Transcript Muons from single top events for DC04

Muons from single top events for DC04
S. Marcellini – INFN Bologna
Single top is produced via the following mechanisms:
a) q’b  t q’’ (x-sec ~ 240 pb)
b) b g  t W
(x-sec ~ 90 pb)
c) q’q’’  t b
(x-sec ~ 10 pb)
These processes are sources of di-muon events, with different
kinematic properties.
Pythia (default) is used, with the complex model for multiple interactions
The process c) was not considered.
Muons from single top: process a)
• The cross section is 242 pb, (about 40 % of the ttbar total cross section).
• Events were required to have at least 2 muons with pt1 > 20 GeV and
pt2 > 10 GeV in the range |h| < 2.5.
• The acceptance of this selection is 0.6 %
• The observed x-sec is therefore 1.5 pb, before any other selection cut
(missing Et, isolation, Minv etc…)
• Events were generated using Pythia with MSEL=0, MSUB(83)=1,
MSTP(7)=6.
Muons from single top: process a)
The Minv > 10 GeV cut
accepts 87 % of the events
The fraction of events with
Wmn (W from top decay)
is 85 %.
The x-sec. of
4 muon events (20,10,3,3)
is 9 fb, before any
selection cut
Muons from single top: process a)
Assuming the same criteria used for the other DC04 sampes (i.e.
enough events to determine the background with 10 % precision)
~ 100 K events with pt1>20 GeV, pt2>10 GeV and Minv> 10 GeV are
needed.
It should take ~ 1 days with the standard 100 machines previously used to
define the needed CPU time.
Due to the small CPU time needed, it is not necessary to force the decay
Wmn.
Muons from single top: process b)
The process g b  t W has a x-sec of ~ 90 pb, but it is a more serious
Background than process a, due to the kinematic and topology of the muon
produced in the final state.
A special request for this
sample has been asked by
M. Dittmar of the Higgs group,
of similar statistics as a)
Process b) has s(20,10)=2pb,
compared to 1.5 pb of process a).
s(20,10,3,3)=20fb compared
To 9 fb of process a)
Minv distribution has longer
tail for process b)
Muon Rates from overlapping events
S. Marcellini and A. Perrotta (INFN Bologna)
1): study whether single muons or di-muons from a single minimum
bias event, and overlapping other types of events containg muons (es:
other MB events, W  mn, Z  mm etc…) can contribute to the di-muon
or the 4-muon background.
2): study how single muons from different min. bias events
occuring at the same BX can contribute to the di-muon background.
An average number of 3 MB events per BX are were considered.
These results are obtained with Pythia. They do not take into
account detector simulation and event selection efficiency, and they
could easily vary by some factors.
The aim of this study is to point out the potentially dangerous cases.
Four muon background
At a given BX, the probability to have a muon with pt > 5 GeV, in Pythia,
is P(5) = 0.75 * 10 - 3 at low luiminosity (3 MB events/BX).
In Pythia the cross-section for 3 muon events with pt > 20, 10, 5, is
s(20,10,5) = 0.09 nb. Therefore the cross section of events with 4 muons
with pt > 20,10,5,5, three from a MB event and one from another
overlapping MB event is s(20,10,5)·P(5)=70 fb.
This has to be compared with the cross section to have 4 muons all from
the same min. bias event, s(20,10,5,5) = 12 pb.
If we believe that the latter will be killed by the Higgs selection, it is
reasonable to believe that the other one will be killed too.
(The visible cross section for the Higgs into 4 muons is ~ 1 fb)
Four muon background
The probability of di-muon events pt1>5,pt2>5 in min. bias events
(N.B. both muons from the same event) is P(5,5)  0.75 * 10 -5
A potential background might come from the superposition of such an event
with a Zmm event, which has an observed cross-section of ~ 1nb.
Therefore the cross section of 4-muon events is
s (Z mm)  P(5,5)  7.5 fb
The cross section is therefore of the same order of magnitude of the signal.
 Check of the properties of (5,5) two muon events, to estimate whether
this could be an important background
Four-muon background
Mu-pair Minv, with no other
selection cuts
Minv + isol(.OR.) + opp. Charges:
reduction factor ~ 5 (from 7.5 to 1.5 fb).
(it goes to ~ 0.5 fb with isol (.AND.)
Requiring muons come from a common vertex gives an additional reduction
factor ~ 10 (V. Bartsch-Higgs group). A dedicated sample of < 1000 events
would be enough to rule-out this potential background
Di-muon Background
The di-muon (20,10) cross section from MB events is 0.6 nb.
This potential background to WWmnmn is expected to be completely
killed by missing Et, isolation, Minv cuts etc…
The cross section of di-muon events from two overlapping MB events is
3.5 pb (not a problem if we assume similar properties to MB di-muon
events, as indeed shown by checks done on fully reconstructed events)
Mixed events Wmn + single mu from M.B:
-5
s(Wmn, pt > 20 GeV) ~ 10 nb (no selection cuts); P(10) = 3*10
Therefore s(20,10) from combined events is ~ 300 fb
The signal visible cross section is ~ 5 fb.
N.B. This background is before any event selection cuts like missing
Et, isol, inv mass etc…
It is reasonable to think that these cuts will kill this background to a
negligible level, but a dedicated sample of few thousand events could be
helpful.
Summary (low luminosity)
Di-Muon Process
X-sec (pb)
Four Muon Process
X-sec (pb)
MB(20,10)
600
MB(20,10,5,5)
12
MB(20)+MB(10)
3.5
MB(20,10,5)+MB(5)
0.07
Wmn (20) + MB(10)
0.3
Zmm (20,10) + MB(5,5)
0.0075
ttbar(20,10)
20
ttbar(20,10,5,5)
0.4
ttbar(20)+MB(10)
0.008
ttbar(20,10,5)+MB(5)
0.003
Higgs signal
0.005
Higgs signal
0.001
might need a small dedicated sample for a more detailed study