Physics Archives – Page 69 of 142

BESIII and the XYZ mystery

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%.

CCbes2_04_14 — Fig. 1. Distribution of π^±J/ψ mass for e⁺e^– → π⁺π^–J/ψ events at a centre-of-mass energy of 4.26 GeV, with a peak visible at 3.9 GeV/c². Dots with error bars are data, the dash-dot curve is the smooth non-resonant background and the smooth curve is the fit.

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, α_QED – which is not constant – at the mass of the Z boson, α_QED(M_Z). The new cross-section measurements shifted the value of α_QED(M_Z) 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 D_s 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´ → π⁰h_c followed by h_c → γη_c 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 η_c, 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 η_c 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´ → γη_c(2S) and measure the mass and width of the η_c(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´.

CCbes3_04_14 — Fig. 2. Distribution of π^±h_c mass for e⁺e^– → π⁺π^–h_c events for the combined data at centre-of-mass energies of 4.23, 4.26, and 4.36 GeV, with a peak visible at 4.02 GeV/c². Dots with error bars are data, the dotted curve is the smooth, non-resonant background and the smooth curve is the fit.

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, f_D+, using the world-average value of the Cabibbo-Kobayashi-Maskawa matrix element |V_cd|, or determination of V_cd using the lattice QCD value of f_D+. 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.

Fig. 3. Distribution of mass recoiling from the π^± for e⁺e^– → π^±(D̅^*D^*)^∓, events at a centre-of-mass energy of 4.26 GeV, with a peak visible at 4.025 GeV/c² on top of the expected broad phase-space distribution and combinatorial background. Dots with error bars are data and the solid curve is the fit.

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 Z_c(3900). The mass and width of the Z_c(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 Z_c(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^–h_c, where h_c → γη_c and η_c decays to 16 exclusive hadronic states (Ablikim et al. 2013b). This is similar to the previous analysis with the J/ψ replaced by the h_c – another charmonium particle. Here again the π^±h_c mass distribution reveals a narrow structure, named the Z_c(4020), as shown in figure 2. The mass and width of the Z_c(4020) are 4022.9±0.8±2.7 MeV/c² and 7.9±2.7±2.6 MeV, respectively. No significant Z_c(3900) is seen in this process.

CCbes5_04_14 — Fig. 4. D⁰D^*– mass distribution for e⁺e^– → π⁺D⁰D^*– events at a centre-of-mass-energy of 4.26 GeV, with a peak visible at 3.885 GeV/c². Dots with error bars are data, the dotted curve is the non-resonant background and the smooth curve is the fit. The corresponding mass distribution of D⁺D̅^*0 looks very similar.

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^±Z_c(4025)^∓, Z_c 4025)^∓ → (D^*D^*)^∓, where the mass and width of the Z_c(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^±Z_c(3885)^∓, Z_c(3885)^∓→ (DD^*)^∓, where the mass and width of the Z_c(3885) are 3883.9±1.5±4.2 MeV/c² and 24.8±3.3±11.0 MeV, respectively. The data prefer that the Z_c(3885) has spin-parity J^P = 1⁺.

Some of the Z_c 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 Z_c states.

CCbes6_04_14 — Fig. 5. π⁺π^–J/ψ mass distribution for e⁺e^– → γπ⁺π^–J/ψ events, with a peak at 3872 GeV/c². Dots with error bars are data and the smooth curve is the fit.

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 Z_c 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.

Tevatron experiments find missing piece in top-quark puzzle

CCnew4_03_14 — Single top-quark s-channel discriminant distribution ranked by expected signal/background. The s-channel signal, all of the backgrounds and the background uncertainty are normalized to their expected value.

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.

CDMS puts new constraints on dark-matter particles

While it is now generally accepted that dark matter makes up the majority of the mass in the universe, little is known about what it is. A favoured hypothesis among particle physicists has long been that dark matter is made of new elementary particles. However, experiments searching for such particles face a serious challenge: neither the particles’ mass nor the strength of their interaction with normal matter is known. So the experiments must cast an ever-widening net in search of these elusive particles.

At the end of February, the Cryogenic Dark Matter Search collaboration announced new results, obtained with the SuperCDMS detector. They expanded their search down to a previously untested dark-matter particle-mass range of 4–6 GeV/c² and a dark-matter nucleon cross-section range of 1 × 10^–40–1 × 10^–41 cm². Their exclusion results contradict recent hints of dark-matter detection by another experiment, CoGeNT, which uses particle detectors made of germanium – the same material used by SuperCDMS.

