum physics/particle physics (some would say obsessively so, I’d say its an appropriate amount of obsession in order to better understand the very subject I teach). Do you have any suggestions, or know of any resources (CERN’s or otherwise), that I could use to quickly “plant the seed” for my students and hopefully inspire them to look further into this fascinating field? I’m very limited time-wise and content-wise (course is almost entirely classical, but the state allows a very brief “window” in which I could do this by including the Big Bang theory in the content.) I show videos (very short, unfortunately, due to said time constraints) that demonstrate the “concept” of particle colliders, but I’ve yet to find one that’s informative enough so as to not be “over their heads” yet exciting enough to keep their attention.

    Long story short: Can you help a struggling teacher inspire students? :) (The idea of what “inspires students”, particularly HS students that just need the credit, is a fundamental mystery that ranks right up there with understanding what gives matter mass!)

    Thanks again for the post,
    Jody D. Porter

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