Production and decay of heavy flavour baryons

Nicola Neri on behalf of the LHCb collaboration presenting also result from Atlas and CMS

INFN - Sezione di Milano

FPCP 2015 25-29 May 2015 - Nagoya, Japan Outline

‣ Introduction to b-baryon physics ‣ Experimental results ‣ Conclusions

Nicola Neri 2 Production and decay of HF baryons - FPCP 2015 Introduction to b-baryon physics

Nicola Neri 3 Production and decay of HF baryons - FPCP 2015 Physics motivations ‣ At LHC b-baryons are produced in unprecedented quantities ➝ opens a new field in flavour physics for precision measurements 0 - Most precise measurement of |Vub| using ⇤ b pµ⌫¯µ decays [arXiv:1504.01568] ! - Mass, lifetimes and branching ratios measurements ‣ b-baryon physics is a relatively unexplored territory: - search for physics beyond the Standard Model (SM) in rare decays and CP violation - useful QCD laboratory in different energy regime with respect to light baryons. Experimental anchor point for QCD models

Nicola Neri 4 Production and decay of HF baryons - FPCP 2015 Beautiful baryons

‣ Baryons are fermions composed by 3 quarks

‣ State function antisymmetric under exchange of equal-mass quarks qqq = color space, spin, flavour | iA | iA ⇥ | iS ‣ SU(4) multiplets for baryons made of (u,d,s,b) observed quarks

- B= bottomness, Y=hypercharge, I3= Isospin z-component

- SU(4) symmetry heavily broken - large b-quark mass

- particles in SU(4) multiplets have same spin and parity

Nicola Neri 5 Production and decay of HF baryons - FPCP 2015 b-baryons states

q2 s, ` S, J

‣ b-baryons ( bq 1 q 2 ) as QCD L = ` + `0 `0 laboratory: q1

- m b m q 1 q 2 simplified dynamics b ground state p + + - b quark in the limit m b j =0 , 1 j = L + s (L = 0) !1(m 4.8GeV) effective static colour field b ⇠ - heavy baryon properties determined by dynamics of diquark system in b- quark color field

- Ground state baryons ( L = 0) with spin-parity J P =1 / 2 + , 3 /2+ characterised by the spin-party of p + + the diquark system j =0 , 1 1 + 3 + J P = (J P = not shown) 2 2

Nicola Neri 6 Production and decay of HF baryons - FPCP 2015 b-baryon rare decays

0 ⇤ pµ⌫¯ ‣ Access to |Vub| CKM element, b ! µ form factors f ( ⇤ b p ) determined by Lattice! QCD. Experimentally very challenging, Vub achieved unprecedented See Greg Ciazerek talk for precision more details 3 V =(3.27 0.23) 10 | ub| ± ⇥ arXiv:1504.01568, submitted to Nature 0 + ‣ Angular analysis of b-baryon ⇤b ⇤µ µ flavour-changing neutral current ! decays, e.g. b s transitions, is sensitive to physics! beyond Standard Model (SM)

arXiv:1503.07138, submitted to JHEP See Christian Linn talk for more details

Nicola Neri 7 Production and decay of HF baryons - FPCP 2015 Experimental results

Nicola Neri 8 Production and decay of HF baryons - FPCP 2015 b-baryon production

‣ Production cross-section strongly depends on pT of b-hadron :

f 0 /fd - different b-quark fragmentation function ratio ⇤ b measured at 0 0 LEP and at LHC, where f ⇤ 0 = P ( b ⇤ b ) and fd = P (b Bd) b ! !

f 0 /fd - measurement of ⇤ b vs pT of b-quark is cleaner to interpret. Expected a slow dependence in that case arXiv:1505.02771

JHEP 08, 143 (2014) PRD85, 032008 (2012)

Note: LEP average not included in the fit. LHCb measurements are not independent

Nicola Neri 9 Production and decay of HF baryons - FPCP 2015 Production kinematic dependence

0 + 0 + ‣ Use clean ⇤ ⇤ ⇡ (45K) , B D ⇡ (106K) exclusive b c ! decays to measure! dependance of f 0 /f d on b-hadron ⇤b kinematics, e.g. pT, pseudorapidity η.

