Exotic Hadrons Bend the Rules

CERN Courier April 2017 Hadron spectroscopy

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The LHCb detector at CERN is one of several experiments where unusual hadrons have appeared in the past few years. Exotic hadrons bend Cerium Bromide (CeBr3) scintillation detectors the rules ● High resolution Your perfect partner C ● No 138La background for quality M ● 76 x 76 mm available vacuum components Y 4% @ 662 keV CM Half a century after the quark model was His primary focus was on the spectroscopy of baryons, which were MY interpreted as bound states of three quarks. Dalitz realised that the restrictions enforced by the Pauli exclusion principle led to a dis- CY devised, a number of hadrons have been tinct pattern of supermultiplets. Today, this simple model remains CMY discovered that appear to challenge its axioms. in excellent agreement with experiments, in particular for mesons K that comprise a quark–antiquark pair. Despite its success in matching empirical data, the theoretical Now available Fifty years have passed since Dick Dalitz presented his explicit underpinning of this non-relativistic model for light hadrons has on our new website constituent-quark model at the 1966 International Conference always been unclear. One of the remarkable features of hadron spec- on High Energy Physics in Berkeley, US. Murray Gell Mann and troscopy is that, half a century after the invention of the constituent- SCIONIX Holland B.V. T. +39.011.0968307 – F. +39.011.0968393 George Zweig independently introduced the quark concept in quark model, the particle data tables are filled with states that fit [email protected] - www.vaqtec.com Tel. +31 30 6570312 1964, and the idea had also been anticipated by André Petermann with a non-relativistic spectrum almost to the exclusion of anything Fax. +31 30 6567563 in a little-known paper received by Nuclear Physics in 1963. But it else. Quarks are but a few MeV in mass, and are therefore surely Email. [email protected] was Dalitz who developed the model and considered excitations of relativistic when confined within the 1 fm radius of a proton, yet the s www.scionix.nl quarks by analogy with the behaviour of nucleons in atomic nuclei. constituent-quark model treats them as if relativity plays no role.

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CCApr17_HADSPEC.indd 31 08/03/2017 15:39 CERNCOURIER www. V o l u m e 5 7 N u m b e r 3 A p r i l 2 0 1 7 CERN Courier April 2017 CERN Courier April 2017 Hadron spectroscopy Hadron spectroscopy Front. Phys. the spectroscopy of QCD is so meagre. Colour singlets also allow (a) D – ππ Y(4660) 101401 combinations of pairs of quarks and antiquarks (“tetraquark” u 4.6

10 d d u u u d u d + d u d u mesons), four quarks and an antiquark (“pentaquark” baryons), in Z(4430) 10 e– ψ 3 101401 c 4.4 (4 S1) 3 + 6 addition to states comprised solely of gluons (“glueballs”). Fur- η 1 3 + 3 = c(4 S0) Y(4360) u 3 Front. Phys. Front. us χ d s d s u s thermore, combinations called “hybrids” in which the gluonic – 1 c2(3 P2) s s u hc(3 P1) χ 3 Y(4360) + c1(3 P1) fields entrapping the quark and antiquark are themselves excited Z2(4250) χ 3 γ 4.2 3 c0(3 P0) are also theoretically possible within QCD (figure 1). Glueballs, c X(4160) ψ(2 D ) s s u 1 tetraquarks and hybrid mesons, predicted in the late 1970s, can – Y(4140) e+ c + form correlations among a meson’s J, P and C quantum numbers Zc(4050) (b) J/ψ 1 3 + 2M d u d u d u 4.0 η (3 S ) ψ(3 31) Zc(4020) D* that are forbidden by the non-relativistic model. Indeed, it is the –c c 0 D ) 3 1 2 χ (2 P ) X(3940) + 3 c2 2 – – – lack of any empirical evidence for such exotic states in the meson Z (3900) Y(3915) χc1(2 P1) 3 3 3 c 3 + – χc0(2 P0) X(3872) MD+MD* spectrum that helped to establish the constituent-quark model in Fig. 2. At 4.6 GeV, e e annihilation produces DD1 at threshold. d s u s d s u s d s d u – – – – 3.8 the first place. It is therefore ironic that searches for such states at The (cu )(cu) content rearranges into (cc)(uu ) → J/ψππ. 3 ψʹʹ(1 D1)

