XYZ particle

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XYZ particles, also referred to as XYZ states, are recently discovered heavy mesons whose properties do not appear to fit the standard picture of charmonium and bottomonium states. [1] They are therefore types of exotic meson. The term arises from the names given to some of the first such particles discovered: X(3872), Y(4260) and Zc(3900), although the symbols X and Y have since been deprecated by the Particle Data Group. [2]

Contents

Theoretical significance

Since 2003 a frontier for the Standard Model (SM) has emerged at low energies through XYZ particle discoveries. The well-established theory of Quantum Chromodynamics (QCD) is tested by many exotic charmonium discoveries since the X(3872) was first identified at the Belle experiment in 2003. [3] The basic model of hadron physics is the assembling of quarks into groups of 3 (baryons) or a quark and anti-quark pair (mesons). A meson with a charm quark and an anti-charm quark is called charmonium, and the same parallels with the bottom quark and bottomonium. More than two dozen previously unpredicted charmonium- and bottomonium-like states have been discovered, and the understanding of heavy quarkonium physics is undetermined. [4] Previously postulated exotic Standard Model states might apply to these new unique particles. One proposed state is the hybrid state of a quark, anti-quark, and a gluon, sometimes mentioned with charm quarks as an excited charmonium. A multi-quark state of 4 or more quarks (tetraquark, pentaquark, etc.) is also proposed as well as a molecule-like state of multiple mesons. [5] [6] While each of these three types of states have had some success of explaining the newly discovered particle, a complete explanation has not been found.

Types of particle

The first charmonium state with an unpredicted mass was X(3872). The Belle collaboration was searching for the B -> K π+ π- J/ψ decay when they discovered a peak in the π+ π- J/ψ invariant energy at 3872 with JCP quantum numbers of 1++. X(3872) was quickly confirmed by BaBar, CDF, and D0. The mass of X(3872) is close to the mass of DD* and makes it a candidate as a meson molecule or a possible tetraquark. In 2005 the BaBar collaboration found Y(4260) from Initial state radiation as well in π+ π- J/ψ production. Again a charmonium-like particle with a large coupling to final states without open charm mesons. Continued search shows a lack of an observation in the inclusive hadronic cross section. The BES III collaboration in 2012 started taking data at 4260 MeV and could observe direct production instead of B decay or Initial State Radiation to continue the study with a higher luminosity. The Zc(3900) state was discovered at BESIII in 2013.

See also

Related Research Articles

In particle physics, a hadron is a composite subatomic particle made of two or more quarks held together by the strong interaction. They are analogous to molecules that are held together by the electric force. Most of the mass of ordinary matter comes from two hadrons: the proton and the neutron, while most of the mass of the protons and neutrons is in turn due to the binding energy of their constituent quarks, due to the strong force.

<span class="mw-page-title-main">Meson</span> Subatomic particle; made of equal numbers of quarks and antiquarks

In particle physics, a meson is a type of hadronic subatomic particle composed of an equal number of quarks and antiquarks, usually one of each, bound together by the strong interaction. Because mesons are composed of quark subparticles, they have a meaningful physical size, a diameter of roughly one femtometre (10−15 m), which is about 0.6 times the size of a proton or neutron. All mesons are unstable, with the longest-lived lasting for only a few hundredths of a microsecond. Heavier mesons decay to lighter mesons and ultimately to stable electrons, neutrinos and photons.

<span class="mw-page-title-main">Charm quark</span> Type of quark

The charm quark, charmed quark or c quark is the third most massive of all quarks, a type of elementary particle. Charm quarks are found in hadrons, which are subatomic particles made of quarks. Examples of hadrons containing charm quarks include the J/ψ meson, D mesons, charmed Sigma baryons, and other charmed particles.

The bottom quark or b quark, also known as the beauty quark, is a third-generation heavy quark with a charge of −1/3 e.

<span class="mw-page-title-main">Pentaquark</span> Human-made subatomic particle

A pentaquark is a human-made subatomic particle, consisting of four quarks and one antiquark bound together; they are not known to occur naturally, or exist outside of experiments specifically carried out to create them.

In particle physics, the baryon number is a strictly conserved additive quantum number of a system. It is defined as

<span class="mw-page-title-main">Tetraquark</span> Exotic meson composed of four valence quarks

A tetraquark, in particle physics, is an exotic meson composed of four valence quarks. A tetraquark state has long been suspected to be allowed by quantum chromodynamics, the modern theory of strong interactions. A tetraquark state is an example of an exotic hadron which lies outside the conventional quark model classification. A number of different types of tetraquark have been observed.

<span class="mw-page-title-main">Exotic meson</span>

Exotic mesons are mesons that have quantum numbers not possible in the quark model; some proposals for non-standard quark model mesons could be:

<span class="mw-page-title-main">J/psi meson</span> Subatomic particle made of a charm quark and antiquark

The
J/ψ
(J/psi) meson or psion is a subatomic particle, a flavor-neutral meson consisting of a charm quark and a charm antiquark. Mesons formed by a bound state of a charm quark and a charm anti-quark are generally known as "charmonium". The
J/ψ
is the most common form of charmonium, due to its spin of 1 and its low rest mass. The
J/ψ
has a rest mass of 3.0969 GeV/c2, just above that of the
η
c
, and a mean lifetime of 7.2×10−21 s. This lifetime was about a thousand times longer than expected.

