Another day, another first for the ALICE detector! This morning, we began taking data from 900 GeV proton collisions again, with not just a handful but ALL of its detectors! This is an incredible achievement for ALICE, and once again the champagne was out. This time we have thousands of events. At some point, we will have to stop marking the “firsts” with champagne, or we will never get any work done! 😉
The progress at the moment is also marking out several “firsts” for the LHC, as more bunches are added to the beam and more protons are added to each bunch, taking us closer to optimal running conditions. There is still a long way to go before 14 TeV collisions every 25 ns of course, but the speed that these developments are coming along is impressive!
For ALICE newbies, just learning about the various detectors can seem daunting. When I first joined the group years ago I can remember being very confused for a while! To give you a feel for how many acronyms float around in a typical ALICE discussion, here’s a quick summary (I will post about their physics another time!):
“Central barrel” detectors (these collectively surround the beam pipe around the collision point, and are inside our large red 0.5 T solenoid). These are:
SPD – Silicon Pixel Detector,
SSD – Silicon Strip Detector,
SDD – Silicon Drift Detector;
These three detectors have two layers each and make up our Inner Tracking System (ITS), providing information about the path of particles passing through them.
TPC – Time Projection Chamber;
Our outer tracking detector – a huge gas filled chamber that helps us to identify particles by mapping their trajectory in three dimensions.
TRD – Transition Radiation Detector,
TOF – Time of Flight,
EMCAL – ElectroMagnetic CALorimeter,
PHOS – PHOton Spectrometer,
HMPID – High Momentum Particle IDentification;
These detectors work in various ways using various new technologies to identify a variety of particles with a range of momenta. I will detail them later, but collectively, ALICE has probably the most powerful particle identification in the world!
ACORDE – ALICE’s COsmic Ray DEtector;
This guy is pretty separate from the rest – it is a collection of scintillator detectors and it sits on top of the L3 magnet and looks for cosmic rays. It isn’t designed for collision measurements, but it has helped the detector alot over the past few years!
“Forward” detectors (Put simply, detectors placed such that they measure particles coming off more horizontally from the collision. They would be missed by the central barrel). These are:
FMD – Forward Multiplicity Detector,
PMD -Photon Multiplicity Detector,
ZDC – Zero Degree Calorimeters.
These provide various important information including timing to identify background events, extra triggers for event selection, and the particle density in this “forward” region (which is rather uncertain at new energies!)
“The ARM”. The muon arm is placed behind the “barrel”, and consists of the largest warm dipole magnet in the world, designed to bend the trajectory of “forward” muons that come from a collision, a thick absorber designed to block anything that isn’t muons, and these detectors:
MUON TRG – Muon TRiGgers,
MUON TRK – Muon TRacKers.
By selecting events when muons pass through the triggers and tracking their trajectory through the dipole magnet with the 5 tracking points, ALICE can spot particles decaying to muons (like the J/Psi).
Multiply that lot of detectors by the challenge each sub-group faces in ensuring their detector is functioning correctly on time for data-taking, and you can see how much of an achievement today is for ALICE.
Just after this great achievement however, the LHC beam was dumped. I have it from my insider knowledge (that could just be rumour so don’t take this as Gospel) that the LHC operator saw that some screens had crashed so they rebooted the system…which simultanously triggered the dump. Oops! As silly as this sounds, it is a fantastic safety feature – I find it reassuring that if for any reason control is lost, the beam will be safely dumped within a few turns of a near-light-speed proton 🙂