mass (GeV/c 2MD modern experiments are now being used to establish the dynamic 3 (c) pentaquark H-dibaryon tetraquark η ʹ(21S ) ψʹ(2 S ) role of gluonic excitations in hadron spectroscopy. structures with a mass of around 4.4 GeV (CERN Courier Sep- c 0 1 3.6 u χ 3 d d u d u Although QCD is well tested to high precision in the per turbative tember 2015 p5). These have normal strong-interaction life- 1 c2(1 P2) hc(1 P1) –s d u u sd s – – χ 3 d s regime, where it is now an essential tool in the planning and inter- times and have been interpreted as clusters of three quarks plus c1(1 P1) pretation of experiments, its implications for the strong-interaction a charm–anticharm pair. Whether these are genuinely compact 3 3.4 χc0(1 P0) diquark–diquark– diquark–diquark– diquark–diantiquark limit are far less understood. Forty years after its discovery, and pentaquarks, or instead bound states of a charmed baryon and a antiquark diquark _ notwithstanding the advent of lattice QCD, hadron physics is still meson or some other dynamic artefact, they do appear to qualify established cc states (d) molecule hybrid glueball led by empirical data, from which clues to novel properties in the as “exotic” in that they do not fit easily into a traditional three- 3.2 predicted, undiscovered g – strong interactions may emerge. The search for exotic hadrons is constituent picture. d 3 neutral XYZ mesons π,... d – J/ψ(1 S1) s u an essential part of this strategy, and in recent years several new There have also been interesting meson sightings at lepton d charged XYZ mesons –u g hadrons have been discovered that do not fit well within the tradi- colliders in recent decades. Electron–positron annihilation above 1 3.0 ηc(1 S0) tional quark model. energies of 4 GeV in numerous experiments reveals a series of 0–+ 1– 1+– 0++ 1++ 2++ Fig. 1. Some of the ways in which QCD allows quarks to arrange peaks in the total cross-section that are consistent with radial JPC into hadrons: (a) a red and blue quark triplet produces a Strange sightings excitations of the fundamental cc– J/ψ meson: the ψ(2S), ψ(4040), magenta (anti-green) antitriplet and sextet; (b) three With hindsight, one of the first clues to the existence of quarks ψ(4160) and ψ(4415), which are non-exotic and fit within the Fig. 3. The spectrum of charmonium and charmonium-like anticoloured diquark antitriplets; (c) exotic, colour-singlet came in the 1950s from measurements of cosmic-ray interactions non-relativistic spectrum. Evidence for exotic mesons has come candidates. states formed from quarks, antiquarks, diquarks and in the atmosphere, which revealed hadrons with unusual pro- from data on specific final states, notably those containing a J/ψ diantiquarks; and (d) other possible multiquark/gluon systems. duction and decay properties. These “strange” hadrons, we now with one or more pions, which have revealed several novel states. channel containing a pion and a charmonium meson. Since it can – know, contain one or more strange quarks or strange antiquarks, Historically, the first clue for an exotic charmonium meson of carry electric charge, this state must contain ud (or du–) in addition In the case of mesons, which fit the quark model arguably even yet history has left us with a perverse convention whereby this type above a mass of 4 GeV came around a decade ago from to its cc– content, and therefore cannot be explained as a bound state better than baryons, this incongruity is especially significant. When strange quarks are deemed to carry negative strangeness, and the BaBar experiment at SLAC in the US. Analysing the pro- of a single quark and antiquark. In principle, these states should be Dalitz spoke in 1966, it made sense to emphasise baryons because strange antiquarks are positive. Thus mesons can have one unit of cess e+e– → J/ψππ, researchers there found a clear resonant-like accessible in decays of B mesons, but there is no sign of them so far. they outnumbered the known mesons at that time. Following the strangeness, in either positive or negative amounts, while baryons structure dubbed Y(4260), which has no place in the qq– spectrum Nonetheless, B decays are a source of further exotic structures. discovery of charm and heavy flavours in the late 1970s, however, can have strangeness –1, –2 or –3 (antibaryons, in turn, can have because its mass lies between the ψ(4160) and ψ(4415) cc– states. For example, the invariant-mass spectrum of B → K π±ψ(2S) con- the spectroscopy of mesons flourished and the correlations among positive strangeness). More remarkably, this state decays into charmonium and pions tains a structure called the Z(4430) observed by Belle and LHCb in a meson’s spin (J), parity (P) and charge conjugation (C) were also A bar yon with positive strangeness (or an antibar yon with nega- with a standard strong-interaction width of the order of 100 MeV the ψ(2S)π invariant-mass spectrum, which contains both hidden found to be in accord with those of a non-relativistic system. tive strangeness) is therefore classed as exotic. The minimal con- rather than 100 keV, which is more typical for such a channel. charm and isospin and hence must contain (at least) two quarks Following Dalitz’s description of the baryon spectrum, Green- figuration for such a baryon would involve four quarks together The clue to the nature of this meson appears to be that the mass of and two antiquarks. These features first need to be established as berg, Nambu, Lipkin and others noted that the model’s ad-hoc cor- with the strange antiquark, giving a total of five and the techni- the Y meson (4260 MeV) is near the threshold for the production of genuine and not artefacts associated with some specific produc-

