442
R. Stock
minimum bias nucleon-nucleon collisions occurring at the microscopic level. Of
course this cannot be a realistic model but for the hard, partonic collisions occurring
during the primordial interpenetration we may still employ it as a standard of
comparison. Nevertheless it was tempting to check whether p + p collisions
revealed any of the effects ascribed to the QGP medium in A + A collisions, such
as grand canonical hadrochemical equilibrium at 160 MeV apparent temperature,
collective hydrodynamic flow in its multifold Fourier harmonics phenomena, jet
attenuation etc., thus making it useless as a “no new physics” standard reference.
This experimental program occupied the first half of this decade, both at RHIC
and LHC. Lacking space for detail, we briefly summarize the main results. Indeed,
minimum bias p + p collisions turned out to be essentially free of the new
physics: ok to employ them in R AA . However, onset behaviour of grand canonical
hadronization [347], of elliptic flow [348], and new modification patterns of jet
production [349] were observed in high multiplicity selections of p + p data
at LHC where the midrapidity charged particle density increases from about 8
in minimum bias mode, up to well beyond 20 in the ALICE study [347], with
extremes ranging up to about 80. No surprise then: these are geometrically small
but energy rich systems which lead to equilibrium conditions due to extreme density
of degrees of freedom. Such that the bulk phenomena, advent of grandcanonical
hadronization [347] and development of anisotropic hydro flow, exhibit onset
patterns. Whereas the hard probes which depend on pathlength might well still
stay unaffected of the medium. No big surprise, but initially the community was
quite smitten by the surfacing of typical QGP signals in an elementary collision.
A different, completely new aspect concerning jet production: the selection of
extreme multiplicity density at midrapidity clearly represents a strong bias as to
the primordial partonic collision generation, possibly selecting for changes in jet
observables [349]. An interesting, unexplored aspect of jet production.
Along with p + p came a wave of p + Au/P b collision studies. Here we
would expect that the projectile side of rapidity space shows essentially p + p
properties whereas the heavy nucleus side shows, firstly, a strong dependence of
impact parameter and participant nucleon number variation. Second, the resulting
primordial fireballs should grow in proportion to N part , leading to clear collective
signals in the bulk production observables, emerging with increasing centrality
(multiplicity), as was indeed observed [347, 350]. However, again the hard probes
did not show significant attenuation as we have seen in Figs. 7.74 and 7.75 for LHC
collisions [318, 319]. It is reassuring to confirm that extended path length in hot QGP
matter is required in order to produce attenuation, whereas cold nuclear spectator
matter has a very low transport parameter [320]. In general p + P b collisions share
the complicating feature, common to all light-on-heavy collisions, that the effective
center of mass for soft production moves away from the corresponding p + p cm
frame that controls hard processes, as N part increases. At the LHC this shift goes
up to two rapidity units for very central collisions, potentially causing more or less
trivial effects in the attenuation of hard partons traversing the soft-produced QGP
medium [351]. Also the effective midrapidity position for soft, bulk production is
now outside the acceptance for ALICE.
R. Stock
minimum bias nucleon-nucleon collisions occurring at the microscopic level. Of
course this cannot be a realistic model but for the hard, partonic collisions occurring
during the primordial interpenetration we may still employ it as a standard of
comparison. Nevertheless it was tempting to check whether p + p collisions
revealed any of the effects ascribed to the QGP medium in A + A collisions, such
as grand canonical hadrochemical equilibrium at 160 MeV apparent temperature,
collective hydrodynamic flow in its multifold Fourier harmonics phenomena, jet
attenuation etc., thus making it useless as a “no new physics” standard reference.
This experimental program occupied the first half of this decade, both at RHIC
and LHC. Lacking space for detail, we briefly summarize the main results. Indeed,
minimum bias p + p collisions turned out to be essentially free of the new
physics: ok to employ them in R AA . However, onset behaviour of grand canonical
hadronization [347], of elliptic flow [348], and new modification patterns of jet
production [349] were observed in high multiplicity selections of p + p data
at LHC where the midrapidity charged particle density increases from about 8
in minimum bias mode, up to well beyond 20 in the ALICE study [347], with
extremes ranging up to about 80. No surprise then: these are geometrically small
but energy rich systems which lead to equilibrium conditions due to extreme density
of degrees of freedom. Such that the bulk phenomena, advent of grandcanonical
hadronization [347] and development of anisotropic hydro flow, exhibit onset
patterns. Whereas the hard probes which depend on pathlength might well still
stay unaffected of the medium. No big surprise, but initially the community was
quite smitten by the surfacing of typical QGP signals in an elementary collision.
A different, completely new aspect concerning jet production: the selection of
extreme multiplicity density at midrapidity clearly represents a strong bias as to
the primordial partonic collision generation, possibly selecting for changes in jet
observables [349]. An interesting, unexplored aspect of jet production.
Along with p + p came a wave of p + Au/P b collision studies. Here we
would expect that the projectile side of rapidity space shows essentially p + p
properties whereas the heavy nucleus side shows, firstly, a strong dependence of
impact parameter and participant nucleon number variation. Second, the resulting
primordial fireballs should grow in proportion to N part , leading to clear collective
signals in the bulk production observables, emerging with increasing centrality
(multiplicity), as was indeed observed [347, 350]. However, again the hard probes
did not show significant attenuation as we have seen in Figs. 7.74 and 7.75 for LHC
collisions [318, 319]. It is reassuring to confirm that extended path length in hot QGP
matter is required in order to produce attenuation, whereas cold nuclear spectator
matter has a very low transport parameter [320]. In general p + P b collisions share
the complicating feature, common to all light-on-heavy collisions, that the effective
center of mass for soft production moves away from the corresponding p + p cm
frame that controls hard processes, as N part increases. At the LHC this shift goes
up to two rapidity units for very central collisions, potentially causing more or less
trivial effects in the attenuation of hard partons traversing the soft-produced QGP
medium [351]. Also the effective midrapidity position for soft, bulk production is
now outside the acceptance for ALICE.