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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 the Exclusive Diffractive Hbb Production at the LHC - to recall the backgrounds to the ED H, WW channels and to the searches of the CP-odd Higgs

FP-420

(based on works : A. Roeck, R. Orava and KMR, EPJC 25:391-403,2002 ; V. Khoze, M. Ryskin and W.J. Stirling, hep-ph/0607134; KMR , EPJC 26:229-236,2002; C34:327-334,2004)

CERN, Sept. 27 2006

☻ 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 !

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.

bb

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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 |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

bb

bb

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 )

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)

4

5

M M

effect. PP lumi33,( ) 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

assuming misidentification probability P(g/b)=1%

(DKMOR-02) 0.2

for forward going protons LO QCD bgd suppressed by Jz=0 selection rule and by the colour, spin and mass resol. (M/M) –factors.

RECALL :

bb

forfor reference purps : SM Higgs (120 GeV)reference purps : SM Higgs (120 GeV)

3 // /gg SSM

MB Br BrS M M

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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 ≈ 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 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 )

No 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

some hopes recently… A.Shuvaev + Durham.

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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

sinmiPP s gM M

bb bbM M

the overall suppression factor :

21(

2/ )PPbbM M ( )2PP JB B

z

Background due to central inelastic production

H/bb

mass balance, again

subprocess is strongly suppressed

produces a small tail on the high side of the missing mass

PPgg Hgg

(DKMOR-02)

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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

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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

+ + …….

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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 ET>10-20 GeV) KKMR-04

pp p + X +H/A +Y +p (CDD)

in LO azim. angular dependence: cos² (H), sin² (A), bgd- flat

challenges: bb mode – difficult bgd conditions -mode- small (QED)bgd, but low Br

A testing ground for CP-violation studies in the CDD processes (KMR-04)

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within the (MS) MSSM, e.g. mh scenarios with =±200 (500) GeV, tan=30-50

CDD(A->bb) ~ 1-3 fb, CDD(A->) ~ 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

CDD results at (RG) >3, ET>20 GeV

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A

in this extreme case : (Agg) Br(Abb) 22-24 MeV at MA=160-200 GeV ,tan 50,

CDD (A bb) is decreasing from 65fb to 25fb (no angular

cuts)

CDD (A) 0.8-0.3 fb

S/B ~ (A->gg) Br (A->bb) / 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

There is no easy solution here, we must work hard in order to find way out .

We have to watch closely the Tevatron exclusion zones

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Proton Dissociative Production (experimental issues)

(thanks to Monika, Michele , Risto & Albert)

Measurement of the proton diss. system with ET of 20 GeV and 3<<5 -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 2*10³³ lumi- borderline

Maybe we can think about adding RPs into the trigger ( no studies so far) Maybe neutrons triggered with the ZDC (Michele )?

Can we discriminate between the cos² and sin² experimentally ?

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 backgroundbb

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

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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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 SS B B

PPL

3( ) (/ )H gg M Br H bb

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 < 120 GeV theor. uncertaint. in of order 2-2.5, decreases with M decreasing (as ).

S

B /M M

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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 ( )²=0.6.

Does the CEDP environment help ?

Mass window M≈3 (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)

b,b b

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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.

PPgg qq

Known Unknowns or Unknown Unknowns ?

Going to higher luminosities (up to 10^34) ? 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 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 done

Further theoretical & experimental studies are needed