LHCb Results on Exotic Meson Spectroscopy Thomas Britton On behalf of the LHCb collaboration Thomas...

LHCb Results on Exotic Meson Spectroscopy

Thomas Britton

On behalf of the LHCb collaboration

Thomas Britton, Syracuse, Hadron Conference, Sep. 2015 1

Pc

Recent Research

• Over the last few years there has been exciting research in exotic spectroscopy, more specifically charmonium and charmonium like states

• There are many new results from LHCb– Quantum number determination of X(3872)– Study of the resonant nature of Z(4430)– Discovery of pentaquarks

• This talk will focus mainly on the determination of X(3872) quantum numbers and the study of the resonant nature of Z(4430).

Z(4430)


X(3872)

The LHCb Detector at the LHC

• Single-arm forward spectrometer designed for precision measurements of CP violating decays, specifically decays involving bottom or charm hadrons

– Covering 25

• Great particle identification– Muons: ~97% with 1-3% of → misidentification– Kaons: ~95% with 5% of →K misidentification

• Good Impact parameter resolution: m

• Good momentum resolution: p/p=0.5% at 20GeV to 0.8% at 100GeV


Int. J. Mod. Phys. A 30,

1530022 (2015)

X(3872)

Charmonium? Tetraquark? Molecule?


X(3872) Quantum Numbers

• X(3872) is a long-lived state (<1.2 MeV) with the mass within the errors of m(D0)+m(D0*). Both conventional (charmonium) and exotic (D0D0* molecule, tetraquark) interpretations have been proposed. Determination of its JPC is important for narrowing down the possible interpretations.

• C=+ by observation of X(3872) J/ (and (2S)) decays• Previous determinations of JP

– all assumed the lowest possible orbital angular momentum in X(3872) J/ decays (Lmin value depends on JP

hypothesis).– CDF 2007:

• ruled out all but 1++ and 2+

– LHCb-PAPER-2013-001 (1 fb-1): • ruled out 2+ • the data is consistent with 1++ (and a pure S-wave decay)

• Significant L > Lmin could invalidate JP=1+ assignment and hint at molecular structure of X(3872)

Data sample: Changes • 2011 (1 fb-1) 2011+12 (3 fb-1) • The same selection except for:

– Now we cut on track ghost probability– pT()> 0.9 0.55 GeV; pT(,K)> 0.25 0.2 GeV– No cut on Q=M(J/)-M(J/)-M() Q < 250 MeV– J/ mass + J/K vertex constraints + B mass + B pointing

to PV constraints

1 fb-1 3 fb-1

31326 signal events171 equivalent bkg free signal events

B+X(3872)K+ events:

101138 signal events 778 equiv.

=2.8 MeV=5.5 MeV

Ns

Nes

Statistical gain 4.5 in equivalent signal yield (2.1 in statistical errors)

LHCb LHCb


LHCb-PAPER-2015-015LHCb-PAPER-2013-001

Thomas Britton, Syracuse, Hadron conference, Sep. 2015 77

L in X(3872)J/and Determinations of X(3872) JP

• All previous determinations of JP (CDF PRL 98(2007)132002, LHCb-PAPER-2013-001 PRL 110 (2013)

222001 1fb-1) assumed the lowest possible orbital angular momentum in X(3872)J/decays

Q<250 MeV selection cut

LHCb data

X(3872)J/ MC

2 m

Orbital angular momentum: L = r p

Conservation of parity: L = Lmin, Lmin+2, ...

LHCb3 fb-1

Q=0

m = mX(3872) – mJ – Q [MeV]low Q

low p of decay products

small r

Standard charmonium

Molecule

large r

low L L = ?

(Lmin value depends on JP)

• Significant L > Lmin could: invalidate the previous JP=1+

assignment hint at molecular structure of

X(3872)

LHCb-PAPER-2015-015 supplemental


Determination of X(3872) JPC: helicity formalism

x,J/

5 independent angles describing the decay in helicity formalism

Matrix element

Relation of the helicity couplings to LS amplitudes

Parity conservation

CDF 2007

LHCb 2013

LHCb 2015

x,

Many more amplitudes to fit

L

B+X(3872)K+, X(3872) J/ J/

LS amplitudes to be determined from the data

– all 5 angles


Determination of X(3872) JPC

• Data unambiguously prefers 1++ hypothesis (new: no assumptions about L)

Likelihood-ratio test: data vs simulated experiments

t =

t t

tdata

LHCb-PAPER-2015-015

Projection of all angles with 1++ hypothesis

D-wave fraction in X(3872)J/for JPC=1++

• Fit to the data:2 2

2 2

10 1

1

0

2 220.0018 0.0042 0.0066 0.0081B B

B B

D-wave amplitudes are consistent with zeroD-wave significance using Wilks theorem applied to the likelihood ratio with/without D wave amplitudes: 0.8

Gaussian part of the likelihood:fD

peak=0.4% (=0.3%)

Non-Gaussian tail

fD<4% at 95% CL5% prob.

