-antibaryon femtoscopy – do we see a hint of a hexaquark in ALICE?

Łukasz Graczykowski

HIRG 8 October 2018 Status of femtoscopy in ALICE

● Previous results – Multipion Bose-Einstein correlations in pp, p-Pb, and Pb-Pb collisions at the LHC, Phys. Rev. C 93 (2016) 054908

– Centrality dependence of freeze-out radii in Pb-Pb collisions at √sNN=2.76 TeV Phys. Rev. C 93 (2016) 024905 – 1D pion, , femtoscopy in Pb-Pb Phys. Rev. C 92 (2015) 054908 – Pion femtoscopy in p-Pb Phys. Rev. C 91 (2015) 034906 – Freeze-out radii extracted from three-pion cumulants in pp, p-Pb and Pb-Pb collisions at the LHC, Phys. Lett. B 739 (2014) 139-151

– Two and Three-Pion Quantum Statistics Correlations in Pb-Pb Collisions at √sNN =2.76 TeV at the LHC Phys. Rev. C 89 (2014) 024911 – Charged kaon femtoscopic correlations in pp collisions at √s=7 TeV, Phys. Rev. D 87 (2013) 052016 – K0sK0s correlations in pp collisions at √s=7 TeV from the LHC ALICE experiment, Phys. Lett. B 717 (2012) 151-161 – Femtoscopy in pp a 0.9 and 7 TeV, Phys. Rev. D 84 (2011) 112004,

– Two-pion Bose-Einstein correlations in central Pb-Pb collisions at √sNN = 2.76 TeV, Phys. Lett. B 696 (2011) 328-337 – Two-pion Bose-Einstein correlations in pp collisions at √s=900 GeV, Phys. Rev. D 82 (2010) 052001

● Newly published papers: – Measuring K0sK± interactions using pp collisions at √s = 7 TeV. Submitted 21 Sep to arXiv (submitted to Phys. Lett. B) – p-p, p-Λ and -and Λ-Λcorrelationsstudiedviafemtoscopyinppreactionsat√s=7TeV,arXiv:1805.1245(submittedtoPhys.Rev.C)Λand correlations studied via femtoscopy in pp reactions at √s = 7 TeV, arXiv:1805.1245 (submitted to Phys. Rev. C)

– Azimuthally-differential pion femtoscopy relative to the third harmonic event plane in Pb-Pb collisions at √sNN = 2.76 TeV, Phys. Lett. B 785 (2018) 320-331

– Azimuthally differential pion femtoscopy in Pb-Pb collisions at √sNN=2.76 TeV, Phys. Rev. Lett. 118 (2017) 222301

– Kaon femtoscopy in Pb-Pb collisions at √sNN = 2.76 TeV, Phys. Rev. C96 (2017) 064613

– Measuring K0sK± interactions using Pb-Pb collisions at √sNN =2.76 TeV, Phys. Lett. B 774 (2017) 64 ● Recent preliminary results: 17 papers in total – Baryon results: ~7.4% of all ALICE papers ● Baryon-antibaryon correlations (pp, and pΛ andΛ-Λcorrelationsstudiedviafemtoscopyinppreactionsat√s=7TeV,arXiv:1805.1245(submittedtoPhys.Rev.C), pΛ, and and Λ-Λcorrelationsstudiedviafemtoscopyinppreactionsat√s=7TeV,arXiv:1805.1245(submittedtoPhys.Rev.C)Λ andΛ-Λcorrelationsstudiedviafemtoscopyinppreactionsat√s=7TeV,arXiv:1805.1245(submittedtoPhys.Rev.C)) from Run1 and Run2 in Pb-Pb (231, 23 Jul 2018) ● Analysis of heavier (eg. pΞ,, pΞ,) – Lambda-K+, Lambda-K-, and Lambda-K0s 4 more coming soon – Pion-kaon, Kaon-proton (under IRC review) 8 October 2018, HIRG Łukasz Graczykowski (WUT) 2/27 Recent changes in the PAG

Tom Hummanic → Laura Fabbietti

Łukasz Graczykowski → Ludmila Malinina

Photos from SAMS (CERN database)

8 October 2018, HIRG Łukasz Graczykowski (WUT) 3/27 Beyond the system size

2 3 * C (q)=∫ S(r)|Ψ(q ,r)| d r q=2⋅k =p1− p2

measured correlation emission function pair wave function (source size/shape) (includes cross section) increase of (anti)correlation = decrease of the radius OR MC simulation increase of the interaction THERMINATOR cross section

25 September 2018, CF Workshop Łukasz Graczykowski (WUT) 4/29 Beyond the system size

25 September 2018, CF Workshop Łukasz Graczykowski (WUT) 5/29 Potential applications ● Input to models with re-scattering phase (eg. UrQMD): – annihilation cross sections only measured for pp, pn, and pd pairs – UrQMD currently guesses it for other systems from pp pairs

