Transcript B-physics with the Atlas detector

B-physics with the initial ATLAS
detector
Aleandro Nisati for the ATLAS
Collaboration
INFN Commissione Scientifica I
February 3rd, 4th 2003
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
2
The initial experiment conditions
Many uncertainties:
1. The LHC luminosity: the target initial
luminosity was doubled to L= 2 x 1033 cm-2 s-1;
2. The detector configuration; in particular the
initial HLT/DAQ system bandwidth and
processing power (resources limitations);
3. The physics rates (uncertainties on p,K and
heavy flavour production cross-sections);
3
The initial experiment conditions
•
•
Results presented here refer to the nominal
detector configuration and L= 1x1033 cm-2 s-1
[Yellow Report 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); however some
preliminary indications on the degradation of the
physics performances will be provided.
4
The ATLAS Physics Programme
1. 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 Bexperiment (LHCb);
– The ATLAS detector allows also a wide programme of
B-physics studies, competitive with LHCb in some 5
channels, “for free”…
Cross-sections and rates
• huge range of cross-section
values and rates
– listed for 1034 cm-2 s-1
– total
 s  100 mb
(109 Hz)
– b production
 s  0.7 mb
(7*106 Hz)
– W/Z production
 s  200/60 nb
(2/0.6 kHz)
– Top production
 s  0.8 nb
– SM Higgs
(80 Hz)
(mH = 150 GeV)
 s  30 pb
(3 Hz)
• With branching ratios included
– W  en
150 Hz
– Z  ee
15
Hz
6
– H  gg
0.003 Hz
B-simulation
• Monte Carlo generator: PYTHIA 5.7/JETSET 7.4;
– Flavour creation, flavour excitation and gluon splitting
included;
– CTEQ2L parton distribution;
– Peterson function eb=0.007
• Full GEANT3 simulation of the detector response;
in some case integrated with fast simulation;
• Total inelastic cross-section: 80 mb; bb cross
section: 500 mb;
7
The ATLAS experiment
Pixel module
Muon Trigger Elx and Algor.
LAr e.m. endcap module
Tile Calorimeter Module(s)
MDT chamber assembly
8
RPC chambers
The ATLAS Trigger/DAQ System
Detectors
Front-end
Pipelines
LVL1
1 GHz interaction rate /
40 MHz bunch-crossing rate
Level 1
– Hardware based
(FPGA and ASIC)
– Coarse calorimeter
granularity
– Trigger muon
detectors: RPCs and
TGCs
2ms latency
<75 (100) kHz
~100 GB/s output data flow
Readout
Drivers
RoI Pointers
Readout
Buffers
Event
Builder
Buffers &
Processing
Farms
Data
Storage
Level 2
O(10) ms latency
O(1) kHz output rate
O(1) GB/s output data flow
HLT
LVL2
–
–
–
EF
–
–
–
O(100) Hz output rate
–
O(100) MB/s output data flow –
Event Filter ~ seconds latency
Region-ofInterest
(RoI)
Specialized algorithms
Fast selection with early
rejection
Full event available
Offline derived algorithms
Seeding by LVL2
Best calibration / alignment
Latency less demanding
9
The Atlas B-Physics Programme
• The main physics processes that can be studied:
– CP violation:
• Asymmetry in B0d  J/Y K0s  measurement of sin2b;
• Asymmetry in B0s  J/Y f; test of the SM;
• Asymmetry in B0d,s  h+ h-  measurement of b+g;
– B0s - B0s oscillations;
– Rare B-decays with dimuons: B0d,sm+ m-, B0dK*0m+
m-, B0sf0m+ m -, …
• Also:
– B-production cross-section measurement;
– Lb polarisation measurement;
– Related to B-physics: direct J/Y, U production
10
The B-trigger -1
• L=1 x 1033 cm-2 s-1;
• Level-1: single muon trigger pT > 6 GeV/c,
|h|<2.5;
– Rate is expected to be about 23 kHz;
– dominated by in-flight decays of p,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 inflight decays;
11
– Rate: about 5 kHz;
The B-trigger -2
•
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.
– ID full scan: unguided search for tracks in all Pixel
system; track extrapolation to the SCT+TRT
(electrons down to 1 GeV);
– RoI approach: consider only regions with calorimeter
activity tagged by level-1 system: example: em cluster
ET>2 GeV; hadronic cluster: ET >5 GeV; it requires
less processing power resources (but less efficient)
12
The B-trigger -3
• J/Psim+ m-: two opposite muons pT1>6
GeV and pT2 >3 GeV (m in TileCal); mass
cuts;
• J/Psie+e -: two opposite-charge electrons
with both pT1 >1GeV; mass cuts; rate @
lvl2: 40 Hz (lvl1 mu8, L=1 x 1033 cm-2 s-1);
• B hadrons: example: B h+ h-: two
opposite tracks with pT>4 GeV; mass cuts;
13
The B-trigger -4
• 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.
