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Exotic Heavy Flavor Hadrons

Exotic Heavy Flavor Hadrons. Stony Brook seminar P. Grannis Apr. 17, 2017. The quark constituents of hadrons.

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Exotic Heavy Flavor Hadrons

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  1. Exotic Heavy Flavor Hadrons Stony Brook seminar P. Grannis Apr. 17, 2017

  2. The quark constituents of hadrons In the original quark model paper, Gell Mann said “… baryons can now be constructed from combinations (qqq), (qqqqq) etc. while mesons are made out of (qq), (qqqq), etc. …” Zweig made a similar comment regarding his ‘aces’. _ _ _ _ Until rather recently no direct evidence for such exotic states existed, although in some cases there are more candidate states than can fit into the available qqmultiplets, implying the need for more complex structures. Also the masses and decay patterns of some states could be hard to accommodate in the ordinary quark model. The spectroscopy of mesons containing heavy quarks (c and b) can provide clearer and more direct evidence for exotic mesons (or baryons) due to their distinctive decays. _ 39

  3. The quark constituents of hadrons Luciano Maiani, in the 2013 Erice school, talked on exotic mesons and recalled the quote from the 1967 Erice School: “… then the data and masses and quantum numbers were examined in detail in order to determine “what is certain, what remains to be confirmed and what should probably be forgotten.” L to R: Bruno Zumino, Sidney Coleman, Nino Zichichi, NicoloCabibbo, Sheldon Glashow, Murray Gell-Mann) In 1967 these words pertained to ordinary mesons, but are valid for exotic mesons today. Many candidates for exotic states exist, but experimental observations are often contradictory and theoretical explanations are controversial. Hadron taxonomy in non-perturbative QCD is difficult! 38

  4. Types of multi-quark states _ _ u u Multiquark exotic mesons may have more complex structures than ordinary mesons, due both to the color combinations and to the binding mechanism. _ _ _ c c c _ c A pair of colorless qq states bound loosely with a colorless pion in van der Waals force (analogous to the deuteron). Expect such a ‘MOLECULAR STATE’ to have a mass quite close to the sum of the two colorless hadrons (here D0 and D0) p __ u A colored diquark and a colored anti-diquark pair, bound with a multicolor gluonic state. The color force binding the diquark (color 3 or 6) and anti-diquark (color 3 or 6) is quite strong so this ‘TETRAQUARK’ state mass can be quite far below the D0D0 threshold. _ _ c u gg __ c A ‘HYBRID’ state of colored quark, antiquark and gluon in an overall color singlet …. or a ‘GLUEBALL’ with no quarks in it And of course more complex structures can be envisioned with Fock space wavefunctions built of superpositions of two or more structures 37

  5. Light quark exotic states The nonet of JPC = 0++ mesons displays a peculiar mass spectrum that is inverted from the pattern for the familiar 1-- or 0-+ nonets: JPC=0++ JPC=1-- _ss _ _us _ _ _ud - (f) _ a0(980) mass f0(980) - (K*) _ _ _ _ _ _ k(800) _ uu±dd - (r,w) - - f0(500) This could be explained (Jaffe) if the 0++ states were tetraquarks with a color 3 diquark bound to a color 3 anti-diquark, giving quark content as shown _ - - _ With this content, the mesons with higher mass are those containing the more massive strange quark. Maiani, et al. assigned a0 and f0 as tetraquarks and predicted analogs with c and b quarks (su)(ds) - - etc. (su)(us) _ _ _ _ _ _ - - (sd)(ud) etc. - - (ud)(ud) 36

  6. First heavy quark exotic state BELLE The first meson to be widely recognized as exotic was the X(3872) seen by BELLE in 2003, originally in B → K X0 → K (p+p- J/y). It has been seen in both e+e- and hadron colliders by BELLE, BaBar, CDF, D0, LHCb, CMS, BESIII and ATLAS. Decays to wJ/y, g J/y, and D0 D*0 have been observed. _ _ X(3872) mass is essentially equal to the sum of the D0 andD*0 masses. Width is consistent with resolution, so GX < 1.2 MeV LHCb measured JPC as 1++ Although the e+e- colliders first saw X as a decay product of B’s, CDF and D0 have shown that prompt production of X(3872) is more copious (84%). 35