‣ Measure

where and x = pT , ⌘

Data sample 1fb-1 at 7 TeV - JHEP08(2014)143

0 + 3.05 < ⌘ < 3.2 0 + ⇤b ⇤c ⇡ B D ⇡ 3.05 < ⌘ < 3.2 ! !

Nicola Neri 10 Production and decay of HF baryons - FPCP 2015 Production kinematic dependence

f 0 /fd ‣ Absolute value of ⇤ b from LHCb semileptonic analysis Phys. Rev. D 85 (2012) 032008

- obtain most precise branching ratio measurement of b-baryon to date (8% precision)

0 + stat. syst. semileptonic (B D ⇡) JHEP08(2014)143 analysis B !

Exponential dependence vs pT Linear dependence vs η

Nicola Neri 11 Production and decay of HF baryons - FPCP 2015 0 ⇤ b production cross-section 0 ‣ Measurement of differential production cross-section for ⇤ b 0 + using ⇤ J/ ⇤ decays with J/ µ µ, ⇤ p⇡ b ! ! ! PLB 714 (2012) 136–157

‣ pT distribution falls faster than measured b-mesons spectra and than predicted spectra from NLO MC POWHEG and leading-order MC PYTHYA

0 0 ‣ Cross-section ratio ( ⇤ b ) / ( ⇤ b ) consistent with 1 and constant vs pT, and rapidity |y|

Nicola Neri 12 Production and decay of HF baryons - FPCP 2015 0 Observation of ⌅b⇤ ‣ New resonant state compatible with 0 3 + ⌅⇤ (usb) J P = ,j =1 b 2 ‣ Signal reconstruction involves three 21 signal events significance >5σ secondary vertices, ⌅ b , ⌅ , ⇤ and a dimuon pair

00 ‣ Theory predicts m ( ⌅ ) m ( ⌅ )

p ⇡ PRL 108, 252002 (2012)

0 ⇤ ⇡ ⌅b m = m(⌅⇤ ) m(⌅) m + = 14.84 0.74 0.28 MeV b b ⇡ ± ± µ 108 14 + 0 µ J/ ± m(⌅⇤ ) = 5945.0 0.7 0.3 2.7(PDG) MeV b ± ± ± ⌅ 0 ⌅ + (⌅b⇤ )=2.1 0.74 MeV b ⇡ ± 0 ⌅b⇤ Nicola Neri 13 Production and decay of HF baryons - FPCP 2015 Two new ⌅ b baryon resonances

1 + 0 (dsb) J P = ,j =1 ‣ Study ⌅ b ⇡ s mass spectrum: 2 ⌅b0 2 3 + new resonances consistent with ( dsb ) J P = ,j =1 ⌅b⇤ 2

121 12 ±

237 24 ±

-1 0 0 3fb data at 7-8 TeV - PRL 114, 062004 (2015) m = m(⌅ ⇡) m(⌅ ) m b b ⇡

Nicola Neri 14 Production and decay of HF baryons - FPCP 2015 ⌅ b helicity angle distributions ‣ Flat distribution if J=1/2 or J>1/2 but zero longitudinal polarisation

‣ Cannot determine J value. However, data are consistent with quark model predictions J=1/2 and J=3/2 (if not or weakly polarised)

PRL 114, 062004 (2015)

0 ⌅⇤ ⌅b b

⇡

a =0.89 0.11 a =0.88 0.11 ± + ± ⌅c ✓ Helicity angle definition h 0 ⇡ (0, ) ⌅b ⇤ ⇡ ⌅b Nicola Neri 15 Production and decay of HF baryons - FPCP 2015 0 ⌅ b lifetime and mass measurements 3fb-1 data at 7-8 TeV - PRL 113, 032001 (2014) ‣ Measure lifetime relative to ⇤ 0 and mass difference m ( ⌅ 0 ) m ( ⇤ 0 ) b b b 0 + + + 0 + + + ⇤ ⇤ ⇡, ⇤ pK⇡ ⌅ ⌅ ⇡, ⌅ pK⇡ b ! c c ! b ! c c !

