Since the quark model was first conceived 50 years ago, physicists have been searching for “exotic” hadrons – strongly interacting particles that are neither quark–antiquark pairs (mesons) nor three-quark states (baryons). Now the LHCb collaboration has published results that for the first time unambiguously demonstrate the exotic nature of one of the candidate exotic hadrons – the Z(4430). At the same time, LHCb’s measurements show that the f0(500) and the f0(980) states cannot be four-quark states (tetraquarks), contrary to what has long been suggested.
The first evidence for the Z(4430) came in 2008 from the Belle collaboration at KEK’s B-factory, KEKB. It appeared as a narrow peak in the ψ΄π– mass distribution in B → ψ΄Kπ– decays. With negative charge, the Z(4430) cannot be a charmonium state, raising the possibility that it could be a multiquark state, for example ccud.
LHCb has now analysed about 25,200 decays of the kind B0→ ψ΄Kπ–, ψ΄ → μ+μ– in data corresponding to an integrated luminosity of 3 fb−1 of proton–proton collisions at the LHC at centre-of-mass energies of 7 and 8 TeV. The collaboration observes the Z(4430) in the ψ΄π– mass distribution with a significance of at least 13.9σ, and determines the quantum numbers JP to be 1+, by ruling out 0–, 1–, 2+ and 2– at more than 9.7σ (LHCb collaboration 2014a). While this emphatically confirms the evidence from Belle, the LHCb analysis also establishes the resonant nature of the observed state. Its Argand diagram (figure 2) shows unambiguously that the Z(4430) really is a particle. Moreover, with a minimal quark content of ccud, it must be a tetraquark state.
In a related analysis, LHCb has also studied the decay B0 →J/ψπ+π−, extracting the invariant mass of the π+π− pairs. While this clearly reveals a peak corresponding to the f0(500) meson, there is no evidence for the f0(980). This rules out at 8σ the production of the f0(980) at the rate expected for tetraquarks, which would lead to a much smaller difference in the production rates for the two f0 mesons. However, the f0(980) is clearly visible in the corresponding π+π− invariant mass distribution for the decay B0s →J/ψπ+π−. The absence of the f0(980) in B0 decays and its presence in B0s decays in addition to the presence of the f0(500) only in the B0 decays is exactly what is expected if these states are normal quark–antiquark states (LHCb collaboration 2014b).
The Standard Model is currently the best theory there is of the subatomic world, but it fails to answer several fundamental questions, for example: why are the strengths of the fundamental interactions so different? What makes the Higgs boson light? What is dark matter made of? Such questions have led to the development of theories beyond the Standard Model, of which the most popular is supersymmetry (SUSY). In its most minimalistic form, SUSY predicts that each Standard Model particle has a partner whose spin differs by ½ and an extended Higgs sector with five Higgs bosons. SUSY’s symmetry between bosons and fermions stabilizes the mass of scalar particles, such as the Higgs boson and also the new scalar partners of the Standard Model fermions at high energy. If, as suggested by some theorists, the new particles have a conserved SUSY quantum number (denoted R-parity), the lightest SUSY particle (LSP) cannot decay and primordial LSPs might still be around, forming dark matter.
Two charginos, χ~±1,2, and four neutralinos, χ~01,2,3,4 – collectively referred to as electroweakinos – are the SUSY partners of the five Higgs and the electroweak gauge bosons. Based on arguments that try to accommodate the light mass of the Higgs boson in a “natural”, non-fine-tuned manner, the lightest electroweakinos are expected to have masses in the order of a few hundred giga-electron-volts. The lightest chargino, χ~±1, and the next-to-lightest neutralino, χ~02, can decay into the LSP, χ~01, plus multilepton final states via superpartners of neutrinos (sneutrinos, ν~) or charged leptons (sleptons, l~), or via Standard Model bosons (W, Z or Higgs). If SUSY exists in nature at the tera-electron-volt scale, electroweakinos could be produced in the LHC collisions.