  7. Interpreting X(3872) _ Only unassigned cc charmonium JP= 1++ state was the first radial excitation of cc1 but the mass splitting from known cc2’ is wrong. The width of X is too small to be charmonium; also the r J/y decay violates isospin for a cc state. So left with exotic 4 quark interpretations with quark content ccqq (q=u or d). X(3872) as a molecular state? The very small binding energy suggests this, but it is hard to see how such a weakly bound object could be directly produced copiously in violent high pThadron collisions. X as a tetraquark? Where are the missing partners (would expect both neutrals cucu and cdcd as well as charged partners such as cucd) that are not seen. Hybrid cc /DD* molecule?? May explain the various puzzles by invoking a state with a more complex Fock space components – e.g. superposition of molecular state and ordinary quark antiquark configurations. The X(3872) puzzles typify the difficulties in deducing the structure of the exotic meson candidates. _ _ _ - - - - - - _ _ 34

  8. XYZ states Many exotic XYZ mesons have been sighted. Often, some experiments see it, others do not, so details of production, backgrounds, interferences etc. seem to be important. (The nomenclature itself is murky!) More or less: Y= neutral 1-- states seen in e+e- s-channel production Z= charged states X= all the others (PDG uses X(mass)) Some of the candidate XYZ’s 33

  9. Another example: Y(4140) CDF In 2009, CDF announced a J/yf resonance, Y(4140), in B+ →Y K+, close to the J/yf threshold. The quark content is thus ccssand the state is manifestly exotic, likely a molecular state. CDF also had evidence for a higher state at 4274 MeV. Shortly thereafter, LHCb, Belle, BaBar and BESIII failed to detect this state. However CMS and D0 did confirm it at the 3 and 5s level and saw the higher mass state. Subsequently, D0 presented strong evidence that Y(4140) is also produced inclusively (not only through the cleaner B decay channel), so interpretation as a reflection not likely. Very recently LHCb, with a larger data set and a sophisticated amplitude analysis confirmed Y(4140) with JPC=1++, Y(4274) as well as two higher states. _ _ D0 The pattern of non-confirmation, then confirmation, ambiguous interpretation, dependence on production modes etc. is typical. 32

  10. D0 Bsp± _ _ _ _ D0 was a multipurpose detector at the Tevatron pp collider (ran 1992 – 2011) _ _ _ _ _ D0 did a search for a resonance in Bsp+p-(quark content bsuu/bsdd) for an analog of known states of csuu/csdd. Nothing was seen but a side look at the Bsp± mass distribution (Bs → J/yf, J/y → m+m-f → K+ K- ) showed a peak slightly above the thresholdof 5506 MeV. Due to rapid mixing, one cannot distinguish between Bsand Bs. The analysis does not separate p+ and p-. The quark content of a state in this channel can be: bsud, bsud, bsud or bsud, Such 4-flavor (charged) mesons have not been seen before, and are manifestly exotic. _ _ _ _ _ _ _ _ _ 31

  11. D0 Bsp± hadronic channel  Phys. Rev. Lett. 117, 022003 (2016), ‘hadronic’ refers to the decay Bs → J/yf as opposed to the semileptonic decay Bs→ Ds+m-X to be discussed later. Dotted lines indicate particles with lifetimes too short to detect. 30

  12. J/y: pTm > 1.5 GeV; M(mm) = (2.92, 3.25) GeV; selected dimuons are constrained to the PDG J/y mass. f: pTK > 0.7 GeV: MKK = (1.012, 1.030) GeV Bs candidate: M(m+m-K+K-) = (5.304, 5.425) GeV; Lxy/s(Lxy) > 3 Additional p±: add a track assumed to be a p with pTp>0.5 GeV; IPxy<0.02 cm, IP3D<0.12 cm pT(Bsp) > 10 GeV and an optional cut on the angular separation of Bs and p± DR = √Dh2 + Df2 < 0.3 (the “cone cut”) The Bsp mass resolution is improved by using M(Bsp) = Mobs(m+m- K+K- p) – Mobs(m+m-K+K-) + MPDG(Bs) Bsp candidates are restricted to 5506(threshold) < M <5900 GeV Bsp± hadronic: Selections D0 Bsp± 29

  13. Bsp± hadronic: Backgrounds Identify two classes of background in J/yf distribution: Real Bs modelled by MC Bs production (Pythia) ‘False’ Bs from combinatorial mm KK. Model these by sidebands >5s from Bs peak, with center of gravity at M(Bs) Obtain Bsp background from combination of Bs (or J/yf sideband candidate) and random p± in same event ‘real’ Bs ‘false’ Bs sidebands Combine real and false backgrounds in appropriate proportion (71%/29%). However the real Bs (data points) and fake Bs (histogram) background distributions are nearly the same. 28

