Beauty 2014, Edinburgh, Scotland, July 14-18, 2014
Beauty Baryon Decays
@ LHCb
Steven Blusk
Syracuse University
On behalf of the LHCb collaboration
Beauty baryons @ LHCb
Beauty baryons are produced copiously in LHCb,
opening up new avenues for study and improved
precision of their properties and decays.
Production
Mass
Lifetime
New decay modes
CP asymmetries
Other studies, for a future BEAUTY meeting…
Rare decays, e.g. LbLm+m-.
1 fb-1 result (PLB 725, 2013) being updated to 3 fb-1.
Probe CKM elements Vcb, Vub in LbLcln and Lbpln decays
Excited b-baryon states
bc-baryons
2
Detector & data sets
 Precision vertex detector (sip~ 20 um), tracking sp/p ≈ 0.5%
 Excellent particle identification, including hadron separation to ~100 GeV
 High bandwidth trigger, ~2-5 kHz to tape, hadronic, EM and muonic final states.
 Wide & flexible physics scope: b, c, quarkonia, W, Z’s, exotics, jets, etc.
 Analyses based on:
 2011, 1 fb-1 @ 7 TeV
 2012, 2 fb-1 @ 8 TeV
3
Menu for this talk
B(LbLcp)
arXiv:1405.6842
Xb- and Wb- lifetimes
arXiv:1405.1543
Xb0 mass, lifetime & production properties
arXiv:1405.7223
Lb/Xb0  D0ph± decays
arXiv:1311.4823
LbJ/ypK- and LbJ/ypp-
arXiv:1406.0755
Search for Lb/Xb0  KSph±
arXiv:1402.0770
First observation of LbLc+ D(s)-
arXiv:1403.3606
4
2.3 m
Reconstructing hyperons from b decay
Average b-momentum in LHCb ~ 100 GeV
E.g. LbJ/yL, 30 GeV L  g≈30  <d> ≈ 2.5 m
5
2.3 m
Reconstructing hyperons from b decay
Average b-momentum in LHCb ~ 100 GeV
E.g. LbJ/yL, 30 GeV L  g≈30  <d> ≈ 2.5 m
 Hyperons reconstructed if they decay upstream of TT.
6
2.3 m
Reconstructing hyperons from b decay
 Going beyond Lb ..
 Example: Xb-J/yX-, X- Lp Get hit 2X …. t(X-)~164 ps, t(L)~260 ps
7
Production, BFs
Mass and
Lifetimes
8
Lb  Lcp BF
• State of affairs, pre-2014:
-3
B(L b0  L cp - )  5.7 +4.0
-2.6  10
LHCb-PAPER-2014-004
arXiv:1405.6842
submitted to JHEP
1 fb-1
– “Golden mode” for Lb decays only known to ≈ 70% precision!
• Use 45,000 fully reco’d LbLcp+
and 106,000 B0D-p+ decays.
9
Lb  Lcp BF
• State of affairs, pre-2014:
LHCb-PAPER-2014-004
arXiv:1405.6842
submitted to JHEP
1 fb-1
-3
B(L b0  L cp - )  5.7 +4.0
-2.6  10
– “Golden mode” for Lb decays only known to ≈ 70% precision!
• Use 45,000 fully reco’d LbLcp+
and 106,000 B0D-p+ decays.
• Anatomy:
N Lcor0 L p - ( pT )
f Lb
B(L b0  L cp - )
B(L c  pK -p  )



 pT 
N Bcor0  Dp - ( pT )
B ( B 0  D p - )
B( D   K -p p  )
fd
b
c
Measured here,
in LbLcp
decays
LHCb, PRD 85,
032008 (2012)
Scale factor between the
two analyses
Measured in
semileptonic
analysis
10
Lb  Lcp BF
• State of affairs, pre-2014:
LHCb-PAPER-2014-004
arXiv:1405.6842
submitted to JHEP
1 fb-1
-3
B(L b0  L cp - )  5.7 +4.0
-2.6  10
– “Golden mode” for Lb decays only known to ≈ 70% precision!
• Use 45,000 fully reco’d LbLcp+
and 106,000 B0D-p+ decays.
• Anatomy:
N Lcor0 L p - ( pT )
f Lb
B(L b0  L cp - )
B(L c  pK -p  )



 pT 
N Bcor0  Dp - ( pT )
B ( B 0  D p - )
B( D   K -p p  )
fd
b
c
Measured here,
in LbLcp
decays
LHCb, PRD 85,
032008 (2012)
Scale factor between the
two analyses
Measured in
semileptonic
analysis
• B(LcpKp) common to both analyses  cancelation of large syst. error
BF measured to 8% precision! Huge leap forward in precision!
