Federica Legger Polarized radiative b decays at LHCb
Federica Legger
Polarized radiative b decays at LHCb
2
Outline
Theoretical motivations Angular distributions Observables b production at LHC Selection of b (x)
events at LHCb Status and perspectives
3
Theoretical motivations
b p
s
d u
b
d
u
b
b (X) p k
s
d
u
b
d
u
b
u
up
k
Electromagnetic penguin b s In the SM the photon is predicted to be left-
handed, but could have a right-handed component in LR symmetric models;
Effective Hamiltonian at LO in s: left right
4
Photon polarization measurements
Melikov, Nikitin, Simula, PLB 442, 381 (1998)
Grossman, Pirjol, JHEP06, 029 (2000)
Atwood, Gronau, Soni, PRL 79, 185 (1997)
Mannel, Recksiegel, JPG: NPP 24, 979 (1998)
LHC
bB
fact
ori
es
Knecht, Schietinger, PLB 634, 403 (2006)
Gronau, Pirjol, PRD 66, 054008 (2002)
B-B interference
First measurements of K* polarization in B->K*l+l- by Belle/Babar
e+e- conversion
Exp. status Theor. Refs.
Latest world averagesin2 = 0.0 ± 0.3
Higher K* resonances
Difficult to disentangle resonance structure (BaBar, hep/0507031)
Gronau, Grossman, Pirjol, PRL 88, 051802 (2002)
Charmonium res. interference
No results so far...
b-baryonsHiller, Kagan, PRD 65, 074038 (2002)
Exploit ang. correlations between polarized initial state and final state. Under study at LHCb
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Polarized b (1115) decays
Angular distributions for b ((1115) p)
depend on photon polarization
and constrain
Hiller, Kagan, Phys Rev D65, 074038 (2002)
PB = b polarization p = weak decay
parameter
Evtgen distribution
6
2 indipendent measurements for r, or
b polarization measurement: a discrepancy with the value measured in semileptonic b
c l X decays would indicate the presence of non-standard right-handed b c currents
Direct CP violation at NLO: O(1%) in SM but 10% if NP! r probes the ratio of CP even contributions to NLO
Hamiltonian
Observables for b (1115)
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(X) resonances
(X) parameters (PDG 2004) + BR(b (X) ) (slide 16)
1690
1520
1670
1600
spin = 1/2
spin = 3/2
From the experimental point of view the decay b (1115) is quite hard to observe (c = 7.89 cm)
Can we increase the statistics by using heavier resonances?
8
Need angular distributions helicity formalism
Helicity formalism for b (px)
b polarization
Polarization density matrix :
Helicity amplitudes b (X) (X) px
+ ½- -½
Jacob, Wick, Ann Phys 7, 404 (1959)
9
Results : J = 1/2
Photon angular distribution depends on photon helicity parameter which is related to |r|
Proton angular distribution flat because of P conservation
Helicity formalism
HQET
10
Results : J = 3/2
helicity can now assume the values: ±1/2, ±3/2
4 helicity amplitudes
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Helicity formalism : J = 3/2
Decay probability:
12
Similar dependence to spin 1/2 resonances
but now depends on the asymmetry of b baryons produced with different helicities
Photon angular distribution
13
Photon helicity is independent of the helicity of the final state formed in the hadronization process
Because of parity conservation in strong interactions, the ratio of baryons produced with helicity 3/2 and 1/2 = ratio of baryons produced with helicity -3/2 and -1/2
Assumptions
14
Photon angular distribution dependence can be factorized with the photon helicity parameter and strong parameter
no theoretical predictions for … but we can extract it from the proton
angular distribution
Results : J = 3/2
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(X) with helicity 3/2 dominates » 1
same amount of 1/2 and 3/2 helicity 1
(X) with helicity 1/2 dominates « 1