14
B0d  J/Y K0s
• J/Y  l+l- reconstruction;
mass resolution: 40 MeV
(muons) and 60 MeV
(electrons);
• K0sp+p-: 4.5 – 7.0 MeV
mass resolution;
• B0d: 3D kinematic fit
applying vertex and mass
constraint; B0d mass
resolution: 19 (26) MeV;
• Background mainly from B
decays with a J/Y in the
finals state; small
contribution from false J/Y ;
B0d reconstruction; CDF has shown
a similar signal; sb/stot and prod. rate
improved at LHC
15
B0d  J/Y K0s
• Flavour tagging:
J/Y(ee)K0s
signal back.
J/Y(m6m3)K0s
signal back.
– Opposite-side tagging:
muon (trigger) or
5800 500
e tags
electron (pt>5 GeV);
Dtag = 0.5;
m tags 14400 900 11900 1100
– Same-side tagging:
B-p algorithm (charged B-p
376100 13700
meson associated with
tags
the B-hadron); Dtag =
0.16;
• Event yeld for 30 fb-1 @
L=1x1033cm-2 s-1
16
B0d  J/Y K0s
Statistical Error:
Estimate of the statistical error of sin2b using a time-dependent
analysis with an integrated luminosity of 30 fb-1.
J/Y(ee)K0s
J/Y(m6m3)K0s
lepton tags
0.018
0.023
B-p tags
-
0.015
Overall statistical error:
• 10 fb-1: 0.018
• 30 fb-1: 0.010
Competitive with LHCb and B-factories
17
B0d  J/Y K0s
Systematic Error:
• analysis of
control samples:
– B+  J/Y(mm)K+
– B0dJ/Y(mm)K*0
– Provide
measurements of
Dtag, and Ap.
Invariant mass distribution for
B0d  J/Y K+ with superimposed the
18
Estimated background.
B0d  J/Y K0s
• Systematic errors
– dDtag/Dtag: 0.003
– dDback/Dback: 0.006
– dAP: 0.0005;
Global systematic error: < 0.01
19
B0d,s  h+ h • Expected to provide
measurements of the CP
asymmetry related to the
angle a.
• ATLAS does not have an
event-by-eveny particle
identification, but can
separate on statistical
basis; the signals from all
significant two-body
decays of b-hadrons will
overlap:
B0dp+ p-, B0dK+ p-,
B0sK- p+, B0sK+ K-,
Lbp p-, Lbp K-;
dACP: about 0.1 .
Can provide cross-check of results
from dedicate B-experiments20
0
B
s
-
0
B
s
oscillations
• Processes considered:
– B0s  Ds-p+ and B0s  Ds-a1+ ; (Ds -fp-; fK+K-)
• Event reconstruction includes vertices and masses
reconstruction;
• Proper time resolution: rms=0.06 ps.
• Event yield in 30 fb-1:
– 7100 B0s  Ds-p+ and 2600 B0s  Ds-a1+ ;
– Background: mainly from B0dDs-a1+, B0dDs-p+
(2200 events) and from the combinatorial background
(11300 events)
21
0
B
•
s
-
0
B
s
oscillations
Dms reach evaluated with
the amplitude-fit technique;
it is measurable with more
than 5s if:
–
Dms < 22.5 ps-1; L=10 fb-1;
– Dms < 29.5 ps-1; L=30 fb-1;
The measurement significance as a
function of Dms for L=30 fb-1;
22
Rare decays with dimuons
• The decays B0sm+ m- and B0d  m+ m- have very
small BR but they can be selected by the atlas
trigger even at the nominal LHC luminosity. With
130 fb-1 of data the reaction B0sm+ m- can be
seen with 4.7s assuming the S.M. BR of 4.9 10-9.
• Another interesting class of reactions are exclusive
decays such as B0sf0m+ m - , B0dr0m+ m- ,
B0dK*0m+ m- , …
• Detailed measurements of the decays can test the
SM  search for new physics (eg AFB in decay)
23
Initial ATLAS configuration
• New radial position of the B-layer since the Yellow report
(CERN 2000-004)
• Limited resources and technical/schedule constraints
– effect: detector staging and TDAQ staging.
• Stage the following components (defer for 1-2 years)
–
–
–
–
–
The middle pixel layer (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
24
Initial detector configuration
• Main effect to the B-physics performance due to the
detector layout 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;
• Effects of the missing pixel layer under study.
25
T/DAQ Deferrals
• Temporary re-allocation of TDAQ sub-system resources
will be used to fund overcosts in common projects
–
Would lead to drastic reduction in initial HLT/DAQ system
if additional funds not obtained (only about 1/2 of the HLT/DAQ
CORE budget remaining!)
• Impact of deferrals on rate capability is difficult to
estimate.
–
–
Evaluation of rate capability versus cost requires understanding
behavior of HLT/DAQ (whose design is not yet complete) as a
function of many parameters
At this time, we use a simplified cost model with significant
uncertainties
26
T/DAQ Deferrals & LHC lum.
• The target initial luminosity was doubled to L= 2 x
1033 cm-2 s-1;
–  increase the low-pT inclusive muon trigger
threshold;
– include the low pt inclusive muon trigger with a low-pT
dimuon trigger;
• Consequences:
– CP violation: dsin2b: 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;
27
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 sin2b comparable to that of LHCb;
– Measurement of the B0s- B0s oscillations;
– Unique opportunity to search for rare B0s decays:
potential indirect evidence of new physics.
28