Resonance production from jet fragmentation
Christina Markert
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for the STAR Collaboration
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University of Texas
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Austin, TX 78712, USA
Short lived resonances are sensitive to the medium properties in heavy-ion collisions. Heavy hadrons have larger probability to be produced within the quark gluon plasma phase due to their short formation times. Therefore heavy mass resonances are more likely to be affected by the medium, and the identification of early produced resonances from jet fragmentation might be a viable option to study chirality. The high momentum resonances on the away-side of a triggered di-jet are likely to be the most modified by the partonic or early hadronic medium. We will discuss first results of triggered hadron-resonance correlations in Cu + Cu heavy ion collisions. Chiral symmetry restoration is one of the fundamental features of QCD at high temperatures and densities of nuclear matter which should occur around the phase transition (0.81.2 Tc) according to lattice QCD calculations [1]. The signature of chiral symmetry restoration could be mass shifts and/or broadenings of the hadronic resonance states inside the medium created in a heavy ion collision. Resonances from jet fragmentation can be produced early, depending on their hadronic formation times and might be medium modified through interactions with either the late partonic (1.2 Tc) or early hadronic (0.8 Tc) medium. To measure this effect, resonances also need to decay inside the medium. The formation time of hadrons from jet fragmentation depends on their mass and momentum. I will discuss the relevant momentum range for different resonances. Furthermore one needs to select the resonances on the away-side of a surface biased (triggered) jet in order to maximize their in-medium interactions. This will be discussed in Sect. 3.
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The formation time of hadrons and resonances from jet
fragmentation can be calculated in a quantum mechanical
approach [2, 3]. An alternative approach, based on string
fragmentation [4], arrives at a similar conclusion for heavy mass
hadrons, light quark objects. A heavier hadron mass leads to
a shorter formation time while a higher momentum causes
a longer formation time. In the theoretical calculations [3]
the probability of high momentum heavy hadron (or
resonance) formation in the partonic medium is finite.
Therefore we expect the resonance to be medium modified,
after its formation, through parton-resonance interactions in
the so called mixed degree of freedom phase of partons and
hadrons. The proper momentum range needs to be defined
by a lower and an upper limit to ensure the formation and
decay of the resonance inside the chiral medium. If the width
of the resonances is broadened in the medium their lifetimes
is shortened which will increase the probability of the
resonances decaying inside the medium. This would also
result in a broader signal width in the invariant mass
spectrum which decreases the statistical significance of the
signal. Figure 2.1 shows the linear dependence of the average
formation time on the transverse momentum of the hadron
(or resonance) integrated over the fragmentation distribution
of all produced jets at RHIC. The yellow shaded area is the
lifetime of the QGP calculated by assuming a longitudinal
Bjorken expansion with Tc 180 MeV for the critical
temperature. The estimated lifetime at RHIC is QGP = 6.2 fm/c
and at the LHC is QGP = 14 fm/c based on an longitudinal
Bjorken expansion [3]. The RHIC QGP lifetime is in
agreement with the partonic lifetime, derived from the resonance
suppression in the hadronic medium [59] and the pion HBT
source lifetime measurement ( = 512 fm/c) [10].
Figure 2.2 shows the corresponding linear dependence of the
mean formation time on the transverse momentum of the
hadron (or resonance) for the LHC.
Fig. 2.1 Transverse momentum dependence of leading hadron
formation times at RHIC energies, s = 200 GeV. Results for both
stable particles and resonances are presented. The shaded area represents
the estimated partonic lifetime at RHIC. Insert shows the meson and
baryon distributions P (z) at a fixed hadron momentum pT = 8 GeV/c.
The shaded area represents the estimated partonic lifetime at LHC
using a Bjorken expansion [3]
Fig. 2.2 Transverse momentum dependence of heavy resonance
formation times at LHC energies, s = 5.5 TeV. Insert shows the
meson and baryon distributions P (z) at a fixed hadron momentum
pT = 32 GeV/c. The shaded area represents the estimated partonic
lifetime at LHC using a Bjorken expansion [3]
3 Resonances in jets
In order to extract the chirally restored resonances from the
partonic and early hadronic medium, a correlation analysis
using a jet or hadron triggered high momentum resonance
is suggested. This also ensures that the resonance signal is
largely unaffected by the late hadronic medium where
regeneration of resonances will dilute it. UrQMD calculations
suggest that the regeneration of resonances is predominant
in the low momentum region (pT < 2 GeV/c) [ (...truncated)