402
R. Stock
c quarks (a few per central Au+Au collision at RHIC, a few tens at LHC) finally
undergo statistical hadronization at T = T c along with all other light and strange
quarks. To deal with the non-equilibrium overall charm abundance an extra charm
fugacity factor γ c is introduced into the statistical model [108] calculation (for
details see [241]). J// and are thus created in non-perturbative hadronization,
with multiplicities proportional to γ 2
c and phase space weights, along with all other
charmed hadrons. This “regeneration” model also agrees with the RHIC data of
Fig. 7.55, albeit within a large systematic uncertainty [241].
We note that the term regeneration is, in fact, misleading. The statistical
hadronization process does not recover the initial, small fraction of correlated cc
pairs that would end up in J// in vacuum. It arises from the total density of
primordially produced c and c, uncorrelated in the hadronizing fireball volume.
The statistical hadronization J// production process, sketched above, thus
has the unfortunate property of providing a trivial background charmonium yield,
unrelated to the deconfinement signal [41] referring to the primordial J// yield.
Only about 1% of the primordial cc yield results in charmonia, in vacuum. The
in-medium deconfinement process breaking up the cc correlation on its way to
charmonia, thus constitutes a mere 1% fraction of the total charmed quark and
anti-quark number. The regeneration process is insensitive to this 1% fraction,
deconfined or not. At T c , charm hadronization reacts only to the total abundance
of c and c, as imprinted into the dynamical evolution by the perturbative QCD cc
production rate of initial nucleon-nucleon collisions. At RHIC, it turns out [241] that
the c and c density is low, giving rise to substantial canonical suppression (recalling
Eqs (7.38)–(7.42) in Sect. 7.3) of the two charm quark charmonia, relative to D
mesons, during hadronization. With a tenfold c, c density at LHC, grand canonical
charmonium production will set in, thus probably overshooting the primordial yield
reference, σ
J //
NN × N coll . Thus we expect R AA > 1 at the LHC. The role of a critical
deconfinement “thermometer” is lost for J// at LHC, but the bottonium Y states
can take over, being deconfined well above T = 300 MeV [242].
The RHIC result [237] for J// in central Au+Au collisions (Fig. 7.55), namely
that R AA → 0.2, represents the lucky coincidence that the initial temperature, T ≈
300 MeV, is high enough to dissolve the correlated cc charmonium precursor states,
while the J// suppression is not yet overshadowed by the trivial hadronization
yield of J//.
7.6.2 Direct Photons
Photons are produced during all stages of the dynamical evolution in A+A collisions. About 98% stem from final electromagnetic hadron decays, not of interest
in the present context, other then by noting that their rate has to be painstakingly
measured experimentally, in order to obtain “direct” photon spectra at low p T by
R. Stock
c quarks (a few per central Au+Au collision at RHIC, a few tens at LHC) finally
undergo statistical hadronization at T = T c along with all other light and strange
quarks. To deal with the non-equilibrium overall charm abundance an extra charm
fugacity factor γ c is introduced into the statistical model [108] calculation (for
details see [241]). J// and are thus created in non-perturbative hadronization,
with multiplicities proportional to γ 2
c and phase space weights, along with all other
charmed hadrons. This “regeneration” model also agrees with the RHIC data of
Fig. 7.55, albeit within a large systematic uncertainty [241].
We note that the term regeneration is, in fact, misleading. The statistical
hadronization process does not recover the initial, small fraction of correlated cc
pairs that would end up in J// in vacuum. It arises from the total density of
primordially produced c and c, uncorrelated in the hadronizing fireball volume.
The statistical hadronization J// production process, sketched above, thus
has the unfortunate property of providing a trivial background charmonium yield,
unrelated to the deconfinement signal [41] referring to the primordial J// yield.
Only about 1% of the primordial cc yield results in charmonia, in vacuum. The
in-medium deconfinement process breaking up the cc correlation on its way to
charmonia, thus constitutes a mere 1% fraction of the total charmed quark and
anti-quark number. The regeneration process is insensitive to this 1% fraction,
deconfined or not. At T c , charm hadronization reacts only to the total abundance
of c and c, as imprinted into the dynamical evolution by the perturbative QCD cc
production rate of initial nucleon-nucleon collisions. At RHIC, it turns out [241] that
the c and c density is low, giving rise to substantial canonical suppression (recalling
Eqs (7.38)–(7.42) in Sect. 7.3) of the two charm quark charmonia, relative to D
mesons, during hadronization. With a tenfold c, c density at LHC, grand canonical
charmonium production will set in, thus probably overshooting the primordial yield
reference, σ
J //
NN × N coll . Thus we expect R AA > 1 at the LHC. The role of a critical
deconfinement “thermometer” is lost for J// at LHC, but the bottonium Y states
can take over, being deconfined well above T = 300 MeV [242].
The RHIC result [237] for J// in central Au+Au collisions (Fig. 7.55), namely
that R AA → 0.2, represents the lucky coincidence that the initial temperature, T ≈
300 MeV, is high enough to dissolve the correlated cc charmonium precursor states,
while the J// suppression is not yet overshadowed by the trivial hadronization
yield of J//.
7.6.2 Direct Photons
Photons are produced during all stages of the dynamical evolution in A+A collisions. About 98% stem from final electromagnetic hadron decays, not of interest
in the present context, other then by noting that their rate has to be painstakingly
measured experimentally, in order to obtain “direct” photon spectra at low p T by