The ATLAS collaboration’s searches for charginos, neutralinos and sleptons use events with multiple leptons and missing transverse momentum from the undetected LSP. The two-lepton (e, μ) search has dedicated selections that target the production of l~ l~, χ~±1χ~∓1 and χ~±1χ~02 through their decays via sleptons or W and Z bosons. Meanwhile, the three-lepton (e, μ, τ) analysis searches for χ~±1χ~02 decaying either via sleptons, staus (the SUSY partner of the τ), W and Z bosons, or W and Higgs bosons. Charginos and neutralinos decaying via Standard Model bosons are more challenging to search for than the decays via sleptons, owing to the smaller branching ratio into leptons. The main backgrounds in the two(three)-lepton search are WZ and Z+jets (tt) production, and these are modelled using Monte Carlo simulation and data-driven methods, respectively.
ATLAS has found no significant excess beyond the Standard Model expectation in either the two or three-lepton SUSY searches. This null result can be used to set exclusion limits on SUSY models, narrowing down where SUSY might exist in nature. For example, the two-lepton analysis sets the first direct limits in a simplified SUSY model of χ~±1χ~∓1, where the chargino decays 100% of the time to a W boson. The selections based on the presence of hadronically decaying τ particles in the three-lepton analysis set exclusion limits for χ~±1χ~02 decaying via W and Higgs bosons.
In some cases, the results of two or more analyses can be combined to strengthen the exclusion limits in a particular SUSY model. This is done for the two and three-lepton searches in a simplified SUSY model of χ~±1χ~02, where the χ~±1 and χ~02 are assumed to decay exclusively via W and Z bosons (figure 1). On its own, the two-lepton analysis excludes χ~±1 and χ~02 masses from 170–370 GeV, while the three-lepton analysis excludes masses from 100–350 GeV. By combining the two searches, the exclusion limit is pushed out much further to χ~±1 and χ~02 masses of 415 GeV for a massless χ~01 (figure 2).
So far, no evidence for SUSY has been observed with the first dataset collected by ATLAS. However, in 2015 the LHC will collide protons at higher energies and rates than ever before. This will be an exciting time as exploration of unchartered territories of higher-mass SUSY particles and rarer signatures begins.
The collaborations working on the world’s leading particle-collider experiments have joined forces, combined their data and produced the first joint result from Fermilab’s Tevatron collider and CERN’s Large Hadron Collider. Scientists from the four experiments involved – ATLAS, CDF, CMS and D0 – announced their joint findings on the mass of the top quark at the 2014 Rencontres de Moriond international physics conference on 19 March. The four collaborations pooled their data-analysis power to arrive at a world’s best value for the mass of the top quark of 173.34±0.76 GeV/c2.
Experiments at the LHC and the Tevatron collider are the only ones that have observed the top quark – the heaviest-known elementary particle. Its large mass makes it one of the most important tools in the quest to understand the nature of the universe.
The CDF and D0 experiments discovered the top quark in 1995, and the Tevatron produced some 300,000 top-quark events during its 25-year lifetime, before it finally shut down in 2011. Now the LHC is the world’s leading top-quark factory, having produced close to 18-million events with top quarks since it started collider physics operations in 2009.
Each of the four collaborations had previously released their individual measurements of the top-quark mass. Combining them together required close collaboration between the four large groups of researchers, and a detailed understanding of each other’s techniques and uncertainties. Each experiment measured the mass of the top quark using several different methods. The analyses involved a variety of top-quark decay channels, employing sophisticated techniques that have been developed and improved over more than 20 years of top-quark research, beginning at the Tevatron and continuing at the LHC.
More than 6000 researchers from more than 50 countries participated in the four experimental collaborations.
While this article was in preparation, the CMS Collaboration released the world’s most precise single measurement of the top-quark mass in the semileptonic decay channel, using the experiment’s full sample of data at 8 TeV. Combined with the previous CMS results, this gives a mass of 172.22±0.73 GeV/c2. More details will appear in the next edition of CERN Courier.
After the discovery of a Higgs boson at the LHC in 2012, all of the measurements of its properties and tests of its spin-parity have proved to be consistent with the predictions of the Standard Model. One important property is its natural width, which is expected to be small in the Standard Model – approximately 4 MeV. A larger width could indicate, for example, additional non-standard Higgs decays into known or unknown particles.
At the 2014 Rencontres de Moriond in March, the CMS collaboration presented new and stronger constraints on the total width of the 125 GeV Higgs boson by applying a novel technique on the data collected at the LHC at a centre-of-mass energy of 8 TeV. Following suggestions from several theorists to measure the ratio of the production rate for Higgs-mediated ZZ events with a mass considerably above the mass of the resonance (larger than approximately 200 GeV) to that on the peak, it is possible to derive precise indications on the maximal size of the Higgs boson’s natural width. For this analysis, CMS exploited two ZZ decay channels of the Higgs boson: H → ZZ → 4 leptons, where the four leptons can be electrons or muons, and H → ZZ → 2 leptons + 2 neutrinos.