Possible scenarios for
3/2 - 1/2 p3/2 1
3/2 1/2 p3/2 -3
3/2 p3/2 0
|r| can be probed by measuring 3/2 and
SM prediction
16
b production at LHC:
bb cross section in pp collision = 500 b 10% of produced bb hadronize in B hadrons b dominates (90%) b produced with transversal polarization
Predictions are PB ~ 20%
ATLAS plans to measure it with a statistical precision better than 1%
BR (b (1115) = 4.15 · 10-5
BR (b (1520) = 1.30 · 10-5
BR (b (1670) = 0.70 · 10-5
BR (b (1690) = 0.70 · 10-5
p1 p2
bn
Ajaltouni, Conte, Leitner, ‘‘Λb into Λ-vector decays’’, Phys Lett B, 614 (2005)
Feasibility of Beauty Baryon Polarization Measurement in b Jdecay channel by ATLAS – Atlas note
Calculations based on Hiller (2002)+PDG2004
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Data sample & Tools
DaVinci v12r16
No particular method to optimize cuts values: PT, IPS (with respect to all primaries) cuts on final states Mass window = 4 for resonances with intrinsic width Cut values chosen to kill bb events while maintaining
higher possible efficiencies
b (1115) pol = long, full300k evts b (1670) pol = long, full300k evts b (1670) pol = transv, full300k evts b (1670) phsp, full300k evts
bb inclusive (DC04-v2) 39M evts
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(1115) p c = 7.89 cm About 14% of interact before decay or decay
after LHCb spectrometer lost 305000 (generated) 262464 (DoI)
T1 T2 T3
VELO
TTT track
Upstream track
Long track
Downstream track
Velo track
BY (T)
z (m)
- 0.2
- 0.4
- 0.6
- 0.8
- 1.0
- 1.2
0
0 2 4 6 8
LL23%
UD1%
UU1%
LU9%
LD3%
DD63%
candidates (associated to MC truth)
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Vertex fit for b (1115)
(1115) vertex: refit the vertex explain apply cut on mass and unconstrained chi
square
b vertex = (1115) + photon fit: PV + the direction Choose the PV with minimum chi square
p
PV
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UP DLL p- > 6 DLL p-k > 0 PT > 500 MeV sIPS > 4
Charged tracks selection ((1115))
p:
DOWN DLL p- > 10 DLL p-k > 8 PT > 2500 MeV sIPS > 3
LONG DLL p- > 6 DLL p-k > 4 PT > 1600 MeV sIPS > 4
UP PT > 250 MeV sIPS > 4
:DOWN PT > 350 MeV sIPS > 3
LONG PT > 350 MeV sIPS > 4
Hard DLL and PT cuts on protons to suppress background
Slow momentum pions
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UL 2 < 2 m < 27 MeV PT > 500 MeV sIPS > 4 FS > 5
(1115) selection
LL 2 < 6 m < 6 MeV PT > 500 MeV sIPS > 4 FS > 4
LD 2 < 2 m < 6 MeV PT > 1500
MeV
DD 2 < 2 m < 11 MeV PT > 2000 MeV sIPS > 3 FD > 300 mm
LD3%
DD18%
LL67%
UL12%
= 1.2 MeV = 2.8 MeV
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b ((1115) p )
LL PT > 2500
MeV 2 < 2 m < 300 MeV (b) < 0.15
UL PT > 500
MeV 2 < 2 m < 300
MeV (b) < 0.15LD PT > 1000 MeV 2 < 1 m < 300 MeV (b) < 0.15
PT > 3200 MeV/c (LL) PT > 3400, 3800 MeV/c for UL, DD, LD
PT (in b direction) [2250, 3000] MeV/c
selection
bselection
DD PT > 2000 MeV 2 < 1 m < 300 MeV (b) < 0.15
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Efficiencies for b (1115)
tot = 0.011 %
no events selected in 39M bb incl
Yield = 747 / year B/S < 42 @ 90 % CL
LD1%
DD9%
LL80%
UL10%
= 78.1 MeV
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
x 10-4
BR measurement
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b (1670) selection
Protons: Only Long tracks DLL p- > 5 DLL p-K > 0
Exclusive DLL selection PT > 600 MeV sIPS > 3
Kaons: Only Long tracks DLL K- > 5 DLL K-p> 0
Exclusive DLL selection PT > 600 MeV sIPS> 3
PT > 2600 MeV 1600 MeV < PT (in b direction) <
2800 MeV
(1670) : 2 < 6, m < 100 MeV PT > 1500 MeV sIPS > 4
b : m < 200 MeV FS > 2; PT > 2000 MeV (b) < 0.01 PV
b
p
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Efficiencies for b (1670)
