Introduction WG4 MPI and low x and diffraction (theory talks) - PowerPoint PPT Presentation

Introduction WG4 — MPI and low x and diffraction (theory talks) Martin Hentschinski ICN-UNAM (Mexico City, Mexico) 
 + BUAP (Puebla, Mexico) 
 + UDLAP (Puebla, Mexico) martin.hentschinski@gmail.com


 MPI & low x — why care? multi-parton interactions become usually important in the • intermediate and low x region (phase space) in this region: standard fixed order DGLAP description (may) • lose its validity. 
 Reason: α S ln(1/x)~ 1 → need to be resumed to all orders within QCD perturbation theory, this is achieved by the BFKL • equation can affect the MPI analysis •

example: forward-backward diets (Mueller-Navelet jets) PDF vs. G PDF picture taken from 1602.01882 for long time one of the standard processes to search for BFKL effects at the LHC → signal: • decorrelation in azimuthal angle with increasing rapidity 
 [Duclou, Szymanowski, Wallon; 1309.3229], [Celiberto, Ivanov, Murdaca, Papa; 1504.08233], … MPIs can/could give such a decorrelation! • recent BFKL studies: extend this to 3 and 4 jets 
 • [Caporale, Chachamis, Murdaca, Sabio Vera; 1508.07711], [Caporale, Chachamis, Gordo Gomez, Murdaca, Sabio Vera; 1606.00574]

low x → high energy factorisation → k T factorization z 1 , k 1 ,T high energy factorisation & BFKL • evolution provide cross-section in the z 2 , k 2 ,T low x region as convolution of 
 a) kT (TMD) dependent coefficients 
 z 3 , k 3 ,T b) kT (TMD) dependent parton distribution functions z 4 , k 4 ,T z 5 , k 5 ,T main advantage: treat kinematics with • higher accuracy (“approximate NLO”), z 6 , k 6 ,T can/could affect size MPI contributions •

Double parton scattering in 4-jet production (Mirko Serino, Cracow) 4 jet production within high energy factorisation (=off-shell initial partons) • study combination of single and double parton contributions • Question: how to maximize the double-parton scattering (DPS) contribution in • four-jet production by selecting kinematical cuts? Automated calculations for MPI (Andreas van Hameren, Cracow) Monte-Carlo code for automatic calculations of hard single- and multi parton • scattering processes both kT factorisation and collinear factorisation, all tree-level matrix elements for • SM processes

At some (very small) x: saturation effects Q 2 s (Y) s a t u r a t i o n Geometric r e g i o n Scaling non-perturbative region HERA data, BFKL: power like rise of the gluon • Y = ln 1/x BK/JIMWLK distribution ~x - λ , 
 unitarity: most slow down & stop at some value of x BFKL mechanism: high density effects; system • DGLAP characterised by correlation length/saturation scale Λ 2 ln Q 2 QCD ~ 1/k T α s ~ 1 α s < < 1 2 ) k T φ (x, k T max. density saturation scale acts as an effective cut- • off of k T -dependence at small k T know how to ? do physics here Q s 1 < 1 α s ∼ Λ QCD α s < k T

Exploring minijets beyond leading power (Piotr Kotko, Penn State) MPI models in Monte-Carlos rely on mini-jet Xsec, derived from collinear • factorization � � ∼ α 2 p 2 d σ 2jet s T . dp 2 p 4 differential Xsec. divergent for small p T • T T the MPI model was constru introduce energy dependent cut-off p T >p T,min (s) • Two questions investigated within high energy factorisation → off-shell initial • patrons! 
 i) cut-off provided by initial off-shell gluons? 
 ii) minijet suppression away from the small p T region?

MPI & low x diffraction ‘cut’ Pomeron: high multiplicity ‘uncut’ Pomeron: elastic/diffractive events (total Xsec.) scattering (amplitude level) A ( s, t ) σ tot = 1 s = m A ( s, t = 0) at cross-section level: multi-parton scattering (if resolved)

Hard diffractive processes in the kT- factorisation approach (Marta Luszczak, Rzeszow) diffractive production of open charm, t • p a p a bottom mesons and diets at the LHC x IP F D g ( x 1 , k 2 1 t , µ 2 ) Y Pomeron: Ingelmass-Schlein model + • k 1 t ̸ = 0 β 1 absorptive corrections Q ¯ Q x 2 parton distributions: kT-factorization • k 2 t ̸ = 0 (from collinear pdfs using KMR p b X description) F g ( x 2 , k 2 2 t , µ 2 )