In the past few years the field of hadron spectroscopy has seen renewed interest due to the pubblication, initially mostly from B-Factories, of evidences of states that do not match regular spectroscopy, but are rather candidates for bound states with additional quarks or gluons (four quarks for tetraquarks and molecules and two quarks and gluons for hybrids). A huge effort in understanding the nature of this new states and in building a new spectroscopy is ongoing. This paper reviews the experimental and theoretical state of the art on heavy quarkonium exotic spectroscopy, with particular attention on the steps towards a global picture

Ablikim

Aubert

Chen

Cotugno

Gell-Mann

Jaffe

Mizuk

R. Faccini

English

Faccini, R.

Scientific Open-access Literature Archive and Repository

DOI 10.1393/ncc/i2011-10800-7Colloquia: IFAE 2010IL NUOVO CIMENTO Vol. 33 C, N. 6 Novembre-Dicembre 2010New hadronic spectroscopyR. FacciniDipartimento di Fisica, Universita` di Roma “La Sapienza” and INFN, Sezione di RomaPiazzale A. Moro 2, I-00185 Rome, Italy(ricevuto l’ 8 Ottobre 2010; pubblicato online il 15 Febbraio 2011)Summary. — In the past few years the field of hadron spectroscopy has seenrenewed interest due to the pubblication, initially mostly from B-Factories, of evi-dences of states that do not match regular spectroscopy, but are rather candidatesfor bound states with additional quarks or gluons (four quarks for tetraquarks andmolecules and two quarks and gluons for hybrids). A huge effort in understandingthe nature of this new states and in building a new spectroscopy is ongoing. Thispaper reviews the experimental and theoretical state of the art on heavy quarkoniumexotic spectroscopy, with particular attention on the steps towards a global picture.PACS 13.25.Hw – Decays of bottom mesons.PACS 14.40.Nd – Bottom mesons (|B| > 0).It is now almost seven years since the first heavy quarkonium exotic state has beenobserved and the number of observed such states has significantly increased. In order toexplain all these states one needs to go beyond the assumption that mesons can only bemade of a quark and an anti-quark and consider the possibility to be observing boundstates of four quarks or two quarks and valence gluons. These novel states of aggregationare not a new idea and it was indeed already in the foundations of the quark model [1].Also, the hypothesis that the mesons of the scalar nonet are actually tetraquarks can bedated back to the ’70s [2]. But only now, with the presence of several exotic candidateswith heavy quarks the possibily to have a global picture is becoming real.The path towards such global picture comprises several steps. First all relevant ex-perimental observations need be considered. Figure 1, from ref. [3], summarizes such ob-servations in the charmonium sector, the richest of exotic candidates. Next, a completeset of theoretical predictions and a systematic experimental search is needed. The workin the theoretical path is progressing although strong interactions are extremely hard tocompute, even in presence of heavy quarks, and therefore predictions are extremely hardand uncertain. Finally, additional states predicted by the theoretical models need to besearched for. This paper sketches the path towards this last goal.The status of the experimental observations is extremely fragmented. Pictorial sum-maries, separated by production mechanisms, are in figs. 2–4. The exotic states havebeen observed always in only one production mechanism and often in only one finalc© Societa` Italiana di Fisica 129130 R. FACCINIFig. 1. – Observed states with hidden charm above the open charm threshold. The theorypredictions are according to the potential models described in ref. [4].state. None of the states has been searched systematically in all final states. Sometimesthe analysis of a given final state for a given production mechanism is missing, but oftenit has either been performed only in a limited mass range or the invariant mass spectrumhas been published without a fit to the possible new state. This is mostly due to thefact that some of the analyses did not show a significant signal themselves and they werepublished before a new state was observed. As an example, the invariant mass spectraof a charmonium and a photon in refs. [5, 6] are focussed on the X(3872) region and noinformation is available outside it. On the other side in ref. [7] the J/ψη invariant massspectrum is published, but not fitted for all the possible new states. Whatever the causeis, the critical point is that building a global picture requires having either an observa-tion or a limit in each final state for each candidate new state: limits allow to quantifythe level at which a decay mode has not been observed and can show whether a signalFig. 