ptribedy's blog

We present measurements of the charge separation across different harmonic event-planes in U+U and Au+Au collisions to search for signatures of the chiral magnetic effect (CME). We explore the dependence of our observables on pseudo-rapidity separation between particles, event centrality, and colliding system. We also include in our analysis charge separation across third harmonic event-plane insensitive to CME. We test commonly used symmetry and factorization assumptions to estimate non-CME background contribution using the charge separation across third harmonic plane and charge-dependent two-particle correlations and find that for our data they are invalid.

We also decompose the charge separation observables into in-plane and out-of-plane components where CME should be most prevalent in the out-of-plane direction. We compare our data to a non-CME background model based on hydrodynamics coupled with local charge conservation and global momentum conservation. Although several features of our data appear to favor an interpretation based on the magnetic field dependence expected for CME, we find the background model captures most of the observed trends.

Motivation:

Glauber calculations indicate that U+U and Au+Au have large B-field difference at large Npart. CME driven signal of charge separation should be sensitive to such difference.

Hydrodynamic simulations with LCC + GMC indicate background to be similar between U+U and Au+Au.
B-field driven charge separation can be contrasted to background driven charge separation

The current dataset (~300M U+U & ~400M Au+Au events) can provide good precision measurements of the charge separation & flow driven background expectations.
Precision differential measurements like Δη-dependence of azimuthal correlations can help isolate non-flow backgrounds assuming a simple ansatz that is purely empirical.

The aim is to quantify different sources of correlations that determine the structure in Δη such as HBT, Coulomb & di-jets

Two particle correlations (short-range subtracted) & charge separation (before & after short range subtraction) in Au+Au & U+U can be quantified this way

Study of Higher harmonics and test of symmetry/factorization and data driven CME background expectations. The following equations demonstrate how such assumptions used in the literature are used to express three particle correlations into product of two-particle correlations. Often such assumptions are used to extract background expectations of CME

Observables :

In this analysis we study the following observables :

We also analyze the higher order cumulants:

In order to demonstrate the symmetry assumptions we express the correlators as follows:

We estimate the factorization co-efficient

We de-compose γ-correlator into in-plane and out-of-plane component

Figures that will go into the paper :

Fig.1 : Signal and background expectations from the Models

Fig.2 : Δη dependence of the charge dependent 3-particle correlations in all centralities showing different components in U+U

Fig.3 : The same as Fig.2 but for Au+Au .

Fig.4 : The systematics of Δη dependence on C112 showing the widths and magnitudes of different components Fig.2 & Fig.3

Fig.5 : The scaled correlator showing different components and their centrality dependence in Au+Au and U+U collisions.

Fig.6 : The scaled correlators in Au+Au and U+U collisions when an η-gap is applied using a sub-event.

Fig.7 : The correlators in Au+Au and U+U collisions compared to models

Fig.8 : The same as Fig.6 but scaled by v_n

Fig.9 : The figure demonstrating the symmetry assumption <sin sin>=0 does not hold at RHIC

Fig.10 : A cartoon representation of Fig.9

Fig.11 : Ratio showing the factorization breaking and it's dependence on harmonics

Fig.12 : In-plane and out-of-plane de-compositions of the γ-correlator as used in the Sergei's original notations

Summary :

Although the cleanest test to distinguish v₂ related background from magnetic field dependent signal will come from collisions of isobars, in this paper we present data from U+U and Au+Au collisions where the different systems provide an opportunity to study events where the system size are similar but the magnetic fields are different.

Hydrodynamic simulations including resonance decays and local charge conservation predict that background contribution to Δγ₁₁₂ scaled by N_part/v₂ is similar in U+U and Au+Au collisions. Therefore, we have two systems to contrast signal and background for CME.

Studying the correlators as a function of the pseudo-rapidity separation between particles “a” and “b”, we find that peripheral collisions are dominated by positive, short-range correlations consistent with jets, quantum interference and other near-side non-flow effects. In mid-central and central collisions, the character of the ∆η dependence changes and the contribution from near-side short-range, non-flow effects decreases significantly. We subtracted those non-flow contributions and presented the residual correlations.

We examine the charge dependence of Δγ₁₂₃, expected to be insensitive to the CME signal. We compare these correlations to model calculations.

It is clear that the assumptions required to express the three particle correlations in terms of flow modulated two particle correlations (factorization and neglecting sin terms in the expansion) do not hold up. Data include charge sensitive correlations dependent on flow axis that can not be factorized into two particle correlations.

In this paper we have focused on the qualitative features of the data. The results do not appear to entirely conform with either expectations of background or with expectations of signal.

Supporting materials:

QM proceedings :

https://arxiv.org/pdf/1704.03845.pdf

Our previous paper on 3-particle correlations :

https://drupal.star.bnl.gov/STAR/content/three-particle-harmonic-decomposition

Presentation in PWG and collaboration meeting :

https://drupal.star.bnl.gov/STAR/system/files/paper_proposal_cmefocusgroup_july2019.pdf

https://drupal.star.bnl.gov/STAR/system/files/Delta_eta_systematics_threeparticle_correlations_uupaper.pdf

https://drupal.star.bnl.gov/STAR/system/files/uupaper_analysis_meeting_dec2018.pdf

https://drupal.star.bnl.gov/STAR/system/files/uupaper_after_comments_model_study.pdf

https://drupal.star.bnl.gov/STAR/system/files/uupaper_after_comments.pdf

https://drupal.star.bnl.gov/STAR/system/files/july10_taskforce_UU_paper.pdf

https://drupal.star.bnl.gov/STAR/system/files/taskforce_UU_paper_draft_presentation_june26.pdf

https://drupal.star.bnl.gov/STAR/system/files/taskforce_UU_paper_draft_presentation_june5.pdf