(180.5 0.5) 103 3775 71 ± ⇥ ±

‣ Ratio of efficiency corrected yields vs time

(1/⌧⇤0 1/⌧⌅0 )t N 0 (t)/N 0 (t)=e b b ⌅b ⇤b Lifetime ratio from a fit using exponential function

Nicola Neri 16 Production and decay of HF baryons - FPCP 2015 0 ⌅ b lifetime and mass measurements

PRL 113, 032001 (2014) 0 ⇤b lifetime measurements ⌧⇤0 =1.479 0.009 0.010 ps b ± ± PLB, 734, (2014) 122

‣ Measure ⌧⇤0 =1.449 0.036 0.017 ps b ± ± first measurement PRD 87, 032002 (2013)

⌧⇤0 =1.503 0.052 0.031 ps b ± ± JHEP 07 (2013) 163

0 0 2 ‣ Measure m ( ⌅ b ) m ( ⇤ b ) = 172 . 44 0 .39 0.17 MeV/c x4± precision± improvement 0 ‣ Measure relative production rate ⇤ b mass measurements m⇤0 = 5619.36 0.26 MeV b ± PRL 112, 202001 (2014)

m⇤0 = 5619.7 0.7 1.1 MeV assuming naive Cabibbo factors b ± ± PRD 87, 032002 (2013) obtain f(⌅0)/f(⇤0) 0.2 b b ⇡

Nicola Neri 17 Production and decay of HF baryons - FPCP 2015 lifetime and mass measurements ⌅b

3fb-1 data at 7-8 TeV - PRL 113, 242002 (2014)

0 0 ‣ Measure lifetime relative to ⇤ and mass difference m ( ⌅ ) m(⇤ ) b b b

0 + + + 0 0 + ⇤ ⇤ ⇡, ⇤ pK⇡ ⌅ ⌅ ⇡, ⌅ pKK⇡ b ! c c ! b ! c c !

(220.5 0.5) 103 1799 46 ± ⇥ ±

Nicola Neri 18 Production and decay of HF baryons - FPCP 2015 ⌅ lifetime and mass results b PRL 113, 242002 (2014) Ratio of efficiency vs decay time Ratio of efficiency corrected yields vs decay time

‣ Measure x2 precision improvement

0 ‣ Using LHCb results relative to ⌅ b we obtain

‣ Results consistent with predictions from heavy quark expansion (HQE)

Nicola Neri 19 Production and decay of HF baryons - FPCP 2015 0 Observation of ⇤ J/ p ⇡ decay b ! - Large interference between tree and penguin amplitudes. 0 JHEP07(2014)103 Measure relative BR wrt ⇤ J/ pK and search for CPV b !

3 3 V V VtbVtd ( Vus = ) / cb cd ⇠ / ⇠ | | 0 0 ⇤ J/ p⇡ 2102 61 ⇤b J/ pK11179 109 b ! ± ! ±

Nicola Neri 20 Production and decay of HF baryons - FPCP 2015 Search for CP violation

A 3fb-1 data at 7-8 TeV ‣ Measurement of CP cancel production and proton JHEP07(2014)103 reconstruction asymmetries 0 0 0 + (⇤ J/ ph)= (⇤ J/ ph)+ (⇤ ) (h )+ (p) Araw b ! ACP b ! Aprod b Areco Areco 0 0 A = (⇤ J/ p⇡) (⇤ J/ pK) CP Araw b ! Araw b ! 0 0 + + = (⇤ J/ p⇡) (⇤ J/ pK)+ (⇡ ) (K ) ACP b ! ACP b ! Areco Areco 0 0 (5.7 2.4 1.2)% 2.2 from zero raw(B J/ K⇤ )=( 1.10 0.32 0.0.6)% ± ± ⇡ A ! ± ± ‣ No indications of large local CP asymmetries in Dalitz plane

‣ Rich resonant structure in m ( p ⇡ ) , no evidence for exotics in m(J/ p),m(J ⇡)

‣ BR compatible with expected value 0.08: CKM x phase space factor

Nicola Neri 21 Production and decay of HF baryons - FPCP 2015 0 (0) Search for ⇤ b ⇤ ⌘ decays

