Physics Archives – Page 85 of 144

LHCb looks forward to electroweak physics

CClhc1_03_12 — The decay of a Z boson to two muons (thick white lines) in the LHCb experiment. The green dots show where the particles pass through the muon chambers. Electroweak physics at LHCb includes measurements of the production of Z and W bosons.

LHCb is one of the four large experiments at the LHC. It was designed primarily to probe beyond the Standard Model by investigating CP violation and searching for the effects of new physics in precision measurements of decays involving heavy quarks, b quarks in particular. At the LHC, pairs of particles (B and B mesons) containing these quarks are mainly produced in the direction of the colliding protons, that is, in the same forward or backward cone about the beam line. For this reason, LHCb was built as a single-arm forward spectrometer that covers production angles close to the beam line with full particle detection and tracking capability – closer even than the general-purpose experiments, ATLAS and CMS. This gives LHCb the opportunity to study the Standard Model in regions that are not easily accessible to ATLAS and CMS. In particular, the experiment has an active and rapidly developing programme of electroweak physics that is beginning to test the Standard Model in several unexplored regions.

Closer to the beam

Particle production at collider experiments is usually described in terms of pseudorapidity, defined as η = –ln(tan θ/2), where θ is the angle that the particle takes relative to the beam axis. The particles tend to be produced in the forward direction: that is crowded into small values of θ, while in terms of η, they are spread more uniformly. The inverse relationship means that the closer a particle is to the beam line, the larger its pseudorapidity. LHCb’s forward spectrometer is fully instrumented in the range 2 < η < 5, a portion of which (2 < η < 2.5) is also covered by ATLAS and CMS. However, the forward region at η > 2.5 – roughly between 10° and 0.5° to the beam – is unique to LHCb, thanks to its full complement of particle detection.

LHCb can explore electroweak physics through the production of W and Z bosons, as well as virtual photons. The experiment can trigger on and reconstruct muons with low momentum p_μ > 5 GeV and transverse momentum p_Tμ > 1 GeV, giving access to low values of the muon-pair invariant mass m_μμ > 2.5 GeV. Specialist triggers can even explore invariant masses below 2.5 GeV in environments of low multiplicity. Coupled with the forward geometry, this reconstruction capability opens up a large, previously unmeasured kinematic region.

CClhc2_03_12 — Fig. 1. The x-Q² kinematic region explored by the LHCb experiment (pink), compared with ATLAS and CMS (light green), as well as previous measurements.

Figure 1 shows the kinematic regions that LHCb probes in terms of x, the longitudinal fraction of the incoming proton’s momentum that is carried by the interacting parton (quark or gluon), and Q², the square of the four-momentum exchanged in the hard scatter. Because of the forward geometry, the momenta of the two interacting partons are highly asymmetric in the particle-production processes detected at LHCb. This means that LHCb can simultaneously probe not only a region at high-x that has been explored by other experiments but also a new, unexplored region at small values of x. The high rapidity range and low transverse-momentum trigger thresholds for muons allow potential exploration of Q² down to 6.25 GeV² and x down to 10^–6, thus extending the region that was accessible at HERA, the electron–proton collider at DESY.

The aim is to probe and constrain the parton-density functions (PDFs) – basically, the probability density for finding a parton with longitudinal momentum fraction x at momentum transfer Q² – in the available kinematic regions. The PDFs provide important input to theoretical predictions of cross-sections at the LHC and at present they dominate the uncertainties in the theoretical calculations, which now include terms up to next-next-to-leading order (NNLO).

Using data collected in 2010, the LHCb collaboration measured the production cross-sections of W and Z bosons in proton–proton collisions at a centre-of-mass energy of 7 TeV, based on an analysis of about 36 pb^–1 of data (LHCb collaboration 2011a). Although only a small fraction of W and Z bosons enter the acceptance of the experiment (typically 10–15%), the large production cross-sections ensure that the statistical error on these measurements is small. The results are consistent with NNLO predictions that use a variety of models for the PDFs. With greater statistics, the measurements will begin to probe differences between these models.

CClhc3_03_12 — Fig. 2. LHCb’s measurement of W charge asymmetry, shown as a function of lepton pseudorapidity, compared with next-to-leading order (NLO) predictions, including results from the ATLAS and CMS experiments.

The uncertainty in luminosity dominates the precision to which cross-sections can be determined, so the collaboration also measures ratios of W and Z production, which are insensitive to this uncertainty, as well as the charge asymmetry for W production, A_W = ( σ_W+ – σ_W– / σ_W+ + σ_W–). Figure 2 shows the results for A_W overlaid with equivalent measurements by ATLAS and CMS. It illustrates how the kinematic region explored by LHCb is complementary to that of the general-purpose detectors and extends the range that can be tested at the LHC. It is also apparent that LHCb’s acceptance probes the region where the asymmetry is changing rapidly, so the measurements are particularly sensitive to the parameters of the various PDF models.

Low-momentum muons

The LHCb collaboration also plans to increase the probing power of the cross-section measurements by improving the uncertainty in the luminosity itself. Work is ongoing to measure the exclusive production of pairs of muons, a QED process that should ultimately yield a more precise indirect measure of integrated luminosity. Although instrumented in the forward region, LHCb has some tracking coverage in the region –1.5< η< –4 because the proton–proton collision point lies a little way inside the main tracking detector. The measurement exploits this acceptance, LHCb’s ability to trigger on muons with low momentum and the low pile-up environment of collisions at LHCb, which allows the identification of these low multiplicity, exclusively produced events. First measurements based on 2010 data show that the measurement is feasible (LHCb collaboration 2011b). Updated measurements based on the 2011 data set are underway.

In high-energy hadron–hadron scattering the production of Z and W bosons, which decay into lepton pairs, occurs as a Drell-Yan process in which a quark in one hadron interacts with an antiquark in the other hadron to produce a W or a Z or a virtual photon, which then produces a lepton pair of opposite charges. With its ability to trigger on and identify muons with low transverse-momentum, LHCb can measure the production of muon pairs from Drell-Yan production down to invariant masses approaching 5 GeV. As figure 1 shows, these measurements probe values of x around 10^–5 and can be used to improve knowledge of the behaviour of gluons inside the proton, building on the knowledge gained at HERA.

