From generation to generation

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by Robert Kowalewski from Nature Physics 11, 705–706 (2015) doi:10.1038/nphys3464


A new measurement from the LHCb experiment at CERN’s Large Hadron Collider impinges on a puzzle that has been troubling physicists for decades namely the breaking of the symmetry between matter and antimatter.

Experimental constraints on the unitarity triangle. Each band shows the allowed region (at 95% confidence level, CL) based on specific measured quantities. The quantities η and ρ are functions of the Cabibbo-Kobayashi-Maskawa (CKM) matrix elements, which allow the triangle to have a base of unit length oriented along the ρ axis. The angles α, β and γ correspond to the blue and tan bands, and are measured from matter-antimatter-violating asymmetries in B meson decay. The circular arcs centred on (10) show the constraints from the mass differences, Δmd and Δms, measured in studies of B-B oscillations. Measurements of matter-antimatter violation in the kaon system determine εK, which is a measure of the admixture of the CP-even eigenstate in the long-lived neutral kaon, and result in the green band. The dark green semi-circle centred on (0,0) shows the constraint from the measurement of the ratio IVubl/IVcbl, where Vub describes the transition of a b quark to a u quark. Image courtesy of the CKMfitter group.

Experimental constraints on the unitarity triangle. Each band shows the allowed region (at 95% confidence level, CL) based on specific measured quantities. The quantities η and ρ are functions of the Cabibbo-Kobayashi-Maskawa (CKM) matrix elements, which allow the triangle to have a base of unit length oriented along the ρ axis. The angles α, β and γ correspond to the blue and tan bands, and are measured from matter-antimatter-violating asymmetries in B meson decay. The circular arcs centred on (10) show the constraints from the mass differences, Δmd and Δms, measured in studies of BB oscillations. Measurements of matter-antimatter violation in the kaon system determine εK, which is a measure of the admixture of the CP-even eigenstate in the long-lived neutral kaon, and result in the green band. The dark green semi-circle centred on (0,0) shows the constraint from the measurement of the ratio |Vub|/|Vcb|, where Vub describes the transition of a b quark to a u quark. Image courtesy of the CKMfitter group.

