semileptonics at lhcb , and the prospects for v ub

Rob Lambert, CERN HQ&L, Oct 2010 1

Semileptonics at LHCb, and the prospects for Vub

Robert W. Lambert

on behalf of the LHCb Collaboration

Acknowledgements Thanks to the LHC and the rest of the LHCb collaboration

Specifically extra thanks to my colleagues: Marina Artuso, Phillip Urquijo, Kim Vervink, Liming Zhang

for direct contributions to this talk


Outline LHC and LHCb

Semileptonics and challenges

Progress towards Vcb and Vub

A little on afs


LHC The LHC is surpassing earlier predictions

We hope for 50 pb-1 by the end of the year


Online Luminosity Estimate

LHC now achieves>1pb-1 per fill!

LHCb LHCb is a dedicated, precision, b-physics experiment

High statistics: we’re in the forward region, and at LHC


Being Timely Proper Time: LHCb Velo precise down to 35 fs! (MC)


Being Precise Particle ID: separation handled by dedicated subdetectors

Two RICHes, Calorimetry and Muon system


RICH 1(Vertical)

RICH 2(Horizontal)

MA

GN

ET

TT

T1-T3

Semileptonic decays LHCb can reconstruct charmed and charmless B-decays

Backgrounds from open charm and other light mesons Since Vcb>>Vub, then the charmed are the first to be seen…

To measure the q2 we also need to reconstruct the neutrino...


Xcb XDb

Neutrino Reconstruction Use kinematics to calculate the neutrino momentum

Two ambiguous solutions, the lower momentum has lower error


Fit low

bb cross-section Second LHCb paper, L=(15.0±1.5)nb-1

sbb from semileptonic decays of the form

ln(IP) distribution used to discriminate prompt contribution


XDb 0

CERN-PH-EP-2010-029arxiv:1009.2731

bb cross-section Second LHCb paper, L=(15.0±1.5)nb-1

sbb from semileptonic decays of the form

Scale to full rapidity:


XDb 0

CERN-PH-EP-2010-029arxiv:1009.2731

b4920284 s Xbbpp

See ConorFitzpatrick’s talk

(yesterday)

Vub

Theoretically interesting: constrains UT and form factors

LHCb can also probe Vub in the Bs system


BABAR 2010arxiv:1005.3288B→pln

Vcb

Vcb is an early-data proxy for Vub, and is the main background Like Vub, need two external inputs: BR and/or Form Factors


Vcb

Vcb is an early-data proxy for Vub, and is the main background However, very little is known in this sector Form factors are expected to differ in q2 shape


BABAR 2010arxiv:1005.3288B→pln

?

BS->Xc(*)ln

Vcb

Vcb is an early-data proxy for Vub, and is the main background However, very little is known in this sector Form factors are expected to differ in q2 shape And The exclusive Ds

(*)(*) branching ratios are not well known

… plus one measurement from D (2009)Rob Lambert, CERN HQ&L, Oct 2010 15

(my copy ofthe PDG 2008)

21 1014.017.020.003.12536 XDB ss PRL 102, 051801

D** branching ratios


**sDBs Measurements of and will constrain

LHCb is investigating…

KD(*)(*)sD

Phys. Rev. D 32, 189–231 (1985)

Vcb

Choose a set of form-factors for the LHCb MC (HQET2/ISGW2)

Set the relative branching ratios from D/D*/D**, ~2.5:~6:~1

0.8 pb-1 examined so far, should become powerful in 50 pb-1


Ds

Ds*Ds**

Ds

Ds*Ds**PromptmKKp SB

MC

An aside: prospects for afs


afs

Great attention thanks to the recent D result Dominated by detector and background asymmetries

They measured:

3.2 s from SM!


hep-ex 1005.2757

fs

fs

-310 (syst)]46.12.51(stat) 57.9[ bA

2

dfs

sfsb aa

A

Xpp

Xpp vs. See Peter

Ratoff’s talk(tomorrow)

afs

At LHCb systematics will dominate exclusive measurements Particularly detector and production asymmetries

We can measure:

Since the detector asym.cancels for the same final states


2,

dfs

sfsds

fs

aaA

45.06.0 105.2

SMfs

fs

XDB ss

XDBd vs.

MC

afs

We have already collected thousands of relevant decays We expect to have sufficient statistics after 100 pb-1


~100k Ds in 5 fb-1 Estimate 100k Ds in 0.1 fb-1

Conclusions LHCb has a broad but precise physics program

We expect 50 pb-1 this year, which makes us competitive with the b-factories and TeVatron

Good prospects for the near future: Vcb, form factors, D**/D*/D ratios Gs, Gd, Gs/Gd

Hopefully also: ms, md

fs&fd

Next year: Afs and Vub


Theory input encouraged!

End Backups are often required


Semileptonic Prospects


50 pb-1

<0.1 pb-1

1 pb-1

10 pb-1

100 pb-1

bb cross-section

ms md

Afs

[D, D*, D**]Branching

Ratios

Form factors

Vub

CERN-PH-EP-2010-029

ratiosfs fd

Gs/Gd

Gs, Gd

Being Timely Proper Time: LHCb Velo precise down to 35 fs! (MC)


Being Exclusive Our forte: exclusive, reconstructed, b-decays

In particular, time-dependent measurements


~100k Ds in 5 fb-1 Estimate 100k Ds in 0.1 fb-1


The simple formula

qqb

q

qqp

qfs

qc

qfsq

fs SB

ttmaa

tA

G

22/coshcos

2222)(

10-3 -> 10 -5

ff

fftAqfs GGGG

)(


The simple formula

Polluting asymmetries are much larger than afs

Detector asymmetry c ~(10-2) Production asymmetry p ~(10-2) Background asymmetry b ~(10-3)

qqb

q

qqp

qfs

qc

qfsq

fs SB

ttmaa

tA

G

22/coshcos

2222)(

1//

1)()(

1)()(

0

0

SBSB

ININ

ff

b

p

i

ic

10 -2 10 -2 10 -310-3 -> 10 -5

Very Complicated

ff

fftAqfs GGGG

)(

Detector Asymmetry, c

Magnet divides +/- charge, allowing +/- asymmetry

by reversing magnet in D0: c reduced from 3% -> ~0.1%


-1

Asymmetry from Long Muon Tracks Reconstructed in MC

Left Right

+ve -ve

Left Right

-ve +ve

Detector Asymmetry, c

Matter detector hadronic interactions are asymmetric

Dominant systematic at order 1%


Kaon PDG cross-section

PDG

0

20

40

60

80

100

120

0.1 1 10 100 1000

momentum in lab frame Plab / GeV c

hadr

onic

cro

ss-s

ectio

n / m

b. K-, pK+, p

-1

Kaon interaction cross-section

K- pK+ p

Resultant charge asymmetry (MC)

Monte Carlo

Production Asymmetry, p

LHC is a proton-proton collider: not CP-symmetric

LHCb is at high rapidity where production asymm. are largest

There is never a simple control channel to measure p


-0.020

-0.015

-0.010

-0.005

0.000

0.005

0.010

0.015

0 50 100 150 200 250 300 350

Energy, E / GeVPr

oduc

tion

Asy

mm

etry

, p

p (B0 and B0)

Valence QuarkScattering

Monte Carlo

Explicitly asymmetric at LHC

The clever method Take Bs/Bd with the same final states ( =KKp )

All production asymmetry is in x2/x3, just throw it away

Measure the difference between Bs and Bd


f

2211,

dfs

sfs

dsdsfs

aaxxA

45.06.0 105.2

SM

semileptonics at lhcb , and the prospects for v ub

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