11
Xb-
and
Wb-
lifetimes
LHCb-PAPER-2014-010
arXiv:1405.1543
submitted to PLB
3 fb-1
• Reconstruct Xb- J/yX-, X- Lp- and Wb- J/yW-, W- LK-, with Lpp-.
313±20
58±8
12
Xb-
and
Wb-
LHCb-PAPER-2014-010
arXiv:1405.1543
submitted to PLB
3 fb-1
lifetimes
• Reconstruct Xb- J/yX-, X- Lp- and Wb- J/yW-, W- LK-, with Lpp-.
313±20
58±8
t (X b- )  1.55-0.10
0.09  0.03 ps
t (Wb- )  1.54-0.26
0.21  0.05 ps
 Best precision to date.
 Consistent with the recent CDF results
CDF, PRD 89, 072014 (2014)
t (X b- )  1.32  0.14  0.02 ps
t (Wb- )  1.66
0.53
-0.40
 0.02 ps
(See talk by J. Rosner)
13
Studies of the
Xb0
baryon - 1
• Until recently, little known about the Xb0 baryon.
LHCb-PAPER-2014-021
arXiv:1405.7223
Accepted in PRL
3 fb-1
– Lifetime unmeasured
– Mass only known to ~2.5 MeV (Lb mass known to ~0.3 MeV)
– Relative production rates ? Are prod. properties (pT, h) similar to Lb ?
14
Studies of the
Xb0
baryon - 1
• Until recently, little known about the Xb0 baryon.
LHCb-PAPER-2014-021
arXiv:1405.7223
Accepted in PRL
3 fb-1
– Lifetime unmeasured
– Mass only known to ~2.5 MeV (Lb mass known to ~0.3 MeV)
– Relative production rates ? Are prod. properties (pT, h) similar to Lb ?
• In Xb, Wb studies, decays with J/y+hyperons have been ‘the work horse’.
– Use CF decays, J/y for triggering, well-displaced vertices from hyperons with narrow
widths. Hadron ID helpful, but not crucial.
– In LHCb, large boosts, large ct leads to a reduction in total efficiency for hyperon reco.
15
Studies of the
Xb0
baryon - 1
• Until recently, little known about the Xb0 baryon.
LHCb-PAPER-2014-021
arXiv:1405.7223
Accepted in PRL
3 fb-1
– Lifetime unmeasured
– Mass only known to ~2.5 MeV (Lb mass known to ~0.3 MeV)
– Relative production rates ? Are prod. properties (pT, h) similar to Lb ?
• In Xb, Wb studies, decays with J/y+hyperons have been ‘the work horse’.
– Use CF decays, J/y for triggering, well-displaced vertices from hyperons with narrow
widths. Hadron ID helpful, but not crucial.
– In LHCb, large boosts, large ct leads to a reduction in total efficiency for hyperon reco.
• To play into LHCb’s strengths, use Cabibbo-suppressed decays … no hyperons!
X  X p , X  pK p
0 CF
b

c
-
 CS
c
-

B(X c  pK -p  )
~ 0.34
B(X c  X -p p  )B(X -  Lp - ) B(L  pp - )
• Despite 3 X lower BF, still win big in total acceptance.
16
Studies of the
Xb0
baryon - 1
• Until recently, little known about the Xb0 baryon.
LHCb-PAPER-2014-021
arXiv:1405.7223
Accepted in PRL
3 fb-1
– Lifetime unmeasured
– Mass only known to ~2.5 MeV (Lb mass known to ~0.3 MeV)
– Relative production rates ? Are prod. properties (pT, h) similar to Lb ?
• In Xb, Wb studies, decays with J/y+hyperons have been ‘the work horse’.
– Use CF decays, J/y for triggering, well-displaced vertices from hyperons with narrow
widths. Hadron ID helpful, but not crucial.
– In LHCb, large boosts, large ct leads to a reduction in total efficiency for hyperon reco.
• To play into LHCb’s strengths, use Cabibbo-suppressed decays … no hyperons!
X  X p , X  pK p
0 CF
b

c
-
 CS
c
-

B(X c  pK -p  )
~ 0.34
B(X c  X -p p  )B(X -  Lp - ) B(L  pp - )
• Despite 3 X lower BF, still win big in total acceptance.
• Excellent normalization mode (identical final state): LbLcp, LcpKp !
– Most systematics cancel at 1st order in relative measurements.
