S.A. Voloshin Strong and Electroweak Matter 2010, McGill University, Montreal, July 29-July 2, 2010 page 1 Sergei A. Voloshin L or B Charge dependent correlations and chiral magnetic effect
S.A. VoloshinStrong and Electroweak Matter 2010, McGill University, Montreal, July 29-July 2, 2010page 1
Sergei A. Voloshin
L or B
Charge dependent correlations and chiral magnetic effect
S.A. VoloshinStrong and Electroweak Matter 2010, McGill University, Montreal, July 29-July 2, 2010page 1
Local strong parity violation and new perspectives in experimental study of non-perturbative QCD
Sergei A. Voloshin
L or BOutline:
QCD vacuum. Chiral Magnetic Effect Anisotropic flow Observable Experimental results Future directions
Summary
S.A. VoloshinStrong and Electroweak Matter 2010, McGill University, Montreal, July 29-July 2, 2010page 1
Local strong parity violation and new perspectives in experimental study of non-perturbative QCD
Sergei A. Voloshin
L or BOutline:
QCD vacuum. Chiral Magnetic Effect Anisotropic flow Observable Experimental results Future directions
Summary
STAR results:
S.A. VoloshinStrong and Electroweak Matter 2010, McGill University, Montreal, July 29-July 2, 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
S.A. VoloshinStrong and Electroweak Matter 2010, McGill University, Montreal, July 29-July 2, 2010page 3
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. VoloshinStrong and Electroweak Matter 2010, McGill University, Montreal, July 29-July 2, 2010page
Anisotropic flow X
Z
XZ – the reaction plane
Picture: © UrQMD
S.A. VoloshinStrong and Electroweak Matter 2010, McGill University, Montreal, July 29-July 2, 2010page
Anisotropic flow X
Z
XZ – the reaction plane
Picture: © UrQMD
E877, PRL 73, 2532 (1994)
S.A. VoloshinStrong and Electroweak Matter 2010, McGill University, Montreal, July 29-July 2, 2010page
Anisotropic flow X
Z
XZ – the reaction plane
Picture: © UrQMD
E877, PRC 55 (1997) 1420E877, PRL 73, 2532 (1994)
S.A. VoloshinStrong and Electroweak Matter 2010, McGill University, Montreal, July 29-July 2, 2010page
Anisotropic flow, RHIC
STAR Collaboration , PRL, 86 (2000) 402
v2 = 〈cos(2(ϕ − ΨRP )〉
STAR Collaboration , PRL, 101 (2008) 252101 Au+Au, 200 GeV
X
Z
XZ – the reaction plane
Picture: © UrQMD
S.A. VoloshinStrong and Electroweak Matter 2010, McGill University, Montreal, July 29-July 2, 2010page
Anisotropic flow, RHIC
STAR Collaboration , PRL, 86 (2000) 402
v2 = 〈cos(2(ϕ − ΨRP )〉
STAR Collaboration , PRL, 101 (2008) 252101 Au+Au, 200 GeV
X
Z
XZ – the reaction plane
Picture: © UrQMD
S.A. VoloshinStrong and Electroweak Matter 2010, McGill University, Montreal, July 29-July 2, 2010page 6
Observable
The effect is too small to see in a single event The sign of topological charge varies and〈a〉= 0 ➔ one has to measure correlations, 〈aα aβ〉, P -even quantity (!)
Consider only first harmonic 〈aα aβ〉 is expected ~ 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
Single particle effective distribution
L or B
Bin ≈ Bout, v1= 0
S.A. VoloshinStrong and Electroweak Matter 2010, McGill University, Montreal, July 29-July 2, 2010page 6
Observable
The effect is too small to see in a single event The sign of topological charge varies and〈a〉= 0 ➔ one has to measure correlations, 〈aα aβ〉, P -even quantity (!)
Consider only first harmonic 〈aα aβ〉 is expected ~ 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
Single particle effective distribution
L or B
Bin ≈ Bout, v1= 0
S.A. VoloshinStrong and Electroweak Matter 2010, McGill University, Montreal, July 29-July 2, 2010page 7
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. VoloshinStrong and Electroweak Matter 2010, McGill University, Montreal, July 29-July 2, 2010page 8
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 (spectator neutrons) 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. VoloshinStrong and Electroweak Matter 2010, McGill University, Montreal, July 29-July 2, 2010page 9
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. VoloshinStrong and Electroweak Matter 2010, McGill University, Montreal, July 29-July 2, 2010page 9
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. VoloshinStrong and Electroweak Matter 2010, McGill University, Montreal, July 29-July 2, 2010page 9
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
LPV theory
LPV theory
“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. VoloshinStrong and Electroweak Matter 2010, McGill University, Montreal, July 29-July 2, 2010page 10
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.