These and other production studies are being updated for the upcoming 20th International Workshop on Deep-Inelastic Scattering that takes place in Bonn on 26–30 March. The first measurements using electron final-states will also be available soon, as will those on the production of Z bosons in association with jets. The latter will open the way to more direct probes of the PDFs, once the jets can tagged by flavour (for example, a measurement of the production of a W boson together with a charm jet will allow constraints to be placed on the behaviour of the strange quark inside the proton).

The forward acceptance of LHCb also provides unexpected advantages for other measurements. The further forward in pseudorapidity that final states are produced, the more likely they are to arise from interactions between a valence quark in one proton and an antiquark in the “sea” of the other proton. This is in contrast to the ATLAS and CMS experiments, which experience predominately sea–sea collisions. The measurement of the forwards–backwards asymmetry of Z bosons, which is sensitive to the electroweak mixing angle, sin²θ_W, benefits from this ability to define a “forward” incoming-quark direction. Studies show that LHCb can identify this correctly in more than 90% of events that have boson rapidities above 3 (McNulty 2011). PDF uncertainties are also reduced in this region. This gives the LHCb experiment the potential to reach the precision of a typical measurement of sin²θ_W at the Large Electron–Positron collider, even with the data set of 1 fb^–1 already recorded.

Studies of the production of the top quark could also benefit from LHCb’s detection system. Although the production rate for top inside LHCb is small at 7 TeV, at 14 TeV the rate should be large enough to make measurements viable. At this centre-of-mass energy, top-pairs are produced by quark–antiquark annihilation twice as often inside the forward region of LHCb’s acceptance as they are in the central region. A measurement of the t-t asymmetry with LHCb could give a direct and comparable cross-check of the recent result from Fermilab’s Tevatron.

Electroweak physics at LHCb may not have been part of the original programme, but the future prospects are bright.

Experiment recreates ‘seeds’ of the universe’s magnetic fields

CCnew5_02_12 — This composite picture shows on the left an image of a laser-produced shock wave; the right side is a simulation of a collapsing shock-wave arising during the pregalactic phase of the universe. Brighter colours correspond to regions of higher density or temperature.
Image credit: F Miniati/ETH.

How did magnetic fields arise in the universe? An experiment using a high-power laser to create plasma instabilities may have glimpsed the processes that created magnetic fields during the period of galaxy formation.

Magnetic fields pervade the cosmos. Measurements of synchrotron emission at radio frequencies from cosmic rays and of Faraday rotation reveal that they exist in Galaxy clusters on the megaparsec scale, with strengths that vary from a few nanogauss to a few microgauss. Intergalactic magnetic fields weave through clusters of galaxies forming even larger-scale structures. In these clusters the temperatures can often be greater than 10⁸ K, making them strong X-ray emitters. It is possible that the energy to heat the plasma comes from the magnetic field through some plasma instability. In general, wherever intergalactic hot matter is detected, magnetic fields with strengths greater than 10^–9 G are also observed – with weaker magnetic fields tending to occur outside galaxy clusters. The magnetic field therefore appears to play a role in the structure of the universe.

The only way to explain the observed magnetization is through a magnetic dynamo mechanism in which it is necessary to invoke a “seed” field – but the origin of this seed field remains a puzzle. Prior to galaxy formation, density inhomogeneities would drive violent motions in the universe, forming shock waves that would generate vorticity on all scales. In 1997 Russell Kulsrud suggested that the “Biermann battery effect” could create seed magnetic fields as small as 10^–21 G that would be amplified by the protogalaxy dynamo. In this effect, proposed by astrophysicist Ludwig Biermann in 1950, electric fields can arise in a plasma as the electrons and the heavier protons respond differently to external pressure and tend to separate. The Biermann battery acts to create the seed magnetic fields whenever the pressure and density gradients are not parallel.

Now, an international team of scientists has performed an experiment to recreate the conditions similar to those in the pregalactic epoch where shocks and turbulent motions form. They used a high-power laser at the Laboratoire pour l’Utilisation des Lasers Intenses in Paris to explode a rod of carbon surrounded by helium gas in a field-free environment. Magnetic induction coils monitored the magnetic fields created in the resulting shock waves. The team found that the explosion generated strong shock waves around which strong electric currents and magnetic fields formed, through the Biermann battery effect, with fields as high as 10–30 G existing for 1–2 μs at 3 cm from the blast. When scaled through 22 orders of magnitude, the measurements matched the predynamo magnetic seeds predicted by theory prior to galaxy formation.

Looking at the top for new physics

Last year one of the properties of top-quark production, the tt charge asymmetry, attracted much interest with the publication of measurements by the CDF and DØ collaborations at Fermilab’s Tevatron (T Aaltonen et al. 2011, V M Abazov et al. 2011). They reported results that were 2σ above the predicted values. This deviation can be explained by several theories that go beyond the Standard Model by introducing new particles that contribute to top-quark production. The CMS collaboration has now measured this top-quark property for the first time at the LHC – and finds a different result.

CCnew6_02_12 — Fig. 1. Event display of a tt̅ candidate event. The t is reconstructed in the forward direction; the t̅ is reconstructed from the three central jets. The difference in pseudorapidity Δ|η| is 2.7.

In the Standard Model, a difference in angular distributions between top quarks and antiquarks (commonly referred to as the charge asymmetry) in tt production through quark–antiquark annihilation appears in QCD calculations at next-to-leading order. It leads to there being more top quarks produced at small angles to the beam pipe, while top antiquarks are produced more centrally (figure 1). As a consequence, the pseudorapidity distribution of top quarks is broader than that of top antiquarks, which makes the difference of the respective pseudorapidities, Δ|η| = |η_t| – |η_t|, a suitable observable for measuring the charge asymmetry (figure 2). The Standard Model predicts a small asymmetry of A_C = 0.0136 ± 0.0008, which translates into an excess of about 1% of events with Δ|η| > 0 compared with events with Δ|η| < 0 (Kuhn and Rodrigo 2011). The existence of new sources of physics could enhance this asymmetry.

