Large Hadron Collider discovers double charm tetraquark – News Home

The LHCb collaboration, which runs at the European Organization for Nuclear Research (CERN)’s Large Hadron Collider, which includes a number of Russian institutes, has announced the discovery of a new particle, an exotic tetraquark. The new particle is very different from the tetraquarks found in many other experiments because it is a so-called double-charm tetraquark—it contains both charm quarks, but not the charmed antiquark. In addition, the particle has a very long lifespan—it lasts one to two orders of magnitude longer than particles of similar mass.

A brief introduction to quarks and hadrons

In general, there are three interactions in the Standard Model of elementary particles that are now generally accepted and described by physics in what is happening in the microscopic world: electromagnetic, weak and strong. The first of these is well known; the carrier of this interaction is the photon. The weak force is called because of its carrier, Electrically neutral Z0– Bosons and charged W±– bosons, are very heavy compared to most elementary particles, and they contribute very little to many interaction processes.The strength of the strong interaction carried by massless gluonas the distance between quark – Elementary particles that carry “strong” charges (also involved in electromagnetic and weak interactions). This leads to the fact that quarks are always bound into composite particles, Hadron. This phenomenon is called confinement quark. Except for quarks and gluons, no particles participate in the strong interaction.

The Standard Model includes six quarks, traditionally known as the following:you — up), down (d – down), enchanting (C – fascinated), strange (s – strange), true (Ton – truth or top) and lovely (b – beautiful or bottom). Their charges and masses are shown in Figure 1. 2. It can be seen from the figure that the mass of quarks varies greatly, from 2.3MElectron Volts for you– Quarks up to 173 GeV Ton– Quark.Usually physicists say you-, d– and s– quarks are light, and C-, b– and Ton– Quarks are heavy.

Hadrons can be composed of all quarks (and antiquarks, which are not the same as the responsible quarks) except Ton– A quark, which is heavy and therefore decays very quickly into other particles, does not have time to form bound states with other quarks, which can be called particles.That’s why Ton-Quarks only participate in the interaction process of elementary particles in the role of virtual particles. According to the standard model of quantum fields of elementary particles, quarks are point particles, but hadrons have a finite size of about 10-13 centimeter.

There are several types of hadrons.The best researched among them is mesonwhich consists of a pair of “quark-antiquark” and has an integer internal rotational moment (called back), and a baryon consisting of three quarks with half-integer spin (the quark itself has a spin of 1/2). For example, baryons include the protons and neutrons that make up the nucleus of an atom.they contain only light you– and d– quarks

In fact, the quantum field theory of the strong interaction is Quantum Chromodynamics (QCD) – Claims that in addition to this “ordinary” quark, called a valence quark, hadrons include an infinite number of gluons that bind valence quarks to each other, as well as virtual quark-antiquark pairs that are constantly being created from the vacuum. These virtual quarks are called “seas” (sea ​​quark). In the framework of this paper, when we speak of hadron composition, we only refer to valence quarks.

There are many known baryons and muons, and they are well studied.However, as early as 1964, Americans Married to Gail Manom and George Zweig It has been proposed that there are hadrons consisting of four or even five quarks – four quarks and pentaquarkSubsequently, their hypothesis was confirmed: so far, 4 pentaquarks and about 20 tetraquarks have been experimentally detected. In total, about 500 hadrons were discovered (including antiparticles, which differ from hadrons by replacing all constituent quarks with antiquarks, and vice versa). At CERN’s Large Hadron Collider alone, 62 new hadrons were discovered, 59 of which were discovered in the past 10 years (Figure 3).

new tetraquark

Recently, the LHCb collaboration at the European Physical Society High Energy Physics Conference (European Physical Society Conference on High Energy PhysicsEPS-HEP) report About the discovery of a new exotic tetraquark.This hadron consists of two heavy C-quarks and light reflections-you– and anti-d– Quark. The strange thing about the new particle is that it is the first tetraquark discovered to have what’s called a “double open charm”: it consists of two charm quarks and no anti-C– Quark.All other experimentally discovered tetraquarks have or “hidden charms” (that is, they contain an equal amount of C– quarks and their antiparticles) or “single charm” (i.e. they include a charm quark).

