by Panos Charitos. Published: 08 July 2012

Professor Adam Kisiel - Warsaw University of Technology.

Adam Kisiel pursued his PhD at the Faculty of Physics of the Warsaw University of Technology and then moved to CERN in 2007 as a post-doc at Ohio State University. He continued his research at CERN when he was granted a research fellowship. He has recently moved back to Poland where he was elected full Professor at the Warsaw University of Technology; at the same he continues working on ALICE with a very active group of students on the study of particle correlations by using femtoscopic techniques.

Alice Matters: I would like to start this interview by asking you about femtoscopy. Could you explain to us what it means and what it shows us?

Adam Kisiel: Interferometry was originally used by astrophysicists to study the magnitude of stars. It was later found that the same kind of interference that you have between photons in astronomy you can actually observe between pions. This technique is based on the convolution of two basic characteristics: the relative momentum distribution and the relative spatial distribution. In principle the correlation function reflects the two so you can measure one and then extract the other.

Astronomers usually measure the spatial distribution and then extract the momentum size of the star which is converted to the angular size of the star. In femtoscopy it is the other way around as you measure the relative momentum distributions and then you can extract the size of the source. This is important since this size is not measurable in any other way. Measurements of flows or spectra are all based in some way on measuring the momentum distributions of particles.

Now with femtoscopy we are also measuring momentum distributions, but our aim is to extract the spatial distributions of sources (of the order of a femtometer = 10-15 meters). It was originally done for pions since they are the most abundant particles. This allowed us to make precise measurements.

Simply measuring the size is not very interesting for us because we know it, for a Pb atom it is around 6 fermi (femtometers). What is interesting is to measure the size as a function of other variables. On that note there was a prediction that If we measure the size as a function of the transverse momentum of the particle we should see the size of the source decreasing and this change is understood in terms of hydrodynamics. It showed that the system behaves as a fluid which means that we can treat it as a piece of matter. If we didn’t have a piece of matter we couldn’t say anything about the state of this matter as there would be only individual particles. That was the next thing that we tried to do, namely, to perform this analysis versus the momentum of the particles.

A.M So have such measurements been done in the past (at RHIC) and what have we learnt so far? Adam Kisiel: Some time ago such measurements were done at RHIC which we repeated here at LHC and we saw that they nicely agreed with the hydrodynamic picture. At the beginning RHIC found that the sizes didn’t match those predicted by hydrodynamics. For a very long time this was a big puzzle because roughly the general size was correct but when you were looking for details in 3D space you could see that things were behaving differently from a purely hydrodynamic model prediction. Specifically you noticed in the model a deviation from the value of 1 for the Rout/Rside ratio, while the data stayed around unity.

Figure 1. Rout/Rside ratio for hydrodynamic models (lines) and RHIC data (points).

This kind of behaviour was predicted for a system that would emit particles for a long time. If we create a big system we would expect it to live long. But then it turned out that the ratio was close to 1 which means that the system may live long but when it starts to fall apart it falls apart immediately and converts to particles right away. This points to the need for changes in the theory and shows us something new that we didn’t know before. Specifically it points to the fact that there is no first order transitions (as it is the case when liquid water turns into ice). In this kind of transition there is a period in which the temperature doesn’t change and the only thing that happens in your system is that water is turning into ice. Nothing changes except the state of matter. For Quark Gluon Plasma this transition is the conversion to individual particles. However, it was found that this phase doesn’t take place in high energy collisions. Instead everything changes very rapidly, the quark gluon plasma turns into hadrons immediately. This also implies that you have a very strong collective movement of your particles. So I would say that there are two things that you have to take into account: the first is the rapid phase change and the second is the strong collective movements. Both of them are extremely important in the study of QGP.

A.M. And what is the difference between doing femtoscopy for baryons and femtoscopy for pions?