● Structure of baryons/search for CPT violation STAR, Nature 527, 345-348 (2015)

● Search for H-dibaryon ALICE, PLB 752 (2016) 267-277

● Hypernuclear structure theory Nucl.Phys. A914 (2013) 377-386 For details see EP seminar by Laura Fabbietti ● star equation of state https://indico.cern.ch/event/749074/ Nucl.Phys. A804 (2008) 309-321

● Relativistic heavy-ion collisions at LHC or RHIC produce very similar number of baryons and antibaryons, “-antimatter pair factories”

8 October 2018, HIRG Łukasz Graczykowski (WUT) 6/27 Baryon-antibaryon correlations

Draft under final steps of approval Paper Committee: ŁG (chair), Adam Kisiel Target journal: Phys. Rev. X (?)

8 October 2018, HIRG Łukasz Graczykowski (WUT) 7/27 Baryon-baryon correlations

Draft under final steps of approval Paper Committee: L. Fabbietti (chair), A. Mathis, B. Hohlweger, D. Mihaylov, O. Doce Target journal: Nature Physics (?)

8 October 2018, HIRG Łukasz Graczykowski (WUT) 8/27 Baryon-antibaryon

pp correlations Explanation of the fitting procedure:

● χ2 is calculated from a “global” fit to all functions: 2 data sets, 3 pair combinations, 6 pΛ+pΛ centrality bins (total 36 functions)

● simultaneous fit accounts for parameters shared between different ΛΛ systems (such as Λ andΛ-Λcorrelationsstudiedviafemtoscopyinppreactionsat√s=7TeV,arXiv:1805.1245(submittedtoPhys.Rev.C)Λ andΛ-Λcorrelationsstudiedviafemtoscopyinppreactionsat√s=7TeV,arXiv:1805.1245(submittedtoPhys.Rev.C) scattering length)

● radii scale with multiplicity for a given 3 system Rinv =a⋅√N ch+b

● for different systems we assume radii

scaling with mT

● Fractions of residual pairs taken from AMPT

PRC 92(2015) 054908

8 October 2018, HIRG Łukasz Graczykowski (WUT) 9/27 Baryon-antibaryon correlations Conclusions from fitting:

● Scattering parameters for all baryon-antibaryon pairs are similar to each other (UrQMD assumption is valid)

● We observe a negative real part of scattering length → repulsive strong interaction or creation of a

● Significant positive imaginary part of scattering length – presence of a non-elastic channel – annihilation

● Can we find a hexaquark?

8 October 2018, HIRG Łukasz Graczykowski (WUT) 10/27 Baryon-antibaryon correlations Conclusions from fitting:

● Scattering parameters for all baryon-antibaryon pairs are similar to each other (UrQMD assumption is valid)

● We observe a negative real part of scattering length → repulsive strong interaction or creation of a bound state

● Significant positive imaginary part of scattering length – presence of a non-elastic channel – annihilation

● Can we find a hexaquark?

8 October 2018, HIRG Łukasz Graczykowski (WUT) 11/27 Other possibilites

25 September 2018, CF Workshop Łukasz Graczykowski (WUT) 12/29 What does it all mean?

inelastic channel ℑ f negative (annihilation) 0 even for non-matching pairs like pΛ

repulsion

spatial separation?

ℜ f0 negative

bound states

8 October 2018, HIRG Łukasz Graczykowski (WUT) 13/27 Bound states ℑ f0 non-zero u d

u ℜ f negative X 0 proton

p Λ̄ Y

u̅ d̅ Z s̅

antilambda

● Negative ℜ f0 – bound state creation, decaying to different than before scattering

(non-zero ℑ f0)

● Binding energy from the scattering length ≈ 7.8 MeV

● System decaying through the strong interaction – baryonium/hexaquark

● Mass would be slightly larger than sum of masses of given BB̄ pairandthe pair and the lifetime should be of the order or 10-24 -10-23 s, corresponding to the resonance width of 0.07 - 0.70 GeV.

8 October 2018, HIRG Łukasz Graczykowski (WUT) 14/27 Possible decay channels We can write down possible combinations

p̄ (ūūpair andtheΛΛ-Λcorrelationsstudiedviafemtoscopyinppreactionsat√s=7TeV,arXiv:1805.1245(submittedtoPhys.Rev.C)d̄ pairandthe uds) dū + ud̄ pairandthe K- + π+ + π- sū uū + d̄ pairandthe K- + γ/j/π /j/π0 + γ/j/π /j/π0

- 0 These channels should sd̄ pairandthe uū + dū K̄ pairandthe 0 + π + γ/j/π /j/π be easy to measure pΛ (uudūand Λ-Λcorrelationsstudiedviafemtoscopyinppreactionsat√s=7TeV,arXiv:1805.1245(submittedtoPhys.Rev.C)̄pairandtheds̄̄ pairandthe pairandthe ) dū + ud̄ pairandthe K+ + π+ + π- us̄ pairandthe uū + d̄ pairandthe K+ + γ/j/π /j/π0 + γ/j/π /j/π0

+ 0 ds̄ pairandthe uū + ud̄ pairandthe K0 + π + γ/j/π /j/π

8 October 2018, HIRG Łukasz Graczykowski (WUT) 15/27 1st step – invariant mass, data ● We can construct the 3- invariant mass distribution for K- + π+ + π- (pp collisions @ 13 TeV – cleaner signal, no flow, etc.)