! arXiV:1505.03295 ‣ b-baryons decays to final states with ⌘ , ⌘ 0 not yet Submitted to JHEP observed. From BR measurements determine ⌘ ⌘ 0 mixing

Dominant EW loop diagram Non-spectator diagram Anomalous diagram 1 ¯ ‣ Consider ⌘ , ⌘ 0 as admixture of light ⌘ q = u u¯ + d d , | i p2| i strange ⌘ s = s s¯ quark states, and gluons gg : | i | i | i ⌘0 cos sin ⌘ + cos cos ⌘ +sin gg | i' G P | qi G P | si G| i ⌘ cos ⌘ sin ⌘ | i' P | qi P | si

0 (0) ‣ Mixing parameters determined in B ( s ) J/ ⌘ decays +1.5 ! P = (43.5 2.8) G =(0 25) JHEP01(2015)024 ± Nicola Neri 22 Production and decay of HF baryons - FPCP 2015 Analysis strategy and event selection

3fb-1 data at 7-8 TeV - arXiV:1505.03295, submitted to JHEP

‣ Measure BR of ⇤ ⇤ ⌘ 0 + and ⇤ ⇤ ⌘ + 0 b ⇡ ⇡(,⌘) b ⇡ ⇡⇡ 0 0! ! relative to B K ⌘0 + S ⇡ ⇡ 0 !+ ‣ Long lived K ⇡ ⇡ and ⇤ p ⇡ are divided in Long and S ! ! Downstream categories if produce hits in the vertex detector or not. Different track resolution and selection optimisation

‣ Full decay chain refitted , primary vertex with tracks not from b- 0 hadron decays, fix to nominal value the mass of ⇤,KS, ⌘, ⌘0 (m) = 30.0 1.5 MeV/c2 (m) = 29.4 1.3 MeV/c2 ± ±

Preliminary Preliminary 188 16 149 14 ± ±

Nicola Neri 23 Production and decay of HF baryons - FPCP 2015 Fit results 3fb-1 data at 7-8 TeV - arXiV:1505.03295, submitted to JHEP

⇤b ⇤⌘0 + ⇤ ⇤⌘0 + ⇡ ⇡ 1.0 4.4 b ⇡ ⇡ ⌘ 4.2 2.3 ! ± ! ±

Preliminary Preliminary Preliminary Preliminary

⇤b ⇤⌘⇡+⇡ ⇡0 5.3 3.8 3 significance ! ±

Preliminary Preliminary Use Feldman, Cousins unified method approach for CL Phys. Rev. D57 (1998) 3873

0 0 ‣ Measure ratio of BR and use known (B K ⌘0) B ! S

68% CL

Nicola Neri 24 Production and decay of HF baryons - FPCP 2015 0 |Vub| measurement with ⇤ pµ⌫¯ b ! µ 2fb-1 data at 8 TeV - arXiv:1504.01568 0 + ‣ Normalise yields to ⇤ b ⇤ c µ ⌫ ¯ µ , Vcb mediated decay, cancel! many systematic uncertainties

‣ Apply tight vertex cut, PID on proton and muon, track isolation to reject 90% of See Greg Ciezarek talk background (using boosted decision tree) for more details ‣ Use corrected mass to reconstruct the signal and retain events with (Mcorr) < 100MeV

2 2 Mcorr = p + Mpµ + p ? ? q 0 ‣ Use ⇤ b flight direction and mass to determine q2 with two-fold ambiguity (neutrino). Require both solutions >15 GeV2, minimise migration to low q2

Nicola Neri 25 Production and decay of HF baryons - FPCP 2015 |Vub| results

2fb-1 data at 8 TeV - arXiv:1504.01568 ‣ Measure: 17687 733 0 ± (⇤ pµ⌫¯ ) 2 2 2 2 B b ! µ q >15GeV Vub = Vcb 0 + RFF | | | | (⇤ ⇤c µ ⌫¯ ) 2 2 B b ! µ q >7GeV world average measured LQCD [1] 3 2 (39.5 0.8) 10 (1.00 0.04 0.08) 10 0.68 0.07 ± ⇥ ± ± ⇥ ± [1] W. Detmold, C. Lehner, and S. Meinel, arXiv:1503.01421

Most precise measurement 3 V =(3.27 0.15 0.17 0.06) 10 | ub| ± ± ± ⇥ exp. LQCD |Vcb|

‣ Background contributions estimated 34255 571 using ad hoc control samples ±

+ + ‣ Largest exp. uncertainty from (⇤ pK ⇡) B c !

