Not Present in the Standard Model
Possible anomalous Zgg/ZZg contribution to ppZgATLAS: Precision
Reach and Couplings
Neutral Triple Gauge CouplingsCouplings specified by 4
parameters: fiV with i=4,5 and V=Z,gPossible anomalous ZZg/ZZZ
contribution to ppZZ:Couplings specified by 8 parameters: hiV with
i=14 and V=Z,gExample: Fit to the pT spectrum of the g=>Again,
the sensitivity is in the tail of the distribution
I will report on some studies made to assess what can be done to
investigate the electroweak sector and measure the top mass at the
LHC. This is a somewhat random selection of SM studies.I will also
discuss the underlying event as this is an area where recent
Tevatron data has been uselfulLHC will operate at luminosities from
1033cm-2s-1 giving to 10fb-1 per year for ~ years 1-3 and the rise
to 1034cm-2s-1 giving integrated luminosities of 100fb-1
SM processes have cross-sections of order of ~nanobarns ~10M
events/yr rates ~1Hz.Therefore for precision measurements we will
have negligible statistical errors systematics limited, but even
this can be helped by large statistics samples of control or
calibration channels. We shall see this in the precision
measurements of W and top-mass.the Tevatron analyses allow us to
estimate the systematics and how they scale with statistics.Also
important to tune and understand limitations theoretical models eg
W-massv important for precision measurements
It also allows searches for rare SM processes such as single-top
and triple gauge couplings
However operating at a high luminosity hadron collider comes at
a price, as I used to say somewhat scathingly a hadron collider is
like throwing two dustbins at each other (trashcans at the Tevatron
!) and hoping that you will get a , there is the underlying event
of the hard-interesting-scatter and accompanying minimum bias
events-soft proton scatters on average ~1.7/crossing at low
luminosity and ~17/crossing at high luminosity. This is an area
where we can learn from recent tevatron studies.
The CMS detector is: a multi-purpose 4 detector (with coverage
up to || = 5) designed to exploit the physics of proton-proton
collisions at a centre of mass energy of 14 TeV over the full range
of luminosities expected at the LHC. CMS is designed to measure the
energy and momentum of photons, electrons, muons and other charged
particles with high precision, resulting in excellent mass
resolution for many new particles.
The interaction region is surrounded by a powerful inner
tracking system based on fine-grained micro-strip and pixel
detectors.
Outside the tracker is the calorimetry, which consists of a
finely segmented Lead Tungstate electromagnetic calorimeter
extending to || = 1.479 in the barrel region and to || = 3.0 in the
endcaps. The sampling hadron calorimeter is made of plastic
scintillator tiles inserted between copper absorber plates.
Outside the calorimetry is the high magnetic field
superconducting solenoid coupled with a multilayer muon system. The
muon spectrometer consists of drift tubes, cathode strip chambers
and resistive plate chambers.
The CMS magnet with a field of 4 Tesla will be the largest
superconducting magnet system in the world: the energy stored into
it, if liberated, will be large enough to melt 18 tons of gold
Precision SM measurement 1The W-mass and top-mass should be
measured as they are fundamental parameters of the SM and because
they can be used to indirectly determine the Higgs mass, this will
not discover the Higgs but it will allow a check of the SM at the
level of radiative corrections.To ensure that one measurement does
not dominate the other in the EW fit, the errors on the two masses
are related by equation above.Given that the top-mass can be
measured ~2GeV, we need to measure the W-mass to ~20MeV. This will
constrain the Higgs mass at the level of 30% -- 30% of what !
Measure the transverse mass in the Wl+nu channel. Get the
neutrino from the recoil against the lepton and underlying event
check this. The mass is then measured by simulating the mt
distribution over a range of masses and comparing to the
experimentally measured distribution. This requires a good
knowledge of the physics and the detector.
A clean W-signal is found by applying the cuts given:The
jet-veto and the recoil cut ensures that we are dealing with low-pt
Ws. For large ptW the mt resolution and the QCD background get
worse.
Very large statistics, we get ~10M events of each lepton
flavour/year, so there is negligible statistical error <
2MeV.
Lepton scale is the most challenging, but there is a chance
thanks to the large statistics Z->ll sample
ATLASCurrently muons are limited by understanding of B-field in
toroids, limits energy scale to 0.1%. Requires Z->ll sample, but
this needs understanding of energy loss of muons in calorimeter at
the level of 10MeV in 4GeV. Electrons will also use Z->ll but
this should be easier.