CCnew7_02_12 — Fig. 2. The reconstructed distribution of the difference of the absolute values of the top quark and antiquark pseudorapidities.

CMS has measured the tt charge asymmetry using data corresponding to an integrated luminosity of 1 fb^–1 (CMS collaboration 2011). A total of 12 757 events were selected in the lepton+jets channel, where one top quark decays into a b quark, a charged lepton (electron or muon) and the corresponding neutrino, while the other top quark decays into three quarks. The background contribution to this dataset is about 20%. The measurement of the charge asymmetry is based on the full reconstruction of the four-momenta of the top quarks, which have to be reconstructed from the observed leptons, jets and missing transverse energy. The dependency of the selection efficiency both on the Δ|η| value and on the smearing of the momenta of the top-quark decay products, because of finite detector resolution, are accounted for when calculating the final result.

The measured value of A_C = –0.017 ± 0.032 (stat.)+0.025 –0.036 (syst.) is consistent with the Standard Model prediction and does not provide any indication for a new physics contribution. CMS also measured the uncorrected charge asymmetry as a function of the invariant mass of the top-quark pair, m_tt. Previous measurements by the CDF collaboration had found an asymmetry that was more than 3σ above the predicted value for large values of m_tt. However, the current analysis by CMS dampens the excitement that the CDF result caused because it reveals no hints for a deviation from the Standard Model predictions.

The hunt for long-lived exotic beasts

The hunt for exotic massive long-lived particles is an important element in the ATLAS collaboration’s programme of searches. The signatures associated with such long-lived objects are particularly striking and experimentally challenging. At the LHC they could appear as slow-moving and highly ionizing objects that could slip into the next bunch-crossing, saturate the read-out electronics and confound the event reconstruction software. An alternative approach to the direct detection of moving long-lived particles is to search for those that stop in the detector and subsequently decay. This is the method used in a recent search by the ATLAS collaboration.

CCnew8_02_12 — Observed (solid line) and expected (dashed line) cross-section limits on the pair-production of gluinos as a function of the gluino’s mass. The predicted cross-section is also shown, as the red line with blue band, allowing a lower mass limit to be inferred from the intersection of the observed cross-section limit with the predicted cross-section.

The new search looks for metastable R-hadrons that would be formed from gluinos and light quarks (ATLAS collaboration 2011a). If produced, some R-hadrons would stop in the dense calorimeter material – following electromagnetic and hadronic interactions. Within the scenario of split-supersymmetry, an R-hadron could decay to a final state of jets and a neutralino. During 2010, the experiment used jet triggers to record candidate decays in empty bunch crossings when no proton–proton collisions were intended. With the subsequent analysis, which required estimations of cosmic and beam-related backgrounds along with the uncertainties on R-hadron stopping rates, ATLAS has set upper limits on the pair-production cross-section for gluinos with lifetimes in the range 10^–5–10³ s. From this, the collaboration has obtained a lower mass limit for the gluino of around 340 GeV at the 95% CL (see figure). Although the search was inspired by split-supersymmetry, the results are generally applicable for any heavy object decaying to jets.

This complex work complements other, more conventional, searches for long-lived particles that interact or decay in the ATLAS detector. These results allow stringent limits to be set on topical models of new physics. Moreover, the collaboration is performing experimentally driven searches up to the limits of the detector’s capability to detect long-lived objects. For example, a search based on early collision data sought exotic particles with large electric charge (up to 17e).

With more data and a continually improving knowledge of the detector response, the ATLAS collaboration is aiming at a set of comprehensive searches for long-lived objects, which possess a range of colour, electric and magnetic charges, and appear as stable objects or decay to a variety of final states.

ALICE unveils mysteries of the J/ψ

Fig. 1. A comparison of the J/ψ suppression between RHIC (PHENIX) and the LHC (ALICE). The ALICE results show a strikingly smaller suppression, in particular for head-on collisions (large N_part), despite the much larger centre-of-mass energy.

The J/ψ meson, a bound state of a charm (c) and an anticharm (c) quark, is unique in the long list of particles that physicists have discovered over the past 50 years. Found almost simultaneously in 1974 – at Brookhaven, in proton–nucleus collisions, and at SLAC, in e⁺e^– collisions – this particle is the only one with two names, given to it by the two teams. With a mass greater than 3 GeV it was by far the heaviest known particle at the time and it opened a new field in particle physics, namely the study of “heavy” quarks.

The charm quark and its heavier partners, the bottom and top quarks (the latter discovered more than 20 years later, in 1995), have proved to be a source of both inspiration and problems for particle physicists. By now, thousands of experimental and theoretical papers have been published on these quarks and the production, decay and spectroscopy of particles containing heavy quarks have been the focus of intense and fruitful investigations.

In conclusion, a particle that has been known for almost half a century continues to be a source of inspiration and progress

However, despite a history of almost 40 years, the production of the J/ψ itself still represents a puzzle for QCD, the standard theory of strong interactions between quarks and gluons. On the one hand, the creation of a pair of quarks as “heavy” as charm (m_c ≈ 1.3 GeV/c²) in a gluon–gluon or quark–antiquark interaction is a process that is “hard” enough to be treated in a perturbative way and therefore well understood by theory. On the other hand however, the binding of the pair is essentially a “soft” process – the relative velocity of the two quarks in a J/ψ is “only” about 0.5 c – and this proves to be much more difficult to model.

J/ψ production

About fifteen years ago, the results obtained at Fermilab’s Tevatron collider first showed a clear inconsistency with the theoretical approach adopted at the time to model J/ψ production, the so-called colour-singlet model. This unsatisfactory situation led to the formulation of the more refined approach of nonrelativistic QCD (NRQCD), which brought a better agreement with data. However, other quantities such as the polarization of the produced J/ψ, i.e. the extent to which the intrinsic angular momentum of the particle is aligned with respect to its momentum, were poorly reproduced. This uncomfortable situation also arose partly because of controversial experimental results from the Tevatron, where the CDF experiment’s results on polarization from Run1 disagreed with those from Run2. Considerable hope is therefore placed on the results that the LHC can obtain for this observable (more on this later).

