Double charm tetraquark discovered at Large Hadron Collider

One of the possible variants of the internal structure of the new tetraquark

The LHCb collaboration, which operates at the Large Hadron Collider at the European Organization for Nuclear Research (CERN) and which includes many Russian institutes, has announced the discovery of a new particle, an exotic tetraquark. The new particle is strongly distinguished from a number of other experimentally discovered tetraquarks by the fact that it is a so-called doubly charmed tetraquark—it contains two charmed quarks at once, but does not contain charmed antiquarks. In addition, the particle is very long-lived – its lifetime is one or two orders of magnitude longer than that of particles with a similar mass.

Briefly about quarks and hadrons

In total, there are three interactions in the Standard Model of elementary particles, which is now generally accepted in physics and describes what is happening in the microcosm: electromagnetic, weak and strong. The first of them is well known to all; the carrier of this interaction is the photon. The weak force is so called because its carriers, electrically neutral Z0-boson and charged W±– boson, are very heavy compared to most elementary particles, and their contribution to many interaction processes is very small. The intensity of the strong interaction carried by massless gluonsincreases as the distance between quarks – elementary particles that carry a “strong” charge (and also participate in electromagnetic and weak interactions). This leads to the fact that quarks always turn out to be bound into compound particles, Hadron. This phenomenon is called confinement quarks. Apart from quarks and gluons, no particles participate in strong interactions.

The Standard Model includes six types of quarks, which are traditionally called as follows: up (u — up), lower (d – down), enchanted (c – charmed), strange (s – strange), true (t – truth or top) and adorable (b – beauty or bottom). Their electric charges and masses are shown in Fig. 2. As can be seen from the figure, the masses of quarks vary greatly, from 2.3 MeV for u-quark up to 173 GeV for t-quark. Usually physicists call u-, d– and s-quarks are light, and c-, b– and t-quarks are heavy.

Rice. 2. Masses and charges of quarks in the Standard Model

Hadrons can be made up of all quarks (and antiquarks, which differ from quarks in charge) except t-quark, which is very heavy, and therefore quickly decays into other particles, not having time to form a bound state with other quarks, which could be called a particle. That’s why t-quark participates in the processes of interaction of elementary particles only in the role of a virtual particle. According to the quantum field Standard Model of elementary particles, quarks are point particles, but hadrons have a finite size of approximately 10−13 cm.

There are several types of hadrons. The most well-researched of these are mesonsconsisting of a pair of “quark – antiquark” and having an integer internal moment of rotation (called back), as well as baryons, consisting of three quarks and having a half-integer spin (the quarks themselves have spin 1/2). Baryons include, for example, protons and neutrons, which make up atomic nuclei. They only contain light u– and d– quarks

In fact, the quantum field theory of strong interactions is quantum chromodynamics (QCD) – claims that in addition to such “ordinary” quarks, called valence quarks, the hadron consists of an indefinite number of gluons that bind valence quarks to each other, and virtual quark-antiquark pairs that are constantly born from vacuum. These virtual quarks are called “sea” (sea quarks). In the framework of this article, speaking of the composition of hadrons, we mean only valence quarks.

There are many known baryons and mesons, and they are well studied. However, back in 1964 the Americans Marry Gell-Mannom and George Zweig it has been suggested that there are hadrons consisting of four and even five quarks – tetraquarks and pentaquarks. Subsequently, their hypothesis was confirmed: at the moment, 4 pentaquarks and about 20 tetraquarks have already been experimentally detected. In total, about 500 hadrons were discovered (together with antiparticles, which differ from hadrons by replacing all their constituent quarks with antiquarks and vice versa). Only at the Large Hadron Collider at CERN, 62 new hadrons were discovered, of which 59 were discovered in the last 10 years (Fig. 3).

Rice. 3. Hadrons discovered in the last 10 years at the Large Hadron Collider

New tetraquark

Recently, the LHCb collaboration at the European Physical Society High Energy Physics Conference (The European Physical Society Conference on High Energy PhysicsEPS-HEP) reported about the discovery of a new exotic tetraquark. This hadron consists of two heavy c-quarks and light anti-u– and anti-d-quark. The exoticism of the new particle lies in the fact that it is the first discovered tetraquark with the so-called “double open charm”: it includes two charm quarks and no anti-c-quark. All other experimentally discovered tetraquarks possess or “hidden charm” (that is, they contain an equal amount c-quarks and their antiparticles), or “single open charm” (that is, they include one charmed quark).

