A lot of the articles in the news talk about the physics we’re trying to learn, but there’s not much discussion of how we do it. I spend most of my day thinking about our detectors, how they work, and how to interpret the data. Our detectors are the way we see, hear, feel, smell, and taste what goes on in proton-proton and lead-lead collisions. Just like the Mars rover explores places that are uninhabitable to people, our detectors explore places that are too small for us.
ALICE is made up of several subsystems, each of which helps us sense collisions in a different way. Below is a picture of ALICE with each subsystem labeled:
Some detectors are like our eyes – they help us see particles coming out of the collision. These are our tracking detectors. The main tracking detector is the Time Projection Chamber (TPC). We also have an Inner Tracking System (ITS), comprising three different silicon detectors, the Silicon Strip Detector, the Silicon Drift Detector, and the Silicon Pixel Detector. The ITS is a bit like our glasses – we can see particles with just the TPC, but with the ITS, the picture comes into sharper focus. Tracking detectors tell us the momentum and spatial location of particles that go through our detector.
Some detectors are our taste buds – they help us determine the flavor of the particles we’re measuring. A lot of different types of particles are produced in both a proton-proton collision and a lead-lead collision. The Time-Of-Flight (TOF), the Transition Radiation Detector (TRD), and the High Momentum Particle Identification Detector (HMPID) are all designed to identify particles. These detectors all work by measuring a particle’s velocity. Momentum is velocity times mass, so if we know the velocity and the momentum of a particle (which we can get from the tracking detectors), we can determine its mass and therefore figure out what kind of particle it is. The TOF measures the velocity of particles by measuring how long it takes for a particle to reach the TOF. Since velocity is the change in distance over time and the distance traveled is known, this measures the velocity of the particle. Here you can see one of our physics performance plots showing different particles identified by the TOF:
The x-axis is the momentum, as measured by the TPC and the ITS, and the y-axis is the ratio of the velocity to the speed of light in a vacuum. Pions (π) are the lightest particle (140 MeV/c2) so at a given momentum, they have the highest velocity. Protons (p) are the heaviest (938 MeV/c2) particle visible in the plot above so at a given momentum they have the lowest velocity.
The HMPID and the TRD both work on the same principle. The speed of light in a vacuum is constant, but the speed of light in a medium can be lower. For example, the speed of light is lower in water than in air – this is why images get distorted when you look through water. If a fast particle moves through a medium faster than the speed of light in that medium, it will emit photons – called Cherenkov radiation – until it slows down to the speed of light in the medium. At a given momentum, lighter particles go faster, so lighter particles will emit photons at a larger angle relative to their path. The medium in the TRD is optimized so that only electrons (0.5 MeV/c2) will radiate photons, so the TRD can be used to distinguish electrons from everything else. The HMPID is a ring imaging Cherenkov detector. The photons emitted by a particle are emitted in a cone and the radius of that cone depends on the velocity of the particle. The HMPID is optimized for distinguishing pions, kaons, and protons. Here you can see the signal from the HMPID:
The x-axis is the momentum and the y-axis is the angle of the cone of light emitted by the particle. At a given momentum, a pion is going faster than a kaon or a proton. The radius of the cone of light emitted by the particle is larger the further the particle’s speed is from the speed of light in the medium, so at a given momentum the pion band is above the kaon band, which is above the proton band.
The tracking detectors, the TPC and the ITS, can also identify particles. They work by measuring how much energy a particle loses as it travels through the detector. A heavier particle will loss more energy than a light particle. Think of one of those ball pits for kids. If you threw a tennis ball in, it would knock some of the balls out of the way. If you threw in a bowling ball, a lot of balls would get knocked around. We know the bowling ball lost more energy than the tennis ball because the lighter balls got knocked around more. We can distinguish between heavier particles and lighter particles like this. If the TOF, the HMPID, and the TRD are the way we taste the particles created in the collision, the ITS and the TPC help us smell them. Below you can see the signal from the TPC:
The x-axis is rigidity, which is the momentum over the charge. Charge is in units of the electron charge. All of the particles here have a charge of ±1. Positively charged particles are on the right and negatively charged particles are on the left. The y-axis is proportional to the energy lost by the particle in the TPC. We see the same three particles we saw before – pions, kaons, and protons – but now we also see deuterons and tritons. At a given momentum, heavier particles lose more energy, so as you go up the y-axis the mass of the particles increases.