2. – Status of the searches of the new states in the processes B → XK (top) and e+e− →XγISR, X → f , for several final states f . Final states where each exotic state was observed(“S”) or excluded (“N/S”) are indicated. A final state is marked as “N” if the analysis has notbeen performed in a given mass range and with “M/F” if the spectra are published but a fit toa given state has not been performed. Finally “N/A” indicates that quantum numbers forbidthe decay and “N/F” if an analysis is experimentally too challenging.NEW HADRONIC SPECTROSCOPY 131Fig. 3. – Status of the searches of the new states in collisions e+e− → XJ/ψ (top) and γγ → X(bottom), X → f , for several final states f . Symbols are explained in the caption of fig. 2.Fig. 4. – Status of the inclusive searches of the new neutral states in pp¯ collisions (top) and ofthe new charged states (bottom) for several final states. Symbols are explained in the captionof fig. 2.132 R. FACCINIcould have been observed in a final state or the efficiency, the branching fraction or thebackground level would make it unobservable. It is also worth remarking that althoughfinding new decay modes of these states might not necessarily cast light on the natureof these particles, they would in any case concur to the evidence of the existence of thestates that suffer from lack of statistics.Looking into this “observational” tables in detail, B decays (top of fig. 2) are themost studied, but there is a significant amount of missing fits (“M/F”). Particularlysevere is the lack of analysis of the baryonic spectra since baryonic decays are a signa-ture of tetraquark states. Some other modes have never been studied, mostly becausethe number of expected events is very low. Nonetheless the study of B → ψ(2S)ππKdecays should be relatively clean, while D∗D¯∗ and above all D(∗)s D¯(∗)s suffer from the lowbranching fractions of the observed states.States produced in conjunction with an initial state radiation (ISR) photon (bottomof fig. 2) have an unambigous JCP assignment and therefore fewer analyses are needed toestablish their properties. Nonetheless it is striking to see that a large fraction of analyseshave been carried out exclusively for the first observed exotic state, the Y (4260). It canalso be noticed that no search is published involving D(∗)s mesons: while the efficiencyis expected to be very low, background should be low as well and surprises can alwaysarise. Finally the Y (4660) has been the object of one of the combined analyses we areadvocating here [8]: two states apparently different, observed in ψ(2S) and ΛcΛ¯c finalstates, if fitted under the same ansatz were found to be consistent with being the sameand interesting ratios of branching fractions were measured.On the recoil of a J/ψ and in γγ interactions (fig. 3) only C = + neutral states can beobserved. This restricts the number of final states of interest. Also, the low multiplicityof these decays and the large missing momentum in the case of γγ decays makes theseanalyses experimentally challenging. On the other side C = + states are the least knownand reinforcing the evidence of the signals would help. It is also interesting to noticethat, mostly due to statistics, the recoil to any other particle but the J/ψ has not beeninvestigated. Given the selection rules the recoil to χc0 or χc2 would be very interesting.Searches at hadron colliders (top of fig. 4) have the advantage to have very largesamples. On the other side backgrounds are very high and therefore final states withtoo high multiplicity and above all with neutral particles are not at reach. Finally,it is hard to extract information on production cross sections and therefore branchingfractions. Nonetheless, it is clear that a systematic search in pp¯ collisions is likely toclarify the picture significantly and it is a pity that it is still missing. This