polarization in heavy ion physics - university of virginia · 2008. 9. 26. · collisions and...
TRANSCRIPT
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
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