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LIB2DHP@LHC (Myths and Reality)
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 25:391,2002 ; V. Khoze, M. Ryskin and W.J. Stirling, hep-ph/0607134; B. Cox et al. EPJC 45:401,2006;
KMR: EPJC 26:229,2002; C34:327,2004 )
FP-420
Dec. 10th, 2006
☻ 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 !
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.
bb
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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 |Jz|=2 production.
NLO radiative contributions (hard blob and screened gluons)
③ 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).
bb
bb
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 )
bb
250 20
3
2 2
22
2
2 22
( ,...) ( , ) ( ,.
(
..)
)
harp f
ff
dp ef
e
MM L M y
LM S L M
y M
focus on 2( ,...)bgd
hard M2( , )effL M y the same for Signal and Bgds
effL
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.
KMR technology (implemented in ExHume)
(Koji’s talk)
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KMR analytical results
( ) 0.8 ( )T TE meas E part
( ) 0.75 ( )T TE meas E part
CDF preliminary
Good ExHume description
(Rjj includes different sources : Centr. Inclus. + soft PP + (rad. ) tail of Centr. Exclus. + experim. smearing)
outside-cone energy (Koji)
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effect. PP lumi 33,( ) 1/SM SMH gg M M
(HKRSTW, work in progress).
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the prolific LO subprocess PPgg gg may mimic bb production
misidentification of outgoing gluons as b –jets
for jet polar angle cut 60 120
(DKMOR WishList)
for forward going protons LO QCD bgd suppressed by Jz=0 selection rule and by colour, spin and mass resol. (M/M) –factors.
RECALL :
bb
forfor reference purps : SM Higgs (120 GeV)reference purps : SM Higgs (120 GeV)
misidentification prob. P(g/b)=0.01 B/S 0.06
23 ( [ ( / ) / 0.01 1](120 / )4 )/ / 4M Ge P gM V b
SM Higgs
(difference in a factor of 2 ? )
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A little bit of (theoretical) jargon
Helicity amplitudes for the binary bgd processes
g gg (
i helicities of ‘active gluons’
q (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)
1 2( ) for Jz=0 the Born HCA vanishes,
(usually, HCA is the dominant helicity configuration.)
for large angles HNCA
Symmetry argumts (BKSO-94)
(Jz=2, HCA)
S – Jz=0, LO B- domint. Jz=2
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Jz=0 suppression is removed by the presence of an additional (real/ virtual) gluon (BKSO-94)
in terms of the fashionable MHV rules (inspired by the behaviour in the twistor space)
only (+ - ; + -) J_z=2, HCA (-+ ; -+ /+-)
0
an advantageous property of the PPgg qq large angle amplitudes
sall HNCA (Jz=0, Jz=2, all orders in ) are suppressed by
all HC ampts ((Jz=0, Jz=2, all orders ) are \propto vanish at
/q TEmPPgg cos 90
recall: we require in order to suppress t-channel singl. in bgd processes, also acceptance of the CD.. an additional numerical smallness ( )
60 120
1/ 5
rotational invariance around q-direction (Jz=2, rotational invariance around q-direction (Jz=2, PPPP-case only)-case only)
LOLO HCHCAA vanishes in the Jz=0 case (vanishes in the Jz=0 case (valid only for the Born amplitudevalid only for the Born amplitude))
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Classification of the backgrounds PPgg qq
|Jz|=2 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.
2cos /1M GeV
≈ ≈ 22 2 22 2// cos32 ( ) sins Fq CNM Cm
result result
dominant contribution at very large masses M
at M< 300 GeV still phenomenologically unimportant due to a combination of small factors
appearance of the factor consequence of supersymmetry2( )F CC N
3 // / SMSM Br BrB S M M
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mass-suppressed Jz=0 contribution
theoretically most challenging (uncomfortably large higher-order effects)
naively Born formula would give 0.06 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 Jz=0 )
Guidance based on the experience with QCD effects in . DL effects can be reliably summed up( FKM-97 , M. Melles, Stirling, Khoze 99-00 ). Complete one-loop result is known (G. Jikia et al. 96-00 )
-complete calculation of SL effects ( drastically affects the result)
2(8 / ) ln ( / ) 1S bM m F=
5 // / SMSM Br BrB S M M
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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.
2 ....)(1(1 2 / )FC FN C
HNC contribution rapidly decreases with increasing M
Fq=
Currently the best bet: :
Fq ≈
with c≈1/2.
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
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PPgg qqNLO radiation accompanying hard subprocess
+ + radiation off b-quarks
potentially a dominant bgd :( ) strongly exceeding the LO expectation.
2/ ( / )S bm M
only gluons with could be radiated, otherwise cancel. with screening gluon ( ).
t tp Q
KRS-06 complete LO analytical calculation of the HC , Jz=0
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.