relation of baryon spins with the constraints of the Pauli principle cally incorrect name of “pentaquark”. A claim to have found such DD1 – the combination of pseudoscalar (D) and axial (D1) charmed tion process. Their appearance and decay in other channels would required some novel degree of freedom, which we call “colour”. a state – the θ(1540) – made mesons (figure 2). This is the first channel in e +e– annihilation where help in this regard, while the observation of analogous signals for The advent of quantum chromodynamics (QCD) in the 1970s pro- headlines nearly two decades charmed meson pairs can be produced with no orbital angular other combinations of flavour may also signpost the underlying – vided the rationale for this concept, explaining the existence of The empirical guide ago but is now widely disre- momentum (i.e. via S-wave processes). Thus at threshold there is no dynamics. If real, these states are the product of charmonium cc quark–antiquark or three-quark combinations in terms of colour- garded. The scepticism was not angular-momentum barrier against a DD1 pair being created effec- and light-quark basis states (a summary of charmonium candidates singlet clusters. But QCD did not explain the non-relativistic pat- of the quark model is that a pentaquark exists, since tively at rest, and rearranging their constituents into the form of J/ψ can be seen in figure 3). tern of states. Feynman, who in his final years devoted his attention so well established QCD can accommodate such and light flavours (the latter then seeding pions). Thus the structure to this issue, asserted: “The [non-relativistic] quark model is cor- that hadrons outside a state, but that it appeared to could simply be a threshold effect rather than a true resonance, or an Proceed with caution rect as it explains so much data. It is for theorists to explain why.” be anomalously stable. More exotic “molecule” made of D and D1 charmed mesons. It is clear that peaks are being found that cannot be interpreted as Today, physicists still await this explanation. Yet the empirical of this straitjacket recently, the LHCb experiment The decay of the Y(4260) into J/ψππ reveals a manifestly exotic qqq or qq– clusters. But one should not leap to the conclusion that guide of the quark model is so well established that hadrons outside are deemed “exotic”. at CERN’s Large Hadron Col- structure. The J/ψπ± channel is electrically charged with a pro- we have discovered some fundamentally novel state built from, say, of this straitjacket are deemed “exotic”. lider (LHC) reported decays nounced peak called Z(3900), as reported by both the BESIII diquarks and antidiquarks or, for baryons, a pentaquark. A qq –qq–

Although the restriction to colour singlets within QCD explains of the Λb pentaquark-like experiment in China and Belle in Japan in 2013. Another sharp peak “tetraquark”, for example, looks less exotic when trivially rewritten s the existence of qq– and qqq hadrons, it raised the question of why baryon that revealed similar observed by BESIII – the Z(4020) – appears in the flavour-exotic as qq– qq–, which is suggestive of two bound conventional mesons.

