Physics Letters B Measuring K K Interactions Using Pb–Pb Collisions

Physics Letters B 774 (2017) 64–77 Contents lists available at ScienceDirect Physics Letters B www.elsevier.com/locate/physletb 0 ± √Measuring KSK interactions using Pb–Pb collisions at sNN = 2.76 TeV .ALICE Collaboration a r t i c l e i n f o a b s t r a c t ± Article history: 0 We present the first ever measurements of femtoscopic correlations√ between the KS and K particles. The Received 22 May 2017 analysis was performed on the data from Pb–Pb collisions at sNN = 2.76 TeV measured by the ALICE Received in revised form 24 August 2017 experiment. The observed femtoscopic correlations are consistent with final-state interactions proceeding Accepted 4 September 2017 via the a0(980) resonance. The extracted kaon source radius and correlation strength parameters for Available online 8 September 2017 0 − 0 + K K are found to be equal within the experimental uncertainties to those for K K . Comparing the Editor: L. Rolandi S S results of the present study with those from published identical-kaon femtoscopic studies by ALICE, mass and coupling parameters for the a0 resonance are tested. Our results are also compatible with the interpretation of the a0 having a tetraquark structure instead of that of a diquark. © 2017 The Author. Published by Elsevier B.V. This is an open access article under the CC BY license 3 (http://creativecommons.org/licenses/by/4.0/). Funded by SCOAP . 1. Introduction nance since the kaon pair is in an I = 1 isospin state, as is the a0, whereas the f0 is an I = 0state. 0 ± Another feature of the K K FSI through the a0 resonance is, Identical boson femtoscopy, especially of identical charged pi- S 0 due to the a having strangeness S = 0 and the K being a linear ons, has been used extensively over the years to study experi- 0 S 0 0 mentally the space–time geometry of the collision region in high- combination of the K and K , energy particle and heavy-ion collisions [1]. Identical-kaon fem- 1 toscopy studies have also been carried out, recent√ examples of 0 = √ 0 + 0 = KS K K , (1) which are the ones with Au–Au collisions at sNN √ 200 GeV 2 0 0 = by the STAR Collaboration√ [2] (KS KS ) and with pp at s 7TeV = 0 + 0 + 0 − 0 − and Pb–Pb collisions at sNN 2.76 TeV by the ALICE Collabora- only the K K pair from KS K and the K K pair from KS K have 0 0 ± ± = tion [3–5] (KS KS and K K ). The pair-wise interactions between S 0 and thus can form the a0 resonance. This allows the pos- 0 0 the identical kaons that form the basis for femtoscopy are for sibility to study the K and K sources separately since they are ± ± 0 − 0 + K K quantum statistics and the Coulomb interaction, and for individually selected by studying KS K and KS K pairs, respec- 0 0 KS KS quantum statistics and the final-state interaction through the tively. An additional consequence of this feature is that only 50% 0 − 0 + f0(980)/a0(980) threshold resonances. of either the KS K or KS K detected pairs will pass through the One can also consider the case of non-identical kaon pairs, a0 resonance. This is taken into account in the expression for the 0 ± e.g. KS K pairs. Besides the non-resonant channels which may be model used to fit the correlation functions. present, e.g. non-resonant elastic scattering or free-streaming of On the other hand, the natural requirement that the source 0 ± the kaons from their freeze-out positions to the detector, the other sizes extracted from the KS K femtoscopy agree with those ob- 0 ± ± ± only pair-wise interaction allowed for a K K pair at freeze out 0 0 S tained for the KS KS and K K systems allows one to study the from the collision system is a final-state interaction (FSI) through properties of the a0 resonance itself. This is interesting in its own the a0(980) resonance. The other pair-wise interactions present right since many studies discuss the possibility that the a0, listed 0 ± for identical-kaon pairs are not present for KS K pairs because: by the Particle Data Group as a diquark light unflavored meson a) there is no quantum statistics enhancement since the kaons are state [6], could be a four-quark state, i.e. a tetraquark, or a “K–K not identical, b) there is no Coulomb effect since one of the kaons molecule” [7–12]. For example, the production cross section of the 0 − − is uncharged, and c) there is no strong FSI through the f0 reso- a0 resonance in a reaction channel such as K K → a should de- − 0 pend on whether the a0 is composed of duor dssuquarks, the former requiring the annihilation of the ss pair and the latter