Exotic Hadronic States Jeff Wheeler May 1, 2018 Abstract Background: The current model of particle physics predicts all hadronic ​ matter in nature to exist in one of two families: baryons or mesons. However, the laws do not forbid larger collections of quarks from existing. Tetraquark and ​ pentaquark particles have long been postulated. Purpose: Advance the particle ​ ​ landscape into a regime including more complex systems of quarks. Studying these systems could help physicists gain greater insight into the nature of the strong force. Methods: Many different particle colliders and detectors of varying ​ ​ energies and sensitivities were used to search for evidence of four- or five-quark states. Results: After a few false alarms, evidence for the existence of both ​ ​ tetraquark and pentaquark was obtained from multiple experiments. Conclusion: ​ Positive results from around the world have shown exotic hadronic states of matter to exist, and extended research will continue to test the leading theories of today and help further our understanding of the laws of physics. I. Introduction Since the discovery of atomic particles, scientists have attempted to determine their individual properties and produce a systematic method of grouping based on those properties. In the early days of particle physics, little was known besides the mere existence of protons and neutrons. With the advent of particle colliders and detectors, however, the collection of known particles grew rapidly. It wasn’t until 1961 that a successful classification system was produced independently by American physicist Murray Gell-Mann and Israeli physicist Yuval Ne’eman. This method was built upon the symmetric representation group SU(3). In this configuration, the spin-1/2 baryons were arranged in an octet (see fig. 1), which is what model. Gell-Mann and Russian-American inspired Gell-Mann to dub the system the physicist George Zweig postulated three new “eightfold way”. With this system, other particles, called quarks, that would constitute categories of particles could be grouped the triplet. With the triplet in place, along with similarly; for example, spin-3/2 baryons and its conjugate, all of the other representations vector mesons were arranged in decuplets and could be constructed [1]. Gell-Mann and Zweig nonets, respectively, where “nonet” is the term had not only produced a theory of hadron for the combined group of the singlet and octet. classification, but also a theory of the hadronic This method of grouping hadrons proved to be constituents: a theory of subatomic particles. very successful, and even led to Gell-Mann The quark model divides hadrons into postulating the existence of a yet undiscovered two groups: baryons and mesons. Baryons - consist of three quarks (qqq) or antiquarks particle, which he named Ω .​ Only three years ​ later, the particle was discovered at (qqq) , and mesons are composed of a Brookhaven National Laboratory [1]! quark-antiquark pair (qq) . However, while all The success of the eightfold way paved of the particles detected prior to the turn of the the road for the next advancement in particle twenty-first century fit into one of these two physics. While Gell-Mann’s method showed categories, they are not exhaustive. In fact, promise categorizing hadrons into singlets, Gell-Mann himself postulated the existence of octets, and decuplets, it was missing an integral particles with a higher quark content [1]. part of the representation group: the triplet. Continued work in the field produced a This issue led to the formulation of the quark new theory of the strong nuclear force, quantum 1 chromodynamics (QCD), using the subatomic [5], but there are some theorists who believe the building blocks postulated by Gell-Mann and particles detected were more of a “meson Zweig along with eight new vector bosons molecule”. This configuration would consist of called gluons. In addition to the configurations two traditional mesons that are loosely joined of quarks given by the quark model, QCD together [3] (see fig. 2); the attractive force calculations shows that particles containing four between the mesons could come from a pion or five quarks could also exist [2]. exchange [1]. Similarly, it would be possible for a pentaquark signature to be created from a II. Exotic Hadron Structure joined system of a baryon and a meson, opposed to a true pentaquark. In QCD, a new quantum property called color charge is introduced. There are three colors, each with a corresponding anticolor: red, green, blue, antired, antigreen, and antiblue. Each quark carries a single color charge, and all particles composed of quarks must be colorless in total. Particles can be colorless by containing all three colors in equal amounts or with equal amounts of color and its anticolor. The rule that all quark particles must be colorless is referred to as color confinement. With color confinement, we recover the two families of hadrons from the quark model with