B-Physics & Trigger at the DØ experiment

- operational experience

Daniela Bauer

Can the experience gained at Fermilab help prepare for

(b-physics at) the LHC ?

Yes.

No. Nothing can prepare you for the LHC.

Maybe, but we'd like to make our own mistakes, thank you.

Motivation

Making the transition from an e+e- to a hadron collider

can be an interesting experience →

b-physics at an e+e- collider

(simplified view)

b-physics at a hadron collider

(simplified view)

Background

b-physics at hadron colliders ↔ b-physics at e+e- machines:

No fixed centre of mass energy.

Triggers ! Triggers ! Triggers !

The mess we refer to as underlying event/additional interactions.

Additionally at DØ:

No particle ID.

Competition with high pT programme.

Talks given at Beauty2006 (Oxford) and

IOP “Tevatron for LHC” meetings.

Overview

The DØ Detector at Fermilab

B-physics Highlights

Trigger/High Luminosity Challenges

Trigger system

Doing b-physics at a multi purpose detector

b-physics triggers

Conclusions

Tevatron

protons

anti-protons

Chicago

Recycler

DØ

Fermilab

60 km

Middle of nowhere

CDF

Run II

ECM= 1.96 TeV

2.5 fb-1

Run I 1992-1995

ECM = 1.8 TeV

125 pb-1

RunIIa

RunIIb

April 02

July 07

The upgraded DØ Detector

Solenoid, CFT, SMT, Preshowers

Shielding

Forward mini

drift chambers

Forward Scintillators

Silicon Microstrip Tracker (SMT)

Hybrid design: 6 barrels with 8 layers (+ Layer 0), 12 F-Disks, 4(2) H-Disks

Essential for b-physics trigger and analysis:

Tracking, primary and secondary vertex reconstruction, impact parameter.

Design provides tracking up to |η| < 3.0, but

Most analyses also require tracks to have hits in the CFT.

H-disks had high rate of failure, most forward disks have now been

decommissioned to make room for Layer 0 readout cables.

Silicon Microstrip Tracker Layer 0

Beam Pipe

Layer 0

30% improvement in impact parameter

resolution vs RunIIa → great news for b-physics

Commissioned and up and running.

Impact parameter resolution from cosmics: 21 μm

IP1 + IP2

pull

Central Fibre Tracker (CFT)

16 doublet layers of scintillating fibres,

arranged in 8 superlayers

Radius 20 – 52 cm

Track reconstruction up to |η| < 2.0

CFT standalone used for triggering at

lowest trigger level.

Tracker Event Displays at 70e30 cm-2 s-1

Tracker Event displays at 140e30 cm-2 s-1

If this was the LHC you probably wouldn't see the detector underneath

all the tracks.

Muon system

Main features:

3 layers of drift tubes.

3 layers of scintillators: triggering, improved

resolution in wire direction, rejection of cosmics

Toroid magnet (1.8 T) after the first layer:

local pT measurement (trigger).

Toroid and solenoid polarities reversed on

regular basis.

Track matched muons up to |η| < 2.2

Mini Drift Tube plane

Scintillator plane

Muon System

Calorimeter

Designed for high pT physics, but

low pT electrons can be used for b-tagging:

E.g. Measurement of Bd mixing using

opposite-side flavour tagging, PRD 74, 112002(2006)

D2() = 1.48%

D2(e-) = 0.21%

Tagging efficiency =Ntag/Ntot

Dilution D=(NR-NW)/(NR+NW)

Tagging power=D2

J/ψ → e+e-

Results

With this detector we have seen all sorts of b.....

Λb

B

*

s2

excited B

Bs

Bd



+ B+, Bc, X(3872), ...

Measurement of the charge asymmetry in semileptonic Bs decays, PRL 98, 151801 (2007)

Lifetime difference and CP violating phase in the Bs system, PRL 98, 121801 (2007)

Measurement of B0 mixing using opposite-side flavor tagging, PRD 74, 112002 (2006)

Measurement of the Bs lifetime in Semileptonic Decays PRL 97, 241801 (2006)

Measurement of the CP-violation parameter of B0 mixing and decay with

ppbar → mumuX data PRD 74, 092001 (2006)

Search for the Rare Decay Bs → Φμ+μ- with the DØ Detector, PRD 74, 031107 (2006)

Direct Limits on the Bs Oscillation Frequency, PRL 97, 021802 (2006)

Measurement of the Upsilon differential cross section..., PRL 94, 232001 (2005)

Measurement of the ratio of B+ and B0 meson lifetimes, PRL 94, 182001 (2005)

Measurement of the Λb lifetime in the decay J/ψ Λ decays..., PRL 94, 102001 (2005)

A search for the flavour-changing neutral current decay Bs→μ+μ-, PRL 94, 071802 (2005)

Measurement of the Bs lifetime in the exclusive decay channel Bs→J/ψ Φ,

PRL, 94, 042001 (2005)

Measurement of the lifetime difference in the Bs system, PRL 95, 171801 (2005)

Measurement of semileptonic branching fractions of B mesons to narrow D** states,

PRL 95, 171803 (2005)

Observation and Properties of the X(3872) Decaying to J/ψ π+π-..., PRL 93, 162002 (2004)

DØ b-physics publications

The most talked about paper in 2006

M = mass matrix

Γ = decay matrix

Bs Mixing

Bs mixing via the dominant top-quark process

Bs (~bs), Bs(~bs) are produced in one of the two possible flavour states.