● No clear signal in the raw distribution

● Remove pairs from known resonance decays:

– those decaying to two :

ρ(770),ω(783),f(770), ω(783),f(783),f0(980),f2(1270),ρ(770),ω(783),f (14

50),f0(1500),ρ(770),ω(783),f3(1690),f0(1710),f4(20 ZOOM 50),K0

– those decaying to kaon and pion:

K*(892),K*(1410),K2*(1430),K*(1680),

K3*(1780),K4*(2045),D0

8 October 2018, HIRG Łukasz Graczykowski (WUT) 16/27 2nd step – invariant mass, model ● We proceed with exactly the same procedure for model, EPOS-LHC in this case

● Blue – data, red – model

● In general model and data overlap, BUT…

● After subtraction some differences are visible

● We fit the 4th order polynomial outside the possible signal region

● Bottom plot: blue – data minus subtraction model, red – polynomial

8 October 2018, HIRG Łukasz Graczykowski (WUT) 17/27 3rd step – Breit-Wigner fit ● Finally, we remove smooth background and fit the Breit-Wigner curve

● The curve fits data well (no difference with relativistic and non- relativistic BW)

● Extracted mass is ≈ 2.130 GeV

● Extracted width is Γ ≈0.125GeV ≈ 0.125 GeV

● The sum of masses of proton and Λ andΛ-Λcorrelationsstudiedviafemtoscopyinppreactionsat√s=7TeV,arXiv:1805.1245(submittedtoPhys.Rev.C) is ≈ 2.054 GeV < 2.130 GeV, which suggests a decay due to strong int.

● The width translates to lifetime of ≈ 5.3 ᐧ1010-24 s, typical strong decay lifetime

● The fit has significance of 5.9 sigma

8 October 2018, HIRG Łukasz Graczykowski (WUT) 18/27 Comparison – , LHCb

● For comparison, pentaquark Pc(4380)+ discovered by LHCb has a mass of 4.380 GeV, and J/Ψ +protonmassis4.035GeV.WidthofP + proton mass is 4.035 GeV. Width of Pc is 0.205 GeV

LHCb, PRL 115, 072001 (2015)

8 October 2018, HIRG Łukasz Graczykowski (WUT) 19/27 Systematic checks

Conclusions from systematic checks:

● different datasets → consistent results ✓

● different background functions → consistent results ✓

● varying 2-body decay cuts → consistent results ✓

● shifting plots may produce some peaks → one should be very careful with that, further investigation needed!

→ more in the backup

8 October 2018, HIRG Łukasz Graczykowski (WUT) 20/27 Possibilities Known particles with mass close to 2130 MeV:

● f0(2100): Γ ≈0.125GeV ≈ 250 MeV, needs confirmation, not sure what are the decay channels

● f0(2200): Γ ≈0.125GeV ≈ 250 MeV, not in summary table, not sure what are the decay channels

● π2(2100): no decay to Kππ, needs confirmation, much wider (≈ 600 MeV)

● ρ(770),ω(783),f(2150): no decay to Kππ, not in summary table, width 100-400 MeV

● φ(2170): nodecaytoKππ,notinsummarytable,width50-150MeV(2170): no decay to Kππ, not in summary table, width 50-150 MeV

● f2(2150): no decay to Kππ, not in summary table, width similar

● other light - nothing decays into Kππ

● nothing with similar mass in heavier quark mesons

● no baryons decaying to Kππ

● K2(2250): close, a bit higher mass. Has Kππ decay, width slightly bigger

● K3(2320): higher mass, similar width - described explicitly as pΛ andΛ-Λcorrelationsstudiedviafemtoscopyinppreactionsat√s=7TeV,arXiv:1805.1245(submittedtoPhys.Rev.C) system

8 October 2018, HIRG Łukasz Graczykowski (WUT) 21/27 K (2250) and K (2320) 2 3 K2(2250): ● This entry contains various peaks in strange systems reported in the 2150–2260 MeV region ● It has another decay channel into

K*(892) and f0(980), which we can see in the Dalitz plot

K3(2320): ● pΛ andΛ-Λcorrelationsstudiedviafemtoscopyinppreactionsat√s=7TeV,arXiv:1805.1245(submittedtoPhys.Rev.C) system, but more like an , decaying back into proton and antilambda, ● It “needs confirmation” and is not listed in the PDG summary table.