To maximize the sensitivity of this analysis, in the 4-lepton channel, CMS took advantage of the kinematic differences between 4-lepton production occurring through gluon–gluon fusion (as for Higgs production) and through quark–antiquark scattering, which constitutes a large background to this analysis. The collaboration employed a matrix-element likelihood discriminant Dgg similar to that used for the standard Higgs analysis to help separate signal from background, and carried out a simultaneous fit of this discriminant versus the 4-lepton mass to measure the cross-section for off-peak production. The figure shows the distribution of the discriminant Dgg for events with high mass.
The 2 lepton + 2 neutrino channel has the advantage of a larger branching ratio, but it comes at the price of more background: owing to the presence of neutrinos, the final state is not fully reconstructed. This channel is based on the presence of large missing transverse energy (MET), and therefore is only sensitive to the off-shell part of the cross-section. In the case of on-peak production, the Z decaying into neutrinos does not have large transverse momentum and does not generate a significant MET. The on-peak cross-section measured from H → ZZ → 4 leptons is used for both channels.
The final result of the analysis is that the two channels have very similar sensitivities. In the Standard Model scenario, each of them is expected to exclude at the 95% confidence level (CL) a Higgs-boson width about 10 times larger than the natural width predicted by the model. The combined result is an exclusion of 17 MeV (35 expected) at 95% CL, which corresponds to 4.2 (8.5 expected) times the width in the Standard Model. Previous direct limits obtained from the measured width of the H → ZZ and H → γγ peaks, which are dominated by the detector resolution, are much weaker (of the order of a few giga-electron-volts).
By the time that the first long run of the LHC ended early in 2013, the LHCb experiment had collected data for proton–proton collisions corresponding to an integrated luminosity of 2 fb–1 at 8 TeV, to add to the 1 fb–1 of data collected at 7 TeV in 2011. The first batch of data allowed the LHCb collaboration to announce a variety of results, many of which have now been updated using the larger data sample and/or by including different decay channels. At the 2014 Rencontres de Moriond conference in March, the collaboration presented more precise results from a number of different analyses.
The flavour-changing neutral-current decay B → K*μ+μ– is an important channel in the search for new physics because it is highly suppressed in the Standard Model. While there are relatively large theoretical uncertainties in the predictions, these can be overcome by measuring asymmetries in which the uncertainties cancel. One of these is the isospin asymmetry, based on the differences in the results of measurements of B0 → K*μ+μ– and B+ → K*+μ+μ–. The Standard Model predicts this isospin asymmetry to be small, which LHCb confirmed in 2011, based on 1 fb–1 of data. On the other hand, a similar analysis for decays in which the excited K* is replaced by its ground state K, showed evidence for a possible isospin asymmetry.
Now, the analysis of the full 3 fb–1 of data, which was presented at the Moriond conference, gives results that are consistent with the small asymmetry predicted by the Standard Model in both the K* and K cases. However, even if this confirms that the difference between B0 and B+ decays is small for this channel, there is a tendency for the differential branching fractions to have lower values than the theoretical predictions, as the figures show.
Another interesting result that LHCb has now refined concerned the exotic state X(3872), which was discovered by the Belle experiment at KEK in 2003. The nature of the X(3872) is puzzling because although it appears charmonium-like, it does not fit in to the expected charmonium spectrum. Exotic interpretations include the possibility that it could be a DD* molecule or a tetraquark state.
With the data from 2011, LHCb unambiguously determined its quantum numbers JPC as 1++. At Moriond the collaboration went further by presenting a measurement of the ratio of the branching fractions for the decay of the X(3872) into ψ(2S)γ and J/ψγ. This ratio, Rψγ, is predicted to be different depending on the nature of the X(3872). LHCb finds Rψγ = 2.46±0.64±0.29, which is compatible with other experiments but more precise. This value does not support the interpretation as a pure DD* molecule.