We learn early that the matter in and around us is made up of three particles: electrons, and the up and down quarks found in nuclei. Add in the electron neutrino and we also account for nuclear fission and fusion and the stellar furnace that fuels life on Earth. But nature is not that simple. It replicates this four-particle structure in ‘generations’ of heavier, but otherwise similar, particles. The first evidence for this was the discovery of the muon in 1936. Other second-generation particles were subsequently discovered, as was another unexpected phenomenon: the violation of matter-antimatter (CP) symmetry in neutral kaons(1). Now, writing in Nature Physics, the LHCb collaboration(2) provides fresh evidence to fuel the ongoing discussion surrounding CP violation.
In 1973, Makoto Kobayashi and Toshihide Maskawa proposed a mechanism whereby mixing between the mass and weak eigenstates of quarks would, if there were three generations, result in an irreducible complex phase that could be responsible for CP violation(3).
The discovery of the first third-generation particle, the tau lepton(4), came a year later, followed in 1977 by the discovery of the third-generation ‘b’ quark(5). With the advent of high-intensity electron-positron colliders at the start of the twenty-first century, studies of CP violation in the decays of B mesons (which contain a b quark) at the BaBar and Belle experiments validated Kobayashi and Maskawas proposal, for which they shared in the 2008 Nobel Prize in Physics.
The CKM matrix – introduced by Kobayashi and Maskawa, following the formative work of Nicola Cabibbo – describes the mixing of quark mass and weak eigenstates in the standard model of particle physics. It is unitary and can be fully specified with four parameters: three real angles and one imaginary phase. This unitarity condition is the basis for a set of testable constraints in the form of products of complex numbers that sum to zero – for example, V*ud Vub + V*cd Vcb + V*td Vtb = 0 where Vub describes the transition of a b quark to a u quark. The triangle in Fig. 1 provides a convenient graphical representation of this equation. The unitarity condition connects a large set of measurable quantities in the standard model, including CP-violating asymmetries, which depend on the imaginary phase, and mixing strengths, which are magnitudes such as |Vub| and |Vcb|. In the standard model, all the bands corresponding to the different measurements in Fig. 1 should overlap at a unique point, which they do at the current level of precision. The presence of new particles or interactions would contribute to these measurable quantities in different ways, resulting in bands that fail to converge at a point. The ratio of matrix elements |Vub|/|Vcb| corresponds to the length of the side of the ‘unitarity triangle’ opposite the angle labelled β, which is well determined from measured CP-violating asymmetries. The precise determination of this ratio is a crucial ingredient in providing sensitivity to new particles and interactions.
Experiments at electron-positron colliders have measured |Vub| and |Vcb| for many years using two complementary methods based on the decays of a B meson to an electron or muon, its associated neutrino and one or more strongly interacting particles. The first method measures exclusive final states whose decay rates are proportional to |Vqb|2 (where q = u, c), and uses lattice quantum chromodynamic (QCD) calculations of form factors to determine |Vqb|. The second inclusive method requires only the presence of an electron or muon and sums over many exclusive final states. These summed rates are also proportional to |Vqb|2, the determination of |Vqb| in this case relies on perturbative QCD calculations and auxiliary measurements. Although these two methods have improved significantly in precision over the years, the values determined for both |Vub| and |Vcb| from the inclusive method persistently exceed those from the exclusive method by two to three standard deviations. This has prompted speculation that the familiar left-handed charged weak interaction has a right-handed counterpart that contributes
to this difference.
With this backdrop, the new measurement of the ratio |Vub|/|Vcb| from the LHCb experiment at CERN’s Large Hadron Collider (LHC) is a welcome addition to the literature(2). It is based on a different exclusive decay mode than can
be measured at the electron-positron collider experiments, namely that of a baryon containing b, u and d quarks (a heavier version of the neutruon) that decays into a proton, a muon and a neutrino. Particle physicists have been surprised that these decays, where the missing neutrino prevents reliance on kinematic constraints, can be distinguished from the huge background inherent in proton-proton collisions at the LHC This new result, which makes use of very precise spatial measurements of the decay vertices of short-lived particles and uses innovative analysis techniques, is a noteworthy achievement.
What have we learned? The new experimental information, instead of resolving the inclusive-exclusive puzzle, deepens it. The measurement and corresponding lattice QCD calculation lead to a value for |Vub|/|Vcb| that is lower than both the pre-existing exclusive and inclusive determinations. The consistency of the three determinations with a single value is only 1.8%, indicating that particle physicists have more work to do in this area. On a more positive note, the LHCb measurement, when combined with previous measurements, strongly disfavours the hypothesis of a right-handed weak interaction.


(1) Christenson, J. H., Cronin, J. W. Fitch, V L. & Turlay, R. Phys. Rev. Lett. 13, 138-140 (1964).
(2) The LHCb collaboration Nature Phys. 11,743-747 (2015).
(3) Kobayashi, M. & Maskawa, T. Prog. Theor Pinys. 49, 652-657 (1973).
(4) Perl, M. L. et al Phys. Rev. Lett 35,1489-1492 (1975).
(5) Herb, S. W. et al Phys. Rev Lett. 39, 252-255 (1977).

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Physics paper sets record with more than 5,000 authors

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by Davide Castelvecchi from Nature doi:10.1038/nature.2015.17567


Detector teams at the Large Hadron Collider collaborated for a more precise estimate of the size of the Higgs boson.

Thousands of scientists and engineers have worked on the Large Hadron Collider at CERN.

Thousands of scientists and engineers have worked on the Large Hadron Collider at CERN.

A physics paper with 5,154 authors has — as far as anyone knows — broken the record for the largest number of contributors to a single research article.
Only the first nine pages in the 33-page article, published on 14 May in Physical Review Letters(1), describe the research itself — including references. The other 24 pages list the authors and their institutions.
The article is the first joint paper from the two teams that operate ATLAS and CMS, two massive detectors at the Large Hadron Collider (LHC) at CERN, Europe’s particle-physics lab near Geneva, Switzerland. Each team is a sprawling collaboration involving researchers from dozens of institutions and countries.
By pooling their data, the two groups were able to obtain the most precise estimate yet of the mass of the Higgs boson — nailing it down to ±0.25%.
Robert Garisto, an editor of Physical Review Letters, says that publishing the paper presented challenges above and beyond the already Sisyphean task of dealing with teams that have thousands of members. “The biggest problem was merging the author lists from two collaborations with their own slightly different styles,” Garisto says. “I was impressed at how well the pair of huge collaborations worked together in responding to referee and editorial comments,” he adds.
Too big to print?
Every author name will also appear in the print version of the Physical Review Letters paper, says Garisto. By contrast, the 2,700-odd author list for a Nature paper on rare particle decays that was published on 15 May(2) will not appear in the June print version, but will only be available online.
Some biologists were upset this week about a genomics paper with more than 1,000 authors(3), but physicists have long been accustomed to “hyperauthorship” (a term credited to information scientist Blaise Cronin at Indiana University Bloomington(4)).
An article published in 2008 about the CMS experiment at the LHC(5), before the machine started colliding protons, became the first paper to top 3,000 authors, according to Christopher King, editorial manager of Thomson Reuters ScienceWatch. The paper that announced the ATLAS team’s observation of the Higgs particle in 2012 had 2,932 authors, of whom 21 were listed as deceased(6).


