workhop on recent developments in high energy physics and cosmology, athens, 17-20 april 2003 th....

Workhop on Recent Developments in High Energy Physics and Cosmology, Athens, 17-20 April 2003

Th. Lagouri, A.U.Th.

B-physics with the Atlas detector

Theodota Lagouri

(for the Atlas Collaboration)

Aristotle University of Thessaloniki



Outline• The initial experiment conditions• The ATLAS Physics Programme• The ATLAS detector & trigger• B-physics potential with the nominal detector @ L=1033 cm-2 s-1;

• Preliminary estimate of the B-physics potential with the initial detector and luminosity;

• Conclusions



The initial experiment conditions

• Results presented here refer to the nominal detector configuration and L= 1x1033 cm-2 s-1

[CERN 2000-004] ;• The analysis with the initial detector layout

(including the change of the B-layer radial position, re-evaluation of the material distribution in the ID) as a function of luminosity and the trigger conditions is on-going (within the Data Challenge project); preliminary results about the physics performance at Initial conditions will be presented.



The ATLAS Physics Programme1. The most prominent issues for the LHC are the quest for

the origin of the spontaneous symmetry-breaking mechanism (SM and MSSM) and the search for new physics: SuSy, Heavy Bosons, etc…

2. ATLAS (and CMS) is a general-purpose experiment optimized to maximize the potential discovery new physics: Higgs boson(s) , SuSy particles, W’ and Z’, etc…

3. However we have to consider that:– The LHC is a beauty factory dedicated B-

experiment (LHCb);– The ATLAS detector allows also a wide programme

of B-physics studies, competitive with LHCb in some channels;



The ATLAS experiment

Tile Calorimeter Module(s)

Barrel toroid coil

LAr Calorimeter Cryostat

LAr e.m. barrel module

Solenoid magnet

MDT chamber assembly



Detectors

Front-endPipelines

ReadoutBuffers

EventBuilder

Buffers &ProcessingFarms

DataStorage

Readout Drivers

1 GHz interaction rate /

<75 (100) kHz

O(1) kHz output rate

O(100) Hz output rate

~100 GB/s output data flow

O(100) MB/s output data flow

O(1) GB/s output data flow

2 s latency

O(10) ms latency

~ seconds latency

40 MHz bunch-crossing rate

Level 1

Level 2

Event Filter

LVL2– Region-of-

Interest (RoIRoI)

– Specialized algorithms– Fast selection with early

rejection EF

– Full event available– Offline derived algorithms– Seeding by LVL2– Best calibration / alignment– Latency less demanding

LVL1– Hardware based

(FPGA and ASIC)– Coarse calorimeter

granularity– Trigger muon

detectors: RPCs and TGCs

The ATLAS Trigger/DAQ System

RoI Pointers

HLTHLT



The Atlas B-Physics Programme

• The main physics processes that can be studied:– CP violation:

• Asymmetry in B0d J/ K0

s measurement of sin2;• Asymmetry in B0

s J/ test of the SM;• Asymmetry in B0

d,s hh measurement of ;

– B0s - B0

s oscillations;– Rare B-decays with dimuons;

• Also: – B-production cross-section measurement;– b polarisation measurement;– Related to B-physics: direct J/, production



The B-trigger• L=1 x 1033 cm-2 s-1; • Level-1: single muon trigger pT > 6 GeV/c, ||

<2.5;– Rate is expected to be about 23 kHz;– dominated by in-flight decays of ,K and heavy flavour

muon production;– Dimuon trigger possibly with lower thresholds; – Raise thresholds for higher luminosities;

• Level-2, step 1: confirm level-1 muon trigger in RoI;– Use precision muon system together with ID for

momentum measurement important rejection of in-flight decays;

– Rate: about 5 kHz;



The B-trigger• Level-2, step 2:

– Specific selections are applied for different channels; in all cases we perform a track reconstruction in the Inner Detector with:1. Either an ID full-scan;2. Or RoI-based ID track reconstruction.

• J/ : two opposite muons pT (1) >6 GeV and pT (2) >3 GeV ( in TileCal); mass cuts;

• J/ee : two opposite-charge electrons with both pT (1) >1GeV; mass cuts; rate @ lvl2: 40 Hz (lvl1 mu8, L=1 x 1033 cm-2 s-1);

• B hadrons: example: B hh: two opposite tracks with pT>4 GeV; mass cuts;



The B-trigger

• Event Filter: track refit, including a vertex fit; decay length and fit quality cuts are applied;

• about a factor of 10 wrt LVL2 can be achieved by exclusive selections.



