Title: Charged kaon femtoscopy in Au+Au collisions at 14.6, 19.6, 27, 39, 62.4, and 200 GeV

PA: Yevheniia Khyzhniak, Michael Lisa, Grigory Nigmatkulov
Contact information: eugenia.sh.el@gmail.com, lisa@physics.osu.edu, nigmatkulov@gmail.com

Target journal: Physical Review C

Talks in PWG:
1) Purity, momemtum resolution, first results
2) ToF edge effect
3) Spherical decomposition, PID influence, first results for energy dependence
4) Momentum resolution correction, first results for systematics, merging effect (selecting good octants)
5) First paper proposal in PWG
6) Updated paper proposal in PWG
7) PWGC presentation -> Software group made some changes in links and spaces in links now "%20" -> "+", so this link does not work
7) PWGC presentation
8) Preliminary request for QM

Preliminary figures:
1) Correlation functions
2) Charge difference for pions
3) Charge difference for kaons
4) 3D radii energy dependence
5) Emission time energy dependence

Abstract:
We present the results of three-dimensional femtoscopic analysis of charged pions and kaons produced in Au+Au collisions at = 14.6, 19.6, 27, 39, 62.4, and 200 GeV recorded by the STAR detector.

Correlation functions are studied as a function of transverse mass, collision centrality and energy using the Bertsch-Pratt ("out", "side", "long") decomposition. Femtoscopic radii (Rout, Rside, Rlong) are extracted from the correlation function fits assuming the Gaussian source profile. Measured femtoscopic pion and kaon radii increase with increasing collision energy. Rout and Rlong radii for kaons are generally larger than that for pions obtained for the same mT range for all collision energies and centrality intervals.

Previous measurements have noted a difference between radii for negatively and positively charged pions source at low energies. With higher statistics and high quality data, we met out the difference between correlation functions themselves for the entire considered range of energies. The difference is stronger for central collisions at low transverse momentum of pion pairs and lower energies. The effect is consistent with the UrQMD calculations. The charge splitting of the source radii was early predicted in the investigation of the combined effects of nuclear Coulomb field, radial flow, and opaqueness on two-pion correlations for Au+Au collisions.

The time of maximal emission of the system is estimated using the three-dimensional femtoscopic analysis for pions and kaons as a function of energy. The measured emission time of kaons is larger than those for pions and increases with increasing energy.

Figure 1:
The top panel shows the particle identification using dE/dx in the TPC. The vertical line shows TPC applicability limit in this analysis. The bottom panel shows particle identification using inverse velocity (1/β) measured by the TOF. Both figures are for Au+Au at 200 GeV collisions.

Figure 2:
Two-dimensional projections of the three-dimensional correlation function in the "long" vs. "side" (upper left panel), "long" vs. "out" (upper middle), and "side" vs. "out" (upper right) planes, respectively, for π−π− pairs with transverse momentum 0.15 − 0.25 GeV/c for 0 − 10% centrality in Au+Au@200GeV. The lower panel of figures corresponds to the same dependence after selection of the relative momentum regions (octants) that are less affected by the track-merging effect.

Figure 3:
One-dimensional projections of the three-dimensional correlation function onto the "out", "side", and "long" axes for π−π− pairs from events in the 0-10% centrality range with transverse momentum 0.15 − 0.25 GeV/c. For each projection qi shown, the other components of relative momentum are integrated over the range |qj| < 50 MeV/c. Green markers show the projections of the correlation function constructed using the octants that are least affected by the merging effect; red markers - using the most affected.

Figure 4:
Energy, centrality and kT dependence of the ratio of one-dimensional correlation functions of positive and negative pions (top row) and kaons (bottom row).

Figure 5:
The ratio of one-dimensional correlation functions of positive and negative pions with a centrality of 0-10% and transverse momentum of the studied pair of particles 0.15-0.25 GeV/c for 19.6 GeV from experimental data (blue markers) and UrQMD calculations (red markers).


Figure 5.1:
The ratio of one-dimensional correlation functions of positive and negative pions (red points) and kaons (blue points) constructed from toy-model data.


Figure 6:
One-dimensional projections of the three-dimensional correlation function onto the "out", "side", and "long" axes for pion (magenta stars) and kaon (blue crosses) pairs from events in the 0-10% centrality range with transverse momentum 0.25 − 0.35 GeV/c (top row) and 0.55 − 0.65 GeV/c (bottom row). For each projection qi shown, the other components of relative momentum are integrated over the range |qj| < 50 MeV/c. The lines of the corresponding colors show the fit of the correlation functions.

Figure 7:
The mT dependence of Rout, Rside , and Rlong for each energy and multiple centralities. Errors are statistical only. Closed markers show the radii obtained from the fit of pion correlation functions, open markers show same for kaons.

Figure 8:
The mT dependence of the ratio and difference of the transverse radii of the pions source (closed markers) and kaons (open markers) for 200 GeV (top row) and 19.6 GeV (bottom row) energy and multiple centralities.

Figure 9:
Energy dependence of the Rout (top), Rside (middle), and Rlong (bottom) for central Au + Au and Pb + Pb collisions at midrapidity and kT ≈ 0.3 GeV/c (closed markers) and kT ≈ 0.5 GeV/c (open markers). Blue markers show pion radii, green markers show kaon radii.

Figure 10:
Rlong vs. mT for kaons and pions. The solid lines show the fit using Eq. (X) for pions and kaons to extract the emission times (τ); the dashed and dotted lines show the fit using Eq. (Y).

Figure 11:
Emission times of the system as a function of beam energy for central Au + Au and Pb + Pb collisions assuming a temperature of T = 0.144 GeV (for Eq. X) and T = 0.120 GeV (for Eq. Y) at kinetic freeze-out.

Conclusions
- Identical charged pion and kaon femtoscopic correlation are measured in Au+Au collisions at = 14.6, 19.6, 27, 39, 62.4, and 200 GeV
- Difference between correlation functions of charged pions themselves for the entire considered range of energies observed. The effect is stronger for central collisions at low transverse momentum of pion pairs and lower energies. The effect consistent with the UrQMD calculations.
- Extracted femtoscopic radii (Rout, Rside, Rlong) increase with collision energy.
- Rlong for kaons is generally larger than that for pions:
    = Larger time of maximum emission that increases with energy
    = Influence of resonance (K*) decays