LIB2DHPLHC

1
CERN, Sept. 27 2006
LIB2DHP_at_LHC (Myths and Reality)
FP-420
V.A. Khoze (IPPP, Durham)
Main aims -to quantify the mjor sources of the
Lumi-Independent Backgrounds,
-to the Exclusive Diffractive H?bb Production at
the LHC - to recall the
backgrounds to the ED H???, WW channels and
to the searches of the CP-odd Higgs
(based on works A. Roeck, R. Orava and KMR,
EPJC 25391-403,2002
V. Khoze, M. Ryskin and W.J. Stirling,
hep-ph/0607134
KMR , EPJC 26229-236,2002
C34327-334,2004)
? CEDP- Main Advantages - Measure the
Higgs mass via the missing mass technique
(irrespectively of the decay channel).
-H ? opens up (Hbb Yukawa coupling)
unque signature for the MSSM sector
-Quantum number/CP filter/analyzer.
-Cleanness of the events in the central
detectors.
? If the potential experimental challenges are
resolved, then there is a very 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?
  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) soft Pomeron
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)
• ? 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

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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
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.
4
(No Transcript)
5
M
M
effect. PP lumi
(HKRSTW, work in progress).
6
RECALL
? for forward going protons LO QCD
bgd ? suppressed by Jz0 selection rule
and by the 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
for jet polar angle cut
assuming misidentification probability
P(g/b)1
? 0.2
(DKMOR-02)
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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
  ?0.06 ?
  0.2
• 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
A lot of further theoretical efforts is needed
some hopes recently A.Shuvaev Durham.
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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
main suppression
lies within
mass interval
bb
the overall suppression factor
?
Background due to central inelastic production
mass balance, again
subprocess is
strongly suppressed produces a small tail on the
high side of the missing mass
H/bb
(DKMOR-02)
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?? 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)
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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
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Hunting the CP-odd boson, A
?(LO) selection rule an attractive feature of
the CEDP processes, but ? the flip side to
this coin strong ( factor of 10²
)suppression of the CED production of the
CP-odd boson. ?A way out to allow incoming
protons to dissociate (E-flow ETgt10-20 GeV)
KKMR-04 pp? p X H/A Y p
(CDD)
in LO azim. angular dependence cos²? (H), sin²?
(A), bgd- flat
A testing ground for CP-violation studies in the
CDD processes (KMR-04)
? challenges bb mode difficult bgd
conditions ??-mode- small (QED)bgd, but low Br
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CDD results at ?? (RG) gt3, ETgt20 GeV
? within the (MS) MSSM, e.g. mh
scenarios with ? 200 (500) GeV, tan?30-50
?CDD(A-gtbb) 1-3 fb, ?CDD(A-gt??) 0.1-03
fb ?CDD(H)-?CDD(A)
max
bb mode challenging bgd conditions (S/B
1/50). ??-mode- small (QED) bkgd, but low
Br situation looks borderline at best

• ? best case (extreme) scenario
• mh with ?-700 GeV , tan? 50, mg
  10³GeV

max
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in this extreme case ?(A?gg) Br(A?bb) ? 22-24
MeV at MA160-200 GeV ,tan? ?50,
?CDD (A ?bb) is decreasing
from 65fb to 25fb (no angular cuts)

? ?CDD (A???) ? 0.8-0.3 fb
A ?
S/B ?(A-gtgg) Br (A-gtbb) /? MCD ? 5.5
/?MCD (GeV) currently ?MCD 20-30 GeV (??12GeV
at 120 GeV) Prospects of A- searches strongly
depend, in particular, on the possible progress
with improving ?MCD in the Rap. Gap
environment
We have to watch closely the Tevatron exclusion
zones
There is no easy solution here, we must work hard
in order to find way out .
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Proton Dissociative Production (experimental
issues)
(thanks to Monika, Michele , Risto Albert)
Can we discriminate between the cos²? and sin²?
experimentally ?
? Measurement of the proton diss. system with ET
of 20 GeV and 3lt?lt5 -probably OK for
studying the azimuthal distributions (HF or
FCAL calorimeters) ? Trigger is no problem if
there is no pile up (Rap Gaps at Level 1)
4jet at 210³³ lumi- borderline
Maybe we can think about adding RPs into the
trigger ( no studies so far) Maybe neutrons
triggered with the ZDC (Michele )?
? From both the theoretical and experimental
perspectives the situation with searches
for the A in diffractive processes looks
at best borderline,
but the full simulation should be performed
before arriving at a definite conclusion.
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Approximate formula for the background
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

? detailed studies of statistical significance
for the MSSM Higgs signal discovery ,
based on the CMS Higgs group procedure in
progress (HKRSTW, hopefully 2006 )

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To gain insight into the H- mass dependence
Crude approximation
? simplified formulae for stat. sign. ?
neglect M-dependence of

(more or less - MSSM with large tan
). ? neglect mass dependence of ?M.

• S/B does not depend on the current (theor.)
  uncertainties in
• ? theor. uncertaint. 1.5
• ? at large masses M 140 GeV, B is
  controllable reliably predicted.
• is practically independent on M.
• ? at low masses M lt 120 GeV
• theor. uncertaint. in of order
  2-2.5,
• decreases with M decreasing (as
  ).

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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.
• In DKMOR we required P(g/b) 0.01 and chose an
  optimistic value ( )²0.6.
• Does the CEDP environment help ?
• ? Mass window ? M3 ? (M)
• ? 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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Unknown unknowns

• a complete computation (at least on the SL
  level ) of the radiative effects for
• the HNC
• ? Experimental perspectives for the CP-odd Higgs
  studies in the proton-dissociation modes.

Known Unknowns or Unknown Unknowns ?
? Going to higher luminosities (up to 1034) ?
Pile-up. ? -an instructive example P. Bussey et
al (long-lived gluinos) hep-ph/0607264
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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