(1) Aad, G. et al. (ATLAS Collaboration, CMS Collaboration) Phys. Rev. Lett. 114, 191803 (2015).
(2) CMS Collaboration & LHCb Collaboration Nature http://dx.doi.org/10.1038/nature14474 (2015).
(3) Leung, W. et al. Genes Genomes Genet. 5, 719–740 (2015).
(4) Cronin, B. JASIST 52, 558–569 (2001).
(5) The CMS Collaboration et al. J. Instrum. 3, S08004 (2008).
(6) ATLAS Collaboration Phys. Lett. B 716, 1–29 (2012).

Proton smasher spots rare particle decays

Standard

by Daria Zieminska from Nature (2015) doi:10.1038/nature14520


The extremely rare decays of particles known as neutral B mesons have been observed at CERN’s Large Hadron Collider. The result may be a glimpse of physics beyond that of the standard model of particle physics.
For more than three decades, physicists have been looking for the decay of the ‘strange B meson’ particle into a pair of muons, the heavy cousins of electrons. The process is incredibly rare, and harder to find than the famous Higgs particle, the discovery of which at the Large Hadron Collider at CERN, near Geneva, Switzerland, was celebrated worldwide in 2012. The standard model of elementary particle physics(1) makes an exact prediction of the number of particle-decay events researchers should observe in an experiment. Anything more than the predicted value means potential trouble for the standard model. In a paper published on Nature‘s website, researchers working on the CMS and LHCb collaborations(2) at the Large Hadron Collider describe a joint analysis of data from proton collisions that set the decay rate of the strange B meson at about three in one billion — in agreement with the standard-model prediction. However, they find that the decay rate of another type of neutral B meson, the ‘non-strange’ B meson, is at odds with the expectation from the standard model.
The standard model is at a crossroads. It has been very successful in describing elementary particles and their interactions, but such particles comprise only 4% of the known Universe. The theory does not provide a candidate for the dark matter that binds galaxies together and makes up one-quarter of the cosmos. Nor does it accommodate dark energy, the remaining, unknown component of the Universe that is causing it to expand at an accelerated rate. It also does not explain the preponderance of matter over antimatter. Lastly, it makes a worrisome warning that the Universe is probably unstable, ready to collapse in a ‘big crunch’.
Many models have been proposed to solve some of these problems. One of the most compelling ideas for unknown physics beyond that of the standard model is supersymmetry(3), affectionately called SUSY. Supersymmetry states that, for every known particle, there is a twin ‘superparticle’ of much higher mass. These superparticles could in principle be produced in colliders. They should quickly decay to lighter superparticles and ordinary particles, except for the lightest superparticle, which should be stable — and that is SUSY’s candidate particle for dark matter.
Physicists have been searching for SUSY superparticles for years, so far with no success. In the absence of direct observations, they watch for discrepancies of measurements of particle properties from standard-model predictions. The decay of neutral B mesons to muons (Fig. 1) is a sensitive test of the standard model because the model predicts the decay rate with good precision. B mesons are made up of one quark and a ‘bottom’ antiquark, the antimatter partner to the quark; quarks are the elementary building blocks of protons and neutrons, and come in six flavours (up, down, strange, charm, top and bottom). There are two kinds of neutral B meson, which have no charge. One type, the strange B meson (Bs0), contains a bottom antiquark and a strange quark. The other, the non-strange B meson (B0), has a bottom antiquark paired with a down quark. The decay of the neutral B mesons to a pair of muons would mean that the bottom antiquark and its quark partner annihilate, and that the energy released in the process is given to the muons.

