Ⅱ-4. Hadron Physics

RIKEN Accel. Prog. Rep. 49 (2016)

Search for exotic dibaryons at LHC-ALICE
K. Terasaki,∗1,∗2 H. Hamagaki,∗2 and T. Gunji∗2ΩΩFig. 1. Decay topology of the N Ω dibaryon.
ΛH dibaryon (uuddss)1) is one of the most investigated dibaryon candidates and its experimental
searches were recently carried out in heavy-ion collisions, where various hadrons are produced abundantly
2,3). Besides exploring a bound state of dibaryons, themeasurements of baryon-baryon correlations provide
us with the information on baryon-baryon interactions
and equation of the state for dense matter.
ALICE is the dedicated experiment to study Quark
Gluon Plasma (QGP), a hot and dense Quantum chromodynamics (QCD) medium, via heavy-ion collisionsat LHC. The two main tracking detectors in the central
barrel are the Inner Tracking System (ITS) and theTime Projection Chamber (TPC). The ITS consistsof six cylindrical layers of silicon detectors for particle tracking and reconstructing the secondary vertex.
The TPC surrounds the ITS and comprises a 90 m3
cylinder ﬁlled with Ne/CO2 /N2 (90/10/5). It is also
used for particle identiﬁcation via the energy deposit(dE/dx) measurement.
As a dibaryon candidate other than H dibaryon, it
is predicted by lattice QCD calculations that the N Ωsystem has enough attraction to form a bound state4) .
The analysis strategy for searching N Ω is based on
topological particle identiﬁcation. The decay pattern
is shown in Fig. 1.
Our assumed decay topology of the N Ω dibaryon is
as follows.
(1) N Ω dibaryon decays into p, K− , and Λ at the1st-V 0 vertex.
(2) Λ decays into p and π − at the 2nd-V 0 vertex.
The reconstruction of a N Ω candidate via this decay
topology starts with the reconstruction of Λ (i.e., thesearch of the 2nd-V 0 vertex in Fig. 1). The V 0 decays
are determined by two tracks that are emitted from a∗1∗2

RIKEN Nishina Center
Center for Nuclear Study, Graduate School of Science, University of Tokyo- 111 -Fig. 2. Invariant mass distribution for ΛpK in Pb-Pb collisions.

secondary vertex and might come close to each other.
The minimum distance between the two tracks is calledthe Distance-of-Closest-Approach (DCA). The V 0 decay point is determined using DCA. The protons andpions are selected from all 2nd-V 0 candidates and theinvariant mass of Λ is calculated. In a similar way, the1st-V 0 vertex is determined by selecting protons andkaons from all V 0 candidates. A criterion for selecting
the appropriate 1st- and 2nd-V 0 candidate is the cuton the pointing angle. It is the angle between the reconstructed ﬂight-path and the reconstructed momentum of the V 0 particle.
Figure 2 shows the invariant mass distribution in the√decay channel N Ω → ΛpK for Pb-Pb data at sN N =2.76 TeV. A 3σ dE/dx cut in the TPC is used for
the PID of p, K, and π. The invariant mass of Λ at
2nd-V 0 is restricted between 1.111 GeV/c2 < mΛ <1.120 GeV/c2 and is combined with the four-vectors ofproton and kaon at the 1st-V 0 vertex. The cuts on theDCA and the cosine of the pointing angle for both V 0vertex are < 1.0 cm and > 0.9, respectively. There isno signiﬁcant signal in the invariant mass distribution
so far, however the cut optimization is ongoing. Otherdibaryon candidates such as ΩΩ, Λ∗ Λ∗ will also beevaluated.
References
1) R. L. Jaﬀe, Phys. Rev. Lett. 38, 195 (1977).
2) N. Shah for the STAR Collaboration, Nucl. Phys. A914, 410 (2013).
3) ALICE Collaboration arXiv:1506.07499 [nucl-ex] CERNPH-EP-2015-069.
4) F. Etminan et al. (HAL QCD Collaboration), Nucl.
Phys. A 928, 89 (2014).