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Shooting through a brick January 17, 2007

Posted by dorigo in physics, science.

High-energy particle detectors have always been ambitious endeavours,  as a whole new technology had to be invented and developed in the course of the last fifty years to study the products of particle collisions. Largely, every new experiment built on the experience of the previous ones, and made the design a bit more daring.

Indeed, if we examine the ATLAS and CMS detectors under construction at CERN, we find several concepts pushed to extremes. Almost any of the features of these giants is awesome: from their sheer mass, which implies not only an industrial-sized production chain for the manufacture of its basic components, but also mind-boggling installation and infrastructure puzzles as the pieces are to be lowered in underground caverns and assembled in confined spaces; to the amount of cutting-edge electronics they are stuffed with, and the issues connected with powering it up and drawing away the heat it generates; to the tens of square meters of silicon microstrip and pixel detectors equipping their core; to the challenge of reading out, processing, and storing the resulting gigabits of information every 25 nanoseconds. 

The basic concept of a collider detector, however, has not changed appreciably since the sixties. In order to be able to completely reconstruct the particles produced in a high-energy collision,  several different devices need to be put together in a onion-like structure which has to be as hermetic as possible, while still allowing the incoming projectiles to enter the collision point. The inner shell is usually a “tracker“, a low-density device which measures the trajectory of charged particles without destroying them; while the outer part is called “calorimeter“, and it measures the energy of both charged and neutral bodies impinging on it by stopping them in sufficient amounts of heavy material. Outside of the calorimeter, “muon detectors” wrap up the whole structure to detect these penetrating particles, the only ones besides the invisible neutrino which can traverse the calorimeters unscathed.

Some things, however, work well only at their natural size, and scaling them up creates troubling situations. Take a giraffe, for instance: it stands tall at 5 meters of height, but its height still allows it to stand up and reach food on the trees, walk, and perform all the tasks that allow it to live a decent giraffe life. However, if you tripled its size, its weight would grow by about a factor of 27, and such elegant legs would not be able to sustain the body anymore. What’s worse, the delivery of its babies would be complex, since these would fall from high enough to have few chances of survival.

The CMS tracker is entirely made of layers of silicon sensors. These are wonderful devices, capable of detecting the passage of charged tracks with a position precision of the order of 10 micrometers. The figure on the left shows a part of the silicon tracker of CMS being assembled in a technical building at CERN. You can see the coaxial layers of silicon sensors crowding the interior of the large tracker barrel.

In previous experiments four or five layers of silicon in the proximity of the particle collision were enough for a precise tracking, and outside of the silicon the detectors usually featured gas chambers, where charged tracks left trails of ions and could survive the trip before hitting head-on with the dense material of the calorimeter.

In CMS the amount of material particles need to traverse before reaching the calorimeter is larger than in previous experiments. The figure on the left shows the “material budget” of the tracker, in units of radiation lengths (the y axis) as a function of particle rapidity (the x axis) – the latter is basically the angle of the particle path with respect to the beam: 0 means orthogonal, -4 or +4 is quite close to the beam in opposite directions. 

A radiation length is the amount of material which causes an electron to reduce its energy by 65%. Even more critically, a radiation length is seven ninths of the mean free path for an energetic photon: what that means is that a sizable fraction of photons produced in the core of CMS do not make it to the calorimeter, converting instead into an electron-positron pair.

A heavy tracking device is not a desirable thing. As a particle crosses it, it is subjected to interactions which scatter it from a straight path – making it hard to track it precisely. When discussing the performance of the new CDF II calorimeter a few years ago, Larry Nodulman said “this time we are shooting through a brick!”. He was referring to the heavier silicon detector the upgraded CDF II detector had with respect to the old CDF, which worsened the performances of electron and photon identification.

Can we manage to live with one radiation length of material in front of the CMS calorimeter ? We can deal with it, but it poses further challenges.

One of the very few ways to detect the production of a Higgs boson if its mass is below 120 GeV in CMS is to reconstruct the decay of the Higgs to a pair of high-energy photons, a rare but clean process. Indeed, the very design of the calorimeter was steered by the need to precisely measure the energy of the two photons. A full radiation length to travel through before being measured makes things harder for the photons needed for the reconstruction of the Higgs decay.

Will the giraffe deliver dead siblings ? Well, the situation is not that dramatic – but it is possible that many cubs will not stay alive when they hit the ground. We will know very soon!  



1. Kea - January 17, 2007

Goodness, Tommaso. You’re becoming a prolific writer. I only just saw your Globular Cluster post. Yes, this CERN business is very, very exciting. Can’t say I’ve seen a giraffe give birth … but I’ll be watching your CMS (+ATLAS etc) posts closely!

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