Adam Kisiel: Baryons in principle are heavier than pions. If the hydrodynamics is correct, then baryons should move in the same way as pions despite the fact they are heavier as they are forming the same type of system. On the other hand you could have other models which are not based on hydrodynamics which could predict different behaviours because of the different mass. That means that it is important to check whether in the case of baryons you are getting the same type of behaviour. This will actually be the topic that ALICE will be presenting at the Quark Matter conference this August. Perhaps I should also mention that we repeated our analysis with Kaons which in terms of mass are sitting in the middle since they are mesons like pions but are heavier as they carry almost half proton’s mass. This is an analysis we did with my colleagues from the University of Ohio and the results will also presented in the Quark Matter conference.

A.M So what exactly are you measuring between these different types of particles (i.e .pions, kaons, baryon)?

Adam Kisiel: What we have to measure is the momentum of the particle (a vector with three components). Equally important is to know what type is this particle. There are many types of measurements that don’t really care whether it is a pion, a kaon or a proton. But for our purpose it is crucial to identify each particle because the effect of femtoscopy – i.e. the symmetrisation of the wave function- happens only for identical particles. That is why it is important to do this analysis in ALICE as ALICE has a very high capability of identifying particles. Starting from a very low pt it has detectors that allow us to thoroughly identify each produced particle. Another important thing is that it can measure each track and say whether it was a kaon, proton or pion. Many experiments can do this only at a statistical level (i.e. they say that out of 10.000 particles 150 were protons, 3000 were Kaons but cannot identify each track). ALICE gives us this capability).

Figure 2a. 3D representations of femtoscopic measurements from pions

Figure 2b. 3D representations of femtoscopic measurements from kaons- Note the assymetry

Then you calculate the relative momentum in two ways: first you do this distribution for pairs of particles from the same event which means that they are correlated or can be correlated. Then you do the baseline distribution combining particles from different events so that they can’t be correlated. Then you divide these two and you get the correlation function. This is the measurement we are actually aiming at. In principle this is very simple although it gets more complicated when you move to three dimensions. The rest has to do with the way you interpret your measurements.

A.M. Where do we stand in terms of determining the emission function and the spatial dimensions of the source?

Adam Kisiel: I will try to answer this question by referring to previous data that were taken in 2001 since our current data haven’t been published yet. So in Figure 3 you can see the data we got from STAR in 2001 and how they were fitted with theoretical calculations done by Scott Pratt in 2009. In his calculations he added many different effects and he managed to reproduce the original data and as you can see it took us many years until we get the correct interpretation from the model.

Figure 3.The experimental data from various experiments (coloured dots) and the black dots which fit them following the theoretical modifications by Scott Pratt (2009)

In principle the results from heavy-ion collisions at LHC are not qualitatively different from those from RHIC. Of course you have a definitely bigger system with higher flows but you still have to include all the effects that we included in analyzing our data from RHIC.

Perhaps you could see that these effects are now stronger since the initial flow is larger but it seems that by taking the same effects that we observed in analysing data from RHIC we can also describe the data from ALICE. We don’t have new effects coming but now we can move to more exotic things like baryons which we can check in more detail.

Another important thing is that ALICE allows us to study better the interaction coefficient for more exotic particles. For example you can study the interaction coefficient between protons and ? or protons and anti-? (which is poorly known) or more exotic particles like ? or ?. We need to study these interactions not only for HEP but also for Astrophysics. In ALICE we have quite a few exotic particles produced as we have a larger system and of course we are getting more particles per event which is another important feature in studying the correlation function. Having three times more particles in each event means that you have nine times more pairs. In this respect ALICE is doing very well: having many more particles of this mass so we are able to measure the correlation function, after only two years of data taking, at the same level as STAR after ten years of data taking. So ALICE is very competitive in that sense and that’s what we are trying to show in Quark Matter conference.

A.M: As far as I am aware there has been a long tradition of science in Poland. How would you describe the current situation of scientific discourse in the Polish public sphere?

Adam Kisiel: There is a long tradition in physics and science in general in Poland. I think that Poland has reasonably good education in science. So there is interest in these issues. However many people who have science skills are moving to other sectors where they can also use their scientific skills. This might be a little bit dangerous. However, I often see news about CERN and LHC and I would say that physics science and hard sciences are highly respected.