holds also andin particular for the charged states—for instance the longly debated Z+(4430) shouldgive a large signal in the J/ψπ+ spectrum—and in the bottomonium—where no otherexperiment is in the position of studying inclusive Υ(nS)π+π− and Υ(nS)π± spectra.Concerning the searches of charged exotic states, the most striking signature of statesmade of more than two quarks, very few searches have been conducted in B decays. Webelieve that for each exotic neutral spectrum the corresponding charged state should besearched for completeness and, as shown in the bottom of fig. 4 only five combinationsof final states and exotic states has been searched for. As an example, no informationhas been extracted from refs. [9, 10] on the charged partner of the X(3872), Z(3870) inour table, which has long been pointed out as a critical state to search for. Moreoveramong the four quark bound states there must be states that contain a single s quark,their mostly distinctive signature being a charmonium plus a charged kaon. These statescould be searched in B decays in association with a ss¯ state or inclusively in pp¯ collisions.NEW HADRONIC SPECTROSCOPY 133Concerning the bottomonium states, the searches are extremely limited and there isno confirmed evidence of exotic states yet [11,12]. The search potential of B-Factories islimited by the fact that bottomonium at masses higher than the open bottom thresholdcan only be produced in conjunction with initial state radiation and that in addition theaccessible mass range is full of threshold openings. This restricts the search to JPC = 1−−states with significant signatures in exclusive final states. Besides the lack of informationabout most of the possible final states, the real contribution could come from hadroncolliders, where the search could be inclusive over Υ(nS)X final states (where X is madeof charged tracks) and extended to exotic states with open bottom, eventually even withopen charm.In conclusion, seven years of discoveries of exotic particles need to be followed by asystematic study of the possible new spectroscopy to be able to give a global and definitepicture. To this aim still a lot can be extracted from existing data of B-Factories andTevatron. Nonetheless in the present generation of experiments statistics is extremelylow and we will not be able to have a convincing picture until the results of ultra-highintensity machines, LHC and the Super Flavour Factories (SuperB and/or SuperBelle),will be available.REFERENCES[1] Gell-Mann M., Phys. Lett., 8 (1964) 214.[2] Jaffe R. L., Phys. Rev. D, 15 (1977) 267.[3] Drenska N., Faccini R., Piccinini F., Polosa A. D., Renga F. et al., Riv. NuovoCimento, 33 (2010) 633.[4] Brambilla N. et al., hep-ph/0412158 preprint (2004).[5] Aubert B. et al., Phys. Rev. Lett., 102 (2009) 132001.[6] Abe K. et al., hep-ex/0505037 preprint (2005).[7] Aubert B. et al., Phys. Rev. Lett., 93 (2004) 041801.[8] Cotugno G., Faccini R., Polosa A. D. and Sabelli C., Phys. Rev. Lett., 104 (2010)132005.[9] Ablikim M. et al., Phys. Lett. B, 656 (2007) 30.[10] Mizuk R. et al., Phys. Rev. D, 78 (2008) 072004.[11] Chen K.-F. et al. (Belle Collaboration), Phys. Rev. Lett., 100 (2008) 112001.[12] Aubert B. et al., Phys. Rev. Lett., 102 (2009) 012001.

New hadronic spectroscopy

Crossref

Nuclear Physics B - Proceedings Supplements

New Hadronic Spectroscopy

In the past few years the field of hadron spectroscopy has seen renewed interest due to the publication, initially mostly from B-Factories, of evidences of states that do not match regular spectroscopy, but are rather candidates for bound states with additional quarks or gluons (four quarks for tetraquarks and molecules and two quarks and gluons for hybrids). A huge effort in understanding the nature of this new states and in building a new spectroscopy is ongoing. This paper reviews the experimental and theoretical state of the art on heavy quarkonium exotic spectroscopy, with particular attention on the steps towards a global picture

FACCINI, Riccardo

Archivio della ricerca- Università di Roma La Sapienza

http://eprints.bice.rm.cnr.it/17101/1/ncc9782.pdf

New hadronic spectroscopy

Abstract

Similar works

Full text

Available Versions

Scientific Open-access Literature Archive and Repository

Crossref

Archivio della ricerca- Università di Roma La Sapienza