There are only small extrapolations required going from Z->ll
energy scale to the W->lnu scale. This is one of the limitations
at the Tevatron where they originally relied on J/Psi->ll and
required a large extrapolation to the W-scale. At run-2 can now use
Z->ll but statistics are limited. However Tevatron experience
does show that such systematic errors do scale with statisitcs.
check
Resolutions from test-beam, MC and in-situ using Z->ll and
E/p from W->e+nu. Used successfully at the Tevatron. Limited by
Z+gamma.
Recoils can be modelled using Z->ll.
W-Pt spectrum, again use Z->ll, use Pt-Z spectrum, may
improve with theoretical work on ratio of W and Z pt
distributions
pdfs enter by modifying the longitudinal boost of the W-system
which can change lepton acceptances. Cannot use the Afb as used at
the Tevatron but look at pseudorapidity distribtuions of leptons
from W and Z decays -check Dittmar refWhy can we not use Z->ll
for pdfs ? Similar longitudinal momenta ?
This is based on W-width being measured to 30MeV at the
Tevatron. Can be improved using LHC data eg fits to high
Mt-tail.
Radiative decays shift W-mass, need to improved theoretical
models
Recent work on studies of W-polarisation have shown that
theoretical uncertainties due to higher order QCD+QED corrections
can shift the W-mass by ~10MeV. Therefore it is important the
models are developed an benchmarked at the Tevatron.Todays combined
measurements from the Tevatron give a top mass of 174.3 5.1
GeV.
Precision measurement of the top quark mass provides several
test of the SM and together with MW sets constraints on the mass of
the Higgs boson.
t t-bar production is a main background to new physics processes
for example the production and decay of the Higgs boson and SUSY
particles.
In addition top events can be used to calibrate the calorimeter
jet scale and precision measurements in the top sector can provide
information on the fermion mass hierarchy.
At low luminosity, 1033, we expect a Next to Leading Order cross
section of around 833 pb, approximately 8 million events. At high
luminosity, we expect around 108 events.
Of these we expect 2.5 million single lepton decays, and 4
hundred thousand di-lepton decays.
The semi-leptonic channel is the best channel for the top mass
measurement at he LHC. We can also use the purely hadronic
channel.Measured in semileptonic channel, with the mass
reconstructed from the invariant mass of the t->jjb system.This
can achieve a precision of ~1.3MeV.
Plot shows top+bg including signal from W->tau+nu, background
dominated byt W-decays.
Similar to the W-mass this relies on getting ~1% for the
jet-energy scale. This can be achieved using Tevatron techniques
such as Z/gamma-jet events supplemented with W->jj from tt
events for light quarks. Can use a further sample of Z+b events for
b-jet energy scale. Get ~0.2GeV from light quarks and ~0.7GeV for
b-jet energy scale assuming this is at 1% level.The remaining main
systematic error is then FSR ~1.0GeV.
60k events eff=2.5%, purity ~65% with single b-tag for +/- 35MeV
around peakThe effect of the final state radiation can be reduced
by doing a constrained kinematic fit. The light-quark jet mass and
the leptonic masses are both constrained to the W-mass, the
combinations with the two identified b-jets are taken to be the
same and equal to the top-mass estimator.The FSR and effect of
energy lost through decay neutrinos result in poor x-sq fits.The
top-mass is estimated by taking samples from slices of x-sq fit and
fitting a gaussian to them. This results in a plot of mass vs x-sq
as shown and the top mass is taken as the intercept at
x-sq=0.Systematic error on b-mass from FSR is reduced to 0.5GeV,
taking it as 20% of the shift between switching FSR on and off.The
error is then dominated by the effect of the b-jet energy
scale.Measure top in high pt-sample~200GeV.Events generated with
m-hard scatter>200GeVLow statistics sample of ~4k events in
10fb-1:Select events with:pass triggerone isolated lepton with
pt>30GeV eta 30GeV>4 jets reconstructed with R=0.4,
pt>40GeV and eta235GeV. This is 2% efficiency and ~3.6k events
for 10fb-1.
Reconstruct the cluster invariant mass by summing calorimeter
cells directly, this is done for dR=0.8-1.8. Get the mass from
fitting a gaussian around each cluster distribution.The mass
increases with cluster size due to underlying event
contribution.