〈 +,+ 〉 and 〈–,–〉 results agree within errors and are combined in this plot and all plots below.
S.A. VoloshinStrong and Electroweak Matter 2010, McGill University, Montreal, July 29-July 2, 2010page 10
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 then move to more “differential” results
〈 +,+ 〉 and 〈–,–〉 results agree within errors and are combined in this plot and all plots below.
S.A. VoloshinStrong and Electroweak Matter 2010, McGill University, Montreal, July 29-July 2, 2010page 11
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. VoloshinStrong and Electroweak Matter 2010, McGill University, Montreal, July 29-July 2, 2010page 12
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. VoloshinStrong and Electroweak Matter 2010, McGill University, Montreal, July 29-July 2, 2010page 13
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. VoloshinStrong and Electroweak Matter 2010, McGill University, Montreal, July 29-July 2, 2010page 14
Charge combinations
The results are independent of the charge of “c” particle. (Note results for 3 particles all of the same charge)
ReversedFull Field
Full Field
S.A. VoloshinStrong and Electroweak Matter 2010, McGill University, Montreal, July 29-July 2, 2010page
PHENIX (from talk of R. Lacey)
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S.A. VoloshinStrong and Electroweak Matter 2010, McGill University, Montreal, July 29-July 2, 2010page
PHENIX: results
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S.A. VoloshinStrong and Electroweak Matter 2010, McGill University, Montreal, July 29-July 2, 2010page
PHENIX/STAR comparison
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S.A. VoloshinStrong and Electroweak Matter 2010, McGill University, Montreal, July 29-July 2, 2010page 18
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. VoloshinStrong and Electroweak Matter 2010, McGill University, Montreal, July 29-July 2, 2010page
Au+Au and Cu+Cu @ 200 GeV
The signal is multiplied by Npart to remove “trivial” dilution due to multiplicity increasein more central collisions.
S.A. VoloshinStrong and Electroweak Matter 2010, McGill University, Montreal, July 29-July 2, 2010page 19
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. VoloshinStrong and Electroweak Matter 2010, McGill University, Montreal, July 29-July 2, 2010page 20
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. VoloshinStrong and Electroweak Matter 2010, McGill University, Montreal, July 29-July 2, 2010page 21
Transverse momentum dependences (AuAu200).
S.A. VoloshinStrong and Electroweak Matter 2010, McGill University, Montreal, July 29-July 2, 2010page 21
Transverse momentum dependences (AuAu200).
Signal persiststo too high pt ?
S.A. VoloshinStrong and Electroweak Matter 2010, McGill University, Montreal, July 29-July 2, 2010page 22
Transverse momentum dependences (AuAu200).
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. VoloshinStrong and Electroweak Matter 2010, McGill University, Montreal, July 29-July 2, 2010page
Two-particle correlations
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Little comments…Where are they coming from?How large is the contribution to them,from the same LPV physics?
Requires differential analysis.
S.A. VoloshinStrong and Electroweak Matter 2010, McGill University, Montreal, July 29-July 2, 2010page
Two-particle correlations
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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>
S.A. VoloshinStrong and Electroweak Matter 2010, McGill University, Montreal, July 29-July 2, 2010page
Two-particle correlations and background
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Solid – 2part correlations
Open -- <a+b-2Psi>
Two “remarkable” cancellations:-- <cos(φ1+φ2)> , opposite sign, very close to zero-- <cos(φ1+φ2)>, same sign, very close to <cos(φ1-φ2)>
S.A. VoloshinStrong and Electroweak Matter 2010, McGill University, Montreal, July 29-July 2, 2010page
Two-particle correlations and background
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Solid – 2part correlations
Open -- <a+b-2Psi>
Two “remarkable” cancellations:-- <cos(φ1+φ2)> , opposite sign, very close to zero-- <cos(φ1+φ2)>, same sign, very close to <cos(φ1-φ2)>
Consider 2-particle correlations of a type ~[a + 2b cos(φ1-φ2)]
It leads to <cos(φ1- φ2)> ≈ b and <cos(φ1+φ2)> = 2 b v2
S.A. VoloshinStrong and Electroweak Matter 2010, McGill University, Montreal, July 29-July 2, 2010page
Summary of STAR/PHENIX results
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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. VoloshinStrong and Electroweak Matter 2010, McGill University, Montreal, July 29-July 2, 2010page 26
Future program
Dedicated experimental and theoretical program focused on study of 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.