B0d J/ K0

s• J/ ll reconstruction;

mass resolution: 40 MeV (muons) and 60 MeV (electrons);

• K0s: 4.5 – 7.0 MeV

mass resolution;• B0

d: 3D kinematic fit applying vertex and mass constraint; B0

d mass resolution: 19 (26) MeV;

• Background mainly from B decays with a J/ in the final state; small contribution from false J/;

B0d reconstruction; CDF has shown

a similar signal; b/tot and prod. rate improved at LHC



B0d J/ K0

s

• Flavour tagging:– Opposite-side tagging:

muon (trigger) or electron (pt>5 GeV)

– Same-side tagging: B- algorithm (charged meson associated with the B-hadron)

• Event yield for 30 fb @ L=1x1033cm s

J/(ee)K0s

signal back.

J/()K0s

signal back.

e tags - - 5800 500

tags 14400 900 11900 1100

B- tags

- - 376100 13700



B0d J/ K0

s

J/(ee)K0s J/()K0

s

lepton tags 0.018 0.023

B- tags - 0.015

Estimate of the statistical error of sin2 using a time-dependentanalysis with an integrated luminosity of 30 fb.

Overall statistical error:• 10 fb-1: 0.018• 30 fb-1: 0.010Competitive with LHCb and B-factories



B0d J/ K0

s

• Systematic error: analysis of control samples:– B+ J/()K+

– B0dJ/()K*0

Invariant mass distribution for B+ J/ K+ with superimposed theestimated background.

Global systematic error: 0.01

Final error: 0.01 (stat) + 0.01(sys)



Rare decays with dimuons

• The decays B0s+ and B0

d + have very small BR but they can be selected by the atlas trigger even at the nominal LHC luminosity. With 130 fb of data the reaction B0

s+ can be seen with 4.7 assuming the S.M. BR of 4.9 109.

• Another interesting class of reactions are inclusive decays such as B0

s0+ , B0d0+ ,

B0d*0+ , …

• Detailed measurements of the decays can test the SM search for new physics (eg AFB in decay)



Initial detector configuration• Limited resources and technical/schedule constraints

– effect: detector staging and TDAQ staging:

• Stage the following components (defer for 1-2 years)– One of the three pixel layers (not the B-layer)

– Outermost TRT wheels, half of the CSC layers

– MDT chambers in transition region (EES, EEL)

– Cryostat gap scintillators, part of high luminosity shielding

– Reduction of Read-Out Drivers for LAr calorimeter



Initial detector configuration

• However the main effect to the B-physics performance wrt to the Yellow Book results comes from the change of the B-layer radial position (from 4.30 cm to 5.05 cm) and from the material in and before that layer (increased thickness of the beam pipe and pixel services); preliminary estimations with DC1 data analysis:– Impact parameter resolution and proper time resolution

degraded by about 30%;– Mass resolutions degraded by about 15%;– Reconstruction efficiencies: no important degradation

found;



Summary of B-physics DC1 validations

• Physics channels and software used in validation

• Results on software validations

• B-performance with new geometry

• Athena reconstruction



Physics channels• Input: TDR Generator events• Masses and proper-times from different

topologies:

BsDsDs d • J/J/ee from Bd J/s and

Bs J/s study yet.• Charged particles from B-events combined

with single particles



Software

• Simulation: atlsim3.2.1, realistic ID field• Reconstruction: Atrecon (xKalman) 4.0.0 and

4.5.0 or Atlsim (xKalman) 4.0.0 and 4.5.0. Output to CBNT

• Vertexing code - still homogenous field• ATL-COM-PHYS-2003-003

B. Epp, V.M. Ghete, A. Nairz, R. Jones, V. Kartvelishvili, M. Smizanska, N. Nikitine, S. Sivoklokov, K. Toms, J.F. Laporte, T. Lagouri, H. Bikolon, M. Testa



Validation of reconstruction software (1)• All reconstructed masses displaced from

generated:• Bs Ds (-7 MeV)• J/ (-4 MeV) in both Bd J/Ks ,

Bs J/• Bd (-3 MeV)… non trivial: -dependent

effect • Bd (-5 MeV) - barrel, Bd (+4 MeV) -

endcap• Ds , (K+K-) (obviously not shifted)

