The history of LHCb

I. Belyaev, G. Carboni, N. Harnew, C. Matteuzzi. F. Teubert
In this paper we describe the history of the LHCb experiment over the last three decades, and its remarkable successes and achievements. LHCb was conceived primarily as a b-physics experiment, dedicated to CP violation studies and measurements of very rare b decays, however the tremendous potential for c-physics was also clear. At first data taking, the versatility of the experiment as a general-purpose detector in the forward region also became evident, with measurements achievable such as electroweak physics, jets and new particle searches in open states. These were facilitated by the excellent capability of the detector to identify muons and to reconstruct decay vertices close to the primary pp interaction region. By the end of the LHC Run 2 in 2018, before the accelerator paused for its second long shut down, LHCb had measured the CKM quark mixing matrix elements and CP violation parameters to world-leading precision in the heavy-quark systems. The experiment had also measured many rare decays of b and c quark mesons and baryons to below their Standard Model expectations, some down to branching ratios of order 10-9. In addition, world knowledge of b and c spectroscopy had improved significantly through discoveries of many new resonances already anticipated in the quark model, and also adding new exotic four and five quark states.
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2 new subatomic particles discovered at CERN

New baryons are six times more massive than protons
Scientists at the world’s largest particle collider said Wednesday they have discovered two new subatomic particles never seen before that could widen our understanding of the universe.

An experiment using the European Organization for Nuclear Research’s Large Hadron Collider found the new particles, which were predicted to exist, and are both baryons. Baryons are particles that are each made up of three tiny elementary particles called quarks. Neutrons and protons, the familiar particles that make up atoms, are also baryons.

In a statement Wednesday, officials at the lab known by its French acronym CERN announced the discovery, which could shed more light on how things work beyond the “Standard Model” physics theory explaining the basic building blocks of matter. The results also were submitted to the publication Physical Review Letters.

Summary of the paper on Arxiv

“Nature was kind and gave us two particles for the price of one,” said one of the CERN collaborators, Matthew Charles, of the CNRS’s LPNHE laboratory at Paris VI University. Continue reading 2 new subatomic particles discovered at CERN

Two beautiful new particles

First excited state of Λb

In beautiful agreement with the Standard Model, two new excited states (see below) of the Λb beauty particle have just been observed by the LHCb Collaboration. Similarly to protons and neutrons, Λb is composed of three quarks. In the Λb’s case, these are up, down and… beauty.

Second excited state of Λb

Although discovering new particles is increasingly looking like a routine exercise for the LHC experiments (see previous features), it is far from being an obvious performance, particularly when the mass of the particles is high. Created in the high-energy proton-proton collisions produced by the LHC, these new excited states of the Λb particle have been found to have a mass of, respectively, 5912 MeV/c2 and 5920 MeV/c2. In other words, they are over five times heavier than the proton or the neutron.

Physicists only declare a discovery when data significantly show the relevant signal. In order to do that, they often have to analyse large samples of data. To obtain its beautiful result, the LHCb Collaboration has analysed the information coming from about 60 million million (6×1013) proton-proton collisions collected during the 2011 data-taking period. In particular, since the excited states only survive for a very short time before decaying, physicists carefully studied the decay products and tracked the whole process back to the decay vertex. The analysis took scientists several months to complete but today they are able to present the discovery with very high statistical significance, namely 4.9 σ for the first excited state and 10.1 for the second one.

Although never observed before, the excited states of the Λb particle were expected to exist according to the Standard Model, the theory that tells us how quarks combine to build particles and matter. The LHCb result is therefore a new confirmation of the success of the theory itself.

 

EXCITED STATES OF MATTER

Matter can be formed in different energy states. The most stable one – that is, the one that survives the longest before decaying – is the so-called “ground state”, in which particles have the lowest possible energy. States with higher energy are called “excited states”. They are still allowed by Nature but they are unstable. The higher the formation energy (i.e. the mass) the more unstable they are.

Read more about this result on the LHCb Public Webpage.

by Antonella Del Rosso

LHCb experiment squeezes the space for expected new physics

Geneva, 5 March 2012. Results presented by the LHCb collaboration this evening at the annual ‘Rencontres de Moriond’ conference, held this year in La Thuile, Italy, have put one of the most stringent limits to date on the current theory of particle physics, the Standard Model. LHCb tests the Standard Model by measuring extremely rare processes, in this case a decay pattern predicted to happen just three times out of every billion decays of a particle known as the Bs (B-sub-s) meson. Anything other than that would be evidence for new physics. Measuring the rate of this Bs decay has been a major goal of particle physics experiments in the past decade, with the limit on its decay rate being gradually improved by the CDF and D0 experiments at Fermilab, LHCb, and most recently CMS at CERN1.

