Collective effects at the LHC
S.M. Troshin
Institute for High Energy Physics,
Protvino, Moscow Region, 142281, Russia
In the talk the discussion on how unitarity saturation could lead to
appearance of a hadron liquid phase at very high temperatures and
how the ridge-like structure in the two-particle correlation function could
result from the rotation of the transient state of matter was given.
Multiparticle production and global observables provide information
on the mechanisms of deconfinement, hadronization as well as on the size
and the temperature of an interacting system.
Discovery of the deconfined state of matter has been announced by four
major experiments at RHIC [1].
Despite the highest values of energy and density have been reached,
a genuine quark-gluon plasma QGP (gas of the free current quarks and
gluons)was not found. The deconfined state reveals the properties of the
perfect liquid, being a strongly interacting collective state and therefore it
was labelled as sQGP.
These results immediately have became a subject for an active
theoretical studies. The nature of this new form of matter is not known
and the variety of models has been proposed to treat its properties. The
importance of this result is in the fact that well beyond the critical values
of density and temperature the matter is strongly correlated
and reveals high degree of the coherence.
The mechanism of particle productions discussed in the talk is sensitive
to the particular direction of rotation and to the rotation plane orientation.
In the proposed explanation of the ridge effect at the LHC energy 7 TeV
a dynamical selection of peripheral region in impact parameter space
responsible for the inelastic processes is the iportant point. This selection
results from the prominent role of the reflective scattering at the LHC
energies [2]. The reflective scattering is a natural interpretation of the
unitarity saturation based on the optical concepts in high energy hadron
scattering. Such reflective scattering can be traced to the
continuous increasing density of the scatterer with energy, i.e. when
density goes beyond some critical value relevant for the black disk limit
saturation, the scatterer starts to acquire a reflective ability. The concept
of reflective scattering itself is quite general, and results from the S-
matrix unitarity saturation related to the necessity to provide the total
cross section growth at asymptotic energies. This picture predicts
that the scattering amplitude at the LHC energies is beyond the black
disk limit at small impact parameters. The consequences for the total,
elastic and inelastic cross-sections have been discussed in [3].
The geometrical picture of hadron collision at non-zero impact
parameters implies that the generated massive virtual quarks in overlap
region could obtain very large initial orbital angular momentum
at high energies. Due to strong interaction between the quarks this orbital
angular momentum leads to the coherent rotation of the quark system
located in the overlap region as a whole in the xz-plane. This rotation is
similar to the rotation of liquid. The particle production mechanism at
moderate transverse momenta is an excitation of a part of the rotating
transient state of massive constituent quarks (interacting by pion
exchanges).
The ridge-like structure observed in the high multiplicity events by the
CMS Collaboration [4] can be considered as an experimental
manifestation of the coherent rotation of the transient matter in hadron
collisions. The narrow two-particle correlation distribution in the
asimuthal angle is the distinctive feature of this mechanism. However,
two-particle correlation could have a rather broad distribution in polar
angle in this mechanism.
Other experimentally observed effects of this mechanism are the directed
flow in hadron reactions, elliptic flow and energy dependence of the
average transverse momentum of secondary particles produced in protonproton collisions.
We start here with mentioning the physical implementations of the
average transverse momentum dependence on the energy. The dynamical
mechanism of the average transverse momentum energy growth
originates in collective effect of transient matter rotation, while dynamics
of average multiplicity growth is related to the mechanism where a
nonlinear field couplings transform the kinetic energy to internal energy
related to mass. According to this difference one could expect divergent
energy dependencies of these two observables at higher energies.
Indeed, formation of a genuine quark-gluon plasma in transient state in
the form of the non-interacting gas of free quarks and gluons
would result in disappearance of the collective effect of rotation of the
transient state. Orbital angular momentum presented in the initial state
would lead then to the global polarization of produced particles.
Several remarks should be made on the elliptic flow in pp-collisions at
the LHC energy region. One should note that, in fact, the rotation gives a
contribution to the x-component of the transverse momentum and does
not contribute to the y-component of the transverse momentum. The
effect of the rotating transient matter contribution into the elliptic flow
was calculated by analogy with the calculation of the average transverse
momentum. The mechanism of anisotropic flow related to rotation might
be a leading one in hadron collisons, since hadrons have smaller
geometrical extension and the probabilty of hydrodynamical generation
of elliptic flow is lower compared to the collisions of nuclei.
In conclusion, we would like to remark that performing studies of the
multiparticle productions in pp--collisions at the LHC under the scope of
the searches for the possible existence of the rotation effects would be of
a significant interest. Such collective effects [5] would be absent when
the genuine quark-gluon plasma (gas of free quarks and gluons) being
formed at the LHC energies because of all collective effects and the
anisotropic flows, in particular, should dissapear in this case.
1. Quark Gluon Plasma. New Discoveries at RHIC: A Case of
Strongly Interacting Quark Gluon Plasma. Proceedings, RBRC
Workshop, Brookhaven, Upton, USA, May 14-15, 2004: D.
Rischke, G. Levin, eds; 2005, 169pp.
2. S.M. Troshin, N.E. Tyurin, Int. J. Mod. Phys. A 22, 4437 (2007).
3. S.M. Troshin, N. E. Tyurin, Eur. Phys. J. C 21, 679 (2001).
4. CMS Collaboration, V. Khachatryan et al., JHEP 1009, 091 (2010)
5. S.M. Troshin, N.E. Tyurin, Mod. Phys. Lett. A, 25, 1315 (2010).