La fisica del quark
top
Fabrizio Margaroli
The last discovered quark
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Top quark was needed
to complete the third
generation of quarks
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20 years long search,
spanning 2 generations,
3 colliders, 2 continents
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Discovered in 1995 at
Fermilab by CDF and D0
collaborations
Fabrizio Margaroli
Getting into XXI
century physics!
Sapienza Università di Roma-INFN Roma 1
2
The last discovered quark
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The Standard Model is a highly predictive theory: we knew (approximately) the top quark
mass far in advance
Fabrizio Margaroli
Sapienza Università di Roma-INFN Roma 1
3
The last discovered quark
•
The Standard Model is a highly predictive theory: we knew (approximately) the top quark
mass far in advance
Fabrizio Margaroli
Sapienza Università di Roma-INFN Roma 1
4
The last discovered quark
Fabrizio Margaroli
Sapienza Università di Roma-INFN Roma 1
5
The top quark is special
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Since 1995, the most massive elementary particle, almost as massive as a gold atom (gold
atom = 79 protons + 118 neutrons + 79 electrons)
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Yes even after the Higgs discovery
Fabrizio Margaroli
Sapienza Università di Roma-INFN Roma 1
6
The top quark is special
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Since 1995, the most massive elementary particle, almost as massive as a gold atom (gold
atom = 79 protons + 118 neutrons + 79 electrons)
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Thanks to its large mass, the top quark has the largest coupling to the Higgs boson
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Yukawa(top) = √2 Mtop/vev (vev=246GeV) = 0.996±0.006
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closeness to one leads to speculation about possible special role of the top quark in the
electroweak symmetry breaking mechanism
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Fabrizio Margaroli
Sapienza Università di Roma-INFN Roma 1
7
How to produce it
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Top quarks produced once in 10^10 collisions at Fermilab (1/10 000 000 000)
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Top quarks produced once in 10^8 collisions at LHC
Fabrizio Margaroli
Sapienza Università di Roma-INFN Roma 1
8
Top quark decays
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top lifetime τ t = 1 / Γ t  5 × 10 −25 s  τ had = 1 / ΛQCD  3 × 10 −24 s
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only quark that decays before hadronizing → no top hadron spectroscopy
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top quark width = 1.3 GeV much smaller than experimental resolution
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BR(t→Wb)=99.8%∝|Vtb|2 (where Vtb is inferred from the knowledge of the other elements
of the CKM matrix, and make use of unitarity)
Fabrizio Margaroli
Sapienza Università di Roma-INFN Roma 1
9
Identifying top quarks final states
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I assume you know from earlier classes how to identify a charged and neutral leptons,
quarks (jets). Here you need one more ingredient: identifying jets originating from b-quarks
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b-quarks form B mesons, that have lifetime long enough to be able to detect its decays
through the reconstruction of displaced tracks with large impact parameter (IP), giving rise
to a secondary vertex (SV)
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typical efficiencies: 40% per Tevatron, 80% LHC. Mistags rates can be <<1%
Fabrizio Margaroli
Sapienza Università di Roma-INFN Roma 1
10
Things are just getting complicated
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Reminder: BR(top→bW)=99.8%
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BR(W→qq)=66.6%, BR(W→leptons)=33%, so final state depends on how the two W
decay.
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If both W bosons decay leptonically “dilepton” channel
Fabrizio Margaroli
Sapienza Università di Roma-INFN Roma 1
11
Things are just getting complicated
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BR(top→bW)=99.99%
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BR(W→qq)=66.6%, BR(W→leptons)=33%, so final state depends on how the two W
decay.
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If only one W decays leptonically and the other hadronically, “semi-leptonic decays”
Fabrizio Margaroli
Sapienza Università di Roma-INFN Roma 1
12
Things are just getting complicated
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BR(top→bW)=99.99%
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BR(W→qq)=66.6%, BR(W→leptons)=33%, so final state depends on how the two W
decay.
