LIB2DHPLHC

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FP-420
LIB2DHP_at_LHC (Myths and Reality)
Dec. 10th, 2006
V.A. Khoze (IPPP, Durham)
Main aims -to quantify the mjor sources of the
Lumi-Independent Backgrounds,
-to Exclusive Diffractive Higgs Production at
the LHC. (based on works A. De Roeck, R.
Orava and KMR, EPJC 25391,2002 V. Khoze,
M. Ryskin and W.J. Stirling, hep-ph/0607134 B.
Cox et al. EPJC 45401,2006 KMR EPJC
26229,2002 C34327,2004 )
? CEDP- Main Advantages - Measure the
Higgs mass via the missing mass technique
(irrespectively of the decay channel).
-H ? opens up (Hbb Yukawa coupling)
unique signature for the MSSM Higgs sect.
-Quantum number/CP filter/analyzer.
-Cleanness of the events in the central
detectors.
? If the potential experimental challenges are
resolved, then there may be a real chance
that for certain MSSM scenarios the
CEDP becomes the LHC Higgs discovery channel !
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• In the proton tagging mode the dominant H?
  in principle can be observed directly .
• certain regions of the MSSM parameter space are
  especially proton tagging friendly
• (at large tan ? and M , S/B
  )

Myths
For the channel LIBs are well known and
incorporated in the (DPE )MCs Exclusive LO -
production (mass-suppressed) gg misident
soft PP collisions.
Reality
The complete background calculations are
still in progress (uncomfortably unusually
large high-order QCD and b-quark mass effects).

• About a dozen various sources known (DKMOR,
  Andy, Marek)
• ? admixture of Jz2 production.
• ? NLO radiative contributions (hard blob and
  screened gluons)
• NLLO one-loop box diagram (mass- unsuppressed,
  cut-nonreconstructible)
• ? b-quark mass effects in dijet events (most
  troublesome theoretically) still incomplete
• (similar problems in photon-photon
  collisions).

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KMR technology (implemented in ExHume)
focus on
? the same for Signal and Bgds
contain Sudakov factor Tg which exponentially
suppresses infrared Qt region ? pQCD
new CDF experimental confirmation, 2006
(Kojis talk)
S² is the prob. that the rapidity gaps survive
population by secondary hadrons ? soft
physics S² 0.026 (LHC), ? S²/b² -weak
dependence on b.
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Good ExHume description
KMR analytical results
CDF preliminary
outside-cone energy (Koji)
(Rjj includes different sources Centr. Inclus.
soft PP (rad. ) tail of Centr. Exclus.
experim. smearing)
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effect. PP lumi
(HKRSTW, work in progress).
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RECALL
? for forward going protons LO QCD
bgd ? suppressed by Jz0 selection rule
and by colour, spin and mass resol. (?M/M)
factors.
for reference purps SM Higgs (120 GeV)
? misidentification of outgoing gluons as b jets
may mimic
production
the prolific LO subprocess
SM Higgs
for jet polar angle cut
misidentification prob. P(g/b)0.01 ? B/S ?0.06
(DKMOR WishList)
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A little bit of (theoretical) jargon
Helicity amplitudes
for the binary bgd processes
g (
g
g
helicities of active gluons
S Jz0, LO B- domint. Jz2
(double) helicities of produced quarks

• ? convenient to consider separately
• q-helicity conserving ampt (HCA) and
  q-helicity non-conserving ampt(HNCA)
• do not interfere, can be treated independently,
• allows to avoid double counting (in particular,
  on the MC stage)

Symmetry argumts (BKSO-94)
?for Jz0
the Born HCA vanishes,
(usually, HCA is the dominant helicity
configuration.) ? for large angles HNCA

(Jz2, HCA)
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? in terms of the fashionable MHV rules
(inspired by the behaviour in the twistor
space) only ( - -) J_z2, HCA
(- - /-)

? an advantageous property of the
large angle amplitudes

• all HNCA (Jz0, Jz2, all orders in )
  are suppressed by
• all HC ampts ((Jz0, Jz2, all
  orders ) are \propto ? vanish at

rotational invariance around q-direction (Jz2,
PP-case only)
recall we require in order to
suppress t-channel singl. in bgd processes,
also
acceptance of the CD.. ? an additional numerical
smallness ( )
? LO HCA vanishes in the Jz0 case (valid only
for the Born amplitude)
? Jz0 suppression is removed by the presence of
an additional (real/ virtual) gluon


(BKSO-94)
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Classification of the
backgrounds
? Jz2 LO production caused by non-forward
going protons. HC process, suppressed by
and by

0.02 ( )
estimate ? NLLO
(cut non-reconstructible) HC quark
box diagrams.
result

• dominant contribution at very large masses M
• at Mlt 300 GeV still phenomenologically
  unimportant due to a combination of small
  factors
• ? appearance of the
  factor? consequence of supersymmetry

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• mass-suppressed Jz0 contribution
• ? theoretically most challenging
  (uncomfortably large higher-order effects)
• naively Born formula would give
  ?0.12 ?
  