Since the observation of a Higgs boson at a mass around 125.5 GeV by ATLAS and CMS in July 2012, both collaborations are making every effort to pin it down and decide if it is indeed the Higgs boson of the Standard Model, or the first member of a somewhat larger family, as predicted by several models that go beyond the Standard Model. Working in this direction, ATLAS used the six million tt pairs produced in Run I of the LHC to look for the possible decay of a top quark or antiquark into a light quark (up or charm) and a Higgs boson, t → qH.
In the Standard Model such decays, which proceed via flavour-changing neutral currents, are highly suppressed, but in more complex models they might be present, albeit with a small branching ratio compared with the dominant t → bW decay. Doing the search using the dominant decay mode of the Higgs boson (H → bb) would lead to final states that are very hard to distinguish from the majority of tt decays. Therefore ATLAS made the choice to use the H → γγ decay mode – which has a clean signature of two photons with high transverse-momentm (pT) clustering as a narrow peak in invariant mass around 125.5 GeV – the power of this decay mode being demonstrated by the Higgs-boson discovery. Unfortunately the use of this decay mode is hampered by a small branching fraction, only 0.23%. Putting numbers together, and taking into account the acceptance of the detector and of the selection, a branching ratio B of 1% for t → qH would lead to about 11 observed events in a topology with two high pT photons and four jets, of which one would be identified as a b-jet. In addition, about three events with two high-pT photons, two jets, a lepton and missing transverse momentum (from the leptonic decay of the W) would also be expected.
After making kinematical cuts to ensure the compatibility of the selected events with the tt final state, ATLAS obtained the diphoton mass-spectrum shown in the figure. This rules out B = 1% immediately because it is clear that there is not an 11-event signal at 125.5 GeV. A detailed statistical analysis gives an expected limit on B of 0.53%. The small, non-significant excess in the 124–128 GeV bin worsens the observed limit to 0.79%, at the 95% confidence level.
This is the first experimental result on this channel and its precision is limited, mainly by the available statistics. When data become available at 13/14 TeV – leading to an increase of the tt- production cross-section of almost a factor of four – and with a larger integrated luminosity, either a much tighter limit will be obtained or, perhaps, a significant signal will show up, giving evidence for physics beyond the Standard Model in the Higgs sector.
The final run of the LHC in January 2013 prior to the start of the current long shutdown provided collisions between a beam of protons and a beam of lead ions, allowing the LHCf experiment to make further studies related to the interactions of cosmic rays in the Earth’s atmosphere. In particular, the collaboration was able to measure the distribution in transverse momentum (pT) for the inclusive production of neutral pions in the very forward region.
Despite several experimental indications at the HERA electron–proton collider at DESY, it is still not well understood how the density of partons (quarks and gluons) in a proton target increases or even saturates when Bjorken-x in the target – essentially the fraction of the proton’s momentum – is extremely small. Such phenomena are known to be visible in events at large rapidities – that is, close to the beam direction. Furthermore, in the case of nuclear targets, the parton density in the target is expected to be larger by about A1/3, where A is the nuclear mass number. In hadronic interactions, partons in the projectile hadron would lose their energy while travelling in the dense QCD-governed matter of the nuclear target, and particle production mechanisms would change accordingly when compared with those in nucleon–nucleon interactions.
The LHCf detector is designed to measure the hadronic production cross-sections of neutral particles emitted at angles close to the beam direction – the “very forward” region – in proton–proton (pp) and proton–lead (pPb) collisions at the LHC. The detector covers a pseudorapidity range larger than 8.4 and is capable of precise measurements of the forward high-energy inclusive-particle-production cross-sections of neutral particles. Now, the collaboration has analysed the data taken in January 2013 on pPb collisions at nucleon–nucleon centre-of-mass energies of √sNN = 5.02 TeV and a beam-crossing angle of 145 μrad, for an integrated luminosity of 0.63 nb−1.
To obtain the soft-QCD component of the forward pion production, which is sensitive to the parton density in target, unavoidable contamination from ultra-peripheral collisions was first calculated using Monte Carlo simulations and then subtracted from the measured pT spectra. Once the ultra-peripheral collisions have been taken into account, the pT spectum measured by LHCf in the rapidity range −11.0 < ylab < −8.9 and 0 < pT < 0.6 GeV (in the detector reference frame) indicates a strong suppression of the production of neutral pions. This leads to a value of the nuclear modification factor value, RpPb, relative to the interpolated pT spectra in pp collisions at √s = 5.02 TeV, of about 0.1–0.4 – a value that is in overall agreement with the predictions of several Monte Carlo simulations of hadronic interactions.