2| |gg qqgM
3 // / SMq SMq g S Br BrB S M M
,
1/t tg d Q
Large-angle, hard-gluon radiation does not obey the selection rules
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How hard should be radiation in order to override Jz=0 selection rule ?
as well known (classical infrared behaviour)
neglecting quark mass
(a consequence of Low-Barnett-Kroll theorem, generalized to QCD )
the relative probability of the Mercedes –like qqg configuration for Jz=0 radiative bgd process becomes unusually large
marked contrast to the Higgs-> bb (quasi-two-jet- like) events.
charged multiplicity difference between the H->bb 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 Jz=2. KRS-06
sinmis g bbM M
b-direction case (HCA)
0.2 ( R/0.5)²
Note : soft radiation factorizes strongly suppressed is not a problem, NLLO bgd numerically small
bbgg
radiation from the screening gluon with pt~Qt :
HC (Jz=2) LO ampt. ~ numerically very small
hard radiation - power suppressed
KMR-02
cos2( / )t tQ p
(R –separation cone size)
bbM
t tM p Q
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Production by soft Pomeron-Pomeron collisions
bb
main suppression : lies within
mass interval
inmiP gP ssM M
bb bbM M
the overall suppression factor :
2 21( (1 )
2/ ) /PP P PPbb bbM g MM M
Background due to central inelastic production
H/bb
mass balance, again
subprocess is strongly suppressed
produces a tail on the high side of the missing mass
PPgg Hgg
(DKMOR-02)
from DKMOR-02 (WishList)
z
PS
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A.Pilkington, CERN27th Sept.2006
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.
What else should be done/checked?
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Not the end of the story…..
Preliminary estimates :We should be fine : B/S(DPE) <0.1-1.
in terms of Feynman graphs: Central Inelastic Soft PP- Fusion (Pomwig)
Studied by KMR 02-06 formally suppressed by 2S
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).
Currently, in the description of the Rjj <0.8 distr.ibution, normalization to models (Pomwig...) is fixed by the CDF data! Normalization at Rjj>0.8 (Exhume) comes from the theory.
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(recall also Pomwig normal. to CDF Run I Dif. Inel data (0.27-0.1) )
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MAKEDPEMC
HISTORY
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WW mode(detailed studies in B. Cox et al. hep-ph/0505240)
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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mode
● Irreducible bgds (QED) are small and controllable. ( >0.1-0.2GeV)
QCD bgd is small if g/- misidentification is < 0.02
(currently ~0.007 for -jet efficiency 0.60)
tp
Lepton Trigger Efficiency Cox et al (e or ) ATALS (e, , 2e, 2, e)
M=120 GeV 8.7% 20.3% M=140 GeV 12.8% 26.9%
M=160 GeV 16.6% 28.8% (Sandra Horwat, MPI)
Lepton trigger efficiency looks optimistic: ATLAS ((e, , 2e, 2, e)
M=120 GeV 22.1% M=140 GeV 25.8% M=160 GeV 28.3%
Pile-Up situation is hopefully bettter
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Approximate formula for the background assuming DKMRO-2002 wishlist inputbb
main uncertn. at low masses
SSM/B1 at ΔM 4 GeV
Four major bgd sources (~1/4 each at M≈120 GeV)
gluon-b misidentification (assumed 1% probability) NLO 3-jet contribution Correlations, optimization -to be studied. admixture of |Jz|=2 contribution + NNLO effectsb-quark mass effects in quasi-dijet events
Recall : large M situation in the MSSM is very different from the SM.
HWW/ZZ - negligible; H bb/- orders of magnitude higher than in the SM
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DKMOR (2002) currently
Four years on
13 GM eV
M=120 GeV
5.76 GeV
WishList
(87% signal catch )
10CDM GeV 30-40 GeV
misidentification prob. P(g/b)=1%
(b-tag efficiency) 0.6
2( )b
2.5% (CMS fast simulation.) Marek1.3% (ATLAS) Andy
2( )b 0.36
no Pile-Up studiesPA (Monika, Marek, Andy, Andrew..)
A.Rozanov (ATLAS)
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MSSM
Recall : large M situation in the MSSM is very different from the SM. HWW/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 GeVhbbM. Tasevsky et al. (preliminary)
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Hbb
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small eff scenario
for the SM Higgs at M = 120 GeV = 0.4 fb, at M= 140 GeV = 1 fb
mh 121-123 GeV
hWW PRELIMINARY
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Conclusion
Luminosity Independent Backgrounds to CEDP of Hbb do not overwhelm the signal and can be put under full control especially at M> 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 doneFurther theoretical & experimental studies are needed
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PU (a naïve theorist’s view)
Overlap background: hard event (bb,WW, ...) +2 SD events in the same bunch Xing
2( 1) / 2 ( 1) ( 2) / inhP dU artN N RP RP
(220 ) / 0.03; (420) / 0.01.t tm
Roughly : *( ) 100 ; ( ) (1) .in
hard hardinbb nb WW nb
(Misha’s talk)
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….)
Nc , cuts (~100, Andy)
veto in T1, T2 detectors - Risto
CN
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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 M≈3 (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)
,b b
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BACKUP
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8
(KMR- based estimates)
(more on the pessimistic side, studies based on the CMS Higgs group procedure –still to come)
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PU (a naïve theorist’s view)
PU?
Overlap background: hard event (bb,WW, ...) +2 SD events in the same bunch Xing
2( 1) / 2 ( 1) ( 2) / inhP dU artN N RP RP
(220 ) / 0.03; (420) / 0.01.t tm Roughly : *( ) 100 ; ( ) (1) .in
hard hardinbb nb WW nb