– Small differences in momentum spectra and lifetime of Xc vs Lc pKp
17
Studies of the
 Precision mass
LbLc+p-
Xb0
baryon - 2
LHCb-PAPER-2014-021
arXiv:1405.7223
Accepted in PRL
3 fb-1
Xb0Xc+p~188,000 LbLc+p~3,800 Xb0Xc+pMass of Xb0 ~5X more precise than prev. best from
LHCb
0
0
M (X b - L b )  172.44  0.39  0.17 MeV
M(X b0 )  5791.80  0.39  0.17  0.26L0 MeV
b
18
Studies of the
 Precision mass
LbLc+p-
Xb0
LHCb-PAPER-2014-021
arXiv:1405.7223
Accepted in PRL
3 fb-1
baryon - 2
Xb0Xc+p~188,000 LbLc+p~3,800 Xb0Xc+pMass of Xb0 ~5X more precise than prev. best from
LHCb
0
0
M (X b - L b )  172.44  0.39  0.17 MeV
M(X b0 )  5791.80  0.39  0.17  0.26L0 MeV
b
 Lifetime of Xb0 baryon measured relative to that of Lb.
 Lb lifetime now known to high precision (see talk by P. Gandini)
 Perform sim. fit in bins of decay time, correct by relative efficiency (from simulation)
N cor (X b0 )
 e t
0
N cor (L b )
=
1
tL
o
b
-
1
tX
o
b
1st measurement of Xb0 lifetime
t (Xb0 ) / t (L b0 )  1.006  0.018  0.010
t (Xb0 )  1.477  0.026  0.014  0.013L ps
0
b
 Lb and Xb0 lifetimes equal to within 2%
 Consistent with expectations from HQE
19
Studies of the
Xb0
baryon - 3
 Can also study the relative production rate vs (pT, h)
LHCb-PAPER-2014-021
arXiv:1405.7223
Accepted in PRL
3 fb-1
Semileptonic
Steep pT dependence
of Lb/B0 production
also in Xb0!
LbLcp+ scaled
(see talk by Maria Zangoli)
 Relative production rate, integrated.
≈1
≈0.1
Consistent with f X0 / f L0  0.2 with above assumptions
b
b
20
Studies of the
Xb0
baryon - 3
 Can also study the relative production rate vs (pT, h)
LHCb-PAPER-2014-021
arXiv:1405.7223
Accepted in PRL
3 fb-1
Semileptonic
Steep pT dependence
of Lb/B0 production
also in Xb0!
LbLcp+ scaled
(see talk by Maria Zangoli)
 Relative production rate, integrated.
≈1
≈0.1
Consistent with f X0 / f L0  0.2 with above assumptions
b
b
 We also have a VERY CLEAN sample of Xc+ decays
Mass of Xc+ ≈4X more precise than WA
M (X c - L c )  181.51  0.14  0.10 MeV
M(X c )  2467.97  0.14  0.10  0.14L MeV
c
21
New decay
modes & CPV
studies
22
Lb/Xb0  D0ph±
 The LbD0pK decay can be used to measure the weak phase g, ala
LHCb-PAPER-2013-056
arXiv:1311.4823
PRD 89, 032001 (2014)
1 fb-1
0
0
*0
B D K .
Usual trick..
• Both diagrams are O(l3)
• Exploit interference for final
states common to D0 and D0.
23
Lb/Xb0  D0ph±
 The LbD0pK decay can be used to measure the weak phase g, ala
LHCb-PAPER-2013-056
arXiv:1311.4823
PRD 89, 032001 (2014)
1 fb-1
0
0
*0
B D K .
Usual trick..
• Both diagrams are O(l3)
• Exploit interference for final
states common to D0 and D0.
 Start with CF D0Kp decay, to first observe these decays.
24
Lb/Xb0  D0ph±
 The LbD0pK decay can be used to measure the weak phase g, ala
LHCb-PAPER-2013-056
arXiv:1311.4823
PRD 89, 032001 (2014)
1 fb-1
0
0
*0
B D K .
Usual trick..
• Both diagrams are O(l3)
• Exploit interference for final
states common to D0 and D0.
 Start with CF D0Kp decay, to first observe these decays.
 Several first observations in Lb and Xb0 decays!
 Good prospects for g with larger data set
25
LbJ/ypK- and LbJ/ypp CP violation not yet observed in the baryon sector.
LHCb-PAPER-2014-020
arXiv:1406.0755|
Submitted to JHEP
3 fb-1
 The decay LbJ/ypK recently observed by LHCb  most precise t(Lb) (see P. Gandini talk).
 The CS decay, LbJ/ypp-, unobserved prior to this analysis, has 2 amplitudes, tree & penguin
with different weak phase  NP can enter in the penguin and give rise to direct CPV.
26
LbJ/ypK- and LbJ/ypp-
LHCb-PAPER-2014-020
arXiv:1406.0755|
Submitted to JHEP
3 fb-1
 CP violation not yet observed in the baryon sector.