This initial state evolves into a time-dependent superposition

of the two states according to:

Observables in Bs Mixing

extract Vts

CP violating phase: The level of CP violation

in the Standard Model is too small to produce

the observed baryon number density

→ looking at all sources of CP violation

→ CP violation in SM is expected to be small

large values could indicate physics

beyond the SM

The Big Picture

Measuring Δms at DØ

Bs →μ+ Ds- X

Ds-→ Φ π-

Φ →K+ K-

signal

tag (→ initial flavour)

Problem.

→ limited reach in Δms

Measuring Bs mixing at DØ

signal

tag

Limited reach in Δms, but ....

Results (DØ and CDF)

semileptonic + hadronic decays

semileptonic decays only

Δms = 17.77 ± 0.10 (stat) ± 0.07 (syst) ps-1

17 < Δms < 21 ps-1

at 90 % confidence level

First !!

Best !!

|Vtd/Vts| = 0.26060 ± 0.0007 (exp) ± (theory)

0.0081

0.0060

scalar → VV decay

 3 amplitudes

L = 0 (even),1 (odd), 2 (even)

described in transversity basis

In J/Ψ restframe:

K+K- plane defines (x,y) plane

K+ defines +y direction

Θ, Ψ polar and azimuthal angles of μ+

φ in Φ restframe: angle(K+, -J/Ψ)

sand s from Bs→J/ 

Bs → μ X asymmetry

Dimuon Asymmetry

World Average: τfs

ΔΓ and Φs

ΔΓs = 0.13 ± 0.09 ps-1

SM prediction*: 0.088 ± 0.017 ps-1

Φs = -0.70 ±

SM prediction*: (4.2 ± 1.4) x 10-3

0.47

0.39

*Lenz,Nierste hep-ph/0612167

Triggers

Go ahead, make my data !!!!!

Data taking rates

L1 Trigger

L2 Trigger

L3 Trigger

2kHz

1kHz

<50Hz>

250kB/ev

Detector

Reconstruction

Farm

1.7 MHz

Calorimeter

Digitization

Rate

SVXII chip

Digitization

Rate

The DØ trigger system

Level 1 triggers

Calorimeter: 0.2x0.2 η-φ triggers towers (+ET)

Central Track Trigger (CTT): uses axial layers of the CFT to find tracks

4 pT bins

Tracks can be confirmed by muon hits.

Muon: Looks for hits (wire & scintillator) consistent with muons.

Level 2 triggers

Refine L1 trigger terms using added event information (e.g. wire and

scintillator times for muons).

Results are combined in a global L2 term.

Silicon Track Trigger for displaced vertices, improved momentum

measurement.

Trigger System: Level 1 & Level 2

Silicon Track Trigger

L1 CTT tracks are used to define roads

into the SMT.

SMT hits are clustered in these roads.

Track is refit within the road.

→ Improved pT measurement wrt L1.

→ Impact parameter measurement.

Under-used by b-physics in RunIIa:

Impact parameter bias difficult to model/analyze.

(Planned) late commissioning: Triggers already well established with

sufficient rate reduction.

No displaced track only trigger due to L1 bandwidth limitations.

RunIIb: b-physics and Higgs group are the main users of the STT.

Trigger System: Level 3

Software based.

Goal: To perform a (partial) reconstruction of the event.

Tools of the trade:

muons

electrons

tracking

taus

jets

missing ET

primary & secondary vertexing

isolation (muons, electrons)

impact parameter (tracks, muons)

invariant mass

... and almost any combination thereof

0.95 1 1.05 GeV

m(KK)

Trigger strategy:

The trigger menu needs to accommodate all physics groups.

Most physics aiming for maximum luminosity on a given trigger.

Most b-physics needs the maximum of b-events.

Store 4022 March 2005

100e30

20e30

non b-triggers

b-triggers

Doing b-physics at a multi-purpose experiment

At the end of RunIIa there were 56 b-triggers (out of ~ 300 triggers total).

The number of triggers was limited by the number of L1/L2 bits (128).

Only L1/L2 bits could be prescaled individually.

b-physics triggers at DØ

300 triggers should be enough for everybody, right ?

Apparently not:

RunIIb has seen L2 oring/splitting and a doubling of the number of triggers:

Needs increasingly sophisticated tools (and databases) to administer this list.

Difficulties in identifying problematic triggers during run time.

Automated performance monitoring helps, but you still need a brain to analyse it.