<span class="mw-page-title-main">LHCb experiment</span> Experiment at the Large Hadron Collider

The LHCb experiment is one of eight particle physics detector experiments collecting data at the Large Hadron Collider at CERN. LHCb is a specialized b-physics experiment, designed primarily to measure the parameters of CP violation in the interactions of b-hadrons. Such studies can help to explain the matter-antimatter asymmetry of the Universe. The detector is also able to perform measurements of production cross sections, exotic hadron spectroscopy, charm physics and electroweak physics in the forward region. The LHCb collaboration, who built, operate and analyse data from the experiment, is composed of approximately 1260 people from 74 scientific institutes, representing 16 countries. Chris Parkes succeeded on July 1, 2020 as spokesperson for the collaboration to Giovanni Passaleva. The experiment is located at point 8 on the LHC tunnel close to Ferney-Voltaire, France just over the border from Geneva. The (small) MoEDAL experiment shares the same cavern.

<span class="mw-page-title-main">Quarkonium</span> Meson whose constituents are a quark and its own antiquark of the same flavor

In particle physics, quarkonium is a flavorless meson whose constituents are a heavy quark and its own antiquark, making it both a neutral particle and its own antiparticle.

<span class="mw-page-title-main">Exotic hadron</span> Subatomic particles consisting of quarks and gluons

Exotic hadrons are subatomic particles composed of quarks and gluons, but which – unlike "well-known" hadrons such as protons, neutrons and mesons – consist of more than three valence quarks. By contrast, "ordinary" hadrons contain just two or three quarks. Hadrons with explicit valence gluon content would also be considered exotic. In theory, there is no limit on the number of quarks in a hadron, as long as the hadron's color charge is white, or color-neutral.

The X(3872) is an exotic meson candidate with a mass of 3871.68 MeV/c2 which does not fit into the quark model because of its quantum numbers. It was first discovered in 2003 by the Belle experiment in Japan and later confirmed by several other experimental collaborations. Several theories have been proposed for its nature, such as a mesonic molecule or a diquark-antidiquark pair (tetraquark).

<span class="mw-page-title-main">Onium</span>

An onium is a bound state of a particle and its antiparticle. These states are usually named by adding the suffix -onium to the name of one of the constituent particles, with one exception for "muonium"; a muon–antimuon bound pair is called "true muonium" to avoid confusion with old nomenclature.

Z(4430) is a mesonic resonance discovered by the Belle experiment. It has a mass of 4430 MeV/c2. The resonant nature of the peak has been confirmed by the LHCb experiment with a significance of at least 13.9 σ. The particle is charged and is thought to have a quark content of
c

c

d

u
, making it a tetraquark candidate. It has the spin-parity quantum numbers JP = 1+.

The Y(4140) particle is an electrically neutral exotic hadron candidate that is about 4.4 times heavier than the proton. It was observed at Fermilab and announced on 17 March 2009. This particle is extremely rare and was detected in only 20 of billions of collisions.

The Y(4260) is an anomalous particle with an energy of 4260 MeV which does not appear to fit into the quark model. It was discovered by the BaBar experiment at Stanford University for the Department of Energy in California and later confirmed by several other experimental collaborations. It being a Charmonium state is unlikely because the Y(4260) is heavier than the threshold for production of two D mesons, yet sits, surprisingly in a dip in the production rate for pairs of D's. It is a possibility that it is a hybrid—a predicted but not-yet-seen type of particle, where a gluon is actually a permanent part of the makeup of the particle, instead of just an ephemeral messenger keeping the quarks bound together.

The Zc(3900) is a hadron, a type of subatomic particle made of quarks, believed to be the first tetraquark that has been observed experimentally. The discovery was made in 2013 by two independent research groups: one using the BES III detector at the Chinese Beijing Electron Positron Collider, the other being part of the Belle experiment group at the Japanese KEK particle physics laboratory.

SooKyung Choi is a South Korean particle physicist at Gyeongsang National University. She is part of the Belle experiment and was the first to observe the X(3872) meson in 2003. She won the 2017 Ho-Am Prize in Science.

The double-charm tetraquark is a type of long-lived tetraquark that was discovered in 2021 in the LHCb experiment conducted at the Large Hadron Collider. It contains four quarks: two charm quarks, an anti-up and an anti-down quark.

References

  1. "BESIII and the XYZ mystery". CERN Courier. 30 April 2014.
  2. "Naming scheme for hadrons" (PDF). Particle Data Group .
  3. Shen Cheng-Ping and Belle collaboration (2010). "XYZ particles at Belle". Chinese Physics C. 34 (6): 615–620. arXiv: 0912.2386 . Bibcode:2010ChPhC..34..615S. doi:10.1088/1674-1137/34/6/001. S2CID   119181870.
  4. "New Vector Particles Observed at BESIII". Institute of High Energy Physics. 10 March 2017.
  5. Pakhlova, Galina V.; Pakhlov, Pavel N.; Eidel'man, Semen I. (2010). "Exotic charmonium". Physics-Uspekhi. 53 (3): 219–241. doi:10.3367/ufne.0180.201003a.0225. S2CID   250860230.
  6. Nielsen, M. (2010). "New exotic charmonium states". Chinese Physics C. 34 (9): 1157–1162. Bibcode:2010ChPhC..34.1157N. doi:10.1088/1674-1137/34/9/002. S2CID   250763143.