The OPERA experiment at the INFN Gran Sasso Laboratory has detected a fourth example of neutrino oscillation, with a muon neutrino (νμ) produced at CERN detected as a τ neutrino (ντ) after travelling a distance of 730 km.
The international OPERA experiment, which involves 140 physicists from 28 research institutes in 11 countries, was designed to observe this exceptionally rare phenomenon, gathering data in the neutrino beam produced by the CERN Neutrinos to Gran Sasso (CNGS) project. Generated by decays of pions and kaons made in the interactions of a proton beam from the Super Proton Synchrotron with a graphite target, the beam consisted mainly of νμ that would pass unhindered through the Earth’s crust towards Gran Sasso. The appearance and subsequent decay of a τ lepton in the OPERA experiment provides the telltale sign of νμ to ντ oscillation through a charged-current interaction.
After the first neutrinos arrived at the Gran Sasso Laboratory in 2006, the experiment gathered data for five consecutive years, from 2008 to 2012, during which the CNGS beam delivered a total of 17.97 × 1019 protons on target, yielding 19,500 neutrino events in the detector. The first ντ was observed in 2010, the second and third ones in 2012 and 2013, respectively.
The detection of the fourth ντ is important confirmation of the events seen previously. It means that the νμ to ντ transition has been seen for the first time with a statistical significance exceeding the 4σ level, so that OPERA can now claim the observation of this extremely rare phenomenon. The collaboration will continue to search for ντ in the data that remain to be analysed.
BESIII is the latest incarnation of an experimental programme that began in 1989 when the Beijing Electron–Positron Collider (BEPC) and the Beijing Spectrometer (BES) detector started operation at the Institute of High Energy Physics (IHEP). The focus is on the physics of charm and the τ lepton, which are accessible at the centre-of-mass energies of BEPC. The BES programme is the only one in the world to focus entirely on this area of particle physics through the collection of record numbers of J/ψ, ψ´, D and τ particles. During the past two decades, thanks to the luminosity available first at BEPC and then at BEPCII, the BES collaboration has made many important, high-precision measurements. More recently, this has led to investigations of new particles – the XYZ particles – that appear not to fit in with the standard picture of charmonium states.
One of the first major contributions of the BES programme came in 1992, when the collaboration made a much more precise measurement of the mass of the τ lepton and cleared up a big disagreement between the particle’s mass, its lifetime and its branching ratio to electrons – quantities that are related by the Standard Model. Then from 1993 to 1997, BEPC and BES were upgraded. BES became BESII and received a new main drift chamber (MDC) and time-of-flight (TOF) system. The collaboration soon embarked on a scan of the ratio of hadron to muon-pair production, which measured the hadronic cross-section at 93 energy points in the range 2–5 GeV and improved the precision in this region from 15–20% to less than 6%.
These cross-section results, together with many different measurements from Fermilab, CERN’s Large Electron–Positron collider and the LHC, are used in stringent tests of the Standard Model. The cross-section measurements are required to determine the value of the fine structure constant, αQED – which is not constant – at the mass of the Z boson, αQED(MZ). The new cross-section measurements shifted the value of αQED(MZ) and also moved the mass of the Higgs boson predicted by the Standard Model to be more in line with the measured lower limits on the mass at that time. BES and BESII also produced many other results on J/ψ and ψ´ hadronic decays, ψ´ transitions, and D and Ds decays.
The upgrade of BEPC to BEPCII began in 2004 and finished in 2008. The facility became a two-ring collider with 93 beam bunches in each ring, superconducting micro-β focusing quadrupole magnets, superconducting RF, and a design luminosity of 1 × 1033 cm–2 s–1. At the same time, a brand new detector – BESIII – was constructed with a small-celled, helium-based MDC, a new TOF system, a CsI(Tl) electromagnetic calorimeter, a resistive-plate-chamber muon identifier and a 1 T superconducting solenoidal magnet.
In the first year of operation, 2009, BESIII accumulated 106 million ψ´ events and 226 million J/ψ events. With the ψ´ data, BESIII was able to observe clearly the process ψ´ → π0hc followed by hc → γηc and measure for the first time the individual branching ratios, which allowed comparison with theoretical predictions.