Only 10% (or so) of all triggers are actually used for analysis.

Manpower ~ triggers2

Yes, I am bitter.

b-physics triggers at DØ

In RunIIa there were 3 major groups of b-physics triggers:

single muons, impact parameter unbiased ('low' lumi)

single muons with impact parameter requirement (all luminosities)

di-muons (all luminosities)

additionally

tri-lepton

electron-muon

muon+jets

Apart from requiring one or more muons, the b-physics triggers also use

the following trigger requirements:

track match for muons: tracks required to have SMT hits

tracks (number of tracks, pt)

impact parameters (for muons and/or tracks)

invariant mass filters: , J/, 

charge (opposite sign)

primary vertex: ± 35 cm

Anatomy of three 'best-of' (late) RunIIa triggers

unbiased single muon trigger (up to 55e30, 100e30 RunIIb)

semileptonic decays, mixing

L1: tight scintillator, loose wire, pT > 3 GeV (from CTT), primary vertex

L2: one medium muon (RunIIb: track match requirement)

L3: track matched, 3-layer muon with pT > 3,4,5 GeV,

|z (primary vertex)| < 35 cm

single muon trigger with impact parameter (all luminosities)

use muon for tagging to avoid IP bias in the signal (hadronic decays)

L1: tight scintillator, loose wire, pT > 5 GeV (from CTT), primary vertex

L2: one medium muon (RunIIb: track match requirement)

L3: track matched 3-layer muons with IP significance > 3 and pT > 5 GeV

|z (primary vertex)| < 35 cm

Beloved by trigger people, hated by analysers data goes unused.

di-muon trigger (all luminosities)

J/ψ (e.g. ΔΓ/Γ),  , Bs →μμ

L1: 2 muons, no pT cut, (RunIIb: one match to a CTT track required)

L2: one or two muons, depending on luminosity

L3: 2 muon system only muons, pT > 2 GeV, one or two muons must have

hits in all 3 layers.

Anatomy of three 'best-of' (late) RunIIa triggers (cont)

Apr01 Apr02 Apr03 Apr04 Apr05 Feb06

Challenges ahead: Increasing instantaneous luminosity

160

120

80

40

320

280

240

200

Luminosity e30

Peak Luminosities RunII

April 01 April 02 April 03 April 04 April 05 April 06 April 07

RunIIa

RunIIb

b-physics (RunIIa)

Reconstruction time vs initial luminosity

GHz-secs

Challenges ahead: Increasing instantaneous luminosity

so far, so good

Reconstruction of the events dominated by track finding.

The same tracking algorithm has to run at all luminosities !

Initial Luminosity

b-physics (RunIIb)

unbiased single muon triggers prescaled

RunIIa

RunIIb

Triggers – timing is (almost) everything

L3 Track CPU consumption vs lumi

<t> in s

L1 track matched muon

first hint of

non-linearity

Rate

RunIIb tracker 3 x faster than RunIIa tracking, but still not fast enough:

→ More CPUs.

increased occupancy

luminosity

b-physics triggers often require low

pT tracks → triggers are intrinsically slow:

optimize trigger ordering

move rate reduction from L3 to L1/L2

(e.g. STT)

Luminosity 1030 cm-2s-1

Remember the event display from the beginning of the talk ?

Offline reconstruction

Level 3

seconds

milliseconds

We can't do everything.

High occupancy will kill your trigger.

pain threshold

Special thanks to R. Beuselinck for making these plots.

Coincidence makes great tracks.

*

* same number of clusters per CFT layer

as before, randomly distributed

More layers: Singlet equations at L1, SMT requirement at L3

Less noise (new AFE boards)

Luminosity levelling ?

So far everything is under control .....

What do we do now ?

No SMT hits.

The RunIIa b-physics programme has been a great success !

By playing to our strengths, i.e. making optimal use of our

wide muon coverage and upgraded tracking system, DØ

published 15 b-physics papers (4 more are submitted,

plus 13 preliminary results)

Results are also available on the web:

http://www-d0.fnal.gov/Run2Physics/WWW/results/b.htm

Increasing luminosity is a challenge and an opportunity.

Layer 0 working as expected.

High expectations for RunIIb.

CKM Matrix

Quarks: Weak Eigenstates  Mass Eigenstates
 CKM Mixing Matrix

3 angles

1 complex phase  CP-violation

Wolfenstein parametrization:

λ = |Vus|

w

e

a

k

m

a

s

s

CP violation

(higher orders)

CKM triangle(s)

ρ = (1-λ2/2)ρ

η = (1-λ2/2)η

Wolfenstein parametrization:

λ = |Vus|

η: CP violation

Triangles identical up to λ3

Δms → |Vts/Vtd|

Inputs:

m(B0)/m(Bs) = 0.9830 (PDG 2006)

ξ= 1.21 +0.047 (M. Okamoto, hep-lat/0510113)

Δ md = 0.507 ± 0.005 (PDG 2006)

-0.035

2006-09-11T12:46:21