8 October 2018, HIRG Łukasz Graczykowski (WUT) 22/27 Dalitz plots pΛ and Λ-Λ correlationsstudiedviafemtoscopyinppreactionsat√s=7TeV,arXiv:1805.1245(submittedtoPhys.Rev.C)-data pΛ and Λ-Λ correlationsstudiedviafemtoscopyinppreactionsat√s=7TeV,arXiv:1805.1245(submittedtoPhys.Rev.C)-model ● Data and model qualitatively similar, but different band widths

pΛ andΛ-Λcorrelationsstudiedviafemtoscopyinppreactionsat√s=7TeV,arXiv:1805.1245(submittedtoPhys.Rev.C) - data pΛ andΛ-Λcorrelationsstudiedviafemtoscopyinppreactionsat√s=7TeV,arXiv:1805.1245(submittedtoPhys.Rev.C) - model

8 October 2018, HIRG Łukasz Graczykowski (WUT) 23/27 Dalitz plots pΛ and Λ-Λ correlations studiedviafemtoscopyinppreactionsat√s=7TeV,arXiv:1805.1245(submittedtoPhys.Rev.C)-(data-model)data-model) ● Actually those resonances are better visible in the model than in the data

● K*(892) It would be good to check parent ID in the model and also check if those particles not listed in the summary tables

(K2, K3…) are included in the D0 model.

ρ(770),ω(783),f(770)

?

8 October 2018, HIRG Łukasz Graczykowski (WUT) 24/27 Peak properties ● If this particle would decay through ρ(770),ω(783),f , f0, K*, D0 - magnitude of the peak should increase if we include them back when calculating invariant mass… Model: always all 2-body decays excluded

Data: ● red: all 2-body decays excluded

● green: ρ(770),ω(783),f (770),f0(980), K*(892), D0 included

8 October 2018, HIRG Łukasz Graczykowski (WUT) 25/27 Conclusions What has been done already: ● Results from analysis of femtoscopic BB̄ pairandthe correlations suggest a possibility of bound state ● Invariant mass analysis of hypothetical BB̄ pairandthe bound state decaying into Kππ has been performed ● After many-steps (and possibly dangerous) procedure, a peak was observed at m≈ 2130 MeV, Γ ≈0.125GeV 125 MeV, ● Evidence for similar particles was reported by experiments running in 80s ● Basic systematic checks were done, so far the result survived all tests

● Dalitz plots were also produced, suggesting cascade decays through ρ(770),ω(783),f and K*, which are visible also in the model ● Analysis on PbPb data needs optimization (timeout error). Probably not very useful due to huge combinatorics

● We tired to look for heavier bound states (Λ and , , Λ-Λ correlationsstudiedviafemtoscopyinppreactionsat√s=7TeV,arXiv:1805.1245(submittedtoPhys.Rev.C)ΛandΛ-ΛC)̄pairthep̄Σ,p̄Ξ…),butdidn’tgetanyΞ,…), but didn’t get to any meaningful results yet.

8 October 2018, HIRG Łukasz Graczykowski (WUT) 26/27 Open questions Opened questions:

● Is the peak in minv an artifact coming from the procedure, or is it really a particle? ● why do I see a peak even though I remove all 2-body decays? Is this direct 3-body decay?

● how does this analysis relate to old K2 and K3 analyses? ● why does model reproduce Dalitz plots qualitatively?

● Is that K2/K3 being implemented in the model? ● Or maybe they come from other processes? ● how to determine what’s the internal structure of such particle (more like baryonium atom, or rather 6 mixed together)?

A few ideas for next steps: ● perform such analysis with some known, similar 3-body decay (e.g. D+) ● validate the procedure with description of known 2-body decays which were excluded ● try with a different model ● verify in the model composition of the background, then try to suppress it with cuts ● try finer z-vertex and multiplicity bins for mixing and use mixed events instead of the model ● rotate events in event mixing so that geometry is kept the same ● try to perform a spin-parity measurement ● examine other decay channels (branching ratios, more difficult to detect)

8 October 2018, HIRG Łukasz Graczykowski (WUT) 27/27 Backup

8 October 2018, HIRG Łukasz Graczykowski (WUT) 28/27 Baryon-baryon correlations

● Constraining lambda-lambda scattering parameters

● First measurement of attractive proton-Xi potential

● Constraining EoS For details see EP seminar by Laura Fabbietti https://indico.cern.ch/event/749074/

8 October 2018, HIRG Łukasz Graczykowski (WUT) 29/27

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8 October 2018, HIRG Łukasz Graczykowski (WUT) 30/27

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