The new pentaquark, illustrated here as a pair of standard hadrons loosely bound in a molecule-like structure, is made up of a charm quark and a charm antiquark and an up, a down and a strange quark (CERN Illustration)
Physicists say they’ve found evidence in data from Europe’s Large Hadron Collider for three never-before-seen combinations of quarks, just as the world’s largest particle-smasher is beginning a new round of high-energy experiments.
The three exotic types of particles — which include two four-quark combinations, known as tetraquarks, plus a five-quark unit called a pentaquark — are totally consistent with the Standard Model, the decades-old theory that describes the structure of atoms.
The LHCb collaboration was launched in 2016 to test explore the events that followed the Big Bang. Credit: CERN
Ever since the discovery of the Higgs Boson in 2012, the Large Hadron Collider has been dedicated to searching for the existence of physics that go beyond the Standard Model. To this end, the Large Hadron Collider beauty experiment (LHCb) was established in 1995, specifically for the purpose of exploring what happened after the Big Bang that allowed matter to survive and create the Universe as we know it.
Since that time, the LHCb has been doing some rather amazing things. This includes discovering five new particles, uncovering evidence of a new manifestation of matter-antimatter asymmetry, and (most recently) discovering unusual results when monitoring beta decay. These findings, which CERN announced in a recent press release, could be an indication of new physics that are not part of the Standard Model.
In this latest study, the LHCb collaboration team noted how the decay of B0 mesons resulted in the production of an excited kaon and a pair of electrons or muons. Muons, for the record, are subatomic particles that are 200 times more massive than electrons, but whose interactions are believed to be the same as those of electrons (as far as the Standard Model is concerned).
The LHCb collaboration team. Credit: lhcb-public.web.cern.ch
This is what is known as “lepton universality”, which not only predicts that electrons and muons behave the same, but should be produced with the same probability – with some constraints arising from their differences in mass. However, in testing the decay of B0 mesons, the team found that the decay process produced muons with less frequency. These results were collected during Run 1 of the LHC, which ran from 2009 to 2013.
The results of these decay tests were presented on Tuesday, April 18th, at a CERN seminar, where members of the LHCb collaboration team shared their latest findings. As they indicated during the course of the seminar, these findings are significant in that they appear to confirm results obtained by the LHCb team during previous decay studies.
This is certainly exciting news, as it hints at the possibility that new physics are being observed. With the confirmation of the Standard Model (made possible with the discovery of the Higgs boson in 2012), investigating theories that go beyond this (i.e. Supersymmetry) has been a major goal of the LHC. And with its upgrades completed in 2015, it has been one of the chief aims of Run 2 (which will last until 2018).
A typical LHCb event fully reconstructed. Particles identified as pions, kaon, etc. are shown in different colours. Credit: LHCb collaboration
Naturally, the LHCb team indicated that further studies will be needed before any conclusions can be drawn. For one, the discrepancy they noted between the creation of muons and electrons carries a low probability value (aka. p-value) of between 2.2. to 2.5 sigma. To put that in perspective, the first detection of the Higgs Boson occurred at a level of 5 sigma.
In addition, these results are inconsistent with previous measurements which indicated that there is indeed symmetry between electrons and muons. As a result, more decay tests will have to be conducted and more data collected before the LHCb collaboration team can say definitively whether this was a sign of new particles, or merely a statistical fluctuation in their data.
The results of this study will be soon released in a LHCb research paper. And for more information, check out the PDF version of the seminar.
A typical LHCb event fully reconstructed. Particles identified as pions, kaon, etc. are shown in different colours. Credit: LHCb collaboration
Since it began its second operational run in 2015, the Large Hadron Collider has been doing some pretty interesting things. For example, starting in 2016, researchers at CERN began using the collide to conduct the Large Hadron Collider beauty experiment (LHCb). This is investigation seeks to determine what it is that took place after the Big Bang so that matter was able to survive and create the Universe that we know today.
According to the research paper, which appeared in arXiv on March 14th, 2017, the particles that were detected were excited states of what is known as a “Omega-c-zero” baryon. Like other particles of its kind, the Omega-c-zero is made up of three quarks – two of which are “strange” while the third is a “charm” quark. The existence of this baryon was confirmed in 1994. Since then, researchers at CERN have sought to determine if there were heavier versions.
The LHCb collaboration team. Credit: lhcb-public.web.cern.ch
And now, thanks to the LHCb experiment, it appears that they have found them. The key was to examine the trajectories and the energy left in the detector by particles in their final configuration and trace them back to their original state. Basically, Omega-c-zero particles decay via the strong force into another type of baryon (Xi-c-plus) and then via the weak force into protons, kaons, and pions.
From this, the researchers were able to determine that what they were seeing were Omega-c-zero particles at different energy states (i.e. of different sizes and masses). Expressed in megaelectronvolts (MeV), these particles have masses of 3000, 3050, 3066, 3090 and 3119 MeV, respectively. This discovery was rather unique, since it involved the detection of five higher energy states of a particle at the same time.