Nevertheless, despite these unresolved mysteries surrounding its production, the J/ψ has an important “application” in high-energy nuclear physics and more precisely in the branch that studies the formation of the state of (nuclear) matter where quarks and gluons are no longer confined into hadrons: the quark–gluon plasma (QGP). If such a state is created, it can be thought of as a hot “soup” of coloured quarks and gluons, where colour is the “charge” of the strong interaction. In the usual world, quarks and gluons are confined within hadrons and colour cannot fly over large distances. However, in certain situations, as when ultrarelativistic heavy-ion collisions take place, a QGP state could be formed and studied. Indeed, such studies form the bulk of the physics programme of the ALICE experiment at the LHC.

The J/ψ is composed of a heavy quark–antiquark pair with the two objects orbiting at a relative distance of about 0.5 fm, held together by the strong colour interaction. However, if such a state were to be placed inside a QGP, it turns out that its binding could be screened by the huge number of colour charges (quarks and gluons) that make up the QGP freely roaming around it. This causes the binding of the quark and antiquark in the J/ψ to become weaker so that ultimately the pair disintegrates and the J/ψ disappears – i.e. it is “suppressed”. Theory has shown that the probability of dissociation depends on the temperature of the QGP, so that the observation of a suppression of the J/ψ can be seen as a way to place a “thermometer” in the medium itself.

Such a screening of the colour interaction, and the consequent J/ψ suppression, was first predicted by Helmut Satz and Tetsuo Matsui in 1986 and was thoroughly investigated over the following years in experiments with heavy-ion collisions. In particular, Pb–Pb interactions were studied at CERN’s Super Proton Synchrotron (SPS) at a centre-of-mass energy, √s, of around 17 GeV per nucleon pair and then Au–Au collisions were studied at √s=200 GeV at Brookhaven’s Relativistic Heavy-Ion Collider (RHIC).

The J/ψ and its suppression can be seen as a thermometer in the medium created in the collision

As predicted by the theory, a suppression of the J/ψ yield was observed with respect to what would be expected from a mere superposition of production from elementary nucleon–nucleon collisions. However, the experiments also made some puzzling observations. In particular, the size of the suppression (about 60–70% for central, i.e. head-on nucleus–nucleus collisions) was found to be approximately the same at the SPS and RHIC, despite the jump in the centre-of-mass energy of more than one order of magnitude, which would suggest higher QGP temperatures at RHIC. Ingenious explanations were suggested but a clear-cut explanation of this puzzle proved impossible.

At the LHC, however, extremely interesting developments are expected. In particular, a much higher number of charm–anticharm pairs are produced in the nuclear interaction, thanks to the unprece¬dented centre-of-mass energies. As a consequence, even a suppression of the J/ψ yield in the hot QGP phase could be more than counter-balanced by a statistical combination of charm–anticharm pairs happening when the system, after expansion and cooling, finally crosses the temperature boundary between the QGP and a hot gas of particles. If the density of heavy quark pairs is large enough, this regeneration process may even lead to an enhancement of the J/ψ yield – or at least to a much weaker suppression with respect to the experiments at lower energies. The observation of the fate of the J/ψ in nuclear collisions at the LHC constitutes one of the goals of the ALICE experiment and was among its main priorities during the first run of the LHC with lead beams in November/December 2010.

The ALICE experiment is particularly suited to observing a J/ψ regeneration process. For simple kinematic reasons, regeneration can be more easily observed for charm quarks with low transverse-momentum. Contrary to the other LHC experiments, both detector systems where the J/ψ detection takes place – the central barrel (where the J/ψ→e⁺e^– decay is studied) and the forward muon spectrometer (for J/ψ→μ⁺μ^–) – can detect J/ψ particles down to zero transverse momentum.

As the luminosity of the LHC was still low during its first nucleus–nucleus run, the overall J/ψ statistics collected in 2010 were not huge, of the order of 2000 signal events. Nevertheless, it was possible to study the J/ψ yield as a function of the centrality of the collisions in five intervals from peripheral (grazing) to central (head-on) interactions.

Clearly, suppression or enhancement of a signal must be established with respect to a reference process. And for such a study, the most appropriate reference is the J/ψ yield in elementary proton–proton collisions at the same energy as in the nucleus–nucleus data-taking. However, in the first proton run of the LHC the centre-of-mass energy of 7 TeV was more than twice the energy of 2.76 TeV per nucleon–nucleon collision in the Pb–Pb run. To provide an unbiased reference, the LHC was therefore run for a few days at the beginning of 2011 with lower-energy protons and J/ψ production was studied at the same centre-of-mass energy of Pb–Pb interactions.

Fig. 2. The two parameters, λ_θ and λ_φ, that express the degree of polarization of the J/ψ, as a function of its transverse momentum, in the helicity and Collins-Soper frames. The values obtained by ALICE are, within errors, compatible with zero, showing an unpolarized J/ψ production at LHC energy.

The Pb–Pb and p–p results are compared using a standard quantity, the nuclear modification factor R_AA. This is basically a ratio between the J/ψ yield in Pb–Pb collisions, normalized to the average number of nucleon–nucleon collisions that take place in the interaction of the two nuclei and the proton–proton yield. Values smaller than 1 for R_AA therefore indicate a suppression of the J/ψ yield, while values larger than 1 represent an enhancement.

The results from the first ALICE run are rather striking, when compared with the observations from lower energies (figure 1). While a similar suppression is observed at LHC energies for peripheral collisions, when moving towards more head-on collisions – as quantified by the increasing number of nucleons in the lead nuclei participating in the interaction – the suppression no longer increases. Therefore, despite the higher temperatures attained in the nuclear collisions at the LHC, more J/ψ mesons are detected by the ALICE experiment in Pb–Pb with respect to p–p. Such an effect is likely to be related to a regeneration process occurring at the temperature boundary between the QGP and a hot gas of hadrons (T≈160 MeV).

