S.A. Voloshin School of Collective Dynamics in High Energy Collisions, LBNL, June 7-11, 2010 page 1...
-
Upload
ursula-megan-henry -
Category
Documents
-
view
214 -
download
1
Transcript of S.A. Voloshin School of Collective Dynamics in High Energy Collisions, LBNL, June 7-11, 2010 page 1...
S.A. VoloshinSchool of Collective Dynamics in High Energy Collisions, LBNL, June 7-11, 2010page 1
Local strong parity violation and new perspectives in
experimental study of non-perturbative QCD
Sergei A. Voloshin
L or B Outline:
QCD vacuum. Chiral Magnetic Effect Anisotropic flow Observable Experimental results Future directions
Summary
LPV in Heavy Ion Collisions: Experiment
STAR results:
S.A. VoloshinSchool of Collective Dynamics in High Energy Collisions, LBNL, June 7-11, 2010page 2
QCD vacuum topology
NCS = -2 -1
0 1 2
Energy of gluonic field is periodic in NCS direction (~ a generalized coordinate)
Instantons and sphalerons are localized (in space and time) solutions describing transitions between different vacua via tunneling or go-over-barrier
Defines the confinement radius, hadron size.
Defines the size of constituent quark, size of the “small” instantons
Quark propagating in the background of the quark condensate obtains dynamical
“constituent quark” mass
Quark interactions with instantons - change chirality (P and T odd),- lead to quark condensate
3 42 2
30
230 MeV , 850 MeV
500 MeV/fm
qq g G
S.A. VoloshinSchool of Collective Dynamics in High Energy Collisions, LBNL, June 7-11, 20103
P-odd domains in heavy ion collisions. Exp.
where α is the fraction of particles originating from the P-odd domain
b
Difference in the orientation of the event planedetermined with positive or negative particles!
This analysis has to be repeated using RHIC data
S.A. VoloshinSchool of Collective Dynamics in High Energy Collisions, LBNL, June 7-11, 2010page 4
EDM of QCD matter
Charge separation along the orbital momentum:EDM of the QCD matter (~ the neutron EDM)(Local Parity Violation)
Chiral magnetic effect:
NL≠NR ⊕ magnetic field or Induction of the electric field parallel to the (static) magnetic field
L or B
The asymmetry is too small to observein a single event, but is measurable by correlation techniques
S.A. VoloshinSchool of Collective Dynamics in High Energy Collisions, LBNL, June 7-11, 2010page
Anisotropic flow
X
Z b
XZ – the reaction plane
Picture: © UrQMD
E877, PRC 55 (1997) 1420
; | | nininn n n n nQ e Q Q e X iY
2/22 MYX
E877, PRL 73, 2532 (1994)
S.A. VoloshinSchool of Collective Dynamics in High Energy Collisions, LBNL, June 7-11, 2010page
Anisotropic flow, RHIC
STAR Collaboration , PRL, 86 (2000) 402
v2 =⟨cos(2( −RP )⟩
STAR Collaboration , PRL, 101 (2008) 252101 Au+Au, 200 GeV
X
Z b
XZ – the reaction plane
Picture: © UrQMD
*,, ,, cos n a n bn a n b a b nu u v vn
S.A. VoloshinSchool of Collective Dynamics in High Energy Collisions, LBNL, June 7-11, 2010page 9
Observable
The effect is too small to see in a single event The sign of Q varies and 〈 a 〉 = 0 (we consider
onlythe leading, first harmonic) one has to measure correlations, 〈 aα aβ〉 , P -even quantity (!)
〈 aα aβ〉 is expected to be ~ 10-4
〈 aα aβ 〉 can not be measured as 〈 sin φα sin φβ 〉due to large contribution from effects notrelated to the orientation of the reaction plane
the difference in corr’s in- and out-of-plane A practical approach: three particle correlations
Effective particle distribution
L or B
Bin ≈ Bout, v1= 0
S.A. VoloshinSchool of Collective Dynamics in High Energy Collisions, LBNL, June 7-11, 2010page 10
Charge separation: expectations/predictions
Same charge particles are preferentially emitted in the same direction, along or opposite to the system orbital momentum and magnetic field.
Unlike-sign particles are emitted in the opposite directions.
“Quenching” in a dense medium can lead to suppressionof unlike-sign (“back-to-back”) correlations.
The effect has a “typical” Δη width of order ~ 1.
The magnitude of asymmetry ~ 10─2 for midcentral collisions 10─4 for correlations.