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If both W bosons decay hadronically, then “all-hadronic” final state
Fabrizio Margaroli
Sapienza Università di Roma-INFN Roma 1
13
In case you are not good at math...
Fabrizio Margaroli
Sapienza Università di Roma-INFN Roma 1
14
How to identify a collision giving a top pair
t→Wb→qq/lnu b thus for each top quark in the event:
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identify the b jet
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identify the other jets/identify a lepton and/or neutrino
Fabrizio Margaroli
Sapienza Università di Roma-INFN Roma 1
15
How to identify a collision giving a top pair
t→Wb→qq/lnu b thus for each top quark in the event:
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identify the b jet
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identify the other jets/identify a lepton and/or neutrino
Fabrizio Margaroli
Sapienza Università di Roma-INFN Roma 1
16
How to identify a collision giving a top pair
t→Wb→qq/lnu b thus for each top quark in the event:
•
identify the b jet
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identify the other jets/identify a lepton and/or neutrino
Fabrizio Margaroli
Sapienza Università di Roma-INFN Roma 1
17
How to identify a collision giving a top pair
t→Wb→qq/lnu b thus for each top quark in the event:
•
identify the b jet
•
identify the other jets/identify a lepton and/or neutrino
Fabrizio Margaroli
Sapienza Università di Roma-INFN Roma 1
18
Backgrounds
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Being a “small” signal -> σ(ttbar)=10-10 σ(total), so anything end up looking just as top
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Of course, it depends on what signature you are looking to
Dilepton: 2 opposite sign leptons,
jets, missing transverse energy
Dilepton
Physics
background
Instrumental
background
Fabrizio Margaroli
Drell-Yan+jets
fake leptons(QCD)
Sapienza Università di Roma-INFN Roma 1
19
Backgrounds
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Being a “small” signal -> σ(ttbar)=10-10 σ(total), so anything end up looking just as top
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Of course, it depends on what signature you are looking to
Lepton+jets: 1 charged lepton,
large missing transverse energy, jets
Physics
background
Instrumental
background
Fabrizio Margaroli
Dilepton
Lepton+jets
All-hadronic
Drell-Yan+jets
W+jets
QCD bb+jets
fake leptons(QCD) fake leptons(QCD)
Sapienza Università di Roma-INFN Roma 1
QCD qq+jets
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Backgrounds
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Being a “small” signal -> σ(ttbar)=10-10 σ(total), so anything end up looking just as top
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Of course, it depends on what signature you are looking to
All-hadronic: many jets,
at least one b-jet
Physics
background
Instrumental
background
Fabrizio Margaroli
Dilepton
Lepton+jets
All-hadronic
Drell-Yan+jets
W+jets
QCD bb+jets
fake leptons(QCD) fake leptons(QCD)
Sapienza Università di Roma-INFN Roma 1
QCD qq+jets
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How to distinguish signal from background
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After you require jets, leptons and missing transverse energy, S/B 1/5 (lepton+jets)
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remember, top is heavy, you need 2XMtop = 350GeV to produce a pair
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backgrounds have lower (MW,MZ) or none (QCD) thresholds - low objects Pt
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Ht=scalar sum of transverse momentum
of all final state objects
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etamax=max eta of the jets
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b-jets are much rarer than light flavor jets
Fabrizio Margaroli
Sapienza Università di Roma-INFN Roma 1
22
One more step in complexity
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Sometime life is harder (all-hadronic channel) and you have to combine the statistical
power of several observables into one - likelihood ratios, neural networks, boosted
decision tree, etc.
Fabrizio Margaroli
Sapienza Università di Roma-INFN Roma 1
23
Ok let’s assume you have good S/B now...
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...how do you know it’s really a top quark?
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start pairing things in order to get the right resonances. How to do this?
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Depends on the final state
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Remember the flow: tt→bWbW→bqq(lnu)bqq(lnu)
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Let’s pick tt→bWbW→blnubqq
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One charged and a neutral lepton uniquely identify a W boson
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If you identify both b quarks, you have two b’s, one matches with the leptonic W.