The picture arises from these observations is consistent with the formation, in Pb–Pb collisions at the LHC, of a deconfined system (QGP) that can suppress the J/ψ meson, followed by a hadronic system in which a fraction of the charm–anticharm pairs coalesce and ultimately give a J/ψ yield larger than that observed at lower energies. This picture should be clarified by the Pb–Pb data that were collected in autumn 2011. Thanks to an integrated luminosity for such studies that was 20 times larger than in 2010, a final answer on the fate of the J/ψ inside the hot QGP produced at the LHC seems to be within reach.

ALICE is also working hard to help solve other puzzles in J/ψ production in proton–proton collisions, in particular by studying, as described above, the degree of polarization. A first result, recently published in Physical Review Letters, shows that the J/ψ produced at not too high a transverse momentum are essentially unpolarized, i.e. the angular distribution of the decay muons in the J/ψ→μ⁺μ^– process is nearly isotropic (figure 2). Theorists are now working to establish if such behaviour is compatible with the NRQCD approach that up to now is the best possible tool for understanding the physics related to J/ψ production.

In conclusion, a particle that has been known for almost half a century continues to be a source of inspiration and progress. However, even if particle and nuclear physicists working at the LHC are confident of being able finally to understand its multifaceted aspects, the future often brings the unexpected. So stay tuned and be ready for surprises.

DIRAC observes dimeson atoms and measures their lifetime

CCdir2_02_12 — This image shows the section between the beam shielding and the magnet, with the vacuum chamber (blue), the beginning of the flat chamber (yellow) and the magnetic screen (orange) in front of the magnet. The beam pipe for the primary protons (silver) lies beneath the blue vacuum chamber.

The study of nonstandard atoms has a long tradition in particle physics. Such exotic atoms include positronium, muonic atoms, antihydrogen and hadronic atoms. In this last category, mesonic hydrogen in particular has been investigated extensively in different experiments at CERN, PSI and Frascati. Dimeson atoms also belong to this category. These electromagnetically bound mesonic pairs, such as the π⁺π^– atom (pionium, A_2π) or the πK atom (A_πK), offer the opportunity to study the theory of the strong interaction, QCD, at low energy, i.e. in the confinement region.

CCdir1_02_12 — A general view of the downstream part of the DIRAC experiment beyond the magnet (orange), with the yellow boxes of the Cherenkov counters in the foreground.

This strong interaction leads to a broadening and a shift of atomic levels, and dominates the lifetime of these exotic atoms in their s-states. The ππ interaction at low energy, constrained by the approximate chiral symmetry SU(2) for two flavours (u and d quarks), is the simplest and best understood hadron–hadron process. Since the bound-state physics is well known, a measurement of the A_2π lifetime provides information on hadron properties in the form of scattering lengths – the basic parameters in low-energy ππ scattering.

CCdir3_02_12 — Fig. 1. Pionium (A_2π), produced in nickel, is detected through its break-up (ionization). Alternatively, the A_2π annihilates (decays). N_A denotes the total number of produced atoms, whereas n_A is the number of “atomic pairs” from atom break-up.

Moreover, the low-energy interaction between the pion and the next lightest, and strange, meson – the kaon – provides a promising probe for learning about the more general three-flavour SU(3) structure (u, d and s quarks) of hadronic interactions, which is a matter not directly accessible in pion–pion interactions. Hence, data on πK atoms are valuable because they provide insights into the role played by the strange quarks in the QCD vacuum.

The experiment

The mesonic atoms A_2π (A_πK) are produced by final-state Coulomb interactions between oppositely charged ππ (πK) pairs that are generated in proton–target reactions (Nemenov 1985). In the DImeson Relativistic Atom Complex (DIRAC) experiment at CERN, they are formed when a 24 GeV/c proton beam from the Proton Synchrotron hits a thin target, typically a 100 μm-thick nickel foil (figure 1). After production, the mesonic atoms travel through the target and some of them are broken up (ionized) as they interact with matter. This produces “atomic pairs”, which are characterized by their small relative momenta, Q < 3 MeV/c, in the centre of mass of the pair. These pairs are detected in the DIRAC apparatus. The remaining atoms mainly annihilate into π⁰π⁰ pairs, which are not detected, or they survive and annihilate later. The number of “atomic pairs” from the break-up of atoms, n_A, depends on the annihilation mean-free-path, which is given by the atom’s lifetime, τ, and its momentum. Thus, the break-up probability, P_br, is a function of the A_2π’s lifetime, τ.

CCdir4_02_12 — Fig. 2. The DIRAC double-arm spectrometer. The microstrip gas chambers (MSGC), the scintillating-fibre detector (SFD) and the scintillation-ionization hodoscope (IH) provide initial track data. Downstream of the spectrometer magnet, drift chambers (DC) measure final tracks to determine momenta. The vertical and the horizontal-scintillation hodoscopes (VH and HH) are used for trigger purposes and time measurement. Cherenkov detectors (Ch) containing nitrogen, preshower detectors (PSh) and scintillation muon detectors (Mu) behind the iron absorber help to distinguish pions from electrons and muons.

The interactions between the protons and the target also produce oppositely charged free ππ pairs, both with and without final-state Coulomb interactions, depending on whether or not the pairs are produced close to each other. This gives rise to “Coulomb pairs” and “non-Coulomb pairs”. The latter includes meson pairs in which one or both mesons come from the decay of long-lived sources. Furthermore, two mesons from different interactions can contribute as “accidental pairs”. The total number of atoms produced, N_A, is proportional to N_C, the number of Coulomb pairs with low relative momenta, N_A = kN_C, where the coefficient, k, is precisely calculable. DIRAC measures the break-up probability for the A_2π, which is defined as the ratio of the observed number of “atomic pairs” to the number of produced atoms: P_br(τ)= n_A/N_A. N_A is calculated from the number of “Coulomb pairs”, N_C, obtained from fits to the data.

The purpose of the DIRAC set-up is to record oppositely charged ππ (πK) pairs with small relative momenta, Q. As figure 2 shows, the emerging pairs of charged pions travel in vacuum through the upstream part where co-ordinate and ionization detectors provide initial track data, before they are split by the 2.3 Tm bending magnet into the “positive” (T1) and “negative” (T2) arms. Both arms are equipped with high-precision drift chambers and trigger/time-of-flight detectors, as well as Cherenkov, preshower and muon counters. The relative time resolution between the two arms is around 200 ps.

