Three More Students Complete Ph.Ds on RHIC Experiments
Three more Ph.D.s were granted for work on RHIC experiments (These are in addition to those announced earlier this year in the September 25 issue of RHIC News). They are:
Yuting Bai - STAR
Advisor: Raimond Snellings
Institute: Nikhef, Amsterdam, The Netherlands
Thesis: Anisotropic Flow Measurements in STAR at the
Relativistic Heavy Ion Collider
In the thesis, the system created in collisions of heavy ions
at the Relativistic Heavy-Ion Collider (RHIC) is studied using
the STAR detector. These heavy-ion collisions offer the unique
opportunity to gain better understanding of the properties of
Quantum Chromo Dynamics (QCD) at very high energy densities,
energy densities which also prevailed in the early universe a
few microsecond after the Big Bang. In non-central heavy-ion
collisions anisotropic flow, in particular elliptic flow,
provides access to the equation of state of the created system.
In fact the observed large elliptic flow at RHIC and its close
agreement with ideal hydrodynamics calculations formed the basis
for the sQGP discovery and the claimed perfect liquid behavior.
In the thesis, using the large number of Au+Au collisions
recorded at sqrt SNN = 200 GeV, robust charged
particle elliptic flow values, from a four particle cumulant
analysis, up to 8 GeV/c are obtained. The still sizable elliptic
flow observed above 6 GeV/c is consistent with parton energy
loss and evidences the formation of very dense matter. With the
particle identification extended to higher transverse momentum
(pt > 2.5 GeV/c) using the relativistic rise of the
specific ionization energy loss in the STAR TPC, we measure the
elliptic flow of pions and protons at intermediate p_t. The
characteristic low-pt mass ordering of the elliptic
flow and its break down at intermediate pt, first
observed for Ks0 and Lamda, is with this
analysis also seen for the pions and protons. These measurements
combined confirm the baryon meson scaling of the elliptic flow
and are in agreement with the constituent quark number scaling.
Furthermore, in the thesis we show that the measured integrated
elliptic flow values as function of center of mass energy,
collision centrality and also the ratio v4/v22
do not follow the predictions from ideal hydrodynamics which
might indicate that the hydrodynamic limit is not yet reached at
RHIC.
Jonathan Bouchet - STAR
Advisor: Sonja Kabana
Institute: Subatech, France
Title: Performance of the Silicon Strip Detector of the STAR
experiment at RHIC
The Silicon Strip Detector (SSD) is a fourth
layer of silicon detectors of the STAR experiment, thus
completes its inner tracking device. The goal of STAR is to
study heavy ions collisions in order to probe the existence of
the QGP, a deconfined state of nuclear matter. Strangeness
enhancement, such as KS0, Λ, Ξ and Ω
particles production, has been proposed to sign the formation of
the QGP. STAR central tracking device is a large cylindrical
Time Projection Chamber (TPC), which provides momentum and
allows particle identification. Closer to the beam axis, the
inner tracking system is dedicated to the localization of the
primary vertex and the reconstruction of secondary particles. It
includes the Silicon Vertex Tracker (SVT) arranged in 3 layers
surrounded by the SSD.
The SSD was proposed to enhance the tracking capabilities at
mid-rapidity by providing a better connection between
reconstructed tracks in the TPC and the SVT. It was developed by
the Laboratoire de Physique Subatomique et des Technologies
Associées (Subatech) in Nantes, and the Institut de Recherche
Subatomique (IreS) in Strasbourg. It uses double-sided silicon
microstrip sensors and consists of 320 detector modules arranged
on 20 ladders, forming a barrel at a radius of 23 cm from the
beam, inserted between the SVT and TPC. The design of the sensor
has been constrained by requiring a good position resolution and
by minimizing the number of ambiguous hits expected in the high
multiplicity environment of central collisions and its compacity
was achieved by using a novel Tape Automated Bonding method to
connect the silicon wafers to the front end electronics.
The work done in this thesis presents the intrinsic performances
of the SSD and its impact on the inner tracking system
performances by studying Cu-Cu collisions occurred at RHIC in
2005. Study of simulated data will also permit a better
comprehension of these results.
Mate Csanad
- PHENIX
Advisor: Tamas Csorgo
Institute: ELTE University, Budapest, Hungary
Thesis: Experimental and Theoretical Investigation of
Relativistic Heavy Ion Collisions at RHIC with Focus on
Non-Central Collisions
Ultra-relativistic collisions, so called
"Little Bangs" of gold nuclei are observed at the experiments of
the Relativistic Heavy Ion Collider (RHIC) of the Brookhaven
National Laboratory, New York. The aim of these experiments is
to create and investigate new forms of matter that existed in
nature a few microseconds after the Big Bang, the creation of
our Universe. An important (though mathematically never proven)
property of the theory of the color degree of freedom of the
quarks and gluons (Quantum Chromo Dynamics, QCD) is that they
are bound into hadrons in a matter of normal temperature and
pressure. In the early Universe, energy density was many orders
of magnitude higher than that, thus deconfined phases of colored
matter might have existed. Quark Gluon Plasma (QGP) was
predicted to be such a possible phase. This type of matter is
searched for at the RHIC experiments.
A consistent picture emerged after the first three years of
running the RHIC experiments: quarks indeed become deconfined,
but also behave collectively, hence this hot matter acts like a
liquid [1], not like an ideal gas theorists had anticipated when
defining the term QGP. The situation is similar to as if
prisoners (quarks and gluons confined in hadrons) have broken
out of their cells at nearly the same time, but they find
themselves on the crowded jail-yard coupled with all the other
escapees. This strong coupling is exactly what happens in a
liquid [2].
Based on elliptic flow measurements and the broad range success
of analytic hydro models, we can make the definitive statement
that in relativistic Au+Au collisions observed at RHIC we see a
perfect fluid [3, 4]. Based on our estimates on the temperature
[5] and energy density [6] we also conclude that the observed
matter is in a deconfined state. We also see a possible signal
of partial restoration of the chiral UA(1) symmetry via the mass
reduction of η' bosons [7]. Future plan is to explore all
properties of the Quark Matter, by analyzing more data and using
higher luminosity. We are after the full map of the QCD phase
diagram, and in order to explore it, we also have to go to
higher energies and compare them to lower energy data. If the
Quark Matter is the New World, then Columbus just realized he is
not in India, but on a new continent.
References
[1] K. Adcox et al., Nucl. Phys. A757, 184 (2005).
[2] M. Riordan and W. A. Zajc, Sci. Am. 294N5, 24 (2006).
[3] M. Csanad et al., nucl-th/0512078.
[4] M. Csanad, T. Csorgo, and B. Lorstad, Nucl. Phys. A742,
80 (2004).
[5] M. Csanad, T. Csorgo, B. Lorstad, and A. Ster, J. Phys.
G30, S1079 (2004).
[6] T. Csorgo, M. I. Nagy, and M. Csanad, nucl-th/0605070.
[7] M. Csanad, Nucl. Phys. A774, 611 (2006).
The Relativistic Heavy Ion Collider at Brookhaven National
Laboratory is a world-class scientific research facility primarily
funded by the U.S. Department of Energy Office of Science. Hundreds
of physicists from around the world use RHIC to study what the
universe may have looked like in the first few moments after its
creation. What physicists learn from these collisions may help us
understand more about why the physical world works the way it does,
from the smallest subatomic particles, to the largest stars.