Exotic Heavy Flavor Hadrons
Stony Brook seminar P. Grannis Apr. 17, 2017 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 qq multiplets, 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 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 L to R: Bruno Zumino, Sidney Coleman, Nino forgotten.” Zichichi, Nicolo Cabibbo, 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 Types of multi-quark states
Multiquark exotic mesons may have more complex structures than ordinary mesons, due both to the color combinations and to the binding mechanism. _ c u _ A pair of colorless qq states bound loosely with a colorless π pion in van der Waals force (analogous to the deuteron). _ Expect such a ‘MOLECULAR STATE’ to have a mass quite__ close u c to the sum of the two colorless hadrons (here D0 and D0)
A colored diquark and a colored anti-diquark pair, bound with a multicolor gluonic state. The color force binding the diquark c u _ _ (color 3 or 6) and anti-diquark (color 3 or 6) is quite strong so gg _ _ this ‘TETRAQUARK’ state mass can be quite far below the__ D0 c u D0 threshold.
c A ‘HYBRID’ state of colored quark, antiquark _ and gluon in an overall color singlet c …. 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 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=1−− JPC=0++
ss- (φ) a (980) _ _ 0 ______f0(980)
_ _us- (K*) _ _ _ _ κ(800)
mass -
_ _ _ud (ρ,ω) _ _ f (500) uu- ±dd- 0
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 (su)(ds)- _ mass are those containing the more massive _ _ _ (su)(us)- - etc.
strange quark. - _ _ _ _ (sd)(ud- ) etc.
Maiani, et al. assigned a and f as tetraquarks 0 0 _ (ud)(ud- - ) and predicted analogs with c and b quarks 36 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 (π+π− J/ψ). It has been seen in both e+e− and hadron colliders by BELLE, BaBar, CDF, D0, LHCb, CMS, BESIII_ and ATLAS. Decays to ω J/ψ, γ J/ψ, and D0 D*0 have been observed.
X(3872) mass_ is essentially equal to the sum of the D0 and D*0 masses.
Width is consistent with resolution, so ΓX < 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 Interpreting X(3872)
_ Only unassigned cc charmonium JP= 1++ state was the first radial excitation of χc1 but the mass splitting from known χc2’ is wrong. The width of X is_ too small to be charmonium; also the ρ J/ψ 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 pT hadron 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 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)) 33 Another example: Y(4140) CDF In 2009, CDF announced a J/ψ φ resonance, Y(4140), + + in B → Y K , close_ _ to the J/ψ φ threshold. The quark content is thus ccss and 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 5σ D0 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. The pattern of non-confirmation, then confirmation, ambiguous
interpretation, dependence on production modes etc. is typical. 32 ± D0 Bs π
D0 was a multipurpose _ detector at the Tevatron pp collider (ran 1992 – 2011)
+ − D0 did a _search_ _ for_ a resonance in Bs π π (quark content_ _ bsuu_ _ /bsdd) for an analog of known states of csuu/csdd. Nothing was seen but a side look at ± + − the Bs π mass distribution (Bs → J/ψ φ, J/ψ → µ µ φ → K+ K− ) showed a peak slightly above the threshold of 5506 MeV. Due to rapid mixing,_ one cannot distinguish between Bs and Bs. The analysis does not separate π+ and π−. 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 ± D0 Bs π hadronic channel Phys. Rev. Lett. 117, 022003 (2016), ‘hadronic’ refers to
the decay Bs → J/ψ φ as opposed to the semileptonic decay + − Bs → Ds µ X to be discussed later.
Dotted lines indicate particles with lifetimes too short to detect.
30 ± ± J/ψ: p µ > 1.5 GeV; M(µµ) = (2.92, 3.25) BD0s π hadronic:Bs π Selections T GeV; selected dimuons are constrained to the PDG J/ψ mass.
K φ: pT > 0.7 GeV: MKK = (1.012, 1.030) GeV
+ − + − Bs candidate: M(µ µ K K ) = (5.304, 5.425) GeV; Lxy/σ(Lxy) > 3
Additional π±: add a track assumed to be a π π with pT >0.5 GeV; IPxy<0.02 cm, IP3D<0.12 cm
pT(Bsπ) > 10 GeV
and an optional cut on the angular separation ± of Bs and π ∆R = √∆η2 + ∆φ2 < 0.3 (the “cone cut”)
The Bs π mass resolution is improved by using + − + − + − M(Bs π) = Mobs(µ µ K K π) – Mobs(µ µ + − K K ) + MPDG(Bs)
Bs π candidates are restricted to 5506(threshold) < M < 5900 GeV 29 ± Bs π hadronic: Backgrounds
Identify two classes of background in J/ψ φ ‘real’ Bs distribution: a) Real Bs modelled by MC Bs production
(Pythia) ‘false’ Bs b) ‘False’ Bs from combinatorial µµ KK. Model these by sidebands >5σ from Bs peak, with center of gravity at M(Bs) sidebands c) Obtain Bs π background from combination of Bs (or J/ψ φ sideband candidate) and random π± in same event
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 ± Bs π hadronic: Background and signal model
Fit the background to a product of polynomial and exponential: 2 3 4 2 FBKD = (C1 + C2m + C3m +C4m )*exp(C5 + C6m + C7m )
Fit is good both with and without the cone cut.