My last post was on the Electromagnetic Calorimeter (EMCal). A calorimeter is used to measure particles’ energy. This is a way of feeling the collision – it’s like laying in the sun. When you lay in the sun, you don’t feel photons hitting you but when photons hit you, they warm you up. Particles hitting the calorimeter do the same thing – they hit the calorimeter and deposit their energy. (Everything loses energy except muons – muons travel right through the calorimeters.) We look at the energy deposited in the calorimeter to determine how much energy the particle had. (See my post on the electromagnetic calorimeter for more details.) We have two more calorimeters in ALICE, the Photon Spectrometer (PHOS) and the Zero Degree Calorimeter (ZDC). The PHOS is optimized to measure photons. The ZDC is a calorimeter very close to the beam pipe far away from the interaction point, at an angle close to zero degrees from the beam pipe. The ZDC is useful in lead-lead collisions for both measuring nucleons which did not participate in the collision. These particles are called “spectators” and are not deflected by the magnetics that keep the beam in the beam pipe because the spectators do not have the same charge to mass ratio as lead nuclei. We can figure out of the collision was head-on or just glancing using this information.
We hear the collision in the VZERO, a scintillator detector. When a particle hits it, the scintillator emits photons and we know there was a collision when we see these photons. Think of it as like a fire alarm – it’s what tells us there was a collision.
There’s a few detectors that don’t really fit into this metaphor but I want to mention them anyways. The Photon Multiplicity Detector (PMD) measures the multiplicity of photons at angles close to the beam pipe. The muon arm measures muons, the heavy cousin of the electron. The ALICE Cosmic Ray Detector (ACORDE) is designed to trigger on cosmic rays so that the rest of ALICE can be used to study cosmic rays. Cosmic rays were used to calibrate ALICE before the first collisions at the LHC.
Each of these detectors helps us understand proton-proton and lead-lead collisions in a different way. When we put them all together, we have a sort of Quantum Chromodynamics rover that helps us explore exotic places – the insides of protons and nuclei – that are near us all the time.
Tags: ALICE, detector, heavy ion physics
“you don’t feel photons hitting you but when photons hit you”
Well, really….we are immerse on a sea of photons and hadrons that populate a cosmic electroweak superconductor- the space.
Is it just me or are some of the Pions in Figure 1 a bit faster than light? Is this a proper Graph? May this be some artifact of the Hardware (not so exact when measuring some particles)?
Best regards
Leon
Hi Leon,
I would speculate that it is not the speed of light in
vacuum but the speed of light in the detector (which is
lower). It is possible that particles (but not electromagnetic waves) travel faster than the mediums speed of light. In this case you get Cherenkov radiation.
“the y-axis is the ratio of the velocity to the speed of light in a vacuum”
I agree with josh222, it can’t be the speed of light in a vacuum, unless the graph is not proper.
I have a quastion. What is that square root of s in the first graph?
Is it the total energy of two colliding particles in the center of momentum frame?
Hi all – sorry for taking so long to reply – I was traveling.
Regarding the first graph – There are measurement errors (see note on “errors” below) on everything – on the momentum of the particle and on the measurement of the speed of the particle. Most things are measured with roughly a Gaussian distribution about their true value, which means that sometimes we will measure the speed of a particle to be faster than the speed of light in a vacuum. This is fine, as long as it is within our experimental resolution.
A large part of the job of a high energy physicist is determining the error on our measurements. Getting a measurement of something is not nearly as hard as knowing how accurate that measurement is.
The TOF is roughly 4m from the center of the beam pipe. (Specifically it goes from 370cm to 399cm from the center of the beam pipe, but it’s easier to do calculations with 4m.) The speed of light is 3×10^8 m/s. This means that it takes something traveling at the speed of light a minimum of 13 nanoseconds (13×10^-9 seconds) to get to the TOF. There is an intrinsic timing resolution of 50 picoseconds (50×10^-12 seconds) in the TOF. This corresponds to an intrinsic resolution of 0.4% of the speed of light – not enough to explain the difference seen on the plot.
