Orbital Momentum Effects in Heavy Ion and Hadron CollisonsSergey
Troshin, IHEP, Protvino
Role of polarization measurements in searches of
quarkgluon plasma, studies of the dynamical nature
of the deconfined state and identification of the
relevant degrees of freedom
Role of orbital angular momentum in peripheral
collisions and related observable effects
Bridge between the ideas and methods in spin and
heavy ion physics
MotivationMotivation
Tool –
stable pattern of hyperon polarization
(lambda, in particular) observed in unpolarized
hadron interactions: energy independent, linear rise
with xF,
1 GeV/cPPΛ
−+→Λ
πp0Polarization is determined through weak
(parity nonconserving) decay
Earlier (mideighties) prediction: isotropic distribution of the parton
momenta in the plasma rest frame.
QGP does not remember the directions of the initial momenta.
Vanishing polarization of hyperons relative to production plane
(due to
parity conservation there could not be longitudinal polarization).
Papers:
P. Hoyer, Phys. Lett. B, 187, 162, 1987;
A.D. Panagiotou, Phys. Rev. C 33, 1999, 1986;
R. Stock et al., Proc. of the Conference on Quark Matter Formation in
HeavyIon Collisions, Singapore 1982.
Vanishing hyperon polarization w.r.t. production plane follows from
various models also
0,0)( →→Λ bbPP b - impact parameter
Reaction plane (definition)
b
)( 2
10)( ≅LHCL
1=h
P. Carruthers, 1983:
“Enormous orbital angular
momentum involved in a high
energy nucleusnucleus collisions”
Reaction plane
What are the observable effects of the large angular orbital
momentum presented in the peripheral heavyion and
hadron collisions?
Burst of polarized vector mesons (P. Carruthers, 1983)?
Strong necessity for the spins of the outgoing particles to
line up parallel to each other in the transverse direction to
the reaction plane (C.N. Yang and T.T. Chou, 1986)?
{ Reaction plane is the plane perpendicular to the orbital momentum }
0)( 21 >⋅ average TT σσ
due to a net deficiency of orbital angular
momentum in the left and rightmoving
outgoing systems
Thus, in a process of high energy hadron
and nuclear collisions large initial
orbital angular momentum can, in principle, be converted into the spin angular
momentum of final particles resulting in their polarization relative to the
reaction plane. We would try to connect this possibility with the nature of
transient strong interaction matter.
Experimental facts
Asymptotic freedom in QCD formation of QGP
gas of free
quarks and gluons [J. C. Collins, M. J. Perry; Phys. Rev. Lett. 34,
1353 1356 (1975)]
Discovery of the deconfined
state of matter has been announced by the four major experiments at RHIC
Despite the highest values of energy and density have been reached, a genuine quarkgluon plasma QGP
was not found. The deconfined
state reveals the properties of the perfect liquid, being strongly interacting
collective state and therefore it was labelled
as sQGP.
How can orbital momentum help in detecting nature and
properties of transient quarkgluon matter produced in the
noncentral collisions?
Weaklycoupled matter (parton
model with final state interactions)
Z.T. Liang, X.N. Wang, Phys. Rev. Lett, 94, 102301, 2005: no
collective rotation of the system, finite transverse gradient of the
average longitudinal momentum per produced in the overlap region
parton. It is claimed that relative OAM in collision of partons
will lead
to global quark polarization due to spinorbital coupling.
Significant (order of tens %) polarization of hyperons relative to
reaction plane was anticipated. Similar ideas were used for the
hyperon polarization in hadron
noncentral collisions, S. Voloshin,
2004, nuclth/0410089v1. The idea to observe circularly polarized
photons as a signal of quark polarization in the QGP was proposed by
A. Ipp, A Di Piazza, J. Evers, C.H. Keitel, arXiv: 0710.5700v1. Also: B.
Betz, M. Gyulassy, G. Torrieri, Phys. Rev. C 76, 044901, 2007
Typical values of global hyperon
polarization relative to reaction plane
were predicted to be
3.0≅= R q
R H PP
The measurements of global polarization and were
performed at RHIC (STAR Collaboration) and upper limit
has been obtained
Λ Λ
GeVsNN 200;4.62=
Global polarization measurements in Au+Au
collisions, B.I. Abelev et al.
Phys. Rev. C 76, 024915 (2007)
)sin(8 * RPp
πα
I. Selyuzhenkov (for the STAR
Collaboration), J. Phys. G 34 (2007) S1099
Global spin alignment for φ, K*0
and global polarization for Λ were
not observed for the different centralities.
=
−−−−
−
−
If - the alignment of vector mesons takes place 3/100 ≠ρ
Interesting to measure polarization and spin alignment w.r.t.
production plane
Thus, at the moment no experimental evidence exists for conversion of
the orbital angular momentum into the spin angular momentum in
nuclear collisions and this conclusion is correlated but not necessarily
follow from the result on the strongly interacting nature of transient
matter observed at RHIC.