 The decay LbJ/ypK recently observed by LHCb  most precise t(Lb) (see P. Gandini talk).
 The CS decay, LbJ/ypp-, unobserved prior to this analysis, has 2 amplitudes, tree & penguin
with different weak phase  NP can enter in the penguin and give rise to direct CPV.
 First observation of LbJ/yppB(L b0  J /y pp - )
 (8.24  0.25  0.42)%
B(L b0  J /y pK - )
 Direct CPV wrt favored mode
(|P/T|SM <<1)
A CP  A(L b0  J /y pp - ) - A(L b0  J /y pK - )
 (5.7  2.3  1.2)%
27
Charmless: Search for Lb/Xb 
0
KSph±
LHCb-PAPER-2013-061
arXiv:1402.0770
JHEP 04 (2014) 087
1 fb-1
Norm. mode
Signal channels
• Prior to this analysis, no 3-body charmless decays of b-baryons had been observed.
Like B decays, 3-body charmless b-baryon decays contains penguin contributions NP can enter.
• Opens up an interesting window to explore CPV in the b-baryon sector across the Dalitz plot.
• BFs normalized to the kinematically similar B0KSp+p- decay.
First
Observation
M(KSpp) [ MeV ]
M(KSpp) [ MeV ]
M(KSpK) [ MeV ]
M(KSpK) [ MeV ]
M(KSpp) [ MeV ]
M(KSpp) [ MeV ]
28
Charmless: Search for Lb/Xb 
0
LHCb-PAPER-2013-061
arXiv:1402.0770
JHEP 04 (2014) 087
1 fb-1
KSph±
Signal channels
• Prior to this analysis, no 3-body charmless decays of b-baryons had been observed.
Like B decays, 3-body charmless b-baryon decays contains penguin contributions NP can enter.
• Opens up an interesting window to explore CPV in the b-baryon sector across the Dalitz plot.
• BFs normalized to the kinematically similar B0KSp+p- decay.
B(L b0  K S0 pp - )
 0.25  0.04  0.02  0.02 fLb / fd
B( B 0  K S0p p - )
B(L b0  K S0 pK - )
 0.07 @90% CL
B( B 0  K S0p p - )
First
Observation
M(KSpp) [ MeV ]
M(KSpp) [ MeV ]
f X0 B(X 0  K 0 pp - )
b
S
b
 0.03 @90% CL
0
0  f d B( B  K S p p )
f X0 B(X 0  K 0 pK - )
b
S
b
 0.02 @90% CL
0
0  f d B( B  K S p p )
See paper for additional Lb BFs
Norm. mode
M(KSpK) [ MeV ]
M(KSpK) [ MeV ]
 Measure direct CPV relative to CF mode
LbLc(pKS)p- (penguins should be negligible)
A CP  A(L b0  K S0 pp - ) - A(L b0  L c ( K S0 p)p - )
 0.22  0.13  0.03
Consistent with zero.
M(KSpp) [ MeV ]
M(KSpp) [ MeV ]
29
LbLc+ D(s)-
 First observation of b-baryons decaying to double-charm final states
LbLc+Ds-
Yield~4,600
LHCb-PAPER-2014-002
arXiv:1403.3606
PRL 112, 202001 (2014)
3 fb-1
LbLc+D-
Yield~260
Consistent with expected Cabibbo suppression
Bonus: Get most precise Lb mass measurement from measured mass difference M(Lb )– M(B0):
30
LbLc+ D(s)-
 First observation of b-baryons decaying to double-charm final states
LbLc+Ds-
Yield~4,600
LHCb-PAPER-2014-002
arXiv:1403.3606
PRL 112, 202001 (2014)
3 fb-1
LbLc+D-
Yield~260
Consistent with expected Cabibbo suppression
Bonus: Get most precise Lb mass measurement from measured mass difference M(Lb )– M(B0):
 B(LbLcDs): Normalize to B(B0D-Ds+), but relative yield strong function of pT.
 Measure double ratio, same pT dependence, which cancels in ratio.
LHCb
L b0  L c DsB 0  D  Ds-
Several world best BF measurements
L b0  L cp B 0  D p -
31
Summary
• b-baryon rate at LHCb very large, 1 Lb for every 2 B0!
• Production, decays, b-baryon properties and CPV are
beginning to be probed.
– Lifetime of Xb0 went from unmeasured  2% precision (t(Lb) known to ~1%)
– Mass of Xb0 went from ±2.5 MeV  ~0.4 MeV, almost on par with Lb!
• Other high yield modes of b-baryons are also being explored that
exploit the excellent PID in LHCb.
• Expect more on rare decays, Vcb, Vub,
excited baryonic states soon..
The future is bright for
b-baryon studies @ LHCb.
32