Nicola Neri 26 Production and decay of HF baryons - FPCP 2015 |Vub| puzzle

~3.5σ tension between exclusive LHCb measurement does not and inclusive measurements support explanation based on right handed current added to SM

PDG Inclusive 2014

PDG 2014 Exclusive arXiv:1501.05373 (RBC/UKQCD) (B→πlν) arXiv:1503.07839 (FNAL/MILC)

LHCb arXiv:1503.01421 0 (RBC/UKQCD) (Λ b→pµν)

0.003 0.0035 0.004 0.0045 0.005

|Vub |

Nicola Neri 27 Production and decay of HF baryons - FPCP 2015 Conclusions

‣ At the LHC, b-baryons represent a new field in flavour physics for precision measurements. However, it is a relatively new territory for experiments and theory ‣ Precision measurements of mass, lifetimes and BR provide experimental anchor points for theory and QCD models. Rare decays and CPV are sensitive to physics beyond SM

‣ |Vub| measurement using Λb is an outstanding example of advancement of both experimental techniques and LQCD calculations, providing a stringent test of SM. Others are foreseen

‣ Be prepared to be surprised by b-baryon physics in the near future!

Nicola Neri 28 Production and decay of HF baryons - FPCP 2015 Backup slides

Nicola Neri 29 Production and decay of HF baryons - FPCP 2015 Parity violation in ⇤0 J/ ⇤ b ! ‣ Parity violation is not maximal in hadron weak decays and depends on hadron constituents. In b-baryons can be predicted by perturbative QCD (pQCD) and heavy quark effective theory (HQET) polarisation 1 w(cos ✓)= (1 + ↵P cos ✓) angle 2 parity-violation 0 ‣⇤ b polarisation allowed only to be perpendicular to production plane, due to parity conservation in pp strong interaction ‣ Use 4 helicity amplitudes to describe the ⇤ 0 J/ ⇤ decay b ! A(⇤, J/ ):a+ = A(1/2, 0),a = A( 1/2, 0), b+ = A( 1/2, 1),b = A(1/2, 1) Nicola Neri 30 Production and decay of HF baryons - FPCP 2015 Parity violation results

4.6fb-1 data at 7 TeV - PRD 89, 092009 (2014) ‣

=0 in a symmetric interval in pseudorapidity

‣ Assume CP conservation and extract ↵ from a simplified angular analysis with 5 independent parameters 1240 40 ± 2 2 2 2 ↵ = a+ a + b+ b | | | | | | | | =0.30 0.16 0.06 ± ±

‣ Consistent with LHCb measurement Negative helicity states for ⇤ are preferred. ↵ =0.05 0.17 0.07 PLB 724, 27 (2013) but not with± pQCD± [-0.17,-0.14] and HQET predictions 0.78

‣ LHCb measured P =0.06 0.07 0.02 ± ±

Nicola Neri 31 Production and decay of HF baryons - FPCP 2015 LHCb detector

T Stations Tracker Turicensis (TT)

VELO

Nicola Neri 32 Production and decay of HF baryons - FPCP 2015 LHCb tracking system TT: 500μm thick, single sided Si strip detector, pitch~100-200μm, vertical and stereo angle strips arrangement (x-u-v-x)=(0∘,-5∘,+5∘,0∘)

Tracker Turicensis~8.4 m2 Si Inner Tracker~4.0 m2 Si

VELO Outer Tracker Gas drift time detector Argon (70%), C02 (30%) 200μm position resolution

B field bending power 3.7Tm

Nicola Neri 33 Production and decay of HF baryons - FPCP 2015 Track definitions at LHCb

Ghost track = is a fake track. For example it can be formed by matching a real track segment in the VELO (VELO seed) with a real track segment in the downstream tracker (T seed)

Nicola Neri 34 Production and decay of HF baryons - FPCP 2015