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● Further reading D* D Fig. 4. “Deusons” arise from anticipate meson molecules (or, by analogy with the deuteron, appearance of a state near the corresponding S-wave threshold. _ _ pion exchange. Pion exchange “deusons”), which would take us beyond the simple quark-model In some of these cases, but not all, the familiar forces of conven- F Close 2017 The New Cosmic Onion: Quarks and the Nature of the Universe c u u c is the source of an attractive spectroscopy. The Y(4260) could be an example of such a state, tional nuclear physics play a role, and the multi-particle events at (Taylor and Francis). – – S L Olsen 2015 Front. Phys. 101401. force between D and D* or D since both DD1 and D*D0 S-wave thresholds lie in this region and the LHC have the kinematic reach to include all combinations of 10 and D* charmed mesons. This pion exchange may play a role in linking the two channels (figure non-strange, strange, charm and bottom mesons. How many of A Petermann 1965 Nucl. Phys. 63 349. also occurs between D and D* 4). If these states are indeed deusons then there should also be these can in practice be identified is the challenge, but identifying analogous to the case of the partners with isospin. Establishing whether these structures are the dynamics of states “beyond qq– ” may depend on it. Résumé deuteron. singletons or have siblings is therefore another important step in In conclusion, these exotic states need to be studied in differ- Les hadrons exotiques font plier les règles identifying their dynamical origins. ent production mechanisms and in a variety of decay channels. A