CCdir5_02_12 — Fig. 3. Q_L distributions of π⁺π^– pairs from data (black dots) and simulation (solid line). Top: the experimental spectrum in comparison with the simulated background components of “Coulomb” (N_C), “non-Coulomb” (N_nC) and “accidental” (N_acc) pairs. Bottom: the residuals for the “atomic pairs” (n_A) – the pionium signal – after background subtraction and the corresponding simulation.

The momentum reconstruction in the double-arm spectrometer uses the drift chamber information from both arms as well as the measured hits in the upstream co-ordinate detectors. The resolution on the longitudinal (Q_L) and transverse (Q_T) components of the relative momentum of the pair, Q, defined with respect to the direction of the total momentum of the pair in the laboratory, is 0.55 MeV/c and 0.1 MeV/c, respectively. A system of fast-trigger processors selects the all-important events with small Q.

Observing and measuring lifetimes

The observation of the A_2π was reported from an experiment at Serpukhov nearly 20 years ago. This was followed at CERN 10 years later with a measurement of the A_2π lifetime by DIRAC (Adeva et al. 2005). Last autumn, DIRAC presented the most recent value for the A_2π lifetime in the ground state, τ = 3.15 × 10^–15 s, with a total uncertainty of around 9%, based on the statistics of 21 200 “atomic pairs” collected with the nickel target in 2001–2003 (Adeva et al. 2011). Figure 3 (overleaf) shows the characteristic accumulation of events at low Q_L from the break-up of the π⁺π^– atom: the A_2π signal appears as an excess of pairs over the background spectrum in the low Q region.

S-wave ππ scattering is isospin-dependent, so this lifetime can be used to calculate a scattering-length difference, |a₀–a₂|, where a₀ and a₂ are the S-wave ππ scattering lengths for isospin 0 and 2, respectively. The measured lifetime yields a result of |a₀–a₂| = 0.253 (m_π^–1) with around 4% precision, in agreement with the result obtained by the NA48/2 experiment at CERN (Batley et al. 2009). The corresponding theoretical values are 0.265 ± 0.004 (m_π^–1) for the scattering-length difference (Colangelo et al. 2000) and (2.9 ± 0.1) × 10^–15 s for the lifetime (Gasser et al. 2001). These results demonstrate the high precision that can be reached in low-energy hadron interactions, in both experiment and theory.

The first evidence for the observation of the πK atom, A_πK, was published by the DIRAC collaboration in 2009 (Adeva et al. 2009). In this case, the mesonic atoms were produced in a 26 μm-thick platinum target and the DIRAC spectrometer had been upgraded for K identification with heavy-gas (C₄F₁₀) and aerogel Cherenkov detectors. An enhancement observed at low relative momentum corresponds to the production of 173 ± 54 πK “atomic pairs”. From this first data sample, the collaboration derived a lower limit on the πK atom lifetime of τ_πK > 0.8 × 10^–15 s (90% CL), to be compared with the theoretical prediction of (3.7 ± 0.4) × 10^–15 s (Schweizer 2004). The ongoing detailed analysis of a much larger data sample aims first to extract a clear signal for the production of the A_πK atom and then to deduce from these data a value for the A_πK lifetime.

Future investigations

Pionium is an atom like hydrogen and the properties of its states vary strongly with their quantum numbers. An illustration of this is the 2s→1s two-photon de-excitation in hydrogen (τ_2s ≈ 0.1s), which is many orders of magnitude slower than the 2p→1s radiative transition (τ_2p = 1.6 ns). In pionium, the situation is similar but opposite: the decay A_2π (2s-state)→2π⁰ (τ_2s = 23.2 fs) is roughly three orders of magnitude faster than the 2p→1s radiative transition (τ_2p = 11.7 ps). The DIRAC collaboration aims to measure Lamb shifts in pionium by exploiting the properties of these specific states and in 2010 started to study the possibility of observing long-lived A_2π states (Nemenov et al. 2002).

CCdir6_02_12 — Fig. 4. (above) Pionium energy-level scheme and 2s–2p energy splitting. The decay widths (Γ_1s, Γ_2s) and the energy shifts (ε_1s, ε_2s, ε_2p) are presented for the states with principal quantum numbers n=1 and 2.

The energy shifts, ΔE_ns–np, for levels with the principal quantum number, n, and orbital quantum number, l, are another valuable source of information. These shifts contain a dominant strong contribution, ΔE^strong _ns–np, together with minor QED contributions, ΔE^QED _ns–np, from vacuum polarization and self-energy effects. The strong s-state energy shift,ΔE^strong _ns–np, is proportional to (2a₀+a₂), i.e. it depends on the same scattering lengths, a₀ and a₂, as the pionium lifetime. As figure 4 shows, for the principal quantum number n=2, the strong and electromagnetic interactions shift the 2s level below the 2p level by ΔE_2s–2p = ΔE^strong _ns–np + ΔE^QED _2s–2p = – 0.47 eV – 0.12 eV = – 0.59 eV (Schweizer 2004).

By studying the dependence of the lifetime of long-lived A_2π (with l≥1) on an applied electric field – the Stark-mixing effect – the DIRAC experiment is in a unique position to investigate the splitting of pionium energy levels. This will allow another combination of pion-scattering lengths to be extracted, so that a₀ and a₂ can finally be determined individually.

Getting excited about the Higgs?

Main auditorium — The main auditorium was packed, having reached its full capacity already three and a half hours before the presentations began.

Tuesday 13 December 2011 is a day that many will remember. There was high anticipation of what the ATLAS and CMS collaborations would have to say about the latest results on the search for the elusive Higgs boson. Only senior management knew what the other collaboration was going to present, for the rest it was a well kept surprise.