The new particle is denoted by \(T_{cc}^+\). The letter “T” means that it is a tetraquark, the symbols “cc” – that it contains two charmed quarks, and the plus sign indicates that the particle has a positive electric charge (it is equal to +1). The mass of the new tetraquark turned out to be approximately 3.875 GeV, which is close to the masses of the other discovered tetraquarks, ranging from about 3 to 7 GeV. An unusual property of a particle (associated with its quark composition) is a very long lifetime (and it, like the vast majority of other hadrons, is unstable, that is, it decays into other particles): it lives several tens and even hundreds of times longer than others hadrons of similar mass (for information on why a doubly charmed tetraquark should be more stable than its counterparts, see the problem Such different tetraquarks). In order of magnitude, the lifetime of a new tetraquark is 10−21 with

Another interesting property \(T_{cc}^+\) is that this tetraquark, as physicists say, is very “loose”, that is, it has a low average density: with a mass slightly larger than the mass of the nucleus of a helium atom, it turned out to be approximately equal to the nucleus of an atom of radium, which is 50 times heavier. From more precise analyzes planned for the future, the researchers hope to understand the new tetraquark’s internal structure. For example, it may look like an “atom” that has a very small and heavy “core” consisting of two charm quarks surrounded by a very large cloud of light antiquarks (Fig. 1). Or it may look like a “molecule” in which two heavy particles D0 and D*+ (D0 is a meson consisting of c– and anti-u-quark, and D*+ comprises c– and anti-d-quark) are located at a distance of approximately 8–10 times greater than the size of each of these particles (Fig. 4). These are the main options, and physicists hope at some point to find out exactly which scenario is realized in nature.

Rice. 4. Another possible variant of the structure of the new tetraquark

There are two mysteries associated with the discovery of \(T_{cc}^+\). First, the mass of the new tetraquark for some reason turned out to be very close to the sum of the masses of the mentioned charmed D0– and D*+-mesons, which aroused great interest among theorists. In addition, there is another mysterious particle \(\chi_{c1}(3872)\) (its mass in MeV is indicated in brackets), which has been known for about 20 years, but scientists still do not know how it works. As you can see, its mass is very close to the mass \(T_{cc}^+\). Whether this is a coincidence or not is still unclear. The difference between these two particles is that the composition of the new tetraquark has two c-quark, and \(\chi_{c1}(3872)\) includes c-quark and anti-c-quark. One gets the impression that these are some kind of “close relatives”.

The experimental data in which the \(T_{cc}^+\) tetraquark was found were collected from 2011 to 2018. During this time, about 200 events of the birth of a new particle were registered. The signal is observed strongly with a statistical significance greater than 20 standard deviations (meaning that the probability that this effect will appear in the data by chance 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 a positively charged \(\pi\) meson consisting of u-quark and anti-dquark) into which it decays.

Rice. 5. Peak in the invariant mass of the system \(D^0D^0\pi^+\)

The discovery of a tetraquark containing two heavy c-quark and not a single anti-c-quark, gives researchers hope that there may be a particle containing a pair of even heavier beautiful b-quarks without corresponding anti-quarks. The lifetime of this hypothetical particle is expected to be approximately 10−13 seconds, which is another 8 orders of magnitude longer than the long-lived \(T_{cc}^+\). It is impossible to calculate the processes of particle interaction on such large time scales in QCD, so the experimental study of the behavior of this as yet undiscovered hadron is of great interest.

1) Observation of an exceptionally charming tetraquark — a short message about the opening of the LHCb collaboration on the website.
2) New tetraquark a whisker away from stability – a note in the publication CERN Courier.
3) I. Polyakov. Reсent LHCb results on exotic meson candidates — report of the LHCb collaboration representative at the EPS-HEP conference.

Andrey Feldman

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