This was made possible thanks to the specialized capabilities of the LHCb detector and the large dataset that was accumulated from the first and second runs of the LHC – which ran from 2009 to 2013, and since 2015, respectively. Armed with the right equipment and experience, the researchers were able to identify the particles with an overwhelming level of certainty, ruling out the possibility that it was a statistical fluke in the data.
The discovery is also expected to shed light on some of the deeper mysteries of subatomic particles, like how the three constituent quarks are bound inside a baryon by the “strong force” – i.e. the fundamental force that is responsible for holding the insides of atoms together. Another mystery that this could help resolve in the correlation between different quark states.
The Large Hadron Collider is the world’s largest and most powerful particle accelerator Credit: CERN
As Dr Greig Cowan – a researcher from the University of Edinburgh who works on the LHCb experiment at Cern’s LHC – explained in an interview with the BBC:
“This is a striking discovery that will shed light on how quarks bind together. It may have implications not only to better understand protons and neutrons, but also more exotic multi-quark states, such as pentaquarks and tetraquarks.“
The next step will be to determine the quantum numbers of these new particles (the numbers used to identify the properties of a specific particle) as well as determining their theoretical significance. Since it came online, the LHC has been helping to confirm the Standard Model of particle physics, as well as reaching beyond it to explore the greater unknowns of how the Universe came to be, and how the fundamental forces that govern it fit together.
In the end, the discovery of these five new particles could be a crucial step along the road towards a Theory of Everything (ToE), or just another piece in the very big puzzle that is our existence. Stay tuned to see which!
Today, CERN announced that the LHCb experiment had revealed the existence of two new baryon subatomic particles. Credit: CERN/LHC/GridPP
With its first runs of colliding protons in 2008-2013, the Large Hadron Collider has now been providing a stream of experimental data that scientists rely on to test predictions arising out of particle and high-energy physics. In fact, today CERN made public the first data produced by LHC experiments. And with each passing day, new information is released that is helping to shed light on some of the deeper mysteries of the universe.
This week, for example, CERN announced the discovery two new subatomic particles that are part of the baryon family. The particles, known as the Xi_b’– and Xi_b*–, were discovered thanks to the efforts of the LHCb experiment – an international collaboration involving roughly 750 scientists from around the world.
The existence of these particles was predicted by the quark model, but had never been seen before. What’s more, their discovery could help scientists to further confirm the Standard Model of particle physics, which is considered virtually unassailable now thanks to the discovery of the Higgs Boson.
Like the well-known protons that the LHC accelerates, the new particles are baryons made from three quarks bound together by the strong force. The types of quarks are different, though: the new X_ib particles both contain one beauty (b), one strange (s), and one down (d) quark. Thanks to the heavyweight b quarks, they are more than six times as massive as the proton.
Cross-section of the Large Hadron Collider where its detectors are placed and collisions occur. Credit: CERN
However, their mass also depends on how they are configured. Each of the quarks has an attribute called “spin”; and in the Xi_b’– state, the spins of the two lighter quarks point in the opposite direction to the b quark, whereas in the Xi_b*– state they are aligned. This difference makes the Xi_b*– a little heavier.
“Nature was kind and gave us two particles for the price of one,” said Matthew Charles of the CNRS’s LPNHE laboratory at Paris VI University. “The Xi_b’– is very close in mass to the sum of its decay products: if it had been just a little lighter, we wouldn’t have seen it at all using the decay signature that we were looking for.”
“This is a very exciting result,” said Steven Blusk from Syracuse University in New York. “Thanks to LHCb’s excellent hadron identification, which is unique among the LHC experiments, we were able to separate a very clean and strong signal from the background,” “It demonstrates once again the sensitivity and how precise the LHCb detector is.”
Blusk and Charles jointly analyzed the data that led to this discovery. The existence of the two new baryons had been predicted in 2009 by Canadian particle physicists Randy Lewis of York University and Richard Woloshyn of the TRIUMF, Canada’s national particle physics lab in Vancouver.
The bare masses of all 6 flavors of quarks, proton and electron, shown in proportional volume. Credit: Wikipedia/Incnis Mrsi
As well as the masses of these particles, the research team studied their relative production rates, their widths – a measure of how unstable they are – and other details of their decays. The results match up with predictions based on the theory of Quantum Chromodynamics (QCD).
QCD is part of the Standard Model of particle physics, the theory that describes the fundamental particles of matter, how they interact, and the forces between them. Testing QCD at high precision is a key to refining our understanding of quark dynamics, models of which are tremendously difficult to calculate.
“If we want to find new physics beyond the Standard Model, we need first to have a sharp picture,” said LHCb’s physics coordinator Patrick Koppenburg from Nikhef Institute in Amsterdam. “Such high precision studies will help us to differentiate between Standard Model effects and anything new or unexpected in the future.”
The measurements were made with the data taken at the LHC during 2011-2012. The LHC is currently being prepared – after its first long shutdown – to operate at higher energies and with more intense beams. It is scheduled to restart by spring 2015.
The research was published online yesterday on the physics preprint server arXiv and have been submitted to the scientific journal Physical Review Letters.