LHCb3 fb-1

The likelihood was profiled as a function of the D-wave fraction:

Phys. Rev. D 92, 011102 (2015)



Importance of Multidimensional Analysis

• JPC sensitivity is in the multidimensional correlations!– 1D fits to helicity angles lack discrimination power between certain JP

assignments: see Belle analysis of BX(3872)K, X(3872) J/PRD84, 052004 (2011)

– Our data and simulations show the larger the number of decay angles included in the likelihood, the larger JP separation.

– When using all decay angles, detector efficiency corrections become JP independent - the analysis becomes simpler when using all angles.

– The smaller the data statistics the more important to analyze all dimensions.

After | cos() | >0.6All events

LHCbLHCb

cos(X) projections of the 5D fitsLHCb-PAPER-2015-015 (supplemental)

Radiative decays of X(3872) in LHCb

Measurement of R=BR(X(3872)2S)/BR(X(3872)J/) a good probe for internal structure of

X(3872)

• The results are not consistent with the expectations for a simple molecular model of X(3872)

• Mounting evidence that X(3872) has a very significant c(23P1++) component

3.6, 3.5

Signal significance: B+X(3872)K+,

X(3872)2S J/

0.4, 4.9 90% CL ULLHCb 2014 4.4, 12.0 LHCb

BR(X(3872)2S)/BR(X(3872)J/)

= 2.48±0.64±0.29

X(3872)2S

LHCb-PAPER-2014-008 NP B886, 665 (2014)

4.4

The LHCb results are consistent with, but more precise than, the BaBar and Belle results

LHCb


Z(4430)

Z(4430)+ previous measurements

18 3013 13

M(

signifi

) 44

( )

33 4

cance

45 MeV

.5

2 MeV

6

Z

Z

1D M(’) mass fit

41 264

22 2822 1

6 35

1

( ) 20

M( ) 448

6.4 (5.6 with sys

5

0 MeV

M V

.

e

)

Z

Z

4D amplitude fit

B’K

JP=1+ preferred by >3.4

non-B bkg bkg

No Z

With Z(4430)+

PRL 100, 142001 (2008) PRD 88, 074026 (2013)

(“K* veto region”) (“K* veto region”) 1D4DBelle

M(’) GeV

BaBar 2009Belle 2008

BaBar did not confirm Z(4430)+

in B sample comparable to Belle.Did not numerically contradict the

Belle results.

PRD 79, 112001 (2009)

Harmonic moments of K*s (2D)reflected to M(’)

Made Ad hoc assumption about the K*K background shape.

’ ≡ (2S) ]

Bkg

. sub

trac

ted

effi

cien

cy c

orre

cted

Almost model independent approach to K*K backgrounds.

Belle 2013

Z(4430)+

K*K bkg.

(2D amplitude fit in 2009)

Model dependent approach to K*K backgrounds.Higher statistical sensitivity.

(subsample with ’ ll)

Z+ K*


Z(4430)+ in LHCb: 4D amplitude analysis (a la Belle)

’ K*

K-

B0

K*

’

mK-

(or mK-m’

Z(4430)+ ’ amplitudeparameterized in angles derivable from the angles in the K* decay chain.

LHCb-PAPER-2014-014, PRL 112 (2014) 222002

• The 2D model independent approach establishes the need for exotic hadron contributions, but cannot be used to characterize their properties:– The quantitative analysis does not formally distinguish between ’+ and ’K

exotics.

– There is no way to fit Z(4430)+ on top of the K* reflections in m’+ distribution because of the Z-K* interferences

• The amplitude analysis is necessary!• Use all dimensions of the decay kinematics (4D):

– The best sensitivity to underlying physics – Avoids biases from non-uniform efficiency in averaged dimensions (for example the

differences between Belle 4D vs 2D fits)

15Thomas Britton, Syracuse, Hadron Conference, Sep. 2015

Amplitude fits with JP=1+ Z(4430)+

• The 4D 2 p-value = 12% (< 2x10-6 when fitting without Z(4430) ).

• The data are well described only when JP=1+ Z(4430)- is included in the fit. • Z(4430)- significances from (-2lnL) is 18.7 (13.9 with systematic

variations e.g. allowing the tail of K*3(1780)).

(“all data”) (log)


LHCb-PAPER-2014-014, PRL 112 (2014) 222002

Is Z(4430)+ really a bound state?

• Does it follow resonant behavior if not forced by the amplitude model?

• Replace the Breit-Wigner amplitude for Z(4430)+ by 6 independent amplitudes in m’

2 bins in the peak region

amplitude

phase

Rapid phase transition at the peak of the amplitude resonance!

The first time the resonant character of a four-quark candidate

wasdemonstrated this way!

