# Synopsis: Pentaquark Discovery Confirmed

New results from the LHCb experiment confirm the 2015 discovery that quarks can combine into groups of five.

Pentaquarks are here to stay. Two new studies from the LHCb collaboration at CERN’s Large Hadron Collider quash any remaining doubts about the discovery of the exotic five-quark particles that was announced last year (see 12 August 2015 Viewpoint). One study demonstrates that the evidence for pentaquarks in the discovery data is model independent. Another reports evidence for exotic hadronic particles—whose properties are consistent with those of the previously observed pentaquarks—in a new particle-decay channel.

Quarks normally aggregate in groups of twos and threes. But in the past two years the LHCb collaboration has confirmed the existence of exotic four-quark and five-quark particles that had long been predicted by theorists. In the pentaquark case, in 2015 the team analyzed data from the decay of the ${\Lambda }_{b}$ particle, which consists of three quarks, into three quark-containing particles: a $J∕𝜓$, a proton, and a charged kaon. They found that sometimes, when ${\Lambda }_{b}$ decays, it turns into an intermediate state comprising a five-quark particle and a kaon. But despite having a whopping statistical significance of $9𝜎$, the result relied on model assumptions about the nature of other intermediate states containing a kaon and a proton.

In one of the new studies, the researchers redid the analysis, eliminating these assumptions, and showed with a significance of over $9𝜎$ that pentaquarks are indeed necessary to explain the data. In the other study, the team sifted through data from another decay channel of ${\Lambda }_{b}$, in which the particle decays into $J∕𝜓$, a proton, and a charged pion. They demonstrated that the data are consistent with the theoretical prediction for decays involving the same type of pentaquarks as those previously detected.

This research is published in Physical Review Letters.

–Ana Lopes

Ana Lopes is a Senior Editor of Physics.

More Features »

### Announcements

More Announcements »

## Subject Areas

Particles and Fields

## Previous Synopsis

Condensed Matter Physics

Mesoscopics

## Related Articles

Particles and Fields

### Synopsis: Material Size Offers Check on Constants

Searching for variations in fundamental constants—which are predicted by some theories of dark matter—might potentially be done by monitoring the sizes of solid crystals. Read More »

Particles and Fields

### Synopsis: Explaining Light Ion Production in High-Energy Collisions

Pions could catalyze reactions between protons and neutrons, allowing the stable production of deuterons in high-energy ion-ion collisions. Read More »

Particles and Fields

### Synopsis: Ultrafast Oscilloscope for Ultrashort Electron Beam

Driving an electron beam into a helical pattern with terahertz electromagnetic pulses allows researchers to measure the beam’s complete shape with femtosecond resolution. Read More »