For their new results, CDMS employed a redesigned cryogenic detector known as iZIP that has ionization and phonon sensors interleaved on both sides of the germanium crystals. This substantially improves rejection of surface events from residual radioactivity, which have limited dark-matter sensitivity in previous searches. The collaboration operated these detectors 0.7 km underground in the Soudan mine in northern Minnesota, to shield them from cosmic-ray backgrounds.

There have been several recent hints for low-mass dark-matter particle detection, from previous data using silicon instead of germanium detectors in CDMS, and from three other experiments—DAMA, CoGeNT and CRESST—all finding their data compatible with the existence of dark-matter particles between 5 and 20 GeV/c². But such light dark-matter particles are hard to pin down. The lower the mass of the dark-matter particles, the less energy they leave in detectors, and the more likely it is that background noise will drown out any signals.

The new CDMS iZIP detectors, with their improved background rejection, are continuing this search at Soudan, and hopefully soon in the lower background environment at SNOLAB. Confirmation of a signal of the direct detection of dark matter, and understanding of the interaction of dark matter with normal matter, is likely to require spotting these particles with different target nuclei in at least two different experiments.

Beauty-quark decays reveal photon polarization

CCnew7_03_14 — Results for the asymmetry in bins of Kππ mass.

The Standard Model predicts that the photons emitted in b → sγ transitions, which can only occur through loop-level processes, are predominantly left-handed. This means that the asymmetry between the amplitudes with right- and left-handed photons – photon polarization – is close to its minimum value of –1. This quantity has never been observed in a direct measurement and remains largely unexplored. As a consequence, there still exist several extensions of the Standard Model that predict a photon polarization significantly closer to zero but have not been ruled out by other measurements of b → sγ transitions.

CCnew6_03_14 — Definition of the upward and downward directions for the photon emission.

The LHCb collaboration has exploited B⁺→K⁺ π^–π⁺γ decays, which are governed by the b → sγ transition, to probe the photon polarization. The “up–down” asymmetry between the number of photons detected above and below the plane defined by the momenta of the kaon and the two pions in their centre-of-mass frame is proportional to the photon polarization. So, a measurement of a nonzero asymmetry implies observation of photon polarization. The investigation is conceptually similar to the experiment that discovered parity violation in 1957 by measuring a nonzero up–down asymmetry for the electrons emitted in the weak decay of ⁶⁰Co nuclei with respect to their spin direction.

Using the full data sample collected with the LHCb detector in 2011 and 2012, the collaboration has reconstructed almost 14,000 B⁺→K⁺ π^–π⁺γ events. Their angular distribution has been studied in four regions of the K⁺π^–π⁺ system’s mass, where different kaon resonances and their interferences can result in different sensitivities to the photon polarization. From determination of the up–down asymmetry, A_ud, in each of these mass regions, LHCb finds a combined significance with respect to the null hypothesis of 5.2σ, and therefore observes photon polarization for the first time in such decays (LHCb collaboration 2014). This important result opens the door to the future determination of the value of the polarization of the photon, which will provide a strong new test of the validity of the Standard Model.

First observation of Z-boson production via weak-boson fusion

The fusion of two weak bosons is an important process that can be used to probe the electroweak sector of the Standard Model. Measurements of Higgs production via weak-boson fusion are crucial for precise extraction of the Higgs-boson couplings and have the potential to help pin down the charge conjugation and parity of the Higgs boson. A similar process, weak-boson scattering, is sensitive to alternative electroweak symmetry-breaking models and to anomalous weak-boson gauge couplings. These processes are extremely rare and the experimental observation of the production of heavy bosons via weak-boson fusion has become possible only recently with the large centre-of-mass energy and luminosity provided by the LHC. Extracting the signals from the huge backgrounds in the high pile-up conditions at the LHC is a major challenge.

CCnew9_03_14 — Fig. 1. Z-boson production via weak-boson fusion.

The production of a Z boson via weak-boson fusion (figure 1) is an excellent benchmark for these rare processes. Weak-boson fusion has the characteristic signature of two low-angle jets, one on each side of the detector. These “tagging” jets typically have transverse momentum of the order of the W mass, because they arise from quarks in each proton recoiling against the two W bosons that fuse to produce the Z boson. Another interesting feature is the lack of colour flow between the tagging jets, which means there is little hadronic activity in that region. These features have been exploited by the ATLAS collaboration to extract the purely electroweak contribution to Z-plus-two-jet production, which includes the weak-boson fusion process.

CCnew10_03_14 — Fig. 2. The dijet invariant-mass distribution in the signal-enhanced search region.