Later, BESIII measured the mass and width of the ηc, taking into consideration for the first time interference between the resonance and the non-resonant background. Previously, the CLEO collaboration had pointed out that the masses and widths of the ηc were different when measured in ψ´ radiative decay and measured in proton–antiproton or two-photon production. Including the interference effect produced results that were consistent with the latter, and the most precise measurements to date. Moreover, BESIII was able to observe for the first time the M1 transition ψ´ → γηc(2S) and measure the mass and width of the ηc(2S) and the branching fraction for this process. With the J/ψ data, BESIII confirmed the X(1835) seen by BESII and observed two new resonances, the X(2120) and the X(2370), in the process J/ψ → γπ+π–η´.
In the following years, BESIII accumulated another 1000 million J/ψ events, 400 million ψ´ events, and approximately 3 fb–1 of data at the ψ(3770) resonance. The ψ(3770) decays more than 90% of the time to quantum-correlated DD pairs, which allow measurement of absolute branching ratios, as well as of DD mixing. The collaboration recently made the most precise determination of the branching ratio of D → μν, which allows determination of the pseudo-scalar decay constant, fD+, using the world-average value of the Cabibbo-Kobayashi-Maskawa matrix element |Vcd|, or determination of Vcd using the lattice QCD value of fD+. The energy region of τ and charm is extremely rich in the variety of physics topics available and BESIII is accumulating world-class data sets to study them.
XYZ physics
The X(3872) was discovered in the decay of B mesons at KEK by the Belle experiment in 2003. This was the first member of a family of exotic particles that do not agree with the predicted masses of charmonium particles in this mass region and decay in a peculiar way. Rather than decaying as expected into a pair of particles with open charm, such as a D meson and its antiparticle D, they decay into π+π–J/ψ. In 2005, the BaBar experiment at SLAC discovered the Y(4260) in initial-state radiation (ISR) production, where much of the electron or positron energy is radiated away leaving the energy remaining at 4260 MeV. Like the X(3872), the Y(4260) has a mass that does not agree with those expected for charmonium and also decays to π+π–J/ψ.
The X(3872) and Y(4260) are members of the XYZ family of particles, which now contains numerous members, although many of them are not yet confirmed. The discovery of the particles, which do not fit into the standard picture, has sparked a great deal of theoretical interest and many theoretical papers.
In December 2012, BESIII jumped into the world of XYZ physics by beginning to take data at 4.26 GeV – the energy of the Y(4260). Running at this energy has the advantage that Y(4260) events might be produced directly rather than indirectly by B decay or ISR production, both of which have a much smaller cross-section.
Analysing the accumulated sample after one month of data taking, the collaboration found 1477 e+e– → π+π–J/ψ, J/ψ → l+l– events – where l is an electron or a muon – and obtained a cross-section consistent with Y(4260) production (Ablikim et al. 2013a). The π±J/ψ mass distribution, shown in figure 1, revealed an unexpected structure that was named the Zc(3900). The mass and width of the Zc(3900) are 3899.0±3.6±4.9 MeV/c2 and 46±10±20 MeV, respectively. The decay contains both charmonium – the J/ψ – and a charged pion, suggesting that the Zc(3900) contains four quarks. The discovery was quickly confirmed by the Belle collaboration and by an analysis of CLEO data. Other charged charmonium-like particles had been found earlier by Belle but never confirmed, so this is the first confirmed Z state.
Data taking continued through to June 2013 at 13 energies between 3.9 and 4.4 GeV, bringing the total luminosity to approximately 2.5 fb–1, and the analysis of four other processes has now been completed. The first is e+e– → π+π–hc, where hc → γηc and ηc decays to 16 exclusive hadronic states (Ablikim et al. 2013b). This is similar to the previous analysis with the J/ψ replaced by the hc – another charmonium particle. Here again the π±hc mass distribution reveals a narrow structure, named the Zc(4020), as shown in figure 2. The mass and width of the Zc(4020) are 4022.9±0.8±2.7 MeV/c2 and 7.9±2.7±2.6 MeV, respectively. No significant Zc(3900) is seen in this process.