New particles are represented by $$T_{cc}^+$$. The letter “T” indicates that it is a tetraquark, the symbol “cc” – indicates that it contains two charm quarks, and the plus sign indicates that the particle is positively charged (equal to +1). The new tetraquark has a mass of about 3.875 GeV, which is close to the masses of other tetraquarks that have been discovered, in the range of about 3 to 7 GeV. An unusual property of a particle (related to its quark composition) is a very long lifetime (it, like most other hadrons, is unstable, that is, it decays into other particles): it can survive Dozens or even hundreds are several times longer than other hadrons of similar mass (see Question So Different Tetraquarks for information on why a twin charm tetraquark should be more stable than its counterpart). By order of magnitude, the lifetime of a new tetraquark is 10-21 and

Another interesting property$$T_{cc}^+$$ is that, as physicists say, this tetraquark is very “loose”, that is, its average density is low: its mass is slightly greater than that of the nucleus A helium atom, which turns out to be roughly equal to the nucleus of a radium atom, is 50 times heavier. With more precise analyses planned for the future, the researchers hope to understand the internal structure of the new tetraquarks. For example, it might look like an “atom” with a very small and heavy “core” consisting of two charm quarks surrounded by a very large cloud of light antiquarks (Figure 1). Or it might look like a “molecule” with two heavy particles in it D0 and D*+ （D0 is a meson, by C– and anti-you-Quark and D*+ Include C– and anti-d-quarks) are approximately 8-10 times the size of these particles (Fig. 4). These are the main options, and physicists hope to at some point pinpoint exactly what scenarios are realized in nature.

The discovery of $$T_{cc}^+$$ has two mysteries. First, for some reason, the mass of the new tetraquark is very close to the sum of the masses of the charm D above.0– and D*+mesons, aroused great interest among theorists. In addition, there is another mysterious particle $$\chi_{c1}(3872)$$ (with its MeV mass in parentheses), which has been known for about 20 years, but scientists still do not know how it is work. As you can see, its mass is very close to mass $$T_{cc}^+$$. Whether this is a coincidence is unclear.The difference between the two particles is that the new tetraquark has two components C-quark, and $$\chi_{c1}(3872)$$ includes C-quarks and anti-C– Quark. The impression is that these are some kind of “close relatives”.

The experimental data for the discovery of $$T_{cc}^+$$ tetraquarks were collected from 2011 to 2018. During this period, about 200 new particle birth events were recorded. The signal was strongly observed to have statistical significance greater than 20 standard deviations (meaning that the probability of this effect occurring by chance in the data due to statistical fluctuations is negligible).The tetraquark is observed as a rather narrow peak in the invariant mass spectrum of the $$D^0D^0\pi^+$$ system ($$\pi^+$$ is positively charged$$\pi$$ Meson contains consisting of you-quarks and anti-dquark), which decays into it.

Four quarks containing two heavy masses found C-quarks instead of a single anti-C-Quarks, giving researchers hope that there may be a particle containing a pair of even heavier beauty b– A quark without a corresponding antiquark. The lifetime of this hypothetical particle is expected to be about 10-13 seconds, which is 8 orders of magnitude longer than the long-lived $$T_{cc}^+$$. Computation of particle interaction processes on such large timescales is not possible in QCD, so experimental studies of this as-yet-undiscovered hadron behavior are of great interest.

source:
1) Observe an exceptionally fascinating tetraquark — A short message on the website about the opening of the LHCb partnership.
2) New tetraquarks one step closer to stability – Notes in publications CERN Messenger.
3) I. Polyakov. Recent LHCb results on exotic meson candidates — Presentation at EPS-HEP meeting by representatives of the LHCb collaboration.

Andre Feldman

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