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If you identify only one b quark, larger number of combinations
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What to do if you have so many combinations? Pick the one that seems most
“reasonable”
Fabrizio Margaroli
Sapienza Università di Roma-INFN Roma 1
24
How do you know it’s really a top quark?
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Choose the combination where the dijet
invariant mass most reasonably match a W
boson invariant mass, the lnu system most
reasonably matches the other W, and the b
quarks rearrange in such a way as to have 2
Mtop distributions reasonably similar. In other
words:
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choose the value of the top mass that
minimizes a chi square
Fabrizio Margaroli
Sapienza Università di Roma-INFN Roma 1
25
Some W and top invariant masses
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From O(10) evts for discovery,
[email protected]
Fabrizio Margaroli
to O(1000) for precision,
[email protected]
Sapienza Università di Roma-INFN Roma 1
to O(10000)
pp@7TeV
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Anatomy of a peak
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Peak is a Breit Wigner. Then distortion by PDF. Add instrumental resolution effects. Top
with combinatorial background, and extra jets coming from initial and final state radiation
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Question, if you approximate the peak with a gaussian, sigma is 27GeV, you have 30 evts
with negligible background, what uncertainty do you expect to measure on Mtop?
Fabrizio Margaroli
Sapienza Università di Roma-INFN Roma 1
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Mtop summary
Fabrizio Margaroli
Sapienza Università di Roma-INFN Roma 1
28
Mtop summary
Fabrizio Margaroli
Sapienza Università di Roma-INFN Roma 1
29
Systematics
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Any idea?
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Let’s keep it simple: everything you
measure has an uncertainty.
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There are experimental uncertainties:
how often you identify a lepton, jet, b-jet,
etc. Also, their energy/momentum.
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There are theoretical uncertainties: how
much you (the theoretician?) understand
the underlying physics.
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Factorize problems: uncertainty on the
perturbative part of the process (qq→tt)
and uncertainty in the non-perturbative
part (q→jet)
Fabrizio Margaroli
Sapienza Università di Roma-INFN Roma 1
30
From the top (and W) mass, to the Higgs
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Just as the value of Mtop was predicted from other SM observables, Mhiggs can be
predicted by using existing precision measurements
Continuous improvements in Mt
and Mw lead to increasing
constraints on MHiggs.
!
Also, SM consistency check in
case of discovery
Fabrizio Margaroli
Sapienza Università di Roma-INFN Roma 1
31
From the top (and W) mass, to the Higgs
•
Just as the value of Mtop was predicted from other SM observables, Mhiggs can be
predicted by using existing precision measurements
Continuous improvements in Mt
and Mw lead to increasing
constraints on MHiggs.
!
Also, SM consistency check in
case of discovery
Fabrizio Margaroli
Sapienza Università di Roma-INFN Roma 1
32
From the top (and W) mass, to the Higgs
•
Just as the value of Mtop was predicted from other SM observables, Mhiggs can be
predicted by using existing precision measurements
Continuous improvements in Mt
and Mw lead to increasing
constraints on MHiggs.
!
Also, SM consistency check in
case of discovery
Fabrizio Margaroli
Sapienza Università di Roma-INFN Roma 1
33
From the top (and W) mass, to the Higgs
•
Just as the value of Mtop was predicted from other SM observables, Mhiggs can be
predicted by using existing precision measurements
Continuous improvements in Mt
and Mw lead to increasing
constraints on MHiggs.
!
Also, SM consistency check in
case of discovery
Fabrizio Margaroli
Sapienza Università di Roma-INFN Roma 1
34
From the top (and W) mass, to the Higgs
•
Just as the value of Mtop was predicted from other SM observables, Mhiggs can be
predicted by using existing precision measurements
Continuous improvements in Mt
and Mw lead to increasing
constraints on MHiggs.
!