  14. Bsp± hadronic: Background and signal model Fit the background to a product of polynomial and exponential: FBKD = (C1 + C2m2 + C3m3 +C4m4)*exp(C5 + C6m + C7m2) Fit is good both with and without the cone cut. The DR < 0.3 cone cut suppresses the events at high mass: The signal X is assumed to be a relativistic s-wave Breit-Wigner with a mass-dependent width G(m) = GX (qm/q0) where qm is the rest frame momentum of Bs at M(Bsp)=m and q0 is the momentum at the central MX value. The BW is convoluted with the Gaussian resolution, s=3.8 MeV Fit data (or MC pseudo-expts) to NX•BW(MX,GX) + (Ntot-NX)•FBKD 27

  15. Bsp± hadronic: Fit result MX = 5567.8±2.9 MeV GX = 21.9 ± 6.4 MeV NX = 133 ± 31 events With DR < 0.3 cone cut, fit for MX, GX and NX, taking mass resolution s = 3.8 MeV, obtain: Non-zero width → strong decay Probability for null hypothesis to reproduce the yield gives local significance of 6.6s. significance = √(-2 Lb/Ls+b) Using prescription of Gross and Vitellsfor Look Elsewhere Effect (LEE) get 6.1s global (statistical ) significance. If the X decay is s-wave (it is close to Bsp threshold of 5506 MeV), JP = 0+ and X is counterpart to a0(980). If this bump were really X→ Bs*p± with Bs*→ Bs+ unseen g (48 MeV) , would have MX =5616.5 MeV. If this decay is s-wave, JP = 1+. 26

  16. Bsp± hadronic: Fit result Fits in two pT(Bs) intervals returns similar X parameters, whereas the background shape changes 15< pT(Bs)<30 GeV 10< pT(Bs)<15 GeV Fraction of Bs that come from X decay is r=8.6±1.9 ±1.4%. (This seems like a large fraction!) 25

  17. Bsp± hadronic: Cross checks To test whether the X is really due to candidates with genuine Bs, we fit for the Bs signal yield in a series of bins of J/yf p mass. The resulting Bs yield as a function of M(J/yf p) gives a peak and width compatible with the primary analysis and NX = 118 ± 22 events. (In this analysis there is no non-Bs background.) • Other checks: • Look at Bdp± to see if a similar peak is generated: (none seen) • Look for peaks in Bsp or BsK (none seen) • Alternate background functions & binning, • Compare p+ and p- (no difference) • Two versions of Pythia Bs background known B1 5568 MeV 24

  18. Bsp± hadronic: Systematic uncertainties With the systematic uncertainties (and LEE), the significance is 5.1s. With the strong decay and the bs content of Bs and ud content of p+, the X flavor content is busd – the first four-quark state built from 4 distinct flavors. _ _ 23

  19. Bsp± hadronic: No cone cut Remove the cone cut, keep the functional form of the background (different parameters) and refit: Fix MX and GX to values for fit with cone cut and obtain NX = 106 ± 23 events, with corresponding local significance of 4.8s. The difference from the 133 events with cone cut is not well explained as a statistical fluctuation. The lower yield without the cone cut led PRL editor to insist on “Evidence” in the title, rather than “Observation”, despite the 5.1s global significance with the cone cut. We note that the yield is sensitive to the choice of background function in the high mass region, 5.7<M<5.9 GeV and alternate choices give higher yields. We also note that there may be states such as Bc → Bs np (n>1) that are not in our MC. The inclusion of added high mass backgrounds would cause the yield in the no-cone-cut case to increase. 22

  20. Can the cut create a peak? Bsp± hadronic: vary cone cut DR<0.2 DR<0.3 DR<0.5 DR cut efficiency Bsp mass Fit data with a series of DR cuts from 0.2 to 0.5 (and no cut). Fitted mass is stable within statistical uncertainty. We see no evidence of the cut sculpting the peak. 21

  21. Bsp± hadronic: Other experiments LHCb (pp collisions at 7 & 8 TeV [PRL 117, 152003 (2016)] sees no evidence for X(5568). Fraction of Bsthat come from X is less than 2.4% (95% CL), compared with D0 (8.6%). CMS (pp at 8 TeV) set limit at fraction of Bs from X less than 3.9% (95% CL) (ICHEP 2017, CMS-PAS BPH-16-002) CDF and ATLAS have yet to report results. So, is the X(5568) dead? Or are there differences due to production channel, energy?? 20