The analysis was carried out using proton–proton collisions at a centre-of-mass energy of 8 TeV recorded by the ATLAS detector in 2012. Events containing a Z boson candidate in association with two high-transverse-momentum jets were selected in the e⁺e^– and μ⁺μ^– decay channels. The electroweak component was extracted by a fit to the dijet invariant mass spectrum in an electroweak-enhanced region that was defined, in part, by a veto on additional jet activity in the interval between the tagging jets. The background model was constrained using data in a signal-suppressed control region that was defined by reversing the jet-veto requirement. This data-driven constraint reduced the experimental and theoretical modelling uncertainties on the background model, allowing the electroweak signal to be extracted with a significance above the 5σ level. Figure 2 clearly demonstrates that the background-only model is inconsistent with the data in the electroweak-enhanced region. The cross-section measured for electroweak Z-plus-two-jet production, σ = 54.7±4.6 (stat.) ^+9.8_–10.4 (syst.) ±1.5 (lumi.)fb, is in good agreement with the Standard Model prediction of 46.1±1.2 fb.

Heavy stable charged particles: an exotic window to new physics

As the LHC experiments improve the precision of their measurements of Standard Model processes, the extent of possibilities for new physics open to exploration is becoming ever more apparent. Even within a constrained framework for new physics, such as the phenomenological minimal supersymmetric standard model (pMSSM), there is an impressive variety of final-state topologies and unique phenomena. For instance, in regions of the pMSSM where the chargino–neutralino mass difference is small, the chargino can become metastable and exhibit macroscopic lifetimes, potentially travelling anywhere between a few centimetres and many kilometres before it decays. An experiment like CMS can identify these heavy stable charged particles (HSCPs) through specialized techniques, such as patterns of anomalously high ionization in the inner tracker, as well as out-of-time signals in the muon detectors.

CCnew12_03_14 — Number (top) and fraction (bottom) of excluded points of the considered pMSSM subspace as a function of the average chargino decay length, for the HSCP (red) and supersymmetry (blue) searches.

The CMS collaboration recently released a reinterpretation of a previously published search for HSCPs that used these techniques to constrain several broad classes of new physics models (CMS 2013a). There are two purposes for this reinterpretation. The first is to provide a simplified description of the acceptance and efficiency of the analysis as a function of a few key variables. This simplified “map” allows theorists and others interested to determine an approximate sensitivity of the CMS experiment to any model that produces HSCPs. This is an essential tool for the broader scientific community, because HSCPs are predicted in a large variety of models and it is important to understand if the gaps in their coverage are still present.

The second purpose is to provide a concrete example of a reinterpretation in terms of the pMSSM. In this analysis, CMS chose a limited subspace of the full pMSSM, requiring, among other things, that sparticle masses extend only up to about 3 TeV. The figure shows the number of points in this restricted pMSSM subspace that are excluded as a function of the average decay length, cτ, of the chargino. The red points are excluded by the HSCP interpretation described here (CMS 2013b). The blue points are excluded by another CMS search dedicated to “prompt” chargino production (CMS 2012a). The bottom panel shows the fraction of parameter points excluded by each of these two searches. Only a few parameter points, with chargino cτ >1 km, are still not excluded. This is because the theoretical cross-section for these parameter points is small – around 0.1 fb.

This analysis demonstrates the power of the CMS search for HSCPs to cover a broad range of models of new physics. By mapping the sensitivity of the analysis as a function of the HSCP kinematics and the detector geometry, it also makes the results from the search accessible for studies by the broader scientific community.

Although this analysis searches for metastable particles, another open possibility is the production of new, exotic particles that traverse a short distance – around 1 mm to 100 cm – before decaying to visible particles within the detector. CMS has also released results from two searches for such particles. One search looks for decays of these long-lived particles into two jets, and another into two oppositely charged leptons (CMS 2012b and 2012c). The results from these searches exclude production cross-sections for such particles as low as about 0.5 fb, depending on the lifetime and kinematics of the decay.

New precision reached on electron mass

Knowledge of the electron mass has been improved by a factor of 13, thanks to a clever extension of previous Penning-trap experiments. A team from the Max-Planck-Institut für Kernphysik in Heidelberg, GSI and the ExtreMe Matter Institute in Darmstadt, and the Johannes Gutenberg-Universität in Mainz, used a Penning trap to measure the magnetic moment of an electron bound to a carbon nucleus in the hydrogen-like ion ¹²C⁵⁺. The cyclotron frequency of the combined system allowed precise determination of the magnetic field at the position of the electron, while the precession frequency allowed for measurement of the mass of the electron.