Effect is likely to be most pronounced at pt ≤ ~ 1 GeV/c, though radial flow can move it to higher pt
Asymmetry is proportional to the strength of magnetic field
“Signature” of deconfinement and chiral symmetry restoration
Kharzeev, PLB 633 260 (2006) [hep-ph/0406125]Kharzeev, Zhitnitsky, NPA 797 67 (2007)Kharzeev, McLerran, Warringa, NPA 803 227 (2008)Fukushima, Kharzeev, Waringa, PRD 78, 074033
S.A. VoloshinSchool of Collective Dynamics in High Energy Collisions, LBNL, June 7-11, 2010page 11
STAR Experiment and Data Set
This analysis used the data taken during RHIC Run IVand based on (after all quality cuts)Au+Au 200 GeV ~ 10.6 M Minimum Bias eventsAu+Au 62 GeV ~ 7 M Minimum Bias eventsCu+Cu 200 GeV ~ 30 M Minimum Bias eventsCu+Cu 62 GeV ~ 19 M Minimum Bias events
Tracking done by two Forward TPCs (East and West) and STAR Main TPC.
Tracks used: | η | < 1.0 (Main TPC) -3.9 < η < -2.9 (FTPC East) 2.9 < η < 3.9 (FTPC West)
0.15 < pT < 2.0 GeV/c(unless specified otherwise)
ZDC-SMD (spectatorneutrons) is used for the first order reaction plane determination
Results presented/discussedin this talk are for charged particles in the main TPC region (|η| < 1.0)
Central Trigger Barrel
Silicon Vertex Tracker
ZDC
Barrel EM Calorimeter
Magnet
Coils
ZDC
FTPC west
Main TPC
FTPC east
S.A. VoloshinSchool of Collective Dynamics in High Energy Collisions, LBNL, June 7-11, 2010page 12
Backgrounds
Event generators: the signal is not zero, but different from expectations (e.g. same charge ~ opp. charge)
(+,+) and (-,-) results are combined as “same charge”HIJING+v2 = added “afterburner” to generate flowMEVSIM: flow as in experiment, number of resonances maximum what is consistent with experiment
“Flowing clusters”/RP dependent fragmentation
Global polarization, v1 fluctuations, …
I. Physics (RP dependent). Can not be suppressed
II. RP independent. (depends on method and in general can be greatly reduced)
?
S.A. VoloshinSchool of Collective Dynamics in High Energy Collisions, LBNL, June 7-11, 2010page 13
Data vs models
Large difference in like-sign vs unlike-sign correlations in the data compared to models.
Bigger amplitude in like-sign correlations compared to unlike-sign.
Like-sign and unlike-sign correlations are consistent with theoretical expectations
… but the unlike-sign correlations can be dominated by effects not related to the RP orientation.
The “base line” can be shifted from zero.
First – let us understand uncertainties and them move to more “differential” results
〈 +,+ 〉 and 〈– ,– 〉 results agree within errors and are combined in this plot and all plots below.
S.A. VoloshinSchool of Collective Dynamics in High Energy Collisions, LBNL, June 7-11, 2010page 14
Testing the technique. (RP from TPC and FTPC)
Testing:
Using ZDC-SMD for the (first harmonic) event plane yields similar agreement, though with larger uncertainties (Run VII – next slide).
Center points: obtained using v2{FTPC}bands: lower limits - using v2{2}, upper – v2{4}; where v2{4} is not available it is assumed that using v2{FTPC} suppresses non-flow by 50%.
| η | < 1.0 (Main TPC) 2.9 < |η| < 3.9 (FTPC)
S.A. VoloshinSchool of Collective Dynamics in High Energy Collisions, LBNL, June 7-11, 2010page 15
Testing the technique. (RP from ZDC-SMD)
Testing: | η | < 1.0 (Main TPC) 2.9 < |η| < 3.9 (FTPC)
G. Wang (STAR),talk at 2010 April APS Meeting
S.A. VoloshinSchool of Collective Dynamics in High Energy Collisions, LBNL, June 7-11, 2010page 16
Uncertainties at small multiplicities
Thick lines: uncertainties due to 3-particle correlations not related to the reaction plane orientation for the case of particle c taken from TPC region (from HIJING; UrQMD givesabout factor of 2 smaller values).
Note: such uncertainty in principle can be decreased, e.g. if use particle c separated from (α,β ) by a rapidity gap.
- Uncertainty is smaller for the same charge signal than for opposite charge.
