Aim Of Exercise
In this exercise you are going to work with real data from the ALICE experiment.
On your computer screen, you will be able to visualise how the events with their many particles appear in the event display.
You will also learn how to identify some very special particles, called strange particles, by observing the way their decay patterns look in the event display.
Click here to see an event display of a pp event at 900 GeV containing a strange particle decay
The aim is to count the total number of strange particles (say in 100 events) and compare them to the total number of non-strange particles (in the same 100 events).
The ratio of strange to non-strange particles, called the "strangeness enhancement factor", is a very interesting measurement. In fact, theory predicts very different values of this "strangeness enhancement factor" in normal matter and in a special kind of matter called Quark Gluon Plasma (QGP).
Click here for more details on the QGP.
Therefore the measurement of this ratio can give us information on the creation of such a matter in some collisions.
The family of strange particles includes particles with one or more strange quarks, such as the ones called K0s, Lambda, Xi, Omega, which you will learn how to identify.
Click here for more details on strange particles
Where Do Strange Particles Come From?
The particles that make up our normal world, like protons for example, are made out of "up and down quarks"; they do not contain strange quarks.
So, where do strange particles come from?
During a high energy collision, several known processes, well described by theory, are at work and some of them create strange particles. These processes are then described in simulation programs which can be used to generate "simulated data" (these programs are called "event generators").
If nothing else but known processes occur during a high energy collision, then, the number of strange particles measured in the "real data" and their ratio to non-strange particles will be identical to the ones we find in the simulated data (produced by the MC event generators).
Strangeness Enhancement As Signature Of QGP
However, according to QCD, the theory of strong interactions, something exotic may happen during some collisions, if the temperature and density rise above certain extreme limits. In fact, a special type of matter, called Quark Gluon Plasma may be created. According to Standard Model theory this plasma came into existence at the very beginning of the universe, some nnn seconds after the Big Bang.
In this type of matter, quarks and gluons are not imprisoned inside particles, but they can move freely; among them plenty of strange quarks (and even charm and beauty quarks). The number of s quarks relative to u and d in such a state of matter is enhanced as compared to normal matter.
This type of matter cannot be kept in a bottle; it expands and cools down rapidly. When the temperature and density drop below the limits, quarks and gluons are forced to combine together and create particles which fly out towards the detectors of the experiment. Among those, are strange particles, which imprison strange quarks.
Where To Look For QGP
In order to obtain conditions of high temperature and density we collide nuclei at very high energy.
The higher the energy and the bigger the colliding nuclei the better the chances of obtaining such conditions. This is why we use the bigger (heavier) nuclei, such as Pb.
To see the simulation of a Pb+Pb collision creating a QGP state of matter click here. In the overlapping region, where temperature and density grow, the QGP is created; the free quarks are represented by the colorful balls (as they carry color). As time passes, they recombine to create neutral color particles, represented by white balls.
However, some very high energy pp collisions where very many particles are produced (high multiplicity events) could also provide the necessary temperature and density conditions for the creation of Quark Gluon plasma.
It is interesting therefore to perform this measurement for (a) normal (minimum bias pp events (b) high multiplicity pp events (c) PbPb events and compare the results (i) across the different systems (ii) with simulated data produced by data generators that describe the known processes (iii) with simulated data produced by data generators that describe a QGP scenario
click here to see a:
Results And Conclusions
The first step is to find out if the ratio of strange to non-strange particles in the real data is in agreement with this ratio in the simulated data produced by an data generator that describes the known processes or if it is enhanced (and by how much).
For the minimum bias pp events at 900 GeV, this ratio is slightly enhanced but not enough to conclude that GQP was created.
Based on these results, theorists will fine tune their knowledge about known processes and better describe them in the simulation programs.
The next step is to do this measurement for high multiplicity pp events and finally for PbPb events
How To Trace And Identify Particles
1.click here to see the different detector elements of ALICE experiment.
2.Here you can see a collision in the center of the experiment and how particles propagate through the detectors leaving their traces.
3. Here, you can take a closer look to the main detector of ALICE, called Time Projection Chamber (TPC).
To understand how to identify strange particles you will need to learn how particles propagate through the detectors and what we can learn from that about their identity.
click here to see a photograph of particles going through a "streamer chamber"
Because each kind of particle has a very special way of leaving its marks going through matter different detectors are optimised to identify different kind of particles such as e, p, muons etc
In general, particles, and in particular charged particles that we are interested in here, leave their traces in the detectors, as they fly through them in a similar way as airplanes sometimes leave a white line in the sky
When we see such a white line in the sky, we can deduce that an airplane went that way; in a similar manner we reconstruct the particles trajectory through the detectors.