CCnew2_01_12 — Left: The expected (dashed line and coloured bands) and observed (black line) 95% confidence-level cross-section limits as a function of the Higgs boson mass over the full mass range from the CMS collaboration. Right: Similar plot from the ATLAS collaboration concentrating on the low mass range. The y-axes are normalized to the Standard Model cross-section, thus an observed value less than 1 corresponds to a mass that is excluded. The coloured bands show the 95% (yellow) and 68% (green) regions around the mean (dashed line) expected limit.</p>

From 8.30 a.m. onwards, physicists flocked into the main auditorium at CERN and by 10.30 a.m. the place was packed – three and a half hours before the talks even started – and no more people were being allowed in. The atmosphere was almost festive, maybe because the wireless network became saturated so that nobody could work. Similarly eager anticipation could be felt in a separate room where journalists representing the many news agencies and TV channels were able to share in the excitement.

But what was all of the excitement about?

The short answer is that the presentations revealed that if the Higgs boson exists in the manner predicted by the Standard Model, then its mass is most likely between 115.5 and 127 GeV. To be more precise, the CMS collaboration rules out at 95% confidence level a Higgs boson with a mass larger than 127 GeV, while the ATLAS collaboration rules it out for masses below 115.5 GeV and larger than 131 GeV (with a small window of 237–251 GeV in mass not yet excluded by ATLAS). The upper limits on the exclusion region are 468 GeV for ATLAS and 600 GeV for CMS.

The ability to exclude the low-mass region was limited in both experiments by an excess of events around 120 GeV.

The ability to exclude the low-mass region was limited in both experiments by an excess of events around 120 GeV. Such excesses could be just background fluctuations or the first indications of a Higgs signal building up. These results are consistent with what is expected from the statistics accumulated so far, whether the low-mass Higgs exists or not.

The 2012 data campaign, during which the LHC is expected to deliver at least twice as many collisions as in 2011, should put to rest the 40-year quest for the Standard Model Higgs boson via either its discovery or its complete exclusion.

Only a month after the end of proton–proton collisions in 2011, both collaborations showed preliminary results using the full statistics of the year, corresponding to an integrated luminosity of 4.6–4.9 fb^–1 – almost twice that shown at the summer conferences. The results unveiled in the presentations in December demonstrate the deep understanding achieved by each collaboration of detector performance and of the numerous backgrounds.

Both spokespeople, Fabiola Gianotti for the ATLAS collaboration and Guido Tonelli for the CMS collaboration, paid tribute in their presentations to the hundreds of physicists – most of them students and young post-docs – who have worked so hard in recent months to improve substantially the understanding of the detectors, in particular under the complex condition of ever increasing pile-up, where as many as 20 interaction vertices were reconstructed in a single event.

As a result of a coin toss by the director-general, Gianotti spoke first. The ATLAS collaboration had concentrated on updating the analyses in the channels that are most sensitive to the low-mass Higgs boson: H→γγ and the “golden channel”, H→ZZ→llll, where l indicates an electron or a muon. An update of the H→WW search with the data collected for the summer conferences was also shown. While in the first two channels the Higgs boson would be seen as a narrow peak on top of a broad background, the presence of the Higgs boson in the third channel would be seen as a more broad excess of events.

Tonelli then took the stage and presented a full array of CMS analyses – all including the full 2011 statistics – starting with the ones sensitive to the highest Higgs masses, H→ZZ →(llqq), (llνν), (llττ), continuing with H→WW, H→ττ, H→bb and finishing with H→γγ and the golden channel H→ZZ→ llll.

The energy and angular resolutions of the electromagnetic calorimeters are the key ingredients in the analysis of H→γγ, which is potentially the single-most sensitive mode in the low-mass region. The better the resolution, the narrower the peak in the invariant-mass distribution of the two photons and the easier it will be to see the Higgs, if it exists.

Despite having two different detector technologies – ATLAS uses a liquid argon sampling calorimeter, while CMS relies on crystals – the mass resolution obtained in both experiments in the channel with the best resolution is 1.4 GeV. For CMS, good mass resolution is possible thanks in particular to the major progress that has been achieved in understanding the calibration of the crystal calorimeter; in the central area of the calorimeter the performance is now close to nominal. The ATLAS calorimeter provides a similar mass resolution to that of CMS, despite intrinsically worse energy resolution. This is thanks to its capability to measure photon angles.

To explore all our coverage marking the 10th anniversary of the discovery of the Higgs boson ...

Click here

LHCb sees first evidence for CP violation in charm decays

Mass-difference spectra for reconstructed D^*+ → D⁰π⁺ decays, with D⁰ → K^–K⁺ (top) and D⁰ → π^–π⁺ (bottom), showing the high-statistics samples of cleanly reconstructed decays in these suppressed modes.

The LHCb experiment was initially designed for the study of B physics (the “b” in its name stands for beauty, or b quark). However, the LHC is also a copious source of particles that contain the charm quark, such as the D meson, which also makes the experiment well suited to their study. The rate at which data are selected by the LHCb trigger and written to storage was therefore increased last year by 50%, to 3 kHz, with the extra capacity dedicated to charm. This has now paid off spectacularly, with one of the most interesting (and unexpected) results to come from the LHC so far: evidence of CP violation in charm decays.

CP symmetry, the combination of charge-conjugation, C, and parity, P, is known to be violated in B and K decays. It is an important property to study because it is a necessary ingredient for explaining the matter–antimatter asymmetry in the universe. The CP violation observed so far in B and K decays is consistent with the predictions of the Standard Model but is far too small to explain the observed matter–antimatter asymmetry. The prediction for D mesons in the Standard Model is that they should have little CP violation, at the level of 10^–3 or less, but this may be enhanced by new physics.

The LHCb collaboration performed its search by measuring the difference in the CP asymmetry for two final states

CP violation can be observed as a difference in the rate of D⁰ decays to a given final state, compared with the rate of the antiparticle D⁰ decays to the same final state. Because the effect being looked for is tiny, the LHCb collaboration performed its search by measuring the difference in the CP asymmetry for two final states, D⁰ → K^–K⁺ and D⁰ → π^–π⁺, which is denoted by ΔA_CP. In this way, systematic uncertainties in the production and detection of particles compared with antiparticles should cancel, while the CP asymmetry, which is expected to have opposite sign for the two modes, should remain visible.