- Even smaller (consistent with zero) for triplets with all particles of the same charge (not shown)
(RP independent background)
S.A. VoloshinSchool of Collective Dynamics in High Energy Collisions, LBNL, June 7-11, 2010page 17
Charge combinations
The results are independent of the charge of “c” particle. (Note results for 3 particles all of the same charge)
RFF FF
S.A. VoloshinSchool of Collective Dynamics in High Energy Collisions, LBNL, June 7-11, 2010
PHENIX (from talk of R. Lacey)
page 18
S.A. VoloshinSchool of Collective Dynamics in High Energy Collisions, LBNL, June 7-11, 2010
PHENIX: results
page 19
S.A. VoloshinSchool of Collective Dynamics in High Energy Collisions, LBNL, June 7-11, 2010
PHENIX/STAR comparison
page 20
S.A. VoloshinSchool of Collective Dynamics in High Energy Collisions, LBNL, June 7-11, 2010page 21
Au+Au and Cu+Cu @ 200 GeV
+/– signal in Cu+Cu is stronger,qualitatively in agreement with “theory”,but keep in mind large uncertainties dueto correlations not related to RP
S.A. VoloshinSchool of Collective Dynamics in High Energy Collisions, LBNL, June 7-11, 2010page 22
Au+Au and Cu+Cu @ 200 GeV
Opposite charge correlations scale with Npart,
(suppression of the back-to-back correlations ?)
Same charge signal is suggestive of correlations with the reaction plane
Opposite charge corr’s are somewhat stronger in CuCu compared to AuAu at the same Npart
The signal is multiplied by Npart to remove “trivial” dilution due to multiplicity increasein more central collisions.
S.A. VoloshinSchool of Collective Dynamics in High Energy Collisions, LBNL, June 7-11, 2010page 23
Δη dependences (AuAu200).
Typical “hadronic” width,consistent with “theory”.
Not color flux tubes?
Correlations at large Δη (and large Δpt, see next slide) --it is not HBT or Coulomb.
S.A. VoloshinSchool of Collective Dynamics in High Energy Collisions, LBNL, June 7-11, 2010page 24
Transverse momentum dependences (AuAu200).
Signal persiststo too high pt ?
S.A. VoloshinSchool of Collective Dynamics in High Energy Collisions, LBNL, June 7-11, 2010page 25
Transverse momentum dependences (AuAu200).
25
The transverse momentum dependence of the signal shown in the previousslide is fully consistent with a picture in which particles from a LPV cluster decay has pt distribution only slightly “harder” than the bulk.
S.A. VoloshinSchool of Collective Dynamics in High Energy Collisions, LBNL, June 7-11, 2010
Two-particle correlations
page 26
Little comments…Where are they coming from?How large is the contribution to them,from the same LPV physics?
Requires differential analysis.
Solid – 2part correlations
Open -- <a+b-2Psi> 1 2 1 2 1 2cos 2 RP c c s s
1 2 1 2 1 2cos c c s s
S.A. VoloshinSchool of Collective Dynamics in High Energy Collisions, LBNL, June 7-11, 2010
Two-particle correlations and background
page 27
Solid – 2part correlations
Open -- <a+b-2Psi> 1 2 1 2 1 2cos 2 RP c c s s
1 2 1 2 1 2cos c c s s
Two “remarkable” cancellations:-- <cos(φ1+φ2)> , opposite sign, very close to zero-- <cos(φ1+φ2)>, same sign, very close to <cos(φ1-φ2)>
Consider correlations of the type ~[a + 2b cos(φ1-φ2)]
It leads to <cos(φ1-φ2)> ≈ b and <cos(φ1+φ2)> = 2 b v2
S.A. VoloshinSchool of Collective Dynamics in High Energy Collisions, LBNL, June 7-11, 2010
Summary of STAR/PHENIX results
page 28
Measured correlations agree with the magnitude and gross features of the theoretical predictions for Chiral Magnetic Effect
Observables are P-even and might be sensitive to non-parity-violating effects.
The observed signal cannot be described by the background models studied (HIJING, HIJING+v2, UrQMD, MEVSIM), which span a broad range of hadronic physics.
S.A. VoloshinSchool of Collective Dynamics in High Energy Collisions, LBNL, June 7-11, 2010page 29
Future program
Dedicated experimental and theoretical program focused on the local parity violation, and more generally on non-perturbative QCD: structure of the vacuum, hadronization, etc.
Experiment:
U+U central body-body collisions
Beam energy scan / Critical point search
Isobaric beams
High statistics PID studies / properties of the clusters
Such collisions (“easy” to trigger on) will have low magnetic field and large elliptic flow – clean test of the LPV effect.