Also, SM consistency check in
case of discovery
Fabrizio Margaroli
Sapienza Università di Roma-INFN Roma 1
35
From the top (and W) mass, to the Higgs
•
Just as the value of Mtop was predicted from other SM observables, Mhiggs can be
predicted by using existing precision measurements
Continuous improvements in Mt
and Mw lead to increasing
constraints on MHiggs.
!
Also, SM consistency check in
case of discovery
Fabrizio Margaroli
Sapienza Università di Roma-INFN Roma 1
36
From the top (and W) mass, to the Higgs
•
Just as the value of Mtop was predicted from other SM observables, Mhiggs can be
predicted by using existing precision measurements
Continuous improvements in Mt
and Mw lead to increasing
constraints on MHiggs.
!
Also, SM consistency check in
case of discovery
Fabrizio Margaroli
Sapienza Università di Roma-INFN Roma 1
37
From the top (and W) mass, to the Higgs
•
Just as the value of Mtop was predicted from other SM observables, Mhiggs can be
predicted by using existing precision measurements
Continuous improvements in Mt
and Mw lead to increasing
constraints on MHiggs.
!
Also, SM consistency check in
case of discovery
Fabrizio Margaroli
Sapienza Università di Roma-INFN Roma 1
38
From the top (and W) mass, to the Higgs
•
Just as the value of Mtop was predicted from other SM observables, Mhiggs can be
predicted by using existing precision measurements
Continuous improvements in Mt
and Mw lead to increasing
constraints on MHiggs.
!
Also, SM consistency check in
case of discovery
Fabrizio Margaroli
Sapienza Università di Roma-INFN Roma 1
39
From the top (and W) mass, to the Higgs
•
Just as the value of Mtop was predicted from other SM observables, Mhiggs can be
predicted by using existing precision measurements
Continuous improvements in Mt
and Mw lead to increasing
constraints on MHiggs.
!
Also, SM consistency check in
case of discovery
Fabrizio Margaroli
Sapienza Università di Roma-INFN Roma 1
Range on x/y axes
now 25% smaller!
40
From the top (and W) mass, to the Higgs
•
Just as the value of Mtop was predicted from other SM observables, Mhiggs can be
predicted by using existing precision measurements
Continuous improvements in Mt
and Mw lead to increasing
constraints on MHiggs.
!
Also, SM consistency check in
case of discovery
Fabrizio Margaroli
Sapienza Università di Roma-INFN Roma 1
Range on x/y axes
now 25% smaller!
41
From the top (and W) mass, to the Higgs
•
Just as the value of Mtop was predicted from other SM observables, Mhiggs can be
predicted by using existing precision measurements
Continuous improvements in Mt
and Mw lead to increasing
constraints on MHiggs.
!
Also, SM consistency check in
case of discovery
!
Mhiggs ∼90±30GeV
Fabrizio Margaroli
Sapienza Università di Roma-INFN Roma 1
Range on x/y axes
now 25% smaller!
42
From the top (and W) mass, to the Higgs
•
Just as the value of Mtop was predicted from other SM observables, Mhiggs can be
predicted by using existing precision measurements
!
!
!
Continuous
improvements in Mt
and Mw lead to increasing
!
constraints
on MHiggs.
!
! SM consistency check in
Also,
case of discovery
!
!
Mhiggs
∼90±30GeV
!
Range on x/y axes
now 25% smaller!
!
!
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Same game can be played in non-SM scenarios. Discovered particle still consistent with
BSM
Fabrizio Margaroli
Sapienza Università di Roma-INFN Roma 1
43
A lot to be measured about the top
Fabrizio Margaroli
Sapienza Università di Roma-INFN Roma 1
44
Up in energy: from Tevatron to LHC
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Fabrizio Margaroli
why W and Z boson cross section increases
by a factor of 5, while ttbar a factor of 25 in
going from [email protected] to pp@7TeV?