D^*± → D⁰π^± decays are used as the source of the neutral D-meson sample because the charge of the pion tags the produced particle as a D⁰ or D⁰ (see figure). The experiment has collected more than a million of these decays, a total more than 10 times greater than in measurements by experiments at B-factories. It is also larger than the data sample used to obtain the previously most precise result, from the CDF experiment at Fermilab’s Tevatron, and benefits from the clean selection made possible by LHCb’s ring-imaging Cherenkov detectors.

LHCb measures the asymmetry to be ΔA_CP = (–0.82 ± 0.21 ± 0.11)%, where the first error is statistical and the second systematic. The significance of the measured deviation from zero is 3.5 σ, giving the first evidence for CP violation in the charm sector, at a level that is higher than was expected. Establishing whether this result is consistent with the Standard Model, or the first hint of new physics, will require the analysis of additional data and improved theoretical understanding. LHCb has already collected almost a factor of two more data, with more to come this year, so this exciting first indication should be clarified soon.

ATLAS discovers its first new particle

The spectrum of the χb states observed in three different ways with peaks, from the left, for the χb(1P), the χb(2P) and the new χb(3P). The plot on the left shows the spectrum for decays involving unconverted photons, while the right-hand plot shows spectra for decays involving converted photons. In the right-hand plot, the upper (red) curve shows the spectrum for χb decays to ϒ(1S), while the lower (brown) curve shows the spectrum for decays to ϒ(2S). (Only the χb(3P) peak appears distinctly in the lower spectrum because it is the only χb state with decays involving enough energy to be detected in this study.)

The ATLAS collaboration has announced the discovery of the χ_b(3P), which is a bound state of a bottom quark and bottom antiquark (bb).

Bound states of a heavy quark and its antiquark – collectively called quarkonium – are the QCD analogues of the hydrogen atom, with each particle corresponding to a different energy level. For bb states, the S states are the well known ϒ particles, while the P states are known as χ_b. As with the hydrogen atom, transitions between these states can be observed through the emission of a photon (γ).

The collaboration discovered the new state through the radiative transitions χ_b(3P) → ϒ(1S) + γ and χ_b(3P) → ϒ(2S) + γ, followed by the decay of the ϒ to two muons. The figure shows the spectrum of the χ states: the leftmost peak is the χ_b(1P), the middle one the χ_b(2P) and the rightmost the new state, χ_b(3P). The photons were detected either by the electromagnetic calorimeter (in which case they remained unconverted) or, if they had interacted with material and converted to an e⁺e^– pair, by the ATLAS tracking detectors.

Usually, a new particle is discovered in one or at most two channels, and the first observation is at the very edge of statistical significance. However, ATLAS has seen the χ_b(3P) with three different signatures, in both the ϒ(1S) and ϒ(2S) channels, and the peaks are unmistakable. The outstanding performance of both the LHC and the ATLAS detector made such a clear observation possible.

Also in analogy with atomic physics, the visible peaks contain internal structure from hyperfine splitting among states of different angular momentum. These could be resolved with future data samples.

Studying the energy levels of quarkonium states provides information about the forces that bind quarks together. One surprise is that the χ_b(3P) is slightly heavier than predicted, implying that the quark–antiquark pair is a little more loosely bound than expected. The χ_b(3P) is just at the very limit of being bound, so the quark and antiquark are about as far apart from each other as they can possibly be.

Hadron spectra probe nature of matter in Pb–Pb collisions

In current understanding, the matter created in heavy-ion collisions – the quark–gluon plasma (QGP) – behaves as a nearly perfect liquid. The confirmation of this hydrodynamic behaviour, previously observed at Brookhaven’s Relativistic Heavy Ion Collider (RHIC), was one of the most eagerly awaited results from the first Pb–Pb collisions at the LHC. One of the crucial measurements for the characterization of the fireball produced in the collisions centres on the spectra of identified hadrons, which encode the collective expansion velocity in the QGP and hadronic stages. Moreover, their overall abundances are believed to be fixed at hadronization.

The ALICE detector was designed to perform these measurements with a unique combination of detectors for particle identification (PID): the silicon inner-tracking-system, the time-projection chamber and the time-of-flight detector. The collaboration has used these to measure the production of pions, kaons and protons in the range in transverse-momentum, p_t, where most of the particles are produced (0.1 to about 3 GeV/c).

The figure shows the results compared with the expectation from a hydrodynamic model, revealing a good agreement with the predicted shapes (Floris 2011). Together with the results on the azimuthal anisotropy also reported by ALICE and the other LHC experiments, this represents the most direct confirmation of the hydrodynamic interpretation at the LHC. On an absolute scale, however, the model calculations shown in the figure significantly over-predict the production of protons – a surprise revealed by the first LHC data.

CCnew9_01_12 — The production of pions (red), kaons (blue) and protons (black) measured by ALICE in Pb–Pb collisions at the LHC. The theoretical curves are from a hydrodynamic model (Heinz *et al.* 2011).

The production of soft hadrons (p_t < 1–2 GeV/c) is generally described in a statistical language: it is assumed that particles are created in thermal equilibrium. This idea, dating back to a classic 1950 paper by Enrico Fermi, has proved successful over a range of collision energies (√s ˜ 2 GeV – 200 GeV) and provides a possible link to the temperature of the hadronization (or deconfinement phase transition).

At present, however, the yield ratios measured by ALICE seem to challenge both previous experiments and theory. While the K/π, Ξ/π and Ω/π ratios are compatible with the expectations from the thermal model with T ≈ 165 MeV, as in previous observations, the p/π ratio points to a significantly lower temperature. On the experimental side, there are indications of a similar effect at lower energies, which call for further investigations. On the theoretical side, a number of different possibilities are being investigated, none of them conclusive at the moment.

The unique PID capabilities of the ALICE experiment will continue to be crucial for the characterization of the deconfined matter produced in Pb–Pb collisions at the LHC. They also pave the way for a rich programme in proton–proton physics, especially in the soft physics domain, e.g. with the forthcoming measurement of fragmentation constraints with identified particles and spectra in high-multiplicity events.

Topics

Reset your password

Registration complete