The result, in atomic-mass units, is 0.000548579909067(14)(9)(2) where the last error is theoretical. This new value for the electron’s mass value will allow comparison of the magnetic moment of the electron to theory – which is good to about 0.08 parts in 10¹² – to better than one part in 10¹².

Jets give clues to the geometry of proton–nucleus collisions

Studies of the centrality-dependence of jet production in proton–lead collisions in ATLAS at the LHC are yielding surprising results.

In both proton–ion and ion–ion collisions, many interesting phenomena are influenced by the initial geometry of the collision system. In proton–nucleus collisions, protons that strike the centre of the nucleus (central collisions) see a thicker nuclear target than those that strike the edge (peripheral collisions) and are more likely to undergo a hard scattering. Traditionally, the geometry of the collisions or “centrality” is characterized by a measure of the event activity. For this measurement in ATLAS, the centrality is defined by the total transverse energy in the forward calorimeter in the direction of the lead beam. A model that describes the expected geometric enhancement of hard-scattering rates is used to relate this experimental measure to a factor T_A (Miller et al. 2007).

CCnew3_02_14 — R_CP for R = 0.4 jets in √s_NN = 5.02 TeV proton–lead collisions. Each panel shows the R_CP value in jets in multiple rapidity bins at a fixed centrality interval. The top row shows R_CP for 0–10%/60–90% and the bottom row shows RCP for 30–40%/60–90%. In the left column, R_CP is plotted versus jet p_T. In the right column, R_CP is plotted against the quantity p_Tcosh(y*), where y* is the midpoint of the rapidity bin. Error bars on data points represent statistical uncertainties, boxes represent systematic uncertainties, and the shaded box on the R_CP = 1 dotted line indicates the systematic uncertainty for that centrality interval.

The quantity R_CP is defined as the ratio of the per-event jet yields in different centrality bins divided by the ratio of corresponding T_A factors, which account for the expected geometric enhancement. The left panels of the figure show R_CP values as a function of the jet transverse-momentum, p_T, for different ranges in jet rapidity, y*, in the centre-of-mass frame. Negative y* indicates the proton-going direction. The normalized jet ratio, R_CP, is suppressed at high p_T and large negative y* compared with the expectation from known nuclear effects, which would correspond to a value near unity in R_CP. The suppression of jets is strongest in the most central collisions (0–10%) at the highest p_T values. Additional studies, independent of the centrality definition, indicate that final-state energy-loss like that observed in ion–ion collisions (jet quenching) is unlikely to be the main source of the effect.

These results suggest either a correlation between hard-jet production and soft-particle production that breaks the traditional geometric paradigm, or that in proton–lead collisions the energy in the forward calorimeter is not a good measure of the centrality of the collision in events with jets.

The right panel of the figure shows the jet R_CP as a function of p_Tcosh(y*), which corresponds closely to the jet’s energy. When recast in terms of this quantity, the R_CP values from different y* intervals all fall along the same line. Whether this is coincidence or related to the underlying dynamics is not yet known.

These observations are among the most striking preliminary ATLAS results from the 2013 proton–lead run of the LHC. Further measurements are needed to uncover the mechanism underlying the observed soft–hard correlations.

CMS observes new single-top production mode

The top quark remains, nearly 20 years after its discovery by the experiments at Fermilab’s Tevatron, the heaviest particle known. Its production and decays continue to be the subjects of extensive studies, both at the Tevatron and at the LHC. While top quarks are predominantly produced in pairs through the strong interaction, the production of single top quarks is also possible by virtue of the electroweak interaction, albeit with a much smaller production cross-section. This latter production mechanism provides a unique window into the dynamics of the top quark.

Three different production channels for single top quarks can be distinguished: the t-channel, the s-channel and the W-associated channel (figure 1). After many years of intensive searches, the Tevatron experiment collaborations first reported the observation of singly produced top quarks in 2009. These initial searches were optimized for the t- and s-channel production modes combined. Later, the t-channel mode was individually established by experiments at the Tevatron and the LHC, while evidence for s-channel production was reported only recently by the D0 collaboration at the Tevatron, in July 2013.

CCnew5_02_14 — Fig. 1. Feynman diagrams for the electroweak production of single top quarks in the s-channel (a), in the t-channel (b), and for W-associated production (c).