be- − E-mail address: [email protected]. ing a direct transfer of the quarks in the kaons to the a0 . The http://dx.doi.org/10.1016/j.physletb.2017.09.009 0370-2693/© 2017 The Author. Published by Elsevier B.V. This is an open access article under the CC BY license (http://creativecommons.org/licenses/by/4.0/). Funded by SCOAP3. ALICE Collaboration / Physics Letters B 774 (2017) 64–77 65 0 − 0 results from KS K femtoscopy might be sensitive to these two dif- reconstructed KS was required to be less than 0.3 cm in all di- ferent scenarios. rections. The required Nσ values for the pions were Nσ TPC < 3 0 ± In this Letter, results from the first study of KS K femtoscopy√ and Nσ TOF < 3for p > 0.8GeV/c. An invariant mass distribu- = + − 0 are presented. This has been done for Pb–Pb collisions at sNN tion for the π π pairs was produced and the KS was defined 2.76 TeV measured by the ALICE experiment at the LHC [13]. The to be resulting from a pair that fell into the invariant mass range ± 0 + − 2 physics goals of the present KS K femtoscopy study are the fol- 0.480 < mπ π < 0.515 GeV/c . lowing: 1) show to what extent the FSI through the a0 resonance ± 0 0 describes the correlation functions, 2) study the K and K sources 2.1.2. K selection to see if there are differences in the source parameters, and 3) test Charged kaon tracks were also detected using the TPC and published a0 mass and coupling parameters by comparisons with TOF detectors, and were accepted if they were within the range published identical kaon results [5]. 0.14 < pT < 1.5GeV/c. In order to reduce the number of secon- daries (for instance the charged particles produced in the detector 2. Description of experiment and data selection material, particles from weak decays, etc.) the primary charged kaon tracks were selected based on the DCA, such that the DCA The ALICE experiment and its performance in the LHC Run 1 transverse to the beam direction was less than 2.4 cm and the (2009–2013) are described in Ref. [13] and Ref. [14,15], respec- DCA along the beam direction was less than 3.2 cm. If the TOF 6 tively. About 22 × 10 Pb–Pb collision events with 0–10% centrality signal were not available, the required Nσ values for the charged class taken in 2011 were used in this analysis (the average cen- kaons were Nσ TPC < 2for pT < 0.5GeV/c, and the track was re- trality in this range is 4.9% due to a slight trigger inefficiency in jected for pT > 0.5GeV/c. If the TOF signal were also available and the 8–10% range). Events were classified according to their cen- pT > 0.5GeV/c: Nσ TPC < 3 and Nσ TOF < 2(0.5 < pT < 0.8GeV/c), trality using the measured amplitudes in the V0 detectors, which Nσ TOF < 1.5(0.8 < pT < 1.0GeV/c), Nσ TOF < 1(1.0 < pT < consist of two arrays of scintillators located along the beamline 1.5GeV/c). 0 ± and covering the full azimuth [16]. Charged particles were recon- KS K experimental pair purity was estimated from a Monte structed and identified with the central barrel detectors located Carlo (MC) study based on HIJING [18] simulations using GEANT3 within a solenoid magnet with a field strength of B = 0.5T. [19] to model particle transport through the ALICE detectors. The Charged particle tracking was performed using the Time Projection purity was determined from the fraction of the reconstructed MC 0 ± Chamber (TPC) [17] and the Inner Tracking System (ITS) [13]. The simulated pairs that were identified as actual KS K pairs input ITS allowed for high spatial resolution in determining the primary from HIJING. The pair purity was estimated to be 88% for all kine- (collision) vertex. Tracks were reconstructed and their momenta matic regions studied in this analysis. were obtained with the TPC. A momentum resolution of less than 10 MeV/c was typically obtained for the charged tracks of inter- 3. Analysis methods est in this analysis. The primary vertex was obtained from the ITS, the position of the primary vertex being constrained along the 3.1. Experimental correlation functions beam direction (the “z-position”) to be within ±10 cm of the cen- ter of the ALICE detector. In addition to the standard track quality 0 ± This analysis studies the momentum correlations of KS K pairs selections, the track selections based on the quality of track re- using the two-particle correlation function, defined as construction fit and the number of detected tracking points in the ∗ ∗ ∗ TPC were used to ensure that only well-reconstructed tracks were C(k ) = A(k )/B(k ) (2) taken in the analysis [14,15].

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