The ∆R < 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 Γ(m) = ΓX (qm/q0) where qm is the rest frame momentum of Bs at M(Bs π)=m and q0 is the momentum at the central MX value. The BW is convoluted with the Gaussian resolution, σ=3.8 MeV
Fit data (or MC pseudo-expts) to NX•BW(MX,ΓX) + (Ntot-NX)•FBKD 27 ± Bs π hadronic: Fit result MX = 5567.8±2.9 MeV
With ∆R < 0.3 cone cut, fit for MX, ΓX and NX, ΓX = 21.9 ± 6.4 MeV taking mass resolution σ = 3.8 MeV, obtain: NX = 133 ± 31 events
Non-zero width → strong decay Probability for null hypothesis to reproduce the yield gives local significance of 6.6σ.
significance = √(−2 Lb/Ls+b) Using prescription of Gross and Vitells for Look Elsewhere Effect (LEE) get 6.1σ global (statistical ) significance.
If the X decay is s-wave (it is close to P + Bsπ threshold of 5506 MeV), J = 0 and X is counterpart to a0(980).
* ± If this bump were really X→ Bs π * with Bs → Bs + unseen γ (48 MeV) , would have MX =5616.5 MeV. If this P + decay is s-wave, J = 1 . 26 ± Bs π hadronic: Fit result
10< pT(Bs)<15 GeV 15< pT(Bs)<30 GeV Fits in two pT(Bs) intervals returns similar X parameters, whereas the background shape changes
Fraction of Bs that come from X decay is ρ=8.6±1.9 ±1.4%. (This seems like a large fraction!)
25 ± Bs π 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/ψφ π mass. The resulting Bs yield as a function of M(J/ψ φ π) 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 Bd π to see if a similar peak is generated: (none seen) Look for peaks in B p or B K (none seen) 5568 MeV known B1 s s Alternate background functions & binning, Compare π+ and π− (no difference)
Two versions of Pythia Bs background 24 ± Bs π hadronic: Systematic uncertainties
With the systematic uncertainties (and LEE), the significance is 5.1σ. _ _ + With the strong decay and the bs content of Bs and ud content of π , the X flavor content is busd – the first four-quark state built from 4 distinct flavors. 23 ± Bs π hadronic: No cone cut Remove the cone cut, keep the functional form of the background (different parameters) and refit:
Fix MX and ΓX to values for fit with cone cut and obtain NX = 106 ± 23 events, with corresponding local significance of 4.8σ. 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 5.1σ 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 We also note that there may be states such as Bc → Bs nπ (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 ± Can the cut create a peak? Bs π hadronic: vary cone cut ∆R<0.2 ∆R<0.3 ∆R<0.5 ∆R cut efficiency Bs π mass Fit data with a series of ∆R 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 ± Bs π hadronic: Other experiments LHCb (pp collisions at 7 & 8 Te V [PRL 117, 152003 (2016)] sees no evidence for X(5568). Fraction of Bs that come from X is less than 2.4% (95% CL), compared with D0 (8.6%). CMS (pp at 8 Te V ) 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 ± Bs π semileptonic channel In preliminary analysis for Moriond QCD2017, D0 undertook to search for X(5568) in a complementary decay channel: ± X(5568) → Bs π − + with Bs → Ds µ ν (+X) − − + − − Ds → φ π → K K π X = possible other particles (& charge conjugates) from Bs Selections: K’s: pT>1.0 GeV, opposite charge, MKK = (1.012, 1.030) GeV µ: pT = (3, 25) GeV; opposite charge to π from Ds; good vertex with Ds Ds and Bs decay vertices well separated from primary vertex π in Bs π decay: pT=(0.5, 25) GeV (same selection as in hadronic channel) pT(µDs) > 10 GeV 4.5 GeV https://www-d0.fnal.gov/Run2Physics/WWW/results/prelim/B/B68/ 19 ± Bs π semileptonic data D0 MC Restricting M(µDs) to be large (Eν low) M(µD )>4 GeV s improves mass resolution for Bs π To further improve mass resolution, define: M(µDs)<4 GeV M(Bsπ)=m(µDsπ)−m(µDs)+mPDG(Bs) M(Bsπ) D0 preliminary Data Bs π mass distribution M(Bs π) After correcting for efficiency/acceptances, expect ~20% more semileptonic X(5568) than hadronic. 18 ± Bs π 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(µ) to agree with data kinematic distributions. Data Data MC no RW MC no RW MC w/ RW MC w/ RW 17 ± Bs π semileptonic – background modeling Fit reweighted MC background with baseline function: 2 3 4 2 Fbkg = (C1∆+C2∆ +C3∆ +C4∆ ) exp(C5∆+C6∆ ) where ∆ =m(Bs π) – mthreshold Three alternate fits of MC background: 2 3 4 2 1) Fbkg = (C1+C2∆ +C3∆ +C4∆ ) exp(C5+C6∆ +C7∆ ) (the functional form used for the hadronic channel analysis) 2 C1 2) Fbkg = m[ (m/mthreshold) −1 ] exp(C2m) (the ‘ARGUS’ function for resonances near threshold 3) The MC histogram itself, smoothed by ROOT. 16 ± Bs π semileptonic – background cross check Also consider two