“The LHCb result on Bs decaying to two muons pushes our knowledge of the Standard Model to an unprecedented level and tells us the maximum amount of New Physics we can expect, if any, in this very rare decay,” explained LHCb spokesperson, Pierluigi Campana. “We know this is an important result for the theoretical community and also nicely complements the direct searches in ATLAS and CMS.”

The Standard Model is a highly successful theory that has been put to the test by experiments over several decades, and come through unscathed. Nevertheless, it is known to be an incomplete theory, accounting for just the 4% of the Universe that is visible to astronomy. New physics is needed to account for the remaining 96%. Such new physics could manifest itself directly, through the production of new particles that would be detected by the ATLAS and CMS experiments, or indirectly through the influence it would have on rare processes of the kind studied by LHCb.

The LHCb particle detector is a highly specialised instrument specifically designed to study short-lived B mesons, and is systematically investigating the rarest decays of these particles. Since the Standard Model gives very precise predictions for such decays, they provide a very sensitive testing ground for new physics. The latest LHCb result constrains the decay rate for Bs to two muons to be less than 4.5 decays per billion Bs decays. That does not rule out new physics, but does start to constrain theoretical models for it, and helps to set the direction for searches in all the LHC experiments.

“Sometimes we feel like Achilles pursuing the tortoise,” said Campana, “we believe our distance from new physics is steadily halving, but we will eventually reach it!”

This result is scheduled to be submitted to the journal Physical Review Letters on 20 March.
Read more:press.web.cern

LHC reveals hints of ‘new physics’ in particle decays

Read also:
1. LHCb uses charm to find asymmetry

2.LHCb has evidence of new physics! Maybe

3. New Physics at LHC? An Anomaly in CP Violation


By Jason Palmer

Large Hadron Collider researchers have shown off what may be the facility’s first “new physics” outside our current understanding of the Universe.

Particles called D-mesons seem to decay slightly more often into one kind of particle rather than another, LHCb physicist Matthew Charles told the HCP 2011 meeting on Monday.

The result may help explain why we see so much more matter than antimatter.

The team stresses that further analysis will be needed to shore up the result.

At the moment, they are claiming a statistical certainty of “3.5 sigma” – suggesting that there less than a 0.5% chance that the result they see is down to chance.

The team has nearly double the amount of data that they have analysed so far, so time will tell whether the result reaches the “five-sigma” level that qualifies it for a formal discovery.

It matters
The LHCb detector was designed to examine particles called mesons, watching them decay through time after high-energy collisions of other fundamental particles.

The LHCb Collaboration was looking at decays of particles called D-mesons, which can in turn decay into kaons and pions.
LHCb, one of the six separate experiments at the Large Hadron Collider, is particularly suited for examining what is called “CP violation” – slight differences in behaviour if a given particle is swapped for its antimatter counterpart.

Our best understanding of physics so far, called the Standard Model, suggests that the complicated cascades of decay of matter particles into other particles should be very nearly the same – within less than 0.1% – as a similar chain of antimatter decays.

Other experiments, notably at the Fermi National Accelerator facility in the US, have found a CP violation of about 0.1%, but with an uncertainty in their measurement that meant the result might just fit within the Standard Model.

Statistics of a ‘discovery’

LHC-beauty, or LHCb, is an enormous detector designed to examine CP violation

  • Particle physics has an accepted definition for a “discovery”: a five-sigma level of certainty
  • The number of standard deviations, or sigmas, is a measure of how unlikely it is that an experimental result is simply down to chance rather than a real effect
  • Similarly, tossing a coin and getting a number of heads in a row may just be chance, rather than a sign of a “loaded” coin
  • The “three sigma” level represents about the same likelihood of tossing more than eight heads in a row
  • Five sigma, on the other hand, would correspond to tossing more than 20 in a row
  • A five-sigma result is highly unlikely to happen by chance, and thus an experimental result becomes an accepted discovery

But the LHCb team is reporting a difference of about 0.8% – a significant difference that, if true, could herald the first “new physics” to be found at the LHC.

“Our result is more significant firstly because it comes out with a [greater difference] and secondly because our precision is improved – somewhat more precise than all of the previous results put together,” Dr Charles told BBC News.

Spotting such a difference in the behaviour of matter and antimatter particles may also finally help explain why our Universe is overwhelmingly made of matter.

“Certainly this kind of effect, a new source of CP violation, could be a manifestation of the physics which drives the matter – antimatter asymmetry,” Dr Charles explained.

However, he stressed there are “many steps in the chain” between confirming the collaboration’s experimental result, and resolving the theory to accommodate it.

“This result is a hint of something interesting and if it bears out, it will mean that, at a minimum, our current theoretical understanding needs improving,” Dr Charles said.

“It’s exactly the sort of thing for which the LHC was originally built.”
http://www.bbc.co.uk