an added twist. In traditional baryons, each quark must have a different color charge in order to be colorless in total; similarly, mesons must be composed of a colored quark and a quark of the corresponding anticolor. However, baryons and mesons are not the only colorless configurations III. Experimental detection of quarks. Colorless four-quark systems can be composed of two colored quarks and their Since their postulation in the antiquarks (qqqq) , or five-quark systems can mid-1960’s, there have been several exist with four quarks and an antiquark experiments attempting to discover them. Here, (qqqqq) . I will focus on a few experiments detecting Despite evidence for tetraquark and tetraquark candidates and the LCHb experiment pentaquark particles discussed in a later section, detecting pentaquark candidates. there is still some debate on the true structure of the detected systems. For example, four-quark systems were observed at the Belle experiment 2 III.1 Tetraquarks researchers claimed to have evidence of a 2 pentaquark of mass 1500 MeV/c .​ Additional ​ In the early 2010’s, two experiments data analyses from several other groups claimed published papers claiming discovery of to have reproduced the findings. These claims four-quark systems. One of these experiments did not hold, however, and after other repeated was the Belle collaboration. The Belle detector experiments of the same type failed to is located at the High Energy Accelerator reproduce the signature in the data, the Research Organization (KEK) in Tsukuba, pentaquark seemed to be ruled out of existence Japan. While analyzing electron-positron [2]. collisions, the detector recorded data consistent Over a decade later, the pentaquark with the decay of a four-quark system. These would make a resurgence. This time, the news systems were determined to consist of two came from researchers at the LHCb experiment charm quarks and two lighter quarks, denoted at the Large Hadron Collider (LHC) at the by Zc(3900) [3]. European Organization for Nuclear Research ​ ​ The Zc(3900) particle was also (CERN) in Geneva, Switzerland. Unlike the ​ ​ independently discovered by the Beijing previous particle colliders mentioned, the LHC Spectrometer III (BESIII) housed at the Beijing studies proton-proton collisions and operates at Electron Positron Collider. much higher energies. This allows the creation As mentioned in the previous section, of higher-mass particles. In 2015, the LHCb there is still some debate regarding the actual announced new evidence for the elusive internal structure of the detected four-quark pentaquark [4]; and this time, they weren’t even systems. Those in favor of true tetraquark looking for it! detection point out that the meson molecule While studying the decay of unstable should split apart upon decay, but this type of particles, researchers found something strange behavior has not been recorded. However, the in their data, collected between 2009 and 2012. 0 uncertainties in the measurements are consistent The surprise came from analyzing Λb decay. with both the molecule and tetraquark structures This the main decay channel is expected to be 0 * [3]. Λb → J/ψΛ (fig. 3a) with the further decay of Λ* → K−p being detected. However, there III.2 Pentaquarks could also be some exotic decay channels, such 0 + − as the one given by Λb → P c K (fig. 3b), Just as the search for tetraquarks had + where P c denotes the pentaquark with raised some skepticism, the history of the composition (uudcc) and referred to as pentaquark discoveries have not always been “charmonium” [2,4]. widely accepted. The first announcement of a While analyzing their data, researchers + pentaquark, named Θ , was put forth in 2002 noticed bump growing sticking out over their from data collected at the SPring-8 synchrotron expected curve (see fig. 4). Interestingly, due to in Harima, Japan. This experiment involved the infamous past of the pentaquark, the colliding high-energy photons and neutrons, and researchers originally ignored this anomalous 3 hump and continued their original research; The baryons and mesons of the quark model no however, they got to a point where they could longer encompassed all that was allowed in the no longer ignore their data. The bump was laws of particle physics, and since the very staring them right in the face, with an introduction of QCD, physicists have pondered astonishing significance of 9-sigma [2]! Not more exotic states of matter than those seen in only did the researchers find evidence for a new everyday life. Despite being postulated in the pentaquark particle, but the data showed mid-twentieth century, it wasn’t until the early evidence for two distinct short-lived objects twenty first that the first signs of tetra- and ​ 2 2 with masses of 4380 Mev/c 4450 Mev/c ,​ much pentaquarks were gathered. Results from 2011 ​ ​ higher masses than the Θ+ claimed to have been to 2015 showed significant evidence of exotic discovered previously [2].