Meanwhile, the third production mechanism, W-associated production, remained out of the Tevatron’s reach because of its small cross-section and the lack of a sufficiently distinctive signature. At the LHC, however, the production rate for top quarks is much higher. This allowed ATLAS and CMS to report evidence of W-associated production already with the data collected at a centre-of-mass energy of 7 TeV in 2011, although these measurements did not reach the 5σ gold standard for the solid observation of a new signal. Now, thanks to the data collected in 2012 with the LHC operating at a centre-of-mass energy of 8 TeV, the CMS collaboration has been able to complete the experimental picture of the family of single top-quark production mechanisms.

The main background to W-associated production comes from pair-produced top quarks, where the decay of one of the top quarks appears W-like, because the b quark into which it decays fails the b-tagging requirements. No single defining feature separates the two processes sharply, so several kinematic properties have been combined into a single multivariate discriminant by a boosted decision tree – a machine-learning technique that is particularly suited to separating tiny signals from overwhelming backgrounds. Figure 2 shows the discriminant from the boosted decision tree.

CCnew6_02_14 — Fig. 2. The discriminant from the boosted decision tree in the final state with two light leptons (electron or muon) and one b-tagged jet.

The amount of signal in the selected sample is inferred by a fit to the discriminant distribution. To constrain tightly the amount of top-quark pair background with the data, two complementary samples with one additional jet, separated into cases with one or both jets b-tagged, are also included in the fit. The relative population of the samples with one or two b-tagged jets – both of which are almost pure in top-quark pair events – proves to be a powerful handle to reduce the uncertainty on the b-tagging efficiency, and therefore improve the precision of the analysis.

The excess of events with respect to the background-only hypothesis is quantified at a significance of 6.1σ (with 5.4±1.4σ expected from simulation), and the cross-section is found to be 23.4±5.4 pb, in agreement with the Standard Model prediction. Two cross-check analyses, less sensitive but relying on fewer modelling assumptions, confirm the result, therefore further supporting the first-time observation of this new single-top production mode.

This result opens the door to future searches for anomalous interactions of the top quark with the W boson, in a production mode that brings complementary information to studies that have so far been performed only with selections optimized for the more abundant top-pair and single top-quark t-channel events.

Charmonium produced in unusual topology sheds light on QCD

The LHCb collaboration has released updated measurements of central exclusive production of the J/ψ and ψ(2S) mesons (LHCb collaboration 2014).

Central exclusive production is a class of reactions in which one or two particles are produced from a beam collision, but the colliding hadrons emerge intact. At the LHC this leads to an unusual and distinctive topology of low-multiplicity events contained in a small rapidity interval with large rapidity gaps on either side. J/ψ and ψ(2S) mesons are produced when a photon emitted from one proton interacts with a pomeron (a colourless combination of gluons) from the other. Measurements of the process can be used to test QCD predictions – to improve our understanding of the distribution of gluons inside the proton – and are also sensitive to saturation effects.

CCnew8_02_14 — The J/ψ and ψ(2S) photoproduction cross-section measured by LHCb as a function of the photon-proton centre-of-mass energy (W).

LHCb’s ability to trigger on low-momentum particles and the low number of proton–proton interactions per beam crossing provide an ideal environment to study these processes with particularly low multiplicity. Using data collected in 2011, around 56,000 central exclusive J/ψ and 1500 ψ(2S) mesons have been identified by reconstructing their decays to pairs of muons. While non-resonant backgrounds are very small, the challenge in the analysis is to estimate the larger background that arises when J/ψ and ψ(2S) mesons are produced and one or both of the colliding protons dissociate. As LHCb is instrumented in the forward region mainly, this effect often cannot be detected directly. Instead the collaboration has developed methods to estimate the background rate from the portion of events that are detected.

The measured cross-sections are compared to theoretical predictions, as well as to photoproduction measurements from the HERA electron–proton collider and from fixed-target experiments. Although these environments are quite different from collisions at the LHC, the underlying process is the same. In the former a photon is emitted from an incoming electron beam, while the latter use photon beams directly.

The figure shows a model-dependent comparison of the LHCb results with those from the other types of experiment. It plots the photoproduction cross-section as a function of the photon–proton centre-of-mass energy (W). There is a two-fold ambiguity in converting LHCb’s proton–proton differential cross-section to a photoproduction cross-section, corresponding to the photon being either an emitter or a target. This is resolved using recent results from the H1 experiment at HERA (H1 collaboration 2013). The data in the figure show broad consistency over two orders of magnitude, but are in marginal agreement with a single power-law distribution expected from leading-order QCD. Better agreement is provided either at next-to-leading order QCD (Jones et al. 2013) or by including saturation effects (Gay Ducati et al. 2013).

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