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

5

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)

7

(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

11

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

17

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

24

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

31

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

35

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

39

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)

40

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

41

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

43

Photon polar angle res. (sel evts)

b (1115) b (1670)