• No shifts in TDR



Validation of reconstruction software (2)• Is the displacement due to Vertex code?• If yes- not due to 2 T instead of 2.085T near the IP• If yes - not only: we did not use vertexing for

masses J/ , Bd • If yes - not only: there are shifts in single - particle

parameters just after Atrecon:

e.q. shift in ‘pull’ distribution of muons pT = 1 GeV

(1/pTrec-1/pTtrue)/(1/pT) (-0.05)

(no shift for high pT muons pT = 200 GeV)



B-performance with new geometry• Despite problems in reconstruction software-first

conclusions about B-performance in new geometry and realistic field versus TDR geometry and homogeneous field.

• Mass resolutions degraded by (10-20 %)• Variables more sensitive to B-layer radius and

material in and before: secondary vertices, proper-times degraded by (25-35 %)

• This matches with degradation (~30%) of impact parameter resolution of charged tracks at PT ~ 5 GeV in < 0.25



Athena ID ReconstructioniPatRec 5.3.0

• DC1 data, detector simulation with atlsim 3.2.1

• J/s J/events

• Reconstruction with iPatRec 5.3.0 in Athena

• Athena ID reconstruction validation

• No mass shift observed!



Athena ID reconstruction validation • Peak position (3096 ± 0.6) MeV Shift (-0.9 1.5) MeV No shift Nominal value: 3069. 9 MeVDC1 Atrecon 4.5.0 (xKalman++): Shift (-4.9 ± 0.3) MeV • Mass resolution ±eV (16 %)TDR 39.3 ± 0.4 MeVDC1 (Atrecon 4.5.0 ) (44.6 ± 0.5) MeV (13

%)



Athena ID reconstruction validation

• Barrel (<1 both muons)Mass resolution: (38.9 ± 0.6) MeV Peak shift: (+0.1 ± 0.5) MeV (No Shift)• Endcap (>1.2 both muons)Mass resolution: (61.7 ± 0.2) MeV Peak shift: (-1.9 ± 1.9) ? MeV (limited statistics) Material implementation corrected for 6.0.2 to

include the insertable pixel layout support structure (significant effect in endcap region)



DC1 Atrecon-xKalman vs TDR

Mean:(3092 ± 0.3 ) MeV

44.6± 0.5 ) MeV

Mean:(3095 ± 0.3 ) MeV

39.3± 0.4 ) MeV

• DC1 Atrecon(4.5.0) -xKalman++

• TDR - iPatRec

Shift: (-1.9 ± 0.3) MeVShift: (-4.9 ± 0.3) MeV



DC1 Athena-iPatRec vs TDR • DC1 Athena (5.3.0) -

iPatRec• TDR - iPatRec

Mean:(3095 ± 0.3 ) MeV

39.3± 0.4 ) MeV

Mean:(3096 ± 0.6 ) MeV

45.5± 0.6 ) MeV

Shift: (-0.9 ± 0.6) MeV Shift: (-1.9 ± 0.3) MeV



Athena-ipatRec Barrel-Endcap• Barrel |<1 • Endcap |

Mean:(3097 ± 0.5 ) MeV

38.9± 0.6 ) MeV

Mean:(3095 ± 1.9 ) MeV

61.7± 0.2 ) MeV

Shift: (+0.1 ± 0.5) MeV Shift: (-1.9 ± 1.9 ) MeV



T/DAQ Deferrals & LHC lum.• The target initial luminosity was doubled to L= 2 x

1033 cm-2 s-1; the low pt inclusive muon trigger will be replaced by a low-pT

dimuon trigger;

• Consequences:– CP violation: sin2: 0.010 0.015 (dimuon trigger

only);– Mixing cannot be studied with dimuon trigger only;– Rare B decays: unaffected;

• Restore the low lumi trigger menu as soon as L approaches values close to 1x1033 cm-2 s-1;



Summary

• Although ATLAS is designed to probe the O(1TeV) energy scale, this experiment can make several useful measurements in the B-physics sector:– Sensitivity to sin2 comparable to that of LHCb;

– Measurement of the B0s- B0

s oscillations;

– Unique opportunity to search for rare B0s decays (not

possible by BaBar, Belle): potential indirect evidence of new physics.

workhop on recent developments in high energy physics and cosmology, athens, 17-20 april 2003 th....

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