• however, various higher-order effects are
  essential
• ? running b-quark mass Single Log effects
  (
  )
• ? the so-called non-Sudakov Double Log
  effects , corrections of order
• (studied in FKM-97 for the case of
  at Jz0 )
• Guidance based on the experience with QCD
  effects in .

F
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• bad news ? violently oscillating leading term
  in the DL non- Sudakov form-factor
• 
  (2.5)
• ? DL contribution exceeds the
  Born term strong dependence on the NLLO, scale,
  
• running mass. effects
• ?No complete SL calculations
  currently available.

Fq
HNC contribution rapidly decreases with
increasing M
Currently the best bet
Fq
with c1/2.
Taken literally ? factor of two larger than the
naïve Born term. Cautiously
accuracy, not better than a factor of 5
Further theoretical efforts are needed
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NLO radiation accompanying hard
subprocess
Large-angle, hard-gluon radiation does not obey
the selection rules
radiation off b-quarks
? potentially a dominant bgd (
) strongly exceeding the
LO expectation.
? only gluons with could be
radiated, otherwise cancel. with screening gluon
( ).
? KRS-06 ?complete LO analytical calculation of
the HC , Jz0 in
the massless limit, using MHV tecnique. Hopefully
, these results can be (easily) incorporated into
more sophisticated MC programmes to investigate
radiative bgd in the presence of realistic expt.
cuts.
,
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How hard should be radiation in order to override
Jz0 selection rule ?
(classical infrared behaviour)
as well known
neglecting quark mass
(a consequence of Low-Barnett-Kroll theorem,
generalized to QCD )
?the relative probability of the Mercedes
like qqg configuration for Jz0
radiative bgd process becomes unusually large
? marked contrast to the Higgs-gt bb
(quasi-two-jet- like) events. ? charged
multiplicity difference between the H-gtbb signal
and the Mercedes like bbg bgd
for M ?120, ?N 7, ?N rises with increasing
M.
?hopefully, clearly pronounced 3-jet events can
be eliminated by the CD, ? can be useful for bgd
calibration purposes. Exceptions ? radiation
in the beam direction ?
radiation in the b- directions.

could be eliminated DKMOR-02

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beam direction case
if a gluon jet is to go unobserved outside the
CD or FD ( )

• violation of the equality
  (limited by the )
• contribution is smaller than the admixture
  of Jz2. KRS-06

b-direction case (HCA)
0.2 ?( ?R/0.5)²
(?R separation cone size)
Note ? soft radiation factorizes ? strongly
suppressed ?is not a problem, ? NLLO
bgd ? numerically small
? radiation from the screening gluon with ptQt
KMR-02
HC (Jz2) LO ampt.
?numerically very small ? hard radiation -
power suppressed
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Production by soft Pomeron-Pomeron collisions
from DKMOR-02 (WishList)
main suppression
lies within
mass interval
z
bb
the overall suppression factor

PS
Background due to central inelastic production
mass balance, again
subprocess is
strongly suppressed produces a tail on the high
side of the missing mass
H/bb
(DKMOR-02)
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A.Pilkington, CERN 27th Sept.2006
What else should be done/checked?
1.New MRW(2006) diffractive pdf should be used,
which are close to H1 fit B much lower gluon
density at large z (MRW fit available in Durham
HEPDATA). 2. Appropriate value of survival
factor. 3.Proper cuts on low ET. .
4. Possibility to veto the Remnant Pomeron
jets (CD extra jets, maybe T1, T2 detectors to
be studied) Risto , Marek.
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Not the end of the story..
in terms of Feynman graphs Central Inelastic
Soft PP- Fusion (Pomwig)
Studied by KMR 02-06 formally
suppressed by

• In theoretical jargon CI - kT-
  factorization, Soft PP collinear
  factorization.
• CDF inclusive diffractive dijet data evidence
  of manifestation of the CI dijet production
• (both RunI and RunII).
  

Preliminary estimates We should be fine
B/S(DPE) lt0.1-1.
Currently, in the description of the Rjj lt0.8
distr.ibution, normalization to models
(Pomwig...) is fixed by the CDF data!
Normalization at Rjjgt0.8 (Exhume) comes from the
theory.
(CMS-TOTEM TDR ???)
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MAKE
HISTORY
DPEMC
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(detailed studies in B. Cox et al.
hep-ph/0505240)
WW mode

• ? No trigger problems for final states rich in
  higher pT leptons.
• Efficiencies 20 (including Br) if
  standard leptonic (and dileptonic)
• trigger thresholds are applied.
• Further improvements, e.g. dedicated
  ? -decay trigger.
• Statistics may double if some
  realistic changes to leptonic trigger
• thresholds are made.
• ? Much less sensitive to the mass resolution.
• ? Irreducible backgrounds are small and
  controllable.
• .
• Recall the h- rate can rise by about a factor
  of 3.5-4 in some MSSM
• models (e.g., small ?eff scenario).

