I am a PHD Candidate from the University of California at Davis interested in the study of the Quark Gluon Plasma. According to wikipedia a quark–gluon plasma (QGP) or quark soup is a “possible” phase of quantum chromodynamics (QCD) which exists at extremely high temperature and/or density. This phase consists of asymptotically free quarks and gluons, which are several of the basic building blocks of matter.
According to me, however, the word “possible” makes the whole story as
interesting as the search for the Higgs with a caveat: while the whole
world mines big data in the search for God’s particle I mine the data
in the search for the absence of Quarkonium (a bound state of heavy
)
(Click on each link to learn more about these areas)
High Energy and Nuclear Heavy Ion Physics
Lattice QCD
Multivariate Analysis
Optimization Algorithms
Machine Learning
Python, iPython, Pandas, Numpy, Scipy, SciKit Learn, MLpy, pyML (Scientific Computing in general)
Big Data Science
Data Modeling
In 1986, Matsui and Satz [1] predicted that quarkonium (bound states of heavy quark-antiquark, , pairs)
production would be suppressed by color screening in a quark-gluon plasma (QGP). Suppression does not imply that
the will not be produced, but rather that the observed quarkonium yield will be depleted relative to the expected
yield either because the pair fails to form a quarkonium state or the state itself is destroyed through its subsequent
interactions.
There are two quarkonium families that have been studied in heavy-ion collisions, charmonium (
) and bottomo-
nium (
). Both are similar in structure to positronium
bound states). At zero temperature, the masses and
radii of the quarkonium states can be modeled by a potential that combines a Coulomb-like term with a confining
term, 
, where 
is a coupling based on one-gluon exchange, 
is called the string tension and is
related to the energy in a rigid string between the 
and
, and 
is the separation between the and .
The first quarkonium studies in heavy-ion collisions were done with the charmonium family. The 
has been
studied in nuclear collisions since the first measurements at the CERN SPS in the late 1980s [2]. Because the
results are not easy to understand, it was hoped that the more massive 
states at the LHC would be easier to
interpret [3]. The CMS results on sequential
suppression suggests that this is the case.
radiative decays. The BB threshold is also shown.
The bottomonium family is shown in Fig. 1, along with the location of
the 
threshold, the sum of the two lowest 
meson masses. States with masses below this threshold cannot decay to B meson pairs but must decay
either by strong or electromagnetic processes. The ground state, 
, decays to a variety of hadronic states but
also has a 2.5% probability to decay to lepton pairs. The higher S states below the threshold, 
and
, can contribute to the total
yield by either direct decays such as 
or indirectly by
electromagnetic transitions such as
and, subsequently, 
.
The total yield thus is only about 50% direct, with the rest coming from feed down from states with masses below the threshold.
States that lie above the threshold decay almost entirely to meson pairs and do not contribute to the yields of the
states below threshold. The masses and radii of the states in the family
can be calculated by solving the Schrodinger equation. The ground state (1S) is the most tightly bound with the largest binding energy and smallest radius. Higher mass states
have smaller binding energies and larger radii with the the most loosely bound sub-threshold state. The values
of the binding energies and radii are given in Table I.
The screening length in a finite-temperature medium, 
, has been calculated in lattice QCD [4]. The screening
length decreases with increasing temperature. If the temperature is such that 
, where ri is the radius of
the bottomonium state, the state will no longer remain bound in the medium and the final-state yield will be reduced
or suppressed. Thus bottomonium states with larger radii and smaller binding energies will break up first while those
with smaller radii and larger binding energies require higher temperatures to be suppressed. The state, with
the largest binding energy of all, requires the largest temperature for direct suppression. While it is not clear that
the temperature is high enough at the LHC, the more massive and should be suppressed and their feed
down contributions to the absent. The suppression of the states closest to threshold first is called sequential
suppression [6]. The LHC is the first machine at which it is possible
to observe sequential suppression of states.
The electromagnetic decays of the quarkonium S states to lepton pairs (dileptons) produce peaks in the dilepton
continuum at the , and masses. These states are also rather long-lived with narrow decay widths.
Indeed, the measured widths reflect the detector resolution rather than the natural widths of the states.
The CMS detector is an ideal device with which to measure sequential suppression because of its excellent dilepton
mass resolution. Three separate peaks can be distinguished above background in pp interactions at center of mass
energy, ps, of 2.76 TeV. The decay muons need momenta greater than 4 GeV/c to reach the muon chambers, low
enough for states with pT 
0 to be detected in CMS.
The standard way to determine the amount of suppression is to compare the yields of the individual states in
Pb+Pb collisions to those in pp collisions at the same energy. In addition to simply counting the states observed
in all Pb+Pb collisions, the data can also be divided into ‘centrality’ bins. The most central Pb+Pb collisions are
almost head on and involve most of the nucleons in both lead nuclei. More peripheral collisions have fewer participant
nucleons. Less suppression due to QGP production is expected in peripheral collisions because the produced system
is smaller and at lower temperature. The maximum suppression should appear in the most central collisions.
The suppression of each state is quantified by the suppression factor,
, which is the ratio of a particular state
in Pb+Pb relative to pp collisions, normalized to account for the centrality of the collision. In addition, CMS also
compares the relative yield ratios, 
with n = 2 and 3, in pp and Pb+Pb collisions. In both measures, a
sigificant depletion of all states is observed.
The peak is not distinguishable from the background in Pb+Pb collisions so that 
is consistent
with zero within the experimental uncertainties. It would thus appear
that the , with its low binding energy,
is completely suppressed by the medium. The also shows large suppression,:math:R_{AA}Upsilon(2S) < 0.3 in peripheral
collisions and < 0.1 in more central collisions. In the most central collisions, 
is consistent with the
absence of excited-state feed down. This is the first measurement of
sequential suppression in the bottomonium system.
Some work remains to be done, however. There are non-QGP effects which could still play a role in Pb+Pb collisions. These ‘cold matter’ effects can be studied in the forthcoming p+Pb run at the LHC. In addition, future higher statistics Pb+Pb data should be able to study the suppression pattern as a function of transverse momentum [7].
| [1] | T Matsui and H. Satz, Phys. Lett. B 178, 416 (1986). |
| [2] | L Kluberg, Eur. Phys. J C 43, 145 (2005); L. Kluberg and H. Satz, arXiv:0901.3831 [hep-ph]. |
| [3] | A D Frawley, T. Ullrich and R. Vogt, Phys. Rept. 462, 125 (2008). |
| [4] | H Satz, J. Phys. G 32, R25 (2006). |
| [6] | S Digal, P. Petreczky and H. Satz, Phys. Rev. D 64, 094015 (2001). |
| [7] | J F Gunion and R. Vogt, Nucl. Phys. B 492, 301 (1997). |