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. VoloshinStrong and Electroweak Matter 2010, McGill University, Montreal, July 29-July 2, 2010page 27
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. VoloshinStrong and Electroweak Matter 2010, McGill University, Montreal, July 29-July 2, 2010page
Central U+U collisions
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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. VoloshinStrong and Electroweak Matter 2010, McGill University, Montreal, July 29-July 2, 2010page
Model
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Magnetic field:
Elliptic flow:
Sum runs over allspectators,Calculated for t=0.
Nuclear parameters:
S.A. VoloshinStrong and Electroweak Matter 2010, McGill University, Montreal, July 29-July 2, 2010page
Central U+U and Au+Au (Nspect<20)
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How to select events with large v2?
S.A. VoloshinStrong and Electroweak Matter 2010, McGill University, Montreal, July 29-July 2, 2010page
Central U+U and Au+Au (Nspect<20)
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How to select events with large v2?
S.A. VoloshinStrong and Electroweak Matter 2010, McGill University, Montreal, July 29-July 2, 2010page
Selecting on multiplicity
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Note the possibility to perform testof CME “working” on fluctuations in Au+Au collisions
S.A. VoloshinStrong and Electroweak Matter 2010, McGill University, Montreal, July 29-July 2, 2010page
Sphaleron clusters
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S.A. VoloshinStrong and Electroweak Matter 2010, McGill University, Montreal, July 29-July 2, 2010page
Sphaleron clusters
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S.A. VoloshinStrong and Electroweak Matter 2010, McGill University, Montreal, July 29-July 2, 2010page 33
Clusters in multi-particle production
S.A. VoloshinStrong and Electroweak Matter 2010, McGill University, Montreal, July 29-July 2, 2010page 33
Clusters in multi-particle production
cluster = sphaleron?
S.A. VoloshinStrong and Electroweak Matter 2010, McGill University, Montreal, July 29-July 2, 2010page
Pomeron out of instantons?
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S.A. VoloshinStrong and Electroweak Matter 2010, McGill University, Montreal, July 29-July 2, 2010page
Pomeron out of instantons?
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We have to identify other features of the “topological bubbles”, e.g. instanton “bubble” - decays isotropically, - into equal number of q-qbar pairs of all flavors.
t’Hooft’s interaction
S.A. VoloshinStrong and Electroweak Matter 2010, McGill University, Montreal, July 29-July 2, 2010page
pp2pp Phase II
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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. VoloshinStrong and Electroweak Matter 2010, McGill University, Montreal, July 29-July 2, 2010page 36
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 LPV 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. VoloshinStrong and Electroweak Matter 2010, McGill University, Montreal, July 29-July 2, 2010page 37
Instantons in QCD
Interacting with instanton quarks change chirality !
S.A. VoloshinStrong and Electroweak Matter 2010, McGill University, Montreal, July 29-July 2, 2010page 37
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.
S.A. VoloshinStrong and Electroweak Matter 2010, McGill University, Montreal, July 29-July 2, 2010page 38
P-odd domains in heavy ion collisions. First look.
where α is the fraction of particles originating from the P-odd domain
Difference in the orientation of the event planedetermined with positive or negative particles!
S.A. VoloshinStrong and Electroweak Matter 2010, McGill University, Montreal, July 29-July 2, 2010page 38
P-odd domains in heavy ion collisions. First look.
where α is the fraction of particles originating from the P-odd domain
Difference in the orientation of the event planedetermined with positive or negative particles!
S.A. VoloshinStrong and Electroweak Matter 2010, McGill University, Montreal, July 29-July 2, 2010page 38
P-odd domains in heavy ion collisions. First look.
where α is the fraction of particles originating from the P-odd domain
Difference in the orientation of the event planedetermined with positive or negative particles!
This analysis has to be repeated using RHIC data