Sapienza Università di Roma-INFN Roma 1
45
Top rediscovered at the LHC
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Comparison in top production rates at the Tevatron (10years [email protected]) and LHC (1
year pp@8TeV)
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TeV
LHC@8TeV
Lumi/hour (pb-1/h)
0.4*
14*
!
sigma(pb)
7.5
220
!
Nevts top/h
3*
3000*
Lumi total
10
16
Nevts top total
?
?
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*numbers refer to Tevatron and LHC best stores respectively
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The LHC is a top
factory!
plus b-tag efficiency is twice larger, and ATLAS/CMS have a factor of two larger eta coverage for leptons/jets
Fabrizio Margaroli
Sapienza Università di Roma-INFN Roma 1
46
What to do with all those tops?
Differential cross section
measurements
Fabrizio Margaroli
Sapienza Università di Roma-INFN Roma 1
47
Interesting processes involving top
Top (@7 TeV)
σ
1,000,000 fb
100,000 fb
10,000 fb
1,000 fb
100 fb
tt
t
ttɣ
ttW ttZ
ttH
10 fb
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Probe strong interactions in the top sector (qq→gluon→ttbar)
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Probe electromagnetic intearactions in the top sector (qq→ttɣ)
Fabrizio Margaroli
Sapienza Università di Roma-INFN Roma 1
48
Single top quark production
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Three main categories of electroweak single top quark production, each one is interesting
in its own way
s-channel
t-channel
Wt channel
Collider
s-ch
t-ch
Wt
Tevatron
LHC@7TeV
1
5
2
65
0.05
16
Do you know why the ratios are so different?
Fabrizio Margaroli
Sapienza Università di Roma-INFN Roma 1
49
Single top observation in 2009
Fabrizio Margaroli
Sapienza Università di Roma-INFN Roma 1
50
Single top observation in 2009
Fabrizio Margaroli
Sapienza Università di Roma-INFN Roma 1
51
The measurements today
Tevatron
LHC
Fabrizio Margaroli
Sapienza Università di Roma-INFN Roma 1
52
Other nice measurements: ttW, ttZ
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ttW and ttZ are very rare SM processes. Interesting per se, they are also backgrounds to
new physics searches.
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How to look for such a rare and complex final state? Same sign leptons!
W
Fabrizio Margaroli
Sapienza Università di Roma-INFN Roma 1
53
Where do we stand
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ttH is difficult: cross section 130±20fb (@8TeV) approx 1/100 of gg→H
?
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CDF/CMS/ATLAS
investigated semileptonic
and dileptonic decays of
ttbar, and bbar decays of H
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Limits ranging from
4.5xSM(CMS) to
10.5xSM(ATLAS) using 5fb
@7TeV
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Ttbar in lepton+jets,
Plus dilepton
Fabrizio Margaroli
Sapienza Università di Roma-INFN Roma 1
54
Conclusions
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Top quark physics is a fascinating topic since 1995
Entered the LHC era, top stats have been increasing one
order of magnitude each year!
Interesting connections to electroweak symmetry breaking
Pivotal for new physics exploration at higher and higher mass
scales
!
!
more on new physics and top quarks next time
Fabrizio Margaroli
Sapienza Università di Roma-INFN Roma 1
55
Some useful reading
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Top is recent so you won’t find much on textbooks
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An exception: you can find the discovery papers on the Goldhaber Cahn
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Review papers are very useful at graduate level (graduate = MS, PhD) if you want to know
more on a specific topic
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A top mass review: L.Galtieri, I.Volobuev, myself - Rept.Prog.Phys. 75 (2012) 056201 arXiv:
1109.2163
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Recent reviews on top quark physics:
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W.Bernreuther, Top quark physics at the LHC. J.Phys. G35 (2008) 083001 http://
arxiv.org/pdf/0805.1333.pdf
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Particle data group http://pdg.lbl.gov/2012/reviews/rpp2012-rev-top-quark.pdf
Fabrizio Margaroli
Sapienza Università di Roma-INFN Roma 1
56