The CMS and LHCb collaborations have accelerated and smashed together beams of protons travelling in opposite directions in the Large Hadron Collider at CERN, near Geneva, Switzerland, producing neutral B mesons, among many other particles. The authors observed the extremely rare decay of the strange neutral B meson (Bs0) to two oppositely charged muons (μ+ and μ−) with high statistical significance.

The CMS and LHCb collaborations have accelerated and smashed together beams of protons travelling in opposite directions in the Large Hadron Collider at CERN, near Geneva, Switzerland, producing neutral B mesons, among many other particles. The authors observed the extremely rare decay of the strange neutral B meson (Bs0) to two oppositely charged muons (μ+ and μ) with high statistical significance.

But the standard model forbids the annihilation of quarks of different flavours, so it predicts the decay of the neutral B mesons into muons through an intermediate process that involves the exchange of a top quark between the quarks and the emission of two W bosons (elementary particles that mediate the weak nuclear force). The decay of the strange B meson is expected to occur by this process in about four parts in one billion, and that of the non-strange B meson in about one part in ten billion. However, if yet-unknown SUSY superparticles are exchanged between the quarks in addition to the top-quark exchange, these decay rates will be greatly enhanced relative to the standard-model rate.
The decay rate of the strange B meson observed by the CMS and LHCb collaborations confirms the standard-model prediction. That is good news for the standard model, but not such good news for physics beyond it. However, the decay of the non-strange B meson, which the authors also observed, albeit with a lower statistical significance than obtained for the strange B meson, exceeded the standard-model expectation by almost fourfold — something to watch in the years to come.
CMS and LHCb are two of seven particle detectors at the Large Hadron Collider. Their designs follow different concepts. CMS is a large cylinder (21.6 metres long and 14.6 metres in diameter) in which two counter-propagating beams of protons collide and give rise to neutral B mesons, among many other particles. LHCb is specifically designed to study B mesons, which tend to stay close to the line of the beam pipe. Unlike the CMS detector, which surrounds the proton collision point, the LHCb detector is a stack of instruments stretching for 20 metres along the beam pipe on one side of the collision point. But the two teams adopted a similar strategy to analyse their data. Both groups selected particle events that involved two oppositely charged muons travelling from a common point, which is displaced by a few hundred micrometres from the point at which the protons collide. The events associated with the decay of a neutral B meson are a small fraction of initial candidates. The rest are random pairs of muons originating from other, more common processes.
To separate the signal of the neutral B meson from background events, the teams each built a ‘decision tree’ — a sequence of binary splits of data into signal-like and background-like parts. The system ‘learns’ to distinguish between signal and background by ‘training’ on a simulated sample of the signal and on a sample of real data representing background events. For the selected signal-like events, the researchers deduced the mass of the parent particles using the momenta and directions of travel of the two muons. They then compared the spectrum of the deduced masses with that predicted for a sum of two bell-shaped curves corresponding to the two kinds of neutral B meson, strange and non-strange, and a smooth background.
The two collaborations had previously performed this type of analysis, and each reported their results in separate publications(4, 5). But it was only the combination of data from the two experiments that allowed the researchers to observe with high statistical significance the decay of the strange B meson. In the process, the researchers identified, and corrected, issues with the previous analyses. In particular, they isolated and subtracted a background from the decay of a particle called a bottom Lambda baryon that mimics the signal of a neutral B meson.
Studies of B-meson decays will continue in the coming years. The Large Hadron Collider has just restarted after a two-year break for upgrades, and will soon accelerate proton beams to an energy of 13 teraelectronvolts (TeV), increased from the 8-TeV level reached before the upgrades. The proton beams will also be more tightly focused and will collide at a higher rate than that achieved so far. Both experiments will collect a large number of rare events, and should eventually find which path away from the standard model nature has chosen.


(1) The Standard Model
(2) CMS Collaboration & LHCb Collaboration. Nature (2015).
(3) Supersymmetry
(4) Chatrchyan, S. et al. (CMS Collaboration) Phys. Rev. Lett. 111, 101804 (2013).
(5) Aaij, R. et al. (LHCb Collaboration) Phys. Rev. Lett. 111, 101805 (2013).