The question arise again: what are the experimental
manifestations of the large orbital angular momentum could be in
the case of strongly interacting transient matter ?
Ideal liquid low viscosity large interaction crosssection (about 22 mb)
Large orbital angular momentum rotation of the transient deconfined
matter in the overlap region as a whole, i.e. all parts have the
same angular
velocity and orbital angular momentum does not convert to spin angular
momentum.
Experimental manifestations – directed flow v
1
What is directed flow
Several experimental probes of collective dynamics in AA interactions: momentum
anisotropies defined by means of the Fourier expansion of
the secondary particle
distribution over the momentum azimuthal
angle. The azimuthal
angle is the angle of
the detected particle with respect to the reaction plane, (spanned by the collision axis z
and the impact parameter vector b
directed along the x axis).
nv φ
S.M. Troshin, N.E. Tyurin,
Int. J. Mod. Phys. E, 2008
φnvn cos=
π
Provided impact parameter is fixed and known, the directed flow
can
be studied in nuclear as well as in hadronic
collisions.
Asimuthal
angle of RP is taken to be
zero
φφ ⇒Ψ− RP
Measurements of the characteristics of multiparticle
production processes in hadronic
collisions at fixed impact parameter should be performed with selection
of the
specific events sensitive to the value and direction of impact parameter.
The relationship of the impact parameter with the final state multiplicity. Chou
Yang approach: one can restore the values of impact parameter from the charged
particle multiplicity.
inel
≅ W. Broniowski, W. Florkowski,
Phys. Rev. C 65 (2002) 024905
c(N) is the centrality of the events with multiplicity larger than N and b(N) is the
impact parameter where mean multiplicity is equal to N.
Centrality and impact parameter
Geometrical picture of hadron
collision has an apparent analogy with collisions of nuclei
and determination of the reaction plane in the noncentral hadronic
collisions could be
experimentally feasible with standard procedure.
Transient state in nuclei and Transient state in nuclei and
hadronhadron collisionscollisions
effQCD LL ⇒
Overlap regionOverlap region
Inclusive crosssection:
∑ ≥ =
2|),(1|),,,(),,,( − ⊥⊥ −= bsiUpybsvpybsv nn
Rotation as origin of directed Rotation as origin of directed
flowflow
Assumed 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) by the one of the valence constituent quarks with
subsequent hadronization of the quarkpion
liquid droplets .
Q
|| 1|| 1
beamyy v
− ∝ nv σ∝1
Weakly increasing energy
dependence of the directed
flow (due to increase of
elastic scattering cross section
P dependence of directed flow T
TpR /1≈
At high transverse momentum an internal structure of
constituent quark is resolved –
vanishing directed flow is expected
Directed flow in nuclear collisions
Au+Au and Cu+Cu at 62.4 and 200 GeV at RHIC (STAR preliminary
data)
Other effects related to OAMOther effects related to OAM
Contribution to elliptic flow
S.M. Troshin, N.E. Tyurin, hepph/0609248
F. Becattini, F. Piccinini, J. Rizzo, Phys. Rev.
C77: 024906, 2008. [arXiv: 0711.1253]
Conclusion: expectations for the Conclusion: expectations for the
LHCLHC
What kind of the deconfined
matter would be produced at the LHC energies?
Vanishing directed flow –
weaklycoupled system, QGP.
What about global polarization? Is this a signal of QGP
formation? Not necessarily, the reason is the reflective
(antishadowing) scattering.
. S.M. Troshin, Phys.Lett.B597:391,2004
The generic geometric picture at fixed energy beyond the black disc
limit can be described as a scattering off the partially reflective
and partially absorptive disk surrounded by the black ring which
becomes grey at larger values of the impact parameter.
The evolution with energy is characterized by increasing albedo due
to the interrelated increase of reflection and decrease of
absorption at small impact parameters.
22 |),(1| ),(Im
Negative imbalance of the orbital
momentum at the LHC energies
This imbalance should be compensated by the total spin of final
particles LS Δ−=
Qualitative features of the transient state in hh
and AA collsions and role of
rotating quarkmeson liquid in the directed flow formation.
Effect of rotation is maximal for peripheral collisions –
centrality dependence.
Vanishing directed flow can serve as a signal of a genuine QGP formation.
If genuine QGP is formed, OAM can be converted to the global polarization of the
secondary particles.
LHC studies of anisotropic flows: global polarization at such high energies can
exists even when sQGP
is formed, it is then results from reflective scattering.
Further studies (theoretical and experimental) are necessary in this field
combining experiences of spin studies with multiparticle
production.
Polarization in Heavy Ion Physics
Transient state in nuclei and hadron collisions
Overlap region
Other effects related to OAM
Conclusion: expectations for the LHC