Heavy Nuclei, From RHIC to The Cosmos Page: 4 of 10
This report is part of the collection entitled: Office of Scientific & Technical Information Technical Reports and was provided to UNT Digital Library by the UNT Libraries Government Documents Department.
Extracted Text
The following text was automatically extracted from the image on this page using optical character recognition software:
4
LUi
- 0.8
0.6
0.4
0.2
C
10O
103
10'
10BBC vs ZDC analog response
-'- y
10-15/
510~
-5%
0 2 0.4 06 0.8 1
0BBC'0BBC
Minimum bias multiplicity distribution
--... at mid-rapidity
. . . . 1, . , . , .. .. . .. ,1 n -
20 40 60 80 100 120 r40 160 r 80
Number of tracksO 200 400 600 dNe/dqIO
Figure 3. (a)Relationship between forward neu-
trons (ZDC energy) and charged multiplicity (the
charge in the beam-beam counters, QBBC), as
measured by the PHENIX collaboration at 130
GeV for 4 impact parameter bins. (b) The over-
all charged particle multiplicity, do/dNca (solid
dots), with calculations of the multiplicity dis-
tribution for the same 4 impact parameter bins.
From Ref. [21].
least second order), and occurs at a temperature
of 150-200 MeV and an energy density ec, 1
GeV/fm3[20]. This calculation is for an infinite
medium with an infinite lifetime; edge effects and
formation time are not considered. Although the
expected system lifetime is only , 10-23s, calcu-
lations indicate that equilibration occurs quickly,
so a clear phase change is possible.
Although it is a key parameter in heavy ion
collisions, the impact parameter b is not directly
observable. We use two classes of observables to
infer the impact parameter. The first is the num-
ber of forward (zero-degree) neutrons. These neu-
trons come from the non-interacting part of the
nucleus. Enough energy propagates from the ini-
tial collision to dissociate the non-interacting part
of the nuclei into neutrons, protons and small nu-clear fragments. The other observables are sensi-
tive to the number of interacting nucleons. Exam-
ples are the charged particle multiplicity or trans-
verse energy. A model is necessary to relate these
observables to the impact parameter. To avoid
systematic uncertainties, events are often sorted
by centrality (i.e. by charged multiplicity), and
divided into classes, such as the 10% most central
(those with the smallest impact parameter).
Figure 3 shows the relationship between the
number of forward neutrons (measured in the
ZDCs) and the charged multiplicity[2 1]. The
charged multiplicity rises continually as the im-
pact parameter decreases. However, the number
of forward neutrons is largest at moderate impact
parameters. In very central collisions, most of the
nucleus interacts, leaving few remnant neutrons,
while in very peripheral collisions, some of the
nucleus remains intact, reducing the number of
forward neutrons, producing the curve in Fig. 3.
5. Thermal Freezeout
The particles present at thermal freezeout are
those observed in the RHIC detectors, and are
relevant for comparison with models of heavy ion
collisions. The charged particle multiplicity is
shown as a function of pseudorapidity y in Fig. 4.
The multiplicity dN/dy is roughly flat for Iyj < 2.
This central plateau shows that there is boost in-
variance. Within this region, the system appears
invariant with respect to the longitudinal boost
(velocity); the expansion may be treated in 2 di-
mensions.
At 130 GeV, the maximum dN/dy is about
570, rising to 650 at 200 GeV. This corresponds
to total multiplicities of about 4100 210 and
4960 250 respectively[22]. These multiplici-
ties are considerably lower than most pre-RHIC
predictions [23], and seem to be best fit by mod-
els based on a combination of hard interactions
(calculated by perturbative QCD) and soft inter-
actions (extrapolated from lower energies). Most
popular cosmic ray air shower codes predict con-
siderably larger multiplicities [24].
The dN/dy per participant (nucleon involved in
the collision) are about 40% higher than at lower
energies, and also 50% higher than in pp collisions
Upcoming Pages
Here’s what’s next.
Search Inside
This report can be searched. Note: Results may vary based on the legibility of text within the document.
Tools / Downloads
Get a copy of this page or view the extracted text.
Citing and Sharing
Basic information for referencing this web page. We also provide extended guidance on usage rights, references, copying or embedding.
Reference the current page of this Report.
Klein, Spencer R. Heavy Nuclei, From RHIC to The Cosmos, report, November 1, 2002; Berkeley, California. (https://digital.library.unt.edu/ark:/67531/metadc738058/m1/4/: accessed March 29, 2024), University of North Texas Libraries, UNT Digital Library, https://digital.library.unt.edu; crediting UNT Libraries Government Documents Department.