  22. Bsp± semileptonic channel In preliminary analysis for Moriond QCD2017, D0 undertook to search for X(5568) in a complementary decay channel: X(5568) → Bsp± with Bs → Ds-m+n (+X) Ds-→ f p- → K+ K-p- X = possible other particles (& charge conjugates) from Bs • Selections: • K’s: pT>1.0 GeV, opposite charge, MKK = (1.012, 1.030) GeV • m: pT = (3, 25) GeV; opposite charge to p from Ds; good vertex with Ds • Ds and Bs decay vertices well separated from primary vertex • p in Bsp decay: pT=(0.5, 25) GeV (same selection as in hadronic channel) • pT(mDs) > 10 GeV • 4.5 GeV<M(mDs)<M(Bs) to minimize effect of final state n • No cone cut is imposed https://www-d0.fnal.gov/Run2Physics/WWW/results/prelim/B/B68/ 19

  23. Bsp± semileptonic data D0 MC Restricting M(mDs) to be large (En low) improves mass resolution for Bsp To further improve mass resolution, define: M(Bsp)=m(mDsp)-m(mDs)+mPDG(Bs) M(mDs)>4 GeV M(mDs)<4 GeV M(Bsp) D0 preliminary Data Bsp mass distribution M(Bsp) After correcting for efficiency/acceptances, expect ~20% more semileptonic X(5568) than hadronic. 18

  24. Bsp± semileptonic – background We generate Pythia Monte Carlo to simulate background, modified to use EVTGEN for b/c hadron decays. Data events with no interactions are superimposed to simulate the effects of pileup. MC events are reweighted in pT(Bs) and pT(m) to agree with data kinematic distributions. Data MC no RW MC w/ RW Data MC no RW MC w/ RW 17

  25. Bsp± semileptonic – background modeling Fit reweighted MC background with baseline function: Fbkg = (C1D+C2D2+C3D3+C4D4) exp(C5D+C6D2) where D =m(Bsp) – mthreshold Three alternate fits of MC background: 1) Fbkg= (C1+C2D2+C3D3+C4D4) exp(C5+C6D+C7D2) (the functional form used for the hadronic channel analysis) 2) Fbkg = m[ (m/mthreshold)2-1 ]C1exp(C2m) (the ‘ARGUS’ function for resonances near threshold 3) The MC histogram itself, smoothed by ROOT. 16

  26. Bsp± semileptonic – background cross check Also consider two alternate background shape choices to the MC: Data sidebands in m(fp±) distribution Data same (wrong) sign Ds± m± events sideband SS MC background shape reproduces the sideband and wrong sign shapes well. points: Dssideband data histogram: Monte Carlo points: wrong sign data histogram: Monte Carlo 15

  27. Bsp± semileptonic – signal modeling The signal is modeled with the same relativistic Breit Wigner s-wave shape as used for the hadronic channel, convoluted with a Gaussian mass resolution whose (mass-dependent) width is taken from MC. This resolution is dominated by the missing n. D0 MC Resolution + n smear 3.8 MeV mass resolution due to track curvature resolution is convoluted with the smearing due to missing n. n only MC of X(5568) signal including resolution effects and the effects of the unseen neutrino shows a narrow peak with a broad flat tail due to an association of a Bs with an incorrect p± 14

  28. Bsp± semileptonic – fit Data are fit to the sum of background and signal distributions with the background parameters fixed to the values from fitting the MC distribution. The signal mass MX and width GX, and signal and background normalizations are obtained from the fit. D0 MC We test the closure of the fit and background model by injecting signals of various sizes in MC ensembles with M=5568, G=21.9 MeV and fitting for MX, GX and NX. Output NX and MX agree with inputs; Fits tend to have small GX for small Ninput. 13

  29. Bsp± semileptonic – systematics Vary energy scale and resolution by 1s; fit signal to p-wave BW; vary resolution smearing due to n. Dominant uncertainty due to background shape, estimated by comparing baseline and alternate background models. 12

  30. Bsp± semileptonic – results Fit results: MX = 5566.7 MeV GX = 6.0 MeV NX = 139 Statistical significance 4.5s; significance with syst: 3.2s We have evidence for X(5568) in the semileptonicchannel, with consistent MX and GX as hadronic channel, +3.6 +1.0 -3.4 -1.0 +9.5 +1.9 -6.0 -4.6 +51 +10.9 -63 -31.5 Fraction of Bs from X(5568) (pT(mDs)>10 GeV) is 7.6±2.9% in SL channel. Compare this with the hadronic fraction of 8.6±2.4% for pT(J/yf)>10 GeV. Finding a resonance in final state with n is novel, enabled by the cut 4.5 GeV<M(mDs)<M(Bs) and the fact that X(5568) is close to Bsp threshold 11