To identify charged particles we insert the detectors in a magnetic field and we make use of the following features: (i) particles of opposite charge bent at opposite directions (ii) faster particles (bigger momentum) bent less; slow particles (small momentum) have big curvature
heavy particles (like protons) leave a thick trace; light particles, like pions leave a fine trace
for details on particle momentum, click here
The ALICE experiment has many different kinds of detectors to identify many different kinds of particles; click here to see the different detector elements of ALICE experiment.
Here you can see a collision in the center of the experiment and how particles propagate through the detectors leaving their traces.
Here, you can take a closer look to the main detector of ALICE, called Time Projection Chamber (TPC) This detector is used to identify charged particles, like protons and pions which are then used to identify the strange particles that you are looking for.
How To Identify Strange Particles
A needle in a haystack!
To identify strange particles we relay on their very characteristic decay patterns which become very clear when the decay occurs in a magnetic field.
Strange particles are rare; they are not produced in every other event.
What's more, they are produced among many other particles.
Click here to see a Xi decay pattern among all other particles in a Pb+Pb simulated event.
To make it more easily visible, on the side of the event with the Xi decay, particles have been removed from the event display.
Strange particles are not stable (they do not live long). They decay finally into other particles that are stable, like protons, pions Different strange particles have different characteristic decay patterns.
K0s, Lambdas and antiLambdas are neutral particles and decay to 2 charged daughter particles; one positive, one negative (charge conservation).
Because in a magnetic field, opposite charged particles go to opposite directions the charged daughter particles give a characteristic V shape; this is why they are called V0s.
So, we look for a V shape (2 charged particles of opposite charge coming from the same point (secondary vertex); the V can be symmetric or asymmetric, as you will see below.
Click here to see the decay patterns of a K0s, a Lambda, an antiLamba, and a Xi
A K0s decays to a pi+ and a pi- because the daughter particles are of the same mass, they bend the same way in the magnetic field, thus giving a symmetric V shape
K0s --> pi- + pi+ symmetric decay
A Lambda decays to a pi- and a proton
because the proton is heavier than the pi-, the V shape is asymmetric
Lambda --> pi- + proton asymmetric decay
An !AntiLambda decays to a pi+ and an antiproton
because the antiproton is heavier than the pi+, the V shape is asymmetric
The difference between the Lambda and !AntiLambda decay pattern is that the heavier particle (proton, antiproton) bends at opposite directions because of the opposite charge.
The Xi (and !AntiXi) is charged and decays into a Lambda and a pion; the Lambda then decays into a pi and proton; this is why it is called "cascade" decay.
br/> Click here to see the decay patterns of a Xi.
In addition to the decay topology pattern, we find the mass of the mother particle; this identifies the strange particle uniquely.
br/> How do we calculate the mass of the mother particle?
We calculate the mass of the mother particle, from the mass and momenta of the daughter particles; relying on energy conservation laws we apply the following formulas.
add formulas here
How do we know the momenta of the daughter particles?
Tracking detectors (like TPC) are used to measure the curvature of particles in the magnetic field and thereby their momentum
br/> Click here to see one of the first pp events in the TPC recorded on 10 Dec 2009.
How do we know the mass (identity) of the daughter particles?
Particle identification detectors (like TPC, TOF etc) are used to measure different particle properties that finally reveal the Particle IDentity (PID)
br/> Click here to see the results of PID from TPC and/or TOF detectors?
Based on this knowledge and using the above formulas we obtain the mass of the mother particle.
Every particle has a well defined mass (reflecting uniquely its identity) and listed in the Particle Data G (for example the PDG mass for a K0 is 497,6 MeV/c2).
So, every time the above calculations give us a result of 497,6 MeV/c2, or close to it, we increase the number of K0s counts.
Precision of measurements (for higher level?)
However, all the above measurements involve some precision. Even if you measure the same piece of wood, one meter long, 1000 times, you will never get the same result a 1000 times. Therefor the mass of the mother particle, say K0s, calculated in different events, may not be exactly the same; but will be almost the same.
On the other hand, by mistake, we may pick up a particle of the event that did not really originate from a strange particle decay; then the above calculations will result in a mass for the mother particle which is not close to the real (PDG) one. This faulty result is known as background.
For every event we analyse, we insert the resulting mass of the mother particle in a histogram. The correctly found K0s will give a peak around its correct PDG mass; the faulty ones will give a (usually flat) background, left and right from the peak.
Click here to see such a histogram for K0s.
Subtracting the background, and counting the number of entries under the peak, we obtain the number of K0s.