Potential for Standard Model physics with CMS at the LHC

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Potential for Standard Model physics with CMS at
the LHC

• Where are we with hard- and software ?
• From the Tevatron/LEP to the LHC
• Top quark, EW, QCD, B physics, ...
• How to search for the Standard Model Higgs ?

Jorgen DHondt (Vrije Universiteit Brussel) on
behalf of the CMS Collaboration HEP-EPS
Conference, Lisbon, 21-27 July 2005
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The CMS detector design sketch
76k pieces
16k pieces
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The CMS detector construction progress

• Large activities are ongoing to make
• previous sketch into reality
• Main underground cavern is ready
• Several sub-detector systems are
• completed or being completed
• Tracker serious speed-up of production,
  overall good quality of modules
• ECAL ? of barrel crystals delivered, first
  SuperClusters for endcap made
• HCAL assembled, start with electronics
  integration, calibration ongoing
• Magnet completed and succesfully tested for
  leaks
• Muons CSCs completed, RPCs being constructed
  and gradually integrated,
• 80 of DTs are completed
• Trigger boards are being produced
• CMS will be closed and ready for beam on 30
  June 2007 (T.Virdee, HCP05 talk)

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The CMS detector becoming a reality
barrel tracker silicon detectors
muon RPCs
magnet
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Example of an Event
SUSY event pT gt 1.0 GeV h lt 2.4
IGUANA low luminosity
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Example of an Event
SUSY event pT gt 1.0 GeV h lt 2.4
IGUANA high luminosity
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Current status of Simulation and Reconstruction

• Still a few years before real data hence all
  based on Monte Carlo simulation
• first data expected in 2007
• Main generator used PYTHIA 6.2
• does not include many features present in
  dedicated generators
• fast simulation the PYTHIA objects are smeared
  to mimic the detector
• the particle interactions are not simulated with
  GEANT
• Results in this presentation studies based on
  fast simulation (FAMOS)
• large efforts have been made to optimize the
  reconstruction code
• large Data-Challenge efforts have been made to
  provide dedicated
• GEANT-4 simulation (created 100M simulated
  events, 1Mb/event)
• in the process of writing a Physics - Technical
  Design Report with this
• accurate simulation and reconstruction tools
  (expected early 2006)
• Current results to be digested as an
  illustration of what can be learned from
• CMS data upon arrival

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From Tevatron/LEP to LHC
Obtaining orders in magnitude in both the
integrated luminosity and the energy, we will
collect a huge amount of Standard Model
benchmarks channels. 109 events/10fb-1 W
(200 per second) 108 events/10fb-1 Z
(50 per second) 107 events/10fb-1 tt
(1 per second) These can be used as
control/calibration samples for searches beyond
the Standard Model, but can also be used to
scrutinize even further the Standard Model.
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Top Quark Physics top quark mass
10 tt pairs per day _at_ Tevatron ? 1
tt pair per second _at_ LHC qq ?tt 85
gg?tt 87
Most important parameter is the top quark mass
(mt), to be estimated with an accuracy of around
Dmt 1 GeV/c2.
width of peak 10 GeV
Golden channel semi-leptonic tt
?bWbW?blvbqq Selection via lepton, miss.ET, 4
jets, 2 b-tags (S/Bgt20) Top mass from hadronic
side t?qqb Main systematics are the jet energy
scale Improve with kinematic fit and more
advanced statistical inference techniques are
ongoing.
fastsim 10fb-1
dmt(stat) 300 MeV (10fb-1)
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Top Quark Physics top quark mass

• From t ? l J/y X decays
• 100 fb-1 gives after selection 1,000 signal
  events (S/B gt 100)
• the large mass of the J/y induces a strong
  correlation with the top mass
• easier to identify (extremely clean sample)
• BR(overall in tt) 5.3 x 10-5
• no jet related systematics !!

CMS fast simulation
hep-ph/9912320
slope
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New method hep-ex/0501043 correlate the b
transverse decay length with mt
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Top Quark Physics top versus anti-top

• Spin correlations
• the top quark does not loss its spin information
  before it is decaying into W and b
• 
  with A 0.431 (gg) and A - 0.469
  (qQ)
• two observables q(q-) angle between t(T)
  direction
• in the tT c.m. frame and the l(l-)
  direction of flight in
• the t(T) rest frame
• fit to double differential distribution
• result (30fb-1) A (stat) 0.035 and A
  (syst) 0.028
• Measuring the difference between mt and mT
• almost all systematics cancel when measuring the
  difference between both
• after several years the precision could be
  around 50 MeV/c2

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Top Quark Physics single-top-channel
Never observed !!

