1 LIB (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/ ; B. Cox et al. EPJC 45:401,2006; KMR: EPJC 26:229,2002; C34:327,2004 ) FP-420 Dec. 10 th, 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.

2 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) ŽNLLO 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).  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 )

3 focus on  the same for Signal and Bgds contain Sudakov factor T g which exponentially suppresses infrared Q t region  pQCD new CDF experimental confirmation, 2006 S² is the prob. that the rapidity gaps survive population by secondary hadrons  soft physics; S² = (LHC),  S²/b² - weak dependence on b. KMR technology (implemented in ExHume) (Koji’s talk)

4 KMR analytical results CDF preliminary Good ExHume description (Rjj includes different sources : Centr. Inclus. + soft PP + (rad. ) tail of Centr. Exclus. + experim. smearing) outside-cone energy (Koji)

5 effect. PP lumi (HKRSTW, work in progress).

6 the prolific LO subprocess may mimic production  misidentification of outgoing gluons as b –jets for jet polar angle cut (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 : for reference purps : SM Higgs (120 GeV) misidentification prob. P(g/b)=0.01  B/S  0.06 SM Higgs (difference in a factor of 2 ? )

7 A little bit of (theoretical) jargon Helicity amplitude s for the binary bgd processes g g g (g ( helicities of ‘active gluons’ (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)  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

8  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 (-+ ; -+ /+-) an advantageous property of the large angle amplitudes all HNCA (Jz=0, Jz=2, all orders in ) are suppressed by all HC ampts ( (Jz=0, Jz=2, all orders ) are \propto  vanish at recall : we require in order to suppress t-channel singl. in bgd processes, also acceptance of the CD..  an additional numerical smallness ( ) rotational invariance around q-direction (Jz=2, PP-case only) LO HCA vanishes in the Jz=0 case (valid only for the Born amplitude) LO HCA vanishes in the Jz=0 case (valid only for the Born amplitude)

9 Classification of the backgrounds  |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. ≈ 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 supersymmetry

10  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 ). Complete one-loop result is known (G. Jikia et al ) -complete calculation of SL effects ( drastically affects the result) F=

11 -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. 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

12 NLO radiation accompanying hard subprocess + + 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, 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., Large-angle, hard-gluon radiation does not obey the selection rules

13 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

14 beam direction case if a gluon jet is to go unobserved outside the CD or FD ( ) v iolation of the equality : (limited by the ) contribution is smaller than the admixture of Jz=2. KRS-06 b-direction case (HCA) 0.2  (  R/0.5)² Note :  soft radiation factorizes  strongly suppressed is not a problem,  NLLO bgd  numerically small  radiation from the screening gluon with p t~Qt : HC (Jz=2) LO ampt. ~ numerically very small  hard radiation - power suppressed KMR-02 (  R –separation cone size)

15 Production by soft Pomeron-Pomeron collisions bb main suppression : lies within mass interval the overall suppression factor : 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 (DKMOR-02) from DKMOR-02 (WishList) z PS

16 A.Pilkington, CERN 27 th Sept 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?

17 Not the end of the story….. Preliminary estimates :We should be fine : B/S(DPE) < in terms of Feynman graphs: Central Inelastic Soft PP- Fusion (Pomwig) Studied by KMR 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). 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.

18 (recall also Pomwig normal. to CDF Run I Dif. Inel data ( ) )

19 MAKE DPEMC HISTORY

20 WW mode (detailed studies in B. Cox et al. hep-ph/ )  No trigger problems for final states rich in higher p T 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 in some MSSM models (e.g., small  eff scenario). KRS ……. Currently : trigger situation is more optimistic.

21  mode ● Irreducible bgds (QED) are small and controllable. ( > GeV) QCD bgd is small if g/  - misidentification is < 0.02 (currently ~0.007 for  -jet efficiency 0.60) 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

22 Approximate formula for the background assuming DKMRO-2002 wishlist input main uncertn. at low masses S SM /B  1 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 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

23 DKMOR (2002) currently Four years on M=120 GeV 5.76 GeV WishList  (87% signal catch )  GeV misidentification prob. P(g/b)=1% (b-tag efficiency) 0.6  2.5% (CMS fast simulation.) Marek 1.3% (ATLAS) Andy 0.36  no Pile-Up studies PA (Monika, Marek, Andy, Andrew..) A.Rozanov (ATLAS )

24 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 )

25 mhmax scenario,  =200 GeV, M SUSY =1000 GeV h  bb M. Tasevsky et al. (preliminary)

26 H  bb

27 small  eff scenario for the SM Higgs at M = 120 GeV  = 0.4 fb, at M= 140 GeV  = 1 fb m h  GeV h  WW PRELIMINARY

28 Conclusion  Luminosity Independent Backgrounds to CEDP of Hbb 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

29 PU (a naïve theorist’s view) Overlap background: hard event (bb,WW, ...) +2 SD events in the same bunch Xing Roughly : (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

30 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  M CD from the Central Detector only (bb,  modes) in the Rap Gap environment? Can we do better than  M CD ~20-30 GeV? Mass dependence of  M CD ? (special cases: inclusive bgds, hunting for CP-odd Higgs)

31 BACKUP

32 8 (KMR- based estimates) (more on the pessimistic side, studies based on the CMS Higgs group procedure –still to come)

33 PU (a naïve theorist’s view) PU? Overlap background: hard event (bb,WW, ...) +2 SD events in the same bunch Xing Roughly :