Call me a geek but I do believe that anyone who thinks that doing physics isn’t cool has never actually seen physics being done.
The purpose of the Tile Calorimeter in ATLAS is simple: estimate the energy of particles entering the calorimeter. These energies are then used to identify the potentially interesting particles (such as the Higgs) produced at the beam’s collision point. It is such a simple statement and yet so hard to achieve in reality. For this purpose, the Tile Calorimeter is designed as a ‘sampling calorimeter’ composed of scintillating tiles sandwiched between plates of iron. In some ways a sampling calorimeter is a lot like trying to watch a dance through a series of photographic snapshots. The dancers move from one picture to the next and although you didn’t directly observe the movement you can derive what the movements were. The faster the snapshots, the more lifelike the dance.
TileCal is similar except we try to measure a particle’s energy not a dancer’s movement and we do that by measuring the amount of light in the scintillating tiles. The scintillator/iron structure is clearly shown in this picture. For a sense of scale, this is only part of one wedge of the calorimeter and there are a total of 256 wedges. The dark lines between the backlit scintillating tiles are layers of iron. Energetic particles coming from beam’s collision will be slowed down in the iron and will produce light in the scintillator. Without the iron, the particles would not slow down very quickly and the calorimeter would have to be even more humungous than it already is. The small circles at the bottom are where the photomultiplier tubes (PMT) sit. Optical fibers are run from each scintillator to a PMT and the PMT measures the total light.
A wedge with all the fibers connected looks like this. The upper row of fibers is just a template for the technicians to know which fiber goes to which PMT. If the fibers are run to the wrong location, we will be confused as to which scintillator tile is being lit by the particles passing through.
When asked what it means for me to ‘do physics’, I could say, ‘we estimate the energy of particles entering the calorimeter’ or I could show these pictures. They both have the same meaning but I think they leave very different impressions. To me the former sounds pretty boring but the latter looks very cool.
Hey Monica,
I’ve been following your blog, along with those of the other US-LHC bloggers for a short while now – it’s a nice view into the other experiments (I’m on ALICE). I was attracted by this title “doing physics”, in a personal way. Yes, hardware is damn sweet, it always is. There is nothing this job gives us that is cooler than the big toys. But there is something which is getting to the point of obsession, something lacking, something I’m starting to crave…. DATA !
I’m obviously not the only one – after the first cosmic runs in December, the mailing lists are going kinda crazy with people demanding reconstructed data from these runs. I mean, yes, there are good reasons to have this data reconstructed and checked urgently, but let’s take a step back for a second : it’s just cosmics ! Not to sound too disparaging – we all know how crucial these early runs are – but I personally cannot wait for the real thing ! Ever had that feeling where you were opening Christmas presents and you had to get through all the small ones from various insignificant aunts, uncles and cousins. to finally get to the big one, the size of the tree in the corner, wrapped in gold paper ? That’s kinda the feeling. Bring On The Data !
Agreed! I too am feeling the data craving. And I feel like the craving for data keeps rising exponentially each day. Imagine how bad it is going to be come this spring/summer!
Monica,
This is Alejandro from Durham, North Carolina. You mentioned that the data collection you will be doing once the LHC is in operation involves the search for particles relating to the Higgs field, so I wanted to ask you a quick question. Since the investigation of Higgs field messengers and other new subatomic particles is one of the more sought after goals of the LHC, how does data priority work into the equation? By this I mean although the data being collected will be sent to the many collaborators involved in the accelerator once analyzed, is there any priority relating to who will receive the data first and who will have publishing rights or will the data be open to public use as soon as it has been gathered? I thank you for your time and any reply would be greatly appreciated.
Sincerely,
Alejandro C.
Distributing the data to the large number of ATLAS collaborations is a challenging and interesting problem. The basic approach is the data is readout from the detector and stored at CERN. Immediately copies of the data are transferred to several other sites. One site is Brookhaven National lab in the US for example. People in the ATLAS collaboration can access the data from CERN or any of these sites. No group of people is given priority to accessing the data and only members of ATLAS can access it.
As for publishing, ATLAS (as well as CMS) has very strict publication rules, which states the process of how analyses are approved for publication. And a publication for a Higgs discovery, for example, would include the full ATLAS author list.
Cool blog
Thanks
Neil
Thanks for the help and I wish you tons of luck with the upcoming experiments.
Sincerely,
Alejandro C.