But remember – we have a charged particle moving in a magnetic field. A charged particle moving in a magnetic field curves. A pion with a momentum of 0.5 GeV/c goes more than 4 meters to get to the TOF. To measure the speed right, we have to measure the momentum right so that we know how far the pion traveled. And the particle is losing energy as it travels through the detector (this is how we identify particles in the TPC and the ITS) so technically the radius of curvature of the particle is changing as it travels through the detector – we have to measure this right to get the speed right too.
On top of that, we have to calculate where the collision happened – where the particles are coming from. Most of the collisions occur near the center of the detector but we get collisions within +/- 10 cm of the center (along the beam pipe.) There is an experimental error on this, too – which leads to an additional error on the distance traveled by particles reaching the TOF.
This is how you can get to the experimental resolution seen in the plot above. There are several measurements which go into the measurement of the velocity and each piece has an error. The fact that we measure some particles to have a speed above the speed of light – a speed which is not physically possible – is an indication of how well we can measure the speed in this way. (Note that the average for any particle is below the speed of light.)
Note that when I say “error” I don’t mean mistake. I mean the resolution – our intrinsic ability to measure a quantity. If I told you I drive 20.23546 miles to work every day, you would not believe me. You might believe I drive 20.2 miles to work, but how could I reasonably measure my trip with an accuracy of 0.00001 miles (about 1/2″)? I would change the distance of my trip by more than that if I change lanes.
And sqrt(s) – this is a bit of jargon. It means the energy in the center of mass. (The origin is this – http://en.wikipedia.org/wiki/Mandelstam_variables). It is very difficult, if not impossible, to show public data plots without any jargon on them at all.
I enjoyed Christine Nattrass’ highly informative ‘Tasting Quark Soup’, and hope this trend continues. The collider data processing is converging toward the threshold of quark-gluon-hadron coherence where quantum effects originate, a big step with vital consequences. Research progress depends on the data density of the atomic topological function used to analyze the structural details of the waves, energy, and force fields being analyzed. Recent advancements in quantum string science have produced the picoyoctometric (10^-36 m), 3D, interactive video atomic model imaging function, in terms of chronons and spacons for exact, quantized, relativistic mechanics. This format returns clear numerical data for a full spectrum of variables. The atom’s RQT (relative quantum topological) data point mapping function is built by combination of the relativistic Einstein-Lorenz transform functions for time, mass, and energy with the workon quantized electromagnetic wave equations for frequency and wavelength.
The atom psi (Z) pulsates at the frequency {Nhu=e/h} by cycles of {e=m(c^2)} transformation of nuclear surface mass to string forcons with joule values, followed by nuclear force absorption. This radiation process is limited only by timespace boundaries of {Gravity-Time}, where gravity is the force binding space to psi, forming the GT integral atomic wavefunction. The expression is defined as the differential series expansion of nuclear output rates with quantum symmetry numbers assigned along the progression to give topology to the solutions.
Next, the correlation function for the manifold of internal heat capacity energy particle 3D string-structural functions is extracted by rearranging the total internal momentum function to the photon gain rule and integrating it for GT limits. This produces a series of 26 topological waveparticle functions of the five classes; {+Positron, Workon, Thermon, -Electromagneton, Magnemedon}, accounting for each energy intermedon of the 5/2 kT J internal energy cloud.
Those 26 energy data values intersect the sizes of the fundamental physical constants: h, h-bar, S.B. delta, nuclear magneton, beta magneton, k (series), 5/2 k, 3/2k. They quantize atomic dynamics by acting as fulcrum particles. The result is the TCD-CRQT exact picoyoctometric, 3D, interactive video atomic model function, responsive to software application keyboard input of virtual photon gain events by shifts of electron, force, and energy field states and positions. This system also gives a new equation for the magnetic flux variable B, which appears as a waveparticle of varying frequency.
Images of the h-bar magnetic energy waveparticle of ~175 picoyoctometers, and the workon, h, are found online at http://www.symmecon.com/GUPPP.docx. CRQT conforms to the unopposed motion of disclosure in U.S. District (NM) Court, 04/02/2001, The Solution to the Equation of Schrodinger.
(C) 2010, Dale B. Ritter, B.A.