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Title: Triangle singularities and XYZ quarkonium peaks

We discuss analytical properties of partial waves derived from projection of a 4-legged amplitude with crossed-channel exchanges in the kinematic region of the direct channel that corresponds to the XYZ peaks in charmonium and bottomonium. We show that in general partial waves can develop anomalous branch points in the vicinity of the direct channel physical region. In a specific case, when these branch points lie on the opposite side of the unitary cut they pinch the integration contour in a dispersion relation and if the pinch happens close to threshold, the normal threshold cusp is enhanced. We show that this effect only occurs if masses of resonances in the crossed channel are in a specific, narrow range. We estimate the size of threshold enhancements originating from these anomalous singularities in reactions where the $$Z_c(3900)$$ and the $$Z_b(10610)$$ peaks have been observed.
Authors:
 [1]
  1. Indiana Univ., Bloomington, IN (United States). Dept. of Physics and Center for Exploration of Energy and Matter; Thomas Jefferson National Accelerator Facility (TJNAF), Newport News, VA (United States). Theory Center
Publication Date:
OSTI Identifier:
1198551
Report Number(s):
JLAB-THY-15-1999; DOE/OR/23177-3264; arXiv:1501.01691
Journal ID: ISSN 0370-2693; PII: S0370269315004517; TRN: US1601127
Grant/Contract Number:
AC05-06OR23177; FG0287ER40365
Type:
Published Article
Journal Name:
Physics Letters. Section B
Additional Journal Information:
Journal Volume: 747; Journal ID: ISSN 0370-2693
Publisher:
Elsevier
Research Org:
Thomas Jefferson National Accelerator Facility, Newport News, VA (United States)
Sponsoring Org:
USDOE Office of Science (SC), Nuclear Physics (NP) (SC-26)
Country of Publication:
United States
Language:
English
Subject:
72 PHYSICS OF ELEMENTARY PARTICLES AND FIELDS