_ _ The first sign of deusons may be expected in the axial-vector genuine resonant state should appear in different modes, whereas 50 ans après la création du modèle des quarks, certains hadrons c u u c channel formed from a pseudoscalar and vector charmed (or bot- a structure that appears in a single production mechanism and a semblent défier ses axiomes. Si la correspondance avec les données _ D D* tom) meson. This is because pion exchange can occur between unique decay channel is suggestive of some dynamical feature that empiriques est une réussite, la fondation théorique de ce modèle non _ a pair of vector mesons or as an exchange force between a is not truly resonant. While interesting in its own right, such a state relativiste pour les hadrons légers n’a toutefois jamais été claire. Les * D D pseudoscalar-vector combination, but not within a state of two is not “exotic” in the sense of hadron spectroscopy. résultats d’expériences telles que BESIII, Belle et LHCb indiquent des pseudoscalars as this would violate parity conservation. The As for truly exotic states, there are different levels of exotic- pics qui ne peuvent pas être interprétés comme des états traditionnels enigmatic state X(3872), which was first observed in B decays ity. For flavoured hadrons: the least exotic are meson analogues à deux ou à trois quarks. Il ne faudrait toutefois pas en conclure – 0 π by Belle in 2003 and occurs at the D0 D* + cc threshold, has of nuclei – “deusons” driven by pion exchange between pairs of hâtivement la découvert d’un état fondamentalement nouveau formé long been a prime candidate for a deuson. If so, there should mesons. Next are “hybrids”: states anticipated in QCD where the de diquarks et d’antidiquarks ou, pour les baryons, un pentaquark. – be analogous states in the BB* as well as charm-bottom flavour gluonic degrees of freedom are excited in the presence of quarks Ces états exotiques doivent d’abord être étudiés dans différents mixtures and perhaps siblings with two units of charm or bot- and/or antiquarks. Finally, the most exotic of all would be colour- mécanismes de production et divers canaux de désintégration. _ D’ici-là, ils garderont le même statut que la vie extraterrestre : même DD* tom. Whether these states have charged partners is one of many singlet combinations of compact diquarks, which are allowed in model-dependent details. That some of these states should occur principle by QCD and would lead to a rich spectroscopy. At pre- si nous imaginons que ces êtres doivent exister dans la richesse de la Indeed, these could be the two mesons in the invariant mass of seems unavoidable, however, and if doubly charmed states exist sent their status is like the search for extraterrestrial life: while nature, ils semblent vouloir rester cachés. which the peak was seen. Unless the peak is seen in different chan- they should be produced at the LHC. one feels that in the richness of nature such entities must exist, nels, and ideally in different production mechanisms, one should Whereas for baryons the attractive forces arise in the exchange they seem reluctant to reveal themselves. Frank Close, University of Oxford, UK. be cautious. or “t channel”, for pairs of mesons there can also be contributions For example, when three or more hadrons are produced in a sin- due to qq– annihilation in the direct s-channel. In QCD this can also gle decay it is common to discover peaks in invariant-mass spec- mask the search for glueballs: for example, the scalar glueball of tra just above the two-body thresholds. These are not resonances, lattice QCD predicted at a mass of around 1.5 GeV mixes with the although papers on the arXiv preprint server are full of models nonet of scalar qq– states in this very region. The pattern of these built on the assumption that they are. Instead, the peaks likely scalars empirically is consistent with such dynamics. arise due to competition between two effects. First, phase space Scalar mesons are interesting not least because the theoretical opens up for the production of the two-body channel, but as the interest in multiquark or molecular states originated in such par- invariant mass increases, the chance of this exclusive two-body ticles 40 years ago, after Robert Jaffe noticed that the chromo- mode dies off because the probability for the wavefunctions of magnetic QCD forces are powerfully attractive in the nonet of the two hadrons to overlap decreases. Any peak seen within a few light-flavoured scalar mesons. Intriguingly, this idea has remained Journal of Physics series hundred MeV of such a threshold is most likely to be the acciden- consistent with the observed nonet of scalars below 1 GeV ever tal result of this phenomenon. Such “cusps” have been proposed since. The main question that remains unresolved is to what extent Celebrating 50 years of leading physics as explanations of several recent exotic candidates, such as the these states are dominantly formed from coloured diquarks and • Join more than 40 Nobel Prize laureates and publish your Z(3900) and Z(10610) spotted at BESIII and Belle, among others. their antidiquarks, or are better described as molecular states latest research in the Journal of Physics series. Whether the tetraquark candidates X(4274), X(4500) and X(4700) formed from colour-singlet π and K mesons. • Articles in the series were downloaded more than 3.4 million recently observed at LHCb, in addition to the X(4140) found by LHCb in particular has shown that it is possible to identify light times in 2015. the CDF experiment at Fermilab in 2009, herald the birth of a new scalars among the decay debris of heavy-flavoured mesons, offer- • Journal of Physics articles were cited more than 114,000 QCD spectroscopy or are examples of more mundane dynamics ing a new opportunity to investigate their nature and dynamics. times in 2015. such as cusps, is also the subject of considerable debate. In short, Indeed, the kinematic reach of the LHC potentially enables a mul- if a peak occurs above a two-body threshold in a single channel: titude of information to be obtained about heavy-flavoured mesons Find out more about the prestigious Journal of Physics series beware. in both conventional and exotic combinations. We might therefore at iopscience.org. hope that information about exotic mesons will be extended into Enter the deuson different flavour sectors to help identify the source of the binding. More interesting for exotic-hadron studies are peaks that lie just below threshold. Such states are well known in the baryon sector, Remarkably robust the deuteron being a good example. The nuclear force driven by In general, the simple qq– picture of mesons appears to remain pion exchange that binds neutrons and protons inside the atomic remarkably robust so long as there are no nearby prominent chan- nucleus should also occur between pairs of mesons, at least for nels for pair production of hadrons in the S-wave channel. “Exotic” those that are stable on the timescale of the strong interaction. Thus mesons and baryons seem to correlate with some S-wave channel on purely phenomenological and conservative grounds, we should sharing quantum numbers with a nominal qq– state and causing the

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CCApr17_HADSPEC.indd 34 08/03/2017 15:40 CCApr17_HADSPEC.indd 35 Journal of Physics08/03/2017 16:36 series CERNCOURIER www. Celebrating 50 years of leading physics V o l u m e 5 7 N u m b e r 3 A p r i l 2 0 1 7 Be in great company Join more than 40 Nobel Prize laureates and publish your latest research in the Journal of Physics series.

Extend the impact of your research Articles in the series were downloaded more than 3.4 million times in 2015.

Raise your visibility amongst your peers Journal of Physics articles were cited more than 114,000 times in 2015.

Find out more about the prestigious Journal of Physics series at iopscience.org. CERN Courier April 2017 CERN Courier April 2017 Hadron spectroscopy Hadron spectroscopy

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Extend the impact of your research Articles in the series were downloaded more than 3.4 million times in 2015.

Raise your visibility amongst your peers Journal of Physics articles were cited more than 114,000 times in 2015.

Find out more about the prestigious Journal of Physics series at iopscience.org. CERN Courier April 2017

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