.
KRS-05
Currently trigger situation is more optimistic.
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Lepton Trigger Efficiency Cox et al (e
or ?)
ATALS (e, ?, 2e, 2?, e?) M120 GeV
8.7
20.3
M140 GeV
12.8
26.9 M160 GeV
16.6
28.8


(Sandra Horwat, Bruce Mellado)
Pile-Up situation is hopefully bettter
?? mode ? Irreducible bgds (QED) are small
and controllable.
( gt0.1-0.2GeV) ? QCD
bgd is small if g/?- misidentification is lt 0.02
(currently 0.007 for ?-jet efficiency 0.60)
Lepton trigger efficiency looks optimistic
ATLAS ((e, ?, 2e, 2?, e?) M120 GeV

22.1 M140 GeV
25.8
M160 GeV
28.3
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Note normalisation of ExclDPE is
dependent ! (factor of 10) .
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Approximate formula for the
background assuming DKMRO-2002 wishlist input
main uncertn. at low masses
SSM/B?1 at ?M ?4 GeV
Four major bgd sources (1/4 each at M120 GeV)
? gluon-b misidentification (assumed 1
probability) ? NLO 3-jet contribution
Correlations,
optimization -to be studied. ? admixture of
Jz2 contribution NNLO effects ?b-quark mass
effects in quasi-dijet events
Recall large M situation in the MSSM is very
different from the SM. H?WW/ZZ -
negligible H? bb/??- orders of
magnitude higher than in the SM

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Four years on
M120 GeV
WishList
DKMOR (2002)
currently
5.76 GeV
(87 signal catch )
?
30-40 GeV
?
?
misidentification prob. P(g/b)1 (b-tag
efficiency) 0.6
2.5 (CMS fast simulation.) Marek 1.3 (ATLAS)
Andy
0.36
A.Rozanov (ATLAS)
PA (Monika, Marek, Andy, Andrew..)
?
no Pile-Up studies
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MSSM
Recall large M situation in the MSSM is very
different from the SM. H?WW/ZZ -
negligible H? bb/??- orders of
magnitude higher than in the SM

? detailed studies of statistical significance
for the MSSM Higgs signal discovery ,
based on the CMS Higgs group procedure in
progress (HKRSTW, hopefully first part of 2007..
)
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mhmax scenario, ?200 GeV, MSUSY 1000 GeV
h?bb
M. Tasevsky et al. (preliminary)
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H?bb
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h?WW
small ?eff scenario
PRELIMINARY
mh? 121-123 GeV
for the SM Higgs at M 120 GeV ? 0.4 fb,
at M 140 GeV ? 1 fb
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Conclusion

• ? Luminosity Independent Backgrounds to CEDP of
  H?bb do not overwhelm the signal
• and can be put under full control especially
  at Mgt 120 GeV.
• ? Luminosity Independent Backgrounds to H? WW
  ?? channels are controllable
• and could be strongly reduced.
• ? The complete background calculation is still
  in progress
• (unusually uncomfortably large
  high-order QCD effects).
• ? Further reduction can be achieved by
  experimental improvements,
• better accounting for the kinematical
  constraints, correlations..
• ? Optimization, complete MC simulation- still
  to be done

Further theoretical experimental studies are
needed
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(Mishas talk)
PU (a naïve theorists view)
Overlap background hard event (bb,WW, ??...) 2
SD events in the same bunch Xing
Roughly
we have to achieve reduction of in 6-7 orders
?correlations between measurements in CD and RPs
(?1, ?2, ?M...), ? correlations in azimuthal
angles, pt- cuts, ?use of fast timing detectors
(40-100) ( Andrew, Monika, Mike.) ? Nc ,
cuts (100, Andy) ? veto in T1, T2
detectors - Risto

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Known (un)knowns

• ?The probability to misidentify a gluon as a
  b-jet P(g/b) and the efficiency of
• tagging ?b.
• Does the CEDP environment help ?
• ? Mass window ? M3 ? (M), any further
  improvement ?
• ? Correlations, optimisation -to be studied
• ? S² (S²/b²), further improvements, experimental
  checks.
• ? Triggering issues
• Electrons in the bb trigger ?
• Triggering on the bb/?? without RP
  condition at M? 180 GeV ?
• ? Mass window ?MCD from the Central Detector
  only (bb, ?? modes) in the Rap Gap environment?
• Can we do better than ?MCD 20-30 GeV?
  Mass dependence of ?MCD ?
• (special cases inclusive bgds, hunting for
  CP-odd Higgs)

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BACKUP
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(recall also Pomwig normal. to CDF Run I Dif.
Inel data (0.27-0.1) )