New LHCb results hint at possible deviations from the Standard Model

18 April 2017

The LHCb collaboration have just announced a new measurement that is causing excitement among particle physicists. The measurement is of a quantity called RK*, which tells us how often B0 mesons decay to a K* meson and two oppositely-charged muons relative to the case where they decay to a K* and two oppositely-charged electrons. Within the Standard Model (SM) the equivalence ('universality') of lepton interaction strengths means that RK* should be almost exactly equal to one. However, the presence of non-SM particles could modify the rate of one of these decays over the other, leading to the breakdown of lepton universality.

The LHCb collaboration has used its Run 1 dataset (from 2011 and 2012) to make the most precise measurement of RK* [1] in two different regions of q2 (the squared-invariant mass of the two lepton system) that show a discrepancy with the concept of lepton universality. The measured values are each around 2.5 standard deviations below the SM predictions as seen in the figure. Previous, less precise, measurements of RK* have been performed by the Belle and BaBar collaborations [2,3] that were consistent with predictions and the new LHCb results.

Although the significance is below the five-sigma threshold usually required in particle physics to claim a discovery, this new information goes in the same direction as a previous indication of lepton non-universality in a similar ratio, RK, which the LHCb collaboration published in 2014 [4]. Together with other measurements related to similar b hadron decays, these measurements could be explained by the existence of non-SM processes.

Dr. Simone Bifani from the University of Birmingham presented the new results in a seminar at CERN on April 18th. He says "The measurements represent a milestone for the LHCb collaboration. We have achieved excellent understanding of potential biases in the results through detailed studies of control samples. When we update the analysis to include data recorded during Run 2 we have the potential to make the first observation of physics beyond the Standard Model at the LHC."

Prof. Tim Gershon from the University of Warwick and spokesperson for LHCb-UK adds "The mood is one of cautious excitement -- no-one is popping any champagne corks yet. Detailed understanding of these deviations requires a long-term programme of measurements that we are now planning. Work is ongoing towards LHCb detector upgrades that will enable the increased sensitivity that is required."

[1] LHCb collaboration, R. Aaij et al., LHCb-PAPER-2017-013
[2] Belle collaboration, J.-T. Wei et al., Phys. Rev. Lett. 103 (2009) 171801
[3] BaBar collaboration, J. P. Lees et al., Phys. Rev. D86 (2012) 032012
[4] LHCb collaboration, R. Aaij et al., Phys. Rev. Lett. 113 (2014) 151601

Five new particles discovered at once

21 March 2017

This week the LHCb collaboration announced the discovery of five new particles known as excited Ωc (Omega_c) baryons. Baryons are composed of three fundamental particles called quarks. Well-known examples of baryons are the protons and neutrons that are found in atomic nuclei. A proton is made of two "up" quarks and one "down" quark, bound together via the strong nuclear force. The Ωc baryons are similar, but they are made from two "strange" quarks and one "charm" quark. These are like heavier (i.e., more massive) versions of the up and down quarks. The Ωc baryons do not exist inside atomic nuclei and can only be produced on earth in certain particle physics experiments such as the CERN Large Hadron Collider.

The first Ωc baryon (the ground-state) was studied over 20-30 years ago. By giving energy to the quarks inside the baryon, it is possible to "excite" the baryon into a new state, somewhat like the way that atoms can be excited into a new state. These states typically have different orbital angular momentum between the constituent quarks and the baryon is more massive. This is what we see in the data, where we have five very distinct peaks at five different values of the mass from 3 GeV up to about 3.1 GeV. For comparison, the proton has a mass of about 0.9 GeV (GeV is a convenient unit used for measuring particle masses).

These five new states were expected to exist, but had never been seen before. It is only with the CERN LHC that we have the ability to produce these states in large numbers, which can then be detected through the excellent performance of the LHCb detector. The next steps will be for LHCb collaborators to try to measure the quantum numbers of these states to see if they match up with theoretical predictions that are based on the theory of the strong force (Quantum Chromodynamics - QCD). This should be able to confirm how we think QCD operates or lead to refinement of the theorists predictions.

This discovery shows the amazing potential of the LHCb experiment to further understand QCD. It will help theorists better understand how quarks and gluons bind together into baryons and, in particular, how the spin correlations between the constituent quarks play a role in that binding. This will have interesting implications as we search for more exotic multi-quark states such as pentaquarks and tetraquarks using data from LHCb.

Dr. Greig Cowan, LHCb physicist at the University of Edinburgh says "What is fantastic is that this observation was performed with only a subset of the currently available data. We have already recorded more and will continue to take data until the end of 2018 (LHC Run 2), meaning that there could be many more surprises waiting to be discovered."

Prof. Tim Gershon, spokesperson for the LHCb-UK collaboration adds "Between 2019-2021 we will upgrade the LHCb detector, ready to start again for LHC Run 3. I anticipate that there will be much more to learn about the strong nuclear force once we get our hands on that data!"

LHCb results presented at ICHEP 2016

3rd August 2016

This week sees the start of the big summer event in particle physics: the International Conference on High Energy Physics (ICHEP). This year it is hosted by the University of Chicago and many LHCb collaborators are now heading west to present the latest results from the experiment.

One of the highlights is the first measurement of the photon polarisation in radiative B decays. This is a crucial measurement as the Standard Model (SM) of particle physics predicts that photons are predominantly left-handed, while many beyond-the-SM models predict enhanced amounts of the right-handed component. Handedness refers to how the photon's spin rotates about its direction of motion in either a clockwise or anticlockwise direction, referred to as right- or left-handed polarisation, respectively. The LHCb collaboration have used the full Run-1 data sample of Bs0 → φγ decays to measure a quantity called AΔ, which is sensitive to the polarisation and is predicted to be AΔSM = 0.047+0.029−0.025 in the SM. LHCb has made the first measurement (LHCb-PAPER-2016-034) of this quantity to be AΔ = −0.98+0.46−0.52+0.23−0.20, which is compatible with the prediction.

Dr Tom Blake, Royal Society University research fellow and convenor of the working group that produced this results states that "This is a major milestone for us. In 2009 we set out a roadmap for six important measurements we wanted to make with the LHCb detector. This is the last measurement to be ticked off that list. With Run-2 data fast coming in we are looking forward to making even more precise tests of the photon polarisation predicted by the Standard Model."

Another new result is one of the first to use data recorded during in 2015 at the start of Run-2 of the LHC. With that dataset LHCb has measured (LHCb-PAPER-2016-031) the b quark production cross-section, which tells us how often b-quark flavoured hadrons are produced at in the 13 TeV proton-proton collisions at the LHC. The measured value is 164.9 ± 2.3 ± 14.6 micro-barns, which means that for every inverse micro-barn of data collected by the LHCb experiment in Run-2, approximately 165 b-hadrons are produced within the LHCb detector acceptance. Given that LHCb has already collected over 1 inverse femtobarn of 13 TeV collisions, this means that the experiment has recorded more than 165 billion b-hadrons! This large sample will allow LHCb to make even more precise measurements of b-quark properties in the future.

Dr Greig Cowan, STFC Ernest Rutherford fellow, helped produced the cross-section result states that "This result is a benchmark for the LHCb experiment that will be used to constrain and influence theoretical models that predict the properties of b-quark production. By measuring this basic quantity we have shown that the LHCb detector is fully operational in Run-2 and can look forward to a host of new measurements in the coming months."

LHCb will present new results from over 20 publications and conference reports. These include observations of extremely rare B meson decay modes, new constraints on CP violation effects in both charm and beauty sectors and first results with LHCb's new HERSCHEL subdetector.