alternate background shape choices to the MC: (a) Data sidebands in m(φπ±) distribution ± ± (b) Data same (wrong) sign Ds µ events sideband SS MC background shape reproduces the sideband and wrong sign shapes well. points: wrong sign data points: Ds sideband data histogram: Monte Carlo histogram: Monte Carlo 15 ± Bs π 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 ν. D0 MC Resolution + ν smear 3.8 MeV mass resolution due to track curvature resolution is convoluted with the smearing due to missing ν. ν 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 π± 14 ± Bs π 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 ΓX, 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, Γ=21.9 MeV and fitting for MX, ΓX and NX. Output NX and MX agree with inputs; Fits tend to have small ΓX for small Ninput. 13 ± Bs π semileptonic – systematics Vary energy scale and resolution by 1σ; fit signal to p-wave BW; vary resolution smearing due to ν. Dominant uncertainty due to background shape, estimated by comparing baseline and alternate background models. 12 ± Bs π semileptonic – results Fit results: M = 5566.7 +3.6 +1.0 MeV X −3.4 −1.0 Γ = 6.0 +9.5 +1.9 MeV X −6.0 −4.6 +51 +10.9 NX = 139 −63 −31.5 Statistical significance 4.5σ; significance with syst: 3.2σ We have evidence for X(5568) in the semileptonic channel, with consistent MX and ΓX as hadronic channel, Fraction of Bs from X(5568) (pT(µDs)>10 GeV) is 7.6±2.9% in SL channel. Compare this with the hadronic fraction of 8.6±2.4% for pT(J/ψ φ)>10 GeV. Finding a resonance in final state with ν is novel, enabled by the cut 4.5 GeV D0 Run II, 10.4 fb-1 250 The hadronic and semileptonic channels agree on mass and 200 width of X(5568), so it makes sense to combine the two. 150 Semileptonic channel 100 Number of events 50 Hadronic channel, no ΔR cut 0 5568 5500 5600 5700 5800 5900 M(Bs π) - MeV 10 ± Combination of Bs π 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 or 9 ± Combination of Bs π hadronic and semileptonic channels Because of the ν, 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)) −7 −4 For phad = 3.8x10 (with cone cut) and pSL = 6.4x10 , we obtain −9 pcomb = 5.6x10 corresponding to 5.7σ significance Combining the hadronic result without cone cut and the semileptonic result, we get a significance of 4.7σ. 8 ± Bs π summary D0 sees a strongly decaying (Γ>0) meson in two quite different channels with different backgrounds etc. The combined significance is 5.7σ (4.7σ 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 Bd K 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 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 + − + Λb→ Pc K with Pc →J/ψ p The higher mass state is narrow (Γ=39 MeV) and the lower one broad (Γ=205 MeV). Detailed analysis reveals the Argand diagram phase variation P − + appropriate for resonances, and J =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/ψ p threshold. _ The minimal quark content is uudcc, manifestly an exotic pentaquark. 6 D0 Pentaquark search Motivated by the LHCb result, D0 searched for strange analogs of the + + + − − LHCb states: e.g. Λb→ Pc φ with Pc →J/ψ Λ , and J/ψ →µ µ , Λ→pπ (and charge conjugates). https://www-d0.fnal.gov/Run2Physics/WWW/results/prelim/B/B69/ 0 − Such Pc states could come from decays of Λb (as in Figure), Ξb , etc. Ta k e search window up to 500 MeV above J/ψ Λ threshold, based on the LHCb states and theoretical models. Event selection is based on that for the D0 Λb lifetime measurement; extended tracking version to increase acceptance for low momentum π from Λ decay. 5 D0 Pentaquark search Divide J/ψ Λ sample into ‘prompt’ and ‘non-prompt’ subsets based on J/ψ decay length significance. See clear Λb signal in non-prompt sample, so focus on this subset. Λb ? 500 MeV search region 4 D0 Pentaquark search Perform a series of fits of the form where Fsig is a Gaussian of width σX 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 D0 Pentaquark search Fit result at the step with maximum significance. MX = 4319.3± 6.6 MeV σX = 15.6 ± 7.7 MeV NX = 142 ± 66 (mass resolution here is 5 MeV) Local significance = 3.45σ Perform 6400 pseudoexperiments to determine the probability of a 3.45σ or higher fluctuation anywhere in the search range. Global significance = 2.8σ. Use of alternate signal model, alternate background function, or J/ψ or Λ sidebands increase the observed yield, so systematic uncertainties do not diminish the significance. Thus no evidence for strange analog of LHCb pentaquark in J/ψ Λ, but at least a pointer on where one might look. 2 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.