  31. Comparison of Bsp± hadronic and semileptonic channels The hadronic and semileptonic channels agree on mass and width of X(5568), so it makes sense to combine the two. Semileptonic channel Hadronic channel, no ΔR cut 5568 10

  32. Combination of Bsp± hadronic and semileptonic channels Since the semileptonic and hadronic fit parameters are consistent, we do not need the look elsewhere effect (LEE) for the semileptonic channel. Can combine hadronic channel with/without cone cut Inputs to combination and or 9

  33. Combination of Bsp± hadronic and semileptonic channels Because of the n, different triggers, and different backgrounds the systematic uncertainties for hadronic and semileptonic channels are nearly independent. For the combination of two uncorrelated measurements with p-values p1 and p2, the combined p-value is: pcomb = p1 p2 (1 – ln(p1 p2)) For phad = 3.8x10-7(with cone cut) and pSL = 6.4x10-4, we obtain pcomb = 5.6x10-9 corresponding to 5.7s significance Combining the hadronic result without cone cut and the semileptonic result, we get a significance of 4.7s. 8

  34. Bsp± summary D0 sees a strongly decaying (G>0) meson in two quite different channels with different backgrounds etc. The combined significance is 5.7s (4.7s when using the hadronic channel without cone cut.) The X(5568) would be the first meson that includes four quark flavors. The X(5568) lies 200 MeV below the Bd K± threshold making a pure BdK± molecular state unlikely. If tetraquark, there should be companion exotic mesons. No other experiment has yet observed X(5568), but LHC and Tevatron production processes might differ. We await the Tevatron analysis from CDF. 7

  35. Coda on Pentaquarks _ _ Just as exotic mesons can be constructed from qqqq, we can have exotic baryons such as qqqqq (pentaquarks). _ In 2015, the LHCb collaboration reported two structures, Pc(4450)+ and Pc(4380)+ in the decay Lb→ Pc+ K- with Pc+ →J/yp The higher mass state is narrow (G=39 MeV) and the lower one broad (G=205 MeV). Detailed analysis reveals the Argand diagram phase variation appropriate for resonances, and JP=3/2- and 5/2+. R. Aaij et al., LHCb Collaboration, PRL 115, 072001 (2015) _ These Pc states are within 400 MeV of the J/y p threshold. The minimal quark content is uudcc, manifestly an exotic pentaquark. 6

  36. D0 Pentaquark search Motivated by the LHCb result, D0 searched for strange analogs of the LHCb states: e.g. Lb→Pc+fwith Pc+→J/yL , and J/y→m+m-, L→pp- (and charge conjugates). Such Pc states could come from decays of Lb0 (as in Figure), Xb-, etc. Take search window up to 500 MeV above J/yL threshold, based on the LHCb states and theoretical models. https://www-d0.fnal.gov/Run2Physics/WWW/results/prelim/B/B69/ Event selection is based on that for the D0 Lb lifetime measurement; extended tracking version to increase acceptance for low momentum p from L decay. 5

  37. D0 Pentaquark search Divide J/yL sample into ‘prompt’ and ‘non-prompt’ subsets based on J/y decay length significance. See clear Lb signal in non-prompt sample, so focus on this subset. Lb ? 500 MeV search region 4

  38. D0 Pentaquark search Perform a series of fits of the form where Fsig is a Gaussian of width sX and MX is incremented in 10 MeV steps from threshold to 4.7 GeV, and Fbgr is the background function Pentaquark significance from scan max local Signif.= 3.58 at MX=4.32 GeV 3

  39. D0 Pentaquark search Fit result at the step with maximum significance. MX = 4319.3± 6.6 MeV sX = 15.6 ± 7.7 MeV NX = 142 ± 66 (mass resolution here is 5 MeV) Local significance = 3.45s Perform 6400 pseudoexperiments to determine the probability of a 3.45s or higher fluctuation anywhere in the search range. Global significance = 2.8s. Use of alternate signal model, alternate background function, or J/y or L sidebands increase the observed yield, so systematic uncertainties do not diminish the significance. Thus no evidence for strange analog of LHCbpentaquark in J/yL, but at least a pointer on where one might look. 2

  40. Summary Exotic mesons and baryons are now definitively observed but their construction is still not well understood D0 has added a new exotic meson, X(5568) to the zoo with sightings in two channels. LHC experiments do not observe it. The recent pentaquark baryons Pc seen by LHCb should have close family relations. The D0 search was negative, but suggests a place to focus attention.

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