Colliding isobaric nuclei (the same mass number anddifferent charge) and by that controlling the magnetic field
Note that such studies will be also very valuable for understanding the initial conditions, baryonstopping, origin of the directed flow, etc.
Ru+ Zr96 9644 40
Look for a critical behavior, as LPV predicted to depend strongly on deconfinement and chiral symmetry restoration
in particular with neutral particles; see also next slides
S.A. VoloshinSchool of Collective Dynamics in High Energy Collisions, LBNL, June 7-11, 2010page 30
Future program. Needs for theory.
Theory:
Confirmation and detail study of the effect in Lattice QCD
Theoretical guidance and detailed calculations are needed: ▪ Dependence on collision energy, centrality, system size, magnetic field, PID, etc. ▪ Understanding physics background ! ▪ Size/effective mass of the clusters, quark composition (e.g. equal number of q-qbarpairs of different flavors?).
S.A. VoloshinSchool of Collective Dynamics in High Energy Collisions, LBNL, June 7-11, 2010
Central U+U collisions
page 31
In both cases the magneticfield is small, but elliptic flow is large in body-body.
All (“physics”) background effects scale with elliptic flow.
Correlations due to chiral magneticeffect scale with (square of ) the magnetic field.
S.A. VoloshinSchool of Collective Dynamics in High Energy Collisions, LBNL, June 7-11, 2010
Model
page 32
Magnetic field:
Elliptic flow:
Sum runs over allspectators,Calculated for t=0.
Nuclear parameters:
S.A. VoloshinSchool of Collective Dynamics in High Energy Collisions, LBNL, June 7-11, 2010
Central U+U and Au+Au (Nspect<20)
page 33
How to select events with large v2?
S.A. VoloshinSchool of Collective Dynamics in High Energy Collisions, LBNL, June 7-11, 2010
Selecting on multiplicity
page 34
Note the possibility to perform test“working” on fluctuations in Au+Au collisions
S.A. VoloshinSchool of Collective Dynamics in High Energy Collisions, LBNL, June 7-11, 201035
Clusters in multi-particle production
cluster = sphaleron?
S.A. VoloshinSchool of Collective Dynamics in High Energy Collisions, LBNL, June 7-11, 2010
Clusters = sphalerons?
page 36
S.A. VoloshinSchool of Collective Dynamics in High Energy Collisions, LBNL, June 7-11, 2010
Pomeron out of instantons?
page 37
S.A. VoloshinSchool of Collective Dynamics in High Energy Collisions, LBNL, June 7-11, 201038
t’Hooft’s interaction. Sphaleron cluster.
We have to identify other features of the “topological bubbles”, and extend experimental search, e.g. instanton “bubble” decay isotropically, it should lead to equal number of q-qbar pairs of all flavors.We can check this.
S.A. VoloshinSchool of Collective Dynamics in High Energy Collisions, LBNL, June 7-11, 2010
pp2pp Phase II
page 39
Look for: Mass distribution, multiplicities, quark flavor content of the clusters (PID correlations, KKπ vs πππ, etc.), angular distributions, unusual behavior in HBT parameters (production by coherent field)
S.A. VoloshinSchool of Collective Dynamics in High Energy Collisions, LBNL, June 7-11, 201040
Conclusions
The physics of the local strong parity violation is not an exotics but an integral part of QCD: quark interactions with topologically non-trivial gluonic configurations - instantons, sphalerons, etc., the same physics as that of the chiral symmetry breaking.
Observable effects are within reach of the experiment !
In non-central nuclear collisions it leads to the charge separation along the system’s orbital momentum/magnetic field.
STAR and PHENIX detect a signal that is in agreement with (mostly qualitative) theoretical predictions.
A dedicated program aimed on a direct experimental study of non-perturbative QCD effects is developing: U+U collisions, beam energy scan, identified particle correlations, isobaric beams…
S.A. VoloshinSchool of Collective Dynamics in High Energy Collisions, LBNL, June 7-11, 2010
Zr+Ru, GSI
page 41
Change in the strength of the magnetic filed ~20% !!
Very interesting reactions to investigatethe baryon stopping and the nature of the directed flow
S.A. VoloshinSchool of Collective Dynamics in High Energy Collisions, LBNL, June 7-11, 201042
HIT, LBNL, May 27, 2008
Instantons in QCD
Interacting with instanton quarks change chirality !
Topological transitions have never been observed directly (e.g. at the level of quarks in DIS). An observation of the local strong parity violation would be a clear proof for the existence of such physics.