• Each channel sensitive to different signals
• heavy W ? s-channel
• FCNC ? t-channel
• H ? Wt-channel
• Also directly related to Vtb to percent level
• (s-channel preferred, t-channel dominated by PDF
  scale uncertainties of 10)

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W polarization in top decays
The large top quark mass allows the W boson to be
longitudinaly polarized q defined as angle
between lepton (in W rest frame) and W (in top
rest frame) Standard Model prediction f0 mt2
/ (2 mW2 mt2) 0.7 and fR 0 (mb0)
LH (1cosq)2
Long sin2q
PYTHIA 5.7, 1 year CMS
only W?ev and W?mv
Expected uncertainty Dstat f0 0.023 Dsyst f0
0.022
estimation of systematic uncertainties
conservative most of the time limited by the
statistical precision of the effect
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Gauge Boson Couplings
Direct measurements of vector boson couplings are
possible via the cross-section measurements of
the processes in which they appear. They test the
non-Abelian nature of the Standard Model gauge
theory. Anomalous couplings or new physics can
be included in the effective Lagrangian at a
fundamental scale L.
pt-spectrum of photon sensitive to anomalous
couplings (L1.5 TeV)
g
W
Limits _at_95CL (L2TeV)
l0.3, k0
Dk,l
l0, k0.95
W
BAUR MC generator
pp?Wg
l0, k0
cross-section enhanced when anomalous couplings
are present
pt(g) (GeV)
for 100 fb-1 (L2TeV) ? Dk lt 0.1
? l lt 0.0009
large improvement for l compared to Tevatron
For ZZg and Zgg couplings both the pT(g) and the
MT(llg) spectrum are sensitive to hiV (VZ,g)
anomalous couplings
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Drell-Yan Production of Lepton Pairs
The Drell-Yan process pp?l l - is a measure for
AFB and hence sin2qefflept
e-,m-
q
Weak-mixing angle sin2qefflept can be determined
to Dsin2qefflept 0.00014 using forward lepton
tagging
Precision will exceed the magnitude of the EW
corrections up to Mll2 TeV
Z/g
q
e,m
inverse of ee- ? qq at LEP
Rel. exp. uncertainty on sll (in )
LHC reaches much higher masses
main systematic uncertainty is the knowledge of
the PDFs
10
5
0
can also use sin2qefflept to constrain the PDFs
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Parton Probability Functions
How to find the partons in the colliding protons
? ? need for precise PDF(x,Q2) Extrapolate
from HERA, but also use the huge LHC data
itself Ratio of W/W- cross-section is related
to u(x)/d(x)
y pseudorapidity
differentiate between several models
0.1 fb-1
DGLAP evolution
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Parton Probability Functions
The PDFs for the heavy quarks can also be
measured
Isolated g with high pT jet including m
Estimate 5-10 accuracy on PDFs limited by
fragmentation functions
Isolated e/m with high pT jet including m
The PDFs can be determined relative to each
other, and therefore depend on the accuracy of
the theoretical calculations. In a similar way
the gluon luminosity function can be obtained
with a 1 accuracy.
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B-physics

• The CMS detector allows a rich B-physics program
  due to its precise tracking and vertexing
• (but no Particle ID detectors, usually triggers
  on high-pT objects)
• CP violation
• measurements of Bs oscillations
• rare decays
• life-time
• Bc mesons
• etc...

all B-physics studies require a profound
knowledge of the detector performance
Example (alternative to lepton-tag method)
CKM angle b via Bd0?J/j Ks0 in B?Bd0()
p The flavour of B0 is tagged with
p Expected precision is D(sin2b)0.022
(10fb-1) (to be repeated with higher trigger
thresholds)
after selection
related to angle b
ICHEP04 sin(2b) 0.725 0.037
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Standard Model Brout-Englert-Higgs boson

• The SM-like scalar Higgs boson can be observed in
  several physics channels (depending on its mass
  mH which is a free parameter of the model, LEP mH
  gt 114.4 GeV _at_95CL)

production cross-section
decay branching ratios
balance between production rate, decay rate and
reconstruction efficiency
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Standard Model Brout-Englert-Higgs boson
qqH ? qqgg
60fb-1
H ? gg
H ? ZZ ? 4l
100fb-1
100fb-1
mH130
mH130,150,170
mH115
signal
signal
signal
qqH ? qqtt
ttH ? ttbb
30fb-1
30fb-1
H ? WW ?lvlv
30fb-1
mH135
mH115
mH140
signal
signal
signal
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Standard Model Brout-Englert-Higgs boson

• Combined discovery potential as a function of mH

zoom in low mass region
5s at 2 fb-1
WW/ZZ
at 10 fb-1
5s at 10 fb-1
gg
at 30 fb-1
at 30 fb-1
at 60 fb-1
gg
WW
ZZ
At 10 fb-1 full 5s coverage from LEP to 800 GeV
LEP limit
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Standard Model Brout-Englert-Higgs boson

• Integrated luminosity needed for 5s discovery as
  a function of mH

zoom in low mass region
30 fb-1
30 fb-1
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Ultimate test of the Standard Model

• Comparing the direct and indirect values of mH

To give mt and mW equal weight DmW 0.7
10-2 Dmt Goal of LHC experiments Dmt lt
1 GeV DmW lt 15 MeV ? DmH/mH lt 25
After a discovery one can use EW measurements to
differentiate between SM or MSSM Higgs bosons
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Outlook for CMS activities

• First fast-simulation studies (presented)
• significant improvement in Standard Model
  measurements
• Higgs boson mass range completely covered after
  10fb-1
• Current progress within the CMS Collaboration
• created large amounts of dedicated/realistic
  simulation
• large effort in optimizing our reconstruction
  methods
• gradually design more advanced analyses
  techniques
• start studies on detector calibration/alignment
  (check influence)
• Outlook for the near future
• write-up of all the above into a
  Physics-Technical Design Report
• this will summarize the physics potential of the
  experiment in great detail
• foreseen by December 2005 (Volume-I) and by
  April 2006 (Volume-II)