This is a report of the low energy and flavour physics working group at WHEPP-8, held at the Indian Institute of Technology, Mumbai, India, during 5–16 January 2004.

The branching ratio for $B_{s} \rightarrow \ell^{+} \ell^{−}$ γ mode is of the same order as $B_{s} \rightarrow \ell^{+} \ell^{−}$, since there is no helicity suppression in the 3-body decay mode. New Physics beyond Standard Model may affect these rates favourably for experimental observation at LHC and simultaneous measurements of the modes $B_{s} \rightarrow \mu^{+} \mu^{−}$ and $B_{s} \rightarrow \mu^{+} \mu^{−} \gamma$ at LHC experiment will indicate the basic nature of the interaction at play. A simulation study has been performed to evaluate the potential of CMS detector to observe the more difficult mode of $B_{s} \rightarrow \mu^{+} \mu^{−} \gamma$. An upper limit of $2.08 \times 10^{−7}$ on the branching ratio is expected to be achieved corresponding to an integrated luminosity of 10 fb^-1.

In several scenarios of Beyond Standard Model physics, the invisible decay mode of the Higgs boson is an interesting possibility. The search strategy for an invisible Higgs boson at the Large Hadron Collider (LHC), using weak boson fusion process, has been studied in detail, by taking into account all possible backgrounds. Realistic simulations have been used in the context of CMS experiment to devise a set of event selection criteria which eventually enhances the signal contribution compared to the background processes in characteristic distributions. In cut-based analysis, multi-jet background is found to overwhelm the signal in the finally selected sample. With an integrated luminosity of 10 fb^-1, an upper limit of 36% on the branching ratio can be obtained for Higgs boson with a mass of 120 GeV/c² for LHC energy of 14 TeV. Since the analysis essentially depends on the background estimation, detailed studies have been done to determine the background rates from real data.

In several scenarios of beyond Standard Model physics a new heavy resonance is invoked which may decay preferentially, to a pair of taus. Identification of the decay of Standard Model $Z$ resonance to tau pairs at LHC via subsequent decays of the taus to leptons as well as hadrons is the first step towards the discovery. A method has been suggested to discriminate $Z$ to tau pair to electron $+$ muon final state against various backgrounds, for early phase of 14 TeV LHC.

As $B_{s}$ -mesons will be produced abundantly at the LHC, the observability of the flavour-changing-neutral-current decay mode $B_{s} \rightarrow \phi \mu^{+} \mu^{-}$ has been studied in CMS at the LHC centre-of-mass energy of 10 TeV. With an integrated luminosity of 100 pb^-1, an upper limit of $6.7 \times 10^{−6}$ on the branching ratio is expected to be obtained. The potential at 7 TeV with a luminosity of 1 fb^-1 is expected to be better .

Drell–Yan process at LHC, $q\bar{q} \to Z/ \gamma^{\ast} \to \ell^+ \ell^-$, is one of the benchmarks for confirmation of Standard Model at TeV energy scale. Since the theoretical prediction for the rate is precise and the final state is clean as well as relatively easy to measure, the process can be studied at the LHC even at relatively low luminosity. Importantly, the Drell–Yan process is an irreducible background to several searches of beyond Standard Model physics and hence the rates at LHC energies need to be measured accurately. In the present study, the methods for measurement of the Drell–Yan mass spectrum and the estimation of the cross-section have been developed for LHC operation at the centre-of-mass energy of 10 TeV and an integrated luminosity of 100 pb^-1 in the context of CMS experiment