The resulting mass needs to be re-scaled, not surprising as no
mass scale has been imposed.Can start with a MC modelling to
determine re-scaling, but apply method to top-decay on Wdecays.
Clean signature that can be used at high luminosity highly
suppressed BR=3x10-5, giving ~1k events/100fb-1The top mass is
partially reconstructed from the lepton and J/Psi from the b-decay.
This is correlated with top-mass using Monte Carlo. The main
problem is how well the production and decay are described by the
MC, but this can be tuned to the data.
The M(lJ/psi) has a better correlation with the top mass
compared to other measures such as isolated lepton+mu-in-jet, as
the J/Psi carries a greater part of the b-momentum.
This requires good knowledge of the physics in the MC to relate
the measured mass to the top mass.Semileptonic channels we have
already discussed.Di-leptons are characterised by tow high pt
isolated leptons, large Et-miss and two b-jets.Get ~400k events for
10fb-1Cuts are:two opposite leptons with pt>35Gev and 25Gev in
eta40GeV and two jets with pt>25GeV.Get 80K signal with a
S/B~10Backgrounds are Drell-Yan, Z->tautau+jets, WW+jets and
bbbar production.To find top mass do kinematic fit using various
input top masses. The weights of the best solution for a given top
mass are found. For each top mass the mean weight over all events
is found, the maximum average weight corresponds to the measured
mass.Efficiency-purity is 97.6%-73%.Constraints are conservation of
pt in ttbar system assuming pt=0; lnu systems are constrained to
Mw; lnuj systems are constrained to Mtop, used as an input.
Multi-jet channel, S/B=3x10-8 at production. Kinematic cuts get
to S/B=1/19 and after kinematic fit and x-sq cut get 1/2.6, and
finally limiting the mass window to 130-200GeV, get 6/1. The
efficiency for ttbar is 0.18% but large statisitcs allow for this
still giving 6660 events/10fb-1.
The top mass can be measured in a number of different ways, that
mostly have different dominant systematic errors so that
combinations of these measurements, and combinations of the expts
should certainly allow the top mass error to get below ~1
GeV.Single top production, which is interesting given the hints
from H1 can be observed, again because of the x-sect.This will
allow a measurement of Vtb, complementing that from BR.
Also sensitive to new physics
Look at deviations from SM couplings using Baur et al MC.ATGCs
will increase xsect but this is very difficult to measure due
luminosity and theoretical uncertainties (and presumably pdfs ?).
However the deviations will typcially appear for high mass pairs
and depends on angles. Pt gamma or Z is the most sensitive
variable.main background mis-id of jets as photonsNote quite
comparing the same thing. The k0 and lamda0 are energy independent
terms that go with k(s) and lamda(s), ie they are independent of a
form factor. The value depends on partonic cms energy and form
factor used.The ATLAS result is based on using a mass cut-off upto
3 TeV which is the scale at which the limits as a fn of mass are
asymptotic. The limits here are unitarity safe and presented
without any cutoff or form factor.Typically sensitive for form
factors >6TeV, results for form factors ~10TeV (need to check
with Paul). For lower form factors the efficiency drops off.CDF
underlying event data, shows that the event activity in the
transverse region. The default of 1.9GeV is too high. Need a value
closer to 2.2GeV, however this breaks the idea of unifying the
soft-physics model of min-bias and the underlying event.The cuts
used for this analysis were:Exactly one isolated lepton of pT >
20 GeVMissing energy of > 20 GeVAt least four jets with ET >
40 GeVExactly 2 b jets with ET > 50 GeVTransverse mass of W
lepton < 100 GeV Here we have the mass of the reconstructed top
after all cuts including the contribution of background processes
as indicated. The contributing fraction of signal events with tau
leptons from the leptonically decaying W boson and the negligible
background are superimposed. Both the signal and background
correspond to the integrated luminosity of 10 fb-1 according to the
LO cross sections. The dominant background process is the W
production.
The dependence of the reconstructed top mass on the generated
top quark mass is shown on the right; it is linear.
The predicted error on the top mass from the semi-leptonic
channel is < 1.3 GeV, from the di-lepton channel it is < 2
GeV and from the J/psi decays, < 1GeV ( this has low statistics,
the Branching ratio is 5x10-5 Looking at dN/deta from CDF and UA5
we can see, somewhat crudely, that the default value of
pt-min=1.9GeV gives the best description of the data. Increasing
pt-min reduces the event activity, as can be seen from the
pt-min=2.1 and 2.3 lines.