The second process analysed is e+e– → π±(D*D*)∓, where a partial reconstruction technique is used that requires the identification of the π±, a charged D from the decay of a charged D*, and one π0 from either the D* or the D* decay (Ablikim et al. 2014a). The analysis is based on 827 pb–1 of data at 4.26 GeV. When the mass recoiling from the π± is plotted, an enhancement is seen, as shown in figure 3, so the process is interpreted as e+e– → π±Zc(4025)∓, Zc 4025)∓ → (D*D*)∓, where the mass and width of the Zc(4025) are 4026.3±2.6±3.7 MeV/c2 and 24.8±5.6±7.7 MeV, respectively.
The third process is e+e– → π±(DD*)∓, where again a partial reconstruction technique is used, requiring that the π± and a D be identified (Ablikim et al. 2014b). The analysis is based on 525 pb–1 of data at 4.26 GeV. When the mass of the (DD*)∓ is plotted an enhancement is seen, as shown in figure 4, so the process is interpreted as e+e– → π±Zc(3885)∓, Zc(3885)∓→ (DD*)∓, where the mass and width of the Zc(3885) are 3883.9±1.5±4.2 MeV/c2 and 24.8±3.3±11.0 MeV, respectively. The data prefer that the Zc(3885) has spin-parity JP = 1+.
Some of the Zc states described above might be the same state. Interference has been neglected in the fitting of the peaks, and it could shift the masses and widths obtained. However, there are probably at least two separate Zc states.
So far the X(3872) has been seen in B decays and hadron collisions only, but its quantum numbers are such that it should be able to be produced in radiative decays of the Y(4260). Figure 5 shows the π+π–J/ψ mass distribution for e+e– → γπ+π–J/ψ events from the combined data at 4.009, 4.229, 4.26 and 4.36 GeV (Ablikim et al. 2014c). The clear peak has a mass of 3872.1±0.8±0.3 MeV, to be compared with the mass m(X(3872)) = 3871.68±0.17 MeV listed in the Particle Data Group tables. Although the events could be produced directly, it is highly plausible that the X(3872) is from radiative decay of the Y(4260).
There are many possible theoretical explanations for the XYZ particles, including the Y(4260) and the recently discovered Zc structures observed by BESIII. They include four-quark models with molecular states comprising charm and anti-charm particles, tetraquark states, and hadro-charmonium, as well as hybrid states (charmonium states with an extra gluon) and a model of initial single-pion emission. More experimental results are necessary to check the predictions of the various models and to decide which ones, if any, describe the physics correctly.
BESIII entered the era of XYZ physics by acquiring about 2.5 fb–1 of data at around 4.26 and 4.36 GeV. Currently, more data are being acquired and many other analyses of the data collected so far are in progress. Future results will help decide among the various models, or rule them all out.
Data from the CDF and D0 experiments at the Tevatron have revealed one of the rarest methods of producing a top quark. The two collaborations announced jointly on 21 February that they have observed s-channel production of single top quarks.
The top quark, t, which was discovered in proton–antiproton collisions at the Tevatron in 1995, is the heaviest elementary particle of the Standard Model, with a mass of 173 GeV. Only the Tevatron and the LHC colliders have so far been capable of making t quarks. In s-channel production, a quark from a proton and an antiquark from an antiproton create a W boson, which decays to a t quark and a b quark. The t quark in turn decays to a new W boson and a b, leading to a final state of Wbb. The production of single t quarks in the s-channel is among the rarest decays of the W boson, given that one of the final state particles (t) has a rest mass more than twice that of the parent W boson.
Selecting a region of high signal-to-background, as shown in the figure, required the development of sophisticated analysis methods. These included identifying jets from the hadronization of the b quarks efficiently and with low background. Between them, the CDF and D0 collaborations analysed more than 2 × 1010 events recorded to tape during operation of the Tevatron as a collider between 2001 and 2011.
Each experiment saw a substantial excess of events – about 40 in total – that could be attributed to single s-channel production of the t quark (CDF 2014 and D0 2013). However, only by combining the results from both experiments to make full use of the Tevatron data set, could the teams push the significance of the observation to 6.3σ, consistent with the discovery of a new process (CDF and D0 2014). The measured production cross-section in the s-channel of 1.29 pb agrees with the Standard Model prediction and so excludes the possibility of a particle other than the W boson, not predicted by the Standard Model, as a source of single t quarks.
Detection of this extremely rare process was one of the final goals of the Tevatron programme to be achieved. However, it is not the end of the story for the top quark because many more studies will continue, both with Tevatron data and at the LHC, to understand fully the heaviest known elementary particle.
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