Various models for Z(4430)+ have been proposed:tightly bound tetraquark (e.g. diquark model)

Breit-W

igner

data

Argand diagram


LHCb-PAPER-2014-014, PRL 112 (2014) 222002

Z(4430)- spin-parity analysis

• JP=1+ now established beyond any doubt

26 PRD 88, 074026 (2013)

Belle

18 using a

conservative approach

Rejection level relative to 1+

Disfavored JP LHCb Belle

0- 9.7 3.4

1- 15.8 3.7

2+ 16.1 5.1

2- 14.6 4.7

Including systematic variations:

or or

Exotictetraquark,molecule,hybrid,

…

Standardcharmonium Standard

D*-D1*0

threshold cusp

JP=0-,1-,2-

d

charged neutral

d

The only other confirmed charged four-quark candidate Z(3900)-

observed by BES-III and Belle in 2013 could be a DD* threshold effect


LHCb-PAPER-2014-014

Possibility of an additional Z+

• Argand diagram for this state is inconclusive, and there is no evidence for it in the model independent approach.

• More data is needed to clarify.

Phys Rev Lett 112, 222002 (2014)

An additional 0- state has 6 significance. 0- is preferred over 1-,2+,2- by 8. It could be 1+, however, Z+ becomes unreasonably wide under that JP assumption


Z(4430)+ in LHCb: 2D model independent analysis

cos(K*) vs. mK+

“Rectangular Dalitz plot”

vs. mK+

Decompose into

Legendre moments

Pass only moments with l not more than lmax=Jmax/2

cos(K*) vs. mK+

“K* Jmax filtered”“K* Jmax filtered”

actual distribution

correlated statistical errorsIn the filtered distribution

spin J

Jmax=2

4D Belle

K*(892)J=1 Dalitz plot K*2(1430)

J=2

Excess of events over the K* Jmax=2 filtered distribution

in the Z(4430)- region is apparent !

Dalitz plot

*

1

1(cos )

dataNU

l l K ii i

P P

Z(4430)- ?

This qualitative analysis was included in the 2014 paper


LHCb-PAPER-2014-014, PRL 112 (2014) 222002

Quantitative results from the model independent approach

LHCb-PAPER-2015-038 in preparation (preliminary results)

K* Jmax=2 lmax=4

K* Jmax=3 lmax=6

K* Jmax=15 lmax=30

mK MeVlmax=2 - 836 3 836-1000 4 1000 -

Allows for a tail of K*3(1780) Allows implausible K* contributionsAllows for K* states up to K*2(1430)

preliminary preliminary preliminary preliminarydatadatadata

data


The outlined procedure from the previous slide is repeated for various Jmax (lmax)

Such high moments only contribute in the presence of Z

Quantitative results from the model independent approach

Test significance of implausible lmax< l <30 cos(K*) moments using the log-likelihood ratio:

Explanation of the data with plausible K* contributions is ruled out at high significance without assuming anything about K* resonance shapes or their interference patterns!

preliminary preliminary preliminary


LHCb-PAPER-2015-038 in preparation (preliminary results)

• While not part of exotic meson spectroscopy I would be remiss if I did not at least mention LHCb’s discovery of pentaquarks as they, along with all other exotic candidates mentioned contain cc

• See plenary talk tomorrow at 11:50: Observation of Pentaquark Resonances in Λb → J/ψKp Decay

– Zhenwei Yang

• Resonant nature clearly seen in Pc(4450)

cc and beyond

With Pc

Without Pc

Pc(4450)Pc(4380)Features in J/p unable to be described by * alone

Best modeled by two Pc states

Thomas Britton, Syracuse, Hadron conference, Sep. 2015

Phys. Rev. Lett. 115, 072001 (2015)

23


Summary• So far LHCb has contributed to the study of neutral candidates for

exotic mesons (X(3872)) and the charged tetraquark candidate of Z(4430)

• X(3872):– With the full 3 fb-1 and after removing the assumption of

X(3872)→J/proceeding by the lowest angular momentum X(3872)’s JP unambiguously determined to be 1+.

• The quantum numbers are consistent with the molecular or tetraquark models or with the c1(23P1++) charmonium state, or their mixture. Inconsistent with the other interpretations e.g. c(11D2-+) proposed previously.

• An upper limit on D-wave fraction was set at 4% with a 95% CL:– Small D-wave fraction is expected in models in which X(3872) is a compact state

(e.g. charmonium)– Since no quantitative predictions for D-wave fraction exist in any model, it is hard to

draw any conclusions

– The measurement BR(X(3872)2S)/BR(X(3872)J/) is pointing to the strong component of c1(23P1++) in X(3872), possibly mixing with the exotic form e.g. D0D0* molecule.

• Z(4430):– Amplitude analysis confirmed Z(4430)+ with overwhelming

significance, together with JP=1+. More precise mass, width measurements.

– The first exotic hadron candidate for which a resonant nature has been demonstrated with an Argand diagram

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LHCb Results on Exotic Meson Spectroscopy Thomas Britton On behalf of the LHCb collaboration Thomas...

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