Phase space tot = 0.225 % Annual yield: 2515
long. pol tot = 0.224 % Annual yield: 2507
trans. pol tot = 0.228 % Annual yield: 2553
no events selected in 39M bb incl. B/S = 18.2 @ 90% CL
TDR, after L0 x L1
Bs , tot = 0.220 % Bd K* , tot = 0.156 %
After HLT Generic
Bs,d /K* , = 64 MeV
= 69.4 MeV
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Photon polarization
b (1670) selected evts.transversally polarized) efficiency corrected (from
unpolarized decays)
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Statistical sensitivity on |r|
Still far from the SM expected value, but interesting if NP is present! LHCb could be the first to measure the photon polarization in b-> s
transitions
b (1115) b (1670)
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Conclusions
Selection for b (1115) andb (1670) ready
BR studies feasible Angular asymmetries studies ongoing:
promising photon polarization measurement Can we separate the (1670) and the (1690)? Still observing these resonances could give
indications on their production mechanism...
LHCb note(s) in preparation
Federica Legger
Backup slides
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Photon polarization (eff. correction)
b (1670) transversally polarized)
b (1670) phase space)
efficiency
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b ((1670) p K)
Charged tracks:
false ptrue p
Protons: Only Long tracks DLL p- > 5 DLL p-k > 0 Exclusive DLL
Kaons Only Long tracks DLL K- > 5 DLL K-p> 0 Exclusive DLL
false Ktrue K
bb inclsignal
bb inclsignal
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b ((1670) p K)
Charged tracks:
bb inclsignal
bb inclsignal
Protons: PT > 600 MeV sIPS > 3
Kaons PT > 600 MeV sIPS> 3
bb inclsignal
bb inclsignal
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b ((1670) p K)
selection:
PT > 2600 MeV/c PT (with respect to b direction) [1600, 2800]
MeV/c
bb inclsignal
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b ((1670) p K)
(1670) selection:bb inclsignal
2 < 6 m < 100 MeV PT > 1500 MeV sIPS > 4
bb inclsignal
bb inclsignal
bb inclsignal
bselection: m < 200 MeV FS > 2 PT > 2000 MeV (b) < 0.0165
PV
b
p
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b (1115) DLL cuts
bb inclsignal
bb inclsignal
bb inclsignal
false ptrue p
false ptrue p
false ptrue p
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b (1115) PT cuts (p and )
bb inclsignal
bb inclsignal
bb inclsignal
bb inclsignal
bb inclsignal
bb inclsignal
37
b (1115) IPS cuts (p and )
bb inclsignal
bb inclsignal
bb inclsignal
bb inclsignal
bb inclsignal
bb inclsignal
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b (1115) FD cuts ()
bb inclsignal
bb inclsignal
bb inclsignal
bb inclsignal
bb inclsignal
bb inclsignal
bb inclsignal
bb inclsignal
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KF unconstrained fit 2
(1115)
2 (LL)
2 (LD)
2 (LU)
Separated cut on chi2 and mass
signal (true )bb incl (true )bb incl (fake )
2 (DD)
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b global fit 2
2 (LL)
2 (DD)
2 (LD)
2 (LU)
Fake PVtrue PV
signal (Fake PV)signal (true PV)
Fake PVtrue PV
Fake PVtrue PV
Global fit distinguishes fake from true PVs
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b (1115) PT cuts (b,)
bb inclsignal
bb inclsignal
bb inclsignal
bb inclsignal
bb inclsignal
bb inclsignal
bb inclsignal
bb inclsignal
42
b mass resolution (true tracks)
Mean = 5606 MeV/c2
Mean = 5601 MeV/c2Mean = 5601 MeV/c2
Mean = 5606 MeV/c2
Mass peak: 20 MeV offset due to photon calibration
90 MeV
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Photon polar angle res. (sel evts)
b (1115) b (1670)